Siliceous mordenite crystals

Aluminum-27 NMR spectra show that after crystallization, all the TOA-containing zeolites exhibit a well resolved resonance at <50 ppm, corresponding to framework Al(IV) atoms. However, following NH.- exchange and calcination in air at 500°C, a new band appears at abdut 0 ppm due to Al(VI) resulting from aluminum removed from the crystal lattice. In general, calculated Si/Al ratio from silicon-29 NMR data are in reasonable agreement with chemical analysis results. Thus, all the aluminum atoms in these siliceous mordenite and Al-rich pentasils are believed to be in tetrahedral coordination and incorporated into the zeolite lattice. 

Fig. 14. Mordenite crystals prepared from mixtures of composition 100 Si02 5.26Al203 22.74 Na20 1545 H2O at 175 °C. Pre-treatment of the silicic acid (silica source) in air for 20 h at a un-treated b 300 °C c 550 °C d 850 °C. Reprinted with permission from Zeolites, vol. 16, Warzywoda J, Dixon AG, Thompson RW, Sacco A, Suib L, The role of the dissolution of silicic acid powders in aluminosilicate synthesis mixtures in the crystallization of large mordenite crystals, (1996),pp. 125-37, Elsevier Science Inc.

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). 

Mordenite and ZSM-5 are synthesized at lower pH values, and it is not surprising that in these conditions silicate species are not usually observed in solution [27,29]. In both cases, the solid initially resembles vitreous silica. In the case of ZSM-5, new vibrations in the solid phase are only observed when ZSM-5 crystals appear [27]. More information concerning the intermediate synthesis steps can be retrieved from mordenite synthesis spectra [29]. In mordenite synthesis, a 495 cm band is observed at an early stage in the spectrum of the solid fraction of the gel. In analogy to what is observed for A and X zeolites, this band is ascribed to 4MR aluminosilicate units. Only at a later stage, broader bands appear at 402 and 465 cm, indicative of the formation of still rather... 

Spectra of chemisorbed pyridine show, as expected, bands characteristic of Bronsted (B, at 1542 cm ) and of Lewis (L, at 1453 cm ) acid. Ghos h, and Curthoys (13) reported an additional Lewis band at 1462 cm in dealuminatetf lnordenites degassed above 400°C this band was not observed in any of the siliceous H-mordenites (containing Al(VI)) examined in this study, lending some support to the correlation between the 1462 cm band and the existence of hydroxyl nests in the crystal lattice proposed by these authors (13). 

As we concluded above, the activation energy of crystallization corresponds to that of crystal growth. The activation energies of different zeolites are close. For example, for mordenite and zeolite A, the calculated values mentioned above are 10 to 11 kcal/mole i.e., activation energy is equivalent to the energy of 2 hydrogen bonds. It can be connected with the necessity of dehydration of the silicate... 

TABLE 11-2 Phase components in dependence on the concentration of PDDA-Cl (P40) and on the crystallisation time. The crystallization temperature is 175 C. The composition of the reaction mixture is 4 Na O 1 AljOj 40 SiOj 1200 H2O x PDDA-Cl. The phase component were denoted as a amorphous, MFl zeolite MFI, Mo mordenite, SH-P40 novel layer silicate, Ke kenyaite-like silicate, Cr cristobalite, and Q quartz. 

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