Surface acidic sites

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

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

Fig. 15.6 Reduction of acidic surface sites on treatment with H2 (left) and concurrent increase in basic surface sites (right). Open symbols Activated carbon Norit, oxidized with 02, Filled symbols Norit loaded with 200 pmol g 1 Pt. Reprinted from Ref. [29], Copyright (1994), with permission from Elsevier.

Kaolinite is the main constituent in china clay used to make porcelain. The layers are largely held together by van der Waals forces. Bentonite is used in cosmetics, as a filler for soaps, and as a plasticizer, and it is used in drilling-muds as a suspension stabilizer. Bentonite and kaolinite clays are used, after treatment with sulfuric acid to create acidic surface sites, as petroleum cracking catalysts. Asbestos also has a layered structure (Section 12.13). 

If one develops this concept further it becomes evident that the general adsorption of the molecules at the surface proceeds continuously to a localized adsorption on distinct and specific surface sites. This appears entirely reasonable as an extreme case, i.e., for ion exchange interaction with basic or acidic surface sites. The consequence, of course, is that the effective area required by an adsorbed sample molecule increases. 

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. 

The study of acidic surface sites capable of donating protons to or accepting electrons from adsorbed molecules is one of the most important... 

This heterogeneous process may constitute an interesting alternative to the classical synthetic route. Catalyst deactivation can be slowed down by deliberately poisoning the acidic surface site with added pyridine in the reactant feed. A feasible operation mode for a continuous heterogeneous process consists of reaction and subsequent reoxidation cycles of the catalyst. 

The solutions for y>(z) and m(z) of the system of eqs 3 and 4 depend not only on the surface charge density but also on the surface dipole densities. The boundary conditions are related to the surface charge and the surface dipoles generated by the association of the cations and H+ with some of the Na acidic surface sites per unit area of the surface and of the anions and OH with some of the Nb basic surface sites per unit area. The details are given elsewhere,11 and the results will only be briefly reviewed here. 

Changes in the catalytic activity of activated alumina surfaces have also been probed by Temperature Programmed Desorption (TPD) and FTIR photoacoustic measurements [74]. Ammonia TPD allows a fast and convenient determination of the overall acidity of a solid surface. The desorption profiles provide information on the distribution, the amount and strengths of the acid surface sites, since molecules adsorbed at weaker sites desorb at lower temperatures than those adsorbed at stronger acidic sites. The activation of y-alumina with CHC1F2 resulted in a sig-... 

In summary, the development of doped catalysts, in both oxide and fluoride families, for catalytic fluorination is at an early stage. Much work remains to be done before a clear picture is obtained, however it is apparent that doping can perturb the crystalline nature of pure phases under some circumstances. When a substantially greater surface area results, a high concentration of acidic surface sites is also observed. There is considerable uncertainty regarding the selectivity and long term stability of doped catalysts and these aspects require further investigation. 

Sant and Varma (183) found that low concentrations of zirconium lowered the temperature required to reach the maximum yield. Various interpretations of this observation have been put forward either the increase in surface area or the increase in oxygen transport rates can be sufficiently altered by the zirconium to result in high yields of MA at lower temperatures. The studies generally agree that aroimd 1.5% zirconium has the most beneficial effect on the activity, and good catalytic performance could be achieved at lower temperatures (172). One of the reasons for this that has been proposed is that zirconium and titanium both create acidic surface sites on the vanadium phosphate surface. These sites prevent the desorption of reaction intermediates (butene, butadiene, and furan) while facilitating the desorption of the acidic MA. 

Carbon monoxide, a soft base, is expected to interact with a soft acidic surface site (19), The octahedral iron cations (+2.5 average oxidation state) are the softer of the acid sites on magnetite and may be expected to provide CO adsorption sites. The initial interaction should result in a carbonyl surface species, and such species have been observed by infrared spectroscopy (20-22)t... 

Bulk TPA presents very strong acidic sites (Ei = 631 mV) (Fig. 3a). TPA-PVA-PEG sample shows acidic surface sites with essentially the same strength (Ei = 640 mV), but a lower total number of acid sites (Fig. 3b), as a result of the lower acidity of the protons engaged to OH groups. On the other hand, the PVA-PEG beads present very weak acidic sites (Ei = -63 mV). 

Waste products are formed from propane on this catalyst primarily via the second formed acrolein. It is postulated that the acrolein, once formed, readily readsorbs on the catalyst surface, and presumably interacts with a second site which might be either a highly acidic surface site (Mo-0 H" ) or a reduced surface site ( Mo ) or simply by interaction, before desorption, with an adjacent overactive surface site. This scenario is particularly strongly implied by the observed Langmuir-Hinshelwood dependence of waste formation from propylene. 

Some occasional information is available concerning the reusability of the SZ catalyst in the benzoylation of anisole with BAN. The catalyst is effectively utilized for a second cycle, but more drastic experimental conditions are needed to achieve a yield comparable to that of the first run. Moreover, detailed spectroscopic analysis of the utilized catalyst confirms that, during the reaction, carbonaceous deposits are formed on the surface, and the number of the available acid surface sites decreases. 

E. Probing Bronsted and Lewis Acid Surface Sites by Adsorbed Molecules... 

Sample N Content (irmoPg) Catalytic Activity (p,moiyg H2S03 g) Acidic Surface Sites (ftmol/g) Basic Surface Sites (itmol/g) ... 

The data collected in Table 2 evidence the unquestionable responsibility of basic and/or one-electron donor centres for the activity exhibited by MgO in CTR. Simultaneously it is clearly shown that the elimination of all acidic surface sites by poisoning with n-propyl-amine did not result in any change of catalyst activity. 

Trimethylphosphine (TMP) and dimethylphosphine (DMP) were protonated by acid surface sites to give the corresponding phosphonium ions, which were observed, for instance, by Bein et al. [747], when they contacted the phosphine vapors with the acid H-Y zeolite. DMP is the weaker base and exhibited some advantages ... 

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