Study On Photocatalytic Conversion Of Lignin Derivatives Using Nickel Modified Titanium Dioxide

Lignin is one of the three major components of biomass and the largest source of aromatic compounds in nature.The depolymerization of lignin to produce small molecular aromatic hydrocarbons can achieve significant economic benefits,reduce environmental pollution caused by lignin incineration,and provide strong support for the country to realize the goal of "carbon peak and carbon neutrality".Traditional thermal catalysis provides the necessary energy to overcome the reaction barrier by heating and activating all the bonds in the substrate indiscriminately.Due to the complex structure of lignin,the thermal catalytic depolymerization of lignin is subject to harsh reaction conditions and suffers from poor selectivity.Photocatalysis generates reducible photoelectrons and oxidizable photo-holes in the catalyst through illumination.By adjusting the oxidation-reduction potentials of the valence and conduction bands,the oxidation-reduction ability of the electron-hole pairs in the photocatalyst can be controlled,thereby achieving the goal of controlling reaction selectivity.Therefore,photocatalysis is expected to depolymerize lignin under mild conditions.Because of its stable chemical properties,non-toxicity,and low cost,TiO2 has attracted much attention.However,its wide bandgap and low quantum yield limit its further use.To overcome these limitations,this study doped Ni@C and NiO into TiO2 to construct metal-semiconductor interfaces and P-N type heterojunctions.This led to a rearrangement of the Fermi level,which reduced the bandgap and enhanced the absorption of visible light sources.Using these catalysts,selective photocatalytic oxidation of benzyl alcohol and styrene,as well as photocatalytic depolymerization of a lignin β-O-4 model compound,were achieved.The main work is as follows:Firstly,Ni@C/TiO2-Z catalysts were synthesized and selective photocatalytic oxidation of benzyl alcohol was achieved by controlling the reaction solvent.Under a 380 nm wavelength for 18 h,the yield of benzaldehyde was 75.3%when using acetonitrile as the reaction solvent,and the yield of benzoic acid was 96.6%when using ethyl acetate.The findings of the photocurrent test show that homogeneous dispersion of Ni@C can enhance the photocurrent intensity of the catalyst and promote the transfer of photogenerated electrons.This provides a theoretical basis for the subsequent preparation of highly dispersed Ni-modified TiO2-based catalysts.Then,a P-N type heterojunction was formed by loading 1.7 nm NiO on 5.7 nm TiO2 to prepare the L-NiO/TiO2 catalyst,and compare the photocatalytic activity of Ni@C/TiO2-Z and L-NiO/TiO2 catalysts for lignin depolymerization.Photocatalytic oxidation experiments were conducted using styrene as a substrate.Results showed that Ni@C/TiO2-Z did not break the C=C bond of styrene as a catalyst,and the highest yields of the main products,styrene oxide,and acetophenone,could reach 95.7%.However,when L-0.4%NiO/TiO2 was used as the catalyst,the C=C bond of styrene was broken,and the yield of the main products,benzaldehyde,and benzoic acid,reached up to 91.1%.By comparing the distribution of photocatalytic products of styrene using L-0.4%NiO/TiO2 and Ni@C/TiO2-Z catalysts,and comparing their band gaps,it was found that constructing P-N type heterojunctions is more beneficial for improving the photocatalytic activity of the catalysts than constructing metalsemiconductor interfaces.Further,L-NiO/TiO2 catalysts with NiO loadings of 0.4%,0.8%,1.6%,and 3.2%were prepared.By comparing the photocatalytic hydrogenolysis efficiency of these catalysts for lignin β-O-4 ketone model compounds,it was found that the photocatalytic activity of the catalysts first increased and then decreased with increasing NiO loading.Research has shown that a small amount of NiO is beneficial to the transfer of photogenerated electrons,inhibits the recombination of photogenerated electrons and photogenerated holes,and improves the photocatalytic activity of the catalyst.However,excessive NiO will become the recombination center of photogenerated electrons and photogenerated holes,reducing the photocatalytic activity of the catalyst.Furthermore,we achieved highly selective photocatalytic depolymerization of lignin β-O-4 ketone model compounds by adjusting the hydrogen source and sodium salt.Using L-0.8%NiO/TiO2 as the catalyst,0.4 mL of benzyl alcohol as the hydrogen source,and 5 mg of NaOAc added,the conversion of the substrate 2-phenoxy-1-phenylethanone reached 99.2%,and the yields of acetophenone and phenol were 78.7%and 96.5%,respectively,after irradiation with a 365 nm wavelength light source for 2 h.Finally,the S-NiO/TiO2 catalyst was prepared by reducing the particle size of NiO and doping it into the lattice of TiO2.The doping of NiO causes lattice distortion of TiO2,resulting in more lattice defects,which further improves the hydrogen evolution ability of the catalyst.Under the irradiation of a 450 nm wavelength light source for 36 h,the lignin β-O-4 alcohol moldings were dehydrogenated to produce lignin β-O-4 ketones,with a conversion rate of up to 99.5%.We present a one-pot strategy that utilizes two NiO/TiO2 catalysts for the photocatalytic depolymerization of β-O-4 alcohols.Specifically,S-1.6%NiO/TiO2 can be used for the photocatalytic dehydrogenation of β-O-4 alcohols,whereas L-0.8%NiO/TiO2 is suitable for the photocatalytic depolymerization of β-O-4 ketones.Under the optimum conditions,the yields of ketone and phenol can reach 89.3%and 88.7%,respectively.Remarkably,these two reactions can be performed in one pot,by simply adding a hydrogen source and switching the light wavelength.In this thesis,the metal semiconductor interface and P-N heterojunction are constructed,which led to a rearrangement of the Fermi level,reduce the band gap,and enhance the absorption ability of visible light sources.It can promotes the transfer of photogenerated electrons,inhibits the recombination of photogenerated electrons and photogenerated holes,improves the yield of light quanta,weakens the photothermal effect of the catalyst,and converts more light energy into chemical energy rather than thermal energy.Moreover,this thesis regulates the NiO loading and loading methods according to different reaction requirements to prepare TiO2-based photocatalysts with high selectivity and catalytic activity.This enriches the research in the field of photocatalytic depolymerization of lignin to prepare aromatic compounds and provides ideas and technical support for clean,high value,and efficient conversion of lignin.