Zeolite frameworks, boron incorporation

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

The acidity of zeolites can be reduced by the incorporation of boron in the zeolite framework [162,163] and therefore B-substitut ZSM-5, ZSM-11 and Beta were teaed [158,164]. Al " " free boron zeolites are inactive, but these zeolites with low levels of Al " " ions which can be obtained by adding AI2O3 binder to the Al free boron zeohte have weak acidity and are moderately active at 500 - 600°C and isobutene selectivities of up to 50 % have been reported. At these conditions the observed activity and selectivity of B/Al-ZSM-5, B/Al-ZSM-11 and B/Al-Beta were. similar and therefore it was concluded that the pore aructure did not play a decisive role in the converaon of n-butene into isobutene [164]. However, A1 which migrates into the pores not only modifies the acidity but also modifies the effective pore diameter. 

The total amount of TPA/u.c. is equal to 3.4-3.8 for samples synthesized with 0.5 moles of H3BO3. For the K-borosilicalite samples, the amount of TPA/u.c. decreases to 3.2 (sample synthesized with 4 moles of H3BO3) and to 2.8 for samples obtained with 6 and 10 moles of H3BO3. The decrease of TPA/u.c. is also indicative of boron incorporation into the MFI structure. Indeed, it was previously observed, that the increase, of A1 [15] or the increase of both A1 and B [16] in the zeolitic framework was accompanied by a decrease of TPA/u.c. Finally, only the high temperature endothermic peak at 470°C remains for the K- and Cs-borosilicalite samples. 

If boron is incorporated in the zeolitic framework, its presence leads to a contraction of the imit cell because the atomic radius of the B atom (0.98 A) is smaller than that of the Si atom (1.32 A). The cell parameters and the unit cell... 

Zeolite beta was among the first zeolites which underwent successful replacaaoent of boixrn for aluminium (1). The main grx>und for inserting boron in zeolitic frameworks is the modulation of the strength of the acid sites (2-5), but structural boron proved to be less stable than aluminium In the activation treatments, especially in hydrothermal conditions (6, 7). Ihis drawback may be turned into advantage Wien a network quite unstable under dealuminating conditions is concerned, as in the case of zeolite beta (8). The milder conditions required for deboration are likely to affect to a lesser extent the lattice stability. B-beta could then represent a suitable precursor of the activated form of the zeolite (9). Moreover the different kinetics of incorporation of boron and aluminium are likely to influence other properties of the solid, like the size and habit of the crystals and the defect patterns (10-12). 

B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites. 

As shown in Figure 1, B MAS NMR spectra demonstrate the boron incorporation into the zeolite framework during the synthesis. The narrow line at -3.6 ppm is due to tetrahedrally coordinated framework boron and the broader line at ca. -2.0 ppm is caus by boron atoms in the amorphous part of the sample. For a crystallization time t >7h the boron is completely incorporated into the framework. 

The B MAS NMR spectrum of the hydrated sample shows one line at -3.3 0.1 ppm which is assigned to boron atoms incorporated into the zeolite framework. Other samples which are not completely crystalline contain boron in the amorphous part, giving rise to a broad line at about -2.0 ppm. Therefore, the intensity of the line at -3.3 ppm may be used to determine the concentration of framework boron atoms. 

Recently, there has been considerable interest in the isomorphous substitution of tetrahedral aluminium in zeolite frameworks with catalytically active elements such as iron, gallium and boron. These materials have acidities Afferent from the corresponding aluminosilicates leading to altered activity, selectivity and stability. Mdssbauer spectroscopy has been used to study the iron incorporated into zeolites during synthesis. Fe(III) can be present on tetrahedral framework sites as Fe " cations acting as counterions and as Fe(III) oxides precipitated in or on the zeolite crystals. The most common iron oxide is a-Fe203 which contains iron only in octahedral coordination. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H. Sulikowski, B. Klinowski, J. T.Phvs.Chem.. (in press)). The substitution of gallium in silicalite-n has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species. 

We prepared boron substituted mordenite by direct synthesis from gel precursors and by post- synthetic substitution into dealuminated mordenite. Direct substitution is favored in aluminum deficient gels, but exacting crystallization requirements for mordenite formation limit the amount of boron that can be incorporated into the framework structure. Higher substitution levels were achieved using a post-synthetic treatment. Boron substituted zeolite Y could not be prepared by a similar direct synthetic method, but post-synthetic methods were effective at providing low substitution levels. This demonstrates the more general utility of post-synthetic substitution methods. The hexane cracking activity of... 

The Co-exchanged zeolites were not effective catalysts for the oxidation of cyclohexane. The cobalt exchanged ions were not stabilized enough by the zeolite interactions and part of these cations were released in the oxidation medium. Thus, we decided to explore the activity of P-zeolites in which cobalt ions were incorporated into the framework. We hoped that the incorporation would increase the stability of the cation within the solid. We studied the catalytic activities of cobalt substituted P-zeolites containing aluminium (Co-Al-BEA) and boron (Co-B-BEA) towards the oxidation of cyclohexane into adipic acid. 

Further catalytic uses of zeolites and related materials include polymerisation of alkenes as well as the development of basic zeolitic materials generated by the incorporation of alkali metal ions. Gallium- and boron-substituted zeolites have already been shown to be useful catalysts in wide variety of reactions (Table 2.2) and undoubtably these will be followed by novel zeotypes including mesopor-ous materials with other catalytically active elements within their frameworks.40... 

Liu and Xu [39] reported on a limited increase in the Si/Al ratio of NH4-Y from about 2.5 to 3.4 upon treatment with 0.1 M aqueous solutions of NH4[Bp4] at 60 °C for 24 h. They suggested that aluminum is released from the framework induced by slow hydrolysis of the boron complex and, subsequently, silicon is incorporated into the lattice vacancies left by dealumination. The silicon involved in the claimed healing process was believed to originate from dissolution of atoms located on the external surface of the zeolite and from amorphous silica (if any). However, in this case, the insertion of silicon into the framework... 

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