Direct Catalytic Methanol-to-Ethanol Photo-conversion via Methyl Carbene

Article abstract of DOI:10.1016/j.chempr.2019.01.005

A photo-driven direct methanol-to-ethanol conversion is reported with a robust gallium nitride catalyst under ambient conditions. This conversion is achieved with no solvent, ligand, additive, heating, atmosphere, or pressurization—just with light irradiation. A methyl carbene reaction intermediate is observed during the conversion, and the method enables access to the more useful (as both fuel and starting material) renewable resource ethanol. As an important effort to secure the sustainable “fossil alternative,” the direct conversion of the more readily available methanol to the more user-friendly, less toxic, and broadly applicable ethanol poses exciting potential as well as a tremendous scientific challenge. Herein, we report the first photo-driven one-step conversion of methanol to ethanol at ambient temperature, catalyzed by an ultra-stable gallium nitride semiconductor. Mechanistic studies revealed that methyl carbene (methylene), one of the most fascinating C₁ building blocks in synthetic chemistry, was generated as a reaction intermediate, which potentially enables a green and novel method for generating carbene. We also found that methanol can be converted to n-propyl alcohol with the same catalyst through a simple change in reaction temperature, giving a unique selectivity and a high-value-added product. Methanol is an easily accessible fossil fuel alternative, but it is classified as hazardous and is also generally less valuable than other sources of carbon. A direct conversion of methanol to ethanol would provide facile access to a renewable starting material for applications as a safer fuel or an intermediate for the synthesis of the most demanded plastic, polyethylene (PE). This article reports the chemical transformation of methanol to ethanol and other higher alcohols, enabled by sp³-C–H methylation. In addition, the underlying chemistry can also be of importance for biochemistry and pharmaceutical chemistry, where methylation plays a pivotal role. The methanol-to-ethanol conversion is achieved in the presence of a robust catalyst and in only one step. No additive, solvent, or hazardous material is required.

Full text of DOI:10.1016/j.chempr.2019.01.005

Article
Direct Catalytic Methanol-to-Ethanol Photo-
conversion via Methyl Carbene
Mingxin Liu, Yichen Wang,
Xianghua Kong, ..., Hong Guo,
Zetian Mi, Chao-Jun Li
zt.mi@umich.edu (Z.M.)
cj.li@mcgill.ca (C.-J.L.)
HIGHLIGHTS
Transformation of a hazardous
fuel into a safer and more useful
renewable fuel
Generation of the versatile
reaction intermediate methyl
carbene from methanol
One-step conversion using only
light and no other additive
Carbon chain elongation could
enable conversions to materials
such as plastic (PE)
A photo-driven direct methanol-to-ethanol conversion is reported with a robust
gallium nitride catalyst under ambient conditions. This conversion is achieved with
no solvent, ligand, additive, heating, atmosphere, or pressurization—just with
light irradiation. A methyl carbene reaction intermediate is observed during the
conversion, and the method enables access to the more useful (as both fuel and
starting material) renewable resource ethanol.
Liu et al., Chem 5, 1–10
April 11, 2019 ª 2019 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2019.01.005

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doi.org/10.1016/j.chempr.2019.01.005
Article
Direct Catalytic Methanol-to-Ethanol
Photo-conversion via Methyl Carbene
Mingxin Liu,1,2,5 Yichen Wang,2,5 Xianghua Kong,3,5 Roksana T. Rashid,2 Sheng Chu,2 Chen-Chen Li,1
1,6,
Zoe Hearne,1 Hong Guo,3 Zetian Mi,2,4, and Chao-Jun Li
*
*
¨
SUMMARY
The Bigger Picture
Methanol is an easily accessible
fossil fuel alternative, but it is
classified as hazardous and is also
generally less valuable than other
sources of carbon. A direct
conversion of methanol to ethanol
would provide facile access to a
renewable starting material for
applications as a safer fuel or an
intermediate for the synthesis of
the most demanded plastic,
polyethylene (PE). This article
reports the chemical
As an important effort to secure the sustainable ‘‘fossil alternative,’’ the direct
conversion of the more readily available methanol to the more user-friendly,
less toxic, and broadly applicable ethanol poses exciting potential as well as a
tremendous scientific challenge. Herein, we report the first photo-driven one-
step conversion of methanol to ethanol at ambient temperature, catalyzed by
an ultra-stable gallium nitride semiconductor. Mechanistic studies revealed
that methyl carbene (methylene), one of the most fascinating C1 building blocks
in synthetic chemistry, was generated as a reaction intermediate, which poten-
tially enables a green and novel method for generating carbene. We also found
that methanol can be converted to n-propyl alcohol with the same catalyst
through a simple change in reaction temperature, giving a unique selectivity
and a high-value-added product.
transformation of methanol to
ethanol and other higher alcohols,
enabled by sp3-C–H methylation.
In addition, the underlying
INTRODUCTION
The urgent need for a sustainable energy source to resolve worldwide reliance on
fossil fuels is motivated by the finite nature of these resources, the greenhouse
gas emissions associated with their combustion, and geopolitical tensions that
influence their availability.1 Methanol (MeOH) and ethanol (EtOH) are promising
alternatives to fossil fuels. However, they are both encountering challenges in their
development and implementation. Although MeOH is easily accessed from sustain-
able sources (CO2, methane, biomass, etc., annually produced into over 300,000
metric tons of renewable MeOH and composing 8% of the total MeOH production
in the United States),2 many major fuel consumers are abandoning MeOH because
of its toxicity and environmental hazards.3,4 On the other hand, despite the fact that
EtOH is more user friendly, broadly applicable, and less toxic than MeOH,5 EtOH is
predominantly produced from crops,6 and its widespread application has been held
back as a result of ‘‘food-competition’’ concerns. A strategy that combines the ad-
vantages while limiting the disadvantages of MeOH and EtOH would greatly
contribute to solving the problems that currently hinder the advancement of sustain-
able fuels.
chemistry can also be of
importance for biochemistry and
pharmaceutical chemistry, where
methylation plays a pivotal role.
The methanol-to-ethanol
conversion is achieved in the
presence of a robust catalyst and
in only one step. No additive,
solvent, or hazardous material is
required.
Among the possibilities, MeOH-to-EtOH conversion chemistry is believed to be
particularly intriguing. Although this conversion holds great potential to address
the above-mentioned problems, the seemingly simple increase of one carbon to
the molecule has proven to be highly challenging. Up until the present, as illustrated
in Figure 1, this conversion has been achieved via three strategies, including carbon-
ylation and hydrogenation (route 1),7–9 direct reductive carbonylation (route 2),10–13
and carbonylation via dimethyl oxalate intermediate (route 3).14,15 However, they
are mostly thermally driven reactions that require high reaction temperatures. In
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addition, toxic materials such as carbon monoxide are extensively used. For this
conversion to be viable practically, a transformation that proceeds under mild
conditions and has high atom economy by avoiding additional reagents and redun-
dant intermediates is of great importance but scientifically challenging. To the best
of our knowledge, there is no previous report on a transformation that fulfills those
conditions. Herein, we report the first one-step MeOH-to-EtOH conversion, driven
by light at ambient temperature and catalyzed by a gallium nitride (GaN)
semiconductor.
RESULTS AND DISCUSSION
Catalyst Design, Synthesis, and Characterization
Previously, we have demonstrated facile and reliable methods to generate well-
defined, thermally and chemically stable (melting point > 2,500ꢀC) GaN nanostruc-
tures by using a molecular beam epitaxy (MBE).16–21 Among them, GaN nanowire
(NW) arrays exhibit exceptional surface (area density < 0.1 mg,cmÀ2), optical
(<365 nm light absorption), and energy band (ꢁ 3.4 eV)22 properties that enable it
to serve as the ideal platform to test photochemical catalytic reactions. Therefore,
we commenced our study by synthesizing a GaN NW catalyst (see Experimental
Procedures and Supplemental Information for experimental details). Electron micro-
scopy results (Figure 2) confirmed the morphology of the generated NW as cylinder-
shaped rods with average diameters of 100 nm and heights of 800 nm. Two types of
surfaces, the polar surface (c-plane) located at the end of the rods and the non-polar
surface (m-plane) located at the side of the rods, were presented in the NW, which
aligns vertically to the Si substrate and gives a brush-like overall appearance. The
GaN can also form an n- or p-type semiconductor by incorporating trace amounts
of dopant, giving a significantly altered surface energy band structure and Fermi
level compared to intrinsic GaN (i-GaN) NW.22
Catalyst Performance
We then investigated the catalytic activity by using only 0.35 mg of the synthesized
GaN NWs for MeOH-to-EtOH conversion in a sealed, evacuated reactor under light
illumination (Figure 3A; see Experimental Procedures and Supplemental Information
for experimental detail). With 25 mL MeOH loaded, it was discovered that 17 mmol of
EtOH was generated in 12 h with p-GaN NWs, giving an impressive 48,500 mmol,gÀ1
or 4,050 mmol,gÀ1,hÀ1 catalyst efficiency. GaN NWs with different dopants were
then examined. The i-GaN NW and n-GaN NW gave approximately half of the
EtOH yield (10 and 8 mmol EtOH generated, respectively), showing that high yields
strongly rely on p-type doping. 0.35 mg commercial GaN powder gave a further
1Department of Chemistry and FRQNT Centre for
Green Chemistry and Catalysts, McGill University,
801 Sherbrooke Ouest, Montreal, QC H3A 0B8,
reduced yield of 7 mmol. Control experiments were also conducted. It was demon-
strated that the reaction was completely shut down without GaN or light. The cata-
Canada
lyst also exhibits no sign of deactivation after 4 consecutive reaction cycles (Fig-
2Department of Electrical and Computer
Engineering, McGill University, 3480 University,
was used to characterize the catalyst before and after light illumination (Figure S10).
Montreal, QC H3A 0E9, Canada
ure 3B). To examine the catalyst stability, X-ray photoelectron spectroscopy (XPS)
The Ga3d peak binding energy showed that the catalyst remains unchanged during
3Department of Physics, McGill University,
the reaction. An isotopic labeling experiment was also conducted with MeOH-d4
(CD3OD), returning fully deuterated CD3CD2OD (Figure S6). This further confirmed
that the EtOH was synthesized from MeOH.
Rutherford Building, 3600 University, Montreal,
QC H3A 2T8, Canada
4Department of Electrical Engineering and
Computer Science, University of Michigan, 1301
Beal Ave, Ann Arbor, MI 48109, USA
Because the reaction is likely to take place on the surface of the GaN NW, it is neces-
sary to determine which surface of GaN NW enables the desired MeOH-to-EtOH
conversion. As previously characterized,16,23 the polar, N-terminated c-plane and
non-polar m-plane are suggested to correspond to the end and side of the NW,
respectively (Figure 3C). The corresponding surface areas of c-plane and m-plane
5These authors contributed equally
6Lead Contact
*Correspondence: zt.mi@umich.edu (Z.M.),
cj.li@mcgill.ca (C.-J.L.)
https://doi.org/10.1016/j.chempr.2019.01.005
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Figure 1. Previous Methods to Convert MeOH to EtOH and Our Work
of our catalyst can also be calculated based on transmission electron microscopy
(TEM) and scanning electron microscopy (SEM) measurements of the diameter,
length, and density of GaN NW.16 In addition, reducing the growth time while main-
taining other growth conditions leads to shorter NW formation. The length reduction
of NW grown for 2 h compared to 4 h was confirmed by SEM analysis (approximately
400 versus 800 nm), which also confirms that c-planes of these two types of NW were
identical. As illustrated in Figure 3D, both these catalysts showed almost the same
reactivity (all approximately 17 mmol EtOH generated). To further examine this phe-
nomenon, we used commercially available GaN thin film with the same N-termi-
nated c-plane to test the conversion. Astonishingly, even with the absence of
m-plane, the GaN film showed a similar EtOH yield of 17 mmol, indicating that the
MeOH-to-EtOH conversion most likely takes place on the polar c-plane of GaN
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Figure 2. Scanning Electron Microscopy (SEM) Characterization of GaN NW Catalyst
(A and B) Low-resolution (A) and high-resolution (B) 45ꢀ-view SEM. The inset in (A) shows a catalyst slice viewed by the naked eye.
(C) High-resolution top-view SEM showing the c-plane located at the end of an NW.
(D) Graphic of an NW showing the average diameter and length.
NW, which can be further testified by the scaling of EtOH yield when a larger GaN
film was used under identical reaction conditions. This is particularly intriguing
because most of our previous GaN-catalyzed reactions of organic molecules took
place on the non-polar m-plane of the NW due to C–H activation by both nitrogen
and gallium,16–18 which is not exposed23 on the suggested N-terminated c-plane,
implying a completely different reaction mechanism.
Mechanistic Study
We then conducted a mechanistic study by carefully reviewing the high-resolution
1H-NMR spectrum of the reaction product. Besides the discovery of water in our re-
action, to our surprise, a small singlet signal at 0.23 ppm was discovered, which sug-
gests the presence of cyclopropane. Cyclopropane purchased commercially also
gave the singlet signal at the same chemical shift (Figures 4A and 4B). These results
suggest that the MeOH-to-EtOH methylation process may proceed via a carbene
mechanism. A methyl carbene intermediate may be generated from the elimination
of water from the MeOH molecule, which may then undergo transformations that are
consistent with classic carbene-type reactivity: either methylation of another MeOH
molecule to give the EtOH product or dimerization and cycloaddition to give the
4
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A
B
C
D
Figure 3. Catalyst Performance Investigation
(A) Performance of catalysts with different doping using the same amount of GaN.
(B) Catalyst recyclability test; the same catalyst was used without further treatment in between each cycle (12 h for each cycle).
(C) Catalyst surface composition adjustment with growth time.
(D) EtOH generation efficiency with catalysts from (C), showing catalyst surface area (gray, m-plane; black, c-plane).
cyclopropane. Such reactivity is highly unique and has made methyl carbene and its
derivatives among the most fascinating single-carbon (C1) building blocks in syn-
thetic chemistry.24 To further test our hypothesis, we introduced ethylene into the
reaction mixture during the reaction and again obtained the same cyclopropane
1H-NMR signal at 0.23 ppm (Figure 4C; note that almost no EtOH was generated
at this time, indicating that ethylene captures the methyl carbene). Using
exactly the same protocol, we conducted a parallel experiment by introducing
ethylene into the MeOH-d4 reaction mixture. The 1H-NMR gave a quintet-like signal
instead of the singlet signal at 0.23 ppm with a coupling constant of 3JD-H = 1.3 Hz
(Figure 4D). This further indicates the capturing of a methyl carbene intermediate
generated from MeOH during our reaction by the introduction of ethylene. The
isotope labeling experiment using MeOH-d2, which consists of two C–D bonds
and one C–H bond, was also conducted (Figure 4E; see also Figure S7), producing
approximately the same quantities of EtOH -d3 and EtOH -d4. The density func-
tional theory (DFT) calculation also indicates the generated carbene to be adsorbed
at the suggested N-terminated GaN surface (Figure 5; see also Table S2).
On the basis of the above experiments, we propose the reaction mechanism
of the MeOH-to-EtOH conversion as follows (Figure 4E): as the energy band of
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A
B
C
A
B
C
D
D
E
F
Figure 4. Mechanistic Experiments and Corresponding High-Resolution Upfield 1H-NMR Results
(A) Commercial cyclopropane as a control.
(B) Methanol-to-ethanol conversion under optimized conditions, generating cyclopropane.
6
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Figure 4. Continued
(C) Introduction of ethylene into the optimized experiment.
(D) Parallel experiment of (C) using methanol-d4.
(E) Methanol-to-ethanol conversion using methanol-d2.
(F) Proposed reaction mechanism.
p-GaN bends downward near the surface,22 the photoexcited electrons can
aggregate on the GaN surface instead of in internal regions (see Figure S11
for more discussions regarding band bending). This results in a region of highly
negatively charged NÀ at the c-plane surface upon light illumination. The NÀ
then attacks the MeOH C–H bond (while the O–H bond is also open to attack,
it does not cause sequential transformations; see also Figure S12), followed by
the elimination of a water molecule to generate the methyl carbene. The methyl
carbene can then methylate the C–H bond of MeOH to give EtOH or cyclopro-
pane via dimerization and cyclopropanation. Hypothetical methyl carbene gener-
ation from MeOH is not without precedent.25 However, in this case, the carbene
did not take part in C–H methylation, dimerization, or cyclopropanation reactions
and was only quenched by the O–H bond of MeOH to give ether, which was not
detected in our study. This difference in reactivity could indicate that the former
carbene is more electron poor, whereas the photo-generated carbene from the
current study might be more electron rich.26,27 Until present, carbene-type C–H
methylation, dimerization, and cyclopropanation transformations were generally
not achieved unless hazardous reagents and/or workups were applied, which
also generates highly volatile carbene equivalents.28–31 The photochemical
method reported here can potentially offer a green and sustainable alternative
to afford such carbene-type reactivities, which can be highly desirable in syn-
thetic chemistry.
Further Applications
At the end of our investigation, we were excited to examine wider applications for
the MeOH photo-methylation system. As an initial design, we envisioned that by
lowering the reaction temperature, the desorption rate of the generated EtOH on
the GaN surface might be reduced, which would potentially enable a second
methyl carbene to attack EtOH on the GaN surface and generate higher alcohols
with increased carbon numbers. To our delight, with an experiment conducted un-
der the optimized reaction conditions except for a reduced reaction temperature
of 0ꢀC (Figure 6A), 2 mmol n-propyl alcohol (n-PrOH) was detected, which had not
been detected in any previous experiments. Further lowering the reaction temper-
ature resulted in reduced conversion of MeOH. Intriguingly, no isopropyl alcohol
(i-PrOH) was detected. This unique regioselectivity is consistent with common car-
bene reactivity:24 the methyl carbene generated on the GaN surface preferentially
attacks the b carbon, also generated on the GaN surface, rather than the a carbon
originated from MeOH and positioned off-surface (Figure 6B). To examine this hy-
pothesis, when MeOH and EtOH were introduced simultaneously to the photo-
conversion conditions, both n-PrOH and i-PrOH were detected (Figure 6C; see
also Figure S5). In this case, the generated methyl carbene can methylate either
the a or b carbon of the introduced EtOH, which gave a significantly different
result compared to the methylation of in-situ-generated EtOH on the GaN surface
(as described in Figure 6B). This result showed that our reaction system may have
the potential to readily carry out a wider range of lower-to-higher-alcohol conver-
sion processes, generating high-value-added products.32,33 In the attempt to
examine large-scale MeOH-to-EtOH conversion, 10 mL MeOH, charged with
20 mg commercial GaN powder, was examined and illuminated for 24 h. To
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Figure 5. DFT Calculation Showing the Fully Relaxed Atomic Structure of Methyl Carbene on the
c-Plane of GaN
Top (A) and side (B) view of the fully relaxed atomic structure of carbene adsorption on GaN (0001)-
wurtzite with a reconstruction of 2 3 2 N adatom (H3). The corresponding adsorption energy is
À7.44 eV.
our surprise, the resulting solution comprised 0.3 vol % EtOH in MeOH (Fig-
ure 6D), indicating the generation of approximately 470 mmol EtOH. The yield
of EtOH can be further increased by using p-GaN NW instead of commercial
GaN powder as the catalyst, giving 0.5 vol % EtOH in MeOH. In addition,
compared with the common alcohol fermentation processes, which usually obtain
EtOH in water solution, the EtOH-in-MeOH solution generated in this report re-
quires much less energy for distillation purifications, which can be appealing
regarding industrialization.
Conclusion
In summary, we have demonstrated the first application of photochemistry in the
direct conversion of MeOH to EtOH catalyzed by the c-plane of GaN. This one-
step photochemical conversion shows high catalytic efficiency and exceptional
atom economy. Mechanistic studies suggested the generation of a methyl carbene
intermediate on the polar c-plane of the GaN surface in this novel catalytic process.
Furthermore, the reported method was also shown to enable MeOH to other higher
alcohol conversion and large-scale conversion with unique selectivity. Further appli-
cations of GaN catalysts toward other transformations are already underway in our
laboratory.
EXPERIMENTAL PROCEDURES
The growth of GaN NW was accomplished with plasma-assisted-MBE (PA-MBE) on
Si wafer as the substrate under nitrogen-rich conditions. The plasma power was
adjusted at 400 W and substrate temperature at 750ꢀC. The NW was grown for
4 h. The GaN can also form an n- or p-type semiconductor by incorporating trace
amounts of tetravalent silicon (Si4+) or divalent magnesium (Mg2+) dopant,
respectively.
MeOH-to-EtOH Photo-conversion
A slice of p-GaN NW (slice size = 3.5 cm2, equivalent to 0.35 mg GaN) was placed at
the bottom of a 120 mL air-tight quartz reactor, which was then evacuated under
high vacuum (< 5 3 10À2 mbar) at 400ꢀC for 30 min to remove adsorbate. The reactor
was then allowed to cool to room temperature before being submerged in a 15ꢀC
chiller. High-performance liquid chromatography (HPLC)-grade MeOH (25 mL or
0.618 mmol, free of any trace of EtOH, confirmed by gas chromatography-mass
spectrometry [GC-MS]) was then injected into the reactor, which was then
8
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A
B
C
D
Figure 6. Investigating Further Applications for the Reported Process
(A) MeOH-to-EtOH and MeOH-to-n-PrOH conversion using our method.
(B) Proposed rational for selectivity of n-PrOH over i-PrOH on the GaN surface.
(C) Conversion of the MeOH-EtOH mixture.
(D) Large-scale MeOH-to-EtOH conversion.
illuminated by a 300 W Xe lamp for 12 h. The reaction product was characterized by
GC-MS.
SUPPLEMENTAL INFORMATION
Supplemental Information includes 12 figures and 3 tables and can be found with
this article online at https://doi.org/10.1016/j.chempr.2019.01.005.
ACKNOWLEDGMENTS
M.L. and C.-J.L. are grateful to the Canada Research Chair (Tier 1) foundation, the
Natural Science and Engineering Research Council of Canada, the Fonds de Re-
cherche du Quebec – Nature et Technologies, the Canada Foundation for Innova-
tion, and McGill University for financial support. M.L. is also grateful to Dr. Robin
Stein from the McGill Chemistry NMR Facility and Dr. Tara Sprules from the
Quebec/Eastern Canada High Field NMR Facility (QANUC) for discussion and
help with NMR studies. Z.M. and Y.W. are grateful to Emission Reduction Alberta
for financial support. X.K. and H.G. acknowledge financial support from the Natural
Science and Engineering Research Council of Canada and thank Compute Canada
and the High Performance Computing Center of McGill University for computation
facilities.
AUTHOR CONTRIBUTIONS
M.L. discovered the reported process, conceived the concept, designed and carried
out all the experiments except for the growth of GaN NW and microscopic charac-
terization, and composed the manuscript. Y.W. designed and grew all the GaN NWs
by using MBE and characterized them by using TEM and SEM. X.K. and H.G. contrib-
uted to the DFT study. R.T.R. contributed to the experimental part. S.C. helped in
the microscopic characterization. C.-C.L. participated in some crucial discussions.
Z.H. helped proofread the entire manuscript. General guidance, project directing,
and manuscript revisions were done by Z.M. and C.-J.L.
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DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: June 26, 2018
Revised: December 21, 2018
Accepted: January 16, 2019
Published: February 14, 2019
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