Steamed zeolites

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

In conclusion, A1 MAS-NMR and IR results show the formation of amorphous silica-alumina during dealumination. It shows stronger acidity than the hydroxyls attached to zeolite framework aluminums. This silica-alumina can account for the superacidity observed by some authors in steamed zeolite samples (12,13). 

K in 6RuNa faujasites with different A1 contents (A1 per unit cell). The 2 samples with lowest A1 content are obtained via dealumination with SiCl. The open point correspond to a steamed zeolite support (ultrastable zeolite). 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

Dealuminated zeolite samples were calcined in air at 540°C for three hours prior to catalytic testing. A portion of each sample was further modified using high-temperature steam. Zeolite samples were placed in a fixed-bed quartz tube 95% steam was passed through the bed at 750°C and atmospheric pressure for 4 hours. 

Catalytic Properties. Catalytic properties were determined for both calcined and steamed zeolites using hexadecane cracking as a test reaction. Hexadecane cracking provides information on zeolite activity and selectivity which can be used to estimate octane production. Data were obtained for both calcined and steamed zeolites at hexadecane conversions from 30% to 70% multiple runs were made for each catalyst. 

Selectivity results at constant 50% conversion are reported in Tables VI and VII for calcined and steamed zeolites, respectively. Product selectivities are divided into light gas (C1-C4), gasoline (C5-C12) and coke. The gasoline fraction is further divided into paraffin, olefin, naphthene and aromatic (PONA) components. 

Calcined AFS and USY zeolites show significant differences in both selectivity ratios whereas steamed zeolites show similar light gas selectivities. However, steamed selectivities are dramatically different from those of calcined zeolites. These results are in good qualitative agreement with results obtained for AFS and USY zeolites by gas-oil cracking (17). 

Figure 7 shows calculated octane numbers from hexadecane cracking as a function of gasoline yield. Calcined and steamed zeolites are represented by open and closed symbols, respectively. The calculated octane number reflects changes in the gasoline molecular weight distribution and, to a lesser extent, composition effects. 

Extraframework aluminum contributes to the observed catalytic behavior in both activity and selectivity. It is interesting to note that although steamed USY contains more extraframework aluminum than steamed AFS, both AFS and USY give similar product selectivities. Observed product selectivities from steamed zeolites are insensitive to the amount of extraframework aluminum present within the composition range investigated in this study. 

Through the use of hexadecane cracking alone, we have been unable in this work to elucidate the role of mesoporosity in the catalytic behavior of calcined or steamed zeolites. Steamed AFS and USY zeolites show differences in mesoporosity but exhibit similar catalytic performance. While mesoporosity may affect diffusion in actual FCC catalysts, larger molecules than hexadecane will be required to determine mesopore effects. 

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. 

A synergistic effect was observed for the hydroisomerization of n-decane with intimately mixed Pt/ZSM-22 and deep-bed steamed zeolite NH4Y (311). The synergism is rationalized in terms of a two-step isomerization mechanism. Pt/ZSM-22 converts decane selectively into 2-methylnonane, even at high conversion. The Y sieve receives this 2-methylnonane as its feed inside this Pt-free sieve a high concentration of monobranched alkenes will build up. This enhances their subsequent conversion to multibranched alkenes. 

A major problem with Al NMR spectroscopy of steamed zeolites (and with dehy-droxylated minerals such as metakaolinite (MacKenzie et al. 1985)) is the inability to detect all the Al present at lower applied fields and slower MAS speeds. The presence... 

Zholobenko et al. used n-hexane cracking, alone and with co-addition of small amount of n-hexene to assay the acidity of steam-dealuminated H-ZSM-5 catalysts. They observed that addition of the olefln increased the activity of the fresh zeolite but had the opposite effect on the steam-dealuminated sample. They concluded that the steamed zeolite had Lewis sites which were enhancing the acidity of the Brpnsted sites but were also readily poisoned by the olefin. They were not able to use this technique to quantify the different sites. 

Lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd) exchanged Y zeolites with different rare earth content, as well as mixed lanthanum-cerium exchanged. eolites, taute been prepared and characterized. sSi and 7A1 MASNMR spectra, surface area, crystallinity and unit cell size data were obtained for fresh and steamed zeolites. The stability of these zeolites toward steam has been investigated. An increase in lanthanum content from about 14 wt% La 03 to over 20 wt% enhances the stability of the Lar zeolite towards dealumination by steam. A similar effect was observed >r NdY and PrY zeolites. The resolution loss of Si- JR sigqia 1 s observed for paramagnetic jpns (Nd3, Pr, ... 

Upon steaming zeolites containing lanthanum-cerium mixtures, dealumination increases and stability decreases with increased cerium content of the zeolite. The differences in observed stability are discussed. XPS data show that steaming induces the migration of both aluminum and rare earths to the surface of the zeolite crystals. 

XPS data for fresh and steamed LaY and CeY zeolites are shown in Table II. The steamed samples show an enrichment in surface aluminum when compared to the fresh samples. This is indicative of steam induced non-framework aluminum migration to the crystal surface. The aluminum enrichment of the surface of steamed zeolites increases in the order Hi-LaYframework dealumination (vide supra). 

Na-, La-, and Re-exchanged zeolites have also been used as catalysts of the Michael reaction between silyl enol ethers and a, 6-unsaturated carbonyl compounds. This study, performed by Sasidharan et al. [87], focused mainly on the catalytic activity of titanium silicalite molecular sieves (TS-1 and TS-2). They found that TS-1 and TS-2 catalyze 1,4-Michael addition of silyl enol ethers and a,y5-unsatu-rated carbonyl compounds under anhydrous conditions. The zeolites tested as catalysts of this reaction, e. g. ReY, LaY, steamed zeolite Y, and cation-exchanged ZnZSM-5, were less active (or inactive). 

B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites. 

High-resolution multiple quantum (MQ) Al MAS NMR of steamed zeolite Y, collected at 600 MHz, with satellite peaks asterisked (left) and ID Al MAS NMR of the same sample collected at three different fields (right). Taken together, these spectra indicate the presence of aluminium in octahedral five-fold (AP ) and two kinds of tetrahedral... 

Figure 26.4. (a) Rate of phenol formation in benzene oxidation with N2O on H-ZSM-5 depending on Fe content, (O) dehydrated zeolite, ( ) steamed zeolite at 600°C. (b) Rate of phenol formation in benzene oxidation with N2O on H-ZSM-5 depending on the concentration of Al-Lewis sites on (O) steamed zeolites and on ( ) in situ dehydroxylated zeolites at 720°C. Adapted from Ref. 9. 

K is the ordinary OH group, and the other giving maximum activity at 853 K is similar to that postulated for the steamed zeolites. Only the latter is active for propane conversion. It was confirmed that 13% of the framework aluminum was dislodged after pretreatment at 853 K. It was shown that the activity for propane conversion was greatly enhanced by steaming. 

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