To evaluate the effect of residence time on products selectivity, some reaction tests at 533 K using different amounts of La₂CuO₄ as catalyst were carried out. The selectivity toward HA, pyruvaldehyde, and glyceraldehyde as a function of glycerol conversion is described in Fig. 2.

In general, it is possible to observe that the selectivity toward HA increases until reaching a maximum value around 76 % at complete glycerol conversion (1000 mg of catalyst). This trend indicates that reaction pathways leading to HA formation from glycerol are thermodynamically favourable, even though the global kinetic reaction is rather slow, since a long contact time is required to obtain high yields. On the other hand, the selectivity toward glyceraldehyde and pyruvaldehyde was relatively low and constant with the glycerol conversion, indicating that although these compounds are coming from reactions that are thermodynamically unfavourable, once they reach their maximum yield, they can participate as intermediate species on consecutive reactions leading to HA formation under catalytic conditions.

Alhanash et al. ^[26] and Kinage et al. ^[11] proposed, on the basis of their observations, that the formation of HA from glycerol can be catalysed not only by acid sites, but also over basic sites. However, an optimum number of acidic and basic sites on catalyst surface must be taken into account to obtain high selectivity toward HA formation. Given this result, it is reasonable to think that on the La₂CuO₄ catalyst used in this study, a balance between both Lewis acid-base sites and Brønsted basic sites should exist.

Figure 2 Source: Own source

It has also been reported that the main reaction pathway that contributes to the selective HA formation from glycerol occurs through a direct dehydration over Lewis acid sites ^[17,^21]. However, other reaction pathways involving sequential dehydrogenation/dehydration steps on Brønsted-basic sites can also contribute to the increase in HA selectivity as reported in Fig. 2. For instance, the glycerol dehydrogenation over Brønsted basic sites leads to the formation of glyceraldehyde which is then dehydrated to form an enol intermediate that is reorganized to give pyruvaldehyde.

Finally, the pyruvaldehyde is hydrogenated to HA or 1,2-propanediol depending on H₂ content as it was proposed by Auneau et al. ^[27] and Kinage et al. ^[11]. But, according to Albuquerque et al. ^[28], the reaction from HA to pyruvaldehyde can also be possible under alkaline conditions in liquid phase, which leads us to believe that this reaction step can be reversible.

However, none of these authors detected the formation of glyceraldehyde or pyruvaldehyde in their studies, probably due to the oxidant reaction conditions used. The experimental results obtained from the evaluation of some reaction pathways discussed above will be presented in the next section to verify their validity.

With the aim to understand possible reaction pathways from glycerol to HA, a series of complementary reactions were carried out using glyceraldehyde and HA as reactants. The reactivity of glycerol under hydrogen and nitrogen as carrier gases was also evaluated. The results of conversion, selectivity, and carbon balance are gathered in Table 1.

All experiments were carried out using 1000 mg of La₂CuO₄ catalyst corresponding to the best result reported in Fig. 2. Entry 1 shows the reference values corresponding to the reaction performed under N₂ atmosphere, condition at which the complete glycerol conversion and higher selectivity towards HA were obtained.

Entry 2 shows how the change of carrier gas from N₂ to H₂ increases the selectivity toward HA followed by a decrease in glyceraldehyde and pyruvaldehyde formation. This result agrees with the mechanism of Auneau et al. ^[29] and others ^[6,¹¹,^17] implying that the hydrogenation of these two species toward HA can proceed as proposed in Fig. 1. However, due to the hydrogen-rich environment used in Entry 2, the extension of hydrogenation reaction can promote the formation of 1,2-propanediol from HA. It is important to highlight that the detection of 1,2-propanediol was carried out by GC and its quantification is within the 18 % of other products. Something similar is expected to occur under N₂ atmosphere, since the H₂ coming from initial dehydrogenation step from glycerol will promote the subsequent hydrogenation reactions, but the extension of the reaction will be limited only toward HA formation since the H₂ content is lower than the one used in Entry 2.

Similarly, the formation of pyruvaldehyde was also experimentally investigated using HA and 10 % glyceraldehyde-glycerol solution as reactants. Entry 3 shows that HA is a very stable compound since only 10 % was transformed mostly to pyruvaldehyde, indicating that the dehydrogenation of HA to pyruvaldehyde is highly selective but kinetically unfavorable under our reaction conditions. In fact, Auneau et al. ^[27] proposed that equilibrium must exist between pyruvaldehyde and HA that in the presence of hydrogen could shift the equilibrium in favor of HA formation. On the other hand, the addition of glyceraldehyde to glycerol as reactant does not significantly modify the selectivity of pyruvaldehyde but increases the selectivity to HA, when compared to the reference values reported in Entry 1. This suggests that the reaction leading to HA from glyceraldehyde can be much more favorable than the one leading to pyruvaldehyde under our reaction conditions.

With the aim to having more detailed information of reaction pathways described above leading to HA formation from glycerol, we performed a computational analysis involving different routes of glycerol transformation at gas phase looking for the same products. Therefore, in terms of elementary reactions, intermediate species, transition states, reaction coordinates, and thermodynamics of the process, we performed a computational analysis that will be described in the next section.

Table 1

Source: the authors

a. Glyceraldehyde dehydration

Although in a previous work ^[22] it was proposed that HA is formed directly from glyceraldehyde, computational results suggest that after the dehydration reaction of glyceraldehyde (structure 2) an intermediate (int2) is formed, which is shown in ( eq. 4 ). From int2, the results show two competitive reactions ( 5),( eq.6)), which correspond to a tautomerism step that leads to the formation of pyruvaldehyde (structure 3) and a hydrogenation step that leads to the formation of int3. The transition state (TS4), which leads to int2, is similar to that for the dehydration of glycerol, i.e. a four-membered cyclic transition structure with an activation energy of ΔG_533,TS4 = 239.3 kJ mol^-1. This result implies that if the dehydration of glycerol is possible to form int1 at 533 K in ( (eq. 1 ), then it is expected that the dehydration reaction of glyceraldehyde to form int2 is also favorable, since the energy barrier is lower.

Additionally, due to the same transition state structure it is possible that the energy barrier for this dehydration reaction could also be affected by the presence of Lewis acid-base pair on the catalyst surface.

( Eq. 5) involves a breaking of an H-H bond in a hydrogen molecule that requires a large amount of energy (ΔG₅₃₃ = 406.7 kJ mol^-1) to proceed. However, if there is a hydrogen molecule near to two electronegative atoms, it is expected that the electron interaction would decrease the dissociation energy compared to that required for the isolated hydrogen molecule. In the same context, it is also possible to suggest that the metal sites on the catalyst surface contribute to the dissociation of the hydrogen molecule. Therefore, this hydrogenation step would be favored through a catalytic effect. In this reaction, a six-membered cyclic transition state (TS5) structure was obtained, with an energy barrier of ΔG_533,TS5 = 236.4 kJ mol^-1, which is lower than that for four-membered cyclic transition states. This result is expected since six-membered cyclic structures have less geometric stress.

On the other hand, the energy barrier for the tautomerism step ((eq. 6)) is ΔG_533,TS6 = 241.0 kJ mol^-1, which is very similar to the hydrogenation step in ( eq. 5). This suggests that both pathways are thermodynamically feasible.

However, it was observed in the experimental results (Table 1) that when hydrogen was used as carrier gas, the selectivity to pyruvaldehyde decreases to nearly zero followed by an increase in the HA selectivity. This result implies that it is possible to avoid the pyruvaldehyde formation when hydrogen content in the reaction medium is high. Another aspect that can suggest the pathway of these two reactions is associated to the exothermicity. For ( eq. 5) ΔH₅₃₃ = -64.0 kJ mol^-1 while for ( eq. 6), ΔH₅₃₃ = -18.4 kJ mol^-1. These results suggest that the hydrogenation reaction that leads to int3 can be slightly favored under low temperatures and a hydrogen atmosphere.

Finally, HA is formed from int3 as the most stable product through a tautomerism reaction as it is shown in eq. ( eq. 7). This reaction has a relatively low energy barrier (ΔG_533,TS7 = 154.4 kJ mol^-1) suggesting that once the int3 is formed, the conversion to HA at 533 K should be rather easy.

Fig. 3 corresponds to a Gibbs free energy plot where all the previous reaction pathways are included. From computational results, it is possible to conclude that the dehydration reaction (grey line) is the most probable pathway to obtain HA as the most favorable product of glycerol transformation. However, it was found that it is possible to obtain HA through a second pathway that involves the formation of glyceraldehyde after dehydrogenation of glycerol (continuous black line), which corresponds to the most difficult step due to its high energy barrier. Nevertheless, once glyceraldehyde is formed it may undergo a dehydration reaction that conducts to an intermediate (int4) from which there are two competitive reactions that lead to pyruvaldehyde (dotted line) formation or to another intermediate (int3) which finally may tautomerize to form HA.

b. Hydrogenation of pyruvaldehyde

We studied the possibility of producing HA from the hydrogenation of pyruvaldehyde as it is proposed in the literature ^[28]. It was found that this hydrogenation step is thermodynamically favorable but kinetically controlled due to the high energy barrier (ΔG_533,TS8 = 356.8 kJ mol^-1) that has to be overcome. This result is in agreement with the experimental result reported in Entry 1 and Entry 2 (Table 1), implying that even in the presence of catalyst, the hydrogenation reaction of pyruvaldehyde toward HA depends on the H₂ partial pressure (Entry 2).

In this way, it is concluded that under our reaction conditions, the hydrogenation of pyruvaldehyde toward HA is unfavorable since the H₂ content was low. A similar analysis for the reverse reaction, from HA to pyruvaldehyde, can be performed. In this case, the dehydrogenation step is much less favorable than the hydrogenation step described above since the energy barrier is higher (ΔG_533,TS8,r = 387.0 kJ mol^-1). This agrees with the experimental result obtained in Entry 3 (see Table 1) indicating that although the selectivity toward pyruvaldehyde was high, the HA conversion was relatively low (10 %).

Figure 3 Source: Own source

The hydrogenation step that leads to the formation of 1,2-propanediol from HA was also studied. It was found that this reaction also has a high energy barrier (ΔG_533,TS9 = 390.8 kJ mol^-1) and agrees with the low selectivity obtained for this product in the experimental results. Particularly, for these two hydrogenation reactions similar transition structures were found as can be observed in ( eq. 8), ( eq. 9), i.e. four-membered transition states (TS8 and TS9). These results compared to those obtained for six-membered cyclic transition state would suggest that the energy barrier of the hydrogenation steps depends strongly on the transition state geometry.