Silicalite-1 Zeolite Research Articles

Subnanometric Pt-Ga alloy clusters confined within Silicalite-1 zeolite for n-hexane dehydrogenation

Dehydrogenation of light naphtha followed by catalytic cracking into high-value end products can meet the high demand in the world’s petrochemical market. In this work, at attempting synthesis of novel catalysts for hexane dehydrogenation, a series of bimetallic Pt-Ga clusters encapsulated within Silicalite-1 (S-1) catalysts were synthesized using the ligand-protected direct H2 reduction method. The reduced Pt-Ga@S-1-H catalysts comprised restricted ultra-fine Pt-Ga alloys with an average size 0.86–0.95 nm, mainly corresponding to the Pt3 clusters with coordinate number 1.5–2.1. Ga doping significantly improved the catalytic stability, depending on the Ga content, which ranged from 0.02 to 0.27 wt%. As confirmed by the XPS and EXAFS analysis, after the Ga addition, electron transfer occurred between the Pt-Ga alloys and the skeleton oxygen in zeolite, stabilizing the Pt clusters on the zeolite, thereby inhibiting the growth of Pt crystals. During the n-hexane dehydrogenation, the 0.40Pt0.04 Ga@S-1-H catalyst exhibited a hexene selectivity of 87.1 %, achieving a hexene formation rate of 101.4 molhexene∙gPt-1∙h−1 at 550 °C with a WHSV of 90 h−1. After 1000 min of reaction, this best catalyst experienced only slight deactivation with a deactivation constant of 0.066 h−1. The catalytic activity remained almost unchanged after consecutive five cycles of H2 regeneration. DFT calculation using the synchronous structural models showed that the Gibbs free energy changes (−ΔG) for the first three steps of hexane dehydrogenation over the 0.4Pt0.04 Ga@S-1-H catalyst were much lower than those obtained on the 0.4Pt/S-1-H catalyst, suggesting that the introduction of Ga energetically favored the hexane dehydrogenation.

Hydrophobic nanosheet silicalite-1 zeolite for iodine and methyl iodide capture

Effective capture of radioactive iodine from nuclear fuel reprocessing is of great importance for public safety as well as the secure utility of nuclear energy. In this work, a hydrophobic nanosheet silicalite-1 (NSL-1) zeolite with an adjustable size was developed for efficient iodine (I2) and methyl iodide (CH3I) adsorption. The optimized all-silica zeolite NSL-1 exhibits an excellent I2 uptake capacity of 553 mg/g within 45 min and a CH3I uptake capacity of 262 mg/g within 1 h. Benefiting from the reduced thickness and enhanced porosity, microporous NSL-1 possesses enhanced iodine adsorption capacity and fast adsorption kinetics, which is a considerable high value among inorganic materials. Unexpectedly, the remarkable characters of high hydrophobicity, acid-resistance and anti-oxidation endow it a higher iodine uptake capacity than traditional aluminosilicate zeolites. More importantly, the high uptake selectivity toward I2 possessed by NSL-1 owing to its hydrophobic skeleton under simulated dynamic conditions. The low cost, facile and scalable synthesis of NSL-1 further highlights great prospects for applications in the nuclear industry. This work provides useful insights for designing efficient adsorbents for iodine capture.

Calorimetric Heats of Intrusion of LiCl Aqueous Solutions in Hydrophobic MFI-Type Zeosil: Influence of the Concentration.

For the first time, we report calorimetric measurements of intrusion of aqueous LiCl solutions in a hydrophobic pure siliceous MFI zeolite (silicalite-1) under high pressure. Our results show that the intrusion heats are strongly dependent on the LiCl concentration. The intrusion process is endothermic for diluted solutions (molar H2O/LiCl = 12) as well as for water, but it becomes exothermic for a concentration close to saturation (molar H2O/LiCl = 4). Analysis of the data in the framework of wetting thermodynamics shows that besides surface wetting, other phenomena occur during intrusion, such as hydrogen-bond weakening and composition change. In all cases, water is preferentially intruded so that the intruded phase becomes more diluted than the bulk solution. In the case of the most diluted solution, only water molecules seemed to be intruded. Furthermore, silicalite-1 is shown to be very stable in the presence of LiCl solution, with no noticeable structural and textural modifications observed after intrusion.

Organic template residues confined in silicalite-1 zeolite membranes for tuning pore structures and boosting H2/CO2 separation

Silicalite-1 zeolite membranes were successfully activated at 300–350 °C in a hydrogen atmosphere. FTIR, TGA, and N2 adsorption-desorption characterizations confirmed that the amount of the organic structure directing agent (OSDA) residue in silicalite-1 zeolite channels was tunable. The H2-calcined membrane showed a lower gas permeance than that of the air-calcined membrane due to the partial pore blockage by the OSDA residue. The permeance of H2 and CO2 increased with increasing temperature, showing an activated diffusion-like mechanism. The activation energies for the permeation of H2 and CO2 were in the range of 1.63–4.15 and 0.76–5.90 kJ mol−1, respectively, which confirmed the reduced average membrane pore size and different pore surface chemistry due to the presence of the OSDA residue in the membrane. Significant interactions between CO2 molecules and OSDA residues confined in the membrane resulted in a notably lower CO2 permeance compared to N2 and CH4, despite CO2 having a smaller kinetic diameter. Enhanced H2/CO2 selectivity in the silicalite-1 zeolite membrane, featuring confined OSDA residue, was attributed to the reduction in membrane pore size and the fine-tuning of pore surface chemistry.

Synthesis, characterization, and evaluation of the CO2 adsorption capacities of beta, ZSM-12, ZSM-48, and silicalite-1 zeolites obtained in the presence of tetraethylammonium hydroxide and 1,6-diaminohexane as organic structure-directing agents

Synthesis, characterization, and evaluation of the CO2 adsorption capacities of beta, ZSM-12, ZSM-48, and silicalite-1 zeolites obtained in the presence of tetraethylammonium hydroxide and 1,6-diaminohexane as organic structure-directing agents

Rapid and Facile Synthesis of Nanosized Silicalite-1 Zeolites from Solid Silica in a Semisolid-like System via a Two-Step Crystallization Method

Rapid and Facile Synthesis of Nanosized Silicalite-1 Zeolites from Solid Silica in a Semisolid-like System via a Two-Step Crystallization Method

Tricoordinated Single-Atom Cobalt in Zeolite Boosting Propane Dehydrogenation.

Propane dehydrogenation (PDH) reaction has emerged as one of the most promising propylene production routes due to its high selectivity for propylene and good economic benefits. However, the commercial PDH processes usually rely on expensive platinum-based and poisonous chromium oxide based catalysts. The exploration of cost-effective and ecofriendly PDH catalysts with excellent catalytic activity, propylene selectivity, and stability is of great significance yet remains challenging. Here, we discovered a new active center, i.e., an unsaturated tricoordinated cobalt unit (≡Si-O)CoO(O-Mo) in a molybdenum-doped silicalite-1 zeolite, which afforded an unprecedentedly high propylene formation rate of 22.6 molC3H6 gCo-1 h-1 and apparent rate coefficient of 130 molC3H6 gCo-1 h-1 bar-1 with >99% of propylene selectivity at 550 °C. Such activity is nearly one magnitude higher than that of previously reported Co-based catalysts in which cobalt atoms are commonly tetracoordinated, and even superior to that of most of Pt-based catalysts under similar operating conditions. Density functional theory calculations combined with the state-of-the-art characterizations unravel the role of the unsaturated tricoordinated Co unit in facilitating the C-H bond-breaking of propane and propylene desorption. The present work opens new opportunities for future large-scale industrial PDH production based on inexpensive non-noble metal catalysts.

Preparation of hierarchical TS-1 zeolites from ordered macroporous-mesoporous carbon templates for efficient chloropropene epoxidation

The combination of titanium silicalite-1 (TS-1) zeolite and H2O2 oxidant provides an excellent alternative to many highly toxic and polluting olefin oxidation processes due to the by-product being water-only and without deep oxidation. However, the diffusion of bulky reactants and products suffers from the inherent micropores of conventional TS-1 zeolites. Herein, we designed a reticulated ordered macroporous carbon material with uniform and coherent mesopores, and successfully synthesized ordered hierarchical TS-1 zeolites containing intracrystalline macroporous-mesoporous-microporous by dry gel method. The fully interconnected intracrystalline macroporous-mesoporous-microporous hierarchical TS-1 improves the accessibility of the active site and the diffusion efficiency of bulky compounds. It showed excellent catalytic performance during the epoxidation reaction of chloropropene, and its epichlorohydrin yield and H2O2 effective utilization were 70.11% and 77.05%, which were 2.2 and 1.9 times of the catalytic performance of commercial TS-1 zeolites, respectively, and significantly better than the ultrafine TS-1 synthesized by conventional methods. The high catalytic performance was maintained after three cycles of use. This offers a good opportunity for the design and synthesis of efficient zeolite catalysts in the future.

Hierarchical porous Pd/HS-1 zeolite as an efficient and reusable catalysts for Suzuki-Miyaura reaction

The Suzuki-Miyaura reaction is one of the most effective routes for the construction of carbon–carbon bonds, due to the obvious advantages of mild reaction conditions, wide range of functional group tolerance, simplicity of operation, and environmental friendliness. However, these reactions highly depend on the development of the active and stable catalysts. Herein, the hollow hexagonal prismatic silicalite-1 (HS-1) zeolite-supported palladium nanoparticles (Pd NPs) were synthesized and applied as a catalyst in Suzuki-Miyaura reaction. The experimental results show that 2.0Pd/HS-1 is an efficient and stable catalyst, which can achieve a high conversion rate (>99.9 %) in a short time (45 min) and can be recycled. Further studies show that 2.0Pd/HS-1 has abundant mesopores, which are favorable for exposing more active sites and promoting the transfer of reactants around the catalysts, thus improving the contact opportunities between the catalysts and the feedstock. Moreover, its proper acid sites as well as metal-zeolite interactions can modulate the electronic state of Pd, and thus improve the conversion of bromobenzene. This work provides new insights into the design, synthesis and applications of highly efficient hollow silicalite-1 zeolite loaded metal catalysts.

An active and stable catalyst of Zn modified Pt nanoparticles encapsulated within silicalite-1 zeolite for dehydrogenation of ethane

The rapid deactivation of catalysts for ethane dehydrogenation (EDH) at considerably higher temperature and the relatively inert feature of ethane makes it urgent to develop highly active and stable catalysts. Herein, a Zn modified Pt nanoparticles confined within silicalite-1 (S-1) zeolite catalyst for EDH reaction was synthesized by hydrothermal synthesis using the PtZn/SiO2 precursor without any protective ligands, which achieved extremely high catalytic performance (e.g., 15.9 % conversion which was close to the thermodynamic equilibrium limitation, 99.5 % ethylene selectivity, and 0.001 h−1 the deactivation rate). Multiple characterizations with XPS, in situ CO-DRIFTS, in situ C2H6-DRIFTS, TG-DTA, and Raman techniques on these catalysts revealed that the electron-rich Pt sites with stable electronic environment formed by Zn promoter introduced, resulting in enhanced anti-reduction and coke-resistance abilities. Furthermore, characterizations with XRD, HRTEM, Py-IR, hydroxyl vibration region of DRIFT spectra and 1H MAS NMR spectroscopic analysis showed that the dilution effect of Zn and the trapping of zinc and platinum species by Si-OH groups in the form of (≡Si-O-Zn)xPt complex could inhibit migration and aggregation, resulting in smaller metal particles and increase anti-sintering ability. The superior catalytic performance as well as the environmental benign preparation route with no-ligand will further contribute to the advantages for future applications.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts.

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Construction of Pd@K-Silicalite-1 via in situ encapsulation and alkali metal modification for catalytic elimination of formaldehyde at room temperature

The Silicalite-1-encapsulated Pd catalyst with alkali metal K modification (Pd@K-Silicalite-1) was prepared via the hydrothermal method with ethylenediamine as ligand and KOH as precursor of K promoter. The introduction of K partially replaced H of silanol in Silicalite-1 and induced the formation of Si-O-K-Pd species, which were beneficial for the improvement of Pd dispersion. The Pd nanoparticles with a size of ca. 1.9 nm were uniformly encapsulated in the matrix of Silicalite-1 zeolite. The catalytic activity of Pd@K-Silicalite-1 was much better than those of Pd/Silicalite-1 and Pd@Silicalite-1 for HCHO oxidation, HCHO conversion could reach 100 % over Pd@K-Silicalite-1 (Pd content = 0.15 wt%) at room temperature under space velocity (SV) of 120,000 mL/(g × h) and relative humidity (RH) of 35 %. Based on the results of various characterization techniques, one can conclude that the improvement in catalytic activity of Pd@K-Silicalite-1 was attributable to high dispersion and small size of Pd NPs and enhanced HCHO adsorption capacities. In addition, we investigated the reaction mechanism of HCHO oxidation over the Pd@K-Silicalite-1 catalyst by in situ DRIFTS spectra. This work provides some guidance on developing high-performance catalysts for HCHO elimination via in situ encapsulation and alkali metal modification.

Investigation on Titanium Silicalite-1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent

Investigation on Titanium Silicalite-1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent

Role of reduction temperature on nanosized silicalite-1 zeolite confined Pd cluster catalysts for oxidation of hydrogen isotopes

Controllable nuclear fusion is a major breakthrough in the future energy field and an important way for mankind to pursue clean and sustainable energy. However, since the fusion reaction raw material tritium is radioactive (β decay, half-life is 12.43 years), the leakage of these hydrogen isotopes under accident conditions will bring the risk of explosion and cause radiation damage to human. Therefore, efficient removal of hydrogen isotopes leaked due to accidents is crucial to the safe operation of nuclear fusion and human health. This study utilizes the metal-support interaction effect to design a series of Pd catalysts for efficient catalytic oxidation of hydrogen isotopes. Using pure silicon zeolite as a carrier, Pd is confined in the channel structure using a ligand-stabilized in situ encapsulation method. The prepared catalyst has nanoscale dispersed metal clusters and has excellent hydrogen isotope catalytic oxidation performance. This study systematically explored the effects of catalyst post-treatment methods, reaction space velocity, and reaction temperature on the hydrogen isotope catalytic oxidation conversion rate. The results show that the catalyst can be fully reduced at 600 °C and can achieve a hydrogen isotope catalytic oxidation conversion rate of greater than 98 % (24,000 mL g–1 h–1) at 100 °C. The conversion rate for hydrogen is higher than 96 % under the same space velocity at 50 °C. After 35,360 min (∼24.5 d) of continuous testing and intermittent operation, the catalyst can also maintain a conversion rate of 99 % (100 °C, 24,000 mL g–1 h–1). A lower reduction temperature will leave some organic matter, which will coordinate and bond on the Pd clusters and thus affect the catalytic activity. Under the same conditions, the hydrogen conversion rate of the catalyst directly reduced at 400 °C was 14 % lower than that of reduction at 600 °C, which proves the criticality of the reduction temperature. This work demonstrates a systematic study of utilizing the confinement effect of molecular sieve pores to obtain Pd cluster catalysts and applying them to the oxidative removal of hydrogen isotopes. It focuses on exploring the impact of the direct reduction temperature of the catalyst on its performance, which may be useful for providing a certain reference for designing relevant catalysts in engineering applications.

The Synergistic Impact of Crystal Seed and Fluoride Ion in the Synthesis of Silicalite-1 Zeolite in Low-Template Systems.

Silicalite-1 zeolites are widely applied in gas adsorption, catalysis, and separation due to their excellent hydrothermal stability and unique pore structure. However, traditional preparation methods have inherent drawbacks such as high pollution, high cost, etc. Therefore, this work proposed a green and efficient route for preparing Silicalite-1 zeolite by adding NH4F (F/Si = 0.1) and seeds (10 wt%) in a much shorter time (8 h) in a low-template system (TPA+/Si = 0.007). It was found that NH4F is beneficial for inhibiting the formation of SiO2. The S-1 seeds could drastically induce the formation of the zeolite skeleton structure. Noteworthy, the morphology of zeolites was determined by the relative content of NH4F and seeds. The crystal morphology is determined by the higher content of the two substances; however, when the content is similar, the crystal morphology is determined by NH4F. The results showed that simultaneous control of NH4F and seeds can suppress SiO2 formation, can improve the relative crystallinity of products, and can be precisely regulated via the synergistic effect of both in zeolite morphology. This work not only provides new ideas for regulating the morphology of silicate-1 crystals but also offers a new path for industrial large-scale production of low-cost and efficient zeolites.

Promoted propane dehydrogenation over Co confined within core–shell silicalite-1 zeolite crystals

Silicalite-1 coating boosted the catalytic activity of Co-loaded silicalite-1 zeolite for propane dehydrogenation reaction.

Highly dispersed Pd-based pseudo-single atoms in zeolites for hydrogen generation and pollutant disposal.

Atomically dispersed metal catalysts with excellent activity and stability are highly desired in heterogeneous catalysis. Herein, we synthesized zeolite-encaged Pd-based pseudo-single atoms via a facile and energy-efficient ligand-protected direct H2 reduction method. Cs-corrected scanning transmission electron microscopy, extended X-ray absorption, and pair distribution function measurements reveal that the metal species are close to atomic-level dispersion and completely confined within the intersectional channels of silicalite-1 (S-1) zeolite with the MFI framework. The Pd@S-1-H exhibits excellent activity and stability in methane combustion reactions with a complete combustion temperature of 390 °C, and no deactivation is observed even after 100 h on stream. The optimized bimetallic 0.8Pd0.2Ni(OH)2@S-1-H catalyst exhibits an excellent H2 generation rate from FA decomposition without any additives, affording a superhigh turnover frequency up to 9308 h-1 at 333 K, which represents the top activity among all of the best heterogeneous catalysts under similar conditions. Significantly, zeolite-encaged metal catalysts are first used for Cr(vi) reduction coupled with formic acid (FA) dehydrogenation and show a superhigh turnover number of 2980 mol(Cr2O72-) mol(Pd)-1 at 323 K, surpassing all of the previously reported catalysts. This work demonstrates that zeolite-encaged pseudo-single atom catalysts are promising in efficient hydrogen storage and pollutant disposal applications.

Tracking carbon dioxide adsorbate intramolecular dynamics in pure silica zeolite Silicalite-1 by in situ Raman scattering.

We report a series of high-quality Raman spectra of carbon dioxide (CO2) adsorbed at room temperature and at various equilibrium pressures, sampling the corresponding adsorption isotherm up to 12bar. The observed splitting in Fermi diad resonance lines, which were additionally split into two well-resolved components, arising from at least two different CO2 species, were compared to the same quantity in high-pressure gas/solid/liquid CO2 phases. Our studies provide material specific spectral data that could be useful in the detection, identification, and dynamical characterization of CO2 deposits, inclusions, or other forms in remote locations and of various origins, e.g. geological, planetary, stellar, and deap-sea sediments.

Encapsulating platinum nanoparticles into multi-hollow silicalite-1 zeolite for catalytic oxidation of volatile organic compounds

Platinum nanoparticles (Pt NPs) embedded in the multi-hollow silicalite-1 zeolite (Pt@MH-S-1) were prepared by the sequential alkali etching and recrystallization of parent microporous silicalite-1 zeolite. Then, toluene and acetone were employed as probe molecules to determine the catalytic behavior of Pt@MH-S-1 for the elimination of volatile organic compounds (VOCs). The results reveal that Pt@MH-S-1 with multi-hollow structure owns large external specific surface area for dispersing the Pt NPs, and abundant mesoporous channels for improving the accessibility of toluene to Pt NPs as well as high reactive oxygen content, excellent toluene adsorption capacity and moderate adsorption strength. Owing to these characteristics, Pt@MH-S-1 exhibits superior performance both in toluene and acetone catalytic oxidations as reflected by 100% conversion acquired at even a low temperature of 137 and 180 °C, respectively. Moreover, Pt@MH-S-1 also shows good stability toward the various conditions, signifying its superiority and potential in practical application for VOCs treatment.

The electron-rich [ZnH]+ species stabilized by titanium silicalite-1 zeolite for propane dehydrogenation

Propane dehydrogenation to propylene has broad application prospects, so it is of great significance to explore low-cost and environmentally friendly catalysts. Metal hydrides, as active sites, exhibit excellent dehydrogenation property. Herein, we report titanium modified [ZnH]+ ([ZnH]+···TiO6) site with high activity (14.49 mmolC3H6·gcat−1·h−1) and regeneration performance, which is derived from the in-situ conversion of [ZnOH]+ in the dehydrogenation reaction. Combined with in-situ spectroscopic characterizations and ab initio molecular dynamics (AIMD) simulations, the unique [ZnH]+···TiO6 moiety was confirmed, and the evolution process from [ZnOH]+ to [ZnH]+ was clarified. XPS and AIMD calculations show that Zn in the [ZnH]+···TiO6 structure is in an electron-rich state with high H-affinity, which can effectively reduce the propyl C-H cleavage barriers based on slow-growth simulations. In general, titanium modification can effectively stabilize the [ZnH]+ species and enrich the electrons density of zinc, which contribute to the enhanced PDH performance.