Catalytic application of Al-MCM-41 in the esterification of acetic acid with various alcohols

Mesoporous Al-MCM-41 molecular sieves in four Si/Al ratios: 25, 50, 75 and 100, were synthesised under hydrothermal condition. They were characterized using powder X-ray diffraction (XRD), FT-IR, BET and thermogravimetric-differential thermal analysis (TGA-DTA). The XRD spectra showed that the materials were of hexagonal mesoporous structure. The FT-IR spectra revealed the order of the hydrophobic character of the catalyst materials when one compared the broad envelopes due to the OH stretching of water in the higher energy region. The acidity of the catalyst was measured by FT-IR using pyridine as the diagnostic base. The BET surface area measurements indicated the surface areas between 900 and 1000 m2 g−1 and pore diameters around 26 Å units. The catalytic activity of the materials was tested for the vapour phase esterification of acetic acid with n-butyl alcohol (NBA), isobutyl alcohol (IBA) and tertiary butyl alcohol (TBA) under autogeneous condition at 100, 150, 175 and 200 °C. NBA conversion was found to be higher than IBA and TBA. The hydrophobic properties of catalyst, alcohols and the stearic properties of alcohols were suggested to play important roles in the esterification. Al-MCM-41 (25) for NBA, Al-MCM-41 (100) for IBA and TBA was found to be more active. The mechanism was Eley-Rideal type. The reaction was also studied over commercially available heteropolyacids and zeolites (HM, Hβ, HZ and ZSM-5). Except for zeolites, other catalysts were found to have nearly identical activity to that of Al-MCM-41. Zeolites showed less activity for NBA and TBA but comparable activity to IBA. The requirement of Brönsted acid sites for the reaction was clearly established by running the reaction in the absence of catalyst. Occurrence of the reaction mainly within the pores was confirmed by running the reaction over the as-prepared catalyst that provides less conversion than the calcined material.