Hydrothermal dealumination

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Figure 1. Reaction mechanism for hydrothermal dealumination and stabilization of Y zeolites.

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

The increase in octane observed using dealuminated faujasite compared to high cell size rare earth exchanged faujasite has been correlated with the Si/AI ratio of the sieve and with the sodium content (3). While the relationship between Si/Al ratio as measured by unit cell is confirmed by pilot unit studies in our laboratory. Figure 1, the relationship with sodium content is more complicated. Figure 2. Sodium added to the catalyst after hydrothermal dealumination reduces activity but does not affect octane, while sodium present before hydrothermal dealumination increases activity but does reduce octane. This result implies that selectivity for octane is related to structures formed during... 

Fig. 26. J9Si MAS NMR spectra (471) at 79.80 MHz of the novel zeolite theta-1 (439) (top) and its hydrothermally dealuminated form (bottom).

Spectral information on several dealuminated zeolites (91), as well as on ZK-4 and TMA-sodalite studied by Jarman (104), who also considered the quantitative relationship between 6 and 6, is given in Table IX. The average values of 0 for hydrothermally dealuminated zeolite Y and dealuminated acid-washed mordenite, as determined by X-ray diffraction, are very close to those in nondealumina ted materials, and Thomas et al. (56) assumed that this is also true for other dealuminated zeolites. They proposed the following linear correlation between 0 and S (see Fig. 30) ... 

Fig. 6 29gi MAi> NMR spectra of hydrothermally dealuminated samples. Temperatures in °C. Reproduced with permission from reference 30a. (Copyright 1985 Royal Society of Chemistry)...

In hydrothermally dealuminated faujasites additional 27A1 lines in the 50-30 ppm range appear, in addition to the tetrahedral and octahedral lines at 60 and 0 ppm. Gilson found a line at 34 ppm in zeolite Y with framework Si/Al = 20 by steaming at 760°C in 100% steam. Freude and coworkers (39,40) have described the use of 1H-and 27A1-MAS-NMR techniques to determine the dealumination mechanism in the hydrothermal and acid treatment of zeolite Y. They determined the framework aluminum from the equivalence of A102 anions in the framework and NH4 + cations using the Kjeldahl method to determine the amount of NH3 in the zeolite. 

The total concentration of OH groups in dealuminated zeolite Y samples was determined using 1H-NMR techniques described (40) and is in reasonable agreement with the concentration of tetrahedrally coordinated extra-framework Al. The concentration of SiOH groups does not change in hydrothermal dealumination, but increases on acid leaching due to removal of framework Al. 

During the course of hydrothermal dealumination (ultrastabilisation) of zeolite Y quadrupole nutation NMR detects four kinds of aluminium sites. In addition to signals from the framework (F), non-framework tetrahedral (NFT) and non-framework octahedral (NFO) aluminium there is a signal which we attribute to distorted framework (DFT) aluminium bonded to hydroxyl nests formed during dealumination. 

A closer look at the mechanism of hydrothermal dealumination (hydrolysis of Si-O-Al bonds) illustrates that a healing process also takes place during this transformation (3), but contrary to the chemical substitution highlighted above, the source of Si originates from the zeolite, and results in a partial destruction of the zeolites and Si migration. The mesoporosity created during the process is beneficial to the diffusion of the large molecules of oil. 

Two sets of chemically and hydrothermally dealuminated zeolites were prepared from separate sources of partially ammonium-exchanged Y zeolite. The first set of samples, designated USY-1 and AFS-1, were prepared from Davison ammonium-exchanged Y zeolite (Y-l). The second set of samples, designated USY-2 and AFS-2, were prepared from Linde LZ-Y62 zeolite (Y-2). Typical physical and chemical properties of the two starting materials are compared in Table I the primary difference between these materials is the extent of soda removal by ammonium exchange. 

The hydrothermally dealuminated zeolites (USY-1 and USY-2) used in this study were plant-grade materials and were used without further exchange. The chemically dealuminated zeolites (AFS-1 and AFS-2) were prepared in our laboratory following the method of Breck and Skeels(12). The AFS samples were washed thoroughly until residual fluoride was no longer detected in the filtrate AFS-1 received additional warm-water washes to reduce further fluoride levels in the solid. AFS samples were prepared so as to have the same extent of framework dealumination as the USY samples. 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

Characterization and catalytic data have been presented for chemically and hydrothermally dealuminated Y zeolites. These data show that zeolite structural differences lead to differences in catalytic behavior. USY and AFS zeolites show distinct structural differences when freshly prepared and after calcination, however, these differences are diminished on steam treatment. As a result, the catalytic behavior of calcined AFS and USY zeolites appears different while that of steamed zeolites is similar. No apparent effects due to source of Y zeolite were observed. 

NPRA Annual Meeting, 25-27 March 90, paper AM-90-42. Beyerlein,R.A., Feng,C.C,Hall,J.B., Higging,B.J. and Ray,G.J. Evolution of NFA Species during hydrothermal dealumination of USY Catalysts... 

Hydrothermally dealuminated PER and sample that were subsequently acid treated exhibited better selectivities for isobutylene formation than an untreated PER catalyst (27). Furthermore, hydrothermally dealuminated PER exhibited a lower activity than untreated PER but higher selectivity for isobutylene 30,62,66). A subsequent acid treatment (with 5% HCl solution) further decreased the conversion and increased the isobutylene selectivity. The hydrothermal treatment created mesoporosity by A1 extraction. The A1 extraframework species were located in the mesopores and/or in the micropores. The HCl treatment removed part of the extraframework Al, leaving part in the micropores. The elimination of extraframework A1 from the mesopores was evidently beneficial for isobutylene selectivity. Evidently, the active sites associated with extraframework Al located in large voids are nonselective in contrast, extraframework Al located in the micropores (and not removed by acid treatment) does not contribute to catalytic activity. The steamed and acid-washed ferrierite exhibits excellent isobutylene selectivity and catalytic stability 30). 

For such treated samples it is not easy to discriminate between two possible effects of dealumination, namely, the removal of some acid sites and the decrease in microporosity due to the deposition of aluminum-containing debris in the pores. Thus, hydrothermally dealuminated FER, hydro thermally dealuminated acid-washed FER. acid-washed FER, and CsFER were compared under the same experimental conditions (62). The results indicate the following order of isobutylene selectivities untreated FER < acid-treated FER < hydrothermally treated FER < hydrothermally acid-treated FER < CsFER 61). These results, obtained with noncoked catalysts, reinforce the interpretation in terms of shape selectivity. The hydrothermally acid-treated sample has acid sites located only in the micropores, and the aluminium debris in the micropores creates an additional constraint playing a role identical to that of Cs" in FER. 

Hydrothermally dealuminated Y type zeolites (HDY s) possess a number of characteristics which make them very useful catalyst components—very high activity for acid-catalyzed reactions and outstanding thermal stability Such zeolites were thus rapidly incorporated into catalysts for two major petroleum refining processes, catalytic cracking [lp2] and hydrocracking, [3,4] which operate at high temperatures, and for which resistance to process upsets and the ability to withstand oxidative regeneration are both important. 

The Si MAS NMR spectrum (Fig, 3) of the untreated sample shows a main signal at about - 107.7 ppm [Si(OSi)4 in the framework] and a minor one at about -101 ppm [Si(OSi)30Al in the framework]. The additional spectral intensities can be assigned to amorphous silica and amorphous aluminosilicate produced in the hydrothermal dealumination process. 

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