Role of porosity on silica immobilized palladium complexes as recyclable catalysts in the methoxycarbonylation of 1-hexene

Immobilization of the palladium complex [Pd(L1)Cl2] (Pd1), where L1 = N,N'E,N,N'E)-N,N'-(3-(3-(triethoxysilyl)propyl)pentane-2,4-diylidene)dianiline, on MCM-41 and SBA-15 supports at 100 degrees C and Pd loading of 10% afforded the respective materials [Pd1@MCM-41] (Pd2) and [Pd1@SBA15] (Pd3) in moderate yields. Calcination temperatures of 150 degrees C and 200 degrees C using SBA-15 support gave the corresponding compounds Pd4 and Pd5 respectively, while using MCM-41 support and Pd loadings of 5% afforded compound Pd6. The characterization of the new materials was achieved by using FT-IR for functional group analyses, ICP-OES, TEM and SEM-EDX and XPS for surface morphology, elemental composition, particle size distribution and oxidation status respectively, PXRD for crystallinity and N2 physisorption measurements for surface area and porosity determinations. All the immobilized materials (Pd2-Pd6) formed active catalysts in the methoxycarbonylation of 1-hexene with moderate catalytic activities (TON of 372) to afford mainly linear esters. The catalytic activities of the materials depended on the nature of support and their physical properties and was largely controlled by their porosity. The SBA-15 supported catalysts, which displayed larger pore sizes, were generally more active than those of supported on MCM-41. The supported catalysts were recycled five times with minimum reduction in catalytic activities.