Oleg Bortnovsky

Exchange of Co(II) ions in H-BEA zeolites: identification of aluminum pairs in the zeolite framework

Abstract The Co(II) ion exchange in NH 4 -, HNa- and H-BEA zeolites (Zeolite beta, BEA) was employed to monitor framework Al pairs in these zeolites balancing the charge of the divalent cations. The character and concentration of unperturbed bridging OH groups or those perturbed via hydrogen bonding, and Lewis sites (defective and tricoordinated framework Al) of NH 4 -, HNa- and H-BEA zeolites, and their changes induced by Co(II) ion exchange were investigated by quantitative analysis of IR spectra in the region of OH groups and vibrations of CN groups of adsorbed d 3 -acetonitrile. It is shown that the Co(II) ions in dehydrated BEA zeolites, requiring two AlO 2 − framework groups for their charge balance, are at cationic sites which were occupied in the parent zeolites by Na ions or by mutually interacting protons or by those exhibiting a high tendency to be dehydroxylated with the formation of Lewis sites. A part of the unperturbed bridging OH groups is not exchanged by divalent Co(II) ions, indicating the far distances of the respective framework aluminum atoms. This indicates that the framework local structures bearing close AlO 2 − groups in BEA represent an unstable environment with weakened framework Al–O bonds. Metal cations, both Na(I) and Co(II) ions greatly prefer occupation of cationic sites containing two close framework AlO 2 − groups. Thus metal ion exchange brings about a high stabilization of the BEA framework, and protects the local environment of the aluminum framework against substantial rearrangements leading to the formation of Lewis sites.

Quantitative analysis of aluminium and iron in the framework of zeolites

Aluminosilicates of ZSM-5, mordenite, ferrierite and BEA structures, and ferrisilicates of ZSM-5 and -22 structures in NH4- and dehydrated/deammoniated H-form were investigated in order to determine the concentration of Al and Fe isomorphously substituted silicon in their frameworks. A comparative study has been made of the chemical composition of zeolites, the quantitative analysis of 27Al NMR spectra, the amount of released ammonia and the intensity of N–H IR vibration (at 1445 cm−1) in NH4-zeolites on the one hand, and the quantitative analysis of IR bands of Si–OH–M groups (M=Al or Fe) at 3605–3630 cm−1 of H-zeolites and the IR band at 2297 cm−1 related to the CN groups of adsorbed d3-acetonitrile interacting with the Bronsted sites of H-zeolites on the other hand. It has been shown that the intensity of N–H vibration at 1445 cm−1 of fully exchanged NH4-zeolites with the determined extinction coefficient, represents a quantitative measure of the concentration of NH4+ ions and, accordingly, the concentration of aluminium or iron in the framework positions of the hydrated zeolites. With dehydrated/deammoniated H-zeolites, the intensity of the IR band of the CN groups of adsorbed acetonitrile gives the concentration of the strong acidic bridging Si–OH–M groups. The characteristic band of adsorbed acetonitrile at 2297 cm−1 detects all the Si–OH–M groups, including those interacting via hydrogen bonding, while the IR bands at 3605–3630 cm−1 of Si–OH–M groups reflect only unperturbed OH groups.

On the necessity of a basic revision of the redox properties of H-zeolites

Negligible activity in NO-NO 2 equilibration and SCR of NOx by propane, as well as benzene hydroxylation and N 2 O decomposition reactions was obtained over laboratory synthesized H-MFI, H-FER and H-BEA zeolites containing Fe with concentration below 50 ppm. On the other hand significant activity was found over H-zeolite samples of the same structural type and similar Si/Al values, but with the content of iron impurities of several hundreds of ppm. The consequence of this evidenced role of such low levels of iron content, i.e. at the level usually present in the commercial samples mostly used for the catalytic studies, and not considered in the analysis of the redox function of these systems, is discussed.

New Generation of Geopolymer Composite for Fire-Resistance

The most popular matrix used for fiber-reinforced industrial composites is organic polymer. The nature flammability of the organic polymer matrix (Marsh, 2002), however, limits the use of these materials in ground transportation (Hathaway, 1991), submarine and ships (Demarco, 1991), and commercial aircraft (Davidovits, 1991), where restricted egress of fire hazard is an important design consideration, although traditional fibers, such as carbon and glass fibers or new developed, high temperature, thermal-oxidative stable fibers from boron, silicon carbide and ceramic are inherently fire resistant (Papakonstantinou et al., 2001). In other word, most of organic matrix composites cannot be used in applications that require more than 200 oC of temperature exposure. In these cases of applications, composites based on carbon matrix or ceramic matrices are being exploited. However, use of these materials is even strongly limited, due to high cost accompany with special and high-thermal processing requirements (Papakonstantinou et al., 2001; Papakonstantinou & Balaguru 2005). In 1978, Joseph Davidovits proposed that binders could be produced by a polymeric reaction of alkaline liquids with the silicon and the aluminum in source materials of geological origin or by-product materials such as fly ash and rice husk ash (Davidovits, 1999). These binders have been coined as term geopolymers since 1979; they are inorganic polymeric materials with a chemical composition similar to zeolites but without defined crystalline structure and possessing ceramic-like features in their structures and properties. The amorphous to semi-crystalline three dimensional of sialate network consists of tetrahedral SiO4 and AlO4 which are linked alternately by sharing all the oxygens to create polymeric Si-O-Al bonds (Davidovits & Sawyer 1985; Davidovits, 1991). Geopolymers are still considered as a new material for coatings and adhesives, a new binder for fiber composites, and a new cement for concrete (Davidovits, 2008). They are mineral polymers and the essence of all mineral polymers is never burn (Davidovits, 2008). Therefore, we can state that geopolymer materials are ideal for high temperature and fire applications. Fiber-reinforced composites based on geopolymer matrix (geocomposite) have been wellknown for over 20 years, since the first Davidovits’ patent was filed (Davidovits et al., 1989).

Square root relationship in growth kinetics of silicalite-1 membranes

Conceivable limiting models of growth kinetics of polycrystalline layers involving diffusion of low molecular silicon containing species, Brownian motion of nanoparticles and their sedimentation are analyzed from the point of view of their application to the preparation of zeolite-based membranes. The activation energies derived using these models were evaluated and the effect of support orientation was quantified. A criterion of a relative importance of colloidal particle sedimentation with respect to the Brownian motion was formulated.