Mechanistic Insights into Aerobic Oxidative Methyl Esterification of

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Mechanistic Insights into Aerobic Oxidative Methyl Esterification of Primary Alcohols with Heterogeneous PdBiTe Catalysts David S. Mannel, Jesaiah King, Yuliya Preger, Maaz S. Ahmed, Thatcher W Root, and Shannon S. Stahl ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02886 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Catalysis

Mechanistic Insights into Aerobic Oxidative Methyl Esterification of Primary Alcohols with Heterogeneous PdBiTe Catalysts David S. Mannela, Jesaiah Kinga, Yuliya Pregera, Maaz S. Ahmedb, Thatcher W. Roota,*, Shannon S. Stahlb,* a

Department of Chemical and Biological Engineering b

Department of Chemistry

University of Wisconsin-Madison, Madison Wisconsin, 53706

TOC Graphic O R OH + O2 + MeOH

R

Key role of BiTe promoters:

+ 2 H2O OMe

PdBiTe

Pd/BiTeOx

O2

active Pd/BiTeOx

Bi and Te prevent inactive PdOx formation inactive O2 PdOx Pd

• TOL Step: β-hydride elimination from surface-bound alkoxide • O2 activation occurs at BiTe promoter sites

ABSTRACT: Aerobic oxidative methyl esterification of primary alcohols is an important chemical transformation that converts a nucleophile (alcohol) into a versatile electrophile (methyl ester). We recently discovered a heterogeneous PdBiTe/C catalyst that exhibits the highest activity yet reported for this transformation. Bi and Te serve as synergistic promoters that enhance both the rate and yield of the reactions relative to reactions employing Pd alone or Pd in combination with Bi or with Te as the sole promoter. Here we report a mechanistic study of the

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oxidative methyl esterification of benzyl alcohol and 1-octanol to provide insights into the overall multistep transformation as well as the role of the Bi and Te in the reaction. The catalytic rates of the oxidative esterification of benzyl alcohol and octanol with Pd, PdBi, PdTe and PdBiTe catalysts exhibit a saturation dependence on [alcohol] and [K2CO3] and a first-order dependence on pO2. Hammett studies of benzyl alcohol oxidation reveal opposing electronic trends for initial oxidation of alcohol to aldehyde (negative r value) and the oxidation of aldehyde to methyl ester (positive r value). These data and complementary kinetic isotope effect data support a Langmuir-Hinshelwood mechanism in which a surface-bound alkoxide or hemiacetal intermediate undergoes rate-limiting b-hydride elimination. Molecular oxygen participates in this process, as revealed by a first-order dependence on pO2. X-ray photoelectron and X-ray absorption spectroscopic methods show that the promoters undergo oxidation in preference to Pd, maintaining the Pd surface in the active metallic state and preventing inhibition by surface Pd-oxide formation. Collectively, these results provide valuable insights into the synergistic benefits of multiple promoters in heterogeneous catalytic oxidation reactions.

Key Words: oxidation, alcohols, catalysis, palladium, oxygen

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Introduction Homogeneous and heterogeneous Pd catalysts have been widely studied for the aerobic oxidation of alcohols to aldehydes, ketones, and carboxylic acids.1,2 Aerobic oxidative methyl esterification of primary alcohols is an appealing complementary transformation that provides an attractive route to methyl esters (Scheme 1), which are common products and intermediates in the pharmaceutical, agrochemical, and fine chemical industries. This reaction has been the subject of a number of recent catalyst development efforts, including Au catalysts for gas-phase applications,3 a number of noble metal catalysts (Au,4 Ag,5 and bimetallic polymer-incarcerated Au-Pd6) and a Co3O4-N@C7 catalyst for liquid phase reactions. The reactions in liquid phase are especially amenable to pharmaceutical and other complex-molecule syntheses and are especially effective with benzylic alcohols. We have recently developed a heterogeneous Pd-based catalyst that incorporates Bi and Te promoters (PdBiTe) and exhibits the fastest rates and broadest substrate scope yet reported for oxidative methyl esterification of alcohols.8 The catalyst is effective with both activated (benzylic) and unactivated (aliphatic) substrates, and it also tolerates heterocycles and other heteroatom-containing functional groups.

R

O

cat., [O] OH

Primary Alcohol

R Aldehyde

OH

+ MeOH - MeOH

R

OMe

Hemiacetal

O

cat., [O] R

OMe

Methyl Ester

Scheme 1. Reaction Pathway for the Aerobic Oxidation of Alcohols to the Methyl Ester.

Aerobic alcohol oxidation reactions that use heterogeneous Pd and Pt catalysts containing promoters have been the subject of considerable investigation, commonly focusing on oxidation of sugars and other primary alcohols to carboxylic acids in aqueous conditions.9 The majority of these examples feature a single promoter element, with bismuth being the most common. The

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PdBiTe catalyst is an important example of a dual-promoter catalyst system,10 in which the two promoter elements exhibit synergistic benefits. This effect is illustrated in Figure 1, which shows that the oxidation of 1-octanol affords excellent yield of methyl octanoate (96%) when the catalyst contains both Bi and Te. In contrast, only modest conversion is observed in the presence of a catalyst with Bi or Te alone as the promoter (46% and 51% yield, respectively), and a very poor outcome is observed (7% yield) with the parent Pd/C catalyst. Preliminary observations also implicated cooperative benefits of the Bi and Te promoters on the catalyst activity and stability.8 In an effort to begin probing the unique features of the PdBiTe catalyst, we have undertaken a kinetic and mechanistic study of PdBiTe-catalyzed oxidative methyl esterification of benzyl alcohol and 1-octanol. The results highlight similarities and differences between the oxidation mechanisms of these representative activated and unactivated primary alcohols and provide the basis for a multistep mechanism for the catalytic reaction. The rate law derived for this mechanism is effective in modeling the experimental data. In addition, X-ray photoelectron and X-ray absorption spectroscopic (XPS and XAS) analysis of different catalysts provides valuable insights into the synergistic role of Bi and Te. 1 mol % Pd 3

1M

OH

60 °C, 1 bar O 2, MeOH 25 mol % K 2CO3, 8 h

O 3

OMe

Figure 1. Yields of methyl octanoate obtained from the aerobic methyl esterification of 1-octanol over the Pd, PdBi, PdTe, and PdBiTe catalysts.8b Catalyst compositions: Pd = Pd/C (5 wt. % Pd), PdBi = PdBi0.35/C (5 wt. % Pd), PdTe = PdTe0.23/C (5 wt. % Pd), PdBiTe = PdBi0.35Te0.23/C (5 wt. % Pd).

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Results Our work was initiated with kinetic studies of the oxidative methyl esterification of benzyl alcohol and 1-octanol, which serve as representative activated and unactivated alcohols, respectively. Kinetic data were acquired by monitoring the time course of the oxidation of these alcohols by gas chromatography. Four different carbon-supported catalysts were tested: a nonpromoted catalyst Pd/C (Pd), two single-promoter catalysts, PdBi0.35/C (PdBi) and PdTe0.23/C (PdTe), and an optimized dual promoter catalyst PdBi0.35Te0.23/C (PdBiTe), wherein the fractional values reflect the molar ratio of the promoters relative to Pd.8b Comparison of Pd, PdBi, PdTe and PdBiTe catalysts in the oxidation of benzyl alcohol. The time courses for the oxidation of benzyl alcohol (Figure 2) show the formation and decay of benzaldehyde en route to the methyl ester product (cf. Scheme 1). The oxidation of benzyl alcohol proceeds slowly over Pd/C with a large buildup of benzaldehyde as the major product after 24 h (63% yield, Figure 2A). Reactions with the Bi-, Te-, and BiTe-promoted Pd/C catalysts proceed with ≥90% conversion of benzyl alcohol after 2 hours (Figure 2B-D), during which the intermediate aldehyde builds up and decays, reaching a maximum concentration of ~20% of the original [alcohol]. The alcohol-to-aldehyde and the aldehyde-to-ester reactions were modeled by treating both steps as pseudo-first order reactions in an A ® B ® C kinetic sequence (eqs 1–3). The results provide effective rate constants keff1 and keff2 for the two steps. The values show that inclusion of Bi and/or Te in the catalyst increases the rate of benzyl alcohol oxidation by an order of magnitude, and increases the rate of benzaldehyde conversion to methyl benzoate by two orders of magnitude, relative to the rates observed with the Pd-only catalyst (Figure 3).

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OH 1M

O

0.1 mol % Pd

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O OMe

60 °C, 1 bar O2, MeOH 25 mol % K 2CO 3

A. Pd

B. PdBi

C. PdTe

D. PdBiTe

Figure 2. Time courses for the aerobic methyl esterification of benzyl alcohol showing conversion of benzyl alcohol to benzaldehyde intermediate and subsequent reaction of benzaldehyde to methyl benzoate over (A) Pd, (B) PdBi, (C) PdTe, and (D) PdBiTe catalysts. Colored lines reflect fits to the data using eqs 1–3, and the corresponding rate constants are depicted in Figure 3.

𝑑 𝑅𝐶𝐻% 𝑂𝐻 = −𝑘+,,- 𝑅𝐶𝐻% 𝑂𝐻 𝑑𝑡

(1)

𝑑 𝑅𝐶𝐻𝑂 = 𝑘+,,- 𝑅𝐶𝐻% 𝑂𝐻 − 𝑘+,,% 𝑅𝐶𝐻𝑂 𝑑𝑡

(2)

𝑑 𝑅𝐶𝑂𝑂𝑀𝑒 = 𝑘+,,% 𝑅𝐶𝐻𝑂 𝑑𝑡

(3)

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Figure 3. Effective rate constants for the oxidation of benzyl alcohol to benzaldehyde (keff1) and benzaldehyde to methyl benzoate (keff2) using eqs 1–3. Fits and experimental conditions are provided in Figure 2. Error bars denote confidence intervals for keff1 and keff2 in the fits shown in Figure 2. All rate constants reported below are given with respect to total Pd present in the catalyst. Results of CO uptake and X-ray diffraction (XRD) analysis of the catalysts, however, show that the Pd/C catalyst has an initial dispersion of about 25% and addition of promoters decreases the Pd surface site concentration by a factor of 2-4 (see Supporting Information, Section 14 for details). Consequently, the observed increase in activity for the promoted catalysts, evident in Figure 3, is even larger if normalized relative to surface Pd sites (Table S5). Comparison of Pd, PdBi, PdTe and PdBiTe catalysts in the oxidation of octanol. In contrast to the oxidation of benzyl alcohol, the oxidation of 1-octanol to methyl octanoate reveals very little build-up of the corresponding aldehyde intermediate (< 3%). This observation implies that keff2 >> keff1, which is chemically reasonable, owing to the increased tendency of aliphatic aldehydes to form a hemiacetal via addition of methanol to the carbonyl group.11 The initial rate of the reaction shows that inclusion of Bi and/or Te in the catalyst increases the rate of 1-octanol oxidation by an order of magnitude relative to the rate observed with the Pd-only catalyst (Figure 4B). The initial rate increases slightly upon inclusion of both Bi and Te in the catalyst but, more importantly, the PdBiTe catalyst exhibits sustained activity that leads to higher yields of methyl octanoate relative to the yields obtained with the single promoter catalysts (Figure 4A).

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3

1M

A.

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1 mol % Pd

O

60 °C, 1 bar O2, MeOH 25 mol % K 2CO 3

3

OH

OMe

B.

Figure 4. Aerobic methyl esterification of 1-octanol to methyl octanoate showing (A) differences in yield (lines are guides to the eye) and (B) differences in initial rates between Pd, PdBi, PdTe, and PdBiTe. Error bars denote confidence intervals for keff1 in fitting data in plot A. Kinetic study of PdBiTe-catalyzed oxidation of benzyl alcohol and octanol. Modeling of the oxidative methyl esterification of benzyl alcohol and 1-octanol with the PdBiTe catalyst under different conditions enabled determination of the kinetic order in alcohol, aldehyde, and K2CO3 concentrations, as well as the pO2 (Figure 5). The data obtained from the oxidation of benzyl alcohol show that the reaction exhibits a saturation behavior in [benzyl alcohol], a slight saturation in [K2CO3], and a first-order dependence on pO2. Kinetic studies were also performed with benzaldehyde as the substrate, and the reaction exhibits a saturation dependence on [benzaldehyde] and a slight saturation dependence on [K2CO3]. The kinetic data obtained with 1octanol as the substrate show similar overall trends, but the rates are approximately 1-2 orders of magnitude lower than those obtained with benzyl alcohol under comparable conditions. The reaction shows saturation behavior in [1-octanol] and [K2CO3] and is first order in pO2. Control experiments show that increasing the catalyst loading at individual oxygen pressures increases the rate of reaction, with a first-order dependence on Pd loading in all cases.12 These observations demonstrate that the reaction is not limited by gas-liquid mass transfer (see Figure S8 in the Supporting Information). Full analysis of the reaction mixture shows that negligible

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background oxidation of the methanol solvent to methyl formate is observed (< 1% relative to [1-octanol]0, corresponding to < 0.01% of the initial [CH3OH]3,8b).

OH

keff1

O

keff2

O keff

OMe 3

O

OH 3

A.

A'.

B.

B'.

C.

C'.

OMe

Figure 5. Kinetic data from the oxidation of benzyl alcohol and 1-octanol by PdBiTe assessing the kinetic dependence on [substrate] (A/A'), [K2CO3] (B/B'), and pO2 (C/C'). Rates were obtained by taking small aliquots and analyzing by GC during catalytic turnover except for (C) where consumption of O2 was monitored by a pressure transducer in the ChemSCAN reactor. Standard reaction conditions are 0.1 mol% PdBiTe, 60 °C, 1 M substrate, 0.25 M K2CO3, and 1 bar O2 in MeOH (1 mL total volume). All experiments start with the alcohol except for benzaldehyde in plots A and B which start with the aldehyde. The curves fit in plots A, A', B, and B' reflect a nonlinear least-squares fit to rate = c1[substrate or K2CO3]/(c2 + [substrate or K2CO3]).

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Hammett studies and kinetic isotope effects. To gain additional insights into the mechanism of the reaction we investigated the oxidation rate of different substituted benzyl alcohols and benzaldehydes (p-methoxy, p-methyl, m-fluoro, and p-trifluoromethyl). Reaction rates were determined both for oxidation of alcohol-to-aldehyde (keff1) and for oxidation of aldehyde-to-methyl ester (keff2), and Hammett plots were generated (Figure 6). The oxidations of the alcohols to the aldehydes exhibit a negative slope (r = -1.2), while oxidations of the aldehydes to methyl esters show a non-linear, but positive, trend reflecting more rapid reaction of electron-deficient aldehydes. A chemical step relevant to conversion of aldehydes to esters is the formation of a hemiacetal species via addition of methanol to the carbonyl group (cf. step 2 in Scheme 1). 1H NMR analysis of methanol solutions of the different substituted benzaldehydes reveal that the aldehydes bearing electron-withdrawing group exhibit a significant equilibrium population of the hemiacetal in solution, which could favor more rapid conversion to the corresponding methyl esters (see section 10 in the Supporting Information).

A.

B. R

OH

O R

Figure 6. Hammett plots derived from independent rate measurements of substituted benzyl alcohols and benzaldehydes: (A) keff1 is rate of alcohol to aldehyde (r = -1.2) and (B) keff2 is rate of aldehyde to methyl ester, starting with the aldehyde. (See Figure 5 caption for reaction conditions). Kinetic isotope effects (KIEs) for the alcohol-to-aldehyde conversion were measured by comparison of the independent rates of RC6H4CH2OH and RC6H4CD2OH (R = p-MeO and p-

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CF3) and by an intramolecular competition experiment with p-MeOC6H4CHDOH (Table 1). The KIE obtained by independent rate comparison of p-MeOC6H4CH2OH and p-MeOC6H4CD2OH gave a normal KIE value of kH/kD = 2.4±0.6 for the oxidation of the alcohol to the aldehyde, while an intramolecular competition using p-MeOC6H4CHDOH exhibited a KIE of 4.0±0.2. The oxidation of p-CF3C6H4CH2OH and p-CF3C6H4CD2OH shows a larger normal KIE for the oxidation of the alcohol to the aldehyde (kH/kD = 4.1±1.4).

Table 1. Kinetic Isotope Effects for PdBiTe Catalyzed Oxidative Methyl Esterificationa A. Intramolecular Competitionb H

H D R

OH

D

+

R

O

R

O

B. Independent Rate Comparisonc H H R

OH

-or-

H

D D R

OH

R

D

-orO

R

O R

O

Experiment

R = 4-MeOC6H 4

A. Intramolecular Competition

4.0±0.2

--d

B. Independent Rates

2.4±0.6

4.1±1.4

OMe

R = 4-CF 3C6H 4

a

Reaction conditions given in Figure 5 caption. bKIE measured from the ratio of deutero to protio aldehyde by 1H NMR. cKIE determined from the difference in the fitted rate (keff1) of disappearance of protio and deutero alcohol. d Insufficient 4-CF3-benzaldehyde intermediate builds up to determine the H/D ratio by 1H NMR analysis.

O2 stoichiometry studies. Further study of the reactivity of O2 in the reaction was performed to determine the O2:product stoichiometry. If water is the final product of the reaction, then each equivalent of ester produced will consume one equivalent of O2, while each equivalent of the aldehyde intermediate will consume 0.5 equivalent of O2. If H2O2 is the final product, then a twofold higher quantity of O2 would be consumed.13 Comparison of the quantity of O2 consumed relative to the amount of product formed (mol ester + mol aldehyde/2) reveals a 1:1 correlation, consistent with full reduction of O2 to H2O under the reaction conditions (Figure 7A). The close correlation between the O2 stoichiometry and the benzyl alcohol-derived products is consistent with the observation that oxidation of the methanol solvent (e.g., via homocoupling to methyl 11


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formate) is a negligible side reaction, as noted above.8b Hydrogen peroxide cannot be excluded as an intermediate in the reduction of O2 to water, however, as independent studies show that all of the catalysts mediate rapid disproportionation of H2O2, with rates much faster than substrate oxidation (TOFH2O2 = 5-9 x 104 h-1, Figure 7B and TOFRCH2OH ~ 1 x 103 h-1). A.

B.

Figure 7. Gas uptake/evolution experiments showing (A) mol product versus mol O2 consumed in the oxidative methyl esterification of benzyl alcohol and (B) moles of O2 produced from the H2O2 disproportionation over the various catalysts. Conditions: (A) 1 M benzyl alcohol and 25 mol% K2CO3 in MeOH (2 mL), 0.1 mol% PdBiTe (1 mM [Pd]), and 60 °C measured on the ChemSCAN. (B) 8 mM [Pd], 0.25 M K2CO3 in 2 mL 9:1 MeOH:H2O. Reaction initiated by injection of 250 µL of 1 M H2O2 in 9:1 MeOH:H2O. X-ray photoelectron and X-ray absorption spectroscopic analysis of the catalysts. In an effort to gain additional insights into the role of the Bi and Te promoters, the kinetic and mechanistic studies described above were complemented by X-ray photoelectron and X-ray absorption spectroscopic (XPS and XAS) analysis of the different catalysts. Previous characterization of the Pd, PdBi, PdTe, and PdBiTe catalysts by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX)8b indicated that Bi and Te co-localize with Pd on the carbon support. In the present study, X-ray photoelectron spectroscopy (XPS) was used to probe similarities and/or differences among the oxidation states of the Pd, Bi, and Te components in the four different catalysts (Pd, PdBi, PdTe, and PdBiTe) at three different conditions. The catalyst samples included (1) "as-prepared" samples of the catalysts obtained

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from the wet-impregnation preparation method (see section 1 of Supporting Information for details), (2) "oxidized" catalysts obtained after stirring an aqueous suspension of the catalysts at 80 °C under 1 atm O2 in the absence of substrate, and (3) "post-reaction" catalysts, obtained by filtration of the catalysts after being used in the oxidation of benzyl alcohol. Similar trends were observed for each of the three sets of catalyst samples. The data for the "oxidized" catalysts are presented below, while data for catalysts from the other two conditions are included in the Supporting Information (see Section 9). Pd XPS data reveal that all four catalysts (Pd, PdBi, PdTe, and PdBiTe) contain a mixture of metallic and oxidized Pd (Figure 8). The catalysts containing Te (i.e., PdTe and PdBiTe) have the lowest fraction of oxidized Pd (PdOx, PdO, and PdO2) with binding energies > 336 eV. The fraction of oxidized Pd in these samples (45-49%) is significantly less than that in PdBi (61%), while the unpromoted catalyst, containing neither Bi nor Te, has the highest fraction of oxidized Pd (73%, Figure 9). For catalysts containing Te, the peak for fully reduced (i.e., metallic) Pd appears at somewhat higher binding energies (~0.3 eV) relative to the metallic Pd peak in the Pd and PdBi samples, suggesting that Te modifies the electronic character of Pd in the former catalysts.

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A. Pd

B. PdBi

C. PdTe

D. PdBiTe

Figure 8. Pd 3d5/2 XPS spectra obtained for (A) Pd, (B) PdBi, (C) PdTe, and (D) PdBiTe catalysts following exposure to 0.25 M K2CO3 in water and 1 bar O2 at 80 °C for 15 h fit with 30% Lorentzian, 70% Gaussian Voigt functions and fixed full width at half maximum (FWHM). Peak reference values obtained from the literature for Pd,14a-d PdOx,14b-d PdO,14a-d and PdO2.14b

Figure 9. Calculated composition % of Pd, “PdO”, and PdO2 oxidation states by XPS for the Pd, PdBi, PdTe, and PdBiTe catalysts. Pd is the most reduced peak at 335.2-335.5 eV, "PdO" is the summation of peaks with binding energies between 336 and 337 eV, and PdO2 is >338 eV.

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XPS analysis of Bi (Figure 10) and Te (Figure 11) in the different catalysts provides complementary insights. The Bi is significantly more oxidized in the PdBi catalyst relative to the PdBiTe catalyst. For example, Bi2O3 and other Bi components with a binding energy > 159 eV correspond to 61% of the Bi present in PdBi, while these components correspond to only 15% of the Bi in PdBiTe (Figure 10 and Figure 12A). Te shows the opposite trend: a lower fraction of oxidized Te is present in PdTe (52% TeO2; binding energy > 575 eV) relative to PdBiTe (74% TeO2) (Figure 11 and Figure 12B). A. PdBi

B. PdBiTe

Figure 10. Bi 4f7/2 XPS spectra measured on (A) PdBi and (B) PdBiTe catalysts after exposure to 0.25 M K2CO3 in water and 1 bar O2 at 80 °C for 15 h. Peak reference values obtained from the literature for Bi,14a,b BiOx(OH)y,14d Bi2O3,14a-d and Bi2O5.14c A. PdTe

B. PdBiTe

Figure 11. Te 3d5/2 XPS spectra measured on (A) PdTe and (B) PdBiTe catalysts after exposure to 0.25 M K2CO3 in water and 1 bar O2 at 80 °C for 15 h. Peak reference values obtained from the literature for Te14a,b,e and TeO2.14a,b,e

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A.

B.

Figure 12. Bi and Te compositions derived from fitting XPS data for PdBi, PdTe, and PdBiTe catalysts. In (A), "Bi" corresponds to the lowest-energy peak at 158-158.5 eV, "BiOx(OH)y" corresponds to the peak at 158.8 eV, and "Bi2O3 +" corresponds to all peaks at >159 eV. In (B), "Te" corresponds to peaks at < 575 eV, while TeO2 corresponds to peaks at > 575 eV. The conditions required for XPS sample preparation and data collection necessitate exposure of the catalyst sample to atmospheric conditions in the absence of substrate and, therefore, prevent direct analysis of catalysts under reducing or standard reaction conditions. Consequently, Pd X-ray absorption spectroscopy was used to analyze Pd and PdBiTe "reduced" and "reaction" catalyst samples, as well as "as-prepared" and "oxidized" samples. The "reduced" catalyst was prepared by stirring the catalyst in the presence of 1-octanol under anaerobic conditions, while the "reaction" catalyst was obtained from a catalytic reaction with 1-octanol that had reached 50% conversion, followed by rapid filtering of the catalyst. Both the "reduced" and "reaction" samples were immediately frozen in liquid nitrogen upon filtration to ensure the catalysts did not undergo further changes. They were maintained at 77 K during XAS data collection. The Pd XAS data for the Pd-only catalyst reveal a progressive increase in the K-edge energy for the "reduced", "reaction", "as-prepared" and "oxidized" samples, while the "reduced" and "reaction" samples exhibit spectra that closely resemble the Pd-foil reference sample (Figure 13A). By contrast, the Pd K-edge energy in the PdBiTe catalyst is essentially the same for all four catalyst samples, and they exhibit an energy similar to the Pd-foil reference (Figure 13B), indicating that the Pd present in these samples is mostly metallic (rather than oxidic).

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Collectively, the XAS data support the conclusions of the XPS analysis, which reveal that the Bi and Te promoters maintain Pd in the metallic state, even under oxidizing conditions.

A. Pd

B. PdBiTe

Figure 13. Normalized XANES spectra collected at the Pd K-edge for (A) Pd-only and (B) PdBiTe catalysts obtained from different conditions. Discussion Mechanistic proposal and analysis of kinetic data. These experimental results provide a rich collection of features to incorporate into a multistep catalytic mechanism for the reaction on the Bi/Te-modified Pd catalyst surface. As a brief summary, the rate studies describe the dependence of both the alcohol and aldehyde conversion steps on reaction conditions, displaying saturation dependence on substrate and base (only slight saturation for base) and a linear dependence on the oxygen concentrations. The Hammett studies show that both steps are sensitive to the electron donating/withdrawing nature of substituted benzylic substrates, although their trends exhibit different behavior. Oxidation of alcohols to aldehydes has a relatively strong negative slope (r = –1.2), while the more rapid oxidation step to methyl esters shows a more complex, but net positive, trend. The primary kinetic isotope effects of 2-4 demonstrate that reaction of the b-hydrogen atom of the alcohol is at least partly turnover-limiting. Meanwhile, 17


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the spectroscopic data provides some preliminary insights into the influence of the Bi and Te components on the catalyst structure and the catalytic mechanism. A mechanism for PdBiTe-catalyzed oxidative methyl esterification of primary alcohols that accounts for the kinetic observations summarized above is shown in Scheme 2. For simplicity, elementary steps that are not kinetically resolved (e.g., those involving fast equilibration or occurring after the turnover-limiting step) are represented as a single combined step. The first step involves Brønsted base-promoted binding of an alcohol to the metal surface as an alkoxide, (Scheme 2, step i). The Brønsted base could be either methoxide or carbonate, and these are shown to be kinetically equivalent in Figure S5 (see Supporting Information). Reversible binding of O2 to the surface (step ii), possibly in a second type of site generated by the promoter, is proposed to initiate turnover-limiting b-hydride elimination from the bound alkoxide, ultimately resulting in formation of the aldehyde and a hydroperoxide intermediate (step iii). The data are consistent with (and cannot distinguish between) direct or surface-mediated transfer of the hydride to the adsorbed O2 species. Oxidation of the aldehyde intermediate to the methyl ester can be rationalized by a similar mechanism, initiated by formation and surface binding of a deprotonated hemiacetal species (step iv). The surface-bound hemiacetal will undergo b-hydride elimination in the presence of the active O2 surface species to form the methyl ester together with another hydroperoxide species (step v) that may undergo O–O dissociation or react with a proton source such as MeOH to form H2O2 which will undergo rapid disproportionation to water and O2 (step vi; see also, Figure 7 and associated text for consideration of the H2O2 disproportionation step).

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RCH 2OH + MeO - + ∗ O2 + ‡ RCH 2O∗ + O2‡ RCHO + MeO - + ∗ RCH(OMe)O∗ + O2‡ MeOH + HOO‡

kalc k-alc KO2 k1 kHemi k-Hemi k2 kox

RCH 2O∗ + MeOH

(i)

O2‡

(ii)

RCHO + HOO‡ + ∗

(iii)

RCH(OMe)O∗

(iv)

RCOOMe + HOO‡ + ∗

(v)

H 2O + 1 /2O2 + MeO - + ‡

(vi)

Scheme 2. Proposed reaction mechanism for the aerobic oxidation of alcohols to methyl esters over PdBiTe. The sequence in Scheme 2 accounts for all of the kinetic data, including a saturation dependence on [alcohol] and [base], a first-order dependence on pO2, as well as the observation of kinetic isotope effects, and diagnostic Hammett correlations. Two Langmuir-Hinshelwood models were evaluated, in which the alcohol/hemiacetal and O2 bind to different or the same sites (Schemes 2 and S2, respectively). Rate laws were derived for both mechanisms assuming steady-state buildup of the surface bound alkoxide and hemiacetal intermediates, equilibrium binding of O2, and irreversible b-hydride elimination as the rate-determining step (see eqs 4–7 for the two-site model and eqs S26–S29 in the Supporting Information for the single-site model). This is comparable to previously proposed mechanisms for alcohol oxidation to carboxylic acids or methyl esters over noble metal catalysts where oxidation proceeds via deprotonation of the O– H bond followed by b-hydride elimination to the surface.3,15 The active oxygen species is shown as a form of molecularly-adsorbed O2. It is clearly not dissociatively adsorbed oxygen in adsorption/desorption equilibrium on Pd, as that would produce a half-order dependence. This molecular oxygen intermediate could be weakly bound on additive-induced sites, such as Bi surface oxides or Te/Pd surface alloy species.16 Either additive alone produces the large rate acceleration over the bare Pd catalyst rate for benzyl alcohol (Fig. 3), while for the more difficult

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aliphatic substrate the combination of both Bi and Te increases the catalytic oxidation rates significantly more than either alone (Fig. 4). The fraction of vacant sites (𝜽∗𝒗 ) on the catalyst is calculated from a site balance (eq 4), and the proposed mechanism results in expressions for time-dependent changes in the concentrations of alcohol, aldehyde and ester, as shown in eqs 5–7.17

𝜽∗𝒗 =

𝟏 𝑹𝑪𝑯𝑶 𝑴𝒆𝑶@ 𝒌𝒂𝒍𝒄 𝑹𝑪𝑯𝟐 𝑶𝑯 𝑴𝒆𝑶@ 𝒌 𝟏+ + 𝑯𝒆𝒎𝒊 𝒌@𝒂𝒍𝒄 𝑴𝒆𝑶𝑯 + 𝒌A 𝟏 𝒑𝑶𝟐 𝒌@𝑯𝒆𝒎𝒊 + 𝒌′𝟐 𝒑𝑶𝟐

(4)

𝒅[𝑹𝑪𝑯𝟐 𝑶𝑯] −𝒌A 𝟏 𝒌𝑨𝒍𝒄 𝑹𝑪𝑯𝟐 𝑶𝑯 [𝐌𝐞𝑶@ ]𝒑𝑶𝟐 𝜽∗𝒗 = 𝒌@𝒂𝒍𝒄 𝑴𝒆𝑶𝑯 + 𝒌′𝟏 𝒑𝑶𝟐 𝒅𝒕

(5)

𝒅[𝑹𝑪𝑯𝑶] 𝒌A 𝟏 𝒌𝑨𝒍𝒄 𝑹𝑪𝑯𝟐 𝑶𝑯 [𝐌𝐞𝑶@ ]𝒑𝑶𝟐 𝜽∗𝒗 𝒌A 𝟐 𝒌𝑯𝒆𝒎𝒊 [𝑹𝑪𝑯𝑶][𝐌𝐞𝑶@ ]𝒑𝑶𝟐 𝜽∗𝒗 = − 𝒌@𝒂𝒍𝒄 𝑴𝒆𝑶𝑯 + 𝒌′𝟏 𝒑𝑶𝟐 𝒌@𝑯𝒆𝒎𝒊 + 𝒌′𝟐 𝒑𝑶𝟐 𝒅𝒕

(6)

𝒅[𝑹𝑪𝑶𝑶𝑴𝒆] 𝒌A 𝟐 𝒌𝑯𝒆𝒎𝒊 [𝑹𝑪𝑯𝑶][𝐌𝐞𝑶@ ]𝒑𝑶𝟐 𝜽∗𝒗 = 𝒌@𝑯𝒆𝒎𝒊 + 𝒌′𝟐 𝒑𝑶𝟐 𝒅𝒕

(7)

The rate expressions were used to fit the experimental data for [substrate], [K2CO3] and pO2 dependence shown in Figure 5, with excellent agreement (Figure S2, Supporting Information). The two-site model produces the best fit; however, the fits to both the one-site and two-site models are not sufficiently different to categorically exclude the one-site model (see Fig. S3, Supporting Information). The rate constants derived from the fits to the data for benzyl alcohol oxidation were then expanded to account for substrate electronic effects by incorporating terms for the Hammett-based s parameters and r values to obtain adjusted rate constants for deprotonation of the alcohol (kalc) and hemiacetal (kHemi) and b-hydride elimination steps (k’1 and k’2) for substituted benzyl alcohols (and intermediate benzaldehydes) (Figure 14).

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A.

B.

Figure 14. Experimental and calculated Hammett plots. Optimized global fit Hammett plots for the oxidation of (A) benzyl alcohol to benzaldehyde and (B) benzaldehyde to methyl benzoate over PdBiTe. Curves from the two site Langmuir-Hinshelwood rate law were fit to the experimental data by minimizing the square of the errors. Experimental data from Figure 6. Reaction conditions from Figure 5.

The fits of the Hammett data provide a clear rationale for the observed electronic trends. The negative slope for the alcohol-to-aldehyde oxidation sequence (Figure 14 A) arises primarily from enhanced rates of b-hydride elimination from electron-rich alkoxides. This enhanced rate arises from increased electron density in the b C-H bond, which facilitates b-hydride elimination. However, for the most electron-rich substrates, steady-state deprotonation and binding of the alcohol to the surface (Scheme 2, step i) is partially rate limiting and accounts for the slight curvature of the plot in Figure 14A. The aldehyde-to-ester oxidation sequence exhibits the opposite electronic trend and more-dramatic non-linearity in the data. For electron-rich aldehydes (i.e., MeO-, Me- and H-substituted derivatives), the rate increases with more facile conversion to the hemiacetal (i.e, positive r value). Electron-deficient aldehydes can exist almost entirely in their hemiacetal form (Figure S6), and eventually the Hammett slope shifts negative, owing to the electronic contribution the b-hydride elimination step. The kinetic model also fits accurately and rationalizes the observed kinetic isotope effects (Table 2). Specifically, electron-rich alcohols exhibit a smaller KIE from independent rate

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measurements for ArCH2OH vs. ArCD2OH. This result may be rationalized by the acceleration of b-hydride elimination for electron-rich substrates, resulting in the alcohol deprotonation step having partial contribution to the rate-determining step.18

Table 2. Comparison of independent rate KIE’s from experimental rates and calculated rates using derived rate law. Substrate 4-MeOC6H4CH2OH 4-CF3C6H4CH2OH

Experimental KIE 2.4±0.6 4.1±1.4

Fitted KIE 2.3 3.8

Bi and Te Promotion Effect Factors. The results obtained herein provide valuable insights into the role of the Bi and Te additives in enhancing the rate of Pd-catalyzed oxidative methyl esterification. For example, the XPS data of Pd/C, PdBi, PdTe, and PdBiTe catalysts show that the promoters help to maintain a more reduced Pd surface, lowering the amount of oxidized Pd (PdO and PdO2) in the catalyst. The amount of reduced Pd increases progressively in the order Pd < PdBi < PdTe < PdBiTe. XAS data further support these observations, as the Pd K-edge energy increases under oxidizing conditions for unpromoted Pd, but does not change significantly for the PdBi, PdTe, and PdBiTe catalysts. These observations are consistent with previous studies showing that the reduced Pd surface is the active form of the catalyst. For example, Besson and coworkers noted that restricting the oxygen supply minimized the deactivation of a PtBi catalyst for 9-decen-1-ol oxidation to the carboxylic acid.19 In-situ X-ray absorption spectroscopic studies by Baiker and coworkers showed that metallic Pd was active for aerobic oxidation of benzyl alcohol while PdO was inactive.20 In a subsequent study of aerobic alcohol oxidation with Bi-promoted Pd and Pt catalysts, in situ X-ray absorption and ATR-IR spectroscopic data indicated that Bi protects Pd and Pt from overoxidation.9k

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Our XPS and XAS data could suggest that the promoters are blocking sites that lead to O2 dissociation and thereby contribute to a mechanism that involves adsorbed molecular O2, as proposed in Scheme 2. But, the data also show that the promoters undergo oxidation in preference to Pd, maintaining the Pd surface in the active metallic state and preventing inhibition by surface Pd-oxide formation. But, the fraction of reduced/oxidized Pd in the different catalysts (cf. Figure 9) does not account for the dramatic difference in activity between the Pd and promoted-Pd catalysts in Figures 3 and 4. These data could suggest that the promoters provide active sites for substrate activation. For example, calorimetric data obtained by Besson and coworkers suggest that O2 preferentially adsorbs on Bi, rather than Pd,9e and Te is even more readily oxidized than Bi.21 The time-course data in Figure 4A further suggest that Bi and Te exhibit a synergistic effect that helps to maintain catalyst activity, possibly by blocking certain sites on Pd to prevent side reactions (e.g. decarbonylation) that poison the catalyst. This synergistic effect is most evident with the less reactive aliphatic alcohol, octanol.

Conclusion In this study we have shown a significant promotion effect on the aerobic methyl esterification of alcohols by Bi and Te addition to a Pd catalyst. Both Bi and/or Te significantly increase the initial rate of the reaction for both benzyl alcohol and 1-octanol oxidation to the methyl ester and, more importantly, the addition of both additives to the catalyst shows significant enhancement in the final yield of methyl octanoate. Kinetic studies identified a Langmuir-Hinshelwood mechanism with an irreversible rate determining step of b-hydride elimination from an adsorbed alkoxide to a molecularly bound oxygen species most likely occupying a second type of adsorption site generated by one of the additives. The change in the

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KIE between electron rich and electron poor substrates is attributed to variation in the adsorption strength of the bound alkoxide. The Bi and Te are proposed to protect the catalyst from overoxidation, keeping the Pd in a reduced state, and thereby increasing the rate of the reaction.

Corresponding Author *E-mail: [email protected], [email protected]

Supporting Information. Experimental description and supplementary experimental data; mechanistic proposals and rate law derivations. These are available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The authors thank Jamie Chen for assistance in obtaining XRD spectra; Joanne Redford, Scott McCann, and Janelle Steves for help with the synthesis and purification of deuterated substrates; Joanne Redford and Scott McCann for help with the gas-uptake and ChemSCAN apparatus; Alison Wendlandt for help with the acquisition of low temperature 1H NMR spectra; and James Gerken for assistance in obtaining XAS data. Financial support for this work was provided by the Dow Chemical Company. Funding for Transmission Electron Microscopy and XPS instrumentation was provided by UW Materials Research Science & Engineering Center and NSF grant DMR-1121288. This research used resources of the Advanced Photon Source, a

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U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Mechanistic Insights into Aerobic Oxidative Methyl Esterification of

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