Ivo Jakubec

Enhancement of Activity and Selectivity in Acid‐Catalyzed Reactions by Dealuminated Hierarchical Zeolites

High-silica zeolites with crystalline aluminosilicate frameworks balance the charge of strongly acidic protons during the processing of oil, in petrochemistry, and increasingly in numerous organic syntheses. The transformation of hydrocarbons is controlled by the concentration and strength of the acid sites and the dimensions and architecture of the inner pores. Zeolite micropores, which have a diameter similar to organic molecules, govern the shape selectivity of the reaction in the inner space, but also result in slow transport of reactants and products, thus limiting the reaction rate. Several approaches have been developed to enhance the mass transport by using zeolite nanosheets and nanocrystals, or zeolites that contain both microand mesopores. The latter hierarchical zeolites were prepared by confined crystal growth, by using polymers as mesoporogens, or through post-synthesis desilication or dealumination processes. The advantage of the presence of mesopores is, however, accompanied by the nonshape-selective environment of the acid sites located in the mesopores. Our interest in the effective formation of secondary mesoporosity through postsynthesis alkaline treatment of conventional zeolites prompted us to study the potential of leaching procedures for the preparation of hierarchical zeolites, preserving the shape-selective environment of the active sites. The main principles of forming mesopores in high-silica zeolites through alkaline leaching have been described by Groen et al. They demonstrated that dissolution of Si depends mainly on the Al concentration in the framework and occurs in the Si-rich areas. Al atoms partly remain at the framework sites and partly form extra-framework Al species in the mesopores. Groen et al. 13] and Caicedo-Realpe and PerezRamirez have shown that the formed Al species can be removed by treatment with mild acid, thus restoring the original Si/Al ratio. This treatment increased the isomerization of o-xylene, however, the selectivity for p-xylene did not reach that of parent microporous zeolites. This study is primarily concerned with the elimination of both the extraframework and framework Al species, and thus the related acid sites from the mesopores of the desilicated zeolites by employing oxalic acid. The advantage of hierarchical zeolites with acid sites predominantly located in the confined reaction space of the micropores is demonstrated on acid-catalyzed reactions controlled by shape-selectivity effects. TEM images of the alkalineand subsequently acidleached zeolites are given in Figure 1. They clearly show that the treatment resulted in the extensive formation of a secondary mesoporous structure, which is characterized by numerous crystal cavities, which are more populated in ZSM-5 (Si/ Al = 22.2) compared to mordenite (MOR, Si/Al = 12.1). The adsorption isotherms of treated ZSM-5 zeolites (Figure 2) indicate adsorption in the zeolite micropores and an H3 hysteresis loop typical for slit-shaped mesopores. But the extensive formation of a mesoporous structure also resulted in a decrease in the micropore volume. Treatment with oxalic acid further extended the mesopore volume and the micropore volume increased, with the final value only slightly lower compared to the parent zeolite. Al plugs, which were formed in the mesopores after desilication and blocked parts of the micropores, were removed by acid leaching, similar to results of Caicedo-Realpe and Perez-Ramirez. With mordenite, alkaline and acid leaching resulted in similar textural changes and led to well-developed secondary mesoporosity with preserved high micropore volumes. The dealuminated zeolite surface was analyzed by XPS monitoring of the relative concentration of Al to Si in the zeolite (sub)surface layers ( 50 ) by the Al 2p and Si 2p electron levels. The surface Si/Al ratio of both desilicated ZSM-5 and mordenite zeolites compared to the bulk composition (Table 1) indicated accumulation of Al species on the external crystal surface. In contrast, zeolites treated with oxalic acid resulted in a slight surface enrichment in Si. Analysis of the Brønsted and Lewis acid sites of dealuminated micro-mesoporous zeolites indicated predominant Brønsted acidity corresponding to the concentration of Al in the framework (Table 1). The population of acid sites in the dealuminated micro-mesoporous (deAlmm) ZSM-5(I) was analyzed using the FTIR spectra of adsorbed 2,6-ditertbutylpyridine (DTBPy), the kinetic diameter of which (10.5 ) does not allow it to penetrate into the [*] Dr. P. Sazama, Prof. Dr. Z. Sobalik, Dr. J. Dedecek, Dr. Z. Bastl, Dr. J. Rathousky, Dr. H. Jirglova J. Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic 18223 Prague 8 (Czech Republic) E-mail: petr.sazama@jh-inst.cas.cz

Structural defects in Cu-doped Bi2Te3 single crystals

The relation between the concentration of free charge carriers and the concentration of copper atoms in Bi2Te3 single crystals doped with copper over a wide range of concentrations has been investigated, with the aim of clarifying the existence of inactive Cu ions. Changes in the concentration of free charge carriers arising from Cu-doping of the melt with that induced by electrochemical intercalation of copper are compared. Models of possible defect structures are proposed for both doped and intercalated single crystals of Bi2Te3.

Chemical oxygen-iodine laser using a new method of atomic iodine generation

The chemical oxygen-iodine laser (COIL) with a new chemical method of atomic iodine production was investigated. In this system, iodine atoms are formed in the COIL cavity by the fast chemical reaction of hydrogen iodide with chlorine atoms that are also produced chemically. It was found that, in the absence of singlet oxygen, the ground state atomic iodine can be produced with a high yield (80%-100%). In gas containing singlet oxygen, a gain on 3-4 electronic transition in iodine atom was achieved (0.35% cm/sup -1/). Both the concentration of atomic iodine and the gain depend substantially on the ratio of reacting gases and the penetration of secondary gases into the primary gas flow. In laser experiments, effects of the flow rate of reacting gases and their penetration on the laser output power were found. The output power of 310 W was attained at chlorine flow rate of 27 mmol/spl middot/s/sup -1/ corresponding to chemical efficiency of 12.7%. This was the first time the gain and laser output power were achieved in the COIL with atomic iodine generated by the proposed method.

Chemical generation of atomic iodine for the chemical oxygen-iodine laser. II. Experimental results

Abstract A new method for the chemical generation of atomic iodine intended for use in a chemical oxygen–iodine laser (COIL) was investigated experimentally. The method is based on the fast reaction of hydrogen iodide with chemically produced chlorine atoms. Effects of the initial ratio of reactants and their mixing in a flow of nitrogen were investigated experimentally and interpreted by means of a computational model for the reaction system. The yield of iodine atoms in the nitrogen flow reached 70–100% under optimum experimental conditions. Gain was observed in preliminary experiments on the chemical generation of atomic iodine in a flow of singlet oxygen.

The accelerating role of water in hydrogen insertion into tungsten trioxide

Abstract Hydrogen insertion into tungsten oxide WO 3 was studied by in situ infrared (IR) and electron spin resonance (ESR) spectroscopic methods. The accelerating role of water in the reaction system is discussed.

Chemical generation of atomic iodine for COIL

A method of the chemical production of atomic iodine aimed for application in COIL was studied experimentally. The method is based on chemical generation of chlorine atoms and their subsequent reaction with hydrogen iodide. Effects of initial ratio of reactants and the way of their mixing were investigated and interpreted by means of the developed model of the reaction system. In optimum conditions, the yield of iodine atoms, related to HI, attained 70 - 100 percent.

Preliminary experimental results on chemical generation of atomic iodine for a COIL

A chemical method of atomic iodine generation with a potential application in chemical oxygen-iodine laser (COIL) was investigated experimentally. The process consists in a fast reaction of gaseous hydrogen iodide with chlorine atoms produced in reaction of gaseous chlorine dioxide with nitrogen oxide. In conditions characteristic for a subsonic mixing region of COIL, atomic iodine was produced with a yield of 20-50 %. This is in a fair agreement with results ofmathematical modeling ofthis complex reaction system.

Insertion of hydrogen into hexagonal ammonium tungsten bronze (NH4)0.3WO3

Insertion of hydrogen into the hexagonal ammonium tungsten bronze was carried out by electrochemical and “hydrogen spill-over” techniques. The coefficient x = 0.06 in the hydrogen bronze Hx(NH4)0.3WO3 and the diffusion coefficient of hydrogen DH = 2.29 × 10−17 m2s−1 were found by the hydrogen insertion in the absence of water.

Atomic iodine generation via F atoms for COIL

Chemical generation of atomic iodine for a Chemical Oxygen-Iodine Laser (COIL) was investigated experimentally. This all-gas process includes atomic fluorine as an intermediate species. In the two-step reaction mechanism, F atoms are produced in reaction of molecular fluorine with NO and react further with hydrogen iodide to iodine atoms. The efficiency of this process was studied in dependence on mixing conditions, flow rate of reacting gases and pressure in the reactor. The maximum concentration of atomic iodine was obtained at approximately equimolar ratio of reacting gases (F2, NO and HI), which agrees with the stoichiometry of the production reactions. A shortage of any of the reacting gases limits the rate of atomic iodine formation. A considerable excess of F2 against NO at a simultaneous deficit of HI had a most detrimental effect on atomic iodine production. Sufficiently high concentrations of atomic iodine (5 to 8 x 1015 cm-3) can be achieved by this method even at pressure 4 - 9 kPa that enable to inject the gas with iodine atoms into the singlet oxygen flow upstream the nozzle throat in the chemical oxygen-iodine laser.

Chemical oxygen-iodine laser with atomic iodine generated via Cl or F atoms

Two alternative chemical methods of atomic iodine generation for a chemical oxygen-iodine laser (COIL) were studied. These methods are based on fast reactions of gaseous hydrogen iodide with chemically produced chlorine and fluorine atoms. Both processes were studied first in small-scale reactors. A yield of atomic iodine in the Cl system and nitrogen (non-reactive) atmosphere exceeded 80%, while in the F system it was only up to 27% related to F2 or 50% related to HI. The process of atomic iodine generation via Cl atoms was employed in operation of the supersonic COIL. A laser power of 430 W at 40 mmol Cl2/s, and the small signal gain up to 0.4%/cm were attained. The proposed methods promise an increase in laser power, easier control of laser operation, and simpler iodine management in comparison with the conventional source of atomic iodine using I2. The experimental results obtained so far with this experimental arrangement did not proved yet increasing COIL chemical efficiency because some process quenching a part of singlet oxygen was indicated. Therefore a modified experimental set-up has been designed and prepared for further investigation.