The synthesis and characterization of novel high-silica zeolites

Abstract

The synthesis and characterization of high-silica molecular sieves is reviewed using molecular recognition phenomena and structure-direction as the unifying themes. A comparative analysis between the synthesis conditions employed and the zeolite structures formed is carried out starting with the synthesis of clathradils or 0-dimensional zeolites, and extending to one-dimensional, multi-dimensional zeosils. The review finishes with the analysis of the combined effects of heteroatoms (Al, B and Zn) and organic structure-directing agents in zeolite product selectivity and thermodynamic stability. Structure-direction phenomena is further investigated using the synthesis and characterization of the pure-silica zeolite SSZ-24, prepared using the chiral molecule N(16)methylsparteinium hydroxide as the structure-directing agent. The material is characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), solid-state NMR spectroscopy, fourier transform IR spectroscopy (FTIR) and physical adsorption experiments. The B-substituted SSZ-24 prepared here is the first example where the isomorphous substitution of B for Si in the SSZ-24 framework is accomplished during synthesis using sodium borate as the source of B. The B can be easily substituted by Al. The Al-substituted SSZ-24 is an active catalyst for the cracking of alkanes and may be potentially useful in refinery and chemical processes. The structure solutions and a detailed structural characterizations of the disordered zeolites SSZ-26 and SSZ-33 is presented. These two materials are the first synthetic zeolites with intersecting open 10- and 12-ring pore systems. SSZ-26 and SSZ-33 are expected to show a combination of reaction activity, selectivity and stability unique among known zeolites. SSZ-26 and SSZ-33 may be very useful for catalytic applications in the petrochemical and refining industries. The feasibility of synthesizing a zeolite whose pore structure has been designed a priori is demonstrated with the zeolite SSZ-26 and its structure-directing agent. The synthesis of a new borosilicate, CIT-1, is described. The proposed structure of CIT-1 is confirmed by a Rietveld refinement of the synchrotron XRD pattern. CIT-1 is demonstrated to be an ordered polymorph of the SSZ-33 zeolites. The catalytic properties of CIT-1 are compared to the catalytic properties of known high-silica zeolites (ZSM-5 and zeolite beta) and CIT-1 is shown to be a very active catalyst for the cracking of n-butane. The synthesis of CIT-1 supports the idea that the chiral polymorph A of zeolite beta can be synthesized using the appropriate structure-directing agent. A combination of molecular modeling and 1H MAS NMR spectroscopy are used to characterize the interactions of structure-directing agents with the zeolite framework. The results indicate that to simulate correctly the energetic interaction and motional properties of structure-directing agents in zeolites, short-range and long-range forces, water molecules, silanol groups and defects need to be considered simultaneously.