Straightforward Processing Route for the Fabrication of Robust Hierarchical Zeolite Structures

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.

Zeolite Nanocrystals Embedded in Microcellular Carbon Foam as a High-Performance CO2 Capture Adsorbent with Energy-Saving Regeneration Properties

Here, the facile synthesis of four-length-scaled (ultramicro-micro-meso-macroporous) hierarchically structured porous carbon nanocomposite by an emulsion-template strategy is reported. This previously unreported combination of zeolite nanocrystals embedded in the walls of microcellular carbon foams gives unique textural and structural properties, which result in their excellent ability to selectively capture CO₂ owing to the presence of ultra-micropores. The zeolite-microcellular carbon foam synergism delivers an adsorbent with a significantly enhanced CO₂ capture capacity of up to 5 mmol g^-1 , CO₂ /N₂ selectivity of up to 80, and an outstanding multi-cycle capture performance under humid conditions (70 % performance retention after 30 regeneration cycles). More impressively, the electrically conductive carbon framework enables Joule heating and cooling, and thus fast and energy-efficient regeneration is possible, with an estimated energy consumption of only about 12 kWh.

Three-Dimensional-Printed Core-Shell Structured MFI-Type Zeolite Monoliths for Volatile Organic Compound Capture under Humid Conditions

Crystalline aluminosilicate zeolites with high sorption capacity and low production cost have been recognized as a promising adsorbent for volatile organic compound (VOC) capture. However, the ubiquitous water vapor in the VOC streams may compete with VOCs during the practical separation process because of the hydrophilic property of aluminosilicate zeolites. Herein, a self-supporting core-shell structured MFI-type zeolite monolith was fabricated by 3D-printing aluminosilicate ZSM-5 zeolites as the core, followed by coating silicalite-1 zeolites as a hydrophobic shell via post-hydrothermal crystallization. Natural sepiolite nanofibers (SNFs) were employed as printing ink additives for reinforcing the mechanical stability of 3D-printed ZSM-5 monoliths. Colloidal silica was also introduced into the printing inks, affording continuous growth of silicalite-1 layers (with a thickness of ∼200 nm) over ZSM-5 crystals. Such core-shell structured MFI-type zeolite monoliths exhibited superior dynamic adsorption performance for toluene at 298 K under humid conditions (relative humidity: 50%), with a saturated adsorption capacity of 44.3 mg/g. This work provides a facile strategy for designing self-supporting zeolite monoliths with core-shell architectures for adsorption/separation and other advanced applications.

Fabricating Mechanically Robust Binder-Free Structured Zeolites by 3D Printing Coupled with Zeolite Soldering: A Superior Configuration for CO2 Capture

3D-printing technology is a promising approach for rapidly and precisely manufacturing zeolite adsorbents with desirable configurations. However, the trade-off among mechanical stability, adsorption capacity, and diffusion kinetics remains an elusive challenge for the practical application of 3D-printed zeolites. Herein, a facile "3D printing and zeolite soldering" strategy is developed to construct mechanically robust binder-free zeolite monoliths (ZM-BF) with hierarchical structures, which can act as a superior configuration for CO2 capture. Halloysite nanotubes are employed as printing ink additives, which serve as both reinforcing materials and precursor materials for integrating ZM-BF by ultrastrong interfacial "zeolite-bonds" subjected to hydrothermal treatment. ZM-BF exhibits outstanding mechanical properties with robust compressive strength up to 5.24 MPa, higher than most of the reported structured zeolites with binders. The equilibrium CO2 uptake of ZM-BF reaches up to 5.58 mmol g-1 (298 K, 1 bar), which is the highest among all reported 3D-printed CO2 adsorbents. Strikingly, the dynamic adsorption breakthrough tests demonstrate the superiority of ZM-BF over commercial benchmark zeolites for flue gas purification and natural gas and biogas upgrading. This work introduces a facile strategy for designing and fabricating high-performance hierarchically structured zeolite adsorbents and even catalysts for practical applications.