Zdenek Bastl

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

Fluorination of Isotopically Labeled Turbostratic and Bernal Stacked Bilayer Graphene

Fluorination of graphene opens up a bandgap, which creates opportunities for optoelectronics, and also paves the way for the creation of extremely thin insulating layers, which can be important for applications in devices. However, in spite of many interesting features offered by, for example, unequally doped layers in multilayered systems, most of the work has concerned the fluorination of graphene monolayers. Here, the fluorination process of graphene bilayers is investigated through high-resolution Raman mapping followed by analysis of more than 10,000 spectra of bilayer graphene. Isotopically labeled bilayers are used, allowing each individual layer in bilayer graphene to be addressed unambiguously. The fluorinated graphene is prepared through exposure to XeF2. Monolayer graphene is found to be significantly more sensitive to fluorination than bilayer graphene. Through comparison of the D/G area ratio and the position of the G band for turbostratic and Bernal stacked (AB) bilayers, it is found that the fluorination process is more effective for turbostratic than for AB-stacked bilayer graphene. The fluorination changes the electronic structure similarly for the top and bottom layers in turbostratic bilayers. However, the top layer is more sensitive than the bottom layer in AB-stacked bilayers.

Chemical vapour deposition of selenium and tellurium films by UV laser photolysis of selenophene and tellurophene

Excimer laser-induced photolysis of gaseous selenophene and tellurophene affords gaseous 1-buten-3-yne and ethyne (as major products) and butadiyne (a very minor product) and results in chemical vapour deposition of selenium and tellurium films. The film properties were characterized by XPS and SEM techniques and by UV spectroscopy. Copyright

Synthesis of C-Doped TiO2 Nanoparticles by Novel Sol-Gel Polycondensation of Resorcinol with Formaldehyde for Visible-Light Photocatalysis

Nanosized C-doped TiO 2 was prepared by a new method based on resorcinol-formaldehyde (RF) gel formation in situ during hydrolysis of Ti precursor (TTIP, titanium tetraisopropoxide). XRD, DRS, EDX, SEM and HRTEM results confirmed that the carbon in the sample existed as free carbon (C–C), carbonate carbon (–CO 3 ) and doped into lattice of TiO 2 (Ti–C), resulting in red shift of optical adsorption edge and band gap narrowing. The photo activity of the C-doped TiO 2 , tested on 2-chlorophenol, revealed that it was more active under visible light (k vis = 3.50 × 10 −3 min −1 ).

ArF laser photo-polymeric films of acetylene

Low intensity ArF laser irradiation of gaseous acetylene affords films of photopolymers with a completely saturated C/H framework, which differs from reactive unsaturated polyacetylene obtainable by traditional catalytic routes. The films were characterized by FTIR and XP spectra and by electron microscopy.

IR laser degradation of some fluoro-polymers

Abstract The degradation of three fluoro-polymers, poly(chlorotrifluoroethene) Kel-F, poly(vinylidene difluoride) (PVDF) and poly(perfluoro-ethene-propene) (FEP) by CW CO 2 laser irradiation in vacuo results in the formation of small fluorocarbons and solid materials. The latter have been characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Exploratory investigations of the ability of selected materials to adsorb/absorb Br 2 and anhydrous HF and the reaction between a PVDF film and aluminium are reported.

Newly Developed Fe-Fe2O3/Polyoxocarbosilane Core-Shell Nanocomposite Prepared by Laser Pyrolysis: Characterization and Sensing Properties

Nanocomposites of iron /iron oxide/ polyoxocarbosilane were prepared by the IR co-pyrolysis of Fe(CO)5 and HMDSO. Their morphologies, chemical content and thermal behavior were studied by different analytical techniques. For examining the sensing capabilities of the low-polymer content nanocomposite thick films, the variation of the electrical resistance to CO and CH4 gases was tested. A promising behavior as concerning the sensitivity and the selectivity was found.

Laser-induced explosive decomposition of Fluoromethylsilanes - reductive chemistry for gas-phase deposition of Carbon, Silicon Carbide and Si/C/F material

Abstract Polyfluoromethylsilanes constituted of fluorocarbon and silane moieties are expected to be susceptible to heterogeneously assisted thermal decomposition in the gas-phase that occurs both with fluorocarbons [1] and silane [2]. A truly homogeneous decomposition mechanism may be induced by tuning IR laser radiation into C-F and/or Si-H vibrational modes [3]. We report that decomposition of (CF 3 ) 2 SiH 2 (I), CF 3 SiH 3 (II), HCF 2 SiH 3 (III) and H 2 CFSiH 3 (IV) can be induced at 1–50 Torr as an explosive reaction initiated by irradiation with a single pulse from a TEA CO 2 laser. The reaction threshold depends on the pressure of the particular fluoromethylsilane and on the wave- number and delivered fluence of the laser radiation. While gaseous silicon-containing products (SiF 4 and SiHF 3 ) are the same in all decomposition processes, the carbon-containing products differ. Thus, solid C, Si/C/F(H) and SiC materials are formed (and deposited from the gas phase on a cold surface) from I, II and IV, respectively, whereas CH 4 is formed from III. The different products can be accounted for by reductive processes involving transfer of fluorine from C to Si and hydrogen from Si to C, the chemistry being specific for each fluoromethylsilane. Possible mechanisms will be inferred from the products arising from the non-explosive decomposition induced by a great number of low-fluence pulses. Analyses of solid deposits by IR spectroscopy, ESCA and SEM techniques will be used to comment on their electrical conductivity and to assess their potential in microelectronics.