Abstract
Abstract A series of acid-activated clay catalysts were prepared from a purified bentonite, rich in structural iron, collected at Serra de Dentro on the island of Porto Santo. The purified bentonite exhibited a surface area of 130 m2 g−1 which increased to values as high as 500 m2 g−1 following activation with 4M HCl at 95 °C for 30 min (SD-4M-95-30). The ability of the activated samples to convert limonene to p-cymene was evaluated using a reaction time of 15 min at 80 °C. The sample prepared using 3M HCl at 95 °C for 30 min (SD-3M-95-30) offered the optimum combination of surface area (470 m2 g−1) and acidity (0.26 mmol g−1) and 95% of the limonene was converted to product. About 15% of the product mixture was p-cymene whilst non-volatile products and polymeric species made up 54% of the product mixture. The presence of iron in the octahedral sheet of the SD bentonite appears responsible for the dehydrogenation activity. TG–MS analysis of acid activated samples, saturated with cyclohexylamine, reflected the dehydrogenation capabilities of the catalysts in that SD-3M-95-30 produced the most benzene and aniline as decomposition/transformation products.
Highlights
Introduction pCymene (1-isopropyl-4-methylbenzene) finds many applications in the chemical industry including its use as a solvent in the ink and varnish industry, for the production of p-cresol and carvacrol, as an intermediate in the synthesis of anti-oxidants (e.g. BHT), in the production of synthetic resins, in perfumery and as a thermal fluid in heat exchanging systems [1,2,3]
Nitrogen adsorption data revealed that Serra de Dentro (SD) exhibits higher surface areas (130 m2 gÀ1) than normally observed for smectites [27]
The cation exchange capacity (CEC) was determined using the method suggested by Rhodes and Brown [28], and a value of 81 mequiv./100 g was obtained
Summary
Six locations were selected and samples collected, after discarding the top 10 cm of superficial materials to avoid weathering contamination. The resulting representative sample was decarbonated with hydrogen peroxide (30 vol.% at 80 8C) to remove all organic matter, centrifuged at 6000 rpm, washed with deionised water and centrifuged again. The material obtained at Serra de Dentro (SD) was selected as the starting material for further work, since it contained the lowest amount of impurities within the group of samples collected [25]. Acid activated (AA) samples were prepared by mixing 10 g of SD with 300 cm aliquots of aqueous HCl (with selected concentrations) at 25 or 95 8C, for different periods of time. The samples were diluted with a large quantity of cold water to effectively terminate the leaching process, centrifuged and washed successively until a stable pH was achieved. SD-6M-25-120 is the material obtained when a 10 g sample of Na-SD was treated with 6M HCl at 25 8C, for 120 min
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