Extra-framework Al-containing species

Martin et al. [429] used the IR/pyridine technique (cf. Sect. 5.5.2.6.2) to look at the elimination of Bronsted acid centers upon hydrothermal treatment of H-ZSM-5 zeolite and evidenced the condensation of extra-framework Al-containing species (Lewis sites). 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The catalytic activities of calcined samples of V-MFI (B), V-MEL (B3) and V-Al-P (3) which possess most of the vanadium in framework positions are presented in Table 4. Additionally, the activity of V-Al-P (1) containing mostly nonframework (extractable) V is also compared. It is noticed that the first three samples containing mostly framework V have similar activities (TON) and exhibit similar product selectivities. V-Al-P (1) with more nonframework ions possesses a lower activity and a different product selectivity, producing more catechol than the others. In the the case of TS-1, the formation of catechol is believed to take place over the active sites at the external surface, while hydroquinone is formed over the sites inside the pore system. The above observation and the rapid extraction of V by H2O2 during the reaction suggest that the extra-framework V-species are present mostly at the external surface of the crystallites. An important conclusion of the studies is that the intrinsic activity of the framework V ion is independent of the structure of the zeolite. This suggests that the framework V species are similar in nature and environment in all the above three systems. 

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 X-ray diffraction patterns of the AlPO and CoAPO samples indicate pure phases, except for some samples of CoAPO —34, which might contain amorphous material. However, in view of the product compositions (Table 1) it must be concluded that many samples are not pure since the (Al+Co)/P ratio is considerably greater than 1. In these cases, part of the aluminium and/or cobalt must be present in extra-framework species. In particular, samples prepared with high cobalt concentrations in the gel and samples of CoAPO -34 exhibit unfavourable element ratios. These samples are also often less homogeneous in that they can contain white particles, whereas the major phase exhibits a blue colour. 

Measurements of adsorption of pyridine into acid zeolites were severely impeded by the strong interaction of the adsorbate molecules with the adsorption sites, that is, centers of Bronsted and/or Lewis acid types, i.e., acidic OH groups and only threefold coordinated framework Al or Al-containing extra-framework species, respectively ( true Lewis sites see Vol. 4, Chap. 1,... 

For the materials derivatised during the synthesis, the effect of copper or silver addition is illustrated by A1 MAS NMR spectra in Fig. 2. The NMR spectra of the calcined samples with largest metal content of 20 wt % shows an intense line, especially for Gu-containing sample, at 8 = 52 ppm firom tetrahedrally coordinated Al. The appearance of a second signal at 8 = 0 ppm indicates that extra-framework aluminium attributed to octahedrally coordinated Al is only present in sample without metal, SiA120. Tetrahedrally coordinated Al was of special interest and highly desirable, because it is incorporated into the framework and therefore is responsible for the acid sites formation, and thus enhance the propene adsorption selectivity and capacity [16]. The octahedral aluminium species are occluded in the pores or exist as an amorphous by-product. 

Additionally, spectrum 3 b in Fig. 12 displayed an intense band at 1446 cm due to pyridine interacting with Ca(OH)+, resulting in a coordination complex with pyridine, and a band at 1455 cm typical of pyridine bound to true Lewis sites (Al-containing extra-framework species). 

The IR spectra (with and without application of pyridine as a probe molecule) of H-ZSM-5 and Co-ZSM-5 prepared via SSIE confirmed the suggestion that in the case of nco/nAi 1.0 the acid Bronsted OH groups were completely consumed. Two types of newly formed acid sites, most likely Lewis acid sites, were indicated, viz., C0CI+ species (vide supra) and possibly true Lewis sites , i.e., Al-containing extra-framework species (cf. [171,172]). 

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