Direct Conversion of Methanol to Ethanol on the Metal-Carbon Interface

Article abstract of DOI:10.1002/cctc.201900437

Direct conversion of one-carbon (C₁) compounds to two-carbon (C₂) and multi-carbon compounds remains a critical challenge for converting non-petroleum resources to valuable chemicals or fuels. The key issue is the selective activation of C₁ compounds, methanol, as well as the controlled formation of carbon-carbon (C?C) bonds. Herein, we achieve the direct electrocatalytic methanol to ethanol, an important chemical and energy candidate, with methanol conversion, ethanol selectivity, and faradic efficiency of 257.0 g ? m^?2 ? h^?1, 95.1 %, and 12.5 %, respectively. Furthermore, the appropriate participation of water, as a by-product from methanol electrocatalysis, in hydrogen evolution reaction (HER) facilitates electrocatalytic reaction of methanol. Mechanistic studies reveal hydroxymethyl and methyl radicals are formed on the electropositive low-valent metal sites and electronegative carbon vacancies, respectively, and then combined with each other to form ethanol at the metal/carbon interface. This work opens a unique route for high-efficient concerted redox conversion of methanol reactant to ethanol.

Full text of DOI:10.1002/cctc.201900437

Accepted Article
Title: Direct conversion of methanol to ethanol on the metal-carbon
interface
Authors: Yunhua Li, Lu Junfeng, Wang Xihui, Zhang Hua, Wu Xuee,
Zhang Kelvin H. L., Ye Jinyu, and Zhan Dongping
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900437
Link to VoR: http://dx.doi.org/10.1002/cctc.201900437
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ChemCatChem
Directconversion of methanol to ethanol on the metal-carbon interface
Yunhua Li*, Junfeng Lu, Xihui Wang, Hua Zhang, Xuee Wu†, Kelvin H. L. Zhang, Binghui Chen, Jinyu Ye,
Dongping Zhan‡
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Centre of Chemistry for Energy Materials, Department
of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering Xiamen University, Xiamen 361005, P. R. China
Abstract: Direct conversion of one-carbon (C1) compounds to two-carbon (C2) and multi-carbon compounds
remains a critical challenge for converting non-petroleum resources to valuable chemicals or fuels. The key
issue is the selective activation of C1 compounds, methanol, as well as the controlled formation of carbon-
carbon (C-C) bonds. Herein, we achieve the direct electrocatalytic methanol to ethanol, an important chemical
and energy candidate, with methanol conversion, ethanol selectivity, and faradic efficiency of 257.0 g·m-2·h -
1, 95.1 %, and 12.45 %, respectively. Furthermore, the appropriate participation of water, as a by-product from
methanol electrocatalysis, in hydrogen evolution reaction (HER) facilitates electrocatalytic reaction of
methanol. Mechanistic studies reveal hydroxymethyl and methyl radicals are formed on the electropositive
low-valent metal sites and electronegative carbon vacancies, respectively, and then combined with each other
to form ethanol at the metal/carbon interface. This work opens a unique route for high-efficient concerted
redox conversion of methanol reactant to ethanol.
With the exhaustion of petroleum resource, the utilization of non-petroleum carbon resources offers an
alternative strategy to produce important petro-chemicals and fuels.[1] Methanol, as an abundant and renewable
C1 resource, can be obtained from carbon dioxide and biomass,[2] as well as natural gas and coal,[3] which has
generally been used to synthesize C2+ hydrocarbon and oxygenates.[4] Among them, selective and direct
methanol coupling to C2 compound, ethanol, has become extremely attractive in catalytic chemistry for both
academia and industry.[5] It’s accepted that the oxygen-hydrogen (O–H) dissociation in methanol is the more
favorable than the carbon-hydrogen (C–H) and carbon-oxygen (C–O) bond scissions. As a result, ethanol has
generally been synthesized from methanol via a carbonylation reaction followed by a reduction process.[6]
Recently, Wang’s group studied the cleavage of C–H bond in methanol into ethylene glycol on photoexcited
holes on CdS. Additionally, Jiang’s group found the ratio of C–O/C–H dissociation in menthol can enhance
by 100 times through exciting the C–O stretching mode using a linearly polarized laser technology. Moreover,
Wang’s group found that ethanol with an selectivity of 65.0 % can achieve from methanol homologation using
*
Corresponding author:E-mail: yunhuali@xmu.edu.cn
xewu@xmu.edu.cn
dpzhan@xmu.edu.cn
†
‡
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10.1002/cctc.201900437
ChemCatChem
a [RuCl2(CO)3]2/Co4(CO)12 bimetallic catalyst in lower reaction temperature, 160 °C ,. [5] However, direct
concerted dissociation of C–H and C–O bond in methanol to ethanol still remains a great challenge.[7]
Metallic cobalt is a conventional catalyst for C-C coupling reactions. It has the advantages of the high
catalytic activity and high selectivity of C2+ hydrocarbons and oxygenates through crystallite size or surface
composition.[8] To date, concerns over mild catalytic conditions have recently been concentrated on
electrocatalysis process, such as HER of water, CO2 reduction and ammonia synthesis (NRR).The controlled
electrocatalytic features of a metal and carbon composite result in an extraordinary synergistic effect between
the electropositive metallic species and electronegative carbon support. On the electrocatalytic material, the
carbon vacancies with the higher electronic density could facilitate electroreduction reactions, such as the
oxygen reduction reaction (ORR) and HER reactions. In the meanwhile, the electropositive metal species
could adsorb hydroxyl to facilitate the electrooxidation reaction, such as the oxygen evolution reaction
(OER).[9] The field generated by this ion-image-charge pair in the electrode also interplays with polar
adsorbates at a longer range to even control the kinetics of electrochemical reactions.[10] When Co and carbon
species are present on either side of an interface of catalysts simultaneously, concerted redox conversion of
methanol may proceed to provide an opportunity to increase ethanol selectivity on catalysts.
Herein, we demonstrate a direct electrocatalytic route from methanol to ethanol with high selectivity at
20°C and atmospheric pressure. There is three remarkable characteristics in this reaction: (1) the specific
adsorption of methanol on the surface of the electrocatalyst; (2) the controllable cleavage of C-H and C-O
bonds at neighbouring metal-carbon active sites of the electrocatalyst; (3) the tailorable participation of by-
product water from electrocatalytic reaction of methanol, in HER, which facilitates the formation of C-C bonds.
In brief, a synergetic mechanism was proposed for the direct electrocatalytic conversion of methanol to ethanol
[11]
on a unique
electrocatalyst. Because of the localized charge distribution at the metal/carbon support
interface under an electric field,[12] the low-valent Co sites are electropositive and the adjacent carbon
vacancies are electronegative. Consequently, the different dissociation mechanisms of methanol on this
electrocatalyst occur to form hydroxymethyl and methyl radicals on neighbouring metal and carbon active
sites, respectively. The C-C coupling between hydroxymethyl and methyl adsorbates produces ethanol at the
interface. A higher ethanol selectivity 95.1 % at 20°C is achieved with methanol conversion of 257.0 g·m-
2·h -1 and faradic efficiency of 12.5 % than conventional methanol homologation with an ethanol selectivity of
65.0 % in 160 °C [5]. The overall reaction can be expressed as equation (1).
2CH3OH→CH3CH2OH+H2+1/2O2
(1)
It should be noted that the conversion of methanol to ethanol according to equation (1) cannot occur at
temperatures lower than 500 °C based on thermodynamic calculations (Figure S1). Electrocatalysis provides
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10.1002/cctc.201900437
ChemCatChem
a potential strategy to convert methanol to ethanol under ambient conditions. In principle, the driving force of
electrocatalysis is the interfacial or surface potential. Under certain potentials, the activations of both C-H and
C-O bond would become possible following methanol adsorption on the surface of the electrocatalyst.[5,13] We
performed electrocatalytic conversion of methanol using a series of carbon-supported Ni, Ru, Rh, Co, and
Co3ZnC catalysts, as listed in Table 1. Fascinatingly, Co3ZnC/NC shows better catalytic performance for direct
conversion of methanol to ethanol than the others. Its performance is even higher than that of the noble metal
catalysts with similar metal loading. The Co3ZnC/NC catalyst presents a methanol conversion of 203.2 g·m-
2·h -1, and an ethanol selectivity of 93.1 % (GC-MS and GC analysis of products in liquid and gas phase are
shown in Figures S2 and S3, respectively).
Table 1. Comparison of methanol conversion and products selectivity over different catalysts[a]
Selectivity[c] (%)
Conversion[b]
(g·m-2·h -1)
EtOH yield
(g·m-2·h -1)
Entry
Catalyst
EtOH Dimethyl ether[d] Ethylene glycol[d]
1
2
3
4
5
10% Ni/ZIF-8-C
5%Ru-RuO2/BP2000[e]
5% Rh/C[e]
68.6
89.6
79.9
97.0
97.7
76.4
93.1
18.3
1.9
2.8
1.1
0.9
5.4
1.3
39.5
62.5
88.0
46.0
136.0
125.4
83.7
1.4
10% Co/BP2000
10% Co3ZnC/NC
18.2
5.6
203.2
[a] Reaction performances were evaluated under catalyst loading of 12.5±0.187, 25±0.285, 25±0.310, 12.5±0.206,
12.5±0.135mg/cm2 for 10% Ni/ZIF-8-C, 5%Ru-RuO2/BP2000, 5% Rh/C, 10% Co/BP2000 and 10% Co3ZnC/NC,
respectively. The corresponding change of potentials was presented in Figure S4 in electrocatalytic reaction process. And
the experiments were repeated for three times. [b] Methanol conversion was calculated based on the electrode area and
reaction time. [c] Selectivity was obtained on a molar carbon basis. [d] Dimethyl ether and ethylene glycol were denoted
as DME and EG, respectively. [e] The 25 mg/cm2 catalyst was used to maintain the same metal loading as the other
catalysts. The ratio between Ru and RuO2 in 5%Ru-RuO2/BP2000 is about 0.11.
By using Co3ZnC/NC, the effects of reaction conditions on the catalytic performance, including
temperature, potential, current, time, electrolyte pH value, stirring speed, and the catalyst amount, were
investigated (Figure S5). The optimal operation parameters are the reaction temperature of 30 °C, the current
of 10 mA/cm2, 4.5 h, pH of electrolyte solution of 11.5, stirring speed of 600 rpm, and 12.5 mg/cm2 loading
of the catalysts. We discovered that, under this optimized condition, the methanol reaction rate and ethanol
selectivity are 257.0 g·m-2·h -1 and 95.1 %, respectively. These are accompanied by the generation of
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hydrogen[14] and an increase in the solution pH in the cathodic chamber, as well as oxygen evolution in the
anodic chamber according to equation (2) and (3). The half-reactions occurred in cathodic and anodic
chambers are expressed as followed:
Cathode: 2퐻2푂 + 2푒− → 2푂퐻− + 퐻2
4MeOH + 2푒− → 2푂퐻− + 퐻2 + 2퐸푡푂퐻
Anode: 2퐻2푂 → 푂2 + 4푒− + 4퐻+
(2)
(3)
(4)
The catalytic behavior of Co3ZnC/NC depends on its specific redox property (unpassivated, reduced or
oxidized form, Table S1). 10 % Co3ZnC/NC exhibited the best electrocatalytic performance for direct
methanol conversion among all the samples with different calcination pretreatments. Thus, these Co3ZnC/N-
based samples were used to further study the structure-property relationships.
To explain the nature of the crystal phases, X-ray diffraction (XRD) patterns of the samples are presented
in Figure S6. The reduced, unpassivated, and oxidized Co3ZnC catalysts exhibit an obvious diffraction peak
at 41.9 ° attributed to the (111) lattice of cubic Co3ZnC, and another peak at 47.6 ° corresponding to the (101)
lattice of hexagonal Co, as well as individual redox characteristic peaks. High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy (EDS)
elemental mapping were employed to determine the positions of elements, the composition, and the particle
size of the unpassivated Co3ZnC/NC catalyst. As shown in Figure 1a and b, Co, Zn, and C are uniformly
distributed and overlap with each other, indicating the presence of the Co3ZnC phase. The (111) interplanar
spacing of 0.216, corresponding to Co3ZnC, was also observed in the high-resolution TEM (HRTEM)
micrograph (Figure 1c). Additionally, the particle size of the Co3ZnC/NC was obtained by the Scherrer
equation to be 11.4 nm (Table S2), which is similar to the result from TEM in Figure 1d. In addition, we
analyzed the composition of the surface elements over Co3ZnC/NC catalysts using X-ray photoelectron
spectroscopy (XPS). The results are presented in Figure S7 and Tables S3 and S4. The Co3ZnC/NC catalyst
contains a certain amount of low-valent Co species, attributed to Co3ZnC and Co metal. The amount of low-
valent Co species decreases in the following sequence: reduced Co3ZnC/NC>unpassivated
Co3ZnC/NC>oxidized Co3ZnC/NC (Table S3). And for the XPS spectra of C 1s, Co3ZnC/NC presents the
higher ratio at 284.4 eV, assigned to metal carbide, which is partially related to the interaction of Co3ZnC with
the carbon support (Table S4). By combining the XRD, TEM and XPS results, it is clear that the nanoparticles
of cubic Co3ZnC and Co are uniformly dispersed on the surface of the carbon support.
The signal centered at 2.0043 for the unpaired electrons indicates the appearance of carbon or zinc
vacancies in Figure 1e.[16] By comparison, the ZIF-8-C support shows a relatively weak characteristic signal,
while Co3ZnC/NC has the highest among all the samples. The formation of more carbon vacancies on the
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10.1002/cctc.201900437
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catalysts is related to the addition of the cobalt precursor and the generation of the low-valent Co3ZnC and Co
crystal phase. Additionally, compared with the support ZIF-8-C, the intensity ratio of the D and G bands (ID/IG)
decreases with the addition of Co in Figure 1f, implying interfacial contact between low-valent Co species
Figure 1. Composition of the Co3ZnC/NC catalyst. a) HAADF-STEM image of the unpassivated Co3ZnC/NC catalyst. b)
Corresponding EDS maps with in the region marked by the red rectangle in the HAADF-STEM image. c) and d) HRTEM
micrographs and the histogram of the particle size. e) Low-temperature ESR spectra of the catalysts. f) and g) Raman spectra
of the catalysts.
and carbon vacancies.[9] Through curve fitting with a Lorentzian function, the value of ID/ID’ is about 5.8 for
Co3ZnC/NC based catalysts (Figure 1g), which is higher than the value of 4.6 measured on the ZIF-8-C
support, indicating that more carbon vacancies are formed over Co3ZnC/NC catalysts[17]. Importantly, these
carbon vacancies are adjacent to low-valent Co species in the graphitization process of Co3ZnC/NC.
Combining the evaluation with characterization results, Co3ZnC/NC, which simultaneously has more low-
valent Co species and carbon vacancies on the support, illustrates excellent electrocatalytic performance
(Table S5). Positive correlation between interface composition and electrocatalytic performance indicates that
the synergy between Co3ZnC or Co and carbon vacancies indeed dominates electrocatalytic performances for
direct methanol to ethanol. The endurance test also indicates that the structures and compositions of
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ChemCatChem
Co3ZnC/NC change slightly during the methanol electrocatalysis process, leading to stable electrocatalytic
performances of methanol (Figure S8 and Figure S9).
It is accepted that Co3ZnC is a potential catalyst for HER in water. In this case, comparison of Tafel
slopes, LSV curves, and electrocatalytic performance of HER from water and methanol conversion was
accomplished to further understand the reaction mechanism, since water is also a possible by-product of the
direct methanol to ethanol conversion as well as from water permeability by N117 membrane from anode
compartment.
The Tafel slopes for pure methanol consumption and HER in a single-chamber reactor (SCR) are 86.5
and 114.6 mV dec-1, respectively, and it is 62.9 mV dec-1 under the conditions of the co-occurrence of methanol
consumption and HER in a two-compartment reactor (TCR) in Figure 2a, indicating that under the certain
potentials, actual methanol conversion is preferred in comparison with HER from water or individual methanol
electrocatalysis over the Co3ZnC/NC catalyst. The relative lower electrocatalytic performances for HER from
water is partially related to hydrophobic properties of Co3ZnC/NC (Figure 2b).[18] On the other hand, as a by-
product from methanol electrocatalysis, water in HER also promotes methanol conversion to some extent. By
comparison with 58.6 g·m-2·h -1 in a SCR, methanol conversion is 203.2 g·m-2·h -1 in a TCR. The LSV for
methanol electrocatalysis in a TCR (Figure 2c) also shows increased electroreduction current in TCR
compared with that in a SCR. The tailorable participation of by-product water from direct methanol
electrocatalysis in HER enhances the occurrence of electrocatalysis of methanol in TCR. However, the
addition of excess water can affect the ethanol yield due to the dilution of reactant methanol.
Figure 2. Electrocatalytic performance of Co3ZnC/NC catalyst in methanol and H2O. a) Tafel slopes. b) Contact angles. c)
LSV curves.
To further examine the reaction mechanism, we investigated the possible radicals generated during the
electrocatalysis process using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping agent[5] (Table
S6). With the addition of DMPO, the formation of ethanol is evidently suppressed, implying that ethanol is
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10.1002/cctc.201900437
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generated via free radical intermediates. Electron spin resonance (ESR) spin trapping is widely used to detect
short-lived free radicals, since ESR is sensitive to a relatively stable paramagnetic nitroxide adduct, formed
by spin traps, DMPO and active free radicals[19]. Therefore, in situ ESR spectroscopic study in the presence
of DMPO was employed to detect the generation of the hydroxymethyl radical (∙CH2OH) and methyl radical
(∙CH3) during the reaction process by comparing the results with and without electricity supply. We observe
DMPO-CH2OH (aN=15.7, aH=21.5) and DMPO-CH3 (aN=16.0, aH=22.3) signals during the electrocatalytic
reaction (Figure 3). This means that methyl and hydroxymethyl coexist during the electrocatalytic process. In
the absence of a spin-trapping agent, the formed hydroxymethyl and methyl radicals might freely collide with
one another to generate ethanol, ethylene glycol, or ethane products. Fascinatingly, the selectivity of ethanol
is 95.1% for electrochemical conversion. In contrast, the selectivities for other products, such as dimethyl
ether and ethylene glycol, are negligible.
As mentioned above, the synergy from low-valent Co species adjacent to carbon vacancies is a critical
factor for methanol electrocatalysis. More importantly, these species on the interface might serve as active
sites for hydroxymethyl and methyl formation,[20] respectively. Once these radicals are generated, they
subsequently migrate and combine to form the product ethanol over the surface of catalysts in Figure 4. The
appropriate participation of water in HER facilitates electrocatalytic conversion of methanol, because water
acts as a by-product in methanol electrocatalysis process. Meanwhile, this participation also enhances the
electrolyte solution pH to contribute to further methanol consumption. As aforementioned, higher pH favors
methanol electrocatalysis.
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Figure 3. In situ ESR spectra of electrocatalytic reaction systems in TCR in the presence of DMPO with or without electricity.
a) Electrochemical reaction conditions were a N2-saturated solution of 0.147 g NaCl in 25 mL methanol containing
Co3ZnC/NC catalyst with DMPO (cathode chamber) and that of 25 mL 0.5 M aqueous Na2SO4 (anode chamber).
Characterization conditions were microwave frequency of 9.44 GHz, microwave power of 20 mW, modulation frequency of
100.00 kHz; b) Reaction and characterization conditions were consistent with a)except for no addition of electricity; c) Related
conditions were consistent with b except for no addition of DMPO in cathode chamber.
Figure 4. Schematic illustration of Co3ZnC/NC catalyst for direct electrocatalytic conversion of methanol to ethanol and the
plausible reaction mechanism.
We also conducted control experiments without water or the electrolyte, NaCl, in SCR (Figure S10) for
in-depth understanding of the electrocatalysis mechanism in TCR. In situ ESR signals for the CH2OH radical
in TCR (Figure 3) are 2.4 fold higher than those without water in SCR (Figure S10a), indicating the
electrocatalytic capacity increases with the addition of a certain amount of water. Moreover, ESR spectra for
DMPO-CH3 disappear without NaCl and water in SCR (Figure S10b). Thus, the more prominent ESR signals
corresponding to high electrocatalytic performance in TCR confirm that methanol electrocatalysis to ethanol
is promoted by the optimized electron polarization of the catalysts induced by an electric field.
In summary, direct methanol electrocatalysis into ethanol with high efficiency was achieved in this work.
An excellent ethanol selectivity of 95.1 %, a yield of 175.6 g∙m-2∙h-1 and faradic efficiency 12.5 % were
obtained under the optimal reaction conditions. The tailorable participation of water in HER contributes to
methanol consumption in the electrocatalytic process. Control DMPO experiments and the corresponding ESR
confirm the generation of methyl and hydroxymethyl during the electrocatalytic process. The synergy between
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10.1002/cctc.201900437
ChemCatChem
electropositive low-valent Co species and adjacent electronegative carbon vacancies facilitates the formation
of hydroxymethyl radical and methyl on Co3ZnC/NC, respectively, which further react to generate ethanol.
This work paves a path for an amazing promotion of catalytic efficiency by electrocatalytic redox conversion
of methanol reactant as well as a highly efficient, nonpetroleum route for the synthesis of ethanol.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 21476188,
21106118).
Conflict of interest
The authors declare no competing interests.
Keywords: methanol electrocatalysis ethanol C-C coupling metal-carbon interface
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