Zeolite Walls Research Articles

Organosilane Surfactant-Directed Synthesis of Mesoporous Zeolites

nnIntroducing substantial mesoporosity in zeolites has recently attracted more interests in both fundamental and applied research, thanks to the integration of the advantages of mesoporous materials and zeolite crystals. This article highlights the recent developments of synthesis, characterization and application of highly mesoporous zeolites by using organosilane surfactants as mesoporogens, which have several attractive features such as large surface area, interconnected mesopores with zeolitic pore walls, pore size control, facile synthesis on a large scale and so on. This review tends to focus more on the application of the hierarchical zeolites for heterogeneous catalysis in acid- and base-catalyzed reactions, redox reactions, coupling reactions and synthesis of fine chemicals, especially when bulky molecules are involved. The superior catalytic performance of the novel hierarchical materials in comparison with pure micro- and mesoporous counterparts, as well as mesoporous zeolites synthesized by some other synthetic approaches, will be discussed in detail.

Dynamic Quantum Molecular Sieving Separation of D2 from H2–D2 Mixture with Nanoporous Materials

Quantum molecular sieving separability of D(2) from an H(2)-D(2) mixture was measured at 77 K for activated carbon fiber, carbon molecular sieve, zeolite and single wall carbon nanotube using a flow method. The amount of adsorbed D(2) was evidently larger than H(2) for all samples. The maximum adsorption ratio difference between D(2) and H(2) was 40% for zeolite (MS13X), yielding a selectivity for D(2) with respect to H(2) of 3.05.

A facile method for the fabrication of IM-5 hollow zeolite sphere in emulsion system

IM-5 hollow zeolite spheres (HZSs) were successfully prepared by CTAB improved emulsion system, with the aid of 1,5-bis(methylpyrrolidinium)pentane (1,5-MPP) as a structure-directing agent. The obtained IM-5 HZSs displayed a uniform size about 5μm with polycrystalline IM-5 zeolite wall of 1μm thickness. It was verified that crystallization time and the concentration of CTAB affected the formation process of IM-5 HZSs. Moreover, the IM-5 HZSs were obtained under the conditions of a CTAB concentration of 1.39mol/L and crystallization time of 10–12days. The characterization methods of XRD, Raman, SEM, TEM, N2 sorption isotherms, 27Al, 29Si MAS NMR and Dynamic light-scattering (DLS) were employed to analyze as-prepared samples. The obtained results clarified the “formation of hydrophobic gel particles and crystallization on its surface” crystallization process. This study may provide a facile method to fabricate HZSs.

Investigating the Role of Zeolite Nanocrystal Seeds in the Synthesis of Mesoporous Catalysts with Zeolite Wall Structure

Mesoporous titania-silica catalysts were prepared by hydrothermal conversion from synthesis solutions containing a zeolite structure directing agent (TPA+) and a porogen (CTA+). Syntheses were carried out with and without addition of zeolite seeds (i.e., TS-1 and Sil-1 nanocrystals). Catalysts prepared in the presence of zeolite nanocrystal seeds have greater proportions of tetrahedrally coordinated titanium, which correlate well with the detection of zeolite SBU fragments and ring-structure by TOF-SIMS and FTIR. The incorporation of titano-silicates and zeolite structural units in the mesopore walls could explain the better activity and stability of these catalysts (Ti-MT1 and Ti-MS1) compared to the catalyst prepared by the conventional method (Ti-MCM-41). The similarity in the reactivity of catalysts prepared from TS-1 and Sil-1 nanocrystal seeds dispels the idea that the seeds are merely a source of the zeolite fragments incorporated in the pore walls but instead suggests that the seeds play an active...

New Insights in the Formation of Combined Zeolitic/Mesoporous Materials by using a One‐Pot Templating Synthesis

AbstractZeolitic growth is often absent or occurs in separate phases when synthetic strategies based on the combination of zeolite templates and mesopore templating agents are applied. In this work, zeolitic growth and mesopore formation have been investigated at different temperatures by applying a one‐pot templating approach, based on a TS‐1 zeolite synthesis whereby part of the microtemplate (tetrapropylammonium hydroxide, TPAOH) is replaced by a mesotemplate (hexadecyltrimethylammonium bromide, CTMABr). Moreover, the synthesis duration and the molar ratio of the microtemplate/mesotemplate have also been studied. The different syntheses clearly show the inherent competitive mechanism between zeolitic growth and mesopore formation. These insights have led to the conclusion that by following a one‐pot templating strategy with standard, nonexotic commercial templates, i.e. CTMABr and TPAOH, it is not possible to develop a “true” hierarchical mesoporous zeolite, meaning a mesoporous siliceous material with highly crystalline zeolitic walls. The resultant materials are instead combined zeolitic/mesoporous composite structures with, however, highly tuneable and controllable porosity characteristics.

A kinetic model of standard selective catalytic reduction of NO by NH3 on single sites

Reduction of NO by NH3 in the presence of O2 may occur on single Fe or Cu atoms or dimers incorporated into the inner walls of zeolite. One of the likely schemes of this reaction implies the formation of N2 and H2O through the reaction of gas-phase NO and adsorbed NH3. The steady-state kinetics corresponding to this scheme was recently analyzed by the author assuming that the reaction runs on single metal atoms. In this work, the author presents a model including two metal atoms. Under the practically important conditions, the kinetics predicted by the one- and two-site models are demonstrated to be similar. In particular, both models allow one to interpret the apparent reaction orders observed experimentally.

Confined PFSA–zeolite composite membrane for self-humidifying fuel cell

Perfluorosulfonic acid (PFSA) polyelectrolyte confined in subnanoliter volume within zeolite cladded walls exhibits higher glass transition temperature and excellent tolerance to high-temperature fuel cell operation under dry conditions, generating an order of magnitude higher power density than standard PEMFC.

Influence of chemical vapour infiltration conditions of acetylene on the structural and textural properties of EMT-type zeolite nanocasted carbon replica

Ordered microporous carbons were synthesized by the nanocasting process using EMC-2 zeolite (EMT structure type) and acetylene as a mould and a carbon precursor, respectively. In this study, the conditions of the synthesis methods for preparing the ordered microporous carbons were examined. Temperature and duration parameters for the chemical vapour infiltration were optimized to yield an ordered carbon replica that displays up the three XRD diffraction peaks. This faithful replica exhibits also a high micropore volume (1.3–1.4 cm3/g) with mainly supermicroporosity, a high specific surface area (>2900 m2/g) and nearly no mesoporosity. The pore size distribution calculated with NLDFT method from nitrogen physisorption data shows three maxima at 0.6, 1.0 and 1.8 nm diameters. The second is due to the zeolite wall dissolution. The first and the third are attributed to different types of default. Compared to the classical two-step procedure, the direct infiltration with acetylene appears an interesting route for the preparation of ordered microporous carbon replicas with high micropore volume.

KINETICS OF CATALYTIC REACTIONS ON A UNIFORM SURFACE AND SINGLE SITES: SIMILARITY AND DIFFERENCE

To illustrate typical similarities and differences in the reaction kinetics occurring on a uniform surface and single sites (e.g. on single metal atoms incorporated into the inner walls of zeolite), we analyze a generic reaction scheme of N2O decomposition running in the presence of oxygen. In both cases, there are three reaction regimes, (i) controlled exclusively by the N2O -decomposition step, (ii) partly inhibited by oxygen, or (iii) limited by oxygen desorption. Regime (i) is kinetically similar (first order in N2O ) in the two cases. The states of the catalytic sites are however different. Regime (ii) exhibits different reaction orders in oxygen (-0.5 for the surface and -1 for single sites). Regime (iii) is fully identical in both the cases.

Molecular recognition in cation‐exchanged zeolites

AbstractThe concepts of confinement‐ and molecular recognition‐driven chemical reactivity of cation‐exchanged zeolites is illustrated by our recent results from periodic and cluster density functional theory (DFT) calculations. The reactivity of alkali‐earth‐ and alkaline‐exchanged low‐silica zeolites for selective photo‐oxidation of alkenes with molecular oxygen and for N2O4 disproportionation is shown to be mainly due to the specific arrangement and the size of the cations in the zeolite cage. An attempt is made to separate the effects of basicity of the framework, the Lewis acidity of the extra‐framework cations and the electrostatic field in the zeolite cage as well as its geometrical properties for the respective reactions. The importance of the favorable adsorption fashion of the reagents controlled by noncovalent interactions with the microporous matrix is shown. The role of the weak interactions with the zeolite walls and the factors, which determine the preference for a particular adsorption mode, are discussed by the example of light alkanes adsorption to Mg‐ and Ca‐exchanged faujasites. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Interactions of amino acids with H-ZSM-5 zeolite: An embedded ONIOM study

The adsorption of glycine and l-alanine on the H-ZSM-5 zeolite has been studied at an embedded ONIOM (MP2/6-31G(d,p):UFF) level of theory. The most stable adsorption structure involves ion-pair interactions between the protonated amino acid and the anionic zeolite framework. The adsorption energy is computed to be −31.3 and −34.8 kcal/mol for glycine and l-alanine, respectively. Two hydrogen bonded complexes are identified: one is the cyclic double hydrogen bonded complex via the interactions of the carboxyl group and the zeolite acid site having adsorption energies of −25.4 and −30.0 kcal/mol for glycine and l-alanine, respectively. The second is the hydrogen bonded complex via the interactions of the hydroxyl group and the zeolite acid site having weak interaction energies, −20.7 and −23.9 kcal/mol for glycine and l-alanine, respectively. The zwitterion form is not found in the acidic H-ZSM-5 due to the acid–base reaction, but the glycine zwitterion is found to be stably adsorbed on the Na-ZSM-5 with the adsorption energy of −24.8 kcal/mol. In addition to the interactions with the Brønsted acid site of the zeolite, the interactions with the nearby oxygen framework and van der Waals interactions with the zeolite walls are also found to be important for stabilizing the adsorbed amino acids and zwitterion, suggesting the role of the zeolite framework as a “solid solvent molecule”.

1H Relaxivity of Water in Aqueous Suspensions of Gd3+-Loaded NaY Nanozeolites and AlTUD-1 Mesoporous Material: the Influence of Si/Al Ratio and Pore Size

The results of a (1)H nuclear magnetic relaxation dispersion (NMRD) and EPR study on aqueous suspensions of Gd(3+)-loaded NaY nanozeolites and AlTUD-1 mesoporous material are described. Upon increase of the Si/Al ratio from 1.7 to 4.0 in the Gd(3+)-loaded zeolites, the relaxation rate per mM Gd(3+) (r1) at 40 MHz and 25 degrees C increases from 14 to 27 s(-)1 mM(-1). The NMRD and EPR data were fitted with a previously developed two-step model that considers the system as a concentrated aqueous solution of Gd(3+) in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results show that the observed increase in relaxivity can mainly be attributed to the residence lifetime of the water protons in the interior of the material, which decreased from 0.3 to 0.2 micros, upon the increase of the Si/Al ratio. This can be explained by the decreased interaction of water with the zeolite walls as a result of the increased hydrophobicity. The importance of the exchange rate of water between the inside and the outside of the material was further demonstrated by the relatively high relaxivity (33 s(-1) mM(-1) at 40 MHz, 25 degrees C) observed for a suspension of the Gd(3+)-loaded mesoporous material AlTUD-1. Unfortunately, Gd(3+) leaches rather easily from that material, but not from the Gd(3+)-loaded NaY zeolites, which may have potential as contrast agents for magnetic resonance imaging.

A novel template-free sol–gel synthesis of silica materials with mesoporous structures and zeolitic walls

Hierarchical porous materials with zeolite structures show great promise in catalysis due to combining the advantages of zeolites and mesoporous materials. Here a novel template-free sol–gel method is developed to synthesize hierarchical porous silica materials. This method involves solvothermal recrystallization of the xerogel converted from uniform silicalite-1 precursor sol via vacuum drying process. The zeolite sol and the solid samples were characterized by laser light scattering, XRD, N2 adsorption/desorption isotherm, FTIR, SEM, TEM and thermal analysis technologies. The results show that we obtain two novel materials with different mesoporous structures and silicalite-1 walls by using different recrystallization media, one of which has irregular arrays of mesopores, the other possesses 3D wormhole mesoporous structure. We speculate that formation of different mesoporous structures results from different nucleation and growth process of materials

An Ordered Mesoporous Aluminosilicate with Completely Crystalline Zeolite Wall Structure

An ordered mesoporous aluminosilicate with completely crystalline zeolite pore wall structure, denoted as OMZ-1, was successfully synthesized by recrystallization of SBA-15 using in situ formed CMK-5 as the hard template. The role of carbon material not only serves as a hard template to preserve ordered mesoporous structure but also kinetically controls the crystallization process to form large crystals.

Influence of adsorption parameters on catalytic cracking and catalyst decay

Adsorption constants and heats of adsorption of different types of hydrocarbons on zeolitic catalysts have been obtained under cracking reaction conditions, by means of kinetic techniques. The heat of adsorption of a naphthene is very close to the alkane with the same number of carbon–carbon bonds, which in turn is defined by the van der Waals interactions between the hydrocarbon and the zeolite walls. The presence of the double bond in the olefin increases the heat of adsorption of the corresponding alkane by ∼40 kJ/mol. Comparison of decalin and tetralin shows the contribution of the aromatic ring to the heat of adsorption. Differences in heat of adsorption not only explain differences in cracking conversion, but can also predict variations in catalyst decay when industrial feeds of different compositions are used.

Enhanced Enantio- and Diastereoselectivity via Confinement and Cation Binding: Yang Photocyclization of 2-Benzoyladamantane Derivatives within Zeolites

Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters. Whereas in solution the ee in the product ketoester is zero, within achiral NaY zeolite, in the presence of a chiral inductor such as pseudoephedrine, ee's up to 28% have been obtained. The influence of zeolite on several chiral esters of 2-benzoyladamantane-2-carboxylic acids has also been examined. Whereas in solution the diastereomeric excess is <15%, in zeolite the delta-ketoesters are obtained in 79% de (best examples). Ab initio computations suggest that enhancement of chiral induction within zeolites is likely to be due to cation complexation with the reactant ketone. Alkali ion-organic interaction, a powerful tool, is waiting to be fully exploited in photochemical and thermal reactions. In this context zeolites could be a useful medium as one could view them as a reservoir of "naked" alkali ions that are only partially coordinated to the zeolite walls.

A Computational Study on the Templating Ability of the Trispyrrolidinium Cation in the Synthesis of ZSM-18 Zeolite

The role of trispyrrolidinium cation as the structure-directing agent of the ZSM-18 zeolite structure is explained by a close matching between the template and the zeolite walls. Two template orientations are proposed, and their respective structure-directing abilities are discussed in terms of template-zeolite energetic interactions. The relation of the template orientation and the Al distribution of the ZSM-18 structure is explained, and the preferential substitution of Al in the three-membered rings of ZSM-18 is explained from the results of the calculations, in accordance with the XRD solution of the structure.

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by acidic mordenite.

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite (Si/Al = 23) is reported. Cluster DFT as well as periodical structure DFT calculations have been performed. Full reaction energy diagrams of the elementary reaction steps that lead to the formation of the three xylene isomers are given. The use of periodical structure calculations allows one to account for zeolite framework electrostatic contributions and steric constraints that take place in zeolitic catalysts. Especially the steric constraint energy contribution has a significant effect on the energies and bond formation paths. The activation energy barrier of p-xylene formation is found to be approximately 20 kJ/mol lower than the corresponding values for the formation of its isomers. Computed host-guest binding energies according to the DFT method need a correction due to the absence of the dispersive interaction with the zeolite wall. Apparent activation energies obtained with this correction are in good agreement with experimental data.

QENS and FTIR studies on binding states of benzene molecules adsorbed in zeolite HZSM-5 at room temperature

Fourier-transform infrared (FTIR) spectroscopy and quasi-elastic neutron scattering (QENS) were employed for monitoring of the binding states of benzene molecules, adsorbed in HZSM-5 zeolite at 300 K and for loadings of 0.6 to 7 molecules per unit cell. While the in-plane combination C–C and C–H stretching bands of adsorbed benzene remained unaffected, a splitting was observed in the out-of-plane C–H bending vibrational bands, a feature reported for the transformation of benzene from liquid to solid phase. Also, the intensity ratio of the in-plane C–C stretching band (ν19) of adsorbed benzene at 1479 cm−1 and the bands in the region 3100–3035 cm−1 due to fundamentals and combination C–C and C–H stretching vibrations indicated a trend observed typically for a condensed phase of benzene. No shift was observed in the frequency of the above-mentioned IR bands when zeolite samples exchanged with Na+ or Ca2+ were employed. QENS results suggest that the benzene molecules occluded in zeolitic pores (∽3 molecules per unit cell) undergo a 6-fold rotation but their translation motion is too slow. Also, a high residence time of 16.5 ps was observed for the benzene entrapped in HZSM-5, compared to a time of ∽2.5 ps reported for the liquid and ∽19 ps for the solid state of benzene. These results reveal again the compression of the benzene molecules on adsorption in zeolitic pores. It is suggested that the benzene molecules confined in cavities experience a strong intermolecular interaction, giving rise eventually to their clustered state depending on the loading. In the clustered state, benzene molecules are packed with their plane parallel to zeolitic walls and interact with each other through π-electron clouds. No electronic bonding is envisaged between these clusters and the framework or the extra-framework zeolitic sites.

Enhanced Reduction of Fe2O3 Caused by Migration of TM Ions out of Zeolite Channels

Theoretical models do not predict significant hydrogen spillover from metal particles to the surface of a nonconducting oxide in the absence of a high concentration of OH groups. This is confirmed by measuring the reduction enhancement of hematite powder that is intimately mixed with H-zeolite-supported Pt, Pd, Rh, or Co. For Pt, Pd, or Rh in various zeolites, marked shifts of the H2 TPR peak to lower temperature are found compared to the same peak for pure Fe2O3. Such reduction enhancement is, however, only observed with precalcined mixtures, never with mixtures that were only reduced after grinding. This eliminates H spillover along zeolite walls as an effective mechanism. Instead, transition metals (TMs) in zeolite cavities form oxide particles which react with the protons to form TM ions during calcination. These ions migrate out of the zeolite to the iron oxide, where they can be reduced to TM metals, now positioned directly on the Fe2O3. This thermodynamically permitted type of H spillover to the oxide thus requires direct contact between the metal and the reducible oxide. The mechanism has general validity; however, marked differences between metals and different zeolites illustrate which factors are crucial for TM ion mobility through zeolite channels and over hematite particles. While migration of TM oxide clusters through zeolite channels is faster than migration of TM ions, ligated ions move faster than naked ions, and their migration can be accelerated by exposure to water vapor.