Molecules In Zeolites Research Articles

ChemInform Abstract: Computer Simulations of the Structure, Thermochemistry and Dynamics of Adsorbed Molecules in Zeolite and Related Catalysts

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Distribution and motion of organic guest molecules in zeolites

Hexamethyltienzene (HMB), adamantane, and naphthalene are adsorbed into zeolite Na-Y by a vapor phase impregnation method. Results from thermogravimetric analyses, Raman, and 13 C NMR measurements indicate that the organic species reside in the supercages of Na-Y. Proton multiple-quantum NMR spectroscopy is used to investigate the distribution of the organic species adsorbed within the cavities of Na-Y and the organic species occluded during the synthesis process into the intracrystalline voids of thecubic and hexagonal polytypes of faujasite and ZSM-18

A theory for two-component diffusion in zeolites

This chapter discusses the assumptions underlying modern atomistic and lattice approaches, and details the techniques and applications of modeling both rapid dynamics and activated diffusion. Practical applications of zeolites are typically run under steady-state conditions, making the relevant transport coefficient the Fickian diffusivity or other related permeability coefficient. However, modeling such steady-state transport through zeolites with atomistic models is challenging, prompting many researchers to simulate self-diffusion, which is the stochastic motion of tagged particles at equilibrium. The wide range of diffusional timescales encountered by molecules in zeolites presents unique challenges to the modeler, requiring that various simulation tools, each with its own range of applicability, be brought to bear on modeling dynamics in zeolites. The goals of simulating molecular dynamics in zeolites with atomistic detail are two-fold: to predict the transport coefficients of adsorbed molecules and to elucidate the mechanisms of intracrystalline diffusion. The chapter summarizes the major findings discovered over the last several years and enumerates future needs for the frontier of modeling dynamics in zeolites. With a rich variety of interesting properties and industrial applications, and with over 100 zeolite framework topologies synthetically available, each with its own range of compositions, zeolites offer size, shape, and electrostatically selective adsorption, diffusion, and reaction up to remarkably high temperatures.

Dynamics of adsorbed molecules in zeolites

Dye molecules adsorbed or embedded in the cages of a microporous zeolite have unique degrees of freedom which are not available to chromophores in e.g. molecular crystals or glasses. We have used low temperature optical spectroscopy and spectral hole burning to study the dynamics of a selected system, namely thionine molecules in a faujasite structure. The spectral properties depend strongly on the water content of the faujasite. Thionine in a hydrated sample shows considerable electron-phonon coupling comparable to the behaviour in a glassy matrix. In a dehydrated sample there exist two stable thionine conformations which can interconvert at low temperatures through ground state as well as excited state reactions. This reversible photocycle explains the appearance of antiholes at the burning wavelength. The electron-phonon coupling in the dehydrated sample is weak, similar to the characteristics of adsorbate/substrate systems.

Aligned molecular sieve crystals

The controlled incorporation of guest molecules in zeolites opens the way to novel materials with desirable optical, magnetic, or electronic properties. The properties of the guests are often direction dependent and therefore the bulk orientation of the host matrixes (see figure) in an electric field can lead to greatly enhanced performance. magnified image

X-ray powder diffraction and t.p.d. study of SO 2 adsorption on type Y zeolite

The forms of SO 2 adsorption and their location in two samples of zeolites Na, HY with differentSi/Al ratio and Na content were studied by X-ray powder diffraction (XRPD) and t.p.d. Two types of adsorbed species were distinguished. Strongly held SO 2 ( T des = 390 K) is formed due to interaction of SO 2 with cations. It was shown by XRPD that strongly held molecules are located in the supercage at a distance of ca. 1.8A˚from the cation occupying the S II site. Weakly held SO 2 species ( T das = 286–300 K) are formed mainly due to interactions between SO 2 molecules in zeolite Y cavities. At low temperature conditions (120 K), they occupy the center of the supercage.

Monte Carlo Simulations of Adsorption of Non-polar and Polar Molecules in ZeoliteX

Abstract Grand canonical Monte Carlo simulations have been performed for single-gas adsorption of Lennard-Jones molecules with point multipole moments in zeolite cavities of type X. Fluid-solid electrostatic interactions are taken into account. Adsorption isotherms, isosteric heats and structural properties have been calculated for Xe, CH4, CO2, C2H4 and i-C4H10 and compared with experiment. The results reveal the importance of adsorbate-adsorbate interactions for sorption in faujasite. The interaction of the quadrupole moment of CO2 with the electric field generated by the zeolite ions constitutes more than half of the total energy of adsorption. Density and energy distributions show that the cavity is either a relatively homogeneous surface or a highly heterogeneous one, depending on the molecular characteristics of the adsorbed gas and the type, position and charge of the zeolite cations.

NMR study of the distribution of aromatic molecules in NaY zeolite

Local and macroscopic distributions of adsorbed benzene, 1,3,5-trimethylbenzene, and hexamethylbenzene (HMB) molecules among the intracrystalline cavities of NaY zeolite samples have been investigated by {sup 129}Xe and multiple-quantum NMR. Xenon-129 NMR, a sensitive probe of macroscopic distributions of molecules in zeolites at room temperature, demonstrates the importance of sample heat treatment in distributing an adsorbate homogeneously throughout a collection of zeolite particles. NaY samples containing organic guests adsorbed at temperatures near the bulk species melting point produce multiple or broad xenon lines characteristic of macroscopic (i.e., interparticle) adsorbate concentration gradients. After thorough heat treatment of the samples at elevated temperatures, the xenon spectrum collapses to a single sharp line, consistent with a macroscopically uniform distribution. For HMB adsorbed in NaY, counting hydrogen clusters by multiple-quantum NMR reveals intraparticle HMB distributions consistent with one molecular per cavity at low loadings (average bulk loading {le} 1 molecule per cavity) and two molecules per cavity at higher loadings.

Molecular dynamics consideration of the mutual thermalization of guest molecules in zeolites

Molecular dynamics (MD) is applied to simulate methane self-diffusion in a model zeolite ZK4. It is found that the mutual interaction of the adsorbed molecules is sufficient to guarantee their thermalization, so that in first-order considerations, the energy exchange with the zeolite framework may be neglected.

Sorption states of isoelectronic linear molecules in zeolites

Sorption states of di- and triatomics in zeolites A containing mono- and divalent ions were investigated, applying an electrostatic interaction model, and compared with experimental data, i.e. heats of sorption, IR band shifts, intensities and interatomic distances obtained from X-ray diffraction. In all cases zeolite cations have to be considered as centers of interaction, the isoelectronic pair CO and N 2 being attached with two possible orientations of CO. Due to their size the isoelectroNic triatomics CO 2 and N 2O interact with two cations simultaneously. The calculated heats of adsorption turned out to be in good agreement with the experiment. Theoretical frequency shifts of CO and N 2 support the assignment of bands. The calculated electric field strength at the sorption site corresponds satisfactorily with values derived from the induced IR intensities of nitrogen. It is concluded that isoelectronic molecules reveal strong similarities in their sorptional behaviour.

Simulation studies of reactive molecules in zeolites

This paper describes the use of Mott–Littleton calculations to study hydrocarbon molecules in zeolites. The energetics and the geometry of adsorption of methanol and ethene molecules inside the pores of silicalite and ZSM-5 zeolite are reported.

Diffusion in High Occupancy Zeolites

AbstractDiffusion of small molecules in zeolites most often takes place in the configurational diffusion region, where the molecular diameter is approximately the same or slightly greater than the channel diameter. Since two molecules may not pass each other in a pore, the random walk based diffusion equation does not apply under high occupancy conditions in zeolites with one-dimensional pores.For zeolites with multi-dimensional pores, such as ZSM-5 and A, one sometimes encounters the counter-intuitive result that diffusivity dramatically rises with occupancy. There are two explanations for this behavior: one from irreversible thermodynamics and the Darken equation, which predicts that diffusivity will always rise with occupancy; the other from a Markov model of random activated jumps between low energy positions, such as pore crossings, which predicts that diffusivity will increase with occupancy if the activation energy of diffusion decreases with occupancy– such as due to swelling.

Mobility of salt molecules in zeolites

The zeolites clinoptilolite (Hector, California) and 4A have been treated with molten chloride, fluoride and nitrate salts. The resulting salt-loaded zeolites were characterized by X-ray powder diffraction, thermal and chemical analyses. Rates of extraction of occluded species were measured, using specific ion-electrodes, and thermodynamic parameters estimated for the elution process which was assumed to be diffusion controlled within the zeolite particles. The results obtained for Cl− and No−3 species were a function of zeolite structure, ion size and ion hydration as were those for F− elution, except that evidence for F− incorporation into the lattice was observed.

Positions of cations and molecules in zeolites with the mordenite-type framework X dehydrated calcium hydrogen mordenite

Prolonged acid treatment of natural mordenite (2M Hc1, 90°C, 2 months) did not remove 1.4Ca atoms which persisted in site I of the final product after dehydration at 300°C. Electron microprobe analysis showed no detectable reduction of Al/Si ratio, but the cell parameters at room temperature ( a 18.058(3) b 20.297(3) c 7.484(2)Å) are lower than for the dehydrated-deammoniated variety, and the mean T-O distance is lower (1.606 vs. 1.617Å). The O-T-O and T-O-T angles are closer generally to those of dehydrated Ca-mordenite than to dehydrated-deammoniated mordenite. Population refinement produced no evidence in favor of vacant tetrahedral sites, but technical considerations cause complications. The present data would not be inconsistent with earlier suggestions that Al is replaced by Si in tetrahedral sites.

Positions of cations and molecules in zeolites with the mordenite framework IX dehydrated H-mordenite via acid exchange

Nearly complete removal of cations was accomplished for natural mordenite by preliminary Na-exchange followed by exposure to 2M HCl for 1 month at 90°C. The electron microprobe analysis (atomic Si/Al 4.7) indicates no detectable extraction of Al. However the cell dimensions after dehydration at 300°C followed by cooling to room temperature ( a 18.178(7) b 20.394(6) c 7.488(4)Å) are lower than for “hydrogen-mordenite” prepared via an ammonium-exchanged intermediary ( a 18.223(7) b 20.465(9) c 7.531(4)Å). Furthermore the tetrahedral distances are smaller (T4 down 0.017Ȧ, T3 0.010, T1 0.008, T2 0.002). Diffractions from the HCl-treated mordenite are sharp, and the positional and population parameters are close to those for the de-ammoniated mordenite. The simplest interpretation is that treatment with HCl results in extraction of Al from tetrahedral sites into other positions in the crystal coupled with migration of Si so that a crystalline structure is retained: however, many subtle complications require cautious interpretation.

Positions of cations and molecules in zeolites with the mordenite-type framework: VIII Dehydrated sodium-exchanged mordenite

Natural mordenite was ion-exchanged in NaCl solution and dehydrated at 300°C [ a 17.92(1) b 20.31(1) c 7.480(7)Å at room temperature]. Dehydration reduced the symmetry from Cmcm to Pbcn. All diffractions violating the C-centering are diffuse, and in one crystal are split into doublets indicating a domain structure with a (true cell) ∼5 a (pseudo-cell). The diffuse diffractions disappear upon rehydration. Diffraction intensities were collected using a wide scan for diffuse diffractions. Site occupancies are: I', 3.1Na; IV, 2.6Na; VI, 1.5Na. Electron microprobe analysis yielded Na 7.3K 0.2Ca 0.03Al 8.3Si 39.9O 96, and some cationic Al, or OH −, or both, may be needed for charge balance. (Na(I') is displaced 0.53Å from its ideal position at (0,0.5,0), perhaps because of electrostatic repulsion between adjacent ions. An ordered model with 3.2Na and 0.8 vacancies in site I' provides a qualitative explanation of the domain structure. Na(IV) is displaced laterally from the center of an eight-ring. The effect of “decationization” on site occupancy and molecular sorption is discussed.

Positions of cations and molecules in zeolites with the mordenite-type framework VII dehydrated cesium mordenite

Natural mordenite was ion-exchanged with CsCl solution and dehydrated at 300°C. The crystal structure [ a 18.194(19) b 20.470(17) c 7.506(9): Cs 7 4Al∼ 8Si∼ 40O 96] probably belongs to space group P2 1cn but was refined in the average space group Cmcm. Site occupancies for Cs are site II 3.8, IV 1.9, VI 1.8 atoms. Combined occupancies of sites IV and VI are less than 4 in dK-, dRb- and dCs- varieties. Cations larger than 1.3Å have not been found in site I. Cations in site VI block diffusion of large molecules in the main channel, and conversion of Na-mordenite from small-port to large-port variety by “decationization” towards H-mordenite might be explained by emptying sites IV and VI while up to 4 Na could remain in site I.

Positions of cations and molecules in zeolites with the mordenite-type framework VI dehydrated barium mordenite

Positions of cations and molecules in zeolites with the mordenite-type framework VI dehydrated barium mordenite

Positions of cations and molecules in zeolites with the mordenite-type framework: V dehydrated Rb-mordenite

Mordenite from Challis Valley, Idaho, was ion-exchanged with RbCl 2 solution to ∼Rb 8Al 8Si 40O 96·nH 2O. After dehydration at 340°C for 1 day, the crystal structure was determined at room temperature. Earlier attempts to dehydrate crystals at higher temperature led to disintegration, and even the present crystal fractured. Although this resulted in poor X-ray data, refinement in Cmcm ( a 18.127(7) b 20.408(6) c 7.463(3)Å) yielded atomic positions similar to those for dehydrated K-mordenite. Rb atoms in sites II (3.7Rb), IV (3.1Rb) and VI (0.7Rb) block most of the small channels (“side pockets”) and may hinder diffusion along the main channels. A few diffractions violating the C lattice were ignored in the refinement but indicate structural distortion similar to that in dehyrated K-mordenite which was refined in Pbcn. However the presence of a few weak diffractions violating the b glide indicates even lower symmetry.

Positions of cations and molecules in zeolites with the chabazite framework. IV Hydrated and dehydrated Cu 2+-exchanged chabazite

Chabazite from Bozen, Tyrol was ion-exchanged with KCl and further exchanged with CuCl 2 solution to Cu 1.8K 0.2Al 3.9Si 8.1O 24.nH 2O. Both the hydrated and dehydrated structures were determined at room temperature. Cell dimensions in R3m are: hydrated a 9.411(2)Å α 95.31(1)°; vacuum-dehydrated at 350°C a 9.310(2)Å α 92.01(2)°. All Cu 2+ ions of d-Cu-chabazite were located near the center of the 6-ring, 1.97Å from three 0(4) and 2.83Å from three 0(3). Displacement of Cu 2+ by 0.15Å into the large cage from a position for triangular coordination to three 0(4) may result from either an attempt to reach a Jahn-Teller distorted tetrahedral coordination, or electrostatic repulsion from another Cu 2+ across the ditrigonal prism, or both. The T-O distances increase as the T-O-T angles decrease. Interpretation of the electron density of h-Cu-chabazite is uncertain, but all Cu 2+ ions and water molecules are in the large cage, perhaps as hydration complexes with Jahn-Teller distortion.