Preparation of nanotubes-clay hybrid systems by iron-catalyzed isobutane decomposition

Nanotubes-clay hybrid systems are synthesized by decomposition of isobutane at 700 °C over as-purchased and iron-loaded montmorillonite catalysts. The changes produced by the variations of total iron content (2-17 wt.%) and of reduction-temperature (500 °C or 700 °C) in catalyst surface properties, as well as, in reaction yield and crystalline quality of C deposits are discussed. A growth mechanism is proposed accounting for the bi-functional behavior of catalyst and the role, played by the clay support acidity with regard to the process selectivity and the resulting crystallinity degree of reaction products, is demonstrated. The decrease of number of catalyst acid sites, by reduction at 700 °C or by sodium exchange reaction, inhibits the formation of highly disordered carbonaceous nanostructures, responsible for metal deactivation and scarce selectivity at small iron loads. It is worth noting that catalysts prepared with sodium-exchanged clay allow obtaining crystallinity degrees higher than those ever reported in literature for similarly grown nanotubes.