Embryonic zeolites: the recovery of an old concept for developing more efficient zeolite catalysts

Abstract

Despite the several outstanding properties of the zeolites caused by the pores and cavities derived from their inherent microporosity, their nanometric pores cause a negative impact in catalytic applications in the cases in which the diameter of the diffusing molecule approaches or overcome the diameter of the zeolite pore size causing restrictive traffic of molecules. Numerous procedures have been described in literature in the last 15 years, such as nano-sized zeolites, zeolite nano-sheets produced by the use of organosilanes and so on, aiming to insert larger pores in zeolites in order to shorten the diffusion path length of reactants and products, and to improve their overall catalytic performances. The last edge in the synthesis of improved zeolites for catalytic application comprises partially formed zeolites (i.e. embryonic zeolites) having their active sites extremely accessible. The first communication of embryonic zeolites occurred in the beginning of the 80's, but only more than 30 years later researches perceived that partially formed zeolites, blind to X-ray diffraction, had a catalytic behavior much superior to the full-crystallized zeolite due to very accessible and well-defined zeolite structural organization. This proposal aims to explore the synthesis and characterization of embryonic zeolites of MFI and FAU structures and to assess their chemical and catalytic properties in methanol dehydration to dimethyl ether. The acid sites accessibility will be determined by means of the accessibility factor followed by chemisorption of pyridine and other bulky molecules including 2,4,6-trimethylpyridine and 2,6-di-tert-butylpyridine. The strength of the hydroxyl groups (Brønsted or Lewis acid sites) formed in the embryonic zeolites with different origins and structures, such as bridging hydroxyl, nest hydroxyl and terminal silanol groups will be measured by different techniques (TPD-NH3, FTIR, pyridine chemisorption and deprotonation energy by computational chemistry). As there is a lack of comprehensive method to characterize the acid sites under catalytic performance due to the complexity associated to the origin of the acid site and confinement effect, Density Function Theory (DFT) protocols will be used to study the acidity of embryonic zeolites and their catalytic activity at the molecular level.