Basicity pyrrole adsorption

A second factor which obviously plays a role in the adsorption of molecules on the hydrogenation sites is the Jt electron density. This factor was considered to be predominant in the study of Moreau et al. [10]. The role of the x electrons can explain the fact that, in the study of Nagai et al [53], cyclohexylamine and piperidine did not fit the linear correlation between the adsorption coefficient and the gas phase basicity the adsorption coefficient of these saturated nitrogen compounds was about twice lower than expected on the basis of the above mentioned correlation. In our experiments, the high adsorption coefficient of the pyrrolic ring can be related to the very high n character of this heterocycle [59]. 

Layered zeolite menbranes LCO, reforming Leaching, MFI Lead removal Lewis acid Lewis acid sites Lewis acidity Lewis basic sites Li MAS NMR Li NMR, FAU adsorbent Li exchange Li,Na-FAU, Na NMR Li,Na-FAU, pyrrole adsorption Li-LSX... 

Barthomeuff has explored the basic site strength in seolites X,Y and Z, mordenite and ZSM-5 using pyrrole adsorption. The results indicate that cationic faujasites have coupled acid-basic sites. In these oxides the strength of acid and basic sites depends on the nature of the cation and upon... 

A combination of column adsorption chromatography on basic alumina and GC of the eluate served for characterization of the trace fraction of nitrogen-containing compounds in hydroprocessed naphtha. These were subdivided into groups of four types, namely pyridines, pyrroles (the most abundant), anilines and indoles125. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

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]. 

Thus, the data obtained show that chloroform and acetylene and its derivatives are suitable IR-spectroscopic probe-molecules for basic centers in zeolites. These probes exhibit the following advantages as compared to the conventionally used molecules, like CO2 and pyrrole (1) the wride ranges of the frequency shifts, which allows one to differentiate the centers of different nature and strength, (2) the easiness and reversibility of adsorption/desorption of these molecules, and (3) the favorable spectral range where the spectral features attributed to adsorbed probes appear. The use of such an approach allows us to shed some light on the nature and properties of basic sites in zeolites. The similar technique will be applied in our future studies devoted to other solid superbases. 

Multinuclear solid state nuclear magnetic resonance (NMR) has been applied to study the interaction of pyrrole with extra framework compensating cations in zeolites LiNaY and LiNaX. Upon adsorption over zeolite LiNaY, Na and Li cations migrate towards accessible positions in the supercage to interact with one molecule of pyrrole. The adsorption over zeolite LiNaX decreases the mobility of SIIT Na cations, while pyrrole molecules do not interact with Li" cations. At lower loading, pyrrole adsorbs over more basic sites, which are associated with Na cations in zeolite LiNaY. 

Heidler et al. [806] investigated and discussed by a combination of the energy equalization method and results of the Monte Carlo technique the observed heterogeneity in the spectra of pyrrole adsorbed on basic faujasite-type zeolites (cf. also [804,805]), arriving at the result that there is only one adsorption site for pyrrole but an effect of the nsi/nAi ratio on the orientation of the pyrrole molecule with respect to the six-membered rings. 

Fig. 50. Adsorption of pyrrole on zeolite K-LTL and the shift of the wavenumber of the NH-stretching band, Av (NH), as a measiue of basicity (adopted from [802])...

The microcalorimetric technique has also been applied to investigate the adsorption of acetonitrile, dimethylether, water, pyrrole and ammonia on ferrierite, a small-pore zeolite [58], This investigation showed that the results of the determination of the site-strength distribution are dependent on the basic strength of the probe, on the acidity and porosity of the acid solid, and on the adsorption temperature. A similar calorimetric investigation of the acidity of de-aluminated Y-type zeolites was performed using the same probes [59],... 

The characterization of basic sites is quite less studied than that of their acidic counterparts. A major problem is the choice of a probe molecule which has to be specific of basicity and should not be decomposed during the study. Several molecules were used (CO2. phenol, pyrrole). Infrared studies of CO2 adsorption showed that it interacts with a large variety of sites including the cations (alkaline, alkaline earth. ..) generating many different adsorbed ill-defined carbonates and bicarbonates (63,64). Due to its easy decomposition phenol can not be used. Pyrrole is preferable if the experiments are conducted in conditions which do not induce a fast decomposition or polymerization. 

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. 

---