Ethyl lactate from dihydroxyacetone by a montmorillonite-supported Pt(II) diphosphane complex
Graphical abstract
Introduction
Ethyl lactate (EtLA) is an environmentally benign solvent which finds its major application: (i) in the coating industry as a result of its high solvency power, high boiling point, low vapor pressure and low surface tension; (ii) as cleaning agent for the polyurethane industry and for metal surfaces, from which the removal of greases, oil adhesives and solid fuels is allowed; and (iii) in the pharmaceutical industry as a solvent for various biologically active compounds [1]. EtLA is conventionally produced via esterification of lactic acid with ethanol in the presence of an acid catalyst. The main drawback of this method is the self-esterification of lactic acid which leads to a mixture of acid and ester monomer and oligomers. Alternative synthetic protocols for EtLA use glycerol [2], C6 [3], [4] and C3-sugars, such as glyceraldehyde or dihydroxyacetone (DHA) [5], [6], [7], [8], [9], [10], [11], [12] as starting material. DHA is a convenient starting compound, since it is obtained by either fermentation of glycerol [13] or selective metal-based glycerol oxidation reactions [14], [15], [16], [17], [18], [19]. Homogeneous metal salt-based catalytic reactions employed for the DHA to alkyl lactate conversion occurred only at a high metal concentration (ca. 10 mol%). In the case of SnCl2, which is one of the most promising Lewis acid-based precatalysts for DHA to EtLA conversion, the resulting chemoselectivities for EtLA are <90% at 90 °C [5]. Sn-modified zeolites and montmorillonite (MMT) showed EtLA chemoselectivity >90% at 90 °C [6] and 150 °C [8], respectively. Pyruvaldehyde diacetal (PADA) was found to be the main side product, which is, in case of Sn-based catalysts, slowly converted to EtLA at high reaction temperatures [8]. The highest chemoselectivity for EtLA (96%), although with a low catalytic activity, has been reported for an amorphous mesoporous alumosilicate with a Si to Al atom ratio of 10 [11]. MMT is an alumosilicate clay mineral well known to constitute a suitable support for Pt-nanoparticles (NPs) [20] and Pt-coordination compounds [21]. These systems have been successfully used as catalysts for hydrogenation of nitro compounds to the corresponding anilines.
We exploited the Lewis-acidity of well-defined cationic Pt(II)(aqua)diphosphane complexes [22], [23], [24] for the catalytic conversion of DHA to EtLA in both homogeneous and heterogeneous (MMT-intercalated) phases. The confinement of the Pt-based catalyst between the lamellae of MMT, led to a significant increase in the catalytic activity and chemoselectivity.
Section snippets
Materials
All reagents used were purchased from Aldrich. Na-MMT (cation exchange capacity of 128 meq./100 g) was purchased from Laviosa Chimica Mineraria (Italy). Meso-2,4-bis(diphenylphosphino)pentane (bdppp) was synthesized as described in the literature [25]. Solvents used for synthesis and catalysis were distilled over an appropriate drying reagent, while deuterated ones were used as received.
Physical methods used
31P {1H} and 1H NMR spectra were acquired on a Bruker Avance 400 MHz spectrometer at 161.98 and 400.13 MHz,
Results and discussion
A series of bis-cationic Pt(II)(aqua)diphosphane complexes, bearing a chelating diphosphane ligand such as 1,1′-bis(diphenylphosphanyl)ferrocene (dppf) (1), 2,4-bis(diphenylphosphanyl)pentane (meso-bdppp) (2), 1,2-bis(diphenylphosphanyl)ethane (dppe) (3), 1,2-bis(diphenylphosphanyl)benzene (benzphos) (4), 1,1-bis(diphenylphosphanyl)methane (dppm) (5) or two triphenylphosphane (PPh3) ligands (6), have been synthesized. The Pt(II)(aqua) complexes were obtained upon two successive reaction steps (
Conclusions
Pt(II)-bis(aqua) complexes bearing various diphosphane ligands have been screened for the catalytic activity in the conversion of dihydroxyacetone to ethyl lactate in neat ethanol at moderate temperatures (50–70 °C), showing for the Pt(dppe)-based catalyst the best performance in terms of activity and chemoselectivity (i.e. TOF = 530 h−1, chemoselectivity, 96% at 70 °C). The bis-cationic Pt(II)(dppe) complex was successfully intercalated between the lamellae of montmorillonite upon a simple cationic
Acknowledgments
The authors thank Dr. Elisa Passaglia for providing Na-MMT. F.B. acknowledges EIT RawMaterials for a grant through FREECATS Nol (Project Number 15054).
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