Systematic synthesis of zeolites that contain cubic and hexagonal stackings of faujasite sheets

Abstract

Synthetic cubic faujasite (FAU) has been known for several decades, and it is one of the most studied and commercially important zeolites. Recently, the hexagonal polytype of faujasite has been synthesized by Delprato et al. We have subsequently confirmed the synthesis procedures used to crystallize this new zeolite and have compared some of its physicochemical properties to those of FAU. Also, we have shown that the hexagonal polytype of faujasite (for our purposes denoted as hex) can be dealuminated by contact with SiCl_4 vapor in a manner similar to that used for the dealumination of FAU. The existence of the cubic and hexagonal polytypes of faujasite leads one to speculate on whether zeolites can be synthesized as intergrowths between these two end members at any ratio of two polytype contents. ZSM-20 is an intergrowth of cubic and hexagonal stackings of faujasite sheets, and ZSM-2, ZSM-3, and ZSM-19 may also be intergrowths. The purpose of our work is to explore the possibility of synthesizing zeolites that are comprised of intergrowths of cubic and hexagonal stackings of faujasite sheets and vary in the ratio of cubic/hexagonal content. From Delprato et al., pure hex and FAU can be synthesized from the reaction mixture R·2.4Na_2O·Al_O_3·10SiO_2·140H_2O, where R is 1,4,7,10,13,16-hexaoxacyclooctadecane (18- crown-6) and 1,4,7,10,13-pentaoxacyclopentadecane (15- crown-5), respectively. Thus, in this work we use the reaction mixture (1 - x)R_1·xR_2·2.4Na_2O·Al_2O_3·10SiO_2• 140H_2O, where R_1 = 18-crown-6, R_2 = 15-crown-5, and x is varied as 0 ≤ x ≤ 1.

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