Measure pyrrole adsorption

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

The results reported in Hgure 17 are the measured distributions of differential heats of adsorption of pyrrole on the same solids. Again it is found that the only peak in the distribution for NaX (117 kJ/mol) is present in the curves of KX, RbX and C and shifted to 123 kJ/mol in LiX. The second component of these diagrams varies regularly with the cation from 110 kJ/mol on LiX to 146 IJ/mol for CsX. 

The adsorption of pyrrole on a series of different oxides and zeolites showed that the shift of the NH infrared vibration to lower values could be used as a measure of the increased basic strength of the sites (65). Infrared spectra can be recorded 10-15 min. after pyrrole is contacted with the zeolite. This is a great advantage over calorimetry for instance which requires a long time to reach equilibrium, which may favor the pyrrole transformation. 

Micro calorimetric measurements of ammonia, pyrrole, dimethylether, and acetonitrile adsorption unveiled various strength distributions among the acid sites population of Y-type zeoUtes with various Si/Al ratios [85]. Ammonia proved to be a reliable probe when only BrOnsted acid sites were investigated. Dimethylether, a very weak base, did not appear to be any better than ammonia to reveal the inhomogeneity of one particular acid sites population, whereas pyrrole appeared as a rather acidic probe which helped visualize the basicity difference between the parent material and the dealuminated samples. Acetonitrile proved to be a reUable probe to monitor quantitatively and qualitatively Lewis acidity. 

The microcalorimetric technique was also apphed to investigate the adsorption of acetonitrile, dimethylether, water, pyrrole, and ammonia at 296 K on a small-pore type ferrierite (Si/Al = 15.4) [84]. The acidity spectra collected from the calorimetric measurements showed that the distribution of site strengths was much dependent on the strong or weak basicity of the probe, on the acidity and porosity of the acid sohd and on the adsorption temperature [84]. 

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