AN NMR-STUDY OF ACID SITES ON CHLORIDED ALUMINA CATALYSTS USING TRIMETHYLPHOSPHINE AS A PROBE

The MAS-NMR spectra of adsorbed trimethylphosphine (TMP) were used to determine the concentration of Bronsted and Lewis acid sites on pure and chlorinated gamma-Al2O3 samples. Chlorination with CHCl3, CCl4 or AlCl3 promoted the formation of Bronsted acid centers, which are characterized by the protonated adduct of TMP. This adduct has a P-31 CheMiCal shift of ca. -3.8 ppm and a J(P-H) scalar coupling of 517 Hz. Additional resonances in the -44 to -54 ppm range are attributed to Lewis acid-base pairs. In some cases a partially resolved J(P-Al) coupling is observed, which confirms the assignment. Upon thermal treatment of a chlorinated sample at temperatures > 200-degrees-C, the concentration of Bronsted acid centers decreased; the concentration of one type of Lewis acid increased and another remained almost constant. In a parallel set of experiments the initial conversion of n-hexane at 150-degrees-C and the yields of cracking and isomerization products were determined. Comparable functional relationships were observed between the loss of Bronsted acid sites and the decrease in yields of both cracking and isomerization products. These results suggest that Bronsted acidity is responsible for the cracking and isomerization of n-hexane over chlorided aluminas at 150-degrees-C.