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

• 2,6-Lutidine was used to determine the acidic centers of MoO3 type catalysts.
• Both MoO3 and Pt/MoO3 possess strong Brönsted and Lewis acid sites.
• XRD and IR results confirmed the formation of acidic Brönsted (MoOx)−(Hy)+.
• The formation of acidic Brönsted (MoOx)−(Hy)+ was only observed on Pt/MoO3.
• Acidic Brönsted (MoOx)−(Hy)+ is an active site 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.

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