Synthesis And Catalytic Performance Of Supported Heteropolyacids And Their Salts Catalysts For Dehydration Of Gas-phase Glycerol To Acrolein

Biodiesel,as a clean fuel,is chemically non-toxic in nature and biodegradable.The yield of biodiesel is increasing year by year and expected to 67 billion gallons by 2020.Glycerol,as a major by-product,could be produced with 10 kg per 100 kg of biodiesel.Therefore,it’s significant to to produce high value-added chemical products with glycerin as raw material.Keggin heteropoly compounds have attracted much attention in the field of catalysis because of their adjustable acidity and environment-friendly properties.The supported heteropoly compound catalyst is selected as the catalyst for glycerol dehydration to acrolein.It is of great theoretical significance to study the influence of the type of both support and heteropoly compounds on the activity and selectivity of glycerol dehydration to acrolein.In this paper,MxH3-xPW@SY(M=NH4+,Cs+,Ag+)catalysts are prepared by in-situ deposition method using ultra-stable zeolites(USY)as the substrate to investigate the reaction laws on the catalysts.Besides,HPW/USY(x),NixH3-2xPW/Y-ASA and NixH4-2xSiW/Y-AS A catalysts are prepared by impregnation method,choosing the ultrastable Y zeolite USY(x)and Y-ASA(ASA:amorphous aluminasilicate)micro/meso-porous composites as the support,respectively.Furthermore,a new series of catalysts are prepared by replacing Ni2+with Ce3+,Mg2+or Zn2+.The catalytic performance for dehydration of gas-phase glycerol to acrolein on these catalysts is investigated.The results are as follows.Acidic and textual properties play an important role in the dehydration of glycerol.USY(x)with decreased acidity and acidic strength and increased mesopores are obtained by hydrothermal treatment for various times,and are further tuned the acidity by supporting HPW.As the results shown,USY(23)and HPW/USY(23)catalysts achieved the highest glycerol conversion at 1 h of reaction(84.45%and 89.01%respectively)due to the largest pore volume and average pore size.It’s demonstrated that the pore structure of catalyst plays an important role in the conversion of glycerol.Supporting HPW on USY(x)decreases the strong acidity and subsequently improves the activity and stability of the catalysts.The glycerol conversion of HPW/USY(23)catalyst reached to 64.39%at 4 h of reaction,17.87%higher than that on USY(x)catalyst,indicating that excessive amount of strong acid would accelerate the deactivation of catalyst.Phosphotungstic acid,in insoluble salt MxH3-xPW,was immobilized on USY by in-situ deposition method.The reaction performance of glycerol on MxH3-xPW@USY(M=NH4+,Cs+,Ag+)catalyst was investigated.As the analysis of characterization said,the immobilization of MxH3-xPW increases the total acidity on USY.NH4+ entering the supercages of USY leads to immobilize(NH4)xH3-xPW in the supercages as well,resulting in reducing the average pore size.While AgxH3-xPW and CsxH3-xPW attach to the surface of USY,blocking holes and increasing the average pore size.Compared to the acid sites covered by AgxH3-xPW and CsxH3-xPW,(NH4)xH3-xPW@USY possesses more acid sites and displays higher glycerol conversion,up to 75.98%at 1 h of reaction.It confirms that the acidity plays a major role in the catalytic activity for MxH3-xPW@USY catalysts.An increasing amount of mesoporous is beneficial to improve the activity of the catalyst.The reason why USY(x)and HPW/USY(x)catalysts deactivate faster and MxH3-xPW@USY poorly activates may be the insufficient mesoporous volume of catalysts.Using Y-ASA micro/meso-porous composites as the supports,the supported phosphotungstic acid and silicotungstic acid catalysts with different content of exchanged Ni2+are prepared by impregnation method.It’s observed that Ni2+exchanging facilitates the generation of new moderate and weak acid sites and an increase of Br(?)nsted and Lewis acid sites.The selectivity to acrolein of both the two series of catalysts is raised as the Br(?)nsted acidity and B/L ratio increase.For the supported nickel phosphotungstate catalyst,the glycerol conversion increased with the increase of the total acidity.When the proportion of Ni2+is 1.0 in phosphotungstic acid,the catalyst possesses the highest total and Br(?)nsted acidity,and achieves the best activity with the glycerol conversion of 81.78%and the selectivity to acrolein of 74.52%.For the supported nickel silicotungstate catalyst,The glycerol conversion increased with the total acidity increasing,passing through a maximum and dropping for catalysts with higher total acidity,because excessive acidity provokes the coke deposition on the catalysts surface during the reaction,reducing the glycerol conversion.When the proportion of Ni2+is 0.5 in silicotungstic acid,the catalyst possesses proper acidity and dispalys the best catalytic activity with the glycerol conversion of 87.80%and the selectivity to acrolein of 74.83%.Calcination treatment enabled to reduce the total acidity of catalysts.With the increment in calcination temperature,the total acidity of the catalyst decreased gradually.The highest glycerol conversion is obtained on the catalyst calcined at 500℃,reaching to 97.24%at 1 h of reaction.The results of reutilization experiment show that the supported nickel silicotungstate catalyst calcined at 500℃ still maintains high activity and selectivity during 5 cycles of 6 h each,which exhibited excellent reusability of the catalyst for the dehydration of gas-phase glycerol to acrolein.Several metal ions with different ionic radius and electronegativity are selected for ion exchange with H+in phospho tungstic acid and silicotungstic acid,which supported on Y-AS A to prepare catalyst.It’s found that the larger the radius and electronegativity of the exchanged metal ions are,the more the total and Br(?)nsted acidity the catalyst possess,which is more favorable to the conversion of glycerol.Among them,Ce phospho tungstate and silicotungstate catalysts achieve the best activity due to the highest total acidity.In addition,from the investigation into the supported silicotungstate catalysts,it’s observed that acid density is also an important factor affecting the activity of the catalyst except for acidity and acid strength.When the acidity is close,the decreased acid density increases the catalytic activity of the catalyst.