APCATA 459-2013-8

Formation of acidic Brönsted (MoOx)−(Hy)+ evidenced by XRD and
2,6-lutidine FTIR spectroscopy for cumene cracking

2,6-Lutidine adsorbed IR spectroscopy has been employed to study the property of acidic sites
on MoO3 and Pt/MoO3. The results showed that both catalysts possess doublet adsorption bands
at 1605 + 1585 cm−1, ascribed to Lewis acid sites, and duo-doublet bands at 1660 + 1650 and
1640 + 1630 cm−1, ascribed to hydroxyl groups; these indicate an OH defect structure of MoO3 and
Mo–OH Brönsted acidic sites. All Brönsted acid sites were strong enough to retain outgassing at 473 K,
while a considerable number of relatively weak and medium acid sites as well as strong Lewis acid
sites existed. The addition of Pt slightly altered the ratio of Lewis/Brönsted acid sites and distribution
of Lewis acid sites. The XRD result confirmed the formation of molybdenum oxyhydride (MoOx)−(Hy)+
on the hydrogen treated Pt/MoO3, whereas the hydrogen adsorption on 2,6-lutidine pre-adsorbed catalysts
showed the formation of protonic acid sites over Pt/MoO3. These results strongly suggested that the
interaction of molecular hydrogen with Pt/MoO3 formed acidic Brönsted (MoOx)−(Hy)+ via a hydrogen
spillover mechanism. In fact, no (MoOx)−(Hy)+ and protonic acid sites were observed on Pt-free MoO3.
The presence of (MoOx)−(Hy)+ enhanced the activity of Pt/MoO3 in the cumene hydrocracking in which
the rate conversion of cumene increased by about 30%, while the apparent activation energy decreased
by approximately 28 kJ/mol.