Superacidity and Catalytic Activity of Sulfated Zirconia

Abstract

The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm -2 , and the adsorption heat of ammonia was ca. 200 kJ mol -1 . This corresponds to -19 of the H 0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm -2 ), an adsorption heat of ca. 160 kJ mol -1 , an H 0 of -12, and Brønsted nature. The former was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butane in gas phase.

Adsorption–desorption equilibrium investigations of n-butane on nanocrystalline sulfated zirconia thin films

“…XPS measurements show that the Zr:S atomic ratio (based on the Zr 3d and S 2p peak areas and reported atomic sensitivity factors [40]) of the activated sulfated zirconia thin film is 5.4. Assuming a homogeneous distribution of sulfur and zirconium within the thin film over the XPS measurement depth, a sulfate surface area of 31 Ų [71] and a zirconia surface 2 x 2 unit cell of 6.425 x 7.284 Å, [8] the sulfate groups are shown to cover ~33% of the surface, which is equivalent to a surface site density of ~1.1x10 18 S atoms/m². Alternatively assuming the sulfate groups are situated on the surface of zirconia crystals, a layer model that accounts for attenuation of the zirconium signal can be used to estimate the sulfate surface density.…”

Styrene and styrene dimer derivatives, cyclohexene, tert-butylbenzene and cumene as test reactions for acid strength measurements of crystalline and amorphous silica-aluminas, sulfated oxides and Amberlyst

“…Katada et al [13] proposed to treat ammonia as a Hammett indicator and they developed a formula relating H O with the heat of ammonia adsorption (DH O ):…”

“…Lavalley et al [15] [17] measured the acid strengths of HZSM-5 zeolites. A similar method based on 13 CNMR measurements of mesitil oxide molecule interaction with acids allowed Haw et al [18] to compare the acid strength of zeolites with that of mineral acids and superacids. The Hall correlation [10] has been also used by Fraenkel et al [19] to link acid strength of acid catalysts with the results of isobutane isomerization at strictly defined condition.…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…Surface sulfate species probably act as a barrier to crystal growth by preventing the olation and oxolation reactions by which small particles can connect during calcination of the hydrous precursor. Estimates of the area occupied by a sulfate group vary between 0.25 nm 2 [64] and 0.5 nm 2 [65]. Assuming each sulfate group requires an area of 0.5 nm 2 [65], 5.7 wt% SO 4 2-would be necessary to form a monolayer on a sample with a surface area of 135 m 2 g -1 .…”