Catalisadores heterogêneos baseados em materiais de carbono sulfonado para aplicação na reação de esterificação

Abstract

Sulfonated carbon-based catalysts are candidates to replace homogeneous catalysts such as H2SO4. Mainly due to the easy separation of the reaction medium, acidic surface, high selectivity and thermal stability. These physicochemical properties depend on the carbon source, method of synthesis, density of acid groups and surface chemistry. In this context, the present work aims to prepare new heterogeneous catalysts of sulfonated carbon by different synthesis methods and carbon precursors for application in the esterification reaction. The first method described in Chapter 2 is related to the preparation of a highly efficient heterogeneous magnetic catalyst for biofuel production. Two abundant residues from the Amazon region were used as low-cost precursors, red mud and acai seed. The catalyst was prepared by the wet impregnation method, followed by carbonization at 400 ⁰C for 3 h and subsequent sulfonation at 80 ⁰C for 3 h. The materials were characterized by XRD, SEM-EDS, TG, FTIR, N2 adsorption-desorption, VSM and acid-base titration. Sample CALVS1-4 showed 88% conversion to methyl oleate under reaction conditions of 5% catalyst loading, molar ratio oleic acid:methanol 1:12, temperature of 100 °C and time of 1 h. At the end of the reaction, the catalyst was easily separated from the reaction medium by a magnet and exhibited good reusability until the third reaction cycle. The second synthesis method inserted in Chapter 3 describes the preparation of a heterogeneous catalyst by the in situ functionalization method of acai seed residue. This sulfonation route has the advantage of carbonizing and sulfonating the material simultaneously, avoiding wasted time and energy. Sample HTAS6-130, simultaneously carbonized and sulfonated at 130 °C for 6 h, showed better performance in the esterification reaction of oleic acid and methanol with a conversion of 95% at 100 °C for 1 h, catalyst loading of 3% and molar ratio of methanol to oleic acid 12:1. The recyclability of HTAS6-130 was satisfactory, reducing the conversion by only 18% after the five reaction cycles. The in situ functionalization method brought together the advantages of rapid synthesis, low energy consumption and high density of active acid sites after use in five consecutive reaction cycles. Chapter 4 brings as a novelty the ultrasound-assisted synthesis of sulfonated mesoporous carbon as an alternative to produce structured carbon materials. The synthesis was prepared by the soft-template method in an ultrasonic processor and subsequently carbonized and sulfonated for application in the esterification model reaction. The functionalization of the samples with sulfuric acid brought about a reduction in porosity, but simultaneously resulted in the generation of functional groups of an acidic nature. Resulting in the CMS5-6h catalyst with sulfonic density of 1.6 mmol g-1, BET surface area 402 m2 g-1 and pore diameter 10.6 nm, remaining in the mesopore range even after acid treatment. The CMS5-6h catalyst showed excellent activity in the esterification reaction with 95% oleic acid conversion. The recyclability of MCS5-6h was satisfactory during five reaction cycles, showing a reduction of only 20% after the last cycle studied. The experiments yielded promising results for obtaining sulfonated carbon materials in a short period of time under mild temperatures and with excellent catalysis performance. In addition, the use of residues from Amazonian biomass in the preparation of catalysts may be the appropriate solution for the destination of these types of residues.