Impact of hierarchical pore structure on the catalytic performances of MFI zeolites modified by ZnO for the conversion of methanol to aromatics
Impact of hierarchical pore structure on the catalytic performances of MFI zeolites modified by ZnO for the conversion of methanol to aromatics
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DOI：
10.1039/c7cy01041a
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发表时间：
2017-08
影响因子：
5
通讯作者：
Xinquan Shen;Jincan Kang;W. Niu;Mengheng Wang;Qinghong Zhang;Ye Wang
中科院分区：
文献类型：
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作者：
Xinquan Shen;Jincan Kang;W. Niu;Mengheng Wang;Qinghong Zhang;Ye Wang
制备了具有双峰和三峰分级孔结构的含ZnO MFI分子筛催化剂，并对其进行了表征和甲醇制芳烃反应研究。用NH4F和NaOH处理H-ZSM-5分别产生平均尺寸约为0.8 nm的较大微孔和平均尺寸为5 - 20 nm的中孔。碱和氟化物处理的组合导致三峰孔结构。H-ZSM-5的处理方法影响ZnO的分散性。氟化物处理有利于ZnO的分散，而碱处理导致大的ZnO颗粒。我们阐明了ZnO的分级孔结构、酸性和分散性在芳烃的形成中起着至关重要的作用。苯、甲苯和二甲苯（BTX）是催化剂上的主要芳烃组分，随着反应时间的延长，BTX的产率下降。较大的布朗斯台德酸浓度有利于BTX生成初期的产率，但不利于BTX生成的稳定性。孔层次的增加抑制了微孔内的焦炭沉积，提高了焦炭耐受性，从而提高了催化剂形成BTX的稳定性。具有较大孔结构的催化剂对芳烃和BTX也表现出较高的选择性。芳烃可以通过氢转移或脱氢途径经由低级烯烃中间体形成。我们建议，ZnO，特别是高度分散的ZnO簇，通过催化脱氢途径提高芳烃的选择性，而分级孔结构有利于反应中间体的转移，从而加速芳烃的形成。
ZnO-containing MFI zeolite catalysts with bimodal and trimodal hierarchical pore structures were prepared, characterized and studied for the conversion of methanol to aromatics. Treatments of H-ZSM-5 with NH4F and NaOH generated bigger micropores with a mean size of around 0.8 nm and mesopores with mean sizes of 5–20 nm, respectively. The combination of alkaline and fluoride treatments resulted in a trimodal pore structure. The method for H-ZSM-5 treatments affected the dispersion of ZnO. The fluoride treatment favoured the dispersion of ZnO, whereas the alkaline treatment led to large ZnO particles. We clarified that the hierarchical pore structure, acidity and dispersion of ZnO played crucial roles in the formation of aromatics. Benzene, toluene and xylenes (BTX) mainly constituted the aromatics over our catalysts, and the yield of BTX decreased with increasing reaction time. A larger density of Bronsted acidity favoured the yield of BTX at the initial stage but was unbeneficial to the stability for BTX formation. The increase in pore hierarchy suppressed the coke deposition inside the micropores and increased the coke tolerance, thus enhancing the catalyst stability for BTX formation. The catalyst with a larger pore hierarchy also showed higher selectivities for aromatics and BTX. Aromatics can be formed via lower olefin intermediates by hydrogen-transfer or dehydrogenation pathways. We propose that ZnO, in particular the highly dispersed ZnO clusters, enhances the selectivity for aromatics by catalysing the dehydrogenation pathway, whereas the hierarchical pore structure facilitates the transfer of reaction intermediates and thus accelerates the formation of aromatics.