Effect of tin on Ru-B/γ-Al2O3 catalyst for the hydrogenation of ethyl lactate to 1,2-propanediol
Introduction
Biomass-based organic acids and esters are important feedstocks for chemical production because they have multiple reactive functionalities and have become increasingly available at low cost [1], [2]. Lactic acid can be produced by fermentation of a number of renewable sources such as carbohydrates derived from agricultural crops and biomass streams. Hydrogenation of lactic acid or lactates to 1,2-propanediol (PDO) provides a green process [3], [4]. 1,2-PDO, a material that has been widely used as a solvent or reagent in pharmaceutical and chemical industries, is produced commercially by the hydration of propylene oxide produced from selective oxidation of propylene. This process involves either hydroperoxidation chemistry or the antiquated chlorhydrin process, whereas direct hydrogenation of lactic acid or lactates to 1,2-PDO provides an environmentally benign alternative to the petroleum-based process.
Hydrogenations of carboxylic acids and esters to the corresponding alcohols are often carried out under vigorous reaction conditions due to weak polarisability and intrinsic steric hindrance of the CO bond of esters [5]. However, for lactates or lactic acid that contain a reactive hydroxyl group, high reaction temperature is undesirable because it would lead to side reactions and consequently to a decrease in selectivity to 1,2-PDO [6], [7]. Therefore, development of active catalysts for the hydrogenation of esters to the corresponding alcohols under mild conditions is of great importance.
Good intrinsic hydrogenation activity for carbonyl compounds makes ruthenium very attractive. Thus, Ru-based catalysts with different supports or promoters or different preparation methods were studied by a number of researchers for the hydrogenation of a wide range of carboxylic acids and esters under different conditions [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Ru-based catalysts showed good reaction performance in hydrogenation of carboxylic acids and esters. Although the nature of active sites and the reaction mechanism are still being debated [16], [17], [19], it is generally accepted that the preparation method and the reduction mode of ruthenium precursor greatly influence the catalytic behavior [14], [20], [21], and that the presence of tin [16], [17] and boron [22], [23] is favorable for the activation of carbonyl groups.
The previous work of our research group [24] showed that the impregnation sequence of reductant and metal salt dramatically influenced the physicochemical properties of the Ni-B/AC catalysts. A catalyst prepared by reductant impregnation had much higher activity than that prepared by metal salt impregnation in 4,4′-dinitrodiphenyl ether hydrogenation to 4,4′-diaminodiphenyl ether. In the present work, γ-Al2O3-supported Ru-B catalyst (denoted as Ru-B/γ-Al2O3) for hydrogenation of ethyl lactate was prepared by using reductant impregnation. The effects of tin on chemical composition, on physicochemical properties, on thermal stability as well as on the catalytic performance of Ru-B/γ-Al2O3 catalyst were studied.
Section snippets
Catalyst preparation
The catalysts were prepared by the following reductant impregnation method. A weighed amount of γ-Al2O3 was immersed into a 3.0 M potassium borohydride solution at 298 K for 15 min. The excessive solution was decanted and a mixed solution of RuCl3 and SnCl2 was poured into the flask containing the impregnated γ-Al2O3 to start the reduction. The KBH4 to (Ru + Sn) molar ratio is 6:1. The mixture was kept undisturbed at 298 K until bubble generation ceased. The resulting black solids were washed with
Effect of tin on the chemical composition and morphology of the Ru-B/γ-Al2O3 catalyst
The chemical compositions and physical properties of the Ru-Sn-B/γ-Al2O3 catalysts containing different amounts of tin are listed in Table 1. It shows that the atomic ratio of Sn/Ru in the final catalysts is lower than that in the precursor solutions, and the atomic ratio of Ru/B decreases with increasing the amount of tin. This seems to imply that the deposition efficiencies of tin and ruthenium on the reductant-impregnated γ-Al2O3 are different, and that the existence of tin influences the
Conclusions
Addition of tin to the Ru-B/γ-Al2O3 catalyst prepared by the reductant impregnation method significantly improves the dispersion of Ru-B particles on the support and the thermal stability of the amorphous Ru-B structure. The B/Ru ratio of the catalyst, the electron density of Ru and thus the strength of hydrogen adsorption on ruthenium increase with increasing tin content. The capacity of H2 adsorption shows a maximum at Sn/Ru atomic ratio of 12%. The reaction investigation shows that Ru-B/γ-Al2
Acknowledgments
This work is supported by the Major State Basic Research Development Program (G2000048009, 2003CB615807), the NSF of China (20203004) and Shanghai Science and Technology Committee (02ZA14006, 03QB14004).
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