Lewis centers

The nature (Bronsted or Lewis centers), the number, and the strength of the acidic sites of the Pd/Al203 and Pd/Zr02 solids have been checked using infixed spectroscopy of adsorbed pyridine and thermoprogrammed desorption of ammonia. 

In order to study he Lewis acidity of the samples, the intensity of the 1450 cm pyridine band was also measured. Sample HYUS-8 shows a high amount of Lewis centers (Fig. 4d), relative thf HYD-400 sampl (Fig. 5c). This agrees with the absence of A1 as observed by A1 MAS-NMR for HYD samples. However, chemical analysis (Table I) indicates that there is more aluminum in this sample than in that from the unit cell constant m i urements. These differences cculd be explained considering that A1 MAS-NMR does not detect octahedral EFAL because of the low symmetry of its environment (i ). If this is so, it is remarkable that this EFAL does not show Lewis acidity as measured by pyridine ad y ption. On the other hand, if indeed thej is a small amount of A1, then the EFAL should be present as Al" outside the zeolite framework. In this case it should be present as amorphous silica-alumina. 

Lewis acid centers, which were thought to be the primary catalytic sites. Boreskova et al. (51) studied the poisoning effect of quinoline on the cracking of cumene over Na, H—Y zeolite and observed a linear decrease in activity with the amount of quinoline added until a constant level of activity was reached. The catalytic activity was attributed to trivalent aluminum centers (Lewis acids), which were poisoned by coordinately bound quinoline. In a similar study of cumene cracking, Turkevich et al. (50) also concluded on the basis of magnetic resonance experiments that Lewis centers were the active sites. 

In both homogeneous and heterogeneous catalysis, carbon monoxide activation involves first the coordinaiive interaction of carbon monoxide with a metal acceptor center. Carbon monoxide, being a weak donor base, does not react with a proton and produces only a vety weak interaction with a hard acid center such as BH3, With less hard Lewis centers, such as CuX, AgX, AuXj etc. (X - halogen), more or less stable carbon monoxide adducts can be isolated. A variety of modes of CO coordination in well characterized organometallic complexes is known. Scheme 1 contains some selected examples. 

Spectroscopic and calorimetric investigations of the adsorption of acetonitrile [122] have shown that the sorbent Si,Al-MCM-41 has strong Lewis (aprotic) acidity and poorly defined Br0nsted (protic) acidity [17]. The presence of strong Lewis centers on Si,Al-MCM-41 was confirmed by flow microcalorimetry for the adsorption of 1-butanol from a solution of n-hexane [123, 124]. Depending on the aluminum content of the structure, the number of strong Lewis centers on the surface of this sample was found to vary in the range of 43 to 70% of the total number of active centers. 

It seemed of some interest to test the ability of a series of REY zeolites to ionize polynuclear aromatics since the oxidizing properties of zeolites were pointed out (8, 16), but the nature of the electron acceptor site is still under discussion. Hall et al. (5), studying dehydroxylated HY zeolites, presumed it to be molecular oxygen trapped in an anion vacancy, while Hirschler (7) asserted that the protons may be the oxidative centers. In a previous work, as stated by Turkevich et al. (16), we concluded that the active sites are Lewis centers, while the chemisorbed oxygen increases their electron affinity (27). In a recent work, Richardson (14) related spin concentration to the electron affinity of the cation, presuming that the electron transfer took place from the anthracene to the cupric ion, but he could not observe any variation of the Cu peak intensities. 

When this happens only one Lewis center is formed with the disappearance of two BrSnsted OH S. Then... 

Sampling Equipment Literature is available from Gilson Company Inc., P.O. Box 200, Lewis Center, OH 43035. 

The selectivity for acylation as well as catalytic activity are lower at high temperatures the decrease in the yield of the acylated product at higher temperatures is due to the conversion of Bronsted acidic centers into Lewis centers. The acylation proceeds largely at the 2-position of the heteroaromatics to yield a somewhat sterically bulkier product. In fact, it is not shape selectivity but electronic and thermodynamic effects that mainly determine the position of the acylation in these substrates. 

While it is well known how the acid strength of Bronsted sites depends on the physicochemical parameters of a zeolite, the situation in the case of Lewis centers is much less established. In particular the influence of the chemical composition and crystalline structure on the more tenuous concept of softness and hardness of the metal sites is a subject of recent interest [1]. 

In the case of silica-alumina, dehydroxylated at temperatures >300 °C, the surface is mainly composed of isolated silanols, which can be close to A1 Lewis centers. Other supports such as magnesia or titanium not only contain hydroxyl groups, but also Lewis acid and basic centers. 

Chap. 1 of the present series (see also Fig. 11). Similarly, the amounts of Bronsted and Lewis centers are often measured via pyridine adsorption, and the strength of such acid sites in zeolites is frequently determined by temperature-programmed desorption of previously adsorbed pyridine (see, e.g., [38]). 

InSect. 5.2.2.10, the work by Kubelkovaetal. [271] onH-Y, Ce,Na-Y and dealuminated Ce, Na-Y was already mentioned, which was carried out employing the DRS/KBr pellet technique. These authors, however, obtained also DRS spectra of the powdered materials and compared their results with those of the conventional IR transmission spectroscopy of self-supporting wafers. They found that some of the OH bands observed by DRS were shifted to lower wavenumbers and attributed this effect to the higher temperature produced by the DRS beam in the powdered samples. In an investigation of hydrothermally treated and, thus, dealuminated H-ZSM-5 Martin et al. [429] observed by the pyridine/IR technique an elimination of Bronsted sites accompanied by a condensation of extra-frame-work Al-containing species (Lewis centers). 

For surface acids a distinction is made between protic (Bronsted centers) and non-protic (Lewis centers). Bronsted centers can release surface protons, while Lewis centers represent surface acceptor sites for electron pairs and thus bind nucleophiles. 

Other methods for determining the surface acidity of a catalyst are also available. For example the sum of Bronsted and Lewis centers can be determined by chemisorption of basic substances such as ammonia, quinoline, and pyridine. 

In comparison with AI2O3, Lewis centers are not so readily formed on the surface of Si02 since the OH groups are very strongly bound, so that Bronsted acidity predominates, albeit in a weak form, comparable to acetic acid. 

The thermodynamic group at NASA Lewis Center in Cleveland, led by Gordon, undertook a long study in order to investigate the problem of chemical equilibrium [76,77]. As a result, a close scrutiny of the polynomialization of the... 

In order to unravel adsorption mechanisms, a detailed knowledge of the composition and reactivity of the adsorption centers on the initial adsorbent is imperative [35-37]. It is well known [37-39] that fractured surfaces can be covered by cations and anions with unoccupied orbitals that act as Lewis acid and Lewis base centers, respectively. After adsorption of water molecules, with the evolution of hydroxyl and hydrogen species, the Lewis centers are transformed into Bronsted centers, which will influence surface properties. In many cases, characterizing the adsorption sites is complicated, and controversy stiU exists in the interpretation of IR spectra of functional groups, even for extensively studied oxides such as silica and alumina [6, 40, 41]. 

Figure 2. IR-spectrums NH3 adsorbed on q-Al203 (1), 7t-Al203 (2) and dissociative adsorption NH3 on coupled Lewis center.

Neil J. Kidner, Sergio Ibanez, Kellie Chenault, Kari Smith, and Matthew. M. Seabaugh NexTech Materials Lewis Center, OH USA... 

It appears that it is often difficult to determine the nature of the adsorbed species, or even to distinguish between the different kinds of adsorbed species from the calorimetric data. In many cases this technique fails to distinguish between cations and protonic sites due to the insufficient selectivity of the adsorption. The experimental data obtained by microcalorimetric measurements of ammonia adsorption are, at low coverages on zeohtes, markedly affected by the circumstance that the differential heats of adsorption on strong Lewis centers and strong Bronsted sites are relatively close. This fact might render it difficult in some cases to discriminate Lewis and Brbnsted sites solely by the adsorption of basic probe molecules and microcalorimetry if no complementary techniques... 

Adsorption of ozone and/or an organic molecule on its surface is the first requirement, but the exact nature of the sites which catalyze the generation of hydroxyl radicals after ozone adsorption is still debated. The suggestions for ozone decomposition sites are (i) surface redox sites (ii) Lewis centers of metal oxides (AI2O3, Ti02, Zr02, etc.) (iii) non-dissociated hydroxyl groups of metal oxides and (iv) basic centers of the catalyst. 

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