Yy V

Fig. 32.25. a) Acetic acid sensor in acyl transfer reactions. b) Representative polystyrene bead simultaneously functionalized with a sensor and a catalyst.

Fig. 32.26. Seven different acylation catalysts used for the chemosensor-based approach to catalyst discovery.

Sequence, Re-synthesize, Re-screen "Hits"

Fig. 32.27. Fluorescent gel system for the detection of bead-supported catalysts.

different catalyst loadings. The reaction progress was monitored as a function of time with a standard fluorescence plate reader, allowing individual reaction rates to be determined. The known superacylation catalysts 4-pyrollidimo-pyridine (PPY) and DMAP were the most active catalysts, producing rapidly growing fluorescent signals. N-methyl-imidazole (NMI) and pyridine gave very low reaction rates. Most significantly, the fluorescent intensities are consistent with the observation that the catalytic activity of A is greater than B and NMI.

A sensor-functionalized polymeric gel for screening pooled catalyst libraries has been developed by Miller and coworkers [222]. The method involves deposition of resin-bound catalysts onto a polymeric matrix that is designed with sufficient permeability such that reagents can diffuse to the beads. The polymer also incorporates (by covalent attachment) a fluorescent probe that signals the presence of reaction products. The method (Fig. 32.27) relies on slow diffusion of reaction products out of the bead into the matrix, which triggers the probe and creates a fluorescent zone around the active catalyst. Miller and coworkers used the same aminomethylanth-racenes as in the previous experiments to investigate acylation reactions of alcohols with acetic anhydride; poly(ethylene glycol)dimethylacrylamide (PEGA) was used for the polymer matrix.

Smotkin, Mallouk, and coworkers have developed an optical screen for electro-oxidation that utilizes a fluorescent pH indicator [157]. In their method, catalyst compositions were applied to Teflon-coated Toray carbon discs such that each 2-mm-diameter catalyst spot contained the same molar concentration of metal at a

quinine

Phloxine B

Fig. 32.28. Fluorescent pH indicators used in an optical screen for electro-oxidation.

loading of approximately 1 mg cm~2 [179]. The catalyst array was analyzed using a three-electrode gas diffusion cell, with the Toray carbon substrate linking the catalyst elements as a working electrode, Pt gauze as the counter electrode, and a reversible hydrogen electrode as a reference. Electrochemical half-cell reactions either generate or consume ions, creating a change in the pH in the location of active catalysts. Utilizing an indicator that is fluorescent in the presence of an acid or conjugate base allows the determination of which elements within the library were most active for a particular anode or cathode reaction. Mallouk and coworkers have used quinine and Phloxine B as fluorescent indicators for neutral pH, and Ni2+ complexed with 3-pyridine-2-yl-(4,5,6)triazolo-(1,5-a)pyridine (Ni-PTP) (Fig. 32.28) for low pH [157]. Using the fluorescence method, Mallouk and coworkers screened ternary libraries of metal alloys and identified novel electrocatalysts for methanol and bifunctional oxygen reduction/water oxidation regenerative fuel cells

Additional optical screening techniques have been developed for a number of different chemical processes. The methods include colorimetric assays where colorless 1-naphthol undergoes an electrophilic aromatic substitution with a diazo-nium salt to give a bright orange azo product [223], indigo [61], and Prussian blue staining of reaction products [224].

32.10.2.2 Resonance-enhanced Multiphoton Ionization (REMPI)

In a proof of concept experiment, Senkan has described high-throughput screening technology for combinatorial catalyst libraries that may supply data on both activity and selectivity for a dehydrogenation reaction [187]. Senkan's approach is

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