Natural Sunlight Driven Oxidative Homocoupling of Amines by a Truxene-Based Conjugated Microporous Polymer

Article abstract of DOI:10.1021/acscatal.8b00623

In this study, we report the synthesis of a metal-free truxene-based conjugated microporous polymer (Tx-CMP) and demonstrate its heterogeneous photocatalytic applications under direct sunlight. We employed Tx-CMP for the oxidative homocoupling of amines.

Full text of DOI:10.1021/acscatal.8b00623

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Article
Natural sunlight driven oxidative homocoupling of amines
by truxene based conjugated microporous polymer
Venugopala Rao Battula, Harpreet Singh, Sunil Kumar, Indu
Bala, Santanu Kumar Pal, and Kamalakannan Kailasam
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00623 • Publication Date (Web): 12 Jun 2018
Downloaded from http://pubs.acs.org on June 12, 2018
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Natural sunlight driven oxidative homocoupling of
amines by truxene based conjugated microporous
polymer
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Venugopala R. Battula,a‡Harpreet Singh,b‡Sunil Kumar,a Indu Bala,b Santanu K. Pal*b and
Kamalakannan Kailasam*a
aInstitute of Nano Science and Technology (INST), Phase 10, SAS Nagar, Mohali 160062, India
bDepartment of Chemical Sciences, Indian Institute of Science Education and Research (IISER)
Mohali, Sectorꢀ81, SAS Nagar, Mohali 140306, India.
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Abstract
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In this study, we reported the synthesis of a metalꢀfree Truxene based conjugated microporous
polymer (TxꢀCMP) and demonstrated heterogeneous photocatalytic application under direct
sunlight. We employed TxꢀCMP for the oxidative homocoupling of amines. This model reaction
describes an effective way to harvest natural sunlight effectively for the organic transformations.
The higher conversion and selectivity of TxꢀCMP has been demonstrated with recyclability for at
least 5 times without any decline in its catalytic activity under direct sunlight.
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KEYWORDS: Truxene based CMP, Conjugated Microporous polymers, Heterogeneous
photocatalysis, Amine homocoupling and Sunlight photocatalysis.
Introduction
Natural sunlight is an abundant and inexpensive energy source which provides continues support
to living beings in the form of food and biomass through photosynthesis.[1] Giacomo Ciamician,
the father of organic photochemistry, had challenged the scientists to vision a chemical industry
that can synthesize chemicals using sunlight resembling the photosynthesis in the plants.[2]
Indeed, very few initial attempts of synthesising fine chemicals by homogeneous catalysis using
organic dyes, metal complexes and simple organic molecules as photosensitizers proved that the
fine chemical synthesis using solar energy is technically achievable and environmentally
sustainable.[3] Despite of many efforts by researchers in the recent years using solar simulator in
the laboratory, reactions in natural sunlight for organic conversion through heterogeneous
catalysis is always a bottleneck which yet to be realised. Hence, the utilization of natural sunlight
having higher luminous efficacy (an ability to produce visible light) rather than simulated
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sunlight (requires significant cost and large infrastructure) to drive the chemical reactions to
produce high value fine chemicals is the “holy grail” in the present scenario, where the
environmental policies are becoming more stringent dayꢀbyꢀday.
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Recently, visible light induced photoredox catalysis using inorganic transition metal complexes
[4] and organic dyes [5] as homogeneous photocatalyst has shown successful results in a variety
of chemical transformations due to their commercial availability, excellent stability and
Photoredox properties. However, their intrinsic drawbacks and postꢀreaction removal cost of
homogeneous catalysts, etc are the huge obstacles for large scale commercial applications.
Therefore, metalꢀfree, heterogeneous, visibleꢀlight driven photocatalysts which are recoverable
and reusable with high stability are most needed to address the aforementioned concerns that are
facing the industrial challenges today.
In this respect, in spite of few reports from other organic polymers, Graphitic carbon nitride
polymers, gꢀCN (commonly known as gꢀC3N4) provided the momentum in this regard and have
been explored for a series of metalꢀfree heterogeneous photocatalytic reactions.[6] Very recently,
another class of heterogeneous photocatalysts, Conjugated Microporous Polymers (CMPs),[7]
with permanent high porosity, photoactive structure, chemical tunability, high thermal stability
and high flexibility are the centre of attraction to explore them as photocatalysts either in the
presence or absence of coꢀcatalysts, The scope of CMPs as a metal free photocatalysts is much
wider, because their optoelectronic properties can be easily engineered at the molecular level
with the help of rich organic chemistry tools.[8] Thus, the CMPs have emerged as an efficient
and stable platform for heterogeneous visible lightꢀpromoted chemical transformations such as
molecular oxygen activation,[9a] selective oxidation of organic sulphides,[9b] CꢀC bond
formation,[9c] reductive dehalogenation reaction,[9d] oxidative hydroxylation of arylꢀboronic
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acids, visible lightꢀinitiated free radical and cationic polymerization, lightꢀinduced hydrogen
evolution[8e,9,7e] etc.[9] However, we have noticed that nearly all the reports addressing the
photocatalytic application use artificial sources, which requires an expensive setup and high
energy light source. To address these issues, we thrived to perform the photocatalytic reactions
under natural sunlight in the effective sunshine hours which is a costꢀeffective way for possible
large scale catalytic applications in future.
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In this report, we have developed a novel Truxene (Tx) based conjugated microporous polymer
(TxꢀCMP) having high specific surface area (SSA) for better harvesting of sunlight and effective
mass transfer during catalytic process. We reasoned the choice of molecular entity, a C3
symmetric, starꢀshaped strong electron donating πꢀconjugated polyarene, to build polymeric
framework for photocatalyst since its extensive use in the optoelectronic devices as
photosensitizers,[10] photoresists,[11] organic lasers,[12] sensors,[13] OLEDs,[14] OFETs,[15]
and OPVs.[16] Truxene is a fused trimer of fluorene molecule, where fluorene is already
employed as a building block for the CMPs to drive various photocatalytic chemical
transformation.[9a, 9d, 9e] With a strong motive that the introduction of Tx molecule in CMP
will provide unique photocatalytic properties, we employed TxꢀCMP as a metalꢀfree sunlight
active heterogeneous photocatalyst for the fundamental organic transformation of amines to
corresponding imines. The imines also called Schiff bases are well known for their versatility as
intermediates in the synthesis of pharmaceutical or biological active compounds and fine
chemicals. [17] As per the available literature and to our best knowledge this is the first report,
where we achieved >99 % conversion with 90% yield for oxidative homocoupling of
benzylamine in natural sunlight by using a heterogeneous metal free organic photocatalyst within
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4 h of reaction time. This, we believe as a major step forward in the field of
organophotocatalysis.
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Results and Discussions
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In Scheme 1, the synthetic pathway of the conjugated porous polymer TxꢀCMP is displayed in
which Tx, a wellꢀknown electron donor, is attached as a 3D centre with phenyl units via
palladiumꢀcatalysed Suzuki crossꢀcoupling reaction to give the microporous polymer (TxꢀCMP).
More detailed synthesis of the monomer is provided in Scheme S1, see supporting information
(SI). For comparison purpose, mesoporous carbon nitride (SGꢀCN) was synthesized according to
the previous reported procedure.[6i] Titanium dioxide, TiO2 (commercial P25) was used as such
for comparing the photocatalytic activity with TxꢀCMP and SGꢀCN.
Scheme 1. Synthesis route and idealized structure of TxꢀCMP; Reaction conditions: (a)
Pd(PPh3)4, K2CO3, DMF/H2O.
Characterization of catalyst
Solidꢀstate 13C CP/MAS NMR spectrum showed signals at 23 and 46 ppm, which are assigned to
the methyl groups and tertiary carbon of Tx units, respectively. The signals between 110 and 160
ppm can be assigned to the aromatic carbons in the polymer backbone (Figure S1, see SI).
Thermogravimetric analysis (TGA) measurements showed that the conjugated network remained
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intact up to ~400 ºC (Figure S2, see SI). The successful establishment of porous framework of
TxꢀCMP is clearly verified by the N2ꢀphysisorption analysis (Figure 1a). The TypeꢀI N2ꢀ
physisorption isotherms at 77 K displayed a steep nitrogen gas uptake at low relative pressure
(p/p0 below 0.1) reflecting the presence of micropores, with hysteresis loop indicating the
presence of mesoporosity. The BET specific surface area was found as 1022 m2/g with average
pore volume 0.7 ccgꢀ1. The pore diameter and distribution were obtained by DFT method as
shown in the inset of Figure 1a showing a major proportion of micropores with dimensions
ranging from 0.7ꢀ1.8 nm and also the evidence of mesoporosity. The diffuse reflectance UVꢀVis
(DRUVꢀVis) spectrum (Figure 1b) of TxꢀCMP showed a broad absorption band with the
absorbance starting around 600 nm with a wide shoulder and the maximum absorbance peak
centred around 400 nm. It showed the broad absorption range of the TxꢀCMP polymers in the
visible and UV light regions which is crucial for their activity in the natural sunlight. The
characteristic broad shoulder in the spectrum of microporous polymer could partially be due to
the light scattering effect originated from the broad pore size range in these porous polymers.[9d]
The inset of Figure 1b showed the band gap of 2.6 eV calculated from the Tauc plot. The
scanning electron microscope (SEM) images of TxꢀCMP showed agglomerated flake type
structures (Figure 1c and Figure S3, see SI). The elemental composition of TxꢀCMP was
estimated by Xꢀray photoelectron spectroscopy (XPS) as shown in Figure S4 indicating carbon is
the majority component presents besides trace amount of oxygen and bromine at the dangling
ends of the polymer. The conduction band minimum (CBM) was calculated as ꢀ0.4 V via Mottꢀ
Schottky electrochemical measurements (Figure S5, see SI). Based on CBM and optical band
gap (2.6 eV), the valence band maximum (VBM) was estimated as 2.2 V (vs Ag/AgCl). It had
been established that the generation of reactive oxygen species (ROS) and
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Figure 1. a) N2ꢀphysisorption isotherms and pore size distribution curve (inset). b) DRUVꢀVis
spectra and Tauc plot to calculate the optical band gap (inset). c) Scanning Electron Microscope
image of TxꢀCMP and d) Band energy diagram of TiO2, SGꢀCN and TxꢀCMP.
the oxidation ability of holes depend on the valence band and conduction band positions of the
semiconductors. As shown in Figure 1d, the VBM position is far positive (+2.4 V vs NHE)
which indicates that the TxꢀCMP is a better oxidant comparable to carbon nitride (+1.4 V). On
the other hand, specific surface area (SSA) of a photocatalyst also can play an active role in the
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photocatalytic activity. The SSA and pore volume of the employed photocatalysts, TiO2 (P25)
and SGꢀCN and TxꢀCMP had been tabulated in Table S1 (see SI).
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Photocatalytic activity of catalyst in natural sunlight
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Due to its high SSA, small optical band gap (2.6 eV) and appropriate band positions, we decided
to explore TxꢀCMP’s photocatalytic activity for photoꢀoxidation of amines in the natural
sunlight. Oxidative homocoupling of amines to corresponding imines were carried out by using
oxygen as an oxidant and acetonitrile as solvent in the presence of sunlight as shown in Figure 2.
Figure 2. The photocatalytic reaction setup for the oxidative homocoupling of amines (Inset
showing closer view of the round bottomed flask kept in sunlight to carryout oxidative
homocoupling reactions ). This photo was taken by one of the authors only.
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The sunlight is largely available, inexpensive and inexhaustible source of energy which contains
approximately 3% of UV, 44% of visible and 53% of NIR. This inspired us to explore the
utilization of natural sunlight to drive the chemical reactions rather than other artificial sources of
light which requires a lot of expenditure when possible large scale applications are concerned.
Initially, the natural sunlightꢀdriven oxidative homocoupling of benzylamine (BAN) was taken
as model reaction with TxꢀCMP, as a metalꢀfree photocatalyst and the optimization conditions
had been shown in Table 1. From entries 1ꢀ3, it is clear that the photocatalyst and sunlight are
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o
indispensable for this reaction. In presence of sunlight (average temp. =35.2 C, solar intensity =
80.2 mW/cm2), >99% conversion was obtained with 90% yield within 4 h of reaction time
(Table 1, entry 4). In order to study the individual effect of UV and Visible light portion of
natural sunlight on the catalytic activity, we carried out a reaction with the simulated sunlight
under fullꢀarc conditions (T =35oC) and under visible light (>420 nm, T=35oC) by using 450 W
Xe lamp operated at 400 W (Height was adjusted to give the intensity of 80 mW/cm2) separately
which gave 88% and 75% yields respectively (Table 1, entries 5 and 6). The reason for the better
yield in fullꢀarc over visible light conditions could be attributed to its absorption in UV region.
The reaction under openꢀair gave 79 % yield only (Table 1, entry 7). Moreover, these results also
suggested that the TxꢀCMP is an efficient natural sunlight active photocatalyst for the oxidative
homocoupling of BANs to corresponding imine. The screen shot of GCꢀMS chromatogram of
reaction mixture at 0 h, after 4 h in dark and sunlight, respectively were provided in Figure S6,
see SI. The average temperature and average solar intensity during these whole studies in natural
sunlight had been given in Table S2.
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Table1. Optimization of the conditions for natural sunlight driven oxidative homocoupling of
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benzylamine.a
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Entry
Catalyst
Amine
conversion
(%)b
Product
selectivity/
yield (%)b
ꢀ
TOFc
1
2
No Catalyst
0
ꢀ
ꢀ
No Catalyst, 0
No light
ꢀ
3
4
5
6
7
TxꢀCMPd
TxꢀCMP
TxꢀCMPe
TxꢀCMPf
TxꢀCMPg
<1
100/ꢀ
91/90
89/88
97/75
80/79
ꢀ
>99
>99
77
12.4
12.4
9.6
12.4
>99
aReaction Conditions: BAN (0.5 mmol), TxꢀCMP = 10 mg, Solvent = 5 mL Acetonitrile,
Reaction time = 4 h in O2 atmosphere, under natural sunlight. Calculated by GCꢀMS. TOF=
mmol of benzylamine converted per gram of catalyst per h. Without light. Fullꢀarc
experiments(T=35 oC). fUnder visible light (>420 nm, T=35 oC). gUnder open air.
b
c
d
e
Further the catalytic activity of TxꢀCMP was compared with commercially available TiO2 (P25)
and mesoporous carbon nitride (prepared by solꢀgel route) SGꢀCN (characterized by several
techniques, Figure S7 in the SI), for the photooxidation of BAN under natural sunlight. The
catalytic activity of these catalysts in sunlight was found to be in the order: TxꢀCMP > TiO2
(P25) > SGꢀCN, which could be explained on the basis of their band position and surface area
parameters as briefed above (Figure 3a). The better catalytic activity of TiO2 (P25) over SGꢀCN
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is due to its more positive valence band position and formation of surface complex with BAN
which acts as an antenna for visible light absorption. [18] Even though, the valence band position
of TiO2(P25) is slight positive than that of TxꢀCMP (Figure 1d), the catalytic performance of Txꢀ
CMP is far superior to TiO2 (P25) essentially due to its high SSA and smaller band gap. Energyꢀ
dispersive Xꢀray spectroscopy (EDX) experiments showed the absence of any traces of
Palladium present in the asꢀprepared TxꢀCMP sample after the Suzukiꢀcoupling reaction (Figure
S8 in the SI) which ruled out the possibility of catalytic activity by palladium. From the above
experiments, it could be inferred that the SSA and band position stimulated the superior
photocatalytic activity of TxꢀCMP as compared to TiO2 (P25) and SGꢀCN in direct sunlight.
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Recyclability of catalyst
To study the durability of TxꢀCMP in natural sunlight, recyclability tests were conducted for five
subsequent cycles. Before using the catalyst for next cycle, the catalyst was recovered by
b
a
Conversion
Selectivity
yield
Conversion
Selectivity
Yield
100
80
60
40
20
0
100
75
50
25
0
1
2
4
5
Number 3of Cycles
Tx-CMP
SG-CN
TiO
2
Figure 3. a) Comparison bar diagram of TxꢀCMP photocatalytic activity with TiO2 and SGꢀCN
for oxidative homocoupling of benzylamine. b) Recyclability test of TxꢀCMP in oxidative
homocoupling of benzylamine. Reaction Conditions: Substrate (0.5 mmol), TxꢀCMP = 10 mg,
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centrifugation, washed with acetonitrile and dried at 120 °C in vacuum oven for overnight. As
shown in Figure 3b, the conversion of BAN was >99% in each cycle with more than 90 % yield.
Thus, the photocatalytic activity of TxꢀCMP remained similar throughout its use in all cycles.
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We carried out the Solidꢀstate C CP/MAS NMR, FTꢀIR, DRUVꢀVis, TGA and SEM analysis
of recovered TxꢀCMP after 5th cycle (Figure S9ꢀS14, see SI). The result shows that TxꢀCMP
retained the similar structural features after being used for 5 consecutive cycles of reaction
course. The SSA of TxꢀCMP catalyst after 5th cycle was measured to be 904 m2/g (Figure S14,
see SI). Only a minor loss in the surface area compared to that of the fresh one and the retention
of the porous structure was observed. This indicated the superior performance and robustness of
TxꢀCMP as a metalꢀfree photocatalyst for natural sunlight driven oxidative homocoupling of
amines.
Mechanistic studies
Based on the observation from the above experimental studies and previous reports, a plausible
mechanistic pathway for the natural sunlightꢀdriven oxidative homocoupling of amines (BAN
has been taken as model) was proposed (Scheme 2). Generally, this reaction was initiated by the
charge carriers, holes and electrons that were generated upon light absorption by TxꢀCMP. The
similar reaction when carried out under N2 atmosphere gave only 7% conversion of BAN to
imine (Table 2, entry 1) which confirmed that the molecular O2 is essential for this reaction.
As reported previously, the molecular oxygen (O2) is converted into reactive oxygen species
.ꢀ
(ROS) by electron or energy transfer process (superoxide radical (O2 ) and singlet oxygen (1O2))
species.[8a, 8b] The holes in the valence band of TxꢀCMP oxidized the BAN to benzylamine
radical cation.[6g] On the formation of benzylamine radical cation the acidity of benzylic CꢀH
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Scheme 2. Proposed mechanism for the natural sunlight driven oxidative homocoupling of
benzylamine catalyzed by TxꢀCMP.
(benzylic hydrogen) adjacent to amine functional group would increase. The formed ROS would
further abstract the benzylic hydrogen from the benzylamine radical cation to form
benzyliminium ion. To investigate the influence of benzylamine radical cation, the hole
scavenger KI was added and found that the conversion of BAN was decreased drastically to 10%
in presence of KI (Table 2, entry 2), which confirmed that the formation of benzylamine radical
cation formation step is crucial for this reaction.
To shed more light on mechanistic pathways, the role of ROS in this sunlight driven oxidative
homocoupling of BAN was investigated. When pꢀbenzoquinone (BQ), a superoxide radical
scavenger was added to the reaction mixture, the homocoupling of BAN decreased drastically to
30% in (Table 2, entry 3) which indicated the vital role of superoxide radicals.[19] The singlet
oxygen scavenger sodium azide (NaN3) was also added to the reaction mixture and it was
observed that the homocoupling of BAN decreased to 77% (Table 2, entry 4) which proved the
role of singlet oxygen in this reaction pathway.[8b] To note that, the benzyliminium ion further
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converted into benzylimine with the formation of H2O2 as side product.[20] The H2O2 further
dissociated into .OH by the UV light present in the sunlight and might played a supportive role in
the oxidation of BAN to corresponding imine. The formation of H2O2 and its dissociation into
OH radicals by UV light present in natural sunlight during the reaction has been proved by a
controlled experiment by using of N,Nꢀdiethylꢀ1,4ꢀphenylenediamine (DPD) (Figure S 15 in SI).
Further, the benzylimine (detected in GCꢀMS at 5.3 min retention time (Figure S6 in the SI))
could react in 3 different ways: 1) Undergo dehydrogenative step to give benzonitrile (detected
in GCꢀMS at 4.6 min retention time) [21]; 2) Converts into benzaldehyde (detected in GCꢀMS at
4.4 min retention time). This further could react with BAN to give corresponding imine. The
formation of benzylimine was confirmed by the fragmentation spectra (Figure S16 in SI) and 3)
Reacts with BAN to give corresponding imine.[8d, 22] In order to prove the formation of above
intermediates, we monitored the reaction for initial 2 h. We observed that the concentration of
benzaldehyde and benzonitrile increased for this time period (Figure S17 in the SI) with
consequent decrease in concentration of reactant. The corresponding increase in the
concentration of both (benzaldehyde and benzonitrile) indicated the formation of benzylimine
intermediate which could undergo dehydrogenation and hydrolysis, respectively to benzonitrile
and benzaldehyde. These additional experiments to understand the mechanism in the natural
sunlight as observed earlier in high energy light sources prove the similar mechanistic pathway
in the current study.
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Table 2. Controlled experiments to predict the mechanism.a
Entry O2
1b
Additive
Amine
Conversion (%)
7
ꢀ
ꢀ
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2
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+
+
Hole scavenger
Superoxide
scavenger
10
30
7
8
9
4
+
Singlet
oxygen 77
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scavenger
aReaction Conditions: Substrate (0.5 mmol), TxꢀCMP = 10 mg, Solvent = 5 mL Acetonitrile,
Reaction time = 4 h under direct sunlight,b N2 atmosphere, KI as hole scavenger, pꢀbenzoquinone
as superoxide scavenger and NaN3 as singlet oxygen scavenger.
Substrate scope of the catalyst
Further to explore the versatility of TxꢀCMP catalyst, various substituted BANs (Table 3) were
used under same reaction condition. When BAN is substituted with electron withdrawing
substituents (ꢀCl & ꢀF), we achieved 77% and 82% conversion with 95% and 92% selectivity,
respectively within 4 h of reaction time (Table 3, entry 2 and 3). However, the BAN substituted
with electron donating substituents (ꢀCH3 & ꢀOCH3) achieved 96% and 97% conversion with
80% and 81% selectivity, respectively, within 4 h reaction time (Table 3, entry 4 and 5). In
addition, the applicability of TxꢀCMP for oxidative homocoupling of heterocyclic amine
(furfurylamine) was explored resulting in the 98% conversion with 86% selectivity (Table 3,
entry 6). But the reaction with aliphatic amines (Dodecylamine and nꢀbutylamine) could not give
corresponding imine under these experimental conditions (Table 3, entry 7 and 8). In this case,
only fragmentation products were observed in GCꢀMS spectrum. This is due to the activation of
multiple reactive centres by reactive oxygen species which results in unselective autoꢀoxidation
of aliphatic amines.[22b] The above experimental results suggested that TxꢀCMP is a highly
promising photocatalyst which can promote oxidative homocoupling of substituted amines and
heterocyclic amines to corresponding imines with high conversion and selectivity within 4 h in
natural sunlight.
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Table 3. Photocatalytic activity of the TxꢀCMP for selective oxidation of various amines.a
Entry Substrate
Product
Conversion
/selectivity (%)
>99/91
TOFb
1
2
12.4
9.6
77/95
82/92
97/80
3
4
10.3
12.1
5
6
96/81
98/86
12.0
12.3
7
8
0/0
0/0
ꢀ
ꢀ
aReaction Conditions: Substrate (0.5 mmol), TxꢀCMP = 10 mg, Solvent = 5 mL Acetonitrile,
Reaction time = 4 h in O2 atmosphere under direct sunlight. bTOF= mmol of amine converted per
gram of catalyst per h.
Conclusion
In summary, we have successfully designed a novel Truxene based conjugated microporous
polymer (TxꢀCMP) linked through highly stable CꢀC bond for the first time. The novel TxꢀCMP
has high surface area, large pore volume, narrow band gap and extraordinary stability. It serves
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as a metalꢀfree photocatalyst for the oxidative homocoupling of amines with optimal conversion
under natural sunlight within 4 h. This model reaction also describes an effective, greener and
sustainable way to harvest natural sunlight effectively for the organic transformations. TxꢀCMP
has shown recyclability without any decline in its catalytic activity. We hope this work opens the
door for the employment of various conjugated microporous polymers as metalꢀfree
heterogeneous photocatalysts for effective harvesting of natural sunlight to drive many organic
transformations.
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Methods
All chemicals and solvents were of analytical grade and used without further purifcation.1ꢀ
indanone, 1,4ꢀphenylenediboronic acid, potassium carbonate and Pd(PPh3)4 were purchased from
Sigma Aldrich (Bangalore, India). Thinꢀlayer chromatography was performed on aluminium
sheets preꢀcoated with silica gel (Merck, Kiesel gel 60, F254). Column chromatographic
separations were performed on silica gel (60–120 mesh). Structural characterisations of the
compounds were conducted by 1HꢀNMR and 13CꢀNMR (Bruker Biospin, Switzerland, Avanceꢀiii
400 spectrometer), Infrared spectroscopy (Perkin Elmer Spectrum AX3) and DRUVꢀVis
spectrophotometer (3000+). NMR spectra were recorded in deuterated chloroform (CDCl3) and
deuterated dimethylsulfoxide (DMSOꢀd6) using tetramethylsilane as the internal standard. SEM
images were recorded on a JEOL 7600F instrument. N2ꢀphysisorption isotherms were recorded
on Autosorb iQ2 instrument (Quantachrome) at 77 K. Before measurements, samples were
degassed overnight under vacuum at 120 °C. The specific surface area was calculated by the
BET Micropore assistant surface area calculations for TxꢀCMP. Density functional theory (DFT)
method was employed to estimate the pore size distribution for TxꢀCMP and BJH method for
SGꢀCN. The total pore volume was calculated from the amount of N2 adsorbed at p/p0 0.9,
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assuming that adsorption on the external surface was negligible compared to that of the pores.
The optical band gap of TxꢀCMP was derived from Tauc plot. Mott−Schottky electrochemical
measurements were carried to get the information about the conductionband potential. Catalyst
ink was prepared by dispersing the catalyst in ethanol, IPA, nafion and then working electrode
was prepared by drop casting catalyst ink on the glassy carbon electrode (GCE) and left to dry
overnight. Electrochemical impedance spectra measurements were carried out by standard three
electrode system using Platinum wire and Ag/AgCl (3M KCl) as counter and reference
electrodes, respectively. The measurement was conducted in a buffer solution (pH= 7) in the
potential range of ꢀ0.6 to 1V. Powder Xꢀray diffraction (XRD) patterns were recorded on a
Bruker D8 Advance diffractometer equipped with a scintillation counter detector, with CuꢀKα
radiation (λ = 0.15418 nm, 2θ = 5─80o) source operating at 40 kV and 40 mA. Diffuse
reflectance UVꢀVisible (DRUVꢀVis) spectra were recorded on a Shimadzu UVꢀ2550
spectrophotometer. The elemental composition of TxꢀCMP was estimated by using the XPS with
AES module (PHI 5000 Versa Probe II, FEI Inc.).
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General procedure for oxidative homocoupling of amines
The oxidative homocoupling of amines was performed in a homeꢀmade photocatalytic vessel
under direct sunlight (Figure 2). In a typical reaction, 0.5 mmol of BAN was added to 5 mL of
acetonitrile solvent and then 10 mg of TxꢀCMP was added. The reaction mixture was kept for
stirring under natural sunlight (In Mohali, India, during May, June and August, 2017) and the O2
atmosphere (1 atm) was maintained during the reaction. The dateꢀwise average temperature and
average solar intensity data was given in (Table S2, Figure S18). The catalyst was separated by
micro centrifuge after the reaction for further characterization. Then, the conversion, selectivity
and yield of the reaction products were quantified by gas chromatography (GCꢀMS Shimadzu
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GCꢀ2014) equipped with an HPꢀ5 capillary column using calibration curves. The same procedure
has been followed for the catalysis using TiO2 and SGꢀCN for better comparison.
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AUTHOR INFORMATION
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Corresponding Author
*kamal@inst.ac.in, *skpal@iisermohali.ac.in
Author Contributions
‡Authors contributed equally to this work.
Notes
Authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information
Synthesis details, IR, Solidꢀstate 13C{1H} CP/MAS NMR spectra, BET, BJH, SEM, XPSꢀsurvey
scan, EDX, XRD, TGA, MottꢀSchottky measurement, Textural properties, Average Temperature
and Solar intensity and GCꢀMS screen shot images along with Sunlight intensity and
Temperature Tables.
ACKNOWLEDGMENT
We thank INST Mohali and IISER Mohali for financial support. Dr K. Kailasam thanks DSTꢀ
Nano Mission NATDP funded Technology Project, File No. SR/NM/NTꢀ06/2016 and DSTꢀ
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SERB for the Early Career Research Award (ECR), File No. ECR/2016/001469. Dr S. K. Pal is
grateful for the financial support of the INSA (bearing Sanction No. SP/YSP/124/2015/ 433) and
CSIR (bearing Sanction No. 02(0311)/17/EMRꢀII). Dr Sunil Kumar would like to acknowledge
DST (India) SERB fellowship under project No. PDF/2016/002227.
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