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ChemComm
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DOI: 10.1039/C7CC00467B
COMMUNICATION
Journal Name
o
(250 C in O2 flow)17, which indicated that the square planar formation of an active site was retarded during O2 activation
Cu2+ center itself is not an adsorption site for O2. Therefore, presumably due to an O-O cleavage of the intermediate site.
the decrease in the absorption at 16,700 cm-1 in our results This mechanistic difference led to the favorable formation of
was caused by a change in the nearby site that conferred the an active copper species during N2O activation at elevated
capability to adsorb oxidants. Reduced Cu+ species are the temperatures as a consequence of the lower negative change
most plausible adsorption site for oxidants, so these results in entropy.
mean that Cu+ species are in close proximity to the square
planar Cu2+ center.
This work was supported by the Center for C1 Gas Refinery
grant funded by the Korean government (Ministry of Science,
Spectroscopic study helped describe the formation of ICT & Future Planning, NRF-2016M3D3A1A01913255). This
copper species during oxidative activation by either N2O or O2 work was also supported by a NRF (National Research
(Scheme 1). The thermal desorption of H2O resulted in the Foundation of Korea) Grant funded by the Korean Government
formation of square planar Cu2+, reduced Cu+, and inactive (NRF-2016-Fostering Core Leaders of the Future Basic Science
copper oxo clusters. Under N2O activation, active copper oxo Program/Global Ph.D. Fellowship Program).
clusters readily formed with the disappearance of the former
two species, regardless of activation temperatures. In contrast,
Notes and references
under O2 activation, an intermediate site was formed, and
further activation of the O-O bond was required to produce
active copper oxo clusters.
1
M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs and R.
A. Schoonheydt, J. Am. Chem. Soc., 2005, 127, 1394.
J. S. Woertink, P. J. Smeets, M. H. Groothaert, M. A. Vance, B.
F. Sels, R. A. Schoonheydt and E. I. Solomon, Proc. Natl. Acad.
Sci., 2009, 106, 18908.
2
From
activation by N2O (Cu + N2O
(ΔΔH = ΔHN2O – ΔHO2 ≈ -80 kJ mol-1) than that by O2 (Cu + 1/2
O2 Cu-O). Therefore, by comparison with O2, it is possible to
a
thermodynamic point of view, the oxidative
↔
Cu-O + N2) is more exothermic
3
4
5
T. Sheppard, C. D. Hamill, a Goguet, D. W. Rooney and J. M.
Thompson, Chem. Commun., 2014, 50, 11053.
M. J. Wulfers, R. F. Lobo, B. Ipek and S. Teketel, Chem.
Commun., 2015, 51, 4447.
M.-L. Tsai, R. G. Hadt, P. Vanelderen, B. F. Sels, R. A.
↔
form active copper species at lower temperatures with N2O.3
In addition, the activation by N2O accompanies a lower
negative change in entropy (ΔS ≈ -28 J mol-1 K-1) than that by
O2 (ΔS ≈ -100 J mol-1 K-1). This made the activation by N2O
feasible even at elevated temperatures, but led to a decrease
in methanol production for O2-activated Cu-MOR.
Schoonheydt and E. I. Solomon, J. Am. Chem. Soc., 2014, 136
3522.
,
6
7
P. Vanelderen, B. E. R. Snyder, M.-L. Tsai, R. G. Hadt, J.
Vancauwenbergh, O. Coussens, R. A. Schoonheydt, B. F. Sels
and E. I. Solomon, J. Am. Chem. Soc., 2015, 137, 6383.
S. Grundner, M. A. C. Markovits, G. Li, M. Tromp, E. A. Pidko,
E. J. M. Hensen, A. Jentys, M. Sanchez-Sanchez and J. A.
Lercher, Nat. Commun., 2015, 6, 7546.
It is also notable that the formation of the intermediate site
via adsorption of O2 on Cu+ species accompanies
a
considerable negative change in entropy (ΔS
≈
- 200 J mol-1 K-
1). Recent theoretical work showed that O2 molecules adsorb
onto Cu+ sites in copper-exchanged zeolites with
approximately -100 kJ mol-1 of adsorption energy.20 According
to these values, the sign of the change in free energy is
8
9
S. Grundner, W. Luo, M. Sanchez-Sanchez and J. Lercher,
Chem. Commun., 2016, 52, 2553.
G. Li, P. Vassilev, M. Sanchez-Sanchez, J. A. Lercher, E. J. M.
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10 N. V. Beznis, B. M. Weckhuysen and J. H. Bitter, Catal.
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11 P. Tomkins, A. Mansouri, S. E. Bozbag, F. Krumeich, M. B.
Park, E. M. C. Alayon, M. Ranocchiari and J. A. VanBokhoven,
Angew. Chem. Int. Ed., 2016, 55, 5467.
12 A. I. Olivos-Suarez, À. Szécsényi, E. J. M. Hensen, J. Ruiz-
Martinez, E. A. Pidko, J. Gascon, ACS Catal., 2016,
13 G. Cruciani, J. Phys. Chem. Solids, 2006, 67, 1973.
o
reversed at 227 C, and thus equilibrium of the process shifts
to Cu+ species over this temperature (see Figure S8). The
equilibrium constant considerably decreases with increases in
temperature, and the value at 600 oC is 3.4 x 10-5. As a
consequence, the intermediate site is scarcely formed at high
6, 2965.
temperatures, which results in
a decrease in methanol
production for O2-activated Cu-MOR. The above descriptions
regarding the differences between N2O and O2 activation were
also supported by the effect of the partial pressure of oxidants
on methanol production (see additional discussion in
supporting information and Figure S9).
14 F. Giordanino, P. N. R. Vennestrøm, L. F. Lundegaard, F. N.
Stappen, S. Mossin, P. Beato, S. Bordiga and C. Lamberti,
Dalton Trans., 2013, 42, 12741.
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16 E. M. C. Alayon, M. Nachtegaal, A. Bodi, M. Ranocchiari and
J. van Bokhoven, Phys. Chem. Chem. Phys., 2015, 17, 7681.
17 P. Vanelderen, J. Vancauwenbergh, M.-L. Tsai, R. G. Hadt, E.
I. Solomon, R. A. Schoonheydt and B. F. Sels,
ChemPhysChem, 2014, 15, 91.
18 E. M. C. Alayon, M. Nachtegaal, A. Bodi and J. A. Van
Bokhoven, ACS Catal., 2014, 4, 16.
19 P. J. Smeets, R. G. Hadt, J. S. Woertink, P. Vanelderen, R. A.
Schoonheydt, B. F. Sels and E. I. Solomon, J. Am. Chem. Soc.,
2010, 132, 14736.
In summary, we found that proper activation conditions
(N2O flow at 600 oC) resulted in enhanced methanol
production from Cu-MOR, thanks to the efficient utilization of
copper species. The N2O-activated Cu-MOR showed an
increasing trend in methanol production with increases in the
activation temperature, whereas O2-activated Cu-MOR
showed a volcano-type plot over a range of temperatures. The
maximum methanol production for N2O activation was 97
μmol g-1, which is 1.5 times higher than that for O2 activation
(69 μmol g-1). UV-vis-NIR spectra showed the direct formation
of an active copper center for N2O activation, whereas the
20 S. Siahrostami, H. Falsig, P. Beato, P. G. Moses, J. K. Nørskov
and F. Studt, ChemCatChem, 2016, 8, 767–772.
4 | J. Name., 2012, 00, 1-3
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