Elsevier

Surface Science

Volume 606, Issues 15–16, August 2012, Pages 1129-1134
Surface Science

Origin of the selectivity in the gold-mediated oxidation of benzyl alcohol

https://doi.org/10.1016/j.susc.2012.03.013Get rights and content

Abstract

Benzyl alcohol has received substantial attention as a probe molecule to test the selectivity and efficiency of novel metallic gold catalysts. Herein, the mechanisms of benzyl alcohol oxidation on a gold surface covered with atomic oxygen are elucidated; the results show direct correspondence to the reaction on gold-based catalysts. The selective, partial oxidation of benzyl alcohol to benzaldehyde is achieved with low oxygen surface concentrations and takes place through dehydrogenation of the alcohol to form benzaldehyde via a benzyloxy (C6H5–CH2O) intermediate. While in this case atomic oxygen plays solely a dehydrogenating role, at higher concentrations it leads to the formation of intermediates from benzaldehyde, producing benzoic acid and CO2. Facile ester (benzyl benzoate) formation also occurs at low oxygen concentrations, which indicates that benzoic acid is not a precursor of further oxidation of the ester; instead, the ester is produced by the coupling of adsorbed benzyloxy and benzaldehyde. Key to the high selectivity seen at low oxygen concentrations is the fact that the production of the aldehyde (and esters) is kinetically favored over the production of benzoic acid.

Highlights

► The mechanism of the oxidation of benzyl alcohol on metallic gold is revealed. ► Selectivity for aldehyde and ester is high at low surface oxygen coverage. ► Aldehyde/ester formation is kinetically more facile than formation of benzoic acid.

Introduction

Gold-based heterogeneous catalysis has been an intense area of research in the recent years [1], [2], [3], [4], [5]. The propitious effort in this area is related to the search for environmentally friendly synthetic processes, where organic solvents are not necessary, innocuous oxidants (e.g. oxygen, air, hydrogen peroxide) are employed, and renewable energy sources can be used (e.g. through photocatalysis). Remarkably, two desired characteristics in modern catalysis, cleaner reactions and less expensive processes, are combined exceptionally for gold-based catalysis.

Although the underlying principles behind the oxygen-assisted activity of gold are emerging [1], [5], [6], the complexity of the supported catalyst still presents challenges for complete understanding. In particular, the details regarding the activation of molecular oxygen (O2) remain unclear for both supported and unsupported gold catalysts. Recent experiments with nanoporous (unsupported) gold show high activity and selectivity for alcohol esterification and suggest that the catalytic activity, in particular the activation of molecular oxygen, may be closely linked to under-coordinated gold surface atoms [7], [8]. The reactions observed were entirely predicted from our previous studies under controlled conditions (using ultra high vacuum methods) [9] and parallel the results obtained in the liquid phase at practical catalytic conditions as well [10].

We have elucidated the mechanism of several gold-mediated oxidation reactions, using gold surfaces pre-covered with atomic oxygen under ultra-high vacuum (UHV) conditions [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The generation of atomic oxygen on the surface provides active oxidant species which activate many substrates containing acidic hydrogen. Under these conditions the atomic oxygen also induces the formation of gold nanoparticles on the originally ordered Au(111) surface [15], [22], [23], [24], [25]. The use of UHV conditions allows for a controlled environment in which the concentration of reactants and the nature of the surface can be monitored with high precision. As noted above these studies correlate well with the results of other investigations conducted under more practical working conditions where direct comparisons are possible [7], [8], [9], [10], [26], [27].

The oxidation of benzyl alcohol is a test reaction for probing the properties of novel gold-based catalysts [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70]. The work reported here focuses on the mechanism for this reaction. Although the use of benzyl alcohol as a probe is partially motivated by the importance of its oxidation products (benzaldehyde, benzoic acid and benzyl esters) in the fragrance and food industries, it possesses unique physical and chemical properties attractive for practical studies. Since this alcohol does not form allotropes with water (as it is the case for most aliphatic alcohols), its purity is advantageous for studying solvent-free catalysis [29], [30], [31], [32], [33], [34], [35], [36], [44], [45], [46], [47], [48], [49], [58], [59]. Further, due to its relatively high boiling point, the reaction can be studied over a broad temperature span without phase changes. Chemically, a consistent pattern of reactivity over a wide variety of gold-based catalysts is observed, characterized by an extremely selective conversion to benzaldehyde (~ 90%) [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [44], [45], [46], [54], [55], [56], [57], [58], [59], [60], [61], [62]. Limited amounts of benzyl benzoate are usually detected, and benzoic acid is often undetected. This fact has been explained by assuming that the acid readily undergoes a condensation reaction with the alcohol, forming the ester and eliminating water, even in solvent-free conditions [30], [31], [32], [33], [45], [46], [47], [59]. While condensation can be expected in solution, the present work demonstrates that in a solvent environment the production of ester and acid take place through different mechanisms, with the latter being kinetically more demanding.

Section snippets

Experimental section

Experiments were performed in a chamber with a base pressure below 2 × 10 10 Torr, equipped with a mass spectrometer (Hiden HAL 3F) and an electron-energy loss spectrometer (LK 2000). In addition, the chamber is outfitted with an Auger electron spectrometer (PHI 15–155) and low-energy electron diffraction optics (PHI 15–120), which are employed to determine the cleanliness and morphology of the Au(111) surface.

The gold surface was routinely cleaned by cycles of ozone (O3) exposure at 200 K followed

The interaction of benzyl alcohol with clean Au(111)

Benzyl alcohol is relatively strongly bound to the gold surface, exhibiting binding energies of 90 and 105 kJ/mol, as deduced from a Redhead analysis of the desorption temperatures of a saturated layer (310 and 360 K, Fig. 1). Strong interactions between aromatic compounds and metal surfaces are common and are usually associated with adsorption geometries where the aromatic ring lies nearly parallel to the surface [76], [77], [78], [79], [80], [81], [82]. In the case of benzyl alcohol, analysis

Conclusions

The remarkable selectivity achieved during the gold-mediated oxidation of benzyl alcohol is associated with conditions in which benzyl alcohol is in excess with respect to surface atomic oxygen. The kinetic barrier for aldehyde formation is found to be ~ 20 kJ/mol lower than that for benzoate formation (which ultimately leads to benzoic acid and combustion gases). This difference is sufficiently large to ensure a selective oxidation to benzaldehyde.

Acknowledgments

R.J.M. and J.C.F.R.-R. gratefully acknowledge the support of the National Science Foundation for this research (NSF CHE 0952790). C.M.F. gratefully acknowledges support of the U.S. Department of Energy, Basic Energy Sciences, grant no. FG02-84-ER13289. Dr. B. Xu is acknowledged for valuable discussions.

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