| Acid-catalyzed reaction is a common type of catalytic reaction in industry,and most of the traditional acid catalysts are homogeneous liquid acids such as sulfuric acid or hydrochloric acid.Although homogeneous liquid acids have the characteristics of fast mass transfer and super high activity and selectivity,they also have many disadvantages,such as difficult separation,unable to recycle,corrosion to equipment and environmental pollution,which limit their industrial applications.Based on this,a large number of researchers have turned their attention to solid acid catalysts to optimize them and improve their catalytic reaction performance to replace the traditional liquid acid catalysts.The alkylation reaction is a classical acid-catalyzed reaction in which benzene(C6H6)and cyclohexene(CHE)are used to produce cyclohexylbenzene which is important for the energy cell industry.Currently,the main problem of this reaction is that the active sites are not uniformly dispersed,while the mechanism of this reaction process is not clear.Keggin-type heteropolyacids have strong Br(?)nsted acids and are expected to catalyze alkylation reactions,but the heteropolyacid molecules are easily soluble in organic solutions,resulting in the loss of active components,and their specific surface area is small.Zeolite,with stable framework,rich microporous and mesoporous structures and high specific surface areas,are expected to reduce the loss of heteropolyacids in the catalytic process and promote the uniform distribution of active components by encapsulating them in zeolite as carriers.Therefore,this paper focuses on the catalytic alkylation of heteropolyacids for the preparation of cyclohexylbenzene,and investigates the performance of zeolite-encapsulated heteropolyacid catalysts,and examines their catalytic mechanisms by infrared and DFT theoretical calculations,mainly as follows.(1)First,zeolite-encapsulated heteropolyacid catalysts were prepared and used for the alkylation of C6H6 with CHE to prepare cyclohexylbenzene,and the catalytic activity and stability of the catalysts as well as the catalytic reaction mechanism were investigated.At a lower C6H6 to CHE feeding ratio(7:1),the encapsulated HPW@USY catalyst showed the best catalytic performance with 100%CHE conversion and 99.9%cyclohexylbenzene selectivity,and the catalytic performance was better than that of the HPW/USY catalyst(conv.88.9%,sel.88.5%)and the mechanically mixed catalyst HPW+USY(conv.52.6%,sel.52.1%)and USY(conv.80.0%,sel.86.6%).The mechanism of the reaction was investigated by DFT theoretical calculations and various characterizations,which confirmed that the catalytic active site of the HPW@USY catalyst was W-OH on the encapsulated HPW unit,which had a strong activation capacity for CHE and promoted the formation and stabilization of the reaction intermediates.the HPW@USY catalyst showed no significant decrease in activity after 8 cycles.The good recycling performance further proved its good industrial potential and prospect.(2)Based on the preparation of phosphotungsten heteropolyacid zeolite catalysts in the first step,the encapsulation of phosphomolybdenum heteropolyacid(HPMo)and silicotungsten heteropolyacid(HSiW)in USY zeolite was attempted.The HPA@USY catalysts outperformed de-USY in both the C6H6 and CHE alkylation reactions,which further demonstrated the encapsulation effect of HPA in the Si-Al backbone.Among them,the catalytic performance of HPW@USY was superior to that of HPMo@USY and HSiW@USY catalysts,which was attributed to the fact that HPW@USY had the highest B/L among the three catalysts and possessed more abundant Br(?)nsted acid active sites.Finally,the alkylation reactions of CHE and different aromatics were tried with the HPW@USY catalyst,and it was found that the encapsulated heteropolyacid zeolite catalyst had excellent catalytic performance in all the alkylation reactions,which indicated its wide universality. |
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