Forming, zeolite powders

Forming Zeolite Powders into Usable Shapes... 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

Some zeolites such as AIPO4-5 (API) have a negative coefficient of thermal expansion over certain temperature ranges [230, 231]. For a zeolite formed into a pellet, one method to verify the presence of a negative coefficient of thermal expansion is by thermal mechanical analysis (TMA). For just the zeolite, powder X-ray diffraction at various temperatures can be used. Such an analysis can be of importance for identifying pellet strength or vessel containment issues. 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

Pulsed-field-gradient NMR (291-292), in which spin echoes are measured in the presence of a time-dependent magnetic field gradient, has been used to determine effective diffusion coefficients, Deff, in beds of zeolite powder. Barrer (35) quotes the expression for the spin-echo amplitude given by Karger (259) in the form ... 

H. Robson Palladium was deposited on the zeolite powder by exchange with Pd(NH3)4Cl2 solution. We did not observe significant differences in product distribution between cationic forms of erionite, but of course we were looking primarily for the disappearance of n-pentane and n-hexane. 

For the IR (Perkin-Elmer 1730 FTIR) spectroscopic studies self-supported wafers were pressed from the zeolite powder with a thickness of 15 mg/cm . The wafers were placed into a sample holder inside a pyrex glass cell with KBr windows, which allowed pretreatment (773 K, 1 hr, vacuum <10 Torr), introduction of probes, after cooling to the desired temperature, and registration of the spectra at 295 K. Three types of IR experiments were run (i) stepwise loading (0.1-6 Torr) the wafers with adsorbates at 295 K, (ii) generation of surface species with increasing temperature in the presence of adsorbate (closed cell), and (iii) loading adsorbate at 453 K followed by detection of surface species formed with time (1-240 min) at the same temperature. After each set of IR measurement the same sample was analyzed by UV-VIS spectroscopy. 

The reaction was carried out in a stainless-steel fixed-bed flow reactor. The zeolite powder was pressed to wafers which were subsequently crushed and sieved. Prior to reaction, the catalyst (0.4 - 0.6 mm particles) was converted in situ under vacuum to the hydrogen form by heating at a rate of 5 K/min to 675 K and then holding at 675 K for 2 hours. 

Ferrierite zeolite samples in their acidic form were prepared in-house or obtained from a commercial supplier. The properties of the base materials are summarized in Table 1. For testing purposes, the zeolite powders were shaped into small particles... 

Extruded silicalite zeolite honeycombs were also prepared by well known procedures (9,10). Silicalite zeolite powder (Union Carbide Corp. S-115) was extruded with a boehmite alumina (Vista Chemical Catapal D) which forms a gamma alumina binder phase after heat treatment. The final composition is 84% silicalite + 16% gamma alumina. These extruded honeycomb samples were ion-exchanged and evaluated in the same way as the washcoated ones. 

Only in very rare cases can zeolites be used direetly in the form in which they were originally synthesized. For many larger-seale industrial applications, for instance, the synthetically obtained zeolite powders must be formed into larger attrition and crush-resistant partieles using inorganic binder materials [33]. In most cases a thermal treatment in air (calcination) is at least required to decompose the organic template, to dehydrate the zeolite and to desorb impurities [35]. This holds particularly true if the proton form of a zeolite is desired from the ammonium form for aeid eatalysis. 

Zeolites are crystalline aluminosiUcates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al, 2006 Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). 

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