Preparation, Characterization And Performance For Aryl Ether Hydrodeoxygenation Of Niobium-based Solid Acids

Lignin, as the most abundant and renewable carbon source composed of aromatic moieties in nature, is a complex polymer consisting of methoxylated or hydroxylated phenylpropanoid units linked by C-O ether bonds and C-C bonds. In the three-dimensional amorphous structure of lignin, two-thirds or more of the total linkages are C-0 ether bonds. Catalytic hydrodeoxygenation technology can effectively achieve the cleavage of C-O bonds in the lignin structure and the removal of oxygen-containing functional groups in the generated products, to produce alternative transportation liquid fuels and aromatic chemicals. Because of the unique acidity, excellent water-resistance and thermostability, niobium-based solid acids have been widely applied in hydrolysis, dehydration, hydrogenolysis, esterification and alkylation reactions. Therefore, a series of nickel nanoparticles supported on niobium-based solid acids as metal-acid bifunctional catalysts have been prepared for hydrodeoxygenation ?HDO? of lignin-derived oxygen-containing aromatic compounds ?mainly including aryl ethers?. Besides, the synergistic effect between metal active sites and acid sites has also been researched. The main research contents and results are listed as follows:?1? High specific surface area bulk Nb-Ni oxide catalysts with different Nb/Ni molar ratio have been prepared by the chemical precipitation method. The amorphous Nb₂O₅ species can effectively promote the dispersion of NiO active components, consequently inhibiting the agglomeration of NiO particles. The bulk Nb-Ni catalysts after reduction in hydrogen have displayed better catalytic performance for HDO of lignin-derived oxygen-containing aromatic compounds than bulk Ni catalyst. The selectivity of deoxygenated products over Ni_0.92Nb_0.08 catalyst increased to 2.5 times of that over bulk Ni catalyst at 160 ? and 3 MPa H₂, which is mainly owing to the catalytic synergistic effect between amorphous Nb₂O₅ species and metal Ni active sites. With futher increasing the reaction temperature to 200 ?, deoxygenation degree can almost reach up to 100%. In addition, the use of supports can further improve the dispersion of metal active sites and acid sites and various catalyst supports indeed play a significant role in deoxygenation of the lignin-derived oxygen-containing aromatic compounds.?2? Niobic acid-activated carbon composite has been successfully synthesized by the ammonia precipitation method with the niobium?V? oxalate hydrate as the niobium source. The niobic acid-activated carbon composite still exhibits good stability and acidity after calcination at 500 ?. Both C-O ether bond cleavage activity and dehydration activity of Ni/AC catalyst are obviously enhanced with the loading of niobic acid and the selectivity of deoxygenated products over Ni/30NbAC catalyst increased to 3.5 times of that over Ni/AC catalyst at 200? and 3 MPa H₂. Besides, higher reaction temperature can obviously improve the activity toward oxygen-removal of Ni/xNbAC catalysts and deoxygenation degree of diphenyl ether ?DPE? increases from 20% to 100%.?3? Layered binary HNbMoO₆ was prepared by simple calcinating Li₂CO₃, Mb₂O₅ and MoO₃ mixture followed by the proton-exchange process in HNMO₃. HNbMoO₆ nanosheets prepared by exfoliation and aggregation of layered HNbMoO₆ through soft-solution process have been successfully supported on ordered mesoporous SBA-15. SEM results suggest that chiffon-like HNbMoO₆ nanosheets are supported on the surface of SBA-15. TEM results show that nickel nanoparticles are better dispersed both in the pores of SBA-15 and on the HNbMoO₆ nanosheets. Nickel nanoparticles supported on these composite materials have exhibited better C-0 bond cleavage ability and dehydration activity with a selectivity to cyclohexane of 88% than blank Ni/SBA-15 catalyst in the HDO reaction of DPE, indicating that the composite HNbMoO₆ nanosheets can modify the surface acidity, water-tolerance and thermostability of the SBA-15 support. In addition, the conversion of DPE favors the production of oxygen-free aromatics by the direct hydrogenolysis process of 4-0-5 aryl ether bond and phenol at the relatively low H₂ pressure. Under the conditions of 320 ? and ambient pressure, the DPE conversion can reach up to 99% and the selectivity of benzene can reach to 90%.?4? Mesoporous ZrNb binary phosphate solid acids and W-Nb complex metal oxides have been synthesized by the sol-gel and hydrothermal synthesis method, respectively. Zr and W promotors can modify the pore structure and surface acidity of niobium phosphate and industrial niobic acid. The corresponding supported nickel catalysts displayed strong synergistic effect between metal active sites and acid sites and better hydrogenolysis ability at low reaction temperature. The selectivity of deoxygenated products over Ni/W₁Nb₃O catalyst at 160 ? can reach to 94% and deoxygenation degree can almost reach up to 100% at 200 ?. The overall process at low hydrogen pressure includes a hydrogenolysis/hydrogenation route and a hydrogenation/hydrogenolysis route. Moreover, excellent hydrogenolysis capacity for the cleavage of C-0 bonds and soft hydrogenationcapacity for restraining the aromatic ring-saturation are key to promote the yield of benzene. Nickel on Zr-doped niobium phosphate has exhibited an excellent activity in the conversion of DPE ?84%? with the selectivity of oxygen-free aromatic up to 62%, and the total HDO selectivity can reach to 92% at the condition of 220 ? and 0.5 MPa.