Simple Scheme to Predict Transition-State Energies of Dehydration

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Simple Scheme to Predict Transition-State Energies of Dehydration Reactions in Zeolites with Relevance to Biomass Conversion Michal Fecí̌ k,† Philipp N. Plessow,*,† and Felix Studt†,‡ †

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Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 18, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: Dehydration of various alcohols over H-ZSM-5 is studied using density functional theory. The activation energies are shown to scale linearly with the van der Waals interaction with the zeolite framework. The van der Waals interaction itself is shown to be a simple function of the number of atoms of the involved alcohol. Consequently, activation barriers for the dehydration of primary alcohols are now easily derived directly from the number of atoms of these alcohols through the obtained scaling relations.

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INTRODUCTION Biomass is increasingly coming into focus for the production of fuels and chemicals in a post-oil society.1,2 The utilization of biomass as a feedstock for chemicals is especially intriguing as the oxygen content needs to be only partially removed and some oxygen-rich chemicals could in principle be obtained without heavy deoxygenation.1 These upgrading processes often involve dehydration steps usually facilitated by acids. In this respect, the catalytic dehydration over acidic zeolites has received wide attention,3−7 as these materials show potential for several reactions including the conversion of fructose to 5HMF,8,9 the dehydration of xylose to furfural,10,11 and the conversion of glycerol to acrolein.12−16 With the majority of these processes still in their infancy, more detailed understanding of the reaction mechanisms on the atomic scale is highly desirable. In particular, an understanding of how different hydroxy groups are reacting with acid sites and what influences their reaction barriers needs to be achieved for a targeted optimization of catalysts and process conditions. Herein, we investigate the first step of the dehydration of a large variety of organic molecules with a hydroxy group (see Scheme 1) over H-ZSM-5 using density functional theory (DFT) calculations. We show how the reaction barriers relate to the interaction between adsorbate and the zeolite cavity through dispersion forces and how these can be easily estimated based on the number of atoms of the reacting molecules. We will show where and why this works and for which molecules our simple approach fails. In turn, we show how these findings can be exploited for the prediction of reaction barriers without the need to perform a single DFT © XXXX American Chemical Society

calculation; something that we believe will be of general interest beyond the theoretical community, as it provides an easy tool for experimentalists as well.

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COMPUTATIONAL DETAILS As a model catalyst, we use H-ZSM-5 with Brønsted acidity introduced by substitution of one Si by one Al atom per unit cell of the zeolite (Si/Al = 95:1). The MFI framework contains 12 crystallographically distinguishable tetrahedral sites (T sites). Herein, we employ the commonly chosen T12 site.17−26 The two most stable O-sites25 located in the intersection of the straight and sinusoidal zigzag channels were considered with the data shown only for the most stable adsorbates and transition states (see the Supporting Information (SI) for all energies). The computational setup is the same as used in earlier work.27 All the DFT calculations were carried out using the Atomic Simulation Environment (ASE)28 and the Vienna Ab initio Simulation package (VASP)29,30 in version 5.4.1, employing the PBE-D331−33 functional with the projectoraugmented-wave method and an energy cutoff of 400 eV. All the atoms in the unit cell were structurally optimized with a convergence criterion of 0.001 eV/Å for the atomic forces. Lattice constants of the MFI framework optimized with an energy cutoff of 800 eV are a = 20.340 Å, b = 19.988 Å, and c = Received: August 7, 2018 Revised: September 14, 2018

A

DOI: 10.1021/acs.jpcc.8b07659 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Investigated Reactantsa

a

The dehydrated hydroxy group is marked, where more than one possibility exists.

13.492 Å. Transition state searches were performed using the dimer method34 and the automated relaxed potential energy surface scan.35 For the vibrational analysis, only part of the zeolitethe involved oxygen with bound Brønsted proton and adjacent Si and Al atomstogether with the adsorbate were considered. Corresponding vibrational frequencies were obtained in the harmonic oscillator approximation using a central finite-difference scheme with displacements of ±0.01 Å. For further information, see the SI.

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RESULTS AND DISCUSSION

Our initial focus lies on the first step of alcohol activation, the formation of the corresponding surface alkoxy species (SAS). In this reaction, the initial state (IS) results from adsorption on the acid site through a hydrogen bond between the acid site and the hydroxy group and van der Waals (vdW) interactions (see Figure 1). The hydroxy group is then protonated, eventually forming water, whereas the alkyl group subsequently forms a bond with the framework oxygen (see Figure 1). Small barriers for protonation/rotation of the alcohols need to be overcome additionally. These are typically negligible as is shown in detail for the case of methanol in Figure S3.1. The reaction of the simplest alcohol, methanol, has been subject to many experimental and theoretical studies, as it comprises the first step of the dehydration of methanol to dimethyl ether36−39 and of the methanol-to-olefin process.27,40−44 We calculate methanol adsorption on the zeolite

Figure 1. (a) Initial state (IS), transition state (TS), and final state (FS) of the reaction of an alcohol in H-ZSM-5 are shown for ethanol. Bond lengths are given in pm. Color code: C = brown, H = white, O = red, Si = orange, Al = blue. (b) An energy diagram is shown for methanol (blue) and ethanol (red).

to −115 kJ/mol, which is in good agreement with the experimental value −115 kJ/mol reported by Lee et al.45 and B

DOI: 10.1021/acs.jpcc.8b07659 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C earlier theoretical studies presented by Nguyen et al.19 (−116 kJ/mol, T12), Svelle et al. 25 (−115 kJ/mol, T12), Ghorbanpour et al.17 (−131 kJ/mol, T12), or Haase and Sauer46 (−139 kJ/mol, T7). Adsorbed methanol can react to form an SAS (in this case also denoted as surface methoxy species (SMS)). This reaction is uphill in energy by 98 kJ/mol and accompanied by an energy barrier of 140 kJ/mol, again in good agreement with the recent literature.47,48 We only discuss the final state where water is already desorbed because water desorption is generally thermodynamically favorable due to entropic effects. However, water adsorption is always energetically favorable and in the case of benzyl alcohol this leads to a final state that is slightly higher in energy than the transition state. We stress here that this situation changes drastically when the pathway is analyzed in terms of free energy where entropic effects are included. Thus, the free energy of the final state after water desorption will be significantly below that of the transition state. When analyzing higher alcohols, we find that energies of their initial states (IS), transition states (TS), and final states (FS) are lower (more stabilized) when compared to the gasphase reference: the IS, TS, and FS of ethanol are 17, 13, and 18 kJ/mol lower in energy, respectively, when compared to methanol. Interestingly, these differences are almost identical to the differences in vdW interaction of these two alcohols with the zeolite framework (19, 13, and 14 kJ/mol for IS, TS, and FS, respectively). An important observation is thus that the differences in IS, TS, and FS energies between methanol and ethanol can be easily derived from the differences in their corresponding vdW interactions with the zeolite framework. In the following, we considered a large range of alcohols to investigate whether this is a common phenomenon. Figure 2 shows the PBE-D3 energies of the IS, TS, and FS of all the alcohols shown in Scheme 1 as a function of the vdW part (D3) of their total energy (PBE-D3 vs D3) referenced to

their gas-phase value. We observe that there is a group of (simple) alcohols that scale with the vdW interaction for all calculated geometries (IS, TS, and FS). These are all primary alcohols (as shown in the upper part of Scheme 1). Interestingly, when using methanol as a reference, we observe that all the primary alcohols are represented by a slope of 1, meaning that their energy differences relative to methanol are entirely described by their differences in vdW interactions with the zeolite framework. Thus, using methanol as a starting point, one would be able to predict the IS, TS, and FS of other alcohols if one would know their differences in the vdW interactions. We will now discuss the outliers that do not follow the observed scaling. These outliers fall into two categories that can be explained either through steric or electronic effects. Sterically crowded alcohols can be expected to have difficulties accessing the active site, leading to less stable IS, TS, and FS. A simple and rigorous criterion to characterize sterical hindrance is to consider all the nonprimary alcohols as sterically hindered. This criterion applies to isopropanol, tert-butanol, glycerol, glyceraldehyde, tartronaldehyde, and lactic acid. As can be seen in Figure 2, we indeed observe that these reactants usually deviate from the scaling line with energies that are higher than expected based on the trends in vdW interactions. Besides steric hindrance, electronic factors are the other source for deviation. This is most obvious for benzyl alcohol, where mesomeric stabilization (+M-effect) leads to a significant stabilization (55 kJ/mol) of the transition state when compared to the scaling line. Notably, this stabilization only influences the transition state that is cationic in character, whereas IS and FS are hardly affected at all. The other primary alcohol that is affected by mesomeric effects is methyl glycolate. Here, a −M-effect leads to the destabilization of the transition state by 38 kJ/mol, whereas IS and FS are again less affected. A −M-effect is also present in glyceraldehyde, tartronaldehyde, and lactic acid. Here, it is difficult to decouple steric effects from −M-effect, as they work in the same direction. An additional factor leading to deviations from the observed scaling is hydrogen bonding that was identified for a few reactants (ethylene glycol, glycerol, and glyceraldehyde). Although it is not straightforward to pinpoint the magnitude of the effect of hydrogen bonding on the energy, the results can, in general, be explained by the stabilization from hydrogen bonding (see the SI for details). Having identified that the differences in the energies for adsorbates and transition states can be described by their differences in the vdW interactions with the zeolite framework, we will now investigate this interaction in more detail. It has been shown earlier in experimental as well as theoretical studies that the vdW interaction of linear alkanes with the zeolite framework follows a linear relationship with the number of carbon atoms (or CH2 groups) of said alkanes.49−52 The slope of this relationship is typically around 15 kJ/mol (HZSM-22),49 14 kJ/mol (H-SSZ-13),52 and 7.5 kJ/mol (AFI)52 per carbon atom. Similar approaches use the total number of atoms in the corresponding molecule or fragment. Here, a slope of 5.6 kJ/mol per atom has been identified for H-ZSM-5, also employing the PBE-D3 functional.27 Recently, Nguyen et al.53 arrived at similar results when studying the adsorption of C1−C4 alcohols and its scaling with respect to the carbon number. We note that scaling relations have also been used to describe reactivity as a function of acidity measures.54,55

Figure 2. The reaction energy, ΔE, is shown as a function of the vdW contribution to the energy, ΔEvdW, for the initial (blue), final (black), and transition (red) states for all the reactants. Linear scaling lines with a slope of 1 have been chosen to pass through the data point for methanol. C

DOI: 10.1021/acs.jpcc.8b07659 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C We tried different approaches to estimate the vdW interaction between H-ZSM-5 and an adsorbate. These included using the total number of all adsorbate atoms, the total number of the adsorbates heteroatoms, in this case, C and O atoms, and the C6 coefficients proposed by Grimme33 as weight factors (see SI Figures S4.1−S4.3). Among those, we find that using the total number of atoms (including the acid site hydrogen) yields the best results with a slope of −5.3 kJ/ mol per atom, similar to the previous results.27 Using the fact that the energies of IS, TS, and FS of the investigated primary alcohols scale linearly with their vdW interaction with the zeolite framework and that this vdW interaction can be approximated by the total number of atoms of the corresponding IS, TS, or FS, we can now plot their total energies as a function of the number of atoms as shown in Figure 3. Note, that we now have a linear dependence of the

Figure 4. Parity plot showing agreement between our simple model ΔEmodel and the full DFT calculations ΔEPBE‑D3. The orange dashed line represents a slope of 1. Only primary alcohols without strong mesomeric effects are shown.

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ad ad ΔExad = ΔEMeOH − ΔvdW ad MeOH + ΔvdW x

(1)

ΔvdW ad x = − 5.3 kJ/mol · Nx

(2)

ad ΔExad = ΔEMeOH − 5.3 kJ/mol ·(Nx − NMeOH)

(3)

CONCLUSIONS As can be seen from Figure 4, our simple model is able to predict the energies of adsorbates and transition states of the primary alcohols fairly accurately with a mean absolute error (MAE) in the range of 5−8 kJ/mol. Importantly, this model is based on the assumption that vdW interactions can be estimated from the number of atoms of the corresponding adsorbates and transition states. It thus provides a simple tool that is widely accessible. As discussed earlier, calculations for methanol as the reactant and the slope of vdW interactions between the adsorbates and the zeolite frameworks are the only other ingredients needed. We expect the presented approach to hold across frameworks, thus allowing to predict a large number of energies for adsorbates and transition states from eqs 1−3.

Figure 3. The reaction energy, ΔE, is shown as a function of the total number of atoms N in the IS, TS, or FS. Lines represent our simple model derived from ΔE (ΔEvdW) (see Figure 2) using the relation identified for ΔEvdW (see Figure S4.4). The calculated slope is −5.3 kJ/mol per atom and the offset is fixed so that the scaling line passes through the data point for methanol. Only primary alcohols without strong mesomeric effects are shown.

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energies of IS, TS, and FS for the investigated primary alcohols that is given by the number of atoms present in the IS, TS, and FS respectively. In summary, we have shown that the interaction of adsorbates with the zeolite (ΔEad x ) is well described by the interaction calculated for methanol (ΔEad MeOH) together with the difference in vdW interaction of methanol and the ad adsorbate in question (ΔvdWad MeOH and ΔvdWx , respectively) ad (eq 1). Because ΔvdWx is found to be fairly well represented when plotted as a function of the total number of atoms Nx in the adsorbate (eq 2), we can predict the energy of any adsorbate in a given zeolite framework by simply counting the number of adsorbate atoms if we know (a) the energies corresponding to methanol as the reactant and (b) the slope with which the vdW interactions scale. Figure 4 shows the outcome of this simple model where the predictions from the model (eq 3, slope identified for H-ZSM-5) are plotted against the full DFT calculations (PBE-D3).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07659. Unit cell of H-ZSM-5; comparison of O-site energies; complete energy profile for methanol dehydration; modelling van der Waals interaction; hydrogen bonding; coordinates of the empty H-ZSM-5 framework; CIF files of studied structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Philipp N. Plessow: 0000-0001-9913-4049 D

DOI: 10.1021/acs.jpcc.8b07659 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Author Contributions

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The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support by the state of BadenWürttemberg through bwHPC (bwunicluster and JUSTUS, RV bw16G001 and bw17D011). Financial support from the Helmholtz Association is also gratefully acknowledged.

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ABBREVIATIONS

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REFERENCES

DFT, density functional theory; vdW, van der Waals; SAS, surface alkoxy species; SMS, surface methoxy species; IS, initial state; TS, transition state; FS, final state; MAE, mean absolute error

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