Preparation Of Molecular Sieve Catalysts Modified By Metal Ion Under Microwave-irradiation And Its Catalytic Performance For MPV Reaction

MPV reaction is the abbreviation of Meerwein-Ponndorf-Verley（MPV）reduction.It is one of the important reactions in organic synthesis.The reduction of carbonyl group can be achieved by hydrogen transfer.The MPV reaction has a wide range of applications in fine chemicals,such as the synthesis of citronellol,1-butene-3-ol,4-tert-butylcyclohexanol,cinnamyl alcohol and other compounds.The production process is simple and easy to operate.With the in-depth research and development of heterogeneous catalysts,it not only improves the efficiency of MPV synthesis alcohol,makes easier to separate the product and catalyst,but also the catalyst can be recycled,which effectively reduces the resource consumption,and realizes the environmentally friendly green production process.Here,we designed and prepared ZrO₂/H-βwith both Zrand Al acid centers and ZrO₂/Na-βwith acid-base double active centers,and applied them to the MPV reaction of cyclohexanone and isopropanol to study the synergistic effect of double Lewis acid centers and acid-base double active centers on the MPV reaction.ZrO₂/H-βand ZrO₂/Na-βwere prepared by traditional heating impregnation method.The morphology,structure and acidic properties of the modified molecular sieve catalyst were analyzed,and the suitable catalyst was determined.At the same time,microwave-assisted technology was applied to the modification of molecular sieves and MPV reaction and the process was optimized.It was found that the activity of modified molecular sieve catalyst ZrO₂/H-β_MW was significantly improved,and the time of cyclohexanone MPV reaction was greatly shortened.It has considerable economic benefits and broad application prospects.ZrO₂/Na-βand ZrO₂/H-βcatalysts were prepared by the reflux impregnation method and applied to the MPV reduction of cyclohexanone and isopropanol.The catalysts were also characterized by XRD,SEM-EDS,NH₃-TPD,N₂ isothermal adsorption and desorption,and ICP-MS.The products were identified by gas chromatography-mass spectrometry and analyzed by gas chromatography.The experimental results show that ZrO₂/H-βexhibits higher catalytic activity than H-β,Na-βand ZrO₂/Na-βfor the MPV reduction of cyclohexanone and isopropanol,and their activity order is ZrO₂/H-β>ZrO₂/Na-β>H-β>Na-β.Compared with ZrO₂/Na-β,Na-βwith acid-base double active center and H-βwith single Lewis acid center,the Lewis acid active center promoted the MPV reaction of cyclohexanone and isopropanol more and significantly improved the catalytic efficiency,while the acid-base double active center did not play a synergistic role.Therefore,it is believed that the Zracid center is more important for the MPV reaction of cyclohexanone and isopropanol.We also found that both sodium-type and hydrogen-typeβmolecular sieves had high specific surface area and pore volume.After loading ZrO₂,the BET specific surface area and pore volume decreased significantly due to the coverage of ZrO₂ on the inner and outer surfaces of the molecular sieve and the collapse of the molecular sieve pores caused by the calcination of Zr⁴⁺skeleton Al³⁺ion exchange.With the increase of ZrO₂ loading,the activity of cyclohexanone and isopropanol increased first and then decreased.H-Zr-β（6）was used as catalyst,the molar ratio of cyclohexanone to isopropanol was 1:20,the amount of catalyst was6%of the total mass of the reactants,the reaction was carried out at 80℃ for 5 h.Based on cyclohexanone,the conversion of cyclohexanone was 91.2%by area normalization.Further study found that the catalyst can be regenerated by calcination.The conversion of cyclohexanone decreased significantly to 79.4%after 5 times of use of the calcined catalyst,while the dried catalyst lost part of its activity after 1 time of use.Molecular sieve supported catalyst ZrO₂/H-β_MW was synthesized by microwave assisted impregnation method and applied to MPV reaction of cyclohexanone and isopropanol under microwave irradiation.The effects of microwave power of catalyst preparation,microwave impregnation time,ZrO₂ loading,microwave power of MPV reaction,microwave MPV reaction time,catalyst dosage and the molar ratio of reactant cyclohexanone to isopropanol on the catalytic MPV reaction were investigated.Under the optimal conditions of catalyst preparation and MPV reaction,the conversion of cyclohexanone was 98.2%based on cyclohexanone.Compared with the traditional method with the conversion of cyclohexanone reached 92.0%after 5 h reaction,this process greatly shortened the reaction time（microwave assisted 20 min）and significantly improved the conversion of cyclohexanone.And the catalyst can be recycled,which effectively saves resources and reduces energy consumption.It is a green and friendly process.In addition,the catalyst ZrO₂/H-β_MW before and after the reaction were also characterized by XRD,SEM,NH₃-TPD,N₂ isothermal adsorption and desorption,ICP-MS,and TG-DSC thermal analysis.TG-DSC thermal analysis of the catalyst before and after the reaction was carried out,and the reuse of the catalyst was studied.We found that under the microwave power of 400 W,the impregnation time was 10 min,the loading amount of ZrO₂ was 10%,and under the microwave power of 400 W,the molar ratio of cyclohexanone to isopropanol was 1:20,and the catalyst with a total mass of 6%of the reactants was used to catalyze the MPV reaction for 20 min.Based on cyclohexanone,the conversion rate was calculated by area normalization method to be 98.2%.The mechanism of MPV reaction of cyclohexanone catalyzed by ZrO₂/H-βwas discussed.The catalyst ZrO₂/H-β_MW prepared by microwave-assisted was applied to the MPV reaction of unsaturated aldehyde ketone compound citronellal and macromolecular 4-tert-butylcyclohexanone.The conversion of citronellal was close to 100%,and the selectivity of citronellol was 83.3%.The conversion of 4-tert-butylcyclohexanone is also close to 100%,and the selectivity of cis-4-tert-butylcyclohexanol is above 97.3%.