Design Of Novel Heteropolyacids Catalyst In Target Catalytic Transformation Of Saccharides

Polyoxometalate, POMs, also called as heteropolyacid, is a kind of strong Br(?)nsted acid catalyst. In the past, the research always concentrates on the Br(?)nsted acid catalyst. The construction of HPAs catalyst with multiple centers which can meet the requirements of different acid catalytic reactions, especially for the requirements of cascade reactions, is always the focus of concern.Hydrolysis reaction of cellulose is a typical cascade reaction. In traditional technology, liquid acid (such as H2SO4 and HC1) is usually used as the catalyst to prepare glucose from cellulose in one-step hydrolysis. Fructose can be obtained from glucose through isomerization with Lewis acids or Lewis bases,5-hydroxymethylfurfural (5-HMF) can be prepared with fructose through dehydration, and levulinic acid (LA) can be obtained from HMF through hydration. Considering the problem that one-step preparation of chemicals using cellulose in one-pot method needs multi-reactive catalytic sites, HPA catalysts with multiple centers were designed and synthesized in this paper, and the type of catalytic center, acid strength and the influence of reaction conditions on the selectivity of target product as well as the conversion rate of cellulose were explored based on the directional hydrolysis reaction from cellulose to target product, so as to obtain the active HPAs which can control the oriented conversion of cellulose. The specific research contents are as follows:(1) The acid strength of HPA HeP2W18O62 with Dawson structure was investigated, and its catalytic activity was tested by the hydrolysis reaction of cellulose. It is shown that H6P2W18O62 is a six-proton acid, of which the acidity is higher than that of H3PW12O40. H6P2W18O62 shows higher activity than H3PW12O40. The reasonsare as follows:on the one hand,H6P2W18O62 has higher content of proton; on the other hand, it has certain oxidability. However, H6P2W18O62 is a homogeneous catalyst, causing some difficulties when separated. The experimental results show that when H6P2W18O64 was selected as the catalyst for the hydrolysis reaction of cellulose, the reaction temperature was 160 ℃ and the reaction time was 7 h, the conversion rate of cellulose was 94.0% and the yield of glucose was 67.6%.(2) Based on the combination of long-chain quaternary ammonium salthexadecyl trimethyl ammonium bromide (CTAB) as hydrophobic groups and HPA H6P2W18O64, a series of micelle-type nanocatalysts(CTA)nH6-nP2W18O62 were designed and synthesized. The catalysts above could solve the separation problem between catalysts and products, and furthermore improve the selectivity of glucose. The results show that the conversion rate of cellulose was 87.2% and the yield of glucose was 69.1% when the catalysts were used in the hydrolysis reaction of cellulose at 160 ℃ for 9 h. It indicated that the selectivity of glucose was improved to some extent. As for the alcoholysis reaction of cellulose (0.1 g) at 160 ℃ for 7 h, the yield of methyl levulinate was 58.5% when the content of catalysts was 0.07 mmol. The catalyst can be reused for 5 times with desirable activity.(3) Based on the combination of ionic liquid choline chloride (ChCl) and HPA H5AlW12O40 with Keggin structure, a new temperature-controlled intelligent catalyst was designed and synthesized. This catalyst can transform gradually from solid to liquid under high temperature to facilitate homogeneous reaction, solving the problem of mass transfer in solid-solid reaction. With gradual decreasing of temperature, the catalyst transforms from liquid to solid and precipitates from the reaction system, realizing the separation between the catalyst and the products. The activities of the catalyst in water and double solvents (MIBK/H2O) were investigated. It shows that the reaction conditions for the hydrolysis reaction of cellulose with glucose as the principal product were as follows:0.1 g cellulose and 0.08 mmol catalyst were added into 7 mL deionized water, and the reaction temperature and the reaction time were 140 ℃ and 3 h, respectively, then the conversion rate of cellulose was 94.6% and the yield of glucose was 75.9%; while the reaction conditions for the hydrolysis reaction of cellulose with LA as the principal product were as follows:0.1 g cellulose and 0.08 mmol catalyst were added into 7 mL solvent (MIBK:H2O = 10:1), then the conversion rate of cellulose was 98.9% and the yield of LA was 74.8%. ChH4AlW12O40, as the catalyst ofthe hydrolysis reaction of cellulose, still had relatively high catalytic activity after used for 6 times.(4) Because ChCl substitutes a H+from H5AIW12O40, the Br(?)nsted acidity of the catalyst is decreased, influencing the catalytic activity of the catalyst in the reaction of cellulose. Lewis center Ce3+ was introduced to increase the tatol acid of the catalyst, and Lewis acid was used to assist Br(?)nsted acid so as to catalyze the hydrolysis reaction of cellulose. Base on this design concept, biomimetic catalyst Ce [(DS)H4AlW12O40]3 was designed and synthesized.-OSO3C12H25 can adsorb cellulose, Ce3+ and H5AIW12O40 facilitate the bond breaking of cellulose, similar to the catalysis principles of cellulose. The research shows that 0.1 g cellulose and 0.06 mmol catalyst dissolved in 7 mL H2O reacted at 150 ℃ for 5 h, then the conversion rate of cellulose was 76.7% and the yield of 5-HMF was 43.0%.(5) Basic groups can prompt the isomerization from glucose to fructose, and Br(?)nsted acidity can facilitate the further dehydration of fructose to generate 5-HMF and LA. [MgO/PMMA]/[H3PW12O40/PMMA] with acid center and basic center was designed and prepared. The catalytic activity for hydrolyzing glucose was explored. The research shows that when10 wt% glucose was hydrolyzed with 120 mg catalyst at 140 ℃ for 6 h, the conversion rate of glucose was 93.2% and the yield of LA was 65.8%. The catalysts can be used for 5 times with relatively high catalytic activity.