Effect of Pore Structure on the Nitridation of Mesoporous Silica with Ammonia

Various mesoporous silicas [MCM-41(M41), SBA-15(SBA15), MCM-48(M48)] and silica gel were nitrided with ammonia in a plug-flow reactor at 1273 K. The nitrogen contents of various (oxy)nitrides obtained were increased with the amount of ammonia supplied per sample weight. The nitridation rates of silica samples were not changed with the pore sizes of the samples but with the pore structures. They were dependent on the surface areas of the parent silicas. The maximum contents of nitrogen in nitrided M41s, SBA15, M48, and silica gel were ca. 38, 35, 39, and 21 wt.-%, respectively, which indicates the great difference between the reactivity of the mesoporous silicas and silica gel. Si-29 NMR and X-ray photoelectron spectroscopy also supported the progress in almost complete nitridation of the former samples. The XRD and the FE-SEM measurements revealed that the mesoporous (oxy)nitrides maintained the original 2D hexagonal or 3D cubic mesopore structures and particle morphology. Nitrogen sorption analysis showed that the pore diameters, the surface areas, and the primary mesopore volumes were varied with the nitridation degrees and the types of pore structures but there occurred no destruction of pore structures. Finally, the stabilities of the silicon (oxy)nitrides prepared were studied. The pore structures and nitrogen contents remained essentially unchanged upon storage under a nitrogen atmosphere, immersion in 1-buthanol or toluene, or by heating under H-2 flow at 1073 K. Heating samples under O-2 flow at 1073 K resulted in the collapse of the pore structures and a decrease in nitrogen content.