Structural and Active Site Characterization of Sulfated Zirconia Catalysts for Light Alkane Isomerization

Abstract

Sulfated zirconia (SZ) is active for light alkane isomerization at temperatures as low as 373 K [1]. The material has been investigated extensively in the past 2 decades [2] but so far no convincing structure-activity relationship has been established. Here, we report on the investigation of two different SZ materials with an interesting combination of properties. Both materials have a sulfate content of 9 wt.%; however, the material with lower specific surface area (SZ-1, 90 m2og-1) displays a maximum n-butane isomerization rate (373-423 K, 1-5 kPa n-butane at 101.3 kPa total pressure) that is about one order of magnitude higher than that of the material with higher specific surface area (SZ-2, 140 m2og-1). Both materials were produced through precipitation from zirconyl nitrate solution, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. Scanning electron microscopy showed typical particle sizes of 5 to 20 µm for SZ-1, and of 1 to 5 µm for SZ-2. X-ray diffraction and Zr K-edge X-ray absorption spectra identified both materials as predominantly tetragonal ZrO2, but SZ-2 exhibited smaller crystalline domains than SZ-1 (7.5 vs. 10 nm). Diffuse reflectance IR spectra taken during catalyst activation (523 K, inert gas) suggest that the sulfate structures on the two materials rearrange in a slightly different way during dehydration. This is tentatively attributed to different sulfate group densities that result from the ratios of sulfate content to surface area. By ammonia adsorption/desorption, the concentration of acid sites was determined to be 0.52 and 0.48 mmolog-1 for SZ-1 and SZ-2, respectively; this result is not reflected by the catalytic activities. Temporal analysis of products measurements indicated that the residence times for n-butane were shorter than for i-butane, and SZ-1 retained both these molecules longer than SZ-2. The activation energy for n-butane desorption was equivalent for both samples, i.e., 40-41 kJomol-1. Calorimetric measurements of the adsorption of reactant and product at 313 K produced the following results. At 0.3 kPa alkane pressure, SZ-1 and SZ-2 adsorbed similar amounts of n-butane (20 and 25 µmol), but very different amounts of i-butane (80 and 25 µmol). At coverages below 2 µmol the differential heats of adsorption of n-butane were much higher on SZ-2 than on SZ-1, while at higher coverages the heats were nearly identical for both samples and decreased from 60 to 40 kJomol-1. The samples did not differ with respect to the strength of interaction with i-butane, the heats decreased with increasing coverage from 60 to 40 kJomol-1. The results demonstrate that (i) typical SZ catalysts have fewer than 100 µmolog-1 sites, rendering identification by spectroscopic techniques difficult, and (ii) product desorption is a critical factor for the catalytic performance. References: [1] M. Hino, K. Arata, J. Chem. Soc. Chem. Comm. (1980) 851. [2] X. Song, A. Sayari, Catal. Rev. Sci. Eng., 38 (1996) 32