Shaping of sulfated zirconia oxide catalyst and its impact on physico-chemical properties and activity in model reactions of n-hexane isomeriation and isobutane alkylation
The present work focuses on investigation of such important catalyst material as sulfated zirconia, which is used in a number of acid-catalyzed chemical reactions including alkylation of isobutane and isomerazation of paraffins practiced on industrial level. In the current work sulfated zirconia catalyst shaping process is discussed from the viewpoint of rheology. In more detail an influence of zeta-potential (charge near to particle’s surface), polyvinylalcohol (PVA) addition and alumina binder content on the mechanical strength, surface acidity and textural characteristics is addressed. Model reactions of isobutane alkylation and n-hexane isomerization were chosen to relate influence of shaping parameters on catalysts' activity.
According to zeta-potential titration measurements of sulfated zirconium hydroxide with PVA additions, the latter is considered as a weak surface-modifying agent. It slightly shifts point of zero charge of particles. Although rheological measurements demonstrated decrease of the yield stress while introducing PVA, as a result from water-polymer interactions thus leading to more prominent slipping of particles in the suspension. However, it was shown that PVA does not modify acidity, surface area or mechanical strength of final catalysts; consequently, it could be used just as an extrusion aid.
The Krieger-Dougherty model was used to fit the experimental data of the dependence of apparent viscosity on suspensions’ solid fraction. Fitted shape-factor constant was very close to the one for the spherical monodisperse particles, what is also confirmed by SEM study.
As a consequence of the fact that sulfated zirconium hydroxide cannot be shaped alone, its colloidal properties was also studied in the presence of boehmite binder. Zeta-potential titration curves demonstrated that there is an interaction between aluminum species and zirconium ones. Fitting rheological data to Krieger-Dougherty equation showed that boehmite and sulfated zirconia hydroxide tends to form secondary aggregated structures.
Choose of zeta-potential value during shaping process influences on pore-size-distribution curves of the final catalysts and crushing strength. Also, it can slightly modify catalysts’ acidity, what can be explained through migration of alumina species on zirconia surface.
Deconvolution results of pore size distribution curves can distinct the presence of two peaks, which are attributed to zirconia and alumina structures. Choose of zeta-potential value during shaping results in the relative changes of the two peaks areas. At higher values of zeta-potential, uniform structure is formed, while at low values bimodal one was obtained.
Catalytic measurements using model reactions showed that catalysts’ shaping parameters have a strong impact on its activity. Thus, catalyst’s scale-up from laboratory level to industrial should be done with great care and account for structure’s and active center’s changes during production process.