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Computational Support for Phillips Catalyst Initiation via Cr-C Bond Homolysis in a Chromacyclopentane Site Anthony Fong, Craig Vandervelden, Susannah L Scott, and Baron Peters ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03724 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Computational Support for Phillips Catalyst Initiation via Cr-C Bond Homolysis in a Chromacyclopentane Site

Anthony Fonga, Craig Vanderveldena, Susannah L. Scotta,b,*, Baron Petersa,b,** a

b

Department of Chemical Engineering, University of California, Santa Barbara CA, USA 93106-5080

Department of Chemistry & Biochemistry, University of California, Santa Barbara CA, USA 93106-9510

* Corresponding author. Tel.: +1 805 893-5606, [email protected] ** Corresponding author. Tel.: +1 805 284-8293, [email protected]

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Abstract

Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from CrII sites exposed to ethylene. Spin-crossing in an abundant quintet bis(ethylene)CrII site leads to cycloaddition to form a chromacyclopentane site. One Cr-C bond then homolyzes to generate a tethered n-butyl radical: [Cr(CH2)3CH2•]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCrIII sites capable of Cossee-Arlman polymerization. The overall computed barrier for this initiation process is 132 kJ/mol, comparable to the 120 kJ/mol we estimated from reported initiation times in industrial reactors. Poisson statistics suggest that this mechanism could activate approximately 35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm2. Pairwise Cr grafting, amplification by complementary initiation reactions, or the creation of dangling bonds that form as the silica support fractures, might explain the apparent increase in per-site activity at lower Cr loadings.

Keywords Phillips catalyst, polymerization, initiation, density functional theory, homolysis

TOC Graphic

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The Phillips catalyst (Cr/SiO2) is used to produce about 30 million tons of high density polyethylene annually. Despite its long history and continuing industrial importance,1-2 the mechanism by which the active sites are initially formed remains incompletely understood. In industrial practice,2 CrVI sites are reduced by either CO3 or ethylene4 to (≡SiO)2CrII sites. Upon exposure to ethylene at ca. 100 °C, the active sites (which are widely believed to be (≡SiO)2CrIIIalkyls)2 form spontaneously without the assistance of an alkylating co-catalyst. For typical industrial catalysts, estimates of the fraction of Cr sites that is eventually activated vary from 7 to 32 %.2 According to McDaniel’s authoritative review of the commercial Phillips catalyst: “The CO-reduced catalyst, much like its hexavalent parent, polymerizes ethylene under commercial conditions, producing similar polymer”.2 Furthermore, McDaniel shows that activation profiles of Phillips catalysts reduced by either CO or ethylene are very similar,2-3 strongly suggesting that they follow the same activation mechanism with the same kinetic barrier. Thus while the distribution of Cr coordination numbers and local environments may differ in Phillips catalysts subjected to different treatments,5-7 and may be manifested in their average spectroscopic properties, there is no evidence that such differences influence reactivity. Based on published observations that approx. one hour is needed to achieve full catalytic activity from either reduced catalyst in an industrial reactor at 373 K,2 and using transition state theory, we estimated the overall barrier for initiation to be approx. 120 kJ/mol.8 Many mechanistic proposals and active site models for the Phillips catalyst have been put forth, some despite ambiguous evidence or even considerable counter-evidence (as discussed extensively by McDaniel).2 Peters et al. emphasized the need to evaluate computational results more critically for consistency, offering the following three guidelines:9 3 ACS Paragon Plus Environment

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(1)

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The complete catalytic cycle should be considered (initiation, propagation, and termination). A viable mechanism and active site model should distinguish between the preliminary reduction of CrVI, slow initiation of the first chain, and chain termination with rapid re-initiation of subsequent chains. In the words of McDaniel,2 “…termination of a typical chain must not leave the site in the virgin state, requiring full re-alkylation.”

(2)

Computational studies should distinguish between plausible and implausible departures from experiment.9 Computationally-predicted polymerization rates and polymer molecular weights are not expected to have “chemical” accuracy, particularly for heterogeneous catalysts whose active site structures are not well-defined. However, benchmark DFT calculations10 suggest that discrepancies in computed rates that exceed factors of ca. 105 are likely due to problems in the active site model and/or proposed mechanism, rather than to inherent inaccuracies in DFT.

(3)

There should be a viable pathway (based on experimental synthesis protocols) to obtain the sites proposed in computational models.11 This point is especially important in mechanistic proposals that invoke highly strained sites/unusual metal coordination environments,12 or support defects.13 Certainly, site variability can influence the properties of metal sites grafted onto amorphous supports.14-15 However, proposals that invoke unusual sites must also demonstrate sufficient site abundance to explain the observed catalyst activity and/or active site counting results. Computational proposals which invoke undetectably rare sites must also quantify their abundance in order to make falsifiable predictions about their kinetic viability.

Recently, Gierada et al.13,

16

examined a proposed mechanism in which hydroxyl ligands

associated with rare (≡SiO)2CrIII-OH sites abstract a proton from ethylene to give 4 ACS Paragon Plus Environment

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(≡SiO)2CrIII(OH2)(vinyl) sites. Since the partial pressure of water is extremely low under polymerization conditions, the subsequent desorption of H2O from such sites would generate polymerization-active (≡SiO)2CrIII-vinyl sites. However, this proposed initiation mechanism is not supported by any experimental evidence for the existence of the putative (≡SiO)2CrIII-OH sites;16 instead, the polymerization activity of the catalyst is inversely correlated with hydroxyl content of the catalyst.2 In another recent paper, Floryan et al.12 revisited a proposed initiation mechanism involving highly energetically unfavorable proton transfer from ethylene to a bridging oxygen atom on a purported (≡SiO)3CrIII site. The authors examined several versions of this site computationally in order to show that very highly strained sites can spontaneously deprotonate ethylene and thereby initiate polymerization. However, the most highly strained sites also have markedly different energetics than the previously proposed site for the same mechanism (e.g., their proton transfer energies differ by ca. 150 kJ/mol, compared to the original study).17 In addition, the strained sites were obtained by computational alchemy,18 i.e., hand replacement of an ≡SiOH site by a CrIII ion.12 In this respect, the model violates guideline (3) above: the authors do not show that their grafting strategy using a molecular Cr(III) complex and mild calcination could generate such highly strained sites on silica, nor do they predict the abundance of the model sites. Furthermore, their computational findings were reported to be based in part on incompletely optimized structures, with multiple imaginary frequencies.12 Our recent studies,8, 19 as well as earlier calculations by Espelid and Børve,20-21 confirmed that alkylCrIII sites interacting with silica via two anionic silanolate ligands (and possibly additional neutral siloxane ligands) are viable propagating sites for olefin polymerization, but the mechanism by which they could form has not yet been fully resolved. Using a simple cluster 5 ACS Paragon Plus Environment

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model (5I in Figure 1),8 our previous work examined several hypothesized initiation pathways that begin with reduced (≡SiO)2CrII sites. Nearly all were excluded on the basis of very high activation barriers and/or rapid termination after initiation. Ethylene disproportionation to give (≡SiO)2CrIV(CH2CH3)(CH=CH2) sites followed by ethyl radical extrusion was found to be consistent with the observed formation of organic radicals during initiation.22-23 Thus ab initio calculations8 as well as experimental observations22-23 and precedents from molecular chromium chemistry24-25 all suggest a probable role for homolysis pathways in the initiation mechanism. However, our previous computational investigation using simple, rigid cluster models predicted that a two-step mechanism involving activation of coordinated ethylene to give an (ethyl)(vinyl)Cr(IV) site followed by homolytic Cr-C bond cleavage would require hemilabile coordination of siloxane ligands, and no suitable arrangement of ligands was found to allow both steps to proceed at appreciable rates.8 In this work, we explore a modified Cr-C bond homolysis mechanism, beginning from readily accessible chromacyclopentane sites. Of course, different homolysis pathways may be favored at various sites in the heterogeneous catalyst due to differences in the local Cr geometry. Our calculations begin with the same cluster model used in our previous works:8 a small chromasiloxane ring with fixed OH capping groups. The small model assumes only minimal information about the local environment around the Cr site. Larger models rely on more detailed assumptions and, for the Phillips catalyst, there is evidence in many cases that they provide similar results.13 Also following our previous work, intermediates and transition states were optimized using the ωB97X-D functional26-27 with a def2-TZVP basis for Cr28-30 and a TZVP basis for all other atoms.31 All free energies include translational, rotational, and vibrational


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contributions. Further details on the cluster model and computational methods can be found in the Supporting Information. The most stable form of the bis(ethylene)CrII complex has quintet spin multiplicity, and is denoted 5III in Figure 1. Its preferred reaction with ethylene is cycloaddition to give a chromacyclopentane. We and others previously reported computational evidence for facile chromacyclopentane formation in Phillips catalyst models by ethylene oxidative addition,8, 21 and many chromacycles have been described in molecular chromium chemistry,32-34 particularly in homogeneous

catalysts

for ethylene oligomerization.35-36

Spectroscopic evidence for

chromacyclopentane formation in the Phillips catalyst has also been reported,37-38 although this intermediate is considered unlikely to be involved in the polymerization mechanism.19,

21, 39

Direct conversion of 5III to the quintet metallacycle 5IV requires a free energy barrier of 177 kJ/mol, while a spin crossing route to the triplet metallacycle 3IV requires only 81 kJ/mol (i.e., 57+24 kJ/mol).8 Non-adiabatic rate estimates from Landau-Zener theory suggest that the spin crossing step (67 kJ/mol) and the adiabatic barrier crossing step (81 kJ/mol) allow both rapid formation of the metallacycle ring IV and its cycloreversion. These paths, shown in Figure 1, include the CrII and CrIV starting points for each of the mechanisms discussed below.


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Figure

1.

Stationary

points

and

minimum

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energy

crossing

points

(MECPs)

in

chromacyclopentane formation. The preferred path is indicated in red (geometries shown in Supporting Information Figure S1). Dangling Si- bonds were capped with OH groups. Free energies (kJ/mol) for all species are given relative to the bis(ethylene)CrII complex, 5III, at 373 K. Numbers next to arrows represent free energies of transition states. Quintet spin species: 5IV, chromacyclopentane;

Triplet

spin

species:

3

III,

bis(ethylene)CrII

complex;

3

IV,

chromacyclopentane.


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The chromacyclopentane site IV can undergo direct homolysis of a Cr-C bond, resulting in a tethered n-butyl radical, V. Homolysis can occur from either 3IV or 5IV, with the latter requiring a spin-crossing through MECP-IV. In either case, the overall effective free energy to form the tethered radical 5V is 123 kJ/mol relative to the bis(ethylene)CrII resting state, 5III, Figure 2. Once created, 5V could react in one of four ways: 1. Ethylene could coordinate and insert into the remaining Cr-C bond, with an additional barrier of 71 kJ/mol (Supporting Information Figure S2), thereby extending the length of the tethered radical chain. However, the high overall barrier (homolysis+insertion), at 194 kJ/mol, makes this pathway unlikely. 2. Gas phase (i.e., uncoordinated) ethylene could react with the dangling n-butyl radical chain end to create an n-hexyl radical chain. We estimate that this pathway requires an additional 80 kJ/mol, based on the barrier computed for reactions between gas phase alkyl radicals and olefins (Supporting Information, Figure S3). This pathway is consequently also unlikely, because of its high overall barrier of 204 kJ/mol. 3. The tethered n-butyl radical in site 5V could reattach to the Cr center that generated it, with no additional barrier. Reattachment is therefore the most likely fate of the tethered n-butyl radical on most sites. 4. If 5V is sufficiently close to another CrII with an open coordination site, the dangling radical could react with that site to form an n-butyl bridge, shown as 3BB (Figure 2). Based on our previous work,19 both of the resulting alkylCrIII sites would be capable of facile ethylene polymerization. Similarly, Espelid and Børve showed that CrIII-O-CrIII sites connected by an n-butyl chain, can incorporate ethylene at both Cr-C bonds.20 As


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shown below, the computed overall barrier for this initiation pathway is comparable to the estimated initiation barrier for the industrial Phillips catalyst. The reactions of the organic radical in 5V with a neighboring CrII site lacking ethylene ligands, or with a mono(ethylene)CrII site (5II), have no barrier. However, our model predicts that most CrII sites exist in the resting bis(ethylene) state (5III) under reaction conditions (i.e., high ethylene pressure). An additional 9 kJ/mol is needed to create the 5II site from 5III at PC2H4 = 1 atm (Figure 2). Thus the overall free energy barrier to create the n-butyl-bridged alkylCrIII sites from two neighboring 5III sites is 132 kJ/mol.


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Figure 2. A tethered n-butyl radical attached to a CrIII site can attack a neighboring CrII site colocated within a distance l to create a pair of CrIII sites connected by an n-butyl chain (see the full free energy path in the Supporting Information Figure S6). Numbers below structures represent free energies; the combined free energies of 5V and 5II provide an estimate of the overall initiation barrier.


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Phillips catalyst initiation mechanisms that involve the formation of bridging alkyl chains (and specifically n-butyl bridges) between Cr sites have been proposed previously. For example, Rebenstorf et al. proposed direct cycloaddition of two ethylene molecules to form an n-butyl bridge across a [CrII-O-CrII] site.40 Espelid and Børve proposed an n-butyl-bridged site could form from a chromacycle connected via an oxo bridge to a CrII site.20 Although they attempted a coordinate-driving calculation, they were not successful in determining the barrier. Our work establishes 132 kJ/mol as a computational upper bound on the free energy barrier to form nbutyl-bridged CrIII sites. The barrier may be further lowered by coordination of nearby siloxanes from the silica surface to the chromacyclopentane site,8 or by sharing of electron density from the dangling n-butyl radical with the Cr orbitals of the neighboring CrII site as the radical migrates from one Cr atom to the other. To further explore the plausibility of initiation by formation of an n-butyl chain between neighboring sites, we estimated the fraction of sites that could be activated by chromacyclopentane homolysis. To form the 3BB site, the two Cr atoms can be no further apart than the distance l. Assuming that all Cr sites are randomly distributed at a surface Cr density σCr [Cr atoms per surface area], the average number of Cr ions within bonding range of the n-butyl chain is given by the Poisson distribution:41

p( m) = exp(−λ)λm / m!

(1)

with Poisson parameter λ = π l 2σ Cr . For industrial Phillips catalysts, a typical loading σCr is 0.4 Cr/nm2 (corresponding to 0.6-1.0 wt% Cr, depending on the silica surface area).2 Based on eq 1, and the ideal distance between Cr atoms in the bridged CrIII cluster of ca. l = 5.8 Å (Figures 2 and S5), we estimate λ ≈ 0.42 . Accordingly, eq 1 predicts that 66% of Cr sites are isolated with 12 ACS Paragon Plus Environment

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zero nearby neighbors, 27% of Cr sites have a single neighbor within distance l, and about 7% of Cr sites have two or more neighbors within distance l. Chromacyclopentane homolysis should be able to activate all of the sites with a single neighbor and some sites with multiple neighbors, i.e. between 27% and 34% of sites. Thus the tethered homolysis mechanism could fully account for the fraction of active sites in the industrial catalyst, which has been estimated to be in the range 7 - 32 %.2 Finally, we developed a simple kinetic model to estimate the time scale for initiation by chromacyclopentane homolysis. Initially, all sites are dormant (D), but an approximate fraction λexp[-λ] of sites (those paired with a single neighbor at a distance l or less, e.g., 27% for a typical industrial catalyst) can become active (A). A full derivation of the kinetic model is provided in the Supporting Information. Here we summarize the main results. According to our DFT calculations, the fraction of activated sites θ A as a function of time t after exposure of reduced CrII/SiO2 sites to ethylene is

θ A = λ exp[−λ ](1 − exp[−t / τ ind ])

(2)

where the induction time τind depends on the overall free energy to reach the approximate transition state configuration, as depicted in Figure 2. Specifically, activation requires conversion of two 5III sites (the most abundant dormant state) into one site 5V, one site 5II, and one gas phase ethylene molecule. The predicted induction time is

(

τ ind = 1 θ

5

θ

IV| D

5

II| D

k 5 IV→5 V

)

(3)


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where θ 5 IV |D and θ 5 II|D are fractions of the dormant site population in states 5IV and 5II, respectively. The rate constant k

5

IV → 5 V

is the frequency of Cr-C bond homolysis in state 5IV.

Inserting the computed free energies and using transition state theory gives the expression

k T



τ ind = PC 2 H 4  B e − (132 kJ / mol )/ k T   h  B

(4)

which predicts an induction time of ca. 100 hours at 373 K and PC2H4 = 1 atm. This estimate is plausible, compared to the observed induction time of ca. 1 hour,2 and given the current limitations of DFT. The dependence on ethylene pressure emerges from the pre-equilibrium between sites 5III and 5II.

Figure 3. Comparison of (a) activity profiles of catalysts pre-reduced by CO (red) or reduced in situ by ethylene (blue), taken from data reported by McDaniel.2 Exponential rise curves are

shown to aid in comparing the data to the simulated kinetic profile, and (b) simulated kinetic profile based on the mechanism of initiation proposed in this work. The fraction of sites that can be activated within the induction time is comparable to active site counting experiments for commercial catalysts.2 14 ACS Paragon Plus Environment

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Our bichromium initiation mechanism predicts that fewer sites should activate at lower Cr loadings. However, the average per-site polymerization activity in Phillips catalysts increases as the Cr loading decreases, down to 0.01 Cr/nm2.2, 42 Most, but not all,20, 40, 43-45 researchers have interpreted this trend to mean that initiation does not involve more than one Cr site. In some studies, the apparent increase of activity at low loading may be due in part to formation of extended Cr2O3 domains at high loadings. Even at low loadings, pairing might result from associations between Cr sites. Our Poisson analysis does not account for the presence of associated sites and/or extended Cr2O3 domains. Alternative explanations for the increased activity per site at low loadings might involve additional initiation pathways that amplify the number of active sites. One such mechanism we considered is chromacyclopentane ring expansion prior to homolysis, which could extend the reach of the radical-terminated chain to more distant CrII sites. This mechanism requires a significant additional barrier beyond the 132 kJ/mol for the required steps in Figure 2, as shown in the Supporting Information (Figure S9). Thus we conclude that ring expansion prior to homolysis is unimportant for initiation, like ethylene insertion into the Cr-C bond of the n-butyl radical, and coupling of the dangling n-alkyl radical with ethylene (see above). A potentially viable way to amplify the number of Cr sites that can be initiated by the tethered homolysis mechanism involves the dangling bonds (≡Si• and/or ≡SiO• radicals) present on silica fracture surfaces. Such sites are likely to be created as a result of stresses caused by early stages of polymer production in the silica pores.46 The forces induced by polymer formation (see Supporting Information) can fracture pores with widths down to ca. 15 nm in silica,47 and these fractures might lead to increased activity48-49 by abstracting hydrogen atoms from ethylene or from polyethylene (see Supporting Information, Figure S10) to create alkyl and/or vinyl 15 ACS Paragon Plus Environment

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radicals. These radicals would readily interact with CrII sites that have not yet been activated, converting them to polymerization-ready alkylCrIII sites. As yet, this possibility is speculative, and similar catalysts on different support materials can initiate without fracturing.2 However, the possible amplification of the active site population as the support fractures should be amenable to experimental50-51 and computational52 study. In conclusion, a viable route to alkylCrIII sites that are capable of ethylene polymerization was identified using density functional theory calculations and small cluster models for the active sites of the Phillips catalyst. According to our calculations, bis(ethylene)CrII sites readily form chromacyclopentane sites, which undergo homolysis to create tethered butyl radicals, and these radicals can react with neighboring CrII sites. The overall barrier for this initiation mechanism is ca. 132 kJ/mol, in approximate agreement with the experimental barrier estimated from the observed induction time. The tethered homolysis mechanism could cause the activation of about 35% of the Cr sites present at a typical Cr loading of 1.0 wt%, assuming the Cr sites are randomly distributed on the silica surface. We emphasize that questions about the details of the initiation mechanism remain. Our previous work proposed that hemilabile siloxane ligands at some CrII sites may decoordinate to allow oxidative addition of ethylene to give (ethyl)(vinyl)CrIV sites, then recoordinate to facilitate Cr-C bond homolysis to give vinylCrIII sites.8 That two-step mechanism is consistent with several key experimental observations. If a suitable ligand coordination environment were found, it might also explain the low temperature initiation of polymerization by an unspecified number of sites reported in academic studies.22, 37 However, it is not necessary to invoke such sites in order to fully account for activation profiles recorded under industrial conditions (Figure 3).2-3 Without hemilability of the siloxanes, the computed barrier always exceeds 200 kJ/mol for 16 ACS Paragon Plus Environment

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one of the two steps. This work proposes an alternative initiation step involving Cr-C bond homolysis in a chroma(IV)cyclopentane ring near a CrII site. This alternative mechanism does not require a change in Cr coordination number, and appears to be viable without invoking any unusual Cr coordination environments, leading to much faster rates due to its lower computed barrier. However, the need for adjacent Cr sites means that it does not explain the higher per-site activity at lower Cr loadings.42 Acknowledgements AF, SLS, and BP were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under the Catalysis Science Initiative (DE-FG-02-03ER15467). CV was supported by the National Science Foundation, award no. CBET-1605867. Supporting Information. Cartesian coordinates, absolute energies, number of imaginary frequencies, spin contamination, derivation of initiation rate, and analysis of alternative pathways. References 1.

Hogan, J. P.; Banks, R. L. Polymers and Production Thereof. US Patent 2,825,721,

March 4, 1958. 2.

McDaniel, M. P. Adv. Catal. 2010, 53, 123-606.

3.

Merryfield, R.; McDaniel, M.; Parks, G. J. Catal. 1982, 77, 348-359.

4.

Baker, L. M.; Carrick, W. L. J. Org. Chem. 1968, 33, 616-618.

5.

Demmelmaier, C. A.; White, R. E.; van Bokhoven, J. A.; Scott, S. L. J. Catal. 2009, 262,

44-56. 17 ACS Paragon Plus Environment

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6.

Page 18 of 21

Zhong, L.; Lee, M.-Y.; Liu, Z.; Wanglee, Y.-J.; Liu, B.; Scott, S. L. J. Catal. 2012, 293,

1-12. 7.

Barzan, C.; Piovano, A.; Braglia, L.; Martino, G. A.; Lamberti, C.; Bordiga, S.; Groppo,

E. J. Am. Chem. Soc. 2017, 139, 17064-17073. 8.

Fong, A.; Peters, B.; Scott, S. L. ACS Catal. 2016, 6, 6073-6085.

9.

Peters, B.; Scott, S. L.; Fong, A.; Wang, Y.; Stiegman, A. E. Proc. Natl. Acad. Sci. U. S.

A. 2015, 112, E4160–E4161.

10.

Harvey, J. N. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2006, 102, 203-226.

11.

Peters, B.; Scott, S. L. J. Chem. Phys. 2015, 142, 104708.

12.

Floryan, L.; Borosy, A. P.; Nunez-Zarur, F.; Comas-Vives, A.; Coperet, C. J. Catal.

2017, 346, 50-56. 13.

Gierada, M.; Handzlik, J. J. Catal. 2017, 352, 314-328.

14.

Demmelmaier, C. A.; White, R. E.; van Bokhoven, J. A.; Scott, S. L. J. Catal. 2009, 262,

44-56. 15.

Goldsmith, B. R.; Fong, A.; Peters, B. Understanding Reactivity with Reduced Potential

Energy Landscapes: Recent Advances and New Directions. In Reaction Rate Constant Computations: Theories and Applications, Han, K., Ed. RSC Publishing: Cambridge, 2013; pp

213-22. 16.

Chakrabarti, A.; Gierada, M.; Handzlik, J.; Wachs, I. E. Top. Catal. 2016, 59, 725-739.

17.

Delley, M. F.; Núñez-Zarur, F.; Conley, M. P.; Comas-Vives, A.; Siddiqi, G.; Norsic, S.;

Monteil, V.; Safonova, O. V.; Copéret, C. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 1162411629. 18.

Straatsma, T. P.; McCammon, J. A. Annu. Rev. Phys. Chem. 1992, 43, 407-35. 18 ACS Paragon Plus Environment

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19.

Fong, A.; Yuan, Y.; Ivry, S. L.; Scott, S. L.; Peters, B. ACS Catal. 2015, 5, 3360-3374.

20.

Espelid, Ø.; Børve, K. J. J. Catal. 2002, 206, 331-338.

21.

Espelid, Ø.; Børve, K. J. J. Catal. 2000, 195, 125-139.

22.

Brown, C. L., A.; Tao, Y.; Peek, N.; Crosswhite, M.; Mileham, M.; Krzystek, J.; Achey,

R.; Fu, R.; Bindra, J.; Polinski, M.; Wang, Y.; van de Burgt, L.; Jeffcoat, D.; Profeta, S.; Stiegman, A..; Scott, S. ACS Catal. 2017, 7, 7442-7455. 23.

Pan, Q.; Li, L.; Shaikhutdinov, S.; Freund, H.-J. J. Catal. 2018, 357, 12-19.

24.

Espenson, J. H. Acc. Chem. Res. 1992, 25, 222-227.

25.

Katsuyama, T.; Bakac, A.; Espenson, J. H. Inorg. Chem. 1989, 28, 339-341.

26.

Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615-6620.

27.

Goerigk, L.; Grimme, S. Phys. Chem. Chem. Phys. 2011, 13, 6670-6688.

28.

Feller, D. J. Comput. Chem. 1996, 17, 1571-1586.

29.

Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li,

J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045-1052. 30.

Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305.

31.

Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571-2577.

32.

Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674-675.

33.

Emrich, R.; Heinemann, O.; Jolly, P. W.; Krüger, C.; Verhovnik, G. P. J.

Organometallics 1997, 16, 1511-1513.

34.

Rozenel, S. S.; Chomitz, W. A.; Arnold, J. Organometallics 2009, 28, 6243-6253.

35.

McGuinness, D. S. Chem. Rev. 2011, 111, 2321-2341.

36.

Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson, V. C. J. Am. Chem. Soc.

2005, 127, 10166-10167. 19 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

37.

Page 20 of 21

Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. J. Catal. 2006, 240, 172-

181. 38.

Ruddick, V. J.; Badyal, J. P. S. J. Phys. Chem. B 1998, 102, 2991-2994.

39.

McGuinness, D. S.; Davies, N. W.; Horne, J.; Ivanov, I. Organometallics 2010, 29, 6111-

6116. 40.

Rebenstorf, B. J. Mol. Catal. 1989, 56, 170-182.

41.

Dekking, F. M.; Kraaikamp, C.; Lopuhaa, H. P.; Meester, L. E. A Modern Introduction to

Probability and Statistics. 1 ed.; Springer: Heidelberg-Berlin, 2005. 42.

Hogan, J. P. In Appl. Ind. Catal., Leach, B., Ed. Academic Press: New York, 1983; pp

149-176. 43.

Xia, W.; Liu, B.; Fang, Y.; Fujitani, T.; Taniike, T.; Terano, M. Appl. Catal., A 2010,

389, 186-194. 44.

Spitz, R. J. Catal. 1974, 35, 345-352.

45.

Groeneveld, C.; Wittgen, P. P. M. M.; Swinnen, H. P. M.; Wernsen, A.; Schuit, G. C. A.

J. Catal. 1983, 83, 346-361.

46.

McDaniel, M. P. ACS Catal. 2011, 1, 1394-1407.

47.

Aguado, J.; Calleja, G.; Carrero, A.; Moreno, J. Microporous Mesoporous Mater. 2010,

131, 294-302. 48.

Schmeal, W. R.; Street, J. R. AIChE J. 1971, 17, 1188-1197.

49.

McKenna, T. F.; Soares, J. B. P. Chem. Eng. Sci. 2001, 56, 3931-3949.

50.

D'Souza, A. S.; Pantano, C. G. J. Am. Ceram. Soc. 1999, 82, 1289-1293.

51.

Musso, F.; Ugliengo, P.; Solans-Monfort, X.; Sodupe, M. J. Phys. Chem. C 2010, 114,

16430-16438. 20 ACS Paragon Plus Environment

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

52.

Rountree, C. L.; Kalia, R. K.; Lidorikis, E.; Nakano, A.; Van Brutzel, L.; Vashishta, P.

Annu. Rev. Mater. Res. 2002, 32, 377-400.


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