(129b) Design of Solid Acid Catalysts for Aqueous Phase Dehydration of Carbohydrates: The Role of Lewis and Brønsted Acid Sites

Aqueous phase acid-catalyzed reactions offer a viable route for the production of liquid fuels and fuel precursors from lignocellulosic biomass.  In particular, dehydration of carbohydrates makes up a principal class of these reactions for the production of renewable chemicals, including furfural and 5-Hydroxymethylfurfural.  These furan based chemicals can be used to make a wide variety of chemical feedstocks¹ and intermediates for production of liquid alkanes, including jet and diesel fuel range alkanes, and other furan-based chemicals.^2-5  We have studied the kinetics of the aqueous phase dehydration of xylose in a biphasic reaction using a homogeneous catalyst.⁶  Xylose undergoes a triple dehydration reaction to form furfural.  The furfural can react further with itself to form solid humins, which are highly polymerized insoluble carbonaceous species.  Furfural can also react with xylose to form humin products, which is the primary pathway to humin formation.  Humin formation is a low activation energy process, whereas xylose dehydration is a high activation energy step.  A biphasic system is able to quickly extract the furfural from the aqueous phase and prevent further decomposition reactions.  Thus high yields of furfural (i.e. 85%) are obtained at high temperatures in biphasic systems.

It would be desirable to obtain solid acid catalysts that exhibit activities and selectivities comparable to homogeneous catalysts for aqueous phase dehydration.  In this respect we have prepared a series of well characterized solid acid catalysts and tested them for aqueous phase dehydration.  These catalysts include Zr-P, SiO₂-Al₂O₃, WO_x/ZrO₂, Al₂O₃, and HY zeolite.  We have characterized the concentration of both Brønsted and Lewis acid sites in these catalysts and compared the catalytic activity and selectivity with that of homogeneous catalysts.  This in turn enables the design and optimization of solid acid catalysts for the aqueous phase dehydration of carbohydrates in general and xylose in particular.  The catalyst selectivity is a function of the Brønsted to Lewis acid ratio for both the heterogeneous and homogeneous reactions.  Lewis acid sites decrease furfural selectivity by catalyzing a side reaction between xylose and furfural to form humins.  Catalysts with high Brønsted to Lewis acid ratios, such as Zr-P and ion-exchange polymer resins, have comparable results with HCl.  Using HY zeolite revealed a low furfural selectivity due to strong irreversible adsorption of the furfural in the pores, causing an increase in the rate of humin formation.  Thus, to design more efficient aqueous phase dehydration catalysts, it is desirable to have a high ratio of Brønsted to Lewis acid sites.  Furthermore, we have shown that gas phase characterization of acid sites can be used to predict catalytic activity in the aqueous phase.

 References

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5.         R. Xing, A. V. Subrahmanyam, H. Olcay, W. Qi, G. P. van Walsum, H. Pendse and G. W. Huber, Green Chemistry, 2010, 12, 1933-1946.

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