Surface properties of Fe-promoted sulfated zirconia catalysts: Probe molecule adsorption studied by microcalorimetry & IR spectroscopy

Sulfated zirconia (SZ) is a catalyst for the industrially important low temperature isomerization of light alkanes, e.g. of n-butane at 373 K [1]. The activity of SZ is increased by 1–2 orders of magnitude through addition of Mn or Fe cations [2]. Sulfur content, zirconia phase composition, and surface area have been found to affect catalyst performance [3]; and these properties depend strongly on the preparation procedure. However, differently active catalysts can be generated by using one precursor material and just varying the batch size for calcination [4]. Such samples exhibit many identical properties, so those that differ are most likely responsible for differences in activity. Here, we probe sites on such a family of samples with neo-pentane, which is closely related to n-butane but will only adsorb and not react. Catalysts “FeSZ”, containing 2 wt% Fe, were calcined at 923 K in batches of 3, 10, and 20 g. The respective maximum isomerization rates relate as 1: 2.3: 8.6. A SETARAM MS70 Calvet calorimeter and a Nicolet Protégé 466 FTIR spectrometer were used. The sample cells that were placed in the calorimeter or the IR beam were part of calibrated volumetric systems. Neo-pentane was adsorbed at 313 K (calorimetry) or 308 K (FTIR). Adsorption isotherms for differently active FeSZ samples indicate no significant variation in the number of sites interacting with neo-pentane molecules. Differential heats of neo-pentane adsorption at coverages >10 mmol/g reveal that the majority of sites produce 55–60 kJ/mol. Differential heats at low coverages