Effective Fabrication of Catalysts from Large-Pore, Multidimensional Zeolites Synthesized without Using Organic Structure-Directing Agents

Abstract

An Al-rich zeolite beta with *BEA topology and a Si/Al ratio as low as 6–7 was synthesized without the use of an organic structure-directing agent (OSDA) and subsequently treated by steam followed by heating with nitric acid for the purposes of dealumination, so as to prepare a catalyst. The steaming process played an important role in stabilizing the *BEA framework, presumably by repairing site defects with migrating silicon species. Steaming at around 700 °C was observed to produce optimal stabilization of the zeolite and allowed subsequent acidic dealumination while maintaining an intact framework. A second demonstration of successful OSDA-free synthesis and effective catalyst fabrication through postsynthetic modification involved the fabrication of a 12–10–10-ring zeolite having an MSE-type framework. This represented the first successful synthesis of an Al-rich MSE-type zeolite (with a Si/Al ratio as low as 6–7) using seed crystals in the absence of any OSDA. The gel composition as well as the crystallization temperature and time were optimized for the purpose of this synthesis such that a pure MSE phase could be obtained in a relatively short crystallization period of only 45 h. Longer crystallization periods and inadequate aging times gave mordenite as an impurity and as a major phase, respectively. These results offer further support for the so-called “composite building unit” hypothesis. As with the zeolite beta, direct dealumination of the MSE-type zeolite by acid treatment resulted in the collapse of the framework, which was avoided by steaming at 700 °C. After stabilization by steaming, acidic dealumination without framework collapse became possible. The dealuminated versions of the Al-rich beta and MSE-type zeolites were shown to be effective catalysts for the hexane cracking reaction, affording propylene in high selectivity. The MSE-type zeolite exhibited a particularly high level of coking resistance in addition to a significant yield of propylene, indicating that zeolites synthesized without using an OSDA show promise for industrial applications as highly selective and long-lived catalysts.