Study On Keggin-type Phosphotungstic Heteropolyacid Catalysts Catalyzing The Reaction Of Glycerol And Acetal Under Mild Conditions

Since the 21st century,with the rapid development of the world economy,fossil energy has been consumed in large quantities,which has caused many serious problems such as energy shortages and environmental pollution.Biodiesel has been widely studied and applied as a new clean energy with potential to replace traditional petroleum-based diesel.While,with the rapid development of biodiesel industry,the by-product glycerol has appeared to exceed supply.Now,many researchers have been focused on how to use the glycerol or convert it into chemicals effectively.In these studies,the acetalization of glycerol with acetone has attracted more and more attention because it could produce solketal,a high-value chemical used as oxygen containing fuel additive,surfactant,food additive,and organic synthesis intermediate and disinfectant.The acetalization of glycerol with acetone is a typical acid-catalyzed exothermic reaction.Therefore,it is important to develop a highly active solid acid catalyst for this reaction.In this paper,a series of phosphotungsten heteropoly salt catalysts were prepared by ion-exchange method and applied in the acetalization of glycerol with acetone.The synthesized catalyst samples were characterized by XRF analysis,N2 adsorptiondesorption measurements,X-ray diffraction,FT-IR,SEM,NH3-TPD,H2O-TPD and ex situ pyridine-adsorbed FT-IR spectroscopy in details.The influence of reaction parameters such as the calcined temperatures,reaction time,the molar ratio of reactants and catalyst amount on the acetalization of glycerol were also studied.The conclusions as follows:1.A series of metal-exchanged heteropoly tungstophosphoric acid catalysts such as Zr0.6H0.6PW,Cu1.2H0.6PW,Fe0.8H0.6PW and Zn1.2H0.6PW have been synthesized by the ion exchange method.The results showed that the Keggin ion structure of these catalysts was retained even after the protons of HPW exchanged with these metals.Exchange of HPW protons also essentially created new Lewis acidic sites.The other(NH4)xZryHzPW catalysts were prepared with retention of Keggin ion structure by incorporated NH4+ions into Zr0.6H0.6PW.It had much higher surface area and smaller particles.The results showed(NH4)0.5Zr0.5H0.5PW exhibited more Lewis and Br(?)nsted acidity compared to Zr0.6H0.6PW due to the incorporation of NH4+.Meanwhile,the number of NH4+ makes the hydrophobic property of the catalyst greatly enhanced,and also affected the total acidic strength of(NH4)xZryHzPW.2.These metal-exchanged heteropoly tungstophosphoric acid catalysts were evaluated for their reactivity during acid-catalyzed condensation of glycerol with acetone to yield solketal.Their activity was in the following order:Zr0.6H0.6PW>Cu1.2H0.6PW>Fe0.8H0.6PW>Zn1.2H0.6PW.Among these catalysts,Zr0.6H0.6PW showed higher catalytic activity toward the acetalization of glycerol compared to that of other samples investigated.It exhibited 74.37%glycerol conversion with 99.58%solketal selectivity under the room temperature in one hour.A significant increase in glycerol conversion and also yield to solketal over(NH4)0.5Zr0.5H0.5PW could be observed when it was used under the same reaction conditons and more than 90%glycerol was converted.In addition,the catalyst reusability studies indicated that the(NH4)0.5Zr0.5H0.5PW sample was regenerable and highly stable in the acetalization of glycerol.