Introduction

The mechanism of catalytic activity in a heterogeneous system is complex [149]. The distribution of active sites in heterogeneous catalysts as well as the phase integrity is in general poorly defined. The phase integrity plays a major role because the extent of deviation from the equilibrium structure under reaction conditions controls the catalytic function. These types of inherent problems severely hamper or even prevent the rational design of a heterogeneous catalyst for a particular reaction. Conventional methods of catalyst discovery are primarily trial-and-error processes where one catalyst at a time is tested for activity followed by numerous modifications to achieve satisfactory activity. The process is therefore time-consuming and laborious, and in most cases only local minima in a predefined parameter space (composition, reaction conditions, etc.) have been identified rather than the absolute minimum. Combinatorial synthesis and screening offer a new dimension for the quick discovery and optimization of heterogeneous catalysts in terms of both composition and process conditions. The high-throughput approach allows rapid exploration of a much larger parameter space than the conventional approach and therefore leads more likely to the discovery of novel materials with significant performance improvements.

In this section, we focus on recent developments in the area of high-throughput synthesis and screening of heterogeneous catalyst libraries. Special emphasis is given to some recently published literature on methods of sample preparation and parallel screening. A number of interesting patents and articles has appeared which deal mostly with the development of methods for high-throughput synthesis and screening of heterogeneous libraries. Integrated synthesis and screening of a plurality of catalysts in library format has been recognized as an essential factor [150]. In 2000, Gennari and coworkers summarized inorganic catalysis in a review on combinatorial technologies for catalyst design and development [20e]. Recently, Senkan published a more detailed review on combinatorial heterogeneous catalysis [20c]. Mirodatos and coworkers discussed the application of combinatorial chemistry to heterogeneous catalysis in terms of current strategies and perspectives on the industrial and academic levels [151].

There are basically two techniques to prepare libraries of heterogeneous catalytic materials: solution-based methods and thin-film deposition methods. The description of both techniques is omitted in this part since they are described in detail in Chapter 34. Impregnation of catalyst components onto a preformed solid support, however, is an important solution-based technique for the preparation of heterogeneous catalysts, but is seldom utilized for other material sciences applications. The solution-based methods are predominant in the field of combinatorial heterogeneous catalysis. The majority of commercial heterogeneous catalysts is manufactured by solution-based techniques such as coprecipitation, impregnation, and their variations [152], and, thus, results from library experiments can be translated more easily into a bulk catalyst using the solution-based techniques. In addition, the solution-based methods present fewer scale-up problems. Furthermore, many liquid-handling robots and inkjet-based liquid-dispensing systems can be employed to prepare the heterogeneous catalyst libraries of microgram scale to gram scale by the solution-based methods. On the other hand, the thin-film deposition methods are performed on custom-designed and assembled high-vacuum instruments that generally require high initial investment. Special care should be taken to ascertain that the phase integrity of each catalyst member is the desired one since the thin-film deposition methods rely on the interlayer diffusion for mixing and the multi-component layers occasionally result in phase separation. The thin-film-based method will perhaps be of choice for fused catalyst libraries, e.g. metal alloys.

In most cases, the catalyst performance depends on the method of preparation. Therefore, it is very important to adopt proper methods to obtain catalysts with expected surface area, uniform metal distribution, and desirable particle size. Newsam and Schuth have described various routes for combinatorial catalyst synthesis that include hydrothermal synthesis, use of fluid precursors, carrier impregnation, precipitation [153]. A number of other research papers have outlined different methods of preparing combinatorial catalyst libraries. These include computer-controlled inkjet deposition of liquid reactants [154], methods based on solution precursors [155-157], sol-gel techniques [158-160], impregnation of solid supports [161], as well as standard methods such as precipitation and coprecipita-tion [162, 163]. Table 32.1 summarizes some examples of heterogeneous catalyst libraries.

With the development of new and efficient technologies for the synthesis of large catalytic libraries, there is an ongoing effort for inventing fast and parallel screening tools to identify active and selective catalysts. Several new techniques have been proposed and demonstrated in the past few years, including the resonance-enhanced multiphoton ionization (REMPI), time resolved and differential IR thermography, scanning mass spectrometry, and colorimetry. While IR thermography and fluorescence or colored dye assays are established techniques, a few other new methods have emerged recently to identify active catalysts in an array. So far, no screening method offers a general solution to the problem of fast screening of libraries together with the complexity involved in parallel detection of the reactants, products, and side products. Therefore, it is very important to develop screening technologies according to the detection requirements of the reactants, products, and side products of the reaction. Accordingly, in some cases, it is essential to have a combination of tools for a better characterization of reaction products. It is also very important to characterize the time-dependent nature of the catalyst library for developing a practical catalyst. Some catalysts have a significant induction period

Tab. 32.1. Examples of combinatorial heterogeneous catalyst libraries.

Catalyst libraries

Preparation method

0.5% Ag, Bi, Co, Cr, Cu, Er, Fe, Gd, Ir, Ni, Pd, Pt, Rh, Ti, V, Zn, on A1203 pellets

37, 1-10% Co, Cr, Cu, Fe, Ir, Mn, Ni, Pd, Pt, Rh, Ru, V, Zn on Si, Ti oxides

100 Na20/Al203/Si02 zeolites with Li and Cs

37 Ti02/Al203/Si02/Ti02/Zr02 zeolites

645 clusters of Pt, Ru, Os, Ir on carbon

120 ternary thin film clusters of Pt, Pd, Rh, and Pd, Rh, Cu

66 ternary combinations of 1% Pt, Pd and In on A1203

16 Au/Co3 04 and Au/Ti02 powders

33 1-6% Ag, Au, Bi, Co, In, Cr, Cu, Fe, Mo, Ni, Re, Rh, Sb, Ta, Te, V, Y on Si, Ti, Zr oxides

50 ternary and quaternary oxides of Co, Cd, Fe, Ga, Ge, In, Mn, Mo, Ni, Nb, V, W, Zn

Solution-based impregnation

Solution-based sol-gel method

Solution-based hydrothermal zeolite synthesis

Solution-based hydrothermal zeolite synthesis

Solution-based coprecipitation

Thin film deposition: sputtering

Solution-based impregnation

Solution-based coprecipitation

Solution-based sol-gel method

Solution-based coprecipitation h2 + o2

Hyrogenation of 1-hexyne a a

Methanol direct fuel cell

CO oxidation, CO + NO

Cyclohexane dehydro-genation to benzene

CO oxidation Propylene oxidation

Ethane and propane dehydrogenation

Infrared thermography 164

Infrared thermography 165

a 166

a 167

Fluorescence acid-base indicator 157

Scanning quadruple mass 168 spectrometry

REMPI 161

Quadruple mass spectrometry 162

Spatially resolved analysis of an 169 array of batch microreactors

Six-parallel gas chromatography 170

Catalyst libraries

Preparation method

30 binary combinations of Na2W04 and Mn on Si02; Au and In on Zr02, Ti02, Si02, MgO, ZnO, Nd203, Y203, Ce02, Mn203

66 ternary combinations of oxides of Mo, V and Nb

Solution-based impregnation, deposition, and precipitation

Solution-based sol-gel deposition

144 ternary combinations of oxides of V/Al/Nb, and Cr/Al/Nb

Solution-based sol-; deposition

66 ternary combinations of 1% Pt, Pd, and In on A1203

Solution-based impregnation

45 3- to 5-element combinations of Pt, Pd, Rh, Ru, Au, Cu, Ag, and Mn on Ti02 and Fe203

56 quaternary combinations of Pt, Pd, In, Na on A1203

V2C>5/Ti02 mixtures

Solution-based impregnation

Solution-based impregnation

Solution-based coprecipitation

Oxidative coupling of methane and CO oxidation

Quadruple mass spectrometry 171

Ethane oxidative dehydrogenation

Ethane oxidative dehydrogenation

Cyclohexane dehydrogenation to benzene

C3H8 total oxidation

Scanning quadruple mass 172

spectrometry, photothermal deflection, gas chromatography

Scanning quadruple mass 173

spectrometry, photothermal deflection

Quadruple mass spectrometry 174

Quadruple mass spectrometry 175

NO reduction by C3H6 Quadruple mass spectrometry 176

Oxidation on naphthalene to naphthoquinone

Laser-induced fluorescence imaging (LIFI)

36 Pt/Zr/V/Al203

Solution-based impregnation

52 Pt/Zr/V/Al203

715 combinations of Pt, Ru, Os, Ir, Rh

280 V2 O5, m0o3, Mn02, Fe203, Ga203, La203, B203, MgO on A1203

32 Ti-silsesquioxanes from 8 RSiCl3/Ti (OiPr)4 ternary composition of cyclopentyl, cyclohexyl, and phenyl-substituted silanes

65 x 1 mol% main group, transition, and rare earth metal dopands on Ti02, Sn02, or W03

Solution-based impregnation onto A1203 electro-chemically formed on aluminum plate

Solution-based deposition Solution-based impregnation Solution-based sol-gel method

Solution-based sol-gel method a These publications describe only the synthesis of heterogeneous catalyst libraries.

Methane oxidation Co oxidation

Oxidative dehydro-genation of isobutane

02 reduction H20 oxidation

Propane dehydrogenation

Epoxidation of 1-octene with tert-butyl-hydroperoxide (TBHP)

Photooxidation of 4-chlorophenol

Scanning quadruple mass 178

spectrometry coupled with monolith multichannel reactor

Scanning quadruple mass spectrometry coupled with 35 parallel microreactors

Fluorescence acid-base indicator 179

Quadruple mass spectrometry 180

Standard GC equipped with 181

sampling robot

Standard HPLC equipped with 182 sampling robot

ST S cm

§f before they become fully active, and most catalysts deactivate over time. These properties will also vary according to the reaction conditions. Some parallel screening schemes offer the capability of monitoring the reactions over time. For example, a multichannel fixed bed reactor coupled with parallel GC and an array micro-reactor equipped with REMPI allow library screening over a prolonged reaction time. Many techniques such as MS, IR, and other optical detection schemes can be applied in both homogeneous and heterogeneous catalysis. Each of these emerging techniques and tools for high-throughput catalyst screening will be described in detail in the screening section (see Chapter 32.10). Finally, the heterogeneous catalysis section will be concluded by case studies (see Section 32.8.2).

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