Pores for thought

A solid, but sponge-like material has been synthesised by chemists in Singapore. The silica-type material has the most complicated pore structure ever reported and is discussed in the latest issue of the new journal Nature Chemistry.

Mesoporous silica is a technologically important material, that extends the basic principle of porous silica to a specific range of pore size: 2 to 50 nanometres. Materials with pores smaller than this are microporous and anything bigger is macroporous. The meso materials in the middle have great potential in catalysis, chemical separation, gas storage, drug delivery and even imaging. All important for particular applications is the precise size and shape of the pores. Different shapes will be responsive to specific small molecules that might enter in different ways and have diverse effects on how the material interacts or changes those small molecules.

The mesoporous IBN-9 structure (Credit: Ying et al/Nature Chem.)

The mesoporous IBN-9 structure (Credit: Ying et al/Nature Chem.)

The size of the pores also endows any such porous material with a huge internal surface area in a given small volume that would if laid flat cover dozens of football pitches. It is this enormous surface to volume ratio – often around one thousand square metres per gram – that endows mesoporous materials with the ability to absorb, or more strictly adsorb, large volumes of small molecules.

Until now, such materials have been limited to a single network of pores, or at most two disconnected pore systems. This gives them enormous potential as sieves for separating molecules if a mixture of large and small molecules is strained through the material only those that fit the pores can be adsorbed. Such porous materials are essential to the process of catalytic cracking in the petroleum industry in which every molecule found in vehicle fuels has essentially passed through such a material.

Various views of the IBN-9 pores (Credit: Ying et al/Nature Chem.)

Various views of the IBN-9 pores (Credit: Ying et al/Nature Chem.)

Now, Jackie Ying, Yu Han, Leng Leng Chng, and Lan Zhao of the Institute of Bioengineering and Nanotechnology, in Singapore, and colleagues Daliang Zhang, Junliang Sun, and Xiaodong Zou of the Berzelii Center EXSELENT on Porous Materials, in Stockholm, Sweden, have created a mesoporous silica that has not two but three interwoven but disconnected pore systems. This added layer of complexity gives chemists an extra variable with which to work in creating pore sizes and shapes tailored to particular molecules and so particular types of chemical separation or catalytic process.

To make their new triply porous material, known as IBN-9, Ying and colleagues designed a new templating agent around which the complex structure formed. The template compound, a surfactant made from N,N-dimethyl-L-phenylalanine in two steps, has large head-groups and long hydrocarbon tails ending with an amine group. The template then acts as a support around which silica can grow and crystallise, dissolving the template leaves behind the mesoporous silica.

By fine-tuning the ratio of molecular size to head-group and the degree of water repellence, or hydrophobicity, of the tail, the researchers can design the pores of the new materials. They suggest that the presence of both long and short channels in fibres of the material could lead to separation or controlled-release applications that offer different diffusion rates in different directions.

Further reading

Nature Chem., 2009, 1, in press
http://dx.doi.org/10.1038/nchem.166

Volcanic lava goes organic

Nanotubes made using volcanic lava as a support and catalyst come of age with a new paper to be published in the journal Advanced Materials, highlighting applications in the area of heterogeneous catalysis with industrial potential.

Dang Sheng Su and his colleagues at the Fritz Haber Institute in Berlin, Germany, and the Rudjer Boskovic Institute in Zagreb, Croatia, turned to igneous rock formed during eruptions of Mount Etna in Sicily to help them mass produce carbon nanotubes.

Volcanic rock can be used to create nanoscopic carbon fibres and nanotubes for industrial catalyst use (Credit: Su et al/Adv Mater)

Volcanic rock can be used to create nanoscopic carbon fibres and nanotubes for industrial catalyst use (Credit: Su et al/Adv Mater)

Carbon nanotubes and nanofibres have become almost indispensable to components of the emerging fields of nanoscience and nanotechnology. Unfortunately, making CNTs in large quantities has remained an obstacle to the widespread adoption of these materials in research and applications. Su and colleagues recently discovered that they could use highly-porous particles of iron oxide from Etna’s residual igneous rocks as a templating system for depositing CNTs and related carbon nanofibres directly.

Previously, the team developed a unique method of making their catalyst. To prepare the volcanic material, the team first pulverizes the igneous rock and heats it to 700 Celsius under an atmosphere of hydrogen gas. This chemically reduces the iron oxide nanoparticles. They next expose the particles to an organic, carbon-containing, gas – ethylene – together with a hydrogen carrier. Carbon atoms from the ethylene are released through CVD, chemical vapour deposition, and condense on the surfaces of the tiny iron oxide particles, much as dew condenses on blades of grass on a cold morning. The result is the formation of tiny fibres and hollow tubes of pure carbon.

Mount Etna’s periodic eruptions could revolutionise nanoscience

Mount Etna’s periodic eruptions could revolutionise nanoscience

The main advantage of this volcanic approach to nanotubes lies in the all-too renewable nature of the template material. Moreover, there are no wet chemical reactions to carry out and the scheme works at relatively moderate temperatures. We have proven the concept of using this useless natural material, lava, for nanocarbon growth, Su told Spotlight.

The team has now shown that they can obtain high yields of CNTs without amorphous carbon impurities. This is an important step to reduce the high cost of CNT synthesis, Su adds. The team used scanning electron microscopy (SEM) to study in minute detail the structure of their carbon products.

The next step was to develop applications for the products. The systems seem to be well suited for the desired purpose of using them as carbon hybrid catalysts to compete with industrially optimized catalytic systems. We have now found that the materials can be used as a catalyst for the production of the important industrial chemical styrene through the oxidative dehydrogenation of ethyl benzene, explains Su, The synthesis is highly selective and the carbon catalyst is stable during the reaction. In addition, the system is also active and stable for synthesis of butadiene from 1-butene by oxidative dehydrogenation.

Su suggests that the realization of volcanic CNT catalysts could provide a highly economical way to produce other important starting materials in the chemical industry. Further challenges could be use other waste or otherwise ‘useless’ natural materials for chemistry, catalysis and nanotechnology, he says.

Further reading

Angew. Chem., 2007, 46, 1823-1824
http://dx.doi.org/10.1002/anie.200604207

Professor Dr Robert Schlögl
http://w3.rz-berlin.mpg.de/ac/em/index.html

Adv. Mater., 2008, published online Aug 27, 2008
http://dx.doi.org/10.1002/adma.200800323

Suggested searches

carbon nanotubes
igneous rocks
scanning electron microscopy
catalysis

Flush with nanoparticles

What happens to carbon-based nanoparticles when they enter groundwater? Can municipal water supplies filter them out? And, if they cannot will they cause health problems? These are crucial questions that need answers now, as nanotechnology grows. Now, a new study by Kurt Pennell, of the Georgia Institute of Technology, and colleagues, suggests that subtle differences in the solution properties of the water carrying such particles can determine their ultimate fate.

Pennell and colleagues have studied the transportation of the archetypal nanoparticle, the soccerball-shaped carbon-60 molecule known as buckminsterfullerene, or the buckyball for short. While C60 is a well-studied molecule little is known about its environmental impact, moreover, little is known about what happens to this material and related materials, such as carbon nanotubes when they enter groundwater.

Kurt Pennell (standing) and Younggang Wang flush with success in water transport experiments on nanoparticles (Georgia Tech Photo: Gary Meek)

Kurt Pennell (standing) and Younggang Wang flush with success in water transport experiments on nanoparticles (Georgia Tech Photo: Gary Meek)

The researchers explain that in slightly salty water, clusters of C60 might aggregate and adhere tightly to soil particles or filtration system particles. However, in water contaminated with natural organic compounds or synthetic surfactants, such as detergents and soaps, isolated C60 particles would be stabilized and so be transported further in water.

In some cases, the nanoparticles move very little and you would get complete retention in the soil,” explains Pennell, But in different solution conditions or in the presence of a stabilizing agent, such as a surfactant, they can travel just like water. The movement of these nanoparticles is very sensitive to the solution conditions.

The buckyball, an archetypal nanoparticle

The buckyball, an archetypal nanoparticle

Pennell and his team have published detailed research funded by the US Environmental Protection Agency into the transport and retention of C60 nanoparticles in the journal Environmental Science and Technology.

We want to figure out now what will happen to these nanoparticles and how toxic they will be in the environment, says Pennell. The team hopes to build up a mechanistic model of what makes nanoparticles flow well in certain conditions and aggregate in others. When we look at real soils with finer particles, we will expect to see more retention, he says.

For municipal drinking water filtration, the sensitivity to solution characteristics means local conditions may play a key role. Under most conditions, you should be able to remove nanoparticles from the water, he adds, But you will have to be careful if the nanoparticles are stabilized by a natural surfactant or humic acid. If those are present in the water, the nanoparticles could go right through.

Further reading

Environ. Sci. Tech., 2008, in press
http://dx.doi.org/10.1021/es800128m

Dr. Kurt D. Pennell
http://people.ce.gatech.edu/~kp48/

Nanotech threat to your safety
https://www.sciencebase.com/science-blog/nanotech-threat-to-your-safety.html

Suggested searches

nanotechnology
fullerenes