Influence of potassium-based catalysts in the pyrolysis of Caupi bean pod (Vigna unguiculata): bio-oil characterization by gas chromatography coupled with mass spectrometry (GC/MS) and comprehensive two-dimensional gas chromatography (GC x GC/TOFMS)

The Caupi bean pod (Vigna unguiculata) is an agricultural residue with the potential to be used in the pyrolysis process, as it contributes to minimize the environmental impact and add value to this material. In situ catalytic pyrolysis, in which the catalysts and feedstock were kept in one reactor, was performed with five different potassium catalysts, and their catalytic effect on the bio-oils composition distribution and phenols enrichment was evaluated. The bio-oils obtained were characterized by gas chromatography coupled with mass spectrometry (GC/MS) and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry GC x GC/TOFMS. The application of GC x GC/TOFMS was important to resolve coeluted peaks from the GC/MS analysis, resulting in a different distribution of compounds by chemical classes. This affected the percentual area of the phenols group, since it ranged from 44.28 to 65.78% with GC/MS analysis, while the area from the same group varied from 59.77 to 73.75% in the GC x GC/TOFMS analysis of the same samples. The bio-oils produced were characterized, and the data showed that K+ and anions affected primary and secondary reactions, modifying the formation of several chemical species. The catalytic pyrolysis of the bean pod showed a strong tendency to produce high yields and selectivities of phenols and their derivatives due to the performance of the acids catalysts. This work demonstrated that the potassium ion (K+) acts to improve the quality of bio-oil from the pyrolysis of cowpea pods (Vigna unguiculata), as well as a greater or lesser synergistic effect of counterions in enhancing the bio-oil quality. This contributes to the formation of commercially valuable compounds, such as phenols and their derivatives.