Influence of synthesis conditions on solid phosphoric acid catalyst structure
The effect of alkaline earth metals and transition metals such as calcium and vanadium on Solid phosphoric acid catalyst (SPA) structure is questionable. The influence of calcium and vanadium doping and other SPA catalyst synthesis conditions on the SPA catalyst structural parameters (that is, free acid, crystallinity, pore volume and mechanical hardness) was investigated. The SPA catalyst was synthesized using the impregnation method at a mixing temperature of 75 ⁰C using both amorphous and synthetic kieselguhr together with phosphoric acid. In addition to that, calcium and vanadium doped SPA catalyst was also synthesized. It was observed that the pre-calcination (drying the synthesis mixture at 200 ⁰C for twenty minutes) step is crucial for the extrudability of the SPA reaction mixture. Different geometries of SPA catalysts such as cylindrical and quadrilobed shaped catalyst were synthesized and calcined at varying temperatures ranging from 400 ⁰C up to 800 ⁰C. The optimal calcination temperature of 500 ⁰C was reported. The synthesized catalyst was characterized using XRD, N2- adsorption, XRF, Mercury Intrusion Technique, TEM, STEM and EDS techniques. Herein, pore volume data from the calcium and vanadium promoted SPA catalyst indicated that the SPA catalysts have the slit-shaped and ink-bottle type of pores. However, the un-promoted SPA catalyst exhibited a narrow distribution of uniform pores connected by constrictions. This forms the novelty part of this study since SPA catalyst porosity data is not available in the open literature. SPA catalyst synthesized from synthetic kieselguhr exhibited higher crystallinity compared to SPA catalyst synthesized from amorphous kieselguhr. However, data from TEM characterization revealed that there is no difference in grain size of the SPA catalyst synthesized from amorphous and synthetic kieselguhr. Nevertheless, SPA catalyst calcium and vanadium doping foster the synthesis of larger crystal grains compared to the grains of un-promoted SPA catalyst. This led to the conclusion that the use of calcium and vanadium as doping agents will not result in the synthesis of SPA catalyst with higher mechanical strength compared to un-promoted SPA catalyst. EDS confirmed the homogeneous distribution of the calcium and vanadium atoms on the SPA catalysts surface.
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2022