Phenomena Affecting Catalytic Reactions at SolidâLiquid Interfaces

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Phenomena Affecting Catalytic Reactions at Solid-Liquid Interfaces Carsten Sievers, Yu Noda, Long Qi, Elise M. Albuquerque, Robert M. Rioux, and Susannah L Scott ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02532 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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Phenomena Affecting Catalytic Reactions at Solid-Liquid Interfaces Carsten Sievers,1,2,3* Yu Noda,4 Long Qi,5,6 Elise M. Albuquerque,1,7 Robert M. Rioux,4,8 Susannah L. Scott5,6,* 1

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States

2

School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, United States

3

Renewable Bioproducts Institute, Georgia Institute of Technology, Atlanta, GA 30332, United States 4

Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802-4400, United States

5

Department of Chemical Engineering, University of California, Santa Barbara CA 93106-5080, United States

6

Department of Chemistry & Biochemistry, University of California, Santa Barbara CA 931069510, United States 7 8

Instituto Militar de Engenharia, 22290-270 Rio de Janeiro, RJ, Brazil

Department of Chemistry, Pennsylvania State University, University Park, PA 16802-4400, United States

*C.S.: Tel: +1-404-385-7685, Fax: +1-404-894-2866, Email: [email protected] S.S.: Tel: +1-805-893-5606, Fax: +1-805-893-4731, Email: [email protected]

Abstract: Interest in liquid phase reactions over heterogeneous catalysts is growing rapidly, partly because of the desire to find efficient methods for biomass conversion to renewable fuels and chemicals. The presence of a solvent can affect reactions at surfaces by competing with reactants and products for adsorption sites, and solvating adsorbed species. Mass transport limitations can also have a pronounced effect on liquid phase reaction rates. Because many heterogeneous catalysts were designed to be stable under gas phase reaction conditions, their 1 ACS Paragon Plus Environment

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operation in liquid reaction media at moderately elevated temperatures can result in unexpected structural changes. In some cases, components derived from the evolving catalyst contribute significantly to the catalytic activity. Solvents, as well as by-products from biomass feedstocks, can also act as homogeneous catalysts to alter the intrinsic reactivity of the heterogeneous catalyst. In this contribution, we discuss each of these phenomena and provide illustrative examples.

Keywords: Biomass, Solvation, Aqueous Phase, Transport, Catalyst Stability

1 Introduction The political, economic, social and environmental consequences of the widespread use of nonrenewable carbon resources have combined to motivate the valorization of non-food biomass via its transformation to renewable chemicals and fuels, and have inspired significant efforts to design efficient heterogeneous catalysts for such processes.1,2 The low volatility of many biomass-derived compounds makes their transport in the gas-phase extremely challenging, therefore catalysts must be able to operate, at moderately elevated temperatures, in the presence of aqueous or organic solvents or mixtures thereof. Heterogeneous catalysts can interact strongly with solvents, and may undergo profound and possibly irreversible changes in their structure, catalytic activity and selectivity due to interfacial adsorption and/or reactions with solvent molecules. Identifying, understanding and controlling the complex relationships between the structure of a heterogeneous catalyst and the behavior of its active sites in contact with a solution are necessary elements of rational catalyst design. Indeed, these newly recognized needs have revived interest fundamental research on (catalytic) solid-liquid interfaces.3 In particular, there is now much greater acknowledgement of issues related to selectivity and stability for inorganic catalysts such as zeolites and metal nanoparticles supported on amorphous oxides, which were originally developed for upgrading petroleum and used in early biomass valorization studies, when they are repurposed for sustained use in liquid phase operating conditions. Our fundamental understanding of reactivity at solid-liquid interfaces4-6 is currently much less well-developed compared to our knowledge of the molecular details of gas-solid reactions.7,8 In part, this is a consequence of a historical emphasis on transformations of volatile hydrocarbons, as described above. The development of appropriate in-situ tools (e.g., spectroscopic methods) to 2 ACS Paragon Plus Environment

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interrogate mass and energy partitioning/transfer at solid-liquid interfaces is challenging because the presence of large numbers of solvent molecules leads to significant background signals in many techniques.9-12 Nevertheless, the combination of in-situ spectroscopy and modeling studies is presently the only conceivable way to obtain the necessary insight.12 In studies of homogeneous catalysis, similar solvent effects are well-documented, and they have been shown to have profound implications for catalytic activity.13 Depending on the catalyst, the active site may be exposed to the solvent, or protected from it by bulky and/or hydrophobic organic ligands. Indeed, organization of the solvent near the active site is partly responsible for the extraordinary activity and selectivity of many enzymes, which restrict solvent access to the active site by locating it in a hydrophobic pocket, or promote access to the active site by positioning nearby peptide residues capable of orienting solvent molecules in key locations. In contrast, there has been far less investigation of the interaction of solvent molecules and solvated reactants with active sites in heterogeneous catalysis. Analogies between gas-phase and liquid-phase catalysis are appealing in their simplicity, but often limited in their utility. For example, Dumesic and co-workers have shown that conventional heterogeneous catalysts (developed for petrochemical transformations) are capable of converting aqueous sugars to chemicals and fuels (so called aqueous-phase reforming).14-17 However, the activity, selectivity and stability of these catalysts are strongly influenced by solvent effects. The presence of water (or other solvents) in biomass-derived feed streams not only has implications for catalyst materials selection, but can influence reactivity and selectivity by adsorbing to and blocking active sites, preferentially solvating certain soluble components of the reaction mixture, (de)stabilizing adsorbed reactants and transition states. It is also possible for water to participate directly in the reaction as a co-catalyst, for example, as a proton shuttle in processes that involve tautomerization.18 Evaluation of a heterogeneous catalyst’s effectiveness at promoting a particular chemical transformation requires a careful assessment of the effect of mass transport on the kinetic behavior. Relative to an analogous gas-phase reaction, the sheer number of solvent molecules can present a formidable barrier to diffusion of reactants and products in nanoporous materials. Analysis is further complicated by solvent partitioning, which can cause the fluid composition inside the pores to differ from that of the bulk phase. The influence of the solvents may also extend to structural changes of the active phase and/or supporting phase, which eventually 3 ACS Paragon Plus Environment

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impacts catalytic behavior. It is possible that the catalytically active species does not even remain associated with the heterogeneous catalyst during a liquid-phase reaction. A standard hot filtration test can demonstrate whether a species leached into solution from the heterogeneous phase is responsible for the observed catalytic activity.19 In this Perspective, we focus on fundamental issues of current interest in heterogeneous catalysis at solid-liquid interfaces. While motivated by the catalytic valorization of biomass, in which water plays a prominent role as primary solvent or solution component, most ideas are also relevant to organic solvent systems. We discuss adsorption and surface reactions at these “buried” interfaces, emphasizing how the presence of a solvent can influence elementary steps in a catalytic cycle (Section 2). The impact of transport effects must also be considered in order to obtain meaningful structure-function relationships (Section 3), considering that mass transfer effects are likely more important than heat transfer effects in such systems. In Section 4, catalyst stability and identification of the active sites are discussed, since these are relevant to prospects for the wider use of heterogeneous catalytic processes in biomass valorization. In particular, oxide-based catalytic supports are highly susceptible to degradation in aqueous solution due to direct reaction between water and the support. Finally, we evaluate the possibilities for release of catalytically active species that are ultimately responsible for the observed activity and selectivity (Section 5). Both structural degradation and active site leaching have major implications for catalyst lifetime and recyclability. Illustrative examples are provided in each section.

2 Adsorption and Surface Reactions of Oxygenates in Liquid Phase Most biomass-derived oxygenates with two or more functional groups have low or negligible vapor pressure. Therefore, many processes for their conversion will be performed with liquid phase feedstocks, in which the reactant is dissolved in a solvent. The presence of solvent molecules can influence the reaction in variety of ways.20 In this section, we will illustrate how they can compete for adsorption sites and stabilize adsorbed reactants on surfaces.

2.1 Competitive Adsorption A major difference between catalytic reactions that take place from the liquid rather than the gas phase is that potential adsorption sites at solid-liquid interfaces are essentially completely 4 ACS Paragon Plus Environment

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covered with reactants, intermediates, products, spectators, or solvent molecules at all times. Therefore, the initial coordination of a reactant to the surface should be understood as involving displacement of one or more other adsorbed species, rather than simply adsorption on an empty surface site. In addition, when they are not adsorbed on the surface, all such molecules engage in multiple interactions with other molecules (e.g., solvent) in the liquid phase. Consequently, the driving force for surface interaction includes not only the free energy change for adsorption of the adsorbate (A) onto a surface site (ΔGads,A), but also free energy changes associated with the desorption of previously adsorbed species (B) (ΔGads,C), interactions of A and B with solvent, both in solution and on the surface (ΔGsolv,solution and ΔGsolv,surface) and sorbate-sorbate interactions for adsorbed A and C (ΔGsorbate-sorbate): ∆𝐺 = ∆𝐺𝑎𝑎𝑎,𝐴 − ∆𝐺𝑎𝑎𝑎,𝐵 − ∆𝐺𝑠𝑠𝑠𝑠,𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐴 + ∆𝐺𝑠𝑠𝑠𝑠,𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐴 + ∆𝐺𝑠𝑠𝑠𝑠,𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐵 − ∆𝐺𝑠𝑠𝑠𝑠,𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐵 + ∆𝐺𝑠𝑠𝑠𝑠𝑠𝑠𝑠−𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐴 − ∆𝐺𝑠𝑠𝑠𝑠𝑠𝑠𝑠−𝑠𝑠𝑠𝑠𝑠𝑠𝑠,𝐵

The manifestation of this complexity was described in an earlier review as “solvent-induced nonideality”.21 Of course, the same reasoning applies separately to the enthalpy and entropy of adsorption, and heats of adsorption measured in such experiments are net values that reflect more than simply the adsorbate-surface interaction.. Unlike in gas phase systems, it is conceivable that adsorption could occur even though the net enthalpy of adsorption is positive (i.e., endothermic), provided the net entropic contribution to ΔG (i.e., -TΔS) is sufficiently negative. It is also important to recall that competitive adsorption is specific to certain sites. Thus, a reaction could take place at a particular site solely because other sites are blocked by adsorbed species other than the reactants. The complex interplay between sorbates, co-adsorbed species, solvents, and different surface sites can alter the relative abundance of surface species. For example, in liquid phase hydrogenation reactions, polar solvents have been reported to enhance the adsorption of nonpolar reactants and vice-versa.21 The presence of water has been reported to either limit22,23 or enhance24 the adsorption of organic oxygenates on different solid catalysts. The limiting effect is typically attributed to water competing for adsorption sites. In contrast, enhancement of oxygenate adsorption on silica was explained by stronger hydrogen bonding between water molecules in solution relative to their interaction with the silica surface or with other dissolved 5 ACS Paragon Plus Environment

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molecules. This effect resulted in preferential adsorption of alcohols on hydrophilic fused silica, as well as on hydrophobic alkylsilane-covered silica surfaces.24 Adsorption of organic molecules from the aqueous phase can also be strongly influenced by the electrolyte concentration and pH of the solution.25 For example, the surfaces of aluminas are positively charged in contact with acidic solutions, and negatively charged in contact with basic solutions.26,27 The highest adsorption capacity for sucrose on alumina was measured close to the isoelectric point of the sorbent (pH = 9.0).26 Another study showed that acetonitrile competes with citronellal for the SnIV sites in Sn-BEA, Sn-SBA-15, and Sn-SiO2.5 The interaction reduces the turnover frequency for cyclization of citronellal compared to the reaction in toluene. This effect is less pronounced in case of Sn-BEA due to the lower concentration of acetonitrile inside the micropores. In addition, the smaller space around the confined SnIV site leads to enhanced stereoselectivity for this catalyst. The etherification of β-citronellene with ethanol over zeolite BEA is also affected by competitive adsorption.28 In this case, the pores of the zeolite are almost completely filled with ethanol, and the reaction between β-citronellene approaching from the bulk liquid and ethanol from within the pores occurs at the pore entrances. The preferential adsorption of ethanol effectively suppresses side-reactions, such as the isomerization of β-citronellene. The adsorption of oxygenates can be influenced by the composition of the solvent system. Thus, while glucose, fructose, and isomaltose are adsorbed into H-BEA from aqueous solution, the sugars desorb readily in the presence of water-ethanol mixtures.29 These observations were explained in terms of favorable hydrophobic interactions between the sugars and ethanol in the liquid phase, although competition between different species for adsorption sites must also be taken into account. This can be a very complex problem, due to the large number of forces that contribute to the surface interactions of each of the components. In one study, 5hydroxymethylfurfural (5-HMF) and levulinic acid were found to adsorb more strongly than fructose or glucose into zeolite H-BEA.30 This behavior has important consequences for the dehydration of sugars catalyzed by H-BEA, since accumulation of the reaction products in the zeolite can reduce the rate of sugar diffusion into the pores, lowering the observed rate of reaction. Co-adsorbed species can also enhance desorption of specific adsorbates. For example, the hydroxyl group of co-adsorbed methanol can interact with the π-electrons of cyclohexene 6 ACS Paragon Plus Environment

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adsorbed on Ru, reducing the strength of the latter’s surface interaction.31 Consequently, an increased selectivity in the hydrogenation of benzene to cyclohexene, rather than the completely hydrogenated cyclohexane, was observed. For a particular adsorbate, different functional groups may compete to interact with the surface. A flexible oxygenate with multiple functional groups can form several linkages with the surface. However, a rigid molecule and/or one with functional groups that are too close to each other may not be able to optimize the surface interactions of each functional group independent of the others. Of course, the outcome of the resulting tradeoff will vary depending on the nature of the surface site(s) involved.

2.2 Effect of Solvents on Adsorbed Species Solvents can affect the rates of catalytic reactions on surfaces by stabilizing or destabilizing surface-bound intermediates and transition states.32-36 In a limited number of studies, these effects have been demonstrated for specific reactions and catalysts. Lefferts et al. used ATR-IR spectroscopy to study adsorption and oxidation of CO over Pt/Al2O3 and Pd/Al2O3 catalysts in contact with either dry CO in the gas phase or CO dissolved in liquid water.37,38 A red shift of the C-O stretching mode in the presence of the aqueous phase, signaling increased π-back-donation to adsorbed CO, was attributed to the higher potential (i.e., electron density) of the metal particles in contact with condensed water.35,37,38 The effect becomes stronger with increasing pH as more hydroxyl anions interact with the surface.37,38 The rate of CO oxidation increased with increasing potential of the metal particle.37 However, increased πback-donation alone does not explain an observed increase in the extinction coefficient for adsorbed CO, because the increase was not observed when the Pt surface was fully covered with CO in the absence of liquid water. Therefore, it was suggested that co-adsorbed water molecules also polarize adsorbed CO in a way that increases its transition dipole moment.37,38 A combination of DFT and MD simulations showed how water organizes around CO on Pt(111), as well as around the methanol- and glycerol-derived dehydrogenated fragments CH2OH and C3H7O3, forming hydrogen bonds with the adsorbed species.39 These hydrogen bonds grow stronger as adsorbates become larger and more polar, as is the case for polyols. Consequently, decarbonylation of hydrophilic polyols to hydrophobic CO reduces the hydrogen bonding interactions. 7 ACS Paragon Plus Environment

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Heyden et al. used a combination of microkinetic modeling and DFT calculations to study the hydrodeoxygenation of propionic acid on Pd(111) and Pd(211) surfaces.33,40 On Pd(111), the presence of nonpolar n-octane does not have a strong effect on the turnover frequency, while polar solvents such as n-butanol and water cause the turnover frequency to increase by a factor of up to 30 via stabilization of key surface intermediates.33 On a Pd(211) surface chosen to represent step sites, water has the opposite effect because it increases the binding strength of CO formed as one of the products.40 The overall reaction rate is lower because the number of readily accessible surface sites is reduced. The adsorption of biomass-derived molecules in zeolites is strongly influenced by van der Waals interactions,41 and the stabilization of adsorbed species and transition states by their interaction with the pore walls of the zeolite can be considered as a type of solvation.42 The relative sizes of the reactant molecules and the zeolite pores affect the extent of this solvation and whether solvent molecules from the bulk liquid can interact with adsorbed reactants. Moreover, the solvent composition can exert a strong influence on adsorption. Thus, the uptake of 5-HMF by hydrophobic zeolites such as dealuminated H-Y or silicate-1 decreases as the DMSO:H2O ratio in the solvent increases, due to competitive adsorption of DMSO.43 In other cases, the solvent exerts a direct effect on the catalytic rate. For example, water stabilizes 1-propanol adsorbed at the Brønsted acid sites in H-ZSM-5 (Si/Al = 26) to a greater extent than it does the transition state for dehydration to propene, resulting in a reduced dehydration rate.44 The same stabilization of 1-propanol adsorbed at the Brønsted acid sites was observed when the amount of 1-propanol present exceeded the number of Brønsted acid sites. In other cases, the nature of the solvent has a strong influence on the reaction path. For example, framework Sn sites in Sn-BEA isomerize glucose to fructose by 1,2-hydride transfer when the reaction is performed in water.45,46 However, when methanol is the solvent, mannose is formed, apparently at the same sites, by a C1-C2 carbon shift.45 The precise origin of these solvent effects on selectivity remains unknown at this time. In addition to modifying the stability of surface species, water can facilitate surface reactions as a mediator. For example, DFT calculations predicted that water assists in transferring protons between neighboring sites during water splitting on a tetrahedral Pt4 cluster.36 It can be challenging to distinguish such solvent-mediated elementary reactions from true surface reactions, in which all intermediates are surface species. 8 ACS Paragon Plus Environment

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While some level of insight into solvent effects has been reached for relatively simple reactions, as described in the examples in the following sections, much more work is needed to extend these studies to more complex reactants and reactions. However, the available information suggests that significant effects exist and that they have a profound influence on catalytic activity and selectivity.

2.3 Surface Reactions of Specific Oxygenates A number of studies have described the adsorption of specific oxygenates from the aqueous phase, particularly in the areas of wastewater treatment and adsorption from natural waters onto mineral surfaces. Both activated carbon47-50 and metal oxides have been studied.24,26,27,41,51-54 Most early work in this area was aimed at removing certain compounds quantitatively from aqueous solutions, with limited attention paid to the nature of the resulting adsorbed species. However, recent studies have provided more insight, including distinction between weak “outersphere” adsorption, often involving hydrogen-bonding, and strong “inner-sphere” adsorption in which covalent bonds to specific surface sites are formed.55 The following sections will highlight representative examples from such studies, with a focus on surface chemical reactions of polyols, carbohydrates, and phenolics.

2.3.1 Polyols Adsorption of polyols from aqueous solutions on hydrophobic zeolites is dominated by van der Waals interactions.41 This behavior is similar to that of alkanes in zeolites, where each additional CH2 group in the sorbate molecule contributes a constant increment to the heat of adsorption.56-58 In contrast, the presence of a hydroxyl group decreases the adsorption constant because it prevents the molecule from assuming the configuration that would allow for the strongest vander-Waals interactions.41 However, the Henry’s constants for the adsorption of propylene glycol, 1,2-butanediol, and ethylene glycol on silicalite-1 increased by factors of up to 30 when defect sites were present in the zeolite.54 Thus, directed interactions (e.g., hydrogen bonds, covalent or ionic bonds) can dominate the adsorption of polyols when even minor amounts of appropriate adsorption sites are present. Directed interactions also play a critical role in polyol adsorption on the polar surfaces of nonmicroporous metal oxides. Polyols can outcompete water for Lewis acid sites, forming strongly 9 ACS Paragon Plus Environment

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chemisorbed multidentate surface species on γ-Al2O3, TiO2, ZrO2, CeO2, and Nb2O5.23,59,60 For example, the most stable surface species derived from glycerol forms on these metal oxides when both primary hydroxyl groups of glycerol interact with the same Lewis acid site (Figure 1).23 While one hydroxyl is deprotonated and forms a bridging alkoxide, the other coordinates without deprotonation to the same site. The secondary hydroxyl group interacts with the surface more weakly, forming a hydrogen bond to a basic surface oxygen atom. In the absence of Brønsted acid sites, these surface species are very stable and prevent hydrolytic attack by hot liquid water61 that would otherwise convert the metal oxide to a hydrated phase (see Section 4.1).62,63 When Brønsted acid sites are present, dehydration of polyols can occur.60 On niobia, where the primary hydroxyl groups of glycerol are activated by interaction with a Lewis acid site, dehydration is catalyzed by a Brønsted acid site and occurs preferentially in the terminal position. However, when dehydration occurs without involvement of a Lewis acid site, the secondary hydroxyl is removed selectively due to the higher stability of the corresponding carbenium ion formed in the transition state. The ability of polyols to form chelates is also important during polyol-mediated synthesis of metal and metal oxide nanoparticles with controlled sizes and shapes.64,65

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Figure 1. Most stable surface species formed from glycerol on γ-Al2O3.23 Reproduced with permission from ref 23. Copyright 2013, American Chemical Society.

The surface chemistry of supported metal catalysts during the oxidation of alcohols to carbonyl compounds and carboxylic acids has received considerable attention recently.66-68 Most likely, the oxidation of primary alcohols begins with formation of a metal alkoxide (Figure 2). It was suggested that this species is formed by deprotonation of the alcohol to the corresponding alkoxide in solution, followed by chemisorption of the latter on the metal surface.67 However, alternative mechanisms, such as the formation of surface alkoxy species by reaction with surface Ru-OH groups (i.e., RuOH + ROH  RuOR + H2O), have also been proposed.69 Surface alkoxides can undergo β-hydride elimination to form the corresponding aldehydes.68 Complete oxidation to the carboxylic acid may occur by hydration of the aldehyde to a geminal diol, 11 ACS Paragon Plus Environment

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followed by a second β-hydride elimination step.70 The role of O2 is to oxidize the resulting hydride species to water.67 An alternative mechanism for glycerol oxidation catalyzed by Au nanoshells in water was proposed based on evidence from surface-enhanced Raman spectroscopy.71 It was suggested that glycerolate does not directly interact with the metal surface, but rather reacts with adsorbed species like activated oxygen or hydroxyl groups. In contrast to the absent glycerolate intermediate, significant quantities of carboxylate products were observed on the surface. Support interactions have a strong influence on product selectivity in glycerol oxidation catalyzed by supported bimetallic AuPt nanoparticles.72 The initial dehydrogenation step to glyceraldehyde is not strongly influenced by the support, but the formation of glyceric acid in the next step and its conversion to tartronic acid proceed significantly faster when the support contains strong base sites. Unfortunately, the exact nature of the surface interactions remains unknown at this time.

Figure 2. Proposed mechanism for alcohol oxidation on a gold surface in the presence of liquid water at high pH.67 Reproduced with permission from ref

67

. Copyright 2010, The American

Association for the Advancement of Science.


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2.3.2 Carbohydrates The chemistry of carbohydrate solutions in contact with mineral surfaces has been studied, motivated originally by the use of polysaccharides such as starch and dextrin in froth flotation. Wang et al. reported that glucose and sucrose are well-dispersed on silica and alumina from their aqueous solutions, thereby maximizing interactions between their hydroxyl groups and the hydroxyls of the oxide surface, as long as the sugar loading remains below one monolayer.73 At higher loadings, sucrose forms crystallites readily while glucose forms amorphous aggregates. Brandao et al. concluded that glucose and starch form complexes with iron ions present on the surface of hematite in the presence of water, based on their IR spectra.74 Tautomerization is a characteristic and facile reaction of carbohydrates, leading to the presence of multiple, equilibrated species in solution and affecting the selectivity of dehydration reactions. Since tautomer distributions depend strongly on the local environment (such as the solvent), they are expected to change as a result of adsorption on a surface or in a porous catalyst. For example, the fructose tautomer distribution in aqueous solution inside SBA-15 silica shifted to favor the furanoses when the mesopores were modified with polyvinylpyrrolidone.75 The effect, which resembles that observed when fructose is dissolved in N-methylpyrrolidone, was attributed to differences in the solvation of the sugar inside the pores. Specific chemisorption interactions can also influence the ratio of adsorbed tautomers. For example, ribose exists primarily (ca. 80%) as pyranose tautomers when dissolved in water, but the fraction of furanose tautomers increases significantly when ribose is adsorbed onto silica which is then dried in air to remove co-adsorbed water, presumably due to a specific interaction between its 5-hydroxymethyl groups and the surface silanols.76 In Sn-BEA zeolite in contact with water, partial hydrolysis of framework Sn4+ sites allows them to bind to hydroxyls at the C1 and C2 carbons of glucose or fructose.77,78 An ensuing ringopening was inferred from the solid-state 13C NMR spectrum of an aqueous fructose solution in contact with Sn-BEA, which showed a ten-fold increase in the relative abundance of the acyclic tautomer.77

2.3.3 Phenolics Aromatic oxygenates exhibit distinctly different adsorption behaviors from solution, compared to polyols and carbohydrates. Hydroxyl substituents on aromatic rings are considerably more acidic 13 ACS Paragon Plus Environment

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than aliphatic alcohols, and the interactions of phenolic hydroxyls with hydrophilic, hydroxylated surfaces are generally stronger than adsorption via the aromatic ring. Phenolic adsorption is further expected to be strongly dependent on the solution pH and ionic strength. The aromatic rings themselves can engage in π-stacking and π-cation interactions, making these the preferred types of adsorption on hydrophobic surfaces. Of course, multiple surface interactions can occur simultaneously. Catalyst deactivation during hydrodeoxygenation of biooils, obtained by pyrolyzing lignocellulose, has been attributed to coking associated with the strong binding of phenolic compounds to oxide surfaces.79 However, strong furfural adsorption on Ru/C inhibits guaiacol hydrodeoxygenation.80 Variously substituted phenols and anisoles adsorbed onto silica from a heptane solution were reported to interact with the silanols via hydrogen-bonding to either the oxygen or the aromatic ring, or both, depending on the steric and electronic influence of other substituents on the aromatic ring.81,82 In contrast, molecularly adsorbed anisole desorbs completely at 373 K, restoring the O-H stretching modes of the surface silanols.79 Adsorbed phenol and guaiacol are retained to a small extent even at temperatures as high as 723 K (typical of HDO reaction conditions), suggesting the formation of covalently-bonded phenolate and methoxyphenolate, respectively.79 On aluminas, phenol itself is extensively deprotonated at room temperature, creating new bridging hydroxyls on the alumina surface. The strongly adsorbed phenolates were proposed to bind to one Lewis acidic Al site or to bridge two Al sites, and many do not desorb even at 723 K.79 Hydrogen-bonding can also occur between acidic surface hydroxyls and the phenolate ring. Adsorbed anisole decomposes on alumina via cleavage of the C6H5O-CH3 bond to give separately adsorbed phenolate and methoxy species. Each of these reactions is depicted in Figure 3. For each phenolic compound, its reactions on amorphous silica-alumina showed characteristics typical of adsorption on silica and alumina, but with less strongly held phenolate derivatives due to the lower number of Lewis acid sites.


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Figure 3. Reactions and proposed structures for (A) phenol; (B) anisole; and (C) guaiacol, adsorbed from the vapor phase onto alumina.79 Reproduced with permission from ref.

79

.

Copyright 2010, American Chemical Society.

Catechol binds more strongly to metal oxides such as alumina than either phenol or resorcinol due to its ability to form 1:1 bidentate complexes with surface Al ions.83 These chelating catecholates readily displace monodentate adsorbates, including water and anions such as chloride and phosphate. Similar catecholate species can form “roadblocks” in zeolites preventing diffusion through the pore windows in which they reside.84 Catechol adsorption on the rutile TiO2(110) surface at room temperature was modeled using DFT in the presence of a co-adsorbed water film. Double deprotonation allows catecholate to displace two bound water molecules and to bind in a bidentate fashion to a penta-coordinated Ti site.85 The stability of adsorbed catechols and phenols decreases in the order TiO2 > Fe2O3 > Al2O3.86 Catechol adsorption is less energetically favorable when water is present, due to the need to displace pre-adsorbed molecules into solution. The possibility of redox reactions during adsorption must be considered, due to the facile oxidation of catechol to semiquinone and o-quinone.87 Furthermore, high catechol concentrations may even cause extraction of metal ions as soluble coordination complexes (see Section 5 below). 15 ACS Paragon Plus Environment

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Since hydrodeoxygenation catalysts often involve metal or sulfide components, phenolic adsorption on these sites must be considered as well. When phenol was adsorbed on sulfided CoMo/Al2O3 from the vapor phase, it interacted predominantly with the alumina support rather than with the metal or sulfide sites.88 Nevertheless, the adsorbed phenolates blocked access of CO to the reactive sulfide edge sites. In direct phenol deoxygenation on Ru/TiO2, water was suggested to be adsorbed on the support near the metal-support interface where phenol is located, and it assists the reaction via hydrogen-bonding to the phenolic hydroxyl group.89 On noble metals, phenol tends to adsorb parallel to the surface and interact with the metal via its aromatic ring (Figure 4).90,91 In bimetallic alloy nanoparticles containing an oxophilic metal component, such as MoPt, the oxophilic metal sites engage in interactions with phenolic hydroxyl groups as well.92 They also alter the relative adsorption energies of keto-enol tautomers. Ab initio molecular dynamics calculations showed that co-adsorbed water changes the electronic structure of the metal surface, lowering adsorption energies for phenol and partially hydrogenated intermediates slightly.93 The presence of co-adsorbed water also accelerates proton transfer reactions, thereby promoting tautomerization of adsorbed enols (Figure 5).

Figure 4. Schematic top view (upper) and side view (lower) of phenol adsorption on Pt(111). Pt is blue; O, H, and C atoms are red, white, and gray, respectively.90 Reproduced with permission from ref. 90. Copyright 2015, American Chemical Society.


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Figure 5. The impact of co-adsorbed water on the energetics of adsorption of phenol and partially hydrogenated intermediates on Pt and Ni surfaces.93 Reproduced with permission from ref. 93. Copyright 2014, American Chemical Society.

2.3.4 Surface interactions of other compounds Carbonyl-containing molecules, such as aldehydes (furfural, hydroxylmethylfurfural, etc.) and carboxylic acids (formic acid, lactic acid, etc.) are frequently present among the conversion products of lignocellulosic biomass. When they remain strongly adsorbed on catalyst surfaces, they restrict the ability of reactants to access the active sites. Carboxylic acid adsorption from solution onto oxides is strongly pH-dependent, being strongest at slightly acidic pH values, which allow the anionic carboxylate to interact with a positively charged surface.55 The carboxylic acid proton may be transferred to a surface hydroxyl group (with release of water into solution, at least when the solvent is polar).94 A high ionic strength in the solution phase decreases the affinity of the carboxylic acid for the surface. Intact (i.e., neutral) carboxylic acids are weakly bound via their carbonyl groups, and are readily washed off with neat solvent. Carboxylates can be present in monodentate or bidentate structures coordinated in either chelating or bridging fashion. Adsorption is enhanced when the solution concentration is high or when the carboxylic acid contains other sites capable of coordination and/or hydrogen-bonding, such as a phenolic or aliphatic hydroxyl group. Thus, neutral salicylic acid was retained on alumina longer than benzoic acid, even though both were strongly adsorbed from cyclohexane in their deprotonated forms.95 Likewise, lactic and glycolic acids were strongly adsorbed from water onto titania, while propanoic acid was not.96 17 ACS Paragon Plus Environment

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Dicarboxylic acids with saturated (e.g., adipic, glutaric, succinic, malonic, and oxalic acids) or unsaturated (e.g., maleic, fumaric, glutaconic, and muconic acids) linkers can be formed by ringopening of phenolic compounds. In general, they are more strongly adsorbed from aqueous solution by oxides such as titania than the corresponding monocarboxylic acids, and the strength of their interaction depends on the length of the carbon chain as well as the pH and the ionic strength.97 The orientation of the two carboxylate groups must allow for simultaneous interaction with the surface, either through close proximity or flexible linkers. Secondary interactions also promote adsorption, as when the carboxylic acids are attached to long alkyl chains, which can organize via van-der-Waals interactions into an aligned hydrophobic array, or when they contain aromatic rings that can associate via π-stacking. On metal surfaces such as Cu(111), formic acid vapor adsorbs molecularly at temperatures below 160 K to form a hydrogen-bonded array (Figure 6), before dissociating by O-H bond cleavage at higher temperatures.98

Figure 6. Adsorption of formic acid on Cu(111), observed by STM.98 Reproduced with permission from ref. 98. Copyright 2015, American Chemical Society.

Adsorbate-adsorbate interactions can have a major impact on selectivity. In catalytic transformations of aldehydes such as 5-HMF/furfural, selectivity for decarbonylation over hydrogenation is affected by surface coverage-induced conformational changes (Figure 7). At 18 ACS Paragon Plus Environment

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low coverages on Pd(111), DFT calculations conducted without coadsorbed solvent showed that the furan ring prefers to orient parallel to the surface, maximizing interactions of the metal surface with the furan ring and the carbonyl group and favoring decarbonylation.99 However, when the surface coverage increases, the furan ring tilts to favor coordination via the aldehyde oxygen or the furan ring, favoring hydrogenation. The addition of a more oxophilic metal such as Zn also changes the adsorption geometry to favor carbonyl bonding to the surface.100

Figure 7. A flat and two tilted geometries of furfural on Pd(111) various coverages.99 Adapted from ref. 99. The original figure was published under an ACS AuthorChoice License.

3 Transport Phenomena in Heterogeneous Catalysis in Liquid Phase 3.1 Mass Transport Effects on the Rates of Surface Reactions Section 2 described adsorption of different types of reactant(s) from the liquid phase onto catalytic site(s) as the first step in a surface reaction. In heterogeneous catalysis, the rate of mass transport of reactant(s) from the gas or liquid phase to the solid surface can significantly influence reaction rates, as well as selectivity and even reaction mechanisms. It is essential to analyze intrinsic reaction kinetics by acquiring kinetic data under conditions that are free from transport limitations. In this section, we examine transport effects in heterogeneous catalysis by briefly reviewing classical techniques (which have predominantly been applied to gas-solid interfaces), then discuss how they can be used in liquid phase systems, with examples from the recent literature on biomass conversion. In principle, both mass and heat transport phenomena can affect rates in heterogeneous catalysis, but heat transport limitations are generally less significant in liquid-phase reactions, because liquids tend to have high thermal conductivity and heat capacity relative to gases. Therefore, we will discuss only mass transport limitations, 19 ACS Paragon Plus Environment

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although heat transport phenomena can be analyzed in a similar fashion based on the analogy between their two constitutive equations: Fourier’s law of heat conduction, and Fick’s law of diffusion.

3.1.1 Basics of Transport Phenomena in Reacting Systems Mass transport is classified as external or internal depending on whether it is occurring in the boundary layer adjacent to the particle surface, or inside the pores of a catalyst particle.101,102 When the rate of diffusion of a component A to the catalyst surface is slow relative to the intrinsic reaction rate at the surface, the difference between the concentrations of A in the bulk of the fluid (gas or liquid) and at the catalyst surface becomes so large that the reaction rate is no longer determined by the intrinsic reaction rate, but by the diffusion rate. This scenario is termed a “diffusion-limited (or diffusion-controlled)” reaction. The other extreme is “reaction-limited”, in which diffusion is fast enough so that the intrinsic reaction rate is observed. (Of course, intermediate regimes are also possible.) The difference is manifested in the temperature dependence of the observed rate, since activation energies are usually much larger for chemical reaction steps than for diffusion. An example of the transition from reaction-limited to diffusionlimited regime in a liquid-phase reaction is shown in Figure 8.103 The magnitude of the activation energy, 104 kJ mol-1, for glucose isomerization in a faujasite zeolite at temperatures below 100 °C is typical for this base-catalyzed reaction. As higher temperatures, the much lower activation energy signals that the rate is strongly limited by mass transport.


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Figure 8. Arrhenius plot for glucose isomerization to fructose catalyzed by NaX zeolite, showing the change from a reaction-limited rate at lower temperatures to a diffusion-limited rate at higher temperatures.103 Reproduced with permission from ref. 103. Copyright 2000, Elsevier.

When a reacting system is “diffusion-limited”, external transport limitations can be reduced by increasing the volumetric flow rate of the fluid, or by increasing the rate of agitation of the fluid, while ensuring that the contact time between the reactants and the catalyst remains constant. If the rate of flow or agitation is changed and the conversion remains constant, the impact of external transport may be assumed to be insignificant. The assessment of internal transport phenomena presents a more difficult challenge, since most heterogeneous catalysts are porous over a wide range of scales (i.e., micropores with dpore < 2 nm; mesopores with 2 nm < dpore < 50 nm; and macropores with 50 nm < dpore 0.1-102 nm). The diffusivity of molecules confined in such pores is much more complex to evaluate.

3.1.2 Criteria to Examine Transport Limitations: 1. Weisz-Prater Criterion A number of criteria have been developed to assess mass transport effects on reaction rates.104 The Weisz-Prater (WP) criterion for the assessment of internal transport limitations is helpfully defined in terms of the observed (measured) reaction rate, rather than the true rate constant. The latter is often difficult to obtain but is required in most other approaches.105 The dimensionless


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Weisz-Prater parameter (ΦWP) is the ratio of the reaction rate (rA, in mol cm-3 s-1) to the rate of internal transport: 𝛷WP

𝑟𝐴 𝑅p 2 = 𝐶s,𝐴 𝐷eff

where RP is the radius of the catalyst particle, in cm, Cs,A is the concentration of A at the external particle surface, in mol cm-3, and Deff is the effective diffusivity of A, in cm2 s-1. Here, it should be noted that the effective diffusivity is the diffusion coefficient through the pore space, which is different from the bulk diffusivity. The relationship between these two diffusion coefficient is discussed in Section 3.2. The critical value of the WP parameter depends on the reaction order. For example, it is 0.3, 0.6, or 6 for 0th-, 1st-, or 2nd-order reactions, respectively. With this range in mind, it is acceptable to consider in general that reactions that are “diffusion-limited” have

ΦWP > 6, whereas the absence of transport limitations is indicated by ΦWP < 0.3, although strictly speaking there is no single critical value for WP criterion. Nonetheless, unity can be used as a useful threshold for the analysis, and is well accepted in the catalysis community. An important requirement when determining the WP parameter is the estimation of Deff in the pores. For reactants in the gas phase, the effective diffusivity is characterized relatively easily, using either molecular diffusion or Knudsen diffusion depending on relationship between the mean free path (λ) and the average pore diameter (d).101,102 In contrast, evaluating Deff for reactants in the liquid phase is more complex due to the presence of solvents (or condensed reactant/product molecules themselves), solvent-solute interactions, and other non-ideal liquidphase behaviors. Furthermore, experimental measurement of Dbulk in solvent systems without any pore confinement may still be challenging for the same reasons. For diffusion measurements, optical techniques have traditionally been a common approach,106-108 while spectroscopic methods involving NMR109,110 and IR111,112 measurements have been developed as well. Meanwhile, direct, microscopic measurement of Deff in pore structures has also become available, mainly with the advancement of pulsed field gradient (PFG) NMR.113,114 For more information on microscopic diffusion measurements, readers may refer to a recent comprehensive review by Kärger and Valiullin.115 In general, diffusivity in a solvent system is several orders of magnitude lower than in the gas phase: for example, the diffusivity of H2 decreases from ca. 1.3 cm2 s-1 in the gas phase to ca. 4.8 22 ACS Paragon Plus Environment

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× 10-5 cm2 s-1 in water.116 The large difference affirms the importance of testing for mass transport limitations in liquid-phase catalytic reactions. In their study of liquid-phase hydrogenation of citral over a Pt/SiO2 catalyst, Mukherjee and Vannice demonstrated the use of the WP criterion and presented detailed assessments of Deff for both dissolved H2 and citral in a number of solvents.117 For example, Deff values in ethanol were determined to be 7.05 × 10-5 and 1.78 × 10-5 cm2 s-1 for H2 and citral, respectively, resulting in ΦWP values of 0.03 and 0.01. Since these values are well below the 0.3 threshold (the critical value for a zeroth-order reaction ), they confirm the absence of mass transport limitation for their experimental conditions. The WP criterion was used recently in liquid-phase biomass conversion chemistry to characterize fatty acid deoxygenation over Pt/C,118 carboxylic acid esterification over ion-exchange resin catalysts,119,120 and levoglucosan hydrogenation over Ru/C.121 In all of these studies, the WP criterion was satisfied, verifying the absence of internal transport limitations. In contrast to reactions catalyzed by supported metals or acidic resin catalysts, reactions in zeolites are more susceptible to mass transport limitations due to the presence of micropores. For example, severe mass transport limitations were observed in liquid phase alkylation reactions of benzene and isobutane, with WP parameters on the order of 101-2 (well above unity).122,123

3.1.3 Criteria to Examine Transport Limitations: 2. Madon-Boudart Test/Koros-Nowak Criterion In the field of catalysis, the most widely used experimental approach to examine mass transport limitations is the powerful Madon-Boudart (MB) test. Unlike the WP criterion, the MB test does not require determination of Deff. Instead, one performs multiple rate measurements, varying only the number of catalytically active sites. In the absence of mass transport limitations, the reaction rate will be proportional to the number of active sites. Madon and Boudart demonstrated the rigor of this test in their study of supported metal catalysts in the liquid phase hydrogenation of cyclohexene over various Pt/SiO2 catalysts.124 Because this test was developed based on the criterion originally proposed by Koros and Nowak, the test is sometimes called the KorosNowak (KN) criterion.125 While Koros and Nowak diluted the catalyst bed by mixing catalyst particles with an inert powder to change the number of active sites, Madon and Boudart varied the Pt loading on the catalyst support. It should be noted that the density of active sites does not change during the KN test, making it incapable of providing information about internal diffusion 23 ACS Paragon Plus Environment

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limitations in micropores, which is addressed in the MB test. In this review, we use the term Madon-Boudart (MB) test for consistency, and discuss the MB test in detail below as the more robust technique. A prerequisite for the MB test is that the surface reaction be structureinsensitive (i.e., the rate is independent of metal dispersion), or that the metal particle size and shape can be assumed constant. 3.1.4 Examples of the Assessment of Mass Transport Effects in Liquid Phase Reactions There are two experimental approaches for applying the MB test to heterogeneous reaction rates: (1) calculation of the turnover frequency, and (2) correlation of the reaction rate (typically expressed per unit catalyst mass) with the number of the catalytically active sites per unit mass. To use the first approach, the turnover frequency (TOF, in s-1) is calculated as moles of reactant converted per s per mole of catalytically active sites (e.g., represented by the moles of surface metal atoms for a supported metal catalyst). If the reaction is free from both external and internal mass transport limitations, the TOF will remain unchanged regardless of the absolute number of active sites. An example of the MB test applied to the liquid-phase hydrogenation of benzene catalyzed by Pd/η-Al2O3 is shown in Table 1.126 The similarity of the TOFs for catalysts with different Pd loadings holds at several temperatures, verifying the absence of heat transport limitations as well. As described above, the original MB test was demonstrated with supported metal catalysts, but there are a few recent works related to biomass chemistry where the test has been extended to zeolite catalysts. For instance, in their study of arabinose epimerization in water over Sn-Beta zeolite catalysts, Román-Leshkov and co-workers applied the MB test by changing the Sn/Si ratio of the zeolite.127 Varying the atomic composition of the zeolite framework amounts to changing the concentration of active sites. In their study, the TOF was constant when the Sn/Si ratio was varied by a factor of four, suggesting the absence of mass transfer limitations under the reaction conditions. In another approach, Resasco and co-workers altered the number of acidic proton sites in HY zeolite catalysts by varying the extent of Na ion-exchange treatment in their study of m-cresol alkylation in decahydronaphthalene.128 No significant change in TOF was observed for the three different acid site concentrations studied. Despite the apparent success in these examples, methods for such extension of the MB test to zeolites or other microporous


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catalysts are not well-established, and the possible impact of catalyst modification on the validity of the MB test to describe per-site reactivity should be taken into consideration.

Table 1. Madon–Boudart test for the absence of heat and mass-transfer effects, via measurement of TOF (s-1) in benzene hydrogenationa over supported Pd catalysts.126 Temp.

a

Catalyst

(K)

0.08% Pd/η-Al2O3

0.94% Pd/η-Al2O3

1.20% Pd/η-Al2O3

388

0.11

0.07

0.13

408

0.23

0.22

0.20

448

0.30

0.26

-

Reaction conditions: 200 psia H2 and 60 mol% benzene in hexane.

Another approach for applying the MB test involves comparing the reaction rate, normalized by catalyst mass, to the number of the catalytically active sites per unit mass of catalyst. This is often practiced in the form of a log-log plot. In the absence of transport limitations, such a plot will have a slope of unity; the slope will be less than one under the influence of mass transport limitations. For example, a value of 0.5 was obtained in Madon and Boudart’s original study of cyclohexane hydrogenation on Pt/SiO2 catalysts when the size of the catalyst particles was larger than 0.074 mm (200 mesh).124 A representative plot for transesterification of waste cotton seed oil catalyzed by a solid base is presented in Figure 9.129 The slopes determined at two different temperatures are close enough to unity to demonstrate that the reaction regime is free from mass transport limitations. In another example, Jean et al.130 applied the MB test to liquid-phase hydrogenolysis of glycerol over ZnO-based catalysts. The slope of ~0.5 in the plot reveals that the system was highly influenced by mass transport limitations over a range of reaction conditions. There may be many situations in liquid-phase biomass reactions where mass transport phenomena have been overlooked, since not all kinetic studies of catalytic biomass conversion or related chemistry check for them. As the number of publications on new heterogeneous catalysts for biomass conversion grows, all researchers are encouraged to pay closer attention to the effect of transport phenomena.


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Figure 9. Madon-Boudart test for transesterification of waste cotton seed oil catalyzed by K‒ CaO, analyzed in terms of the amount of K+ (expressed as fw, the mass fraction of K+ per gram of catalyst.129 Reproduced with permission from ref. 129. Copyright 2012, Elsevier.

3.2 Pore Size and Solvent Effects on Transport Phenomena 3.2.1 Pore Size Effect on Diffusivity In the previous section, use of Weisz-Prater criterion to assess mass transport limitations required an estimation of effective diffusivity Deff of reactants in the catalyst pores. The conventional model for Deff is

𝐷𝑒𝑒𝑒 = 𝐷𝑏𝑏𝑏𝑏

ε τ

where ɛ is catalyst porosity and τ is tortuosity.102 However, this model is too simplistic for molecular solute diffusion in liquid-filled catalyst pores; in such cases, Deff also critically depends on the relative sizes of the relevant molecules and the pores. Estimation of Deff for liquid-phase solute diffusivity in pores can therefore be complex, necessitating the use of multiple parameters.131,132 However, the simplest expression developed by Ternan still describes experimentally-measured values well:133


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𝐷𝑒𝑒𝑒

(1 − 𝜆)2 = 𝐷𝑏𝑏𝑏𝑏 1 + 𝑃𝑃

where λ is the molecule-to-pore radius ratio, and P is an empirical parameter. Figure 10 presents Deff values obtained experimentally for aromatic molecules dissolved in cyclohexane interacting with γ-aluminas of varying pore diameter,134 as well as the fit obtained by Ternan.133 Deff decreases exponentially with increasing λ, and is less than half of Dbulk for λ > 0.1. The λ values are typical for catalytic biomass conversion processes in micropores and even some mesopores, when the reactants (e.g., carbohydrates, polyols, and aromatics) are C3-C6 species or larger. Studies of liquid-phase hydrogenation reactions also showed that mass transfer limitations may be present in mesoporous catalyst systems.135,136

Figure 10. Variation of diffusivity ratio (effective-to-bulk) with molecule-to-pore radius ratio, adapted from ref. 133. Experimental data are from Chantong and Massoth.134

3.2.2 Solvent Effects on Diffusivity In liquid-phase reactions, diffusivity depends not only on the size of the diffusing molecules and the catalyst pore radius, but also on the nature of the solvent. As discussed in other sections, 27 ACS Paragon Plus Environment

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solvent effects are a key feature of liquid-phase reactions that impact important phenomena in catalysis such as adsorption, surface reaction rates, and catalyst stability, in addition to transport phenomena. Illustrating the effect of solvent on transport, Deff for H2 increases by almost an order of magnitude when the solvent is changed, from 1.69 × 10-5 cm2 s-1 in cyclohexanol to 14.9 × 10-5 cm2 s-1 in n-hexane.117 This large difference in Deff influences the WP parameter, demonstrated in citral hydrogenation over Pt/SiO2 catalysts. An important property of the solvent that affects diffusivity of a solute is the viscosity. An empirical expression developed for estimation of the diffusivity a dilute solute (1) in a solvent (2) is a function of solvent viscosity η2:137

𝐷12 = 4.4 × 10

−15

𝑣𝑣𝑣 1/2

𝑇 𝑉2 1/6 𝐿2 � � � 𝑣𝑣𝑣 � 𝜂2 𝑉1 𝐿1

where T is the temperature, Vi is the molar volume, and Livap is the enthalpy of vaporization of component i. The numerical constant has units of [Pa∙cm2∙K-1]. Figure 11 shows that Deff for dissolved H2 is inversely correlated with solvent viscosity, as expected. A similar trend was reported for citral.117 Of course, the solvent effect on transport is convoluted with many other potential solvent effects on adsorption, reactivity, and/or catalyst stability.


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Figure 11. Effect of solvent viscosity on the effective diffusivity of H2 dissolved in various organic solvents. Plotted using data reported by Mukherjee and Vannice.117

3.3 Catalyst Design to Enhance the Rate of Mass Transport Enhancing the rate of a heterogeneous-catalyzed reaction may require a faster surface reaction, faster mass transport, or a combination of both. When tests such as the WP or MB criteria suggest that the rate is strongly impacted by transport phenomena, adjusting the process parameters and/or the catalyst pore structure may be appropriate. Below, we briefly discuss how catalyst design can be used to minimize the impact of transport phenomena, using zeolites as a class of catalytic materials to illustrate some of the important principles.

3.3.1. Mesoporous Zeolites There are three major approaches to improving the rate of mass transport in zeolite catalysts: (1) decreasing the size of the zeolite particles;138,139 (2) replacing one zeolite by another with larger pore openings;140,141 and (3) creating larger pores in an existing microporous zeolite.142 In smaller zeolite particles, the intracrystalline diffusion path length is reduced, with a corresponding decrease in the diffusion barrier. While the synthesis of nanocrystalline zeolites 29 ACS Paragon Plus Environment

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has been extensively studied, several challenges remain, including imperfect crystallization and lower hydrothermal and mechanical stability, which can adversely impact catalytic performance.143,144 The synthesis of zeolites with larger pore sizes has proved fruitful, with some showing improved performance as refining catalysts.140,141,145 The third approach typically involves creating mesopores in known zeolites using new synthesis methods146 (e.g., carbon black or polymer templating),147-149 or post-synthetic treatments (e.g., hydrothermal, acid, alkaline, or other chemical treatments).141,142 This new class of materials also includes unique morphologies such as zeolite nanosheets (or two-dimensional zeolites)150,151 and hierarchical zeolites.152,153 For example, a house-of-cards arrangement of nanosheets with mesoporosity was recently achieved using a bottom-up synthetic approach called repetitive branching.154 A comprehensive review on synthetic approaches to mesoporous zeolite materials was published by Tao et al.155, and Möller and Bein provided a recent tutorial review.156 As discussed above, the extent to which the effective diffusivity deviates from the bulk diffusivity is greatly influenced by the pore structure and the dimensions of diffusing molecules. Therefore, incorporation of mesopores is expected to enhance the rate of diffusion of reactant(s) and product(s). Figure 12 illustrates the concept of enhancing the rate of transport using mesopores, and the larger fraction of the catalyst that is effectively utilized as a result.157 Indeed, the beneficial effect of mesopores on catalytic activity has been demonstrated in many zeolitecatalyzed reactions. Some examples include benzene alkylation over ZSM-5,158 cyclohexene epoxidation over TS-1,159 cracking of alkanes,160 and isomerization of n-hexadecane.161 Studies involving both gas-phase158,160 and liquid-phase149,159,161,162 reactions have been reported. In the area of zeolite nanosheets and their application in catalysis, Ryoo and coworkers pioneered many new structures and reported improved catalytic performance.152,163-165 While these studies illustrate the promise of mesoporous zeolites with better transport properties, the incorporation of mesopores may also impact the chemical nature of the material, regardless of the synthetic route. Because the acid strength of a zeolite depends on the type of framework, atomic composition, and pore size, a change in the catalytic performance of a new zeolite material may be a result of a changes in intrinsic reactivity, in addition to transport effects.


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Figure 12. Schematic illustration of conventional (microporous) and mesoporous zeolites: (A) Comparison of reactant concentration profiles, showing the greater impact of internal diffusion limitations in the conventional zeolite; (B) Comparison of reaction zones, showing that only a small fraction of the sub-surface region may be utilized in conventional zeolites when severe diffusion limitations are present.157 Reproduced with permission from ref.

157

. Copyright 2007,

Elsevier.

3.3.2. Chemical modification of zeolite pores In liquid-phase systems, a reactant/product is constantly surrounded by solvent molecules and/or other reactant/product molecules, and as mentioned above, the diffusivity in such an environment is several orders of magnitude smaller than in the gas phase. A versatile way to impact transport phenomena, particularly in the liquid phase, is chemical modification of pores in order to create a partitioning effect based on polarity. One well-explored method is silylation, which was first applied to porous materials designed for use in metal extraction.166-168 Resasco and co-workers silylated HY zeolite169-171 as a means of enhancing catalyst stability in hot liquid water (see Section 4.1), and studied its use in liquid-phase reactions such as m-cresol alkylation with 2propanol as a model for biofuel upgrading.170 The external surface of HY zeolite was functionalized with organosilanes (e.g., octadecyltrichlorosilane) to increase its hydrophobicity. In one case, the conversion of m-cresol in a biphasic (water-decalin) solvent system more than doubled for the functionalized zeolite compared to the untreated zeolite. The authors suggested that diffusion of liquid water molecules from the bulk liquid phase to the interior of the catalyst particles may be hindered by the presence of the hydrophobic functional groups. However, they 31 ACS Paragon Plus Environment

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also found that the adsorption profile of water vapor was unaffected by silylation.171 In a review of hydrophobic porous catalysts used for biomass conversion in liquid water, Gounder pointed out that functionalization of external and internal zeolite crystalline surfaces can result in a variety of effects.172 For example, hydrophobic external surfaces may affect diffusion of liquid water within void spaces without preventing water adsorption in the pores, while internal surface functionalization alters the environment within the pores and may impact events such as adsorption, solvation, and reaction at the active sites. Further studies are needed to assess catalytic rates in hydrophobically-modified porous materials, and the effect on diffusion of such modifications.

4. Structural Transformations and Deactivation of Catalysts in Liquid Phase In many cases, biomass conversion processes occur in the liquid phase in the presence of water at elevated temperatures and pressures. Depending on the process, the pH may vary over a wide range. Under these conditions, the dissociation constant of water, Kw, is significantly higher than under ambient conditions, while the dielectric constant and extent of hydrogen bonding are decreased.173 Specifically, Kw increases by three orders of magnitude as the temperature increases from 25 to 250 °C. Since most common catalytic materials were designed for vapor-phase reactions that pose very different challenges to catalyst stability, such materials can suffer modifications when subjected to hydrothermal reaction conditions. Consequently, the hydrothermal stability of the catalyst must be considered explicitly in any study of aqueous or semi-aqueous phase processes for biomass conversion. Of course, the same precaution should be taken as a matter of course when conducting reactions in other solvents. Several recent review articles have addressed hydrothermal stability of solid catalysts in detail.174-176 The presence of a solvent can affect solid catalytic materials by forming solvated phases, weakening porous structures, leaching and/or dissolving part(s) of the catalyst, or facilitating the agglomeration of supported metal particles. Moreover, dissolved species derived from biomass, or impurities present in the biomass, can lead to fouling (e.g., formation of carbonaceous deposits), or poisoning of active sites. In most cases, these transformations result in the destruction of catalytically active domains and loss of activity. However, formation of new


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active sites under reaction conditions is also a possibility. The following section will discuss each of type of transformation, and provide illustrative examples.

4.1. Hydrolysis and Structural Collapse A very common problem encountered during the use of metal oxide-based catalysts in aqueous phase reactions is the conversion of these materials to hydroxides or oxyhydroxides, with concomitant structural, morphological, and textural modifications. For example, alumina-based materials undergo structural modifications in the presence of water, such as the hydration of γAl2O3 to a boehmite phase (i.e., AlOOH) with a significant decrease in its surface area and acidity.61-63,177,178 During this transformation, supported metal particles can agglomerate or become encapsulated, resulting in their deactivation.62 At 200 °C, phase-pure γ-Al2O3 is completely transformed within 10 h, although structural modification is delayed in the presence of supported metal particles (e.g., Ni or Pt). This suggests that the transformation starts with a hydrolytic attack on the Lewis acid sites of the support, which are occupied when metal particles are present.62 Interestingly, the Lewis acid sites can also be blocked by biomass-derived oxygenates (e.g., polyols and lignin fragments),61,179 particularly when such compounds are chelating (i.e., binding to the surface via multiple functional groups).23 While leaching of aluminum cations has been observed for some alumina-supported catalysts, it appears to be independent of the formation of boehmite.63 However, the catalytic activity of leached species must be considered (see Section 5.1). Like solid acids, solid base catalysts can also be affected by hydrolysis. This occurred for CaO during the conversion of glycerol to lactic acid in the presence of 5-30 wt% of water.180 In the presence of even small amounts of water (e.g., 10 wt%), CaO is converted to Ca(OH)2, which has significantly lower basicity leading to reduced lactic acid yield. In addition, leaching of Ca2+ was observed (see Section 4.2). Many solid base catalysts are prepared by calcination of layered double hydroxides (LDH), which undergo dehydration to the corresponding oxides. Rehydration restores the LDH structure,181,182 which is described as a memory effect.183 The resulting transformation converts the oxide to one or more phases with lower basicity. Both compact crystalline182 and lamellar182,183 structures have been reported.181,182 Not surprisingly, this can influence the activity due to changes in the numbers of defects and anchoring sites.182 Moreover, the selectivity of 33 ACS Paragon Plus Environment

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catalytic reactions can be influenced due to modification of active sites. For example, in the conversion of hydroxyacetone over LDH-derived catalysts, the selectivity shifted from lactic acid to pyruvic acid as the reaction proceeded, due to rehydration of the catalyst.181 In addition to modifying the reactivity of affected surfaces, hydrolytic transformations can lead to structural collapse. Although porous materials are commonly used in catalysis because they possess large surface areas that maximize the accessibility of active sites per unit mass of catalyst, their structures are intrinsically metastable. Thus, their susceptibility to transformation is not surprising. For example, silica-based materials such as MCM-41 have high thermal stability, but their stability in water is limited.184 Both MCM-41 and MCM-48 lose their ordered pore structures completely in boiling water as a result of silicate hydrolysis.185 Galarneau et al. reported that dissolution and redeposition of silica from SBA-15 is favored when pores have a small positive curvature, reminiscent of the Kelvin effect186 that describes the dependence of particle solubility on their curvature.187 The same behavior was observed by Pollock et al. after treating SBA-15 in water for 3 h at temperatures from 115 to 155 °C (Figure 13).188


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Figure 13. Representation of silica dissolution and redeposition in SBA-15. Dissolution occurs preferentially in pores with low, positive curvature. (A) Fresh SBA-15, (B) elimination of micropores by redeposition of silica, (C) silica redeposition at entrances to the secondary pore network.188 Reproduced with permission from ref.

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. Copyright 2012, American Chemical

Society. Some zeolite structures collapse within a few hours in presence of hot liquid water.189,190 Ravenelle et al. showed that commercial ZSM-5 (Si/Al = 15-40) is stable in hot liquid water at 200 °C and autogenous pressure, while the degradation of commercial zeolite Y is fast and increases with increasing Si/Al ratio.189 This degradation was attributed to hydrolysis of Si-O-Si bonds, rather than dealumination, with hydroxyl anions playing a key role.189,191,192 Negative 35 ACS Paragon Plus Environment

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framework charges resulting from the incorporation of Al atoms can repel these species to a certain extent. Zhang et al. concluded that the presence of silanol-terminated defects is responsible for the limited stability of many types of zeolite, which are stabilized when these sites are capped.190 A few studies indicate that mixed metal oxides are more resistant to hydrolysis than the corresponding single-component metal oxides. For example, silica and niobia both undergo significant transformation individually in hot liquid water, but remarkably stable materials were obtained when mesoporous silica was coated with niobia, or when small amounts of silica were incorporated into a niobia phase.193,194 Similar stabilization at the interface between different metal oxide domains has been reported for amorphous silica-aluminas.195 For example, coreshell materials with a silica-rich core and an alumina-rich shell, prepared by depositionprecipitation, develop boehmite domains on their external surfaces, but the transformation stops at the interface with the silica-rich core (Figure 14). As a result, changes in surface area and the concentration of acid sites are relatively small. Amorphous silica-aluminas prepared by cogelation contain more evenly mixed domains, and while they undergo some reduction in surface area and acid site concentration in hot liquid water, these changes are much less pronounced relative to pure γ-Al2O3. The materials seem to achieve a meta-stable state with surface areas of 80-150 m2/g and acid site concentrations of ca. 0.1 mmol/g after a few hours in hot liquid water. Thus, it may be feasible to use these materials as robust catalysts for liquid-phase reactions after appropriate hydrothermal pretreatment.


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Figure 14. Transformation in hot liquid water of amorphous silica-aluminas prepared by either co-gelation or deposition-precipitation.195 Reproduced with permission from ref.

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While most hydrothermal transformations of catalysts are detrimental for catalytic activity, they can sometimes lead to the generation of active sites under reaction conditions. One such case is the formation of Brønsted acid sites on Ru clusters supported on carbon during its exposure to liquid water above 200 °C.196 These sites appeared to be active for the hydrolysis of cellulose to glucose. The conversion of metal oxides to new phases is not limited to hydrothermal environments. For example, Shanks et al. demonstrated that CeO2 is entirely converted to an acetate phase when exposed to acetic acid in either the vapor or liquid phase between 150 and 300 °C.197

4.2. Leaching Leaching of catalyst components can play an important role in the deactivation of heterogeneous catalyst in the liquid phase.198-200 A particular challenge is recognizing and accounting for leaching, since many academic studies of aqueous phase processes for biomass conversion are performed in batch reactors, where leached species are retained as part of the reaction mixture, and thus may continue to participate in the reaction as soluble (i.e., homogeneous) catalysts (see Section 5.1). While catalyst recycle studies may provide some indication of the role of such species (provided conversions are intentionally kept low), other methods, such as hot filtration, must be used for rigorous assessment.201 For example, relatively small amounts of a leached species may be highly active, and the corresponding heterogeneous catalyst may act as a reservoir for the active species, showing similar performance even after a significant number of recycles.202 One such highly active solubilized species is Pd(0), used in C-C coupling reactions.19 Reactions in flow reactors will likely play a critical role in the large-scale production of chemicals and fuels from biomass.203 These operating conditions cause leached species to be transported out of the reactor, resulting in observable catalyst deactivation. In addition to affecting the activity of the catalyst that undergoes leaching, solubilized species can deactivate chemical and biochemical catalysts used in downstream processes, and contaminate final products. 37 ACS Paragon Plus Environment

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Many factors can influence the leaching of catalysts, such as the pH and redox potential of the reaction medium, polarity of the solvent, reaction temperature, as well as the presence of certain ions (e.g., Cl-, NO3-, SO42-) and compounds with multiple functional groups, which can act as chelating ligands.176,198,204 For example, leaching of Al3+ ions occurs upon hydrolysis of the SiO-Al bonds in zeolites in acidic solvents,199 in a process related to the well-known dealumination of zeolites that occurs upon exposure to steam at elevated temperatures.205,206 Leaching is often enhanced at high pH because the formation of soluble ionic species is favored. For example, the silica and alumina supports present in Cu/SiO2 and CuO/Al2O3 catalysts, respectively, dissolved completely in aqueous NaOH (1.2 mol/L) at 240 °C.207 Interestingly, ICP analysis indicated that less than 0.1% of the copper was solubilized, but this might be due to the formation of a precipitate. The thermodynamic stability of metals is represented by a Pourbaix diagram. A typical example is shown in Figure 15, which reveals that metallic Ni, Co, and Fe are oxidized in liquid water at 200 °C even in the presence of 1 atm H2 (corresponding to the H2O/H2 line), whereas Cu, Ru, Pd, and Pt are stable as reduced metals under the same conditions.175

Figure 15. Pourbaix diagram illustrating the stability of different metals in hot liquid water at 200 °C as a function of pH and electrochemical potential.

175

Reproduced with permission from

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The strong influence of the solvent on leaching is typically attributed to its polarity.176 However, conflicting results in the literature suggest that a single parameter may not suffice to describe the effect. For example, leaching of Cu species from Cu/ZrO2 was reported to be almost completely suppressed when hydrogenation of levulinic acid was performed in methanol rather than in water.208 In contrast, during the isomerization of dihydroxyacetone, leaching of Sn species from MFI- and BEA-type zeolites was much more pronounced in methanol than in water.209 Many biomass-derived compounds contain multiple functional groups and can act as chelating agents for surface species (see Section 2.3). These molecules can form strongly adsorbed surface species that poison active sites. In addition, the coordination of a chelating ligand to a surface metal atom or ion can provide a driving force for leaching.199 For example, the presence of even small amounts of gluconic acid enhanced leaching of Ni cations from a Raney Ni catalyst210 as well as Fe cations from reactor walls 211 during the hydrogenation of glucose to sorbitol. The extent of leaching can be influenced by the choice of solvent and reaction temperature. However, these parameters can also influence the desired reaction. A few studies have reported catalyst modifications that improve resistance against sintering. For example, leaching of Re during the hydrogenolysis of 2-(hydroxymethyl)tetrahydropyran was reduced by increasing the reduction temperature of the ReOx-promoted Rh/C catalyst, forming a more stable rhenium oxide phase.212 Another approach is to use bimetallic catalysts, such as supported Au/Pt or Au/Pd, although additional studies are needed to explain the extent to which the stabilization is the result of electronic or geometric effects.213-215

4.3. Agglomeration of Supported Metal Particles Supported metal particles are very common in heterogeneous catalysis. The high cost of the catalytically active phase often dictates their use as small (i.e., highly dispersed) nanoparticles, so that the largest possible fraction of metal atoms is accessible to the reactants. A frequent concern during high temperature, gas phase reactions is their agglomeration or sintering (Figure 16).216 This can occur by migration of intact crystallites, or single atoms (i.e., Ostwald ripening).217 Agglomeration of metal particles is often observed when the Tamman temperature (i.e., half the temperature of the melting point of the metal) is exceeded. However, the rate of agglomeration also depends on the reactive atmosphere, and the type and surface area of the support. Since sintering processes are known to be accelerated by water vapor,216 agglomeration 39 ACS Paragon Plus Environment

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is a possibility in liquid phase reactions even though these reactions are typically performed at relatively low temperatures.198

Figure 16. Model for catalyst sintering by (A) atomic migration or (B) crystalline migration.218 Reproduced from ref.

218

. The original figure was published under an Open Access Creative

Commons Attribution License.

Water-induced sintering of metal particles was reported for Ni/Al2O3 during aqueous-phase reforming of glycerol, ethylene glycol and sorbitol at 210 °C.219,220 The process was slower for supported bimetallic NiSn particles and Sn-promoted Raney-nickel, which consists of larger, more stable, particles.220 Slower sintering was also reported for Pt/Al2O3 under the same conditions.219 Sintering of supported Pd particles has been reported in liquid-phase reactions such as acetone hydrogenation to isopropyl alcohol,221 conversion of γ-valerolactone to pentanoic acid (by acidcatalyzed ring opening, followed by hydrogenation),194 and hydrogenation of 1-hexene.222,223 Sintering of Pd particles on niobia was reduced significantly by addition of small amounts of silica, which prevented the niobia support from crystallizing in hot liquid water.194 The stability of Pd particles on silica was also improved by deposition of a carbonaceous overcoat created by pyrolysis of adsorbed sugars.221 The stabilizing effect was explained as a combination of reduced atomic migration leading to Ostwald ripening of the Pd nanoparticles, and hydrolytic stabilization of the support. Similarly, atomic layer deposition (ALD) of Al2O3 onto Cu/Al2O3 stabilized the catalyst against sintering and leaching during liquid phase hydrogenation of furfural in the solvent 1-butanol.224 While the ALD treatment initially resulted in a dense Al2O3 layer covering the Cu particles, 40 ACS Paragon Plus Environment

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pores were created during calcination. The overlayer was suggested to stabilize the Cu component of the catalyst by interacting selectively with under-coordinated Cu atoms at edges, corners, and defects. Addition of Pd to Cu/ZrO2 reduced agglomeration of the Cu nanoparticles during hydrogenolysis of sorbitol at 220 °C.225 The authors suggested that the stabilizing effect arises due to electron transfer from Pd to Cu. Biomass-derived oxygenates, such as polyols and sugars, can effectively promote Ostwald ripening via chelation of metal species.198 The chelated species may be redeposited on larger, more stable metal particles, or permanently removed from the reactor (i.e., carried away in the product stream of a flow reactor, or eliminated during the recycling procedure for a batch reactor). In some rare cases, the mobility of supported metals can lead to a decrease in the average particle size.179 This phenomenon was reported when Pt/Al2O3 was exposed to lignin oligomers in a mixture of water and ethanol. The authors proposed that deposition of lignin oligomers on the Al2O3 creates additional binding sites for metal particles.

4.4. Fouling, Coking, and Humin Formation Deactivation may still occur even when the active sites are themselves stable. A common cause for such deactivation processes is fouling, i.e., deposition of high molecular weight organic compounds that block access to the active sites of the catalyst.226 A specific type of fouling is coking, in which oligomeric organic species are formed from smaller reactant molecules.227 “Hard coke” typically consists of polycyclic aromatics.227,228 While coking is observed in many hydrocarbon conversion processes, formation of hard coke typically requires considerably elevated temperatures (e.g., above 350 °C). However, highly reactive biomass-derived oxygenates can oligomerize under much milder conditions. These undesired oligomers are typically referred to as humins.229,230 An initial but not unambiguous indication of humin deposition is a color change of the catalyst from white to yellow or brown.226 Due to the diversity of functional groups in biomass-derived oxygenates, there are many reaction paths for the formation of humins. Several papers on the production or further transformation of levulinic acid have reported the formation of humins.226,231-235 For example, polymerization of 5HMF and 2,5-dioxo-6-hydroxyhexanal via aldol addition and condensation reactions was proposed as the mechanism of humin formation during the acid-catalyzed conversion of 5-HMF 41 ACS Paragon Plus Environment

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to levulinic acid and formic acid (Figure 17).226,235 High-boiling polar solvents (e.g., dimethyl sulfoxide, dimethylformamide, and dimethylacetamide) appeared to enhance the formation of humins during similar reactions.226 An alternative reaction path for humin formation from HMF under acidic conditions is nucleophilic attack of the carbonyl group of 5-HMF on the α- or βposition of the furanic ring of another 5-HMF molecule.236 Interestingly, this process is suppressed when DMSO is used as a co-solvent along with water. Thermochemical conversion of sugars involves complex reaction networks including dehydration, condensation, and rearrangement.226 The combination of these reactions is commonly referred to as caramelization. Interestingly, cellulose-derived humins produced in an ionic liquid gave a 13C NMR spectrum dominated by peaks typical of cellulose, indicating sugars present as abundant building blocks.237

Impurities in sugar feeds can also contribute to

fouling.231 Fouling of a catalyst consisting of 4-ethylbenzenesulfonic acid groups immobilized on silica during sunflower oil transesterification with methanol (i.e., biodiesel production)238 was associated with deposits containing saturated aliphatic domains as well as C=C bonds. The deposits produced in the reaction were distinct from those obtained from the individual reactants and products, indicating that multiple precursors must be involved in their formation. The most common way to remove coke from catalysts used in conventional hydrocarbon conversion is combustion. This strategy is not applicable to many catalysts used in liquid-phase processes.226 For example, polymeric resins like the Amberlyst materials,232 or inorganic-organic hybrid materials,238 have limited stability under strongly oxidizing conditions at elevated temperatures. Treatment with an aqueous H2O2 solution,232 extraction with an organic solvent such as γ-valerolactone,232 or lowering the reactant concentration in the feed of a continuous process,231 have all been demonstrated as alternative approaches for catalyst regeneration.


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Figure 17. Proposed reaction path for humin formation by condensation of 5-HMF and 2,5dioxo-6-hydroxyhexanal.235 Adapted from ref. 235.

Certainly more research is needed to elucidate the chemical nature of humins, and reaction pathways for their formation. This will allow for the development of more effective mitigation strategies, which may include the selective removal of certain compounds from the feed that are particularly prone to deposit formation, and the identification of catalysts and process conditions that avoid undesirable side reactions.

4.5. Poisoning Catalyst poisoning refers to the strong adsorption of a contaminant or product on the active sites (Figure 18).239-241 Sulfur poisoning of metal catalysts has long been a concern for processes involved in the conversion of oil- and coal-based feedstocks.239 This section will illustrate that a variety of poisons can deactivate catalysts during processes for biomass conversion. In this context, it is important to point out that liquid feeds are much denser than gaseous feeds. Therefore, active sites may be exposed to a significantly larger amount of poison per processed volume.


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Figure 18. Representative image for catalyst poisoning.

Acid catalysts can be deactivated by alkali- and alkaline earth-based components of biomass. Specifically, it was estimated that 100 g of certain grasses contain enough of these basic compounds to neutralize 5 g H2SO4.175 Likewise, the acetic acid released during hydrolysis of acetyl groups from hemicellulose neutralizes basic catalysts.242 Basic CaO used as a transesterification catalyst for biodiesel production was even poisoned by CO2 and H2O it adsorbed from the air during handling prior to reaction, presumably by formation of carbonate and hydroxyl species on the surface.243 In addition to S-, N-, and Cl-containing impurities, other metals and organic oxygenates can poison metal catalysts.175,211 For example, supported Ru catalysts used in glucose hydrogenation were deactivated by components such as Fe244 and Cr, and Si245 that leached from the reactor walls. Increased Fe accumulation with increasing Ru content of the catalyst suggested selective Fe adsorption on the supported Ru particles.244 The effect was enhanced when the iron was complexed by a gluconic acid impurity in the feed. Au and Pt catalysts used in the oxidation of glycerol by O2 were inhibited by the presence of certain organic oxygenates.66 In particular, ketones and their condensation products were found to be much stronger poisons than carboxylic acids. In principle, poisoning may be avoided by removing the offending components with an appropriate ion exchanger or guard bed, or by chemically converting them to less harmful compounds.175 However, more study is needed to develop approaches suitable for specific poisons and processes. Leaching of metals from reactors can be avoided by coating with materials like Teflon, or modifying the composition of the metal alloys used in their construction.211 There are also strategies to regenerate poisoned catalysts. Organic poisons and sulfur can often be removed by calcination, although high temperatures can lead to undesirable 44 ACS Paragon Plus Environment

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transformations of the catalyst.232,238 In certain cases, appropriate ion exchange procedures may be capable of regenerating specific active sites. However, much work remains to be done to develop suitable protocols. Prevention appears to be the most reliable poisoning mitigation strategy at this time.

5. Unexpected Contributions of Soluble Species to Heterogeneously-Catalyzed Reactions Previous sections have described how solvents can influence reactions, how catalyst components can leach into solution, and how reaction intermediates and products compete with the reactants for adsorption sites. In light of this complexity, the true identity of the catalyst in liquid phase reactions with a nominally heterogeneous catalyst may be obscured. This possibility was anticipated many years ago in studies of redox molecular sieves for heterogeneous catalytic reactions in the liquid phase. Sheldon et al. warned that such materials are often “Trojan horses”, which release metal ions into solution under the reaction conditions.246 In this section, we address contributions by reactive species other than the intended catalyst.

5.1 Catalysis by Species Leached from Heterogeneous Catalysts Deactivation by leaching of active sites from heterogeneous catalysts was discussed in Section 4.2. It is also possible that leached species are responsible for some or all of the observed catalytic activity. Thus, V- and Cr-substituted zeolites and mesoporous silicas, as well as V complexes encapsulated in zeolites, evolve to form soluble, catalytically active metal ions in the presence of solubilizing oxidants such as H2O2.247 The acetic acid solvent leaches metal ions from Co-substituted aluminophosphates, and these dissolved ions then catalyze the homogeneous oxidation of cyclohexane; even in non-polar solvents, the adipic acid product extracts cobalt ions.248 Efforts to develop heterogeneous catalysts based on Earth-abundant 3d transition metals for biomass transformations, such as the oxidation of glycerol to glyceric acid or 5-HMF to 2,5furandicarboxylic acid, have to contend with the solubility of their metal complexes under reaction conditions. For example, an unspecified combination of Fe, Cr and Ni ions leached from a Hasteloy-C reactor by aqueous NaOH at 240 °C showed catalytic activity for the conversion of glycerol to lactic acid.207 Strongly alkaline solutions typically used in such reactions can enhance solubility by dissolving solid oxide hosts, causing them to release transition metal ion sites slowly into solution. Sheldon 45 ACS Paragon Plus Environment

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et al. pointed out that the failure to observe a decline in activity over multiple catalyst recycles is not a reliable way to exclude leaching of active sites.246 When Zhang et al. used a combination of Raney Ni and different solid tungsten-based catalysts to convert cellulose to ethylene glycol, water-soluble HxWO3 species were formed at elevated temperatures, and were shown to catalyze the hydrolysis of cellulose to glucose.249 Since less than 2% of the tungsten was dissolved in each batch reaction, the catalyst was successfully recycled about 15 times, although an eventual decay in activity might be related to depletion of tungsten. Furthermore, the finding that leached species are present at very low levels is not grounds to exclude them from consideration: the relationship between concentration and activity can be highly non-linear, as exemplified by “homeopathic” cross-coupling catalysis by extremely low concentrations of soluble and/or colloidal Pd.250 In addition to leaching of metal ions, other heterogeneous catalyst components can also be lost. For example, organosulfonic acids attached to solid supports, such as silicas or organosilicas, can be solubilized by hydrolysis of the anchoring Si-O bonds at elevated temperatures. The resulting homogeneous organosulfonic acids can continue to catalyze reactions, such as fructose dehydration to 5-HMF, when they are conducted in batch reactors. In contrast, loss of heterogeneous catalytic activity due to leaching is readily apparent in a packed bed reactor.251 During aerobic oxidations of glycerol and 5-HMF, the hydrotalcite (HT) support of Au/HT dissolved, increasing the pH of the solution and enhancing the conversion to the respective diacids.252 In the case of H-BEA zeolite, soluble reaction products such as levulinic and formic acids, which arise from 5-HMF rehydration, can cause dealumination. The resulting soluble Lewis acidic Al3+ ions may contribute further reactivity, depending on the reaction conditions.253 Tessonnier et al. showed that the chloride ions enhance the dissolution of Al3+ from H-ZSM-5 (Si:Al = 18) at pH 2.5.254 The zeolite filtrate converted glucose to 5-HMF at 170 °C, even though glucose is too large to enter the zeolite pores. This result is significant because raw biomass can contain significant amounts of naturally-occurring chloride. Dissolution of alumina from Pt/alumina is also accelerated by chloride, in this case present as a residue from the use of H2PtCl6 during catalyst preparation.63 The soluble Al3+ ions catalyzed cellulose hydrolysis to smaller oligomers and glucose.


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5.2 Solvocatalytic Activity Solvents can contain or generate impurities that participate in catalytic reactions. For example, one of the most frequently used polar aprotic solvents in biomass conversion, DMSO, decomposes when heated in air to methanesulfonic acid and sulfuric acid.255 As early as 1982, Moody and Richards observed that traces of acidic species in DMSO can induce sucrose hydrolysis, although they can be removed by neutralization/adsorption on solid bases, such as BaO or CaH2.256 Horváth and coworkers estimated the acid concentration to be in the range 10-6 10-5 mol L-1 after 2 h at 120 °C. This is acidic enough to catalyze fructose dehydration to 5HMF.257 Another polar aprotic solvent, dimethylformamide (DMF), decomposes to CO and dimethylamine. As a base, dimethylamine catalyzes aldo-keto isomerization,258 while CO is a strong reducing agent and that can alter the solubility and reactivity of metal ions.259 Solvents can also contain metal residues from catalysts used in their synthesis. For example, ionic liquids that have been widely investigated as solvents in biomass processing are difficult to purify due to their lack of volatility. While performing catalytic carbohydrate conversions in [EMIM]Cl, Zhang and coworkers discovered that activity and selectivity were strongly affected by solvent purity.260 Elemental analysis revealed ppm levels of Cr, Cu, Fe, Ni, Na, etc., in addition to traces of Brønsted acidity, all of which contributed to the direct conversion of cellulose. Little production of glucose or 5-HMF was observed in the high purity form of the ionic liquid (HP: 99.5 %), but the medium purity solvent (MP: 98 %) produced a significant amount of 5-HMF, while the low purity solvent (LP: 93 %) gave much more glucose (Figure 19).


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Figure 19. Effect of ionic liquid purity on cellulose depolymerization activity. Reactions were conducted at 180 °C for 5 min.260 Reproduced with permission from ref.

260

. Copyright 2012,

Springer.

Direct solvent participation in biomass reactions must be considered as well. For example, solvolysis of lignin by polar protic solvents (notably, alcohols) occurs readily at elevated temperatures, often in parallel with catalytic depolymerization.261 Imidazolium-based ionic liquids can be deprotonated at the C2 position and react further with aldoses (such as glucose and the reducing end of cellulose) to form Baylis-Hillman adducts.262 An example is shown in Scheme 1, in which adduct formation with glucose proceeds without additional reagents and is reversible upon addition of an organic amine as catalyst.263 The reaction is also known for 5HMF; in this case, the adduct undergoes further reaction in the presence of chloride (Scheme 1).264


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Scheme 1. Reactions of n-butylmethylimidazolium with glucose263 and 5-HMF.264 Adapted from refs. 263 and 264.

5.3 Autocatalytic Behavior Soluble products of catalytic reactions such as carboxylic acids can act as homogeneous catalysts that enhance the rate of a heterogeneously catalyzed reaction. For example, formic acid (FA, pKa = 3.74), acetic acid (pKa = 4.75), and levulinic acid (LA, pKa = 4.59) are all capable of catalyzing the hydrolysis of sucrose, maltose, and polysaccharides. The rate of dehydration of carbohydrates such as glucose and fructose, which yields carboxylic acids, can therefore be autocatalytic.265-267 Likewise, acetic acid is released when acetyl groups are hydrolyzed from hemicellulose.268 Since formation of organic acids causes the pH of the reaction solution to decrease, neutralizing acid species in situ or buffering the solution can dramatically lower the production of 5-HMF from fructose.266 Reactions starting with biomass can be autocatalytic for other reasons. The hydrolysis of soluble hemicellulose was studied using heterogeneous acidic resins like Amberlyst 15 and Smopex 101 as catalysts.269 An increase in the reaction rate with conversion was attributed to an increased rate of mass transfer as the molecular weight and branching of the biopolymer decreased. Biomass also contains a wide range of trace metals, including Zn, Pb, Cd, Mn, Cr, Ni, Co, Cu, Sn, Sb, Se, V, Mo, and As.270 Although little studied at this time, these metal contaminants can potentially contribute to catalysis. Acid pretreatment of raw biomass to increase the accessibility of biomass can liberate, mobilize or remove metallic species and thereby alter their contributions.


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6 Summary and Outlook The low vapor pressure of many biomass-derived oxygenates has stimulated considerable interest in heterogeneously catalyzed reactions in the liquid phase. These reactions are significantly more complex than heterogeneously catalyzed reactions of gaseous reactants. Solvents can compete for surface sites and stabilize or polarize adsorbed intermediates and transition states. In addition, certain key elementary reactions can occur in the bulk liquid or be mediated by solvent molecules. Additional complexity exists in converting biomass-derived species with multiple functional groups that can interact simultaneously with specific surface sites. Often these interactions are not independent, and a tradeoff between the possible interactions determines the surface species that is ultimately formed. Mass transport limitations can be much more pronounced in liquid phase reactions, especially in viscous solvents or microporous solids. Modification of the structure and hydrophilicity/hydrophobicity of porous materials can be used to enhance rates of diffusion. Many types of structural transformations (e.g., structural collapse, leaching, and agglomeration of supported metal particles) of solid catalysts have been reported, when these materials were used in liquid phase reactions at moderately elevated temperatures. Protection of “points of attack” with overlayers and/or addition of promoters to supported metal particles are among the viable strategies to improve the stability of solid catalysts in liquid phase reactions. Of course, deactivation by formation of carbonaceous deposits or poisons must also be considered, and pretreatment of feedstocks may be necessary to inhibit these processes. The added complexity in liquid phase reactions also creates a risk of attributing observed reactivity to surface reactions by mistake when, in fact, soluble species derived from catalysts, solvents, or feedstocks are the true catalytic species. This is a particular danger in batch reactions, where soluble species remain in the reactor until the end of the experiment. While studies of the individual properties of catalysts can provide valuable insight, the catalytic “trinity” of activity, selectivity, and stability is usually not readily decoupled. Thus, it is necessary to develop catalytic processes by optimizing all three simultaneously. This Perspective aims to provide a better understanding of critical phenomena to offer guidance for such efforts.


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Acknowledgements We thank Kara Yogan for designing the Table of Contents Figure. S.S. and L.Q. acknowledge funding from the U.S. National Science Foundation CBET Program (award number: 1512228). Y.N. and R.M.R. acknowledge funding from the U.S. National Science Foundation CBET Program (award number: 1067384). E.M.A. thanks CAPES for a fellowship.


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ToC Figure


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