Bransted acidity determination

The influence of hydrothermal treatment on the concentration of Bransted sites, which are directly related to tetrahedrally framework aluminum, has been studied by FTIR spectroscopy in dependence on the Al content. A previously calcined and dehydrated H,A1-MCM-41, containing only ca. 20% of the initial Bransted acid sites, was in situ treated with water vapour at 80°C in the IR cell. After subsequent removal of loosely bound water by evacuation, which was controlled by the disappearance of the deformation mode of water at 1640 cm 1, ammonia was loaded to determine the change of the Bransted acidity. Based on the intensity of the BS band at 1450 cm 1, a distinct increase of the Bransted site content is found (Fig. 2). In comparison with the ammonium exchanged MCM-41, the amount of Bransted sites rises to 53% of the initial value observed with the ammonium exchanged MCM-41. 

After sulfidation and reduction, pyridine adsorption was performed to determine acid site density (Table 4) the number of Bronsted acid sites increased with the extent of the H2S treatment, e.g. the number of Lewis acid sites increased from 0 to 139 with increasing in S/Pt, while at higher extents of sulfiding no significant changes were observed. In particular the BR. signal attributed to the presence of Bransted acid sites reached a maximum at 1545 cm in S/Pt=0 and 2 samples, while a new band at 1553 cm"... 

The ferrierite structure was later solved in 7222 space group [01Y2] - Fig. 9a,b. As different from Immm structure [66V1], the angle T41-05-T42 was 168°. The Na and H2O extra-framework sites were also determined - Table 2. The 03, 04, 06, and 072 (or 071) should be Bransted acid sites. The 5.8 A1 atoms were located in the unit cell. These were distributed on T1 site (3.2 A1 atoms - 55 %), T2 site (1.6 A1 atoms - 28 %), and T42 or T41 (1 A1 atom - 17 %). 

Through lack of an unambiguous method for direct determination, the acidity constants of carbon acids have, for many years, been estimated from the rate of proton abstraction by means of rate-equilibrium relationships. Thus, Bell (1943) (see also Hibbert, 1977) estimated the acetaldehyde, acetone and acetophenone acidity constants (19.7,20.0 and 19.2, respectively) by assuming that the rate constants for proton abstraction from several mono- and dicarbonyl compounds to a single base (A-) with pAHA = 4.0 obey a Bransted equation in its differential form (47). By taking curvature into account, the... 

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