Mechanistic Investigation On The Co-Conversion Of Methanol And Alkane Over Acidic Zeolites

Methanol to hydrocarbons(MTH)can convert clean energy methanol into specific hydrocarbon products by using mature processes.On this basis,the co-conversion of methanol and alkanes can realize both the conversion of methanol and the activation of inert alkanes under the condition of thermal neutrality.At present,the researches on the co-conversion of methanol and alkanes mainly focus on the development of chemical process and the design of new catalysts.To date,the mechanistic investigation of the co-conversion of methanol and alkanes,especially in the initial stage,is still scarce.Based on this,this dissertation focus on the mechanism of the co-reaction between methanol and low-carbon alkanes(ethane,propane,butane).In this study,isotopic-labeled methanol(¹³CH₃OH)was used to co-react with normal alkanes on acidic zeolites.Then the co-reaction system was monitored by in-situ solid-state NMR spectrometer to preliminarily identify the products.Next,off-line GC-MS technology was used to monitor the gaseous products in the system,and Microsoft Excel 2007 programming solution was used to simulate the isotopic labeling of the products.Combined with the results of NMR and GC-MS,the reaction path was speculated and the mechanism of the corresponding reaction were clarified.By studying the co-reaction of methanol and alkanes under different zeolites systems and different low-carbon alkanes,the main results and contributions of this dissertation are as follows.1)By identifying the reaction path in the co-reaction of methanol and alkanes at initial stage,our research fills the mechanism gap in this field and provides mechanism reference for the efficient co-conversion of methanol and alkanes.In general,there are three reaction paths in the reaction system:co-reaction between methanol and alkanes,self-decomposition of methanol and the cracking of alkanes.The proportion of these three reaction paths in the system is related to the size of alkane molecules and the reaction time.For example,when ethane is used as the co-reactant,the main reaction paths in the system are the co-reaction of methanol and ethane and the self degradation of methanol.When butane is used as co-reactant,the main reaction paths in the system are the co reaction of methanol and butane and the cracking of alkanes.2)By analyzing the mechanism and transition state of the co-reaction between methanol and alkanes,the universality and applicability of carbocation chemistry theory in homogeneous and heterogeneous systems are revealed.In the process of co-reaction between methanol and alkanes,the surface methoxy species generated by methanol will attack the C-H bond of alkanes and form a non-classical carbonium ion transition state.This transition state can produce hydride transfer products or methylation products.The selectivity of these two paths is depend on the stability of the products and the zeolite structure.It is worth to note that this transition state is similar to the non-classical carbonium ion in the homogeneous superacid system,that is,there is a theoretical unity between the homogeneous system and the heterogeneous system.Both of the theoretical basis is the carbocation chemistry theory established in the superacid system.3)By monitoring the formation,transformation and degradation of surface methoxy species in the co-reaction system,the carbenium nature of surface methoxy species was revealed,which provided a basis for the establishment of relevant theories in heterogeneous systems.The intrinsic characteristic of the co-reaction of methanol and alkanes is the co-reaction between surface methoxy species and alkanes.In this reaction,methanol is firstly transformed into surface alkoxy species,and then surface methoxy species undergo hydride transfer or alkylation to produce corresponding products.This reactivity indicates that the surface alkoxy species has the carbenium nature and undergo the carbenium reaction even if the surface methoxy species are bonded to the zeolite surface.By revealing this reactivity of surface alkoxy species,it is not only helpful to establish the theory of Physical Organic Chemistry in heterogeneous system,but also provides a new possibility for the reaction network of the first C-C bond formation in MTO.4)By exploring the influence of catalyst structure on the co-reaction,the particularity of carbocation chemistry theory in heterogeneous system is clarified.At the same time,it provides an experimental basis for further understanding the structure-activity relationship on solid acid catalyst and establishes a basis for the design of new catalysts in the future.Based on the above experiments,the next study focus on the solid acid catalysts with different structural characteristics,in order to investigate the differences of co-reaction paths and mechanisms on different catalysts.Furthermore,it is hope to explore the effects of acidity and pores on the reaction products and the transition state.According to the related results,due to the weak acidity and pore confinement of zeolite,the carbocation intermediate/transition state is more difficult to be stable on acidic zeolite,which makes the overall activation energy of the reaction higher than that of the homogeneous system.At the same time,in different from the polarized complexes or stable carbenium ions in the homogeneous system,the active precursors observed on the zeolite are surface methoxy species,which is due to the differences caused by the characteristics of the catalyst structure in heterogeneous system.