Highly efficient and selective methanol production from paraformaldehyde and water at room temperature

Article abstract of DOI:10.1021/acscatal.8b00321

An efficient catalytic system using a water-soluble iridium complex, Cp?IrL(OH₂)²⁺ (Cp? = pentamethylcyclopentadienyl, L = 2,2′,6,6′-tetrahydroxy-4,4′-bipyrimidine), was developed for highly selective methanol production at room temperature (initial turnover frequency of 4120 h^-1) with a very high yield (93%). This catalytic system features paraformaldehyde as the sole carbon and hydride source, leading to a record turnover number of 18200 at 25 °C. A step-by-step mechanism has been proposed for the catalytic conversion of paraformaldehyde to methanol on the basis of density functional theory (DFT) calculations. The proposed pathway holds the potential capacity to extend the scope of indirect routes for methanol production from CO₂.

Full text of DOI:10.1021/acscatal.8b00321

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Article
Highly Efficient and Selective Methanol Production from
Paraformaldehyde and Water at Room Temperature
Lin Wang, Mehmed Z. Ertem, Kazuhisa Murata, James T. Muckerman, Etsuko Fujita, and Yuichiro Himeda
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00321 • Publication Date (Web): 01 May 2018
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Highly Efficient and Selective Methanol Production from
Paraformaldehyde and Water at Room Temperature
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†
‡
†
‡
‡
Lin Wang, Mehmed Z. Ertem* , Kazuhisa Murata, James T. Muckerman , Etsuko Fujita and
Yuichiro Himeda*†
†
Research Institute of Energy Frontier, Department of Energy and Environment
National Institute of Advanced Industrial Science and Technology
Tsukuba Central 5, 1ꢀ1ꢀ1 Higashi, Tsukuba, Ibaraki, 305ꢀ8565, Japan
‡Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973ꢀ5000, USA
2+
ABSTRACT: An efficient catalytic system using a waterꢀsoluble iridium complex, Cp*IrL(OH2) (Cp*
=
pentamethylcyclopentadienyl, L = 4,4′,6,6′ꢀtetrahydroxyꢀ2,2′ꢀbipyrimidine), was developed for
ꢀ
1
highly selective methanol production at room temperature (initial turnover frequency of 4120 h ) with
a very high yield (93%). This catalytic system features paraformaldehyde as the sole carbon and
hydride source, leading to a record turnover number of 18200 at 25 °C. A step–by–step mechanism
has been proposed for the catalytic conversion of paraformaldehyde to methanol based on density
functional theory (DFT) calculations. The proposed pathway holds the potential capacity to extend
the scope of indirect routes for methanol production from CO2.
KEYWORDS: iridium complexes, proton-responsive ligand, methanol production, transformation of
paraformaldehyde, homogeneous catalysis
INTRODUCTION
Methanol is an extremely important building block in the chemical industry that can also serve as a
sustainable and efficient energy carrier (12.6 wt% H ) in the future based on the proposed “methanol
2
1,2
economy” concept. Moreover, methanol is a sustainable source of liquid fuels and fuel additives
which can be converted into highꢀoctane gasoline (methanolꢀtoꢀgasoline process), dimethyl ether,
3
oxymethylene ethers, as well as other valueꢀadded petrochemicals. Currently, the industrialꢀscale
production of methanol is by the conversion of fossil feedstockꢀbased synthesis gas (CO, H , and
2
CO ) in the presence of heterogeneous catalysts (Cu/ZnO/Al O ꢀtype) under high pressure (50ꢀ250
2
2
3
4
bar) and elevated temperature (>200 °C).
In recent years, attractive ways to produce methanol from CO hydrogenation have been intensely
2
investigated. This would slash dependence on fossil feedstocks with prospects as an effective
approach for CO utilization. Several research groups have investigated the conversion of CO and
2
2
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H to produce methanol in the presence of homogeneous catalysts. However, these systems
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always suffer from high pressures and high operating temperatures, the use of organic solvents, and,
in most cases, low turnover numbers (TONs) and frequencies (TOFs). To minimize energy
consumption, improve the transformation efficiency, and provide a greener methanol economy,
methanol production at a low reaction temperature and in water is highly desired.
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Alternatively, significant recent research has aimed at an indirect approach for the conversion of
CO to methanol. For example, the use of readily available CO derivatives, such as carbamates,
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(
methyl/dimethyl/cyclic) carbonates, urea derivatives, formamides, and formic acid, as substrates to
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produce methanol as the product, which in certain cases also results in the formation of secondary
byproducts. Very recently, we reported another promising route for efficient methanol production
from the hydrogenation/disproportionation of formic acid using a Cp*Ir complex bearing
5
,5′ꢀdimethylꢀ2,2′ꢀbipyridine with a TON of 1314 and selectivity up to 47.1% at low temperature
6j
(
60 °C) under 5.2 MPa of H2. In this promising pathway for methanol production, more efficient
catalysts should be developed to compete against the more favorable formic acid dehydrogenation.
Formaldehyde is also one of the important C building blocks that is widely applied in the chemical
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industry and has been reported to be produced by catalytic hydrogenation of CO2, room
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temperature reduction of CO by borane, and the electrocatalytic reduction of CO and water.
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Moreover, an energy efficient lowꢀtemperature formaldehyde production from syngas in water
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(
Scheme 1, left) was also reported. Similar to formic acid and methanol, formaldehyde could be
12
envisioned as a new liquid organic hydrogen carrier (LOHC). Lately, H production from aqueous
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2a,13
formaldehyde or paraformaldehyde has received considerable attention.
Miller and Goldberg et
al. investigated the transfer hydrogenation of paraformaldehyde into methanol and methyl formate
ꢀ1
using a highly concentrated formic acid solution (TON = 7760; TOF = 240 h ) catalyzed by
[
Cp*Ir(2,2′ꢀbipyridine)(H O)][OTf] in a supplementary study of the disproportionation of formic acid
2
2
6g
to methanol. Prechtl et al. first reported selective methanol production from paraformaldehyde and
water catalyzed by [Ru(pꢀcymene)Cl ] with a high yield of 93% in the absence of an external hydride
2
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source at 80 °C. Although attractive, this catalytic system suffers from low durability with a
maximum TON of 493 after 3 cycles (i.e., all volatiles were removed in vacuo after each cycle, and
the reaction was repeated). Therefore, more efficient and durable catalysts under milder conditions
should be developed for this promising route for methanol production. High requirements for the
equipment and energy input are needed for the direct hydrogenation of CO to methanol. However,
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the indirect hydrogenation of CO based on the use of a CO derivative (formaldehyde) is beneficial
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because of its lower activation barrier. Furthermore, no more H was required for the transformation
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of formaldehyde to methanol, thus enabling milder reaction conditions and less energy input for
methanol production. In this context, methanol production from paraformaldehyde would not only be
of fundamental interest, but also extend the scope of indirect routes for methanol production from
CO2.
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Herein we report a waterꢀsoluble Cp*Ir catalytic system for selective methanol production from
paraformaldehyde and water in an open system with argon flow at room temperature, affording high
TON and TOF. The overall reaction pathway does not proceed by a Cannizzaroꢀtype reaction under
the applied reaction conditions (Scheme 1). We also present a step–by–step mechanism for the
conversion of paraformaldehyde to methanol, consistent with experimental observations, obtained
via theoretical calculations.
This Work
Selective methanol production
H2/boranes
2n CH OH + n CO
3 2
ON
CO2
CO
/electrocatalysis
3(HCHO)n
H2 + H2O
1.5n CH OH + 1.5n HCOOH
3
Low temperature
Cannizzaro reaction
OFF
Scheme 1. Highly selective methanol production from paraformaldehyde and H O at room
2
temperature.
RESULTS AND DISCUSSION
Chart 1. All the catalysts screened in this study.
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In the present study, a family of Ir, Rh, and Ru complexes were evaluated for their catalytic activity
towards methanol production (Chart 1). The complexes were prepared according to previously
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described procedures. In a typical reaction, the catalyst (0.083 mol%), 3 mmol paraformaldehyde,
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and 10 mol% K PO were dissolved in 5 mL H O (pH 12) and stirred at 30 °C under argon gas.
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Product yields were determined by H NMR spectroscopy utilizing acetonitrile as an internal
standard. The catalyst screening results are summarized in Table 1. First, control reactions were
carried out to test the catalytic capabilities (entries 1 and 2), indicating that K PO alone could not
3
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catalyze the reaction, and almost no Cannizzaro reaction occurred under the applied conditions. To
examine the catalytic activities systematically, multifunctional ligands (complexes 2ꢀ7) with varied
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ꢀ
ꢀdonor substituents at the 6 and 6' (or the 4 and 4') positions on the bipyridine ligand backbones
were adopted. We observed remarkable enhancement in the methanol production yield with donor
+
–
ligands (ꢀp < 0), especially with ꢀOMe and ꢀO substituents, compared to the unsubstituted complex
1
(entry 3 vs entries 4ꢀ8), which indicates the dependence of the catalytic activity on the bipyridine
ligand’s ꢀꢀdonor power. Interestingly, similar methanol yields were observed for the 4,4'ꢀdihydroxy
substituted complex (5, entry 7, 36%) and the corresponding 6,6' complex (6, entry 8, 34%), which
illustrates that the catalytic activity is not dependent upon the position of the substituent. Notably,
complex 7 bearing four hydroxyl groups displayed significantly higher activity than the other iridium
complexes (entry 9, 78%). The rhodium and ruthenium analogues (8 and 9) were also tested for
catalytic activity (entries 10 and 11), but lower yields were observed after 10 h (15% and 8%).
a
Table 1. Catalyst screening results for methanol production from paraformaldehyde and water
b
b
entry
catalyst
K
3
PO
4
/mol%
HCOOH yield /%
MeOH yield /%
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ꢀ
ꢀ
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ꢀ
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<1
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<1
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a
Reaction conditions: paraformaldehyde (3 mmol, 90 mg), catalyst (2.5 ꢀmol), H
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O (5 mL), time (10 h), 30 °C,
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b 1
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under Ar atmosphere. H NMR yield, average of at least two runs, CH CN as the internal standard. All values
are normalized to the reaction stoichiometry, only allowing a maximum of 67% methanol.
Catalyst 7 was selected for further studies as a model catalytic system. Various additives were
tested to determine the optimal conditions and to gain a better understanding of the factors
influencing the catalytic reaction (Table 2). It is clearly observed that the base additives could
promote high conversion of paraformaldehyde into methanol (entries 1ꢀ6), and the additive K CO3
2
exhibited the highest methanol yield (pH 11) (entry 2, 80%). However, only a trace amount of
methanol (<1%) was observed in the presence of the 10 mol% weak base NaOAc (pH 7.4) (entry 7)
and no activity for the formation of methanol was detected in the presence of H SO (pH 1.2) (entry 8)
2
4
indicating the requirement for highly basic conditions.
a
Table 2. Additive screening for methanol production from paraformaldehyde and water
b
entry
additive/mol%
PO /10%
CO /10%
CO /10%
KHCO /10%
NaHCO /10%
yield /%
1
2
3
4
5
6
7
8
K
3
4
78
K
2
3
80
Na
2
3
76
3
72
3
70
NaOH/10%
NaOAc/10%
78
<1
H
2
SO
4
/10%
0
9
K
2
CO
3
/5%
/10%
/20%
59
c
1
0
K
K
2
2
CO
3
3
93
c
1
1
CO
89
a
Reaction conditions: paraformaldehyde (3 mmol, 90 mg), catalyst 7 (2.5 ꢀmol), 30 °C, 10 h, all the reactions
b
c
were carried out under Ar atmosphere. HPLC yield. Run for 22 h.
Next, we examined the effect of the amount of K CO on the formation of methanol. Decreasing
2
3
the amount of K CO to 5% led to a much lower yield (59%, entry 9). Most notably, with the extension
2
3
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of the reaction time to 22 h for the full conversion of paraformaldehyde, a 93% yield of methanol was
obtained with 10% K CO (entry 10). On the other hand, a lower methanol yield of 89% was
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11) and under this condition, the Carnnizzaro reaction occurred (producing a 5% final formate yield),
thus decreasing the methanol selectivity. Therefore, 10 mol% of K CO was chosen as the optimal
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amount. The results of further experiments on the optimization of other reaction conditions such as
paraformaldehyde concentration and temperature are presented in Tables S1 and S2.
Figure 1. Reaction progress of methanol production in the presence of catalyst 7. Conditions:
2 2 3
paraformaldehyde (3 mmol, 90 mg, 94% purity), catalyst 7 (2.5 ꢀmol), H O (5 mL), K CO (10 mol%), 30 °C. All
the data were determined by HPLC.
It is known that formaldehyde can be released from paraformaldehyde, which is subsequently
12a,13a,13c,17
hydrated to produce methanediol in basic aqueous solutions.
In order to gain mechanistic
insights into the transformation to methanol, we monitored the reaction progress in the presence of
complex 7 via the highꢀperformance liquid chromatography (HPLC) technique (Figure S1). The
simultaneous conversion of formaldehyde and the yield of methanol are clearly displayed in Figure 1.
Interestingly, a clear change in the trend of formate concentration can be observed during the
catalytic process. After 15 min the yield of formate achieved the highest level (9% yield). However,
the formed formate finally was almost completely consumed (0.19% yield) after 22 h. In situ analysis
of the pH of the reaction process under the optimal conditions indicated a clear change (gradually
2
decreasing from pH 11 to 7) as seen in Figure S2, which arises from consumption of the base K CO3
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by the formed formic acid and CO . In our investigations herein, a basic condition is very important
2
for this reaction. In combination with the pH changes during the reaction process, after the fast initial
reaction stage, one major reason for the decreasing rate of methanol production is due to the
decreasing pH. Indeed, the paraformaldehyde was almost fully converted after 20 h with a yield
of >92.5%. Furthermore, the released gases during the reaction were analyzed by gas
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chromatography (GC). Only a trace amount of H (∼55 ppm) was detected; and no other gases were
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observed except the major product CO . In 2015, Fukuzumi et al. reported catalytic H and CO
2
2
2
+
2
production from paraformaldehyde using Cp*IrL′(OH2)] (L′ = 4ꢀ(1Hꢀpyrazolꢀ1ꢀylꢀκN )benzoic
3
13a
acidꢀκC ) in basic solution. In contrast, our catalytic system produces methanol with very good
selectivity.
In order to investigate the catalytic activity of catalyst 7 in more detail, the time course of the TON
for methanol production at room temperature (25 °C) was carried out (Figure 2). A very high TOF of
ꢀ1
4
120 h (average TOF for initial 30 min) was achieved. Remarkably, catalyst 7 is highly robust under
the applied conditions. A maximum TON of 18200 was obtained after 380 h, affording a very good
yield of >91%. These results, to our knowledge, represent the highest TON and TOF yet reported at
room temperature. We also examined the catalytic activities for 5, 6 and 7 under the optimal
conditions obtained for catalyst 7 and observed significantly less catalytic activity for 5 and 6
compared to 7 (Figure S3), which is attributed to less electron donating ability of ligands in 5 and 6.
Figure 2. Highly robust methanol production from a paraformaldehyde–water solution. Reaction conditions:
paraformaldehyde (15 mmol), 0.5 ꢀmol catalyst 7, K
2
3 2
CO (10 mol%), 25 mL H O, 25 °C.
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Mechanistic Studies
Based on the results obtained above and previous investigations on the transformation of
12a,13a,13c,14,18,19
paraformaldehyde
as well as our computational mechanistic investigation (see below),
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a plausible mechanism for methanol production from paraformaldehyde and water is proposed in
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Scheme 2. We performed density functional theory (DFT) calculations at the M06 level of theory in
21
conjunction with the SMD aqueous continuum solvation model (see computational methods in the
SI for details) and propose a stepꢀbyꢀstep mechanism for the conversion of paraformaldehyde to
methanol by catalysts 1, 6 and 7 in basic aqueous solutions. Catalysts 6 and 7 involve
protonꢀresponsive OH groups that can exist in protonated and deprotonated forms with distinct
hydrogen bonding modes and electron donating ability. Therefore, in order to examine the methanol
22
formation mechanisms, we applied a speciation approach in which the relative free energies of the
intermediates and transitionꢀstate structures of relevant protonation states of the complexes are
computed, and the changes in relative free energies with changes in pH are tracked. The results are
summarized in Scheme 2 and Table 3 for only the most stable protonation states of complexes 1, 6
and 7 at pH 12.0 (i.e., all OH groups are deprotonated), and detailed information on the energetics of
the other protonation states of the complexes is provided in the SI. Total charges of various Cp*Ir
complexes are omitted in the following discussions for clarity.
Scheme 2. Proposed mechanism for methanol production from a paraformaldehydeꢀwater solution. Total
charges of various Ir complexes in the scheme are omitted.
Here we describe the proposed mechanism for MeOH generation by complex 7 at pH 12 (Scheme
2), but energetics associated with complex 1 and 6 along with complex 7 are presented in Table 3. In
basic solution, the Cp*Ir complexes bearing an aqua ligand are predicted to be in equilibrium with
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the hydroxo complexes (ꢁG1 = −6.3 kcal/mol), either of which could react with methanediol
(
H C(OH) ) produced from paraformaldehyde and H O. The displacement of the aqua ligand by
2 2 2
methanediol (H C(OH) ) is uphill by only ꢁG = 3.4 kcal/mol. Then βꢀhydride elimination from
2
2
2
‡
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
4
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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8
9
0
1
2
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7
8
9
0
1
2
3
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
Ir−HOCH OH] (ꢁG = 15.0 kcal/mol) (Figure 3a) affords an iridium hydride complex, [Ir−H] (ꢁG =
2
1
3
–
−
15.6 kcal/mol), with simultaneous production of formate (HCOO ) (Scheme 2). The [Ir−H] complex
–
could react with H C(OH) , H C(OH)O or alternatively with formaldehyde (H CO) to produce the
2
2
2
2
methoxide intermediate [Ir−OCH ]. The optimized transition state structure for the reaction of the
3
[
Ir−H] complex with H C(OH) involves three additional water molecules to stabilize the newly
2
2
–
‡
forming OH anion (Figure 3b) and is associated with an inhibitively high activation free energy (ꢁG
2
=
45.2 kcal/mol) (see SI for details on optimized TSs with different numbers of explicit H O molecules
2
and details on the effects of inclusion of explicit solvent molecules on solvation of reacting species
‡
–
and computed ꢁG s). Another possibility is the reaction of [Ir−H] with H C(OH)O , which could exist
2
23
in equilibrium with H C(OH) (pK = 13.3) under high pH conditions. The optimized TS structure
2
2
a
–
‡
with H C(OH)O (Figure 3c) features a significantly lower activation free energy of ꢁG = 21.3
2
kcal/mol. We also considered the reaction of H CO with [Ir−H] (Figure 3d) which proceeds with a
2
‡
very low free energy of activation (ꢁG 3 = 7.8 kcal/mol). Although H CO is predicted to react with
2
H O to generate H C(OH) (ꢁG = −4.2 kcal/mol) in aqueous solutions, the formation of H CO inside
2
2
2
2
the solvent cage of [Ir−H] reacting with H C(OH) cannot be ruled out. The common product of all
2
2
three possible pathways investigated is the methoxide intermediate [Ir−OCH ], which could further
3
react with a H O molecule to generate methanol (CH OH) and [Ir−OH] (ꢁG = −4.2 kcal/mol)
2
3
6
followed by protonation to regenerate the initial aqua complex [Ir−OH ] (ꢁG = −6.3 kcal/mol).
2
1
–
In addition, the aqua complex can also react with the formed HCOO to generate a formato
complex [Ir−OCHO] (ꢁG = 5.0 kcal/mol), which then could evolve CO by βꢀhydride elimination
7
2
‡
(
ꢁG = 11.5 kcal/mol) (Figure 3e) to produce the [Ir−H] complex (ꢁG = −2.5 kcal/mol). Similarly,
4
8
–
methanol could be produced from the reaction of the [Ir−H] complex with H C(OH)O or H CO
2
2
(
Scheme 2). Furthermore, the formation of the hydride complex [Ir−H] in the proposed catalytic cycle
1
has been supported by HꢀNMR analyses as shown in Figure S4 (see SI for general procedure for
the formation of IrꢀH species).
‡
Table 3. Computed freeꢀenergy changes (ꢁGs) and activation free energies (ꢁG s) at pH 12 for the
chemical steps in the proposed mechanism for the methanol production in Scheme 2 for the most
stable protonation states of complexes 1, 6 and 7
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6
7
ꢁ
ꢁ
G
1
2
3.5
−4.8
1.8
−6.3
3.4
G
1.0
0
1
2
3
4
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6
7
8
9
0
1
2
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1
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0
1
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9
0
1
2
3
4
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6
7
8
9
0
‡
ꢁ
ꢁ
ꢁ
G
1
27.0
−28.4
18.4
15.0
ꢁ
G
3
−18.3
−15.6
‡
a
a
b
a
b
G
2
45.1
46.3 (22.2)
45.2 (21.3)
ꢁ
G
4
−2.5
−
−2.7
8.5
−3.7
7.8
‡
G
3
ꢁ
G
5
G
6
G
7
−6.7
−0.1
−5.6
11.3
−7.0
−7.0
−2.4
2.2
−7.9
−4.2
5.0
ꢁ
ꢁ
‡
ꢁ
G
4
11.7
−3.9
11.5
−2.5
ꢁ
G
8
a
The computed activation free energies are with respect to [Ir–H] and [H
2
C(OH)
2
(H
2
O)
3
] complexes as
b
–
reactants.
The computed activation free energies are with respect to [Ir–H] and [(H
2
C(OH)O )(H
2
O)
3
]
complexes as reactants.
(
a)
(b)
(c)
(d)
(e)
2ꢀ
3ꢀ
Figure 3. Optimized transition state structures of 7 for (a) βꢀhydride elimination from [Ir−HOCH
2
OH] , [Ir−H]
–
3ꢀ
reaction with (b) H
2
C(OH)
2
, (c) H
2
C(OH)O (d) H
2
CO and (e) βꢀhydride elimination from [Ir−OCHO] .
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The proposed mechanism based on the computations agrees favorably with several experimental
observations. Firstly, isotope and proton NMR studies of complex 7 show that during the conversion
of (HCHO) in D O only CH OD was observed, but no CH DOD, CHD OD, or CD OD (determined
n
2
3
2
2
3
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
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9
0
by H NMR spectroscopy, see Figure S5). In contrast, when (DCDO) was used in H O, only CD OH
n
2
3
(
no CH OH) formed. These results demonstrate that the existing C–H (or C–D) bond of methanediol
3
is preserved through the reduction; moreover, indicating that paraformaldehyde acts as the sole
carbon and hydride source in line with the computed mechanism as shown in Scheme 2 (note that
the hydrogen atoms originating from the C–H group of methanediol are colored red). Secondly, we
carried out a deuterium kinetic isotope effect (KIE) study with catalyst 7. The reaction rates over the
initial 15 min were investigated using deuterated substrates and solvents as shown in Figure S6.
Moreover, the relative KIE results based on the reaction rates obtained from Figure S6 are shown in
Table S3. The KIE experiments suggest that the deuterated formaldehyde substrate (KIE: 1.6, entry
3) is more influential than deuterated water (KIE: 1.1, entry 2) on the reaction rate. This is in line with
the proposed catalytic cycles in Scheme 2 and the computed data in Table S4, which show the
important role of [Ir−H], with the possible rate determining step of [Ir−H] to [Ir−OCH ] in both cycles A
3
and B for methanol production. Thirdly, 1 shows only a negligibly small methanol yield and our
computations show that the βꢀhydride elimination from the [Ir−HOCH OH] intermediate involves
2
‡
‡
‡
1
quite a high ꢁG of 27.0 kcal/mol compared to those of 6 (ꢁG = 18.4 kcal/mol) and 7 (ꢁG = 15.0
1
1
kcal/mol), which is predicted to be the limiting factor for methanol production for complex 1 as other
reaction steps display energetics similar to 6 and 7 (Table 3). On the other hand, for complexes 6
and 7 the methoxide intermediate ([Ir−OCH ]) production is predicted to be rate limiting and for all
3
–
three possible pathways investigated for the reaction of H C(OH) , H C(OH)O or H CO with the
2
2
2
2
[
Ir−H] intermediate, complex 7 exhibits more favorable kinetics and thermodynamics compared to
‡
those of 6, in line with the observed improved activity of 7. Finally, the computed ꢁG s indicate that
‡
βꢀhydride elimination from [Ir−OCHO] (ꢁG 4 = 11.5 kcal/mol for 7) to be much faster compared to
‡
βꢀhydride elimination from [Ir−HOCH OH] (ꢁG = 15.0 kcal/mol for 7) so that the formate product
2
1
which builds up from the latter reaction is predicted to be consumed at a fast rate in line with
experimentally observed formate concentration profiles during the course of methanol production
(
Figure 1).
CONCLUSIONS
The first example of the robust and selective catalytic production of methanol from
paraformaldehyde and water at room temperature has been developed. For the case of complex 7, a
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very high yield of methanol with a high TOF and TON was achieved. Furthermore, a stepꢀbyꢀstep
mechanism for the production of methanol from paraformaldehyde and water is provided based on
density functional theory calculations. On the basis of current rapid development of CO2
0
1
2
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4
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0
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transformations and establishment of a methanol economy, we herein add a promising pathway to
the important field of C chemistry, which may eventually open up the possibility of the renewable
1
production of methanol from CO at room temperature and low pressures (ideally atmospheric
2
pressure).
ASSOCIATED CONTENT
Supporting Information.
Experimental section including general methods, NMR studies and computational details. This
material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
*
Yuichiro Himeda, Eꢀmail: himeda.y@aist.go.jp
Mehmed Z. Ertem, Eꢀmail: mzertem@bnl.gov
ORCID
Lin Wang: 0000ꢀ0001ꢀ5644ꢀ8865
Mehmed Z. Ertem: 0000ꢀ0003ꢀ1994ꢀ9024
Kazuhisa Murata: 0000ꢀ0002ꢀ5857ꢀ7668
James T. Muckerman: 0000ꢀ0001ꢀ5169ꢀ8929
Etsuko Fujita: 0000ꢀ0002ꢀ0407ꢀ6307
Yuichiro Himeda: 0000ꢀ0002ꢀ9869ꢀ5554
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported by ENEOS Hydrogen Trust Fund. The work at BNL was carried out with
support from the U.S. Department of Energy, Office of Science, Division of Chemical Sciences,
Geosciences & Biosciences, Office of Basic Energy Sciences under contract DEꢀSC0012704.
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