TiO2-supported heteropoly acids for low-temperature synthesis of dimethyl ether from methanol

From CO2 to DME: Enhancement through Heteropoly Acids from a Catalyst Screening and Stability Study

The direct synthesis of dimethyl ether (DME) via CO₂ hydrogenation in a single step was studied using an improved class of bifunctional catalysts in a fixed bed reactor (T _R: 210-270 °C; 40 bar; gas hourly space velocity (GHSV) 19,800 NL kg_cat ^-1 h^-1; ratio CO₂/H₂/N₂ 3:9:2). The competitive bifunctional catalysts tested in here consist of a surface-basic copper/zinc oxide/zirconia (CZZ) methanol-producing part and a variable surface-acidic methanol dehydration part and were tested in overall 45 combinations. As dehydration catalysts, zeolites (ferrierite and β-zeolite), alumina, or zirconia were tested alone as well as with a coating of Keggin-type heteropoly acids (HPAs), i.e., silicotungstic or phosphotungstic acid. Two different mixing methods to generate bifunctional catalysts were tested: (i) a single-grain method with intensive intra-particular contact between CZZ and the dehydration catalyst generated by mixing in an agate mortar and (ii) a dual-grain approach relying on physical mixing with low contact. The influence of the catalyst mixing method and HPA loading on catalyst activity and stability was investigated. From these results, a selection of best-performing bifunctional catalysts was investigated in extended measurements (time on stream: 160 h/7 days, T _R: 250 and 270 °C; 40 bar; GHSV 19,800 NL kg_cat ^-1 h^-1; ratio CO₂/H₂/N₂ 3:9:2). Silicotungstic acid-coated bifunctional catalysts showed the highest resilience toward deactivation caused by single-grain preparation and during catalysis. Overall, HPA-coated catalysts showed higher activity and resilience toward deactivation than uncoated counterparts. Dual-grain preparation showed superior performance over single grain. Furthermore, silicotungstic acid coatings with 1 KU nm^-2 (Keggin unit per surface area of carrier) on Al₂O₃ and ZrO₂ as carrier materials showed competitive high activity and stability in extended 7-day measurements compared to pure CZZ. Therefore, HPA coating is found to be a well-suited addition to the CO₂-to-DME catalyst toolbox.

Direct Synthesis of Dimethyl Ether from CO2: Recent Advances in Bifunctional/Hybrid Catalytic Systems

Dimethyl ether (DME) is a versatile raw material and an interesting alternative fuel that can be produced by the catalytic direct hydrogenation of CO2. Recently, this process has attracted the attention of the industry due to the environmental benefits of CO2 elimination from the atmosphere and its lower operating costs with respect to the classical, two-step synthesis of DME from syngas (CO + H2). However, due to kinetics and thermodynamic limits, the direct use of CO2 as raw material for DME production requires the development of more effective catalysts. In this context, the objective of this review is to present the latest progress achieved in the synthesis of bifunctional/hybrid catalytic systems for the CO2-to-DME process. For catalyst design, this process is challenging because it should combine metal and acid functionalities in the same catalyst, in a correct ratio and with controlled interaction. The metal catalyst is needed for the activation and transformation of the stable CO2 molecules into methanol, whereas the acid catalyst is needed to dehydrate the methanol into DME. Recent developments in the catalyst design have been discussed and analyzed in this review, presenting the different strategies employed for the preparation of novel bifunctional catalysts (physical/mechanical mixing) and hybrid catalysts (co-precipitation, impregnation, etc.) with improved efficiency toward DME formation. Finally, an outline of future prospects for the research and development of efficient bi-functional/hybrid catalytic systems will be presented.

Controllable coordination of a phosphotungstic acid-modified carbon matrix for anchoring Pt species with different sizes: from single atoms and subnanoclusters to nanoparticles

Pt species with different sizes were uniformly dispersed on phosphotungstic acid-modified carbon, and Pt SAs_0.5/PTA-C exhibited outstanding catalytic performance.

Effects of support and reaction pressure for the synthesis of dimethyl ether over heteropolyacid catalysts

Dimethyl ether (DME) is an advanced second-generation biofuel produced via methanol dehydration over acid catalysts such as γ-Al₂O₃, at temperatures above 240 °C and pressures above 10 bar. Heteropolyacids such as tungstosilicic acid (HSiW) are Brønsted acid catalysts with higher DME production rates than γ-Al₂O₃, especially at low temperatures (140–180 °C). In this work, we show that the performance of supported HSiW for the production of DME is strongly affected by the nature of the support. TiO₂ and SiO₂ supported HSiW display the highest DME production rates of ca. 50 mmol_DME/h/g_HSiW. Characterization of acid sites via ¹H-NMR, NH₃-isotherms and NH₃-adsrobed DRIFT reveal that HSiW/X have Brønsted acid sites, HSiW/TiO₂ showing more and stronger sites, being the most active catalyst. Methanol production increases with T until 200 °C where a rapid decay in methanol conversion is observed. This effect is not irreversible, and methanol conversion increases to ca. 90% by increasing reaction pressure to 10 bar, with DME being the only product detected at all reaction conditions studied in this work. The loss of catalytic activity with the increasing temperature and its increasing with reaction pressure accounts to the degree of contribution of the pseudo-liquid catalysis under the reaction conditions studied.

Heteropolyacid Salt Catalysts for Methanol Conversion to Hydrocarbons and Dimethyl Ether: Effect of Reaction Temperature

Phosphotungstic and silicotungstic acid salt catalysts (CuPW, CuSiW, FePW, FeSiW) were synthesized by substitution of protons with ferric and copper ions through a simple replacement reaction. The structure and thermal stability were characterized by IR, XRD and TG, and the salts showed a keggin structure and a thermal tolerance near 450 °C. Temperature programmed reactions indicated that the four catalysts showed similar trends in the change of methanol conversion, DME selectivity, and light olefins selectivity at 100–400 °C. Copper salt catalysts showed a 100% DME selectivity at temperatures ranging from 100–250 °C, while FeSiW and FePW catalysts had a 100% DME selectivity near 250 °C. Moreover, the heteropolyacid salt catalysts also produced a certain number of light olefins at the temperature ranging from 250–350 °C, and the CuSiW catalyst exhibited the highest ethylene and propylene selectivity of 44%. In the stability test evaluated at 200 °C, the catalysts showed different tendencies during the induction period and the same trends during the reduction period for the methanol conversion to DME, due to the differences in the strengths of the strong acid sites. Finally, the silicotungstic acid salt catalysts showed the longest lifetime of 120 h, much longer than the heteropolyacids. This approach provides an effective way to synthesize hydrocarbons through methanol, especially DME, at different temperatures using one catalyst.

Cycloaddition of CO2 with epoxides and esterification reactions using the porous redox catalyst Co-POM@MIL-101(Cr)

The catalytic activity of the Co-POM@MIL-101(Cr) composite in solvent-free cycloaddition of CO₂ to epoxides and esterification of acetic acid with alcohols is due to an outer-sphere electron transfer mechanism using the Co(iii)/Co(ii) redox pair.

CO₂ Recycling to Dimethyl Ether: State-of-the-Art and Perspectives

This review reports recent achievements in dimethyl ether (DME) synthesis via CO₂ hydrogenation. This gas-phase process could be considered as a promising alternative for carbon dioxide recycling toward a (bio)fuel as DME. In this view, the production of DME from catalytic hydrogenation of CO₂ appears as a technology able to face also the ever-increasing demand for alternative, environmentally-friendly fuels and energy carriers. Basic considerations on thermodynamic aspects controlling DME production from CO₂ are presented along with a survey of the most innovative catalytic systems developed in this field. During the last years, special attention has been paid to the role of zeolite-based catalysts, either in the methanol-to-DME dehydration step or in the one-pot CO₂-to-DME hydrogenation. Overall, the productivity of DME was shown to be dependent on several catalyst features, related not only to the metal-oxide phase-responsible for CO₂ activation/hydrogenation-but also to specific properties of the zeolites (i.e., topology, porosity, specific surface area, acidity, interaction with active metals, distributions of metal particles, …) influencing activity and stability of hybridized bifunctional heterogeneous catalysts. All these aspects are discussed in details, summarizing recent achievements in this research field.

Keggin H3PW12O40 pore blockage by coke can be reversible in the gas phase methanol-to-DME reaction

Flowing methanol induces a cracking of H₃PW₁₂O₄₀ particles which acts against pore blockage by coke in the methanol-to-DME reaction. As a result, coking does not necessarily decrease the performance.

Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman

To activate an HPA's bulk in the methanol-to-DME reaction: 1) dehydrate it, 2) pre-expose it to methanol at 25 °C.

TiO2-supported heteropoly acid catalysts for dehydration of methanol to dimethyl ether: relevance of dispersion and support interaction

TiO₂-supported heteropoly acids are very active catalysts for DME production from methanol. Due to the interaction with the support, the most active catalysts are obtained when a monolayer of HPA is deposited on the support.