Acidity zeolitic adsorbents

Fig. 6 Normalized quadrupole coupling constants, Cq/Cq(0) as a function of proton affinity of adsorbed probe molecules. Cq(0) is the quadrupole coupling constant of the unloaded acidic zeolite (assumed to be loaded with N2 molecules, proton affinity of494 kJ/mol). Data are taken from DFT calculations of Ehresmann [234] and Koller [232], and NMR measurements of Jiao [233], and Marthala [235]...

Starting from the Pt-cinchona modified system, more recently an interesting concept has been developed by Feast and coworkers [144], A chiral acidic zeolite was created by loading one molecule of iM,3-dithianc-l-oxide per supercage of zeolite Y, either during or after the zeolite synthesis. Other chiral zeolites were formed by adsorbing ephedrine as a modifier on zeolites X and Y for the Norrish-Yang reaction [145],... 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

Utilization of zeolite acidic strength in Cg-aromatics (xylens) systems is illustrated next. In the presence of strong acids, xylene isomers have varying basicity (Table 6.5), with m-xylene being the most basic and p-xylene the least basic among the Cg-aromatics [27]. Based on the basicity of the xylenes, the acidity of each zeolite can be properly adjusted to selectively adsorb m-xylene or p-xylene. As demonstrated in Figure 6.4, a more acidic zeolite such as NaY will selectively adsorb m-xylene from other Cg-aromatics [28, 29], In contrast. Figure 6.5 shows that a weaker acidic zeolite such as KY will selectively adsorb p-xylene from other Cg-aromatics [30, 31]. In both systems, toluene was used as the desorbent. 

Acid-base interactions between zeolitic adsorbents and adsorbates do not always correctly predict the trend of adsorbent selectivity. This is illustrated by the adsorptive separation of durene from isodurene. Pulse test experiments indicated that the adsorbent selectivity for durene/isodurene increases from KX < NaX < LiX, shown in Table 6.6 [32], Because isodurene is a stronger base than durene (Table 6.5), one would expect that the results for adsorbent selectivity... 

The cyclocondensation of l,3-amino alcohols with carboxylic acid derivatives is a method often applied for the synthesis of 5,6-dihydro-4/7-l,3-oxazines <1996CHEC-II(6)301 >. Ebsorb-4, a weakly acidic zeolite-type adsorbent with 4 A pore size, proved an efficient catalyst of the cyclization of benzoic acid and 3-aminopropanol <2002TL3985>. In the presence of zinc chloride as a catalyst, the expulsion of ammonia drove the reactions of 3-aminopropanol with nitriles to completion, affording 2-substituted 5,6-dihydro-47f-l,3-oxazines in good yields... 

Trapped by a suitable compound, a transient intermediate can be converted into a more stable species for unequivocal identification. Stepanov and Luzgin (82) investigated the reaction of acetonitrile with 1-octene or tert-butyl alcohol on acidic zeolite HZSM-5 ( 2si/ Ai = 49) at 296 K by in situ MAS NMR spectroscopy under batch reaction conditions. Upon coadsorption of acetonitrile and 1-octene, a C MAS NMR signal at 108 ppm was observed, indicative of TV-alkylnitrilium ions 2 in Scheme 3. As depicted in Scheme 3a, the formation of these cations was explained by trapping the chemically unstable alkylcarbenium ions (formed from the adsorbed... 

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

At reaction temperatures of r< 523 K, the conversion of methanol on acidic zeolites is dominated by a dehydration of methanol to DME 210). Two mechanisms have been proposed for the formation of DME. In the indirect pathway (Eqs. (27a, b)), methanol molecules adsorbed on bridging OH groups react first to give methoxy species (ZOCH3), which subsequently react with another methanol molecule to give DME 280,281) ... 

The previous two sections of this review deal with classical simulation methods. A description of the activation of adsorbates by acidic sites, together with any bond breaking or bond formation that may take place, is the realm of quantum mechanical (QM) simulations. These types of calculations are particularly well-suited to zeolite-adsorbate systems when the cluster approximation is used. The active acidic site in the zeolite is modeled by a molecular cluster, formed by cutting out a small portion of... 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

Fig. 4. 45-MHz 2H spectra of CD3I on zeolites CsX, HY, and HZSM-5. On CsX the adsorbate forms a framework-bound CD3 group. Methyl iodide is much less reactive on acidic zeolites. It tumbles isotropically in HY and shows restricted motion in HZSM-5. (Courtesy of Larry W. Beck.)...

The feasibility of acid-catalyzed direct hydroamination has been demonstrated. Acidic zeolites afford, at low conversions, highly selective formation of ethyla-mine,297,298 isopropylamine,298 and ferf-butylamine,298-301 in the reaction of ammonia with ethylene, propylene, and isobutylene, respectively. Amine formation is explained as a reaction of surface carbocation intermediates with adsorbed or... 

Pollution Control. Zeolite adsorbents can effectively remove pollutants such as S02, H2S, and NO from industrial off-gas streams at near ambient temperature (54-57). Since water vapor usually exists along with these acidic compounds, an acid-stable or acid-resistant zeolite adsorbent is necessary for a long service life. Union Carbide announced three new processes for pollution control recently. They are the PuraSiv-Hg process for mercury vapor removal, the PuraSiv-N process for NO removal from nitric acid plant off-gas, and the PuraSiv-S process for S02 removal from... 

The first two examples both involved the creation of cationic species on an acidic zeolite. In both cases we did not need to model the interaction of the cation with the zeolite framework good agreement was obtained with just calculation of the isolated cation. Apparently, the cation is not strongly perturbed by the presence of the zeolite. Such fortunate circumstances are rare. Here we show an example of how theoretical NMR calculations can help elucidate chemistry on a basic metal oxide surface, in particular, the adsorption of acetylene on MgO (26). For this study we needed to model the active sites of the catalyst, for which there are many possibilities. It is assumed the reactive sites are those in which Mg and O are substantially less coordinated than in the bulk. Comer sites are those in which Mg or O are three-coordinate, whereas Edge sites have four-fold coordination. These sites are where the strongest binding of the adsorbates are obtained. 

By contrast, acidic zeolites allow the formation of invert sugar under mild operating conditions, for high concentrations in the starting sucrose, and with an efficient control of the degree of coloured materials due to their adsorbent properties. 

The oxidation of coke molecules begins by their hydrogen atoms with formation of oxygenated compounds which can undergo various reactions decarbonylation, decarboxylation, condensation. The greater the density of the acid sites the faster the oxidation of coke. Radical cations formed through reaction of molecular oxygen with coke molecules adsorbed on protonic sites would be intermediates in coke oxidation on acid zeolites. 

A Rideal type mechanism in which phenol from the gas phase reacts with methanol adsorbed on acid sites was proposed for O- and C-methylation of phenol over acid zeolites (46) ... 

The first tests were carried out to evaluate the behavior of different catalysts under gas phase conditions (Table 15.1). It was observed that conversion of 18 decreased rapidly in the presence of various acidic zeolites H-B-ZSM-5, H-ZSM-5 and H-US-Y. This behavior was even more distinctive with BP04 and Nb205. The low service times of the catalysts are assumed to be caused by strongly adsorbed compounds as well as coke precursors blocking the acidic sites. Surprisingly, a silica catalyst having gentle acidity showed the best performance. With selectivity to 19 of about 40% there was no drop in conversion after 8 h TOS, even at 230 °C. 

A number of inoiganic pollutants are removable by TSA processes. One of the major pollutants requiring removal is S02 from flue gases and from sulfuric acid plant tail gases. The Sulfacid and Hitachi fixed-bed processes, the Sumitomo and BF moving-bed processes, and the Westvaco fluidized-bed process all use activated carbon adsorbents for proven S02 removal (58). Zeolites with high acid resistance, such as mordenite and clinoptilolite, have proven to be effective adsorbents for dry S02 removal from sulfuric acid tail gas (59), and special zeolite adsorbents have been incorporated into the UOP PURASIV S process for this application (54). 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

Coadsorption of both reactants resulted in the formation of a bimolecular complex between methanol and toluene at the acid sites of HZSM5. This complex involves the interaction of the protonated hydroxyl group of a methoxonium ion with the aromatic ring of toluene. Methanol is preferentially adsorbed over toluene on acidic zeolites. Under the reaction conditions employed, the rate of methylation of toluene was found to be directly proportional to the surface concentration of the activated methanol species. The concentration of the bimolecular complex, which is expected to be a precursor to the transition state in the rate determining step is very low under reaction conditions. 

Surface reactions of adsorbed thiophene occur at room temperature over highly acidic zeolites only. The most important primary reactions are polymerization and ring opening. These reactions as well as the adsorption of thiophene are irreversible and produce coke and weakly adsorbed H2S (SH groups) on the surface of the acidic zeolites. 

Richardeau et al. investigated thiophene adsorption from liquid solutions containing hydrocarbons over HFAU zeolites in a stirred batch system at room temperature.144 They found that the maximum number of thiophene molecules adsorbed per gram of zeolites is equal to their concentration of acidic sites and considered that the acidic sites are the adsorption sites. They further found that the presence of toluene causes a large decrease in the removal of thiophene, and when the concentration of thiophene is high (27.7 wt%), an acid-catalyzed condensation of thiophene occurs to form dimers, trimers, and tetramers, which remain trapped on the zeolite. They concluded that thiophene removal by adsorption on acidic zeolites could only be carried out from diluted solutions containing no olefinic compounds. 

We now report how theoretical methods can be used to provide information on the adsorption, diffusion, and reactivity of hydrocarbons within acidic zeolite catalysts. In Section A, dealing with adsorption, the physical chemistry of molecules adsorbed in zeolites is reviewed. Furthermore, in this section the results of hydrocarbon diffusion as these data are obtained from the use of the same theoretical methods are described. In Section B we summarize the capability of the quantum-chemical approaches. In this section, the contribution of the theoretical approaches to the understanding of physical chemistry of zeolite catalysis is reported. Finally, in Section C, using this information, we study the kinetics of a reaction catalyzed by acidic zeolite. This last section also illustrates the gaps that persist in the theoretical approaches to allow the investigation of a full catalytic cycle. 

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