Isomerization and Dimerization of Pinene using Al-Incorporated MCM-41 Mesoporous Materials

A series of Al-incorporated mobile crystalline materials (Al-MCM-41) were prepared by hydrothermal synthesis and characterized by XRD, N2 adsorption (isotherms), inductively coupled plasma (ICP) analysis, 27Al magic angle spinning (MAS) NMR spectroscopy, and in situ pyridine adsorption (IR analysis). All samples exhibit an ordered structure with the majority of Al species in framework positions, except those with an SiO2/Al2O3 ratio =10. The concentration of acid sites is closely related to the Al content in the unit cell. In the presence of Al-MCM-41, both a- and beta-pinenes are quickly transformed into isomerized products, which then couple to form homo- and heterodimers. Lewis acid sites are active in triggering both the isomerization and dimerization reactions, but weak Bronsted acid sites seem inactive for the dimerization. Al-MCM-41 has a greater activity than many other microporous and layered materials, and can be easily regenerated. The activity correlates well with the concentration of acid sites, and the sample with an SiO2/Al2O3 ratio of 20 shows the highest activity.