Formic Acid [PDF]

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Formic Acid JUKKA HIETALA, Eastman Chemical Company, Helsinki, Finland ANTTI VUORI, Eastman Chemical Company, Helsinki, Finland PEKKA JOHNSSON, Eastman Chemical Company, Helsinki, Finland ILKKA POLLARI, Eastman Chemical Company, Helsinki, Finland WERNER REUTEMANN, BASF Aktiengesellschaft, Ludwigshafen, Germany HEINZ KIECZKA, BASF Aktiengesellschaft, Ludwigshafen, Germany



1. 2. 3. 3.1. 3.2. 3.3. 4. 4.1. 4.1.1. 4.1.2. 4.1.3. 4.2. 4.2.1. 4.2.2. 4.3. 4.4. 4.5. 4.6. 5. 6. 7. 8. 9. 10. 10.1. 10.1.1. 10.1.2. 10.2.



Introduction . . . . . . . . . . . . . . . . Physical Properties . . . . . . . . . . . Chemical Properties. . . . . . . . . . . Stability . . . . . . . . . . . . . . . . . . . Carboxylic Acid Reactions . . . . . . Other Reactions. . . . . . . . . . . . . . Production . . . . . . . . . . . . . . . . . Methyl Formate Hydrolysis . . . . . Kemira–Leonard Process . . . . . . . . BASF Process . . . . . . . . . . . . . . . USSR Process . . . . . . . . . . . . . . . Production from Formates . . . . . . Formates as Polyol Byproducts . . . . Formates from Carbon Monoxide . . Formic Acid from Carbon Dioxide Formic Acid from Biomass . . . . . Other Processes . . . . . . . . . . . . . . Recovery of Formic Acid . . . . . . . Environmental Protection . . . . . . Quality Specifications. . . . . . . . . . Analysis . . . . . . . . . . . . . . . . . . . Storage and Transportation . . . . . Legal Aspects . . . . . . . . . . . . . . . Uses . . . . . . . . . . . . . . . . . . . . . . Biomass Preservation. . . . . . . . . . Silage . . . . . . . . . . . . . . . . . . . . . Animal Biomass . . . . . . . . . . . . . . Leather . . . . . . . . . . . . . . . . . . . .



1 2 4 4 4 5 5 6 7 8 9 9 10 10 10 11 11 11 11 12 12 12 13 13 13 13 14 14



1. Introduction Methanoic acid, better known as formic acid [6418-6], HCOOH, Mr 46.03, is a colorless, corrosive liquid with a pungent odor. It is completely miscible with water and many polar solvents but only partially miscible with hydrocarbons.  2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007.a12_013.pub3



10.3. 10.4. 10.5. 10.6. 10.6.1. 10.6.2. 10.6.3. 10.6.4. 10.6.5. 10.6.6. 11. 12. 12.1. 12.1.1. 12.1.2. 12.1.3. 12.2. 12.2.1. 12.2.2. 12.2.3. 12.3. 13.



Textiles . . . . . . . . . . . . . . . . . . . . Feed Additives. . . . . . . . . . . . . . . Pharmaceuticals and Food Additives. . . . . . . . . . . . . . . . . . . Other Uses . . . . . . . . . . . . . . . . . Rubber Coagulation. . . . . . . . . . . . Gas Desulfurization . . . . . . . . . . . . Well Acidifiers . . . . . . . . . . . . . . . Formic Acid as Source of Hydrogen and Carbon Monoxide . . . . . . . . . . Cleaning Agents . . . . . . . . . . . . . . Solvent Use . . . . . . . . . . . . . . . . . Economic Aspects . . . . . . . . . . . . Derivatives . . . . . . . . . . . . . . . . . Salts . . . . . . . . . . . . . . . . . . . . . . Sodium Formate . . . . . . . . . . . . . . Potassium Formate . . . . . . . . . . . . Other Formate Salts. . . . . . . . . . . . Esters . . . . . . . . . . . . . . . . . . . . . Methyl Formate . . . . . . . . . . . . . . Ethyl Formate. . . . . . . . . . . . . . . . Other Esters . . . . . . . . . . . . . . . . . Performic Acid . . . . . . . . . . . . . . Toxicology and Occupational Health. . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . .



14 14 15 15 15 15 15 15 16 16 16 16 16 16 17 17 17 18 18 18 19 19 20



Formic acid derives its name from ants (lat. Formica) from which it was first obtained by dry distillation. The first scientific study on its properties, “Concerning Some Un-Common Observations and Experiments Made with an Acid Juyce to be Found in Ants” was published as early as 1670 [1].



2



Formic Acid



Formic acid and its salts are used primarily in the feed industry, grass silage, leather tanning, and anti-icing. Other applications include textile dyeing and finishing, food additives, natural rubber, drilling fluids, and various chemical processes. The worldwide production of formic acid was about 621 000 t/a in 2012 [2]. It is produced mainly by hydrolysis of methyl formate. The other important method is acidolysis of formate salts.



Formic acid, mp 8.3°C [3], bp 100.8°C (at 101.3 kPa) [4], is a colorless, clear, corrosive liquid with a pungent odor. It is the strongest unsubstituted alkyl carboxylic acid (pKa 3.74). The temperature dependence of the density of formic acid is given in Table 1. The density of formic acid–water binary mixtures as a function of formic acid concentration is shown in Table 2. The freezing point diagram for the formic acid–water binary mixture exhibits a eutectic point (Table 3) at 48.5°C and 41.0 mol% of formic acid [3]. Formic acid does not increase in volume when it solidifies and has a tendency to undergo supercooling. Table 4 shows the vapor pressure curve of pure formic acid. The vapor of formic acid deviates considerably in behavior from an ideal gas because the molecules dimerize partially in the vapor phase. At room temperature and normal pressure, 95% of the formic acid vapor consists of dimerized formic acid [7]:



Table 1. Density of pure formic acid as a function of temperature [5] t, °C



ϱ, g/cm3



t, °C



ϱ, g/cm3



0 10 15 20 25 30



1.244∗ 1.232 1.226 1.220 1.214 1.207



40 50 60 70 80 90 100



1.195 1.182 1.169 1.156 1.143 1.130 1.117



Supercooled liquid.



Formic acid content, wt%



ϱ, g/cm3



Formic acid content, wt%



ϱ, g/cm 3



2 5 10 20 30 40



1.003 1.011 1.024 1.048 1.072 1.095



50 60 70 80 90 100



1.118 1.141 1.163 1.185 1.204 1.221



Table 3. Freezing points of various formic acid–water mixtures [3]



2. Physical Properties



∗



Table 2. Density of aqueous formic acid solutions at 20°C [6]



Formic acid molar fraction, % 100.0 80.0 74.8 69.5 64.5 59.0 54.2 50.0 44.6 42.0



fp, °C 8.3 5.6 8.2 13.0 17.3 23.2 28.3 35.3 42.0 46.7



Formic acid molar fraction, % 41.0 39.8 37.0 31.7 28.0 23.1 20.0 12.0



fp, °C 48.5 46.8 43.0 33.5 29.3 22.8 19.7 10.3



Table 4. Vapor pressure of pure formic acid [8] Liquid



Solid



t, °C



p, kPa



5.23 0.00 8.25 12.57 20.00 29.96 39.89 49.93 59.98 70.04 79.93 100.68 110.62



1.083 1.488 2.392 3.029 4.473 7.248 11.357 17.347 25.693 37.413 52.747 101.667 135.680



t, °C 5.07 0.00 8.25



p, kPa 0.664 1.096 2.392



The enthalpy of the gas-phase dimerization is 63.8 kJ/mol [7]. The thermodynamic properties of monomeric and dimeric formic acid have been investigated [9]. The ring-type dimeric structure exists both in the vapor phase and in solution. Liquid formic acid consists of long chains of molecules linked to each other by hydrogen bonds. Solid formic



Formic Acid



acid can also be isolated in two polymorphic forms (α and β) [10]:



3



Table 6. Dynamic viscosity of pure formic acid as a function of temperature [16] t, °C



η, mPa
s



10 20 30 40 50



2.262 1.804 1.465 1.224 1.025



Table 7. Specific heat capacity of formic acid as a function of temperature at constant pressure [17]



The thermodynamic properties of monomeric and dimeric formic acid have been investigated [10]. Vapor–liquid equilibrium data for formic acid–water mixtures and for mixtures of formic acid with organic compounds have been collected in [11]. Formic acid and water form a maximum-boiling azeotropic mixture whose boiling point is 107.6°C at 101.3 kPa; it consists of 77.6 wt% formic acid and 22.4 wt% water. The composition and boiling point of formic acid–water azeotropic mixtures are shown as a function of pressure in Table 5. Formic acid can form azeotropic mixtures with many other substances [12]. The variation of dynamic viscosity with temperature is shown in Table 6. The dynamic viscosity of formic acid–water mixtures decreases approximately linearly as the water content of formic acid increases [15]. The thermal conductivity of formic acid is markedly higher than that of comparable liquids, because of its pronounced polarity. The variation of specific heat capacity with temperature is shown in Table 7. The specific heat capacity of formic acid–water mixtures decreases approximately linearly as the concentration of formic acid increases [6]. Table 5. Azeotropic data for the formic acid–water system p, bar



bp of azeotropic mixture, °C



Formic acid content, wt%



Reference



0.093 0.267 1.013 2.026 3.140



48.6 72.3 107.6 128.7 144



66.2 70.5 77.6 84 85



[13] [13] [13] [14] [14]



t, °C



cp, J g



Solid 150 100 75 50 25 0



0.921 1.114 1.193 1.285 1.411 1.800



Liquid 20 50 100



2.169 2.202 2.282



1



K



1



t, °C



cp, J g



Vapor 25 100 200 300 400 600 800 1000 1200



1.058 1.192 1.348 1.480 1.589 1.757 1.870 1.953 2.037



1



K



1



The physical properties of formic acid are listed below: Heat of fusion Heat of vaporization (at bp) Dielectric constant Liquid (at 20°C) Solid (at 10.1°C) Refractive index n20 D Surface tension σ (at 20°C) (at 40°C) (at 60°C) (at 80°C) Heat of formation ΔH 0f Liquid (at 25°C) Vapor (at 25°C, monomer) (at 25°C, dimer) Heat of combustion ΔH 0c Liquid (at 25°C) Entropy S0 Liquid (at 25°C) Vapor (at 25°C, monomer) (at 25°C, dimer) Heat of neutralization



276 J/g [8] 483 J/g [8] 57.9 [18] 11.7 1.37140 [19] 37.67×10 35.48×10 33.28×10 31.09×10



3



N/m [20] N/m 3 N/m 3 N/m 3



425.0 kJ/mol [21] 378.57 kJ/mol [7] 820.94 kJ/mol 254.8 kJ/mol [21] 129.0 J K 1 mol 1 [10] 248.88 J K 1 mol 1 [7] 332.67 J K 1 mol 1 [7] 56.9 kJ/mol [22]



4



Formic Acid



Critical data pcrit Tcrit ϱcrit Thermal conductivity λ Liquid (at 20°C) (at 60°C) (at 100°C) Vapor (at 50°C) (at 100°C) (at 200°C) Electrical conductivity (at 25°C) Coefficient of cubic expansion α (at 30°C)



7.279 MPa [23] 581 K [23] 0.392 g/cm3 [23]



unstable compound which has two major alternative routes for decomposition [29]:



0.226 W m 1 K 1 [23, 24] 0.205 W m 1 K 1 0.185 W m 1 K 1 0.0136 W m 1 K 1 0.0176 W m 1 K 1 0.0267 W m 1 K 1 6.08×10 5 Ω 1 cm 1 [25] 0.001 [4]



HCOOH→CO ‡ H2 O …ΔH r ˆ ‡10:5 kJ=mol†



…1†



HCOOH→CO2 ‡ H2



…2†



The important physical properties of formic acid and other low molecular mass fatty acids as a function of temperature are reported in [23].



3. Chemical Properties A comprehensive review of the chemistry of formic acid and its simple derivatives was published in 1969 [26]. More recent developments in the use of formic acid and its salts in synthetic chemistry are reviewed in [27, 28].



3.1. Stability Even though formic acid is relatively stable at room temperature, it is actually a thermally



…ΔH r ˆ 31:0 kJ=mol†



Dehydration reaction (1), which is predominant in the liquid phase, is favored in the presence of oxide catalysts or strong acids, while dehydrogenation reaction (2) is promoted by metal catalysts [30]. Formic acid becomes particularly unstable in concentrations near 100%, and this must be taken into account while storing highly concentrated formic acid. In general, the decomposition of formic acid is a function of both acid concentration and temperature (Fig. 1) [31, 32].



3.2. Carboxylic Acid Reactions Formic acid exhibits many of the typical chemical properties of the aliphatic carboxylic acids, e.g., esterification and amidation, but, as is common for the first member of a homologous series, there are distinctive differences in the properties of formic acid and its higher homologues [29].



Figure 1. Decomposition of formic acid (FA) Left: At 40°C as a function of acid concentration (♦ 90, ■ 98, ▵ 99 wt%) and storage time, showing that FA is stabilized by water. Right: As a function of temperature (♦ 20, ■ 30, ▵ 40°C) and storage time for 99% FA. Decomposition rates were calculated on the basis of published CO formation data [33].



Formic Acid



Formic acid forms esters with primary, secondary, and tertiary alcohols. The high acidity of formic acid makes the use of mineral acid catalysts unnecessary in simple esterifications. Primary and secondary alcohols are esterified in pure formic acid 15 000–20 000 times more rapidly than in pure acetic acid [34]. The rate of esterification of primary, secondary, and tertiary alcohols in formic acid has been determined [35, 36]. Formic acid also adds to the double bonds of olefins to form esters. Acetylenes react with formic acid in the vapor phase to yield vinyl formates [37]. Formic acid reacts with most amines to form formylamino compounds. For example, in the production of formamides, formic acid is used to achieve formylation by dehydration. With certain diamines imidazole formation occurs. Both reaction types have synthetic utility. Formic acid and tertiary organic bases form addition compounds (3 : 1 and 2 : 1 ratio of formic acid : base). Addition compounds of formic acid and trimethylamine or triethylamine can be used as liquid reducing agents in many selective reductions. The reduction of sulfur dioxide to sulfur is approximately quantitative [38]. Formic acid dissolves iron and zinc and corrodes most types of steels. The reaction with iron yields FeII formate and hydrogen gas: 2 HCOOH ‡ Fe→Fe…OOCH†2 ‡ H2



…3†



Among common metals and alloys, aluminum, copper, and nickel show some resistance. Formic acid is most corrosive in relatively dilute solutions in water. For mild steel, the highest corrosion rate is observed around 20% solution concentration, at which the conductivity of the solution is also the highest [39]. Some corrosion inhibitors can give very good protection [40]. Formic acid reacts with hydrogen peroxide in the presence of an acidic catalyst to form unstable performic acid [107-32-4] (HCOOOH).



3.3. Other Reactions The formyl hydrogen atom exhibits some aldehydic character, and formic acid reduces some salts and oxides to metals. It can be used as a



5



reducing agent (hydride donor) also in many organic reactions, replacing H2. The reduction of imines (Schiff bases) by formic acid has been known for a long time. Primary amines can be prepared from ketones, ammonia, and formic acid (Leuckart reaction). Formic acid can also be used as a source of carbon monoxide. Highly branched carboxylic acids can be prepared from olefins and formic acid in the presence of sulfuric or hydrofluoric acid (Koch carboxylic acid synthesis). The reaction proceeds via addition of CO. O R



+ HCOOH R’



R



OH



…4†



R’



4. Production The formic acid processes practiced today are based mainly on two main routes: methyl formate hydrolysis and preparation of free formic acid from formates. The methyl formate based process route is currently dominant. Approximately 90% of the installed capacity is based on this on-purpose process. The economic disadvantages of the methods earlier practiced led to the development of a process specifically dedicated to the production of formic acid with no undesirable byproducts. In the 1970s, the hydrolysis of methyl formate to methanol and formic acid was developed commercially by various companies into an economically feasible method. This process involves carbonylation of methanol and subsequent hydrolysis of the methyl formate produced. The methanol resulting from this process is returned to the first stage. Formic acid plants based on this process were started up at BASF (Federal Republic of Germany) in 1981 and Kemira (Finland) in 1982. More recent large-scale producers using this route are the Chinese companies Feicheng Acid Chemicals and Luxi Chemical Group. The other current production method involves formation of the free acid from its salts. Mainly sodium formate [141-53-7] and calcium formate [544-17-2] are used for this purpose. The acidolysis is normally carried out with sulfuric acid or phosphoric acid. Sulfate or



6



Formic Acid



Table 8. Formic acid production capacity at the end of 2014 (without standby plants; Eastman assessment)



Table 9. Calculated equilibrium concentrations of formic acid at different temperatures and pressures [42]



Process



Capacity, t/a



Percentage of total capacity, %



Hydrolysis of methyl formate Acidolysis of alkali formates Total



770 000 180 000 950 000



81 19



phosphate salts are produced as byproducts. The share of the formate salt based process has also decreased and today it is mainly practiced by Perstorp in Sweden and smaller Chinese producers. Formic acid used to be a byproduct in the production of acetic acid [64-19-7] by liquidphase oxidation of butane or naphtha (→ Acetic Acid). For many years, oxidation of hydrocarbons was the most important method of producing acetic acid. However, the preferred process today is carbonylation of methanol [41], in which formic acid is not formed. The production of formic acid by hydrolysis of formamide [75-12-7] played an important role in Europe until the 1970s.; However, the consumption of ammonia and sulfuric acid, along with the unavoidable production of ammonium sulfate, has made this process economically inferior. Although other methods for producing formic acid have been patented, they do not appear to have been implemented industrially. Table 8 shows the estimated world capacity for various formic acid production processes.



4.1. Methyl Formate Hydrolysis The simplest theoretically possible method of producing formic acid is the reaction of carbon monoxide with water: CO ‡ H2 O→HCOOH



…5†



Below 150°C, the reaction is very slow, and although equilibrium is reached rapidly at higher temperature, the pressure must be increased drastically to obtain acceptable formic acid concentrations (see Table 9). The thermodynamic hurdles can be bypassed if methyl formate is formed as an intermediate.



Carbon monoxide pressure, MPa at 25°C



at 100°C



at 217.9°C



0.027 0.30 0.72 3.7 108



0.63 7.15 16.95 85.8 2540



6.95 79.1 187 950 28170



Formic acid concentration, wt%



1 10 20 50 90



The liquid-phase carbonylation of methanol to methyl formate (Eq. 6) in the presence of a basic catalyst such as sodium or potassium methoxide (NaOCH3 or KOCH3) and further hydrolysis to formic acid (Eq. 7) has been practiced industrially already since the early 1980s. Potassium methoxide is more soluble in methyl formate than sodium methoxide, and correspondingly gives a higher reaction rate. The methanol liberated in the second stage is returned to the first stage: CH3 OH ‡ CO→HCOOCH3



…ΔH r ˆ 29 kJ=mol†



HCOOCH3 ‡ H2 O→HCOOH ‡ CH3 OH



…6†



…ΔH r ˆ ‡16:3 kJ=mol† …7†



The methyl formate process is utilized, e.g., by Eastman Chemical Company, BASF, Kemira, Luxi Chemical Group, and Feicheng Acid Chemicals. A number of variants of the process have been patented [43]. The reaction conditions for the first-stage carbonylation step are broadly similar in each, but they differ in their approach to the hydrolysis stage. Carbonylation. According to the published data, typical reaction conditions appear to be 80°C, 4.5 MPa pressure, and 2.5 wt% sodium methoxide catalyst. Under these conditions the methanol conversion is around 30%, and the conversion of carbon monoxide is about 95%. Nearly quantitative conversion of methanol to methyl formate can be achieved by recycling the unconverted methanol. The carbonylation rate can be increased by increasing the temperature, the CO partial pressure, the catalyst concentration, or the interface between gas and liquid phases.



Formic Acid



As side reactions, the catalyst produces formate salts. MOCH3 ‡ HCOOCH3 →HCOOM ‡ CH3 OCH3



…8†



MOCH3 ‡ H2 O ‡ HCOOCH3 →HCOOM ‡ 2 CH3 OH



…9†



M ˆ Na; K



Formate salts are less soluble in methyl formate than in methanol. Hence, the risk of encrustation and blockage due to precipitation of catalyst must be managed. The carbon monoxide must not contain a significant amount of carbon dioxide; otherwise, the catalytically inactive carbonate is precipitated. Undecomposed alkali metal methoxide in the methyl formate must be neutralized to avoid the reverse reaction, that is, decomposition of methyl formate into methanol and CO, when heated [44]. Hydrolysis. The hydrolysis equilibrium of methyl formate is relatively unfavorable, but is dependent on the water concentration in a way that favors the use of high stoichiometric excess of water, with consequent problems in finding an energy-efficient method of removing the excess water. Formic acid acts as the hydrolysis catalyst (autocatalysis). One way to overcome the unfavorable position of the hydrolysis equilibrium is to hydrolyze methyl formate in the presence of an additive, e.g., a tertiary amine. The base forms a saltlike adduct with formic acid (Eq. 10); therefore, the concentration of free formic acid decreases and the hydrolysis equilibrium is shifted in the direction of the products. In a subsequent step, formic acid can be distilled from the base without decomposition. Screening of the applicability of more than 70 potential complexing agents in methyl formate hydrolysis is described in [45].



7



4.1.1. Kemira–Leonard Process A 20 000 t/a formic acid plant based on a method developed by the Leonard Process Co. [46] was built at Kemira in Finland and put into operation in 1982. The process has been developed further by Kemira, and licenses for it have been issued in Korea, India, and Indonesia. In the Kemira–Leonard process, carbonylation is carried out at about 4 MPa and a temperature of approximately 80°C, with additivecontaining alkoxides used as catalyst [47]. Hydrolysis is carried out in two reactors with different operating conditions. Methyl formate and water react in the preliminary reactor in approximately equimolar proportions. The formic acid produced catalyzes the hydrolysis in the main reactor. An excess of methyl formate is employed in the main reactor, and hydrolysis is carried out at ca. 120°C and 0.9 MPa. The reactor discharge is brought to atmospheric pressure in a flash tank; re-esterification is largely prevented by the cooling that occurs during flash evaporation. Methyl formate and methanol are separated under vacuum. Although the reverse reaction is attenuated further as a result, condensation of the low-boiling methyl formate in vacuo presents a problem. The formic acid is dehydrated by distillation. If 85 wt% formic acid is desired, dehydration must be carried out at ca. 0.3 MPa. Even higher concentrations can be achieved by connecting an additional dehydration column downstream under atmospheric pressure, and formic acid concentrations up to ca. 98 wt% can then be drawn off as the distillate. Process Description (see Fig. 2). Compressed carbon monoxide and methanol are converted to methyl formate in reactor (a). Catalyst is fed into the reactor in a methanol solution. The amount of methanol introduced in this way makes up for methanol losses in the process. The discharge from reactor (a) is flashed and fed into the methyl formate column (b), from which methyl formate is drawn off as the distillate. Methanol and the dissolved catalyst are returned to the reactor; inactivated catalyst (primarily sodium formate) is crystallized and discharged. Off-gas from column (b) and waste gas from reactor (a) are burned. Methyl formate reacts partially with water in the preliminary reactor



8



Formic Acid



Figure 2. Production of formic acid (Kemira–Leonard process) a) Methyl formate reactor; b) Methyl formate column; c) Preliminary reactor; d) Main reactor; e) Flash tank; f) Recycle column; g) Acid separation column; h) First product column; i) Second product column. See [46–48].



(c), and discharge from the preliminary reactor is fed into the main reactor (d) along with recycled methyl formate, methanol, and water. The contact time in this reactor is largely sufficient for equilibrium to be established. Reactor discharge is flashed to approximately atmospheric pressure in the flash tank (e); methyl formate, methanol, and small quantities of formic acid evaporated in this process are recycled to the main reactor (d). Methyl formate and methanol are distilled in vacuo in the acid separation column (g). Contact time is minimized by internals with a small liquid holdup. The distillate is separated into methyl formate and methanol in the recycle column (f). Formic acid can be concentrated under pressure in a column; if the pressure is ca. 0.3 MPa, 85% formic acid is drawn off as the bottom product. Distillation in two columns is more industrially controllable. Water is distilled overhead in the first product column (h). The bottom product is concentrated further in the second product column (i), and formic acid with a maximum concentration of ca. 98 wt% is drawn off as distillate. The bottom product is recycled to the first product column.



4.1.2. BASF Process A 100 000 t/a formic acid plant began operating in Ludwigshafen (Germany) in 1981. In this plant, a technology for the hydrolysis and dehydration was used for the first time. The production of methyl formate by carbonylation of methanol has been carried out on a large scale for many years at BASF [49, 50]. The carbonylation stage is largely identical to that of the Kemira–Leonard process, but the hydrolysis stage and the dehydration of formic acid are noticeably different. In the classical BASF process, hydrolysis is carried out with a large excess of water (about 5 mol of water per mole of methyl formate) to shift the equilibrium in the direction of formic acid. Much of the water is separated by liquid–liquid extraction with a secondary amide. Process Description (see Fig. 3). Carbon monoxide and methanol react in the methyl formate reactor (a) in the presence of sodium methoxide. Methyl formate is fed as a distillate from the methyl formate column (b) into the formic acid



Formic Acid



9



Figure 3. Production of formic acid (classical BASF process) a) Methyl formate reactor; b) Methyl formate column; c) Hydrolysis reactor; d) Low-boiler column, e) Extraction unit; f) Dehydration column; g) Pure acid column



reactor (c) together with recycled methyl formate. Methanol and dissolved catalyst are drawn off from the bottom of column (b) and returned to reactor (a); catalyst decomposition products are discharged by crystallization. In reactor (c), methyl formate is hydrolyzed with excess water at elevated temperature and increased pressure. The reaction product is flashed into the low-boiler column (d). Methyl formate is removed as the distillate, with methanol as a side stream, and dilute aqueous formic acid is drawn off from the bottom into the extraction unit (e). Here, the formic acid and some of the water are extracted by a secondary amide. Most of the water (largely free of formic acid) is recycled to reactor (c). The extract—a mixture of extractant, formic acid, and some water—is distilled in the dehydration column (f). Enough water is distilled via the head for the required formic acid concentration to be obtained in the pure acid column (g). This column is operated in vacuo. The extraction agent is recycled from the bottom of column (g) to the extraction unit. BASF has developed new technology in which a complex former such as a tertiary amine is used to shift the hydrolysis equilibrium and to reduce the amount of water that needs to be



evaporated. For details of one version of the modified BASF process, see [51]. 4.1.3. USSR Process Two 40 000 t/a formic acid plants were designed in the 1980s for two projects in the Soviet Union, based on the process described in [52], one in Saratov (Ukraine), and the other in Lesogorsk (North-West Russia). According to [52], hydrolysis is carried out under atmospheric pressure in a column that is divided into two zones. Water is fed into the upper zone, which contains a fixed bed of strongly acidic ion-exchange resin at 60–100°C. Methyl formate is fed to the lower zone, which is kept at 100–107°C. Hydrolysis is catalyzed by the resin in the upper zone and by formic acid in the lower zone. Most of methanol and unconverted methyl formate are taken out from the top, and formic acid and water from the bottom.



4.2. Production from Formates The reaction of formate salts with mineral acids (usually sulfuric acid) is the oldest commercial



10



Formic Acid



process for the production of formic acid, and it still has industrial importance. Perstorp is the largest producer of formic acid by acidolysis of formate salts. Sodium formate [141-53-7], potassium formate [590-29-4], and calcium formate [544-17-2] are available industrially from the production of polyhydric alcohols. The acidolysis of formate salts is technically straightforward, but the unavoidable production of coproduct salts such as sodium and calcium sulfate (Eqs. 11a and 11b) is a clear disadvantage of this route. 2 HCOONa ‡ H2 SO4 →2 HCOOH ‡ Na2 SO4



(11a)



…HCOO†2 Ca ‡ H2 SO4 →2 HCOOH ‡ CaSO4



(11b)



2 HCOOH ‡ Ca…OH†2 →…HCOO†2 Ca ‡ 2 H2 O



Also, the excess alkali in the process is neutralized by formic acid. The pentaerythritol product is isolated by stepwise concentration and fractional crystallization. 4.2.2. Formates from Carbon Monoxide Formate salts can be made by a direct process based on the reaction of CO with a base such as NaOH (Eq. 16) and KOH (Eq. 17) [54, 58–60]. NaOH…aq† ‡ CO→HCOONa…aq:† KOH…aq† ‡ CO→HCOOK…aq†



Some Chinese patents describe preparation of formic acid from CO and alkali metal or alkaline earth metal hydroxides, followed by acidification with phosphoric acid [54, 55]. Carbon monoxide can come from phosphorus production. The resulting phosphate salts have use as fertilizers.



4.2.1. Formates as Polyol Byproducts Formate salts occur as byproducts in the production of pentaerythritol [115-77-5], trimethylolpropane [77-99-6], and 2,2-dimethyl-1,3propanediol [126-30-7] (neopentyl glycol). For example, pentaerythritol is produced by reaction of formaldehyde with acetaldehyde in aqueous alkaline medium (Eq. 12) [56, 57]. The intermediate aldehyde reacts further with formaldehyde by Cannizzaro reaction to yield pentaerythritol and formic acid (Eq. 13). The formic acid formed reacts immediately with the alkali present giving the corresponding formate salt as the final coproduct (Eqs. 14 and 15). 3 HCHO ‡ CH3 CHO→…HOCH2 †3 CCHO



…12†



…HOCH2 †3 CCHO ‡ HCHO ‡ H2 O→C…CH2 OH†4 ‡ HCOOH …13† (Reactions 12) and (13): overall ΔHr =



92 kJ/mol)



HCOOH ‡ NaOH→HCOONa ‡ H2 O



…ΔH r ˆ 69:4 kJ=mol† …14†



…ΔH r ˆ 93:4 kJ=mol† …15†



…ΔH r ˆ 108:6 kJ=mol†



…ΔH r ˆ 86:1 kJ=mol†



…16† …17†



4.3. Formic Acid from Carbon Dioxide Hydrogenation of carbon dioxide in alcohol produces formic acid in an almost water-free environment, which is an advantage in product recovery. Soluble Ru complexes are the preferred catalysts, and a complexing agent is used to make the reaction thermodynamically favorable.



The technology was first introduced by BP Chemicals in the 1980s [61] and has been developed by BASF [62, 63]. The reaction takes place in a mixture of tertiary amine (usually n-trihexylamine) and alcohol (e.g., methanol) at 50–70°C and 10–12 MPa. Some water is added to ease phase separation. The formic acid–amine complex is thermally dissociated at 150–185°C. The process must keep the expensive transition metal complex catalyst active but avoid even traces of it being present in active form in formic acid distillation, because it can catalyze decomposition of the acid. Possible catalyst residues can be reversibly inactivated with CO.



Formic Acid



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Scheme 1. Formation of levulinic acid (3) and formic acid (4) from a C6 sugar (1) via 5-hydroxymethylfurfural (2) [66].



Q1



Several electrochemical processes for formic acid or formate salt production from carbon dioxide are in development [64]. The process of CO2 electrolysis is basically running a fuel cell in reverse. In an electrolyzer CO2 is reduced at the cathode, while the oxygen evolution reaction takes place at the anode. In recent years great improvements in the catalyst systems and electrodes have resulted in much higher yields, Faradic efficiencies, and fuel efficiencies.



4.4. Formic Acid from Biomass Virtually any plant biomass can be oxidized to formic acid, CO2, and water with high selectivity to formic acid by using polyoxometallates as oxygen-delivering catalysts [65]. Mineral acid hydrolysis of cellulose and hemicellulose around 200°C produces a mixture of levulinic acid, formic acid, and furfural [66–68]. The total reaction from C6 sugars to levulinic acid and formic acid is shown in Scheme 1.



the higher boiling point at high pressure also increases the decomposition rate of formic acid. A combination of distillation under pressure followed by vacuum distillation can produce practically all desired concentrations. A number of entrainers have been proposed for azeotropic distillation [4]. Extraction. Several technologies have been developed in which reactive extraction is used to transfer formic acid to a separate organic phase or formic acid is bound to a weak complexing agent in water phase. In both cases the azeotrope can be avoided. When phase separation is involved, it is possible to reduce the total amount of water that needs to be evaporated. Usually, secondary amides or tertiary amines are used. In extractive distillation formic acid is extracted in a distillation column by means of a basic extracting agent introduced countercurrently and fed into the bottom of the column. If this mixture is heated in a column downstream, the formic acid is liberated from the saltlike adduct and distilled.



4.5. Other Processes Other possible processes include oxidation of formaldehyde [53] and coupling of methanol [69].



4.6. Recovery of Formic Acid Azeotropic Distillation. Formic acid–water mixtures cannot be concentrated to more than the azeotropic composition by simple distillation. The concentration of formic acid in the azeotropic mixture increases if distillation is carried out under pressure (see Table 5), but



5. Environmental Protection Like most other simple organic acids, formic acid can be degraded rapidly and completely by biological methods. For this reason, formic acid is totally mineralized in a short time in purification plants and streams. The ecological equilibrium of the streams can, however, be disturbed by a change in pH. Some environmental values related to formic acid are listed in Table 10 [70]. Formic acid vapor can be removed from exhaust streams by washing with water. Fog



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Formic Acid Table 10. Environmental values for formic acid BOD COD5 COD20 Biodegradation



350 mg/g 86 mg/g 250 mg/g > 99%



LC50 (96 h), fish EC50 (48 h), invertebrate EC50 (72 h), algae



130 mg/L 540 mg/L 1240 mg/L



OECD guideline method 301 C; if an additional carbon source is available, degradation proceeds more quickly. OECD guideline 203, Danio rerio OECD guideline 202, Daphnia magna OECD guideline 201, Pseudokirchnerella subcapitata



results when formic acid vapor is emitted during periods of high humidity. For emission rates of 0.1 kg/h or higher, the formic acid concentration must be