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Applied Catalysis A: General 246 (2003) 193196

Perspective

Heterogeneous catalysis: what lies ahead in nanotechnology

In the past few years, there has been a huge in-

crease in the interest in nanotechnology, a term that is

virtually unheard of a decade ago. As nanotechnology

is increasingly embraced by industrial leaders, theeducated general public, and the government (espe-

cially the funding agencies), catalysis researchers and

practitioners begin to employ this term to describe

their activities. In fact, the length scale of importance

in heterogeneous catalysis has been known by re-

searchers to be nanometer or smaller for many years,

be it the crystallite size of a well-dispersed supported

noble metal catalyst, the cavity or channel size of a

zeolite, the ligand size of a coordination complex, or

the active center of an enzyme. Thus, in many ways,

nanotechnology has been part of catalysis technology,

which, when coupled with macroscale reaction engi-

neering technology, constitute the chemical process

industry as we know to date.

However, the phenomenal rise in interest in nan-

otechnology is not a result of new developments in

catalysis research. Instead, it results from develop-

ments in understanding the behavior of ensembles

of molecules or particles (e.g. self-assembly) and in

instrumentation and techniques that permit manipu-

lating and observing matters, molecules, and atoms at

that scale. Thus, it is fitting to ask how may catalysis

benefit from this rapid increase in the knowledge ofnanoscience and nanotechnology? Others have of-

fered their perspectives (e.g.[1]),here we offer ours

for discussion.

One area that could receive immediate benefit is

combinatorial catalysis or high-throughput testing.

Here, nanotechnology can help miniaturize the equip-

ment and develop analytical methods to facilitate

detection of trace quantities of products or occurrence

of reactions reliably and rapidly. Another potentially

promising area is catalytic reaction engineering. In

recent years, various forms of microchannel reactors

have been fabricated and demonstrated to offer sub-

stantial advantages in heat transfer characteristic andother attributes [2,3]. Can one reduce a reactor fur-

ther to nano-size? Imagine a nano-size box-like batch

reactor with a lid that could be opened and closed

on demand. Such a reactor, which is of a dimension

in the order of the mean free path of a molecule,

may be just large enough to accommodate one set of

reactant molecules (e.g. one A and one B molecule

for the reaction A + B C). Then, the only pos-

sible reaction that can take place in the reactor is

between them. The selectivity would be 100% at very

high conversions of the limiting reagent, which is a

highly desirable goal. Such reactors could be used in

new application areas such as those that could ben-

efit from the ability to synthesize and deliver small

quantities of products to specific locations. An ex-

ample would be drug delivery into a human body or

treatment of microbial infection. One could imagine

adapting the molecular switching technology for the

openclose motion of the lid. Fabricating such reac-

tors will be a research challenge, and producing a

large ensemble of them would be a manufacturing

challenge.

Designing and synthesizing a highly selective cat-alyst has been an intriguing and challenging goal in

catalysis research. The desired properties of a catalyst

comprise of an active site with the correct ensemble of

metal atoms, metal ions, or other active components

such as oxides, carbides, etc., and a cavity around the

active site that can change configuration to facilitate

binding of a specific reactant to the active site and ex-

pulsion of the product. Much effort has been spent in

attempts to achieve control of these properties. How

0926-860X/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0926-860X(03)00023-1


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194 Perspective / Applied Catalysis A: General 246 (2003) 193196

may nanoscience and nanotechnology lead us to the

next level? Here are some possibilities.

Zeolites are important catalysts due to their acid-

ity associated with the framework Al ions and shapeselectivity consequent to the molecular-size channels

and pores. Yet, their synthesis is still based largely

on empirical knowledge. At present, the specific re-

lationship between synthesis condition and zeolite

structure has not been rigorously established and the

positioning of the Al ions in the framework is not

well controlled. The formation of the zeolite struc-

tures is a self-assembly mechanism of aluminate

and silicate ions around the structure-directing agent.

Advances in nanoscience may help improve our un-

derstanding of the arrangement of these ion clusters

and the structure-directing agent during synthesis

and how they lead to the final zeolite structure. They

could improve our ability to design better zeolites

or other new materials that possess similar desirable

properties. Rapid advances in material synthesis in

nanotechnology may offer know-how to incorporate

Al ions at specific locations.

Supported metals are another class of widely used

catalysts. It has been known for many years that

not all surface atoms of a metal crystallite are alike

in a catalytic reaction. That is, a reaction may be

structure-sensitive. The degree of structure sensitivitydiffers, from being rather minor for reactions such as

hydrogenation on noble metals, to being quite sub-

stantial for reactions such as ammonia synthesis and

hydrogenolysis of hydrocarbons [4]. For the latter

type of reactions, the detailed arrangement of atoms

on the surface of crystallites of the active compo-

nent is very important, in addition to the crystallite

size. Therefore, a desirable goal for catalyst synthesis

would be to control the surface atomic arrangements

so as to optimize the desired structure and density

of the active sites. This would include the ability to,among others, manipulate the deposition of atoms to

a specific location at a specific rate and order (espe-

cially for multicomponent catalysts). However, there

are a number of obstacles to practical applications of

such an atom-by-atom approach of catalyst synthe-

sis. If one synthesizes the active sites one at a time,

and sites of the order of 10161017 are needed for

bench scale testing, an extremely efficient synthesis

method will be needed to prepare sufficient samples

for testing in a reasonable time. Lithography has been

used to deposit metal clusters in a pattern [5], and

deposition of small clusters using a STM tip has been

demonstrated [6]. There is also the possibility for

scale-up using the millipede technique [7], perhapscoupled with the dip-pen technology. Yet, these tech-

niques do not offer fine control at the atomic level.

Thus, new techniques with atomic control that can be

readily scaled up are needed.

Even if one succeeded in achieving atomic level

control during the preparation of metal clusters, there

is the uncertainty of the stability of the clusters or

surface structures after exposure to the working envi-

ronment. If the deposited and desired structure is not

stable, would rapid structural transformation negate

the advantages of such an approach? This is a ques-

tion that awaits an answer. An alternate approach is to

invent new methods to stabilize the metastable struc-

ture against restructuring. This is feasible in principle.

As an example, zeolites are metastable structures.

Recently, there is increasing recognition of the

important role that the surrounding environment of

an active site can play in heterogeneous catalysis.

Thomas and his coworkers demonstrated that a hex-

ane molecule can achieve an orientation by nonbond-

ing interaction with the cavity wall of a Co-AlPO-18

molecular sieve so as to favor oxidation of the terminal

carbons and, thus, high selectivity for the adipic acidproduct [8]. Increased selectivity was also reported

using catalysts prepared with the molecular templat-

ing technique which enhances favorable interaction of

the desirable reaction intermediate with the active site

[9]. The explosion of newly discovered self-assembled

structures offers unprecedented choice of templates

for forming cavities of various shapes and sizes. The

formation of these well-organized, self-assembled

moieties has been extensively studied. For example,

a review article by Seidel and Stang [10] outlined

detailed methodology for forming high-symmetry co-ordination cages using well-defined building blocks

of specific geometries. Nonetheless, further research

is needed to adapt such knowledge to catalysis. Then,

nanotechnology could be used to construct cavities

that promote certain orientations or conformations of

reactants and reaction intermediates, thereby facilitat-

ing activation of particular bonds in the molecule by

the active site. This situation is similar to that of an

enzyme in which the protein provides an environment

for specific interaction with the reactant molecules.


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Perspective / Applied Catalysis A: General 246 (2003) 193196 195

Computational studies will be valuable to provide sug-

gestions of cavity shapes and size for various reactions.

The concept behind molecular templating has been

used in conventional zeolite synthesis, namely theuse of structure-directing agents. These agents are

relatively small molecules and the resulting microp-

orous zeolites contain regular channels and cavities

of molecular size. Larger templates such as micelles

of surfactants or triblock polymers are used for the

synthesis of mesoporous materials, such as MCM-41

and SBA-5 [11,12]. More recently, materials con-

taining regular macropores have been synthesized

using ordered polystyrene spheres [13]. Formation

of chiral silica [14] and TiO2 [15] nanotubes have

been demonstrated using organogel templates. Thus,

there are techniques to prepare materials contain-

ing regular channels or cavities of a wide range

of sizes.

The meso- and macroporous materials offer much

lower resistance to molecular diffusion than microp-

orous solids. Their thin walls are more advantageous

for rapid solid state transformation than bulk solids.

Introduction of organic functional groups into the

channel walls such as in the synthesis of periodic

mesoporous organosilicas [16] greatly enhances the

potentials of these materials as useful catalysts. How-

ever, at present, there is no systematic method to in-troduce or position functional groups that can be used

to orient a reactant molecule or reaction intermediate,

as it is the case in an enzyme. New developments are

needed to overcome this limitation by offering more

versatile templates and new synthetic techniques.

New methods to induce self-organization of the tem-

plates using stronger and controllable forces should

prove to be very beneficial in providing better control

of the shape and size of the channels and cavities of

the final catalyst.

Another characteristic feature of a protein in anenzyme is its mobility. Proteins often change confor-

mation upon binding of a reactant and release of a

product. Such flexibility renders the system the ability

to accommodate and bind large molecules at multiple

sites. As of to date, there is practically no inorganic

system that offers such a flexible framework. A new

way of thinking will be needed to achieve the effect

of flexibility. Perhaps concepts, knowledge, and tech-

niques developed for molecular switching could be

beneficial here.

Up until now, we described various possibilities

that nanoscience and nanotechnology could impact

on heterogeneous catalysis and catalytic reaction en-

gineering. We have identified a number of technicaldevelopments or inventions needed to bring them to

fruition. In order to make these possibilities a prac-

tical reality, the cost of production must also be con-

sidered. Although any cost estimate in the absence of

the technology is highly speculative, one possibility is

that nanotechnology in catalysis would follow a sim-

ilar path as microelectronics. Thus, the initial impact

would be on specialty applications of low volume and

high cost. With proven benefits, increasing demands,

and improved techniques, the cost of manufacturing

would drop rapidly with maturity of the technology.

Then, the practice of catalysis and catalytic reaction

engineering would reach a new state unimaginable a

few decades ago.

Acknowledgements

This work was supported by the Department of En-

ergy, Office of Science, Basic Energy Sciences. Very

insightful comments by Professors Robert Davis and

Raymond Gorte are also acknowledged.

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Harold H. Kung, Mayfair C. Kung

Department of Chemical Engineering

Northwestern University, 2145 Sheridan Road

Evanston, IL 60208-3120, USACorresponding author.E-mail address: [email protected]

(H.H. Kung)

1 December 2002

01 - 2003 - kung, ap cat a gen, v246, p193.pdf

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