Low Temperature Direct Conversion of Methane using a Solid Superacid

Article abstract of DOI:10.1002/cctc.201801310

The direct conversion of methane to higher hydrocarbons and hydrogen can be catalyzed using “superacids”: nCH₄→C_nH_m+xH₂. The first report of catalytic oligomerization of methane using superacids was that of Olah et al., who demonstrated the superacidity of FSO₃H?SbF₅, which is a liquid. More recently, Vasireddy et al. showed that gas-phase HBr/AlBr₃ was an active superacid. The only reported solid superacid for methane oligomerization is sulfated zirconia (SZ). Here, we report a new class of Br-based solid superacids, AlBr_x/H-ZSM-5 (“ABZ-5”, x=1 or 2). ABZ-5 is based on gas-phase HBr/AlBr₃, with the objective of synthesizing a heterogeneous analogue of the gas-phase superacid HBr/AlBr₃. The results show that ABZ-5 is significantly more active than SZ. Perhaps more significantly, results here show methane conversions of ～1 % at 300 °C using ABZ-5. By comparison with SZ, 350 °C is the lowest temperature reported in the literature at which measurable conversions are shown, and the corresponding methane conversions were <0.15. Here, we demonstrate direct conversion of methane using a solid superacid catalyst, AlBr_x/H-ZSM-5. This solid catalyst is synthesized using a vapor-phase process in which AlBr₃ vapor is grafted on to solid H-SZM-5. This catalyst is characterized using NH₃-TPD, XRD, and DRIFTS. Hydrocarbon products observed in the temperature range of 200–400 °C include both C₂–C₆ hydrocarbons and aromatics.

Full text of DOI:10.1002/cctc.201801310

Accepted Article
Title: Low Temperature Direct Conversion of Methane using a Solid
Superacid
Authors: Swarom R Kanitkar, James H Carter, Graham J Hutchings,
Kunlun Ding, and James Spivey
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201801310
Link to VoR: http://dx.doi.org/10.1002/cctc.201801310
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Low Temperature Direct Conversion of Methane using a Solid
Superacid
Swarom Kanitkar,[a] James H. Carter, [b] Graham J. Hutchings,[a,b] Kunlun Ding, [a] James J Spivey,*[a]
Abstract: The direct conversion of methane to higher hydrocarbons
Activation of methane typically requires very high temperatures
[3]
and hydrogen can be catalyzed using “superacids”:
and oxidants like O2 (partial oxidation/oxidative coupling)
,
.
CO2/H2O (reforming) [4], S2 (sulfidation) [5], or Br2 (bromination) [6]
푛퐶퐻4푠→푢푝푒푟푎푐푖푑 퐶푛퐻푚 +푥퐻2
These reactions produce intermediates like syngas or
CH3OH/CH3SH/CH3Br, which are subsequently converted into
hydrocarbons or oxygenates. In these reactions, methane
activation at elevated temperatures leads to another challenge:
complete dehydrogenation of methane into elemental carbon or
coke.
The first report of catalytic oligomerization of methane using
superacids was that of Olah et al., who demonstrated the superacidity
of FSO3H-SbF5, which is a liquid. More recently, Vasireddy et al.
showed that gas-phase HBr/AlBr3 was an active superacid. The only
reported solid superacid for methane oligomerization is sulfated
zirconia (SZ). Here, we report a new class of Br-based solid
superacids, AlBrx/H-ZSM-5 (“ABZ-5”, x=1 or 2). ABZ-5 is based on
Direct (non-oxidative) conversion of methane is an
alternative to processes that require intermediates such as
syngas. Catalysts involving direct conversion include lattice-
gas-phase HBr/AlBr3, with the objective of synthesizing
a
[7]
[8]
confined single Fe sites
or modified-zeolites
at very high
heterogeneous analogue of the gas-phase superacid HBr/AlBr3. The
results show that ABZ-5 is significantly more active than SZ. Perhaps
more significantly, results here show methane conversions of ~1% at
300⁰C using ABZ-5. By comparison with SZ, 350⁰C is the lowest
temperature reported in the literature at which measureable
conversions are shown, and the corresponding methane conversions
were <0.15. Here, we demonstrate direct conversion of methane
using a solid superacid catalyst, AlBrx/H-ZSM-5. This solid catalyst is
synthesized using a vapor-phase process in which AlBr3 vapor is
grafted on to solid H-SZM-5. This catalyst is characterized using NH3-
TPD, XRD, and DRIFTS. Hydrocarbon products observed in the
temperature range of 200 – 400oC include both C2-C6 hydrocarbons
and aromatics.
temperatures (700–1100°C). One particularly promising
approach for the direct conversion of methane at lower
temperatures is based on superacids (Figure 1). These catalysts
were first demonstrated by Olah [9], using liquid “magic acid”
(FSO3H-SbF5). Other superacids have been reported for the
oligomerization of methane: (SbF5-HF)[10], (HBr-AlBr3) [11], and
sulfated zirconia [12]
.
Introduction
Methane conversion to higher-value products has been a topic of
research for decades, but the “shale gas revolution” has made
low-cost natural gas readily available. This has presented a
unique opportunity to take advantage of the immense, previously
inaccessible reserves of natural gas. This has already had major
Figure 1. Current and proposed process for converting methane into olefins
and fuels. At present, methane is reformed into syngas and then transformed
into higher hydrocarbons through the Fischer-Tropsch synthesis. This indirect
process requires high temperatures and pressures. Methane oligomerization
using solid superacids offers a direct route to converting methane into higher
hydrocarbons under mild conditions.
impacts on the energy and chemical industries [1]
.
Methane is a thermodynamically stable, non-polar molecule
[2]
containing strong, equivalent (C-H) bonds (434 kJ/mol)
.
HBr-AlBr3 is aparticularly promising gas-phase superacid catalyst.
This is a proven strong superacid, and has been compared to the
strongest known superacid, “magic acid” [13]. Recently [11b], this
gas-phase superacid has been shown to convert CH4 into higher
hydrocarbons (C2 – C26) and H2 at temperatures as low as 200-
400°C. Although CH4 conversions were relatively high, separating
the catalyst from the gaseous products was difficult, making it
practically impossible to carry out a carbon balance, or to envision
a practical process based on a gas-phase superacid. This could
be addressed by incorporating the proven gaseous HBr-AlBr3
superacid onto a solid support.
[a]
[b]
M.S.-Chem. Eng., S. Kanitkar, Dr. K. Ding, Dr. J. J. Spivey, Dr. G.
Hutchings
Cain Department of Chemical Engineering
Louisiana State University
Baton Rouge, LA – 70803, USA
E-mail: jjspivey@lsu.edu
Dr. J. Carter, Dr. G. Hutchings
Cardiff Catalysis Institute
Cardiff University
Cardiff, UK CF10 3AT
Any process based on direct conversion of methane using
superacids at the temperatures reported here (200-400 oC) must
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201801310
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address the equilibria limitations. At the conditions of interest here,
equilibrium methane conversion is ~ 0.5 to 12 % (with coke
formation allowed) depending on the product composition,
including hydrogen, alkanes/alkenes, polynuclear aromatics,
among others. It is outside the scope of our study here to
determine if a feasible process based on superacid catalysis of
methane oligomerization is practical. However, we recognize that
a solid superacid could be an essential element of a practical
process.
Table 1 shows that results for ABZ-5 at the temperature of 300 °C,
while reported studies were carried out at higher temperatures,
350-450 oC. SV values in the literature were 0.44 - 4.5 L gcat-1 h-1,
-1
while here the space velocity was 18 L gcat h-1, a factor of 4-40
times greater than those reported here. Table 1 suggests that
ABZ-5 is more active than SZ. In order to test this, a direct
comparison was carried out.
Direct comparison of ABZ-5 with other solid superacids
We are aware of no report of a solid superacid based on
bromine for methane oligomerization, despite the potential
significance of an active solid superacid based on the gas-phase
HBr/AlBr3 catalyst sites. Our study here describes the synthesis,
characterization and direct conversion of methane to higher
hydrocarbons using a solid superacid, AlBrx/H-ZSM-5 (“ABZ-5”, x
= 1 or 2). Though the oligomerization of methane is the focus here,
Table 2 directly compares the activity of four solid superacids in
CH4 oligomerization at identical conditions: H-ZSM-5 (as-
received), and three catalysts synthesized here (SI
(supplementary information), 1.2): sulfated zirconia (SZ), AlBr3
supported on SiO2 (ABSi) and AlBr3 supported onZSM-5 (ABZ-5).
An intermediate temperature of 300 oC was selected to compare
solid superacid product selectivities directly.
there are clear significant opportunities in related acid-catalyzed
[14]
processes, e.g. alkylations and acylations
;
alkane
isomerization and cracking [14c, 15]; and polymerization [14c]
.
Table 2. Time on stream (TOS) activity of ABZ-5 catalyst and its comparison
against various catalysts for CH4 oligomerization (300 oC, 1 atm, 9 Lgcat-1h-1,
CH4 conv. ~ 1% for ABZ-5)
Product Selectivity (%)[e]
Results and Discussion
TOS
(hr)
TOF
(hr-1)
Cat.
X
T[b] EB[c] (o,m
,p)[d]
Comparison with previous research
C2=
C3
C4
B[a]
Table 1 compares the literature on direct conversion of methane
using solid superacids with a representative result on ABZ-5
reported here (300 oC).
ABZ-5
ABZ-5
ABZ-5
1
2
3
0.1
3.0
8.2
7.9
2.3 0.5 18.5 59.6
8.1
9.5
0.015
0.004
14.9
17.8
-
-
-
-
26.1 41.3
Table 1. Comparison of performance of ABZ-5 with other solid superacids
Space
13.5
38
-
-
-
30.6
Temp[a]
Catalyst
velocity (L Conv. (%)
gcat-1 h-1)
Notes
Ref.
(°C)
Blank
(Quartz
Wool)
1
0
-
-
-
-
-
[12b]
SZ/Fe/Mn
350-450 0.76
< 0.15
NR*
Flow reactor
Pulsed and
flow exp.
[16]
SZ/Fe/Mn[c]
450
450
450
450
450
400
400
300
4.5
4.5
4.5
4.5
4.5
0.6
0.6
18
ABSi
1
1
1
0
0
0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Pulsed and
flow exp.
[16]
[16]
[16]
[16]
[17]
[17]
SZ/Co/Mn[c]
SZ/Cu[c]
NR
NR
NR
NR
H-
ZSM-5
Pulsed and
flow exp.
SZ
Pulsed and
flow exp.
SZ/Ni[c]
[a] Benzene [b] Toluene [c] Ethylbenzene [d] Xylene (ortho, meta, para)
Pulsed and
flow exp.
[e]Selectivity is calculated based on total number of observed products.(-) =
not detected
[c]
SZ/Al2O3
0.35 after
5 h
recirculating
reactor
SZ[d]
Table 2 shows that only ABZ-5 is active, while there are no
measurable products from any of the other catalysts at these
conditions. A decrease in TOF with TOS for ABZ-5 indicates
deactivation. With time, there is a general increase in ethylene
and propane selectivity. Selectivity to aromatics is significant,
even when methane conversion is ~1%. The aromatic selectivity
also changes on-stream: after 1 h, ethylbenzene and toluene are
the major products, but after 3 h, no ethyl benzene was observed
and the selectivity to xylene and toluene increased significantly.
Despite low methane conversions, these results provide
clear evidence of methane activation at temperatures as low as
300 °C and demonstrates that ABZ-5 is a strong solid superacid.
1.9 after 5
h
recirculating
reactor
[d]
SZ/Al2O3
Fixed
bed
Current
Work
ABZ-5
~ 1
reactor
Table 1 shows that methane conversions in the literature are not
typically given. For studies in which conversions are reported,
values vary from 0.15-1.9%. However, a direct comparison of
~1% conversion for the ABZ-5 catalyst with the literature is not
possible because of the different reaction conditions. For example,
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Table S1 (SI) shows that there is some bromine loss during the
reaction, but at least the Brønsted sites due to zeolite should
remain because they are stable. The presence of coke on the
spent catalyst, as indicated by the decrease in TOF with time
(Table 2), is likely responsible for blocking access to the acid sites.
Acidity of both ABZ-5 and H-ZSM-5 was also characterized
using NH3-TPD. Figure 2b shows the results. Both catalysts show
Acidity measurements
The acidity of these superacids was characterized using pyridine-
DRIFTS, and NH3-TPD (Figure 2) and the crystallinity was
characterized using XRD (SI, Figure S3). Pyridine-DRIFTS uses
pyridine as a probe molecule to identify Brønsted and Lewis acid
sites. Figure 2[a] compares the IR spectra of H-ZSM-5 and ABZ-
5 catalysts, which shows the effect on the IR spectra when AlBr3
is added to H-ZSM-5.
two principal peaks, one at
~ 150-200 °C that can be
characterized as a low temperature (LT) or weakly bound
ammonia site [19], and the second one, at ~ 370-400 °C,
attributable to a high temperature (HT) or strongly bound
ammonia site [19], which probably is able to protonate NH3 to form
+
NH4 . Similar TPD peaks were also observed when AlCl3 was
grafted on mesoporous MCM-41 zeolites[20]. The low temperature
peaks are often identified as ammonia adsorbed through
hydrogen co-ordinate bond [19b]. The literature suggests that this
weak acidity has little catalytic significance [21]. In the case of ABZ-
5, both the LT and HT peaks are present but shifted to
temperatures higher than H-ZSM-5. The LT, weak acid peak for
ABZ-5 is centered at ~ 210°C compared to ~165°C for H-ZSM-5.
The HT, strong acid peaks are very similar: ~400°C for ABZ-5
compared to ~390°C for H-ZSM-5. Both peak shifts, and the total
acidity, as measured by the areas under the NH3-TPD curves,
clearly indicate an increase in acidic strength due to the presence
of AlBr3 [22]. Note that the difference in total acidity between ABZ-
5 and H-ZSM-5 is almost entirely due to that of the strong, HT
acidity, even though the acid strength, as measured by the shift
in the peak temperature, is comparable (~ 10 oC shift for HT peak).
For spent catalyst, NH3-TPD also showed significantly fewer acid
sites and the loss was seen in both the LT and HT regions. Table
3 shows that ABZ-5 clearly has more acid sites, which can be
attributed to the AlBr3 grafting, which generated new acid sites in
the H-ZSM-5. The spent ABZ-5 catalyst on the other hand, did
show a loss in number of acid sites (Figure 2[b] and Table 3). This
could either be due to the loss of Br from ABZ-5 during reaction
or due to lack of access to the acid sites from coke.
Table 3: Amount of acid sites on zeolite catalysts using NH3-TPD
Figure 2. [a] Comparison of DRIFTS spectra at 300 oC for (a) Fresh ABZ-5, (b)
H-ZSM-5, and (c) Spent ABZ-5. Region corresponding to (L) = Lewis acid sites,
(B) = Brønsted acid sites; [b] NH3-TPD for (a) Fresh ABZ-5, (b) H-ZSM-5, and
(c) Spent* ABZ-5.
Conc. of acid sites
Catalyst
(mmol/g)
H-ZSM-5
1.19
1.55
0.85
*Spent catalyst refers to ABZ-5 recovered after CH4 oligomerization reaction
ABZ-5 (fresh)
ABZ-5 (spent)
ran at 300oC, 1 atm, and 9 L.gcat-1 hr-1 for 16 hrs
DRIFTS spectra of both catalysts at 300 oC show the presence of
strong Brønsted acid sites (~ 1545-1550 cm-1). Although there are
very few differences between fresh ABZ-5 and H-ZSM-5, one
noticeable difference is a new shoulder that corresponds to a
Brønsted acid site at 1540 cm-1 in the case of ABZ-5. This
shoulder is apart from the main peak at 1547 cm-1, which must
have been generated due to the grafting of AlBr3 on H-ZSM-5.
Also, the Brønsted (“B”) acid sites appeared to be stable at high
temperatures, although the concentration of Lewis acid sites
decreased with increasing temperatures (Figure S5, SI). Acid
catalysts having peaks in the same range of wavenumbers are
reported to be solid superacids [18]. The spent ABZ-5 catalyst
shows significantly lower intense peaks, indicating either a loss of
acid sites or lack of access to the acid sites. Figure S7 (SI) shows
that H-ZSM-5 maintains acid sites up to at least 400 oC, and it is
unlikely that there is a loss of Brønsted acid sites from this zeolite.
Effect of temperature
Reactions were run on ABZ-5 at three temperatures from 200 to
400 °C. The effect of temperature on CH4 oligomerization is
shown in Table 4. ABZ-5 showed activity at each temperature.
Higher temperatures (400 °C) resulted in higher rate, as
measured by TOF. There is a general increase in total selectivity
to aromatics with temperature, with toluene selectivity being the
only clear trend with temperature. These aromatic products are
thought to be precursors for polynuclear aromatic compounds and
coke. Formation of these products at a faster rate at higher
temperatures likely leads to faster deactivation. Similar
deactivation is also typically observed in methane aromatization
[23]
.
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TPO results for the 300°C catalyst show a small peak at ~120°C
that is not shown in the others. In addition, there is more coke at
the 420°C peak than the other two runs. This is perhaps
consistent with greater aromatic selectivity and TOF for the 300°C
catalyst (Table 4). [The anomalous TPO peak at ~120°C for the
fresh catalyst may be due to trace amounts of CO2 adsorbed in
handling].
Table 4: Effect of temperature on methane oligomerization over ABZ-5
catalyst (1 atm, 9 L gcat-1 h-1). (-) = not detected; measured at 1 hr
Product selectivity (%)
Temp.
(°C)
TOF
(h-1)
X
C2=
C3
C4
B
T
EB
M
(o,m,p)
200
300
400
0.01
0.1
19.7
3
34
6.0
2.3
1.4
35.4
0.49
9.0
10.8
18.6
35.6
-
-
-
-
-
7.9
14.4
59.6
24.6
8.1
7.1
0.08
7.8
Effect of space velocity
Space velocity (SV) significantly affects hydrocarbon selectivity.
ABZ-5 was studied at three different space velocities (3.6, 9 and
18 L gcat-1 h-1). The corresponding activity results are presented in
Table 5. The total aromatic selectivity is relatively high at all
conditions tested here, consistent with coke formation. TOF
increases at lower SV, as expected. However, there is no clear
trend of total aromatic selectivity with SV, nor with any specific
aromatic compounds. The only clear trend in selectivity is that of
propane selectivity, which increases with increasing SV,
indicating that propane is an intermediate. However, there is no
obvious relationship between propane and any measurable
aromatics.
Table 5: Effect of space velocity on methane oligomerization over ABZ-5
catalyst (1 atm, 300 oC). Samples are taken at 1 hr time on stream
Space
Velocity
Product selectivity (%)
TOF
(h-1)
Figure 3: TPO comparison for ABZ-5 after CH4 oligomerization runs (1 atm, 9
L gcat-1 h-1) at various temperatures on ABZ-5 catalyst (a) fresh catalyst, (b) 200
oC, (c) 300 oC, (d) 400 oC (after running reaction at respective temperatures for
~ 1000 min).
-1
X
(L gcat
C2=
C3
C4
B
T
EB
M
h-1)
3.6
9
(o,m,p)
0.16
0.1
4.2
3
0.85
7.9
2.1
1.5
5.5
2.0
0.5
4.6
8.4
18.8
17
20.1
60.6
30.3
5.6
8.3
-
57
-
18
0.02
17.0
23.5
-
Conclusions
Vapor grafted AlBr3 on to H-ZSM-5 has been used to synthesize
AlBrx/H-ZSM-5 catalyst (“ABZ-5”). Grafting of AlBr3 creates new
Brønsted acid sites in the H-ZSM-5 framework, as observed from
the NH3-TPD and pyridine-DRIFTS. These acid sites, in synergy
with the existing acid sites from the H-ZSM-5 framework, are likely
the cause for the protonation of methane via an oligo-
condensation mechanism [9], producing higher hydrocarbons.
This catalyst has been shown to be active in the oligomerization
of methane at temperatures as low as 200°C.
TPO
Results in Tables 2, 4, and 5 clearly show the relatively high total
selectivity to aromatics. Figure 3 compares the amount and
reactivity of coke analyzed in the four catalysts of interest here:
fresh catalyst and spent catalysts after being run at three reaction
temperatures (200 °C, 300 °C, and 400 °C). Peak TPO positions
were similar for all runs, ~ 420°C, indicating amorphous carbon
on the surface of all three spent catalysts [24]. TPO results (Table
6) show slightly different levels of coke deposition depending on
reaction temperature.
Experimental Section
Table 6: Carbon deposition from oligomerization reactions
Carbon deposited
Catalyst
(mmol/g)
Materials
Fresh ABZ-5
2.53
3.00
AlBr3 (anhydrous, 98%) and NH4-ZSM-5 (SiO2/Al2O3
= 50:1) were
Spent ABZ-5
(after 16 hrs reaction at 200 oC)
Spent ABZ-5
purchased from Alfa Aesar. SiO2 gel was purchased from PQ Corporation.
Zr(OH)4 (97%) was purchased from Sigma Aldrich Inc. and H2SO4 (95-
98%) was purchased from Malinckrodt Chemicals Inc. Silver Nitrate
(AgNO3), 0.0141 N solution was purchased as is from Macron Fine
Chemicals. Pyridine (50 ppm) in gas phase (balance Argon) was
3.60
3.47
(after 16 hrs reaction at 300 oC)
Spent ABZ-5
(after 16 hrs reaction at 400 oC)
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purchased from Praxair Inc. 10% NH3/He (ultra-high purity), 10% CH4/Ar
(ultra-high purity) were purchased from Airgas Inc.
temperature, catalyst was saturated with pyridine vapors for 45 min. The
saturated sample was then treated under He flow for 30 min. Sample was
then treated at 100 °C for 10 min and cooled to room temperature and the
actual spectrum was recorded. Similar spectra were recorded at room
temperature after 10 min. treatments were carried out at 200 °C, 300 °C,
and 400 °C to check the thermal stability of the acid sites on catalysts.
FTIR of hydroxyl region was carried out in the same aforementioned
DRIFTS cell under Helium flow and at various temperatures. Some IR
catalyst samples were also recorded using Bruker Alpha IR spectrometer
using Diamond ATR crystal in the range of 4000 – 400 cm-1.
Catalyst Preparation
ABZ-5 was prepared as follows: H-ZSM-5 was prepared by calcining NH4-
ZSM-5 at 500 °C for 3 h in a muffle furnace. In order to optimize the –OH
concentration on the surface of the as-prepared H-ZSM-5, it was washed
in three alternate cycles of HBr (1 M) and deionized water and then dried
in a vacuum at 80 oC for 72 h before being exposed to the room
atmosphere[25]
.
TPO
Then, in separate vials, AlBr3 (1 g) and the washed H-ZSM-5 (1 g)
were loaded into a Teflon lined SS autoclave (Parr Instruments Inc., Moline,
IL) inside a glovebox. The autoclave was then sealed and put inside an
isothermal oven at 180 °C for 72 h in order for AlBr3 to vaporize and react
with the hydroxyl groups on the surface of the zeolite. The excess pressure
generated during the synthesis (HBr formation) was released by opening
the autoclave in a fume hood. The autoclave was then returned to the
glovebox where the catalyst was recovered. Masses of all the materials
were recorded in order to check for the weight gain by the H-ZSM-5 sample.
ABSi catalyst was also prepared using the same methodology, but
substituting the H-ZSM-5 for the SiO2 gel.
Temperature programmed oxidation (TPO) experiments were carried out
in an Altamira AMI-200 catalyst characterization system in conjunction with
Ametek Dycor Quadlink residual gas analyzer. Typically, 30 mg of spent
catalyst was loaded in the reactor tube and the temperature was ramped
up from 30 °C to 900 °C at a rate of 10 K min-1 under a flow of 10%O2/He.
Masses (m/z) that were analyzed for this procedure included 44 (CO2), 28
(CO), 18 (H2O), 32 (O2), 4 (Helium).
Sulfated zirconia (SZ) was prepared using a previously reported
literature procedure [26]. Briefly, Zr(OH)4 gel was washed with H2SO4 (0.5
M) solution and the washed product was dried at 110 °C overnight in an
isothermal oven. Then the material was subsequently heated to 550 °C at
a ramp rate of 5 °C min-1 and calcined at this temperature for 4 h under
static air. The solid sulfated zirconia powder obtained was then stored for
further use. Formation of sulfated zirconia was confirmed by XRD on the
prepared sample, which showed a diffraction pattern similar to that
XRD
X-ray diffraction studies were conducted at Shared Instrument Facility
(SIF) at LSU using PAN Analytical EMPYREAN diffractometer with a
generator voltage of 45 mV and a tube current of 40 mA. The scan range
chosen was from 5 to 90° at a step size of 0.02626.
previously reported [26]
.
XPS
XPS analysis of all the samples was carried out using ScientaOmicron
ESCA 2SR XPS/Auger instrument at Shared Instrument Facility (SIF) at
LSU. All the samples were ran at a characteristic energy of 1486.7 eV and
with an acquisition time of 2s.
Ammonia-TPD
Ammonia-TPD was carried out using AMI-200 reactor system (Altamira
Instruments Inc. Pittsburgh, PA) in conjunction with an Ametek LC-D Mass
Spectrometer. Typically 25 mg of catalyst was weighed and loaded in a
quartz tube reactor. The catalyst was pretreated at 100 °C for 30 min under
He flow to clean the catalyst surface. After pretreatment, the sample was
cooled down to 50 °C, and ammonia was adsorbed by flowing it through
the catalyst bed for 1 h. After ammonia adsorption, 25 ml min-1 of He was
flowed for 40 min to remove any physisorbed/residual ammonia. Then, the
Mass Spectrometer and TCD detectors were turned on and the
temperature was ramped up at 10 °C min-1 from 50 °C to 500 °C. Masses
that were analyzed included 16 (NH3), 17 (NH3), 18 (H2O), 81 (HBr).
Reaction Studies
CH4 oligomerization was carried out in a custom built reactor equipped with
a Hamilton ¼” OD glass lined SS tube. Typically, the supported catalyst
(0.10 g) was loaded into the reactor tube and was heated to the desired
reaction temperature at which time methane was passed over the catalyst.
Downstream of the reactor, a caustic wash tank was fitted (aq. KOH, 0.1
N) to neutralize HBr (when used) during the treatment of the catalyst.
Products were analyzed using Shimadzu GC2014 (FID) equipped with
Restek RT-Q-Bond column (30 m x 0.53 mm x 20 μm). A schematic could
be found in Figure S1, SI. Downstream of the reactor the outlet line going
to the GC (for analysis), was not heated because we didn’t observe any
condensation because analytes of interest were few ppms.
Pyridine-DRIFTS
DRIFTS experiments using pyridine as a probe molecule were carried out
using Thermo Scientific Nicolet 6700 FTIR equipped with Harrick Praying
Mantis reaction cell fitted with KBr windows. Spectra of all the samples
were recorded with a spectral resolution of 4 cm-1 in region going from
4000 – 650 cm-1. In a typical experiment, the IR cell was loaded with the
catalyst sample inside the glovebox. The sample was then transferred to
the spectrometer and pretreated at 100 °C for 30 min. under He flow (30
sccm) to clean surface of the catalyst. After pretreatment, the sample was
cooled down to 25 °C and a background spectrum was recorded. At this
Silver nitrate (AgNO3) test
AgNO3 is a common test for identifying halide ions and can also be used
to quantify the amount of halides present in a sample. This test was used
in the estimation of bromine (Br) content as well as to confirm the presence
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10.1002/cctc.201801310
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of bromide ions in the HBr dissolved aqueous sample. In a typical test,
measured amount of catalyst (0.2 g) was dissolved in water (although its
moisture sensitive, here only dissolution of bromide ions in the aqueous
phase was desired), then using 1 M potassium chromate (K2CrO4)
indicator solution, it was titrated against 0.0141 N silver nitrate to the
reddish-brown endpoint. For the HBr dissolved water samples, it was
directly titrated with silver nitrate solution using potassium chromate
indicator solution.
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Authors would like to thank Chevron for Chevron Innovation
Research Fund, LSU ORED (Office of Research and Economic
Development) and NSF EAGER grant (# 1644895)
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Swarom Kanitkar, James Carter,
Graham Hutchings*, Kunlun Ding,
James J. Spivey*
Page No. – Page No.
Low Temperature Direct Conversion
of Methane using a Solid Superacid
Superacid catalyst for methane conversion: Methane can be activated through
variety of ways including use of oxidants, without oxidants. Superacidic systems
possess high enough acidity to protonate methane and start oligomerization of it
through oligo-condensation mechanism. Products can include C2, C3 feedstocks as
well as aromatics.
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