Design of Novel Catalysts Based on Acridine Derivatives Immobilized on Carbon Materials for Molecular Hydrogen Production

Article abstract of DOI:10.1134/S1070363219050104

Immobilization of acridine and its derivatives (9-phenylacridine and N-methyl-9-phenylacridinium iodide) has been studied and quantitative data on physical adsorption have been obtained. The adsorption equilibrium constants K, the parameters A_∞

Full text of DOI:10.1134/S1070363219050104

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 5, pp. 918–923. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Authors(s), 2019, published in Zhurnal Obshchei Khimii, 2019, Vol. 89, No. 5, pp. 726–732.
Design of Novel Catalysts Based on Acridine Derivatives
Immobilized on Carbon Materials
for Molecular Hydrogen Production
a
a
a
a
E. V. Okina *, O. V. Tarasova , A. V. Balandina , K. A. Grigoryan ,
a
a
Yu. M. Selivanova , and A. V. Gorbunova
a
N. P. Ogarev National Research Mordovia State University, ul. Bol’shevistskaya 68, Saransk, 430005 Russia
e-mail: iman081@gmail.com
*
Received November 22, 2018; revised November 22, 2018; accepted November 29, 2018
Abstract—Immobilization of acridine and its derivatives (9-phenylacridine and N-methyl-9-phenylacridinium
iodide) has been studied and quantitative data on physical adsorption have been obtained. The adsorption
equilibrium constants K, the parameters A and ∆Gads have been calculated from the Langmuir adsorption
∞
isotherms. The electrochemical properties of organic compounds immobilized on a carbon material surface
have been studied by means of cyclic voltammetry.
Keywords: acridine, adsorption, immobilization, heterogeneous catalyst, hydrogen
DOI: 10.1134/S1070363219050104
Despite significant progress achieved recently in
the field of hydrogen energetics, practical application
of the developed technologies has remained extremely
limited due to low efficiency of synthetic catalysts
This report contains the data on acridine and its
derivatives adsorption on carbon materials as well as
electrochemical properties of the obtained systems.
The main goal of this study was determination of
optimal conditions for the process of organic
compounds adsorption on carbon supports with the
aim to obtain innovative types of catalytic materials
based on organic molecules immobilized on a surface
via strong physical adsorption (π–π-stacking of
[1, 2]. Unique properties of the carbon-based materials
make them promising for hydrogen energetics [3–9]. It
has been recently shown that metal-free catalysts based
on carbon structures exhibit high catalytic activity
towards the reaction of molecular hydrogen production
2
[10–19]. It has been shown that electrocatalysts based
aromatic rings with regions of sp -hybridized carbon at
on metal complexes containing polyaromatic groups
can be efficiently immobilized on the surface of the
working glassy-carbon electrode [19]. Heterogeneous
catalysts obtained via this method are of new
promising class of heterogeneous catalysts for
molecular hydrogen production.
the surface). Carbon paper (Sigraset GD39), activated
carbon (VulcanXC-72, VU), and glassy-carbon were
used as carbon materials. Acridine 1, 9-phenylacridine
2, and N-methyl-9-phenyl-acridine 3 (Scheme 1) were
chosen for the investigation of the features of organic
compounds adsorption at the carbon materials.
It has been shown earlier that electrocatalytic system
based on acridine, 9-phenyl-N-phenylacridinium
iodide (PhAcrI), can efficiently catalyze the process of
molecular hydrogen production using aprotic solvents
in the presence of chloric acid, as well as using water
with different рН [20, 21]. Hence, it can be assumed
that the immobilization of acridine and its derivatives
Scheme 1.
+
N
N
N
(
Scheme 1) on the surface of carbon materials will
−
afford highly efficient heterogeneous catalytic systems
for molecular hydrogen electrochemical production.
CH I
3
1
2
3
9
18

DESIGN OF NOVEL CATALYSTS BASED ON ACRIDINE DERIVATIVES
Elucidation of the major factors affecting adsorption of
(a)
919
model compounds should allow the control of the
adsorption of more complex compounds of the same
series and perform targeted modification of the
surface to impart desired properties.
The presence of extended conjugated regions in the
structure of compounds 1–3 makes their immo-
bilization possible due to strong physical adsorption on
the surface of carbon support via π–π-stacking.
However, carbon paper and glassy-carbon were not
found to be suitable for the investigation of adsorption
processes. Adsorption of the test organic compounds
on them did not occur even at high concentrations of
adsorbate in the solution. That was the reason why
further investigations were conducted using VulcanXC-
(
b)
7
2 material which showed excellent adsorption
properties. The investigation of adsorption rate of
compounds 1–3 in the presence of VulcanXC-72 was
performed via conventional methods.
(
c)
The time to attain equilibrium sorption changed in
the following order: 1 > 2 > 3. Compound 1 possessed
planar structure which should be ideal for the adsorp-
tion on the carbon support surface via π–π-stacking.
Probably, its high sorption rate was due to the same
reason, since compound 2 had a substituent at position
9
, which, according to quantum-mechanical simula-
tion, was orthogonal to the π-system of acridine
scaffold. Such molecule geometry would be less
favorable for immobilization on the support surface.
The structure of molecule 3 was more distorted in
comparison with compounds 1 and 2. In order to
confirm the obtained results, we performed quantum-
mechanical simulation to model the process of
adsorption of compounds 1–3 on graphene support, the
graphite-like regions were simulated using the DFT
method with the B3LYP/6-31+G basis; the obtained
vibrational frequencies confirmed that those structures
corresponded to the energy minimums (Fig. 1). A
model of graphene surface consisting of 28 conjugated
cycles was used in the simulation. Free valences of the
terminal carbon atoms of graphene surface were
saturated with hydrogen atoms.
Fig. 1. Equilibrium states for adsorption of compounds
(a) 1, (b) 2 and (c) 3 on graphene surfaces obtained via
B3LIP/6-311+G(2d,p) simulation.
energies were low and close to each other [ΔEads
2.1603 (1), –0.28 (2), and –1.30 kJ/mol (3)]. Since
=
–
the energy of stacking-interaction strongly depends on
the degree of overlapping, it could be assumed that the
orbitals of interacting fragments were marginally
overlapping. The reason for such weak interaction was
a prominent difference between the energies of the
boundary orbitals of graphene surface and of
compounds 1–3, meaning that strong difference in
energy did not lead to stabilization of a system during
adsorption. As can be seen from the simulation results,
planar molecule of acridine had the highest ΔEads
value, the HOMO–LUMO values and the distance
between interacting fragments of compound 1 and 2
being close. Hence, the orbitals of polyaromatic planar
systems could overlap better than those of nonplanar
ones.
Weak interactions between the molecules and the
surface with parallel face-to-face orientation and
distance between planes of the interacting fragments
close to 3 Å were found in those three cases. The
obtained values coincided with the X-ray crystallo-
graphy data for the systems known to exhibit π–π-
stacking (3 to 3.5 Å). The calculated adsorption
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 5 2019

9
20
OKINA et al.
Calculated parameters of acridine and its derivatives adsorption on carbon-containing sorbent VulcanXC-72
Parameter
, mol/kg
tanα
Acridine
0.143
9-Phenylacridine
0.125
9-Phenyl-10-methylacridinium iodide
А
∞
0.08
0.0006
20833
–24.6
0.0014
4995
0.0018
3
K, m /kmol
G°ads, kJ/mol
4444
∆
–21.1
–20.8
To investigate the adsorption process in more
detail, we plotted the Langmuir isotherms were plotted
and determined adsorption constant (K), limiting
reflecting the process efficiency was higher for
compound 1, than for compound 2, being significantly
lower for compound 3. The increase in the size the
molecule volume of compound 3 led to almost twofold
adsorption value (А ), and free energy of adsorption
∞
process (∆G° ). The sorption isotherms of the
decrease in the maximal adsorption (А ) in comparison
ads
∞
investigated organic compounds on carbon-containing
sorbent corresponded to Langmuir monomolecular
adsorption and showed convex shape. That equilibrium
could be described by Langmuir equation (1):
with compound 1. The obtained data coincided with
the conclusion that adsorption of compounds 1–3 was
a structure-dependent process, strongly affected by the
presence and the size of the substituents in acridine
core. It is interesting to notice that the introduction of
phenyl substituent (2) at position 9 of the parent
Kc
.
(1)
A = A
∞
1
+ Kc
acridine 1 led to minor decrease in parameter А , while
∞
Here А being the adsorption value, mol/kg; А being
the limiting monomolecular adsorption (capacity of
the introduction of methyl group at the nitrogen atom
∞
led to the decrease in parameter А by almost 1.5 times
∞
monolayer); K being the adsorption equilibrium
in comparison with compound 2. Such unusual
behavior could not be explained solely by steric
3
constant, m /kmol (ratio of adsorption and desorption
rate constants characterizing the energy of adsorbate-
factors. Also it is worth notice that the А value being
∞
adsorbent interaction); c being concentration of
adsorbate in the solution, kmol/m .
the lowest (for compound 3), the K value was almost 5
times higher than that for compounds 1 and 2, and the
3
∆
G°ads values for compounds 1 and 2 were therefore
The experimental adsorption data were processed
using the Langmiur equation in linearized form (2).
significantly higher. In our opinion, high value of the
constant for compound 3 was related to its positive
charge allowing the interaction and stabilization at the
surface of carbon support bearing different functional
groups (COOH, COH, OH). The presence of
functional groups at the surface was confirmed by the
data of IR spectroscopy for that carbon support. Its IR
spectrum contained characteristic signals of carboxyl,
carbonyl, and hydroxyl groups. The obtained data
coincided with the reference ones [22] where the
presence of organic functional groups at the surface of
VulcanXC-72 was evidenced by means of X-ray
photoelectron spectroscopy (XPS) analysis. Hence,
compound 3 adsorbed on the surface of the support
was stabilized by coulomb interaction with the surface
groups and formed the surface layer partially blocking
channels and pores of carbon support, thus reducing
the efficiency of sorption process in terms of maximal
adsorption. That assumption was in line with the
1
A
1
A
1
1
.
.
(2)
=
+
A K C
∞
∞
The plotted dependences 1/A = f(1/C) should be
linear with the intercept being the reciprocal of
monolayer capacity (1/A ) and the slope being 1/(A K)
∞
∞
which gave the equilibrium adsorption constant K.
The experimental isotherms were transformed in
the linear form, and tgα, K (adsorption equilibrium
constant), and ∆G° (change of Gibbs adsorption
ads
energy) were calculated using equations (3)–(5).
1
,
tan α =
(
3)
A K
∞
1
,
(4)
(5)
K =
A∞tgα
G°ads = –RTlnK.
∆
The calculated sorption parameters are given in
significant decrease in the А parameter for compound
∞
the table. As it can be seen, the A , parameter
3 in comparison with compound 2, their structures
∞
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 5 2019

DESIGN OF NOVEL CATALYSTS BASED ON ACRIDINE DERIVATIVES
921
being very similar. Besides that, the ∆G° values were
(a)
ads
significantly higher than the theoretically obtained
values of ΔE . Similar values of ∆G° were obtained
ads
ads
in the study of adsorption of clathrate-chelate cobalt
complexes containing polyaromatic groups in apical
position on mesoporous carbon materials [21]. It has
been suggested by the authors that the sorption process
has occurred in the sorbent pores. Taking that fact in
the account, we concluded that the structure-dependent
adsorption of compounds 1-3, strongly affected by the
structure of adsorbate (the amount and type of substi-
tuents in acridine molecule) also occurred as physical
adsorption in material pores rather than π–π-stacking.
E, V
The determinative parameter affecting the adsorp-
tion of polyaromatic compounds via π–π-stacking
mechanism is the compatibility of HOMO and LUMO
energies of the adsorbate and surface; therefore, to
make adsorption via π–π-stacking mechanism more
efficient, it is necessary to control the energies of
frontier orbitals of the interacting fragments, which can
be achieved via chemical modification of the surface
or adsorbate structure. In this study, the decrease in the
energy of LUMO of the adsorbate was achieved by the
introduction of electron-acceptor substituents in the
structure of compound 1, which allowed to
significantly enhance ΔEads value.
(
b)
E, V
Hence, one can conclude that nonpolar molecules
of 1 and 2 compounds were adsorbed in the carbon
support pores via physical interactions rather than via
π–π-stacking mechanism. The mechanism of sorption
of compound 3 was complicated and could involve
physical adsorption as well as chemical one.
(
c)
Electrochemical properties of the adsorbed com-
pounds were studied via cyclic voltammetry method.
CVAs of compounds 1–3 immobilized on carbon
material VulcanXC-72 showed a single one-electron
wave in at cathodic potentials close to those of the
compounds in the solution (Fig. 2). It is important to
notice that during cycling of compounds 1 and 2
between 0 to –1.8 V and compound 3 (50 cycles)
between 0 to –0.8 V, the peak current of compounds 1
and 2 was not decreased, while for compound 3 the
peak current was decreased. That could correspond to
partial washing out to the solution of the molecules
adsorbed on the surface via coulomb stabilization by
surface functional groups. The constant peak current
values for compounds 1 and 2 evidenced the electrode
stability during electrochemical reactions and no
desorption from the electrode surface. It is worth
notice that the electron transfer from electrode to
E, V
Fig. 2. CVA curve for compounds (a) 1, (b) 2 and (c) 3
immobilized on carbon support Vulcan 72. Conditions:
methylene chloride, V = 20 mV/s, Ag/AgCl/KCl.
catalytic centers at the electrode surface should be fast
and not limited by the rate of electron transfer between
redox sites to obtain an efficient electrocatalytic
system. The linear log Ipvs/log ν dependence with the
slope close to unity was found for the immobilized
compounds, which is common of the systems where
electron transfer from electrode surface to catalyst is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 5 2019

9
22
OKINA et al.
2
the limiting stage rather than charge transfer between
redox sites. Hence, electrochemical data showed that
compounds 1–3 immobilized on carbon support could
be used as heterogeneous catalytic systems for mole-
cular hydrogen production.
with the active surface area 0.125 cm . Scanning rate
was 20 mV/s. Platinum electrode was used as the
counter electrode, standard silver-chloride electrode
(E = 0.41 V in CH CN, vsFc/Fc ) was used as the
reference electrode. The solutions were deaerated with
argon.
0
+
3
In summary, the process of organic heterocyclic
compounds (acridine, 9-phenylacridine, and N-methyl-
CONFLICT OF INTEREST
No conflict of interest was declared by authors.
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