Catalytic activity of copper(II) benzenetricarboxylate (HKUST-1) in reactions of aromatic aldehydes condensation with nitromethane: Kinetic and diffusion study

Article abstract of DOI:10.1016/j.ica.2014.11.018

Catalytic activity of copper(II) 1,3,5-benzenetricarboxylate, HKUST-1, in reactions of condensation of aromatic aldehydes with nitromethane was studied. It was shown that HKUST-1 catalyzed condensation of salicylaldehyde with nitromethane leading to formation of trans-nitrovinylphenol, while 9-anthracenecarbaldehyde did not react in the same conditions. Catalytic activity of HKUST-1 was consistent with its sorption capacity in respect to the reagents. The difference in catalytic activity of HKUST-1 in reactions with salicylaldehyde and 9-anthracenecarbaldehyde can be caused by sieving effect (molecules discrimination by size). Kinetics of salicylaldehyde condensation with nitromethane, catalyzed by HKUST-1, as well as kinetics of salicylaldehyde sorption by HKUST-1 were studied. Accessibility of pores of HKUST-1 to salicylaldehyde and condensation product (trans-nitrovinylphenol) was evaluated by molecular mechanics. Catalytic activity (reaction rate and salicylaldehyde diffusion rate) of HKUST-1 is compared with Fe^III analog, role of diffusion rate is shown.

Full text of DOI:10.1016/j.ica.2014.11.018

Accepted Manuscript
Catalytic activity of copper (II) benzenetricarboxylate (HKUST-1) in reactions
of aromatic aldehydes condensation with nitromethane: kinetic and diffusion
study
Svetlana A. Sotnik, Konstantin S. Gavrilenko, Anton S. Lytvynenko, Sergey V.
Kolotilov
PII:
S0020-1693(14)00713-0
http://dx.doi.org/10.1016/j.ica.2014.11.018
ICA 16301
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
15 July 2014
12 November 2014
20 November 2014
Please cite this article as: S.A. Sotnik, K.S. Gavrilenko, A.S. Lytvynenko, S.V. Kolotilov, Catalytic activity of
copper (II) benzenetricarboxylate (HKUST-1) in reactions of aromatic aldehydes condensation with nitromethane:
kinetic and diffusion study, Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.11.018
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Catalytic activity of copper (II) benzenetricarboxylate (HKUST-1) in reactions of aromatic
aldehydes condensation with nitromethane: kinetic and diffusion study
a
b
a
a
Svetlana A. Sotnik, Konstantin S. Gavrilenko, Anton S. Lytvynenko, Sergey V. Kolotilov *
a
L. V. Pisarzhevskii Institute of Physical Chemistry of the National Academy of Sciences of
the Ukraine, Prospekt Nauki 31, Kiev, 03028, Ukraine
b
Research-And-Education Chemicobyologycal Center, National Taras Shevchenko
University of Kyiv, Chervonotkackaya str, 61, Kiev, 03022, Ukraine
*
Corresponding author. Fax +38 044 5256216. E-mail svk001@mail.ru
Abstract
Catalytic activity of copper (II) 1,3,5-benzenetricarboxylate, HKUST-1, in reactions of
condensation of aromatic aldehydes with nitromethane was studied. It was shown that
HKUST-1 catalyzed condensation of salicylaldehyde with nitromethane leading to formation
of trans-nitrovinylphenol, while 9-anthracenecarbaldehyde did not react in the same
conditions. Catalytic activity of HKUST-1 was consistent with its sorption capacity in respect
to the reagents. The difference in catalytic activity of HKUST-1 in reactions with
salicylaldehyde and 9-anthracenecarbaldehyde can be caused by sieving effect (molecules
discrimination by size). Kinetics of salicylaldehyde condensation with nitromethane,
catalyzed by HKUST-1, as well as kinetics of salicylaldehyde sorption by HKUST-1 were
studied. Accessibility of pores of HKUST-1 to salicylaldehyde and condensation product
(trans-nitrovinylphenol) was evaluated by molecular mechanics. Catalytic activity (reaction
III
rate and salicylaldehyde diffusion rate) of HKUST-1 is compared with Fe analogue, role of
diffusion rate is shown.
Keywords: metal-organic frameworks; catalysis; Henry reaction; kinetics; diffusion;
molecular mechanics.
1
. Introduction
Metal-organic frameworks (MOFs), which possess different functional properties,
have been recently attracting attention as promising catalysts of organic reactions [1–8], for
example, condensation of carbonyl compounds [9–12], cyanosilylation [13–15], oxidation of
alcohols and polyphenols [16], alcoholysis of epoxides [17]. In many cases catalytic activity
of MOFs arise due to the presence of accessible coordinatively-unsaturated metal sites, which

have the properties of Lewis acidic centers [15, 18]. Though catalytic activity of MOFs in
many cases is lower compared to homogeneous catalysts, the advantages of MOFs as
catalysts include the possibility to vary the composition and structure of these compounds, in
particular, characteristics of pores, which can allow to modify activity and selectivity of the
catalyst [19–21]. Selectivity of MOF in catalytic reaction can be caused by sieving effect, i.e.
accessibility of pores to small molecules and non-accessibility to large ones [11, 12, 15]. At
the same time, the role of porous structure in catalytic activity of MOFs is obvious not in all
cases, because potential active sites are located both in pores and on the "external surface" of
microcrystals of the catalyst. Detailed studies of catalytic activity of MOFs is important for
understanding of mechanisms of catalytic reactions as well as for development of new
selective catalytic systems.
III
Recently
we
showed
that
Fe
1,3,5-benzenetricarboxylate,
III
III II
3-
[
Fe (OH) (H O) (btc) ]Cl (isostructural to [Fe Fe (H O) (btc) ]Cl [22], where btc
2 0,3 2 1,7 4/3 1,7 2 2 4/3
=
1,3,5-benzenetricarboxylate) catalyzed Henry reaction – condensation of salicylaldehyde
and nitromethane [23, 24]. The aim of the present study was to compare catalytic activity of
III
Cu (btc) (H O) ,
known
as
HKUST-1
[25,
26],
and
Fe
analogue,
3
2
2
2
III
[
Fe (OH) (H O) (btc) ]Cl in the same reaction, in order to reveal the role of the nature
2 0,3 2 1,7 4/3 1,7
of metal ion and the role of reagents diffusion rate in catalyst performance. In addition, size-
selectivity of reaction catalyzed by HKUST-1 was examined and accessibility of pores to
reagents was checked by sorption measurements and molecular mechanics calculations.
HKUST-1 [25, 26] has been attracting attention due to simplicity of preparation [27,
2
8], high thermal stability [13], accessibility of pores to comparatively large molecules [29]
and known catalytic activity in different reactions, for example, cyanosilylation of aldehydes
13-15], isomerization of terpenes [30], condensation of carbonyl compounds [10]. Crystal
[
-
-
lattice of HKUST-1 can be considered as [Cu (COO ) (H O) ] binuclear units (where COO is
2
4
2
2
3
-
carboxy-group of btc ), linked by with 1,3,5-benzene residues [25]. There is 3D system of
pores in crystal lattice of HKUST-1, possessing diameter about 7 Å [31].
This paper presents the results of the study of salicylaldehyde or 9-
anthracenecarbaldehyde condensation with nitromethane along with kinetic peculiarities of
the first reaction. Salicylaldehyde was chosen due to its relative stability to aerial oxidation
(better compared to benzaldehyde or other substituted benzaldehyde), while 9-
anthracenecarbaldehyde was used as a molecule of large size in order to see if there is any
sieving effect in studied catalytic system. Sorption of reagents by HKUST-1 was measured,
and kinetics of salicylaldehyde diffusion in pores of HKUST-1 was studied in order to see if
there is significant contribution of diffusion rate into reaction rate. Accessibility of pores to
2

salicylaldehyde and condensation product (trans-nitrovinylphenol) was evaluated by
molecular mechanics.
2
. Experimental
.1. Reagents
2
Commercially available reagents and solvents (ABCR, UkrOrgSintez, Synbias) were
used as received, except the cases listed below. Salicylaldehyde (Merck) was distillated before
use. Anhydrous nitromethane and toluene were prepared from commercially available reagents
(UkrOrgSintez) using standard methods [32].
HKUST-1 was synthesized mechanochemically in planetary mill Pulverisette 6 (Fritsch)
by grinding of 1,3,5-benzenetricarboxylic acid and copper(II) acetate monohydrate during 30
min following published method [27]. The product had composition Cu (btc) (H O) ∙5H O
3
2
2
3
2
according to data of C, H analysis [33]. Prior to catalytic experiments HKUST-1 was desolvated
by heating in vacuum (0.1 Torr) over P O at 150 °C during 2 days. Hereinafter desolvated
2
5
sample is denoted as 1.
2
.2. Equipment and methods
CHN-analyses were performed on Carlo Erba 1106 instrument. Powder X-ray diffraction
1
experiments were performed on Bruker D8 instrument with CuK radiation (λ = 1.54056 Å). H
α
NMR spectra were measured using Bruker NMR500 spectrometer. UV, Vis-spectra were
measured on Specord 210 spectrophotometer. SEM image was obtained using LMU Mira3
Tescan instrument. N adsorption by 1 was measured by Sorptomatic-1990 instrument at 77 K, for
2
-2
the measurements sample was desolvated in 10 Torr vacuum at 150 °C.
For measurements of sorption of salicylaldehyde, 9-anthracenecarbaldehyde or
nitromethane from toluene solutions at 373±1 K by 1 hermetic vials, containing weighed
samples of 1 and solutions of corresponding substrate of certain concentration, were mixed
during 2 h to adsorption equilibrium. Then the equilibrium concentration of substrate in solution
was measured by spectrophotometry at 330 nm and 405 nm for salicylaldehyde and 9-
anthracenecarbaldehyde, respectively. Efficient concentration of 1 in suspension was determined
as the ratio of a quantity of suspended 1 (in moles) to the volume of solution. Studies of kinetics
of salicylaldehyde adsorption and kinetics of condensation were carried out in similar way. In
experiments on sorption kinetics the range of initial concentrations of salicylaldehyde was 0.053
–
0.381 M, in experiments on condensation kinetics the initial concentrations of salicylaldehyde
varied between 0.152 M and 0.710 M, while initial concentrations of nitromethane were in range
from 0.7 M to 8 M at efficient concentration of 1 was equal to 0.05 M (counting on Cu formula
2
3

unit). The reaction mixtures were continuously stirred at 373 K (100 °C) during certain time
1
period. In some experiments compositions of the reaction mixtures were studied by H NMR.
For this purpose stirring of the reaction mixtures was stopped, particles of 1 precipitated, aliquots
of the clear reaction mixtures (without particles of 1) were taken, toluene with nitromethane were
1
evaporated, the residues were quantitatively dissolved in CDCl and H NMR spectra were
3
measured.
Adsorption of nitromethane from gaseous phase was measured gravimetrically, using a
quartz microbalance (293 K). Each point on the adsorption isotherm corresponds to equilibrium
-1
S
conditions (no change of sample weight at certain P·P ). Sample was desolvated, as indicated
-3
above, and additionally heated at 393 K (120 °C) at 10 Torr. Sorption capacity in respect to
salicylaldehyde from gaseous phase was evaluated by change of sample's weight in air,
containing saturated vapor of salicylaldehyde at 293 K. Isotherm of this aldehyde sorption from
gaseous phase was not determined because of its low saturated vapor pressure.
2
.3. Molecular mechanics modeling
Molecular mechanics modeling was performed similarly to the procedure, described
previously [34]. OpenBabel 2.2.3 [35] engine (managed by self-written Python scripts) was used
for modeling. Universal force field (UFF) [36] was used as implemented in OpenBabel due to
2
+
presence of Cu ions in the model particle. The model particle was a fragment of HKUST-1,
built using true coordinates determined by X-ray diffraction [25]. The size of this model particle
was chosen in order to reproduce the structure of single channel and corresponded to a/2  a/2 
2
.5a, where a is unit cell parameter (Fig. S1, Supporting Information). The model particle can be
described as six Cu (btc) fragments of C local symmetry, located along one crystallographic
8
12
4
axis; the pore axis coincides with C axis of these fragments. Total amount of atoms in the model
4
was 1344. Coordinates of the atoms of the model particle, except hydrogen atoms, were fixed in
all calculations, because positions of atoms in discrete model particle (a part of infinite crystal)
do not correspond to the minimum of crystal lattice energy and molecular mechanics is known to
be poor while dealing with coordination bonds [37].
The values of in-pore adsorption energy and diffusion activation barrier were calculated
using a relaxed surface scan. Substrate molecule was trailed through the pore towards a fixed
dummy atom (an atom of He) located in fixed position opposite to one exit from the channel on
C axis going through this channel at the distance ca. 20 Å from the closest atoms of the model
4
particle (Fig. S1, Supporting Information). A substrate molecule (salicylaldehyde or trans-
nitrovinylphenol) was located opposite to the another exit on the same C axis. The distance
4
from the dummy atom to the carbonyl oxygen atom of salicylaldehyde or oxygen of nitro-group
4

of trans-nitrovinylphenol (r) was set to 100 Å and constrained. The geometry was optimized
with above mentioned constraints. Then the distance r was consequently changed from 100 to
1
0 Å (with 0.5 Å step) and the geometry was optimized within each consequently obtained
geometry with above mentioned constraints. The optimizations were provided with convergence
-6
tolerance ΔE = 10 kJ/mol and up to 2000 steepest descent iterations followed by up to 2000
conjugate gradient iterations.
3
. Results and discussion
Identity of powder X-ray diffraction patterns of HKUST-1, its desolvated form 1 and
sample, isolated from the reaction mixture after catalytic experiments, evidences for stability of
crystal lattice of this MOF upon desolvation and catalysis (Fig. S2, Supporting Information).
Powder XRD patterns for these samples were similar to the one, expected for X-ray structure of
HKUST-1 [25]. N adsorption isotherm was typical for microporous compounds (type I
2
according to IUPAC classification [38]), SBET value, estimated from N adsorption data, was
2
2
-1
equal to 1300 m g , and micropores volume, estimated by Dubinin-Radushkevich model, was
3
-1
0
.49 cm g (Fig. S3). These values fall in the range of adsorption characteristics, reported for
samples of HKUST-1 prepared by different methods [39]. Size distribution of particles of 1, used
for catalytic experiments, was estimated by optical microscopy. The sample consisted of
particles with wide distribution on size, average size is about 1.8 μm (Fig. S4). However, these
particles are not uniform: large particles are aggregates of small particles with size about 0.1 -
0
.2 μm, as evidenced by SEM image (Fig. S5).
Condensation of salicylaldehyde with nitromethane catalyzed by 1 (Henry reaction) led to
1
formation of trans-nitrovinylphenol as the sole product (Fig. 1), which was confirmed by H-
III
III
2
NMR. Similar condensations catalyzed by Fe analog [Fe (OH) (H O) (btc) ]Cl
1,7
0,3
2
1,7
4/3
III
(
desolvated form, i.e. [Fe (OH) (btc) ]Cl , hereinafter is denoted as 2) led to formation of
2 0,3 4/3 1,7
the same compound as a sole reaction product. No condensation products were observed in
reaction mixture, containing salicylaldehyde and nitromethane without 1, which confirms
catalytic activity of this compound. In separate experiment the reaction mixture, containing
salicylaldehyde, nitromethane and 1, was heated during 90 h; 7 % conversion of aldehyde was
achieved. Then solid catalyst was filtered off and the filtrate was heated during additional 210 h.
No additional conversion of salicylaldehyde was detected, thus, it can be concluded that catalytic
effect was provided by solid 1 rather than possible soluble products of catalyst destruction.
Conversion of salicylaldehyde in reaction with nitromethane, catalyzed by 1, increased
with time reaching 80 % after 2000 hours (Table 1; concentrations of reagents are indicated in
5

III
table caption). In the case of catalysis by Fe analog, compound 2, the same conversion was
achieved after 80 h or reaction. Assuming, that all metal ions in the catalyst are accessible for the
reaction, these conversions correspond to turnover numbers at the level of 5.6 moles per one
II
III
mole of Cu and for 1 and for 5.6 moles per mole of Fe for 2 (turnover frequency TOF =
-3
-1
-2 -1
2
.8·10 h and 8.0·10 h for 1 and 2, respectively). It can be concluded that MOFs 1 and 2 are
equally efficient according to TON criterion, but reaction is much slower on 1. We would like to
note, that these values of TON and TOF can be considered only as an estimation, because metal
ions located in the centers of catalyst micro particles can be not accessible due to long diffusion
pathway. In any case, the contribution of metal ions located close to pore entrance is higher
compared to metal ions in particle center.
The reaction mixture, containing 9-anthracenecarbaldehyde, nitromethane and 1
remained unchanged after 60 h at 373 K. Conversion of 9-anthracenecarbaldehyde reached 4 %
after 130 h or reaction, and heating of the reaction mixture during 450 h led to 8 % conversion
(Table 1). Selectivity of 1 in reactions of 9-anthracenecarbaldehyde and salicylaldehyde
conversion was also examined in the experiment, when both aldehydes were simultaneously
present in the reaction mixture at the same concentrations (0.07 M of each aldehyde, 8 M of
nitromethane and 0.05 M of 1, calculated per Cu unit, which is equivalent to 0.1 M of active
2
II
sites provided that all Cu in 1 are accessible). After 350 h of reaction conversion of
salicylaldehyde was equal to 26 %, while conversion of 9-anthracenecarbaldehyde was 4 %;
after 450 h of reaction conversion of salicylaldehyde increased to 33 %, while conversion of 9-
anthracenecarbaldehyde remained almost unchanged.
It can be concluded that reaction of 9-anthracenecarbaldehyde and nitromethane is very
slow compared to reaction of salicylaldehyde and nitromethane, and it can be caused by
inaccessibility of pores in 1 to 9-anthracenecarbaldehyde (which is larger compared to
salicylaldehyde). Sorption measurements are consistent with this supposition – it was found that
sorption of 9-anthracenecarbaldehyde by 1 in toluene solution at 373 K was negligibly low. Non-
II
zero conversion of 9-anthracenecarbaldehyde can be caused by catalytic action of Cu ions on
the external surface of microcrystals. Notably, lower reactivity of 9-anthracenecarbaldehyde
compared to salicylaldehyde can contribute to difference between conversion of these aldehydes
[
40], but zero sorption of 9-anthracenecarbaldehyde by 1 favors for conclusion about sieving
effect in reaction described here.
Accessibility of pores of 1 to other reagents, used in this work, was studied by sorption
measurements. Isotherm of nitromethane sorption by 1 from gaseous phase (Fig. S6) is typical
for adsorption in microporous sorbents (type I according to IUPAC classification [38]), which is
completely consistent with its crystal structure [25] and reported isotherms of other substrates
6

adsorption at room temperature [39]. Sorption capacity of 1 in respect of nitromethane at 293 K
3
from gaseous phase is 4.2 moles per mole of Cu (0.56 cm /g at P/P = 0.9). Compound 1 also
2
S
3
adsorbs 2.1 moles of salicylaldehyde per Cu unit (0.54 cm /g at P = P at presence of air). The
2
S
values of pores volume, accessible to nitromethane and salicylaldehyde, agree with the result of
theoretical estimation, performed by Platon software [41], which gives solvent-accessible pores
3
3
volume 0.70 cm /g for probe molecule with r = 2.5 Å or 0.59 cm /g for probe molecule with r =
.5 Å (the latter approximately corresponds to kinetic radius of molecules similar to
3
nitromethane and aromatic molecules with two substituents, similar to salicylaldehyde,
respectively [42]).
Isotherms of salicylaldehyde and nitromethane adsorption by 1 from toluene solution at
3
73 K (Fig. 2a and 2b) can be fitted by Langmuir model. In this case the values of maximal
sorption capacity amax and sorption constants K373 for nitromethane are almost two times higher,
than corresponding parameters for salicylaldehyde (Table 2). The ratio of sorption capacity of 1
on nitromethane to sorption capacity on salicylaldehyde in toluene solution is higher, than
similar ratio in gaseous phase (3.2 and 2.0, respectively), this difference can be explained by
competing sorption of toluene.
Thus, it was shown that pores of 1 were accessible both for salicylaldehyde and
nitromethane, while sorption of larger 9-anthracenecarbaldehyde was negligibly low. Different
sorption capacities of 1 in respect to these aldehydes can be responsible for different catalytic
activity of 1 in their reactions. It can be concluded that 1 showed size-selectivity in aromatic
aldehydes condensation with nitromethane.
Parameters of nitromethane and salicylaldehyde sorption by 1 (both from solution and
gaseous phase) are significantly higher than similar parameters for these compounds sorption by
2
(Table 2) [24]. This difference is consistent with the difference between crystal structures of
these compounds, because part of pores volume in 2 is occupied by chloride ions.
Kinetics of salicylaldehyde condensation with nitromethane in presence of 1 was studied.
It was found by the method of initial rates that the reaction had the first order on salicylaldehyde
concentration and zero order on nitromethane concentration, and at permanent effective
concentration of 1 (quantity of 1 in suspension per unit of volume) the reaction rate r could be
described by equation (1).
1
r = k [SA] ,
(1)
t
1
where [SA] is concentration of salicylaldehyde at the moment of time t, k is effective rate
t
1
constant of this reaction, catalyzed by 1. The values of k were determined for different initial
7

concentrations of reagents (and permanent effective concentration of catalyst 1, equal to 0.05 M
per Cu unit) from linear regressions of ln([SA] /[SA] ) vs. t dependencies, where [SA] is initial
2
0
t
0
concentration of salicylaldehyde (Table 3, Fig. 3). Obtained dependencies are linear up to 80 %
conversion of salicylaldehyde, which is achieved after 2000 hours (Fig. 3), and average value of
1
-5
-1
k was equal to 1.43·10 min .
1
The dependency of rate constant k on effective concentration of 1 is linear (Fig. S7),
which can evidence that metal sites participate in formation of intermediate state.
Previously we proposed a mechanism for similar reaction, catalyzed by 2 [23]. In line
with this mechanism, activation of salicylaldehyde upon coordination to coordinatively-
unsaturated metal ion was supposed to be limiting stage (equation 2), and product formation rate
was described by equation (3). Other reported data are also consistent with the importance of
II
aromatic aldehyde (benzaldehyde) coordination to Cu ions in 1 for its activation in catalytic
reaction [13].
k2
1
SA +Fe-btc-Fe
SA
Fe-btc-Fe
2
-
k
1
(2)
(3)
2
r = k [SA][2],
1
2
where Fe-btc-Fe means structural unit of 2, k is rate constant for process of salicylaldehyde
1
III
-3
activation by 2 (adsorption in pores and coordination to Fe ), which was found to be 7.98·10
-1
2
min ·l·(mole Fe ), k is rate constant of reverse process (aldehyde decoordination and
2
-1
desorption). Assuming that the mechanism of condensation is the same in the case if 1 is used as
1
1
the catalyst, and given that k = k [1], equation (1) can be re-written in the following form
1
(equation 4):
1
r = k [SA] [1],
(4)
1
t
1
where k 1 is the constant of salicylaldehyde activation by 1, which is rate-limiting stage for
product formation.
1
-4
-1
The value of k was found to be 2.67·10 min ·l·(mole Cu ), and it is 30 times lower
1
2
2
than previously found k [23]. Two reasons can be responsible for the difference between these
1
III
II
constants: (i) different Lewis acidity of Fe and Cu , as well as (ii) different rate of
salicylaldehyde diffusion in pores of MOF. In order to evaluate contribution of diffusion rate,
kinetics of salicylaldehyde sorption by 1 from toluene solution at 373 K was studied.
8

Kinetic curve of salicylaldehyde sorption by 1 abruptly grew at the beginning of sorption
Fig. 4) and reached saturation after ca. 3 minutes. Sorption process can be considered as a
(
sequence of several stages, such as diffusion of the substrate from the solution to an area,
surrounding the particle, diffusion from this area to particle's surface (so-called "external
diffusion"), and diffusion from the surface to the internal sites (so-called "internal diffusion"),
which can be completed by substrate binding by active sites (chemosorption) [43, 44]. Some
conclusions regarding the nature of diffusion (external or internal) at sorption of substrate can be
made on the basis of possibility of sorption kinetic curves fitting by different equations.
For the case, when sorption rate is controlled by the external diffusion, experimental data
can be fitted by equation (5) [45].
ln(1 – a /amax)= –k't,
(5)
t
where a is the quantity of salicylaldehyde, adsorbed by one mole of 1 at time t, a
is the
t
max
maximal sorption capacity. It was found, that the curves, corresponding to this equation (Fig.
(a)), had two linear sections, and the second section was almost parallel to ordinate axis, which
5
is typical for mixed-diffusion processes.
Mixed-diffusion sorption processes can be fitted by equation (6) [45].
1/2
a = k ∙ t + a ,
(6)
t
d
0
where k is the constant of internal diffusion rate, and a is the quantity of salicylaldehyde,
d
0
1
/2
adsorbed at initial moment of time (at t → 0). The first linear section of a vs. t curve
corresponds to the process of quick external diffusion of salicylaldehyde (Fig. 5(b)), while the
second section is responsible for slow internal diffusion process. Rate constant of internal
diffusion can be determined from the inclination of the second section of the curve. The value of
-3
-3
-1/2
this constant k is 1.8·10 ±0.2·10 mole·min ·(mole Cu ), and it is almost 3 times lower
d
2
compared to corresponding constant of internal diffusion rate in the process of salicylaldehyde
-3
-3
-1/2
sorption by 2, which was found to be equal to 5.0·10 ±0.5·10 mole·min ·(mole Fe ) [24].
2
Diffusion kinetics can also be fitted by the pseudo-second order equation (equation 7); a
possibility to fit data using this model was considered to be an evidence of chemosorption of
substrate, associated with the process of internal diffusion [46]. In some cases fitting of diffusion
kinetics by the pseudo-second-order model was shown to be more adequate compared to the
pseudo-first-order model, for example, in case of organic dye xylenol orange (XO) diffusion in
porous chromium-benzenedicarboxylate MOF (MIL-101(Cr)) [47].
9

t
1
t


2
(7)
at k2a
a
max
max
where k is the rate constant of the pseudo-second order reaction (Fig. S8).
2
-1
-1
The value of k , calculated from equation (7), is 5.7 ± 0.4 mole(Cu )∙mole(SA) ·min ,
2
2
which slightly exceeds the value of similar constant for the rate of salicylaldehyde sorption by 2
-1
-1
(
4.3 ± 0.3 mole(Fe )∙mole(SA) min ), it can be concluded that data treatment by this model do
2
not show significant difference between diffusion rates in 1 and 2 [16]. Nevertheless, these
kinetic data do not contradict to supposition that chemosorption of salicylaldehyde takes place in
1
. The values of k , determined for sorption of salicyladehyde in 1 and 2, fall in range of k ,
2 2
measured for diffusion of xylenol orange in MIL-101(Cr) (between 3 and 90
-1
-1
mole(Cr )∙mole(XO) min , depending on dye concentration) [47].
2
In order to get additional information about accessibility of pores to reagents and the
possibility of product removal from the catalyst, molecular mechanics calculations of
salicylaldehyde and trans-nitrovinylphenol molecules movement along the pore were carried out.
The molecule of guest was located in certain position in pore and the system was optimized
(detail of optimization and restrains are presented in Experimental section). Similar approaches
for evaluation of pores accessibility to substrate were reported for diffusion in MOF [34] or
zeolites [48]. Calculation of nitromethane diffusion was not carried out because its size is smaller
compared to salicylaldehyde, and there are no doubts that the pore is accessible to the former if it
is accessible to the latter.
Normalized values of system's energy depending on guest molecule position are
presented on Fig. 6. The state when guest molecule (both salicylaldehyde and trans-
nitrovinylphenol) is located in the pore in any position is characterized by negative energy (in
respect to a state where guest molecule is located far from the channel), indicating that
adsorption occurs with evolution of heat. Guest molecule movement inside pore occurs with
energy barriers equal to ca. 30 kJ/mole for salicylaldehyde and ca. 45 kJ/mole for trans-
nitrovinylphenol. These values are higher than the values calculated for diffusion activation
energies for alkylbenzenes in large pore zeolite catalysts [48], which is consistent with the ratios
of pore size and guest molecule size.
Thus, molecular mechanic calculations confirm that salicylaldehyde can enter pore of 1
and trans-nitrovinylphenol can leave the pore without significant barrier, which is consistent
with the above conclusions that reaction of salicylaldehyde with nitromethane occurs in pores.
1
0

4
. Conclusions
It was found in the present study that condensation of salicylaldehyde with nitromethane,
catalyzed by 1, led to formation of only one product, trans-nitrovinylphenol. At the same time
condensation of 9-anthracenecarbaldehyde and nitromethane catalyzed by 1 was significantly
slower. Taking into account that sorption of 9-anthracenecarbaldehyde by 1 was found to be
negligible, different catalytic activity of 1 in reactions of condensation of these aldehydes can be
caused by sieving effect. Accessibility of pores of 1 to salicylaldehyde and nitromethane was
confirmed by sorption measurements, and in addition molecular mechanic calculations showed
that diffusion of salicylaldehyde or trans-nitrovinylphenol in pores of this MOF could occur
without significant activation barrier.
The rate of reaction between salicylaldehyde and nitromethane, catalyzed by 1, could be
described by the equation of the first order on concentration of salicylaldehyde and the first order
on effective concentration of the catalyst 1 (the second order in total), similarly to the case of
III
Fe analog, compound 2 [24]. Though MOFs 1 and 2 are equally efficient according to TON
1
criterion, rate constant k (catalysis by 1) appeared to be 30 times lower than previously found
1
2
k
(rate constant of the same reaction, catalyzed by 2). It was found that the rate of
1
salicylaldehyde diffusion in pores of 1 was governed by the internal diffusion stage, which was
-3
-3
-1/2
characterized by k = 1.8·10 ± 0.2·10 mole·min ·(mole Cu ). This value of k was several
d
2
d
times lower compared to similar k for salicylaldehyde diffusion in 2. These data can be an
d
experimental confirmation that there is significant contribution of reagents diffusion rate in pores
into reaction rate, and diffusion rate is an important factor which controls catalyst's efficiency,
along with the nature of active sites (metal ions in this case).
We believe that the results of this study will be helpful for development of selective
catalysts based on porous systems.
Acknowledgement
The study was supported, in part, by joint grant of the National Academy of Sciences of
Ukraine and Russian Foundation for Basic Research (No. 03-03-14 and 14-03-90423). The
authors thank to Dr. R.A.Polunin for the measurement of nitromethane sorption from gaseous
phase.
1
1

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1
4

FIGURE CAPTIONS
Fig. 1. Reaction of salicylaldehyde and nitromethane, catalyzed by 1.
Fig. 2. Isotherms of salicylaldehyde (a) and nitromethane (b) sorption by 1 from solutions in
toluene. On all figures a is the quantity of adsorbed compound per mole of 1 (Cu formula unit).
2
Solid lines are Langmuir fits, see text. [SA] means concentration of salicylaldehyde.
Fig. 3. Graphs of ln([SA] /[SA] ) vs. t dependencies for the cases [SA] = 0.152 M (a) and [SA]
0
0
t
0
=
0.440 M (b).
Fig. 4. Dependency of salicylaldehyde adsorption a (moles per 1 mole of Cu units) on time t.
2
1/2
Fig. 5. Dependency of –ln(1 - a /amax) on time t (a) and dependency of a on t (b), where a is
t
salicylaldehyde adsorption.
Fig. 6. Energy profiles for diffusion of salicylaldehyde and trans-nitrovinylphenol in pore of 1
calculated by molecular mechanics. The scales of crystal lattice of 1 and 0x axis are the same.
1
5

TABLES
Table 1
Conversion of aldehydes in reaction with nitromethane catalyzed by 1 and 2. Initial
-1
II
concentrations for all cases: c(CH NO ) = 8 M, c(catalyst) = 0.05 (mole M )·L (M is Cu or
3
2
2
III
Fe in the cases of 1 and 2, respectively), c(aldehyde) = 0.7 М. T = 373 K.
Catalyst
Aldehyde
Time, h
28
Conversion, %
Ref.
this work
a)
1
1
2
2
13
23
80
0
0
4
5
8
.7
.5
.2
.0b)
1
salicylaldehyde
b)
a)
b)
this work
1
salicylaldehyde
60
1
1
1
1
1
1
1
1
salicylaldehyde
salicylaldehyde
salicylaldehyde
130
300
2000
28
this work
this work
this work
this work
this work
this work
this work
this work
23, 24
b)
b)
a)
9-anthracenecerbaldehyde
9-anthracenecerbaldehyde
9-anthracenecerbaldehyde
9-anthracenecerbaldehyde
9-anthracenecerbaldehyde
salicylaldehyde
a)
a)
a)
a)
60
130
350
450
28
b)
2
2
44
67
b)
salicylaldehyde
56
23, 24
Notes.
1
a) estimated by H NMR;
b) estimated from UV spectra
Table 2
Parameters of salicylaldehyde and nitromethane sorption by 1 and 2.
Sorption from gaseous phase
at 293 K
Sorption from solution in toluene at 373 K
a)
a)
K , (mole M )/mole
amax , mole/(mole M2)
3
73
2
amax, mole/(mole M2)
SA
CH NO2
SA
CH NO2
SA
CH NO
3
3
3
2
1
0.78
0.22
2.5
71
26
133
25
2.1
4.20
1.6
b)
2
0.24
0.93
Notes
a) parameter of Langmuir fit;
b) data from ref. [24].
1
6

Table 3
Rate constants k (Equation 1) for reaction of salicylaldehyde and nitromethane catalyzed by 1
effective concentration of 1 is 0.05 M per Cu unit)
1
(
2
1
-1
[
SA] , M
[CH NO ] , M
k , min
0
3
2 0
-
5
-7
-7
0
0
0
0
0
.152
.440
.710
.710
.710
8.00
1.22·10 ±4·10
-5
8.00
8.00
0.70
1.70
1.37·10 ±4·10
-5
-7
-7
-7
1.49·10 ±3·10
-5
1.45·10 ±6·10
-5
1.61·10 ±7·10
1
7