Environmental Engineering Reference
In-Depth Information
corrosive gas and catalyst poison. Recent legislation also emphasizes the need for
reducing the sulfur content in oil-derived fuels and for controlling the emissions of SO 2
in industrial operations. Desulfurization and denitrification of nonrenewable energy
sources will be an effective way for NPS pollution control.
Catalytic Desulfurization and Denitrification. For sulfur-containing
compounds, C 4 H 4 S is particularly difficult to be desulfurized. Therefore, an extensive
effort has been focused on the search for chemical compounds with a high efficiency for
breaking the S-O bonds in SO 2 and the C-S bonds in C 4 H 4 S. Molybdenum compounds
can be useful in desulfurization processes, which are the typically industrial catalysts for
hydrodesulfurization (HDS). Molybdenum sulfide under reaction conditions is partially
transformed into carbide (MoC x S 2-x ) where the HDS reactions occur (Chianelli and
Berhault, 1999). Molybdenum carbide catalysts have shown the potential to replace
sulfided Mo catalysts in HDS reactions because the carbide generally is twice greater
than those of commercial catalysts. For the desulfurization process, the direct
participation of the C sites in the desulfurization process could lead to a much more
complex situation. An important issue is how the desulfurization activity of carbide
varies when going from nanoparticles to bulk surfaces. In practical applications,
nanoparticles of molybdenum carbide are dispersed on catalyst supports, and size effects
could induce special chemical properties that are not seen for the bulk compounds
(Furimsky, 2003).
With recent improvements, the density functional theory (DFT) is capable of
providing qualitative and quantitative insights into surface science and catalysis. A set of
DFT calculations was carried out to systematically study the adsorption properties of
SO 2 and C 4 H 4 S on Mo carbides, including bulk surfaces such as -Mo 2 C(001) and -
MoC(001) as well as the metallocarbohedrene (metcar) Mo 8 C 12 nanoparticle (Liu et al.,
2004). On the nanoparticle, steric repulsion between the C-2 groups and thiophene
overcomes the high reactivity of the Mo atoms in corner or edge sites. The results
illustrate the interplay of ligand and steric effects in nanoparticles of metal compounds.
Pellet-Rostaing et al. (2005) synthesized functional nanomaterials of great
effectiveness for asymmetric catalysis, deep desulfurization from gasoline feed and
separation of ions. Photochemical desulfurization and denitrification of the light oil were
studied based on oil/H 2 O 2 aqueous solution two-phase extraction and photochemical
desulfurization using benzophenone-modified silica gel (SG-BZP) as heterogeneous
triplet photosensitizer (Shiraishi and Hirai, 2003). Dibenzothiophenone (DBT) dissolved
in tetradecane was effectively desulfurized by photoirradiation of the oil/H 2 O 2 aqueous
solution in the presence of SG-BZP. The indirect photoexcitation of DBT with SG-BZP
followed by oxidation of photoexcited compounds by H 2 O 2 proceeded effectively. At
photoirradiation time 60 h, the sulfur content of the light oil decreased from 0.18 to 0.05
wt%. Many desulfurizing processes also are denitrogenating to some extent.
Search WWH ::




Custom Search