Bronsted protonic zeolites

This work will be continued by examining the X-ray patterns of materials that had been subjected to the heat treatment. Undoubtedly the Lewis acid form (decationated zeolite) has a different X-ray pattern than the Bronsted (protonated zeolite) because of the charge separation... 

In aU three cases the protonated zeolite, HZ, is created containing Bronsted sites, which, when heated above 550 °C, lose water to prodnce Lewis sites, viz. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

Another example is the extensive study by Haase and Sauer, whose calculations were carried out for a methanol molecule with a number of cluster models, the most sophisticated of which was sufficiently large to represent a portion of the faujasite lattice formed by three 4-R rings and one 6-R. All structures were fully optimized at the SCF and/or MP2 level. The authors found that, for all cases studied, the IP complex was a transition state, whereas the HB complex corresponded to a minimum on the PES of the system. However, the adsorption energies for both structures were very close to one other, with a barrier for proton transfer of only a few kilojoules per mole. A strong H-bond between the Bronsted proton and the O atoms of methanol, and a long and weak H-bond between the H atom of the molecule and the O atom of the zeolite... 

Figure 30 Proton-exchange reaction between methanol molecule and the Bronsted proton of sodalite lattice. Solid and dotted lines show distances between the methanol oxygen and the methanol and zeolite protons and respectively.

The structure work consists of synthesizing the faujasite material and characterizing it by X-ray diffraction. The mechanism of the synthesis has been studied and an investigation has been made of the nature of the replacement of the sodium ion by the ammonium ion and of the details of the process of the decomposition of the ammonium ion into the protonic zeolite (Bronsted acid) and decationated zeolite (Lewis acid). (We shall call the material in which the cation has been displaced by a proton and then heated to remove the proton as water, the decationated material.)... 

The nature of this catalytically active zeolite can be either Bronsted (proton donor) or Lewis (electron-pair acceptor) acidic, with protons attached to the framework tetrahedra or not (Figure 7.19). 

In the absence of water, typical vibrational frequencies were found for the protonated zeolite but an unanticipated degree of heterogeneity was also discovered. With the addition of one water molecule per Bronsted site, single-load, the ZSM-5 spectrum was considerably modified and strongly resembled earlier work on mordenite. The interpretation of this spectrum was, however, quite different fi-om the previous work and lead to its reassessment. A difference spectrum showing only the INS of water was compared with the results of two calculated spectra based on either a H-bonded water molecule or a HsO. The comparison clearly favours the H-bonded water molecule model and the presence of hydroxonium ions was discounted. 

Acetic acid sorbed on H+ZSM-5 held at 150°C was studied by tga, td/tas and FTIR. It was found that one molecule of acetic acid was sorbed per zeolite acid site with only a partial transfer of the Bronsted proton to the acetic acid. 

The Bronsted proton had been transferred to the acetic acid with two broad bands centred at -2870 and 2480 cm-1 observed. Similar bands have been observed for HgO sorbed at 80°C [9,10], but not for sorbed NH3. In the case of ammonia sorption the proton is completely transfered giving NH4+ and no broad bands are observed. The broad bands probably result from protons with a broad range of energies shared between the zeolite and the acetic acid. 

Acetic acid is strongly sorbed on to H+ZSM-5 held at 150°C. Tga showed a one-to-one association with the zeolite acid sites. The acetic acid molecule remained intact with very little or no further reaction at this temperature. FTIR spectra showed that the Bronsted protons cure only partially transfered to the acetic acid molecules and that they occupy a wide range of energy states. No behaviour was observed that could indicate the presence of basic sites. 

Because of their Bronsted acidity, zeolites can, in principle, be used as catalysts for any organic reaction subject to conventional proton catalysis. Table 6.2 lists the acidity levels of several classes of acid catalysts. Note that zeolites with a Hammett acidity function in the range of -13.6 to —12.7 are quite high in the acidity range. The only limitation would be the bulkiness of the molecule to be generated inside the zeolite cavity and desorbed from there. 

The resulting LH type model builds on this concept by considering that adsorbed NH3 reacts with surface NOx species. It is noted that NH3 adsorption on Fe-zeolite systems is not inhibited by the co-adsorption of water. A simple interpretation of this key observation is that the adsorption of NH3 and H2O occur on different sites. NH3 adsorption on protonated zeolites is known to occur on the Bronsted acid sites, and that has led Tronconi, Nova, and coworkers among others to propose for vanadia-based catalysts the exchange of NH3 between two types of sites. Applying this concept for Fe-zeolite catalysts gives ... 

The most straightforward hypothesis about the Bronsted acidic center in H-[A1]-MTS, as advanced even recently [43,44], is to assume that it coincides with that of protonic zeolites, the hydroxyl group bridging between an Si and an A1 atom. This acidic group will be designated subsequently as bridging OH species and represented as Si(OH)Al a model is given in Scheme 1. 

The role of Lewis acidity is particularly clear in the reaction of H-[A1]-MCM-41 with propene (Bonelh B, Garrone E, unpublished results). This reaction was studied to assess whether Bronsted acidity was strong enough to promote polymerization, a reaction clearly visible in the IR. Basically, the result was that, imlike protonic zeolites, H-[A1]-MTS systems do not polymerize propene. Traces of polymerization products observed increased with temperature of pretreatment, thus suggesting that the reaction occurs on Lewis and not on Bronsted sites. 

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites... 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

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