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

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

Crystal Growth Kinetics as a Tool for Controlling the Catalytic Performance of a FAU-Type Basic Catalyst

This study reports on the catalytic performance of nanosized zeolite X crystals and their precursors in the reaction of benzaldehyde with ethyl cyanoacetate. Crystal growth kinetics of FAU-type zeolite is studied at low temperature (35 °C) in order to discriminate different crystallization stages. First X-ray crystalline material is detected after 6 days of hydrothermal treatment. The formation of the crystalline phase is preceded by changes in the ring structure of an aluminosilicate precursor as revealed by the combined Raman–HEXRD–solid-state NMR analyses. The set of experimental data shows that these changes are related to the reorganization of the gel structure and the formation of zeolite units. Prior to the appearance of crystalline material, the apparently amorphous solid exhibits chemical composition and short-range order organization similar to that of a crystalline FAU-type zeolite. Knoevenagel condensation was used to test the catalytic activity of a series of zeolite intermediates and nanosized zeolite crystals. The amorphous precursor obtained after 5 days of hydrothermal treatment showed the highest yield of ethyl α-cyanocinnamate. Superior catalytic performance of this material was attributed to the combination of strong basic sites and less restricted and more accessible structure of the semicrystalline zeolite units. Thus, the crystal growth kinetics of FAU-type zeolite can be used as a tool to tune the properties of a catalyst used in Knoevenagel condensation

Facile Fabrication of ZSM‑5 Zeolite Catalyst with High Durability to Coke Formation during Catalytic Cracking of Paraffins

Post-synthetic HNO₃ treatment of ZSM-5 zeolite synthesized in the absence of organic structure-directing agent removes framework Al selectively from the external surface, producing a unique ZSM-5 zeolite catalyst that has very few acid sites on its external surfaces. The resulting external acid sites can be readily deactivated at the early stage of the reaction, giving a potentially long-life catalyst as a result of a high durability to coke formation during the cracking of hexane or other paraffin molecules