Titanium Dioxide Reinforced Metal-Organic Framework Pd Catalysts: Activity and Reusability Enhancement in Alcohol Dehydrogenation Reactions and Improved Photocatalytic Performance

Article abstract of DOI:10.1002/cctc.201500747

Porous coordination polymers or metal-organic frameworks have been proposed as promising catalyst materials since their discovery. A fundamental problem associated with MOF-based catalysts is the stability during their catalytic performance, especially in liquid-phase catalysis. Herein, we report on the controlled incorporation of nanoscale palladium and titanium dioxide inside MIL-101 (Cr). The introduction of the metal species was accomplished by metal-organic chemical vapor deposition and can be varied over a large weight-percentage range. The enhanced catalytic activity and the improved reusability of the resulting Pd/TiO₂@MIL-101 composite materials were demonstrated in hydrogenation and dehydrogenation reactions. Furthermore, the presence of TiO₂ (amorphous) allowed observing an enhanced photocatalytic activity. Catalysis on the inside! The controlled incorporation of nanoscale Pd and TiO₂ inside MIL-101 (Cr) by means of sublimation is reported. The resulting Pd/TiO₂@MIL-101 composite materials are efficient catalysts in hydrogenation and dehydrogenation reactions. In addition, an enhanced photocatalytic activity is observed.

Full text of DOI:10.1002/cctc.201500747

DOI: 10.1002/cctc.201500747
Full Papers
Titanium Dioxide Reinforced Metal–Organic Framework Pd
Catalysts: Activity and Reusability Enhancement in Alcohol
Dehydrogenation Reactions and Improved Photocatalytic
Performance
[
a]
Dominic Tilgner, Martin Friedrich, Justus Hermannsdçrfer, and Rhett Kempe*
Porous coordination polymers or metal–organic frameworks
have been proposed as promising catalyst materials since their
discovery. A fundamental problem associated with MOF-based
catalysts is the stability during their catalytic performance, es-
pecially in liquid-phase catalysis. Herein, we report on the con-
trolled incorporation of nanoscale palladium and titanium di-
oxide inside MIL-101 (Cr). The introduction of the metal species
was accomplished by metal–organic chemical vapor deposition
and can be varied over a large weight-percentage range. The
enhanced catalytic activity and the improved reusability of the
resulting Pd/TiO @MIL-101 composite materials were demon-
2
strated in hydrogenation and dehydrogenation reactions. Fur-
thermore, the presence of TiO (amorphous) allowed observing
2
an enhanced photocatalytic activity.
Introduction
[
1]
Porous coordination polymers (PCP) or metal–organic frame-
of the catalyst by the metal oxide. The incorporation of titani-
um dioxide in PCP/MOFs has already been reported for a few
[
2]
works (MOF) are highly ordered crystalline materials built
from multidentate ligands (linkers) and metal ions or clusters
[
8]
examples. To the best of our knowledge, the selective load-
(
connectors). The coexistence of large specific surface areas,
ing of PCP/MOFs with TiO and noble MNPs is a not yet solved
2
[
6h]
well defined pore or cavity structures, and variable functionali-
ties enables the application of MOFs in a variety of fields such
as gas storage and separation, drug delivery, bioimaging, elec-
problem.
We have a rather long ongoing interest
[
6f,j,m,n,9]
in PCP/MOF
synthesis and catalysis and report here on the combined incor-
poration of titanium dioxide and palladium nanoparticles into
[3]
trochemistry, sensing, and catalysis. With regard to catalytic
applications, PCP/MOFs are diversely used. Catalytically active
sites can result from the connectors (metal complex catalysis),
from organic functionalities of the linkers (organo catalysis), or
from the introduction of (well defined) metal nanoparticles
MIL-101 (Cr). The Pd/TiO @MIL-101 composite materials are
2
highly efficient catalysts in hydrogenation and dehydrogena-
tion reactions. A superior performance in comparison to classic
Pd catalysts like Pd@AC (AC=activated carbon), Pd@Al O , or
2
3
[
4]
(
MNP). The solvent-free loading by metal–organic chemical
Pd@SiO has been observed. Besides the very good activity,
2
vapor deposition (MOCVD) is an elegant method for the intro-
stability, and reusability, an enhanced photocatalytic activity
[5]
[10]
duction of such MNPs. It offers a high degree of control, the
addressing of broad MNP weight contents and has been used
intensively in recent years to design MNP@PCP/MOF cata-
has been noticed for Pd/TiO @MIL-101. MIL-101 was chosen
2
as host material because of its stability towards moisture and
its impressive use in liquid-phase catalysis aroused from differ-
[
5,6]
[11]
[12]
lysts.
Beside the generation of highly active MNP inside the
ent incorporated monometallic species (Au,
Cu,
Fe O /
2
[17]
3
[
13]
[6j,m,n,14]
[6f,14o,15]
[16]
cavities of MOFs, the additional incorporation of a nanoscale
semiconducting material such as titanium dioxide seems to be
a promising goal with regard to (i) the outstanding photoelec-
Fe O , Pd,
Pt,
and Rh ), bimetallic alloys, or
3
4
[
18]
MNP-free functional sites.
[7]
tric properties of titania and (ii) a potential stabilizing effect
Results and Discussion
The loading of the Ti precursor titanium(IV) isopropoxide [Ti(O-
+
[
a] D. Tilgner, M. Friedrich, Dr. J. Hermannsdçrfer, Prof. Dr. R. Kempe
Anorganische Chemie II – Catalyst Design
Universität Bayreuth
[8a]
iP) ] into MIL-101 was accomplished by sublimation. The size
4
of the synthesized MIL-101 crystallites was adjusted by the
amount of HF and repetitive centrifugation steps to obtain
a narrow size range centered at a diameter of 180 nm (Sup-
porting Information, Figure S1). Such crystallite size is optimal
with regard to activity (preferred as small as possible) and easy
Universitätsstraße 30, 95440 Bayreuth (Germany)
Fax: (+49)921-55-2157
E-mail: kempe@uni-bayreuth.de
+
[
] Current Affiliation:
INM - Leibniz-Institut für Neue Materialien
Stuhlsatzenhausweg 3, 66123 Saarbrücken (Germany)
[
6n]
separation of the catalyst (preferred as large as possible).
A
2
À1
specific surface area of 2900 m g and the characteristic cavi-
ties of MIL-101 were observed for these crystallites (Figure S2).
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500747.
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2 2
Table 1. ICP–OES analysis of TiO @MIL-101 and Pd/TiO @MIL-101 com-
pounds with different metal contents (wt%). The adjusted Pd and Ti con-
tents achieved by MOCVD loading of the MOF MIL-101 were conform to
the theoretical adjusted contents of 10, 15, 20, or 30 wt%.
Sample
Pd content
Ti content
[wt %]
[wt %]
TiO
TiO
Pd/TiO
Pd/TiO
2
2
@MIL-101
@MIL-101
–
–
9.8
11.4
20.2
33.5
15.3
20.2
2
2
@MIL-101
@MIL-101
Scheme 1. Preparation of TiO
2 4
@MIL-101. The Ti-precursor [Ti(O-iP) ] was infil-
trated into MIL-101 by MOCVD and thermally decomposed to titania under
water atmosphere. The dry TiO
loading with Pd.
2
@MIL-101 materials enabled further metal
The general synthesis strategy for TiO @MIL-101 is illustrated in
2
Scheme 1. The volatile Ti-precursor titanium(IV) isopropoxide
was infiltrated into MIL-101 under dynamic vacuum
À4
(
10 mbar) at 358C for 20 h. The gas-phase loading procedure
enabled the homogeneous disposition of the precursor inside
the porous structure of the metal–organic host system. The hy-
drolysis of [Ti(O-iP) ] led to the disposal of 2-propanol and the
4
formation of TiO . The removal of precursor recesses and ad-
2
sorbed water under vacuum resulted in titania-loaded MIL-101
that permitted the further incorporation of palladium nanopar-
ticles (vide infra). The amount of titanium(IV) isopropoxide was
varied for several gas-phase loadings to find the maximal load-
ing capacity. Loading of MIL-101 with Ti contents ꢀ30 wt%
led to the formation of titania agglomerates outside the PCP/
MOF crystallites (Figure S6). To achieve a homogeneous disper-
sion of TiO2 inside the MIL-101 crystallites, Ti loadings of
Figure 1. XRD analysis of TiO
2
@MIL-101 with different Ti contents (5, 10, 15,
20, and 30 wt%) in comparison to MIL-101. The intensified noise signal of
crystalline MIL-101 in the range from 2 to 308 (2q) caused by infiltrated
amorphous titanium dioxide correlates with rising Ti contents.
phous TiO . The compounds with high Ti contents exhibited in-
2
tensified noise signals caused by the amorphous TiO2
2
0 wt% and lower seemed appropriate. The subsequent gen-
(Figure 1). The hydrolysis of pure [Ti(O-iP) ] in analogy to the
4
eration of noble MNPs was also accomplished by gas-phase
preparation procedure of TiO @MIL-101 resulted in the forma-
2
5
3
loading. The infiltration of volatile [(h -C H )Pd(h -C H )] was
tion of amorphous TiO , too (Figure S9). The unsupported
5
5
3
5
2
followed by the hydrogenolysis of the Pd precursor at 708C
amorphous titanium dioxide generated this way has a specific
[6j,m,n]
2
À1
and 50 bar H and led to Pd/TiO @MIL-101
with approxi-
surface area of 500 m g (Figure S7a,b). The additional incor-
2
2
mately 10 wt% of Pd. The incorporation of TiO resulted in the
poration of palladium into TiO @MIL-101 led to regularly ar-
2
2
reduction of the specific surface area of the composites.
ranged Pd particles with a mean diameter of 2.2 nm and
a narrow particle-size distribution as visualized by transmission
electron microscopy (TEM). High-resolution transmission elec-
tron microscopy (HRTEM) indicates the presence of Pd nano-
particles based on a distance of 0.224 nm between adjacent
lattice planes, which is the characteristic value for the (111)
planes of cubic Pd (Figure 2). Electron energy loss spectrosco-
TiO @MIL-101 materials with Ti contents of 10 wt% and
2
2
0 wt% exhibited surface-area decreases of 38% and 59%, re-
spectively. It indicates a precursor deposition in the cavities of
MIL-101. Further loading of TiO @MIL-101 with Pd decreased
2
the remaining specific surface area of the composite materials
as well (Supporting Information, Table S1). Inductively coupled
plasma optical emission spectroscopy (ICP–OES) provides in-
py (EELS) analysis of TiO @MIL-101 (20 wt% Ti) shows the char-
2
sight into the exact metal contents of TiO and Pd nanoparti-
acteristic energy loss peaks for incorporated titanium, chromi-
um, and oxygen corresponding to the composition of MIL-101
(Figure 3a). A qualitative investigation of the chemical compo-
sition was also performed by energy-dispersive X-ray spectros-
copy (EDX) analysis. The signals of Pd and Ti, as well as the ele-
ment signals of the metal–organic framework (Cr, C, and O),
were identified by the characteristic elemental peaks (Fig-
2
cles. The theoretical metal contents (assuming all precursor
material is incorporated) are consistent with the measured
values for titanium and palladium with minor deviations
(
Table 1). X-ray powder diffraction analysis (XRD) of the
TiO @MIL-101 materials with Ti contents from 5 wt% to
2
3
0 wt% displays the characteristic pattern of MIL-101 from 2
to 208 (2q), which supports the integrity of the metal–organic
ure 3b). The incorporation of Pd and TiO in MIL-101 affects
2
host structure after the incorporation of TiO . No peaks were
the light absorption behavior of the MOF. The UV-light absorp-
2
found at the calculated reflex positions for crystalline titanium
dioxide (anatase or rutile) indicating the presence of amor-
tion of TiO @MIL-101 was enhanced compared to pure MIL-
2
101 (Figure S4). Pure amorphous TiO prepared in analogy to
2
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Figure 4. Degradation of RhB by MIL-101, TiO
2
@MIL-101 (20 wt% Ti) and Pd/
TiO
2
@MIL-101 (10 wt% Pd, 20 wt% Ti) under irradiation of UV light with nor-
0
malized catalyst loading referred to the MIL-101 amount. c and c are the in-
tensities of the absorbance peak at 552 nm at the reaction time and the ini-
tial time, respectively.
comparison to the activity of pure MIL-101 could be verified
by the rate constants for the degradation of RhB in aqueous
solution in the presence of external irradiated UV light
2
Figure 2. a) TEM images of Pd/TiO @MIL-101 (10 wt% Pd, 20 wt% Ti) confirm
the integrity of the metal–organic host system and the absence of larger ti-
tanium dioxide aggregates outside the MIL-101 crystallites. b) The highly
(
Figure 4). The decrease of the dye concentration determined
by UV/Vis measurements at 552 nm approximately followed
pseudo-first-order kinetics. Pd/TiO @MIL-101 offered the largest
controlled Pd loading of TiO
2
@MIL-101 enabled narrow Pd NP size distribu-
2
À3
À1
tions. c,d) Homogeneous palladium loading can additionally be illustrated
by HRTEM analysis including the characteristic Pd lattice planes marked by
white lines in (d). The inset in (d) shows the diffractogram.
rate constant of 3.7Æ0.1 (”10 min ) and a half-live time of
À3
À1
1
85 min. TiO @MIL-101 [2.9Æ0.1 (”10 min )] also showed
2
faster degradation of RhB than MIL-101 [1.9Æ0.1
À3
À1
(
”10 min )].The adsorption of the dye molecule inside the
porous structure of the used
MIL-101 based photocatalysts re-
duced the concentration of the
RhB solution. To distinguish be-
tween the photochemically in-
duced degradation of RhB and
the reduction of the dye concen-
tration through adsorption by
the porous hosts, the reaction
mixture was stirred for 2 h under
exclusion of light prior to photo-
catalysis. Significant dye adsorp-
tion was observed during this
Figure 3. a) EELS analysis of TiO
cates the incorporation of Ti. b) EDX analysis of Pd/TiO
incorporated in MIL-101.
2
@MIL-101 (20 wt% Ti) with characteristic energy loss peaks for Ti, O, and Cr indi-
pretreatment. The degradation
of the RhB concentration was
also measured without UV-light
irradiation to prove the stability
2
@MIL-101 with characteristic element peaks for Pd and Ti
the synthesis protocol of TiO @MIL-101 resulted in a colorless
of the dye under the reaction conditions. No degradation of
RhB was observed (Figure S11). Furthermore, no decrease of
the RhB concentration was determined if it was exposed to UV
light in the absence of the catalyst materials (Figure S12). The
photochemically induced degradation of RhB was also investi-
gated in presence of amorphous TiO and Pd@TiO as photoca-
2
powder and featured light absorption in the UV-light range
from 200 to 380 nm. Additional loading of TiO @MIL-101 with
2
palladium nanoparticles resulted in gray to black materials (de-
pending on the Pd content) with almost constant light absorp-
tion over the entire irradiated range from 200 to 800 nm (Fig-
ure S5).
2
2
talysts. These TiO2 based materials showed a significantly
slower degradation of the dye. The reduction of the RhB con-
centration under the same conditions as applied above was
29% for TiO and 45% for Pd@TiO (Figure S14). The reusability
The combined introduction of nanoscale Pd and TiO into
2
MIL-101 led to a multifunctional catalyst material. A preliminary
investigation of the photocatalytic activity was performed by
the decoloration of the organic dye rhodamine B (RhB). The in-
creased photoactivity of TiO @MIL-101 and Pd/TiO @MIL-101 in
2
2
of Pd/TiO @MIL-101 was investigated for two additional runs
2
pointing out the stability of the catalyst system under the UV-
2
2
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light conditions (Figure S15). Steady rate constants and conver-
sions were observed.
In addition to the photocatalytic activity, the performance of
Pd/TiO @MIL-101 in hydrogenation and dehydrogenation reac-
2
tions was investigated. We were especially interested in analyz-
ing the long-time stability and the reusability of the novel cat-
alysts introduced here. First, the hydrogenation of acetophe-
none in combination with a thermal treatment was investigat-
ed. Pd/TiO @MIL-101 (20 wt% Ti) and Pd@MIL-101 were ther-
2
mally treated at 1508C for 20 h prior to each catalytic run and
reused several times. A stable conversion was observed for Pd/
TiO @MIL-101 for five runs. In contrast, Pd@MIL-101, showing
2
the same conversion for the first run, continuously decompos-
es indicated by lower conversion numbers from run to run
Figure 6. Conversion for the reduction of benzophenone to diphenylmetha-
nol (1408C, 30 bar H , 5 h, 600 rpm, 0.01 mol% Pd) in the presence of Pd/
2
(
Supporting Information and Figure 5).
TiO
excellent recyclability of the stabilized catalysts in comparison to Pd@MIL-
01.
2 2
@MIL-101 for two different TiO loadings with constant conversions and
1
Figure 5. Conversion for the reduction of acetophenone to 1-phenylethanol
508C, 30 bar H , 20 h, 600 rpm, 0.05 mol% Pd) with thermal treatment of
the two catalysts before each catalytic run (1508C, 20 h).
(
2
Figure 7. Conversion for the dehydrogenation of diphenylmethanol to ben-
zophenone (1308C, 60 h, 600 rpm, 0.3 mol% Pd) in the presence of Pd/
TiO
2
@MIL-101 (20 wt% Ti), unstabilized Pd@MIL-101, Pd@TiO
, Pd@AC, and Pd@SiO ).
2
, and commer-
cial available Pd-catalysts (Pd@Al
2
O
3
2
Second, Pd/TiO @MIL-101 and Pd@MIL-101 were used as cat-
2
alysts for the hydrogenation of benzophenone at 1408C. Pd/
TiO @MIL-101 with 10 wt% and 20 wt% of Ti were compared
as Pd@AC, Pd@Al O , and Pd@SiO . Finally, the dehydrogena-
2 3 2
2
with Pd@MIL-101. A very good long-time stability and reusabil-
tion of several alcohols was performed by using Pd/TiO @MIL-
2
ity was observed for the TiO -loaded catalysts. The catalytic ac-
101 at the relatively low temperature of 908C in combination
with small catalyst loadings (0.3 mol% Pd). Efficient acceptor-
less alcohol dehydrogenation under neutral conditions and
below 1008C is rare and has been observed for homogenous
2
tivity of Pd@MIL-101 clearly decreases over five runs (Figure 6).
The structural integrity of the Pd/TiO @MIL-101 catalyst with
2
2
0 wt% Ti after five runs of hydrogenation of benzophenone
[
20]
at 1408C was demonstrated by XRD analysis displaying the
representative reflexes of MIL-101 (Figure S10). Third, we inves-
tigated our catalysts in dehydrogenation reactions. The accept-
or-less dehydrogenation of alcohols generating reactive car-
bonyl groups and dihydrogen is an attractive reaction in terms
catalysts mainly.
Very good conversions and selectivities
have been observed for secondary alcohols (Table 2).
Conclusions
[19]
of organic synthesis and hydrogen production. The dehydro-
genation of diphenylmethanol to benzophenone using Pd/
We generated multifunctional metal–organic framework (MOF)
catalysts by incorporating noble-metal nanoparticles (Pd) and
TiO @MIL-101 could be performed with considerably higher
nanoscale TiO . The volatile precursor titanium(IV) isopropoxide
2
2
conversions than using Pd@MIL-101 (Figure 7). In addition, Pd/
was sublimed into MIL-101 in different amounts. Subsequent
TiO @MIL-101 showed only a slightly decrease of the activity
hydrolysis led to the formation of TiO @MIL-101. Further quan-
2
2
over five runs. Pd@MIL-101 decomposes significantly faster.
titative gas-phase loading of [(C H )Pd(C H )] gave rise (after re-
5
5
3
5
Furthermore, the Pd/TiO @MIL-101 catalyst system enabled
duction by hydrogen) to Pd/TiO @MIL-101 composite materi-
2
2
a higher catalytic activity and considerable better reusability
als. Pd/TiO @MIL-101 materials are efficient catalysts in hydro-
2
than Pd@TiO or the commercially available Pd catalysts such
genation and dehydrogenation reactions. A superior activity
2
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tor (below 0.1 ppm of oxygen and water). Manipulations with light
sensitive substances were carried out under exclusion of light. n-
hexane and THF were distilled from sodium benzophenone ketyl.
Table 2. Acceptor-less dehydrogenation of aromatic alcohols through
CÀH activation by Pd/TiO
2
@MIL-101 (908C, 0.3 mol% Pd).
1
-Phenylethanol C H O, 1-(2-methoxyphenyl)ethanol C H O , 1-(2-
8 10 9 12 2
Entry Substrate
Product Conversion Selectivity
[
a]
[a]
[
%]
[%]
methylphenyl)ethanol C H O, 1-(4-fluorophenyl)ethanol C H FO,
9 12 8 8
1
-(4-methoxyphenyl)ethanol C H O , 1-(4-methylphenyl)ethanol
9 12 2
C H O, 1-phenyl-1-pentanol C H O, 1-phenyl-1-propanol C H O,
9
12
11 16
9
12
[b]
[c]
[c]
1
2
3
98
98
94
97
>99
>99
4
-methylbenzyl alcohol C H O, acetophenone C H O, allylpalla-
8 10 8 8
dium(II) chloride dimer [(C H ) Pd Cl ], benzophenone C H O,
3
5 2
2
2
13 10
benzyl alcohol C H O, chromium(III) nitrate nonahydrate
7
8
[Cr(NO ) ·9H O], diphenylmethanol C H O, ethanol C H O, hydro-
3
3
2
13 12
2
6
fluoric acid HF, n-dodecane C H , Pd@AC, Pd@Al O , Pd@SiO , rho-
12
26
2
3
2
damine B C H ClN O , sodium cyclopentadienide NaC H , tereph-
28
31
2
3
5
5
thalic acid C H (COOH) , and titanium(IV) isopropoxide
6
4
2
[Ti(OCH(CH ) ) ] were obtained commercially from Sigma–Aldrich,
3
2 4
ABCR or Acros Organics and used without further purification. The
5
3
[21]
[6n]
products [(h -C H )Pd(h -C H )]
and MIL-101 (Cr)
were pre-
5
5
3
5
pared according to published procedures.
[
d]
b]
4
5
91
96
>99
Characterization
[
97
1
H NMR spectra were recorded with a Varian Unity 300 MHz spec-
trometer. Chemical shifts given in ppm were recorded at 258C and
are referenced using CDCl3 as internal standard. EDX measure-
ments were performed with a Zeiss 1540 EsB Cross beam,
equipped with a Thermo Noran System Six EDX-system. Elemental
analysis was performed by standard protocols employing micro-
[b]
6
93
92
À
+
wave assisted digestion in HNO /HCl/H PO /(BF ) H and induc-
3
3
4
4
[
c]
tively coupled plasma optical emission spectrometry using a Varian
7
8
95
91
>99
>99
Vista-Pro radial. FTIR measurements were performed at a Perkin–
À1
Elmer FTIR-Spectrum 100 over
5
a
range from 4000 cm
to
À1
00 cm . GC analyses were performed using an Agilent Technolo-
[d]
gies 6850 gas chromatograph equipped with a flame ionization de-
tector (FID) and a MN Optima 17 capillary column (30.0 m”
0
.32 mm”0.25 mm) using n-dodecane as internal standard and THF
[
b]
[
[e]
9
1
96
97
58
67
as solvent. Gas mixtures were analyzed using a Agilent special Plot
and Molsieve capillary column (30.0 m”0.32 mm”0.25 mm). GC–
MS analyses were performed using an Agilent Technologies 6890
gas chromatograph with a MN–MS HP-5 capillary column (30.0 m”
c]
[f]
0
0
.32 mm”0.25 mm) and a coupled mass spectrometer as detector.
[
a] Determined by GC. [b] 24 h. [c] 40 h. [d] 48 h. [e] Byproduct benzyl
Nitrogen physisorption isotherms were determined at À1968C
benzoate. [f] Byproduct 4-methylbenzyl 4-methylbenzoate.
using a Quantachrome Nova 2000e apparatus. Specific surface
areas were calculated by using p/p -values from 0.05–0.25 by the
0
BET method. Specific total pore volumes were measured by the
and a very good reusability were observed for Pd/TiO @MIL-
DFT calculations (N at À1968C on silica (cylindr. pore, NLDFT equi-
2
2
1
01 in comparison to Pd@MIL-101, Pd@TiO , and commercially
librium model)). TEM measurements were conducted by using
a LEO 922O (Carl Zeiss, 200 kV). The samples were suspended in
chloroform and sonicated for 5 min. A volume of 2 mL of the sus-
pension was placed on a CF200-Cu-grid (Electron Microscopy Sci-
ences) and allowed to dry. HRTEM measurements were performed
by using a CM 300 UT (Philips, 300 kV). The samples were suspend-
ed in chloroform and sonicated for 5 min. A volume of 2 mL of the
suspension was placed on a CF200-Cu-grid with a lacey carbon
film (Electron Microscopy Sciences) and allowed to dry. TGA analy-
2
available Pd-catalysts such as Pd@Al O , Pd@AC, and Pd@SiO .
2
3
2
The relevance for catalysis in general is the superior per-
formance (activity and reusability) of the catalyst system intro-
duced here in comparison to classic Pd catalysts. The MOF–
TiO host combines excellent and stable Pd dispersion as well
2
as an efficient accessibility of the active sites. Furthermore, Pd/
TiO @MIL-101 is an efficient photocatalyst.
2
À1
sis were performed from 308C to 4508C (58Cmin ) using a Mettler
TGA/SDTA 85 under air. UV/Vis spectra (liquid) were taken for su-
pernatants of the reaction solutions in the range of 400–600 nm
using a CARY 300 (Agilent Technologies) with Scan-software and
PS-cuvettes (12.5 mm”12.5 mm”45 mm). Solid-state UV/Vis spec-
tra were measured using a CARY 300 with an Ulbricht sphere in
the range of 200 nm to 800 nm. XRD diffractograms were recorded
by using a STOE STADI-P-diffractometer (CuKa radiation, 1.54178 Š)
in q-2q-geometry with a position sensitive detector.
Experimental Section
Reactants and solvents
All manipulations of air-sensitive products were performed under
dry and oxygen-free argon atmosphere (Schlenk techniques) or in
a nitrogen-filled glovebox (mBraun) with a high-capacity recircula-
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Syntheses and Reactions
(4.50 mg, 10 wt% Pd, 20 wt% Ti), and 20 mL RhB aqueous solution
À5
À1
(
2·10 moll ) were mixed in a borosilicate glass vessel and stirred
Synthesis of MIL-101: [Cr(NO ) ·9H O] (640 mg, 1.60 mmol), H BDC
3
3
2
2
for 2 h under exclusion of light at 208C. The catalysts were weight-
ed accurate to 0.01 mg. UV-light catalysis was performed by using
a UV-F 400F lamp (450 W Hg) with transmission from 290 nm to
(
265 mg, 1.60 mmol), HF 40 wt% (10 mL, 0.228 mmol) and deion-
ized H O (8.00 mL) were combined and sealed in a 23 mL Teflon-
2
lined hydrothermal autoclave. The mixture was heated for 1 h at
4
20 nm. The distance between the lamp and the borosilicate glass
À1
8
08C (heating rate: 2.78Cmin ) and for 7.45 h at 2208C
vessels was 20 cm. At given irradiation intervals during the UV-light
irradiation, a volume of 1.00 mL of the solution was taken out. The
catalyst was separated by centrifugation at 8000 rpm for 8 min.
UV/Vis spectra were measured from the supernatant in the range
of 400–600 nm. The catalysts were purified in water three times
between the runs of recyclability studies.
À1
(
4.78Cmin ). The reaction mixture was cooled down fast to 1608C
À1
and slowly to 308C (cooling rate: 5.48Ch ). Excessively crystallized
H BDC was removed by filtration over a pore 3 filter. The green
2
product was separated from the reaction solution by centrifuga-
tion (2000 rpm, 45 min). The MIL-101 was heated to reflux in etha-
nol/water (90/10 vol.%) for 14 h and centrifuged (1800 rpm,
4
5 min) three times to remove grimaldiite (a-CrOOH) impurities.
Repetitive differential centrifugation in ethanol/water (1800 rpm,
min) was also used to separate MIL-101 crystals with different
size distribution. The different MIL-101 fractions were dried under
Hydrogenation of aromatic ketones: All reduction experiments
were performed for 5 h or 20 h in a Parr Instruments steel auto-
clave with 30 bar H . The catalysts amounts between 0.5 mg and
2
15 mg were weighted accurate to 0.01 mg. Conversion numbers
and purity were determined by GC and GC–MS. The catalysts were
purified in THF three times between the runs of recyclability stud-
ies.
5
À4
2
À1
vacuum (10 mbar, 858C). (BET: 2900 m g ; XRD (2q (8), intensity
%)): 2.78 (56), 3.26 (100), 3.41 (44), 3.94 (19), 4.30 (11), 4.84 (16),
(
5
8
1
.13 (32), 5.59 (16), 5.85 (22), 6.24 (5), 6.48 (5), 8.10 (7), 8.40 (22),
.58 (11), 8.86 (15), 9.02 (28), 9.71 (5), 9.86 (5), 10.30 (6), 11.22 (4),
6.50 (6)).
Dehydrogenation of aromatic alcohols: All dehydrogenation ex-
periments were performed for 24, 40, 48, or 60 h in a pressure
tube (inner diameter 25.4 mm, length 20.3 cm, volume 28 mL)
5
3
Synthesis of [(h -C H )Pd(h -C H )]: A solution of NaCp (1.50 g,
5
5
3
5
1
7.1 mmol) in 40 mL of THF was added dropwise to a solution of
under dry inert gas atmosphere. The generated H was allowed to
2
[
(C H ) Pd Cl ] (2.50 g, 6.83 mmol) in 50 mL of THF at À788C. The
leave the reaction system. The catalyst amounts between 0.5 mg
and 15 mg were weighted accurate to 0.01 mg. Conversion num-
bers and purity was determined by GC and GC–MS. The catalysts
were purified in THF three times between the runs of recyclability
studies.
3
5 2
2
2
reaction mixture was stirred for 10 min at À788C, warmed up to
room temperature and stirred for another 15 min at room temper-
ature. The solvent was removed under vacuum and the resulting
residue was dissolved in n-hexane. The NaCl was removed by can-
nula filtration and the solvent was removed under vacuum. The
À4
red residue was sublimated under vacuum (10 mbar) and the ob-
tained red crystals were stored under exclusion of light and air at
Acknowledgements
1
À308C. (Yield 2.50 g, 11.76 mmol (86%)). H NMR (300 MHz, CDCl ):
3
3
3
d=1.74 (d, J =9.83 Hz, 2H, CH ), 3.11 (d, J =6.10 Hz, 2H, CH ),
The authors thank the Deutsche Forschungsgemeinschaft (DFG,
SFB 840, B1) for funding. Further the help of Florian Puchtler
HH
2
HH
2
4
.47 (m, 1H, CH), 5.23 ppm (s, 5H, C H ).
5 5
(
XRD measurements), Dr. Christine Denner (EDX measurements),
Synthesis of amorphous TiO : The precursor compound [Ti(O-iP) ]
2
4
and Andreas Gollwitzer (graphical modifications) is gratefully ac-
knowledged. We would like to thank Prof. Dr. Erdmann Spiecker,
Dr. Mirza Mackovic, and Yolita Eggeler (Center for Nanoanalysis
and Electron Microscopy (CENEM), University of Erlangen-Nurem-
berg) for providing the HRTEM CM 300 UT (Philips, 300 kV).
was processed in hydrolysis to yield amorphous TiO . The hydroly-
2
sis of [Ti(O-iP) ] was performed by thermal treatment under H O at-
4
2
À4
mosphere at 808C. The material was evacuated (10 mbar) at
58C for 18 h to remove former metal ligand recess and water.
8
Synthesis of TiO @MIL-101: MIL-101 powder and [Ti(O-iP) ] were
2
4
placed into a two-chamber-tube separated by a glass frit. The gas-
À4
phase loading occurred at 358C in a 10 mbar (diffusions pump)
Keywords: dehydrogenation
nanoparticles · palladium · photocatalysis
· metal–organic framework ·
dynamic vacuum for 16 h. The resulting bright green powder was
instantly processed in hydrolysis to yield TiO @MIL-101. The hydrol-
2
ysis of [Ti(O-iP) ] was performed by thermal treatment under H O
4
2
À4
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3921
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Full Papers
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Received: July 1, 2015
Published online on October 7, 2015
ChemCatChem 2015, 7, 3916 – 3922
www.chemcatchem.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim