Mordenite, large crystals

The silica/alumina mole ratios of the zeolites studied were as follows ZSM-5, 62 large crystal ZSM-5, 75 high silica ZSM-5, 1670 ZSM-11, 78 and dealuminized mordenite, 61. Aluminum analysis was by atomic absorption spectroscopy. Zeolite crystallite sizes were generally less than O.ly, except for the large crystal ZSM-5 sample which was larger than in. 

Most of the reports on the syntheses of mordenites were involved with an increase of silica to alumina ratio [91K4, 93S1,02S2], or in the production of very large crystals of mordenites (7 to 500 pm) [95S1, 96W1, 98G2, OlSl]. 

Alkylation of chlorobenzene with methanol over ZSM-5 at 523 K gives a mixture of 0- and -chlorotoluene, the amount of the meta isomer being less than a few percent. The selectivity depends on the crystal size of the zeolite, high para selectivity (90%) being obtained over large crystals (220 /tm). Dealuminated mordenite gave a selectivity of about 40% for p-chlorobenzene. 

Three dimensional surface pores of small crystals of zeolites A, X and Y can be observed in the SEM images, as depicted by black spots in Fig. 2.6b, e, f [3, 8], whereas one dimensional large crystals can be seen in the zeolites, Mordenite, as... 

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. 

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... 

Figure 4 presents the isothermal phase transformation diagram of the template-free syntheses in which the SiCL/Alo ratio and the time t of crytallization are varied. The Siof/Nafcr and l O/SiCL ratios are 10 and 30, respectively. The pentasil phase could only be synthesized for n = SiC /A O, =30-50 and t = 36 - 72 h. Outside of this area amorphous material, mordenite, sheet structures similar to kenyaite, quartz and crystobalite can be found. For values of n less than 25 the crytalline product is mordenite. For 30 < n < 50 a yield of 95% (related to the SiC content) ZSM-5 type, which was proved by X-ray diffraction pattern, could be found. Depending on n and the crystallization time, t, a more or less large amount of amorphous material is produced. This is shown in Figure 5. A long crystallization time causes recrystallization and is harmful to the yield of ZSM-5 products.

Many more stories can be told in this regard. The chaimel structure of natural mordenite was not foreseen on the basis of sorption experiments because a relatively small number of blockages or dislocations quite adequately blocked the large charmels. In an experiment in my laboratory, sodium LTA failed completely to sorb tetrachloroethylene (both fully anhydrous) because of its size (The crystal structure of a single crystal in an atmosphere of ca 100 torr of C2CI4 for more than a week was exactly that of fully dehydrated LTA.) we should have honored a simple mechanical calculation of size apparently no alternative mechanism existed. In contrast, statements that Cs ions should not be able to enter sodalite cavities in LTA or FAU, have repeatedly been shown crystallographically to be incorrect such entry of Cs" " appears to occur both in the presence and absence of water. 

Mordenite zeolites are widely used as commercial acid catalysts. Two types of mordenites can be synthesized small port or large port. The channel system for the large port mordenite is two dimensional with a large channel with dimensions of 0.67 x 0.70 nm and a perpendicular small pore with dimensions of 0.29 x 0.57 nm separated by 0.28 nm restrictions (162). The channel system for the small-port mordenite is one dimensional for the diffusion of large molecules owing to diffusion blocks produced by crystal stacking faults or the presence of amorphous material or cations in the channels. 

Electron diffraction patterns indicate that heavily and weakly faulted mordenite crystals were obtained weakly faulted mordenites patterns resemble those of commercially-available large port mordenites. The absence of odd order spots or streaks in the hOO rows of the ZSM-11 diffraction pattern has been taken as evidence of the presence of ZSM-11 crystals essentially free from ZSM-5 intergrowths. The (Na,K,T0A)ZSM-5 crystal morphology is different from that of typical (Na,TPA)ZSM-5 the relative lengths of straight and tortuous channels are interchanged (J ). 

In the present study, by use of a combination of thermal and acid treatments, a series of large-port mordenites has been prepared down to a material with essentially no residual aluminum but which retains the mordenite crystal structure as determined by x-ray powder diffraction... 

The selective alkylation of biphenyl and naphthalene is also possible, and is of commercial interest because 4,4 -diisopropylbiphenyl and 2,6-diisopropyl-naphthalene are used in the synthesis of speciality chemicals, for example for application as liquid crystal polymer materials for displays. For these larger molecules, large-pore zeolites such as mordenite give the best performance for isopropylation of naphthalene, introducing shape selectivity to linear (or more linear) dialkylated products over dialkylated products. 

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