Influence Parameters on Liquid Phase Reaction Preparing Terephthalic Acid from P- Xylene over Metal Complexes-MCM-41 Catalysts

摘要

The ordered mesoporous silica MCM-41 was anchored with metal-Shiff base complexes to produce metal- Schiff base- MCM-41 catalyst (Mn-Sal-APTES-MCM-41 and Co-Sal-APTES-MCM-41). The manganese or colbalt-Schiff base was synthesized from salicylaldehyde (Sal), (3-aminopropyl)triethoxysilane (APTES) in ethanol and manganese(II) acetate or cobalt (II) acetate. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), nitrogen adsorption – desorption analyses confirm the retaining of support properties after anchoring. The incorporation of Schiff base complex into the matrix causes a decreasing in the mesoscopic order, the volume of adsorbed nitrogen, the pore volume, the specific surface area, the pore diameter and a increasing in the wall thickness. The DTA-TGA and the FT-IR revealed the incorporation of the manganese or cobalt-Shiff base complex within the pores of MCM-41. On these catalysts, the liquid-phase oxidation reactions of p-xylene to terephthalic acid with hydrogen peroxide over obtained Shiff base complexes showed that distribution of p-xylene oxidation catalyzed by manganese - cobalt complexes (8:2) greatly differs from that by cobalt or manganese complex. Especially, cobalt-manganese complexes catalysts show higher activity and selectivity than neat metal acetate salts for the oxidation reaction of the methyl group under mild condition. At the reaction conditions of heterogeneous catalyst: p-xylene: H₂O₂: solvent molar ratio = 1:9:5 (mol/mol/mol); heterogeneous catalyst Mn-Co (8: 2); p-xylene: catalyst weight = 1:5 (mol/g); Me: Br molar ratio = 1: 6 (mol/mol); reaction temperature: 100°C; drop speed of H₂O₂ = 25 μL/min; reaction time: 72 hours.