Arnošt Zukal

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.

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.

Mesoporous silica with controlled porous structure and regular morphology

Abstract A new procedure for the synthesis of mesoporous silica with controlled porous structure and regular morphology was developed. It is based on the precipitation from a homogeneous environment using cetyltrimethylammonium bromide as a structure directing agent. The decrease in pH, which causes the formation of solid particles, is achieved by the hydrolysis of ethyl acetate. The procedure enables to obtain not only the MCM-41 mesoporous molecular sieve with a very high degree of pore ordering and phase purity, but also materials of a new type, viz. bimodal silicas containing both the MCM-41 mesopore system with a pore size of about 3 nm and a system of larger mesopores with sizes ranging from 10 to 30 nm. Owing to their structural properties and regular worm-like morphology, bimodal silicas are promising materials for applications in separation processes or as supports for bulky molecules or nanoparticles.

Mesoporous thin film TiO2 electrodes

Thin layer electrodes (200–500 nm) of highly organized nanotextured anatase were prepared by hydrolysis of TiCl4 in the presence of poly(alkylene oxide) block copolymer, Pluronic P-123, acting as a structure-directing agent. Electrochemical insertion of Li was studied in LiN(CF3SO2)2 + ethylene carbonate + dimethoxyethane. Cyclic voltammograms of these electrodes showed unusual occurrence of two pairs of cathodic/anodic peaks at 1.5 and 1.6 V vs. Li/Li+. These peaks can serve as indicators of structural perfection of the mesoscopic skeleton; they diminish after mechanical or thermal destruction of the organized nanotexture.

Adsorption on MCM-41 mesoporous molecular sieves. Part 1.—Nitrogen isotherms and parameters of the porous structure

Nitrogen adsorption isotherms have been measured on a series of aluminosilicate and titanosilicate MCM-41 Molecular sieves, whose mean pore radius varied from ca. 0.9 to ca. 2 nm. By means of comparison plots the nature of the adsorption on these materials was found to depend strongly on their pore size. With pores of radius of around 1 nm multilayer coverage of the pore walls occurs. If the radius is increased to 1.5–1.8 nm the mechanism of adsorption changes into a two-stage one, i.e.. the multilayer coverage of the pore walls is suceeded by the spontaneous filling of the pore volume by capillary condensation without hysteresis. In even greater pores the usual capillary condensation with hysteresis occurs.The estimation of pore structure parameters was based on standard methods of adsorption isotherm processing, including the calculation of the pore size distribution from the desorption branch of the hysteresis loop. With the smaller pore materials, where the Kelvin equation does not hold, the estimation was based on the cylindrical pore model. A detailed knowledge of the pore structure of MCM-41 materials was thus obtained.

Controlling of morphology and characterization of pore structure of ordered mesoporous silicas

A new procedure based on the precipitation of MCM-41 particles due to a gradual decrease in pH was developed, enabling control of the morphology of mesoporous molecular sieves. As the decrease in pH is achieved owing to the hydrolysis of ethylacetate, local variations of pH do not occur and the formation of a rigid structure due to silica polymerization is delayed. By a judicious adjustment of reaction conditions, materials with markedly different morphologies were obtained. An MCM-41 molecular sieve with a high degree of internal mesopore ordering is distinguished by an almost perfectly uniform particle shape and size. The mesopore structure of a bimodal mesoporous silica with a worm-like morphology consists of mesopores, whose size is typical for an MCM-41 material, and larger ones with a mean diameter of 18 nm. A tentative mechanism explaining how the morphogenesis is controlled by the synthesis strategy used is suggested. A new method based on nitrogen adsorption data was developed for the quantitative determination of the content of amorphous phase in calcined samples. The consistency of the structure parameters determined by physically differing methods was checked using a geometrical model of the honeycomb structure.

Adsorption on MCM-41 mesoporous molecular sieves. Part 2.—Cyclopentane isotherms and their temperature dependence

The effect of temperature on adsorption isotherms of cyclopentane on mesoporous MCM-41 molecular sieves with different pore size has been determined at 243–333 K. The occurrence of adsorption hysteresis upon capillary condensation in mesopores depends on the pore critical temperature, which is lower than the bulk critical temperature and decreases with decreasing pore diameter. If the pore critical temperature is within the range of experimental temperatures of adsorption, capillary condensation on a single sieve may occur both reversibly (hypercritical region) and irreversibly (subcritical region).

The role of the extra-framework cations in the adsorption of CO2 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.

Controlling the Adsorption Enthalpy of CO2 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.

Nitrogen adsorption study of organised mesoporous alumina

As the arrangement of pores generally achieved in organised mesoporous alumina is not completely regular, gas adsorption methods appear important for characterisation of this material. One of the promising approaches to adsorption data analysis is based on the comparison of the adsorption isotherm of nitrogen at −196 °C on the porous solid under study with the standard adsorption isotherm of the same adsorbate on a reference non-porous solid. To obtain reliable standard nitrogen adsorption data for characterisation of the alumina porous structure, nitrogen adsorption isotherms on Degussa Aluminiumoxid C and α-alumina were carefully measured at −196 °C with ASAP 2010 and modified Accusorb 2100E instruments. Standard nitrogen adsorption data (adsorbed amount divided by surface area) were listed for 41 points in the relative pressure range 0.001 to 0.95. Their applicability was tested on organised mesoporous aluminas of various structural parameters using the comparison plot method. Although some samples of organised mesoporous alumina were calcined at relatively high temperatures, only the standard data on Degussa Aluminiumoxid C proved suitable for the comparative analysis of adsorption isotherms. The standard adsorption data on Degussa Aluminiumoxid C transformed to the statistical film thickness curve were used for calculation of Barrett–Joyner–Halenda (BJH) pore size distributions. The determined material parameters of the organised mesoporous aluminas were also supported by X-ray diffraction data and were consistent with the conditions of synthesis and heat treatment of these materials.