Investigation of thermal decomposition of phosphonic acids

Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

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Investigation of thermal decomposition of phosphonic acids T. Hoffmann ∗ , P. Friedel, C. Harnisch, L. Häußler, D. Pospiech Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069, Dresden e.V., Germany

a r t i c l e

i n f o

Article history: Received 15 August 2011 Received in revised form 10 February 2012 Accepted 3 March 2012 Available online 19 March 2012 Keywords: Phosphonic acid ATMP HEDP Thermal decomposition

a b s t r a c t The paper compares building and decomposition pathways of two phosphonic acids, amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP). The theoretical formation reactions were composed using elementary reactions and compared to reaction routes published in literature. As result, summary reaction pathways for both phosphonic acids are proposed which only vary in the number of reaction steps required. These reaction steps include carbonyl reactions, SN 2-reactions, and ionic reactions. The synthesis of ATMP proceeds in three reaction steps, whereas HEDP is formed in one reaction. The thermal decomposition of both phosphonic acids in solid state was examined by combination of thermogravimetry coupled with infrared spectroscopy as well as pyrolysis gas chromatography coupled with mass spectrometry. Decomposition mechanisms were deduced and compared to the theoretical findings resulting in the conclusion that the decomposition processes of ATMP and HEDP follows the formation mechanism. Thus, the suitability of theoretical considerations for the understanding of thermal decomposition processes is shown. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Thermal and hydrolytic stability of chemical structures are important properties that determine the application of materials. For example, phosphonates are hydrolytically more stable than phosphates due to their C P bond as compared to the C O P bond of phosphates [1]. Therefore, theoretical considerations to predict the thermal and hydrolytic stability of compounds and the comparison to experimental results can help to determine the application profile of materials. The work presented here aimed at understanding the solid state properties of phosphonic acids, in particular amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene1,1-diphosphonic acid (HEDP). The phosphonic acids used can be divided by their chemical structure as vicinal-substituted amino methylene phosphonic acid) and geminal diphosphonic acid. The chemical structure of both phosphonic acids illustrates Scheme 1. Whereas ATMP possesses three phosphonic acid groups and a central nitrogen atom, a central carbon atom and two phosphonic acid groups characterize the chemical structure of HEDP. In HEDP, the phosphonic acid groups are linked directly to the central carbon atom, together with a hydroxyl and a methyl group. Therefore, different thermal decomposition pathways may be expected, which will be followed by the combination of thermogravimetry coupled with infrared spectroscopy (TGA/FTIR) as well as pyrolysis gas

∗ Corresponding author. Tel.: +49 351 4658 1212; fax: +49 351 4658 290. E-mail address: [email protected] (T. Hoffmann). 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2012.03.001

chromatography coupled with mass spectrometry (pyrolysis GC–MS) and compared to theoretical considerations. In addition, the effect of nitrogen in ATMP (P C N) in comparison with the P C P bond in HEDP was evaluated with respect to the application as flame retardant. Both phosphonic acids have good chemical stability. ATMP is a stronger complexing agent for divalent cations than HEDP [2]. Thus, they can be applied in aqueous solution and oilfield water pipelines to control the scale formation and to inhibit corrosion of metal equipment and pipelines [3]. The hydrolytic stability of organophosphonic acids was studied exemplarily in aqueous solution of ethylene diamine tetra (methylene phosphonic acid) (EDTMP) [4]. Similar results were found by Kaslina et al., which examined aqueous solutions of ATMP at elevated temperature (T > 423 K) and different pH-values [5]. They described the decomposition of ATMP via ABMP (amino bimethylene phosphonic acid) and AMMP (amino monomethylene phosphonic acid) to hydroxymethyl phosphonic acid, ammonia and phosphoric acid. Depending on the pH-value the decomposition of ATMP runs either via P C or N C bond scission [5]. While the behavior in aqueous solutions is well characterized, the knowledge about solid-state properties is rather weak but necessary to evaluate applications as flame retardant additives. For example, aluminum salts of such phosphonic acids alone [6] or in combination with pentaerythritol can be used as flame retardants in polymers [7]. This paper presents the thermal analysis of amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene-1,1diphosphonic acid (HEDP) under inert atmosphere and describes

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T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

(a)

HO O

OH P

P N

HO

O

+ R1 δ+ δδ R5 δ R3 + H Y C O + H X R6 R2 R4

R1

R3

R5

X C Y R4

R2

+ H2O R6

OH ATMP: R1 = R2 = H, X = N, Y = PO, R3 = R4 = C-R1-R2-Y(R5, R6), R5 = R6 = OH

P O

OH OH

amino tris (methylene phosphonic acid) (ATMP)

+ R1 δ+ δδ R5 C O+ 2H Y R2 R6

R3

R1

R5

Y C Y R4

R2

+ H 2O R6

HEDP: R1 = CH3, R2 = OH, Y = PO, R3 = R4 = R5 = R6 = OH

(b) O HO

P

H 3C HO

Scheme 2. Summary reaction pathway for the synthesis of ATMP and HEDP.

OH OH

P

OH

O 1-hydroxyethane-(1,1-diphosphonic acid) (HEDP) Scheme 1. Chemical structure of ATMP (a) and HEDP (b).

their thermal stability and gaseous decomposition products. The goal was to establish thermal decomposition pathways for both phosphonic acids and to compare these pathways with the corresponding formation reactions. Furthermore, theoretical formation mechanisms were proposed based on the respective starting materials used. The experimental and theoretical results were compared.

2. Experimental 2.1. Materials The phosphonic acids were synthesized according to refs. [8,9], were kindly supplied by Zschimmer & Schwarz Mohsdorf GmbH & Co KG (Burgstädt, Germany) and used as received. The chemical structures were characterized by NMR spectroscopy and the phosphorus content was determined and compared to the phosphorus content calculated from the chemical structure (Scheme 1): ATMP: 1 H NMR, duplet at 3.47 ppm; 31 P NMR, singlet at 9.24 ppm; P (calc.): 31.10 wt.%, P (exp.): 31.15 wt.% HEDP: 1 H NMR, triplet at 1.31 ppm; 31 P NMR, quadruplet at 20.59 ppm; P (calc.): 30.09 wt.%, P (exp.): 30.54 wt.%

2.2.3. Thermogravimetric analysis coupled with FTIR spectroscopy (TGA–FTIR) The thermal decomposition was followed by thermogravimetric analysis (TGA) using a TGA Q5000 (TA Instruments, USA) in nitrogen atmosphere (25 mL min−1 ) from 313 K to 1023 K at a scan rate of 10 K min−1 . The evolution of the gaseous products was studied by coupling the TGA Q5000 with the FTIR-spectrometer Nicolet 380 (Thermo Electron, USA). The information resulting from the TGA/FTIR study was represented by means of Gram–Schmidt curves. 2.2.4. Pyrolysis gas chromatography coupled with mass spectrometry (pyrolysis GC–MS) Pyrolysis experiments were performed in a Pyroprobe 5000 pyrolyzer (CDS Analytical, Inc., USA) using platinum filament pyrolysis instrument equipped with a coil probe for pyrolysis of samples in quartz tubes. The pyrolyzer was coupled with a GC–MS setup (GC7890A and MSD 5975C, Agilent Technologies) in which the MS detector was applied in the electron impact mode at an electron energy of 70 eV with the electron source being kept at about 503 K. The pyrolysis temperature was 773 K being hold for 10 s and achieved from 373 K at a heating rate of 10 K ms−1 . The pyrolysis products were separated on a HP-5MS capillary column (Agilent Technologies, Inc., USA), dimensions: length of 30 m, an inner diameter of 0.25 mm and a film thickness of 0.25 ␮m under Helium flow of 1 ml/min. The heating program of the column was 2 min at 313 K, temperature increase to 573 K at a rate of 10 K min−1 and 10 min at 553 K. The injector temperature was 523 K. MS–detector worked in the mass range of 15–550 amu (unified atomic mass unit). MS identification was carried out using a NIST library, or if the spectrum was not included in the library by constructing the ion decomposition pattern for the best fit with the mass spectrum. The pyrolysis temperatures were performed stepwise at different temperatures on the phosphonic acids as determined by TGA.

2.2. Methods 3. Results and discussion 2.2.1. NMR spectroscopy The chemical structure was characterized by 1 H and 31 P NMR spectroscopy. NMR spectroscopy was carried out on a DRX 500 spectrometer (Bruker, Germany) operating at 500.13 MHz for 1 H and 202.40 MHz for 31 P using D2 O as solvent. The spectra were referenced to the solvent signal (ı(1 H) = 4.78 ppm) and external phosphoric acid (ı(31 P) = 0 ppm), respectively.

2.2.2. Quantification of phosphorus content The analytical determination of the phosphorus content was performed by Mikroanalytisches Labor Kolbe (Mülheim a. d. Ruhr, Germany) by means of digestion in sodium peroxide and precipitation with ammonium heptamolybdate.

3.1. General considerations about the formation reactions The first theoretical consideration assumes that the summary reaction pathway in Scheme 2 is valid both for the synthesis of ATMP as well as for the synthesis of HEDP according to literature [8,9]. The synthesis of ATMP proceeds by a Mannich-type reaction using phosphonic acid, formaldehyde, and ammonia as reactants [8]. HEDP was synthesized using acetic acid, water and PCl3 [9]. The substituents R1 –R6 , X, and Y shown in Scheme 2 represent chemical groups of the phosphonic acids. A different number of formation reaction steps are necessary for the two different phosphonic acids. Whereas one reaction step leads directly to HEDP, three reaction steps are the precondition

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

45

Table 1 Substituents in the summary reaction pathway to ATMP and HEDP. Substituents

AMMPa ABMPb ATMP HEDP a b

R1

R2

X

R3

R4

Y

H H H CH3

H H H OH

N N N P O

H H CH2 OH

H CH2 CH2 OH

P P P P

O O O O

R5

R6

OH OH OH OH

OH OH OH OH

1st step 2nd step 3rd step –

AMMP: amino monomethylene phosphonic acid = end product of the first reaction step on the way to ABMP (amino bimethylene phosphonic acid). ABMP: amino bimethylene phosphonic acid = end product of the second reaction step on the way to ATMP (amino trimethylene phosphonic acid).

+ H

Q

Q'

AMMP

+

H

described in Schemes 4–6 and represent carbonyl reactions, SN 2 reactions, and fast ionic reactions. Within the carbonyl and the SN 2 reactions of ATMP and HEDP rearrangement reactions can occur that are marked by (‘). The formation of the intermediate (G) (hydroxymethane phosphonic acid) within the reaction pathway to ATMP (Scheme 4) was already proven during the investigation of the hydrolytic stability of organophosphonic acids [4]. The intermediate (Q) in Scheme 6 represents amino methyl phosphonic acid (AMMP) as the end product in the first reaction step towards ATMP, as shown in Scheme 3. The fast ionic reactions in Scheme 6 can not only lead to the desired final product, but also to by-products (intermediates (P) and (R)). Another side reaction represents the rearrangement of the end product (Q) to the intermediates (R) and (P). The elementary reactions 6 and 8 in Scheme 5 represent parallel reactions resulting from intermediates (I) and (K). The formation of HEDP proceeds in one reaction step with 11 elementary reactions, in which 16 intermediates are involved, as shown in Scheme 7. Scheme 8 describes an alternative synthesis route to HEDP leading to a side product (intermediate (Q)) by dehydration of dihydroxyethane phosphonic acid (intermediate (C)). A rearrangement reaction, as described in the reference [10] can also occur in HEDP. If the intermediate (C) = (G) in Scheme 8 is rearranged according this mechanism, acetyl phosphate arises and therefore, a transformation of a phosphonate to a phosphate and an increase of the oxidation state of the phosphorus. The phosphoric acid appearing in the proposed decomposition pathway of HEDP could be also explained with that. From this theoretical approach it can be concluded that the formation reaction of ATMP proceeds in three steps and HEDP was formed directly in one reaction step. These reaction steps are classified by elementary reactions that are consisting of carboxylic, SN 2,

Q'' ATMP

ABMP

Scheme 3. Constitution reaction of ATMP via AMMP and ABMP using the reaction between (H) and (Q) as well as (Q ).

for the synthesis of ATMP. Table 1 represents the substituents and reaction steps necessary for the formation of ATMP and HEDP. For the synthesis of ATMP three reaction steps are necessary and each reaction step contains 14 elementary reactions with 18 intermediates. Each reaction step includes carbonyl reactions, SN 2 reactions, and fast ionic reactions. These elementary reactions needed for the first reaction step to ATMP and for the reaction step to HEDP are arranged in a matrix, as shown in Table 2. Intermediates, shown as letters A to R which are consumed are assigned as (−1) and those are formed as (+1), respectively. In the first two reaction steps amino mono (methylene phosphonic acid) (AMMP) as well as amino bis (methylene phosphonic acid) (ABMP) as final products were formed by reaction of the intermediates (H) and (N) to AMMP (Q), as well as (Q) the end product of the first reaction step with the intermediate (H) to ABMP (Q ), the end product of the second reaction step. The formation of ABMP was also reported during the decomposition of aqueous solution of ATMP [3]. ATMP was finally formed in the third reaction step by the reaction of (Q ) with (H), as shown in Scheme 3. Due to of the comparability to the first reaction step the following two steps to ATMP via ABMP were not considered, since both reactions follow the same mechanism. As mentioned before, the synthesis route for ATMP follows three reaction steps, whereas HEDP needs only one reaction step. In case of ATMP, the elementary reactions in each reaction step are Table 2 Elementary reactions in the formation mechanism of ATMP and HEDP (grey marked). Elementary reactions

A B C D E F G H I J K L M N O P Q R

1

2

3

4

5

6

7

8

9

10

11

12

13

14

−1 −1 +1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 −1 +1 +1 0 0 0 0 0 0 0 0 0 0 0 0 0

−1 0 0 0 0 −1 +1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 +1 0 −1 +1 0 0 0 0 0 0 0 0 0 0

0 0 −1 0 0 −1 0 0 +1 0 0 0 0 0 0 0 0 0

0 0 −1 0 0 −1 0 0 0 +1 0 0 0 0 0 0 0 0

0 −1 0 0 0 0 −1 0 0 0 +1 0 0 0 0 0 0 0

0 −1 0 0 0 0 −1 0 0 0 0 +1 0 0 0 0 0 0

0 −1 0 0 −1 0 0 0 0 0 0 0 +1 +1 0 0 0 0

0 0 0 0 −1 −1 0 0 0 0 0 0 +1 0 +1 0 0 0

0 0 0 −1 0 0 0 0 0 0 0 0 0 −1 0 +1 0 0

0 0 0 −1 0 0 0 0 0 0 0 0 0 0 −1 0 +1 0

0 0 0 0 0 0 0 −1 0 0 0 0 0 −1 0 0 +1 0

0 0 0 0 0 0 0 −1 0 0 0 0 0 0 −1 0 0 +1

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T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

H

H δ+ δ− δ+ δ− H C O + H N H H

1

H

H

2

HO C N H

formaldehyde

ammonia

H methanolamine

A

B

C

H

C N

+

OH

H

H

H'

E

2' H

H

H

2'

H C O N H

H

H

H C

O N

+

H

H

methyl hydroxylamine D'

C'

H δ+ δ− δ− δ+ OH C O + H P O OH H

H 3

formaldehyde phosphonic acid A

H C

O

H

E''

H

OH P O OH

4'

H C

OH +

O

P O

H

OH

D

E'

methoxy phosphonic acid G'

F 4

H HO C

OH P O OH

H 4

H hydroxymethane phosphonic acid

C H

OH P O + OH

H

G

OH

E

Scheme 4. Carbonyl reactions within the ATMP reaction scheme.

and ionic reactions. During these elementary reactions intermediates arise that are representative for the respective phosphonic acid.

3.2. Characterization of the thermal decomposition 3.2.1. Thermogravimetric analysis Thermogravimetric analysis (TGA) indicated 3 and 6 decomposition steps for ATMP and HEDP, respectively. However, only 2 steps for ATMP and 3 steps for HEDP with a mass loss of >∼15 wt.% are important for the identification of the main decomposition products, as shown from Table 3. Fig. 1 illustrates the thermal decomposition of HEDP and ATMP followed by TGA. The decomposition curves of ATMP and HEDP differ significantly from each other, even if the temperature ranges of the major decomposition processes are comparable. Both samples show the

main weight loss at around 486 K and at 780 K. The main step of weight loss was found at 785 K for ATMP and 773 K for HEDP. Table 3 summarizes the thermal decomposition behavior of ATMP and HEDP in nitrogen atmosphere. The bold-marked steps of weight loss are the main decomposition steps. The gases evolved at these steps were investigated by FTIR spectroscopy and pyrolysis GC–MS. The formation of a non-combustible residue at 1023 K occurs in both phosphonic acids. ATMP forms nearly twice as much residue than HEDP which can be discussed in terms of both a higher thermal stability of N C P bonds in ATMP than of P C P bonds in HEDP and in terms of the ability to form aromatic condensed decomposition products. The phosphorus content in the residue after TGA is significantly lower (ATMP: 2 wt.%; HEDP: 3 wt.%) than in the starting compounds (ATMP: 11.4 wt.%, HEDP: 30.5 wt.%). This means, in both phosphonic acids the major part of phosphorus has left as gaseous decomposition products and only the minor part remains in the residue.

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

H

H

N C OH + H H

H

methanolamine

H

H OH N C P O H H OH OH H

5

H OH P O HO C OH H N H H H

6

phosphonic acid

C

HO O P HO

OH P O OH

47

F

H

I

H

C OH + H N

7

H

H

hydroxymethane phosphonic acid G

J

H H HO H O P C N HO H OH H

ammonia

H N HO C H H P OH HO O

aminomethyl phosphonic acid K=Q

B

H

H

8

L

Scheme 5. SN 2 reactions within the ATMP-reaction scheme.

3.2.2. FTIR study of volatile decomposition products The gaseous decomposition products of ATMP and HEDP were identified by FTIR spectroscopy at the corresponding steps of weight loss found by TGA. Gram–Schmidt curves are used to show information resulting from TGA–FTIR study and arise by integration of the spectra over the entire wave length region. Therefore, Gram–Schmidt curves represent the absorption behavior of the volatile decomposition products as a function of the temperature or the time and describe consequently the change of the concentration of the gases evolved. Chemigrams were obtained by integration of the wave length regions that are characteristic for the functional groups and by plotting as function of the time. The FTIR chemigrams of the gaseous decomposition products of ATMP and HEDP reflect the chronological sequence of the IR spectra of the decomposition products in selected wave length regions [11]. Fig. 1 summarizes the course of the weight loss for ATMP and HEDP and the absorption behavior of the resulting volatile decomposition products represented by the respective Gram–Schmidt curves. The Gram–Schmidt curves of both phosphonic acids indicate a multi-step decomposition (Fig. 1). For ATMP, the first absorption maximum of the Gram–Schmidt curve at 19 min (489 K) represents water (3852 cm−1 ). The second absorption maximum at 46 min (785 K) are CH2 /CH3 compounds = mixture of small

compounds which contain methylene and methyl groups (2900 cm−1 ), smaller amounts of ammonia (965 cm−1 ) and weak traces of carbon dioxide (668 cm−1 ). In the broad and slow increase at 30 min (579 K) with low absorption intensity ammonia (965 cm−1 ) and CH2 /CH3 compounds (2900 cm−1 ) were proven (Fig. 2). In addition, phosphorus-containing decomposition products arise within the second absorption maximum at 46 min and the broad and slow increase at 30 min. Absorption bands at 1300 and 1225 cm−1 correspond to P O fragments. Furthermore, the occurrence of bands at 929 cm−1 and 850 cm−1 can be ascribed to the formation of P O and P C fragments. The absorption maximum of the Gram–Schmidt curve for HEDP at 19 min (483 K) characterizes mainly acetic acid (1796 cm−1 ), CH2 /CH3 compounds (2900 cm−1 ) and small amounts of water (3852 cm−1 ). The first area with low absorption intensity at 14 min (423 K) indicates only water (3852 cm−1 ) and the second area at 46 min (773 K) stands only for CH2 /CH3 compounds (2900 cm−1 ). The presence of small CH2 /CH3 compounds could indicate the formation of acetaldehyde which was analyzed by pyrolysis GC–MS. Absorption bands at 1200 cm−1 interpreted as phosphate were found at 483 and 773 K. By the FTIR analysis absorption bands (at 1300, 1225, 929, and 850 cm−1 in ATMP as well as at 1200 cm−1 in HEDP) are found

Table 3 Thermal decomposition behavior of ATMP and HEDP and characterization of the phosphorus contents in the initial state and in the residue after TGA. Phosphonic acids

TDTG (K)

Tend (K)

Weight loss (wt.%)

ATMP

489 579 698 785 888c

548 598 715 863

7.5 2.2 14.9 55.7 7.2

HEDP

393 423 463 483 503 593 773 808

408 443 467 497 551 615 786 1023

1.6 4.4 3.0 15.3 5.0 2.4 40.4 20.9

a b c

Performed by Mikroanalytisches Labor Kolbe, Mülheim, Germany. Related to the content of phosphorus in the initial phosphonic acid. Shoulder was detected.

Residue at 1023 K (wt.%)

Steps (Fig. 1)

Content of phosphorus (wt.%)a Virgin

Residue

31.15

11.43 2.15b

29.88

30.54 3.00b

1 12.4

2 3 1 2

6.4

3 4 5 6

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T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

N C H

H2O + M

H

N

N

H

OH

H

O P

C

HO

H

HO

H

HO 13 fast

H

O

P

14

O OH O

fast

HO O P HO

Intensity (a.u.)

H

H

C N H

H

H C H

P

0,3 0,0 10

20

30

40

50

60

OH O OH

R methane diphosphonic acid (side reaction)

Scheme 6. Fast ionic reactions within the ATMP-reaction scheme.

which were assigned to phosphorus-containing volatile products, according to the references [12,13]. It is supposed that these phosphorus-containing fragments represent phosphine oxide and phosphate which was proved also in the pyrolysis GC–MS of both phosphonic acids (compare with Schemes 9 and 10). The presence of CH2 /CH3 compounds (2900 cm−1 ) could indicate also the formation of acetaldehyde in the case of HEDP which was analyzed by pyrolysis GC–MS as well. In addition, a P O fragment (at 929 cm−1 ) also was indicated in the diammonium salt of ATMP ((NH4 )2 ATMP). It should be noted that this P O fragment also was detected during the thermal decomposition of alumina trisphosphinate, a compound that is used as commercial flame retardant (Exolit OP 1230) in polyamides or polyesters [12]. 3.2.3. Pyrolysis GC–MS Pyrolysis GC–MS is used to determine the gaseous decomposition products of both phosphonic acids at temperatures corresponding to the main decomposition steps detected by TGA. Fig. 3 shows the pyrogram for ATMP (a) and HEDP (b). The pyrograms of the phosphonic acids with the decomposition products of the subsequent steps and the main step of weight loss are shown in Fig. 3(a) and (b). The gaseous

70

Time (min)

1,00

3

3

ATMP HEDP

0,50

1 0,00 300

500

DTG

2

4 1

2

400

5 6 600

700

800

900

1000

Temperature (K) TG

ATMP HEDP

80 60 40 20 0 300

Q = AMMP

OH +

P

HO

N

H H

H

amino monomethylene phosphonic acid Q = AMMP

H

H

C

fast

0,6

100

OH N C P O OH H H

12

N

+

H

H

H

O

HO

H

methane diamine P

P O OH

+

H

Gram-Schmidt

ATMP HEDP

0

N C N

fast

H

D

P

11

H

+

H

HO

H

H

D

N C

OH P O OH O

10

F

H

H

H N

M

H

H

N

Deriv. Weight (%/K)

H

OH H P O OH + OH

O

H2O +

B

E

H

Weight (%)

E

0,9

9

400

500

600

700

800

900

1000

Temperature (K) Fig. 1. Thermal decomposition of ATMP and HEDP followed by TGA and TGA–FTIR.

compounds evolved were identified by MS and the peak assignments are summarized in Table 3. Typical gaseous decomposition products of ATMP at 812 K as outlined in Fig. 3(a) are trimethylamine (peak 1), trimethylphosphine oxide (peak 2), tetramethylpyrrole (peak 4), 4-dimethylaminophenol (peak 5), and multiple substituted benzene compounds (peaks 6 and 7). It was assumed that these compounds could be formed from intermediate products (P) and (R), and from final product (Q), as shown in the reaction pathway in Scheme 6. The total ion chromatogram for the first step of weight loss of HEDP indicates only water as decomposition product (Fig. 3(b)). The comparison of the results obtained by TGA–FTIR and pyrolysis GC–MS enables to assign main decomposition products to the important steps of weight loss. As shown for ATMP in Table 4, the weight loss at 495 K is caused by water, and in the main step of

300

400

500

Temperature (K) 600 700

800

900

1000

7,0 CH /CH compounds 2 3 0,0 0,01 Intensity (a.u.)

H OH + H N

CO2

0,00 0,2

CH3COOH

0,0 0,1

H2O

0,0 0,2

ATMP HEDP

NH3

0,0

0

10

20

30 Time (min)

40

50

60

Fig. 2. Release rate of volatile products of ATMP at T = 525 K and 837 K (a) and HEDP at T = 525 K (850 K) (b) as a function of time as detected by FTIR spectroscopy.

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

49

(a) H3C δ+ δ− δ− δ+OH C O + H P O HO OH

H

1

HO C O H3C

acetaldehyde phosphonic acid A

OH P O OH

OH

H

2

HO C

+

O

P O

H3C

OH

hydroxy ethoxy phosphonic acid D=H

C=G

B=F

E'

2' CH3

HO O

P

HO

2'

C OH

HO O

P C

HO

OH

CH3 +

OH

OH

1,1'-ethanediol phosphonic acid D' = H'

C' = G'

E

(b) HO O

P

HO

OH

CH3 C OH + H OH

1,1'-ethanediol phosphonic acid

P

CH3 H OH HO H P O O P C HO HO OH OH

3

O

OH

4

CH3 H OH P O HO C HO P O OH HO O H

phosphonic acid

C=G

B=F

I=K

J=L

(c) OH OH + H

P O

5 H2 O +

OH E HO O

B=F CH3

P C

HO

OH

D' = H'

+

M

OH P O OH N=O

OH P O OH

CH3 OH HO O P C P O fast HO OH OH 1,1'-hydroxyethane diphosphonic acid (HEDP)

N=O

P

6

Scheme 7. Reaction scheme toward HEDP including carbonyl reactions (a), SN 2 reactions (b), and fast ionic reactions (c).

weight loss at 812 K ammonia and 4-dimethylaminophenol were evolved. The letter (a) in Table 4 corresponds to Schemes 4–6). In HEDP, the first step of weight loss at 491 K characterizes the release of water and acetic acid. In the main step of weight loss at 808 K acetaldehyde as well as acetic acid and phosphorus (P4 ) were identified by pyrolysis GC–MS [14]. The mass spectrum of phosphorus released from the pyrolysis of HEDP at 808 K is represented in Fig. 4. The letter (b) in Table 4 corresponds to Schemes 7 and 8.

Characteristic intermediate products proposed by the theoretical approach were compared to products that are verified by thermal analysis and confirmed intermediate products are summarized and highlighted in Table 5. Intermediate products which are characteristic for the constitution reaction for ATMP and HEDP are shown in Table 5 and compared to products identified during the thermal analysis of the decomposition processes. This comparison leads to the conclusion,

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T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

CH3 H H H OH C O P P O H OH HO OH

CH3 H OH P O HO C HO P O OH HO O H

HO

C OH

HO

HO -H2O

O

OH

P

CH3 C

HO

O

CH3 OH HO O P C P O HO OH OH 1,1'-hydroxyethane diphosphonic acid (HEDP) P

CH3

P

P

-H2O

J=L HO

O

CH3

1,1'-ethanediol phosphonic acid acetyl phosphonic acid C=G Q

I=K

O

HO -H2O

CH3

HO

C OH

O

OH

C

P O

rearrangement of 1,1’-ethanediol phosphonic acid to acetyl phosphate [9]

O + 2H

HO

1,1'-ethanediol phosphonic acid C=G

acetyl phosphate R

Scheme 8. Dehydration as an alternative synthesis route to HEDP.

that educts of the formation reactions can be identified also during the thermal decomposition of both phosphonic acids. Based on the findings of our calculations as well as the thermal analysis of the decomposition of ATMP and HEDP, models for the decomposition pathways were derived and compared to products proven by TGA–FTIR (bold framed) and pyrolysis GC–MS (normally framed). The thermal decomposition of ATMP is characterized by intramolecular elimination of water as well as by scission of carbon–phosphorus- as well as nitrogen–carbon-bonds leading to the formation of orthophosphate or hydroxymethyl phosphonate and to the gaseous by-products water, ammonia (identified by TGA–FTIR) and trimethylamine trimethylphosphine oxide and substituted pyrrole as a possible indication for the presence of

tetramethylpyrrole (identified by pyrolysis GC–MS). Trimethylamine occurs during decomposition of ATMP by scission of P C bonds in amino bis(methylene phosphonic acid) (ABMP), a typical intermediate of ATMP. Methylphosphine from ATMP and phosphine from HEDP are formed in the same manner by reduction and change in the oxidation state. The formation of tetramethylpyrrole occurred only during decomposition of ATMP and is based on the dipolar cycloaddition of azomethine ylides [15]. Nevertheless, pyrrole could not be detected. The reaction of the tetramethylpyrrole with water can serve as explanation for the formation of 4-dimethylaminophenol by rearrangement during the thermal decomposition of ATMP. Thus, it can be concluded that the thermal decomposition of ATMP starts either by disproportionation reaction (N C bond

Table 4 Identification of the evolved gases of ATMP (a) and HEDP (b) by pyrolysis GC–MS at different steps of weight loss (Fig. 3). Phosphonic acid

(a) ATMP

(b) HEDP

T (K)

Identified compounds/Ret.time (min)

495 711 812 491 808

Predicted by simulation Phosphonic acid

(a) ATMP

(b) HEDP Predicted by simulation a b

T (K)

495 711 812 491 808

Water

Phosphine

Acetaldehyde (3) 1.57

Methyl phosphine (4) 1.52

(1) 1.84/0.42/1.66/1.57

(2) 1.51

+ + − + + +

− − − − + +

− − − − + +

− + − − − +

Subst. pyrrolea

Phosphorus (P4 )

(9) 2.09

Trimethyl phosphine oxide (10) 6.47/6.10

(11) 7.98/8.22/8.72

− − + − − −

− + + − − +

− +b +b − − +

1-Hexene

2-Propane amine) (5) 1.68

Trimethyl amine (6) 1.61

Acetic Acid (7) 2.27/1.98

Trimethyl phosphine (8) 1.78

− + − − − +

− − + − − +

− − − + + +

− + − − − +

(12) 7.58

Triethyl phosphine oxide (13) 9.96

Dimethyl amino phenol (14) 9.78/9.75

Pentamethyl benzene (15) 10.67

Hexamethyl benzene (16) 12.64

− − − − + −

− − − − + +

− + + − − (+)

− − + − − −

− − + − − −

Structural isomers of tetramethylpyrrole. Within the thermal decomposition of ATMP several peaks with the molar mass of 123 g mol−1 were detected, who are not distinguishable from the mass spectrum.

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

51

N-C bond scission: HO

OH

O P HO

III OH

3H 3 H3C

P O N

O

OH

P

disproportionation

O

HO

O +

P

P OH OH

+ P + 1/2 O2 + HO CH2 CH3 H3C

+ 1/2 O2

-

O

H3C

P

CH3

O

CH3

trimethylphosphine

1) HO

O

CH3

CH3 H3C

P

OH dimethyl phosphine hydroxymethane phosphonic ac

P

trimethylphosphine oxide

OH

P N

6H

OH

+ 4 H 2O

3 HO

P

O + N CH3 CH3

OH

P O

CH3

OH

P O

H 2O O

H

OH

C N

2

1)

NH3

OH

OH methyl phosphonic acid

V OH

-III H

V OH

This structure has been indicated only exemplarily. Intermolecular condensation to polyphosphoric acid is possible.

C

azomethine ylide

+

(dipolaric cyclo addition)

N

N

H

H pyrrole

tetramethylpyrrole

OH +

H2O + 2 H2

(rearrangement)

N H

N

tetramethylpyrrole

4-dimethylaminophenol

P-C bond scission: OH HO

OH

O P HO

O

O

P O N

H3C P

OH

O +

HO O

P OH OH

P O N

+3

OH

HO HO P

CH3

H2O

N CH3

+ 3

CH3

P OH OH

trimethyl amine

methylamino bis (methylene phosphonic acid)

O

HO

orthophosphate/ phosphoric acid

Scheme 9. Proposed decomposition pathway of ATMP starting via N C or P C bond scission by means of thermoanalytic results (bold framed: proven by TGA–FTIR; normally framed: proven by pyrolysis GC–MS; dotted framed: on the basis of TGA–FTIR possibly).

Table 5 Products identified by thermal analysis (bold) compared with intermediate products proposed by the theoretical formation reaction of ATMP and HEDP. SN 2 reactions (Scheme 4)

Carbonyl reactions (Scheme 3)

ATMP

Formaldehyde Ammonia Phosphonic acid

A B F

Methanolamine Hydroxy methane phosphonic acid

C G

Amino monomethylen phosphonic acida – Methanol amine (NH3 + HCHO) Hydroxy methane phosphonic acid – SN 2 reactions (Scheme 6b)

Carbonyl reactions (Scheme 6a)

HEDP

a b c

Acetic acid 1,1 -Ethanediol phosphonic acidc Phosphonic acid Acetyl phosphonic acid

Ionic reactions (Scheme 5)

A C B D

Phosphonic acid – – –

By amination of the hydroxyl group. Side product is not stable and decomposes to methyl phosphonic acid (re-elimination of R). Not stable, decompose in acetaldehyde and water.

K – C

Amino monomethylen phosphonic acida Diamino methane –

K P –

G

– Methane diphosphonic acidb

–

onic reactions (Scheme 6c) B – – –

Acetyl phosphonic acid Hydroxymethane diphosphonic acid – –

D P – –

52

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

P-C bond scission: O HO P OH H3C

H+

OH

H3C

CH2

HO P OH O

V OH

III OH

OH

P O + HO

P O

OH

OH

disproportionation

2 HO

PH3 + 3 H3C CH2

P O

+ 1/2 O2

P O + PH3 + 1/2 O2 OH

phosphoric acid

CH3 OH

-III

CH2 H3C CH2

OH

phosphine

OH

P O + 3H

P O

CH2

OH

CH3 triethyl phosphine oxide

phosphine

O HO P OH H3C

O HO P

- H2O

OH

H3C

HO P OH O

OH

O O

4 H2O H3C

HO P OH O

C

+ 2 HO

OH

P O + 4 H+ OH

acetic acid

phosphoric acid

Scheme 10. Proposed decomposition pathway of HEDP starting via P C bond scission by means of thermoanalytic results (bold framed: proven by TGA–FTIR; normally framed: proven by pyrolysis GC–MS; dotted framed: on the basis of TGA–FTIR possibly).

a

7

5x10

6 14

7

4x10

Abundance

at 812 K

14

10

11

9

15

16

7

3x10

4 5

7

2x10

at 711 K

11

8 10

1 7

1x10

at 495 K 1

0 0

2

b 8,0x106

4

8

10

12

3

2

6

12

14

at 808 K

scission) or by intramolecular separation of water (P C bond scission), as illustrated in Scheme 9. Furthermore, besides the formation of cyclic P O P containing products by intramolecular separation of water the formation of linear P O P containing products is possible based on the condensation mechanism of phosphoric acid to polyphosphate leading to cross-linked P O P compounds. Additionally, phosphorus was proved in the residue after the thermal decomposition of ATMP and HEDP. The intermediate products highlighted in Schemes 9 and 10 were proven by pyrolysis GC–MS and the bold ones by TGA–FTIR. The products with dotted frames represent phosphorus-containing products which can be assumed regarding absorption bands of P O (1300/1225 cm−1 ) and P O fragments (929 cm−1 ). The thermal decomposition of HEDP differs from the decomposition mechanism of ATMP. Deviating from the described mechanism acetic acid and diethyl phosphine oxide are formed (Scheme 10).

7 1

P4 13 6

at 491 K 2

5

5,0x10

P2

0,0 0

2

4

6 8 Time (min)

10

12

P

14

Fig. 3. Pyrograms for ATMP (a) at the subsequent (T = 495 K, 711 K) and at the main step (T = 812 K) of weight loss and for HEDP (b), at the subsequent (T = 491 K) and at the main step (T = 808 K) of weight loss, respectively.

31.0

1

P3

92.9

6

2,0x10

6

1,0x10

62.0

4,0x10

Abundance

Abundance

6

1,5x10

123.9

6

6,0x10

0,0 0

30

60

90

120

m/z Fig. 4. Mass spectrum showing phosphorus (P4 ) released by the pyrolysis of HEDP at 808 K (peak 4 in Fig. 3(b)).

T. Hoffmann et al. / Journal of Analytical and Applied Pyrolysis 96 (2012) 43–53

It should be noted that the temperature conditions during the formation of the phosphonic acids (T ∼ 390 K) are different to the thermal conditions during the thermal decomposition (T up to 812 K). 4. Conclusion This paper studied the thermal decomposition behavior of amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) as typical representatives of organic phosphonic acids. The comparison of the experimental results found by the study of the thermal decomposition using TGA–FTIR and pyrolysis GC–MS with modeling of the formation reactions showed the usefulness of the latter to predict possible decomposition products. The authors try to answer the question whether on the decomposition mechanism, postulated by experimental data, corresponds to that of the formation mechanism. Gaseous decomposition products containing phosphorus were evolved from both phosphonic acids, while the residue after decomposition had only a reduced content of phosphorus. Differences exist between the gaseous products identified by pyrolysis GC–MS and TGA–FTIR, particularly with regard to aromatic structures (e.g. tetramethylpyrrole, 4-dimethylaminophenol) found in the thermal decomposition of ATMP. Based on the identified intermediate products decomposition pathways which consider the respective starting point of the thermal decomposition (P C or N C bond scission) were derived for both phosphonic acids. By comparison of the experimental results with the theoretical considerations the hypothesis can be established that the decomposition process should follow the formation mechanism, i.e. the thermal decomposition can be likely understood as inverse formation reaction of the phosphonic acids. Thus, the suitability of theoretical considerations for the understanding of thermal decomposition processes is shown. Acknowledgements Financial support for this investigation has been provided by the Sächsische Aufbaubank Dresden (Grant-No: 12298/1989). Furthermore, the authors would like to thank Dr. Stephan Liebsch and Mrs.

53

Juliane Kaltofen (Zschimmer & Schwarz Mohsdorf GmbH & Co KG, Burgstädt, Germany) for the kind delivery of the phosphonic acids and helpful discussions, Mrs. Kerstin Arnhold for the TGA–FTIR measurements, Dr. Dieter Fischer for his assistance regarding the evaluation of the FTIR spectra, and Dr. Hartmut Komber for the NMR investigation (all from Leibniz-Institut für Polymerforschung Dresden e.V., Dresden, Germany). References [1] N. Moszner, U. Salz, J. Zimmermann, Chemical aspects of self-etching enameldentin adhesives: a systematic review, Dent. Mater. 21 (2005) 895–910. [2] V. Deluchat, J.C. Bollinger, B. Serpaud, C. Caullet, Divalent cations specification with three phosphonate ligands in the pH-range of natural waters, Talanta 44 (1997) 897–907. [3] Treatment of Cooling Water, 2009, Springer-Verlag, ISBN: 978-3-642-01984-5, pp. 111–113. [4] G. Tschäbunin, G. Schwedt, P. Fischer, Zur Analytik von Polymethylenphosphonsäure, Fresenius Z. Anal. Chem. 333 (1989) 123–128. [5] N.A. Kaslina, I.A. Polyakova, A.V. Kessenikh, B.V. Zhadanov, M.V. Rudomino, N.V. Churilina, M.I. Kabachnik, Study of the thermal decomposition of ATMP in aqueous solution, J. Gen. Chem. USSR 55 (1985) 472–475. [6] N. Richardson, R.J. Dellar, Phosphonsäuresalze enthaltende flammfeste Polymer-Zusammensetzungen, EP 0245207 B1 Ciba-Geigy AG, 1992. [7] W. Shiming, S. Dwight, Flame retardant phosphonate additives for thermoplastics, EP 1651737 Rhodia, 2004. [8] K. Moedritzer, R.R. Irani, The direct synthesis of ␣-aminomethylphosphonic acids, Mannich-type reactions with orthophosphorous acid, J. Org. Chem. 31 (1966) 1603–1607. [9] J.B. Prentice, O.T. Quimby, R.J. Grabenstetter, D.A. Nicholson, Interaction of acylating agents and phosphorus (III) sources. I. Intermediacy of condensed species in the formation of ethane-1-hydroxy-1,1-diphosphonic acid, J. Am. Chem. Soc. 94 (1972) 6119–6124. [10] W. Lorenz, A. Henglein, G. Schrader, The new insecticide O,O-dimethyl2,2,2-trichloro-1-hydroxyethylphosphonate, J. Am. Chem. Soc. 77 (1955) 2554–2556. [11] H. Güntzler, H.-U. Gremlich, IR-Spektroskopie, Wiley-VCH, Weinheim, 2003. ISBN-13: 978-3-527-30801-9. [12] U. Braun, B. Schartel, M.A. Fichera, C. Jäger, Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass–fibre reinforced polyamide 6,6, Polym. Degrad. Stab. 92 (2007) 1528–1545. [13] A.I. Balabanovich, D. Pospiech, A. Korwitz, L. Häußler, C. Harnisch, Pyrolysis study of a phosphorus-containing aliphatic-aromatic polyester and its nonocomposites with layered silicates, Polym. Degrad. Stab. 94 (2009) 355–364. [14] F. W. McLafferty, D.B. Stauffer, The Wiley/NBS Registry of Mass Spectral Data, vol. 1, 1989, p. 140. [15] H. Waldmann, E. Bläser, M. Jansen, H.-P. Letschert, Asymmetrische Synthese hochsubstituierter Pyrrolidine durch 1,3-Dipolare Cycloaddition von Azomethin-Yliden an N-Acryloylprolinbenzylester, Angew. Chem. 106 (6) (1994) 717–719.