(132c) Etherification of Glycerol and Other Biomass-Derived Polyols: New Routes to Valuable Bulk Chemicals

New catalytic routes have to be developed to transform renewable highly oxygenated platform molecules to valuable bulk and fine chemicals. The polyols constitute an important class of these biomass-derived oxygenates, with glycerol being a prime example. Glycerol is produced in large quantities as a by-product of biodiesel production, i.e. for every 1000 kg of fatty acid esters around 100 kg of glycerol is formed. Although glycerol itself has a lot of commercial applications, e.g. in the cosmetic and pharmaceutical industries, new attractive applications should be introduced to improve the economics of the biodiesel process. Furthermore, glycerol is regarded as a potential platform molecule since it can be directly produced from sugars or sugar alcohols, which are considered as the cornerstones of future biorefinery schemes. The development of new catalyst systems for glycerol valorization is therefore of great importance. In this respect, etherification of glycerol or, indeed, other biomass-derived polyols represents an important application as the products can be used as fuel additives, intermediates in the pharmaceutical industry, agrochemicals or as non-ionic surfactants. In this contribution we will discuss the recent efforts of our group which have focused on two distinct approaches for glycerol/polyol valorization [1-3] through etherification. We have developed efficient heterogeneous catalyst systems for both routes.

The first approach aims at the synthesis of di- and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. We have shown that the alkaline earth metal oxides CaO, SrO and BaO are highly active and selective as solid base catalysts for glycerol oligomerization (> 90 % selective at 60 % conversion) [1]. Importantly, glycerol oligomerization was not accompanied by any significant acrolein formation. Catalyst surface area and number of basic sites, but also Lewis acid strength turned out to be key parameters. Catalyst stability tests revealed the spontaneous formation of colloidal CaO particles which, when isolated, turned out to have superior etherification activity. A DFT study provided additional insight on glycerol absorption and reactivity in the oligomerization mechanism and the role of surface basicity and Lewis acidity therein [2]. The theoretical results were found to be in good agreement with the experimental data on catalyst reactivity. In addition, theoretical evidence was found for the possible direct involvement of Lewis acid sites in the catalytic etherification of bio-derived alcohols.

In our second approach, we have extended these studies to the etherification of glycerol with long linear alkenes, e.g. 1-octene, using solid acid catalysts [3]. Remarkably, the catalytic etherification route of direct nucleophilic addition of glycerol to linear, long-chain olefins has been little explored. We have successfully developed efficient and selective solid acid catalysts systems for the etherification of glycerol and other biomass-derived polyols with 1-octene over solid acid catalysts. This provides a direct route to long alkyl chain ethers with potential surfactant application. To this extent, various heterogeneous acid catalysts were screened in the etherification of neat glycerol with 1-octene. Zeolites showed modest conversions compared to Amberlyst 70 or pTSA, but superior selectivities towards the most valuable monoether products. The highest activity was obtained with H-Beta zeolites, which gave excellent selectivities for the mono and di-octyl ethers of glycerol of always > 85 %. H-Beta with a Si/Al ratio of 12.5 gave the highest conversion (15 %) and the highest selectivity to the mono-octyl ether (94 %). Several factors were found to influence both the etherification activity and the selectivity. Hydrophilic properties and porous structure of the catalyst turned out to be the critical parameters. Other parameters like reaction time, 1-octene:glycerol molar ratio, reaction temperature and the addition of an inert gas were investigated in order to improve the etherification activity of H-Beta zeolite. Catalysts deactivation due coke formation was investigated as well. In addition, the catalyst was recovered and re-used in 3 reaction cycles without any loss of activity or selectivity. The scope of the catalyst system could be successfully extended to other bio-based polyols. Very promising results were obtained in the etherification of, for instance, ethyleneglycol and 1,2-propanediol with 1-octene, as good conversions (around 60 %) and excellent selectivities (around 85 %) were obtained.

We would like to thank the ASPECT-ACTS Program for financial support.

[1] Ruppert, A. M.; Meeldijk, J. D.; Kuipers, B. W. M.; Erne, B. H.; Weckhuysen, B. M. Chem. Eur. J. 2008, 14, 2016-2024

[2] Calatayud, M.; Ruppert, A. M.; Weckhuysen, B. M. Chem. Eur. J. 2009, in press

[3] Ruppert, A. M.; Parvulescu A. N.; Arias, M.; Hausoul P. C.; Bruijnincx P. C. A.; Klein Gebbink, R. J. M.; Weckhuysen, B. M. J. Catal. 2009, submitted for publication