Deactivation of Cu3(BTC)2 in the synthesis of 2-phenylquinoxaline

Article abstract of DOI:10.1007/s10562-015-1497-4

Cu₃(BTC)₂ deactivates by deterioration of its crystal structure during the use of this metal organic framework as catalyst in the synthesis of 2-phenylquinoxaline from phenacylbromide and o-phenylenediamine at room temperature. The m

Full text of DOI:10.1007/s10562-015-1497-4

Catal Lett
DOI 10.1007/s10562-015-1497-4
Deactivation of Cu3(BTC)2 in the Synthesis
of 2-Phenylquinoxaline
•
Amarajothi Dhakshinamoorthy Mercedes Alvaro
•
Hermenegildo Garcia
Received: 15 December 2014 / Accepted: 5 February 2015
Ó Springer Science+Business Media New York 2015
Abstract Cu3(BTC)2 deactivates by deterioration of its
crystal structure during the use of this metal organic
framework as catalyst in the synthesis of 2-phenylqui-
noxaline from phenacylbromide and o-phenylenediamine
at room temperature. The material resulting from the use of
Cu3(BTC)2 as catalyst was characterized by powder XRD,
UV–Vis diffuse reflectance spectra and EPR showing that
the crystal structure collapses and Cu2? becomes reduced
under the reaction conditions.
Keywords Heterogeneous catalysis Á Metal organic
frameworks Á Heterocycle synthesis Á Catalyst stability
1 Introduction
Metal–organic frameworks (MOFs) are crystalline materi-
als in which the lattice is constituted by metal nodes held in
place by rigid linkers [1, 2]. The high metal content, the
possibility of having some coordinatively unsaturated sites
around the metal atoms, their large surface area and por-
osity have made MOFs as one of the most promising ma-
terials for heterogeneous catalysis [3–9]. There is by now a
considerable number of examples showing that MOFs can
exhibit very high catalytic activity to promote a large va-
riety of organic reactions of interest in the synthesis of fine
chemicals [10] among other processes.
Graphical Abstract
One of the main advantages of MOFs as heterogeneous
catalysts is their easy and reliable synthesis from readily
available precursors. In this context, a convenient and facile
procedure for the room-temperature synthesis of HKUST-1
from copper(II) hydroxide in aqueous ethanolic solution has
been reported [11]. One of the major concerns to be addressed
when using MOFs as catalysts is stability under reaction
conditions[12]. There are examples showing thatsome MOFs
are initially active as catalysts but, however, deactivate during
the course of the reaction due to the incompatibility of MOF
structure with solvents or reagents under the conditions re-
quired for the catalysis [12]. Specifically, one MOF with large
interest as heterogeneous catalyst, but that has been found to
become destroyed in some reactions is Cu3(BTC)2 (BTC:
1,3,5-benzenetricarboxylate)[13]. This MOF is amply usedas
heterogeneous catalysts in many different types of reactions
including cyanosilylation [14], epoxide rearrangement [15],
ring opening of epoxide [16], acetalization of aldehydes [17],
A. Dhakshinamoorthy (&)
School of Chemistry, Madurai Kamaraj University,
Madurai 625 021, Tamil Nadu, India
e-mail: admguru@gmail.com
A. Dhakshinamoorthy Á M. Alvaro Á H. Garcia (&)
´
Instituto de Tecnologıa Quımica CSIV-UPV, Av. De los
Naranjos s/n, 46022 Valencia, Spain
´
e-mail: hgarcia@qim.upv.es
M. Alvaro
Department of Chemisry, Unviersidad Politecnica de Valencia,
46022 Valencia, Spain
H. Garcia
Centre of Excellence for Advanced Materials Research, King
Abdulaziz University, Jeddah, Saudi Arabia
123

A. Dhakshinamoorthy et al.
N
oxidation of xanthene [18] and Knoevenagel condensations
[19]. However, in several cases, it has been observed that
Cu3(BTC)2 is not stable enough to survive the reaction con-
ditions, as for instance in the case of the aerobic oxidation of
thiols to disulfides [20] and aerobic oxidation of benzyl al-
cohol to benzaldehyde [21]. Considering the catalytic prop-
erties of Cu3(BTC)2, it is important to delineate further for
which type of reactions this commercially available MOF can
stand the reaction conditions and also to provide information
on those other cases in which Cu3(BTC)2 is not stable as solid
catalysts.
O
NH2
NH2
Cu3(BTC)2
Br
N
+
Scheme 1 Condensation of OPD and phenacyl bromide to form
2-phenylquinoxaline
It has been already shown unambiguously that Cu3
(BTC)2 possesses Lewis acid sites which are responsible in
promoting chemical transformations. This could be
achieved by the removal of coordinated solvent molecules
from Cu nodes, thus, creating coordinated unsaturated sites
or Lewis acid sites. Under the present experimental con-
ditions, OPD reacts with phenacyl bromide and interacts
simultaneously with the Lewis acid sites in Cu3(BTC)2,
thus, leading to the reduction of Cu2? to the lower oxida-
tion state or metallic Cu. This reduction process collapses
the crystal structure by leaching of Cu2? and trimesic acid
to the solution (Scheme 2). Analysis of the reaction mix-
ture by GC–MS clearly indicates the presence of trimesic
acid, proving the breakdown of metal–ligand coordination
bond present in MOF. It is interesting to mention here that
care must be given to maintain catalyst stability as well as
to select the reactants which should not disturb the crys-
talline nature of the catalyst. It is relevant to mention here
that the collapse of MOFs crystal structures is mainly due
to the nature of reagent and temperature. In the present
case, OPD interacts with Cu2? possibly through a strong
coordinative bond competing that responsible for the
crystal structure of Cu3(BTC)2. Cu2? is known to form
strong bonds with diamines [29–31]. There are precedents
in which amines as reagents produce the collapse of the
structure of the MOF acting as catalyst. For instance,
methylation of n-hexyl amine by dimethyl carbonate has
been reported using with Al(OH)(BDC) (BDC: 1,4-ben-
zenedicarboxylate) at 170 °C observing together with
methylated and carbamoylated products the presence of
dimethyl terephthalate [32]. Formation of dimethyl
terephthalate was interpreted as indicating that the coor-
dinative bond between Al3? and BDC linker is distroyed
by the amine liberating free carboxylate groups that later
undergo methylation by dimethyl carbonate. In another
precedent, the crystal structure of Cu3(BTC)2 was col-
lapsed during the aerobic oxidation of thiophenol at 70 °C
Recently, MOFs are being used as one of the convenient
heterogeneous catalysts for various organic transformations
with very high activity compared to other porous solids [9,
22]. In particular, Cu3(BTC)2 has been reported as hetero-
geneous solid acid catalyst for the synthesis of quinoline
derivatives in high yields under solvent-free reaction con-
ditions. The reaction between 2-aminobenzophenone and
acetylacetone in the presence of Cu3(BTC)2 resulted in 80 %
of 3-acetyl-2-methyl-4-phenylquinoline in 1 h and the re-
action was completed in 2 h [23]. It was also shown that
Cu3(BTC)2 can be recovered without any significant struc-
tural changes. The advantage of using MOFs as solid acid
catalysts is that it shows higher activity compared with ho-
mogeneous catalysts. Furthermore, it can be reused in con-
secutive cycles. Quinoxaline is one of the nitrogen
containing heterocycles that has been synthesised using
¨
many homogeneous Lewis acid [24] catalysts, Bronsted acid
[25] catalysts and heterogeneous solid catalysts [26–28].
In this context, our aim is to use Cu3(BTC)2 as hetero-
geneous catalyst for the synthesis of 2-phenylquinoxaline
from phenacyl bromide and o-phenylenediamine (OPD) at
room temperature. Unfortunately, it was observed that the
catalyst is not stable under these mild conditions, resulting
structural deterioration. Structural analysis of the material
resulting after the use of Cu3(BTC)2 as catalyst show that
OPD interacts with Cu2? ions in Cu3(BTC)2 leading to
transformation of the lattice and reduction of Cu2? ions.
2 Results and Discussion
In our continuous effort of using MOFs as heterogeneous
solid catalysts, we were interested in studying the catalytic
activity of Cu3(BTC)2 for the synthesis of 2-phenylqui-
noxaline by condensation of phenacyl bromide and OPD in
ethanol at room temperature (Scheme 1). It was observed
that under the present reaction conditions Cu3(BTC)2
promotes the reaction towards the formation of the desired
product in 57 % yield after 3 h. However, Cu3(BTC)2 be-
comes deactivated as noticed by the change in the colour
from blue to yellow and the fact that the reaction did not
progress further.
in acetonitrile medium after 10 min due to the strong Cu2?
-
thiol bond [13]. We note, however, that in the present case,
the experimental conditions are milder, since the reaction is
performed at room temperature and the reaction times are
comparatively shorter. It appears that OPD interacts
strongly with Cu2?, resulting in the collapse of the crystal
structure. It seems that the effect of OPD should be general
for other MOFs whose lattice is based on Cu2?-carboxylate
interactions.
123

Deactivation of Cu3(BTC)2
Scheme 2 Decoordination of
Cu3(BTC)2 to trimesic acid and
Cu2? by OPD. L: ligands that
can be coordinating species
present in the medium including
other OPD, Br- or solvent
molecules
Currently, there is a need to delineate the stability of
MOFs compared with zeolites and other related metal ox-
ides with the aim to clarify if MOFs could be used as
industrial heterogeneous catalysts. We have already
demonstrated that Cu3(BTC)2 undergoes major changes in
its crystal structure by forming Cu nanoparticles when re-
acted with aliphatic/aromatic thiols [13]. Similarly, de-
struction of Cu3(BTC)2 framework was observed in the
oxidation of CO [33]. In another precedent, the structure of
Pt-doped HKUST-1 collapsed in the presence of dissoci-
ated hydrogen due to the higher reduction potential of Cu
compared with H, the instability of the divalent copper in
HKUST-1 increasing with temperature. Unlike HKUST-1,
MIL-53 and ZIF-8 maintained their structures in processes
that involve both dihydrogen and dissociated hydrogen at
temperatures up to 150 °C [34]. Also, Cu3(BTC)2 with an
incorporated Keggin polyoxometalate has been shown to
be stable under steaming conditions up to 210 °C, while the
isostructural HKUST-1 degrades and transforms into
[Cu2OH(BTC)(H2O)]nÁ2nH2O from 70 °C onwards
[35]. Hence, it is of interest to determine in more detail the
cause of Cu3(BTC)2 deactivation in the quinoxaline syn-
thesis. In the present study, we provide spectroscopic in-
formation showing that by interaction with OPD,
Cu3(BTC)2 undergoes a transformation in its crystal
structure.
3000
2500
2000
1500
1000
500
(d)
(c)
(b)
(a)
0
10
20
30
40
50
2θ
Fig. 1 Powder XRD patterns of a fresh Cu3(BTC)2, b Cu3(BTC)2
treated with 100 mg of OPD, c Cu3(BTC)2 treated with 175 mg of
OPD and d the solid material derived from Cu3(BTC)2 used as
catalysts after the reaction. Note that diffractrogram ‘‘a’’ and ‘‘d’’ are
totally different
be seen in Fig. 1 that the stability of the crystal structure is
mainly dependent on the concentration of OPD that is the
reagent causing damage of the Cu3(BTC)2 crystallinity.
XRD indicates that the crystal structure of Cu3(BTC)2 is
relatively unaffected when the concentration of OPD is low
(100 or 175 mg) that correspond to less than one OPD
equivalent per Cu2? in Cu3(BTC)2. When the OPD
(250 mg) concentration exceeds with Cu2? amount
(OPD:Cu2? mol ratio 1.12), the crystal structure of Cu3
(BTC)2 undergoes major changes in its diffraction pattern
(Fig. 1). In contrast to the behaviour of Cu3(BTC)2 in the
presence of OPD, XRD and the colour of Cu3(BTC)2 was
maintained when the MOF was treated with phenacyl
bromide under identical reaction conditions. In any case,
the crystal structure was completely collapsed when
Cu3(BTC)2 was used as catalyst for the synthesis of
2-phenylquinoxaline from phenacyl bromide and OPD at
room temperature. Thus, XRD clearly establishes that most
of the characteristic peaks in the fresh catalyst have de-
creased/disappeared in the solid material derived from
Cu3(BTC)2 used as catalysts supporting our claim based on
literature precedents of the possible interaction of OPD
substrate with the dimeric Cu nodes in the MOFs structure,
Thus, after the reaction, the solid material derived from
Cu3(BTC)2 used as catalysts was washed with excess
ethanol and it was characterized by powder XRD, UV–Vis
diffuse reflectance spectroscopy (DRS) and EPR. This
study shows that Cu3(BTC)2 stability must be surveyed
when any catalytic reaction involves the use of OPD as
reactant.
Figure 1 shows the powder XRD patterns of fresh
Cu3(BTC)2 and the solid material derived from Cu3(BTC)2
used as catalysts after the treatment with phenacyl bromide
and OPD. As it can be seen there, the characteristic peak
pattern and peak intensity of Cu3(BTC)2 changes com-
pletely and decreases much in its intensity when treating
Cu3(BTC)2 with these reactants. Also, Fig. 1 shows a new
peak at low angle which is absent in the fresh material
indicating that there is a structural change of the solid
material derived from Cu3(BTC)2 used as catalysts. It can
123

A. Dhakshinamoorthy et al.
transforming it into another crystal structure and leading to
deactivation. Overall, XRD shows that Cu3(BTC)2 under-
goes structural transformation in the presence of OPD by
destroying its crystal structure probably into another inac-
tive form.
corresponding to the presence of Cu2?. In contrast, the
solid material derived from Cu3(BTC)2 used as catalysts
showed a decrease in its intensity and a change in the
coupling constant of the EPR signal indicating a decrease
in the population of EPR active copper species (Cu2?).
This decrease in the intensity of Cu2? agrees well with the
results from DRS and must be related to the collapse of the
crystal structure observed by powder XRD. Furthermore,
literature data showing that Cu2? promotes spontaneous
oxidation of OPD can explain why Cu2? becomes reduced
under the reaction conditions [36].
Figure 2 provides DRS of fresh and deactivated Cu3
(BTC)2 after the condensation reaction leading to 2-phe-
nylquinoxaline formation. The fresh Cu3(BTC)2 exhibits
two absorption bands around 284 and 700 nm. The first
absorption band corresponds to the aromatic ligand and the
latter one is due to the d–d transition of Cu2?. Interestingly,
the d–d band completely diminishes in the solid material
derived from Cu3(BTC)2 used as catalysts after the cat-
alytic reaction. It is interesting to note that the decrease in
the intensity of d–d band was dependent on the concen-
tration of OPD. When the concentration of OPD increased
gradually, the intensity of d–d band diminished completely
even at room temperature. However, the solid material
derived from Cu3(BTC)2 used as catalysts still shows the
absorption band corresponding to the ligand due to the
existence of ligand even after the structural change. Fur-
thermore, Fig. 2 also shows that the absorption between
200 and 300 nm characteristic of aromatic linkers has re-
duced considerably its intensity in the solid material
derived from Cu3(BTC)2 used as catalysts. The residual
absorption in this UV region probably indicates that some
linker is still present in the solid material derived from
Cu3(BTC)2 used as catalysts. From powder XRD it was
concluded that the structure of Cu3(BTC)2 has completely
changed and DRS shows the gradual disappearance of
Cu2? as a function of OPD concentration.
Thus, in view of the moderate strength of the Cu2?
-
carboxylate interaction responsible for MOF structure, and
the high affinity of Cu ions for thiols and amines, it can be
anticipated that Cu3(BTC)2 should not be considered an
appropriate solid catalyst for those reactions that involve
the type of reagents with high affinity as Cu ions ligands.
Interestingly, the synthesis of 2-phenylquinoxaline has
been reported by the oxidative cyclization of a-hydrox-
yketone and OPD using Cu(BDC) as heterogeneous cata-
lyst in quantitative yield at 100 °C in toluene under air
atmosphere in 3 h [37]. Furthermore, the reaction using
Cu2(BDC)2(DABCO) as catalyst resulted in 87 % con-
version, while MOF-118 was found to be even more active
by giving 92 % conversion after 3 h. On the other hand,
Cu3(BTC)2 encapsulated heteropolyacid exhibited 84 %
conversion. A slightly lower yield was achieved with
MOF-199 as catalyst under identical conditions. These
previous results are in good agreement with the present
work and can be interpreted as derived from the instability
of MOF-199 whose structure is similar to Cu3(BTC)2. A
very interesting point from this precedent is that among the
various Cu-based catalysts tested for the synthesis of
2-phenylquinoxaline, the crystal structure of Cu(BDC) was
not damaged. Interestingly, Cu(BDC) exhibited 97 %
conversion even after eight runs and powder XRD of the
recovered Cu(BDC) after its use as catalyst revealed that its
EPR spectra were measured for Cu3(BTC)2 and the solid
material derived from Cu3(BTC)2 used as catalysts after the
reaction. The observed results are presented in Fig. 3. The
fresh catalyst Cu3(BTC)2 shows the EPR spectrum
30
20
10
0
1500000
1000000
500000
0
-500000
-1000000
-1500000
-2000000
-2500000
300
400
500
600
700
800
Wavelength/nm
2250
3000
3750
4500
G (T)
Fig. 2 DRS of fresh Cu3(BTC)2 (black line), Cu3(BTC)2 treated with
100 mg of OPD (blue line), Cu3(BTC)2 treated with 175 mg of OPD
(red line) and the solid material derived from Cu3(BTC)2 used as
catalysts (green line)
Fig. 3 EPR spectra of fresh Cu3(BTC)2 (solid line) and the solid
material derived from Cu3(BTC)2 used as catalysts (dotted line)
123

Deactivation of Cu3(BTC)2
Scheme 3 Contrasting frame work stability of various Cu MOFs in the synthesis of 2-phenylquinoxalines
crystallinity was maintained with slight difference in the
diffractogram. This stability was proposed to derive from
the flexibility of Cu(BDC) structure and the non-isotropy
during fresh sample preparation. Scheme 3 compares the
herein observed data for Cu3(BTC)2 with those of related
precedents.
4 Experimental Section
4.1 Materials
Cu3(BTC)2 also commercially known as Basolite C 300
MOF, OPD and phenacyl bromide used in the present study
was purchased from Sigma Aldrich and used as received.
Solvents used in the present study were purchased from
Sigma Aldrich.
Comparing the present data with that of the results re-
ported in the literature [37] it can be concluded that the
catalyst stability must be surveyed after any catalytic re-
action. Furthermore, the observed results are applicable
only to the crystal structure of Cu3(BTC)2 under a com-
bination of reagents and conditions and it cannot be gen-
eralized to other Cu-based MOFs. Based on the results
from DRS and EPR, it can be assumed that Cu exists in the
metallic state either as Cu nanoparticles or in the form of
Cu (I)-complexes. Further studies are required to identify
in detail the composition of the solid material derived from
Cu3(BTC)2 used as catalysts.
4.2 Characterization Techniques
Powder XRD diffraction patterns were measured in the
refraction mode using a Philips X’Pert diffractometer using
˚
the CuKa radiation (k = 1.54178 A) as the incident beam,
PW3050/60 (2 theta) as Goniometer, PW 1774 spinner as
sample stage, PW 3011 as detector, incident mask fixed
with 10 mm. PW3123/10 for Cu was used as
a
monochromator. PW3373/00 Cu LFF was used as X-ray
tube with power scanning of 45 kV and 40 mA current.
The sample powder was loaded into a holder and levelled
with a glass slide before mounting it on the sample
chamber. The specimens were scanned between 2° and 70°
with the scan rate of 0.02°/s. UV–Vis diffuse reflectance
spectra were measured using a Cary 100 G UV–Vis-NIR
spectrophotometer using an integrating sphere accesory.
EPR spectra were recorded using a Bruker EMX, with the
typical settings: frequency 9.80 GHz, sweep width 30.6 G,
time constant 80 ms, modulation frequency 100 kHz,
modulation width 0.2 G, microwave power 200 mW.
3 Conclusions
The present work highlights that Cu3(BTC)2 undergoes
structural deactivation in the presence of OPD which re-
sults in drastic structural changes. The observed results
indicate that Cu3(BTC)2 as heterogeneous solid catalyst is
not stable in reactions that involve OPD as one of the
reagents. This may be due to the strong coordination of
OPD groups with the coordinatively unsaturated Cu2? sites
which subsequently decomposes the crystal structure even
at room temperature and can promote Cu2? reduction. This
study illustrates that the active sites are decomposed or
changed into another inactive material as evidenced by
powder XRD, DRS and EPR and contrasts with related
precedents. Hence, it can be concluded that the catalyst
stability mainly depends on the reaction temperature, na-
ture of reagents, contact time of catalyst with reagents and
solvent and predictions on MOF stability can be
misleading.
4.3 Experimental Procedure
To a stirred solution of 0.250 mg of OPD and 480 mg of
phenacyl bromide in 20 mL of ethanol, 350 mg of Cu3
(BTC)2 (activation at 150 °C for 3 h) was charged and the
mixture was stirred at room temperature under air at at-
mospheric pressure. As the reaction progresses, the colour
of the heterogeneous mixture turned from blue to yellow.
123

A. Dhakshinamoorthy et al.
15. Alaerts L, Seguin E, Poelman H, Thibault-Starzyk F, Jacobs PA,
De Vos DE (2006) Chem Eur J 12:7353
16. Dhakshinamoorthy A, Alvaro M, Garcia H (2010) Chem Eur J
16:8530–8536
After 2 h, the pale yellow solid was filtered, washed with
ethanol (50 mL) in two portions, dried.
Acknowledgments A.D.M. thanks University Grants Commission
(UGC), New Delhi for the award of Assistant Professorship under its
Faculty Recharge Programme. ADM also thanks Department of
Science and Technology, India for the financial support through Fast
Track project (SB/FT/CS-166/2013). Financial support by the Spanish
Ministry of Economy and Competitiveness (CTQ-2012-32315 and
Severo Ochoa) and Generalidad Valenciana (Prometeo 2012-014) is
gratefully acknowledged. The research leading to these results has
received partial funding from the European Community’s Seventh
Framework Programme (FP7/2007–2013) under Grant Agreement
No. 228862.
17. Dhakshinamoorthy A, Alvaro M, Garcia H (2010) Adv Synth
Catal 352:3022–3030
18. Dhakshinamoorthy A, Alvaro M, Garcia H (2009) J Catal
267:1–4
19. Opanasenko M, Dhakshinamoorthy A, Shamzhy M, Nachtigall P,
Horacek M, Garcia H, Cejka J (2013) Catal Sci Technol
3:500–507
20. Dhakshinamoorthy A, Alvaro M, Garcia H (2010) Chem Com-
mun 46:6476–6478
21. Dhakshinamoorthy A, Alvaro M, Garcia H (2011) ACS Catal
1:48–53
22. Dhakshinamoorthy A, Opanasenko M, Cejka J, Garcia H (2013)
Catal Sci Technol 3:2509–2540
ˇ
´
23. Perez-Mayoral E, Cejka J (2011) ChemCatChem 3:157–159
24. Cai J-J, Zou J-P, Pan X-Q, Zhang W (2008) Tetrahedron Lett
49:7386
References
1. Ferey G (2008) Chem Soc Rev 37:191–214
2. Corma A, Garcia H, Llabres i Xamena FX (2010) Chem Rev
110:4606–4655
3. Farrusseng D, Aguado S, Pinel C (2009) Angew Chem Int Ed
48:7502–7513
25. Brown DJ (2004) The chemistry of heterocyclic compounds
quinoxalines: supplement II. Wiley, New Jersey
26. Venkateswara Rao KT, Sai Prasad PS, Lingaiah N (2009) Mol
Catal A 312:65–69
27. Brahmachari G, Laskar S, Barik
3:14245–14253
P (2013) RSC Adv
4. Liu J, Chen L, Cui H, Zhang J, Zhang L, Su CY (2014) Chem Soc
Rev 43:6011–6061
28. Paul S, Basu B (2011) Tetrahedron Lett 52:6597
29. Collman JP, Zhong M (2000) Org Lett 2:1233–1236
30. Nakajima M, Miyoshi I, Kanayama K, Hashimoto S, Noji M,
Koga K (1999) J Org Chem 64:2264–2271
31. Strieter ER, Blackmond DG, Buchwald SL (2005) J Am Chem
Soc 127:4120–4121
32. Dhakshinamoorthy A, Alvaro M, Garcia H (2010) Appl Catal A
Gen 378:19–25
33. Qiu W, Wang Y, Li C, Zhan Z, Zi X, Zhang G, Wang R, He H
(2012) Chin J Catal 33:986–992
34. Chen H, Wang L, Yang J, Yang RT (2013) J Phys Chem C
117:7565–7576
35. Mustafa D, Breynaert E, Bajpe SR, Martens JA, Kirschhock CE
(2011) Chem Commun 47:8037–8039
36. Ohde H, Hunt F, Wai CM (2001) Chem Mater 13:4130–4135
37. Dang GH, Vu YTH, Dong QA, Le DT, Truong T, Phan NTS
(2015) Appl Catal A 491:189–195
5. Lin Z-J, Lu¨ J, Hong M, Cao R (2014) Chem Soc Rev
43:5867–5895
6. Na K, Choi KM, Yaghi OM, Somorjai GA (2014) Nano Lett
14:5979–5983
7. Zhao M, Ou S, Wu C-D (2014) Acc Chem Res 47:1199–1207
8. Zhu Q-L, Xu Q (2014) Chem Soc Rev 43:5468–5512
´
9. Gascon J, Corma A, Kapteijn F, Llabres i Xamena FX (2014)
ACS Catal 4:361–378
10. Sengupta A, Su C, Bao C, Nai CT, Loh KP (2014) Chem-
CatChem 6:2507–2511
´
´
11. Majano G, Perez-Ramırez J (2013) Adv Mater 25:1052–1057
12. Dhakshinamoorthy A, Alvaro M, Corma A, Garcia H (2011)
Dalton Trans 40:6344–6360
13. Dhakshinamoorthy A, Alvaro M, Concepcion P, Garcia H (2011)
Catal Commun 12:1018–1021
14. Schlichte K, Kratzke T, Kaskel S (2004) Microporous Meso-
porous Mater 73:81–88
123