Outstanding response of carbon nitride photocatalysts for selective synthesis of aldehydes under UV-LED irradiation

Article abstract of DOI:10.1016/j.cattod.2019.03.050

The photocatalytic conversion of alcohols into the corresponding aldehydes constitutes an important reaction in organic synthesis. Optical semiconductors based on carbon nitride were prepared by thermal condensation of dicyandiamide followed by a thermal

Full text of DOI:10.1016/j.cattod.2019.03.050

Accepted Manuscript
Title: Outstanding response of carbon nitride photocatalysts
for selective synthesis of aldehydes under UV-LED irradiation
Authors: Joana C. Lopes, Maria J. Sampaio, Raquel A.
Fernandes, Maria J. Lima, Joaquim L. Faria, Cl a´ udia G. Silva
PII:
S0920-5861(18)31656-0
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.03.050
CATTOD 12072
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
30 November 2018
11 March 2019
21 March 2019
Please cite this article as: Lopes JC, Sampaio MJ, Fernandes RA, Lima MJ,
Faria JL, Silva CG, Outstanding response of carbon nitride photocatalysts for
selective synthesis of aldehydes under UV-LED irradiation, Catalysis Today (2019),
https://doi.org/10.1016/j.cattod.2019.03.050
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Outstanding response of carbon nitride photocatalysts for selective synthesis
of aldehydes under UV-LED irradiation
Joana C. Lopes, Maria J. Sampaio, Raquel A. Fernandes, Maria J. Lima, Joaquim L. Faria, Cláudia
G. Silva*
Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials
(LSRE-LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-
465 Porto, Portugal
*Corresponding author e-mail address: cgsilva@fe.up.pt (C.G. Silva)
Graphical abstract
Highlights:




gCN-based photocatalysts are efficient for the synthesis of aromatic aldehydes.
Anisyl alcohol was selectively converted to anisaldehyde using gCN-T.
gCN-TS presents enhanced photocatalytic activity for anisaldehyde production.
gCN-TS-coated glass rings show high stability under continuous mode reactions.
Abstract
1

The photocatalytic conversion of alcohols into the corresponding aldehydes constitutes an
important reaction in organic synthesis. Optical semiconductors based on carbon nitride were
prepared by thermal condensation of dicyandiamide followed by a thermal post-treatment (gCN-
T) and further sonication (gCN-TS). The efficiency of these materials was evaluated on the
selective conversion of different aromatic alcohols to the corresponding aldehydes, namely
anisaldehyde, benzaldeyde, tolualdehyde, piperonal and vanillin. The best performance of gCN-T
-1
(
1 g L ) was found for the conversion of anisyl alcohol to anisaldehyde (>99% conversion and
selectivity in 45 min of UV-LED irradiation). The alcohol conversion and yield for aldehyde
formation was related with the number, electronic nature and position of the functional groups in
the aromatic ring. The use of gCN-TS, constituted by carbon nitride nanosheets, also produced
>99% conversion and selectivity towards anisaldehyde, while reducing the reaction time to 30 min
-1
and the catalyst load to 0.2 g L . The immobilization of gCN-TS in glass rings revealed to be a
promising strategy for continuous production of aldehydes without the need of a catalyst separation
step.
Keywords: selective oxidation; photocatalysis; carbon nitride; aromatic aldehydes; glass rings.
1. Introduction
Photocatalytic technologies have been attracting wide attention of the scientific community in
different fields, including degradation of organic compounds [1-4], hydrogen production [5-8],
reduction of CO [9, 10], organic synthesis [11, 12], among others. In recent years, selective
2
conversion of different alcohols into the corresponding aldehydes has emerged as a hot topic of
research, due to their importance in organic synthesis reactions and fine chemical industry [13, 14].
In general, conventional industrial production of aldehydes usually requires the use of hazard
oxidants/reductants and harsh operation conditions, such as high temperature and pressure.
Moreover, during their industrial synthesis high quantities of wastes are produced, usually
presenting severe toxicity. Thus, the development of efficient alternatives operating under mild and
2

environmentally friendly conditions (room temperature, atmospheric pressure and the use of water
as solvent) is a topic of major concern.
Polymeric carbon nitride (C
semiconductors widely used in photocatalytic processes [15-19] , which is due to the low band gap
~2.7 eV) when compared to metal oxides (e.g., TiO and ZnO). Moreover, the position of valence
and conduction bands of C (+1.58 V and 1.12 V, respectively) in water [14, 20], makes this
material a promising catalyst for selective oxidation of aromatic alcohols [21]. Other applications
of C based materials in photocatalytic organic synthesis include C-H amination [22],
3
N
4
) based materials have been reported as highly efficient optical
(
2
3
N
4
3
N
4
dehydrogenation of N-heterocycles [17], cycloaddition, esterification, and transesterification
reactions [23]. Carbon nitride has also been widely used for photocatalytic elimination of organic
pollutants such as tetracycline [24, 25], sulfamethazine [26, 27], and oxytetracycline hydrochloride
[
28], and for photocatalytic fuel production such as hydrogen from water splitting, and
hydrocarbons from CO reduction [29-35].
The synthesis of C may be accomplished by thermal polymerization of a variety of precursors
such as urea, thiourea, melamine, dicyandiamide, cyanamide, and guanidine hydrochloride in the
temperature range of 500 – 600 ºC under air or inert atmosphere [10]. Nevertheless, bulk C
2
3
N
4
3
N
4
materials usually exhibit low specific surface area, being a drawback due the small amount of active
sites available to promote photocatalytic redox reactions. Some strategies have been employed
seeking for enhanced performance of C
3
4
N materials namely by exposing this material to thermal,
mechanical and chemical treatments [36, 37]. Remarkable performance of a mesoporous C
3
4
N ,
obtained by templating with silica nanoparticles, has been reported by Su et al. [38] for the
photocatalytic synthesis of aldehydes under visible light. Yet, reactions were carried out using
organic solvents (such as acetonitrile and toluene), and under high pressure and temperature (8 bar
of O
2
and 100 ºC, respectively). The performance of the C
3
N
4
material was ascribed to its high
ability to activate oxygen (generating superoxide radicals, O 
) and to the basic character of the
2
3

material facilitating the deprotonation of the alcohol, followed by reaction with positively charged


hole and recombination with O
2
to form the aldehyde. In addition, the authors also observed the
. More recently, several works
formation of H , resulting from two-electron reduction of O
2
O
2
2
reported the synthesis of aromatic aldehydes in aqueous medium at mild conditions of temperature
and pressure [36, 39-43]. For instance, Bellardita et al. [40] used different precursors (melamine,
urea and thiourea) to prepare bulk C
3
4
N . The materials were subjected to a second thermal treatment
in the presence of a phosphorus precursor, producing P-doped catalysts with increased surface area.
These materials were used for the photocatalytic synthesis of piperonal, benzaldehyde and
tolualdehyde, under aerated conditions, using fluorescent lamps as irradiation source, resulting in
maximum selectivity of 69%, 72% and 26% for benzaldehyde, tolualdehyde and piperonal,
respectively after 4 h of irradiation.
In a previous work by our group [36], a C
3
4
N material obtained by thermal exfoliation of bulk
carbon nitride at 500 ºC (here denoted as gCN-T) was tested for the synthesis of benzaldehyde
under deoxygenated conditions using a UV-LED system as light source. The exfoliation process
leads to an increase in benzyl alcohol conversion (from 18 to 66 % in 4 h of reaction), keeping the
selectivity for benzaldehyde above 90%, this feature being attributed to the higher surface area of
the exfoliated photocatalyst.
In the present work, we have extended the use gCN-T [36], to the photocatalytic production of
benzaldehyde and other aromatic aldehydes, including anisaldehyde, tolualdehyde, piperonal and
vanillin, under oxygenated conditions.
Due to the several practical problems arising from the use of powder materials (e.g., separation of
the catalyst), C
3
N
4
nanosheets (gCN-TS), obtained by sonication of gCN-T, on glass rings, which
were used for the photocatalytic synthesis of anisaldehyde in continuous flow. To the best of our
knowledge, this is the first study using immobilized C
aldehydes.
3
4
N for the photocatalytic synthesis of
4

2. Experimental
2.1 Materials
Dicyandiamide (DCN, 99 %), benzyl alcohol (BA, > 99%), benzaldehyde (BAD, > 99%),
piperonyl alcohol (PA, 98%), piperonal (PAD, 99%), p-anisaldehyde (AAD, 98%), polyvinyl
alcohol (PVA), titanium (III) sulfate solution (Ti
2
(SO
4
3
) ) and sulfuric acid (purity ≥ 95-97%) were
purchased from Sigma-Aldrich. Anisyl alcohol (AA, 98%), vanillyl alcohol (VA, > 98%) and
vanillin (VAD, 99%) were supplied from Alfa Aesar. p-toluyl alcohol (TA, 98%) and p-
tolualdehyde (TAD, 97%) were obtained from Acros Organics. Formic acid (98%), methanol
(99.6%), 2-propanol (100%) and acetone (100%) were purchased from Fluka. Titanium dioxide
®
Aeroxide P25 was acquired from Evonik Industries. Ultrapure water (UP) was obtained by a
Direct-Q Milipore system.
2
.2 Catalyst preparation and characterization
Polymeric carbon nitride was prepared by thermal decomposition of DCN, as described elsewhere
36]. Briefly, a certain amount of DCN was placed in a porcelain-capsule inside a muffle furnace
[
under static air conditions for 4 h at 550 ºC. The resulting material (here labeled as gCN), was
washed and dried at 110 ºC for 12h. Then, the gCN was exposed to a second thermal step for 2 h
at 500 ºC and labeled as gCN-T.
Additionally, an aqueous suspension containing a certain amount of gCN-T was sonicated
(ultrasonic processor UP400S, 24 kHz) for 50 min. The sonicated suspension was centrifuged for
15 min at 3000 rpm to isolate the unexfoliated gCN-T material from the supernatant. The obtained
colloidal suspension was denoted as gCN-TS.
Glass rings (diameter = 3 mm, length = 3 mm) were cleaned as described elsewhere [44]. Before
the coating with the gCN-TS suspension, the glass rings were treated with a 2% (w/V) aqueous
solution of PVA by dip-coating. This procedure was directly followed by a second dip-coating of
5

the glass rings in the gCN-TS suspension. The resulting glass rings were dried under ambient
temperature for 48 h.
The specific surface area (SBET) of the powder materials was determined by multipoint analysis of
nitrogen adsorption isotherms at -196 °C in relative pressure range from 0.05 to 0.2 using the
Brunauer-Emmett-Teller method in a Quantachrome NOVA 4200e apparatus. The optical
absorption was determined by diffuse reflectance UV–Vis (DRUV-Vis) spectroscopy using a
JASCO V-560 spectrophotometer equipped with an integrating sphere attachment (JASCO ISV-
469). The results were recorded to equivalent absorption Kubelka–Munk units and used to calculate
the optical bandgap energy of the photocatalysts. The morphology of the materials (as powders and
immobilized) was examined by scanning electron microscopy (SEM) using a FEI Quanta 400 FEG
ESEM/EDAX Genesis X4M (15 keV). Transmission electron microscopy (TEM) micrographs
were conducted on a JEM 220FS microscope (Jeol, Japan), equipped with a LaB6 electron gun
operating at 200ꢀkV.
2.3 Photocatalytic experiments
The photocatalytic efficiency of the different photocatalysts for the synthesis of different
aldehydes, was firstly evaluated using a batch reactor. In a typical experiment, a borosilicate reactor
was filled with 50 mL of a 1.5 mM of an aromatic alcohol aqueous solution. The catalyst load was
-1
fixed at 1.0 g L by taking into consideration our previous studies using similar catalysts for
benzaldehyde production [36]. The suspensions were continuously stirred and saturated with an air
flow. Before turning on the irradiation, a 30 min period in dark was performed to establish the
adsorption-desorption equilibrium between the aromatic alcohol aqueous solution and the
photocatalyst. Then, the photocatalytic synthesis of the aldehydes was conducted for 4 h using a
four LED system with a maximum wavelength at 370 nm as radiation source. The nominal intensity
6

-2
of the irradiation of each LED was c.a. 450 W m , which was determined using an UV-Vis
spectroradiometer (USB2000+, OceanOptics, USA).
Experiments using the gCN-TS suspension were performed using the system describe above.
Photocatalytic experiments in continuous mode were carried out in a borosilicate cylindrical reactor
(internal diameter = 2.7 cm; length = 7.0 cm) with 38.5 mL of useful volume and packed with 115
glass rings coated with gCN-TS. This reactor configuration was used for the synthesis of synthesis
of AAD from AA. An AA solution (0.5 mM) contained in a reservoir was introduced in the reactor
-1
using a peristaltic pump at a constant flow rate (0.70 mL min ). The irradiation source was the
same one used in batch reactions. Reactions were extended for 24 h.
High Performance Liquid Chromatography (HPLC) was used for identification and quantification
of aromatic alcohols and aldehydes using a Shimadzu Corporation apparatus equipped with a Diode
Array Detector (SPD M20A), a solvent delivery pump (LC-30AD) and a KinetexTM F5 1.7 µm
100 Å column (100 mm × 2.1 mm). The temperature of the autosampler and column oven were
fixed at 4.0 ºC and 35 ºC, respectively. A mixture of 0.1% v/v of formic acid and methanol with
-1
volume ratio of 30:70, respectively, was isocratically eluted for 15 min at 0.150 mL min .
3. Results and discussion
3.1 Catalyst characterization
Detailed characterization of gCN and gCN-T materials can be found in a previous work by our
group [36]. The thermal treatment of gCN promotes the exfoliation of the material by separating
the layers of the bulk material (gCN) which are stacked together by weak van der Waals forces.
According to the results obtained from the nitrogen adsorption-desorption isotherms, the textural
properties were changed by the thermal treatment applied to the gCN material. The specific surface
area of gCN and gCN-T, obtained by the Brunauer-Emmett-Teller method (SBET), were 6 and 79
2
-1
m g , respectively, corresponding to an increase of more than 13 times promoted by the thermal
treatment. For most photocatalytic semiconductors, an increase in the SBET is recognized as an
7

advantage, since more active sites are available for the occurrence of redox reactions at the catalyst
surface.
Different morphologies were observed by microscopy analyses of gCN and gCN-T (Fig. 1). While
the gCN material (Fig. 1a) presents a compact stacked lamellar structure, SEM micrographs of the
thermal-exfoliated material (gCN-T) revealed the presence of thinner layers (Fig. 1b). TEM
micrographs of the gCN and gCN-T materials (Fig. 1a and 1b, inset) confirm the formation of a
less dense material after submitting gCN to the thermal post-treatment (gCN-T). These
observations have been well discussed in the literature [16, 36, 45], and corroborate the results
obtained from the SBET measurements, i.e., the thermal exfoliation promote the increase in the
surface area of gCN-T by causing the splitting of the interlayer bonds.
Representative SEM images of gCN-TS-coated glass rings show a relatively homogeneous surface
at microscale (Fig. 1c and 1d). gCN-TS particles of smaller dimensions than gCN-T can be
observed at higher magnification (Fig. 1d), which resulted from the intense ultrasonication
treatment applied to gCN-T. The colloidal nature of the gCN-TS suspension was demonstrated by
the scattering observed when the suspension is crossed with the red light of a laser beam – Tyndall
effect (Fig. 1e). The nanometric dimensionality of the sample is in line with the TEM analysis of
the colloidal suspension (Figs. 1f-g), in which is clearly observed the ultrathin lamellar morphology
of the material.
8

Figure 1. (a-b) SEM micrographs of gCN (a) and gCN-T (b) powders (insets are the corresponding
TEM micrographs); (c-d) SEM micrographs of glass rings coated with gCN-TS at different
magnifications (see bar scale for appropriate dimensions); (e-f) Tyndall effect for gCN-TS colloidal
suspension (recipient on the right) under illumination with red laser beam (e), in comparison with
artificial light (f); (g) TEM micrograph of gCN-TS.
3.2 Photocatalytic synthesis of aromatic aldehydes using gCN-T
In our previous work, gCN-T (designated in that work as T500 [36]) was tested in the photocatalytic
synthesis of benzaldehyde under deoxygenated conditions and using radiation with an excitation
maximum at  = 392 nm. In the present work, the main goal was the study of the photocatalytic
performance of the gCN-T based materials for the synthesis of five aromatic aldehydes selected
9

based in their importance as high value-added compounds: benzaldehyde (BAD), anisaldehyde
AAD), tolualdehyde (TAD), piperonal (PAD) and vanillin (VAD). In this case, the reactions were
(
carried out starting from the respective aromatic alcohols, under oxygenated conditions and using
radiation with max at 370 nm.
The photochemical stability of the aromatic alcohols, in absence of catalyst, was examined. Under
these conditions, the initial concentration of each alcohol remained constant after 4 h of reaction
(data not shown). These results were expected since the absorption spectra of the selected
molecules do not overlap the emission spectrum of the radiation source used (Fig. SD1). Reactions
under dark conditions was performed for 2 h in order to establish the adsorption-desorption
equilibrium between gCN-T and each aromatic alcohol. Negligible adsorption was observed
disregarding the aromatic alcohol used, as exemplified for the case of AA (Fig. SD2).
The concentration profiles of the five aromatic aldehydes selected in this study during the
photocatalytic reactions using gCN-T as catalyst are depicted in Fig. 2a. Comparing the results
obtained for the synthesis of the different compounds, the maximum yields obtained followed the
order AAD > TAD > BAD > PAD > VAD (Fig. 2a). It is noteworthy that the maximum yield to
AAD (99%) was obtained in merely 45 min of reaction with a selectivity of 99% (Fig. 2b). TAD
and BAD yields of 60 and 50%, with selectivity of 80%, were obtained at the end of 45 and 120
min, respectively. Finally, very low yields of PAD and VAD were obtained under the used
conditions (9 and 2%, respectively) in reactions with low selectivity for these compounds (Fig. 2b).
1
0

Figure 2. (a) Concentration profiles of the aldehydes BAD, AAD, TAD, PAD and VAD after 4 h
reaction using gCN-T catalyst. (b) Conversion (C) of the aromatic alcohols, selectivity (S) and yield
(
Y) for aldehydes formation (bars) and reaction time for the maximum yield of each aldehyde
).
(
In heterogeneous photocatalysis, the reactivity of benzene derivatives is widely influenced by the
number, electronic mature and position of substituent groups in the aromatic ring [12, 44, 46].
1
1

Thus, based on the principles of photocatalytic oxidation of aromatic compounds, the relative
facility of selectively synthesize the aromatic aldehydes here studied may be rationalized by the
relative ability of the parent compound to be converted. The chemical structures of AA, PA, TA
and VA derive from BA, the difference being the type and position of substituent groups in the
aromatic ring (Fig. SD3). The photocatalytic conversion of such molecules may be accelerated or
hindered by the presence of electron-donating (activating) or electron-withdrawing (deactivating)
groups in the aromatic ring, respectively. In molecules with more than one functional group, this
effect results from the overall contribution of each functional group. Thus, it may be expected that
the relative conversion extent of each aromatic alcohol can be related with the number, position
and nature of the substituent groups.
Among the five alcohols tested, the AA showed the fastest conversion and the high yield for AAD
formation. The presence of CH
donors, respectively) in the aromatic ring of AA may explain the higher production of AAD. The
combination of CH OH and OCH functional groups made the AA more prone to be selectively
oxidized to AAD. TA, with two weak (CH OH and CH ) activating groups, was also easily
converted to TAD. The highest selectivity (80%) to this compound was obtained at the end of 45
min of reaction. On the other hand, VA with the presence of one very strong (OH), and (CH OH
weak” and OCH “less strong”) activating groups was the alcohol which showed the lowest yield
2
OH and OCH
3
functional groups (weak and strong electron-
2
3
2
3
2
“
3
and selectivity for VAD production. The combination of these functional groups has resulted in the
prevalence of the greatly activating nature of the OH substituent, leading to faster VA conversion.
In the case of the reactions using BA (having in common the CH OH group in all the studied
2
molecules), the maximum yield for BAD production (~50%) was obtained at 120 min. In a different
case, the low yield toward PAD production may be attributed to the presence of dioxole as electron-
withdrawing group, which inhibit the reactivity of PA. Additionally, the low selectivity observed
1
2

for the reactions using VA and PA result from the overoxidation of the aldehydes into vanillic acid
and piperonylic acid, respectively.
3.3 Photocatalytic synthesis of AAD using gCN-TS
Two-dimensional nanosheets constituted of few atomic layers of C
3
4
N have been attracting great
attention due to their unique electronic and optical properties [8, 47, 48]. In this work, gCN-TS was
prepared by sonication of gCN-T in water, producing a colloidal suspension with a catalyst load of
-1
0
.7 g L . To avoid a useless excess of catalyst and to ensure total absorption of efficient photons
[
2, 49], reactions using different gCN-TS load were carried out by diluting the original suspension
-1
in order to obtain catalyst loads in the 0.05-0.70 g L range. The suspensions were used for the
conversion of AA into AAD. The kinetic results showed that the photocatalytic conversion of AA
follows a pseudo-first order model (Fig. SD4). The values of the apparent rate constants (kapp
)
obtained for AA conversion with different gCN-TS loads are plotted in Fig. 3.
Figure 3. Effect of catalyst load in the kapp for photocatalytic oxidation of AA using gCN-TS
suspensions.
-1
It was observed that kapp increases with the catalyst load up to 0.10 g L , confirming the
heterogeneous nature of the photocatalytic process. This behavior can be ascribed to an increase in
1
3

the available active sites for alcohol adsorption and conversion. A plateau was observed for catalyst
-1
loads between 0.10 and 0.50 g L with a negligible effect on kapp, meaning that the reaction rate
-1
becomes independent of the catalyst load. A further increase in the catalyst load to 0.70 g L (i.e.,
using the original gCN-TS suspension) leads to a decrease in kapp, which may be assigned to light
scattering resulting from a surplus amount of catalyst particles in the suspension.
Figure 4. (a) Concentration profiles of the AAD varying the initial concentration of AA after 4 h
-1
reaction using gCN-TS (0.2 g L ). (b) Conversion (C) of the AA, selectivity (S) and yield (Y) for
AAD formation (bars); and reaction time for the maximum yield of each aldehyde ().
1
4

-1
Based on the previous results, we used gCN-TS suspensions with a catalyst load of 0.20 g L for
studying the influence of the initial concentration of AA (0.50 - 7.0 mM) in the efficiency of AAD
production.
The maximum concentration of AA was set as 7.0 mM by taking into consideration the solubility
of both AA and AAD in water (~ 15 mM at 20 ºC [50]). As expected, higher AAD production was
achieved by increasing the initial concentration of AA (Fig. 4). Above an initial concentration of
AA of 3 mM, the reactions tend to be slower, due to an increase in the amount of AA molecules
per active site. For the reaction with the highest initial AA concentration (7.0 mM) a maximum
yield of 5.56 mM of AAD was obtained at the end of 90 min with a selectivity of 82% in 90 min,
which is indicative of the high potential of gCN-TS as catalyst to produce this high added-value
product.
Table 1 summarizes the results reported in the literature on the selective photocatalytic production
of AAD from AA and includes the results obtained in this work with gCN-T and gCN-TS.
Table 1. Results reported in the literature and obtained in the present work on the selective
photocatalytic production of AAD from AA.
Catalyst
Solvent
Irradiation
conditions
Catalyst
Reaction
C
S
Ref.
load (g/L) time (h) (%) (%)
TiO
TiO
2
2
P25
P25
Water
Water
Simulated solar light
UV (Hg lamp)
0.2
2
50
65
15
14
[51]
[13]
0.02
13.8
UV (125 W Hg lamp,
TiO
2
Water
0.5
3.8
50
32
[52]
λ =360nm)
TiO
2
2
Water
Water
Vis (λ>420 nm)
2.5
0.4
3
33
50
92
56
[14]
[53]
TiO
Brookite
UV (Hg lamps)
4.3
UV (Hg lamp,
λ=360 nm)
TiO
2
Rutile
Water
0.4
2.36
50
60
[54]
1
5

TiO
graphene
2
-
BTF
Vis
0.8
0.2
20
3
70
54
100
86
[55]
[56]
TiO
DIOL/Ag
2
UV (fluorescent
lamps, λ=365 nm)
Water
/
UV (450 W Hg lamp,
WO
3
/TiO
2
Water
1
6
50
64
[57]
λ>350 nm)
CdS
Water
Water
Vis (λ>420 nm)
2.5
3
4
25
60
72
[14]
[40]
Vis (100 W halogen
lamp)
3
g-C N
4
4
0.33
100
3
g-C N
Water
Water
Vis (λ>420 nm)
2.5
2.5
3
3
10
56
96
89
[14]
g-C
3
3
N
N
4
4
Vis (λ>420 nm)
[14]
[58]
g-C
ACN
Water
Water
UV (300 W Xe lamp)
UV-LEDs (370 nm)
UV-LEDs (370 nm)
2
9
85.7 >99
This
study
This
gCN-T
1.0
0.2
0.75
0.5
>99
>99
>99
>99
gCN-TS
study
The photocatalytic production of AAD from aqueous solutions of AA using the materials
developed in this work appear to be very competitive when compared with those reported in the
literature (Table 1), as high conversion and selectivity are achieved in a very short reaction time
and using UV-LEDs as high-efficient and low-cost radiation source.
Moreover, using gCN-TS, we were capable to decrease the catalyst load to 1/5 of that used with
gCN-T and decrease the reaction time from 45 to 30 min for obtaining >99% conversion of AA
with a >99% selectivity to AAD.
In order to clarify the mechanism and the nature of the species involved in the photocatalytic
reactions, specific scavengers were added to the reaction medium and its effect in the kinetics of
AA conversion to AAD using gCN-TS was assessed (Fig. SD5). In the presence of tert-butanol (t-
BuOH), the AA conversion profile remained practically unchanged, revealing that hydroxyl
radicals are not involved in its conversion into AAD. Yet, a decrease in the kinetics of AA
conversion was observed in the presence of ethylenediaminetetraacetic acid (EDTA), indicating
1
6

that holes play an important role in the mechanism of AA conversion. Another important
observation is that a relatively high amount of H is formed during the photocatalytic reaction
Fig. SD5), which occurs due to two-electron reduction of oxygen at the conduction band of gCN-
TS, hindering the formation of superoxide radicals (resulting from one-electron reduction of O ).
2
O
2
(
2
3.4 Immobilization of gCN-TS
From a technological point of view, immobilization of photocatalysts is highly desired to facilitate
reuse, avoid costs related to separation and to permit continuous operation. In this view, gCN-TS
was immobilized on glass rings previously treated with PVA. Before starting the photocatalytic
reaction, a 0.50 mM AA solution was passed through the reactor for 2 h to establish hydrodynamic
equilibrium (residence time of 55 min). Negligible AA adsorption was observed after this period.
A maximum production of ~ 0.27 mM AAD was observed at 90 min, this production being
maintained for at least 24 h (Fig. 5). The stability of the gCN-TS-coated glass rings was assessed
by repeating the reaction for two more times starting with fresh AA solutions. Before each run, a
stream of ultrapure water was passed through the reactor containing the gCN-TS-coated glass rings
for 4 h. The results showed that the performance of the immobilized system was maintained
constant, which evidence the high stability of the gCN-TS-coated glass rings under the reaction
conditions used.
1
7

Figure 5. Concentration profiles of AA and AAD using gCN-TS immobilized on glass rings during
4 h reaction; each point corresponds to the average of 3 independent runs using the same gCN-TS
2
coated rings.
Globally, the results show that gCN-based materials are highly promising photocatalysts for the
synthesis of aromatic aldehydes. Moreover, the possibility of immobilizing gCN-TS nanosheets
opens the possibility of producing these important chemicals in continuous-flow reactors, reducing
operation costs and allowing large-scale production.
Conclusions
Carbon nitride materials, gCN-T and gCN-TS, prepared by thermal treatment of bulk gCN and
further sonication, respectively, were successfully used for the photocatalytic synthesis of a
series of aromatic aldehydes from the respective alcohols under UV-LED radiation.
The best photocatalytic performance using gCN-T was observed for the synthesis of anisyl
alcohol from anisaldehyde (C, S and Y> 99% at 45 min of reaction). The relative extent in the
conversion of the alcohols was correlated with to the electronic nature (electron-donor/or
withdraw), number and position of the respective functional groups.
1
8

Sonication of gCN-T in water resulted in a colloidal suspension of carbon nitride nanosheets (gCN-
TS), with enhanced photocatalytic activity towards the conversion of anisyl alcohol to anisaldehyde
(C, S and Y> 99% at 30 min of reaction).
gCN-TS nanosheets were successfully immobilized over glass rings, which revealed to be highly
stable during consecutive utilizations for the synthesis of anisaldehyde in a continuous-flow
reaction system.
Acknowledgments
This work was financially supported by Associate Laboratory LSRE-LCM
-
UID/EQU/50020/2019 - funded by national funds through FCT/MCTES (PIDDAC) and by
projects POCI-01-0145-FEDER-031398 and POCI-01-0145-FEDER-030674, funded by European
Regional Development Fund (ERDF) through COMPETE2020 - Programa Operacional
Competitividade e Internacionalização (POCI) – and by national funds through FCT - Fundação
para a Ciência e a Tecnologia. C.G.S. and M.J.L. acknowledge the FCT Investigator Programme
(
IF/00514/2014) with financing from the European Social Fund and the Human Potential
Operational Programme. R.A.F. acknowledges the PhD fellowship funded by Project NORTE-08-
369-FSE-000028, supported by Norte Portugal Regional Operational Programme (NORTE 2020),
5
under the PORTUGAL 2020 Partnership Agreement, through the European Social Fund (ESF).
We are indebted to Dr. Carlos Sá and the CEMUP team (Portugal) for technical assistance with
SEM measurements.
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