Determination in zeolites

The next problem to investigate was whether the cationic positions could also be determined in zeolite A and faujasites by semiempirical methods. From this aspect the MNDO method is the best choice, since most of alkali and alkaline earth elements are parameterized within this approximation. 

Zeolites modified with transition metals were used as catalysts under microwave irradiation [183]. Two main dielectric mechanisms of microwave heating were determined in zeolites rotational polarization phenomenon and interfacial polarization. The selective catalytic oxidation of styrene with resulted in benzaldehyde formation. 

The more permeable component is called the. st ga.s, so it is the one enriched in the permeate stream. Permeability through polymers is the product of solubihty and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 22-23 after Breck, Zeolite Molecular Sieves, Wiley, NY, 1974, p. 636). 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

The reported surface area is the combined surface area of zeolite and matrix. In zeolite manufacturing, the measurement of the zeolite surface area is one of the procedures used by catalyst suppliers to control quality. The surface area is commonly determined by the amount of nitrogen adsorbed by the catalyst. 

For adsorption in zeolites, the biased Monte Carlo method as developed by Smit is an excellent method to determine the free energies of molecules adsorbed on zeolites [9bj. This method can be used to compute the concentration of molecules adsorbed on zeolites, as we discuss below. 

Hat matrix and leverages in charge density refinements example of atomic net charges determination in a natural zeolite, the scolecite... 

Determination of Si-Al connectivities in zeolites with 2D Al-Si RAPT CPMAS CPMG HETCOR NMR techniques. 

Determination of zeolite closed porosity in (ID) channel systems (AFI and MTW types ). 

The optimal activation temperature of zeolites in the range 300-600°C was determined in preliminary experiments. The temperature 500°C was selected for all catalysts but for H-MFI-28 (400°C). DSC and TG analyses show that at 500°C, NH4+ forms of zeolites are totally converted into H-MOR-20 and H-FER-20, respectively. Table 2 shows alcohol conversion (XR0H), selectivity to ether (SROr), to olefin (Soiefm) and branched ethers (SrOR>), and ether yield (YROr) at 180°C after 6 h reaction course. 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

Blatter, F., Moreau, F. and Frei, H. (1994). Diffuse reflectance spectroscopy of visible alkene-02 charge transfer absorptions in zeolite Y and determination of photooxygenation quantum efficiencies. J. Phys. Chem, 98(50), 13403-13407... 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

Finally, new developments in zeolite catalyst and adsorbent manufacture will be outlined in the form of a survey of recent open and patent literature. Patents are extensively cited as reference materials for this chapter however no effort is made to identify specific manufacturing techniques actually used by any particular company to manufacture zeolites or any catalytic or adsorbent products. This is because there is no way to determine whether manufacturing patents are practiced as is or whether further refinements have been accomplished subsequent to the filing of the patents that may be held by specific companies as trade secrets. 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

The choice of probe molecule for acidity determination is largely dictated by the sensitivity of the nuclei, and the high sensitivity of P nuclei has been exploited to quantify acidity in zeolites. The use of phosphorus containing bases as probe molecules to measure solid acidity circumvents insensitivity problems associated with (1.1% abundance) and N (0.4% abundance) in bases such as ammonia. 

Loss of crystalline structure as a function of temperature can also be measured ex situ by performing the heat treatment and then determining the change in zeolite content by X-ray diffraction. Such an analysis is shown in Figure 4.43 for... 

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