A Study On Copper-Based Catalyst For Methanol Nonoxidative Dehydrogenation To Methyl Formate

Methanol is an important raw material in chemical industry.With the global overcapacity of methanol,it needs to transform methanol into high value-added downstream chemicals.Methyl formate,as one of important C1 chemical,has both aldehyde groups and ester groups in the molecule,which can be used to synthesize methyl acrylate,methyl glycolate and dimethylformamide.Due to the great supply and demand gap of methyl formate in China,it is urgent to develop a suitable route of methyl formate synthesis based on methanol.There are four routes to produce methyl formate from methanol:methanol esterification method,methanol carbonylation method,methanol oxidative dehydrogenation method and methanol nonoxidative dehydrogenation method.Among them,methanol anaerobic dehydrogenation method is the simplest,cheapest,and cleanest process.The hydrogen as by-product is an important and valuable gas in industry.In general,supported copper-based catalysts is widely used in the anaerobic dehydrogenation of methanol.However,the Cu/SiO2 catalyst prepared by the traditional method usually has a complexed Cu active sites,resulting in unsatisfactory methyl formate selectivity.In addition,the Cu nanoparticles are easily sintered during the reaction process,leading to the deactivation of catalyst.Taking the nonoxidative dehydrogenation of methanol to methyl formate as the model reaction,this thesis carefully investigated the catalytic performance of Cu/SiO2 catalysts prepared by different preparation methods.H2-TPR shows the Cu/SiO2-AE catalyst prepared by ammonia-evaporation method exhibited a lower reduction temperature,which implied a weaker interaction between Cu andSiO2.Then,it is easier to be reduced to metallic copper than the comparison catalysts.On the other hand,the literature and experimental results demonstrated that Cu0 is an important active center for the anaerobic dehydrogenation of methanol to methyl formate.The dehydrogenation process is mainly taken place on Cu0 to generate HCHO as intermediate,then reacted with CH3OH to produce methyl formate and H2.If there is excess Cu+on the catalyst,the adsorbed HCHO intermediate will transfer from Cu0 to Cu+,followed by decomposing into CO and H2.Therefore,the regulation of surface Cu species to form more metallic Cu is of great importance to achieve high methyl formate selectivity.The Cu/SiO2-AE is selected as model catalyst and treated in5%H2-95%N2 under the temperature ranging from 300 to 800℃.XRD results showed the Cu nanoparticle size is approximately 4.2 nm when the reduction temperature is lower than 550 ℃.Further increasing the reduction temperature leads to a significant aggregation of Cu nanoparticles size over 12 nm.Meanwhile,XPS,CO-TPD and CO-FTIR characterization results show that Cu+ratio of Cu/SiO2-AE exhibits a downward trend with increasing reduction temperature.Combined with the characterization and catalytic performance,it is speculated that both the valence state and nanoparticle size of Cu govern the catalytic performance of Cu-based catalyst.When the Cu nanoparticle size is less than 9.5 nm,the TOF is positively related to the nanoparticle size,and then decrease while Cu nanoparticle larger than 9.5 nm.In summary,the conversion and methyl formate selectivity remained up to 38%and 90%during 100 h under the optimal reaction conditions of 200 ℃,0.1 MPa,n(N2)/n(CH3OH)=2.6,and WHSV=4 h-1 over Cu/SiO2-AE-750 catalyst.