Base Tandem Catalytic System For Carbon Oxide Hydrogenation

CO₂ is a greenhouse gas,and its emission has led to many environmental problems,such as global warming,seawater acidification,and extreme climates.In 2020,China has proposed the goal of"carbon dioxide peaking and carbon neutrality".So,reducing CO₂ emissions is an urgent problem to be solved.Based on the utilization of resource,CO₂ is both a carbon resource and a carbon oxide resource.Recycling CO₂ is one of the important ways to reduce CO₂ emissions.CO₂ with high stability（Δ_fG^o=-393.5kJ/mol）could be hydrogenated to produce high value-added chemicals,such as CO,formic acid,methanol,olefins,and aromatics.The green H₂,which is produced from renewable resources,was used in the CO₂hydrogenation process to further realize the reducing CO₂ emissions and CO₂ utilization.Aromatic compounds containing sp~3C-H bonds could be used to synthesize high value-added chemicals can through sp~3C-H bond alkylation.In these reactions,formaldehyde has usually been used as a methylating reagent,but its poor stability and high toxicity limit the application of formaldehyde.Besides,methanol could produce formaldehyde by dehydrogenation,which means it can also be used as a methylating agent to replace formaldehyde for the side-chain alkylation.However,the decomposition of methanol also limits its utilization in side chain alkylation.Based on the above,we developed the composite metal oxide-basic molecular sieves tandem catalytic system.The metal oxides could promote the CO₂ hydrogenation to produce carbon-oxygen intermediates,which are used as methylation reagents to react with aromatic compounds through alkylation of sp~3C-H bond,catalyzed by alkaline molecular sieves.Realizing the tandem reaction could promote the utilization of CO₂and reduce CO₂ emissions.1.A PVP-Zn_xZr_100-xO_y/metal ion-modified molecular sieve tandem catalytic system was developed to promote the tandem reaction of CO₂ hydrogenation coupled with 4-methylpyridine.Under the optimized reaction conditions,the optimal catalyst PVP-Zn₄₀Zr₆₀O_y/CsX(m(PVP-Zn₄₀Zr₆₀O_y):m（CsX）=1:6)could realize the transformation of 4-methylpyridine（MEPY）to 4-ethylpyridine（ETPY）efficiently.Under the condition of the volume hourly space velocity（VHSV）of 0.6 mL·g^-1·hour^-1,H₂/CO₂=3:1,the gas hourly space velocity（GHSV）of 6000 mL·g^-1·hour^-1,reaction temperature of 380℃,2 MPa,the catalyst shows a 19.6%conversion and selectivity82%for ETPY.The tandem catalyst integrates the advantages of composite metal oxides that can efficiently catalyze CO₂ hydrogenation to methanol at high temperature and the advantages of alkali metal ion-modified X-type zeolite for efficient activation of sp~3C-H bonds,and finally realizes a tandem reaction for CO₂ hydrogenation and MEPY side-chain alkylation.The synergy of the tandem catalyst system on the spatial scale and the intermediates of CO₂ hydrogenation are the key factors to realize the kinetic coupling of the tandem reaction.In the MEPY side-chain alkylation reaction,the conversion of the CO₂ hydrogenation system was 6.5 times higher than that of the system using methanol as the methylating reagent.Based on the results above,it can be concluded that the key intermediate of the tandem reaction is the CH_xO,which is generated during the CO₂ hydrogenation.2.Influenced by the electronic properties,the sp3 C-H bond of the toluene is more inert than the sp3 C-H bond of the MEPY,so the alkylation of the toluene side chain to provide ethylbenzene is more challenging.Therefore,the activation of the sp~3C-H bond in toluene requires a higher reaction temperature.According to the thermodynamics of the CO₂ hydrogenation to methanol,the equilibrium conversion of CO₂ hydrogenation to methanol is close to 0 at temperatures higher than 400℃.Therefore,in order to realize the side chain alkylation of toluene,CO should be employed to replace CO₂.Under the optimized reaction conditions,the optimal catalyst m（ZnCrAlO）/m（CsX-H）=25:75 can realize the process of CO hydrogenation coupled with toluene to prepare ethylbenzene.Under the conditions of T=470℃,H₂/CO=2:1,GHSV=3000 mL·g^-1·hour^-1,P=2.0 MPa,VHSV=1.0 mL·g^-1·hour^-1.A 6.1%conversion of toluene and a41%selectivity of ethylbenzene were obtained with the tandem catalyst,and this tandem catalyst shows good stability and no obvious deactivation detected after 96 h.It can be concluded that CO₂/CO hydrogenation coupling MEPY/toluene can efficiently realize the alkylation of side-chain sp~3C-H bonds by constructing a composite metal oxide/alkali metal ion modified molecular sieve tandem catalyst.The construction of this system provides a theoretical and experimental basis for the activation of sp~3C-H bonds and the coupling of C-C bonds through CO₂/CO hydrogenation.