Pli: s0141-3910(97)00250

Furfuraldehyde resin pyrolysis

R. Sbnchez,“* C. HernHndezb & J. Rieumont”

Polwwr Degradation and Srabiliry 61 ( 1998) 5 13-5 17 C> 1998 Elsevier Science Limited. All rights reserved

Prmted in Cfkeat Britain 3910/98!%-see front matter 4 0141

in a closed system

aState University of North Fluminense, Center of Science and Technology, Campos, RJ, CEP 28015-620, Brazil ‘National Center for Scientific Researches, Havana, Cuba

(Received 3 September 1997; accepted 20 October 1997)

The pyrolysis of two insoluble resins obtained by acid catalysis from furfur- aldehyde and furfuraldehydedimethylketone was carried out in a closed system at 400°C. The products of the pyrolysis were extracted under their vapor pres- sure. The compositions of the pyrolyzates were analyzed by mass spectrometry using a direct inlet probe technique under an electron impact of 70eV. Thus, the main structural sequences of both resins were determined. Trifurylic sequences are the main component for the furfuraldehyde resin but the furfuraldehyde- dimethyl ketone resin has two types of structural units: furfurylidene and fur- ylmethylene. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

Thermodegradative methods have been used widely in the study of resins and polymer struc- turesrA by means of continuous5 and pulse”* techniques of heating, that require a proper choice of the pyrolysis temperature and the quantity and homogeneity of the sample. These procedures guarantee the analytical character of the thermo- degradative method.9

The most-used techniques for the identification of the pyrolysis products are masslo and infrared spectrometry I’ Pyrolysis with direct introduction into the ionization chamber of the mass spectro- meter (DI) is extremely useful because it provides the initial characterization of the polymer or resin as its ‘fingerprint’, as well as determining the molecular weights of the fragments.

However not all the structural features are iden- tified from these results alone. Therefore com- plementary techniques are required. One possible alternative is the use of a closed system with con- tinuous heating and distillation of the products as they are formed.

It is well known that recombination products are formed in these systems making the analysis more difficult. Nevertheless it is possible to relate the

*To whom correspondence should be addressed.

513

composition of the pyrolyzate with the mass spec- trum obtained by DI and subsequently to deter- mine the most significant sequences present in the structure of the polymer or resin under study.

The purpose of this work was to study the pyr- olysis of two resins, obtained from furfuraldehyde and from a mixture of furfuraldehyde-dimethyl ketone, by catalysis with a Briinsted acidr2,i3 at 400°C in a closed reactor with the aim of estab- lishing the most characteristic sequences in both insoluble and infusible resins.

2 EXPERIMENTAL

The resins were obtained from furfuraldehyde (FI) and furfuraldehyde and dimethylketone (FAH) in bulk from fresh vacuum distilled monomers, using a strong mineral acid (sulphuric acid) as the cata- lyst.i2,13 The resins were purified thoroughly until the monomer and catalyst were eliminated and dried under vacuum (10e5 Pa). The elemental ana- lysis of resins was performed by a semi-micro method in a Carlo Erba Analyzer.

The pyrolysis reactor used is shown in Fig. 1. A sample of 0.2g of the resin with a particle size ranging between 0.1 and 0.15 mm was put in the flask (a) and sealed under high vacuum (10 Pa). Thermodegradation was carried out using a tubu- lar oven at 400* 10°C. Flask (a) was put in the

514 R. Shchez et al.

d

b

Fig. 1. Thermodegradative reactor used by pyrolysis in a closed system.

central part of the oven and the flask (b) was cooled in liquid nitrogen.

The pyrolysis mixture was analyzed in a coupled GC/MS system with a Jeol Mass Spectrometer

The degradation temperature was previously selected from the differential thermograms (DTA) corresponding to both resins.14 The thermo-

DX300. The volatile fraction of the mixture was

degradation time was approximately 6-8 h. Flask (a) was removed from the oven and the reactor was

analyzed by connecting (d) to the mass spec-

closed in at (c) keeping flask (b) under liquid nitrogen.

trometer and breaking the glass seal (f). The liquid fraction formed two immiscible lay-

ers. It was dissolved in dimethyl ketone and injec- ted into the coupled system.

The chromatographic separation was carried out using a capillary column of fused silica, impreg- nated with a free fatty acid phase (FFAP) (50m, 0.1 mm inner diameter). Helium was used as the carrier gas at a flow rate of 30cm3min-’ and iso- thermic heating (SO’C). Mass spectra were obtained in electron impact mode (70eV) with the ionization chamber at a temperature of 250°C and a pressure of 1.3 x lo4 Pa.

The scanning was in the range of 3CWlOO mass units and the spectra were recorded on a hard disk (4.56 M) and processed by a JMA-3 100 Mass Data Analysis System.

3 RESULTS AND DISCUSSION

The elemental analysis of FH and FAH resins obtained (Table 1) showed a slight difference in

Fig. 2. Mass spectrogram of the gas fraction obtained from the FAH resin pyrolysis.

Table 1. Elemental analysis of furfuraldehyde (FH) and furfur- aldehydedimethylketone (FAH) resins

Resins %C % H %0

FH 65.1 3.8 31.1 FAH 66.7 4.5 28.8

composition. The oxygen content of FAH was lower than FH as expected from the initial oxygen composition for the condensation products between furfuraldehyde and dimethylketone.

The gas fraction formed in the pyrolysis of the FH resin was a mixture of carbon dioxide, furan and 2-methyl furan (Fig. 2).

The compounds obtained for longer retention times must give better information to establish the characteristic sequences of the resin if, as the pre- vious ones, they are not markedly affected by re-

In the liquid fraction seven components were identified (Fig. 3 and Table 2). Acetic acid was detected in peak 1, near the solvent, followed by

arrangement and recombination reactions.

furfuraldehyde and furylmethylketone. These compounds were also detected, at the same tem- perature, by direct introduction of the sample12T14 using the chemical ionization technique with methane (DI/CH4), in which recombinations and re-arrangements are not favored. Thus, the pres- ence of these compounds in the pyrolyzate is due to direct fragmentation.

At peak 4 (Fig. 4) a fragmentation was observed that is characteristic of the 2-[5-aceto fury11 fur- ylmethane with m/z= 190. It implies an a! breaking that gives rise to the fragment m/e 175 (main peak). This interpretation was confirmed by the presence of the peak M + 1 in the ID/CH4.15

6

Furfuraldehyde resin pyrolysis in a closed system 515

%I

4

7

2 (J_3 ,u

~,,,,,,,,,,,.,.,,,,._

0 500 loo0 1500 2000 Scan

Fig. 3. Mass chromatogram of the FH resin pyrolyzate.

Table 2. Detection of pyrolysis products obtained in a closed reactor for FH resin

Peaks Characteristic fragments

Assignments

60,45,43 Acetic acid 96,95,67,39 Furfuraldehyde 110,95,68,39 2-furylmethylketone 190,176,175,43 2-[5 acetofuryI]furyl

methane . ..tail of peak 4

204,190,189,43 2-[5 acetofuryl] 5 methylfuryl methane

242,200,199,171,157 Di 2-[5 methyl fury112 furyl- methane

256,241,214,213,171,43 Tri2-[Smethyl furyllmethane

0

50 100 150 200 M/Z

Fig. 4. Mass spectrogram of the component present at peak 4 in the FH resin.

It can be assumed that this sequence is derived from the direct fragmentation of the resin and by its relative abundance in the pyrolyzate must con- stitute an important structural unit.

Peak 6 with m/z 204 of lesser abundance con- firms the previous identification because it presents a similar fragmentation pattern but the fragment M-43 is more abundant. It corresponds to the homologue methylated at the carbon C-5 position of the furan ring considering the loss of acetyl groups as a characteristic feature of these com- poundsi

Its origin can be related to a more complex sequence that includes methylene groups. Its frag- mentation probably seems to be the origin of peak 4 of greater relative abundance in the pyrolyzate. Thus its formation from coupling reactions with methyl radicals formed during the pyrolysis can be ruled out.

Peak 7 which was relatively abundant and had m/z 242 was identified as di:2-(Smethyl furyl)2- fury1 methane and is shown in Fig. 5. It forms a very stable molecular ion and its fragmentation results in the formation of the acetyl ion.

1+

K-J- CH- 0

m/e 242

Peak 8 is responsible for a fragmentation pattern of a methylated homologue of the compound from

2 TI%

10

6

2

0

50 100 150 250 300 M/Z

Fig. 5. Mass spectrogram of the component present at peak 7 in the FH resin.

516 R. Sbzchez et al.

peak 7 that consequently will give a greater con- tribution to the fragment m/e 43. Due to the poor relative abundance of this compound in the pyr- olyzate its presence can be analyzed in a similar way to peaks 4 and 6. Thus these four compounds are related to the products of the same fragmenta- tion of a trifurylic-type sequence and taking into account the composition and abundance of the mixture of products in the pyrolyzate it must con- stitute the fundamental sequence of the FH resin:

Structures of this kind show a pattern very dif- ferent from their linear homologues that are char- acterised by the abundance of the fragments m/e 8 1 and m/e 91.

de 91

The pyrolysis of the FAH resin gives a gas frac- tion with a composition similar to that obtained with the FH resin. However, the liquid fraction yields a mass chromatogram (Fig. 6 and Table 3) markedly different. The outstanding number of components (15) seems to indicate that the basic sequence of the FAH resin is composed of several different structural units. This feature indicates a sharp difference in the relative abundance of the pyrolysed components for both resins and confirms that FAH resin does not possess a predominant sequence as the FA resin.

The analysis of the products allowed peaks 6, 8 and 9 to be identified as benzofurans because of their characteristic fragmentation pattern’5,16 with

%I ) ,

intense signals corresponding to the species M and M- 1. The presence of these condensed compounds is a specific characteristic of this pyrolyzate but must not be associated with sequences created during the synthesis of the resin. The reaction of furfuraldehyde and dimethylketone in the presence of a Brijnsted acid takes place through condensa- tion and alkylation but not giving aromatic struc- tures in any case. ” This kind of structure is also produced in the pyrolysis of other polymers like polybutadiene3 that do not have aromatic units.

Peak 10 was identified as furfurylidenacetone

MHz -

de 94

H-CH-COCH,

de 136

from its fragmentation that has a relatively stable molecular ion and by the presence of the m/e 121 (main peak) as a consequence of the loss of the alkyl fragment typical of ketonic structures. On the other hand the presence of the fragment m/e 65 associated with cyclopentadiene ions is character- istic of furan compounds with a double bond on the side group. This species is the more abundant in the pyrolyzed mixture Fig. 7 and indeed may be related to peak 15 of m/z 246 assigned to species:

1 TM

65

10

136

43

brddd&

94 I

i

5

205 I m

so 1.0 150 ztm 21 M/Z

Fig. 7. Mass spectrogram ofthe component present at peak 10 in the FAH resin. Fig. 6. Mass chromatogram of the FAH resin pyrolyzate.

Furfuraldehyde resin pyrolysis in a closed system 517

Table 3. Detection of pyrolysis products obtained in a closed reactor for FAH resin

Peaks

1 2 3 4 5 6 7 8 9 10 11 12 13 14

15

Characteristic fragments

60,45,43

96,95,67,39 110,109,94,67,66,65,53,3 148,120,119,91,81 132,131,103,95 162,147,91,43 146,145,131,117,115,65 146,145,115 136,121,94,65,43 176,175,161,160,145,134,119,109,105,77,43 190,176,175,43 204,190,189,43 242,200,199,171,157,43 246,131,174,121,108

Assignments

Acetic acid Not assigned Furfuraldehyde 5-methyl furfuraldehyde Difurylmethane Methyl benzofuran 2-[5 methylfuryl] fury1 methane Dimethyl benzofuran 2-benzofuryl carboxi-aldehyde Furfurylidenketone 1,2 difuryl ethene, bifurane 2-[5-aceto fury11 fury1 methane 2-[5-aceto furyl][5-methyl fury11 methane Di 2-[5-methyl fury11 2-furylmethane 5-[(5-methyl furyl) hydroxy methyl furfurylidenketone

.-

that gives the fragment m/z 121 characteristic of this kind of furfurylidene structures. The detected species from peaks 7, 12, 13 and 14 are difur- ylmethylene derivatives, though however they were less abundant than the ones observed for FH. They form, with the furfurylidene structures, the main chain sequences of the FAH. From these results it can be confirmed that the basic sequence of the FAH resin is formed mainly from units of this type:

CH=CH-CO-

4 CONCLUSIONS

Pyrolysis in a closed system was suitable for obtaining precise structural information on inso- luble and infusible resins. The FH resin was characterized as a structure formed mainly by tri- furylic species. For the FAH resin, the pre- dominant sequences were of furfurylidene nature associated with furylmethylene units.

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