Zeolites phases

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

As the gel progressively dissolves to yield Si-richer zeolitic phases, the EDX and PIGE Si/Al ratios become closer. However, even for a 100 % crystalline phase, less A1 is still probed by EDX. This suggests that the A-synthesis ZSM-5 crystals must still contain some Al-rich amorphous phase, deeply embedded within the large particles, as to partially escape the EDX probing. For the same reason, the XPS also continuously probes the outer Si-rich layer of the growing particles. However, at the end of the crystallization, more A1 is detected on the outer rim than in the... 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Small-pore zeolite Nu-6(2) has a NSI-type structure and two different types of eight-membered-ring channels with limiting dimensions of 2.4 and 3.2 A [54]. Gorgojo and coworkers developed mixed-matrix membranes using Nu-6(2) as the dispersed zeolite phase and polysulfone Udel as the continuous organic polymer phase [55]. These mixed-matrix membranes showed remarkably enhanced H2/ CH4 selectivity compared to the bare polysulfone membrane. The H2/CH4 selectivity increased from 13 for the bare polysulfone membrane to 398 for the Nu-6(2)/ polysulfone mixed-matrix membranes. This superior performance of the Nu-6(2)/ polysulfone mixed-matrix membranes is attributed to the molecular sieving role played by the selected Nu-6(2) zeoHte phase in the membranes. 

Another difficulty presented by precipitation of secondary phases is that other aqueous species, such as those of Al, can be affected, as stated above. Under certain conditions the activity of Al(OH)4 in mildly alkaline solutions can drop to near zero due to precipitation of zeolite phases. The question is how to represent this change in aluminate activity using a rate equation based on chemical affinity concepts. For example, in equation (1), the activity of the... 

Let us now combine the zeolite phases related in Figures 32a-c with those phyllosilicates commonly found in the larger K-Na-Al-Si system. Representation of the clay minerals associated with zeolites can be made... 

Barrer and Mainwaring (20) report the use of metakaolin as the aluminosilicate raw material for reaction with the hydroxides of K and Ba as well as the binary base systems Ba-K and Ba-TMA to form zeolites. Zeolite phases previously synthesized in the analogous hydrous aluminosilicate gel systems were crystallized with KOH, including phillipsite-, chabazite-, K-F-, and L-type structures. The barium system yielded two unidentified zeolite phases (Ba-T and Ba-N) and a species Ba-G,L with a structural resemblance to Linde zeolite L. Ba-G,L was reported previously by Barrer and Marshall (21) as Ba-G. Similar phases were formed in the Ba-K system and in the TMA-Ba system where, in addition, erionite-type phases were formed. The L-type structures are said to represent aluminous analogs of the zeolite L previously reported (22). 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Information published during thepast few years about the faujasite class of zeolites indicated that they present a possibly unique system in which the necessary conditions might be met. Sherry (4, 5) reported that rare earths, as compared with alkali or alkaline earth metals, are readily exchanged into Linde X from dilute aqueous solutions, and that they strongly favor the zeolite phase. When such an exchanged zeolite is dehydrated by heating to 350-700° C, the lanthanide ions move into the small pore system (6>, 7) after which they are not readily exchanged back out of the crystal. Smith (8) has reviewed the structure of lanthanide X and Y zeolites. 

Products were recovered by batch elution, using a variety of salt solutions for eluents and a large liquid-to-zeolite phase ratio (within the range 10 to 200). In a few cases, the exchanged zeolites were eluted without prior heat treatment to determine the behavior of the metal before it was fixed in the small pore structure. In most cases, the zeolite was ignited at 500-700°C to fix the metal before subsequent manipulations, including irradiation. 

Kd is the desorption coefficient for product D. The first-order desorption term should be strictly Kd(Cdp — H Cdg), allowing for an equilibrium backpressure, where H is an equilibrium adsorption constant relating mole fractions in the gas and zeolite phases. H Cdg was shown empirically to be small compared with Cdp under our conditions. 

In equation 3 the terms of fNa+ and 7H + are the rational activity coefficients of exchanging cations in the zeolite phase and the terms yNa+ and XM + are the molal single ion activity coefficients in the solution phase. Equation 4 can be rewritten as equation 5 when the two salts, NaX and MX2 have a common anion. The mean molal activity coefficients usually can be estimated from literature data. The corrected selectivity coefficient includes a term that corrects for the non-ideality of the solution phase. Thus any variation in the corrected selectivity coefficient is due to non-ideality in the zeolite phase (see equation 3). 

Finally, a note of caution. The patterns are useful in helping to establish the structural purity of a zeolite phase, yet they may not always allow one to readily and unambiguously determine the framework type of the sample. This assignment is often not straightforward and may require more sophisticated analyses. W. J. Rohrbaugh and E. W. Wu review the factors affecting the diffraction characteristics of zeolite materials (ACS Symposium Series 411 279-302 (1989)). 

Table 3.6 shows that zeolites Na-X and Na-Y have been produced depending on the reaction time. These materials are similar in phase composition to commercial Na-X and Na-Y zeolites, because, are composed of about 80 wt % of the zeolitic phase. In commercial zeolites because part of the phases are binders [124], the amount of zeolite is about 80 wt %. Applications of these type of materials are described elsewhere [11,25,52,125,126],... 

In the case of mixtures, Gonzales et al. [30,31,39,40] have applied the methodology to minerals, for example, laterites [40] and zeolites [44], In Table 4.1, [44] the results of the phase analysis of several natural zeolites are reported. In Table 4.2, the elemental compositions, in oxide wt %, of the same natural zeolite rocks are reported [44], To calculate the absolute quantity of the zeolite phase in the samples used as standards, it was necessary to use the adsorption method (see Chapter 6). 

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