Modelling Diffusion in Zeolites by Molecular Dynamics Simulations

Abstract

The diffusion of molecules sorbed in zeolites is of growing interest for understanding the mechanisms of chemical processes with regard to selectivity and reactivity [1].

MD simulations give insight into physical systems on the molecular level allowing to study and visualize the motion of molecules even beyond the possibilities of experiments [2,3]. Single system parameters can easily be varied to study their influence, also those parameters that are fixed in reality (e.g., the size of particles). We present a cross section of our recent work to illustrate the capabilities of MD: The self diffusion coefficients (D) of a mixture of methane and xenon in silicalite show remarkable deviations from those of the pure species. This is shown and confirmed by PFG NMR experiments [4].

Simulating ethane in zeolite A the mechanism of diffusion has been studied. The effects of rotation on the diffusion lead to cases where D decreases with growing temperature [5].

The independence of self diffusion on lattice vibrations is proven even for zeolites with windows of guest particle size comparing simulations with rigid and vibrating zeolite lattice [6].     

Treatment of Nippostrongylus brasiliensis in normal and SPF rats using tetramisole loaded into zeolite.

Administration to rats of tetramisole loaded into zeolite was more successful in killing adults of Nippostrongylus brasiliensis than the administration of tetramisole alone. The most successful treatment occurred in SPF (Specific Pathogen Free) rats dosed with tetramisole loaded into zeolite and no worms were present in this group at autopsy eight days post-infection. It is concluded that the slow release of drug from the zeolite matrix improved its efficacy, especially in removing worms from low-grade infections.     

Bactericidal activity of Ag-zeolite mediated by reactive oxygen species under aerated conditions

The bactericidal activity induced by the introduction of silver ions into zeolite was studied. Escherichia coli was used as the test microorganism. Silver ions were loaded into zeolite by the ion-exchange method. Silver-loaded zeolite was demonstrated the strong bactericidal activity. Dissolved oxygen was an essential factor for the occurrence of the bactericidal activity because the activity was observed only under aerated condition. Superoxide anions, hydrogen peroxide, hydroxyl radicals and singlet oxygen were formed. Scavengers of these each reactive oxygen species (ROS) inhibited the bactericidal activity. This means that all ROS contributed to the activity.     

Evaluation of kinetic and mass transfer parameters for adsorption of clavulanic acid into natural and synthetic zeolites

This work investigates the adsorption of clavulanic acid using natural six cationic forms (Na⁺, Ca⁺², Ba⁺², Sr²⁺, K⁺, and Mg²⁺) of the X and NZ zeolites in a stirred tank reactor since the separation is an important step of the biomolecule production. A mathematical model was proposed taking into account the transport of CA molecules from the liquid phase to the surface of the adsorbent and after diffusion into the particles. The estimated kinetic and mass transfer parameters were used to evaluate adsorption rates and mass transfer resistances involved in the separation of clavulanic acid from the broth. It has been shown that mass-transfer phenomena were a limiting step in the clavulanic acid adsorption process and that the adsorption rate should be considered to evaluate the system. Amongst the materials, the synthetic zeolite NaX was selected as the most appropriate material to separate clavulanic acid because this material presented the highest values for the observed reaction rate, compensating for the external mass transfer resistance. Modeling and simulation of clavulanic acid purification using the zeolite NaX showed a satisfactory fitting of experimental data. The model was used to simulate the process and it was evaluated for its technical and economical viability by comparisons considering the influence of the solid:liquid ratio on the adsorption equilibrium time and on the hydrolysed mass of biomolecule.     

The reactivity of tetracarbonylnickel encapsulated in zeolite X. A case history of intrazeolite coordination chemistry.

Zeolite X shows a high capacity for tetracarbonylnickel (up to 28 weight percent) such that complete pore filling with ‘liquid like’ material takes place. The adsorbed material may be removed simply by evacuation at room temperature. Partial decomposition of the Ni(CO)₄ occurs on standing at room temperature under N₂. The resultant orange species is highly reactive and has spectroscopic properties consistent with a coordinatively unsaturated ‘Ni(CO)₃’. Complete and irreversible decomposition by heating to 200 °C in vacuo gives a black zeolite, with an undefined metal phase, which is unreactive towards carbon monoxide. Reaction of the zeolite supported Ni(CO)₄ with various phosphorus ligands is highly dependent on the original loading level as well as the physical size of the ligands involved. At low loadings two kinds of reactivity are observed: 1) With ligands too large to gain access to the zeolite crystal interior, reaction occurs only in solution and so drags the Ni(CO)₄ from the zeolite: 2) With smaller ligands, reaction takes place inside the zeolite cages leading to well-defined, encapsulated, ship-in-bottle complexes which have a stoichiometry dictated by the available space in the cages. At high loading levels, pore blocking phenomena lead to inhomogeneous distributions of encapsulated complexes wherein a complete shell of phosphorous ligand substituted nickel carbonyl species forms at the crystal surface layers and prevents further reaction deeper inside the crystal. The reactivity with large phosphines has been used to study the diffusion of Ni(CO)₄ from the zeolite. Monitoring the appearance of the Ni(CO)₃L (where L = phosphine) by 31-P NMR of the supernatant solution shows that Ni(CO)₄ leaves the zeolite with a first order rate constant of at least 2 × 10^?2 sec^?1 at 298 K.     

Antimicrobial and mechanical properties of acrylic resins with incorporated silver-zinc zeolite - part I

Objective: The purpose of this in vitro study was to evaluate the antimicrobial activity of acrylic resins containing different percentages of silver and zinc zeolite, and to assess whether the addition of zeolite alters the flexural and impact strength of the resins. Background: The characteristics of acrylic resins support microorganism development that can threaten the health of the dentures user. Material and methods: A microwave‐polymerised (Onda‐Cryl) and two heat‐polymerised (QC20 and Lucitone 550) acrylic resins were used. The materials were handled according to the manufacturers’ instructions. Fifty rectangular‐shaped specimens (8 × 10 × 4mm) were fabricated from each resin and assigned to 5 groups (n = 10) according to their percentage of Irgaguard B5000 silver–zinc zeolite (0%– control, 2.5%, 5.0%, 7.5% and 10%). Flexural strength and Izod impact strength were evaluated. The antimicrobial activity against two strains of Candida albicans and two strains of Streptococcus mutans was assessed by agar diffusion method. Data were analysed statistically by one‐way anova and Tukey’s test at 5% significance level. Results: The addition of 2.5% of Irgaguard B5000 to the materials resulted in antimicrobial activity against all strains. Flexural strength decreased significantly with the addition of 2.5% (QC20 and Lucitone 550) and 5.0% (Onda‐Cryl) of Irgaguard B5000. The impact strength decreased significantly with the addition of 2.5% (Lucitone 550) and 5.0% (QC20 and Onda‐Cryl) of zeolite. Conclusion: The addition of silver–zinc zeolite to acrylic resins yields antimicrobial activity, but may affect negatively the mechanical properties, depending on the percentage of zeolite.     

An investigation of plant growth in an organo-zeolitic substrate and its ecological significance

This work concerns a series of experiments designed to test and understand the effect of ammoniated zeolite on plant growth. The affinity of the zeolite mineral clinoptilolite for NH₄ ⁺ is utilised in organo-zeolitic substrates to enhance plant growth. By comparing plants grown in substrates with and without ammoniated zeolite, an increase in plant dry weight of some 19% was shown to be due to the presence of the zeolitic NH₄ ⁺–N. In this study, experimental work has shown that in an organically enriched substrate an exponential diffusion of NH₄ ⁺ occurs as a non-equilibrium reaction. It is suggested that ion-exchange is taking place in which soil Ca⁺² is exchanged for lattice bound NH₄ ⁺. Nitrifying bacteria, utilising the diffusing NH₄ ⁺, appear to protect seedlings from the effect of ammonium toxicity and in so doing act as a biological buffer, allowing the plant to take up nitrogen at a rate which is most advantageous throughout its growth. Leaching experiments confirm the presence of very high soil nitrate concentrations indicating that a large population of nitrifying bacteria is established. The ionic mobility of major cations is also greatly increased in the organo-zeolitic substrates. This behaviour is already known to produce beneficial effects in the rhizosphere increasing aeration by flocculating colloidal clay particles and enabling the diffusion of metal ions to occur. Research reported elsewhere demonstrates that plants grown in organo-zeolitic substrates on toxic waste sites exhibit low up-take of toxic metals and it would, therefore, appear that the unique features of organo-zeolitic substrates have both nutritional and ecological value.     

Influence of force fields on the selective diffusion of para-xylene over ortho-xylene in 10-ring zeolites

A molecular dynamics study of diffusion of p-xylene and o-xylene has been performed over three different pure silica 10-ring zeolites, MFI, SFG and TUN. The shape selective properties of the frameworks of these three materials have been tested using four different types of force fields commonly used based on united atom, rigid-ion and core-shell approximations. The performance of each force field is analysed in order to find which force fields can give sufficiently accurate estimations that allow to select appropriate zeolites for selective separation of para/ortho xylene. This performance was found to depend on the quality of the structural properties of the zeolite, in particular the size and shape of the 10 rings which act as bottlenecks for the diffusion. The computational results allow us to define some optimum characteristics for the selective diffusion of p-xylene.     

A comparative study of the adsorption of water and methanol in zeolite BEA: a molecular simulation study

Grand canonical Monte Carlo simulations were carried out to study the equilibrium adsorption concentration of methanol and water in all-silica BEA zeolite and HBEA zeolites with different Si/Al ratios over a wide range of temperatures and loadings. These zeolites have oval-shaped channels with one side longer than the other. Water sorption into the hydrophobic BEA zeolite had a sharp transition with its sorption going from zero to near full capacity over a very small pressure range. Methanol sorption was much more gradual with respect to pressure. With the addition of hydrophilic sites for the HBEA zeolites by decreasing the Si/Al ratio, adsorption at lower pressures increased significantly for water and methanol. At higher loadings, water and methanol adsorption were found to behave in fundamentally different ways. Water structures in the zeolite channels formed hydrogen-bonded chains while maximising contact with the surfaces on the longer edges of the zeolite channels. Methanol molecules, in contrast, formed very few hydrogen bonds between themselves, with their hydroxyl groups primarily binding with surface of the shorter edge of the zeolite channels and their methyl groups located near the middle of the zeolite channels. The addition of hydrophilic groups in the HBEA zeolites strongly influenced positions of the methanol hydroxyl groups at high loadings, but did not have a significant effect on water structure.     

Ultra-stable zeolites--a tool for in-cell chemistry

Ultrastable zeolite particles were used as vehicles to carry low molecular bio-active substances and macromolecules as proteins into viable cells. Zeolite particles that can be used for internalisation by phagocytosis were obtained from the non-sedimenting fraction of a commercially available zeolite preparation after 1 x g sedimentation. Protein adsorbed on the zeolite surface was shown to enter the endosomal pathway after phagocytosis and could be cleaved by the endosomal proteases. As a model of a low molecular weight bio-active molecule, the inhibitor of the cellular synthesis of nitrogen oxide, N-nitro-L-arginine methyl ester (L-NAME), was used. A partial inhibition of the cellular NO production was shown after utilizing zeolites as vehicles to introduce the inhibitor into the cells. A targeting of the intra-cellular enzymes that was at least 10 times more efficient was obtained by the use of zeolites as a carrier of the inhibitor, as opposed to addition of the inhibitor to the culture medium.     

Molecular modeling of chiral-modified zeolite HY employed in enantioselective separation

Insight into enantioselective separation utilizing chiral-modified zeolite HY could be useful in designing a chiral stationary phase for resolving pharmaceutical compounds. A model was employed to better understand the enantioseparation of valinol in zeolite HY that contains (+)-(1R;2R)-hydrobenzoin as a chiral modifier. This model incorporates the zeolite support and accounts for the flexible change. Results from grand canonical Monte Carlo and molecular dynamics simulations indicate that the associated diastereomeric complex consists of a single (+)-(1R;2R)-hydrobenzoin and a single valinol molecules located in the zeolite HY supercage. Supercage-based docking simulation predicted an enantioselectivity of 2.6 compared with that of 1.4 measured experimentally. Also, the supercage-based docking simulation demonstrated a single binding motif in the S complex, and two binding motifs in the R complex. The multiple binding modes in the R complex resulted in its lower stability. This is hypothesized to be the origin of the weaker binding between (-)-(R)-valinol and the chiral modifier, and explains why (+)-(R)-valinol is retained more in the chiral-modified zeolite system studied.     

Assessment of a UASB reactor with high ammonia concentrations: Effect of zeolite addition on process performance

The UASB process for wastewater treatment has been extensively studied, but the use of zeolite to improve UASB reactor performance has rarely been explored. In this study, a UASB reactor modified with natural zeolite operating at high nitrogen concentrations (0.5, 0.7 and 1 g/L) was evaluated. Two laboratory bioreactors, one with zeolite and one without, were operated at ambient temperatures ranging between 18 °C and 21 °C. The experimental phase had a start-up period of 21 days. In the reactor with zeolite, the pH was found to be between 7.9 and 9.1, with a COD removal efficiency of about 60% after 80 days of operation at ammonia concentrations of between 0.229 and 0.429 g/L in the effluent. In the reactor without zeolite, the pH was between 8.3 and 9.3, and the COD removal efficiency was about 40% at ammonia concentrations between 0.244 and 0.535 g/L in the effluent. The addition of zeolite also decreased the volatile suspended solids (VSS) concentration in the effluent, generating a biomass with larger granules and higher settling rates as compared to a UASB reactor without zeolite. Taking the lower ammonia concentration, the higher COD removal and the improved granulation into account, it can be concluded that natural zeolite positively influenced the behavior and performance of the UASB reactor operating with high nitrogen concentrations.     

Evaluation of Pathological Effects in Broilers During Fumonisins and Clays Exposure

This study was conducted to evaluate the possible protector effect of bentonite and zeolite in Bovans chicks fed a diet containing 59 mg kg(-1) of fumonisin B1 (FB1) during 3 weeks. A total of 200 one-day-old male chicks were treated varying the amount of bentonite and zeolite. Chick weight was registered weekly. At the end of the experiment, all the chicks were killed, and the livers were analyzed for gross examination and histopathological changes. Plasmatic activity of alanine amino transferase and aspartate amino transferase (AST) were also determined. Sphinganine and the sphinganine-to-sphingosine ratio in serum were evaluated. Both, bentonite and zeolite showed a protector effect against FB1 adsorption in the digestive tract of chicks. Chicks fed with FB1-contaminated feed, amended either with zeolite or bentonite, were heavier, and no macroscopic lesions were observed in the livers. AST activity might be considered as an indicator for FB1 exposition because AST levels were affected when only FB1 was present in the basal diet. These results indicate that both, zeolite and bentonite can be added into feed to diminish the effects of FB1.     

Fabrication of silver ion exchanged zeolite using scoria and its antibacterial activity

In this study, we examined the antibacterial activity of silver ion exchanged zeolite synthesized from Cheju Scoria. We synthesized zeolite in various NaOH concentrations, but zeolite synthesized in 4 M NaOH was most similar to type A zeolite. Using the synthesized zeolite, we prepared a silver ion exchanged zeolite for studies of antibacterial activity. Antibacterial tests using agar cultures of Escherichia coli (E. coli), with silver ion exchanged zeolite showed a zone of inhibition colonies bacteria did not grow near silver ion exchanged zeolite. Furthermore, spectrophotometry demonstrated a significantly low absorbance of E. coli culture mediums when silver ion exchanged zeolite was included indicating that E. coli propagation was prevented. Through results of these experiments, we conclude that synthesized products with sodalite crystal can be synthesized from Scoria, and these are suitable to produce silver ion exchanged zeolite with antibacterial activity.     

Zeolite catalyzed selective deprotection of di- and tri-O-isopropylidene sugar acetals

H-Beta zeolite, a microporous solid acid, is demonstrated to be an efficient catalyst for the selective deprotection of cyclic as well as acyclic O-isopropylidene sugar acetals derived from d-glucose, D-xylose, D-mannose, and D-mannitol in aqueous MeOH at room temperature. A notable observation is the conversion of d-mannitol triacetonide into 1,2:3,4-di-O-isopropylidene-D-mannitol (48%) and 3,4-O-isopropylidene-D-mannitol (36%) brought about in 6h by H-beta zeolite and the non-occurrence of any hydrolysis in the case of H-ZSM-5 catalyzed reaction in 24h under the same conditions.     

Optical spectroscopy of inorganic-organic host-guest nanocrystals organized as oriented monolayers

Oriented and densely packed zeolite L monolayers were prepared on a glass support. The one-dimensional channels of zeolite L, being all oriented perpendicular to the glass and parallel to each other, were sequentially filled by ion exchange with two strongly fluorescent dye molecules. First N-methylacridine (MeAcr⁺) was inserted followed by 3,3′-diethylthiacarbocyanine (DTC⁺). The shorter MeAcr⁺ is oriented perpendicular to the channel axis while the longer DTC⁺ is parallel, due to the constraints imposed by the geometry of the zeolite L channels, as deduced from fluorescence anisotropy of single MeAcr⁺-zeolite L and DTC⁺-zeolite L crystals. The dye molecules can enter the channels only from the top side of the monolayer, since the entrances on the bottom are blocked by the glass support. The resulting ordering has been observed by fluorescence microscopy of single DTC⁺, MeAcr⁺-zeolite L crystals. Conditions were found to suppress the pronounced Rayleigh scattering of zeolite monolayers. Thus high quality absorption spectra of DTC⁺, MeAcr⁺-zeolite L monolayers on glass could be measured at different angles between the incident light and the layer. The results deliver a direct proof that microscopic ordering of the dyes in the channels of zeolite L as well as macroscopic organization of the dye-zeolite L monolayer on the glass support was achieved. Thus a high level of organization was obtained by controlled assembly of the zeolite L crystals into oriented structures followed by subsequent insertion of strongly luminescent dyes.     

Diffusivity of CH4 In Model Silica Nanopores: Molecular Dynamics and Quasichemical Mean Field Theory

Equilibrium molecular dynamics and dual control volume grand canonical molecular dynamics experiments were carried out aiming at the investigation of the dependence of transport diffusivity upon the adsorbent pore size and sorbate concentration of CH₄ in cylindrical silica nanopores at 298?K, calibrated with respect to experimental data of zeolite VPI 5; the results of simulation were elaborated on the basis of the quasichemical mean field approximation via a theoretical model for surface diffusion. Our mapping procedure between simulation and quasichemical theory reveals that sorbate–sorbate energetics emerge as the physical reason for the variation of corrected (Darken) and hence transport diffusivity with respect to pore size and sorbed phase fractional occupancy.     

改性沸石粉吸附去除微污染水中Ni(Ⅱ)的试验研究

用壳聚糖改性天然沸石粉,制备了改性沸石粉吸附材料,对微污染水源水中微量镍(Ⅱ)的吸附去除进行了研究。考察了吸附时间、沸石粉投加量、镍(Ⅱ)初始浓度以及原水pH 值等因素对镍(Ⅱ)去除效果的影响。结果表明,改性沸石粉对镍(Ⅱ)的去除效果明显优于天然沸石粉;当镍(Ⅱ)初始浓度低于0.2 mg/L 时, 投加1.5 g/L 壳聚糖改性沸石粉,经20 min 吸附后,水中镍(Ⅱ)的剩余浓度低于 0.02 mg/L,可满足《生活饮用水卫生标准》(GB5749-2006)中的要求;提高水的pH 值有利于两种沸石粉对镍(Ⅱ)的去除。另外,两种沸石粉对镍(Ⅱ)的吸附过程均符合Freudlich 等温吸附模型,且改性沸石粉的吸附指数更小,其吸附能力更强。     

Ammonium exchange and bioregeneration of bio-flocculated zeolite in a sequencing batch reactor

Natural zeolite was added to the sequencing batch reactors to assess its role in ammonium exchange. Even though the biofilm was fully developed on the zeolite surface, ammonium removal and the biological regeneration of the zeolite occurred constantly during the anoxic-fill phase and the reaction phase, respectively. However, the specific nitrification rate of the bio-flocculated zeolite was lower than that observed in the control on account of the limited ammonium release to the liquid phase.