Framework of mordenite

The local structures of Zn (II) and Fe (III) in the lattice framework of mordenites have been characterized by means of X-ray Absorption Fine Structure. The main absorption structure of the XANES reveals the covalent bonding between the heteroatom and the lattice oxygen atom. The pre-edge structure appeared in XANES spectra of (Si, Fe)-MOR suggests a tetrahedral structure of Fe, which confirms the incorporation of Fe into the zeolite framework. Furthermore, the tetrahedral structure of the heteroatoms in the framework and their coordination distances are determined by using EXAFS technique. 

The concentration of A1 and Fe isomorphously substituting silicon in the frameworks of mordenite, ferrierite, ZSM-5, and ZSM-22, as well as in other zeolites were determined in NH4 and dehydrated/deammoniated forms by IR and Al NMR [01B3]. It has been shown that the intensity of N-H vibration at 1445 cm" of fully exchanged NH4-zeolites with the determined extinction coefficient represented a quantitative measure of the concentration of NH4 ions and, accordingly, the concentration of A1 and Fe in the fiamework positions of the hydrated zeolites [01B3]. 

The channel system of mordenite is formed by parallel elliptical 12-rings of 0.67 x 0.70 nm and 8-rings of 0.29 x 0.57 nm. The parallel channels are interconnected via small side pockets of 0.29 nm [178]. The porous framework of mordenite behaves as a two-dimensional system for small molecules and as a one-dimensional system for large molecules. 

Structure of mordenite framework as viewed along c-axis (66). 

Ba-Modenite s selectivity to MX is higher than OX, but the opposite is true for BaY. This reversal in selectivity is a result of differences in adsorbent framework characteristics mordenite has higher acid strength compared to Y zeolite. Adsorption and desorption rates of xylenes are expected to be faster in BaY compared to Ba-Mordenite because Mordenite is a one-dimensional channel system while Y zeoUte is a three-dimensional channel. With the reason stated, a three-dimensional channel ZeoUte is the preferred mass separating agent of choice compared to one-or two-dimensional channels for the liquid adsorption separation. 

The nature of the acidity of mordenite and its relation to catalytic activity have been investigated by Benesi (757), Lefrancois and Malbois (227) and Eberly et al. (225). Eberly et al. observed two absorption bands in the hydroxyl region of the infrared spectrum of H-mordenite. A band at 3740 cm-1 was attributed to silica-type hydroxyl groups, and a lower frequency band, 3590 cm-1, was thought to arise from hydroxyl groups associated with aluminum atoms in the structure. Acid extraction of the aluminum atoms from the framework, although leaving the structure intact resulted in a loss of the lower frequency hydroxyl band. 

The desire to synthesize molecular sieve compositions containing other than the typical silicon and aluminum atoms is evidenced by the large number of efforts, primarily in the primary synthesis area. The Secondary Synthesis process has now been extended to include substitution of both Fe3t and T1 + into the frameworks of a number of zeolites. This paper will describe substitution of iron or titanium ions into the frameworks of zeolites Y, L, W, mordenite, ZSM-5 and LZ-202. Zeolites Y, L, W and mordenite were obtained from Union Carbide Corporation. Zeolon, a synthetic mordenite, was obtained from The Norton Company. ZSM-5 was synthesized according to the procedures described by Argauer et al., (8). LZ-202 is an omega type zeolite, synthesized without the... 

Figure 1. The proposed structural framework of ECR-1, showing alternate sheets of mazzite and mordenite, the 26.5A repeat unit and the single 12-ring channel.

Steaming usually produces extra framework aluminum, but even in the case of leaching some extra framework aluminum may be left on the mordenite. In some cases they can be removed by ammonium ion exchange, if desired, followed by calcination. However, depending on the amount and location of the extra framework aluminum, as the case may be, these species may increase or decrease the catalyst activity. An early review on the dealumination of mordenite was given by Karge and Weitkamp (14). In particular for hydroisomerization of paraffins, this effect has been studied by A. Corma et al. (15, 16). 

The proposed structure for ECR-1 was solved by accumulating evidence from many "traditional" sources, such as the synthesis phase relationships, powder x-ray diffraction (PXD) and electron diffraction (ED), molecular probe sorption, infra-red analysis (IR) and electron microscopy (EM). Initial unsuccessful models were based on extended merlinoite frameworks, followed by modifications based on mordenite. The observation of crystal overgrowths of mazzite in high resolution lattice images was the key to recognizing the compatibility of mordenite and mazzite structural layers, and that intimate intergrowth of the two structures was possible. 

MAS NMR data revealed that upon dealumination of mordenite with phosgene a part of the framework Al remains in the channels as octahedrally coordinated aluminium compounds. As a result of aluminium removal, framework vacancies and internal silanol groups are generated. The transformation of SiCl groups to SiOH groups is due to hydrolysis by ambient moisture. Framework reconstruction occurs during dealumination. 

Sawa et al. have compared the framework Al determined by Al NMR with the acid amount determined by NH3 TPD for a series of mordenite zeolites. They found... 

Usually, inorganic and organic acids can be used for framework dealumination of zeolites, and the acids include hydrochloric acid, nitric acid, formic acid, acetic acid, and so on. According to its acid-resistance ability, hydrochloric acid can be used for high-silica zeolites such as mordenite, clinoptilolite, erionite, etc. We will take mordenite as an example to describe this dealumination method (Table 6.4). The first step in the treatment of mordenite using hydrochloric acid is to convert the zeolite into H-type, and further acid treatment can then enlarge the pore diameter through dealumination. After partial dealumination, the Si/Al ratio of the zeolite is increased and the heat-resistance, water-resistance, and acid-resistance abilities are enhanced. 

Figure 9.3 Section of framework structure of mordenite, showing the various extraframework cation sites (represented as spheres).

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