Improved light olefin yield from methyl bromide coupling over modified SAPO-34 molecular sieves

Article abstract of DOI:10.1039/c0cp01985b

As an alternative to the partial oxidation of methane to synthesis gas followed by methanol synthesis and the subsequent generation of olefins, we have studied the production of light olefins (ethylene and propylene) from the reaction of methyl bromide over various modified microporous silico-aluminophosphate molecular-sieve catalysts with an emphasis on SAPO-34. Some comparisons of methyl halides and methanol as reaction intermediates in their conversion to olefins are presented. Increasing the ratio of Si/Al and incorporation of Co into the catalyst framework improved the methyl bromide yield of light olefins over that obtained using standard SAPO-34.

Full text of DOI:10.1039/c0cp01985b

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PAPER
Improved light olefin yield from methyl bromide coupling over modified
SAPO-34 molecular sieves
a
a
a
a
Aihua Zhang, Shouli Sun, Zachary J. A. Komon, Neil Osterwalder,
a
a
a
b
Sagar Gadewar, Peter Stoimenov, Daniel J. Auerbach, Galen D. Stucky* and
c
Eric W. McFarland
Received 30th September 2010, Accepted 1st December 2010
DOI: 10.1039/c0cp01985b
As an alternative to the partial oxidation of methane to synthesis gas followed by methanol
synthesis and the subsequent generation of olefins, we have studied the production of light
olefins (ethylene and propylene) from the reaction of methyl bromide over various modified
microporous silico-aluminophosphate molecular-sieve catalysts with an emphasis on SAPO-34.
Some comparisons of methyl halides and methanol as reaction intermediates in their conversion
to olefins are presented. Increasing the ratio of Si/Al and incorporation of Co into the catalyst
framework improved the methyl bromide yield of light olefins over that obtained using
standard SAPO-34.
1
2
to activity and selectivity, however to date, low yields or other
practical limitations of most of these potential pathways have
prevented their translation into practical industrial processes.
One high-yield approach uses the low activation energy of
methane halogenation to activate methane by reaction with
Introduction
Methane is among the most important fossil resources today
and, based on known available reserves, will continue to be so
in the future. Methane is available not only as a fossil resource,
a major component of natural gas, but also from a variety of
renewable sources (e.g. biogas). The conversion of methane
from renewable sources to higher hydrocarbons is potentially a
large and important component of technologies for the
production of sustainable fuels and chemicals.
halogens under relatively mild conditions to produce a
13,14
reactive methyl halide intermediate.
Under controlled
conditions, the major products are mono-halogenated
methane (CH X) and di-halogenated methane (CH ). From
3
2 2
X
a molecular structure and bond strength standpoint, mono-
halogenated methane compounds are similar to methanol.
Therefore, many of the pathways for converting methanol to
The cost-effective and efficient conversion of methane to
higher valued and more easily transportable liquid products
remains a significant chemical and engineering challenge. The
only significant commercial process to convert methane to liquid
chemicals practiced today relies on the partial oxidation of
higher hydrocarbons can be applied analogously to mono-
15–18
halomethane.
The primary focus has been on reactions
13,19
with chlorine and bromine.
Looking at the bond strengths
methane to synthesis gas (CO and H ) followed by CO
2
of different halogens, bromination of methane has significant
advantages over chlorination primarily due to the weaker Br–C
20
hydrogenation to liquid hydrocarbons using Fischer–Tropsch
1
synthesis or methanol synthesis. At present, the process
bond, relative to Cl–C. This gives greater selectivity for
economics require these plants to be very large with high
2
capital investments.
monobromomethane and the resulting final hydrocarbon
products. As it is a liquid at room temperature, bromine
management has practical advantages over chlorine in terms
of storage and delivery. As with any halogen based process, the
need to use high-alloy (and high-cost) materials of construction
to resist corrosion is an important engineering consideration.
Although there are structural and chemical similarities
between methanol (C–OH) and mono-halogenated methane
Looking for better solutions, researchers have investigated a
3–5
variety of different approaches to selectively activate the C–H
bonds of methane at low temperatures including transition
4
,6,7
8
mercury complexes, zeolites,
5,9
metal complexes,
and
novel high-performance polymeric solid state frameworks
1
CTF) as catalysts. The sulfuric acid based approaches of
0
(
7,11
Periana et al.
10
¨
and Schuth et al. are promising with respect
(
C–X), the reaction thermodynamics, microkinetics, and
practical process design details for methyl halide conversion
a
15–17
GRT, Inc., Santa Barbara, California, USA
are different than for methanol.
of CH Cl conversion to hydrocarbons,
CH
There are several reports
b
University of California, Department of Chemistry, Santa Barbara,
California 93106 – 5080, USA. E-mail: stucky@chem.ucsb.edu
University of California, Department of Chemical Engineering,
Santa Barbara, California CA 93106, USA
21,22
3
as well as reports of
2
2,23
c
Br conversion to both light olefins
and aromatic-rich
22
3
2
3
fuel mixtures. Very relevant to this work is a study
2
550 Phys. Chem. Chem. Phys., 2011, 13, 2550–2555
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discussing the conversion of methyl halides to hydrocarbons
over SAPO-34 which showed that SAPO-34 is an active,
selective, and structurally stable catalyst for the conversion
of methyl chloride and methyl bromide to olefins. However, in
these studies the single-pass conversion was low due to
limitations of the catalyst material.
Experimental setup and procedure for coupling reactions
Coupling reactions were conducted in an atmospheric pressure
fixed bed flow reactor system (Fig. 2). CH Br was fed to a glass
tube reactor (id 1 cm) through a micro liquid delivery pump
M6 Pump, VICI). The feed was diluted by nitrogen to achieve
3
(
lower partial pressures and stable flow. The reaction
temperature was varied in the range of 400 to 500 1C. The
reaction residence time was varied over the range of 0.1 to 2.0 s.
The effluent stream from the reactor was passed through a
series of Teflon trap vessels each containing 4 ml of 4 M NaOH
solution and 4 ml organic solution (8 wt% decane in dibutyl
phthalate) where the HBr coproduct was neutralized and the
organic products were captured. All remaining gas phase
products were collected in a gas bag after passing through a
final base trap (30 ml of 4 M NaOH solution) to prevent any
residual HBr from entering the bag. For most of the
experiments, reactions were run for one hour with all
products trapped and analyzed. The coupling products were
analyzed with three GCs, which measured (1) unconverted
CH Br, (2) gaseous hydrocarbon products C –C , and (3)
In this report we describe improvements to SAPO-34
catalysts that give higher yield for methyl bromide coupling
to light olefins. We also explore how the product distribution
and yields depend upon the process parameters and catalyst
structure.
Experimental
Synthesis of SAPO-34 and Co-SAPO-34
The silicoaluminophosphate molecular-sieve SAPO-34
samples were prepared following a procedure described by
1
7
Keil with minor modifications. A solution of 12.6 g of 85%
phosphoric acid, 1.6 g of 37% HCl and 20.3 g of DI water
was added to 27.2 g of aluminium isopropoxide in a
polyethylene (PE) bottle. 4.0 g of Ludox SM-30 colloidal
silica (Sigma Aldrich) was then added to the bottle followed
by 1 min vigorous stirring. Subsequently, 56.2 g of 35%
tetraethylammonium hydroxide (TEAOH, Aldrich) and
3
1
6
liquid hydrocarbons including C +, benzene, toluene,
4
xylenes (BTX), and other products.
After one hour of reaction, the feed gas was switched to air
and the reactor temperature raised to 500 1C in order to decoke
9
.1 g DI water were added and the bottle was stirred again
2
the catalyst for at least 2 h. CO released from decoking was
for 1 min. The mixture was then transferred to a Teflon-lined
autoclave, and agitated for 48 h at room temperature.
The approximate composition of the resulting gel
captured in another trap prefilled with excess saturated Ba(OH)
2
solution. The amount of carbon deposited on the catalyst during
coupling was determined by gravimetric analysis of the dried
2
4
is TEAOH : Al O : 0.89P O : 0.3SiO : 0.2HCl : 64H O.
2
2
3
2
5
2
BaCO precipitate. Based on the quantitative analyses, CH Br
3 3
The temperature was then increased to 215 1C, for 100 h
without agitation. After washing the resulting white
precipitate with DI water 10 times, the product was
transferred to a porcelain cup, dried at 120 1C and then
calcined at 600 1C for 6 h to yield a white powder. The
presence of a pure SAPO-34 phase (CHA) was confirmed
by XRD phase analysis (Fig. 1a). Samples with partial
framework substitution of Co were prepared by mixing
cobalt nitrate with the starting mixture solution with a mole
ratio of Co/Al O approximately equal to 0.02 (Fig. 1b).
conversions, product selectivities (C mol base) and product
yields (conversion multiplied by selectivity) were calculated.
For all of the experiments, carbon balances were calculated
based on the total carbon delivered to the reactor and the sum of
all carbon measured in the traps and as coke. Experiments with
carbon balances of 100 Æ 5% were considered to be acceptable
and reportable; however, most of the results had a carbon
balance of 100 Æ 2%.
Temperature dependency of coupling of CH Br over
3
2
3
SAPO-34 and Co-SAPO-34. Product samples were collected
Fig. 1 XRD patterns for the synthesized SAPO-34 (a) and CoSAPO-
4 (b).
3
3
Fig. 2 Experimental setup for CH Br coupling reaction.
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selectivity to propylene decreases and that to ethylene
increases with increasing temperature. Fig. 3 shows the
results of similar coupling experiments with Co-SAPO-34.
The substitution of Co into the SAPO-34 framework results
in a significantly higher CH Br conversion at the same
3
temperatures as the pure SAPO-34. However, the LO yields
are unchanged and the coking is slightly increased.
As shown in Table 1, higher temperatures promote higher
ethylene production, however, there is increased methane and
decreased higher hydrocarbons production, C4–6, which is
consistent with increased hydrocarbon cracking. Brominated
species, RBr, are decreased with increasing temperature as
dehydrohalogenation takes place.
Co-SAPO-34 showed higher catalytic activity at lower
Fig. 3 Conversion of CH
3
Br and yields of light olefin and coke over
temperature than regular SAPO-34; CH Br conversion for
3
SAPO-34 and CoSAPO-34 as a function of temperature.
Co-SAPO-34 was over 90% at 400 1C. Compared to regular
SAPO-34, coke formation is higher, LO yields and C2 /C3
=
=
ratios are slightly higher at low temperature. Comparing
product distributions, a major difference is that larger olefin/
paraffin ratios were measured at low temperature from
Co-SAPO-34 than observed from SAPO-34. All of these
findings suggest that the incorporation of Co into the
SAPO-34 framework likely changes the acidity as well as the
surface and pore structures, resulting in a more favorable
catalyst to perform the coupling reaction of CH Br to LO
3
and other products.
Effect of residence time over SAPO-34
Coupling reactions of methyl bromide were run over SAPO-34 at
475 1C and 0.2 atm partial pressure. The residence time was varied
from 0.5 to 4.0 s. The results are summarized in Fig. 5 and 6.
CH Br conversion increases non-linearly with increasing
3
=
2
=
residence time. Maximum LO yield near 60% was obtained
=
Fig. 4
C
/C
3
ratio (by weight) over SAPO-34 and CoSAPO-34.
=
at 2 s residence time as shown in Fig. 4. The C
2
/C
3
molar
ratios did not change with residence time (Fig. 6). Coke
amounts increase with the increased residence time. At very
short residence time (o2 s), conversion increased faster than
coking (exponential vs. linear dependency).
in gas bags and liquid traps over 1 h at each temperature over
the range of 400–500 1C in 251 increments. As shown in Fig. 3,
3
the conversion of CH Br over SAPO-34 increases significantly
with temperature from 58 to 90% and the yield for LO
The dependence of product formation on residence time is
=
(
ethylene + propylene) increases from 34 to 60%. By
=
=
not the same for all species. C2 and C3 selectivities decrease
with residence time while CH , C H , C H , and BTX
=
3
plotting the ratio C
2
/C
(by weight) as shown in Fig. 4,
it was found that the ratio increases almost linearly
with temperature. Furthermore, as shown in Table 1, the
4
2
6
3
8
selectivities increase with residence time (Fig. 6). Light olefins
Table 1 Product distribution for SAPO-34 at different temperatures
Catalyst, SAPO-34
Condition, 2.0 s, 0.2PCH
3
Br
C mol selectivity, %
400 1C
425 1C
450 1C
475 1C
500 1C
CH
4
1.3
19.4
0.3
64.7
39.3
7.0
5.6
14.0
2.8
8.9
7.0
2.1
24.3
0.4
60.8
35.9
5.2
6.9
13.3
4.4
3.2
29.7
0.6
49.5
32.5
4.8
6.8
10.4
3.7
4.8
34.4
0.8
43
27.1
3.6
7.5
9.6
3.1
7.9
8.6
8.2
40.2
1.0
40.3
24.1
2.4
10.1
6.5
3.7
5.1
C
C
C
C
C
C
C
2
2
2
3
3
3
H
H
H
H
H
H
4
6
4
6
8
6
/C
2
3
H
6
8
/C
H
4–6
BTXM+
RBr
Coke
9.1
5.3
9.2
5.9
Fig. 5 Dependence of CH
3
Br coupling on residence time over
Br.
8.9
SAPO-34 at 475 1C with 0.2 atm CH
3
2
552 Phys. Chem. Chem. Phys., 2011, 13, 2550–2555
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3
Fig. 8 Effect of partial pressure on CH Br coupling.
Fig. 6 Product selectivity of major compounds vs. retention time.
Table 2 Effect of partial pressure on CH Br coupling
3
=
2 3
/C
=
PMeBr Conv, % LO yield C
(wt) Coke, % C- balance, %
SAPO-34, 500 1C, 2.0 s
0
0
0
0
.2
.3
.4
.5
91.4
91.7
90.8
88.1
58.7
52.6
50.2
46.9
1.67
1.64
1.69
1.90
8.1
9.6
10.1
7.3
100.3
112.7
109.2
105.6
Fig. 7 Proposed mechanism converting CH
3
Br to light olefins.
CoSAPO-34, 500 1C, 2.0 s
0
0
0
0
.2
.3
.4
.5
97.9
96.4
96.3
94.1
60.2
51.8
48.7
47.0
1.67
1.59
1.62
1.67
11.7
14.4
15.1
13.9
100.4
115.3
100.2
108.4
are thought to be produced as an intermediate product at the
initial reaction stage and they are subsequently converted to
other products such as paraffin, BTX and finally coke as the
reaction is allowed to proceed over the catalysts.
Catalyst stability and reproducibility
Oligomerization of light olefins to higher olefins,
aromatization of olefins, alkylation of aromatics, further
condensation to larger aromatics and ring condensation, and
the eventual formation of coke are all considered part of the
reaction pathway. On the other hand, as some amounts of
highly active hydrogen species will be released in these
reactions, these active hydrogen species will hydrogenate
olefins to paraffins.
To test the catalyst stability and reproducibility, 10 reaction-
decoke cycles were run on SAPO-34 including 1 h of reaction
and overnight decoking. Coupling reactions were run at 475 1C
with 2 s residence time and 0.2 atm CH Br while decoking was
3
3
À1
performed at 500 1C overnight with a flow of 5 cm min air.
As shown in Fig. 9, the catalysts show excellent stability and
reproducibility in terms of CH
=
3
Br conversions, LO yields,
=
C
2 3
/C
ratios, and coke amounts. No notable catalyst
Fig. 7 illustrates a suggested CH Br coupling mechanism
3
performance decay was observed under these conditions and
no structure changes were measured from XRD analysis of the
catalyst after the runs.
which follows from analogous intermediates observed and
1
predicted in MeOH coupling.
5
Influence of partial pressure on CH Br coupling over SAPO-34
3
and Co-SAPO-34
Fig. 8 illustrates the influence of partial pressure on yield and
conversion from the coupling reactions of methyl bromide over
SAPO-34 and Co-SAPO-34. Table 2 contains a summary of
the product distribution observed. The reactions were run for
one hour at 500 1C with a residence time of 2.0 s.
As the partial pressure was increased from 0.2 atm to
0
.5 atm, the CH
=
3
Br conversion decreased slightly, the
=
C
2 3
/C
ratios remained constant, and the LO yields
decreased. The yields of other products including CH
4
,
3 8
C H , BTX and RBr are minimally increased (not shown).
The same trends were observed with both SAPO-34 and
Co-SAPO-34. In spite of the negative effect caused by
increased CH Br partial pressure, the light olefin yields still
3
Fig. 9 Catalyst stability test over SAPO-34 at 475 1C, 2 s and 0.2 atm
reached approximately 47% at 0.5 atm for both catalysts.
3
CH Br.
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Conclusion
In general, it has been found that temperatures of
approximately 475 1C and relatively short residence times
(
1–2 s) favor the formation of light olefins with a minimum
amount of coke. Increasing the Si/Al ratio or incorporation of
Co into the SAPO-34 framework promotes CH Br conversion
3
but also leads to higher rates of coke formation. Increasing
CH Br partial pressure does not significantly affect CH Br
3
3
conversion; however, the product distribution shifts towards
=
=
heavier components. The C2 /C3 ratio almost exclusively
depends on coupling temperature. Light olefins are recognized
as the primary coupling products at the initial reaction stage,
which if given longer product–catalyst residence times undergo
a series of sequential reactions and are converted to byproducts
such as BTX, light alkanes, and coke.
2 3
Fig. 10 Effect of SiO content on CH Br conversion, LO yield and
coke formation.
This work was supported in part by the IUCRP University
of California Discovery Grant program in cooperation with
GRT, Inc.
Effect of SiO content in SAPO-34 on coupling reactions
2
As previously shown, changing the Si/Al ratio in the SAPO-34
structure changes the concentration of acid sites available for
2
5
catalysis. We synthesized five different Al/Si catalysts: S1,
Si0.06AlP0.97O ; S2, Si0.10AlP0.94O ; S3, Si0.15AlP0.89O ; S4,
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