Modification Of Co²⁺-containing Hydrotalcite-like Compounds And Their Catalytic Performance Of Oxidizing Ethyl Benzene

Acetophenone is an important organic chemical raw material,it is usually synthesized by the oxidation of ethylbenzene.However,most of the catalytic systems used for the oxidation of ethylbenzene have strong toxicity,poor conversion and safety,which is contrary to the concept of green chemistry.In this paper,cheap and readily available cobalt is used as active center for the catalytic oxidation,and then,based on the theory that hydrotalcite compounds of cations in layers and interlayer anions can be adjusted,a series of efficient,cheap,segregative,environmentally friendly hydrotalcite catalysts containing cobalt species by roasting or adjusting its composition have been developed studied in the catalytic oxidation of ethylbenzene.Based on the structural characteristics of hydrotalcite materials,catalysts with regular surface structure and surface basicity were obtained by the calcination method,and the influence of calcination temperature on the catalytic performance of the materials was investigated.The results of catalyst screening showed that Co₂Al-LDO-250 had the highest catalytic activity.Using Co₂Al-LDO-250 as catalyst,a range of conditions were optimized for the aerobic oxidation of ethylbenzene.The optimal reaction conditions were obtained as follows:ethylbenzene 1 mmol,NHPI 0.1 mmol,Co₂Al-LDO-2500.1 g,trifluorotoluene 3mL,reaction temperature 100℃,O₂ 5 mL/min,after 8h the conversion was 75.3%,acetophenone selectivity was 93.2%.Through experiments and systematic characterization,the metal oxides obtained after roasting showed stronger alkalinity.At the same time,more cobalt active sites and moderately strong alkaline sites were found on the surface after roasting at250℃.Based on the adjustability of cations,a series of cobalt-containing hydrotalcite materials with different trivalent cations were prepared by the coprecipitation method,and the influence of different trivalent cations on the catalytic performance was investigated.Using Co₂Fe-LDH as catalyst,the reaction conditions were optimized for the aerobic oxidation of ethylbenzene.The optimal reaction conditions were obtained as follows:ethylbenzene 1mmol,NHPI 0.1 mmol,Co₂Fe-LDH 0.1 g,trifluorotoluene 3mL,80℃,O₂5mL/min,the conversion was 97.6%and the selectivity of acetophenone was94.9%after 10 h of reaction.It was discovered that there was a significant synergistic effect between Co and Fe species in Co₂Fe-LDH.Fe species could stabilize Co at bivalent and improved its catalytic activity.During the reaction,electron transfer occurred between Co and Fe species to maintain the stability of catalytic activity.In the meantime,Fe could also improve the selectivity of acetophenone.On the basis of the adjustability of interlayer anions,Co₂Al-LDH intercalated with terephthalic acid（TA-Co₂Al-LDH）has been prepared.Then a variety of conditions for the aerobic oxidation of ethylbenzene were optimized using TA-Co₂Al-LDH as the catalyst.The optimal reaction conditions were obtained as follows:ethylbenzene 1 mmol,NHPI 0.1 mmol,TA-Co₂Al-LDH 0.1 g,acetonitrile 3mL,80℃,O₂ 5 mL/min,after 6 hours of reaction,the conversion of raw materials was 65.8%,acetophenone selectivity was 93.2%,significantly higher than that of Co₂Al-LDH intercalated with carbonate.Different from the previous two systems,the activity of the TA-Co₂Al-LDH catalyst was less affected by its cation compositions and surface microstructure.The expansion of the layer spacing was favorable for the diffusion of medium size substrate molecules,and simultaneously,the lipophilicity of prepared TA-Co₂Al-LDH was higher than that of Co₂Al-LDH.