Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol

Raktim Sen, Alain Goeppert, Sayan Kar, and G. K. Surya Prakash*

Communication

Hydroxide Based Integrated CO Capture from Air and Conversion

to Methanol

Cite This: https://dx.doi.org/10.1021/jacs.9b12711

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sı Supporting Information

ABSTRACT: The ﬁrst example of an alkali hydroxide-based system for CO capture and conversion to methanol has been

established. Bicarbonate and formate salts were hydrogenated to methanol with high yields in a solution of ethylene glycol. In an

integrated one-pot system, CO was eﬃciently captured by an ethylene glycol solution of the base and subsequently hydrogenated to

CH OH at relatively mild temperatures (100−140 °C) using Ru-PNP catalysts. The produced methanol can be easily separated by

distillation. Hydroxide base regeneration at low temperatures was observed for the ﬁrst time. Finally, CO capture from ambient air

and hydrogenation to CH OH was demonstrated. We postulate that the high capture eﬃciency and stability of hydroxide bases

make them superior to existing amine-based routes for direct air capture and conversion to methanol in a scalable process.

arbon dioxide, a potent greenhouse gas, is the main

component of industrial eﬄuents and automobile

emissions. Over the past two centuries, we witnessed a

major rise of anthropogenic CO emissions leading to an

exponential increase in atmospheric CO concentration from

about 280 ppm to more than 415 ppm, currently. This

phenomenon has already led to a substantial average global

temperature rise of ∼0.8 °C compared to preindustrial levels.

Related problems such as global sea rise, ocean acidiﬁcation,

and more extreme and unpredictable weather patterns have

also emerged. In an united eﬀort to curb global warming, the

Kyoto Protocol followed by the Paris Agreement were signed

by most countries with the aim of curbing greenhouse gas

emissions and transition to renewable energy alternatives.

Among various proposed solutions, direct air capture of CO₂

DAC) has recently attracted momentous attention. This

(

process allows the point of capture to be independent from the

emission sources. Hence, it enables capture stations to be free

of geographical constraints and allows the capture of CO from

small and dispersed point sources responsible for about half of

total emissions. Once CO is captured, it can be desorbed,

pressurized, and sequestered under earth’s surface in what is

known as Carbon Capture and Sequestration (CCS). An

alternate and value-added route is the utilization of the

captured CO (Carbon Capture and Utilization, CCU) as a C1

source by conversion into various products including chemical

Figure 1. Reported integrated CO capture and conversion systems

compared to the present study

feedstocks and polymers. CO can be hydrogenated with H

generated through H O electrolysis using renewable energy.

Among possible hydrogenation products, methanol is one of

the most attractive. With a current annual global production of

amines as capturing agents. These amine systems can suﬀer

from volatility issues, oxidative degradation, and notable

over 100 billion liters, CH OH is already a C1 feedstock for a

multitude of chemicals and products and a superior fuel for

internal combustion engines and fuel cells. CH OH is also an

attractive liquid hydrogen carrier (12.6 wt % of H₂).

Received: November 25, 2019

Recently, the integration of CO capture from air and

subsequent conversion to methanol was spearheaded by us as

well as others (Figure 1). Until now, such systems used

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Communication

toxicity, making them less suitable for DAC in a scaled-up

Analogous to metal hydrogen(bi) carbonates, metal alkyl

process. Alternate CO scrubbing agents used on a large scale

carbonates are formed when CO reacts with metal alkoxides.

are alkali hydroxide solutions. Hydroxides have higher capture

While this concept is well established, its role in the ﬁeld of

carbon capture has not been well explored. In 2016, Wee et

eﬃciency from dilute CO sources than amines. However,

recycling of the base through a conventional causticization−

al. demonstrated that CO

capture in an ethanolic NaOH

calcination−slaking process leads to a high energy penalty with

solution formed sodium ethyl carbonate having minimal

5c,12

solubility in ethanol (Table 1, entry 1). For the present

required temperatures of 700 °C and above.

On the other

hand, practical utilization of metal carbonates as C1-building

blocks is not well established. In our previous work on alkali

hydroxide-based capture and conversion, we captured CO₂

from air and synthesized formate salts. However, further

Table 1. CO Capture by Alcoholic Hydroxide Solutions

CO captured

CO /

Solubility of

carbonate

entry

Base

(mmol)

hydrogenation of the formate to CH OH was found to be

NaOH

KOH

2.5

1.5

∼1

∼0.6

insoluble

soluble

3b,14

ineﬀective.

^A^l^c^o^h^o^l^-^a^s^sⁱ^s^t^e^d^C^O2 hydrogenation to methanol via

formate ester has been studied in great detail.

10h,15

Hence,

Ca(OH)₂

insoluble

we speculated that alcohols should be able to mediate the

hydrogenation pathway from formate salt to methanol through

the key formate ester intermediate. It was also shown that

formamides are essential intermediates for amine assisted

Capture conditions: base (2.5 mmol), ethylene glycol (5 mL),

stirring (800 rpm), rt, t = 3 h. Captured CO amounts determined

gravimetrically. Moles of CO₂captured per mole of hydroxide.

Ethylene glycol replaced by EtOH. 1.25 mmol of Ca(OH) was

10f

hydrogenation of CO to CH OH (Figure 1). Herein, we

used. Calculations error ±5%.

report a catalytic system for CO capture and conversion to

CH OH involving alkali-metal hydroxides (capturing agent)

and alcohol (formate ester facilitator). This process has

numerous advantages, including the following: (1) alkali

study, the alcohol should have the following properties: (1)

high boiling point for easy separation of methanol by

distillation, and (2) high dielectric constant enabling facile

dissolution of the hydroxide base and the formed metal alkyl

carbonate for eﬃcient hydrogenation. Hence, ethylene glycol

hydroxides have high eﬃciency for DAC of CO ; (2)

widespread availability of the hydroxide bases; (3) low toxicity,

volatility, and higher stability over amines; (4) hydrogenation

of ester intermediates is more facile than formamide

(

bp = 197.6 °C, ε = 37) was selected for our one-pot CCU

system. Interestingly, a solution of NaOH in ethylene glycol

intermediates.

Additionally, for the ﬁrst time, this system

captured CO quantitatively and the obtained carbonates were

oﬀers easy integration with already existing hydroxide-based

completely soluble (entry 2). The capture capacity of a KOH

solution was similar to that of NaOH (entry 3). With

Ca(OH) , the CO capture was signiﬁcantly lower.

CO scrubbing industries to utilize the captured CO to

produce value-added methanol.

To validate our proposed alcohol assisted pathway, we

Following the capture, the CO -loaded solutions were

initially explored the yet unreported hydrogenation of formate

13a,15g

hydrogenated under 70 bar of H in the presence of Ru-

salt to methanol.

Ethylene glycol was chosen as the

Macho-BH (C-1) (Table 2). This catalyst was already well-

alcohol solvent for its high boiling point and relative

nontoxicity. In accordance with our hypothesis, quantitative

studied by our group and others for CO hydrogenation to

methanol. KOH was chosen as the model alkali hydroxide due

conversion of HCOOK to CH OH was observed under 70 bar

to fast dissolution in ethylene glycol. At 140 °C, with 0.5 mol

of H at 140 °C in the presence of catalyst Ru-Macho-BH (C-

catalyst loading, full conversion of the captured CO was

). Similarly, KHCO₃under identical conditions formed

achieved with a methanol yield of 80% within 10 h. When the

reaction time was extended to 20 h, quantitative yield of

CH OH with a 92% yield as observed by H NMR (Scheme

). The formation of ester could not be detected by NMR after

CH OH was obtained. Ru-Macho (C-2) displayed an activity

reaction completion, suggesting that the formate salt is

thermodynamically favored, which is typical under alkaline

conditions.

similar to C-1 with 92% CH OH yield. When the P-

substituents in the pincer catalyst were replaced from R =

Ph to R = Pr, Bu, or Cy (C-3 to C-5), the CH OH yield

dropped drastically. Similarly, a meager 13% CH OH yield was

Scheme 1. Proof of Concept: Hydrogenation of Bicarbonate

and Formate Salts to Methanol

observed when the N−H moiety of C-2 was replaced with N-

Me in C-6 suggesting the involvement of a Ru−N cooperative

mechanism. With NaOH as the base, CH OH was produced

with 93% yield. CO -loaded Ca(OH) solution was not

hydrogenated eﬃciently. When the C-1 catalyst loading was

lowered to 0.25 and 0.1 mol %, the CH OH yield decreased to

0% (TON = 360) and 48% (TON = 480), respectively. Upon

lowering the temperature to 120 °C CH OH was obtained

with 99% yield after 72 h. Further lowering the temperature to

00 °C resulted in a CH OH yield of 31% and 60% at 0.5 and

mol % catalyst loading, respectively. From these results, it is

clear that the glycol mediated Ru-PNP based catalytic system is

signiﬁcantly active for CH OH synthesis even at relatively low

temperatures.

Reaction conditions: substrate (2.5 mmol), ethylene glycol (5 mL),

0 bar H , 140 °C, catalyst (0.5 mol %), 48 h. Yields determined by

During the hydrogenation reactions, formation of potassium

H NMR ( BuOH as internal standard).

glycolate (PG) was observed in addition to CH OH (Table

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Communication

Table 2. Tandem Hydrogenation of Captured CO in Ethylene Glycol

entry

MOH

KOH

cat. (mol %)

T (°C)

time (h)

formate (%)

MeOH (%)

TON_M_e_O_H

C-1 (0.5)

C-2 (0.5)

C-3 (0.5)

C-4 (0.5)

C-5 (0.5)

C-6 (0.5)

C-1 (0.5)

C-1 (0.25)

C-1 (0.1)

C-1 (0.5)

C-1 (1)

140

120

100

160

200

184

186

360

480

198

KOH

NaOH

Ca(OH)₂

KOH

Reaction conditions: solutions from Table 1 (as speciﬁed) were directly hydrogenated. Captured CO = 2.5 mmol, H = 70 bar. Yields

determined by H NMR ( BuOH as internal standard). Captured CO = 1.5 mmol. Yield calculations error ±5%. TON = moles of methanol per

mol of catalyst.

Scheme 2. Insights into the Reaction Sequences and Base Regeneration

S3). To investigate this unforeseen observation, a set of control

experiments were performed (Scheme 2B). It was revealed that

under the hydrogenation conditions, a solution of KOH in

ethylene glycol was converted to PG. This phenomenon was

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.9b12711.

Communication

not observed in the absence of catalyst. Dehydrogenation of

alcohols using such catalytic systems have been well

documented. Yet, our observation of dehydrogenation at

such high H₂pressure is entirely unprecedented. The

formation of PG during the hydrogenation reactions indicates

that only a fraction of the base remained available for

eﬃciency of the capturing species and developing a ﬂow

system to achieve continuous CO capture and methanol

production. This report is essentially the ﬁrst key step toward

employing hydroxide-based CO scrubbing and economical

regeneration of the base in addition to the production of value-

added methanol with the goal of achieving a sustainable and

carbon neutral cycle.

subsequent CO capture (Scheme 2A). To conﬁrm the partial

regeneration of the base, a scaled-up reaction was performed

with 20 mmol of captured CO and a catalyst loading of 0.1%

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge at

■

(

Scheme 2C). 9.6 mmol of CH OH, 10.4 mmol of HCOOK,

sı

and 3.3 mmol of PG were produced after 72 h. When this

reaction solution was directly subjected to subsequent CO₂

capture, only about 5 mmol of CO was captured by the

General information and experimental details, including

Figures S1−S12 and Tables S1−S3 (PDF)

regenerated base.

Finally, the eﬃcacy of the system was tested for DAC of

CO and conversion to CH OH. Following our reported

protocol,

indoor air was bubbled through a solution of 5

AUTHOR INFORMATION

Corresponding Author

■

mmol of KOH in 10 mL of ethylene glycol. After 48 h, 3.3

mmol of CO was captured in the form of carbonate and alkyl

G. K. Surya Prakash − Loker Hydrocarbon Research Institute

and Department of Chemistry, University of Southern

California, Los Angeles, California 90089-1661, United States;

orcid.org/0000-0002-6350-8325; Email: gprakash@

usc.edu

carbonate salts (Table 3). Additionally, there was an

Table 3. CO Capture from Ambient Air and Conversion to

CH OH

Authors

Raktim Sen − Loker Hydrocarbon Research Institute and

Department of Chemistry, University of Southern California,

Los Angeles, California 90089-1661, United States;

orcid.org/0000-0001-7992-3956

Alain Goeppert − Loker Hydrocarbon Research Institute and

Department of Chemistry, University of Southern California,

Los Angeles, California 90089-1661, United States;

orcid.org/0000-0001-8667-8530

Sayan Kar − Loker Hydrocarbon Research Institute and

Department of Chemistry, University of Southern California,

Los Angeles, California 90089-1661, United States;

orcid.org/0000-0002-6986-5796

entry captured CO (mmol) time (h) formate (%)

MeOH (%)

3.3

100

Reaction conditions: 140 °C. Yields determined by ¹H NMR

(^tBuOH as internal standard). Yield calculations error ±5%.

accumulation of around 3 mL of H O from moisture in air.

When the resulting solution was hydrogenated, CH OH was

obtained with a 25% yield after 20 h. When the reaction was

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.9b12711

extended to 72 h, the captured CO was completely converted

to CH OH.

Notes

In conclusion, we have developed a novel amine-free system

The authors declare no competing ﬁnancial interest.

for integrated CO capture and conversion to methanol. The

alkali hydroxide captured CO quantitatively in ethylene glycol,

ACKNOWLEDGMENTS

Support of our work by the Loker Hydrocarbon Research

Institute, USC is gratefully acknowledged.

■

forming alkyl carbonate salts, which were eﬃciently hydro-

genated using Ru-PNP catalysts. With Ru-Macho-BH,

quantitative conversion to CH OH was achieved within 20 h

at 140 °C. The catalyst showed signiﬁcant activity for CH OH

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