Quasi in-situ Adsorptive Microcalorimetric Characterization of Sulfated Zirconia Catalyst for n-Butane Isomerization with n-Butane and Isobutane as Probe Molecules

Sulfated zirconia (SZ with 0.94 mmol/g sulfur and 100 m2/g) is an active catalyst for the industrially important low temperature (373 K) isomerization of light alkanes, e.g. of n-butane [1]. The catalytic activity exhibits a multicomponent profile along time-on-stream (TOS) including an activation period and a maximum followed by rapid deactivation. This observation suggests a continuous change of the catalyst under reaction conditions, especially, of the sites that gain activity during the induction period. It has been found that a freshly prepared catalyst shows some characteristics that are far from those of the catalyst during reaction. Also it is known that probe molecules such as NH3 and pyridine, which are often used for catalyst characterization by adsorptive microcalorimetry, interact with the active sites much stronger than the reactant. It is hence compulsory to do characterization in-situ using probe molecules with the same characteristics as the reactant. Therefore, we developed a quasi in-situ adsorptive microcalorimetric method in order to characterize the active sites on SZ. The calorimeter cell was used as a fixed bed flow reactor, in which the catalytic reaction of n-butane isomerisation was carried out (0.5 g SZ pellets, 378 K, 1 kPa n-butane in N2). The feed was introduced through a capillary. Conversion was monitored on-line by GC. The reaction was stopped after various TOS, the cell was evacuated at 378 K, and placed in a SETARAM MS 70 calorimeter equipped with a volumetric system that allows dosages of < 0.01 µmol [2]. Adsorption of n- or isobutane was performed at 313 K [3]. The n- and isobutane adsorption isotherms of the SZ catalyst at different TOS indicate that the number of sites interacting with educt- (n-butane) and product (isobutane) molecules decreases with TOS, especially, during the induction period. Only a modified and not the simple Langmuir model fites these adsorption isotherms [4]. The order of adsorption decreases with the increasing catalytic activity, e.g. n-butane adsorption from 1.7 (TOS = 0) to 1.1 (TOS = 120 min). This indicates a more complicated, maybe activated adsorption process. Differential heats of butanes adsorption at coverages > 2 mmol/g show that the majority of sites produce 40 – 50 kJ/mol. These stable sites are probably related to the steady state activity of SZ beyond the activity maximum. Differential heats at < 20 mmol/g show that only a minority (< 2% of sulfur species) of sites change their character during the induction period. It seems that only these sites determine the induction process. The state of highest activity is characterized by a strong interaction of n-butane with the active sites (75 kJ/mol at < 2 mmol/g). However, the weak interaction of isobutane (50 kJ/mol at < 2 mmol/g) indicates an increasing easiness of product-desorption from the surface sites. The adsorbed amount of n-butane and isobutane is comparable (ca. 20 mmol/g at 6 mbar). In the state of highest activity the catalyst reacted with n-butane in the calorimeter cell (additional heat evolution, gas phase products). [1] M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. [2] L.C. Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053. [3] S. Wrabetz, X. Yang, F.C. Jentoft, R. Schlögl, in preparation. [4] I. Langmuir, J. Am. Chem. Soc. 38 (1916) 2221