Lewis add sites

When H-EDTA-MOR was exposed to 0.3 kPa of NH3 at 473 K and evacuated at 473 K (Figure 6E), only protonated ammonia (NH4 ) was observed. The NH4" deformation band builds up at 1443 cm- together with a small amount of coordinated NH3 bonds (deformation) that attached to Lewis add sites at 1624 cm L... 

The major addic sites on H-MOR are Bronsted sites determined by pyridine adsorption studies above 80 % of addic sites are Br0nsted sites and the rest are Lewis add sites [4,5]. After adsorption of NH3, 0.3 kPa of EA are admitted on H-EDTA-MOR at 473 K (Figure 6F) adsorbed NH3 is easily replaced by EA to produce deformation bands of NH3+ (1597 cm-i, 1497 cm-t), CH2 (1460 cm-i). This spectrum is quite the same as the spectrum in Figure 6A. The results suggest that adsorption of EA is much stronger than that of NH3. When adsorbed EA is heated up to 573 K (Figure 6G-6H), the spectra are almost the same as the spectra in Figure 6B and 6C. 

The recovery of the Lewis add sites and the decrease in the protonic acid sites to the original values by outgassing gas phase hydrogen are rather slow and require a high temperature. The addition of Pt/Si02 did not affect much the restoration of Ae Lewis add sites of H-ZSM-5 by outgassing gas phase hydrogen. 

Periodic density functional investigation of lewis add sites in zeolites relative strength order as... 

Ligands Susceptible to Reaction with Lewis Add Sites of Inorganic Oxides... 

Acetonittile CH3-CN CH2—CN anions, previous coordination by nitrogen on Lewis add sites... 

Silica is the most widely used commercially available normal-phase packing in various forms, having standard particle sizes ranging from 3 to 10 /im and surface areas from 200 to 800 m /g [1]. It is classified as an acidic adsorbent because its surface consists of acidic hydroxyl groups that are covalently bound to the Si atoms (i.e., silanol groups). Silica contains weak Brpn-sted acidic sites and does not contain any Lewis-add sites. 

The surfaces of zeolites can thus display either Bronsted or Lewis acid sites or a combination of the two depending on how the zeolite is prepared. Bronsted acid sites are converted into Lewis add sites as the temperature is increased above approximately 500 °C and water is driven off. The strength of the add sites is directly related to the framework composition of the zeolite. Zeolites with a high Si Al ratio have the strongest add sites.7... 

Sihcates with MFI structure were synthesized in alkali- and template-free medium by Wallau et al. [521] and characterized, inter alia, via IR spectroscopy. The zeohtes synthesized from template-containing gels showed almost exclusively Bronsted addity, whereas the samples produced from template-free synthesis mixtures possessed a large number of Lewis add sites. 

The effect of the synthesis medium (OH - or F -containing) on the acidity of [Si,Al]ZSM-5 was investigated by IR spectroscopy with and without pyridine as a probe in the work by Joly et al. [681 ]. These au ors found in the case of H-MFl synthesized in the presence of OH" only Bronsted add sites, indicated by the OH band at 3600 and the pyridinium band at 1545 cm, whereas synthesis in F"-con-taining media resulted in materials with more Lewis add sites (Py—>L bands at 1622 and 1455 cm" ), depending on the nsi/n i ratio. 

Surface hydroxyl groups coordinated to pairs of AP" cations appear on the plane perpendicular to the crystallographic c axis in gibbsite (Fig. 1.2). This basal plane makes up most of the surface of the mineral, but it appears that the hydroxyl groups bound to Lewis add sites on the edge plane perpendicular to the basal plane are more reactive.These Lewis acid sites comprise an Al " cation coordinated to a single water molecule. 

NH3 coordinated to Lewis add sites (coordinatively unsaturated metal cations) ... 

When sodium cations in the mont interlayer are exchanged with Sc + using an aqueous solution of Sc(OTf)j, a monomeric aqua complex, [Sc(H20)g] +, was formed in the interlayer space [104]. The Sc " -mont could efHdently catalyze the Michael reaction of 1,3-dicarbonyl compounds with enones in water solvent. The Michael addition reaction of 1,3-dicarbonyl compounds with enones provides access to 1,5-dioxo synthons, which can be transformed easily into cyclohexanone derivatives for use as important intermediates in steroid and terpenoid syntheses. Initially, both the 1,3-dicarbonyl compound and the enone coordinate with the Sc + center, which acts as a Lewis add site. Subsequently, successive carbon-carbon bond formation produces an intermediate Sc-alcoholate, followed by protolysis to afford the Michael adduct and regenerate the initial Sc species, [Sc(H20)g] +. The importance of coordination of both the substrates is evidenced by IR spectrometry. 

In addition to group V elements (Nb and Ta), mesoporous titanium nanoparticles with Lewis add sites are elTective for HMF formation from glucose [180, 181]. 

Fundamentally, these reactions are base catalyzed via proton elimination. First, the primary and secondary hydroxyl groups of glycerol gravitate toward the base sites, then the more reactive primary alcohol group electrophilically attacks the carbon of the carbonyl. When Lewis add sites coexist on the catalyst, it is able... 

Table 2. Types and concentrations of Lewis add sites according to FITR spjectroscopy of adsorbed CO...

Catalyst Vco, cm-1 Lewis add sites, pimol/ g Lewis acid sites, pimol/ g Total Lewis acid sites, pmol/g Total Bronsted acid sites, pmol/ g... 

Supported liquid-phase (water-ethylene glycol) catalyst containing Pd complexes was used for hydrogenation of stilbene in toluene [57]. It was found that zeolite CaY acted, in the presence of Bronsted acid sites, either as a reagent for reducing stilbenes to 1,2-diarylethanes or as a catalyst for isomerizing cis-stilbenes to the more stable trans form [58]. In contrast, the Lewis add sites generated by the activation process yield radical cations from stilbenes, but these did not yield any stable products. 

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