Catalyst design criteria and fundamental 
limitations in the electrochemical synthesis of 
dimethyl carbonate

Manuel Šarić, Bethan Jane Venceslau Davies, Niels Christian Schjødt, Søren Dahl, Poul Georg Moses, María Escudero-Escribano, Matthias Arenz and Jan Rossmeisl     


Scientific Article Accepted for Publication in Green Chemistry

Database and plotting scripts

DOWNLOAD FILES

Requirements

python - tested on arch linux, version 3.6.2
numpy - tested on arch linux, version 1.13.1
matplotlib - tested on arch linux, version 1.13.1
ase - installed from source, version 3.14.1

Database description (dmc.db)

The database is sorted using the keyword type.
type="molecule" contains all molecules
type="surface" contains all surfaces
type="adsorbate" contains all adsorbates

type="molecule" additional keywords:

  • product=True if the molecule is a product (DMC, DMO etc.)
  • molecule=<name> where <name> is "H2", "DMC" etc.
  • energy calculated 0 K energy
  • free calculated free energy
  • data["ensemble"] 0 K ensemble energies
  • data["free_ensemble"] ensemble free energies
  • reaction_energy reaction energy at 0 K (requires product=True)
  • reaction_free_energy reaction free energy (requires product=True)
  • uncertainty one standard deviation of the reaction energy ensemble (requires product=True)

type="surface" additional keywords:

  • element=<name> where <name> is "Au", "Cu" etc.
  • energy calculated 0 K energy
  • data["ensemble"] 0 K ensemble energies

type="adsorbate" additional keywords:

  • adsorbate=<name> where <name> is "CO", "CH3O" etc.
  • element=<name> where <name> is "Au", "Cu", etc.
  • energy calculated 0 K energy
  • free calculated free energy
  • data["ensemble"] 0 K ensemble energies
  • data["free_ensemble"] ensemble free energies
  • adsorption_energy adsorption energy at 0 K
  • adsorption_free_energy adsorption free energy
  • uncertainty one standard deviation of the adsorption energy ensemble

Useful examples

Reproduce all the plots (in a shell):

$ bash plots.sh

OR

$ chmod +x plots.sh
$ ./plots.sh

Working with the database (in a python interpreter):
> # import the required functions
> from ase.db import connect
> # connect to the database
> db = connect("dmc.db")
> # get the structure of Au 111
> au = db.get_atoms(type="surface", element="Au")
> # let's view it!
> from ase.visualize import view
> view(au)
> # get the calculated energy of Au 111 (eV)
> e_au = db.get(type="surface", element="Au").energy
> # get the ensemble of Au 111
> ens_au = db.get(type="surface", element="Au").data["ensemble"]
> # let's get the reaction free energy of DMC
> rfe_dmc = db.get(type="molecule", molecule="DMC").reaction_free_energy
> # what about getting the reaction free energies of all products
> for row in db.select(type="molecule", product=True):
        print(row.molecule, row.reaction_free_energy)
> # get the structure of CO on Cu
> co_cu = db.get_atoms(type="adsorbate", adsorbate="CO", element="Cu")
> # get the adsorption free energy of CO on Cu
> g_co_cu = \
  db.get(type="adsorbate", adsorbate="CO", element="Cu").adsorption_free_energy

Abbreviations

Abbreviation Meaning Formula
M Methoxy CH3O*
HM Hydroxy methyl CH2OH*
MF Methyl formate CH3OCO*
GA Glycol aldehyde CH2OHCO*
DMC Dimethyl carbonate CH3OCOOCH3
DMO Dimethyl oxalate CH3OCOCOOCH3
DMP Dimethyl peroxide CH3OOCH3
EG Ethylene glycol HOCH2CH2OH
DHA Dihydroxy acetone HOCH2COCH2OH
DHBDO 1,4-dihydroxybutane-2,3-dione HOCH2COCOCH2OH
MM methoxy methanol CH3OCH2OH
MG methyl glycolate CH3OCOCH2OH
MHOP methyl-3-hydroxy-2-oxopropanoate CH3OCOCOCH2OH
DME dimethyl ether CH3OCH3

Description

Dimethyl carbonate (DMC) is an environment-friendly chemical that is used as a precursor in various chemical processes. DMC can replace phosgene and other toxic precursors in carbonylation and methylation reactions. Another possible application of DMC is as a fuel or fuel additive.

DMC was traditionally produced by a phosgenation reaction. Nowadays DMC is produced in various processes involving hazardous mixtures of CO and O2 in order to avoid phosgenation. An electrochemical pathway could make it possible to produce DMC by using a potential difference between two electrodes in order to activate the reactants and provide a driving force for DMC production, thus avoiding the use of hazardous precursors or mixtures.

The electrochemical process studied (from a theoretical standpoint) herein is described by the following oxidation reaction (occurring at the anode):

2CH3OH + CO ⇄ CH3OCOOCH3 + 2(H+ + e-)

Density functional theory calculations were performed to get insight in the stability of various surface intermediates on metallic surfaces as well as the reaction free energies (driving force) towards DMC and other possible co-products.

In order to find a suitable catalyst for the reaction we take into account 3 requirements that it should satisfy:

  1. Energy efficiency (low potential to activate methanol).
  2. No surface poisoning (DMC should form and leave the surface).
  3. Selectivity towards DMC (DMC should be produced and other products should be avoided).

Energy efficiency

We assume that the activation of methanol determines the potential needed to drive the synthesis of DMC. Methanol is activated by adsorbing either as methoxy (CH3O), methyl formate (CH3OCO), hydroxy methyl (CH2OH) or glycol aldehyde (CH2OHCO). The activation on methanol to either of these surface species is accompanied by the release of a proton-electron pair, making this a potential dependent redox reaction. The 4 surface species mentioned above as well as adsorbed CO are the species that most likely populate the surface during the electrosynthesis of DMC. Therefore, in order to analyze the reaction, it is necessary to take into account 5 different parameters. It is possible to reduce the complexity of this analysis to only 2 parameters (CO and methoxy) due to the scaling relations found between CO and the forms of activated methanol that bind on surfaces through a carbon atom.

scaling relations

Scaling relations between adsorbates binding on surfaces through a carbon atom. The left column is shown using the reactants as reference while in the right column, the corresponding species adsorbed on copper is used as reference. The colored zones show covariance ellipses of the BEEF ensemble values.

 

thermodynamic analysis

Thermodynamic analysis for the electrosynthesis of DMC. The catalysts lying in the grey zone below the red dashed line have surface species poisoning the surface thus inhibiting the formation of DMC. The catalysts found far to the left are poisoned by CO while the ones found far to the right are not able to adsorb CO. The catalysts found in the green zone produce DMC selectively over DMO. In the cyan zone and orange zone, DMO is produced selectively over DMC at lower potentials and an overpotential is required to produce DMC. In the orange zone it is more favorable to activate methanol as hydroxy methyl than methoxy which could cause additional selectivity issues. The vertical distance between the purple triangles line and a data point represents the potential (in volts) required for DMC production.

On the thermodynamic analysis plot, the vertical distance between the data points representing different catalysts and the purple triangles line represents the potential needed to drive the reaction (volts). This shows that Cu requires ~1 V less than Au in order to produce DMC.

No surface poisoning

Catalysts found in the grayed out zone below the red dashed line of the thermodynamic analysis plot have surface species that are not leaving the surface as DMC, i.e. the equilibrium will be shifted towards surface species (grayed out zone below the red line). Catalysts far to the left are poisoned by CO while the ones far to the right do not bind CO at all. The ideal catalysts are thus lying in the green, blue and orange zones of the thermodynamic analysis plot.

Selectivity towards DMC

Assuming that the surface is covered by hydroxy methyl, methoxy, methyl formate and glycol aldehyde a wide range of molecules can be produced. The reaction energies plots shows the calculated reaction energies for products resulting in simple combinations of the mentioned surfaces species plus dimethyl ether (DME) which requires the cleavage of a C-O bond (this might be limited by unfavorable kinetics).

reaction energies

Reaction energies of products resulting from simple combinations of surface species assumed to be present on the surface during the electrosynthesis of DMC (plus dimethyl ether).

It can be seen from the reaction energies plot that, besides DME, DMC is the most favorable product which should help with selectivity.

In the thermodynamic analysis plot, for the catalysts found above the blue dash-dot line (cyan and orange zone) the formation of methyl formate is more favorable than the formation of methoxy. We assume that it is possible for methyl formate to adsorb on the surface without the intermediate step of adsorbing methoxy. If the surface is covered only by methyl formate it is not possible to produce DMC and DMO is produced selectively. This means that the catalysts found above the blue dash-dot line (cyan and orange zone) are selective towards DMO at lower potentials. By applying additional potential methoxy is stabilized and DMC production starts. On the other hand, on catalysts found below the blue dash-dot line (green zone) it is more favorable to adsorb methoxy than methyl formate. This indicates that the catalyst found in the green zone are selective to DMC at lower potentials. This theory is further visualized by the DMC vs DMO free energy diagram.

DMC vs DMO

Free energy diagrams showing the selectivity between DMC and DMO on Au and Cu at various applied potentials (needed to activate methanol as methoxy or methyl formate). The full line leads to DMC with the intermediate step of adsorbing methoxy. The dashed line leads to DMO with the intermediate step of adsorbing methyl formate. On Au it is more favorable to adsorb methyl formate leading to selective production of DMO at lower applied potentials. Applying additional potential stabilizes methoxy and enables the producition of DMC. On Cu it is more favorable to adsorb methoxy compared to methyl formate leading to selective production of DMC at lower potentials.

Conclusion

From the results shown it can be seen that using Cu instead of Au as electrocatalyst leads to a lowering in the potential required by ~1 V as well as selective production of DMC versus DMO.