Selective, room-temperature transformation of methane to C1 oxygenates by deep UV photolysis over zeolites

Article abstract of DOI:10.1021/ja204559z

Methane can directly be transformed into liquid C₁ oxygenated products with selectivities above 95% at 13% conversion by deep UV photocatalysis, in the presence of H₂O and air. Pure silica zeolites, and more specifically, beta zeolite with a large number of internal silanol groups is active and selective, while amorphous silica with no micropores is much less efficient. Irradiation produces the homolytic cleavage of surface hydroxyl groups, leading to silyloxyl radicals that will generate methyl radicals from methane. The selectivity arises from the occurrence of the reaction in a confined space restricting the mobility of the radical intermediates that will be mostly attached to the solid surface. Energy consumption of the process is in the order of 7.2 Gcal × mol^-1 that compares very favorably with the energy required for transforming methane to synthesis gas (15.96 Gcal × mol^-1).

Full text of DOI:10.1021/ja204559z

ARTICLE
pubs.acs.org/JACS
Selective, Room-Temperature Transformation of Methane to C1
Oxygenates by Deep UV Photolysis over Zeolites
Francesc Sastre, Vicente Fornꢀes, Avelino Corma,* and Hermenegildo García*
Instituto Universitario de Tecnología Química CSIC-UPV, Universidad Politꢀecnica de Valencia, Av. De los Naranjos s/n,
46022 Valencia, Spain
S Supporting Information
b
ABSTRACT: Methane can directly be transformed into liquid C1 oxygenated products
with selectivities above 95% at 13% conversion by deep UV photocatalysis, in the
presence of H2O and air. Pure silica zeolites, and more specifically, beta zeolite with a
large number of internal silanol groups is active and selective, while amorphous silica
with no micropores is much less efficient. Irradiation produces the homolytic cleavage of
surface hydroxyl groups, leading to silyloxyl radicals that will generate methyl radicals
from methane. The selectivity arises from the occurrence of the reaction in a confined space restricting the mobility of the radical
intermediates that will be mostly attached to the solid surface. Energy consumption of the process is in the order of 7.2 Gcal  molÀ1
that compares very favorably with the energy required for transforming methane to synthesis gas (15.96 Gcal  molÀ1).
’ INTRODUCTION
conversion level of 13% in minutes. Interestingly the energy
required for 13% methane conversion is one-half of the current
energy used for the industrial conversion of methane to syn gas.
Methane is the major component of natural gas and the most
abundant feedstock.1À4 One of the main problems encountered
in the current exploitation of natural gas is that reserves are
generally in remote places and to be processed it has to be
transported as compressed gas over long distances. Currently,
methane is directly used in power plants, in refineries to produce
H2 by methane reforming, and to produce synthesis gas for
methanol production or to feed FischerÀTropsch units.5À11 It
would be of much interest to directly convert methane into liquid
fuels and particularly into methanol and C1 oxygenates.
Most of the work related to the direct conversion of methane
into liquid fuels involves catalytic reactions that are carried out at
temperatures well above 500 °C. The three major novel pro-
cesses currently under investigation are methane aromatization
with molybdenum containing zeolites,12,13 the oxidative coupling of
methane,14À16 and direct oxidation of methane into methanol.17À19
Although significant advances have been made in these pro-
cesses, product selectivity, or catalyst life are still important issues
to be solved.1,12,13,20
It is possible to functionalize alkanes (methane), even at room
temperature, in the gas phase by means of a radical chain
mechanism mediated by hydroxyl radicals.21À24 Unfortunately,
such a process results in a very low selectivity, giving rise to
extremely complex mixtures.21À24 The reason for this poor
selectivity is that as the reaction progresses, the primary oxyge-
nated products are more reactive than methane and undergo a
network of undesired secondary reactions.
’ RESULTS AND DISCUSSION
Chemistry of the Process. The present process for room
temperature methane activation is based on the general ability of
hydroxyl radicals to activate methane. However, to avoid the low
selectivity associated to hydroxyl radicals in the gas phase, the
(hydr)oxyl radicals generated are attached to a solid surface in a
confined micropore system, scavenging the initial methyl radicals
to disfavor consecutive reactions. Our process involves the synthesis
of zeolites with a large number of internal silanol groups (and possible
coadsorbed water). When the zeolites are subjected to deep UV
irradiation (165 or 185 nm), there is a homolytic cleavage of OÀH
bonds generating a hydrogen atom and a surface bound silyloxyl
radical (and hydroxyl in case water is present). Our hypothesis is that
the surface bound silyloxyl radicals shown in eq 1 should also be able
to abstract a hydrogen atom from methane to form methyl radicals or
to couple with methyl radicals to form silyl methyl ethers (eq 2) as it
was known to occur for free OH radicals in the gas phase.
3
In the present work we report an original photocatalytic
process that converts methane into C1 oxygenates at room-
temperature based on deep UV irradiation (λ < 200 nm) of
methane in the confined spaces of zeolites containing internal
OH groups in the presence of some oxygen. In this way C1
oxygenates are obtained with 95% selectivity at a methane
While OÀH chromophores are transparent to visible and UV
irradiations, these OÀH groups attached to the zeolite exhibit
Received: May 18, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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Table 1. Analytical and Textural Properties of the Zeolites Used in the Present Work
zeolite
Si/Al
pore size (Å)
surface area (m2/g)
pore volume (cm3/g)
population of silanol groupsa
silica
∞
∞
∞
22
∞
no micropores
5.5 Â 5.1
7.1 Â 6.6
100
10
20
22
33
30
31
silicalite
386
481
503
490
540
607
0.203
0.22
0.23
0.22
0.24
0.17
beta (Si,F)
beta (Al, F)
beta (Si,OH)
beta (Al, OH)
commercial beta
22
12.5
a Relative population of silanol groups.
a high absorption coefficients for wavelengths below 200 nm,
the limit of the UV region. Therefore, it is possible to activate
photochemically surface OH groups by irradiation at wave-
lengths shorter than 200 nm (deep UV). Prior work has
reported that the direct photolysis of water by irradiation
with deep UV generates hydroxyl radicals with a quantum
yield approaching unity.25 If this is so, we can then assume
that eq 1 will also occur with high quantum yields. In a
preliminary test of the concept, irradiation of the zeolite has
led to the detection of hydrogen in the gas phase, thus,
providing experimental support to the occurrence of such a
photochemical homolytic OÀH cleavage.
Nature of the Catalyst. To prove this novel concept of
deep UV functionalization of methane in a confined space, a
series of six zeolites was selected. The set of samples includes
two different structures (medium and large pore) containing
or not framework aluminum and varying the population of
internal silanol groups. A nonporous amorphous silica was
used for comparison purposes. Then, their activity and selec-
tivity for the photochemical functionalization of methane into
C1 oxygenates was tested. The characteristics of the different
zeolites used in this work are presented in Table 1. In this
Table 1, beta zeolite samples have been denoted indicating the
presence or absence of framework Al and the nature of the
mineralizing agent .26 Notice that the 12 membered ring large
pore zeolite (beta) has been synthesized with two different
mineralizing agents (F Àor OH+). In the case of the beta
zeolite synthesized in fluoride media the sample has lower
number of hydroxyl silanol groups (see 29Si MAS NMR and IR
spectroscopy in the Supporting Information, Figure 1S), than
the same zeolite synthesized in OHÀ media (Supporting
Information, Figure 1S). A second type of hydroxyl groups,
that is, bridging tAl(OH)Sit hydroxyls, have also been
generated in a Beta zeolite synthesized in OHÀ media, by
introducing Al in framework positions (see Supporting Infor-
mation, Figure 2S). With the materials prepared we should
be able to explore the potential role of the pore/cavities size,
the influence of the number of “active sites” (hydroxy groups),
as well as the impact of the different OH (bridged and silanols)
types on activity and selectivity of the photocatalyst for
methane transformation by deep UV irradiation.
Figure 1. 13C NMR spectrum recorded for zeolite beta (Al, F) after
1 h 165 nm photolysis at roomÀtemperature of 13C-labeled methane
containing 8 vol% O2.
difference being that the presence of oxygen reduces con-
siderably the contribution of the products in the gas phase,
decreasing the proportion of light hydrocarbons and increas-
ing significantly the percentage of methanol and C1 oxyge-
nates. Besides the products observed in the gas phase, a
significant amount of products were also present within the
pores, being the amount of the products in the gas phase very
low compared to the material present in the solid (see
Table 2). The total amount of organic compounds absorbed
in the catalyst after irradiation in the presence of oxygen was
determined by combustion chemical analysis. Meanwhile, the
nature of the products adsorbed in the solid phase was
determined by MAS 13C NMR spectroscopy using 13CH4 as
substrate. A representative example of the 13C NMR spectra
recorded for the solids after irradiation in the presence of
oxygen is presented in Figure 1. As it can be seen there, in the
presence of oxygen the 13C NMR spectra show signals
corresponding to physisorbed methanol (48 ppm), methoxy
groups (54 and 58 ppm) as well as formaldehyde acetals (from
97 to 99 ppm) and formic acid (168À175 ppm). The presence
of adsorbed methanol and methoxysilyl groups support the
mechanistic proposal based on eqs 1 and 2. Notably 13C NMR
spectra reveal the absence of ethane and light alkanes ad-
sorbed in the solid.
Deep UV Photolysis. Control experiments revealed that
almost negligible conversion takes place upon deep UV irradia-
tion in the absence of any solid in a chamber saturated with
water vapor.
When performing the photochemical reaction in the pre-
sence of zeolites, either in the absence or presence of oxygen,
light alkanes and hydrogen are detected in the gas phase, the
2D 13C NMR correlation spectroscopy confirmed that the
peaks from 167 to 169 ppm assigned to formic acid are no
associated to any methyl group. On the other hand, the fact
that the peaks at δ about 55 ppm correspond to methyl groups
was confirmed by recording off-resonance 13C NMR spectra
and observing quartets because of the residual coupling with
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Table 2. Activity Data and Product Distribution Observed after 5 min for the Room-Temperature 185 nm Photolysis of Methane
and 20 vol % Oxygen in the Presence of Silicates
solid
silica gel
silicalite
beta (Si F)
beta (Al F)
beta (Si OH)
beta (Al OH)
no catalyst
total conversion (%)a
0.50
82
0.85
91
0.86
99
1.63
>99
2.01
>99
1.66
98
0.49b
0
percentage in the solid phase
products in the gas phase (%)
C2
54.2
29.1
--
53.3
8.9
--
--
100
--
--
100
--
84.2
15.8
--
73.5
3.8
H2
20.7
79.3
CH3OH
37.8
5.1
products in the solid phase (%)
formic acid
formaldehyde
methanol
19.6
28.3
52.1
15.9
37.6
46.5
23.8
25.5
50.7
25.8
24.4
49.8
21.2
30.8
48.0
16.4
28.7
54.9
23.9
25.8
50.3
a Mass balances were higher than 95% The initial methane amount in these experiments is 3.7 mmol. b Mass balance of 85%
three protons. Furthermore, 13C NMR spectroscopy clearly
proves that in the presence of some oxygen, the selectivity
toward methanol and oxygenates is over 99% at conversions
over 10%, a value that is remarkable and without precedent.
Accordingly, the percentage of oxygen in the gas phase
decreases from the initial 8% to 6%, indicating that some
oxygen is consumed during the photolysis
Table 3. Methane Conversion and Other Catalytic Data As a
Function of the Amount of Commercial Beta 811 Zeolite
Employed as Photocatalyst for the Room-Temperature
185 nm Photolysis of Methane in the Presence of 20 vol %
Oxygen after 5 min Irradiation
mass photocatalyst (g)
total conversiona,
0.1
1.8
1
3.9
2
6.20
1b
13
The advantage of having a micropororous system like in
zeolites over a surface lacking microporosity is clearly deduced
from the fact that photolysis over silica gives the lowest
conversion of the series, though the amount of silanol groups
is much larger on the amorphous silica catalyst. Concerning
the influence of the zeolite composition and the presence of
framework Al in the selectivity, 13C NMR spectroscopy shows
that the presence of framework Al promotes over oxidation of
methanol to formaldehyde and formic acid, while purely
siliceous zeolites exhibit higher selectivity toward methanol
at similar methane conversion.
products in the gas phase (%) (less than 1% of total)
C2
32.15
53.12
H2
100 100 100
CH3OH
products in the solid phase (%) (over 99% of total)
formic acid
formaldehyde
methanol
23.9
25.8
50.2
25.4
24.3
50.3
17.7
25.8
56.5
36
22.5
40.5
a Mass balances higher than 90% b After 1 h irradiation.
Following the reaction mechanism presented in eqs 1À3 it
appears that the larger number of internal silanol groups
within the zeolite pores, the larger should be the activity for
methane transformation upon methane irradiation. This can
indeed be observed in the results given in Table 2 in where
methane conversion follows the order beta(OHÀ) > beta
(Al F) > beta(F), that it is in line with the relative amount
of internal silanols within the samples. Therefore, we can
already conclude that with respect to the solid catalyst,
microporous materials appears to be better than amorphous
silica, and among the zeolites, the larger the number of
internal silanol groups, the higher the catalyst activity. If this
is so, the conversion should be increased by increasing the
zeolite to CH4 ratio up to the point in which all the photons
entering into the system are absorbed.
One point of concern was to determine the influence that
the amount of adsorbed water may have on the product
distribution arising from deep UV photolysis of methane
over zeolites. Considering that zeolites tend to adsorb more
water when the number of silanol groups is larger we have
carried out the deep UV photolysis with ambient equili-
brated hydrated Beta (Si,OH) zeolite. To address the influ-
ence of coadsorbed water, twin experiments in which Beta
(Si, OH) was used in the hydrated form or after thermal
dehydration were carried out. Although some minor variations in
the proportion of the products adsorbed in the zeolite were
observed, it is safe to say that the water content in the zeolite
is not relevant at the conversion levels achieved in our
experiments.
In most of the experiments carried out in the presence of
oxygen the products adsorbed on the photocatalyst account
for most of the 99% of all methane converted. Combustion
chemical analysis to determine the carbon content of the
zeolite wafers shows that the carbon content also varies
depending on the zeolite structure, the irradiation wave-
length, and the irradiation time. Concerning the irradiation
wavelength, although selectivity and product distribution
were very similar in both cases, it was found that the
185 nm setup was more efficient and higher conversions were
obtained in less time and lower energy consumption than
when photolysis was performed with the 165 nm system.
Precedents in the literature have estimated that the quantum
yield of the homolytic OÀH bond breaking using photons in
the deep UV region was essentially the unity25 and the same
can be implied here for either 165 or 185 nm, photolysis.
Therefore the difference observed in our case should reflect
more differences in the design of the 165 or 185 nm system
than real differences in quantum yields. In addition, it was
found that methane conversion increases with the amount of
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photocatalyst present in the photoreactor. (See results in
Table 3). This influence can be simply rationalized consider-
ing that the population of silanols needed in eqs 1À3 in-
creases with the amount of photocatalyst. It is remarkable
that methane conversions above 13% could be obtained in 1 h
at 185 nm irradiation with selectivity over 99% toward
oxygenated products. Blank controls consisting on the irra-
diation of methane in the presence of moisture-saturated
oxygen atmosphere at 185 nm show negligible conversion,
thus, reinforcing again the need of silanol groups to promote
methane conversion.
Two-Step Cycle. On the basis of the characterization of the
reaction products, it is possible to device a two-step cycle
consisting of (a) irradiation of methane over the zeolite
to form oxygenated species and (b) desorption of these
oxygenated compounds formed and regeneration of the
solid. To assess the feasibility of the two-step process, the
beta (Al, OH) zeolite was submitted to three cycles involving
each time photolysis in a methane/air atmosphere followed
by washing the solid with water to remove the adsorbed
products. A summary of the MAS 13C NMR spectra recorded
for this series of consecutive irradiation/desorption experi-
ments are shown in Figure 2. It can be seen there that the
aqueous phase after washing the Beta (Al, OH) zeolite clearly
contains methanol, formol and formic acid (spectra b and e in
Figure 2), while the resultant solid is almost free of organic
compounds (spectra c and f in Figure 2). In addition, these
experiments also indicated that the zeolite can be reused
(at least 3 times) without observing changes in the behavior
of the material.
Reaction Mechanism. Concerning the reaction mechanism,
we propose that deep UV irradiation will produce the homo-
lytic splitting of silanol groups into silyloxyl radicals and a
hydrogen atom (Scheme 1) as it has been reported in the
literature for water splitting.25 These oxyl radicals will gen-
erate methyl radicals from methane as it is well-known in
vapor phase radical chemistry for hydroxyl radicals. Depend-
ing on the presence or absence of oxygen these methyl radicals
will evolve giving oxygenated species or hydrocarbons, re-
spectively. Selectivity will arise from the occurrence of the
reaction on a surface and in a confined space, restricting the
mobility of the radical intermediates that will be mostly
attached to the solid surface. As commented before, the
presence of water does not influence selectivity, at least, in
the range of conversion studied here. In contrast, the presence
of oxygen is crucial to scavenge methyl radicals, stopping the
formation of hydrocarbons (see Scheme 1).
Energy Consumption. A first calculation of the energy
required for the room temperature methane activation based
on deep UV photolysis over a solid support, was carried out by
determining the energy consumption of the lamp to achieve a
certain methane conversion. The results indicate that while
the transformation of 1 mol of methane to syn gas requires
Figure 2. 13C NMR spectrum recorded at room temperature for
(a) zeolite beta (Si, OH) after 11 h 165 nm photolysis at room
temperature of 13CH4 containing 8 vol% O2, (b) room temperature
spectrum of the water solution after desorbing the products at 150 °C for
1.5 h, (c) photolyzed zeolite beta (Si,OH) after being submitted to water
desorption, (d) zeolite beta (Si, OH) after a second 11 h 165 nm photolysis
at roomÀtemperature of methane containing 8 vol% O2, (e) room
temperature spectrum of water used to desorb products at 150 °C after
1.5 h, and (f) zeolite Beta (Si,OH) after a second water desorption.
about 15.9 Gcal  mol
,
À1 27 to achieve 13% conversion using
the 185 nm lamp (60 min irradiation), the energy consump-
tion is 7.16 Gcal  molÀ1
.
Scheme 1. Proposed Mechanism for the Deep UV Transformation of Methane on Silica Surface
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’ CONCLUSIONS
deep UV photoreactors and emission spectra of the lamps, and
determination of the mass balances. This information is available
free of charge via the Internet at http://pubs.acs.org/.
Using 13C labeled methane and determining the product
distribution by solid state 13C NMR spectroscopy, it has been
shown that deep UV photolysis of methane over Beta zeolites
containing internal silanol groups allows the room-temperature
transformation of methane into C1 oxygenates. When a small
amount of oxygen is present, the product selectivity toward C1
oxygenated products, that is, methanol, formaldehyde, and
formic acid is over 95%. The nature of the zeolite plays a role
in the conversion, the most appropriate material being the
all-silica beta zeolite prepared in OHÀ media to maximize the
number of internal silanol groups. In addition, it has been found
that the presence of coadsorbed water does not alter conversion
or product distribution in the experimental range studied here.
The estimated energy consumption of the process to convert one
mole of methane is about one-half than the energy required for
the conventional methane steam reforming process.
’ AUTHOR INFORMATION
Corresponding Author
acorma@itq.upv.es; hgarcia@qim.upv.es
’ ACKNOWLEDGMENT
Financial support by the Spanish Ministry of Science and
Innovation (MICINN Grants CTQ-2009-0585 and CONSOLI-
DER INGENIO MULTICAT) is gratefully acknowledged.
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’ ASSOCIATED CONTENT
Supporting Information. MAS 29Si NMR spectra of
S
b
beta(Si, F), beta(Si, OH), and beta(Al, OH), blank control of
deep UV photolysis of commercial beta zeolite, schematic of
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