advances in the partial oxidation of methane to synthesis … · advances in the partial oxidation...

Journal of Natural Gas Chemistry 13(2004)191–203

Advances in the Partial Oxidation of Methane to Synthesis Gas

Quanli Zhu1,2∗, Xutao Zhao1, Youquan Deng2

1. Petrochemical Research Institute of Lanzhou Petrochemical Company, China National Petroleum Corporation,

Lanzhou 730060, China; 2. State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou

Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

[Manuscript received November 10, 2004; revised November 26, 2004]

Abstract: The conversion and utilization of natural gas is of significant meaning to the national economy,even to the everyday life of people. However, it has not become a popular industrial process as expecteddue to the technical obstacles. In the past decades, much investigation into the conversion of methane,predominant component of natural gas, has been carried out. Among the possible routes of methaneconversion, the partial oxidation of methane to synthesis gas is considered as an effective and economicallyfeasible one. In this article, a brief review of recent studies on the mechanism of the partial oxidation of

methane to synthesis gas together with catalyst development is wherein presented.

Key words: methane partial oxidation, synthesis gas, catalyst, reaction mechanism

1. Introduction

Natural gas, which is mainly composed of

methane, is an abundant resource found over the

world and is predicated to outlast the oil reserves

by a significant margin [1]. Most of these reserves,

however, are situated in the areas far away from the

markets of highest energy consumption, and the ex-

pensive cost of compression, transportation and stor-

age, makes the utilization of natural gas as an unpre-

possessing proposition. Contrary to it, petroleum is

relatively cheap and it can be conveniently disposed.

In order to make the utilization of natural gas more

economically viable, a large amount of investigation

into the conversion of methane to liquids or higher hy-

drocarbons has been carried out in the past decades.

Unfortunately, the productive rate in these processes

is still lower than what is expected, because these

products resulted from methane partial oxidation are

usually more chemically active than methane, which

limits methane converting to the expected products.

For example, in the process of direct oxidative cou-

pling of methane to ethylene, the highest productive

rate was no more than 30% [2,3], and in the pro-

cess of direct oxidation of methane to methanol [4] or

formaldehyde [5], the highest productive rate was 8%

and 4%, respectively. It was recently reported that a

50% of methanol productive rate was achieved by a

pilot plant in homogeneous catalysis, but still lower

than what is expected [6,7]. On the other hand, the

mercury and concentrated sulfuric acid was used in

this process, and the resulted sulfuric dioxide should

to be re-oxidized for recycle. Although there are in-

dustrial processes of the direct conversion of methane,

such as the oxidation of methane and ammonium or

amine to cyanide [8] and the pyrolysis of methane to

acetylene [9], their marked disadvantage is that these

processes are needed to operate at very high tempera-

ture, usually above 1300 K. Because of these reasons,

it is very difficult for natural gas to compete with

petroleum at present.

In order to elevate the additional value of

methane, the utilization of methane can be theoret-

ically carried out via two pathways: one is the di-

rect conversion of methane, such as, as mentioned

above, the oxidative coupling of methane to ethy-

∗ Corresponding author. E-mail: [email protected]

192 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004

lene, the direct oxidation of methane to methanol

or formaldehyde, etc. It is impossible for these pro-

cesses to be applied to mass production unless great

breakthroughs of these technologies are achieved. The

other is the indirect conversion of methane, namely,

converting methane to any other products via syn-

thesis gas, which is the mainly practical route for the

methane conversion at the present time.

Nowadays, there are three methods for produc-

ing synthesis gas from methane: the steam reform-

ing, the dry reforming and the partial oxidation, of

methane. Compared with the former two, the partial

oxidation of methane (POM) possesses characteristics

as follows: (1) POM reaction is a mild exothermic

reaction, while the former two are endothermic reac-

tions. Thus, the industrial process based upon POM

is energy saving. In view of this, the utilization of

POM combined with steam reforming or dry reform-

ing is more effective. (2) The molar ratio of H2 to

CO in the resulted synthesis gas is close to 2 if POM

reaction is carried out according to stoichiometric ra-

tio. This kind of synthesis gas containing little CO2 is

an ideal feedstock for downstream processes, such as

methanol synthesis, etc. (3) POM can be carried out

under the condition of very high gas hourly space ve-

locity (GHSV), which makes the process require less

investment and less production scale to achieve the

same or larger capacity.

However, carbon depositing over catalyst bed was

unavoidable even if POM was carried out precisely

under the condition of 2:1 of the molar ratio of CH4

to O2, or less than 2:1, and the coke formation was

even worse at higher temperature. POM, thus, did

not get proper attention until the oil crisis. Since the

eighties’ of last century, Green and co-workers [10, 11]

have done much work for the renaissance of study on

POM. They used noble metals as POM catalysts, and

obtained the synthesis gas with compositions close

to thermodynamic equilibrium. POM is, henceforth,

paid much attention in the catalytic cycle around the

world.

2. Brief thermodynamic analysis of methane

partial oxidation

Possible pathways via which methane is converted

are shown in Figure 1. At high temperature, the main

reaction products between methane and oxygen are,

however, limited to CO, CO2, H2O and H2, [12,13],

apart from some intermediates. Product distribution

depends to a great extent upon the employed catalyst,

temperature, pressure and the ratio of methane to

oxygen in feedstock, as well as kinetic factors. Three

reactions possible to occur during POM are briefly

expressed in Figure 2, wherein some thermodynamic

information is included.

Figure 1. Possible route for methane conversion

Figure 2. Thermodynamic representation of POM

The calculated product gas distribution at the

thermodynamic equilibrium under the condition of

atmospheric pressure and input methane-to-oxygen

ratio of 2:1 versus temperature, based the reactions

in Figure 2, is shown in Figure 3 [14], regardless of

carbon deposition [14]. It can be observed in Figure

3 that the selectivity to CO and H2 increases with

increasing reaction temperature. In fact, very high

methane conversion (>90%) and selectivity (>90%)

to synthesis gas can be obtained above 1000 K. Like-

wise, the equilibrium gas compositions versus pressure

were done by Lunsford and co-workers [15]. Their

results indicated that the partial pressure of CH4,

CO2 or H2O in the equilibrium gas compositions in-

creased with elevated total pressure, which means

that high pressure is unfavorable to POM to synthesis

gas. However, elevated temperature can compensate

this pressure effect. In other words, from the point of

view of thermodynamics it is feasible for the reaction

Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 193

of methane and oxygen to synthesis gas at increased

pressure to be commercially used.

Figure 3. Equilibrium gas compositions for

methane partial oxidation, 1 bar, 2:1

of CH4:O2

(1) p(H2), (2) p(CH4), (3) p(CO), (4) p(CO2), (5) p(H2O)

It is generally accepted that POM could be cat-

alyzed to, or almost, to the thermodynamic equilib-

rium by group VIII metal catalysts [16,17]. How-

ever, Choudhary and his co-workers [18–20] obtained

the yields of synthesis gas much higher than that

calculated according to the thermodynamic equilib-

rium, over the metal oxide-supported nickel or cobalt,

within the temperature range from 723 to 773 K,

under very short residence time. They also investi-

gated the steam reforming and the dry reforming un-

der the same condition, and it was found that the

result was higher than that predicted by thermody-

namics. Based upon these data, they regarded that

POM reaction occurred under non-equilibrium con-

dition, and the mechanism was different from that

under equilibrium condition [10,11]. Green and co-

workers [14] studied the phenomenon described by

Choudhary et al, and it was found that the temper-

ature of catalyst bed increased with gas flow rate.

Therefore, this result was consistent with the pred-

ication by thermodynamics if the real reaction tem-

perature was taken into account.

3. Catalysts for POM to Synthesis Gas

POM reaction to synthesis gas can be carried out

without employment of a catalyst, but it occurs at

very high temperature, usually above 1400 K in the

flame [21]. The employment of catalyst can facilitate

the light-off of POM and promote it to thermody-

namic equilibrium. The catalysts for POM to synthe-

sis gas can be divided into three groups: Ni, Co and

Fe, noble metal and early transition metal carbide.

3.1. Ni, Co and Fe catalysts

The earliest work on the catalytic partial oxida-

tion of methane to synthesis gas was performed by

Liander [22], Padovani and Franchetti [23] and Pret-

tre et al. [24], who obtained high yields of synthe-

sis gas with ca. 2:1 of H2/CO molar ratio, within

the temperature range from 1000 to 1200 K, at at-

mospheric pressure. They proposed that a sequence

of reactions including total oxidation and reforming

reactions of methane was taking place over nickel cat-

alyst. The calculated equilibrium gas compositions

based upon those reactions shown in Figure 2 gave a

good agreement with the observed exit gas composi-

tions, which implied that the thermodynamic equilib-

rium was established in all cases, if carbon deposition

was ignored.

Vermeiren et al. [25] reported the results of ox-

idizing methane with air over nickel catalyst, and

drew similar conclusions to Prettre’s [24]. They com-

pared the POM activity with methane steam reform-

ing over the similar nickel catalyst and found that

POM was 13 times faster than the latter. Thus, they

presumed that there were extra reaction pathways,

which greatly accelerated the methane converting in

methane/oxygen mixture.

Lunsford and co-workers [15] studied in detail

the alumina-supported nickel catalyst bed exposed to

POM atmosphere using XRD and XPS techniques,

and found that three zones were formed in catalyst

bed. The outer zone was made up of NiAl2O4 phases,

which was moderately active for total oxidation of

methane. The mid zone included NiO and Al2O3 par-

ticles, which was thought to complete the total oxi-

dation of methane. The inner zone contained metal-

lic nickel particles, it was suggested that at this zone

the reforming reaction was catalyzed to thermody-

namic equilibrium. XPS studies indicated that only

products of CO2 and H2O were formed, without for-

mation of carbon deposition over the catalyst sur-

face below 973 K. At 1023 K, the surface carbon de-

position increased to monolayer companied with the


higher methane conversion. Their results also indi-

cated that the amount of surface carbon deposition

was affected by the ratio of methane to oxygen in

feedstock, a higher ratio resulting in more carbon de-

position, and vice versa.Nickel is active component for POM, but the

nickel species with different oxidative state plays a

different role in surface reaction steps, as mentioned

above. In general, metallic nickel is beneficial to

the production of synthesis gas, while nickel species

with oxidative number ≥2 trends to catalyze the to-

tal combustion of methane. The distribution of sur-

face nickel species with different oxidative number

depends upon the support properties and synthesis

procedure. When referring to nickel-based catalysts,

its activity and stability are indispensable topics to

be touched. In order to enhance the POM activity

and stability of nickel-based catalyst, one approach is

to choose suitable supports from miscellaneous ma-

terials. Choudhary and co-workers [18–20] carefully

studied catalysts of nickel supported on Yb2O3, MgO,

CaO, TiO2, ZrO2, ThO2 and UO2, as well as alumina

doped with rare earth oxides. It was found that the

catalyst containing CaO, MgO, rare earth oxide and

alumina had higher activity under the condition of

short residence time. For nickel catalyst containing

ThO2, UO2 and ZrO2, the activity had the order as

follows, NiO/ThO2 > NiO/UO2> NiO/ZrO2. TiO2

or SiO2, as support, was not suitable for POM to syn-

thesis gas due to the easier sintering of nickel oxide

and the inertness of binary metal oxide at high tem-

perature.

Ruckenstein et al. [26–28] carefully studied

NiO/MgO catalyst system. It was found that the

high activity originated from the formation of solid

solution, nickel atoms evenly dissolved in the crystal

lattices of MgO. In addition, MgO, due to its weak

basicity, can prevent catalyst from carbon depositing

to some extent [29,30]. These functions of alkaline

earth oxide were also observed in other catalyst sys-

tems [31–35]. The alkaline earth oxide can improve

the dispersion of nickel due to the strong interaction

between nickel and alkaline earth oxide, and also it is

the strong interaction, the highly dispersed nickel par-

ticles, once formed can be prevented from agglomer-

ating, and can be stabilized. Although alkaline earth

oxide supported or modified nickel catalyst exhibited

high POM activity, the deactivation is unavoidable

due to the carbon deposition and the loss of nickel at

high reaction temperature [19].

Among the nickel-based catalysts, it was found

that nickel supported on the perovskite-sturctured

materials, e.g. Ni/Ca0.8Sr0.2TiO3, prepared by cit-

rate method, exhibited good ability to restrain itself

from carbon depositing. Negligible amount of carbon

deposition was found over its surface after 150 hours’

run [36–40]. It was claimed that this kind of sup-

port could control the size of dispersed active phases

below the threshold value needed to generate car-

bon deposition. Therefore, oxygen species over cat-

alyst surface can react with carbon deposition, which

leads to nickel particles prevent from being covered

[41]. Another kind of active support is hydrotalcite-

structured mixed oxide. It is also claimed that this

support can control the dispersed nickel particles size

[42,43]. However, the activity and selectivity for this

kind of catalyst depended to a great extent upon the

reducibility, concentration of nickel oxide and resi-

dence time of reactants. The reducibility of catalyst

relates to the properties of precursor, such as compo-

sition and pretreatment etc. High Mg/Al ratio leads

to less formation of spinel-type species, which is less

active for POM [43].Some other materials, for example, CaAl2O4,

AlPO4-5 and calcium phosphate/hydroxyapatite, etc.

[33,44,45], were tried to be used as the support of

POM catalysts. CaAl2O4 and calcium phosphate sup-

ported nickel catalysts showed excellent performance

of sintering and carbon depositing resistance, and

therefore they showed higher methane conversion and

selectivity to H2. AlPO4-5 also gave good catalytic

performance; however, the phase transformation to

tridymite-structured species caused the specific area

of catalyst and activity to be lost quickly.

Another approach to elevate activity and stability

is the modifying support. The employment of alka-

line earth oxide [35,40,44,46,47] usually leads to the

formation of solid solution. In this case, active com-

ponent is highly dispersed. There is also very strong

interaction existed between alkaline earth oxide and

active phase due to its chemical activity, which results

in anchoring of dispersed active particles, further pre-

vents these particles from agglomerating. The weak

basicity of alkaline earth oxide can restrain the car-

bon depositing to some extent. For example, carbon

deposition was hardly observed over the catalyst af-

ter 500 hour’s run [46]. Other effective modifier is

rare earth oxide [48–53]. These catalysts exhibited

long-term stability [51]. The promotion of rare earth

oxide may be resulted from its oxygen storage/release

capacity, which lands itself to oxidizing surface de-

posited carbon. It was also believed that it could


restrain catalyst from sintering at high temperature

because of its strong interaction with active compo-

nent. It was also reported that the improved activity

was attributed to the enhanced reducibility of active

component after the addition of rare earth oxide [53],

because it usually accepted that metallic component

is highly active for POM to synthesis gas.Furthermore, other active component such as Fe,

Co, Pd, Rh, etc. were tried to modify the nickel-

based catalyst for the purpose of improving the sta-

bility and activity. Provendier and co-workers [54]

found that Fe could stabilize nickel catalyst. Us-

ing sol-gel method, They synthesized LaNixFe(1−x)O3

(0< x <1), perovskite-structured mixed oxide, a pre-

cursor of highly active catalyst for POM to synthesis

gas. For these catalysts, stability was improved as

the amount of added iron increased, owing to the re-

versible migration of nickel from the bulk to surface.

Choudhary et al. [20] pointed out that cobalt addition

to Ni/Yb2O3, NiO/ZrO2 or NiO/ThO2 catalyst can

reduce the formation rate of carbon deposition and ac-

tivation temperature of catalyst, which resulted from

improved reducibility of nickel species by the cobalt

addition. The addition of noble metal, although it is

very active for the POM, led to the change of nickel

chemical state or the distribution of nickel species

with different oxidative state. Thus, the tempera-

ture distribution along the longitudinal catalyst bed

was also changed, usually the hot spots disappeared.

The variation of nickel species with different oxidative

state resulted in reasonable distribution between the

total combustion and reforming of methane occurred

along the longitudinal catalyst bed during the process

of POM to synthesis gas [55].

The synthesis procedure of catalyst can affect its

activity and selectivity to a great extent. Xu et al.

[56] prepared alumina supported nickel catalyst us-

ing microemulsion method, and its stability was im-

proved due to an increased coking resistance. Li et al.

[57] prepared Ni/SiO2 catalyst using monodisperse

silica sol and rather high POM activity was achieved,

though SiO2 is an inferior support. Highly coking

resistant nickel catalyst is also synthesized using co-

precipitation method [58]. In general, an important

step in preparation is to improve the dispersion of ac-

tive phase and the interaction between active phases

and support. Both of these factors determine the

chemical state of active component, namely, the activ-

ity of catalyst. So the pretreatment of catalyst, like

the determining step of synthesis procedure, is very

crucial to the activity and selectivity.

Studies on Fe- or Co-based catalysts showed that

the activity of these catalysts was inferior to Ni-based

catalyst because they show higher activity for the to-

tal combustion of methane [49,59,60]. For these sup-

ported catalysts, the activity for POM to synthesis

gas has the order as follows: Ni>Co>Fe. It was re-

ported by Swaan et al. [61] that the cobalt based

catalyst was active only when it was promoted by the

substance that can facilitate its reduction, and this

was the reason that cobalt catalyst with higher load-

ing had higher activity [62]. The support plays an

important role in determining the activity and stabil-

ity of catalyst. Wang et al. [63] found that MgO is

an effective among the alkaline earth oxide supported

cobalt catalysts, and that the calcination temperature

threw a great impact upon the activity and stability.

The employment of cobalt together with noble metal

may be a good idea because cobalt is difficult to sin-

ter. Pt-Co catalyst system showed high activity [64].

It was usually thought that the active species of cobalt

catalyst was metallic cobalt, and that the stability

depended upon its preparation. Moreover, the deac-

tivation of cobalt containing catalyst resulted from

the sintering of active components and formation of

CoAl2O4 [63]. In fact, it was important to choose the

support, for instance, Co/ZrO2 showed high activity,

while Co/La2O3 deactivated rapidly [65].

Comprehensive investigation into Ni- or Co-based

catalyst has been performed, while less attention has

been paid to Fe-based catalysts. The deactivation of

Ni- based catalyst mainly attributed to carbon de-

position and nickel loss at high temperature and high

GHSV. The utilization of cobalt and iron, due to their

higher melting points, if substituting for nickel, may

give better performance. Other elicitation from a ref-

erence [17] on steam reforming is to control the size

of active phase and introduce some modifier into cat-

alysts for the purpose of improving the stability of

catalyst.

3.2. Noble metal catalysts

Green and co-workers obtained high yields of syn-

thesis gas over all noble metal catalysts, as well as

over the rare-earth ruthenium pyrochlores. These

catalysts catalyzed methane conversion to synthesis

gas with yields and selectivity closely approaching to

the thermodynamic calculations. For palladium cat-

alyst, like nickel, substantial carbon deposition was

observed, while for iridium and rhodium catalyst, no

macroscopic carbon deposition was observed [10,11].


Poirier and co-workers [66] carried out POM to

synthesis gas experiment at extremely high GHSV

(0.893 molCH4/(kg·s), CH4/O2/He=8/4/3), namely

under the condition that products were dominated

by kinetics, and it was found that Rh was more ac-

tive than nickel, even though at very low loading

(0.015wt%Ru/Al2O3).

Hochmuth et al. [67–70] studied the monolith

supported noble metal catalysts for POM to syn-

thesis gas. The results of pilot plant test at high

GHSV showed that Pt or Pd and the like, was ex-

tremely effective for the production of synthesis gas

from methane. They drew the conclusion that the

complete oxidation of methane had carried out at the

foreside before reforming reaction occurred at the rear

part of catalyst bed. Schwiedernoch et al. [71] also

drew the similar conclusions.

The activity of noble metal for POM to synthe-

sis gas not only depends upon noble metal itself, but

also relates to the preparation procedure and support

properties. Basile and co-workers [72] used anion clay

as precursors of noble metal based catalysts for the

activation of methane and found that the synthesis

gas production activity increased according to the or-

der Rh>Ru∼Ir�Pt>Pd. The best catalytic perfor-

mance was observed for a 1% Rh content (atomic ra-

tio) and Rh content above 1% did not increase the

activity, unlike Ru based catalysts. The results by

Yan et al. [73] indicated that the conversion and se-

lectivity were relative stable over the Rh based cat-

alyst, while they were changeful over Ru based cat-

alyst. Furthermore, the pulse reaction showed that

only CO was formed as carbonaceous product over

Rh catalyst, while CO and CO2 were formed over

Ru catalyst. This implies that the reaction mech-

anisms over these two catalysts are different. As for

the Ir based catalysts during POM reaction to synthe-

sis gas, the activity order of supports was as follows:

TiO2 ≤ZrO2 ≤Y2O3 >La2O3 >MgO≤Al2O3 >SiO2

[74]. A series of rare earth supported noble metal

catalysts was studied, among them, Pt/Gd2O3 and

Pd/Sm2O3 gave preferable catalytic performance [75].

Nevertheless, the selectivity to CO was higher than

that to H2, and it was ascribed to the reverse reaction

of steam reforming. They thought that alkali earth or

rare earth metal oxide played the double roles: one is

to disperse noble metal and the other is to improve

the selectivity.Ruckenstein and coworkers [76–78] investigated

into the effect of different structured magnesia, rare

earth metal oxide, as well as other stable supports

on Rh based catalysts. It was found that the com-

pound formation between Rh and support depended

upon the calcination temperature. No Rh compound

was formed over γ-Al2O3 and SiO2, while LaRhO3,

MgRh2O4, YRhO3 and RhTaO4 were formed over

La2O3, MgO, Y2O3, Ta2O5, respectively, if calcined

at properly high temperature. Among them, La2O3

can provide better catalytic activity and selectivity af-

ter adequate calcinations. The catalyst stability and

the interaction between active metal and support can

be improved at higher reaction temperature. Among

the non-reductive metal oxide supported catalysts,

the activity decreased according to the order as fol-

lows: La2O3 < γ-Al2O3 ≤MgO, after 100 hours’ run.

Rh catalysts supported on variedly structured mag-

nesia hardly showed visible difference, though three

Rh species, Rh2O3, MgRh2O4 and a compound of Rh

and MgO, were observed over the used catalysts. The

higher stability of MgO supported catalyst was as-

cribed to the stronger interaction between the active

metal and support. Clausen et al. [79] investigated

into the local structure of dispersed Rh particles us-

ing in situ X-ray adsorption fine structure technique,

and it was found that metal particle size increased

significantly during the treatment in hydrogen, while

no structure change was observed during POM re-

action, and not influenced by the residence time of

reagents. In the other hand, methane conversion and

selectivity depend upon the residence time. Ruck-

enstein et al. [80] also investigated into the effect

of Rh content in alumina supported catalysts on the

catalytic performance, and it was found that almost

the same methane conversion and selectivity was pro-

vided when Rh content was within the range from 0.5

to 5.0wt%.Jones and co-workers [81] studied the performance

of Eu2Ir2O7 catalyst using in situ X-ray diffraction

and Mass spectrometry and found that the pyrochlore

structure of iridate catalyst was destroyed at the out-

set of catalysis, giving an active catalyst that was

shown to comprise particles of iridium metal of about

3 nm in diameter supported on europium oxide. The

sudden increase of synthesis gas, monitored by mass

spectrometry, corresponded to the onset of reduction

of pyrochlore. Ashcroft et al. [82] drew similar con-

clusions in a study of iridium pyrochlore catalysts us-

ing in situ energy dispersive X-ray diffraction by syn-

chrotron radiation.

Kunimori et al. [83] found excellent catalytic

properties of RhVO4/SiO2 and un-promoted Rh/SiO2

catalysts for the POM to synthesis gas, above 90% of


methane conversion at 973 K. They also compared

the activity of the two catalysts over temperature

range of 573–973 K at ambient pressure, using a feed

of CH4/O2 with a molar ratio of 2 diluted with ni-

trogen. It was found that the onset of activity oc-

curred around 773 K for RhVO4/SiO2 catalyst, while

for Rh/SiO2 the catalyst exhibited activity at tem-

perature above 873 K. The examination of the used

catalysts indicated that the average Rh particle size

in RhVO4/SiO2 catalyst was smaller than that in

the un-promoted catalyst, Rh/SiO2. Therefore, the

difference of activity at low temperature was ascribed

to the active metal particle size, morphology and its

interaction with the support.The activity of the catalysts of 1% Pd supported

on oxides including IIIA-IVA metal oxide and rare

earth oxide, were investigated at 1023 K, using GHSV

of 5000 h−1 and CH4/O2 ratio of 8:1 [84]. The

methane conversion varied from 33.4% to 66.9%, but

surprisingly they all gave more than 99% of selectivity

to CO, with no data for hydrogen selectivity. How-

ever, the methane conversion in all cases exceeded the

theoretical maximum (25%) for synthesis gas produc-

tion under the fixed ratio of methane to oxygen. In

addition, the GHSV was set at 5000 h−1 in their work,

which is very small as compared with those used by

other researchers. It is thus probable that carbon de-

position over palladium catalysts is responsible for the

difference in methane conversion.

Platinum supported on alumina doped with zir-

conia gave very excellent performance, which was as-

cribed to the improved oxygen mobility brought by

the introduction of zirconia [85]. The properties of

support significantly affect the activity and selectivity

of Pt-based catalyst via adjusting interaction between

support and active component [86]. The studies on Pt

catalyst revealed that the deactivation of Pt catalyst

was mainly due to the agglomeration of dispersed Pt

particles and carbon deposition [87,88].

The modification of support can affect the oxida-

tive state of active component, which is the key factor

to determine the activity and selectivity. Elmasides

et al. [89] reported that for the Ru/TiO2 catalyst,

the introduction of W6+ into TiO2 led to stabiliza-

tion of oxidative ruthenium, which resulted in lower

conversion and selectivity, while the introduction of

Ca2+ cation led to the formation of metallic ruthe-

nium, which resulted in higher conversion and selec-

tivity.

Recently, noble metal based membrane reactor

has attracted a lot of attention. Using this kind of

membrane reactor, POM to synthesis gas can be car-

ried out at lower temperature, while higher CO and

H2 selectivity can be achieved. Armord et al. [90]

found that Pd based membrane reactor can elevate

H2 production during methane converting to synthe-

sis gas or liquid fuel. Kikuchi et al. [91] found that us-

ing noble metal based membrane catalysts POM can

be carried out at temperature below 773 K if the feed-

stock is poor in oxygen. The methane conversion and

CO selectivity can be elevated through removing H2

from the reactor. It was also found that carbon depo-

sition occurred after steam was depleted, while it can

be avoided by replenishing steam. The nickel-based

membrane reactor was also reported lately [92] and

high methane conversion and selectivity was reached.

Monolith reactor or monolith supported catalysts,

similar to the membrane reactor, was tried to be ap-

plied to POM to synthesis gas [93,94]. For rhodium

impregnated foam monoliths, very high methane con-

version (>90%), CO selectivity (>90%) and complete

conversion of oxygen was achieved during POM to

synthesis gas under the adiabatic condition, using ex-

tremely short residence times of between 10−4 and

10−2 seconds and the feedstock with stoichiometric

ratio. Under the same condition, H2 selectivity for

Pt based catalyst decreased to 70%, whilst the Pd

catalyst promoted the carbon deposition.

Another interesting noble metal based catalyst

system is a mixed oxide (Ba3NiRuTaO9) with per-

ovskite structure [95]. At 1173 K, it can provide 95%

of methane conversion and 98% of H2 selectivity. At

1070 K, it can catalyze the complete conversion of

ethane, obtaining 94% of synthesis gas, but there is no

transformation of perovskite structure post-catalysis

and no carbon deposition formed.

3.3. Early transition metal carbides and other

catalysts

Early transition metal carbides, particularly of

molybdenum and tungsten, exhibited excellent cat-

alytic performances in a large number of reactions,

which were usually catalyzed by noble metal based

catalysts. York et al. [96,97] obtained high yields of

synthesis gas using supported molybdenum or tung-

sten carbides at elevated pressure and temperature.

But it was deactivated rapidly at ambient pressure, re-

sulting in metal dioxide (MO2). In addition, if POM

was carried out under stoichiometric conditions, no

carbon deposition was observed on the used catalysts.

A study of the relative activity of molybdenum car-


bide to the noble metals demonstrated that it had an

activity similar to iridium, both per active site and per

gram [96], while high space velocity is unfavorable to

the stability of carbide catalysts. The deactivation of

the catalyst may result from the oxidation of carbides

into oxides, followed by vaporization of oxides under

ambient pressure. However, under the elevated pres-

sure, the vaporization was choked up, since high pres-

sure prevented the carbide, possessing much higher

boiling point than its oxide counterpart, from being

transformed into oxides, especially in the reducing at-

mosphere. The atmosphere imposes influence greatly

upon the stability of carbide, particularly at high tem-

perature. The atmosphere of steam and CO2 goes

against the retention of oxides, but hydrogen and CO

favor retaining carbides [98]. Recent study shows that

addition of some transition metals can significantly

increase the catalyst activity and stability [99]. With

the addition of transition metal promoters, the cata-

lyst activity can be as high as the noble metal catalyst

even at very high space velocity and pressure condi-

tions, but there is much less carbon deposition over

the carbide catalysts. Other example is β-SiC with

medium surface area used as support [100]. β–SiC

supported nickel catalyst showed stable and high ac-

tivity for POM, and the hot spots usually occurred on

alumina supported catalysts were removed due to the

high thermal conductivity of β–SiC, and the carbon

nanofilament growth was scarcely observed.

Besides above POM catalysts, it was reported by

Otsuka et al. [101] that cerium dioxide could trans-

form methane into synthesis gas with a molar ratio of

2 for H2 to CO within temperature range from 873

to 1073 K. It was demonstrated that during the re-

dox cycle of ceria, methane is directly converted to

H2 and CO. Carbon dioxide resulting from the oxida-

tion of methane in gas phase is reduced by partially

reduced cerium cation and CO is given as the only

product. The addition of Pt to cerium dioxide cata-

lyst accelerated the formation of synthesis gas, while

the reduction of catalyst over 60 minutes led to the

synthesis gas with a molar ratio of H2 to CO higher

than 2. The latter case implies the formation of car-

bon deposition after a period of reduction. However,

cerium is used as a modifier at more time to improve

the oxygen mobility.

4. Methane partial oxidation mechanism

The mechanism of methane partial oxidation to

synthesis gas was dealt with and debated widely in

the literature. As far as it goes, it can be divided into

two categories: one is the indirect oxidation mecha-

nism involving methane total combustion, and steam

and dry reforming reactions, which is often referred to

as the “Combustion and Reforming Reactions Mecha-

nism” (CRR); the other is the direct oxidation mech-

anism in which surface carbon and oxygen species re-

act to form primary products, known as the “Direct

Partial Oxidation Mechanism” (DPO).

4.1. CRR mechanism

First mention of the CRR mechanism was made

by Prettre et al. [24]. Their experiments, later re-

peated by Vermeiren et al. [25], indicated that the

longitudinal temperature profile of the catalyst bed

was not uniform, namely, markedly higher tempera-

ture of the front part of catalyst bed than that of the

rear part and the furnace temperature, as shown in

Figure 4[25].

Figure 4. Schematic representation of the tempera-

ture in POM catalyst beds

Choudary et al. [59] carried out the POM reac-

tion using Ni/MgO catalyst under high GHSV con-

dition in order to get non-equilibrium product dis-

tribution. When York et al. [102] repeated the ex-

periment reported by Choudary et al, it was found

that the hot spots were formed. These results indicate

that exothermic reactions occurred at first and then

followed by endothermic reactions. As depicted in

Figure 2, the total combustion of methane, and then

followed by steam and dry reforming of methane. In

fact, nickel and noble metal are very effective catalyst

for the steam shift and steam reforming of methane.

According to CRR mechanism, synthesis gas is sec-

ondary product. Green et al. [11] in their latter ex-

periment investigated into the effect of reaction con-

ditions on the product distribution in the process of

POM to synthesis gas, and explained why there is

lower selectivity of synthesis gas and higher selectivity


of CO2 and H2O under the condition of higher GHSV

or higher ratio of O2/CH4, using CRR mechanism. At

the same time, they pointed out that hydrogen and

carbon monoxide were formed as secondary products.

4.2. DPO mechanism

Hickman and Schmidt et al. [68–70] considered

H2 and CO as primary products during POM to syn-

thesis gas under adiabatic condition at very short

residence time. When they doubled the residence

time, the conversion and selectivity were improved.

When substituting Pt-10% Rh wire net for monolith

supported catalyst, the conversion and selectivity in-

creased with increasing the gas velocity of feedstock.

This phenomenon is conflicted with the case in Ref.

[11]. If the gas velocity is fixed and the layer number

of Pt-Rh wire net is increased (not less than 3), no

difference of conversion and selectivity was observed,

and product distribution was away from the equilib-

rium of steam shift or steam reforming reaction, com-

panied with a lower ratio of H2/CO than that calcu-

lated according to thermodynamic equilibrium. All

these facts cannot be explained by CRR mechanism.

In order to elucidate the phenomena mentioned above,

the DPO mechanism was put forward. In this mecha-

nism, synthesis gas is produced as a primary product.

CH4 = C(ads) + 4H(ads)

C(ads) + [O]s = CO(ads) = CO(g)

2H(ads) = H2(g)

They constructed a model incorporating the ele-

mentary adsorption, desorption and surface reaction

steps involved in a mechanism, of which some of the

most important steps are shown in the above equa-

tions. According to this model, the product selectiv-

ity over Pt or Rh catalyst can be forecasted.

Recently, the studies of POM specific mechanism

under specific conditions have become popular. When

Weng et al. [103–105] investigated into the reduc-

tion of Rh and Ru catalyst using FTIR technique, it

was found that CO was formed as a primary prod-

uct of POM reaction over reduced or really working

Rh/SiO2 catalyst, DPO pathway was the main route

of formation of synthesis gas over Rh/SiO2 catalyst.

In contrast to this, CO2 was a primary product of

POM reaction over Ru/Al2O3 or Ru/SiO2 catalyst

[73]. Synthesis gas was formed over Ru-based cata-

lyst by way of CRR mechanism. Of course, the oxygen

content in feedstock can alter the reaction direction.

Transient response and isotope exchange tech-

nique have also been used to investigate into the POM

mechanism. Nakagawa et al. [106,107] reported that

synthesis gas was formed over Ir/TiO2 and Rh/SiO2

catalysts via CRR mechanism. However, the en-

dothermic reaction, methane decomposed to hydro-

gen, carbon and dehydrogenated methane group, ini-

tially occurred, followed by a reaction: carbon and

the dehydrogenated methane group oxidized by oxy-

gen to COx species. As for POM performed over sup-

ported Rh catalyst, its reaction pathway depended

greatly upon the properties of the support. Ruck-

enstein et al. [108,109] reported that during POM

reaction over Ni/SiO2 catalyst, CH, CH2 and CH3

species were formed, which means that methane is ac-

tivated via dissociation, and the amount of methane

taking part in isotope exchange was larger than the

amount of methane converting to CO and CO2. It was

concluded that methane dissociating is not the rate-

limiting step. Over the un-reduced Ni/SiO2 catalyst,

methane directly reacted with oxygen without disso-

ciation of methane. Jin et al. [110] drew a similar

conclusion regarding alumina supported nickel cata-

lyst. Temperature can throw influence upon the POM

pathway. Within the temperature from 973 to 1023

K, POM proceeds mainly via the pathway of the dis-

sociation of methane, whilst at the temperature of

1123 K, CRR mechanism makes a rather contribu-

tion to POM [26]. Li et al. [111] made a point of

producing a high yield of synthesis gas requiring the

catalyst with reduced state. As reported in many

References. [45,55,73,112–114], metallic active com-

ponent, not only nickel but also noble metal, was very

effective to produce H2 and CO. Surface state deter-

mines reaction mechanism, and plays an important

role in determining conversion and selectivity.Judging by current evidences for the partial oxida-

tion of methane to synthesis gas, it is possible for the

two mechanisms to occur over nickel or noble metal

catalysts, but the real pathway depends upon the real

conditions. The pivotal factor is the chemical state of

active component element: metallic state prefers to

methane dissociation reaction, followed by surface re-

action with oxygen species to synthesis gas; while the

active component element with higher oxidative num-

ber facilitates deeper oxidation of H2 and CO, as ob-

served over Ru catalyst by Balint et al [115]. However,

the oxidative state of active component relates to the

properties of support, modifiers, pretreatments, re-

action temperature and oxygen partial pressure, etc.

Other important reason is the active surface oxygen


species and its mobility. Of course, kinetic factors

may exert an influence on it, even changes its direc-

tion. This is the reason why two reaction mechanisms

seem to be possible over all catalysts.

The two mechanisms are also applied to elucidate

the POM to synthesis gas over carbide catalysts [116].

(1) DPO type mechanism: this involves surface

species similar to those shown for the DPO mecha-

nism discussed earlier. However, it is likely that syn-

thesis gas is not a primary product over the carbide

catalysts, and that CO2 and H2O are important re-

action intermediates.

(2) Redox mechanism: O2, CO2 or H2O in the

reactor can react with surface carbide carbon species,

generating vacancies and oxide species. These vacan-

cies can then react with carbon from methane disso-

ciation, returning the site back to the carbidic. This

is shown below for the reaction of CH4 and CO2.

Mo2C + 5CO2 = 2MoO2 + 6CO

2MoO2 + 5CH4 = Mo2C + 4CO + 10H2

The most probable mechanism over carbide cata-

lysts is the redox mechanism according to the results

obtained by Xiao et al. [99] using in situ Raman and

pulse techniques. A possible model for the reaction is

given in Figure 5 [99].

Figure 5. Model of 2CH4+O2 reaction to synthesis

gas over molybdenum carbide catalyst

Oxygen first reacts with the carbide, producing

CO; the oxide or oxycarbide surface is then reduced

by methane to produce CO and H2. Because the re-

carburization of the oxide surface by methane is a

slow process and endothermic reaction, the carbide

catalyst is stable only under a condition of high pres-

sure and low space velocity [99].

5. Problems and future studies

Non-catalytic homogeneous partial oxidation of

methane to synthesis gas is well established. For ex-

ample, in Sarawak, Malaysia, Shell have been suc-

cessfully operated a highly selective process for pro-

ducing synthesis gas at high temperatures, typically

above 1400 K, and pressures of around 5–7 MPa,

as a part of the Middle Distillate Synthesis process

(SMDS) [117]. Out of question, employment of cat-

alysts would markedly lower the operating temper-

ature required for the production, which makes the

process more economically attractive [118]. However,

more work should be done to solve the following main

problems for the practical application of this process.

(1) Carbon deposition over the reactor and cata-

lyst bed. There are two possible routes for the forma-

tion of carbon, namely methane decomposition and

the Boudouard reaction.

CH4 = C(s) + 2H2(methane decomposition reaction)

2CO = C(s) + CO2(Boudouard reaction)

Studies by Claridge et al. [119] demonstrated that

both reactions are thermodynamically favorable un-

der reaction conditions typically for methane partial

oxidation, but that the source of carbon deposition is

mainly resulted from the methane decomposition. Ev-

idence for this was given by observing the amount of

carbon deposited on a nickel catalyst in pure methane

or carbon monoxide atmosphere; at high tempera-

tures the pure methane gave much more deposited

carbon than pure carbon monoxide, while support-

ing evidence arose from the fact that carbon built up

from the front of the catalyst bed, where methane

partial pressure was at its highest. Two types of car-

bon are formed on the partial oxidation catalysts as

shown in Figure 6[14]: (a) encapsulate carbon, which

envelops the nickel particles resulting in deactivation,

and (b) whisker carbon, which grows from the face of

the nickel particles and does not alter the rate of syn-

thesis gas formation, but is likely to eventually result

in reactor clogging. Detailed studies on the carbon

formation mechanism have been carried out for the

related steam reforming reaction [120–123]. To sup-

press the carbon deposition, more work needs to be

done on the catalyst preparation and reaction condi-

tion optimization. For example, some steam may be


added to the feedstock to eliminate the hot spots in

the catalyst bed, and also may be helpful to suppress

the carbon deposition.

Figure 6. Micrograph showing carbon deposited

over a nickel catalyst after POM

(2) Active component loss during the POM to

synthesis gas, particularly nickel catalyst. The over-

all POM reactions are mildly exothermic, while it may

occur by two steps, initial combustion and then dry

and wet reforming. In the first step, a large amount

of heat is given off, which can melt the active metal,

leading to peeling the active metal off the support.

Because nickel has a lower melting point, lower than

other active components, such as noble metal or Co

and Fe, thus, it may be easier to deactivate. However,

this can be improved by strengthening the interaction

between support and metal, and carrying out the re-

action at milder temperature.

(3) Noble metal catalysts have shown superior ad-

vantages to nickel metal catalyst in carbon deposition-

resistance, but the carbon deposition is still unavoid-

able at high temperature, because of the acidic sup-

port and the methane decomposition, etc. As re-

ported by Albertazzi et al. [87], carbon deposition,

together with sintering of noble metal particles, re-

sulted in the deactivation. Also a high loading of no-

ble metal is required to sustain the high activity, thus

the cost of the catalyst is very high. The combina-

tion of a small amount of noble metal with transition

metals such as Co, Ni or Fe may be a wise way to

decrease the catalyst cost and improve catalyst activ-

ity and stability. New alternative catalysts such as

transition metal carbide are expected to decrease the

catalyst cost and improve the catalyst stability.

(4) Because of the thermodynamic restriction, the

POM reaction under high pressure often gives rise to

more CO2 and H2O formation. To improve the prod-

uct distribution, the feedstock needs to be optimized.

New technology such as membrane catalyst and re-

actor are expected to lead the reaction to a dynamic

state, and thus to release from the thermodynamic

restriction. The combination of POM and steam re-

forming as an alternative, to increase CH4 conversion

and synthesis gas production, is also possible. In addi-

tion, a more stable catalyst system being able to resist

carbon depositing under excessive methane feedstock

is very important to increase the selectivity to CO

and H2. A high CH4/O2 ratio is also desired from the

view of safety, because lower CH4/O2 (≤1.5) ratio

increases the danger of explosions, and this is of par-

ticular importance when high pressure is employed.

6. Conclusions

As mentioned above, a great number of chemicals

and fuel can be obtained from methane by way of

synthesis gas, while the direct conversion of methane

is economically infeasible due to its intrinsic barrier.

Therefore, the partial oxidation of methane to syn-

thesis gas is likely to become more important in the

future, particularly when alternative sources of energy

are required. Early work showed that nickel was an

active catalyst for this reaction. Now it has been fol-

lowed up, particularly in the past 2–3 decades. At

present, a number of potential alternative catalysts

have been discovered for carbon free methane partial

oxidation, including the noble metals and molybde-

num and tungsten carbide. However, there are also

some problems such as carbon deposition for nickel

catalyst, the stability for transition metal carbide and

so on, to be resolved. In order to make POM to syn-

thesis gas become popular industrial process, further

study on this subject is required.

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