Zeolites proton release

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. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

When calcination is carried out at 5(X)°C and reduction is at 2(X)°C, the original nuclearity n is 1. For room temperature and zeolite Y the final nuclearity an is 13. The migration and coalescence of the mobile primary Pd carbonyl clusters leading to the formation of PdnfCO) , clusters, with concomitant release of zeolite protons, are schematically illustrated in Fig. 8. 

The release of zeolite protons due to the cutting of the original anchors has been independently detected by FTIR. It was found that the intensity of the supercage OH band between 3640 and 3650 cm increases on admission of dry CO to the reduced sample, in agreement with Eq. (7) and Fig. 8 (752). The effect of CO on the migration of monoatomically dispersed Pd in NaHY is clearly demonstrated by the EXAFS functions shown in Fig. 9 and their Fourier transforms shown in Fig. 10. 

Sachtler proposed 87) an explanation of the CO release and concomitant changes in the IR band characteristic of zeolite O—H vibrations involving chemical interaction of zeolite protons and Pd carbonyl clusters. 

Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites.

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Whereas the synthesis of zeolite occurs in nature and in the laboratory under strongly basic conditions (pH 9-11), they are widely used as catalysts in hydrocarbon chemistry under their acidic form. In order to obtain acidic zeolites, the alkali cations (K, Li, Na, Ca, etc.) are first exchanged by NH4+C1 followed by heating which, after release of ammonia, leaves the proton loosely attached to the framework on the Si-O-Al bridging group (Figure 2.19). 

Protons are released upon heating which in part balance the negative charge of the host clay layers. A number of review articles have recently appeared which summarize the synthesis and physical properties of metal oxide pillared days derived fix>m the intercalation of polyoxocations of aluminum, zirconium, chromium and many other metals [10-12]. The Lewis acid sites provided by coordinatively unsaturated metal ion sites on the pillar and the Bronsted addity formed upon thermolysis imparts novel chemical catalytic properties [13,14]. Since the pores between pillars often are larger than those foimd in conventional zeolites, there is considerable interest in the use of metal oxide pillared clays for the processing of large organic molecules, espedally petroleum [14-17]. 

Above 800 K only the release of HCl was observed. Obviously, ions from the NH. -zeolite were replaced. Some NH Cl, which may have formed intermittently, would be thermally decomposed into NHj and HCl. At higher temperatures, H" " from the deammoniated material was exchanged for La ions from the solid LaClg. Since the protons were consumed by this solid-state reaction with LaClj, the dehydroxylation peak observed with pure NH -Y around 950 K should not occur upon heating of the LaClj/NH -Y mixture. As can be recognised from Figure 8, this peak is indeed absent in the TPE spectrum of LaClj/NH -Y. 

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