Lewis Bronsted acids and

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

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. 

Identify Bronsted and Lewis acids and bases in a chemical reaction (Self-Test 10.2). 

The thermal decompositions are catalyzed by Bronsted and Lewis acids [68]. In general, when M is electron poor and Lewis acidic, the thermal decompositions occur efficiently and at low temperatures (typically between 100 and 200 °C, but sometimes at lower temperature). The addition of a catalytic amount of a Lewis or Bronsted acid (i.e., AICI3 or HCl) has been observed to accelerate the ehmination of isobutylene and the formation of three-dimensional network structures [64,124-126]. Pioneering studies on pyrolyses of various metal alkoxides by Bradley and others have also shown that alkene eliminations represent a primary decomposition pathway [104]. 

Table 1 Bronsted and Lewis acidity of fresh proton-form catalysts.

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]. 

C. On Fe-Beta-300 the dimethyl ether selectivity was maximum at 240 °C and the hydrocarbons (mostly LC4) selectivities increased in addition to some constant amount of formaldehyde between 240-360 °C (Fig. la). This result confirms the presence of light Bronsted and Lewis acid sites (Fig. 2a) in accordance with FTIR results by pyridine [6], The methyl iodide started to convert to hydrocarbon (C1-C4) at 240 °C but dimethyl ether was not detected. 

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

Chatteijee, A. 2006. A reactivity index study to rationalize the effect of dopants on Bronsted and Lewis acidity occurring in MeAlPOs. J. Mol. Graphics Model. 24 262-270. 

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. 

Homogeneous Chemical Catalysis of the Reduction of Carbon Dioxide. Synergistic Effect of Bronsted and Lewis Acids... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

Unlike Arrhenius, Lowiy-Bronsted and Lewis acids and bases, the Usanovich s concept in a much broader sense includes all the oxidizing agents as acids and the reducing agents as bases, e.g.,... 

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.

Figure 4.40 P MAS NMR chemical shift ranges for Bronsted and Lewis acid sites in trimethyl phosphine (IMP) loaded zeolites.

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. 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

Based on the above results, we conclude that the diazomethane modification of zeolites is an effective method to change selectively the amount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature 300 0 catalytic reactions. 

The atom-planting method for the preparation of several metallosilicates with MFI structure was studied. By the treatment of silicalite or ZSM-5 type zeolite with metal chloride vapor at elevated temperatures, metal atom could be introduced into the zeolite framework. From the results of alumination of silicalite it is estimated that the metal atoms are inserted into defect sites, such as hydroxyl nests in zeolite framework. The metallosilicate prepared had both Bronsted and Lewis acid sites with specific acid strength corresponding to the kind of metal element. 

C is constant on the three La zeolites heated at 900° C in dry air whereas the number of Lewis sites decreases with increasing La content. The Na-8.7 sample heated at 900° C in dry air has no acidity. Accordingly, the Bronsted and Lewis acid sites on the Na-8.7 sample heated at only 800° C are plotted in Figure 3 for comparison with the 900°C-pretreated La zeolites. The plots indicate that the introduction of La3+ ions in the lattice increases the number of Bronsted sites even for a higher temperature... 

Table II. Bronsted and Lewis Acidities (in Arbitrary Units) of the La-7 Zeolite Pretreated at Indicated Temperatures and Rehydrated...

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. 

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