Studies On Friedel-Crafts Acylation Over Supported Heteropolyacid(Polyoxometalates)

Friedel-Crafts acylation is the primary method for the preparation of aromatic ketones,which are widely used as intermediates in fine chemicals and pharmaceutical industries.Up to now,the homogeneous metal chlorides or protonic acid used as the main catalysts for Friedel-Crafts acylation in industry.Nevertheless,application of those catalysts in industries have many disadvantages,such as high dosage,non-regeneration and environment pollution.Therefore,the study of developing clean and green catalysts for the Friedel-Crafts acylation becomes important and critical.Heteropoly acids(HPA:Tungstosilicic acid,SiW;Tungstophosphoric acid,PW)directly supported on siliceous MCM-41 molecular sieve(PW/M,SiW/M)or supported on zirconia-grafted siliceous MCM-41 molecular sieve(PW/Zr02/M,SiW/Zr02/M),respectively,was prepared through the impregnation method.Co2+ or Cs+modified phosphotungstate catalysts(Co0.5PW,Cs2.5PW)were prepared and supported on siliceous MCM-41 molecular sieve(Co0.5PW/M,Cs2.5PW/M),respectively,was prepared.Their catalytic activities in the anisole Friedel-Crafts reaction were investigated and compared to bulk HPA.At the same time,the samples were characterized by XRD,N2 adsorption-desorption,TG-DSC,IR,NH3-TPD,in order to study the interactions between HPA and ZrO2,support on their catalytic activities.The results showed that the catalytic activities of HPA/M were decreased compared to bulk HPA,due to the distortion of the Keggin structure of HPA caused by the interaction between HPA and support.The reduction in activity of PW/M was more than that of SiW/M,owing to the greater distortion of Keggin-PW compared to Keggin-SiW.Compared to SiW/M,when SiW was supported on zirconia-grafted siliceous MCM-41,the obvious increasing in catalytic activity was observed ascribed to the increasing in the amount of middle and strong acidic sites on the surface of catalyst.Inversely,when PW was supported on zirconia-grafted siliceous MCM-41,the catalytic activity was significantly decreased compared to PW/M,attributed to the reduction of the amount of middle and strong acidic sites on the surface of catalyst.The catalytic activities of cation-modified phosphotungstate were decreased compared to bulk HPA,but the catalytic activity of Cs2.5PW was higher than that of Co0.5PW.When supported on MCM-41,the activity of PW/M was lower than bulk PW,while the activity of Co0.5PW/M has obvious increased compared to Co0.5PW,even higher than PW/M,attributed to the Co0.5PW finely dispersed on the support and the interaction between Co0.5PW and support increased the amount of acidic sites on the surface of catalyst.In the contrary,Cs2.5PW/M has little catalytic activity,due to the obvious decrease in the amount and strength of acidic sites caused by the interaction between Cs2.5PW and support.