Improved Efficiency for Partial Oxidation of Methane by Controlled Copper Deposition on Surface-Modified ZSM-5

Article abstract of DOI:10.1002/cctc.201500980

The mono(μ-oxo) dicopper cores present in the pores of Cu-ZSM-5 are active for the partial oxidation of methane to methanol. However, copper on the external surface reduces the ratio of active, selective sites to unselective sites. More efficient catalysts are obtained by controlling the copper deposition during synthesis. Herein, the external exchange sites of ZSM-5 samples were passivated by bis(trimethylsilyl) trifluoroacetamide (BSTFA) followed by calcination, promoting selective deposition of intraporous copper during aqueous copper ion exchange. At an optimum level of 1-2 wt % SiO₂, IR studies showed a 64 % relative reduction in external copper species and temperature-programmed oxidation analysis showed an associated increase in the formation of methanol compared with unmodified Cu-ZSM-5 samples. It is, therefore, reported that the modified zeolites contained a significantly higher proportion of active, selective copper species than their unmodified counterparts with activity for partial methane oxidation to methanol.

Full text of DOI:10.1002/cctc.201500980

DOI: 10.1002/cctc.201500980
Full Papers
Improved Efficiency for Partial Oxidation of Methane by
Controlled Copper Deposition on Surface-Modified ZSM-5
Thomas Sheppard, Helen Daly, Alex Goguet, and Jillian M. Thompson*[a]
The mono(m-oxo) dicopper cores present in the pores of Cu-
ZSM-5 are active for the partial oxidation of methane to meth-
anol. However, copper on the external surface reduces the
ratio of active, selective sites to unselective sites. More efficient
catalysts are obtained by controlling the copper deposition
during synthesis. Herein, the external exchange sites of ZSM-5
samples were passivated by bis(trimethylsilyl) trifluoroaceta-
mide (BSTFA) followed by calcination, promoting selective dep-
osition of intraporous copper during aqueous copper ion ex-
change. At an optimum level of 1–2 wt% SiO2, IR studies
showed a 64% relative reduction in external copper species
and temperature-programmed oxidation analysis showed an
associated increase in the formation of methanol compared
with unmodified Cu-ZSM-5 samples. It is, therefore, reported
that the modified zeolites contained a significantly higher pro-
portion of active, selective copper species than their unmodi-
fied counterparts with activity for partial methane oxidation to
methanol.
Introduction
Partial oxidation of methane to methanol represents a signifi-
cant challenge in modern chemistry.[1] Accomplishing this diffi-
cult reaction would facilitate the production of valuable chemi-
cal raw materials from methane abundant in natural reserves.
In addition, conversion of natural gas into liquid form would
simplify the issue of storage and transportation from remote
or offshore sources.[2] Although interest in this process has
been high among the scientific community for many years, the
partial oxidation of methane has proven difficult owing to the
extreme stability of the methane molecule and the energy re-
quired to activate the CÀH bond. Additionally, as the products
of methane oxidation are inevitably more reactive than meth-
ane, total oxidation is a concern in any reaction system. Nu-
merous examples of partial methane oxidation are available in
the literature, but in many cases these require high tempera-
tures, harsh solvents, or expensive reagents to facilitate the
process. Catalytic oxidation to formaldehyde was demonstrat-
ed by Herman et al. over V2O5/SiO2, but with high temperature
(6008C) and low product selectivity towards CH2O (16.3%) and
CH3OH (2.5%).[3] Periana et al. used bipyrimidyl PtII complexes
dissolved in concentrated H2SO4 to protect the product from
further oxidation, obtaining methyl bisulfate with 81% selectiv-
ity, yielding methanol and regenerating the catalyst by hydrol-
ysis.[4] However, the corrosive medium and homogeneous
system remained problematic for product isolation. Later,
Schüth et al. immobilised similar platinum catalysts on a poly-
mer framework in a heterogeneous system allowing simple
separation of products, but the corrosive acid medium was re-
tained.[5] Despite numerous advances, to date an efficient and
viable method for direct conversion of methane to methanol
remains elusive.
In the last decade, interest has shifted towards metal-ex-
changed zeolites for the partial oxidation of methane to meth-
anol, following the identification of a reactive copper species
now characterised as the bent mono(m-oxo) dicopper core.[6–8]
The core was found to facilitate methane oxidation under mild
conditions and can be stabilised in the framework of ZSM-5
and other zeolites. The same copper species has been linked
to the active site of the enzyme partial methane monooxyge-
nase (pMMO), which offers an ideal mild reaction pathway for
methane to methanol conversion in nature.[9,10] Besides Cu-
ZSM-5, other zeolite frameworks including FER, BEA, MOR,[11]
and also artificial zeolites such as SSZ-13 and SAPO-34,[12] are
known to be active for partial methane oxidation following
copper exchange. In particular, Cu-FER and Cu-MOR show vari-
able activity for methane oxidation at different temperatures,[11]
leading to the assumption that other unidentified active
copper cores may be present in various zeolite frameworks.
Cu-MOR is one of the most widely studied examples owing to
its relatively high product yield and as yet undefined reaction
mechanism,[13,14] although it is also known to contain the
mono(m-oxo) dicopper core.[15,16] A growing body of research
suggests a rich variety of species that may be active for partial
[a] Dr. T. Sheppard,+ Dr. H. Daly, Prof. A. Goguet, Dr. J. M. Thompson
School of Chemistry and Chemical Engineering
Queen’s University
Stranmillis Road, Belfast, BT9 5AG (Northern Ireland)
Fax: (+44)028-90974687
E-mail: jillian.thompson@qub.ac.uk
[+] Current Address:
Institute of Catalysis Research and Technology (IKFT)
Karlsruhe Institute of Technology (KIT)
Kaiserstrasse 12, 76131 Karlsruhe (Germany)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500980.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
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methane oxidation, demonstrating the versatility of zeolites in
supporting and stabilizing such species.[15]
This work describes efforts to reduce the proportion of
either the unselective or inactive copper within the structure
of Cu-ZSM-5 through selective modification of the zeolite sur-
face. Ichikawa et al. showed that interaction of a silylating
agent with Brønsted acid zeolite exchange sites followed by
high temperature treatment leads to passivation, forming
Lewis acid SiOx species and rendering the sites inactive for
cation exchange.[28] Passivation of zeolite exchange sites has al-
ready been applied for selective ion exchange on a number of
metal–zeolite systems, including ZSM-5. For example, Lercher
et al. showed that the acidity of the external surface of ZSM-5
could be decreased by modifying the external surface hydroxyl
groups with bulky silylating agents.[29] Alternatively, Iglesia
et al. used the same technique to reduce the exchange of
MoOx species on the external surface of ZSM-5, resulting in
higher selectivity for the desired low-order aromatics as
a result of reactions occurring in the pores of the catalyst, as
well as higher conversion owing to improved dispersion.[30]
Surface modification of various zeolites for selective catalysis
on a variety of organic species is well established,[31–33] but to
the best of our knowledge has not previously been applied to
the partial oxidation of methane to methanol.
In the present work, we focus on Cu-ZSM-5 as the earliest
known example of a catalyst for partial methane oxidation to
methanol with a copper-exchanged zeolite. The original reac-
tion method over Cu-ZSM-5 developed by Schoonheydt et al.
is well documented.[11] The zeolite is first activated in O2 at
high temperature (350–6508C); however, this can also be ach-
ieved at lower temperatures by using different oxidants such
as NO[17] or N2O.[11] After activation, the reaction with methane
proceeds under milder conditions at 1508C to form methanol
stoichiometrically with >98% selectivity. This can be partially
recovered ex situ by washing the catalyst with water or in situ
by using steam, allowing an entirely gas-phase process.[14,18]
Following the interaction of methane with the activated cata-
lyst, the product is thought to form initially as a methoxy inter-
mediate, requiring a protic solvent in order to desorb.[8,14,15]
However, liquid-phase extraction is known to be inefficient
and some product remains trapped within the zeolite struc-
ture, possibly as a result of strong interactions with the numer-
ous adsorption sites present and poor diffusion though the
narrow channels of Cu-ZSM-5.[11,19] This behaviour has also
been observed for Cu-MOR,[12] suggesting that the zeolite
structure may play a role.[15,20] Alternatively, product extraction
with steam at elevated temperatures (150–2008C) has been
shown to improve methanol yields in the case of Cu-ZSM-5,[17]
and allows complete desorption of products in the case of Cu-
MOR.[18] It should be emphasised that partial methane oxida-
tion over copper zeolite systems is stoichiometric,[11] although
batch-type systems for recurring yields of methanol have been
demonstrated.[17,18] Recently, Hutchings et al. detailed both
batch and continuous-flow liquid-phase processes over bimet-
allic Fe/Cu-ZSM-5 (in which iron is the active species) by using
H2O2 as the oxidant; however, the relatively expensive oxidiz-
ing agent and low conversion are typically problematic for par-
tial methane oxidation.[21,22]
Herein, selective functionalization of external zeolite ex-
change sites was performed with the bulky organic silylating
agent bis(trimethylsilyl) trifluoroacetamide (BSTFA). The de-
crease in the total available exchange sites is intended to mini-
mise formation of unselective copper nanoparticles on the ex-
ternal zeolite surface as well as promote exchange of copper
within the zeolite pores during wet ion exchange, owing to an
increased concentration gradient. The modified catalysts pro-
duced were structurally characterised and tested for activity to-
wards partial methane oxidation. A number of unmodified cat-
alysts were also prepared for comparison. The overall goal was
to produce catalysts with an increased ratio of active and se-
lective intraporous copper sites to unselective or inactive
copper sites on the external zeolite surface, while retaining ac-
tivity towards partial methane oxidation.
Studies of Cu-ZSM-5 involving selective adsorption of IR
probe molecules have identified at least two distinct copper
environments; those on the external zeolite surface and those
within the microporous framework.[19] Only the latter have Results and Discussion
shown activity for low temperature, selective partial methane
Diffuse reflectance infrared Fourier transform spectroscopy
oxidation, indicating a relatively low proportion of active
copper sites in the zeolite structure, with estimates at around
5%.[11] In the case of Cu-ZSM-5, it is thought that the numer-
ous unselective or inactive copper sites contribute to ineffi-
cient product extraction and are potentially active for the total
oxidation of methane to CO2.[7,8] Large volumes of inactive
metal also complicate direct characterization by techniques
such as X-ray absorption spectroscopy (XAS), which measure
the bulk metal species and thus have difficulty in distinguish-
ing between suspected active sites and inactive metal
sites.[23,24] The latter point is particularly relevant given that
active copper sites across a range of zeolite types are still
being discovered and characterised.[13,14,25–27] Similarly, the pre-
cise activation mechanism of Cu-ZSM-5 with various oxidizing
agents is still being debated.[15,20] The need for further detailed
in situ or operando studies is therefore clear.
(DRIFTS) analysis of silylated zeolites
A range of modified Na-ZSM-5 samples were synthesised with
SiO2 loadings ranging from 0.2 to 3 wt% (Table 1). The label
‘SCZ-Y’ indicates a silylated Na-ZSM-5 zeolite with ‘Y’ loading
of SiO2. Figure 1 (and Figure S4.1 in the Supporting Informa-
tion) show the DRIFTS spectra of the samples following silyla-
tion, drying at 808C and calcination at 5008C, in comparison
to unmodified Na-ZSM-5. On treatment with BSTFA and drying,
the carbonyl stretching band at 1745 cmÀ1 appeared with in-
creasing intensity as silylation was increased, indicating func-
tionalization of zeolite surface sites with BSTFA.
Silylated Na-ZSM-5 samples were then calcined at 5008C to
remove the organic precursors and complete the surface modi-
fication process. Thermal decomposition of the organic species
was noted by the removal of all carbonyl bands observed
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Brunauer–Emmett–Teller (BET) analysis of the unmodified
Cu-ZSM-5 samples showed approximate surface areas and
pore volumes of 300–320 m2 gÀ1 and 0.109–0.115 cm3 gÀ1, re-
spectively. This was within the expected range for microporous
zeolites. For the modified Cu-ZSM-5 samples, a reduction in
both surface area and pore volume was noted at higher (>
1 wt% SiO2) levels of silylation (Figure 2). As no specific trend
was identified for the unmodified samples (Section S3 in the
Supporting Information), the trends observed are proposed to
be a result of surface silylation. As BSTFA is larger than the
pores of the zeolite (Figure S8.6), the decrease in surface area
combined with lower Cu loading indicates that large amounts
of newly formed SiO2 species on the zeolite surface reduce
both the availability of the external exchange sites through
passivation, and possibly intraporous exchange sites through
steric blocking. However, the reduction in cumulative pore
volume with increasing SiO2 wt% suggests increased exchange
of copper within the zeolite channels. TEM performed on un-
modified CZ-2.12 revealed the presence of copper particles,
which were visibly less numerous than for modified SCZ-1 (Fig-
ure S3.1). Owing to the small channel size of ZSM-5 (5.2–5.5 Š),
visible particles of several nanometres were likely indicative of
external surface species.[19] Energy-dispersive X-ray spectrosco-
py (EDX) analysis revealed the presence of copper where none
was physically visible, indicating high dispersion of metal
nanoparticles for the silylated sample.
Table 1. Synthesis and physical characteristics of Cu-ZSM-5 zeolites.
Zeolite[a]
[Cu solution]
[moldmÀ3
Cu
[wt%]
BSTFA
[mol]
SiO2
[wt%]
]
CZ-0.7
5.00”10À4
1.00”10À3
1.25”10À3
1.50”10À3
2.00”10À3
2.50”10À3
3.00”10À3
5.00”10À3
2.00”10À3
2.00”10À3
2.00”10À3
2.00”10À3
2.00”10À3
2.00”10À3
0.70
1.52
1.88
2.12
2.65
2.68
2.88
2.95
2.65
2.08
2.45
2.39
2.11
1.94
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
CZ-1.52
CZ-1.88
CZ-2.12
CZ-2.65
CZ-2.68
CZ-2.88
CZ-2.95
CZ-2.65
SCZ-0.2
SCZ-0.6
SCZ-1
0
0
3.36”10À5
1.01”10À4
1.67”10À4
3.36”10À4
5.04”10À4
0.2
0.6
1
2
3
SCZ-2
SCZ-3
[a] Labels: CZ-X, CZ=copper-exchanged zeolite, X=Cu wt% (determined
by ICP); SCZ-Y, SCZ=silylated copper-exchanged zeolite, Y=SiO2 wt%
(theoretical).
during prior functionalization with BSTFA (Figure 1b). Copper
ion exchange was then performed on the modified Na-ZSM-5
samples to form Cu-ZSM-5, followed by treatment in air at
5008C to remove organic species (see Figure S4.2). Copper ex-
change was accompanied by a colour change in the catalyst
from white to pale blue.
Structural characterization
FTIR analysis of copper distribution
The Cu-ZSM-5 samples were analysed by inductively coupled
plasma (ICP) techniques, which revealed a range of total
copper loadings from 0.7 to 2.95 wt% Cu for the unmodified
(CZ-X) series (Table 1). The silylated catalysts were prepared
with equal concentrations of copper solution to the unmodi-
fied CZ-2.65 sample and a uniform reduction in Cu wt% was
observed, which was generally more prominent at higher
levels of silylation. The reduced copper loading with increased
silylation could be expected from the decrease in the availabili-
ty of zeolite exchange sites owing to surface passivation as
well as decreased diffusion in the precursor.
As ICP results are only indicative of the total copper loading,
the relative proportion of copper sites on the external surface
and within the pores of Cu-ZSM-5 was estimated by using
transmission IR. IR studies were performed with sequential ad-
sorption of two probe molecules onto Cu-ZSM-5; firstly, pivalo-
nitrile (PVN), which was considered too bulky to enter the zeo-
lite channels and therefore adsorbs onto Cu on the external
surface, followed by NO, which could access the remaining
available copper sites in the channels. These probe molecules
have previously been shown to be selectively indicative of ex-
ternal and intraporous copper species, respectively, with the
Figure 1. Zeolite overtones and carbonyl region (1700–2100 cmÀ1) of the DRIFTS spectra of BSTFA-modified Na-ZSM-5 (a) after drying at 808C and (b) after
calcination at 5008C. Unmodified Na-ZSM-5 is shown for reference.
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Figure 2. BET analysis of BSTFA-modified Cu-ZSM-5 (0.2–3 wt% SiO2) and unmodified CZ-2.65 (0 wt% SiO2, indicated by an unfilled circle): (a) total surface
area; (b) cumulative pore volume.
amount of intraporous Cu proportional to the amount of
methanol formed.[19] Given that only copper sites in the zeolite
channels were shown to be potentially active, the ratio of the
integrated NO–Cu2+ to PVN–Cu2+ band intensities was used as
an indicator of selective formation of intraporous copper spe-
cies, with a maximised ratio being more favourable.
On exposure to PVN, a complex absorption feature was ob-
served from 2210 to 2315 cmÀ1 (Figures S6.2 and S6.3). Contri-
butions from PVN on the zeolite support were determined by
following PVN adsorption on standard Na-ZSM-5. The PVN–
Cu2+ absorption band was therefore assigned as that at
2280 cmÀ1 in accordance with Bitter et al.[19] NO adsorption re-
sulted in a second complex band at approximately 1900 cmÀ1
(Figure S6.4), which was isolated as above to identify the NO–
Cu2+ band as that at 1907 cmÀ1. The key absorption bands at
2280 and 1907 cmÀ1 were deconvoluted and integrated to
quantify the surface area of copper on the external and intra-
porous zeolite exchange sites, respectively. IR analysis results
for the unmodified and modified catalyst series are shown in
Figure 3 with the data summarised in Table 2.
Examination of the PVN–Cu2+ and NO–Cu2+ absorption
peaks for the unmodified catalysts (CZ series) as a function of
copper loading (Figure 3a) indicates a similar peak area for
PVN with increasing Cu loading, which implies similar volumes
Figure 3. Integrated IR absorption peaks of PVN–Cu2+ (2280 cmÀ1) and NO–
of Cu on the external zeolite surface, along with a significant
Cu2+ (1907 cmÀ1) following sequential adsorption of PVN then NO onto:
(a) unmodified CZ series catalysts; (b) modified SCZ series catalysts (includ-
increase in NO peak area, signifying an increase in the intrapo-
rous copper species. It has been shown that deposition of Cu
onto the surface of the zeolites occurs rapidly during ion ex-
change (Figure S8.5) and so the relatively constant PVN signal
can be attributed to rapid saturation of the external exchange
sites, with slower diffusion of Cu into the pores. The increase
in NO signal with higher Cu loading is likely due to the in-
creased concentration gradients from the more concentrated
Cu solutions, allowing more Cu to diffuse through to the interi-
or zeolite channels. As the response factor of the probe mole-
cules is unknown, it is unclear the extent to which the overall
increase in the NO/PVN ratio observed is due to increased
copper loading in the zeolite channels and how much is from
a greater response factor for NO compared with PVN. Regard-
less, it is clear that there is an increase in the amount of Cu in
the intraporous region of the zeolite with increasing Cu
loading.
ing CZ-2.12 as 0 wt% SiO2 blank).
The modified catalyst series showed a simultaneous reduc-
tion in the PVN–Cu2+ signal and a general increase in the NO–
Cu2+ signal with increasing silylation (Figure 3b). As a constant
copper concentration was used during preparation, these ob-
servations can therefore be attributed to the effects of silyla-
tion. Notably, a two-fold increase in the NO/PVN ratio was ob-
served for SCZ-2 in comparison with the unmodified analogue
CZ-2.65. However, as the average copper loading of the modi-
fied catalysts was 2.19 wt% Cu, it is more appropriate to com-
pare the structure and activity of the modified catalysts with
that of CZ-2.12, where a 2.8-fold increase in NO/PVN was ob-
served compared with SCZ-2. It is apparent, therefore, that al-
though the overall Cu loadings are similar, silylation has altered
the distribution of Cu on the internal and external surfaces of
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Table 2. Catalytic activity and FTIR data for Cu-ZSM-5 zeolites for partial
methane oxidation.
Zeolite
Cu
PVN–Cu NO–Cu NO/
UV/vis
[K–M] GC
Yield [mmolgÀ1
TPO
]
[wt%] [a.u.]
[a.u.]
PVN
CZ-0.7
0.70
11.1
10.5
10.2
15.9
11.1
–
17.2
35.6
31.6
62.3
54.8
–
1.55 0.14
0.00
1.32
1.97
3.04
2.56
2.70
2.75
2.66
3.04
1.13
1.64
1.01
1.42
1.51
1.71
1.19
–
3.59
10.4
–
–
14.2
3.59
2.73
6.32
CZ-1.52 1.52
CZ-1.88 1.88
CZ-2.12 2.12
CZ-2.65 2.65
CZ-2.68 2.68
CZ-2.88 2.88
CZ-2.95 2.95
CZ-2.12 2.12
SCZ-0.2 2.08
SCZ-0.6 2.45
3.40 0.54
3.10 0.95
3.92 1.39
4.92 2.20
–
–
2.10
2.67
–
–
15.3
15.9
17.5
102.8
62.3
40.9
51.4
69.8
61.4
66.3
6.73 2.48
3.92 1.39
2.34 0.52
5.75 1.12
8.53 0.69
8.94
SCZ-1
SCZ-2
SCZ-3
2.39
2.11
1.94
8.18
5.61
8.45
13.0
11.1
6.56
10.9
0.70
7.85 0.53
the zeolite. An ideal silylation level of 1 to 2 wt% SiO2 is there-
fore suggested for this catalyst for maximizing the NO/PVN
ratio to the implied optimum of active, selective copper sites
compared with unselective copper sites.
Figure 4. Comparison of modified and unmodified Cu-ZSM-5 samples:
(a) UV/Vis intensity at 440 nm versus total copper loading (Cu wt%);
(b) methanol yield after aqueous product extraction versus UV/Vis absorb-
Activity testing for partial methane oxidation
The activity of the modified Cu-ZSM-5 samples towards partial
methane oxidation was assessed. The catalysts were activated
in oxygen and the presence of the UV/Vis band at 440 nm, rep-
resentative of the bent mono(m-oxo) dicopper core, was deter-
mined.[11] Following activation and reaction with methane, the
strongly adsorbed reaction products were extracted with de-
ionised water and analysed by GC. The results of the UV/Vis
and GC analysis for both unmodified and modified catalyst
series are shown in Table 2 along with the peak areas (arbitrary
units) for adsorbed NO and PVN from the IR analysis. Yields are
presented as mmolgÀ1 of zeolite used, unless stated otherwise.
The 440 nm band was observed for all modified and un-
modified catalysts tested following activation in O2 at 5008C.
For the unmodified CZ series, the intensity of the core band in-
creased linearly with increasing copper loading (Figure 4a), as
observed by Schoonheydt et al.[11] No methanol was observed
below a minimum copper loading of 0.7 wt% Cu (0.14 K–M,
Figure 4b) and the methanol yield reached a maximum at
2.12 wt% Cu, (1.39 K–M, Figure 4b) before decreasing slightly
at higher copper loadings. Rather than limited formation of
methanol, this plateau at high copper loadings has been at-
tributed to the strong adsorption of product methanol to the
catalyst and the high concentration of adsorption sites limiting
aqueous methanol extraction.[8,11]
^
*
^
ance intensity at 440 nm. =CZ series, =SCZ series, =CZ-2.12 (0 wt%
SiO2 blank).
methanol yields in line with expectations from UV/Vis absorb-
ance of the unmodified catalysts (Figure 4b). This analysis
shows that although the distribution of Cu changed after sily-
lation, the modified catalysts were still active for partial meth-
ane oxidation. In addition, the ICP data indicate that the modi-
fied catalysts contained less Cu in total. Despite some samples
giving a similar or greater NO–Cu2+ signal intensity compared
with CZ-2.12, implying greater amounts of copper in the pores,
the UV/Vis data may suggest a lower than expected amount of
core species was formed. However, the product extracted was
in line with that expected from the amount of core species ob-
served by UV/Vis.
Examining the aqueous product yield in relation to specific
copper distribution (Figure S8.1) confirmed that the externally
based copper sites characterised by PVN adsorption have no
correlation with the methanol yield for either unmodified[19] or
modified zeolites. In contrast, intraporous copper sites charac-
terised by NO adsorption have a clearly defined relationship
with product yield for the unmodified series. Notably, the NO/
PVN ratio for unmodified catalysts (Figure 5a) reflects the rela-
tionship between the UV/Vis absorbance and yield shown pre-
viously (Figure 4b). As the NO–Cu2+ peak increases in intensity
and the NO/PVN ratio increases to above a critical value of
about 4 (approximately 2.12 wt% Cu), the methanol yield
reaches a plateau. Again, this is possibly due to hindered prod-
uct desorption or the increased number of potential adsorp-
tion sites.[8,11] For the modified series, no trend was observed
for either PVN or NO absorption, and in turn no trend between
the NO/PVN ratio and product yield (Figure 5b) was observed.
For the modified SCZ series, a general decrease in UV/Vis
signal intensity was observed compared with the unmodified
CZ-2.12 sample (Figure 4a). Despite the small changes in Cu
loading, a strong correlation with the methanol yield was not
observed, rather a range of yields were observed from 1.0 to
1.6 mmolgÀ1. UV/Vis absorbance has previously been shown to
be proportional to the amount of aqueous extracted prod-
uct,[11,19] and the modified series also showed appropriate
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Figure 5. Methanol yield from aqueous extraction as a function of specific copper distribution on Cu-ZSM-5, characterised by integrated IR peaks following
PVN and NO adsorption. Yields shown per moles of Cu for: (a) CZ series; (b) modified SCZ series.
Care must be taken when comparing the performance of
the modified and unmodified samples using UV/Vis and GC
analysis. As mentioned previously, limitations to aqueous prod-
uct desorption have been observed on ZSM-5 at very high
copper loadings based on temperature-programmed oxidation
(TPO) analysis,[11] and studies involving product desorption by
steam.[17,18] Here, the Cu loadings of the modified catalysts are
mostly above the limit where there is a linear relationship
range between CH3OH extracted and Cu loading, as shown by
the plateau in Figure 4. It is widely accepted that at higher Cu
loadings outside this range, aqueous extraction is limited and
does not accurately represent the amount of product
formed.[8,12,15] In addition, formation of methanol in a similar
process over zeolites other than ZSM-5 shows varying re-
sponse with UV/Vis analysis. Cu-MOR gives a greatly reduced
UV/Vis signal compared with the amount of methanol formed,
whereas Cu-BEA and Cu-FAU show activity for low temperature
methane oxidation but no significant UV/Vis peak at all.[11,12,18]
To test the validity of the trends observed with GC and UV/
Vis analysis, TPO experiments were performed to evaluate the
potential amount of adsorbates present on the catalyst surface
following activation and reaction with methane. Combustion
products were observed by mass spectrometry (MS) beginning
at 270–2808C for all catalysts. CO2 and CO were of particular
interest in quantifying the adsorbed organic species. For the
unmodified CZ-series catalysts, the CO2 detected from blank
TPO experiments (catalysts pre-treated in Ar, see Section S7)
was subtracted from the final CO2 yield. Given the high tem-
perature pre-treatment of all catalyst samples, it was assumed
that any CO2 or CO observed from activated catalysts was,
therefore, a result of strongly adsorbed methanol formed
during reaction with methane.
Figure 6 shows the amount of methanol observed by GC fol-
lowing aqueous extraction compared with that observed by
TPO-MS as evolved CO2. For the unmodified series (Figure 6a),
below 2.12 wt% Cu the CH3OH and CO2 yields observed were
very similar. However, at higher copper loadings a clear dispari-
ty was observed, whereby CO2 evolved was in excess of the
aqueous desorbed products. For the modified series, a similar
increase in product detected by TPO was observed compared
with aqueous extraction (Figure 6b). This confirms the limita-
tion of aqueous product removal also for the silylated catalysts.
At Cu loadings above approximately 1.9–2.1 wt% Cu, the
volume of aqueous extracted product appears not to be an ac-
curate representation of catalytic activity. Notably, CO or meth-
anol desorption was not observed for any of the catalysts
Figure 6. Activity data as a function of total copper loading. CH3OH yield after aqueous extraction (unfilled points) and CO2 evolved during TPO at 5008C in
O2 (solid points): (a) unmodified CZ series; (b) modified SCZ series.
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tested, indicating complete combustion of the adsorbed prod-
ucts in O2. CO2 and CH3OH yields are, therefore, considered as
molar equivalents.
level of silylation of the zeolite (Figure 7a and Figure S8.3), it
becomes clear that TON values are optimised at 1–2 wt% SiO2.
Furthermore, this correlates well with the maximum value of
NO/PVN observed through transmission FTIR analysis (Fig-
ure 7b). As the volume of intraporous copper characterised by
NO adsorption was found to correlate with the volume of
product observed and TON (Figure S8.2), the activity of Cu-
ZSM-5 for partial methane oxidation is therefore confirmed to
be dependent only on the volume of copper in the zeolite
channels. In agreement with the work of Bitter et al.,[19] exter-
nal copper sites were found to be inactive. It is apparent that
the large increase in TON observed at 1–2 wt% SiO2 occurs as
a direct result of silylation, which in turn offers an optimised
amount of intraporous copper species that are potentially
active.
In Figure 6, product yields are expressed per moles of Cu
present on the catalyst; this can effectively be considered as
a turnover number (TON). However, it should be emphasised
that the process shown here operates stoichiometrically; there-
fore, TON values are necessarily low and also the amount of
copper does not represent the amount present in the active
cores. However, comparing catalysts with similar copper load-
ings under this assumption permits useful comparison be-
tween the unmodified and modified series, as summarised in
Table 3. The modified series, therefore, exhibited a maximum
three-fold increase in TON at an ideal silylation level of 1–
2 wt% SiO2, and a two-fold increase at 3 wt% SiO2. The former
two catalysts outperformed even the most productive unmodi-
fied sample, CZ-2.95. Considering the near identical copper
loading observed between SCZ-2 and CZ-2.12, this increase in
catalyst efficiency is particularly notable, indicating a significant
increase in the amount of active catalytic sites present.
In summary, for Cu-ZSM-5, the amount of aqueous extracted
methanol does not accurately describe product formation
above a certain Cu loading (2.12 wt% Cu for CZ-series) and,
therefore, the UV/Vis signal intensity does not correlate with
yield above this value (Figure 4b). However, for unmodified
catalysts, the amount of CO2 evolved does correlate with the
intensity of the NO and UV/Vis signal (Figures S8.2 and 8.4), in-
dicative of active copper species. For the modified SCZ series
catalysts, only the volume of CO2 evolved was found to corre-
late with the number of intraporous copper sites (Figure 7b
and Figure S8.2). As the CO2 evolved was in excess of the
aqueous methanol yield, the CH3OH yield was not regarded as
an accurate representation of product formation for any of the
samples tested, any correlation with methanol yield was there-
fore not considered significant. Taking the CO2 evolved as
being representative of the amount of product formed, the
lack of correlation with UV/Vis data (Figure S8.4) shows this as
not representative of the active species in the case of the
modified catalysts. Examining the CO2 evolved together with
FTIR analysis thereby provides a relationship between the in-
traporous Cu species and the amount of product formed, the
effect of silylation was, therefore, assessed by using these
parameters.
It is also important to note that for the modified samples,
no specific trend was observed between yield or TON and the
total copper loading (Figure 6b). However, by considering the
Table 3. Effect of silylation on TON and catalyst efficiency.
Zeolite
Cu
[wt%]
TPO yield[a]
TON[b]
[mmolmolÀ1
]
Increase in
TON [%][c]
[mmolgÀ1
]
CZ-0.7
CZ-1.52
CZ-2.12
CZ-2.65
CZ-2.95
SCZ-0.2
SCZ-0.6
SCZ-1
0.7
1.71
1.19
3.59
10.4
14.2
2.73
6.32
13.0
11.1
6.56
–
–
–
–
–
–
1.52
2.12
2.65
2.95
2.08
2.45
2.39
2.11
1.94
4.97
10.8
24.9
30.6
8.33
16.4
34.5
33.4
21.5
–
51.8
320
309
199
SCZ-2
SCZ-3
[a] Yield defined as mmolgÀ1 zeolite used. [b] TON defined as mmol CO2
observed per mol Cu present. [c] Compared with CZ-2.12 as 0 wt% SiO2
blank.
For the modified SCZ series, a clear influence was observed
for the amount of CO2 evolved depending on silylation. At an
Figure 7. Activity data for modified SCZ series compared with CZ-2.12 (0 wt% SiO2 blank, unfilled point). CO2 evolved during TPO at 5008C in O2 in relation
to: (a) % of silylation; (b) NO/PVN ratio as determined by integrated FTIR peaks.
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ideal level of between 1–2 wt% SiO2 present on the zeolite
surface, the CO2 yield reached a maximum, indicating a large
amount of adsorbed methanol present within the zeolite
pores, although product desorption in the aqueous phase re-
mained significantly hindered, similar to the unmodified cata-
lysts tested. The resulting modified zeolites, therefore, consti-
tute more active catalysts than unmodified Cu-ZSM-5 based on
the total copper loading present. The large increase in CO2
yield and TON at 1–2 wt% SiO2 is consistent with the structural
characterization, which showed a greatly increased NO/PVN
ratio during gas adsorption studies, marking the point at
which the maximum intraporous copper sites and minimum
external copper sites was observed. It should be noted that
the trends observed began to reverse with extensive silylation
of 3 wt% SiO2, which is possibly due to the maximum silylation
level of the zeolite surface already being attained.[29]
unmodified catalysts with comparable copper loading. Al-
though the methane oxidation process demonstrated is stoi-
chiometric, this indicates the increased efficiency of silylated
catalysts for partial methane oxidation. The surface functionali-
zation technique used is flexible and may be applied to
a range of zeolite types and silylating species, advancing the
prospect of developing a material consisting of a high propor-
tion of active metal sites. This may help to facilitate direct char-
acterization of the core species. In addition, application to zeo-
lites with larger frameworks and pore diameters may contrib-
ute to developing more active and efficient catalysts for this
important chemical process.
Experimental Section
The synthetic parameters and characteristics of all zeolites pro-
duced are summarised in Table 1. The suppliers and purity of re-
agents and precursors are detailed in Section S1 in the Supporting
Information. Cu-ZSM-5 samples were prepared by a standard wet
ion exchange method.[11] NH4-ZSM-5 (Si/Al=12, 1 g) was added to
aqueous NaNO3 (1 g, 150 mL) and stirred for 24 h at room temper-
ature (approximately 20 8C). Samples were filtered, washed and
dried at 1008C. Sodium exchange was performed three times to
form Na-ZSM-5. Na-ZSM-5 (1 g) was added to solutions of CuII ace-
tate monohydrate of various concentration in deionised water
(250 mL) and the mixture stirred at room temperature for 24 h.
Samples were filtered, washed and treated at 5008C for 3 h to
remove organic precursors before further use. Surface-modified
Cu-ZSM-5 samples were prepared by silylation of Na-ZSM-5 fol-
lowed by copper ion exchange. Na-ZSM-5 (1 g) as prepared above
was added to dry hexane (25 mL, 3 Š molecular sieve) and the mix-
ture heated to 408C under a controlled nitrogen atmosphere. Vary-
ing concentrations of silylating agent BSTFA (3.73”10À4 to 3.73”
10À3 molLÀ1) in dry hexane were added and the mixture was
heated at reflux for 1 h. Silylated samples were filtered, washed
with dry hexane and dried at 808C for 1 h. Samples were then
treated in air at 5008C for 6 h to decompose the organosilane spe-
cies and form silica-modified Na-ZSM-5. Copper exchange was per-
formed on the modified Na-ZSM-5 precursors as above, aiming to
give a Cu loading similar to that of CZ-2.65 (Table 1). Cu-ZSM-5
samples were treated again in air at 5008C for 3 h before further
use. Details of calculations for metal and BSTFA concentrations
used are shown in Section S2 in the Supporting Information.
This presents a strong case for applying silylation to selec-
tively control copper deposition, showing that catalysts pro-
duced through surface passivation have a greater proportion
of active, selective to non-selective copper sites present than
comparable unmodified samples. The increase in product yield
can be directly linked to a change in specific copper distribu-
tion induced by silylation. The potential volume of product
formed was easily determined by adsorption of gaseous probe
molecules, a process that can be flexibly applied to other sup-
ports. A clear advantage of the silylation process described
here lies in its applicability to other zeolite materials and
framework types, along with the abundance of different silylat-
ing agents available. Application of this method to other
framework types with partial methane oxidation activity, partic-
ularly those with greater pore diameters such as mordenite,
may alleviate the problem of product desorption while result-
ing in a functional and more efficient catalyst. The increased
proportion of active and selective copper present may also
assist with further characterization of the active site.
Conclusions
It has been shown that ZSM-5 functionalised with the silylating
agent BSTFA can facilitate passivation of specific zeolite ex-
change sites, allowing selective control over copper exchange
during synthesis. A range of 1–2 wt% SiO2 was found to be
ideal, effectively favouring deposition of copper within the
zeolite channels at the expense of the external surface sites.
With the observation that copper sites based on the external
surface are inactive for low temperature partial methane oxida-
tion, this indicates a significant increase in potentially active
and selective copper within Cu-ZSM-5. Modified zeolites were
found to be active for partial methane oxidation, showing
product yields comparable with unmodified Cu-ZSM-5 samples
based on UV/Vis analysis of the active copper species. TPO in-
dicated that a large amount of methanol was formed for 1–
2 wt% SiO2 catalysts, in direct proportion to the number of in-
traporous copper sites present. However, the product was not
directly recoverable by aqueous extraction. Considering the
CO2 yield in relation to the moles of Cu present, silylation of 1–
3 wt% SiO2 provided a two- to three-fold increase in TON over
More details on the characterization techniques used and methods
of deconvolution of peaks are shown in Sections S3–S6 in the Sup-
porting Information. The Cu content of samples was determined
by inductively coupled plasma optical emission spectrometry (ICP-
OES) by using a PE Optima spectrometer. BET surface areas and
pore volumes of zeolite samples were determined by nitrogen ad-
sorption at À1968C by using a Micrometrics Tristar II instrument.
TEM was performed by using an FEI Tecnai F20 electron micro-
scope operated at 200 kV, following adherence of the samples to
a copper microgrid. DRIFTS was performed at various stages
during the synthesis to monitor the progress of silylation: (a) fresh
Na-ZSM-5 before modification by silylation, (b) modified Na-ZSM-5
after drying, (c) modified Na-ZSM-5 after calcination, (d) modified
Cu-ZSM-5 after copper exchange. Spectra were recorded at ambi-
ent temperature by using a Bruker Tensor 27 IR spectrometer at
a resolution of 4 cmÀ1. Each spectrum was the result of 128 scans,
with dry KBr used as the background. Spectra were baseline cor-
rected and normalised to the zeolite overtones (1950–2050 cmÀ1).
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active copper species in Cu-ZSM-5 during activation and reaction
with methane.[11] Pelletized samples (0.7 g, 250–500 mm pellets)
were placed in a quartz plug flow reactor and activated in oxygen
at 5008C overnight. Spectra were recorded by a diffuse reflectance
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into self-supporting discs, placed in a Specac gas exchange cell
and outgassed under vacuum at 1508C for 2 h. The sample discs
were then exposed to PVN vapour (30 min, ~8 mbar) and subse-
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periments were also performed by adsorption of PVN then NO to
the parent zeolite Na-ZSM-5. During probe adsorption, spectra
were recorded in situ in transmission mode on a Bruker Tensor 27
IR spectrometer at a resolution of 4 cmÀ1. Each spectrum was the
result of 128 scans, with dry KBr used as the background. Spectra
were baseline corrected and normalised to the zeolite overtones
(1950–2050 cmÀ1). A deconvolution process was applied to isolate
the absorption bands of interest (see Section S6 in the Supporting
Information).
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towards partial methane oxidation (Section S7 in the Supporting
Information). Cu-ZSM-5 samples (0.7 g, 250–500 mm pellets) were
placed in a quartz plug flow reactor and activated overnight in
oxygen (50 mLminÀ1, 5008C). Samples were cooled to room tem-
perature, flushed with argon and then exposed to methane
(50 mLminÀ1, 1% in Ar) at 1508C for 1 h (108CminÀ1). The products
remained adsorbed to the catalyst surface. Two different analytical
methods were used on spent samples following activation and re-
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and combustion products monitored by MS. By the GC method,
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a FID and a Supelco Carbowax Amine column (30 m”530 mm ID).
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The authors would like to thank the EPSRC for financial support
through the CASTech (EP/G012156/1) project, Süd-Chemie for pro-
viding ZSM-5, ASEP for ICP and BET measurements and Stephen
McFarland for assistance with TEM. Thomas Sheppard would like
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Keywords: Cu-ZSM-5
oxidation · silylation
·
methane
·
methanol
·
partial
Received: September 8, 2015
Revised: October 10, 2015
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[2] C. Hammond, S. Conrad, I. Hermans, ChemSusChem 2012, 5, 1668–
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Published online on December 4, 2015
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