2.3. Catalyst Test with Pt/BC
The pathway leading to the products observed during furfural hydrogenation can be simplified as suggested in Figure 2
. The main products were FA, tetrahydrofurfuryl alcohol (THFA), furan (F), methyltetrahydrofuran (MTHF), and methylfuran (MF). Table 1
summarizes the obtained results from the reaction of furfural over Pt/BC 3 wt % catalyst in correlation to the operation parameters.
Hydrogenation rates have been previously reported to increase proportionally with temperatures [24
] however, no clear tendency of this nature was observed for the actual experiments. For the experiments involving the catalyst with a 3 wt % platinum loading, the optimal temperature was reached at 210 °C, showing a maximum conversion of 60.8% with 79.2% selectivity to FA after 2 h (Entry 9, Table 1
). The yield of FA drops from 48.2% to 16.1% (Entry 8) when temperature decreases from 210 °C to 170 °C. Even if a higher conversion value is observed at 320 °C (Entry 13), the selectivity toward FA is 16.8% and the then FA yield decreases from 48.2% to 11.8%. Moreover, results from Entries 10 and 11 also showed that increasing the reaction time only leaded to a slight modification of the conversion rate and an increased portion of furan, resulting in a decrease in selectivity to FA. This behavior was observed as well after addition of a higher amount of catalyst (Entry 5, Table 1
Variation of the temperature in either direction from 210 °C impacts conversion and selectivity to FA (Entries 8, 9 and 12–14, Table 1
) thus increasing the formation of byproducts as MF, furan, as well as other unknown compounds. One hypothesis explaining this phenomenon is probably related to catalyst selectivity that can be affected by adsorption of furfural and/or FA and by products on the surface of the catalyst’s active sites.
pressure from 500 psi to 1500 psi was shown to increase the conversion of furfural from 20.1% to 60.8% (Entries 1, 3 and 9, Table 1
). The formation of byproducts at higher pressures of hydrogen does not appear to significantly affect the selectivity to FA, the latter increasing to 79.2% at 1500 psi conditions that were considered optimal for this catalyst and support. Results also showed that almost no THFA was formed for most of the tests performed with the Pt/BC catalyst, indicating that the latter was not intrinsically selective to C–C double bonds thus prioritizing reduction of C=O or C–OH bonds. The tests using different amounts of catalyst (Pt/BC 1 and 5 wt %) were performed in the conditions that were shown optimal for a 3 wt % loading of catalyst (toluene as solvent, 210 °C, 1500 psi, for 2 h and 4 h), as presented in Table 2
Using a higher percentage of Pt in the catalyst but overall using the same amount of platinum based catalyst did not favor an increased conversion of furfural to FA compared with 3 wt % catalyst (Entries 9 and 10, Table 2
). Furthermore, the use of higher quantity of impregnated metal (5 wt %) was shown to reduce conversion of approximately 20% in both cases (Entries 15 and 16, Table 2
). The reduced homogeneity of Pt distribution, being a consequence of the greater impregnation concentration, results in non-favorable conditions for furfural hydrogenation. Selectivity to FA was also shown to be significantly reduced and chromatograms showed the presence of new unknown byproducts, which did not occur at lower (3 wt %) Pt content.
It should be noted that biochar per se was not inert in furfural hydrogenation, which was elucidated by testing only support at the same experimental conditions as above. Hydrogenation of furfural with the support alone results in conversion of 3.3% and selectivity to FA of 21.3%, using the optimal conditions (identified by the tests on a 3 wt % loading of platinum). The biochar carbon surfaces and pores possess complex structures containing metals and oxygen groups, as well, the biochar has a tendency for fixation of metallic ion thus leading to a concentration of the plant inorganic content. As well, this functionality also allows the biochar to be a very efficient support for other types of metals, either for catalytic purposes or for soil remediation [27
]. Overall, both the textural properties and the trace metal content in the biochar could be generating some activity. Furthermore, it is important to note that the support is stable under the reaction conditions and it does not interact with the solvent. Duplicate experiments were performed showing the rates and selectivity to the various products to be within an error margin of around 5%, thus confirming reproducibility of the results.
The most important solvent effects in the hydrogenation of α,β-unsaturated aldehydes are usually related to solvent polarity, solubility of hydrogen, and interactions between the catalyst and the solvent as well as solvation of reactants in the bulk liquid phase [28
]. Further and as noted previously, both the support and the type of solvent can affect selectivity. As for this work, the solvent effect on the furfural hydrogenation was investigated with the Pt/BC 3 wt % at 210 °C and 250 °C with a H2
pressure of 1500 psi for 2 h using toluene, isopropanol, isobutanol, and hexane. Results obtained for the hydrogenation of furfural using other solvents are shown in Table 3
Utilization of isopropanol has led to the formation of dimerization products (Figure 3
) that did not occur in toluene. The selectivity toward these products was close to 18% and 10% at 250 °C and 210 °C, respectively. However, in both cases, the main product remained FA. Although conversion did increase, the difference was not significantly higher than what was noted for toluene at 250 °C (Entry 17 Table 3
, and Entry 12 Table 3
), and the selectivity to FA was shown to decrease, as a result of significant quantities of MF as well as dimerization products formed. At 210 °C (Entries 9 and 18 Table 3
), conversion decreased to 23% in isopropanol although selectivity remained close to 80% in both cases. Isopropanol has a significant effect on the product’s selectivity and formation of by-products is more pronounced. A hypothesis that could explain such observation would be that the greater availability of hydrogen in the media could lead to furfural and/or FA dimerization via a higher availability of radicals that could generate the end products depicted in Figure 3
below (identified by gas chromatograph coupled to mass spectrometer (GC-MS)).
Utilization of bulky alcohols (as isobutanol) are supposed to lead to greater hydrogen solubility although it was not reported to correlate with an increased hydrogenation rate. The % conversion in isobutanol seems to corroborate such assumptions. At 250 °C, an increase in both conversion and selectivity toward alcohol was observed (Entry 19 Table 3
). However, at 210 °C, the selectivity was reduced by about 20%.
Hexane (Entry 22, Table 3
) gave results that were comparable to toluene at 210 °C (Entry 9, Table 3
) however, two liquid phases were obtained following the reaction and the possibility of finding products in both phases thus hinders quantification. The increased conversion noted at 250 °C (Entry 21, Table 3
) over 210 °C in hexane as compared to the other solvents is probably a result of the increased solubility of furfural in hexane as well as the formation of other unidentified condensation products.
All of the solvents that were used in this research lead to the production of MF and furan. However, dimerization and/or condensation products formation was affected by the solvent type and occurred essentially with a polar solvent.