Dealumination of zeolite

Figure C2.12.8. Schematics of tlie dealumination of zeolites. Water adsorbed on a Br( msted site hydrolyses tire Al-O bond and fonns tire first silanol group. The remaining Al-0 bonds are successively hydrolysed leaving a silanol nest and extra-framework aluminium. Aluminium is cationic at low pH.

Ga-MOR was hydrothermally synthesized by using tetraethylammonium bromide as a template. Ga content was controlled by removing Ga through HCl treatment in the same way as the dealumination of zeolite [1], The numeral at the end of catalyst name stands for Si02/Ga203 ratio in Ga-MOR thus prepared. Ga203/Ga-M0R catalyst was prepared by... 

Figure 15, Dealumination of zeolite Na-Y using SiClk vapor studied by 29Si-MAS NMR at 79.8 MHz. Key top, parent Na-Y zeolite and bottom, after treatment with SiClk. (Reproduced from Ref. 30. Copyright 1982, American Chemical Society.)...

Inevitably, the zeolites with a Spaciousness Index between 2 and 16 have a higher Si/Al ratio than zeolites Y or ZSM-20. To separate the effects of the Si/Al ratio and the pore width, two dealuminated samples of zeolite Y, designated YDl and YD2, were tested which resembled in their Si/Al ratios the Beta and EU-1 sample, respectively. Table 1 gives the isomerization selectivities at 40 % conversion. While dealumination of zeolite Y brings about some improvement in selectivity, H-Beta is clearly superior to HYDl, and H-EU-1 is a much better catalyst than HYD2, which indicates that the pore width in the appropriate range is of prime importance. 

Fig. 46. Dealumination of zeolite Na-Y using SiCl4 vapor studied by 29Si MAS NMR spectroscopy at 79.80 MHz (206). (a) Parent material (Si/Al = 2.61) (b) after complete dealumination (corresponding to the 27A1 MAS NMR spectrum in Fig. 47d) (Si/Al = SS).

Fig. 47. Dealumination of zeolite Na-Y using SiCl4 vapor studied by 27A1 MAS NMR spectroscopy at 104.22 MHz (207). (a) Parent zeolite Na-Y (b) dealuminated material before washing (c) after washing with dilute acid (d) after repeated washing. Note that the aluminum jettisoned from the zeolitic framework is first bound tetrahedrally as NaAlCl4 (see text), but after washing adopts octahedral coordination.

Figure 4 Main steps involved in dealumination of zeolites by hydrothermal treatment.

Dealumination of zeolite NaY using SiCl. vapour studied by Z7A1 MASNMR at 104.22 MHz. ... 

Dealumination of zeolites can be achieved using several other methods such as extraction with EDTA (e.g., 35, 76), high temperature steaming (e.g.,... 

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. 

The dealumination of zeolite MCM-22 by calcination or hydrothermal treatment has been investigated by conventional multinuclear solid-state NMR and checked by ultrahigh-field NMR experiments (19.6 T) with a fast spin rate (19.1 kHz). The presence or variation of different species, sueh as silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminium, during or after dealumination has been detected, and their... 

Using Al MAS and Al MQ MAS NMR, dealumination of zeolite H- 3 has been observed at specific T-positions in the framework. " Al MQ MAS NMR was able to resolve aluminium in the Ti and Tj positions from the other (T3-T9) positions in the framework. A quantitative analysis of the Al MQ MAS NMR spectra has shown that aluminium atoms in positions Ti and T3 resist dealumination and do not adopt an octahedral coordination. It has also been shown that a heat treatment of NH4-P gives a single type of fairly symmetric framework octahedral aluminium, which can be reconverted into framework tetrahedral aluminium by ammonia treatment. 

Modification of Zeolites. The purpose of any modification is to improve catalytic properties. The most common modification is the dealumination of zeolites which has been shown to increase crystalline thermal stability. Three dealumination procedures can be used (35) chemical dealumination, thermal or hydrothermal dealuminatTon and combination of thermal and chemical dealuminations. 

In order to increase their thermal stability and the strength of their acid sites, high A1 content zeolites are submitted to various dealumination treatments ( 7.19.53-55 I.Some procedures will be illustrated by the dealumination of zeolite mordenite, where the Si-NMR results are shown to lead to the determination of A1 distribution in the samples, as well as to the proposition of a dealumination mechanism (19). 

Infrared Analysis of the lattice vibrations has proved to be a particularly convenient, rapid and informative technique to monitor changes in the chemical composition of zeolites, in particular following dealumination of zeolites (22). In addition special structural features such as Five-, Six-, Double six- and Ten- membered rings were identified on the basis of lattice vibrations (12). More recently, particular absorptions were ascribed to the presence of centred tetrahedra in iron-MFI, although the precise assignement was uncertain (23). 

Wang, Q., Giannetto, G. and Guisnet, M. (1991) Dealumination of zeolites. Ill Effect of extraframework aluminium species on the activity, selectivity and stability of Y zeolites in n-heptane cracking, submitted to publication. 

Wang QL, Giannetto G, Trarealba M et al (1991) Dealumination of zeolites D, kinetic study of the dealumination by hydrothermal treatment of a NRiNaY zeolite. Journal of Catalysis 130 459-470... 

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