•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that the autoregulative time to switch the gas flow direction will drop quickly to zero if a large amount of hot gas is withdrawn from the reactor by using the traditional method.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.

•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

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

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

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.

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.

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

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.

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.

The mechanism of olefin elimination in the process of methanol-to-olefins (MTO) over a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by DFT-D calculations, which is a crucial step that controls the MTO product distribution. The results demonstrate that the manners of olefin elimination are related to the pore structure of zeolite catalyst and the interaction between proton transfer reagent (water or methanol) and zeolite acidic framework. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with large pores, as suggested by the energy barrier of rate-determining step and the potential energy surface (PES), but is unfavorable over HZSM-5 with medium-sized pores due to the steric hindrance of spiro intermediates. Over various zeolites, water and methanol perform differently in proton transfer to form the spiro intermediates; over HMOR and HBEA with strong acidity, water is superior to methanol in promoting propene elimination, whereas over HMCM-22 with relatively weaker acidity, methanol is more favorable as a proton transfer reagent.

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.

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.

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.

Polyoxymethylene dimethyl ethers (PODEn or DMMn) were synthesized by the condensation of methanol and trioxymethylene over the catalysts of several molecular sieves like HY, HZSM-5, Hβ, and HMCM-22; the effect of their acidic properties on product distribution was investigated. The results indicated that the acidic molecular sieves, especially HMCM-22, are catalytically active for the condensation of methanol and trioxymethylene to form DMMn. Over HY, the main product is dimethoxymethane (DMM), with a selectivity of 92.87%. Over HZSM-5 and Hβ, the main products turn out to be DMM1-3 and the yields of DMM3-8, which are ideal additives for diesel fuel, reach 6.40% and 13.78%, respectively. With HMCM-22 as the catalyst, the formation of long chain DMMn products is further enhanced and the yield of DMM3-8 attains 29.39%. The results of NH3-TPD demonstrated that the product distribution is related to the surface acidic properties of the catalyst used; short chain DMM may be primarily formed on weak acidic sites, while the acidic sites of medium strength can enhance the formation of diesel fuel additive components DMM3-8.

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.