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411
complexes [32,38]. Until now, several support materials have
been tested. They include materials such as silica, alumina, zeolite,
polymeric organic as well [32,39–45]. However, there are some
limits of supports in this heterogeneous hydroformylation due to
the loss of activity, low thermal stability or complicated procedure
to synthesize the catalyst [38].
In this study, the rhodium supported MIL-101catalyst, Rh@MIL-
101, is investigated in the hydroformylation of different olefins. In
detail, n-alk-1-enes of different chain lengths are used. Further-
more, some bulky and less flexible olefins are included in order
to evaluate the influence of structure of the porous metal–organic
framework. The purpose is to check the catalytic performance of the
Rh@MIL-101 catalyst using hydroformylation as a test reaction.
CHO
CHO
R2
+
R
R
R
H2/CO
R1
Fig. 1. Scheme of the hydroformylation of n-alk-1-ene with synthesis gas to n- and
i-aldehydes and formation of double bond shifted internal alkene side product.
carried out also over Brønsted acidified catalyst [13]. The Lewis
acidity of MIL-101 has been used in the cyanosilylation of benzalde-
hyde with trimethylsilylcyanide [8].
MIL-101 catalysts have been shown to be active also in oxida-
tion reactions like the selective oxidation of sulfides with hydrogen
peroxide to the corresponding sulfoxides, which has been assigned
to the activity of coordinative unsaturated metal sites (CUS) [20].
the selective oxidation of tetralin to tetralone using tert-butyl
hydroperoxide using a MIL-101 coated monolith catalyst [22], the
epoxidation of alkenes with H2O2 over POM supported MIL-101
MIL-101 (Cr, Fe) in liquid-phase processes as the selective oxida-
tion of hydrocarbons with green oxidants like O2 and tert-butyl
hydroperoxide as well as the coupling reaction of organic oxides
with CO2 has been reviewed recently [26].
The catalytic performance in the hydrogenation has been tested
using palladium supported MIL-101 catalysts. MIL-101 has been
found to be a remarkably stable support for palladium in hydro-
genation of styrene and cyclooctene showing significantly higher
activity than e.g. of palladium supported activated carbon cat-
catalyst in the one-step synthesis of methyl isobutyl ketone by
multi-step hydrogenation reaction starting with acetone [27], the
cyclization of citronellal to isopulegol, and the one-pot tandem iso-
merization/hydrogenation of citronellal to menthol [28].Recently,
been also tested in the synthesis of secondary arylamines, quino-
lines, pyrrols, and 3-arylpyrrolidines [29]. High palladium loaded
MIL-101 catalysts containing different sized palladium nanoparti-
cle have been applied for the selective hydrogenation of ketones to
the corresponding alcohols [30]. Highly dispersed single site plat-
inum loaded NH2–MIL–101(Al) catalysts have been prepared by
deposition of the metal on the phosphotungstic acid pre-loaded
metal–organic framework. The catalyst showed high activity in the
ported alumina [31].
scale, the hydroformylation is one of the most important homoge-
neous catalyzed reactions [32,33], in which synthesis gas is added
to an alkene to produce linear and branched aldehydes as shown in
Fig. 1 [34]. Linear aldehyde, a more valuable product, can be used
for the production of alcohols. Approximately 9 million metric tons
of aldehydes and alcohols are annually produced using the hydro-
formylation reaction. They are starting materials for the synthesis
well as agrochemicals [5,35,36].
Both cobalt and rhodium complexes are used in the industrial
homogeneous catalyzed hydroformylation. The enhancement of
reaction rate and selectivity by ligand design as well as process
optimization has received great attention [37]. Even though the
traditional hydroformylation is effective, there is an interest
in the heterogenization of organic synthesis processes. There-
fore, research efforts are aimed at the immobilization of metal
2.1.1. MIL-101 synthesis
MIL-101 was hydrothermally synthesized in the presence of
TMAOH (tetramethylammonium hydroxide) based on literature
[46] using an improved work up procedure.
As starting materials Cr(NO3)3·9H2O (chromium(III) nitrate),
H2BDC (terephthalic acid), and 0.05 M TMAOH were used. Typically,
0.62 g of H2BDC (Merck, ≥98%) was added to 18.75 mL of aque-
ous 0.05 M TMAOH (Sigma-Aldrich, ≥97%) and vigorously stirred
for 30 min at room temperature. Then 1.5 g of Cr(NO3)3·9H2O was
added to this mixture and stirred for further 1 h. Next, this reac-
tion mixture was transferred into a 120 mL Teflon-lined autoclave.
It was heated at the rate of 2 ◦C/min up to 180 ◦C and maintained
at this temperature for 24 h under static condition.
After reaction, the autoclave was allowed to cool down to room
temperature. The green reaction product was recovered by cen-
trifugation at 4000 rpm for 25 min. Thereafter, the precipitate was
suspended in water. The white large elongated unreacted H2BDC
crystals were separated from the reaction product by centrifuga-
tion at 1600 rpm for 5 min. The remaining opaque green mixture
was further centrifuged at 4000 rpm for 25 min to recover the MIL-
101. The obtained product was worked up four times using the
same procedure. Finally, the obtained green MIL-101 sample was
dried at 90 ◦C.
2.1.2. Rhodium loading
An amount of 1.85 g MIL-101 was added to a solution con-
taining 15.7 mL of acetonitrile (Baker), 11.2 mL of toluene (Merck),
and 5.61 mg of [(acetylacetonato)(1,5-cyclooctadiene)] rhodium(I)
with a rhodium content of ca. 33%. The mixture was stirred and
slowly heated to ca. 70 ◦C in order to evaporate the solvents gradu-
ally. The material became dry after ca. 2.5 h. Next, the green powder
was washed three times with 5 mL of toluene. After washing, the
obtained Rh@MIL-101 supported catalyst was dried at 90 ◦C.
2.2. Material characterization
MIL-101 and its rhodium loaded form, Rh@MIL-101, were char-
acterized by XRD, XPS, SAXS, IR, SEM, TEM, AAS, and nitrogen
sorption measurements. The XRD investigation was carried out
on a STADI-P X-ray diffractometer (STOE) using monochromatic
˚
CuK␣ radiation (ꢀ = 1.5418 A). The rhodium content of the Rh@MIL-
101 supported catalyst was determined by atomic absorption
spectrometry with an AAS-Analyst 300 device (Perkin Elmer). A
nitrous oxide/acetylene or air/acetylene mixture was used for the
burner system. XPS measurements were done at an ESCALAB220iXL
spectrometer (Thermo Fisher) with monochromatic Al K␣ radia-
tion (E = 1486.6 eV). The samples were fixed on a stainless steel