Catalytic Cleavage Of Lignin C-O Bonds Over MOFs Derived Mo₂C Catalysts

Lignin constitutes one of the three major components of lignocellulosic biomass,of which the other two components consist of cellulose and hemicellulose.It is mainly an amorphous tridimensional polymer of three primary units:sinapyl(3,5-dimethoxy4-hydroxycinnamyl),coniferyl(3-methoxy 4-hydroxycinnamyl),and p-coumaryl(4-hydroxycinnamyl)alcohols,joined by C-O and C-C linkages.Lignin is the most stable compound among the three components in lignocellulose due to the various structure and linkages.Selective cleavage of C-O bonds is a necessary path for the conversion of lignin to monocyclic small molecule compounds.The C-O bonds in the lignin structure are aryl carboxy(C_aryl-O)and alkyl carboxy(C_alkyl-O).In this work,two molybdenum carbide catalysts were synthesized from molybdenum-based metal organic framework materials(Mo-MOF,Ni-ZIF-8 and Co-ZIF-8)precursors and the catalytic performance in the cleavage of lignin and lignin model compounds was investigate in detail,the main contents are as follows:1.N-doped Mo₂C(N-Mo₂C@C-T)catalysts with a one-dimensional structure were synthesized by a"pyrolysis-carbon thermal reduction"process using Mo-MOF as the precursor,and the effects of carbon thermal reduction temperatures(650,700,and 750°C)on the Mo valence state and surface acidity of N-Mo₂C@C were investigated.The cleavage of C-O bonds in lignin and dimeric model compounds such as diphenyl ethers(DPE),benzyl phenyl ethers(BPE)and phenylethyl phenyl ethers(PPE)over N-Mo₂C@C-T were also investigated.The results showed that the content of Mo⁵⁺increased first and then decreased with the increasing of carbon thermal reduction temperature,while the acidity gradually increased.N-Mo₂C@C-700 showed the best catalytic performance in the C-O depolymerization of DPE,BPE and PPE,among which the conversion of DPE,BPE and PPE reached100%under the reaction conditions of 280°C and 6 h,and the selectivity of phenol were 44%,37%and 44%,respectively.2.N-Mo₂C@C-700 was employed in the depolymerization of enzymatic hydrolysis lignin,and the composition and structural properties in the liquid-phase products were analyzed by gel percolation chromatography(GPC),two-dimensional nuclear magnetic resonance technique(2D-NMR),and Fourier infrared spectroscopy(FT-IR).The highest lignin liquefaction ratio reached 93.5%at 300°C for 6 h,and the liquid-phase products mainly consist of C₆-C₁₁monophenols,such as guaiacol,sinapyl alcohol,p-coumaryl alcohol and their derivatives.the total yield of monooxygenated phenols was 10.1 wt%.The Weight-average Molecular Weight(Mw)of the liquid phase product was 573 and the polydispersity index(PDI)was 1.57,which showed a narrow molecular weight distribution.The 2D-NMR results indicated that the?-O-4 bond of lignin has been broken under this reaction system.3.The highly active Ni-Mo_xC/C and Co-Mo_xC/C catalysts were synthesized from Ni-ZIF-8 and Co-ZIF-8 precursors via a high temperature calcination method at700 ^oC,and the effect of Ni and Co in the Mo_xC catalysts on the cleavage of C-O bonds in dimeric model compounds were investigated.The results showed that the Ni-Mo_xC/C showed a high dispersion of Ni and high catalytic performance in the cleavage of C-O bonds in DPE,PPE and BPE under mild reaction conditions,while the Co-Mo_xC/C showed a low dispersion of Co and the catalytic activity is relatively poor.The conversion of DPE,PPE and BPE were 100%over Ni-Mo_xC/C at 200°C,6h and the selectivities for phenol was 49%,52%and 51%,respectively.