Adsorption and Diffusion of C1 to C4 Alkanes in Dual-Porosity Zeolites by Molecular Simulations.

We employ grand canonical Monte Carlo and molecular dynamics simulations to systematically study the adsorption and diffusion of C1 to C4 alkanes in hierarchical ZSM-5 zeolite with micropores (∼1 nm) and mesopores (>2 nm). The zeolite is characterized by a large surface area of active sites on the microporous scale with high permeability and access to the active sites, which arises from the enhanced transport at the mesoporous scale. We model this zeolite as a microporous Na+-exchanged alumino-sillicate zeolite ZSM-5/35 (Si/Al = 35) in which cylindrical mesopores with a diameter of 4 nm have been built by deleting atoms accordingly. We use the TraPPE and Vujic-Lyubartsev force fields along with the Lorentz-Berthelot combining rules to describe adsorbate-adsorbate and adsorbate-adsorbent interactions. The performance of the force fields is assessed by comparing against experimental single-component adsorption isotherms of methane and ethane in microporous ZSM-5/35, which we measured as part of this work. We compare the adsorption isotherms and diffusivities of the adsorbed alkanes in the dual-porosity zeolite with those in microporous ZSM-5/35 and discern the specific behavior at each porosity scale on the overall adsorption, self-diffusion, and transport behavior in zeolites with dual micro/mesoporosities.

A novel zinc(ii) metal-organic framework with a diamond-like structure: synthesis, study of thermal robustness and gas adsorption properties.

A solvothermal reaction of Zn(ii) salt with methanetetrabenzoic acid (H4MTB) and 1,4,8,11-tetraazacyclotetradecane (cyclam, CYC) created a new microporous metal-organic framework {[Zn2(µ4-MTB)(κ(4)-CYC)2]·2DMF·7H2O}n (DMF = N,N'-dimethylformamide). Single crystal X-ray diffraction showed that the complex exhibits a four-fold interpenetrated diamond-like structure topology with 1D jar-like channels with sizes about 14.1 × 14.1 and 2.4 × 2.4 Å(2). The stability of the framework and activation conditions of the compound have been studied by high-energy powder X-ray diffraction during in situ heating, thermogravimetric analysis coupled with mass spectrometry and infrared spectroscopy performed at different temperatures. The gas adsorption behaviour of {[Zn2(µ4-MTB)(κ(4)-CYC)2]·2DMF·7H2O}n was studied by adsorption of Ar, N2, CO2 and H2. Nitrogen and argon adsorption showed that the activated sample exhibits Brunauer-Emmet-Teller (BET) specific surface areas of 644 m(2) g(-1) (N2) and 562 m(2) g(-1) (Ar). The complex adsorbs carbon dioxide with a maximum storage capacity of 10.5 wt% at 273 K and 101 kPa. The observed hydrogen uptake was 1.27 wt% at 77 K and 800 Torr, which is the highest value reported for the compounds containing a MTB(4-) linker. The adsorption heats of carbon dioxide and hydrogen, calculated according to the Clausius-Clapeyron equation, were in the range 22.8-22.4 kJ mol(-1) for CO2 and 8.9-3.2 kJ mol(-1) for H2, indicating weak interactions of the gases with the framework.

High-resolution adsorption analysis of pillared zeolites IPC-3PI and MCM-36.

The porous structure of pillared zeolites IPC-3PI and MCM-36 and their precursors IPC-3P and MCM-22P, respectively, has been investigated by means of a high-resolution adsorption analysis. The analysis was based on argon adsorption isotherms measured at 87 K from the relative pressure of 10(-6). The isotherms were processed by means of the t-plot method, which made it possible to distinguish adsorption in micropores from adsorption in mesopores. The pore size distribution was evaluated from argon isotherms using Non-Local Density Functional Theory. The obtained results have shown that the microporous structure of the MWW layers is preserved in both pillared zeolites. In contrast to precursors IPC-3P and MCM-22P, pillared samples are characterized by the formation of a porous structure belonging to the lower mesopore region. The distribution of mesopores in the zeolite IPC-3PI is broader and is shifted to larger widths in comparison with the zeolite MCM-36.

A family of zeolites with controlled pore size prepared using a top-down method.

The properties of zeolites, and thus their suitability for different applications, are intimately connected with their structures. Synthesizing specific architectures is therefore important, but has remained challenging. Here we report a top-down strategy that involves the disassembly of a parent zeolite, UTL, and its reassembly into two zeolites with targeted topologies, IPC-2 and IPC-4. The three zeolites are closely related as they adopt the same layered structure, and they differ only in how the layers are connected. Choosing different linkers gives rise to different pore sizes, enabling the synthesis of materials with predetermined pore architectures. The structures of the resulting zeolites were characterized by interpreting the X-ray powder-diffraction patterns through models using computational methods; IPC-2 exhibits orthogonal 12- and ten-ring channels, and IPC-4 is a more complex zeolite that comprises orthogonal ten- and eight-ring channels. We describe how this method enables the preparation of functional materials and discuss its potential for targeting other new zeolites.

The application of electrospun titania nanofibers in dye-sensitized solar cells.

Titania nanofibers were fabricated using the industrial Nanospider(TM) technology. The preparative protocol was optimized by screening various precursor materials to get pure anatase nanofibers. Composite films were prepared by mixing a commercial paste of nanocrystalline anatase particles with the electrospun nanofibers, which were shortened by milling. The composite films were sensitized by Ru-bipyridine dye (coded C106) and the solar conversion efficiency was tested in a dye-sensitized solar cell filled with iodide-based electrolyte solution (coded Z960). The solar conversion efficiency of a solar cell with the optimized composite electrode (η = 7.53% at AM 1.5 irradiation) outperforms that of a solar cell with pure nanoparticle film (η = 5.44%). Still larger improvement was found for lower light intensities. At 10% sun illumination, the best composite electrode showed η = 7.04%, referenced to that of pure nanoparticle film (η = 4.69%). There are non-monotonic relations between the film's surface area, dye sorption capacity and solar performance of nanofiber-containing composite films, but the beneficial effect of the nanofiber morphology for enhancement of the solar efficiency has been demonstrated.

Controlling the adsorption enthalpy of CO(2) in zeolites by framework topology and composition.

Zeolites are often investigated as potential adsorbents for CO(2) adsorption and separation. Depending on the zeolite topology and composition (Si/Al ratio and extra-framework cations), the CO(2) adsorption heats at low coverages vary from -20 to -60 kJ mol(-1), and with increasing surface coverage adsorption heats either stay approximately constant or they quickly drop down. Experimental adsorption heats obtained for purely siliceous porous solids and for ion-exchanged zeolites of the structural type MFI, FER, FAU, LTA, TUN, IMF, and -SVR are discussed in light of results of periodic density functional theory calculations corrected for the description of dispersion interactions. Key factors influencing the stability of CO(2) adsorption complexes are identified and discussed at the molecular level. A general model for CO(2) adsorption in zeolites and related materials is proposed and data reported in literature are evaluated with regard to the proposed model.

Polyacetylene-type networks prepared by coordination polymerization of diethynylarenes: new type of microporous organic polymers.

Microporous organic polymers (MOP) of a new type have been synthesised in high yields by a simple coordination polymerization of 1,3-diethynylbenzene, 1,4-diethynylbenzene and 4,4'-diethynylbiphenyl catalysed by [Rh(cod)acac] and [Rh(nbd)acac] complexes. The new MOPs are non-swellable polyacetylene-type conjugated networks consisting of ethynylaryl-substituted polyene main chains that are crosslinked by arylene linkers. Prepared MOP samples have a mole fraction of branching units (by (13)C CP/MAS NMR) from 0.30 to 0.47 and exhibit the BET (Brunaer-Emmett-Teller) surface up to 809 m(2) g(-1) and hydrogen uptake up to 0.69 wt% (77 K, H2 pressure 750 torr).

Control of CO2 adsorption heats by the Al distribution in FER zeolites.

Proper combination of template and optimized reaction conditions provides zeolite FER with homogeneous distribution of Al in the framework; this results in a new zeolite adsorbent exhibiting a constant heat of CO(2) adsorption.

Mutable Lewis and Brønsted acidity of aluminated SBA-15 as revealed by NMR of adsorbed pyridine-15N.

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Brønsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Brønsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.

Thermodynamics of CO2 adsorption on functionalized SBA-15 silica. NLDFT analysis of surface energetic heterogeneity.

Siliceous SBA-15 mesoporous molecular sieves were functionalized with different amounts of 3-aminopropyl-trimethoxysilane. To obtain a more detailed insight into the material properties of the prepared samples, their textural parameters were combined with results of thermal analysis. Adsorption isotherms of carbon dioxide on parent and functionalized SBA-15 were measured in the temperature range from 273 to 333 K. From the temperature dependence of CO(2) isotherms the isosteric adsorption heats of CO(2) were determined and discussed. Information about the surface energetic heterogeneity caused by tethered 3-aminopropyl groups were obtained from CO(2) adsorption energy distributions calculated using the theoretical CO(2) adsorption isotherms derived from the non-local density functional theory. The values of isosteric heats and the energy distributions of CO(2) adsorption detect highly energetic sites and enabled quantification of their concentrations.

The role of the extra-framework cations in the adsorption of CO(2) on faujasite Y.

A purely electrostatic picture predicts that small, more polarizing cations adsorb more strongly than the large ones. In the case of the adsorption of CO(2) on faujasite Y, however, the inverse order is found at low pressure: CsY and KY adsorb stronger than NaY and LiY. This trend cannot be explained by a simple monopole-dipole or monopole-quadrupole interaction of CO(2) with a single cation. Therefore, we have conducted a combined adsorption, IR and DFT study in order to shed light on this phenomenon. Our results show that a simultaneous interaction of CO(2) with the cations (located in the hexagonal window between supercage and sodalite cage) and framework oxygen atoms (in the 12-ring connecting two supercages) can explain the strong adsorption of CO(2) on CsY and KY. We also discuss the effect of van der Waals interactions. Although there is a redistribution of the charge of three framework oxygen atoms in the 12-ring towards the carbon atom of CO(2), the geometry of CO(2) remains almost linear. The adsorption mode can, therefore, be interpreted as a carbonate precursor. In zeolite Y, this acid-base like interaction is more important than polarization of CO(2) by the cation only. Furthermore, evidence for an adsorption mode where CO(2) binds simultaneously to two cations in the supercage was detected by DFT, as already reported for Na-ferrierite. This adsorption mode is, however, strongly dependant on the distance between two cations in the supercage and is only favorable in the case of KY.

Experimental and theoretical determination of adsorption heats of CO(2) over alkali metal exchanged ferrierites with different Si/Al ratio.

The adsorption of CO(2) in Li-, Na-, and K-FER was investigated by a combination of volumetric adsorption experiments, FTIR spectroscopy, and density functional theory. Experimental isosteric heats of CO(2), Q(st), depend significantly on the cation size, cation concentration, and on the amount of adsorbed CO(2). The differences observed in experimentally determined isosteric heats were interpreted at the molecular level based on good agreement between experimental and calculated characteristics. The highest interaction energies were found for CO(2) adsorbed on so-called "dual cation sites" in which CO(2) is bridged between two alkali metal cations. The formation of CO(2) adsorption complexes on dual cation sites is particularly important on Na-FER and K-FER samples with higher cation concentration. On the contrary, the differences in Q(st) observed for Li-FER samples are due to the changes in the Li(+) coordination with the framework. The DFT/CC calculations show that the dispersion interactions between CO(2) and the zeolites framework are rather large (about -20 kJ mol(-1)).

Alkali metal cation doped Al-SBA-15 for carbon dioxide adsorption.

Mesoporous aluminosilicate adsorbents for carbon dioxide were prepared by the grafting of aluminium into SBA-15 silica using an aqueous solution of aluminium chlorohydrate. As the ion exchange sites are primarily associated with the presence of tetrahedrally coordinated aluminium, extra-framework aluminium on the SBA-15 surface was inserted into the silica matrix by a treatment with an aqueous solution of NH(4)OH. Synthesized mesoporous aluminosilicate preserving all the characteristic features of a mesoporous molecular sieve was finally modified by the alkali metal cation exchange. To examine carbon dioxide adsorption on prepared materials, adsorption isotherms in the temperature range from 0 °C to 60 °C were measured. Based on the known temperature dependence of adsorption isotherms, isosteric adsorption heats giving information on the surface energetics of CO(2) adsorption were calculated and discussed. The comparison of carbon dioxide isotherms obtained on aluminosilicate SBA-15, aluminosilicate SBA-15 containing cations Na(+) and K(+) and activated alumina F-200 reveals that the doping with sodium or potassium cations dramatically enhances adsorption in the region of equilibrium pressures lower than 10 kPa. Therefore, synthesized aluminosilicate adsorbents doped with Na(+) or K(+) cations are suitable for carbon dioxide separation from dilute gas mixtures.

Functionalization of delaminated zeolite ITQ-6 for the adsorption of carbon dioxide.

Novel functionalized adsorbents for CO(2) separation were synthesized by grafting 3-aminopropyl, 3-(methylamino)propyl, or 3-(phenylamino)propyl ligands in the delaminated zeolite ITQ-6. On the basis of the texture parameters determined from nitrogen adsorption isotherms recorded at 77 K and the results of chemical analysis, physicochemical properties of functionalized ITQ-6 were evaluated and compared with those of mesoporous SBA-15 silica functionalized with the same ligands. To examine carbon dioxide adsorption on functionalized materials, adsorption isotherms at 293 K were measured. To obtain information on the surface energetics of CO(2) adsorption on selected samples, isotherms were taken in the temperature range from 273 to 333 K and adsorption isosteres were calculated. Isosteric adsorption heats determined from the slope of adsorption isosteres proved that all of the 3-aminopropyl ligands in ITQ-6 take part in CO(2) adsorption. It was found that in the whole region of CO(2) pressures the efficiency of the amine ligand, defined as the number of adsorbed CO(2) molecules per one amine ligand, is higher for functionalized ITQ-6 than for functionalized SBA-15 silica.

The role of crystallization parameters for the synthesis of germanosilicate with UTL topology.

The investigation of the critical synthesis parameters of germanosilicate of UTL topology, possessing 14- and 12-rings, has been carried out in detail. (6R,10S)-6,10-Dimethyl-5-azoniaspiro[4.5]decane hydroxide was used as the structure-directing agent (SDA). The kinetics of the synthesis, the role of the Si/Ge ratio in the synthesis mixture, and the effect of the calcination procedure were investigated in relation to the crystallinity and textural properties of the synthesized material. The optimum synthesis time was found to be six days for Si/Ge and (Si+Ge)/SDA molar ratios of 2 and 1.7, respectively. The UTL zeolite crystallizes as small sheets of 10 mum in size. The micropore volume of the best crystals is 0.22 cm(3) g(-1) with a micropore diameter of 1.05 nm, based on DFT and Saito-Foley analyses of adsorption data.

Organized mesoporous TiO2 films exhibiting greatly enhanced performance in dye-sensitized solar cells.

The Pluronic P123 templated mesoporous TiO2 film was grown via layer-by-layer deposition and characterized by a novel methodology based on the adsorption of n-pentane. Multiple-layer depositions did not perturb the mesoporous structure significantly. Our TiO2 film was sensitized by a newly developed Ru-bipyridine dye (N945) and was applied as a photoanode in dye-sensitized solar cell. The 1-microm-thick mesoporous film, made by the superposition of three layers, showed enhanced solar conversion efficiency by about 50% compared to that of traditional films of the same thickness made from randomly oriented anatase nanocrystals.

Solvent-induced textural changes of as-synthesized mesoporous alumina, as reported by spin probe electron spin resonance spectroscopy.

The effect of solvent used during the synthesis and postsynthesis treatment on textural properties of organized mesoporous aluminas was investigated and related to the behavior of spin probes studied by electron spin resonance (ESR) spectroscopy. It was found that the structure of surfactant aggregates serving in the as-synthesized precipitates as templates could be easily modified by treatment with different solvents. This treatment induces corresponding variations in surface areas, mesopore volumes, and mesopore diameters of the final products. The ESR spectrum of 5-doxyl stearic acid spin probe properly reflects the changes in template structure based on changes of the solvent used and represents an early indicator of the corresponding textural modifications of the mesoporous alumina.

High-resolution adsorption of nitrogen on mesoporous alumina.

Nitrogen adsorption isotherms on organized mesoporous aluminas prepared by several different synthesis procedures were analyzed by means of comparative plot method using Aluminiumoxid C (Degussa) and alpha-alumina as reference adsorbents. To secure the high-resolution ability of this method, all the adsorption measurements were carefully performed in a relative pressure range from 10(-6) to 0.99. Although some samples of organized mesoporous alumina were treated at temperatures up to 1000 degrees C, only the Aluminiumoxid C has proved to be suitable as a reference adsorbent. The comparative analysis of isotherms on activated aluminas has shown that this method allows the determination of very small amounts of microporosity. The standard nitrogen adsorption data for Aluminiumoxid C and alpha-alumina are presented in a tabulated form, which consists of 91 points for each adsorbent.