9
2
M.N. Timofeeva et al. / Applied Catalysis A: General 471 (2014) 91–97
the cross-aldol reaction due to the close occupation of Zr-formed
UiO-66 sample was synthesized following a procedure previ-
Lewis acid sites and basic NH -groups inside the cages.
ously reported in Ref. [7]. Mixtures of ZrCl , TPA, DMF and HCl were
crystallized at 80 C for 2 days. After reaction, the autoclave used
for the synthesis was cooled to room temperature in air. The result-
ing white product was filtered off, washed with DMF to remove the
excess un-reacted TPA, then repeatedly washed with methanol and
2
4
◦
Effect of functionalized 1,4-benzene-dicarboxylate linkers on
catalytic properties of UiO-66 was also demonstrated in the
citronellal cyclization [13]. Combination of catalytic and computa-
tional molecular modeling indicates that type of functional groups
presented in the linker units can alter the Lewis acidic properties
and also induce additional stabilizing/destabilizing effects on the
reactants depending on their electronic properties. Thus, in the
citronellal cyclization, the initial reaction rate in the presence of
UiO-66-NO2 was at least 56 times higher in comparison with UiO-
dried at room temperature. UiO-66-NH samples were synthesized
2
following procedures previously reported in Ref. [7]. Mixtures of
◦
ZrCl , amino-TPA, DMF and H O were crystallized at 80 C for 2
4
2
days. UiO-66-NO2 sample was synthesized analogously by replac-
ing H N-H BDC with the equivalent molar amount of O N-H BDC
2
2
2
2
6
6 due to the low free energies of the adsorbed state and of the
[8]. The designation of the samples, the reaction conditions of this
synthesis, textural properties and chemical composition of samples
are presented in Table 1 and Figs. S1–S4 (Supplementary Informa-
tion).
transition state. To the best of our knowledge, the applications of
UiO-66 materials in catalysis are limited to the above mentioned
three reports.
In this work, we investigated the effect of two linker ligands con-
taining electron-donating NH -groups and electron-withdrawing
2
2.2. Instrumental measurements
NO -groups (O N-H BDC and H N-H BDC) on acid–base proper-
2
2
2
2
2
ties, and therefore, catalytic properties of a family of isoreticular
MOFs, based on the UiO-66 structure in the reaction of acetaliza-
tion of benzaldehyde (BA) with methanol (Eq. (1)) which is often
used by organic chemists to protect the aldehyde group:
The porous structure of the materials was determined from the
◦
adsorption isotherm of N2 at −196 C on a Micromeritics ASAP
2
400 equipment. The specific surface area (SBET) was calculated
from adsorption data over the relative pressure range between
.05 and 0.20. The total pore volume (Vꢀ) was calculated from the
0
CH O OCH3
3
amount of nitrogen adsorbed at a relative pressure of 0.99. The X-
ray diffraction patterns were measured on a X-ray diffractometer
CHO
CH
(
ThermoARL) with Cu-K␣ (ꢁ = 1.5418 A˚ ) radiation. The TGA patterns
were measured under nitrogen flow with a thermal analyzer (SDT
+
CH OH
3
◦ ◦
Q600 V20.9) up to 800 C (the heating rate was 5 C/min).
(1)
Garcia et al. [13] demonstrated that Fe(BTC), Al (BDC)3 and
2
2.3. IR spectroscopic measurements
Cu (BTC) can be applied as Lewis acid catalysts for acetalization
3
2
of aldehydes by methanol at room temperature due to their mild
acid strength and large hydrophobicity. Cu (BTC) possessed the
The samples were pressed into self-supporting wafers
3
2
2
(
7–20 mg/cm ) and pre-treated within an IR cell by heating
higher activity in comparison with Fe(BTC) and Al (BDC) . Thus,
2
3
◦
at 200 C under vacuum for 3 h before the adsorption experi-
conversions of BA in acetalization of BA with methanol to the cor-
ments. FT-IR spectra were recorded on a Shimadzu FTIR-8300S
responding dimethyl acetal in the presence of Fe(BTC), Al (BDC)3
2
−1
spectrometer with a resolution of 4 cm
.
and Cu (BTC) for 24 h were 71%, 66% and 88%, respectively. It was
3
2
For the analysis of the surface basicity the samples were exposed
suggested that this difference can reflect the relative Lewis acidity
of the materials. Garcia et al. [14] also compared catalytic perfor-
mance of Fe(BTC) and MIL-100(Fe) in this reaction. It was found
that the catalytic properties of Fe(BTC) and MIL-100(Fe) are very
similar to each other in spite of the fact that amounts of Lewis
acid sites were 1.1 mmol/g and 2.0 mmol/g, respectively. Therefore,
effect of Lewis acidity on activity of metal-benzenetricarboxylates
to saturated CDCl vapors for 3 min at room temperature. The spec-
3
tra were obtained both before and after CDCl3 adsorption, and the
difference was calculated. The strength of the base sites was esti-
mated from the shift of ꢂC-D using the following Eq. (2) [15]:
log(ꢃvC-D) = 0.0066PA − 4.36
(2)
where ꢃꢂC-D is the shift, in cm 1, of C-D vibration and PA is the
−
(
M-BTCs) is opened and needs further investigation. In our inves-
tigation we are going to reveal effect of Lewis acidity on activity of
MOFs. In this manner, we have established relationships between
preparation conditions, acid–base properties and catalytic activi-
ties of a family of isoreticular MOFs based on the UiO-66 structure
proton affinity.
For studies of Lewis acidity, the samples were exposed to satu-
rated 5-nonanone vapors for 2 h at room temperature. The spectra
were obtained both before and after 5-nonanone adsorption, and
the difference was calculated.
with two linker ligands containing electron-donating NH -groups
2
and electron-withdrawing NO -groups, such as H N-H BDC and
2
2
2
O N-H BDC.
2
2
2.4. Catalytic test
The reaction of acetalization of BA with methanol was carried
2
. Experimental
◦
out at 30 C in a glass reactor equipped with magnetic stirrer.
◦
Before the reaction, all samples were evacuated at 200 C for
2.1. Materials
4
h to remove residual water in the samples. Then 0.94 mmol
of BA and 3 ml methanol, 0.01 mmol nitrobenzene (as internal
standard) and 50 mg of catalyst were added into the reactor
and the reaction mixture was vigorously stirred (1000 revolu-
tions per minute). At different time intervals aliquots were taken
from reaction mixture and analyzed. A gas chromatograph (Agi-
lent 7820) equipped with a flame ionization detector and HP-5
capillary column HP-5 was used to analyze products quantita-
tively.
Zirconium(IV) chloride, terephthalic acid (TPA, H BDC),
2
were purchased form Sigma–Aldrich. 2-Nitro-terephthalic
acid (nitro-TPA, O N-H BDC) and 2-amino-terephthalic acid
2
2
(
amino-TPA, H N-H BDC) were procured from Alfa-Aeser.
2 2
N,N-dimethylformamide
dichlorobenzene were obtained from Samchun Chemical.
-Nonanone (Aldrich–Sigma) was dried under molecular sieves
NaX.
(DMF)
and
benzaldehyde,
1,2-
5