Dimethoxymethane as a Cleaner Synthetic Fuel: Synthetic Methods

Review Cite This: ACS Catal. 2019, 9, 1298−1318

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Dimethoxymethane as a Cleaner Synthetic Fuel: Synthetic Methods, Catalysts, and Reaction Mechanism Ruiyan Sun, Irina Delidovich, and Regina Palkovits*

ACS Catal. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/15/19. For personal use only.

Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany ABSTRACT: The increasing concerns regarding exhaust and CO2 emissions from fossil-based transportation fuels have propelled intensive research aimed at finding alternative fuel candidates to realize a clean and renewable fuel system. In this context, dimethoxymethane and its derivatives oxymethylene ethers, a class of oxygenated synthetic fuel, have recently attracted increasing interest because of their fascinating characteristics as a diesel blend compound to significantly reduce soot and nitrogen oxide formation. At present, dimethoxymethane production primarily relies on an established two-step process comprising methanol oxidation and methanol condensation with formaldehyde. Several new synthetic routes based on methanol or CO2/H2 have been proposed by adopting a reaction coupling strategy, which enables the production of dimethoxymethane in one step. A large variety of bi- and multifunctional catalysts have been developed for each synthetic route. This Review comprehensively summarizes the latest advances in synthetic approaches, catalyst systems, structure−activity relationships, and reaction mechanism for the catalytic synthesis of dimethoxymethane. Comparisons regarding the features and limitations of different synthetic approaches as well as the related catalytic materials are also provided in order to indicate possible directions for future research, especially on the rational design of catalysts, a vital factor for the commercial production of dimethoxymethane. KEYWORDS: fuel additive, diesel, dimethoxymethane, oxymethylene ether, bifunctional catalyst, structure−activity relationship, reaction mechanism

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effective method to reduce soot emissions.8−10 However, it is very desirable to find an appropriate blend which is completely miscible with diesel fuel and exhibits comparable physical properties. Thus, a modification of the diesel engine itself could be avoided. Unfortunately, simple alcohols or ethers do not meet these requirements. Oxymethylene dimethyl ethers (OMEs), a class of oligomers with the repeat unit of −(O−CH2)n− and two ending groups of −O−CH3 (methoxyl) and −CH3 (methyl), have become the focus in the field of fuel additives in recent years. It was demonstrated that no matter if solely used as an alternative fuel to diesel or blended into diesel fuel, OMEs could significantly suppress soot formation due to their high oxygen content and the absence of C−C bonds in their structures.11−17 OMEs are therefore considered as promising oxygenated compounds to solve the issue of diesel exhaust. The chain length determines the physicochemical properties of OMEs. The medium-chain length of OME (n = 3−5) are especially preferred, because they hold very similar properties to standard diesel fuel in terms of viscosity, cetane number, and vapor pressure.18 OME3−5 could allow for replacing conventional diesel or blending into diesel with limited modifications of engine and fuel infrastructure needed.16

INTRODUCTION Diesel fuel, commonly used to power vehicles and construction machinery, plays a significant role in the transportation sector ever since its first application in the compression-ignition engine in 1895 by Rudolf Diesel. As a major transportation fuel, approximately 94% freight relies on diesel fuel because of the greater energy density as well as the higher fuel efficiency of diesel engines.1,2 Additionally, diesel engines produce a lower amount of CO and CO2 emissions than gasoline engines. This made the diesel engines especially attractive after the Kyoto Protocol entered into force in the 1990s. The share of diesel passenger vehicles in Western Europe rose from 14% in 1990 until 50% nowadays. However, legislative bodies of many European cities are about to ban diesel vehicles in the near future. These actions are taken after manifesting cytotoxicity of diesel exhausts. The latter were considered by the International Agency for Research on Cancer as probable human carcinogenic in 1989;3 their cytotoxicity was confirmed by the World Health Organization (WHO) in 2012.4 The exhaust gas contains large amounts of nitrogen oxides (NOx) and particulate matter (soot), which pose adverse effects on human health and ecosystem. These disclosures propelled intensive research aimed at removal of NOx and soot from diesel exhausts.5−7 It was shown that the addition of oxygencontaining compounds such as dimethyl ether (DME), ethanol, and dimethyl carbonate to diesel fuel presents an © XXXX American Chemical Society

Received: November 5, 2018 Revised: December 19, 2018

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DOI: 10.1021/acscatal.8b04441 ACS Catal. 2019, 9, 1298−1318

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Figure 1. Overview of the synthetic routes and applications of DMM

this Review paper, we aim to provide an overview exclusively focused on DMM addressing synthetic methods, catalyst families, and reaction mechanisms. The emphasis is placed on the reported catalytic materials for each catalytic route, their catalytic performances, and the structure−activity relationships.

The simplest compound in the OME family is OME1, also named dimethoxymethane (DMM) or methylal. Currently, DMM is used predominantly as a solvent in industry, and it is also considered as a potential feedstock to produce concentrated formaldehyde (FA, monomeric H2CO) (Figure 1). It has a lower boiling point (42 °C), cetane number (29), and viscosity (0.36 mm2/s) compared with standard diesel fuel.18 Consequently, when DMM is blended into diesel fuel (DMM-blend), significant modifications of the engine are required. However, DMM presents an important platform for providing high yields of OME3−5 when reacted with FA. The increased interest in the field of OME3−5 synthesis is reflected in a great number of research articles and patents published in the past few years.19−24 A method for producing OME3−5 based on DMM and paraformaldehyde in a fluidized-bed reactor was already industrialized by Shandong Yuhuang Chemical Co. in 2015.25 Sustainability of DMM for producing fuel additives presents a vital question.26 An analysis of DMM synthesis based on CO2 produced from biogas and H2 supplied by water electrolysis has been performed by Deutz et al. recently using a life cycle assessment.27 The authors considered a blend of diesel fuel with DMM, as the most readily available representative of OMEs. This study showed that a DMMblend could substantially reduce the impact of global warming besides the reduction in soot and NOx emissions compared to fossil diesel. The share of renewable energy in electricity proved essential for the environmental impact. Under appropriate conditions, DMM bears a very high potential for a global production of environmentally benign additives to diesel fuels and as intermediate in other chemical value chains. Nowadays, DMM is produced on the basis of fossil feedstocks via a two-step process (Figure 1). The first step presents a well-established methanol oxidation to FA in gas phase, followed by acetalization with methanol to yield DMM in liquid phase.28,29 At the same time, the direct synthesis of DMM (i.e., one-step oxidation of methanol or DME to DMM in gas phase) was also intensively studied. It presents potential benefits of using fewer operation units and avoiding steps for recovery and purification of the intermediate.30 Most recently, renewed interest arose with the report of two novel routes: the reductive synthesis of DMM directly from CO2 and H2 and the dehydrogenative synthesis of DMM directly from methanol.31,32 The ongoing investigations on DMM aim at the development of highly active catalysts and efficient technical processes, respectively. Although there are a number of research papers on DMM synthesis, only few reviews were published. The first review focusing on DMM was published in 2016 by Thavornprasert et al.,30 who systematically summarized the one-step production of DMM from methanol in gas phase. Baranowski et al. reviewed the production of longer OMEs by different catalytic routes not focusing on DMM.19 In

1. INDIRECT SYNTHESIS OF DMM 1.1. Chemical Reactions. Currently, the indirect synthetic route predominates in the commercial production of DMM. This process consists of two consecutive steps. In the first step, FA is synthesized either by a methanol oxidation over iron molybdate or a methanol dehydrogenation over an Ag catalyst. Both processes for FA synthesis are used in industry (eq 1).

The second step (eq 2) of DMM synthesis concerns acetalization of FA with methanol. The acetalization presents a reversible reaction.33 Reaction 2 is believed to proceed as follows: one mole of FA first reacts with one mole of methanol to produce one mole of hemiacetal (eq 3). The latter condenses with another mole of methanol to yield one mole of DMM and one mole of water (eq 4).33,34 Reaction 3 can even take place without acidic catalysts, but for reaction 4 an acidic catalyst is required. Kinetic investigations indicated that reaction 4 is the rate-determining step in the overall reaction, since reaction 3 is very fast and its equilibrium is reached nearly instantaneously.35 The reversibility of acetalization results in a slow reaction progress when equilibrium conversion is reached as well as a thermodynamically limited DMM yield. A possible approach to overcome the thermodynamic restrictions relates to an in situ removal of DMM by reactive distillation.28,36,37 This is complicated by the formation of a low-boiling azeotrope composed of 92.2% DMM and 7.8% methanol, which is typically formed in the light boiler fraction.38 Thus, further purification steps are required to obtain DMM. For example, extractive distillation and pressure swing distillation were reported to be efficient.38,39 Based on the integrated process of 1299

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Amberlyst 15 in DMM synthesis in a plug flow reactor.47 Owing to the inclusion of the relevant oligomerization of FA into this model and the usage of actual intermediate concentrations for calculation, the parameters of this model matched the experimental results well (Figure 2).

reactive and extractive distillation, a technology for commercial manufacture of DMM was established in 1993 by Asahi Chemical Industry Co. This process utilized sulfonated crosslinked polystyrene as a catalyst and produced DMM in 99% purity at nearly 100% FA conversion.40 1.2. Acid Catalysts. In the indirect synthetic process of DMM, a mixture of liquid methanol with aqueous FA solution is commonly used as the substrate. This reaction usually proceeds in liquid phase in a batch reactor or a continuous reactor coupled with a distillation column at a temperature below 100 °C, at atmospheric pressure and a molar ratio of methanol/FA equal or greater than 2. A gas-phase synthetic approach based on the condensation of gaseous methanol with FA has been recently reported by the research group of Ferdi Schüth.41 Generally, an acid catalyst is required to accelerate the DMM synthesis, especially for the acetalization step. A number of liquid or solid acids with different types of acidic centers (Lewis or Brönsted) have been described in scientific papers and patents as acidic catalysts for DMM synthesis. For example, traditional soluble acids such as H2SO4, HCl, pTsOH and FeCl3 were the first investigated catalysts and showed high activity for the production of DMM.42−44 Damiri et al. compared the activities of p-TsOH and H2SO4 with Amberlite resins varying the catalyst amount.42 The different catalysts possessed the same kinetic behavior when compared at an equivalent proton concentration. It was therefore concluded that their performance was proportional to the amount of protons released by the catalysts, which indicated that protons played the main catalytic role. Also ionic liquids with strong Brönsted acidity show activity in catalyzing the formation of DMM.45 Sun et al. found that the catalytic performance of ionic liquids depended on the carbon chain length of the alkyl groups. Over the optimal acidic ionic liquid with a carbon chain length of 6 ([C6ImBS][HSO4]), 55% conversion of FA was reached after 4 h at 60 °C, comparable to that of concentrated sulfuric acid under identical reaction conditions.46 Importantly, the ionic liquid [C6ImBS][HSO4] can be easily separated from the reaction mixture by distillation and successfully recycled five times. Compared with the soluble acids, solid acids including ion exchange resins, sulfonated tetrafluoroethylene resin, heteropolyacids, and zeolites are advantageous for DMM synthesis,28,37,38,41,47−49 due to their recyclability, easy handling, low corrosion potential, and facilitated product purification. Solid acids were primarily applied in reactive distillation. Therein, studies focusing on solid acids catalyzed DMM synthesis mostly regard engineering aspects, simulating and optimizing the reaction-distillation process with only few examples on the catalytic performance. Sharma et al. investigated the factors influencing the chemical equilibrium of DMM production.36 They improved the equilibrium conversion of FA from 47 to 81% by increasing the ratio of methanol to FA from 2 to 6 in the presence of the cationexchange resin Indion 130. Zhang et al. examined Amberlyst 15 for the synthesis of DMM based on methanol and trioxane in a batch reactor. The equilibrium was reached with 62% yield of DMM after 3 h at 70 °C applying a methanol/trioxane molar ratio of 2.2.48 In contrast, when producing DMM by reaction-distillation, a nearly complete conversion of formaldehyde as well as a high purity of DMM (>90%) can be achieved in a single pass, irrespective of the applied solid acid. Besides, a pseudohomogeneous reaction kinetic model was proposed by Hasse et al. to describe the catalytic behavior of

Figure 2. Comparison of concentrations of methanol (□), water (Δ), DMM (x), and formaldehyde (O) between experimental (symbols) and model data (lines). Reproduced with permission from ref 47. Copyright 2012 Elsevier.

A solid acid catalyst featuring high porosity, abundant macropores, appropriate mechanical strength, and good proton-donating ability is required for practical application in reactive distillation. Zhang et al. studied a polystyrene-based macroporous cation-exchange resin (D72) in a catalytic distillation column on pilot scale.38 The effects of operation parameters such as molar ratio of methanol to formaldehyde, feed rate, reflux ratio, catalyst weight, and extractant feeding position were systematically investigated. Under the optimal operating conditions with a methanol to formaldehyde molar ratio of 2.5, a catalyst weight of 0.51 kg, a feed rate of 3.1 kg/h, a reflux ratio of 5 and using aqueous FA as extractant fed at midway of the rectifying section, the conversion of formaldehyde and the purity of DMM in the distillate were up to 99.6% and 92.1%, respectively. A very recent work by Grünert et al. addressed the structure−performance relationship for the gas-phase synthesis of DMM catalyzed by diverse zeolites in a plug-flow reactor.41 They discovered that a lower number of Brönsted acid sites (at a high silica/alumina ratio) or extra framework Al sites (Lewis acid) are beneficial for achieving a high selectivity of DMM. An excess of acidic sites resulted in a significant reduction in DMM selectivity due to the favored formation of side products such as methyl formate and dimethyl ether, which are usually observed in trace amount in liquid phase. Interestingly, a pure silica zeolite silicalite-1, possessing very weak Brönsted acid strength and a negligible number of acidic sites provided by silanol groups, facilitated the highest single-pass DMM yield of 40% among the examined zeolites. This is markedly different from the liquid-phase synthesis of DMM and longer chain length of OME, for which medium/strong Brönsted acidity usually exhibited the best performance.50 1.3. Reaction Mechanism. The indirect synthesis of DMM is realized through a stepwise acetalization of form1300

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ACS Catalysis aldehyde with methanol, which follows the reaction mechanism of nucleophilic addition.36 Hence, based on the classical proton-catalyzed nucleophilic addition, a reaction mechanism of DMM synthesis from methanol and FA is proposed in Scheme 1. The carbonyl group in FA is first protonated leading

Scheme 2. Proposed Reaction Network for Methanol Oxidation to DMM. Adapted with Permission from Ref 51. Copyright 1997 Elsevier

Scheme 1. Reaction Mechanism for the Synthesis of DMM from Methanol and Formaldehyde in the Presence of a Brönsted Acid36

Bifunctional catalysts with two types of active sites are widely used in this one-step process: oxidizing sites (also called basic site) for oxidation of methanol to FA and acidic sites for in situ acetalization of formaldehyde with methanol to DMM. A complicated reaction network is involved in the synthesis of DMM. As depicted in Scheme 2, the composition of the products is influenced by the acidity (vertical direction) and redox property (horizontal direction) of the catalyst. If the catalyst exhibits high acidity, DME is produced as the primary product (Scheme 2). On the other hand, a high oxidative potential of the catalyst results in a high selectivity of oxidation products such as FA, formic acid, and COx (red box zone in Scheme 2). DMM or MF will become the main products provided that both catalytic acid and redox centers are present (blue box zone in Scheme 2). The related selectivity toward DMM or MF depends primarily on the relative strength and surface concentration of acidic and redox sites. DMM and MF are both competitively produced from the same intermediate hemiacetal, which can be oxidized to MF or condense with methanol to form DMM. Thus, to maximize the yield of DMM, an appropriate balance between oxidizing and acidic functions is required for an efficient bifunctional catalyst. However, the design of a catalyst with an optimum nature and concentration of active sites together with a well-tailored ratio between acid and redox properties still remains a challenge, which has stimulated substantial research efforts. A great variety of bifunctional catalysts has been proposed for the one-step oxidation of methanol to DMM. Generally, the reported catalysts can be classified into two groups: noble and non-noble metal catalysts. Noble metal catalysts mainly include Re-based and Ru-based catalysts. Mo-based and Vbased catalysts account for the majority of non-noble metal systems. Each catalyst group including their preparation methods, reaction conditions, and catalytic performances will be reviewed individually in the following sections. Insights into the active sites as well as the available insights into structure− activity relationships will be also discussed. Methanol conversion and DMM selectivity are commonly used to quantitatively describe the performance of a catalyst in the one-step oxidation of methanol to DMM. However, the reported results are typically normalized to neither the amount of catalytic sites nor the contact time. In this Review, catalytic activities will be compared in terms of the DMM formation rate in order to compare the site-time-yield obtained in the presence of various catalysts. The reported yields of DMM in this Review were converted to the DMM formation rates based

to the formation of activated FA. The subsequent nucleophilic addition of methanol to the activated FA generates the protonated hemiacetal, which further deprotonates to the hemiacetal assisted by water. Then, the hydroxyl group of the hemiacetal undergoes sequential protonation and water elimination to form an intermediate containing an activated carbonyl group. DMM is finally obtained through the nucleophilic addition of another methanol to the activated carbonyl group and the following deprotonation.

2. DIRECT SYNTHESIS OF DMM VIA METHANOL OXIDATION 2.1. Chemical Reactions. This section specifically addresses the one-step production of DMM from methanol in gas phase using molecular oxygen as an oxidant (eq 5). In

essence, the one-step route can be considered as a combination of methanol oxidation to FA (eq 1) and the subsequent acetalization of FA with methanol (eq 2) without an intermediate isolation of FA. Owing to high reactivity of all the compounds present in the reactor, including methanol, formaldehyde and hemiacetal, the oxidation of methanol to DMM is always accompanied by several side reactions.51 As shown in Scheme 2, DME is formed via methanol dehydration catalyzed by acids. Formaldehyde can be oxidized to formic acid which either reacts with methanol to form methyl formate (MF) or further decomposes to COx (CO and CO2). In addition, MF can be produced from FA via Tishchenko reaction. 2.2. Bifunctional Catalysts. The direct oxidation of methanol to DMM is usually implemented in a fixed-bed tubular reactor using gaseous methanol (vaporization required) and air as substrates under atmospheric pressure. To avoid the explosive risk of methanol in air, the applied methanol concentration in the feed is either below 7% or higher than 36%. The selection of other operating conditions such as temperature and gas hourly space velocity (GHSV) depends on the applied catalyst as discussed in the following. 1301

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ACS Catalysis Table 1. Noble Metal Catalysts for the One-Step Oxidation of Methanol to DMMa selectivity (%) CH3OH concn (mol %)

GHSV (mL/h*g)

temp (°C)

conv. (%)b

FA

DME

MF

DMM

CO+CO2

DMM formation rate (h−1)

ref

4 4 4 4 4 4 4 4 4 4 4 40 40

10000 40000 40000 40000 40000 40000 40000 40000 40000 40000 40000 n.a. n.a.

300 240 240 240 240 240 240 240 240 240 240 275 275

6.5 53.7 59.5 21.5 35.8 15.5 48.4 15.1 16.3 9.1 4 23

1.9 4.1 2 2 2.4 1.3 2.8 37 6

6.3 0.7 1.1 4.3 trace 1 1 trace trace 19 30 11

1.2 9.1 11.7 7.6 6 4.6 11.9 5.9 13 13

92.5 83.1 78.5 93.7 89.4 90.5 91 60.7 88.3 80 17 67

5.2 4.6 2 1 0.5 1 26.1 2.9 1 3 2

0.1 18.1 18.9 8.2 12.9 5.7 17.9 3.7 5.8 3.0 n.a. n.a.

54 56 56 56 56 56 56 56 56 56 56 57 57

4

26000

260

32.3

5.2

8.3

21.7

54.8

10

6.8

59

4

26000

260

44.3

1.2

12.1

7.8

76.7

2

13

59

4.7 4 4 4

26000 n.a. n.a. n.a.

240 120 120 120

0.8 34.9 12.4 11.6

6 n.a. n.a. n.a.

2.2 53.5 31 30.1

89.1 2.2 56.1 57.4

1.9 9.4 0.6 1

10.7 0.02 1.6 2.9

61 53 53 53

4.1%RuO2/ZrO2

4

n.a.

120

6.6

n.a.

70.7

5.6

16.8

0.3

53

2.2%RuO2/TiO2

4

n.a.

120

25.2

n.a.

69.9

4.1

0.9

0.2

53

4.1%RuO2/SnO2

4

n.a.

120

20

n.a.

60.7

15.5

3.8

1.6

53

4.7% Na(RuO2)Ad

4

n.a.

120

35

n.a.

52

6

7

3.4

62

9.5%RuO2/CNTe

6.9

n.a.

120

17

n.a

34

49

-

25.8

63

9.5%RuO2/CNTf

6.9

n.a.

120

27

n.a

66

6

-

1.6

63

RuCl3g

n.a.

n.a.

120

40.6 ∼20 (2.3 h−1) ∼20 (8.3 h−1) ∼20 (14.9 h−1) ∼20 (17.6 h−1) ∼20 (14.5 h−1) ∼20 (30.6 h−1) ∼20 (171 h−1) 13−25 (158 h−1) 13−25 (79 h−1) 49.8

n.a

n.a

23.2

76.8

n.a

52.3

64

catalyst SbRe2O6 10%Re/TiO2-rutile 10%Re/TiO2-anatase 10%Re/V2O5 10%Re/ZrO2 10%Re/α-Fe2O3 10%Re/γ-Fe2O3 10%Re/SiO2 10%Re/α-Al2O3 10%Re/Sb2O3 10%Re/MoO3 2%Re/TiO2-anatase 10%Re/TaOx/TiO2 -anatase 6.9%Re/TiO2-anataseFc 6.9%Re/TiO2-anataseKc 10.5%Re/SiO2 Bulk RuO2 4.3%RuO2/SiO2 4.4%RuO2/Al2O3

a

n.a. = not available; FA = formaldehyde; DME = dimethyl ether; MF = methyl formate; selectivity reported on carbon basis; all reactions were conducted in a continuous reactor in gas phase under atmospheric pressure unless otherwise stated. bData in the parentheses are methanol conversion rate, calculated according to the moles of methanol converted per total metal site per hour. cF and K denote Hombikat F01 and Hombikat K03, the type number of TiO2, respectively. dMethanol conversion rate was calculated based on surface Ru site. eAnnealing at 100 °C. f Annealing at 400 °C. gLiquid phase reaction in a batch reactor. Twenty mL of methanol, 3 MPa O2, 0.1 g of RuCl3, 2.5 h.

2.2.1. Noble-Metal Catalysts. 2.2.1.1. Re-Based Catalysts. The term “Re-based catalysts” refers to materials bearing rhenium oxides (ReOx). These materials are the first catalysts uncovered for methanol oxidation to DMM. Re-based catalysts are generally used at temperatures around 240 °C, GHSV = 40 000 mL/h*g, and a low methanol concentration of 4%. Most Re-based catalysts exhibit high DMM selectivities of 80 to 94% but achieve significantly varying methanol conversions in the range of 6 to 60%. The relatively high reaction temperature required for Re-based catalysts also brings about a fast deactivation owing to the sublimation of ReOx, which is problematic for large-scale application. Table 1 summarizes the catalytic performances of Re-based catalysts as well as other noble metal catalysts for the direct synthesis of DMM by methanol oxidation. In 2000,54,55 Yuan et al. discovered a binary metal oxide composed of Re and Sb (SbRe2O6) showing high selectivity toward DMM (92.5%) when applied for catalytic methanol oxidation. Experimental and characterization results suggested the lattice oxygen species of SbRe2O6

on the concept of productivity. Specifically, the DMM formation rates (h−1) reported here were calculated by eq 6, where αCH3OH denotes the conversion of methanol, δDMM denotes the selectivity of DMM, FCH3OH denotes the feeding rate of methanol (mol/h*g catalyst), Nmetal atoms denotes the total amount of metal atoms per gram of catalyst (mol/g catalyst), and the factor 3 is included to account for the reaction stoichiometry. It is generally recognized that the formation rate of DMM is principally controlled by the methanol oxidation to formaldehyde over oxidizing sites,51−53 although DMM is produced by a concerted catalysis of acidic with oxidizing catalytic centers. We therefore adopt the total amount of metal atoms, that is, oxidizing sites in the catalyst as the amount of active sites for DMM yield recalculation. DMM formation rate =

αCH3OH × δ DMM × FCH3OH 3 × Nmetal atoms

(6) 1302

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ACS Catalysis

was accompanied by a decreased selectivity of oxidation products. This indicated that the redox property of Re/SiO2 prepared by the sol−gel method was much lower than that prepared by incipient wetness impregnation, and the reduced redox property of Re was more favorable for DMM synthesis. Operando Raman spectroscopy evidenced the presence of a peculiar structure of ReO x , that is, a hydrated ReO 4 tetrahedron formed by reversible binding of Re species with water. The authors proposed that the markedly increased activity can be ascribed to this weak interaction between ReOx and water resulting in a moderate redox property. 2.2.1.2. Ru-Based Catalysts. In view of thermodynamics and kinetics, a low reaction temperature is more favorable for DMM synthesis because of the exothermic nature of DMM formation and the reduced rates of side reactions at low temperatures. The outstanding catalytic activity of RuO2 for oxidations enables methanol oxidation to DMM at low temperatures (60−120 °C). Liu et al. dispersed RuO2 domains on various metal oxides to attain bifunctional Ru-based catalysts.53 It was found that product distribution is decided by the nature of the metal oxide. Metal oxides including SnO2, ZrO2, and TiO2 with redox and amphoteric sites directed the reaction toward MF formation (60.7−70.1% selectivity). An explanation relates to the flexible valence states of these oxides potentially facilitating the redox cycles of RuO2 clusters and therewith contributing to a strong redox ability of RuO2. Over acidic supports such as Al2O3 and SiO 2, DMM was preferentially formed with selectivities of 57.4 and 56.1%, together with MF with selectivities of 30.1 and 31% (Table 1), respectively. The redox ability of RuO2 is dependent on its particle size and the thermal treatment temperature. Encapsulating RuO2 into the cage of zeolite-A (Na) allowed a high dispersion with a uniform particle size of ca. 1 nm.62 Indeed, this catalyst provided an even higher methanol conversion rate of 171 h−1 and DMM formation rate of 3.4 h−1 compared to supported RuO2 domains on metal oxides. Hao Yu et al. used a homogeneous oxidation precipitation method to prepare hydrous RuO2 (RuO2·xH2O) anchored onto functionalized carbon nanotubes (RuO2/CNT).63 The oxygen-containing species on the surface of functionalized CNT contributed to the formation of highly dispersed RuO2 clusters. With annealing temperatures increasing from 100 to 400 °C, DMM selectivity significantly decreased from 49 to 6%. Correspondingly, the DMM formation rate also decreased from 25.8 to 1.6 h−1. Upon thermal treatment, RuO2·xH2O transformed into anhydrous RuO2 by dehydration revealing significantly lower activity compared to RuO2·xH2O. This finding is in good agreement with the reported high activity of hydrated Re/SiO2.61 The annealing treatment was proposed to promote electron transfer from CNT to RuO2 clusters. Thus, the nucleophilicity of anhydrous RuO2 increased favoring the production of MF. Interestingly, ruthenium chloride (RuCl3), an inorganic salt soluble in methanol, was found to be effective for the one-step oxidation of methanol to DMM in a batch reactor.64 The formation rate of DMM was up to 52.3 h−1 within 2.5 h at 120 °C under 3 MPa of O2, the highest value among all reported noble metals. However, the reaction mechanism of DMM formation in the presence of RuCl3 remained unclear. The outstanding performance was tentatively attributed to the high intrinsic activity of the homogeneous Ru3+ species, which probably served as both acidic and redox centers.

to be the active site. The DMM selectivity was quite encouraging, but only 6.5% methanol was converted, and the DMM formation rate was as low as 0.1 h−1 (Table 1), probably caused by the low specific surface area of SbRe2O6 (1 m2/g). To improve the activity of SbRe2O6, ReOx was supported on various metal oxides such as TiO2-rutile, TiO2-anatase, V2O5, ZrO2, α-Fe2O3, γ-Fe2O3, SiO2, α-Al2O3, and MoO3.56 These supported ReOx all exhibited similar DMM selectivities (80− 93%) but higher methanol conversions (9.1−59.5%) than SbRe2O6 except for Re/SiO2 (Table 1). The highest DMM formation rate of ca. 18.9 h−1 was achieved over Re/TiO2anatase with the largest surface area (50 m2/g). Interestingly, Re/SiO2 with a specific surface area of 36 m2/g only provided a low DMM formation rate of ca. 3.7 h−1. Besides, introducing acidic promoter such as tantalum oxide to the supported ReOx catalyst was attempted by Nikonova et al., who used rhenium and tantalum alkoxide complexes as precursors to synthesize the composite catalyst Re/TaOx/TiO2.57 Under methanol-rich condition (40%) (Table 1), the modified catalyst showed an improved DMM selectivity (67 vs 17%) compared with the unmodified catalyst. The enhanced acidity by doping TaOx was responsible for the higher DMM selectivity over Re/ TaOx/TiO2. Well-dispersed ReOx species are regarded as the active centers for DMM production. Based on X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, H2-temperature-programmed reduction (TPR), and NH3-temperature-programmed desorption (TPD), it was found that ReOx species possessed both acidity and redox properties.58,59 The redox role was fulfilled by the oxidation state change of Re during reaction, which was clearly observed by feeding methanol to Re-based catalysts without oxygen conducted by Yuan et al.58 After 10 pulse experiments, an increase of the peak intensity of Re4+ along with a decrease of the peak intensity of Re6+−Re7+ was observed in XPS spectra. After reintroducing oxygen, Re4+ was oxidized to Re6+−Re7+ again. This indicated that the redox sites on ReOx species originated from the redox couple of Re6+−Re7+ and Re4+. The nature of the acidic sites on ReOx was proven by pyridine adsorption to be Lewis acid with Re7+ as active site. The nature of supported-Re species was studied for Re/αFe2O3 by extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD).58 At low loading of Re (below 0.7%), monolayer ReO4 species were formed in a tetrahedral geometry with an oxidation state of +7. A high loading of Re led to aggregation of ReO4 monolayers to form ReOx clusters composed of Re7+ and Re6+, as well as ReO2 crystallites. For the supported monolayer ReO4 and ReOx clusters, Re−O−Fe bonds were generated between Re species and Fe2O3, thus hampering the reduction of Re6+−Re7+ to Re4+. Comparing the catalytic performance of Re/α-Fe2O3 at different Re loading, ReOx clusters were considered as the most active species for methanol oxidation to DMM in terms of DMM yield. ReO2 crystallites were less active than other Re species. This correlation between catalytic activity and the structure of Re species was also confirmed for Re/TiO2 prepared by the oxidative redispersion of metallic Re by Secordel et al.59,60 The hydration degree of ReOx species strongly influences the catalytic performance of Re/SiO2.61 Compared with Re/ SiO2 prepared by incipient wetness impregnation, Re/SiO2 prepared by a one pot sol−gel method presented an increased DMM selectivity and DMM formation rate of 89.1% and 10.7 h−1, respectively (Table 1). The increase of DMM selectivity 1303

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ACS Catalysis Table 2. Non-Noble Metal Catalysts for the One-Step Oxidation of Methanol to DMMa selectivity (%)

entry

catalyst

CH3OH concn (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13c 14 15 16 17 18c 19c 20 21 22b 23 24 25 26 27c 28 29 30c 31c 32c 33 34 35 36 37 38 39 40 41 42d 43

10%V2O5/TiO2-Ti(SO4)2 10%V2O5/TiO2-H2SO4 5%V2O5/TiO2-Ti(SO4)2 15%V2O5/TiO2-Ti(SO4)2 10%VOx/TiO2-Ti(SO4)2 30%VOx/TiO2-Ti(SO4)2 30%VOx/TiO2-Ti(SO4)2 16%VOx/TiO2 11.7%VTiS-RC 26.8%VTiS-CP 22.8%VTiS-SG 24.8%VTiS-MG VTiS-CTAB 20%V2O5/TiNT-Ti(SO4)2 SVTiNT-673 PVTiNT-673 WVTiNT-673 VOx/TS-1-SO42− VOx/TS-1-PO43− VTiS-673 VTiS-773 VTiSw50-673 5%VTiS 15%VTiS 25%VTiS VTiSi VTiAl SO42− -V2O5/VCeTiO VTiSiS V2O5-MoO3/Al2O3 AlP10V20O V2O5/ZrO2-Al2O3 V2O5/CeO2 SbVOx FeMo H3PMo12O40/SiO2 H4PVMo11O40/SiO2 H5PV2Mo10O40/SiO2 MoOx/TiO2 MoOx/MCM-41 Mo12V3W1.2Cu1.2Sb0.5Ox NbOx Cu/ZSM-5

5.3 5.3 5.3 5.3 5.3 5.3 5.3 44 6 5.3 5.3 5.3 28.6 5.3 5.3 5.3 5.3 9.1 9.1 5.3 5.3 5.3 5.3 5.3 5.3 6.5 9.1 5.3 5.3 5.3 9.8 50 6 4 40 4 4 4 7.5 7.7 7.5 n.a. 4

GHSV (mL/h*g)

temp (°C)

conv. (%)

FA

DME

MF

DMM

11400 11400 11400 11400 11400 11400 11400 3750 5000 11400 11400 11400 924 11400 11400 11400 11400 4000 4000 11400 11400 11400 11400 11400 11400 12000 8000 11400 11400 4000 4000 1120 5000 80000 22000 n.a. n.a. n.a. 26000 25500−34000 22000 n.a. n.a.

160 160 160 160 140 140 160 150 130 150 150 150 120 140 130 130 130 150 150 150 150 130 150 150 150 140 120 150 220 120 110 165 160 150 280 220 220 220 ∼170 230 280 120 130

60 61.7 48.2 60 16 41 75 52 48.6 61 33 15 53.4 83 58 31 44 45.6 56.7 43 19 74 28 52 58 51 48.9 72 75 54.2 55.6 11.1 ∼17 12 55.7 68.5 68.2 63.3 ∼5 4.6 63 58 ∼1

1.2 1.7 0.1 16 9 0 6 5 2.0 1 2 n.a 5 2 14 26 1.7 1 12 4 3 5 13 0.1 8 11 4.2 7.6 3.2 4.5 ∼8 15.3 4 -

0.4 0.5 2.4 1 7 1 2 0.6 1 1 7 n.a 1 6.9 11 2 1 1 19 3 2 1 5.7 1 2 5.6 6.2 n.a. 5.3 33.3 31.1 35.4 ∼9 10.7 3.2 ∼2

9.8 12.6 6.0 12 2 9 24 11 5.6 12 6 1 n.a 24 6 1 19 10.9 16.2 5 7 13 1 39 31 4.3 14 4 2.3 12.3 9.2 0.7 11.9 5.6 4 ∼2 2.5 1.7 ∼18

88.6 85.1 91.6 71 83 90 68 83 91.8 86 93 90 92.8 70 92 85 55 80.5 72.7 92 80 83 77 53 54 99 89.9 85 86 92.1 81.5 89 89.8 100 89.7 41 58.1 54 ∼80 70.9 89.2 ∼100 ∼78

CO+CO2

DMM formation rate (h−1)

ref

n.a n.a n.a 0.1 n.a 0.1 n.a. n.a. 1 0.1 5.5 1 0.3 0.6 1.8 ∼2

4 3.9 6.6 2.1 1 0.9 1.3 5.5 1.4 1.5 1 0.4 n.a 2.2 2.1 1 0.9 n.a n.a 1.3 0.5 2 3.2 1.4 0.9 3.3 3.2 2.6 1.2 0.9 n.a. n.a. 0.4 1.4 23.5 n.a. n.a. n.a. ∼1.3 n.a. n.a. n.a. n.a.

70 70 70 71 73 73 73 86 78 74 74 74 72 87 89 89 89 88 88 79 79 79 83 83 83 75 91 92 93 99 97 98 96 95 104 110 110 110 112 113 115 116 117

a

n.a. = not available; FA = formaldehyde; DME = dimethyl ether; MF = methyl formate; VTiSiS = silica- and sulfate-modified V2O5/TiO2; TiNT = TiO2 nanotube; VTiNT = V2O5/TiO2 nanotube; SVTiNT = sulfuric-acid-modified V2O5/TiO2 nanotube; WVTiNT = phosphotungstic-acidmodified V2O5/TiO2 nanotube; PVTiNT = phosphoric-acid-modified V2O5/TiO2 nanotube; selectivity is reported on carbon basis; 673 and 773 (after catalyst) represent calcination temperature; all reactions were conducted in a continuous reactor in gas phase under atmospheric pressure unless otherwise stated. bw50: washed with 50 mL of water. cThe unit of GHSV is h−1. dLiquid-phase reaction in batch reactor; methanol/H2O2 (50% aqueous solution) = 100/1, 0.01 g of catalyst.

The superior activity of Ru-based catalysts at low temperatures along with the reasonable costs makes it a potential catalyst for industrial production of DMM. However, its excellent redox ability always leads to the coproduction of MF in significant amounts which potentially increase the energy consumption and capital cost of subsequent purification. Thus, future research efforts should be directed to improve DMM

selectivity by either increasing catalyst acidity or weakening the redox ability of RuO2. 2.2.2. Non-Noble Metal Catalysts. 2.2.2.1. VanadiumBased Catalysts. Table 2 summarizes the catalytic performances of non-noble metal catalysts for the direct synthesis of DMM by methanol oxidation. Vanadium-based catalysts, mainly composed of vanadium oxide (V2O5), are well-known for their catalytic application in a wide range of industrial 1304

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ACS Catalysis oxidation processes for a long time.65 Especially for methanol oxidation, the catalytic behavior of vanadium pentoxide was well investigated.66 The good catalytic performance of vanadium oxide (VOx) in methanol oxidation to FA suggests a certain potential as catalyst in the direct methanol oxidation to DMM. In fact, DMM was often observed as a byproduct in methanol oxidation to FA and MF over TiO2-supported V2O5 (VTi),67,68 which fostered interest into VTi for the selective production of DMM from methanol. In general, VTi can only enable the selective production of DMM at a relatively low conversion of methanol (below 15%). With increasing methanol conversion, DMM selectivity decreased associated by an increase in the selectivity of FA and MF (Figure 3).69

to be more dependent on the redox property, and DMM selectivity is mainly determined by acidity. For the overall DMM formation rate, the concerted transformation on both acidic and redox sites is important. The predominant chemical nature of VOx species on oxide supports is considered as a key factor determining the redox property. According to literature,65,79,80 three types of vanadium species exist on oxide supports: isolated monomeric VOx species featuring three V−O bonds connected to the support with one terminal VO bond, oligomeric or polymeric VOx species featuring V−O−V bonds, and crystalline V2O5 particles. All of them are considered active for methanol oxidation to DMM. These monomeric, oligomeric, and polymeric VOx species are collectively called amorphous VOx species, since they hold as common structural motif a terminal VO bond and possess low crystallinity. An agreement related to the structure− activity relationship in methanol oxidation to DMM was reached that amorphous VOx species are usually more active than crystalline V 2 O 5 owing to their superior redox abilities.51,79,81 Nonetheless, the individual contributions of monomeric, oligomeric, and polymeric VOx species to the catalytic performance is not fully understood yet.79 The different forms of VOx species are largely dependent on calcination temperature and V2O5 loading. XRD, Raman, and UV−vis spectroscopy allowed studying the transformation of isolated VOx species to polymeric VOx species and further to crystalline V2O5 particle with increasing V2O5 loading or calcination temperature.79,82,83 For instance, Zhao et al. systemically investigated the effects of calcination temperature on the structure and activity of VTiS.79 According to XRD and Raman analyses, polymeric VOx species with terminal VO bonds formed for catalysts calcined at 673 K (VTiS-673) and crystalline V2O5 particles started to form for calcination temperatures above 673 K. As expected, VTiS-673 with amorphous V2O5 presented much higher DMM formation rate and methanol conversion than VTiS-773 (1.3 vs 0.5 h−1 and 43 vs 19%, Table 2, entries 20−21). A study conducted by Guo et al. also stressed the importance of calcination temperature for catalytic performance.84 Another work of Zhao et al. clearly illustrated an increasing methanol conversion with the V2O5 loading increasing from 5 to 25 wt %, whereas the DMM formation rate decreased from 3.2 to 0.9 h−1 (Table 2, entries 23−25).83 The declining DMM formation rate was assigned to a suppressed formation of amorphous VOx species with increasing V2O5 loading, evidenced by Raman spectroscopy. Employing cetyltrimethylammonium bromide (CTAB) as a protective agent for the synthesis of VTiS catalysts also favored the formation of highly dispersed VOx species in amorphous form by inhibiting the aggregation to VOx particles.72 The average particle size of VTiS-CTAB could be maintained at 6 nm even for a calcination at 400 °C. Already at 120 °C, VTiSCTAB exhibited high catalytic activity reaching 53.4% conversion of methanol and 92.8% selectivity to DMM (Table 2, entry 13), which was usually achieved at 140−160 °C on VTiS. Although the addition of sulfate species was proven to be effective in improving DMM production, the performance is strongly affected by the content of the acidic site, the calcination temperature and the type of acidic site (Brönsted or Lewis). Guo et al. studied the effect of sulfur content.77 A comprehensive catalyst characterization revealed well-dispersed sulfate species below a sulfate content of 4 mol % and aggregation above. The highest DMM selectivity of 91.8%

Figure 3. Dependence of DMM selectivity on methanol conversion over 10% V2O5/TiO2 and 10% V2O5/TiO2-Ti(SO4)2. Reproduced with permission from ref 70. Copyright 2007 Royal Society of Chemistry.

The group of Shen overcame this trade-off between methanol conversion and DMM selectivity modifying V 2O 5/TiO 2 catalysts with acidic additives such as Ti(SO4)2 and H2SO4 via a simple impregnation method.70 Owing to the enhanced acidity, sulfate-modified V2O5/TiO2 catalysts (VTiS) greatly accelerated the acetalization step resulting in a high DMM selectivity of ∼90% (Table 2, entries 1−3). In contrast to unmodified VTi, DMM selectivity could be maintained at around 90% over the sulfate modified VTi even at a high methanol conversion of 60% (Figure 3). The formation rate of DMM catalyzed by 5% V2O5/TiO2−Ti(SO4)2 reached up to 6.6 h−1 (Table 2, entry 3), which is comparable to several noble metal catalysts (e.g., Ru and Re). Following this work, various synthetic approaches such as coprecipitation (CP),71,72 evaporation-induced self-assembly (EISA),73 sol−gel (SG),74,75 ultrasonic-assisted impregnation,76 rapid combustion (RP),77,78 and mechanical grinding (MG)74 were applied to prepare acid-promoted VTi catalysts. Among these methods, catalysts prepared by mechanical grinding showed the lowest activity,74 probably due to insufficient mixing of TiO2 and VOSO4 (precursor for V). Other methods resulted in comparable or slightly lower catalytic activities (Table 2, entries 4−13) than the conventional impregnation method in terms of DMM formation rate. It is noteworthy to mention that VTi synthesized by rapid combustion possessed an excellent stability, and no deactivation was observed throughout a 200 h durability test in a fixedbed reactor.77,78 According to the reported catalytic activity data (Table 2), the addition of sulfate species to VTi catalysts usually improved DMM selectivity but did not significantly alter methanol conversion. The intrinsic activity of VTi (i.e., methanol conversion) in methanol oxidation to DMM appears 1305

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ACS Catalysis

the redox properties by partially poisoning VOx species. Zhao et al. also reported such a negative effect of excess sulfate on catalytic activity for VTiS-MG and VTiS-SG.74 Motivated by the excellent catalytic activity of the welldispersed VOx species, investigations focused on high specific surface area support materials such as mesoporous TiO2,86 TiO2 nanotubes (TiNT), and titanium silicalite (TS-1).87−90 In particular, the open tubular structure of TiNT presents an attractive material to reach high V2O5 loadings.87 Nearly monolayer dispersion of V2O5 was obtained on TiNT even at V2O5 loadings as high as 20%. The sulfated V2O5/TiNT catalyst displayed a superior DMM formation rate of 2.2 h−1. At very high conversion of methanol (83%), DMM selectivity still reached 70% (Table 2, entry 14). In addition, the effect of a doping with different types of inorganic acids onto V2O5/ TiNT was further studied (e.g., utilizing H2SO4, H3PO4, and H3PW12O40).89 The DMM formation rate followed the order: SVTiNT > PVTiNT ≈ WVTiNT (Table 2, entries 15−17). This activity order was assigned to weakened redox ability since some VOx species were covered by H3PO4 and H3PW12O40 during calcination. TS-1, a typical porous material for oxidative processes, also served as a support to synthesize bifunctional VOx/TS-1 catalysts (containing 2.5 wt % Ti) for methanol oxidation to DMM.88 Catalyst acidity and reducibility were both enhanced by doping of H2SO4 and H3PO4. H2SO4 and H3PO4 modified VOx/TS-1 catalysts provided 36.7 and 41.2% yield of DMM, respectively (Table 2, entries 18− 19). Apart from doping sulfate species, another frequently applied approach to increase the acidic sites of VTi catalysts is the incorporation of metal oxides such as Al2O3, SiO2, ZrO2, and CeO2 into the structural matrix of TiO2. This approach creates more acidic sites and allows tuning the acidic strength because of the distorted coordination geometries.75,91−93 At the same time, this incorporation alters the redox property. For instance, silica-modified VTi (VTiSi) enabled much higher activity and DMM selectivity compared to the unmodified VTi catalyst (Table 2, entry 26).75 According to XPS and H2-TPR, a large amount of V4+ was generated upon the addition of Si potentially offering more oxygen vacancies to facilitate the electron transfer between active phase and support; hence, methanol conversion improved. FT-IR and NH3-TPD revealed that incorporating Ti4+ into Si−O−Si bonds increased the number of weak Brönsted acidic sites. Based on the observed linear relationship of DMM yield and surface density of weak Brönsted acid sites, this weak Brönsted acidity was identified to be crucial for improved DMM selectivity.75 A study of Wang et al. on Al2O3-doped VTi (VTiAl) has also emphasized the important role of weak acid sites in improving DMM selectivity (Table 2, entry 27).91 DMM selectivity is very sensitive to reaction temperature. The optimal window of reaction temperature for VTiS is generally between 120 and 160 °C, as presented in Table 2. Above 160 °C, a drop in DMM selectivity occurred due to pronounced side reactions (e.g., to MF, DME, and COx). Sun et al. introduced SiO2 to VTiS shifting the optimum reaction temperature to 220 °C, at which DMM selectivity was still maintained at 86% with a methanol conversion up to 75% (Table 2, entry 29).93 The addition of SiO2 greatly reduced acidity (both strength and amount) and redox property of VTiS, as supported by the results of micro calorimetric adsorption of ammonia and H2-TPR. Though, this result contradicts the aforementioned studies, which claimed

(Table 2, entry 9) was obtained with 4 mol % sulfate, adjusting the redox property and acidity of VTiS-RC well. Therefore, an appropriate content of sulfate species combined with amorphous VOx species are crucial to maximize DMM production. Studies also addressed the role of calcination temperature on acidity for VTiS-RC.84 Different calcination temperatures led to a distinct variation in the acid site concentration, while acid strength remained almost unaltered. For a maximum number of acid sites at a calcination temperature of 723 or 773 K, the highest DMM selectivity could be reached. As indicated by H2-temperature-programmed reduction mass spectrometry (H2-TPR-MS) and thermogravimetric analysis (TGA), bidentate sulfate species formed on VTiS calcined at 723 or 773 K, whereas aggregated pyrosulfates were present on VTiS calcined above 773 K. The effect of Brönsted and Lewis acid sites is frequently discussed for the synthesis of longer chain OME (n = 3−5); however, in methanol oxidation to DMM, this point was hardly addressed. In the conventional synthesis of OME3−5, Brönsted acid possesses much higher activity than Lewis acid.50,85 For VTiS-catalyzed methanol oxidation to DMM, no obvious relationship was found between DMM yield and the relative concentration of Lewis acid sites originating from the coordinatively unsaturated Ti4+ ions. In contrast, a nearly positive correlation could be established between the relative concentration of Brönsted acid sites and DMM yield (Figure 4); though, this correlation was only applicable to VTiS with

Figure 4. Correlation between DMM yield and the integrated intensity of Brönsted acid sites (based on the pyridine adsorption band at 1537 cm−1 by FTIR) with different sulfate loading. Reproduced with permission from ref 79. Copyright 2010 Elsevier.

low loading of sulfate species.79 As mentioned above, adding sulfate species primarily promotes DMM selectivity with little influence on catalytic activity. Interestingly, Lu et al. and Zhao et al.82,83 discovered that sulfate doping could at the same time improve DMM selectivity and methanol conversion over VTiS with a high dispersion of V2O5. Further insights into the origin of this finding appear promising. On the other hand, adding sulfate species could also bring about a detrimental effect on catalytic activity. For instance, Zhao et al. reported a much higher DMM formation rate (2 vs 1.3 h−1) over VTiS subjected to an additional water-washed procedure during preparation versus the unwashed catalyst (Table 2, entries 20 and 22).79 Washing significantly reduced the sulfur content from 2.6% in VTiS-673 (unwashed) to 0.8% in VTiSw50-673 (washed) but also decreased H2 consumption measured by H2-TPR. Higher sulfate content possibly weakens 1306

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ACS Catalysis enhanced redox property and acid sites after SiO2 addition. The reason for decreased acidity and redox property in the latter example remain unclear. Alternatively, V-based catalysts could be fabricated by the rational combination of VOx with acidic supports such as Al2O3, SiO2, ZrO2, CeO2, MoO3, Sb2O4, NbOx instead of TiO2.94−101 These TiO2-free V-based catalysts proved active for the direct oxidation of methanol to DMM, but most of them provided an inferior activity to the aforementioned VTiS catalysts in terms of DMM formation rate. For example, Meng et al. prepared a ternary V2O5-MoO3/Al2O3 catalyst affording better catalytic performance than the corresponding V2O5/ Al2O3 and MoO3/Al2O3, respectively.99 Under the optimized conditions, methanol conversion, DMM selectivity, and formation rate reached 54.2%, 92.1%, and 0.9 h−1, respectively (Table 2, entry 30). Based on XRD, Raman, FT-IR, XPS, H2TPR, and O2-TPD, a synergistic effect between V2O5 and MoO 3 was proposed as the reason for an enhanced performance. This synergistic effect is based on two aspects (i.e., the enhanced redox ability owing to the newly formed V− O−Mo phase bonded to Al2O3 together with weaker Brönsted acid sites produced by the addition of MoO3). As illustrated in Figure 5, the V−O−Mo phase could facilitate the redox cycle

Figure 6. Correlation between DMM yield and weak acidic sites determined by ammonia TPD over V2O5-MoO3/Al2O3. Reproduced with permission from ref 99. Copyright 2014 Elsevier.

heteropolyacids, that is, molybdophosphoric acid (H3PMo12O40). FeMo is typically composed of a mixed oxide phase Fe2(MoO4)3 with a large excess of MoO3. In 1930, Adkins and Peterson first claimed the high activity and selectivity of FeMo for methanol oxidation to formaldehyde.102 Currently, FeMo is still employed in the industrial manufacture of formaldehyde under methanol-lean conditions, which is usually accompanied by the coproduction of trace amounts of DMM.103,104 Accordingly, FeMo possesses the potential for methanol oxidation to DMM. In 2010, the research group of Dumeignil found that shifting the reaction conditions from methanol-lean to methanol-rich remarkably boosted the production of DMM in the presence of FeMo.104 Under methanol-lean conditions, DMM selectivity, as expected, was as low as 2.8% with FA as the main product; whereas, DMM selectivity improved to 89.7% upon increasing methanol concentration to 40 mol % (Table 2, entry 35). The strong dependence of DMM selectivity on methanol concentration is in agreement with the trimolecular nature of methanol oxidation to DMM (eq 5), which is more favorable at higher concentration of methanol. High methanol concentration and space velocity enable high DMM productivity of ca. 4.6 kg DMM h−1 kgcat−1. DMM formation rate reaches 23.5 h−1, to the best of our knowledge the highest value achieved to date among non-noble metal catalysts. A follow-up investigation by the same group focused on gaining insights into the relationship between the excellent performance of FeMo and its properties utilizing a comprehensive in situ characterization of FeMo.105 Interestingly, a close relation between DMM yield and Fe content in FeMo became evident. The optimal Fe/MT (Fe + Mo = MT) ratios were between 0.22 and 0.26, which gave the highest DMM yield of about 40−50% at a methanol conversion of about 45% (Figure 7). Pure Fe2O3 (Fe/MT = 1) facilitated 80% selectivity of methyl formate without any formation of DMM at a methanol conversion of 7%, whereas pure MoO3 (Fe/MT = 0) enabled the formation of DMM with nearly 80% selectivity but in a very low yield of 5.6%. Seemingly, both redox and acidic properties are necessary for methanol oxidation to DMM as present on MoO3. Obviously, there was a synergistic effect between Mo and Fe in promoting methanol conversion and DMM selectivity. This synergy was attributed to the enhanced redox properties and acidities due to the presence of a special catalytic architecture on the FeMo catalyst, namely, the combination of MoO4 tetrahedra with FeO6 octahedra via oxygen bridge bonds (Figure 8). Therein, Fe3+ rather than Mo6+ was considered as the redox site, as

Figure 5. Proposed redox cycle over binary V−Mo catalyst. Reproduced with permission from ref 99. Copyright 2014 Elsevier.

through electron transfer between lattice oxygen and metal cations (V5+ + Mo6+ ↔ V4+ + Mo5+), which plays an important role in the formation of oxygen vacancy and replenishment of active lattice oxygen. The underlying reaction mechanism will be discussed in Section 2.3. Importantly, the yields of DMM correlated well with the amount of weak Brönsted acid site (Figure 6), in line with the above-mentioned studies on VTiAl and VTiSi. Chen et al. reported a mesoporous phosphatemodified V2O5/Al2O3 (AlP10V20O) derived by coprecipitation of the corresponding precursors of Al, P, and V.97 The highest DMM yield of 45.3% was achieved at 110 °C under 10 mol % P and 20 mol % V with balanced acidity and redox ability (Table 2, entry 31). The authors suggested that introducing P increases the concentration of V4+, potentially facilitating oxygen adsorption and lattice oxygen migration. Besides, they uncovered that strong acid sites promote the production of MF and DME. This indirectly emphasized the previous hypothesis that weak acid sites are beneficial for DMM synthesis. 2.2.2.2. Mo-Based Catalysts. In the last decades, studies on Mo-based catalysts were mainly dedicated to commercial iron molybdate catalysts (FeMo) as well as Mo-containing 1307

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ACS Catalysis

mesoporous carbon as support to immobilize H5PMo10V2O40.109 DMM selectivity remained at medium level (50%) probably due to nonoptimized acidic and redox properties. Liu et al. systematically investigated the performance of SiO2-supported H3+nPVnMo12−nO40 (n = 0−4) Keggin clusters in the oxidation of methanol and dimethyl ether to DMM.110 The oxidation of dimethyl ether to DMM will be discussed in the following Section 3. DMM presented the main product in methanol oxidation over all H3+nPVnMo12−nO40 (n ≥ 1) catalysts reaching 54 to 60% selectivity at methanol conversions of 63 to 69% (Table 2, entries 36−38). The partial replacement of Mo atoms by V gave rise to the reduction of MF selectivity in favor of higher DMM selectivity. The methanol conversion rate of supported heteropoly acids was approximately 40 times higher than that of the bulk heteropoly acids (250.7 vs 5.1 mmol/gmetal/h), owing to the improved accessibility of redox and acid sites on supported catalysts. The fraction of Brönsted acid sites on H5PV2Mo10O40/SiO2 could be regulated by the thermal dehydroxylation of the Keggin structures, which was reversible up to 400 °C and leads to the irreversible formation of crystalline MoO3 above 400 °C. DMM selectivity therefore followed a volcano pattern as a function of the pretreatment temperature with a maximum (∼70%) at 400 °C. Unfortunately, the DMM formation rate could not be compared because the exact value of space velocity was not available. To further improve DMM selectivity, the authors reported the selective titration of protons in heteropoly acids by an organic base, which could inhibit the competitive methanol dehydration to DME.111 As mentioned above, the formation of DME was more sensitive to the number of Brönsted acid sites than DMM, and thus, this method enhanced DMM selectivity to more than 80%. The effect of MoOx species dispersion on catalytic activities was addressed over MoOx/TiO2 with variable Mo loadings by Faye et al.112,113 Analogously to the above-described VTi catalyst, the dispersion of MoOx on the support can influence both acidic and redox properties. Only amorphous MoOx species were present on catalysts containing 1 to 14 wt % MoOx, proven by XRD and IR-Raman. With increasing MoOx content from 1 to 14 wt %, the highest amount of redox sites was obtained at 8 wt %, according to H2 consumption measured by H2-TPR. With regard to acidity, the addition of MoOx made new acidic sites available through the formation of Mo−O−Ti bonds.114 However, the authors found that increasing MoOx loading led to a decrease in the overall number of acidic sites, mainly medium and strong acidic sites which might be preferentially covered by MoOx. The highest yield of DMM was obtained at an optimum MoOx loading of 8 wt %, corresponding to a submonolayer dispersion of MoOx with appropriate acidic and redox sites, which exhibited 80% selectivity to DMM at 5% conversion of methanol (Table 2, entry 39).112 2.2.2.3. Miscellaneous Catalysts. A polymetallic composite oxide with the general formula Mo12V3W1.2Cu1.2Sb0.5Ox (also called AR01) developed by Arkema has been reported to act as a highly active bifunctional catalyst for DMM production.115 AR01 was synthesized by a simple coprecipitation method. The material possessed low specific surface area (2) when CD3OD was used instead of CH3OD as reactant.53 In contrast, no KIE was found when CH3OH was replaced by CH3OD. They thus concluded that breaking the C−H bond presented the kinetically relevant step, in line with the conclusion drawn in methanol oxidation to FA. Theoretical calculations by Li et al. verified the proposed mechanism of methanol oxidation to DMM on VTi and VTiS.52 Their density functional theory (DFT) calculations, based on the cluster model, suggested that the dehydrogenation of the adsorbed methoxy group to FA possessed the highest energy barriers of 1.29 eV on VTi and 1.18 eV on VTiS within the overall reaction (see Figure 10). Accordingly, H abstraction from the methoxy group to yield FA was predicted to be the rate-determining step on both catalysts, which is consistent with the aforementioned isotopic results. The calculated energy barrier of the rate-determining step on VTiS (1.18 eV) was slightly lower than that on VTi (1.29 eV), indicating that adding sulfate species, to some extent, could enhance catalyst activity as well. For the acetalization of methanol and hemiacetal, two pathways initiated by either methanol chemisorption or hemiacetal chemisorption at the VO bond were considered to calculate reaction energies. The acetalization step preferentially starts with methanol

Figure 10. Energy barriers (ΔE0 K, eV) for the elementary steps involved in methanol oxidation to FA (black), MF (blue), and DMM (red) over VTi catalysts. Calculations were performed at the B3LYP and PBE levels with the cluster (black) and slab (red) models, respectively. Reproduced with permission from ref 52. Copyright 2017 Royal Society of Chemistry.

chemisorption rather than hemiacetal chemisorption, as indicated by the lower energy barrier of methanol chemisorption (1.05 vs 1.88 eV). On the other hand, based on the reaction rate constants estimated from the energy barriers, their calculations predicted that introducing sulfate primarily promoted the selectivity toward MF rather than DMM at low temperatures, which disagreed with experimental results. These DFT calculations are informative for a better understanding of the complex reaction mechanism, but to achieve a realistic simulation of DMM production, future theoretical studies should consider a more accurate catalyst model and reaction model including the influence of kinetic factors. 2.4. Methanol Oxidation to DMM using CO2 as Oxidant. The strategy of replacing O2 with CO2 as oxidant has been applied to various oxidative dehydrogenation reactions. It reduces CO2 emissions and enables a valuable chemicals production simultaneously.129 Recently, this strategy was also used for methanol oxidation to DMM catalyzed by a basic ionic liquid BmimOH in a batch reactor (eq 7).130 At

150 °C and 3 MPa CO2, DMM selectivity reached up to 85% at a methanol conversion of 2.4%. An interesting finding was that trioxane was also produced with a selectivity of 13.1%. Accordingly, FA trimerized during reaction, possibly because of the accumulation of FA resulting from the relatively slow acetalization step under basic conditions. A much lower DMM selectivity of 31.7% with pronounced formation of overoxidized byproducts (carboxylic acid esters) was achieved 1310

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oxidized to CH3OCH2 groups over a redox center in the presence of O2. The combination of CH3OCH2 with a methoxy group generates DMM. In addition, there is another possible pathway to produce DMM via the intermediates of FA and methanol, both of which are formed by DME oxidation and decomposition, respectively (Scheme 4). The DME conversion to DMM is an oxidative gas-phase reaction, which usually proceeds in a continuous-flow fixed-bed reactor. Only one mole of H2O is produced per mole of DMM in this reaction, but two moles of H2O are formed per mole of DMM when using methanol as substrate. In comparison to liquid methanol, DME is gaseous at ambient conditions, and so does not need to be vaporized. The above advantages can reduce the energy consumption and simplify the manufacturing process of DMM. 3.2. Bifunctional Catalysts. Analogously to the direct oxidation of methanol to DMM, the one-step oxidation of DME to DMM is also catalyzed by a bifunctional catalyst with redox and acidic properties. Nevertheless, due to the high chemical stability of DME, the conversion of DME at low temperature is very low, while higher reaction temperatures usually promote the formation of CO and CO2 resulting in a low DMM selectivity. Therefore, numerous efforts have been made to develop efficient bifunctional catalysts to address this challenge, mainly including heteropoly acids, zeolite-supported VOx, and modified carbon nanotube (CNT). In 2003, the first direct oxidation of DME to DMM was reported by Liu et al. using a SiO2-supported H5PV2Mo10O40 catalyst, which led to a DME conversion of 1.8% and a DMM selectivity of 55% at 240 °C.110 The authors underlined the promoting effect of water, namely, DMM selectivity increasing to 68.3% upon the addition of H2O to the reactant stream. Likewise, DME conversion rate increased by a factor of 2 (90 vs 45.5 mmol/gmetal-h). As stated previously, the formation of DMM was more favorable at a high concentration of methanol. The enhanced catalytic activity was therefore attributed to the increasing concentration of methanol from an acid-catalyzed DME hydrolysis accelerated by adding H2O. Following this study, Zhang et al. reported 39.1% selectivity toward DMM at a DME conversion of 8.6% on a MnCl2 modified H4SiW12O40/SiO2 catalyst at 320 °C.131,132 In contrast, DMM selectivity attained over the physically mixed catalyst of MnCl2/SiO2 and H4SiW12O40/SiO2 was as low as 8.1%. Obviously, the acidic and redox sites needed to be spatially close for a high selectivity of DMM. NH3-TPD and H2-TPR profiles disclosed that a MnCl2 modification reduced the acidic sites and increased the redox sites on MnCl2H4SiW12O40/SiO2, and thus led to a balance of acidic and redox sites. Nevertheless, MnCl2-H4SiW12O40/SiO2 underwent fast deactivation within few hours, accompanied by a steady decrease of DMM selectivity. The deactivation was probably caused by a loss of acidic and redox sites as well as carbon deposition, according to the characterization results of the spent catalyst. The same authors later reported that doping Sm2O3 to MnCl2-H4SiW12O40/SiO2 could improve DMM selectivity to 60.3% without compromising DME conversion (9.5%).133 This was ascribed to the increased number of weak acid sites and Mn4+ species (redox sites) on Sm2O3-MnCl2H4SiW12O40/SiO2, as evidenced by XPS and NH3-TPD. Importantly, Pyridine-IR disclosed that the increased number of weak acid sites mainly resulted from Lewis acid sites. Together with the results that DME was mainly converted to CH3OH and CO over strong Brönsted acid site such as

when O2 was used instead of CO2. Such a big difference in product selectivity highlighted the advantage of using CO2 as a moderate oxidant, that overoxidation reactions can be suppressed owing to its medium oxidative capability. In this method, three methanol molecules were incorporated into one DMM molecule accompanied by the reduction of CO2 to formic acid (eq 7), thus producing one less H2O than the traditional O2-involved method (eq 5). In addition, thermodynamics calculation has been conducted based on the eq 7, and the Gibbs energy was estimated to be 68.2 kJ/mol under standard conditions (298 K, 1 atm), which will probably decrease to negative under reaction conditions (150 °C, 4.5 Mpa) due to the endothermic and volume-reduced nature of this reaction. This indicates that the formation of DMM is more favorable at elevated temperatures and pressures from a thermodynamic point of view. The suggested reaction mechanism is shown in Scheme 3. First, CO2 and CH3OH Scheme 3. Proposed Reaction Mechanism for the Conversion of CO2 and Methanol to DMM Catalyzed by a Basic Ionic Liquid BmimOH. Adapted with Permission from Ref 130. Copyright 2016 Elsevier

are activated by BmimOH to form a bent CO2(OH)− anion and an adduct of methanol interacting with BmimOH, respectively. The formed CO2(OH)− anion can function as a soft oxidant to abstract the hydrogen atom in the activated methanol. As a result, methanol is oxidized to FA and concurrently CO2(OH)− is reduced to formic acid, which can be transformed to MF in the presence of methanol. The formed FA undergoes acetalization with methanol to yield DMM.

3. DIRECT SYNTHESIS OF DMM VIA DME OXIDATION 3.1. Chemical Reactions. DME, easily obtained from methanol or syngas, is regarded as a potential feedstock for the one-step production of DMM. The possible reaction pathways as well as the involved intermediates for DME conversion to DMM are summarized in Scheme 4 according to literature.131 The dissociation of DME over acidic center leads to the formation of methoxy groups as capping agent. DME can be Scheme 4. Proposed Reaction Pathways for the Direct Oxidation of DME to DMM. Adapted with Permission from Ref 131. Copyright 2008 Elsevier

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ACS Catalysis H4SiW12O40, the authors proposed Lewis acid sites to play a more important role than Brönsted acid in DME oxidation to DMM. In another study, CNT served as support to prepare a composite 5%Re-30%H3PW12O40/CNT catalyst facilitating a DMM selectivity of 55% at a DME conversion of 8.9% without CO or CO2 formation at relatively low temperature (240 °C).134 Surprisingly, OME2 formation occurred despite a very low selectivity (4%). Generally, it is very difficult to produce longer OMEs in gas-phase continuous-flow reactions owing to the short contact time compared to batch liquid-phase reactions. This is probably also the major reason that the production of longer OMEs by methanol oxidation in continuous gas-phase reaction has rarely been reported so far. According to the proposed chain growth mechanism of OME2,19 a higher selectivity of OME2 in gas-phase requires sufficient monomers and capping agents, such as methanol, DME, and DMM. Indeed, through cofeeding a mixture of O2, DMM, and DME, OME2 selectivity was further boosted to 60% and 89.4% on Re-H3PW12O40/TiO2 and Ti(SO4)2/CNT, respectively.135,136 Apparently, Re and H3PW12O40 work as redox and acidic centers, respectively. Yet, a large number of Lewis acid sites were actually present on Re-H3PW12O40/TiO2. They were proposed to be more significant for the formation of OME2, in line with the conclusion drawn for Sm2O3-MnCl2H4SiW12O40/SiO2. Regarding Ti(SO4)2/CNT, the presence of highly dispersed sulfate species was proven to increase the number of weak acid sites important for OME2 formation. The defect sites on the surface of CNT were considered as redox sites. O2-TPD-MS and DME-TPSR-MS (temperature-programmed surface reaction) suggested that the introduction of Ti(SO4)2 could also promote O2 activation on CNT to form more active oxygen species by the electronic interaction between SO42− and CNT. Though, the activation mechanism of O2 and substrate as well as the specific redox center remain unclear. A series of Hβ-supported V2O5 catalysts with different content of V2O5 were also examined for DME oxidation to DMM and OME2.137 The best catalytic performance was obtained over 15%V2O5/Hβ, showing a total DMM and OME2 selectivity of 46% at a DME conversion of 12.4%. Based on the experimental results, the authors proposed a possible reaction pathway for DME oxidation to OME2 in which DMM acts as the precursor of OME2.135 The terminal C−H bond of DMM could be cleaved on redox sites (ReOx or CNT) to generate a CH3OCH2OCH2 group. The CH3O group is obtained from DMM or DME decomposition over acid sites (mainly weak Lewis acid) (Ti(SO4)2, ReOx, or H3PW12O40). OME2 is finally formed by the combination of CH3OCH2OCH2 and CH3O groups.

Scheme 5. Proposed Reaction Pathway for Methanol Dehydrogenation to DMM

synthesis of DMM can be potentially implemented in either a continuous reactor in gas phase or a batch reactor in liquid phase. From a thermodynamic viewpoint, implementing this process in liquid phase seems to be less challenging than gas phase. In gas phase, in order to achieve considerable methanol conversion, methanol dehydrogenation typically proceeds at 500−800 °C,138 which is not compatible with the reaction temperature for acetalization (usually below 300 °C). However, this big temperature gap can be reduced in liquid phase, since the formed H2 can be separated from the liquid phase to shift the reaction equilibrium toward DMM, thus allowing the reaction to proceed at lower temperatures. Only few studies addressed the liquid-phase methanol dehydrogenation to DMM, while a gas-phase reaction has not been reported until now, probably due to the challenges indicated above. For the liquid-phase reaction, the reported catalysts were mainly based on Ru and Cu, which possess excellent dehydrogenation activity. In fact, DMM is often observed as a byproduct in liquid-phase decomposition of methanol to H2 in the presence of homogeneous Ru complexes or supported Ru/C;139−142 however, DMM selectivity remained usually low owing to the prevailing formation of FA and MF in the liquid phase. The research group of Shinoda studied the catalytic behaviors of Cu-ZnO/SiO2 in the liquidphase methanol dehydrogenation reaction.143 The catalyst calcined under air exhibited 100% selectivity toward MF, which is quite common for Cu-based catalysts in gas-phase methanol dehydrogenation.144 Surprisingly, a catalyst calcined in N2 achieved 100% selectivity toward DMM in the liquid phase. XRD and XPS studies revealed that calcination under N2 generated metallic Cu particles with oxidized surface layers of Cu2O, in contrast to the formation of CuO by calcination under air. The exclusive production of DMM was tentatively attributed to the weak adsorption of the intermediate FA on Cu2O allowing an easy desorption of FA into the liquid phase containing substantial amounts of methanol trapping released FA to yield DMM. Most recently, Wu et al. performed the dehydrogenation of methanol to DMM over Cu/SiO2 in a batch reactor at 240 °C.32 They initially investigated the hydrogen production through methanol decomposition, and a large amount of DMM was unexpectedly found in the liquid phase with almost 100% selectivity. Based on XRD and XPS, the authors hypothesized a possible reaction pathway, where the supercritical methanol first underwent dehydrogenation to yield H2 and FA on the interfaces between Cu2O and metallic Cu species. Then FA reacted with methanol to form DMM, in line with the previous findings over Cu-ZnO/SiO2. To sum up, both works suggest that Cu2O species play an important role in shifting the catalyst selectivity toward DMM in liquid-phase reactions. It should be pointed out that the nature of the acidic site on the catalyst and the detailed mechanism of DMM

4. DIRECT SYNTHESIS OF DMM VIA METHANOL DEHYDROGENATION The dehydrogenative synthesis of DMM involves the nonoxidative dehydrogenation of methanol to yield the FA intermediate coupled with the following acetalization reaction (Scheme 5). The major distinction from the oxidative synthesis of DMM is that the coproduced H2 can be directly used for methanol synthesis, instead of being oxidized to form H2O. Thereby, an H2 circulation could become possible (see Scheme 5), potentially driving this approach more efficient and cost-effective than the oxidative approaches in terms of energy consumption. In addition, safety of the reaction increases compared to methanol oxidation. The dehydrogenative 1312

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methane] in hydrogenating both CO2 and carboxylic acids makes it a potential candidate for the application in CO2 hydrogenation to DMM. The authors first evaluated the physical combination of [Ru(triphos)(tmm)] with a homogeneous acid catalyst. A screening of acid catalysts showed that the optimum combination was [Ru(triphos)(tmm)] with the Lewis acid catalyst Al(OTf)3, on which DMM was obtained with a TON of 214 at 80 °C after 18 h.31 CO2 hydrogenation to DMM is very sensitive to reaction temperature. A significant decrease in the TON of DMM occurred when the temperature increased above 100 °C, probably because of the enhanced formation of side products such as methanol and dimethyl ether. The possible intermediates of this route were identified by an isotopic labeling experiment using13CD3OD and nonlabeled CO2 as substrates. The formation of MF and hemiacetal with the expected 12C at the formal and methylene groups, respectively, was observed by NMR, which corroborated the hypothesis that CO2 followed a sequential hydrogenation via the intermediates MF and hemiacetal to generate DMM (Scheme 6). More recently, using the same catalyst design strategy, the authors discovered that the combination of a molecular cobalt catalyst (Co(BF4)2/Triphos) with the acidic catalyst HNTf2 was also active for this reaction.146 At 100 °C and a partial pressure H2/CO2 of 60/20 bar, Co(BF4)2/ Triphos/HNTf2 provided a TON of 92. The influence of variations on the phenyl group of the Triphos ligand on the catalytic performance was investigated. Compared with the unchanged Triphos ligand, the sterically more demanding and electron-richer ligand TriphosTol (1,1,1-tris(bis(4methylphenylphosphino)methyl)ethane) facilitated the highest TON of 157, which is comparable to that over Ru/Triphos. In spite of the superior catalytic activities toward DMM over these homogeneous catalyst systems, catalyst separation and recycling remain challenging for these homogeneous complexes and acids. In line, future studies on more active catalysts should be accompanied by strategies of catalyst immobilization as well as the search for alternative solid catalysts.

formation are still unclear, which calls for more efforts on this dehydrogenative route, especially, to gain insight on the molecular level.

5. DIRECT SYNTHESIS OF DMM VIA CO2 HYDROGENATION 5.1. Chemical Reactions. From the viewpoint of building a renewable fuel system to reduce the dependence on fossil fuel and reach carbon-neutrality, it is of great interest to synthesize DMM in a sustainable manner, that is, employing renewable CO2 and H2 (from water electrolysis) as substrates. The key step for this novel approach is the catalytic reduction of CO2 with H2 to produce FA, the basic unit of DMM, and then the in situ formed FA can be easily trapped by methanol to form DMM on an acid catalyst. Methanol and formic acid are known to be favorably formed in CO2 hydrogenation because they are more thermodynamically stable than FA. Thus, a selectivity control toward formaldehyde or its derivative bearing a CH2 unit remains a great challenge. The group of Klankermayer put forward a cascade pathway of CO 2 hydrogenation to DMM through the intermediates methyl formate and the hemiacetal in one pot (eq 8).31 The whole

process comprehends three successive steps. To be specific, as shown in Scheme 6, hydrogenation of CO2 first yields formic Scheme 6. Proposed Reaction Pathway for the Reductive Synthesis of DMM from CO2 and H2. Reproduced with Permission from Ref 31. Copyright 2016 John Wiley and Sons

6. SUMMARY AND OUTLOOK DMM is an intriguing chemical for versatile applications, especially serving as a synthetic fuel additive to upgrade conventional diesel fuel and as precursor of the drop-in fuels OME3−5. The prevailing DMM manufacture relies on an indirect method through the established two-step process of methanol oxidation to FA and acetalization of FA with methanol, which suffers from complex operations, equipment corrosion, and high energy consumption. In this context, remarkable advances have been made in developing novel synthetic routes and catalytic materials for energy-efficient production of DMM. Three direct synthetic routes have been reported up to now (Table 3). These direct synthetic routes enable DMM production within one step without separating the intermediate FA, by means of the combination of FA production and the subsequent acetalization in the presence of a bifunctional catalyst. The present Review summarized the current status of catalytic synthesis of DMM in order to provide a complete picture of DMM production including the features of each synthetic route, the applied catalysts, structure−activity relationship, and insights into reaction mechanism. In comparison to the dehydrogenative and reductive routes (Table 3), the direct oxidation of methanol to DMM holds the greatest promise to be used in the near future on commercial

acid followed by esterification with methanol to produce MF. The further hydrogenation of MF leads to the hemiacetal, which eventually condenses with methanol to yield DMM. Considering that methanol can be produced by CO 2 hydrogenation as well, the synthesized DMM derived by this route potentially becomes carbon neutral. 5.2. Multifunctional Catalysts. Recently, Peters et al. conducted a feasibility study for this reductive route from a thermodynamic perspective,145 and concluded that a tailored catalyst which could selectively catalyze CO2 hydrogenation to formaldehyde and concurrently suppress the formation of methanol and dimethyl ether, played a crucial role in synthesizing DMM directly from CO2 and H2. Besides, this reductive route consisting of three consecutive reactions requires the efficient cooperation of three types of catalytic functions (i.e., CO2 hydrogenation, ester (MF) hydrogenation, and acidic functions). All these requirements suggest that a suitable multifunctional catalyst should be specifically tailormade for this reductive route. Only two homogeneous catalytic systems composed of triphos-based catalysts were reported by the group of Klankermayer until now.31,146 The excellent catalytic performance of [Ru(triphos)(tmm)], [triphos = 1,1,1-tris(diphenylphosphinomethyl) ethane, tmm = trimethylene 1313

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limitations

batch reaction, equilibrium limitation

low cost, mature technologies 120−280 °C, atmospheric pressure, high yield, high 4−10 or 40 mol % CH3OH efficiency, continuous-flow 240 °C, 2 MPa N2, pure CH3OH, 4 h high selectivity, H2 recycling, no oxidative step 80 °C, CO2/H2 (20/60 bar), reduced carbon pure CH3OH, 18 h footprint, no oxidative step

advantages

scale. Major drivers are the comprehensive understanding gained in extensive studies and the available bifunctional catalysts possessing excellent performance such as Re-based, Ru-based, V-based, and Mo-based catalysts. Ru-based systems facilitate the highest activity (25.8 h−1) in terms of DMM formation rate, indicating the high inherent redox ability of Ru. Nevertheless, Ru-based catalysts generally present only moderate selectivity toward DMM because of the formation of oxidation products. In terms of productivity, FeMo exhibited the best catalytic performance (4.6 kg DMM h−1 kgcat−1) under industrially relevant conditions (40% CH3OH). The low cost and commercial availability of FeMo catalysts are certainly advantageous. Importantly, FeMo is currently used in the commercial Formox process, which appears to be also applicable for the production of DMM, with minor change of the feeding stream and operation conditions. Thereby, such FeMo catalyst and the established process offer a fairly promising candidate for commercializing the methanol oxidation to DMM in the near future. V-based catalysts are another potential candidate with outstanding performance, especially acid modified V-based catalysts show a DMM selectivity above 90% even at high methanol conversion. They have received high attention by fundamental studies, in which molecular understandings of the elementary steps involved in methanol oxidation to DMM and insights into the catalytic roles of V-based catalysts were gained. This knowledge is crucial for the design of the next generation of bifunctional catalysts. The newly emerging dehydrogenative and reductive routes enrich the available pathways toward DMM. Both innovative routes are featured by circumventing the oxidative process, via methanol dehydrogenation or CO2 hydrogenation to produce FA followed by acetalization to yield DMM. For the dehydrogenation of methanol to DMM, Cu/SiO2 demonstrated high selectivity but the inferior activity as well as operating in batch mode present obstacles hindering a practical application (Table 3). Due to the grand challenge in directly reducing CO2 to FA, harsh conditions and homogeneous catalysts were currently applied for CO2 hydrogenation to DMM (Table 3), leading to moderate catalytic activity. Therefore, this appealing reductive route is still far from a possible industrial application. It should be noted that investigations on these two routes are still in an early stage, and call for more efforts with respect to catalyst synthesis, structure−activity relationship, reaction mechanism, and process design. Particular emphasis should be placed on designing more efficient and selective catalysts, since the low catalytic performance is the main bottleneck for both routes. We are convinced that with increasing interest on DMM from the academic and industrial community, new catalytic materials with improved performance of practical interest will be discovered for the dehydrogenative and reductive routes in the future. Additionally, all routes were discussed with regard to catalyst design. Certainly, considering process economics, capital costs as well as life-cycle assessment are essential for a successful market entry.

214 (TON of DMM) CO2, H2, CH3OH reductive route

[Ru(triphos)(tmm)], Al(OTf)3

99% selectivity Cu/SiO2 CH3OH

RuO2, VTiS, FeMo oxidative route

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

direct synth.

dehydrogenative route

catalytic performance representative catalyst

FeMo or Ag + liquid acid

starting material

CH3OH, aqueous FA CH3OH, O2

synthetic route

indirect synth.

Table 3. Comparison of Various Synthetic Routes for DMM Synthesis

azeotropic DMM (93%) from reactive distillation 50−80% conv., 80−100% selectivity (DMM formation rate 25.8 h−1)

-

reaction conditions

two steps, corrosive waste, energy-intensive explosive risk, producing large amounts of H2O

ACS Catalysis

ORCID

Irina Delidovich: 0000-0002-2948-5553 1314

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Regina Palkovits: 0000-0002-4970-2957 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the German Federal Ministry of Education and Research (BMBF) within the Kopernikus Project P2X: Flexible use of renewable resourcesexploration, validation and implementation of “Power-to-X” concepts.

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DOI: 10.1021/acscatal.8b04441 ACS Catal. 2019, 9, 1298−1318