Selective methoxylation of limonene over ion-exchanged and acid-activated clays

tIn this study, we report the use of clay-based catalysts in the methoxylation of limonene, for the selectivesynthesis of �-terpinyl methyl ether. Na-SAz-1, Ca-SWy-2 and Sap-Ca source clays and a montmorillonite(SD) from Porto Santo, Madeira Archipelago, Portugal were modified by (i) ion-exchange with Al, Fe, Ni andNa and (ii) acid activation, to produce catalysts with markedly different acidic and textural properties. Thelack of activity of Ni2+-SAz-1 (with Lewis acidity maximized), provided evidence that the process occurspreferentially on Brönsted acid sites. The catalysts based on the high layer-charge SAz-1 montmorilloniteproved to be the most active. Ion-exchange with Al3+, followed by thermal activation at 150◦C, affordedthe highest number of Brønsted acid sites located in the clay gallery and this coincided with the maximumcatalytic activity. The influence of various reaction conditions, to maximize limonene conversion andselectivity, was studied over Al-SAz-1. When the reaction was performed for 20 h at 40◦C, the conversionreached 71% with 91% selectivity to the mono-ether. Mild acid activation (1 M HCl, 30 min, reflux) of theraw SAz-1 clay leads to a material with a good catalytic behaviour (slightly inferior to Al-SAz-1), whileany increase in the severity of the acid-treatment (6 M HCl, 30 min, reflux), caused a marked decrease incatalytic activity.