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Abstract

Two series of carbons obtained by carbonization of porous copolymer of 4,4′-bis(maleimidodiphenyl) methane (50 mol%) and divinylbenzene (50 mol%) with and without phosphoric acid (impregnation ratio 1.1) at temperatures 400–1000℃. The carbons were characterized using elemental analysis, nitrogen adsorption, potentiometric titration, Fourier transform infrared and Raman spectroscopy. It has been shown that phosphoric acid causes structural and chemical changes in polyimide copolymer as compared to thermally treated carbons. Structural changes: phosphoric acid promotes transformation of polyimide copolymer to carbon structure at lower temperatures as compared to thermally treated carbons. Phosphoric acid is responsible for formation of highly developed micro/mesoporous structure that is different from that of thermally treated carbons. Chemical changes: phosphoric acid causes elimination of hydrogen and nitrogen, introduction of phosphorus and oxygen as phosphate-like structure. Significant amount of phosphorus imparts acid properties to carbon.

Keywords

Phosphorus-containing carbon carbon structure Fourier transform infrared Raman

Introduction

Interaction of carbonaceous materials with phosphoric acid is important for manufacture of carbon adsorbents (Marsh and Rodríguez-Reinoso, 2006) and developing fire retardant formulations (Bourbigot et al., 1997). Earlier studies on preparation of activated carbons from lignocellulosic materials (Benaddi et al., 1998; Jagtoyen and Derbyshire, 1998; Solum et al., 1995) and coals (Jagtoyen et al., 1992, 1993) showed that the effect of phosphoric acid is to facilitate structural transformation via bond cleavage and cross-linking fragments of carbonaceous precursor. Phosphoric acid considerably reduces volatiles evolution, increases the yield of carbon and decreases the temperature of carbon structure formation during heat treatment of lignocellulosic materials and coals.

The purpose of this study is to characterize the structural transformation of polyimide copolymer to carbons obtained by pyrolysis at different temperatures with and without phosphoric acid.

Materials and methods

Carbons

Phosphorus-containing carbons have been obtained by carbonization of porous copolymer of 4,4′-bis(maleimidodiphenyl) methane (50 mol%) and divinylbenzene (50 mol%), abbreviated as BM-DVB, (Matynia et al., 1996) in presence of phosphoric acid (impregnation ratio 1.1) at temperatures 400–1000℃ (Gawdzik et al., 2004; Puziy et al., 2002, 2007a, 2007b, 2013, Sobiesiak et al., 2006a, 2006b). After carbonization, carbons were extensively washed with hot water in Soxhlet extractor until neutral pH of wash waters. To reveal the role of phosphoric acid in transformation of the polymer to carbon, a second series of carbons were obtained without addition of phosphoric acid at the same temperature range.

Characterization

Elemental analysis

Elemental analysis (CHN) was performed using a Perkin Elmer CHN 2400 analyser (Palo Alto, CA, USA). The sample weight was about 2 mg. Oxygen content was calculated by difference.

Phosphorus content was measured by energy-dispersive X-ray fluorescence method using ElvaX analyser (Elvatech, Ukraine).

Porous structure

Porous structure of carbons was characterized by nitrogen adsorption measured at −196℃ using Autosorb-6 adsorption analyser (Quantachrome, USA). Pore size distribution was calculated by Autosorb-1 software (Quantachrome, USA) using Quenched Solid Density Functional Theory (QSDFT) method and slit/cylindrical pore model (Gor et al., 2012; Landers et al., 2013; Neimark et al., 2009). Brunauer-Emmett-Teller (BET) surface area, A_BET, was calculated by BET method using nitrogen adsorption data in the relative pressure range chosen by recently proposed procedure (Rouquerol et al., 2007). The total pore volume, V_tot, was calculated by converting the amount of nitrogen adsorbed at a maximum relative pressure to the volume of liquid adsorbate. The micropore, V_mi, and mesopore, V_me, volumes were calculated from the cumulative pore size distribution as the volume of pores with sizes less than 2 nm and between 2 and 50 nm, respectively.

Surface groups

Surface chemistry of nanoporous carbons was investigated by potentiometric titration performed in thermostatic vessel at 25℃ using a 672 Titroprocessor combined with 655 Dosimat (Metrohm, Herisau, Switzerland). To prevent contamination with CO₂, the flow of pure argon was used throughout the titration. The proton concentration was monitored using an LL pH glass electrode (Metrohm, Switzerland). Solution equilibria and a correction for possible carbonate and silicate contaminations were calculated using EST software (Del Piero et al., 2006).

Proton affinity distributions, F(pK), were calculated from proton-binding isotherms (Q versus pH) by solving the adsorption integral equation using the CONTIN method (Provencher, 1982a, 1982b; Puziy et al., 1999, 2001).

Fourier transform infrared (FTIR)

Attenuated total reflectance spectra were obtained using a Bruker FTIR spectrophotometer TENSOR 27.

Raman

Raman spectra were recorded in air at room temperature using an inVia Reflex (Renishaw, UK) Raman microscope equipped with Ar ion laser (514.5 nm, nominal power 5.5 mW) and magnification objective ×50. To avoid destruction of carbon samples 5% (0.275 mW) of laser power was used. The spectra were recorded in 60–3200 cm⁻¹ with accumulation of four spectra. The spectral resolution was 4 cm⁻¹.

Results and discussion

Yield

Heat treatment of polyimide copolymer results in gradual decreasing of the mass and the volume of the copolymer (Figure 1). The greatest mass change occurs in 400–600℃ and volume change in wider range 400–800℃. Addition of phosphoric acid greatly changes the variation of mass and the volume of the polyimide copolymer during pyrolysis.

Figure 1.

Mass (a) and volume (b) evolution of polyimide copolymer during carbonization with and without phosphoric acid.

Initial mass loss at temperatures below 400℃ reflects cleavage of macromolecules while increase in yield in the range from 400 to 500℃ shows enhancement of cross-linking process. At temperatures higher than 500℃ the mass yield is greater for carbons obtained with phosphoric acid. At this stage, cross-linking reactions begin to dominate over bond cleavage and depolymerization reactions.

Transformation of polyimide copolymer to activated carbon is accompanied by significant volume changes. Initial contraction is followed by expansion of the material, the maximum being observed at 600℃.

Similar effect of phosphoric acid was observed for wood (Jagtoyen and Derbyshire, 1993, 1998). This phenomenon was shown to be due to the dual role of phosphoric acid during carbonization: first, it catalyses cleavage of macromolecules into smaller fragments; second, it promotes formation of cross-links between molecules resulting in rearrangement of carbon material with larger structural units, thus leading to formation of a rigid cross-linked solid.

Porosity development

There is evidently a direct connection between porosity development and the process of dilation. The expansion of carbon material in temperature range 400–600℃ (Figure 1(b)) corresponds to the development of porous structure with high surface area (Figure 2(a)). At higher temperatures contraction of the material leads to decreasing of specific surface area.

Figure 2.

Temperature evolution of BET surface area (a) and pore size distributions (b) in parent polyimide copolymer (BM-DVB) and carbons obtained with and without phosphoric acid at 600℃.

Pore size distributions (Figure 2(b)) show that carbons obtained by heat treatment of polyimide copolymer without phosphoric acid inherited mesoporous structure with pore size greater than 20–40 nm from parent copolymer. While porous structure of phosphoric acid-activated carbons characterized by more developed microporous structure and mesoporous structure with size 2–4 nm due to redistribution of carbon material by the action of phosphoric acid.

Elemental analysis

Heat treatment of BM-DVB copolymer without phosphoric acid results in increasing carbon content and elimination of foreign atoms like hydrogen, nitrogen and oxygen (Table 1).

Table 1.

Elemental analysis (in mass %) of parent polyimide copolymer and carbons obtained with and without phosphoric acid at different temperatures.

Temperature (℃)	C	H	N	O^a	P	O/P atomic ratio
Parent copolymer	76.21	4.95	5.73	13.10
H₃PO₄ series
400	75.24	3.68	3.89	13.75	3.44	7.74
500	60.41	3.18	2.61	27.54	6.26	8.52
600	56.96	3.44	2.75	27.52	9.33	5.71
700	57.46	2.51	3.08	26.13	10.82	4.68
800	55.51	2.29	3.15	26.86	12.19	4.26
900	57.45	2.05	2.32	28.04	10.14	5.35
1000	63.41	1.79	1.61	24.30	8.89	5.29
Thermal series
400	71.8	3.89	5.88	18.43	0
500	75.85	3.54	6.17	14.44	0
600	83.32	2.91	5.41	8.36	0
700	84.7	2.18	4.49	8.63	0
800	82.61	1.62	3.88	11.89	0
900	82.4	1.43	3.73	12.44	0
1000	82.26	1.37	3.26	13.11	0

Determined by difference.

For phosphoric acid-activated carbons, the content of carbon decreases with temperature. The decrease of carbon content is due to increasing contribution of foreign atoms like oxygen and phosphorus because of enhancing reaction with phosphoric acid. The increase of oxygen content is most likely due to formation of phosphorus compounds containing oxygen-like phosphates. Phosphoric acid activation also promotes elimination of nitrogen at all carbonization temperatures.

Carbonization of polyimide copolymer with phosphoric acid results in carbons containing phosphorus compounds. The maximum phosphorus content is achieved at 800℃. At higher temperatures, the content of phosphorus decreases due to thermal destruction of phosphorus compounds. The composition of phosphorus compounds may be estimated by oxygen-to-phosphorus atomic ratio. O/P atomic ratio decreases with temperature and reaches minimum 4.3 at 800℃. The value of oxygen-to-phosphorus ratio is close to that of phosphoric acid, which gives grounds to expect that phosphorus compounds are structurally similar to phosphates. At higher temperatures, the ratio slightly increases indicating the destruction of phosphocarbonaceous structure.

FTIR

Overall intensity of infrared spectra decreases with temperature for both series of carbons indicating thermal degradation of surface functionalities (Figure 3). The decrease is much more pronounced for thermal series of carbons, suggesting less thermal stability of surface functionality than in the case of phosphoric acid-activated carbons.

Figure 3.

FTIR spectra of polyimide-derived carbons obtained with (P400, P600, P800, P1000) and without (C800) phosphoric acid at different temperatures. FTIR: Fourier transform infrared.

All phosphoric acid-activated carbons show broad absorption band centred at 3435 cm⁻¹, which is due to stretching vibration of hydrogen-bonded hydroxyl groups from carboxyls, phenols or alcohols. Whereas only free hydroxyl groups are present in thermal series carbons which is evidenced by sharp peak at 3633 cm⁻¹ (Figure 3).

All low temperature carbons show stretching vibration bands of C = O in imide linkage (1775 and 1700–1707 cm⁻¹ (Silverstein et al., 2005; Socrates, 2001)) that rapidly disappears as temperature rises up to 600℃ (Figure 3). Formation of polyaromatic structure is evidenced by C = C stretching enhanced by polar functional groups at 1599–1585 cm⁻¹ which shifts to lower frequencies 1555–1533 cm⁻¹ for carbons obtained at high temperatures.

FTIR spectra of phosphoric acid-activated carbons show bands which could be assigned to P = O stretching vibration at 1153 cm⁻¹ and P–O–C linkage at 1059 cm⁻¹ in differently substituted phosphates (Socrates, 2001). Formation of phosphonates at temperatures up to 800℃ is evidenced by vibration of C–P linkage at 490 cm⁻¹ (Thomas, n.d.).

Surface groups

Rich surface functionalities of phosphoric acid-activated carbons are evidenced by potentiometric titration. Proton-binding isotherms show that carbon obtained by heat treatment of BM-DVB copolymer without phosphoric acid has neutral surface with PZC at about pH 7 and contains very low amount of surface groups (Figure 4(a)). On the contrary, carbons obtained by phosphoric acid activation show acidic surface with PZC at about pH 2 with high concentration of acidic surface groups. Calculated distribution of acidic surface groups shows the presence of phosphate groups, obviously, originated from phosphoric acid; carboxylic groups; second dissociation constant of phosphate groups, enol and phenol groups (Figure 4(b)).

Figure 4.

Proton-binding isotherms (a) and proton affinity distributions (b) of polyimide-derived carbons obtained with and without phosphoric acid at 700℃.

Raman

Raman spectroscopy becomes increasingly attractive for investigation of the structure of disordered carbon materials (Dresselhaus et al., 2010). Raman spectra of carbons of both thermal and acid series (Figure 5) are characteristic to low crystallinity highly disordered carbon materials. There are two broad and heavily overlapped peaks in the first-order Raman spectra. The peak at around 1600 cm⁻¹ is characteristic to perfect graphite and a second peak around 1352 cm⁻¹ is present in disordered carbons (Ferrari and Robertson, 2000). This fact indicates that carbon material with turbostratic structure is formed at temperatures as low as 400℃. Formation of polyaromatic backbone of carbon material at 400℃ is confirmed by aromatic ring stretching vibration at 1580–1560 cm⁻¹ in FTIR (Figure 3). Second-order Raman spectra are represented by a small bump indicating the lack of stacking order like in graphite-like structure.

Figure 5.

Raman spectra of polyimide-derived carbons obtained with (a) and without (b) phosphoric acid at different temperatures.

The ratio of intensities of D band to G band is often used as a measure of structural order of carbon materials. Area ratio I_D/I_G increases with temperature for thermal series of carbons. This fact allows assigning the carbonization of polyimide copolymer to stage two of the three-stage model relating Raman parameters to the sp² nanostructure of disordered carbons (Ferrari and Robertson, 2000). On the contrary, the ratio of D and G bands of phosphoric acid-activated carbons is almost the same for all temperatures indicating that structural changes have occurred at earlier temperatures as compared to acid-free samples. In-plane crystallite size (L_a) calculated according to proposed by Ferrari and Robertson equation (Ferrari and Robertson, 2000) shows increasing diameter of carbon crystallite from 0.46 to 0.93 nm with temperature for thermal series of carbons (Figure 6). On the contrary, crystallite diameter of phosphoric acid-activated carbons is almost constant at all temperatures and equals 0.97 ± 0.04 nm. This fact shows that phosphoric acid promotes formation of carbon crystallites already at low temperatures.

Figure 6.

Temperature evolution of crystallite size of polyimide-derived carbons obtained with and without phosphoric acid at different temperatures.

Conclusions

Phosphoric acid causes structural and chemical changes in polyimide copolymer as compared to thermally treated carbons.

Structural changes: Phosphoric acid promotes transformation of polyimide copolymer to carbon structure at lower temperatures as compared to thermally treated carbons. Phosphoric acid is responsible for formation of highly developed micro/mesoporous structure that is different from that of thermally treated carbons.

Chemical changes: Phosphoric acid causes elimination of hydrogen and nitrogen, introduction of phosphorus and oxygen as phosphate-like structure. Significant amount of phosphorus imparts acid properties to carbon, which could be useful in areas where acidity is desired, like metal ion adsorption and in catalysis.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Assessment of the structural evolution of polyimide-derived carbons obtained by phosphoric acid activation using Fourier transform infrared and Raman spectroscopy

Abstract

Keywords

Introduction

Materials and methods

Carbons

Characterization

Elemental analysis

Porous structure

Surface groups

Fourier transform infrared (FTIR)

Raman

Results and discussion

Yield

Porosity development

Elemental analysis

FTIR

Surface groups

Raman

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References