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Hydrothermal synthesis is the state-of-the-art technique for the preparation of zeolites and related porous solids. However, when it comes to the preparation of nanosized zeolites, this technique is limited by low yields, separation problems and high amounts of waste. In this work, we utilized the strengths of a combination of spray drying and steam-assisted crystallization (SAC), also known as dry gel conversion, to reduce these problems. At spray drying temperatures between 300 and 400 °C, it was possible to convert all the amorphous material via SAC into zeolite particles without extra addition of template. Kinetic studies of SAC revealed that about 4 to 8 days are needed to achieve the formation of a 100% crystalline product. The newly formed crystalline phase was crystallized on the surface of the nanosized zeolites and led to a slight increase in the primary particle size while the macroscopic morphology of the spray-dried aggregates was preserved. This work demonstrates that the combination of spray drying and SAC are useful tools in supplementing the hydrothermal synthesis of nanosized zeolites.

The direct synthesis of dimethyl ether (DME) from synthesis gas via methanol as an intermediate is a promising process for realizing a decentralized, small-scale Power-to-X concept. The efficient realization of this coupled process depends to a large extent on the appropriate catalyst configuration, i.e. the combination of the catalyst for methanol formation with the one for the subsequent dehydration step. In this work, two catalyst configurations were compared in terms of CO-conversion and DME-selectivity using modelling and experiments. Catalysts for methanol formation (Cu/ZnO/Al2O3, CZA) and dehydration (Zeolite H-ZSM-5, Z) prepared via flame spray pyrolysis and hydrothermal synthesis, respectively, were combined in the form of hybrid pellets (CZA&Z) and CZA-core@zeolite-shell (CZA@Z) particles. During the synthesis of the CZA@Z system, alteration of the CZA core in terms of CuO-reducibility and activity in methanol synthesis were found after individual steps of calcination and hydrothermal shell synthesis. In contrast, the preparation of the CZA&Z system presents an easy and tunable method with promising catalytic properties in terms of CO-conversion and DME-selectivity, once the proper mass ratio of the two catalysts was set. From modelling, for the same CZA/Z ratio in general higher CO-conversion was found for the CZA&Z system as compared to the CZA@Z, while the latter system shows higher DME-selectivity.

A microfluidic reactor enabled continuous co-precipitation synthesis of CuO/ZnO/Al2O3 catalysts for methanol production.

A continuous-flow reactor and a continuous-flow setup compatible with operando x-ray absorption spectroscopy (XAS) were designed for safely studying liquid-phase reactions on solid high atomic number transition metal catalysts (e.g., Au, Pd, and Pt) under pressures up to 100 bars with temperatures up to 100 °C. The reactor has a stainless-steel body, 2 mm thick polyether ether ketone (PEEK) x-ray windows, and a low internal volume of 0.31 ml. The rectangular chamber (6 × 5 × 1 mm3) between the PEEK x-ray windows allows us to perform XAS studies of packed beds or monoliths in the transmission mode at any position in the cell over a length of 60 mm. A 146° wide-angle beam access also allows recording complementary x-ray fluorescence or x-ray diffraction signals. The setup was engineered to continuously feed a single-phase liquid flow saturated with one or more gaseous reactants to the liquid–solid XAS reactor containing no free gas phase for enhanced process safety and sample homogeneity. The proof of concept for the continuous-flow XAS cell and high-pressure setup was provided by operando XAS measurements during the direct synthesis of hydrogen peroxide at room temperature and 40 bars using a 35 ± 5 mg catalyst (1 wt. % Pd/TiO2) and inline near-infrared spectroscopy. The experiments prove that the system is well suited to follow the reaction in the liquid phase while recording high-quality XAS data, paving the way for detailed studies on the catalyst structure and structure–activity relationships.

Heterogeneous catalysis, a process in which the reaction of gaseous or liquid chemical reagents is facilitated at the surface of a solid material, is responsible for the majority of industrial-scale chemical and fuel production reactions. The energy required to drive these reactions has historically been derived from the combustion of non-renewable fossil fuels and carries an unavoidably large carbon footprint. More recently, the development of environmentally responsible and sustainable chemical industries is increasingly motivated by greenhouse gas-induced climate change, thus creating demand for eco-friendly heterogeneous catalytic processes. This includes innovative approaches enabled by renewable forms of energy, such as the electrification of chemical and petrochemical processes, utilization of CO2 as a feedstock and the incorporation of light into catalytic reactions. Herein we review the conversion of solar energy to chemical energy using CO2, and describe how the photophysical and photochemical properties of nanostructured metal oxide photocatalysts have been engineered to efficiently incorporate light into heterogeneous gas–solid CO2 hydrogenation reactions. Realizing high photonic and energy efficiencies in these systems has demanded innovation in not only photocatalyst engineering, but also photoreactor and process engineering. Rather than exclusively providing an in-depth discussion of the chemistry and science within each individual study, this Tutorial Review highlights the multidisciplinary character of photocatalysis studies by covering the four essential components of a typical research work in this field (materials engineering, theoretical modelling, reactor engineering and process development) via case studies of the archetypal indium oxide catalyst materials. Through advances in these four components, progress has been made towards the ultimate goal of industrializing the production of CO2-derived chemicals and fuels.

The direct synthesis of hydrogen peroxide over TiO2-supported mono- and bimetallic Pd, PdSn, and PdIn nanoparticles (NPs) was performed in a continuous plug-flow reactor at 80 bar in ethanol with H2 : O2 ratios varied from 10 : 1 to 1 : 10. At the same time the catalysts were monitored by operando X-ray absorption spectroscopy (XAS). The setup optimized for XAS allowed productivities that are among the highest reported up to now. A rate of up to 580 mmolH2O2 gcat−1 h−1 and a H2O2 concentration of 80 mmol l−1 were obtained which were only limited by the supply of reactants. During H2O2 synthesis, the studied NPs revealed a face centered cubic (fcc) Pd(Sn/In) metal (alloy) structure at H2 : O2 ratios equal to or smaller than 1 and the corresponding β-hydride structure at H2 : O2 > 1. Under all conditions, additional SnO2/In2O3 species were observed for the bimetallic catalysts. XAS supported by DFT calculations showed that alloying Pd with In or Sn limited the H2 uptake capacity and the corresponding lattice expansion of the bimetallic NPs. Different catalysts performed best at different H2 : O2 ratios. All catalysts were stable at H2 : O2 > 1. Significant leaching of the active Pd and PdIn species could be observed for H2 : O2 ≤ 1 (quantified by XAS), while PdSn was relatively stable under these conditions. The higher stability of PdSn NPs is proposed to be due to a SnO2 shell providing strong bonding between the NPs and the titania support.

The integration of specific sensors into microfluidic reactors and devices is crucial for the optimization of controllable variables such as flow, temperature, energy input (light, microwaves etc.). In this review, we highlight the state of the art for the integration of in situ sensors.

ACS catalysis 8, 2546 - 2557 (2018). doi:10.1021/acscatal.7b03514, The direct synthesis of $H_{2}O_2$ is a dream reaction in the field of selective oxidation and green chemical synthesis. However, the unknown active state of the catalyst and the lack of a defined catalytic mechanism preclude the design and optimization of suitable catalysts and reactor setups. Here direct synthesis of $H_{2}O_2$ over $Pd–TiO_2$ in water was investigated in a continuous flow reactor setup utilizing undiluted oxygen and hydrogen to increase aqueous-phase concentrations safely at ambient temperature and a pressure of 10 bar. In this experiment operando X-ray absorption spectroscopy (XAS) and online flow injection analysis for photometric quantification of $H_{2}O_2$ were combined to build catalyst structure–activity relationships in direct $H_{2}O_2$ synthesis. XAS at the Pd K absorption edge was used to observe the oxidation state and local Pd structure together with $H_{2}O_2$ production for three reactant ratio ($H_{2}O_2$ ) regimes: hydrogen rich (>2), hydrogen lean (, Published by ACS, Washington, DC

Ceria-based catalysts are widely used in oxidation or oxidation-reduction reactions in the field of environmental science. Their catalytic functions are determined by their ability to exchange oxygen species with oxidants. The enhancement of oxygen release is desired since it is often the rate-determining step in redox cycles. Herein, we developed a lattice oxygen distortion method to enhance oxygen activation by quenching the Ce-Zr oxide nanoparticles formed from an extremely high temperature. This process can ensure the formation of solid solutions as well as avoiding atomic rearrangement during calcination, retaining the lattice oxygen at a metastable and disordered state without vacancies. Reduction, vacuum or metal deposition will easily induce oxygen release accompanied by vacancy creation. The metastable oxides can provide about 19 times more oxygen vacancies than traditional ones in a CO atmosphere. CO oxidation rates increased with increasing Zr content from 25 to 75% and achieved a new level, which is attributed to the acceleration of oxygen circulation via promoting oxygen release and supplying plenty of oxygen vacancies for redox cycles. This strategy is expected to be applied in the design and fabrication of improved oxygen storage materials.

A one-stage syngas-to-fuel process was carried out by integrating Fischer-Tropsch synthesis (FTS) with hydrocracking (HC) in a micro-reactor to verify the feasibility of a related process simplification with a view to small-scale X-to-liquid (XtL) plants. Different approaches to the integration of the two process steps and the influence of the operating conditions were investigated targeting liquid fuel synthesis (C5–C20). It was proven that FTS plays the determining role in the integrated process and HC only exhibits a promoting function. Good performance was obtained in both sequential and mixed-bed systems. With an increase of temperature the liquid fuel selectivity reaches a plateau value of around 70% at 230–250 °C, depending on the integration pattern and syngas space velocity. H2/CO ratio and pressure only have limited influence on the fuel selectivity. FTS was enhanced in the mixed-bed configuration. Meanwhile, consistent hydrocarbon distribution was obtained for both integration patterns when reaching the plateau fuel selectivity. The catalyst stability in both configurations was analyzed, and a reasonable and comparable long-term stability was obtained.

Drop-on-demand inkjet printing was used to apply nanoparticulate GaPd2 catalyst as thin and uniform wall coatings into micro-channels. Several inks were formulated and evaluated concerning their stability and printability. Three different printing approaches were investigated and the obtained layers were examined regarding their homogeneity, adhesion and catalytic performance. After a specific catalyst activation step, semi-hydrogenation of acetylene was conducted at 523 K and a feed gas composition of 0.5 vol.% C2H2, 5 vol.% H2, 50 vol.% C2H4 in He. Printed GaPd2/a-Al2O3 showed the best catalytic results and a selectivity of 76% for the hydrogenation to ethylene with a high activity of 480 mol(C2H2)/mol(Pd)/h.

Process simplification by integrating Fischer–Tropsch synthesis (FTS) with hydrocracking (HC) in a microstructured reactor was studied experimentally and by modeling and simulation. A simplified model was created to investigate the experimentally observed FTS and HC performance in the presence of larger amounts of liquid. It is assumed that part of the catalyst would be blocked if the liquid fraction exceeds a threshold. The liquid blocking would cause enlarged diffusion resistance or even inaccessible regions in the catalyst bed, particularly in microstructured reactors with broad flat channels such as the annular geometry of this study. Significant mass transfer limitation was observed for Fischer–Tropsch synthesis in the assumed liquid-filled spots. Hydrocracking exhibits limited response to internal diffusion limitation. Good agreement with experimental data was obtained here by assuming inaccessible local regions in the packed bed. Identical cross section-averaged effectiveness factors of 0.6–0.7 wer...

A Cu/ZnO/Al2O3@ZSM-5 core@shell catalyst active for one-step conversion of synthesis gas to dimethyl ether (DME) was imaged simultaneously and in situ using synchrotron-based micro X-ray fluorescence (μ-XRF), X-ray diffraction (μ-XRD), and scanning transmission X-ray microscopy (STXM) computed tomography (CT) with micrometer spatial resolution. An identical sample volume was imaged stepwise, first under oxidizing and reducing atmospheres (imitating calcination and activation processes), and then under model reaction conditions for DME synthesis (H2:CO:CO2 ratio of 16:8:1, up to 250 °C). The multimodal imaging methods offered insights into the active metal structure and speciation within the catalyst, and allowed imaging of both the catalyst core and zeolite shell in a single acquisition. Dispersion of nanosized Cu species was observed in the catalyst core during reduction, with formation of a metastable Cu+ phase at the core–shell interface. Under DME reaction conditions at 1 bar, the coexistence of Cu0 i...

The integration of Fischer-Tropsch synthesis (FTS) with direct hydrocracking (HC) was investigated in a two-stage reactor system, where the influence of FTS performance (concentration of residual H 2 and CO as well as amount of heavy hydrocarbons) on HC was studied and the overall performance of the integrated process was analyzed regarding the liquid fuel (C 5 -C 20 ) selectivity. Sufficient cracking of the C 21+ fraction was achieved under FTS conditions and primary cracking was proven as dominant. Higher temperature has a positive effect on HC, but too high temperature would turn out to increase the C 1-4 selectivity and a maximum of 70 wt.% C 5 -C 20 was obtained. The cracking of FTS hydrocarbons could be divided into two categories, cracking of heavy molecules via liquid phase and cracking of light molecules depending on gas phase. The cracking of lights was slightly suppressed when raising the H 2 partial pressure, while the cracking of heavy molecules showed no response. Differently, an increase of the CO partial pressure obviously inhibited the cracking performance of both fractions. The wax flow rate study revealed a quite selective cracking of FTS wax, since the cracking of light hydrocarbons is partial pressure-limited. As long as enough wax covers the catalyst along the bed, a further increase in the catalyst amount has no effect on the final product distribution.

Pollutants accumulating in natural and drinking water systems can cause severe effects to the environment and living organisms. Photocatalysis is a promising option to degrade such pollutants. When immobilizing the photocatalyst, additional catalyst separation steps can be avoided. Among various reactor types, the use of microreactors in photocatalysis has proven advantageous regarding process intensification. However, so far the local conditions are not well understood and described in literature and there is little quantitative understanding of the relevant phenomena. In this work, inkjet-printing was used to immobilize TiO$_{2}$ as a thin film with a precisely tuneable thickness and catalyst loading. In a batch reactor, the degradation of rhodamine B (RhB) as a model pollutant was performed for different initial concentrations and catalyst layer thicknesses. By employing the Langmuir–Hinshelwood model and a light irradiation model, the kinetic parameters were determined. The influence of the light intensity at different positions inside the immobilized photocatalyst on the reaction kinetics is quantified. RhB degradation was tested under defined operational conditions using an in-house developed continuous-flow microreactor with advanced fiber optics for precise light introduction. The models derived from batch experiments were used to simulate the degradation in the continuous-flow microreactor. Results show that the simulation allows prediction of the performance with less than 20% deviation to the experimental data. An analysis of mass transport effects on the reaction rate indicates that external mass transfer is a limiting factor in the microreactor experiment. This study further demonstrates the potential of the new reactor system (microreactor, fiber optics and printed catalyst) for detailed investigations on photocatalytic reaction kinetics.

The direct synthesis of dimethyl ether (DME) from synthesis gas (STD) via methanol as an intermediate is a promising option for implementation of the Power-to-X concept, involving the storage of renewable electrical energy via hydrogen or synthesis gas in synthetic fuels or chemicals. The STD reaction shifts the equilibrium conversion dictated by thermodynamics to higher values compared to methanol synthesis alone at given conditions. Notwithstanding, proper catalyst materials and a suitable configuration have to be found that would support high CO-conversion as well as high DME-selectivity. In this work, different catalyst configurations obtained by combining CuO/ZnO/Al2O3 (CZA) for methanol synthesis with zeolite H-ZSM-5 (Z) for its dehydration are investigated via simulation based on a crystallite-pore network model, able to describe e.g. the polycrystalline anisotropic zeolite. In this way, hybrid particles with proximity of the two catalysts in both, the micrometer-scale (medium proximity, CZA+Z) and in the sub-micrometer-scale (close proximity, CZA&Z) as well as structured core@shell particles (CZA@Z) were investigated for a tubular reactor. Moreover, their planar (coated) counterparts, namely the hybrid layered systems with medium (CZA#Z) and close proximity (CZA&/Z) and a structured double layer system (CZA//Z) were investigated for a wall-coated microchannel reactor. The performance of these systems in the STD reaction was studied focusing on the influence of the CZA/Z ratio and the operational parameters (temperature, GHSV, feed composition). According to this study, hybrid particles with close proximity of the two catalysts as well as the double layer structures showed the best performance in terms of CO-conversion and DME-selectivity.

A continuous microfluidic setup was developed to study colloidal synthesis of gold nanoparticles using tetrachloroauric acid as precursor, sodium borohydride as reducing agent and PVP as stabilizer. The setup consists of pressurized vessels that allow pulsation-free flow of reactants and a microfluidic chip with integrated micromixers essential for efficient mixing with small mixing time (2 ms) followed by a meandering microchannel. The microfluidic chip enables recording X-ray absorption spectra (XAS) in situ at different positions along the microchannel at high flow rates approaching turbulent mixing conditions and thus to correlate reaction time with changes in the nanoparticle structure. Significant contributions of oxidized gold could be observed after the first 6 ms of the reaction, whereas after 10 ms principally all gold appeared to be in a metallic state. The nanoparticles obtained were characterized ex situ by various complementary techniques. The resulting nanoparticles had average diameter of 1.0 nm and narrow size distributions compared with those produced in a batch reactor. Depositing the nanoparticles on TiO2 resulted in catalysts with two different Au loadings (0.7 and 1.7 wt% Au/TiO2) which exhibited good CO oxidation activity.

A highly active catalyst is a necessity for a compact microchannel membrane reactor for methane steam reforming (MSR) due to the limitation of catalyst loading per membrane area and the deliberately reduced working temperature compared to a conventional reformer. Two series of Rh/Al2O3 catalysts with nominal Rh loadings in the range of 0.2–5 wt.% were prepared by flame spray pyrolysis (FSP) using flame made and preformed support particles. For the flame made support, Rh and Al precursors were premixed and sprayed simultaneously. In the other case, the Rh precursor was mixed with a preformed boehmite particle suspension and then sprayed into the flame. Their catalytic activity was investigated for MSR in a fix-bed reactor. The nominal 1 wt.% Rh catalyst supported on preformed Al2O3 exhibited the highest CH4 conversion of all catalysts at W/F = 0.374 gcat h/molCH4 and 600 °C. The turnover frequency (TOF) over both types of catalysts was much higher than the values of reported Rh reforming catalysts in that temperature range. In addition, the TOF values of catalysts based on preformed Al2O3 particles exceeded those over the ones with in situ formed Al2O3 by 29–39%. It is proposed that the preformed Al2O3 support promotes the catalytic activity, possibly enhancing H2 spillover which is considered to facilitate the rate determining steps in MSR. The successful synthesis of catalysts supported by preformed materials opens new possibilities for preparation of nano-structured catalysts from a multitude of support materials via flame spray pyrolysis.

The concept of hydrogen storage via liquid organic hydrides in cycles of hydrogenation and dehydrogenation with preferably low amount of reaction enthalpy is well known under the abbreviation LOHC, Liquid Organic Hydrogen Carrier. However, our study addresses the dehydrogenation of methylcyclohexane in a prototype microstructured membrane reactor, which is mainly devoted to the heat storage in a so-called LORC, a Liquid Organic Reaction Cycle. While using the same cycles of hydrogenation and dehydrogenation, heat shall be stored on the contrary by high reaction enthalpy in the LORC. A novel planar membrane module with an integrated catalytic fixed-bed has been developed. It is equipped with structured supports to reduce the mechanical stress on the Pd-membrane foil. Experiments investigating the membrane separation performance were conducted with operational parameters up to 400 °C and 13 bar. The ability of shifting equilibrium conditions to product side and pure H2 recovery was shown. Catalyst deactivation and regeneration was tested in order to analyze stability of the Pd-membrane. An experimental parameter study was performed to obtain a future optimized design of a large-scale prototype reactor.

The approach of producing hydrogen peroxide (H2O2) by direct catalyzed reaction of H2 and O2 has been extensively studied over the past decades. The process offers the potential to establish a “green” alternative to the conventional large-scale anthraquinone process. However, the implementation of this reaction still faces major hurdles including safety, selectivity and productivity of the reaction. In this work, we report results on the development of a novel intensified suspension-flow membrane micro reactor system for direct synthesis of hydrogen peroxide as well as on a related flexible micro reactor system for transfer hydrogenation which served as a starting point. In the new reactor system for H2O2 direct synthesis safety issues are greatly reduced by the introduction of a membrane to separate the gaseous reactants. Furthermore productivity is maximized by increased mass transfer in micro channels and constant reactant feeding over the length of the reactor. Finally, the selectivity can be optimized by controlling the reactant distribution in the liquid phase. A rigorous two-dimensional single-phase model based on ANSYS Fluent was adopted to study the influence of the design on the reaction. This model had already been validated in a previous study. By expanding the model to describe the multiphase phenomena we were able to get further insight on the behavior of the overall system.

In this work, the kinetics of CO2 methanation reaction is investigated on a 17 wt% Ni3Fe/ɣ-Al2O3 catalyst under technical operation conditions of Power-to-Gas processes. A short-contact time microstructured packed bed reactor with internal cross-flow cooling channel was designed particularly for kinetic studies of the highly exothermic methanation reaction. The influence of temperature, pressure, reactant and product composition as well as residence time on the product composition and conversion of the reactants was carefully studied. In total more than 160 data points were collected and fitted to three literature models applying a non-isothermal reactor model. Based on the state-of-the-art kinetic models and the experimental observations, a new LHHW rate equation for the consecutive CO2 methanation reaction was developed. This new model has reasonably lower number of parameters and improves description of the reaction compared to literature models with regard to the Ni3Fe catalyst system. Special attention was paid to the selectivity behavior and the influence of carbon monoxide and water on the reaction.

We performed studies on the oxygenation of a copper(I) complex targeting the intrinsic kinetics of this reaction in a continuous flow setup. It is an example reaction for the metal-based activation of dioxygen. For these experiments we used a SuperFocus mixer, a customised micro mixer with fast mixing behaviour. By means of this setup, we were able for the first time to detect the formation and decay of a thermally very sensitive bis(μ-oxo)dicopper species at ambient temperature. Comparing these data to results from a stopped-flow setup we could confirm the performance of the SuperFocus mixer setup. The rate constant for the decay of the bis(μ-oxo)dicopper species was determined to be 0.90 s−1 using the SuperFocus mixer and to be 1.57 s−1 by stopped-flow at slightly different temperatures.

The recent literature on direct synthesis of H 2 O 2 is reviewed with respect to two important directions. The best catalysts to date are assessed with a view to their composition, e.g., alloys, and regarding the role of the support material and solvent used. Moreover, a number of novel reactor concepts that have been proposed in literature for direct synthesis of H 2 O 2 are analyzed with a focus on their potential for practical implementation. These include: (i) microchannel process technology for safe mixing and conversion of the reactants in undiluted gaseous state, (ii) dense palladium-based membranes acting as a means to prevent gas-phase contact of the reactants while providing a catalytically active interface, (iii) asymmetrically porous catalytic membranes operated in wetted state as an interphase contactor where one reactant is introduced in a controlled way from the gas phase into the liquid reaction medium, (iv) microreactors for multiphase operation with improved mass transfer efficiency compared to conventional reactor technologies, and (v) electrocatalytic synthesis, both in electrolysis mode and in fuel cell mode, which also promise improved safety by avoiding gas-phase mixing of hydrogen and oxygen and increased selectivity due to improved reaction control. Concerning the first aim, an acid-pretreated 5 wt.% Pd/C catalyst appears to be the best choice for methanol as solvent whereas a Pd-Au exchanged Cs-containing heteropolyacid catalyst is the current benchmark for water as solvent. With proper acid pre-treatment of the Pd/C catalyst or acid function incorporated into the support (heteropoly acids), no halides and no inorganic acids are needed in the solvent to prevent hydrogen peroxide decomposition. However, the H 2 O 2 hydrogenation activity in pure water is still rather high which limits the selectivity. Among the different reactor concepts, microchannel process technology seems to offer key advantages in terms of high productivity, selectivity and process safety. The electrocatalytic approach also looks promising. However, major achievements are still needed, e.g., successful scale-up of these concepts to relevant throughputs confirming the good performance obtained in lab scale.

Despite intensive research carried out by academia and industry over the past decades, the liquid phase direct hydrogen peroxide synthesis still faces a lot of challenges including safety, selectivity and productivity of the reaction. One facet of this multiphase reaction is the mass transport of the gaseous reactants, hydrogen and oxygen, in the liquid phase and inside of the catalyst. However, this has not been receiving much attention although, in many of the experiments reported in literature it seems to have affected to a large extent the results obtained. In this paper we report an analysis of performance data from a catalytic membrane reactor system published by Pashkova et al. in 2010 [1] . A rigorous two-dimensional model based on ANSYS Fluent was adopted to study the effects of an inert packing installed in the membrane reactor. The simulation results reveal a massive influence of mass transport limitation on the performance of the experimental system and also indicate significant potential for optimization of productivity and selectivity by adopting micro process technology combined with membranes for reactant dosage.

The importance of the direct synthesis process for the production of hydrogen peroxide from hydrogen and oxygen is apparent from the numerous publications and patents on the subject since the beginning of the 20th century. The prospects it would open up for various hydrogen peroxide applications are undeniable. However, the challenges with which the process has to cope are not to be underestimated. Besides the development of different reactor concepts, one attempt to address the issues concerning safety, activity, selectivity and cost, is the search of alternative solvents. Deploying dense carbon dioxide as solvent is a possibility investigated in recent years. Carbon dioxide would be an environmentally friendly alternative and would also offer benefits in terms of enhanced solubility for the reactants as well as diffusivity. This review summarizes the proposed concepts for CO 2 as medium for the direct synthesis process of hydrogen peroxide and the challenges going along with it. Relevant properties of CO 2 for this particular process together with some economic aspects will be discussed as well.

A spherical microsized core-shell catalyst (CuO/ZnO/Al2O3@H-ZSM-5) was synthesized for direct dimethyl ether synthesis from syngas in a micro packed bed reactor. To assist in the preparation and optimization of the core-shell catalyst, a 1D heterogeneous model was developed to simulate diffusion and reactions within the catalyst particles. Using the data obtained from the tubular reactor, kinetic parameters for the catalyst were determined first. Furthermore, simulation showed that thickness and activity of shell have significant effect on the catalytic performance of the core-shell catalyst.

Producción Científica, A novel plasmonic reactor concept is proposed and tested to work as a visible energy harvesting device while allowing reactions to transform CO2 to be carried out. Particularly the reverse water gas shift (RWGS) reaction has been tested as a means to introduce renewable energy into the economy. The development of the new reactor concept involved the synthesis of a new composite capable of plasmonic activation with light, the development of an impregnation method to create a single catalyst reactor entity, and finally the assembly of a reaction system to test the reaction. The composite developed was based on a Cu/ZnO catalyst dispersed into transparent aerogels. This allows efficient light transmission and a high surface area for the catalyst. An effective yet simple impregnation method was developed that allowed introduction of the composites into glass microchannels. The activation of the reaction was made using LEDs that covered all the sides of the reactor allowing a high power delivery. The results of the reaction show a stable process capable of low temperature transformations.

Inkjet printing technology was applied for the precise deposition of alumina nanoparticles in microchannels as a catalyst support layer. Several alumina nanoparticle containing inks were prepared and tested for inkjet printability. It could be shown that additives such as ethylene glycol and polyethylene glycol are needed for aqueous inks up to a concentration of 50 wt.% for continuous drop generation. The printing was conducted both in semicircular and rectangular microchannels. The coating thickness was controlled by repetitive printing of each channel, and the generated layers were uniform in thickness throughout the microchannel foil. The printed alumina layers were impregnated with rhodium nitrate after calcination. A high metal loading of 15 wt.% was applied to enable a high reaction rate per coated area despite the low thickness of the layer, and the catalytic activity was demonstrated for methane steam reforming (MSR). The prepared catalyst layers were highly active, and conversions exceeding 98% were obtained at 973 K and a W/F (catalyst weight/CH4 feed rate) ratio of 19.7 ( kgcat s ) / mol CH 4 .

In the future, (electro‐)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power‐to‐chemical processes require a shift from steady‐state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well‐known that the structure of catalysts is very dynamic. However, in‐depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time‐resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

In terms of tons per day SO3 is one of the most important chemicals in the world. It is produced via the contact process in a gas phase catalytic oxidation reaction. The contact process consists of multiple catalyst beds, inter-stage cooling and an absorption step in the case of sulphuric acid production. Transferring this process to a microstructured reactor with its unique heat and mass transport properties should make it possible to develop a small scale one pass synthesis well suited for on-site SO3 production, e.g., for surfactant making. In a test reactor experiments were performed with different molar ratios of sulphur dioxide to oxygen, hydrodynamic residence time and reaction temperature. Numerical simulations with ANSYS FLUENT 12.0 were performed for a single channel configuration. For this purpose a detailed model of the flow, the heat distribution and the surface chemistry was used. For the surface chemistry a detailed kinetic model based on elementary steps occurring on the catalyst surface was used. The assumed multi-step mechanism involves coordination of SO2 on two surface sites. The simulations are compared to the experimental results. Good agreement between the model predictions and the experimental results is achieved. A sensitivity analysis was performed to get an idea of the rate determining steps.

The process of sulphur trioxide production could be conducted as a one-step process in microreactors with forced decreasing temperature profile due to their unique heat and mass transfer capabilities. The choice of a very active catalyst in such reactors is one of the key issues to avoid excessive microfabrication and consequently reactor costs. Comparing vanadium to platinum, we found that platinum is the preferred catalyst based on calculations of the ratio of catalyst to reactor volume combined with the specific activity of the catalysts. To demonstrate the feasibility of the one-step process experimentally, we prepared catalysts with 9 wt.% Pt on TiO 2 on microstructured titanium foils by means of a sol–gel method followed by wet impregnation. The catalyst was investigated by SEM, EDX, adsorption measurements and microprobe analysis to study the Pt distribution. We found almost equal distribution of platinum in the TiO 2 layer, both with respect to the axial position in the channels as well as regarding the concentration profile of Pt in depth of the support layer. Pores might at least partially be blocked with Pt. Furthermore, we investigated the influence of the start-up behaviour on the lifetime of the catalyst, finding that starting up with the SO 2 –O 2 mixture gives the most stable operation. Humidity, which cannot be avoided in the laboratory set-up and the graphite-sealed microreactor, is assumed to be the main reason for deactivation during shut-downs.

The surface of hydrogen-permeable PdCu membranes acting as a catalyst for direct hydroxylation of benzene to phenol in the gas phase in a novel catalytic double-membrane reactor was modified by sputtering on it different catalytic layers with the aim to increase the formation rate and selectivity to phenol. Three different systems are described: a 1 μm thick compact layer of Pd 90 Au 10 (10 wt.% Au), a 5 μm thick compact layer of PdGa (50 at.% Ga) and a thin film of Pd 90 Au 10 deposited on a discontinuous V 2 O 5 layer. The different systems were characterized by SEM, EDX, and mainly in terms of their catalytic properties for benzene hydroxylation. The formation rate and the selectivity to phenol could be increased substantially through the catalytic modification. With a maximum phenol selectivity of 67% at 150 °C and a maximum phenol formation rate of 1.67 × 10 −4 mol h −1 m −2 at 200 °C, PdAu reached the best performance in double-membrane operation mode. PdGa showed even more promising results compared to PdAu in kinetic experiments in co-feed operation mode, but suffers from the very low hydrogen permeability of PdGa which stands against its use as a continuous layer in the catalytic membrane reactor.

A continuous process for safe direct synthesis of H2O2 from H2 and O2 over Pd-catalysts in a gas/liquid membrane contactor is proposed. As a prerequisite for the process a method was developed for coating of Pd based catalysts into the fine porous layer on the inside of asymmetric tubular membranes. The method reached uniform distribution and high dispersion of the Pd nano particles with ca. 11 nm size for a range of membrane geometries and lengths and a variety of membrane materials. For the selection of the best catalytic system additional experiments with supported catalysts were performed. They were prepared using the same materials and coating method as for the manufacture of the catalytic membranes. The principle of the membrane contactor was verified with 10 cm long single channel membranes with stable operation of the continuous system for more than 10 h at differential pressures up to 4.5 bar in methanol. Further, the role of key process parameters such as solvent type, system pressure and flow regime were assessed. Productivities up to 1.7 mol H 2 O 2 g pd − 1 h − 1 ( 6.1 mol H 2 O 2 m − 2 h − 1 related to the geometric surface area of the membrane) with a selectivity of 83% towards H2O2 were achieved.

A study of the catalytic membrane contactor operated in pore-flow-through (PFT) mode was carried out for hydrogenation of α-methylstyrene (AMS) to cumene over palladium as a test reaction. By applying a metalorganic chemical vapour deposition method, the catalyst was deposited as nanoparticles on the pore walls of porous alumina capillaries. Experiments were performed with up to six individual Pd-activated capillaries in a laboratory-scale reactor set-up and with capillary bundles in a small pilot plant. The influence of the operating parameters on the reactor performance such as temperature (303–333 K), hydrogen pressure (5–10 bar), transmembrane flux (up to 200 l·m−2·min−1), pore size of the capillaries (0.6–3.0 µm) and concentration of AMS in n-heptane used as solvent was investigated. Moreover, the performance of the PFT-system was compared to that of conventional reactors (e.g. fixed bed, trickle bed, bubble column, stirred tank) and novel reactor designs (catalytic membrane contactor in diffuser mode, monolith reactor) based on published data. It is shown that the PFT principle enables very high activity, comparable with a suspended powder catalyst in a stirred tank reactor and, at the same time, an excellent space time yield. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Asymmetric porous alumina membranes with palladium nanoparticles deposited into the thin surface layer are used as a structured catalyst to perform the direct synthesis of hydrogen peroxide with separate supply of the reactants in a catalytic membrane contactor. Parameters such as mode of supply of the gaseous species, type of liquid medium used, as well as differential and liquid side pressure, have been investigated and their influence on the productivity and selectivity of the reaction is discussed. The presented concept, which targets local hydrogen peroxide production, offers improved safety of the process compared to conventional methods for direct synthesis described in the literature. By optimization of the reactant supply, high productivity (up to 16.8 mol h −1 m −2 in methanol) and high selectivity (between 80% and 90%) to hydrogen peroxide could be demonstrated in this reaction for the first time with catalytic membranes.

A novel method of Pt catalyst direct deposition on polypyrrole (ppy)-modified Nafion composite membranes for direct methanol fuel cell (DMFC) is proposed in this paper. Proton conductivity and methanol permeability of polypyrrole-modified Nafion composite membranes were investigated. The Pt particles were directly deposited on the PPy-modified Nafion composite membranes by using chemical reduction of platinic chloride at room temperature. Scanning electron microscopy measurements showed that Pt particles with a porous structure uniformly exist in the Pt/PPy/Nafion electrodes. The roughness factor and the active surface area of the Pt catalyst increased with increasing PPy loading. The performance of the Pt/PPy/Nafion electrodes was evaluated in a single DMFC.

The feasibility of using poly(3,4-ethylenedioxythiophene) (PEDOT) as Pt catalyst support for direct methanol fuel cell (DMFC) anodes was investigated. Measurements with freshly prepared Pt-PEDOT/C electrodes showed poor activity for methanol oxidation in a half-cell and a DMFC. A substantial enhancement in that activity was evident after either electrochemical over-oxidation of PEDOT or long-time storage of the Pt-PEDOT/C gas diffusion electrode (GDE) in air. Both procedures led to a reorganization and increase in porosity of the reaction layer, which obviously contributed to better methanol accessibility to Pt catalyst active centres. The effects of electrochemical activation and long-time storage in air on the morphology and elementary composition of the Pt-PEDOT layer were investigated by means of Hg porosimetry and SEM/EDAX. It was found that the increase in porosity was due to degradation of PEDOT characterized by a significant depletion of sulphur and oxygen in the conducting polymer matrix.

Cu/ZnO/Al2O3 catalysts were synthesized by flame spray pyrolysis (FSP). The effect of different metal precursor types, i.e. metal nitrates and organometallics, on the catalytic properties was investigated. Organometallic precursors are commonly used for flame spray pyrolysis because small nanoparticles can be produced. In this study, we have obtained nanosized copper and zinc oxide clusters also from the nitrate precursors. Characterization was applied to reveal the difference between the clusters obtained from the different precursor types. Both precursors allowed the formation of well-ordered Cu/ZnO/Al2O3 particles with similar size according to TEM investigations. However, the catalyst from metal nitrate precursors possessed a lower reduction temperature, a higher active copper surface area, and a lower overall BET surface area than the one from the organometallic precursor. The catalytic performance of the obtained catalysts was investigated in the direct DME synthesis from synthesis gas. Methanol dehydration catalyst, H-ZSM-5, was therefore admixed to the FSP powders in a pre-defined amount; the FSP powders served as methanol synthesis catalyst in the mixture. The catalyst from metal nitrate precursors showed higher conversion of syngas than the catalyst from the organometallic precursors at same reaction conditions. This effect can be explained mainly by the higher copper surface area. Catalysts with different Cu/Zn ratio were also tested and the best catalyst was further studied by variation of the reaction conditions. In conclusion, we have demonstrated an efficient utilization of less expensive precursor materials for flame spray pyrolysis for production of Cu/ZnO/Al2O3 catalysts.

An H-ZSM-5 membrane was used in this work for xylene isomerization and for separation of p-xylene from the other isomers. MFI membranes (silicalite-1 and H-ZSM-5) were prepared by seeded growth on porous stainless steel supports. The membranes were tested for separation of the xylene isomers in the temperature range 100–400 °C. The H-ZSM-5 membrane showed higher p-xylene permselectivity (βp/o = 4.3 at 300 °C) and lower permeance to p-xylene (3.55 × 10−8 mol m−2 s−1 Pa−1 at 300 °C) compared to the silicalite-1 membrane (βp/o = 2.1, permeance to p-xylene 7.4 × 10−7 mol m−2 s−1 Pa−1 at 300 °C). However, both were not completely defect-free. The kinetics of xylene isomerization on the catalytically active H-ZSM-5 membrane and, for comparison, on a conventional H-ZSM-5 powder catalyst was studied in a gradient-free recycle reactor (Berty-type). Thereby the membrane showed higher activity than the conventional catalyst. The reaction kinetics was best described by a general triangular scheme. The H-ZSM-5 membrane was then used in a membrane reactor for xylene isomerization and the results were compared to those obtained in a packed-bed reactor using the conventional H-ZSM-5 catalyst. At 400 °C, a relative increase of the conversion of m-xylene by 15% was observed in the membrane reactor combined with higher p-xylene selectivity.

A method to calculate the steady-state multicomponent mass transfer in heterogeneous structures is presented. For this, different transport mechanisms represent different regions inside the composite system. The solving scheme allows the calculation of the molar flow rates along the different transport pathways and the estimation of unknown transport parameters based on experimental data. The steady-state mass transfer of hydrogen and sulfur hexafluoride through a sinter metal supported MFI zeolite membrane is examined as a model system. The support is simulated using the dusty gas model, the molecules passing the zeolite layer may follow transport mechanisms like Knudsen diffusion, surface diffusion or activated gas diffusion. The configuration used, with data from single gas experiments, can predict the binary system only at high temperatures. The problems arising at low temperatures are due to the oversimplified assumption that adsorbed and desorbed molecules do not interact on their way through the MFI zeolite micropores. However, because of the modular structure of the approach, it can be easily extended to more advanced transport models to account for interactions between the different molecules. And the method can be applied equally well to other systems, such as palladium composite membranes or membranes with catalytic activity.

This paper summarises the main ideas of how to use membranes with built-in catalytic function for performing catalysed gas/liquid reactions. Mass transfer effects on catalyst performance and other decisive factors for the design of industrial gas/liquid and gas/liquid/solid reactors are briefly reviewed. Various concepts for applying catalytic membrane layers in multiphase reactors are introduced and the motivation behind their use is explained. What follows is an assessment of the present achievements in using such concepts for gas/liquid reactions. The results available in literature are critically evaluated, focussing mainly on the field of liquid-phase hydrogenations and oxidations, which constitute the major areas where catalytic membranes have been applied to date to gas/liquid systems. The aim of such treatment is to point out the targets, to elaborate on how far the whole field has developed and what challenges remain, rather than to cover a multitude of specific aspects of the many different studies reported in literature.

Catalytically active membranes can be applied for three-phase reactions (liquid, gas and solid catalyst) and have advantages over conventional particle catalysts. Catalytically active components are deposited in the thin fine-porous membrane layer of an asymmetrical ceramic membrane. One reactant is dissolved in the liquid and diffuses through the porous structure of the membrane to the active inner surface, the other reactant is fed through the support to the catalytic layer from the other side of the membrane. Thereby, an effective contact between the two reactants and the solid catalyst is established. Under these conditions catalytically active membranes can typically be applied for hydrogenation or oxidation processes. Alternatively, if pore diffusion needs to be eliminated the dissolved reactants can be pumped through an asymmetric ceramic membrane or just a ceramic support coated with catalytically active metals. Thus, a very short contact time can be achieved. The following paper compares these two concepts—the catalytic diffuser and the forced through flow concept—and discusses their application for hydrogenation processes, like the catalytic nitrate/nitrite reduction in water and the dechlorination of chlorinated hydrocarbons. The developed catalytic membranes are not limited to these reactions, but are applicable for a number of multiphase reactions. A preparation method in order to make ceramic membranes catalytically active and different characterization methods will be described.

Membrane reactors applied to catalytic reactions are currently being studied in many places world-wide. Significant developments in membrane science and the vision of process intensification by multifunctional reactors have stimulated a lot of academic and industrial research, which is impressively demonstrated by more than 100 scientific papers on catalytic membrane reactors being published per year. Palladium as a noble metal with exceptional hydrogen permeation properties and, at the same time, broad applicability as a catalyst, first of all for hydrogenation, is part of many of these developments. This paper discusses two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst. Dense palladium composite membranes can be used for hydrogen separation from packed-bed catalysts in gas-phase hydrocarbon dehydrogenation reactions. Mesoporous membranes containing dispersed bimetallic Pd/X-clusters can be employed as so-called catalytic diffusers for liquid-phase hydrogenation, e.g. of nitrate and nitrite in water. The principles of both concepts are introduced, recently obtained experimental data are evaluated in connection with literature results, and the perspectives for further development are highlighted.

Palladium composite membranes were prepared by high velocity oxy-fuel flame spraying (HVOF) and electroless plating, respectively. Palladium was deposited onto porous stainless steel and alumina tubes. High-temperature permeation tests (350–650°C) were carried out to determine the permeation properties as well as the thermal and mechanical stability of the prepared membranes. The membrane morphology was characterized by scanning electron microscopy. Thicknesses of the Pd-layers ≥3 (electroless plating) and ≥50 μm (HVOF) were necessary to get defect-free films. The membranes showed a good hydrogen permeability and selectivity for both preparation methods. However, the experimental results indicate that the separation behaviour — for membranes prepared by electroless plating — depends on the thickness of the Pd-layer. The N2-permeation of membranes with Pd-layers less than or equal to about 7 μm thickness increased significantly in presence of hydrogen at temperatures above 400°C. In contrast membranes having a Pd-layer well above 7 μm thickness proved to be stable at high temperatures, which is one of the prerequisites for employment of these membranes in industrially relevant dehydrogenation reactions. This was evident from a comparison of the data of permeation runs with H2/N2-mixtures and single gases.

A catalytic membrane contactor for selective hydrogenation of nitrate in water to nitrogen is discussed as a promising new approach to develop a technically feasible catalytic process for nitrate reduction from ground and surface water. Metal organic chemical vapour deposition (MOCVD) was used to place catalytically active metals, i.e., palladium and tin, inside the porous top-layer of asymmetric ceramic membranes (e.g. alumina) with different pore sizes and thicknesses of the top-layer. The influence of the membrane properties, i.e., the material, pore size and thickness of the catalytic layer, and the influence of the MOCVD process parameters on the deposition of the metals was studied and catalytic membranes with different palladium loading and palladium/tin ratio were prepared. These membranes were characterised with respect to the pore structure and the distribution of the active metals. Their catalytic performance in the hydrogenation of nitrate in water was investigated in a laboratory stirred tank membrane reactor under continuous flow conditions to explore the influence of important process parameters. The results show that the catalytic membranes offer a high activity and a fair selectivity to nitrogen which may be further improved by an optimisation of the preparation procedure.

Different methods for the preparation of hydrogen permselective palladium composite membranes on asymmetric ceramic and porous stainless-steel tubes were tested. Electroless plating, electroplating, chemical and physical vapor deposition, as well as high velocity oxy-fuel spraying were investigated. Electroless plating was confirmed to be a suitable method for coating ceramic supports. Promising methods for the preparation of composite palladium–steel membranes are high velocity oxy-fuel spraying and a combined method of electroplating and electroless plating. Successfully coated membranes were employed for hydrogen separation in lab-scale membrane reactors during the dehydrogenation of ethylbenzene to styrene and propane to propylene. In both cases, the removal of hydrogen significantly increased the olefin yield, as compared to the corresponding conventional packed-bed reactor results. A relative increase of styrene yield above 15% was observed using a palladium–ceramic membrane. The relative increase of propylene yield was close to 18% for the same type of membrane. Employing a stainless steel-based membrane even further increased the yield of propylene. However, in contrast to the virtually defect-free ceramic-based membranes, a certain amount of other components passed the palladium–steel membrane along with hydrogen, amounting to roughly 20% of the total exit gas flow.

Hydrogenation of nitrate using supported bimetallic Pd-Cu or Pd-Sn catalysts provides a promising new alternative for denitrification of drinking water with several advantages over established technologies such as reverse osmosis or biological denitrification (Sell et al., 1992). One of the major problems of this technology is the formation of undesired side products, i.e. ammonium and nitrite, both subject to a limiting value far below the admissible nitrate content. Previous studies have shown that the particle size, among other effects, is a key parameter determining the selectivity of the catalyst, pointing to a detrimental effect of pore diffusion limitation (Hahnlein et al., 1998). Moreover, activity and selectivity depend on the concentration of dissolved hydrogen, i.e. high concentration means high activity, but at the same time favoring of ammonium formation due to further hydrogenation. In this paper a porous catalytic membrane acting as a hydrogen diffuser is proposed as a means to solve these problems by creating an efficient three-phase contact between the catalytic surface, dissolved nitrate, and hydrogen gas. Such a design offers the potential to control the activity and selectivity of the process through controlled dosage of hydrogen. Experimental data on the hydrogenation of aqueous nitrate using commercial ceramic membranes modified by insertion of palladium and copper or tin into the pore-structure through impregnation and MOCVD (metalorganic chemical vapor deposition) are presented and discussed together with characterization results obtained by SEM/EDAX and XPS.

The catalytic dehydrogenation of ethylbenzene to styrene was studied in a tubular palladium membrane reactor using a commercial styrene catalyst. A mathematical model of the membrane reactor is presented which takes into account the different mass transport mechanisms prevailing in the various layers of the membrane, that is, multicomponent diffusion in the stagnant gas films on both faces of the membrane, combined effective multicomponent diffusion, effective Knudsen diffusion, and viscous flow in the macroporous support, Knudsen diffusion in the microporous intermediate layer, and Sieverts' law of hydrogen transport through the Pd-film. A kinetic model from the literature [1–3] was adjusted to match conversion and selectivity observed during experiments with a commercial catalyst in a laboratory fixed-bed reactor. Simulation calculations based on the resulting effective kinetics were carried out for industrially relevant operating conditions and various process configurations, that is, use of inert sweep gas, evacuation of the permeate gas, and oxidation of the permeated hydrogen. The results demonstrate that with the present catalyst used under typical process conditions ( T, P, WHSV, S/O ) removal of hydrogen through the membrane gives only a small increase of styrene yield. However, the model predicts that by increasing the reaction pressure in the membrane reactor the kinetic limitation can be overcome and ethylbenzene conversion can be increased to above 90% without markedly decreasing styrene selectivity.

Microreactor technology with high heat transfer in combination with stable catalysts is a very attractive approach for reactions involving major heat effects such as methane steam reforming and to some extent, also the high temperature water gas shift (WGS) reaction. For this study Rh/ceria catalysts and an ultrathin hydrogen selective membrane were characterized in situ in a microreactor specially designed for x-ray absorption spectroscopic measurements under WGS conditions. The results of these experiments can serve as a basis for further development of the catalysts and membranes.