A study of the oxidehydration of 1,2-propanediol to propanoic acid with bifunctional catalysts
Graphical abstract
Introduction
Propanediols (PD) are interesting bio-based building blocks for the synthesis of a variety of chemicals [[1], [2], [3]]. Both 1,2-PD and 1,3-PD can be obtained from biomass, e.g., by sugar fermentation or by chemo-catalytic hydrodeoxygenation (HDO) of glycerol or lactic acid [[4], [5], [6], [7], [8], [9], [10]]. In the latter case, various catalytic systems are known to catalyse efficiently the HDO to 1,2-PD, whereas the synthesis of 1,3-PD is more challenging, requiring catalysts based on Rh(Ir)/Re in order to achieve acceptable selectivity.
Amongst the various compounds which can be synthesized from 1,2-PD, eg, propionaldehyde (PAL) by dehydration [[11], [12], [13], [14]], propanol [15], pyruvaldehyde [16], and propanoic acid (PAC) [[17], [18], [19]], the latter might be the intermediate for the synthesis of bio-based methacrylic acid (MAA) and methylmethacrylate (MMA), the monomer for polymethylmethacrylate [[20], [21], [22]]. The glycerol-to-MAA pathway would thus include the steps shown in Scheme 1.
The second step (i.e., the acid-catalysed dehydration of 1,2-PD) and the third step (the oxidation of PAL to PAC) may be carried out with a one-pot process by using a single but bifunctional catalyst, showing both acid and oxidizing properties [[23], [24], [25]].
Indeed, it is well-know that on acidic catalysts, 1,2-PD can also transform into acetone and allylic alcohol, depending on which hydroxyl group is involved into the dehydration process [11,13,26]. Referring to literature, it is interesting to note that PAL generally is the main dehydration product with most of the acid materials so far studied for 1,2-PD dehydration. The latter behavior was explained by Zhang et al [11] on the basis of the mechanism reported in Scheme 2.
1,2-Diols are known to undergo the pinacol rearrangement to give the corresponding aldehyde [27], hence the authors proposed that protonation of either of the hydroxyl groups and rearrangement can generate three different reactive carbenium intermediates which yield acetone, PAL and allyl alcohol, respectively. The secondary carbenium ion, leading to PAL, is the more stable thus it is expected to have the higher concentration. Despite this, acid/base features of the catalysts might considerably influence 1,2-PD conversion [26].
In previous works, we reported about the reactivity of hexagonal tungsten bronzes (HTBs) for the oxidehydration (ODH) of glycerol to acrylic acid, with intermediate formation of acrolein [[28], [29], [30], [31], [32], [33], [34]]. The glycerol-to-acrylic acid reaction is supposed to be similar to 1,2-PD-to-propanoic acid, because of the similar molecules and reaction steps involved, and HTB oxides appear to possess the proper acid properties to perform the selective dehydration of 1,2-PD to PAL, both in terms of acid strength and type of acid sites, where the preponderance of Brønsted sites was proved to be beneficial for the reaction. Here we report about the use of HTBs for the ODH of 1,2-PD to PAC, via intermediate formation of PAL.
Section snippets
Catalyst preparation
W-V-O, W-Nb-O and W-Mo-V-O catalysts, with hexagonal tungsten bronze structure (HTB), were prepared hydrothermally at 175 °C for 48 h, according to a previously reported preparation procedure [[28], [29], [30]]. The catalysts precursors were finally heat-treated at 600 °C for 3 h in an inert atmosphere. They will be named as WV-1, WNb-2 and WMoV-3, respectively.
A Mo-V-W-O catalyst, with Mo/V/W molar ratio of 8/2/0.5, was prepared by coprecipitation from an aqueous solution (with ammonium
Physico-chemical characteristics of catalysts
Table 1 summarizes the characteristics of catalysts used. They showed surface area in the range of 9 to 40 m2 g−1, depending on the composition and the catalyst preparation procedure.
The tungsten-based metal oxides (i.e. WV-1, WNb-2 and WVMo-3) showed the typical XRD diffraction patterns of the hexagonal tungsten bronze (HTB) phase (JCPDS: 85–2460) (Fig. 1, patterns a to c) [30,31].
The MoVW-4 sample presented diffraction peaks similar to those reported previously for catalysts presenting a Mo5O
Conclusions
Hexagonal tunsgten bronzes (HTBs) were tested as catalysts for the oxidehydration of 1,2-propanediol into propionic acid, via intermediate formation of propionaldehyde; the latter is an intermediate compound in the multi-step transformation of glycerol into methylmethacrylate. Mixed metal HTBs containing W, V and Mo were active in the reaction, but the best propionic acid yield achieved was no higher than 13%. In fact, even though the acid properties of the HTB allowed to efficiently catalyse
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