Zeolites boron-containing

The SSZ-33 zeolite was synthesized using the template molecule derived from 8-keto tricycle [5.2.1.0] decane. The boron containing SSZ-33 was converted to Al-SSZ-33 by a one-step reflux in 1 M aluminum nitrate. The SSZ-35 zeolite was prepared with the template camphorquinone. SSZ-35 zeolite was then subjected to three-fold ion exchange in ammonium nitrate. 

At present, this liquid-solid substitution approach has been extended to the secondary synthesis of zeolites containing Si, Fe, Sn, Ti, Cr, and other heteroatoms. An exception is that BF4 cannot be used for liquid-phase dealumination and boron-addition of zeolites to prepare boron-containing zeolites.[33] In the following galliation of NH4Y will be taken as an example to discuss the isomorphous substitution secondary synthesis technique. 

Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

Microporous silicates synthesised with isomorphous substitution of elements such as boron and gallium for silicon show similar demetallation behaviour, where the heteroatoms leave the structure more readily than aluminium atoms. In particular, the behaviour upon calcination of boron-containing solids has been examined by and Si MAS NMR. Boron is observed to move from tetrahedral to trigonal coordination upon the formation of the protonic boro-silicate form, and studies on the protonated form of zeolite B-Beta have shown that the boron may be removed from the framework stepwise by hydrolysis of Si-O-B bonds, ultimately giving boric acid. This is lost from the structure if put into contact with aqueous solution. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Other framework structures based on zeolites have also been synthesized which contain atoms other than aluminium and silicon, such as boron, gallium, germanium, and phosphorus, which are tetrahedrally coordinated by oxygen. Such compounds are known as zeotypes. Pure aluminium phosphate, commonly called ALPO, and its derivatives, can take the same structural forms as some of the zeolites such as sodalite (SOD), faujasite (FAU), and chabazite (CHA) (e.g., ALPO-20 is isostructural... 

From all the above information, we can describe the important modification during the boronation of zeolites (3 as follows very limited boron atoms are inserted into the (3 framework by treating the sample with an alkaline solution containing boron species. Accompanied by this insertion, a considerable amount of silicon atoms are extracted from the lattice, resulting in the micropores in crystallites are enlarged into the mesopores and the smaller mesopores are developed into larger intracrystalline mesopores. Meanwhile, the corrosion of outer layer of crystallite makes the size of crystal particle reduce. 

Figure 4a is a 11B MAS NMR spectrum of a mordenite sample prepared from an aluminum deficient gel which contained B2O3. The sharpness of the peak indicates a tetrahedral boron location, and the chemical shift agrees with previously reported values for boron in a zeolite framework (8). In contrast, extra- lattice boron in mordenite (vide infra) gives a broad resonance, as shown in Figure 4b. 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

We report (i) isomorphous substitution of boron, by secondary synthesis, into silicalite and into highly siliceous (Si/Al>400) ZSM-5 and (ii) an improved direct synthesis of zeolite (Si,B) -ZSM-5. The chemical status of B in die boronated products depends upon reaction conditions. Careful control of the concentration of the base, the borate species and of die duration of treatment, allows the preparation of samples containing only 4-coordinated B or a mixture of 3- and 4-coordinated B in various relative concentrations. The products were characterized by magic-angle-spinning (MAS) NMR and infrared (IR) spectroscopies and by powder x-ray diffraction (XRD). 

We believe the use of a direct gaseous phase fluorination process for modifying the surface and structure of zeolites to be a new process (18). The literature does contain references to the use of hydrogen fluoride (20, 22, 23), boron trifluoride (21, 24), aluminum monofluoride (sic) (19) and silica difluoride (sic) (19) to treat the surface of a zeolite to obtain higher catalytic activity. However, the use of fluorine gas to modify both surface and structure has not been reported before. The purpose of this paper is to report results of fluorination of zeolites and to describe the process involved in such a treatment. Detailed results on fluorine-treated zeolites and their unusual properties, both adsorptive and catalytic, will be discussed in forthcoming papers. 

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