Silica hybrid gel catalysts containing Group(VIII) transition metal complexes: Preparation, structural, and catalytic properties in the synthesis of N,N-dimethylformamide and methyl formate from supercritical carbon dioxide

Sol-gel derived hybrid materials containing group(VIII) transition metal complexes in a porous silica network were prepared and tested for the catalytic synthesis of N,N-dimethylformamide (dmf) and methyl formate (mf) from supercritical carbon dioxide, hydrogen, and dimethylamine and methanol/triethylamine, respectively. Bifunctional silylether phosphines X = Ph2P(CH2)(2)Si(OEt)(3) and Y = Me2P(CH2)(2)Si(OEt)3 were used as ligands for the preparation of complexes of the type RuCl2X3, RuCl2Y3, MClX3 (M = Ir, Rh), and MCl2X2 (M = Pt, Pd). The silylether complexes were anchored in a silica matrix by co-condensation with Si(OEt)(4). The textural and structural properties of the hybrid gels were characterized by means of P-31 and Si-29 NMR spectroscopy, extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen and argon physisorption. P-31 NMR spectroscopy confirmed nondestructive immobilization of the Ru, Pt, Pd, Rh, and Ir complexes. TFEM and XRD analyses proved the homogeneity and noncrystallinity of the materials. The degree of condensation of the gels and the molecular mixing of the components has been investigated by solid state Si-29 NMR spectroscopy. Textural characterization showed that all gels were micro- to mesoporous. EXAFS measurements indicated no metal-metal interactions, confirming that the organometallic complexes were immobilized as monomers. From all catalysts silica matrix stabilized ruthenium complexes exhibited highest activities at 100% selectivity in dmf synthesis from CO2, H-2, and dimethylamine. The corresponding turnover frequency (TOF) of 1860 h(-1) exceeded those reported so far for heterogeneous catalysts by a factor of 600. In methyl formate synthesis, TOFs up to 115 h(-1) were reached, (C) 1998 Academic Press.