Research On The Preparation Of Coal Tar Pitch Based Multifunctional Solid Acid And Its Catalytic Behavior For Hydrolysis Of Cellulose

Biomass is one of the most abundant and renewable carbon-based resources in the world.How to efficiently convert these resources into high value-added biomass-based platform chemicals and biofuels,such as glucose and hydroxymethylfurfural,has a great significance research topic in the world.Cellulose is a polymer comprising D-glucopyranose molecular chains throughβ-1,4-glycosidic bonds linkages,accounting for 40%～60%of biomass.Cellulose contains numerous intermolecular and intramolecular hydrogen bonds in the structure,and exhibits high degree of polymerization（DP）and crystallinity index.This structure severely hinders the interaction between cellulose and catalyst.Hence,an effective pretreatment approach can significantly reduce the DP and crystallinity index of cellulose and enhance the accessibility between cellulose and catalyst.In comparison with liquid acid and enzymatic,solid acid catalyst has been widely investigated due to the superior performances,such as easy separation of production,less damage to the equipment,reusability,and environment-friendly.Among these,carbonaceous solid acid possesses some superior performances,such as good catalytic activity,thermal stability,the resistance of acid and alkali,and reusability,which has triggered a vast discussion in the word.Compared with the biomass-based carbonaceous solid acid（about 30 wt%of carbon yield）,coal tar pitch（CTP）is a cheap and abundant by-product of the coal chemical industry,which contains mainly carbonaceous polycyclic aromatic hydrocarbons and has a high carbon yield.Therefore,CTP is a promising precursor for fabricating multifunctional carbon-based solid acid catalyst with high acid density,which can be applied to the field of catalytic hydrolysis of cellulose.Moreover,the development of new functional solid acid has potential application value and theoretical significance.In this work,combination effect of hydrogen peroxide,ultrasonic wave and metal ions on the DP of microcrystalline cellulose（MCC）were evaluated,and the depolymerization mechanism and kinetics also were investigated.Meanwhile,a series of multifunctional carbon-based solid acid catalyst was prepared,and its catalytic hydrolysis mechanism and effect on the total reducing sugars（TRS）were studied.The main works and results include the following five aspects:（1）The combination effect of hydrogen peroxide,ultrasonic wave and metal ions（O-M/U-MCC）on the DP of MCC were investigated.Five transition metal ions（M）,namely,Ni²⁺,Cu²⁺,Co²⁺,Mn²⁺,and Fe²⁺,were selected.Results indicated that the DP decreased by 16.6%,32.8%,and 47.9%using U-MCC,Ni²⁺/U-MCC,and O-Ni²⁺/U-MCC,respectively.This phenomenon indicated that the O-Ni²⁺/U-MCC can effectively reduce the DP of MCC.The degrading ability of metal ion on DP was as follows:Ni²⁺>Cu²⁺>Co²⁺>Fe²⁺>Mn²⁺,which was attributed to the smallest ion radius and hydrated ionic radii of Ni²⁺.Ni²⁺can easily enter the internal structure of cellulose to form a stable intermediate complex with glucose unit,which can destroy the hydrogen bonding between cellulose molecules and reduce the DP of cellulose.According to the analysis of kinetic,the kinetic of depolymerization process for MCC obeyed the equation:DP=102.2+93.8e^-0.000798t,with activation energy of 17.33 k J/mol.（2）Mesocarbon microbeads-based solid acid（MCMB-SO₃H）with high surface acid density and good catalytic performances was prepared by using mesocarbon microbeads（MCMB）as raw materials.Under the different sulfonation conditions,the maximum total surface acid density and sulfonic acid（–SO₃H）density were 4.36 mmol/g and 2.18 mmol/g,which was much higher than that of activated carbon-based solid acid.MCMB-SO₃H was used to catalyze the hydrolysis of MCC（DP=195）and pretreated cellulose（DP=101.5）.When the reaction temperature,reaction time,and catalyst dosage were 140°C,4 h,and 0.5 g,the yield of total reducing sugar（TRS）and conversion ratio（Cr）for pretreated cellulose can reach 66.7%and 68.8%.However,under the same hydrolysis conditions,the yield of TRS and Cr of MCC was only 54.3%and56.5%.Therefore,the low DP of cellulose is beneficial for the hydrolysis of cellulose.In addition,the TRS yield still reached 54.1%after the solid acid catalyst recycling six times.Through the first and secondary regeneration for the solid acid catalyst,the TRS yield still reached 63.2%and 62.5%,demonstrating excellent regeneration ability and recyclability of the solid acid catalyst.（3）Due to the mass transfer limitation between the cellulose and single functional-sulfonated solid acid,the multifunctional solid acid（Cl-MCMB-SO₃H）catalyst containing both cellulose-binding domains（–Cl）and catalytic domains（–SO₃H）was prepared using the MCMB with numerous aromatic constituents.The effect of different Firedel-Crafts reaction conditions and sulfonation reaction on the–Cl and–SO₃H densities of the Cl-MCMB-SO₃H were investigated.Under the optimal preparation conditions,the acidic densities of–SO₃H and–Cl in the as-prepared Cl-MCMB-SO₃H reached 1.98 mmol/g and 1.72 mmol/g,respectively.The TRS yield of 70.3%was achieved at 130°C for 3 h in distilled water with Cl-MCMB-SO₃H solid acid as a catalyst.Moreover,the TRS yield still reached 70.1%and 69.7%through the first and second solid acid catalyst regeneration,respectively,demonstrating the excellent regenerability of the solid acid catalyst.The analysis of hydrolytic kinetic model showed that the activation energy（E_a）of cellulose hydrolysis by using Cl-MCMB-SO₃H was 89.4 k J/mol,which was considerably lower than that of MCMB-SO₃H（109.2 k J/mol）.Therefore,the existence of–Cl group was aided in reducing the E_a value of cellulose hydrolysis.（4）Given the difficult separation between solid acid and residual cellulose,a magnetic MCMB precursor（Fe₃O₄/MCMB）was synthesized by loading the active groups on the magnetic MCMB derived from the co-calcination of coal tar pitch and ferroferric oxide.The optimum processing conditions for the Fe₃O₄/MCMB preparation revealed that the CTP should be thermally treated at410°C for 4 h with 9 wt%Fe₃O₄,and the particle size of Fe₃O₄/MCMB was about 20μm.A novel magnetic MCMB-based solid acid（Fe₃O₄/Cl-MCMB-SO₃H）with–Cl group and–SO₃H group was successfully prepared.The acidic densities of–SO₃H and–Cl in Fe₃O₄/Cl-MCMB-SO₃H reached 1.77 mmol/g and 1.32 mmol/g,respectively.The saturation magnetization intensity values for Fe₃O₄/MCMB and Fe₃O₄/Cl-MCMB-SO₃H were 34.4 emu/g and 29.4 emu/g,respectively,which was beneficial in the catalyst separation from the residue with an external magnet.The TRS yield of68.6%can be obtained from cellulose at 140°C for 3 h in distilled water by using Fe₃O₄/Cl-MCMB-SO₃H as the catalyst.The TRS yield still reached 61.1%after the catalyst was used six times.（5）The N-doped carbon precursor(PCP_4-1-1.5)was prepared through the co-calcination of the low-cost coal tar pitch,polyvinyl chloride,and melamine.The double-adsorption multifunctional solid acid catalyst(PCP_4-1-1.5-SO₃H)containing–Cl and–SO₃H groups was prepared through the further sulfonation reaction.The N and Cl group in the solid acid can facilitate the formation of hydrogen bonds with cellulose,which is helpful for the adsorption between cellulose and catalyst.The–SO₃H and total acidic density in the PCP_4-1-1.5-SO₃H solid acid were 1.95 mmol/g and 3.38 mmol/g,and the N and Cl content in the PCP_4-1-1.5-SO₃H solid acid reached 4.7 wt%and 1.27 mmol/g,respectively.In addition,the PCP_4-1-1.5-SO₃H solid acid exhibits excellent catalytic performance and regenerability.The TRS yield of 71.2%was obtained at 130°C of 3 h in distilled water.The maximum TRS yield of 70.8%can be obtained using the regenerated catalyst,demonstrating the excellent regenerability of solid acid catalyst.Through the analysis of kinetic,the E_a value of cellulose hydrolysis and TRS decomposition with PCP_4-1-1.5-SO₃H catalyst was 39.0 k J/mol and 93.6k J/mol,which was much lower than that with sulfuric acid and other carbonaceous solid acids.