Selective Oxygen-Assisted Reactions of Alcohols and Amines

Subscriber access provided by University of Otago Library

Review

Selective oxygen-assisted reactions of alcohols and amines catalyzed by metallic gold: Paradigms for the design of catalytic processes Michelle L Personick, Robert J. Madix, and Cynthia M. Friend ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02693 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Selective oxygen-assisted reactions of alcohols and amines catalyzed by metallic gold: Paradigms for the design of catalytic processes Michelle L. Personick1, Robert J. Madix2*, and Cynthia M. Friend2,3* 1

Department of Chemistry, Wesleyan University, Middletown, CT 06459, USA

2

Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University,

Cambridge, MA 02138, USA 3

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138,

USA *Corresponding authors: [email protected]; [email protected] Abstract Metallic gold has emerged as a highly successful catalyst for selective oxygen-assisted coupling reactions of alcohols and amines to yield industrially important classes of molecules, including esters and amides. In expanding the substrate scope of this class of reactions, fundamental mechanistic principles determined for simple model systems have been used to predict new reactive pathways for other more complex molecular transformations. Given the importance of these fundamental reaction paradigms, this review aims to consolidate the understanding of oxygen-assisted coupling mechanisms on metallic gold catalysts across a broad range of reported studies, including gas phase and liquid phase systems. Further, the review indicates areas where additional understanding is still needed, and where collaboration between the gas phase and liquid phase catalysis communities would be instrumental in the elucidation of detailed reaction mechanisms. Keywords: gold, heterogeneous catalysis, alcohols, selective oxidation, coupling, esters, amides 1. Introduction 1.1

Industrial importance of coupling products

The synthesis of esters is a key industrial process and esters are used extensively as solvents, plasticizers, and surfactants.1 In addition, ester bonds are found in a number of important products in the food (ex. flavors), cosmetics (ex. fragrances), and textile (ex. polyvinyl acetate) industries.1 Further, esters are common synthetic intermediates in the pharmaceutical industry, 1

ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 53

such as in the synthesis of aspirin.1 Other closely related organic species, including amines and imines, also have a significant role in pharmaceutical production and various industrial applications. Nearly 16% of all reactions carried out in medicinal chemistry are acylations of amines with activated carboxylic acids to yield amides, making acylation the most common reaction in pharmaceutical synthesis.2-3 Like amides, esters are also commonly produced via coupling reactions with carboxylic acids, however, in the case of esters, the coupling is between a carboxylic acid and an alcohol, rather than amine.4 While effective, one disadvantage of these coupling reactions with carboxylic acids is that the condensation reaction between the carboxylic acid and the alcohol or amine generally requires a stoichiometric amount of a toxic coupling agent to activate the carboxylic acid. As a result, recent efforts have focused on developing more sustainable approaches to the synthesis of these important chemicals, in particular, the synthesis of esters and amides through the direct aerobic coupling of two alcohols or the coupling of an alcohol and an amine, respectively. Achieving high yields of esters or amides using this route in traditional synthetic organic methodology requires the use of an acid or base catalyst and consequently still yields toxic and caustic by-products. Alternatively, the introduction of a heterogeneous nanostructured gold (Au) catalyst can significantly improve the efficiency of this direct coupling under both liquid phase and gas phase reaction conditions while also producing only benign by-products, such as water. The use of heterogeneous noble metal catalysts in industrial catalysis has a strong historical precedent; indeed, silver (Ag) metal and platinum (Pt) metal powders or meshes have been used for large-scale industrial processes such as ethylene oxidation5 and sulfur dioxide (SO2) oxidation to sulfur trioxide (SO3) for sulfuric acid production.6 Understanding of the fundamental elementary steps in oxygen-assisted coupling reactions of alcohols and/or amines can guide the design of the most active and selective Au-based catalysts. Historically, reaction mechanisms for newly discovered reactions and new catalytic systems have often been defined by analogy to existing synthetic mechanisms. Because the direct oxidative coupling of alcohols was first carried out by synthetic organic chemists without a heterogeneous catalyst, the mechanisms of this reaction in the liquid phase upon addition of a heterogeneous Au catalyst have largely been drawn in analogy to the solution phase organic chemistry. However, the fundamental difference between the heterogeneously catalyzed reaction and the uncatalyzed 2


Page 3 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reaction, both in the liquid phase, is the presence of a gold surface. Thus, it is also necessary to consider the potential for new surface-mediated pathways on the Au that may not have been possible without the catalyst, and which might be more favorable than standard solution phase chemistry. These alternative routes can be explored by analogy to gas phase oxidative coupling reactions, which have extensively studied reactivity on the Au surface both in model systems and on working nanostructured Au catalysts (Figure 1). The same surface studies also elucidate the chemistry behind the unexpected catalytic activity of Au.

Figure 1. The mechanisms of oxygen-assisted coupling on Au in gas phase and in liquid phase reactions can be determined by drawing analogies to surface-based and solution phase chemical reactions known from fundamental surface science experiments and from organic chemistry precedents, respectively. These mechanisms are further enhanced by studying reactivity at multiple length scales and under different reaction conditions. Clockwise from top: transmission electron microscopy (TEM) image of the atomic structure at the surface of a Au catalyst; TEM image of Au nanoparticles supported on titania (TiO2); scanning electron microscopy (SEM) image of the nanoscale surface structure of nanoporous gold (np(Ag)Au), an unsupported Au catalyst; SEM image of the microstructure of a hollow np(Ag)Au shell; computational model of 3


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 53

oxygen adsorption on a Au surface, scanning tunneling microscopy (STM) image of oxygen atoms on a Au surface. Adapted with permission from references 7, 8, and 9. Copyright 2007 and 2016 Elsevier, Inc. and 2015 American Chemical Society. 1.2 The fundamental basis for oxygen-assisted catalysis on gold surfaces It is common knowledge that metallic gold is relatively chemically inert. Ironically, it is this property that imbues gold with the capacity for very selective catalytic chemistry, provided that the surface can be induced to activate the substrates of interest. It was clearly shown in the 1980’s that atomic oxygen on an otherwise clean surface of gold would facilitate such activation for a host of substrates, including CO, alcohols, amines, unsaturated hydrocarbons, aldehydes and ketones.10-12 The intermediates formed from this specific activation reaction then react in predictable fashion due to the limited reactivity of the gold itself. Coincident with this discovery it was found that gold particles supported by reducible oxides would catalyze the oxidation of CO13-14 and propylene.15 The principles governing these surface reactions are simple and powerful, and pertain particularly to reactions on silver and gold surfaces. They can be cast into concise chemical language. First, on these metals adsorbed atomic oxygen acts as a Brønsted base; it generally reacts specifically with acidic hydrogens in the reactant, (using the gas phase acidity as a guide). Similar reactivity pertains to adsorbed OH. Overall, designating the substrate as BH, 2BH + e- + O¯ads → 2B¯ads + H2O,

(1)

where the charges are taken to be formal charges. Subsequent reactions are then determined by the fate of the adsorbed conjugate base, B¯ads, that then determine the overall product selectivity of these oxygen-assisted reactions. For example, hydride elimination from B¯ads leads to the oxidative dehydrogenation of the substrate BH. As will be described below, alcohols are readily transformed to aldehydes. Moreover, B¯ads itself can be nucleophilic, leading to its attack of coadsorbed electrophiles and, subsequently, a rich set of coupling reactions. The second important reaction of adsorbed O derives from its nucleophilicity, which leads to the attack of electron deficient centers in other substrates. For example, 4


Page 5 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

O¯ads + H2CO → H2CO2¯ads → HCOO¯ads + Hds → CO2 + H2 + e,

(2)

(Again the charges are indicated to preserve balance; whereas, in reality they are determined by the bonding to the surface of the metal, which acts as an infinite source and sink for electrons). For heterogeneously catalyzed reactions in the gas phase these principles govern the oxygenassisted catalysis of metallic gold. For reactions in solution there is interplay between reactions on the surface and homogeneous processes. This review addresses this fascinating subject, endeavoring to consolidate the understanding of oxygen-assisted reactions on metal-based gold catalysts across the wide spectrum of research to date. 2. Deriving a fundamental mechanism from model studies The sustainability of chemical synthesis can be improved by designing catalytic systems that operate at moderate temperatures, exhibit high selectivity to the desired product, and avoid the use of environmentally incompatible reactants or by-products. Oxygen-activated Au is selective toward partial aerobic oxidation of alcohols at low temperatures and thus is an optimal platform for the design of highly selective reactive pathways toward methyl ester synthesis. As a result, significant effort has been devoted to developing mechanistic models to predict the reactivity of alcohols on Au catalysts. A model system for such studies is methyl formate synthesis over Au via the self-coupling of methanol studied at low pressure on single-crystalline gold. The Au(111) surface was chosen initially because the facets of supported Au nanoparticle catalysts and of nanoporous gold (see section 3.1.1) are also primarily of (111) orientation. Because bulk Au does not dissociate molecular oxygen, for the model studies the Au(111) surface was precovered with prescribed amounts of atomic O (denoted as ‘O/Au(111)’) by using an oxygen source with a higher chemical potential, e.g. ozone,16 gaseous O atoms,17-19 or electron stimulated dissociation of NO2.20-22 The mechanism of ester synthesis determined using this model system has been applied to predict activity on materials with far greater materials complexity, such as nanoporous gold (np(Ag)Au) (an unsupported porous AgAu alloy- section 3.1.1). Analogous mechanisms have also been shown to be operative in selective oxidative coupling on supported Au catalysts, in the coupling of longer chain alcohols and benzylic alcohols, and in competitive coupling reactions between different alcohols. Therefore, we will first discuss this model system to illustrate the fundamental mechanisms that define oxidative coupling of alcohols on Au, and then extend the 5


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 53

discussion to the application of these mechanisms to more complex reactants and catalyst materials. In accord with the basic reactivity of adsorbed O outlined in section 1.2, the initiating step is the activation of the alcohol O-H bond in methanol by the adsorbed oxygen atom to yield adsorbed methoxy.23 The presence of the adsorbed methoxy intermediate was confirmed using a combination of vibrational and kinetic spectroscopies (see below) and isotopic labeling. On the O-precovered Au(111) surface vibrational (high resolution electron energy loss: HREEL) spectroscopy revealed a peak with a frequency of 370 cm-1, which is characteristic of oxygen chemisorbed at three-fold sites on the Au(111) surface (Figure 2).24-25 Following dosing of methanol onto the O/Au(111) surface at 160 K, this peak was extinguished and a strong feature appeared at 1060 cm-1 due to the formation of adsorbed methoxy.24,26 Concomitantly water is formed, immediately desorbing from the surface. With further heating, reaction with residual oxygen leads to the evolution of gaseous methyl formate and formaldehyde and the appearance of adsorbed formate due to secondary oxidation.

Figure 2. Vibrational (HREEL) spectra of intermediates formed during the reaction of methanol with O/Au(111). Spectra from bottom: O/Au(111) prepared by depositing 0.1 monolayer (ML) of atomic oxygen from ozone at 200 K; surface after dosing of methanol at 160 K; surface after heating the methanol layer to 200 K, 255 K, and 600 K. The methoxy signal decreases with 6


Page 7 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

increasing temperature, indicating the formation of gaseous products. At 255 K some formation of surface adsorbed formate is observed as a result of secondary oxidation of formaldehyde due to the high oxygen coverage. After annealing at 600 K, the clean Au(111) surface is regenerated. The peak at 0 cm-1 is the elastic peak, to which the vibrational losses are referenced. Adapted with permission from reference 24. Copyright 2009 Wiley-VCH. After formation of the adsorbed methoxy the second critical reaction step in the catalytic cycle is the formation of adsorbed formaldehyde via C-H bond cleavage in methoxy (here defined as ‘βhydrogen elimination’, since this carbon is two atoms from the surface). This is shown clearly by the evolution of formaldehyde in the temperature-programmed reaction spectra (TPRS) in which isotopic labeling of certain functional groups within the reactant(s) was employed to obtain critical information for understanding the mechanism of the surface reactions.27-28 This βhydrogen elimination is the rate-limiting step in methanol self-coupling on Au surfaces.24 Comparison of the results with the two isotopes of methanol clearly shows that the ester results from the overall reaction of two methoxy species. Reaction of formaldehyde itself with the adsorbed methoxy further confirms the mechanism.29 The surface-bound formate is the result of the secondary oxidation of formaldehyde due to the higher surface oxygen coverages used in these experiments, which is in sufficient excess that it can be seen to recombine at 480 K in Figure 3a. Because of the low diffusion barrier for the adsorbed formaldehyde on gold, it readily accesses an adsorbed methoxy whence by nucleophilic attack it forms a metallated hemiacetal— referred to here as a hemiacetal alcoholate—and then, again by H-elimination, the coupled product, methyl formate (Figure 3).30 Theoretical analysis indicates that this coupling between methoxy and formaldehyde to give the ester is essentially barrierless.30 The formic acid, carbon dioxide, and water observed are due to further reactions of the adsorbed formate.24 Selectivity to methyl formate is maximized at low oxygen coverages on the Au(111) surface, while combustion to CO2 and water is the primary reaction pathway at high oxygen coverages. At very low oxygen precoverages the formation of the secondary oxidation products can be eliminated. The catalytic cycle is illustrated in Figure 4.

7


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 53

Figure 3. Temperature-programmed reaction (TPRS) spectra for (a) methanol (CH3OH) and (b) deuterated methanol-d3 (CD3OH) on O/Au(111). The O/Au(111) surface was prepared via ozone dosing at 200 K and the heating rate was approximately 10 K s-1. No reaction was detected on clean Au(111). The formation temperature for [D4]methyl formate is 10-15 K higher than that for [D0]methyl formate, indicating that a C-H (or C-D) bond is broken in the rate-limiting step. Reprinted with permission from reference 24. Copyright 2009 Wiley-VCH. Such studies confirm that β-hydrogen elimination from the adsorbed alkoxy to form the aldehyde is the rate-limiting reaction in the formation of the adsorbed hemiacetal alcoholate intermediate, which then leads to the formation of low molecular weight esters (Figure 4).24 However, for higher molecular weight esters desorption of the product can be the rate-limiting step.29 For such higher molecular weight species, methods are then available for predicting both the activation energy and preexponential factor for ester evolution.31 In the case of methanol self-coupling, methanol (formed from adsorbed methoxy and hydrogen liberated in the formation of methyl formate) and methyl formate are evolved from the Au(111) surface at the same temperature (220 K), indicating that they result from the same intermediate (methoxy) in a common rate-limiting step. Further, [D4]methyl formate forms at a temperature that is about 10-15 K higher than [D0]methyl formate, and this kinetic isotope effect is consistent with a rate-limiting step that includes C-H(D) bond breaking (Figure 3).24,

32

Kinetic isotope experiments and Hammett

studies of the partial oxidation of para-substituted benzyl alcohols to yield benzaldehydes over a supported Au/TiO2 catalyst in the liquid phase also concluded that the rate limiting step in 8


Page 9 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

aldehyde formation is β-hydrogen elimination, illustrating the correspondence across very different conditions and catalytic materials.33

Figure 4. Mechanism of oxygen-assisted coupling of methanol on Au to yield methyl formate. To simplify the complex interplay of simultaneous reactions taking place across the entire metal surface, the reactions in the figure are not mass-balanced. Detailed discussion of the elementary reaction steps is presented in reference 30. As in the initial activation of the methanol hydroxyl group, surface oxygen species may facilitate β-hydrogen elimination. Transition metals such as Pd bind hydrogen strongly, which leads to more facile cleavage of C-H bonds, resulting in dehydrogenation of alkoxy groups in the absence of adsorbed O.34 In contrast, binding of hydrogen atoms to Au is very weak, and therefore Au does not easily activate C-H bonds in the absence of other proton acceptors.30 Adsorbed atomic oxygen is calculated to lower the activation energy of β-hydrogen elimination in methoxy from 62 kJ/mol to 47 kJ/mol (Table 1).30, 34-35 In low pressure reactions, OH and methoxy can also facilitate β-hydrogen elimination via a bimolecular reaction on the surface, though atomic oxygen has the lowest calculated barrier of the three species. Further, OH adsorbed on Au(111) is unstable toward disproportionation, and therefore the steady-state coverage of OH is expected to be very low.36 As a result, OH is unlikely to be the primary facilitating species in the gas

9


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 53

phase catalyzed reactions.

The relative role of the Au itself in facilitating β-hydrogen

elimination remains an open question. Table 1. Calculated activation and thermodynamic barriers for possible reaction pathways for the oxidative coupling of methanol to give methyl formate on Au(111).a

a b

c

Adapted with permission from reference 30. The activation barrier (EB) is defined as the difference between the energy of the transition state (ETS) and the energy of the initial state (Ei); ie., EB = ETS - Ei. Thermodynamic energy (ETh) is defined as the difference between the energy of the final state (Ef) and the energy of the initial state (Ei); ie., ETh = Ef – Ei.

We note that while the active oxygen species for the gas-solid heterogeneous process is adsorbed atomic oxygen, the activating agent may sometimes differ in the liquid phase catalyzed reactions, 10


Page 11 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

particularly in basic solution (see below). However, an oxygen species acting as a Brønsted base is required for dissociation of the methanol O-H bond in both systems because there is a high energy barrier for transfer of the activation of the O-H bond directly by the Au surface.30 Specifically, the computed activation energy for bond cleavage via direct activation by Au is 152 kJ/mol compared to 39 kJ/mol for transfer to adsorbed O (Table 1).30, 34-35 In basic solution the concentration of OH can be increased by raising the pH of the reaction solution.35 Thus, in liquid phase reactions, adsorbed OH and OOH have been proposed as the key facilitators of β-hydrogen elimination, and are also believed to be the species responsible for initial activation of methanol.35, 37-38 In general, the same mechanistic principles are involved. While much remains to be understood in comparing mechanisms in the gas and liquid phases, the fundamental transformations of the alcohol remain consistent, and the initial step in the catalytic cycle for the coupling reaction is the formation of surface-adsorbed alkoxy. There have been previous suggestions that undercoordinated Au atoms on small particles (< 2 nm) can promote dissociation based on theoretical studies although there is no experimental evidence that establishes this possible role.39-41 Recent approaches have instead introduced lower free energy pathways for delivering O to the Au via the support or via impurities, such as Ag. Computations lend further insight into the distinctive selectivity of Au for the oxidative coupling of alcohols for the production of esters rather than toward other products such as CO2, CO, or formaldehyde. For example, in order for methoxy to react with formaldehyde via nucleophilic attack to form the coupled ester, the methoxy oxygen must move out of its preferred binding site on the Au(111) surface. This movement of methoxy requires partial bond breaking of the metaloxygen bond in the transition state. Scaling relationships based on metal-C or metal-O bond strengths have been utilized to predict trends in reactivity for identical reactions on different metal surfaces.42 As a result, the barrier for this transition state correlates with metal-oxygen bond strength, and because Au binds O weakly compared to other transition metals, this reaction step is most facile on the Au surface.43 Therefore, given proximity, formaldehyde is more likely to couple with an adsorbed methoxy before it desorbs from the surface or is oxidized further to CO2, leading to high selectivity in the Au-catalyzed ester synthesis. Each of the characteristics of the Au surface that make it selective for partial oxidation of methanol to methyl formate—high mobility of surface species, weak metal-oxygen binding 11


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 53

strength, and lack of facile activation of C-H bonds—is a result of the relative nobility of Au compared to other transition metals. One major consequence of this is that the mechanism and products of ester synthesis via alcohol oxidation are only minimally affected by moving from gas phase to liquid phase reaction conditions, or from low pressure to atmospheric pressure. In the liquid phase, some steps of the reaction, such as the formation of the hemiacetal, can take place in solution via a variation on the mechanism described above, but the chemistry that takes place on the Au surface is analogous to that in the gas phase. Even when using different alcohols as starting reactants, the mechanism of reaction on Au remains as: (1) formation of adsorbed alkoxy; (2) β-hydrogen elimination to form the aldehyde; and (3) nucleophilic attack of an alkoxy species on the carbonyl group of the aldehyde to form the ester. When reactant mixtures are more complex, such as in the case of coupling reactions between dissimilar alcohols, competitive adsorption comes into play in determining selectivity, but the reaction follows the same mechanism. Importantly, the acid-base principles of this mechanism can even be used to predict novel reactive pathways based on the paradigm of Brønsted acid-base chemistry followed by nucleophilic attack of a negatively polarized adsorbate on an electron-deficient carbon. This predictive capability enables the design of previously unachievable catalytic pathways for the formation of high-value products, such as the synthesis of amides through oxidative coupling of amines (see sections 7.1-7.2).44-47 3. Adding materials complexity: Alloys and supported catalysts The development of catalysts that facilitate these coupling reactions under practical conditions on Au necessitates materials that activate molecular oxygen, while retaining the selectivity of the gold itself. Though ozone provides a convenient source of adsorbed atomic oxygen for model studies and determining the mechanisms of the catalytic cycle, it is an impractical oxidant for industrial processes. Because bulk Au does not dissociate molecular oxygen, additional materials complexity is required; the desired functionality can be introduced in a variety of ways, such as alloying Au with metals which dissociate dioxygen or distributing nanoscale Au particles on reducible oxide supports which supply the necessary oxygen.48-51 Often multiple approaches are combined to produce a single catalyst material. In each case, the goal is to facilitate highly selective oxidative coupling chemistry. 3.1 Au alloys 12


Page 13 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

3.1.1 Unsupported nanoporous gold (np(Ag)Au) Nanoporous Au (np(Ag)Au), as commonly synthesized, is a dilute alloy containing 1-3% Ag in a gold matrix and is an excellent catalyst material for these purposes.52-54 It is an unsupported catalyst made by dealloying a bulk alloy of approximately 70:30 Ag:Au through either free corrosion in nitric acid or via electrochemical means. The dealloying process removes the majority of Ag from the starting alloy and yields a porous and highly tortuous structure composed of nanoscale (~10-80 nm) ligaments, which consist of large single crystal domains (Figures 5 and 6).55 Due to the curvature of the ligaments, the surface of these catalysts consists primarily of highly stepped (111) crystalline planes of Au (Figure 6).56 The diameter of the ligaments is determined by the dealloying protocol. This mesoscale material combines nanoscale ligament structure with an overall architecture that is on the microscale or even the macroscale. Its structure provides mechanical stability that minimizes sintering of the material under working conditions. The flexibility in size and morphology of np(Ag)Au enables catalysts to be tailored for use in different catalytic conditions (Figure 5). For example, (1) larger ingots (Figure 5a,b) are suitable for study under ultrahigh vacuum conditions, using the appropriate methods of surface analysis,57-60 (2) micron-scale hollow shells (Figure 5c,d) are ideal for reactor studies since they consist of a thin layer of filamentous material such that pore diffusional limitations are mitigated,8, 49, 61 and (3) thin foils are ideal for high-resolution transmission electron microscopy (TEM) studies.56, 62

13


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 53

Figure 5. Photograph (a) and scanning electron microscopy (SEM) images of (b-d) np(Ag)Au ingots and (c,d) np(Ag)Au hollow shells. The SEM images in (b) and (d) are higher magnification images of the materials in (a) and (c) and show that the nanoporous structure is the same across different material architectures. Adapted with permission from reference 8. Copyright 2015 American Chemical Society.

Figure 6. High-resolution scanning transmission electron microscopy (STEM) images of the surface of the ligaments in np(Ag)Au obtained using a high-angle annular dark-field (HAADF) detector. The images show terraces of atoms with (111) orientation. Adapted with permission from reference 56. Copyright 2012 Nature Publishing Group. Due to the residual silver in the np(Ag)Au, molecular oxygen is readily activated in the absence of an oxide support. Ag is known to dissociate oxygen, but is also otherwise relative inert and is itself a catalyst for some partial oxidation reactions.49 Indeed, np(Ag)Au proves to be highly active and selective for the oxidative coupling of methanol to form methyl formate at temperatures as low as 293 K.48 The selectivity to the ester is nearly 100% under optimized flow conditions at atmospheric pressure (see Figure 7a, 100% methanol),8, 48 which indicates that this nanoporous material retains the selective oxidative chemistry observed on Au(111), while providing adsorbed O.48 The residual silver content in these highly selective npAg(Au) materials is 1-3%, and no systematic study has been conducted to determine the optimal Ag concentration. In contrast, only combustion products—resulting from secondary oxidation of formaldehyde— are observed using supported pure Ag nanoparticles under similar conditions. The selectivity of the nanoporous Au is independent of the ligament size over the range of catalysts studied.

14


Page 15 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Remarkably, the dependence of the product selectivities for oxygen-assisted coupling of dissimilar alcohols on the npAu catalysts under flow conditions at atmospheric pressure on reactant composition is nearly identical to results observed from codosing the two alcohols at different ratios on O-activated Au model catalysts in ultrahigh vacuum (Figure 7a,b and section 5.2).63-64 This correspondence between selectivity on model systems and working catalysts holds not only for the coupling of primary alcohols to yield methyl esters, but also for the synthesis of unsaturated methyl esters from unsaturated alcohols, such as allyl alcohol (Figure 7 c,d and section 6.1).59-60 Well-controlled experiments on model Au surfaces in ultrahigh vacuum provide the mechanisms of ester formation on complex catalytic Au materials under ambient conditions in the gas phase and predict product selectivity with a high degree of accuracy in these systems. These model studies also show that the selectivity for coupling of dissimilar alcohols is structure insensitive based on comparison of Au(111) and Au(110).65 Further, the same elementary steps appear to be applicable to the catalytic cycle operative on supported Au catalysts in the liquid phase.

Figure 7. Graphs of product selectivity as a function of methanol mole fraction, which illustrate the remarkable correspondence between product selectivity on model single crystal Au surfaces and on np(Ag)Au catalysts under working catalytic conditions in the gas phase. (a,b) Product 15


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 53

selectivity in the coupling of the dissimilar alcohols methanol and ethanol and (c,d) product selectivity in the coupling of allyl alcohol (an α,β unsaturated alcohol) and methanol. The np(Ag)Au catalysts in a flow reactor at 1 atm (b,d) exhibit nearly identical selectivity to the various coupling products as that observed on single crystal Au surfaces (a,c) in low pressure (ultrahigh vacuum) conditions. Adapted with permission from references 63 (a), 64 (b), and 60 (c,d). Copyright 2010 (a) and 2016 (c,d) American Chemical Society and 2015 Elsevier (b). Increasing the Ag content in np(Ag)Au shifts the selectivity in methanol oxidation on np(Ag)Au towards combustion.48 At 10% Ag, CO2, which results from the over-oxidation of formaldehyde, is the sole product; at the higher Ag content np(Ag)Au becomes more Ag-like. Similarly, increasing the O2 partial pressures at a fixed Ag content (< 1 %) also drives selectivity toward combustion, as do higher temperatures, particularly at higher O2 partial pressures.48 These effects are compatible with the accumulation of excess O at the surface due to the higher oxidizing potential of the environment. The higher surface Ag concentration leads to excess O, which leads to oxidation of formaldehyde. The dependence of product selectivity on O2 partial pressure can be predicted directly from model studies involving thin films of Ag on Au single crystals, where low O coverage favors the coupling reaction to yield methyl formate and some formaldehyde, while high O coverages lead to combustion of methanol.66 These trends in selectivity are also as expected for high O coverages based on the mechanism defined on Au(111), where formaldehyde, the critical intermediate for ester formation, reacts readily with excess adsorbed O to form formate, which decomposes to CO2 (Figure 4).24 Interestingly, no formaldehyde is observed in the oxidation of methanol over np(Ag)Au at ambient pressure flow conditions, suggestive of very low surface coverage of adsorbed atomic oxygen in the steady state reaction.8, 63

The active site density and the rate constants for the basic steps in the oxidative esterification of methanol on np(Ag)Au were determined using transient pressure pulses of reactants over the catalyst.49,

61

The density of active sites for oxygen activation was determined by quantitative

titration with O2 to be 0.1% of the total number of surface sites, commensurate with the low concentration of Ag at the surface. The O2 is activated with a probability of 10-4 per collision with these sites; the activation energy is 5.0 kcal/mol. The activation of methanol can be further interrogated by reacting a pulse of methanol with preadsorbed oxygen. The reaction probability 16


Page 17 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of methanol when striking an adsorbed oxygen atom is 10-8, which leads to a turnover number for the catalytic self-coupling of methanol over np(Ag)Au of 160 s-1 at 425 K.61 Further, these studies suggest that in the catalytic steady state, formaldehyde desorbs from the surface and reacts facilely with adsorbed methoxy by readsorption from the gas phase. The studies under all sets of conditions, low pressure, Knudsen transient flow, and the catalytic flow reactor point to the same underlying mechanism, and thus understanding gained from model results can be used to optimize the performance of np(Ag)Au under flow conditions. Catalyst materials often require an activating pretreatment, and the reproducible activation of catalysts is a challenge. The approach initially used to activate np(Ag)Au was to expose the catalyst to a flow of reactants (O2 and methanol and/or CO) in the reactor until the catalytic activity stabilized.48 Nanoporous Au catalysts in the form of ingots activated in this way showed stable conversion of methanol to methyl formate for over seven days at 303 K.48, 67 However, in general, this activation procedure proved to be erratic with ingots as well as the other forms of np(Ag)Au, such as hollow shells and foils,8, 68 necessitating the development of an alternative method. Studies with ingots in ultrahigh vacuum showed the necessity for removing adventitious carbon in order to achieve stable activity.58 Ozone doses to the surface followed by heating to 450 K were employed to remove this carbon, after which O2 could be readily activated by the catalyst.58 Notably, in addition to removing carbon, ozone treatment also leads to an enhancement of Ag at the surface of the np(Ag)Au and the formation of a surface AgAu alloy as shown by x-ray photoelectron spectroscopy (XPS).69 Recent environmental TEM and ambient pressure XPS show that the material restructures in addition to changing composition during ozone treatment (Unpublished data).

The same procedure was adapted to atmospheric pressure flow conditions in the reactor; pretreatment in a flowing stream of 3% ozone in helium (He) at 423 K yielded reproducibly active np(Ag)Au ingots, hollow shells, and foils.8 To complete the activation, the catalyst was then purged with He or clean, dry N2 and cooled to room temperature before being gradually heated to the catalytic working temperature of 423 K in the reactant mixture of methanol and O2. A small amount of CO2 was transiently produced by this procedure due to further removal of residual carbon from the catalysts. Ozone pretreatment reproducibly activates np(Ag)Au materials of all architectures (ingots, shells, and foils). This is particularly notable given that the shells and foils are likely to contain significant amounts of incorporated carbon as a result of 17


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 53

their method of synthesis, and the reproducible activation of these materials illustrates the versatility of this approach. Importantly, these ozone-activated np(Ag)Au catalysts are functionally different from the onstream-activated materials.8 First, the ozone-activated catalysts do not oxidize CO to form CO2, where reactant-activated np(Ag)Au is reported to be active for this conversion.53, 70 This inability to oxidize CO may be beneficial for maintaining selectivity in CO-containing reactant streams, as it limits CO2 production. In addition, the ozone-activated catalysts function at 150 °C, while the reactant-activated np(Ag)Au operates, with some irreproducibility, at temperatures as low as 293 K.48 However, at 423 K, the ozone-treated materials are consistently active, maintain their activity for over a month on stream, and remain active after being stored in air for extended periods of time.8 Thus, the stable activity of np(Ag)Au significantly counterbalances the need for higher operating temperature. Finally, the ozone-activated materials effectively catalyze the homo-coupling of the longer-chain alcohols ethanol and 1-butanol to give ethyl acetate and butyl butyrate,8 while the reactant-activated materials only coupled ethanol with low and variable selectivity, and do not couple 1-butanol.67 Mechanistic details regarding control of selectivity in the coupling of higher alcohols are discussed in section 4.1, below.67 3.1.2 Supported Au-Pd nanoparticles Supported Au-alloy nanoparticles have also been used in gas-phase methyl ester synthesis, however, the activity of these alloy systems is often attributed to reaction on the other (non-Au) metal. The effect of these other metals as dopants in Au remains to be studied. Au is instead used to modify the electronic or binding properties of the other metal to improve selectivity to methyl ester formation. One metal that is commonly alloyed with Au in these catalysts is Pd due to the ability of Au and Pd to form solid solutions over a range of compositions.71 Further, Pd(100) surfaces have shown activity for esterification of methanol to methyl formate under low-pressure conditions.72 Gas-phase experiments at atmospheric pressure showed that Janus Au-Pd nanoparticles73 and alloyed AuPd nanoparticles are both active for the conversion of methanol to methyl formate.74 The Au-Pd Janus particles were composed of an Au particle and a Pd particle joined at a (111) plane to form a twinned nanostructure and were supported on graphene (Figure 8).73 Analysis of the catalyst materials via X-ray photoelectron spectroscopy gave binding energies for Au and for Pd that were lower than those of the metals on their own, indicating a 18


Page 19 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

synergistic electronic effect between Au and Pd. In addition, these binding energies are lower than for other Au-Pd catalysts, which suggests a strong interaction of the catalyst with the graphene support. Au can potentially act as a promoter for the transfer of π electrons from the support to electron-deficient Pd, reducing PdO to Pd0 and thus preventing formation of the Pd oxide.73 The influence of support materials on activity and selectivity in methyl ester synthesis is further discussed below in Section 3.2. The maximum catalytic performance for these Au-Pd Janus particles was achieved with a Au2-Pd/graphene catalyst at 70 °C and resulted in 100% selectivity to methyl formate with a 90.2% conversion of methanol. Similarly, studies of AuPd nanoparticles supported using a polymer incarceration approach in basic solution with an Au:Pd ratio ranging from 3:1 to 4:1 converted benzyl alcohol to methyl benzoate with 98% selectivity, illustrating that synergistic interactions between Au and Pd in these alloys can be translated to the liquid phase as well.74-75

Figure 8. Electron microscopy images of Janus Au2-Pd particles supported on graphene. (a) TEM image with the two halves of the Janus particles indicated by circles and (b) highresolution TEM image of a single Janus particle showing the lattice spacings of Au(111) and Pd(111) on the two halves of the particle. Adapted with permission from reference 73. Copyright 2013 The Royal Society of Chemistry. 3.2 Supported Au nanoparticles in the liquid phase The mechanistic insights provided by the low pressure model studies and the gas phase catalytic reactors provide a guide for understanding similar reactions in the condensed phase. The vast majority of work on Au-catalyzed methyl ester synthesis in the liquid phase has been carried out using Au nanoparticles immobilized on support materials. While size effects have not been systematically examined in this catalytic system, most Au nanoparticles used in these studies are 5 nm or less in diameter. Small metal nanoparticles are highly susceptible to aggregation and 19


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 53

sintering under reaction conditions, and thus must be stabilized through immobilization on a support. Even then sintering of the gold is a problem.76 Due to the need for an active oxygen species for initiating and driving methyl ester synthesis, the most common support materials for the Au nanoparticles are metal oxides such as titania (TiO2),33,

77

silica (SiO2),77-78 ceria

(CeO2),51, 77 or mixed metal oxides.79 Other work focusing on recyclability has employed more exotic structured supports, such as mesoporous silica and mesoporous alumina.80-81 A key feature of the metal oxide supports is that they are chemically inert toward ester formation in the absence of Au, so that the characteristic selectivity of Au catalysts toward partial oxidation products is maintained. The exact mechanism of O2 activation on supported Au nanoparticles is still a subject of debate in the field.38, 82-86 A detailed discussion of the mechanism of O2 activation on different supported Au catalysts is beyond the scope of this review, but this topic is currently being reviewed elsewhere.87 However, it is commonly proposed that active oxygen is generated at the interface between the metal oxide support and the Au nanoparticles.88 Given this framework, methyl ester synthesis over supported catalysts in the liquid phase can be discussed in the context of the fundamental mechanisms outlined in the Section 2 above with some important differences due to the presence of the liquid phase. The Brønsted acid-base nature of the esterification reaction provides the primary framework for understanding catalyst function. Specifically, the mechanism commonly invoked in the liquid phase synthesis of methyl esters over supported Au catalysts involves three steps: (1) formation of an aldehyde from an alcohol on Au; (2) reaction in solution of the aldehyde and methanol to form a hemiacetal; and (3) oxidation of the hemiacetal on Au to give the ester. The first step is found to be rate-limiting and is analogous to the gas phase mechanism on Au. However, model studies in the gas phase and mechanistic studies in solution show that this “step” is actually composed of two fundamental reactions: the deprotonation of the alcohol to give surface adsorbed alkoxy and the subsequent β-hydrogen elimination from the alkoxy to give the aldehyde. In solution, as in the gas phase, it is the β-hydrogen elimination that is rate-limiting,24,

33

while the subsequent

formation of the hemiacetal and its oxidation to the final methyl ester are rapid, and can take place under mild conditions at temperatures as low as -70 °C on supported Au catalysts.89 The next step proposed in solution is the direct reaction of the aldehyde and methanol in solution to give a hemiacetal, rather than reaction of the aldehyde with surface-adsorbed methoxy to yield a hemiacetal alcoholate. However, the relative contributions of these two competing processes in 20


Page 21 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

the liquid phase catalyzed conversion has not actually been determined. Indeed, it is possible that the competing formation of the hemiacetal alcoholate on the Au surface from the aldehyde and surface-adsorbed methoxy contributes to the reaction in the liquid phase.90 Much of the work in the design of supported Au catalysts for methyl ester synthesis in the liquid phase has focused on facilitation of hemiacetal formation, and this work will be discussed below. The final oxidation of the hemiacetal in solution to form the ester on Au most likely follows the mechanisms determined from the model studies in the gas phase, except that an initial removal of a proton from the hydroxyl group of the hemiacetal is required to form the hemiacetal alcoholate. 3.2.1 Supported Au nanoparticles in basic conditions The formation of the hemiacetal occurs in equilibrium in a solution of aldehyde and alcohol and this equilibrium is reached more rapidly in acidic or basic solution. Under pH neutral conditions the system reaches equilibrium relatively slowly, which greatly disfavors hemiacetal formation. For the hemiacetal in solution to play a significant role in the catalytic cycle for methyl ester formation, conditions must be found to increase the rate of hemiacetal formation. As a result, the liquid phase oxidative esterification of alcohols over supported Au is generally carried out under basic conditions, in the presence of a soluble base such as K2O3, NaOCH3, and KOH.78, 91 The use of liquid base in these reactions originates from the use of weak bases, such as carbonates, for proton extraction in similar reactions in organic synthesis, including ester hydrolysis to form alcohols and the alkylation of phenols to give ethers. The addition of base promotes the presence of adsorbed hydroxyls, thereby facilitating the esterification reaction by increasing the rate of hydrogen abstraction from the alcohol and the rate of β-hydrogen elimination to form the aldehyde.35 In basic solution the role of the oxygen is proposed to be regenerating hydroxide ions, which are formed via the catalytic decomposition of a peroxide intermediate.35 The base is also proposed to facilitate hemiacetal formation by deprotonating methanol to give methoxy in solution, a strong nucleophile, which attacks the carbonyl of the aldehyde. The resulting species would then have to adsorb on the Au and form the ester by β-hydrogen elimination. This sequence, though possible, seems unnecessarily complex, given the simplicity of the surfacedominated process. In fact, the methoxy generated by the deprotonation in the solution can readily adsorb and contribute to the catalytic cycle itself. Again, the relative roles of the homogeneous and heterogeneous processes need to be assessed. The two possible routes in 21


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 53

solution are compared in Figure 9, including a side reaction to form the carboxylic acid, which occurs in the liquid phase when water is present in the solution.92-94

Figure 9. Potential reaction pathways for the conversion of ethanol to ethyl acetate (blue) or acetic acid (red) over a Au/TiO2 catalyst: (a) one mechanism proposed for reaction in the liquid phase, where hemiacetal formation takes place in solution and (b) the competing reaction pathway where intermediate formation takes place on the Au surface. In many supported Au catalytic systems under aqueous conditions the amount of base can be stoichiometric, or even in excess, but more recent studies have focused on carrying out the esterification reaction using less base or a solid base.78, 91, 95 Early work in this area using Au nanoparticles on SiO2 yielded 100% conversion in the esterification of benzyl alcohol in methanol (Χmethanol = 0.95) to the methyl ester with 100% selectivity in the presence of a substoichiometric amount of base (K2CO3).78 While these reactions were carried out using less base, which is an important step toward more environmentally sustainable reaction conditions, elevated temperatures (403 K) and high pressures (3 atm O2) were still required for maximum conversion and selectivity. In contrast to gas phase studies where methanol coupling to form methyl formate serves as a test reaction, on supported catalysts in the liquid phase, esterification of benzyl alcohol with methanol to yield methyl benzoate is more commonly studied. Studies of the reactivity of benzyl alcohol on well-defined surfaces provided by ultrahigh vacuum suggest that the ease of this

22


Page 23 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reaction is likely due to the facile β-hydrogen elimination from adsorbed benzyloxy compared to methoxy as well as the higher affinity of the benzyl alkoxy for Au as a result of its π-system.96-97 In comparison, methanol is observed to be less easily oxidized on supported Au catalysts in the liquid phase and methyl formate production is generally not observed,33, 98 though low yields of methyl formate have been detected from methanol coupling over supported Au in a few control experiments.99 The origin of this difference may lie in the greater difficulty for β-hydrogen elimination from the adsorbed methoxy. Further, as noted in Section 5.1.1, benzyl alcohol can easily displace methoxy on the catalyst due to the strong affinity of benzyloxy for the gold surface if both alcohols are present in solution, which accounts for the absence of methyl formate product in the liquid phase when both alcohols are present in the liquid phase.65 It is also important to note that the production of methyl formate from methanol does take place over unsupported np(Ag)Au catalysts in the liquid phase.100-101 The origin of these differences requires further examination. The quantity of base required to achieve high conversion and selectivity can be decreased to a stoichiometric amount by increasing the basicity of the oxide support.91 In one example, a more basic support was synthesized through hydrothermal treatment of commercial TiO2 nanoparticles with potassium hydroxide (KOH) to yield potassium metal titanate (K2Ti6O13) nanowires. Using methanol as a solvent, a catalytic amount of KOH, and Au nanoparticles supported on K2Ti6O13, a >99% conversion was achieved in the esterification of benzyl alcohol with methanol, with a 93% selectivity to the ester, methyl benzoate. Importantly, this reaction was also carried out at room temperature and atmospheric pressure. The authors suggested a potential additional role for the soluble base and attributed the promotional effect of the KOH in this system to the neutralization of carboxylic acid byproducts, in this case benzoic acid, which can poison the catalyst surface and prevent complete conversion of benzyl alcohol.50, 91, 97 This also provides an explanation of why stoichiometric amounts of base are required to favor ester formation in aqueous reactions, as carboxylic acid products form more readily during oxidative esterification of alcohols in water.93-94 In contrast, in methanol as a solvent, the methyl ester is the primary product, and thus less base is required.91 In a related approach, a separate study employed a hydrotalcite (Mg6Al2(OH)16CO3·nH2O) support with tailorable basicity to increase selectivity toward methyl ester formation in the absence of a soluble base.95

23


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 53

3.2.1 Supported Au nanoparticles in acidic conditions Hemiacetal formation and hence esterification can also be facilitated by acidic oxide supports.102103

In base-free methanol at 363 K and 5 atm O2, the selectivity to methyl benzoate formation

from reaction of benzyl alcohol over Au/β-Ga2O3, which is known to have increased surface Lewis acidity when in a nanocrystalline form, was 96.5% with 98% conversion.102 This selectivity was attributed to Lewis acid sites on the β-Ga2O3, which can withdraw electron density from the carbonyl of the aldehyde, thus making it more susceptible to nucleophilic attack, promoting hemiacetal formation. Similarly, on Au/Fe2O3 some formation of benzyl benzoate was observed in the oxidation of benzyl alcohol to benzaldehyde.77 It is possible that the role of the acid sites is similar in the two materials. (Figures 3 and 9). Overall, the literature regarding methyl ester synthesis in the liquid phase over supported Au catalysts suggests strong parallels with the gas phase reaction mechanisms, particularly in the formation of aldehydes from alcohols and the oxidation of hemiacetals to esters (Figures 3 and 9). However, because reactions can take place both in solution and on the catalyst surface in the liquid phase, there are competing pathways, primarily in the formation of the hemiacetal intermediate, pointing to opportunities for additional research to better understand their relative importance. The significant mechanistic correlation between the gas and liquid phases is also evident in more complex esterification reactions, such as those involving longer chain alcohols or the coupling of dissimilar alcohols, which are now discussed. 4. Ester formation from longer chain primary alcohols: β-H elimination and selectivity to coupling vs. aldehyde formation Benzyl alcohols and long chain primary alcohols, such as ethanol and 1-butanol, also undergo oxygen assisted homocoupling to form esters over nanoporous and supported Au catalysts (benzyl benzoate, ethyl acetate, and butyl butyrate, respectively) (Figure 10). These larger alcohols follow the same pattern of reactivity as methanol, and differences in selectivity amongst the alcohols are critically dependent on weak interactions of reactants and intermediates with the Au surface. In particular, comparison of selectivity in the homocoupling of alcohols with different chain lengths provides an excellent illustration of the role of β-hydrogen elimination in controlling selectivity between ester formation and aldehyde production. Reactions of longer

24


Page 25 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

alcohols yield both the ester and aldehyde products in model studies on oxygen-activated Au(111) and in flow reactors at atmospheric pressure.8, 57, 67, 104-105

Figure 10. Generalized mechanism of oxygen-assisted homo-coupling of a representative alcohol on Au to yield a homo-coupled ester. To simplify the complex interplay of simultaneous reactions taking place across the entire metal surface, the reactions in the figure are not massbalanced. A more detailed description of the individual reaction steps can be found in reference 106. 4.1 Model studies Model studies on O/Au(111) showed that the relative yield of the ester vs. the aldehyde depends strongly on alcohol chain length, with more aldehyde forming from longer chain alcohols.106 Further, these studies confirmed that the formation of aldehydes from longer chain alcohols follows the same reactive pathway as methanol: hydroxyl bond cleavage facilitated by adsorbed O followed by β-hydrogen elimination,106-108 the latter becoming more rapid for longer chain alkoxide intermediates. The relative production of ester and aldehyde in the model experiments depends on the relative rates of hemiacetal alcoholate formation vs. desorption of the aldehyde into the vacuum. In turn, alcoholate formation requires that the adsorbed aldehyde diffuse on the surface to the adsorbed alkoxy and react by attack at the carbonyl carbon. The rate of diffusion is expected to decrease with increasing molecular weight, and, further, larger aldehydes pose 25


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 53

higher steric restrictions to formation of the hemiacetal alcoholate. Similarly, less combustion to CO2 was observed with longer chain alcohols than was seen in model studies at relatively high surface oxygen coverages. The concentration of surface oxygen plays a major role in the product selectivity.104 At higher O coverages, the formation of CO2 increased due further oxidation of the aldehyde before it could desorb or react to form the hemiacetal alcoholate.104, 106 In the case of ethanol coupling other secondary oxidation products, such as acetic acid and ketene, were also observed at high O coverages. The highest selectivity to the ethyl acetate was obtained at low O coverages, where secondary oxidation of the aldehyde is suppressed and acetaldehyde readily couples with ethoxy to give ethyl acetate with essentially 100% selectivity.104 4.2 Coupling of longer alcohols on unsupported np(Ag)Au These low pressure studies led to the prediction that the np(Ag)Au catalyst would be active for the gas phase formation of the esters ethyl acetate and butyl butyrate from ethanol and 1-butanol, respectively, with minimal combustion to CO2. Indeed, np(Ag)Au catalysts coupled ethanol to give ethyl acetate; acetaldehyde was also formed and, as predicted, there was no production of CO2.8,

67

The ozone-activated np(Ag)Au catalyst showed consistent activity and selectivity.8

Importantly, however, only the ozone-activated catalyst was active for the coupling of 1-butanol to butyl butyrate.8,

67

The catalytic functionality of the ozone-activated np(Ag)Au is

fundamentally different from the reactant-activated material even though both have the same bulk composition and structure.8 Further, the selective coupling of longer-chain alcohols by ozone-treated np(Ag)Au to give the homo-coupled esters in addition to formation of the aldehydes confirms the Au-like activity of this alloyed material, as the selective oxidation of ethanol and 1-butanol over pure Ag yields only the aldehydes. 4.3 Homo-coupling on supported catalysts in the liquid phase Mixtures of coupled esters and aldehydes were also observed in the oxidation of longer chain alcohols over supported Au catalysts in the liquid phase, and the product distributions are in agreement with the catalytic cycle discussed above (Figure 10).51,

77, 105, 109

In the selective

oxidation of benzyl alcohol over AuPd/TiO2, the primary product was benzaldehyde due to the facile β-hydrogen elimination from benzyl alcohol and the consequent rapid depletion of surface adsorbed benzyloxy.77, 109 Only trace amounts of benzyl benzoate were observed, along with a small amount of toluene. In addition, oxidation of ethanol under aqueous conditions over 26


Page 27 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Au/TiO2 and Au/MgAl2O4 exhibited increasing selectivity to the ethyl acetate with increasing ethanol concentration.7 The other product formed under these conditions was acetic acid. Higher concentrations of ethanol in solution lead to a lower ratio of surface O to ethoxy, limiting conversion of acetaldehyde to acetate.104 Decreased production of acetate limits the conversion of ethanol to acetic acid, as adsorbed acetate is an intermediate in that reactive pathway. Interestingly, 1-octanol can be converted to octyl octanoate with a selectivity of ~50% over Au/Fe2O3 (albeit with a low conversion of 2.1%), with the remaining product being the aldehyde, octanaldehyde.77 In contrast, no conversion was obtained using SiO2, TiO2, or carbon supports, reflecting the role of support effects in facilitating formation of the hemiacetal between the alkoxy and the aldehyde. 5. Coupling of dissimilar alcohols: Competitive adsorption, weak interactions, and the kinetic role of β-H elimination Perhaps the most remarkable example of the ability of low pressure model studies to predict selectivity under working catalytic conditions is in the complex, competitive synthesis of esters from mixtures of dissimilar alcohols. In this oxygen-assisted coupling process there are four potential ester products: two homo-coupled esters and two mixed esters (Figure 11). For example, in the coupling of methanol and ethanol, the potential homo-coupled products are methyl formate and ethyl acetate, and the mixed esters are methyl acetate and ethyl formate.63-64 In fact, ethyl formate is not observed experimentally, the reasons for which will be discussed below. These mixed coupling reactions proceed by the same fundamental mechanism understood for homo-coupling (Figure 12). In the model studies on O/Au(111), the selectivity to the different esters varies unusually with the mole fraction of the alcohols dosed to the surface, peaking at very high mole fractions of the species of lower molecular weight (Figure 7); this dependence is, remarkably, mirrored under catalytic flow conditions on unsupported np(Ag)Au.59-60,

63-64

Further, the synthesis of methyl esters over supported Au catalysts in the

liquid phase, which is commonly carried out under conditions where methanol is a solvent for a second alcohol, generally exhibits an increasing selectivity to the methyl ester product with increasing methanol excess.98 This trend can be explained by low pressure model studies and is a consequence of the unequal competition of different alcohols for binding to the Au surface.96

27


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 53

Figure 11. Potential ester products in the coupling of dissimilar alcohols. Not all of the possible products form experimentally. Adapted with permission from reference 110. Copyright 2014 American Chemical Society.

Figure 12. Mechanism of the coupling of methanol and ethanol on Au to yield the mixed ester, methyl acetate. To simplify the complex interplay of simultaneous reactions taking place across the entire metal surface, the reactions in the figure are not mass-balanced. A more detailed description of the individual reaction steps can be found in reference 63. 5.1 Competitive adsorption and displacement In the synthesis of the methyl ester methyl acetate, optimum selectivity to methyl acetate over the homo-coupled methyl formate or ethyl acetate is expected when the relative surface coverage 28


Page 29 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of the two alkoxides on the Au surface is equal.63 With gas phase reactants this equal surface coverage of methoxy and ethoxy would be expected to occur with a 50:50 vapor phase mixture of methanol and ethanol if predicted solely from scaling relationships of metal-oxygen bond strength,42 as methanol and ethanol would be predicted to have nearly identical binding strengths to the surface. Surprisingly, low pressure studies on single crystal Au model surfaces conducted by condensing prescribed mixtures of methanol and ethanol on 0.05 monolayers of preadsorbed O on Au(111) in vacuum showed that unequal, competitive binding occurs between the two alcohols according to the reaction in Equation 3 due to unequal binding strengths (Figure 13).96 CH3Oads + ROHgas → ROads + CH3OHads

(3)

The deviation of the scaling relationships based simply on the Au-O bond strength arises from weak van der Waals interactions which dictate the relative surface stabilities of the adsorbed alkoxides;96 consequently, ethoxy binds more strongly to Au, shifting the equilibrium distribution of the alkoxy species to favor ethoxy in an equimolar gas phase mixture. Because rapid exchange between the adsorbed alkoxides and the ambient phase alcohols can occur, competition among the species for active sites arises, and equal concentrations of the adsorbed alkoxys does not occur at equal concentrations of the alcohols in the reactant phase.63, 96 As a result, the ideal ratio of methanol to ethanol for achieving high selectivity to methyl acetate is not 50:50, but rather 85:15 (Figure 7a,c).63

Figure 13. Schematic illustration of the surface binding equilibrium between adsorbed methoxy and higher molecular weight alcohols and their corresponding alkoxides. Methoxy is displaced by higher molecular weight alcohols. Adapted with permission from 63. Copyright 2010 American Chemical Society. 5.1.1 Model studies of competitive adsorption

29


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 53

Competitive adsorption experiments on O/Au(111) in ultrahigh vacuum conditions reveal the extent to which one alcohol displaces the alkoxide of another alcohol on the Au surface (Equation 4, Figure 13).96 BHgas + B′¯ads → B¯ads + B′Hgas

(4)

This competition can be defined by an equilibrium expression. For example, in the competition between methanol and another alcohol: 𝐾!"!! !"

!!! !"

=

!!"!! !(!) !!!! !" !!!! !(!) !!"!! !"

(5)

where θ is the surface coverage of each alkoxy species and CROH is the concentration of the alcohol in the ambient phase. In these experiments, two dissimilar alcohols were dosed on an O/Au(111) surface, and the relative contribution of each alcohol to the product distribution obtained from the alkoxides was then determined using TPRS methods. Also, the displacement reactions could be followed using vibrational spectroscopy to identify the adsorbed species (Figure 14).63, 96 The relative stabilities of the adsorbed alkoxides were ranked to generate a hierarchy of binding strengths of the different alkoxides to the single crystal Au surface.96 This hierarchy exhibits a strong correspondence between the increasing stability of the adsorbed alkoxides and the gas phase acidity of their respective alcohol precursor (Table 2).96, 111-112 Notably, the relative binding strengths of different alkoxides do not correlate with the homolytic bond dissociation energies of their parent alcohols.96 Gas phase acidities are known for a wide range of molecules, and thus provide a practical guide for predicting selectivity in competitive coupling reactions among homologous reactants, such as alcohols.

30


Page 31 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 2. Relative stabilities of surface intermediates, their gas phase acidity, and the bond dissociation energy of their parent acid.a,b,c

a b c

d

e

Modified with permission from reference 96. Copyright 2014 American Chemical Society. Lower values of gas phase acidity (ΔH) indicate stronger gas phase acids. Intermediates whose relative surface stability is not accurately predicted by gas phase acidity alone are in bold text. Gas phase acidity (taken from the NIST database113) is defined as ΔH for BH(g) → B¯(g) + H+(g) (kJ/mol). The homolytic bond dissociation energy is defined as ΔH for BH → B(g) + H(g) (kJ/mol). The recommended values from the Comprehensive Handbook of Chemical Bond Energies are included here.114

31


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 53

Figure 14. Vibrational (HREEL) spectra showing the displacement of adsorbed methoxy by deuterated ethanol (CD3CD2OD) on a O/Au(111) surface. (a) Spectrum for adsorbed methoxy generated from the introduction of methanol to O/Au(111). (b) Spectrum of adsorbed species on the Au surface after introduction of deuterated ethanol (CD3CD2OD), which shows that methoxy has been displaced by ethoxy. (c) Spectrum of adsorbed species on the Au surface after subsequent introduction of methanol, illustrating that comparable doses of methanol do not displace ethoxy on the Au surface. The peak at 0 cm-1 is the elastic peak, which has a full width at half maximum (FWHM) of 60 cm-1 in each of the spectra. Adapted with permission from reference 63. Copyright 2010 American Chemical Society. 5.1.2 Computational studies and the role of van der Waals forces The relative surface stabilities of these species are actually explained by the significant influence of weak van der Waals interactions on the binding of the adsorbates on the Au surfaces.96 Computational studies using density functional theory (DFT) provide convincing evidence of the role of these weak interactions, as the experimental trends in binding affinity are only correctly reproduced by theory if van der Waals interactions are included (Table 3).96 DFT shows that each of the alkoxides binds to the Au surface with nearly exactly the same metal-oxygen bond strength and that the differences in the stabilities of the alkoxides result from differences in van 32


Page 33 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

der Waals interactions with the surface. For 1-butoxy on Au(111), DFT calculations showed that van der Waals interactions account for 25% of the overall bond energy.65,

96

Because even

relatively small differences in binding strength can lead to large differences in the equilibrium surface coverages by the respective alkoxides, van der Waals interactions have a significant effect on the reaction selectivity for coupling of dissimilar alcohols. For linear alkoxides, the DFT calculations and gas phase acidities predict the same trends in binding strength because the van der Waals interactions trend positively with the polarizability of the alcohol. This provides additional molecular insight into understanding the correlation between gas phase acidity and surface stability, and together these results point to the importance of considering weak interactions of adsorbates with the surface in heterogeneous catalysis as a whole where competitive adsorption may be a factor. There are, however, some minor deviations from this correlation between gas phase acidity and binding strength, such as for fluorinated alcohols in which the fluorine creates repulsive forces with the surface, reducing the strength of the surface bond.96 Table 3. Impact of van der Waals forces on the calculated adsorption energies of different alkoxides on Au surfaces.a, b

a b

Adapted with permission from reference 65. Calculated hierarchy of the stability of intermediates on Au(110) and Au(111) obtained with PBE+vdWsurf and PBE methods using the reaction: 2BH(g) + O(ads) à 2B(ads) + H2O(g), to establish consistent reference for comparison between alkoxides. 33


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 53

5.2 Selectivity in the coupling of dissimilar alcohols These fundamental principles, which predict the surface stability of the alkoxides and the relative surface coverages of species as a function the ambient phase composition of the alcohols, account very well for the selectivity of oxygen-assisted coupling of dissimilar alcohols on both O/Au(111) at low pressure and np(Ag)Au at atmospheric pressure (Figure 7). Further, they provide insight into the success of methyl ester synthesis over supported Au catalysts in the liquid phase. The compositional dependence of the selectivity to different ester products in the coupling of methanol with ethanol and, separately, with 1-butanol is remarkably similar in model studies with O/Au(111) and on np(Ag)Au under catalytic conditions despite obvious differences in reaction conditions (Figures 7a,b and 14). For the coupling of methanol and ethanol in both cases, the highest yield of the methyl ester, methyl acetate, was achieved at a methanol mole fraction of around 0.85-0.9.63-64 Production of the homo-coupled product of ethanol, ethyl acetate, decreased sharply with increasing methanol mole fraction, while methyl formate was only observed at high excesses of methanol (χmethanol > 0.95). Similarly, with methanol and 1butanol maximum selectivity to the mixed ester took place at a methanol mole fraction of 0.90.95 and the production of the homo-coupled esters followed the same trends with increasing methanol mole fraction as in the coupling of methanol with ethanol (Figure 15). Further, in liquid phase reactions, optimal yields of methyl esters were also reached at higher excesses of methanol in coupling reactions with 1-hexanol,98, 115 1-octanol,80, 95, 102 as well as with other long chain alkyl alcohols80, ethylene glycol,105 and a series of benzylic alcohols.102 In each case of methyl ester synthesis, the mole fraction of methoxy must be in excess of the other alcohol (χmethanol > 0.95) to compensate for the stronger binding of the longer alcohols to the Au surface as a result of van der Waals interactions. It is important to note that formation of the aldehyde of the larger alcohol has a lower activation energy due to the easier cleavage of the β-CH bond in the respective adsorbed alkoxy moieties; the aldehyde of this alcohol reacts with the adsorbed methoxy to yield the surface-bound hemiacetal alcoholate, which reacts further to the ester. Thus, a greater excess of methanol is needed to compete effectively and reach a 50:50 coverage of the two alkoxides on the surface.63, 96 The equilibrium constant for competitive adsorption of methanol/1-butanol is higher than that for methanol/ethanol (Equation 5).63-64 Therefore, a greater proportion methanol is required in the vapor phase with 1-butanol than with ethanol to 34


Page 35 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reach optimal coupling between the dissimilar alcohols. Experiments in the liquid phase also showed that the selective coupling of benzyl alcohol with ethanol and n-propanol to form ethyl benzoate and propyl benzoate, respectively102 requires a large excess of ethanol or n-propanol relative to the benzyl alcohol for effective coupling of dissimilar alcohols.

Figure 15. Graphs of product selectivity as a function of methanol mole fraction in the coupling of butanol and methanol on (a) O/Au(111) and (b) np(Ag)Au. Adapted with permission from references 63 (a) and 64 (b). Copyright 2010 American Chemical Society (a) and 2015 Elsevier (b). Notably, for both model studies on O/Au(111) at low pressure and steady state catalytic reaction conditions at one atmosphere on np(Ag)Au, the methyl ester and the homo-coupled esters are observed, but the corresponding alkyl formates (ethyl formate and butyl formate) are not formed. This absence of alkyl formates is a consequence of the slow rate of β-hydrogen elimination from adsorbed methoxy.63 As stated earlier, the activation energies for β-hydrogen elimination from the different primary alkoxides decrease in the order methoxy, ethoxy, butoxy.63 This trend is consistent with previous studies on Ag and on Cu.116-117 Because the aldehydes of the longer alcohols, such as ethanol and 1-butanol, form more readily from their alkoxides than does formaldehyde from methanol, the rapid formation of the hemiacetal alcoholate via reaction between methoxy and acetaldehyde or butyraldehyde takes place preferentially. Thus, no ethyl formate or butyl formate is observed, and methyl formate is produced only at large methanol excesses.63 5.3 Coupling of alcohols and aldehydes

35


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 53

Clearly, direct coupling of methanol and aldehydes would increase the efficiency of methyl ester formation both in the gas phase29,

48, 118

and in the liquid phase102 by circumventing the rate-

limiting β-hydrogen elimination step. Such experiments further establish the kinetic favorability of the nucleophilic reaction of adsorbed methoxy with the aldehyde over β-hydrogen elimination from the adsorbed methoxy to yield the aldehyde followed by coupling to give methyl formate.29 Indeed, in the catalytic coupling of methanol and acetaldehyde over reactant-activated np(Ag)Au at 343 K and 1 atm pressure, methyl acetate was produced with near 100% selectivity even in vapor phase reactant mixtures containing high mole fractions of methanol.118 In contrast, significant formation of methyl formate (> 5% selectivity) was only observed when the methanol mole fraction was greater than 0.97. In agreement with the general mechanism established on the model Au surfaces, the product selectivities followed the same trend for the coupling of methanol and butyraldehyde over np(Ag)Au, and methyl butyrate was formed as the sole product except at extremely high methanol mole fractions. In addition, direct reaction with the aldehyde suppresses the secondary oxidation to organic acids and CO2 because the supply of O on the surface is limited and reaction with adsorbed methoxy prevails.29, 118 In the model studies with O/Au(111) less than 5% of organic acids (formic acid, acetic acid, or benzoic acid) was observed and no acids were formed over np(Ag)Au at atmospheric conditions.29, 118 No CO2 is produced under steady state reactor conditions. 6. Coupling of allylic alcohols: competitive oxidation of O-H and C=C bonds To selectively oxidatively esterify unsaturated alcohols to yield unsaturated methyl esters, catalysts must couple the alcohols without activating the C-C double bond (Figure 16). Unlike other transition metals, such as Pd, metallic Au does not itself activate C=C bonds. On Pd, for example, hydrogenation119-121 and combustion122 readily occur. On the other hand, model studies also demonstrate that metallic Au facilitates epoxidation of olefins, so some competitive side reactions might be expected on Au catalysts.123-124 Recent work shows in fact that Au catalysts are particularly well suited for the reactions. Model studies at low pressure have shown that Au cleanly facilitates the oxygen-assisted dehydrogenation of unsaturated alcohols to give unsaturated aldehydes.123, 125-126

36


Page 37 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 16. Schematic representation of the potential competing reaction pathways for unsaturated alcohols, such as allylic alcohol, on Pd and Au surfaces. Reprinted with permission from reference 127. Copyright 2007 De Gruyter. 6.1 Coupling of unsaturated alcohols (or aldehydes) in the gas phase The coupling of unsaturated alcohols with methanol follows essentially the same mechanism as that described above for saturated alcohols (Figure 12). As with the saturated alcohols discussed earlier, the relative surface coverages of different alkoxide species control selectivity in these reactions. The increased interaction strength of the adsorbed allyloxy with the surface due to the double bond shifts the reactant composition for optimum conversion to the mixed methyl ester to even higher methanol mole fraction.60 The equilibrium constant (Equation 5) for competition between methanol and allyl alcohol on np(Ag)Au is ten times greater than that for methanol and ethanol, and three times greater than the equilibrium constant for methanol and 1-butanol (Table 4). As a result, a maximum selectivity (approximately 20%) to form methyl acrylate occurs at ΧMethanol = 0.97 on Au(110) model surfaces and at ΧMethanol = 0.99 on np(Ag)Au under atmospheric pressure flow conditions at 423 K (Figure 7c,d).59-60 No methyl acrylate was observed in the model studies on Au(110) below a methanol mole fraction of 0.75 or below a methanol mole fraction of 0.8 in atmospheric pressure experiments, the primary product being the aldehyde (acrolein), as methanol cannot compete effectively for binding sites on the Au surface. The impact of molecular structure is even more dramatic for methyallyl alcohol. The 37


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 53

equilibrium constant for competitive binding on the Au surface for methanol and methyallyl alcohol is 4.5 times that of allyl alcohol (Table 4) as a consequence of the additional methyl group. As a result, an even greater methanol mole fraction is required to achieve optimum selectivity in the aerobic esterification of methyallyl alcohol to its corresponding methyl ester, methyl methacrylate. This again illustrates the key role of weak van der Waals interactions in controlling selectivity in the synthesis of methyl esters. Table 4. Equilibrium constants for the displacement of methoxy by different alkoxides on np(Ag)Au at 423 K.

Interestingly, for unsaturated alcohols in the gas phase, replacing the allyl alcohol (or methallyl alcohol) with its respective aldehyde does not simply increase the efficiency of coupling with methanol59 in contrast to the reactions of primary and benzylic alcohols described above.29, 48, 118 On np(Ag)Au under atmospheric pressure flow conditions at 423 K, a selectivity near 100% was achieved for the coupling of acrolein and methanol at a methanol mole fraction of 0.90 to form methyl acrylate.59 However, the conversion of acrolein was only around 3%. A maximum yield of methyl acrylate of 9.1% was obtained at a methanol mole fraction of 0.97. Similarly, for methacrolein and methanol, 100% selectivity to methyl methacrylate was observed at a methanol mole fraction of 0.92, but the conversion of methacrolein was only 1.2%. The yield of methyl methacrylate reached a maximum of 9.0% at a methanol mole fraction of 0.98. These low yields of the methyl esters are a result of adsorption site-blocking by acrylates, as shown by model studies on Au(110).59

38


Page 39 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Acrylate formation from reaction of the aldehyde with adsorbed O is facile, occurring as low as 240 K, which is well below the reaction temperature of 423 K employed for np(Ag)Au under catalytic conditions. Once formed, these acrylates cannot be displaced by methoxy,96 and do not react with methoxy to give methyl esters. Instead, they decompose at higher temperatures; for acrolein, the oxidation product is carbon suboxide, and for methacrolein it is methacrylic acid, both of which are stable up to 420 – 450 K.59 Therefore, when np(Ag)Au is exposed to both methanol and the unsaturated aldehyde simultaneously at 423 K under flow conditions, these acrylates block active sites on the surface, resulting in the low conversions observed. Some acrylate does decompose at 423 K, and thus there is still a low steady-state conversion of the unsaturated aldehyde. Increasing the reactor temperature to 498 K increases the rate of decomposition of the acrylates and significantly improves the conversion of the unsaturated aldehyde, confirming the negative influence of the acrylates on activity. However, at these higher temperatures selectivity shifts toward the homo-coupling of methanol, producing methyl formate. 6.2 Coupling of unsaturated alcohols (or aldehydes) in the liquid phase Esterification reactions of unsaturated aldehydes with methanol in the liquid phase over supported Au catalysts have higher reported conversions than reactions in the gas phase, which suggests that the acrylate surface poisons are either less stable or less predominant in the liquid phase. Au/β-Ga2O3 catalysts exhibited excellent conversion and selectivity to methyl ester formation at 363 K under 5 atm of O2 for a variety of aldehydes including acrolein, crotonaldehyde, and cinnamaldehyde.102 Notably, for acrolein and crotonaldehyde, both the conversions and the selectivities improved when coupling the aldehydes directly with methanol as compared to the coupling of the respective alcohols (allyl alcohol and crotyl alcohol) with methanol. This differs from the results of these reactions in the gas phase,59 and indicates that the fundamental mechanisms of the esterification of unsaturated alcohols in the liquid phase are qualitatively similar to those in the gas phase, they are quantitatively different. In addition, on supported catalysts such as Au/β-Ga2O3, where some reaction steps are proposed to take place on the oxide support or in the solution phase, reactivity and selectivity will differ from that on unsupported catalysts, such as np(Ag)Au.

39


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 53

A number of other examples of methyl ester formation from unsaturated aldehydes or unsaturated alcohols over supported catalysts in the liquid phase further illustrate the quantitative differences between the liquid and gas phases, while highlighting similarities. Under nonoptimized conditions, Au/TiO2 was also reported to couple acrolein and methanol with 87% selectivity and 97% conversion at 318 K and atmospheric pressure using methanol as a solvent in the presence of base.89 In the oxidative esterification of methacrolein with methanol, Au/CeO2 and Au/MgO both exhibit 99% selectivity to methyl methyacrylate with high conversion of methacrolein (98%).115 Au nanoparticles supported on basic hydrotalcite also give high conversion (99%) and selectivity (91%) for this same reaction.115 The reactions with methacrolein were conducted at ~2 atm of O2 and 70 °C using methanol as a solvent in the absence of base, and thus the improved conversion using Au/MgO and Au/HT was attributed to the increased basicity of these supports. The solid base is proposed to facilitate hemiacetal formation and deprotonation through a base-catalyzed pathway. In each of these examples methanol is in great excess relative to acrolein and methacrolein (Χmethanol = 0.98-0.99), conditions that are qualitatively similar to the reactant mixtures required for high selectivity to the coupling products in the gas phase.59-60 Coupling of cinnamyl alcohol with methanol to yield methyl esters has also been carried out at high conversions (>99%) over polymer-incarcerated Au92 and Au supported on hydrotalcite with atmospheric pressures of O2.95 In addition, homocoupling of allyl alcohol to form allyl acrylate has been observed over Au-ZnO catalysts with low conversion (23%) and moderate selectivity (76%).105 Overall, the oxidative coupling of unsaturated alcohols with methanol in the presence of base over supported Au in the liquid phase is very successful in yielding methyl esters with high selectivity. Further studies to understand the precise mechanistic similarities and differences in the oxidation coupling of unsaturated alcohols over Au in the gas and liquid phases would be instrumental in designing catalytic operating conditions that minimize catalyst poisoning by acrylate, as base-assisted reactions with unsaturated alcohols in the liquid phase appear less susceptible to this type of catalyst deactivation. 7. N-compounds: translating mechanisms to other reactions The use of model studies on single crystals under well-controlled conditions not only provides mechanistic understanding of working catalytic systems but also leads to the prediction of new 40


Page 41 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

patterns of surface reactivity that broadly advance catalyst design.44 As shown above, the mechanistic understanding of alcohol esterification gained from model studies led to accurate predictions of activity on working catalysts, with a remarkable correspondence in selectivity between the models and the working catalysts for complex reaction mixtures.63-64 Further, generalization of this mechanism to its underlying fundamental principles enabled the extension to other novel reactive pathways, including acylation and cyclization, which were subsequently validated by model studies and extended to working catalysts.44-47, 100, 128 In the most generalized version of the reaction mechanism, the synthesis of esters over Au catalysts involves the attack of a nucleophile on a molecule with an electron-deficient carbonyl carbon to yield an adsorbed coupled intermediate, which then undergoes β-hydride elimination to give the final product. In ester synthesis specifically, the nucleophile is a surface-bound alkoxy (in the formation of the hemiacetal alcoholate on the Au surface), and the electrophile is an aldehyde. The analogous reaction in solution is an alcohol reacting with the aldehyde to form the hemiacetal in solution. However, the scope of this reactive scheme need not be limited to ester synthesis, as the nucleophile and electrophile may be other species derived from oxygen-assisted activation on the Au surface which then access novel reaction pathways. 7.1 Mechanisms of amine coupling with alcohols on Au An excellent example of this generality is the oxygen-assisted synthesis of amides from alcohols and amines utilizing metallic Au catalysts.44-46,

100, 128

Early work demonstrated that oxygen-

activated Au activated N-H bonds in ammonia; 129 by analogy to the similar chemistry observed on metallic silver,130-131 similar results were expected for amines. Indeed, the N-H bond of the amine is activated by atomic oxygen adsorbed on Au to form surface-bound amides.44, 128 Thus in the presence of alcohol in the reactant mixture, the nucleophilic adsorbed amide can attack the electron-deficient carbonyl carbon of a co-adsorbed aldehyde produced by oxygen-assisted dehydrogenation of the alcohol to create an adsorbed hemiaminal alcoholate, which subsequently undergoes β-hydride elimination to give the coupled amide (Figure 17).44, 128 The aldehyde can also be introduced directly.44, 46, 128 For example, in model studies on Au single crystal surfaces the coupling of dimethylamine and formaldehyde readily produced dimethylformamide, while the coupling of dimethylamine and ethanol gave dimethylacetamide.44, 46, 128

41


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 53

Figure 17. Mechanism of oxygen-assisted coupling of methanol with dimethylamine on Au to yield dimethyl formamide. To simplify the complex interplay of simultaneous reactions taking place across the entire metal surface, the reactions in the figure are not mass-balanced. A more detailed description of the individual reaction steps can be found in reference 44. 7.2 Formylation of amines in the liquid phase over Au alloy catalysts Subsequent to these model studies, directly analogous results were observed on np(Ag)Au in the liquid phase in the absence of acid or base.100-101 At oxygen pressures greater than or equal to one atmosphere, dimethyl amine and methanol coupled in excess methanol with 100% selectivity to form dimethylformamide at 313 K.100 The authors proposed that this reaction takes place on the Au surface and follows the mechanism defined by the model studies on O/Au(111) (Figure 17).44, 128 Importantly, the conversion of the alcohol also increased with increasing alcohol chain length in studies with methanol, ethanol, and n-propanol, which mirrors the reactivity of the respective alcohols in ester formation on Au, where the relative ease of β-elimination to form the aldehyde increases with increasing alkoxy chain length.106 A more extensive study of the formylation of amines over np(Ag)Au in the liquid phase in excess methanol showed reactivity for a broad range of amine substrates, including primary and secondary amines as well as cyclic amines and benzylic amines.101 The authors proposed two 42


Page 43 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

possible mechanisms, which include reactions in solution. First, formaldehyde is formed from methanol on np(Ag)Au and then couples in solution with the amine to give a hemiaminal intermediate, which is then oxidized on np(Ag)Au to yield the amide. This mechanism is analogous to that proposed for alcohol coupling in solution and, as noted previously, this reaction could also take place on the Au surface, assisted by adsorbed O.44, 128 Second, they proposed that methyl formate, produced by coupling of formaldehyde and methoxy on np(Ag)Au, undergoes aminolysis in solution to produce the amide.101 They do not report the formation of methyl formate in their experiments, however, as might be expected. This mechanism is also postulated in other studies of alcohol-amine coupling reactions over supported Au.132-133 In a control experiment, the reaction of formaldehyde with octylamine over np(Ag)Au in the presence of O2 gave a “complex mixture” of products, while the reaction of methyl formate with octylamine in the absence of catalyst under N2 atmosphere gave a 97% yield of N-octylformamide,101 leading the authors to propose that the pathway involving the methyl formate intermediate was more likely. This mechanism, however, requires further consideration and it is clear that there are discrepancies regarding the proposed mechanism in the published literature. Other investigators report facile formation of methyl formate over npAu in the absence of amine but no methyl formate in its presence. They conclude that there is no direct reaction between methyl formate and the amine under aerobic conditions over np(Ag)Au.100 Thus the route via the hemiaminal and/or adsorbed hemiaminal alcoholate seems preferable. Second, displacement reactions similar to those reported in Table 2 show amines, particularly those with large pendant groups such as those attached to the amine function in the species studied76 are expected to readily displace methoxy, such that population of the surface by the conjugate base of the amine (its amide) would be expected to dominate the surface relative to methoxy. Thus, formaldehyde formed from methoxy decomposition would readily find a neighboring amide with which to react. Also, though formaldehyde and the adsorbed amide intermediate would be expected to readily couple, the introduction of the aldehyde to the reactant stream is expected to easily form surface formate and perhaps formic acid, which would complicate the reaction network, leading to unexpected products. The mechanism derived from the model studies remains a viable route.

43


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 53

Moreover, there are numerous reports of the coupling of alcohols and amines over a variety of supported Au catalysts in the solution phase which exhibit activity for an impressive scope of alcohol and amine reactants and, importantly, which come to similar conclusions about the mechanism of reactivity.81, 99, 132-138 In general it is commonly agreed that the aldehyde produced from the alcohol is an intermediate in the process, and that hemiaminal formation followed by its reaction on the Au is the final step to produce the amide. Competing dehydration of the hemiaminal may produce the imine as well.134 It is proposed that the aldehyde remains on the surface, and the hemiaminal must form as the adsorbed hemiaminal alcoholate for the amide to be produced.81 This claim regarding the importance of surface adsorbed aldehyde relative to free aldehyde in solution is supported by additional studies regarding alcohol-amine coupling over Au-Pd alloys and Au-Co mixed metal catalysts in the liquid phase.135-136 In general agreement with these results, high selectivity to amides is observed over Au6Pd catalysts supported on an ion-exchange resin, while similarly supported AuPd4 catalysts favor coupled imine production; the authors attribute this difference to the relative binding strength of the aldehyde to each catalyst.135 They propose that on the Au-rich catalysts in the coupling of benzyl alcohol and aniline, the aldehyde (benzaldehyde) is strongly adsorbed, due to the affinity of the Au for the carbonyl group. This favors reaction of the adsorbed aldehyde with the deprotonated amine on the Au surface (adsorbed amide) and formation of the amide (benzanilide). In contrast, they suggest that on Pd-rich catalysts, binding of the aldehyde is weaker, which leads to desorption of benzaldehyde and subsequent formation of the coupled imine (benzalaniline) from free aldehyde and amine in solution. However, it is well known from studies on clean metal single crystals that the binding of aldehydes on Pd is significantly stronger than on Au due to d-π bonding.139-140 It is possible that dehydration of the hemiaminal occurs readily on the Pd rich surface, whereas on the Au-rich material only dehydrogenation to the amide can occur. It is also possible that on the Pd-rich material O2 is more readily dissociated, leading to higher concentrations of adsorbed O. Under such conditions, the amine may be completely deprotonated, leading to a different reaction intermediate produced by reaction with the adsorbed alkoxy. In support of the hypothesis regarding the role of aldehyde adsorption strength, however, no free benzaldehyde was detected during the coupling of benzyl alcohol and aniline over Au6Pd/resin, while free aldehyde was detected in the same reaction over AuPd4/resin.135 Further, the imine can be formed from benzaldehyde and aniline in the absence 44


Page 45 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of catalyst, confirming that this side product can be generated in a solution based reaction. Clearly further studies are needed to resolve the mechanistic details. 7.3 Other reactions with amines Overall, understanding of the mechanism behind reactions with amines in complex liquid phase systems can be guided by fundamental principles of reactivity defined by model studies. In addition to the coupling of amines and alcohols, the specificity for the reaction between an amine and oxygen adsorbed on Au to give the nucleophilic conjugate base of the amine can also be postulated as a reactive step in a number of other reactions of amines on Au. For example, a number of studies have been conducted regarding oxidative reactions of amines over nonnanoparticulate Au (Au powder) including the oxidative hydrogenation of primary and secondary amines to imines141-142 and the oxidative reaction of amines with isocyanides143 and, separately, with carbon monoxide.144-145 In each case, activation of the N-H of the amine by oxygen on the Au surface is a reasonable mechanistic step. Further, the reaction of amines with carbon monoxide produces ureas, which is a directly analogous reaction to carbonylation of methanol to form dimethyl carbonate, discussed below (section 7.4), except that amines are used instead of methanol.47,

145

While detailed discussion of each of these reactions and their

mechanisms is beyond the scope of this review, the possibility that the fundamental reactivity of oxygen-activated gold gleaned from model surfaces on single crystal surfaces could reveal and predict a broader scope of reactions in the liquid phase highlights an opportunity for additional experiments to understand the precise reactivity that is driving these systems. 7.4 Analogous coupling reactions: carbonylation to dimethyl carbonate In a separate but analogous class of coupling reactions, the electron-deficient carbonyl of the aldehyde can be replaced by a carbon dioxide (CO) molecule, which undergoes sequential nucleophilic attack by two surface-adsorbed methoxy species to yield dimethyl carbonate (Figure 18).47 The development of an environmentally sustainable route to dimethyl carbonate production is industrially relevant, because dimethyl carbonate it is itself used as a more sustainable solvent,146 transesterification reagent in biodiesel production,147 and methylation reagent, replacing toxic methyl halides and phosgene (COCl2).148 The current industrial process for dimethyl carbonate synthesis generates caustic hydrochloric acid as a byproduct,149 and thus

45


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 53

the above method for direct synthesis from methanol and CO over Au is a significant advance in sustainability.47

Figure 18. Mechanism of oxygen-assisted coupling of methanol with carbon monoxide on Au to yield dimethyl carbonate. To simplify the complex interplay of simultaneous reactions taking place across the entire metal surface, the reactions in the figure are not mass-balanced. A more detailed description of the individual reaction steps can be found in reference 47. 8. Conclusion and Outlook The broad scope of the heterogeneous Au-catalyzed oxygen-assisted coupling reactions described in this review underscores the utility and potential of this class of catalytic reactions for improving the sustainability of industrial ester, amide, and imine synthesis. Further, the breadth of new reactive pathways that have been predicted through analogy to the fundamental principles of acid-base chemistry defined on the Au surface emphasizes the importance of establishing an intimate understanding of the elementary steps involved in heterogeneously catalyzed reactions on Au, both in the gas phase and in the liquid phase. This detailed knowledge of reaction mechanisms then enables extrapolation of specific mechanisms for a particular set of substrates to broader classes of reactions, thus opening up exciting new avenues of reactivity.

46


Page 47 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Further, a critical comparison of the mechanisms proposed for oxygen-assisted coupling over Au catalysts in liquid phase and gas phase reactions highlights key areas of opportunity for collaboration between the gas phase and liquid phase catalysis communities to more precisely define mechanisms of reactivity in complex solution-based catalytic systems. The heterogeneously catalyzed reactions in solution clearly involve an interplay between surface reactivity and reactions in solution. The balance of these processes has not been clearly determined. It is clear from the work to date that the knowledge of surface reactivity gathered from controlled, well-defined studies of reactions on surfaces would be a positive addition to the lexicon of organic chemists interested in the use of heterogeneous catalysts for synthesis. Acknowledgments This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award #DE-SC0012573. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Riemenschneider, W.; Bolt, H. M., Esters, Organic. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: 2000. Roughley, S. D.; Jordan, A. M. J. Med. Chem. 2011, 54, 3451-3479. Pattabiraman, V. R.; Bode, J. W. Nature 2011, 480, 471-479. Larock, R. C., Comprehensive organic transformations: a guide to functional group preparations. 2nd ed. ed.; Wiley-VCH: New York, 1999. Rebsdat, S.; Mayer, D., Ethylene Oxide. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: 2000. Müller, H., Sulfuric Acid and Sulfur Trioxide. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: 2000. Jorgensen, B.; Egholmchristiansen, S.; Dahlthomsen, M.; Christensen, C. J. Catal. 2007, 251, 332-337. Personick, M. L.; Zugic, B.; Biener, M. M.; Biener, J.; Madix, R. J.; Friend, C. M. ACS Catal. 2015, 5, 4237-4241. Hiebel, F.; Montemore, M. M.; Kaxiras, E.; Friend, C. M. Surf. Sci. 2016, 650, 5-10. Outka, D. A.; Madix, R. J. Surf. Sci. 1987, 179, 351-360. Outka, D. A.; Madix, R. J. Surf. Sci. 1987, 179, 361-376. Outka, D. A.; Madix, R. J. J. Am. Chem. Soc. 1987, 109, 1708-1714. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405-408. Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175-192. Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566-575. Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270-282. 47


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 53

17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48.

Canning, N. D. S.; Outka, D.; Madix, R. J. Surf. Sci. 1984, 141, 240-254. Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 523, 89-102. Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Am. Chem. Soc. 2004, 126, 1606-1607. Wang, J.; Voss, M. R.; Busse, H.; Koel, B. E. J. Phys. Chem. B 1998, 102, 4693-4696. Deng, X.; Min, B. K.; Guloy, A.; Friend, C. M. J. Am. Chem. Soc. 2005, 127, 9267-9270. Min, B. K.; Deng, X.; Pinnaduwage, D.; Schalek, R.; Friend, C. M. Phys. Rev. B 2005, 72, 121410(R). Liu, X.; Madix, R. J.; Friend, C. M. Chem. Soc. Rev. 2008, 37, 2243-2261. Xu, B.; Liu, X.; Haubrich, J.; Madix, R. J.; Friend, C. M. Angew. Chem. Int. Ed. 2009, 48, 4206-4209. Min, B. K.; Alemozafar, A. R.; Biener, M. M.; Biener, J.; Friend, C. M. Top. Catal. 2005, 36, 77-90. Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366-386. Falconer, J. L.; Madix, R. J. Surf. Sci. 1975, 48, 393-405. Wachs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208-227. Xu, B.; Liu, X.; Haubrich, J.; Friend, C. M. Nat. Chem. 2010, 2, 61-65. Xu, B.; Haubrich, J.; Baker, T. A.; Kaxiras, E.; Friend, C. M. J. Phys. Chem. C 2011, 115, 3703-3708. Campbell, C. T.; Árnadóttir, L.; Sellers, J. R. V. Zeitschrift für Physikalische Chemie 2013, 227, 1435–1454. Madix, R. J.; Telford, S. G. Surf. Sci. 1995, 328, L576-L581. Fristrup, P.; Johansen, L. B.; Christensen, C. H. Catal. Lett. 2007, 120, 184-190. Griffin, M. B.; Rodriguez, A. A.; Montemore, M. M.; Monnier, J. R.; Williams, C. T.; Medlin, J. W. J. Catal. 2013, 307, 111-120. Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Science 2010, 330, 74-78. Quiller, R. G.; Baker, T. A.; Deng, X.; Colling, M. E.; Min, B. K.; Friend, C. M. J. Chem. Phys. 2008, 129, 064702. Chang, C.-R.; Yang, X.-F.; Long, B.; Li, J. ACS Catal. 2013, 3, 1693-1699. Shang, C.; Liu, Z. P. J. Am. Chem. Soc. 2011, 133, 9938-9947. Remediakis, I. N.; Lopez, N.; Nørskov, J. K. Angew. Chem. Int. Ed. 2005, 44, 1824-1826. Lopez, N.; Nørskov, J. K. J. Am. Chem. Soc. 2002, 124, 11262-11263. Jiang, T.; Mowbray, D. J.; Dobrin, S.; Falsig, H.; Hvolbæk, B.; Bligaard, T.; Nørskov, J. K. J. Phys. Chem. C 2009, 113, 10548-10553. Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nat. Chem. 2009, 1, 37-46. Fernández, E. M.; Moses, P. G.; Toftelund, A.; Hansen, H. A.; Martínez, J. I.; AbildPedersen, F.; Kleis, J.; Hinnemann, B.; Rossmeisl, J.; Bligaard, T.; Nørskov, J. K. Angew. Chem. Int. Ed. 2008, 47, 4683-4686. Xu, B.; Friend, C. M.; Madix, R. J. Faraday Discuss. 2011, 152, 241-252. Siler, C. G.; Xu, B.; Madix, R. J.; Friend, C. M. J. Am. Chem. Soc. 2012, 134, 1260412610. Xu, B.; Madix, R. J.; Friend, C. M. Chem. Eur. J. 2012, 18, 2313-2318. Xu, B.; Madix, R. J.; Friend, C. M. J. Am. Chem. Soc. 2011, 133, 20378-20383. Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Bäumer, M. Science 2010, 327, 319-322. 48


Page 49 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73.

Wang, L.-C.; Friend, C. M.; Fushimi, R.; Madix, R. J. Faraday Discuss. 2016, 188, 5767. Abad, A.; Corma, A.; García, H. Chem. Eur. J. 2008, 14, 212-222. Abad, A.; Almela, C.; Corma, A.; García, H. Tetrahedron 2006, 62, 6666-6672. Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450-453. Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42-43. Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M. ACS Catal. 2015, 5, 6263-6270. Petegem, S. V.; Brandstetter, S.; Maass, R.; Hodge, A. M.; El-Dasher, B. S.; Biener, J.; Schmitt, B.; Borca, C.; Swygenhoven, H. V. Nano. Lett. 2009, 9, 1158-1163. Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; Ishikawa, Y.; Asao, N.; Yamamoto, Y.; Erlebacher, J.; Chen, M. Nat. Mater. 2012, 11, 775-780. Stowers, K. J.; Madix, R. J.; Biener, M. M.; Biener, J.; Friend, C. M. Catal. Lett. 2015, 145, 1217-1223. Stowers, K. J.; Madix, R. J.; Friend, C. M. J. Catal. 2013, 308, 131-141. Karakalos, S.; Zugic, B.; Stowers, K. J.; Biener, M. M.; Biener, J.; Friend, C. M.; Madix, R. J. Surf. Sci. 2016, 652, 58-66. Zugic, B.; Karakalos, S.; Stowers, K. J.; Biener, M. M.; Biener, J.; Madix, R. J.; Friend, C. M. ACS Catal. 2016, 6, 1833-1839. Wang, L.; Personick, M. L.; Karakalos, S. G.; Fushimi, R.; Friend, C. M.; Madix, R. J. J. Catal. 2016, DOI: 10.1016/j.jcat.2016.08.012. Fujita, T.; Tokunaga, T.; Zhang, L.; Li, D.; Chen, L.; Arai, S.; Yamamoto, Y.; Hirata, A.; Tanaka, N.; Ding, Y.; Chen, M. Nano. Lett. 2014, 14, 1172-1177. Xu, B.; Madix, R. J.; Friend, C. M. J. Am. Chem. Soc. 2010, 132, 16571-16580. Wang, L.-C.; Stowers, K. J.; Zugic, B.; Personick, M. L.; Biener, M. M.; Biener, J.; Friend, C. M.; Madix, R. J. J. Catal. 2015, 329, 78-86. Karakalos, S.; Xu, Y.; Cheenicode Kabeer, F.; Chen, W.; Rodríguez-Reyes, J. C. F.; Tkatchenko, A.; Kaxiras, E.; Madix, R. J.; Friend, C. M. J. Am. Chem. Soc. 2016, 138, 15243-15250. Xu, B.; Siler, C. G.; Madix, R. J.; Friend, C. M. Chem. Eur. J. 2014, 20, 4646-4652. Kosuda, K. M.; Wittstock, A.; Friend, C. M.; Baumer, M. Angew. Chem. Int. Ed. 2012, 51, 1698-1701. Nyce, G. W.; Hayes, J. R.; Hamza, A. V.; Satcher, J. H. Chem. Mater. 2007, 19, 344-346. Schaefer, A.; Ragazzon, D.; Wittstock, A.; Walle, L. E.; Borg, A.; Bäumer, M.; Sandell, A. J. Phys. Chem. C 2012, 116, 4564-4571. Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Bäumer, M. Angew. Chem. Int. Ed. 2006, 45, 8241-8244. Sankar, M.; Dimitratos, N.; Miedziak, P. J.; Wells, P. P.; Kiely, C. J.; Hutchings, G. J. Chem. Soc. Rev. 2012, 41, 8099-8139. Jorgensen, S. W.; Madix, R. J. Surf. Sci. 1987, 183, 27-43. Wang, R.; Wu, Z.; Chen, C.; Qin, Z.; Zhu, H.; Wang, G.; Wang, H.; Wu, C.; Dong, W.; Fan, W.; Wang, J. Chem. Commun. 2013, 49, 8250-8252.

49


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 53

74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105.

Whiting, G. T.; Kondrat, S. A.; Hammond, C.; Dimitratos, N.; He, Q.; Morgan, D. J.; Dummer, N. F.; Bartley, J. K.; Kiely, C. J.; Taylor, S. H.; Hutchings, G. J. ACS Catal. 2015, 5, 637-644. Kaizuka, K.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc. 2010, 132, 15096-15098. Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811-814. Enache, D. I.; Knight, D. W.; Hutchings, G. J. Catal. Lett. 2005, 103, 43-52. Oliveira, R. L.; Kiyohara, P. K.; Rossi, L. M. Green Chem. 2009, 11, 1366-1370. Smolentseva, E.; Costa, V. V.; Cotta, R. F.; Simakova, O.; Beloshapkin, S.; Gusevskaya, E. V.; Simakov, A. ChemCatChem 2015, 7, 1011-1017. Hao, Y.; Chong, Y.; Li, S.; Yang, H. J. Phys. Chem. C 2012, 116, 6512-6519. Chng, L. L.; Yang, J.; Ying, J. Y. ChemSusChem 2015, 8, 1916-1925. Haruta, M. Nature 2005, 437, 1098-1099. Boronat, M.; Corma, A. Dalton Trans. 2010, 39, 8538-8546. Min, B. K.; Friend, C. M. Chem. Rev. 2007, 107, 2709-2724. Klyushin, A. Y.; Greiner, M. T.; Huang, X.; Lunkenbein, T.; Li, X.; Timpe, O.; Friedrich, M.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. ACS Catal. 2016, 6, 3372-3380. Roldán, A.; González, S.; Ricart, J. M.; Illas, F. ChemPhysChem 2009, 10, 348-351. Montemore, M. M.; Madix, R. J.; Friend, C. M. In preparation. Farnesi Camellone, M.; Marx, D. J. Phys. Chem. C 2014, 118, 20989-21000. Marsden, C.; Taarning, E.; Hansen, D.; Johansen, L.; Klitgaard, S. K.; Egeblad, K.; Christensen, C. H. Green Chem. 2008, 10, 168-170. Yasukawa, T.; Miyamura, H.; Kobayashi, S. Chem. Asian J. 2011, 6, 621-627. Klitgaard, S. K.; DeLa Riva, A. T.; Helveg, S.; Werchmeister, R. M.; Christensen, C. H. Catal. Lett. 2008, 126, 213-217. Miyamura, H.; Yasukawa, T.; Kobayashi, S. Green Chem. 2010, 12, 776-778. Christensen, C. H.; Jorgensen, B.; Rass-Hansen, J.; Egeblad, K.; Madsen, R.; Klitgaard, S. K.; Hansen, S. M.; Hansen, M. R.; Andersen, H. C.; Riisager, A. Angew. Chem. Int. Ed. 2006, 45, 4648-4651. Corma, A.; Domine, M. E. Chem. Commun. 2005, 4042-4044. Liu, P.; Li, C.; Hensen, E. J. M. Chem. Eur. J. 2012, 18, 12122-12129. Rodriguez-Reyes, J. C.; Siler, C. G.; Liu, W.; Tkatchenko, A.; Friend, C. M.; Madix, R. J. J. Am. Chem. Soc. 2014, 136, 13333-13340. Rodríguez-Reyes, J. C. F.; Friend, C. M.; Madix, R. J. Surf. Sci. 2012, 606, 1129-1134. Nielsen, I. S.; Taarning, E.; Egeblad, K.; Madsen, R.; Christensen, C. H. Catal. Lett. 2007, 116, 35-40. Ishida, T.; Haruta, M. ChemSusChem 2009, 2, 538-541. Wichmann, A.; Bäumer, M.; Wittstock, A. ChemCatChem 2015, 7, 70-74. Tanaka, S.; Minato, T.; Ito, E.; Hara, M.; Kim, Y.; Yamamoto, Y.; Asao, N. Chem. Eur. J. 2013, 19, 11832-11836. Su, F.-Z.; Ni, J.; Sun, H.; Cao, Y.; He, H.-Y.; Fan, K.-N. Chem. Eur. J. 2008, 14, 71317135. Abad, A.; Concepción, P.; Corma, A.; García, H. Angew. Chem. Int. Ed. 2005, 44, 40664069. Liu, X.; Xu, B.; Haubrich, J.; Madix, R. J.; Friend, C. M. J. Am. Chem. Soc. 2009, 131, 5757-5759. Hayashi, T.; Inagaki, T.; Itayama, N.; Baba, H. Catal. Today 2006, 117, 210-213. 50


Page 51 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138.

Xu, B.; Friend, C. M. Faraday Discuss. 2011, 152, 307-320. Gong, J.; Mullins, C. B. J. Am. Chem. Soc. 2008, 130, 16458-16459. Gong, J.; Flaherty, D. W.; Yan, T.; Mullins, C. B. ChemPhysChem 2008, 9, 2461-2466. Meenakshisundaram, S.; Nowicka, E.; Miedziak, P. J.; Brett, G. L.; Jenkins, R. L.; Dimitratos, N.; Taylor, S. H.; Knight, D. W.; Bethell, D.; Hutchings, G. J. Faraday Discuss. 2010, 145, 341-356. Xu, B.; Madix, R. J.; Friend, C. M. Acc. Chem. Res. 2014, 47, 761-72. Barteau, M. A.; Bowker, M.; Madix, R. J. Surf. Sci. 1980, 94, 303-322. Barteau, M. A.; Madix, R. J. Surf. Sci. 1982, 120, 262-272. Lias, S. G.; Bartmess, J. E., Gas-phase ion thermochemistry. National Institute of Standards and Technology: Gaithersburg, MD. Luo, Y. R., Comprehensive handbook of chemical bond energies. CRC Press: Boca Raton, FL, 2007. Wan, X.; Deng, W.; Zhang, Q.; Wang, Y. Catal. Today 2014, 233, 147-154. Wachs, I. E.; Madix, R. J. Applications of Surface Science 1978, 1, 303-328. Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 531-558. Wang, L.-C.; Stowers, K. J.; Zugic, B.; Biener, M. M.; Biener, J.; Friend, C. M.; Madix, R. J. Catal. Sci. Technol. 2015, 5, 1299-1306. Vasquez, N.; Madix, R. J. J. Catal. 1998, 178, 234-252. Ide, M. S.; Hao, B.; Neurock, M.; Davis, R. J. ACS Catal. 2012, 2, 671-683. Shekhar, R.; Barteau, M. A. Surf. Sci. 1994, 319, 298-314. Guo, X.-C.; Madix, R. J. J. Am. Chem. Soc. 1995, 117, 5523-5530. Deng, X.; Min, B. K.; Liu, X.; Friend, C. M. J. Phys. Chem. B 2006, 110, 15982-15987. Deng, X.; Friend, C. M. J. Am. Chem. Soc. 2005, 127, 17178-17179. Mullen, G. M.; Zhang, L.; Evans, E. J., Jr.; Yan, T.; Henkelman, G.; Mullins, C. B. Phys. Chem. Chem. Phys. 2015, 17, 4730-4738. Mullen, G. M.; Zhang, L.; Evans, E. J., Jr.; Yan, T.; Henkelman, G.; Mullins, C. B. J. Am. Chem. Soc. 2014, 136, 6489-6498. Abad, A.; Corma, A.; García, H. Pure and Applied Chemistry 2007, 79, 1847-1854. Xu, B.; Zhou, L.; Madix, R. J.; Friend, C. M. Angew. Chem. Int. Ed. 2010, 49, 394-398. Deng, X.; Baker, T. A.; Friend, C. M. Angew. Chem. Int. Ed. 2006, 45, 7075-7078. Thornburg, D. M.; Madix, R. J. Surf. Sci. 1990, 226, 61-76. Thornburg, D. M.; Madix, R. J. Surf. Sci. 1989, 220, 268-294. Kegnaes, S.; Mielby, J.; Mentzel, U. V.; Jensen, T.; Fristrup, P.; Riisager, A. Chem. Commun. 2012, 48, 2427-2429. Klitgaard, S. K.; Egeblad, K.; Mentzel, U. V.; Popov, A. G.; Jensen, T.; Taarning, E.; Nielsen, I. S.; Christensen, C. H. Green Chem. 2008, 10, 419-423. Wang, W.; Cong, Y.; Zhang, L.; Huang, Y.; Wang, X.; Zhang, T. Tetrahedron Lett. 2014, 55, 124-127. Zhang, L.; Wang, W.; Wang, A.; Cui, Y.; Yang, X.; Huang, Y.; Liu, X.; Liu, W.; Son, J.Y.; Oji, H.; Zhang, T. Green Chem. 2013, 15, 2680-2684. Soulé, J.-F.; Miyamura, H.; Kobayashi, S. Chem. Asian J. 2013, 8, 2614-2626. Soulé, J.-F.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc. 2011, 133, 18550-18553. Wang, Y.; Zhu, D.; Tang, L.; Wang, S.; Wang, Z. Angew. Chem. Int. Ed. 2011, 50, 89178921.

51


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 53

139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149.

Shekhar, R.; Barteau, M. A.; Plank, R. V.; Vohs, J. M. J. Phys. Chem. B 1997, 101, 79397951. Silva, T. A. G.; Teixeira-Neto, E.; López, N.; Rossi, L. M. Sci. Rep. 2014, 4, 5766. Zhu, B.; Angelici, R. J. Chem. Commun. 2007, 2157-2159. Zhu, B.; Lazar, M.; Trewyn, B. G.; Angelici, R. J. J. Catal. 2008, 260, 1-6. Lazar, M.; Angelici, R. J. J. Am. Chem. Soc. 2006, 128, 10613-10620. Angelici, R. J. Catal. Sci. Technol. 2013, 3, 279-296. Zhu, B.; Angelici, R. J. J. Am. Chem. Soc. 2006, 128, 14460-14461. Miao, X.; Fischmeister, C.; Bruneau, C.; Dixneuf, P. H. ChemSusChem 2008, 1, 813-816. Fabbri, D.; Bevoni, V.; Notari, M.; Rivetti, F. Fuel 2007, 86, 690-697. Tundo, P.; Selva, M. Acc. Chem. Res. 2002, 35, 706-716. Keller, N.; Rebmann, G.; Keller, V. J. Mol. Catal. A: Chem 2010, 317, 1-18.

TOC Graphic

52


Page 53 of 53

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Selective Oxygen-Assisted Reactions of Alcohols and Amines

Recommend Documents