Kinetic studies of vapor-phase hydrogenolysis of butyl butyrate to butanol over Cu/ZnO/Al2O3 catalyst

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Abstract

Kinetic investigations on the hydrogenolysis of butyl butyrate to butanol over a commercial Cu/ZnO/Al2O3 catalyst were conducted. The catalytic measurements at atmospheric pressure showed that the rate of hydrogenolysis was approximately 0.67 order with respect to butyl butyrate and had a positive effect for hydrogen. The activation energy of this reaction was measured to be about 62 kJ/mol, and D2 isotope studies corroborated that the hydrogenolysis of butyl butyrate proceeds via dissociative adsorption of ester producing C3H7CO and C4H9O fragments. In addition, kinetic studies, interpreted by applying the basis of the Langmuir–Hinshelwood model, suggested that the rate-determining step involves the dissociative adsorption of butyl butyrate. Finally, the rate expression derived in this study provided precise and reasonable fitting results, and an activation energy close to the value obtained from the power law equation.

Research highlights

▶ The rate of hydrogenolysis of butyl butyrate was more dependent on its concentration. ▶ C4H9O and C3H7CO surface fragments were produced via dissociative adsorption. ▶ The rate-determining step involves the dissociative adsorption of butyl butyrate.

Introduction

Since the first description of the catalytic hydrogenolysis of esters (R1COOR2) to two alcohols (R1CH2OH and R2OH) by Flokers and Adkins in 1931 [1], this reaction has been of considerable industrial importance. In particular, many researchers including Trimm and co-workers in the 1980s thoroughly investigated the hydrogenolysis reaction mechanisms of methyl formate and ethyl acetate over copper-based catalysts [2], [3], [4], [5], [6]. Since then, little attention has been paid to this reaction owing to the relatively well-established reaction scheme.

However, the ester hydrogenolysis has attracted much industrial interest in recent years, because ester, which is a raw material for bio-alcohols such as bioethanol and biobutanol, can be produced from organic acids available in fermentation broths. This process is named “indirect fermentation”. Eggeman and Verser in Zeachem Inc. [7], [8] and Holtzapple at Texas A&M University [9] proposed that the energy ratio, defined as the ratio of renewable energy produced divided by the fossil fuel energy input, would be higher in this process than that in the conventional ethanol fermentation process. From this point of view, the process scheme for the biobutanol production can be developed such that 1 mol of butyric acid is esterified with 1 mol of butanol to form butyl butyrate which is subsequently hydrogenated to 2 mol of butanol, where one part of butanol may be recycled for the esterification of butyrate while the other part of butyric acid will come out through the product stream.

The overall reaction for the catalytic hydrogenolysis of butyl butyrate (BB) is as follows:C3H7COOC4H9+2H22C4H9OHand the reaction was carried out in the vapor phase over a commercial Cu/ZnO/Al2O3 catalyst. Although numerous catalytic studies on the hydrogenolysis reaction of methyl formate or ethyl acetate were conducted previously [2], [3], [4], [5], [6], [10], [11], [12], no research work has been reported regarding BB hydrogenolysis. Thus, this study addressed the kinetic mechanism of the vapor-phase hydrogenolysis of butyl butyrate to butanol. In order to investigate if elementary steps include a dissociative adsorption of butyl butyrate, we utilized D2 instead of H2 for the reaction, and we characterized the resulting D-labeled compounds by 1H nuclear magnetic resonance (NMR) spectroscopy and gas chromatography–mass spectrometry. In addition, a kinetics model was developed, on the basis of the mechanism proposed by the experimental study, to determine the rate-determining step.

Section snippets

Reaction kinetic study

Commercial CuO/ZnO/Al2O3 catalyst designated as ICI 83-3M was employed in this study; the copper surface area measured by N2O chemisorption is 9.85 m2/g. Its BET surface area is 78 ± 2 m2/g with a type II isotherm, and its t-plot showed a straight line with an intercept of almost zero (micropore area = 4.5 m2/g), which is indicative of a nonporous material. Thus, the effect of internal mass transfer was negligible. Moreover, the catalyst pellet was crushed into the size of less than 90 μm since the BB

Kinetic study

Since the main reaction presented in Eq. (1) is unfavorable at high temperatures due to its exothermic characteristics (ΔHrxn,298.15 K = −24.9 kJ/mol), equilibrium BB conversions (Xe) were calculated under various H2/BB ratios and reaction temperatures in which no side reactions occur. For the calculation of thermodynamic values for given conditions specified in this study, the constants of heat capacity (Cp) and ΔGf° for butanol and butyl butyrate were taken from the database available in the

Conclusions

The kinetic study for the vapor-phase hydrogenolysis of butyl butyrate to butanol over a commercial Cu/ZnO/Al2O3 catalyst was carried out. The catalytic experiment was conducted far from equilibrium, and several experimental conditions, such as the partial pressures of two reactants, BB and H2, and the reaction temperature, were varied. In the empirical power law equation, the rate of hydrogenolysis had a positive effect with respect to both BB and hydrogen, and was dependent more on the BB

Acknowledgment

The authors would like to acknowledge funding from Korea Ministry of Environment as “Converging Technology Project (202–101–006)”.

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