Bronsted site

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

The catalyst acid sites are both Bronsted and Lewis type. The catalyst can have either strong or weak Bronsted sites or, strong i)i weak Lewis sites. A Bronsted-type acid is a substance capable of donating a proton. Hydrochloric and sulfuric acids are typical Bronsted acids. A Lewis-type acid is a substance that accepts a pair of electrons. Lewis acids may not have hydrogen in them but they are still acids. Aluminum chloride is the classic example of a Lewis acid. Dissolved in water, it will react with hydroxyl, causing a drop in solution pH. 

A carbonium ion, CHj, is formed by adding a hydrogen ion (H ) to a paraffin molecule (Equation 4-6). This is accomplished via direct attack of a proton from the catalyst Bronsted site. The resulting molecule will have a positive charge with 5 bonds to it. 

Both the Bronsted and Lewis acid sites on the catalyst generate carbenium ions. The Bronsted site donates a proton to an olefin molecule and the Lewis site removes electrons from a paraffin molecule. In commercial units, olefins come in with the feed or are produced through thermal cracking reactions. 

The preparation was performed on a commercial microcrystalline beta zeolite. The zeolite was treated with the Fenton s reagent and less than 0.3 wt% of carbon remained after the treatment. The porosity was fully developed as revealed by the pore-size distribution. Elemental analysis combined with TPR did confirm the high degree of Fe-exchange (98%) on the Bronsted sites. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

The amount of BrOnsted sites was evaluated by measuring the surface of the characteristic band at 1540 cm . Corrections have been made to take into account the differences in weight and surface of the wafers. After each test, the wafer was weighted and its cross section was measured with a planimeter. The results were corrected to represent those of a "standard wafer" (Ag) of 5 mg and 25 units of area, according to the following expression ... 

Figure 3 shows the amount of Bronsted sites, as measured by the surface of the characteristic IR peak at 1540 cm after outgassing at 523 K, as a function of the composition of the mechanical mixtures. The dashed lines represent the addition of the contribution of the pure phases, calculated as in Equation 3. An enhancement of the amount of Bronsted sites on the mixtures, when compared to the theoretical values, is observed. This effect is not very clear in SA6 series, but it is more evident in SA12 and SA60 series. The reproducibility of the experiments has been checked the variation between different wafers of the same sample was always inferior to 10%. 

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. 

Pairs of Lewis acid and Lewis base sites are formed. The Lewis acid site is an ion that is co-ordinatively unsaturated. A water molecule will transform a Lewis site into a Bronsted site (Fig. 3.22). 

Figure 3.22. Tran.sformation of Lewi.s site into Bronsted site.

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Figure 2 Model for the Bronsted sites in the supercage of a dealuminated HY, depending on the nature and location of the extraframework aluminic phase (the drawing of the extraframework phase is only schematic). In the center is the unperturbed Bronsted site.

Mossbauer spectra of calcined samples (Table 1). The Fe3+(3) and Fe3+(4) components are probably located in tetrahedral (framework) positions. The charge distribution around the Fe3+(3) is asymmetric (large QS), thus here the charge compensation is probably provided by Fl+, i.e. indicating the existence of Bronsted sites. The charge symmetry around Fe3+(4) is more symmetric, thus the counterion is probably Na+ or Fe(OFl)+. Fe2+ ions are probably located outside of the framework (due to their larger ionic radius). Thus, in the hydrogen a small part of Fe3+ is reduced to Fe2+, and is probable removed to extra-framework sites. 

Figure 2. (A) IR spectra of CO adsorbed on the Fe-TON zeolites of different Si/Fe ratio at very low CO coverage required for the detection of the Lewis sites., (B) Difference spectra of CO adsorbed on Fe-TON of different Si/Fe ratio recorded upon the saturation of all Bronsted sites.

Figure 3. Dependence of an average strength of the Bronsted sites on the Si/Fe ratio...

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

Parry (344) determined the infrared spectrum of pyridine adsorbed on rj-alumina dehydrated at 450°. Characteristic differences in the 1400-1700 cm region exist in the spectra of pyridine adsorbed via hydrogen bonds, pyridinium ions, and pyridine coordinately bonded to electrophilic sites. Pyridinium ions are characterized by a strong band at 1540 cm and a very strong band at 1485-1500 cm" coordinately bonded pj ridine has a strong absorption at 1447-1460 cm". No evidence was found for the existence of Bronsted sites on the alumina surface. 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

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... 

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