A solid-state NMR, FT-IR and TPD study on acid properties of sulfated and metal-promoted zirconia: Influence of promoter and sulfation treatment

The acid properties of various sulfated and metal-promoted zirconium oxide (ZrO₂) catalysts have been studied by solid-state ³¹P MAS NMR, FT-IR and TPD using the adsorbed trimethylphosphine oxide (TMPO), pyridine and ammonia as the probe molecule, respectively. Sulfated zirconia (SZ) catalysts having varied sulfur contents and metal promoter (M = Al, Ga and Fe) were prepared by sol-gel method. Effects of sulfation and promoter on the detailed qualitative and quantitative information of acid sites, viz. types (Brønsted versus Lewis), strengths and distributions, on various synthesized SZ and M/SZ were examined by ³¹P MAS NMR of adsorbed TMPO in conjunction with elemental analyses by ICP-MS. By comparison, pyridine-IR and NH₃-TPD methods are capable of providing only qualitative information of the overall acidity. It was found that while the parent ZrO₂ possesses only weak Lewis acidities, elaborated sulfation treatment leads to formation and coexistence of strong Brønsted (B) and Lewis (L) acid sites whose variations can readily be followed. On the other hand, incorporation of different metal promoters onto SZ, led to simultaneous formation/elimination and variations of B- and L-sites with varied strengths and distributions. As a result, Ga/SZ was found to possess more B- than L-sites, whereas an opposite trend was observed for Al/SZ. As for Fe/SZ, a pronounced increase in both concentration and strength of acid sites were found. With an exception of Ga/SZ, the 'very strong' acid sites observed for various SZ and M/SZ catalysts were found to be associated more to L- than B-sites.