Understanding Diffusion in Hierarchical Zeolites with House-of-Cards Nanosheets

Boosting the Performance of Low-Platinum Fuel Cells via a Hierarchical and Interconnected Porous Carbon Support

The design of a low-platinum (Pt) proton-exchange-membrane fuel cell (PEMFC) can reduce its high cost. However, the development of a low-Pt PEMFC is severely hindered by the high oxygen transfer resistance in the catalyst layer. Herein, a carbon with interconnected and hierarchical pores is synthesized as a support for the low-Pt catalyst to lower the oxygen transfer resistance. A H2-air fuel cell assembled by Pt/hierarchical porous carbon shows 1610 mW/cm2 peak power density, 2230 mA/cm2 current density at 0.60 V, and only 18.4 S/m local oxygen transfer resistance with 0.10 mgPt/cm2 Pt loading at the cathode, which far exceeds those of various carbon black supports and commercially used Pt/C catalysts. Both the experimental and simulation results have shown the advancement of hierarchical pores toward the high efficiency of oxygen transportation.

Intergrowth Zeolites, Synthesis, Characterization, and Catalysis

Microporous zeolites that can act as heterogeneous catalysts have continued to attract a great deal of academic and industrial interest, but current progress in their synthesis and application is restricted to single-phase zeolites, severely underestimating the potential of intergrowth frameworks. Compared with single-phase zeolites, intergrowth zeolites possess unique properties, such as different diffusion pathways and molecular confinement, or special crystalline pore environments for binding metal active sites. This review first focuses on the structural features and synthetic details of all the intergrowth zeolites, especially providing some insightful discussion of several potential frameworks. Subsequently, characterization methods for intergrowth zeolites are introduced, and highlighting fundamental features of these crystals. Then, the applications of intergrowth zeolites in several of the most active areas of catalysis are presented, including selective catalytic reduction of NOx by ammonia (NH3-SCR), methanol to olefins (MTO), petrochemicals and refining, fine chemicals production, and biomass conversion on Beta, and the relationship between structure and catalytic activity was profiled from the perspective of intergrowth grain boundary structure. Finally, the synthesis, characterization, and catalysis of intergrowth zeolites are summarized in a comprehensive discussion, and a brief outlook on the current challenges and future directions of intergrowth zeolites is indicated.

Hyperloop-like diffusion of long-chain molecules under confinement

The ultrafast transport of adsorbates in confined spaces is a goal pursued by scientists. However, diffusion will be generally slower in nano-channels, as confined spaces inhibit motion. Here we show that the movement of long-chain molecules increase with a decrease in pore size, indicating that confined spaces promote transport. Inspired by a hyperloop running on a railway, we established a superfast pathway for molecules in zeolites with nano-channels. Rapid diffusion is achieved when the long-chain molecules keep moving linearly, as well as when they run along the center of the channel, while this phenomenon do not exist for short-chain molecules. This hyperloop-like diffusion is unique for long-chain molecules in a confined space and is further verified by diffusion experiments. These results offer special insights into molecule diffusion under confinement, providing a reference for the selection of efficient catalysts with rapid transport in the industrial field.

Steering interface effect of H-ZSM-5 zeolites with tailored surface barriers to improve their catalytic performances

An efficient zeolite interface with optimized surface barriers was tailored by passivating the hydroxyl-group defects at surfaces or near pore mouths. The surface permeability of the modified zeolite was almost 90% greater than that of the pristine one, leading to remarkable improvements in C=2-3 selectivity and an anti-inactivation rate of 75% for the catalytic cracking reaction.

Insights into Sorption and Molecular Transport of Aqueous Glucose into Zeolite Nanopores

Liquid-phase heterogeneous catalysis using zeolites is important for biomass conversion to fuels and chemicals. There is a substantial body of work on gas-phase sorption in zeolites with different topologies; however, studies investigating the diffusion of complex molecules in liquid medium into zeolitic nanopores are scarce. Here, we present a molecular dynamics study to understand the sorption and diffusion of aqueous β-d-glucose into β-zeolite silicate at T = 395 K and P = 1 bar. Through 2-μs-long molecular dynamics trajectories, we reveal the role of the solvent, the kinetics of the pore filling, and the effect of the water model on these properties. We find that the glucose and water loading is a function of the initial glucose concentration. Although the glucose concentration increases monotonically with the initial glucose concentration, the water loading exhibits a nonmonotonic behavior. At the highest initial concentration (∼20 wt %), we find that the equilibrium loading of glucose is approximately five molecules per unit cell and displays a weak dependence on the water model. Glucose molecules follow a single-file diffusion in the nanopores due to confinement. The dynamics of glucose and water molecules slows significantly at the interface. The average residence time for glucose molecules is an order of magnitude larger than that in the bulk solution, while it is about twice as large for the water molecules. Our simulations reveal critical molecular details of the glucose molecule's local environment inside the zeolite pore relevant to catalytic conversion of biomass to valuable chemicals.

Synthesis of nanosheet epitaxial growth ZSM-5 zeolite with increased diffusivity and its catalytic cracking performance

The introduction of microporous substrate in the nanosheet zeolite reduces the “acid wall” barrier. The diffusional time constant of RP-120 is increased by 32%, and its TOF is increased by 54%.

Enhanced Adsorption and Mass Transfer of Hierarchically Porous Zr-MOF Nanoarchitectures toward Toxic Chemical Removal

Zirconium-based metal-organic frameworks (Zr-MOFs) have shown tremendous prospects as highly efficient adsorbents against toxic chemicals under ambient conditions. Here, we report for the first time the enhanced toxic chemical adsorption and mass transfer properties of hierarchically porous Zr-MOF nanoarchitectures. A general and scalable sol-gel-based strategy combined with facile ambient pressure drying (APD) was utilized to construct MOF-808, MOF-808-NH₂, and UiO-66-NH₂ xerogel monoliths, denoted as G808, G808-NH₂, and G66-NH₂, respectively. The resulting Zr-MOF xerogels demonstrated 3D porous networks assembled by nanocrystal aggregates, with substantially higher mesoporosities than the precipitate analogues. Microbreakthrough tests on powders and tube breakthrough experiments on engineered granules were conducted at different relative humidities to comprehensively evaluate the NO₂ adsorption capabilities. The Zr-MOF xerogels showed considerably better NO₂ removal abilities than the precipitates, whether intrinsically or under simulated respirator canister/protection filter environment conditions. Multiple physicochemical characterizations were conducted to illuminate the NO₂ filtration mechanisms. Analysis on adsorption kinetics and mass transfer patterns in Zr-MOF xerogels was further performed to visualize the underlying structure-activity relationship using the gravimetric uptake and zero length column methods with cyclohexane and acetaldehyde as probes. The results revealed that the synergy of hierarchical porosities and nanosized crystals could effectively expedite the intracrystalline diffusion for the G66-NH₂ xerogel as well as alleviate the surface resistance for the G808-NH₂ xerogel, which led to accelerated overall adsorption uptake and thus enhanced performance toward toxic chemical removal.

Understanding the Catalytic Activity of Microporous and Mesoporous Zeolites in Cracking by Experiments and Simulations

Porous zeolite catalysts have been widely used in the industry for the conversion of fuel-range molecules for decades. They have the advantages of higher surface area, better hydrothermal stability, and superior shape selectivity, which make them ideal catalysts for hydrocarbon cracking in the petrochemical industry. However, the catalytic activity and selectivity of zeolites for hydrocarbon cracking are significantly affected by the zeolite topology and composition. The aim of this review is to survey recent investigations on hydrocarbon cracking and secondary reactions in micro- and mesoporous zeolites, with the emphasis on the studies of the effects of different porous environments and active site structures on alkane adsorption and activation at the molecular level. The pros and cons of different computational methods used for zeolite simulations are also discussed in this review.

High purity separation of n-pentane from neopentane using a nano-crystal of zeolite Y

A method for the separation of a mixture of n-pentane and neopentane using a nano-crystallite of zeolite Y is reported. This method judiciously combines two well-known, counter-intuitive phenomena, the levitation and the blowtorch effects. The result is that the two components are separated by being driven to the opposite ends of the zeolite column. The calculations are based on the non-equilibrium Monte Carlo method with moves from a region at one temperature to a region at another temperature. The necessary acceptance probability for such moves has been derived here on the basis of stationary solution of an inhomogeneous Fokker-Planck equation. Simulations have been carried out with a realistic and experimentally relevant Gaussian hot zone and also a square hot zone, both of which lead to very good separation. Simulations without the hot zones do not show any separation. The results are reported at a loading of 1 molecule per cage. The temperature of the hot zone is just ∼30 K higher than the ambient temperature. The separation factors of the order of 10¹⁷ are achieved using single crystals of zeolite, which are less than 1 μm long. The conditions for including the hot zone may be experimentally realizable in the future considering the rapid advances in nanoscale thermometry. The separation process is likely to be energetically more efficient by several orders of magnitude as compared to the existing methods of separation, making the method very green.

Effect of External Surface Diffusion Barriers on Platinum/Beta-Catalyzed Isomerization of n-Pentane

We have developed a generalizable strategy to quantify the effect of surface barriers on zeolite catalysis. Isomerization of n‐pentane, catalyzed by Pt/Beta, is taken as a model reaction system. Firstly, the surface modification by chemical liquid deposition of SiO₂ was carried out to control the surface barriers on zeolite Beta crystals. The deposition of SiO₂ leads to a very slight change in the physical properties of Beta crystals, but an obvious reduction in Brønsted acid sites. Diffusion measurements by the zero‐length column (ZLC) method show that the apparent diffusivity of n‐pentane can be more than doubled after SiO₂ deposition, indicating that the surface barriers have been weakened. Catalytic performance was tested in a fixed‐bed reactor, showing that the apparent catalytic activity improved by 51–131 % after SiO₂ deposition. These results provide direct proof that reducing surface barriers can be an effective route to improve zeolite catalyst performance deteriorated by transport limitations.

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems.

Transport Behavior of Oil in Mixed Wettability Shale Nanopores

Shale oil reserves play an important role in the oil & gas industry. The investigation of oil transport behavior in shale nanopores is crucial in the successful exploitation of shale oil reservoirs. However, the transport mechanisms of oil in shale nanopores are still not understood. In this paper, a model for oil transport through a single nanopore was established by considering mixed wettability, surface roughness, varying viscosity, and the effects triggered by adsorbed organic matter. The organic surface ratio of a single nanopore was used to quantify mixed wettability, while the effects of adsorbed organic matter were estimated by the surface coverage and the adsorption thickness. The entire mathematical model was simplified into several equations to discuss the contributions of each mechanism. The results showed that to accurately predict the oil transport properties in mixed wettability shale nanopores, it is necessary to consider varying viscosity, wettability alteration, and the oil molecule structure. Adsorbed organic matter led to increase in oil flow capacity by altering the surface wettability. However, the oil flow capacity was greatly reduced when varying viscosity was considered. Additionally, the contributions of each mechanism varied with the pore type. Furthermore, increasing surface roughness significantly reduced the oil flow capacity in both organic and inorganic nanopores. This work provides a better understanding of oil transport behavior in mixed-wettability shale nanopores and a quantitative framework for future research.

Understanding the Loading Dependence of Adsorbate Diffusivities in Hierarchical Metal-Organic Frameworks

Using atomistic simulations, we studied the diffusion of n-hexane in a series of isoreticular hierarchical metal-organic frameworks (MOFs) NU-100x. Nonmonotonic diffusivity-loading relationships that depend on the pore sizes were observed, which can be explained by the spatial distribution of adsorbates at different loadings. For one of the MOFs in the series, NU-1000-M, the diffusivity-loading relationship is almost identical to the previously reported results of n-hexane diffusion in the hierarchical self-pillared pentasil (SPP) zeolite. Detailed analysis revealed that the similarity results from their similar micropore and window sizes, which was confirmed by free-energy mapping. The effects of temperature and adsorbate chain length on the diffusion were also studied, which supported our conclusion that the diffusivity in hierarchical nanoporous materials is primarily controlled by the sizes of the micropores and the connecting windows, particularly at relatively low loadings.

An amino acid-assisted approach to fabricate nanosized hierarchical TS-1 zeolites for efficient oxidative desulfurization

A facile amino acid-assisted approach coupled with two-step rota-crystallization has been developed to prepare nanosized hierarchical TS-1 zeolites, which are free of anatase TiO₂ and possess abundant secondary meso-/macropores.

Understanding the Role of Internal Diffusion Barriers in Pt/Beta Zeolite Catalyzed Isomerization of n-Heptane

Applications of zeolites in catalysis are plagued by strong diffusion resistance, which results from limitations to molecular transport in micropores, across external crystal surfaces, but also across internal interfaces. The first type of diffusion resistance is well understood, the second is receiving increasing attention, while the diffusion barriers at internal interfaces remain largely unclear. We take Pt/Beta catalyzed isomerization of n-heptane as the model system to explore the role of internal diffusion barriers in zeolite catalysis. The two as-synthesized Pt/Beta catalysts have an identical Pt loading, similar Beta particle size and acidity, but different internal structures. A Pt/Beta crystal with no observable internal interfaces can be 180 % higher in activity and 22 % higher in selectivity than its counterpart with numerous internal interfaces. This can only be attributed to the strong transport barriers across internal interfaces, as supported by directly comparing the apparent diffusivities of the two Beta samples.

Adsorptive Nature of Surface Barriers in MFI Nanocrystals

Zeolite nanocrystals with characteristic diffusion lengths of nanometers are widely used in molecular applications to overcome diffusion limitations. However, with a large fraction of external surface area, mass transport in these materials is often limited by the presence of a surface barrier, which limits their overall potential in catalytic or separation applications. Herein, silicalite-1 crystals of varying sizes were synthesized, and the adsorption and diffusion characteristics of four molecules (ethylcyclohexane, methylcyclohexane, cyclohexane, and cis-1,4-dimethylcyclohexane) were measured to mechanistically evaluate the mass transfer surface barrier. The results observed in this study support the presence of a nonstructural surface resistance associated with the strong enthalpic interaction between the diffusing molecule and zeolite surface. Further analysis indicates that the contributions of structural and nonstructural surface barriers to the mass transport vary greatly with the heat of adsorption. This work suggests that when diffusing molecules have a weak heat of adsorption on the zeolite surface, strategies to mitigate the surface barrier should focus on the structural modification of the zeolite surface using methods such as surface etching to remove pore blockages. When the heat of adsorption is strong, strategies should focus on tuning the adsorbate/adsorbent surface interaction by methods such as depositing a mesoporous silica overlayer to reduce surface adsorption or adding a secondary external surface to minimize re-entering of the micropores.

A large-surface-area TS-1 nanocatalyst: a combination of nanoscale particle sizes and hierarchical micro/mesoporous structures

By a simple sequent process of dry-gel steam-assisted crystallization and following a top-down alkali-etching treatment, hierarchically structured TS-1 nanozeolites (nanoTS-1_D) with abundant micro/mesopores have been synthesized for the first time, and they exhibit remarkably high specific surface area of 606 m² g⁻¹ and pore volume of 0.86 cm³ g⁻¹. Characterization by XRD, FTIR, UV-vis and EM confirm the exclusive incorporation of titanium species in zeolite frameworks. More importantly, compared with the microporous TS-1 nanocrystal material with identical particle sizes of 80 nm (nanoTS-1) and submicrometer-sized mesoporous TS-1 material (mesoTS-1), here the reported nanoTS-1_D catalyst shows greatly improved performance in the model reaction of 1-hexene epoxidation. 40.9% olefin conversion and 96.3% epoxide selectivity are achieved and its high stability is verified by the 6 recycling–regeneration tests.

By dry-gel steam-assisted crystallization and top-down alkali-etching treatment, hierarchically structured TS-1 nanozeolites with abundant micro/mesopores were synthesized for the first time, with high specific surface area of 606 m² g⁻¹ and total pore volume of 0.86 cm³ g⁻¹.

Adsorption and Diffusion of C1 to C4 Alkanes in Dual-Porosity Zeolites by Molecular Simulations

We employ grand canonical Monte Carlo and molecular dynamics simulations to systematically study the adsorption and diffusion of C₁ to C₄ alkanes in hierarchical ZSM-5 zeolite with micropores (∼1 nm) and mesopores (>2 nm). The zeolite is characterized by a large surface area of active sites on the microporous scale with high permeability and access to the active sites, which arises from the enhanced transport at the mesoporous scale. We model this zeolite as a microporous Na⁺-exchanged alumino-sillicate zeolite ZSM-5/35 (Si/Al = 35) in which cylindrical mesopores with a diameter of 4 nm have been built by deleting atoms accordingly. We use the TraPPE and Vujić-Lyubartsev force fields along with the Lorentz-Berthelot combining rules to describe adsorbate-adsorbate and adsorbate-adsorbent interactions. The performance of the force fields is assessed by comparing against experimental single-component adsorption isotherms of methane and ethane in microporous ZSM-5/35, which we measured as part of this work. We compare the adsorption isotherms and diffusivities of the adsorbed alkanes in the dual-porosity zeolite with those in microporous ZSM-5/35 and discern the specific behavior at each porosity scale on the overall adsorption, self-diffusion, and transport behavior in zeolites with dual micro/mesoporosities.