Spectroscopic study and catalytic evaluation of mesostructured Al-MCM-41 and Pt/H₃PW₁₂O₄₀/Al-MCM-41 catalysts

Mesostructured Al-MCM-41 materials (referred as CSAn, where n = Si/Al molar ratio = 50, 30 and 10, respectively) and 12-tungstophosphoric acid promoted Pt/Al-MCM-41 catalysts were investigated by a variety of spectroscopic techniques. Aluminum ions were distributed within the resultant solid having 4, 5 or 6-coordination, in different proportion, which correlate with the formation of Brönsted acidity. Grafting the H₃PW₁₂O₄₀ heteropolyacid (referred as HPW) on the Al-MCM-41 solids not only noticeably enhanced, by 2-3 times, the number of Brönsted acid sites, but also greatly improved the acidity strength, with respect to that of the bare host. The primary structure of the Keggin units of the supported heteropolyacid was largely preserved after calcination at 350 °C; however, small parts of the Keggin units were strongly distorted or deformed due to the strong interaction between the heteropolyanions and the hydroxyl groups of the support. Catalytic evaluation confirmed that 1 wt.%Pt/25 wt.%HPW/Al-MCM-41 (denoted as Pt/HPW/CSAn) catalysts were very active for n-heptane isomerization in the presence of hydrogen under an atmospheric pressure condition. High yield of multibranched isoheptanes was produced, which was dependent on the pore diameter and the Brönsted acidity of the catalysts. The molar ratio of multibranched to monobranched isoheptanes is much higher than that reported in the literature under similar reaction conditions using zeolite-containing catalysts, showing the potential of producing high octane number fuel through the approach of n-heptane hydroisomerization by using our novel mesostructured catalysts.