Fe-based heterogeneous catalysts for the Fischer-Tropsch reaction: Sonochemical synthesis and bench-scale experimental tests

Sonoprocessing: From Concepts to Large-Scale Reactors

Intensification of ultrasonic processes for diversified applications, including environmental remediation, extractions, food processes, and synthesis of materials, has received attention from the scientific community and industry. The mechanistic pathways involved in intensification of ultrasonic processes that include the ultrasonic generation of cavitation bubbles, radical formation upon their collapse, and the possibility of fine-tuning operating parameters for specific applications are all well documented in the literature. However, the scale-up of ultrasonic processes with large-scale sonochemical reactors for industrial applications remains a challenge. In this context, this review provides a complete overview of the current understanding of the role of operating parameters and reactor configuration on the sonochemical processes. Experimental and theoretical techniques to characterize the intensity and distribution of cavitation activity within sonoreactors are compared. Classes of laboratory and large-scale sonoreactors are reviewed, highlighting recent advances in batch and flow-through reactors. Finally, examples of large-scale sonoprocessing applications have been reviewed, discussing the major scale-up and sustainability challenges.

The effect of microwave irradiation on heterogeneous catalysts for Fischer–Tropsch synthesis

The work that has been carried out on microwave irradiation applied in catalyst preparation for drying, calcination or postsynthesis methods, and as a heating source for the Fischer–Tropsch reaction has been reviewed. It has been found that microwave irradiation can, in some cases, greatly enhance the performance of heterogeneous catalyst systems for Fischer–Tropsch synthesis. We have also summarized the advantages and drawbacks of using microwave irradiation in Fischer–Tropsch catalyst preparation and postsynthesis, and identified opportunities for future investigation.

Ultrasound-assisted impregnation for high temperature Fischer-Tropsch catalysts

A fraction of the petroleum extracted from oil reservoirs contains associated natural gas. Rather than building infrastructure to recover low volumes of this natural gas, the industry flares or vents it to the atmosphere, which contributes to atmospheric greenhouse gas emissions but also reduces the air quality locally because it contains gaseous sulphur and nitrogen compounds. Converting the natural gas (NG) to hydrocarbons with a small-scale two-step gas-to-liquids process, is an alternative to flaring and venting. In the first step, NG reacts with oxygen to form syngas (Catalytic Partial Oxidation) and in the second step the syngas reacts over metallic catalysts to form higher paraffins at 210 °C to 300 °C-Fischer Tropsch synthesis (FT). For the first time, we synthesize bimetallic FeCo FT catalysts with ultrasound. An ultrasonic horn agitates the solution during the entire impregnation process. The active phase dispersion of the sonicated catalysts was superior to the catalyst synthesized without ultrasound, while reducing the impregnation time by a factor of three. We tested our catalysts in a lab-scale, fixed-bed reactor at 270 °C and 300 °C, and achieved 80% conversion over 3-days on stream and a 40% yield of C₂₊.

Ultrasound assisted synthesis of Ag-decorated TiO2 active in visible light

Titanium dioxide is the most popular photocatalyst to degrade organic pollutants in air, as well as in water. The principal drawback preventing its commercial application lies in its limited absorption of the visible light (400-700nm), while it is active under UV irradiation (≤387nm). Supporting noble metals in the form of nanoparticles on TiO₂ increases its activity in the visible range. However, both the synthesis of noble metal nanoparticles and their deposition on TiO₂ are multi-step processes that often require organic solvents. Here, we deposit Ag nanoparticles from AgNO₃ on the surface of micrometric TiO₂ with H₂O as a solvent and under ultrasound irradiation at 30Wcm^-2. Ultrasound increases the surface amount of Ag on TiO₂ with heterogeneous size distribution of Ag nanoparticles, which are bigger and overlaid (1-20nm vs. 0.5-3nm) compared to the sample obtained in traditional conditions (TEM images). While this change in morphology had no effect on acetone photodegradation under UV light, the 5%, 10%, and 20% Ag-TiO₂ degraded 17%, 20% and 24% acetone under visible light, respectively. The 10% by weight Ag-TiO₂ sample obtained in absence of ultrasound only degraded 14% acetone in 6h, while the bare TiO₂ was not active.