Ag-Cu-BTC prepared by postsynthetic exchange as effective catalyst for selective oxidation of toluene to benzaldehyde

Article abstract of DOI:10.1016/j.catcom.2014.09.047

A mixed-node MOF catalyst Ag-Cu-BTC was prepared by postsynthetic exchange (PSE) method. It is believed that PSE method can realize isomorphous replacement of Ag ion to framework Cu ion in Cu-BTC successfully. The catalytic performance of Ag-Cu-BTC was in

Full text of DOI:10.1016/j.catcom.2014.09.047

Catalysis Communications 59 (2015) 92–96
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ag–Cu–BTC prepared by postsynthetic exchange as effective catalyst for
selective oxidation of toluene to benzaldehyde
a
a
b
Zhiguo Sun a,b, Gang Li a, , Yue Zhang , Hai-ou Liu , Xionghou Gao
⁎
a
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, No. 2 Ling-gong Road, Dalian 116024, China
Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina Company, Ltd., Lanzhou 730060, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2014
Received in revised form 24 September 2014
Accepted 26 September 2014
Available online 5 October 2014
A mixed-node MOF catalyst Ag–Cu–BTC was prepared by postsynthetic exchange (PSE) method. It is believed
that PSE method can realize isomorphous replacement of Ag ion to framework Cu ion in Cu–BTC successfully.
The catalytic performance of Ag–Cu–BTC was investigated via selective oxidation of toluene to benzaldehyde
by molecular oxygen in the absence of solvent and initiator. This catalyst exhibits good catalytic performance:
on the premise of keeping highly selective catalysis of Cu–BTC for toluene oxidizing to benzaldehyde, the intro-
duction of Ag (Ag content is 2.76 wt.%) can promote toluene conversion from 6.5% to 12.7%.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Metal-organic frameworks
Postsynthetic exchange
Ag–Cu–BTC
Toluene selective oxidation
Benzaldehyde
1. Introduction
Recently, metal-organic frameworks (MOFs) have generated substan-
tial attention for the use in catalysis [9,10]. They are porous materials
The selective oxidation of hydrocarbons into useful oxygenated
chemicals is an important family of chemical transformations [1–3].
Among them, the direct oxidation of toluene by oxygen to produce
benzaldehyde is an attractive process. Benzaldehyde is a versatile
intermediate in the manufacture of perfumes, pharmaceuticals, dyes
and so on [4–6]. Commercially, benzaldehyde is mainly produced by
the chlorination of toluene followed by the hydrolysis process, which
generates large amounts of toxic acidic/basic discard solution, leading
to equipment corrosion and environmental pollution. Even worse, the
benzaldehyde produced by this route is not qualified to synthesize
some high-quality compounds such as perfumes or pharmaceuticals
because the product contains chlorine [7,8]. Therefore, there is a clear
need to develop selective oxidation of toluene as an alternative route
to produce benzaldehyde. There have been many attempts to find an
effective heterogeneous catalyst for this reaction. It was reported
that Cu–Fe/γ-Al2O3 catalyzed solvent-free toluene oxidation with
O2 to produce benzaldehyde with 85.9% selectivity at 7.3% conver-
sion in the presence of pyridine(463 K, 1.0 MPa and 2 h) [4]. Au–Pd
nanoparticles supported on carbon gave 63.4% selectivity to benzalde-
hyde at 1.5% conversion (433 K, 1.0 MPa and 7 h) [6]. Obviously, a cata-
lyst with high selectivity and improved activity for this process is greatly
demanded.
composed of metal nodes connected by organic ligands, allowing for
great chemical and structural diversity. These characters as well as
functionizable crystalline framework provide multiple opportunities to
create desirable active sites and determine MOFs as excellent candidates
for catalysis application [11–13]. Among reported MOF catalytic systems,
node metal as catalytic center is the most common case [14–16]. These
node metals provide uniform, high density and non-leaching catalytic
centers and exhibit versatile catalytic properties. However, the investiga-
tions mainly focus on single-node MOF catalysts and the explorations of
mixed-node MOF catalysts are very limited so far. The development of
mixed-node MOF catalyst is of special interest because the incorporation
of two or more kinds of metal ions can add different functionalities. A
mixed-node MOF may realize more complicated catalysis such as multi-
ple catalysis or cooperative catalysis.
A useful strategy to prepare mixed-node MOF is to use different
metal ions in the synthetic system [17]. But this method is not widely
applicable and needs strict control to synthetic conditions, since the ad-
dition of another metal ion may affect the formation of target structure.
An alternative strategy is to substitute some of the metal nodes in a
pre-formed MOF with another metal ion by postsynthetic exchange
(PSE) [18]. Unlike other porous materials such as zeolites, mesoporous
silicas and porous carbons, one distinct feature of MOFs is their excel-
lent postfunctionalization ability [19]. A variety of functional groups
can be introduced to MOFs through postsynthetic approaches, such as
postsynthetic modification (PSM) [20] and postsynthetic deprotection
(PSD) [21]. PSE is a recently reported postsynthetic approach to
⁎
Corresponding author. Tel./fax: +86 411 84986113.
E-mail address: liganghg@dlut.edu.cn (G. Li).
http://dx.doi.org/10.1016/j.catcom.2014.09.047
1566-7367/© 2014 Elsevier B.V. All rights reserved.

Z. Sun et al. / Catalysis Communications 59 (2015) 92–96
93
diffraction (XRD) patterns were employed to identify the structures of
prepared samples. The XRD patterns were recorded on Rigaku D/Max
2400 diffractometer employing Cu Kα radiation. Raman spectra were
recorded using DXR micro Raman spectrometer from Thermo Scientific
Company. The 532 nm wavelength laser source was adopted to investi-
gate the M–O bonding situation in MOFs. X-ray photoelectron spectros-
copy (XPS) was conducted to determine the states of elements in
material using a Thermo ESCALAB 250 X-ray photoelectron spectrome-
ter. Al–Kα acted as light source and Cls (284.6 eV) was used to correct
XPS peaks of other elements. The scanning electron microscopy (SEM)
images were obtained on a NOVA NANOSEM 450 field-emission scanning
electron microscopy using a 20 kV energy source under vacuum. Oxford
energy dispersive X-ray analysis (EDAX) and Inca software were used to
determine elemental mapping of particle surfaces at a working distance
of 5 mm. The BET surface areas of the samples were measured by N2 phys-
ical adsorption–desorption at 77 K on a Quantachrome AUTOSORB-1
apparatus.
Scheme 1. The preparation of Ag–Cu–BTC.
produce functionalized MOFs [22]. It denotes a process that the node
metals or ligands in a MOF crystallite can be readily replaced with
other ones through an exchange fashion while keeps the structure of
MOF intact. This approach provides a generally applicable means to pre-
pare mixed-node MOFs, in which the proportions of metals can be
adjusted.
Herein, we report a mixed-node MOF denoted as Ag–Cu–BTC, which
was prepared by PSE method. The catalytic performance of this material
was investigated via selective oxidation of toluene to benzaldehyde by
molecular oxygen in the absence of solvent and initiator.
2.3. Catalytic test
Typically, the selective oxidation of toluene was carried out in a
100 mL stainless steel autoclave with a polytetrafluoroethylene liner.
10 mL toluene and 0.1 g solid catalyst were added into the reactor.
The autoclave was charged with 1.0 MPa oxygen, stirred and heated to
reactive temperature and kept on reacting for 4 h. Afterward, the reactor
was cooled down to room temperature and the pressure was released
slowly. The products were diluted with acetone and transferred to a
50 mL volumetric flask totally. After enough mixing, the sample was an-
alyzed by Agilent GC-6890 N (HP-5 capillary column, Flame Ionization
Detector).
2. Experimental
2.1. Catalyst preparation
The nitrates were purchased from Sinopharm Chemical Reagent
Company, Ltd. Trimesic acid was purchased from J&K Chemical
Company. All the chemicals were used as received without further
purification.
For the recyclability tests, the reactions were performed under the
same reaction conditions, except using the recovered catalyst. To
study the leaching of exchanged ion during the reaction, the reaction
mixture was treated by centrifugation and the solution was analyzed
by ICP.
MOFs were synthesized and activated according to the literatures
[23–26]. Ag–Cu–BTC was prepared through a PSE method. Typically,
0.064 g AgNO3 was dissolved in the mixture of 48 mL deionized
water and 48 mL ethanol in 125 mL stainless steel autoclave with a
polytetrafluoroethylene liner. 0.4 g prepared Cu–BTC was added
into and the autoclave was put in a 358 K oven. The ion exchange was
carried out for 1 day. Then, the reactor was cooled down to room tem-
perature. After filtration, the cyan solid was washed with excess ethanol
and deionized water. Finally, the product was dried under vacuum at
373 K over the night (Scheme 1).
3. Results and discussion
3.1. Catalytic performances of MOFs with different node metals
In order to investigate the catalytic performances of different node
metals for toluene selective oxidation, MOFs prepared by trimesic acid
(H3BTC) coordinated with different metal ions (Cr, Al, Fe, Cu) were
taken as catalysts. The concentrations of Cr, Al, Fe, and Cu in the samples
are 19.53, 16.15, 21.64 and 29.82 wt.%, respectively. These data imply
that the amounts of active centers are enough and the catalytic perfor-
mance of sample is mainly determined by the nature of node metal.
The reaction results are listed in Table 1 (entry 2 to 5). Contrasting the
catalytic performances of MOFs with different node metals, we can see
2.2. Catalyst characterization
The element contents of the samples were determined by inductive-
ly coupled plasma (ICP) on Optima 2000DV instrument. Powder X-ray
Table 1
The catalytic performances of different catalysts in toluene selective oxidation.a
Entry
Catalyst
Conv./%
Benzyl alcohol select./%
Benzaldehyde select./%
Benzoic acid select./%
Others/%
1
2
3
4
5
6
7
8
9
None
Cr–BTC
Al–BTC
Fe–BTC
Cu–BTC
AgNO3
4.2
12.4
9.0
5.9
6.5
20.5
9.8
12.7
13.0
20.1
15.8
0
3.5
17.0
0
41.8
0
0
47.6
49.5
53.1
58.4
99.5
41.4
99.1
99.0
98.2
90.6
33.7
47.5
41.8
24.3
0
13.6
0.5
0.7
2.9
3.0
1.6
0.3
0.5
3.2
0.4
0.3
0.7
1.5
Ag–Cu–BTC(0.69)b
Ag–Cu–BTC(2.76)b
Ag–Cu–BTC(4.15)b
Ag–Cu–BTC(4.15)b,c
0
0
1.1
7.9
10
a
Reactive condition: catalyst 0.1 g; O2 pressure 1.0 MPa; reaction temperature 433 K; reaction time 4 h.
Data in bracket are weight percentage contents of Ag.
The test was performed for 8 h while other conditions kept unchanged.
b
c

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Z. Sun et al. / Catalysis Communications 59 (2015) 92–96
Table 2
Electronic binding energies of various Ag species.a
Orbit
Ag
Ag2O
AgO
Ag–Cu–BTC
Ag 3d3/2
Ag 3d5/2
373.4–374.2
367.9–368.4
373.9
367.7–368.4
–
374.9
368.8
367.3–368.0
a
The data of Ag, Ag2O and AgO come from NIST XPS Database.
which can exclude the cases that Ag species is existed in new crystal
form or loaded on framework in AgNO3 form. Fig. 2A presents Raman
spectra of Cu–BTC and Ag–Cu–BTC with different Ag exchange degrees.
The most notable change for Ag–Cu–BTC spectra is that a new peak
appears at 230 cm−1 and the intensity of this peak increases with the
Ag exchange degree. This indicates that the peak directly relates to the
newly generated Ag species and can be attributed to the characteristic
peak of Ag\O bond. Fig. 2B shows high resolution XPS of Ag 3d region
for Ag–Cu–BTC. It can be seen that the electron binding energies corre-
sponding to Ag 3d3/2 and Ag 3d5/2 photoelectron peaks are 374.9 and
368.8 eV, respectively. These numerical values are bigger than that of
common chemical states of Ag element (Table 2). This is owing to the
Ag ion in Ag–Cu–BTC which is linked with four oxygen atoms of carbox-
ylic ligands. The oxygen atoms with large electronegativity make share
electron pair deviate from Ag ion, which leads to the increase of inner
electron binding energy. Scanning electron microscopy and energy
dispersed X-ray analysis (SEM-EDAX) also confirmed the presence of
both Cu and Ag in Ag–Cu–BTC particle (Fig. 3). The even distribution
of Ag element implies that Ag species is located in the crystal lattices
of Ag–Cu–BTC. All above results together provide substantial evidence
to that Ag–Cu–BTC was prepared successfully.
Fig. 1. Powder XRD patterns of samples: (a) Cu–BTC; and (b) Ag–Cu–BTC.
that node metals have huge influence on toluene selective oxidation.
When Cr–BTC acts as catalyst, the conversion rate of toluene is highest
among the single-node MOF catalysts. However, the products are mix-
ture of benzaldehyde and benzoic acid, leading to the selectivity of
benzaldehyde is only 49.5%. For Cu–BTC catalyst, although the conver-
sion rate of toluene is not high, the selectivity of benzaldehyde can
reach to 99.5%. The exploitation of high selective chemical reaction
can reduce the waste of raw material, save energy consumption in prod-
ucts separation and protect environment. Therefore, we take Cu–BTC as
main body catalyst for selective oxidation of toluene to benzaldehyde
and explore method to improve its activity. Then, Ag–Cu–BTC was pre-
pared and its catalytic performance in toluene selective oxidation to
benzaldehyde was studied.
3.3. Catalytic performance of Ag–Cu–BTC in toluene selective oxidation
The catalytic performances of AgNO3 and Ag–Cu–BTC with different
Ag exchange degrees for toluene selective oxidation to benzaldehyde
are listed in Table 1 (entry 6 to 10). It can be seen that when AgNO3 is
used as catalyst, the conversion rate of toluene is comparatively high,
which indicates that Ag ions can boost the activation of toluene. Howev-
er, the selectivity of benzaldehyde is not satisfactory. When Ag–Cu–BTC
acts as catalyst, comparing with Cu–BTC, the conversion of toluene can
be greatly improved while the selectivity of benzaldehyde has no
obvious decrease. In order to compare selectivity of Ag–Cu–BTC with
that of AgNO3 at the similar level of conversion, the conversion rate of
toluene was raised to 20.1% by increasing reaction time to 8 h. It was
found that the selectivity of benzaldehyde is still above 90%. The BET
specific surface areas of Cu–BTC and Ag–Cu–BTC are both N1000 m2/g
while that of AgNO3 is b1 m2/g. Big specific surface area means a large
3.2. Characterization of Ag–Cu–BTC
The most direct means to judge whether Ag was introduced to MOF
matrix is elemental analysis. The ICP analysis indicates that the actual Ag
content of Ag–Cu–BTC prepared by described method is 2.76 wt.%. In
addition, Ag–Cu–BTC was treated with nitric acid and the filtrate was
analyzed by ICP. There is no detectable Ag in filtrate discloses that Ag el-
ement does not exist in the form of Ag2O or AgO on the Cu–BTC surface.
The XRD patterns were employed to identify the Cu–BTC samples
(Fig. 1). The patterns of the Ag–Cu–BTC and Cu–BTC are very similar,
which indicates that the ion exchange process is isomorphous replace-
ment and crystal structure of Cu–BTC is kept intact under adopted
conditions. No any new peak appears in the pattern of Ag–Cu–BTC,
Fig. 2. The Raman spectra of samples (A): (a) Cu–BTC; (b) Ag–Cu–BTC(Ag content 2.76%); (c) Ag–Cu–BTC(Ag content 4.15%) and high resolution XPS of Ag 3d region for Ag–Cu–BTC(B).

Z. Sun et al. / Catalysis Communications 59 (2015) 92–96
95
Fig. 4. Recyclability of Ag–Cu–BTC for toluene selective oxidation.
Ag–Cu–BTC exhibits good recyclability: no noticeable decreases of tolu-
ene conversion or benzaldehyde selectivity can be observed during
4 cycle run. The ICP analyses can't detect any Ag or Cu in the solution
of reaction products. It indicates that Ag–Cu–BTC holds outstanding
stability and both framework Cu ions and exchanged Ag ions do not
leach out. Ag–Cu–BTC has much better stability than other ion ex-
changed catalysts because the exchanged ion was introduced to matrix
by coordinate bond rather than physical force.
4. Conclusions
In summary, a mixed-node MOF catalyst Ag–Cu–BTC was prepared
by PSE method and used in toluene selective oxidation to benzaldehyde.
Ag–Cu–BTC exhibits good catalytic performance: on the premise of
keeping highly selective catalysis of Cu–BTC, the introduction of Ag
greatly promotes toluene conversion. It demonstrates that PSE is a prac-
tical and powerful route to develop mixed-node MOF catalysts.
Acknowledgments
The financial support from National Basic Research Program of China
(2011CB201301) is gratefully acknowledged.
Fig. 3. SEM-EDAX mapping of Ag–Cu–BTC single particle: SEM image (A) and the corre-
sponding elemental distributions of Cu (B, green) and Ag (C, red). (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of
this article.)
References
[1] Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343 (2001) 393–427.
[2] R.K. Grasselli, Top. Catal. 21 (2002) 79–88.
[3] R. Curci, L. D'Accolti, C. Fusco, Acc. Chem. Res. 39 (2006) 1–9.
[4] F. Wang, J. Xu, X.-Q. Li, J. Gao, L.-P. Zhou, R. Ohnishi, Adv. Synth. Catal. 347 (2005)
1987–1992.
[5] J.-G. Lv, Y. Shen, L.-M. Peng, X.-F. Guo, W.-P. Ding, Chem. Commun. 46 (2010)
5909–5911.
[6] L. Kesavan, R. Tiruvalam, M.H. Ab Rahim, M.I. bin Saiman, D.I. Enache, R.L. Jenkins, N.
Dimitratos, J.A. Lopez-Sanchez, S.H. Taylor, D.W. Knight, C.J. Kiely, G.J. Hutchings,
Science 331 (2011) 195–199.
number of accessible active sites. So it can be concluded that single atom
activity of Ag is much higher than that of Cu. This may account for the
obvious increase of toluene conversion on Ag–Cu–BTC. A small amount
of framework Ag ions can promote the conversion of toluene. However,
the raise of toluene conversion becomes very slow when Ag exchange
degree achieves a certain value. Moreover, the selectivities of by-
products also increase with Ag exchange degree of catalyst. So there is
an appropriate Ag exchange degree to obtain satisfactory catalytic per-
formance. When Ag content is 2.76 wt.%, the conversion of toluene
reaches to 12.7% while the selectivity of benzaldehyde keeps at 99.0%
over Ag–Cu–BTC catalyst. The above results demonstrate that the intro-
duction of Ag indeed enhances the catalytic ability of Cu–BTC for toluene
selective oxidation to benzaldehyde.
The recyclability is also a vital character for a practical solid catalyst.
Other ion exchange catalysts such as ion exchange zeolite and ion ex-
change resin, usually deactive in reuse since the leaching of exchanged
ions. In this work, we examined the recyclability of Ag–Cu–BTC (Ag
content is 2.76 wt.%) in toluene selective oxidation by using the recov-
ered catalyst under the same reaction conditions. Fig. 4 reveals that
[7] W. Partenheimer, Catal. Today 23 (1995) 69–158.
[8] L.-H. Jia, S. Zhang, F.-N. Gu, Y. Ping, X.-F. Guo, Z.-Y. Zhong, F.-B. Su, Microporous
Mesoporous Mater. 149 (2012) 158–165.
[9] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 48 (2012) 11275–11288.
[10] K. Leus, Y.-Y. Liu, P. Van Der Voort, Chem. Rev. 56 (2014) 1–56.
[11] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38
(2009) 1450–1459.
[12] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606–4655.
[13] M. Ranocchiari, J.A. Bokhoven, Phys. Chem. Chem. Phys. 13 (2011) 6388–6396.
[14] C.N. Kato, M. Hasegawa, T. Sato, A. Yoshizawa, T. Inoue, W. Mori, J. Catal. 230 (2005)
226–236.
[15] Y.K. Hwang, D.-Y. Hong, J.-S. Chang, S.H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi,
C. Serre, G. Férey, Angew. Chem. Int. Ed. 47 (2008) 4144–4148.
[16] J. Kim, S. Bhattacharjee, K.-E. Jeong, S.-Y. Jeong, W.-S. Ahn, Chem. Commun. (2009)
3904–3906.
[17] S. Nayak, K. Harms, S. Dehnen, Inorg. Chem. 50 (2011) 2714–2716.

96
Z. Sun et al. / Catalysis Communications 59 (2015) 92–96
[18] A.D. Burrows, CrystEngComm 13 (2011) 3623–3642.
[23] M. Latroche, S. Surblé, C. Serre, C. Mellot-Draznieks, P.L. Llewellyn, J.-H. Lee, J.-S.
Chang, S.H. Jhung, G. Férey, Angew. Chem. Int. Ed. 45 (2006) 8227–8231.
[24] T. Loiseau, L. Lecroq, C. Volkringer, J. Marrot, G. Férey, M. Haouas, F. Taulelle, S.
Bourrelly, P.L. Llewellyn, M. Latroche, J. Am. Chem. Soc. 128 (2006) 10223–10230.
[25] P. Horcajada, S. Surblé, C. Serre, D.-Y. Hong, Y.-K. Seo, J.-S. Chang, J.-M. Grenèche, I.
Margiolaki, G. Férey, Chem. Commun. (2007) 2820–2822.
[19] Z.-Q. Wang, S.M. Cohen, Chem. Soc. Rev. 38 (2009) 1315–1329.
[20] S.M. Cohen, Chem. Rev. 112 (2012) 970–1000.
[21] R.K. Deshpande, G.I.N. Waterhouse, G.B. Jameson, S.G. Telfer, Chem. Commun. 48
(2012) 1574–1576.
[22] M. Kim, J.F. Cahill, H.-H. Fei, K.A. Prather, S.M. Cohen, J. Am. Chem. Soc. 134 (2012)
18082–18088.
[26] M. Hartmann, S. Kunz, D. Himsl, O. Tangermann, Langmuir 24 (2008) 8634–8642.