Zeolite NaHY

In the same experimental conditions (lh, 160°C), the protonic zeolite NaHY gives rise to quantitative percentages of conversion, being the rate 2/3 = 33 65, while at 130°C, 3h, the conversion is of a 15% and the main product the ketone 3. 

The acid properties of NaH zeolite (NaHY, NaHMOR, NaHMFI) series with different Na contents were investigated with adsorption microcalorimetry23. Whatever the zeolite, the initial heat of base adsorption (NH3, pyridine, etc.) as well as the total acidity increases with the exchange of Na+ by FT Thus for a series of NaHMOR zeolites24, the initial heat of ammonia adsorption passes from 145 to 184 kJ mol"1 when the exchange rate passes from 33 to 98 % and the number of acid sites with adsorption heat greater than 110 kJ mol 1 passes from 0.4 to 9.3 1020 g"1 i.e. from 7% of the theoretical number of protonic sites to 56 %. 

Oxidative redispersion of Pd and Cu aggregates in zeolite NaHY has been reported (76,143,205). First the metal (M = Pd or Cu) is oxidized to oxide particles ... 

Fig. 37. Transmission electronic spectra of zeolite NaHY loaded with 1.33 kPa propene at room temperature after 5 min (a), 1 h (b), 2 h (c) at 320 K after 1 h (d) after evacuation for 1 h at room temperature (e), at 400 K (/) and at 570 K (g). Reprinted from [119] with kind permission of Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands...

Only scant information is available about the influence of coke formation on the alkylation mechanism. It has been proposed that, similar to the conjunct polymers in liquid acids, heavy unsaturated molecules participate in hydride transfer reactions. However, no direct evidence was given for this proposition (69). In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced from an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

Barthomeuf and Beaumont (25) have found that the activity for iso-octane cracking remains unchanged for NaHY zeolites in which up to 33 percent of Al has been extracted with chelating agents. This corresponded to a dealumination from 56 to 37 Al/u.c. Further dealumination resulted in a decrease in... 

Influence of Temperature. Data concerning the thermal stability of the catalytic activity are given in Figure 2 and Table I. The thermal stability of the starting materials Na-8.7 and D.Na-5.4 is discussed first. The limit of stability of the Na-8.7 sample appears to be higher than for the NaHY zeolites studied previously (3, 6, 27, 28). Nevertheless, this sample cannot be considered ultrastable since neither its structural data nor the thermal stability of its OH groups are characteristic of ultrastable zeolites (17). This increase in the stability may be explained by dry air heating and subsequent rehydration. 

In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced firom an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

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 term electron deficiency was introduced by Dalla Betta and Bou-dart to account for the anomalously high hydrogenation activity of small Pt particles in zeolite Y (50). The electron deficiency was ascribed to an electron transfer from small Pt particles to the zeolite. X-Ray absorption has been applied to measure the Pt Lm white line area as an indication of the electron deficiency because the white line is related to the number of unoccupied electronic states in the 5d and the 6j bands (273). For reduced Pt/NaHY it appeared that the white line area, and hence the electron deficiency of Pt particles, are closely related to the proton concentration of the zeolites. For example, the relative white line areas for Pt/H4gY, Pt/ H19Y, and Pt foil are 1.6, 1.2, and 1, respectively. White line areas at the Liii X-ray absorption threshold to determine the if-band occupancy of supported metal catalysts were first reported by Lytle 274). The use of the white line area as an indication for electron deficiency has been questioned by Lewis, who argues that a decrease of the metal particle size will also lead to an increase of the white line area (275). 

For Pd/NaX, it was found with ESR and XPS that reduction of Pd at low temperature results in Pd and small charged clusters Pd " (y > j ) (278,279). Likewise, monopositive Pd" was observed with XPS in zeolite Y after mild reduction (780). As shown in Fig. 18, the binding energy of Pd 3ds/2 in Pd/NaHY is significantly shifted to higher values with respect to the Pd reference. This shift is, in part, due to the small particle size. However, neutralization of Pd/NaHY at low temperature will leave the particle... 

The product distribution was found to be different for Pd on different supports, indicating shape selectivity. Higher hydrocarbons were detected in appreciable concentrations over Pd/NaHY at elevated pressure, but they were totally missing from the product obtained over Pd in zeolite 5A at 11 bar. On Pd/5A the only products are methane, methanol, and dimethyl ether. 

I lG. 57. Tirae/teniperature-scaled Pt spin lattice relaxation data at the surface peaks of nominally clean platinum particles in six different zeolites. Only the data for Pt/NaX zeolite (one dispersion 0,65) have actually been obtained at different temperatures (between 80 and 250 K) all other data are at 80 K only. The triangles represent data for three different dispersions (0.56, 0.64, and 0.77) in Pt/NaY zeolite the squares represent tw o dispersions (0.72 and 0.79) in Pt/NaHY zeolite. The small black dots with error bars represent a doubleexponential fit and its errors. Apparently, the relaxation varies with zeolite type but much less with dispersion. In general, the circles represent a faster relaxation than the squares therefore, in Pt/NaX zeolite the F,f LDOS on the platinum surface is higher than that in Pt/NaHY zeolite. An increase in zeolite acidity decreases the metal surface Ef LDOS. Fhe double-exponential fits (curves) are used to quantify this statement (Fig. 58). 

Catalytic Activity. The catalytic properties of NaHY zeolites were studied. Results are shown in Figure 3. A semilogarithmic scale is chosen to show more clearly the variation of activity for an exchange including 17-37 atoms. 

Fig. 4. Differential heat of pyridine adsorption on NaHY zeolites at 473 K with different Na contents (Adapted from Ref. 151.)...

A direct comparison with other Pt-zeolites seems to be difficult owing to the very large conversion differences. Hence, Y[Cg], i.e. the summarized yields of skeletal products was calculated for three catalysts the present one, Pt-HY and Pt-NaHY, the latter two possessing different acidity [13]. The distribution of these individual products is shown in Table 1 at a value of Y[C0] being - 1%. The... 

Isomerization and fragmentation selectivities over the other acidic Pt-zeolite catalysts (Pt-HY and Pt-NaHY [13] as well as on a mixtiu e of HY and Pt/SiOo [15]) have been found to be roughly equal. The highest abimdance of 2,3DMB among the Cg products over Pt-HZSM (Table 1) indicated that the importance of acid catalyzed isomerization route was most important over this catalyst. 

High activity and selectivity in the pinacol rearrangement of simple diols were observed in subsequent studies. Y zeolites (HY [27], LaHY [26,28], CaHY [28], NaY, and NaHY [29,30]) had the highest activity, and they were also more selective than X zeolites [29,30]. Though HZSM-5 was less active, its activity remained constant whereas substantial deactivation was observed for Y zeolites [28]. This is explained by the higher resistance of HZSM-5 to coke formation. 

NaHY, NaY and HY zeolites (PQ Corporation and Conteka) and mordenite (Norton 900H) were commercial samples. HZSM5 (Si/AI=40) was synthetised according to Mobil patents. Zeolites were calcinated at 520 °C under a dry air flow, for 12 hours. The characteristics of the NaHY and HY samples are given in Table 1. 

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