Zeolite Monoliths Research Articles

Coaxial 3D Printing Enabling Mn/CHA@Pd/CHA Zeolite‐Based Core–Shell Monoliths for Efficient Passive NOx Adsorbers

AbstractPd‐based zeolites are extensively used as passive NOx adsorbers (PNA) for cold‐start NOx emissions to meet stringent emission regulations. However, optimizing adsorber design to reduce Pd usage with substitution by non‐noble metals that are prone to suffer from H2O remains a significant challenge. Herein, the core–shell Mn/CHA@Pd/CHA zeolite monoliths based on non‐noble metal/zeolite core are constructed using coaxial 3D printing technology and identified as efficient passive NOx adsorbers for the first time. In the Mn/CHA@Pd/CHA monolith, the Pd/CHA shell effectively protected the Mn active sites in the core from H2O, while the integration of the Mn/CHA core not only introduced efficient storage sites but also facilitated NOx desorption, thereby achieving comparable adsorption properties and increased the NOx desorption efficiency by 35% at 350 °C compared with that of Pd/CHA monolith. Furthermore, some non‐noble metal‐based zeolites (e.g., Co/CHA, Mn/MFI, Mn/BEA) and Pd‐based zeolites (e.g., Pd/AEI) are also employed as cores and shells respectively to fabricate a series of core–shell zeolite monoliths via coaxial 3D printing, highlighting the benefits of incorporating non‐noble metals into Pd‐based zeolites for improving adsorption and desorption behaviors. This work provides a promising strategy for designing cost‐effective PNA materials and contributes to improving the exhaust after‐treatment technology.

Tailoring and Visualizing Macropores of the Zeolite Monolith to Reveal the Coke Resistance Performance for Bio-oil Hydrodeoxygenation

Tailoring and Visualizing Macropores of the Zeolite Monolith to Reveal the Coke Resistance Performance for Bio-oil Hydrodeoxygenation

3D-printed zeolite 13X gyroid monolith adsorbents for CO2 capture

Self-standing 3D-printed gyroid monoliths of zeolite 13X were fabricated for the first time for carbon dioxide adsorption. Gyroid sheet-based triply periodic minimal surface (TPMS) lattices provide high surface area per unit volume, smooth surfaces with no discontinuities, and low pressure drop compared to powder and beads, which are essential in gas adsorption. A photopolymer resin and zeolite 13X powder slurry was 3D-printed by digital light processing followed by debinding to remove the polymer and sintering to diffuse and consolidate the zeolite 13X particles. The sintered adsorbents were mechanically stable, and structural and morphological analyses revealed the interconnected nature of the gyroid cells permitting CO2 molecules to easily diffuse into the printed structure, thus ensuring sufficient adsorbent/adsorbate contact. The gyroid zeolite monoliths reached an equilibrium adsorption capacity of 3.5 mmol g−1 in 77 min at 25 °C and 1 bar, whereas the beads and powder required 96 and 100 min, respectively, while after 20 pressure swing adsorption (PSA) cycles, the zeolite monolith showed sustainable performance compared to the powder. The CO2/N2 selectivity ranged from 328 (at 50 mbar) to 51 (at 1 bar) at 25 °C for the zeolite monolith, whereas the powder exhibited selectivity values in the range of 294 (at 50 mbar) to 33 (at 1 bar). Dynamic breakthrough experiments using CO2/N2 mixtures confirmed the fast kinetics and separation performance of the monoliths, while the pressure drop was also significantly lower than the powder and the beads. The novel topology of the developed self-standing gyroid zeolite monoliths eliminate the need for structuring of powdered adsorbents and exhibit competitive performance, enhanced kinetics and cyclability, and reduced pressure drop toward large-scale carbon capture application.

Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling.

The self-diffusivity of cyclohexane and n-octane adsorbed in hierarchical zeolite monoliths has been investigated by using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity combines with a complex macroporous network composed of aggregated zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity apparently decreases, reaches a minimum, and then starts increasing upon further increasing the temperature. Such striking, i.e., non-Arrhenius, temperature dependence is not observed for n-octane in the same samples and for cyclohexane adsorbed in purely microporous FAU-X. Through thermodynamic modeling, we show that this anomalous behavior can be rationalized by considering the evolution in the adsorbate populations when changing the temperature. In more detail, we show that the slow and fast diffusing species present in the microporosity and secondary porosity arising from the packing of zeolite nanocrystals vary significantly with a strong impact on the effective diffusivity. Applying the temperature evolution of their relative fractions to a simple two-phase diffusion model helps obtain insights into the physicochemical factors responsible for the complex behavior of effective self-diffusivity in hierarchical zeolites.

Sulfonated hierarchical ZSM-5 zeolite monoliths as solid acid catalyst for esterification of oleic acid.

Sulfonated hierarchical H-ZSM-5 monoliths were synthesized via ice-templating as solid acid catalysts for biodiesel production. Characterization confirmed successful -SO3 group grafting, increasing acid density. The catalysts achieved over 90% conversion of oleic acid in esterification, with easy recovery and reuse, and the relationship between porosity and acid density was explored.

3D printed zeolite monoliths as open-source spare parts for oxygen concentrators

Oxygen concentrators are essential medical devices for basic emergency care, surgery and treatment for chronic and acute respiratory diseases. Despite their reliability and accessibility, they require spare parts which can be difficult to source in low resource settings, where the extreme environmental conditions and the overloads often make failures more frequent. The solution to this shortcoming can be their local production, enabled by 3D printing (3DP) technologies. Here we describe an extrusion-based 3DP process to produce zeolite monolith structures for use in the air adsorption process. The printed structures were physically and mechanically characterized. Compared to the material in pelletized form, the 3D printed zeolite monoliths have better transport properties (30 times increased permeability), comparable adsorption (being comparable with standard pellets), and are easier to handle. They are therefore the more appropriate technology for higher performance, easier maintenance, and lower weight oxygen concentrators in low resource settings.

Construction of high performance binder-free zeolite monolith

Compared to conventional spheres, strips, and granules, structural adsorbents offer shaping of the adsorbent materials. In this work, we report a straightforward procedure to shape binder-free zeolite monolith (BF-ZM). During the synthesis, hydroxypropyl methylcellulose was applied as a binder, and the zeolite precursor material halloysite nanotubes and Y zeolites were mixed to produce the initial zeolite monolith. The binder was removed by high-temperature calcination to gain preliminary mechanical strength, followed by hydrothermal crystallization to convert the binder into Y zeolites to obtain a 100% pure Y zeolite structure. The as-synthesized BF-ZM exhibits excellent mechanical properties, showing negligible powdering after 1.34 × 105 cycles of accelerated pressurization/depressurization test; in comparison, ordinary packed columns using zeolite beads suffered from 3.51% loss due to powdering. This result indicates that BF-ZM is favorable for filling large pressure swing adsorption columns and can significantly alleviate the issue of adsorbent chalking and slabbing caused by the reciprocal pressure swing of the gas flow. The BF-ZM also achieved a high CO2 adsorption capacity of 6.4 mmol·g−1 at 1.2 bar and 273 K, superior to commercial Y zeolite pellets. The results of the breakthrough experiments showed a CO2 adsorption capacity of 4 mmol·g−1 at 333 K and 1.25 bar before the permeation started. The selectivities (CO2/N2:15/85) of BF-ZM have been calculated using the IAST method, showing 74 excellent selectivity at 333 K and 1 bar. This study provides a straightforward approach to the design and fabrication of high-strength binder-free adsorbents for large-scale industrial gas separations.

Microchannel zeolite 13X adsorbent with high CO2 separation performance

• Microchanneled zeolite monoliths for removal of CO 2 through phase separation. • CO 2 dynamic uptake of 3.4 mmol g −1 was obtained for 13X monoliths. • The monoliths present markedly low pressure drop under fast flow rate. • Mass transfer resistance in monolith significantly lower than those for beads. • Ultra high mechanical strength and thermal conductivity for zeolite adsorbent. Uniform 13X films with thicknesses of 3 and 11 µm were grown on supports in the form of steel monoliths with a cell density of 1600 cpsi and microchannels width of 0.5 mm. Sharp breakthrough fronts and a dynamic uptake of 3.4 mmol CO 2 g −1 zeolite were observed in the forwarding step of breakthrough experiments for a feed of 10% CO 2 in N 2 with a high flow rate at 293 K and 1 bar. Numerical modeling showed that the adsorption process was very fast and that the transport of CO 2 in the thin zeolite layer was the rate-limiting step, the mass transfer resistance for the 11 µm film is 2.2 times lower than zeolite 13X pellets and 100 times lower than zeolite 4A beads. Axial dispersion, pressure drop, and gas film resistance were shown to be negligible. The steel monolith support provides good mechanical strength and excellent thermal conductivity for the 13X films. The combination of properties makes this adsorbent a good performer when compared with other types of structured zeolite adsorbents in reported literatures. This microchannel adsorbent is a promising alternative to traditional adsorbents in processes of fast CO 2 separation with short cycle times.

Binderless zeolite monoliths production with sacrificial biopolymers

3D printing has emerged as an attractive way of formulating structured adsorbents, as it imparts lower manufacturing costs compared to hydraulic extrusion while also allowing for unprecedented geometric control. However, binderless structures have not been fabricated by 3D printing, as ink formulation has previously required clay binders which cannot be easily removed. In this study, we report the development of a facile approach to shape engineer binderless zeolites. 3D-printed inks comprised of 13X, 5A, ZSM-5, and experimental South African zeolites were prepared using gelatin and pectin as binding agents along with dropwise addition of various solvents. After printing, the dried monoliths were calcined to remove the biopolymers and form 100% pure zeolite structures. From N2 physisorption and CO2 adsorption measurements at 0 °C, all monoliths showed narrowing below 1 nm from their powders, which was attributed to pore malformation caused by intraparticle bridging during calcination. The various adsorption isotherms indicated that this narrowing led to varying degrees of enhanced adsorption capacities for all three gases, as the slightly smaller pores increased electrostatic binding between the sorbent walls and captured species. Analysis of CO2 adsorption performance revealed comparable diffusivities and adsorption capacities to the commercial bead analogues, implying that biopolymer/zeolite printing can produce contactors which are competitive to commercial benchmarks. The binderless monoliths also exhibited faster diffusivities compared to zeolite monoliths produced by conventional direct ink writing – on account of an enhancement in macroporosity – highlighting that this new method enhances the kinetic properties of 3D-printed scaffolds. As such, the sacrificial biopolymer technique is an effective and versatile approach for 3D printing binderless zeolite structures.

Syngas to higher alcohols synthesis over 3D printed KMoCo/ZSM5 monolith

The catalytic conversion of syngas into high carbon number alcohols (ethanol, propanol, butanol) has opened an avenue for the development of sustainable aviation fuels. The commercial production of higher alcohols from syngas remain challenging because of several constraints including poor selectivity, harsh operating conditions, and unavailability of suitable technology. The present study is based on metal impregnated zeolite-based catalysts for catalytic conversion of syngas into higher alcohols. Following the superior catalytic performance of powdered ZSM5 (Si/Al 100), a structured monolith was engineered via 3D printing (Hyrel 3D – Engine SR). The catalyst precursors (K–Mo–Co) were in situ grown (by hydrothermal treatment) onto channels of zeolite monolith. A comparison between monolithic and powdered structures revealed a similar catalytic performance at lower space velocities (< 3000 h−1). With an increasing space velocity at 6000 h−1 and above, the structured catalyst retained its catalytic performance, whereas, carbon monoxide conversion dropped significantly (~20%) over powdered catalyst. Moreover, the by-products formation (methane and carbon monoxide) were reduced over structured monolith. The development of zeolite-based monolith has provided an opportunity to overcome diffusional limitations for better utilization of intrinsic kinetics of Mo-Co based catalysts for higher alcohols synthesis.

Gamma-FeOOH based hierarchically porous zeolite monoliths for As(V) removal: Characterisation, adsorption and response surface methodology

γ-FeOOH based hierarchical zeolite LTA monoliths as sorbents for As(V) removal were synthesized by modifying hierarchical zeolite LTA monoliths with HCl and FeCl3 solutions. The obtained γ-FeOOH based monoliths (abbreviated as Fe-HZ) maintain the interconnected macropores network structure, which offers a practical solution to the diffusion limitations associated with traditional agglomerated zeolites. Moreover, the self-supported sorbents can be easily recycled after treatment. The derived γ-FeOOH based monolith exhibits higher BET surface area while slightly decreased thermal stability compared with those unmodified sample (labeled as HZ). The sorption performance of HZ and Fe-HZ are evaluated, taking the initial/equilibrium pH, adsorption time and initial concentration into account. Fe-HZ shows an optimal adsorption over the pH range 2–7, and the uptake capacity is 5.11 mg/g at pH of 6 with the equilibrium time of 240 min. Whereas only 0.67 mg/g can be attained for HZ. The dynamic data follows pseudo-second-order model, demonstrating As(V) species chemisorbed on Fe-HZ, and the chemisorption is ascribed to ligand exchange reactions between As(V) species and hydroxyl functional groups. Freundlich and Langmuir isotherms are both acceptable for the equilibrium data of arsenic adsorption on Fe-HZ. In addition, response surface methodology is applied for modeling and optimization monoliths preparation parameters. Results show that the importance of variable factors influencing arsenic adsorption follows the sequence: FeCl3 concentration > PMMA/MK mass ratio > FeCl3 modification time > HCl modification time.

Self-standing zeolite foam monoliths with hierarchical micro-meso-macroporous structures.

The zeolite monoliths were synthesized by a facile polymer scaffold template assisted hydrothermal method. The selected foam-shaped template of a polyurethane (PU) foam monolith, was used to prepare the self-standing zeolite foam (ZF) monolithic materials. The obtained ZF products can preserve the same size, shape and macroporous network structure of the original PU foam scaffold template, although the zeolite nano-crystallites had been fully substituted for the PU template to form the new skeleton struts and walls. The as-synthesized ZF products demonstrated abundant hierarchical porosity (involving triple micro-, meso- and macropores). Meanwhile, compared with the conventional zeolite powders, the self-standing ZF monolithic materials exhibited greater total pore volume and nearly three times higher mesopore volume, suggesting wider applications as catalysts, catalyst supports and adsorbents in industry.

Comparison of MIL-101(Cr) metal-organic framework and 13X zeolite monoliths for CO2 capture

Abstract A comparative study was conducted to determine the pore properties and adsorptive performance of monoliths containing either the MIL-101(Cr) metal-organic framework or 13X zeolite for carbon dioxide (CO2) capture. Although there have been a great deal of previous work on CO2 adsorption onto zeolites and MOFs, there has been far fewer studies on structured adsorbents such as monoliths. The results indicate that MIL-101(Cr) monoliths have 1.3 times higher porosity than 13X zeolite monoliths. Increasing CO2 partial pressure in the gas mixture shortens breakthrough and equilibrium times and increases the breakthrough and equilibrium adsorption capacities of CO2. MIL-101(Cr) monoliths show better mass transfer of CO2 in the adsorbent bed with shorter breakthrough and equilibrium times of about 20% and 35%, respectively, than 13X zeolite monoliths. The adsorption capacity of CO2 on MIL-101(Cr) monoliths is higher by about 37% (based on weight in mmol/g) at breakthrough and slightly lower by about 7% at equilibrium when compared to 13X zeolite monoliths. MIL-101(Cr) monoliths were found to be 1.5 times more efficient for CO2 adsorption than 13X zeolite monoliths. The effects of regeneration temperature after CO2 adsorption on MIL-101(Cr) and 13X zeolite monoliths were studied and results showed an increase in CO2 adsorption capacity as the regeneration temperature was increased. In summary, the study showed MIL-101(Cr) monoliths have better CO2 adsorption properties than 13X zeolite monoliths.

Controlled Crystallization of Hierarchical Monoliths Composed of Nanozeolites

Micro/macroporous and meso/macroporous amorphous silica monoliths have been prepared and transform into ZSM-5 nanosheets zeolitic macroporous monoliths. The hierarchized zeolite monoliths have been...

Structured Zeolite Monoliths with Ultrathin Framework for Fast CO2 Adsorption Enabled by 3D Printing

3D printing provides an excellent solution for the preparation and structure optimization in producing structured zeolite monoliths. However, the developed 3D printed zeolite monolith structures ar...

The effects of cell density and intrinsic porosity on structural properties and adsorption kinetics in 3D-printed zeolite monoliths

Herein, we report the evaluation of dynamic performance of 3D-printed zeolite monoliths for CO2/N2 separation at 5 bar 25 °C. Monoliths with various cell densities (200, 400, 600 cpsi) and porosities (0.23–0.46) were printed and the effects of cell density, wall porosity, and inlet gas velocity on their separation performance were investigated. Dynamic breakthrough tests with 10% CO2/N2 revealed that increasing cell density gives rise to gas throttling, inadequate time for molecular mass transfer, and broad wavefronts. For 200 cpsi monoliths, upon increasing wall porosity from 0.23 to 0.46, the mass transfer zone (MTZ) length decreased from 0.40 cm to 0.07 cm at the feed velocity of 1.8 cm/s. The mass transfer coefficients estimated from modeling the breakthrough profiles were found to decrease steadily with both velocity and monolith cell density. In the macroporous samples, the best mass transfer coefficient was found to be 0.049 s−1 for the 200 cpsi monolith with kaolin binder substitution. Both increasing the plasticizer concentration and substituting a macroporous binder promoted mass transfer rate, however, the former method increased the number of zeolite-bentonite bonds around large surface defects during burnout and reduced the CO2 adsorption capacity by 27% from the other formulations. On the basis of adsorption capacity and kinetics, utilization of a macroporous binder was found to be the best method of developing 3D-printed zeolite monoliths because it could reduce intraparticle resistance without compromising adsorption capacity or mechanical integrity.

Nuclear wastewater decontamination by 3D-Printed hierarchical zeolite monoliths.

The selective removal of radioactive cationic species, specifically 137Cs+ and 90Sr2+, from contaminated water is critical for nuclear waste remediation processes and environmental cleanup after accidents, such as the Fukushima Daiichi Nuclear Power Plant disaster in 2011. Nanoporous silicates, such as zeolites, are most commonly used for this process but in addition to acting as selective ion exchange media must also be deployable in a correct physical form for flow columns. Herein, Digital Light Processing (DLP) three-dimensional (3D) printing was utilized to form monoliths from zeolite ion exchange powders that are known to be good for nuclear wastewater treatment. The monoliths comprise 3D porous structures that will selectively remove radionuclides in an engineered form that can be tailored to various sizes and shapes as required for any column system and can even be made with fine-grained powders unsuitable for normal gravity flow column use. 3D-printed monoliths of zeolites chabazite and 4A were made, characterized, and evaluated for their ion exchange capacities for cesium and strontium under static conditions. The 3D-printed monoliths with 50 wt% zeolite loadings exhibit Cs and Sr uptake with an equivalent ion-capacity as their pristine powders. These monoliths retain their porosity, shape and mechanical integrity in aqueous media, providing a great potential for use to not only remove radionuclides from nuclear wastewater, but more widely in other aqueous separation-based applications and processes.

Solvent-free preparation of hierarchical 4A zeolite monoliths: Role of experimental conditions

A series of novel self-supporting hierarchically porous zeolite monoliths with micropores and macropores were successfully prepared using polymethyl methacrylate (PMMA) microbeads as sacrificial template via solvent-free strategy. The effects of several synthesis parameters, such as Na/Al molar ratio, H2O/metakaolin (MK) mass ratio, MK/PMMA mass ratio and curing time on the structure and morphology of hierarchical zeolite monoliths were systematically investigated. Multiple techniques, including SEM, XRD, FTIR, XRF, BET, MIP and TG-DTG were used to test the properties of resultant products. The experimental results show that hierarchical 4A zeolite monoliths could be obtained at Na/Al molar ratio of 1.2:1, H2O/MK mass ratio of 1.1:1, MK/PMMA mass ratio of 50/50, curing temperature of 70 °C for 24 h and calcination temperature of 550 °C for 2 h. The SiO2/Al2O3 molar ratio and BET specific surface area were found to be 1.92 and 57.67 m2/g, respectively. The diameter of macropore pores were mainly in the range of 840–4130 nm.

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.