Síntese e caracterização de carvões ativados sulfonados obtidos de bagaço de cana-de-açúcar para conversão de xilose em furfural em meio aquoso

Abstract

Furfural is one of the compounds that can be synthesized from hemicellulosic monosaccharides, and it is considered a platform molecule due to its potential for conversion into other chemicals. In the quest for developing technologies and processes that convert lignocellulosic residues in a green and sustainable manner into biofuels and high-value chemicals, the present study aimed to produce sulfonated activated carbons (AC-S) from sugarcane bagasse for application as heterogeneous catalysts in the dehydration reaction of xylose for furfural production in an aqueous medium. Activated carbons (Acs) were synthesized from sugarcane bagasse impregnated with ZnCl2, NiCl2, or CuCl2 and carbonized at 550°C for 3 hours under nitrogen flow. The sulfonation process was carried out with sulfuric acid (98%), a mass/volume ratio of 1:10, a reaction temperature ranging from 103 to 217 °C, and a reaction time of 70 to 410 minutes. The catalytic activity of the ACs before and after sulfonation was evaluated with a catalyst/xylose mass percentage of 10%, temperature ranging from 128 to 212 °C, and reaction times from 11 to 139 minutes. The catalysts were characterized by nitrogen physical adsorption (BET and BJH), Raman spectroscopy, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), elemental analysis, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and total acidity (TA). The textural properties determined by BET and BJH methods showed that the activated carbons with ZnCl2 are classified as microporous materials with high surface areas (971 m2 g-1), while the ACs produced with NiCl2 and CuCl2 have lower SBET and are mesoporous materials. After the sulfonation process, all catalysts showed increased sulfur content due to incorporating sulfonic groups (SO3H) into the carbon matrix. DRX analysis revealed that the ACs possess an amorphous structure. SEM-EDS and Raman's analyses indicated that the sulfonation process did not cause significant structural changes in the ACs. The best experimental conditions determined in the central composite rotational design (CCRD) for maximizing the total acidity (TA) of CA-S/ZnCl2 and CA-S/CuCl2 occurred at a temperature of 217 ºC and a time of 240 minutes; however, for CA-S/NiCl2, the best TA was achieved at 120 °C and 120 minutes. The experimental conditions obtained in the CCRD for xylose conversion, furfural yield, and selectivity to furfural, for all sulfonated catalysts, were obtained at 200 °C and 120 minutes. Among the synthesized acidic catalysts, the CA-S/CuCl2 exhibited the highest furfural yield and selectivity (57.03% and 62.97%, respectively), highlighting that the best catalytic activity of this acid solid is likely related to the combination of a small amount of Lewis acid sites (Cu0) with Brönsted acid sites (-SO3H). Finally, it is important to highlight that, to the best of our knowledge, this work represents the first reported approach in the literature to synthesize AC-S without the leaching step of the activating agent before functionalization with H2SO4, thereby enabling the acquisition of catalysts with both Lewis and Brönsted acid sites in a single step. The yields and selectivity obtained with CA-S/CuCl2 are higher than those of various acidic catalysts reported in the literature, making it a promising alternative for furfural production in an aqueous medium.