Heterogeneous Catalysis: A Central Science for a Sustainable Future

Mar 21, 2017 - Developing active, selective, and energy efficient heterogeneous catalytic processes is key to a sustainable future because heterogeneo...
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Heterogeneous Catalysis: A Central Science for a Sustainable Future Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Cynthia M. Friend*,† and Bingjun Xu*,§ †

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States Department of Chemical and Biological Engineering, University of Delaware, Newark, Delaware 19716, United States

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ABSTRACT: Developing active, selective, and energy efficient heterogeneous catalytic processes is key to a sustainable future because heterogeneous catalysis is at the center of the chemicals and energy industries. The design, testing, and implementation of robust and selective heterogeneous catalytic processes based on insights from fundamental studies could have a tremendous positive impact on the world.

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design of catalysts with high activity and selectivity relies on the fundamental understanding of key parameters (descriptors) of catalysts that determine the surface mediated bond activation and formation of reactants and intermediates. In this regard, it is key to establish structure−activity relations using model and platform chemicals, for example, alcohols, based on which sustainable production of a wide spectrum of chemicals and fuels can be developed. Historically, heterogeneous catalysts have been developed by trial and error. The development of the ammonia synthesis process in the early 1900s illustrates how empirical screening can yield a robust catalytic process.4,5 This example also illustrates the huge potential impact of heterogeneous catalysis on the world; in this case, the dramatic increase in agricultural production enabled by synthetic fertilizers was estimated to support almost half of the world population in 2008.6 Fundamental understanding of catalytic processes has subsequently been used to further optimize catalysts and process conditions. For example, the development of the catalytic converter for automobiles benefitted from fundamental understanding of reaction kinetics for CO oxidation and reduction of NOx on Pt single crystals.7 Likewise, improvements in hydrodesulfurization catalysts were made using guiding principles from fundamental studies and from in situ studies of structure and mechanism.8,9 Major advances in experimental10−12 and theoretical13,14 methodology and a strong foundation of principles for understanding surface reactions are now driving forward the design of new catalytic processes. At the same time, major advances in synthesis of new materials (well-defined nanoparticles, enabling control of shape and size, to hierarchical and hybrid materials) provide a library of potential catalysts. There

eterogeneous catalysis is one of the pillars of the chemicals and energy industries and will be a central science in driving the transition to their eventual carbon neutral operation. The design of active and robust catalytic processes to improve the world is the ultimate goal. Ideally, materials with self-regenerating active sites under functioning conditions can be designed at the atomic level for critical catalytic processes. The pursuit of rational design in catalysis was one of the “holy grails” in the 1995 ACR articles by Breslow1 and by Bard and Fox;2 the quest continues but with a greater urgency. The increased urgency of catalyst development for key processes, for example, biomass upgrading, CO2 reduction, water splitting, and light alkanes activation, is due to the soaring demand for energy, chemical products, and food and the rise in anthropogenic CO2 emissions worldwide (Figure 1a). Catalytic processes are central to the production of chemicals, which accounts for approximately 25% of industrial energy use,3 and the most energy-intensive processes rely on heterogeneous catalysis. Hence, there is a major opportunity to decrease energy consumption and reduce environmental impact by increasing the efficiency of catalytic processes. Among them are development of CO2-neutral processes, efficient and sustainable production of H2 (one of the most energy intensive processes now), selective oxidation of methane, and electrochemical production of ammonia. In this Commentary, we use two examples related to the sustainable production of chemicals that illustrate the value of fundamental studies. We specifically show the importance of using advanced spectroscopy and imaging to interrogate catalytic systems under functioning conditions. Sustainable production of chemicals by converting renewable biomass to major chemicals is an area where new catalytic processes are of paramount importance. The oxygen-rich and multifunctional nature of biomass feedstocks suggest that more direct and costeffective routes could be developed to produce oxygenates than the oxygen-lean hydrocarbon feedstocks (Figure 1b). Rational © 2017 American Chemical Society

Received: October 12, 2016 Published: March 21, 2017 517

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Figure 1. (a) US energy consumption by sectors (data from eia.gov). (b) The differences in the composition and structure of fossil carbon sources and biomass dictate that the strategy for and economics of their processing be fundamentally different. Oxygen-lean fossil carbon sources are ideal for fuels and require selective oxidation to produce key chemicals.

Figure 2. (a) Selective dehydration of methylate requires the protection of the carboxyl group and removal of in situ generated H+. (b) Cooperative metal−Lewis acid catalysis is needed to achieve high selectivity in converting furfural to 2-methylfuran.18 (c) Schematic of the ATR flow cell for probing adsorbed species at solid−liquid interfaces. (d) Vibrational spectra of pyridine adsorbed on dehydrated NaY under vacuum (bottom spectrum) and NaY in the presence of liquid water (top spectrum).

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EXAMPLE 1. SELECTIVE BIOMASS CONVERSION: MULTIFUNCTIONAL SUBSTRATES AND CATALYSTS The interrogation of catalytic processes under functional conditions is extremely challenging for biomass conversion processes, which often are conducted in liquid or multiphase media. As the only substantial source of renewable carbon, biomass is the preferred feedstock for producing renewable liquid fuels and chemicals compatible with the existing infrastructure.17 The “holy grail” of biomass upgrade is breaking down a wide range of biomass feedstocks into a manageable number of platform compounds before upgrading them to desired products. Gasification of biomass to syngas by cleaving all C−C and C−H bonds fails to take advantage of structures and functional groups, for example, aromatic rings, shared between the biomass and desired products. Depolymerizing biomass into molecular building blocks and reassembling them to valuable products should be more efficient. In the complex environment of biomass upgrading reactions, one-to-one correlations between model systems and function-

have also been major advances in the use of theoretical modeling to design effective catalysts,15,16 as reviewed previously. Experimental design of catalytic processes can be achieved by first establishing detailed reaction mechanisms on well-defined model systems that apply to functioning catalytic conditions. This approach enables generalization to predict new reactions through the development of simple chemical paradigms. The key to this approach is to select conditions such that the dominant pathways in the model studies are the same as those under operative catalytic conditions. Specifically, the concentrations and nature of surface species in the model studies must mirror those under catalytic conditions. The predominance of similar atomic scale structures of the model system and the functioning catalyst is also an important factor. An exhaustive survey of rational catalyst design is outside the scope of this Commentary, and we will illustrate the successes and challenges of “catalysis by design” with two specific examples from our laboratories. 518

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Figure 3. Illustration of multiscale approach to understanding catalyst function for oxidative coupling of dissimilar alcohols. Model systems (right) provide an atomic scale understanding of bonding and reaction mechanism. Atomic scale imaging of adsorbed O on Au(110), the active site, is combined with density function functional theory (DFT) calculations to model bonding. The reaction mechanism is also established by model studies. Nanoporous Au catalysts with 3% Ag are used to test the model under steady-state flow conditions. The dependence of selectivity for formation of the methyl ester from oxidative coupling of allyl alcohol and methanol are nearly identical for the single crystal and under catalytic conditions as shown on the bottom two panels.26−28

tandem reactions. Detailed isotopic labeling studies demonstrated that the coexistence of Lewis acidic and metal sites on Ru/RuOx/C was required to enable the tandem hydrogenation and hydrogenolysis of furfural to 2-methylfuran.18 In this case, the cooperativity between the metallic Ru and Lewis acidic RuOx is achieved through the mixing of these sites on the atomic level (Figure 2b): the hydrogenation of the carbonyl group in furfural proceeds through a Lewis acid mediated Mervin−Ponndorf−Verley (MPV) mechanism while the hydrogenolysis of the resulting C−OH group occurs on the metal site. Moreover, the crucial roles of oxygen vacancy on the RuOx phase and the solvent were proposed via complementary density functional theory and microkinetic calculations, leading to a more comprehensive mechanistic picture of this complex system.21,22 Such molecular level understanding will be the basis for the rational design of hydrodeoxygenation catalysts for biomass-derived furanic compounds. The presence of solvent in many biomass upgrading processes makes the understanding of surface mediated reaction mechanisms in the liquid phase even more challenging.23 It is critical to characterize catalytic sites in the presence of solvents to establish structure−activity relations because interactions with solvents could drastically change the properties or even the structure of active sites, as well as the interaction between the substrate and the catalytic site. To this end, attenuated total reflection (ATR) based techniques have been shown to be effective in selectively probing the adsorbed species at the liquid−solid interface.24 With the customized flow ATR cell (Figure 2c), we recently showed that the nature of acid sites on NaY, a purely Lewis acidic material, were modified in the presence of liquid water.25 Only vibrational

ing catalysts are far from established. Nevertheless, concepts derived from fundamental studies can provide clues to effective catalytic biomass conversion. The well-defined structure of zeolite catalyst is an excellent platform for understanding biomass conversion. The multifunctional nature of biomass feedstocks and platform chemicals necessitates tailored catalysts to selectively activate specific bonds while preserving others; for example, the α-hydroxy group needs to be removed without impacting the adjacent carboxyl group in the dehydration of lactic acid to produce acrylic acid. Our recent work demonstrates that the dissociative adsorption of methyl lactate and the formation of sodium lactate on Na-exchanged FAU zeolite (NaY) is key to selectivity control by protecting the carboxylate group from decarbonylation (Figure 2a).19,20 In situ transmission FTIR spectroscopy investigations show that Brønsted acid sites are formed during the during the dissociative adsorption of methyl lactate on NaY in the presence of water, which are known to catalyze the decarbonylation reaction of methyl lactate to acetaldehyde. This led to the hypothesis and validation that the removal of in situ generated protons by a base in the feed, for example, pyridine, could suppress the Brønsted acid catalyzed decarbonylation reaction and enhance the selectivity for desired dehydration products (Figure 2a).19,20 Although the introduction of pyridine to the methyl lactate feed is unlikely to be industrially feasible due to the downstream separation cost, these results set the stage for the rational design of catalysts with intrinsic base functionalities to quench the unwanted sites generated in the reaction. Multiple bond cleavages and formation are frequently needed to reach the desired product, necessitating catalysts that enable 519

DOI: 10.1021/acs.accounts.6b00510 Acc. Chem. Res. 2017, 50, 517−521

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processes. In the future, further advances are necessary that can determine composition and structure at higher resolution, but also to measure functionality locally under operating conditions.

bands corresponding to pyridine adsorbed on Lewis acid sites were observed on dehydrated NaY under vacuum; however, a vibrational band for protonated pyridine, that is, pyridinium, at 1545 cm−1 was observed in the presence of liquid water (Figure 2d). These observations clearly highlight the importance of molecular level understanding of solvent effects under functional conditions on a model system in establishing the structure−activity relations in heterogeneous catalytic reactions carried out in the condensed phase.

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SUMMARY The design, testing, and implementation of new catalytic processes is a tremendous opportunity for positive impact on the world. We have used two examples here to illustrate the need for development of appropriate models that can be translated into realistic conditions and to probe catalysis under functional conditions, but there are many challenges in catalysis that need to be addressed in the future. From a chemist’s viewpoint, the goal of catalysis by design is to map out catalytic steps at the molecular level and to develop simple paradigms for classes of reactions. However, catalytic processes are multiscale in their nature. Specific chemical transformations occur on the molecular scale; whereas, materials transport is at larger scales. As we look ahead, investigation of catalyst structure and mechanism over multiple length scales and time scales simultaneously using multiple techniques will be essential. Theory also has an important role to play in multiscale modeling that includes more accurate electronic structure calculations, multiscale modeling of reaction kinetics, including kinetic Monte Carlo and microkinetic modeling, and grafting to larger scale macroscopic engineering models.

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EXAMPLE 2: BRIDGING THE PRESSURE AND MATERIALS GAPS IN SELECTIVE OXIDATION CATALYSIS Once platform chemicals produced from biomass are available, their selective transformation to an array of chemicals is required. Selective partial oxidation of molecules such as alcohols is a critical class of catalytic reactions for which high selectivity is essential. An example of model studies that provide the foundation for predicting how to achieve high selectivity under functioning catalytic conditions is our study of ester synthesis from selective oxidative coupling of dissimilar alcohols on Au (Figure 3). A detailed model of the oxidative coupling of alcohols was developed, first from foundational studies of methyl formate production from methanol, followed by generalization to longer-chain alcohols. Control of the selectivity for coupling of dissimilar alcohols has the added complexity that reaction selectivity depends strongly on the competition of the key alkoxide intermediates for active sites created by Oads. Hence, the selectivity for production of the desired methyl ester depends on the mole fraction of the two different alcohols in the gas phase.26,27,29 The selectivity was highest for an excess of methanol. Theoretical studies demonstrated that van der Waal’s interactions are critical in determining the relative stability of these alkoxides on Au.30 The selectivity trends were essentially the same for Au single crystals with low concentrations of Oads and for a functioning nanoporous Au (npAu) catalyst operating under flow conditions at atmospheric pressure (Figure 3).26−28 This case also illustrates the need for investigation of catalytic materials under functioning catalytic conditions in order to probe the nature of the active material and the robustness of the catalyst; the application of advanced imaging and spectroscopy methods to catalytic processes has been recently reviewed demonstrating the state-of-the-art in this area.31 The catalytic function of npAu has been illuminated through a combination of in situ imaging with transmission electron microscopy (TEM) and determination of compositional changes under reaction conditions using ambient pressure Xray photoelectron spectroscopy (AP XPS).32 npAu is a dilute alloy of ∼3% Ag in Au that is highly crystalline. The in situ work demonstrates that the surface of the active catalyst is enriched in Ag and that the composition changes depending on whether the gas phase is net oxidizing or reducing. The Ag−Au nanostructures are critical for the initial dissociation of O2 to form Oads, the active site for reaction. Theoretical studies show that Ag structures below the surface reduce the barrier for O2 dissociation to a value similar to that measured experimentally.33,34 Although silver is essential for creation of Oads, the ensuing reactions are characteristic of the majority gold component yielding highly selective oxidative processes. Advances in spectroscopic and imaging methods that can be used under functioning catalytic conditions in the vapor phase have already increased our understanding of heterogeneous

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Cynthia M. Friend: 0000-0002-8673-9046 Bingjun Xu: 0000-0002-2303-257X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.M.F. and B.X. acknowledge the helpful discussion with Dr. Robert J. Madix. C.M.F. also acknowledges the support provided through the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award Number DE-SC0012573. B.X. acknowledges the support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001004, and the National Science Foundation, CBET, under Grant Number CBET-1437129.

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