c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a11 619

Formaldehyde 1

FormaldehydeGunther Reuss, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chaps. 1, 2, 4, and5)Walter Disteldorf, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chaps. 3, 6 9)Armin Otto Gamer, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 10)Albrecht Hilt, Ultraform GmbH, Ludwigshafen, Federal Republic of Germany (Chaps. 11, 12)

1. Introduction . . . . . . . . . . . . . . 22. Physical Properties . . . . . . . . . . 22.1. Monomeric Formaldehyde . . . . . 22.2. Aqueous Solutions . . . . . . . . . . 33. Chemical Properties . . . . . . . . . 54. Production . . . . . . . . . . . . . . . . 64.1. Silver Catalyst Processes . . . . . . 74.1.1. Complete Conversion of Methanol

(BASF Process) . . . . . . . . . . . . . 84.1.2. Incomplete Conversion and Distilla-

tive Recovery of Methanol . . . . . . 94.2. Formox Process . . . . . . . . . . . . 114.3. Comparison of Process Economics 124.4. Distillation of Aqueous Formalde-

hyde Solutions . . . . . . . . . . . . . 134.5. Preparation of Liquid Monomeric

Formaldehyde . . . . . . . . . . . . . 14

5. Environmental Protection . . . . . 146. Quality Specications and Analysis 176.1. Quality Specications . . . . . . . . 176.2. Analysis . . . . . . . . . . . . . . . . . 177. Storage and Transportation . . . . 188. Uses . . . . . . . . . . . . . . . . . . . . 199. Economic Aspects . . . . . . . . . . . 2010. Toxicology and Occupational

Health . . . . . . . . . . . . . . . . . . 2111. Low Molecular Mass Polymers . . 2211.1. Linear Polyoxymethylenes . . . . . 2211.2. Cyclic Polyoxymethylenes . . . . . . 2511.2.1. Trioxane . . . . . . . . . . . . . . . . . 2511.2.2. Tetraoxane . . . . . . . . . . . . . . . . 2811.2.3. Higher Cyclic Polyoxymethylenes . 2812. Formaldehyde Cyanohydrin . . . . 2813. References . . . . . . . . . . . . . . . . 29

1. Introduction

Formaldehyde occurs in nature and is formedfrom organic material by photochemical pro-cesses in the atmosphere as long as life con-tinues on earth. Formaldehyde is an importantmetabolic product in plants and animals (includ-ing humans), where it occurs in low but measur-able concentrations. It has a pungent odor andis an irritant to the eye, nose, and throat evenat a low concentration; the threshold concen-tration for odor detection is 0.05 1 ppm. How-ever, formaldehyde does not cause any chronicdamage to human health. Formaldehyde is alsoformed when organic material is incompletelycombusted; therefore, formaldehyde is foundin combustion gases from, for example, auto-motive vehicles, heating plants, gas-red boil-

ers, and even in cigarette smoke. Formalde-hyde is an important industrial chemical andis employed in the manufacture of many in-dustrial products and consumer articles. Morethan 50 branches of industry now use formalde-hyde, mainly in the form of aqueous solutionsand formaldehyde-containing resins. In 1995,the demand for formaldehyde in the three majormarkets Northern America, Western Europe,Japan was 4.1 106 t/a [1].

History. Formaldehydewas rst synthesizedin 1859, when Butlerov hydrolyzed methy-lene acetate and noted the characteristic odorof the resulting solution. In 1867, Hofmannconclusively identied formaldehyde, which heprepared by passing methanol vapor and airover a heated platinum spiral. This method, but

2 Formaldehyde

with other catalysts, still constitutes the principalmethod of manufacture. The preparation of pureformaldehyde was described later byKekule in1882.

Industrial production of formaldehyde be-came possible in 1882, when Tollens disco-vered a method of regulating the methanol va-por : air ratio and affecting the yield of the reac-tion. In 1886 Loew replaced the platinum spiralcatalyst by a more efcient copper gauze. TheGerman rm, Mercklin und Losekann, startedto manufacture and market formaldehyde ona commercial scale in 1889. Another Germanrm, Hugo Blank, patented the rst use of a sil-ver catalyst in 1910.

Industrial development continued from 1900to 1905, when plant sizes, ow rates, yields,and efciency were increased. In 1905, Badis-cheAnilin&Soda-Fabrik started tomanufactureformaldehyde by a continuous process employ-ing a crystalline silver catalyst. Formaldehydeoutput was 30 kg/d in the form of an aqueous30wt% solution.

The methanol required for the production offormaldehyde was initially obtained from thetimber industry by carbonizingwood.Thedevel-opment of the high-pressure synthesis of meth-anol by Badische Anilin & Soda-Fabrik in 1925allowed the production of formaldehyde on atrue industrial scale.

2. Physical Properties

2.1. Monomeric Formaldehyde

Formaldehyde [50-00-0], CH2O,Mr 30.03, is acolorless gas at ambient temperature that has apungent, suffocating odor and an irritant actionon the eyes and skin.

Formaldehyde liquees at19.2 C, the den-sity of the liquid being 0.8153 g/cm3 at 20 Cand 0.9172 g/cm3 at 80 C. It solidies at118 C to give a white paste. The liquid andgas polymerize readily at low and ordinary tem-peratures up to 80 C. Pure formaldehyde gasdoes not polymerize between 80 and 100 C andbehaves as an ideal gas. For the UV absorptionspectra of formaldehyde, see [2]. Structural in-formation about the formaldehyde molecule isprovided by its uorescence [3], IR [4], Ra-man [5], and microwave spectra [6]. Following

are some of the thermodynamic properties ofgaseous formaldehyde:Heat of formation at 25 C 115.9 6.3 kJ/molGibbs energy at 25 C 109.9 kJ/molEntropy at 25 C 218.8

0.4 kJmol1 K1Heat of combustion at 25 C 561.5 kJ/molHeat of vaporization at19.2 C 23.32 kJ/molSpecic heat capacity at 25 C, cp 35.425 Jmol1 K1Heat of solution at 23 Cin water 62 kJ/molin methanol 62.8 kJ/molin 1-propanol 59.5 kJ/molin 1-butanol 62.4 kJ/mol

Cubic expansion coefcient 2.83103 K1Specic magnetic susceptibility 0.62106Vapor density relative to air 1.04

The vapor pressure p of liquid formaldehydehas beenmeasured from109.4 to22.3 C [7]and can be calculated for a given temperature T(K) from the following equation:p (kPa) = 10[5.0233(1429/T )+1.75logT0.0063T ]

Polymerization in either the gaseous or theliquid state is inuenced by wall effects, pres-sure, traces of humidity, and small quantities offormic acid. Formaldehyde gas obtained by va-porization of paraformaldehyde or more highlypolymerized-polyoxymethylenes, which is ca.90 100% pure, must be stored at 100 150 Cto prevent polymerization. Chemical decompo-sition is insignicant below 400 C.

Formaldehyde gas is ammable, its ignitiontemperature is 430 C [8]; mixtures with air areexplosive. At ca. 20 C the lower and upper ex-plosive limits of formaldehyde are ca. 7 and72 vol% (87 and 910 g/m3), respectively [9].Flammability is particularly high at a formalde-hyde concentration of 65 70 vol%.

At a low temperature, liquid formaldehydeis miscible in all proportions with nonpolar sol-vents such as toluene, ether, chloroform, or ethylacetate. However, solubility decreases with in-creasing temperature and at room temperaturepolymerization and volatilization occur, leavingonly a small amount of dissolved gas. Solutionsof liquid formaldehyde in acetaldehyde behaveas ideal solutions [10]. Liquid formaldehyde isslightly miscible with petroleum ether and p-cymene [11].

Polar solvents, such as alcohols, amines oracids, either catalyze the polymerization offormaldehyde or react with it to form methylolcompounds or methylene derivatives.

Formaldehyde 3

2.2. Aqueous Solutions

At room temperature, pure aqueous solutionscontain formaldehyde in the form of methyleneglycolHOCH2OH[463-57-0] and its oligomers,namely the lowmolecular mass poly(oxymethy-lene) glycols with the following structureHO (CH2O)nH (n = 1 8)

Monomeric, physically dissolved formaldehydeis only present in low concentrations of up to0.1wt%. The polymerization equilibrium

HOCH2OH+ n CH2OHO (CH2O)n+1H

is catalyzed by acids and is shifted towardthe right at lower temperature and/or higherformaldehyde concentrations, and toward theleft if the system is heated and/or diluted [12],[13] (see also Section 11.1).

Dissolution of formaldehyde in water isexothermic, the heat of solution ( 62 kJ/mol)being virtually independent of the solution con-centration [14]. Clear, colorless solutions offormaldehyde in water can exist at a formalde-hyde concentration of up to 95wt%, but thetemperature must be raised to 120 C to ob-tain the highest concentrations. Concentratedaqueous solutions containingmore than 30wt%formaldehydebecomecloudyon storage at roomtemperature, because larger poly(oxymethy-lene) glycols (n 8) are formed which then pre-cipitate out (the higher the molecular mass ofthe polymers, the lower is their solubility).Table 1. Calculated distribution of oligomers of methylene glycol,HO (CH2O)nH, in an aqueous 40wt% formaldehyde solution at35 C [12]n Proportion, % n Proportion, %

1 26.80 7 3.892 19.36 8 2.503 16.38 9 1.594 12.33 10 0.995 8.70 > 10 1.586 5.89

Equilibrium constants have been determinedfor the physical dissolution of formaldehyde inwater and for the reaction of formaldehyde togive methylene glycol and its oligomers [12].These parameters can be combined with otherdata to calculate the approximate equilibria atany temperature from 0 to 150 C and at aformaldehyde concentration of up to 60wt%

[13]. Table 1 gives the calculated oligomer dis-tribution in an aqueous 40wt% solution offormaldehyde.

A kinetic study of the formation of methy-lene glycol from dissolved formaldehyde andwater shows that the reverse reaction is 5103to 6103 times slower than the forward reac-tion [15], and that it increases greatly with theacidity of the solution. This means that the dis-tribution of the higher mass oligomers (n> 3)does not change rapidly when the temperature isincreased or the solution is diluted; the methy-lene glycol content then rises at the expenseof the smaller oligomers (n = 2 or 3). In aque-ous solutions containing 2wt% formalde-hyde, formaldehyde is entirely monomeric.

Methylene glycol can be determined by thebisulte method [16] or by measuring the par-tial pressure of formaldehyde [17]. Molecularmasses and monomer contents can be deter-mined by NMR spectroscopy [13], [18].

The approximate amount of monomericformaldehyde present as formaldehyde hemifor-mal and methylene glycol in aqueous solutionscontaining formaldehyde and methanol, can becalculated from data at 25 80 C [19] by usingthe following equation:

Monomer (mol%) =

100 12.3F+ (1.44 0.0164F )M

where F is the formaldehyde concentration(7 55wt%) and M is the methanol concentra-tion (0 14wt%).

The partial pressure pF of formaldehydeabove aqueous solutions has been measured byLedbury and Blair and computed byWalkerand Lacy [20]. The parameter pF for solutionsin which F is in the range 0 40wt% can becalculated with a relative error of 5 10% in thetemperature range T = 273 353K by using thefollowing equation :

pF (kPa) = 0.1333F eF(a0+a1/T+a2/T2)

= 0.08760 0.00950a0= 12.0127 0.0550a1= 3451.72 17.14a2= 248257.3 5296.8

Results of such calculations are given in Table 2and agree well with the measured values.

Table 3 gives the partial pressures and con-centrations of formaldehyde in the liquid and

4 FormaldehydeTable 2. Partial pressure pF of formaldehyde (kPa) above aqueous formaldehyde solutions

t, C Formaldehyde concentration, wt%

1 5 10 15 20 25 30 35 40

5 0.003 0.011 0.016 0.021 0.025 0.028 0.031 0.034 0.03710 0.005 0.015 0.024 0.031 0.038 0.043 0.049 0.053 0.05615 0.007 0.022 0.036 0.047 0.057 0.066 0.075 0.083 0.09020 0.009 0.031 0.052 0.069 0.085 0.099 0.113 0.125 0.13725 0.013 0.044 0.075 0.101 0.125 0.146 0.167 0.187 0.20630 0.017 0.061 0.105 0.144 0.180 0.213 0.245 0.275 0.30435 0.022 0.084 0.147 0.203 0.256 0.305 0.353 0.398 0.44240 0.028 0.113 0.202 0.284 0.360 0.432 0.502 0.569 0.63445 0.037 0.151 0.275 0.390 0.499 0.604 0.705 0.803 0.89950 0.047 0.200 0.371 0.531 0.685 0.833 0.978 1.119 1.25855 0.059 0.262 0.494 0.715 0.929 1.137 1.341 1.541 1.74060 0.074 0.340 0.652 0.953 1.247 1.536 1.820 2.101 2.37865 0.093 0.437 0.852 1.258 1.657 2.053 2.443 2.831 3.21870 0.114 0.558 1.104 1.645 2.182 2.717 3.250 3.780 4.310

gaseous phases of aqueous formaldehyde solu-tions. The partial pressures and concentrationswere measured at the boiling points of the solu-tions at a pressure of 101.3 kPa [21].Table 3. Concentration and partial pressure of formaldehyde mea-sured at the boiling points (101.3 kPa) of aqueous formaldehydesolutions [21]Formaldehyde concentration, wt% Partial pressure

Liquid phase Gaseous phase (pF), kPa(Fl) (Fg)

3.95 3.68 2.358.0 7.3 4.7512.1 10.6 7.015.3 13.2 8.6520.1 16.95 11.225.85 21.45 14.4530.75 24.9 16.835.65 27.4 18.842.0 30.5 21.447.5 33.1 23.449.8 34.0 24.1

Aqueous Formaldehyde Methanol Solu-tions. Technical-grade formaldehyde solutionscontain a small amount of methanol as a resultof the incomplete methanol conversion duringformaldehyde production. The amount of meth-anol present depends on the production processemployed. The presence of methanol is oftendesirable in aqueous solutions containing morethan 30wt% formaldehyde because it inhibitsthe formation of insoluble, higher mass poly-mers. Methanol concentrations of up to 16wt%stabilize the formaldehyde.

The approximate density (in grams per cu-bic centimeter) of aqueous formaldehyde solu-

tions containingup to13wt%methanol at a tem-perature of 10 70 Ccan be calculated by usingthe following equation [22]:

=a+0.0030 (F b)0.0025 (M c)104 [0.055 (F30) +5.4] (t20)

whereF = formaldehyde concentration in wt%M =methanol concentration in wt%t = temperature in Ca, b, and c = constants

The following values can be assumed whenF is in the range 0 48: a = 1.092, b = 30, andc = 0. The corresponding values in the rangeF = 48 55 area = 1.151,b = 50.15, and c = 1.61.

The boiling points of pure aqueous solu-tions containing up to 55wt% formaldehydeare between 99 and 100 C at atmospheric pres-sure [23]. In dilute aqueous solutions, formalde-hyde lowers the freezing point of water. If so-lutions containing more than 25wt% formalde-hyde are cooled, polymer precipitates out be-fore the freezing point is reached. According toNatta [22], the approximate refractive indexn18D of aqueous 30 50wt% formaldehyde so-lutions containing up to 15wt% methanol canbe calculated from the following equation:

n18D = 1.3295 + 0.00125F+0.000113M

where F and M are wt% concentrations offormaldehyde and methanol, respectively.

In close agreement with measurements ofcommercial solutions, the dynamic viscosity

Formaldehyde 5

of aqueous formaldehyde methanol solutionsmay be expressed by the following equation[24]:

(mPas) = 1.28 + 0.039F+0.05M0.024t

This equation applies to solutions containing30 50wt% formaldehyde and 0 12wt%methanol at a temperature t of 25 40 C.

Detailed studies on chemical reactions,vaporliquid equilibria and caloric propertiesof systems containing formaldehyde, water, andmethanol are available [216226].

3. Chemical Properties

Formaldehyde is oneof themost reactive organiccompounds known and, thus, differs greatlyfrom its higher homologues and aliphatic ke-tones [25], [26]. Only the most important of itswide variety of chemical reactions are treatedin this article; others are described in [27]. Fora general discussion of the chemical proper-ties of saturated aldehydes, see Aldehydes,Aliphatic and Araliphatic, Chap. 2.

Decomposition. At 150 C, formaldehydeundergoes heterogeneous decomposition toform mainly methanol and CO2 [28]. Above350 C, however, it tends to decompose into COand H2 [29]. Metals such as platinum [30], cop-per [31], chromium, and aluminum [32] cat-alyze the formation of methanol, methyl for-mate, formic acid, CO2, and methane.

Polymerization. Anhydrous monomericformaldehyde cannot be handled commercially.Gaseous formaldehyde polymerizes slowly attemperatures below 100 C, polymerization be-ing accelerated by traces of polar impurities suchas acids, alkalis, or water (see paraformalde-hyde, Section 11.1). Thus, in the presence ofsteam and traces of other polar compounds, thegas is stable at ca. 20 C only at a pressure of0.25 0.4 kPa, or at a concentration of up to ca.0.4 vol% at ca. 20 C and atmospheric pressure.

Monomeric formaldehyde forms a hydratewith water; this hydrate reacts with furtherformaldehyde to form polyoxymethylenes (seeSection 2.2). Methanol or other stabilizers, suchas guanamines [33] or alkylenebis(melamines)

[34], are generally added to commercial aqueousformaldehyde solutions (37 55wt%) to inhibitpolymerization.

Reduction and Oxidation. Formaldehydeis readily reduced to methanol with hydrogenover a nickel catalyst [27], [35]. For example,formaldehyde is oxidized by nitric acid, potas-sium permanganate, potassium dichromate, oroxygen to give formic acid or CO2 and water[27], [36].

In the presence of strong alkalis [37] or whenheated in the presence of acids [38], formalde-hyde undergoes a Cannizzaro reaction with for-mation of methanol and formic acid [39]. In thepresence of aluminum or magnesiummethylate,paraformaldehyde reacts to formmethyl formate(Tishchenko reaction) [27].

Addition Reactions. The formation of spar-ingly water-soluble sodium formaldehyde bisul-te is an important addition reaction offormaldehyde [40]. Hydrocyanic acid re-acts with formaldehyde to give glycolonitrile[107-16-4] [27]. Formaldehyde undergoes anacid-catalyzed Prins reaction in which it forms-hydroxymethylated adducts with olens [24].Acetylene undergoes a Reppe addition reactionwith formaldehyde [41] to form 2-butyne-1,4-diol [110-65-6]. Strong alkalis or calcium hy-droxide convert formaldehyde to a mixture ofsugars, in particular hexoses, by a multiple aldolcondensation which probably involves a glyco-laldehyde intermediate [42], [43]. Mixed aldolsare formed with other aldehydes; the productdepends on the reaction conditions. Acetalde-hyde, for example, reacts with formaldehydeto give pentaerythritol, C(CH2OH)4 [115-77-5](Alcohols, Polyhydric, Chap. 4.).

Condensation Reactions. Important con-densation reactions are the reaction of formalde-hyde with amino groups to give Schiffs bases,as well as the Mannich reaction [27]. Aminesreact with formaldehyde and hydrogen to givemethylamines. Formaldehyde reacts with am-monia to give hexamethylenetetramine, andwithammonium chloride to give monomethylamine,dimethylamine, or trimethylamine and formicacid, depending on the reaction conditions [44].Reaction of formaldehyde with diketones andammonia yields imidazoles [45].

6 Formaldehyde

Formaldehyde reacts with many compoundsto produce methylol (CH2OH) derivatives. Itreacts with phenol to give methylolphenol, withurea to give mono-, di-, and trimethylolurea,with melamine to give methylolmelamines, andwith organometallic compounds to give metal-substituted methylol compounds [27].

Aromatic compounds such as benzene, ani-line, and toluidine combine with formalde-hyde to produce the corresponding diphenyl-methanes. In the presence of hydrochloric acidand formaldehyde, benzene is chloromethylatedto form benzyl chloride [100-44-7] [46]. Thepossible formation of bis(chloromethyl)ether[542-88-1] from formaldehyde and hydrochlo-ric acid and the toxicity of this compound arereported elsewhere (Ethers, Aliphatic).

Formaldehyde reacts with hydroxylamine,hydrazines, or semicarbazide to produceformaldehyde oxime (which is spontaneouslyconverted to triformoxime), the correspondinghydrazones, and semicarbazone, respectively.Double bonds are also producedwhen formalde-hyde is reacted with malonates or with primaryaldehydes or ketones possessing a CH2 groupadjacent to the carbonyl group.

Resin Formation. Formaldehyde condenseswith urea,melamine, urethanes, cyanamide, aro-matic sulfonamides and amines, and phenols togive a wide range of resins (Amino Resins;Phenolic Resins;Resins, Synthetic).

4. Production

Formaldehyde is produced industrially frommethanol [67-56-1] by the following three pro-cesses:

1) Partial oxidation and dehydrogenation withair in the presence of silver crystals, steam,and excess methanol at 680 720 C (BASFprocess, methanol conversion = 97 98%).

2) Partial oxidation and dehydrogenation withair in the presence of crystalline silver orsilver gauze, steam, and excess methanolat 600 650 C [47] (primary conversionof methanol = 77 87%). The conversion iscompleted by distilling the product and re-cycling the unreacted methanol.

3) Oxidation only with excess air inthe presence of a modied iron mo-lybdenum vanadium oxide catalystat 250 400 C (methanol conver-sion = 98 99%).Processes for converting propane, butane

[48], ethylene, propylene, butylene [49], orethers (e.g., dimethyl ether) [50] into formalde-hyde are not of major industrial signicance foreconomic reasons. Processes that employ par-tial hydrogenation of CO [51] or oxidation ofmethane [52] do not compete with methanolconversion processes because of the lower yieldsof the former processes.

The specications of the methanol, usedfor formaldehyde production according to pro-cesses 1 3 are listed in Table 4. However,crude aqueous methanol obtained by high- [54],medium-, or low-pressure [55] synthesis canalso be used for process 1. This methanol con-tains low concentrations of inorganic impuri-ties and limited amounts of other organic com-pounds. The methanol must be rst subjected topurication processes and preliminary distilla-tion to remove low-boiling components.Table 4. Specications of commercial methanol (grade AA) usedfor the production of formaldehyde [53]Parameter Specication

Methanol content > 99.85wt%Relative density, d 204 0.7928 g/cm

3

Maximum boiling point range 1 CAcetone and acetaldehyde content < 0.003wt%Ethanol content < 0.001wt%Volatile iron content < 2g/LSulfur content < 0.0001wt%Chlorine content < 0.0001wt%Water content < 0.15wt%pH 7.0KMnO4 test, minimum 30mindecolorization time

4.1. Silver Catalyst Processes

The silver catalyst processes for convertingmethanol to formaldehyde are generally carriedout at atmospheric pressure and at 600 720 C.The reaction temperature depends on the excessof methanol in the methanol air mixture. Thecomposition of the mixture must lie outside theexplosive limits. The amount of air that is used isalso determinedby the catalytic quality of the sil-ver surface. The following main reactions occurduring the conversion of methanol to formalde-hyde:

Formaldehyde 7

CH3OHCH2O+H2 H =+84 kJ/mol (1)

H2+ 1/2O2 H2O H=243 kJ/mol (2)

CH3OH+1/2O2 CH2O+H2O H =159 kJ/mol (3)

The extent towhich each of these three reactionsoccurs, depends on the process data.

Byproducts are also formed in the followingsecondary reactions:

CH2OCO+H2 H=+12.5 kJ/mol (4)

CH3OH+3/2O2 CO2+ 2H2O H =674 kJ/mol (5)

CH2O+O2 CO2+H2O H=519 kJ/mol (6)

Other important byproducts aremethyl formate,methane, and formic acid.

The endothermic dehydrogenation reaction(1) is highly temperature-dependent, conver-sion increasing from 50% at 400 C to 90% at500 C and to 99% at 700 C. The temperaturedependence of the equilibrium constant for thisreaction Kp is given by

logKp= (4600/T )6.470

For detailed thermodynamic data of reactions(1) (6) see [56]. Kinetic studies with silver ona carrier show that reaction (1) is a rst-order re-action [57]. Therefore, the rate of formaldehydeformation is a function of the available oxygenconcentration and the oxygen residence time onthe catalyst surface:

dcFdt

=kco

where

cF = formaldehyde concentrationcO = oxygen concentrationk = rate constantt = time

A complete reaction mechanism for the con-version of methanol to formaldehyde over a sil-ver catalyst has not yet been proposed. How-ever, some authors postulate that a change inmechanism occurs at ca. 650 C [58]. New

insight into the reaction mechanism is avail-able from spectroscopic investigations [227229], which demonstrate the inuence of dif-ferent atomic oxygen species on reaction path-way and selectivity. The synthesis of formalde-hyde over a silver catalyst is carried out understrictly adiabatic conditions. Temperature mea-surements both above and in the silver layershow that sites still containingmethanol are sep-arated from sites already containing predomi-nantly formaldehyde by only a few millimeters.

The oxygen in the process air is shared bet-ween the exothermic reactions, primarily reac-tion (2) and, to a lesser extent depending on theprocess used, the secondary reactions (5) and(6). Thus, the amount of process air controls thedesired reaction temperature and the extent towhich the endothermic reactions (1) and (4) oc-cur.

Another important factor affecting the yieldof formaldehyde and the conversion of metha-nol, besides the catalyst temperature, is the ad-dition of inert materials to the reactants. Wateris added to spent methanol water-evaporatedfeed mixtures, and nitrogen is added to air andair off-gas mixtures, which are recycled to di-lute the methanol oxygen reaction mixture.The throughput per unit of catalyst area providesanother way of improving the yield and affect-ing side reactions. These twomethods of processcontrol are discussed in [59].

The theoretical yield of formaldehyde ob-tained fromReactions (1) (6) can be calculatedfrom actual composition of the plant off-gas byusing the following equation:

Yield (mol%) =

100[+ r +

(%CO2)+(%CO)

0.528(%N2)+(%H2)3(%CO2)2(%CO)

]1

Percentages signify concentrations in vol% andr is the ratio of moles of unreacted methanolto moles of formaldehyde produced [60]. Theequation takes into account the hydrogen andoxygen balance and the formation of byprod-ucts.

4.1.1. Complete Conversion of Methanol(BASF Process)The BASF process for the complete conversionof methanol to formaldehyde is shown schemat-ically in Figure 1 [61]. A mixture of methanol

8 Formaldehyde

and water is fed into the evaporating column.Fresh process air and, if necessary, recycled off-gas from the last stage of the absorption columnenter the column separately [60].Agaseousmix-ture of methanol in air is thus formed in whichthe inert gas content (nitrogen, water, and CO2)exceeds the upper explosive limit. A ratio of60 parts of methanol to 40 parts of water withor without inert gases is desired. The packedevaporator constitutes part of the stripping cy-cle. The heat required to evaporate the metha-nol and water is provided by a heat exchanger,which is linked to the rst absorption stage of theabsorption column [62]. After passing througha demister, the gaseous mixture is superheatedwith steam and fed to the reactor, where it owsthrough a 25 30mm thick bed of silver crys-tals. The crystals have a dened range of particlesizes [63] and rest on a perforated tray, which iscovered with a ne corrugated gauze, thus per-mitting optimum reaction at the surface. The bedis positioned immediately above a water boiler(cooler), which produces superheated steam andsimultaneously cools the hot reaction gases to atemperature of 150 C corresponding to that ofthe pressurized steam (0.5MPa). The almost drygas from the gas cooler passes to the rst stageof a four-stage packed absorption column,wherethe gas is cooled and condensed. Formaldehydeis eluted countercurrent to water or to the circu-lating formaldehyde solutions whose concentra-tions increase from stage to stage.

The product circulating in the rst stagemay contain 50wt% formaldehyde if the tem-perature of the gas leaving this stage is keptat ca. 75 C; this temperature provides suf-cient evaporation energy for the feed stream inthe heat exchanger. The nal product contains40 55wt% formaldehyde, as desired, with anaverage of 1.3wt% methanol and 0.01wt%formic acid. The yield of the formaldehyde pro-cess is 89.5 90.5mol%. Some of the off-gas isremoved at the end of the fourth stage of the col-umn [60] and is recycled due to its extremely lowformaldehyde content (Fig. 1, route indicatedby dashed-dotted lines). The residual off-gas isfed to a steam generator, where it is combusted[64] (net caloric value = 1970 kJ/m3). Prior tocombustion the gas contains ca. 4.8 vol% CO2,0.3 vol% CO, and 18.0 vol% H2 as well as ni-trogen, water, methanol, and formaldehyde. Thecombusted off-gas contains no environmentally

harmful substances. The total steam equivalentof the process is 3 t per tonof 100wt%formalde-hyde.

Figure 1. Flowchart of formaldehyde production by theBASF processa) Evaporator; b) Blower; c) Reactor; d) Boiler; e) Heatexchanger; f) Absorption column; g) Steam generator;h) Cooler; i) SuperheaterRecycling schemes : off-gas, formaldehydesolution.

In an alternative procedure to the off-gasrecycling process (Fig. 1, dashed lines) theformaldehyde solution from the third or fourthstage of the absorption tower is recycled to theevaporator; a certain amount of steam is usedin the evaporation cycle. The resulting vapor iscombined with the feed stream to the reactor toobtain an optimal methanol : water ratio [65]. Inthis case, the temperature of the second stage ofthe absorption column is ca. 65 C.

Formaldehyde 9

The yields of the two processes are similarand depend on the formaldehyde content of therecycled streams.

The average life time of a catalyst bed de-pends on impurities such as inorganic materi-als in the air and methanol feed; poisoning ef-fects caused by some impurities are partiallyreversible within a few days. The life time ofthe catalyst is also adversely affected by longexposure to excessively high reaction tempera-tures and high throughput rates because the sil-ver crystals then become matted and cause anincrease in pressure across the catalyst bed. Thiseffect is irreversible and the catalyst bedmust bechanged after three to four months. The catalystis regenerated electrolytically.

Since formaldehyde solutions corrode carbonsteel, all parts of the manufacturing equipmentthat are exposed to formaldehyde solutions mustbe made of a corrosion-resistant alloy, e.g., cer-tain types of stainless steel. Furthermore, tubesthat convey water or gases must be made of al-loys to protect the silver catalyst against metalpoisoning.

If the throughput and reaction temperaturehave been optimized, the capacity of a formalde-hyde plant increases in proportion to the diam-eter of the reactor. The largest known reactorappears to be that of BASF in the Federal Re-public of Germany; it has an overall diameterof 3.2m and a production capacity of 72 000 t/a(calculated as 100wt% formaldehyde).

4.1.2. Incomplete Conversion andDistillative Recovery of Methanol

Formaldehyde can be produced by partial oxida-tion and distillative recovery of methanol. Thisprocess is used in numerous companies (e.g.,ICI, Borden, and Degussa) [66]. As shown inFigure 2, a feed mixture of pure methanol va-por and freshly blown-in air is generated in anevaporator. The resultingvapor is combinedwithsteam, subjected to indirect superheating, andthen fed into the reactor. The reaction mixturecontains excess methanol and steam and is verysimilar to that used in theBASF process (cf. Sec-tion 4.1.1). The vapor passes through a shallowcatalyst bed of silver crystals or through lay-ers of silver gauze. Conversion is incompleteand the reaction takes place at 590 650 C, un-

desirable secondary reactions being suppressedby this comparatively low temperature. Immedi-ately after leaving the catalyst bed, the reactiongases are cooled indirectly with water, therebygenerating steam. The remaining heat of reac-tion is then removed from the gas in a coolerand is fed to the bottom of a formaldehyde ab-sorption column. In the water-cooled section ofthe column, the bulk of the methanol, water, andformaldehyde separate out. At the top of the col-umn, all the condensable portions of the remain-ing formaldehyde and methanol are washed outof the tail gas by countercurrent contact withprocess water. A 42wt% formaldehyde solutionfrom the bottom of the absorption column is fedto a distillation column equipped with a steam-based heat exchanger and a reux condenser.Methanol is recovered at the top of the columnand is recycled to the bottomof the evaporator. Aproduct containing up to 55wt% formaldehydeand less than 1wt% methanol is taken from thebottom of the distillation column and cooled.The formaldehyde solution is then usually fedinto an anion-exchange unit to reduce its formicacid content to the specied level of less than50mg/kg.

If 50 55wt% formaldehyde and no morethan 1.5wt%methanol are required in the prod-uct, steam addition is restricted and the processemploys a larger excess of methanol. The ratioof distilled recycled methanol to fresh metha-nol then lies in the range 0.25 0.5. If a diluteproduct containing 40 44wt% formaldehydeis desired, the energy-intensive distillation ofmethanol can be reduced, leading to savings insteam and power as well as reductions in cap-ital cost. The off-gas from the absorption col-umn has a similar composition to that describedfor the BASF process (in Section 4.1.1). Theoff-gas is either released into the atmosphereor is combusted to generate steam, thus avoid-ing environmental problems caused by residualformaldehyde. Alternatively, the tail gas fromthe top of the absorber can be recycled to the re-actor. This inert gas, with additional steam, canreduce the excess methanol needed in the reac-tor feed, consequently providing amore concen-trated product with less expenditure on distilla-tion. The yield of the process is 91 92mol%.

Process variations to increase the incompleteconversion of methanol employ two-stage oxi-dation systems [67]. The methanol is rst partly

10 Formaldehyde

converted to formaldehyde, using a silver cata-lyst at a comparatively low temperature (e.g.,600 C). The reaction gases are subsequentlycooled and excess air is added to convert the re-maining methanol in a second stage employingeither a metal oxide (cf. Section 4.2) or a furthersilver bed as a catalyst.

Formaldehyde solutions in methanol with arelatively low water content can be produced di-rectly by methanol oxidation and absorption inmethanol [68]. Anhydrous alcoholic formalde-hyde solutions or alcoholic formaldehyde solu-tionswith a lowwater content can be obtained bymixing a highly concentrated formaldehyde so-lution with the alcohol (ROH) and distilling offan alcohol water mixture with a low formalde-hyde content. The formaldehyde occurs in thedesired solutions in the form of the hemiacetalsRO (CH2O)nH.

Figure 2. Flowchart of formaldehyde production with re-covery of methanol by distillationa) Evaporator; b) Blower; c) Reactor; d) Boiler; e) Distil-lation column; f) Absorption column; g) Steam generator;h) Cooler; i) Superheater; j) Anion-exchange unit

4.2. Formox Process

In the Formox process, a metal oxide (e.g., iron,molybdenum, or vanadium oxide) is used asa catalyst for the conversion of methanol toformaldehyde. Many such processes have beenpatented since 1921 [69]. Usually, the oxidemixture has an Mo : Fe atomic ratio of 1.5 2.0,small amounts of V2O5, CuO, Cr2O3, CoO, andP2O5 are also present [70]. Special conditionsare prescribed for both the process and the acti-vation of the catalyst [71]. The Formox processhas been described as a two-step oxidation reac-tion in the gaseous state (g) which involves anoxidized (KOX) and a reduced (Kred) catalyst[72]:

CH3OH(g) +KOX CH2O(g) +H2O(g) +Kred

Kred + 1/2 O2(g)KOX H =159 kJ/mol

CH2O+1/2 O2CO+H2OH =215 kJ/mol

In the temperature range 270 400 C, con-version at atmospheric pressure is virtuallycomplete. However, conversion is temperature-dependent because at >470 C the followingside reaction increases considerably:

CH2O+1/2 O2CO+H2OH =215 kJ/mol

The methanol oxidation is inhibited by watervapor. A kinetic study describes the rate of re-action to formaldehyde by a power law kineticrate expression of the form [230]

r=kPxCH3OHPvO2P

zH2O

where x = 0.94 0.06; y = 0.10 0,05 and z =0.45 0.07. The rate is independent of theformaldehyde partial pressure. The measuredactivation energy is 98 6 kJ/mol.

As shown in Figure 3, the methanol feed ispassed to a steam-heated evaporator. Freshlyblown-in air and recycled off-gas from the ab-sorption tower are mixed and, if necessary, pre-heated by means of the product stream in a heatexchanger before being fed into the evaporator.The gaseous feed passes through catalyst-lledtubes in a heat-exchanging reactor. A typical re-actor for this process has a shell with a diameterof ca. 2.5m that contains tubes only 1.0 1.5m

Formaldehyde 11

in length. A high-boiling heat-transfer oil circu-lates outside the tubes and removes the heat ofreaction from the catalyst in the tubes. The pro-cess employs excess air and the temperature iscontrolled isothermally to a value of ca. 340 C;steam is simultaneously generated in a boiler.The air methanol feed must be a ammablemixture, but if the oxygen content is reduced toca. 10mol% by partially replacing air with tailgas from the absorption tower, themethanol con-tent in the feed can be increased without form-ing an explosive mixture [73]. After leaving thereactor, the gases are cooled to 110 C in a heat-exchange unit and are passed to the bottom ofan absorber column. The formaldehyde concen-tration is regulated by controlling the amount ofprocesswater added at the topof the column.Theproduct is removed from the water-cooled cir-culation system at the bottom of the absorptioncolumn and is fed through an anion-exchangeunit to reduce the formic acid content. The -nal product contains up to 55wt% formalde-hyde and 0.5 1.5wt% methanol. The resultantmethanol conversion ranges from 95 99mol%and depends on the selectivity, activity, and spottemperature of the catalyst, the latter being inu-encedby theheat transfer rate and the throughputrate. The overall plant yield is 88 91mol%.

Well-known processes using the For-mox method have been developed by Per-storp/Reichhold (Sweden, United States, GreatBritain) [74], [75], Lummus (United States)[76], Montecatini (Italy) [77], and Hiag/Lurgi(Austria) [78].

The tail gas does not burn by itself because itconsists essentially of N2, O2, and CO2 with afew percent of combustible components such asdimethyl ether, carbonmonoxide,methanol, andformaldehyde. Combustion of Formox tail gasfor the purpose of generating steam is not eco-nomically justiable [79]. Two alternativemeth-ods of reducing atmospheric emission have beendeveloped. The off-gas can be burned eitherwithadditional fuel at a temperature of 700 900 Cor in a catalytic incinerator at 450 550 C.However, the latter system employs a heat ex-changer and is only thermally self-sufcient ifsupplementary fuel for start-up is provided andif an abnormal ratio of oxygen : combustiblecomponents is used [80].

Figure 3.Flowchart of formaldehyde production by the For-mox processa) Evaporator; b) Blower; c) Reactor; d) Boiler; e) Heatexchanger; f) Formaldehyde absorption column; g) Cir-culation system for heat-transfer oil; h) Cooler; i) Anion-exchange unit

4.3. Comparison of Process Economics

Considering the economic aspects of the threeformaldehyde processes in practice, it becomesobvious that the size of the plant and the cost ofmethanol are of great importance. Generally, theFormox process proves to be advantageous re-garding the attainable formaldehyde yield.How-ever, in comparison with the silver process thisprocess demands a larger plant andhigher invest-ment costs. For the purpose of a cost compari-son, a study was undertaken based on the costof methanol being $ 200 /t and a plant produc-tion capacity of 20,000 t/a of 37% formaldehyde(calculated 100%) [1]. Table 5 summarizes theeconomic data. According to these data the sil-ver process, without the recovery of methanol(cost of formaldehyde $ 378/t), offers the mostfavorable production costs, followed by the For-mox process ($ 387/t) and the silver processwith

12 FormaldehydeTable 5. Comparison of economic factors in formaldehyde production processes [1]

Complete methanol conversion(BASF process)

Incomplete conversion andmethanol recovery

Formoxprocess

Total capital investment,$ 106

6.6 8.6 9.6

Methanol consumption,t/t

1.24 1.22 1.15

Raw materials, $/t 255 252 227Methanol 250 247 232Catalyst and chemicals 5 5 7Byproduct credit (steam) not mentioned not mentioned 12

Utilities, $/t 12 20 13LP Steam 3.4 9.5Power purchased 3.4 4.3 8.0Cooling water 2.9 2.8 4.0Process water 2.4 3.3 1.0

Variable costs, $/t 267 272 240Direct xed costs, $/t 27 29 30Total allocated xedcosts, $/t

18 20 21

Total cash cost, $/t 312 321 291Depreciation, $/t 33 43 48Cost of production, $/t 345 364 339Return of total capitalinvestment (ROI), $/t

33 43 48

Cost of production andROI, $/t

378 407 387

recovery ($ 407/t). The two latter processes pro-duce a product with < 1% methanol whereasthe methanol content in the silver process with-out recovery lies between 1 5%.

The study takes into consideration the benetof the production of steam only in the case of theFormox process. If the production of steam is in-cluded in the silver process (3 t per tonne CH2Owithout and 1.5 t per tonne CH2O with meth-anol recovery) better results than demonstratedin Table 5 can be obtained (costs per tonne $ 24and $ 12 lower, respectively). The proven ca-pacity limits of a plant with only one reactorare about 20 000 t/a (calculated 100%) with themetal oxide process and about 72 000 t/a withthe silver process.

The key feature of the BASF process for theproduction of 50wt% formaldehyde is a liq-uid circulation system in which heat from theabsorption unit of the plant is transferred to astripper column tovaporize themethanol waterfeed. Therefore, the process produces excesssteam, with simultaneous savings in cooling wa-ter.

Plant operation and start-up are simple; theplant can be restarted after a shutdown or aftera short breakdown, as long as the temperaturesin the stripping cycle remain high. The BASF

process has several other advantages. Formalde-hyde is obtained from a single pass of the meth-anol through the catalyst. If a lower formalde-hyde concentration is needed (e.g., 40wt%) theyield can be increased by employing a feed-stock of suitably pretreated crude aqueousmeth-anol instead of puremethanol (cf. Section 4.1.1).Deacidication by means of ion exchangers isnot necessary. The off-gas does not present anyproblems because it is burned as a fuel gas inpower stations to generate steam or steam andpower. The catalyst can be exchanged within8 12 h of plant shutdown to restart and can beregenerated completelywith little loss. The plantis compact due to the small volume of gas that isused and the low space requirements; both fac-tors result in low capital investment costs.

Formaldehyde production processes basedon incomplete methanol conversion employ anal distillation column to recover the methanoland concentrate the formaldehyde. As shown inTable 5, this means that more steam and cool-ing water is consumed than in the BASF pro-cess. The BASF process has a somewhat loweryield but all other aspects are roughly compa-rable. Other distinctive features of the incom-plete conversion of methanol are the relativelylarge amount of direct steam introduced into the

Formaldehyde 13

feedstock and the lower reaction temperature,which give a somewhat larger amount of hydro-gen in the off-gas with a net caloric value of2140 kJ/m3. The additional ion-exchange unitalso increases production costs.

The Formox process uses excess air in themethanol feed mixture and requires at least13mol of air per mole of methanol. A amma-ble mixture is used for the catalytic conversion.Even with gas recycling, the process must han-dle a substantial volume of gas, which is 3 3.5times the gas ow in a silver-catalyzed process.Thus, the equipment must have a large capacityto accommodate the higher gas ow. The maindisadvantage of the Formox process is that theoff-gas is noncombustible, causing substantialcosts in controlling environmental pollution. Toreduce air pollution to the levels obtained in thesilver-catalyzed processes, a Formox plant mustburn the tail gas with sulfur-free fuel, with orwithout partial regeneration of energy by meansof steam production. Advantages of the processare its very low reaction temperature, which per-mits high catalyst selectivity, and the very simplemethod of steam generation. All these aspectsmean in easily controlled process. Plants basedon this technology can be very small with an-nual capacities of a few thousand tons. As a re-sult, plants employing Formox methanol oxida-tion are most commonly encountered through-out the world. However, if higher capacities arerequired and a small number of reactors must bearranged in parallel, the economic data favor theprocesses employing a silver catalyst.

Although approximately 70% of existingplants use the silver process, in the 1990s newplant contracts have been dominated by themetal oxide technology [1].

4.4. Distillation of AqueousFormaldehyde Solutions

Since formaldehyde polymerizes in aqueous so-lutions, the monomer content and thus the vaporpressure of formaldehyde during distillation aredetermined by the kinetics of the associated re-actions.

Vacuum distillation produces a more concen-trated bottom product and can be carried out at alow temperature, an extremely low vapor pres-sure, and an acid pH value of 3 3.4 [81]. How-

ever, the distillation rates are low, making thisprocedure uneconomical.

High-pressure distillation at 0.4 0.5MPaand above 130 C with long columns producesa relatively concentrated overhead product. Ef-ciency is high, but yields are limited due to theformation of methanol and formic acid via theCannizzaro reaction [82].

If formaldehyde solutions are subjected toslow distillation at atmospheric pressure with-out reuxing, the distillate has a lower formalde-hyde content than the bottom product [21]. If thecondensate is reuxed, the ratio of condensate(reux) to distillate determines the formalde-hyde content of the distillate removed [81].

In the case of aqueous formaldehyde solu-tions that contain methanol, a virtually metha-nol-free product can be obtained by using dis-tillation columns with a large number of platesand a relatively high reux ratio. The product istaken from the bottom of the column [83].

4.5. Preparation of Liquid MonomericFormaldehyde

Twomethods have been described for the prepa-ration of liquid monomeric formaldehyde fromparaformaldehyde, the rst was developed byF. Walker [11] and the second by R. Spence[84]. In Walkers method, liquid formaldehydeis prepared by vaporizing alkali-precipitated -polyoxymethylene. The resultant vapor is thencondensed and the crude liquid condensate isredistilled. The process is performed in an ap-paratus made of Pyrex glass. A vaporizing tubeis charged to about one-half its height with thepolymer. The thoroughly dried system is thenushed with dry nitrogen. The vaporizing tubeis heated to 150 C in an oil bath and the con-densing tube is chilled in a bath of solid carbondioxide and methanol. The polymer is vapor-ized in a slow stream of nitrogen by graduallyraising the temperature. Formation of polymeron the tube walls is minimized by winding wireround the tubes and heating with electricity. Thecrude liquid product, which is opalescent due toprecipitated polymer, is then distilled in a slowcurrent of nitrogen.

According to the method of Spence,paraformaldehyde is dried over sulfuric acid in

14 Formaldehyde

a vacuum desiccator and introduced into a dis-tillation ask. This ask is connected to a glasscondenser via glass tubes with relatively longhairpin turns designed to separate traces ofwater(Fig. 4). The system is rst evacuated by meansof a mercury diffusion pump, and the distillationask is then heated to 110 C in an oil bath toremove traces of oxygen. The distillate is heatedelectrically to 120 C when it ows through theupper parts of the hairpin turns; in the lower partsof the loops, it is cooled to78 Cbymeans of acooling bath. After the valve to the pump is shutand the condenser ask is cooled in liquid air,a colorless solid product condenses. The inletand outlet tubes of the condenser ask are thensealed with a ame. The contents of the con-densing ask liquefy when carefully warmed.The procedure can be repeated to obtain an evenpurer substance. The liquid formaldehyde that isprepared does not polymerize readily and, whenvaporized, leaves only very small traces of poly-meric product.

Figure4.Apparatus for the preparationof liquidmonomericformaldehydea)Distillation ask; b)Glass tubewith hairpin turns; c) Con-denser; d) Glass wool

5. Environmental Protection

As already stated, formaldehyde is ubiquitouslypresent in the atmosphere [85]. It is released intothe atmosphere as a result of the combustion,degradation, and photochemical decompositionof organic materials. Formaldehyde is also con-tinuously degraded to carbon dioxide in pro-cesses that are inuenced by sunlight and bynitrogen oxides. Formaldehyde washed out ofthe air by rain is degraded by bacteria (e.g.,Escherichia coli, Pseudomonas uorescens) toform carbon dioxide and water [86].

The major source of atmospheric formalde-hyde is the photochemical oxidation and incom-plete combustion of hydrocarbons (i.e., methaneor other gases, wood, coal, oil, tobacco, andgasoline). Accordingly, formaldehyde is a com-ponent of car and aircraft exhaust fumes andis present in considerable amounts in off-gasesfrom heating plants and incinerators. The mainemission sources of formaldehyde are summa-rized in Table 6.Table 6. Sources emitting formaldehyde into the atmosphere [87]

Emission source Formaldehyde level

Natural gas combustionHome appliances andindustrial equipment 2400 58 800g/m3

Power plants 15 000g/m3Industrial plants 30 000g/m3

Fuel-oil combustion 0.0 1.2 kg/barrel oilCoal combustionBituminous < 0.005 1.0 g/kg coalAnthracite 0.5 g/kg coalPower plant, industrial,and commercialcombustion 2.5mg/kg coal

Refuse incineratorsMunicipal 0.3 0.4 g/kg refuseSmall domestic 0.03 6.4 g/kg refuseBackyard (garden refuse) up to 11.6 g/kg refuse

Oil reneriesCatalytic cracking units 4.27 kg/barrel oilThermofor units 2.7 kg/barrel oil

Automotive sourcesAutomobiles 0.2 1.6 g/L fuelDiesel engines 0.6 1.3 g/L fuelAircraft 0.3 0.5 g/L fuel

The formaldehyde in the exhaust gases ofmo-tor vehicles is produced due to incomplete com-bustion of motor fuel. Formaldehyde may beproduced directly or indirectly. In the indirectroute, the unconverted hydrocarbons undergosubsequent photochemical decomposition in theatmosphere to produce formaldehyde as an in-termediate [88]. The concentration of formalde-hyde is higher above densely populated regionsthan above the oceans as shown in Table 7 [89].According to a 1976 report of the EPA [89], theproportions of formaldehyde in ambient air arederived from the main emission sources as fol-lows:

Exhaust gases from motor vehicles andairplanes (direct production) 53 63%

Photochemical reactions (derived mainlyfrom hydrocarbons in exhaust gases) 19 32%

Heating plants, incinerators, etc. 13 15%Petroleum reneries 1 2%Formaldehyde production plants 1%

Formaldehyde 15

Formaldehyde in conned areas comes fromthe following sources:

1) Smoking of cigarettes and tobacco products[88], [90], [91]

2) Urea--, melamine--, and phenolformaldehyde resins in particle board andplywood furniture

3) Urea formaldehyde foam insulation4) Open replaces, especially gas res and

stoves5) Disinfectants and sterilization of large sur-

faces (e.g., hospital oors)

Table 7. Geographical distribution of formaldehyde in ambient air

Location Formaldehydeconcentration (max.), ppm

Air above the oceans 0.005Air above land 0.012Air in German citiesnormal circumstances 0.016high trafc density 0.056

Air in Los Angeles (before 0.165the law on catalytic com-bustion of exhaust gasescame into effect)

1 ppm= 1.2mg/m3

Sources generating formaldehyde must bedifferentiated into thosewhich release formalde-hyde for a dened period, cases (1), (4), and(5) and those which release formaldehyde gascontinuously, i.e., decomposition of resins as incases (2) and (3). Many regulations have beenissued to limit pollution of the atmosphere withformaldehyde in both general and special ap-plications [92]. Protection against pollution ofthe environment with formaldehyde must be en-forced with due attention to its sources.

The most effective limitation of atmosphericpollution with formaldehyde is the strict obser-vation of the maximum allowable concentrationindoors and outdoors. A maximum workplaceconcentration of 0.5 ppm (0.6mg/m3) has, forexample, been established in the Federal Re-public of Germany [93]. Other limit values andguide values have been specied for formalde-hyde levels in outdoor and indoor air. Emissionlimits for stationary installations have also beenestablished and regulations for specic productshave been formulated. Table 8 gives a survey ofregulations valid in some countries of the West-ern world in 1987.

In the Federal Republic of Germanyformaldehyde levels and emissions are subjectedto stringent regulations. Plants operating withformaldehyde must conform to the plant emis-sion regulations introduced in 1974 which limitformaldehyde in off-gases to a maximum of20mg/m3 formass ow rates of 0.1 kg/h ormore[94]. This presupposes a closed handling proce-dure. For example, industrial lling and transferof formaldehyde solutions is carried out by usingpressure compensation between communicatingvessels. Discharge of formaldehyde into waste-water in Germany is regulated by law since it en-dangers water and is toxic to small animals [95].Formaldehyde is, however, readily degraded bybacteria in nonsterile, natural water [96].

A maximum limit of 0.1 ppm formalde-hyde in indoor living and recreation areas hasbeen recommended by the BGA (German Fed-eral Health Ofce) [97]. To avoid unacceptableformaldehyde concentrations in room air, theGerman Institute for Structural Engineering hasissued guidelines for classifying particle boardinto emission categories E1, E2, and E3, classE3 having the highest emission [98]. The lowestclass (E1) is allowed a maximum formaldehydeemission of 0.1 ppm and a maximum formalde-hyde content of 10mg per 100 g of absolutelydry board (as measured by the DIN EN-120perforator method) [99]. Furthermore, the usesand applications of urea formaldehyde foams,which are used to some extent for the heat insu-lation of cavity walls, have been controlled byDIN 18 159 [99] since 1978. No formaldehydeemission is permitted after the construction hasdried.

Cigarette smoke contains 57 115 ppm offormaldehyde and up to 1.7mg of formaldehydecan be generated while one cigarette is smoked.If ve cigarettes are smoked in a 30m3 room,with a low air-change rate of 0.1 (i.e., 10%) perhour, the formaldehyde concentration reaches0.23 ppm [88], [91].

The best protection against accumulationof formaldehyde in conned spaces is, how-ever, proper ventilation. The strong smell offormaldehyde is perceptible at low concentra-tion and thus provides adequate warning of itspresence. If all manufacturing and applicationregulations are strictly observed, possible emis-sion of formaldehyde from consumer products

16 FormaldehydeTable 8. International regulations restricting formaldehyde levels

Country Emission limit Product-specic regulations

Outdoor air, ppm Indoor air, ppm

Canada 0.1 (1982) Urea formaldehyde (UF) foam insulation prohibited. Voluntary program ofparticle board manufacturers to reduce emission, no upper limit. Registrationof infection control agents

Denmark 0.12 (1982) Guidelines for particle board: max. 10mg/100 g of absolutely dry board(perforator value). Guidelines for furniture and in situ UF foam. Cosmeticregulations. Prohibited for disinfecting bricks, wood, and textiles if there iscontact with food

FederalRepublic ofGermany

0.02 (MIKD, 1966) a0.06 (MIKK, 1966) b

0.1 (1977) Particle board classication. Guidelines (GefStoffV, Gefahrstoffverordnung)for wood and furniture: upper emission limit 0.1 ppm, corresponding to10mg/100mg of absolutely dry board (perforator value); detergents, cleaningagents, and conditioners: upper limit 0.2%; textiles: compulsory labeling ifformaldehyde content>0.15%. Guidelines for in situ UF foam: upper limit0.1 ppm. Cosmetic regulations

Finland 0.12 0.24 for pre1983 buildings (1983)

Upper limit for particle board: 50mg/100 g absolutely dry board (perforatorvalue). Prohibited as an additive in hairsprays and antiperspirants. Guidelinesfor cosmetics, but as yet (1987) no EEC directives

Great Britain Upper limit for particle board : 70mg/100 g of absolutely dry board(perforator value)

Italy 0.1 (1983) Cosmetic regulations (July 1985)Japan Prohibited as an additive in foods, food packaging, and paints. Guidelines for

particle board, textiles, wall coverings, and adhesivesTheNetherlands

0.1 obligatory forschools and rentedaccommodation(1978)

Particle board quality standard on a voluntary basis: upper limit 10mg/100 gof absolutely dry boad (perforator value). Particle board regulations inpreparation

Sweden 0.4 0.7 (1977) Particle board and plywood quality standards: upper limit 40mg/100 g ofabsolutely dry board (perforator value)

Switzerland 0.2 (introduced 1984,came into force 1986)

Particle board quality standard on a voluntary basis: upper limit 10mg/100 gof absolutely dry board (perforator value, Oct. 1985); quality symbol LignumCH 10

Spain Regulations for in situ UF foam (1984): upper limit 1000g/m3 = 0.8 ppm, 7days after installation; 500g/m3 = 0.4 ppm, 30 days after installation

United States 0.4 (Minnesota,1984) c 0.4(Wisconsin, 1982) c

UF foam insulation prohibited in Massachusetts, Connecticut, and NewHampshire; upper limit for existing UF-insulated houses in Massachusetts0.1 ppm (1986). FDA limit for nailhardening preparations:5%. Department of housing and urban development (HUD) guidelines foremission from particleboard and plywood for the construction of mobilehouses: upper limit 0.3 ppm.

a MIKD=Maximum allowable concentration for constant immission (mean annual value).b MIKK=Maximum allowable concentration for short-term immission (30 min or 24 h).c Replaced by HUD product standards, 1985.

is very low and will not therefore constitute ahuman health hazard.

Formaldehyde concentrations in cosmeticproducts have been limited since 1977, theymust be appropriately labeled if they contain> 0.05wt% formaldehyde [100]. Below thislevel, formaldehyde does not cause allergic re-actions even in sensitive subjects.6. Quality Specications andAnalysis6.1. Quality SpecicationsFormaldehyde is commercially available pri-marily in the form of an aqueous (generally30 55wt%) solution, and in solid form as

paraformaldehyde or trioxane (cf. Chap. 11).Formaldehyde solutions contain 0.5 12wt%methanol or other added stabilizers (seeChap. 7). They have a pH of 2.5 3.5, the acidreaction being due to the presence of formicacid, formed from formaldehyde by the Canniz-zaro reaction. The solutions can be temporarilyneutralized with ion exchangers. Typical prod-uct specications for formulations on the Euro-pean market are listed in Table 9. Other manu-facturers specications are described in [102108].

6.2. AnalysisThe chemical reactivity of formaldehyde pro-vides a wide range of potential methods for its

Formaldehyde 17Table 9. Typical specications of commercial formaldehyde solutions [101]

Formaldehyde Methanol content Formic acid Iron content Density Added Stabilizer

content, wt% (max), wt% content (max), (max), mg/kgmg/kg t, C g/mL

30 1.5 150 0.8 20 1.086 1.09037 1.8 200 1 20 1.107 1.11237 8 12 200 1 20 1.082 1.093 Methanol37 1.8 200 1 20 1.108 1.112 Isophthalobisguanamine,

100 mg/kg50 2.0 200 1 55 1.126 1.12950 2.0 200 1 40 1.135 1.138 Isophthalobisguanamine,

200 mg/kg

qualitative and quantitative determination in so-lutions and in the air.

Qualitative Methods. Qualitative detectionof formaldehyde is primarily by colori-metric methods, e.g., [109], [110]. Schiffsfuchsin bisulte reagent is a general reagentused for detecting aldehydes. In the presence ofstrong acids, it reacts with formaldehyde to forma specic bluish violet dye. The detection limit isca. 1mL/m3. Further qualitative detectionmeth-ods are described in [111].

Quantitative Methods. Formaldehyde canbe quantitatively determined by either physicalor chemical methods.

Physical Methods. Quantitative determina-tion of pure aqueous solutions of formalde-hyde can be carried out rapidly by measuringtheir specic gravity [27]. Gas chromatogra-phy [112], [113] and high-pressure liquid chro-matography (HPLC) [114116] can also be usedfor direct determination.

Chemical Methods. The most importantchemical methods for determining formalde-hyde are summarized in [111]. The sodiumsulte method is most commonly used. Thismethod was developed by Lemme [117] andwas subsequently improved by Seyewetz andGibello [118], Stadtler [119], and others. Itis based on the quantitative liberation of sodiumhydroxide produced when formaldehyde reactswith excess sodium sulte:

CH2O+Na2SO3+H2OHOCH2SO3Na+NaOHThe stoichiometrically formed sodium hydrox-ide is determined by titration with an acid [27].

Formaldehyde in air can be determined downto concentrations in the L/m3 range with the

aid of gas sampling apparatus [120], [121]. Inthis procedure, formaldehyde is absorbed froma dened volume of air by a wash liquid and isdetermined quantitatively by a suitable method.The quantitative determination of formaldehydein air by the sulte/pararosaniline method is de-scribed in [122].

A suitable way of checking the workplaceconcentration of formaldehyde is to take a rele-vant sample to determine the exposure of a par-ticular person and to use this in combinationwith the pararosaniline method. The liquid testsolution is transported in a leakproof wash bot-tle [111]. A commercial sampling tube [123],[124] can also be used, in which the formalde-hyde is converted to 3-benzyloxazolidine duringsampling. Evaluation is carried out by gas chro-matography.

Continuous measurements are necessaryto determine peak exposures, e.g., by thepararosaniline method as described in [125].

7. Storage and Transportation

With a decrease in temperature and/or an in-crease in concentration, aqueous formaldehydesolutions tend to precipitate paraformaldehyde.On the other hand, as the temperature increases,so does the tendency to form formic acid. There-fore, an appropriate storage temperature mustbe maintained (Table 10). The addition of sta-bilizers is also advisable (e.g., methanol, etha-nol, propanol, or butanol). However, these alco-hols can be used only if they do not interferewith further processing, or if they can be sepa-rated off; otherwise, efuent problems may beencountered.

18 Formaldehyde

The many compounds used for stabiliz-ing formaldehyde solutions include urea [126],melamine [127], hydrazine hydrate [128], me-thylcellulose [129], guanamines [130], and bis-melamines [33]. For example, by adding as littleas 100mgof isophthalobisguanamine [5118-80-9] per kilogram of solution, a 40-wt% formalde-hyde solution can be stored for at least 100 dat 17 C without precipitation of paraformalde-hyde, and a 50-wt% formaldehyde solution canbe stored for at least 100 d at 40 C [32].Table 10. Storage temperatures for commercial formaldehyde solu-tions

Formaldehyde Methanol Storagecontent, wt% content, wt% temperature, C

30 1 7 1037 < 1 3537 7 2137 10 12 6 750 1 2 4550 1 2 60 65

Stabilized with 200mg/kg of isophthalobisguanamine

Formaldehyde can be stored and transportedin containers made of stainless steel, aluminum,enamel, or polyester resin. Iron containers linedwith epoxide resin or plastic may also be used,although stainless steel containers are preferred,particularly for higher formaldehyde concen-trations. Unprotected vessels of iron, copper,nickel, and zinc alloys must not be used.

The ash point of formaldehyde solutionsis in the range 55 85 C, depending on theirconcentration and methanol content. Accord-ing to German regulations for hazardous sub-stances (Gefahrstoffverordnung, Appendix 6)and Appendix 1 of the EEC guidelines for haz-ardous substances, aqueous formaldehyde solu-tions used as working materials that contain 1 wt% of formaldehyde must be appropriatelylabeled. The hazard classications for the trans-port of aqueous formaldehyde solutions with aash point between 21 and 55 C containing> 5wt% formaldehyde and < 35wt% metha-nol are as follows [131]:GGVS/GGVE, ADR/RID Class 8, number 63 cCFR 49: 172.01 ammable

liquidIMDG Code (GGVSee) Class 3.3UN No. 1198

Formaldehyde solutions with a ash point> 61 C and aqueous formaldehyde solutions

with a ash point> 55 C that contain> 5wt%formaldehyde and< 35wt%methanol are clas-sied as follows:GGVS/GGVE, ADR/RID Class 8, number 63 cCFR 49: 172.01 combustible

liquidIMDG Code (GGVSee) Class 9UN No. 2209

8. Uses

Formaldehyde is one of themost versatile chem-icals and is employed by the chemical and otherindustries to produce a virtually unlimited num-ber of indispensable products used in daily life[132].

Resins. The largest amounts of formalde-hyde are used for producing condensates (i.e.,resins) with urea, melamine, and phenol and,to a small extent, with their derivatives (seealso Amino Resins; Phenolic Resins;Resins, Synthetic). The main part of theseresins is used for the production of adhesivesand impregnating resins, which are employedformanufacturing particle boards, plywood, andfurniture. These condensates are also employedfor the production of curable molding materi-als; as raw materials for surface coating and ascontrolled-release nitrogen fertilizers. They areused as auxiliaries in the textile, leather, rub-ber, and cement industries. Further uses includebinders for foundry sand, rockwool and glass-wool mats in insulating materials, abrasive pa-per, and brake linings. A very small amount ofurea formaldehyde condensates are used in themanufacture of foamed resins (Foamed Plas-tics, Chap. 4.4.,Foamed Plastics, Chap. 4.5.,Foamed Plastics, Chap. 4.6.) that have appli-cations in the mining sector and in the insulatingof buildings.

Use as an Intermediate. About 40% of thetotal formaldehyde production is used as an in-termediate for synthesizing other chemical com-pounds, many of which are discussed under sep-arate keywords. In this respect, formaldehydeis irreplaceable as a C1 building block. It is,for example, used to synthesize 1,4-butanediol[110-63-4], trimethylolpropane [77-99-6], andneopentyl glycol [126-30-7], which are em-ployed in the manufacture of polyurethane

Formaldehyde 19

and polyester plastics, synthetic resin coat-ings, synthetic lubricating oils, and plasticiz-ers. Other compounds produced from formalde-hyde include pentaerythritol [115-77-5] (em-ployed chiey in raw materials for surface coat-ings and in permissible explosives) and hexa-methylenetetramine [100-97-0] used as a cross-linking agent for phenol formaldehyde con-densates and permissible explosives).

The complexing agents nitrilotriacetic acid[139-13-9] (NTA) and ethylenediaminetetra-acetic acid [60-00-4] (EDTA) are derived fromformaldehyde and are components of moderndetergents. The demand for formaldehyde forthe production of 4,4-diphenylmethane diiso-cyanate [101-68-8] (MDI) is steadily increasing.This compound is a constituent of polyurethanesused in the production of soft and rigid foamsand, more recently, as an adhesive and for bond-ing particle boards.

The so-called polyacetal plastics(Polyoxymethylenes) produced by polymer-ization of formaldehyde are increasingly beingincorporated into automobiles to reduce theirweight and, hence, fuel consumption. They arealso used for manufacturing important func-tional components of audio and video electron-ics equipment [232].

Formaldehyde is also a building block forproducts used to manufacture dyes, tanningagents, dispersion and plastics precursors, ex-traction agents, crop protection agents, animalfeeds, perfumes, vitamins, avorings, and drugs.

Direct Use. Only a very small amount offormaldehyde is used directly without furtherprocessing. In the Federal Republic of Germany,ca. 8000 t/a are used in this way, which corre-sponds to ca. 1.5% of total production. It is useddirectly as a corrosion inhibitor, in the metal in-dustry as an aid in mirror nishing and electro-plating, in the electrodeposition of printed cir-cuits, and in the photographic industry for lmdevelopment. However, formaldehyde as suchis used mainly for preservation and disinfection,for example, in medicine for disinfecting hospi-tal wards, preserving specimens, and as a disin-fectant against athletes foot (Disinfectants).

Modern hygiene requires preservatives anddisinfectants to prevent the growth of microor-gansims which can produce substances that maybe extremely harmful to man. As an antimicro-

bial agent, formaldehyde displays very few sideeffects, but has a broad spectrum of action. Allalternative agents have unpleasant or even dan-gerous side effects. Moreover, their toxicity hasnot been as thoroughly investigated as that offormaldehyde, and their spectrum of action islimited (i.e., they do not provide comprehen-sive disinfectant protection). Another advantageof formaldehyde is that it does not accumulatein the environment since it is completely oxi-dized to carbon dioxide within a relatively shorttime. In the cosmetics industry, formaldehydeis employed as a preservative in hundreds ofproducts, for example, soaps, deodorants, sham-poos, and nail hardening preparations; in someof these items, upper limits have been set forthe formaldehyde concentration due to its sen-sitizing effect (cf. Table 8). Formaldehyde solu-tions are also used as a preservative for tanningliquors, dispersions, crop protection agents, andwood preservatives. Furthermore, formaldehydeis required in the sugar industry to prevent bac-terial growth during syrup recovery.

9. Economic AspectsFormaldehyde is one of themost important basicchemicals and is required for the manufacture ofthousands of industrial and consumer products.It is the most important industrially producedaldehyde.

Formaldehyde can seldom, if ever, be re-placed by other products. Substitutes are gener-ally more expensive; moreover, their toxicitieshave been less thoroughly investigated than thatof formaldehyde.

Worldwide capacity [1], [231] is approxi-mately 8.7 106 t/a in 1996 (see Table 11; thevalues are based on 100% formaldehyde); theve largest manufactures account for ca. 25%of this capacity:

Borden 0.66106 t / aBASF 0.444 106 t / aHoechst Celanese 0.38 106 t / aGeorgia Pacic 0.38 106 t / aNeste Resins 0.37 106 t / a

The three leading countrieswith a capacity shareof about 45% are:

United States 1.77 106 t/aGermany 1.46 106 t/aJapan 0.65 106 t/a

20 Formaldehyde

Table 11. Worldwide formaldehyde production capacities in 1996[1], [231]

Country Total capacity, 103 t/a

Western Europe 3119

Germany1464

Italy389

Spain265

United Kingdom197

France126

Sweden124

Netherlands115

Others439

Eastern Europe 1850North America 2008

United States1772

Canada236

South America 253

Mexico65

Chile63

Brazil48

Argentina44

Others33

Japan 651

Formaldehyde consumption is ca.6 106 t/a, although present data about capacityuse in Eastern Europe are not available. The de-mand and the estimated average annual growthrate in the Western hemisphere is summarizedin Table 12.Table 12.Consumption of formaldehyde in the United States, West-ern Europe, and Japan in 1995 [1], [231]

UnitedStates

WesternEurope

Japan

Consumption, 106 t/a 1.37 2.22 0.52Use, %

Urea formaldehyde resin32 50

27

Melamine formaldehyde resin4 6

Phenol formaldehyde resin24 10 6

Polyoxymethylenes10 10 24

1,4-Butanediol11 7

MDI5 6 4

Others14 11 39

Average annual growth rate, % 2.5 1.5 2.0

Formaldehyde and its associated products areused in ca. 50 different branches of industry, asdescribed in Chapter 8.

In the mid 1980s the sales of industrial prod-ucts derived from formaldehyde was more thanDM300 109 in the Federal Republic of Ger-many [132]. At least 3 106 people in the Fed-eral Republic of Germany work in factories thatuse products manufactured from formaldehyde.

10. Toxicology and OccupationalHealth

Formaldehyde toxicity was investigated exten-sively during the last decades and comprehen-sive reviews are available [233235]. Formalde-hyde is an essential intermediate in cellularmetabolism inmammals and humans, serving asa precursor for the biosynthesis of amino acids,purines and thymine. Exogenously administeredformaldehyde is readily metabolized by oxida-tion to formic acid or reacts with biomoleculesat the sites of rst contact. Inhalation exposureof rats, monkeys and humans to irritant con-centrations did not increase blood formaldehydelevels, which were found to be around 80mM(= 2.4 ppm) in these species.

Formaldehyde gas is toxic via inhalationand causes irritation of the eyes and themucous membranes of the respiratory tract.Concentration response relationship followinghuman exposure is given in Table 13.Table 13. Dose response relationship following human exposureto gaseous formaldehyde [133], [134]Effect Exposure level, ppm

Odor threshold 0.05 1.0Irritation threshold in eyes, 0.2 1.6nose, or throat

Stronger irritation of upper 3 6respiratory tract, coughing,lacrimation, extreme discomfortImmediate dyspnea, burning in 10 20nose and throat, heavy coughing andlacrimationNecrosis of mucous membranes, > 50laryngospasm, pulmonary edema

Aqueous formaldehyde solutions cause concen-tration dependent corrosion or irritation and skinsensitization.There is no evidence for Formalde-hyde to cause respiratory allergy [236].

Formaldehyde 21

In chronic inhalation studies with rats, mice,hamsters, and monkeys no systemic toxicityoccurred in irritant concentrations. Upper res-piratory tract irritation ceased at concentra-tions < ca. 1 ppm. At concentrations above1 2 ppm changes in the nasal mucosa (res-piratory epithelium) occur. At high concentra-tions (15 20 ppm) olfactory epithelium, laryn-geal mucosa, and proximal parts of the trachealepithelium might also be affected. The lesionsare characterized by epithelial hyperplasia andmetaplasia. Studies using other routes of admin-istration also failed to show systemic toxicity orreproductive effects.

Formaldehyde was genotoxic in several invitro test systems. In animals, there are someindications of in vivo genotoxicity in tissues ofinitial contact (portal of entry) but not in re-mote organs or tissues. In workers exposed toformaldehyde no systemic genotoxicity and noconvincing evidence of local genotoxicity wasfound.

No evidence of systemic carcinogenicitywasfound after oral dermal and inhalative adminis-tration of formaldehyde. Several chronic inhala-tion studies in rats showed development of nasaltumors starting at concentrations at or above6 ppm, causing in addition severe chronic epithe-lial damage in the nasal epithelium [237]. Thenonlinear concentration response curve shows adisproportionately high increase in tumor inci-dence at concentrations of 10 and 15 ppm. Thesame nonlinear concentration response was ob-served for DNA protein cross-link (DPX) for-mation in nasal mucosa, which is a surrogate offormaldehyde tissue dose, and for increase incell proliferation in nasal epithelium. This leadsto the suggestion that increased cell proliferationis a prerequisite for tumor development [237].Chronic inhalation studies inmice failed to showstatistically signicant increases in tumor inci-dences at similar concentrations while in ham-sters no nasal tumors were found. This may beattributed to differences in local formaldehydetissue dose or lower susceptibility of the speciesfor nasal tumor formation.

In humans numerous epidemiological stud-ies failed to give convincing evidence of car-cinogenicity [235]. IARC [234] concluded thatthe epidemiologic data available represent lim-ited evidence of carcinogenicity and classiedformaldehyde as probably carcinogenic to hu-

mans (Group 2A). The European Union cate-gorizes the compound as possibly carcinogenicto humans (Class 3).

Current occupational exposure limits in dif-ferent countries vary between 0.3 and 2 ppm[238]. Proposed limit values for indoor air arein the range of 0.1 ppm [239].

11. Low Molecular Mass Polymers

The ability of formaldehyde to reactwith itself toform polymers depends directly on the reactivityof formaldehyde as a whole. Two different typesof formaldehyde polymers are possible and arebased on the following structural elements:1) CH2O2) CH (OH)The polyhydroxyaldehydes consist of the struc-tural unit (2). The highest molecular mass re-presentatives of this group are the sugars. Al-though these substances can be made by aldolcondensation of formaldehyde, they do not re-vert to formaldehyde on cleavage, and are notdiscussed in this article.

The representatives of group (1), the realformaldehyde polymers (polyoxymethylenes),revert to formaldehyde on cleavage and, there-fore, can be considered as a solid, moisture-free form of formaldehyde. If these linear orcyclic compounds contain no more than eightformaldehyde units, they are dened as lowmolecular mass polymers. The high molec-ular mass substances are the real polymers(paraformaldehyde, acetal plastics, see alsoPolyoxymethylenes). Chemical and physi-cal analyses of these low and high molecularmass compounds as well as investigation oftheir chemical reactions led to the elucidation ofthe molecular structure of polymers in general[135].

11.1. Linear Polyoxymethylenes

Apart from the poly(oxymethylene) gly-cols, also called poly(oxymethylene) di-hydrates or simply polyoxymethylenes,of the formula HO(CH2O)nH, deriva-tives such as poly(oxymethylene) di-acetates CH3COO(CH2O)nCOCH3 and

22 Formaldehyde

poly(oxymethylene) dimethyl ethersCH3O(CH2O)nCH3 should be mentioned.Some of their physical properties are given inTable 14. The n values of the real low molecularmass polyoxymethylenes are 2 8; the n val-ues of paraformaldehyde are 8 100. However,high polymers with a degree of polymerizationn 3000 are also obtained. The polyoxymethy-lenes are also classied as , , , , and polymers which are of historical importance.They differ in their degrees of polymerizationand in their chemical structures (Table 15). Theirtoxicology is the same as that of formaldehyde(see Chap. 10).

The lower poly(oxymethylene) glycols arecolorless solids with melting points between 80and 120 C (Table 14). In contrast to the highmolecular mass materials, they dissolve in ace-tone and diethyl ether without or with only slightdecomposition; they are insoluble in petroleumether. When dissolved in warm water, they un-dergo hydrolysis to give a formaldehyde solu-tion. The low molecular mass polymers con-stitute a homologous series, whose propertieschange continuously with the degree of poly-merization.

A freshly prepared, aqueous formaldehydesolution polymerizes to the lower polymerswhen allowed to stand (see also Section 2.2). In-deed, formaldehyde exists in dilute solution asdihydroxymethylene (formaldehyde hydrate),which in turn undergoes polycondensation toyield low molecular mass poly(oxymethylene)glycols:

CH2O+H2OHOCH2OH+ nHOCH2OHHOCH2O(CH2O)nH+ nH2O

Equilibrium is attained under the inuenceof a hydrogen ion catalyst. At a low tempera-ture and a high concentration, equilibrium fa-vors formation of high molecular mass poly-mers. However, the major product is of lowermolecular mass when the system is heated. Thepolymers partially separate out, crystallize, andslowly undergo further condensation polymer-ization [140]. The low molecular mass sub-stances can be further precipitated and isolatedby concentrating the solution at low temperatureunder vacuum conditions; the polymers can befurther precipitated by evaporation [141]. Theresulting mixture can be separated into the in-

dividual substances by exploiting their varyingsolubilities in different solvents [135].

The transformation of poly(oxymethylene)dihydrates to diacetates and, above all, to di-ethers produces a remarkable increase in ther-mal and chemical stability. This is because theunstable hemiacetals at the ends of the chains areeliminated through saturation of the hydroxylgroups. The diethers are stable up to 270 Cin the absence of oxygen and up to 160 C inthe presence of oxygen. These diethers and di-acetates are resistant to hydrolysis under neu-tral conditions, the diethers are also stable inthe presence of alkali. Similar to the dihydrates,the properties of the diacetates and diethers alsochange continuously as the degree of polymer-ization increases (see Table 14).

Poly(oxymethylene) diacetates are producedby the reaction of paraformaldehyde with aceticanhydride [135]. Pure products are isolated byvacuumdistillation, solvent extraction, and crys-tallization.

The formation of poly(oxymethylene) di-methyl ethers involves the reaction of poly-(oxymethylene) glycols or paraformaldehydewith methanol at 150 180 C in the presence oftraces of sulfuric or hydrochloric acid in a closedvessel [135]. Alternatively, they can be synthe-sized by the reaction of formaldehyde dimethylacetal with either paraformaldehyde or a con-centrated formaldehyde solution in the presenceof sulfuric acid. This synthesis can be varied bysubstituting other formaldehyde dialkyl acetalsfor the dimethyl compound [142].

Paraformaldehyde [30525-89-4] wasrst produced in 1859. This polymer, atrst mistakenly called dioxymethylene andtrioxymethylene, consists of a mixture ofpoly(oxymethylene) glycols HO(CH2O)nHwith n = 8 100. The formaldehyde contentvaries between 90 and 99% depending on thedegree of polymerization n (the remainder isbound or free water). It is an industrially impor-tant linear polyoxymethylene.

Properties. Paraformaldehyde is a colorless,crystalline solid with the odor of monomericformaldehyde. It has the following phys-ical properties: fp 120 170 C, dependingon the degree of polymerization; heat ofcombustion 16.750 kJ/kg (product containing98wt% formaldehyde); energy of formation

Formaldehyde 23Table 14. Physical properties of low molecular mass poly(oxymethylene) glycols HO(CH2O)nH and their derivatives

n Poly(oxymethylene) Poly(oxymethylene) Poly(oxymethylene)glycols diacetates dimethyl ethers

fp, C Solubility in acetone fp, C bp, C , g/m3 fp, C bp, C , g/cm3(13 Pa,24 C)

(101.3 kPa) (25 C)

2 23 39 40 1.1283 82 85 Very soluble in the cold 13 60 62 1.158 69.7 105.0 0.95974 82 85 Very soluble in the cold 3 84 1.179 42.5 155.9 1.02425 95 105 Very soluble in the cold 7 102 104 1.195 9.8 201.8 1.0671

(decomp.)6 17 124 126 1.204 18.3 242.3 1.10037 Soluble in the cold ca. 15 180 1908 Soluble in the cold ca. 15 180 1909 115 120 Soluble when heated 32 34 1.216

(decomp.)

Value at 13 Pa and 36 C.

Table 15. Structure and synthesis of polyoxymethylenes [136]

Polymer Formula Synthesis

Paraformaldehyde HO (CH2O)nH from aqueous formaldehyde solution [137]n = 8 100

-Polyoxymethylene HO (CH2O)nH from aqueous formaldehyde solution [137]n> 100

-Polyoxymethylene HO (CH2O)nH by heating paraformaldehyde [138]n> 200

-Polyoxymethylene H3CO (CH2O)nCH3 from a methanolic paraformaldehyde solution in the presence ofn = 300 500 sulfuric acid [139]

-Polyoxymethylene H3CO [CH2OC(OH)HO]nCH3 by prolonged boiling of -polyoxymethylene with water [138]n = 150 170

-Polyoxymethylene HO (CH2O)nH by sublimation of 1,3,5-trioxane [138]n> 300

177 kJ/mol formaldehyde (product containing93wt% formaldehyde); ash point 71 C; ig-nition temperature of dust 370 410 C; lowerexplosion limit of dust 40 g/m3 (the last threevalues stronglydependon theparticle size);min-imum ignition energy 0.02 J.Table 16. Vapor pressure of formaldehyde (p) released fromparaformaldehyde

t, C 10 21 25 33 37p, kPa 0.112 0.165 0.193 0.408 0.667

t, C 43 47 51 58 65p, kPa 0.943 1.096 1.376 1.808 20.8

t, C 80 90 100 110 120p, kPa 32.8 44.1 49.6 53.5 78.3

Values up to 58 C according to [143] and from 65 120 Caccording to [144].

Even at ambient temperature, paraformalde-hyde slowly decomposes to gaseous formalde-hyde (Table 16), a process which is greatly ac-celerated by heating. Depolymerization is basedon a chain unzipping reaction which starts

at the hemiacetal end groups of the individualmolecules. The rate of decomposition thereforedepends on the number of end groups, i.e., onthe degree of polymerization.

Paraformaldehyde contains only a fewacetone-soluble components (lower diglycols).It dissolves slowly in cold water, but readily inwarm water where it undergoes hydrolysis anddepolymerization to give a formaldehyde solu-tion. Although this solution is generally cloudyas a result of impurities, these can be removed byltration. Indeed, the resulting solution is iden-tical with the solution obtained on dissolvinggaseous formaldehyde in water. Not only heat,but also dilute alkali or acid increase the sol-ubility of paraformaldehyde. Alkali catalyzesformaldehyde cleavage at the chain ends, acidcauses additional splitting at the oxygen bridges.The rate constant of dissolution passes througha minimum between pH 2 and 5, it increasesrapidly above and below this pH range [145].The situation becomes more complex at higherconcentrations of formaldehyde in the solution

24 Formaldehyde

due to the incipient back reaction. Solubilityin nonaqueous solvents is also improved in thepresence of acidic or alkaline agents as a resultof the onset of depolymerization. Paraformalde-hyde also dissolves in alcohols, phenols, andother polar solvents, in which depolymerizationand solvation can occur.

Production. Paraformaldehyde is preparedindustrially in continuously operated plants byconcentrating aqueous formaldehyde solutionsunder vacuum conditions. At rst, colloidal,waxy gels are obtained, which later become brit-tle. The use of a fractionating column throughwhich gases were passed dates from about 1925[146], [147].

Paraformaldehyde is currently produced inseveral steps which are carried out at low pres-sure and various temperatures [148], [149].Highly reactive formaldehyde is produced un-der vacuum conditions starting with solutionsthat contain 50 100 ppm of formic acid andalso 1 15 ppmofmetal formateswhere themet-als have an atomic number of 23 30 (e.g., Mn,Co, and Cu) [150]. The solutions are processedin thin-layer evaporators [151] and spray dryers[152].

Other techniques such as fractional conden-sation of the reaction gases in combination withthe formaldehyde synthesis process [153] andvery rapid cooling of the gases [154] are also ap-plied. Alternatively, formaldehyde-containinggas is brought into contact with paraformalde-hyde at a temperature that is above the dew pointof the gas and below the decomposition temper-ature of paraformaldehyde [155]. The productis obtained in the form of akes when a highlyconcentrated formaldehyde solution is pouredonto a heated metal surface. The hardened prod-uct is subsequently scraped off and thoroughlydried [156]. Paraformaldehyde beads are pro-duced by introducing a highly concentratedmeltinto a cooling liquid (e.g., benzene, toluene, cy-clohexane [157]. Acids [158] and alkalis [148],[159] are also added; they apparently accel-erate polymerization and lead to the forma-tion of higher molecular mass but less reactiveparaformaldehyde.

Highly soluble, highly reactiveparaformaldehyde with a low degree of poly-merization is very much in demand. It is pro-duced from concentrated, aqueous alcoholicformaldehyde solutions [160], [161].

Stabi