14 research outputs found

    Highly Efficient Metal Sulfide Catalysts for Selective Dehydrogenation of Isobutane to Isobutene

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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
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