Equilibrium calculations for direct synthesis of dimethyl ether from syngas

Thermodynamic analysis of single‐step synthesis of dimethyl ether (DME) from syngas over a bi‐functional catalyst (BFC) in a slurry bed reactor has been investigated as a function of temperature (200–240°C), pressure (20–50 bar), and composition feed ratio (H₂/CO: 1–2). The BFC was prepared by physical mixing of CuO/ZnO/Al₂O₃ as a methanol synthesis catalyst and H‐ZSM‐5 as a methanol dehydration catalyst. The three reactions including methanol synthesis from CO and H₂, methanol dehydration to DME and water–gas shift reaction were chosen as the independent reactions. The equilibrium thermodynamic analysis includes a theoretical model predicting the behaviour and a comparison to experimental results. Theoretical model calculations of thermodynamic equilibrium constants of the reactions and equilibrium composition of all components at different reaction temperature, pressure, and H₂/CO ratio in feed are in good accordance with experimental values.     

Synthesis of dimethyl ether from syngas obtained by coal gasification

The characteristics of a tubular fixed-bed reactor for the direct synthesis of dimethyl ether (DME) from syngas obtained by coal gasification have been developed. DME synthesis test was conducted with a hybrid DME synthesis catalyst (CuO/ZnO/Al₂O₃ for methanol forming, γ-alumina for methanol dehydration) to understand the performance under the conditions of 6.0MPa, 260°C and GHSV=3,000 l/kg-cat·h. The H₂ conversion and CO conversion were 85-92%, 37-45%, respectively. About 68-80% of DME selectivity was observed. DME synthesis reactor also operated at the productivity of 4.6-4.9 mol/kg-cat·h, which is slightly higher than that in the Peng’s prediction results in case of H₂ : CO=0.5.     

Specifics of dimethyl ether synthesis from syngas over mixed catalysts

Dimethyl ether (DME) synthesis from syngas over a mixture of a methanol synthesis catalyst (ZnO, 25.10 wt %; AuO, 64.86 wt %; Al₂O₃, 10.04 wt %) and a methanol dehydration catalyst (γ-A1₂O₃) has been investigated for one-, two-, and three-layer catalyst beds. There is a common regularity for these three variants: with an increasing temperature, the total CO conversion decreases, the CO-to-methanol conversion decreases, and the CO-to-DME conversion increases. The largest values of DME selectivity and DME yield have been attained with the three-layer bed. The highest DME yield has been obtained at 250–285°C. Use of a mechanical mixture of the methanol synthesis catalyst and alumina makes it possible to efficiently obtain DME from syngas ballasted with nitrogen (20 vol %) at an H₂/CO ratio of 1, which is unfavorable for methanol synthesis. The DME yield on the syngas input basis in this case with the ballast gas (nitrogen or CO₂) taken into account can be about 10 wt %.     

Roles of interaction between components in CZZA/HZSM-5 catalyst for dimethyl ether synthesis via CO2 hydrogenation

The roles of interaction between two catalyst components in CuO–ZnO–ZrO₂–Al₂O₃ (CZZA)/HZSM-5 bifunctional catalyst for dimethyl ether (DME) synthesis via carbon dioxide hydrogenation were investigated. It was found that CZZA catalyst showed excellent stability during methanol (MeOH) synthesis for 100 h, while there was a severe loss of catalytic activity in the bifunctional catalyst for DME synthesis. Hence, the effects of different degrees of intimacy of two catalyst components were studied for DME synthesis, including mixed and separated modes. For the mixed mode, the particle size of catalysts and the amount of reaction intermediates were proven to influence the catalyst deactivation. For the separated mode, the catalysts showed rapid deactivation within a short time. Various characterizations indicated that the remarkable deactivation of separated mode was mainly caused by the decrease of copper active centers (e.g., sintering and oxidation) and blockage of acid sites via increased coke deposition on HZSM-5.     

Direct synthesis of dimethyl ether from carbon-monoxide-rich synthesis gas: Influence of dehydration catalysts and operating conditions

Various dehydration catalysts were studied in the synthesis of dimethyl ether (DME) directly from carbon-monoxide-rich synthesis gas under a series of different reaction conditions. The investigated catalyst systems consisted of combinations of a methanol catalyst (CuO/ZnO system) with catalysts for methanol dehydration based on γ-Al₂O₃ or zeolites and γ-Al₂O₃ was identified as the most favorable dehydration catalyst. Various reaction parameters such as temperature, H₂/CO ratio and space velocity were studied. The impact of water on Cu/ZnO/Al₂O₃-γ-Al₂O₃ catalysts was investigated and no deactivation could be observed at water contents below 10% during running times of several hours. A running time of several days and a water content of 10% led to a significant increase of CO conversion but the water gas shift reaction became dominating and CO₂ was the main product. After termination of water feeding significant deactivation of the catalyst system was observed but the system returned to high DME selectivity. Catalyst stability and the influence of CO₂ in the gas feed were studied in experiments lasting for about three weeks. The presence of 8% of CO₂ caused an approximately 10% lower CO conversion and an about 5% lower DME selectivity compared to the reaction system without CO₂.     

Liquid phase methanol and dimethyl ether synthesis from syngas

The Liquid Phase Methanol Synthesis (LPMeOH^TM) process has been investigated in our laboratories since 1982The reaction chemistry of liquid phase methanol synthesis over commercial Cu/ZnO/Al₂O₃ catalysts, established for diverse feed gas conditions including H₂-rich, CO-rich, CO₂-rich, and CO-free environments, is predominantly based on the CO₂ hydrogenation reaction and the forward water-gas shift reactionImportant aspects of the liquid phase methanol synthesis investigated in this in-depth study include global kinetic rate expressions, external mass transfer mechanisms and rates, correlation for the overall gas-to-liquid mass transfer rate coefficient, computation of the multicomponent phase equilibrium and prediction of the ultimate and isolated chemical equilibrium compositions, thermal stability analysis of the liquid phase methanol synthesis reactor, investigation of pore diffusion in the methanol catalyst, and elucidation of catalyst deactivation/regenerationThese studies were conducted in a mechanically agitated slurry reactor as well as in a liquid entrained reactorA novel liquid phase process for co-production of dimethyl ether (DME) and methanol has also been developedThe process is based on dual-catalytic synthesis in a single reactor stage, where the methanol synthesis and water gas shift reactions takes place over Cu/ZnO/Al₂O₃ catalysts and the in-situ methanol dehydration reaction takes place over -Al₂O₃ catalystCo-production of DME and methanol can increase the single-stage reactor productivity by as much as 80%. By varying the mass ratios of methanol synthesis catalyst to methanol dehydration catalyst, it is possible to co-produce DME and methanol in any fixed proportion, from 5% DME to 95% DMEAlso, dual catalysts exhibit higher activity, and more importantly these activities are sustained for a longer catalyst on-stream life by alleviating catalyst deactivation.     

Flame-made Cu/ZnO/Al2O3 catalyst for dimethyl ether production

For the single step synthesis of dimethyl ether (DME) from synthesis gas a Cu/ZnO/Al₂O₃-catalyst has been prepared using flame-spray pyrolysis. The resulting powder was co-mixed with γ-alumina to give an admixed system for DME production. The flame-made catalyst was analyzed using the BET method, in situ XRD, N₂O decomposition, TPR and XPS. These studies unraveled that the catalyst exhibited a high Cu surface area including good contact with zinc oxide and alumina as well as small Cu particles resulting in high catalytic activity and product selectivity, also in comparison to a commercially available catalyst.     

Effect of water on liquid phase DME synthesis from syngas over hybrid catalysts composed of Cu/ZnO/Al2O3 and γ-Al2O3

DME synthesis from syngas via methanol has been carried out in a single-stage liquid phase reactor. Cu/ ZnO/Al₂O₃ and γ-Al₂O₃ were used together as methanol synthesis catalyst and dehydration catalyst, respectively. The influence of water on the catalytic system was investigated mainly. Water affected the activity of methanol dehydration catalyst as well as methanol synthesis catalyst. Thus, removal of water from the reaction system, by adding a dehydrating agent or controlling methanol formation rate by the reaction parameters, was efficient in maintaining the high catalytic activity and stability. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Characterization and catalytic application of MnCl2 modified HZSM-5 zeolites in synthesis of aromatics from syngas via dimethyl ether

The conversion of syngas to aromatics via dimethyl ether was investigated over MnCl₂ modified HZSM-5 zeolites. The results demonstrated that 2%MnCl₂ modified HZSM-5 (SiO₂/Al₂O₃ = 38) exhibited higher p-xylene selectivity than other catalysts and further decreased 1,2,4,5-tetramethylbenzene selectivity. The CO conversion was obviously increased after 5%MnCl₂ modification to HZSM-5. The catalysts were characterized by XRD, BET, XPS, FT-IR, NH₃-TPD, SEM, element analysis and O₂-TPO. The loading amount of MnCl₂ affected the adsorption and reaction of DME molecules on zeolites. Appropriate amount of MnCl₂ introduction could adjust the acidity and pore volume of HZSM-5 to increase p-xylene selectivity and CO conversion.     

An integrated biomass-derived syngas/dimethyl ether process

A Cu-Zn-Al methanol catalyst combined with HZSM-5 was used for dimethyl ether (DME) synthesis from a biomass-derived syngas containing nitrogen. The syngas was produced by air-steam gasification of pine sawdust in a bubbling fluidized bed biomass gasifier with a dry reforming reaction over ultra-stable NiO-MgO catalyst packed in a downstream reactor for stoichiometric factor (H₂, CO, CO₂) adjustment. It constantly gave syngas with H2/CO ratio of 1.5 and containing trace CH₄ and CO₂ during a period of 150 h. The obtained N₂-containing biomass-derived syngas was used directly for DME synthesis. About 75% CO per-pass conversion and 66.7% DME selectivity could be achieved under the condition of 533 K, 4MPa and 1,000-4,000 h^-1. The maximized DME yield, 244 g DME/Kgbiomass (dry basis), was achieved under a gasification temperature of 1,073 K, ER (Equivalence Ratio) of 0.24, S/B (Steam to Biomass Ratio) of 0.72 and reforming temperature of 1,023 K with the addition of 0.54 Nm³ biogas/Kg_biomass (dry basis).     

Effect of H2O on Cu-based catalyst in one-step slurry phase dimethyl ether synthesis

One-step dimethyl ether (DME) synthesis in slurry phase was catalyzed by a hybrid catalyst composed of a Cu-based methanol synthesis catalyst and a γ-Al₂O₃ methanol dehydration catalyst under reaction conditions of 260 °C and 5.0 MPa. It was found that instability of the Cu-based catalyst led to rapid deactivation of the hybrid catalyst. The stability of the Cu-based catalyst under DME synthesis conditions was compared with that under methanol synthesis conditions. The results indicated that harmfulness of water, which formed in DME synthesis, caused the Cu-based catalyst to deactivate at a high rate. Surface physical analysis, elemental analysis, XRD and XPS were used to characterize the surface physical properties, components, crystal structures and surface morphologies of the Cu-based catalysts. It was found that Cu⁰ was the active component for methanol synthesis and Cu₂O might have less activity for the reaction. Compared with methanol synthesis process, crystallite size of Cu became bigger in DME synthesis process, but carbon deposition was less severe. It was also found that there was distinct metal loss of Zn and Al caused by hydrothermal leaching, impairing the stability of the catalyst. In slurry phase DME synthesis, a part of Cu transformed into Cu₂(OH)₂CO₃, causing a decrease in the number of active sites of the Cu-based catalyst. And some ZnO converted to Zn₅(OH)₆(CO₃)₂, which caused the synergistic effect between Cu and ZnO to become weaker. Crystallite size growth of Cu, carbon deposition, metal loss of Zn and Al, formation of Cu₂(OH)₂CO₃ and Zn₅(OH)₆(CO₃)₂ were important reasons for rapid deactivation of the Cu-based catalyst.     

Intrinsic kinetics study of LPDME process from syngas over bi-functional catalyst

The intrinsic kinetics of the three-phase dimethyl ether (DME) synthesis from syngas over a bi-functional catalyst has been investigated in a agitated slurry reactor at 20–50 bar, 200–240 °C and H₂/CO feed ratio from 1 to 2. The bi-functional catalyst was prepared by physical mixing of CuO/ZnO/Al₂O₃ as methanol synthesis catalyst and H-ZSM-5 as methanol dehydration catalyst. The three reactions including methanol synthesis from CO and H₂, methanol dehydration and water gas shift reaction were chosen as the independent reactions. A kinetic model for the combined methanol and DME synthesis based on a methanol synthesis model proposed by Graaf et al. [G.H. Graaf, E.J. Stamhuis, A.A.C.M. Beenackers, Kinetics of low pressure methanol synthesis, Chem. Eng. Sci. 43 (12) (1988) 3185; G.H. Graaf, E.J. Stamhuis, A.A.C.M. Beenackers, Kinetics of the three-phase methanol synthesis, Chem. Eng. Sci. 43 (8) (1988) 2161] and a methanol dehydration model by Bercic and Levec [G. Bercic, J. Levec, Intrinsic and global reaction rate of methanol dehydration over γ-Al₂O₃ pellets, Ind. Eng. Chem. Res. 31 (1992) 399–434] has been fitted our experimental data. The obtained coefficients in equations follow the Arrhenius and the Van’t Hoff relations. The calculated apparent activation energy of methanol synthesis reaction and methanol dehydration reaction are 115 kJ/mol and 82 kJ/mol, respectively. Also, the effects of different parameters on the reactor performance have been investigated based on the presented kinetic model.     

Low-temperature synthesis of DME from CO2/H2 over Pd-modified CuO–ZnO–Al2O3–ZrO2/HZSM-5 catalysts

Three series of Pd-modified CuO–ZnO–Al₂O₃–ZrO₂/HZSM-5 catalysts were prepared and characterized by BET, XRD and TPR analysis. The catalytic system was evaluated in the development of direct synthesis of dimethyl ether (DME) from carbon dioxide hydrogenation at low temperature (T=200 °C, P=3.0 MPa). The results indicated that the addition of palladium markedly enhanced the DME synthesis and retarded the CO formation. An explanation of this promoting effect of Pd on the DME synthesis could be attributed to the spillover of hydrogen from Pd⁰ to the neighboring phase.     

On the mechanisms and the selectivity determining steps in syngas conversion over supported metal catalysts: An IR study

An IR study of syngas and methanol conversion has been performed over Cu–ZnO–Al₂O₃ methanol synthesis catalyst, Ni–Al₂O₃ methanation catalyst and Co–Al₂O₃ Fischer Tropsch catalyst. The data, obtained at low pressure, provide unequivocal evidence of the existence of a way via oxygenated intermediates (formates, possibly dioxymethylene, methoxy groups) in the three cases. In the selectivity determining step, methoxy groups desorb associatively as methanol on Cu–ZnO–Al₂O₃. Methoxy groups are selectively hydrogenolyzed to methane over Ni–Al₂O₃. Over Co–Al₂O₃ oxygenated surface species may be involved in the chain growth to give C₂₊ compounds. It is possible that this mechanism coexists with the via-carbide all-metallic catalysis reported for methanation and FT synthesis, on the basis of studies performed on pure metals.     

A new method of low-temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2

A new synthesis method of low-temperature methanol proceeded on Cu/ZnO/Al₂O₃ catalysts from CO/CO₂/H₂ using 2-butanol as promoters. The Cu/ZnO/Al₂O₃ catalysts were prepared by co-impregnation of r-Al₂O₃ with an aqueous solution of copper nitrate and zinc nitrate. The total carbon turnover frequency (TOF), the yield and selectivity of methanol were the highest by using the Cu/ZnO/Al₂O₃ catalyst with copper loading of 5% and the Zn/Cu molar ratio of 1/1, which precursor were not calcined, and reduced at 493 K. The activity of the catalysts increased due to the presence of the CuO/ZnO phase in the oxidized form of impregnation Cu/ZnO/Al₂O₃ catalysts. The active sites of the Cu/ZnO/Al₂O₃ catalyst for methanol synthesis are not only metallic Cu but also special sites such as the Cu–Zn site, i.e. metallic Cu and the Cu–Zn site work cooperatively to catalyze the methanol synthesis reaction.     

Hydrogenation of CO2 to dimethyl ether on La-, Ce-modified Cu-Fe/HZSM-5 catalysts

Cu–Fe–La/HZSM-5 and Cu–Fe–Ce/HZSM-5 bifunctional catalysts were prepared and applied for the direct synthesis of dimethyl ether (DME) from CO₂ and H₂. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density of Cu and improved the reduction ability of the Cu–Fe catalyst in comparison to the Cu–Fe–Zr catalyst, which may increase the selectivity for DME. The Cu–Fe–Ce catalyst had a greater specific surface area than the Cu–Fe–La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the DME selectivity and the CO₂ conversion. The catalysts were stable for 15 h.     

A Novel, Low Temperature Synthesis Method of Dimethyl Ether Over Cu–Zn Catalyst Based on Self-Catalysis Effect of Methanol

A new DME synthesis route from syngas at a relatively low temperature (443 K) has been developed for the first time by the combination of a conventional DME synthesis catalyst (Cu/ZnO:HZSM-5 catalyst) with methanol as a catalytic solvent. The addition of methanol to the reaction system is the key to the success of DME synthesis at this temperature. Indeed, a CO conversion of 29 and 43% with a DME selectivity of 69 and 68% were achieved at 443 or 453 K, respectively, and 4 MPa, when methanol was used as a catalytic solvent. Importantly, no other by-products including methanol and hydrocarbons were observed in the DME product attained, suggesting no significant subsequent purification stages. Assuming no scale up problems, this process potentially provides a high purity of DME with less energy consumption, and so offers an opportunity for the economically viable future sustainable production of DME.     

Cu–Zn–Cr2O3 Catalysts for Dimethyl Ether Synthesis: Structure and Activity Relationship

The CuO dispersed on ZnCr₂O₄ catalysts derived from Cu–Zn–Cr hydrotalcite like layered double hydroxide precursors with varying Zn/Cr ratios have been synthesized, characterized by BET—Surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), electron spin resonance (ESR), N₂O titrations and the activities were evaluated for single step dimethyl ether (STD) synthesis from syngas. It is observed that the copper species were in highly dispersed state over Cu–ZnO–Cr₂O₃ at high Zn/Cr ratios while the copper cluster were present at low Zn/Cr ratios. The ESR analysis revealed signals due to isolated Cu²⁺ at high Zn/Cr ratios and clustered Cu²⁺ at low Zn/Cr ratio in fresh catalysts and only Cr³⁺ species in used catalysts. The TPR results indicated that the reduction peak shifted to high temperatures with an increase in chromium content due to large copper crystallites, which was supported by XRD analysis. The conversion of syngas to DME was well correlated with the copper metal surface areas, indicating that STD synthesis can be controlled by methanol synthesis rate.     

Research on the Acidity of the Double-function Catalyst for DME Synthesis from Syngas

Two kinds of HZSM-5 zeolite (SiO₂/Al₂O₃ = 50,300) were introduced into the STD (syngas-to-DME) reaction and the double-function catalysts containing CuO/ZnO/Al₂O₃ and HZSM-5 were investigated by activity evaluation and NH₃-TPD. It was found that the acidity of HZSM-5 played a critical role in the performance of STD catalyst, and an appropriate acidic amount was required to obtain the best activity of STD catalyst; more and less acidic amount were both unfavorable for DME yield.     

CO2 Hydrogenation to Methanol Over Cu/ZnO/Al2O3 Catalysts Prepared by a Novel Gel-Network-Coprecipitation Method

A novel gel-network-coprecipitation process has been developed to prepare ultrafine Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from CO₂ hydrogenation. It is demonstrated that the gel-network-coprecipitation method can allow the preparation of the ultrafine Cu/ZnO/Al₂O₃ catalysts by homogeneous coprecipitation of the metal nitrate salts in the gel network formed by gelatin solution, which makes the metallic copper in the reduced catalyst exist in much smaller crystallite size and exhibit a much higher metallic copper-specific surface area. The effect of the gel concentration of gelatin on the structure, morphology and catalytic properties of the Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from hydrogenation of carbon dioxide was investigated. The Cu/ZnO/Al₂O₃ catalysts prepared by the gel-network-coprecipitation method exhibit a high catalytic activity and selectivity in CO₂ hydrogenation to methanol.