Density functional calculations have been performed on initial copper deposition and its growth over ZnO nonpolar (101̅0) and (112̅0) surfaces. On (101̅0), our results demonstrate that copper atoms first interact with dangling bonds or bonding orbital above ZnO. As the deposition increases, they form a zigzag structure along U-shaped gaps, then the surface layers fluctuate, and finally they develop into three-dimensional clusters on the surface. On (112̅0), we show that copper atoms also initially interact with dangling bonds. As the deposition increases, however, they form strips in the U-shaped gaps, then surface layers, and finally two-dimensional cages. In either case, our results fit well with experimental observations. Our results also indicate that the bonding of copper with ZnO substrate and that among adsorbed copper atoms are competitive, in the sense that copper (3d104s1) has limited bonding ability. Further, our results indicate that some copper atoms that strongly interact with ZnO become positively charged.

Density-functional theory together with large surface clusters is applied to study elementary processes of the catalytic sulfidation of the MoO3(010) surface. For all sites, surface oxygen is found to bind more strongly with its substrate environment than the corresponding sulfur substitute with binding distances that are shorter for oxygen than for sulfur. Sulfur−oxygen exchange reactions are energetically preferred over sulfur adsorption at MoO3(010). The first and second sulfur substitution takes place preferentially at the terminal oxygen site O(1) where the two steps are energetically similar. Further, sulfur binding is found to be facilitated by the existence of surface oxygen vacancies where sulfur substitution takes place preferentially at the terminal oxygen sites O(1) and O(1)′. On the basis of the theoretical results, different sulfidation schemes are considered. They indicate that sulfidation of the MoO3 surface is facilitated by hydrogen participating in the reaction.

•Crystal size of H-ZSM-5 is regulated by adding proper amount of silicalite-1 seeds.•Zn state and catalytic performance of Zn/H-ZSM-5 are related to the crystal size.•Overall acidity of H-ZSM-5 and Zn/H-ZSM-5 is less influenced by the crystal size.•Linear correlation lies between the amount of ZnOH+ and selectivity to aromatics.•Small crystal Zn/H-ZSM-5 with more ZnOH+ exhibits high selectivity to aromatics.H-ZSM-5 zeolites with a uniform crystal size from 0.25 to 2 μm were obtained by adding colloidal silicalite-1 seed in the synthesis gel; with H-ZSM-5 as the support, Zn/H-ZSM-5 was prepared by incipient wet impregnation. The influence of crystal size on the state of Zn species and its relation to the catalytic performance of Zn/H-ZSM-5 in the conversion of methanol to aromatics (MTA) was then investigated. The results illustrated that the state of Zn species and catalytic performance of Zn/H-ZSM-5 are closely related to the crystal size, though the crystal size has little influence on the overall acidity. There exist mainly two types of zinc species, viz., ZnO and ZnOH+; Zn/H-ZSM-5 with smaller crystal size is provided with more ZnOH+ species. The selectivity to aromatics and catalyst stability can be improved greatly by using small crystal Zn/H-ZSM-5. A good linear correlation is observed between the amount of ZnOH+ species and the selectivity to aromatics, suggesting that ZnOH+ species plays an important role in enhancing the dehydrogenation of alkanes and aromatization of alkenes to aromatics. As a result, small crystal Zn/H-ZSM-5 with large portion of ZnOH+ species exhibits high selectivity to aromatics and long lifetime in MTA.The state of Zn species and catalytic performance of Zn/H-ZSM-5 are closely related to the crystal size; small crystal Zn/H-ZSM-5 with large portion of ZnOH+ species exhibits high selectivity to aromatics in MTA.Download high-res image (291KB)Download full-size image

•Pilot scale reverse flow reactor demonstrated for ventilation air methane mitigation•A kind of non-noble metal catalyst was used and tested in the reverse flow reactor.•Hot gas withdrawal was used to recover part of heat from methane oxidation.•Reactor control and heat recovery schemes proposed previously were verified.•Switch time affects heat recovery efficiency; hot gas removed impacts reactor stability.The mitigation and utilization of ventilation air methane was demonstrated in a pilot scale catalytic reverse flow reactor. A kind of non-noble metal oxide catalyst of 1.8 m3 was loaded and lean methane with a concentration of 0.2–1.0 vol% and a maximum feed flow rate of 800 m3/h was processed. The schemes of reactor control and heat recovery, viz., a simple logic-based controller plus hot gas withdrawal from reactor center, as proposed previously by simulation, were verified in this pilot scale reactor. The results prove that the autoregulative time to switch the gas flow direction will drop quickly to zero if a large amount of hot gas is withdrawn from the reactor by using the traditional method. The switching time has a great influence on the heat recovery efficiency, whereas the amount of hot gas removed out of the reactor impacts significantly on the reactor stability. All these experimental observations are in line with the simulation results. The long term operation proves the feasibility of hot gas withdrawal with a heat recovery efficiency of about 56% and the reliable performances of the non-noble metal catalyst in lean methane oxidation with a methane conversion over 90%. These results prove that the catalytic reverse flow reactor and control schemes used in this work are quite effective in the mitigation and utilization of lean methane.A pilot scale catalytic reverse flow reactor was demonstrated for the mitigation and utilization ventilation air methane; the schemes of reactor control and heat recovery proposed previously by simulation were verified.Download high-res image (300KB)Download full-size image

Nowadays, it is widely recognized that the performance of many catalytic materials is closely related to their particle size; however, it is still a great challenge to produce metal particles on a sub-nanoscale at low cost and in a facile way. In this work, uniformly dispersed palladium sub-nanoclusters were controllably decorated on reduced graphene oxide (Pd/rGO) by a facile modified impregnation method without using any protecting agents; a mean palladium cluster size of as low as 0.7–0.9 nm can be achieved by impregnation with PdCl2 as the precursor, ethanol or acetone as solvent and calcination in hydrogen at low temperature. The Pd/rGO catalyst exhibits high activity in the aerobic oxidation of benzyl alcohol, with almost a complete alcohol conversion and selectivity of 100% to benzaldehyde at 60 °C; moreover, it also displays much higher stability against deactivation from the aggregation of Pd particles than the Pd catalyst with active carbon as the support. The superior performance of the Pd/rGO catalyst can be ascribed to the small size and high valence of Pd sub-nanoclusters and the coordination of chloride ions, as well as the strong interaction between Pd species and the rGO support, as demonstrated by various characterization measures. These results help to clarify the relationship between the preparation, structure and performance of the supported Pd/rGO catalyst in the selective oxidation of alcohols, which brings forward an effective approach to obtain metal sub-nanoclusters on rGO with superior catalytic performance.

As the conversion of methanol to olefins (MTO) over a zeolite catalyst is conducted on acid sites derived from framework aluminum (AlF), it is possible to enhance the catalytic performance by altering the siting of AlF if one knows the catalytic behavior of specified AlF located at certain sites. In this work, two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were synthesized with silica sol and tetraethyl orthosilicate, respectively, as the silicon source. Both series of H-ZSM-5 zeolites exhibit similar acidity, morphology, and textual properties. However, they are quite different with respect to AlF siting, as determined by UV–vis–DRS of Co(II) ions and 27Al MAS NMR; AlF of S-HZ-m is enriched in the sinusoidal and straight channels, whereas AlF of T-HZ-m is concentrated in the channel intersections. When they are used as the catalyst in MTO, T-HZ-m gives higher selectivity to ethene and aromatics and a larger hydrogen transfer index (HTI) than S-HZ-m, whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. Moreover, the 13C/12C-methanol-switching experiments indicate that the incorporation of 12C into pentamethylbenzene and hexamethylbenzene is faster on T-HZ-m, whereas the scramble of 12C for C3–C5 olefins is speedier on S-HZ-m. All of these illustrate that AlF in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of the aromatic-based cycle, whereas AlF in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. These results help to clarify the catalytic behavior of given framework acid sites of H-ZSM-5 in MTO and then bring forward an effective approach to improving the catalytic performance by regulating the framework aluminum siting.Keywords: acid site; alkene-based cycle; aromatic-based cycle; framework aluminum siting; H-ZSM-5; methanol to olefins

As the process of methanol to hydrocarbons (MTH) is catalyzed by acid sites, the regulation of framework aluminum siting and acid distribution in a zeolite catalyst to enhance its performance in MTH is an important and challenging task. In this work, the regulation of framework aluminum siting in H-MCM-22 was achieved through boron incorporation; the relation between catalytic performance and acid distribution was investigated. The results illustrate that the distribution of framework aluminum and Brönsted acid sites among three types of pores in H-MCM-22 can be regulated through adjusting the content of boron incorporated during synthesis, due to the competitive occupancy of various framework T sites between boron and aluminum, whereas the textural properties and overall acid types and amounts are less influenced by boron incorporation. Incorporating a proper content of boron can concentrate the Brönsted acid sites in the sinusoidal channels rather than in the surface pockets and supercages. The acid sites located in the surface pockets and supercages are prone to carbonaceous deposition, whereas those acid sites in the sinusoidal channels are crucial for MTH in the steady reaction stage. Moreover, the acid sites in the sinusoidal channels are favorable to the olefin-based cycle that produces preferentially higher olefins. As a result, the incorporation of proper content of boron delivers the H-MCM-22 zeolite much greater stability and higher selectivity to higher olefins such as propene and butene in MTH than previously reported. These results help to clarify the relation between the catalytic performance of H-MCM-22 in MTH and its acid distribution and then bring forward an effective approach to develop better MTH catalysts by regulating the acid distribution.Keywords: acid distribution; aluminum siting; boron incorporation; catalytic stability; H-MCM-22; methanol to hydrocarbons

Graphene oxide (GO) prepared by a modified Hummers' method exhibits excellent catalytic performance in the synthesis of polyoxymethylene dimethyl ethers (PODEn) from methanol (MeOH) and trioxymethylene (TOM), owing to a synergy between the sulfonic groups and the hydroxyl and carboxyl groups present on the surface of GO with a unique layered structure.

•The (1 0 0), (0 0 1) (0 1 0), (1 1 0) and (1 1 1) surfaces can be fully oxidized by O2.•With increasing O coverage, the portion of the (1 1 1) surface increased largest.•CO desorption is more favorable at saturated oxygen coverage than low coverage.•O diffusion to carbon vacancy is always favorable for the (0 1 1) and (1 0 1) surfaces.Oxygen chemisorption on β–Mo2C surfaces, the subsequent CO/CO2 desorption and oxygen diffusion to the carbon vacancy have been investigated by density-functional theory. The most stable structures together with the energetics of oxygen stepwise adsorption, CO/CO2 desorption and oxygen diffusion to the carbon vacancy were identified. We examined the effect of oxygen coverage on the morphology of β–Mo2C by plotting the equilibrium crystal shape. Thermodynamic effect of temperature and reactant or product pressure on the CO/CO2 desorption were investigated. The CO/CO2 desorption is more favorable at the saturated oxygen coverage than the low oxygen coverage thermodynamically. The subsequent oxygen diffusion to the carbon vacancy after CO/CO2 desorption may happen depending on the surfaces and oxygen coverage.

H-MCM-22 zeolite is a potential catalyst for the conversion of methanol to olefins (MTO). Previous studies indicated that three types of pores in H-MCM-22, viz., the supercages, sinusoidal channels, and pockets, are different in their catalytic action; however, the evolution of aromatic species in the supercages and its effect on MTO are still highly controversial. In this work, density functional theory considering dispersive interactions (DFT-D) was used to investigate the evolution of aromatic species including their formation, reactivity, and deactivation behavior in the supercages; the active role of the supercages in catalyzing MTO was elucidated. The results demonstrated that benzene can be generated in the supercages through aromatization of light olefins; after that, polymethylbenzenes (polyMBs) are formed through methylations, in competition with the construction of naphthalenic species. Both polyMBs (e.g., hexamethylbenzene) and polymethylnaphthalenes (polyMNs, e.g. dimethylnaphthalene) exhibit high reactivity as the hydrocarbon pool species in forming light olefins. Owing to the appropriate electrostatic stabilization and space confinement effects, naphthalenic species in the supercages are inclined to serve as the active intermediates to produce light olefins rather than act as the coke precursors in the initial period of MTO; as a result, the supercages contribute actively to the initial activity of H-MCM-22 in MTO, though they may be prone to deactivation in the later reaction stage in comparison with the sinusoidal channels. The insights shown in this work help to clarify the evolution of aromatic species and the active role of the supercages in MTO over H-MCM-22, which is of benefit to the development of better MTO catalysts and reaction processes.

The catalytic performance of HZSM-5 zeolite in the synthesis of polyoxymethylene dimethyl ethers (PODEn) from dimethoxymethane (DMM) and trioxymethylene (TOM) is closely related to its Si/Al ratio; HZSM-5 with a high Si/Al ratio exhibits high PODE2–8 yield and excellent stability and reusability.

H-MCM-22 zeolite bears three types of pores, supercages, sinusoidal channels, and pockets, and exhibits excellent catalytic performance in the process of methanol to olefins (MTO); however, the catalytic role that each type plays in MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the contributions of various pores in H-MCM-22 to MTO. The results demonstrated that these three types of pores are different in their catalytic action on MTO, because of the large differences in pore size and shape that determine the space confinement and electrostatic stabilization effects. The formation of propene is predicted to take place in the supercages, where propene can be effectively produced through both polyMB and alkene cycles, with a relatively low free energy barrier as well as low enthalpy barrier and entropy loss for the rate-determining steps. In the sinusoidal channels, the free energy barrier of the methylation and cracking steps is elevated due to the space confinement and the reactivity of alkenes is also markedly depressed in the narrow channels, in comparison with those in the supercages; as a result, the contribution of the sinusoidal channels to the entire propene formation is minor. Meanwhile, the pockets are probably detrimental to MTO, as certain large intermediates such as 1,1,2,6-tetramethyl-4-isopropylbenzenium cations are easily formed in the pockets but are difficult to decompose due to the lack of an electrostatic stabilization effect from the zeolite framework, which elevates the total free energy barrier and may lead to a rapid deactivation of these active sites. In comparison with the difference in pore size and structure, the difference of various pores in the acid strength of the active sites exhibits an insignificant effect on their catalytic behaviors in MTO. The theoretical insights in this work are conducive to a subsequent investigation on the MTO mechanism and the development of better MTO catalysts and reaction processes.Keywords: alkene cycle; density functional theory; H-MCM-22; methanol-to-olefins; pockets; polyMB cycle; sinusoidal channels; supercages

Au–Pd nanoparticles supported on graphene–carbon nanotube hybrid exhibit high catalytic activity in selective oxidation of methanol to methyl formate at low temperature, owning to the strong interaction between graphene and Au–Pd as well as the spacing and bridging effect of nanotube inserter on the hybrid three-dimensional structure.

Polymethylbenzene (polyMB) and alkene cycles are considered as two main routes forming light olefins in the process of methanol to olefins (MTO); however, the contribution that each cycle makes to MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the catalytic roles that the polyMB and the alkene cycles may play in forming ethene and propene from methanol in MTO over H-ZSM-5. The results demonstrated that ethene and propene can be produced in nearly the same probability via the polyMB cycle, as they have a very close free energy height as well as a similar free energy barrier for the rate-determining steps. Via the alkene cycle, however, propene is the dominant product, because the methylation and cracking steps to get propene have a much lower free energy barrier in comparison with those to form ethene. As a result, ethene is predominantly formed via the polyMB cycle, whereas propene is produced via both the polyMB and the alkene cycles. The contribution of the alkene cycle is probably larger than that of the polyMB cycle, resulting in a high fraction of propene in the MTO products. Meanwhile, both cycles are interdependent in MTO, as the aromatic species generated by aromatization via the alkene cycle can also serve as new active centers for the polyMB cycle, and vice versa. Moreover, the catalytic activity of H-ZSM-5 zeolite is directly related to its acid strength; weaker acid sites are unfavorable for the polyMB cycle and then enhance relatively the contribution of the alkene cycle to forming light olefins. These results can well interpret the recent experimental observations, and the theoretical insights shown in this work may improve our understanding of the MTO mechanism, which are conducive to developing better MTO catalysts and reaction processes.

•Zn is introduced in ZSM-5 by impregnation, ion exchange, physical mixing & direct synthesis.•Lewis acid sites of ZnOH+ are formed at the expense of silanol hydroxyl and proton sites.•Catalytic performance of Zn/ZSM-5 in MTA is influenced by the method to introduce Zn.•Direct synthesis gives longest lifetime, while ion exchange the best selectivity to aromatics.•A linear correlation lies between the amount of ZnOH+ species and selectivity to aromatics.Zn-containing HZSM-5 zeolites (Zn/ZSM-5) were prepared by four methods including impregnation (IM), ion exchange (IE), physical mixing with ZnO (PM), and direct synthesis (DS); the influence of preparation method on the catalytic performance of Zn/ZSM-5 in the process of methanol-to-aromatics (MTA) was investigated. The results indicated that Lewis acid sites of zinc species (ZnOH+) are formed by introducing zinc into HZSM-5, at the expense of the silanol hydroxyl and proton acid sites. The distribution of acid sites and the nature of zinc species as well as the subsequent catalytic performance of Zn/ZSM-5 in MTA are significantly influenced by the preparation method for introducing zinc. In Zn(PM)/ZSM-5, zinc is mainly present as macro ZnO particles and trace ZnOH+ is formed by solid state reaction; in Zn(IM)/ZSM-5, ZnOH+ is the main ingredient, together with nano ZnO particles dispersed in the zeolite channel; in Zn(IE)/ZSM-5 and Zn(DS)/ZSM-5, however, only ZnOH+ species are observed. There is a linear correlation between the amount of ZnOH+ species and the selectivity to aromatics for MTA over the Zn/ZSM-5 catalysts prepared by different methods; ZnOH+ species may promote the dehydrogenation of light hydrocarbons to aromatics and suppress the hydrogen transfer reaction and the formation of alkanes by depressing the Brønsted acidity. Zn(DS)/ZSM-5 with small particle size and high mesoporous volume exhibits the longest catalytic lifetime, whereas Zn(IE)/ZSM-5 with high fraction of surface ZnOH+ species gives the highest selectivity to aromatics in MTA.The acidity and Zn species state as well as the subsequent catalytic performance of Zn/ZSM-5 in MTA are significantly influenced by the preparation method for introducing Zn.

Graphene supported Au–Pd bimetallic nanoparticles exhibit high catalytic activity in methanol selective oxidation, with a methanol conversion of 90.2% and selectivity of 100%, to methyl formate at 70 °C, owing to the synergism of Au and Pd particles as well as the strong interaction between graphene and Au–Pd nanoparticles.

A composite support of CoOx–SiO2 was obtained through modifying silica with cobalt of different loadings; with CoOx–SiO2 as the support, a series of Pd/CoOx–SiO2 catalysts were prepared for the combustion of lean methane at low temperature. The effects of CoO loading and palladium precursor on the catalytic performance of Pd/CoOx–SiO2 were investigated. The Pd/CoOx–SiO2 catalyst containing 20 wt% CoO and 1 wt% Pd with acetate as palladium precursor performs excellently in lean methane combustion; over it a complete conversion of methane can be achieved at 420 °C and the activity does not show any decline in a long-term test of 120 h. Various characterizations suggested that cobalt species are highly dispersed on the silica of high surface area and there exists a synergetic interaction between palladium and cobalt species, which improves the redox properties of Pd/CoOx–SiO2 and contributes to the excellent catalytic performance in lean methane combustion.

The control system of a catalytic flow reversal reactor (CFRR) for the mitigation of ventilation air methane was investigated. A one-dimensional heterogeneous model with a logic-based controller was applied to simulate the CFRR. The simulation results indicated that the controller developed in this work performs well under normal conditions. Air dilution and auxiliary methane injection are effective to avoid the catalyst overheating and reaction extinction caused by prolonged rich and lean feed conditions, respectively. In contrast, the reactor is prone to lose control by adjusting the switching time solely. Air dilution exhibits the effects of two contradictory aspects on the operation of CFRR, i.e., cooling the bed and accumulating heat, though the former is in general more prominent. Lowering the reference temperature for flow reversal can decrease the bed temperature and benefit stable operation under rich methane feed condition.

Dehydrogenation of ethylbenzene (EB) to styrene (ST) in the presence of carbon dioxide (CO2) was carried out over silica-supported vanadium catalysts (VOx/SiO2) to investigate the role of CO2 played in this reaction coupling process. A prominent promoting effect of CO2 on EB dehydrogenation is observed; over VOx/SiO2 with a vanadium loading of 0.8 mmol/g-SiO2, ST yield at 550 °C in CO2 is 2.05 times higher than that in an inert atmosphere of nitrogen and the catalyst also deactivates much more slowly in CO2. CO2 as a soft oxidant can effectively keep/regain high valence vanadium species that are highly active for EB dehydrogenation, which is then conducive to enhancing EB conversion and suppressing catalyst deactivation. Both carbonaceous deposition and deep reduction of the active vanadium species contribute to the catalyst deactivation; however, CO2 is only effective on alleviating the catalyst deactivation by protecting the high valance vanadium species from deep reduction, but is invalid in suppressing coke formation.Graphical abstractFor ethylbenzene (EB) dehydrogenation in CO2 over VOx/SiO2 catalyst, a prominent promoting effect of CO2 is observed. CO2 can effectively keep/regain high valence vanadium species active for EB dehydrogenation.Highlights► CO2 has a prominent promoting effect on ethylbenzene (EB) dehydrogenation. ► EB dehydrogenation over VOx/SiO2 in CO2 may follow a redox mechanism. ► CO2 can effectively keep/regain high valence active vanadium species. ► CO2 can alleviate catalyst deactivation, but cannot suppress coke formation. ► Coke quantity deposited is only related to the amount of EB converted.

The CuO/CeO2 catalysts are prepared by an improved incipient wetness impregnation method with ammonia as chelating agent for CO preferential oxidation (PROX). The effects of CuO loadings of the catalysts and the presence of H2O and CO2 in the reaction stream on the catalytic performance are investigated. The CuO/CeO2 catalyst with 10.0 wt.% CuO loading has high activity and stability for the CO-PROX. In the long-term stability test under the realistic reaction condition with 10% H2O and 15% CO2 in the reactant stream, 100% CO conversion can maintain for 1600 h at 140–150 °C with 85–75% selectivity. The catalysts are characterized by means of XRD, H2-TPR, and CO-TPR. The results show that the high activity of the CuO/CeO2 catalyst is closely related to the fine-dispersed CuO species strongly interacting with CeO2 support.

A bifunctional V2O5/ZrO2–Al2O3 catalyst of oxidation and dehydration was prepared by wet impregnation method and used in methanol oxidation. The redox, acidity and texture properties of the catalyst were characterized by N2 sorption, H2-TPR, NH3-TPD, and XRD. Under mild reaction conditions, V2O5/ZrO2–Al2O3 possesses reasonable acidic and redox properties and exhibits excellent catalytic performance in methanol oxidation with dimethoxymethane, methyl formate and formaldehyde as main products. The catalytic performance of V2O5/ZrO2–Al2O3 is dependent on the catalyst compositions and reaction temperature. V2O5(16.5 wt.%)/ZrO2(8.5 wt.%)–Al2O3 exhibits a good synergistic effect of the redox and acidic properties in the methanol oxidation; over it the selectivity to dimethoxymethane reaches 89.0% with the methanol conversion of 11.1% at 165 °C, while the selectivity to methyl formate is 30.0% with the methanol conversion of 66.1% at 215 °C.

A novel strategy to synthesize a size-selective catalyst consisting of a Pd-containing silica core and an outer silica shell with controllable pore size, was developed. Such intended structures were confirmed with N2 sorption, XRD, TEM and SEM. The pore sizes on the shell could be further tailored through silylation with organosilanes with variable chain lengths. This intriguing nanostructured catalyst showed a high activity in the aerobic oxidation of alcohols. Impressively, when the pores on the shell were tailored to particular sizes the catalyst exhibited size-selective catalysis, and the substrate molecules with only a slight difference in molecular size could be discriminated. This study potentially supplies a new approach for constructing size-selective catalysts.

Oxygen chemisorption on β-Mo2C surface and its oxidation have been investigated by using the density functional theory with the periodic models. Two surfaces of (011) and (101) were chosen to perform the calculations and the most stable surface structures together with the energetics of oxygen stepwise adsorption were identified. Thermodynamic effect of temperature and reactant pressure on the chemisorption and surface oxidation was investigated. The results suggest that the (101) surface is more active than the (011) surface towards the oxygen adsorption. The (101) surface can be fully oxidized by O2 at PO2/P0 of 10− 21–104 and temperature of 100–700 K. For the (011) surface with O2 as the oxidant, the most stable structure is that with 1/2 ML or 7/8 ML oxygen coverage, depending on the temperature and PO2/P0 value. The increase of gaseous oxidant pressure or decrease of temperature can enhance the oxidation of β-Mo2C surface and lead a more negative reaction Gibbs free energy. High temperature and low oxidant pressure may hinder the surface oxidation process.Highlights► We study oxygen chemisorption on β-Mo2C surface and its oxidation by DFT. ► (101) surface is more active than the (011) surface towards oxygen adsorption. ► (011) surface can be covered by 1/2 or 7/8 ML oxygen. ► (101) surface can be fully oxidized to be 1 ML by oxygen. ► Low temperature and high oxidant pressure favor the surface oxidation.

A degradable polymer was prepared from α-angelica lactone, a five-membered unsaturated lactone, by ring-opening polymerization (ROP). The polymerizability of α-angelica lactone was explained by a DFT calculation. The degradability of the resultant polymer and the reaction kinetics of α-angelica lactone ROP were also considered. Owing to the presence of a CC bond in α-angelica lactone, the ROP of five-membered cyclic lactone becomes feasible under moderate conditions and the resultant polyester exhibits good degradability under light or acidic/basic circumstances. Since α-angelica lactone can be easily obtained from the commercially available green bio-platform chemical levulinic acid, its ROP may provide a potential route to produce functionalized aliphatic polyesters from renewable resources.

The adsorption and subsequent dissociative reaction of methanol on the bald Mo-edge, 50% Mo-edge, and 50% S-edge of MoSx clusters were investigated by using density functional theory (DFT). The calculation results showed that the adsorption of methanol molecule through its oxygen atom prefers the corner sites to the edge sites of MoSx surfaces. The pathways of methanol dissociation via C–H, C–O and O–H bond scissions are considered and O–H bond scission is found to be the most favorable pathway on the MoS2 surface; the activation barrier is 0.45 eV on the bald Mo-edge and about 1.0 eV on the 50% Mo-edge and 50% S-edge. Among the intermediates formed from methanol dissociation, CH3O is the dominant surface species after the MoS2 surface is exposed to methanol.Graphical abstract. When MoS2 surface is exposed to methanol, O–H bond scission is found to be the most favorable pathway for methanol dissociation and CH3O is the dominant surface species.Research highlights► Adsorption and dissociation of methanol on MoS2 surface were investigated by DFT. ► Methanol molecule prefers being adsorbed through its oxygen atom on the corner sites. ► O–H bond scission is the most favorable pathway for methanol dissociation. ► CH3O is the dominant surface species upon the exposure of methanol on MoS2.

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V2O5/TiO2 catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W  (SO42-) = 15%) and V2O5 (W(V2O5) = 15%) components loaded in the acid-modified V2O5/TiO2 catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H2SO4-modified V2O5/TiO2 catalyst gives a much higher DMM yield at 150 °C than the unmodified one.

Desulfurization of diesel fuel was conducted via reactive adsorption over a coprecipitated Ni/ZnO adsorbent. A negative effect of the residual sodium in Ni/ZnO adsorbent on its adsorption performance was observed. The desulfurization ability of Ni/ZnO adsorbent is markedly weakened with the increase in the residual sodium content. This negative effect can be attributed to the fact that the residual sodium decreases the adsorbent surface area and pore volume, suppresses the interaction between Ni and ZnO, and leads to an increase in the crystallite size of the active species. Moreover, the residual sodium is enriched on the adsorbent surface upon calcination and reduction treatment, which may promote the formation of the catalytically inactive Ni−Zn and NaZn(OH)3 species.

The role of a Co promoter in a water-gas-shift reaction on Co-MoS2 has been investigated on the basis of density functional theory computation. On the basis of the computed adsorption energy of the reaction intermediates and H2O dissociation barriers, the active catalyst is the Mo edge with 25% Co substitution and 25% sulfur coverage, while the S edge with 25% Co substitution and 50% sulfur coverage is not active. On the basis of the computed reaction barriers, the redox mechanism (CO + H2O → CO + O + 2H; CO + O + 2H → CO2 + H2) is the preferable reaction path, and the rate-determining step is the second step dissociation of OH into surface O and H, while the reaction path from carboxy (CO + OH → COOH; COOH → CO2 + H) is not favored due to its high dissociation barrier. In addition, formate (HCOO) is a side product from gas phase CO2 and surface H and does not participate directly in the reaction mechanism. Detailed comparisons reveal that the Co promoter is not an active center in H2O dissociation and CO oxidation but changes the adsorption configuration of the reaction intermediates and reduces the reaction barriers. The Co promoter plays the role of a textual promoter in creating more active sites and accelerating the reaction rate.

Adsorption and dissociation of CO and NO molecules at the Mo- and C-terminated β-Mo2C(0001) surfaces has been investigated systematically using density functional theory methods together with cluster models. The calculations yield stable CO and NO adsorption for both surface terminations, suggesting strong adsorbate binding. Molecular adsorption of CO exhibits similar stability for the two terminations, while the molecular NO adsorbate prefers Mo termination over C termination. Computed vibrational frequencies of CO and NO are compared with data from infrared (IR) spectroscopy, allowing a detailed interpretation and assignment of the different features in the experimental spectra. C, N, and O atoms are quite strongly bound at the β-Mo2C surface, where at the Mo-terminated surface, hollow sites are energetically preferred. For the C termination, only oxygen adsorbs near carbon sites, whereas C and N stabilize above Mo substrate atoms or in hollow sites. Dissociative adsorption of NO is energetically preferred over molecular adsorption, while for CO, the two types are energetically similar. Dissociation barriers of adsorbed NO are lower than those for CO, which is consistent with the experimental results. The barrier calculations show also that dissociation prefers the Mo-terminated over the C-terminated surface.

Density functional theory calculations have been performed on the structure and stability of β-Mo2C bulk and the corresponding low-index surfaces. The eclipse configuration with a Mo–C–Mo–C stacking is the most stable, followed by the structure with a Mo–C–Mo–Mo–C stacking where there is an empty carbon layer every fourth layer. For (0 0 1) and (1 0 0) surfaces, the pure C terminations are more stable than the pure Mo terminations. For (0 1 0) and (1 1 1) surfaces, the Mo terminations are more stable than the C terminations. For the (0 1 1) surface, the mixed Mo/C termination is a little more stable than the Mo termination. Relaxation of these surfaces is moderate with no relaxation degree exceeding 12.8%. Among these surfaces, the mixed Mo/C termination of the (0 1 1) surface is the most stable with the lowest surface free energy, followed by the (1 0 1) surface and the TMo-2 termination of the (0 1 0) surface.

Lanthanum oxides were real time modified to ZSM-5 during the hydrothermal synthesis of the zeolite. The addition of lanthanum oxides does not change the basic structure of ZSM-5, but obviously decrease the crystallinity and BrÖnsted acid amount. The catalysis tests show that modification of ZSM-5 with lanthanum oxides significantly suppresses the coke formation on the catalyst surface and improves the catalyst stability, which is likely associated with the decrease of BrÖnsted acid amounts and a promotional function of lanthanum oxides for the reaction between coke and water.

In order to describe the structural and energetic properties of Al-MCM-22 zeolite effectively and reasonably, ONIOM2 and ONIOM3 models as well as full B3LYP/6-31G(d) optimization on different sized clusters have been tested. On the basis of the computed bond lengths, substitution energies, proton affinities and O–H stretching frequencies for the Al1–O3H–Si4 and Al2–O9H–Si5 acid sites, three-layered ONIOM schemes (B3LYP/6-311G(d,p):HF/3-21G(d):MNDO) with 8T high-layer up to cluster sizes of 10 Å have the nearly same results as compared with those obtained from full B3LYP/6-31G(d) optimizations on 8 Å clusters. The computed O–H stretching frequencies, adsorption energy of ammonia, and Al···H1 distances agree well with the available experimental data.

The small particle-sized molecular sieves CoAPO-5 and CoAPO-20 have been synthesized using surfactant-assisted method hydrothermally. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and ultraviolet visible spectrometer (UV–vis). The results show that the obtained CoAPO-5 and CoAPO-20 have AFI and SOD structure respectively. The CoAPO-5 are bundles of acicular crystal and its sizes vary from 0.1 × 2 μm to 0.1 × 5 μm and the CoAPO-20 show spheric morphology with size distribution about 300–800 nm. It is proved that the surfactant-assisted method is an effective route to synthesize small particle-sized molecular sieves.

Critical temperatures and pressures of nominal reacting mixture in synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide (quaternary mixture of carbon dioxide + methanol + water + DMC) were measured using a high-pressure view cell. The results suggested that the critical properties of the reacting mixture depended on the reaction extent as well as its initial composition (initial ratio of carbon dioxide to methanol). Such information is essential for determining the reaction conditions when one intends to carry out the synthesis of DMC with CO2 and methanol under supercritical conditions.

The dehydrogenation of isobutane (IB) to produce isobutene coupled with reverse water gas shift in the presence of carbon dioxide was investigated over the catalyst Cr2O3 supported on active carbon (Cr2O3/AC). The results illustrated that isobutane conversion and isobutene yield can be enhanced through the reaction coupling in the presence of carbon dioxide. Moreover, carbon dioxide can partially eliminate carbonaceous deposition on the catalyst and keep the active phase (Cr2O3), which are then helpful to alleviate the catalyst deactivation.

Nano-sized Beta zeolites, with a crystal size of 80–100 nm, were synthesized via surface wet method. The nano-sized HBeta zeolites exhibit much higher activity and stability in the Friedel–Crafts acylation of anisole and toluene with acetic anhydride than the conventional zeolites of large particle size. The small crystal size of nano-sized zeolites may bring on more accessible active sites and then enhance the catalytic activity. The exposed pore openings in nano-sized zeolites allow a fast desorption of heavy products from the catalyst and can then reduce the occupancy of active sites by the adsorption of products; this can then alleviate the catalyst deactivation and improve the catalyst stability.

The two-layered ONIOM method (B3LYP/6-31G(d,p):HF/3-21G) is used to study the interaction of amines (NH3, MeNH2, Me2NH and Me3N) with H-[Ga]MOR. The optimization of the local structure of H-[Ga]MOR cluster leads to two stable bridging hydroxyl sites (O10H and O2H) in the zeolite framework, being different from that of H-[Al]MOR. In the adsorption complexes, all amines are protonated by the acidic proton of H-[Ga]MOR, and the protonated amines (HNR3+) are stabilized by hydrogen bonds between the negatively charged zeolite oxygen atoms and the hydrogen atoms of the NH and CH bonds in the adsorbates. This interaction is confirmed by the structure of the adsorption complexes as well as the calculated IR stretching frequencies. The calculated adsorption energies of amines agree reasonably with the available experimental data. It is found that NH3 prefers to adsorb at the O2H Brønsted site, while Me2NH and Me3N prefer to adsorb at the O10H site, and MeNH2 can be in equilibrium between O2H and O10H. The relative order of the basicity of amines on the basis of the computed adsorption energies agrees well with the experiments, but differs from those in the gas phase (proton affinity) and in solvents (pKa).The adsorption of amines (NH3, MeNH2, Me2NH and Me3N) in two stable bridging oxygen sites (O10H and O2H) in H-[Ga]MOR clusters is investigated by using the ONIOM2 method. All amines are protonated and the formed [HNR3]+ are stabilized by the H-bonds between the negatively charged zeolite oxygen atoms and the hydrogen atoms of the NH and CH bonds in the adsorbates. On the basis of the calculated adsorption energies, it is concluded that the relative basicity of amines follows the order of Me2NH > Me3N > MeNH2 > NH3. Due to the effect of introduced methyl groups and their different basicity and structures, NH3 prefers to adsorb at the O2H Brønsted site (Fig. a), while Me2NH and Me3N prefer to adsorb at the O10H site (Fig. b), and MeNH2 can be in equilibrium between O2H and O10H.

The sorption isotherms of xylenes in CoAlPO4-5 were measured by the gravimetric technique at 342 K. The isotherms of para- and meta-xylenes followed typical type-I and could be well fitted by the Langmuir model; while the isotherm of ortho-xylene did not show a distinct saturation with the increase of pressure and could be then described by the Freundlich equation. In opposition to the adsorption in AlPO4-5, the isotherms of para- and meta-xylene in CoAlPO4-5 lay above that of ortho-xylene. The free energy change (ΔG) of adsorption increased with the loadings, while the diffusivities of xylenes decreased with the loadings. Moreover, the diffusivities of ortho-xylene were slower than those of other two isomers.

The zincosilicate analog of zeolite mordenite was hydrothermally synthesized in the presence of citric acid and characterized with several spectroscopic techniques. The zeolite thus prepared had a higher crystallinity and Zn concentration in the framework compared with the one obtained in the absence of citric acid. XRD and FTIR provided evidence for the incorporation of Zn in the framework. Results of XAFS indicated a tetrahedral structure of Zn in the lattice framework with a Zn–O distance of 0.1938 nm. It is speculated that the citric acid might decrease the concentration of Zn2+ in the synthesis mixture, thereby preventing the unfavorable formation of oxide or hydroxide species.

Dehydrogenation of isobutane to isobutene in the presence of carbon dioxide was carried out over NiO/Al2O3 catalyst and the effect of K2O additive on its catalytic behavior was investigated. Compared with the reaction in an inert atmosphere, the dehydrogenation of isobutane is coupled with the reverse water-gas shift by conducting the reaction in carbon dioxide, which can then enhance the isobutene yield. The addition of K2O in the NiO/Al2O3 catalyst can decrease its acidity, alleviate the deep reduction of NiO active species during the reaction and then suppress the side reactions like cracking and coke formation; these are helpful to enhance the isobutene yield and to improve the catalyst stability.

Low temperature CO oxidation was carried out over CeO2-TiO2 composite oxide and thereon supported Pd catalysts. The effects of Ce/Ti ratio and pre-treatments of calcination and reduction on the catalytic behaviour were investigated. The CO oxidation starts at about 220 °C over CeO2-TiO2 and the pre-reduction treatment has little influence on the catalytic activity. Pd supported on CeO2-TiO2 (Pd/CeO2-TiO2) exhibits high activity for CO oxidation and a complete conversion of CO to CO2 can be achieved even at ambient temperature, which suggests a synergistic effect between Pd and CeO2-TiO2. The activity and stability of Pd/CeO2-TiO2 can be further improved by the pre-reduction treatment. Ce/Ti ratio influences the catalytic behaviour significantly; the catalyst Pd/CeO2-TiO2 with a Ce/Ti mole ratio of 0.20 (Pd/Ce20Ti) owns the highest activity and stability, which suggests an optimization of the Pd-Ce-Ti interaction in Pd/Ce20Ti. The calcined Pd/CeO2-TiO2 with a Ce/Ti mole ratio higher than 0.10 shows a distorted light-off profile with the temperature, which implies an alternation of the reaction mechanism with increasing temperature.

Reactive adsorption desulfurization (RADS) of diesel oil was conducted over a Ni/ZnO adsorbent; the transfer of sulfur species in the RADS process was investigated by the sulfur K-edge X-ray absorption near-edge structure (XANES) and X-ray diffraction (XRD). The results indicted that the organic sulfur compounds in the diesel oil are first decomposed on surface Ni of Ni/ZnO to form Ni3S2, followed by the reduction of Ni3S2 to form H2S, and then H2S is stored in the adsorbent accompanied by the conversion of ZnO into ZnS.

The origins of the deactivation of a Au/CeO2–Co3O4 catalyst during CO preferential oxidation (PROX) are investigated in detail by means of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction of hydrogen (H2-TPR), temperature-programmed oxidation of oxygen (O2-TPO), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A possible mechanism involving –OOH intermediate is proposed and used to explain the deactivation in the long-term stability test of CO PROX. The aggregation or sintering of the Au particles is excluded from the origins of deactivation by HRTEM and XRD. The deactivation of the catalyst is mainly due to an intrinsic transformation in the chemical state of the gold species and the support oxides in the Au/CeO2–Co3O4 catalyst. The XPS, XRD, and H2-TPR results demonstrate the reduction of ionic Au to metallic Au and of cobalt oxide to cobaltous compound. The changes of the chemical states imply a structure reordering of the catalyst surface, which will suppress the supplement of active oxygen and the formation of –OOH species, inhibit the CO oxidation reaction, and lead to the deactivation of the catalyst. The accumulation of carbonates and H2O on the deactivated catalyst is revealed by XPS, DRIFTS, O2-TPO, and a regeneration test. They are responsible for the complete deactivation of the catalyst. The hydration of the catalyst surface may play a more important role than the formation of carbonates in the deactivation of the catalyst.The catalyst loses its activity at various rates in the different periods of the CO PROX process, implying that the origins of deactivation are different.Download high-res image (131KB)Download full-size image

The mechanism of thiophene cracking catalyzed by Brønsted acidic zeolites was computed at the level of B3LYP density functional theory. It was found that this catalytic reaction involves two major steps: (1) protonation of thiophene associated with an electrophilic aromatic substitution to another thiophene in a concerted way to form 2-(2,5-dihydrothiophen-2-yl) thiophene, and (2) CS bond dissociation in 2,5-dihydrothiophene promoted by further protonation. The intermediate, 4-mercapto-1-(thiophen-2-yl)but-2-en-1-ylium, was found to have a CH2 group close to a CC bond and a SH group, in agreement with the experimental findings. A strong stabilization effect of the zeolite framework on the transition states was found by embedding the 5T cluster into the larger 34T and 56T clusters. The rate-determining step is the electrophilic aromatic substitution.

A novel strategy to synthesize a size-selective catalyst consisting of a Pd-containing silica core and an outer silica shell with controllable pore size, was developed. Such intended structures were confirmed with N2 sorption, XRD, TEM and SEM. The pore sizes on the shell could be further tailored through silylation with organosilanes with variable chain lengths. This intriguing nanostructured catalyst showed a high activity in the aerobic oxidation of alcohols. Impressively, when the pores on the shell were tailored to particular sizes the catalyst exhibited size-selective catalysis, and the substrate molecules with only a slight difference in molecular size could be discriminated. This study potentially supplies a new approach for constructing size-selective catalysts.

Graphene supported Au–Pd bimetallic nanoparticles exhibit high catalytic activity in methanol selective oxidation, with a methanol conversion of 90.2% and selectivity of 100%, to methyl formate at 70 °C, owing to the synergism of Au and Pd particles as well as the strong interaction between graphene and Au–Pd nanoparticles.

Graphene oxide (GO) prepared by a modified Hummers' method exhibits excellent catalytic performance in the synthesis of polyoxymethylene dimethyl ethers (PODEn) from methanol (MeOH) and trioxymethylene (TOM), owing to a synergy between the sulfonic groups and the hydroxyl and carboxyl groups present on the surface of GO with a unique layered structure.