Synthesis, Characterization and Isomerization Activity Studies for Modified Sulfated Zirconia Catalysts

Sulfated zirconia (SZ) is an attractive alternative for use in industrial processes because of its good alkane activation potential as well as being a non-toxic replacement for liquid acid catalysts currently in use. Over the past 10 years, numerous studies have attempted to synthesize non-deactivating SZ-based catalysts by adding modifiers and/or promoters. In this work, a synthesis method is presented that is used to prepare a Pt-modified SZ catalyst, which shows exceptionally high and stable activity for n-pentane isomerization at atmospheric pressure. A 3-step method consisting of calcining a sulfated zirconium hydroxide at high temperature prior to adding platinum, reduction of platinum followed by additional pretreatments is presented. The activity of such an SZ catalyst remains stable for up to 4 hours with conversion of n-pentane exceeding 70%. EXAFS studies showed that the active catalyst contains relatively large crystallites of platinum. Presence of Pt-S in the inactive catalyst gives evidence for the cause of deactivation of the samples prepared in the conventional manner. XANES analysis of the data also shows that the state of platinum in the active catalyst is that of a metallic platinum (Pt0). Using XPS data, it is shown that the surface undergoes rearrangements during the preparation. Use of operando DRIFTS showed the sulfur-oxygen groups on the surface to be more complex than previously reported as species different than just sulfates, were present. During the reaction, a particular species of OH group disappear. This species is replenished by the atomic hydrogen provided by the metallic platinum. The experimental results give evidence of the changes in the catalyst surface during the preparation, pretreatment and the reaction itself. Based on the findings, a Site-Juxtaposition hypothesis is presented. This hypothesis is based on the hydrogen spillover occuring on the metallic crystallites, and the capability of this atomic hydrogen to hydrogenate coke species, thus arresting deactivation. The deactivation suppression is brought about by juxtaposition of the metal sites and the acid sites during the oxidation-reduction pretreatments. A simple math model is included that attempts to corroborates this phenomenon.