Tandem Photocatalysis Protocol for Hydrogen Generation/Olefin Hydrogenation Using Pd-g-C3N4-Imine/TiO2Nanoparticles

Hydrogenation Using Pd-g ‑C N ‑Imine/TiO Nanoparticles

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Tandem Photocatalysis Protocol for Hydrogen Generation/Oleﬁn

Maasoumeh Jafarpour,* Fahimeh Feizpour, Abdolreza Rezaeifard, Narges Pourmorteza,

and Bernhard Breit*

Cite This: Inorg. Chem. 2021, 60, 9484 −9495

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sı Supporting Information

ABSTRACT: An unprecedented visible-light-driven photocata-

lytic system consisting of Pd nanoparticles stabilized on g-C N -

imine-functionalized TiO₂nanoparticles was discovered for

photoassisted hydrogen generation followed by oleﬁn hydro-

genation under mild conditions. The structural integrity of the as-

synthesized photocatalyst was corroborated by Fourier transform

infrared spectroscopy, X-ray powder diﬀraction, energy-dispersive

X-ray spectroscopy, inductively coupled plasma atomic emission

spectroscopy, X-ray photoelectron spectroscopy, ultraviolet-diﬀuse

reﬂectance spectroscopy, Brunauer−Emmett−Teller measure-

ments, and thermogravimetric analysis (TGA). Transmission

electron microscopy and high-resolution scanning electron

microscopy revealed the nanoscopic nature of the catalyst. The

photocatalyst promoted several diﬀerent transformations in a one-

pot reaction sequence: hydrogen evolution through photocatalytic acceptorless formation of benzimidazoles as important

therapeutic agents followed by visible-light-driven photocatalytic reduction of oleﬁns with a high hydrogen utilization eﬃciency of up

to 92% under mild conditions. A signiﬁcant volume of H was produced under blue light-emitting diode (LED) irradiation during

the selective formation of benzimidazole, while the selectivity reduced signiﬁcantly under a Xe lamp or in the dark. The in situ-

generated H could be activated by the as-prepared Pd-C N -imine/TiO photocatalyst to eﬀectively hydrogenate oleﬁns under mild

conditions at appropriate time exposed to blue LED irradiation. The light-dependent photocatalytic performance of the title catalyst

was assessed using action spectra by calculating the apparent quantum eﬃciency (AQE), which exhibited the maximum AQEs at 410

and 550 nm, at which the highest performance for styrene hydrogenation was obtained. The improved photoredox activity of the

title nanohybrid could be caused by the synergistic eﬀects of the heterojunction of carbon nitride-Pd on TiO nanoparticles

evidenced by photoluminescence spectra and catalytic reactions. The catalyst proved to be air-stable, robust, recyclable, and very

active in the absence of any undesirable additives and reducing agents. Thus, this work presents a new protocol for improving the

photocatalytic properties of semiconducting materials for various photocatalytic applications under environmentally friendly

conditions.

INTRODUCTION

reliable, and cost-eﬀective H production is of extreme interest

to tackle the energy and environmental challenges. The use of

■

The excessive consumption of fossil fuels and relevant

environmental issues are one of the grand challenges of the

1st century and have made a great impetus for the discovery

renewable water and sunlight for hydrogen production through

water splitting emulating the natural photosynthesis would

create a virtually limitless energy source which is capable of

−3

of clean, renewable, cheap, and sustainable energy sources.

−8

replacing fossil fuels and severely reducing carbon emission.

In this line, hydrogen as a clean alternative fuel producing

water as the only byproduct is of extreme demand. It is a

renewable and sustainable energy source, which is currently

produced from fossil fuels, biomass, and water. Hydrogen is

mainly produced by steam reforming at high temperature or

coal and biomass gasiﬁcation or electrolysis of water. These

industrial protocols mostly use fossil fuel as an energy source

and sometimes hydroelectricity, both of which are energy

intensive and not always environment friendly and economic.

Therefore, the development of innovative methods for clean,

Accordingly, much attention has been devoted to develop

photocatalytic H -production methods, and semiconductors

Received: February 28, 2021

Published: June 16, 2021

2021 American Chemical Society

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Article

Scheme 1. Synthetic Route of the Pd-g-C N -imine/TiO Nanocatalyst

proved to be the best photocatalysts for driving this important

transportation of H gas are highly dangerous and costly and

−10

23,24

reaction in a green and sustainable manner.

To date,

require special facilities.

Thus, performing the hydro-

hundreds of semiconductors comprising metal and/or non-

genation reaction using in situ-generated H (dehydrogenation

11,12

metal elements such as Ta N ,

TaON, GaN/ZnO solid

CoO, and graphitic carbon nitride (g-

have been successfully fabricated and applied in

and hydrogenation process) is in extreme demand. Photo-

catalytic hydrogenation of unsaturated bonds, especially those

lacking polarity such as oleﬁns, is a major challenge in organic

synthesis. They are usually poorly converted in hydrogenation

processes because of the more negative reduction potential of

many organic compounds than the conduction-band-edge

potential. Nevertheless, positive holes oxidize or decompose

most of the organic substrates and accordingly a vast amount

of studies has been devoted to partial or total oxidation of

organic compounds. Therefore, photocatalytic hydrogenation

15,16

solution, sulﬁde,

8,19

C N )

6,20

photocatalytic hydrogen production. However, most photo-

catalysts still have poor quantum eﬃciency, photostability, and

visible-light absorption for hydrogen evolution.

In addition to using hydrogen as a fuel, it has various other

uses, such as a hydrogenation agent, as an oxygen scavenger,

and as a coolant in electrical generators. The hydrogenation

processes are vital reactions in both organic synthesis and

industrial chemistry including petroleum, food, and ﬁne

5−28

(or reduction) is basically restricted to carbonyl

groups,

and nitro

1,22

29−36

chemistry.

need reducing reagents and high temperature, and hydrogen

H ) gas has attracted a considerable amount of attention to

Nevertheless, the commonly used systems

and photocatalytic reductions of other functional

groups have been scarcely investigated. Considering these

limitations, one-pot oleﬁn hydrogenation using in situ-

generated H₂through an acceptorless dehydrogenation

(

the area of catalytic hydrogenation. However, storage and

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Figure 1. (A) FT-IR spectra and (B) XRD pattern of (a) nanostructure TiO , (b) g-C N , (c) g-C N -imine/TiO , and (d) Pd-g-C N -imine/

TiO nanohybrid. (C) Adsorption−desorption and (D) pore size-distribution curves of the Pd-g-C N -imine/TiO nanohybrid.

reaction at room temperature features a sustainable method,

which is signiﬁcantly safer and operationally simpler than that

using an external hydrogen gas source. However, examples of

catalysts describing activity of both dehydrogenation and

hydrogenation are exceedingly rare.

In the present study, we wish to report an unprecedented

visible-light-driven photocatalytic system for the one-pot

sonicated for 5 h followed by drying at 70 °C until a dry gel was

formed. The solid product (g-C -imine) was washed with ethanol

to remove unreacted chemicals. Finally, the isolated product was dried

at 100 °C for 12 h in a vacuum oven.

37,38

Preparation of g-C

TiO Nanohybrid. To a suspension of TiO

mL) was gradually added a suspension of g-C

-Imine/TiO

and the Pd-g-C

NPs in ethanol (0.1 g/2

-imine in ethanol

N -Imine/

3 4

hydrogenation of oleﬁns using photogenerated H through

(0.1 g/3 mL) at 60 °C under ultrasonic agitation. The mixture was

reﬂuxed for 8 h and after that the product (g-C N -imine/TiO ) was

centrifuged and washed with ethanol followed by air drying. Next, to

acceptorless dehydrogenation of the benzimidazoline inter-

mediate. A novel Pd catalyst supported on g-C N -imine-

functionalized TiO nanoparticles (NPs) activated by using

load Pd on the surface of the g-C N -imine/TiO heterostructure, 3

visible light catalyzes three sequential reactions in one pot at

room temperature: (I) synthesis of benzimidazole derivatives

as biologically and pharmaceutically important materials

through condensation of ortho-arylenediamines with aldehydes,

mL of a suspension of Pd(OAc) in ethanol (4.2 mg/mL) was added

to a suspension of 0.1 g of g-C N -imine/TiO in 2 mL of ethanol.

After sonication for 1 h, the mixture was reﬂuxed for 3 h. The gray

precipitates were collected by centrifugation and washed with ethanol

repeatedly followed by air drying.

(

II) production of H as an ideal fuel from an ecological

viewpoint, and (III) reduction of oleﬁns under mild reaction

conditions using in situ-generated hydrogen. As far as we

know, there are no reports so far on the use of such a Pd

photocatalyst capable of facilitating the above-room temper-

ature photochemical reactions in one pot, making our work

unique and novel and opens the scope for introducing a new

one-pot oleﬁn-hydrogenation photocatalyst.

General Procedure for the Photocatalytic Reduction of

Oleﬁns. In a typical experiment, 2 mg of Pd-g-C N -imine/TiO

nanohybrid was added to a test tube containing 4-chlorobenzaldehyde

0.2 mmol) and 1,2-phenylenediamine (0.22 mmol) dissolved in

(

EtOH (3 mL). The tube was sealed with a septum cap, and the

reaction mixture was deaerated for 5 min (Ar). Afterwards, 30 μL of

oleﬁn was injected into the reaction mixture and the tube was

transferred into a reactor chamber equipped with a magnetic stirrer.

The mixture was stirred at room temperature for 5 h under a blue

light-emitting diode (LED) (40 W) or Xe lamp (300 W) while the

EXPERIMENTAL SECTION

■

Instrumental details and experimental procedures for the preparation

of TiO NPs and g-C N are given in the Supporting Information.

light intensity reaching the liquid level was 100 mW/cm . At the end

Preparation of Imine Derivative of g-C N (g-C N -Imine).

of the reaction, the solid catalyst was removed by centrifugation and

the yield and characterization of the products were determined by gas

chromatography (GC)−mass spectrometry (MS) analysis using

chlorobenzene as an internal standard.

To a suspension of g-C N in ethanol (0.1 g/10 mL) was gradually

added 0.4 mL of (3-oxopropyl) trimethoxysilan (2.5 mmol) in

ethanol (10 mL) at 70 °C under ultrasonic agitation. Then, the

reaction mixture was kept under the same conditions for 2 h. It was

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Inorg. Chem. 2021, 60, 9484−9495

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Figure 2. XPS spectra of the Pd-g-C N -imine/TiO nanohybrid: Peak deconvolution of (a) Ti 2p, (b) O 1s, (c) N 1s, (d) C 1s, (e) Pd 3d, and (f)

Si 2p.

RESULTS AND DISCUSSION

XRD patterns depicted in Figure 1B elucidate the phase and

structural parameters. The characteristic diﬀraction peaks of

■

Catalyst Fabrication and Structural Analysis. TiO

pristine TiO

Figure 1B(a) at 25.4° (101), 37.7° (004), 48.2°

200), and 54.1° (105) corroborate the anatase phase of TiO₂

JCPDS no. 21-1272). Two diﬀraction peaks at 13.1 and 27.4°,

and g-C N

were synthesized according to reported

(

procedures with minor modiﬁcations (Supporting Informa-

tion). Condensation of the −NH group of g-C N with

attributed to the (100) plane and (002) plane of g-C N ,

3 4

organosilicon aldehyde prepared by our group produced a

novel g-C N /imine organosilicon compound prone to attach-

Figure 1B(b), are characteristic of in-planar structural packing

of aromatic systems and inter-planar stacking, respectively

JCPDS no. 87-1526). The observed changes in the diﬀraction

ment to titanium dioxide, which gave the g-C N -imine/TiO

(

nanocomposite as a catalyst precursor. Finally, the desired Pd-

containing catalyst (Pd-g-C N -imine/TiO ) was prepared by

pattern of g-C N -imine/TiO shown in Figure 1B(c), conﬁrm

the successful fabrication of the composite. The peak at 13.1

corresponds to the (100) plane of C N disappeared due to the

addition of Pd(OAc) to g-C N -imine/TiO under ultrasonic

agitation (Scheme 1 ). (For experimental details, see the

Supporting Information).

Characterization techniques such as Fourier transform

infrared spectroscopy (FT-IR), X-ray powder diﬀraction

decrease in the planar size of g-C N and conﬁrms the

45−47

successful exfoliation of g-C N .

Besides, the peaks at

7.4° correspond to the (002) plane of C N shifted to higher

3 4

θ angles, suggesting a lower distance between the planes and

(

XRD), Brunauer−Emmett−Teller measurements (BET), X-

consequently increases the planar pressure stress in the

ray photoelectron spectroscopy (XPS), inductively coupled

plasma atomic emission spectroscopy (ICP-AES), transmission

electron microscopy (TEM), scanning electron microscopy

48−50

nanosheets.

Moreover, the broad peak at 22.5° ascribes

to SiO in the heterostructure. For Pd-g-C N -imine/TiO , the

XRD pattern depicted in Figure 1B(d) reveals a coexistence of

phase structures after the process. The peak at 27.4 ascribed to

the (002) plane of C N is not observed, which may be due to

(

SEM), energy-dispersive X-ray spectroscopy (EDS), ultra-

violet-diﬀuse reﬂectance spectroscopy (UV-DRS), and ther-

mogravimetric analysis were used to investigate the structure

and morphology.

relatively overlapping by the titania matrix, owing to the low

content of g-C N in the specimen and lower intensity of C N

1−55

The FT-IR spectra of TiO , g-C N , g-C N -imine/TiO ,

than TiO₂.

Additional peaks in the XRD pattern of Pd-g-

and Pd-g-C N -imine/TiO nanocomposites are shown in

C N -imine/TiO appeared at 40.2, 46.95, and 68.85°,

Figure 1A. The typical TiO bands can be clearly observed in

demonstrating the formation of face-centered cubic Pd NPs

−

56−58

Figure 1A(a,c,d). The major bands between 450 and 750 cm₄

with high crystallinity (JCPDS no. 46-1043).

The surface and pore structure of the Pd-g-C N -imine/

are attributed to the stretching vibrations of Ti-O groups.

The broad peaks at 3400 and 1625 cm⁻¹correspond to the

stretching and bending modes of adsorbed water molecules on

TiO nanohybrid was characterized using nitrogen adsorp-

tion−desorption isotherms. Figure 1C displays the type IV

the surface, respectively. In the spectrum of pristine g-C N ,

isotherm with a H3 hysteresis loop, characteristic of slit-shaped

−

Figure 1A(b), the sharp absorption band at 810 cm shows

the typical breathing mode of triazine units, and the peaks

pores. The BET speciﬁc surface area is 94.56 m /g with an

average diameter of ∼5.3 nm based on pore size-distribution

curves depicted in Figure 1D.

−

located at 1241−1644 cm are rationalized to the typical

stretching modes of the C−N bonds in heterocycles. As seen

in Figure 1A(c), strong absorption peaks from 1000−1130

XPS analysis was carried out to explore the chemical

compositions and states of the elements of the title catalyst.

The high-resolution XPS spectra of Ti 2p, O 1s, N 1s, C 1s, Pd

3d, and Si 2p of the sample are depicted in Figure 2a−f,

respectively. The Ti 2p peaks centered at binding energies of

458.7 and 464.1 eV (Figure 2a) are assigned to Ti 2p₃_/₂and Ti

−

cm are associated with the Si−O bands. Furthermore,

characteristic peaks of g-C N and TiO appeared in Figure

A(c,d), conﬁrming their presence in the as-prepared

composite.

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Figure 3. (a) TEM, (b) HRSEM images, (c−g) elemental mapping images, and (h) EDX analysis of the Pd-g-C N -imine/TiO nanohybrid.

p₁_/₂of anatase TiO . The O 1s spectra in Figure 2b can be

respectively) in the as-prepared catalyst compared to the

ﬁtted into two peaks at 529.9 and 532.7 eV, corresponding to

the crystal lattice oxygen (Ti−O) and the external OH group

or H O molecules adsorbed on the TiO surface, respectively.

reference values (335.1 and 340.4 eV) indicates that the Pd

in the title supported catalyst is more electron rich than that in

pure NPs. Also, as illustrated in Figure 2f, high-resolution

spectra of Si 2p at binding energies of 102.4 and 103.4 eV

indicate the covalent bonding of Si in the Ti−O−Si

environment of the catalyst.

The Pd content of the catalyst was found to be 0.4 mmol/g

based on ICP-AES analysis. The TEM (Figure 3a) and high-

resolution scanning electron microscopy (HRSEM) (Figure

3b) images revealed uniformly distributed nanocomposites

with an average size of 8−12 nm without any agglomeration.

Elemental mapping at the microstructural level by SEM with

EDS is given in Figure 3. The uniform distribution of Ti, O, Si,

and Pd can be clearly observed in the system.

The N 1s spectrum was ﬁtted into three peaks centered at

98.7, 399.8, and 401.5 eV, which can be assigned to the sp -

hybridized nitrogen (CN−C), tertiary nitrogen (N−(C) ),

and amino functional groups (C−N−H), respectively (Figure

c). Moreover, the peaks of N 1s (Figure 2c) shifted 0.5 eV

toward higher binding energy compared to pristine g-C N

(398.2, 399.3, and 400.9 eV), testifying the chemical change

in the environment, resulting from the strong interaction

between g-C N with Pd NPs and TiO . The Figure 2d shows

four deconvoluted spectra of C 1s at 284.5, 286.1, 287.8, and

91.9 eV. Two former peaks can be assigned to the

adventitious carbons C−C and C−CN, whereas the third

The UV−vis diﬀuse reﬂectance spectra of TiO , g-C N , g-

3 4 3 4 2 3 4 2

composites were recorded to assess the visible light-absorption

peak corresponds to sp bonded carbons (N−CN). The

C N -imine, g-C N -imine/TiO , and Pd-g-C N -imine/TiO

61,62

peak at 291.9 eV can be assigned to π-excitation.

Besides, the Pd 3d spectra of the title catalyst (Figure 2e)

could be ﬁtted into asymmetric peaks, indicating the existence

of two states of Pd species. The prominent Pd 3d₅_/₂peak at

abilities and band gap values. The UV−vis diﬀuse reﬂectance

spectra along with Tauc plots of the abovementioned

materials are given in Figure S1. An enhancement in visible-

light absorption for Pd-g-C N -imine/TiO than for parent

34.7 eV and the Pd 3d₃peak at 339.9 eV are assigned to

metallic Pd . A small shift to a lower binding energy observed

materials was observed, which can be attributed to the

for Pd 3d electrons (0.4 and 0.5 eV for 3d₅_/₂and 3d₃_/₂

synergistic eﬀects between TiO NPs, g-C N -imine, and the

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at 355 nm are depicted in Figure 4 . For the pristine g-C N

Article

doped Pd NPs. The Tauc plots exhibit the band gap values of

TiO , g-C N , g-C N -imine, g-C N -imine/TiO , and Pd-g-

imine/TiO can be used as a composite photocatalyst at a

relatively high temperature of 300 °C.

C N -imine/TiO at ∼3.15, 2.7, 2.68, 2.91, and 2.8 eV,

Catalytic Activity. Hydrogen Evolution. To seek an in situ

photogenerated hydrogen source for oleﬁn reduction, accept-

orless dehydrogenation of the benzimidazoline intermediate

under visible-light irradiation was examined. The acid- and

oxidant-free synthesis of benzimidazole using 4-chlorobenzal-

dehyde (0.2 mmol) and 1,2-phenylenediamine (0.22 mmol)

was chosen as a model reaction under mild condition (25−27

°C). The data for systematic investigation of solvents, given in

Figure S3a , revealed that water, 2-propanol, dichloromethane,

and n-hexane as well as solvent-free conditions were not

suitable for the reaction to proceed in the presence of Pd-g-

C N -imine/TiO . Nevertheless, in alcohol solutions (meth-

respectively. Thus, the Pd-g-C N -imine/TiO semiconductor

material with a relatively narrow band gap strongly absorbs the

visible light and is expected to facilitate eﬀectively the

sequential photocatalytic reactions, if the eﬀective separation

of carriers occurs. It can be evaluated by photoluminescence

PL) spectroscopy. A lower PL intensity is a general indication

of a lower recombination rate of carriers, resulting in higher

photocatalytic activity. The comparative PL spectra of g-C N ,

(

TiO , g-C N -imine/TiO , and Pd-g-C N -imine/TiO excited

anol and ethanol), complete condensation of starting diamine

and aldehyde occurred. We chose ethanol for further

experiments just for the advantage of being ecofriendly.

Next, the amount of solvent and dose of the catalyst were

examined. Based on the data presented in Figure S3b,c, the

reaction needs 2 mg of catalyst and 3 mL of ethanol to reach to

the highest performance.

To prove the production of H gas, a new experiment under

optimized conditions was performed and H generation during

photocatalytic benzimidazole formation was monitored by gas

chromatography (Scheme 2).

For this, the Pd-g-C N -imine/TiO nanocomposite (0.1

mol %) was added to a septum-sealed vial containing 4-

chlorobenzaldehyde (0.2 mmol) and 1,2-phenylenediamine

(

0.22 mmol) dissolved in EtOH (3 mL). After purging with Ar

for 3−5 min, it was subjected to a radiation source (blue LED

0 W) with an intensity of 1.1 W/cm . The 100 μL gas

samples from the headspace of the vessel were analyzed by gas

chromatography at an interval of 30 min. The volume of H₂

formed was determined by means of a calibration curve

depicted in Figure S4 as a function of the exposure time. A

Figure 4. PL spectra of pristine g-C N , TiO , g-C N -imine/TiO ,

and Pd-g-C N -imine/TiO .

larger H production was observed under blue LED irradiation

than using a 300 W Xe lamp and a much lower in dark. It can

and TiO samples, the higher PL intensity testiﬁes the rapid

recombination of the photoinduced electron−holes, leading to

low photocatalytic activity. Nevertheless, the PL spectra

quenched noticeably by the combination of imine-g-C N

also be seen that the production of H could only be detected

after a lag time of around 30, 60, and 120 min under blue LED,

Xe lamp, and in dark, respectively (Figure S4). Moreover, a

high conversion and excellent selectivity of the benzimidazole

product (A in Scheme 3) were obtained under a blue LED,

while the selectivity reduced signiﬁcantly with a Xe lamp or in

dark, resulting from an over-condensation reaction (product B

in Scheme 3) with benzimidazole (product A).

with TiO NPs, featuring the eﬀective separation of carriers.

A further reduction in PL intensity for the Pd-g-C N -imine/

TiO hybrid heterojunction stressed that the incorporation of

Pd into g-C N -imine/TiO suppressed eﬀectively the

recombination of photoinduced electron−holes, promoting

its photocatalytic activity as will be shown in next sections.

To assess the thermostability of the composite, the TG−

DSC experiment was performed from room temperature to

It should be noted that no hydrogen gas evolution occurred

under catalyst-free conditions or over bare TiO

-imine, g-C -imine/TiO , Pd/TiO , Pd(OAc)

(OAc) + g-C -imine/TiO , and Pd(PPh . These negative

, g-C N

, g-

, Pd-

3 4

00 °C at a heating rate of 10 °C/min, and the results

presented in Figure S2 revealed that Pd-g-C N -imine/TiO is

)

3 4

stable up to about 300 °C. There is an endothermal peak

between 250 and 350 °C, which can be attributed to the

decomposition of the organosilicon phase. A 12% weight loss

in the temperature range of 500−650 °C is related to the

decomposition of g-C N in this region. Thus, Pd-g-C N -

results highlight the critical role of Pd NPs stabilized in g-

C N -imine grafted on titanium oxide NPs. Furthermore, the

hydrogen production ceased by removing any component of

the reaction, that is, diamine or aldehyde, conﬁrming that

hydrogen gas just originated from photocatalytic dehydrogen-

Scheme 2. Photocatalytic Generation of Hydrogen Gas from the Catalytic Condensation of Phenylendiamines and 4-

Chlorobenzaldehyde

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the formation of benzimidazole (Figure 5 ). As depicted in

Article

Scheme 3. Photocatalytic Condensation of Phenylendiamines and 4-Chlorobenzaldehyde

ation of the benzimidazoline intermediate. It is noteworthy

that no evidence for partial oxidation of ethanol was detected

by GC−MS analysis, excluding the involvement of solvent in

hydrogen production.

Table 1. Control Experiments for Photocatalytic Styrene

Reduction

Oleﬁn Hydrogenation. Then, we tried to utilize this

photogenerated hydrogen gas for one-pot hydrogenation of

oleﬁns to produce hydrocarbons, which is of extreme interest

21−24

in modern synthetic chemistry.

For this, several control

experiments using styrene as a model substrate were

performed. In the ﬁrst experiment, 4-chlorobenzaldehyde

entry

catalyst

condition

ethylbenzene %

(

0.2 mmol) was reacted with 1,2-phenylenediamine (0.22

Pd-C N -Imine/TiO

Blue LED

300 W Xe

Dark

Blue LED

mmol, under blue LED irradiation). We injected 30 μL of

styrene (0.26 mmol) in the reaction mixture at the time of the

maximum hydrogen production (27 μL at 225 min), during

Pd-C N -Imine/TiO

TiO₂

g-C N₄

g-C N -Imine

Pd-g-C N -Imine

g-C N -Imine/TiO

Pd/TiO₂

Pd(OAc)₂

Pd(PPh₃)₄

Pd(OAc) + g-C N -Imine/TiO

The reactions containing 4-chlorobenzaldehyde (0.2 mmol); 1,2-

phenylendiamine (0.22 mmol), and styrene (30 μL, 0.26 mmol) were

run in ethanol (3 mL) at room temperature for 5 h. The yields are

determined by GC−MS analysis. Byproduct B was produced (Figure

S5).

catalyst precursors including bare TiO , g-C N , g-C N -imine,

Pd-C N -imine, g-C N -imine/TiO , Pd/TiO , Pd(OAc) , and

even their mixture such as Pd(OAc) + g-C N -imine/TiO

Figure 5. Time course of photoassisted hydrogen evolution during

were negative (Table 1, entries 5−12). Whereas, the as-

benzimidazole formation over the Pd-g-C N -imine/TiO nanohybrid

before and after injection of styrene under blue LED irradiation.

prepared Pd-C N -imine/TiO photocatalyst exhibited re-

markable reactivity to furnish ethylbenzene in 38 and 78%

yield after 3 and 5 h, respectively. A prolonged reaction time of

8 h did not improve the yield of the corresponding

hydrogenation product (ethylbenzene, 78%).

To prove the necessity of light in the hydrogenation

reaction, a control experiment was conducted in the dark and

the yield of ethylbenzene reached ultimately to 6% after 5 h

(Table 1, entry 4). It was further supported by the light−dark

cycle in the hydrogenation of styrene. The production of

ethylbenzene ceased in the dark and restored upon blue LED

light irradiation based on GC analysis (Figure 6).

Figure 5, the amount of hydrogen gas declined rapidly after

adding styrene and ethylbenzene was detected in the reaction

mixture alongside benzimidazole as a major product and a

minor percentage of product B (Scheme 3) was detected by H

NMR (Figure S5 ) and GC−MS analysis. Nevertheless, when

styrene was injected at shorter times of the reaction (t = 3, 2, 1,

and 0 h), the selectivity of the benzimidazole product

enhanced. Thus, for next experiments, styrene and other

oleﬁns were added in the mixture of aromatic diamines and

benzaldehydes at the beginning, albeit after 5 min of purging

with Ar (see the Supporting Information).

Finally, to elucidate the light-dependent photocatalytic

performance of the title catalyst, action spectra were also

66,67

For the clear evaluation of catalytic activity of Pd-g-C N -

recorded.

The action spectrum of photocatalytic reduction

imine/TiO , a catalyst-free reaction was carried out. Using

of oleﬁns was determined based on the apparent quantum

eﬃciency (AQE) recorded using a full-spectrum CFL lamp

equipped with di ﬀ erent cut-oﬀ ﬁlters as mentioned in the

Supporting Information. The AQE versus the respective

wavelengths of the reaction is plotted for the hydrogenation

styrene as a substrate under optimized reaction conditions, no

hydrogenation product was observed even after 24 h under

visible-light irradiation (Table 1, entry 2). The results of

control experiments in the presence of starting materials and

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Inorg. Chem. 2021, 60, 9484−9495

Article

Note that in our experiments, oleﬁns were added in far

excess (0.26 mmol) compared to 4-chlorobenzaldehyde (0.2

mmol) and 1,2-phenylendiamine (0.22 mmol) as substrates for

the synthesis of pertinent benzimidazole. According to this,

ultimately 0.2 mmol H₂could be produced during the

synthesis of benzimidazole, ideally leading to the hydro-

genation of 0.2 mmol of oleﬁn. In other words, the hydrogen

utilization eﬃciencies (HUEs), deﬁned as HUE %= (mmol of

ethylbenzene/mmol of aldehyde) × 100, are 78, 92, 85, 85,

and 64 for styrene and its derivatives 4-Cl, 4-CH , 2-CH , and

allyl alcohol, respectively. These results testify that oleﬁn

hydrogenation in this photocatalytic system consumes the

majority of the produced hydrogen during the synthesis of

benzimidazole.

Mechanistic Aspects of Photocatalysis with Pd-g-

C N -Imine/TiO . It is well known that the eﬀective

separation of photogenerated electron−hole pairs at the

heterojunction interfaces is a critical factor for promoting the

photocatalytic performance.

Figure 6. Comparison between light−dark cycles (gray cycles) and

continuous irradiation (blue square) using blue LED light. The yields

are determined by GC analysis.

Scheme 4 shows the visible light-driven electron−hole

separation process and the conduction band (CB) and valence

band (VB) potentials of the TiO and g-C N used in the

product at diﬀerent wavelengths in the visible-light region

(

>400 nm). The AQE was calculated using the following

equation: AQE (%) = [(Y_v_i_s− Y_d_a_r_k) × 2(/photon number

entered into the reaction vessel)] × 100, where Y and Y_d_a_r_k

fabrication of Pd-g-C N /TiO . The CB and VB edge positions

light

can be estimated using the Mulliken electronegativity formulas

are the amounts of products formed under light irradiation and

dark conditions, respectively. As shown in Figure 7 , the

EVB = Χ − E + 0.5Eg, ECB = EVB − Eg

where E_V_Bis the VB edge potential, X is the electronegativity of

the semiconductor, E is the energy of free electrons on the

hydrogen scale (∼4.5 eV), and E is the band gap energy of the

semiconductor. According to the DRS analysis, the band gap

energy of TiO and g-C N was estimated to be 3.15 and 2.70

eV, respectively. The E_C_Band E_V_Bvalues of g-C N were

obtained as −1.12 and +1.15 eV, respectively, in agreement

68,69

with the literature values.

Furthermore, the CB and VB

edge potentials of TiO were −0.27 and 2.88 eV, respectively.

The more negative CB and VB edge potentials of g-C N than

those of TiO favours the transfer of the g-C N ’s photo-

induced electrons to TiO , followed by reduction of H to H .

In addition, the Pd atoms loaded on the surface trap the

electrons and facilitate the separation of electron−hole pairs.

Accordingly, a possible reaction mechanism for hydrogen

evolution via photocatalytic dehydrogenation of benzimidazo-

line intermediates followed by photocatalytic reduction of

Figure 7. Photocatalytic action spectrum for reduction of styrene

associated with synthesis of 2-[4-chlorophenyl] benzimidazole using

the Pd-g-C N -imine/TiO photocatalyst.

styrene as a model substrate over Pd-g-C N -imine/TiO is

proposed (Scheme 4). Under visible-light irradiation, both g-

−

C N and TiO are excited to produce (e ) in the CB and (h )

maximum AQEs were observed at 410 and 550 nm, at which

we obtained the highest performance for styrene hydro-

genation. These results conﬁrm that light is necessary to trigger

the reaction.

in the VB due to narrow band gaps. Owing to the

heterojunction structure formed between g-C N and TiO

and the fact that CB and VB edge potentials of g-C N are

more negative than those of TiO , photogenerated electrons in

The scope of title methodology for the photocatalytic

hydrogenation of a variety of oleﬁns under a blue LED in the

presence of the Pd-g-C N -imine/TiO nanocomposite at

the CB of g-C N tend to diﬀuse into the CB of TiO via the

interface, whereas photogenerated holes migrate to the VB of

g-C N . This process could eﬀectively enhance the life time of

room temperature is summarized in Table 2. Styrenes (4-H, 4-

Cl, 4-CH , and 2-CH ) and allyl alcohol (entry 5) showed

good reactivity toward photohydrogenation under optimized

conditions with good to excellent product yields. Nevertheless,

α, β-unsaturated ketones (entries 6 and 7) showed modest

yields for hydrogenation and non-conjugated allylbenzene

the electron−hole pair. Subsequently, benzimidazoline inter-

mediates, resulting from condensation of aldehyde and 1,2-

phenylenediamin, produces proton (H ) in the presence of h

−

followed by reduction to H₂using e in the CB. In

continuation, the in situ photogenerated H molecules are

activated on the Pd surface to form the active hydrogen species

(as H−Pd) for saturation of the C−C double bond of an oleﬁn

molecule like styrene.

Reusability Tests. To assess the recycling eﬃciency of the

presented method, ﬁve successive experiments were performed

(entry 8) was inactive under diﬀerent conditions. Remarkable

chemoselectivity was observed in the presented method. The

oleﬁn moiety was successfully hydrogenated without disturbing

aromatic and carbonyl groups.

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Inorg. Chem. 2021, 60, 9484−9495

Inorganic Chemistry

Article

Table 2. Photocatalytic Reduction of Oleﬁnes from Condensation of 1,2-Phenylenediamines and 4-Chlorobenzaldehyde in the

Presence of Pd-g-C N -imine/TiO

The reactions containing 4-chlorobenzaldehyde (0.2 mmol); 1,2-phenylendiamine (0.22 mmol), and styrene (30 μL, 0.26 mmol) were run in

ethanol (3 mL) containing 2 mg of Pd-g-C N -imine/TiO at room temperature for 5 h. GC yield based on chlorobenzene as an internal standard

(Figure S6). Second use. Third use. Fourth use. Fifth use.

for one-pot photohydrogenation of styrene through synthesis

of 2-[4-chlorophenyl] benzimidazole under optimized reaction

conditions. After each experiment, the photocatalyst was

recovered by centrifugation followed by decantation (3 × 5

mL ethanol) and dried overnight at 80 °C in an oven. It was

found that the Pd catalyst maintained its catalytic activity

under photochemical conditions and almost the same product

yield was observed for ethylbenzene after each run even the

ﬁfth run (78−76%, Table 2, entry 1) (Figure S7). The ICP-

AES analysis of the ﬁltrate excluded any leakage of palladium,

meaning that the palladium NPs were held tightly by the g-

C N -imine/TiO support, featuring the heterogeneous nature

organic transformations: (1) photocatalytic hydrogen evolu-

tion, (2) acceptorless benzimidazole synthesis, and (3) mild

reduction of oleﬁns. The avoidance of a high-pressure external

hydrogen gas source and high temperature in this work are

important beneﬁts which highlight the merit of this method-

ology. Performing the reaction under heterogeneous con-

ditions in ethanol as a green solvent using perfectly safe light

sources along with recyclability and ease of product separation

are additional beneﬁts of our work. Thus, the present system

which qualiﬁes all the signiﬁcant conditions will open up a new

perspective for H as an important environmentally friendly

fuel as well as for metal-catalyzed one-pot sequential organic

transformations.

of the title photocatalytic system.

Advantage over Other Catalytic Systems. The hydro-

gen generation and reduction of oleﬁn has been accomplished

CONCLUSIONS

■

with a number of catalysts based on Pd, TiO , or g-C N in the

In summary, the photo-eﬃciency of novel Pd NPs stabilized on

literature. Nevertheless, the catalysts are unable to perform

both processes in a single pot and required an external

hydrogen source, a reducing agent, or special reactors for oleﬁn

reduction. The reaction conditions and their catalytic perform-

ance for hydrogen production or styrene reduction are

summarized in Table S1 . Some of the mentioned methods

suﬀer from substrate diversity and are limited to one oleﬁn

substrate. Long reaction times, harsh and tedious work-up

procedures, and the use of hazardous solvents and reagents are

further drawbacks of these methodologies which cannot be

ignored for industrial implementations. Whereas, in the present

study, the as-prepared Pd-composite drives one-pot sequential

g-C N -imine-functionalized titanium oxide NPs was explored.

The as-prepared heterojunction photocatalyst promoted

several types of transformations in a sequential one-pot

strategy by combining photocatalytic and catalytic actions:

(1) photocatalytic production of hydrogen as a future fuel, (2)

acceptorless synthesis of important therapeutic agents, such as

benzimidazole, and (3) photocatalytic reduction of oleﬁns

under mild reaction conditions as a challenging reaction. The

present photocatalytic dehydrogenation/hydrogenation system

is sustainable, eﬃcient, and mild. The removal an external

hydrogen gas source in this method is an extraordinary

advantage to provide a safe and an operationally simple

492

Inorg. Chem. 2021, 60, 9484−9495

Inorganic Chemistry

Germany; orcid.org/0000-0002-2514-3898 ;

Article

Scheme 4. Proposed Photocatalytic Mechanism for One-Pot Oleﬁn Reduction Using In Situ Photogenerated H During

Benzimidazole Formation Over Pd-g-C N -Imine/TiO Under Visible Light Irradiation

alternative. The improved photoactivity of the nanocomposite

predominantly beneﬁts from the synergistic eﬀects of carbon

Email: bernhard.breit@chemie.uni-freiburg.de

nitride-Pd and TiO NPs in a heterojunction structure, which

Authors

endows the photoredox capability of the nanocatalyst. The

present system which performs three important sequential

processes well in the same reaction vessel and employs an air-

stable, very active, robust, and recyclable photocatalyst in green

ethanol solvent under a visible-light source qualiﬁes the

signiﬁcant conditions for implementation in the industry.

This strategy will open up a new outlook for better use of

semiconducting materials in photochemical applications and

Fahimeh Feizpour − Catalysis Research Laboratory,

Department of Chemistry, Faculty of Science, University of

Birjand, Birjand 97179-414, Iran

Abdolreza Rezaeifard − Catalysis Research Laboratory,

Department of Chemistry, Faculty of Science, University of

Birjand, Birjand 97179-414, Iran; orcid.org/0000-0002-

8717-9036

Narges Pourmorteza − Catalysis Research Laboratory,

Department of Chemistry, Faculty of Science, University of

Birjand, Birjand 97179-414, Iran

has a promising scope for safe H generation and sequential

organic transformations.

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00603.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.1c00603

■

Notes

The authors declare no competing ﬁnancial interest.

Full experimental procedures, characterization data and

required ﬁgures, and GC−MS traces (PDF)

ACKNOWLEDGMENTS

■

Partial support for this work by the Research Council of

University of Birjand, the Iran Science Elites Federation

(ISEF), and the “Iran National Science Foundation” (grant no.

96013225) is highly appreciated. M.J. also thanks the staﬀ of

the Institute of Organic Chemistry of Albert-Ludwigs-

University of Freiburg for support of this work and some

people in the Institute of Inorganic and Analytical Chemistry

for their kind collaboration, Prof. Dr. Philipp Kurz for the

partial access to the solar simulator and GC instrument for

hydrogen evolution studies, and Prof. Dr. Anna Fischer for the

partial access to a light source Lumatec Superlite S04 and a

AUTHOR INFORMATION

Corresponding Authors

Maasoumeh Jafarpour − Catalysis Research Laboratory,

Department of Chemistry, Faculty of Science, University of

■

Birjand, Birjand 97179-414, Iran; Institut fur Organische

Chemie, Albert-Ludwigs-Universität-Freiburg, Freiburg im

Breisgau 79104, Germany; orcid.org/0000-0002-9946-

013 ; Email: mjafarpour@birjand.ac.ir

Bernhard Breit − Institut fur Organische Chemie, Albert-

Ludwigs-Universität-Freiburg, Freiburg im Breisgau 79104,

493

Inorg. Chem. 2021, 60, 9484−9495

Inorganic Chemistry

Raybaud, P.; Toulhoat, H.; Farrusseng, D. Periodic Trends in the

Article

HR-SEM instrument funded by the German Federal Ministry

of Education and Research (BMBF, project EDELKAT,

Multifunctional Photocatalytic Materials for Energy; Elsevier, 2018; pp

15−236.

21) Corvaisier, F.; Schuurman, Y.; Fecant, A.; Thomazeau, C.;

(

03X5524). M.J. also thanks Yasin Rezaeifard for designing

Selective Hydrogenation of Styrene over Silica Supported Metal

Catalysts. J. Catal. 2013, 307, 352−361.

the graphical abstract.

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