Biodiesel Processing Using Sodium and Potassium Geopolymer Powders as Heterogeneous Catalysts

Abstract

This work investigates the catalytic activity of geopolymers produced using two different alkali components (sodium or potassium) and four treatment temperatures (110 to 700 °C) for the methyl transesterification of soybean oil. The geopolymers were prepared with metakaolin as an aluminosilicate source and alkaline activating solutions containing either sodium or potassium in the same molar oxide proportions. The potassium-based formulation displayed a higher specific surface area and lower average pore size (28.64–62.54 m²/g; 9 nm) than the sodium formulation (6.34–32.62 m²/g; 17 nm). The reduction in specific surface area (SSA) after the heat treatment was more severe for the sodium formulation due to the higher thermal shrinkage. The catalytic activity of the geopolymer powders was compared under the same reactional conditions (70–75 °C, 150% methanol excess, 4 h reaction) and same weight amounts (3% to oil). The differences in performance were attributed to the influences of sodium and potassium on the geopolymerization process and to the accessibility of the reactants to the catalytic sites. The Na-based geopolymers performed better, with FAME contents in the biodiesel phase of 85.1% and 89.9% for samples treated at 500 and 300 °C, respectively. These results are competitive in comparison with most heterogeneous base catalysts reported in the literature, considering the very mild conditions of temperature, excess methanol and catalyst amount and the short time spent in reactions.

Leachability and basicity of Na‐ and K‐based geopolymer powders and lattices used as biodiesel catalysts

“…Figure 1 shows the XRD pattern for powdered geopolymers heat treated at 110 and 700°C. Patterns for samples treated at 300 at 500°C are omitted as they showed no difference with the ones treated at 110°C 10 . The samples treated at the lowest temperature show the amorphous structure typical of geopolymers, with a hump centered at approximately 27°; few peaks are present in all spectra and relate to the impurities of the metakaolin raw material, that is, quartz, anatase, and muscovite.…”

“…Similarly, the potassium content was reduced 20.2% for K‐GP (from 21.51 to 17.16 g/100 g) and 13.6% for Na.K‐GP (from 11.37 to 9.82 g/100 g). The gradual decrease in leaching with increasing heat treatment temperature was most likely caused by a decrease in the SSA 10 and solvent accessibility within the solid particles.…”

“…It is worth noting that Na‐GP and K‐GP powders treated at temperatures ranging from 110 to 700°C were recently tested for methanolysis of soybean oil using a ratio of 3 g of geopolymer to 100 g of oil and a 150% methanol excess for a 4‐h reaction at 75°C 10 . In that study, all powders tested yielded some biodiesel, even those treated at 700°C (51.4% for Na‐GP and 16.7% for K‐GP).…”

“…Additive Manufacturing, namely Direct Ink Writing, was used to fabricate the lattices which were shaped as short cylinders with a diameter of 25 mm and a height of 9.6 mm. All powders and lattice materials were successfully tested for methanolysis of soybean oil, and the results were published in our previous articles 8–10 . Table 1 summarizes the theoretical compositions of the powder and lattice geopolymers tested.…”

Biomass: A Renewable Source of Fuels, Chemicals and Carbon Materials

“…Biodiesel (i.e., long-chain alkyl esters) is typically produced by the transesterification of triglycerides found in vegetable oils with methanol and homogenous basic catalysts (e.g., NaOH). The utilization of heterogeneous basic catalysts provides an alternative for cleaner and more sustainable biodiesel production, as reported by Botti et al [8]. Valeric biofuels (esters of valeric acid, which can be obtained from biomass-derived levulinic acid) were reported to be obtained via esterification over lipases immobilized by a sol-gel method [9].…”