A Versatile Class of Carbon-Silica Composites for Tackling Biomass Challenges (2011)

Introduction We here present a new class of carbon-silica composite materials (CSM) with tunable bimodal porosity obtained by the synthesis of nano-sized graphitic crystallites inside the mesopores of a silica template. Besides the synthesis, their use in heterogeneous catalytic reactions will be highlighted in particular with emphasis on the valorisation of sugars to lactates. This value-added biomass conversion requires multiple catalytic functions and ideally demonstrates the versatility and tunability of this new class of composite materials. Experimental Section First, micelle-templated spherical Si-MCM-41 particles are synthesized according to a modified Stöber method1. Lewis acidic Sn is then incorporated via grafting and subsequent calcination in air. This mesoporous silica is impregnated with a furfurylalcohol solution and transferred into a quarts flow reactor. The organic precursor is polymerized and pyrolyzed under He at temperatures ranging from 673 to 1173 K. Post-synthesis oxidation of the carbon component is done at 573K under oxygen flow. The material was fully characterized using XRD, pyridine-adsorption-IR, TPD, ICP-OES, TGA, XPS, 31P-NMR and N2-physisorption. Catalytic tests with sugars were conducted in batch in alcoholic media. Results and Discussion Interestingly, the porosity characteristics of the CSM are readily controlled by the pyrolysis conditions and the amount of carbon precursor. The porous materials can be tuned from microporous over biporous to mesoporous composites. While past research by molecular probing techniques already demonstrated the shape selective properties of CSMs, we here focus on its usage in catalysis. The true unique advantage of this composite is the ability to introduce and tune various types of catalytic active functions independently onto both composing parts. The CSMs here described are bifunctional: weak Brønsted acidity arrives from oxygen built into the carbon (e.g. COOH) while Lewis acidic Sn is grafted on the silica surface. A schematic of the catalyst can be seen in figure 1. The synthesis of alkyllactates or lactic acid from sugars is an important biomass route towards valuable industrial products. Short chain alkyllactates are considered green solvents, and long chain esters are used in health care and cosmetics. The reaction mechanism starting from sucrose, C6 or C3-sugars is shown in figure. Starting from dihydroxyaceton, the first step can be both Lewis or Brønsted acid catalyzed, while the second one needs Lewis acidity for selectivity towards alkyllactates. Strong Brønsted acidity will imply acetal formation. The alcoholic solvent determines the alkylchain of the formed ester. In earlier work, the dihydroxyaceton conversion was proven to work with zeolites containing extra framework Lewis Al. It is however difficult to tune Lewis and Brønsted acidity separately with zeolites and the selectivity and activity were therefore quite low. Hencefore, the bifunctional CSMs with weak proton acidity were designed. Table 1 summarizes some catalytic results. Nafion, a strong Brønsted acid catalyst, shows high conversion but low selectivity to lactates. The bare Sn-MCM-41 (entry 2), displays low conversion and high selectivity. By introducing carbon with weak Brønsted acid oxygen functionalities in the mesopores of the silica, the conversion can be boosted, while the selectivity remains good. Entry 3 to 6 have been synthesized with an increasing carbon amount. Its effect is clear from the conversion. Entries 4, 7, 8 and 9 show CSMs synthesized at different pyrolysis temperatures. This determines the nature of the carbon. Pyrolyzed at 673 K, the carbon is rather poly-aromatic, with non-acidic oxygen groups and therefore low lactate yields are achieved. Pyrolyzing at 1173 K again gives a drop in conversion. The carbon here is more graphitic with less oxygen defects (evidenced with TPD) and more steric constraints. A simple oxidation procedure for introducing more oxygen defects and thus acid groups can further enhance the conversion to almost a 100% (entry 10). This catalytic activity is the highest reported to date for this reaction. The catalytic trends can be linked to the characterization of the acid sites and the carbon. These catalysts also perform well in long chain alcoholic media or starting directly from C6 sugars. An extra retro-aldol (and isomerization) is needed but up to 40 % yield of methyllactate could be obtained straight from sucrose at 433K in methanol. In conclusion, a new class of porous carbon-silica materials is addressed with easily adaptable pore architecture of benefit in catalytic reactions. The new material allows for an easy introduction of different active sites especially useful for one-pot multifunctional catalytic approaches such as the conversion of sugars to lactates.