Bronsted aluminium sites

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

It has been reported that aluminium can be introduced into the framework of silicalite with MFI structure by the treatment with AICI3 vapor at elevated temperatures [4-8]. By such treatment, not only Bronsted acid sites but Lewis acid sites are also generated, because aluminium atoms are introduced not only into the framework sites but alkso into the non-framework sites [6-8]. It is expected that this method can be applied to prepare some metallosilicates with MFI structure. Namely, by treating silicalite with metal chloride vapor at... 

Chemical modification of the ALPOs is required to create a new class of catalysts. In the metalloaluminophosphates (MeALPOs), the framework contains metal (Me), aluminium and phosphorus. Thus, it becomes possible to produce a wide range of active catalysts with Lewis and Bronsted acid sites and redox properties by the partial replacement of Al3+ by Me2+ ions (e.g. Co, Cu, Mg, Zn, etc.) in an ALPO framework (Thomas, 1995 Martens etal, 1997). 

The mesoporous solids developed by Mobil group in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen... 

It is seen from the data in Table 3 that the (Si/Al)s ratio is higher than the bulk value (2.33). This suggests an aluminium depleted surface region. The low values observed for the ratio (N/AI)s reflect the fact that only part of the BrOnsted acid sites are accessible to pyridine. The pyridine molecule kinetic diameter of 5.9 A does not allow it to enter the sodalite cages with 2.2 A openings. Thus only the acid sites protruding in the supercages can chemisorb pyridine. The number of these molecules is estimated to be 24 per unit cell [41] and since the Y zeolite with a Si/Al ratio of 2.33 has 57 A1 atoms per unit cell, the maximum N/Al ratio is 0.42. This value is reasonably close to the 0.38 value obtained after calcination at 300°C. 

Sastre et aL [112] studied the isomerization of m-x>iene over OfBretite and observed monotonical increase in m-xylene conversion upon exdumge of the K -cations. This was ascribed to the increase of the concentration of the protons and the increase in accessibility of the pores, which resulted in a higher selectivity for the isomerisation reaction at the e q>aise of the disproportionation reactioiL Only a sHght increase in the p-xyloie in the fraction of oitho-and para-xylene was observed. Over Beta a maximum activity for the xylene isomersation was observed and this was explained by either a pos le existence of a eigistic effect between extra-framework aluminium and the fitunework Bronsted acid sites or a concentration effect [113]. 

Similarly, the low frequency overtone at 6950 cm-1 associated with acidic OH vanishes, while the silanol overtone band develops at 7325 cm-1 (9) and the ( v + 6) combination shifts to 4540 cm-1. These observations are consistent with the creation of silicon defects in the structure of dealuminated Y zeolites (10) while the weak overtone band at 7240 cm 1 is probably related to hydroxylated aluminium species extracted from the lattice (11, 12). Thus, the near-IR spectra give evidence for the decrease of the number of Bronsted acid sites as a result of dealumination. 

The expected result of dealumination is a decrease in the total number of Bronsted acid sites, whereas the number of strong Bronsted acid sites increases relative to the number of aluminium atoms because of an increase in the number of isolated Al. The number of Lewis acid sites also increases because of an increase in the non-framework aluminium content.21... 

In zeolites, tetrahedral framework aluminium can be distinguished from non-tetrahedral extra-framework species by means of Al MAS NMR. However, some AlPOs are known to contain 5- or 6-coordinated Al in the framework, which complicates a quantitative determination [4—6]. Quantitative methods for the monitoring of substituting metals Me are, therefore, required. In the case of transition metal ions, possible changes in the oxidation state must also be taken into account, since the charge n of [Me02] building units should directly affect the number of Bronsted acidic sites. 

The acid sites of porous solids are distributed evenly over their internal surfaces. As a consequence, they are amenable to study by bulk structural methods. Bronsted acid sites may be observed by proton NMR, vibrational spectroscopy and, in favourable cases, by neutron diffraction. Furthermore, Al-H distances can be determined by NMR spectroscopy and, as described in Chapter 3, the local environment of aluminium ions associated with Bronsted acid sites can be measured by aluminium EXAFS. " The structural picture for the dehydrated acid sites on H-ZSM-5 is given in Figure 8.2. 

The IR study of the Bronsted acid sites in aluminium-deficient NaHY zeolites" Bull. Acad. Pol. Sci., 23, 445. 

Fig. 4 shows clearly that the steaming treatment conditions have a considerable impact on the variation of the acidic properties of ZSM-5 zeolites. Notably, ZSM-5-ST only possesses residual Bronsted acidity. ZSM-5-MT exhibits a lower number of pyridine molecules chemisorbed on Bronsted acid sites at room temperature, compared to ZSM-5-P. As steaming posttreatment extracts part of the framework aluminium species (acting as Bronsted acid sites) and converts these into extra-framework aluminium, (acting as Lewis acid sites), a deerease in the number of Bronsted acid sites is expected. A close look to the band at 1545 cm of ZSM-5-MT at 573 K and 673 K reveals a higher number of pyridine molecules attached to... 

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