In the department of inorganic chemistry and catalysis at Utrecht University, for example, chemistry professors Diederik C. (Diek) Koningsberger, Krijn P. de Jong, and Bert M. Weckhuysen focus on heterogeneous catalysis. The team emphasizes its three-pronged approach to catalysis research, which is based on making advances in synthesis, characterization, and catalytic performance.
De Jong's area of expertise is what he calls assembly--putting together catalytic materials with three-dimensional control on the nanometer scale. The idea is to be able to finesse the composition, structure, and location of active phases to produce effective catalysts. Koningsberger and Weckhuysen specialize in developing analytical methods for catalyst characterization.
THE GROUP DRAWS upon zeolites and other porous materials, solid acids and bases, and carbon nanofibers for use as catalyst supports. Metal particles and other active components are brought together with support materials by way of a variety of deposition techniques such as impregnation, ion exchange, precipitation, and other wet-chemistry methods. The researchers also use vapor deposition techniques and other processes to prepare catalysts.
"If you want to be able to assemble materials on the nanometer scale in three dimensions, then you need to find suitable ways to study them," de Jong asserts. Working with Abraham J. Koster of Utrecht's faculty of biology, that's just what de Jong and coworkers have done by adapting electron tomography methods for studying solid catalysts.
Transmitting an electron beam through a 3-D object results in a 2-D projection, de Jong explains. But by using a specially designed stage to tilt a specimen to numerous prescribed angles relative to the beam, and collecting images at all of those angles, Utrecht scientists are able to construct high-resolution 3-D images of specimens with the help of digital image-processing techniques. What's more, the group can turn, flip, and roll the images at will--allowing study of the specimens in great detail. The method was used to reveal the locations of metal nanoparticles in a Ag/NaY-zeolite catalyst and the nature of mesopores in zeolite Y [J. Phys. Chem. B, 104, 9368 (2000)].
In an investigation of solid-base aldol condensation catalysts known as hydrotalcites, in situ X-ray absorption methods developed by Koningsberger revealed details of structural transformations and electronic changes made to the catalysts by heat treatments and other procedures. "These types of studies help us understand why a particular structure leads to a certain type of catalytic performance," Koningsberger notes.
In other work, Koningsberger and coworkers developed data analysis methods used to extract pertinent chemical information from a spectral region that traditionally has been overlooked. Known as atomic X-ray absorption fine structure (AXAFS) spectroscopy, the methods have led to new insights in interactions between catalyst supports and metal and metal-oxide particles (C&EN, Jan. 21, page 37).
Describing the essence of his research thrust, Weckhuysen states, "We're trying to push the boundaries to probe real catalysts under working conditions." In one example of that type of work, the newly appointed Utrecht professor studied destructive adsorption of chlorinated compounds using in situ vibrational spectroscopy. By following the complex set of reactions involved in decomposing the chlorinated species as they occur, Weckhuysen was able to deduce the reaction mechanism, which led his team to discover and patent highly effective new catalysts for the process. The catalysts, which belong to the lanthanide oxide family, may be useful to manufacturers of fungicides, herbicides, and similar products.
Recently, Weckhuysen began using optical fiber technology "to bring the spectrometer to the reactor" instead of the other way around, using makeshift reactors as many researchers have done. The marked decrease in data collection times in UV-Vis spectroscopy measurements (milliseconds versus minutes) permits researchers to follow catalyst changes and to study reaction kinetics.
Meanwhile, on the other side of campus, homogeneous catalysis and metal-mediated synthesis are studied in a research group headed by chemistry professor Gerard van Koten. The group concentrates on designing, synthesizing, and characterizing coordination and organometallic compounds useful as catalysts or synthesis reagents.
Using parallel synthesis methods and computer modeling, the Utrecht group discovered a family of fluorous ligands that can be tacked onto catalyst molecules to improve their solubility in fluorocarbon solvents. Van Koten remarks that it's instructive for students to use parallel synthesis methods because it teaches them that there are new ways of achieving synthetic results quickly.
With close ties between the chemical industry and university scientists the norm in Holland, it's no surprise that van Koten's group includes a chemist who's an industry employee. Atofina's Berth-Jan Deelman joined the Utrecht group in a collaboration agreement as a senior scientist to study the synthesis of organo-tin compounds. Monobutyltin trichloride is widely used in coating processes by the glass industry, yet conventional routes to producing the tin compound are not particularly selective. Van Koten notes that Deelman and students developed a catalyst so promising that pilot plant catalyst-manufacturing studies are now under way. "The collaboration has been very successful," van Koten remarks.
Other research topics studied in van Koten's group include pincer systems and dendrimers. Molecules with pincerlike ligands that can hold metal atoms in place could be used to blend the best features of homogeneous and heterogeneous catalysis, explains Gerard P. M. van Klink, an assistant professor in the group. One end of the molecules can be attached easily to silica and other materials--a boon to the catalyst separation and recycling issue--and the other end can be tailored chemically to ensure high activity and selectivity. Similarly, dendrimers with arms that can be functionalized are being studied for use as reusable complex support systems by assistant professor Robertus J. M. Klein Gebbink and coworkers.
Catalysis and reactor engineering are the main topics of research in Jacob A. Moulijn's group at Delft University of Technology (TUD). One of the group's research projects focuses on the ceramic bricks known as monoliths that are widely used in automotive catalysis but used in few other applications. The porous bricks, which are prepared for catalysis by coating with active catalyst components, catalytic promoters, and other materials, serve jointly as catalyst supports and chemical reactors.
"A monolith is really just a collection of parallel microreactors," Moulijn remarks. "It provides a way of applying the microreactor concept to an industrial setting."
Delft's Laboratory for Applied Organic Chemistry & Catalysis is headed by Thomas Maschmeyer. His research program aims to develop new catalytic processes that make efficient use of raw materials and energy while generating a minimum of waste. The main areas of investigation include porous solids, renewable materials, chiral cataly-sis, combinatorial and high-throughput methods, and molecular modeling.
Maschmeyer's group recently synthesized a new mesoporous foam that has attracted commercial interest. Named for the university, TUD-1 can be combined with zeolites to make catalysts that--for some processes--are 10 times more active and significantly more selective than zeolites without TUD-1. According to Maschmeyer, plans are under way to make 10-ton batches of the material this year.
Another success reported by Maschmeyer's group is a zeolite membrane reactor that isomerizes linear hydrocarbons and separates the branched alkanes from unbranched products. Hydroisomerization is used to boost the octane ratings of fuels. In other projects, the group developed a new catalyst for one-pot conversion of inulin--a polysaccharide derived from plants--to alditols, and a lanthanide-based magnetic resonance imaging contrast agent that boasts four times greater efficiency than today's commercial products.
At Eindhoven University of Technology, researchers with an interest in catalysis are clustered in the Schuit Institute of Catalysis. Collectively, the group studies surface science, spectroscopy, computational chemistry, topics in applied physics, and other subjects.
Fundamental surface science is J. W. (Hans) Niemantsverdriet's bailiwick. The professor of chemistry and chemical engineering applies ultra-high-vacuum methods to study elemental processes relevant to hydrotreating, automotive emissions cleanup, and other problems in catalysis. The group is particular about selecting model catalysts that have much in common with standard industrial catalysts.
NO PRESSURE Eindhoven's Niemantsverdriet (left) discusses experiment strategies and fine points of high-vacuum equipment with students.
Niemantsverdriet's research group relies on techniques such as X-ray photoelectron spectroscopy and secondary-ion mass spectrometry--methods that probe the topmost layers of a solid specimen. Especially useful for studying catalysis fundamentals are techniques that enable the surface scientist to follow--in real time--elementary molecular reactions such as association, dissociation, and desorption. "The strength of surface science is that we have a whole battery of techniques to apply to problems in combination," he says.
Rutger A. van Santen often studies basic catalytic processes while seated behind a computer. The Eindhoven chemistry professor has used quantum mechanical methods and other techniques to probe reactions of organic molecules in zeolites, alkylation of toluene, and other reactions. In collaboration with Eindhoven mathematics professor Peter A. J. Hilbers and others, van Santen has conducted Monte Carlo simulations to understand surface diffusion processes and the nature of CO electrooxidation on platinum-ruthenium surfaces.
Although somewhat off the beaten path, nuclear reactions work their way into catalysis studies in the Netherlands. Martien J. A. de Voigt, an applied physics professor at Eindhoven, uses positron emission profiling methods as an in situ catalysis imaging tool. Often working with Eindhoven's van Santen or Niemantsverdriet, de Voigt uses tiny quantities of 11C-, 13N-, and 15O-labeled compounds to probe catalytic reactions.
BY DELIVERING PULSES of radiolabeled compounds into a reactor and observing the metal-piercing g radiation derived from positron-electron annihilation events, the physicist can map--in time and position--concentrations of reactants, intermediates, and products as they flow through a reactor's interior. Information of this type, which is inaccessible by nearly every other technique, is useful for studying reaction modeling, reactor engineering, and transient processes.
Eindhoven chemistry and chemical engineering professor Dieter Vogt anchors his group's research in coordination chemistry and homogeneous catalysis, but notes that exciting research takes place at the interface of that field with heterogeneous catalysis, industrial catalysis, kinetics, and other subjects. In the area of immobilizing and recycling homogeneous catalysts, Vogt and coworkers study advanced polymers, organic and inorganic nanoparticles, and dendrimers.
As is common throughout research in the Netherlands, Vogt often teams up with scientists at other institutions. For example, Vogt and the University of Amsterdam's Piet W. N. M. van Leeuven joined forces on a project using immobilized phosphine ligands in asymmetric hydroformylation reactions. And studies on selective hydrovinylation of styrene using carbosilane dendrimers in membrane reactors were carried out in collaboration with van Koten of Utrecht University.
"Following all those details gives us a good sense of how a catalyst is performing and which way to tailor a catalyst in order to improve selectivity and activity," Vogt remarks. Measurements of that type also reveal mechanistic information and offer important clues about reaction pathways. A lot of pertinent information is missed, he says, by limiting measurements and analysis to overall performance parameters such as yields and selectivities of a single target product. The automation work is carried out in conjunction with Avantium Technologies.
Based in Amsterdam, Avantium is a young company that specializes in high-throughput technology used to provide product and process R&D services to the chemical and life sciences industries. Ian E. Maxwell, Avantium's chief executive officer, says the company focuses on developing novel equipment and software for rapid experimentation and computer visualization, analysis, as well as modeling methods for managing enormous quantities of data.
To screen large numbers of catalysts in search of a few promising candidates for key reactions, Avantium uses in-house-designed instrumentation that combines 64 parallel fixed-bed flow reactors, for example, or 160 parallel high-pressure batch reactors. And polymorph screening for new drug compounds is carried out with equipment designed to collect and analyze X-ray diffraction data from many specimens rapidly and automatically. Maxwell points out that these types of operations require custom-designed integrated software that controls analytical equipment and automation processes and is compatible with databases.
According to Maxwell, DSM Research was looking for catalysts that would boost selectivity in certain homogeneous reactions. Using conventional equipment, DSM was screening 10 catalysts per week. Avantium recently began conducting contract research for DSM, "and now we're doing 1,000 screens per week under realistic and scalable conditions," Maxwell says. In another project, Avantium's high-throughput methods provided a pharmaceutical company with a catalyst and method for aromatic hydrogenations that boosted selectivity while reducing reaction pressure and time from 100 bar and two days to 5 bar and 20 hours.
In some ways, Avantium could be thought of as a microcosm of Dutch catalysis. The company began as a project at Shell--one of the grandfathers of Dutch catalysis--and now is an independent two-year-old company. And just as Dutch research groups are known for combining diverse sets of skills and developing collaborations between academia and industry, so too Avantium's ranks are filled with chemical, mechanical, and software engineers, and the company operates a large laboratory at Delft University, where students conduct research.
The prevailing attitude in the Netherlands about catalysis research is summed up by Delft's Kapteijn, who says: "Catalysis is multidisciplinary. Succeeding in the field requires addressing all of the related topics and recognizing that you can't do it alone."
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