Probes for acidity and basicity

Knozinger, H. and Huber, S. (1998) IR spectroscopy of small and weakly interacting molecular probes for acidic and basic zeolites, J. Chem. Soc., Faraday Trans., 94, 2047. 

As was mentioned in the preceding section, methanol, benzene, simple benzene derivatives and alkanes were employed, to some extent, like other small-molecule compounds as probes for acidity and basicity of the adsorbents. However, IR... 

Relative extraction efficiencies of polar polymeric neutral, cation, and anion exchange sorbents (HLB, MCX, and MAX) for 11 beta antagonists and 6 beta agonists in human whole blood were probed.109 Initial characterization of MCX and MAX for acidic and basic load conditions, respectively, showed that both the agonists and antagonists were well retained on MCX, while they were recovered from MAX in the wash with either methanol or 2% ammonia in methanol (see Table 1.6). Blood samples were treated with ethanol containing 10% zinc sulfate to precipitate proteins and the supernatants loaded in 2% aqueous ammonium hydroxide onto the sorbents. After a 30% methanol and 2% aqueous ammonia wash, the analytes were eluted with methanol (HLB), 2% ammonia in methanol (MCX), or 2% formic acid in methanol (MAX). The best recoveries were observed with MCX under aqueous conditions or blood supernatant (after protein precipitation) spiked sample load conditions (see Table 1.7). Ion suppression studies by post-column infusion showed no suppression for propranolol and terbutaline with MCX, while HLB and MAX exhibited suppression (see Figure 1.6). 

Lercher, J.A., Jentys, A., and Brait, A. (2008) Catalytic test reactions for probing the acidity and basicity of zeolites. Mol. Sieves, 6,153-212. 

Another interesting aspect lies in the capacity of nitrogen containing molecules such as basic NH3 and pyridine and acidic (amphotenic) pyrrole, to probe respectively acidic and basic sites in zeolites. This allows among other things to monitor acidic and basic sites in the outer surface of the crystals, a region in which the control of the density and strength of such sites is often critical for catalysts selectivity and deactivation. 

Catalytic Test Reactions for Probing the Acidity and Basicity of Zeolites... 

In the following, we will first discuss catalytic test reactions sensitive to probe acid sites in zeolites, which is by far the most studied type of reaction due to widespread use of zeolites as solid acids. Then, we will address bimolecular reactions, which require adjacent acid and base sites to proceed. Test reactions for probing the acid and basic properties of zeolites via product selectivity will be discussed subsequently. The last part will deal with reactions probing basic sites in zeolites, which is still not as well explored as probing acidity. 

Lercher JA, Jentys A, Brait A (2008) Catalytic Test Reactions for Probing the Acidity and Basicity of Zeolites. 6 153-212... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

---