Study On The Modified CuZnAl Catalysts In Catalyzing Aldehydes To Produce Carboxylic Acids

Industrial chemicals from biomass are an important way to utilize renewable biomass resources.The main characteristic of biomass-based chemicals is that most of them are oxygen-containing compounds of aldehydes,so the conversion between oxygen-containing functional groups is an important research content in biomass chemical industry.Aldehyde water shift reaction can achieve high selectivity of alcohols and aldehydes into carboxylic acids,and water as a mild oxidant,so it is an environmentally friendly and pollution-free green conversion process.In this paper,CuZnAl catalysts modified by Mn and Ni were prepared by co-precipitation method and used to catalyze the water-water shift reaction of aldehydes in the heterogeneous system.High catalytic activity and selectivity of propionic acid were obtained when propionic aldehyde was used as the model compound.The effects of temperature,water-aldehyde ratio and space velocity on the catalytic reaction were investigated.The results show that the Mn CuZnAl and Ni CuZnAl catalysts have the highest catalytic activity when the molar ratios of Cu/Mn and Cu/Ni are 1/1 and 2/1,respectively.The catalysts were characterized by XRD、BET、XPS、UV-Vis、CO₂-TPD、H₂-TPR and other analytical methods.The results show that the doping of metal elements can improve the specific surface area of CuZnAl catalyst and the surface dispersion of Cu,and can improve the reducibility of Cu species in different degree.The higher the reducibility of the catalyst,the higher the catalytic activity,indicating that the reaction may follow the surface redox mechanism.Another effect of Mn and Ni doping is to reduce the proportion of Cu⁰/Cu⁺species in the total copper content on the surface,thus inhibiting the hydrogenation activity of aldehyde,reducing the generation of propanol as a by-product,and improving the selectivity of carboxylic acid.The catalyst showed slow deactivation behavior during 120 h stability experiment,and carbon deposition may not be the main reason for the deactivation.The suitability of the aldehyde water transformation reaction was studied on the preferred Ni CuZnAl catalyst.The results show that the aldehyde water shift reaction is very sensitive to the properties of the substrate.Under the reaction conditions,fatty alcohols have a higher conversion rate,but the selectivity of carboxylic acids is low.The main products are aldehydes obtained by direct dehydrogenation of alcohols.When aliphatic aldehydes were used as substrates,the conversion of aldehydes was low,but the selectivity of carboxylic acids was close to 100%at the highest.However,the conversion rate of furfural and cinnamaldehyde compounds with conjugated structure is very low or even not converted.