Ir/Pt-HZSM5 for n-pentane isomerization: Effect of Si/Al ratio and reaction optimization by response surface methodology

The effects of Si/Al ratio on the properties of Ir/Pt-HZSM5 and n-pentane isomerization were studied. XRD results indicated that the increasing Si/Al ratio increased the percentage crystallinity of the catalysts, whereas, FTIR results showed that the increasing Si/Al ratio decreased the number of strong Brönsted and Lewis acid sites which led to decrease the catalytic activity towards n-pentane isomerization. Ir/Pt-HZSM5 with Si/Al ratio of 23 showed highest activity towards n-pentane isomerization, and the operating condition was further optimized by using response surface methodology (RSM). The RSM experiments were designed by using face-centered central composite design (FCCCD) by applying 24 factorial points, 8 axial points and 2 replicates, with three response variables (n-pentane conversion, isopentane selectivity and isopentane yield). The Pareto chart indicated that the reaction temperature have largest effect for all responses. The optimum condition of n-pentane isomerization over Ir/Pt-HZSM5 was at treatment temperature of 723 K, treatment time of 6 h, reaction temperature of 548 K and F/W of 500 ml g−1 min−1 in which the predicted value for the n-pentane conversion, isopentane selectivity and isopentane yield was 63.0%, 98.2% and 61.9%, respectively.

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► High activity of catalyst was observed for Ir/Pt-HZSM5 with Si/Al ratio of 23.
► Isomerization of n-pentane over Ir/Pt-HZSM5 was optimized by RSM.
► The largest effect on n-pentane isomerization was reaction temperature.
► The least important effect on n-pentane isomerization was F/W.
► The yield of isopentane could reach 61.9% under the optimum conditions.