Detecting Important Intermediates in Pd Catalyzed Depolymerization

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Detecting Important Intermediates in Pd Catalyzed Depolymerization of a Lignin Model Compound by a Combination of DFT Calculations and Constrained Minima Hopping Pemikar Srifa, Maxim V. Galkin, Joseph S. M. Samec, Kersti Hermansson, and Peter Broqvist J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05622 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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The Journal of Physical Chemistry

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Detecting Important Intermediates in Pd Catalyzed 8

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Depolymerization of a Lignin Model Compound by 12 13 15

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a Combination of DFT Calculations and Constrained 17

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Minima Hopping 21

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Pemikar Srifa,† Maxim V. Galkin,† Joseph S. M. Samec,*,‡, Kersti Hermansson,*,§ Peter 27

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Broqvist*,§ 28

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Department of ChemistryBMC, Uppsala University, Box 576, 75123 Uppsala, Sweden.

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Department of Organic Chemistry, Stockholm University, 106 91 Stockholm, Sweden.

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Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 75121 Uppsala,

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Sweden. 39 40 42

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ABSTRACT 43 4 45

Density functional theory (DFT) calculations, combined with a constrained minima hopping 48

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algorithm (global minimum search while preserving the molecular identity), have been 50

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performed to investigate important reaction intermediates for the heterogeneously catalyzed 51 53

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O4’ bond cleavage in lignin derivatives. More specifically, we have studied the adsorption 5

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properties of a keto tautomer (1methoxypropan2one) and its enol form on a catalytically 56 57 58 59 60 ACS Paragon Plus Environment

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active Pd(111) surface. In agreement with experiments, we find that for the gas-phase molecules 5

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the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the 6 7

Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on 10

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the catalyst surface. The global minimum complex found on the metal surface corresponds to an 12

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enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its pi13 15

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electrons from the CC region to two adjacent palladium atoms. The actual formation of a 17

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chemical bond to the surface in the case of the enol molecule could be the key to understanding 18 19

why the enol derivative is needed for an efficient O4’ bond cleavage. 21

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INTRODUCTION 25

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Lignin is the second most abundant biopolymer on earth and constitutes around 30 weight% of 29

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the lignocellulosic biomass.13 Furthermore, due to its lower O/C ratio compared to 30 32

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carbohydrates, lignin comprises more than 50% of the energy of the total biomass. The lignin 34

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polymer is built from methoxy-substituted phenolic subunits of irregular structural complexity 35 36

and interlinked by different bonds (Figure 1). The O4’ bond is the most abundant of the 39

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interlinkages (more than 50%) between monomeric units in the lignin polymer and is therefore 40 41

the obvious target for lignin depolymerization strategies (Figure 1, blue). Today, lignin is 42 4

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generated in pulp mills, where it is burnt to a low value to produce process heat and for the 46

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regeneration of process chemicals.4,5 Along with the increasing environmental awareness 47 48

concerning the use of petroleum (non-renewable) sources as a raw material for energy 51

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production and in chemical industries, a world-wide strive to find renewable and sustainable 53

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alternatives has emerged.6 In this context, lignin is a potential candidate material that can 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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accommodate the demand for renewable aromatic compounds, and currently, many efforts have 5

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been made to valorize this biopolymer.712 6 7 8

An efficient and selective lignin depolymerization reaction is the key to utilizing lignin as a 1

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renewable raw material in chemical transformations. Lately, several studies have used 13

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heterogeneous catalysts for the lignin depolymerization reaction.1315 Here, the Pd/C catalyst has 14 16

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been shown to be an efficient catalyst to cleave the O4’ bond in lignin (Scheme 1).1620 Due 18

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to the complexity of natural lignin polymers, it is common to use model compounds (lignin 19 21

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derivatives) when trying to understand possible reaction mechanisms. From such studies, it has 23

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been proposed that an initial transfer dehydrogenation of the benzylic alcohol in the O4’ 24 25

motif (I) to the resulting ketone (II) is a key step in the overall transformation to generate the 28

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cleaved products IV and V in Scheme 1. From theoretical calculations17 of the bond dissociation 30

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energies (BDE), it has been found that the CO bond in the dimeric ketone II indeed is lower 31 3

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(61.2 kcal/mol) than that of the alcohol (I) (72.2 kcal/mol). This result is consistent with the 35

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proposed mechanism, i.e. that the CO bond of the intermediate keto form in Scheme 1 is easier 36 37

to break.16 39

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We have in a previous experimental study investigated the reaction mechanism of the 43

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transformation of I to IV and V in Scheme 1, and found support for a reaction mechanism that 4 46

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proceeds through the ketone intermediate. For example, we demonstrated by a competition 48

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experiment that the keto dimer II’ was more reactive than substrate I (Scheme 2). By reacting I 50

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in the presence of II’, the expected outcome, calculated from rate differences between the two 51 53

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substrates, would be a 70% conversion of the more reactive substrate II’ (2.3 times faster) as 5

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compared to substrate I. However, the observed conversion (96.5%) of keto dimer II’ is much 56 57 58 59 60 ACS Paragon Plus Environment

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higher than the expected one (70%), which provides support for a reaction mechanism where the 5

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keto dimer is the reactive intermediate in the transformation of I to IV and V in Scheme 2. 6 8

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The lower BDE value for substrate II as compared to substrate I may not be the only reason for 9 1

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the higher reactivity of the keto intermediate. A hypothesis that has been brought forward is that 13

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the keto intermediate may tautomerize to its corresponding enol form (III in Scheme 1), which is 14 15

more reactive than ketone II in the transformation to products IV and V. We have previously 16 18

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experimentally studied the importance of the keto intermediate tautomerization. For example, 20

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when substrate I’’, in which the hydrogen in the β-positions have been exchanged for methyl 21 2

groups, was reacted under the employed reaction conditions, only the transfer dehydrogenation 25

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occurs and no cleavage reaction was observed (Scheme 3).19 This experiment demonstrates that 27

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the initial transfer dehydrogenation reaction to generate the keto intermediate is not enough for 28 30

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the cleavage of the C–O bond. A possible explanation for this finding is that enol (substrate III 32

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in Scheme 1) has a higher affinity to the catalyst due to the softer nature of the olefin as 34

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compared to the relatively harder oxygen on the carbonyl group according to the HSAB rule. 35 37

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Additionally, it was experimentally found that the transfer hydrogenolysis only operated under 39

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slightly basic conditions where it is known that bases promote the tautomerization. 40 41 42

The abovedescribed experiments provide support for a reaction mechanism that proceeds 43 45

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through an initial transfer dehydrogenation reaction to generate a ketone intermediate that can 47

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easily tautomerize to its enol form. The reason for this intermediate step is not known, and to 48 50

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understand and provide further insight to the importance of the ketotoenol tautomerization, we 52

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herein use density functional theory (DFT) calculations to study the interaction of these 53 54

molecules with the Pd catalyst. We specifically aim at characterizing the interaction between the 5 57

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catalyst surface and the inter-linkage atoms, i.e. the O4’ bond, which is the actual part that is 58 59 60 ACS Paragon Plus Environment

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breaking in the depolymerization reaction. We are aware that the aryl groups will have a strong 5

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affinity to the surface,2124 but have for the reason stated above in this study chosen to use a more 6 8

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simple lignin derivative model with the inter-linkage atoms terminated with methyl groups on 10

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both sides. This choice of model allows us to study the interaction of the inter-linkage atoms with 1 12

the surface to identify key steps leading to the inter-linkage CO bond cleavage. 13 14 16

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Only a few previous theoretical studies related to reactions of lignin can be found in the 18

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literature. DFT calculations have been used to explain the role of transition metals (i.e. Ni, Rh) in 19 20

the homogeneously catalyzed transformation of biaryl ether, which is another structural motif in 23

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lignin.25-27 In case of heterogeneously catalyzed reactions, Willock et al. have used DFT 25

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calculations to study the hydrogenation of acetone to propan2ol over a Pt(111) surface.28,29 In 26 28

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these reports, they concluded that acetone first tautomerize to its enol form and adsorb onto the 30

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Pt surface by the C=C bond instead of the C=O bond, before the hydrogenation occurred. This 31 32

result is highly relevant for our present study, as this tautommerization could occur in a similar 35

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way for the molecules studied in the present reaction scheme, given that the rate of the 37

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hydrogenation reaction of the keto (II) species in Scheme 1 is slower than its tautomerization 38 39

rate. 41

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Theoretically, identifying reaction intermediates on a catalytic surface is a complex task, and it 45

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should be noted that the complexity increases with the number of atoms in the system. For large 46 47

molecules, there are generally more local minima structures, and it is difficult to “guarantee” that 50

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obtained adsorption configurations are “truly” global minimum structures. However, there exists 52

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algorithms that aids in the search for a ground state, such as simulated annealing,30,31 basin 53 54

hopping,32 or Monte Carlo minimization.33 Among these, the minima hopping algorithm 56

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developed by Goedecker in 200434 is particularly suitable for structure prediction of molecular 5

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adsorbates on a surface and constitutes a powerful tool in finding global minimum structures. 6 8

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In this work, we have used DFT calculations in combination with the constrained minima 9 1

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hopping algorithm to investigate the adsorption properties of the model keto and enol molecules 13

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(II and III in Scheme 1 above) with the palladium catalyst, represented by a Pd(111) surface. 14 15

Our goal is to determine the most stable adsorption configurations for both tautomers on the 16 18

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surface and provide support for our previous experimental interpretation of the heterogeneously 20

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catalyzed lignin depolymerisation reaction, which involves the tautomerization of the keto 21 2

intermediate as a key step. 24

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COMPUTATIONAL DETAILS 27 29

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A. Electronic structure calculations. The electronic structure calculations performed in 31

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this work are all based on the DFT and the exchange correlation functional proposed by Perdew 32 3

et al.35 In the study, we have used two different implementations. At first, we used an all-electron 36

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implementation with a local basis set for the expansion of the electron wavefunctions (the 38

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Gaussian09 program36) to search for the global minimum structures of the bare gas phase 39 41

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molecules. In this search, we used the triple zeta cc-pVTZ basis set.37 Increasing the basis to a 43

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quadruple base did not affect the results. Thereafter, for the adsorption studies and the metal 4 45

catalyst, we used an implementation with pseudopotentials and a plane-wave expansion of the 48

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electron wavefunctions (Vienna Ab initio Simulation Package, VASP).38-41 Here, we used 50

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pseudopotentials of the Projected Augmented Wave (PAW) type as proposed by Blochl et al.42 51 52

With this method, we treated explicitly 10 electrons for Pd (5s04d10), 4 electrons for C (2s22p2), 1 5

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electron for H (1s1), and 6 electrons for O (2s22p4), respectively. The wavefunctions were 56 57 58 59 60 ACS Paragon Plus Environment

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truncated using a plane-wave energy cutoff of 500 eV. In the adsorption calculations, the 5

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Brillouin-zone was sampled with a 2×2×1 Monkhorst-Pack grid. To speed up the SCF 6 7

convergence, we additionally used a Gaussian smearing of 0.2 eV. All presented structures were 10

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optimized until the force on each atom was less than 0.01 eV/Å. To test the performance of the 12

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pseudopotential description, we additionally recalculated all the stable structures from the local 13 14

basis set calculations using the periodic density functional theory implementation with the 17

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molecules in a periodic cell of dimensions 20x20x20 Å, sampling only the gamma point of the 19

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Brillouin zone. The lowest-energy structure was selected to be the reference for the adsorption 20 2

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energy calculations. 24

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All reported energies are given without zero-point energy contributions. To justify this 25 26

procedure, we have calculated, on the one hand, the difference in zero-point energies between 27 29

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the two geometry-optimized gas phase molecules and, on the other hand, the difference in zero31

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point energies between the bare molecules fixed in its respective geometry as obtained when 32 3

adsorbed on the Pd surface. The zero-point energy difference, ZPE(enol) – ZPE(keto), for the 36

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gas-phase molecules amounts to 1.4 kcal/mol while it is -0.2 kcal/mol for the adsorbed 38

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geometries. Thus, the inclusion of the zero-point energies will not alter the conclusions we will 39 41

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present in the Results section. 42 4

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B. Model systems. The very large and complex structures targeted in this study are difficult to 46

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handle efficiently using DFT. Therefore, to gather insight in to the interactions between lignin 47 48

derivatives and the catalyst in this pioneering study, we have introduced model systems to mimic 51

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the behavior of the larger systems when it comes to the details of the structure we want to study. 53

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In the following, we will briefly describe and motivate the model structures we have chosen. 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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Lignin derivative model 4 6

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To study the isolated CO bond cleavage of lignin, it is common to use lignin derivatives, as 7 8

discussed in the introduction. In this study, we have further simplified such a model by removing 1

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the aryl rings, which enables us to isolate the binding of the inter-linkage atoms to the surface 13

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and study the effect of keto-enol tautomerization. The keto and enol tautomers were thus 14 16

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represented by 1methoxypropan2one, and 1methoxyprop-1en2ol, respectively (Keto 18

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and Enol in Scheme 4). To find the ground state structures of the isolated intermediates, we first 20

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used the method of Rodríguez et al.43, to make sure that the alignment of the substituent of 23

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acetophenone is taken in to account in a systematic way. In this work, the alignment of the 24 25

substituents on the alpha carbon (C) and the beta carbon (C) were considered (Scheme 4). In 26 28

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the keto intermediate, two possible configurations where the methoxy group points out of and 30

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into the CC bond are defined as Keto1 and Keto2, respectively. 31 32 34

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For the enol tautomer, the orientations of functional groups on the carbon double bond CC 36

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were taken into consideration. Either the hydroxyl group or the methoxy group can here possibly 37 38

align endo or exo to the double bond. This leads to twice the number of possible structures for 41

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enols compared to keto forms. The Keto1 and Keto2 can be converted into four enol isomers 42 43

(c.f. Scheme 4). 4 45 47

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Pd catalyst 48 50

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In real applications, the CO cleavage of lignin molecules is catalyzed by Pd metal particles 52

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supported on a carbon support. To model this catalyst, we here use the most stable palladium 53 5

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surface, i.e. the close-packed Pd(111) surface. The surface model was cleaved from the geometry 57

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optimized bulk structure. More details regarding the modeling is presented in the results section. 58 59 60 ACS Paragon Plus Environment

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C. Constrained minima hopping for searching the most stable adsorption complex. To find 5

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the most stable adsorption complexes, we have used the minima hopping method as developed 7

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by Goedecker.34 This method combines ab initio molecular dynamics (AIMD) at high 10

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temperature with geometry optimizations to screen different possible adsorption sites. The 12

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procedure is as follows: The minima hopping is initiated by first optimizing the initial adsorbate 13 14

structure. Thereafter, a molecular dynamics (MD) simulation in the microcanonical (NVE) 17

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ensemble at high temperature is initiated to search for new minima. After the MD has been 19

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equilibrated, a new geometry optimization is performed and the resulting structure and energy is 20 2

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compared to previously obtained minima. The routine is cycled until no new local minima are 24

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found. 25 27

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The initial temperature for the AIMD and the energy difference to differentiate between different 28 30

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local minima for the constrained minima hopping were set to 3500 K and 0.05 eV, respectively. 32

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The high initial temperature ensures that the molecule diffuses over the surface to search for new 34

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local minima. During the search, the MD temperature is however adjusted on-the-fly depending 35 36

on whether the minimum found is a new one, or one that is already discovered. 34 At these high 39

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temperatures, we need to introduce constraints in order to keep the molecular identity of the keto 40 41

and enol molecules, as well as to ensure that the Pd surface stays intact. Following the work of 4

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Peterson, 44 we used a class of Hookean constraints, which works well with the minima hopping 46

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method. The Hookean constraints were used both on the substrate PdPd bonds, and on the CH 47 49

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bonds of the adsorbates. We used the constrained minima hopping method as implemented in the 51

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Atomistic Simulation Environment (ASE, version 3.7.0).45 The parameters are given in Table 1. 52 54

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D. Properties. Adsorption energies were calculated according to the following formula: 5 56 57 58 59 60 ACS Paragon Plus Environment

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Eads=  [E(molecule/Pd slab)  E(Pd slab)  E(keto ) ] 4

(1)

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where E(Pd slab), E(keto) and E(molecule/Pd slab) are the total energies of optimized structures 7 8

of the bare Pd surface, the isolated keto molecule, and the combined adsorption system 1

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(molecule + slab) as explained in section B, respectively. We have here chosen to use a common 13

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reference (i.e. the Pd slab and the keto gas phase molecule) to enable direct comparison of 14 15

energies between the adsorbed tautomers. In this form, the adsorption energy becomes a reaction 18

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energy where positive Eads signals exothermic processes. To minimize computational errors due 20

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to different sampling of the unit cells, all energies are obtained using the same periodic box 21 23

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dimensions. 24 26

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In addition, we have quantified the charge transfer between the adsorbates and substrate by 28

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calculating the charge density difference ((r)), defined as: 29 30 32

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(r) = (molecule/Pd slab)  (Pd slab)  (molecule). 3

(2)

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The charge differences gives insight related to the adsorption induced electron rearrangement 36 37

and can be used to characterize the adsorbate-surface interaction. It should be noted that in the 38 40

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calculations of these charge densities (all atomic coordinates for the three terms in (2) were 42

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set to be the same as in the optimized structure for the adsorbed molecule on the surface. 43 4 45

RESULTS AND DISCUSSION 47

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3.1 Gas phase keto and enol forms. To find the global minimum structures of the gas phase 51

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keto and enol forms, we initiated a scan of the precise torsion angle for each conformation based 53

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on the Klyne-Prelog nomenclature.46 The dihedral angle () of OHCCOCH3 in enols and 56

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OCCOCH3 in keto forms were calculated from  = 0º to  = 300º using the allelectron 57 58 59 60 ACS Paragon Plus Environment

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DFT in the implementation with a local basis set as described in the computational details 5

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section. All stable structures found (local minima, see Figure 2.) were reoptimized using the 6 7

DFT method in the implementation with plane-waves and pseudopotentials, which is the method 10

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that we use in the adsorption studies. The relative energies between the different local minima 12

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using the two methods are given in Table 2. As expected, the two different DFT 13 14

implementations agree very well for the bare molecule isomers (differences are less than 1.2 17

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kcal/mol). In the following, we will only discuss energies and structures obtained using the DFT 19

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in the implementation with plane-waves and pseudo-potentials. 20 21 2

The global minimum (comparing both the keto and enol forms) corresponds to an (E)25

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configuration of the keto tautomer (KetoS1 in Figure 2) in which the oxygen of CO1 and 26 27

the oxygen of the ether linkage (CO2) are trans to each other with a perfect dihedral angle of 30

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180º. In contrast, the minimum found for the enol tautomer corresponds to a (Z)configuration, 32

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which contains an intra-molecular hydrogen bond between O1H…O1CH3 with a distance of 35

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2.163 Å (EnolS1). Here, it becomes important to establish whether our method (the PBE 37

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density functional theory) is capable of describing the energetics for hydrogen bonds, for 38 40

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example as PBE does not implicitly include non-local correlation effects. Here, we use literature 42

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data for the water dimer as an example. 47 In Ref. 47 it is shown that the PBE density functional 43 4

is indeed capable to describe both the minimum configuration and the potential energy curve for 45 47

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the water dimer when compared to CCSD(T) calculations.47 48 50

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To conclude, as free molecules, the keto forms are always more stable than the enol forms. The 51 52

most stable keto isomer (KetoS1) is 5.1 kcal/mol more stable than the most stable enol form 5

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(EnolS1). This relative ordering is in agreement with experimental observations.48 56 57 58 59 60 ACS Paragon Plus Environment

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3.2 Modeling adsorbed keto and enol on Pd(111). To represent the catalyst surface, we used a 5

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Pd(111) surface modeled in a p(4x4) supercell structure. The supercell geometry was constructed 6 7

from Pd bulk with the geometry optimized crystallographic lattice parameter of 3.94 Å at the 10

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PBE level (expt. 3.89 Å)49 and with an additional vacuum gap of 10 Å normal to the surface to 12

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ensure a minimal interaction between periodically repeated slabs and adsorbates. Tests for keto 13 14

and enol adsorption showed that increasing this distance to 15 Å did not change the calculated 17

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adsorption energies. In the calculations, molecules were only adsorbed on one side of the slab, 18 19

and the bottom Pdlayer in the supercell was kept fixed at bulk positions while all other atoms 20 2

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were allowed to relax. Tests using different slab thicknesses were performed and are reported 24

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below. In the case of a onelayer slab, all Pd atoms were kept fixed at bulk positions. 25 26 27

The adsorption calculations performed in this study are of two types. At first, we used a “small 30

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model” to explore the potential energy landscape and search for stable adsorption structures 32

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using the constrained minima hopping algorithm discussed above. Here we searched for the 3 35

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global minimum structures for both the keto and the enol adsorption on Pd(111). Secondly, we 37

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used a “large model” for the most stable structures found at the first stage, to obtain accurate and 38 39

converged adsorption energies. To allow for direct comparison of reported energies, all 40 42

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adsorption energies were calculated with the keto gas phase molecule as reference, as described 4

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in the computational details section. 45 47

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To be able to perform as many searches as possible in the constrained minima hopping it is 48 50

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necessary to have as small model as possible, but still one that is capable of describing the 52

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relative energetics between the different adsorption structures. To determine the minimum 53 54

number of layers needed in our model, we therefore investigated the convergence with respect to 5 57

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number of Pd layers in the slab for keto (KetoS1) and enol (EnolS1) adsorption. The results 58 59 60 ACS Paragon Plus Environment

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from these tests are given in Figure 3. As seen in the figure, there is an initial strong odd-even 5

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dependence in the adsorption energy for the enol form (compare 1 and 3 to 2 and 4 layers of Pd 6 7

in the figure). This dependence is not seen for the keto adsorption energy, which is similar in all 10

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cases above one layer of Pd(111). These results suggest that the two molecules interact very 12

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differently with the surface, and to shed light on this finding, we characterized the bonding of the 13 14

two molecules to the Pd surface using charge difference analysis. The results for the enol 17

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molecule adsorbed on 1 and 2 layers of Pd are given in the upper and lower panels, respectively, 19

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of Figure 4(a). In the upper panel, we clearly see that enol adsorption leads to the breaking of 20 2

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the CC bond to form two CPd bonds. This di- pattern of a bonding has earlier been 24

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observed by Haubrich et al. for ethene molecules on Pt(111).50 However, for the 2-layer slab 25 26

(lower panel) the effect is seen to be much smaller. The results for the keto molecule, which 29

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mainly interacts through electrostatic interactions with the surface, are quite different from those 31

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of the enol molecule. This is clearly seen from a comparison of the upper and lower panels of 32 34

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Figure 4(b); for the keto molecule the various bonding features for the 1- and 2-layer slabs are 36

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rather similar. The difference in slab thickness sensitivity for the enol and keto tautomers is even 37 38

more evident when looking at the integrated charge difference (Figure 4), where the charge 39 41

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reorganization is much larger in case of enol adsorption. From this analysis, we conclude that the 43

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bonding behavior is completely different for a chemisorbed (enol) and a physisorbed (keto) 4 45

molecule, which is also reflected in the interaction energies, where we calculate a stronger 48

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interaction of the enol molecule with the surface compared to the keto form (recall that keto is 50

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5.1 kcal/mol more stable in gas phase). This analysis further explains why the 1layer slab is 51 53

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overbinding the enol, which allows for optimal charge reorganization in z-direction. The 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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oddeven convergence behavior diminishes however fast, and beyond three layers of Pd atoms, 5

4

it is negligible. 6 7 8

Based on these convergence tests we find that a 2layer slab is able to qualitatively describe the 1

10

9

relative energetics between the adsorbed keto and enol forms on the surface, but to ensure 13

12

converged results, we prefer to use thicker slabs. Thus, in the following we have used the 14 16

15

Pd(111) p(4x4) 2layer slab (32 Pd atoms) for the constrained minima hopping calculations and 18

17

the Pd(111) p(4x4) 4layer slab (64 Pd atoms) for the refinement calculations of the most stable 19 21

20

structures. 2 24

23

3.3 Global Minimum search. To find the minima structures for our adsorbatePd systems, we 25 26

have used the constrained minima hopping method as discussed above. The MD simulations 29

28

27

started at high temperature (3500 K) using a single keto or enol molecule on the surface. To 31

30

make sure that the adsorbed molecules keep their identity at this high temperature, we have used 32 34

3

Hookean constraints for all PdPd and all CH bonds (see Table 1). 35 37

36

Initial structures 38 40

39

The global minima search began from six initial adsorption complexes (two keto forms and four 41 42

enol forms), chosen based on the stable bare gas phase molecular structures of the adsorbates 45

4

43

found during the initial test calculations (see Figure 2). To choose the initial adsorption 47

46

configurations, we further made use of Polanyi’s theory,51 i.e. that organic molecules prefer to 48 50

49

adsorb on a metal catalyst surface through their reactive functional groups. We additionally 52

51

hypothesized that the (E)configuration Enol-S1 can bind at a bridging site on the surface by 53 5

54

using either its CC bond (namely, position 1: P1) or O1---O2 linkage pointing up (position 56 57 58 59 60 ACS Paragon Plus Environment

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2: P2) or down (position 3: P3) to any two adjacent Pd atoms (EnolS1P1, EnolS1P2, 5

4

EnolS1P3). Also for the (Z)-configuration Enol-S2, the CC was positioned across two Pd 6 8

7

atoms in the surface (EnolS2P1). The KetoS1P1 and KetoS2P1 are characterized 10

9

according to their geometric (E) and (Z) isomerism. All initial structures (before optimization) 1 13

12

are depicted in Figure 5. 14 16

15

Enol adsorption 17 19

18

The four different initial enol starting configurations are shown in Figure 5 a)-5d). In total, we 20 2

21

performed 53 different search steps (including MD and geometry optimizations). The energies 24

23

for all these structures are given in the left panel of Figure 6. Among these 53 structures, we 26

25

found 31 different local minima according to our energy criteria (with energy differences larger 27 29

28

than 0.05 eV). After visual inspection of the different minima, we found that these structures in 31

30

fact corresponded to two different adsorption cases characterized either as an (E) or a (Z)enol 32 34

3

binding across a PdPd bridge on the Pd(111) surface (see Figure 7 a)-d)). 35 37

36

The lowest energy structure found corresponds to an (E)enolate form, named EnolS1M1 in 38 39

Figure 7a), with an adsorption energy of 10.0 kcal/mol (taking the Pd(111) slab and the most 42

41

40

stable gas-phase keto structure as reference). In the most stable local minimum, the adsorbed 43 4

tautomer forms bonds to the Pd surface in a bridge position, breaking the CC double bond to 47

46

45

form two CPd bonds. This results in a bond elongation between the C and C atoms (from 48 49

1.345 Å in the isolated molecule to 1.503 Å), which confirms the breaking of the double bond 50 52

51

forming a single bond. The distances of CPd and CPd are 2.118 Å and 2.111 Å, 54

53

respectively. Furthermore, in the adsorbed complex, we found an intramolecular hydrogen bond 5 57

56

between O2 and the hydrogen atom at O1 with a distance of 2.060 Å (see Figure 7a)). The 58 59 60 ACS Paragon Plus Environment

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hydrogen bond was also found for the adsorbates in complex EnolS1M2 and EnolS1M3 5

4

from the constrained minima search starting from the initial structures EnolS1P2 and 6 8

7

EnolS1P3. The adsorption conformer EnolS2M1 in Figure 7d) has a less negative 9 10

adsorption energy (weaker interaction) than that of EnolS1M1 (1.89 kcal/mole). This energy 1 13

12

difference demonstrates the role of hydrogen bonding on the stabilization of the enol adsorption 15

14

complex. 16 17 18

Keto adsorption 19 20 2

21

The two different initial keto starting configurations are shown in Figure 5e)-5 f). For the keto 24

23

adsorption, 21 different local minima were found based on the energy criteria from 28 steps of 26

25

global minima search. The energies for all these structures are given in the right panel of Figure 27 29

28

6. After visual inspection of these 21 structures, only one conformer was found. The global 31

30

minimum corresponds to the complex KetoS1M1 shown in Figure 7e). Furthermore, a self32 3

rotation of one of the initial structures [(E)KetoS2P1] to form its (Z)conformer was found, 36

35

34

see KetoS2M1 in Figure 7f). 37 39

38

Both keto complexes were found to adsorb in the so called η1(O) configuration,52 binding 42

41

40

through physisorption via its CO1 functional group pointing towards one Pd atom in the surface 43 4

with a O1Pd distance of 2.246 Å and 2.237 Å, respectively. When adsorbed, the CO1 bond 47

46

45

length is slightly elongated (1.244 Å for KetoS1M1) and 1.241 Å for KetoS2M1 compared 48 49

to 1.220 Å of the KetoS2 gas phase structure). Furthermore, the CC bond was found to be 52

51

50

1.502 Å. Upon adsorption, this bond stays unaffected. Even though the KetoS2M1 and 53 54

KetoS1M1 looks very similar, they differ in adsorption energy by 1.4 kcal/mol. The 57

56

5

adsorption energy of the most stable complex KetoS1M1 was calculated to –9.1 kcal/mol. 58 59 60 ACS Paragon Plus Environment

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Another configuration for cellulosic aldoses adsorption on metal surfaces called η2(C, O) has 5

4

been reported in the literature.52 These molecules were found to bind to the surface through the 6 7

pi orbitals of the C=O functional. This structure was not found in our global minimization. 10

9

8

Starting geometry optimizations from such geometry in the case of the keto molecule always 12

1

returned to the more stable η1 (O) adsorption complex. One explanation for this behavior could 13 15

14

be that the keto model used in this study does not contain any hydrogen atom at C position 17

16

which helps to stabilize the η2(C, O) adsorption complex. 18 20

19

3.4 The most stable structures on 4 layers of Pd. In the final step, the most stable keto and 21 23

2

enol adsorption complexes found in the screening process, namely EnolS1M1 (Figure 7a)) 25

24

and KetoS1M1 (Figure 7e)), were fully optimized using a four layer Pd(111) slab (with the 26 28

27

bottommost layer kept frozen at bulk distances); this was the slab thickness where the adsorption 30

29

energies can be considered to be converged (see Figure 3). Upon adding the two extra Pd layers 31 32

(i.e. going from 2 to 4 layers), the adsorption complexes undergo only small relaxation in the 35

34

3

optimization (compare Figures 7 and 8). The different bond lengths changed less than 0.05 Å. 37

36

The largest change was found for the hydrogen bond in the EnolS1M1 complex, which was 38 40

39

elongated from 2.060 to 2.109 Å in EnolS1G1. 41 43

42

As seen already in Figure 3, we do not expect any dramatic changes in the adsorption energy 4 45

upon adding the two extra Pd layers, but instead give the correct adsorption behavior without any 48

47

46

oddeven effects and allow for comparison between the two adsorbed molecular forms. For the 49 50

keto and the enol adsorption on the 4layer slab, we find that the enol form is 1.9 kcal/mol 51 53

52

(compared to the 0.9 kcal/mole for the 2-layer slab) more stable than the keto form. 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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CONCLUSIONS 4 6

5

From previous experiments, it has been suggested that the first step in the Pd/C catalyzed lignin 8

7

depolymerization via O4’ cleavage is an initial dehydrogenation, forming a keto 9 1

10

intermediate. In this study, we have used DFT calculations and the constrained minima hopping 13

12

algorithm to investigate the interaction of enol and keto tautomers when adsorbed on a Pd(111) 14 15

surface. Whereas the keto tautomer is 5.1 kcal/mol more stable in the gas phase, the enol 18

17

16

tautomer is found to be 1.9 kcal/mole more stable when adsorbed on the surface. The origin of 20

19

the increased stability for the adsorbed enol tautomer is traced back to the two molecules very 21 23

2

different interaction to the Pd surface. While the enol form is found to form a chemical bond to 25

24

the surface in a bridge configuration with two Pd-C covalent bonds, the keto form is found to 26 27

only physisorb through its CO1 functional group, pointing towards one Pd atom. The actual 30

29

28

formation of a chemical bond to the surface in the case of the enol molecule could be the key to 32

31

understanding why the enol derivative is needed for an efficient depolymerization as experiments 3 35

34

have shown that model substrates that cannot undergo tautomerization to the enol form are not 37

36

reactive in O4’ bond cleavage. Adding aryl groups to the model would significantly increase 38 39

the adsorption energies53 and this will be further studied in the future. 41

40

43

42

The results also support using the minima hopping method together with Hookean constraints to 45

4

find reactive intermediates involving metal surfaces and organic molecules and we hope that this 46 47

will inspire other researchers to use this methodology in the future. 49

48 50 51 52 53 54 5 56 57 58 59 60


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ACKNOWLEDGMENT 5

4

The Swedish Energy Agency, the Swedish Research Council (VR) and the National Strategic e6 7

Science program eSSENCE are gratefully acknowledged for funding this study. P.S. thanks the 10

9

8

Erasmus Mundus Action 2 (EXPERTS4Asia program). The simulations were performed on 12

1

resources provided by the Swedish National Infrastructure for Computing (SNIC) at UPPMAX 13 14

and NSC. 16

15

18

17

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FIGURES 4 5 6 7 8 9 10 1 12 13 14 16

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Figure 1. Representation of the lignin polymer (the -O-4’ bonds are indicated in blue). 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 35

34

Figure 2. Stable ground state configurations of the keto and enol conformers in the gas phase. 36 37

The geometrical data are taken from the geometry optimized structures using DFT in the 40

39

38

implementation with plane-waves and pseudopotentials. Given geometrical parameters are in Å. 42

41

Grey balls are carbon, red balls are oxygen, and white balls are hydrogen. 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 3

32

Figure 3. Convergence test for the adsorption energy of keto and enol adsorption on Pd slabs 34 36

35

with varying number of Pd layers. The bottom layer is always kept fixed at bulk distances. For 38

37

the one layer slab, the Pd surface is kept fixed at bulk distances. 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 45

4

Figure 4. Charge difference analysis for a) enol adsorption and b) keto adsorption on one and 47

46

two layer slabs of Pd(111). The left figures show structural models and isosurface (0.08 e-/Å3), 50

49

48

where pink and yellow isosurfaces signals charge depletion and charge gain, respectively. The 52

51

right figures show the integrated charge differences along the z-axis, normal to the surface. In the 53 5

54

structural model, grey balls are carbon, red balls are oxygen, white balls are hydrogen and blue 57

56

balls are palladium. 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 48

47

Figure 5. Initial adsorption configurations for enol and keto tautomers on Pd(111). a)-d) are 49 50

different enol conformers and e) and f) are different keto conformers adsorbed on the surface. 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 2

21

Figure 6. Adsorption energy vs constrained minima hopping steps for enol (left) and keto (right) 24

23

adsorption on a two layer Pd(111) slab. The legends refer to the initial starting configurations 25 27

26

shown in Figure 5. For the enol tautomer: a) EnolS1P1, b) EnolS1P2, c) EnolS1P3, d) 29

28

EnolS2P1. For the keto tautomer: e) KetoS1P1, f) KetoS2P1. 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 47

46

Figure 7. Minimum energy structures from the constrained minima hopping on a 2 layer of 48 50

49

Pd(111) slab. The structures are presented following their initial starting configurations as shown 52

51

in Figure 5. Given geometrical parameters are in Å. 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 32

31

Figure 8. Global minimum configurations of a) enol (EnolS1G) and b) keto (KetoS1G) on 34

3

a 4 layer Pd(111) slab. Given geometrical parameters are in Å. 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 3

2

SCHEMES 4 6

5

Scheme 1. The Pd/C catalyzed redox neutral cleavage of model I to products IV and V, via an 7 8

initial transfer dehydrogenation to generate intermediates tautomers II and III. 10

9 1 12

trans fer dehydrogenation

13

tautomerization

14 OH

16

15

O Ar I model of -O-4' bond

Ar

20

19

18

17

O

Pd/C -H2

Ar

II

OH O

O Ar III Ar model of enol tautomer

Ar

model of keto tautomer

C-O bond cleavage H2 Pd/C

O

Ar OH IV V model of model of keto product phenol product Ar

21 2 23 25

24

Scheme 2. Competition experiment showing that the keto intermediate II’ reacts faster than 26 27

expected in the Pd/C catalyzed reaction. 28 29 30 31

O 32

Ph 35

34

3

O II' OH

37

36

Ph I 39

38

O

+

Ph Ph

F

O

Pd/C

F

IV

V'

+ HO Ph OH V

42

41

40

Observed conversion

100%

96.5%

3.5%

45

4

Expected conversion

100%

70%

30%

43

46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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Page 36 of 39

1 3

2

Scheme 3. This experiment shows that the hydrogens in β-position are essential for C–O bond 5

4

cleavage to occur. When the hydrogens in I have been exchanged for Me groups (I’’) only a 6 7

transfer dehydrogenation to generate II’’ occur without cleavage. 9

8 10 1

OH 12

O Ar I'' Ar Me Me 15

14

13

O

Pd/C

no further reaction

O

Ar II'' Ar Me Me

redox neutral conditions

16

(5)

17 18 19 20 2

21

Scheme 4. Conformational analysis of keto and enol forms. 23

H

25

24 O

26

C

27 28 Me

30

29

Me

O

O

C

Keto-1

O C

H

H

Me

Me

H

O C

C

Enol-1

C

Me

H

Me

O

Enol-3

31

H

32 3 34 35

Me

36 O

37

C Me

42

41

H

Keto-2 43

H

Me O

C

C

40

Me

O

O

38 39

H

H

O

C

Me

O C

H

C

Me

Enol-2

H

Enol-4

4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 3

2

TABLES 4 6

5

Table 1. Parameters for setting up the Hookean constraints used in the minima hopping 7 8

calculation. rt is the threshold distance and k is the spring constant. 10

9 1 12

Moiety 14

13

element-element

19

18

17

16

15

CH3(terminal) O2CH3(terminal)

3 x (CH) CO 3 x (CH)

1.59 1.79 1.40

7 5 5

CH2O2(keto)

CO 2 x (CH)

1.58 1.59

10 7

C=O1(keto)

C=O

1.58

10

CHO2 (enol)

CO CH

1.58 1.59

10 7

C-O1H(enol)

CO OH

1.79 1.40

5 10

Pd surface

PdPd

0.94

15

20 2

21 23 24

28

27

26

25

29 30 32

31

Identity Constraint rt (Å) k

3 35

34 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

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1 3

2

Table 2. Relative energies of the stable keto and enol conformers in gas phase. 4 5 6 8

7

Stable 9 1

10

VASP calculation

Ediff (kcal/mol)

Ediff (kcal/mol)

Keto-S1

0.000

0.000

Keto-S2

2.340

2.312

Enol-S1

6.097

5.143

Enol-S2

10.171

9.551

Enol-S3

11.138

10.385

Enol-S4

11.224

10.028

Enol-S5

12.067

11.269

Enol-S6

13.068

12.405

Enol-S7

15.922

15.064

Conformer 12

Gaussian calculation

13

18

17

16

15

14

3

32

31

30

29

28

27

26

25

24

23

2

21

20

19

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1 3

2

AUTHOR INFORMATION 4 6

5

Corresponding Author 8

7

Joseph S. M. Samec*,‡, tel: +46 70 559 2511 9 10 1

Peter Broqvist*,§, tel: +46 18 471 3714 13

12

15

14

Present Addresses 18

17

16

†

Department of ChemistryBMC, Uppsala University, Box 576, 75123 Uppsala, Sweden,

20

‡

Department of Organic Chemistry, Stockholm University, 106 91 Stockholm, Sweden,

§

Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 75121 Uppsala,

19

23

2

21

25

24

Sweden. 26 28

27

Author Contributions 30

29

The manuscript was written through contributions of all authors. All authors have given approval 31 3

32

to the final version of the manuscript. 34 35 36 38

37

Table of Contents Graphic 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

39

Detecting Important Intermediates in Pd Catalyzed Depolymerization

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