Mordenite dealumination

Phosgene proved to be an appropriate reactant to remove aluminium from the zeolite framework [1,2]. Detailed measurements have been carried out to investigate the adsorption of phosgene [3], the mechanism of dealumination by infrared spectroscopic and thermogravimetric methods [4], structural consequences [5], adsorption and catalytic properties of modified mordenites [6]. In addition an NMR study has been carried out, to measure the thermal stability of so-called hydroxy nests in H-mordenite dealuminated with phosgene [7]. 

These results, which indicate the presenee of extra-lattice A1 (Aivi) even in the mordenites dealuminated by acid treatment, are in agreement with our previous results obtained with 2 a1 MAS NMR (3). This suggests that the amorphous material formed by the extraction process of lattice alumina w is not eliminated. This difficulty in eliminating the extra-lattiee A1 altogether has also been observed in other studies (7,8). 

Figure 5. J9Si-NMR spectra of mordenite dealuminated by acid leaching and subsequent steaming. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)...

Table VI Evolution of the different Si configurations and tne number of SiOH defect groups generated per extracted Al during mordenite dealumination by acid...

Fig. 10. Signals of A1 2p kft) and A1 KLL (right) of mordenites dealuminated by different procedures (steaming-leaching combinations). (Al/(Al+Si))t, U =0.083 (a),0.025 (b),0.014 (c), 0.013 (d), 0.011 (e), 0.009 (f). Taken from RemyMJ, Genet MJ.Notte PP, Lardinois PF.Poncelet G (1993) Microporous Mater 2 7, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands...

H.-M. Kao, Y.-C. Chen, C.-C. Ting, P. T. Chen, J.-C. Jiang, Characterization of extraframework aluminum in H-mordenite dealuminated with ammonium hexafluorosilicate, Catal. Today, 97, 13-23 (2004). 

Traditionally, aromatic nitrations have been performed with mixtures of concentrated nitric and sulphuric acids leading to considerable effluent problems associated with the spent acid. A heterogeneous alternative for. sulphuric acid has been reported (Bertea et al., 1995), namely dealuminated Mordenite, which is sufficiently robust to be able to catalyse the vapour-phase nitration of benzene with 65% aqueous nitric acid. 

To a 500 g sample of acidic, dealuminized mordenite (CBV-20A from PQ Corporation, 1.5 mm diameter extrudates that had been calcined at 540 °C, overnight) was added a solution of 33 ml of 48% HF in 1633 ml of distilled water, and the mix cooled in ice, stirred on a rotary evaporator overnight, then filtered to recover the extruded solids. The extrudates were further washed with distilled water, dried in vacuo at 100 °C, and then calcined at 540 °C, overnight. Analyses of the treated mordenite showed 1.2% fluoride, 0.49 meq/g acidity. Samples were charged to the reactive distillation unit either as 20/40 mesh granules, or as ca. 1.5 mm extrudates. 

Klinowski, J., Anderson, M.W., Fyfe, C.A. and Gobbi, G.C. (1983). Dealumination of mordenite using silicon tetrachloride vapor. Zeolites 3, 5-7... 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Zeolite dealumination was first reported by Barrer and Makki (1), who progressively removed aluminum from clinoptilolite by treating the zeolite with hydrochloric acid of different strengths. Subsequent dealumination studies were carried out primarily on mordenite (2-5) and Y zeolites. 

Reactions with acids. Hydrochloric acid was used in the dealumination of clinoptilolite (1), erionite (14) and mor-denite (2,3,15,92). In the case of Y zeolite, dealumination with mineral acids was successful only after conversion of the zeolite into the ultrastable form (vide infra). Barrer and Makki (1) were the first to propose a mechanism for the removal of aluminum from mordenite by mineral acids. It involves the extraction of aluminum in a soluble form and its replacement by a nest of four hydroxyl groups as follows ... 

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

When the two-step process is repeated on the same material, the thermal treatment following the chemical dealumina t ion results in further expulsion of aluminum from the framework into zeolite cages or channels. The solubilization of non-framework aluminum during the first chemical treatment appears to facilitate further framework dealumination during the subsequent thermal treatment due to the altered steric and electrostatic parameters in the zeolite channels. The newly formed non-framework aluminum species can be readily solubilized by acid treatment. This cyclic method has allowed the almost total removal of aluminum from mordenite (5). 

The spectrum of the aluminum-deficient mordenite shows no signal for Si(2 Al) and a considerable decrease in intensity of the Si(l Al) signal, due to framework dealumination. 

Stability. Being a fairly high-silicious zeolite, mordenite generally has high thermal stability. It was reported (77) that progressive acid dealumination results in an increase in thermal stability, followed by a decrease. Maximum stability was reached for a Si09/Al90 ratio of about 19. 1 1 J... 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

The formation of such bonds during the heat treatment of dealuminated mordenite has also been suggested by Rubinshtein et al. (72-74), in some instances without the intermediate formation of SiOH groups. The hydrophobic nature of the zeolite also increases with progressive dealumination. Chen (71) has shown that aluminum-deficient mordenite zeolites with SiO /Al O ratios over 80 absorb little or no water at low pressure. These highly silicious zeolites are truly hydrophobic and in this respect are similar to highly silicious zeolites prepared by direct synthesis (e.g. ZSM-5) (75). 

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