•The competitive adsorption mechanism of benzene/thiophene was proposed.•The competitive adsorption relationships for the whole loadings were revealed.•Intrinsic reasons for the transformation in adsorption mechanism were found.•Three practical factors on thiophene adsorption were analyzed.To explore the whole process of competitive relationship changing with increasing adsorption amount, the competitive adsorption mechanism of benzene/thiophene in siliceous faujasite (FAU) zeolite from infinite dilution to saturation adsorption was analyzed through grand canonical ensemble Monte Carlo simulations for the first time. Results showed that the competitive adsorption mechanism transferred from “ideal-displacement adsorption” to “insertion-displacement adsorption” with an inflection point of 40 molecule/UC, as the total loading amount of benzene and thiophene grew. At “ideal-displacement adsorption” stage, both benzene and thiophene molecules adsorbed ideally on S and W sites. Meanwhile, as the total loading closed to 40 molecule/UC, increasing amount of thiophene on favorable S adsorption sites was displaced by benzene and migrated to W sites. Comparatively, at “insertion-displacement adsorption” stage, benzene molecules continued displacing some thiophene adsorbed on S sites when the total loading increased. The displaced thiophene inserted near the center of the supercage. The transformation in this competitive adsorption mechanism was due to the interaction energy. Besides, higher thiophene concentration, adsorption temperatures and ratios of Si/Al to some extent contributed to the increase in the selectivity for thiophene.Download high-res image (89KB)Download full-size image

•Electronic characteristics determined adsorption characteristics of transition metals.•Cobalt has the similar adsorption ability of thiophene as nickel.•Adsorption capacity of Cr and Mo was extremely fierce, while Cu has the potential ability for adsorbing thiophene.•The preference adsorption site for thiophene was hollow site on all the seven surface.To explore characteristics of active metals for reactive adsorption desulfurization (RADS) technology, the adsorption of thiophene on M (100) (M = Cr, Mo, Co, Ni, Cu, Au, and Ag) surfaces was systematically studied by density functional theory with vdW correction (DFT + D3). We found that, in all case, the most stable molecular adsorption site was the hollow site and adsorptive capabilities of thiophene followed the order: Cr > Mo > Co ≈ Ni > Cu > Au ≈ Ag. By analyzing the nature of binding between thiophene and corresponding metals and the electronic structure of metals, the excessive activities of Cr and Mo were found to have a negative regeneration, the passive activities of Au and Ag were found to have an inactive adsorption for RADS adsorbent alone, while Ni and Co have appropriate characteristics as the active metals for RADS, followed by Cu.Download high-res image (93KB)Download full-size image

The potential mechanism of sulfur-resistant CO methanation was theoretically investigated via density functional theory (DFT + D) calculations. Comparisons were made between modified Co–MoS2 and pure MoS2 catalysts and we highlighted the distinguished CO methanation pathway in the presence of Co-promoter. Multiple intermediates were formed at different catalytic sites during the reaction, which further increased the mechanism complexity. The results obtained from Co–MoS2 imply that the CH3OH species could be formed along the most feasible reaction pathway on Mo catalyst termination; the subsequent dissociation of CH3OH into CH3 and OH was found to be the rate determining step with a reaction barrier of 29.35 kcal mol−1 at 750 K. On the S edge of Co–MoS2, the CH2OH intermediate could be formed as a result of CH2O reacting with adsorbed hydrogen, and subsequent CH2OH dissociation was noted to release CH2. Afterwards, consecutive hydrogenation of CH2 led to the final CH4 yield. On S catalyst termination, it was suggested that the CHO intermediate formation played a key role as the rate-determining step with the reaction barrier of 19.56 kcal mol−1 at 750 K. By comparing the CO methanation energy profiles over different samples, it was discovered that the Co-promoter did possess promoting effects at both the Mo edge and the S edge of the catalyst; note that this enhancement at the Mo edge was superior to that at the S edge, especially for larger scale applications. Moreover, after doping with Co, the OH species was easier to remove in terms of H2O molecules, which created enough vacant active sites for a continuous reaction.

•It was necessary to convert NdCl3·2H2O to NdCl3 after comparing different properties of three complexes.•MEP surface revealed that the molecular reaction site should be located around the Cl atom.•The HOMO and LUMO orbitals found the electron flow in the NdCl3·3C3H8O·2H2O.•The electron redistribution of complexes had been studied in the analysis of mulliken atomic charges.The catalytic activity of NdCl3-ROH-AlR3 is closely related to the amount of water in NdCl3. Usually NdCl3·6H2O has been dehydrated step by step to NdCl3·2H2O, however, the further dehydration process will be very difficult. In this work, we investigated the effect of added water molecules on structures and properties of NdCl3·3C3H8O, NdCl3·3C3H8O·H2O, and NdCl3·3C3H8O·2H2O, including bond length, angle, mulliken atomic charge, molecular electrostatic potential (MEP), HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy, and natural bond orbital (NBO). All properties were calculated by density functional theory (DFT) using B3PW91 functional and basis set of 6-31G++**/SDDALL. It was found that the bond length of NdCl bond and bond angles of Cl3-NdCl2, O9-NdO5, Cl3-NdCl4 could be directly affected due to the addition of water molecule. The MEP analysis revealed that the molecular reaction site should be located around Cl atom. Furthermore, analysis of mulliken atomic charges showed that charge of Nd atom as the active center of the reaction changed from 0.956 to 1.238 and then to 1.301 after addition of one water molecule and two water molecules respectively. HOMO and LUMO orbitals were carried out to investigate the stability of the system, the addition of two water molecules in the system enhanced the electron flow in the system. Also, NBO analysis was further performed to supply an in depth insight into the electronic structure, the distribution of valence electrons and bond orders, which all changed after the addition of water molecule. Therefore, it was necessary to convert NdCl3·2H2O to NdCl3 in order to achieve a higher catalytic activity.Download high-res image (163KB)Download full-size image

•A comprehensive method was proposed to evaluate stability, accessibility, and strength.•LASA was proposed to measure the space accessibility at the atomistic level.•The accessibility depends on the OH orientation and hydrogen bonds.•The strength of the Brønsted sites decreases as bridging OH > nest OH > terminal OH.•Both the pore geometry and hydrogen bonds can enhance the acid strength.Catalytic reactions of zeolites occur mainly at their acid sites; however, there is a lack of comprehensive method to characterize the acid sites by multiple aspects due to their complexity associated with both the host structures and the origins of formation. Here we propose a comprehensive computational approach, from both thermodynamic and kinetic points of view, to evaluate jointly the energetic stability, space accessibility, and acid strength (SAS) critical to the catalytic performance of acid sites in zeolites. In this work, we evaluated the SAS systematically for various potential Brønsted acid sites in zeolites, using density functional methods and surface area calculations. The total energy was used to compare the relative stabilities of various acid sites of the same type. The deprotonation energy (DPE) was calculated to represent the acid strength. Moreover, we proposed a concept of local accessible surface area (LASA) to measure the accessibility of a localized surface region of molecular sites of complex shape. The SAS were evaluated comprehensively for various Brønsted acid sites including bridging hydroxyl (BH), nest hydroxyl (NH), and terminal silanol (TS) in the internal pores and on the external surfaces of MFI-type zeolite. We found the most stable acid sites are the Al2/H2, Al9/H2, and Al1/H1 sites, respectively, for intra-crystalline BH, and BHs on the (0 1 0) and (1 0 0) external surfaces. The most stable NH sites are located at the T4, T7, and T5 defective sites, respectively, in the internal pores, and on the (0 1 0) and (1 0 0) external surfaces. The accessibility of an acid site depends on the OH orientation at the studied acid sites. Most of the NH sites are difficult to access due to the connected hydrogen bonds. The acid strength of the studied Brønsted acid sites decreases as BH > NH > TS. Both the pore geometry and hydrogen bonds can enhance the acid strength. The first-principles computation of the SAS provides a comprehensive approach to characterize acid sites that facilitates evaluation of catalytic performance for the design of zeolite catalysts.Download high-res image (77KB)Download full-size image

An interesting two-stage adsorption mechanism was first proposed for the benzene/HY system by Metropolic Monte Carlo (MMC) simulations at loadings below and above an “inflection point”, and were composed of processes labeled “ideal adsorption” and “insertion adsorption”, respectively. Below the inflection point (from infinite dilution up to 32 molecule/UC for all Si:Al ratios), benzenes were located on the sorption sites inside the supercages with an ideal adsorption geometry configuration, which is in accordance with previous studies. Above the inflection point, the benzene molecule tended to insert into the space between existing adsorbed benzenes, and no obvious rearrangement was observed for previously adsorbed benzenes. It was found that the proposed adsorption mechanism existed independently of the Si:Al ratio, while the inflection point shifted to a higher loading for zeolite with a lower Si:Al ratio. This is due to increased utilization of the 12-T ring caused by the contribution of the H1 site in zeolite with a lower Si:Al ratio, which result in less crowed adsorption at loadings approaching saturation.

In desulfurization-related researchers, thiophene is widely studied in adsorption, separation, and catalysis processes as a typical sulfur-containing compound. However, the adsorption behavior of thiophene for the very first step of all processes still remains ambiguous. In this study, we proposed the loading dependence of the adsorption mechanism of thiophene in siliceous faujasite (FAU) zeolite using Monte Carlo simulations combined with the research of adsorption isotherms, density distributions, concentration profiles, radial distribution functions, and interaction energies. The results revealed that the thiophene adsorption mechanism in the whole loading range could be divided into two parts: “ideal adsorption” and “insertion adsorption”, with the inflection point of the loading at 40 molecules/UC, which was similar to the adsorption of monoaromatics in zeolite. Below the inflection point, adsorbed thiophene distributed broadly and mainly occupied S and W adsorption sites ideally; after the inflection point, newly adsorbed thiophene molecules entered into the space near the center of the supercage with no influence on previously adsorbed ones. As the loading of thiophene increased, the adsorption amount on the S sites went up consistently over the entire loading range. By contrast, the adsorption amount on the W sites grew first and then dropped gradually for loadings below the inflection point. Finally, it decreased noticeably with loading close to saturated adsorption. In addition, the occurrence of the inflection point was due to the change of the dominated interaction energy.

Monte Carlo simulations are performed to study the adsorption of aromatic molecules (toluene, styrene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, and naphthalene) in all-silica faujasite (FAU) zeolite. For monoaromatics, a two-stage “ideal adsorption” and “insertion adsorption” mechanism is found by careful inspection of locations and distributions of the adsorbed toluene molecules. The validity of this mechanism is confirmed for all monoaromatics considered in the current study. Remarkably, the number of C atoms per unit cell corresponding to the inflection point of adsorbate loading (CI-P) is defined as a valid and convenient characterizing factor in the packing efficiency of monoaromatics in the FAU zeolite. For the case of naphthalene, a type of diaromatic, the three-stage mechanism is proposed, which consists of the first two stages and a third stage of “overideal adsorption”. The so-called overideal adsorption is labeled because the naphthalene molecules start to occupy the S site nonideally at loadings that approach saturation, leading to a more localized feature of the adsorbates. The explicit adsorption mechanism can be used to understand the loading dependence of isosteric adsorption heat for the aromatics concerned.Keywords: adsorption; aromatics; FAU zeolite; Monte Carlo simulation;

The detailed hydrogenation processes of 2-ethylhexenal on Pd(111) toward 2-ethylhexanol were investigated by density functional theory (DFT) calculations to understand the hydrogenation mechanism of 2-ethylhexenal. Several adsorption modes of 2-ethylhexenal on Pd(111) were studied. The adsorption of cis-conformers on the Pd(111) surface was found to be more stable than the trans-conformers; however, cis-isomers are less stable in the gas phase. Both E-η3-trans and E-η4-trans modes were used to probe the hydrogenation of 2-ethylhexenal, although the former plays a primary role in hydrogenation reactions. Several plausible reaction pathways were calculated. For E-η3-trans mode, C3 → C2 → O → C1, C3 → C2 → C1 → O, C2 → C3 → O → C1, and C2 → C3 → C1 → O routes are feasible to produce saturated alcohol. However, for the E-η4-trans mode, the formation of the 2-ethylhexanal intermediate, which is a saturated aldehyde, appeared to be easy through both C2 → C3 and C3 → C2 pathways because of low active barriers. Only 2-ethylhexanal generated via the C3 → C2 route was presumed to be available to generate 2-ethylhexanol on E-η4-trans mode. In general, the consecutive hydrogenation reaction of 2-ethylhexenal to 2-ethylhexanal and then to 2-ethylhexanol on the Pd(111) surface determines the whole reaction process and even becomes the rate-limiting step.

Monte Carlo (MC) simulations were performed to study the influence of framework protons on the adsorption sites of the benzene molecule in HY zeolite with different Si:Al ratios. Eleven types of adsorption sites were observed including five reported sites (H1, H2, U4, U4(H1), and W) and six newfound sites (W(2H1), U4(2H1), H1(H2), U4(H1,H1), H1(H2,H1), and U4(H1,H1,H1)), which were “supersites” with more than one proton. The stability order of the sites found in the 28Al model can be expressed as U4(H1,H1,H1) > U4(H1) > H1(H2,H1) > W(2H1) > U4(H1,H1) > H1(H2) > H1 > H2 > U4 > U4(2H1) > W. Increasing number of zeolite protons resulted in an increasing proportion of supersites, which enhanced adsorption energies of sites. For HY zeolite models containing different numbers of protons with the same ratio of H1:H2, the amount of the most stable adsorption sites containing H1 proton increased, while the amount of the most stable adsorption sites containing H2 decreased, with increasing number of protons.

The diffusion of o-, m-, and p-xylene in a FAU zeolite at 300–900 K was investigated using molecular dynamics simulations. Calculated self-diffusion coefficients of xylene isomers showed that the mobility of p-xylene was the fastest, m-xylene the second fastest, and o-xylene the slowest in the FAU zeolite at the same temperature. The diffusion activation energy of o-xylene, m-xylene and p-xylene was, respectively, determined to be 9.04, 7.45 and 6.44 kJ mol−1 within the temperature range of 400 to 900 K, while to be 14.12, 13.59 and 15.47 kJ mol−1 within the temperature range of 300 to 400 K. Xylene density profiles and orientational analysis suggested that this can be attributed to the xylene molecules that diffuse in the FAU zeolite by two different mechanisms at high and low temperatures. The behavior of motion for xylene in the FAU zeolite exhibits a “fluid-like” mode at high temperatures and exhibits a “jump-like” mode at low temperatures.

•The formations of sulfur vacancy at different MoS2 edges were studied.•The influence of sulfur vacancy on DDS and HYD pathways was investigated.•The interaction energies were assessed by dispersion corrected methods (DFT + D).•Sulfur vacancy at Mo-edge was favored to DDS pathway with lower reaction barriers.•A detailed reaction network of thiophene HDS was proposed.The detailed hydrogenation (HYD) and direct desulfurization (DDS) pathways of thiophene over the sulfur vacancy of different MoS2 edge structures were investigated by density functional theory (DFT) calculations. The interaction energies were evaluated by dispersion corrected methods (DFT + D). Innovatively, a detailed thiophene hydrodesulfurization (HDS) reaction network over the sulfur vacancy was proposed, involving most of products which could be detected in the experiments. Taking the influence of sulfur vacancy into consideration, it could be found that the sulfur vacancy at Mo-edge was more beneficial for the formation of intermediates and products contained in DDS pathway by comparing the reaction barriers of DDS and HYD pathways. The HYD reaction pathway, which involved hydrogenation to 2-hydrothiophene followed by hydrogenation to 2,3-dihydrothiophene and 2,5-dihydrothiophene, could proceed with a mild reaction barrier at the S-edge with the creation of sulfur vacancy. The results also showed that butane could be formed at both S and Mo edges with relatively high reaction barriers of 52.60 kcal/mol (Mo-edge, DDS), 53.99 kcal/mol (S-edge, DDS) and 58.07 kcal/mol (Mo-edge, HYD), however, the formations of 1-butene and 2-butene were much more favored with energy barriers of 33.45 kcal/mol (S-edge HYD) and 35.20 kcal/mol (Mo-edge DDS), respectively. These results demonstrated that the sulfur vacancy at the different edges of MoS2 catalysts had a great impact on the overall HDS reaction routes. Based on the systematic calculations, the contribution of sulfur vacancy to the formation of certain intermediates and products was clearly orientated, which provided theoretical guidance for designing highly active catalysts for HDS technology.Download high-res image (154KB)Download full-size image

NiY and KNiY were successfully prepared by impregnation method and characterized by X-ray diffraction (XRD), N2 sorption (BET), scanning electron microscope (SEM), infrared spectrum (IR) and X-ray Photoelectron Spectroscopy (XPS). The competitive adsorption mechanisms of adsorbents were studied by in situ FTIR to explain different desulfurization performance which was evaluated in a miniature fixed-bed flow by gasoline model compounds with 1-hexene or toluene. NiY and KNiY adsorbents showed better desulfurization performance than HY zeolite due to the high selectivity of loaded active metals. Especially, KNiY adsorbent showed its advantages in desulfurization performance with 5 vol% olefins or 5 vol% aromatics involvement. It could be assigned that introduced K cation enhanced dispersion and content of active Ni species on the surface which made Ni species reduce easily. On the other hand, adsorption mechanisms showed that the protonation reactions of thiophene and 1-hexene occurred on the Brönsted acid sites of NiY, which resulted in pore blockage and the coverage of adsorption active centers. By doping K cation on NiY, the amount of the Brönsted acid sites of NiY was decreased and protonation reactions were weaken. Therefore, the negative effects of Brönsted acid sites were reduced.

The diffusion of o-, m-, and p-xylene in a FAU zeolite at 300–900 K was investigated using molecular dynamics simulations. Calculated self-diffusion coefficients of xylene isomers showed that the mobility of p-xylene was the fastest, m-xylene the second fastest, and o-xylene the slowest in the FAU zeolite at the same temperature. The diffusion activation energy of o-xylene, m-xylene and p-xylene was, respectively, determined to be 9.04, 7.45 and 6.44 kJ mol−1 within the temperature range of 400 to 900 K, while to be 14.12, 13.59 and 15.47 kJ mol−1 within the temperature range of 300 to 400 K. Xylene density profiles and orientational analysis suggested that this can be attributed to the xylene molecules that diffuse in the FAU zeolite by two different mechanisms at high and low temperatures. The behavior of motion for xylene in the FAU zeolite exhibits a “fluid-like” mode at high temperatures and exhibits a “jump-like” mode at low temperatures.