Preparation, Acidity, and Catalytic Activity of Synthetic Montmorillonites in α-Pinene Isomerization

A series of montmorillonites with different Mg and Al contents (Na_2x(Al_2(1–x),Mg_2x)Si₄O₁₀(OH)₂·nH₂O, where 0 < x ≤ 1) were prepared by the hydrothermal method and characterized by MAS NMR, XRD, N₂ adsorption–desorption, FTIR with pyridine, TPD-NH₃, SEM, and thermal analysis techniques. The material obtained without aluminum (x = 1), or with a minimum amount of it (x = 0.9) is a trioctahedral saponite becoming a fully dioctahedral montmorillonite at its highest content (x = 0.1). The introduction of aluminum into the montmorillonite structure during its synthesis leads to the appearance of pronounced Brønsted acidity, the highest concentration of which was in montmorillonite with x = 0.1. In contrast, the Al-free sample is practically a Lewis acid (L/B of 71.3). The studied materials possessed catalytic activity in the isomerization of α-pinene, which correlated with the strength of the acid sites. Al-containing montmorillonites yielded camphene with a selectivity of 50–53%, and the presence of medium and strong Brønsted sites favored its formation. In contrast, Mg-montmorillonite with almost exclusively Lewis acidity demonstrated an unusually high selectivity to limonene (44.5%). To the best of our knowledge, this is the first work describing the catalytic activity of synthetic Al–Mg montmorillonite. However, the ease of varying the nature and strength of acid sites in synthetic materials opens up the prospect of their use in a number of reactions, including those for renewable terpene compounds.