Structure of Sulfated Metal Oxides and Its Correlation with Catalytic Activity

Abstract

Sulfated metal oxides (SMO's) were prepared by thermal decomposition of metal sulfate salts at various temperatures in an attempt to relate their physicochemical properties with the decomposition temperature and the resultant catalytic activity. Work centered on SMO's of Ti, Zr, Fe, Sn, and Al, which have been claimed in the literature to possess superacidity. In all cases, the BET surface area (SA) passed through a maximum as a function of sulfate decomposition temperature. Up to that maximum, a linear correlation was found in the case of Ti and Zr between SA and catalytic activity derived from literature cumene cracking data. Above the maximum, a close interrelation was revealed between SA, the average crystallite size (Dav), and the sulfate level (e.g., wt % SO4) by applying a simple, straightforward geometric model. From the specific surface area (SSA), i.e., surface area occupied by a single sulfate species, two saturated surface states of the sulfate have been found: a dilute state corresponding to an SSA of about 0.50 nm2, in the case of Ti, Zr, and Al, and a dense state (“close-packed”) of about 0.14 nm2, in the case of Fe and Sn. The results are discussed in light of the known literature and believed to indicate that SMO's have well-organized surface structures and distinct sulfate centers that may act as strong acid catalytic sites. Specifically, sulfated zirconia was found (by elemental analysis and XPS) to have a surface metal-to-sulfur ratio of 7−8, and the sulfate groups can be modeled in terms of (O−)3SO tripods, whether single or chained (e.g., surface “polysulfates”), as previously proposed based on IR and Raman studies.