Ind. Eng. Chem. Res. 1996, 35, 4801-4803
4801
Infrared Thermographic Screening of Combinatorial Libraries of Heterogeneous Catalysts F. C. Moates,† M. Somani,† J. Annamalai, J. T. Richardson, D. Luss, and R. C. Willson* Department of Chemical Engineering, University of Houston, Texas 77204-4792
A combinatorial library of catalyst candidates, each consisting of a different metal element supported on γ-alumina, is screened for hydrogen oxidation catalytic activity. Heat liberated on the surface of active catalysts by the catalyzed reaction is detected by noninvasive IR thermography. A 16-candidate library identifies four distinctly active pellets, which correspond to active formulations known from the literature. A higher density library shows similar results, but heat and mass transport effects influence the pellet temperatures. This method may be used to screen and optimize catalyst formulations more efficiently and quickly than current methods and may also be useful for study of operational lifetime, resistance to poisons, and regenerability. Introduction Heterogeneous catalysis plays a vital role in the modern chemical industry, and important economic and environmental benefits may be attained by optimization of catalyst formulation. Despite extensive study of the properties and underlying mechanisms of catalysts, many improvements in catalyst formulations arise from extensive screening of candidate compositions. Testing of candidate formulations remains an expensive, laborious, and time-consuming process. High-throughput screening has created a revolution in pharmaceutical development and has been extended to optimization of materials (Xiang et al., 1995). A combinatorial approach to the development of catalytically-active metal-complexing polymers has recently been described (Menger et al., 1995). In this work poly(allylamine) was modified with combinations of functionalized carboxylic acids and complexed with one of three metals, giving rise to a large number of potential polymeric catalysts. The catalytic activity of each candidate for hydrolysis of the chromogenic reactant p-nitrophenyl phosphate was determined using spectrophotometric techniques. In the present work we describe the use of IR imaging thermography for screening of libraries of heterogeneous catalyst formulations. Infrared thermography has been used to monitor the dynamics of reactions on solid surfaces (Pawlicki and Schmitz, 1987; Lobban et al., 1989), and we have adapted this approach to identifying superior catalyst formulations from libraries of many candidates in a highly-parallel manner. Materials and Methods 99% γ-alumina pellets (4 mm length × 3 mm diameter) were obtained from Alfa (Ward Hill, MA). Precursor compounds were prepared in concentrations (4.424 g/L) chosen to give 0.5 wt % metal in the pellets, assuming complete deposition of all metal salt. Precursors (bismuth(III) nitrate pentahydrate, chromium(III) nitrate nonahydrate, cobalt(II) nitrate hexahydrate, copper(II) nitrate hemipentahydrate, erbium oxalate * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (713) 743-4308. † These authors contributed equally to this work.
S0888-5885(96)00476-9 CCC: $12.00
hydrate, gadolinium(III) oxalate hydrate, iron(III) nitrate nonahydrate, nickel(II) nitrate hexahydrate, palladium(II) nitrate hydrate, silver nitrate, ammonium titanyl oxalate monohydrate, ammonium metavanadate, zinc oxide, hydrogen hexachloroplatinate(IV) hydrate, hydrogen hexachloroiridate(IV) hydrate, rhodium(III) nitrate) were from Aldrich (Milwaukee, WI). 10 wt % HCl was added where necessary to ensure complete dissolution. Ten pellets were added to 10 mL of each solution, and the water was evaporated at 120 °C in air for a period of 10 h. The impregnated pellets were then calcined at 600 °C for 6 h. Visual inspection of fractured pellets revealed rather uniform deposition in the exterior 0.2 mm. Weighing of the residual materials after evaporation indicated deposition efficiencies ranging from 0.80 to 0.98. The calcined pellets were exposed in the reactor chamber to pure hydrogen at 400 °C for 12 h. Prepurified-grade hydrogen (99.99%) and extra-drygrade oxygen (99.6%) from Linde were desiccated after mixing by an in-line purifier (Molecular Sieve 4A, Linde). Gas flow rates were set by a thermal mass-flow controller (Tylan). The hydrogen flow rate was 600 mL/ min, while the oxygen flow rate was varied but kept below 30 mL/min to avoid formation of explosive mixtures. The reactor was a cylindrical aluminum vessel with an inner diameter of 7.0 cm and a depth of 4.8 cm (Lane et al., 1993; Somani et al., 1996). Feed gases were premixed and fed to the reactor through four angularly symmetric inlet ports near the bottom of the reactor chamber, and product gases exited through four similar ports near the top. The catalyst pellets were arranged on an alumina disk placed midway between the inlet and outlet ports. An infrared-transparent sapphire window (Union Carbide Crystals Division) allowed viewing of the catalyst pellets. The reactor was operated at atmospheric pressure. It was placed inside an oven heated by a 1600 W cylindrical electrical heating element, and its temperature was controlled (to (0.2 K) by a PID temperature controller (Omega CN2041). A Radiance PM (Amber, Goleta, CA) 256 × 256 pixel, Indium Antimonide focal-plane-array camera sensitive to 3-5 µm radiation was used to measure temperatures in the pellet array. Radiation levels were measured with 16-bit accuracy, giving a temperature sensitivity © 1996 American Chemical Society
4802 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
of 0.025 °C. A 100 mm focal length lens was used for experiments conducted at lower temperatures (35-200 °C); a 50 mm lens with a 1% transmission neutral density filter was used for higher temperatures (200300 °C). We adjusted the camera parameters (emissivity, object distance) such that the temperature indicated by the camera was the same as that measured by the reactor thermocouples, while no reaction occurred on the pellets. Despite their differing appearances in visible light, the effective IR emissivities of the lightly-loaded pellets showed only minor emissivity variations in the range 35-300 °C under nonreactive conditions (see below). Images were displayed at 60 frames/s on an LCD panel attached to the camera and the NTSC signal was carried to videotape. Digital images were saved on-thefly on a PCMCIA Type-1 memory card for later analysis, while a 24-bit video capture interface (Snappy; Play Inc., Rancho Cordova, CA) was used to transfer images from video to digital format (tiff). Results Pellets were placed in the reactor at 10 mm spacing as shown in Figure 1a. With O2-free H2 flowing at 600 mL/min, the reactor was scanned over temperatures from 35 to 350 °C and pellets were imaged in the absence of reaction. As illustrated by the results obtained at 200 °C (Figure 1a), the pellets differed in emissivity by less than 1%, although emissivity varied by 3% over the temperature range studied. The temperature was then equilibrated at 35 °C followed by introduction of 5 vol % O2 into the H2 feed stream. The temperature of three candidates (Pd, Ir, and Pt) increased detectably within 10 s of O2 introduction. The reactor temperature was then increased (2.2 °C/min) to 300 °C. The Rh-loaded pellet ignited at about 82 °C. Parts b and c of Figure 1 show the array just before and after ignition of the Rh-loaded pellet. All other pellets remained inactive to 300 °C. The temperatures of all ignited pellets were within a few degrees of each other and remained 90 ( 5 °C above the reactor temperature while the reactor temperature was increased, suggesting that the pellets were mass-transfer limited. Rearrangement of the 4 × 4 pellet array produced similar results, suggesting that temperature and gas phase compositional nonuniformities do not affect the results at this density. Experiments with a more closely spaced (5 mm) 7 × 7 pellet array confirmed the qualitative results obtained at lower density but the apparent activities depended on the position in the array, indicating heat- and mass-transfer effects. Discussion Catalysts having activity for hydrogen oxidation were readily identified both by higher, steady-state, local temperatures and by ignition at lower temperatures during heating in the presence of reactants. The elements identified as possessing significant hydrogen oxidation activity (Pt, Pd, Rh, and Ir) were among those known from the literature (Norton, 1982; Golodets, 1983). The number of candidates examined was limited in this work by the reactor dimensions and could readily be expanded by moving the camera (e.g., Lobban et al., 1989) across a larger reactor. Sample density could likely be increased by optimization of reactor design, and the screening experiments could be of very short dura-
Figure 1. IR thermographic image of candidate catalyst formulations at (a) 200 °C under nonreactive conditions (O2-free H2 feed), (b) before ignition of the Rh-loaded pellet (reactor temperature, 80 °C), and (c) after the ignition of the Rh-loaded pellet (reactor temperature, 85 °C).
tion. High-throughput screening would require at least partial automation of sample preparation, as well as informatics approaches to experimental planning and data management. Potential limitations of the applicability of the method to identification of catalysts useful in large-scale processes include mass- and heat-transfer limitations in packed beds, competing reactions, and variations in preparation methods with scale. Even a qualitative indication of activity would be useful, however, in broad screening of candidate formulations. Rapid screening
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should also prove useful in assessing operational lifetime, resistance to deactivation, and regenerability. Acknowledgment We thank D. Bergbreiter and D. Ables for helpful discussions. This work was supported by grants from the University of Houston Energy Laboratory and PEER program and the Hoechst Celanese Corp. We thank the Leake Co. and Amber Scientific for loan of the Radiance PM camera. Literature Cited Golodets, G. I. Heterogeneous Catalytic Reactions Involving Molecular Oxygen; Elsevier: Amsterdam, The Netherlands, 1983. Lane, S. L.; Graham, M. D.; Luss, D. Spatiotemporal Temperature Patterns During Hydrogen Oxidation on a Nickel Disk. AIChE J. 1993, 39, 1497. Lobban, L.; Philippou, G.; Luss, D. Standing Temperature Waves on Electrically Heated Catalytic Ribbons. J. Phys. Chem. 1989, 93, 733.
Menger, F. M.; Eliseev, A. V.; Migulin, V. A. Phosphatase Catalysis Developed via Combinatorial Organic Chemistry. J. Org. Chem. 1995, 60, 6666. Norton, P. R. in The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, The Netherlands, 1982. Pawlicki, P. C.; Schmitz, R. A. Spatial Effects on Supported Catalysts. Chem. Eng. Prog. 1987, 86, 40. Somani, M.; Liauw, M. A.; Luss, D. Hot-Spot Formation on a Catalyst. Chem. Eng. Sci., 1996, 51, 4259. Xiang, X.-D.; Sun, X.; Briceno, G.; Lou, Y.; Wang, K.-A.; Chang, H.; Wallace-Freedman, W. G.; Chen, S.-W.; Schultz, P. G. A Combinatorial Approach to Materials Discovery. Science 1995, 268, 1738.
Received for review August 5, 1996 Revised manuscript received September 19, 1996 Accepted September 19, 1996X IE960476K X Abstract published in Advance ACS Abstracts, November 1, 1996.
