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adsorption properties of porous phenolic-formaldehyde and melamine-formaldehyde polymers
Materials Chemistry and Physics 77 (2002) 276–280
Absorption/adsorption properties of porous phenolic-formaldehyde and melamine-formaldehyde polymers J. Goworek∗ , A. Deryło-Marczewska, W. Stefaniak, W. Zgrajka, R. Kusak Faculty of Chemistry, Department of Physical Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland Received 20 March 2001; received in revised form 5 November 2001; accepted 20 November 2001
Abstract The synthesis of porous rigid resins which are chemically stable to organic solvents over a wide pH range and which have a desired porosity and surface area has substantial commercial importance. Two types of porous phenolic- and melamine-formaldehyde polymers were synthesized using colloidal silica as a porous matrix. The porosity of the materials obtained was characterized on the basis of adsorption/desorption isotherms of nitrogen at −195 ◦ C. For each sample the specific surface area, pore size distribution, and total pore volume were estimated. The porous polymers used in the experiments exhibit various pore size distributions and relatively high specific surface areas. The obtained porous polymers were tested as column packings in gas chromatography. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous polymers; Adsorption; Porosity
1. Introduction The preparation of various types of copolymers with well-defined porous structures is a subject of industrial and analytical interest. Polymeric sorbents are used as supports in chromatography, ion exchangers or for selective removal of organic solvents. Porous polymers are widely used owing to their high surface area and stability over a wide pH range. Well known members of mesoporous organic polymers are materials containing polystyrene cross-linked with divinyl benzene (PS–DVB). Porous copolymer beads are produced usually by suspension copolymerization in the presence of an inert diluent [1,2]. Another method to produce the porosity is a polymerization at the presence of an inorganic matrix of desired shape, porosity and surface area in the reacting mixture. After polymerization, the inorganic template is removed by dissolution in alkaline solutions without destruction of polymerizate. Organic sorbents of this type expose a highly developed surface area and pore structure. Porous polymers exhibit a variety of specific surface areas, pore volumes and pore diameters. The separation of various types of gas or liquid mixtures in chromatography is an effect of adsorption or absorption. The retention depends on the character of the adsorbate and its relation to the adsorbent. In most cases retention is the result of a combined adsorption ∗
Corresponding author. Tel.: +48-81-537-5716; fax: +48-81-533-3348. E-mail address: [email protected] (J. Goworek).
and absorption [3–5]. The share of both these processes in separation of organic mixtures is determined by specific interactions of the adsorbate molecules with the surface and the degree of swelling of the organic sorbent in contact with gas and liquid media [6–8]. Generally, one can say that porous polymers are specific sorbents for some groups of adsorbates, which demonstrate an extremely high adsorption affinity to the surface and skeleton of organic sorbent [9]. To achieve high selectivities of these sorbents it is necessary to control the specific interactions responsible for adsorption as well as their porosity. In the present paper are presented absorption/adsorption properties of two types of cross-linked porous polymers obtained in the process of condensation of formaldehyde with phenol and melamine. As a template forming appropriate pore structure of the resulting polymer colloidal silica particles were used. The most important factor influencing the size and distribution of the pore dimensions is the amount of silica modifier present in reacting mixture. The aim of the present paper is to investigate the absorption/ adsorption properties of porous polymers in relation to water and various organic substances. 2. Experimental Two porous polymers ME-A (melamine-formaldehyde) and PF-A (phenolic-formaldehyde) and two nonporous polymers ME-0 and PF-0 were used in the experiment.
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 5 6 - 9
J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
In preparing the resins, a reacting mixture was formed of the melamine or bis-phenol A and paraformaldehyde at controlled pH. The resins were prepared by the conventional method in the presence of colloidal silica as an inorganic matrix. The reacting mixture in the form of paste was aged during 3 days and heated up to 180 ◦ C for 3 h. Upon completion of the polymerization reaction, the template silica was removed from the cross-linked copolymer by dissolution in alkaline solution (NaOH) having a pH ≈ 12. Nonporous samples were prepared in the absence of the silica matrix. The textural characterization of the obtained resins was performed by low-temperature adsorption of nitrogen. The adsorption/desorption isotherms of nitrogen at −195 ◦ C were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, SBET , were calculated from the linear form of the Brunauer–Emmett–Teller (BET) equation, taking the cross-sectional area of the nitrogen molecule to be 16.2×10−20 m2 . The pore size distributions were calculated in the standard manner by using the Barrett–Joyner–Halenda (BJH) method [10]. The thermogravimetric (TG) experiments were performed using the derivatograph C (MOM, Hungary). The gas chromatographic experiments were performed with a Hewlett-Packard 5890 apparatus using hydrogen as the carrier gas and thermal conductivity detector. The column used had a length of 1.5 m and an inner diameter of 2 mm.
3. Results and discussion The thermal stability of the porous polymers was tested by thermogravimetry using a linear heating program. The changes of weight of the porous samples against temperature are illustrated in Fig. 1. As can be seen, the loss of weight within temperature range 20–300 ◦ C is small and the destruction of the polymer begins at about 300 ◦ C. The decrease of mass at lower temperatures is probably connected with the desorption of a small amount of water present in the polymer skeleton. It is well known that polymeric adsorbents swell appreciably in water and many organic solvents. Thus, reliable
Fig. 1. Weight loss of ME-A and PF-A polymers against temperature.
277
information about the porous structure of organic resins is more difficult to obtain in comparison to rigid inorganic adsorbents. The low-temperature adsorption of nitrogen polymers demonstrates some tendency to crash and deform. In the case of mercury porosimetry, high pressure leads to a structural deformation of the organic skeleton. The textural characteristic, derived from low-temperature nitrogen adsorption data, when the polymer skeleton is in dry state, is usually different from that obtained by other methods. The thermal desorption of liquids indicates among others that the pore radii of melamine- and phenolic-formaldehyde polymers wetted with water are higher than that estimated from the nitrogen isotherm [11]. Moreover, the textural parameters depend on the chemical character of the wetting liquid used in the TG experiment. Absorption/adsorption of water was investigated for both nonporous ME-0 and PF-0 samples. The polymer beads 0.05–0.07 mm were wetted before the experiment with pure water. Next, the samples in the form of paste were placed in a platinum crucible and heated using a quasi-isothermal program. The desorption curves of water from both investigated polymers are shown in Fig. 2. During the desorption experiment carried at quasiisothermal conditions the temperature and the heating rate are not constant [12]. If the evaporation of the liquid is slow, the fixed weight loss level (in our case 0.5 mg min−1 ) regulating the run of the program is not exceeded. As a result, the linear increase of temperature within this measurement range is realized. At the certain temperature when intensive evaporation occurs the above mentioned level is exceeded and isothermal conditions are established. In the TG experiment the sharp evaporation of the bulk liquid corresponds to its boiling point. In the case of porous solids the pores
Fig. 2. Thermal desorption of water from nonporous polymers. I: desorption of the bulk liquid; II: desorption of water taking part in swelling process.
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J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
are emptied at higher temperatures depending on the pore dimensions. For emptying pores the temperature should be higher when the pore dimensions become smaller. The desorption of the liquid taking part in the swelling process requires also a higher temperature. Thus, to eliminate the effect of the porosity in the study of the swelling process of melamine- and phenolic-formaldehyde resins the nonporous samples were selected. Segment I of the desorption curves in Fig. 2 represents the evacuation of the bulk water from the interparticle space. This part of water is evacuated at its normal boiling point. However, segment II above the boiling point corresponds to the desorption of water from the polymer surface and interior of the polymer skeleton. From the data presented in Fig. 2 one can calculate that ME-0 and PF-0 samples absorb about 30% of water. It follows from the above-presented data that the TG experiment is reliable to estimate the amount of liquid, which takes part in the swelling process. For comparative purposes the adsorption and desorption of nitrogen at −195 ◦ C on the investigated polymer samples was measured. The low-temperature nitrogen isotherms for ME-A and PF-A porous polymers are shown in Fig. 3. The quantity of gas adsorbed is expressed as a dependence of its volume at standard conditions of temperature and pressure, STP, against relative pressure p/p0 , where p is the equilibrium pressure and p0 the saturation pressure of nitrogen at −195 ◦ C. The pore size distributions derived from these data are presented in Fig. 4. It can be seen that the nitrogen isotherms for both samples featured at higher p/p0 a well-shaped hysteresis loop characteristic for mesopores. It should be mentioned that the adsorption of nitrogen for the ME-A sample is completely reversible.
Fig. 4. Pore size distribution for porous polymers ME-A and PF-A.
The specific surface areas SBET , the total pore volumes Vp , and the mean pore radii Rp for the sample studied are collected in Table 1. It follows from this table that melamine resin is characterized by relatively high specific surface area. The pore size distribution for this sorbent is symmetrical. The pore size distribution for the PF-A sample is of bimodal type, showing two peaks corresponding to pore radii of about 2 and 9 nm. Further, our investigations were devoted to the evaluation of the mesoporous polymers as the packings for gas adsorption chromatography. Figs. 5 and 6 show the results of chromatograhpic tests performed for two types of adsorbents. As testing mixtures were chosen n-alkanes (n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane) and a ternary mixture of methanol, water and ethanol. In the case of PF-A polymer a good resolution of n-alkanes is observed. However, the retention times are relatively long. In the case of ME-A polymer the separation of the same components is poor and the retention times are shorter. Generally, one can say that retention of various substances on porous polymers depends on the chemical character of the sorbate molecule as well as on its dimension. In the case of both melamine and phenolic sorbents absorption and/or adsorption is caused by nonspecific interactions. As mentioned above, in the presence of organic vapours not only adsorption occurs but also the molecules of the sorbate can penetrate into the polymer skeleton. The swelling Table 1 Parameters characterising the pore structure of the samples investigated
Fig. 3. Adsorption/desorption isotherms of nitrogen at −195 ◦ C for porous polymers ME-A and PF-A.
Sample
SBET (m2 g−1 )
Vp (cm3 g−1 )
Rp (nm)
ME-A PF-A
222 105
0.45 0.32
3.65 2.0; 8.85
J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
279
Fig. 5. Characteristic chromatograms for melamine-formaldehyde polymers as packing materials: (a) ME-A (200 ◦ C); (a ) ME-0 (100 ◦ C); 1—water; 2—methanol; 3—ethanol; (b) ME-A (200 ◦ C); (b ) ME-0 (100 ◦ C); 1—n-hexane; 2—n-heptane; 3—n-octane; 4—n-nonane; 5—n-decane; 6—n-undecane. Thermal conductivity detector, carrier gas hydrogen, column 150 mm × 2 mm, flow rate 25 cm3 min−1 .
Fig. 6. Characteristic chromatograms for phenolic-formaldehyde polymer PF-A as a packing material: (a) 1—water; 2—methanol; 3—ethanol; (b) 1—n-hexane; 2—n-heptane; 3—n-octane; 4—n-nonane; 5—n-decane; 6—n-undecane. Thermal conductivity detector, carrier gas hydrogen, column 150 mm × 2 mm, flow rate 25 cm3 min−1 .
process starts simultaneously with the sorption before pores of the dry sorbent are filled. Long retention times of hydrocarbons in the case of phenolic-formaldehyde resin indicate that the separation is a result of combined adsorption and absorption. Due to weak nonspecific interactions of n-alkane molecules with the polymer surface one can assume that the retention is caused mainly by absorption in the bulk polymer material. The absorption mechanism is also suggested by chromatograms of n-alkanes on nonporous melamine-formaldehyde resin ME-0, S BET = 2 m2 g−1 (see Fig. 5b ). Separation of the members of hydrocarbon homologous series on the PF-0 sample at 100 ◦ C is similar although the retention times are markedly shorter. Thus, in the case of hydrocarbons one can assume that the differences in the retention times for various polymers depend mainly on their porous structure. The porosity and surface area play the main role especially in the dynamic conditions of a gas chromatography (GC) experiment. Similar effects are observed also in static conditions when the swelling ratio and swelling time are dependent on the bead size and porous structure of the polymeric material [6]. Interesting results were observed for ternary mixture of polar components. In the case of the porous ME-A sample the separation at 200 ◦ C is reasonably good. It may indicate a smaller share of absorption and higher share of adsorption
in the separation process. For the nonporous ME-0 sample the retention even at 100 ◦ C is very weak and identical for the polar components. The adsorption mechanism of these components may be also confirmed by smaller retention and a poor separation on the porous PF-A sample characterized by a lower specific surface area in comparison to the ME-A polymer. Both melamine and phenolic sorbents have nitrogen or oxygen heteroatoms which permit adsorption of water and alcohols through polar interactions.
4. Conclusions The studies demonstrate that the porous melamine- and phenolic-formaldehyde polymers prepared in the presence of colloidal silica as a modifier producing their porosity are thermally stable sorbents with a high sorption capacity. Both polymeric sorbents swell markedly at the presence of water and organic solvents. The GC studies with a series of hydrocarbons and polar components for the porous and nonporous sample of the same polymer provided experimental evidence for the absorption and adsorption mechanism of the retention. Our further investigations will be devoted to chemical and textural modifications of melamine and phenolic-formaldehyde adsorbents for obtaining materials of the desired chromatographic resolution.
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J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
Acknowledgements Financial support from the State Committee for Scientific Research (KBN, Warsaw), Project No. 3T09B00717 is gratefully acknowledged. References [1] M.P. Tsyurupa, A.S. Shabaeva, L.A. Pavlova, T.A. Mrachkovskaya, V.A. Davankov, in: B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing, K.K. Unger (Eds.), Characterization of Porous Solids IV, The Royal Society of Chemistry, Cambridge, 1997, p. 398. [2] T. Balakrishnan, W.T. Ford, J. Appl. Polym. Sci. 27 (1982) 133. [3] M. Gassiot-Matas, B. Monrabal-Bas, Chromatographia 3 (1970) 547.
[4] O. Dufka, J. Malinsky, J. Churaˇcek, K. Komarek, J. Chromatogr. 51 (1970) 111. [5] M. Ousalem, X.X. Zhu, J. Hradil, J. Chromatogr. A 903 (2000) 13. [6] S. Kiatkamjornwong, S. Traisaranapong, P. Prasassarakich, J. Porous Mater. 6 (1999) 205. [7] H. Omidian, S.A. Hashemi, P.G. Sammes, I. Meldrum, Polymer 26 (1998) 6697. [8] L.D. Belyakova, A.V. Kiselev, N.D. Platonova, T.I. Schevchenko, Adv. Colloid Interfac. Sci. 21 (1984) 55. [9] B.J. Brune, J.A. Koehler, P.J. Smith, G.F. Payne, Langmuir 15 (1999) 3987. [10] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [11] J. Goworek, W. Stefaniak, W. Zgrajka, Mater. Chem. Phys. 59 (1999) 149. [12] F. Paulik, J. Paulik, J. Therm. Anal. 5 (1973) 253.
Absorption/adsorption properties of porous phenolic-formaldehyde and melamine-formaldehyde polymers J. Goworek∗ , A. Deryło-Marczewska, W. Stefaniak, W. Zgrajka, R. Kusak Faculty of Chemistry, Department of Physical Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland Received 20 March 2001; received in revised form 5 November 2001; accepted 20 November 2001
Abstract The synthesis of porous rigid resins which are chemically stable to organic solvents over a wide pH range and which have a desired porosity and surface area has substantial commercial importance. Two types of porous phenolic- and melamine-formaldehyde polymers were synthesized using colloidal silica as a porous matrix. The porosity of the materials obtained was characterized on the basis of adsorption/desorption isotherms of nitrogen at −195 ◦ C. For each sample the specific surface area, pore size distribution, and total pore volume were estimated. The porous polymers used in the experiments exhibit various pore size distributions and relatively high specific surface areas. The obtained porous polymers were tested as column packings in gas chromatography. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous polymers; Adsorption; Porosity
1. Introduction The preparation of various types of copolymers with well-defined porous structures is a subject of industrial and analytical interest. Polymeric sorbents are used as supports in chromatography, ion exchangers or for selective removal of organic solvents. Porous polymers are widely used owing to their high surface area and stability over a wide pH range. Well known members of mesoporous organic polymers are materials containing polystyrene cross-linked with divinyl benzene (PS–DVB). Porous copolymer beads are produced usually by suspension copolymerization in the presence of an inert diluent [1,2]. Another method to produce the porosity is a polymerization at the presence of an inorganic matrix of desired shape, porosity and surface area in the reacting mixture. After polymerization, the inorganic template is removed by dissolution in alkaline solutions without destruction of polymerizate. Organic sorbents of this type expose a highly developed surface area and pore structure. Porous polymers exhibit a variety of specific surface areas, pore volumes and pore diameters. The separation of various types of gas or liquid mixtures in chromatography is an effect of adsorption or absorption. The retention depends on the character of the adsorbate and its relation to the adsorbent. In most cases retention is the result of a combined adsorption ∗
Corresponding author. Tel.: +48-81-537-5716; fax: +48-81-533-3348. E-mail address: [email protected] (J. Goworek).
and absorption [3–5]. The share of both these processes in separation of organic mixtures is determined by specific interactions of the adsorbate molecules with the surface and the degree of swelling of the organic sorbent in contact with gas and liquid media [6–8]. Generally, one can say that porous polymers are specific sorbents for some groups of adsorbates, which demonstrate an extremely high adsorption affinity to the surface and skeleton of organic sorbent [9]. To achieve high selectivities of these sorbents it is necessary to control the specific interactions responsible for adsorption as well as their porosity. In the present paper are presented absorption/adsorption properties of two types of cross-linked porous polymers obtained in the process of condensation of formaldehyde with phenol and melamine. As a template forming appropriate pore structure of the resulting polymer colloidal silica particles were used. The most important factor influencing the size and distribution of the pore dimensions is the amount of silica modifier present in reacting mixture. The aim of the present paper is to investigate the absorption/ adsorption properties of porous polymers in relation to water and various organic substances. 2. Experimental Two porous polymers ME-A (melamine-formaldehyde) and PF-A (phenolic-formaldehyde) and two nonporous polymers ME-0 and PF-0 were used in the experiment.
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 5 6 - 9
J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
In preparing the resins, a reacting mixture was formed of the melamine or bis-phenol A and paraformaldehyde at controlled pH. The resins were prepared by the conventional method in the presence of colloidal silica as an inorganic matrix. The reacting mixture in the form of paste was aged during 3 days and heated up to 180 ◦ C for 3 h. Upon completion of the polymerization reaction, the template silica was removed from the cross-linked copolymer by dissolution in alkaline solution (NaOH) having a pH ≈ 12. Nonporous samples were prepared in the absence of the silica matrix. The textural characterization of the obtained resins was performed by low-temperature adsorption of nitrogen. The adsorption/desorption isotherms of nitrogen at −195 ◦ C were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, SBET , were calculated from the linear form of the Brunauer–Emmett–Teller (BET) equation, taking the cross-sectional area of the nitrogen molecule to be 16.2×10−20 m2 . The pore size distributions were calculated in the standard manner by using the Barrett–Joyner–Halenda (BJH) method [10]. The thermogravimetric (TG) experiments were performed using the derivatograph C (MOM, Hungary). The gas chromatographic experiments were performed with a Hewlett-Packard 5890 apparatus using hydrogen as the carrier gas and thermal conductivity detector. The column used had a length of 1.5 m and an inner diameter of 2 mm.
3. Results and discussion The thermal stability of the porous polymers was tested by thermogravimetry using a linear heating program. The changes of weight of the porous samples against temperature are illustrated in Fig. 1. As can be seen, the loss of weight within temperature range 20–300 ◦ C is small and the destruction of the polymer begins at about 300 ◦ C. The decrease of mass at lower temperatures is probably connected with the desorption of a small amount of water present in the polymer skeleton. It is well known that polymeric adsorbents swell appreciably in water and many organic solvents. Thus, reliable
Fig. 1. Weight loss of ME-A and PF-A polymers against temperature.
277
information about the porous structure of organic resins is more difficult to obtain in comparison to rigid inorganic adsorbents. The low-temperature adsorption of nitrogen polymers demonstrates some tendency to crash and deform. In the case of mercury porosimetry, high pressure leads to a structural deformation of the organic skeleton. The textural characteristic, derived from low-temperature nitrogen adsorption data, when the polymer skeleton is in dry state, is usually different from that obtained by other methods. The thermal desorption of liquids indicates among others that the pore radii of melamine- and phenolic-formaldehyde polymers wetted with water are higher than that estimated from the nitrogen isotherm [11]. Moreover, the textural parameters depend on the chemical character of the wetting liquid used in the TG experiment. Absorption/adsorption of water was investigated for both nonporous ME-0 and PF-0 samples. The polymer beads 0.05–0.07 mm were wetted before the experiment with pure water. Next, the samples in the form of paste were placed in a platinum crucible and heated using a quasi-isothermal program. The desorption curves of water from both investigated polymers are shown in Fig. 2. During the desorption experiment carried at quasiisothermal conditions the temperature and the heating rate are not constant [12]. If the evaporation of the liquid is slow, the fixed weight loss level (in our case 0.5 mg min−1 ) regulating the run of the program is not exceeded. As a result, the linear increase of temperature within this measurement range is realized. At the certain temperature when intensive evaporation occurs the above mentioned level is exceeded and isothermal conditions are established. In the TG experiment the sharp evaporation of the bulk liquid corresponds to its boiling point. In the case of porous solids the pores
Fig. 2. Thermal desorption of water from nonporous polymers. I: desorption of the bulk liquid; II: desorption of water taking part in swelling process.
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J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
are emptied at higher temperatures depending on the pore dimensions. For emptying pores the temperature should be higher when the pore dimensions become smaller. The desorption of the liquid taking part in the swelling process requires also a higher temperature. Thus, to eliminate the effect of the porosity in the study of the swelling process of melamine- and phenolic-formaldehyde resins the nonporous samples were selected. Segment I of the desorption curves in Fig. 2 represents the evacuation of the bulk water from the interparticle space. This part of water is evacuated at its normal boiling point. However, segment II above the boiling point corresponds to the desorption of water from the polymer surface and interior of the polymer skeleton. From the data presented in Fig. 2 one can calculate that ME-0 and PF-0 samples absorb about 30% of water. It follows from the above-presented data that the TG experiment is reliable to estimate the amount of liquid, which takes part in the swelling process. For comparative purposes the adsorption and desorption of nitrogen at −195 ◦ C on the investigated polymer samples was measured. The low-temperature nitrogen isotherms for ME-A and PF-A porous polymers are shown in Fig. 3. The quantity of gas adsorbed is expressed as a dependence of its volume at standard conditions of temperature and pressure, STP, against relative pressure p/p0 , where p is the equilibrium pressure and p0 the saturation pressure of nitrogen at −195 ◦ C. The pore size distributions derived from these data are presented in Fig. 4. It can be seen that the nitrogen isotherms for both samples featured at higher p/p0 a well-shaped hysteresis loop characteristic for mesopores. It should be mentioned that the adsorption of nitrogen for the ME-A sample is completely reversible.
Fig. 4. Pore size distribution for porous polymers ME-A and PF-A.
The specific surface areas SBET , the total pore volumes Vp , and the mean pore radii Rp for the sample studied are collected in Table 1. It follows from this table that melamine resin is characterized by relatively high specific surface area. The pore size distribution for this sorbent is symmetrical. The pore size distribution for the PF-A sample is of bimodal type, showing two peaks corresponding to pore radii of about 2 and 9 nm. Further, our investigations were devoted to the evaluation of the mesoporous polymers as the packings for gas adsorption chromatography. Figs. 5 and 6 show the results of chromatograhpic tests performed for two types of adsorbents. As testing mixtures were chosen n-alkanes (n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane) and a ternary mixture of methanol, water and ethanol. In the case of PF-A polymer a good resolution of n-alkanes is observed. However, the retention times are relatively long. In the case of ME-A polymer the separation of the same components is poor and the retention times are shorter. Generally, one can say that retention of various substances on porous polymers depends on the chemical character of the sorbate molecule as well as on its dimension. In the case of both melamine and phenolic sorbents absorption and/or adsorption is caused by nonspecific interactions. As mentioned above, in the presence of organic vapours not only adsorption occurs but also the molecules of the sorbate can penetrate into the polymer skeleton. The swelling Table 1 Parameters characterising the pore structure of the samples investigated
Fig. 3. Adsorption/desorption isotherms of nitrogen at −195 ◦ C for porous polymers ME-A and PF-A.
Sample
SBET (m2 g−1 )
Vp (cm3 g−1 )
Rp (nm)
ME-A PF-A
222 105
0.45 0.32
3.65 2.0; 8.85
J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
279
Fig. 5. Characteristic chromatograms for melamine-formaldehyde polymers as packing materials: (a) ME-A (200 ◦ C); (a ) ME-0 (100 ◦ C); 1—water; 2—methanol; 3—ethanol; (b) ME-A (200 ◦ C); (b ) ME-0 (100 ◦ C); 1—n-hexane; 2—n-heptane; 3—n-octane; 4—n-nonane; 5—n-decane; 6—n-undecane. Thermal conductivity detector, carrier gas hydrogen, column 150 mm × 2 mm, flow rate 25 cm3 min−1 .
Fig. 6. Characteristic chromatograms for phenolic-formaldehyde polymer PF-A as a packing material: (a) 1—water; 2—methanol; 3—ethanol; (b) 1—n-hexane; 2—n-heptane; 3—n-octane; 4—n-nonane; 5—n-decane; 6—n-undecane. Thermal conductivity detector, carrier gas hydrogen, column 150 mm × 2 mm, flow rate 25 cm3 min−1 .
process starts simultaneously with the sorption before pores of the dry sorbent are filled. Long retention times of hydrocarbons in the case of phenolic-formaldehyde resin indicate that the separation is a result of combined adsorption and absorption. Due to weak nonspecific interactions of n-alkane molecules with the polymer surface one can assume that the retention is caused mainly by absorption in the bulk polymer material. The absorption mechanism is also suggested by chromatograms of n-alkanes on nonporous melamine-formaldehyde resin ME-0, S BET = 2 m2 g−1 (see Fig. 5b ). Separation of the members of hydrocarbon homologous series on the PF-0 sample at 100 ◦ C is similar although the retention times are markedly shorter. Thus, in the case of hydrocarbons one can assume that the differences in the retention times for various polymers depend mainly on their porous structure. The porosity and surface area play the main role especially in the dynamic conditions of a gas chromatography (GC) experiment. Similar effects are observed also in static conditions when the swelling ratio and swelling time are dependent on the bead size and porous structure of the polymeric material [6]. Interesting results were observed for ternary mixture of polar components. In the case of the porous ME-A sample the separation at 200 ◦ C is reasonably good. It may indicate a smaller share of absorption and higher share of adsorption
in the separation process. For the nonporous ME-0 sample the retention even at 100 ◦ C is very weak and identical for the polar components. The adsorption mechanism of these components may be also confirmed by smaller retention and a poor separation on the porous PF-A sample characterized by a lower specific surface area in comparison to the ME-A polymer. Both melamine and phenolic sorbents have nitrogen or oxygen heteroatoms which permit adsorption of water and alcohols through polar interactions.
4. Conclusions The studies demonstrate that the porous melamine- and phenolic-formaldehyde polymers prepared in the presence of colloidal silica as a modifier producing their porosity are thermally stable sorbents with a high sorption capacity. Both polymeric sorbents swell markedly at the presence of water and organic solvents. The GC studies with a series of hydrocarbons and polar components for the porous and nonporous sample of the same polymer provided experimental evidence for the absorption and adsorption mechanism of the retention. Our further investigations will be devoted to chemical and textural modifications of melamine and phenolic-formaldehyde adsorbents for obtaining materials of the desired chromatographic resolution.
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J. Goworek et al. / Materials Chemistry and Physics 77 (2002) 276–280
Acknowledgements Financial support from the State Committee for Scientific Research (KBN, Warsaw), Project No. 3T09B00717 is gratefully acknowledged. References [1] M.P. Tsyurupa, A.S. Shabaeva, L.A. Pavlova, T.A. Mrachkovskaya, V.A. Davankov, in: B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing, K.K. Unger (Eds.), Characterization of Porous Solids IV, The Royal Society of Chemistry, Cambridge, 1997, p. 398. [2] T. Balakrishnan, W.T. Ford, J. Appl. Polym. Sci. 27 (1982) 133. [3] M. Gassiot-Matas, B. Monrabal-Bas, Chromatographia 3 (1970) 547.
[4] O. Dufka, J. Malinsky, J. Churaˇcek, K. Komarek, J. Chromatogr. 51 (1970) 111. [5] M. Ousalem, X.X. Zhu, J. Hradil, J. Chromatogr. A 903 (2000) 13. [6] S. Kiatkamjornwong, S. Traisaranapong, P. Prasassarakich, J. Porous Mater. 6 (1999) 205. [7] H. Omidian, S.A. Hashemi, P.G. Sammes, I. Meldrum, Polymer 26 (1998) 6697. [8] L.D. Belyakova, A.V. Kiselev, N.D. Platonova, T.I. Schevchenko, Adv. Colloid Interfac. Sci. 21 (1984) 55. [9] B.J. Brune, J.A. Koehler, P.J. Smith, G.F. Payne, Langmuir 15 (1999) 3987. [10] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [11] J. Goworek, W. Stefaniak, W. Zgrajka, Mater. Chem. Phys. 59 (1999) 149. [12] F. Paulik, J. Paulik, J. Therm. Anal. 5 (1973) 253.