Zeolites surface dehydroxylation

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

The photonics of Nph molecules on the surfaces of the zeolite X as well as on aerosil has been studied. Nph in zeolite voids is present as physically adsorbed molecules and bound in donor-acceptor complexes (CTC). Water vapor admission to an adsorbed Nph causes oxidation under formation of luminescent products of a-naphthoquinon on the Na-forms and its mixture with b-naphthol in presence of the alkaline-earth cations. Assumption about participation of oxygen chemisorbed on acid centers in an oxidation reaction is substantiated. In presence of transition metal ions the Nph luminescence is totally quenched. The Nph molecules adsorbed on the dehydroxylated aerosil surface form bimolecular excimer-like associates with a stable ground state and resistance to UV irradiation. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

Of great interest is the question of the role of trigonal aluminum, which is usually assumed to act as a LAS. Such a center should be quite typical of A1203, where it may appear as a result of surface oxygen vacancy formation. These vacancies may either develop due to dehydroxylation or be of a biographical nature. A similar situation may take place in the case of such mixed oxides as amorphous aluminosilicates. Uytterhoeven, Cristner, and Hall 123) have concluded that trigonal aluminum could also appear as a LAS upon dehydroxylation of H forms of zeolites. Their scheme was criticized, however, by Kiihl 124), who has undertaken X-ray fluorescence studies of the dehydroxylated forms of faujasites and found that the dehydroxylation was accompanied by dealumination of a zeolite framework with formation of extralattice aluminum which could also exhibit the Lewis acidity. 

Furthermore, results obtained after heating at 550°C are in good agreement v ith the postulated dehydroxylation of La-H-Y zeolite under heating (3). According to this idea, increasing the temperature beyond 300°—400° C eliminates one proton from the surface then the slope... 

X-Ray photoelectron spectroscopy data showed an increase of about 20% in the Al/Si ratio as the outgassing temperature was increased from 773 to 1173 K, indicating that the surface concentration of A1 increased (169). Treat-ment with HCl to dissolve nonstructural aluminum produced a 15% decrease in the aluminum content of the sample calcined at 1173 K with respect to the 773 K sample. Subsequently, a small extent of dealumination occurred during the dehydroxylation at high temperature. It was suggested that aluminum from the lattice was extracted on calcination and resulted in an aluminalike species that stayed within the cavities of the zeolite. 

The water adsorbed on external surfaces and the zeolitic water from the nanoporous tunnels is removed at relatively low temperatures. The elimination of coordinated, structural water starts when the zeolitic water is lost and is completed when dehydroxylation begins. Folding of the sepiolite crystals occurs when some structural water has been removed. This process, reversible for temperatures below 350°C, becomes irreversible once all the structural water molecules are removed and partial dehydroxylation has occurred, forming an anhydride form. Finally, the remaining Mg-OH hydroxyl groups are released at 850°C. 

Flgare 4-5. Schematic representation of the removal of water from the internal surface of a hydrogenform zeolite sudi as HY zeolite. Hie process is called dehydroxylation and converts two Br0nsted add sites (OH groups) into a Lewis add site (exposed Al cation). 

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