High Methane Selectivity Research Articles

Effect of reduction and carburization pretreatment on iron catalyst for synthesis of light olefins from CO hydrogenation

Effect of reduction and carburization pretreatment on iron catalyst for CO hydrogenation to light olefins was investigated. The bulk structure and surface composition of the catalysts during pretreatment were characterized by means of X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS) and high-resolution transmission electron microscopy(HRTEM). The results indicated that phase transformation of iron phases involved α-Fe2O3→Fe3O4→α-Fe both in the bulk and on the surface layers in hydrogen atmosphere. However, α-Fe2O3 was firstly transformed to Fe3C and then to Fe5C2 in CO atmosphere, while in syngas atmosphere directly to Fe5C2. As carburization pretreatment time was prolonged, the degree of carburization on the surface increased, and the increase degree of iron catalyst carburization in CO pretreatment was stronger than that in syngas pretreatment. It inferred that surface carbon species was more easily formed in syngas pretreatment instead of iron carbide in CO pretreatment. After hydrogen pretreatment, when the catalyst was reduced to a mixture of magnetite and metallic iron, the selectivity to light olefins was relatively low. Under the joint effects of the active sites from surface iron carbide and surface carbon layers blocking the active sites, longer CO pretreatment time resulted in higher methane selectivity and less light olefins. The selectivity to methane on H2-pretreated catalyst was lower than that of CO-pretreated catalyst.

Implications of methanol disproportionation on catalyst lifetime for methanol-to-olefins conversion by HSSZ-13

The effects of methanol space velocity and inlet methanol partial pressure on lifetime and selectivity of methanol-to-olefins catalysis are examined and interpreted to elucidate reaction parameters and propose intermediates and reactions relevant to catalyst deactivation. The propensity of active centers in HSSZ-13 to turn over for methanol-to-olefins catalysis increases when the methanol partial pressure local to organic co-catalysts confined within the inorganic chabazite cages is lower either by decreasing methanol space velocity or inlet methanol partial pressure. High initial methane selectivity reveals methanol disproportionation, to methane and formaldehyde, a primary reaction, and continual methane formation implicates persistent participation of methanol in bimolecular hydrogen transfer reactions throughout the catalyst lifetime. Methane selectivity correlates positively with inlet methanol partial pressure reflecting enhanced relative rates of formaldehyde formation with increasing methanol partial pressure. Subsequent alkylation reactions of olefins- and aromatics-based CC chain growth carriers by formaldehyde accelerate the relative rates of hydrogen transfer and proliferate, apparently, the precursors mediating the transformation of active hydrocarbon pool participants to those inducing catalyst deactivation.

First-principles study of structure sensitivity of chain growth and selectivity in Fischer–Tropsch synthesis using HCP cobalt catalysts

Co (0001) prefers the CO insertion mechanism with high methane selectivity, but Co (101̄1) prefers the carbide mechanism with high C2-hydrocarbon selectivity.

Fischer–Tropsch synthesis on impregnated cobalt-based catalysts: New insights into the effect of impregnation solutions and pH value

The Co-based catalysts were prepared with different cobalt acetate solutions. Effects of pH value were studied deeply on Fischer–Tropsch synthesis (FTS) through a semi-batch reactor. Among all impregnation solutions (water, butanol, amyl alcohol, acetic acid, nitric acid and ammonium nitrate), the catalyst prepared by NH4NO3 solution showed the highest catalytic activity due to its small particle size and high reduction degree. However, the catalyst with the smallest particle size derived from water as impregnation solution exhibited low activity as well as high methane selectivity since it was difficult to be reduced and inactive in FTS. According to FT-IR spectra results, the low intensity of absorbed CO on the catalyst prepared from water solution resulted in low FTS activity. Whereas, the high activity of catalysts prepared from NH4NO3 solution could be explained by the high intensity of absorbed CO on the catalysts. The cobalt species on the catalysts prepared under lower pH conditions exhibited smaller particle size distribution as well as lower CO conversion than those prepared at higher pH value.

Effect of the structure of Ni/TiO2 catalyst on CO2 methanation

CO2 methanation over supported nickel catalysts attracts an increasing attention. However, the mechanism with it is still in debate. It is confirmed here that the structure of Ni/TiO2 catalyst has a significant effect on CO2 methanation. The catalyst with Ni (111) as the principal exposing facet shows higher activity with higher methane selectivity. The maximum activity for CO2 methanation is obtained at 350 °C with a CO2 conversion of 73.2% and a CO2 conversion rate of 1.56 molCO2gcat−1h−1 at a high GHSV of 60,000 h−1. This rate is higher than those reported CO2 conversion rates in the literature. According to the FTIR analysis, CO2 methanation over catalyst with Ni (111) as the principal exposing facet follows the mechanism via CO intermediate, while the catalyst with multi-facets takes the pathway of the direct hydrogenation of formate, with which nickel is only functional for hydrogen dissociation.

A Single-Event MicroKinetic model for the cobalt catalyzed Fischer-Tropsch Synthesis

The Single-Event MicroKinetic methodology has been successfully extended from Fe to Co catalyzed Fischer-Tropsch Synthesis. A total of 82 experiments were performed in a plug flow reactor with a H2 to CO molar inlet ratio between 3 and 10, a temperature range from 483 to 503K, CO inlet partial pressures from 3.7 to 16.7kPa and space time varying between 7.2 and 36.3(kgcats)mol−1. Via regression, statistically significant and physicochemically meaningful estimates were obtained for the activation energies in the model and the H, C and O atomic chemisorption enthalpies as required for the UBI-QEP method. A reaction path analysis allowed relating the observed deviations from the Anderson-Schulz-Flory distribution, i.e., a high methane and low ethene selectivity, to the symmetry numbers and a higher chemisorption enthalpy of the metal methyl species compared to the other metal alkyl species. Simulations at industrially relevant conditions show that, as a catalyst descriptor, the H atomic chemisorption enthalpy crucially determines both the CO conversion and the C5+ selectivity. The higher FTS activity of Co compared to Fe is explained via the higher oxygen atomic chemisorption enthalpy on the latter compared to the former.

Highly active and selective catalyst for synthetic natural gas (SNG) production

A novel nickel-based methanation catalyst has been prepared and tested for CO and CO2 hydrogenation to synthetic natural gas under various conditions. The catalysts before and after reaction have been characterized using XRD, Laser Raman and IR spectroscopes. It is showed that the novel catalyst can give high methane selectivity and yield at a very broad ranges of temperature and H2/CO ratios, although the selectivity to methane increase with the H2/CO ratio, which, however, increases with the temperature rising from 200 to 350 °C, then gradually decrease. The characterization results showed that the there is surface carbons forming over the catalyst surface, and the nickel crystalline structure changes during the reaction. Despite this, the catalyst still gives the same performance, which suggested that the catalyst can operate in a very broad range conditions.

Palladium–Silver-Activated ZnO Surface: Highly Selective Methane Sensor at Reasonably Low Operating Temperature

Metal oxide semiconductors (MOS) are well known as reducing gas sensors. However, their selectivity and operating temperature have major limitations. Most of them show cross sensitivity and the operating temperatures are also relatively higher than the value reported here. To resolve these problems, here, we report the use of palladium-silver (70-30%) activated ZnO thin films as a highly selective methane sensor at low operating temperature (∼100 °C). Porous ZnO thin films were deposited on fluorine-doped tin oxide (FTO)-coated glass substrates by galvanic technique. X-ray diffraction showed polycrystalline nature of the films, whereas the morphological analyses (field emission scanning electron microscopy) showed flake like growth of the grains mainly on xy plane with high surface roughness (107 nm). Pd-Ag (70-30%) alloy was deposited on such ZnO films by e-beam evaporation technique with three different patterns, namely, random dots, ultrathin (∼1 nm) layer and thin (∼5 nm) layer as the activation layer. ZnO films with Pd-Ag dotted pattern were found show high selectivity towards methane (with respect to H2S and CO) and sensitivity (∼80%) at a comparatively low operating temperature of about 100°C. This type of sensor was found to have higher methane selectivity in comparison to other commercially available reducing gas sensor.

Dilute gas methanation of synthesis gas from biomass gasification

Synthetic natural gas (SNG) based on biomass is a secondary energy carrier with a lot of advantages. The methane synthesis reaction conditions are mild and the process is uncomplex. The whole natural gas infrastructure can be used for storage, distribution and application. In addition, methane is much easier to handle than hydrogen. In the SNG process synthesis gas will be converted to mainly methane and after a processing unit it could be fed into the natural gas grid. Air gasification technology is state-of-the-art and a low cost gasification technology. The synthesis gas out of an air gasifier is highly diluted with nitrogen and thus just used for combined heat and power production (CHP). The current SNG concepts need synthesis gas from a much more complex gasification with oxygen or steam. Because no literature could be found which analyzes the effect of nitrogen in the methanation unit, this paper will show first results in this field. Synthesis gas with different nitrogen content was tested in a fixed bed methanation reactor at different temperature levels, using a commercial 20wt.% Nickel catalyst. The nitrogen content has the positive effect to limit the temperature increase by the strong exothermic reaction. Full carbon monoxide conversion and high methane selectivity was observed. It can be shown, that the methane selectivity is independent up to 50Vol.% of nitrogen in the feed gas. An occurred side reaction is the ammonia production. The highest ammonia concentration, 614ppm, was observed at the lowest nitrogen content. The same experiment shows the highest methane selectivity of 90% and methane content of 30Vol.%. With increasing dilution, the ammonia formation falls off. The shown influence of nitrogen on the methanation reaction should point out the chances and risks for methanation of a diluted synthesis gas derived from biomass gasification, especially in small sizes, like decentralized air-gasification plants.

Fischer–Tropsch Synthesis: Impact of H2 or CO Activation on Methane Selectivity

The effect of CO conversion on methane selectivity of a 10 % Co/TiO2 catalyst was investigated using a 1L continuously stirred tank reactor at 493–503 K, 2.4 MPa, H2/CO = 2 and GHSV = 0.4–6 Nl g cat −1 h−1. The cobalt catalysts were activated by H2 and CO, respectively, in two separate runs. For the H2 activated catalyst, the methane selectivity decreased (15–6 %) and C5+ selectivity increased (80–91 %) linearly with increasing CO conversion from 10 to 60 %. The C2–C4 hydrocarbon selectivity decreased slightly with CO conversion. The enhancement in C5+ selectivity was mainly due to the decrease in methane selectivity. The CO activated cobalt catalyst was initially in a carburized form after activation, and then converted to a form with greater metallic character when exposed to syngas at Fischer–Tropsch conditions. The CO activated catalyst at pseudo steady-state had higher methane selectivity in general as compared to the H2 activated one, possibly due to the presence of the cobalt carbide phase. Two kinetic regions appear to exist for the CO activated catalyst. When CO conversion is lower than 20 %, the methane selectivity increased exponentially with CO conversion, which is attributed to enhanced hydrogenation. When CO conversion is higher than 20 %, the CO activated catalyst behaves more like a H2 activated catalyst, with a decrease in methane and an increase in C5+ selectivity with CO conversion.

XAS Study on Calcination Effect of Silica Supported Cobalt Catalysts for Fischer-Tropsch Synthesis

The precalcined and calcined silica supported cobalt catalysts at 15, 20, and 25%Co were investigated by X-ray Absorption Spectroscopy (XAS) including the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS). The results showed the phase of Co(NO3)2.6H2O in all precalcined catalysts, which corresponded to the XRD measurement. When increasing the amount of cobalt in the precalcined catalysts, there was the presence of ordered Co(NO3)2.6H2O phase. After calcination in Ar at 600°C for 6 h, the Co3O4phase was presented in all calcined catalysts. For the catalytic performance testing, the selected 20%Co/Aerosil_wi_calcined catalyst was reduced at 450°C in H2and operated at 190°C with a total pressure of 10 bar and H2/CO flow rate of 20:10 ml/min for Fischer-Tropsch synthesis. After reaction testing, the used 20%Co/Aerosil_wi catalyst showed the main phase of Co3O4. The result showed high methane selectivity at the beginning of reaction. By increasing of reaction time, the methane selectivity tended to decrease, whereas the C2-C4and C5+selectivity was increased.

Fe–Mo interactions and their influence on Fischer–Tropsch synthesis performance

Fe–Mo interactions and their effect on Fischer–Tropsch synthesis (FTS) performance were investigated over Fe–Mo catalysts. The interaction between Fe and Mo is in the form of Fe–O–Mo structure in as-prepared catalysts and remains largely undestroyed after activation. This interaction leads to increased charge transfer between Fe and Mo species and electron-deficient Fe species with increasing Mo content. The electron-deficient Fe species is responsible for the poor reduction and high methane selectivity in FTS reactions. Meanwhile, Mo incorporation increases the dispersion of iron oxides. The Mo dispersion effect inhibits the aggregation of active iron particles in the heat-treatment. Under the combined action of the electron transfer effect and dispersion effect, the FTS activity of catalysts exhibits a parabola-like trend as a function of Mo content and passes through a minimum at Mo/Fe ratio of 10/100. In contrast, the methane selectivity demonstrates an opposite tendency to FTS activity.

Characteristics of hydrogen production by a plasma–catalyst hybrid converter with energy saving schemes under atmospheric pressure

This paper investigated the production of hydrogen from methane under atmospheric pressure using a plasma–catalyst hybrid converter with emphasis on energy conservation. A spark discharge was used to ionize the hydrocarbon fuel and air mixture with a catalyst to enhance hydrogen production using two energy saving schemes, namely, heat recycling and heat insulation. The experimental results showed that higher methane feeding rate resulted in higher reformate gas temperature and a corresponding increase in methane conversion efficiency. The energy saving systems also enabled the oxygen/carbon ratio to be decreased to reduce oxidation of hydrogen and carbon monoxide and thereby improving the concentrations of hydrogen and carbon monoxide. By heat recycling, a lower methane feeding rate showed an 8.7% improvement in methane conversion efficiency whilst improvement was not apparent with higher methane supply rates due to the already high conversion efficiency. Moreover, it was shown that hydrogen production increased significantly with the reaction from water–gas shifting under the same operation parameters but with high methane selectivity. The best combination resulting in a total thermal efficiency of 77.11% was 10 L/min methane feeding rate and 0.8 O 2/C ratio. With water–gas shifting (S/C ratio=0.5), an 86.26% hydrogen yield, equating to 17.25 L/min hydrogen production rate could be achieved. The equilibrium production rate was calculated using the commercialized HSC Chemistry software ( ©ChemSW Software, Inc.). Good correlation was obtained between the calculations and the experimental results.

Effects of Impregnation Solvents on Catalytic Performance of Co-Ru-ZrO2/γ-Al2O3 Catalyst for Fischer-Tropsch Synthesis

Used ZrO2 modified γ-Al2O3 as support, Co-Ru catalysts were prepared by incipient impregnation method. The effects of impregnation solvents on the performances of catalysts were examined. The catalyst was prepared with ethanol solution and high Co dispersion was obtained, exhibiting highest activity of CO hydrogenation, very low methane selectivity, and high heavy hydrocarbon C5 + selectivity. The catalysts were prepared with aqueous solution and methanol solution, and the reaction behaviors were similar. The solvent isopropanol caused the lowest catalytic activity and highest methane selectivity. Increasing the reaction temperature enhanced the CO hydrogenation rate, and the CO conversion slightly increased the CO2 selectivity and favored the formation methane and light hydrocarbons, while the chain growth probability decreased. For the catalyst prepared with ethanol, the CO conversion, the CH4 selectivity, and the C5 + selectivity were 94.16%, 5.65%, and 88.2%, respectively, and the chain growth probability was 0.87 at 493 K, 1.5 MPa, 800 h−1, and n(H2):n(CO) = 2.0 in feed.

Hydrothermal post-synthesis of HZSM-5 zeolite to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization reaction: Reconstruction of pore structure and modification of acidity

Abstract Hydrothermal post-synthesis of a commercial HZSM-5 zeolite in Al(NO3)3 aqueous solution resulted in the creation of a uniform microporous network and the formation of Bronsted acid sites with suitable acidic strengths through the dynamic extraction and incorporation of aluminum species between the solution and the framework. NMR and N2 adsorption–desorption isotherms measurements revealed that a dynamic extraction and incorporation of aluminum from the framework occurred during the post-synthesis treatment. XRD, XP spectra and TPR measurements of the resulting Mo/HZSM-5 catalyst indicated that the dispersion of Mo species was greatly promoted over the post-synthesized HZSM-5 zeolite, significantly facilitating the migration of Mo species into the channels. As a result, the Mo/HZSM-5 catalyst showed rather high methane conversion and selectivity of aromatics by effectively inhibiting the formation of coke during the methane dehydroaromatization reaction. Comparatively, the Mo/HZSM-5 catalyst, where the HZSM-5 zeolite was obtained by conventional hydrothermal post-synthesis, exhibited lower methane conversion and heavy coke deposits, apparently due to the substantial loss of the Bronsted acid sites and the creation of secondary pores.

Studies on MCM-48 supported cobalt catalyst for Fischer–Tropsch synthesis

MCM-48 molecule sieve was used as a support of cobalt catalyst for Fischer–Tropsch synthesis (FTS). Co/MCM-48 catalysts were prepared using incipient witness impregnation method with cobalt loadings of 5, 10 and 15 wt.%, respectively. The catalysts were characterized by N 2 physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), hydrogen temperature programmed desorption (H 2-TPD) followed by pulse oxygen titration and transmission electron microscopy (TEM). The catalytic properties for FTS were tested in a fixed bed reactor. TPR and oxygen titration indicated different extent of overall reduction of Co cobalt species in different cobalt content catalysts. With increasing Co loading, CO conversion and C5+ selectivity first increased remarkably and then remained nearly invariable. Lower reducibility of smaller cobalt particles on 5Co/MCM-48 is likely to be one of the reasons responsible for the lower CO conversion and highest methane selectivity. Co loading exceeding 10 wt.% had no significant effect on FTS properties of the catalysts.

Hydrodechlorination of carbon tetrachloride over PtNaX zeolite: Deactivation studies

Platinum-containing NaX zeolite showed high activity and high methane selectivity in the hydrodechlorination of carbon tetrachloride. The catalyst showed fast deactivation. To identify the possible causes of deactivation, the fresh and used catalysts were studied by thermogravimetric analysis, X-ray diffraction, thermoprogrammed oxidation, infrared spectroscopy and diffuse reflectance spectroscopy. The results indicate that the deactivation is probably caused by the attack of the HCl produced during the reaction to the zeolite structure forming an amorphous aluminosilicate. The HCl adsorption can also be pointed as a cause for deactivation. It is worthy to mention that no coke formation (oligomers) was observed, which is not consistent to some other observations made for other platinum-containing catalysts.

A highly active and stable Fischer–Tropsch synthesis cobalt/silica catalyst with bimodal cobalt particle distribution

Silica-supported cobalt (20 wt%) catalysts were prepared by incipient wetness impregnation of silica with different cobalt nitrate solution. The catalyst prepared from dehydrated ethanol solution exhibited highest activity and very low methane selectivity. The catalyst prepared from cyclohexanol had the lowest activity and highest methane selectivity. The catalyst prepared from aqueous solution, a most conventional catalyst, exhibited moderate reaction behavior. The catalyst prepared from dehydrated ethanol had cobalt particles with two different size where the large particles showed low bulk density with cluster-like structure.

Chemical Effect and Spatial Effect of New Bimodal Catalysts for Fischer-Tropsch Synthesis.

A catalyst support with both small pores and large pores, as well as a distinct bimodal pore structure, has excellent advantages in industrial solid-catalysis reactions because the large pores provide pathways for rapid molecular transportation and the small pores serve a large area of active surface. A simple preparation method of bimodal supports was developed by introducing SiO2 or ZrO2 sol into large pores of an SiO2 gel pellet directly. The pores of the obtained bimodal supports distributed distinctly as two kinds of main pores. On the other hand, the increased BET surface area and decreased pore volume, compared to those of original silica gel, indicated that the obtained bimodal support formed according to the designed route. The obtained bimodal support loaded with cobalt was applied in slurry-phase Fischer–Tropsch synthesis (FTS). The bimodal catalyst presented the best reaction performance in slurry-phase FTS with a high reaction rate and low methane selectivity, because the spatial promotional effect of the bimodal structure and the chemical effect of the porous zirconia were available inside the large pores of original silica gel.

Promotional SMSI Effect on Supported Palladium Catalysts for Methanol Synthesis

High-temperature reduction (HTR) of palladium catalysts supported on some reducible oxides, such as Pd/CeO2, and Pd/TiO2 catalysts, led to a strong metal-support interaction (SMSI), which was found to be the main reason for their high and stable activity for methanol synthesis from hydrogenation of carbon dioxide. But low-temperature-reduced (LTR) catalysts exhibited high methane selectivity and were oxidized to PdO quickly in the same reaction. Besides palladium, platinum exhibited similar behavior for this reaction when supported on these reducible oxides. Mechanistic studies of the Pd/CeO2 catalyst clarified the promotional role of the SMSI effect, and the spillover effect on the HTR Pd/CeO2 catalyst. Carbon dioxide was decomposed on Ce2O3, which was attached to Pd, to form CO and surface oxygen species. The carbon monoxide formed was hydrogenated to methanol successively on the palladium surface while the surface oxygen species was hydrogenated to water by spillover hydrogen from the gas phase. A reaction model for the hydrogenation of carbon dioxide was suggested for both HTR and LTR Pd/CeO2 catalysts. Methanol synthesis from syngas on the LTR or HTR Pd/CeO2 catalysts was also conducted. Both alcohol and hydrocarbons were formed significantly on the HTR catalyst from syngas while methanol formed predominantly on the LTR catalyst. Characterization of these two catalysts elucidated the reaction performances.