Methyl formate as a new building block in C1 chemistry

Applied Catalysis, 57 (1990) l-30 Elsevier Science Publishers B.V., Amsterdam -

Printed in The Netherlands

Review

Methyl Formate as a New Building Block in C, Chemistry JAE S. LEE*, J.C. KIM and Y.G. KIM Department of Chemical Engineering, Pohang Institute ofSciences and Technology (POSTECH), and Research Institute of Industrial Sciences and Technology (RISTj, 125, Pohang (Korea)

P.O. Box

(Received 26 June 1989, revised manuscript received 23 August 1989) CONTENTS

1.

2. 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 3. 3.1. 3.2. 3.2.1. 3.2.2. 3.3. 3.4. 4. 4.1. 4.1.1. 4.1.2. 4.1.3. 4.1.4. 4.2, 4.2.1. 4.2.2. 4.2.3. 4.2.4. 4.3.

2 Abstract .................................................................................................. 2 Introduction ........................................................................................... 4 Synthesis of methyl formate.. ............................................................... 4 Carbonylation of methanol ................................................................... 5 Dehydrogenation of methanol.. ............................................................ 6 Oxidative dehydrogenation of methanol.. ........................................... 7 Dimerization of formaldehyde.. ............................................................ Direct synthesis from synthesis gas.. ................................................... 7 Hydrocondensation of carbon dioxide with methanol.. ..................... 8 Methyl formate for separation, storage and transport of synthesis 8 gas ............................................................................................................ Current technologies for separation and recovery of synthesis gas .. 8 Separation of synthesis gas by methyl formate synthesis.. ............... 10 10 Use of a methanol carbonylation process.. .......................................... 11 Use of a methanol dehydrogenation process.. ..................................... Recovery of CO from a gas mixture containing N 2 ............................ 11 Methyl formate as a synthesis gas carrier.. ......................................... 13 Methyl formate as a versatile chemical intermediate.. ...................... 13 Reactions involving breakage of C-O-C bonds ................................... 14 14 Hydrolysis .............................................................................................. Hydrogenolysis ...................................................................................... 15 Homologation.. ....................................................................................... 16 Carbonyl transfer ................................................................................... 17 Reactions involving activation of f&my1 hydrogen ........................... 18 Coupling with carbonyl compounds.. ................................................... 18 Coupling with olefins ............................................................................ 20 Halogenation.. ........................................................................................ 21 Oxidation.. .............................................................................................. 21 Isomerization of methyl formate: synthesis of acetic acid.. ............... 22

0166.9834/90/$03.50

0 1990 Elsevier Science Publishers B.V.

2

5. 5.1. 5.2. 6.

Examples of integrated C, chemistry complexes based on methyl formate.. .................................................................................................. Example 1: production of chemicals from steel-mill off-gases .......... Example 2: generation of electricity from synthesis gas.. .................. Concluding remarks .............................................................................. References ..............................................................................................

24 24 25 26 27

ABSTRACT Methyl formate has been proposed as a building block molecule in C, chemistry. This paper examines the potential of this concept by reviewing the processes of synthesis of the molecule and the chemical reactions that it undergoes. Methyl formate can be produced by a variety of routes from a number of feedstocks. The reaction between methanol and carbon monoxide is an efficient process, used commercially. Combination of an efficient synthesis of methyl formate and its facile decomposition allows the molecule to be used as a means for separation, storage and transport of synthesis gas. The number of reactions that convert methyl formate to other chemicals is large. In particular, the synthesis of large volume chemicals such as methanol, acetic acid and ethylene glycol deserves serious consideration, Examples are provided of applications in the chemical and energy industries involving methyl formate. The reactions involved in the synthesis and transformation of methyl formate are mostly catalytic in nature. Many currently known catalytic systems are not efficient to compete with conventional routes involving methanol or synthesis gas. Fundamental research to understand the catalytic chemistry involved is highly desirable in order to improve the performance of the catalytic systems. 1. INTRODUCTION

C, chemistry refers to processes that convert molecules with one carbon atom such as carbon monoxide, methanol, formaldehyde and methane into organic compounds with increased carbon number. The synthesis gas can be produced from practically any carbonaceous materials including coal and biomass, and other C, chemicals can easily be derived from it. Stimulated by recent concern over the cost and availability of petroleum feedstocks in the future, C, chemistry has been an active area of research in academic and industrial laboratories. Although direct routes to products from synthesis gas are preferred, they are often nonselective or require drastic reaction conditions in order to obtain high yields. Therefore, methanol has been considered as a building block in the synthesis of various chemicals from synthesis gas [l-4]. The synthesis of methanol from CO and H, is an efficient and established technology and was the topic of recent reviews [ 5-61. A large number of important chemicals can be derived from methanol [l-4]. The use of methanol instead of synthesis gas itself also provides a great deal of convenience in storage, transport and handling. Methyl formate has also been proposed for a similar role of a building block in C, chemistry. An integrated chemical industry complex involving methyl formate could come into existence in the future. In order for a compound to

3

play such a role, it must possess the following attributes: (1)The synthesis process of the compound should be efficient in large scale production. (2) The compound should be convenient to handle, store and transport. A stable liquid at moderate conditions is preferred. (3 ) Efficient and versatile synthetic routes should be available from the compound to other chemicals, especially to large volume commodity chemicals. Methyl formate is an ester of formic acid and can be synthesized relatively easily in various ways. The physical properties of methyl formate are summarized in Table 1. It is a colorless and transparent liquid soluble in water and most organic solvents. It is volatile with a boiling point of 315°C at atmospheric pressure and has a specific gravity of 0.97. It is slightly corrosive. These properties indicate that the chemical could be handled, stored and transported in a manner similar to liquefied petroleum gas (LPG). Since a molecule of methyl formate, HCOOCHB, contains one more CO than methanol, methyl formate could be more efficient for the transport and storage of CO and H,. Methyl formate is toxic and paralyses the central nervous system. It can be used as a solvent and an insecticide as it is. But the main industrial use of methyl formate at present is in producing formic acid by hydrolysis. In recent years, there has been a rapid advance in chemical synthesis based on methyl formate. However, it appears that this interesting branch of C, chemistry has been receiving relatively limited attention among catalytic chemists or engineers. This is partly because much of the available literature is in patents. A few reviews of limited scope have been published [ 7-91. The present review attempts to analyse the potential of methyl formate as a new building block molecule in C1 chemistry and to provide some conceptual examples of implementation. Rather than an extensive literature survey, only the literature directly related to the review is cited. TABLE 1 Physical properties of methyl formate Boiling Point at 1 bar Melting Point Specific Gravity ($” ) Refractive Index Surface Tension at 20” C Heat of Comhustion of Liquid at 25” C Heat of Formation of Liquid at 25 ‘C Flash Point, Tag Open Cup Ignition Temperature Flammable Limits in Air Source: ref. 7.

315°C -99’C 0.9742 1.3433 25.08 dyne cm-’ - 979.5 kJ mol-l -378.2 kJ mol-’ -2O’C 450cc 5.0-20% ester by volume

4

2. SYNTHESIS OF METHYL FORMATE

Methyl formate can be manufactured by many processes as summarized in Fig. 1; ( 1) carbonylation of methanol, (2 ) dehydrogenation of methanol, (3 ) oxidative dehydrogenation of methanol, (4) dimerization of formaldehyde, (5) direct synthesis from CO and Hz, and (6) hydrocondensation of carbon dioxide with methanol. Base-catalysed methanol carbonylation represents an established, state-of-the-art commercial technology. The synthesis of methyl formate by dehydrogenation of methanol followed by pyrolysis to produce pure CO was once considered for commercial operation by Mitsubishi Gas Chemicals Co. (MGC) in Japan. Because of the rising price of methanol, however, the plan was abandoned. 2.1. Carbonylation of methanol The oxygen-hydrogen bond of methanol is readily activated by a base, generating a methoxide ion which then undergoes attack by CO. In practice, this is achieved by using an alkaline metal methoxide catalyst dissolved in methanol at 60-120” C and with 20-70 bar of CO [lo-111 : CH30H+CO+HCOOCH,,

dHO,= -29.1 kJ mol-’

(1)

The nucleophilic attack of methoxide ion on carbon monoxide was proposed by Christiansen in 1942 [ 121 and supported by a recent kinetic study [ 131. The process was first patented by BASF in 1925 as a part of formic acid synthesis process [ 141. Improved processes which can use dilute CO have been developed by Leonard Process Co. [ 151 and Scientific Design/Bethlehem Steel [ 16-171. Typically, conversion of CO and methanol are 95% and 30% respectively, and selectivity to methyl formate is 99%. Synthesis gas streams that contain as little as 50% CO can be economically used as long as contaminants such as H,O, CO*, 0, and sulfur compounds have been reduced to low-ppm levels. It is said that both processes employ improved catalysts containing proCH,OH +CO 2C0 +2H2

2CH20

HCOOCH3

PCH3OH C& + Hz t CHzOH

Fig. 1. Synthesis routes for methyl formate.

5

prietary additives to increase the reaction yield and lower the pressure [ 111. Moisture leads to formation of insoluble sodium formate and causes plugging, while H, and N, present no problems. Although the current processes are quite satisfactory, the extreme sensitivity of alkali methoxide catalyst to minute contaminants, especially moisture and CO2 has caused some operational problems. Some recent work in methyl formate synthesis from methanol and carbon monoxide aims at developing more robust catalyst systems that could avoid the problems experienced in current processes. This includes homogeneous ruthenium [ 18,191, platinum [ 201 and tungsten [ 211 complexes and non-metallic catalysts (guanidines) [ 221. Unlike processes requiring the activation of carbon-oxygen bonds, these transition metal complex catalysts do not need halide promoters to form methyl formate by activation of the oxygen-hydrogen bond of methanol. A base-catalysed carbonylation of methanol has been described using a pendant ion-exchange resin with quaternary ammonium group which serves as a counter-ion for the catalytic OH- species [ 231. These newer catalytic systems, however, are either in the stage of fundamental research or not as efficient as the current homogeneous sodium methoxide catalyst system. 2.2. Dehydrogenation of methanol Dehydrogenation of methanol over copper catalysts to yield methyl formate has been known since the 1920s [ 24-251: 2CH3 OH-+HCOOCHB + 2Hz, dHg = + 98.9 kJ mol-’

(2)

In addition to copper [ 9,26-271, silver [ 281 and tungsten carbide [ 29-301 have been reported to be efficient catalysts for the reaction. Since silver and copper catalysts are used in the production of formaldehyde and dehydrogenation of methanol [lo], formation of methyl formate via a formaldehyde intermediate is a possible pathway. However, no formaldehyde is cited as a product, even in trace amounts. Furthermore, at the low reaction temperature employed for the methyl formate synthesis, a simple thermodynamic calculation shows that equilibrium between CH,OH, HCHO and H, predicts a formaldehyde concentration too low to explain the high methanol conversion obtained. Therefore, other possible routes must be considered. As mentioned, the process was considered for commercial operation by MGC to produce pure CO by a subsequent pyrolysis step. The catalyst system employed by MGC comprises copper-zinc-zirconium and aluminium as an option [ 9,261. This combination of promoter is said to give better yields and greater catalyst life than a simple copper-zinc catalyst. The dehydrogenation of methanol is carried out in the gas phase over a fixedbed catalyst, at temperatures of 150-300” C, atmospheric pressure, and gas space velocity of 500-30 000 h-l. The yield of methyl formate is nearly 50% at

6

a methyl formate selectivity of about 90%. This yield is subject to thermodynamic constraints on equilibrium ratios of methanol and methyl formate at the temperature of interest, and the competing reaction of total decomposition to synthesis gas. The performance was stable enough in a pilot test to prove the possible commercial viability of the process. Based on the performance data reported by MGC, synthesis of methyl formate from methanol by a dehydrogenation route appears to be technically feasible. The MGC process uses methyl formate as an intermediate for pure CO using readily available methanol as a feed stock. For this purpose, the mixture of methanol and methyl formate can be fed to a subsequent pyrolysis reactor without distillation, making the process remarkably simple. However, this route has a drawback relative to the methanol carbonylation route, i.e. a mole of methanol is more expensive than a mole of carbon monoxide, and thus the efficient use of by-product hydrogen is required to make the process economically viable. 2.3. Oxidative dehydrogenation of methanol Methyl formate has also been synthesized by oxidative dehydrogenation of methanol to achieve a thermodynamically more favorable process: 2CH,OH + O2 -+ HCOOCH, + H, 0, dHg = - 472.8 kJ mol-1

(3)

Due to the higher exothermic nature of the reaction, a liquid phase process at elevated pressure has been proposed [ 311. Soluble chromium compounds [ 311 or ruthenium complexes [ 321 are used as catalysts. A gas phase process based on bimetallic oxide catalysts, generally MOO, or WO, as one component shows a high selectivity of methyl formate [ 331. In the best case, a SnO,-Moos (Sn-to-MO= 7/3) catalyst gives 90% of methyl formate selectivity at 72% of methanol conversion at 160’ C. However, methyl formate throughput calculated from the data in the work [ 331 is extremely low at 0.06 g/g catalyst/h. Unlike the dehydrogenation route, the major co-product of the reaction is formaldehyde, which is believed to be the primary reaction product which converts in the further reaction to methyl formate. CHBOH+ 1/202 +HCHO+H20 2HCHO+HCOOCH3

(4)

It appears that bimetallic combinations are essential for the reaction. A single component SnOz is inactive for the reaction, while pure MOO, produced mostly formaldehyde [ 341. Compared to the dehydrogenation of methanol described earlier, the oxidative dehydrogenation makes the overall reaction exothermic, and lowers the reaction temperature. However, it loses Hz, a potentially valuable by-product,

7

as water. Also, the reaction appears to be less selective due to competing side reactions. 2.4. Dimerization of formaldehyde Dimerization of formaldehyde to give methyl formate is a Tischenko type reaction (intermolecular oxidation-reduction) with the following stoichiometry: BHCHO +HCOOCHB, dH& = - 146.4 kJ mol-’

(5)

However, a Cannizaro reaction followed by esterification of formic acid with methanol also yields HCOOCH3. 2HCHO+HzO+CHBOH+HCOOH

(6)

CH30H+HCOOH-,HCOOCH3+Hz0

(7)

Both homogeneous and heterogeneous catalytic systems have been reported. The reaction between paraformaldehyde in n-dibutyl ether in the presence of the complex RhCl( CO) (Ph,P), gives methyl formate along with methanol and synthesis gas [ 351. The reaction can be carried out more efficiently in the vapor phase over Cu-Zn [36], Cu/SiO, [37] or Sn02-WO, [38]. Both acidic and basic sites are found to be necessary to catalyse the reaction. It has been proposed that a hemiacetal intermediate CH2 (OH) OCHB is involved in both reactions 5 and 2 [ 391. 2.5. Direct synthesis from synthesis gas The direct synthesis of methyl formate from synthesis gas can be achieved in a high-pressure liquid-phase reaction in the presence of homogeneous transition metal catalysts. 2C0 + 2Hz + HCOOCH,, LIH”,= - 157.2 kJ mol-l

(3)

Methyl formate and methanol are selectively produced with complexes of cobalt [40-431, ruthenium [42-451, or iridium [42] as a catalyst. Typical reaction conditions are 220-250°C and l-2*10’ Pa with throughputs of l-3 mol HCOOCH,/mol catalyst/h. Methanol is the dominant product with Hz-rich synthesis gas, while increasing CO partial pressure decreases the activity and increases the yield of HCOOCHB. The systems have been investigated in a small scale continuous unit [ 461. A recent patent describes a catalyst system comprising an acetate of nickel, palladium or cobalt, a Group 6 metal carbonyl, and an alkali metal hydride [ 471. Good yields of methyl formate and methanol are claimed under mild conditions of 50-100°C and 7-20~10~ Pa. A heterogeneous system of alkalized Cu-ZnO-Alz03 is also reported [ 481.

8

Although direct synthesis of methyl formate is the most desired process, reported performance data are not good enough for industrial realization. However, this is an area of active research and significant improvements may be achieved in the future. 2.6. Hydrocondensation of carbon dioxide with methanol The hydrocondensation of carbon dioxide with alcohol has been described relatively recently. Methyl formate has been synthesized from methanol, CO, and H2 in benzene in the presence of a catalyst composed of ruthenium, iridium, osmium or platinum complexes and BF, [ 491. CO, +H, +CHBOH+HCOOCH3

+H,O

(9)

A low-valent complex of palladium, ruthenium, rhodium or iridium and a tertiary amine [50-511, anionic ruthenium carbonyl clusters [52], and anionic group 6B metal carbonyl [53] are among recently reported catalysts. These catalysts show much greater activity for methyl formate formation from CO and CH30H. Furthermore, the same catalysts promote the water-gas shift reaction. There is a possibility that the reaction of CO2 and H, proceed via CO produced by the reverse of the water-gas shift reaction. This possibility, however, is not consistent with the fact that the presence of CO suppresses the C0.JH2 reaction and with the results of an isotope study which demonstrated that only H12COOCH, was formed in the initial stage of the reaction when a catalyst precursor with 13C0 ligands was employed [ 52-531. The use of carbon dioxide as a feedstock for the synthesis of methyl formate is an interesting idea. However, the process requires H2 which is, in many cases, more expensive than CO. Furthermore, the activity for methyl formate formation is quite low. For example, with 17*105 Pa each of CO2 and Hz, and at 125’ C, turnover rates of 3.8-7.3 per 24 h are observed for HRu3 (CO) 11- [ 521. 3. METHYL SYNTHESIS

FORMATE

FOR SEPARATION,

STORAGE

AND TRANSPORT

OF

GAS

3.1. Current technologies for separation and recovery of synthesis gas When synthesis gas is used as a chemical feedstock, it is often necessary to separate CO and Hz or to adjust the ratio between them. Sometimes CO and/ or H, have to be recovered from non-conventional sources such as vent streams from the chemical industry or off-gases from the steel industry. Before discussing the use of methyl formate as a means of separation or recovery of synthesis gas, currently available technologies are briefly reviewed. They are cryogenic separation, chemical solvent absorption, physical/chemical adsorption and membrane separation [ 541.

9

These processes differ in the degree of establishment, and yet possess their own advantages and limitations. For example, cryogenic separation cannot be used to obtain pure CO if N, is present in the synthesis gas due to the close physical properties of N, and CO. However, both CO and H, are obtained in high purity by this method. The cryogenic process is the most widely employed, but is a highly capital and energy intensive process. The best known chemical absorption process is the Cosorb process which utilizes a cuprous aluminium chloride (CuAlCl,) in toluene to separate CO from synthesis gas [ 551. The solvent absorbs CO to form a carbonyl complex and releases it upon mild heating. CuAlCl,.C,H,

+COF?CuAlClq.CO+C7Hg

(10)

The solvent is inert to N2, H, and COZ, but extremely sensitive to moisture, H,S and ammonia, thus causing operational difficulties. Pressure swing adsorption (PSA) to recover pure H, using a selective adsorbent such as a zeolite has long been established by Union Carbide and others [ 561. Recent developments of new adsorbents, Cu’+-exchanged zeolite or alumina, has made it possible to recover high-purity CO by PSA even in the presence of N,. Japanese steel companies such as NKK [ 571, Kawasaki Steel [58] and Kobe Steel [59] have developed their own processes. The PSA processes for CO recovery are remarkably simple and energy-efficient compared to Cosorb. However, for the production of large volumes of CO, the size of the vacuum pump is a limiting factor in scale-up. Thus 3000 Nm3h is considered to be the current ceiling of CO produced from a single unit of CO-PSA [ 571. The development of a selective membrane for synthesis gas separation is TABLE

2

Comparison

of CO separation

technology PSA

Principle

Product CO Purity CO recovery (% ) Operating Temp.

(% ) ( o C)

Operating Pressure Optimum Scale

( X lo5 Pa)

Capital Investment Operating Cost Operability No. of Commercial Source: refs. 57-59.

Plants

COSORB

Physical/chemical

Chemical

Adsorption

Absorption

99 70-85

99 99 50-80

Ambient-80 0.2-l

Cryogenic solvent

Distillation

98 98 -200 High

Small

2-3 Medium

Low

Medium

Low Easy 1

Medium-High Complicated

High

> 10

Many

Large High Complicated

10

one of the most active areas of research and development [ 601. In Japan, it is one of the more successful areas in the national C,-chemistry project [54]. The process suffers from low selectivity, but could be economically used for the adjustment of CO/H2 ratios. Table 2 compares some processes for the recovery of CO. A brief survey of current technologies of synthesis gas separation presented here reveals that there still exists much room for improvement. The use of methyl formate synthesis for the separation of synthesis gas adds a new category to the separation technology, i.e. separation by chemical synthesis. 3.2. Separation of synthesis gas by methyl formate synthesis 3.2.1. Use of a methanol carbonylation process The fact that synthesis gas containing as little as 50% CO could be economically used to synthesize methyl formate in recent versions of base-catalysed methanol carbonylation [ 111 enables the process to be used to recover CO from synthesis gas streams. A possible scheme is depicted in Fig. 2. The synthesis gas stream is first passed to gas purifiers (not shown ) wherein impurities such as entrained solids and a variety of sulfur containing gases are removed by conventional methods. The cleaned synthesis gas is then enriched in CO in an enrichment unit which can be a “once through” methanol synthesis unit (without recycle of unreacted gas) or a unit for the removal of H, from the synthesis gas by diffusion through a gas semi-permeable membrane, or the like [ 611. After exiting from the enrichment unit, any remaining gaseous sulfur compounds that would be damaging to the catalysts of methyl formate synthesis are removed in gas purification units. The CO-rich synthesis gas then enters the methyl formate synthesis unit where it reacts with methanol in the presence of a base catalyst. The CO is recovered in the form of methyl formate which, after separation from the reaction mixture, is stored or transported for further processing including decomposition to produce pure CO. The vent stream from the reactor is now rich in HZ, and could be used for other purposes. Hz-rich

1 or semipermeable membrane 1

CH30H

Recycle

Fig. 2. Separation of synthesis gas by methyl formate synthesis: base-catalysed carbonylation of methanol.

11

3.2.2. Use of a methanol dehydrogenutionprocess Synthesis gas can also be separated by methanol synthesis followed by the production of methyl formate through dehydrogenation of methanol as shown in Fig. 3. If the methyl formate is decomposed to methanol and CO, complete separation of synthesis gas into pure H, and pure CO is achieved. The original purpose of the MGC process was to separate synthesis gas by this scheme [ 91. The H, produced by this process is only of 90-92% purity, while the CO is more than 98% pure. 3.3. Recovery of CO from a gas mixture containing N2 Among steel-mill off-gases, the converter (basic oxygen furnace or BOF) gas, commonly called LDG (Lintz-Donawhitz Gas), appears to be the most interesting as a potential source of CO for the chemical feedstocks since it is rich in CO and nearly free of HP, sulfur compounds and hydrocarbons, as shown in Table 3. The other components of the gas are mostly N2 and CO,. The CO content varies depending on steel refining conditions, but stays between 60 and 70%. With bottom O2 blowing, instead of the more conventional top blowing, the CO content could increase to as high as 85% [8]. Since roughly 100 Nm3 of the converter gas is generated per ton of crude steel, 1.0.10’ Nm3/year or 1.2*106 MT/year of the gas is generated from a steel mill with 1.0.107 MT/ year of crude steel capacity. Steel mills also produce the coke oven gas (COG) with high H, content (Table 3). Thus it is interesting to envision a chemical industry complex using steel-mill off-gases as a feedstock. The candidates for the possible location would be Japan, Korea, Europe and the United States where large steel mills are located. Because of the high CO content in LDG, the synthesis of methyl formate by carbonylation of methanol could be applied to recover CO from the gas. Fig. 4 shows an example of such a system from a converter with bottom blowing [ 62 1. The LDG for chemical applications comes from a separate gas holder (ll), which contains LDG with a higher CO content than in the main holder (8). After removal of COa, moisture, and other impurities in a pretreatment system (12), LDG now containing CO and N, is compressed by a compressor (13) Hz (90-92%)

co 098%) t

r co ‘Ii2

MeoH synthesis

--r--l

CHaOH *

[Cu znl

I

I I

HCOOCHJ synthesis

HCOOCH3 decomposition

[Cu al

[KCI Cl

Zrl

-..A

I

I

unconverted

gas recycle Fig. 3. Separation

of synthesis

gas by methyl formate synthesis:

dehydrogenation

of methanol.

12

TABLE 3 Characteristics of Steel-Mill Off-Gases”

Composition (Vol. % )

Impurity

Exit Temperature (‘C) Exit Pressure ( x lo5 Pa) Moisture Density (kg Nm3) Heat Content (kcal Nm3)

CO,

CJLb 02 co CH, H2 N2

Tar H,S SO, NH, Naphthalene HCN Dust

COG

LDG

3.1 2.9 0.3 8.4 26.6 56.4 2.3 IO-40 mg Nm3 500-800 mg Nm3 _

17.8 -

50-120 mg Nm3 180-260 mg Nm3 1000 mg Nm3 30-40 1.66 Saturation 0.44-0.51 4400

0.1 64.2 _ 2.0 15.9 1.5 ppm 20 ppm 3 ppm I ppm 30 mg Nm3 30-40 1.53 Saturation 1.34-1.40 2000

“Source: Actual operating data of Pohang Iron and Steel Co., Pohang, Korea. bMainly saturated C&-C, hydrocarbons.

23

Fig. 4. Recovery of CO from a converter of a steel plant: 1. Converter; 2. O2 blowing port; 3. Auxiliary lance; 4. Hood; 5. Shield gas; 6. Venturi; 7. Main LDG recovery system; 8. Main LDG holder; 9. Auxiliary LDG recovery sytem; 10. CO analyser; 11. Auxiliary LDG holder; 12. Gas pretreatment system; 13. Compressor; 14. Methyl formate synthesis reactor; 15. HCOOCH,/ CH,OH distillation tower; 16. Methyl formate holder; 17. Methyl formate decomposition reactor; 18. Methanol recycle; 19. Purified CO; 20. Unreacted gas; 21. Pipeline; 22. Truck; 23. Train; 24. By-product separator (adapted from ref. 62 ).

13

and fed to a reactor (14) where methyl formate is synthesized by a reaction with methanol in the presence of a base catalyst. Unreacted methanol is separated in a distillation tower (15) and purified methyl formate is stored in a holder (16). This methyl formate could be pumped or transported by rail or truck to the end users including a producer of pure CO by decomposition (17). 3.4. Methyl formate as a synthesis gas carrier As demonstrated by the MGC process [ 91, methyl formate can be decomposed below 200” C to give CO and methanol. HCOOCH, +CHBOH + CO, dHO,= + 29.lkJ mol-l

(II)

The reaction is carried out over a fixed catalyst bed of alkali metal salts such as KC1 or Na,SO, supported on activated carbon. The catalyst can selectively decompose methyl formate even when excess methanol is present. With KC1 supported on activated carbon, for example, 99.5% of methyl formate is converted to CO and methanol with a selectivity better than 99%. The combination of reactions ( 1) and ( 11) would make methyl formate a convenient CO carrier. The gas obtained from the decomposition contains CO (over 98% ) , H, (l-2% ) , and CH4 and COT (0.3-l% ) . Pressurized CO gas can be obtained by carrying out the process under pressure. If the decomposition temperature of methyl formate is raised to 300-350°C over a catalyst bed of alkali or alkaline earth metal oxides, the mixture of CO and H2 is obtained according to the reaction 12 [ 81: HCOOCH3 +2CO+

2H2, AH”, = + 157.2kJ mol-l

(12)

The synthesis gas obtained has the purity and stoichiometry suitable as a feed to, for example, an 0x0 unit. Also, by combination of this reaction with a method of methyl formate synthesis, methyl formate can serve as a synthesis gas carrier. 4. METHYL

FORMATE

AS A VERSATILE

CHEMICAL

INTERMEDIATE

Methyl formate has been reported to undergo a variety of reactions, and thus could serve as a versatile chemical intermediate. However, the reactivity of the molecule is barely understood at the fundamental level except for some gasphase, homogeneous reactions [ 63-651. Scattered information reported in the literature on the catalytic chemistry of methyl formate is not enough to reveal the fundamental nature of the chemistry. Methyl formate, as an ester of formic acid, undergoes typical reactions of carboxylic acid esters. Furthermore, it contains a formyl C-H bond that is aldehyde-like yet different from the usual aldehyde due to the influence of the C-OR bond. Catalytic reactions of methyl formate could be divided into two types according to the molecular structure of the products: (1) reactions involving the activation of the formyl C-H bond, keeping the ester part intact,

14

and (2 ) reactions involving the fission of a formyl-oxygen bond (HC (0 )-0) or methyl-oxygen bond (0-CH,). The conversion of methyl formate to acetic acid is apparently an isomerization, and is discussed separately. It should be noted that this classification does not necessarily indicate the mechanistic details of the elementary steps involved. The manufacture of formic acid and formamide are commercial processes. 4.1. Reactions involving breakage of C-O-C bonds In the reactions of this type, the formyl group in methyl formate is mainly decarbonylated or decarboxylated. Carbonylation and hydrogenation take place in the case of homologation reactions. The simple decarbonylation to CH,OH and CO was discussed already. Methyl formate can also decompose to two molecules of formaldehyde, the reverse of the reaction (5)) or be decarboxylated to CO2 and CH,. These reactions, of little synthetic value, are not discussed. 4.1.1. Hydrolysis Traditionally, formic acid has been produced by the ammonolysis of methyl formate to give formamide followed by hydrolysis by sulfuric acid. HCONHz + 1/2H, SO, + H2 O+HCOOH+

l/2 (NH,), SO,

(13)

This process developed by BASF, is hampered by the co-production of economically unattractive ammonium sulfate, and is being replaced by the direct hydrolysis of methyl formate. HCOOCH:, +HaO+HCOOH+CH30H

(14)

The reaction is limited by thermodynamic equilibrium under usual operating conditions. To drive the reaction to the formic acid side, processes developed by Halcon SD/Bethlehem Steel Corp. and by the Leonard Process Co. [ lO,ll] employ an excess of water and reduced contact time between HCOOH and CH30H during work-up. The reaction is carried out at 8O”C, 3 x lo5 Pa, and about 1 h of residence time, with the product formic acid acting as the catalyst [ 151. In another mode of methyl formate hydrolysis, the BASF process uses the stoichiometric amount of water and protects the formic acid by forming an adduct with an amine of weak basicity such as 1-pentylimidazole [ 66-671. The reaction of methyl formate, water and the imidazole occurs at 130°C 10. lo5 Pa and ca. 0.67 h of residence time. After decomposition of the HCOOH/imidazole adduct, pure formic acid is obtained with 99% selectivity at 64% methyl formate conversion. If hydrolysis of methyl formate is carried out with HCl or HCN, instead of water, methyl chloride or acetonitrile is obtained along with formic acid [ 91.

15

HCOOCH,+HCl+HCOOH+CH,Cl

(15)

HCOOCH3 +HCN+HCOOH+CH3CN

(16)

When methyl formate is reacted with HCl at 60 “C! in the presence of a catalyst system of ZnClz and H3P04, a formic acid selectivity of 87% and a CH,Cl selectivity of 99% is obtained at a methyl formate conversion of 69%. The reaction between methyl formate and HCN is carried out in the presence of a ZnCl, catalyst. At 3-4 x lo5 Pa and 70’ C, CH&N is produced with a yield of 95% and formic acid, 88%. Another patent [68] reports the hydrolysis of methyl formate in the presence of CO. HCOOCH3+CO+HpO+CH3COOH+HCOOH

(17)

With a Rh-I catalyst and at 180” C, 30 x lo5 Pa, a yield of 88% is claimed. These methods thus enable two products to be generated from a single reactor. 4.12. Hydrogenolysis Methyl formate can undergo hydrogenolysis to give two moles of methanol in the reverse step of reaction (2). HCOOCH3 +2H,+2CH,OH

(18)

Reaction (18) was originally described by Christiansen [ 691 as an alternative two-step production of methanol from CO and H,. As outlined in Fig. 5, methanol is first carbonylated to methyl formate as described earlier. The subsequent hydrogenolysis step produces two moles of methanol. Since one mole of methanol must be recycled to the first step, net one mole of methanol is produced from CO and HZ. Alcohols heavier than methanol could also be employed as recycling alcohol. Compared to the well-established direct synthesis of methanol from CO and HS, the two-step process offers advantages of milder reaction conditions and favorable thermodynamic equilibrium of product formation. Owing to this advantage of equilibrium, the two-step process could

HCOOR

ROH

Fig.% Methanol synthesis through alkyl formate intermediate (adapted from ref. 17).

16

avoid a large recycle of unreacted synthesis gas experienced in the direct route. Instead, the two-step process requires a large recycle of methanol and thus larger reactor and related equipment. Furthermore, while the conventional methanol synthesis uses a CO,-containing feedstock, the two step synthesis requires a rigorous removal of CO, in order to avoid a damage to the catalyst of methyl formate synthesis. The hydrogenolysis of methyl formate was carried out in the gas phase over a copper-based catalyst [70]. Evans et al. [71] showed that methanol was formed with a selectivity in excess of 90% at a methyl formate conversion of 70-80%, at 140°C and atmospheric pressure over a copper chromite catalyst. The major by-product was CO which appeared to originate from the decarbonylation of methyl formate. In the case of ethyl formate, an almost quantitative yield to ethyl and methyl alcohols was observed at 200” C and 6-11*105 Pa [ 721. The amounts of by-product CO, CO, and ethyl acetate observed at atmospheric pressure were reduced by operation at higher hydrogen pressures. In the liquid-phase hydrogenolysis of methyl formate, Sorum et al. [73] reported that copper-chromite type catalysts promoted by barium and manganese were found to have high activity for the selective production of methanol, and that the dissociative chemisorption of methyl formate appeared to be the rate-determining at hydrogen pressures above 70~10~ Pa. 4.1.3. Homologation The homologation of methyl acetate to produce ethylidene diacetate [ 741, or ethyl acetate [ 75-761 has been a subject of extensive research. Homologation of other esters including formic esters, however, has received little attention. HCOOR+CO+2H,-CH,COOR+H,O

(19)

A ruthenium carbonyl iodide system is active for the hydrogenation of the formyl moiety of methyl formate to methyl derivatives and in their carbonylation and homologation to acetyl and ethyl compounds at 200’ C and a CO/ H, pressure of 150-200*105 Pa [ 17,77-781. A patent [ 791 describes an Fe-NR3 catalyst system which gives ethanol with 26% yield at 220°C and 300*105 Pa. When a CoI, is used as a catalyst in a polar solvent such as N-methyl pyrrolidine at 200” C and 2CO/H2 pressure of 300-400. lo5 Pa, acetaldehyde, formic acid and acetic acid are formed after 1.5 h of reaction time with yields of 50%, 71% and 18%, respectively [SO]. More recently, a better acetaldehyde yield of 80% has been reported with (Ph,P),Rh (CO)Cl and an iodide promoter at 18O’C and 80*105 Pa [81]. Homologation of methyl formate is, in general, poorly described and little understood. Many products are formed from the complicated reaction network. In addition to hydrogenation of the formyl group, carbonylation, hom-

17

ologation, decarbonylation, or decarboxylation of the group seems to occur simultaneously as well as homologation of the methyl group. 4.1.4. Carbanyl transfer The reactions of this type belong to the decarbonylation of methyl formate. However, they do not produce gaseous CO, but carbonylate other substrates. For example, dimethylformamide can be produced almost quantitatively by applying some heat (50” C ) and pressure (5 *lo5 Pa) without a catalyst [ 821. HCOOCH3 +NH (CH,), -tHCON(CH3)2

+CH30H

(20)

The process is being commercially practised in the U.S. and Europe. The reaction of methyl formate and ammonia to obtain formamide is carried out on a large scale mainly to produce formic acid via the BASF process. HCOOCH3 + NH3 -+HCONH, + CH, OH

(21)

The reaction is typically carried out at 80-100°C and 4-6. lo5 Pa with use of liquid ammonia. Alternatively, the Du Pont process employs gaseous ammonia at atmospheric pressure and 20-30°C [83]. A process is described for the synthesis of HCN from this formamide [ 841. The examples suggest that, for proper substrates, carbonylation with methyl formate as a CO source could be a promising direct application of methyl formate. Furthermore, it should be noted that the reaction conditions are much milder than with gaseous CO as a carbonylating agent, as shown in Table 4. The carbonylation of both ammonia and NH (CH, )* with gaseous CO employs sodium methoxide dissolved in methanol as a catalyst as in the synthesis of methyl formate by methanol carbonylation. It appears that carbonylation with CO proceeds through the methyl formate intermediate, and that formation of the intermediate requires more severe conditions than for subsequent ammonolysis. TABLE Reaction

4 conditions

Amide

for the synthesis

of amides with CO or methyl formate

Carbonylating

T

P

agent

(‘Cl

( X 10’ Pa)

Formamide

co

Dimethylformamide

HCOOCH, CO HCOOCH,

Catalyst

Ref.

80-100

100~200

NaOCH,

84

80-100 110-150

4-6 15-25

None NaOCH,

83 84

None

82

50

5

18

4.2. Reactions involving activation of formyl hydrogen It is known that the formyl proton in methyl formate is about as acidic as that in water [ 851. It can be activated by strong bases, acids, or organometallic compounds. In reactions of this type, the whole molecular structure of methyl formate except the formyl hydrogen is maintained in the product molecule. 4.2.1. Coupling with carbonyl compounds Reaction of methyl formate with paraformaldehyde or trioxane yields methyl glycolate (hydroxyacetic acid methyl ester) in an acid-catalysed process. HCOOCH3 +HCHO+HOCH2COOCHB

(22)

As shown in Table 5, typical conditions for this synthesis are 90°C and atmospheric pressure with sulfuric or an organic sulfonic acid as catalyst [ 861, or 110°C and 60*105 Pa with solid Lewis acids or a cation exchange resin [87]. The reaction is related to Du Pont’s glycolic acid synthesis process from formaldehyde and CO, which was practised commercially until 1968. The carbonylation of formaldehyde requires, however, more severe conditions of 200°C and 700*105 Pa of CO [lo]. Lately, this process is receiving fresh attention with the advent of efficient catalysts which make the carbonylation reaction possible under mild conditions [ 88-911. The glycolic acid of the Du Pont process is, after esterification with methanol, hydrogenated to give ethylene glycol. If methyl formate is reacted with formaldehyde, the methyl ester is obtained directly. The hydrogenation of methyl glyoclate is carried out in the presence of a copper-based catalyst. HOCH2COOCH3 +2H2+HOCH2CH20H+CH30H

(23)

The reaction was carried out in either gas phase at 200-225 ’ C and ca. 30. lo5 Pa [92] or liquid phase at ca. 400. lo5 Pa in the Du Pont process. Due to the facile decomposition of the glycolate, it was difficult to obtain ethylene glycol TABLE Synthesis

5 of methyl glycolate from methyl formate

Catalyst

H,SO, CIS03H

T

Glycolate

Yield

Ref.

(“C)

yX105Pa)

(%o)

70-200

1

24-69

86

110

60

56

87

CH,SOsH CH,,C,H,SO,H Diaion” Montmorilonite “A cation-exchange

resin.

19

in yields better than 30% in the old process. However, significant progress in the catalyst performance has been made lately, as seen in Table 6 [ 88,94-971. Although detailed studies are not available, one can conceive the synthesis of some important derivatives of methyl glycolate. The carbonylation by CO leads to malonate ester, an important intermediate for pesticides or pharmaceuticals. HOCH2COOCH3 +CO+HOOCCH2COOCH3

(24)

The methyl glycolate when reacted with ammonia forms DL-glycine, the simplest amino acid. HOCH2COOCH3 + NH3 +H,NCH,

COOH + CH, OH

(25)

The hydrolysis of methyl glycolate yields glycolic acid and methanol. The former can undergo self-polycondensation to give polyglycolate. HOCH,COOCH,+H,O+HOCH,COOH+CH,OH

(26)

nHOCH,COOH+

(27)

(-0-CH,-CO-),+nH,O

This new polymer could become important as a surgical suture because it is easily hydrolysed in vivo. The coupling of formaldehyde and methyl formate to form methyl glycolate followed by synthesis of its derivatives provides more prospective applications of methyl formate. Serious attention should be paid to production of ethylene glycol via this route. Because the coupling reaction is much easier than direct carbonylation of formaldehyde with CO and because of recent improvements in the performance of catalysts in the hydrogenation of methyl glycolate, it is TABLE

6

Hydrogenation

catalysts of methyl glycolate P (X105Pa)

Ethylene

(“C)

180-190

200-335

40-96

94-96

180-250

170-230

80-90

94-96

200

70-140

78-96

94-96

172

200

97

97

T

Catalyst

glycol yield

Ref.

(So)

Co-ThO, Co-Cr oxide Co-Fe-Ni-Cu Co-ThO,mMgO Co-Zn-Cu

oxide

Co-Zn

oxide

CuXr

oxide

Cu-Cr oxide Co-Cu oxide Pd-Re Rh-Re/Kieselguhr Ru-Re

20

interesting to consider satellite industries producing small volume chemicals such as malonic ester and its derivatives, polyglycolic acid, etc., attached to an ethylene glycol production facility which employs the methyl formate route. 4.2.2. Coupling with olefins The C-H bond of formyl group in methyl formate is activated by oxidative addition to organometallic complexes [ 98-1041. L,M+

HCOOCH, -IL,-M-COOCH,

(28)

The complex could further react with substrates such as olefins yielding esters. Reaction of ethylene with methyl formate gives methyl propionate in the presence of ruthenium [ 10%1071 or iridium [ 1081 complexes. CHz=CH2+HCOOCH3+CH3CH2COOCH,

(29)

With RuClz ( PPh3), (0.14 mmol ) as catalyst precursor dissolved in 80 ml of HCOOCH,, and under 10 x lo5 Pa of C,H, and 10 x lo5 Pa of N, at 190-200°C Isnard et al. [ 1051 obtained 40 mmol of methyl propionate and 55 mmol of CO in 18 hours. Under the same conditions, ethyl formate produced only CO. They suggested that the reaction with the ruthenium catalyst involves an initial fragmentation of the formate to free or ligand CO. Keim and Becker [ 1071 obtained propionate in yields as high as 92% in 20 hours with various Ru carbony1 complexes (0.1 mmol) as catalyst precursor and at 23O”C, 90 x lo5 Pa of Na, 0.5 mol of HCOOCH3 and 0.5 mol of C&H4in 35 ml of toluene. Addition of CO diminished the ester yield. They proposed a mechanism involving a formy1 complex without decarbonylation of HCOOCH,. The discrepancy between the two investigations appears to be due to the difference in catalyst precursor employed. An interesting comparison was made by Keim and Becker [ 1071 between HCOOCH3 and a mixture of CH,OH and CO as a hydroesterification agent. As shown in Table 7, HCOOCH3 was found to be a better agent. With propylene or 1-butene in the presence of a ruthenium catalyst, only low yields of the corresponding ester are obtained with the normal isomer being TABLE

7

Hydroesterification

of ethylene

using HCOOCH,

and CH,OH/CO

as methoxycarbonylation

reagents” HCOOCH3

CH,OH

CO

Methyl propionate

(mol)

(mol)

(X105Pa)

(%)

0.5 _

_

_

92.5

0.5

90

29.7

“T = 230 ‘C; P = 90 x lo5 Pa of CO or N2 respectively; toluene; 2 h. Adapted from ref. 107.

yield

0.1 mmol Ru,

(CO)12; 0.5

mol C2H,; 35 ml

21

predominant in both cases [ 1061. Strong acid catalysts give better yields with propylene but form mainly methyl isobutyrate. CH3CH=CH2

+HCOOCHB + (CH3)2CHCOOCH3

(30)

With a HF catalyst at 50°C and 50. lo5 Pa, a methyl isobutyrate yield of 66% was claimed [ 1091. The hydroesterification of butadiene to adipic ester is of industrial importance and close to industrial realization [ 1 lo]. CH2=CHCH=CH2+2CH30H+2CO+CH300C(CH2)4COOCH3

(31)

In addition, methyl esters of pentenoic acid, CH&H = CHCH2COOCH3 and C, telomers, CH2= CH (CH,),CH = CHCH2COOCH3, are produced. With PdClz as catalyst, Keim and Becker [ 1071 found that HCOOCH3 gives methyl esters of pentenoic acid, C, telomers and butadiene dimers. They found it essential that CO was added. 4.2.3. Halogenation Monochlorination [ill] mate have been reported.

and complete chlorination

[112] of methyl for-

HCOOCH3 + Cl, -+C1COOCH3 + HCl

(32)

HCOOCH3 + 4C12+C1COOC13 +4HCl

(33)

The complete chlorination product, trichloromethylchloroformate (diphosgene), is formed under ultraviolet irradiation and in the presence of PCl, catalyst. It is a liquid at room temperature (boiling point 128” C ), yet is easily converted to phosgene thermally or when brought into contact with activated carbon or iron oxide. It is more convenient to handle and transport than phosgene and could be used as a substitute for the latter in the synthesis of fine chemicals. Methyl chloroformate can react with many compounds to yield methoxy carbonyl compounds as in the following examples. ClCOOCH, + CH2 = CH2 +CH, = CHOOCH3 + HCl

(34 1

C1COOCH3 +CgH5NH2 -,C,H5NHCOOCH3

(35 1

+HCl

4.2.4. Oxidation Methyl formate can undergo oxidative coupling to yield oxalic acid ester. 2HCOOCHB + 1/202 + (COOCH3)2 +H20

(36)

A very low yield of 8% was reported at 150°C and 60~10~ Pa with a Pd-HNO, catalyst system [ 1131. If the yield is considerably improved, the reaction could become important since the oxalate can be reduced to yield ethylene glycol [114].

22

(COOCH3)2+4H2-+HOCH2CH20H+2CH30H

(37)

Dimethyl carbonate is formed with a yield of 37%) when metallic selenium and sodium methoxide are added to a solution of methyl formate and tetrahydrofuran under mild conditions while 0, is supplied [ 1151. HCOOCH,+CH,ONa+1/20,~CH,OCOOCH,+NaOH

(38)

A more desirable method of dimethyl carbonate synthesis may be the use of methanol instead of sodium methoxide. HCOOCH,+CH30H+1/202~CH30COOCH3+HZ0

(39)

Again, obtaining a high yield would be a challenge. Dimethyl carbonate is considered to be a potental substitute for phosgene in carbonylation reactions, and for dimethyl sulfate in methylation reactions [ 116,117]. Oxidation reactions involving methyl formate and O2 show, in general, very poor selectivity to desired products relative to combustion of the reactants. Only limited potential for practical applications exists. 4.3. Isomerization of methyl formate: synthesis of acetic acid Methyl formate and acetic acid are formally isomeric and both can be synthesized from methanol by carbonylation. While acetic acid is formed by insertion of CO between the carbon-oxygen in methanol, methyl formate is produced by insertion into the oxygen-hydrogen bond. Of the two, acetic acid is thermodynamically more stable. Although the “isomerization” of methyl formate to acetic acid does not consume CO stoichiometrically, it is usually carried out under CO pressure in a liquid phase. HCOOCH3+CHBCOOH

(49)

As summarized in Table 8, catalysts consisting of a transition metal complex from rhodium [ 118-1191, iridium, [ 1201, cobalt [ 1211, nickel [ 122-1231, ruthenium [ 124-1251, palladium [ 124-1251, and an iodine promoter have been employed. A carboxylic acid is usually used as a solvent. Rhodium and iridium are the most active and selective catalysts. Nickel is less active, but has excellent selectivity to acetic acid when an organic nitrogen compound is employed as an additional promoter. The reaction has been known since 1929 [126], but systematic studies are still lacking. Mechanistic reaction pathways appear to vary depending on the type of catalyst used. For rhodium, Bryant et al. [ 1271 proposed that the reaction proceeded via decarbonylation of methyl formate to methanol and CO which then recombine to form acetic acid in the presence of CO. HCOOCHB+[CHBOH+CO]-+CH3COOH

(41)

23 TABLE 8 Catalytic systems for the isomerization of methyl formate to acetic acid Catalyst

Solvent

CO-pressure”

T

Acetic acid*

Activity’

(X lo5 Pa)

(“Cl

yield

(h-l)

Ref.

(%) Ru(acac)s/MeI/Ph,P

185

36

220

NMP

250 150

200 200

86 94 4

Ch,COOH CH,COOH

RuClJMeI Cal, Co(OAc),/MeI

2od

CH,COOH

200

RhCl(Ph,P),/MeI RhCl(Ph,P),/MeI

CH,COOH

15 33

[Ir(COD)Cl],/MeI IrClJMeI

C,H,COOH CH,COOH

Ni/MeI/2.6-Lutidine

PhCOMe

70 10

CH,COOH

50 70

Pd(OAc)JMeI/PhsP Pd(acac),/MeI

230

200 190 235 180 200 230

173 15 43 4

95 94

448 235

76 100 95

168 500 14 42

86 29

5

124 125 121 125 119 125 120 125 122-123 124 125

“at ambient temperature; %ncludes methyl acetate; ‘mol acetic acid/m01 transition metal X hour; ?Syngas (CO/H,=

1)

This mechanism, however, has been challenged by Schreck et al. [ 1191 when LiI is used as a promoter. The following scheme has been proposed: HCOOCHB + LiIGCH,I + HCOOLi

(42)

CH&OOH+HCOOCH3~CH3COOCHB+HCOOH

(43)

CH,COOCH,

(44)

+ LiI*CH,I+CH,COOLi

CH31+CO+CHBCOI

(45)

The LiI cleaves methyl formate to CHJ and the corresponding lithium formate HCOOLi. The CHJ is carbonylated to acetyl iodide CH,COI by the rhodium catalyst. In the step following reaction (45 ), reaction of acetyl iodide with lithium formate yields the mixed anhydride, CH,COOCCOH, which at the elevated temperatures employed decomposes into acetic acid and CO. The mechanism accounts for the effect of gaseous CO and the absence of methanol at any stage of the reaction. In the case of iridium, the reaction proceeds smoothly without CO pressure. Pru&t et al. [120] proposed that in the presence of an iridium complex, an organic iodide and a carboxylic acid solvent, the first reaction occurring was transesterification (reaction 43). The transesterification was also known to occur as a side reaction with less active catalysts [ 1251. The formic acid is

24

transformed into acetic acid through a P-hydride elimination and methyl migration of the iridium complex.

FH3 HCOOH+L.~freI,L-:I I

F%

p -OCH+HI

(461

I

A bench scale continuous operation of methyl formate isomerization to acetic acid has been carried out [ 1191. Gas-phase isomerization by the use of supported rhodium catalysts has also been reported [ 1281. Although the synthesis of acetic acid from methyl formate requires an additional process step when compared with direct methanol carbonylation, it could be economically attractive under some circumstances if purification, storage, and transport of synthesis gas are considered together. 5. EXAMPLES

OF INTEGRATED

C, CHEMlSTRY

COMPLEXES

BASED ON METHYL

FORMATE

As discussed above, methyl formate can be synthesized from many feedstocks by various methods. A facile synthesis of methyl formate makes is possible for methyl formate to be used as a means of gas separation or recovery under favorable conditions. As a liquid, it is handled, stored and transported relatively easily. It can also be transformed into a variety of other chemicals. These attributes of methyl formate commend consideration of integrated C, chemistry complexes revolving around methyl formate. Two examples are discussed to demonstrate the potential. 5.1. Example 1:production of chemicals from steel-mill off-gases As mentioned earlier, steel-mill off-gases could be an ideal feedstock if processed effectively using methyl formate as the key intermediate. These gases are by-products of steel-making processes and are currently mainly used as fuels. They are in large enough quantity to be supplied as a feedstock for several bulk chemicals. Since CO and H2 originate from different sources, there is no need to generate synthesis gas by expensive processes such as steam reforming or partial oxidation, The only problem is the economic recovery of CO or H, in the presence of other components of the gases (Table 3 ) . Fig. 6 shows schematically a system which could be implemented as a sat-

25

Gas

PI&eat.

Nz+COMethyl *

r’-‘-Formate --) Methy’ Formate Synthesis storage

lsomerization

Acetic Acid

Methanol

Fig. 6. Production of chemicals from steel-mill off-gases.

ellite chemical complex attached to a steel plant [ 1291. The system allows production of important commodity chemicals such as acetic acid, methanol, ethylene glycol and formic acid together with some small volume chemicals by using only steel-mill off-gases as feed stocks. The system features the use of methyl formate as a means of CO recovery and storages as well as a starting material for the synthesis of those chemicals. The LDG first passes through a series of pretreatment units to remove components detrimental to the catalyst of methyl formate synthesis such as sulfur compounds, ammonia, CO, and moisture. The gas stream, now containing mainly CO and Nz, is contacted with recycled methanol and a NaOCH, or KOCH3 catalyst to form methyl formate. The reaction conditions are maintained so that most of the CO in the gas stream is converted, thus gas phase, consisting mostly of Nz, is easily separated from the liquid product phase and vented. As discussed earlier, with the synthesis of methyl formate in this way, there is no need to install a rather expensive CO-N, separation unit such as Cosorb. The methanol required for the synthesis of methyl formate can be produced by hydrogenolysis of methyl formate. The H, for the hydrogenolysis is recovered from COG by means of H,-PSA or other separation techniques. The amount of methyl formate to be produced should be large enough to be used in conversion to other chemicals. 5.2. Example 2: generation of electricity from synthesis gas A difficulty in electric power generation from direct coal combustion or from combustion of synthesis gas obtained from coal gasification is to meet greatly fluctuating demands for power. In the case of synthesis gas combustion, this objective could be satisfied by producing a liquid chemical from the excess synthesis gas during the off-peak period and storing it for the peak demand period when it is cornbusted in separate gas turbines as is or after gasification.

Acid Gases

l

CO

Gas

Enrichment

Turbine Stack Gas Power

Fig. 7. Production

of peak electricity

from synthesis gas (adapted

from ref. 61).

The liquid could be methanol, dimethyl ether, or methyl formate. The last case is discussed here [ 611. As shown in Fig. 7, synthesis gas produced in a coal gasifier and cleaned of impurities can be concentrated in CO by a “once-through” methanol synthesis, or by utilization of a semi-permeable membrane as described earlier. The resulting gas, now rich in CO, is reacted with methanol to form methyl formate. The methyl formate is separated from the reactants and catalysts and passed to a suitable storage zone. The remaining synthesis gas is used to fuel the base load requirement in power generating gas turbines. In periods of power demand, methyl formate is catalytically dissociated to supply CO gas, to be fed to additional gas turbine power generation units, and methanol, which is recycled to the methyl formate synthesis unit. Make-up methanol for methyl formate synthesis can be recovered from the CO enrichment unit when the once-through methanol synthesis is employed for CO enrichment. Alternatively, a hydrogenolysis unit can be installed between methyl formate synthesis and the base load power generator. Many variations are possible depending on the composition of synthesis gas feed or the difference between the peak and the average demand for electricity. The fundamental idea, however, is to capture, store and release CO by forming and dissociating methyl formate. The process is effective and flexible in matching the precise CO/H2 ratio presented by the selected gasification process without resorting to a shift reaction or compression of synthesis gas usually done for utilization of methanol as an energy storage molecule. 6. CONCLUDING

REMARKS

Methyl formate can be produced by a variety of routes using a number of feed stocks. In general, processes involving only CO, CO/H, or C02/H2 as a

27

raw material are of practical value. Where a low-cost synthesis gas is not available as in Japan, the MGC process based on imported methanol could be a choice. Only the process of methanol carbonylation has an extended track record of commercial operation. However, because the market for formic acid is relatively small, the plants based on this process have not been built on a scale of usual commodity chemicals. The process should be subjected to engineering scrutiny to identify potential obstacles to scale-up prior to its possible application for a large scale C, industrial complex based on methyl formate. Furthermore, it is highly desired to find a catalyst which is more tolerant of moisture and is capable of promoting the synthesis in the presence of CO,. Most of the other processes need further research to improve the performance of the catalysts before commerical implementation is considered. The survey of reactions that convert methyl formate to other chemicals shows that the chemistry is quite rich. Some of them could become practicai processes in the future. The synthesis of large volume chemicals such as methanol, acetic acid and ethylene glycol deserve serious consideration. Compared to the conventional synthesis from synthesis gas, methyl formate routes usually require one more step, the synthesis of methyl formate itself. In order for a methyl formate route to be competitive, advantages of the route such as simplified gas separation and convenient handling, storage and transport should compensate for the cost of this additional step. That many methyl formate routes require milder reaction conditions or simplified processing than required for conventional routes is a factor in their favor. However, the potential of this branch of C, chemistry has not been sufficiently explored. In particular, most of the reactions revolving around methyl formate are catalytic in nature. Current understanding of the catalytic systems, however, is limited and fundamental research to understand the catalytic chemistry involved is highly desired. ACKNOWLEDGEMENT

This work has been supported by Pohang Iron and Steel Co. Ltd. through Contract 8119A. We thank Prof. W. Keim for providing us with his manuscripts before their publication.

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