Aqueous-phase hydrodeoxygenation of sorbitol: A comparative study of Pt/Zr phosphate and Pt-ReOx/C

In this paper, we have studied the aqueous-phase hydrodeoxygenation of sorbitol over two fundamentally different types of bifunctional catalysts: Pt/Zr-P and Pt-ReOx/C in a continuous flow reactor. During the reaction, the Pt/Zr-P catalyst undergoes phase transformations of amorphous into crystalline Zr-P having a rhombohedral framework along with a 97% loss of surface area, 86% loss of Pt surface sites, and 95% loss of surface acid sites. Pt-ReOx/C has a higher hydrothermal stability than that of Pt/Zr-P and only lost 17% of its surface area during the reaction. The number of Pt surface sites increased by a factor of 3.5 for the Pt-ReOx/C catalyst after reaction. Products were classified into three major categories: (1) light gases, (2) gasoline-range products, and (3) aqueous-phase products. The light gases included CO2 and C1-C4 alkanes. The gasoline-range products included C5-C6 alkanes, C2-C6 alcohols, tetrahydrofurans, tetrahydropyrans, and small amounts of C2-C6 aldehydes, ketones, and organic acids. The aqueous-phase products included isosorbide, mannitol, sorbitan, hexanetriol, C2-C6 diols, glycerol, and methanol. The gasoline-range yield of each catalyst was 66.8% and 44.4% for the Pt/Zr-P and Pt-ReOx/C catalysts, respectively. The Pt-ReOx/C catalyst had a 34 times higher reaction rates than Pt/Zr-P on a Pt mass basis. However, the turnover frequency of both catalysts was similar based on the number of surface Pt sites measured after reaction. The acid sites on the Pt-ReOx catalyst are able to isomerize sorbitol into mannitol, whereas no mannitol was observed with the Pt/Zr-P catalyst. Mannitol had a lower rate of coke formation than sorbitol for APHDO on the Pt/Zr-P catalyst. The Pt-ReOx/C had a higher selectivity toward secondary alcohols than Pt/Zr-P which produced all primary alcohols. The Pt-ReOx/C had a higher CO2 selectivity than Pt/Zr-P, suggesting the Pt-ReOx/C has a higher rate of decarbonylation than Pt/Zr-P. Pt/Zr-P had a higher rate of C-O bond cleavage than Pt-ReOx/C, leading to higher C6 product selectivity (35.8% vs. 11.6%). (C) 2013 Elsevier Inc. All rights reserved.