Monoliths as multiphase reactors: A review

Recent Advances in Regulating Ceramic Monolithic Catalyst Structure for Preferential Oxidation of CO in H2

Preferential oxidation of CO (CO-PROX) has tremendous significance in purifying hydrogen for fuel cells to avoid catalyst poisoning by CO molecules. Traditional powder catalysts face numerous challenges, including high pressure drop, aggregation tendency, hotspot formation, poor mass and heat transfer efficiency, and inadequate thermal stability. Accordingly, ceramic monolithic catalysts, known as their excellent thermal stability, high surface area, and superior mass and heat transfer characteristics, are gaining increasing research attention. This review examines recent studies on ceramic monolithic catalysts in CO-PROX, placing emphasis on the regulation of active sites (e.g., precious metals like Pt and Au, and non-precious metals like CuO and CeO2), monolith structures, and coating strategies. In addition, the structure-catalytic performance relationships, as well as the potential and limitations of different ceramic monolithic catalysts in practical application, are discussed. Finally, the challenges of monolithic catalysts and future research prospects in CO-PROX reactions are highlighted.

Leave No Photon Behind: Artificial Intelligence in Multiscale Physics of Photocatalyst and Photoreactor Design

Although solar fuels photocatalysis offers the promise of converting carbon dioxide directly with sunlight as commercially scalable solutions have remained elusive over the past few decades, despite significant advancements in photocatalysis band-gap engineering and atomic site activity. The primary challenge lies not in the discovery of new catalyst materials, which are abundant, but in overcoming the bottlenecks related to material-photoreactor synergy. These factors include achieving photogeneration and charge-carrier recombination at reactive sites, utilizing high mass transfer efficiency supports, maximizing solar collection, and achieving uniform light distribution within a reactor. Addressing this multi-dimensional problem necessitates harnessing machine learning techniques to analyze real-world data from photoreactors and material properties. In this perspective, the challenges are outlined associated with each bottleneck factor, review relevant data analysis studies, and assess the requirements for developing a comprehensive solution that can unlock the full potential of solar fuels photocatalysis technology. Physics-informed machine learning (or Physics Neural Networks) may be the key to advancing this important area from disparate data towards optimal reactor solutions.

Additive manufacturing of microstructured reactors for organometallic catalytic reactions

The use of Additive Manufacturing for the fabrication of chemical reactors for flow chemistry is a promising field as it can lead to several improvements over more standard equipment. In this work, two different reactors were fabricated and compared: a Honeycomb monolith reactor with straight channels and a Periodic Open Cell Structure reactor. The Honeycomb monolith reactor was used as an example of a standard reactor (not necessarily additive manufactured) while the Periodic Open Cell Structure is a promising new type of reactor, which improves some key features, such as contact surface area and porosity. The two reactors were manufactured by Stereolithography technology with a high temperature resin and their internal surfaces were chemically activated by the grafting of palladium. For the surface activation, a two-step procedure was developed, firstly using NaOH and in a second step an aqueous solution of Na₂PdCl₄. After activation, a heterogeneous catalytic reaction was used to characterize the performance of the two fabricated reactors. The chosen reaction was the Suzuki-Miyaura reaction, which is commonly used in the pharmaceutical industry. The experimental results showed that, for equal contact surface area, the new designed reactor had better performance compared to the standard geometry.

Review on Additive Manufacturing of Catalysts and Sorbents and the Potential for Process Intensification

Additive manufacturing of catalyst and sorbent materials promises to unlock large design freedom in the structuring of these materials, and could be used to locally tune porosity, shape and resulting parameters throughout the reactor along both the axial and transverse coordinates. This contrasts catalyst structuring by conventional methods, which yields either very dense randomly packed beds or very open cellular structures. Different 3D-printing processes for catalytic and sorbent materials exist, and the selection of an appropriate process, taking into account compatible materials, porosity and resolution, may indeed enable unbounded options for geometries. In this review, recent efforts in the field of 3D-printing of catalyst and sorbent materials are discussed. It will be argued that these efforts, whilst promising, do not yet exploit the full potential of the technology, since most studies considered small structures that are very similar to structures that can be produced through conventional methods. In addition, these studies are mostly motivated by chemical and material considerations within the printing process, without explicitly striving for process intensification. To enable value-added application of 3D-printing in the chemical process industries, three crucial requirements for increased process intensification potential will be set out: i) the production of mechanically stable structures without binders; ii) the introduction of local variations throughout the structure; and iii) the use of multiple materials within one printed structure.

Zeolites on 3D-printed open metal framework structure: metal migration into zeolite promoted catalytic cracking of endothermic fuels for flight vehicles

Selective laser melting printed metal lattice structures functionalised using zeolites have been shown as a promising new generation of catalysts. The unprecedented catalytic activity can be explained by metal migration (most likely chromium) from the support into the zeolite phase making it a promising candidate for endothermic fuels for high-speed flight vehicles.

Electrical Capacitance Volume Tomography (ECVT) for Characterization of Additively Manufactured Lattice Structures (AMLS) in Gas-Liquid Systems

Against the background of current and future global challenges, such as climate change, process engineering requires increasingly specific solutions adapted to the respective problem or application, especially in gas–liquid contact apparatuses. One possibility to adjust the conditions in this kind of apparatuses is an intelligent and customized structuring, which leads to consistent fluid properties and flow characteristics within the reactor. In the course of this, the interfacial area for mass transfer, as well as residence times, have to be adjusted and optimized specifically for the respective application. In order to better understand and advance the research on intelligent customized additively manufactured lattice structures (AMLS), the phase distributions and local gas holdups that are essential for mass transfer are investigated for different structures and flow conditions. For the first time a tomographic measurement technique is used, the Electrical Capacitance Volume Tomography (ECVT), and validated with the volume expansion method and a fiber optical needle probe (A2PS-B-POP) for an air-water system for different modes of operation (with or without co-current liquid flow in empty or packed state). The ECVT proved to be particularly useful for both in the empty tube and the packed state and provided new insights into the phase distributions occurring within structured packings, which would have led to significantly underestimated results based on the visual reference measurements, especially for a densely packed additively manufactured lattice structure (5 mm cubic on the tip). Particularly for the modified structures, which were supposed to show local targeted differences, the ECVT was able to resolve the changes locally. The additional use of a pump for co-current flow operation resulted in slightly higher fluctuations within the ECVT data, although local events could still be resolved sufficiently. The final comparison of the empty tube at rest data with a fiber optical needle probe showed that the results were in good agreement and that the local deviations were due to general differences in the respective measurement techniques.

Another Critical Look at Three-Phase Catalysis

Three-phase catalysis, for example, hydrogenation, is a special branch of chemical reactions involving a hydrogen reactant (gas) and a solvent (liquid) in the presence of a metal porous catalyst (solid) to produce a liquid product. Currently, many reactors are being used for three-phase catalysis from packed bed to slurry vessel; the uniqueness for this type of reaction in countless processes is the requirement of transferring gas into liquid, as yet there is not a unified system of quantifying and comparing reactor performances. This article reviews current methodologies in carrying out such heterogeneous catalysis in different reactors and focuses on how to enhance reactor performance from gas transfer perspectives. This article also suggests that the mass transfer rate over energy dissipation may represent a fairer method for comparison of reactor performance accounting for different types/designs of reactors and catalyst structures as well as operating conditions.

Photocatalytic Methane Reforming: Recent Advances

Methane reforming is an important potential technology for solving both environmental and energy problems. This technology is important because methane is counted as a greenhouse gas, but on the other hand, it can be reformed into industrially valuable compounds. More research has focused on photocatalytic methane reforming, which has a higher activity than thermal catalysts under dark conditions. The reaction selectivity toward specific products in photocatalytic methane reforming is sometimes different from thermal catalyst systems. Herein, we discuss recent advances in photocatalytic methane reforming to provide various strategies for reforming.

Syngas Derived from Lignocellulosic Biomass Gasification as an Alternative Resource for Innovative Bioprocesses

A hybrid system based on lignocellulosic biomass gasification and syngas fermentation represents a second-generation biorefinery approach that is currently in the development phase. Lignocellulosic biomass can be gasified to produce syngas, which is a gas mixture consisting mainly of H2, CO, and CO2. The major challenge of biomass gasification is the syngas’s final quality. Consequently, the development of effective syngas clean-up technologies has gained increased interest in recent years. Furthermore, the bioconversion of syngas components has been intensively studied using acetogenic bacteria and their Wood–Ljungdahl pathway to produce, among others, acetate, ethanol, butyrate, butanol, caproate, hexanol, 2,3-butanediol, and lactate. Nowadays, syngas fermentation appears to be a promising alternative for producing commodity chemicals in comparison to fossil-based processes. Research studies on syngas fermentation have been focused on process design and optimization, investigating the medium composition, operating parameters, and bioreactor design. Moreover, metabolic engineering efforts have been made to develop genetically modified strains with improved production. In 2018, for the first time, a syngas fermentation pilot plant from biomass gasification was built by LanzaTech Inc. in cooperation with Aemetis, Inc. Future research will focus on coupling syngas fermentation with additional bioprocesses and/or on identifying new non-acetogenic microorganisms to produce high-value chemicals beyond acetate and ethanol.

Monolith Metal-Oxide-Supported Catalysts: Sorbent for Environmental Application

The emission of untreated environmental harmful gases such as sulfur and nitrogen oxide (SOx and NOx) emissions is considered old fashioned, since industries are compelled by governments and legislations to meet the minimum threshold before emitting such substances into the atmosphere. Numerous research has been done and is ongoing to come up with both cost-effective equipment and regenerable catalysts that are adsorbent—or with enhanced sorption capacity—and with safer disposal methods. This work presents the general idea of a monolith/catalyst for environmental application and the technicality for improving the surface area for fast and efficient adsorption–desorption reactions. The chemical reactions, adsorption kinetics, and other properties, including deactivation, regeneration, and the disposal of a catalyst in view of environmental application, are extensively discussed.

Potential of metal monoliths with grown carbon nanomaterials as catalyst support in intensified steam reformer: a perspective

A monolithic catalytic support is potentially a thermally effective system for application in an intensified steam reforming process. In contrast to ceramic analogues, metal monoliths exhibit better mechanical strength, thermal conductivity and a thermal expansion coefficient equivalent to that of the reformer tube. A layer of carbon nanomaterials grown on the metal monolith’s surface can act as a textural promoter offering sufficient surface area for hosting homogeneously dispersed catalytically active metal particles. Carbon nanomaterials possess good thermal conductivities and mechanical properties. The future potential of this system in steam reforming is envisaged based on hypothetical speculation supported by fundamental carbon studies from as early as the 1970s, and sufficient literature evidence from relatively recent research on the use of monoliths and carbon in catalysis. Thermodynamics and active interaction between metal particle surface and carbon-containing gas have resulted in coke deposition on the nickel-based catalysts in steam reforming. The coke is removable through gasification by increasing the steam-to-carbon ratio to above stoichiometric but risks a parallel gasification of the carbon nanomaterials textural promoter, leading to nickel particle sintering. We present our perspective based on literature in which, under the same coke gasification conditions, the highly crystallised carbon nanomaterials maintain high chemical and thermal stability.

Confined multiphase swirled flows in chemical engineering

Exploration of confined swirled flows in the former USSR and present-day Russia has a long history and is presented by numerous publications, mostly written in Russian. The obtained results have been put to practical use in different areas of chemical, energy, and processing industries. In view of the process intensification concept, such characteristics of confined swirled flows may be considered unique: high centrifugal gravity, elongation of trajectories, the presence of internal separation zones, energy separation phenomena, and efficient mass/heat transfer in the absence of any moving parts. For instance, high gravity provides an excellent opportunity for multiphase flow stabilization, preventing the appearance of nonuniformities and stability loss, while enhancement of various driving forces acting on the reaction medium takes place. This review is devoted to outlining the main research trends and to discuss the most essential practical implementations in the subject matter done up until now. It is not foreseen to embrace the whole area of swirled flow investigations in the world, but only covers most of the significant ideas and applications contributed in by former Soviet and Russian scientists and engineers. Shortcomings and difficulties of using swirled flows are also briefly discussed.

Process intensification in chemical engineering: general trends and Russian contribution

Minimization of the costs with simultaneous increase in the raw materials and energy use efficiency is a challenge for the modern world. One of the most effective tools to solve this task is the use of process intensification (PI), first proposed by Ramshaw C. The incentive for process intensification, Proceedings, 1st Intl. Conf. Proc. Intensif. for Chem. Ind., 18, BHR Group, London, 1995, p. 1. and then extended by Stankiewicz AI, Moulijn JA. Process intensification: transforming chemical engineering. Chem Eng Prog 2000: 22–34. In the presented review, some principles of PI in chemical engineering and their application for wide variety of processes is discussed. The role of the Russian scientist with a research background is carried out in other countries.

Macroporous Gel with a Permeable Reaction Platform for Catalytic Flow Synthesis

We mimic a living system wherein target molecules permeate through capillary and cells for chemical transformation. A monolithic porous gel (MPG) was easily prepared by copolymerization of gel matrix, tertiary amine, and cross-linking monomer in one-step synthesis. Interconnected capillaries existed in the MPG, enabling flow application with high permeability. Because the capillaries were constituted of polymer gel, Pd(0)-loaded MPG provided another permeable pathway to substrates in a gel network, contributing to its much high turnover number after 30 days of use, compared with that of Pd(0)-loaded inorganic supports. Interestingly, the gel network size of the MPG influenced the catalytic frequency. Diffusivities of the substrates and product in the gel networks increased with increasing network sizes in relation to catalytic activities. The MPG strategy provides a universal reactor design in conjunction with a practical process and precisely controlled reaction platform.

Monolithic substrate support catalyst design considerations for steam methane reforming operation

This paper reviews the research undertaken to study the design criteria that address the monolithic support structure requirements in steam reforming operation for the effective mass transfer of process gases to the active sites and effective conductive heat transfer through tube wall to the active catalytic areas, as well as low pressure drop operation. Design considerations include selection of substrate materials that possess good mechanical strength to withstand the severe reaction conditions and prevent catalyst crushing that would lead to carbon formation and catalyst deactivation, and excessive heating of the tube that results in hot spots which is fatal to tube lifetime. The support’s mechanical properties are listed for the purpose of providing guidelines on verifying the structure durability. The practical aspect of packaging and stacking the monolith structures in the reformer tube for ease of loading and discharge is discussed to understand its readiness in industrial application.

Aligned macroporous monoliths with intrinsic microporosity via a frozen-solvent-templating approach

Aligned macroporous monoliths of an organic cage, a polymer of intrinsic microporosity (PIM-1), and a metal–organic framework (HKUST-1) are prepared by a controlled freezing approach. In addition to macropores, all the monoliths contain the intrinsic micropores.