Design Of Attapulgite-supported Solid Catalysts For The Catalytic Conversion Of Lignin

Among the three constituents of lignocellulosic biomass,cellulose and hemicellulose are the source of substrates for biological conversion into bio-ethanol,however,lignin in the biorefinery process is usually discarded as non-cellulosic wastes and has been utilized predominately as an agricultural waste for the production of steam to run the process.The insufficient utilization of lignin is mainly due to its recalcitrant nature and heterogeneous structure.Catalytic conversion of lignin via acid catalysts attracts considerable attention due to the high yield of monophenols,however,the condensation and repolymerization of intermediates and poor selectivity of products restrict the industrial application of this strategy.In view of the fact,a new strategy based on the design of efficient and stable mesoporous attapulgite?ATP?-supported solid acid catalysts was proposed to catalyze lignin depolymerization for the directional synthesis of high value-added aromatic esters and phenols.Moreover,interactions between separation method,structural characteristics and liquefaction performance of lignin were investigated for the efficient depolymerization and directional liquefaction of lignin.In addition,the interaction mechanism between catalytic functional groups and structural units of lignin was revealed by the analysis of catalytic liquefaction products.The study can lay the theoretical foundation for the green transformation of lignin.?1?The ATP mineral was first used as catalyst/catalyst support for lignin conversion,exhibiting high catalytic activity to depolymerize alkaline extracted lignin?AEL?.This could be related to the special chain-layered structure of ATP,the rich acid sites in ATP and the excellent thermal stability of ATP.Different ATP-supported catalysts such as Co-Mo/ATP,STA/ATP,ZrO₂-ATP,SO₄^2-/ZrO₂,SO₄^2-/ZrO₂-diatomite,SO₄^2-/ZrO₂-Al₂O₃,SO₄^2-/ZrO₂-MCM41 and SO₄^2-/ZrO₂-ATP were prepared and used to catalyze the depolymerization of AEL.The results demonstrated that the addition of ATP-supported catalysts was favorable for controlling the distribution of valuable depolymerization products.A high bio-oil yield of 60.5%and the lowest residue yield were obtained for SO₄^2-/ZrO₂-ATP when AEL was depolymerized in ethanol.The molecular weight distributions of the oligomers in bio-oil were monitored by MALDI-TOF-MS.Negligible peaks intensities of oligomers in the range of m/z>378?trimers?were observed when AEL was depolymerized over SO₄^2-/ZrO₂-ATP,indicating the outstanding activity of the ATP-supported catalyst.?2?Lignin depolymerization using acid catalysts suffers from char formation and repolymerization of the oligomeric products,therefore,an ATP-supported magnetic dual acid-base catalyst?AMDC?was designed and synthesized by incorporation of acid sites,basic sites and magnetic properties using natural attapulgite clay as a precursor via a simple and inexpensive process.Lignin depolymerization experienced different reaction pathways in the presence of acid sites and basic sites,moreover,the unique chain structure of ATP provided a peculiar reaction location for lignin depolymerization since lignin or intermediates could move in and out through the mesopores surface.The resulting catalyst displayed high catalytic activity for lignin conversion to phenolic monomers,which resulted in 1.32%4-ethylphenol yield,1.26%4-ethylguaiacol yield,1.51%2,6-dimethoxyphenol yield and 10.4%total monomers yield at250°C for 2 h with a catalyst loading of 50%and a solvent-solid ratio of 20:1 in 50%C₂H₅OH.The total monomers yield decreased to 2.69%after eight successive reaction runs.After being calcined at 600°C for 3 h,the recovered catalyst provided a total monomers yield of 6.74%in the ninth reaction run.?3?A mixture solvent of ethanol/1,4-dioxane/formic acid?FA?is firstly reported to efficaciously depolymerize industrial lignin to produce high-value added phenolic monomers,in which 1,4-dioxane acts as lignin solvent,ethanol acts as solvent,alkylating agent and in situ hydrogen donor,and FA acts as acid catalyst and in situ hydrogen donor.The effects of solvent composition and reaction conditions on the lignin conversion and product yields were explored,resulting in a low residue yield of 6.57%and a high phenolic monomers yield of 22.4%at 300°C for 2 h when Kraft lignin was depolymerized in the mixture solvent of ethanol/1,4-dioxane/FA?10:10:2,v/v?.The selectivity of saturated 2-methoxy-4-propylphenol reached to 36.2%,indicating the directional conversion of lignin.Possible reaction mechanism was illustrated,suggesting a favorable synergistic effect among the three components of the mixture solvent.To study the feedstock adaptability of the synergistic solvents,depolymerization of other industrial lignins such as alkali lignin?AL?,cellulolytic enzyme lignin?CEL?and concentrated acid hydrolysis lignin?CAHL?was conducted under the optimum conditions,resulting in phenolic monomers yields of 13.2%,18.3%and 4.28%,respectively.The possible reusability of the synergistic solvents was also examined,implying that the liquefaction capacity of the mixture solvent can be almost completely recovered by simple addition of FA since an 18.1%phenolic monomers yield was obtained after solvent regeneration?4?Catalytic system based on ATP and solvent system based on the mixture solvent of ethanol,1,4-dioxane and formic acid?10:10:2,v/v?was integrated,indicating that the catalytic liquefaction process could be improved by increasing operational temperature,moreover,a high phenolic monomers yield of 24.6%could be obtained over SO₄^2-/ZrO₂-ATP under 300°C which was higher than ATP with 18.6%and AMDC with 19.6%.?5?There was no significant change in the distribution of main products when Kraft lignin was pyrolyzed with ATP-supported solid catalyst SO₄^2-/ZrO₂-ATP and without catalyst,however,the formation of oligomers could be promoted and the yield of phenolic monomers could be increased by using the catalyst.The phenolic monomers yield was 9.35%,which was higher than AMDC with 8.71%,ATP with 7.96%and no catalyst with 6.37%.The purity and the yield of phenolic monomers in the bio-oil from lignin liquefaction in the mixture solvent of E-D-FA could be improved by the addition of ATP-supported solid catalyst,which laid a good foundation for the further value-added utilization of bio-oil.