(186a) Interactions of Sulfur with High Surface Area Carbides and Nitrides

Carbides and nitrides are active for a variety
of reactions including hydrodesulfurization and hydrodenitrogenation.  In
addition, some materials in this class have been reported to be tolerant to
sulfur. The basis for their sulfur tolerance, however, is not well understood.
In this work, we studied the interaction of a variety of organosulfur compounds
with early transition metal carbides and nitrides as part of a more
comprehensive investigation of the sulfur tolerance of these materials.

The materials were prepared from
their corresponding oxide precursors using temperature program reaction
methods. The carburization or nitridation was carried out using either a
mixture containing 15% CH₄ in H₂ or NH₃,
respectively. Interactions of the carbides and nitrides with organosulfur
compounds including thiophene, benzohiophene (BT), dibenzothiophene (DBT), and
tetrahydrothiophene (THT) were investigated. In addition, solvent effects were
examined for benzene, and isooctane. The interactions of carbides and nitrides
with thiophene were investigated at different temperatures. Isotherms showed
that the adsorption of sulfur compounds on Mo₂C and Mo₂N
was Langmuirian, suggesting that the heat of adsorption did not vary
significantly with coverage.  The heats of adsorption for thiophene on Mo₂N
and Mo₂C were 30 and 49 KJ/mol, respectively. Characterization
indicated that the interaction with sulfur was limited to the surface. These
materials showed more affinity to non-aromatic sulfur compounds (THT) as
compared to aromatic sulfur compounds. Additional results regarding competitive
adsorption and surface characterization of the carbides and nitrides will be
described.