Mechanistic Studies of Alcohol Selective Oxidation

Abstract

IntroductionCatalytic selective oxidation (selox) is an important class of clean chemical transformations employed in the synthesis of valuable chemical intermediates, and a test bed for many fundamental concepts within heterogeneous catalysis and surface science. The selox of alcohols, carbohydrates, and aromatics is especially challenging in terms of understanding the dynamics of chemical reactions at the liquid-solid-gas interface, and requires new spectroscopic tools and analytical protocols to provide quantitative spatiotemporal information on structure-function relationships in order to optimize reaction conditions and design next-generation selox catalysts. Advances in inorganic methodologies to synthesize tunable nanostructures, and synchrotron science and the parallel development of multidimensional spectroscopies, afford new possibilities for understanding the operation of catalysts under working conditions (operando), and thereby nanoengineering the active site for improved activity, selective, and lifetime in selox chemistry. Applications of Selective OxidationThe oxidative dehydrogenation of alcohols represents key steps in the synthesis of aldehyde, ketone, ester, and acid intermediates employed within the fine chemical, pharmaceutical, and agrochemical sectors, with allylic aldehydes in particular high-value components used in the perfume and flavoring industries [1]. For example, crotonaldehyde is an important agrochemical and a valuable precursor for the food preservative sorbic acid, while citronellyl acetate and cinnamaldehyde confer rose/fruity and cinnamon flavors and aromas, respectively. There is also considerable interest in the exploitation of biomass-derived feedstocks such as glycerol (a by-product of biodiesel synthesis from plant or Heterogeneous Catalysts for Clean Technology: Spectroscopy, Design, and Monitoring, First Edition. Edited

Dynamic Behavior of Pd/P4VP Catalyst during the Aerobic Oxidation of 2-Propanol: A Simultaneous SAXS/XAS/MS Operando Study

“…Oxidation of the catalyst was achieved by flowing in the reactor a 15% O 2 in He flow (20 mL/min of total flow) at 50 °C and successively increasing the temperature to 200 °C (ramp 2 °C/ min). The aerobic oxidation of 2-propanol was conducted on the catalysts (1) prereduced in 2-propanol, (2) 48,52,53 The catalyst powder was placed in a 2 mm glass capillary, having upstream and downstream two small pieces of quartz wool. The capillary was connected to the BM26A gas rig with massflow controllers for gas delivery, 54 and heated with a heat gun.…”

“…1−6 Albeit significant progress has been achieved in understanding the role of Pdbased catalysts by using in situ or operando spectroscopic 10−12 and microscopic 13,14 tools, the mechanism of the reaction is still a matter of discussion. 15−17 It has long been accepted that the reaction proceeds following an oxidase-style mechanism consisting of two steps: (1) a Pd-mediated oxidation of the alcohol by dehydrogenation, with the formation of the corresponding aldehyde and of a Pd-hydride intermediate; and (2) the aerobic oxidation of the reduced catalyst. The two stages occur independently and in sequence.…”

“…The two stages occur independently and in sequence. [1][2][3][4][5][6]18,19 According to this mechanism, molecular oxygen is not directly involved in the substrate oxidation, but has the double role of reoxidizing the Pd-hydrides (supported by the observation that O 2 can be replaced by a hydrogen acceptor), 20,21 and suppressing the decarbonylation of the oxidation products over metallic Pd. 1−5 Competing reactions may occur at specific surface sites 19 during the catalytic cycles (such as the decarbonylation or the overoxidation of the products, the overoxidation of the catalyst, and the aggregation of Pd species to inactive bulk metal), with a consequent decrease in selectivity and deactivation of the catalyst.…”

“…Selective aerobic oxidation of alcohols to their corresponding aldehydes over noble-metal-based catalysts is an environmentally benign process in fine chemistry, but also a reaction particularly demanding, because it requires the activation of molecular oxygen and C–O bonds in close proximity, at temperatures typically below 160 °C. − The economic and environmental advantages of molecular oxygen as a chemical oxidant are readily apparent: oxygen is abundant, inexpensive, and thermodynamically potent. However, effective solutions to this problem must overcome the intrinsic reactivity and selectivity challenges posed by the chemistry of O 2 : O 2 is a four-electron oxidant when it is reduced to water but most desired reactions are 2-electron oxidations, and partially reduced oxygen species are typically more reactive and potent oxidants than O 2 itself. − Both heterogeneous and homogeneous Pd-based catalysts are largely employed in selective alcohol oxidations, on account of the Pd ability to perform selective oxidations at temperatures typically between 60 and 160 °C and atmospheric oxygen pressure. − Albeit significant progress has been achieved in understanding the role of Pd-based catalysts by using in situ or operando spectroscopic − and microscopic , tools, the mechanism of the reaction is still a matter of discussion. − …”

Bimetallic PdCu Nanoparticle Catalyst Supported on Hydrotalcite for Selective Aerobic Oxidation of Benzyl Alcohol

“…Especially in this area, precious metals (i.e. Au, Pd, Pt, Ru etc) based catalysts have been studied widely for the oxidation reaction [5], however, utilization of the precious metal needs to be minimized in order to replace these precious catalysts for the classical process in industrial plant scales.…”