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

A series of montmorillonites with different Mg and Al contents (Na₂(Al_2(1-,Mg₂)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.