An alternative route to synthesize acrylic acid: the dehydration of lactic acid and lactates over alkaline earth phosphates
Résumé
Introduction
Acrylic acid is a key platform molecule whose main applications are superabsorbent polymers (diapers), paints, coatings, adhesives and flocculating agents for water treatment. It is produced at 4.5 Mt/year by selective oxidation of propene, whose price is growing quickly because of increasing demand and rarefaction of petroleum. Several alternative sustainable routes are investigated at the present time. Among them, dehydration of lactic acid and lactates are interesting since these reactants can be yielded both by dehydrogenation of glycerol, a by-product of bio-diesel production and directly by fermentation of sugars.
In spite of first study in 1958 [1], dehydration of lactic acid (LA) and derivatives have been mostly investigated since recent years with the rise of biomass valorization. High yields to acrylic acid (AA) and derivatives were obtained using modified zeolites [2] or calcium phosphates [3-4]. However, zeolites suffer from hydrothermal instability.
In this work, different alkaline earth phosphates were prepared and evaluated for dehydration of lactic acid to acrylic acid. Because of high LA reactivity, the ethyl lactate (EL) conversion was also investigated. The origin of catalytic efficiency was explained from acido-base properties and the nature of the active sites present in such catalysts.
Materials and Methods
Alkaline earth ortho, pyrophosphates and hydroxyapatites were prepared by co-precipitation method. Their labeling was described elsewhere [5]. The solids were characterized by XRD, FTIR spectroscopy, BET measurements and ICP analysis. Furthermore, surface characterization was obtained crossing XPS, 1H-31P CP-MAS NMR, TEM and IR techniques. Acid-Base properties were measured by NH3/CO2-TPD respectively and adsorption of probe molecules followed by FTIR.
Gas phase dehydration of ethyl lactate or lactic acid was conducted in a fixed bed reactor at atmospheric pressure. Solutions of reactant were vaporized at 160-170°C and diluted with N2 before sending to the top of the reactor. After trapping, the condensed molecules were analyzed off-line with a GC chromatograph while gas products were analyzed on line.
Results and Discussion
In first step, the influence of reaction parameters was investigated: selectivity to acrylic acid strongly depends on the reaction temperature but not on the contact time. At optimized temperature of 380°C, values ranging from 19 to 49% were measured for the different prepared phosphates catalysts that are stable for at least 24 h. The best yield (43%) was obtained from barium orthophosphate [5].
Acidâbase properties determined from NH3 and CO2 TPD measurements revealed that such phosphates contain high proportion of acidic and basic sites with same weak strength. Furthermore, correlation between selectivity to acrylic acid and the acidâbase balance was clearly established: it was 50% for balance close to 1 and decreased increasing this parameter [5].
Dehydration of ethyl lactate appeared as promising reaction in spite of lower conversions. Indeed, selectivity values in dehydration products (AA and ethyl acrylate (EA)) were much higher with maxima of 87%. The evolution of selectivity sets with the conversion revealed that AA is mainly formed by simultaneous dehydration/hydrolysis reaction. However, the catalysts were unstable vaporizing pure EL solution because of polymerization of EA leading to coke formation. Interestingly, it was shown that deactivation can be inhibited adding water to the gas phase [6].
XPS characterization of the catalysts evidenced the presence at the surface of P-rich phase that corresponds to amorphous overlayer of few nanometers as evidenced by TEM. From XPS, 1H-31P CPMAS and DRIFT spectra, it was concluded that such phase contains high quantity of POH species and could correspond to a mixture of mono and dihydrogenophosphates or polyphosphates. In situ DRIFT spectra have revealed that POH species are formed under water vapor whereas they are consumed or modified under reaction mixture for both reactants suggesting that they involved in one step of the reaction mechanism. Finally, NH3 and CO2-TPD measurements followed by in situ DRIFT spectroscopy revealed that acid base pairs (M2+; P=O) and P=O basic sites are respectively probed by these techniques. The acid-base balance of 1 obtained for the most selective catalysts was explained by the involvement of (M2+; P=O) pairs in the rate-determining step of the reaction mechanism. Finally, the monitoring of POH isotopic labeling under reaction feed revealed proton transfer to the methyl group of lactic acid. From all these results, it was proposed that (M2+; P=O) pairs would constitute the adsorption site of lactic acid which then dehydrate in acrylic acid by an E2 mechanism involving POH species.
Conclusions
Selectivity to acrylic acid is limited for dehydration of lactic acid over alkaline earth phosphates. Very high selectivity to dehydration products can be obtained using ethyl lactate as reactant but high specific surface area is required to reach high yield. Water vapor has to be added to the reaction feed in good proportion to avoid catalyst deactivation and reactant hydrolysis leading to lower selectivity. Finally, crossing different techniques and from in situ experiments, it was proposed that (M2+; P=O) pairs are the active and selective sites in such catalysts. These findings will allow designing more efficient catalysts.
Acknowledgements
This work was supported by French ANR Program Chimie Durable â Industries â Innovation (CD2I) GALAC, a joint project between IRCELYON, UCCS, LC/ENS-Lyon and Novance company.
References
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