Pyridines Bronsted basicity

The increased rate of epoxidation observed using pyridine as an additive has been studied by Espenson and Wang and was to a certain degree explained as an accelerated formation of peroxorhenium species in the presence of pyridine [62]. A stabilization of the rhenium-catalyst through pyridine coordination was also detected, although the excess of pyridine required in the protocol unfortunately led to increased catalyst deactivation. As can be seen above, MTO is stable under acidic conditions but at high pH an accelerated decomposition of the catalyst into perrhenate and methanol occurs. The Bronsted basicity of pyridine leads to increased amounts of HO2 which speeds up the formation of the peroxo-complexes and the decomposi-... 

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. ... 

Two types of probe molecules have been used for the detection of Lewis and Bronsted acid sites. The first involves the adsorption of relatively strong basic molecules such as pyridine, ammonia, quinoline, and diazines. The second kind involves the adsorption of weak base molecules such as CO, NO, acetone, acetonitrile, and olefins. The pioneering works of Parry27 and Hughes and... 

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... 

Describe the spectroscopic methods for detection of Lewis acidity/basicity and Bronsted acid-ity/basicity of metal oxides, and explain why pyridine (in spite of its toxicity and low volatility) is a popular choice as an adsorbate molecule. 

A variety of concave pyridines 3 (Table 1) and open-chain analogues have been tested in the addition of ethanol to diphenylketene (59a). Pseudo-first-order rate constants in dichloromethane have been determined photometrically at 25 °C by recording the disappearance of the ketene absorption [47]. In comparison to the uncatalyzed addition of ethanol to the ketene 59a, accelerations of 3 to 25(X) were found under the reaction conditions chosen. Two factors determine the effectiveness of a catalyst basicity and sterical shielding. Using a Bronsted plot, these two influences could be separated from one another. Figure 4 shows a Bronsted plot for some selected concave pyridines 3 and pyridine itself (50). 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

A key issue in the chemistry and catalysis of basic molecules reacting in acid zeolites is the extent to which proton transfer occurs from the Bronsted site to the basic molecule. For strongly basic molecules like ammonia or pyridine, infrared spectroscopy clearly identifies the protonated adduct (NH4+ or PyH+) from its characteristic vibrational frequencies. For trimethylphosphine, also a strong base, both infrared and NMR evidence for complete proton transfer are convincing[37]. For molecules which are less strongly basic, the question is not so easily answered. 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

The Bronsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, C2H4,... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

---