Protonated faujasite

K. P. Schroder, J. Sauer, M. Leslie, C. R. A. Catlow, and J. M. Thomas, Chem. Phys. Lett., 188, 320 (1992). Bridging Hydroxyl Groups in Zeolitic Catalysts A Computer Simulation of Their Structure, Vibrational Properties and Acidity in Protonated Faujasites (H-Y Zeolites). 

Partially protonated faujasite X is far superior to amorphous silicas and to other zeolites in terms of efficiency and regioselectivity (Table 2.3). Advantages of /-butyl hypochlorite over other chlorinating reagents have been demonstrated.41 The methodology is applicable to a range of substituted benzenes with... 

Schroder KP, Sauer J, Leslie M, Callow CRA, Thomas JM (1992) Bridging hydrodyl groups in zeolitic catalysts a computer simulation of their structuie, vibrational properties and acidity in protonated faujasites (H-Y zeolites). Chem Phys Lett 188 320... 

Figure 2. Structure of the Faujasite right part, a natural Faujasite with location of cations (l,r, II, ir and U) on symmetry axis. Left part, a protonated Faujasite with dots 1,2,3 and 4 showing the possible locations of -OH groups.

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Then, contrary to our previous hypothesis, the reaction proceeds via a Bai2 displacement of aniline on DMC. The product, mono-A -methyl aniline (PhNHMe), plausibly adsorbs into the zeohte in a different way with respect to anihne, because different H-bonds (N H — O-zeolite) take place with the solid. As recently reported by Su et al., A-methyl amines also may interact with NaY by H-bonding between the protons of the methyl group and the oxygen atoms of the zeolite this probably forces the molecule a bit far from the catalytic surface in a fashion less apt to meet DMC and react with it. This behavior can account for the mono-A-methyl selectivity observed, which is specific to the use of DMC in the presence of alkali metal exchanged faujasites in fact, the bis-A-methylation of primary aromatic amines occurs easily with conventional methylating agents (i.e., dimethyl sulfate). ... 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

Selective hydroxylation of phenol with hydrogen peroxide was reported on acid zeolite catalysts [91-92]. Peroxonium ions, formed by H2O2 protonation, are the oxidizing species. When the reaction is carried out on a faujasite catalyst, a mixture of hydroxybenzenes and tars is obtained [91]. In the presence of H-ZSM-5 on the other hand, no tar formation was mentioned (which does not necessarily mean that it was absent) and p-selectivities close to 100% were reported for the hydroxylation [92]. These superior selectivities reflect the shape selective properties of ZSM type zeolites. 

Partially proton-exchanged Na faujasite X, in turn, is the best catalyst for selective monochlorination with tert-butyl hypochlorite.258 NaX, NaY, and NaKL zeolites used in the chlorination of toluene with sulfuryl chloride undergo rapid deactivation because of the accumulation of polychlorinated toluenes in the pores of the catalysts and dealumination.259, 260 Direct electrophilic fluorination of aromatics can be effected by using Selectfluor in the presence of triflic acid.261 Electrophilic fluorination may also be carried out by R2NF and R3N+FA reagents.262 Elemental fluorine may also act as a powerful electrophile in acidic media (sulfuric acid, trifluoroacetic acid, or formic acid), but monosubstituted aromatics give isomeric mixtures.263-265... 

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-... 

Regarding the reaction of Cp2Fe with an acid faujasite, it is possible to exclude a simple acid-base reaction, yielding protonated... 

HMordenite, HFaujasite-780, HFaujasite 720 and Na-Faujasite zeolites. Among the different catalysts, HFaujasite-720 was the most active and selective catalyst towards 2,4-dinitrotoluene, achieving a yield of dinitrotoluenes of 92 % with a ratio of 2,4- to 2,6- isomers of 4.3 1 in 3 min reaction time. Using this zeolite, l-chloro-2-nitrobenzene and pyrazole were also nitrated regioselectively to obtain l-chloro-2,4-dinitrobenzene in a l-chloro-2,4-dinitro l-chloro-2,6-dinitro ratio of 30 1, and 1,4-dinitropyrazole in 80% yield, respectively. The authors proposed a nitration mechanism in which the protons in the zeolite are replaced by nitronium ions derived from N2Os in a fast pre-equilibrium process. This produces active sites for transfer of nitronium ion from faujasite to aromatic in the rate-controlling step. 

Although several methods have been reported in recent literature concerning the preparation of 5-hydroxymethylfurfural by dehydration of fructose, we have shown that microporous catalyts in their protonic form, for instance, Mordenites, Beta, Y-faujasites and ZSM-5 zeolites constituted a convenient alternative route to the catalysts used up to now, namely mineral acids, oxides or ion-exchange resins/27,281... 

In the faujasite framework there are also four crystallographically distinct oxygens. The experimental proton occupation in H-faujasite is 8 2 4 for the... 

Discrimination between surface and intrazeolite sites is often difficult. The chemical reactivity of zeolite-bound complexes to reagents of different sizes is helpful in determining the location of an irmnobilized complex. For example, in rhodium faujasites prepared from Rh(aUyl)3 and H-Na-X (i.e. the faujasite Na X in which a fraction of the sodium ions have been replaced with protons), catalytic activity for hydrogenation can be limited to the intrazeolite sites only when P(n-Bu)3 is used to poison the surface rhodium sites. Tri-n-butylphosphine is too large to penetrate the zeolite pores, thus the rhodium complexes within the zeolite remain catalytically active. 

The decomposition of Co2(CO)8 in faujasites has been studied in some detail. Low-temperature spin-echo ferromagnetic nuclear resonance spectroscopy shows that very small Co particles are formed in supercages of zeolite NaX by microwave plasma activation at low temperatures (86). In situ far-infrared spectroscopy revealed that adsorbed Co2(CO)s interacts with accessible supercage cations in NaY and CoY (239). Carbonyl complexes of different Co nuclearity, such as Co4(CO)i2 and Co(CO)4, are also formed (227,228). In HY the Co atoms are oxidized to Co ions by the zeolite protons. 

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