Siliceous Zeolite Research Articles

Cu nanoparticles confined in siliceous MFI zeolite for methanol steam reforming

The Cu@S-1 catalyst synthesized by the ligand-stabilized strategy shows high stability and activity in the methanol steam reforming (MSR) reaction.

A fluoride-free siliceous STW-type zeolite synthesized using a designed organic structure-directing agent.

A pure silica STW zeolite is synthesized with no impurities under a wide range of synthesis conditions with and without fluoride by using easily available 1-methyl-1,5-diazabicyclo[4.3.0]non-5-ene (MDBN) as a template. MDBN having an appropriate size and geometry fits well in the STW cage, leading to its high specificity in structure-directing formation of zeolite STW.

Density functional theory study of hydrophobic zeolites for the removal of triclosan from aqueous solution

The adsorption of triclosan in highly siliceous zeolites was studied with electronic structure calculations, delivering insights into the impact of pore size, shape, and hydrophobicity on the affinity towards this emerging organic contaminant.

Direct Synthesis of Highly Siliceous ZnO-FAU Zeolite with Enhanced Performance in Hydrocarbon Cracking Reactions

Direct Synthesis of Highly Siliceous ZnO-FAU Zeolite with Enhanced Performance in Hydrocarbon Cracking Reactions

Computational investigation of the structures and energies of microporous materials

We present a comprehensive study of calculated lattice and cohesive energies for pure silica zeolites and pure microporous alumino-phosphates (ALPOs). Molecular mechanical and quantum mechanical methodologies based on Density Functional Theory (DFT) are employed to calculate respectively lattice and cohesive energies, whose values relative to those of the dense α-Quartz (SiO2) and Berlinite (AlPO4) phases are compared to experimental values. The results confirm that the siliceous zeolites and microporous ALPOs are all metastable with respect to α-Quartz and Berlinite with the energy differences between the microporous and dense phases, calculated by the DFT methods for the siliceous systems being closer to experiment than those with the interatomic potential based methods; although calculations based on shell model potentials gave values closer to experimental values than those based on the rigid ion model and can reproduce the trends observed in both DFT and experiment at a low computational cost. For the zeolitic structures, interatomic potential based calculations tend to overestimate lattice energies which may arise from inadequacies in the modelling of charge transfer which can be modelled by the DFT studies. For the ALPO systems, DFT gives higher energies than the interatomic potential based methods which deviate appreciably from the experimental data. Possible origins of the discrepancy are discussed.

Rivet of cobalt in siliceous zeolite for catalytic ethane dehydrogenation

•A catalyst with isolated cobalt in siliceous zeolite was prepared •This catalyst catalyzed the EDH at thermodynamically limited conversion levels •Superior productivity of ethylene was achieved •The catalyst was coking resistant with constant performances over a long period Catalytic dehydrogenation of ethane (EDH) is promising for the utilization of shale gas to produce ethylene, but the current processes mostly exhibit space-time productivity of ethylene lower than 1.5 KgC2H4 Kgcat−1 h−1 because of the insufficient catalyst activity in the non-oxidative route. We reported a catalyst with isolated cobalt in siliceous zeolite derived from a mechanically assisted spontaneous dispersion. This material can operate as a non-oxidative EDH catalyst at thermodynamically limited conversion levels under rapid gas feeding, resulting in ethylene productivity at 13.4 KgC2H4 Kgcat−1 h−1. This result is superior to that of the previous non-noble metal catalysts and even outperforms the precious PtSn/Al2O3 catalyst. The isolated cobalt sites are preserved upon calcination at high temperatures and during EDH operation under harsh conditions, resulting in superior durability in a long reaction period with negligible coke formation. Catalytic dehydrogenation of ethane (EDH) is promising for the utilization of shale gas to produce ethylene, but the current processes mostly exhibit space-time productivity of ethylene lower than 1.5 KgC2H4 Kgcat−1 h−1 because of the insufficient catalyst activity in the non-oxidative route. We reported a catalyst with isolated cobalt in siliceous zeolite derived from a mechanically assisted spontaneous dispersion. This material can operate as a non-oxidative EDH catalyst at thermodynamically limited conversion levels under rapid gas feeding, resulting in ethylene productivity at 13.4 KgC2H4 Kgcat−1 h−1. This result is superior to that of the previous non-noble metal catalysts and even outperforms the precious PtSn/Al2O3 catalyst. The isolated cobalt sites are preserved upon calcination at high temperatures and during EDH operation under harsh conditions, resulting in superior durability in a long reaction period with negligible coke formation.

Preparation of Confined One-Dimensional Boron Nitride Chains in the 1-D Pores of Siliceous Zeolites under High-Pressure, High-Temperature Conditions.

Low-dimensional boron nitride (BN) chains were prepared in the one-dimensional pores of the siliceous zeolites theta-one (TON) and Mobil-twelve (MTW) by the infiltration, followed by the dehydrocoupling and pyrolysis of ammonia borane under high-pressure, high-temperature conditions. High-pressure X-ray diffraction in a diamond anvil cell and in a large-volume device was used to follow in situ these different steps in order to determine the optimal conditions for this process. Based on these results, millimeter-sized samples of BN/TON and BN/MTW were synthesized. Characteristic B-N stretching vibrations of low-dimensional BN were observed by infrared and Raman spectroscopies. The crystal structures were determined using a combination of X-ray diffraction and density functional theory with one and two one-dimensional zig-zag (BN)x chains per pore in BN/TON and BN/MTW, respectively. These 1-D BN chains potentially have interesting photoluminescence properties in the far ultraviolet region of the electromagnetic spectrum.

Catalytic performances in methane combustion over Pd nanoparticles supported on pure silica zeolites with different structures

Catalytic combustion of CH4 to CO2 and H2O was studied over Pd nanoparticles supported on different siliceous zeolites (MFI, BEA and CHA) with γ-Al2O3 as the reference support, and it was found that as supports, zeolites exert remarkable influence on the catalytic performance of methane oxidation, with the Pd catalyst (1 wt% loading) supported by siliceous MFI (ZSM-5-Si) presenting the highest activity. Over the exceptional catalyst, a complete conversion of methane can be achieved at around 380 °C, which is about 120 °C lower than the temperature needed for the Pd nanoparticles supported on γ-Al2O3 (Pd/γ-Al2O3), one of the best catalysts for the methane combustion. Physicochemical characterizations including X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), temperature programmed desorption of methane (CH4-TPD) and temperature programmed reduction of methane (CH4-TPR) techniques were used to acquire an idea what is behind the discrepancy in the catalytic performances of methane combustion over these Pd-based catalysts. Based on these results, it is proposed that the different methane adsorption and the oxygen migration capability are mainly related to the difference in the catalytic performances of methane combustion over these catalysts. Among these factors, the difference in the methane adsorption is critical for the different catalytic performances, as evidenced by the kinetic measurements.

Performance of BEA, FAU, LTL, MFI, and MOR zeolites in the removal of ethyl mercaptan traces from natural gas by Monte Carlo molecular simulation

Selective removal of ethyl mercaptan from natural gas is presently challenge in the gas industry. Nanoporous zeolite frameworks with different topologies have been known as the most appropriate compounds for the adsorption of ethyl mercaptan. Grand Canonical Monte Carlo (GCMC) simulation has been employed to predict the adsorption and separation properties of mixtures containing ethyl mercaptan under difficult experimental conditions. The adsorption isotherms of ethyl mercaptan in pure form and mixtures with methane on purely siliceous zeolites including BEA, FAU, LTL, MFI, and MOR have been obtained at the temperature of 298 K over the pressure range of 0–100 kPa. The results show that the adsorption capacity is a function of the pore diameter of zeolites. FAU has appeared as an adsorbent with high adsorption capacity. In contrast, it has shown considerably lower adsorption affinity for methane in comparison with ethyl mercaptan, confirming the competitive adsorption between them. Therefore, FAU could be the promising candidate for industrial desulfurization of natural gas. The effects of temperature, pressure, initial bulk concentration, and pore structure have been examined. Distribution snapshots displays the preferred positions of zeolites for the adsorption of sulfur species.

Solvent-free and low template content synthesis of Ti-Beta zeolite via interzeolite transformation for oxidative desulfurization

Solvent-free and low-template synthesis of zeolite materials is the future direction of zeolite production, which meets the requirements of green chemistry. Ti-Beta zeolite with opened channel systems of the 3-dimensional 12-membered ring is a promising industrial catalyst in the field of selective oxidations. Herein, nanosized Ti-Beta zeolites were successfully synthesized via interzeolite transformation of siliceous MOR zeolite using sample and commercially available tetraethylammonium hydroxide (TEAOH) as organic structure-directing agents under solvent-free and low-template-content (TEAOH/SiO2 = 0.2) conditions within a short crystallization time. The crystallization mechanism and Ti coordinated state were thoroughly investigated by numerous characterization techniques. It was found that the hydrothermal synthesis of Ti-Beta-M zeolites obeyed a liquid crystallization mechanism, during which pre-dissolution of the silicon source MOR zeolite was a critical step in achieving successful crystallization. Ti4+ ions mainly existed in the form of tetrahedrally coordinated states in zeolite framework. Thus Ti-Beta zeolites showed remarkable activity and reusability during the oxidative desulfurization of fuel oil. Moreover, the deactivated mechanism and regenerated method were also studied in detail. This work provides new guidance for solvent-free and low template content synthesis of metallosilicate zeolites.

Interventions to the Spontaneous Fabrication of Hierarchical ZSM-5 Zeolites by Fluorination-Alkaline Treatment

The sequential fluorination-alkaline treatment protocol has been applied for the tailoring of siliceous ZSM-5 zeolite. The original spontaneous growth of mesoporosity in alkaline medium is altered due to the antecedent fluorination step. The outcome is demonstrated by the apparent delay in the mesoporosity growth, whose essential duration for the well-defined mesoporosity is therefore extended from 30 min to 60 min. A low fluorination level decelerates the mesoporosity growth, whereas a high fluorination level enables the achievement of the mesoporosity. These impacts are closely linked with the alteration to the states of Al sites as the function of fluorination level. Compared to the states of Al sites in the pristine and steamed zeolites, the electronic and steric consequences on the environment of Al species by fluorination is proposed for the interplay with the alkaline medium for the mesoporosity growth.

Accurate large-scale simulations of siliceous zeolites by neural network potentials

The computational discovery and design of zeolites is a crucial part of the chemical industry. Finding highly accurate while computational feasible protocol for identification of hypothetical siliceous frameworks that could be targeted experimentally is a great challenge. To tackle this challenge, we trained neural network potentials (NNP) with the SchNet architecture on a structurally diverse database of density functional theory (DFT) data. This database was iteratively extended by active learning to cover not only low-energy equilibrium configurations but also high-energy transition states. We demonstrate that the resulting reactive NNPs retain DFT accuracy for thermodynamic stabilities, vibrational properties, as well as reactive and non-reactive phase transformations. As a showcase, we screened an existing zeolite database and revealed >20k additional hypothetical frameworks in the thermodynamically accessible range of zeolite synthesis. Hence, our NNPs are expected to be essential for future high-throughput studies on the structure and reactivity of siliceous zeolites.

Atomic Insight into the Local Structure and Microenvironment of Isolated Co-Motifs in MFI Zeolite Frameworks for Propane Dehydrogenation.

Embedding metal species into zeolite frameworks can create framework-bond metal sites in a confined microenvironment. The metals sitting in the specific T sites of zeolites and their crystalline surroundings are both committed to the interaction with the reactant, participation in the activation, and transient state achievement during the whole catalytic process. Herein, we construct isolated Co-motifs into purely siliceous MFI zeolite frameworks (Co-MFI) and reveal the location and microenvironment of the isolated Co active center in the MFI zeolite framework particularly beneficial for propane dehydrogenation (PDH). The isolated Co-motif with the distorted tetrahedral structure ({(≡SiO)2Co(HO-Si≡)2}, two Co-O-Si bonds, and two pseudobridging hydroxyls (Co···OH-Si) is located at T1(7) and T3(9) sites of the MFI zeolite. DFT calculations and deuterium-labeling reactions verify that the isolated Co-motif together with the MFI microenvironment collectively promotes the PDH reaction by providing an exclusive microenvironment to preactivate C3H8, polarizing the oxygen in Co-O-Si bonds to accept H* ({(≡SiO)CoHδ- (Hδ+O-Si≡)3}), and a scaffold structure to stabilize the C3H7* intermediate. The Co-motif active center in Co-MFI goes through the dynamic evolutions and restoration in electronic states and coordination states in a continuous and repetitive way, which meets the requirements from the series of elementary steps in the PDH catalytic cycle and fulfills the successful catalysis like enzyme catalysis.

Nickel encapsulated in silicalite-1 zeolite catalysts for steam reforming of glycerol (SRG) towards renewable hydrogen production

Valorisation of crude glycerol via steam reforming, i.e., SRG, is a promising method to produce sustainable hydrogen. However, catalyst deactivation under harsh SRG conditions is still a main challenge which hinders the further development of practical SRG. In this work, the encapsulated Ni catalyst in siliceous silicalite-1 zeolite (Ni@Si-1) were developed to show the improved performance and enhanced anti-deactivation potentials in catalytic SRG as compared with the conventional impregnated Ni catalysts (i.e., Ni/Si-1). Importantly, the post-synthetic treatment of Ni@Si-1 using TPAOH solution formed the encapsulated Ni catalyst with the mesoporous hollow structure (i.e., Ni@HolSi-1), which demonstrate even better performance in SRG with glycerol conversion of >95%, H2 yield of ~70%, H2/CO2 molar ratio of >2.33 and CO/CO2 molar ratio of <1 at 750 °C. Specifically, highly dispersed ultrasmall encapsulated Ni particles were retained within the hollow crystals of siliceous silicalite-1, as confirmed by XPS and HRTEM characterisation. The activation energy for glycerol conversion over Ni@HolSi-1 (i.e., Ea = ~ 19 kJ mol−1) was much lower than that of Ni/Si-1 and Ni@Si-1. 100-h longevity tests over the three catalysts were investigated at 750 °C, and the Ni@HolSi-1 catalyst exhibited an excellent stability and activity, as well as insignificant coke deposition, which could be due to the enhancement of highly dispersed yet accessible Ni NPs within the hollow Si-1 crystals. The findings of the work show the promise of the encapsulation strategy and mesoporous zeolites for developing the future reforming catalysts.

Impact of residual sodium cations in azonia-spiro templates on the formation of large and extra-large pore zeolites

Azonia-spiro compounds have been reported as organic templates for synthesizing many extra-large pore high silica zeolites, often isomorphically substituted by germanium. However, these templates are usually obtained in the presence of sodium hydroxide, leading to persisting sodium ions in the final products. Sodium cations may compete with azonia-spiro organics as templates during zeolite synthesis. Therefore, effect of any residual sodium cations on the synthesis results needs to be investigated. This work documents the effect of sodium ions on zeolite formation using four structurally similar azonia-spiro compounds as templates. Different silicogermanate zeolites (UTL, an unknown zeolitic phase, etc.) were formed in the absence of sodium cations. Notably, the syntheses of silicogermanate UTL and siliceous MTW zeolites using azonia-spiro[4,6] and [6,6] are reported for the first time. Controlled amounts of sodium chloride were added to zeolite crystallization mixtures without residual sodium hydroxide. In the presence of 0.005–0.1 mol/L Na+, silicogermanate zeolites were the dominant phases. Further increasing the Na+ concentration to 0.152 and 0.317 mol/L, formation of UTL and an unknown silicogermanate phase were inhibited in favor of a dense sodiumgermanate (Na4Ge9O20). Moreover, sodium ions next to azonia-spiro organic templates also influence the morphologies of the MTW zeolites. The observed high sensitivity of zeolite formation in the presence of even traces of sodium therefore has impact on synthesis protocols of large pore silicogermanate zeolites. This work highlights how small, often overlooked, factors in inorganic material synthesis can drastically change the outcome.

Dynamics of Topology-dependent Water Purification by Siliceous Zeolite Membranes

The efficiency of pollutant rejection (PR, %) and water purification of the zeolite membranes is highly dependent on their topology, i.e., porosity and chemical composition. Nevertheless, mechanisms controlling such a physicochemical phenomenon have been limitedly discussed. Herein, we provide insights into the physics and dynamics of water purification by zeolite nanosheets in terms of topological variables. The molecular dynamics (MD) simulation pictured the molecular-lever water purification having heavy metal ions (Co2+ and Pb2+) as typical water contaminants. Three siliceous zeolites having different topologies, i.e., LTA, FAU, and MFI zeolites, were positioned in the simulation space. MD simulation was served under pressure and temperature of 24 MPa and 300 K, respectively, applying the Canonical ensemble (NVT) condition. The intermolecular interactions between contaminant ions, zeolite, and graphene were tracked by hybridizing Tersoff and Lenard-Jones potentials, while the TIP3P model of the CHARMM force field was used to simulate water dynamics. The LAMMPS software and Visual Molecular Dynamics (VMD) were used to collect data and graphics over 15 ns, with early 5 ns devoted to reaching equilibrium.Interestingly, we demonstrated by the aid of potential of the mean force (PMF) pollutant ions which encounter the zeolite pores smaller than their kinetic diameter consume more energy for permeation across the membrane, -reason why LTA appeared selective against Pb2+, Co2+, and Cl- ions, with PR of 40, 60, and 100%, respectively. The absolute water flux values were monitored, and LTA and FAU fluxes were ∼ 50% higher than MFI zeolite, but 5 ns after equilibrium, LTA flux was overtaken sharply by the simulation because of the larger pore size. Overall, the electrostatic forces showed absolute control over the purification but were governed by the zeolite topology.

Toward accurate ab initio modeling of siliceous zeolite structures.

Structures of purely siliceous materials in the International Zeolite Association database were investigated with four different theoretical methods ranging from the empirical approaches, such as the distance least squares and force fields to the computationally demanding dispersion-corrected density functional theory method employing the generalized gradient approximation-type functional. The structural characteristics were first evaluated for dense silica polymorphs, for which reliable low-temperature experiments are available. Due to the significant errors in experimentally determined atomic positions of siliceous zeolites, lattice parameters and the cell volume were proposed as reliable descriptors for the structural assessment of zeolite frameworks. In this regard, the most consistently performing (systematically underestimating/overestimating) methods are the Sanders-Leslie-Catlow (SLC) force field and the PBEsol density functional. The best overall agreement with the experiment is observed for PBEsol-D2. However, it is a result of fortuitous error cancellations rather than improved description upon adding dispersion correction. We proposed two approaches to estimate accurate cell volumes of siliceous materials from theoretical data: (i) using the SLC and PBEsol volumes as lower and upper bounds and (ii) using a structural response to the dispersion correction along with the SLC compressibility as an additional criterion.

Ultrafast green synthesis of sub-micron Silicalite-1 zeolites by a grinding method

Sub-micron Silicalite-1 zeolite is traditionally synthesized by a prolonged synthesis approach which takes several days to complete. We herein demonstrate a rapid solvent-free synthesis route for sub-micron Silicalite-1 zeolite using TPAOH as an organic template to control the triple stage crystal growth rate and crystal morphologies of Silicalite-1 without the assistance of crystal seeds. This procedure is only achieved through simple mixing and grinding of raw materials and crystallization at 180 ​°C for 45 ​min. The fast synthesized Silicalite-1 zeolite proved to have higher crystal phases compared with the conventionally synthesized product. Through the absence of using additional solvents, all reactants were ultimately utilized without any waste, resulting in not only effectively reducing the cost of siliceous zeolites synthesis but also minimizing its environmental impact.

Alloyed PdCu Nanoparticles within Siliceous Zeolite Crystals for Catalytic Semihydrogenation.

Selective hydrogenation of acetylene to ethylene is an industrially important process to purify the raw ethylene stream for producing high-grade polyethylene. The supported Pd catalyst exhibits superior activity for acetylene hydrogenation but suffers from poor ethylene selectivity because of the easy overhydrogenation to produce ethane. Here, we report that the PdCu alloy nanoparticles within siliceous zeolite crystals effectively tuned Pd-catalyzed overhydrogenation into semihydrogenation. This catalyst displayed an ethylene selectivity of 92.9% with a full conversion of acetylene. Mechanism studies reveal that the zeolite fixation stabilized the alloyed structure, where the electron-enriched Pd surface benefits the rapid ethylene desorption to hinder the deep hydrogenation. This work provides an efficient strategy for a rational design of bimetallic metal catalysts for selective hydrogenations.

Compressibility, Phase Transition, and Argon Insertion in the Siliceous Zeolite Mobil-Twelve at High Pressure

The siliceous zeolite Socony Mobil-twelve (ZSM-12) or MTW (Mobil-TWelve) with a one-dimensional pore system was studied at high pressure by synchrotron X-ray powder diffraction in nonpenetrating DAPHNE7474 oil and in a penetrating argon pressure medium. A phase transition from the space group C2/c to P2/n is observed in the nonpenetrating medium at close to 1.5 GPa with a 4% volume decrease and a strong increase in compressibility in the ac plane corresponding to partial collapse of the pores. Strong decreases in diffracted intensity are observed with further compression, and the diffraction pattern contains broad features characteristic of an amorphous material above 10 GPa. Distinct behavior is observed when this material is pressurized in argon. Argon fills the pores with 9 ± 1 Ar atoms per unit cell. In this medium-pore zeolite, the quantity of inserted argon was not found to vary with pressure. Argon-filled MTW is 35% less compressible than the corresponding empty form.