The Effect Of Transition Metal And Alkali/acid Number On The Oxidation Of Glycerol Catalyzed By AuPt

The development and utilization of the new green energy biodiesel have led to the overproduction of by-product glycerol and the sharp decline in market price,thus restricting the development of biodiesel industry.Therefore,exploring effective catalysts,converting glycerol into high value-added fine chemicals have become a hot topic in this field.In this experiment,AuPt bimetallic catalysts supported on MgM-HTs(M=Al,Ti,Ce,Cr,Fe,Ni,Zn,Co and Cu)and Au supported on CuMgM-HTs catalysts were prepared,their structures were characterized by physical adsorption,X-ray diffraction,field emission transmission,CO2(or NH3)temperature programmed desorption,X-ray photoelectric spectroscopy and other techniques.The synergy and anti-synergy of basic sites(Mg-OH),acidic sites(M-OH),and transition metal ions(Mn+)in glycerol oxidation by AuPt catalysts was studied.The catalytic mechanism of the catalyst in glycerol oxidation was explored.The effect of Cu+ and basic sites of Au/CuMgAl-HTs over Au for glycerol oxidation was also studied.The main conclusions are as follows:Transition metal ions(M)exhibit the properties of acidic sites,as well as d orbital characteristics.When M was a pre-transition metal ion(M=Ti,Ce and Cr),the properties of acidic sites were dominant,the synergy of acidic sites of M-OH and basic sites of Mg-OH facilitated the activity of AuPt/MgM-HTs,Mg/Ti(Mg/Ce)ratio increased from 1:9 to 9:1,the conversion of glycerol increased from 15%(19%)to 63%(64%).The d orbital characteristics were dominant when M was a late transition metal ion(M=Fe,Ni,Zn,Co and Cu),the d orbital characteristics suppressed the synergy between acidic and basic sites.with the increase of Mg/M,the glycerol conversion changed slightly,about 30%,for AuPt/MgCu-HTs,the glycerol conversion was low,only 3%.The selectivity of the catalysts toward glyceraldehyde decreased with the molar ratio of basic to acidic sites.The selectivity toward products generated from the sequential oxidation of glyceraldehyde increased with the molar ratio of basic to acidic sites.The d orbital characteristics favored the over-oxidation of products,which yielded a higher proportion of C2 and C1 products.Especially,the cobalt catalysts are active in converting glycerol to glycolic acid and the obviously high selectivity of FA is detected.The basic and acidic sites activate O-H groups of glycerol,and the chemisorption/desorption of the products.AuPt activate the glycerol C-H groups and O2.The empty d orbitals of M sites decreased the electron density of AuPt,the presence of M sites also affected the chemisorption/desorption of the products,especially when M was a late transition metal ion(Fe,Ni,Zn,Co and Cu),the product desorption is suppressed,which resulted in C-C fissure.Au/CuMgAl-HTs catalyst exhibited high activity and the high selectivity toward dihydroxyacetone.It indicated that the basic sites and Cu+ sites of catalysts promoted the selectivity to dihydroxyacetone by Au for glycerol oxidation.Under optimal conditions,the Au/CuMgAl-HTs catalyst with a Cu:Mg:Al molar ratio of 1:9:1,exhibited superior performances with a 53%conversion of glycerol and a 72%dihydroxyacetone selectivity in a glycerol/metal molar ratio up to 1000 for glycerol oxidation under base-free conditions.