Adsorption Lewis/Bronsted sites

Catalyst Pyridine adsorption /mggcf Bronsted sites / % Lewis sites / %... 

The differential heats of adsorption on Bronsted sites and Lewis sites (cationic species) are not easily comparable [53]. For the former it is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of ammonia, while for Lewis sites the differential heat of adsorption represents the energy associated with the transfer of electron density towards an electron deficient, coordinatively imsaturated site, and probably an energy term related to a relaxation of the strained smface. Micro calorimetric studies of several zeohtes (H-mordenite, USY, H-ZSM-5) treated in such a way as to contain a noticeable amount of extra-framework aluminum have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the Lewis sites (Freund-lich isotherm) and hnear for the Brbnsted sites (Temkin isotherm) [53]. 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

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. 

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

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. 

The NH4-Beta-300 (Zeolyst International, number denote Si02/Al203 molar ratio) was transformed to corresponding proton form using a step calcination procedure at 500 °C. H-Beta-300 was partially modified with Fe by repeated ion-exchange method (Fe(III)nitrate). The surface areas as well as acidities (Bronsted and Lewis acid sites) of Fe-Beta (iron content - 0.1 wt %) were determined by nitrogen adsorption and pyridine desorption at 250, 350 and 450 °C using FTIR spectroscopy [6]. 

White28 indicated that the adsorption of pyridine molecule can be used to determine the concentration of Bronsted and Lewis acid sites. When IR is used in conjunction with thermal desorption, an estimation of the acid strength distribution can be obtained. 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

The pyridine adsorption/desorption infrared spectra (Fig. 6.6) of JML-I40 at various temperatures showed that, as the desorption temperature was increased, the intensity of the peak at 1449 cm (Lewis acidic site) was almost unchanged, whereas that at 1545 cm (Bronsted acidic site) decreased significantly. However, both of these bands still co-existed at 450°C, and the intensity was about 50 to 60% of that at room temperature. These results suggest that both Lewis acid and Bronsted acid coexist in JML-1, and that the Lewis acidic sites are dominant. 

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. 

Figure 4.26 IR spectra showing absorbance bands due to pyridine adsorption on a H-FAU sample. PyrH is protonated pyridine on Bronsted sites, Pyr L is pyridine coordinated to Lewis acid sites, Pyr phys is physisorbed pyridine. All spectra recorded at 25 °C.

Figure 4.28 Effect of steaming and calcination on Bronsted and Lewis acid site strength distributions of a FAU-type zeolite as determined by pyridine adsorption/desorption IR.

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

Post-synthesis alumination using A1(N03)3 as the precursor improves the acidity of siliceous MCM-41 materials significantly. FTIR results show that both Bronsted and Lewis acid sites are increased upon alumination. The number of acid sites increases with the Al content on MCM-41. NH3-TPD reveals the mild strength of these created acid sites. Due to the improved acidity, the catalytic activity for dehydration of isopropanol to propylene over these alumina-modified MCM-41 materials is considerably promoted by post-synthesis alumination. The results of XRD and N2 adsorption show that the enhancement of acidity for siliceous MCM-41 by postsynthesis alumination does not cause any serious structural deformation of the resulting material. 

Since the acidity of porous materials is important in catalytic applications, a characterization of this interesting property is carried out by adsorption of the probe molecule acetonitrile CD3CN. Acetonitrile-d3, a weak base, can be applied to investigate Bronsted and Lewis acid sites and to discriminate between both types of sites [9,10], The analysis is based on the study of the C=N stretching region by infra-red spectroscopy. 

Pyridine adsorption experiments have showed that the nickel containing smectites have Lewis acid sites and do not have Bronsted acid sites [9]. The Ni2+-substituted smectite catalysts have large surface areas even after 873 K treatment because many small fragments with the same smectite structure are intercalated in the interlayer region. The activities of the Ni2+ substituted catalysts are derived from Ni2+ Lewis acid sites located on the edge framework. 

Py adsorption measurements did not reveal significant differences in the acidic properties of the samples. No Bronsted acid sites were detected and about the same amount of Lewis acid sites were available for Py adsorption in the different catalysts. However, significant differences were found in the reactivity of the surfaces towards Py (Table 1). 

The strength of acid sites can be approximated by determining the heat of adsorption using the Van t Hoff isochore. In this method the pressures P required to give a fixed surface coverage at a series of temperatures T is determined. The derivative of In P with respect to T is related to the isosteric heat of adsorption. This technique is not used routinely. Nor is that of IR spectroscopy, where it is reported that Lewis and Bronsted sites can be differentiated by using substituted compounds. 

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

Furan itself can be used as the starting material for the synthesis of 1-methylpyrrole <2002MI179>. 7-AI2O3 was found to be an effective catalyst for the dehydration reaction between furan and methylamine to afford 1-methylpyrrole. A yield of 57.6% was achieved under the experimental conditions of a reaction temperature of 400 °C, a methylamine/ furan molar ratio of 1.5, and the molar flow rate of furan approximately 3-3.5 mmol/h. Furan was adsorbed onto Bronsted acid sites on the catalyst, while the methylamine was adsorbed onto Lewis acid sites. With this heterogeneous catalyst, the rate determining step of the mechanism was suggested to be the adsorption of furan on the Bronsted acid sites to form a ring-opened species, which is followed by the insertion of the adsorbed methylamine to form secondary amine intermediates. Further dehydration at the Lewis acid sites would yield 1-methylpyrrole. 

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