Fluoride-free synthesis of high-silica, medium-pore zeolites PST-22 and PST-30

Abstract

We recently reported the synthesis of three high-silica (Si/Al ∼ 10) zeolites with new framework structures (i.e., PST-21, PST-22, and PST-30) via an excess fluoride approach which uses synthesis mixtures with HF/OSDAⁿ⁺ = 2/n (n = 1 or 2). However, the use of toxic and corrosive fluoride ions should be avoided for the green manufacturing of zeolites. Here we examine the structure-directing abilities of eight diazolium-based mono- and dications in hydroxide media which could crystallize PST-21, PST-22, or PST-30 under excess fluoride conditions. Among them, 1,1′-(1,4-butanediyl)bis(2,4-dimethyl-1H-pyrazol-2-ium) (14DMP-C₄²⁺) and 1,1′-(1,4-butanediyl)bis(2,5-dimethyl-1H-pyrazol-2-ium) ions were found to direct the synthesis of PST-22 and PST-30 in the presence of Na⁺ ions, respectively. Powder X-ray diffraction and Rietveld analyses of as-made PST-22 reveal that while the pyrazolium moieties in 14DMP-C₄²⁺ are located near the center of t-pww cages, the tetramethylene chain runs through a 10-ring window connecting two t-pww cages. The proton forms of PST-22 and PST-30 synthesized here exhibit better performance in 1-butene skeletal isomerization than H-ferrierite, one of the most selective catalysts for this reaction.