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Energy-Efficient Syngas Production through Catalytic Oxy-Methane Reforming Reactions.

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Reviews
V. R. Choudhary and T. V. Choudhary
DOI: 10.1002/anie.200701237
Syngas Production
Energy-Efficient Syngas Production through Catalytic
Oxy-Methane Reforming Reactions
Tushar V. Choudhary and Vasant R. Choudhary*
Keywords:
fuels · heterogeneous catalysis ·
methane activation ·
partial oxidation · syngas
Angewandte
Chemie
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2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
Angewandte
Chemie
Syngas Production
The considerable recent interest in the conversion of stranded methane
into transportable liquids as well as fuel cell technology has provided a
renewed impetus to the development of efficient processes for the
generation of syngas. The production of syngas (CO/H2), a very
versatile intermediate, can be the most expensive step in the conversion
of methane to value-added liquid fuels. The catalytic oxy reforming of
methane, which is an energy-efficient process that can produce syngas
at extremely high space–time yields, is discussed in this Review. As
long-term catalyst performance is crucial for the wide-scale commercialization of this process, catalyst-related studies are abundant.
Correspondingly, herein, emphasis is placed on discussing the different
issues related to the development of catalysts for oxy reforming.
Important aspects of related processes such as catalytic oxy-steam,
oxy-CO2, and oxy-steam-CO2 processes will also be discussed.
1. Introduction
From the Contents
1. Introduction
1829
2. COMR Catalysts
1830
3. Reaction Mechanisms
1837
4. Combined Reforming
1839
5. Summary and Outlook
1842
that it represents the predominant cost
associated with the indirect methane
conversion processes.[27] As a result,
extensive studies are being undertaken
in academia as well as in industry to
develop energy-efficient processes for
syngas generation. Steam methane reforming, carbon dioxide
methane reforming, and oxy(gen) methane reforming are the
three basic processes for the production of syngas (Table 1).
Other technologies such as combined reforming and autothermal reforming are derived from these processes. The
steam methane reforming process, which was first commercialized in 1930s, is currently the most widely used process for
methane conversion.[28] As observed from Table 1, the steam
methane reforming process is highly endothermic (energy
The efficient upgrading of methane,[1–8] the major component of natural gas, has been a longstanding challenge for the
scientific community. Although there are abundant reserves
of methane, these are unfortunately located in remote areas.
For efficient transportation, the stranded methane needs to be
transformed physically or chemically onsite to easily transportable fuels.[9–12] Methane, which is a very refractory
compound, can be chemically converted into higher
hydrocarbons/liquid fuels by either indirect routes or
low-yielding direct routes.[13] The direct routes involve
processes such as oxidative coupling of methane,
high-temperature methane coupling, methane aromatization, and two-step methane homologation,
whereas the indirect routes involve methane upgrading via intermediates (e.g. synthesis gas (syngas),
which is a mixture of hydrogen and carbon monoxide)
formed from the reaction of methane with oxygen,
steam, and so on. Unfavorable thermodynamics result
in low product yields for the direct methane conversion routes which makes them commercially less
viable.[2, 13, 14] Instead, the indirect syngas-based routes
are considered to be promising from a commercialization viewpoint.
One of the most widely touted technologies for
the methane-upgrading process is the gas-to-liquids
Figure 1. Schematic of methane applications via syngas as an intermediate.
(GTL) process, which involves conversion of methane
into syngas in the first step followed by conversion of
syngas to hydrocarbon liquids in the second step via the
[*] Prof. Dr. V. R. Choudhary
Fischer–Tropsch process.[15–17] A simplistic overview of methChemical Engineering and Process Development Division
ane upgrading via syngas is shown in Figure 1. The first step
National Chemical Laboratory
involves the reaction of methane with oxygen and/or steam
Pune-411008 (India)
and/or carbon dioxide to syngas. Syngas, which is a very
Fax: (+ 91) 202–590–2612
[18]
versatile intermediate,
can then be efficiently converted
E-mail: vr.choudhary@ncl.res.in
into a variety of value-added products or used in fuel cell
Dr. T. V. Choudhary
applications or power plants.[19–26]
ConocoPhillips Company
Unfortunately, the generation of syngas from methane is a
Bartlesville Technology Center
cost-intensive process (large capital investment), so much so
Bartlesville, OK 74004 (USA)
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1829
Reviews
V. R. Choudhary and T. V. Choudhary
Table 1: Basic processes for syngas production from methane.
Process
Reaction
H2/CO ratio DH1173 K
[kJ mol1]
Steam reforming CH4 + H2O!CO + 3 H2
3:1
CO2 reforming
CH4 + CO2 !2 CO + 2 H2 1:1
(dry reforming)
Oxy reforming
CH4 + 1/2 O2 !CO + 2 H2 2:1
(partial oxidation)
225.7
258.8
23.1
intensive) and produces 3 mol hydrogen per mole of methane
consumed. If hydrogen production is the goal (e.g. at
refineries), the amount of hydrogen produced can be further
increased by utilizing the water gas shift reaction, wherein
carbon monoxide is reacted with steam to produce carbon
dioxide and hydrogen. The carbon dioxide methane reforming process, on the other hand, produces the least amount of
hydrogen per mole of methane consumed. Although this
process consumes two greenhouse gases, it suffers from two
major disadvantages: extremely high energy costs and rapid
catalyst deactivation through carbon deposition.[29]
From an energy efficiency viewpoint, the oxy-methane
reforming (partial oxidation of methane) process is most
promising amongst the three basic processes for syngas
production. The H2/CO ratio (2:1) of the syngas produced
by this method is also suitable for the synthesis of a variety of
value-added chemicals. Oxy reforming of methane can be
carried out homogeneously or catalytically. The homogeneous (non-catalytic) oxy-methane reforming process for syngas
generation has been commercially demonstrated in a GTL
plant at Sarawak, Malaysia.[30] This process requires very high
temperatures (> 1573 K) for obtaining high methane conversion and minimizing soot formation. The non-catalytic
oxy-methane reforming reaction is also used in the autothermal reforming technology, which is currently considered an
important technology for syngas generation in the GTL
business.[31–34] The autothermal reforming process may be
simplistically described as a combination of non-catalytic oxymethane reforming and catalytic steam and CO2 methane
reforming processes. In this Review, we will focus on catalytic
oxy-methane reforming (COMR).
One of the other main advantages of the COMR process is
that high methane conversions can be obtained with excellent
syngas selectivity at extremely high space velocities (contact
time on the order of milliseconds).[35, 36] However, despite
favorable thermodynamics and fast reaction kinetics, the
COMR technology has yet to deal with significant challenges
before it can be widely commercialized. The high space
velocities coupled with high conversions can result in high
local temperatures on the surface of the COMR catalyst
which can result in catalyst deactivation due to sintering or
formation of catalytically inactive phases by solid–solid
reactions and carbon deposition. Moreover, catalyst deactivation can decrease syngas selectivity and make the process
highly exothermic, thereby raising safety concerns.
These issues can however be circumvented by developing
thermally stable and carbon-resistant catalysts. As catalyst
development is critical for this process, a major share of the
COMR research has been focused on catalyst-related issues.
Consequently, a significant portion of this Review will be
devoted towards addressing key issues in COMR catalyst
development with emphasis on recent studies (Section 2). As
catalyst development benefits from a strong fundamental
understanding of the reaction system, several studies have
been undertaken to gain insights into mechanistic issues
related to COMR; key aspects of these studies will be
discussed in Section 3. The prior-mentioned issues which
afflict the COMR process can also be mitigated by employing
a combined oxy-steam, oxy-CO2, or oxy-steam-CO2 methane
reforming process.[37, 38] The combined process involves coupling of the exothermic COMR reactions with the endothermic steam methane reforming and/or carbon dioxide methane
reforming carried out simultaneously on the same catalyst.
These studies will also be addressed in considerable detail in
Section 4.
2. COMR Catalysts
2.1. Overview of COMR Catalyst Development
While the first report related to COMR research was
published almost 80 years ago,[39] it was only in the 1990s that
research in this area really propelled forward. Following the
work of Ashcroft et al. in 1990,[2] which showed that COMR
using noble-metal catalysts provided high methane conversions with excellent syngas selectivity, a deluge of studies was
Tushar V. Choudhary obtained his PhD from
Texas A&M University (USA) in 2002 on
work carried out with D. W. Goodman
exploring novel COx-free hydrogen production routes for low-temperature fuel-cell applications. He has worked in a number of
catalysis areas ranging from methane/loweralkane activation to hydrotreating petroleum
feedstocks, and has authored more than 45
papers and holds or has filed 10 patents. He
is currently working as a research scientist at
the Bartlesville Technology Center, ConocoPhillips (USA).
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Vasant R. Choudhary completed his PhD in
physical chemistry at Pune University (India)
in 1972. During his career, he has mainly
worked at the Chemical Engineering and
Process Development Division, National
Chemical Laboratory (Pune), and spent
several months as a visiting scientist/professor at different research institutions in
Europe, Japan, and the USA. His research
interests include methane/lower-alkane conversion, zeolites, and green processes. He
has published more than 360 papers and
holds more than 75 US/Indian patents. He
is currently Emeritus scientist at the
National Chemical Laboratory, Pune.
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
Angewandte
Chemie
Syngas Production
undertaken in this area. Chief among these studies was the
work undertaken by two independent research groups,[35, 36]
which showed that the COMR process could result in high
syngas production rates at extremely high space velocities.
Using space velocities of over 500 000 cc g1 h1 (cc: cumulative pore volume in cm3 ; an order of magnitude higher than
used earlier[2]), Choudhary et al. obtained high (> 90 %)
methane conversions and excellent syngas selectivity over a
Ni/MgO catalyst.[35] Around the same time, Schmidt and
Hickman also reported near-complete methane conversion
and over 90 % selectivity for H2 and CO at extremely short
reactor residence time, albeit over Pt and Rh surfaces
supported on a porous alumina foam.[36] The capability of
the COMR process to obtain high syngas yields at extremely
high space velocities is a very attractive feature of this
technology.[40–43] In contrast, the energy-intensive steam
reforming and CO2 reforming processes have to be operated
at significantly lower space velocities to obtain high methane
conversions.
Both noble- and non-noble-metal-based catalysts have
been extensively investigated for the COMR process.
Although several noble metals have been used as catalysts
for COMR, most studies have focused on Rh, Ru, and Pt,[14]
among which Rh is considered to be the most active for the
COMR process (Table 2).[36, 44–46] In the case of the non-nobleTable 2: Activity ranking for noble metals for the COMR process.
Support/matrix
Activity ranking
Ref.
ceramic monoliths
hydrotalcite-type
alkaline- and rare-earth-metal oxides
calcium oxide/alumina (12:7)
Rh > Pt
Rh @ Ru Ir @ Pt > Pd
Pt > Pd
Pt > Ru > Pd
[36]
[44]
[45]
[46]
metal-based catalysts, those based on Ni have attracted the
most attention, although other systems have also been
explored.[14, 29, 47–50] The noble-metal-based catalysts are, in
general, more active and stable than their non-noble-metal
counterparts. However, as noble metals are expensive and in
relatively short supply, there is a considerable incentive to
develop non-noble-metal-based catalysts with comparable
activity and stability for COMR applications. Also, from a
cost standpoint it is desirable to minimize the metal content of
the catalyst while retaining high COMR activity/selectivity.[44, 51, 52] Several research groups have investigated the effect
of metal loading on the COMR performance to optimize the
metal content. In general, a large enhancement in COMR
performance is observed with increasing metal content until a
certain threshold;[44, 46, 53] however, exceptions have been
noted for certain catalyst systems.[54, 55]
One of the major advantages of the CMOR process is its
ability to provide high methane conversion at extremely high
space velocities.[35, 56, 57] As the COMR reaction exhibits
extremely rapid kinetics, facilitating the interaction between
the reactants and the active components of the catalyst is
expected to improve the process performance,[58–60] especially
at higher operating pressures where appreciable intraparticle
mass transport limitations are expected.[61] Easy access for the
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
reactants to the active phase can be facilitated by utilizing an
eggshell catalyst design, wherein the active component is
selectively deposited in the outer regions of the catalyst
support.[62, 63] Recently, a significantly improved COMR
performance for an eggshell Ni/Al2O3 catalyst was observed
as compared to a Ni/Al2O3 catalyst with a homogeneous
distribution of Ni.[64] The eggshell catalyst was prepared with
acetone as the solvent for nickel nitrate, while the uniformly
distributed Ni catalyst was prepared by using water.
Pure oxygen, which is a requirement for the COMR
process, necessitates expensive air-separation units. Efforts
are therefore being undertaken to develop membranes
(perovskite-like ceramics, etc.) that can afford oxygen separation and syngas generation in a single step.[65–75] Moreover,
this would also assist in increasing the safety of the COMR
process. Interesting synergistic effects have been observed on
application of the membrane in the presence of a COMR
catalyst for the COMR process.[70, 76] A recent study of the
performance of Ba0.15Ce0.8FeO3d with and without a Ni-based
COMR catalyst showed that the COMR performance and
oxygen permeability were both significantly improved in the
presence of the COMR catalyst.[76] Unfortunately, although
interesting advances have been made in this area, as a result
of issues related to membrane stability, scalability, and oxygen
permeability flux (low syngas yields), the commercial use of
membranes for the short-contact-time COMR process is less
likely in the near future.
A significant fraction of the literature concerning COMR
is devoted towards developing more active catalysts. Owing to
the industrial importance of this process, the work on COMR
catalyst development has been extensively documented in
papers[77–89] as well as patents.[90–97] The strategies employed to
develop more active catalysts involve the introduction of
multiple active components in the catalyst[82, 93, 95, 97–99] and the
use of novel preparation methods,[77, 78, 91, 92] superior supports,[83, 85, 94] novel precursors,[86–90] and novel pretreatment
methods.[100, 101] Choudhary et al. observed a dramatic effect in
the starting COMR reaction temperature (Ts) for a Ni/Al2O3
catalyst on addition of small amounts of noble metals (Pt, Pd,
and Ru).[99] The decrease in Ts was found to strongly depend
on the noble metal and its content (Table 3) and was
attributed to a greatly enhanced reduction rate of the
NiAl2O4 produced by the reaction over the noble metal and
spillover of highly active atomic hydrogen from the noble
metal to NiAl2O4. Eriksson et al.,[85] on the other hand, found
Table 3: Influence of the nature and content of noble metals on the startof-reaction temperature (Ts) of Ni/Al2O3 catalysts (CH4/O2 = 2:1; gas
hourly space velocity (GHSV) = 5.6 D 105 cc g1 h1).[99]
Catalyst
Noble-metal content [wt %]
Ts [K]
0
0.1
0.5
2.5
0.1
0.5
2.5
2.5
1063
878
803
693
793
733
673
803
Ni/Al2O3
Pt-Ni/Al2O3
Pt-Ni/Al2O3
Pt-Ni/Al2O3
Pd-Ni/Al2O3
Pd-Ni/Al2O3
Pd-Ni/Al2O3
Ru-Ni/Al2O3
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1831
Reviews
V. R. Choudhary and T. V. Choudhary
that the Ts for Rh-based catalysts was weakly dependent on
Rh content but strongly influenced by the support. The results
were attributed to significantly superior dispersion of Rh on
the Ce-ZrO2 support and high oxygen mobility.
The focus of the above-described studies has been to
develop COMR catalysts with enhanced activity; however,
from a commercial viewpoint, it is also critical that the
catalysts can sustain their activity over a long term. COMR
catalysts, unfortunately, have a tendency to deactivate as a
result of sintering/consumption of catalytically inactive
phases by solid–solid reactions and carbon deposition
during extended time-on-stream runs. Studies related to the
development of high-temperature stable and carbon-deposition-resistant catalysts are individually discussed in the
following sections.
Figure 3. Effect of GHSV on COMR methane conversion (black
column), H2 selectivity (gray column), and CO selectivity (white
column) over a NiCoMgOx/SZ5564 catalyst. Temperature = 1123 K;
CH4/O2 = 1.8; precalcination temperature = 1673 K; reduction = 1173 K
in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[103]
2.2. Thermally Stable COMR catalysts
2.2.1. Effect of Process Parameters on the COMR Process
Before delving further in catalyst development, it would
be beneficial to discuss the effect of process parameters on the
COMR performance. This will assist in identifying the lower
bound temperature constraints associated with the process for
obtaining economically attractive syngas yields and also
enunciate the necessity for developing thermally stable
catalysts. As expected, increasing the process temperature
has a considerable positive influence on the methane
conversion (Figure 2), whereas increasing the space velocity
Figure 2. Effect of temperature on COMR methane conversion (~), H2
selectivity (gray column), and CO selectivity (white column) over a
NiCoMgOx/SZ5564 catalyst. GHSV = 120 000 cc g1 h1; CH4/O2 = 1.8;
precalcination temperature = 1173 K; reduction: 1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[102]
has a detrimental effect (Figure 3).[102, 103] The most important
process parameter for the COMR process is perhaps the
operating pressure. Owing to pressure requirements of downstream applications such as Fischer–Tropsch and methanol
synthesis, process economics dictate that the COMR reactor
should be commercially operated at modestly high pressures
(0.5 to 4 MPa).[104] Unfortunately, the thermodynamics for the
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COMR process become increasingly unfavorable with
increasing reactor pressure. Calculations indicate that the
equilibrium methane conversion at 1223 K decreases from
almost 100 % to about 85 % when the pressure is increased
from 0.1 to 1 MPa.[61]
Despite the obvious importance of the pressure dependence for the COMR process, scarce attention has been paid to
this aspect. Investigating the effect of pressure on the COMR
process (0.2 to 0.8 MPa), Lyubovsky et al.[104] observed that
the negative effect on methane conversion due to increasing
pressure could be partially offset by operating the reaction at
higher oxygen-to-carbon ratios. An increase in pressure
resulted in a decrease in COMR peak temperature, which
allowed the operation of the reactor at higher O/C ratios.
Operating at higher O/C ratios (> 1.2) afforded conversions
above 90 % even at 0.8 MPa with over 90 % process
selectivity. The authors believe that the trend observed in
their study (continual decrease in peak temperature with
increasing pressure) would assist in allowing successful
operation of the COMR process even at pressures above
0.8 MPa.
The upshot from the above discussion on the process
parameters is that for obtaining industrially relevant methane
conversion, the COMR process needs to be operated at
temperatures above 1273 K. Furthermore, as this process
involves high methane conversion (> 90 %) coupled with very
low contact times (even at very high selectivity for CO and
H2) a large amount of heat is produced in a small catalyst
zone. This can result in the catalyst surface being exposed to
higher local temperatures.[105–110] During commercial operation, due to changes in feed composition and/or unit upsets,
the catalyst can be exposed to even higher temperatures. In a
recent laboratory-scale COMR catalyst life-test study, a
significant rise in temperature (maximum catalyst temperature over 1373 K) was observed in the middle of the test as a
result of the failure of the air compressor controller
system.[111] Similar unit upsets are not uncommon in an
industrial setting and hence the COMR catalysts should be
developed assuming that they would be exposed to temperatures of 1373 K during commercial operation.
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
Angewandte
Chemie
Syngas Production
2.2.2. Development of Thermally Stable COMR Catalysts
From the previous section, it is apparent that high reaction
temperatures are critical for the practical operation of the
COMR process. As high temperatures have a detrimental
effect on the catalyst activity, thermal stability is a prime
requirement for sustaining long-term performance of the
COMR catalysts. Thermal catalyst deactivation can occur by
the following mechanisms: a) sintering of the active components; b) sintering of the support; and c) chemical interaction
between the active component and the support to form an
inactive phase.[112, 113] As the non-selective methane oxidation
reactions [Eq. (1) and Eq. (2)] are much more exothermic
than the selective COMR process [Eq. (3)], even a small
decrease in syngas selectivity can result in greatly increased
reaction exothermicity. Catalyst deactivation can thus also be
a major concern from a safety standpoint.
1
CH4 þ 1:5 O2 ! CO þ 2 H2 O DH 1173 K ¼ 520:6 kJ mol
ð1Þ
CH4 þ 2 O2 ! CO2 þ 2 H2 O DH 1173 K ¼ 802:5 kJ mol1
ð2Þ
CH4 þ 0:5 O2 ! CO þ 2 H2 DH 1173 K ¼ 23:1 kJ mol1
ð3Þ
The effect of catalyst calcination on the COMR performance can be effectively used to study the thermal stability of
the catalyst. Previous studies have shown that certain catalysts
show significantly inferior performance when calcined at
temperatures relevant to practical process conditions.[53, 55]
Such catalysts obviously do not qualify as thermally stable
catalysts. One approach to develop thermally stable catalysts
is to identify appropriate support materials for the active
component. From this viewpoint, MgO is considered to be a
preferred COMR catalyst support.[35, 114–120] Interestingly, a
continual improvement in COMR performance was observed
on increasing the calcination temperature of the Ni/MgO
catalyst from 1023 to 1473 K.[116]
The excellent resistance to sintering for the Ni/MgO
catalysts was attributed to the strong interaction between NiO
and MgO in the NiO/MgO solid solution. X-ray photoelectron spectroscopy studies have shown a lower Mg(2p) and
a higher Ni(2p3/2) binding energy for the NiO-MgO solid
solution than MgO or NiO alone.[121] This indication of
electron transfer from NiO to MgO alludes to a strong NiO–
MgO interaction. Other independent studies have also noted
an activity improvement on increasing the calcination temperature of the Ni/MgO catalyst.[54] On the basis of X-ray
diffraction, temperature-programmed reduction, and X-ray
photoelectron spectroscopy results, the improvement in the
performance was attributed to formation of an enhanced solid
solution between NiO and MgO at the higher calcination
temperature.
Although Ni/MgO catalysts show high thermal stability
and good COMR activity, due to their hygroscopic nature
their pellets have poor mechanical strength (attrition resistance and crushing strength). As appropriate mechanical
strength is necessary for commercial catalysts, Choudhary
et al. precoated high-mechanical-strength, low-surface-area
porous silica alumina catalyst carriers with MgO and deposAngew. Chem. Int. Ed. 2008, 47, 1828 – 1847
ited Ni on these MgO-precoated supports.[122, 123] The Ni
catalysts prepared in this manner showed significantly
superior activity compared to those prepared on the same
supports without MgO precoating (Table 4).[124] The enorTable 4: COMR performance of supported Ni catalysts with and without
precoating of the support by MgO. Reaction temperature = 1073 K;
GHSV = 5.1 D 105 cc g1 h1; CH4/O2 = 1.86:1. The main component of
SA5205, SC5232, and SS5231 are Al2O3, SiC, and SiO2, respectively.[124]
Catalyst[a]
CH4 conv. [%] H2 sel. [%] CO sel. [%]
NiO(12.0)/SA5205
3.0
NiO(12.2)/MgO(5.6)/SA5205 94.7
NiO(18.0)/SC5232
3.3
NiO(15.8)/MgO(7.5)/SC5232 91.1
NiO(11.4)/SS5231
5.9
NiO(10.3)/MgO(7.8)/SS5231 91.8
11.1
97.5
26.4
98.9
39.1
95.8
17.0
95.6
15.0
95.1
10.0
95.1
[a] Numbers in brackets refer to wt %.
mous beneficial effect of precoating the support was attributed to the drastic reduction in the propensity for formation
of inactive Ni2SiO4 and NiAl2O4 phases during the hightemperature calcination step. The activity of the Ni deposited
on precoated support catalysts was found to be comparable
(or slightly superior in certain instances) to the Ni/MgO
catalysts.[122] Unfortunately, these catalysts were only stable
until a calcination temperature of 1273 K.[123] From a practical
viewpoint (as was emphasized earlier), the COMR catalyst
should be stable well above 1273 K and should be able to
accommodate high-temperature shocks.
The same research group recently modified their catalyst
synthesis strategy to develop high-temperature stable and
mechanically strong catalysts.[125] The new approach involved
deposition of Ni on a commercial low-surface-area macroporous sintered zirconia-hafnia support material (SZ5564)
along with several other components (Co and/or Mg and/or
Ce and/or Zr). The catalysts prepared by this method showed
exceptionally high thermal stability. The COMR performance
of the catalysts precalcined at 1673 K is shown in Figure 4.[126]
As the NiCoMgOx/SZ5564 and NiCoMgCeOx/SZ5564 catalysts showed the best performance, these were selected for
additional studies. To further investigate the thermal stability
of these catalysts, an accelerated thermal deactivation/shock
test was devised. The accelerated test involved torching the
catalyst in an oxygen-acetylene flame ( 2273 K) for several
minutes. The shock test entailed a repeated 0.5-min exposure
(six times) of the catalysts to the oxygen-acetylene flame after
every 10 min. As observed from Table 5, the catalysts retained
high COMR performance despite being exposed to enormously stringent temperature conditions. Expectedly, both
the catalysts showed stable activity and syngas selectivity for a
50 h COMR test.[126]
As silicon carbide has high mechanical strength, thermal
conductivity, and resistance to oxidation, it has also been
considered as a catalyst support for the COMR process.[127, 128]
Studies have shown a superior stability for the Ni/SiC
catalysts as compared to Ni/Al2O3 catalysts,[127] however, the
thermal stability of the catalyst has not been truly tested as
the catalyst was not exposed to temperatures above 1273 K.
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V. R. Choudhary and T. V. Choudhary
Figure 4. Comparison of supported Ni-Co catalysts (calcined at 1673 K
for 4 h) on methane conversion (~), H2 selectivity (gray column), and
CO selectivity (white column) for the COMR process. Temperature = 1173 K; GHSV = 62 000 cc g1 h1; CH4/O2 = 1.8; reduction:
1173 K in the presence of 1:1 H2/N2 for 1 h before the start of the
reaction.[126]
Table 5: COMR performance of NiCoMgOx/SZ5564 and NiCoMgCeOx/
SZ5564 catalysts (precalcined at 1173 K) after exposure to hightemperature schocks by torching in an oxy-acetylene flame; temperature = 1123 K, GHSV = 1.2 D 105 cc g1 h1, CH4/O2 = 1.80:1.[126]
Catalyst
Exposure time
to flame [min]
CH4 conv.
[%]
H2 sel.
[%]
CO sel.
[%]
NiCoMgOx/SZ5564
NiCoMgOx/SZ5564
NiCoMgCeOx/SZ5564
NiCoMgCeOx/SZ5564
NiCoMgCeOx/SZ5564
NiCoMgCeOx/SZ5564
0
30 (once)
0
15 (once)
30 (once)
0.5 (six times)
95.7
95.6
98.0
98.3
96.9
97.2
97.4
96.1
97.6
96.9
96.7
97.2
98.4
98.0
98.4
96.7
98.2
98.4
Hexaaluminate-based catalysts, owing to their high thermal
stability, are considered to be promising candidates for the
high-temperature catalytic methane combustion reactions,[129]
and therefore are also obvious candidates for the COMR
reaction.[130] Interestingly, a Ni-hexaaluminate catalyst calcined at 1673 K was found to be more stable than a Ni/Al2O3
catalyst calcined at just 1273 K.[131] The higher stability of the
former catalyst was attributed to formation of finely dispersed
Ni particles following high-temperature reduction of the
homogeneous mixed-oxide phase that formed during the
calcination step.
The crystallite thermal stability of the noble metals (Ru,
Rh, and Pt) is expected to be higher than that of Ni as the
crystallite stability is known to correlate directly with the
melting point of the metal in a reducing environment.[113]
However, note that the thermal stability can be altered by
controlling the metal–support interactions. From a thermal
deactivation viewpoint, the noble-metal-based catalysts have
been investigated to a much lesser extent than the Ni-based
catalysts. A recent study has addressed the thermal stability of
different noble-metal-based hexaaluminate catalysts.[132] The
BaMxAl12xO19& (M = Ru, Pd, Pt) hexaaluminate com-
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pounds used in this study were synthesized by the alkoxide
method and calcined at temperatures ranging from 1373 to
1673 K. At a calcination temperature of 1673 K, the Ru-based
catalysts significantly outperformed the Pd- and Pt-based
catalysts in terms of methane conversion activity. In case of
Ru-based catalysts, methane conversion was found to be
almost independent of the calcination temperature (1373–
1673 K). In contrast, the Pd-based catalysts showed a
continual decrease in activity with increasing calcination
temperature due to sintering.
Efforts are also being undertaken to modify existing
supports and identify/develop supports with novel thermalresistant properties.[111, 133–135] Recently, the COMR reaction
over Rh has been investigated on a support characterized by
very high heat and mass transfer coefficients.[111] Although
temperatures approaching 1373 K were observed for a short
period during the test, the catalyst was found to be stable over
the 500 h operation and showed good COMR activity. As
mentioned earlier, the long-term catalyst stability is determined by its thermal stability as well as its resistance to
carbon formation. The next section discusses catalyst development studies related to carbon formation during the
COMR process.
2.3. Development of Carbon-Resistant COMR Catalysts
The resistance of the catalyst to form carbon is influenced
by a number of factors: a) the nature of introduction of the
active component or the catalyst preparation method; b) the
nature of the active component; c) dispersion of the active
component; d) modification of the active component through
addition of a promoter; e) the nature of the support;
f) modification of the support by promoter addition; and
g) the preparation method of the support. Because of
interdependency between the factors, herein the key studies
related to factors a–d have been grouped together under the
broad heading “influence of the active component”, while
those addressing the remaining factors have been classified
under the general heading “influence of the support”.
2.3.1. Influence of the Active Component
The noble metals in general are known to exhibit a lower
proclivity for carbon deposition in methane reforming
processes than Ni-based catalysts. In particular, for the
COMR process the relative carbon formation tendency of
the metals was found to increase in the following order: Re Ir < Rh < Pd < Ni.[136, 137] The carbon deposition in the COMR
process is expected to occur as a result of methane decomposition and CO disproportionation;[136–140] however, studies
suggest that methane decomposition is possibly the major
mechanism for carbon formation at COMR-relevant (higher)
temperature.[136] It is important to note that the carbon
deposition rate is not only a function of the nature of the
active component but also depends on metal dispersion, the
nature of the support, and the process conditions. The
influence of temperature on the relative propensity of the
metals to form carbon from CH4/H2 (95:5) mixtures can be
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clearly observed from thermogravimetric analysis studies.[141]
The carbon deposition rate was found to increase in the
following order at 773 K: Pd < Pt < Ru < Ir < Rh < Ni; at the
higher temperature (more relevant to the COMR process) of
923 K, it increased in the following order: Ru < Pt < Ir <
Rh < Pd < Ni.
Note that the trend observed in this methane decomposition study is qualitatively similar to the relative carbon
deposition rate observed earlier in a COMR study;[136] this
indicates that simple methane decomposition experiments
could be used as an accelerated preliminary screening tool for
distinguishing the coke-forming propensity between COMR
catalysts.
The difference in carbon formation over Ni-based catalysts as compared to noble-metal-based catalysts would seem
exaggerated if low Ni dispersion catalysts are employed. Low
metal dispersion is expected to favor carbon deposition
because coke/graphite formation requires large metal ensembles.[29, 142–146] The strong dependence of coking on metal
crystallite size, which has an inverse relationship with
dispersion, is apparent from Figure 5.[146] Due to significantly
dispersion can be influenced by the metal content, preparation method, and pretreatment procedure.[139, 146–149] In these
studies, the catalysts exhibiting superior Ni dispersion showed
a significantly more stable COMR performance due to
increased resistance to coking.
The COMR carbon formation rates can also be profoundly influenced by promoter addition to the active
component.[138, 150–153] Studies have shown that Co addition to
Ni-based systems can drastically reduce carbon formation
during the COMR process.[138, 150] The beneficial effect of Co
was attributed to increased activity for carbon oxidation and/
or decreased activity for graphitic carbon formation. Similarly, recent work has shown that addition of Au to a Ni/YSZ
catalyst (YSZ: yttria-stabilized zirconia) can inhibit graphite
formation during methane decomposition.[154] Previous DFT
calculations also concur that Au addition can significantly
reduce graphite formation in the case of Ni-based catalysts.[155]
Recent studies have shown boron to be a promising
promoter for diminishing the rate of carbon deposition on Nibased catalysts.[152] The boron promotion resulted in a
decrease in the average coking rate from 1.1 (for unpromoted
Ni catalyst) to 0.68 mgC gcat h1. For comparison purposes, the
coking rate under identical process conditions for a 1 % Rh/
Al2O3 catalyst was found to be 0.39 mgC gcat h1. The boronpromoted catalysts showed similar COMR activity and
stability as the 1 % Rh/Al2O3 catalyst in a short time-onstream COMR study. Methane decomposition studies followed by temperature-programmed oxidation showed that a
large fraction of the carbon formed on the boron-promoted
catalyst could be oxidized at a much lower temperature than
the carbon formed on the unpromoted catalyst, indicating
that the presence of boron decreased the coking rate by
minimizing the formation of the hard-to-remove graphitic
carbon.
Figure 5. Influence of Ni crystallite size (dNi) on the average coking
rate (rc) during the COMR process. Dispersion measured after
calcination at 973 K for 12 h; reaction temperature = 923 K; CH4/
O2 = 1.89; GHSV = 8400 cc g1 h1.[146]
2.3.2. Influence of the Support
lower metal content usage and lower tendency to sinter, it is
relatively facile to obtain highly dispersed noble-metal
catalysts. Catalyst development studies related to superior
dispersion are therefore more focused on the Ni-based
catalyst systems, wherein it is more challenging to obtain
highly dispersed catalysts. As shown in Table 6, the Ni
Table 6: Parameters which affect the Ni dispersion in catalysts.
Parameter
Comment
Ref.
Metal content higher dispersion for Ni/Ce(La)Ox catalysts with
5 atom % Ni than for those containing 10 or
20 atom % Ni
Preparation
higher dispersion by water-in-oil microemulsion
method
than by sol-gel method or impregnation
Preparation
higher dispersion by sol-gel method as compared
method
to conventional wet impregnation
Pretreatment higher dispersion following plasma pretreatment
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[139]
[146]
[148]
[149]
There is considerable experimental evidence which suggests that coke formation during the COMR process is also
influenced by the nature of the catalyst support;[147, 156, 157] that
is, the dispersion of the active component cannot alone
explain the observed carbon formation rates (Figure 6).[156] A
large number of supports have been investigated to ascertain
their role in determining carbon deposition. A study involving
a series of rare-earth oxides in combination with NiO showed
that NiO–La2O3 showed the lowest rate of coke deposition
(Figure 7).[158] Although large amounts of carbon were
deposited on other catalysts, there was no significant decrease
in the time-on-stream catalyst activity/selectivity. This was
attributed to the formation of filamentous carbon, whereby
the active component is located at the top of the carbon
filament and can still participate in the reaction.[159, 160]
Although there was no activity loss, a large pressure drop
across the reactor was noted for the catalysts which showed
carbon deposition. Rapid carbon filament formation, as
observed in the study above, should be avoided to maintain
the mechanical integrity of the catalyst and to minimize
pressure drop issues.
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Figure 6. Influence of the metal dispersion (XNi, white column) in
different supports on the total carbon accumulation (mC, ~) during a
4 h time-on-stream COMR process. Temperature = 1023 K; CH4/
O2 = 2.5, GHSV = 53 000 cc g1 h1; precalcination temperature = 773 K).[156]
Figure 7. Comparison of the average rate of carbon deposition (rC) for
different NiO–rare-earth oxide catalysts during the COMR process.
Temperature = 833 K, GHSV = 5.2 D 105 cc g1 h1; CH4/O2 = 2.03; catalysts used without prereduction.[158]
The coke-forming tendency of the COMR catalysts can
also be decreased by modifying commonly used supports with
promoters.[152, 161–163] Cao et al. found that addition of 2 %
La2O3 to NiO/Al2O3 catalyst significantly decreased carbon
deposition as well as enhanced the COMR activity.[162] The
carbon deposition rate was found to be about 25 % lower for
the 2 % La2O3-promoted catalyst. Further decrease in carbon
deposition rate was observed for the CaO–2 % La2O3
promoted Ni/Al2O3 catalyst. The beneficial effect of Ca
addition has also been observed for the CMOR reaction at
higher pressures (0.7 MPa).[152] While the initial COMR
performance in this case was similar for the promoted and
unpromoted catalysts, the coking rate for the Ca-promoted
catalyst was found to be about 80 % lower than that over the
unpromoted catalyst.
A significant improvement in Pt catalyst stability and
activity was observed on modification of Al2O3 support by
ceria or ceria–zirconia.[163, 164] The procedure by which ceria–
zirconia was deposited on the Al2O3 support played an
important role in determining catalyst stability; the catalyst
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for which ceria and zirconia were deposited by an impregnation procedure was significantly more stable than the catalyst
wherein a precipitation method was used.[163] This was
attributed to higher coverage of ceria–zirconia over alumina
by the impregnation procedure, which in turn minimized
direct contacting between the active component (Pt) and
alumina. Furthermore, a ceria loading of 12 wt % was
required for obtaining high stability for Pt/CeO2/Al2O3.[164]
While MgO-based catalysts have been widely investigated
for developing thermally stable catalysts, CeO2-based catalysts have been favored for developing carbon-resistant
catalysts.[163, 165–168] The interest in CeO2 is related to its
excellent oxygen storage capacity, ionic conductivity, and
high basicity as these properties are known to inhibit carbon
formation.[20, 29, 169, 170] The oxygen storage properties of CeO2
can be further modified by doping it with other
oxides.[163, 171–174] Studies have shown that CeO2-doped ZrO2based catalysts show high resistance to catalyst deactivation
due to inhibition of carbon deposition.[163, 175] A Pt/
Ce0.14Zr0.86O2 catalyst was found to be considerably more
stable than a Pt/ZrO2 catalyst with similar Pt loading.[175, 176]
While the former catalyst lost less than 9 % catalyst activity
during a 24 h time-on-stream COMR reaction, the latter lost
more than 66 % of its initial activity.
In general, a combination of high metal dispersion and
good oxygen storage/release properties are catalyst requirements for achieving a high resistance to carbon deposition in
the COMR process. A recent study compared a series of Pt/
CexZr1xO2 catalysts with Pt/CeO2 and Pt/ZrO2.[177] The
oxygen storage capacities of the different catalysts along
with metal dispersion data are shown in Figure 8. If metal
dispersion was the only important factor, then Pt/
Ce0.14Zr0.86O2 would have the highest stability, whereas if the
oxygen storage capacity was the only critical parameter, then
the Pt/Ce0.5Zr0.5O2 would be the most stable catalyst. However, in reality the stability of the catalyst was found to
decrease in the order: Pt/Ce0.75Zr0.25O2 Pt/Ce0.25Zr0.75O2 >
Pt/Ce0.5Zr0.5O2 > Pt/Ce0.14Zr0.86O2 > Pt/ZrO2 > Pt/CeO2. That
is, the Pt/Ce0.75Zr0.25O2 and Pt/Ce0.25Zr0.75O2 catalysts, which
were characterized by high dispersion (increased metal-
Figure 8. Oxygen uptake (~) and Pt (XPt) dispersion (white column)
for various Pt-based catalysts.[177]
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support interface) as well as good oxygen storage properties,
showed the highest resistance to carbon deposition during the
COMR process.
As mentioned earlier, MgO and doped-CeO2 based
supports are considered promising from viewpoints of thermal stability and carbon resistance, respectively. Promotion
with Co is also known to drastically reduce carbon formation.
Recently, a Ni catalyst has been synthesized with all the
components mentioned above (SZ5564).[126, 178] This catalyst
showed exceptional thermal stability (as described in a
previous section) and showed no deactivation during a 50 h
time-on-stream run.[126] As much larger carbon deposition
rates are expected in the CO2 methane reforming reaction,
this was used as an accelerated test reaction to determine the
catalyst resistance to carbon-related deactivation. As
expected, due to the presence of Co and high mobility of
lattice oxygen (CeO2-ZrO2), no coking-related deactivation
was observed for a 20 h time-on-stream CO2 methane
reforming reaction. Similarly, this catalyst also showed
significantly higher resistance to sulfur-related deactivation
as compared to a NiCoMgOx/ZrO2-HfO2 catalyst.[179]
2.4. Possible Standardized Approach for Future COMR Catalyst
Evaluation
From the previous sections, it is apparent that interesting
work has been undertaken to develop superior COMR
catalysts. However, as explained below, these studies are
rather preliminary and hence it is arduous to directly glean
commercially relevant information from them. There is
significant scope for developing superior COMR catalysts,
which would be an important step towards wide-scale
commercialization of this process. This section suggests a
possible standardized approach for future studies.
The following important features can be discerned from
previous studies:
a) A major requirement for COMR catalysts is high temperature stability ( 1373 K).[123, 126, 131]
b) High temperatures can have a strong negative effect on
the dispersion of the active components and/or supports.[53, 55, 132]
c) The carbon resistance of the COMR catalysts is very
sensitive to active component(s) dispersion and oxygen
storage capacity; catalysts with higher dispersion and
oxygen storage capacities have higher carbon resistance.[139, 146–148, 175–177]
d) Most COMR applications would require the process to be
operated at moderately high pressures.[61, 104]
While a large body of catalyst development work related
to carbon resistance has been undertaken, in the majority of
these studies the catalysts have not been exposed to temperatures above 1173 K; in fact, in most cases the maximum
exposure temperature for the catalyst has been 1073 K. As the
catalyst development strategy for most studies is to maximize
dispersion, to truly distinguish between catalysts it is critical
to ascertain the dispersion of the catalysts after exposure to
industrially relevant temperatures (> 1300 K). The mobility
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of the lattice oxygen can also be influenced by the reaction
temperature (additionally there could also be support sintering, phase segregation, etc. for certain systems), therefore it
would also be beneficial to measure oxygen storage/release
properties of the catalysts at relevant COMR temperatures.[169] Moreover, it is also known that carbon deposition
rates can be strongly influenced by pressure.[152, 180]
Now that the previous work has collectively defined the
practical requirements for the COMR catalysts, the COMR
development studies in the next phase can be carefully
planned such that these efforts would potentially make direct
contributions to the wide-scale commercialization of the
COMR process. Although long-term catalyst stability is
critical, for preliminary screening of proposed COMR
catalysts it would not be practical, especially in academia, to
perform long (> 1000 h) time-on-stream tests. Instead, efforts
should be undertaken to determine “accelerated” thermal
stability and carbon-resistance tests. Some of the studies
described in the previous sections suggest interesting possibilities for these tests. Accelerated high-temperature thermal
stability tests could potentially include pretreatment of
catalyst (calcination, etc.) at temperatures of 1373 K and
above, and thermal shock treatments (exposure to oxyacetylene flame) prior to the reaction.[126] The accelerated
carbon-resistance test could potentially be high-temperature,
moderate-pressure methane decomposition tests (see Section 2.3.1) and/or CO2 methane reforming and/or varying fuel
composition COMR tests.[126] The suitability of these tests
would depend on the catalyst system and previous studies
could assist in providing appropriate information for selecting
or identifying new accelerated tests for the proposed catalyst
system. Well-defined, mechanistic tests related to catalyst
deactivation would also assist in identifying appropriate
accelerated tests.
Following accelerated tests, a short 24 h time-on-stream
test at a COMR reaction temperature of 1273 K, moderate
pressure, in the presence of pure O2 and short contact times,
would facilitate comparison between the proposed catalyst
and a reference catalyst (e.g. noble-metal based, for example,
Rh) that has previously demonstrated good COMR performance. The long-term (> 1000 h) time-on-stream test needs
to be done only after the appropriate catalyst (which satisfies
a predetermined criteria) is identified by the above process.
3. Reaction Mechanisms
There has been a considerable debate on the reaction
mechanisms over COMR catalysts.[14, 29] Two major mechanisms have been proposed for the COMR process: a direct
mechanism wherein CO is the primary product and an
indirect mechanism wherein CO2 is the primary product
(Figure 9). In the direct mechanism, CO is formed as a
primary product through the reaction of adsorbed/lattice
oxygen with carbonaceous species formed from methane
decomposition. The indirect mechanism, on the other hand,
involves combustion of methane in the first step to produce
steam and CO2, which subsequently react with excess
methane to form CO and H2. An excellent summary of
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Figure 9. Simplified representation of the direct and indirect mechanisms for the COMR process.
previous COMR mechanistic studies (prior to 2003) may be
found elsewhere.[14, 29]
In theory, it should be rather straightforward to distinguish between direct and sequential mechanisms by varying
the residence time over the catalyst (reactant space velocity)
in continuous flow reactor experiments. Due to significantly
slower kinetics for the CO2 and steam methane reforming
reactions, at extremely short contact times only the combustion products would predominantly be observed in the
products if the reaction proceeded through an indirect
combustion-reforming mechanism. Unfortunately, as large
space velocities coupled with high conversion can cause a
significant rise in local temperatures at the catalyst surface, it
can be quite complicated to interpret continuous flow data.
Therefore, along with continuous flow reactor data, several
research groups have also used pulse reactors for elucidation
of the COMR reaction mechanism. In the case of pulse
reactor studies, only small quantities of reactants are pulsed
onto the catalyst surface resulting in two important advantages: a) the activity data can be directly related to the
catalyst composition/structure and b) activity data can be
collected under isothermal conditions.[29, 181]
Confusion about the COMR mechanism is apparent from
recent studies on Rh-based catalysts. While the COMR
reaction was observed to occur by an indirect pathway over a
Rh/Al2O3 catalyst,[182] the reaction occurred via a direct
pathway over the Rh/SiO2 catalyst;[183] interestingly, the
reaction was found to proceed through an indirect pathway
over a Ru/SiO2 catalyst.[183, 184] To further complicate matters,
the reaction over Rh supported on ZrO2 and other novel
supports seemed to have contribution from both the reaction
pathways.[111, 185] The temperature was also found to influence
the reaction pathway; while the reaction over a Rh/Al2O3
proceeded by a direct pathway at low reaction temperatures,
it was found to favor an indirect pathway at higher reaction
temperatures.[186] In contrast over a Rh/MgO catalyst, low
reaction temperatures favored the indirect mechanism,
whereas high reaction temperatures favored the direct pathway.[117] Recently, the calcination temperature was also shown
to have a strong influence on the CMOR products over Rh/
Al2O3 catalysts.[187, 188] Using an array of analytical methods,
the authors concluded that the difference in the nature of the
products for the samples calcined at 873 K and 1173 K was
related to the formation of different Rh species.
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Similar differences in reaction pathways have also been
observed over Ni-supported catalysts. The COMR reaction
mechanism over the Ni/TiO2 catalysts was found to change
from an indirect mechanism (oxidized) to a direct mechanism
(reduced or partially reduced) depending on the oxidation
state of Ni.[189] While an indirect mechanism was observed for
LaNi1xCoxO3 catalysts,[190] direct mechanisms were reported
over Ni foam,[191] Ni-Cr alloys,[191] Ni/Al2O3[192] and reduced Ni
calcium hydroxylapatite catalysts.[193] At short contact times a
Ni/La2O3 catalyst reacted through a direct mechanism, while
the same catalyst proceeded by a combination of direct and
indirect mechanisms at higher contact times.[53] Similar to Rhbased catalysts, the reaction mechanism was also strongly
influenced by temperature over Ni-based catalysts. Over NiO/
MgO catalysts, the COMR reaction pathway switched from a
direct mechanism at 973 and 1023 K to a combined direct and
indirect mechanism at 1123 K.[115]
A mechanism, different from the pyrolysis (direct) and
indirect (combustion-reforming) mechanisms, has been proposed specific to yttrium-stabilized zirconia catalysts.[194] This
mechanism involved a reaction through formation of surface
formaldehyde,[195] which subsequently either converted into
syngas or into CO2, CO, H2, and H2O via a formate
intermediate. Although adsorbed oxygen was also observed,
lattice oxygen was found to be the active oxygen species
under reaction conditions over both ZrO2 and yttriumstabilized ZrO2.[196]
As an indirect mechanism involves the highly exothermic
methane combustion reaction, it is expected to be detrimental
from the viewpoint of catalyst stability. The high space
velocity and high conversion coupled with large exothermicity would generate enormous heat at the inlet zone of the
catalyst bed and thereby significantly increase the probability
of thermal sintering/volatization of the catalyst. From a
practical viewpoint, it would therefore be desirable to design
catalysts such that COMR preferentially proceeded by the
direct reaction pathway. However, in order to accomplish this,
as a first step it is important to obtain reliable COMR
mechanistic information over different catalyst systems. One
of the disturbing features of most mechanistic studies is the
absence of spatially resolved information. Obviously, due to
the inherent nature of the COMR reaction, spatially resolved
investigations would provide significantly more reliable and
relevant information on the reaction fundamentals,[111, 197–200]
and would also assist the detailed modeling of COMR.[201, 202]
Very recently, an elegant method has been developed to
determine temperature profiles and measure the axial
distributionwith excellent spatial resolution (0.3 mm) for
short contact time reactions.[198] This method, which can be
used to follow the formation and consumption of the different
species along the reactor axis, was used to investigate the
COMR mechanism over Rh-coated alumina foam monoliths.
Based on the spatially resolved studies and numerical
simulations, a two-zone (oxidation and steam reforming
zones) mechanism was proposed for the Rh-based catalyst.[203]
The study showed complete oxygen conversion within 2 mm
of the catalyst inlet under all investigated conditions. While
some H2 and CO was formed in the oxidation zone, more H2
and CO formation occurred in the reforming zone through
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the steam reforming reaction. CO2 was mainly formed in the
oxidation zone and its amount was found to remain constant
thereafter in most cases. Interestingly, CO2 methane reforming reaction was not observed under the investigated experimental conditions. It would be very interesting to obtain
similar insights into other important COMR catalyst systems
(e.g. Ni-based catalysts) under varying process conditions.
Recent work has shown that an optically accessible channel
reactor can also be used to obtain COMR information with
high spatial resolution.[199]
4. Combined Reforming
Although the COMR process is highly energy-efficient, it
can also be potentially hazardous from the viewpoint of
hotspot formation and reaction runaway. This is related to the
exothermic nature of methane oxidation reactions coupled
with poor heat transfer. Although steam and CO2 methane
reforming processes do not suffer from major safety issues,
they have other distinct disadvantages; they are highly
energy-intensive and do not provide favorable H2/CO ratio
required for important downstream syngas conversion applications.
An interesting approach is to exploit the advantages of the
different syngas generation processes; this entails the combination of the exothermic COMR reactions with endothermic
steam and/or CO2 reforming reactions.[37, 38] A combined
process is expected to be safer than the COMR process, as the
heat generated by the exothermic reactions can be used by the
endothermic reactions, thus precluding hotspot formation and
temperature runaway. The process can be operated thermoneutrally and is thus much more energy-efficient than
individual steam and CO2 methane reforming reactions.
Moreover, based on methane it can provide 100 % selectivity
for H2 (for steam-COMR), or CO (for CO2-COMR), or both
(for steam-CO2-COMR). The combined process also affords
some flexibility to tune the H2/CO ratio such that it favors the
downstream applications for syngas conversion. As this
process replaces some oxygen with steam and/or CO2, there
is also some reduction in O2 costs. However, as compared to
the COMR process, the combined process requires significantly higher contact times to achieve practical methane
conversions due to significantly lower rates of steam and CO2
methane reforming. In the previous sections, our discussion
was focused on issues related to COMR; herein the discussion
will be extended to include steam-assisted (steam-COMR),
CO2-assisted (CO2-COMR), and steam–CO2-assisted (steamCO2-COMR) COMR.
4.1. Steam-COMR
4.1.1. Effect of Process Parameters on Heat of Reaction and
H2/CO Ratio
Steam-COMR involves the combination of the COMR
reaction with steam methane reforming reaction over the
same catalyst. The reactions described in Equations (1)–(3)
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and (4)–(6) can occur simultaneously or consequently in the
steam-COMR process:
CH4 þ H2 O ! CO þ 3 H2 DH 1123 K ¼ þ225:5 kJ mol1
ð4Þ
CO þ H2 O ! CO2 þ H2 DH 1123 K ¼ 33:64 kJ mol1
ð5Þ
CH4 þ CO2 ! 2 CO þ 2 H2 DH 1123 K ¼ þ255:2 kJ mol1
ð6Þ
As the process variables can affect the contribution from
each of the above reactions, they can be used to manipulate
the heat of the reaction as well as the H2/CO ratio. The
influence of H2O/CH4 ratio on the heat of the reaction was
recently investigated over a NiCoMgCeOx/SZ5564 catalyst
(Figure 10) at a reaction temperature of 1123 K.[178] The net
Figure 10. Effect of H2O/CH4 ratio on the net heat (DH) of reaction
(~) and H2/CO ratio [experimental (~) and equilibrium (dotted line)]
for the steam-COMR process over NiCoMgCeOx/SZ5564 catalyst.
Temperature = 1123 K; GHSV = 49 000 cm3 g1 h1, CH4/O2 = 2.0; precalcination temperature = 1673 K; reduction = 1173 K in the presence
of 1:1 H2/N2 for 1 h before the start of the reaction.[178]
heat of reaction (DH) for the overall process was estimated by
subtracting the heat of formation (at the process temperature)
of the components in the feed from that of the components in
the products stream. Depending on the feed concentration,
the reaction was found to switch from being mildly exothermic (at lower H2O concentration) to being moderately
endothermic (at higher H2O concentration). The increasing
endothermicity of the reaction with increasing steam concentration was related to the increasing contribution from the
endothermic steam reforming process. Similarly, the H2/CO
ratio was also found to be significantly influenced by the
change in the steam concentration of the feed. The methane
conversion, however, increased only slightly with increasing
H2O/CH4 ratio. As the steam methane reforming reaction
produces three moles of H2 per mole of methane consumed,
while the COMR process only contributes to two moles of H2
per mole of CH4 consumed, the H2/CO ratio in case of the
steam-COMR reaction is expected to increase with increasing
contribution from the steam methane reforming reaction.
However, it is generally not straightforward to anticipate the
change in H2/CO ratio as it can also be influenced by the
water gas shift reaction or other reactions that can occur
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during the process.
As the reaction temperature is also expected to influence
the contribution from different reactions occurring during the
steam-COMR process, it can be used to manipulate the heat
of the reaction as well as the H2/CO ratio. A recent study
showed that the heat of reaction over a NiCoMgCeOx/SZ5564
catalyst (Figure 11) at a constant feed concentration (CH4/
Figure 11. Influence of the reaction temperature on the heat (DH) of
reaction (~) and H2/CO ratio [experimental (~) and equilibrium
(dotted line)] during the steam-COMR process over NiCoMgCeOx/
SZ5564 catalyst. GHSV = 46 000 cm3 g1 h1,CH4/O2 = 2.0; H2O/
CH4 = 0.17; precalcination temperature = 1673 K; reduction = 1173 K in
the presence of 1:1 H2/N2 for 1 h before the start of the reaction.[178]
O2 = 2.0, H2O/CH4 = 0.17) switched from being moderately
exothermic at 1023 K to being mildly endothermic at
1173 K.[178] Although the H2/CO ratio was also influenced
by the reaction temperature, no clear trend was observed.
However, a clear increasing trend of methane conversion was
observed with increasing temperature. For the NiCoMgCeOx/
SZ5564 catalyst, depending on the process conditions, high
methane conversions (> 95 %) coupled with excellent syngas
selectivity (> 90 %) could be obtained under thermoneutral
conditions. As the extent of contribution from different
reactions is expected to depend on the catalyst system, the
effect of the process parameters is also expected to be
catalyst-dependent. However, studies have shown that the
general trend observed for the changes in the heat of the
reaction with varying process parameters are similar for
different catalyst systems.[103, 204–206] While small contributions
from water gas or reverse water gas shift reactions due to their
lower heat of reactions are not expected to significantly affect
the net heat of the reaction, they can, however, significantly
affect the H2 and CO selectivities. The influence of the
catalyst system on the H2/CO ratio is therefore more
unpredictable.
4.1.2. Steam-COMR Catalysts
To minimize hotspot formation during the steam-COMR
reaction, it is important that the exothermic methane
oxidation and the endothermic steam reforming reactions
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occur simultaneously. One of the approaches to validate this is
by evaluating the proposed catalyst for the individual steam
reforming reaction under steam-COMR process conditions.[126, 206] Only those catalysts which provide high activity
for the steam reforming reaction should be considered as
good candidates for the steam-COMR reaction. IR thermography, which provides information related to surface temperature during the reaction, can also be used to evaluate the
coupling efficiency between the exothermic oxidation and
endothermic reforming reactions. Based on IR thermography
studies on alumina-supported catalysts, Rh was found to be
significantly superior to the Pt- and Pd-based catalysts for the
efficient heat transfer from the exothermic reactions to the
endothermic reactions.[207] In another IR thermography study,
a Pt/Al2O3 catalyst was found to be superior to a Ni/Al2O3
catalyst due to the significantly higher reforming activity of
the Pt catalyst.[208]
As seen previously for COMR catalysts, the support can
also profoundly influence the steam-COMR performance of
the catalyst systems.[209–211] In a recent study, a Pt/ZrO2/Al2O3
catalyst was found to be significantly more stable than Pt/
ZrO2 and Pt/Al2O3 catalysts.[211] The higher stability for the Pt/
ZrO2/Al2O3 catalyst was attributed to its higher resistance to
coke formation due to enhanced Pt–Zrn+ interaction at the
metal–support interface.[209] A similar better stability for the
Pt/ZrO2/Al2O3 catalyst was also reported for the CO2
methane reforming reaction.[212] Compared to Pt/ZrO2 and
Pt/Al2O3 catalysts, a Pt/CeO2 catalyst was also found to
exhibit superior steam-COMR performance due to enhanced
metal–support interactions and higher mobility of lattice
oxygen.[210] The effect of catalyst preparation can also
influence the activity/stability of the steam-COMR catalyst.[213] Ni-based catalysts prepared from Mg-Al hydrotalcite
precursors using a coprecipitation method were found to be
more active for the steam-COMR reaction as compared to Ni
catalysts prepared by impregnation on Mg-Al mixed oxides or
alumina or MgO. Moreover the time-on-stream steamCOMR stability of the hydrotalcite-based catalysts was also
significantly superior to the Ni-based catalysts impregnated
on Mg-Al mixed oxides.
Several groups have also investigated the addition of
promoters to improve the steam-COMR performance
(Table 7). The promoters can improve the steam-COMR
performance in one or more of the following ways
a) enhanced interaction between active component and the
support, b) enhanced reduction of the active component,
c) inhibition of oxidation of the active component and
providing a more reducing environment, and d) decreased
carbon formation rates.[214–223] Considerable promotion of NiTable 7: Common catalyst promoters for the steam-COMR process.
Base catalyst
Promoter(s)
Ref.
Ni/ZrO2
Ni/Al2O3
Ni/SiC
Pt/oxide ion conducting support
Ni/MgO
Ni/MgO
CaO and/or CeO2
MgO or CaO
Rh
Rh
Pd
Rh or Pt
[216]
[215]
[217]
[218]
[220, 222]
[221, 223]
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Syngas Production
based catalysts can be achieved with even tiny amounts of
noble metals,[221, 222] as the promoter can be preferentially
localized on the surface.[208]
4.2. CO2-COMR
4.2.1. Effect of Process Parameters on the Heat of Reaction and
H2/CO Ratio
The CO2-COMR process involves the combination of the
COMR reaction with the CO2 methane reforming reaction
over the same catalyst.[37, 224, 225] While the same reactions that
are described in Section 4.1.1 (steam-COMR) also occur
during this process, the contribution from CO2 reforming
[Eq. (6)] is expected to be larger than from steam reforming
[Eq. (4)] in the case of the CO2-COMR process. As one of the
main advantages that the CO2-COMR process offers is
flexibility in managing the reaction energetics and the H2/
CO ratio, several studies have been undertaken to study the
effect of different process variables on these properties.[226–231]
While the absolute effect of the process variables was
found to be different for the different catalyst systems, in
general similar trends were observed for the change in heat of
the reaction and H2/CO ratio with change in reaction
temperature, CO2/CH4 ratio, and space velocity of the CO2COMR process. The net heat of reaction (DH) for the overall
process was estimated by subtracting the heat of formation (at
the process temperature) of the components in the feed from
that of the components in the products stream while the H2/
CO ratio was determined from product analysis.[229] When the
reaction temperature was increased, in general, the overall
CO2-COMR process became more endothermic, methane
conversion increased, and the H2/CO ratio decreased.[229–231]
This was related to the large increase in CO2 conversion with
increasing reaction temperature. As the CO2 reforming
reaction is very endothermic and produces the lowest H2/
CO ratio, an increase in this reaction results in increasing the
overall endothermicity of the process, while decreasing the
H2/CO ratio of the overall process. Similarly, an increase in
space velocity and decrease in CO2/CH4 ratio decreases the
individual contribution from the CO2 reforming process and
thereby decreases the process endothermicity and increases
the H2/CO ratio.[226, 229, 231]
However, note that the absolute values of the heat of
reaction and the H2/CO ratio are profoundly influenced by
the catalyst system.[226–231] The different influence of the CoOx/
MgO/SA5205 and CoOx/CeO2/SA5205 catalyst systems on
the net heat of reaction and H2/CO ratio (plotted on the same
scale) with change in reaction temperature may be observed
from Figure 12.[229, 231] Although there are similarities in
certain trends (for example, the methane conversion
increased with temperature) for both the catalysts, the
catalyst system effectively determines the extent of contribution from the different reactions and thereby the absolute
values for the heat of reaction and H2/CO ratio.
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
Figure 12. Effect of temperature on the heat of reaction (DH, ~) and
H2/CO ratio [experimental (~) and equilibrium (dotted line)] during
the oxy-CO2 reforming of methane over a) CoOx/MgO/SA5205 catalyst[229] and b) CoOx/CeO2/SA5205 catalyst.[231]
GHSV = 46 000 cc g1 h1; CH4/O2 = 2.5; CH4/(O2+0.5CO2) = 1.87; precalcination temperature = 1173 K; reduction = 1173 K in the presence
of 1:1 H2/N2 for 1 h before the start of the reaction.
4.2.2. CO2-COMR Catalysts
There are two important issues from the viewpoint of
catalyst development for the CO2-COMR process:
a) The catalyst should be effective for coupling the exothermic methane oxidation reactions with the highly endothermic CO2 reforming reaction. In other words, the
catalyst should have a high activity for the individual CO2methane reforming reaction.[229]
b) The catalyst should be thermally resistant and have high
resistance towards carbon deposition, as CO2-reforming is
plagued by rapid deactivation due to carbon deposition.[229, 231, 232]
A recent study involving the CO2-COMR process over a
Rh/LaCoO3 catalyst has suggested the absence of any
significant contribution from the CO2 reforming process;
instead, the CO2 conversion occurred through the mildly
endothermic reverse water gas shift reaction.[233] Obviously,
the catalyst system under the investigated process conditions
was not adequately effective for exploiting the advantages
offered by the CO2-COMR process. This study has demon-
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1841
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V. R. Choudhary and T. V. Choudhary
strated the importance of evaluating the proposed catalyst
system for the individual CO2-reforming reaction. Only
catalysts with high activity for the CO2-reforming reaction
should be considered as potential catalysts for the CO2COMR process.[231, 234] As mentioned earlier, an elegant
approach for verifying the effectiveness of the proposed
catalyst system in coupling the exothermic and endothermic
reactions is by following the temperature profile generated
during the combined process.[208, 235] Measurements of bed
profile temperatures have shown that a Pt/Al2O3 catalyst was
more effective at transferring the heat from the methane
oxidation reactions to the endothermic CO2 reforming
reaction as compared to a Ni/Al2O3 catalyst.[235] The superiority of the Pt/Al2O3 catalyst was attributed to its resistance to
oxidation in the presence of oxygen.
A study based on noble-metal catalysts prepared from
hydrotalcite-type precursors showed that Ru provided higher
syngas yields than Rh and Pt for the CO2-COMR process at
1123 K.[236] A study of the effect of Ru loading revealed a
similar steady-state activity for a 2 % and 0.1 % Ru loading
catalyst, which is important from the viewpoint of minimization of noble-metal content. Along with the nature of the
active component, the support also plays an important role in
determining the catalyst performance for the CO2-COMR
process.[237] For the three supports investigated in the CO2COMR process at 1073 K (Al2O3, ZrO2, ZrO2/Al2O3), the Pt/
ZrO2/Al2O3 system showed the highest stability (maximum
resistance to coke formation), while the Pt/ZrO2 showed
maximum deactivation. The higher stability of the Pt/Al2O3/
ZrO2 was related to the enhanced Pt–Zrn+ interaction at the
metal–support interface.[237]
One of the commonly used strategies for developing
superior CO2-COMR catalysts is to introduce promoters
(Table 8) into the catalyst system.[178, 226, 238–244] As coking-
Table 8: Common catalyst promoters for the CO2 -COMR process.
Base catalyst
Promoter
Ref.
NiCoMgOx/SZ5564
Ni/MgO
Ni/SiO2
Ni/SiO2
Ni/SiO2
Pt/ZrO2
LaFeO3
Ni/Al2O3
CeO2
Co
SrO
CaO
La2O3
CeO2
Co
Pt
[178]
[226]
[238]
[239]
[240]
[241]
[242]
[243, 244]
4.3. Steam-CO2-COMR
4.3.1. Effect of Process Parameters on the Heat of Reaction and
H2/CO ratio
The steam-CO2-COMR process involves the combination
of the COMR reaction with CO2 and steam methane
reforming reaction over the same catalyst.[38, 227, 228, 245, 246] As
can be imagined based on the variety of reactions that could
occur to different extents during this process, the net heat of
the reaction and the H2/CO ratio obtained by this process is
extremely sensitive to process conditions, especially the ratios
of the different components in the feed.[227, 245] The increase in
reaction temperature and decrease in O2/CH4 increases the
conversion of CO2 and steam and thereby increases the
endothermicity of the reaction.[245] However the H2/CO ratio
depends strongly on the extent of CO2 reforming and steam
reforming reactions, as each produces significantly different
H2/CO ratios.[247]
4.3.2. Steam-CO2-COMR Catalysts
As for the steam-COMR and the CO2-COMR processes,
only those catalyst systems that efficiently couple the
exothermic and endothermic reactions can take maximum
advantage of the coupled process. A commonly used
approach to select the catalyst system involves the evaluation
of the catalyst for individual steam and/or CO2 reforming
reactions under relevant process conditions.[123, 205, 228, 248–250]
Only those catalysts that show considerable activity for the
endothermic reforming reactions should be considered as
potential candidates for the steam-CO2-COMR process. As
compared to steam-COMR and CO2-COMR, fewer catalyst
development studies have been undertaken on the steamCO2-COMR system. In general, catalyst systems that show
promising results in the steam and CO2-COMR processes
should also be interesting candidates for the steam-CO2COMR process. Recently, the steam-CO2-COMR process has
been suggested as a potential approach to process flue gases
from natural gas and coal fired power plants; use of such a
process would not require preseparation of CO2 and is
therefore interesting.[251, 252] Unfortunately, the flue gas from
existing power plants contains large amounts of nitrogen
which makes the process economically less attractive from the
view of downstream application of the syngas. However, this
may not be a significant issue in the future if power plants
switch to using oxygen-enriched air or oxygen for combustion.
5. Summary and Outlook
related deactivation is a concern for any process involving
CO2 reforming, most studies have used promoters to minimize carbon formation.[178, 226, 239–241] However, promoters have
also been used to enhance the coupling of the exothermic and
endothermic reactions occurring in the CO2-COMR process.[243, 244] From a catalyst development viewpoint, accelerated deactivation tests involving the determination of carbon
deposition rate during a CO2-reforming only reaction could
potentially provide an important indication of the long-term
stability of proposed catalysts for the CO2-COMR process.
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Amongst the different processes, COMR has a unique
advantage in that it can generate syngas at exceptionally high
space time yields. However, due to relatively stringent
operating conditions, COMR catalysts can undergo significant deactivation during extended time-on-stream operations.
As long-term catalyst stability is extremely important from a
process yield as well as a safety viewpoint (reaction runaway,
etc.), COMR research has been dominated by catalyst-related
studies. Depending on the catalyst system the following
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Syngas Production
factors were found to be important in defining its long-term
performance: location, dispersion, and content of the active
component; synergistic effects from promoters; level of
chemical interaction between active component and support;
and oxygen storage/release properties of the support. Different preparation methods and appropriate supports have been
identified to obtain catalysts with the desirable catalyst
attributes. However, there is concern about the direct
applicability of a major fraction of these studies for development of practical catalysts. Although there are two very
important and thoroughly interlinked mechanisms for catalyst deactivation (thermal and fouling by carbon deposition),
most studies have only considered one individual aspect of
deactivation. However, based on the collective knowledge
gained from the previous studies the scientific community is
now in a position to launch studies that can directly assist in
developing superior commercial catalysts (a possible standardized evaluation procedure is described in Section 2.4).
As catalyst development can strongly benefit from an
enhanced understanding of the process fundamentals, several
studies have been undertaken to obtain mechanistic information for the COMR process. These studies suggest that the
COMR reaction mechanism is a complex function of the
process conditions and the catalyst system. As the COMR
reaction exhibits extremely fast kinetics and changing catalyst
bed temperature profiles, spatially resolved information is
important for developing a fundamental understanding for
the process over a given catalyst system. Unfortunately, most
of the proposed mechanisms are based on studies that provide
no spatially resolved information. Very recently, an elegant
approach has been proposed to obtain temperature profiles
and concentration of species with excellent resolution;[198] it
would be astute to exploit such studies for obtaining realistic
mechanistic information concerning the COMR system under
investigation. To develop accelerated deactivation
approaches for catalyst development, it would also be
beneficial to develop an enhanced understanding of the
carbon deposition mechanism over COMR catalysts under
relevant operation conditions.
Studies related to steam-COMR, CO2-COMR, and
steam-CO2-COMR have revealed that there are some distinct
advantages (minimization of safety concerns while maintaining high energy efficiency, lowering oxygen separation costs,
and manipulating H2/CO ratio) to combining the exothermic
oxy reforming reactions with endothermic steam and/or CO2
reactions. Although the number of studies on this topic has
been steadily increasing, these studies can be described as
preliminary in nature owing to lack of practical information
such as pressure dependence, extended time-on-stream
performance, and so on under relevant process conditions.
In terms of catalyst development, issues that are applicable to
COMR are also relevant to the combined processes.
In summary, the significant body of work undertaken in
this very challenging area of research has provided the
necessary information for undertaking future investigations
that can more directly contribute to the development of
superior practical catalysts. Development of vastly enhanced
catalysts is expected to significantly assist the widespread
commercialization of oxy reforming technology for syngas
Angew. Chem. Int. Ed. 2008, 47, 1828 – 1847
generation. Few studies have also considered using advanced/
alternative reactor technology (fluidized bed reactor, microchannel reactors, etc.) for minimizing some of the safety and
operational issues related to the oxy reforming processes.[253–259] However, considerably larger efforts need to be
undertaken to appropriately evaluate alternative reactor
technologies. Breakthrough improvements (scale-up and
reliability, permeability flux and stability) in membrane
technology would also greatly assist the methane oxy
reforming processes by significantly decreasing the oxygen
separation cost. Some of the above-described improvements
might require substantial development time. Fortunately,
unlike the environmental regulation driven technologies
(sulfur regulations, etc.), which are stringently bound by
time, the syngas generation technologies have considerable
flexibility in terms of process development time. The importance of methane as a critical energy resource is not expected
to diminish in the foreseeable future and as such there will be
room for efficient competitive methane conversion technologies.
Received: March 20, 2007
Published online: January 10, 2008
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