Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic–organic hybrids: homogeneity of sol–gel derived hybrid gels

Abstract

The structures of sol–gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC 2H 5) 3, R = CH 3, C 5H 11, C 8H 17 and C 6H 5) mixtures (TEOS : RTES : CH 3CH 2OH : H 2O : HNO 3 = 0.5 : 0.5 : 10 : x : 0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of 29Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q 4 ( Si(OSi) 4) units were successfully simulated by assuming distributed cross relaxation times ( T SiH) and an intrinsic 1H spin-lattice relaxation time in the rotating frame ( T 1 ρ H ); the latter was different from the apparent T 1 ρ H value estimated by assuming single T SiH. The distribution ranges of T SiH for the TEOS–RTES gels broadened as the water content increased, suggesting that the Q 4 units tended to be separated from the T units and that the local concentration of 1H spins around the Q 4 unit tended to decrease.