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Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects Pierre Schwach,† Xiulian Pan,*,† and Xinhe Bao*,†,‡ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China Chemistry Department, Fudan University, Shanghai 200433, P.R. China

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‡

ABSTRACT: The quest for an efficient process to convert methane efficiently to fuels and high value-added chemicals such as olefins and aromatics is motivated by their increasing demands and recently discovered large reserves and resources of methane. Direct conversion to these chemicals can be realized either oxidatively via oxidative coupling of methane (OCM) or nonoxidatively via methane dehydroaromatization (MDA), which have been under intensive investigation for decades. While industrial applications are still limited by their low yield (selectivity) and stability issues, innovations in new catalysts and concepts are needed. The newly emerging strategy using iron single sites to catalyze methane conversion to olefins, aromatics, and hydrogen (MTOAH) attracted much attention when it was reported. Because the challenge lies in controlled dehydrogenation of the highly stable CH4 and selective C−C coupling, we focus mainly on the fundamentals of C−H activation and analyze the reaction pathways toward selective routes of OCM, MDA, and MTOAH. With this, we intend to provide some insights into their reaction mechanisms and implications for future development of highly selective catalysts for direct conversion of methane to high value-added chemicals.

CONTENTS 1. Challenges of C−H Activation and Methane Conversion 1.1. Introduction 1.2. Properties of Methane and Methyl Group 1.3. Gas-Phase C−H Activation 1.4. Heterogeneous Conversion of Methane to Hydrocarbons and Scope of This Review 2. Direct Conversion of Methane to Olefins and Valuable Chemicals 2.1. C−H Activation under Oxidative Conditions (OCM) over Oxide Catalysts 2.1.1. Doped Oxides: Homolytic C−H Activation 2.1.2. Lewis Acid−Base Pairs: Heterolytic C−H Activation 2.1.3. Example of Methane Activation over Pure MgO 2.1.4. Oxidative Coupling of Methane 2.2. C−H Activation under Nonoxidative Conditions (MDA) over Bifunctional Catalysts 2.2.1. C−H Activation on Brønsted Acid Sites 2.2.2. Role of Brønsted Acid Sites in Molybdenum Speciation and Methane Activation 2.2.3. Activation through σ-Complex Formation 2.2.4. Methane Dehydroaromatization 2.3. C−H Activation under Nonoxidative Conditions (MTOAH) over Single Site of Iron (Fe©SiO2) © 2017 American Chemical Society

2.3.1. C−H Activation on Single Site of Metals 2.3.2. Heterolytic Formation and Homolytic Conversion of Methyl Radicals 3. Comparison of OCM, MDA, and MTOAH for Selective Conversion of Methane to Hydrocarbons 3.1. Thermodynamic Considerations 3.2. Reaction Pathways 4. Concluding Remarks Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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1. CHALLENGES OF C−H ACTIVATION AND METHANE CONVERSION

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1.1. Introduction

With wildly fluctuating prices and declining reserves of crude oil, natural gas is attracting increasing attention as an important source of clean fossil energy and as a feedstock for chemicals. In particular, large reserves of shale gas, coalbed methane, and

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Special Issue: CH Activation Received: October 21, 2016 Published: May 5, 2017

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DOI: 10.1021/acs.chemrev.6b00715 Chem. Rev. 2017, 117, 8497−8520

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methane hydrate have been recently discovered.1 Unfortunately, most of these methane reserves are located in depopulated areas. Transportation of methane over a long distance is not economically viable. Therefore, there is an urge, since decades motivated initially by the successive oil crises in the 1970s, to convert methane on-site and on a large scale to transportable high-density energy sources or high value-added chemicals such as olefins, aromatics, and hydrogen. Light olefins such as ethylene, propylene, and benzene are important building blocks for a wide range of commodities such as cosmetics, lubricants, detergents, and polymers. As shown in Scheme 1, methane can be converted to these chemicals by an

ionization potential (IP) of 12.6 eV.8 The electron affinity (EA) of methane was calculated to be about −1.9 eV,9 meaning that the methane anion (CH4−) is less stable than the neutral methane. Methane is a very weak acid with a high pKa (estimated at ca. 48)10 and a modest proton affinity (543.9 kJ·mol−1).11 These properties render unfavorable reactions involving electron transfer and proton transfer with methane, such as oxidation or reduction reactions. IR spectrum of the free methane molecule (methane in the gas phase) is characterized by a triply degenerated antisymmetric stretching mode ν3 at 3019 cm−1 and a 2-fold degenerated H−C−H deformation mode ν4 at 1306 cm−1. Two other vibration modes are infrared-forbidden: the symmetric stretching vibration ν1 (Raman band of free methane molecule at 2917 cm−1) and the degenerated deformation mode ν2 with a Raman band at 1533 cm−1.12 Activation of methane leads to the abstraction of a hydrogen atom from methane yielding a methyl group, either a radical, a cation, or an anion depending on the catalytic system. Methyl radical has a planar structure13 with the central carbon atom in sp2 hybridization, a bond length of 1.079 Å,14 and a BDE of 461.5 kJ·mol−1.15 The IP of CH·3 (9.84 eV)14 to form methyl cation CH3+ is lower than IP(CH4). This characteristic is useful for differentiating CH·3 from CH4 with certainty by threshold ionization technique.16 Alkyl substitution stabilizes both C-radicals and carbenium ions, so in both radical and electrophilic reaction, the reactivity follows the C−H bond basicity order: tertiary > secondary > primary > methane. Contrary to methyl group, which loses a hydrogen, if catalyzed by a super acid, methane can gain a hydrogen and become hypercoordinated. The methonium ion (CH5+) is the smallest nonclassical alkane carbonium and was suggested by Olah and Schlosberg to play an important role in electrophilic and acidcatalyzed transformation of alkanes.17,18 The structure of CH5+ carbonium consists of a pyramidalized CH3+ unit strongly bound to an H2 molecule, resulting in a formal, three-center, twoelectron bonding arrangement.19,20 Li et al.21 interpreted the difficulties in activation of methane in terms of molecular orbitals. Because the level of lowest unoccupied molecular orbital (LUMO) is high and that of highest occupied molecular orbital (HOMO) is low, it is difficult to donate an electron to the LUMO and to remove an electron from the HOMO. However, a strong interaction with a catalyst surface can reduce the Td symmetry structure, as shown by the appearance of the infrared-forbidden band ν1 mode, corresponding to the symmetric stretching vibration. Upon adsorption on an oxide, the ν3 vibration frequency and ν1 modes are red-shifted to lower wavenumber, as a result of the Stark effect associated with the local electric field of the adsorptive ion.22 The distortion of the methane symmetry changes the relative position of LUMO and HOMO and hence facilitates the activation of methane.

Scheme 1. Pathway of C−C Coupling Reaction from Methane to Olefins and Aromatics

indirect route involving multiple steps, via synthesis gas (a mixture of CO and H2, known as syngas). From syngas a variety of products such as olefins, gasoline, and diesel, as well as oxygenates, can be obtained using the well-established technology of Fischer−Tropsch synthesis (FTS).2,3 Alternatively, syngas can be converted first to methanol and then from methanol to chemicals using technologies such as methanol-toolefins (MTO),4 methanol-to-gasoline (MTG), and methanolto-aromatics (MTA).5 In addition to the drawbacks that syngas production is energy-intensive and capital costing, both approaches require either CO or H2 to remove oxygen from CO when it comes to production of hydrocarbons. This will either reduce the carbon atom utilization efficiency or consume valuable H2 resources. More than frequently, H2 is produced from fossil resources, which is accompanied by CO2 emission. Thus, the indirect route is characterized with addition of oxygen atom into carbon to form syngas and then removal of oxygen again out of CO to form hydrocarbons. Therefore, direct conversion of methane to hydrocarbons without going through the intermediate syngas production step would be potentially more economical and more environmentally friendly. However, this remains a grand challenge of chemistry because methane is rather stable and difficult to activate, which will be discussed in the following section. 1.2. Properties of Methane and Methyl Group

Thermochemical properties of methane explain the high stability of the molecule and the difficulties in its activation and conversion. The molecule of methane has a tetrahedral geometry with four equivalent C−H bonds due to the sp3 hybridization of the central carbon atom with a C−H bond length of 1.087 Å and a H−C−H bond angle of 109.5°. The absence of dipole moment and a rather small polarizability (2.84 × 10−40 C2·m2·J−1)6 imply that methane needs a relatively high local electric field in order to be polarized and to allow nucleophilic or electrophilic attack. Methane exhibits the highest C−H bond strength among all alkanes with the first bond dissociation energy (BDE) of 439.3 kJ·mol−1 (at standard condition),7 meaning that methane is the least reactive alkane. Methane also exhibits a high

1.3. Gas-Phase C−H Activation

At high temperatures, the activation of methane frequently involves methyl groups and methyl radicals, as reported for OCM, MDA, and newly emerging MTOAH technologies. Although the exact reaction mechanisms are still in debate, gasphase reactions are frequently involved in the processes running at high temperatures (>900 K). Therefore, we set out to consider the C−H bond dissociation in gas phase for comparison. In the gas phase, homolytic bond dissociation occurs preferentially. Because of the large IP and small EA of methane, heterolytic bond dissociation energy is rather high. For instance, the noncatalytic deprotonation reaction of methane (in the gas 8498


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phase) has a standard enthalpy of reaction of ∼1745 kJ·mol−1, forming a methyl anion and a proton, i.e., ΔrH° = D(CH−3 , H+) = BDE(CH4) + IP(H) − EA(CH3). Dissociation to a methyl cation and a hydride in the gas phase is only slightly less endoenergetic at ∼1460 kJ·mol−1. Noncatalytic methane conversion necessitates harsher experimental conditions. For example, methane is converted in an oxidative atmosphere at a temperature similar to the catalytic OCM reaction but at a higher pressure (>2.5 bar).23−25 According to the thermodynamic data, methane starts to be unstable above 773 K.26 However, its conversion remains negligible below 1200 K at 1 bar, and most noncatalytic methane pyrolysis plants run above 1450 K.27 Activation of methane via reaction 1 or 2 creates free radicals in the gas phase, forming a so-called “radical pool”. Many of these radicals may abstract a hydrogen atom from methane via bimolecular radical reaction 3. Under oxidative conditions, the main radicals include HO·, HO·2, CH3O·2, CH3O·; in nonoxidative reaction, radicals (e.g., H) and higher hydrocarbon fragments (e.g., alkyl, vinyl, and aryl radicals (e.g., C2H·3, C4H·5, C6H·5)) may play an important role in the activation of methane and formation of desirable products. CH4( +M) → CH·3 + H·( +M)

(1)

CH4 + O2 → CH·3 + HO·2

(2)

CH4 + R· → CH·3 + HR

(3)

Table 1. Enthalpy Change of Reaction (ΔH) between CH4 and RH and Activation Energy Ea for the Reactions CH4 + R· → CH·3 + RH R

ΔHa [kJ·mol−1]

Eab [kJ·mol−1]

HO·2 CH3O·2 CH3CO· C2H·5 H· CH·3

73.2 69 65.3 18.8 3.5 0

77.7c 77.3c 116.8c 90.9c 64.7c 60.7d

R

ΔHa [kJ·mol−1]

Eab [kJ·mol−1]

CH3O· C2H·3 C6H·5 HO· C2H·

−0.9 −24.9 −32.9 −57.8 −118.5

37c 60.0c 36e 31.1c 2.08c

a

Bond dissociation energy taken from CRC Handbook of Chemistry and Physics.40 bParametrized reaction rate in the form k = ATn exp(−Ea/RT) where reevaluated with a standard Arrhenius equation in the temperature range 850−1500 K in order to obtain an activation barrier comparable with experimental values. cFrom Tsang and Hampson.35 dFrom Arthur and Bell.41 eFrom Heckmann et al.42

activation barrier for reaction 3 but rather the repulsive force in the form of antibonding interaction in the transition state and the polar effect in addition to the exothermicity of the reaction are important.37,38 The effect of polarity on the activation energy of reaction 3 can be illustrated by comparing hydrogen abstraction from CH4 by CH·3 and CH3O·. Both reaction are thermoneutral (or almost); however, the activation energy for the reaction involving CH·3 is nearly twice that for CH3O· (see Table 1). Note that the activation energies given in Table 1 are to be interpreted more in a qualitative way than quantitatively and may vary significantly depending on the references considered. A detailed description of the kinetics of bimolecular reaction can be found in the work of Truhlar and co-workers.39 However, in the presence of a catalyst, methane can dissociate heterolytically on its surface, due to the presence of local electric field. The heterolytic dissociation of methane requires the transfer of an electron from the methyl anion to the catalyst or from the catalyst to the methyl cation in order to form a methyl radical, which is the precursor in C−C coupling in the gas phase. The ability of the catalyst to transfer electrons is probably the key factor in its efficiency to activate methane and form methyl radical.

Unimolecular dissociation/recombination reactions depend on the collision frequency and the efficiency of collision energy transfer. Therefore, the total pressure28,29 and the nature of ambient atmosphere30 of the reaction system have a significant influence on the reaction rates especially in the so-called “lowpressure” regime, where the rates are linearly dependent on the pressure. At higher pressures, collision efficiency is not a determining factor any more and the reaction rates depend only on the temperature. This pressure dependency is represented by the notation (+M) in eq 1, where M represents an unspecified collision partner that supplies energy to break a C−H bond in methane and is referred to as “third body” or “bath gas”. The collision efficiency depends on the nature of the third body species M.30 Troe and co-workers described in detail the theoretical calculation of the rate of unimolecular dissociation.31−34 Surprisingly, information on bimolecular methane dissociation with oxygen (reaction 2) is rather sparse experimentally as well as theoretically, even if reaction 2 is included in many kinetic models in the domain of combustion and oxidation of light alkanes. The kinetic parameters used for reaction 2 were mostly based on the values reported by Tsang and Hampson35 or Baulch et al.36 Radicals are especially reactive species and can easily abstract a hydrogen atom homolytically from methane. If the activation barrier is small enough, reaction 3 (CH4 + R· → CH·3 + RH) will be controlled thermodynamically and will proceed rapidly if the reaction is exothermic, i.e., if the BDE of the new R−H bond formed exceeds that of the CH3−H bond broken. Zavitsas and Melikian determined the activation energy of several reactions involving hydrogen abstraction by free radicals based on a semiempirical approach taking into account the BDE, bond length, and bond-stretching frequency parameters.37,38 Table 1 collect the enthalpy of formation and activation energy for common free radicals possibly formed during oxidative and nonoxidative activation of methane. Zavitsas and Melikian concluded that the BDE is not a critical factor in the determination of

1.4. Heterogeneous Conversion of Methane to Hydrocarbons and Scope of This Review

Despite the challenges and difficulties of activating methane, there is significant progress in the development of direct conversion of methane to chemicals, particularly for the production of hydrocarbons. Pioneering work can be dated to Keller and Bhasin in the 1980s, converting methane to C2+ hydrocarbons, known as oxidative coupling of methane (OCM).43 This initiated a worldwide surge of research efforts in this direction.44−46 To date, hundreds of catalytic materials have been synthesized and tested for OCM, including metal oxides and doped oxides. Wideranging studies show that the main challenge lies in the selectivity control because almost all intermediates and/or products including hydrocarbons and oxygenates are more reactive than methane. They are readily overoxidized in the presence of oxygen if they are not protected by forming a more stable compound, thus leading to poor product selectivity and low carbon efficiency. As shown in Scheme 1, an alternative direct conversion technology is methane dehydroaromatization (MDA) under nonoxidative conditions over zeolite-supported metal catalysts, which was first proposed in 1993.47 MDA can avoid CO2 formation and gives a high selectivity of benzene (>80%). 8499


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However, its application is hampered by its relatively low methane conversion (10−15%) and the limited lifetime of the acidic zeolite-based catalysts caused by extensive coke deposition. More recently, a catalyst with iron single sites embedded in a nonacidic silica matrix was developed, which enables direct conversion of methane nonoxidatively to olefins, aromatics, and hydrogen (named as MTOAH).48 The conversion is as high as 48% at 1363 K, while the total hydrocarbon selectivity reaches >99%. Because a number of excellent reviews have been published covering the general aspects of direct methane valorization concepts in methane chemistry,49 from ambient temperature activation over enzyme to high-temperature activation;50 heterogeneously catalyzed methane conversion;51,52 to the development of catalysts and related processes for OCM53−57 and MDA,58−62 this paper mainly focuses on the fundamentals of C−H activation, which is the key for direct conversion of methane. Particularly, we compare the reaction pathways for OCM, MDA, and MTOAH, trying to determine the similarities and differences between them. With this, we intend to provide some insights into their reaction mechanisms and implications for future development of highly selective catalysts.

centers depends on the distribution of the charge and of the spin on the O atom and its surroundings.70 Assuming a localized hole bound to O2−, the resulting O− center is a strong Lewis acid and can capture electrons easily. If methane is present in the gas phase, electrons are supplied to the O− center in form of hydrogen atom (H· ≡ e− + H+), forming a hydroxyl group according to reaction 4. [O−] + CH4 → [OH−] + CH·3

(4)

Reaction 4 is sometimes referred to as hydrogen atom transfer (HAT) or more commonly in heterogeneous catalysis to homolytic bond splitting or homolytic activation. Thus, methane splits, forming a hydroxyl group with the O− center and releasing a methyl radical to the gas phase. Note that, during this process, methane does not adsorb on the oxide surface; more precisely, methane is not adsorbed through its central carbon atom and only the abstracted hydrogen atom is in interaction with the O− centers. Gas-phase reaction between methane and O− center71,72 as well as methane reaction with oxygen-centered radical over atomic clusters studied by mass-selected reagents under near single-collision conditions73,74 has helped foster better understanding of the phenomena in reaction 4. However, the picture is somewhat more complicated (see Figure 1) as compared to what

2. DIRECT CONVERSION OF METHANE TO OLEFINS AND VALUABLE CHEMICALS 2.1. C−H Activation under Oxidative Conditions (OCM) over Oxide Catalysts

2.1.1. Doped Oxides: Homolytic C−H Activation. Charge transfer from the adsorbate molecule to the catalyst surface and vice versa is a key factor in the activation of molecules. Therefore, the electronic structure of a catalyst is of primordial importance63−66 and controls its reactivity with surrounding molecules. For example, OCM requires an electron-acceptor catalyst (p-type conductors) to activate methane and form a methyl radical.67 On the other hand, the activation of molecular oxygen, which is a critical step in OCM, maybe even more so than the abstraction of H atom from methane, necessitates electrons from the system (i.e., electron donor or n-type catalyst).68 The conductivity of oxides can be modified and tuned by doping. The change in the electronic properties of an oxide (i.e., Fermi level and electrical conductivity) is influenced mainly by the valence of the cationic dopant and the host matrix, as well as the location of the dopant, i.e., interstitial or substitutional position. Donor dopants raise the Fermi level and act as “traps” (i.e., localized center) for free holes, while acceptor dopants lower the Fermi level and are able to trap free electrons. The following discussion refers only to the substitutional doping. Lowvalence dopants (LVDs) create an excess of electrons (with respect to the ideal lattice), e.g., one extra electron for each Li+ cation replacing a Mg2+ cation in Li-doped MgO. This excess of electrons is compensated either by the formation of electron holes in the host matrix or by lattice oxygen vacancy (denoted as •• V•• ′ . On O in the Krö ger−Vink notation), e.g., one VO for two LiMg the contrary, high-valence dopants (HVDs) induce a deficiency of electron, which is compensated either by the formation of free or bounded electrons in the host matrix or by cationic vacancies. If an electron hole is strongly bonded to a lattice oxygen (adjacent to the LVD cation), an O− center is formed with a strong radical character localized to the O atom, as in the case of Li-doped MgO, forming [Li+O−] centers.69 The reactivity of O−

Figure 1. Potential-energy surface, given in kJ·mol−1, for the reaction of [MgO]·+ with CH4 calculated at the MP2/6-311 + G(2d,2p) level of theory; selected bond lengths are given in Å. The encircled structures depict the rearrangements occurring along the reaction coordinate. Reproduced with permission from ref 75. Copyright 2006 Wiley-VCH.

is generally assumed in OCM according to the so-called Lunsford mechanism.46 C−H abstraction over [Mg2+O·−] cluster occurs via an indirect HAT reaction involving a long-lived [H3C···Mg− OH] intermediate (see Figure 1).75 Lithium-doped MgO (denoted as Li/MgO) is one of the most studied catalysts in OCM, and its reactivity was attributed to the presence of [Li+O−] center on the catalyst surface.46 Although the presence of [Li+O−] during the OCM reaction catalyzed by Li/MgO was questioned recently,51,76,77 it is undeniable that O− center can be formed in MgO.78 O− center can be generated by UV radiation79,80 or by compensation effect from cation vacancies (V″Mg) in a similar way as LVD ([OxOV″MgOxO]” ≡ [O•OVMg ″ O•O]x).81,82 The overall reaction of methane on O− center (located in the lattice of the MgO oxide, not an activated 8500


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(formal charge −1, hydride or methyl anion) or a Lewis base (formal charge +1, proton or methyl cation) (Scheme 2).

oxygen adsorbed on the surface) follows reactions 4−6 according to the Lunsford mechanism.46,53 2OH− → O2 − + V •• O + H 2O

(5)

− 2O2 − + 2V •• O + O2(g) → 4O

Scheme 2. Methane Adsorption and C−H Bond Polarization on Basic Oxide Surface

(6) −

The homolytic splitting of methane on O center may occur already at room temperature.75,80 The latest theoretical calculation gave an energy barrier as low as 12 ± 6 kJ· mol−1 for the HAT reaction over [Li+O−] center.69 The problem comes rather from the regeneration of the active O− center (reactions 5 and 6). Reaction 5 requires a significant proton conductivity of the oxide surface in order to be efficient. Unfortunately, the role of proton conductivity was never addressed in OCM. This disproportionation reaction also requires the oxide to be able to form oxygen vacancies. Baerns and co-workers found that LVDdoped La2O3 catalyst with a higher anionic conductivity (high concentration of oxygen vacancies) exhibits a lower apparent activation energy in the OCM reaction.67,83 Derk et al. correlated the apparent activation energy of OCM over LVD- and HVDdoped La2O3 to the oxygen vacancy formation energy.84 The reaction rate of the reoxidation (reaction 6) in the Lunsford mechanism is probably enhanced if the catalyst exhibits a p-type conductivity because 4 electron holes are formed and have to be • x transferred during the reaction (2V•• O + O2(g) ≡ 2OO + 4h (in Kröger−Vink notation)). Despite the recent debate on the Lunsford mechanism,51,76,77 a remarkable scientific work lies behind the development of the mechanism, especially considering the complexity of the reaction. Thanks to the development and application of electron spin resonance (ESR) spectroscopy in heterogeneous catalysis, the determination of paramagnetic species as reactive centers, the matrix-isolated electron spin resonance (MIESR) technique to characterize radical intermediates, and the concept of heterogeneous−homogeneous catalytic system emphasizing the role of radical reactions in the gas phase were and still are groundbreaking ideas in heterogeneous catalysis over oxides. Molecular oxygen adsorbs and reacts on the surface of an oxide doped with HVD.68,82,85,86 The doped oxide exhibits an excess of electrons (representing a Lewis base character), and the Lewis acid oxygen reacts through electron transfer. Depending on the amount of charge transferred, the molecular oxygen forms electro- or nucleophilic entities. Methane can react with the adsorbed (and activated) oxygen following an Eley−Rideal reaction mechanism.87 Lunsford and co-workers invetigated the reaction of methane with adsorbed O−2 oxygen,88 O− oxygen,89 and O−3 oxygen.90 Generally, it is assumed that electrophilic oxygen species activate methane via an homolytic C−H bond activation while nucleophilic oxygen species (typically O2−) form methoxy groups that lead to unselective reaction pathway and later decompose to CO and H2.68 2.1.2. Lewis Acid−Base Pairs: Heterolytic C−H Activation. Over 30 years’ development, a series of metal oxides are shown to be active for OCM.91−95 It is well-recognized that their basicity plays an essential role, which allows adsorption of the weakly acidic methane.91−95 Adsorption of methane on basic oxide results in a negatively charged fragment coordinated to the metal cation and a positively charged one coordinated to the basic lattice oxygen of the oxide. Depending on the basicity strength of the oxide, methane dissociates, yielding a methyl anion and a proton. Both fragments are amphoteric; i.e., depending on their formal charge, they can act as either a Lewis acid

The group of Metiu examined the role of the emphoteric character of CH3 and H in the dissociative chemisorption of CH4 on planar irreducible oxides (La2O3(001), MgO(001), or CaO(001)).96 To avoid confusion, their definition of Lewis acid and base is reiterated here:85 “A molecule whose electron charge increases during a reaction is a Lewis acid; the one that loses electrons is a Lewis base”. If CH3 or H adsorbs alone on the surface, they preferentially adsorb to a lattice oxygen and act as a Lewis base (electron donor). If coadsorbed, one will bind to a site where it can function as a Lewis base and force the other to adsorb to a site as a Lewis acid. In such a case, the formation of an acid−base pair is expected to lower the energy of dissociative adsorption much below the sum of the energies of binding of the fragments alone.85 As a result, the energy of dissociative adsorption (ΔE[CH4]) is the lowest when H is adsorbed to a lattice oxygen and CH3 is adsorbed to a cation on La2O3, CaO, and MgO.96,97 However, it has also the highest desorption energy of the CH3 fragment to form a CH·3 radical (by 2.33 eV),96 in agreement with the Brønsted−Evans−Polanyi rule. Atomic hydrogen adsorbs to an oxide surface, forming a hydroxyl group (a proton adsorbed on the lattice oxygen), and a polaron is located on the oxide surface. On such a surface, the Lewis acid−base pair “cooperation” will occur and the energy for the dissociative adsorption of methane ΔE[CH4] will be significantly lowered, yielding a hydride adsorbed to a La cation and releasing a methyl radical in the gas phase.98 The valence state of the oxide does not change during the activation of methane, and the catalyst does not need to be reoxidized to adsorb a new methane. This is different from the abstraction of H from methane by the O− center, which is formed due to an electron deficiency, as mentioned in the previous section. When methane adsorbs on the oxide surface as a methyl anion, its desorption requires the loss of an electron to form a methyl radical. Over pure, irreducible oxide, oxygen (or any other oxidant) may play the role of electron acceptor. However, it is not clear if the extra electron is directly transferred to the oxidant or first transferred to the oxide as a polaron and then to the oxidant. However, the final result is the same: yielding a superoxide (O−2 ), often strongly coupled to the neighboring proton on the surface of the oxide, if the oxidant is O2, as observed by ESR spectroscopy.78 Yet some theoretical work mentioned that the adsorption of methane on planar (001) surface of irreducible oxide is very weak (if at all), e.g., La2O3(001)99 and MgO(001).100 Methane is less likely to be activated on planar (001) surface but rather on a coordinatively unsaturated site (cus). cus ions exhibit higher Lewis basicity (or acidity) due to their lower coordination and are therefore more reactive. Indeed, the Lewis acidity strength of an oxide correlates strongly with the cation electronegativity (i.e., electron-accepting capability) and cation coordination.101,102 Most basic oxides used in OCM (e.g., MgO, CaO, and La2O3) have an ionic character due to the low electronegativity of the cation. Levine and Mark investigated the surface state of ionic 8501


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Figure 2. Structure of dehydroxylated pure MgO. (A) TEM images of MgO dehydroxylated at 1173 K. Adapted with permission from ref 108. Copyright 2015 Elsevier. (B) Schematic representation of cus sites on the surface of MgO. Adapted with permission from ref 109. Copyright 2006 American Chemical Society.

Figure 3. (Left) Adsorption of methane on MgO (red line) and after preadsorption of CO (black line). (Right) Methane adsorption and coadsorption of CO and CH4 on MgO edges and corners. Adapted with permission from ref 110. Copyright 2015 Elsevier.

defect and surface stability, and charge transfer and oxygen activation. Experimental21,110 and theoretical studies111−113 both97,100 showed that methane adsorbs with a dihapto configuration through C and H atoms of methane on Lewis acid−base pairs present on edges and steps, as well as with a monohapto configuration through H atoms on Lewis basic lattice oxygen anions present on corners and kinks (see Figure 3). In the presence of O2, oxygen can act as an electron acceptor and facilitate the formation of a methyl radical according to the reaction represented in Scheme 3. As a result, a superoxide radical O·2 − forms,

crystals based on the coordination number of the metallic cation.103 However, fully dehydroxylated alkaline earth oxides, for example, exhibiting only {100} facet and higher index planes (like {110} or {111} facets or facet-like) are stabilized only upon hydroxylation.104,105 Low coordination ions exist in the form of topological (or morphological) defects on the cubic {100} facetlike steps, corners, or kinks (see Figure 2). They exhibit a higher reactivity due to their reduced Madelung constant, i.e., lower coordination and therefore higher Lewis basicity or acidity strength for cus anion and cation, respectively, as shown by Garrone et al. for alkaline earth oxide.106 On an ideally pure and point-defect-free, insulating, and irreducible oxide (alkaline earth or rare earth), the heterolytic activation of methane results from the interaction of electrostatic potential generated by surface morphological defects (cus).107 The bond splitting occurs close to the surface, where a high local field exists and can polarize the molecule, elongate the C−H bond, and weaken it. Consequently, the reaction catalyzed by Lewis-basic oxide is structure-sensitive, and the stability of the morphological defects (cus) impacts directly on the reaction. Therefore, knowledge about the nature, stability, and reactivity of cus is relevant for catalyst design in order to quantify the reactive sites and to increase their concentration. 2.1.3. Example of Methane Activation over Pure MgO. MgO-based material is a typical Lewis basic oxide catalyst for OCM. The activation of methane in the OCM reaction over pure MgO was already comprehensively reviewed by Schlögl.56 However, it may be interesting to resume the key features such as the roles of Lewis acidity−basicity, morphological

Scheme 3. C−H Bond Dissociation by Intermolecular Electron Transfer with Molecular Oxygen

adsorbing on MgO in close vicinity to a proton. Interestingly, the formation of superoxide already can be observed at room temperature or below, indicating that the high temperature necessary for OCM is not because of the C−H activation110,114,115 but rather is to regenerate the active sites.116 The superoxide and the proton can desorb in the form of perhydroxyl radicals, but it can also form water or hydroxyl radicals. The presence of reactive species renders the overall reaction mechanism complicated because molecular oxygen, superoxide, perhydroxyl, or hydroxyl radicals can react further, leading to a nonselective pathway. 8502


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Figure 4. Sintering of primary MgO particle during OCM at 1073 K. Adapted with permission from ref 108. Copyright 2015 Elsevier.

Scheme 4. Simplified Scheme of the OCM Reactiona

It was shown that careful IR experiments allow semiquantitative analysis of the magnesium cus cations and their Lewis acidity strength depending on their coordination.117 Semiquantitative IR studies of methane adsorption on MgO catalysts with different morphologies and structures combined with kinetic studies108 indicated a strong correlation between the rate of methane conversion and the relative concentration of (i) Lewis acid−base pairs present on the oxide and (ii) monatomic steps on the surface. It was observed that not only does the Lewis basicity strength play a key role in methane activation but also the local structure of the Lewis sites is important. The specific structure of monatomic steps allows the reaction of methane and oxygen without itself supplying or accepting electrons for the activation of methane. In this case, MgO acts as a “marriage broker” between methane and oxygen, as stated by Schlögl.56 Therefore, OCM over MgO is structure-dependent, and the stability of the structure is of primordial importance. However, in OCM, a nonnegligible quantity of hydroxyl group and water form and MgO undergoes sintering at the reaction temperature, which leads to vanishing of the highly active monatomic steps and hence eventual deactivation (Figure 4). 2.1.4. Oxidative Coupling of Methane. OCM was first proposed by Keller and Bhasin43 in 1982 followed by the work of Hinsen and Baerns in 198344 and Lunsford’s group in 1985.45,46 These early milestone works opened the perspective to the production of ethylene from methane. The scientific community widely agrees, in an oversimplified view, that the selective formation of ethylene occurs via three steps: (i) activation of methane to methyl radical through a C−H bond breaking and a hydrogen abstraction, (ii) homogeneous coupling of two methyl radicals to ethane in the gas phase, and (iii) oxidative dehydrogenation of ethane to ethylene. At each step, nonselective (homogeneous and/or heterogeneous) oxidation may happen, giving products COx (see Scheme 4). Therefore, OCM can be written as 4CH4 + O2 → 2C2H6 + 2H 2O

(7)

2C2H6 + O2 → 2C2H4 + 2H 2O

(8)

a

Adapted with permission from ref 54. Copyright 2008 Wiley-VCH.

Recently, Zavyalova et al.118 analyzed the catalysts reported for OCM in more than 400 publications since 1982 and generated over 1850 data sets on catalyst compositions. The results in Figure 5 show that all high-performance catalysts exhibit strong basicity and are mainly based on Mg- and La-doped oxides.118 For example, doping of the host oxides with alkali (Li, Cs, and Na)

Figure 5. Elemental compositions of OCM catalysts with Yield(C2+) ≥ 25% reported in the literature. All the catalysts were tested in a fixed-bed reactor in the cofeed mode under atmospheric pressure at temperatures from 943 to 1223 K, p(CH4)/p(O2) = 1.7−9.0, and contact times from 0.2 to 5.5 s. Reproduced with permission from ref 118. Copyright 2011 Wiley-VCH. 8503


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and alkaline-earth (Sr and Ba) metals increases the selectivity toward C2 hydrocarbons, whereas dopants such as Mn and W increase methane conversion. Most active catalysts give a C2 selectivity of 72−82% and a C2 yield of 16−26%. In addition, Mn/Na2WO4/SiO2 catalysts reported by Wang et al. typically exhibit a high yield for C2+ hydrocarbons (>25%).119 However, on a mechanistic point of view, only little is understood to date, and therefore, the activation of methane on such catalysts will not be discussed in this Review. For more information, please refer, for instance, to reports by Takanabe and Iglesia,120 Takanabe,121 and Arndt et al.122 Beside oxygen, N2O123 and S2124 are used as oxidants in OCM. The use of “softer” oxidants may alleviate to some degree but cannot avoid completely deep oxidation of hydrocarbon molecules.54 With elemental sulfur as the oxidant, the reaction with 5% CH4 balanced by Ar yields a 15% conversion and a selectivity of 19% toward C2H4 over the best catalyst (PdS supported on ZrO2) at a rather high temperature (1323 K).124 In addition, major safety and technical issues due to the formation of H2S as a byproduct must be considered, especially for large-scale applications. A wide variety of oxides (reducible−nonreducible, doped− pure, supported−bulk, acid−basic, ionic−covalent, halogenated) used as catalysts in OCM may explain the absence of a unified mechanism so far. Indeed, there is still debate on how methane is activated (homolytically or heterotytically), how oxygen is activated, which properties activated oxygen needs to have for a selective reaction (i.e., nucleophilic or electrophilic), which pathway of the reaction is heterogeneous, and which pathway is homogeneous.54 Despite these contradictions and unsuccessful attempts to industrialize the process so far, the research efforts in the last 3 decades have led to remarkable progress. This includes (i) synthesis and characterization of solid solution mix oxide, (ii) doping chemistry,125 (iii) point defects chemistry,82 (iv) characterization of radical intermediates, and (v) activation of oxygen126,127 and their roles in alkane activation.88−90 On the basis of these advances, some common features can be drawn: • Most active catalysts exhibit strong basicity and low surface areas.93 • The C−H bond cleavage is the rate-determining step (based on product distribution of isotopic labeled experiments and kinetic isotope effect (KIE)).128−130 • CH·3 is produced upon CH4 activation and is involved in the methane coupling pathway (MIESR46,131 and vacuum ultraviolet soft photoionization molecular-beam mass spectrometry132 (VUV-SPI-MBMS)). • Activation of oxygen is important and controls the selective pathway.68 Although some general criteria were proposed for designing an active and selective catalyst,67 a rational development of a highly efficient OCM catalyst failed so far, likely due to complex relations between heterogeneous and homogeneous reaction networks. Therefore, it is of interest to understand the roles of gas-phase reactions in the whole process. A recent study combining in situ characterization of the formaldehyde intermediate, gas-phase kinetic measurements, and modeling under industrially relevant reaction conditions emphasized that methyl radical coupling in the gas phase to C2 products is indeed possible.25 In this context, comprehensive approaches that include reaction engineering are important to fully understand the catalytic process.133

The efforts include the one at the Unicat cluster hosted at the Technical University Berlin134 led by Driess, Schomäcker, and Wozny, which covered catalyst synthesis, characterization, and engineering aspects of the OCM reaction. Construction, within Unicat, of a miniplant operating with a fluidized-bed reactor for a better heat transport135 or a packed-bed membrane reactor to avoid deep oxidation of methane136 illustrates the complexity of the downstream process on which the determination of optimal experimental condition of catalysts may depend.137 Recently, Siluria Technologies138 also attempted to demonstrate and scale up the OCM technology based on their nanowire-like catalysts,139 although there was only little information released about their technology. 2.2. C−H Activation under Nonoxidative Conditions (MDA) over Bifunctional Catalysts

2.2.1. C−H Activation on Brønsted Acid Sites. It is known that methane dehydrogenation and methane hydrogen exchange reaction are catalyzed by strong Brønsted acid (also called sometimes superacid).140 Solid-state superacid, with the most known examples of zeolites,141 is widely used in catalysis such as hydrocarbon cracking. Its acidity strength is closely related with the concept of doping and Lewis acidity (see section 2.1.1). Most solid Brønsted acids have a silica matrix, which represents an ideal host because of its irreducible property, a Lewis acidic character, and high valence cation consequently easily doped by LVD. For example, Al3+ cations induce formation of electron holes (h·) with the catalytic active center being equivalent to [AlO4h]0. If the hole is delocalized over the 4 oxygen atoms surrounding the LVD cation in SiO2, it will bind a compensating H atom, weakly forming a more labile proton.142 [AlO4h]0 is too reactive and readily compensated by an electron donor like H atom, forming [AlO4H]0. Density functional theory (DFT) calculations demonstrated that the same Brønsted acidity strength can be achieved on zeolite and on α-quartz doped with Al3+.142 However, because all LVD Al cations in zeolite are located on the surface, zeolites have a much higher concentration of Brønsted acid sites available for catalytic reaction than the doped α-quartz, for the same amount of dopant. Controlling the position of LVD cation on a host matrix like α-quartz by synthesis is challenging (if possible at all). IR investigation showed that methane adsorbs on SiO2,143 Al2O3,21 and ZSM-5144 surfaces in a similar manner, mainly through hydroxyl groups and/or cus lattice oxygen. Similar to basic oxides, the Td symmetry of methane is disturbed upon adsorption, as evidenced by IR. H/D exchange between CD4 and the surface hydroxyl group of silica, alumina, and ZSM-5 can be observed above 773 K (or even 573 K for η-Al2O3).145 DFT calculations146−148 (Figure 6) showed that methane is coordinated through one H atom to a lattice oxygen anion near the aluminum heteroatom and the central C atom is coordinated to the protonic acid (Figure 6a). Thereby both C−H and O−H bonds are elongated and the bond distance depends on the strength of the Brønsted acidity.147 One oxygen, next to the aluminum cation, acts as a Brønsted acid, donating a proton to methane, while the other acts as a Lewis and Brønsted base, receiving the hydrogen atom from methane. The methyl group has a anionic character (with a charge of −0.573 e) over a H2OAl(OH)H2 model cluster.146 Methane dehydrogenation can be regarded as a methyl cation coordinated with molecular hydrogen, forming a CH5+ intermediate coordinated to a lattice oxygen (Figure 6b).146 In both H-exchange and dehydrogenation, the strength of the Brønsted 8504


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Figure 7. Correlation of the aromatics formation rate during catalytic MDA reaction with the quantity of different molybdenum species in catalysts with varying Mo loadings. The solid line is the best linear fit of the data for the exchanged Mo species, while the dashed lines are nonlinear, i.e., the best polynomial fit of the data, for the total Mo species (triangle) and MoO3 crystallite (circle), respectively. Reproduced with permission from ref 149. Copyright 2008 American Chemical Society.

(framework or extraframework); therefore, it is related to the ring structure and the size of the zeolite pores.151 At a low Si/Al ratio (typically 6−10 wt% (depending on the Si/Al ratio), Brønsted acid sites are insufficient for the ionexchange reaction and MoO3 will locate on the zeolite extraframework. This will affect the catalytic activity directly, as monomeric dihapto Mo species located inside the zeolite framework are reported to be the most active species.153 Therefore, it is essential to control the distribution of Mo oxide species and limit their anchoring positions. Distortion of Al Brønsted site by Mo was revealed by ultrahigh-field 27Al MAS NMR, confirming the strong interaction of Mo with zeolite host.154 However, these MoO3 species are not the active site responsible for the activation of methane to C2Hx. The MDA reaction exhibits an induction time prior to the formation of benzene.155−157 During this induction time, the Mo species are first partially reduced and carburized to molybdenum oxycarbide (MoOxCy) species by methane, releasing CO, CO2, and H2O as products.158−161 The MoOxCy species are able to convert CH4 to C2+ product, although not to benzene. Benzene only starts to form when molybdenum is completely carburized to MoCx.161 During the reduction and carburization process, anchored monomeric or dimeric species may agglomerate to MoxCy clusters in the size range of 0.6−1 nm. Some of those Mo clusters also migrate from the channels to the zeolite extraframework. DFT calculations suggested that MoxCy clusters153 and anchored Mo1,2Cy (mono- or dimeric) species162,163 are active for methane activation. A similar conclusion was obtained experimentally by the group of Iglesia.164,165 Over MoxCy clusters, a σ-bond metathesis is suggested whereby the methyl group bound to a Mo cation and the proton attached to a carbon atom. The reaction is

Figure 6. Transition state structure for methane reaction over zeolite: (a) hydrogen-exchange reaction and (b) dehydrogenation reaction. Reproduced with permission from ref 147. Copyright 2006 Elsevier.

acidity influences the configuration of the transition states and the activation barriers. Methane hydrogen exchange exhibits a significantly lower activation barrier (estimated between 134 and 150 kJ/mol) than methane dehydrogenation (368−393 kJ/mol) depending on the acidity strength.147 Direct observation of methane dehydrogenation was not reported. However, a conversion of 1.4% was observed by feeding methane directly to ZSM-5 at 973 K, exclusively forming benzene and probably coke (although it was not reported then).47 Wang et al. proposed that methane was activated on “pure” ZSM-5, forming methonium cations (CH+5 ), which decomposed to H2 and CH3+ and subsequently to ethylene according to eq 9.18 +H +

−H2

+CH4

−H2

−H +

CH4 ⎯⎯⎯⎯→ CH+5 ⎯⎯⎯→ CH+3 ⎯⎯⎯⎯⎯→ C2H+7 ⎯⎯⎯→ C2H+5 ⎯⎯⎯⎯→ C2H4 (9)

2.2.2. Role of Brønsted Acid Sites in Molybdenum Speciation and Methane Activation. Upon loading Mo species onto zeolite supports, activation and conversion of methane can be significantly enhanced. It is generally agreed that the Brønsted acidic sites play several important roles in MDA although their actual mechanisms are still in debate.60 They serve as anchoring points for the molybdenum species inside or outside of the zeolite pores. They are also considered to be the active sites for the oligomerization and cyclization of C2Hx to benzene, as well as the formation of coke. Ultrahigh-field NMR revealed that molybdenum atoms migrate into the zeolite’s channels and anchor onto the acidic aluminum sites during catalyst preparation, thereby forming Al−O−Mo linkages.149,150 The actual location and nature of the Mo species, which control the reaction (Figure 7), depend on the location and density of the Brønsted acid sites. This in turn is related with the Si/Al ratio and the distance between two aluminums and their locations 8505


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[AlO4]− center, rendering the Zn2+ cation a stronger Lewis acid. At the same time, a strong electrostatic field is created between the Zn2+ cation and the distant second [AlO4]− center.

energetically more favorable on framework cluster stabilized by one or more [AlO4]− centers than on extraframework sites stabilized by terminal SiO groups.153 The presence of Mo species stabilized on extraframework and in framework [AlO4]− centers was determined by ESR experiment.166 Pretreatment and carburization process have a strong influence in the MDA reaction over Mo/zeolite catalysts.167,168 Over Mo1,2Cy, Xing et al.162 and Zhou et al.163 suggested that the carbidic molybdenum is first hydrogenated to form molybdenum carbene and methylene bridge (in the case of dimeric species). MoCH2 represents a Lewis acid−base pair, which can polarize and activate methane, forming a negatively charged methyl group bound to the Mo cation, and a positively charged methyl group is formed by reaction of the carbene with the resulting proton from the C−H bond scission. The two CH3 groups undergo C−C coupling,. eliminating H2 to form an ethylene ligand. The ethylene group desorbs after the activation of a third methane molecule to replace the original carbene ligand incorporated in the formed ethylene. Ethylene then reacts further to benzene in the zeolite channels via successive hydrogenation/dehydrogenation reactions catalyzed by Brønsted acids. 2.2.3. Activation through σ-Complex Formation. Upon coadsorption of CO molecule with methane, adsorbed methane molecules on ZnO169 or α-Cr2O3170 are completely displaced, indicating that methane adsorbed solely on cations, forming a H3C−H···Mn+ coordination with low-coordinated Lewis acid cations and not with the lattice oxygen anions (O2−). The interaction between methane and the transition metal oxide was explained by a strong electrostatic field centered at the cations, without excluding the possible contribution of orbital overlapping between the σ orbitals of a C−H bond and d orbitals of the transition metal cation stabilizing the interaction of methane with the surface,170 similar to the formation of a σ-bond complex in organometallic chemistry.171 The bathochromic shift of the ν1 symmetric stretching mode reported for ZnO at 2872 cm−1 is stronger than that observed for the other oxides (e.g., MgO, SiO2, and Al2O3), indicating a strong interaction between methane and the ZnO surface. For comparison, molecular hydrogen dissociates readily on ZnO at 77 K, forming a ZnH hydrid and a hydroxyl group.169 In metal complex chemistry, an agostic bond, a C−H moiety coordinated to a metal center,172 is characterized by the donation of σ electron from the C−H bond to the metal acceptor orbital accompanied by a C−H bond stretching, thus enhancing the acidity and electrophilicity of the C−H moiety because of the reduced electron density of the C−H fragment. The M−H−C interaction is strengthen and stabilized further by backbonding of metal dπ orbitals to the σ* antibonding orbital of the C−H moiety. However, when the metal π-back-donation to the C−H σ* becomes too strong, the C−H bond is cleaved and an oxidative addition to the metal occurs.171,173,174 On a solid oxide, the σ-bond donation would acidify the C−H bond and facilitate the hydrogen abstraction by a neighboring or distant basic lattice oxygen anion. The results of C−H bond splitting via σ bonding or interaction with a Lewis base cannot be distinguished. Both reactions yield, depending on the emphoteric character of the methyl goup and the hydrogen atom, either a methyl anion bond to the surface cation and a proton bond to a lattice oxygen or the opposite. For example, in Zn-modified HZSM-5 zeolite prepared by ionic exchange with zinc vapor, all acidic proton are replaced by Zn2+ cation according to eq 10.175 Thereby the charge of the bivalent Zn2+ cation is only partially compensated by an adjacent

2[AlO4 ]− H+ + Zn 0 → [AlO4 ]− Zn 2 + + [AlO4 ]− + H 2 (10)

Adsorption of methane on such a ZnZSM-5 zeolite exhibits a rather unusual large bathochromic shift of the ν1 symmetric stretching vibration of Δν1 = −100 cm−1.176 In comparison, on Y zeolite, the charge compensation of Zn2+ is more complete because of the smaller ring and closer proximity of the 2[AlO4]− center to the bivalent cation,177 and Δν1 is only −56 cm−1;175 and on ZnO oxide, Δν1 = −45 cm−1.169 At 473 K, methane dissociates on ZnZSM-5, forming an irreversible IR band assigned to methyl anion bond to Zn2+ cation and hydroxyl group. Those observations were confirmed later by a solid-state NMR experiment with the detection of dissociated methane on ZnZSM-5 even at room temperature.178 2.2.4. Methane Dehydroaromatization. Since it was first reported in 1993,47 a variety of catalysts based on metal ions dispersed on various zeolites have been tested.179−182 These metal ions include V, Cr, Fe, Mo, W, Re, Zn, Cu, Mn, Ga, etc. Mo remains the most active. Among the studied zeolites, e.g., ZSM-5, ZSM-8, MCM-22, and MCM-49, ZSM-5 and MCM-22 as the supports give the highest activity and selectivity to benzene.59,183 The combination of the acidity and the pore architecture with Mo species constitutes the bifunctionalties of MDA catalysts. The zeolite channel size,183 the concentration of Brønsted acid sites,151,184 the type of metals used,179 and different preparation methods such as impregnation, sublimation, and solid-state reaction can also influence significantly the activity and selectivity in MDA.185 Ma et al. demonstrated the importance and synergetic effect between zeolite size ring, Brønsted acidity and its role on the Mo speciation, and Mo Lewis acidity and hence the structure−performance relationship.186 It was demonstrated that the MCM-22-supported Mo catalyst exhibits a similar activity but much higher selectivity toward benzene, and less naphthalene and coke than Mo/ZSM-5 with the same Mo loading.182,183 This can be attributed to the unique pore architecture of MCM-22. It has a large three-dimensional 12-member ring supercage, which is interconnected by the 10-member ring windows with a size of 4.0 × 5.5 Å. The presence of such a supercage accounts for the higher tolerance of MCM-22 to coke deposition.182,183 Consequently, the catalyst lifetime is extended significantly over the MCM-22-supported catalyst compared to that with ZSM-5. These results reflect the shape selectivity effects of zeolites in methane conversion. MDA is a thermodynamically limited reaction.187 The extensive studies over the years have led to significant progress. Methane conversion has now almost reached the equilibrium value. For example, Mo/ZSM-5 gives a methane conversion of 16.7% with benzene yield ∼10.1% and naphthalene yield 1.4% at 1003 K.188 However, in this temperature range, the greatest challenge of this process is the limited catalyst lifetime due to extensive coke deposition, which blocks the pores and covers the active sites. Actually, carbon plays an important role in MDA, and at least three types of carbon are recognized: carbidic carbon in molybdenum carbide, molybdenum-associated coke, and aromatic-type coke on acid sites.154,189,190 It is generally accepted that deep dehydrogenation of CHx on the metal catalyst (oxide, carbide, or metallic) yields amorphous coke deposition (soft coke), while polymerization of C2Hx and C6H6 on the Brønsted acid sites of the zeolite via proton exchange191 gives 8506


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polyaromatic carbonaceous deposits (hard coke),190,192 as proposed in Scheme 5. Coke accumulates with time on stream.

zeolite channels and a reduction of graphite-like coke on the zeolite surface.195 In the mean time, the selectivity to C2H4 increases with increasing H2 concentration in the feed due to coke hydrogenation and benzene selectivity is almost unchanged. However, this is at an expense of lower methane conversion, due to a shift in the thermodynamic equilibrium.195 Therefore, wide efforts have been made to search for approaches to extend the lifetime of the catalyst and to explore regeneration methods.196−198 As a result, a variety of methods have been developed to modify the acidity and pore structure of zeolites, e.g., dealumination of zeolite via steam treatment,199,200 silanation,165,168,201,202 or chemical vapor deposition of tetraethyl orthosilicate to shield the external acid sites, creating mesoporosity. All these were observed to be effective to improve the stability with additional benefits of enhanced activity and/or selectivity. For instance, steam treatment of Mo/ZSM-5 reduces the concentration of Brønsted acid sites due to the migration of Al cations to the extraframework of the zeolite and subsequent elimination with acid washing during the preparation, reducing coke formation. However, further steam treatment is not beneficial because Al2(MoO4)3 species or its hydrated form are generated, reducing the Mo available for methane conversion.199,200 In addition, the lifetime of Mo-based zeolite catalysts can also be improved by cofeeding small amounts of other reactants,203 or by coupling MDA with other reactions mostly involving weak oxidation in the presence of COx, H2O, and NOx.192,204 For practical applications, it is essential to develop effective regeneration methods for the deactivated catalysts. To this end, a variety of atmospheres had been tried. For example, oxygen can effectively remove coke by forming COx, which, however, also facilitates the sublimation of MoOx and/or formation of inert Al2(MoO4)3. Catalyst regeneration in oxygen atmosphere can turn molybdenium carbide back to the oxide phase and redisperse the Mo nanoparticles into an isolated single Mo atom

Scheme 5. Possible Route for Aromatics and Coke Formationa

a

Reproduced with permission from ref 192. Copyright 1999 Elsevier.

Studies with X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), thermogravimetry (TG)/differential thermogravimetry (DTG), high-resolution transmission electron microscopy (HRTEM), and differential thermal analysis (DTA) showed that the sp2/sp3 ratio of the coke increases with time on stream.193 This also suggested that the polyaromatic type carbon is likely the main reason for the catalyst deactivation. At the same time, coke formation also leads to the sintering of molybdenum carbide particles present at the external surface of the zeolite due to a weak interaction between MoxCy particles with the zeolite surface.190 Studies show that deactivation of Mo/ZSM-5 catalyst occurs in three stages.194 Benzene selectivity decreases only during the last deactivation stage simultaneously with the increase of polyaromatic coke formation in the zeolite channels. During this last stage, selectivity of C2H4 increases, and Song et al. suggested that aromatictype coke formed in the zeolite channels originates mainly from C2H4 cracking.194 With coke blocking the zeolite pores and covering the acidic sites, the catalyst deactivates rather quickly, within a few hours or even less, and the selectivity switches to higher light olefins. This is a major obstacle to commercial applications of this process. Cofeed of hydrogen and methane leads to a reduced amount of aromatic-type coke inside the

Figure 8. Long-term stability of Mo/ZSM-5 catalysts in MDA at 1033−1073 K with a periodic switch of CH4/N2 to H2. The switching time was 15− 45 min, and the space velocity for H2 was 25−40 mL·min−1. Varied temperatures and additional hydrogenation regeneration time are indicated in the figure. (1) Methane conversion, (2) Benzene yield, (3) Aromatic yield, (4) Ethylene selectivity, (5) Benzene selectivity, (6) Naphthalene electivity, (7) Coke selectivity. Adapted with permission from ref 207. Copyright 2015 Elsevier. 8507


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until iron carbide has been formed. Tan concluded from XPS and XRD results that carburized Fe is the active species for aromatic formation, while metallic iron is active in methane activation but leads to the “over”-dehydrogenation of methane.220 Then the author proposed a mechanism for the formation of C2 products via iron carbene (C−FeCH2).220 Recently, Lai and Veser elaborated the nature and speciation of iron on Fe/ZSM-5 catalysts. Particularly, it was observed that benzene selectivity of the Fe-ZSM-5 catalyst is strongly correlated with the presence of highly dispersed Fe cations on the HZSM-5 micropores like Fe3+ in framework positions or in cationic positions in the zeolite channels or binuclear iron complexes in extraframework positions (Figure 9).221 However, strong coke formation still occurs

anchored to Brønsted acid site, as present in the fresh catalyst treated in O2. Too long exposure to O2 at 753 K was observed to lead to the migration of Mo on the extraframework of the zeolite and a decrease of the benzene yield.153 Cyclic oxidative regeneration applying 1.5 h reaction, 0.5 h regeneration in O2, and a recarburization period significantly enhances the activity and benzene yield in comparison with a single-run reaction; however, the activity of the catalyst decreases over an 18 h period, indicating that some irreversible changes occur during the regeneration.205 A recent study presented a new approach of periodic pulsing of oxygen into the methane feed in order to tackle the coke-formation issue in MDA over Mo/ZSM-5 catalyst.206 The strategy was based on the presumption that soft coke can be burned away with oxygen. Thus, by supplying periodically short pulses of oxygen at an optimized frequency allowed substantial stabilization of methane dehydroaromatization at 973 K. In addition, the cumulative benzene yield was more than two times higher than a reference test with a methane-only feed.206 In this process, the only nonhydrocarbon side-product was syngas. CO2 is supposedly formed during the process but reacts further with coke via Boudouard chemistry (C + CO2 = 2CO) and therefore helps to enhance coke removal. The authors believed that pulsed O2 process combined with cyclic regeneration could be a powerful method toward a viable MDA process.206 In comparison, hydrogen treatment was also studied and approved to be effective via hydrogenation of coke and the catalyst lifetime is significantly extended.196,197,207 A nanosized HZSM-5-supported Mo catalyst was tested in a 1000 h time on stream at 1033−1073 K with a periodic switch of CH4/N2 and H2.207 This was achieved in a specially designed four-channel, fixed-bed microreactor. Figure 8 shows that the performance is rather stable, with methane conversion in a range of 13%−16% and yield to aromatics (i.e., benzene and naphthalene) exceeding 10%, which is similar to the initial activity in the normal operation. Unfortunately, hydrogenation of coke is a slow reaction and coke cannot be removed completely, as revealed by TG and temperature-programmed oxidation (TPO) studies.207 Even with the periodic oxygen combustion, coking cannot be completely suppressed.206 Therefore, further work is still needed to explore a more effective regeneration approach. Beside molybdenum, iron was also found to be active in methane activation. For instance, mono- or dinuclear FeO centers analogue to the active center in methane monooxygenase (MMO) were observed over Fe/ZSM-5 catalysts.208,209 Therefore, Fe/ZSM-5 was widely studied as a catalyst for the partial oxidation of methane to formaldehyde and methanol.210−212 In addition, Fe/ZSM-5 can also catalyze methane conversion under nonoxidative conditions, similar to Mo/ZSM-5.213 MDA reactivity over Fe/ZSM-5 depends strongly on iron species and the Brønsted acidity of the zeolite. Weckhuysen et al. reported a benzene selectivity of 74% at a methane conversion of ∼4% over an impregnated Fe/ZSM-5 catalyst.179,213 The catalyst was observed to undergo an induction period in methane feed prior to the formation of products (benzene, toluene, and naphthalene). The study demonstrated that the activity was based on a unique balance between formation of iron clusters on the surface of the zeolite and the presence of a sufficient number of Brønsted acid sites.179,213 Over Fe/ZSM-5 catalysts, a series of different iron species have been identified.214−219 X-ray photoelectron spectroscopy evidenced that Fe2O3 is first reduced to metallic iron (Fe°) followed by the formation of iron carbide during the MDA reaction.220 Aromatic products are not observed

Figure 9. Representation of (a) Fe3+ in framework positions (isomorphously substituted), (b) Fe3+ in cationic positions in the zeolite channels, (c) binuclear and, in general, oligonuclear iron complexes in extraframework positions, (d) iron oxide FeOx nanoparticles, and (e) large iron oxide particles Fe2O3. Reproduced with permission from ref 216. Copyright 2002 Elsevier.

via secondary reactions of aromatic products over Brønsted acid sites and hence limits the aromatics yield.221 In light of the recent work on Mo/ZSM-5153,161 and Fe/ZSM220,221 a striking similarity can be drawn for the two catalysts in 5, the MDA reaction: • The role of metal speciation and especially the importance of isolated cations leading to active and selective catalysts; • The role of Brønsted acid sites for the speciation of metal, the aromatization of C2 intermediates, and the formation of coke due to secondary reaction by proton transfer; and • Induction time prior to the formation of aromatics during which metal is carburized to form active species. The above studies reveal the importance of highly dispersed Fe cation on the HZSM-5 micropores, which are selective in methane activation. However, the presence of Brønsted acid sites leads to coke formation. In this respect, it is reasonable to understand that the atomically dispersed iron species within the matrix of silica can activate methane and that nonacidic silica could avoid severe coke deposition in the newly developed process of methane to olefins, aromatics, and hydrogen (MTOAH). For more details on the methane dehydroaromatization (MDA) reaction, readers are invited to read more specialized reviews on the subject, for instance, Ismagilov et al.,59 Ma et al.,60 or Vogt et al.62 and references therein. 2.3. C−H Activation under Nonoxidative Conditions (MTOAH) over Single Site of Iron (Fe©SiO2)

2.3.1. C−H Activation on Single Site of Metals. The MDA and OCM processes for methane conversion have their 8508


Chemical Reviews

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A lifetime test at 1303 K for 60 h on stream showed that methane conversion remains at ∼20% throughout this run, with selectivity to C2, benzene, and naphthalene remaining >99%. Furthermore, the product selectivity toward C2 or aromatics is tunable by varying the types of sweep gases and their flow rates. For instance, the selectivity to naphthalene increases with increasing He flow rate while that to C2 and benzene product increases with increasing H2 flow rates.227 2.3.2. Heterolytic Formation and Homolytic Conversion of Methyl Radicals. Such lattice-confined iron single sites behave distinctly differently from iron nanoparticles (NPs).48 For comparison, iron was dispersed over a high surface area SiO2 support (348 m2·g−1) with the same loading via the impregnation method (0.5% Fe/SiO2), which exhibits a narrow particle size distribution in a range of 2−5 nm. Under the same reaction conditions in MTOAH, this catalyst gives a large amount of coke with a selectivity of >98%. Even by varying the support materials, the preparation methods, and the iron loadings, the coke formation cannot be completely avoided over catalysts prepared by impregnation. The in situ X-ray absorption fine structure spectroscopy (XAFS) indicates that the impregnated Fe/SiO2 catalyst exhibits a metallic state and only Fe−Fe bonding is observed under reaction conditions, reflecting the presence of NPs.48 It is known that iron NPs are widely used catalysts for growth of carbon nanotubes and nanofibers with methane as the carbon source.228,229 That process is generally accepted to involve catalytic cleavage of all four C−H bonds of methane leading to formation of carbon atoms and hydrogen, followed by the dissolution of carbon species into the iron lattice forming iron carbide. With carefully controlled conditions, recrystallization from the supersaturated carbide solid solution and C−C coupling on iron NP surface drive the growth of nanotubes. Depending on the size of Fe NPs, nanotubes with various diameters can be produced.229−231 With the size of Fe NPs decreasing to ∼1 nm, graphene flakes and carbon deposit may dominate. This may explain the extensive carbon deposition over the Fe NPs catalysts. In contrast, iron single sites can avoid catalytic C−C coupling of methyl groups. Thereby, the formed methyl radicals over Fe©SiO2 undergo a series of gas-phase reactions, forming thermodynamically stable products such as ethylene, benzene, and naphthalene. In comparison, noncatalytic pyrolysis of CH4 has been extensively studied for light hydrocarbon synthesis. However, the product was dominated with acetylene accompanied by severe coke formation.27,232,233 It was noted previously that the quartz wall of the reactor may have the capability to activate methane.234−236 The following defect centers may be present in the quartz wall of the reactor such as Si·, Si−O·, Si-OO·, or peroxide groups. These sites were proposed to be active in activating methane via methylation, giving (Si−O)2Si(OH)(CH3) groups as products. The following study by the same authors showed that the main channel for the pyrolysis of (Si− O)2Si(OH)(CH3) groups was their decomposition, giving a methane molecule.237 However, no conversion of methane was reported in those reports. In the study of Guo et al.,48 it was demonstrated that a blank experiment without a catalyst gave a CH4 conversion of only 2.5% and 95% of the products was coke (see Figure 10), in contrast to the performance given by 0.5% Fe©SiO2 under the same conditions. However, it is not clear yet if this 2.5% conversion in the blank reactor was attributed to pure pyrolysis of methane or catalyzed by the quartz defect sites or by the traces of metal impurities such as Fe, Ti, and Ca in a range of few hundred ppm contained in the quartz reactor material

pros and cons. To make methane conversion much more efficient with less CO2 emission and coke deposition, new catalysts and new concepts are obviously needed. The high activity of coordinatively unsaturated iron sites toward the C−H bond of CH4 was already reported previously.49,125,222 Fe stabilized at cation-exchanged sites associated with framework Al atoms can catalyze methane oxidation with N2O as the oxidant.223 Just recently, Impeng et al. reported based on DFT calculations that Fe- and FeO-embedded graphene and boron nitride sheets are highly active for methane C−H bond cleavage and methane oxidation to methanol.224,225 As discussed above, highly dispersed Fe cations on the HZSM-5 micropores have been shown to be active in methane dehydroaromatization under nonoxidative conditions, but accompanied by severe coke deposition.179,213,220,221,226 By using nonacidic silica lattice-confined single-iron sites, methane can be activated and intensive coke deposition can be avoided.48 Iron species were embedded within the lattice of silica by the following method.48 Commercial SiO2 with a BET surface area SiO Groups. Kinet. Catal. 2004, 45, 265−272. (238) Bao, X.; Guo, X.; Fang, X.; Pan, X.; Meng, J.; Yu, Q.; Dali, T. A design of catalytic reactor for methane conversion to olefins, aromatics and hydrogen under non-oxidative conditions. CN Patent App. CN201610286107.6, 2016. (239) Lee, M.-t.; Greif, R.; Grigoropoulos, C. P.; Park, H. G.; Hsu, F. K. Transport in packed-bed and wall-coated steam-methanol reformers. J. Power Sources 2007, 166, 194−201. (240) Berger, R. J.; Kapteijn, F. Coated-Wall Reactor ModelingCriteria for Neglecting Radial Concentration Gradients. 2. Reactor Tubes Filled with Inert Particles. Ind. Eng. Chem. Res. 2007, 46, 3871−3876. (241) Berger, R. J.; Kapteijn, F. Coated-Wall Reactor ModelingCriteria for Neglecting Radial Concentration Gradients. 1. Empty Reactor Tubes. Ind. Eng. Chem. Res. 2007, 46, 3863−3870. (242) Li, J.; Zhou, S.; Zhang, J.; Schlangen, M.; Weiske, T.; Usharani, D.; Shaik, S.; Schwarz, H. Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2]+ (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer. J. Am. Chem. Soc. 2016, 138, 7973−7981. (PMID: 27241233). (243) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Müller, J.; Lippard, S. J. Dioxygen Activation and Methane Hydroxylation by Soluble Methane Monooxygenase: A Tale of Two Irons and Three Proteins. Angew. Chem., Int. Ed. 2001, 40, 2782−2807. (244) Dooley, S.; Burke, M. P.; Chaos, M.; Stein, Y.; Dryer, F. L.; Zhukov, V. P.; Finch, O.; Simmie, J. M.; Curran, H. J. Methyl formate oxidation: Speciation data, laminar burning velocities, ignition delay times, and a validated chemical kinetic model. Int. J. Chem. Kinet. 2010, 42, 527−549. (245) Sinev, M. Y.; Margolis, L. Y.; Korchak, V. N. Heterogeneous freeradical reactions in oxidation processes. Russ. Chem. Rev. 1995, 64, 349. (246) Casavecchia, P. Chemical reaction dynamics with molecular beams. Rep. Prog. Phys. 2000, 63, 355. (247) Balucani, N.; Leonori, F.; Casavecchia, P. Crossed molecular beam studies of bimolecular reactions of relevance in combustion. Energy 2012, 43, 47−54. (Second International Meeting on Cleaner Combustion (CM0901-Detailed Chemical Models for Cleaner Combustion)).

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Direct Conversion of Methane to Value-Added ... - ACS Publications

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