Ga₄B₂O₉:a Structure-induced Solid Base Catalyst And Its Catalytic Properties In Knoevenagel Condensation Reaction

Solid-base catalysts play a decisive role in green chemistry by providing an environmentally benign and sustainable route for the syntheses of fine chemicals and chemical intermediates.In this paper,a mullite-type Ga₄B₂O₉ catalyst,as an intrinsic Lewis solid base,has been successfully synthesize through a combination of boric acid melting method and high-temperature solid-phase method.The cell parameters,surface morphology,specific surface area,basic site distribution,and thermal stability of the synthesized samples was characterized by XRD,SEM,TEM,BET,CO₂-TPD and heat treatment.Le Bail fitting to the XRD data showed that no impurity diffraction peak were observed for the as-synthesized Ga₄B₂O₉sample;SEM image showed that the crystalline grains of Ga₄B₂O₉ are clustered-like and are comprised of some compact aggregates of nano-scaled rods.CO₂-TPD results showed that the presence of numerous basic sites and its basic sites belongs to medium-strong base;heat treatment shows that it has excellent structural stability.The synthetic Ga₄B₂O₉ solid base catalyst were assessed using benzaldehyde（BA）and malononitrile（MN）as substrates in Knoevenagel condensation reaction.The operation parameters,including solvent medium,temperature,molar ratio of reactants and catalyst type were investigated in order to obtain an optimal reaction condition.A maximum product yield of 90%was obtained in the optimal conditions for the reaction.The performance of Ga₄B₂O₉ catalyst was compared with other basic oxides and the results shown that Ga₄B₂O₉ has the moderate amount of basic sites,but exhibited the optimal catalytic activity.The kinetic analysis results indicating Knoevenagel condensation reaction over Ga₄B₂O₉ obeys the second-order reaction kinetics,and the final calculated Ea is～61.6 k J/mol.Examining the reusability and stability of Ga₄B₂O₉catalyst by the repetitive usage under the same reaction conditions,the results show that its catalytic activity no any decrease after 10 cycles.The powder XRD pattern of the recovered Ga₄B₂O₉ remained almost the same with the fresh catalyst,confirming an excellent structure stability of Ga₄B₂O₉ in Knoevenagel condensation reaction.A series of benzaldehyde and malononitrile containing substituents on the benzene ring as the reaction substrate for substrate expansion have obtained higher yields and 100%product selectivity,confirming the solid base catalyst Ga₄B₂O₉for this type The Knoevenagel condensation reaction with aromatic aldehydes as substrates has good universality.The CO₂-TPD results showed that the surface of Ga₄B₂O₉ has basic sites,which bind to the acidic probe molecule CO₂.The result of the poisoning method by adding hydrochloric acid to the reaction system can further confirmed the existence of basic sites on the surface;As the temperature rises,there is a phase transition from Ga₄B₂O₉→Ga BO₃→β-Ga₂O₃.By comparing its basic structural unit and its catalytic activity under different structure coordination catalysts,the results confirmed that its basic source is the special Ga O₅ Coordination in Ga₄B₂O₉.Consequently,the relationship between the structure of Ga₄B₂O₉ and its catalytic performance is established and a possible alkaline reaction mechanism is proposed.This study proves that gallium-based solid-phase materials have good structure-performance correlations,which can help researchers to deeply understand the structure-efficiency relationship of catalysts in catalytic reactions,and open a new way for the design of new intrinsic solid-phase catalysts.