Study of the Keggin structure and catalytic properties of Pt-promoted heteropoly compound/Al-MCM-41 hybrid catalysts

Aluminum-containing mesoporous molecular sieves (referred as WSAn, where n = Si/Al molar ratio = 50, 30 and 10) were synthesized via a surfactant templated approach by using fumed silica and aluminum sulfate as Si and Al precursors, respectively. When the 12-tungstophosphoric acid was grafted onto the surface of WSAn, a high dispersion of the heteropolyacid was achieved on the heteropoly compound/WSAn hybrid catalysts. The Keggin structure of the dispersed 12-tungstophosphoric acid was primarily preserved without destruction, but it was distorted in some degree, as confirmed by FTIR, ³¹P NMR-MAS and UV-vis spectroscopic characterizations. The surface Brönsted acidity of the catalysts was greatly enhanced by several times in comparison to that of the bare WSAn support. In the hydroisomerization reaction of n-heptane, the Pt/H₃PW₁₂O₄₀/WSA30 catalyst exhibited the highest catalytic activity and the best isomerization selectivity among the catalysts tested, which can be generally correlated with its large number of Brönsted acid sites and high structural regularity. After the n-heptane hydroisomerization reaction, a high molar ratio of multibranched to monobranched isohexanes was obtained, indicating that Pt-promoted heteropoly compound/WSAn hybrid catalysts have a great potential for the hydroisomerization of long carbon chain hydrocarbons.