EFAL specie

In addition to the Bronsted acidity in zeolites, in these materials the Lewis acidity is present as well. According to Lewis, an acid is an electron pair acceptor, a definition which is broader than that given by Bronsted, since a proton is a particular case of an electron pair acceptor. Then, the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only a particular type of process [128], The Lewis acidity is related to the existence of an extra-framework A1 (EFAL) species formed during the zeolite dealumination process [128],... 

In zeolite-based catalysts, the Lewis acidity is related to the existence of extra-framework A1 (EFAL) species formed during the zeolite dealumination process [18], It occurs frequently in zeolite activation, for example, during the calcination process... 

In accord with this proposal, the removal of these EFAL species by acid treatment (17) or with ammonium hexafluorosilicate causes a significant decrease in acidity and catalytic activity. 

The zeolite composition and structure, which can affect hydrogen transfer activity, are important parameters determining the activity, selectivity, and stability of the zeolite during isobutane alkylation. In the case of USY zeolites, a maximum initial 2-butene conversion was observed for a framework Si/Al ratio of about 6 (63). However, the TMP/DMH ratio, which can be taken as a measure of the alkylation/oligomerization ratio, continuously increased when decreasing the framework Si/Al ratio. On the other hand, the amount and nature of extraframework Al (EFAL) species also affected the alkylation properties of USY zeolites (64). 

Zeolite Beta has also been studied for isobutane/butene alkylation (65, 66), but it was less selective to the desired TMP than USY, suggesting some diffusional limitations for these highly branched products at the relatively low reaction temperatures used. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite (66). As for USY zeolites, the activity, selectivity and deactivation rate of Beta zeolite were influenced by the presence of EFAL species (67). Medium pore zeolites, such as ZSM-5 and ZSM-11 were also found active for alkylation, but at temperatures above 100°C (68, 69). Moreover, the product obtained on ZSM-5 and ZSM-11 contained more light compounds (C5-C7), and the Os fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products. 

For the parent material XRF analysis gave aluminium contents that were noticeably larger than the framework aluminium concentration indicating the presence of large amounts of extralattice or non-framework aluminium. A comparison of the results from Table 1 clearly demonstrate that at relatively low dealumination levels, i.e. 16 and 32%, the AHFS treatment preferentially removed EFAL species while both EFAL and FAL were removed when the AHFS concentration was increased [14,15]. Hence, for H-Y(d5o%) sample, almost 100% EFAL and 30% FAL was extracted from the parent material. Indeed, it was found for this sample that the framework Si/Al ratio obtained from the unit cell size was very close to the value obtained by XRF measurements. Our observations were consistent with the results... 

The physicochemical properties of the different beta samples (before impregnation of platinum) are presented in Table 1. It can be seen that the chemically dealuminated HP c nd HP p r samples have a similar bulk Si/Al ratio, which would be very close to the framework Si/Al ratio, as both samples are practically free of EFAL species. By ccHitrast, the steamed HP, sample has the same bulk Si/Al ratio than the parent Hp sample, as it contains all the EFAL generated, but the framework Si/Al ratio should be very close to that of the chemically dealuminated catalysts, as mdicated by the Si MAS-NMR analysis. The XPS results indicate that steaming of HP sample did not produce an... 

The extra-framework aluminium species (EFAL) generated by hydrothermal treatment of zeolite Y have a strong effect on catalytic activity and selectivity upon cracking, isomerization and alkylation reactions of hydrocarbons [14-17]. The effect of EFAL species on the liquid phase isomerization of a-pinene has also been studied [18]. 

In this work we study the effect of EFAL species in steam dealuminated USY zeolites with different framework compositions, on the hydration reactions of a-pinene and camphene, at 328 K. Scheme 2... 

The increase of activity observed for increasing dealuminations is not only due to the increase of the acid strength of the Bronsted acid sites due to an inductive effect of EFAL species [14,15], but also to the increase of the hydrophobicity of the zeolite s surface. 

In the extraframework octahedral aluminium (EFAL ) (Fig. 2) there are species appearing at 1.4 and higher ppm s whereas it is usually reported from -0.5 to 0 ppm (6). This is an indication that these EFAL species are polymerized, forming an alumina type material. Therefore, in order to explain the cracking activity and selectivity of highly dealuminated zeolites we have to take into... 

It is well known that the hydroxyl groups associated with FAL show two bands in the i.r. region at 3630 cm (HF) and 3555 cm- (LF). EFAL species such as the non-crystalline silico-alumina, which can be responsible for Bronsted sites other than that corresonding to the HF and LF hydroxyl bands, should appear in the same region, and would interact with pyridine. Indeed, very recently (8, 10) we have shown that in dealuminated Y zeolites some of the... 

When zeolites are dealuminated by steam-calcination part of the framework A1 is extracted and generates extra-framework species (EFAL) that can be cationic, anionic or neutral. Some of these EFAL species can act as Lewis acid sites [19] or can influence the Brpnsted acidity, by either neutralizing Brpnsted acid sites by cation exchange, or by increasing the acidity by a polarization effect and/or by withdrawing electron density from lattice oxygens [20-22]. However, under mild steaming the A1 can also become partially, and reversibly, disconnected from the lattice [23]. This opens the way to Lewis acid catalysis by the A1 [24]. 

The influence of the acid pretreatment of BEA on ifs acfivity in the model reaction between 2-MN and AAN was analyzed in depth/ The contribution of the inner and outer surfaces of the catalyst was examined by considering the selectivity with respect to the bulky product 28 and the linear product 29, which are assumed to be formed on fhe outer and on both the inner and outer surfaces of fhe catalyst, respectively/ " It is shown that the production of EFAL species located in the micropores of BEA subjected to high heating rate calcination provokes the increase in the selectivity of the less hindered 29 because the formation of the bulky 28 is sterically hampered. Indeed, when the external surface of zeolite BEA is passivated by coating with amorphous silica, a significant increase in the selectivity of 29 is observed, and fhis resulf is a clear example of shape-selective acylation with zeolite catalyst. On the contrary, acid treatment increases the catalytic activity of fhe oufer surface due to the extraction of the catalytically active EFAL species out of the micropores, leading to the preferential formation of fhe bulky 28. 

Aluminum hydroxyls connected to EFAL species in zeolites absorb at 3665 cm The CO-induced shift of (OH) of EFAL species present in H-zeolites was reported to span a wide interval, —190 to —260cm. The presence of alkali metal cations in zeolites decreases the acidity of the residual hydroxyls (215,221,249). Similarly, potassium cations on alumina decrease the hydroxyl acidity (260). On the contrary, sulfation results in an increase of acidity. 

Analysis of the pubHshed results reveals the following general picture of the location of hydroxyls in zeofttes. The bridging hydroxyls are located in the zeoHte pores. On the contrary, the silanol groups are mainly located at the external surface (typical band at 3748 cm ). Hydroxyls associated with EFAL species (bands around 3780 (159) and 3650 cm ( 96)) are also found to be located at the internal surfaces (in contrast to the EFAL species responsible for Lewis acidity). 

The extraframework alumina species in zeohtes are also of significant interest. They are primarily characterized by two groups of bands, around 3780 and 3665 cm, respectively. It was estabhshed that, in contrast to the EFAL species responsible for Lewis acidity, the hydroxylated species are located inside the zeolite pores (i96). 

Table 2.31 reports the frequencies of hydroxyl groups bound to cations that were introduced into zeolites by ion exchange. These hydroxyls can he formed according to the reaction of Equation (2.24) and, as a rule, are unstable. In some instances, the reported frequencies are very close to the frequencies of hydroxyls attached to EFAL species, and the assignments should be viewed with caution. With very few exceptions (217,709), the hydroxyl groups of this type are poorly characterized. 

A synergetic effect of cationic EFAL species on the T-O-T hydroxyl groups, enhancing their acidity and catalytic activity on H-Mordenite and H-ZSM-5 mildy steamed has been presented (93,94). 

However it must be pointed out that EFAL means nothing unless the different types are considered. Indeed, when steaming a NH, Y zeolite to generate a USY sample, the amount, nature and distribution of the EFAL species along the crystallite size depends on temperature and steaming conditions (90,91,96,97). 

At low steaming temperatures, and low steam partial pressure most of the EFAL is of cationic nature and it compensates T-O-T hydroxyl groups. Meanwhile EFAL is well dispersed across the zeolite. When the steaming temperature increases some of the EFAL condensates giving alumina type, pentacoordinated and tetrahedrally coordinated EFAL species, while a surface migration occurs. 

---