This study highlights the potential of natural sepiolites, abundant fibrous hydrated magnesium silicates, as possible eco-friendly acid catalysts. By investigating materials with different aluminum contents, their thermal behavior up to 600 °C and leaching by water or acid treatments, the role of Al 3+ substituting Si 4+ or Mg 2+ in the tunning of acid properties of sepiolites is discussed. The amount, strength and strength distribution of the acid sites were determined by NH 3 adsorption microcalorimetry. Besides, Fourier Transform InfraRed and Nuclear Magnetic Resonance spectroscopy techniques were used to get insight into the nature of the acid sites when low amount of Al is incorporated in the sepiolite structure. The density of acid sites, key parameter for high catalytic activity, increased with the Al content. Bridging silanol (Si–OH + -Al) Brønsted acid sites were evidenced in thermally treated samples. Isopropanol conversion was studied between 100 and 300 °C in a pulse system for its practical importance for green propylene production and as test reaction to probe the acid properties of solids. The edge Al VI species able to reversibly reduce their coordination number under the reaction conditions and medium strengths sites (80 to 150 kJ/mol as determined by NH 3 adsorption microcalorimetry) are at the origin of active acid sites. • Sepiolite-type clays are active catalysts in isopropanol dehydration to propylene. • The density of acid sites increases with the Al content. • Bridging silanol (Si–OH + -Al) and unsaturated Al V sites are evidenced. • Partially dehydrated Al VI species with medium-strong strengths acting as active sites.
Nguyen et al. (Wed,) studied this question.