Study On Preparation And Structure-Performance Relationship Of High-Performed Catalysts Base On CoAl-LDH Material For Selective Oxidation Of Alcohols

Selective oxidation of alcohols to their corresponding aldehydes or ketones is an important class of functional group transformation reaction. Carbonyl compounds, as raw materials or intermediates of fine chemicals, have been widely used in the field of plastics, detergents, paints, cosmetics, food additives and drugs. Currently, molecular oxygen is used in selective oxidation of alcohols as the green oxidant. However, molecular oxygen has high activation energy and slow oxidation rate, which is very essential to introduce suitable catalyst to reduce the activation energy and promote the oxidation rate. In recent years, supported RuO₂·xH₂O catalysts have received extensive concern, because of its unique structure as "electronic proton buffer" material which can combine with O into unsaturated ligand to activate O and enhance the catalytic performance. However, RuO₂·xH₂O catalysts are usually prepared by traditional methods such as impregnation and deposition-precipitation which usually have weak interaction with support and undergo aggregation leading to the deactivation in the oxidation of alcohols. Thus, adopting a novel synthesis strategy for RuO₂·xH₂O immobilized on suitable support is potentially the most interesting and fruitful way to obtain highly dispersed Ru-based catalyst.Hydrotalcites （LDHs） are a class of two-dimensional layered inorganic functional materials, which has been widely used in the field of heterogeneous catalysis as catalyst carrier, the precursor or new catalytic materials. In this paper, based on the structural characteristics and properties of LDH including cation-tunability of the brucite-like layers, surface alkaline and high adsorption capacity for nanoparticles, RuO₂·xH₂O supported on CoAl-LDH catalyst was synthesized by the co-precipitation （CP） method and the deposition-precipitation （DP） method for selective oxidation of alcohols. The catalyst prepared by CP method exhibited higher activity compared with that obtained by DP method due to stronger interaction between RuO₂ and CoAl-LDH support as well as slightly smaller particle size of RuO₂ nanoparticles. The influence of the temperature pretreatment on catalytic performance was then investigated. Among the catalysts pretreated by different temperature, RuO₂/CoAl-LDH treated at 200℃ showed the highest activity with the TOF of 142 h-1, which was nearly 55% higher than that of the untreated catalyst. It could be related to not only the suitable amount of RuO₂· xH₂O for β-H cleavage, but also the presence of Co3+ species for the activation of O₂ molecules and storage of the resulting active O* species. Furthermore, the strong interaction between RuO₂ and support was revealed to promote the adsorption and activation of benzyl alcohol and thus enhanced the catalytic performance. Significantly, RuO₂/CoAl-LDH treated at 200℃ was found to selectively oxidize various alcohols to the corresponding aldehydes and ketones with respectable activity and had greater advantage comparable to that of some Ru catalysts. In addition, according to the above results, this paper proposed a possible reaction mechanism of selective oxidation of benzyl alcohol for RuO₂·xH₂O/CoAl-LDH catalysts.Due to the low natural reserves, high prices, and low metal utilization of noble metal catalyst, the preparation of new and efficient non-noble metal catalysts to replace noble metal catalyst has become an inevitable trend. Based on preliminary findings, it is found that the transition metal Co shows considerable catalytic performance in the selective oxidation of alcohols. Yet although the high dispersion containing Co-LDHs can be prepared on account of uniform dispersion of cations in the LDHs layers, conventional LDHs nanomaterials are made of the arranged and stacked two-dimensional layers which lead to poor accessibility and low utilization. Hence, the development of a novel method for the preparation of high utilization Co-based catalyst is of great significance.In this paper, based on the advantages of LDH including uniform dispersion of cations in the layers, anionic exchangeability and exfoliation of LDHs plates, a series of CoAl-LDH sheets/HAP composites were prepared by using CoAl-CO₃-LDH as catalyst precursor, which undergone ion exchange, delamination and immobilization process. Crystal structure, micro morphology, surface basicity and valence states of the catalysts were characterized by means of XRD, SEM, CO₂-TPD and XPS. Subsequently, we discussed the influence of Co/Al ratio on the exfoliation degree of LDH and the catalytic performance in selective oxidation of benzyl alcohol. The results show that, the catalytic performance of CoAl-LDH sheets/HAP composite was preferable than that of CoAl-CO₃-LDH catalyst, wherein when CoAl molar ratio of 3:1, the conversion of benzyl alcohol was up to 50.83% and 2.5 times than that of Co3Al-CO₃-LDH. The high activity of composites was attributed to the enhanced accessibility of active sites in each layers of LDH and increased utilization of Co species. In addition, a large amount of defect sites were produced in the delamination process which resulted in unstable Co2+ species oxidated to Co3+. Surface oxygen vacancies caused from unsaturated coordination of Co3+ and O species could accelerate the activation of oxygen molecules and store the resulting active oxygen to further enhance the catalytic activity.