Design Strategies for the Molecular Level Synthesis of Supported Catalysts

Abstract

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble averages and therefore do not isolate the catalytic function of individual components within the mixture. New synthetic approaches are required to more controllably tailor supported catalyst structures. In this Account, we review advances in supported catalyst synthesis and characterization developed in our laboratories at Northwestern University. We first present an overview of traditional synthetic methods with a focus on supported vanadium oxide catalysts. We next describe approaches for the design and synthesis of supported polymerization and hydrogenation catalysts, using anchoring techniques which provide molecular catalyst structures with exceptional activity and high percentages of catalytically significant sites. We then highlight similar approaches for preparing supported metal oxide catalysts using atomic layer deposition and organometallic grafting. Throughout this Account, we describe the use of incisive spectroscopic techniques, including high-resolution solid state NMR, UV-visible diffuse reflectance (DRS), UV-Raman, and X-ray absorption spectroscopies to characterize supported catalysts. We demonstrate that it is possible to tailor and isolate defined surface species using a molecularly oriented approach. We anticipate that advances in catalyst design and synthesis will lead to a better understanding of catalyst structure and function and, thus, to advances in existing catalytic processes and the development of new technologies.

The societal significance of catalysis and the growing practical importance of single-site heterogeneous catalysts

“…So far as the first category of SSHCs is concerned-that is, those in which heteroatoms such as Al III , Ti IV or Sn IV ions introduced into a zeolitic molecular sieve such as silicalite (figure 4) so as to generate either Brønsted or Lewis acid active centres-it is possible by X-ray absorption, UV-visible, diffuse-reflectance, high-resolution NMR spectroscopy and other techniques described (see Marchese et al 1997;Wegner et al 2012;Zecchina & Groppo 2012) to determine the precise atomic environment of the active centre. (Seldom is it feasible to do so with those heterogeneous, solid catalysts which are composed of close-packed structures, although there are a few exceptions-see Zambelli et al 1996.…”

Supported Single-Site Organometallic Catalysts for the Synthesis of High-Performance Polyolefins

“…However, understanding structure/reactivity/ selectivity relationships in these systems is severely complicated by uncertainties in the active site structure(s) and the percentage(s). For all these reasons, grafting welldefined, single-site homogeneous molecular catalysts on surfaces has emerged as a powerful tool to create new catalysts, to characterize surface species, and thus to establish unambiguous structure/activity relationships [68][69][70][71][72][73]. For example, it has been shown that well-defined zirconium hydrides supported on alumina are highly active catalysts for alkane hydrogenolysis (Fig.…”

Improving Heterogeneous Catalyst Stability for Liquid-phase Biomass Conversion and Reforming

“…The overcoat was then deposited by 45 successive cycles of Atomic Layer Deposition (ALD). Among various catalytic design techniques, [49] ALD has been extensively used to tune properties at the molecular level. [50] ALD allows deposition of thin layers (< 100 nm) with high level of conformality.…”