X-Ray amorphous zeolite

Figure 1. High-resolution electron micrograph and corresponding optical transform (inset) of an x-ray amorphous zeolite-Y specimen that has undergone ion-exchange with a solution containing U022+ ions. The microcrystalline regions are rendered visible by the locally ordered U022+ ions. ( See text.)...

Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore RWan has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures [28], Ex situ methods (with separation of solid and liquid) and in situ methods have been applied. In studying the liquid phase [10-11], one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typic ly 0.05 - 0.1 M, [15-27, 29] (ii) that the cross-section of the Al(OH)4 species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. 

Samples of zeolite beta varying in XRD crystallinity were prepared and used to catalyze the reaction of methanol and isobutene to form MTBE (methyl /cr/-bulyl ether). Considerable high catalytic activity and selectivity to MTBE were maintained over partially crystalline zeolite beta. The maximum conversion of isobutene over 30% crystalline sample was only slightly lower than that of fully crystalline one, but the selectivity to MTBE was much higher. It was evidenced further by TPDT method that the strong interaction between template and aluminosilicate had occurred at the early stage of crystallization, the presence of X-ray amorphous zeolitic species, the smaller crystallite and lower Si/Al ratio may account for the observed catalytic characteristics. 

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

Preparation of X-ray amorphous ZSM-5 crystallites according to procedure BT It is important that the gel formation takes place as homogeneously as possible. Because of the particular sensitivity of various silica and alumina species to the pH (63,64), the pH range between 4.5 and 8.5 was avoided. Nucleation was performed at pH 3.5-4, where a low viscous gel containing essentially monomeric silica species is rapidly formed (65).The, pH is theii raised to about 9, in order to form tetrahedral A1(0H) entities and to favour the further A1 incorporation within the zeolitic framework. NaCl was used to increase the (super)saturation of the gel, which will flocculate into a macromolecular colloid (V) and initiate the nucleation. This procedure yields 100 % crystalline zeolite after... 

Thus, in many syntheses the gel ageing (8-11) or the addition of the "crystal direction agent" (aged, X-ray amorphous aluminosilicate gel) (7,12-14) is a necessary step needed for the obtaining of the desired type of zeolite at the desired reaction rate. 

A second finding is that crystallinity loss is not due to extensive zeolite dissolution nor to simple collapse to a dense, amorphous material In no case was more than 0.2 wt % of a zeolite dissoived, and in general, even X-ray amorphous products retained significant surface areas ... 

The results of stability teats made with pH B and 10 NH4NO3 solutions are given in Table IV It should firat be noted that because the pH 6 solutiona had very little buffering capacity ([NHj]/[NH +J Q.03B), the zeolite slurries actually became acidic over the two-hour treatment period. Hone of the products were thus affected by the treatments except for the highly siliceous cbV-760, which became X-ray amorphous and lost 40% of its surface area in even this near-neutral tnedium. 

Molodetsky, 1, Navrotsky A, Lajavardi M, Brune A (1998) The energetics of cubic zirconia from solution calorimetry of yttria- and calcia-stabilized zirconia. Z Physik Chem 207 59-65 Molodetsky 1, Navrotsky A, Paskowitz MJ, Leppert VJ, Risbud SH (2000) Energetics of X-ray-amorphous zirconia and the role of surface energy in its formation. J Non-Crystalline Solids 262 106-113 Moloy EC, Davila LP, Shackelford JF, Navrotsky A (2001) High-silica zeolites a relationship between energetics and internal surface area. Microporous Mesoporous Materials (submitted)... 

Three groups of materials can be distinguished (a) non-crystalline (X-ray amorphous) (b) zeolite type and (c) crystalline, non zeolite type. 

It has been shown that pulsed UV-lasers can be used to ablate certain fragments from a zeolite target these fragments can be deposited on a substrate and subjected to a subsequent hydrothermal treatment resulting in crystalline zeolite layers. The initially deposited layers appear to be X-ray amorphous, although they can still be used to nucleate crystals of the original zeolite target material. [73,74,75]... 

Inspection of the calculated Si/Al ratios for most of the fresh materials show that they are in reasonably good agreement with chemical analyses data (Table III). Furthermore, aluminum-27 MAS NMR spectra show only a single signal at about 52 ppm corresponding to the presence of lattice Al(IV) atoms. Evidence for the presence of possible impurities such as NaAlO- (normally seen at <11 ppm), dehydroxylated A1(OH)3 or x-ray amorphous gel could not be found in any of the spectra examined. Thus, all the aluminum atoms are probably in tetrahedral coordination and directly incorporated in the zeolite lattice framework only minor amounts of Si-OH defects (26-28) exist in these TOA containing crystals. In the case of sample 4, (Na.TOA) mordenite, the calculated Si/Al ratio is 5.6 if... 

Solid phase (X-ray amorphous aluminosilicate) separated from the hydrogel having the batch composition 3.23 Na20 Al203-1.93 Si02-128 H2O, was calcined at different temperatures (100-800 C) for Ih and then transformed into zeolite A by heating in a 2M NaOH solution at SO C. Analysis of both precursors and products (zeolite A) has shown that the calcination of the precusor lowers the concentration of nuclei in the gel matrix and hence influences the particulate properties of the zeolite obtained during the hydrothermal treatment of the precursors... 

A rather complex mixture is that constituted by the Italian tuffs [8, 53, 54], which contain various hydrated phases, namely three zeolites (phillipsite, chabazite and analcime, Table 1), unreacted glass (pumice, glass fragments, scoriae), hydrated ferric oxides and an X ray amorphous gel-like alumino-silicate, in addition to some non-hydrated phases, such as sanidine and biotite crystals. The most concentrated phases are phillipsite and chabazite, the total content of which usually amounts to 50% or more. Since Italian tuffs, simply because of the elevated contents of the above zeolites, are gaining a pre-eminent position in many industrial, agricultural and environmental applications, it is of great interest to have a rapid and reliable method available for evaluating zeolite content in the rock. 

Microporous materials possess pore structures in which the major fraction of the total void colxjune is contained in the pore size region below 20 A. The structures of microporous materials can span the entire range from X-ray amorphous compounds (e.g., small pore oxides xero-gels) and highly disordered materials (e.g., carbon molecular sieves) to quasi-crystalline compounds (e.g., pillared clays) and regular, 3-dimensional crystalline structures (e.g., zeolites). 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

Crystallization was followed by analyzing the solid product quantitatively by x-ray powder diffraction. Prepared mixtures of a standard sample of mordenite and the amorphous substrate of mordenite composition were used to establish a calibration curve for the quantity of mordenite based on the summation of x-ray peak intensities. For zeolites A and X, the unreacted aluminosilicate gel was used to prepare mixtures with standard samples of zeolites A and X for quantitative phase identification. 

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