One-Pot Conversion of Methane to Light Olefins or Higher Hydrocarbons through H-SAPO-34-Catalyzed in Situ Halogenation

Article abstract of DOI:10.1021/jacs.7b10725

Methane was converted to light olefins (ethene and propene) or higher hydrocarbons in a continuous flow reactor below 375 °C over H-SAPO-34 catalyst via an in situ halogenation (chlorination/bromination) protocol. The reaction conditions can be efficiently tuned toward selective monohalogenation of methane to methyl halides or their in situ oligomerization to higher hydrocarbons. The presence of C5+ hydrocarbons in the reaction products clearly indicates that by using a properly engineered catalyst under optimized reaction conditions, hydrocarbons in the gasoline range can be produced. This approach has significant potential for feasible application in natural gas refining to gasoline and materials under moderate operational conditions.

Full text of DOI:10.1021/jacs.7b10725

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
One-Pot Conversion of Methane to Light Olefins or Higher
Hydrocarbons through H‑SAPO-34-Catalyzed in Situ Halogenation
Patrice T. D. Batamack, Thomas Mathew, and G. K. Surya Prakash*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California
90089-1661, United States
S
* Supporting Information
ABSTRACT: Methane was converted to light olefins (ethene and
propene) or higher hydrocarbons in a continuous flow reactor below
375 °C over H-SAPO-34 catalyst via an in situ halogenation
(chlorination/bromination) protocol. The reaction conditions can be
efficiently tuned toward selective monohalogenation of methane to methyl
halides or their in situ oligomerization to higher hydrocarbons. The
presence of C5+ hydrocarbons in the reaction products clearly indicates
that by using a properly engineered catalyst under optimized reaction
conditions, hydrocarbons in the gasoline range can be produced. This
approach has significant potential for feasible application in natural gas refining to gasoline and materials under moderate
operational conditions.
Scheme 1. Three-Step Conversion of Methane to
Hydrocarbons
INTRODUCTION
■
The reserves of natural gas are expected to grow significantly
through 2040 with the advances in exploration techniques,
horizontal drilling, and hydraulic fracturing technologies
coupled with processes for reducing flaring and venting of
the gases associated with petroleum.1 Therefore, methane, the
most abundant component of natural gas, may soon become
the main raw material. Today, methane is mostly burned to
produce energy. Another major industrial application of
methane is the high-temperature and cost-intensive steam
reforming to produce synthesis gas (CO and H2), which can be
converted to methanol and higher hydrocarbons.2a−d The
direct heterogeneous conversion of methane to useful
chemicals and fuels, generally achieved at temperatures above
500 °C, has so far resulted in limited yields due to severe
kinetic limitations.2e,3 Therefore, an economic large-scale direct
functionalization of methane to useful chemicals still remains a
big challenge to researchers because of the high stability of
methane and the higher reactivity of the desired products.
In liquid superacids, the conversion of methane to higher
hydrocarbons was achieved at low temperatures albeit in low
yields.4a In 1985, Olah et al.4a,b proposed a three-step approach
for converting methane to hydrocarbons, which involved
selective halogenation (chlorination or bromination) of
methane to methyl halide with chlorine or bromine as oxidant
followed by its hydrolysis to methanol. Its subsequent
transformation led to higher hydrocarbons (Scheme 1).
Later, Noceti et al.4c,d and others4e,f,5 presented a two-step
process consisting of the oxidative chlorination of methane on
Cu-based or rare-earth metal based catalysts followed by the
condensation of the methyl chloride to hydrocarbons over a
silicoaluminate zeolite (Scheme 2).
Scheme 2. Two-Step Conversion of Methane to
Hydrocarbons
A two-step approach was developed by Lorkovic et al.6 to
convert methane to methyl bromide/methylene bromide at
high temperature followed by condensation to oxygenates and
higher hydrocarbons over a solid basic oxide as a catalyst and a
source of oxygen. Recently, EuOBr/ZrO2 was shown to be an
effective catalyst for the oxybromination of methane to CH3Br
and CH2Br2 at 500 °C and is exceptionally stable for the
oxidation of HBr to Br2.7 Osterwalder and Stark8 also described
a two-step process in which methane could be selectively
converted to methyl bromide that was subsequently oligo-
merized to higher hydrocarbons in a pressurized reactor using
AlBr3 as a catalyst at temperatures between 160 and 400 °C.
The regeneration of the catalyst and the hydrogenation of the
carbonaceous deposits formed during the reaction required a
significant amount of hydrogen. Under superacidic HBr/AlBr3
conditions, methane was converted to higher hydrocarbons in a
continuous flow reactor at atmospheric pressure between 200
and 400 °C.9 Under these conditions, AlBr3, a highly corrossive
Received: October 8, 2017
© XXXX American Chemical Society
A
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and environmentally unsustainable solid Lewis superacid, poses
many practical difficulties in the downstream for its separation
from the liquid hydrocabon products and make its recyling
tedious. Therefore, large-scale conversion of methane to higher
hydrocarbons under these conditions is not feasible. The direct
conversion of methane at low pressure (10−50 Torr) via a
microwave plasma process10 was shown to produce C2
hydrocarbons. Oxidative coupling of methane to produce
ethene and ethane requires high temperatures, generally 500−
1000 °C which affects the yields and the stability of the catalyst
significantly.3e,j,k Ethene undergoes subsequent oligomerization
to produce higher hydrocarbons. Recently, nonoxidative
reaction of methane above 1000 °C yielded selectively ethene
and aromatics.3j,k High yields of methyl esters can be obtained
under batch conditions using concentrated sulfuric acid or
trifluoroacetic acid that need careful hydrolysis to produce
methanol and require catalyst regeneration.11 Therefore, direct
conversion of methane to higher hydrocarbons under flow and
moderate reaction conditions, i.e., below 400 °C and ambient
pressure remains a challenge and a long sought goal.
In our continuing effort to achieve selective monohalogena-
tion of methane, H-SAPO-34, a silicoaluminophospahte zeolite
that is isostructural of the chabazite framework topology with a
cavity window diameter of 3.8 Å × 3.8 Å and a cavity diameter
of 7.3 Å × 12 Å, was used as a catalyst searching for appropriate
reaction conditions. It is an excellent catalyst for the methanol-
to-olefin and methyl halide-to-olefin processes and quite stable
in the presence of HCl or HBr unlike silicoaluminate zeolite.
Both chlorination and bromination of methane were conducted
at a relatively high dilution level (methane to bromine molar
ratio of 10, where deactivation due to carbon deposits is low)
and moderate temperatures, safe enough to produce selectively
the methyl halide or to expect its conversion directly to higher
hydrocarbons.
as the reaction continued, it was found to remain constant (as
noticed during the 27 h on-stream, see Figure 1). The
Figure 1. Chlorination over 5 g of H-SAPO-34 at 350 °C with time
on-stream in the presence of moisture. CH4/Cl2/N2/H2O =
10:1:1.3:0.3. Total flow: 600 mL/h·g.
selectivity to methyl chloride was remarkably improved (above
98%) and also remained constant. Water seemed to
considerably suppress the formation of coke by probably
poisoning strong active sites. No carbon oxides were detected
in the products.
By decreasing the space velocity from 600 (5 g of catalyst) to
420 mL/h·g (7 g), the contact time was increased from
approximately 40 to 60 s. At 40 s, the only products observed
were methyl chloride (98.7%) and a small amount of methylene
chloride (1.3%), though the catalyst turned slightly gray after
the reaction.
With a contact time of 60 s, 2 h on-stream, methyl chloride
was observed as the sole product (Table 1). However,
Table 1. Chlorination over 7 g of H-SAPO-34 at 350 °C in
a
the Presence of Moisture
RESULTS AND DISCUSSION
■
The thermal chlorination is a well-known radical process
yielding all chloromethanes as the “reproportionation” reaction
with chloroform does not proceed at significant levels at 550 °C
while dichloromethane remains inert.12 In our present work,
the thermal chlorination of methane went to completion at 350
°C. The product distribution was almost constant and limited
to methyl chloride (87%) and methylene chloride (13%) after
24 h on-stream. The extent of the chlorination was limited to
the two first derivatives by the relatively high methane to
chlorine ratio employed under a relatively short contact time
(about 2.6 s). Initially, in the presence of the catalyst, the
conversion of chlorine decreased from 92.3% after 1 h on-
stream to 71.5% after 22 h on-stream. The selectivity also
dropped from 96.5% methyl chloride to 91.2% during the same
period of time, which is probably due to some coke formation.
However, after the regeneration of the catalyst at 550 °C with
oxygen, the initial conversion was almost restored and the
selectivity of methyl chloride was steady at 94.1% after the first
4 h on-stream. The Brønsted acid sites (bridging OH groups)
and the acid sites (formed by the reversible interaction of HCl
on the Al−O−P groups yielding Al−Cl and P−OH) in the
catalyst are capable of polarizing the chlorine molecules to
effect the electrophilic chlorination of methane as suggested by
Olah et al.4a,b,13,14
product
distribution
(mol %)
time (h) conversion Cl2 (mol %) CH3Cl CH2Cl2 carbon balance (%)
0.5
1
87.3
88.9
91.7
83.0
81.0
100
100
100
60.3
52.9
100
99
2
97
b
b
18
22
0.4
0.3
96
96
a
b
CH4/Cl2/N2/H2O = 10:1:1.3:0.3. Total flow: 420 mL/h·g. The rest
is made of higher hydrocarbons; see Figure 2.
remarkably, with longer reaction times, conversion of methyl
chloride to hydrocarbons was achieved. Ethene (22.7%),
propene (13.6%), and C4 compounds (5.8%) were the major
higher hydrocarbons formed after 18 h on-stream (Figure 2).
This is a significant breakthrough in methane activation
especially in the direct conversion of methane to hydrocarbons
using H-SAPO-34 as a catalyst at this relatively much lower
temperature of 365 °C. To the best of our knowledge, this
approach is found to be the most effective among all other
methods reported so far on the transformation of methane to
higher hydrocarbons over a solid catalyst. Zhang et al.15 found
that methyl chloride was converted to ethene (25%), propene
(31%), and butenes (15%) at 450 °C over H-SAPO-34 using
equimolar CH3Cl/N2 mixture. Svelle et al.16 reported that
approximately after 1 h on-stream at 350 °C with 1 bar of MeCl
The conversion of chlorine was found to be lower in the
presence of moisture than that observed in the absence of
moisture despite the increase of the catalyst loading. However,
B
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Article
Figure 2. Hydrocarbon phase composition in the chlorination of
methane over H-SAPO-34 at 350 °C after 18 and 22 h on-stream in
the presence of moisture; 7 g of catalyst. CH4/Cl2/N2/H2O =
10:1:1.3:0.3. Total flow: 420 mL/min.
and a flow rate of 10 mL/min a product distribution of ethene
(20%), propene (38%), C4 (25%), and C5+ (7%). Herein, we
report a direct protocol for the conversion of methane to higher
hydrocarbons through the halide over a solid catalyst at a
relatively low temperature of 365 °C.
The product distribution is sensitive to the reaction
conditions. The presence of methyl chloride as the sole
product during the first 2 h (Table 1) suggested an induction
period for the production of higher hydrocarbons as observed
by Svelle et al.16 in the conversion of methyl halides to
hydrocarbons. The GC of the product mixture also clearly
shows the selective formation of methyl chloride. After 18 h on-
stream, carbon deposits were visible on the catalyst as reported
by several authors,15,16 which explains the drop in the carbon
balance.
Figure 4. 13C NMR of the product mixture in CDCl3 (with CH2Cl2 as
internal standard) after bromination of CH4 over of H-SAPO-34 at
144 mL/g·h, 365 °C, 3 h on-stream.
contact time of 83 s, the conversion of bromine was 38.6% with
methyl bromide as the sole product (Scheme 3).
Scheme 3. Methane Bromination over H-SAPO-34
The bromination of methane was carried out at 345 and 365
°C at different H-SAPO-34 loadings with a methane flow rate
between 43 and 216 mL/g·h and methane to bromine molar
ratio of 10.3:1. The conversion of bromine and the selectivity of
the products were affected by both the catalyst loading and the
temperature. Reaction with 3 g of catalyst at 365 °C for 3 h
At 365 °C, the conversion of bromine increased from 41.2%
to 48.9% and 61.8% with a change in the amount of catalyst
from 2 g of 1:1 catalyst−silica gel mixture to 3 g of catalyst and
10 g of catalyst, respectively. The selectivity remained 100% for
both reactions using 1 g of catalyst (mixed with 1 g of silica,
contact time 17 s) and 3 g of catalyst (contact time 25 s)
(Scheme 3). As the amount of catalyst was increased to 10 g, a
mixture of higher hydrocarbons was formed including liquid
ones. As in the case of chlorination, methane could be directly
converted to higher hydrocarbons at 365 °C via the in situ
formed methyl bromide using bromine as an oxidant and H-
SAPO-34 as a catalyst near atmospheric pressure. H-SAPO-34
contains bridging OH groups {SiO(H)Al} acting as Brønsted
acid sites, which are responsible for the activity of the
silicoaluminophosphates.18,19 As suggested in the case of the
electrophilic chlorination of methane with chlorine over
aluminosilicate zeolites,13 bromine is also activated on the
Brønsted acid site in the pores of the zeolites to form the
reactive electrophilic bromooxonium ion species through a
transitional form in which the bromine atom would be
covalently coordinated to the oxygen, making it capable of
reacting with methane to yield methyl bromide.
1
gave methyl bromide as the sole product as seen from H and
13C NMR of the product mixture (Figures 3 and 4). However, a
blank reaction in the absence of the catalyst produced methyl
bromide, methylene bromide, and bromoform, typical of a free
radical process.12,17 In the presence of the catalyst, the
conversion of bromine increases with temperature, and at a
given temperature, the conversion increases with an increase in
the amount of catalyst. At 345 °C with 10 g of catalyst and a
The presence of Lewis acid sites in the SAPO cannot be
excluded. These Lewis acid sites are extra-framework of
aluminum or aluminum still partially bonded to the frame-
work,18−22 and under the reaction conditions they can readily
be formed by the action of the hydrogen bromide produced
during the reaction on the framework and nonframework
aluminum.15,16
Figure 3. 1H NMR of the product mixture in CDCl3 (with CH2Cl2 as
internal standard) after bromination of CH4 over H-SAPO-34 at 144
mL/g·h, 365 °C, 3 h on-stream.
C
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IR studies of NH3 adsorption on H-SAPO-34 revealed the
presence of Brønsted and Lewis acid sites of similar strength.20
These Lewis acid sites can also polarize the bromine molecules
and induce electrophilic bromination as suggested by Olah et
al.4b for the halogenation of methane over oxyhalides,
supported Lewis superacid catalysts, and superacidic sulfated
zirconia-based catalysts.14 When the contact time was increased
to 83 s (10 g of catalyst) at 365 °C, methyl bromide formed in
the course of the reaction was partially transformed to higher
hydrocarbons (Table 2, Figures 5 and 6, and Scheme 3).
products. In this case, propane (27.6%), ethene (15.0%), and
propene (8.3%) were the major products in contrast to the
mixture of ethene and propene obtained when a mixture of
methyl bromide and helium was passed over H-SAPO-34 at 0.1
bar.15b A hydrocarbon pool mechanism has been invoked to
explain the formation of the products observed in the methanol
or methyl halides to hydrocarbon reactions.4a,15a,20,23−25 Wei et
al.15b using FTIR found that the first step in the chloromethane
conversion to hydrocarbons over H-SAPO-34 was the
formation of surface methoxy groups from the dehalogenation
of methyl chloride on the Brønsted acid site to give HCl and
methyl species bonded to the oxygen. While the carbon balance
was almost 100% at 345 and 365 °C for amounts of catalyst
from 0 to 3 g, it fell to 98% for 10 g of catalyst and carbon
deposits were visible on the catalyst after the reaction. Svelle et
al.16 found that the deactivation of the catalyst due to coke and
polyaromatics depends on the reaction temperature in the
conversion of methyl chloride and methyl bromide to
hydrocarbons over H-SAPO-34. They observed a slow
deactivation after exposure of the catalyst to the methyl halides
at 350 °C, and methylbenzene isomers predominated inside the
cavities of the catalyst with about 3% of coke deposits using
10% methyl bromide in helium after 0.5 h of reaction. At high
temperature, the deactivation was severe and naphthalene
derivatives were present in large amounts. Highly substituted
benzene and cyclopentadiene derivatives were also found in the
hydrocarbon mixture during the condensation of methyl
bromide over AlBr3 in a closed vessel.8 In the latter case,
propane was among the major products formed.
a
Table 2. Bromination of Methane over H-SAPO-34
product distribution
(mol %)
catalyst
wt (g)
conv. Br2
(mol %)
carbon
T (°C)
CH3Br CH2Br2 CHBr3 balance (%)
10
0
345
365
365
365
365
38.6
57.8
41.2
48.9
61.8
100
81.0
100
100
36.4
100
99
12.6
6.4
b
1
100
100
98
3
c
10
a
CH4/Br2 molar ratio 10.3:1. Methane flow rate 7.2 mL/min. Three h
b
c
on-stream. 1 g of H-SAPO-34 mixed with 1 g of silica gel. The rest is
made of higher hydrocarbons; see Figures 5 and 6.
Under the reaction conditions studied, H-SAPO-34 with a
catalyst loading of 3 g exhibited good stability with time. No
carbon deposits were observed at 345 °C, though the catalyst
turned slightly gray at 365 °C after 15 h on-stream. The
increase of bromine conversion with temperature may be
explained by a decrease in the activation energy and an increase
in the formation of Lewis acid sites as already mentioned and/
or coke formation, which induces a long contact time. It was
shown in the case of MTO reaction over H-SAPO-34 that coke
reduced the diffusivity of dimethyl ether and enhanced the
formation of olefins.25
Figure 5. Hydrocarbon phase composition in the bromination of
methane over H-SAPO-34 at 43.2 mL/g·h (10 g of H-SAPO-34), 365
°C, 4 h on-stream.
CONCLUSION
■
In conclusion, we have shown that at ambient pressure and
moderate temperatures (345−365 °C) using chlorine or
bromine as oxidant H-SAPO-34 is an effective and stable
catalyst capable of converting methane to methyl halide or
directly to higher hydrocarbons in one step (Scheme 4) simply
Scheme 4. One-Step Conversion of Methane to
Hydrocarbons over H-SAPO-34
Figure 6. FID chromatogram of the bromination of CH4 over of H-
SAPO-34 at 43.2 mL/g·h (10 g of H-SAPO-34), 365 °C, 4 h on-
stream.
by altering the reaction conditions. In previous cases, the
conversion of methane to useful chemicals and fuels over solid
catalysts proceeded at high temperatures or at least in two steps
that require very different reaction conditions. When the
activation of methane is achieved at high temperatures, the
desired products, which are more reactive, undergo further
Linear alkanes (ethane, propane, n-butane), branched alkanes
(2-methylbutane, 2,3-dimethylbutane, 3-methylpentane), and
olefins (ethene, propene, 2-butene) were among the identified
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transformations to an undesired mixture of compounds. In this
new protocol, the direct transformation to higher hydrocarbons
is made possible through one-pot halogenation of methane to
methyl halide and its oligomerization to hydrocarbons
successively over H-SAPO-34 at the same temperature.
Formation of methyl halide and hydrocarbons can be fine-
tuned by changing the contact time. Short contact times
favored the formation of the methyl halide, while long contact
times allowed the in situ formation of hydrocarbons mainly in
the C2−C6 range.
In the future, this approach would promote significant
operational cost savings with further improvements in the
conversion of methane to hydrocarbons in high yields with
feasible low-cost reoxidation of the acid halide to halogen.
Detailed studies are underway to explore more in depth the use
of zeolites including metal-modified ones, particularly alumino-
silicate phosphates for the direct conversion of methane to
valuable chemicals via halogenation/oxidative condensation
protocol.
AUTHOR INFORMATION
Corresponding Author
*gprakash@usc.edu
ORCID
Thomas Mathew: 0000-0002-2896-1917
G. K. Surya Prakash: 0000-0002-6350-8325
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This paper is dedicated to the cherished memories of Professor
George A. Olah. The work was supported by the Loker
Hydrocarbon Research Institute, University of Southern
California and the Department of Energy (DOE). We thank
UOP (Honeywell) for providing the zeolite sample and Dr.
Laxman Gurung for helping with the BET surface area and
porosity analysis.
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■
Materials. H-SAPO-34 (Al + P/Si = 5.57) was generously
provided by UOP in the powder form. The preparation of the catalyst
has been described earlier.26 The chemical composition of catalyst was
(Si0.14 Al0.55 P0.30) O2 with 50% of the particles smaller than 1.2 μm.
The acid density of 1.38 acid site per cage was determined by NH3-
TPD adsorption. The surface area (438 m2/g) of the catalyst degassed
at 673 K for 12 h was measured using NOVA 2200e Surface Area and
Pore Size Analyzer; the pore diameter was 37.2 Å, and the pore
volume was 0.20 cc/g.
Activation of Catalyst and Halogenation Procedure. Experi-
ments were performed using a vertical 620 mm long 9 mm i.d. quartz
tubular reactor. In a typical experiment, 3 g of catalyst was placed in
the reactor between two quartz wool plugs. All parts were shielded
from light. The catalyst was heated in air from room temperature to
500 °C at a rate of 3 °C/min and kept for 1 h at that temperature.
Then the catalyst was cooled down to the reaction temperature under
nitrogen flow. Methane flow, controlled by a calibrated mass flow
controller (Aalborg), entrained bromine vapor from a bromine
reservoir maintained at 2 °C to give a continuous supply of
methane−bromine mixture in a ratio of 10.3:1. For bromination
reactions, after 1 h on-stream, the products from the exit of the reactor
were collected for 2 h in a solution of CDCl3 kept at around −60 °C
and analyzed by NMR (AS400 MHz Oxford) using dichloromethane
as an internal reference. The connection from the exit of the reactor to
the sample collector was heated at 150 °C to avoid condensation of
the products. The flow out from the CDCl3 trap was passed in a 5 M
NaOH aqueous solution and measured using a flow meter. Samples
were also taken at the entrance of the reactor and at the exit before and
after the aqueous sodium hydroxide solution trap and analyzed off-line
on two ThermoFiningan Trace GCs: one equipped with FID and
TCD detectors using a 30 m Carboxen 1010 PLOT column from
Supelco and the other with a FID detector using a 30 m GS-GASPRO
column from Agilent. For chlorination reactions, N2 was used as an
internal reference and samples were only analyzed and quantified by
GC. When moisture was added during chlorination, methane was
passed into the water reservoir maintained at 22 °C before combining
with chlorine. For catalyst loadings of 5 g and above, the maximum
pressure observed in the system did not exceed 30 psi.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b10725.
(7) Paunovic,
Hauert, R.; Per
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