Acidic zeolite sites

Conversion of TIPB in dependence on W content, activation temperature (623 and 773 K) and time on stream (TOS) is shown in Figures 6 and 7. It follows that the impregnation of H-ZSM-5 with heteropolyacid has altered the catalytic properties of the external zeolite surface. Decomposition of TIPB distinctly differs in dependence on the activation temperature. All samples exhibited very low initial activities (TOS =15 min) after activation at 623 K. Activation at 773 K primarily enhanced the initial activity of the zeolite itself (sample OW-ZSM-5) but, nevertheless, activities of the modified samples were also considerably increased, even after 55 min time on stream. Obviously, blockage of acidic zeolite sites is mostly removed by calcination at 773 K, possibly due to aggregation of dispersed heteropolyacid but, additionally, this aggregated heteropolyacid should partly contribute to the TIPB conversion because the activity of the modified samples 2W-ZSM-5 and 4W-ZSM-5 exceeds that of the unmodified ZSM-5. 

The strength of acidic zeolite sites at the addition of a small Mo amount changes insignificantly, which is pointed out by close temperatures of peak maximums. This confirms the facts that Mo introduction does not lead to the formation of new acidic sites, and that the concentration decrease results Ifom their blocking by different Mo forms generated during calcination at the preparation of catalysts. 

Based on the thermodynamic evaluation by Bonura et al. [217], the conversion of methanol to DME on the acidic zeolite sites rapidly approaches the equilibrium level at all the investigated temperatures because it is a fast reaction. Moreover, they also observed a promoting effect of the reaction temperature on the relative rate of methanol synthesis from CO2 rather than CO, on the basis of thermodynamics. Hence, based on their findings, the authors have proposed the reaction in Scheme 7.32. It turns out that, apart from the methanol dehydration reaction, 3 which takes place very fast at any temperature, reactions 1-2-4 affect the methanol formation depending on their relative reaction rates. The thermodynamic analysis of the main reaction paths involved in the synthesis of DME by CO2 hydrogenation reveals that, under kinetic conditions, the CO concentration dramatically increases with temperature therefore, low reaction temperamre and recycling unreacted COx/H2 mixtures could be a solution to improving the methanol/DME productivity. 

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

An active matrix provides the primary cracking sites. The acid sites located in the catalyst matrix are not as selective as the zeolite sites, but are able to crack larger molecules that are hindered from entering the small zeolite pores. The active matrix precracks heavy feed molecules for further cracking at the internal zeolite sites. The result is a synergistic interaction between matrix and zeolite, in which the activity attained by their combined effects can be greater than the sum of their individual effects [2J. 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

Figure 3 Conversion of acid surface sites in zeolites treated by steam calcination...

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

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. 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

E. Baburek and J. Novakova, Isomerization of n-butane over acid zeolites, role of Brpnsted and Lewis acid sites, Appl. Catal. A Gen., 185 (1999) 123-130. 

The differences in selectively between REY and USY zeolites have been related to the lower total acidity (lower site density) in USY zeolites (63,65). A similar interpretation can be applied to explain the selectivity characteristics observed for DAY zeolites (64). 

Acidic properties of zeolitic materials have widely been investigated in the past fifteen years ( 1), particularly for Y-type zeolites. The presence of strong acid sites, as it is usually the case for acid zeolites, results in the presence of only weak basic sites, if any. Therefore, up to now, majority of the studies has dealt with the characterization of acid+rather than basic properties. The acid sites (Bronsted s H, Lewis Al) and basic sites (0, OH ) may be characterized directly by using physical... 

Pd supported overlarge-pore tridimensional acidic zeolites such as HFAU are the more active and selective catalysts for the synthesis of bulkier ketones. Thus, in a 0.2% Pd-HFAU catalyst, yield and selectivity from cyclohexanone of 23 and 75% can be obtained in cyclohexylcyclohexanone synthesis. Furthermore, the S5mthesis of aldehydes can only be made selective by joining the hydrogenating metaUic sites (Pd) to basic sites (instead of acidic sites). Thus, 2-ethylhexanal, which is a component of perfumes and fragrances, can be synthesized with high yield and selectivity (64 and 91%, respectively) on a PdKX zeolite. Much lower yields and selectivities are obtained over nonzeolitic materials, such as Pd/MgO. 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

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. 

NMR has been extensively applied to carbonyl compounds in acidic zeolites and other solid acids. The unshared pairs of electrons on the oxygen can interact with either Brpnsted or Lewis sites, and aldol condensation reactions are commonly observed. Acetone was first studied on a zeolite by Bosacek and co-workers (146) followed by Haw and co-workers (147) and later by Gorte and co-workers (148). The conclusion of an earlier acetone paper of Gorte and co-workers (149) was that acetone forms a static complex on the Brdnsted site of HZSM-5 at room temperature, but this claim was later revised (150) upon the realization that molecular motion in the complex is not halted except at appreciably lower temperatures. 

Complementing this contribution, Haw and Xu present a detailed assessment of the nature of acidic surface sites (most in zeolites) and their interactions with probe molecules, as assessed in NMR experiments. Their comprehensive approach sheds light on a number of timely issues in acid-base catalysis and demonstrates how successfully NMR spectroscopy has been used recently to understand surface and catalytic phenomena. 

Some observations indicate that the zeolite framework is a prerequisite for good catalyst performance via physical constraints and proximity of acid-base sites within the molecular sieves environment.431 Other observations show that rather microporosity plays an important role in the alkylation.432... 

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