Aqueous-phase hydrodeoxygenation of carboxylic acids to alcohols or alkanes over supported Ru catalysts

Abstract

For the aqueous-phase hydrodeoxygenation (APHDO) of carboxylic acids over the Ru/C, Ru/ZrO₂ and Ru/Al₂O₃ catalysts, the CO hydrogenation and C–C bond cleavage reactions were studied by collecting reaction kinetics data and the measures of DRIFTS. The C–C bond cleavage was improved at high temperature and with high metal loadings. The acidic supports in combination with Ru metal can favor the CO hydrogenation of carboxyl. The C–C bond cleavage derived from the decarbonylation of acyl on the catalyst was studied by the measures of DRIFTS. The APHDO and DRIFTS results demonstrated that the C–C bond cleavage was favored in the order of Ru/C > Ru/ZrO₂ > Ru/Al₂O₃. The catalysts were characterized by multiple methods (H₂-TPR, NH₃-TPD, CO-FTIR and DRIFTS of propanoic acid). It is concluded that the effect of support on the reaction routes may be attributed to these factors of catalysts, i.e., surface acidity, metal–support interaction and electronic state of Ru species.

Introduction

Biomass is a promising candidate to serve as a sustainable source of organic carbon for the production of industrial chemicals [1]. The available strategies for transforming biomass to bio-fuels and/or chemicals have been attracted more interestingly. In future, the range of block chemicals from biorefineries is extensive and is expected to become greater with further research [2], [3], [4], [5]. Generally, these block chemicals, i.e., the so-called platform molecules, are defined as chemicals containing multiple functional groups (e.g., acids, ketones, alcohols, amines, etc.). It is imperative to develop more efficient processes for catalytic upgrading of these block chemicals. However, lots of potential feedstocks are produced along with water from biomass by fermentation and/or hydrolysis routes [4]. However, removal of water from these compositions would be time-consuming and costly, and water has many advantages over more conventional solvents that present problems with toxicity and difficulties with handling and disposal. Considering these factors, the aqueous-phase hydrodeoxygenation (APHDO) reactions [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] are a crucial component of a number of strategies for the conversion of biomass-derived feedstocks into fuels and chemicals, e.g., hydrogenation of targeted functionalities of biomass including acids, sugars, aldehydes and furans. Many groups have extensively worked on these (e.g., Dumesic [8], [9], [10], [11], [15], [20], [22], Miller and Jackson groups [6], [7], [12], [13], [23], Huber [17], [20], [22], [24], [25], [26], [27], [28], Corma [29], Clark and Luque groups [30], [31]), including alcohol production from organic acids [6], [12], [17], [23], gasoline production from bio-oils [28], and alkane production from carbohydrates [8], [11], [20], [22].

The APHDO of carboxylic acids to alcohols has been investigated as a possible pathway for production of novel, high-valued products from bio-derived feedstock [23]. The direct APHDO of organic acids offers an alternative that is atom economical and amenable to continuous processing, and that obviates the need for intermediate esterification, use of organic solvents, and byproduct waste streams [23]. Recently, Miller and Jackson groups have studied the APHDO of propanoic acid and lactic acid over the Ru/C catalyst [6], [12], [13]. They demonstrated that production of alcohol was improved at lower temperature and higher hydrogen pressures, and the methane, ethane and propane were detected as the main by-products. In addition, many degraded alkane byproducts were produced in the hydrogenolysis of glycerol to propylene glycol over the Ru-base catalysts [32], [33], [34], [35]. In these reactions, the cleavage of C–C bonds and the formation of methane were proposed to occur primarily through a metal-catalyzed reaction on Ru [33]. Thus, selectively removing oxygen from the biomass-derived compounds is one of the key challenges in converting renewable biomass resources into fuels and chemicals. The oxygen is removed from the biomass with the APHDO by a combination of the CO hydrogenation, the C–O bond cleavage and the C–C bond cleavage reactions. If the larger alkanes or upgraded oxygenates are desired, then the C–C bond cleavage should be inhibited and the CO hydrogenation and the C–O bond cleavage should be strengthened. If the degraded alkanes are desired, then the C–C bond cleavage should be enhanced. Therefore, it is necessary to understand the CO hydrogenation and C–C bond cleavage reactions over the supported Ru catalysts, since the ruthenium is a promising catalyst for the conversion of biomass feedstock to various chemicals [6], [12], [13], [31], [32], [33], [34], [35], [36], [37].

In this work, the APHDO of carboxylic acids (C₂–C₄), especially propanoic acid served as a probe molecule, was studied to understand the catalytic performance of several supported Ru catalysts (Ru/C, Ru/ZrO₂ and Ru/Al₂O₃). The goal was to identify key reaction intermediate and to reveal the reaction mechanisms of the CO hydrogenation and C–C bond cleavage on supported Ru catalysts. The different reaction routes (CO hydrogenation of carboxyl and C–C bond cleavage) were investigated in detail. In particular, the DRIFTS of propanoic acid were performed to understand the mechanism of the C–C bond cleavage. The effect of support on the hydrodeoxygenation routes was discussed based on the characterizations of catalysts (H₂-TPR, NH₃-TPD, CO-FTIR and DRIFTS of propanoic acid). In the present contribution, the APHDO reaction can be tuned to make a targeted product (alkanes or upgraded oxygenates) from biomass-derived chemicals by the improvement in the catalyst design.