Foundational techniques for catalyst design in the upgrading of biomass-derived multifunctional molecules
Graphical abstract
Introduction
Motivated by concerns about anthropogenic climate change, major advances have been made in the last decade-plus toward the synthesis of fuels and commodity chemicals from non-edible lignocellulosic biomass, which is the only substantial source of renewable carbon [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Abundant and transportable petroleum-derived hydrocarbons have been used to synthesize fuels and chemicals for the last century, and heterogeneous catalytic research has historically been performed in the context of upgrading these fossil carbon resources [15], [16], [17], [18], [19]. Industrial processes and catalytic materials that efficiently mediate the selective oxidation of hydrocarbons have been established and improved over decades to synthesize partially oxidized platform chemicals and monomers [20], [21], [22]. Replacing polluting fossil carbons with renewable biomass-based raw materials entails the development of new production methods that account for the substantial differences in the chemical composition between the two types of feedstocks (particularly the oxygen content, which is 0.05–1.5 wt% in fossil hydrocarbons vs. 30–50 wt% in biomass feedstocks), and the resulting contrasts in physicochemical properties such as thermal stability and polarity [16], [23], [24], [25]. Selective reduction and deoxygenation, rather than oxidation, are required for the synthesis of drop-in platform chemical substitutes from biomass feedstocks [3], [9], [10], [26], [27]. Efficient heterogeneous catalysis is necessary to upgrade biomass on the industrial scale, but new fundamental understanding is first required to develop the novel catalytic materials and systems that will facilitate renewable chemicals production [7], [14], [26], [28], [29], [30], [31], [32], [33]. In this review, we discuss the application of key research strategies used to gain molecular-level insights into the catalytic chemistry of biomass valorization, and how those insights are applied to the design of active, selective, and durable catalysts (Fig. 1.1).
While multiple strategies have been proposed for upgrading biomass resources to fuels and specialty chemicals, direct catalytic transformation is the most promising approach because it has the potential to meet the stringent technical, economic, and sustainability targets [9], [13], [14], [28], [34], [35], [36], [37]. Lignocellulosic biomass has three main constituent parts: hemicellulose (25–35 wt%) and cellulose (40–50 wt%) are composed of polymerized sugars, and lignin (15–20 wt%) is an amorphous polymer consisting of methoxylated phenylpropane units [1], [2], [23], [38], [39]. Early valorization proposals focused on complete gasification or pyrolysis of raw biomass to produce syngas for established petrochemical processes such as the Fischer–Tropsch and the alcohol synthesis processes [40], [41], [42], [43], [44], [45], [46]. This approach deconstructs the starting material through complete C–H and C–C bond cleavage followed by inefficient molecular-level reassembling of the most basic building blocks (synthesis gas) in capital-intensive chemical plants. Due to high transportation costs, widely distributed biomass resources are not well-suited to the centralized processing required for upgrading through gasification and pyrolysis. These processes also destroy the potentially valuable structural units of lignocellulose [3], [36], [37], [47], [48], [49]. By contrast, direct catalytic transformations utilize these molecular building blocks to synthesize value-added chemicals and monomers, and can be performed in a modular and distributed manner [1], [9], [24], [50]. Upstream processing steps such as fractionation and depolymerization are applied to generate a variety of oxygenated platform molecules, e.g., short-chain polyols/carboxylic acids, substituted furanics, and phenolics [6], [51], [52], [53], [54], [55]. These multifunctional molecules can be converted to polymer precursors such as alkylated furans, aromatics, and α,ω-diols through selective hydrodeoxygenation (HDO) and/or dehydration, accompanied by hydrogenation, hydrogenolysis, cyclo-addition, or isomerization steps as needed to attain the final product [3], [12], [27], [32], [56], [57], [58]. This review focuses on these downstream transformations, particularly the challenges of upgrading multifunctional molecules, as well as developing multifunctional catalytic materials that enable these processes.
The most significant challenge in direct conversions of multifunctional biomass-derived platform molecules is selectivity control. Selectivities for critical reactions have substantially improved in recent years as a direct result of improved mechanistic understanding. Targeting specific functional groups in multifunctional molecules for reduction or deoxygenation typically requires catalysts with tailored combinations of active sites [27], [31], [50], [59], [60], [61], [62], [63]. The complexity of these materials poses challenges in establishing clear structure-activity relations and gaining insight into reaction mechanisms, both of which are prerequisites for catalyst design. Rather than a comprehensive review of the biomass upgrading field, this article aims to provide the authors’ perspective on the central methodologies utilized in mechanistic investigations of biomass-to-chemicals reactions, and how the insights gained aid in the rational development of active, selective, and durable catalysts for upgrading biomass-derived feedstocks. We focus on the application of: (1) spectroscopic techniques; (2) multi-scale computational methods; (3) rigorous kinetic investigations; and (4) controlled materials synthesis methods (Fig. 1.1). Readers interested in more general surveys of biomass conversion are referred to recent review articles [10], [12], [13], [27], [31], [32], [49], [55], [56], [64], [65], [66], [67], [68], [69].
A common theme of the case studies included in this review is the use of a technique or approach to obtain unique insights that are difficult to acquire otherwise. Comprehensive mechanistic understanding of a particularly system usually requires a combination of complementary techniques because a specific technique, however powerful it may be, only reveals one or a few aspects of the process. A long-sought goal in the catalysis community is to have predictive computational models that efficiently survey a broad parameter space to identify promising materials or systems for more detailed experimental study. With more powerful hardware and efficient algorithms than were previously available, such models are being actively pursued. Meanwhile, accurate and efficient models must be calibrated and trained with reliable experimental data and appropriate catalyst structure information, especially in the case of semi-empirical methods. Thus, intimate interplays between computational and experimental investigations are needed, which will be reflected in the discussion of case studies as a crosscutting trend. Through discussions of specific techniques and reaction mechanisms in case studies, this contribution aims to elucidate general themes in the rational design of selective catalysts for biomass valorization, including: (1) designing multifunctional catalytic surfaces, with atomically mixed active sites mediating tandem reactions; (2) tuning surface site functionalities, by either modifying the composition/structure of the active sites or introducing strongly adsorbed species; and (3) tailoring the binding of targeted functional groups on the surface. As this review is focused on the technical advances and challenges of the design, synthesis, characterization and evaluation of catalytic systems, recent progress on the economic and process intensification aspects of biomass upgrading will not be covered in detail [59], [63], [68], [70], [71], [72], [73], [74], [75], [76], [77].
Section snippets
Spectroscopic techniques
Spectroscopic techniques, especially those conducted under in situ or operando conditions, can yield unparalleled molecular-level information regarding the interactions of reactants and intermediates with the catalyst surface [78], [79]. The compositional and structural complexity of both the catalyst and substrate in biomass reactions makes spectroscopic insights particularly helpful in establishing structure-activity relations [80], [81], [82], [83]. Spectroscopic techniques are generally
Computational techniques
Studies that incorporate computational modeling of surface phenomena have increasingly become an indispensable part of heterogeneous catalytic research in general and investigations of biomass-to-chemicals valorization in particular [175], [176], [177], [178]. Experimental techniques can provide snapshots of ensemble behavior and properties of a catalytic system in a top-down manner, but this is typically insufficient to construct a complete molecular-level mechanistic picture. Thus, proposed
Reaction kinetics
Kinetic analysis is an informative technique for probing catalytic reaction mechanisms in complex systems where obtaining detailed information from other techniques is challenging, such as the liquid- or multi-phase catalytic reactions common to biomass upgrading. Biomass-to-chemicals conversions are often conducted in liquid or multiphasic media with solvents due to the high boiling point and/or low decomposition temperature of biomass-derived platform molecules [3], [11], [282], [283].
Synthesis of catalysts with tailored composition and structure
Developing synthetic methods to prepare materials with controlled composition and structure has two vital roles in catalyst development: (1) realizing precise molecular arrangements based on catalyst design parameters; and (2) aiding in the establishment of reliable structure-activity relations. Recent developments in materials synthesis have enabled structural control at both the micro- and meso-scales, from molecular construction of surface composition and configuration to crystalline micro-
Perspective
In the last few decades, focus on catalytic biomass upgrading has led to significant improvements in selectivity for reactions converting multifunctional biomass-derived feedstocks to bulk and specialty chemicals. Case studies examined in this review highlight that in situ/operando spectroscopies, predictive computational modeling, rigorous kinetic investigations, and controlled materials synthesis have been the foundational techniques in the development of state of the art catalytic systems.
Acknowledgments
We gratefully acknowledge the support of the National Science Foundation (NSF), CBET, under grant number CBET-1437129. The authors declare no competing financial interest.
Brian Murphy received his B.S. in Chemical Engineering at the University of Virginia in 2013, and is currently a graduate student in the Department of Chemical & Biomolecular Engineering at University of Delaware.
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Brian Murphy received his B.S. in Chemical Engineering at the University of Virginia in 2013, and is currently a graduate student in the Department of Chemical & Biomolecular Engineering at University of Delaware.
Bingjun Xu received his B.S. in Chemistry at Fudan University in 2004 and his Ph.D. in Physical Chemistry from Harvard University in 2011 under the guidance of Profs. Friend and Madix. His Ph.D. thesis focused on oxidative coupling reactions on Au surfaces. Bingjun did his postdoctoral research with Prof. Davis at California Institute of Technology on the topic of developing a low-temperature thermochemical cycle. He is currently an Assistant Professor in the Department of Chemical & Biomolecular Engineering at University of Delaware. His research interests include heterogeneous catalysis, electrocatalysis and renewable energy.