Solid acid catalysts adsorbed basic probe molecules

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... 

Temperature-Programmed Desorption. Also called thermal desorption spectroscopy (TDS), the temperature-programmed desorption of basic probe molecules has been developed as a powerful tool for the characterization of solid acids (73). In this method, a strong base is isothermally pre-adsorbed on an acidic catalyst and then exposed to a stream of inert gas (eg. He). Heating by a... 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

Microporous solids play a special role due to their large area/volume ratio providing the opportunity for adsorption of large amounts of guest molecules on very small amounts of substrate, due to the very high specific area of the adsorbent. In particular, powdered solids with controlled pore size solids may have specific uses such as molecular sieves, microreactors of controlled microsize, or catalysts. The surface-probe interaction may simply be of electrostatic nature, of hydrogen bonding, or in terms of an acid or basic behavior (Brbnsted or Lewis). Substrates may also interact with the adsorbed probe as electronically active supports and have an important role in redox processes (as in advanced oxidative processes for persistent pollutant destruction) [1-5]. 

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