Methane dehydroaromatization over Mo/HZSM-5 catalysts in the absence of oxygen: effects of silanation in HZSM-5 zeolite
Methane dehydroaromatization over Mo/HZSM-5 catalysts in the absence of oxygen: effects of silanation in HZSM-5 zeolite
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DOI：
10.1016/j.cattod.2004.05.014
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发表时间：
2004-09-01
期刊：
影响因子：
5.3
通讯作者：
Xu, YD
中科院分区：
文献类型：
--
作者：
Liu, HM;Li, Y;Xu, YD
以正硅酸乙酯（TEOS）为外来硅源，采用气相化学气相沉积（CVD）法对HZSM-5分子筛进行改性。在甲烷无氧芳构化反应中，硅改性的Mo/HZSM-5催化剂比未改性的HZSM-5催化剂具有更高的活性和稳定性。用FT-IR、NH_3-TPD和H-1 MAS NMR研究了硅烷化对酸中心的影响以及钼物种与酸中心之间的相互作用。外来硅物种不仅选择性地覆盖和消除了HZSM-5分子筛外表面上几乎所有的Bronsted酸中心，而且还影响了靠近孔道开口的内部Bronsted酸中心。这导致每个晶胞中的布朗斯台德酸位点的数量明显减少。在MDA反应中，Mo/HZSM-5催化剂的B酸中心不仅为活性中间体的聚合和芳构化提供了活性中心，而且也为积炭的形成提供了活性中心，积炭的形成是导致Mo/HZSM-5催化剂失活的重要因素。只有一小部分的布朗斯台德酸网站，约。0.6每单元电池需要约100个酸中心才能完成芳构化，而多余的游离布朗斯台德酸中心在温度高达973 K的非氧化条件下会导致严重的碳质沉积。(C)2004 Elsevier B. V.保留所有权利。
TEOS as the exotic silicon source was used to modify HZSM-5 zeolite by chemical vapor deposition (CVD) in a vapor phase flow system. Mo/HZSM-5 catalysts prepared by using silicon-modified zeolite showed higher activity and stability than those prepared by using non-modified HZSM-5 zeolite in methane dehydro-aromatization (MDA) under non-oxidative conditions. The effect of silanation on the acid sites and the interaction between the Mo species and the acid sites have been studied by using FT-IR, NH3-TPD and H-1 MAS NMR. The exotic silicon species not only selectively covered and eliminated almost all of the Bronsted acid sites on the external surface of HZSM-5 zeolite, but also affected the internal Bronsted acid sites, which are close to the channel openings. This results in an obvious decrease in the number of Bronsted acid sites per unit cell. In the MDA reaction, the Bronsted acid sites provided active sites not only for the polymerization and aromatization of the active intermedia, but also for the formation of carbonaceous deposits, which is a crucial factor leading to the deactivation of Mo/HZSM-5 catalyst. Only a small fraction of Bronsted acid sites, ca. 0.6 per unit cell, was required to accomplish the aromatization, and the superfluous free Bronsted acid sites would cause severe carbonaceous deposits under non-oxidative condition at a temperature as high as 973 K. (C) 2004 Elsevier B.V. All rights reserved.