摘要
Abstract
The addition of urea as a crystal growth inhibitor to the gel system for synthesis of ZSM-5 molecular sieve can significantly reduce the scale along the b-axial direction of the ZSM-5 molecular sieve, and the lamellar structure of molecular sieve is beneficial to mass transfer, inhibiting the side reaction and extending the life of the catalyst in methanol to propylene (MTP) reaction. The effects of urea dosage on the synthesized molecular sieve and its MTP reaction performance were investigated. The results showed that the ZSM-5 molecular sieve sample synthesized without urea had the same contents of isolated Al and paired Al (50%, respectively), and its Al content on surface was lower than that in bulk; the addition of urea increased the isolated Al content of the sample to about 95%, and made the sample surface rich in Al; the samples synthesized with different amounts of urea had different amounts of defect sites, and, compared with the sample synthesized with a urea/SiO2molar ratio of 0.1, the samples synthesized with the urea/SiO2molar ratios of 0.5 and 1, respectively, had increasing B acid sites (Si-OH-Al) and decreasing internal silicon hydroxyl group and silicon hydroxyl nest defects; and whether or not hydrothermal treatment was conducted, the catalytic activity and product selectivity of the ZSM-5 catalyst synthesized with addition of urea were significantly better than that of the catalyst synthesized without urea. Comprehensively analyzing the differences in the length of diffusion path of ZSM-5 molecular sieves, acidity, Al distribution, and the kinds and distribution of defect sites, it can be known that the crystal size, the contents of isolated Al and paired Al are the key factors in improving the performance of ZSM-5 molecular sieve synthesized in presence of urea, and the optimal urea/SiO2molar ratio is in the range of 0.5 to 1.关键词
ZSM-5分子筛/尿素/Al分布/酸性/缺陷位/甲醇制丙烯Key words
ZSM-5 molecular sieve/urea/Al distribution/acidity/defect site/methanol to propylene分类
化学工程
