NaA zeolite

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

Figure 10.2 NaA zeolite membrane (a) cross-section and (b) top view [11]. (Reprinted with permission of Elsevier.)...

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

It was postulated that organosilver radicals were formed by reaction of CH2OH radicals with Ag+ cations. In order to solve a problem of organosilver radicals structure we carried out the experiments with 109Agi-NaA zeolite exposed to 13CH3OH. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

Another type of mixed-matrix membranes for alcohol/water pervaporation applications was developed utilizing hydrophiUc poly(vinyl alcohol) (PVA) and ZSM-5. The ZSM-5/PVA mixed-matrix membranes demonstrated increased selectivity and flux, compared to pure PVA, for the water/isopropyl alcohol separation [97]. This type of mixed-matrix membranes, however, may have membrane swelling issue due to the hydrophilic nature of the PVA polymer. Mixed-matrix membranes comprising modifled poly(vinyl chloride) and NaA zeolite have shown both enhanced flux and selectivity for the ethanol/water separation at high NaA loadings [98]. 

The present study deals with the application of RDF to the sodium aluminosilicate hydrogels, which were formed by mixing, aging and heating of sodium silicate solution with sodium aluminate solution under stirring, in order to produce NaA-zeolite crystal. 

Ag-NaA zeolites Silver clusters Silver clusters, formed in X-ray irradiated samples at 77 K and annealed at 280 K, were characterized by electron spin resonance spectroscopy 556... 

M. Kondo, M. Komori, H. KitaandK.-I. Okamoto, Tubular-type Pervaporation Module with NaA Zeolite Membrane, J. Membr. Sci. 133, 133 (1997). 

Minachev et al. [76] studied oxidative dehydrogenation of cyclohexane on zeolite cationic forms at 300-475 °C, the main reaction product of which is cyclohexene. Cyclohexadiene and C02 are also formed, and at long-term contacts benzene is detected. Cyclohexene yield and selectivity of the reaction depend on zeolite structure and composition, reaction temperature and oxygen cyclohexane ratio in the reaction mixture. Among alkaline cationic forms of zeolite, the highest cyclohexene yield (21%) is observed for NaA zeolite (66% selectivity). 

For NaA zeolite the composition was found to be nearly unity across the crystal. For Na-ZSM-5, three types of chemical profiles were found. Small particles of Si Al=10 showed either enrichment of Si near the surface or a nearly homogeneous composition. 

This paper describes chemical analyses at points across individual zeolite crystals in the size range 0.1-2.0pm. The technique employed was x-ray emission spectroscopy in the scanning transmission electron microscope (STEM). Two ZSM-5 preparations were made with Si Al ratios about 10 and 40. Many particles were examined carefully to detect chemical segregation. To check the analysis procedure, particles of NaA zeolite were examined as a control. 

Crystals of NaA zeolite showed strong cubic habits typical of these structures (Figure 3). The surface morphology of the 0.3pm crystals of ZSM-5(9) was usually irregular (Figure 4) and seldom exhibited crystal facets. The 2pm crystals of ZSM-5(40) showed facets typical of ZSM-5 of moderate aluminum content (Figure 5). 

Chemical Profiles Across Particles It is well known that NaA zeolite has a Si Al ratio close to unity because it is considered a stoichiometric compound with little, if any, range in solid solubility. Since the chemical profile across the NaA zeolite crystal in Figure 6 is flat and near unity, this profile is in effect a control specimen for the analysis of the ZSM-5 particles. 

Figure 2. Evidence of beam damage in NaA zeolite at high beam currents. Mass loss is evident at places where a high intensity electron beam was stationary for several minutes.

Figure 3. Surface scanning electron microscope (SEM) image of NaA zeolite crystals showing a cubic habit.

Figure 6. Chemical profile across a particle of NaA zeolite.

ENHANCEMENT OF IR ABSORPTION OF CO ADSORBED ON PALLADIUM NANOPARTICLES PREPARED BY SHIP-IN-A-BOTTLE IN SUPERCAGES OF NaA ZEOLITE... 

Palladium nanoparticles (nm-Pd) were synthesized by ship-in-a-bottle technique in supercages of NaA zeolite. The behaviors of electrodes of thin film of nm-Pd accommodated in NaA zeolite were characterized by cyclic voltammetry. The results illustrated that the nm-Pd possess particular properties for hydrogen reaction, i.e. in contrast to hydrogen absorption on massive palladium electrode, the surface processes of hydrogen adsorption-desorption become the dominant reaction on electrodes of thin film of nm-Pd. The processes of adsorption and desorption of carbon monoxide on the electrodes were studied using in situ electrochemical FTIR reflection spectroscopy. It has been revealed that in comparison with CO adsorbed on a massive Pd electrode, the IR absorption of CO adsorbed on nm-Pd particles accommodated in NaA zeolite has been enhanced to about 36 times. 

A value of 36 has been calculated using equation (2). The variation of the center of the COb band (vb) versus E for massive Pd electrode and Pd Ec-NaA/GC is plotted in Figure 3a and 3b. Two straight lines can be observed in the case of Pd°Ec-NaA/GC. One is for E below -0.6 V, which yields a Stark shift rate (dva / dE) of 52 cm v. The second linear part is observed in the potential range between -0.6 V to -0.2 V, from which a Stark shift rate of 16 cm v has been evaluated. In comparison with the value of Stark shift rate of 47 cm v on massive Pd electrode, the small values of Stark shift rate in Figure 3 a may be attributed to the structure of Pd nanoparticles and geology in NaA zeolite. 

In the present study, Pd nanoparticles were synthesized in the supercages of NaA zeolite. Novel IR optical properties of COad adsorbed on electrodes of Pd Ec-NaA/GC were revealed for the first time. In comparison with CO adsorbed on a... 

Figure 16 Process for the production of NaA zeolite. (Reproduced from E. Roland, Zeolites as Catalysts, Sorbents and Detergent Builders , eds. H.G. Karge and J. Weitkamp, 1989, p. 650 by permission of E. Roland)...

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