Properties and catalytic evaluation of nanometric X zeolites containing linear alkylammonium cations

Abstract

The reduction of zeolite particle size and the development of methods to enhance the basic properties of catalysts are some of the important research fields currently explored. In light of this, this work aimed at evaluating how the combination of crystallite size reduction and ion exchange of micro- and nanocrystalline NaX zeolites with different chain-length linear alkylammonium cations affects their overall properties, Lewis basicity and catalytic activity. Scanning electron microscopy (SEM) analyses showed that micro- and nanocrystalline NaX possessed octahedral and undefined crystalline forms, respectively, both being polycrystalline. 27Al and 29Si MAS-nuclear magnetic resonance (NMR) analyses revealed that nanocrystalline zeolites presented lower long-range organization, forming a less rigid structure with more accessible sites, which is a desirable feature for catalytic applications. These characteristics were confirmed using a range of techniques: nitrogen physisorption, thermogravimetric analysis and catalytic evaluation. Nanocrystalline zeolites exhibited higher exchange degrees than the microcrystalline zeolites, because the smaller crystal sizes and the higher mesoporosity of the former enhance the accessibility to the exchangeable sites and reduce steric hindrance. Catalytic evaluation of the zeolites via Knoevenagel condensation reaction between butyraldehyde and ethyl cyanoacetate confirmed the advantages of both zeolite modification methods Ion-exchanged zeolites presented higher catalytic activity than NaX zeolites. Zeolites with nanometric crystals showed greater conversions, and higher reaction rates than the micrometric. In addition, most of the nanosized catalysts presented higher turnover frequencies at zero reaction time.