Catalyst Design for Sustainable Chemistry

Developing catalysts that can efficiently cleave strong bonds like C─H or O─H is not only inherently fascinating but has the potential to enable entirely new chemical transformations or streamline important processes. Unfortunately, however, chemical transformations that start with unreactive materials, such as alkanes, and introduce desirable functional groups, such as alcohols or multiple bonds, bring with it an inherent selectivity challenge. The incentive for transforming, for example, methane to methanol, is to convert an unreactive gaseous molecule into an easy-to-transport liquid that can serve a building block for a myriad of other materials. Quite a large number of catalysts are available to oxidize unreactive methane to methanol, but efficiently preventing the more reactive methanol from being oxidized further constitutes an unsolved challenge.

We are interested in the design of catalysts that reverse traditional definitions of reactivity. We aim to achieve unusual selectivities in oxidation reactions through the development of catalysts associated with unusual reaction mechanisms where classically unreactive molecules are preferentially oxidized.

We are interested in the development of cooperative catalysis, where two or more catalysts interact with the same substrate to facilitate a challenging elementary step. While introducing multiple catalyst-substrate interactions has an obvious beneficial effect on the enthalpy of activation, the need for three or more molecules to be arranged in a particular orientation in the transition structure can render the entropic cost of cooperative catalysis forbidding. Whereas chemical catalysis struggles to effectively control entropic effects, enzyme catalysis highlights that the cooperative effect of multiple interactions with a transition structure can be extremely powerful, when not held back by entropic barriers. A crucial feature of enzyme catalysis is a three-dimensional environment where multiple residues are pre-organized in a favorable orientation to provide transition state stabilization. Metal-organic frameworks (MOFs), by virtue of providing pore environments that can be engineered with atomic-level precision can present both open metal sites as well as organic moieties in a precisely defined arrangement to incoming substrates.

We are interested in the development of MOF-based catalysts that serve as functional enzyme mimics. Preorganization of suitable catalytic centers within the pore space of MOFs will minimize the entropic cost of cooperative catalysis.

An overarching goal of the work in the Neumann lab is the development of catalysts for transformations that involve difficult elementary steps, such as the homolytic cleavage of strong and highly polarized bonds. The O─H bond in water, for example, can be heterolytically cleaved with ease to yield hydroxide and a proton, and proceed with a low activation barrier in the presence of an acid or base. Homolytic cleavage of the same bond, however, poses a formidable challenge. A small number single site transition metal catalysts are capable of O─H bond activation, but the difficulty of the O─H bond activation step makes it difficult to incorporate into a catalytic cycle: the properties of the catalyst have to be finely honed to accomplish a very challenging oxidative addition so that the following reduction or reductive elimination step may be rendered unfavorable. Sharing the work of cleaving a strong bond, particularly a highly polarized strong bond, between two or more atoms makes less stringent demands on the properties of any particular metal center. Alloy nanoparticles are promising candidates for the activation of strong, polar bonds because the components of the alloy can be individually adjusted to interact effectively with the two fragments resulting from bond cleavage.

We are interested in the efficient synthesis and stabilization of alloy nanoparticles composed of metals with unfavorable mixing enthalpies. Exploration of the reactivity of such nanoparticles is focused on the effect of the disparate components and their interfaces on the activation barrier of challenging elementary steps in catalytic reactions.

Extensive research efforts are devoted to honing catalytically active materials to deliver the best possible performance. Whatever shape, size and composition of the active catalyst is selected, however, the majority of heterogeneous transition metal catalysts require a support material, such as carbon, alumina or silica. Despite the fact that the support material makes up a large fraction of the total weight of the active catalyst, there are comparatively few good options from which to select. Common materials such as carbon, alumina, silica or ceria can offer high stability to temperature, high surface areas and some variation in the polarity and basicity of the support surface. Pairing an active catalyst with a suitable support material is crucial for maximizing the stability and activity of catalyst. The interface between transition metal particles and support, in particular, can be a key contributor to catalytic activity and the metal-support interactions are crucial for preventing the leaching or agglomeration of active catalyst particles.

We are interested in the design of support materials that not only lead to long-lived highly active catalysts, but take an active role in controlling the outcome of a catalytic transformation. Given the ubiquitous nature of catalyst support materials in heterogeneous catalysis, the development of novel materials that enhance the intrinsic selectivity of the catalytically active material can impact a wide range of catalytic processes.