Mordenite Subject

Many books, reviews and treatises have been pubUshed on related subjects [1-7]. Thus the objective of this chapter is the deUneation of the key features of the catalytic surface and the process conditions which enable better control of the reaction pathways for more efficient and environmentally friendly processes and minimal utiHzation of precious natural resources. As it stands today, hundreds of known framework types have been synthesized and scaled-up [8], but only a handful have found significant application in the hydrocarbon processing industries. They are zeolite Y and its many variants, ZSM-5, Mordenite and zeohte Beta. Other very important crystalline materials (including aluminophosphates (ALPOs),... 

Bates et al. (172-174) considered the energetics, locations, and conformations of alkanes ranging from butane to decane in a variety of different all-silica zeolites. Calculations similar to those described already were performed for alkanes in mordenite, zeolite rho, faujasite, ferrierite, and zeolite A. A linear increase in the calculated heat of adsorption with increasing carbon number was found for all zeolites. Less experimental information is available to compare with the calculated heats of adsorption, and thus the performance of the technique and parameters cannot be subjected to quite the same scrutiny as the results for silicalite (111). Nonetheless, where... 

The siting of cations in mordenite is generally less well understood than that in the zeolites described above. Smith and co-workers (12) have, however, in recent years carried out a number of single-crystal X-ray analyses on various cation-exchanged forms of mordenite. These workers correctly emphasize, however, that the cation population densities are subject to unknown errors due to pseudosymmetry. The alkali metal ions are distributed over four major sites, namely ... 

The chemical shifts are taken relative to the aluminium of a fully ionised salt. i.e. the chemical state [A1(H20)5]. With this convention, hexa-co-ordinated aluminium resonates between -15 and +15 ppm and tctra-co-ordinated aluminium between 40 and 75 ppm. Some authors identify penta-co-ordinated aluminium atoms in the region of 40 ppm but this attribution remains subject to caution. It thus appears possible, using aluminium NMR, to monitor, qualitatively, the distribution of these various species during dealumination (self-steaming. acid attack, washing), for example, in a mordenite H (cf. spectrum in Fig. 13.7). 

Mass-transfer Effects. - Zeolites possess a large internal surface area and are necessarily subject to mass-transfer effects, although there have been relatively few studies of these. Swabb and Gates observed that for H-mordenite at low temperatures (155°C), rate was independent of crystallite size for methanol dehydration, but at higher temperature rate variation was consistent with a Thiele model. 

Table 2 shows the results of ethylene diffusivity values together with NOx conversion for the different H-mordenites studied in this work. Note that while D/R2 decreases an order of magnitude in the deactivated sample (if compared with the fresh one), it remains approximately constant for the samples subjected to acidic dealumination. This result suggests that the aluminum extracted from the lattice of the mordenite during of the mordenitic structure along the for the deactivation observed in the... 

The titank and mordenite based samples were tested for their performance by subjecting them to NOx reduction and SO2 oxidation in a microreactor (Chemical Data System s Dual catalytic reactor system Model 810-CD-HP). Technovation make NO SO2 analysers were used for monitoring NOx and SO2 at the inlet and outlet. 

The dealumination of mordenite by different treatments has been the subject of numerous studies [21-24]. Many of these dealumination studies observed marked differences from sample to sample [25]. Alteration of the lattice symmetry and reduced thermal stability has been observed in acid-dealuminated mordenite heated above 650 K [26]. The variety of changes involved in the dealumination of mordenite, and the problems associated with reproducibility, crystallographic symmetry changes, and the lack of specific knowledge relating to sample source and treatment conditions make interpretation of the literature on this subject a challenge. The introduction of SI is an attempt to help clarify differences between data on mordenite and mordenite catalysts. 

Microcalorimetric studies of several zeolites (H-mordenite, USY, H-ZSM-5), treated in such a way as to contain a noticeable amount of extra-framework aluminum, have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the I wis sites (Freundlich isotherm) and linear for the Brdnsted sites (Temkin isotherm) [82,83]. In most cases, the catalytic activity is related to the number of Brdnsted sites rather than Lewis acid sites. However, the influence of acidic Lewis sites in catalytic reactions over zeolites is still subject to controversy and cannot be neglected. 

The Ga-modified mordenite has been obtained from the H-MOR (Si/Al = 5) by impregnation with a Ihrrm nitrate aqueous solution [11S2]. A ratio of Ga/Al = 0.5 was found. The sartqrles were dry calcirrated and subjected to a reduction treatment in hydrogea... 

In some instances the materials were also repeatedly subjected to this two-step treatment to enhance the degree of dealumination, e.g., in [89]. In the case of zeolites resistant to acids due to high framework Si/Al ratios, e.g.,mordenite and ZSM-5, acid leaching may also precede the hydrothermal treatment [ 10,23]. In this case, healing of framework hydroxyl nests left by aluminum release is the lattice-stabilizing process. The hydrothermal stability of steamed Y zeolites was found to increase upon removal of extra-framework aluminum by extraction with acids [94]. 

Lee and Ha [132] treated Na-mordenite (Si02/Al203 weight ratio = 6.5) with a 6 M HCl solution at 90°C for 3-16 h or a 0.5 M HF solution at room temperature for 192 - 528 h. HCl was appHed in large excess, HF in amounts about equivalent to the aluminum in the sample subjected to acid leaching. After washing with... 

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