The Role of Copper Speciation in the Low Temperature Oxidative Upgrading of Short Chain Alkanes over Cu/ZSM-5 Catalysts

Article abstract of DOI:10.1002/cphc.201701046

Partial oxidative upgrading of C₁–C₃ alkanes over Cu/ZSM-5 catalysts prepared by chemical vapour impregnation (CVI) has been studied. The undoped ZSM-5 support is itself able to catalyse selective oxidations, for example, methane to methanol, using mild reaction conditions and the green oxidant H₂O₂. Addition of Cu suppresses secondary oxidation reactions, affording methanol selectivities of up to 97 %. Characterisation studies attribute this ability to population of specific Cu sites below the level of total exchange (Cu/Al<0.5). These species also show activity for radical-based methane oxidation, with productivities exceeding those of the parent zeolite supports. When tested for ethane and propane oxidation reactions, comparable trends are observed.

Full text of DOI:10.1002/cphc.201701046

Accepted Article
Title: The role of copper speciation in the low temperature oxidative
upgrading of short chain alkanes over Cu/ZSM-5 catalysts
Authors: Robert Armstrong, Virginie Peneau, Nadine Ritterskamp,
Christopher Kiely, Stuart Taylor, and Graham John Hutchings
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10.1002/cphc.201701046
ChemPhysChem
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The role of copper speciation in the low temperature oxidative
upgrading of short chain alkanes over Cu/ZSM-5 catalysts
Robert D. Armstronga, Virginie Peneaua, Nadine Ritterskampa, Christopher J. Kielyb, Stuart. H. Taylora
and Graham J. Hutchingsa*
Abstract: Partial oxidative upgrading of C1 – C3 alkanes over
Cu/ZSM-5 catalysts prepared by chemical vapour impregnation (CVI)
has been studied. The undoped ZSM-5 support is itself able to
catalyse selective oxidations, for example, methane to methanol,
using mild reaction conditions and the green oxidant H2O2. Addition of
Cu suppresses secondary oxidation reactions, affording methanol
selectivities of up to 97 %. Characterisation studies attribute this ability
to population of specific Cu sites below the level of total exchange
(Cu/Al < 0.5). These species also show activity for radical- based
methane oxidation, with productivities exceeding those of the parent
zeolite supports. When tested for ethane and propane oxidation
reactions, comparable trends are observed.
In nature, methane monooxygenase (MMO) oxidises n-alkanes
selectively to alcohols under ambient conditions, over a μ-oxo
bridged diiron active site [7, 8]. Synthetic approaches to directly
oxidise these lower alkanes have been reported, and typically fall
into three categories; (i) gas phase systems – typically at high
temperatures and/ or pressures [9-16], (ii) biomimetic- structural
models of MMO active site [17, 18] and (iii) oxidation in acidic
media to produce methyl/ ethyl esters [19-23]. It should be noted
that transition metal- containing materials have also been
activated in N2O/O2 and subsequently used to oxidise alkanes,
though not in a truly closed catalytic cycle [24, 25].
It has recently been reported that ZSM-5 shows high intrinsic
activity in the low temperature oxidation of C1 – C3 alkanes with
hydrogen peroxide as oxidant [26-29]. Hammond et al. attributed
catalytic activity to trace amounts of iron, present as impurities
within the zeolite [26, 30]. These are shown to form catalytically
active extra-framework diiron- μ- oxo- hydroxo or oligomeric iron
complexes upon high temperature activation [26, 30]. Addition of
Cu2+ was shown to afford high selectivity to methanol (ca. 90%)
by inhibiting further oxidation to formic acid, with no overall
change in catalyst productivity [26]. This effect was shown when
Cu was introduced as a homogeneous additive, a supported co-
catalyst or supported upon ZSM-5 itself. Through EPR
spectroscopy experiments, this effect was shown to be due to a
lack of hydroxyl radicals when Cu2+ was present [26, 31]. A shift
in reaction selectivity was also reported for ZSM-5 (SiO2/Al2O3 =
30) catalysed ethane oxidation upon addition of Cu2+, though
rather than affording high ethanol selectivity as might be expected
from C1 studies, this led to lower acetic acid and high ethene
selectivity (> 30 %) [27]. Mechanistic studies showed that ethane
is activated to yield two primary products; ethylhydroperoxide and
ethanol, with ethene also indicated as a potential primary product.
These were shown to undergo catalytic oxidation to acetic acid,
with C-C scission yielding C1 oxygenates; formic acid,
methylhydroperoxide and methanol [27]. Oxygenate selectivity
was consistently high (ca. 95 %) [27]. Concordantly, high propene
selectivities were observed when the same catalysts were tested
for propane oxidation under comparable conditions [32].
Introduction
Finite crude oil reserves and increasingly environmentally-
conscious legislation are driving the search for alternate
feedstocks and routes for the synthesis of bulk chemical products.
Proposed as a transitional fuel for the move away from a
petroleum- derived economy, the valorisation of natural gas as a
chemical feedstock is therefore
a key field of research.
Comprising mainly methane and ethane, the direct oxidation of
these lower alkanes is hindered by their kinetically inert nature,
with C-H bond energies of 439.57 and 423.29 kJ mol-1 [1]. As a
result, commercialised technologies often use indirect routes and
are energy intensive. For instance, syngas is required for the
methanol synthesis process, and is produced through steam
reforming of methane [2]. Through carbonylation at high
pressures of carbon monoxide, methanol then yields an estimated
75% of global acetic acid (ca 7.8 Mt/annum) via the BP Cativa and
Celanese processes [3-5]. Meanwhile steam cracking of ethane
yields ethene, a precursor to ethanol, acetaldehyde, acetic acid
and polyethylene amongst other higher value products [6]. The
direct oxidation of short chain alkanes to partially oxygenated
products is therefore economically and environmentally
preferable to current practices.
[a]
[b]
Cardiff Catalysis Institute
School of Chemistry, Cardiff University
Park Place, Cardiff, UK, CF10 1AQ
Corresponding E-mail: Hutch@Cardiff.ac.uk
Department of Materials Science and Engineering,
Lehigh University, 5 East Packer Avenue, 18015-3195, Bethlehem,
Pennsylvania, USA
The aim of the present work is to study the role that Cu speciation
plays in effecting the performance of Cu/ZSM-5 catalysts. To this
end we characterise Cu/ZSM-5 catalysts using DR-FTIR, TEM,
TPR and NH3-TPD to identify specific Cu sites and correlate these
with catalyst performance. Through EPR trapping studies we then
aim to rationalise testing and characterisation data to determine
structure – function relationships. We aim to determine whether
Cu sites perform in the same way during the catalytic oxidation of
methane, ethane and propane with H2O2.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cphc.201701046
ChemPhysChem
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studies, a metal loading of 2.5 wt% Cu was employed [26], whilst
the SiO2/Al2O3 ratio was varied within the range of 23 – 280.
Reaction data at t = 0.5 h are presented in Table 1 (Entries 1-4).
Of the catalysts tested, 2.5% Cu/ZSM-5 (SiO2/Al2O3 = 23) showed
relatively high activity, with 42.8 μmol products formed and 97.4 %
selectivity towards methanol. This is compared with the previously
reported 2.5 % Cu/ZSM-5 (30), which yielded 8.4 μmol products
at 78.5 % methanol selectivity. Further increasing the SiO2/Al2O3
to 50 or 280 (at a fixed loading of 2.5 wt. % Cu) led to further
decreases in catalyst activity. Higher SiO2/Al2O3 showed a
significant change in colour post- reaction, with 2.5% Cu/ZSM-5
(30-280) turning yellow (Figure S2). This is consistent with
formation of reduced Cu species, potentially Cu-OOH. Notably,
2.5% Cu/ZSM-5 (23) showed no apparent colour change.
Alumina- free 2.5 % Cu/Silicalite-1 (Table 1, Entry 13) showed
relatively low productivity (4.3 μmol) and methanol selectivity
(52.9 %) despite a higher rate of H2O2 conversion. To rationalise
these data, proton form H-ZSM-5 (SiO2/Al2O3 = 23 or 30) were
assessed for activity (Table 1, Entries 5 and 9). Of these
unmodified zeolites, H-ZSM-5 (30) showed markedly higher rates
Results and Discussion
Studies into the selective oxidation of methane to methanol using
supported ZSM-5 (30) catalysts have reported increased reaction
selectivity in the presence of Cu2+, with no loss of reaction rate. In
that system EPR studies showed that Cu2+ was either (i)
suppressing the propagation of •OH or (ii) catalytically
sequestering ·OH and therefore affording high primary product
selectivity [26, 31]. Our previous studies into ZSM-5 (30) to
catalysed ethane oxidation determined that ethene was formed
under reaction conditions, with particularly high selectivity
observed upon testing of 2.5 Cu/ZSM-5 (30) and 1.25% Fe 1.25%
Cu/ZSM-5 (30) catalysts (30.2 and 34.2
%
selectivity
respectively)[27]. It was concluded that ethene could form through
two pathways; (i) direct H-abstraction from the alkane or (ii) a fully
concerted mechanism over [Fe=O]+ [27]. It was suggested that
ethane undergoes activation at the iron sites in ZSM-5 to yield
ethene. Ethene can then undergo oxidation or, in the presence of
Cu2+ desorb from the zeolite surface and diffuse to the liquid- gas
interface. Indeed, ethene was shown to undergo catalytic
oxidation under reaction conditions, yielding the reaction
products; formic acid, COx and CH3OOH/ CH3OH [27].
-1
of methane conversion (2.1 mol kgcat h-1) than H-ZSM-5
(23)(0.51 mol kgcat-1 h-1) which is consistent with previous studies
[26]. This despite ICP-MS analysis showing the calcined zeolites
to contain comparable levels of Fe contamination (140 and 120
ppm for ZSM-5 23 and 30 respectively). The trend towards
increasing oxygenate yield with decreasing SiO2/Al2O3 in Table 1,
indicates a potential role of the zeolite support in effecting not only
the catalytic activity but also selectivity. Indeed, this would be
consistent with the proposed extra-framework diiron- μ- oxo-
hydroxo active site being stabilised/ sited at AlO4- exchange sites
[26], the concentration of which increase with decreasing Si/Al.
The speciation of Cu would also be expected to differ upon
varying the composition of these MFI zeolites, as has been
extensively reported [35-37]. Copper species are known to
catalyse a myriad of reactions in the presence of H2O2 [38, 39],
therefore data in Table 1 might also indicate formation of a Cu
species which is capable of propagating radical- based methane
oxidation. To better understand the role which Cu sites might play
in effecting the differing rates of methane conversion and
methanol selectivities observed for Cu/ZSM-5 catalysts in Table
1, the physicochemical properties were fully characterised.
The reactivity of ethene under test conditions is further explored
in Table S1. Addition of Cu to either H-ZSM-5 (30) or 1.25%
Fe/ZSM-5 (30) led to no significant change in the rate of H2O2
conversion. However, when Cu was present, the rate of ethene
oxidation typically decreased. For example, deposition of 2.5 wt%
Cu onto H-ZSM-5 (30) effected a decrease in the rate of ethene
oxidation from 1.2 to 0.6 molEthene
kgcat h-1 (Table 1,
-1
converted
Entries 2 and 4). Also, whilst selectivity favoured C1 products, Cu-
containing catalysts afforded appreciable yields of acetic acid
(Table S1, Entries 5 and 7). Given that the oxidation of ethene to
acetic acid is well documented [33, 34] it can be concluded that
ethene is a primary product in this reaction. Indeed, the reactivity
of ethene in this system is comparable to that of methanol under
analogous conditions.
ZSM-5 catalysed alkane oxidation reactions
Transmission electron micrographs for 2.5% Cu/ZSM-5 catalysts
are shown in Figure 1. A strong support effect was observed for
this group of catalysts. With increasing SiO2/Al2O3 ratio we
observed a steady decrease in the mean particle size of
supported Cu species, from 9.0 nm for ZSM-5 (23) to 2.0 for ZSM-
5 (280). Across the same range the particle size distribution
Cu/ZSM-5 catalysts prepared by CVI were tested for the oxidation
of methane under mild aqueous conditions, utilising hydrogen
peroxide as the oxidant. Although higher rates of methane
activation might be attained through co-impregnation of the ZSM-
5 supports with Fe, the presence of Fe species would prevent
specific characterisation of Cu sites [26]. In line with previous
Table 1 Catalytic data for methane oxidation catalysed by Cu/ZSM-5 of varied SiO2/Al2O3 and Cu/Al ratios.
Product / μmol
MeOOH HCOOH
Total Product /
S(MeOH)
/ %
97.38
78.49
0.00
19.55
16.50
93.74
96.09
31.16
18.26
73.08
86.01
59.36
52.89
Entry
SiO2/ Al2O3
% wt Cu
Cu/Al ratio
χ H2O2 / %
μmol
42.82
8.37
0.52
2.2
MeOH
41.7
6.57
0.00
0.43
CO2
0.55
0.51
0.38
0.34
0.34
0.38
0.60
0.52
1.01
2.82
2.52
0.44
0.90
1
2
3
4
5
6
7
8
9
10
11
12
13a
23
30
50
280
23
23
23
23
30
30
30
30
n/a
2.5
2.5
2.5
2.5
0
0.4
1.25
5
0.41
0.59
0.76
4.08
n.a
0.07
0.20
0.82
n.a
0.09
0.29
1.18
n/a
0.57
1.29
0.14
1.43
5.43
2.14
1.43
0.43
14.57
9.29
4.29
1.71
1.14
0
0
0
0
0
0
0
0
7.43
0
8.68
2.48
1.83
11.40
0.08
1.08
2.10
6.76
13.14
7.96
2.10
11.52
20.55
6.91
1.14
40.23
51.89
1.38
28.15
44.96
48.83
5.29
37.71
49.86
0.43
0
5.14
0.4
1.25
5
32.86
42.00
3.14
0
0
0
2.5
4.33
2.29
Test conditions; 27 mg catalyst, 0.5 h, P(CH4) = 30 bar (0.03 mol), 50 oC, 1500 rpm, [H2O2] = 0.5 M (5000 µmol). a Catalyst – 2.5% Cu/Silicalite-1 (CVI)
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10.1002/cphc.201701046
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Figure 1 Representative transmission electron micrographs and corresponding particle size distributions for 2.5 wt% Cu supported on; ZSM-5 (23)
(a,b,c), ZSM-5 (30) (d,e,f), ZSM-5 (50) (g,h,i) and ZSM-5 (280) (j,k,l).
converged, with a standard deviation in particle size of 4.20 nm
and 0.44 nm for ZSM-5 (23) and ZSM-5 (280) respectively. Such
an effect has not, to our knowledge, been reported for Cu/ZSM-5
catalysts. We attribute this relationship between alumina content
and copper oxide particle growth to varying degrees of interaction
between the hydrophobic Cu (acac)2 precursor and the zeolite
surface. With increasing SiO2/Al2O3 ratio, ZSM-5 becomes more
hydrophobic and this would lead to a stronger interaction with Cu
(acac)2. Owing to the lack of solvent, and associated diffusion and
pH effects, this might therefore be unique to the CVI methodology.
Such particle size- control presents an intriguing avenue for future
research, assuming that it might be generalised to other supports
and metal- acetylacetonates. Average particle size appears to
correlate with the activity and selectivity of these catalysts,
however a role for exchanged cationic copper species cannot be
excluded.
Figure 2. All samples exhibit a low temperature reduction (R1) at
ca. 183 – 213 C, which has been reported to comprise of two
o
simultaneous reductions (i) reduction of isolated Cu2+ ions and
Cu2+ oxocations to Cu+ and (ii) reduction of Cu2+ in CuO to Cu0
[40-45]. 2.5% Cu/ZSM-5 catalysts present a second high
temperature peak (R2), not shown for the Silicalite-1 catalyst,
which is attributed to the reduction of Cu+ ions to Cu0 [40, 41, 44-
47]. The maxima of R1 and R2 shifted to a higher temperature with
decreasing SiO2/Al2O3. This is generally ascribed to a change in
dispersion and increasing interaction between Cun+ species with
framework oxygen as the zeolite matrix becomes more negatively
charged (higher Al content) [41, 44, 46]. The increasing breadth
of R2 with decreasing SiO2/Al2O3 suggests the presence of
oligomeric Cu species of varying nuclearity [45] and is consistent
with the broadening particle size distribution of surface oxides
shown in Figure 1.
As H2-TPR studies suggested the presence of exchanged Cu2+
species, the occupation of exchange sites was confirmed through
H2-TPR was employed to probe the reducibility of Cu species in
the 2.5% Cu/MFI zeolite catalysts. H2 uptake plots are shown in
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10.1002/cphc.201701046
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Cun+ speciation in Cu/ZSM-5 has long been studied due to its
activity in the catalytic decomposition or reduction of NOx, with
highly reducible, isolated Cu2+ found to be the preferred active site
[35, 37, 56-58]. In probing ion exchanged Cu sites with NO,
spectroscopically distinct N-O IR stretching bands have been
identified, and shown to arise from binding to Cu2+ in differing
coordination environments [35, 37, 47, 59-63]. Four principle N-
O-Cu2+ binding modes have been described, differing in geometry
(e)
(d)
(c)
(b)
(a)
-
and the number of local AlO4 exchange sites. These consist of
-
exchanged divalent Cu2+ of; square pyramidal (AlO4 pair, 1913
-
cm-1, denoted Cu-I), square planar (single AlO4 , 1895 cm-1, Cu-
-
II), square pyramidal (AlO4 pair, 1921 cm-1, Cu-III) and
unassigned (low intensity, 1906 cm-1, Cu-IV) geometries [36, 59].
Additional bands at 1813 cm-1 (monovalent Cu+-NO species) and
1875 cm-1 (low intensity, gaseous NO) have also been reported
[35, 36, 64].
50
150
250
350
450
550
650
750
Temperature / oC
Figure 2 H2-TPR profiles for 2.5% Cu/ MFI zeolite catalysts prepared by
chemical vapour impregnation. (a) ZSM-5 (23), (b) ZSM-5 (30), (c) ZSM-5 (50),
(d) ZSM-5 (280) and (e) Silicalite-1
NH3-TPD and FTIR in the O-H stretching region. TPD plots and
FTIR spectra for H-ZSM-5 and 2.5% Cu/ZSM-5 catalysts
(SiO2/Al2O3 = 23, 30, 50 and 280) are presented in Figure 3. Two
key NH3 desorptions were observed for all catalysts in Figure 3i,
centred at ca. 270 and 450 oC. The lower temperature desorption
is attributed to adsorption at weak acid sites (Brønsted and Lewis)
with the high temperature desorption unequivocally assigned to
NH3 chemisorbed at strongly acidic Brønsted sites [48-54]. All
zeolites showed decreased NH3 capacity following Cu
impregnation, averaging a loss of 26.0 % (± 5.4 %) of total strong-
Brønsted sites. This was supported by DRIFTS spectra in Figure
3ii which showed decreased intensity of the peak at 3607 cm-1
(OH groups coordinated to Td framework Al3+) for each ZSM-5
following Cu impregnation [55]. NH3-TPD and DRIFTS therefore
suggest siting of Cun+ at AlO4- exchange sites.
Figure 4 IR bands of NO adsorbed onto 2.5 wt% Cu/ZSM-5 of varying
SiO2/Al2O3. SiO2/Al2O3 being (a) 23, (b) 30, (c) 50 and (d) 280.
3653
(i)
(ii)
Spectra acquired following NO saturation of 2.5% Cu/ZSM-5
(SiO2/Al2O3 = 23-280) are shown in Figure 4. The absolute
absorbance decreased with increasing SiO2/Al2O3, suggesting a
decreasing concentration of exchanged Cu species. Indeed, this
was consistent with NH3-TPD results which showed occupation of
ca. 26% of strong Brønsted sites for all zeolites, independent of
total acidity. Curved fitted spectral bands for 2.5% Cu/ZSM-5 (23)
(a) and 2.5% Cu/ZSM-5 (30) (b) are presented, whilst absorbance
in (c) and (d) wasn’t sufficiently intense to allow for objective fitting.
Four bands were identified in (a) and (b), centred at; 1915, 1895,
1874 and 1811 cm-1. The distribution of Cu sites shifted with an
increasing Cu/Al ratio (0.41 and 0.59 for ZSM-5 23 and 30
respectively) with increased relative intensity of bands at 1895 cm-
3736
3607
(h)
(g)
(f)
(e)
(d)
(c)
1
and 1811 cm-1 over that at 1913 cm-1. This is consistent with
-
previous reports which showed that Cu-I type sites (AlO4 pair,
1913 cm-1) are favourable at low Cu/Al ratios, with Cu-II (1895 cm-
1) becoming dominant when overexchanged at Cu/Al > 0.5. It has
also been reported that Cu-II type species undergo more facile
reduction due to the low local negative charge, which results from
being bonded to an isolated AlO4- site [35, 65]. In fact, the band
at 1811 cm-1 (Cu+-NO) is believed to result from autoreduction of
Cu-II (1895 cm-1), resulting from dehydration during the pre-
treatment [35, 66].
(b)
(a)
200
400
600
800
3900
3600
3300
3000
Temperature / oC
Wavenumber / cm-1
Figure 3 NH3-TPD traces (i) and DRIFTS spectra in the OH vibrational region
(ii) for (H-ZSM-5, 2.5% Cu/ZSM-5) respectively where SiO2/Al2O3 = 23 (a,b), 30
(c,d), 50 (e,f) and 280 (g,h). For (i) dashed lines – peak fit of high temperature
desorption.
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10.1002/cphc.201701046
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ARTICLE
These materials were tested for catalytic activity in methane
oxidation and testing data is shown in Table 1. At Cu loadings of
0.4 and 1.25 wt. %, ZSM-5 (23) and ZSM-5 (30) catalysts show
comparable productivities. Indeed, at 1.25 wt. % Cu, 51.9 μmol
and 48.8 μmol of products were observed for ZSM-5 (23) and
ZSM-5 (30) respectively. Methanol selectivity was higher for the
more aluminous catalyst (96.1 % vs 86.0 %). Methane conversion,
though low, increased with Cu loading for both zeolites which is
consistent with copper’s ability to catalyse H2O2 conversion [69-
71]. However once the Cu/Al ratio exceeded 0.5, both methane
conversion and methanol selectivity were observed to decrease
significantly. To determine whether the step change in activity
observed between loadings of 2.5 and 5 wt. % Cu was due to pore
blocking by Cu species, N2 adsorptions studies were carried out
(Table S2). A steady decrease in surface area was observed with
increasing Cu loading, from 423 m2g-1 for the parent zeolite to 259
m2g-1 at 5 wt. % Cu loading (Table S2, Entries 1 and 5). Despite
showing markedly different catalytic performance, 2.5% and 5 %
Cu/ZSM-5 (23) catalysts show similar micropore volumes (0.10
and 0.09 cm3g-1 respectively), suggesting that pore blockage is
not a key factor.
Figure 5 H2-TPR profiles for Cu/ ZSM-5 of varying Cu/Al. (a) 0.4% Cu/ZSM-5
(23), (b) 1.25% Cu/ZSM-5 (23), (c) 2.5% Cu/ZSM-5 (23), (d) 5% Cu/ZSM-5 (23),
(e) 0.4% Cu/ZSM-5 (30), (f) 1.25% Cu/ZSM-5 (30), (g) 2.5% Cu/ZSM-5 (30), (h)
5% Cu/ZSM-5 (30)
Clearly the nature of supported Cu species, both surface oxides
and ion exchanged, varied greatly with SiO2/Al2O3 ratio (at a
constant 2.5 wt % loading). This made determining the exact
catalytic site difficult. Added to this, one had to consider the
hydrophobic/ hydrophilic nature of the support itself as rapid
diffusion of products out of the zeolite pores is expected to afford
higher selectivity towards primary products. Indeed it has been
shown that a hydrophobic-hydrophilic shift occurs at a SiO2/Al2O3
ratio of ca. 20 [67, 68]. To decouple the effects of zeolite
hydrophilicity, surface oxide dispersion and exchange capacity,
H-ZSM-5 with a SiO2/Al2O3 of 23 and 30 were impregnated with
0.4 – 5 wt % loadings of Cu.
3653
3653
3736 3607
3736 3607
(a)
(b)
5%
5%
2.5%
2.5%
Figure 7 IR bands of NO adsorbed onto Cu(II) and Cu(I) sites in Cu/ZSM-5 of
varying Cu/Al. (a) 1.25% Cu/ZSM-5 (23), (b) 1.25% Cu/ZSM-5 (30), (c) 2.5%
Cu/ZSM-5 (23), (d) 2.5% Cu/ZSM-5 (30), (e) 5% Cu/ZSM-5 (23) and (f) 5%
Cu/ZSM-5 (30).
1.25%
1.25%
0.4%
0%
0.4%
0%
Cu/ZSM-5 (23) and (30) catalysts of increasing Cu wt. loadings
were studied using H2-TPR (Figure 5), at constant molCu. As in
Figure 2, two main reduction peaks were observed, at ca. 194 –
3900
3600
3300
3000
3900
3600
3300
3000
o
o
308 C (R1) and 310 – 507 C (R2), which may be assigned as
previously. Consistent with Figure 4, the Tmax for both reduction
peaks shifted to a lower temperature with increasing Cu/Al (either
through increased Cu loading or decreased Al content). At the
highest loading (5 wt % Cu) a third reduction event was observed,
with Tmax of 256 and 233 oC for ZSM-5 (23) and ZSM-5 (30)
respectively. This is assigned as a low temperature Cu+- Cu0
Wavenumber / cm-1
Wavenumber / cm-1
Figure 6 DRIFTS spectra in the OH vibrational region for Cu/ZSM-5 catalysts
of varying Cu loading on (a) H-ZSM-5 (23) and (b) H-ZSM-5 (30). (30), (g) 2.5%
Cu/ZSM-5 (30), (h) 5% Cu/ZSM-5 (30)
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10.1002/cphc.201701046
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reduction where Cu2+ of type Cu-II have undergone auto-
reduction to Cu+ during the pretreatment. Cu/ZSM-5 (23) catalysts
of 0.4 and 5.0 wt. % Cu loading were analysed using HRTEM.
Representative micrographs are shown in Fig. S3. At 0.4 wt. %
Cu loading (Fig S3a) no discreet nanoparticles were observable.
However, upon prolonged exposure to the electron beam (Fig S4)
apparent nanoparticle formation was observed. This indicates (a)
sintering of Cu- species of a size below the sensitivity of the
microscope (sub 1 nm), or (b) migration of nanoparticles from
deep within the zeolite pores. At 5.0 wt. % Cu loading, a mean
particle size of 4.78 nm was observed (Fig. S3a/c). Consideration
of particle size data from Figures 1 and S3 alone, with catalyst
performance in Table 1 is paradoxical. At a fixed 2.5 wt. % Cu
loading, decreasing average NP size (increasing SiO2 / Al2O3)
correlated with decreasing catalytic activity (Fig. 1). Conversely,
a decrease in activity is observed with increasing Cu loading
despite apparent increases in average NP size (Fig S3). This
contradiction further indicates that the nature/ distribution of ion
exchanged Cu- species is the key feature which effected the
trends observed in Table 1 and not Cu-oxide crystallite size.
To better understand the mechanism by which Cu2+ sites affect
both methane conversion and methanol selectivity, EPR
spectroscopy radical trapping studies were performed. EPR
revealed that significant quantities of hydroxyl radicals are
produced over 2.5 % Cu/ZSM-5 (23) (Figure 8a). This is surprising,
as previous studies had correlated termination of such radicals
over Cu sites with high methanol selectivity. Spectra for reactions
over 2.5 % Cu/ ZSM-5 where SiO2/Al2O3 = 30 and 280 are shown
in Figures 8b and 8c. Spectrum 8b is consistent with previous
EPR DMPO trapping studies over the same catalyst [31], showing
-
weak signals, which were previously attributed to superoxide (O2 )
radicals. As previously discussed, the hydrophobic/ hydrophilic
nature of the represented ZSM-5 isomorphs differ significantly. To
further explore the relationship between supported species and
radicals generated, without varying hydrophobicity, the spin trap
(DMPO) was added to methane oxidation reactions catalysed by
1.25 % Cu/ZSM-5 (30). This catalyst, with a Cu/Al ratio = 0.2
showed significantly higher methane conversion rates (3.3 mol
kgcat-1 h-1) and methanol selectivity (86.0 %) than 2.5 % Cu/ZSM-
5 (30) (Si/Al = 0.59, 0.6 mol kgcat-1 h-1, 78.5 % methanol selectivity)
at near iso-conversion of H2O2 (Table 1, Entries 2 and 11). The
EPR spectrum for methane oxidation catalysed by 1.25 %
Cu/ZSM-5 (30) is shown in Figure 9a. This is comparable to that
of 2.5 % Cu/ZSM-5 (23) in Figure 8a, showing significant DMPO-
·OH adduct formation. EPR studies therefore suggest that at a
Cu/Al of < 0.5, supported Cu/ZSM-5 catalysts behave differently
to the previously reported 2.5 % Cu/ZSM-5 (30) (Cu/Al = 0.59)
system. When the Cu/Al is less than 0.5 (Cu-I species dominant),
addition of copper has a beneficial effect upon both methane
conversion rates and methanol selectivity, with EPR radical
trapping studies showing these systems to contain appreciable
quantities of hydroxyl radicals. These data suggest that (i) ·OH
are generated at and relatively stable over Cu-I sites and that
either (ii) Cu-I species act as active sites for selective methane
oxidation or (iii) Cu-I sites propagate generation of ·OH, with
methane activation occurring on a secondary active site within the
catalyst. Conversely, the previously reported DFT- derived
mechanism for ZSM-5 catalysed methane oxidation with H2O2
involved a concerted, molecular active site with no input of oxygen
based radicals [26].
DRIFTS spectra for these catalysts are shown in Figure 6. These
clearly show a decrease in the intensity of the peak at 3607 cm-1
(OH groups coordinated to Td framework Al3+) with increasing Cu
loading, and this is consistent with a gradual increase in the
degree of cation exchange/ population of Cun+ species [55].
(c)
(b)
(a)
(b)
(a)
3300
3320
3340
[G]
3360
3380
Figure 8 DMPO spin- trapping data obtained during a methane oxidation
reaction catalysed by (a) 2.5 % Cu/ZSM-5 (23), (b) 2.5 % Cu/ZSM-5 (30) and
(c) 2.5% Cu/ZSM-5 (280) Reaction conditions; 27 mg catalyst, 50 oC, 5 min,
[H2O2] = 0.5 M (5,000 μmol), 1500 rpm P(CH4) = 30 bar.
As before, the distribution of Cun+ sites was probed using NO-
DRIFTS. Spectra for loadings of 1.25, 2.5 and 5 wt % Cu/ ZSM-5
(23 and 30) are shown in Figure 7. For both ZSM-5 (23) and (30)
an increase in Cu loading was accompanied by increased
absorbance and a shift in the distribution of Cun+ species.
Increased Cu/Al led to a decrease in the ratio of Cu-I/Cu-II (1915
cm-1 / 1895 cm-1) and an increase in the population of Cu+ sites
(1811 cm-1, attributable to reduced Cu-II type sites). In fact, above
a Cu/Al ratio of 0.5, population of these highly reducible square
3300
3320
3340
3360
3380
[G]
Figure 9 DMPO spin- trapping data obtained during a methane oxidation
reaction catalysed by (a) 1.25 % Cu/ZSM-5 (30) and (b) 2.5 % Cu/ZSM-5 (30)
Reaction conditions; 27 mg catalyst, 50 oC, 5 min, [H2O2] = 0.5 M (5,000 μmol),
1500 rpm P(CH4) = 30 bar.
-
planar Cu2+ sites (associated with a single AlO4 site) became
dominant. The shift in speciation of Cu corresponds with the drop
in methane conversion and methanol selectivity (shown in Table
1) at Cu/Al > 0.5.
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10.1002/cphc.201701046
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ARTICLE
Table 2 Catalytic data for ethane oxidation catalysed by Cu/ZSM-5 of varied SiO2/Al2O3 and Cu/Al ratios.
χ
Product Selectivities / %
SiO2/
Entry
wt%
Cu
Cu/Al
ratio
χ H2O2
/ %
C2H6
/ %a
0.11
0.07
0.04
0.06
0.01
0.08
0.10
0.04
0.06
0.20
0.23
0.06
0.09
Al2O3
CH3COOH
EtOH
CH3CHO
EtOOH
MeOOH
MeOH
HCOOH
C2H4
CO2
1
2
23
30
50
280
23
23
23
23
30
30
30
30
n/a
2.5
2.5
2.5
2.5
0
0.4
1.25
5.0
0
0.4
1.25
5.0
2.5
0.41
0.59
0.76
4.08
n.a
0.07
0.20
0.82
n.a
0.09
0.29
1.18
n/a
6.0
13.4
19.0
23.2
4.9
30.2
34.1
19.0
10.5
34.6
22.8
25.7
38.7
24.0
24.7
27.5
24.0
23.9
8.2
13.7
44.4
47.6
14.8
5.1
4.9
6.2
36.7
7.7
7.9
0.6
1.0
3.2
7.0
14.8
4.2
0.0
1.6
4.0
4.1
0.00
1.3
0.7
2.3
2.5
0.7
0.3
3.0
1.5
0.1
6.2
1.6
1.7
5.0
1.3
1.4
1.6
2.5
3.7
3.2
3.6
1.5
0.0
0.0
4.0
2.3
4.9
0.4
0.4
0.8
2.5
2.4
2.8
1.8
1.1
51.7
27.1
0.9
0.9
2.4
2.7
2.0
10.0
2.2
1.5
3.6
1.3
0.7
0.5
2.9
1.5
33.2
33.9
18.4
31.9
14.2
6.9
14.1
33.7
25.3
17.8
14.0
33.2
47.1
3
4
5
6
0.6
10.6
53.4
55.0
14.3
2.3
45.0
41.6
22.6
12.8
9.3
7.7
7
8
9
2.8
18.6
17.7
14.6
14.1
20.4
16.4
13.9
10.1
0.4
2.1
9.6
10
11
12
13a
12.0
35.8
5.2
a Conversion based on carbon. b 2.5% Cu/Silicalite-1 prepared by CVI. Test conditions; 27 mg catalyst, 0.5 h, P(C2H6) = 20 bar, 50 oC, 1500 rpm, [H2O2] =
0.5 M (5000 µmol).
propane = 0.36 %), no propene was observed in the GC-FID
(sensitive to < 1 ppm), with selectivity favouring IPA (38.4 %
To further study the structure- function relationships in these Cu-
MFI catalysts, they were assessed for activity in ethane oxidation,
which our previous studies showed to be mediated at least in part
by oxygen-based radicals [27]. Trends in Table 2 are consistent
with those in Table 1. At a fixed 2.5 wt. % Cu loading, ZSM-5 (23)
affords the highest rate of C2H6 conversion (1.6 mol kgcat-1 h-1) and
relatively high ethene selectivity (51.7 %). These metrics
decrease with increasing SiO2/Al2O3. As with methane, for a
specific ZSM-5 support, the rate of ethane conversion increases
with increasing Cu loading until Cu/Al = 0.5. Ethane conversion
then decreases significantly upon over- exchange with Cu, as
does ethene selectivity (from 55.0 % to 14.3% in Table 2 Entries
7 and 8). Trends in selectivity/ reaction rate are observed at longer
times on line (Figure S5). This further implicates ethene as a
primary product of ethane oxidation under these reaction
conditions.
selectivity) and the rate of H2O2 conversion was an order of
magnitude greater. As with ethane, the catalytic oxidation of
propane under these reaction conditions was previously shown to
be at least partially dependent upon oxygen-based radicals [32,
72].
The catalysis and characterisation studies reported herein show
strong trends between Cu/ZSM-5 catalysed oxidative upgrading
of C1-C3 alkanes in water using hydrogen peroxide as oxidant. It
is clear that Cu speciation plays a key role in achieving high
primary product selectivity, be it to methanol, ethene or propene.
Whilst H-ZSM-5 (SiO2/Al2O3 = 30) shows the highest intrinsic
activity of proton form zeolites, following modification with Cu,
ZSM-5 (23) shows appreciable rates of alkane oxidation and,
crucially, high reaction selectivity. The production of ethene/
propene in this way using H2O2 is academically interesting, and
provides mechanistic insight into the performance of these
catalysts under methane oxidation conditions. The performance
of the copper catalysts exceeds that of previous reports [26, 30],
crucially due to use of catalysts with a Cu/Al ratio of < 0.5.
Spectroscopic and temperature programmed studies show that
this favours formation of Cu-I sites and this correlates with
enhanced catalyst productivity (relative to the parent proton-form
of the zeolite). Over exchanging ZSM-5 (Cu/Al > 0.5) leads to
population of spectroscopically distinct Cu-II sites. This shift from
a majority of Cu-I to Cu-II species is associated with: lower rates
of alkane oxidation, lower primary product selectivity and
increased rates of H2O2 conversion. This relationship between
Cu/Al (at fixed 2.5 wt. % Cu) and reaction productivity for the
catalytic oxidation of short chain alkanes is illustrated in Figure 10.
It is therefore clear that Cu-I sites have bi-functionality:
When assessed for activity in the partial oxidation of propane,
under similarly mild conditions, trends were again consistent with
those in Tables 1 and 2. Indeed Entries 1-4 in Table 3 show similar
trends to entries 1-4 in Table 2. At a fixed 2.5 wt. % Cu loading,
ZSM-5 (SiO2/ Al2O3 = 23) showed the highest propane conversion
(0.39 %) and relatively high alkene selectivity (S(C3H6 = 59.7)).
Both metrics decreased at a Cu/Al > 0.5, with 2.5 % Cu/ZSM-5
(30) affording 0.26 % propane conversion and markedly lower
propene selectivity (20.8 %). At longer times on line, and
consequently higher ethane conversion, the same trends in
ethane conversion rate and reaction selectivity were observed as
shown in Figure S4. From a mechanistic perspective it is
interesting to note that whilst 2.5% Cu/Silicalite-1 showed
comparable propane conversion to 2.5 % Cu/ZSM-5 (23) (χ
Table 3 Catalytic data for propane oxidation catalysed by 2.5% Cu/ MFI zeolites of varied SiO2/Al2O3 ratios.
C3 Selectivities/ %
C2 Selectivities
C1 Selectivities
Formic
χ
χ
SiO2/
Al2O3
Entry
C3H8
/ %
H2O2
/ %
2.7
1.7
8.6
Propanoic
Acetic
Acid
1.1
1.0
6.7
7.7
0.0
Other
Acetone
IPA
PrOH
C3H6
EtOH
C2H6
C2H4
MeOH
CO2
a
Acid
2.3
3.2
0.0
0.0
7.9
Acid
0.1
4.0
0.0
0.0
4.2
1
2
3
23
30
50
280
n.a
0.39
0.26
0.06
0.06
0.36
8.4
9.3
18.6
25.3
19.7
0.0
19.8
0.0
18.6
18.8
6.9
59.7
20.8
9.7
11.8
0.0
0.0
1.4
7.4
5.8
12.6
33.3
2.3
1.7
2.1
0.3
0.6
1.0
1.0
2.5
1.9
4.4
0.6
1.1
1.2
9.4
5.4
1.5
0.4
3.2
5.8
5.3
0.9
4
0.0
19.6
17.7
14.8
26.5
5b
38.4
18.3
0.0
7.6
Test conditions; 27 mg catalyst, 0.5 h, P(C3H8) = 4 bar, P(Total/ C3H8/ Helium) = 20 bar, 50 oC, 1500 rpm, [H2O2] = 0.5 M (5000 µmol). a Selectivity (CO + CH4), b
Catalyst – 2.5% Cu/Silicalite-1 (CVI)
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10.1002/cphc.201701046
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ARTICLE
suppressing over-oxidation of methanol to formic acid and
increasing catalyst productivity. Once population of Cu-II sites
becomes preferable, over Cu-I, further oxidation of methanol is
still suppressed, though these sites are associated with
decreased rates of methane oxidation coupled with increased
H2O2 conversion rates.
chemical vapour impregnation. The procedure for preparation of 2.5 wt%
Cu/ZSM-5 through CVI is as follows; a known mass of H-ZSM-5 (typically
3.5 g) was dried at 150 oC for 2 h under continuous vacuum. Once dried,
the desired mass of H-ZSM-5 (1.95 g) was added to a Schlenk flask and
Cu(II) acetylacetonate (Cu (acac)2) (Sigma Aldrich, 99.9% purity, 0.206 g,
0.787 mmol) added. Following physical mixing of the metal precursor and
zeolite, the dry mixture was heated to 140 oC under continuous vacuum
(ca. 10-3 mbar) for 1 h. The sample was then allowed to cool to ambient
temperature and calcined (550 oC, 20 oC min-1, 3h) in static air.
Conclusions
Alkane oxidation
Cu/ZSM-5 catalysts are active for liquid phase oxidative
upgrading of short chain alkanes, with H2O2 under mild conditions.
In the present work, we have shown that both the activity and
selectivity of these catalysts correlates strongly with the Cu/Al
ratio, and thereby the degree of ion exchange. Catalytic
performance correlates strongly with the speciation of exchanged
Cu- species as determined by characterisation studies. Optimal
conversion and primary product selectivity (that is; methanol,
ethene and propene for C1, C2 and C3 alkanes respectively) were
observed when the speciation of Cu is dominated by so- called
Cu-I sites. These are favoured when the zeolite catalyst’s Cu/Al
ratio is below the level of total exchange (Cu/Al < 0.5). Once over-
exchanged, catalyst performance and selectivity decreases, and
this coincides with formation of spectroscopically distinct, Cu-II
sites. This study provides insight as to the nature of the Cu sites
within ZSM-5 that afford the high methanol selectivities, which
have previously been reported and goes further, identifying Cu
sites that are capable of effecting increased reaction yields.
Indeed, 1.25 wt. % Cu/ZSM-5 (23) yielded 49.86 μmol MeOH in
0.5 h. This equated to 96.1 % selectivity towards methanol and a
methanol productivity of 3.7 mol kgcat-1h-1. Furthermore,
accounting for the intrinsic activity of the ZSM-5 (23) support, this
catalyst showed an apparent TOF (based on Cu) of 16.9 mol
Catalyst performance assessed in a 50 mL Teflon lined Parr autoclave
reactor. A procedure for ethane oxidation reactions is described here. The
reactor was charged with H2O2 (0.5 M, 5000 µmol) and catalyst added (27
mg). Once sealed, the reactor was purged 3 times with the reactant gas
(20 bar) to remove residual gasses. The reactor was then charged with
one of; methane (30 bar, CH4, Air Products 99.9%, 0.03 mol CH4), ethane
(20 bar C2H6, Air Products 99.9%, 0.02 mol C2H6) or propane (P C3H8 = 4
bar, P He = 16 bar, BOC, 0.004 mol). The reactor was then heated to the
reaction temperature (50 oC) under vigorous stirring (1500 rpm). Once the
set point temperature was reached, the reaction was carried out for the
desired time. The autoclave was then cooled to ca. 10 oC to minimise loss
of volatile products. Post reaction, unreacted hydrogen peroxide was
quantified through titration with acidified Ce (SO4)2 (8 x 10-3 mol dm-3) of
known concentration, with a Ferroin indicator. Aqueous products were
quantified with solvent suppressed 1H NMR at ambient temperature on a
Bruker Ultrashield 500 MHz spectrometer using a TMS/ CDCl3 internal
standard. Gaseous products were quantified using a Varian 450-GC fitted
with a CP-Sil 5CB capillary column (50m length, 0.32mm diameter, carrier
gas = He), a methaniser unit and both FID and TCD detectors.
For EPR radical trapping studies, the reactor was charged with methane
as previously described, with the addition of 5,5-Dimethyl-1-pyrroline N-
oxide (0.05 g, Sigma Aldrich, > 97%). Reactions were carried out for 5 min
at 50 oC, after which time the reactor was depressurised, opened and
solution filtered prior to analysis. EPR spectra were collected at 298 K in a
high sensitivity cavity (Bruker ER 4119 HS) Bruker EMX spectrometer,
field modulation 100 kHz, microwave power 1 mW.
molCu-1h-1. Owing to the complexity of reactions
methane converted
between Cu and H2O2, the specific mechanisms through which
copper sites effect the radical- reactions involved has not been
addressed in this article but will form the basis for a future
publication.
Catalyst Characterisation
H2-TPR was carried out using a TPDRO 1100 series analyser. Samples
(equivalent to 0.03 mmol Cu) were pre-treated for 1 h at 130 oC (20 oC min-
1) in a flow of Argon (20 ml min-1). Following this the gas flow was changed
to 10% H2 / Argon and the temperature was ramped to 800 oC (10 oC min-
1). Hydrogen uptake was monitored using a TCD.
Acknowledgements
We thank Cardiff University for financial support. We would like
to thank the Cardiff Electron Microscopy Facility for the HRTEM
analysis.
NH3-TPD was carried out using a CHEMBET TPR/TPD chemisorption
analyser, Quantachrome Industries fitted with a TCD. 50 mg of sample
o
was pre-treated for 1 h at 130 oC (15 C min-1) in a flow of helium (80 ml
min-1). The sample was then cooled to ambient temperature and ammonia
flowed through for 20 min to ensure saturation. The system was then
heated 1 h at 100 oC (15 oC min-1) under a flow of helium (80 ml min-1) to
remove physisorbed ammonia. Subsequently, chemisorbed ammonia was
desorbed by heating to 900oC (15 oC min-1) in a flow of helium (80 ml min-
1) during which period desorbed ammonia was monitored using a TCD,
current 180 mV, attenuation 1.
Experimental
Catalyst Preparation
Supported metal catalysts were prepared by chemical vapour
impregnation (CVI). This methodology has previously been discussed in
detail [27, 73].
TEM images were collected on a JEOL JEM-2100 transmission electron
microscope fitted with a LaB6 filament. Samples were prepared by
dispersing powdered catalyst in high purity ethanol, before adding a drop
NH4 ZSM-5 (Zeolyst) was calcined in a flow of air (550 oC, 20 oC min-1, 3h)
to yield H-ZSM-5. The proton form zeolite was then either; activated in
static air prior to testing (550 oC, 20 oC, 3h), or impregnated with Cu via
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10.1002/cphc.201701046
ChemPhysChem
ARTICLE
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Entry for the Table of Contents (Please choose one layout)
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ARTICLE
Coppers vs Alkanes: Partial
oxidation of short chain alkanes is a
key challenge for the chemical
Robert D. Armstrong, Virginie Peneau,
Nadine Ritterskamp, Christopher J.
Kiely, Stuart. H. Taylor and Graham J.
Hutchings*
sciences. Recently, a number of
articles have reported C1-C3 alkane
activation over Cu/ZSM-5 catalysts
with H2O2. In this article, the specific
Cu- sites which afford high product
selectivity are identified through
correlation of catalytic performance
with the distribution of Cu-sites, as
determined by spectroscopic and
temperature- programmed analyses.
Page No. – Page No.
The role of copper speciation in the
low temperature oxidative
upgrading of short chain alkanes
over Cu/ZSM-5 catalysts
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