Synthesis, characterization and high pressure propene oligomerization over heteropoly acids, SAPO-11 molecular sieves and MeAPSO-11 molecular sieves
Heteropoly acids, HPAs, and SAPO-11 and MeAPSO-11 molecular sieves have been proposed to be active for the oligomerization of propene to high quality distillate fuels. In this study these catalysts have been synthesized, characterized, modified and screened for the oligomerization of propene to distillate fuels. Heteropoly acids consist of high molecular weight poly-oxoanions, counter cations and water of hydration and are strongly acidic. The following salts of 12-tungstophosphoric acid, a 12-type HPA with the Keggin anion structure, were synthesized: AI, Fe, Ce, Ni, Co, Cu, K and NH₄, as well as the AI and NH₄ salts of 12-tungstosilicic acid. Physicochemical characterization of the HPA salts revealed that, depending on the counter cation, these salts exhibited different thermal characteristics and surface areas. The salts were classified into two types, viz.: Type A:- low surface area salts, typically less than 15m²/g, with multiple endothermic mass losses (e.g. AI, Fe, Ce, Ni, Co, Cu) and; Type 8:- high surface area salts, typically 35-150 m²/g, with a single endothermic mass loss (e.g. K and NH₄). The Type A salts were multi-crystalline, consisting of more than one phase, and exhibited a non-uniform morphology. The Type B salts, on the other hand, were uniformly crystalline and consisted of small rounded cubic crystallites. Ammonia desorption, monitored by both TCD and FT-IR, revealed that both the Type A and B salts were strongly acidic with the majority of the ammonia being released only upon decomposition of the salt. However, as ammonia and other polar molecules are known to readily absorb into the bulk of HPAs, the ammonia TPD results did not correlate with catalytic activity for surface type reactions such as alkene isomerization and oligomerization. The Type B salts were active for cracking and skeletal isomerization reactions which suggested strong acid sites were present on their surface. The Type A salts only exhibited double bond shift isomerization activity, suggesting sites of only weak to medium acid strength.
Networked Digital Library of Theses and Dissertations