Glycerol etherification to dimers using basic and acidic catalysts

Etherification is a promising way for valorization of glycerol. Besides conversion to fuel blending components (e.g. tert.-butyl ethers [1]), glycerol etherification to di- and polyglycerols yields attractive products required for manufacturing of biodegradable surfactants, lubricants, cosmetics, food additives and others.
Earth alkaline oxides [2,3] or alkaline modified zeolites and mesoporous materials [4,5] are reported to be promising solid catalysts for glycerol etherification to diglycerol. A theoretical study on the role of surface basicity and Lewis acidity is described in [3].
The current contribution will give an overview on the base- and acid-catalyzed reaction pathways including application of heterogeneous and homogeneous catalysts.
Recently, we have shown [6] that the etherification reaction using Na and Cs modified zeolites X, Y and Beta can favorably be accomplished at 260 °C under atmospheric pressure. Activity (glycerol conversion after 8 h reaction time) was found to be highest on CsX, with 63 % glycerol conversion and selectivity to linear diglycerol of ca. 80 %.
However, XRD and SEM analysis of spent catalysts proved that CsX zeolites suffered a structural collapse during reaction with release of Cs into solution (as confirmed by XRF). This inspired us to look for the role of alkali metal salts that can be considered as homogenous catalysts because of their facile solubility in glycerol. We studied the reaction using MeHCO3 (Me = Na, K, Cs) as well as CsAn (An = OH-, CH3COO-, NO3-) for clarifying the influence of catalyst basicity on the reaction. Results were compared with Cs ion exchanged zeolites (the zeolite aluminosilicate structure might be viewed as complex inorganic anion).
Whereas the influence of the cations was not significant, the conversion of glycerol decreased using CsOH, CH3COOCs and CsNO3. The highest conversion was observed for CsOH, one of most powerful base in aqueous solution.
Alternatively, an acidic reaction route has been investigated, to avoid the drawback of separating the desired products from solid and dissolved catalyst components. The acid-catalyzed glycerol etherification was carried out in a tube reactor under reflux vacuum conditions at ca. 150-180 °C [7,8] using a super-acidic polymer resin as catalyst. The glycerol had only short contact time with the resin catalyst where the etherification proceeded in a liquid film running across the catalyst foil. The di-/polyglycerol mixture was accumulated at the bottom of the reactor construction. This reaction technology allowed maintaining high selectivity of digylcerol at nearly complete glycerol conversion.

References
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[2] A.M. Ruppert, B.M. Weckhuysen, J.D. Meeldijk, B.W.M. Kuipers, B.H. Erné, Chem. Eur. J. 14 (2008) 2016.
[3] M. Calatayud, A.M. Ruppert, B.M. Weckhuysen, Chem. Eur. J. 15 (2009) 10864.
[4] J.-M. Clacens, Y. Pouilloux, J. Barrault, C. Linares, M. Goldwasser, Stud. Surf. Sci. Catal. 118 (1998) 895.
[5] F. Jérôme, Y. Pouilloux, J. Barrault, ChemSusChem 1 (2008) 586.
[6] Y.K. Krisnandi, R. Eckelt, M. Schneider, A. Martin, M. Richter, ChemSusChem 1 (2008) 835.
[7] M. Richter, R. Eckelt Y.K. Krisnandi, A. Martin, Chem. Ing. Techn. 80 (2008) 1573.
[8] Patent DE 10 2007 042 381 B3, LIKAT.