to transformthe methane oxidation to methanol

Fuel Processing Technology

journalhomepage: www.elsevier.com/locate/fuproc

Review

Synthesis of methanol from methane: Challenges and advances on the multi-step (syngas) and one-step routes (DMTM)

Marcio Jose da Silva

Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais 36570-000, Brazil

article info

abstract

Article history: To transform the methane oxidation to methanol in a selective, straight, economically attractive, and less energy- Received 26 October 2015

intense process is a goal pursued by the industry since its discovery. Methane is the main constituent of shale and Received in revised form 13 January 2016

natural gas while methanol is either a fuel as feedstock in the chemical industry. Thus, to develop a technology Accepted 19 January 2016

that combines an affordable raw material with a strategic product became pivotally important for the chemical Available online xxxx

industry. Currently, the industrial route for methanol production from methane is accomplished via a syngas pro- Keywords:

cess, passing by stream catalytic reform of products (i.e., CO and H 2 ). This is a costly route due to its high-energy Methane

consumption. Alternatively, methane partial oxidation to methanol (i.e. DMTM route) in a single step can consti- Methanol

tute a more economically viable strategy. Toward achieving this goal, different approaches were proposed; none- Syngas

theless, until now, any was industrially feasible. In this review, we paid particular attention to the methane direct Natural gas

oxidation processes to methanol carried out in a gas phase under homogeneous or heterogeneous conditions. In Solid catalysts

general, heterogeneous processes are solid-catalyzed in the gas phase, while homogeneous processes occur with- out a catalyst. We too assessed the advances achieved in the traditional route to producing methanol from syngas, as well as recent developments of syngas production from methane.

© 2016 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

1.1. Natural gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2. Syngas-based multistep process for the production of methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.1. A brief overview of methane conversion to syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3. Oxidation of methane to methanol in a multistep processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.1. Brief background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2. Conventional production route of methanol from syngas: from BASF to Synetix process . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.3. Cu–Zn/Al 2 O 3 catalyst: Synetix process for methanol production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.4. Main challenges of the conventional production process of methanol from syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4. Oxidation of methane to methanol in a one-step process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.1. Main challenges of direct methane partial direct oxidation to methanol routes (DMTM routes) . . . . . . . . . . . . . . . . . . . . . . 49

4.2. Activation of C–H bonds by enzymatic complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.3. DMTM route (i): free-catalyst process of methane direct partial oxidation to methanol by oxygen (i.e. homogeneous process) . . . . . . . . 50

4.3.1. The use of other oxidants in catalyst-free process of methane direct partial oxidation to methanol . . . . . . . . . . . . . . . . . 50

4.4. DMTM route (ii): solid-catalyzed processes of direct oxidation of methane to methanol . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.4.1. Copper–zinc/alumina catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.4.2. Zeolite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.4.3. Molybdenum catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.4.4. Iron sodalite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.4.5. Other solid catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

E-mail address: silvamj2003@ufv.br .

http://dx.doi.org/10.1016/j.fuproc.2016.01.023 0378-3820/© 2016 Elsevier B.V. All rights reserved.

M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Fig. 1. Conversion route of natural gas to chemicals and fuels [3] .

1. Introduction highly accessible consumed it as fuel for industrial and domestic heating, or electric power generation. Consequently, almost all methane

1.1. Natural gas produced has become an energy source [4] . Rather than be burned as fuel, natural gas is potentially useful as Natural gas is a clean and effective energy source because its com-

an industrial feedstock. Although there are different approaches to bustion generates fewer greenhouse gases than coal or petroleum liquid

using the methane present in the natural gas as a raw material, the fuels. However, to store and transport natural gas from remote sites up-

most explored and economically viable is its transformation to syn- holds a great challenge, making it difficult to make the prices of its de-

gas (i.e., CO and H 2 ). Industrially, methane has been upgraded to rivatives (i.e., chemicals or liquid fuels) competitive in relation to

syngas throughout multistep processes that use steam and/or those of fossil oil [1] . Nevertheless, the inevitable depletion of petroleum

oxygen (i.e., steam reforming, autothermic reforming, or partial oxi- reserves and the recent discovery of natural gas sites may make it more

dation) [5] .

economically attractive, and fulfill society's demand for alternative en- As shown in Fig. 1 , syngas is a starting material to valuable chemicals ergy sources [2] .

(i.e., dimethyl ether, formaldehyde, acetic acid) or liquid fuels (i.e., via Different from shale gas, which contains other hydrocarbons, the

Fischer–Tropsch catalysis). Nevertheless, the most attractive routes natural gas comprises only methane as a major component. Currently,

are those that involve a single step, such as oxidative coupling of meth- the most of the methane is consumed as fuel, resulting in greenhouse

ane (i.e., OCM), homologation, aromatization, and methane partial oxi- gas (i.e., CO and CO 2 ). Indeed, methane itself is too more deleterious

dation to methanol (i.e., DMTM). This latter constitutes the focus of this molecule to the atmosphere, and also contributes to the greenhouse ef-

review.

fect. Nonetheless, it can also provide clean fuels, such as hydrogen or In this work, we paid special attention to two processes of methanol methanol.

production from methane: the first, the syngas-based multistep pro- Methanol itself can be converted to gasoline and valuable fine

cess; and the second, the single-step process, which directly transforms chemicals throughout catalytic processes ( Fig. 1 ) [3] . Natural gas con-

methane into methanol (i.e. DMTM route). sumption in this century has exponentially increased compared to other energy sources ( Fig. 2 ) [4] . Indeed, countries, where methane is

2. Syngas-based multistep process for the production of methanol

2.1. A brief overview of methane conversion to syngas After exhaustive research, Green et al. grouped three main types of

catalysts as the more effective for the conversion of methane to syngas: (1) supported nickel, cobalt, or iron catalysts, (2) supported noble metal and (3) transition metal carbide catalysts [6,7] . However, herein we will focus only on supported Ni catalysts.

The reforming of methane over Ni catalysts with steam, carbon diox- ide, or oxygen, separately, generates syngas at different stoichiometries [8] . Table 1 displays these reactions and their enthalpy obtained at 1173 K [8] .

Currently, the syngas route has been the only economically viable process for the conversion of methane to chemicals or high value-

Table 1

Process for syngas production from natural or shale gas a .

Process

Reaction

H 2 :CO molar ΔH 1173 K ratio

(kJ mol −1 )

3:1 225.7 Fig. 2. Projection of world energy consumption by fuel type 2000 to 2020.

Steam reforming

CH 4 +H 2 O→3H 2 + CO

1:1 258.8 Adapted from U.S. Energy Information Administration/International Energy

CO 2 reforming

CH 4 + CO 2 →2H 2 + 2 CO

2:1 − 23.1 Outlook, 2005 [4] .

Oxygen reforming

CH 4 + 0.5 O 2 →2H 2 + CO

a Adapted from Ref. [8] .

44 M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Fig. 3. Production of syngas from traditional methane steam reforming. Adapted from Ref. [10] .

added liquid fuels; nevertheless, syngas production is unquestionably Significant advances in the use of promoters (i.e., B, La, and Rh) have the most expensive step of this production chain ( Fig. 3 ) [8] .

outstandingly improved the activity of Ni catalysts on the syngas gener- The reaction of methane present in the natural or shale gas with

ation process [16] . On the other hand, the replacing of alumina supports steam, oxygen, or a mixture of them, provides syngas at industrial

by ceria, zirconia, and silica, all doped with rhodium nanoparticles, sig- scale ( Fig. 3 ). Lunsford et al. studied the activity of alumina-supported

nificantly enhanced the process of syngas production through steam nickel on conversion of methane to syngas in the temperature ranging

methane reforming [17] .

of 720 to 1173 K [9] . Several alternative routes have tried to circumvent the high costs as- Industrially, in this method, steam reforming of methane over

sociated with large-scale methanol plants. The Topsøe Group developed Ni/−Al 2 O 3 catalyst heated to 1173 K occurs in a primary reactor. The

a technology that employs the concept known as “two-step reforming” unreacted methane is then reformed with oxygen and steam in a sec-

for syngas production ( Fig. 5 ) [1] . This conventional process makes

methanol from syngas since the last century. ( Fig. 3 ) [10] .

ondary reactor, producing a mixture of CO and H 2 in equilibrium

As depicted in the layout of Fig. 5 , this traditional route includes adi- During the next two steps, steam is then added to the syngas gener-

abatic pre-reforming, tubular reforming, and oxygen-blown secondary ated under milder thermal treatment than earlier (ca. 673 K), over iron

reforming. In this process, oxygen is the source for internal combustion oxide or copper catalysts. Throughout these water gas shift stages, the

of hydrocarbons. The two-step reforming process combines a fired tu-

bular reforming and oxygen-fired adiabatic reforming allows obtaining required for the further use of syngas [10,11] . In general, the syngas

addition of steam may then adjust the H 2 :CO molar ratio to ones

syngas at most suitable composition. On methanol synthesis process, stoichiometry is handled toward its use as feedstock for diesel synthesis

the quantity of hydrogen and carbon oxides in the syngas should be

properly balanced, to reduce the consumption per unit methanol pro- (Cu–ZnO/Al 2 O 3 catalyst) [11] . Although not shown, these reaction con-

via Fischer–Tropsch process (Fe 2 O 3 catalyst) or methanol synthesis

duction rate.

ditions produce coke, resulting in catalyst deactivation. The use of Ni In two-step reforming process, the primary reforming produces ex- supported over rare earth oxides or alkaline metals minimizes this un-

cess of hydrogen, while secondary reforming has use this excess hydro- desirable reaction, and improves the lifetime of the catalyst restricting

gen on step of hydrogenation of CO to produce methanol. The the carbon deposit [12–15] .

combination of the two types of reforming creates an energy efficient Alternatively, methane can be reformed with a steam and oxygen

creation of synthesis gas.

mixture heated to a high temperature (ca. 2273 K) without catalyst. These conditions favor radical reaction commonly called as “homoge-

50 bar = 90.7 kJ mol −1 ) neous oxidation” ( Fig. 4 ). The following step occurs in another reactor, where the mixture ob-

CO + 2 H 2 → CH 3 OH (−ΔH 298 K ,

On the other hand, the use of stand-alone autothermal reforming tained is then reformed over Ni catalysts resulting in syngas and water,

(ATR technology) together with a suitable selection of reaction condi- which can be processed under water gas shift reaction conditions.

tions and suitable catalysts can produce methanol with high availability

Fig. 4. Production of syngas from methane autothermal reforming. Adapted from Ref. [18] .

M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Fig. 5. Methanol process layout with Topsøe's two-step reforming. Adapted from Refs. [18, 19] .

conditions and viability of operation. The ATR technology operates at ATR Technology has great advantageous such as avoidance of the supply low steam to carbon molar ratio, thus decreasing the flow through the

or dissipation of thermal energy to or from the reaction [18] . Arguably, plant and diminishing the investment.

this characteristic makes ATR suitable to industrially provide syngas Stand-alone autothermal reforming (ATR) technology merges par-

with high quality in large-scale plants.

tial combustion and catalytic steam reforming in one compact refracto- Stand-alone autothermal reforming (ATR) technology comprises a ry lined reactor to yield syngas for methanol production in large scale.

stand-alone reformer and oxygen-fired reformer. The autothermal

Fig. 6. Syngas production equipped with Topsøe's ATR stand-alone reforming. Adapted from Refs.

46 M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Scheme 1. Equations of reactions to produce methanol and syngas. Adapted from Ref. [31] .

reformer comprises a burner, a combustion zone, and a catalyst bed

IV. The energy releasing should be well employed; in a refractory lined pressure vessel. The burner mixes the feed and

V. It is required that reactants and residual aftermaths can be ade- the oxidant. In the combustion zone, the feed and oxygen are

quately disposed.

burned. The catalyst bed brings the steam reforming and shift conversion of reactions to equilibrium. By adjusting of low molar ratio of steam to

The main positive figures of the stand-alone ATR reforming process carbon, ATR plants can run similar to a two-step reforming plant but

are the high rates of methanol reaction (i.e. due to syngas with high with a single incoming stream.

CO/CO 2 proportion), the low steam throughput and low necessity of Nowadays, ATR technology replaces “two-step reforming process”

steam that reduce investment cost and operation of large scale plants. by a Topsøe's optimized solution, aiming large-scale methanol pro-

duction. The route based on “stand-alone ATR technology” avoids the use of a tubular reformer, required in the old two-step process

3. Oxidation of methane to methanol in a multistep processes ( Fig. 6 ). Topsøe's ATR consists of a fixed bed reactor (i.e. Ni-based on ceramic

3.1. Brief background

rings and pellets), where the reforming process takes place. Therefore, unlike the two step process, in a stand-alone ATR reforming process

Giulio Natta, who won the Nobel Prize in Chemistry in 1963, stated, the natural gas is pre-reformed and straightly sent to an ATR reformer,

“ among the main industrial organic reactions, the synthesis of methanol where a mixer/burner burning all the hydrocarbons with oxygen and

is an outstanding example of practical importance of catalytic process- steam. In the combustion chamber, partial combustion reactions

es” [22] . Undeniably, until the half of the twenty century, most metha- occur, followed by methane steam reforming reaction and shifting of

nol had a natural origin (i.e., waste woods distillation). Conversely, the equilibrium conversion over the catalyst bed. Because tubular reformer

petrochemical industry today is responsible by the entire methanol is not required, in a stand-alone reforming process the addition of

consumed.

steam to the feed stream is drastically decreased [20] . Therefore, Although its high toxicity or lower energy content than other liquid the stand-alone ATR technology operates at low ratio molar steam

hydrocarbon fuels, methanol has employ as a pure liquid fuel or gaso- to carbon.

line blend [3,23] . Moreover, it is also a feedstock to synthesize commod- However, though the syngas generated from stand-alone ATR

ities, such as methyl terc-butyl ether (ca. 28%), formaldehyde (ca. 34%), reforming process is more reactive than that obtained from two-step

acetic acid (ca. 7%), and other chemicals, such as methyl acrylate or sol- reforming, it is less reactive than syngas produced from gasification

vents (ca. 31%) [24–27] . Moreover, MTO (i.e., methane to olefins) and route. For this reason, currently the most of industrial units produce

MTG (i.e., methane to gasoline) processes that produce light olefins or methanol from syngas obtained through gasification process. The low

gasoline, respectively, start from methanol [28,29] . reactivity is not the main challenge of manufacturing methanol from

Industrially, methanol has been produced throughout the syngas syngas obtained via stand-alone ATR. Indeed, the integration between

route, which involves two consecutive steps: the first, in which meth- the reforming units and the methanol synthesis is the greatest hamper.

ane is steam reforming over Ni/Al 2 O 3 catalysts resulting in syngas To overcome these drawbacks, we should considerer some points

( Fig. 4 ), and the second, in which the syngas is then converted to meth- [21] ;

anol over Zn–CuO/Al 2 O 3 catalyst (see the next sections). Currently, few industrial processes synthesize methanol (i.e., Jonhson

Matthey, Lurgi, Mitsubishi, or Kellogg processes; Fig. 5 ). Indeed, all of ratio;

I. The process will operate with syngas having a high CO/CO 2 molar

them employ Zn–CuO/Al 2 O 3 catalyst, and operate under pressures of

50 to 100 atm over a range of temperature of 473 to 573 K. Although temperature;

II. The process is highly exothermal and should be operated at low

are highly intensive-energy routes, this catalytic process produces meth-

III. Side-product formation should be reduced; anol with 99% selectivity and energy efficiency higher than 70% [30] .

Scheme 2. Methanol production from syngas route.

M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

3.2. Conventional production route of methanol from syngas: from BASF to sensitive to poisoning by chloride and mainly sulfur, and thus re- Synetix process

quires pre-purification steps, mainly when the feedstock is the shale gas.

Three equations describe the methanol production through the syn- From a mechanical perspective, the methanol synthesis over Cu– gas route (Eqs. (1)–(3); Scheme 1 ). Methanol synthesis (Eqs. (1) and

ZnO/Al 2 O 3 via the syngas route has many issues that are currently the (2)) is an exothermic process that involves a decreasing number of gas-

subject of some controversy. For instance, identifying the nature of ac- eous mol. Consequently, the reactions carried out at low temperatures

tive sites or the methanol precursor (i.e., CO or CO 2 ) and the main and high pressures reach the maximum conversion [31] .

adsorbed intermediate species, as well as determining the reaction The BASF process (named as a “high pressure process”) convert syn-

pathway or the rate-determining step, are actively discussed topics [37] .

Kinetic studies, besides investigations about the adsorption of sures (ca. 250 to 350 bar) and in a temperature range of 573 to 673 K

gas to methanol over ZnO–Cr 2 O 3 catalysts, operating under high pres-

isotopic-labeled species on the surface of different catalysts could be [32] . This catalyst is highly tolerant to sulfur, a poison present in large

useful to understand the methanol formation from the CO 2 and H 2 amounts in shale gas, which was a feedstock widely used in the begin-

adsorbed in the catalyst surface [38–40] . The combination of these ning of the twentieth century.

adsorbed molecules provides the adsorbed format (H 2 COO ads ) [38–40] . Nonetheless, the BASF process employs drastic reaction conditions

The most probable rate-limiting step is the hydrogenolysis of the for methanol production. For this reason, several researchers intensively

adsorbed format (i.e., H 2 COO ads ) to give methoxy species (i.e., CH 3 O ads ) worked to achieve milder reaction conditions. During the first thirty

that capture a proton, which then generates methanol [16] . years of the past century, copper oxide catalysts permitted a lowering

On the other hand, determining how to increase the catalysts' toler- of the temperature and pressure of the BASF process. However, copper

ance to the poisons (i.e., chloride or sulfur) or thermal sintering is still a catalysts possessed a major drawback, i.e., high sensitivity to poison

great challenge. The poisoning and sintering of catalyst result in the de- by sulfur.

activation and consequent stopping of the process. In this regard, during Imperial Chemical Industries, Ltd. (ICI) developed syngas purifica-

methanol synthesis, the catalyst undergoes deactivation even in the ab- tion systems. They discovered that Cu–ZnO catalyst was much more ac-

sence of a poison; catalyst activity drops to less than 30% after the first tive than ZnO–Cr 2 O 3 , even though the former continued to be easily

thousand hours of work [41] . Studies revealed that even in the absence poisoned by sulfur [33,34] . The development of effective purification

of steam or carbon dioxide, Cu/ZnO catalyst was irreversibly deactivated systems and active catalysts resulted in the process currently used,

due to the reduction of CuO catalyst to Cu 2 O [42] . Therefore, increase the

which works over Cu–ZnO/Al 2 O 3 catalyst to convert “metgas”

catalyst lifetime is a key issue that determines economic feasibility of

(i.e., syngas with an adequate molar ratio between CO and H 2 ) to meth-

the process.

anol under pressure of 50 to 100 bar and temperatures in the range of Although it is easily synthesized by alkaline co-precipitation of metal 513 to 533 K ( Scheme 2 ) [10] .

nitrates (i.e., Cu, Zn, and Al), it is difficult to reproduce the industrially used catalyst (i.e. Cu/ZnO). It complicates evaluating of catalyst stability

3.3. Cu–Zn/Al 2 O 3 catalyst: Synetix process for methanol production at the laboratory scale. In addition, there is another experimental prob- lem. The heating to 450 to 510 K activates catalyst oxides under a reduc-

This process was the first route commercialized for methanol ing mixture diluted with inert gas (i.e., natural gas and N 2 ). This mixture production from syngas at low pressure, and was labeled as the “ICI

commonly contains hydrogen, generated during the manufacture of process”; however, it is currently named as the “Synetix process” [10] .

syngas. Furthermore, the presence of hydrogen reduces only CuO Initially, it was accepted that the Cu(0) species constituted the active

oxide to metallic copper, whereas to ZnO or Al 2 O 3 that remains as sites. Nonetheless, study showed that other phases also played impor-

oxides [27] .

tant role in the activity and catalyst lifetime. Indeed, Nonneman and The catalyst activity depends on the high copper surface area or on Ponec demonstrated that pure Cu is an inactive catalyst in methanol

the small crystallite size, which are responsible by the contact between synthesis [35] . They concluded that Cu(I) ions are formed throughout

copper and the promoter zinc oxide on the alumina support [43] . Al-

though the true role of zinc and copper oxides in the mechanism of the Cu(0) surface, which is supplied by adsorbed hydrogen atoms

the process and are stabilized by promoters (i.e., ZnO, CsCO 3 ) on

the methanol synthesis remains unclear, Spencer et al. proposed that (i.e., H ads ).

only when adsorbed hydrogen concentration on copper phase is low,

the hydrogen dissociation on zinc oxide is important [36 , 44] . thesis has been goal of study. Zinc oxide alkalinity may minimize the

The role of zinc oxide on Cu–ZnO/Al 2 O 3 -catalyzed methanol syn-

The tolerance to the poisoning and the thermal stability are features action of the acidic sites on the alumina phase, which would pro-

that affect the activity and lifetime of catalysts. Spencer et al. reported mote methanol conversion to dimethyl ether, and under certain

that when the methanol is synthesized over Cu–ZnO/Al 2 O 3 catalysts conditions, hydrogen supply by spillover from ZnO to the CuO [35] .

containing promoters, the formation of coking or the poisoning of cata- This could be highly desirable for methanol synthesis over Cu–ZnO

lyst are minimized; therefore, only thermal sintering provokes catalyst

catalysts [36] . It is noteworthy that Cu–ZnO/Al 2 O 3 catalyst is

deactivation [45] .

Scheme 3. Methane steam reforming coupled to metgas synthesis. Adapted from Ref. [46] .

48 M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Scheme 4. Block diagram of the conventional production process of methanol. Adapted from Ref. [47] .

An adequate adjustment of the syngas stoichiometry to generate serious drawbacks, such as high consumption of energy and requiring metgas constitutes an important stage throughout the conventional pro-

expensive infrastructure. Moreover, the large amount of steam needed duction process. While syngas is produced by steam methane reforming

results in the reactors' corrosion and difficulty with handling. For at a CO:H 2 molar ratio equal to 3:1, methanol synthesis requires a

these reasons, the price of liquid methanol (i.e., produced via the syngas

route) compared to petroleum derivative liquid fuels tends not to be The metgas has been manufactured from methane after an additional

2:1 molar proportion between CO and H 2 (see Scheme 3 ) [46] .

competitive [30] . Indeed, the syngas process is responsible for 60 to step, in which the homogeneous methane partial oxidation with oxygen

70% of methanol production costs [50] .

(i.e., POX) and metal-catalyzed methane steam reforming (i.e., SR) are consecutively performed ( Scheme 2 , step III). This extra step implies

4. Oxidation of methane to methanol in a one-step process adding additional costs to the process [46] . The straight conversion of methane present in natural gas into meth- anol is a process that could avoid the highly energy-dependent conven-

3.4. Main challenges of the conventional production process of methanol tional route, reducing the number of stages, and thus avoiding the large from syngas

capital investment required to build a syngas industrial plant. This route may constitute an economically viable technology and a more environ-

Scheme 4 presents a block diagram of the conventional production mentally benign process than methanol production via the syngas path.

Undeniably, such development could be an achievement that could First, it is important to highlight that although methanol synthesis

process of methanol from syngas over Cu–ZnO/Al 2 O 3 catalyst [47] .

notably expand the production of methanol-derivatives and influence from syngas is exothermic, the overall enthalpy of the reactions 1 to 3

the planet's economy [48] . However, until today, these processes are (Eqs. (1)–(3), Scheme 5 ) is not favorable in terms of feasibility of meth-

far from being considered as established or industrially practical due anol synthesis from syngas under standard conditions [48] .

to numerous reasons.

Replacing steam by CO 2 could minimize process costs because

Extensive research has been devoted to the direct oxidation of meth-

ane to methanol, which comprises the following technologies cited in Scheme 5 ). However, the enthalpy variation of this reaction is higher

syngas is provided at the equimolar amount of CO and H 2 (Eq. (4),

items (i) to (v) [51] .

than that of steam methane reforming (Eq. (1), Scheme 5 ). Really, this process is inefficient and requires high temperatures (ca. as high as

(i) Free-catalyst homogeneous processes at high temperatures 1000 K), which promote CO disproportionality and result in the unde-

based on radical reactions in the gas phase; sirable formation of coke. All these aspects become the replacement of

(ii) Solid-catalyzed processes in the gas phase; steam by CO 2 economically infeasible [48,49] .

(iii) Solid-catalyzed processes in the liquid phase; Despite large capital investment to build industrial plants, the syn-

(iv) Homogeneous catalytic processes in the liquid phase, in the gas route constitutes the most important process to convert methane

presence of soluble catalysts;

to clean fuels, such as methanol [49] . Nevertheless, it presents some

(v) Enzymatic catalytic processes.

Scheme 5. Standard enthalpy of methanol synthesis from syngas. Adapted from Ref. [48] .

49 Throughout this review, we will attempt to pay special attention to

M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

improve our understanding about how replicate this reaction at indus- methane partial oxidation to methanol in “free-catalyst processes”

trial scale [56] .

(i.e., homogeneous processes based on radical chain reactions in the Methanotrophic bacteria have two methane monooxygenase gas phase), and “catalytic processes” (i.e., activation of C–H bonds by

(MMO) enzymes that convert methane to methanol at room tempera- bioinorganic complexes, solid metal catalyzed-methane partial oxida-

ture; soluble MMO (sMMO) and particulate MMO (pMMO) during the tion in the gas phase).

first step of metabolic cycle (Eq. (5) ) [57] .

4.1. Main challenges of direct methane partial direct oxidation to methanol CH 4 þO 2 NADH þ H þ MMO → CH 3 OH þ H 2 O NAD þ ð5Þ routes (DMTM routes)

The reaction is thermodynamically spontaneous even at room tem- Literature describes that pMMO is a metalloenzyme that con- perature ( Scheme 4 , Eq. (3)). However, since methanol is less stable

tains multi-copper active sites that are found in the plasma of all than the over oxidation products, to develop a direct route to make

methanotrophic bacteria, responsible by its activity. Several au- methanol from methane under room conditions has been referred to

thors have proposed that active sites might contain copper at a

as the “holy grail” due to its immense difficulty wide range of stoichiometries (i.e., two or three mono- or divalent Methane reactivity is lower than methanol due to its stronger C–H

copper ions) [58–60] . Indeed, the level of copper ions is determin- bonds. The dissociation energy of the C–H bond of methanol is

[52] ( Table 2 ).

ing to decide which enzyme will be expressed; if low concentration 393 kJ mol −1 ; whereas of methane, where it is 440 kJ mol −1 . Therefore,

of copper cations is feasible, the enzyme expressed is sMMO; it is easier to oxidize methanol to stable products of over oxidation

whereas, at high copper concentrations the pMMO enzyme is the (i.e., CO or CO

2 ) than to oxidize methane itself. It becomes difficult to Conversely, only the action mechanism of sMMO is well known. A control the selectivity of the one-step process of methane oxidation to

active site [61,62] .

methanol ( Table 2 ). Thus, because the methanol yield is always low, large number of spectroscopy study show that the sMMO enzyme con- the direct conversion route of methane to methanol is commercially in-

tains a carboxylate-bridged diiron unit [61–63] . The atmosphere oxygen feasible [53] . Table 2 presents the effect of temperature on the straight

is the oxidant used by both enzymes to oxidize methane to methanol. oxidation of methane into methanol [48] .

Kinetic parameters suggest that pMMO is more specific for methane Thermodynamically, it is possible to oxidize methane to methanol at

oxidation than sMMO enzyme.

room temperature [48] . Indeed, the highest conversion theoretically Nowadays, there is a growing interest on the pMMO-mediated oxi- achieved when the equilibrium is reached at room temperature

dation reactions which involves the reactivity of copper oxygenase en- (i.e., 298 K) is near 33%, calculated based on Gibbs free energy [43] . Con-

zymes. Particularly, the developing of catalysts for oxidation of versely, the best selectivity was obtained after maximum conversion is

methane to methanol at industrial scale can be improved by under- near 5%. This constitutes a greatest challenge because the carbon yield

standing how pMMO enzyme works. Consequently, several biomimetic studies have been carried out. The protein comprises three distinct sub-

of the conventional syngas process is quite superior to this value (ca.

Deserves highlight that utilizing of molecular oxygen as oxidant is a On this sense, Chan et al. proposed that the catalytic site of pMMO key aspect to do competitive any commercial DMTM (i.e., direct meth-

70 to 75%) [54] .

units (i.e., PmoA, PmoB, and PmoC).

enzyme might be a trinuclear copper cluster, and developed a series of ane to methanol) process. Molecular oxygen is an inexpensive, environ-

tricopper complexes that are capable of catalytic oxidize the methane mentally benign, and highly affordable reactant, which are essential

to methanol under ambient conditions [64,65] . In according with their features for large-scale methane conversion to liquid fuels or chemicals

experiments, those authors reported that two tricopper complexes con- [55] . However, because the molecules of methanol and methane

verted methane to methanol: a tricopper-peptide complex derived possess singlet ground states, their oxidation by oxygen is a spin-

from pMMO and a biomimetic model tricopper complex [66] . forbidden reaction, and thus requires an adequate catalyst to

Chan et al. assessed the oxidation of methane to methanol medi- ated by the tricopper complex [Cu 1+ – Cu 1+ – activate them 1+ [52] . Cu (7-N-Etppz)] 1+

In this regard, a solid catalyst could be an attractive option. Nonethe- in acetonitrile solutions. The ligand (7-N-Etppz) correspond to the less, methanol has a dipole moment higher than methane. Therefore, it

3,3′-(1,4-diazepane-1,4-diyl)bis[1-(4-ethyl[piperazine-1-yl)propan-2- ol] is showed in Fig. 9 , besides catalytic cycle proposed by those authors

is preferentially activated over solid metal catalyst surface and is totally oxidized, compromising thus the reaction yield. In addition, this strong

Chan et al. reported that a turnover number equal to 0.92 was adsorption implies in additional recovery steps, where a polar solvent to

extract the adsorbed methanol. achieved when tricopper complex was activated by oxygen molecular in the presence of methane excess. Nonetheless, the process becomes catalytic by addition of hydrogen peroxide, which recovery the spent

4.2. Activation of C–H bonds by enzymatic complexes catalyst, after tricopper complex transfer oxygen atom to the methane ( Fig. 10 ).

Nature has a notable ability in to do that the chemists pursue since Chan et al. pointed that because tricopper-peptide complex is insol- long time. For these reasons, an extensively work has been done aiming

uble in aqueous buffer, it was encapsulated in mesoporous carbon. XANES and EPR spectra demonstrated that copper(I) cations on the

Table 2 tricopper peptide were quickly reoxidized by air or oxygen, similarly

Temperature effects on Gibbs free energy for products oxidation of methane a .

to the observed on putative tricopper cluster in pMMO. Therefore, it was clearly established that Cu 1+ – Cu 1+ – Cu 1+ – peptide complex is

Reaction

ΔG values

able to mediate transfer of oxygen atom from molecular oxygen to

Temperatures (K)

methane at room temperature [64–66] . This biomimetic approach

allows formulating either homogeneous or heterogeneous catalysts to CH 4 + 0.5 O 2 → CH 3 OH

oxidize methane to methanol [67] .

CH 4 +O 2 → HCHO

Inspired by these findings, a number of researchers have tried to CH 4 + 1.5 O 2 → CO + 2 H 2 O

build active sites similar to those of sMMO or pMMO enzymes into CH 4 +2O 2 → CO 2 +2H 2 O

cavity of zeolites. In the near brief future, Cu–ZSM-5 zeolite could poten- a Adapted from Ref. [48] .

tially serve as a model for the building of selective active sites to

50 M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Scheme 6. Enthalpy variation of the reactions involved in methane and methanol oxidation at 298 K [72] .

understand the working mechanisms of enzymes [68] . For instance, re- equilibrium controlling variables such as temperature and reactant actions carried out over Cu–ZSM-5 zeolite achieved high methanol se-

pressure, aiming shift selectivity toward methanol. High methane pres- lectivity (ca. 98%) [69] .

sure can prevent deep oxidation if adequate temperatures and pressure Another highlighted instance was recently described by Lercher

are employed [72–74] .

et al., which demonstrated that single-site trinuclear copper oxygen Zhang et al. described that other process control parameters, includ- clusters in mordenite was an efficient catalyst for selective conversion

ing reactor type, temperature, feed oxygen concentration, and gas flow of methane to methanol [70] . Copper-exchanged mordenite zeolite

also affect the efficiency of the catalyst-free gas phase reactions [75] . mimics both nuclearity and reactivity of pMMO enzyme active sites.

Those authors verified that pressure exerts a pronounced effect on Lercher et al. verified that multiple Al framework atoms of the

methanol selectivity. Under methane pressure equal to 5.0 MPa and mordenite microporous stabilize trinuclear copper-oxo clusters (i.e.

heating to a temperature of 703 or 743 K, reactions gave 30 and 40% se- [Cu 3 (μ-O) 3 ] 2+ ), which are able to activate carbon–hydrogen bond in

lectivity at conversions of 5 and 10%, respectively. They inhibited the the methane, and its conversion to methanol. They demonstrated that

production of CO 2 , insulating the ringed gap between the inner quartz reversible rearrangements of the trinuclear clusters occurring during

line and the reactor tube. Indeed, they realized that both reactants and the selective transformations of methane to methanol in both enzymat-

products must be isolated from contacting the metal wall of the reactor; ic and copper-exchanged mordenite catalysts [70] . These results corrob-

for this reason, the best result was obtained when the methane partial orate with those reported by Chan et al. [64–66] .

oxidation to methanol is done in quartz and Pyrex glass-lined reactors [47,75] .

4.3. DMTM route (i): free-catalyst process of methane direct partial oxida- There is experimental and theoretical evidence that demonstrates tion to methanol by oxygen (i.e. homogeneous process)

that it is impossible to achieve high methanol yields in these systems, because an increase on the conversion always produces a decrease on

The relevance of homogeneous processes for methane oxidation the selectivity [76–78] . Consequently, the methanol production through the radical chain in the gas phase is undeniable. The radical re-

throughout homogeneous oxidation of methane will be viable at indus- actions can occur as a consequence only of working temperature. Hence,

trial scale only if the price of raw material (natural gas) is sufficiently in- depending on the temperature, its contribution to the reaction yield

expensive to compensate the high investment cost. Arutyunov et al. even when solid catalysts are present could be significant.

assessed the technological prospects and applications of the direct con- In the absence of catalysts, methane oxidation to methanol can also

version of methane to methanol via free-catalyst reactions in the gas occur over the reactor wall, in the gas phase, or in both. Since the ener-

phase and concluded that these technologies can be useful if employed gies involved are quite close, distinguishing between the two ways be-

at least for low-scale and local applications, mainly if natural gas is comes difficult. After an initiation step, a series of radical reactions

abundant and located in remote sites, where storage and transportation govern entirely the reaction course, which comprises of propagation

challenges exist [79] .

and termination steps (or removal of active species). In general, the main topics of interest to research in the field of ho-

4.3.1. The use of other oxidants in catalyst-free process of methane direct mogenous oxidation in the gas phase are as follows: (i) optimization

partial oxidation to methanol

of high-pressure conditions (ca. higher than 10 bar), aiming an ade- Methane oxidation to methanol with nitrogen oxides has been quate conversion; (ii) effect of additives; (iii) modeling mechanistic

widely described in the literature [79,80] . While in the absence of NO, studies; (iv) design of novel reactors and (v) use of photochemical ini-

DMTM gave only 1% methane conversion at 966 K and room pressure, tiators [69] .

the addition of NO (ca. 0.5%) led to a remarkable increase in methane The methane partial oxidation to methanol under free-catalyst con-

conversion to 10%, even at 808 K. The selectivity to C 1 -oxygenates was ditions involves more than 1000 elementary reaction steps with numer-

high, but methanol, methyl nitrite, and formaldehyde were always in ous reactive species [69,71] . Verma et al. developed a complete kinetic

relation near 1:0.5:1. The optimum pressure for methanol production model for these reactions in the gas phase at a temperature range of

was 1.0 MPa at 4% conversion of CH 4 in gas phase reactions with 308 to 973 K [72] .

0.50% NO, which gave 30% methanol selectivity and virtually no formal- Those authors show that the rate-limiting step of the partial oxida-

dehyde [79,80] . However, the presence of NO 2 promoted an increase of tion of methane is the abstraction of the first hydrogen atom to form methyl radical. Consequently, photochemical initiators could decrease the activation energy of this step, favoring the overall reaction [72] .

The data of Scheme 6 reveal that while a great amount of energy is needed to activate the methane, the succeeding reaction also produces other larger quantities. Clearly, since further steps are highly exother- mic, it is difficult to stop the reaction where methanol was the only product. Any attempt to drive the reaction toward high conversions leads to poor selectivity. The energy gap between reactions suggests that it is probable to obtain methanol and formaldehyde individually, al- though from an experimental perspective, this requires a strict reaction

Scheme 7. Process scheme for the net oxidation of methane to methanol (X = OSO 3 H). control. Therefore, it becomes imperative to adjust the reaction

Adapted from Ref. [81] .

51 Table 3

M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Effects of reaction pressure on product selectivity in the presence or absence of Cu–ZnO/Al 2 O 3 catalyst a .

Adapted from Ref. [82] . Exp.

0.5 24.5 0 55.9 15.1 1.6 2.9 a Catalyst, Cu:Zn:Al = 39.7:40.8:19.5; flow rate 120 cm 3 ; reaction time = 180 min; catalyst bed temperature 523 K; feed gas composition = CH 4 (77.5%); O 2 (5.8%); NO (0.5%); Ar (16.2%).

CO selectivity, caused by over oxidation of methane, methanol, and competes with other DMTM routes that may also performed in liquid mainly formaldehyde. However, the applicability of this process com-

phase (i.e., solid-catalyzed DMTM reactions in the liquid phase or even pared to the process that employs only oxygen is considerably limited.

with soluble catalysts). We summarized the main findings by grouping Periana et al. described a high-yield system for the low-temperature

the solid catalysts in accordance with the metal employed. conversion of methane to methanol based on Pt(II)-bipym catalyst (i.e., bpym = bipyrimidine ligand) that operate in liquid sulfuric acid, which act as oxidant. A positive aspect of this system is stability of meth-

4.4.1. Copper–zinc/alumina catalysts

anol formed to suffer overoxidation in this reaction medium [81] . Con- The high selectivity of Cu–ZnO/Al 2 O 3 catalyst when used in metha- nol production via the syngas route also motivated its use in the

versely, the major shortcoming of this solvent system is the difficulty of separating the methanol from the sulfuric acid. Periana et al. have

DMTM reactions. Tabata et al. investigated the optimization of methanol yield achieved via DMTM reactions with O 2 and NO over Cu–ZnO/Al 2 O 3

found that platinum(II) catalysts were more efficient than Hg(II) cata- catalysts [82,83] . They demonstrated that although the reactions lysts previously described by them, and oxidize C–H bond of methane reached conversions of close to 5.5% with or without catalyst at temperatures as low as 100 °C. Those authors showed that

2 O 3 , the selectivity and methanol its deactivation.

( Table 3 ), in the presence of Cu-ZnO/Al

bipyrimidine ligand stabilize the reduced species of platinum avoiding

yield increased.

The best result was obtained under 4 bar of pressure and CH 4 /O 2 = The positive features of this system are facile hydrolysis of the meth-

yl bisulfate, the use of recyclable oxidant (i.e., SO is easily oxidized to

8.0 (ca. 2.3% methanol yield). Previously, those authors suggested that

2 the formation of CH OH in reactions over Cu–ZnO/Al O -catalysts SO 3 ), and the use of stable complex catalyst ( Scheme 7 ).

occur through the hydrogenation of HCHO, and the water gas shift reac- tion (WGSR) [84] .

4.4. DMTM route (ii): solid-catalyzed processes of direct oxidation of meth- Actually, in a subsequent work, Tabata et al. suggested that three ane to methanol

steps: (i) formaldehyde hydrogenation, (ii) formaldehyde dissociation, and (iii) the water gas shift reaction, simultaneously occur in methane

It is clear that, in terms of competitiveness, any route of direct oxida- oxidation to methanol over Cu–ZnO/SiO 2 catalyst, when CH 4 – O 2 – NO tion of methane to methanol (DMTM route) based on a solid-catalyzed

feed is used [85] . Moreover, they verified that when heating the Cu– process poses a major challenge to operating at lower costs without a

ZnO/SiO 2 catalyst bed to 423 to 623 K, the methanol selectivity almost loss of selectivity. Additionally, it is desirable that the solid catalysts to

did not change during the reactions, unlike that observed in reactions

be used in solid-catalyzed DMTM reactions carried out in the gas over alumina as support. They suggested that this indicates that SiO 2 phase should be active under milder reaction conditions than those

was a more favorable support to enhance methanol selectivity than alu- used in the free-catalyst DMTM processes [82] .

mina at these temperatures [86] .

Compared to the OCM process, which is another solid-catalyzed pro- cess for the direct conversion of methane in chemicals (i.e., ethane and

4.4.2. Zeolite catalysts

ethylene), the DMTM process has been relatively less explored. The mo- Currently, studying the support effect, as well as the role of Cu cata- tive for examining DMTM is that, unlike the OCM reactions that are fea-

lyst on DMTM reactions, has assumed great significance due to the dis- sible only in the gas phase, DMTM route via solid-catalyzed process

covery of a selective route to oxidize methane to methanol. Groothaert

Table 4

Methane partial oxidation with nitrous oxide over Cu–Fe-ZSM-5 catalysts a .

Adapted from Ref. [97] . Exp.

CH 4 :N 2 O molar ratio

Space velocity × 10 −4 (h −1 )

Reaction temperature (K)

CH 4 conversion (%)

Product selectivity b (%C)

CH 3 OH HCHO Olef. 1 80:20

b Data taken after 60 min on stream. Olef. = mixture of C 2 and C 3 olefins; n.a. = not available.

52 M.J. da Silva / Fuel Processing Technology 145 (2016) 42–61

Table 5 steps, as follows. First, Cu–ZSM-5 zeolite should be activated, which

2 or NO), Adapted from Ref. [98] .

Methane partial oxidation with oxygen over various Fe–NaZSM-5 catalysts a .

can be performed by flowing the adequate oxidant (i.e., O and then generating the active sites. The second step comprises meth-

Exp. Si:Fe molar ratio

CH 4 conversion (%)

Product selectivity (%)

ane oxidation over zeolite active sites. Finally, the third step is the ex-

CH 3 OH

HCHO

CO 2 traction of methanol formed delivering the active sites to a new

1 b 45 0.06 74.37 15.65 15.20 activation step. Unfortunately, this third step has been impractical and

2 34 0.07 69.13 14.39 16.48 hampered in turning the whole process into a catalytic cycle [93,94] .

3 22 0.10 63.47 14.84 21.68 Recently, a breakthrough in the chemistry of copper zeolites was de- a Reaction conditions: temperature (623 K); contact time (0.5 s); oxygen (15.5 vol.%).

scribed; the ability of another Cu zeolite (i.e., named as Cu–MOR) to b Temperature (663 K).