Isomerization of n-hexane over Pt-H3PW12O40/SBA-15 bifunctional catalysts: Effect of the preparation method on catalytic performance

摘要

Pt-H3PW12O40 based bifunctional catalysts immobilized on SBA-15 were prepared either by mechanically mixing of acidic (HPW/SBA-15 or HPW@SBA-15) and metallic (Pt/SBA-15) monofunctional materials or by incorporation of the two active phases within the same composite using dual encapsulation (Pt@HPW@SBA-15) and encapsulation/impregnation (Pt/HPW@SBA-15) methodologies. The phase structure, chemical composition and surface physico-chemical properties were characterized in details by XRD, FT-IR, P-31 MAS NMR, N-2 adsorption-desorption and HRTEM, STEM and EDX microscopy techniques. All hybrid materials showed highly ordered mesostructuration of the SBA-15 matrix with homogeneous dispersion of both metallic and HPW phases. In the case of the monophase bifunctional catalysts, Pt/HPW@SBA-15 and Pt@HPW@SBA-15, both functions were found to coexist in close vicinity, with the HPW crystallites mainly located in the silica walls whereas Pt was found either in the channels or as part of the walls depending on the preparation route. Their catalytic performance was then evaluated in the isomerization of n-hexane. The catalysts prepared by mechanical mixing exhibited higher activity than those where the Pt and HPW were integrated within a single solid. These results were interpreted by assuming that the carbocations in the latter case were hydrogenated faster on the Pt particles thus decreasing their preponderance at the steady-state compared to the mechanical mixture. (C) 2016 Elsevier B.V. All rights reserved.