Controlled Preparation Of VO₂、VM_xO_y Catalysts By Hydrothermal Method And Their Catalytic Ammoxidation Properties

Since twenty century,green chemical reactions have received increasing attention with environmental degradation and resource depletion.Ammoxidation of hydrocarbon compounds to the corresponding nitriles is an important green chemical reaction,in which the development of highly efficient catalysts is a key factor.In this paper,VO₂、VM_xO_y catalysts were hydrothermally synthesized and applied for ammoxidation of dichlorotoluene（DCT）to the corresponding dichlorobenzonitrile（DCBN）.The aim was to explore the influence of their crystal and morphological structures、pore structures、specific surface area and surface vanadium properties on the catalytic performance,which would provide useful guidance for the design and preparation of high-efficiency catalysts.The main contents and results are as follows:Different crystal structures of VO₂（B）、VO₂（M）in ammoxidation showed that all the used samples were consisted of mixed phases of VO₂（M）、V₂O₅ and V₆O₁₃.Sample VO₂（B）showed better catalytic performance and the average valence of vanadium was+4.4 after it used in ammoxidation.The obtained yield of 3,4-DCBN and 2,6-DCBN was 65.6%and54.6%,respectively.On the basis of optimized crystal structures,different morphologies of VO₂（B）and 5%Cr-doped VO₂（B）,including nanobelts,urchins,and carambolas were hydrothermally synthesized and investigated in the ammoxidation of DCT.The catalytic results showed that VO₂（B）nanobelts and Cr-doped VO₂（B）nanobelts all exhibited the better catalytic behaviours with the yield of 2,6-DCBN respectively reaching up to 65.6%and68.5%.In order to further improve the yields of DCBN,V-Al-O and V-W-O composite oxide catalysts were hydrothermally prepared and their catalytic properties were investigated.It was found that V-Al-O nanobelts showed the yields of 78.0%、77.1%and 73.6%for 3,4-DCBN、2,4-DCBN and 2,6-DCBN,respectively;the yields of 3,4-DCBN、2,4-DCBN and 2,6-DCBN achieved over V-W-O nanobelts respectively reached up to 80.7%,77.3%and 75.1%.In addition,the catalytic behaviours of V-Si-O、V-Co-O、V-Mo-O、V-Fe-O、V-Ti-O、composite oxide catalysts prepared by hydrothermal method were studied.The results showed that the V-Si-O catalyst prepared with silica sol as a precursor showed large-sized pore structures with well mass transfer performance.The obtained yield of 2,6-DCBN was 67.5%.V-Fe-O catalyst exhibited Fe VO₄ crystal phase with microsphere structures.The yield of 2,6-DCBN and3,4-DCBN obtained over the V-Fe-O reached up to 68.3%and 72.2%,respectively.The hydrothermally synthesized V-Ti-O oxides were consisted of lots of nanoparticles with pore size of 29.7 nm and surface area of 90.2 m²g^-1.Vanadium was highly dispersed in amorphous state on anatase Ti O₂.The V-Ti-O catalyst showed the yields of 3,4-DCBN、2,4-DCBN and2,6-DCBN respectively up to 77.7%、79.6%and 84.4%at reaction temperature of 320～330℃.Diverse morphologies of monoclinic Bi VO₄ including rods and branches were hydrothermally prepared by adjusting the H₂O₂ addition and the correlations of crystal structure and catalytic activity were investigated.It was found that the greater was the degree of crystal structural distortion,the easier was the breakage of V-O bond,thus leading to the higher catalytic activity.Because of the greater degree of structural distortion,the branch-like Bi VO₄ catalyst displayed better catalytic behaviors with the yields of 73.1%、72.2%and 70.8%for 3,4-DCBN,2,4-DCBN and 2,6-DCBN,respectively.Furthermore,branch-like Bi VO₄ also exhibited better thermochromic properties.