Improved asymmetrical honeycomb monolith catalyst prepared using a 3D printed template

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.

Recent Progress and Perspectives of Direct Ink Writing Applications for Mass Transfer Enhancement in Gas-Phase Adsorption and Catalysis

Conventional adsorbents and catalysts shaped by granulation or extrusion have high pressure drop and poor flexibility for chemical, energy, and environmental processes. Direct ink writing (DIW), a kind of 3D printing, has evolved into a crucial technique for manufacturing scalable configurations of adsorbents and catalysts with satisfactory programmable automation, highly optional materials, and reliable construction. Particularly, DIW can generate specific morphologies required for excellent mass transfer kinetics, which is essential in gas-phase adsorption and catalysis. Here, DIW methodologies for mass transfer enhancement in gas-phase adsorption and catalysis, covering the raw materials, fabrication process, auxiliary optimization methods, and practical applications are comprehensively summarized. The prospects and challenges of DIW methodology in realizing good mass transfer kinetics are discussed. Ideal components with a gradient porosity, multi-material structure, and hierarchical morphology are proposed for future investigations.

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.

Catalytic Materials by 3D Printing: A Mini Review

Catalytic processes are the dominant driving force in the chemical industry, proper design and fabrication of three-dimensional (3D) catalysts monoliths helps to keep the active species from scattering in the reaction flow, improve high mass loading, expose abundant active catalytic sites and even realize turbulent gas flow, greatly improving the catalytic performance. Three-dimensional printing technology, also known as additive manufacturing, provides free design and accurate fabrication of complex 3D structures in an efficient and economic way. This disruptive technology brings light to optimizing and promoting the development of existing catalysts. In this mini review, we firstly introduce various printing techniques which are applicable for fabricating catalysts. Then, the recent developments in 3D printing catalysts are scrutinized. Finally, challenges and possible research directions in this field are proposed, with the expectation of providing guidance for the promotion of 3D printed catalysts.

Structured Reactors Based on 3D Fe/SiC Catalysts: Understanding the Effects of Mixing

The application of structured reactors provides a number of advantages in chemical processes. In this paper, two different three-dimensional (3D) Fe/SiC catalysts with a square cell geometry have been manufactured by Robocasting: monoliths (D = 14 and H = 15 mm) and meshes (D = 24 and H = 2 mm) and studied in the catalytic phenol oxidation by hydrogen peroxide (H₂O₂) for the sustainable production of dihydroxybenzenes (DHBZ). The fluid dynamics, catalytic performance, reaction rates, external mass transport limitation, and catalyst stability have been compared in three different reactors, monolithic fixed-bed reactor, multimesh fixed-bed reactor, and monolithic stirrer reactor, at selected operating conditions. The results show that the mechanical stirring of the 3D Fe/SiC monoliths avoids the external mass transfer limitation caused by the presence of oxygen bubbles in the channels (produced from the HO _x · species in autoscavenging radical reactions). In addition, the backmixing has a positive effect on the efficient consumption of H₂O₂ but an adverse effect on the phenol selectivity to DHBZ since they are overoxidized to tar products at longer contact times. On the other hand, the wall porosity, and not the backmixing, affects the susceptibility of the 3D Fe/SiC catalyst to the Fe leaching, as occurs in the mesh structures. In conclusion, the monoliths operating under plug-flow and external mass transfer limitation in the monolithic fixed-bed reactor (MFB) provide an outstanding phenol selectivity to DHBZ and catalyst stability.

In Search of an Effective Workability Zone during the 3D Printing of Polymeric Periodic Open Cellular Structures Potentially Useful as Microreactors

The question of how easy the transition is between design and manufacturing by the 3D printing of periodic open cellular structures occurs from the analysis of cases in which additive manufacturing and heterogeneous catalysis merge. The synergy between these two fields suggests that one of the great advantages that the catalysis of this manufacturing methodology can take advantage of is the obtaining of advanced designs that would allow improving the processes from the geometry of the reactors. However, not all 3D-printing techniques offer the same degree of resolution, and this uncertainty grows when using more complex materials to work with, such as ceramics or metals. Therefore, the present work seeks to answer this question by finding experimentation strategies, starting with a simple case study inspired by the additive manufacturing–catalysis combination, in which a ceramic polymer resin of high thermal resistance is used to obtain POCSs that are potentially useful in thermochemical or adsorption processes. This exploration concludes on the need to define limits for what we have called an “effective work zone” that combines both design criteria and the real possibility of printing and manipulating the pieces, making sweeps in structural parameters such as cell size and the diameter of struts in the POCS. Similarly, the possibility of coating these systems with inorganic oxides is explored, using a generic oxide (Al2O3) to analyse this scenario. Finally, a cartridge-type assembly of these systems is proposed so that they can be explored in future processes by other researchers.

Adsorption film with sub-milli-interface morphologies via direct ink writing for indoor formaldehyde removal

In-situ thermally regenerated flexible adsorption films are superior for long-term purification of indoor low-concentration volatile organic compounds (VOCs). To further improve the adsorption kinetics of the films, the surface morphology of adsorption films was suggested in hierarchical channel structure. However, such structure is far from practical applications because of its complicated fabrication method and limited flexibility. In this study, we proposed a convenient and fast method named direct ink writing (DIW) based 3D printing to fabricate flexible adsorption films. Inks were prepared to have appropriate rheological properties and good printability. Three types of adsorption film (flat, straight finned, and trough-like finned) were constructed on flexible polyimide circuit substrates by DIW. We utilized the printed adsorption films for indoor level (1 ppm) formaldehyde removal. The trough-like finned film achieved the best performance among the three printed films, showing a 275% longer penetration time and 252% larger effective adsorption capacity than the flat film. By conducting a 7-cycle adsorption-desorption experiment (more than 12 h), we verified that the films' adsorption performance could effectively recover via in-situ heating. This work could dance around the complicated coating process, increase the structural flexibility and reduce the adsorbent interfacial modification cost.

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.

Skeletal Nickel Catalyst for the Methanation Reaction Developed by Laser-Engineered Net-Shaping Technology

Here, we report a skeletal nickel catalyst prepared by cumulative processing. The Ni, Al, and CoCrMo multi-component alloys were printed by a dual-powder laser-engineered net-shaping system, and alloy samples with different components were obtained through high-throughput design. After leaching in 5 mol/L NaOH at 40 °C for 2 h, the specific surface area of the catalyst increased with increasing Al content. Increasing the leaching temperature and prolonging the leaching time also effectively increased the specific surface area of the catalyst. After leaching at 80 °C for 12 h, the specific surface area was 42.36 m2/g. After cleaning and hydrogen-reduction treatment at 400 °C, the catalyst showed high catalytic activity. The highest conversion rate of CO reached 89.56%, and the selectivity of CH4 remained above 98% for a long time.

Recent Advances in the Development of Highly Conductive Structured Supports for the Intensification of Non-adiabatic Gas-Solid Catalytic Processes: The Methane Steam Reforming Case Study

Structured catalysts are strong candidates for the intensification of non-adiabatic gas-solid catalytic processes thanks to their superior heat and mass transfer properties combined with low pressure drops. In the past two decades, different types of substrates have been proposed, including honeycomb monoliths, open-cell foams and, more recently, periodic open cellular structures produced by additive manufacturing methods. Among others, thermally conductive metallic cellular substrates have been extensively tested in heat-transfer limited exo- or endo-thermic processes in tubular reactors, demonstrating significant potential for process intensification. The catalytic activation of these geometries is critical: on one hand, these structures can be washcoated with a thin layer of catalytic active phase, but the resulting catalyst inventory is limited. More recently, an alternative approach has been proposed, which relies on packing the cavities of the metallic matrix with catalyst pellets. In this paper, an up-to-date overview of the aforementioned topics will be provided. After a brief introduction concerning the concept of structured catalysts based on highly conductive supports, specific attention will be devoted to the most recent advances in their manufacturing and in their catalytic activation. Finally, the application to the methane steam reforming process will be presented as a relevant case study of process intensification. The results from a comparison of three different reactor layouts (i.e. conventional packed bed, washcoated copper foams and packed copper foams) will highlight the benefits for the overall reformer performance resulting from the adoption of highly conductive structured internals.

3D-Printed Fe/γ-Al2O3 Monoliths from MOF-Based Boehmite Inks for the Catalytic Hydroxylation of Phenol

The synthesis of dihydroxybenzenes (DHBZ), essential chemical reagents in numerous industrial processes, with a high degree of selectivity and yield from the hydroxylation of phenol is progressively attracting great interest in the catalysis field. Furthermore, the additive manufacturing of catalysts to produce 3D printed monoliths would provide additional benefits to enhance the DHBZ synthesis performance. Herein, 3D cellular Fe/γ-Al₂O₃ monoliths with a total porosity of 88% and low density (0.43 g·cm^-3) are printed by Robocasting from pseudoplastic Fe-metal-organic frameworks (Fe-MOF)-based aqueous boehmite inks to develop catalytic monoliths containing a Fe network of dispersed clusters (≤5 μm), nanoclusters (<50 nm), and nanoparticles (∼20 nm) into the porous ceramic skeleton. The hydroxylation of phenol in the presence of hydrogen peroxide is carried out at different reaction temperatures (65-85 °C) in a flow reactor filled with eight stacked 3D Fe/γ-Al₂O₃ monoliths and with the following operating conditions: C_phenol,0 = 0.33 M, C_phenol,0/C_H2O2,0 = 1:1 molar, W_R = 2.2 g, and space time (τ = W·Q_L^-1) = 0-147 g_cat·h·L^-1. The scaffolds present a good mechanical resistance (∼1 MPa) to be employed in a catalytic reactor and do not show any cracks or damage after the chemical reaction. DHBZ selectivity (S_DHBZ) of 100% with a yield (Y_DHBZ) of 32% due to the presence of the Fe network in the monoliths is reported at 85 °C, which represents an improved synthesis performance as compared to that obtained by using the conventional Enichem process and the well-known titanium silicalite-1 catalysts (S_DHBZ = 99.1% and Y_DHBZ = 29.6% at 80 °C). This printing strategy allows manufacturing novel 3D structured catalysts for the synthesis of critical chemical compounds with higher reaction efficiencies.

Arming wood carbon with carbon-coated mesoporous nickel-silica nanolayer as monolithic composite catalyst for steam reforming of toluene

Steam reforming is an effective measure for biomass tar elimination as well as H₂-rich syngas (H₂ + CO) production. However, the granular or powdery Ni-based catalysts are prone to deactivation, which is caused by inappropriate mass transfer and clogging of catalyst bed. Herein, monolithic wood carbon (WC) with low-tortuosity microchannels is armed with a carbon-coated mesoporous nickel-silica nanocomposite (Ni-SiO₂@C) layer via an evaporation-induced self-assembly and calcination procedure for toluene (tar model compound) steam reforming. The quality of the Ni-SiO₂@C layer growing on the surface of WC microchannel is affected by the molar ratios of Si/Ni feed. A uniform thin-layer coverage is obtained on the Ni-15SiO₂@C/WC (Si/Ni = 15) catalyst, where highly dispersed Ni nanoparticles (average size of 6.6 nm) with appropriate metal-support interaction and remarkable mechanical strength are achieved. The mass transfer, coke resistance, and hydrothermal stability of the Ni-15SiO₂@C/WC catalyst were significantly improved by the multilevel structure assembled from the WC microchannels and the secondary ordered SiO₂ mesopores. A stable toluene conversion over 97% with an H₂ yield of 135 μmol/min was obtained at 600 °C on the Ni-15SiO₂@C/WC catalyst. This work opens a new window for facilely constructing high-performance wood carbon-based monolithic tar reforming catalyst.

Customizable Heterogeneous Catalysts: Nonchanneled Advanced Monolithic Supports Manufactured by 3D-Printing for Improved Active Phase Coating Performance

Three-dimensional (3D)-printed catalysts are being increasingly studied; however, most of these studies focus on the obtention of catalytically active monoliths, and thus traditional channeled monolithic catalysts are usually obtained and tested, losing sight of the advantages that 3D-printing could entail. This work goes one step further, and an advanced monolith with specifically designed geometry has been obtained, taking advantage of the versatility provided by 3D-printing. As a proof of concept, nonchanneled advanced monolithic (NCM) support, composed of several transversal discs containing deposits for active phase deposition and slits through which the gas circulates, was obtained and tested in the CO-PrOx reaction. The results evidenced that the NCM support showed superior catalytic performance compared to conventional channeled monoliths (CMs). The region of temperature in which the active phase can work under chemical control, and thus in a more efficient way, is increased by 31% in NCM compared to the powdered or the CM sample. Turbulence occurs inside the fluid path through the NCM, which enhances the mass transfer of reagents and products toward and from the active sites to the fluid bulk favoring the chemical reaction rate. The nonchanneled monolith also improved heat dispersion by the tortuous paths, reducing the local temperature at the active site. Thus, the way in which reactants and products are transported inside the monoliths plays a crucial role, and this is affected by the inner geometry of the monoliths.

3D Printing in Heterogeneous Catalysis-The State of the Art

This paper describes the process of additive manufacturing and a selection of three-dimensional (3D) printing methods which have applications in chemical synthesis, specifically for the production of monolithic catalysts. A review was conducted on reference literature for 3D printing applications in the field of catalysis. It was proven that 3D printing is a promising production method for catalysts.

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.

3D-Printed metal-organic frameworks within biocompatible polymers as excellent adsorbents for organic dyes removal

Three-dimensional (3D) printing technique has received exceptional global attention as it can create a myriad of high-resolution architectures from digital models. In the present study, 3D-printed metal-organic frameworks (MOFs) were shaped into several geometries via direct ink writing, which overcomes the instability and high-pressure drop of powdery MOF during the flow of gas or liquid streams. The inclusion of a blend of calcium alginate and gelatin (CA-GE) as biocompatible binder allowed for easy writing and an enhanced mechanical property. Besides, it was found that the printing geometry (square, hexagon, and circle), MOF loading amount, and MOF size also greatly influenced the adsorptive performance. For instance, the methylene blue adsorption efficiency of CA-GE scaffolds without MOF was only 43.6%, while the printed MOF/CA-GE sample exhibited 99.8% adsorption efficiency at 20 min. Both the inherent microporous structure of MOFs and meso/macroporous structures of the 3D matrix contributed to the excellent adsorption properties towards a variety of organic dyes and their mixtures. Furthermore, the 3D-printed adsorbents can be easily regenerated in dilute acid solution and reused for at least 7 times without performance loss. In contrast, the powdery MOF can only be repeatedly used for at most 2 times.