IR study of iridium bonded to perturbed silanol groups of Pt-HZSM5 for n-pentane isomerization

The Ir/Pt-HZSM5 catalyst was prepared by impregnation of iridium on Pt-HZSM5. The activity of Ir/Pt-HZSM5 was tested for n-pentane isomerization under hydrogen stream. The introduction of iridium did not change the bands observed at 3740, 3665 and 3610 cm−1 indicating that neither non-acidic terminal silanol groups nor acidic bridging hydroxyl groups interacted with the iridium. Additionally, the peaks corresponding to the perturbed silanol groups at 3700 and 3520 cm−1 decreased significantly. X-ray photoelectron spectroscopy (XPS) analysis revealed that the iridium is in the form of IrO2. 27Al MAS NMR confirmed the elimination of distorted tetrahedral aluminum. The presence of iridium slightly increased the acidity of Pt-HZSM5 and its selectivity for iso-pentane. Hydrogen adsorption FTIR indicated that iridium enhanced the formation of protonic acid sites which may participate in the isomerization, and inhibited the formation of hydroxyl groups at 3380, 3600 and 3680 cm−1 which may participate in the enhancement of the cracking reaction.

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► Iridium in the form of IrO2 bonded with perturbed silanol groups.
► Iridium did not change the lattice structure of HZSM5.
► Iridium increased the number of Brönsted and Lewis acid sites
► Iridium did not interact with terminal SiOH group or Si(OH)Al group.
► Iridium increased iC5 selectivity and suppressed C1–C4 cracking products.