Ultrasonic Preparation of Catalysts for Dimethyl Ether (DME) Conversion

Dimethyl ether (DME) is a favourable alternative fuel, which can be synthesised from methanol, CO₂ or syngas through catalysis. For the catalytic conversion to DME, potent catalysts are required. Nano-sized mesoporous catalysts such as mesoporous acidic zeolites, decorated zeolites or nano-sized metal catalysts such as aluminum or copper can improve the DME conversion significantly. High-intensity ultrasound is the superior technique for the preparation of highly reactive nano-catalysts. Learn more about how to use ultrasonication for the production of micro- and mesoporous catalysts with excellent reactivity and selectivity!

Bifunctional Catalysts for Direct DME Conversion

The production of dimethyl ether (DME) is a well-established industrial process that is divided into two steps: first, the catalytic hydrogenation of syngas into methanol (CO / CO₂ + 3H₂ → CH₃OH + H₂HO) and secondly, a subsequent catalytic dehydration of the methanol over acid catalysts to produce (2CH₃OH → CH₃OCH₃ + H₂O). The major limitation of this two-step DME synthesis is related to the low thermodynamics during the phase of methanol synthesis, which results in a low gas conversion per pass (15-25%). Thereby, high recirculation ratios as well as high capital and operating costs are occurring.
In order overcome this thermodynamic limitation, direct DME synthesis is significantly more favourable: In the direct DME conversion, the methanol synthesis step is coupled with the dehydration step in a single reactor
(2CO / CO₂ + 6H₂ → CH₃OCH₃ + 3H₂O).

The direct synthesis of DME allows for increasing the levels of conversion per step, up to 19%, which means significant cost reductions regarding the investment and operational production cost of DME. Based on estimations, the DME production cost in direct synthesis are reduced by 20-30% when compared to the conventional two-step conversion process. In order to operate the direct DME synthesis pathway, a highly efficient hybrid bifunctional catalytic system is necessary. The required catalyst must offer the functionality for CO / CO2 hydrogenation for the methanol synthesis and the acidic functionalities, which assist the dehydration of the methanol. (cf. Millán et al. 2020)

Synthesis of Highly-Reactive Catalysts for DME Conversion using Power-Ultrasound

The reactivity and selectivity of catalysts for dimethyl ether conversion can be significantly improved via ultrasonic treatment. Zeolites such as acid zeolites (e.g., aluminosilicate zeolite HZSM-5) and decorated zeolites (e.g., with CuO/ZnO/Al₂O₃) are the main catalysts that are used successfully for DME production.

Chlorination and fluorination of zeolites are effective methods to tune the catalytic acidity. The chlorinated and fluorinated zeolite catalysts were prepared by the impregnation of zeolites (H-ZSM-5, H-MOR or H-Y) using two halogen precursors (ammonium chloride and ammonium fluoride) in the study by the research team of Aboul-Fotouh. The influence of ultrasonic irradiation was evaluated for optimizing both halogen precursors for production of dimethylether (DME) via methanol dehydration in a fixed bed reactor. Comparative DME catalysis trial revealed that the halogenated zeolite catalysts prepared under ultrasonic irradiation show higher performance for DME formation. (Aboul-Fotouh et al., 2016)
In another study, the research team investigated all important ultrasonication variables encountered during carrying out the dehydration of methanol on H-MOR zeolite catalysts to produce dimethylether. For their Sonication eperiments, the research team used the Hielscher UP50H probe-type ultrasonicator. Scanning electron microscope (SEM) imaging of the sonicated H-MOR zeolite (Mordenite zeolite) have clarified that methanol by itself used as an ultrasonication medium gives the best results concerning the homogeneity of particle sizes compared to the untreated catalyst, where large agglomerates and non-homogeneous clusters appeared. These findings certified that ultrasonication has a deep effect on the unit cell resolution and hence on the catalytic behavior of the dehydration of methanol to dimethyl ether (DME). NH3-TPD shows that ultrasound irradiation has enhanced the acidity of H-MOR catalyst and hence it is catalytic performance for DME formation. (Aboul-Gheit et al., 2014)

Almost all commercial DME is produced by the dehydration of methanol using different solid-acid catalysts such as zeolites, sillica–alumina, alumina, Al₂O₃–B₂O₃, etc. by following reaction:
2CH₃OH <—> CH₃OCH₃ +H₂O(-22.6k jmol^-1)

Koshbin and Haghighi (2013) prepared CuO–ZnO–Al₂O₃/HZSM-5 nanocatalysts via a combined co-precipitation–ultrasound method. The research team found “that employing of ultrasound energy have great influence on the dispersion of CO hydrogenation function and consequently the DME synthesis performance. The durability of ultrasound assisted synthesized nanocatalyst was investigated during syngas to DME reaction. The nanocatalyst loses negligible activity over the course of reaction due to coke formation on copper species.” [Khoshbin and Haghighi, 2013.]

An alternative non-zeolite nano-catalyst, which is also very efficient in promoting the DME conversion, is a nano-sized porous γ-alumina catalyst. Nano-size porous γ-alumina was successfully synthesized by precipitation under ultrasonic mixing. The sonochemical treatment promotes the nano particles synthesis. (cf. Rahmanpour et al., 2012)

Why Are Ultrasonically Prepared Nano-Catalysts Superior?

For the production of heterogeneous catalysts often high value-added materials such as precious metals are required. This makes catalysts expensive and therefore, efficiency enhancement as well as life cycle extension of the catalysts are important economic factors. Among preparation methods of nanocatalysts, the sonochemical technique is considered as a highly efficient method. The ability of ultrasound to create highly reactive surfaces, to improve mixing and to increase mass transport makes it a particularly promising technique to explore for catalyst preparation and activation. It can produce homogeneous and dispersed nanoparticles without need for expensive instruments and extreme conditions.
In several research studies, the scientists come to the conclusion that ultrasonic catalyst preparation is the most advantageous method for the production of homogeneous nano-catalysts. Among preparation methods of nanocatalysts, the sonochemical technique is considered as a highly efficient method. The ability of intense sonication to create highly reactive surfaces, to improve mixing and to increase mass transport makes it a particularly promising technique to explore for catalyst preparation and activation. It can produce homogeneous and dispersed nanoparticles without need for expensive instruments and extreme conditions. (cf. Koshbin and Haghighi, 2014)

High Performance Ultrasonicators for the Synthesis of Mesoporous Catalysts

Sonochemical equipment for the synthesis of high-performance nano-catalysts is readily available at any size – from compact lab ultrasonicators to fully-industrial ultrasonic reactors. Hielscher Ultrasonics designs, manufactures, and distributes high-power ultrasonicators. All ultrasonic systems are made in the headquarter in Teltow, Germany and distributed from there all around the world.
The sophisticated hardware and smart software of Hielscher ultrasonicators are designed to guarantee reliable operation, reproducible outcomes as well as user-friendliness. The Hielscher ultrasonicators are robust and reliable, which allows to be installed and operated under heavy duty conditions. Operational settings can be easily accessed and dialled via intuitive menu, which can be accessed via digital colour touch-display and browser remote control. Therefore, all processing conditions such as net energy, total energy, amplitude, time, pressure and temperature are automatically recorded on a built-in SD-card. This allows you to revise and compare previous sonication runs and to optimize the synthesis and functionalization of nano-catalysts to highest efficiency.
Hielscher Ultrasonics systems are used worldwide for sonochemical synthesis processes and are proven to be reliable for the synthesis of high-quality zeolite nano-catalystss as well as zeolite derivatives. Hielscher industrial ultrasonicators can easily run high amplitudes in continuous operation (24/7/365). Amplitudes of up to 200µm can be easily continuously generated with standard sonotrodes (ultrasonic probes / horns). For even higher amplitudes, customized ultrasonic sonotrodes are available. Due to their robustness and low maintenance, our ultrasonicators are commonly installed for heavy duty applications and in demanding environments.
Hielscher ultrasonic processors for sonochemical syntheses, functionalization, nano-structuring and deagglomeration are already installed worldwide on commercial scale. Contact us now to discuss your nano-catalyst manufacturing process! Our well-experienced staff will be glad to share more information on the sonochemical synthesis pathway, ultrasonic systems and pricing!
With the advantage of the ultrasonic synthesis method, your mesoporous nano-catalyst production will excel in efficiency, simplicity and low cost when compared to other catalyst synthesis processes!

The table below gives you an indication of the approximate processing capacity of our ultrasonicators: