Herein, Cu–Mg/Ti-SBA-15 catalysts were prepared through the modification of Cu and Mg to mesoporous Ti-SBA-15 zeolites with different Ti/Si ratios and used for the synthesis of isobutyraldehyde (IBA) from methanol and ethanol. The catalysts were characterized via various techniques including XRF, XRD, TEM, N2 sorption, CO2-TPD, FT-IR, and XPS. With an increase in Ti content, CuO was well dispersed accordingly, and the amounts and strength of the basic sites were reduced. However, an excess introduction of Ti led to the accumulation of single TiO2 crystals, inducing a decrease in the surface area and a deviation from the regular pattern such that the binding energies of Cu 2p, Mg 2p, and Si 2p shifted to lower values. This precisely affected the catalytic behaviors of the prepared catalysts synergistically. The catalyst stability was improved with the increasing Ti content accordingly, and over the catalyst with a Ti/Si ratio = 4/15, the IBA selectivity, after 24 h reaction, could still reach 25%, which was the best durability ever reported for IBA synthesis from methanol and ethanol. The catalytic performance test conducted using a regenerated catalyst and IR measurement of the spent catalyst indicated that carbon deposition on the catalyst surface could be depressed to some extent with the increasing Ti content.

•ZnCr catalysts are prepared by sol-gel method to explore role of hydroxyl groups.•Lower calcination temperature causes more structure defects and hydroxyl groups.•The ZnCr prepared at pH = 2 exhibits high total alcohol and isobutanol selectivity.•The hydroxyl groups could be consumed by CO, resulting in more oxygen vacancies.A series of ZnO, ZnCr catalysts were prepared by a sol-gel method and ammonia solution was used to adjust the pH of the sol solution. The catalysts were characterized by XRD, in-situ FR-IR, NH3-TPD, in-situ XPS, HRTEM. The results show that lower calcination temperature is helpful to reduce the crystal size and crystallinity of ZnCr nanocrystal, as well as forming a certain amount of structure defects and hydroxyl groups. The hydroxyl groups could be consumed by CO under the interaction between ZnO and ZnCr spinel, resulting in more exposed oxygen vacancies. We find the proper calcination temperature and Zn/Cr molar ratio for the ZnCr catalysts preparation are 400 °C and 1.0, respectively. The Zn1Cr1–400 ∼ 2.0 catalyst prepared with the pH value of 2 shows more hydroxyl groups and small particle size, exhibiting the best catalytic performance both on the CO conversion (20.9%) and isobutanol selectivity (24.2 wt%).Download high-res image (142KB)Download full-size image

Nano-CeO2 was prepared through the calcination of Ce(OH)3 precursor in different atmospheres (H2, Ar, air, O2), which was prepared by a hydrothermal method, and then used as catalysts in the direct synthesis of dimethyl carbonate (DMC) from methanol and CO2. The results indicated that the catalyst calcined in O2 (CeO2-O2) showed an optimum catalytic performance, and the yield of DMC reached to 1.304 mmol/mmolcat. In addition, reaction temperature and weight of catalyst were optimized. Based on characterizations of the catalysts, the ratio of Ce(IV)/Ce(III) and Lewis acid-base property of nano-CeO2 catalyst could be adjusted through different calcination atmosphere treatment. It was determined that the higher activity of CeO2-O2 catalyst is mainly attributed to its higher ratio of Ce(IV)/Ce(III) as well as abundant and moderate intensity Lewis acid base sites.

A kind of core–shell catalyst with Fe–Zn–Zr as the core and a zeolite (HZSM-5, Hbeta, and HY) as the shell was synthesized by a simple cladding method. The catalyst has an obvious confinement effect on the synthesis of isoalkanes by CO2 hydrogenation. Especially, the Fe–Zn–Zr@HZSM-5–Hbeta catalyst with a double-zeolite shell exhibits an extraordinary high i-HC/t-HC ratio.

The hydrothermal synthesis method was adopted to prepare a highly active Ga2O3–Al2O3 catalyst (GA-HS), which displayed superior catalytic performance for dehydrogenation of propane to propylene in the presence of CO2 (DHP-CO2). The highest propane conversion on GA-HS was 35.2%, which was much higher than that from the catalysts prepared using the grind-mixture method (8.7%) or coprecipitation method (26.2%). Moreover, propylene selectivity over the GA-HS catalyst was higher than that over the other catalysts in a period of 9 h of reaction. These catalysts were characterized by N2 physical adsorption, ICP-AES, XRD, TGA, TEM, SEM, DRIFT, Py-FTIR, NH3-TPD, XPS, 27Al MAS NMR and 71Ga MAS NMR techniques. The characterization data indicated that the hydrothermal treatment increased the surface area, expanded the pore size and promoted the formation of more tetrahedral Ga ions and the generation of more medium-strong Lewis acid sites. Furthermore, this catalyst mainly displayed an amorphous sponge-like morphology as well as a new morphology whereby some pieces were covered with amorphous nanoparticles. The superior activity of GA-HS was attributed to the higher surface area of this catalyst and the larger amount of tetrahedral Ga ions (Ga3+ and probably Gaδ+δ < 2) related to the medium-strong Lewis acid sites.

With increasing degrees of Mo–Sn interface contact, the molar ratio of methyl formate (MF) to methanol (MeOH) and formaldehyde (FA) was found to linearly increase at the same time, which indicates that a high degree of MoO3 and SnO2 interface contact has a strong positive effect on the conversion of the methoxyl intermediate to MF in the dimethyl ether (DME) oxidation reaction. XRD and Fourier transform EXAFS indicate that the structure of molybdenum oxide starts to change from MoO3 clusters to oligomer MoOx domains with an increase in the degree of Mo–Sn interface contact, and the Mo–O–Sn structure begins to dominate at the contact interfaces when the degree of interface contact increases to 0.0256 and 0.0384 nm2 SnO2/Mo atom. HRTEM also confirms the presence of the MoOx domains on the surface of the catalyst possessing the smallest molybdenum oxide content, the signals of which were even undetected by XRD and Raman. Moreover, ESR, XPS and XAFS suggest that Mo5+ species exist in the Mo–Sn contact interfaces rather than on the surface of the catalysts. The evaluation results of MoO3-SBA-15 further verify the key role of Mo5+ in the formation of MF. Therefore, Mo5+ is reasonably proposed to be the active center for the formation of MF in the DME oxidation reaction. Based on the catalytic characterization results, the reaction pathway of DME oxidation to MF via methoxyl intermediates was proposed on the Mo(V) sites. First, DME was absorbed and consequently dissociated to become an absorbed methoxy-Mo(V) species with the assistance of acidic sites and lattice oxygen. Then, the β-H was eliminated, and an absorbed FA-Mo(IV) species was formed. Consequently, the absorbed methoxy-Mo(V) species reacted with the neighboring absorbed FA-Mo(IV) species, producing MF, and then, the Mo(IV) species was regenerated to become an Mo(V) species by the oxidation of O2 so it could catalyze another reaction cycle. The deeper understanding of this investigation has substantial meaning for the preparation of a catalyst with a specific molybdenum oxide structure that can be applied for transforming the methoxyl intermediate to produce other high-value products through the DME oxidation reaction.

To improve the selectivity of 1,2-propandiol (PDO) by modifying the structure and morphology of the MoO3/SnO2 catalyst, orthorhombic (α), monoclinic (β) and hexagonal (h) MoO3 crystalline phases were prepared to investigate the rational design requirements of the MoO3–SnO2 structure that are beneficial for the reaction of glycol dimethyl ether (DMET) to PDO. With an increase in the reaction temperature, the highest PDO selectivity of the oxidation reaction of glycol dimethyl ether to 1,2-propandiol was always obtained over the h-MoO3–SnO2 catalyst and the lowest PDO selectivity was always obtained over the β-MoO3–SnO2 catalyst. The MoO3 bulk structure, the interaction between SnO2 and MoO3 and the surface properties of these three catalysts could account for this distinctive difference. Hexagonal MoO3 is dispersed more homogeneously over the h-MoO3–SnO2 catalyst due to the hexagonal crystalline tunnel structure existing in the h-MoO3–SnO2 catalyst, and the weak interaction between MoO3 and SnO2; besides, the more hydrated surface of the h-MoO3–SnO2 catalyst can lead to more Brønsted acid sites being present on the catalyst surface and favor the dissociation of the C–O bond in DMET and association of the C–C bond to form PDO with the assistance of the redox and basic sites, which can explain why the highest PDO was obtained over the h-MoO3–SnO2 catalyst. The lattice strain and oxygen vacancies in the β-MoO3–SnO2 catalyst, induced by the substitution of Sn4+ ions with the smaller sized Mo6+ ions, enhance the oxidation ability of the β-MoO3–SnO2 catalyst, and consequently more CH3O· can be formed and transformed to formaldehyde (FA) and methyl formate (MF), which can explain why the total selectivity of FA and MF was highest while the selectivity of PDO was lowest over the β-MoO3–SnO2 catalyst at the same time. These findings are pretty significant for further investigation of the rational design of the MoO3–SnO2 catalyst structure, applied to the conversion of DMET to PDO.

A new preparation method for MoO3–SnO2 catalysts precipitated by HNO3 was developed to selectively synthesize industrially useful chemicals formaldehyde and methyl formate via oxidation of environmentally friendly feedstock dimethyl ether. By adjusting the structure of MoO3–SnO2, the selectivity to formaldehyde and methyl formate can reach as high as 95.0% and 82.4%, respectively, under different conditions. The conclusion has been drawn, based on the experimental results, that the formation of tetrahedral molybdenum oxide species and SnMoO4 species favors the reaction of dimethyl ether to formaldehyde, and that the formation of MoOx domains favors the direct oxidation reaction of dimethyl ether to formaldehyde. The XRD and Raman results suggest that the formation of tetrahedral molybdenum oxide species has a positive correlation with the high selectivity to formaldehyde and indicates that the formation of the MoOx domains favors the high selectivity to methyl formate. The XPS results of the MoO3–SnO2 catalysts with different SnO2 contents demonstrate that formaldehyde is not readily desorbed from Mo1Sn2 and Mo1Sn3 because the electron-poor Mo cations of the domains supported on the surface of the catalysts strengthen their affinity for binding electron-donating dimethyl ether-derived intermediates and formaldehyde, which favors the subsequent reaction of formaldehyde to methyl formate. These interesting findings reveal that tetrahedral molybdenum oxide species and MoOx domains play an important role in the selective oxidation of dimethyl ether to formaldehyde and methyl formate, and give insight into the rational design of the MoO3–SnO2 catalyst structure for dimethyl ether oxidation in future studies.

Mesoporous ZnZSM-5 zeolites were synthesized by introducing zinc directly into an alkaline and surfactant solution. The characterizations reveal that the presence of CTAB is favorable for the recrystallization of zeolite structural units. The amount of strong acid sites of mesoporous zeolites decreased, while the amount of medium acid sites of mesoporous zeolites (especial zinc-containing) increased. The amount of Lewis acid sites increased while the amount of Brønsted acid sites obviously decreased. For mesoporous ZnZSM-5, the emergence of a new species (ZnOH+) further increased the amount of Lewis acid sites. Both the external surface area and mesopore volume of mesoporous ZnZSM-5 gradually decreased with increasing zinc content. Most of zinc species introduced during desilication and reassembly dispersed on the surface of zeolites, but the addition of zinc species had no obvious influence on the zeolite morphology. The catalytic performance of the obtained materials was investigated via aromatization of methanol. The results show that BTX (benzene, toluene, and xylene) selectivity over mesoporous ZnZSM-5 gradually increases with increasing zinc content, and is much higher than that of mesoporous HZSM-5. However, the BTX selectivity of mesoporous HZSM-5 is obviously lower than that of HZSM-5 due to its much lower strong acid sites and larger pore size. The strong Brønsted acid sites, the Zn-Lewis acid sites and mesoporous channels have a synergistic effect on methanol aromatization over mesoporous ZnZSM-5 catalysts. Additionally, compared with HZSM-5, improvement in catalyst lifetime of MHZSM-5 and MZnZSM-5-2 is achieved by introducing additional mesoporous channels and decreasing the amount of strong acid sites.

The selective oxidation of dimethyl ether (DME) to methyl formate (MF) was conducted in a fixed-bed reactor over the MoO3–SnO2 catalysts with different Mo/Sn ratios. The MF selectivity reached 94.1% and the DME conversion was 33.9% without the formation of COx over the MoSn catalyst at 433 K. The catalysts were deeply characterized by NH3-TPD, CO2-TPD, BET, XPS and H2-TPR. The characterization results showed that different compositions of catalysts obviously affected the surface properties of the catalysts, but the valence of the metal hardly changed with the Mo/Sn ratios. Raman spectroscopy, XRD and XAFS were further used to characterize the structure of the catalysts. The results indicated that the catalyst composition exerted a significant influence on the structure of MoO3. The formation of oligomeric MoO3 and the appropriate coordination numbers of Mo–O at 1.94 Å are the main reasons for the distinct high catalytic activity of the MoSn catalyst.

A simple method for the preparation of a Ga/ZSM-5 catalyst for propane aromatization was established by formic acid impregnation and in situ treatment. The catalyst prepared by this novel method showed remarkably superior activity of propane aromatization. Under the conditions T = 540 °C, P = 100 kPa, WHSV = 6000 ml g−1·h−1 and with a N2/C3H8 molar ratio of 2, the highest propane conversion and selectivity to BTX (benzene, toluene and xylene) achieved on H–Ga/SNSA catalyst was 53.6% and 58.0%, respectively, much higher than that of the catalyst prepared using the traditional impregnation method (38.8% and 48.2%). The catalysts were characterized by nitrogen physical adsorption, ICP-AES, DRIFT, py-FTIR, NH3-TPD, H2-TPR, XPS and 27Al MAS NMR techniques. The characterization data indicated that this facile methodology enhanced the dispersion of the Ga species and promoted the formation of highly dispersed (GaO)+ species, which could exchange with the acidic protons (Brønsted acid sites) of the zeolite framework, contributing to the strong Lewis acidity. The super catalytic behavior was attributed to the synergistic effect between the strong Lewis acid sites generated by the (GaO)+ species and the Brønsted acid sites.

A series of Zn–Cr oxides nanoparticles were prepared by a coprecipitation procedure. The structure of different catalysts was investigated by X-ray Absorption Fine Structure (XAFS), X-ray photoelectron spectroscopy (XPS), temperature programed reduction of hydrogen (H2-TPR) and in situ infrared spectrum (in situ IR). Both EXAFS and XANES demonstrated the cation disorder distribution became more serious with decreasing annealing temperature and increasing Zn/Cr molar ratios. The cation distribution also affected the oxygen state on the surface over Zn–Cr spinel. The population of surface hydroxyl species increased with more serious cation disorder distribution and they facilitated the formate formation which was a significant intermediate C1 species for alcohol synthesis. This study was the first time to investigate the situation of cation distribution in Zn–Cr spinel by XAFS and related it to catalyst performance. The results revealed that the isobutanol productivity presented a linear relationship to the level of cation disorder distribution in Zn–Cr spinel, unambiguously revealing the real active sites and structure-activity relationship.

Ga-modified HZSM-5 precursors, containing 1 wt% Ga, were first prepared using the incipient wetness impregnation method, and then subjected to one or three-times consecutive treatment by cycles of reduction in hydrogen and re-oxidation in air. The resulting Ga/HZSM-5 catalysts were characterized by N2 physical adsorption, ICP-AES, DRIFT, Py-FTIR, NH3-TPD, H2-TPR, XPS, DRIFT-TPSR and MS-TPSR techniques to obtain clear mechanistic details that the acidity of these Ga/HZSM-5 catalysts affected their activity for propane aromatization. The characterization data suggested that the impregnated introduction of Ga to ZSM-5 zeolite and subsequent reduction–oxidation treatment led to a great decrease in the number of Brønsted acid sites (BAS) and promoted the formation of strong Lewis acid sites (LAS) attributed to highly dispersed Ga species. The formed strong LAS specifically promoted the dehydrogenation steps during propane aromatization, whereas the original BAS were responsible for the whole aromatization process. The TPSR results suggested that propane was converted to propylene through the dehydrogenation on BAS and strong LAS, and simultaneously converted to ethylene through the β-scission on BAS at a low temperature. With the elevation of temperature, the generated propylene and ethylene on the strong BAS and the strong LAS were further converted into BTX aromatics accompanied by hydrogen release. It was plausible that the highly efficient synergy between BAS and strong LAS could result in lower aromatization temperatures and more product of benzene (β state) over the Ga/HZSM-5 catalysts than that over the Ga-modified HZSM-5 precursors.

•Cu/ZrO2 catalysts promoted by K have good performance for isobutanol synthesis.•Addition of La facilitates the dispersion of CuO and improves Cu–Zr interaction.•Under relative low reaction temperature and high H2/CO ratio (2.5), high selectivity of isobutanol (32.8%) is obtained.A series of K–Cu/ZrO2 catalysts with different La loadings for isobutanol synthesis from CO hydrogenation were prepared by a co-precipitation method. The catalysts were characterized by XRD, reactive N2O adsorption, XPS, H2-TPR, in situ IR and Raman techniques, and then tested for isobutanol synthesis from CO hydrogenation at the conditions of 360 °C, 10.0 MPa, GHSV = 3000 h−1 and H2/CO = 2.5. These characterization results showed that incorporation of La into the K/Cu–Zr catalyst resulted in increase of BET surface area and dispersion of catalyst particles. Especially, medium La addition facilitated the synergistic effect between Cu–Zr interactions, improved the reducibility of catalyst and increased the amounts of dispersed copper. An enrichment of C1 intermediates on catalyst surface related to high dispersed CuO is better for chain growth. In the isobutanol synthesis reaction, the conversion of CO and selectivity of isobutanol increased first and then decreased as the increase of La concentration. Among these La-promoted K/Cu–Zr catalysts, the 3% La/Cu/Zr catalyst exhibited the highest alcohol selectivity of 32.8 wt%, which can be radically attributed to the highest Cu surface area.

A chain growth scheme for the synthesis of alcohols from carbon monoxide and hydrogen is proposed based on the chemical enrichment method on ZrO2-based catalyst. Methanol addition has no obvious effect on the STY of C2+ alcohols, indicating that COH→CCOH is a slow initial growth step. Addition of ethanol and propanols can enhance the STY of isobutanol, especially n-propanol, revealing that n-propanol is largely the precursor of isobutanol. Results of large alcohols addition further reveal the relationship between small alcohols and large alcohols of formation. Also, addition of aldehydes has a similar effect on the formation of higher alcohols, indicating that alcohols exist in the form of aldehydes before desorption. Anisole are introduced into syngas for confirmation of predicted intermediates and the result indicates that formyl species is participated both in the formation of methanol and higher alcohols. Reaction temperature has a significant effect on the chain growth of alcohols synthesis. Under low temperature, chain growth occurs with CO insertion and alcohols are linear products. Isobutanol appears and becomes the main product during C2+ alcohols undergo an aldo-condensation reaction at high temperature.

An efficient rhenium oxide-modified H3PW12O40/TiO2 catalyst is found for a new synthesis of dimethoxy dimethyl ether from dimethyl ether oxidation. The effects of Re loading, H3PW12O40 content and different feedstocks on the performance of Re–H3PW12O40/TiO2 were investigated. The results showed that DMM2 selectivity was significantly improved up to 60.0%, with 15.6% of DME conversion over 5% Re–20% H3PW12O40/TiO2. NH3-TPD, NH3-IR, Raman spectra, H2-TPR, XPS and TEM were used to extensively characterize the structure and surface properties of the catalysts. The introduction of H3PW12O40 significantly affected the structure and reducibility of surface rhenium oxide species, in addition to increasing the acidity of the catalyst. The increased number of Lewis acid sites and weak acid sites and the optimal ratio of Re4+/Re7+ of Re–H3PW12O40/TiO2 were favorable for the formation of DMM2 from DME oxidation. The possible reaction pathway of DME oxidation to DMM2 was proposed.

Dimethyl ether oxidation was conducted over MoO3–SnO2 catalysts prepared from different Sn salt precursors. Over the MoO3–SnO2 catalyst prepared from SnCl4, methyl formate selectivity reached 94.1% at 433 K without the formation of COx. The performance of the catalyst was determined by the existence form of MoO3 and the different surface bonding of Mo and O of the catalyst.

A series of Cu-ZnO-based catalysts modified with Al, Zr, and Ce for the low-temperature methanol synthesis were prepared through co-precipitation and characterized by N2 sorption, H2-TPR, CO2-TPD, N2O titration, XRD, and high-resolution TEM; the effect of various modifiers and calcination temperature on their catalytic performance in methanol synthesis at 170°C was investigated. The results showed that the Cu-ZnO-based catalyst modified with ZrO2, among the various modifiers, exhibits the highest activity. Meanwhile, a lower calcination temperature is propitious to get a higher Cu dispersion, a smaller Cu crystal size, and a higher low temperature activity for methanol synthesis; as a result, the uncalcined catalyst exhibits excellent catalytic performance, with a productivity of 106.02 g/(kg·h) and a selectivity of 87.04% to methanol.

In this paper, the K-promoted Cu/Zn/La/ZrO2 catalysts were prepared by co-precipitation methods, and the hydrogenation of CO was used as the probe reaction to investigate the catalytic performance. The structure and surface properties of K-Cu/Zn/La/ZrO2 catalysts were characterized by XRD, TG, BET, NH3-TPD, H2-TPR, Raman and XPS techniques respectively. The results indicated that the relationship between the structure, specific surface and catalyst activity was not significant, and the strong acidic sites of catalyst surface were not suitable for the formation of isobutanol. But the results indicated that the synergy among Cu-Zn-Zr would have an important effect on the activity of K-Cu/Zn/La/ZrO2 catalysts. When the calcinations temperature was 450°C, the synergistic effect of Cu-Zn-Zr was strongest so that the CuO could be reduced easily. Under the optimum calcination temperature, the conversion of CO reached 69.47%, the selectivity of isobutanol approached 19.09%, and the primary products were methanol and isobutanol that was 95.02% of the alcohol products.

Ni-ZrO2 and Ni-Mg-ZrO2 catalysts were prepared by a co-precipitation method, and then were characterized by BET, XRD, H2-TPR and CO2-TPD techniques. The performances of catalysts in the tri-reforming of coal bed methane to syngas were studied in a fixed bed reactor. The reaction conditions (temperature and feed gas composition) were mainly investigated. The results showed that at t=800°C, atmospheric pressure, CH4/CO2/H2O/O2/N2=1.0/0.45/0.45/0.1/0.4, GHSV=30000 mL·g−1·h−1, about 99% of CH4 conversion, 65% of CO2 conversion, and VH2/VCO of 1.5 could be achieved during 58 h of reaction, which are related to the strong metal-support interaction, the good thermal stability and basic nature of the catalyst. Furthermore, high temperature favors the tri-reforming of methane. By adjusting the feed gas composition, a specific VH2/VCO could be achieved.

Effects of reaction atmospheres (nitrogen, carbon monoxide, carbon dioxide, syngas and water vapor) on stability of the catalyst, selectivities to propylene and byproducts for dimethyl ether conversion to propylene (DTP) over Ca/ZSM-5 were investigated in a continuous flow fixed-bed reactor. Besides, the regenerated catalyst was also studied for DTP process. The results show that the catalyst exhibits best stability, highest propylene selectivity and lowest selectivities to byproducts in carbon dioxide atmosphere, followed by nitrogen, carbon monoxide and syngas atmosphere, and water vapor atmosphere exerts worst effect on DTP. The catalytic performance of the regenerated catalyst also demonstrates that carbon dioxide as reaction atmosphere is most beneficial to DTP process.

Low-temperature oxidation of dimethyl ether (DME) to methyl formate (MF) with high selectivity was realized in a continuous flow fixed-bed reactor over the multifunctional MoO3-SnO2 catalysts designed and prepared intentionally. The effect of the preparation methods including mechanical mixing, co-precipitation and co-precipitation-impregnation on the catalyst activity was investigated. The results showed that the selectivity to MF reaches 94.1% at 160°C over the catalyst prepared by co-precipitation-impregnation, with DME conversion of 33.9% and absence of COx in the products. The results of NH3-TPD, CO2-TPD and H2-TPR characterizations indicated that the catalysts prepared by various methods are also obviously different in the surface acidic, alkaline and redox properties. The results of Raman, XRD and TEM revealed that MoO3 structure and particle sizes have a significant influence on the catalyst activity; small particle size and oligomeric MoO3 may be responsible for the high activity of the MoO3-SnO2 catalysts from co-precipitation-impregnation in the selective oxidation of DME to MF at low temperature.

Deactivation of composite catalyst for one-step dimethyl ether (DME) synthesis in slurry reactor was studied under reaction conditions of 260°C and 5.0 MPa. It was found that instability of Cu-based methanol synthesis catalyst led to rapid deactivation of the composite catalyst. Deactivation rate of the Cu-based catalyst in slurry reactor was compared with that in fixed-bed reactor. The results indicated that harmfulness of water, which is formed in the synthesis of DME, caused the Cu-based catalyst to deactivate at a high rate in slurry reactor. Techniques of TPR, XRD, and SEM-EDS were used to characterize the reduction behaviors, crystal structures, and surface properties of the catalyst. The results showed that carbon deposition and grain growth of Cu were important reasons for the rapid deactivation of the Cu-based catalyst, and no obvious metal loss of Cu was found.

The direct oxidation of dimethyl ether (CH3OCH3, DME) to ethanol (CH3CH2OH) was carried out over WO3/HZSM-5 catalysts in a continuous flow type fixed-bed reactor. The effects of different zeolite types, WO3 loading and reaction temperature on the reaction were investigated. Our results showed that modification of HZSM-5 with 10% WO3 significantly improved the ethanol selectivity from 0.6% to 19.7%. The surface and structure of the catalysts were characterized by NH3-TPD, BET, XRD and in situ FT-IR. Ethanol can be mainly synthesized from DME direct oxidation under the cooperation of trifunctional reactive sites offered by WO3 modified HZSM-5 catalysts.The synthesis of ethanol via direct oxidation of DME catalyzed by metal oxides modified zeolites was investigated. Our results showed that WO3 modified HZSM-5 yielded markedly improved ethanol selectivity from 0.6% to 19.7%. This could be attributed to the synergistic effects of acid sites, redox sites and pore structure of WO3/HZSM-5. Fig. A IR spectra of DME and O2 adsorption and desorption on 10% WO3/HZSM-5 catalyst.Download full-size imageHighlights► Ethanol can be synthesized from direct-oxidation of dimethyl ether over WO3/HZSM-5. ► Ethanol selectivity was improved up to 19.7% from 0.6% over 10%WO3/HZSM-5. ► The effects of different zeolite types and metal oxides loading were investigated. ► Ethanol was formed under the cooperation of acid-redox sites and pore structure.

•Smaller CNT channel offers superior space restriction and electronic interaction.•Encapsulated nanoparticles within channel are better for HAS than outer ones.•Interaction between metals and tubes is enhanced upon decreasing CNT wall thickness.•Excellent HAS selectivity is got over CuCoCe catalyst with the narrowest CNT channel.A series of carbon nanotube (CNT)-supported copper–cobalt–cerium catalysts were prepared and investigated for higher alcohols synthesis. The superior selectivity for the formation of ethanol and C2 + alcohols achieved using the CuCoCe/CNT(8) catalyst was 39.0% and 67.9%, respectively. The diameters of CNTs considerably influence the distribution of metal particles and the electronic interaction between the tube surface and the active species. The electronic effect between the encapsulated Co species and the inner surface is greatly improved in the narrowest CNT channel, which is expected to facilitate the reduction of cobaltous oxide and promote the alcohols yield remarkably (291.9 mg/gcath).Download high-res image (204KB)Download full-size image

•Co–CeO2−x site terminates carbon chain growth effectively for alcohols synthesis.•Separation of Co and Co–CeO2−x sites may cause the long chain hydrocarbon formation.•Geometric structure plays an important role in determining product distribution.Two series of supported Co-based catalysts were prepared using a facile co-impregnation method. The catalysts were examined in a fixed bed for their ability to selectively convert syngas into higher alcohols. Co–Cu and Co–CeO2−x sites have an excellent synergistic effect, which could terminate the growth of the carbon chain effectively for the synthesis of alcohols. Furthermore, the Co–CeO2−x site is more beneficial for the synthesis of higher alcohols than the Cu–Co centre. The selectivity for the formation of ethanol and C1–C3 alcohols achieved using the 10Co5Ce/carbon nanotubes catalyst were 29.7% and 89.5%, respectively. In addition, the separation of the Co and CeO2−x sites weakens the synergistic effect, and thus, causes the formation of long chain hydrocarbons. The structure–performance correlation studies suggest that the geometric structure of the catalyst likely plays an important role in determining the formation of the Co–CeO2−x site, which influences the selectivity towards higher alcohols and higher aliphatic hydrocarbons.

A kind of core–shell catalyst with Fe–Zn–Zr as the core and a zeolite (HZSM-5, Hbeta, and HY) as the shell was synthesized by a simple cladding method. The catalyst has an obvious confinement effect on the synthesis of isoalkanes by CO2 hydrogenation. Especially, the Fe–Zn–Zr@HZSM-5–Hbeta catalyst with a double-zeolite shell exhibits an extraordinary high i-HC/t-HC ratio.

With increasing degrees of Mo–Sn interface contact, the molar ratio of methyl formate (MF) to methanol (MeOH) and formaldehyde (FA) was found to linearly increase at the same time, which indicates that a high degree of MoO3 and SnO2 interface contact has a strong positive effect on the conversion of the methoxyl intermediate to MF in the dimethyl ether (DME) oxidation reaction. XRD and Fourier transform EXAFS indicate that the structure of molybdenum oxide starts to change from MoO3 clusters to oligomer MoOx domains with an increase in the degree of Mo–Sn interface contact, and the Mo–O–Sn structure begins to dominate at the contact interfaces when the degree of interface contact increases to 0.0256 and 0.0384 nm2 SnO2/Mo atom. HRTEM also confirms the presence of the MoOx domains on the surface of the catalyst possessing the smallest molybdenum oxide content, the signals of which were even undetected by XRD and Raman. Moreover, ESR, XPS and XAFS suggest that Mo5+ species exist in the Mo–Sn contact interfaces rather than on the surface of the catalysts. The evaluation results of MoO3-SBA-15 further verify the key role of Mo5+ in the formation of MF. Therefore, Mo5+ is reasonably proposed to be the active center for the formation of MF in the DME oxidation reaction. Based on the catalytic characterization results, the reaction pathway of DME oxidation to MF via methoxyl intermediates was proposed on the Mo(V) sites. First, DME was absorbed and consequently dissociated to become an absorbed methoxy-Mo(V) species with the assistance of acidic sites and lattice oxygen. Then, the β-H was eliminated, and an absorbed FA-Mo(IV) species was formed. Consequently, the absorbed methoxy-Mo(V) species reacted with the neighboring absorbed FA-Mo(IV) species, producing MF, and then, the Mo(IV) species was regenerated to become an Mo(V) species by the oxidation of O2 so it could catalyze another reaction cycle. The deeper understanding of this investigation has substantial meaning for the preparation of a catalyst with a specific molybdenum oxide structure that can be applied for transforming the methoxyl intermediate to produce other high-value products through the DME oxidation reaction.

A simple method for the preparation of a Ga/ZSM-5 catalyst for propane aromatization was established by formic acid impregnation and in situ treatment. The catalyst prepared by this novel method showed remarkably superior activity of propane aromatization. Under the conditions T = 540 °C, P = 100 kPa, WHSV = 6000 ml g−1·h−1 and with a N2/C3H8 molar ratio of 2, the highest propane conversion and selectivity to BTX (benzene, toluene and xylene) achieved on H–Ga/SNSA catalyst was 53.6% and 58.0%, respectively, much higher than that of the catalyst prepared using the traditional impregnation method (38.8% and 48.2%). The catalysts were characterized by nitrogen physical adsorption, ICP-AES, DRIFT, py-FTIR, NH3-TPD, H2-TPR, XPS and 27Al MAS NMR techniques. The characterization data indicated that this facile methodology enhanced the dispersion of the Ga species and promoted the formation of highly dispersed (GaO)+ species, which could exchange with the acidic protons (Brønsted acid sites) of the zeolite framework, contributing to the strong Lewis acidity. The super catalytic behavior was attributed to the synergistic effect between the strong Lewis acid sites generated by the (GaO)+ species and the Brønsted acid sites.

The hydrothermal synthesis method was adopted to prepare a highly active Ga2O3–Al2O3 catalyst (GA-HS), which displayed superior catalytic performance for dehydrogenation of propane to propylene in the presence of CO2 (DHP-CO2). The highest propane conversion on GA-HS was 35.2%, which was much higher than that from the catalysts prepared using the grind-mixture method (8.7%) or coprecipitation method (26.2%). Moreover, propylene selectivity over the GA-HS catalyst was higher than that over the other catalysts in a period of 9 h of reaction. These catalysts were characterized by N2 physical adsorption, ICP-AES, XRD, TGA, TEM, SEM, DRIFT, Py-FTIR, NH3-TPD, XPS, 27Al MAS NMR and 71Ga MAS NMR techniques. The characterization data indicated that the hydrothermal treatment increased the surface area, expanded the pore size and promoted the formation of more tetrahedral Ga ions and the generation of more medium-strong Lewis acid sites. Furthermore, this catalyst mainly displayed an amorphous sponge-like morphology as well as a new morphology whereby some pieces were covered with amorphous nanoparticles. The superior activity of GA-HS was attributed to the higher surface area of this catalyst and the larger amount of tetrahedral Ga ions (Ga3+ and probably Gaδ+δ < 2) related to the medium-strong Lewis acid sites.

A new preparation method for MoO3–SnO2 catalysts precipitated by HNO3 was developed to selectively synthesize industrially useful chemicals formaldehyde and methyl formate via oxidation of environmentally friendly feedstock dimethyl ether. By adjusting the structure of MoO3–SnO2, the selectivity to formaldehyde and methyl formate can reach as high as 95.0% and 82.4%, respectively, under different conditions. The conclusion has been drawn, based on the experimental results, that the formation of tetrahedral molybdenum oxide species and SnMoO4 species favors the reaction of dimethyl ether to formaldehyde, and that the formation of MoOx domains favors the direct oxidation reaction of dimethyl ether to formaldehyde. The XRD and Raman results suggest that the formation of tetrahedral molybdenum oxide species has a positive correlation with the high selectivity to formaldehyde and indicates that the formation of the MoOx domains favors the high selectivity to methyl formate. The XPS results of the MoO3–SnO2 catalysts with different SnO2 contents demonstrate that formaldehyde is not readily desorbed from Mo1Sn2 and Mo1Sn3 because the electron-poor Mo cations of the domains supported on the surface of the catalysts strengthen their affinity for binding electron-donating dimethyl ether-derived intermediates and formaldehyde, which favors the subsequent reaction of formaldehyde to methyl formate. These interesting findings reveal that tetrahedral molybdenum oxide species and MoOx domains play an important role in the selective oxidation of dimethyl ether to formaldehyde and methyl formate, and give insight into the rational design of the MoO3–SnO2 catalyst structure for dimethyl ether oxidation in future studies.

To improve the selectivity of 1,2-propandiol (PDO) by modifying the structure and morphology of the MoO3/SnO2 catalyst, orthorhombic (α), monoclinic (β) and hexagonal (h) MoO3 crystalline phases were prepared to investigate the rational design requirements of the MoO3–SnO2 structure that are beneficial for the reaction of glycol dimethyl ether (DMET) to PDO. With an increase in the reaction temperature, the highest PDO selectivity of the oxidation reaction of glycol dimethyl ether to 1,2-propandiol was always obtained over the h-MoO3–SnO2 catalyst and the lowest PDO selectivity was always obtained over the β-MoO3–SnO2 catalyst. The MoO3 bulk structure, the interaction between SnO2 and MoO3 and the surface properties of these three catalysts could account for this distinctive difference. Hexagonal MoO3 is dispersed more homogeneously over the h-MoO3–SnO2 catalyst due to the hexagonal crystalline tunnel structure existing in the h-MoO3–SnO2 catalyst, and the weak interaction between MoO3 and SnO2; besides, the more hydrated surface of the h-MoO3–SnO2 catalyst can lead to more Brønsted acid sites being present on the catalyst surface and favor the dissociation of the C–O bond in DMET and association of the C–C bond to form PDO with the assistance of the redox and basic sites, which can explain why the highest PDO was obtained over the h-MoO3–SnO2 catalyst. The lattice strain and oxygen vacancies in the β-MoO3–SnO2 catalyst, induced by the substitution of Sn4+ ions with the smaller sized Mo6+ ions, enhance the oxidation ability of the β-MoO3–SnO2 catalyst, and consequently more CH3O· can be formed and transformed to formaldehyde (FA) and methyl formate (MF), which can explain why the total selectivity of FA and MF was highest while the selectivity of PDO was lowest over the β-MoO3–SnO2 catalyst at the same time. These findings are pretty significant for further investigation of the rational design of the MoO3–SnO2 catalyst structure, applied to the conversion of DMET to PDO.