The Catalyzed Hydrolysis And Reaction Kinetics Of Cellulose In Subcritical Water

Biomass is a renewable resource, and it has been focused on as an alternative energy for fossil fuels. Therefore, technologies that can convert biomass to valuable liquid fuels and chemicals will be important for solving our energy and environmental problems. Cellulose is an important biomass consisting of hundreds glucose molecules combined withβ-1,4-glycosidic bound and its high degree of crystallinity makes it difficult to hydrolysis. Subcritical water can act as an acid or base catalyst which makes it has a unique advantage to convert the biomass. Cellulose can be hydrolyzed in subcritical water and converted to useful chemicals by using the excellent solubility and acid catalytic function. Although the conversion of cellulose reaches a high value, the glucose yields still show relatively low values, comparing to those obtained by acid-catalyzed processes. In order to raise the yield of glucose, some suitable catalysts that can enhance the hydrolysis of cellulose or inhibit the decomposition of glucose are needed. Focusing on the promotion of the glucose yield, following studies are done in this paper.1. The hydrolysis kinetics and process of microcrystalline cellulose in subcritical water at the temperature ranging from 260℃to 320℃are investigated. The results show that the main products of the microcrystalline cellulose hydrolysis are acids, oligosaccharides (cellobiose et al), glucose, fructose, pyruvaldehyde, glyceraldehyde, dihydroxyacetone, 1,6-anhydroglucose and 5-HMF. Temperature and time are the main factors that affect the cellulose hydrolysis and glucose degradation. The maximum yield of glucose 14.3% is reached at 280℃and 60s. The microcrystalline cellulose hydrolysis kinetics follows a consecutive, homogeneous and first-order reaction model. The reaction rate constants k for cellulose hydrolysis at 260℃and 280℃are 0.00189s-1 and 0.00845s-1. The reaction rate constants k1 for glucose formation and k2 for glucose decomposition at 260³00℃are 0.00098s-1, 0.00661s-1, 0.01638s-1 and 0.00618s-1, 0.02889s-1, 0.14876s-1 respectively. The activation energy for the formation and decomposition of glucose are 176.02kJ/mol and 198.81kJ/mol respectively. And the pre-exponential factors are 2.19×1014 and 1.98×1017 respectively. XRD analysis shows that the residues of hydrolysis are still the cellulose ? crystal.2. The effects of metal chlorides (ZnCl₂, FeCl₃, CuCl₂ and AlCl₃) on the hydrolysis of microcrystalline cellulose in the subcritical water are investigated. The results show that metal chlorides can accelerate the hydrolysis of cellulose and the decomposition of glucose simultaneously. The influence of these four kinds of metalchlorides on k of the order of CuCl₂233, on k1of the order of ZnCl₂323, and on k2 of the order of AlCl₃223. AlCl₃ existence can raise the glucose yield due to the higher k and k1 than k2. The maximum yield of glucose 46.05% is obtained at 260℃, 120s. The glucose yields of cellulose hydrolysis with ZnCl₂, CuCl₂ and FeCl₃ are lower due to the higher k2 than k1. XRD analysis shows that metal chlorides do not change the type of cellulose crystal. The residues of hydrolysis are still the cellulose ? crystal.3. The hydrolysis kinetics of microcrystalline cellulose in subcritical water with AlCl₃ at the temperature ranging from 220℃to 260℃are investigated. The experimental results show the hydrolysis of cellulose in subcritical water with AlCl₃ follows a parallel, consecutive and first-order reaction kinetics model. The reaction rate constants k for cellulose hydrolysis at 220²60℃are 0.00099s-1, 0.00386s-1, 0.00628s-1, 0.01314s-1. The reaction rate constants k1 for glucose formation and k2 for glucose decomposition at 220²60℃are 0.00083s-1, 0.00306s-1, 0.00545s-1, 0.00999s-1 and 0.00359s-1, 0.00601s-1, 0.00638s-1, 0.00794s-1 respectively. The activation energy Ea for the hydrolysis of cellulose, Ea1 for the glucose formation and Ea2 for the glucose decomposition are 138.66kJ/mol, 135.44kJ/mol and 42.22kJ/mol respectively. The pre-exponential factors are 4.86×1011,1.86×1011 and 1.1×102. XRD analysis shows that AlCl3 destroys the hydrogen bonds of cellulose significantly, but do not change the type of cellulose crystal. The residues of hydrolysis are still the cellulose ? crystal.4. The hydrolysis kinetics and process of peanut shells in subcritical water with AlCl3 at the temperature ranging from 180℃to 240℃are investigated. The experimental results show that AlCl₃ can significantly accelerate the peanut shells hydrolysis, and raise the reducing sugar yield. The maximum yield of reducing sugar can be reached 40.4% under the conditions: 220℃, 180s, and 0.02% AlCl₃. The hydrolysis of peanut shells in subcritical water with AlCl3 follows a consecutive and first-order reaction kinetics model. The reaction rate constants k1 for the formation and k2 for the decomposition of reducing sugar are 0.00164s-1, 0.00427s-1, 0.00636s-1, 0.00824s-1 and 0.00329s-1, 0.00601s-1, 0.0051s-1, 0.00669s-1 respectively. The activation energy Ea1 for the formation and Ea2 for the decomposition of reducing sugar is 135.44kJ/mol and 42.22kJ/mol respectively. The pre-exponential factors are 2.89×103 and 1.26. XRD analysis shows that AlCl3 does not change the type of cellulose crystal at 220℃. The residues of hydrolysis are still the cellulose ? crystal.