Study on the mesoscale mechanism of coking and deactivation of ZSM-5 catalyst in methanol to propylene reaction

doi:10.11949/0438-1157.20211795

Abstract

Abstract:

Methanol to propylene (MTP) is the key technology in coal chemical industry. Coke is considered to be one of the important reasons for the inactivation of catalysts. In this study, the carbon-deposited zeolite was taken as the research object, and the structure-activity relationship among methanol adsorption behavior, surface acidity of molecular sieve, coke composition and methanol reactivity in MTP reaction was investigated by various characterization methods such as IGA, FTIR and TG. It was found that the adsorption capacity of methanol is decreased with the increase of carbon deposition, the decreasing rate is directly proportional to the methanol conversion. The main components of carbon species retained on the catalysts are light hydrocarbons, BTX aromatics, active coking and carbon deposition, and carbon deposition is the main reason for the deactivation of zeolites. Compared with the fresh catalyst, the completely inactivated catalyst still retains certain adsorption ability for methanol, and it is speculated that the carbon deposition mainly exists near the center of acid active site. The hydroxyl and bridge hydroxyl of B acid center are firstly occupied by carbon deposition, followed by extra-framework Al—OH; the methanol conversion was linearly correlated with B acid and L acid in zeolites. In addition, based on the proportional relationship between the deactivation rate of the catalyst and the conversion rate, combined with the reaction kinetics, the mathematical expression of the deactivation curve was deduced, which theoretically explained the mesoscale mechanism of carbon deposition deactivation during the MTP reaction.

Key words: methanol to propylene, ZSM-5, adsorption, coking and deactivation, mesoscale, catalysis

摘要：

甲醇制丙烯（MTP）是当前煤化工领域亟需发展的关键催化技术，积炭被认为是导致催化剂失活的重要原因之一。以积炭分子筛为研究对象，通过IGA、FTIR及TG等多种表征手段，考察甲醇的吸附行为、分子筛表面酸性、积炭成分与MTP反应中甲醇反应活性之间的构效关系。研究结果表明，甲醇的吸附量随催化剂的失活而降低，其下降速率与甲醇转化率成正比。催化剂上滞留的碳物种的主要成分为轻烃、BTX芳烃、活性结焦和积炭，而其中积炭是引起分子筛失活的主要原因。完全失活的催化剂与新鲜催化剂相比仍保留一定的甲醇吸附能力，推测积炭主要存在于酸活性中心周围。积炭首先覆盖的是B酸中心的羟基和桥式羟基，随后是非骨架Al—OH；而催化剂的甲醇转化率与分子筛中可接触的B酸和L酸数量成正比。另外，基于催化剂的失活速率与转化率存在的正比关系，结合反应动力学，推导出了失活曲线的数学表达式，理论上解释了MTP反应过程中的积炭失活介尺度机制。

关键词: 甲醇制丙烯, ZSM-5, 吸附, 积炭失活, 介尺度, 催化

CLC Number:

TQ 032.4

Liyuan LI, Jianqiang WANG, Yi CHEN, Youdi GUO, Jian ZHOU, Zhicheng LIU, Yangdong WANG, Zaiku XIE. Study on the mesoscale mechanism of coking and deactivation of ZSM-5 catalyst in methanol to propylene reaction[J]. CIESC Journal, 2022, 73(6): 2669-2676.

李丽媛, 王建强, 陈奕, 郭友娣, 周健, 刘志成, 王仰东, 谢在库. 甲醇制丙烯反应中ZSM-5分子筛催化剂积炭失活介尺度机制研究[J]. 化工学报, 2022, 73(6): 2669-2676.

Figures/Tables 10

Fig.1 Methanol conversion changed with reaction time in ZSM-5 zeolite for MTP reaction

Fig.2 Isotherms of methanol on coked zeolites at 373 K

Fig.3 Saturated adsorption capacity of methanol on coked zeolites at 373 K

Fig.4 The relationship of methanol adsorption decline rate and methanol conversion for coked zeolites

Fig.5 FTIR spectra of coked zeolites (1500—4000 cm-1)

Fig.6 Differential FTIR spectra of coked zeolites (1300—1800 cm-1)

Fig.7 Differential infrared spectra of pyridine adsorption for coked zeolites

Fig.8 Content of Br?nsted acid sites (1540 cm-1) and Lewis acid sites (1450 cm-1) for coked zeolites changed with time

Fig.9 The DTG curves of coked zeolites

Fig.10 Changes of active coking and carbon deposition with reaction time

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