Cellular honeycomb ceramics have been used in a variety of applications, including support materials for catalytic conversion, adsorptive separations, membrane supports, cross-flow filtration and fluid filtration. The honeycomb design offers a remarkably high geometric surface area per unit volume and improves the chemical residence times for reactions to reach completion in high-flow conditions. Extrusion is a well-understood and highly scalable ceramic forming process, and extruded cellular honeycomb monoliths provide unmatched structural rigidity and robustness.

Existing Systems Face Limitations

Figure 1. In the flow-through configuration, cross-flow through the walls is extremely limited.

One of the best-known uses of honeycomb materials is in gasoline automobile catalytic converters. Since the late 1970s, every car on the road in the U.S. has been equipped with a honeycomb catalytic converter under the chassis. These converters provide a complex chemical reactor to reduce emissions of regulated gas species from gasoline engines. Ceramic flow-through honeycombs have been used in converters because of their high mechanical and chemical durability (see Figure 1). Typically, these ceramic honeycombs are made by extruding powder-based materials like alumina, mullite and silicon carbide. The most common material used in these applications is cordierite, which features a low coefficient of thermal expansion (CTE).

The cellular geometry of the honeycomb provides a high surface area per unit volume for deposition of catalysts or absorptive materials. For example, in the case of a gasoline-powered car, a layer of high-surface-area alumina washcoat is placed on the wall, and precious metal catalysts are dispersed finely on the washcoat bed. At high temperatures, the reactions occur quickly but are typically mass-flow limited in a laminar-flow regime. In the flow-through configuration, cross-flow through the walls is extremely limited.

Figure 2. Alternate channels are blocked in the wall-flow design, leading to turbulent flow and forced convection of fluid through the permeable walls of the honeycomb.

While the flow-through configuration of honeycomb ceramics works well for laminar-flow catalytic conversion, its utility is limited in filtration. Unlike gasoline engines, diesel engines emit gas- and condensed-phase toxic emissions. When these engines became popular for use in the European light-duty market, the emission control industry recognized a need for soot nano-particle filtration functionality. As a result, the wall-flow design was added to increase filtration functionality while retaining the benefits of the cellular honeycomb structure (see Figure 2).

Alternate channels are blocked in the wall-flow design, leading to turbulent flow and forced convection of fluid through the permeable walls of the honeycomb. As the fluid is forced through the wall, filtration of suspended particulate matter takes place while fluid passes through unobstructed. Depending on the size of the particles, diffusion, inertial and interception mechanisms lead to the entrapment of suspended particles on the wall.

In order to create a gas-permeable wall microstructure out of the standard flow-through honeycomb material, the internal wall porosity of powder-based chemistries was increased. Since then, however, the microstructure has been deemed ill-suited for such applications. Unfortunately, the obstruction to flow is relatively high in powder-based honeycomb filters, which leads to backpressure and poor filtration efficiency.

Studies of this obstruction of flow affirm that powder-based media, however porous, are not ideal for the filtration of particles in the nanometer and micrometer size ranges. Advanced filters in applications as diverse as high-efficiency particulate air (HEPA) filtration and air-oil separation have typically relied on fiber-based filtration media. The cross-linked microstructure formed by intersecting and overlapping fibers improves filtration performance without causing significant backpressure. However, until now, this desired microstructure was not available in a robust and durable extruded honeycomb form. Pleated and rolled fiber-paper sheets do not allow for robust filters, and they often exhibit a telescoping failure mode, among other problems.

Developing a New Alternative

Figure 3. A top view of two parallel channels in an extruded porous honeycomb with the patented cross-linked microstructure.

New extruded ceramic honeycombs* have been developed with a unique cross-linked pore microstructure in multiple chemistries. The resulting honeycombs have a high cell density, which leads to high geometric surface area. In addition, high permeability of the wall microstructure results in low filtration backpressure, and the interconnected pore architecture enables high filtration efficiency.

Depending on the fabrication parameters, filters can capitalize on diffusion flow to achieve extremely high nano-particle filtration efficiencies. In addition, benefits such as structural rigidity, robustness and low cost of production are also achieved. By altering the fiber and bonding composition chemistry and processing conditions, high-porosity cellular honeycombs with mullite, silicon carbide, cordierite, alumina and steel alloys have been developed.

Figure 3 shows a top view of two parallel channels in an extruded porous honeycomb with the patented cross-linked microstructure. The filtration media in the cellular honeycomb closely resembles the kind of surface texture and structure expected in the typical fiber-based membranes or candle-type tube and bag filters-but in an extruded, monolithic honeycomb geometry. The requirements and processing conditions for mixing, extruding, drying and firing the materials into numerous desired honeycomb configurations have been determined. Furthermore, based on application needs, similar internal cross-linked wall microstructures can be created in several chemistries by altering material and processing parameters.

*Developed and patented by GEO₂ Technologies.

Figure 4. The size-specific filtration efficiency of a wall-flow filter located downstream of a soot generator source.

The patented, cross-linked microstructure of the new honeycomb substrates and processes provide several advantages over conventional powder-based ceramics. The fiber-based structure exhibits a mechanical strength similar to low-porosity, powder-based filters (~ 45% porous), but at a porosity of greater than 65%. Depending on the pore size distribution, > 99% filtration efficiencies for nano-particles (10-470 nm) have been observed.

For example, Figure 4 shows the size-specific filtration efficiency of a wall-flow filter measured using a scanning mobility particle sizer (SMPS, 135-second scans across the diameter range) downstream of a soot generator source. The filter had an average pore size of 15 µm with a narrow distribution curve. As can be seen, not only is the initial particle filtration efficiency very high, but the filtration efficiency also continues to improve as a fine layer of soot cake builds up on the filter.

Figure 5. In wall-flow honeycombs for use in turbulent flow fixed-bed chemical conversion reactors, the catalytic material can be dispersed throughout the internal porosity of the walls.

The internal pore microstructure of the honeycombs provides additional "open" and "available" porosity for the placement of reactive or catalytic species in the pore volume. For example, in the case of wall-flow honeycombs for use in turbulent flow fixed-bed chemical conversion reactors, the catalytic material can be dispersed throughout the internal porosity of the walls (see Figure 5). This is achieved without causing significant obstruction to the flow of fluids through the wall.

This scenario has been tested in the case of automotive emissions control, where low backpressures were measured on a filter and compared to a standard particle-based cordierite filter. Despite having five times the typical washcoat loading, the filter exhibited lower backpressure and higher catalyst efficiency than the standard filters.

In the case of a wall-flow configuration, fluids such as gases from engine exhaust are forced through the wall structure where they come into direct contact with the residing catalyst. Instead of the gas-phase reactions being rate-limited by mass transfer in the channels, as is the case in flow-through honeycombs, the reactions become pore-diffusion limited, which is much more effective. The induced turbulence leads to an overall increase in reaction rates and better utilization of catalyst loadings.

Figure 6. Honeycomb structures with a highly permeable, cross-linked pore microstructure can be used as membrane support structures for adsorptive reaction, catalytic conversion or cross-flow filtration-type applications.

Finally, honeycomb structures with a highly permeable, cross-linked pore microstructure can also be used as membrane support structures for adsorptive reaction, catalytic conversion or cross-flow filtration-type applications (see Figure 6). Membranes, including zeolite-based or polymeric types, placed as a layer on top of the honeycomb structures can act as the active layers while the honeycomb provides a robust and permeable structure for high-flow-rate and otherwise harsh operating conditions.

Additional Applications on the Horizon

The new honeycomb structures in wall-flow and flow-through configurations are already in use for several applications, including diesel particulate filters, air-oil separators, selective absorption of contaminants in gases and multi-functional catalytic conversion. Additional applications are planned in a wide variety of areas, including gas separations, alcohol-water separations, water filtration and bio-filtration.  

For more information regarding cellular honeycomb ceramics, contact GEO₂ Technologies Inc., 12-R Cabot Rd., Woburn, MA 01801; (781) 569-0559; fax (781) 721-6301; e-mail info@geo2tech.com; or visit www.geo2tech.com.