低温加氢裂解脱除高硅ZSM-5分子筛内TPAOH模板剂

分子筛膜的合成和应用是近年来的研究热点, 特别是具有独特孔道结构的MFI 型分子筛膜. 但由于膜内有机模板剂在高温脱除时会导致膜产生缺陷, 进而影响分子筛膜的应用. 所以分子筛膜及分子筛晶体中有机模板剂的低温脱除工艺一直是研究者们致力解决的问题之一. 本文系统考察了高硅ZSM-5分子筛晶体内有机模板剂(四丙基氢氧化铵, TPAOH)在H₂/N₂气氛下的低温裂解脱除规律, 采用低温加氢裂解工艺, 在350 ℃以下可有效脱除分子筛晶体孔道内的有机模板剂. 通过对裂解后分子筛晶体的比表面积(BET)、热失重(TG)、傅里叶变换红外(FTIR)光谱和拉曼光谱表征证实, 相比于空气和氮气气氛, 含氢还原性气氛更有利于模板剂的低温脱除, 脱除率随温度的升高而增加; 280 ℃时, 加氢裂解后晶体的BET比表面积已达到252 m²·g^-1, 仍有少量有机残余物; 350 ℃时, 加氢裂解后晶体的BET比表面积可达到399 m²·g^-1, 仅有微量无机碳残余物. 此外, 低温加氢裂解后的分子筛表面相对洁净, 且氨气程序升温脱附(NH₃-TPD)结果表明低温加氢裂解后的ZSM-5 分子筛晶体具有相对较多的酸性位.

TPAOH Template Removal from High-Silica ZSM-5 by Low-Temperature Hydrocracking

Zeolite membranes, especially the MFI-type zeolite membranes, have attracted significant attention for decades because of their special properties. While organic templates such as tetrapropylammonium hydroxide (TPAOH) have typically been used for the synthesis of ZSM-5 zeolite and zeolite membranes, the templates remain trapped in the as-synthesized zeolite crystals. A common method for removing organic templates and generating porous frameworks is calcination; however, during this process, the channel structure may be affected. In particular, for ZSM-5 membranes, membrane defects may be produced and the separation efficiency therefore may decrease to some extent. In this study, the low-temperature hydrocracking of TPAOH in ZSM-5 zeolite crystals was studied under H₂/N₂, while N₂ adsorption, thermogravimetric (TG) analysis, Fourier transform infrared (FTIR) spectroscopy, temperature-programmed desorption of ammonia (NH₃-TPD), and Raman spectroscopy were used to characterize zeolite samples. The results show that the organic template in the pores of ZSM-5 can be effectively removed below 350 ℃ by low-temperature hydrocracking. Characterization analyses by BET specific surface area, TG, FTIR, and Raman spectroscopy demonstrated that a reducing atmosphere containing H₂ was more conducive to template removal at low temperature than atmospheres of air or N₂. The degree of template removal increased with temperature increasing. The BET surface area of the crystal after hydrocracking at 280 ℃ reached 252 m²·g^-1, although a small amount of organic residue remained. Furthermore, after hydrocracking at 350 ℃, the BET surface area reached 399m2·g^-1, and only trace amount of inorganic carbon residue remained. In addition, the introduction of hydrogen at low temperatures could prevent coke deposits on acid sites and thus ZSM-5 zeolite crystals had greater numbers of acidic sites after low-temperature hydrocracking.