Low-Cost Protection For Biomass-Processing Catalysts

Suitable catalysts for converting biomass to chemicals and fuels at low cost are not generally available today. A lot of petrochemical refining catalysts could do the job—but biomass refining conditions destroy them.

Researchers have now found a way around this problem, by enhancing the stability and durability of the supports on which catalytic metals are dispersed (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201206675).

Alumina and silica—and other oxide materials widely used to support metal catalysts for petroleum refining—work well under gas-phase reaction conditions. But transforming biomass-derived compounds, such as carbohydrates and other highly oxygenated compounds, generally calls for high-temperature and high-pressure liquid-phase aqueous conditions.

Those conditions often ruin porous oxide supports, which make petroleum refining catalysts unsuitable for biomass refining. For example, γ-alumina, a high-surface-area material, undergoes a phase change near 200 °C to the low-surface-area boehmite phase. And the well-ordered internal pore structure of SBA-15, a common form of silica, collapses when exposed to liquid water at that temperature.

A research team, including Hien N. Pham, Amanda E. Anderson, and Abhaya K. Datye of the University of New Mexico, has found that the supports can be protected by coating them with a thin carbon layer, which the team forms by decomposing simple sugars such as sucrose. The group, which also includes coworkers at Iowa State University, showed that a carbon film wards off most or all of the structural damage caused by exposing uncoated samples to high-temperature water.

They also conducted preliminary catalysis tests based on selective hydrogenation of acetylene to ethyl­ene, a key industrial reaction, to determine whether the coated oxides are not just hydrothermally durable but also retain the same abilities as uncoated oxides to serve as catalyst supports. Indeed, they found that Pd-based catalysts on coated supports outperformed those on uncoated supports. Now the team is preparing to use the coated materials to study biomass conversion.

Describing the work as “an elegant and relevant study,” catalysis specialist Bert M. Weckhuysen of Utrecht University, in the Netherlands, says the carbon-coating approach is a clever way to make common oxides compatible with the demanding aqueous-phase conditions needed for processing biomass. “The study provides clear leads for producing a new generation of more stable catalytic solids,” he adds.