14 research outputs found
Highly Efficient Metal Sulfide Catalysts for Selective Dehydrogenation of Isobutane to Isobutene
Metal
sulfide catalysts were highly efficient in the activation
of C–H bond for isobutane dehydrogenation, and the dehydrogenation
performance was better than that of the commercial catalysts Cr<sub>2</sub>O<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub> and Pt–Sn/Al<sub>2</sub>O<sub>3</sub>, providing a class of environmentally friendly
and economical alternative catalysts for industrial application
Adsorption and Separation Mechanism of Thiophene/Benzene in MFI Zeolite: A GCMC Study
Selective removal
of thiophene from aromatic components is one
of the key challenges facing the petrochemical industry. The adsorption
and separation mechanism from the molecular viewpoint can guide and
upgrade the relative adsorption based technology. Therefore, we performed
the grand canonical ensemble Monte Carlo (GCMC) simulation to investigate
the adsorption performance and mechanism of competitive adsorption.
Density distribution and radial distribution functions (RDF) analysis
give a more detailed description of the adsorption sites. For pure
component adsorption, donut-shaped adsorption sites were obtained
for both benzene and thiophene from the straight channel point. From
the viewpoint of the zigzag channel, the sorbates follow the straight
line shape distribution at low loading and the S shape distribution
at high loading. As for the binary component adsorption, more benzene
adsorbs in the zeolite than thiophene at low pressure; however, thiophene
competes successfully at high pressure. This can be explained by the
key factor: at low pressure, the size effect plays an important role.
While the pressure increases, the interaction energy dominates the
process. Analyzing RDFs of the binary adsorption, we observed that
when benzene competes with thiophene, the preferential adsorption
sites do not change; however, the emergence possibility of benzene
gets smaller
Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of <i>N</i>‑Octane over HY and HZSM‑5 Zeolites: A DFT Study
Bimolecular and monomolecular
cracking mechanisms of alkanes simultaneously
occur and have a competitive relationship, which strongly influences
the product distribution. In this work, the density functional theory
(DFT) calculation is first carried out to elucidate two cracking mechanisms
in HZSM-5 and HY zeolites. It is found that the overall apparent reaction
barrier for the monomolecular cracking reaction at 750 K in the HZSM-5
zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for
bimolecular cracking reaction, indicating that the monomolecular mechanism
is predominant in the HZSM-5 zeolite. In contrast, the bimolecular
mechanism is predominant in the HY zeolite because of a lower apparent
reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking
reaction than that (24.34 kcal/mol) for the monomolecular cracking
reaction. Moreover, the intrinsic reason for the different mechanisms
is further elucidated. The confinement effect can effectively decrease
the energy barrier when the size of transition states is comparable
to the pore size of zeolite. The insights in this work will be of
great significance to the understanding of confinement on catalytic
cracking mechanism and to the design of highly efficient cracking
catalysts
Multifunctional Two-Stage Riser Catalytic Cracking of Heavy Oil
The continuous deterioration of feedstocks, the increasing
demand
of diesel, and the increasingly strict environmental regulations on
gasoline call for the development of fluid catalytic cracking (FCC)
technology. To increase the feed conversion and the diesel yield as
well as produce low-olefin gasoline, the multifunctional two-stage
riser (MFT) FCC process was proposed. Experiments were carried out
in a pilot-scale riser FCC apparatus. Results show that a higher reaction
temperature is appropriate for heavy cycle oil (HCO) conversion, and
the semispent catalyst can also be used to upgrade light FCC gasoline
(LCG). The synergistic process of cracking HCO and upgrading LCG in
the second-stage riser can significantly enhance the conversion of
HCO while reducing the olefin content of gasoline at less expense
of gasoline yield. Furthermore, the novel structure riser reactor
can increase the conversion of olefins in gasoline. Because of the
significant increase of HCO conversion, the fresh feedstock can be
cracked under mild conditions for producing more diesel without negative
effects on the feed conversion. Compared with the TSR FCC process,
in the MFT FCC process, the increased feed conversion, diesel and
light oil yields can be achieved, at the same time, the olefin content
of gasoline decreased by approximately 17 wt %
Fluid Catalytic Cracking Study of Coker Gas Oil: Effects of Processing Parameters on Sulfur and Nitrogen Distributions
To
investigate the effects of operating conditions and the catalyst
activity on the transfer regularity of sulfur and nitrogen during
the cracking process of coker gas oil (CGO), the CGO was catalytically
cracked in a pilot-scale riser fluid catalytic cracking (FCC) apparatus
at different test environments. Then the cracked liquid products were
analyzed for sulfur and nitrogen distributions with boiling point,
from which the sulfur and nitrogen concentrations of gasoline, light
cycle oil (LCO), and heavy cycle oil (HCO) fractions were determined.
The sulfur and nitrogen compounds in each product cut, and their possible
reaction pathways were reviewed and discussed. The results show that
sulfur-containing species are easier to crack but more difficult to
be removed from the liquid product, while nitrogen compounds are easier
to form coke, then be removed from the liquid product. The sulfur
distribution of CGO is different from that of conventional feedstocks.
Different processing parameters can significantly affect the sulfur
and nitrogen distribution yields and concentrations in liquid products.
Increasing the reaction temperature and the catalyst-to-oil ratio
as well as shortening the residence time cannot only increase the
light oil yield but also improve the product quality and reduce the
SO<sub><i>x</i></sub> and NO<sub><i>x</i></sub> emissions in the regenerator
Promoting Effect of Sulfur Addition on the Catalytic Performance of Ni/MgAl<sub>2</sub>O<sub>4</sub> Catalysts for Isobutane Dehydrogenation
Ni/MgAl<sub>2</sub>O<sub>4</sub> catalysts with high NiO loadings
were highly active for isobutane cracking, which led to abundant formation
of methane, hydrogen and coke. The results of activity testing and
XRD characterization jointly revealed that large ensembles of metallic
nickel species formed during reaction notably catalyzed cracking instead
of dehydrogenation. However, after introduction of sulfur into Ni/MgAl<sub>2</sub>O<sub>4</sub> catalyst through impregnation with ammonium
sulfate, undesired cracking reactions were effectively inhibited,
and the selectivity to isobutene increased remarkably. Totally, up
to ∼42 wt % isobutene could be obtained at 560 °C in a
single pass after the modification. From the characterization results,
it was also concluded that, after sulfur introduction, NiO particles
became much smaller and better dispersed on the catalyst surface.
NiS species, formed during the induction period of the reaction, not
only facilitated isobutene desorption from the catalyst, but also
constituted the active sites for isobutane dehydrogenation. In addition,
due to the appearance of NiS species, Ni/MgAl<sub>2</sub>O<sub>4</sub> catalyst after H<sub>2</sub>S/H<sub>2</sub> sulfuration exhibited
a high initial activity without experiencing an induction period,
further confirming the crucial role that introduced sulfur played
In Situ Upgrading of Light Fluid Catalytic Cracking Naphtha for Minimum Loss
The key to reducing the olefin content
in fluid catalytic cracking (FCC) gasoline is to upgrade the olefin-rich
light FCC naphtha (LCN). To minimize the naphtha loss, several parameters
were investigated in a pilot-scale riser FCC apparatus. The results
indicate that, besides the reaction temperature, the catalyst-to-oil
ratio, and the catalyst type, the boiling range and the olefin content
of LCNs also have significant influence on the upgrading effect. Moreover,
a relatively short residence time is beneficial for efficiently upgrading
LCNs. In addition, the influence of the reactor structure should be
brought to our attention. When a novel structurally changed reactor
with a multinozzle feed system was used, significantly increased olefin
conversion and decreased naphtha loss can be achieved. The calculation
of hydrogen balance indicates that, because of the decrease of dry
gas and coke yields, more hydrogen in the feed can be distributed
into the desired products
Theoretical Investigation of the Reaction of Mn<sup>+</sup> with Ethylene Oxide
The potential energy surfaces of Mn<sup>+</sup> reaction with ethylene oxide in both the septet and quintet states are investigated at the B3LYP/DZVP level of theory. The reaction paths leading to the products of MnO<sup>+</sup>, MnO, MnCH<sub>2</sub><sup>+</sup>, MnCH<sub>3</sub>, and MnH<sup>+</sup> are described in detail. Two types of encounter complexes of Mn<sup>+</sup> with ethylene oxide are formed because of attachments of the metal at different sites of ethylene oxide, i.e., the O atom and the CC bond. Mn<sup>+</sup> would insert into a C–O bond or the C–C bond of ethylene oxide to form two different intermediates prior to forming various products. MnO<sup>+</sup>/MnO and MnH<sup>+</sup> are formed in the C–O activation mechanism, while both C–O and C–C activations account for the MnCH<sub>2</sub><sup>+</sup>/MnCH<sub>3</sub> formation. Products MnO<sup>+</sup>, MnCH<sub>2</sub><sup>+</sup>, and MnH<sup>+</sup> could be formed adiabatically on the quintet surface, while formation of MnO and MnCH<sub>3</sub> is endothermic on the PESs with both spins. In agreement with the experimental observations, the excited state a<sup>5</sup>D is calculated to be more reactive than the ground state a<sup>7</sup>S. This theoretical work sheds new light on the experimental observations and provides fundamental understanding of the reaction mechanism of ethylene oxide with transition metal cations
Structure and Composition Changes of Nitrogen Compounds during the Catalytic Cracking Process and Their Deactivating Effect on Catalysts
The comprehensive
structure and composition changes of the nitrogen
compounds during the catalytic cracking processes of coker gas oil
and vacuum residue are investigated using electrospray ionization
combined with Fourier transform ion cyclotron resonance mass spectrometry.
These experiments were conducted over different cracking materials
under the reaction temperatures of 500/520 °C, the weight hourly
space velocity of 18 h<sup>–1</sup>, and the catalyst/oil ratio
of 5. The results show that the diffusion resistance in the micropores
of the zeolite is the key factor affecting the interaction between
the nitrogen compounds and the acid sites. The basic N1 and N2 class
species with double bond equivalence (DBE) values smaller than 10
can easily diffuse into the micropores of the zeolite and are preferentially
adsorbed onto the acid sites. These adsorbed nitrogen compounds generally
conduct condensation reactions and hydrogen transfer reactions to
form coke deposited on the cracking catalysts. The basic N1 and N2
class species with DBE values larger than 10, other basic nitrogen
compounds other than N1 and N2, and the non-basic nitrogen compounds
seldom interact with the acid sites of the zeolite. They usually undergo
side chain thermal cracking on the surface of the matrix, which can
reduce their carbon numbers but cannot change their DBE values. The
basic N1 class species with DBE values smaller than 10 are the main
compounds that poison the cracking catalysts. In comparison to the
SL-CGO catalytic cracking, the nitrogen-poisoning effect on the catalysts
is much less during the SL-VR catalytic cracking process because the
main poisoning compounds (the basic N1 class species with DBE values
smaller than 10) are much fewer