Cover Art

The Catalytic Mechanics of Dynamic Surfaces: Stimulating Methods for Promoting Catalytic Resonance

ACS Catalysis Cover Art 2020, Dauenhauer, et al.

ACS Catal. 2020, 10, 21, 12666–12695

Publication Date:September 25, 2020

Transformational catalytic performance in rate and selectivity is obtainable through catalysts that change on the time scale of catalytic turnover frequency. In this work, dynamic catalysts are defined in the context and history of forced and passive dynamic chemical systems, with the classification of unique catalyst behaviors based on temporally relevant linear scaling parameters. The conditions leading to the catalytic rate and selectivity enhancement are described as modifying the local electronic or steric environment of the active site to independently accelerate sequential elementary steps of an overall catalytic cycle. These concepts are related to physical systems and devices that stimulate a catalyst using light, vibrations, strain, and electronic manipulations including electrocatalysis, back-gating of catalyst surfaces, and introduction of surface electric fields via solid electrolytes and ferroelectrics. These catalytic stimuli are then compared for the capability to improve catalysis across some of the most important chemical challenges for energy, materials, and sustainability.


Finned zeolite catalysts

Nature Materials Cover October 2020Nature Materials, volume 19, pages 1074–1080, (2020)

Dai, H., Shen, Y., Yang, T. et al. Finned zeolite catalysts. Nat. Mater. 19, 1074–1080 (2020).


There is growing evidence for the advantages of synthesizing nanosized zeolites with markedly reduced internal diffusion limitations for enhanced performances in catalysis and adsorption. Producing zeolite crystals with sizes less than 100 nm, however, is non-trivial, often requires the use of complex organics and typically results in a small product yield. Here we present an alternative, facile approach to enhance the mass-transport properties of zeolites by the epitaxial growth of fin-like protrusions on seed crystals. We validate this generalizable methodology on two common zeolites and confirm that fins are in crystallographic registry with the underlying seeds, and that secondary growth does not impede access to the micropores. Molecular modelling and time-resolved titration experiments of finned zeolites probe internal diffusion and reveal substantial improvements in mass transport, consistent with catalytic tests of a model reaction, which show that these structures behave as pseudo-nanocrystals with sizes commensurate to that of the fin. This approach could be extended to the rational synthesis of other zeolite and aluminosilicate materials.

Dehydra-decyclization of 2-methyltetrahydrofuran to pentadienes on boron-containing zeolites

Green Chemistry, Cover Art, 2020, Dauenhauer et alGreen Chemistry, 2020,22, 4147-4160

26 March 2020


1,3-Pentadiene (piperylene) is an important monomer in the manufacturing of adhesives, plastics, and resins. It can be derived from biomass by the tandem ring-opening and dehydration (dehydra-decyclization) of 2-methyltetrahydrofuran (2-MTHF), but competing reaction pathways and the formation of another isomer (1,4-pentadiene) have limited piperylene yields to <60%. In this report, using detailed kinetic measurements of 2-MTHF dehydra-decyclization on zeolites with disparate acidities (boro-, and alumino-silicates) and micropore environments (MFI, MWW, and BEA), weakly acidic borosilicates were shown to exhibit ca. 10–30% higher selectivity to dienes at about five-to-sixty times lower proton-normalized rates than aluminosilicates (453–573 K). Dehydra-decyclization site time yields (STYs) were invariant for aluminosilicates within the investigated frameworks, indicating the absence of pore-confinement influence. However, individual site-normalized reaction rates varied by almost an order of magnitude on borosilicates in the order MWW > MFI > BEA at a given temperature (523 K), indicating the non-identical nature of active sites in these weak solid acids. The diene distribution remained far from equilibrium and was tuned towards the desirable conjugated diene (1,3-pentadiene) by facile isomerization of 1,4-pentadiene. This tuning capability was facilitated by high bed residence times, as well as the smaller micropore sizes among the zeolite frameworks considered. The suppression of competing pathways, and promotion of 1,4-pentadiene isomerization events lead to a hitherto unreported ∼86% 1,3-pentadiene yield and an overall ∼89% combined linear C5 dienes’ yield at near quantitative (∼98%) 2-MTHF conversion on the borosilicate B-MWW, without a significant reduction in diene selectivities for at least 80 hours time-on-stream under low space velocity (0.85 g reactant per g cat. per h) and high temperature (658 K) conditions. Finally, starting with iso-conversion levels (ca. 21–26%) and using total turnover numbers (TONs) accrued over the entire catalyst lifetime as the stability criterion, borosilicates were demonstrated to be significantly more stable than aluminosilicates under reaction conditions (∼3–6× higher TONs).

Resonance-Promoted Formic Acid Oxidation via Dynamic Electrocatalytic Modulation

ACS Catalysis Cover Art - Abdelrahman, et al.ACS Catal. 2020, 10, 17, 9932–9942, 

Publication Date: July 13, 2020

It is a truth universally acknowledged that faster catalysts enable more efficient transformation of molecules to useful products and enhance the utilization of natural resources. However, the limit of static catalyst performance defined by the Sabatier principle has motivated a dynamic approach to catalyst design, whereby catalysts oscillate between varying energetic states. In this work, the concept of dynamic catalytic resonance was experimentally demonstrated via the electrocatalytic oxidation of formic acid over Pt. Oscillation of the electrodynamic potential between 0 and 0.8 V NHE via a square waveform at varying frequency (10–3 < f < 103 Hz) increased the turnover frequency to ∼20 s–1 at 100 Hz, over one order of magnitude (20×) faster than optimal potentiostatic conditions. We attribute the accelerated dynamic catalysis to nonfaradaic formic acid dehydration to surface-bound carbon monoxide at low potentials, followed by surface oxidation and desorption to carbon dioxide at high potentials.


Principles of Dynamic Heterogeneous Catalysis: Surface Resonance and Turnover Frequency Response

ACS Catalysis 2019 Cover Art

ACS Catal. 2019, 9, 8, 6929–6937, DOI:  10.1021/acscatal.9b01606

Acceleration of the catalytic transformation of molecules via heterogeneous materials occurs through design of active binding sites to optimally balance the requirements of all steps in a catalytic cycle. In accordance with the Sabatier principle, the characteristics of a single binding site are balanced between at least two transient phenomena, leading to maximum possible catalytic activity at a single, static condition (i.e., a “volcano curve” peak). In this work, a dynamic heterogeneous catalyst oscillating between two electronic states was evaluated via simulation, predicting catalytic activity as much as three-to-four orders of magnitude (1000–10 000) above the Sabatier maximum. Surface substrate binding energies were varied by a given amplitude (0.1 < ΔU < 3.0 eV) over a broad range of frequencies (10–4 < f < 1011 s–1) in square, sinusoidal, sawtooth, and triangular waveforms to characterize surface dynamics impact on average catalytic turnover frequency. Catalytic systems were shown to exhibit order-of-magnitude dynamic rate enhancement at “surface resonance” defined as the band of frequencies (e.g., 103–107 s–1) where the applied surface waveform kinetics were comparable to kinetics of individual microkinetic chemical reaction steps. Key dynamic performance parameters are discussed regarding industrial catalytic chemistries and implementation in physical dynamic systems operating above kilohertz frequencies.

A Universal Descriptor for the Entropy of Adsorbed Molecules in Confined Spaces

ACS Central Science"A Universal Descriptor for the Entropy of Adsorbed Molecules in Confined Spaces," Paul J. Dauenhauer, Omar Abdelrahman.  ACS Central Science.  2018, 4, 9, 1235-1243.  DOI:  10.1021/acscentsci.8b00419

Publication Date:  Sept. 7, 2018

Confinement of hydrocarbons in nanoscale pockets and pores provides tunable capability for controlling molecules in catalysts, sorbents, and membranes for reaction and separation applications. While computation of the enthalpic interactions of hydrocarbons in confined spaces has improved, understanding and predicting the entropy of confined molecules remains a challenge. Here we show, using a set of nine aluminosilicate zeolite frameworks with broad variation in pore and cavity structure, that the entropy of adsorption can be predicted as a linear combination of rotational and translational entropy. The extent of entropy lost upon adsorption is predicted using only a single material descriptor, the occupiable volume (Vocc). Predictive capability of confined molecular entropy permits an understanding of the relation with adsorption enthalpy, the ability to computationally screen microporous materials, and an understanding of the role of confinement on the kinetics of molecules in confined spaces.

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Activation of Cellulose via Cooperative Hydroxyl-Catalyzed Transglycosylation of Glycosidic Bonds

ACS Catalysis Cover 2019 MarchACS Catalysis 2019, 9, 3, 1943–1955


The thermal activation of cellulose by initial glycosidic bond cleavage determines the overall rate of conversion to organic products for energy applications. Here, the kinetics of ether scission by transglycosylation of β-1,4-glycosidic bonds was measured using the “pulse-heated analysis of solid reactions” (PHASR) method from 400 to 500 °C. Levoglucosan (LGA) formation from cellulose was temporally resolved over the full extent of conversion, which was interpreted via a coupled reactant–product evolution model to determine an apparent barrier of LGA formation of 27.9 kcal mol–1. In parallel, LGA formation from the glucose monomer of cellobiosan was measured at temperatures between 380 and 430 °C by isotopically labeling the 13C1 carbon; an apparent activation energy of LGA formation was measured as 26.9 ± 1.9 kcal mol–1. The unusually low activation barrier for LGA formation at lower temperature is in agreement with previous PHASR studies for cellulose breakdown and is indicative of catalytic rather than thermal C–O bond activation. A catalytic mechanism was proposed wherein vicinal hydroxyl groups from neighboring cellulose sheets promote transglycosidic C–O bond activation. First-principle density functional theory (DFT) calculations showed that these vicinal hydroxyl groups cooperatively act to create an environment that (a) stabilizes charged transition states and (b) aids in proton transfer, thus leading to reduced activation barriers for transglycosylation. Models incorporating intrasheet H bonding of cellulose were also used to establish their influence on kinetics. The calculated apparent barrier (29.5 kcal mol–1) agreed well with the experimental apparent activation energy (26.9 ± 1.9 kcal mol–1) and establishes the dominant mode for cellulose activation and subsequent levoglucosan formation at lower temperatures (<467 °C) as site-specific, vicinal hydroxyl-catalyzed transglycosylation.


Micro-Ratcheted Surfaces for a Heat Engine Biomass Conveyor

Energy & Environmental ScienceChristoph Krumm, S. Maduskar, A.D. Paulsen, A. Anderson, N. Barberio, J. Damen, C. Beach, Satish Kumar, Paul Dauenhauer, Energy & Environmental Science 2016, 9, 1645-1649.

DOI: 10.1039/C6EE00519E

Publication Date:  March 7, 2016

Cellulosic particles on surfaces consisting of microstructured, asymmetric ratchets (100 by 400 μm) were observed to spontaneously move orthogonal to ratchet wells above the cellulose reactive Leidenfrost temperature (>750 °C). Evaluation of the accelerating particles supported the mechanism of propelling viscous forces (50–200 nN) from rectified pyrolysis vapors, thus providing the first example of biomass conveyors with no moving parts driven by high temperature for biofuel reactors.

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Millisecond Pulsed Films Unify the Mechanisms of Cellulose Fragmentation

Chemistry of MaterialsChristoph Krumm, J. Pfaendtner, Paul Dauenhauer, Chemistry of Materials 2016, 28(9), 3108.

DOI: 10.1021/acs.chemmater.6b00580

Publication Date (Web): March 4, 2016

The mechanism of crystalline cellulose fragmentation has been debated between classical models proposing end-chain or intrachain scission to form short-chain (molten) anhydro-oligomer mixtures and volatile organic compounds. Models developed over the last few decades suggest global kinetics consistent with either mechanism, but validation of the chain-scission mechanism via measured reaction rates of cellulose has remained elusive. To resolve these differences, we introduce a new thermal-pulsing reactor four orders of magnitude faster than conventional thermogravimetic analysis (106 vs 102 °C/min) to measure the millisecond-resolved evolution of cellulose and its volatile products at 400–550 °C. By comparison of cellulose conversion and furan product formation kinetics, both mechanisms are shown to occur with the transition from chain-end scission to intrachain scission above 467 °C concurrent with liquid formation comprised of short-chain cellulose fragments.

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Highly Efficient Mechano-Catalytic Depolymerization of Crystalline Cellulose by Formation of Branched Glucan Chains

Green ChemistryP. Dornath, H.J. Cho, A.D. Paulsen, P.J. Dauenhauer, W. Fan, Green Cheemistry 2015, 15, 440-447.

DOI: 10.1039/C4GC02187H

Publication Date:  November 28, 2014

Selective hydrolysis of cellulose into glucose is a critical step for producing value-added chemicals and materials from lignocellulosic biomass. In this study, we found that co-impregnation of crystalline cellulose with sulfuric acid and glucose can greatly reduce the time needed for ball milling compared with adding acid alone. The enhanced reaction time coincides with the rapid formation of branched α(1→6) glycosidic bonds, which have been shown to increase water solubility of β(1→4) glucan oligomers. Co-impregnation of glucose was crucial for the rapid formation of the α(1→6) branches, after which a carbon-based catalyst can rapidly hydrolyze the water-soluble glucan oligomers to 91.2% glucose yield faster than conventional approaches.

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Tuning Cellulose Pyrolysis Chemistry: Selective Decarbonylation via Catalyst-Impregnated Pyrolysis

Catalysis Science & TechnologyM.S. Mettler, A.D. Paulsen, D.G. Vlachos, P.J. Dauenhauer, Catalysis Science and Technology 2014, 4, 3822-3825.

DOI: 10.1039/C4CY00676C

Publication Date:  June 6, 2014

Widespread adoption of biomass pyrolysis for lignocellulosic biofuels is largely hindered by a lack of economical means to stabilize the bio-oil (or pyrolysis oil) product. In this work, impregnation of supported metal catalysts provides a new approach to selectively decarbonylate primary pyrolysis products within intermediate cellulose liquid to targeted gasoline-like molecules with enhanced energy content and stability. Selective deoxygenation of hydroxy-methylfurfural (HMF) and furfural (F) to 88% yield of stable furans occurred over carbon-supported Pd, with negligible loss in overall bio-oil yield or furanic content.

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Fast Pyrolysis of Wood for Biofuels: Spatiotemporally-Resolved Diffuse Reflectance in situ Spectroscopy of Particles (STR-DRiSP)

ChemSusChemAlex D. Paulsen, Blake, R. Hough, C. Luke Williams, Andrew R. Teixeira, Daniel T. Schwartz, J. Pfaendtner, P.J. Dauenhauer, ChemSusChem 2014, 7(3), 765-776.

DOI:  10.1002/cssc.201301056

Publication Date:  February 20, 2014

Fast pyrolysis of woody biomass is a promising process capable of producing renewable transportation fuels to replace gasoline, diesel, and chemicals currently derived from nonrenewable sources. However, biomass pyrolysis is not yet economically viable and requires significant optimization before it can contribute to the existing oil-based transportation system. One method of optimization uses detailed kinetic models for predicting the products of biomass fast pyrolysis, which serve as the basis for the design of pyrolysis reactors capable of producing the highest value products. The goal of this work is to improve upon current pyrolysis models, usually derived from experiments with low heating rates and temperatures, by developing models that account for both transport and pyrolysis decomposition kinetics at high heating rates and high temperatures (>400 °C). A new experimental technique is proposed herein: spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP), which is capable of measuring biomass composition during fast pyrolysis with high spatial (10 μm) and temporal (1 ms) resolution. Compositional data were compared with a comprehensive 2D single-particle model, which incorporated a multistep, semiglobal reaction mechanism, prescribed particle shrinkage, and thermophysical properties that varied with temperature, composition, and orientation. The STR-DRiSP technique can be used to determine the transport-limited kinetic parameters of biomass decomposition for a wide variety of biomass feedstocks.

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Microexplosions in the upgrading of biomass-derived pyrolysis oils and the effects of simple fuel processing

ACS Sustainable Chemistry & EngineeringA.R. Teixeira, R.J. Hermann, J.S. Kruger, W.J. Suszynski, L.D. Schmidt, D.P. Schmidt, P.J. Dauenhauer, ACS Sustainable Chemistry & Engineering 2013, 1, 341-348.

DOI:  10.1021/sc300148b

Publication Date:  January 18, 2013

The development of biofuels produced from biomass-derived pyrolysis oils (bio-oil) requires a deeper understanding of the bio-oil vaporization required for catalytic hydrodeoxygenation, reforming and combustion processes. Through the use of high-speed photography, bio-oil droplets on a 500 °C alumina disk in nitrogen gas were observed to undergo violent microexplosions capable of rapidly dispersing the fuel. High speed photography of the entire droplet lifetime was used to determine explosion times, frequency and evaporation rates of the bio-oil samples that have been preprocessed by filtering or addition of methanol. Filtration of the oil prior to evaporation significantly reduced the fraction of droplets that explode from 50% to below 5%. Addition of methanol to bio-oil led to uniform vaporization while also increasing the fraction of droplets that exploded. Experiments support the necessity of dissolvable solids for the formation of a volatile core and heavy shell which ruptures and rapidly expands to produce a violent bio-oil microexplosion.

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Revealing pyrolysis chemistry for biofuels production: Conversion of cellulose to furans and small oxygenates

Energy & Environmental ScienceM.S. Mettler, S.H. Mushrif, A.D. Paulsen, A.D. Javadekar, D.G. Vlachos, P.J. Dauenhauer, Energy and Environmental Science 2012, 5, 5414.

DOI: 10.1039/C2EE21305B

Published:  Nov. 21, 2011

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify α-cyclodextrin as an appropriate small-molecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

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The Role of Water in the Aqueous and Vapor-Phase Adsorption of Oxygenated Aromatics on Pt and Pd

Journal of Computational ChemistryJ. Yang, P.J. Dauenhauer, A. Ramasubramaniam, Journal of Computational Chemistry 2012, 34(1), 60-66.

DOI: 10.1002/jcc.23107

Publication Date: September 1, 2012

Catalytic processing of biomass-derived oxygenates to valuable chemical products will contribute to a sustainable future. To provide insight into the conversion of processed sugars and lignin monomers, we present density functional theory studies of adsorption of phloroglucinol, a potentially valuable biomass derivative, on Pt(111) and Pd(111) surfaces. A comprehensive study of adsorption geometries and associated energies indicates that the bridge site is the most preferred adsorption site for phloroglucinol, with binding energies in the range of 2–3 eV in the vapor phase. Adsorption of phloroglucinol on these metal surfaces occurs via hybridization between the carbon pz orbitals and the metal d and dyz orbitals. With explicit solvent, hydrogen bonds are formed between phloroglucinol and water molecules thereby decreasing binding of phloroglucinol to the metal surfaces relative to the vapor phase by 20–25%. Based on these results, we conclude that solvent effects can significantly impact adsorption of oxygenated aromatic compounds derived from biomass and influence catalytic hydrogenation and hydrodeoxygenation reactions as well. © 2012 Wiley Periodicals, Inc.

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Reactive Boiling of Cellulose for Integrated Catalysis through a Liquid Intermediate

Green ChemistryP.J. Dauenhauer, J.L. Colby, C.M. Balonek, W.J. Wieslaw, L.D. Schmidt, Green Chemistry 2009, 11, 1555.

DOI:  10.1039/B915068B

Publication Date:  August 14, 2009

Advanced biomass processing technology integrating fast pyrolysis and inorganic catalysis requires an improved understanding of the thermal decomposition of biopolymers in contact with porous catalytic surfaces. High speed photography (1000 frames per second) reveals that direct impingement of microcrystalline cellulose particles (300 μm) with rhodium-based reforming catalysts at high temperature (700 °C) produces an intermediate liquid phase that reactively boils to vapors. The intermediate liquid maintains contact with the porous surface permitting high heat transfer (MW m−2) generating an internal thermal gradient visible within the particle as a propagating wave of solid to liquid conversion. Complete conversion to liquid yields a fluid droplet on the catalyst surface exhibiting a linear decrease in droplet volume with time leaving behind a clean surface absent of solid residue (char). Under specific interfacial conditions, conversion with large cellulosic particles on the length-scale of wood chips (millimeters) occurs continuously as generated liquid and vapors are pushed into the porous surface.

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Millisecond Reforming of Solid Biomass for Sustainable Fuels

Angewandte ChemieP.J. Dauenhauer, B.J. Dreyer, N.J. Degenstein, L.D. Schmidt Angewandte Chemie 2007, 119, 5968-5971.

DOI: 10.1002/ange.200701238

Publication Date:  July 3, 2007

Koksfrei: Thermische Zersetzung und katalytische partielle Oxidation wurden zu einer effektiven Methode für die Umwandlung von fester Biomasse wie Cellulose in Synthesegas gekoppelt (das Foto zeigt die heiße Oberfläche eines Rh-Katalysators). Der Prozess ist schnell, und er verläuft ohne die Bildung von Koks, der Katalysatoroberflächen desaktivieren und Oberflächenreaktionen blockieren würde.

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