Investigation Of Functionalized Metal-Organic Frameworks PTA@MIL-101-NH₂-Cl Applied To Selective Conversion Of Cellulose Into Glucose

Plant fiber is considered as the renewable biomass raw material with developing prospects. As the main component of plant fiber, cellulose hydrolyzes to glucose and then glucose transforms into important energy materials and basic platform chemicals. The conversion of cellulose into glucose is a necessary process in plant fiber utilization. The catalyst is needed to accelerate the reaction rate of cellulose hydrolysis because of its high activation energy. The reported catalysts show poor catalytic selectivity leading to the wide range of hydrolysates and low glucose selectivity. Moreover, the high leaching rate of the reported catalysts reduces the reusability. Therefore, for selective conversion of cellulose into glucose, it is meaningful to design a catalyst which is capable of promoting the selective transformation of cellulose and maintaining the acidity.In this paper, a catalyst denoted as PTA@MIL-101-NH₂-Cl in which PTA served as acid catalysis sites and metal-orgainc framworks grafted both amino groups and chlorine groups served as the carrier was designed according to the demand for selective conversion of cellulose. The influence of synthesis condition on crystal structure of the catalyst was studied to obtain the catalyst with stable MIL-101-type structure. The relationship between reaction conditions and catalytic performace was discussed. The fuctions and mechanism of amino groups as acidity immobilization sites and chlorine groups as cellulose adsorption sites were investigated in cellulose hydrolysis. After investigation, the conclusions were drawn as below:Because of its strong organic ligand dissolving ability and the formation of Cl-H-DMF, DMF promoted the synthesis of PTA@MIL-101-NH₂-Cl with stable MIL-101-type crystal structure. Metal cation concentration kinetically controlled the synthesis process and low metal cation concentration was beneficial to obtain MIL-101 crystals. Extending reation time helped to mature the MIL-101 crystals and led to the transformation of MIL-101 crystals into MIL-53 crystals in the same time.Increasing reaction temperature, time and catalyst dosage fastened both the cellulose hydrolysis and glucose degradation. The increase of the reactant concentration resulted in the rise of cellulose density and the contact frequency between cellulose and the catalyst while was unbeneficial to the cellulose conversion from the prospect of reaction equilibrium.PTA remained its complete Keggin structure in synthesis process. The acidity of PTA decreased caused by the double electrostatic interaction between PTA and amino groups as well as positively charged surface of MIL-101-NH₂-Cl. This electrostatic intereaction immobilized PTA and effectively reduced the PTA leaching rate. The PTA stability enhanced with the increase of amino groups.The catalyst selectively adsorbed cellulose and weakened the hydrogen bond of cellulose by forming hydrogen bond between chlorine groups and hydroxyls from cellulose. As a result, cellulose selectively hydrolyzed to glucose by preventing the glulcose degration. The conversion efficiency increased by reducing the cellulose difficulty in hydrolysis.