Application Of In-situ Synchrotron Radiation Photoionization Mass Spectrometry On Mechanism Study Of Methanol To Hydrocarbons

Olefins are important bulk chemical materials,which can be used to produce various chemical products and are closely related to the national economy and people's livelihood.The raw materials for olefins production mainly come from imported crude oil.In order to make olefin products get rid of the heavy dependence on petroleum resources,China has vigorously developed the olefin production technology with coal as raw material in recent ten years.The most critical step in coal to olefins technology is methanol to olefins process.In order to improve the selectivities of the target products,enhance the conversion ability of the catalyst and prolong catalyst lifetime,scientists have been committed to the study of methanol to hydrocarbons(MTH)reaction mechanism.Scientists have been studying the MTH reaction mechanism for decades and have made many important achievements,but there are still some scientific problems that have not been clarified.In recent years,scientists have found that formaldehyde(HCHO)is an important active intermediate in MTH reaction.They believed that HCHO not only participated in the formation of the first C-C bond in MTH reaction,but also played an important role in the formation path of aromatics,and accelerated the deactivation of catalyst.However,on the one hand,due to the high activity of HCHO,it is easy to be converted by secondary reactions in MTH,so the equilibrium concentration of HCHO is very low.On the other hand,because the traditional chromatography detector has a weak signal response to HCHO,researchers can only use indirect methods to verify the presence and function of HCHO in MTH reaction.Therefore,in the study of MTH reaction mechanism,there has been a lack of direct experimental technologies to in situ observe the formation and transformation of HCHO in MTH.In this dissertation,the self-developed in-situ catalytic reactor combined with synchrotron radiation photoionization mass spectrometry(in-situ SR-PIMS)was utilized to systematically study the formation and conversion of HCHO in MTH reaction over HSAPO-34 and HZSM-5,and the formation and function of HCHO in MTH reaction over Ga-modified HZSM-5.The dissertation includes the following chapters:The first chapter described the important role of methanol to olefins technology in China's energy structure,and introduced the research progress of MTH reaction mechanism.The mechanism study of HCHO involved in MTH was summarized in detail,and the advanced in-situ SR-PIMS technology applied to catalysis research was introduced emphatically.The research objectives and contents of this dissertation were proposed.The second chapter mainly introduced the experimental devices and analysis methods used in the research work of this dissertation.The self-developed in-situ catalytic reactor combined with synchrotron radiation photoionization mass spectrometry and the bubber feeding system for MTH reaction were introduced.The experimental method using the detection system of in-situ SR-PIMS was introduced in detail.In the third chapter,the formation and conversion mechanism of HCHO in MTH were studied by in-situ SR-PIMS.Compared with traditional methods,in-situ SR-PIMS can directly observe HCHO in MTH reaction,and can perform quantitative analysis.Based on the excellent sensitivity and time resolution of in-situ SR-PIMS,the formation and fate of HCHO during the induction,steady-state reaction and deactivation periods in MTH were studied by using HSAPO-34 and HZSM-5.The time-evolved profiles of product real-time yields showed that the formation trends of HCHO and methane were very similar,and they also had a close correlation in yields,indicating that HCHO was produced by disproportionation of methanol at acid sites.Since Y2O3 can decompose HCHO into CO and Hz,Y2O3 was added to catalysts to eliminate HCHO in MTH reaction.The decrease in HCHO yield also had a significant impact on some other products.The obtained results indicated that HCHO participated in the hydrogen transfer processes of olefins into aromatics and aromatics into cokes in MTH reaction,and HCHO production can control the contribution of the aromatic-based cycle in double-cycle mechanism,thus affecting the formation of ethylene.The fourth chapter mainly introduced the mechanism research of Ga-modified HZSM-5 to promote the formation of aromatics using in-situ SR-PIMS.The impregnation method was used to prepare Ga-modified HZSM-5(Ga(IM)HZSM-5),and Ga(IM)HZSM-5 was found to produce more formaldehyde than the parent HZSM-5.By adding Y2O3 to Ga(IM)HZSM-5,the HCHO in MTH was eliminated,and the aromatics yields decreased to the level of parent HZSM-5,which indicated that the main reason why Ga(IM)HZSM-5 can produce higher aromatics selectivity was that more HCHO promoted the HCHO-induced aromatics formation pathway.Through comparative experiments with HZSM-5 with different Ga loadings,it was found that HCHO was generated from the dehydrogenation of methanol on Ga2O3 in Ga(IM)HZSM-5.Compared with Ga(IM)HZSM-5,Ga(IM)HZSM-5(redox)obtained by reduction-oxidation treatment can produce more aromatics,and interestingly,it also produced a larger amount of HCHO.Upon reduction-oxidation treatment,Ga2O3 on the external surface of catalyst would be transformed into Ga2O highly dispersed in the zeolite channels.Based on analysis and discussion,it was found that Ga2O had a stronger dehydrogenation ability for methanol than Ga2O3,and therefore produced more HCHO at a faster rate,thereby promoting the HCHO-induced aromatics formation pathway.The fifth chapter introduced the main conclusions of this dissertation,and looked forward to the future research works.