Selective Oxidation Of Toluene To Benzaldehyde Catalyzed By Immobilized NHPI/Co(Cu)Using Molecular Oxygen

Benzaldehyde is considered as an important chemical intermediate applied in food,pharmaceutical,pesticide and other fields.In current industrial practice,benzaldehyde is produced dominantly via chlorination/hydrolysis of toluene.The process suffers from sewage treatment,equipment corrosion and the residual chlorine in the product that cannot be used in the food and pharmaceutical trade.In contrast,preparation of chlorine-free benzaldehyde by the direct oxidation of toluene using molecular oxygen in the liquid phase is an economic and environmentally friendly process that has potential in the industrial application.It is of great significance to develop and optimize the catalytic system for selective oxidation of toluene to benzaldehyde with high selectivity under mild conditions,in which catalysts,at the same time,can be recycled and reused.In this dissertation,N-hydroxyphthalimide(NHPI)and its initiator,Co2+ or Cu2+,were immobilized to prepare heterogeneous catalysts for tolueIe oxidation.The composition,structure,morphology and chemical state of the prepared catalysts were characterized while the structure-activity relation of the catalysts and the mechanism of toluene oxidation reaction were probed.1.CuO/SiO2 prepared by chemical vapor deposition as a heterogeneous catalyst for selective oxidation of toluene to benzaldehydeCuO/SiO2 catalysts were prepared by chemical vapor deposition using silica gel for column chromatography(SiO2)as support while CuO/SNS using self-made silica nanospheres(SNS).Highly dispersed CuO nanoparticles were successfully supported on the surface of SiO2 with a uniform diameter of 4 nm and monoclinic crystal structure.The content of CuO in CuO/SiO2 decreased slightly after recycling four times,but the crystal structure remained unchanged.Both CuO/SiO2 and CuO/SNS exhibited much higher catalytic activity than bulk CuO in the selective oxidation reaction of toluene to benzaldehyde in the presence of NHPI,but CuO/SiO2,by contrast,showed more excellent catalytic performance.As reaction solvent,fluorinated alcohol can not only promote the conversion of toluene but also inhibit the overoxidation of benzaldehyde,and toluene can be oxidized by NHPI and CuO/SiO2 with a selectivity over 60%to benzaldehyde at a conversion up to 34.0%in fluorinated alcohol.The catalytic performance of CuO/SiO2 remained satble after 4 times of continuous recyclation.Thus,CuO/SiO2 catalyst prepared by chemical vapor deposition showed high activity and selectivity for toluene oxidation to benzaldehyde and reliable stability in the reaction in the presence of fluorinated alcohol and NHPI.2.Preparation of immobilized NHPI catalyst for selective oxidation of toluene to benzaldehydeNHPI was grafted onto SiO2 with different sequence to prepare catalysts SiO2-GPTMS-NHPI and NHPI-GPTMS-SiO2 using pyromellitic anhydride(PMDA),column chromatography silica gel(SiO2)and 3-(glycidoxypropyl)trimethoxysilicane(GPTMS)as precursors,carriers and graft transition materials,respectively.N,N-dihydroxypyromellitimide(NDHPI)was prepared by imidization of PMDA.Then NHPI-GPTMS-SiO2 was synthesized by two-step grafting reaction:preparation of NHPI-GPTMS intermediate by grafting NDHPI on GPTMS and preparation of the catalyst by grafting NHPI-GPTMS on SiO2-Under the same preparation conditions,NHPI grafting density of NHPI-GPTMS-SiO2 was higher than that of SiO2-GPTMS-NHPI prepared by the opposite order of grafting.The grafting of NHPI caused reduction in specific surface area of SiO2 support.After NHPI-GPTMS-SiO2 was recycled four times,there was no obvious loss of NHPI,which suggested the grafted covalent bond C—O—N bond did not break.With Co(OAc)2 as an initiator,immobilized NHPI catalysts displayed good catalytic performance for toluene oxidation,and NHPI-GPTMS-SiO2 was superior to SiO2-GPTMS-NHPI.The optimized temperature for the selective oxidation of toluene was around 90? under the applied experimental conditions.Toluene presented a low conversion at a relatively low reaction temperature,but too high temperature caused reduction in the selectivity to benzaldehyde.At the optimal conditions,the conversion of toluene catalyzed by NHPI-GPTMS-SiO2 and Co(OAc)2 in fluorinated alcohol was 14.1%with good selectivity to benzaldehyde(73.1%).NHPI-GPTMS-SiO2 had the potential for recycling.Briefly,NHPI-GPTMS-SiO2 prepared by grafting NHPI onto SiO2 performed good catalytic activity for selective oxidation of toluene in fluorinated alcohol.3.Preparation of NHPI-GPTMS-CoOx/SiO2 catalyst for toluene oxidation to benzaldehydeNHPI was grafted onto CuO/SiO2 prepared by chemical vapor deposition and CoOx/SiO2 synthesized by coprecipitation,respectively,to prepare NHPI-GPTMS-CuO/SiO2 and NHPI-GPTMS-CoOx/Si02-The content of cobalt in NHPI-GPTMS-CoOx/SiO2 was 4 mmol/g,and highly dispersed cobalt oxide formed Si—O—Co bond with SiO2 support.During the preparation and catalytic reaction of NHPI-GPTMS-CoOx/SiO2,no significant loss of cobalt was observed.NHPI was linked to the SiO2 using 3-(glycidoxypropyl)trimethoxysilicane(GPTMS)as the linker by means of the formation of the covalent bonds C—O—N and Si—O—Si.NHPI-GPTMS-CoOX/SiO2 can be reused as catalyst for the toluene oxidation without severing the grafting covalent bonds,and the grafting density of NHPI remained essentially unchanged.The activity of NHPI-GPTMS-CoOx/SiO2 was higher than that of NHPI-GPTMS-CuO/SiO2.Catalyzed by NHPI-GPTMS-CoOx/SiO2,the conversion of toluene was 42.5%while the selectivity to benzaldehyde was 52.6%.NHPI-GPTMS-CoOx/SiO2 can be recycled without significant loss of NHPI and CO2+.Therefore,NHPI-GPTMS-CoOx/Si02 prepared by immobilizing NHPI and its Co2+ initiator together on SiO2 exhibited good catalytic activity for selective oxidation of toluene in fluorinated alcohol and can be easily separated and recycled for reusing.In conclusion,CuO was supported on SiO2 by chemical vapor deposition while NHPI by covalent bond grafting.Both homogeneous catalyst NHPI with its heterogeneous initiator CuO/SiO2 and immobilized NHPI with its soluble initiator Co(OAc)2 displayed excellent catalytic activity in the selective oxidation of toluene to benzaldehyde.Then NHPI-GPTMS-CoOx/Si02 containing both NHPI and cobalt ions,NHPI-GPTMS-CuO/Si02 containing both NHPI and copper ions,were synthesized successfully.The catalytic performance in toluene oxidation of NHPI-GPTMS-CoOx/SiO2 was more excellent than that of NHPI-GPTMS-CuO/SiO2-There was no obvious loss of NHPI or Co2+detected when the catalyst NHPI-GPTMS-CoOx/SiO2 was reused several times,which proved the catalytic stability was excellent.NHPI-GPTMS-CoOx/SiO2 may have potential for industrialization and catalyzing the oxidation of toluene to benzaldehyde through a fixed bed flow reactor.