Catalysis Today, 8 (1991) 305-335 Elsevier Science Publishers B.V., Amsterdam

305

PROGRESS IN THE PARTIAL OXIDATION OF METHANE TO METHANOL AND FORMALDEHYDE

M.J. Brown and N.D. Parkyns

British Gas plc, Research and Technology Division, London Research Station, London SW6 2AD, tireat Britain

SUMMARY The recent literature on the direct partial oxidation of methane to

methanol and formaldehyde is reviewed, both for nominally heterogeneous and for homogeneous reactions. Emphasis is placed on the interaction of surface and gas phase chemistry in the reactivity of methane. At present, reported yields of oxygenates are only a few per cent at most for catalysed reactions : much higher yields have been claimed for homogeneous or heterogeneously- initiated reactions. The review concludes with some remarks about the economic viability of direct oxidation processes and some future directions of research.

INTRODUCTION

The existence of vast World-wide reserves of natural gas, of which

methane is the chief constituent, has focussed attention (ref. 1) on to the

possibility of converting it directly to other forms of fuel and chemicals.

The need for this arises, at least partially, from the remoteness of the

location of the natural gas wells and the relative costliness of transporting

gaseous fuel over long distances, as compared with liquids. Besides

hydrocarbon liquids, methanol is well established as a valuable product

(ref. 21, both in its own right and as a source of derived chemicals, as well

as the source material for the Mobil MTG process to produce transport fuel.

The need for a process to convert methane to methanol or formaldehyde

directly has long been perceived. At present, the method of breaking it down

to synthesis gas, followed by the highly selective synthesis of methanol over

copper-based catalyst holds sway. but it is a relatively costly and

energy-intensive process. A method of direct conversion would be highly

attractive, if this could be devised. This aspect will be dealt with later in

this Review.

As a result of all these pressures, interest in the direct oxidation

route has been at a high level for more than a decade. The chemical problems

are, however, formidable and no clear path is yet visible. The subject was

reviewed extensively in the mid-1980s by Pitchai and Klier (ref. 31, by Garcia

and Lijffler (ref. 4), by Foster (ref. 5), and by Gesser. Hunter and prakash

(ref. 6), the latter two reviews concentrating specifically on methanol

0920-5861/91/$10.85 0 1991 Elsevier Science Publishers B.V.

306

formation. Scurrell (ref. 7) and Mimoun (ref. 8) have considered the subject

of methane conversion more generally. Recently, Sinev, Korshak and Krylov

(ref. 8) have published a review, which is especially valuable, as it provides

a summary of the extensive Russian work over the past 20 years.

In the face of this impressive literature, we do not propose to review

the subject afresh, but rather to look at progress made over the past 5 years,

referring to earlier work, where necessary, to amplify a point.

Interaction of homogeneous and heterogeneous reactions

The low reactivity of methane and the need to activate a carbon-hydrogen

bond to initiate reaction has tended to mean that, even for gas-solid

heterogeneous reactions, relatively high temperatures (450 - 65O’C) are used

to get reasonable conversions of reactant. Generally, radicals are generated

either at the surface of the reactor or in the gas phase and are able to

initiate further reaction

CH. + 0, * CH, + HO, ( 1)

In the case of homogeneous oxidation of methane to methanol, this reaction

takes place in the gas phase. Thereafter, the course of the reaction is

governed entirely by a series of radical reactions, as described later.

Just as for the oxidative coupling of methane, to give ethane and ethene,

reactions leading to insertion of oxygen may also be initiated at the

surface.

CH. + [surface1 + CH, + [surface.Hl ( 2)

Thereafter, subsequent radical reactions may take place, either on the

catalyst surface, or near to it in the gas phase , and it may be difficult to

distinguish between these alternatives.

A general feature in all reactions is the relatively high energy required

to activate the methane molecule, compared to the ease with which subsequent

reactions may take place, i.e.

CH, + 101 + CH,OH [g’ HCHO + CO + H, ( 3)

The energy diagram given below (Fig. 1) refers to gas phase reactions but the

same tendency is also true for supposedly heterogeneous reactions. The

consequence is, naturally, that it is difficult to stop at the earlier,

desired stages and that any attempt to drive the reaction to high conversions

leads to poor selectivities. This aspect is clearly shown in the literature.

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Enthalcw (kJ/mol) 0

-200

-400

-600

-800

-1000 -

Fig. 1. Enthalpy changes, at 298 K, for successive oxidation reactions of

methane

A slightly surprising feature of the attempts to find selective catalysts for

oxygenated products has been that it does seem possible to produce methanol

and formaldehyde separately, reasonably selectively. The energy diagram shown

above, and the fact that many MO-containing catalysts are known to oxidise

methanol to formaldehyde, would suggest that a mixture of products would be

found . The formation of formaldehyde might be expected to go through methanol

as an intermediate, and methanol is sometimes reported in varying amounts

during formation of formaldehyde, although, as will be discussed, the evidence

is not conclusive.

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HOMOGENEOUS GAS PHASE OXIDATION

The chemistry that occurs in the partial oxidation of methane is similar

to that of most general combustion processes (refs. 10,111. Combustion

studies have, accordingly, produced much of the basic understanding of the

reasons why methanol or formaldehyde are formed at all (ref. 12). Methane,

like other hydrocarbons, will react in a number of phenomenologically-

different ways, depending on the reaction conditions. The conversion process

involves an initiation step, propagation and termination (or removal of

active species). Since the previous reviews, research in the area of

homogeneous gas phase oxidation has concentrated on :

. optimisation of the high-pressure conversion

. effect of additives

. use of modelling to determine the overall reaction mechanism

. use of novel reactor design

. photochemical initiation

High-pressure (>lO bar) conversion

Early studies found that high pressures encouraged the formation of

methanol (see, for example, ref. 13). Recent work has tried to find optimum

conditions for this conversion, and a general consensus has been reached,

though the selectivity to methanol remains a disputed point. All studies used

flov-tube reactors, either premixing the gases prior to the reactor (refs. 14,

15). or using jets at stages to add preheated methane to a cent.ralised air

flov (ref. 16). However, results should be comparable if the reactants are

well mixed. tias chromatographic methods are used for analysis, either by

direct sampling of the products from the reactor, or by using a cold finger to

trap any condensable products. High methane to oxygen ratios are used to

prevent full oxidation.

A switch from oxygenated products to coupled species (ethane, ethene and

>C, hydrocarbons) is observed at increasing temperatures because of the

equilibrium :

CH, + 0, +CH,O, ( 4)

The equilibrium is dependent on temperature, pressure and oxygen

concentration, parameters which are , accordingly, important in governing the

conversion to methanol.

There are, however, discrepancies in methanol selectivities and methane

conversions, when comparing results from apparently similar studies in a 4-mm

diameter Pyrex-lined tube. Table 1 shows how, under closely-related

conditions, the results differ markedly in two different laboratories.

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TABLE 1

Comparison of results from two methane-to-methanol high-pressure

conversion studies (refs. 14,151

Temp. Pressure Feed Composition (X) Residence CH, conv. CH,OH sel. I’C /atm CR, 0, N* time fs /mol(B) /mol(%)

450 49 97.5 2.5 0 200 ca. 5 40 (a> 450 49 93.4 6.6 0 200 ca. 5 36 (a) 450 49 50.0 2.5 47.5 300 ca. 5 38 (b) 451 50 93.3 6.7 0 208 (e) 9.5 76 (cl 452 25 89.3 8.7 2 75 (e) 13.3 55 (d) 456 65 94.9 5.1 0 232 (e) 8.0 83 (c)

(a) from ref. 15 Table 1 (b) from ref. 15 Figure 3(a) (c) from ref. 14 Table 2 (d) from ref. 14 Table 1 (e) calculated from given flow rates (at n.t.p.) assuming ideal gas

There may be a number of reasons to explain these discrepancies. For

example, temperature measurement in flow reactors is something of a problem,

only one study attempting to characterise a full temperature profile for the

reactor, taking into account the exothermicity of the oxidation reaction

(ref. 16). Thermocouples to measure the reaction temperature have been

placed in the reaction zone (ref. 14) or mounted externally to the reactor

(ref. 15). The internal diameter of the reactors used ranges from 4 - 20 mm

and the surface-to-volume ratios (S/V) range from 5.0 to 1.0 cm-*. It is

always possible, of course, that there is efficient heat transfer in the

smaller diameter tubes to ensure that the measured temperature, external to

the reactor, approximates to the actual gas temperature.

Other explanations may lie in the use of a needle-valve immediately

before the reactor, or, as previously mentioned, a thermocouple in contact

with the reaction mixture. Yarlagadda et (ref. 14) use an 0.8 mm diameter

thermocouple inserted into the reactor tube. This will both perturb the

laminar flow giving it annular flow character and also may provide extra

active surfaces to influence the gas phase chemistry. The valve is thought to

break up laminar flow patterns and produce a more turbulent environment, akin

to plug-flow reactors (ref. 17) or, possibly introducing more back-mixing.

Calculations of Reynolds Number from the known experimental data do, however,

give extremely low values, -5 - 10. The flow is, accordingly, purely laminar

and even if disturbed by a valve, should quickly recover that characteristic

downstream of the valve.

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On the other hand, highly laminar flow leads to a marked velocity profile

such that the gas near the reactor wall has a longer residence time in the

tube than that in the centre of the flow. Small differences in reactor

dimensions may have a disproportionately large effect on this velocity profile

so that apparently similar reactors have quite different flow characteristics.

Moreover, there are problems in taking an accurate, mass-averaged sample for

gas chromatography under these conditions, because of the velocity profile.

Apart from the difference in the magnitude of the selectivity of

conversion to methanol, there is a discrepancy in the trend of selectivity

with the amount of oxygen present in the feed gas. Yarlagadda et (ref. 14)

find the selectivity strongly oxygen concentration-dependent, whereas Burch

et al (ref. 15) find only a slight dependence. As Burch et remark, they

find that methanol selectivities can be larger than 50% but this occurs

without full oxygen consumption and at low methane conversions : thus, the

yield (selectivity multiplied by conversion) is low.

An interesting feature of the partial oxidation of methane is temperature

oscillations. Oscillations, attributed to cool-flame phenomena, were observed

at 400 bar in some early work (ref. 18). Similar oscillations have been

reported under conditions which are favourable to methanol production at 25 -

35 bar (ref. 19). They occur in the temperature range 410 - 45O’C for a

methane-oxygen mixture containing 8% oxygen. With oxygen levels lower than

5X, no oscillations were observed. The highest selectivity to methanol was at

a temperature just low enough to eliminate oscillations, suggesting that

dynamic behaviour of this sort may have an adverse effect on selectivity.

Effect of additives

If the partial oxidation of methane is to be utilised commercially, it

will take, as its feedstock, natural gas which contains small amounts of

higher hydrocarbons as well as methane. The effect of these hydrocarbons on

the oxidation process has been studied by adding them to the reactant methane

(ref. 15), or by using natural gas rather than pure methane (refs. 20,21).

For example, adding small amounts of ethane to methane lowers the conversion

temperature (ref. 22) by about 50°C, while a ‘real’ natural gas has a

conversion temperature of the order of 1OO’C lower than pure methane

(ref. 21). In both of these studies, no benefit in terms of selectivity is

noted. A recent study (ref. 22) has looked in a more systematic way at the

effect of additives, or sensitisers, as they are sometimes called. The

evaluation of the sensitisers was made relative to an unsensitised 90%

methane/IO% oxygen mixture at 45O’C, 10 atm and a flow-rate of 40 - 80 ml/min

(NTP). The selectivity to methanol was found to be between 41 and 52X, the

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selectivity to formaldehyde being zero in the absence of sensitisers. These

conditions were deliberately chosen not to be those of maximum selectivity.

An extensive selection of sensitisers was tested, including saturated,

cyclic, unsaturated and aromatic hydrocarbons, ethers, aldehydes, ketones,

thiols, amines. and water, and di-t-butyl peroxide. The criteria used to

assess these sensitisers were :

* the effect on the methanol (and formaldehyde) selectivity;

- the effect on the minimum temperature for complete reaction (MTCR).

The majority of the additives reduced the MTCR to some extent, and many

apparently gave improvements to the selectivity to methanol. A few produced

high selectivity to formaldehyde. However, the percentage of sensitiser in

some experiments reached 10% and this level is sometimes equivalent to (or

greater than) the amount of oxygen present, the additive behaving more like a

co-reagent than a sensitiser. This causes problems in the calculation of

methanol selectivity, as the sensitiser may be the major source of products.

The general points appear to be :

* additives may help to attain high methanol selectivity by reducing the

operating temperature;

* additives may have a slight beneficial action in terms of overall

methanol selectivity.

Modelling partial methane oxidation

In recent years, modelling of complex gas phase reactions has become

more widely used, both as a method of explaining experimental phenomena in

terms of the chemistry occurring, and of predicting them. With all reaction

models, it is necessary to validate the scheme by ensuring that the model

correctly describes some already-established experimental data.

Russian workers have looked at partial methane oxidation at pressures

above 50 bar (refs. 23 - 26). They have produced a model containing 61

elementary reactions involving 18 species. Within the 61 reactions are 6

approximate heterogeneous wall reactions, 4 of vhich have no specified

products. This view of wall reactions may be an over-simplification if they

contribute significantly to the overall reaction. The model assumes that

conditions are isothermal, whereas the experiments are not (ref. 13). This

may introduce discrepancies, though these may be minor as temperature

variations are small. Eight extra reactions were added (ref. 26) when the

model was used to fit experimental data on ignition delay (ref. 27). The

model no longer assumes isothermal conditions and it was used to investigate

the dependence of ignition delay on the initial temperature and the initial

partial pressure of oxygen.

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Simplified reaction mechanisms to explain major features have been

proposed elsewhere (see. for example, ref. 14), but some features of models as

a whole are questionable, as the chemistry may be over-simplified and

important features omitted. There is little dispute that the major source of

methanol is the reaction of methoxy radical with a hydrogen donor, methane,

formaldehyde, hydrogen peroxide, or other species :

CH,O + RH + CH,OH + R ( 5)

Yarlagadda et (ref. 14). propose that the sole source of methoxy

radicals is the decomposition of methyl hydroperoxide :

CH,OOH + CH,O + OH ( 6)

and that the methyl hydroperoxide is formed by hydrogen abstraction from

methane by methylperoxy radicals.

The major point of debate is the fate of the hydroxyl radical. This is

very efficient at abstracting hydrogen atoms to form water, principally from

methane, present in large excess. In terms of a hydrogen balance, there are

four hydrogen atoms in both methane and in methanol, but some hydrogen must be

used to form water. Hence, not all the carbon converted can.form methanol.

If balanced equations are written, there is a theoretical u’,per limit of 2/3

for the selectivity. A reaction which could increase the selectivity limit is

the direct combination of methyl and hydroxyl-radicals.

CH, + OH + CH,OH ( 71

As the concentrations of these two radicals are always small and there is

direct competition for hydroxyl radicals through reaction with methane, this

should alter the selectivity only slightly. Also, other channels exist for

the reaction of methyl with hydroxyl, and their relative importance is the

focus of a certain amount of debate (ref. 28). Another reaction that could

increase the selectivity is the self-reaction of methylperoxy radicals :

CH,OO + CH,OO + CH,O + CH,O + 0, ( 8)

as the methoxy radicals are then able to react further with CH, to produce

methanol. This reaction bypasses the production of hydroxyl radicals and if

it were the dominant source of methoxy radicals, then the theoretical

selectivity limit could reach 100X. The Russian modelling study (ref. 23)

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takes account of all of the above-mentioned points, but does not try to find

the optimum conditions for methanol (or formaldehyde) production.

We are ourselves in the course of formulating a model to find the optimum

conditions for methanol production (ref. 29). The reaction mechanism is based

on one used for ignition modelling (ref. 30). with some extra reactions of

methanol and hydroxymethyl (CH,OH) added. It resembles the Russian model, and

correctly predicts the experimental observations (ref. 15) that pressures of

>20 bar , temperatures of about 4OO*C, high methane-to-oxygen ratios and short

residence times, are needed to form significant amounts of methanol. It

predicts a maximum selectivity of about 40 - 50% for a flow-reactor in plug

flow mode.

Some mechanisms attempt to explain the formation of formaldehyde at lov

conversions by postulating a reaction between methyl radicals and 0,,

competing with the addition reaction (4) :

CH, + 0, + HCHO + OH ( 9)

The reaction is often split into two parts : first, an addition, to form a

peroxy radical, followed by a re-arrangement and decomposition, to give the

products. This reaction has been widely postulated but is now thought not to

occur at low temperatures (ref. 31). Even at temperatures used in oxidative

coupling reactions, the reaction may not be at all important. The reaction

of methyl radicals with molecular oxygen has another possible route :

CH, + 0, + CH,O + 0 (IO)

though this reaction is very slow at these temperatures, as it is highly

endothermic.

Novel reactor and reaction systems

Since work by Brockhaus (ref. 32), it has been apparent that the design

of the reactor can alter the selectivity to oxygenates by influencing the

chemistry. The influence on methanol production of small differences in the

design of the high-pressure flow reactors (refs. 14.15) may be further

evidence of the critical nature of reactor design. The majority of

small-scale experimental work has used either static or flow-tube reactors.

However, methanol has been produced in small yield by passing a methane-oxygen

mixture through alternating electric fields produced in a 0.06” (1.5 mm) gap

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between cylindrical plates (ref. 33). A quenched-pulsed combustion reactor

(ref. 34) has also been used to produce methanol selectively. It consists of

a tube through which a fuel (typically hydrogen) - air mixture flows. This

mixture is ignited by a spark, and methane (or ethane) is subsequently added

to this ignited mixture. Still further down the tube, the reaction may be

quenched by an inert gas. Conversions of methane of up to 6.1% are quoted,

with methanol selectivities of 29%.

Photochemical conversion

Ogura and his co-workers have studied the photochemical conversion of

methane to oxygenated compounds (refs. 35 - 38). These studies are carried

out at relatively low temperature, 50 - 100°C, at atmospheric pressure, and

involve the photolysis of vater vapour as a radical source. The reactions are

performed on mixtures of methane and water vapour, but air can also be added.

The reaction yields many different products, but methanol and formaldehyde are

formed in relatively high amounts, with conversions of methane between 3 and

18%. Methoxy radicals have been detected (ref. 37) using ESR and radical

trapping. The reaction mechanism involved is similar to those at higher

temperature, though parts of the published schemes may be questioned. It

proceeds along different paths according to whether air is present or not.

The common initiation reaction is the photolysis of water vapour by 185 nm UV

light :

H,O + H + OH (11)

In the absence of air, both of the resulting radicals abstract hydrogen from

methane, to produce methyl radicals. The methyl radicals can couple, to

produce the major product, ethane, or react with hydroxyl radicals to form

methanol. In this reaction system, unlike the high-pressure flow-tube

oxidation, the magnitudes of the concentrations of methyl and hydroxyl

radicals are large enough to ensure that the rate for this process is

relatively fast. Other reactions quoted as sources of methanol are the

reaction of methyl radical with water vapour, and the reaction of hydroxyl

radical with methane :

CH, + H,O l CH,OH + H (12)

CH, + OH l CH,OH + H (13)

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but these reactions will not occur, to any great extent, at the operating

conditions used here.

When air (or oxygen) is present, the chemistry is dominated by peroxy

radicals, since the hydrogen atoms and the methyl radicals will be converted

to hydroperoxy and methylperoxy radicals respectively. The formation of

ethane is prevented almost completely when air is present. Self-reactions of

methylperoxy radicals will produce both methanol and formaldehyde in

significant amounts by a combination of equations (8) and (4).

CATALYTIC OXIDATION

Production of methanol

Attempts to oxidise methane catalytically have fallen into two classes

one, aiming at improving the yield from the homogeneous oxidation reaction;

the other, at a fully-heterogeneous reaction. In neither case is the

literature since 1985186 extensive, although some of the ambiguities in

results reported earlier have been resolved by the more recent work.

Both Hunter, Gesser and co-vorkers (ref. 22). and the group led by Burch

(ref. 151, have addressed the problem of improving the yield of methanol from

the high pressure homogeneous reaction. Both groups have looked at the

influence of the reactor wall and concluded that metals are, in general,

undesirable as leading to at least some degree of over-oxidation, although

Burch et al claim that, for pressures >40 bar, there was little difference

betveen Pyrex and stainless steel. Nonetheless, both groups agree that a

relatively inert non-metallic surface, like quartz or Pyrex, is necessary to

obtain the best yield.

Hunter, Gesser et al (ref. 22) have also examined the effect of passing

the methane/oxygen mixtures over various catalytically-active solids. These

included Cu/SiO,, Co/Al,O,, Hopcalite (a copper manganite), TiO, aerogel and

SnO,. They used a constant pressure of 30 atm. temperatures in the range

247 - 407°C and residence times 17 - 50 sets. (By comparison, the residence

time for the mixture in an empty reactor was 174 seconds under the same

conditions, a fact that should be borne in mind in making comparisons with the

catalytically-active reactor.)

Copper, between 300 - 35OOC. had little effect on the course of the

reaction and the selectivity towards methanol remained very high at 92 - 96X,

although the conversion of methane was lower (621, compared to the gas phase

reaction (8%) at longer residence time.

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Of the oxide catalysts, only SnO, showed any promise but the best result,

obtained at 2470C, gave a selectivity to methanol of 83% at only 4% CH,

conversion. Moreover, the authors remark that the activity fell off,

irreversibly, shortly after reaction started. All the other catalysts

produced only oxides of carbon.

Burch et (ref. 15) have looked at the stability of methanol under

reacting conditions to see whether its decomposition accounted for the lower

yield they obtained, compared to the Canadian group. Their results showed,

quite unambiguously, that methanol/oxygen mixtures are surprisingly stable!

Even at 5OO’C. a Pyrex-lined reactor failed to decompose more than a few per

cent of the methanol present. Copper turnings, on the other hand, decomposed

the methanol entirely for T > 400°C under the conditions used. Burch et al

conclude, not surprisingly, that use of copper, despite earlier claims, is

entirely inappropriate in the attempts to make methanol from methane.

The other work on use of catalysts for direct methanol production has

used low pressures (preactants c 1 bar) and low conversions of methane (<10X)

in an attempt to maintain reasonable selectivity, although even the best

results are not encouraging. Kowalak and Moffat (ref. 39) have produced some

interesting results by using a highly-acidic mordenite. They believed that

they could encourage activation of methane through protonation :

CH, + H+ + [CH,]’ + CH,’ + H, (14)

To this end, they used mordenite in the H-form (HM) and increased the acidity

by incorporation of fluorine (HXF). The Hammett acidity function of the H?iF

form was measured to be -13, well in the ‘superacid’ region.

Selectivity was encouraged by the use of nitrous oxide, rather than

dioxygen as oxidant, and appreciable conversion of methane occurred at

temperatures as low as 350 - 425aC. Below 400°C, there was little formation

of oxides of carbon, but most of the products were due.to oxidative coupling,

C,H,, C,H,, C,/C, : methanol did, however, constitute 10% of the products.

The distribution of products was similar for both HM and HMF mordenites, the

main influence of fluorination being to increase reactivity, although it

should be noted that conversion of methane was only -1.0% at 425’C for HMF.

If the temperature was taken above this value, formation of CO + CO, increased

rapidly and dominated the products.

The authors suggest that there is a link between superacid sites and

methanol formation, but a reaction mechanism beyond the initial activation

step was not put forward. One has to comment, however, that a radical

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mechanism cannot be dismissed. The bulk of the products are those of

oligomerisation of CH, and the relatively lov reaction temperature ensures

that oxygenated radical species, such as CH,O, are sufficiently stable to

participate in the reaction scheme.

Il’chenko and colleagues (ref. 40) have also examined mordenites as

partial oxidation catalysts. They used dioxygen as oxidant at much higher

temperatures (700°C) than Kovalak and Hoffatt. As a result, methane

conversions were of the order of 15 - 25% and the bulk of the products was

co + co, (70 - 80%). vhile C,H, +‘C,H, (12 - 25%) made up the rest. However,

even under these conditions, very small amounts of methanol (0.1 - 0.9%) were

found, together vith larger amounts (0.8 - 3.0%) of formaldehyde. At the

lowest temperature (600aC) investigated, the C, hydrocarbon yield fell to

negligible amounts, while the selectivity of methanol and formaldehyde

increased somewhat, CO + CO, remaining dominant, however.

It is difficult to compare these two sets of results. The Russian group

make no suggestions as to how the oxygenates are formed, although they do

comment that de-alumination of mordenites increased the yield of oxygenated

product. Their principal interest does, however, seem to have been in the

oxidative coupling products.

Anderson and Tsai (ref. 41) have also investigated the possibilities of

zeolites for partial oxidation but, in this case, one of low acidity, to

discourage the further conversion of methanol to aromatic hydrocarbons. A

copper-exchanged FeZSM5 zeolite, where most of the structural Al”+ ions had

been replaced by Fe’*, was found to have the appropriate properties. Nitrous

oxide was, again, used to improve selectivity and, at 510 - 615 K conversions

of 0.25 - 1.52% CH,, with selectivities of 78 - 38% MeOH were obtained,

together with small amounts of formaldehyde, the balance being oxides of

carbon. Small amounts of C, and C, olefin were also observed, and the authors

comment that failure to convert these to aromatics was characteristic of the

very low acidity of the Cu-FeZSM5. The formation of methanol was unique to

this combination, which Anderson and Tsai attribute to some kind of

synergism.

The earlier vork of Somorjai (ref. 42) and of Lunsford (ref. 43) has

already been discussed at length in the previous reviews but it may be

mentioned that they investigated MO- and V-based oxide catalysts, to obtain

methanol and formaldehyde at similar levels of conversion and selectivity to

the more recent work received above. Nitrous oxide was used as oxidant.

A rather different catalyst system was used by Il’chenko et (ref.44),

who exchanged ionisable groups on a Russian carbon (AP-3) with a whole series

of transitional metal ions. As seems to be invariably the case, the least

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active catalyst gave the best selectivity for reaction at 773 K, with a

CH,/N,O ratio of 1.33, diluted substantially with helium. Tib+-exchanged

carbon gave 1.8% conversion of CH,, with a selectivity of 13.8% of methanol -

no formaldehyde - the remaining products being CO,. Cr’*, Mn*+, Fez* and Fe’*

gave similar yields of methanol, but higher conversions and lower

selectivities; Zn** and Bi*+ gave smaller amounts of methanol, while Cu2* gave

equal (small) amounts of methanol and formaldehyde.

Production of formaldehyde

Formaldehyde is produced during incomplete combustion of methane and has

been reported in trace quantities during studies on the catalysed complete

oxidation. Industrially, direct conversion of methane to formaldehyde in a

single stage would be a desirable goal as, currently, around a half of the

World’s methanol production is converted to formaldehyde for use in the paint

and polymer industries.

However, the progress towards a yield that would be industrially

attractive has been very slow and, with one or two exceptions, the position

remains much the same as when Pitchai and Klier reviewed the subject in 1986

(ref. 3). Sinev et (ref. 9), with the extensive Russian work in mind,

remark that the practical useful yields on stable catalysts do not exceed 5 -

8% for formaldehyde, or 30% for methanol, unless gas-phase inhibitors

(generally halogen-containing compounds) are added.

Nearly all published work has used oxide catalysts. Pure metals are

generally felt to be too specific towards complete oxidation to be of use, but

one has to remember that Cullis, Keene and Trimm (ref. 45). using Pd/ThO,, and

Mann and Dosi (ref. 46) with Pd/Al,O,, were able to inhibit the complete

oxidation almost completely by adding halocarbons to the CH,/O, feed. At the

same time, the residual activity was very largely for formaldehyde production,

with yields of 7.5% HCHO at 34% (ref. 46) selectivity. This work was

discussed extensively by Pitchai and Klier (ref. 3). and it is a little

surprising to find that this type of modification of the catalytic activity of

metal surfaces has not been pursued further,

In view of the fact that standard industrial catalysts for oxidising

methanol to formaldehyde are based either on silver or on the Fe/MO/oxide

system (ref. 47). it is not surprising that several workers have incorporated

MO, Cr and its congener, W, into their experimental catalysts. However,

silica and other oxides, for which changes in valence state are not easy, have

also been used with some success.

319

Otsuka and Hatano (ref. 48) have measured the activity of a range of

oxides, and tried to correlate it with the electronegativity of the respective

cation. The pointed out that a conceptual scheme for methane oxidation shown

in Fig. 2 required abstraction of hydrogen in step 1. but insertion of oxygen

is step 2, which would imply quite different properties in catalytically-

active sites.

CH+ CH,_x$-HCHO -co,co *

Fig. 2. Reaction sequence for methane oxidation, proposed by Otsuka and Hatano (ref. 48)

One might, therefore, expect that a compromise oxide with sites having a

little of the character needed for both steps might be best. Thus, the

highest activity for methane conversion to ail products, including CO and CO,,

vas at a maximum for Ca,O, and Bi,O,, which lay in the middle of the

electronegativity plot. On the other hand, maximising HCHO yield implies

minimising the rate of step 3, compared with step 2. and the selectivity for

HCHO was encouraged by the most highly-electronegative oxides, those of W, B

and P.

Arguing on these semi-empirical lines, Otsuka and Hatano developed a

binary oxide mixture of Be with B supported on silica. At 873 K and a ratio

of CH,/O, 3.0, they obtained up to 1% yield of HCHO with a conversion of

2.8% methane for a W/F of 0.42 g. hr. L-l, a not untypical value for oxide

catalysts.

Some selected recent data are gathered together in Table 2. In most

cases, where a range of results is quoted, the best data are used i.e. those

with the highest selectivity or best yield. As can be seen, there is a

considerable range of conditions with CH,/oxidant stoichiometry varying

between 3 - 0.2, although temperatures have been kept within the range 723 -

973 K as at the lover value reaction is extremely slow while above 873 K, CO

and CO, formation tends to become much more dominant. Schvank’s results

(ref. 49) are exceptional here because of the high temperatures used, but this

is touched on later. The pseudo-contact time (mass/flow) also clusters in a

relatively narrow range spanning an order of magnitude, with the exception of

some work (ref. 49) on Cr,O,/Ai,O,, where an extremely high mass/flow ratio

was used, probably because of the low temperature employed (593 K).

1.0 813 0.19

1.0 m 0.23

1.0 m 0.B

1.0 123 0.56

1.0 m 0.56

0.66 2 0.33 0.66 0.33 E z E 0.66 863 0.33

1.0

t

1.m 73 2: 1.0 787 2.2

1.0

2: z ::: 1.0 z :::

0.0 “.a.

I I U.0

2.9 n... al.0

0.08 “A. a.0

0.45 “4. 8.0

f---t-

E . . :.:. li ::: * * i.:. El 3.0 n.a. 17

1.0 2: 8.4 :

0.7 1.3 10

2 ::: : 3.3 5.0

~

1.5 6.2 :

0.26 “A.

0.95 n.,. 2 0.13 . . 27 0.13 Ft.:. 0.31 n.s. El

n... ::: n.a.

48 24

I I

‘.O -I G” I 5 1.0 . . I I 15 4.t I. 6.4 n.r. 2 ::: . . K 2 0.7 . . 3.7 :.:. 2: 1.2 “4. 4.6 +F “.a.

;:: n.* 11 ..a: E

82

0.5 “.a. 66.1

H

0.6 ( Kz

0.34

0.6 0.P hdar) 02 Infed

O.Ol

0.66 I.45

::: 0.52

2 -I 1.3

l.2 3.6

- hf.

-

18

-

Es?

-

68

-

a

- 69

-

56

-

n

-

52

-

YI

-

55

-

58

-

10

-

69

-

64

-

321

On the whole, the relatively restrictive conditions used represent the

necessary compromise between the energy needed to activate the methane

molecule and the need to minimise over-oxidation of desirable products. In

only a few cases do the yields of formaldehyde (mole per cent of methane

converted to desired product) exceed I - 2 per cent. Tabie 2 demonstrates how

attempts to force higher yields by increasing methane conversion is, in most

cases, self-defeating, as the selectivity falls sharply under such

conditions.

The use of nitrous oxide as oxidant requires some comment. Just as in

the case of attempted methanol formation , it is hoped that the possibly milder

oxidising power of surface oxides, compared to that of dioxygen, may tend to

partially oxidised products, There is very slight evidence to support these

hopes. Molybdenum oxide-containing catalysts have been used almost

exclusively with N,O as oxidant. In the work of Spencer (refs. 51,52), vhere

he used a MOO, catalyst supported on silica of very low Na content, he obtains

rather higher yields of HCHO with oxygen that any of the other groups do with

N,O. It may be pertinent to note that Barbaux _et (ref. 53) have commented

that NE0 does actually give O- species on MOO, and that these lead to total

oxidation : Ox-, on the other hand, was active for partial oxidation to

formaldehyde.

In a contribution by Krupa et (ref. 541, N,O was produced in situ by

co-feeding NH,, 0, and CH, into two reactors in series. In the first, run at

523 K, the catalyst converts the ammonia into N,O, the reaction mixture then

passing through the second reactor at 673 - 723 K. The authors claim that

with a V,O,/SiO, catalyst, the yield of HCHO is much greater compared with the

direct oxidation reaction of 0, vith methane. The degree of conversion at

723 K is only 0.1% CH,, however, and despite the 99.51 selectivity obtained,

the advantage over direct oxidation (ref. 55) is scarcely overwhelming.

Spencer’s work (ref. 52) on oxidation of CH,fO, mixtures over a

MoO,/SiO, catalyst to give formaldehyde is among the more successful. He lays

great stress on the purity of the silica used as support, and especially the

sodium level, vhich should be reduced as much as possible, preferably <lo ppm.

[ref. 51). The results show very clearly how selectivity to formaldehyde is

inversely proportional to methane conversion (Fig. 3).

322

,.(_...,.. . . . . . . . . . . . . . . ..-f 0 1 2 3 4 5 6 Y

% Methane Conversion

(a) (b)

Fig. 3. Selectivity/conversion relationship for formation of HCHO during oxidation of methane with oxygen at different temperatures : (a) Catalyst MoO,/SiO, (ref. 52). (b) Catalyst V,O,/SiO, (ref. 55)

In the case of MoO,fSiO,, the selectivity/conversion data could be

plotted on a common curve, being independent of space velocity, and only

slightly temperature-dependent (Figure 3(a)). By contrast, a silica-supported

V,O, catalyst showed a marked dependence on temperature (Fig 3(b)).

A number of authors have pointed out that silica itself has measurable

activity for conversion of methane to formaldehyde , albeit at a lower level of

activity than most other catalysts. KaszteIan and Moffat (ref. 561 have shown

that up to 4.5% of the methane co-fed with 0, can be converted at 866 K at

relatively high contact times, but the selectivity is poor (8X). An

interesting point from their vork is that substituting oxygen by N,O gave no

formaldehyde. Schwank et (ref. 57) noted that the Vycor, or quartz walls

of the reactor tubing, had a discernible activity for formaldehyde formation.

At the higher temperatures they used (s893 K), coupled products (mainly C,H.)

were observed also, vhich points to a significant production of CH, radicals.

The activity, but not selectivity, could be enhanced by use of bulk silicic

acid but alumina and magnesia at the same temperature produced only CO and

CO,, confirming the specificity of silica for the partial oxidation.

Quite remarkably good results were obtained by Guliev et (ref. 59)

using two Russian silicas, KSM and KSK-2. Using a rather higher space

velocity than Schwank et (ref. 57) (6000 hr-r compared to -1800), they

obtained much higher conversions at 873 K at reasonable selectivity. The

reasons for this are not clear, although the authors lay great stress on the

surface texture of their silicas and the need to maintain the surface area

above 20 rnp g-1. Carbon atoms deposited on the surface were invoked as being

reaction intermediates.

323

The effect of sodium on reactivity and selectivity of silica and

silica-supported MOO, has already been remarked on. Spencer <ref. 521, found

that sodium had little effect on the behaviour of silica as a catalyst but

reduced the selectivity of HoO,/silica at all levels. MacGiolla Coda et

(ref 591, working on slightly lower temperatures (<873 K) and using N,O as

oxidation, found the effects less clear-cut, suggesting that small levels of

Na* in the support allowed high levels of MOO, in the catalyst without losing

selectivity. The most recent work of Spencer and his co-workers (ref. 60)

suggests that sodium may cause undesirable strong interactions between the

silica and molybdena functions and interfere with the Ho~~/Mo~~ oxygen shuttle

mechanism.

There are three references to work where the yield of formaldehyde

approaches or exceeds 10%. The first of these really uses a manipulation of

reactor design to increase the yield (ref. 61). The catalysts are not

specified explicitly but a relevant Patent (ref. 62) refers to AS-37, a

commercial Russian aluminosilicate catalyst. The work would seem to be a

follow-up of a programme carried out over many years (ref. 8). Although

conversion per pass is probably as low as for many of the catalysts reviewed

above, the products are extensively recycled but with CO and CO, being

selectively removed, as these are shown to depress both conversion and yield.

At the relatively high temperature of 973 K and an overall space velocity of .

8000 hr-‘, a recycle ratio of 50 increased the yield of HCHO from -2% to just

under 38%. When CO and CO, were removed from the recycled gas, the yield of

formaldehyde rose to 42% for the same recycle ratio, while the so-called

reactor efficiency (gHCHO/volume catalyst/hour) increased greatly. Whether

such extensive recycle could form the basis of a commercial process, is,

however, a different matter.

The outstanding conversion and yield for a single pass is that reported

by Kastanas. Tsigdinos and Schwank (ref. 49). Both the non-stoichiometric

iron molybdate Fe,O,(MoO,),.,, and WO, gave HCHO in high selectivity, although

the former was much more active for methane conversion. Pressures between 3 -

5 bar were used, although the oxygen was heavily diluted (1:9) with argon.

Optimum temperatures were -7OWC. Other catalysts, notably Pe,O, and MOO,,

gave much inferior results, the former leading almost exclusively to total

oxidation. Subsequent investigation suggests that although catalysts of the

correct formulation are vital to obtain good yields, the reaction may be

partially occurring in the gas phase in a quasi-cool-flame region (ref. 64).

The last reference to catalysts giving high yields of formaldehyde appear

in a Japanese patent (ref. 64). This is somewhat economical with details but

describes catalysts giving a selectivity of 66% at a conversion of 10.5%

methane at 600°C. The patentees lay stress on the part that water plays in

324

achieving high selectivities, without suggesting what the mechanism is by

which it does this. For example, the results given above can only be obtained

by saturating the methane/air reaction mixture with water vapour at room

temperature, The best catalyst was a mixture of the oxides (10% v/v of each)

of Fe (IIX), Ni (II), Bi, Mg and MO, the remainder of the catalyst (50% v/w)

being a silica support. MO oxides are implied to be the active phase in the

sense that a similar catalyst, without the MOO,, gave only 1.5% methane

conversion at 90.3% selectivity under the same reaction conditions.

Two interesting preliminary communications have recently appeared, which

my lead to new avenues of research. In the first, Amir Ebrahimi and Rooney

(ref. 65) examine, de novo, the problem of developing partial oxidation

catalysts and combining the ideas of methyl radical generation from an Maa0

centre with elements of olefin metathesis catalysts. The result was a series

of catalysts, based on MoCl,/R,Sn, where R is an alkyl group, supported on

silica which plays an important part in the reaction. The best of the series

of catalysts had exceptional activity for formaldehyde formation from methane,

giving 20% conversion with 80% selectivity at 700°C using a 1~1 methane/air

mixture, a conversion rate of 5.35 m mof, CH,/hour.

The second paper (ref. 66) used a combination of excitation by UV light

and moderate temperatures to obtain a very selective oxidation of methane TV

formaldehyde over a 5% MoO,fSiO, catalyst. As low a temperature as 450 R gave

a yield of -5 nmol. HCHO/hour, no oxides of carbon being observed. Other

reaction details were WfF 0.62 g. hr. mol-1, CH,fO, ratio 3.0. Under these

conditions, some methanol, about 3% of the formaldehyde, was noted. This

HCHO yield corresponds to 0.2 m mol. hr-* g-1 of catalyst and compares with a

figure of 0.07 nmoi. hr-1. g-1 for a 3X MoO,lSiO, catalyst under conventional,

non-activated conditions at 843 K (ref. 67) or -3 m mol. hr-1 g-1, using 0,

over a 1.8% Mo/SiO, catalyst at 923 R (ref. 52).

Mechanism of partial oxidation

We have already pointed out that the mechanism for the homogeneous gas

phase reaction that leads to oxygenated species, and especially methanol, is

fairfy well understood, although some of the kinetic data for some of the

steps may yet need further refinement. The initiating step :

CH, + 0, + CH, f HO, t 11

becomes more important as the pressure is increased, partly because of the

increased mass action effect, and partly because surface reactions become

relatively less important. Thereafter, the chain propagation reactions

proceed, controlling the overall chemistry, as shown in Fig. 4.

325

CH 4

WWW02~02

Fig. 4. Schematic block diagram of the methane oxidation mechanism.

It is difficult to make direct comparisons of the gas phase work with the

heterogeneous because there is both a ‘pressure gap’ and a ‘temperature gap’.

Only, in a very few cases has catalytic work been done at other than

sub-atmospheric pressures and, in order to obtain measurable conversions, the

minimum temperature used has generally been >6OO*C (873 K), far above the

highest temperatures used in purely gas phase partial oxidation.

Where catalysts have been included in the reaction volume at high

pressure (refs. 15,21,?2), the results have been inferior to those obtained in

their absence, in every case. This can be attributed to the over-oxidation of

the desired product (ref. 15).

A ‘virtual mechanism’ for selective oxidation of methane vas presented in

I968 by Dowden, Schnell and Walker (ref. 711, shown in Fig. 5.

CH 4teci-” (fH3 (+ ~&+CO’+ CO,

ads ads 1

P

HCHO,,, + OH

A- I

I ads

0 -0orO 0 Z(Q)

I II OCH, 0 I I

ads ads ads ads ads

Fig. 5. ‘Virtual mechanism* for partial oxidation of methane over oxide catalyst (slightly adapted from Dowden et al (ref. 71).

326

This is a classical Langmuir-Hinshelwood mechanism , where all the reactions

take place on the surface. The authors point out that the dissociation of

methane and the activation (and probable dissociation of dioxygen) require

quite different sites, a point re-stated slightly differently by Otsuka and

Hatano (ref. 48) more recently. This led them to suggest the use of

bi-functional catalysts subsequently developed by both them (ref. 72) and

Stroud (ref. 73). with some slight success.

Consideration of the Dowden mechanism suggests that the catalytic cycle

could be closed by elimination of water from neighbouring -OH groups, to

re-generate the active sites for oxygen dissociation. The possible

participation of -OCH, as a precursor species for formaldehyde is also a

feature of subsequent work (see review by Sinev et (ref. 9)).

Another feature of the mechanism is that it bypasses the formation of

methanol as an intermediate and, in fact, methanol production would require a

reductive interaction of adsorbed hydrogen vith -OCR,. Whether methanol is an

intermediate in formaldehyde production , is still an unresolved question, and

current work does not give a clear answer.

A relatively simple reaction scheme, as in Fig. 5, does not, however,

take account of possible gas phase reactions that may occur. The whole

thinking in the area of methane reactions over surfaces has been drastically

changed by the increasing reaiisation that methyl radicals may be produced

relatively easily on a catalyst, and that they may desorb to react further in

the gas phase. Driscoll, Campbell and Lunsford (ref. 74) have reviewed

progress in the field and pointed out that quite reasonable yields of CR, may

be trapped out from the gas phase by passing methane over oxides, like MgO, at

temperatures as lov as 4OO’C. For example, Lin et al (ref. 75) have used _

REHPI (Resonance-Enhanced Multi-photon Ionisation) to detect CH, generated

over Li/MgO surfaces at 400 - 6OO*C. The apparent activation energy for CH,

formation depends on the concentration of Li sites but is considerably lower

than the equivalent value (206 kJ. mol-1) for the gas phase reaction (1). The

CH, radicals react with each other to form coupled products (C, and higher

hydrocarbons) observed at temperatures >700°C. Garibyan and Margolis

(ref. 76) have also emphasised the important rBle of radicals, both on the

surface and in the gas phase in the nominally heterogeneous oxidation of

hydrocarbons, including methane.

If the oxide surface, besides generating methyl radicals, can activate

dioxygen according to the Dowden mechanism, then the gas phase CH, may react

with Oads -species by a Rideal-Eley mechanism, to produce desired products or,

more often, oxides of carbon.

321

We do, therefore, have to consider possible gas phase reactions that may

occur in the region 600 - 7OO’C. Over this temperature range, both the gas

phase and surface initiation processes may occur :

CH, c 0, + CH, + HO,

‘X i- COlsurf + CH, f COHlsurf

In the case of gas phase initiation , the radicals ideally appear spontaneously

over the vhole reaction volume. For surface initiation, however, the

relatively low rate of diffusion of the radicals, coupled with their high

reactivity, may ensure that all subsequent reactions take place at a boundary

layer close to the surface.

Once formed, methyl radicals may react in different ways, according to

the pressure, temperature and oxygen concentration. The flow diagram (Fig. 4)

emphasises the vital rale of the equilibrium :

CH, + 0, * CH,O, ( 4)

as this controls the concentration of the methylperoxy radicals and, hence,

the whole course of subsequent reactions of oxygen-containing radicals

that may lead to methanol and formaldehyde. The value of the equilibrium

constant, K, for equation (4) is strongly dependent on temperature, the

logarithm being 30.4 atm-i at 300 K, 16.4 at 500 K and 10.6 at 700 K.

Thus, for T ) 700 K, the equilibrium increasingly begins to favour the

left-hand side and the concentration of methyl radicals increases to the

extent that bimolecular reactions lead to the coupled products C,H,, C,H, and

higher homologues, at the expense of oxygenated products.

To extend the reaction scheme shown in Fig. 4 to try to take into account

reactions which may occur on the catalyst surface, involving more longer-lived

species, requires an understanding of the possible processes that will take

place on the catalyst. This information is not readily available, although

highly desirable. Approximate treatments do exist and combustion chemists

have been including such reactions in models for some time. The fates of

methyl hydroperoxide, hydrogen peroxide, methylperoxy radical and hydroperoxy

radicals are of particular importance. Most combustion studies assume surface

destruction of these species in a termination-type step, and few try to

characterise the actual products. We suggest that studies of radical/surface

interactions is an area where more work needs to be done.

328

The amount of mechanistic work done on surface processes has been limited

but some common features are discernible for Ho-containing catalysts. AS

already mentioned, N,Q has tended to be used to attempt to control

over-oxidation of products. It is generally believed to form 0- species by

dissociative adsorption but there seems to be a consensus that this leads to

CO and CO, production (refs. 53,67,77). Work by Yang and Lunsford (ref. 78)

on the catalysed oxidation of methanol to formaldehyde, over MoO,/SiO,, shows

that N,O is actually far less selective than 0,. The catalysts vere very

active for methanol oxidation, especially with O,, and converted >90% of the

reactant at 350°C : the conversion rate at 203*C vas 14.5 m mol. hr-1 g-l, SO

it is not surprising that methanol is not observed as an intermediate at 600°C

in the oxidation of methane to formaldehyde. Yang and Lunsford suggested a

mechanism whereby a switch in valence between MO ‘“/Mo”L was responsible for

the oxidation of methanol to formaldehyde, the species for this oxidation step

being a bridging-oxygen, formally corresponding to O*-, between two Ho(V)

atoms. (Fig. 6).

“\\ /-go\ do MO

0 / \ /Mo<

0 0

I CH,OH

Fig. 6. Interaction of methanol molecules with MOO, catalyst surface. (ref. 78)

The other feature that seems common to alf the work on MO-containing

catalysts is the attempted correlation of structure with selectivity-

Kasztelan and Moffat (refs. 67,771 have published a series of papers on

molybdenum in the form of phosphomolybdic and silicomolybdic acids. For

ordinary Moo,-catalysts, supported on silica, both they and Barbaux et

(ref. 53) have correlated increasing selectivity with the formation of

silicomolybdates (SMA) on HoO,/SiO, catalysts, as characterised by XPS and

329

Raman spectroscopy. They attribute the increasing selectivity to

stabilisation on the SMA-phase of the the desirable Oz- species, as opposed to

0- on bulk MOO,, which leads to CO and CO, formation. Spencer (ref. 52) has

pointed out that, at high space velocities, HCHO and CO, are the primary

products of the methane and oxygen reaction, CO being a secondary product,

together with traces of methanoi. He was unabfe to say whether methanol was a

reaction intermediate on the way to formaldehyde or not. A simplified scheme

of the overall reaction is shown in Fig. 7.

02 CH 4- [CH,OH] -

1 k, HCHOy CO

2

ic oxidat Fig. 7. Reaction scheme for catalyt catalysts (ref. 52).

Data were determined for the rate constants k %, k, and k,, which, taken into

consideration with the Thiele modulus, were able to predict the selectivity

towards different products at any degree of methane conversion (ref. 79).

STRATEGIC AND PROCESS IMPIJCATIONS

Economic assessments

As was stated at the beginning of this review, much of the driving-force

ion of methane over MoO,/SiO,

for work in this area has come from the perceived need to an alternative for

the steam reforming with synthesis route to methanol, or other products. For

example, Shell’s recently-announced process for middle distillates (SMDS),

which is currently under construction in Malaysia, uses a relatively

non-selective Fischer-Tropsch stage with conditions and catalyst selected to

encourage hydrocarbon chain growth, followed by a highly-selective

hydrocracking catalyst, to produce the desired kerosine-type product.

(ref. 60). Nonetheless, the first stage is still syngas production from

natural gas, in this case, by partial oxidation rather than steam reforming.

The Shell process offers an alternative to steam reforming, followed by

methanol synthesis, and then Mobil’s methanol to distillate stage, using a

wider-pore zeolite catalyst than ZSM5.

330

A process that could produce methanol in one stage directly from methane

would look attractive, at least superficially, and Edwards and Foster

(ref El> set out to try to quantify possible advantages. They compared the

cost of a conventional methanol plant using, as feed, a rather expensive

source of natural gas ($3.5 MJ-*I with that of a conceptual plant, using a

catalytic partial oxidation reactor producing methanol in one stage. They

assumed that such a stage might run at 10 MPa (100 bar), 4OO*C, and with a

conversion of 10% methane per pass. They assumed, also, that the heat of

oxidation could be removed from the reactor by conventional means, as it is

greater than, but of the same order as, the beat of the synthesis of methanol.

Pure oxygen was used as oxidant : Edwards and Foster dismissed the use of air

because of the build-up of nitrogen in the recycle loop, and of nitrous oxide

as being quite unrealistically expensive for commercial purposes.

One important feature that arose from this, admittedly approximate,

cost-study was the critical importance of selectivity in the methane stage,

the other side-reaction being, of course, the total oxidation of methane to

co,. With 100% selectivity, the cost of methanol was calculated to be

$172. tonne-z, as opposed to $248 tonne-z, for the conventional route. The

cost rose more or less linearly as selectivity fell until, at a value of 77X,

on the assumptions made in the study, the costs of the two routes were equal.

The extra costs arise from the need to provide plant for CO, removal

(Benfield process), increased heat duty on the converter stage, falling

thermal efficiency leading to increased natural gas consnmption, and increased

energy demand to remove water from the methanol process.

It is immediately clear that no catalyst yet investigated for the partial

oxidation of methane to methanol, comes any way near the conversion/

selectivity requirements postulated by Edwards and Foster, nor do they give

much prospect of being developed to do so. On the other hand, the best

published results of Hunter, Gesser and co-workers (refs. 14,171 for the

homogeneous reaction do have some promise, and have been used as the basis of

an evaluation by Kuo and Ketkar of Mobil R and D in a study sponsored by the

U.S. Department of Energy (ref. 81). They compared three cases : one using

the conventional route to methanol, one using the Ilunter-Gesser data for a

direct partial oxidation process (DPOM) and, Finally, the oxidative coupling

of methane (OCM) to athene and hence to gasoline.

They concluded that, on the basis of the Hunter-Gesser data, 7.5%

conversion of methane per pass at 90% selectivity at 50 bar pressure, the DPOM

route showed considerable savings over the other two. The thermal efficiency

would be 70X, compared to 65X for the conventional route and the investment in

heat transfer plant needed to be under a halE. Kuo and Ketkar, like their

331

Australian counterparts, stressed the sensitivity of process cost to

selectivity in the partial oxidation process. They concluded by recommending

further work to confirm the Hunter-Cesser data before proceeding further. In

the absence of such confirmation, further work remains in abeyance.

It may be worth remarking that the use of pure oxygen supplied from an

air liquefaction plant is not excessively expensive. Edwards and Foster

costed it at +30X of the total fixed costs of their conceptual partial

oxidation process and there are prospects of reducing this by future

developments in separation technology.

Despite the reservations described here , the direct oxidation route has

had some application on a development scale at a remote gas field in Russia

(refs. 83.84). The plant is a flow-reactor with a capacity of 200 tonnes/

year. The reactor operates at pressures up to 100 atm, with oxygen present to

a level of 2.5 - 3.0X, and methanol selectivity typically between 35 - 50%

(with total selectivities to liquid organic products being over 60%). These

results fit qualitatively with the flow-tube reactor studies of Burch et al

(ref. 15). The methanol produced by this process is fed directly into the

natural gas pipeline and is used to prevent methane hydrate formation.

The potential economics of other routes seem to be so unquantifiable at

the present time as not to be worth the attempt. This is especially so for

photocatalytic processes which, despite very promising preliminary results

(ref. 66), are totally dependent on commercial developments of applications of

visible/UV light. Especially, one feels that the potential of this type of

process will become clearer as some of the present research on harnessing

solar power comes to fruition.

Future prospects and research areas

It is clear from progress, so far, that there are no catalysts in

prospect that will convert methane to methanol in high yield (say, 10%) in a

single pass. The homogeneous oxidation at high pressures still holds the best

prospects for future process developments, but confirmatory laboratory data

are still needed. In the absence of a breakthrough in present technology,

thoughts are now turning to novel reactor designs where low conversions at

high selectivity may be more effectively used.

At least two such projects are in their initial stages. In one, workers

at Bath University, U.K., Department of Chemical Engineering, are attempting

to apply the concept of a Pressure Swing Reactor to enhance methanol yields.

As the name implies, the reactor operates under an alternating pressure

regime, initially at high pressure in the presence of an oxidising catalyst

which has some proven performance (refs. 72,73), for methanol formation. The

332

reactor will also contain a zeolite. which would preferentially adsorb the

desired product, from which the methanol might be recovered on the

depressurisation stage of the cycle. A second proposal, from the Department

of Chemical Engineering at the University of Colorado, U.S.A., envisages using

a porous membrane reactor (ref. 85) to separate the methanol as it is

produced. A ceramic tube with pores of controlled size between 4 - 5 nm is

proposed. In one variant, a mixed oxide catalyst, similar to that envisaged in

the Bath work, would be coated on the inside of the tube : methanol would

migrate preferentially to the outside of the tube by surface diffusion and be

collected continuously. Of course, no catalyst at all need be used, as the

reaction rates reported for the purely homogeneous reaction (ref. 14) are very

high.

Both these products are at a very early stage and no results are yet

available, but it would certainly seem that this will be the direction that

others are also taking.

The catalytic oxidation to formaldehyde looks more encouraging, with two

or three different groups claiming yields of more than 10% per pass at high

selectivity. These offer opportunities for exploitation and optimising yields

but it is clear that obtaining good results is not easy and may require very

careful and critical design of reactors. Schwank eE (ref. 631, for

example, obtain their results only on carefully-prepared catalysts and using a

reactor which quenches the reaction products immediately after contacting the

catalyst.

ACKNOWLEDGEMENTS

The Authors would like to thank British Gas plc for permission to publish

this paper. They also acknowledge helpful discussions with Professor R. Burch

and with Professor J. Schwank. who provided details of unpublished results.

Dr. N.D. Spencer also provided material in advance of publication.

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