Efficient and stable catalyst converts xylose to xylitol

Suschem student finds “missing” ruthenium

22 September 2015

MSc student Carlos Hernandez-Mejia and his supervisor Dr Raveendran Shiju of the UvA Research Priority Area Sustainable Chemistry have discovered the secret to stable and efficient ruthenium-based xylose hydrogenation catalysts. It lies in crystal lattice matching: when ruthenium was impregnated on matching rutile titania, product yield soared to 98%. The striking result has recently been published in Catalysis Science & Technology.

Converting lignocellulosic biomass into useful chemicals is a fundamental challenge with important societal applications. One such example is the conversion of beechwood xylan hemicellulose to xylose and ultimately to xylitol. This most popular sweetener used in sugar-free chewing gum has an annual market value of $340m.

Supported ruthenium catalysts are the most effective in the last hydrogenation step from xylose to xylitol. But ruthenium is a rare and expensive metal, so the catalyst must be extremely stable to be a viable alternative to the cheaper nickel based catalyst that is currently used.

Lattice matching

Such a stable and efficient ruthenium-based xylose hydrogenation catalyst has now been developed by MSc student Carlos Hernandez-Mejia and his supervisor Raveendran Shiju. They impregnated ruthenium on rutile titania and achieved a product yield of 98%.

Photo of Shiju

Dr Raveendran Shiju. Photo by Roberto Calderone.

In the recent open-access paper in Catalysis Science & Technology they explain that the secret of this success lies in crystal lattice matching. The ruthenium crystal lattice fits perfectly on the rutile titania lattice. Conversely, on anatase titania (another crystalline form of the otherwise identical material) the ruthenium catalyst “disappears” and the product yield drops to a mere 10%.

Working in collaboration with researchers at TU Delft and at the University of St. Andrews, the Amsterdam team succeeded in finding the "missing" ruthenium catalyst. Due to the weak bonds with the non-matching crystal surface of the anatase titania support, the metal nanoparticles migrated and sintered into “giant” particles that could only be seen when a large part of the surface was probed with transmission electron microscopy. On rutile titania, however, the excellent lattice matching helped the ruthenium oxide nanoparticles stay in place, without sintering (see figure).

Lattice matchine of ruthenium catalyst on titania

The crystal structures of ruthenium oxide (on top) and rutile titania (the support) are a perfect fit. Image: HIMS.

Remarkable result

The most remarkable aspect, according to group leader Prof. Gadi Rothenberg, is that the strong interaction between the catalyst and the surface occurs in an intermediate step of the catalyst synthesis, yet changes the final result completely. “This is like altering the taste of chocolate by pruning branches off a cacao tree”. 

Lattice Matching of Ruthenium on Titania

Article

Carlos Hernandez-Mejia, Edwin S. Gnanakumar, Alma Olivos-Suarez, Jorge Gascon, Heather F. Greer, Wuzong Zhou, Gadi Rothenberga and N. Raveendran Shiju:  Ru/TiO2-catalysed hydrogenation of xylose: the role of the crystal structure of the support. Catal. Sci. Technol., 2015, Advance Article, First published online 24 Aug 2015. DOI: 10.1039/C5CY01005E

Carlos Hernandez-Mejia came to Amsterdam as an exchange student from Mexico and completed both his BSc and MSc at the UvA. He is now working as a PhD student at Utrecht University in the group of Prof. Krijn de Jong.

Published by  HIMS