Enhancement and limits of the selective oxidation of methane to formaldehyde over V-SBA-15: Influence of water cofeed and product decomposition

Article abstract of DOI:10.1016/j.catcom.2021.106317

The possibility of a selective catalytic oxidation of methane to formaldehyde has been known for decades, and positive influences of water added to the reaction mixture and ultra-short contact times have been reported. In the present work, the complexity of interdependencies has been revealed. Specific parameter variations can increase conversion and selectivity of the target product. Surprisingly, formaldehyde formation over VO_x species and its decomposition in gas phase were equally dependent on the partial pressure of the added water, so that the sweet spot can only be found by varying the residence time.

Full text of DOI:10.1016/j.catcom.2021.106317

Catalysis Communications 155 (2021) 106317
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Enhancement and limits of the selective oxidation of methane to
formaldehyde over V-SBA-15: Influence of water cofeed and
product decomposition
*
Benny Kunkel, Sebastian Wohlrab
Leibniz-Institut für Katalyse e.V. an der Universit a¨ t Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
The possibility of a selective catalytic oxidation of methane to formaldehyde has been known for decades, and
positive influences of water added to the reaction mixture and ultra-short contact times have been reported. In
the present work, the complexity of interdependencies has been revealed. Specific parameter variations can
Selective oxidation
Methane
Formaldehyde
V-SBA-15
x
increase conversion and selectivity of the target product. Surprisingly, formaldehyde formation over VO species
and its decomposition in gas phase were equally dependent on the partial pressure of the added water, so that the
sweet spot can only be found by varying the residence time.
Water cofeed
1
. Introduction
Methane is the most inert alkane due to its symmetry and stability of
trendsetting. Only water presents an attractive option due to its availability
and non-toxicity. Its influence when cofed already became evident in three
differentways:i)equilibriaareshiftedtolessoxidizedC1-compoundswhen
water is a reaction product, ii) the structure of the catalyst is altered by
forming additional Brønsted acid sites via hydrolysis, and iii) water com-
petes for adsorption on active catalytic sites [22].
the low polarized C-H bonds in combination with the higher reactivity of
its oxidized products (e.g., methanol, formaldehyde, and formic acid).
These properties cause methane to be the most difficult alkane to up-
grade [1]. Hence, processes directly yielding oxygenates from methane
are challenging and often referred as the holy grail of heterogeneous
catalysis. Within the possible oxygenates which can be directly ob-
tained, formaldehyde can be produced from methane and oxygen with a
We have previously reported on the impact of different reaction pa-
rameters on the selective oxidation of methane to formaldehyde over an in
situ prepared V-SBA-15 catalyst [2]. Therein, the addition of water to the
reaction mixturewas demonstrated to have ahighinfluence on the reaction
as it drastically increased methane conversion, while improving the selec-
tivitytoformaldehyde. Therefore, weinvestigatedtheinfluenceofwateron
the selective oxidation of methane to formaldehyde in more detail and how
water alters the consecutive oxidation of the desired product of formalde-
hyde. Herewith, we provide new information of how the yield of the target
product can be increased by adding water, and what one needs to know
about the stability of the same under the reaction conditions applied.
ꢀ 1
ꢀ 1
highest productivity of 13.6 kgCH2O kgcat h . When using a selective V-
SBA-15 catalyst, it is the ultra-short residence time that becomes deci-
sive in achieving such high values [2].
Short residence times, however, cause low conversions. A reduction in
ꢀ 1
gas-hourly space velocity (GHSV, h ) consequently leads to an increase in
conversion, but the longer contact time is responsible for further oxidation
and thus reduction in selectivity for formaldehyde [3]. Hence, different
attempts aiming to improve the selectivity of reaction were made and re-
ported. In previous studies, optimized catalysts as well as reaction condi-
tions were discussed [4–7], which made use of new reactor set-ups, like
continuous recycling [8,9] and rapid quenching [10,11], or the addition of
cofeeds. Especially, the latter offers promising opportunities due to the
2. Experimental
2.1. Catalyst preparation
variety of possible additives, namely, H
O [17], HCl [18], CH Cl [19,20] or H
action network in different ways. But not all of these molecules are really
2
O [5,12–14], NO
2
2
[15], NO [16],
For the synthesis of the V-SBA-15 catalyst, an upscaled version of a
previously described procedure was used [2]. In short, 16 g Pluronic
P123 (Sigma Aldrich) were dissolved in 480 g of deionized water at
N
2
x
y
[21], which all do affect the re-
*
Corresponding author.
E-mail address: sebastian.wohlrab@catalysis.de (S. Wohlrab).
https://doi.org/10.1016/j.catcom.2021.106317
Received 19 February 2021; Received in revised form 15 April 2021; Accepted 17 April 2021
Available online 21 April 2021
1
566-7367/© 2021 The Author(s).
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/4.0/).

B. Kunkel and S. Wohlrab
Catalysis Communications 155 (2021) 106317
room temperature under stirring. Afterwards, 0.59 g NH
4
VO
3
(Merck,
nitrogen sorption curves as well as UV–visspectra of the catalyst at ambient
conditions and after dehydration are given in Figs. S4 and S5 (ESI). A more
in-depth characterization and interpretation of this material can be found
elsewhere [2].
9
3
9%) were added followed by further stirring for 1 h. Subsequently,
4.7 g tetraethyl orthosilicate (Sigma Aldrich, 98%) were added drop-
wise. The pH value was adjusted to 2.5 using an aqueous HCl and
checked with a pH meter (Multilab 540, WTW, Weilheim, Germany).
The influence of water cofeeding on the selective oxidation of
◦
◦
The reaction mixture was then heated to 40 C. After stirring for 24 h,
methane was investigated in the 560–640 C range as shown in Fig. 1,
the suspension was transferred into a PTFE inlet for hydrothermal
and the partial pressure of water was varied in the 0–10 kPa range. At
◦
◦
treatment at 100 C for 48 h. Finally, the precipitate was filtered,
560 C and 0 kPa of water cofeed, the observed methane conversion was
◦
◦
washed with water, dried at 80 C overnight and calcined at 625 C for
0.1% and increased to 0.2% at pH2O = 1.7 kPa, followed by a further
decrease to 0.02% at pH2O = 10 kPa. While the overall conversion of
methane increases with increasing reaction temperature, the maximum
in activity was broadened and shifted to higher amounts of water cofeed
ꢀ
1
1
6 h (heating rate 1 K min ) yielding 10 g of V-SBA-15.
Plain SBA-15 was synthesized by dissolving 16 g Pluronic P123 in 480 g
2
M HCl with subsequent stirringat room temperature. When dissolved, the
◦
◦
solution was heated to 40 C followed by dropwise addition of 34.7 g tet-
in the reaction gas mixture. At 580 C, the maximum in activity was
◦
◦
raethyl orthosilicate. After further stirring at 40 C for 24 h, the suspension
observed at pH2O = 3.4 kPa, and at 600 C at pH2O = 7.2 kPa. Slightly
◦
was transferred into a PTFE inlet for ahydrothermal treatment at 100 C for
above this temperature the maximum lies out of the investigated range.
As reported in previous studies, the addition of water leads to the
hydrolysisof V–O–Si andV–O–Vbondsinthecatalyststructure, forming
V–OH groups which were reported to be highly active in methane
oxidation [23,24]. Upon further addition of water, more and more of
these moieties are blocked by water molecules leading to the observed
decrease in activity - and increased selectivity to oxygenates - at constant
reactiontemperature. Sincehydrolysisandblockingoftheactivesitesare
no entropically favored processes, the sweet spot between hydrolysis and
blocking by water is shifted to higher partial pressures of water with
increasingtemperature. Previoustheoreticalstudiesalreadyinvestigated
24 h. Subsequent steps were equal to the synthesis of V-SBA-15.
2
.2. Catalyst characterization
The amount of vanadium in V-SBA-15 was determined via induc-
tively coupled plasma-optical emission spectrometry (Varian 715-ES
spectrometer). Prior to analysis, the sample was dissolved by micro-
◦
wave irradiation at 200 C and 80 bar in a mixture of aqua regia and
hydrofluoric acid. Powder XRD data were collected using the X’Pert Pro
(
Panalytical) diffractometer with Ni-filtered Cu-K
α
radiation. Nitrogen
◦
sorption was carried out after removal of water at 400 C under reduced
pressure in ASAP2010 (Micromeritics) apparatus, and analyzed by the
BET method. Ultraviolet-visible spectra were measured using an Ava
Spec-2048 spectrometer equipped with a FCR-7UV400C-2 reflection
probe. V-SBA-15 was diluted with three parts SBA-15 and measured
the thermodynamics of the hydration of VO
Justliketheconversion,thewatercofeedhasaninfluenceonthedifferent
production rates of CH O, CO, CH OH and CO as shown in Fig. S6 (ESI).
x
species on silica [25,26].
2
3
2
In catalytic reactions, however, it is crucial to increase the selec-
tivity, while maintaining the reactant conversion. In an isothermal
catalytic performance test series, both the GHSV and the water content
were specifically varied to demonstrate the decisive influences in this
◦
prior and after dehydration at 350 C for 1 h in flowing synthetic air
ꢀ 1
(
100 mL min ) with BaSO
.3. Catalytic oxidation of methane and formaldehyde
Measurements of the catalyst performance were carried out using a
4
as white standard.
◦
main criterion. Fig. 2 shows selectivity versus conversion plots at 640 C
2
for all products (the set of conditions is given in Table S1, ESI). At this
temperature, the positive influence of water increases with increasing
addition of water. In Fig. 2, curve a, the partial pressure of water was
ꢀ 1
ꢀ 1
homemade fixed-bed plug-flow quartz reactor. The reactor had an inner
diameterof8mmthatdecreasedto4mminthemiddleatwhichthecatalyst
bed was placed and kept in place by quartz wool. The temperature was
measured and controlled at the middle of the catalyst bed. The amount of
catalyst was 25 mg diluted with 225 mg of quartz sand. The total flow rate
varied at a constant GHSV of 480,000 L kg
h . With increasing partial
pressure of water, the conversion increases from 1.4 to 4.7% but the
selectivity decreases from 59 to 42%, which is obviously due to the
consecutive oxidation/combustion of formaldehyde.
ꢀ 1
ꢀ 1 ꢀ 1
was 200 mL min resulting in a GHSV of 480,000 L kg h . Effects of
residencetimevariationwereinvestigatedbyvaryingthetotalvolumeflow
rate and catalyst mass (see Table S1, ESI). The composition of the detected
C1 components in the product gas stream during the experiments are given
in Fig. S1 (ESI). The massflowratesof the respectivegaseswereadjustedby
means of mass flow controllers (MKS-Instruments, Andover, USA). Water
was added by means of a syringe pump which injected liquid water into a
◦
heated (150 C) transfer line. Formaldehyde was added via aheatedreactor
filled with paraformaldehyde through which amixture of nitrogen/ oxygen
was passed. Product analysis was performed by means of IR gas analyzers
(Matrix MG01, Bruker, Ettlingen, Germany). Spectra of the carbonyl
stretching region with and without water cofeed are given in Fig. S2 (ESI)
along with an example illustrating the C1-balance over the course of an
experiment (Fig. S3, ESI). The residence time for the experiments in the
empty reactor was calculated as the ratio of the reactor volume in the
isothermal zone (6.6 mL) to the respective flow. Further formulae for the
calculation of catalytic properties are given in the ESI as well as a scheme of
the used experimental set-up (Scheme S1, ESI).
3
. Results and discussion
◦
Fig. 1. Methane conversion over V-SBA-15 (1.74 wt% V) in the 560–640 C range
3
.1. Influence of water on the oxidation of methane
at water levels between 0 and 10 kPa. Reaction conditions: GHSV = 480,000
ꢀ 1
ꢀ 1
h , 20 kPa CH
L kg
4 2 2 2
, 10 kPa O , x kPa H O and 70-x kPa N . The squares show
The V-SBA-15 catalyst used in this study exhibits a Vanadium content of
the concentration of water with the maximum methane conversion at the
respective reaction temperature.
2
ꢀ 1
1
.74 wt% and specific surface area of 762 m
g
. Powder XRD data,
2

B. Kunkel and S. Wohlrab
Catalysis Communications 155 (2021) 106317
◦
Fig. 2. Selectivity toward CH
2
O, CO, CH
3
OH and CO
2
versus methane conversion at 640 C, 20 kPa CH
4
and 10 kPa O
; (b) GHSV = 168,000–480,000 L kg
2
. Dotted lines represent a visual guidance.
2
in the feed obtained over V-SBA-15 (1.74 wt
ꢀ 1
ꢀ 1
ꢀ 1 ꢀ 1
%
V). Reaction conditions: (a) GHSV = 480,000 L kg
h
, x kPa H
2
O, 70-x kPa N
2
2 2
h , 0 kPa H O and 70 kPa N ; (c) GHSV
ꢀ 1
ꢀ 1
, 10 kPa H
=
480,000-1,500,000 L kg
h
2
O and 60 kPa N
In Fig. 2, curve b and c, the GHSV was varied at constant partial
do not continue to react as strongly as when no water is added in the
feed. Even significant amounts of methanol are formed with ultra-short
residence times. Probably, methanol is the direct precursor of formal-
dehyde. Due to the catalytic suitability of the V-species, methanol is
oxidized to formaldehyde [29,30], especially with increasing residence
time. However, the latter cannot be prevented in any way, as the
following series of experiments on formaldehyde stability under reac-
tion conditions showed.
pressure of water, either 0 or 10 kPa. At a conversion of 4.7%, the
selectivity without water is 33% leading to a yield of 1.55%, while at
p
H2O = 10 kPa, a yield of 1.97% formaldehyde was reached. Moreover,
at the lowest probed conversion of 1.4%, the difference in selectivity is
6
% in favor of water-rich conditions.
These findings are somewhat contradictory to previous studies,
which reported no strong effect of selectivity enhancement for the V-
SBA-15 systems [5,13], while it was stated the opposite for example, for
silica supported iron-phosphate [12] or molybdenum [27,28]. Obvi-
3
.2. Influence of water on the oxidation of formaldehyde
ously, the addition of water can increase the selectivity of VO
toward formaldehyde at the same conversion level of methane. The fact
that a molecular change of the VO centers could be causal for this
behavior has already been shown by Launay et al. [24], who detected
the hydroxylation of the VO species by infrared spectroscopy and
x
species
Besides of the activation of methane, the preservation of the pro-
x
duced formaldehyde presents the major challenge in the selective
oxidation. Hence, the stability of formaldehyde was probed under re-
action conditions over V-SBA-15 and in an empty reactor (Fig. 3) as well
as over SBA-15 alone (Fig. S7, ESI).
x
postulated an improved electron delocalization between the V–O bond
initiated by this. Thus, the prerequisite for a much more favored
methane activation is given, namely, electrophilic oxygen species favor
As shown in Fig. 3 (a), there is a relatively sharp drop in the con-
version of formaldehyde over V-SBA-15 between 0 and 1 kPa of water
partial pressure. This can be explained based on the competitive
adsorption of formaldehyde and water on the same active sites, which
has already been shown via IR spectroscopy with the example of
homolytic C–H cleavage and lead to the formation of VOCH
3
species. It
can be seen that under increased water partial pressure the oxygenates,
which are thermodynamically unstable under these reaction conditions,
3

B. Kunkel and S. Wohlrab
Catalysis Communications 155 (2021) 106317
◦
Fig. 3. (a) Formaldehyde conversion in the 560–640 C range over V-SBA-15 (1.74 wt% V) and in an empty reactor at water levels between 0 and 10 kPa. Reaction
ꢀ
1
ꢀ 1
. (b) Formaldehyde conversion in the 560–640 ◦C range at different
conditions: GHSV = 480,000 L kg
h
, 1 kPa CH
2
O, 10 kPa O
2
, x kPa H
2
O and 89-x kPa N
2
residence times. Reaction conditions: 1 kPa CH
2
O, 10 kPa O
2
, 10 kPa H
2
O and 79 kPa N .
2
molybdenum oxide surfaces [31]. Moreover, since methane activation is
increased by water addition and formaldehyde oxidation is suppressed,
one can conclude that both processes require different sites, and the
addition of water promotes the formation of active centers for methane
mechanism of methane activation and transformation of the oxygenates
toward CO when high water partial pressures are applied. On the other
hand, the formation of CO
. Conclusions
In the present study, the influence of water cofed in the selective
2
from direct total oxidation is suppressed.
◦
activation. Below 600 C, this picture seems valid over the whole pH2O
4
range, namely, the conversion slowly decreases with increasing water
partial pressure. However, at higher temperatures, this statement loses
its validity, therefore, there must be another effect to counteract it.
oxidation of methane to formaldehyde was investigated over a wide
temperature range and water partial pressures in the 0–10 kPa range. A
temperature-dependent maximum in activity was found for methane
oxidation which was shifted to higher water partial pressures as the
temperature was increased. In contrast, formaldehyde conversion over
V-SBA-15 was suppressed with low water addition, suggesting the
presence of different active sites for methane and formaldehyde oxida-
tion. At high water partial pressures, an enhanced contribution of gas
phase formaldehyde oxidation was observed. The interplay between
activity enhancement in methane activation and lower formaldehyde
decomposition rate can lead to an improved but limited selectivity in the
investigated reaction.
The cause is evident from stability experiments without catalyst
◦
(
empty reactor). The conversion, while being unaffected at 560 C, in-
creases at elevated temperatures by the addition of water. Remarkably,
◦
at 640 C the conversion in the empty reactor exceeds the conversion
over V-SBA-15 at pH2O ≥ 5 kPa. This observation suggests for a pre-
dominance of gas phase decomposition at high water partial pressure,
which may proceed via the formation of highly labile methanediol in-
termediate species, offering new reaction paths with subsequent dehy-
drogenation to formic acid and dehydration toward carbon monoxide
and water formation. Indeed, formic acid was observed in small quan-
tities, up to 50 ppm in formaldehyde oxidation experiments, and for
formic acid dehydration, water was identified as a catalyst that signifi-
cantly reduces activation barriers, as shown by previous DFT studies
[
32–34]. This gas phase reaction can only be suppressed by lowering the
Declaration of Competing Interest
residence time as shown in Fig. 3 (b). However, it should be stated that
the results from formaldehyde decomposition experiments are not
completely transferable to the methane oxidation case, since formalde-
hyde fed can be decomposed over the whole length of the reactor. In
contrast to that, in methane oxidation, formaldehyde is first produced
within the catalyst bed and decomposed within and after the catalyst
bed at the exit of the reactor. Hence, the formaldehyde decomposition is
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
somewhat overestimated in these experiments. To complete, at pH2O
<
The authors would like to thank Dr. Henrik Lund, Kathleen Schubert,
Anja Simmula and Reinhard Eckelt for analytical support. The authors
are grateful for funding to the Deutsche Forschungsgemeinschaft (DFG,
grant number 351914377, WO 1531/4-1) and our partners at the Fed-
eral Institute of Materials Research and Testing (BAM), Dr. Ana Buza-
nich and Dr. Anke Kabelitz.
2
.5 kPa, SBA-15 alone (Fig. S7, ESI) converts more formaldehyde than
the empty reactor. Similarly, for the V-SBA-15, a decrease of formal-
dehyde conversion is observed at low water partial pressure as well as a
nearly linear increase at higher partial pressure.
In all experiments, carbon monoxide was detected as main product,
and carbon dioxide as by-product. The respective formation rates over
V-SBA-15 are given in Fig. S8 (ESI). As main product, the carbon mon-
oxide formation rates follow the trend of formaldehyde conversion.
Contrary to that, carbon dioxide production is continuously suppressed
with increasing water addition. This result also shows a consecutive
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106317.
4

B. Kunkel and S. Wohlrab
Catalysis Communications 155 (2021) 106317
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