Mordenite crystallization

Fig. 3. Model of the crystal structure of the mineral mordenite showing the main channel formed by 12-membered ring and small channels which contain some of the sodium cations. Synthetic types of mordenite exhibit the adsorption behavior of a 12-membered ring, whereas the mineral does not, probably...

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 all have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. 

This may be partly the result of increased steric crowding in the transition state of transalkylation. Another contributory factor to the increased selectivity in ZSM-5 is the higher diffusion rate of ethylbenzene vs m-/o-xylene in ZSM-5 and hence a higher steady state concentration ratio [EB]/[xyl] in the zeolite interior than in the outside phase. Diffusional restriction for xylenes vs ethylbenzene may also be indicated by the better selectivity of synthetic mordenite vs ZSM-4, since the former had a larger crystal size. 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

A previous examination of a synthetic calcium mordenite 15) revealed an orthorhombic cell. A synthetic strontium mordenite 16) had a C-centered orthorhombic cell although Kerr 12) reported that a few crystals giving electron diffraction patterns corresponding approximately to the body-centered structure Immm) have been synthesized hydro-thermally from aluminosilicate gels containing strontium similar to those gels which yielded a strontium-mordenite. ... 

Many mineral and synthetic samples examined by Bennett and Gard 17,18) gave typical C-centered orthorhombic diffraction patterns and with few exceptions had streaks in the hOl section, indicating an incomplete c-glide plane. A few crystals gave electron diffraction patterns having diffuse maxima in the hOl streaks, which could be interpreted as representing an I-centered mordenite structure. 

Sr mordenite synthesized by Barrer and Marshall (16) gave the Cmcm x-ray pattern and was C-centered orthorhombic by electron diffraction. However, other electron diffraction studies, also by Kerr (12), revealed some synthetic Sr mordenite crystals of the Immm structure prepared at similar synthesis conditions. Future work should examine synthetic Sr and Ca mordenite specimens further for the possible presence of other framework structures, and correlation of structures with Sr2+/Ca2+ ion exchange selectivity. 

Mordenite minerals vary in the ease with which they may be converted to the large port type by chemical treatment (20). The presence of Cmmm as in the Rome, Ore., mordenite should enhance the ease of the chemical conversion by providing greater access to the internal regions of the crystals. Sand (22, 28) reported that mordenite from Rome, Ore., can be easily converted to the large port variety by chemical treatment. Thus the... 

In zeolite systems chosen for study diffusion in the liquid phase and crystal growth on the crystal-liquid interface were the two major steps in converting gels to mordenite, zeolites A and X, the former being the rate-determining step for mordenite and the latter for zeolite X crystallization. In the mordenite system the effect of seed crystals, with surface areas per unit mass different by an order of magnitude, demonstrated the mechanism of nucleation on the seed crystal surfaces. The data support the hypothesis that crystal growth of the zeolite occurs from the solution phase rather than in the gel phase. 

Figure 1. Crystallization curves of mordenite from a batch composition 8.5 N avO-AWz-85 SiD2 182 H20 as a function of temperature and NaCl content filled symbols, no NaCl open symbols, 4.5 moles of NaCl/mole Al2Oz)...

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. 

Figure 2. Mordenite single crystals of uniform morphology (3 X 3X8 fimeters) used as seed. Crystaiized from a batch com--position of 8.5 NazO-AkO -SS SiO -m H2O-4.5 NaCl at 120°C and 9 days.

The fast conversion rate of amorphous batch into mordenite, once the crystallization has started, indicates that the rate-limiting step in the overall process is the nucleation. To substantiate this, mordenite was crystallized with the same batch composition but with the addition of seed... 

Figure 8. The effect of seeding on crystallization rates of mordenite from a hatch composition of 8.6 NazO-AWr-85 SiOz-182 HzO 4-5 NaCl as a function of temperature (8X2X8 imeter seed crystals)...

Figure 5. Acicular mordenite crystals grown at 200°C and 16 hours using the single-crystal seeds shown in Figure 2. (scanning electron micrographs courtesy of AMR Corp. Burlington, Mass.)...

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