Microporous Zeolite Research Articles

Influence of Brønsted Acid-Site Density on Reaction-Diffusion Phenomena that Govern Propene Oligomerization Rate and Selectivity in MFI Zeolites

Brønsted acid zeolites catalyze propene oligomerization to higher-molecular-weight alkenes. Previous studies that only consider kinetic factors have reached contradictory conclusions regarding the dependence of oligomerization rates on H+-site density in MFI zeolites. Herein, we account for both the reaction and diffusion phenomena that govern the rates and selectivity of propene oligomerization and interrogate suites of MFI samples synthesized with widely varying H+-site density (0.3–5.7 H+/unit cell; Si/Al 13–250) and crystallite size (0.03–2.65 μm). Heavier alkene products become occluded within zeolitic micropores during reaction and impose intrazeolite diffusional restrictions for all MFI samples over wide ranges of reaction conditions (483–523 K, 7–615 kPa C3H6). This occluded organic phase becomes heavier in composition at higher propene pressures and lower reaction temperatures, which favor the growth of higher-molecular-weight oligomers, resulting in effective diffusivities of propene and product alkenes that systematically decrease with increasing propene pressure. Analysis of product selectivity reveals that MFI samples of lower H+-site density possess higher rate constant ratios for trimerization relative to dimerization, leading to the formation of heavier alkenes within their micropores and, in turn, to a greater decrease in the effective diffusivity of propene and product alkenes with increasing propene pressure. As a result, measured dimerization rates on MFI samples, which convolve the influences of kinetic factors and intrazeolite diffusional constraints imposed by the occluded organic phase, exhibit an increasingly negative-order dependence on propene pressure with decreasing H+-site density. The combined influences of kinetic factors and intrazeolite diffusional constraints result in different dependences of oligomerization rate on H+-site density when measured at different propene pressures, reconciling apparent contradictions among previous reports. Overall, these findings reveal the strong influences of H+-site density in MFI zeolites on both the kinetics of propene oligomerization reactions and the composition of the product-derived intrazeolite organic phase that influences effective diffusivities, and consequently measured propene oligomerization rates and product selectivity.

Estimation of CO2 Separation Performances through CHA-Type Zeolite Membranes Using Molecular Simulation.

Chabazite (CHA)-type zeolite membranes are a potential material for CO2 separations because of their small pore aperture, large pore volume, and low aluminum content. In this study, the permeation and separation properties were evaluated using a molecular simulation technique with a focus on improving the CO2 separation performance. The adsorption isotherms of CO2 and CH4 on CHA-type zeolite with Si/Al = 18.2 were predicted by grand canonical Monte Carlo, and the diffusivities in zeolite micropores were simulated by molecular dynamics. The CO2 separation performance of the CHA-type zeolite membrane was estimated by a Maxwell-Stefan equation, accounting for mass transfer through the support tube. The results indicated that the permeances of CO2 and CH4 were influenced mainly by the porosity of the support, with the CO2 permeance reduced due to preferential adsorption with increasing pressure drop. In contrast, it was important for estimation of the CH4 permeance to predict the amounts of adsorbed CH4. Using molecular simulation and the Maxwell-Stefan equation is shown to be a useful technique for estimating the permeation properties of zeolite membranes, although some problems such as predicting accurate adsorption terms remain.

Effects of aging and hydrothermal treatment on the crystallization of ZSM-5 zeolite synthesis from bentonite.

In the present study, Lam Dong bentonite clay was utilized as a novel resource to effectively synthesize microporous ZSM-5 zeolite (Si/Al ∼ 40). The effects of aging and hydrothermal treatment on the crystallization of ZSM-5 were carefully investigated. Herein, the aging temperatures of RT, 60, and 80 °C at time intervals of 12, 36, and 60 h, followed by high temperature hydrothermal treatment (170 °C) for 3-18 h were studied. Techniques such as XRD, SEM-EDX, FTIR, TGA-DSC, and BET-BJH were applied to characterize the synthesized ZSM-5. Bentonite clay showed great benefits as a natural resource for ZSM-5 synthesis and is cost efficient, environment friendly, and has a large reserves. The form, size, and crystallinity of ZSM-5 were greatly influenced by aging and hydrothermal treatment conditions. The optimal ZSM-5 product had high purity, crystallinity (∼90%), and porosity (BET ∼380 m2 g-1) as well as thermal stability, which are beneficial for adsorptive and catalytic applications.

Nitrogen rejection in natural gas using NaZSM-25 zeolite.

Natural gas reservoirs usually contain considerable amounts of nitrogen (N2). Methane (CH4) as the main component in natural gas must be purified before transferring to the pipeline or storing as liquified natural gas (LNG). Currently, energy-intensive cryogenic distillation is the only industrial approach for N2 rejection in natural gas. The adsorption process based on a N2-selective adsorbent can minimize the separation cost. However, the search for an adsorbent that can selectively reject N2 in natural gas has lasted for decades. Here, we report a microporous zeolite called NaZSM-25 capable of adsorbing N2 over CH4 with an exceptional selectivity of 47 at room temperature that outperforms all previously known N2-selective adsorbents. At 295 K and 100 kPa, the N2 and CH4 uptakes on NaZSM-25 were 0.25 and 0.005 mmol g-1, respectively. CH4 showed negligible external surface adsorption in the whole temperature range of 273-323 K. Theoretical studies through replica exchanged Monte Carlo, molecular dynamics, and ab initio density functional theory (DFT) proved the diffusion limitation of CH4 as a result of 8-membered ring (8MR) pore opening deformation by Na+ cation. The DFT results showed the diffusion energy barriers of 63 and 96 kJ mol-1 for N2 and CH4, respectively, when passing an 8MR occupied with a Na+. NaZSM-25 is a promising adsorbent to be utilized in a pressure swing adsorption process at room temperature to minimize the energy consumption in N2 rejection units.

Clinoptilolite based zeolite-geopolymer hybrid foams: Potential application as low-cost sorbents for heavy metals

Clinoptilolite based zeolite-geopolymer foams (abbreviated as CFs) were prepared from natural clinoptilolite and calcined clinoptilolite, using H2O2 solution as pore former through a straightforward process. Natural clinoptilolite and CFs are characterized by analytical techniques including optical microscope, XRF, FTIR, XRD, BET, MIP and SEM. The obtained CFs possesses micropores of zeolite and meso/macropores of geopolymer matrix. The porosities range from 66.7 to 69.5%. Clinoptilolite (partially dissolved) and impurity minerals (montmorillonite, illite and albite) contribute to the formation of geopolymer. CFs shows a good static sorption performance for toxic heavy metals at pH = 5 and sorption time of 24 h. Results show that the adsorption amount of CFs for Cr3+, Pb2+, Ni2+, Cu2+ and Cd2+ in the 50 mg/L working solutions are 6.21 mg/g, 6.11–6.13 mg/g, 5.92–6.07 mg/g, 5.53–5.93 mg/g and 5.44–5.79 mg/g, respectively. In addition, CFs could reach a high removal rate (Cr removal rate >80% and Cd > 60%) for different heavy metals after three cycles. The elimination order of toxic metals is Cr3+ > Pb2+ > Ni2+ > Cu2+ > Cd2+. The sequence is in accordance with Hard-Soft-Acid-Base principle, it is also related to the speciation and the ionic radii of the hydrated metal ions. This research provides a feasible approach for preparation of promising foams sorbent based on natural zeolite for wastewater management.

Direct dehydrogenation of propane over Co@silicalite-1 zeolite: Steaming-induced restructuring of Co2+ active sites

The chemical state of the Co-relevant active site determines the catalytic performance of the Co-containing zeolite catalysts in non-oxidative propane dehydrogenation (PDH) reaction. Herein, we demonstrate that the moderate steaming pre-treatment is an available strategy to enhance catalytic activity of silicalite-1 zeolite-supported Co catalyst through tuning the Co state. The steaming-treated Co@S-1-steam catalyst exhibits sevenfold increased specific reaction rate (0.44 s−1) than the untreated Co@S-1 catalyst and maintains 95 % propylene selectivity in the PDH reaction at 590 °C. By comprehensive characterizations on the structural properties of the catalysts, multiple roles of steaming treatment are discovered. Steaming treatment can induce the reductive restructuring of the Co catalytic sites in zeolite micropores to form tetrahedral Co2+ species with enhanced dispersion and unsaturation degree, and significantly accelerate the selective activation of propane CH bonds. Furthermore, it also reduces the surface acidity and consequently promotes the desorption of propylene intermediates for enhancing the catalytic reaction rate and anti-coke stability.

The behavior of aluminum sites in H-[Al]-RUB-18 catalysts: A theoretical-experimental investigation

Functional acidic sites can be generated in the structure of layered silicates by the isomorphic substitution of silicon by aluminum atoms, leading to suitable heterogeneous catalysts. H-[Al]-RUB-18 samples were synthesized with Si/Al molar ratios of 15, 30, and 60. Experimental characterizations showed that two different aluminum sites were obtained: one tetrahedrally coordinated (framework-Al), very similar to those found in microporous zeolites; and the other octahedrally coordinated, analogous to silanol groups (hydrated aluminol sites). Computational simulations based on Density Functional Theory (DFT) were used to test the atomic positions of these aluminum sites and confirmed, in terms of electronic energies, the tetrahedral and octahedral positions already predicted in the experimental characterizations. In addition, ethanol dehydration reaction was used to probe the acidic sites and evaluate the performance of the different [Al]-RUB-18 samples. Although a widely known reaction for zeolites and other porous solids, the chemical analysis in this work indicated that the Al-substituted layered silicates are promising candidates to be used in heterogeneous catalysis. The experimental catalytic data showed that the presence of aluminum atoms improved the reaction conversion. Moreover, MEP calculations also presented, in each step of ethanol dehydration, Alframework sites with the lowest electronic energy barriers.

Catalytic methane halogenation by bromine over microporous SAPO-34 zeolite material towards methyl bromide, dibromomethane and hydrogen bromide

Background:Bromination-based methane conversion is one of the promising separate ways of CH4 valorisation. Both methyl bromide/dibromomethane that are formed can be subsequently transformed into more useful compounds. SAPO-34 could also improve the selectivity (S) towards bromomethane during bromination.Methods:To evaluate the performance of SAPO-34 in this reaction process, the influence of time, temperature (T) and weight hourly space velocity (WHSV) rate was investigated in a packed-bed reactor. T was in the range of 310–420 ∘C, while WHSV was 0.22–1.20 h−1. The molar CH4 to bromine ratio was 5.4:1. GC–MS was used for analysis of products. Material was characterized by XRD, SEM, NH3-TPD, N2 physisorption.Significant findings:In the material used for the reaction crystallinity was decreased, solid surface area was reduced, and the density of Brønsted acid sites was also reduced compared to the fresh material. Changes can be attributed to Al2O3 loss or also the breakage of Al–O–P bonds, caused by hydrogen bromide ions that are produced. A presence of coke was observed on the zeolite at T higher than 360 ∘C. CH2Br2 decomposes, while CH3Br is released in intermediate gas phase. Catalytic yield increase compared to homogeneous one was not notable, which indicates on the prevalence of elementary radical mechanism. The highest S for CH3Br was around 90% at 310 ∘C with the WHSV of 1.20 h−1.

NH4F salt inclusion approach for auxiliary mesopores in zeolite Y

This study presents NH4F salt inclusion approach to creating additional mesopores within microporous zeolite Y. Etching with NH4F salts embedded in the supercages perfomrs the localized dissolution within the zeolite crystals to effectively create additional interconnected mesopores without deteriorating crystallinity. Our work provides a novel idea for tailoring pore architecture to design desirable porous materials.

Maximum Impact of Ionic Strength on Acid-Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment.

The intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous-phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol-hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Maximum Impact of Ionic Strength on Acid‐Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment

AbstractThe intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous‐phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol‐hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Ammonium Hexafluorosilicate (AHFS) Modified HMCM‐22 As Efficient Catalysts for Alkylation of Phenol with Methanol

AbstractA series of xAHFS/HMCM‐22 (x denotes the amount of AHFS treatment on HMCM‐22, AHFS is the abbreviation for ammonium hexafluorosilicate) were prepared and evaluated in the catalytic synthesis of p‐cresol by alkylation of phenol with methanol. Several characterization techniques including N2 adsorption‐desorption, XRD, FT‐IR, 27Al MAS NMR, Pyridine‐IR, and NH3‐TPD have been used to analyze the physicochemical properties of the xAHFS/HMCM‐22 catalyst. The conversion of phenol decreased with increasing x of xAHFS/HMCM‐22 catalyst; while the selectivity for p‐cresol maximized at x=0.07. The 0.07AHFS/HMCM‐22 catalyst was identified as the most efficient catalyst, showing the highest p‐cresol selectivity (53.3 %) for longer than 24 h under optimized reaction conditions (325 °C, WHSV=2 h−1), which is much higher than that of parent HMCM‐22 catalyst (37.1 %). The high p‐cresol selectivity of the catalyst is attributed to AHFS removing part of the acid sites on the external surface of the HMCM‐22, and preserving the acid sites in the micropores of zeolite. By contrast, the decrease in catalyst activity is due to the decrease in total acid sites.

Possibilities of Microporous and Hierarchical MFI Zeolites in the Synthesis of Nitrogen Heterocyclic Compounds

Possibilities of Microporous and Hierarchical MFI Zeolites in the Synthesis of Nitrogen Heterocyclic Compounds

Novel and emerging concepts related to cationic species in zeolites: Characterization, chemistry and catalysis

This review surveys our efforts to form and characterize cationic species located in the micropores of various zeolites, both for metal and non-metal cations, with specific emphasis on their novel, emerging properties, and reactivity. Herein, it is described how we realized synthesis of such species with a focus on both cation identity as well as zeolite architecture, and synthesis methods. Well-defined cationic species could be produced by careful choice of metal incorporation methods, Al distribution tuning and synthesis parameters. Previously unknown phenomena in zeolite and cation-zeolite chemistry were discovered by employing these well-defined systems and will be covered in this review article. Methodology to dramatically improve stability of cations in zeolite for adsorption and catalytic applications, and usefulness of these materials in elucidating hitherto unknown catalytic intermediates and mechanisms will be discussed.

Constructing core-shell structured Au/Snβ@mesosilica composite for one-pot base-free conversion of glycerol to methyl lactate

Herein, core-shell structured Snβ@mesosilica (MS) composite was constructed by oriented assembly of mesoporous silica on microporous Snβ zeolite. The mesoporous channels in the shell were verified vertically aligned to the Snβ core, rendering sufficient mass transfer between the shell and core. The resultant Snβ@MS was thereafter employed as the scaffolds for anchoring Au nanoparticles to develop a bifunctional Au/Snβ@MS catalyst for one-pot base-free conversion of glycerol to methyl lactate (ML), which is regarded as a promising and sustainable route to ML production. Under optimum reaction conditions, a remarkable activity with 98.2% glycerol conversion and 91.9% ML selectivity was achieved. More importantly, the introduction of mesoporous silica shell can efficiently modify the Lewis/Brønsted acid ratio of Snβ@MS, thus inhibit the side reactions. While the confinement effects could protect the highly dispersed Au nanoparticles against sintering. Accordingly, a high stability and recyclability was achieved over the Au/Snβ@MS catalyst.

Separation of oil vapour by polyether block amide composite membrane modified with porous materials.

The ability of membranes to separate oil vapour is affected by their permeance and selectivity. This study modifies polyether block amide (PEBA) composite membranes with a microporous zeolite, Silicalite-1, or a mesoporous zeolite, MCM-41. The results show that when PEBA composite membranes are modified with these zeolites, the selective layer of the composite membrane is coated more thinly, resulting in a higher flux of organic gas. Silicalite-1 increases the hydrophobicity of the membrane, which facilitates the adsorption of organic vapour on the membrane surface, thus improving the membrane selectivity. In the separation of oil vapour, both modified membranes can effectively increase the gas permeabilities and selectivities. The main mechanism governing gas transport in the MCM-41-modified membrane is Knudsen diffusion, so the selectivity for small molecules is improved more significantly. By contrast, the dissolution-diffusion mechanism is dominant in the Silicalite-1-modified membranes, which considerably increases the selectivity for large molecules.

Hydrocarbon Products Occluded within Zeolite Micropores Impose Transport Barriers that Regulate Brønsted Acid-Catalyzed Propene Oligomerization.

Brønsted acid zeolites catalyze alkene oligomerization to heavier hydrocarbon products of varied size and branching. Propene dimerization rates decrease monotonically with increasing crystallite size for MFI zeolites synthesized with fixed H+-site density, revealing the strong influence of intrazeolite transport limitations on measured rates, which has gone unrecognized in previous studies. Transient changes in dimerization rates upon step-changes in reactant pressure (150-470 kPa C3H6) or temperature (483-523 K) reveal that intrazeolite diffusion limitations become more severe under reaction conditions that favor the formation of heavier products. Together with effectiveness factor formalisms, these data reveal that product and reactant diffusion, and consequently oligomerization rates and selectivity, are governed by the composition of hydrocarbon products that accumulate within zeolitic micropores during alkene oligomerization. This occluded organic phase strongly influences rates and selectivities of alkene oligomerization on medium-pore zeolites (MFI, MEL, TON). Recognizing the coupled influences of kinetic factors and intrazeolite transport limitations imposed by occluded reaction products provides opportunities to competently tailor rates and selectivity in alkene oligomerization and other molecular chain-growth reactions through judicious selection of zeolite topology and reaction conditions.

Boosting molecular diffusion following the generalized Murray's Law by constructing hierarchical zeolites for maximized catalytic activity.

Diffusion is an extremely critical step in zeolite catalysis that determines the catalytic performance, in particular for the conversion of bulky molecules. Introducing interconnected mesopores and macropores into a single microporous zeolite with the rationalized pore size at each level is an effective strategy to suppress the diffusion limitations, but remains highly challenging due to the lack of rational design principles. Herein, we demonstrate the first example of boosting molecular diffusion by constructing hierarchical Murray zeolites with a highly ordered and fully interconnected macro-meso-microporous structure on the basis of the generalized Murray's Law. Such a hierarchical Murray zeolite with a refined quantitative relationship between the pore size at each length scale exhibited 9 and 5 times higher effective diffusion rates, leading to 2.5 and 1.5 times higher catalytic performance in the bulky 1,3,5-triisopropylbenzene cracking reaction than those of microporous ZSM-5 and ZSM-5 nanocrystals, respectively. The concept of hierarchical Murray zeolites with optimized structural features and their design principles could be applied to other catalytic reactions for maximized performance.

Recent progress in catalytic deoxygenation of biomass pyrolysis oil using microporous zeolites for green fuels production

Biomass pyrolysis is one of the cleaner ways to produce bioenergy focusing on bio-oil. The high oxygen content of oxygen in bio-oil limits its application in transportation applications. The deoxygenation of bio-oil using various catalyst systems is required to upgrade the bio-oil. Herein, we presented the scientometric analysis of microporous zeolites for deoxygenation of biomass-derived bio-oil. The state of the art review of biomass catalytic deoxygenation using zeolite-based materials is elucidated. A special focus on the role of catalyst physicochemical properties and degree of deoxygenation is Furthermore, the reaction pathways for different zeolites for bio-oil upgradation are presented. Finally, the technology readiness level is assessed and future recommendations are also presented.

Zeolite/polymer core-shell hybrid nanoparticles with hierarchical micro/meso-pores

Here, we report a new approach for preparing zeolite-polymer core-shell-like materials with hierarchical porosity via photopolymerization of trifunctional acrylic monomer (TMPTA) under UV–visible irradiation using a bifunctional Silane-based Photoinitiator (SPI-1) as coupling agent. The free radicals generated on the surface of zeolites initiate the polymerization of the diluted monomers, creating an external and mesoporous polymer shell layer. The efficiency of the photopolymerization process and the textural properties of synthesized materials are investigated using different techniques, including PXRD, TGA, UV–Vis, FTIR, SEM, and TEM. The accessibility to the pores of the zeolite core is investigated by N2 physisorption at 77 K and other probe molecules using IR spectroscopy. Interestingly, the results demonstrate a good covering of the zeolite surfaces by polymers shell. The new approach is highly repetitive and reproducible, and the accessibility to the zeolite's micropores is preserved at around 90%, which is not reported previously. The hybrid materials show higher hydrophobicity and higher ethanol adsorption capacities compared to the parent material and are potential candidates for ethanol dehydration, demonstrating a new and easy way for preparing zeolite/polymer hybrid materials for various applications such as ethanol dehydration.