Desenvolvimento de catalisadores heterogêneos suportados em carvões ativados e aplicações na síntese de biodiesel

Abstract

The development of heterogeneous catalysts useful in the reaction of biodiesel synthesis constitutes a subject of high interest from the environmental and technological points of view. This work describes the preparation and characterization of a series of heterogeneous catalysts supported on activated carbons and their use in the synthesis of biodiesel from soybean oil. In a first stage of the work, different materials containing Ca and Na compounds dispersed in a commercial activated carbon were prepared by wet impregnation followed by heat treatments. Three different wet impregnation routes were employed. The first two routes involved the co-precipitation method, with Ca(NO3)2.4H2O as the source of calcium and NaOH (route 1) or NH4OH (route 2) as the precipitating agent, which were mixed with the activated carbon in aqueous suspension; in the third method (route 3), NaOH and CaO were mixed directly with the activated carbon in aqueous suspension. The prepared materials were heat treated at 800 ºC for 3 h under argon flow. The changes caused by heat treatments in the temperature range from 500 to 800 ºC were also investigated in the case of the catalyst prepared by route 3, with the characterization of the Na- and Ca-containing phases formed at each temperature. The catalytic efficiency of the prepared materials was analyzed by evaluating their performance in the production of biodiesel by the transesterification reaction of soybean oil with methanol. The best catalytic efficiency was achieved for the Na,Ca-based catalyst prepared by route 3, composed mainly by a mixture of CaO and Na2CO3 crystals dispersed in the porous carbon network. Catalytic recycling tests were also carried out to assess the possibility of reusing the catalyst, which revealed a drop in activity after three cycles; however, the reused catalyst still showed significant activity, attributed to the permanence of Na+ ions in the porous carbon structure. The analysis of the Ca and Na contents in the biodiesel samples synthesized in the three reaction cycles revealed relatively low metal amounts leached into the biodiesel. In another stage of the work, heterogeneous catalysts containing Na, Li and K carbonates dispersed in a char derived from coffee husk were produced from the reaction between the char and the respective alkali hydroxides during heat treatment up to 700 ºC. The catalysts containing Li and K carbonates showed high activity only in the first use, since the effects associated with the leaching of the active phases or the permanence of reaction byproducts in the catalyst surface caused a significant drop in the biodiesel conversion yields in successive reuse cycles. The best results were achieved for the catalysts containing Na2CO3 particles, which exhibited high catalytic activity even after three reuse cycles; this feature was attributed to the permanence of the Na2CO3 active phase in the char support. The results obtained show that the chosen routes are promising for the development of heterogeneous carbon-supported Na,Ca-based catalysts, with considerable surface area and good catalytic activity in the transesterification reaction for biodiesel production, being easily recycled and reused in several reaction cycles.