Porous Supports

With so much attention focused on custom designing catalysts and preparing active sites with just the right structure, sometimes catalyst supports get lost on the sidelines. Yet the properties of support materials can be crucial to catalyst performance.

Researchers now have demonstrated that monolithic (one-piece) catalyst supports prepared from a new high-porosity mullite ceramic (a mixture of alumina and silica) outperform supports made from conventional cordierite (magnesium silicate) monoliths in test reactions using immobilized enzymes [Chem. Eng. Sci., published online Sept. 2]. The finding may be useful in fine and specialty chemical manufacturing, which rely increasingly on enzymes fixed on solid supports as catalysts.

Many types of catalysts are supported on solids that take the form of powders, turnings, or other small pieces. Some catalysts, though, are deposited onto the surfaces of ceramic monoliths--large blocks of ceramic that feature numerous parallel channels. Similar in structure to a neatly packed assemblage of drinking straws (but with hundreds of channels per square inch), monoliths made from cordierite are used worldwide in automobile emission catalysis. Despite their widespread use, the texture of cordierite monoliths is less than optimal for catalyst deposition, and some adsorbed catalyst species are inaccessible to reagent molecules.

Karen M. de Lathouder, Freek Kapteijn, Jacob A. Moulijn, and coworkers in the reactor and chemical engineering group at Delft University of Technology, in the Netherlands, and Dow Chemical researcher Sten A. Wallin prepared monolithic supports using a family of new mullite ceramics made by Dow that are twice as porous as conventional cordierite and feature an open structure with micrometer-sized pores that can be adjusted during synthesis.

Mullite supports with average pore sizes of 5, 9, and 16 &micro;m were functionalized with polyethyleneimine and various types of carbon. Then they were treated with lactase and lipase and used in hydrolysis test reactions with p-nitrophenylpropionate and o-nitrophenyl-<img src="http://pubs.acs.org/images/entities/beta.gif" alt="" border="0">-galactopyranoside. Catalytic activity was determined by monitoring hydrolysis products using UV-Vis spectrometry. The results were compared with measurements made using similarly prepared cordierite monoliths.

Based on hydrodynamic tests, the group concludes that, unlike in conventional monoliths, in those made from the new ceramic, liquid-phase reagents diffuse into the interior volume of the support walls and exchange readily with the bulk liquid in the channels. The enhanced flow of liquids through the mullite material, relative to cordierite, leads to greater catalyst deposition on the support and higher catalytic activity (by a factor of about 1.5 to 3) for all catalyst preparation methods evaluated in the study.

Curiously, mullite-supported catalysts were also found to be significantly more stable than their cordierite counterparts. "It remains unclear whether the morphology or chemical composition of the mullite leads to the improved stability of the biocatalysts," the team says.