Study On Construction Of Solid Superacids Based On Silica-isolated Or Loaded Biomass Carbon Nanosheets And Their Acid Catalysis

With the development of modern chemical industry,people are facing two serious problems,energy shortage and environmental pollution.Acid catalysis is one of the key conversion technologies for the production of various fuels and chemicals in chemical industry,the development of efficient and environment-friendly catalytic conversion technology based on solid acids plays an important role in the field of modern green chemicals.Among them,solid carbon sulfonic acid(BCSAs)derived from abundant and renewable biomass has become one of the hotspots in the field of catalysis because of its low cost,easy availability,and its superior performance over commercial sulfonic acid resins in some important acid catalytic reactions.However,traditional BCSAs still have natural defects in dense layered structure,acid stability,acid strength,acid position accessibility and mass transfer efficiency,which restrict their wide application in industry.In order to solve these shortcomings of BCSAs comprehensively,this paper studies the strategy of structural reorganization engineering(SRE).Using bamboo powder from agricultural and forestry processing wastes and low-cost water glass as main raw materials,a series of new structurally reconstituted biocarbon-based solids sulfonic acid were prepared by using two SRE strategies:using silica gel to isolate and support the basic structural unit of BCSAs-biomass carbon nanosheets/quantum dots(CNSs/BCQDs).Their structures,acidity and pore properties,as well as the characterization and evaluation of acid catalytic properties in some typical esterification,etherification,hydrolysis and alcoholysis reactions were systematically studied.The main contents and results are as follows:1.Dense layered bamboo carbon(BC)obtained by hydrothermal catalytic carbonization of bamboo powder was used as raw material,after pre-swelled by sodium hydroxide,chemical stripping was carried out with hexadecyl trimethyl ammonium bromide(CTMAB)solution,it was then treated with acid silica sol,silica insulated carbon nanosheets(SIBC)were obtained by dehydration and exchange removal of CTMAB,a series of silica gel-isolated BCSAs(SIBCSA-1 to 4)were prepared by sulfonation with concentrated sulfuric acid.The results of TEM,STEM-EDS,FT-IR,TGA,BET and ³¹P MAS NMR characterization showed that the state of silica gel-isolated CNSs and the stacking structure of their nanoparticles are controlled by dehydration temperature.Only the highly isolated CNSs obtained under 200-250 ^oC dehydration condition have loose nanoparticle stacking structure.When subsequently sulfonated into SIBCSA-1 and-2,two-dimensional localization(2D)can be constructed between them to the three-dimensional wide-area(3D)super hydrogen bond network(SHBN),this is mainly created by the introduction of sulfonic groups.SIBCSA-1 and SIBCSA-2 have remarkably improved structural and chemical stability,abundant mesoporous-macroporous channels and high external surface area,especially highly exposed superacid sites(stronger than 100%H₂SO₄)and other excellent comprehensive properties.In typical acid-catalyzed reactions such as esterification,etherification,and hydrolysis,the structurally more stable SIBCSA-2has an overwhelming advantage in catalytic activity and stability by comparing with the traditional BCSA and the commercial Amberlyst-15.This groundbreaking work not only provides a new SRE strategy for the construction of biomass carbon-based solid superacids,but also overcomes the bottlenecks that BCSA poses in industrial applications due to its poor structure,acid strength and porosity.2.Studies on the structure and acidity of SIBCSA-2 were controlled by hydrothermal treatment and tube furnace heat treatment.The results of TGA showed that the thermal stability of SIBCSA-2 sulfonate group could be reversibly regulated by the two methods mentioned above.The change of thermal properties of SIBCSA-2 sulfonate group probably originated from the change of hydrogen bonding strength,so it should be corresponding to the change of acid strength.One sample heat treated with water,SIBCSA-2-120-10,had a sulfonic acid decomposition temperature of up to 400 ^oC,which is 119 ^oC higher than its precursor SIBCSA-2,and is also significantly higher than Amberlyst-15(323 ^oC),this is the most stable solid sulfonic acid reported at present.The results of FT-IR,TEM,STEM-EDS and BET characterization showed that the hydrothermal treatment can break the Si-O-C bond of SIBCSA-2,driving silica gel to surface migrate and sulfonated CNSs to be close to each other,thus increasing the density of particles and reducing the void fraction between particles,and significantly enhancing the hydrogen bonding between sulfonic acid groups by increasing the spatial proximity of CNSs.The probe reaction of sucrose hydrolysis confirmed the compactness and super acidity of the structure.The alcoholysis of furfuryl alcohol to n-butyl levulinate showed that the catalyst had much higher low temperature activity than SIBCSA-2,especially BCSA,which further confirmed its super acidity.In addition,it has good reusability in furfuryl alcohol alcoholysis.3.The structural supercritical acid SLBCSA with silica gel-supported sulfonated biomass carbon quantum dots was prepared by a series of steps such as hydrothermal stripping,acidification,dehydration and sulfonation using cheap bamboo carbon and water glass as main raw materials.The newly constructed SLBCSA has abundant mesoporous channels(2-11 nm)and macroporous channels(>114 nm)and high external surface area(about 491 m²·g^-1),most of the SBCQDs are highly dispersed on the surface of silica and can undergo some unique rearrangements,thus forming a hydrogen bond network from2DL to 3DS between their sulfonic groups.The structure and acid stability of SLBCSA were significantly improved,especially the superacid sites with full exposure and strength higher than 100%sulfuric acid.SLBCSA is superior to SIBCSA-2 in preparation strategy,porosity,stability and proportion of superacid.SLBCSA has prominent advantages in mass transfer due to its unique dual-pore distribution,high external surface area and a large number of fully exposed superacid sites.Among the three typical acid catalytic reactions of etherification,esterification and hydrolysis,SLBCSA has overwhelming advantages in catalytic activity,catalytic efficiency and stability compared with traditional BCSA and commercial Amberlyst-15.This provides a new and more advantageous SRE strategy for the construction of biomass carbon-based solid superacids with excellent comprehensive properties and broad application prospects.4.In order to further improve the structural stability and acid density of SLBCSA,the preparation,characterization and catalytic hydrolysis of cellulose with SLBCSA modified by Na H₂PO₄,H₃BO₃,Na BF₄ and Na₂B₄O₇ were studied.The results of XPS showed that the structural stability of SLBCSA can be improved to varying degrees by the treatment of these compounds containing P or B,probably due to the bonding between P and B with silica gel and sulfonated carbon quantum dots.In addition,by having little effect on the thermal stability of sulfonic group,but great influence on the morphology of SLBCSA particles,the introduction of B source can obviously increase the density of sulfonic group.In cellulose hydrolysis promoted by microwave irradiation,especially SLBCSA modified by B compound showed higher activity and stability.For example,when examining two typical SLBCSA catalysts modified with H₃BO₃ and Na H₂PO₄,the reducing sugar yields(RSs)of the first reaction were 68.3%and 45.12%,respectively,and after four times the RSs yields were 59.98%and33.32%,respectively.5.The synthesis of 5-hydroxymethylfurfural(HMF)from cellulose is one of the important transformation technologies for the large-scale utilization of cellulosic biomass in the future.A preliminary study of the polyamide-modified biomass carbon sulfonic acid(BCSA-PA)designed by our group demonstrated high activity and good HMF selectivity in catalyzing this conversion reaction,so in this paper we studied the mechanism of bifunctional synergistic catalysis of-SO₃H and-PA acid-base.By comparing the catalytic performance of BCSA,BCSA-PA,PS-SO₃H and its amin-modified PS-NH₂-SO₃H in microwave-assisted conversion of cellulose,glucose and fructose,combining the adsorption experiments of glucose,cellobiose and cellulose,the reasons for the high activity of BCSA-PA in cellulose hydrolysis and the selectivity of HMF were revealed as follows:(1)BCSA-PA and BCSA showed strong affinity for the?-1,4-glycoside bonds of cellobiose and cellulose,and BCSA-PA with acid-base sites showed stronger affinity than BCSA with only acid sites.More importantly,this advantage was more prominent with the increase of adsorption temperature,indicating that BCSA-PA had obvious acid-base synergism in improving affinity for?-1,4-glycoside bonds,which would be very advantageous,which will be very beneficial to improve its catalytic cellulose hydrolysis activity;(2)Based on the study of intermediates in the analysis of BCSA-PA and BCSA catalyzed glucose conversion to HMF by HPLC-MS,only the BCSA-PA catalytic system detected two key intermediates,so an acid-base synergistic catalytic mechanism of glucose conversion to HMF via non-isomerized fructose pathway is presumed.This mechanism can reasonably explain the difference in catalytic performance of BCSA-PA in catalyzing the conversion of cellulose,glucose and fructose into HMF.