FAU zeolite

Romero Sarria, F., Blasin-Aube, V., Saussey, J. et al. (2006) Trimethylamine as a Probe Molecule To Differentiate Acid Sites in Y-FAU Zeolite FTIR Study, J. Phys. Chem. B, 110, 13130. 

Kwak, J.H., Szanyi, J. and Peden, C.H.F. (2004) Non-thermal plasma-assisted NOx reduction over alkali and alkaline earth ion exchanged Y, FAU zeolites, Catal. Today 89, 135—41. 

The acidic character of 5A zeolite as a function of the calcium content has been explored by different techniques propylene adsorption experiments, ammonia thermodesorption followed by microgravimetry and FTIR spectroscopy. Propylene is chemisorbed and slowly transformed in carbonaceous compounds (coke) which remain trapped inside the zeolite pores. The coke quantities increase with the Ca2+ content. Olefin transformation results from an oligomerization catalytic process involving acidic adsorption sites. Ammonia thermodesorption studies as well as FTIR experiments have revealed the presence of acidic sites able to protonate NH3 molecules. This site number is also correlated to the Ca2+ ion content. As it has been observed for FAU zeolite exchanged with di- or trivalent metal cations, these sites are probably CaOH+ species whose vas(OH) mode have a spectral signature around 3567 cm"1. 

Figure 2.10 Framework structure for FAU zeolite formed by linking sodalite cages through double six-rings.

Figure 4.22 IR spectra of NHJ-FAU zeolite (Si/Al2 = 5.5) after 1 h treatment in flowing He at various temperatures. Spectra collected at 25°C. HF-OH is in supercages, LF-OH in sodalite...

Fig. 2. Spin-spin decay of signals of (A) (A1)-FAU and (B) (Fe)-(A1)-FAU zeolites. Lines 1-4 refer to Si (3A1)-Sl (OAl) signals respectively.

However, there have been only a few reports about the synthesis of the micro-mesoporous materials. Bekkum group have reported that the FAU zeolite overgrown with small content of... 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

Adsorption experiments The method developed for the analysis of carbonaceous compounds formed and trapped within the zeolite micropores during catalytic reactions1581 can be adapted for determining the occupancy of micropores by reactant, solvent and product molecules. However, this method cannot be used with compounds sensitive to hydrolysis, such as AA, because of the step of dissolution of the zeolite in a hydrofluoric acid solution necessary for the complete recovery of the organic molecules located within the zeolite micropores.[58] This method was used to determine the composition of the organic compounds retained within the micropores of three different zeolites [H-BEA (zeolite Beta), H-FAU (zeolite Y), and H-MFI (zeolite ZSM-5)] after contact in a stirred batch reactor at 393 K for 4 min of a solution containing 20 mmol of 2-methoxynaphthalene (2-MN), 4 mmol of l-acetyl-2-methoxynaphthalene (1-AMN) and 1 ml of solvent (sulfolane or nitrobenzene) with 500 mg of activated zeolite.[59 61] From the comparison of... 

Whatever the zeolite, 1-AMN is rapidly isomerized within the micropores into 2-acetyl-6-methoxynaphthalene (2-AMN). The 2-AMN/l-AMN ratios in the RM and AM mixtures are quite similar with H-FAU zeolite but smaller in RM than in AM with H-BEA and especially H-MFI (Table 2.2). This indicates limitations in the desorption of the linear 2-AMN molecules from the micropores of the latter two zeolites. In the case of H-BEA, these unexpected limitations were... 

The bulky and polar compounds trapped within the BEA micropores during anisole acetylation were shown to block the access of nitrogen (hence, obviously, also of the reactants) to these micropores[55] and this process is therefore responsible for deactivation. These compounds can only be eliminated from the zeolite by oxidation treatment under dry-air flow at high temperatures (at least 773 K), i.e. under the common pretreatment conditions. By means of this treatment the activity of a H-FAU zeolite for veratrole acetylation was totally recovered.[82]... 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

During the first 2h of reaction, a decrease in AcOH conversion (from 48 to 43 %) for benzene acetylation at 523 K with an increase in selectivity to the monoacetylated product (from 80 to 90%) can be observed. The only problem involves the low catalyst activity 1.5 mmolh 1g 1 of acetophenone, which corresponds to a TOF value of 2.2 h-1. This means that less than 0.2 g of this acetylated arene can be produced per hour and per gram of catalyst under the operating conditions (i.e. 10 times less than in the liquid phase acetylation of anisole with AA). The kinetic study of the reaction shows an increase in the selectivity with the substrate/acetic acid ratio, but no increase in yield, an increase in acetic acid conversion with the reaction temperature with a significant decrease in selectivity due to a greater formation of diacetylated products.[62,63] HFAU and RE-FAU zeolites do... 

Otherwise, by impregnating a Pd precursor onto a basic K-exchanged FAU zeolite a highly selective bifunctional catalyst is obtained for the low-pressure one-step synthesis of 2-ethylhexanal (a component of perfumes and fragrances) from M-butyraldehydc and H2 in a fixed-bed reactor/12,131 Under optimum reaction... 

The most important process involving a zeolite, Fluid Catalytic Cracking (FCC) uses a catalyst containing an acid FAU zeolite (Chapter 5). Other examples of processes using acid zeolite catalysts will be examined in this book, like Methanol to Olefins (Chapter 12), Acetylation (Chapter 14) etc. 

The most common way to track down the fate of the A1 is by measuring the (cubic) unit cell content of the FAU a direct measure of the framework A1 content is obtained by the so-called Breck curve, Figure 3.3. The FAU zeolite can then be fine-tuned to cover most of its applications in FCC (gasoline yield with high unit cell size materials, octane yield with low unit cell size materials) and Hydrocracking (high middle distillate yields with low unit cell size materials). 

Oxidized V (Oxidation state +5) has a greater mobility due to its higher vapor pressure, and the vanadic acid produced is very destructive towards the zeolite. This is a fundamental difference between Ni and V deactivation while Ni promotes undesirable side reactions (H2 production and coke), V is also lethal for the zeolite. In the case of the FAU zeolite, the following irreversible reaction takes place and destroys the zeolite ... 

The participation of protonic acid sites in xylene isomerization is clearly demonstrated by correlations between the isomerization rate and the concentration of protonic sites of silica alumina with various alumina contents (13), alkaline-earth and rare earth FAU zeolites (14, 15), MFI zeolites (16), etc. Evidence is also provided by the fact that protonic sites participate in alkylbenzene disproportionation. On the other hand, it seems most unlikely that Lewis acid sites play a direct role in xylene isomerization and disproportionation (8). 

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

It should, however, be emphasized that new catalysts with zeolites other than MOR or MFI which give higher paraxylene yields were recently developed for the isomerization of the C8 aromatic cut. Moreover, adsorption on FAU zeolites is now the main technique used for paraxylene separation (Chapter 10). 

For paraxylene separation, both kinds of selectivity can be observed. In the MFI structure, the aperture of the pores is sufficiently close to the dimensions of the molecules to make shape selectivity appear. However, the kinetic diameters of paraxylene and of ethylbenzene are identical, so that the selectivity is not effective for these two components. Moreover, the capacity of MFI zeolites is weak compared to other structures. More open structures which provide the opportunity to use equilibrium selectivity are preferred. The problem is that the selectivity is mainly due to interactions between the zeolite and the aromatic ring which are identical for all the xylenes. It will be shown in the following sections that this problem can be solved by using chosen FAU zeolites. 

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