Reactor Designs for Safe and Intensified Hydrogenations and Oxidations: From Micro- to Membrane Reactors

The current and pressing environmental challenges are leading towards an important paradigm shift within the chemical industry. Green chemistry can be performed by using selective catalysts, and renewable and environment-friendly feedstock. For this reason, it is essential to provide scientists with platform tools that can allow safe and reliable catalyst testing, screening and studies. At the same time, as the use of green feedstock, such as oxygen and hydrogen, can pose new hazards, the design of intensified reactors can represent an unmissable opportunity to drive this green shift within the safe and scalable production of valuable molecules. This thesis reports reactor design solutions of different scale that have been devised to guarantee safe and intensified catalytic hydrogenations and oxidations for catalyst testing and continuous production purposes. Starting with the aim of studying a catalyst under realistic operating conditions, a silicon microfabricated reactor was designed and tested for the gas phase combustion of methane and carbon monoxide over palladium and platinum catalysts. Owing to its small volume and to its isothermal temperature profile, this microreactor proved to be a safe and effective tool for performing information-rich experiments, while exhibiting a plug-flow behaviour with negligible external and internal mass transfer resistances. Reactions were performed in combination with X-ray absorption and IR spectroscopy, allowed by the detailed microfabrication reactor design, to investigate the catalyst structure-activity relationships in steady-state and transient experiments. Boosting the catalyst activity can be achieved using catalytic nanoparticles, which offer an increased surface area compared to their bulk equivalents and hence an improved reaction rate. However, accessibility of the reactants to supported nanoparticles can be limited by the diffusion phenomena occurring around and inside a catalyst support. A recent trend of supporting nanoparticles onto surfaces modified using polyelectrolyte assemblies has attracted attention owing to the low temperature, ease and environmentally friendly preparation process. Finely tuned ex situ synthesised palladium nanoparticles were adsorbed on the inner surface of a tubular Teflon AF-2400 membrane, which was modified with polyelectrolytes in a layer-by-layer configuration. The membrane was used as a tubular reactor inside an outer tube with pressurised hydrogen, and nanoparticles of different size and shape were tested in the continuous hydrogenation of nitrobenzene to aniline. The observed reactivity depended on the different nanoparticle size and on the palladium oxidation state. The use of a tube-in-tube membrane reactor ensured process safety owing to the small volume of gas stored in the tube annular section and to the continuous processing. Alcohol oxidations using molecular oxygen can be dangerous due to the risk of creating explosive mixtures with the organic substrate. Two slurry loop reactors were developed using the same Teflon AF-2400 membrane in different configurations: a tube-in-tube and a flat membrane configuration for scalable reactions. These were designed and tested to carry out safe aerobic oxidation of alcohols. The membrane separated the oxygen from the organic phase and allowed a controlled dosing of the gaseous reactant. In order to boost the turnover frequency, the catalyst was used in the form of a slurry which was recirculated in a loop where it contacted the membrane saturator and a crossflow filter. This allowed the withdrawal of the liquid products from the loop. The reactors could be operated continuously, and provided improved process safety and comparable catalyst turnover frequency to conventional batch processes. When scaling up reactors, inadequate mixing can occur impacting on process safety and product quality. A Taylor-vortex membrane reactor is presented for the first time, combining the benefits of a flexible baffle structure inside a Taylor-vortex system that can hinder axial dispersion, and a supported tubular membrane for safe gas-liquid reactions. Stable conversion and product selectivity were achieved in the homogeneously catalysed continuous aerobic oxidation of benzyl alcohol. No pervaporation of organics through the membrane was detected during reaction, making this reactor a safe and a scalable tool for continuous gas-liquid reactions

Aerobic oxidation of alcohols using a slurry loop membrane reactor

A loop reactor was designed and employed in the safe oxidation of alcohols using a flat polymeric membrane for controlling oxygen dosing, a circulating catalyst slurry for efficient catalyst usage and an extendable loop volume for higher productivity

Taylor‐vortex membrane reactor for continuous gas–liquid reactions

A unique Taylor-vortex membrane reactor (TVMR) design for continuous gas–liquid reactions is presented in this work. The reactor consists of a cylindrical rotor inside a stationary concentric cylindrical vessel, and a flexible system of equispaced baffle rings surrounding the rotor. This restricts the annular cross section to a small gap between the baffles and the rotor, and divides the annulus into 18 mixing zones. The baffles support a 6 m long PFA tubular membrane that is woven around the rotor. At 4 mL/min inlet flow rate, the TVMR showed a plug-flow behavior and outperformed the unbaffled reactor, having 5–12 times lower axial dispersion. The continuous aerobic oxidation of benzyl alcohol was performed for 7 h using the Pd(OAc)2/pyridine catalyst in toluene at 100 °C and 1.1 MPa oxygen pressure. A stable conversion of 30% was achieved with 85% benzaldehyde selectivity, and no pervaporation of organics into the gas phase

Slurry loop tubular membrane reactor for the catalysed aerobic oxidation of benzyl alcohol

A novel reactor that combines a catalyst slurry flowing inside a loop configuration, incorporating a tubular membrane (Teflon AF-2400) for controlled oxygen delivery was designed and employed for the aerobic oxidation of benzyl alcohol using a 1 wt% Au-Pd/TiO2 powdered catalyst. This reactor keeps the liquid phase saturated with oxygen, while avoiding the creation of bubbles in it, thus enhancing operation safety. Experimental results in batch mode compared with those of a conventional autoclave demonstrated that the slurry loop membrane reactor reached a similar oxidation turnover frequency (20,000–25,000 h−1) with comparable values of benzaldehyde selectivity (∼70%). Continuous operation was achieved by using a crossflow filter connected to the loop to keep the catalyst from exiting the reactor. Using a 60 cm long tubular membrane, with the slurry flow circulating at 10 mL/min, continuous reaction was performed at 100–120 °C, 0–5 bar oxygen pressure and 1.2–5.0 g/L catalyst loading. Selectivity to benzaldehyde increased by either decreasing the reaction temperature or increasing the external oxygen pressure. The oxygen consumption rate decreased linearly with the catalyst loading, suggesting negligible gas-liquid mass transfer resistance. This was further evidenced by doubling the length of the tubular membrane, which had no effect on the oxidation turnover frequency. The slurry loop membrane reactor showed significantly better performance than packed-bed membrane microchannel reactors, and similar performance as that of a trickle-bed capillary reactor. This reactor can be implemented for a wide range of applications, which rely on the use of powder catalyst, are limited by gaseous reactant availability and require safe operation

Aerobic Oxidation of Benzyl Alcohol in a Continuous Catalytic Membrane Reactor

© 2018, The Author(s). A catalytic membrane reactor with a Au–Pd catalyst, impregnated at the inner side of the membrane, was studied in the catalytic oxidation of benzyl alcohol in flow. The reactor comprised of four concentric sections. The liquid substrate flowed in the annulus created by an inner tube and the membrane. The membrane consisted of 3 layers of α-alumina and a titania top layer with 5 nm average pore size. Oxygen was fed on the outer side of the membrane, and its use allowed the controlled contact of the liquid and the gas phase. Experiments revealed excellent stability of the impregnated membrane and selectivities to benzaldehyde were on average > 95%. Increasing the pressure of the gas phase and decreasing liquid flowrates and benzyl alcohol concentration resulted in an increased conversion, while selectivities to benzaldehyde remained constant and in excess of 95%.status: publishe

Aerobic Oxidation of Benzyl Alcohol in a Continuous Catalytic Membrane Reactor

A catalytic membrane reactor with a Au–Pd catalyst, impregnated at the inner side of the membrane, was studied in the catalytic oxidation of benzyl alcohol in flow. The reactor comprised of four concentric sections. The liquid substrate flowed in the annulus created by an inner tube and the membrane. The membrane consisted of 3 layers of α-alumina and a titania top layer with 5 nm average pore size. Oxygen was fed on the outer side of the membrane, and its use allowed the controlled contact of the liquid and the gas phase. Experiments revealed excellent stability of the impregnated membrane and selectivities to benzaldehyde were on average > 95%. Increasing the pressure of the gas phase and decreasing liquid flowrates and benzyl alcohol concentration resulted in an increased conversion, while selectivities to benzaldehyde remained constant and in excess of 95%

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

Two Zr-modified alumina supports were synthetized containing the same amount of Zr but a different distribution of this modifier over the alumina surface. These supports, together with the unmodified alumina carrier, were used to prepare three cobalt-based catalysts which were characterized and tested under relevant Fischer–Tropsch conditions. The three catalysts presented very similar porosity and cobalt dispersion. The addition of Zr nor its distribution enhanced the catalyst reducibility. The catalyst activity was superior when using a carrier consisting of large ZrO islands over the alumina surface. The use of a carrier with a homogeneous Zr distribution had however, a detrimental effect. Moreover, a faster initial deactivation rate was observed for the Zr-promoted catalysts, fact that may explain this contradictory effect of Zr on activity. Finally, the addition of Zr showed a clear enhancement of the selectivity to long chain hydrocarbons and ethylene, especially when Zr was well dispersed.The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2013) under Grant Agreement No. 308733. The authors are thankful to the Spanish Ministerio de Economía y Competitividad—MINECO (references: BES-2013-062806, ENE2013-47880-C3-2-R and ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European Union

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

Two Zr-modified alumina supports were synthetized containing the same amount of Zr but a different distribution of this modifier over the alumina surface. These supports, together with the unmodified alumina carrier, were used to prepare three cobalt-based catalysts which were characterized and tested under relevant Fischer–Tropsch conditions. The three catalysts presented very similar porosity and cobalt dispersion. The addition of Zr nor its distribution enhanced the catalyst reducibility. The catalyst activity was superior when using a carrier consisting of large ZrO2 islands over the alumina surface. The use of a carrier with a homogeneous Zr distribution had however, a detrimental effect. Moreover, a faster initial deactivation rate was observed for the Zr-promoted catalysts, fact that may explain this contradictory effect of Zr on activity. Finally, the addition of Zr showed a clear enhancement of the selectivity to long chain hydrocarbons and ethylene, especially when Zr was well dispersed.Ministerio de Economía y Competitividad—MINECO (BES-2013-062806, ENE2013-47880- C3-2-R and ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European UnionEuropean Union Seventh Framework Programme (FP7/2013) under Grant Agreement No. 30873

Control of catalytic nanoparticle synthesis:general discussion

International audienc

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

Two Zr-modified alumina supports were synthetized containing the same amount of Zr but a different distribution of this modifier over the alumina surface. These supports, together with the unmodified alumina carrier, were used to prepare three cobalt-based catalysts which were characterized and tested under relevant Fischer–Tropsch conditions. The three catalysts presented very similar porosity and cobalt dispersion. The addition of Zr nor its distribution enhanced the catalyst reducibility. The catalyst activity was superior when using a carrier consisting of large ZrO2 islands over the alumina surface. The use of a carrier with a homogeneous Zr distribution had however, a detrimental effect. Moreover, a faster initial deactivation rate was observed for the Zr-promoted catalysts, fact that may explain this contradictory effect of Zr on activity. Finally, the addition of Zr showed a clear enhancement of the selectivity to long chain hydrocarbons and ethylene, especially when Zr was well dispersed.Ministerio de Economía y Competitividad—MINECO (BES-2013-062806, ENE2013-47880- C3-2-R and ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European UnionEuropean Union Seventh Framework Programme (FP7/2013) under Grant Agreement No. 30873