Solar Light Driven H2O2 Production and Selective Oxidations Using a Covalent Organic Framework Photocatalyst Prepared by a Multicomponent Reaction [original]

Covalent Organic Frameworks Hot Paper

Solar Light Driven H2O2Production and Selective Oxidations Using a

Covalent Organic Framework Photocatalyst Prepared by a

Multicomponent Reaction

Prasenjit Das,* Jérôme Roeser, and Arne Thomas*

Abstract: A chemically stable 2D microporous COF

(PMCR-1) was synthesized via the multicomponent

Povarov reaction. PMCR-1 exhibits a remarkable and

long-term stable photocatalytic H2O2production rate

(60 h) from pure and sea water under visible light. The

H2O2production is markedly enhanced when benzyl

alcohol (BA) is added as reductant, which is also due to

a strong π–π interaction of BA with dangling phenyl

moieties in the COF pores introduced by the multi-

component Povarov reaction. Motivated by the concom-

itant BA oxidation to benzaldehyde during H2O2

formation, the photocatalytic oxidation of various

organic substrates such as benzyl amine and methyl

sulfide derivatives was investigated. It is shown that the

well-defined micropores of PMCR-1 enable size-selec-

tive photocatalytic oxidation.

Introduction

Hydrogen peroxide (H2O2) is one of the most important

oxidants and widely utilized for many relevant applications

in wastewater treatment, paper bleaching, disinfection,

chemical synthesis and also in fuel cells.[1–4] This has

increased its global annual demand to currently 4 million

tons and to estimated 5.7 million tons by 2027.[5,6] Further-

more, due to the comparable energy density between com-

pressed hydrogen and aqueous H2O2along with the facile

storage and transportation of H2O2, it has become an

alternative source to store energy. Currently, the anthraqui-

none (AQ) process is mainly utilized for industrial H2O2

production involving the hydrogenation of AQ over Pd and

subsequent oxidation reaction with O2. This process is

energy intensive and produces hazardous waste.[7] As an

alternative, H2O2can be synthesized directly from H2/O2

mixtures using Pd or AuPd metal catalysts which however

requires several safety measures.[8] For a sustainable, eco-

friendly and cost-efficient H2O2production, alternative

technologies are thus demanded. In this context, photo-

catalytic H2O2production from water and oxygen through

molecular oxygen reduction has become a sustainable

alternative.[9–12] In this process light irradiation triggers the

generation of oxygen-containing intermediates such as

superoxide (*O2), singlet oxygen (1O2) or hydroxyl radicals

(*OH) summarized as reactive oxygen species (ROS).[13]

These ROS act as vital intermediates in various chemical

and environmental applications such as degradation of toxic

pollutants and chemical oxidation reactions.[14,15] Therefore,

the development of efficient photocatalysts for ROS

production is required for several chemical processes.

Organic semiconductors with tunable optical and elec-

tronic properties are promising photocatalysts for solar-

driven H2O2production. Materials like carbon nitrides,[16–20]

linear conjugated polymers,[21] conjugated microporous

polymers,[22,23] and covalent triazine frameworks[24–26] have

been investigated for photocatalytic H2O2generation. In

recent years, significant advances have been made to

develop entirely organic, crystalline materials formed by

covalent linkages via reversible reactions, named covalent

organic frameworks (COFs).[27,28] Recently, several COFs

have been reported for photocatalytic H2evolution[29–33] and

also a few for H2O2generation.[34–38] However, their often-

low stability in water, especially at higher or lower pH

values, make their applicability for long-term photocatalytic

processes challenging. To develop chemically robust COFs,

an intriguing linkage chemistry has been developed recently

including reversible CC-binding reactions,[39–41] post-syn-

thetic modification[42,44] and multi-component reactions

(MCRs).[36,45–51] The latter MCR process enables a facile

synthetic approach by combining multiple components in

one pot to form robust aromatic linkages and thus chemi-

cally and temperature stable COFs.

Beside hydrogen and hydrogen peroxide production,

COFs have also been applied as photocatalysts for organic

transformation reactions.[51–56] To synthesize diverse fine

chemicals, selective oxidation of both aliphatic and aromatic

compounds are elemental reactions. The conventional

oxidation processes carried out industrially employ harsh

conditions and hazardous oxidants, yielding production of

waste byproducts and increased operational cost with drastic

[*] P. Das, J. Roeser, A. Thomas

Department of Chemistry, Functional Materials,

Technische Universität Berlin

10623 Berlin (Germany)

E-mail: [email protected]

[email protected]

published by Wiley-VCH GmbH. This is an open access article under

the terms of the Creative Commons Attribution License, which

permits use, distribution and reproduction in any medium, provided

the original work is properly cited.

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How to cite: Angew. Chem. Int. Ed. 2023,62, e202304349

doi.org/10.1002/anie.202304349

impact on the environment. Therefore, the use of stable and

recyclable photoactive COFs for the synthesis of value-

added organic products is also demanded for a sustainable

production of chemicals.

In this work, the one-pot synthesis of COFs by using the

multicomponent Povarov reaction (PMCR-1) is reported.

This process yields a novel quinoline-linked microporous

MCR COF with phenyl moieties pointing into the pores.

PMCR-1 produces high amounts of H2O2in both water and

sea water under natural sunlight over a long period of time

(60 h) without loss in catalytic activity. When benzyl alcohol

(BA) is added as sacrificial agent, the production of H2O2is

markedly increased together with a selective oxidation of

BA to the corresponding aldehyde. Such a selective

oxidation is also achieved with other organic compounds

such as amines and sulfides. Furthermore, a size-dependent

substrate-selectivity in the aerobic oxidation process is

observed.

Results and Discussion

A MCR-COF with quinoline linkages and phenyl groups

pointing into the pores was synthesized via the three-

component one-pot Povarov reaction, which involves alde-

hyde, amine and substituted vinyl compounds (Figure 1A).

The suggested reaction course is verified using similar

conditions with molecular model compounds with 3,4-

dimethoxy aniline, salicylaldehyde and styrene (Scheme S1

and Figures S1–S2). PMCR-1 was synthesized via the three-

component reaction, using 2,4,6-Tris(4-aminophenyl)triazine

(TAT), 4,4’,4’’-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde

(TTA) and styrene in a solvent mixture of o-dichloroben-

zene/n-butanol (1:1), in presence of BF3.Et2O, AcOH (6 M)

and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at

120°C for 72 h. The 4-phenyl-quinoline-linked COF was

collected as a golden yellow colored powder in 95% yield

(Figure 1 and Scheme S2).

The crystallinity of PMCR-1 was confirmed by powder

X-ray diffraction (PXRD) analyses. PMCR-1 exhibits an

intense reflection in the low-angle region at 4.0 degrees 2θ

(Figure 1C). An additional set of lower symmetry reflections

that could not be assigned to the starting monomers

indicated the formation of a crystalline framework. The

experimental pattern showed a similar set of reflections

(Figure 1C), suggesting the formation of the expected 2D

layered structure. Hexagonal layered structural models with

hcb topology and with different stacking sequences were

constructed and geometrically optimized. Comparison of the

theoretical with the experimental PXRD pattern suggested

the formation of 2D hexagonal layers with an eclipsed AA

stacking arrangement in the c direction (Figure S3). Pawley

fitting over the full profile was carried out to refine the final

unit cell parameters (Table S1). PMCR-1 was fully charac-

terized by solid-state cross-polarization magic-angle-spin-

ning (CP-MAS) 13C NMR and Fourier transform infrared

(FTIR), and X-ray photoelectron spectroscopy (XPS). The

CP-MAS 13C NMR spectrum of PMCR-1 revealed the

Figure 1. Synthesis, structure and characterization of PMCR-1: (A) Mechanism of the Povarov reaction; (B) Synthesis Scheme for PMCR-1;

(C) PXRD pattern for PMCR-1: experimental (black dotted line) and Pawley refined (red line) patterns and corresponding difference plot (green

line). The theoretical PXRD pattern for an eclipsed AA stacking model of PMCR-1 is provided as blue line. Top and side view of AA stacking in

PMCR-1. (Color code: C, gray spheres; O, red spheres; N, blue spheres, H, pale yellow spheres).

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generation of new signals characteristic for quinoline groups

at 156 and 149 ppm which is well corroborated by the

13C NMR spectrum of the model monomer, where these

peaks appear at 155.9 and 148.6 ppm, respectively (Fig-

ure 2A and S1).

Also, the signals that can be assigned to the benzene ring

pointing into the pores are observed in the aromatic region

(115–130 ppm) of the 13C NMR spectrum. The presence of

the strong characteristic peaks at 1579 cm1in the FTIR

spectrum corresponding to pyridyl stretching frequencies

from quinoline of PMCR-1 (Figure S4) also confirmed the

formation of the expected heterocyclic aromatic linkage.

XPS spectra further validated the formation of quinoline

linkages in PMCR-1 with the characteristic peak of N 1s

present at 400.1 eV (quinoline N) (Figure S5).[36,46] The

porosity of PMCR-1 was investigated by N2sorption experi-

ments at 77 K. N2sorption displayed a type-I sorption

isotherm from which an apparent BET surface area of

1756 m2g1was calculated. The measured value suggests

that PMCR-1 reaches its maximum theoretical uptake which

is calculated to be 1739 m2g1(Figure 2B). The pore size of

PMCR-1 was found to be 7.0 Å and 10.0–14.0 Å using DFT

method which is well correlated with the simulated structure

(Figure S6). From the TGA profile a minimal weight loss is

observed until a temperature of 500°C measured at N2

atmosphere (Figure S7). The chemical stability was deter-

mined by immersing PMCR-1 in 6 M HCl, 3 M NaOH, 1 M

Na2S2O5and 0.5 M H2O2for 4 days and PXRD analyses of

the recovered samples. Quantitative recovery of PMCR-1

and no sign of a loss in crystallinity in the collected PXRD

patterns confirmed the high chemical stability of the

synthesized COF (Figure 2C).

Field emission scanning electron microscopy (FESEM)

image of PMCR-1 showed the formation of branched rod-

like structures from aggregated crystallites (Figure S8).

Transmission electron microscopy (TEM) images confirmed

these aggregated structures while the high-resolution TEM

(HRTEM) image displayed crystalline lattice fringes (Fig-

ure S9).

The UV/Vis reflectance spectra of PMCR-1 displayed an

absorption starting at approx. 475 nm with a maximum at

400 nm (Figure S10). Based on solid-state UV/Vis and

valence band (VB) XPS, the VB and CB of PMCR-1 was

calculated to be 6.49 and 3.78 eV vs. vacuum, respec-

tively, which is consistent with the calculated HOMO and

LUMO energy obtained from DFT calculation (Figur-

es S11–S13).[57] The calculated frontier orbitals of PMCR-1

indicate that triazine groups and phenyl quinoline moieties

act as electron-withdrawing and electron-donating centers,

respectively. To prove the charge separation efficiency,

photocurrent and solid-state electron paramagnetic reso-

nance (EPR) CB spectra (g=2.005) were measured in the

dark and after illumination with visible light. Both photo-

current and solid-state EPR displayed a significant intensity

increase when the light is switched on indicating the efficient

photogeneration of charge carriers via the electron transfer

from the VB to the CB (Figure 3A and 3B). Also, from the

band position it can be anticipated that the reduction of O2

to *O2and oxidation of H2O to H2O2should be possible

because their redox potentials are in between the CB and

VB levels of PMCR-1. Consequently, photocatalytic H2O2

production was tested using PMCR-1 as photocatalyst

(10 mg of COF in 22 mL water or 10:1 water:alcohol

mixtures, measured after 1 h at 25°C and λ>420 nm, O2

purge 10 min, see Supporting Information for details, Fig-

ure S14). A H2O2production of 1294 μmolh1g1was

observed in pure water and air, which can be slightly

increased (1445 μmolh1g1) when pure oxygen is used

(Figure 3C). When sacrificial hole scavengers like ethanol or

isopropanol (IPA) were added (10:1, v/v water/alcohol), the

production of H2O2increased. This can be due to the more

efficient hole utilization of the added reductants and also to

a better dispersion of the rather hydrophobic COF, with its

pendant phenyl groups. PMCR-1 showed H2O2generation

Figure 2. (A) Solid-state 13C NMR spectrum for PMCR-1; (B) N2sorp-

tion at 77 K; (C) Chemical stability of PMCR-1 after 4 days treatment

with 6 M HCl, 3 M NaOH, 1 M Na2S2O5and 0.5 M H2O2.

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rates of 1941 and 2265 μmolh1g1using ethanol and IPA as

sacrificial agents, respectively. However, when benzyl alco-

hol (BA) is used, H2O2production by PMCR-1 is markedly

increased (5500 μmolh1g1) (Figure 3C). It can be assumed

that this high activity is due to a benzene-benzene

interaction between the pending phenyl groups within the

pores and BA, leading to efficient transfer of holes. The

interaction of BA with PMCR-1 was confirmed by TGA

measurements (Figure S15). Furthermore, a two-phase

(liquid-liquid) system if formed from water and BA. Recent

reports illustrated that metal organic framework (MOF) and

COF photocatalysts can achieve better H2O2production

from such binary phase system.[12,36–38,58]

The two-phase system of BA/water allows the dispersion

of PMCR-1 in the organic phase, thus that H2O2production

presumably occurs at the BA/water interphase while the

produced H2O2is dissolved in the water phase (Figure S16).

This avoids the back reaction by H2O2removal, and

separation from the catalyst which plays a key role for

efficient H2O2production and also inhibit H2O2

decomposition.[12,36,37] An apparent quantum yield (AQY) of

14% at 420 nm in water/BA was found for PMCR-1 in

presence of O2(Figure 3D). The activity of PMCR-1 in

H2O2production is favorably comparable to other reported

COFs in the literature (Figure 3E and Table S2).

Motivated by this H2O2production activity, the practical

applicability of PMCR-1 was tested for long-term H2O2

production in pure and seawater in presence of BA.

Production of H2O2from the water/BA two-phase mixture

steadily increased over time (226,450 to 625,790 μmolg1

after 24 to 60 h, respectively) (Figure 3F). Also, under

natural sunlight a high and steady production of H2O2is

observed reaching 129,028 μmolg1after 24 h. The catalyst

can be easily isolated from the organic phase of the reaction

mixture by filtration. After washing and drying, PXRD and

XPS pattern showed almost no change in the structure of

PMCR-1 after the long-term activity test (Figures S17 and

S18). HRTEM images displayed the retained morphology

and crystallinity after long-term photocatalysis (Figure S19).

1H NMR analysis of the filtrate confirmed the formation of

benzaldehyde (Figure S20). The amount of benzaldehyde is

slightly higher than the amount of H2O2produced indicating

the partial decomposition of H2O2in the long-term produc-

tion process.[37,58]

To gain more insight into the photocatalytic H2O2

production mechanism, different conditions were applied

during the tests. Under Ar atmosphere, the production of

H2O2was nearly not detectable suggesting that it occurs via

oxygen reduction. In absence of light, no H2O2production

was observed even after 48 h proving the photocatalytic

process.

Furthermore, different trapping agents such as benzoqui-

none (BQ) as superoxide radical (*O2), t-butyl alcohol

(TBA) as hydroxyl radical (*OH), and Ag(NO3) as electron

scavenger, respectively, were used to further reveal the

mechanism of the photocatalytic reaction. Addition of BQ

quenched the H2O2production, suggesting that *O2is

involved in the process. H2O2production also decreased

Figure 3. (A) Photocurrent measurement of PMCR-1; (B) EPR conduction band (CB) electron spectra of PMCR-1 in the dark and after illumination

(λ>420 nm, 300 W Xe lamp); (C) Photocatalytic H2O2production using PMCR-1 in the presence of water and different sacrificial donors (10 mg of

COF in 22 mL water or 10:1 water:alcohol mixtures, at 25°C, λ>420 nm, O2purge 10 min); (D) Wavelength dependent AQY measurement in

water/BA; (E) Comparison of H2O2production and AQY with other COF and polymer based photocatalysts; (F) Long-term photocatalytic H2O2

production of PMCR-1 in water in presence of BA (10 mg of COFs in 10:1 water:BA (22 mL) at 25°C and λ>420 nm, black arrow sign

corresponding to O2flow for 10 min).

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after adding Ag(NO3), indicating that photogenerated

electrons play a crucial role in the oxygen reduction

reaction. On the other hand, addition of TBA has not much

influence on the H2O2production, which shows that *OH

radicals are not taking part in the photocatalytic process

(Figure S21). To elucidate the formation of the ROS (e.g.

*O2and *OH), EPR measurements were performed. 5,5-

dimethyl-1-pyrrolidine N-oxide (DMPO) was used as a spin

trap to detect the ROS. A strong signal of *O2was

distinguished, which indicates reduction of O2upon illumi-

nation while no signal can be seen in absence of light

(Figure 4A). Finally, possible O2interaction sites and bind-

ing energies (BE) on PMCR-1 were investigated by

configurational bias Monte Carlo (CBMC) simulation (Fig-

ure S22). The triazine (CH···O: 2.36–2.93 Å) and quinoline

moities (CH···O: 2.68–2.94 Å), as well as the pendent phenyl

group (CH···O: 2.68–2.94 Å) of PMCR-1 seem to be

preferred binding sites for the O2molecules. These results

strongly suggest that photocatalytic H2O2production is

indeed occurring by reduction of O2as initial step and

produced superoxide radicals along with another electron

and two protons (from alcohol to aldehyde) to generate

H2O2(Figure S23).

To gain further insight into the interaction of the COF

with the sacrificial reductant, alcohols with a different

sterical hindrance were investigated. When the bulky

sacrificial donor 4-t-butylbenzylalcohol (TBBA) was used,

the production of H2O2was found to be much lower (700

μmolh1g1) than for pure BA (Figure 4B). After addition

of different amounts of BA to TBBA (0:1, 0.25:0.75,

0.5:0.5, 0.75:0.25, 1:0 of BA:TBBA), the H2O2production

increased nearly linearly (from 700 to 5500 μmolh1g1)

(Figure 4B). This result indicates that π-π interactions of the

COF with the SA might be an important factor for an

efficient hole transfer, as such interactions will be much

reduced when a sterically hindered alcohol is used.

Benzyl alcohol to benzaldehyde oxidation should be

however rather regarded as model oxidation reaction as,

even when coupled to H2O2production, the economic

feasibility of this process is questionable. However, this

might change for light driven, selective oxidations of other

alcohols and further chemicals. Therefore, different benzyl

amines and thiols were applied to investigate the versatility

of the presented photocatalytic system. Prior works[54,60]

have reported that benzyl amine generates N-benzyl-1-

phenylmethanimine when applied as sacrificial agent via

phenylmethanimine (PhCH=NH) in photocatalytic H2O2

production (Figure S23). Here, the same reaction was

attempted in the binary phase system water/benzyl amine

while H2O2was detected in the water phase. Notably, when

natural sunlight was used as a light source, already with a

very low amount of benzyl amine in water (0.01:1), a H2O2

production of 4529 μmolg1was observed after 1 h under O2

atmosphere. Similarly, thioanisole in water (0.1:1) produces

a two-phase system but in this case H2O2production is lower

(905 μmolh1g1) compared to BA or benzyl amine.

Inspired by these findings, the aerobic oxidation of

further molecular compounds was attempted, as the photo-

catalytic oxidation produce valuable products for the

chemical and pharmaceutical industry.[59] Recently, various

heterogeneous catalysts were used for such photocatalytic

selective oxidations, however, stability and thus reusability

are critical issues in the presence of highly reactive ROS. In

this respect it can be assumed that the strong covalent bonds

in PMCR-1 will also be beneficial to ensure its stability in

presence of ROS. At first, oxidation of different benzyl-

amine derivatives was tested using PMCR-1 as catalyst

(Table 1). PMCR-1 was dispersed in CH3CN and the amine

and purged with O2for 10 min. For pure benzyl amine

complete conversion was observed after 4 h. Under inert

conditions (Ar purge) a much lower conversion was

observed (12% after 4 h, note that O2cannot be completely

removed from the reaction system). For the other amines,

substrates with electron-donating groups (EDGs) show

better activity compared to electron-withdrawing groups

(EWGs) (Table 1). Again, sterically more demanding groups

yield to lower catalytic conversions. Additionally, the

oxidation of sulfides was tested using PMCR-1. Also, for

this reaction a higher activity for substrates with small and

EDGs groups is observed (Table 2).

These results are pointing to a size-selective effect as

bulkier substrates oxidize much slower than smaller ones.

For larger substrates like t-butylbenzylamine (Entry 8) and

phenylbenzylamine (Entry 9), the conversion is comparably

low (Table 1). When a larger EWG substrate e.g. 3,5-

bis(trifluoromethyl)benzylamine (BTBA) was used the con-

version is even lower (Entry 7, 8, Table 1). Similar results

Figure 4. (A) EPR trapping experiment with DMPO which showed the

formation of DMPO-*OO spin adduct (g=2.006; AN=13.2, and

AH=10.1; (B) H2O2production from sacrificial donor BA:TBBA

mixtures.

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