H-Y zeolite

Dihydropyran, H-Y Zeolite, hexane, reflux, 60-95% yield. H-Rho Zeolite can also be used as a catalyst. "... 

Large-pore zeolites such as Y zeolites are efficient for the hydroamination of several olefins. For example, propene reacts with NH3 over SK-500 (a pelleted lanthanum-exchanged zeolite) or La-Y or H-Y zeolites with 6-15% conversion to give i-PrNHj with high selectivity (95-100%) (Eq. 4.5) [50]. 

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

Keywords Two-Dimensional correlation, Infrared spectroscopy, Dealuminated H-Y zeolite, Bronsted acidity, MQ-MAS NMR. 

The liquid-phase dehydration of 1-hexanol and 1-pentanol to di-n-hexyl ether (DNHE) and di-n-pentyl ether (DNPE), respectively, has been studied over H-ZSM-5, H-Beta, H-Y, and other zeolites at 160-200°C and 2.1 MPa. Among zeolites with a similar acid sites concentration, large pore H-Beta and H-Y show higher activity and selectivity to ethers than those with medium pores, although activity of H-ZSM-5 (particularly in 1-pentanol) is also noticeable. Increased Si/Al ratio in H-Y zeolites results in lower conversion of pentanol due to reduced acid site number and in enhanced selectivity to ether. Selectivity to DNPE is always higher than to DNHE... 

Figure 4.35 IR spectrum of pyridine adsorbed on acidic sites of H-Y zeolite. (Reprinted from Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Vol. 58, J.H.C. van Hooff, J.W. Roelofsen, Techniques of Zeolite Characterization, pp. 241-283. Copyright 1991. With permission from Elsevier.)...

Comparison of the catalytic properties of H-Beta and H-Y zeolites for cracking n-heptane and gasoil shows that zeolite Beta should have more than one type of channel with different dimensions. For gas-oil cracking, zeolite Beta is less active and produces more coke and less gasoline than zeolite HY. 

NHj adsorption microcalorimetry was used by Shannon et al. [225] to follow the changes in acid sites of a H Y zeolite during dehydroxylation, framework dealumina-tion, and the formation of nonframework aluminum species. 

Similar results were also obtained by S tach et al. [229] who studied the acidity of H Y zeolites with different Si/Al ratios (2.4, 5.6, and 12.0) by calorimetric measurements... 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

The relationship of Lewis and Brpnsted acid site concentrations on H—Y zeolite was explored further in a study by Ward (156) of the effect of added water. At low calcination temperatures (<500°C) only a small increase in the Brpnsted acid site concentration occurred upon addition of water to the sample. Rehydration of samples dehydroxylated by calcination above 600°C resulted in a threefold increase in the amount of Brpnsted-bound pyridine. However, no discreet hydroxyl bands were present in the infrared spectrum after rehydration. Thus, the hydroxyl groups reformed upon hydration must be in locations different from those present in the original H—Y zeolite, which gave rise to discreet OH bands at 3650 and 3550 cm-1. 

Lewis acid centers, which were thought to be the primary catalytic sites. Boreskova et al. (51) studied the poisoning effect of quinoline on the cracking of cumene over Na, H—Y zeolite and observed a linear decrease in activity with the amount of quinoline added until a constant level of activity was reached. The catalytic activity was attributed to trivalent aluminum centers (Lewis acids), which were poisoned by coordinately bound quinoline. In a similar study of cumene cracking, Turkevich et al. (50) also concluded on the basis of magnetic resonance experiments that Lewis centers were the active sites. 

In order to distinguish clearly between the contributions of Lewis and Br0nsted acid sites to the cumene cracking activity of H—Y zeolite, a method of selective poisoning was applied by Jacobs and Heylen (44). As was shown previously (54), 2,6-dimethylpyridine (DMPy) selectively ad-... 

Initial Cumene Cracking Activity per Active Site of H—Y Zeolite (169) (% Conversion per... 

Active Sites of H— Y Zeolite for Cumene Cracking as Titrated with Pyridine and 2,6-Dimethylpyridine (Sites g1 x lO mY... 

Iso-octane Cracking Activity of Aluminum-Deficient Na, H—Y Zeolites (300°C, Hi... 

The spectra of alkaline earth ion-exchanged samples, with the exception of the barium form (211), have hydroxyl absorption bands at 3645 and 3540 cm-1, similar to those found in H—Y zeolite. The barium form behaves like the alkali-exchanged zeolites. The similarity of the spectra of the alkaline earth forms with that of the hydrogen form suggests that the acidic hydroxyls are associated with the same structural features (151). Band frequencies in the region of 3600 to 3560 cm-1 vary with the cations and are thought to result from hydroxyl groups associated with the divalent cations (211). They are weakly acidic or inaccessible to adsorbate molecules since the band intensity is not affected by adsorption of pyridine (209). 

Alkaline earth-exchanged samples examined by Ward (211) were more resistant to thermal dehydroxylation hydroxyl bands were present in the spectra after dehydration at 500°C. The concentration of OH groups was, however, much smaller than found in H—Y zeolite, and was dependent on the cation type. An almost linear inverse relationship was found between the alkaline earth cation radius and the concentration of acidic hydroxyl groups (210). 

Ward measured the o-xylene isomerization activities of Na, Mg, RE, and H—Y zeolites and found the rare earth form to be intermediate in activity between the magnesium and hydrogen forms as shown in Table IX (212). The sodium form was essentially inactive. He interpreted the activity relationship RE—Y > Mg—Y to result from the formation of two acidic structural hydroxyl groups per trivalent rare earth cation. The formation of acidic structure hydroxyl groups by exchange of sodium ions with protons in the rare earth solution, as proposed by Bolton (218), may also account for the greater activity of the rare earth-exchanged zeolite. 

The preparation of ammonium ion exchanged Y zeolites has long been known to be a precursor step in the preparation of H Y zeolites. The former materials have been characterized by thermo-gravimetric experiments, while the latter zeolites that contain the H atoms as hydroxyl groups have been intensively examined by infrared spectroscopy (1 .2 A systematic description of the... 

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