Zeolites protonation

Figure 2. Transition state complex in the ethanol + 2-pentanol 8, 2 reaction activated by the proton at the chaimel intersection of H21SM-5 [14]. The zeolite pore structure is represented as a wire-frame section of the intersecting channels produced by the MAPLE V software package. The zeolite proton that activates the 2-pentanol molecule is marked with. ...

Operando DRIFTS examination of the working zeolite catalysts shows adsorbed hexane but do not support the presence of bound alkoxide/olefin/carbenium ion species. Data substantiate that alkanes may be activated without full transfer of zeolite proton to the alkane, i.e., without generation of any kind of real carbocation as transition state or surface intermediate. 

S. R. Blaszlowski and R. A. van Santen, Quantum chemical studies of zeolite proton catalyzed reactions. Top. Catal. 4, 145-156 (1997). 

When both 68263 (or Zn) and H are present within the catalyst, one can assume that the polar propyl carbenium species produced by reaction (3) over 63263 phase will possibly exchange with a zeolite proton through an alkyl surface migration... 

A series of zeolite-Y hosts containing different proton concentrations has been used for MTO encapsulation [80], and the resulting materials were studied for 1-hexene metathesis. The MTO molecule was activated by intra-zeolite protons, and simultaneously blocks their isomerisation activity. The ability to tune intra-zeolite acidity and the doping levels of the intact MTO precatalyst permits control over selectivity in the metathesis reaction. 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

In considering the activation of C-C bonds, we limit ourselves to discussing the activation of ethene and the cracking of ethane, butane, and isobutane. Some of the earliest calculations characterizing the activation of olefins were performed by Kazansky and Senchenya (258-260) and Pelmenschikov et al. (261). In these calculations, it was shown that ethene could interact with a zeolite proton to form either a it- or [Pg.101]

Fig. 18. Interaction of ethene with a zeolite proton, (a) shows the j7-bonded complex, (b) the transition state, and (c) the cr-bonded complex (260).

Chemisorption of acetic anhydride on the zeolite protonic sites with formation of acylium ions and of acetic acid (Step 1). 

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). 

We also illustrate this for water protonation. The zeolite proton will attach to the water oxygen atom, The proton of water will bind to the basic lattice oxygen anion ... 

Fig. 1.2 Parameters determining the acid strength of the zeolite protonic sites...

The results on tables 9 and 10 show that increasing the size of the cluster and/or the basis set does not change appreciably either the structure of the TS s or the activation energies obtained at a lower level of calculation (3T cluster and DZP basis set). Irrespective of the substrate and level of calculation employed, the results show that the protolytic cracking involves the attack of the zeolitic proton to a... 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

However, EXAFS data show that in reality the average Pd particle size is larger in Pd/NaY than in Pd/HY. This observation is clearly related to the interaction of small Pd particles with zeolite protons, which will further be discussed in Section VIII. A change of Pt particle morphology in zeolite L has also been reported due to H2 (26). This effect has tentatively been attributed to the support, though its chemical nature remains to be clarified. 

Recent uses of EXAFS, X-ray scattering, and FTIR spectroscopies have revealed that exposure of zeolite-entrapped Pd or Rh often induces thorough reorganizations of the metal atoms. The combination of CO and zeolite protons can sometimes even lead to changes of the oxidation state of the entrapped metal. 

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. 

The second step involves reaction of zeolite protons with the metal oxides ... 

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. 

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

The interaction of metal clusters in zeolites with protons has been studied by isotope exchange with D2 280-283). The presence of metals or metal impurities is essential for this exchange to occur at moderate temperatures absolutely no exchange between D2 and zeolite protons in metal-free HY is detected at room temperature. In the presence of Pt or Pd, however, exchange is fast and includes all protons in the zeolite, which are detectable by their O—H vibration bands in FTIR. 

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