Synthesis, characterization, and thiophene desulfurization activity of unsupported γ-Mo2N macrocrystalline catalysts

Macrocrystals of gamma-Mo2N were synthesized by temperature-programmed reaction of macrocrystalline MoO3 and NH3 or N-2/H-2 mixtures, X-ray diffraction analyses indicate macrocrystalline Mo2N is an aggregate of particles in crystallographic alignment with diameters ranging from 4.6 to 18 nm, depending on synthesis conditions. Scanning tunneling microscopy (STM) supports these observed diameters. Based on diffraction data and surface area measurements, it is concluded that the particles in each sample are found with a range of diameters, each flattened in the [200] direction with either an amorphous surface phase or polycrystalline interior. The highest crystal BET surface area achieved is 44 m(2)/g. TGA was used to monitor the temperature-programmed reaction of MoO3 and N-2/H-2 mixtures. The lowest reduction temperatures were observed in syntheses employing high H-2 concentrations and slow temperature ramping rates. The rate of the Mo2N macrocrystal synthesis reaction was observed to be slower than the equivalent powder reaction, which could possibly be attributed to the effects of solid-state diffusion. The specific thiophene hydrodesulfurization (HDS) activity (units of mol/s m(2)) of the Mo2N macrocrystalline catalyst at 673 K was found to be higher than the powder form by a factor of 2.5. Weight-specific HDS activities and reaction product fractions over Mo2N macrocrystal and powder catalysts were roughly the same. An analysis of reaction products over a range of conversions indicates thiophene desulfurizes to form predominantly 1-butene with smaller amounts of other C-4 hydrocarbons also present. (C) 1999 Academic Press.