Chemistry of Materials
Article
°C for 4 h resulting in a gelled paste, which was briefly sonicated and
then vacuum-evaporated and dried. The resulting dry powder was
suspended in excess hexane and extracted in a conventional 50-mL
jacketed Soxhlet extractor (Sigma-Aldrich Corp.) for 8 h with three
changes of hexane. The solvent was removed from the solids under
vacuum. The resulting solid was subjected to NMR analysis in DMF-
d7. Elemental Analysis, Calcd. (for C20H33Al2NO8): C, 51.2; H, 7.09;
Al, 11.5; N, 2.98; Found: C, 50.4; H, 7.35; Al, 11.2; N, 3.06.
conceptualized not only to sorb aldehydes by our function-
alized MOFs, but simultaneously to convert aldehydes to esters.
This reaction is ideal for the bio-oil upgrading process as it can
efficiently create carbon−carbon bonds between the low
molecular weight aldehydes, so that they are not eliminated
as potential liquid fuels during hydrodeoxygenation. Aluminum
alkoxides including Al-i-Pro are typically employed as
homogeneous catalysts of the Tischenko reaction, and an
excess of the Al-i-Pro compound is commonly required.16 In
order to convert Al-i-Pro into a heterogeneous catalyst that
affords reuse, the aluminum alkoxide moieties have been
grafted onto mesoporous silicate (MCM-41) via siloxide
linkages, producing materials with enhanced catalytic activity
in the MPVO reduction of cyclic ketones.23 In the present
work, we adopted a similar concept, wherein amino-MOFs
served as a solid support for the Al-i-Pro. We serendipitously
discovered that a simple impregnation of the MOFs with a
hydrocarbon solution of Al-i-Pro followed by heating to 60 °C
results in the Al-i-Pro attachment to the MOF, leading to
heterogeneous catalysts of acceptable chemical stability. These
catalysts are capable of quantitative dimerization of acetalde-
hyde at room temperature, as described below.
Methods. General. MOF surface area and pore parameters were
measured using a Micromeritics ASAP 2020 Accelerated Surface Area
and Porosimetry Analyzer (Micromeritics Corp., Norcross, GA) and
an Autosorb-iQ Automated Gas Sorption Analyzer (Quantachrome
Instruments, Boynton Beach, FL) at 77 K. The samples were
outgassed and activated prior to the measurements at 60 °C for 5 h
and at 80 °C for 10 h. In the measurements on Autosorb-iQ Gas
Sorption Analyzer, the surface areas were calculated using a multipoint
BET method for relative pressures of 0.05 and 0.30. The pore size
distributions were calculated using Quantachrome ASiQwin software,
where the nonlocalized DFT method (NLDFT) was applied using the
N2 sorption on silica at 77 K with cylindrical pores as the model
kernel. Surface areas resulting from the DFT calculations correlated
well with the values obtained using BET method.
1H NMR spectra were collected at 25
0.5 °C using a Bruker
Avance-400 spectrometer operating at 400.01 MHz. FTIR spectros-
copy was performed with a Nicolet 8700 FTIR spectrometer (Thermo
Scientific Inc.). For attenuated total reflection (ATR) FTIR measure-
ments, a Golden Gate ATR accessory (Specac Ltd., Cranston, RI) was
EXPERIMENTAL SECTION
■
applied, with 64 scans performed at a resolution of 1 cm−1
.
Materials. 2-Aminoterephthalic acid (99%, 2-ATA), aluminum
chloride hexahydrate (99%), acetaldehyde (≥99.5%), acrolein (99%),
butyraldehyde (99%), aluminum isopropoxide (Al-i-Pro, ≥98%), N,N-
dimethylformamide (DMF, 99.9%), N-nitroso-di-n-butylamine (99%)
were all obtained from Sigma-Aldrich Chemical Co. and used as
received. All other chemicals, solvents and gases were of the highest
purity available and were received from commercial sources.
Thermogravimetric analysis (TGA) and simultaneous differential
scanning calorimetry (DSC) were conducted using a Q600 TGA/
DSC instrument (TA Instruments, Inc.). Concentration of aluminum
in organic solutions was measured in the 5−50 mg/L concentration
range with a PinAAcle 900 Atomic Absorption Spectrometer
(PerkinElmer, Inc., Shelton, CT), after solvent evaporation and
sample redissolution in HCl/HNO3 mixture using a wavelength of
309.3 nm, according to the EPA Method 202.1.
Capture and Condensation of Aldehyde Vapors on MOF
Materials. For aldehyde capture, borosilicate glass vials, each
containing a weighed amount of dry powder of a solid sample were
placed next to an open 5-mL wide mouth jar containing 2 g of liquid
aldehyde, initially poured into the dish at −20 °C. Both the vials and
the jar were situated in a small glass desiccator, which was sealed
immediately after pouring liquid aldehyde into the jar. Aldehyde
evaporated from the jar at room temperature, with the vapors
contained within the sealed desiccator. The open vials were kept in the
desiccator for 24 h to 14 days at room temperature, and the uptake of
the aldehyde was determined periodically by withdrawing the vials
from the desiccator, immediately sealing them and measuring their
weight. Liquid aldehyde was added into the jar within the desiccator
each time the desiccator was opened for the sample withdrawals, to
maintain the saturated vapor atmosphere inside the desiccator.
Equilibrium weight uptake was reached after 24 h, at which point
no further weight increase of the vials was observed. The weight
uptake, measured in triplicate, was calculated as follows:
MOF Synthesis. NH2MIL101(Al). A solution of aluminum chloride
hexahydrate (0.51 g, 2 mmol) and 2-ATA (0.56 g, 3 mmol) in DMF
(40 mL) was kept at 130 °C for 72 h in a Teflon-lined autoclave bomb.
Then the solids were separated from the solution by centrifugation
(5000 g, 10 min) and washed with DMF under sonication for 20 min.
This was followed by washing with methanol at room temperature,
washing with excess hot (70) methanol for 5 h, and drying under
vacuum at 80 °C until constant weight was achieved. Elemental
analysis, Calcd. (for unit cell, Al816C6528H4896N816O4352): Al, 11.8; N,
6.13%; Found: Al, 12.1; N, 6.34%. The resulting MOF was designated
NH2MIL101(Al)auto
.
NH2MIL53(Al). This material was synthesized by thermal treatment
of the MOF components through autoclaving. A solution of aluminum
chloride hexahydrate (2.55 g, 10 mmol) and 2-ATA (2.8 g, 15.5
mmol) in DMF (99.9%, 40 mL) was kept at 130 °C for 72 h in a
Teflon-lined autoclave bomb. The solids were separated from the
solution by centrifugation (5000 g, 10 min) and washed with DMF
under sonication for 20 min. This was followed by washing with
methanol at room temperature, washing with excess hot (70 °C)
methanol for 5 h, and drying under vacuum at 80 °C until constant
weight was achieved. Elemental analysis, Calcd. (for unit cell,
Al4C32H24N4O20): Al, 12.1; N, 6.27%; Found: Al, 12.7; N, 6.75%.
WU, % = 100 ×(sample weight after equilibration
− initial sample weight)/initial sample weight
The resulting MOF was designated NH2MIL53(Al)auto
.
Amino-Containing MOF Functionalized by Aluminum
Isopropoxide (Al-i-Pro). NH2MIL101(Al)auto or NH2MIL53(Al)MW
MOF (0.88 g) was mixed with a solution of 0.88 g (4.3 mmol) of
aluminum isopropoxide in 10 mL toluene. The suspension was kept at
60 °C for 4 h, briefly sonicated and analyzed for the solvent
composition by HS-GC (see below), which detected formation of
acetone. The suspension was then dried at 70 °C in a vacuum oven
until constant weight. The resulting dry powder was suspended in
excess hexane with brief sonication, separated by centrifugation (5000
g) and dried. Elemental analysis is reported in the text.
Analysis of Acetaldehyde and Products. Acetaldehyde and
products of its chemical conversion on MOF and MOF/Al-i-Pro
composites were quantified using headspace sampling coupled with
gas chromatography (HS−GC). The MOF samples were immersed in
acetonitrile at 10 mg/mL solid concentration. The suspensions were
sonicated briefly, solids were separated by centrifugation (15000 g, 1
min), and the supernatant was diluted with deionized water
(acetonitrile/water ratio, 1:2 v/v), placed into Perkin-Elmer headspace
vials and sealed with PTFE/butyl rubber septa. The vials were placed
into a headspace sampler (TurboMatrix HS-40 Trap) and kept at 60
°C for 1 h. The products were detected using a Clarus 600 GC−MS
(PerkinElmer, Inc.). The gas chromatograph was equipped with flame
Reaction between 2-Aminoterephthalic Acid and Al-i-Pro. 2-
ATA powder (0.44 g) was suspended in a solution of 0.44 g Al-i-Pro in
5 mL toluene with a 5-min sonication. The suspension was kept at 60
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dx.doi.org/10.1021/cm400021g | Chem. Mater. XXXX, XXX, XXX−XXX