Preparation Of Composite Solid Acid Catalysts And Their Catalytic Abilities For The Conversion Of Biomass Into Furfural

Lignocellulosic biomass has been intensely investigated as a promising candidate for solving the problems derived from the progressive depletion of fossil fuel reserves and the increasing environmental pressures due to its abundant, inexpensive, eco-friendly and renewable characteristics. Cellulose, hemicelluloses and lignin are the three major components of lignocellulosic biomass. Hemicelluloses, which comprise roughly 22-41% of plant cell walls, are the second most abundant biopolymer next to cellulose. Therefore, developing ideal techniques for the efficient conversion of hemicelluloses is of great significance for the utilization of lignocellulosic biomass.D-xylose is a natural 5-carbon sugar（pentose） from xylan, which represents the major component of hemicelluloses in plant cell walls. Xylose is considered as a suitable feedstock for the production of a variety of high-value added chemicals such as furfural through the dehydration reaction. Furfural is industrially synthesized from pentose-rich lignocellulosic materials using mineral acids as catalysts. However, equipment corrosion, waste disposal and low production yields have strongly diminished its competitiveness with petroleum-based alternatives in the global market. Solid acid catalysts, which possess properties such as thermal stability and recyclability, are deemed to benefit for the furfural production process. In this paper, three types of composite solid acid catalysts were synthesized via coprecipitation, impregnation and ion-exchanged methods, and their catalytic abilities for the conversion of xylose, xylan-type hemicelluloses and corncob into furfural were investigated in the monophasic and biphasic systems.1. Preparation of SO4/TiO₂-ZrO₂/La and its catalytic ability for the conversion of xylose, hemicelluloses and corncob into furfural（1） SO₄^2-/TiO₂-ZrO₂/La³⁺（STZL） was prepared by coprecipitation and impregnation methods. Physical and chemical properties of STZL were detected by XRD, FT-IR, TGA, BET, UV-vis, NH₃-TPD and pyridine FT-IR. The influences of illuminate, impregnated concentrations of sulfate and lanthanum-loaded amounts on the catalytic ability of STZL for the conversion of xylose into furfural were also investigated. Results showed that STZL presented the good acidity and thermal stability. Ultraviolet irradiation could efficiently enhance its catalytic performance under mild conditions. A furfural yield of 3.26% with 33.0% xylose conversion efficiency was achieved using（1.0）STZL（1.0） as the catalyst at 120 oC for 4 h under ultraviolet irradiation（365 nm）.（2）（1.0）STZL（1.0） was used as the solid acid catalyst for the conversion of xylose into furfural in a biphasic system. The influence of aprotic organic solvents such as DMSO, DMF and DMI in the aqueous phase and the impact of 2-butanol in the MIBK phase on the furfural production were discussed. Furthermore, the reaction parameters were investigated to optimize the reaction conditions. Results showed that the addition of DMI and 2-butanol could greatly increase the furfural yield and xylose conversion efficiency. A furfural yield of 53.5% with 97.9% xylose conversion efficiency was obtained from xylose at 180 oC for 12 h in a modified biphasic system（8:2 water:DMI）:（7:3 MIBK:2-butanol）.（3） The relationship between the structural characteristics of alkali-soluble corncob hemicelluloses and the production of furfural was established. A graded ethanol precipitation technique was employed to obtain different hemicellulosic fractions from alkali-extracted corncob liquid. Detail chemical composition and structural characteristics of the hemicellulosic fractions were measured by HPLC, GPC, FT-IR, TGA, XRD, ¹H and ¹³C NMR spectroscopies. Furthermore, the influences of their structural features on the furfural production were also investigated by the heterogeneous catalysis using（1.0）STZL（1.0） as the catalyst in a biphasic system. Results showed that alkali-soluble corncob hemicelluloses mainly consisted of glucuronoarabinoxylans and L-arabino-（4-O-methylglucurono）-D-xylans. Furthermore, alkali-soluble corncob hemicelluloses with higher xylose content, lower branch degree, higher polydispersity and crystallinity contributed to the furfural production. A highest furfural yield of 41.60% with the xylose conversion efficiency of 92.14% and the furfural selectivity of 44.50% was obtained from the oven-dried hemicellulosic fractions precipitated at the 30%（v/v） ethanol concentration at 190 oC for 2.5 h in the（8:2 water:DMI）:（7:3 MIBK:2-butanol） system.（4） One-pot selectively catalytic hydrothermal pretreatment of corncob into xylose and furfural was developed using the solid acid catalyst（1.0）STZL（1.0）. Results showed that the addition of the catalyst during the hydrothermal pretreatment process promoted the release of hemicelluloses from corncob and further hydrolyzed them into monosaccharides followed by the dehydration into furfural. Hemicelluloses were much easier to hydrolyze than cellulose in lower temperature. The surface area, higher heating value（HHV） and energy enhancement factor（EEF） of the corncob residues increased with the increment of reaction temperature and time, while the value of O/C, crystallinity and the hemicelluloses content decreased. The highest furfural yield（61.80 mg/g） could be obtained in the presence of（1.0）STZL（1.0） at 180 oC for 120 min with 68.00 mg/g xylose yield when the corncob/water ratio was 10:100 g/m L.2. Preparation of tin-loaded montmorillonite and its catalytic ability for the conversion of xylose, hemicelluloses and corncob into furfural（1） Tin-loaded montmorillonite（Sn-MMT） was synthesized via the ion-exchanged method under microwave irradiation and used as a solid catalyst for the one-pot conversion of xylose and corncob hemicelluloses into furfural in a biphasic system（SBP/Na Cl-DMSO）. Chemical and physical characteristics of Sn-MMT were detected, and the effect of Lewis acids and Br?nsted acids on the isomerization and dehydration reactions of xylose was discussed. The influence of the solvent composition and the reaction conditions on the furfural production was also investigated. Furthermore, the origin of the humins was explored. Results showed that the co-existence of Lewis acids and Br?nsted acids in Sn-MMT could improve the furfural yield and selectivity. A furfural yield of 76.79% with 93.13% xylose conversion efficiency and 82.45% furfural selectivity was obtained from xylose at 180 oC for 30 min using Sn-MMT as a catalyst in a biphasic system（SBP/Na Cl-DMSO）. The black solid humins was probably derived from the condensation reaction between xylose and furfural. Furthermore, the highest furfural yield of 45.41% with the xylose conversion efficiency of 99.06% and the furfural selectivity of 45.84% was obtained from the oven-dried hemicelluloses precipitated at the 30%（v/v） ethanol concentration at 180 oC for 2.5 h in the SBP/Na Cl-DMSO system.（2） A two-step process was proposed to yield furfural from corncob using the microwave-assisted hydrothermal pretreatment as the first stage and the heterogeneously catalyzed conversion of the hemicelluloses-derived sugars in hydrolysates for the furfural production over Sn-MMT in the SBP/Na Cl-DMSO system as the second stage. The dissolution and depolymerization of hemicelluloses from corncob by the microwave-assisted hydrothermal pretreatment were assessed, the optimum microwave-assisted hydrolysis conditions were identified, and the relationship between the hydrolysate composition and the furfural yield was investigated. Results showed that about 86.67% yield of xylose presented as monosaccharide and oligosaccharides（DP≤6） after the microwave-assisted hydrothermal pretreatment at 160 oC for 60 min. During the second stage, the highest furfural yield（57.80%） was obtained from the hydrolysates with the maximum xylose content at 190 oC for 10 min. Moreover, controlled experiments showed that furfural yields from corncob hydrolysates were higher than those from the pure xylose solutions, and lower initial xylose concentration may be in favor of the furfural production.3. Preparation of SO₄^2-/SiO₂-Al₂O₃/La³⁺ and its catalytic ability for the conversion of xylose, hemicelluloses and corncob into furfural（1） SO₄^2-/SiO₂-Al₂O₃/La³⁺（SSAL） was prepared by coprecipitation and impregnation methods. Physical and chemical properties of SSAL were detected, and the influences of the synthesis conditions as well as the reaction parameters on the catalytic ability of SSAL for the conversion of xylose and corncob hemicelluloses into furfural were also investigated. Results showed that SSAL presented the good acidity and thermal stability. The strongest acidity of SSAL could be obtained when the calcination temperature was 450 oC and the calcination time was 4 h. Catalytic experiments showed that the highest furfural yield of 57.69% with 100% xylose conversion efficiency and 57.69% xylose selectivity could be achieved from xylose using（1.0）SSAL（1.0） as the catalyst at 190 oC for 30 min in the SBP/Na Cl-DMSO system. Furthermore, the highest furfural yield of 59.52% with the xylose conversion efficiency of 97.09% and the furfural selectivity of 61.30% was obtained from the oven-dried hemicelluloses precipitated at the 30%（v/v） ethanol concentration over（1.0）SSAL（1.0） in the SBP/Na Cl-DMSO system at 190 oC for 30 min.（2） Effect of different mechanical pretreatments（ball milling, ultrasonic, and mixed ball milling of corncob and solid acids） on the catalytic hydrothermal conversion of corncob into furfural was investigated. Results showed that the yields of glucose, xylose, arabinose, acetic acid and furfural were enhanced after the single or combined pretreatments in the presence of（1.0）SSAL（1.0）. The highest yield of carbohydrates in the hydrolysates was obtained after the combined mixed ball milling and ultrasonic pretreatments. Higher reaction temperature was favor of the furfural production, while lower temperature was benefit for the xylose formation. The highest furfural yield（197.76 mg/g） could be achieved at 190 oC for 30 min from corncob pretreated by the hydrothermal treatment and combined mixed ball milling（400 rpm/min, 6 h） and ultrasonic（30 min） pretreatments.In conclusion, clean and efficient approaches for the pretreatment of lignocellulosic biomass were proposed, and the new composite solid acid catalysts with high selectively were developed. A highly efficient heterogeneous catalytic system for the furfural production was brought forward. And the effect of the hemicelluloses structure on the furfural yield was clarified. It was further confirmed that catalysts and the catalytic systems did not affect the impact trends of xylan structure on furfural yields. In comparision with the one-step approach, the two-step process was more beneficial to the recycle ability of solid acid catalysts, the enhancement of the furfural yield and the reusable of furfural residues during the conversion of corncob into furfural.