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There is a pressing need for a breakthrough that will transform catalysis into a more quantitative and predictive science while avoiding the problems described by James Wei, author of the Second Paradigm (1). The Second Paradigm links catalyst performance directly to quantitative information. This link would permit faster and more cost-effective progress in catalyst development and process design, revitalize catalyst research, and improve the rate of return on R&D investment.

Why the stagnation in catalysis?

The multibillion-dollar chemical synthesis and process industries depend on the use of catalysts and are constantly searching for improvements that will lower costs, increase productivity, and reduce unwanted byproducts. These industries have invested heavily in catalyst research, including modeling studies and mechanistic investigations. Attempts at quantification have been driven by the fact that without sophisticated kinetic models, it is difficult to optimize reactor design, and nearly impossible to understand the links between catalyst activity or selectivity and the kinetic parameter changes engendered by changes in catalyst formulation.

Although progress has been made in developing new catalysts and processes, it has remained largely empirical and heuristic. In terms of developing models and mechanisms, the results have been to a great extent unconvincing and often contradictory. The accumulation of all results in catalysis, although large, is for the most part idiosyncratic—made up of a variety of empirical studies that do not lend themselves to integration into a consistent and quantitative whole.

The disappointing returns from R&D investment in catalysis have resulted in the closing or downsizing of many catalyst research facilities in the chemical and petrochemical industries. Catalyst and process development have been increasingly outsourced to dedicated R&D organizations, which, to be cost-competitive, must concentrate on quick, narrowly focused studies. Little general or fundamental understanding comes from such work.

How to rejuvenate catalysis

There is a growing interest in the revitalization of catalysis research (2, 3) and in making the contributions of catalysis more visible (3, 4). Historically, two initiators have played major roles in the rejuvenation of stagnating disciplines:

• new theoretical understanding of the field and
• new instrumentation, which makes it possible to gather previously unavailable data and increase the rate of data acquisition.

Current isothermal experimentation models can take weeks to make a single-point evaluation of a new catalyst. What if the time to evaluate each new formulation could be significantly reduced to a day or less? In a catalyst development setting, such a capability would give a much broader view of the performance of each catalyst sample. On subsequent days, incremental changes in the catalyst could be associated directly with changes in activity and selectivity, the two principal measures of catalyst performance. Evaluating each new formulation over a range of industrially important conditions would turn catalysis into a true predictive science and facilitate rapid advances, not just in the fundamental understanding of the kinetics and mechanisms of catalytic reactions, but also in industrial applications, catalyst formulations, treatments, and process conditions.

In a research setting, this capability would yield, in one day, enough data to discriminate among many alternative mechanistic rate expressions for a given catalyst sample. Short, preliminary, fundamental studies could then be performed to identify a broadly applicable, perhaps mechanistic, rate expression for the routine evaluation of catalyst formulations in a development program.

Research methods as obstacles to rejuvenation

A major obstacle to advances in this field is the difficulty of catalyst evaluation under realistic conditions: the full range of pressures, temperatures, and compositions found in industrial reactors. Another obstacle is the problem of making a meaningful connection between catalyst performance and catalyst formulation. Current testing procedures for determining the activity and selectivity of new catalyst formulations are costly, slow, tedious, only partially illuminating the behavior of the catalyst; and in many cases, they are not even quantitative. For example, using an isothermal test reactor, it takes about a week in most laboratories before the activity of a new catalyst sample, under a single “standard” condition, is communicated to the chemist responsible for the sample.

Over the past 20 years, there was hope that the high- vacuum techniques used for surface studies in catalysis would satisfy the two criteria for the rejuvenation of this field. Unfortunately, these procedures are difficult. They result in sparse data; phenomena are investigated at conditions far removed from those of practical interest; and results tend to consist of qualitative interpretations of observations. Some progress has been made, but at a high financial cost. Gains in theoretical understanding based on high-vacuum data have been few and lacking in generality, and quantitative kinetic information under industrial conditions has been even more rare.

Requirements for improving experimental methods

Borrowing from the jargon of statisticians, catalytic rate expressions are “ill-conditioned”, which means that a large amount of data must be gathered over a broad range of operating conditions to identify a unique mechanistic rate expression and its parameters. This requires a research method that can generate and convert large amounts of reaction data into useful information quickly, quantitatively, consistently, and reproducibly. At SE Reactors, we developed a method for interpreting large amounts of kinetic data that will clear the main obstacles to a theoretical understanding of catalysis. This method can be used alone or, even better, in combination with information on catalyst properties and surface chemical events obtained from high-vacuum techniques. As a consequence, catalyst development can be transformed into a more systematic and quantitative science and permit faster and more cost-effective catalyst and process development.

Breakthrough in kinetics research instrumentation

The temperature-scanning reactor (TSR) is a new kinetics research instrument that can collect and interpret masses of data in catalytic and noncatalytic kinetic studies (5–7). With a TSR, thermal steady state is not necessary to generate useful results. For example, by using a temperature-scanning plug-flow reactor (TS-PFR), we examined the oxidation of CO on a Pt/Al₂O₃ catalyst. In one 5-h experiment, we evaluated the kinetics of this reaction on one catalyst, at one pressure and feed composition, at temperatures of ~200–300 °C.

The satisfactory kinetic rate expression governing the reaction was identified by using ~3500 conversion–temperature– rate triplets (X, T, r) from the unlimited mass of raw data collected from this one experiment. We initially examined two rival models of a mechanistic rate equation using Langmuir– Hinshelwood kinetics—a molecular adsorption model and a dissociative adsorption model (DAM). We used these models to identify the species whose properties affect the rate of reaction and to in corporate them into the rate expression.

Both models were fitted over the range of conversions and temperatures examined in the experiment. The DAM model provided an excellent fit. We continue to examine the same data to determine whether another equation, derived from another mechanism, will give an even better fit.

Breakthrough in kinetics research methodology

The TSR is the first “kinetics instrument”; it is as significant and versatile in its field as the gas chromatograph is in analytical instrumentation. A temperature-scanning (TS) experiment in a fully automated TSR gathers data for 3-D mapping of the surfaces that offer a view of reaction phenomena. Like a mapping satellite, which surveys the terrain in terms of latitude– longitude–elevation, a TSR collects data on the temperature– space-time–conversion (T,, X) surface along well-defined traverses. To interpret the raw data, the TS algorithms convert it into quantitative data consisting of conversion– temperature– rate triplets (X, T, r), suitable for reactor design or rate expression fitting.

To understand how one TS experiment can generate the large amounts of data needed to determine the applicable rate expression, consider reaction rates that correspond to slopes (i.e., dX/d) on the experimental surface (T, , X). Once this surface is well defined, an unlimited number of (X, T, r) triplets are available from a single experiment. The TSR is programmed to determine the shape of the (X, T, r) surface at the desired precision level by taking as many traverses over the kinetic surface as one wishes. Each traverse, or run, takes about 30 min, and 10 runs are usually enough to map the surface. An 8-h day is therefore more than enough to map a kinetic surface in satisfactory detail. By contrast, a traditional isothermal run under standard conditions provides only one point on this surface.

Fundamental and practical advantages of the TSR

The surfaces for two catalyst samples may in fact intersect along a line within the range of (X, ) values of interest. In that case, at some (T, ) values (the x–y positions on the surface), the conversion on a new catalyst will lie above that of the old catalyst, whereas elsewhere, the reverse will be true. A standard run could, by chance, lie in either of these (T, ) regions; if it lies where conversions are lower on the new catalyst, under estimation of its potential will result. In any case, because the two surfaces have different shapes, the optimum commercial operating condition will be different for each catalyst, a fact that a standard isothermal comparison cannot reveal.

Mathematical procedures involved in TS data gathering and interpretation may sound daunting, but as in all sophisticated instruments of this level of sophistication, the mathematical procedures are internalized and fully automated: from operation of equipment, to visualization of data, to interpretation of results. The results provide immediate, detailed information on catalyst activity and selectivity over a broad range of industrially relevant operating conditions.

Flexibility of the TSR

The TS technique is broadly applicable to all kinds of research reactors: including plug-flow, batch, continuously stirred thermal reactor, and stream-swept. It can be used in simple or complicated reactions; for homogeneous or catalytic studies; and in gas, liquid, or solid-phase reactions. In short, if the kinetics can be measured, temperature scanning can be applied.

A single TS-PFR experiment provides rates of reaction over and beyond the full range of commercial conditions, between the limits set by pressure drop at one extreme and the exit from the turbulent-flow regime at the other. Thereafter, for that set of (T, ) conditions, the experiment need never be repeated. All experimentally accessible (T, , X) data for that reactor configuration are gathered in a single experiment and stored as raw data on disk, where they are instantly retrievable and available for rate extraction at any point on the (T, , X) surface or graphical presentations of surfaces of interest.

The TS-PFR used in our experiments can quantify up to 16 components in the output stream, deconvoluting the mass spectrometer signals in real time and providing real-time analysis of the products. The rates of production of up to 15 products can therefore be ascertained in one experiment. Runs under other than TS conditions can easily be performed as well, to observe catalyst stability, transient response, or even isothermal operation. With a TS-PFR, much of this quick-look information is available in real time; the full “kinetics instrument” capability of the same TS-PFR is available with a few keystrokes after the experiment is completed.

Catalyzing catalysis

Two kinds of scientists work in the field of catalysis: qualitative and quantitative. Both contribute to progress in the field, but it is probably fair to say that they keep one another at a distance. The TSR can make both happy. For the qualitatively inclined, a TSR offers various curves in two dimensions and representations of kinetic surfaces in three. The selection of 2-D curves includes all the familiar presentations, such as conversion versus space–time, as well as many pairs of coordinates not often viewed before, such as isothermal rates versus conversion, isokinetic rates in conversion versus temperature coordinates, and conversion versus outlet temperature as inlet temperature is varied. Volumes of such information are instantly accessible in picture form from each experiment.

The 3-D representations of (X, T, r) or (T, , X) can be rotated in three dimensions to offer detailed views and to reveal where they are steep, shallow, or indented, or contain other features. Qualitative examination of these presentations will reveal a great deal to those with an intuitive grasp of the behavior of kinetic systems.

As a bonus, the rate parameters that the software extracts from the data can be examined as a function of whatever changes the chemist is making in the catalyst formulation. All this can be done by using the preprogrammed features of TSR operation and data interpretation. The operator initially selects the procedures that are optimal for the system under study. Once selected, the procedures are automated and require no further modification.

With a TSR, the whole area of reaction space can be examined in the same length of time it takes to obtain a single data point using isothermal methods. Along with satisfying data volume requirements, the features that make the TSR well suited for model discrimination include built-in options for data smoothing to minimize errors, selection of theoretical expressions to fit to the data, and advanced procedures for fitting rate data. The ability to acquire and interpret volumes of data for testing various rate expressions leads to quantitative and well-documented theoretical work. The described capability of TS methods is an essential part of the rejuvenation of catalytic studies.

References

1. Wei, J. CHEMTECH 1996, 25 (5), 16–18.
2. Schilling, L. B. Catalysis and Biocatalysis Technologies; National Institute of Standards and Technology, Advanced Technology Program: Gaithersburg, MD, 1995; p 3.
3. Armour, J. Visibility of Catalysis. North American Catalysis Society Newsletter, Sept 1996, 30 (2), 2.
4. For example, the Florida Catalysis Conference opened its 1997 conference with a keynote panel discussion titled “What can we do to catalyze catalysis in the USA?”. Florida Catalysis Conference, University of Florida, Palm Coast, FL, April 21–25, 1997.
5. Wojciechowski, B. W. Catal. Today 1997, 36, 167–190.
6. Rice, N. M.; Wojciechowski, B. W. Catal. Today 1997, 36, 190–207.
7. Asprey, S. P.; Wojciechowski, B. W. Catal. Today 1997, 36, 209–226.

Bohdan W. Wojciechowski is president of SE Reactors Inc. (2050 Imperial Circle, Naples, FL 34110; fax: 941-598-2410; bohdanw@aol.com).