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Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels
Accepted Manuscript Title: Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels Authors: Eva Kinnertov´a, V´aclav Slov´ak PII: DOI: Reference:
S0040-6031(17)30327-1 https://doi.org/10.1016/j.tca.2017.12.017 TCA 77899
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
31-8-2017 8-12-2017 12-12-2017
Please cite this article as: Eva Kinnertov´a, V´aclav Slov´ak, Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels, Thermochimica Acta https://doi.org/10.1016/j.tca.2017.12.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels Eva Kinnertová, Václav Slovák Faculty of Science, University of Ostrava, 30. dubna 22, 701 03, Ostrava, Czech Republic Email: [email protected], [email protected] Highlights Catalyst amount affects the structure and properties of organic and carbon RF xerogels. Less catalyst – better adsorption properties, microporosity. More catalyst – better mechanical properties, ink-bottle shaped mesopores. No effect of catalyst on pyrolysis and oxyreactivity.
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Abstract
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The effect of catalyst amount on properties of resorcinol-formaldehyde xerogels was investigated. The samples were prepared by sol-gel polycondensation of resorcinol with formaldehyde at three different levels of catalyst (Na2CO3, molar ratios of resorcinol/catalyst R/C = 1000, 2000, 3000). The influence on mechanical, thermal and thermomechanical properties of organic xerogels and on properties of carbon xerogels (surface, adsorption and oxyreactivity) was evaluated. The higher R/C ratio leads to increased particle size of the materials, which affects mechanical strength, residual humidity and shrinkage during heating (all parameters decrease). The R/C ratio does not affect significantly pyrolysis process and microporosity of the materials, the effect on oxyreactivity was negligible. Contrary, higher catalyst content (R/C = 1000) lead to relative high volume of ink-bottle shaped mesopores, but this pores are practically unavailable for adsorption of large molecules of methylene blue or hydrated chromate anions, which negatively influenced adsorption of both adsorbates from aqueous solution on the carbon xerogels. Keywords: Resorcinol-formaldehyde xerogels; catalyst amount; mechanical properties; surface; adsorption; oxyreactivity.
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1. Introduction
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Resorcinol-formaldehyde (RF) carbon xerogels are monolithic amorphous materials with the interesting properties such as high porosity, high surface area, low density, and chemical resistance [1]. These materials are promising for many applications due to the possibility of controlling their properties via the synthesis, drying or pyrolysis conditions [2]. The high specific surface areas and electric conductivity make these materials good candidates for the elaboration of electrodes for supercapacitors, batteries or fuel cells [3–5]. Their low thermal conductivity is interesting for the development of thermal insulators [6]. The ability to tailor the pore texture of the carbon materials is a great advantage in the fields of adsorption [7– 9], catalysis [1] including photocatalysis [10,11] or hydrogen storage [12]. RF xerogels are produced by sol-gel polycondensation of resorcinol with formaldehyde in the presence of alkaline or acidic catalyst. The concentration of reactants (R and F) and the initial pH of aqueous solution [13] are the most important factors affecting the properties of organic gels in this stage of synthesis. The dried polymers, xerogels, are formed by evaporation of the solvent at the ambient pressure. However, the presence of the capillary pressure inside the pores during the drying step may cause changes/degradation of the pore structure of xerogels 1
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[1]. To produce the carbon xerogels, the dried gels are carbonized to form a highly porous carbon network. One of the most important factor influencing properties of resorcinol-formaldehyde xerogels is the molar ratio of resorcinol and used catalyst (R/C), which is in relation with initial pH of polycondensation mixture. In general, low R/C ratio (high catalyst content) results in polymeric gels, which consist of small particles in a well-connected configuration. High R/C ratios (low catalyst content) give rise to colloidal gels containing spherical particles connected by narrow necks [1]. It was reported that increasing R/C leads to higher values of pore volumes and BET surface area [14], but also opposite trend has been observed [15]. Other authors found out that mesoporosity of xerogels increases [16] whereas mesoporosity of aerogels decreases [17] with increasing R/C ratio. The increasing of R/C leads also to increase of aerogels microporosity [17]. The influence of catalyst amount on the mechanical properties of gels was also observed. Some authors reported that the shrinkage of xerogels (during drying [18]) and aerogels (during drying and carbonization [17]) increases with decreasing ratio R/C. In terms of the adsorption properties, the adsorption capacity of xerogels for methylene blue increases with increasing R/C [19]. Although a studies demonstrating ability of carbon xerogels for adsorption of heavy metals from aqueous media exist [8,9], the effect of R/C ratio in this field has not been described yet. Most of the presented (and many other) literature sources deals with effect of R/C to specific properties of the carbon xerogels, but there is a lack of information enabling assessment of the effects on the complex set of properties (often mutually interconnected). That is the reason for our investigation of the influence of catalyst amount (ratio R/C = 1000 - 3000) on a wide range of properties of resorcinol-formaldehyde xerogels (mechanical, thermal, thermomechanical, surface, adsorption, oxyreactivity) presented in this work. 2. Experimental procedures 2.1 Preparation of xerogels
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Preparation of resorcinol-formaldehyde xerogels was based on the sol-gel polycondensation of resorcinol (R) (99 %, p. a., Penta) and formaldehyde (F) (38 %, p. a., Mach Chemikálie s.r.o.) described elsewhere [20]. Resorcinol (16.5 g) was dissolved in 38% formaldehyde (24.32 g) and appropriate amount of 0.1% solution of Na2CO3 (C) (anhydrous, Lachema) and demineralized water was added to obtain molar ratios R/C = 1000, 2000 and 3000 (Table 1). The resulting solution was mixed and poured into the glass tubes (diameter 10 mm) which were sealed by rubber stoppers. The glass tubes containing the RF solution were kept at 21 °C for 11 days. Then the wet organic gels were cut to small cylinders and dried at room temperature for 48 hours. Part of the obtained organic xerogels was pyrolyzed in the atmosphere of nitrogen (30 minutes at 100 °C, 1 hour at 500 °C, heating rate 10 K/min).
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2.2 Mechanical properties of xerogels Softness of organic xerogels was measured by Setsys Evolution, Setaram with thermomechanical (TMA) module. The minimal (5 g) and maximal (150 g) load was applied on the samples (cylinders size 2 – 4 mm) by quartz hemispherical probe at room temperature. The movement of the probe between both used loads was recorded as the measure of softness. The measurement of softness was carried out three times, the average values are presented. 2.3 Thermal and thermomechanical analysis
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The dimensional changes of xerogels were monitored during heating. Measurements were carried out using Setsys Evolution, Setaram with thermomechanical (TMA) module with flat quartz probe. The organic xerogels in the form of cylinders (height about 3 mm) were analysed in the flow of argon (20 mL/min). The samples were tested with load of 5 g. Temperature program consisted of heating from 15 °C to 150 °C (dimensional changes during drying) or 900 °C (dimensional changes during pyrolysis) with the heating rate of 1 K/min followed by cooling back to 15°C with the same rate. Pyrolysis of organic xerogels was studied by the simultaneous thermogravimetric analysis and differential scanning calorimetry (TG/DSC) using Setsys Evolution. Experiments were carried out in the alumina crucibles without standard in the inert argon atmosphere with the flow rate of 20 mL/min. About 20 mg of sample was used for analysis. The samples were equilibrated at 15 °C for 30 minutes and then were heated with a rate of 10 K/min to the final temperature 1000 °C. Oxidative reactivity of the samples was observed thermoanalytically. The simultaneous TG/DSC oxidative experiments were performed using STA 449 C Jupiter, Netzsch. Oxidation was carried out in the alumina crucibles without standard in the dynamic atmosphere containing 20 % O2 and 80 % N2 with the total flow rate of 100 mL/min. The mass of samples was in the range 7.5 – 9.0 mg. The samples were kept at 30 °C for 30 minutes and then they were heated with a rate of 10 K/min to 1000 °C. 2.4 Adsorption from aqueous solution
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Adsorption of methylene blue (MB), iodine, and hexavalent chromium was tested in aqueous solutions. 10 mL of MB solution with various initial concentrations was added to 10 - 12 mg of powdered carbon xerogel sample. The initial concentration of solutions ranged from 0.25 to 2.5 mmol/L. Adsorption was realized at room temperature in the plastic tubes for 0.5 hours to reach equilibrium. The concentration of MB was determined using spectrophotometry (Spectronic 200, Thermo Fischer Scientific) at wavelength of 665 nm. The adsorbed amount of MB was calculated according to equation (1), (𝑐0 −𝑐𝑅 )∙𝑉 𝑚𝑎𝑑𝑠
(1)
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𝑎=
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where a is the adsorbed amount of adsorbate [mmol/g], c0 is the initial concentration of adsorbate [mmol/L], cR is the equilibrium concentration [mmol/L], V is the volume of solution used for adsorption [L] and mads is the mass of adsorbent [g].
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Iodine number (IN) was determined as the amount of iodine adsorbed on 1 g of sample under specific conditions. Experiment was carried out as follows: 25 mL of iodine solution (I2 + KI) (c = 0.0243 mol/L) was mixed with 0.2 g of carbon sample, Erlenmayer flask with this mixture was slowly shaken for 90 minutes at room temperature, then the solution was filtered and 20 mL of filtrate was titrated with Na2S2O3 (p. a., Sigma Aldrich) standardized solution (c = 0.0395 mol/L) using starch as indicator. The value of iodine number was calculated according to the equation (1). Chromium(VI) was chosen as a representative of the toxic heavy metal ions. The stock Cr(VI) solution with concentration of 1 mmol/L was prepared from potassium chromate (99 %, p. a., Lachema), 0.01 M hydrochloric acid (p. a., Mach Chemikálie s.r.o.) was used as a solvent to obtain pH about 2. Adsorption of Cr(VI) was carried out by adding 250 mg of the carbon sample 3
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to 50 mL of Cr(VI) solution (1 mmol/L). This mixture was stirred using a magnetic stirrer for 24 hours to equilibrium. The stirring was stopped and after sedimentation of adsorbent 0.5 mL of the supernatant was taken for analysis. 0.5 mL of Cr(VI) solution (100 mmol/L) was then added to the mixture and this process was repeated several times. The equilibrium concentration was in the range 1 – 8 mmol/L, the pH was during the whole experiment in the range 2.0 – 2.5. The concentration of metal solution was determined using atomic absorption spectrophotometer (FS 240, Varian). The adsorbed amount was calculated according to the equation (1). The data from adsorption of MB and Cr(VI) were interpolated by Langmuir isotherm and the maximal adsorption capacities were calculated. 2.5 Characterization of surface properties
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The surface morphology of samples was investigated using scanning electron microscope (SEM) JSM-6610LV, JEOL. Samples were coated with gold before observation. The accelerating voltage was 30 kV.
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The porosity and surface characteristics of samples were determined by nitrogen adsorptiondesorption isotherms analysis at 77 K using 3Flex Surface Characterization Analyser, Micromeritics. The cumulative mesopore surface area Smeso and mesopore volume Vmeso were obtained from desorption branch of the N2 isotherms using BJH method [21]. The micropore volume Vmicro,N2 and micropore surface area Smicro,N2 were derived by the t-plot method using Carbon Black STSA thickness curve [22].
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3 Results and discussion 3.1 Softness of organic xerogels
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The micropore surface area Smicro,CO2 and micropore volume Vmicro,CO2 were determined by adsorption of CO2 at 30 °C using sorption analyser PCTPro E&E, Hy Energy, Setaram. The surface area of micropores was evaluated from Medek’s equation and volume of micropores from Dubinin-Radushkevich equation, calculation details are described elsewhere [23].
The mechanical properties of organic xerogels determine possibilities of their application in various physical forms as films, cylinders, spherical particles etc.
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Different amount of catalyst used during preparation of samples affected the mechanical properties and appearance of organic xerogels. The samples RC2000 and RC3000 were softer and with a similar appearance of ochre colour. The RC1000 sample was brown and substantially stiffer.
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The simple test of softness of the prepared samples based on measurement of hemispherical indenter movement after applying a force was used. The Fig. 1 illustrates that the softness of the organic xerogels proportionally increased with decreasing of the catalyst content (increasing the R/C ratio). This result is in agreement with general expectation that higher amount of catalyst enables higher conversion of reactants (in the same time of reaction) leading in our case to the higher degree of crosslinking and higher hardness.
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3.2 Dimensional changes of organic xerogels during heating
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Fig. 1 Dependence of intrusion of probe into the RF xerogels on the catalyst content
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Many of the xerogels applications expect their use under the different temperature conditions (e.g. electrodes for supercapacitors or adsorption columns). Therefore it is important to know the behaviour of these materials during the temperature changes.
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The shrinkage of organic xerogels with different catalyst content was measured during heating up to 150 °C. During heating, the shrinkage of xerogels is caused by capillary tension inside the pores of wet material [1]. The Fig. 2 illustrates that the shrinkage of the organic xerogels increased with increasing catalyst content (decreasing the R/C ratio) in the range 3.3 - 4.6 %. Using more catalyst, the gel microstructure becomes finer, composed of the smaller particles and pores, which leads to an increase of the capillary drying stresses and to the collapse of the organic structure [1]. In order to confirm the influence of moisture in the samples on their shrinkage, the samples were dried at 70°C for 24 hours in the oven, kept in the desiccator with dry silica gel (denoted as “dry”) and examined in the same manner. The dimensional changes of the dried samples were minimal and the influence of the catalyst content was negligible in this case (Fig. 2).
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Comparison of the shrinkage and moisture content (determined by thermogravimetric experiments, see Fig. 4) for all three samples (Fig. 3) shows that the relation between these parameters is evident. Increasing the catalyst content during preparation (decreasing the R/C ratio) leads to the higher content of residual moisture after drying at the ambient conditions, which causes more intense shrinkage of the materials during consequent heating. This behaviour can be explained by the differences in the structure of materials prepared with the different amount of catalyst. The higher concentration of catalyst (RC1000) leads to more compact and closed structure which keeps more water during drying at the ambient conditions. In this case, evolution of the moisture (and the beginning of shrinkage) becomes at the higher temperature in comparison with RC2000 or RC3000 (Fig. 2).
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Fig. 2 Shrinkage of the organic xerogels during heating to 150 °C
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Fig. 3 Dependence of shrinkage on the content of moisture 3.3 Pyrolysis of organic xerogels
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Pyrolysis of organic xerogels is one of the key steps of their transformation to carbonaceous materials. The process is accompanied with increasing of carbon content, chemical and thermal stability and enhancement of surface properties at the micropore level [14].
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The process of pyrolysis of the organic xerogels is illustrated in Fig. 4 (mass changes) and Fig. 5 (dimensional changes). The release of residual moisture (up to 200 °C) is the first step of process and was discussed together with the dimensional changes in chapter 3.2. The pyrolysis process continues with two reaction steps recorded as two peaks on DTG curves (300 – 450 °C and 490 – 800 °C). The dimensional changes had a similar course but with different intensity. The mass change (Fig. 4) in the first step of pyrolysis was more significant than the dimensional change (Fig. 5). Therefore it can be assumed that this step occurs without the considerable structural changes of carbonaceous skeleton of the organic xerogels and only the surface groups and heteroatoms are released.
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The carbonization process occurs at the second step of pyrolysis. This is associated with the massive shrinkage and changes of mass (changes occur in the carbonaceous skeleton of xerogel). From the TMA measurements it can be presumed that the second pyrolysis step is at least a two-stage process because the derived thermomechanical peaks indicate two maxima.
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The influence of the R/C ratio on the mass or the dimensional changes during pyrolysis of the organic xerogels is not significant. The mass loss slightly increases with decreasing R/C ratio, which is in agreement with more compactly crosslinked structure in the presence of higher concentration of catalyst.
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Fig. 4 TG and DTG curves of pyrolysis of the organic xerogels
Fig. 5 TMA and derivation TMA curves of pyrolysis of the organic xerogels 3.4 Surface properties of carbon xerogels The surface characteristics of carbonaceous samples are included in Table 2, the pore size distribution is depicted in Fig. 6. It is clear that surface characteristics of mesopores (surface 7
Smeso and volume Vmeso) for sample RC1000 are considerably higher than these values of the other samples. In fact, only the RC1000 sample (highest content of catalyst during condensation) displays significant mesoporosity. The characteristics of microporosity (surface Smicro,N2 and Smicro,CO2, volume Vmicro,N2 and Vmicro,CO2) are nearly the same for all samples. These results indicate that all samples have very similar microporosity, which does not depend on catalyst content. The microporous structure is formed during pyrolysis of xerogels (not in the polycondensation step) and therefore the micropores are independent on the R/C ratio.
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Surprisingly, the iodine number is considerable lower for RC1000 sample. The higher values of Smeso and Vmeso and considerably lower adsorption of iodine for RC1000 in comparison with other samples can be caused probably by different shape of pores. To understand the situation, the kinetics of nitrogen adsorption (time required to reach the equilibrium) was inspected in details for all three materials. It was found that the adsorption of nitrogen took three times longer time for RC1000 than for the other samples. This finding suggests that mesopores present in RC1000 sample are poorly accessible and they have probably ink-bottle shape with very narrow openning at micropore level. Due to this shape the pores are partially blocked for nitrogen molecules, which require longer time for adsorption and attainment of equilibrium during measurement. The mesopores openning has to be so small that it acts as a barrier for iodine molecules (under conditions used for iodine number determination), which are not able to be adsorbed in mesopores and micropores connected with them. Thus the iodine number is smaller for RC1000 sample despite the fact that its micropore characteristics are very close to samples RC2000 and RC3000.
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The different size and shape of pores caused by catalyst amount (as in this case) can be the reason for contradictory observation of the influence of R/C ratio to surface characteristics of carbon xerogels (e.g. [14,15]).
Fig. 6 The pore size distribution of studied samples determined by desorption branch of the N2 adsorption isotherm by BJH analysis SEM images of the surface morphology of the carbon xerogels are shown in Fig. 7. It was observed that the size of particles increased with increasing molar ratio R/C. This dependence 8
is in agreement with literature [1]. The sample with the highest catalyst content was characterized by compactly crosslinked structure and the particles are small, not observable at the given magnification. Reduction of the catalyst amount resulted in the increase of particle size. The particles exhibited size from several tens to hundreds of nanometers in diameter for the sample RC2000. The highest R/C value (RC3000) leads to the particles about a micrometer size. b)
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Fig. 7 SEM images of the carbon xerogels a) RC1000, b) RC2000, c) RC3000 3.5 Adsorption properties of carbon xerogels The adsorption properties of prepared RF carbon xerogels were tested on adsorption of cationic dye methylene blue (MB) and chromate anions from aqueous media (Fig 8).
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The observed similarity of the adsorption of both adsorbates indicates a similar mechanism of adsorption based probably on the same active sites on the xerogels surface. The comparison with the surface characteristics (see chapter 3.4) and expected porosity of the samples shows that both adsorbates are adsorbed in large micropores. The reason for lower adsorbed amount of MB and Cr(VI) on the sample RC1000 is connected with expectation that this sample has the micropores partly present in ink-bottle shaped mesopores which are probably unavailable for studied adsorbates (at least part of them).
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The methylene blue is adsorbed in the form of cations with size 0.400 x 0.793 x 1.634 nm [24], which can be hydrated by up to 3 water molecules. Contrary, anion Cr(VI) is much smaller, but intensively hydrated – the first hydration shell contains 14 molecules of water and its diameter is 1.05 nm [25]. Considering the anion aggregation in acidic conditions (the adsorption experiments were performed at pH 2) and formation of bichromate anion Cr2O72– leads to conclusion that the size of adsorbed Cr(VI) particles exceeds the size of methylene blue molecules. Thus, the Cr(VI) particles are not able to enter all microporosity occupied by better sized and shaped MB molecules and its adsorption is at slightly lower level for samples RC2000 and RC3000. The size of adsorbates can also enable to understand the more distinct difference of adsorption of MB and Cr(VI) on RC1000 – smaller molecules of MB are able to enter and be adsorbed in part of ink-bottle mesopores with wider entrance while this surface is unavailable for bigger hydrated bichromate particles.
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According to [19], the adsorption capacity of alkaline catalyzed xerogels increases with the increasing R/C ratio in the range of R/C 50 - 1000. The increasing of the R/C ratio of our samples led to growth of adsorption capacity for series RC1000 - RC2000. This findings are in agreement with the changes of porosity type when the catalyst content is decreased from R/C = 1000 to R/C = 2000. Further increasing of the R/C ratio did not cause improvement of the adsorption properties of the carbon xerogels, the materials stays mostly microporous.
Fig. 8 Adsorption capacities of the carbon xerogels for methylene blue (white) and Cr(VI) (black) 3.6 Reactivity of carbon xerogels with oxygen
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The heterogeneous catalysis is another field where the carbon xerogels are applied. The carbon xerogels can be used either as catalyst support or as catalysts on their own [26]. The high stability (low reactivity) of materials is required for these applications. Thus, the reactivity of the carbon xerogels with oxygen was investigated. Oxidation reactivity of the carbon xerogels was studied by heating of the samples in the oxidation atmosphere using thermal analysis (Fig. 9). The release of negligible amount of residual moisture occurred bellow 200 °C. Further heating lead to a reasonable increase of mass (detail in Fig. 9) caused by interaction of oxygen with carbon surface and creation of the oxygen 10
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containing surface groups (chemisorption of oxygen). The extent of oxygen chemisorption increased in a series RC3000 – RC1000 – RC2000. The mass increase did not correlate with the surface characteristics of the xerogels and the influence of the catalyst concentration on chemisorption of oxygen was not observed. The process continued as normal oxidation (combustion) with the mass decrease above 350°C. The onset temperature of burning corresponded about 354 °C for all samples. The burning was completed at 535 – 565 °C and the rate increased with the decrease of catalyst content. Slightly higher oxidation resistance of sample RC1000 is connected with expected ink-bottle shaped mesopores which are worse available for interaction with oxygen due to diffusion barrier under dynamic heating.
Conclusions
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Fig. 9 TG curves of oxidation of xerogels in 20 % O2
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In this work the influence of the different catalyst amount (R/C ratio in a range 1000 3000) on the properties of resorcinol-formaldehyde xerogels was investigated. The effect of R/C especially on mechanical, thermal, thermomechanical, surface, adsorption properties and oxyreactivity of xerogels was evaluated. The increasing of the R/C ratio led to the increase of the xerogels particle size from nanoscale level at R/C = 1000 to micrometer size for R/C = 3000. The observed morphology is in agreement with mechanical strength of the material. Small and well connected particles form very compact and rigid structure, while increasing of particle size leads to not so stiffer material. The morphology is also related to shrinkage of organic materials (before carbonization) during thermal treatment. The extent of shrinkage is proportional to moisture content after drying at ambient conditions. Decreasing size of particles (connected with increasing catalyst content) leads to higher content of residual moisture and higher shrinkage of the materials during heating. Contrary, the influence of the R/C ratio on the mass or dimensional changes during pyrolysis of the organic xerogels was not significant. The mass loss slightly increased with 11
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decreasing R/C ratio, which is in agreement with more compact structure in the presence of higher concentration of catalyst. The catalyst amount had negligible effect on microporosity of the materials, but considerable mesoporosity was found only at high catalyst content R/C = 1000. Comparing the surface characteristics from gas sorption and adsorption from aqueous media (iodine, methylene blue and Cr(VI)) lead to conclusion that the mesopores of RC1000 sample are of ink-bottle shape with very narrow openings. The mesopores are mostly unavailable for adsorption from aqueous solutions and the adsorption capacity of RC1000 sample is the lowest. Decreasing of the catalyst content (RC2000 and RC3000) lead to disappearing of the mesoporosity and only available micropores are present. The differences in surface of samples RC1000 and the others have negligible effect on the interaction with oxygen during heating, where only micropores plays important role. Acknowledgements This work was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic in the “National Feasibility Program I”, project LO1208 “TEWEP”, by EU structural funding Operational Programme Research and Development for Innovation, project No. CZ.1.05/2.1.00/19.0388 and by students grant no. SGS01/PřF/2017 (University of Ostrava). The authors gratefully acknowledge Miloslav Lhotka from the Department of Inorganic Technology (University of Chemistry and Technology Prague) for performing the surface characterization of studied samples by nitrogen adsorption. References
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http://search.proquest.com/openview/1b88d2e2677a0c23eade2dd4fb9e8783/1?pqorigsite=gscholar&cbl=2040555 (accessed August 30, 2017). M. Osińska, Removal of lead(II), copper(II), cobalt(II) and nickel(II) ions from aqueous solutions using carbon gels, J. Sol-Gel Sci. Technol. 81 (2017) 678–692. doi:10.1007/s10971-016-4256-0. F. Orellana-García, M.A. Álvarez, M.V. López-Ramón, J. Rivera-Utrilla, M. SánchezPolo, M.Á. Fontecha-Cámara, Photoactivity of organic xerogels and aerogels in the photodegradation of herbicides from waters, Appl. Catal. B Environ. 181 (2016) 94–102. doi:10.1016/j.apcatb.2015.07.044. J.J. Salazar-Rábago, M. Sánchez-Polo, J. Rivera-Utrilla, R. Leyva-Ramos, R. OcampoPérez, F. Carrasco-Marin, Organic xerogels doped with Tris (2,2′-bipyridine) ruthenium(II) as hydroxyl radical promoters: Synthesis, characterization, and photoactivity, Chem. Eng. J. 306 (2016) 289–297. doi:10.1016/j.cej.2016.07.053. S. Schaefer, V. Fierro, A. Szczurek, M.T. Izquierdo, A. Celzard, Physisorption, chemisorption and spill-over contributions to hydrogen storage, Int. J. Hydrog. Energy. 41 (2016) 17442–17452. doi:10.1016/j.ijhydene.2016.07.262. N. Rey-Raap, A. Arenillas, J.A. Menéndez, A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels, Microporous Mesoporous Mater. 223 (2016) 89–93. doi:10.1016/j.micromeso.2015.10.044. A.M. El Khatat, S.A. Al-Muhtaseb, Advances in Tailoring Resorcinol-Formaldehyde Organic and Carbon Gels, Adv. Mater. 23 (2011) 2887–2903. doi:10.1002/adma.201100283. L.C. Cotet, V. Danciu, V. Cosoveanu, I.C. Popescu, A. Roig, E. Molins, Synthesis of meso-and macroporous carbon aerogels, Rev. Roum. Chim. 52 (2007) 1077–1081. E.A. Oyedoh, A.B. Albadarin, G.M. Walker, M. Mirzaeian, M.N.M. Ahmad, Preparation of Controlled Porosity Resorcinol Formaldehyde Xerogels for Adsorption Applications, Chem. Eng. Trans. 32 (2013) 1651–1656. doi:10.3303/CET1332276. J. Feng, J. Feng, C. Zhang, Shrinkage and pore structure in preparation of carbon aerogels, J. Sol-Gel Sci. Technol. 59 (2011) 371–380. doi:10.1007/s10971-011-2514-8. A. Léonard, S. Blacher, M. Crine, W. Jomaa, Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying, J. NonCryst. Solids. 354 (2008) 831–838. doi:10.1016/j.jnoncrysol.2007.08.024. B.S. Girgis, A.A. Attia, N.A. Fathy, Potential of nano-carbon xerogels in the remediation of dye-contaminated water discharges, Desalination. 265 (2011) 169–176. doi:10.1016/j.desal.2010.07.048. M. Wiener, G. Reichenauer, T. Scherb, J. Fricke, Accelerating the synthesis of carbon aerogel precursors, J. Non-Cryst. Solids. 350 (2004) 126–130. doi:10.1016/j.jnoncrysol.2004.06.029. E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380. ASTM D6556-14, Standard test method for carbon black—total and external surface area by nitrogen adsorption, ASTM International, West Conshohocken, PA. (2014). T. Zelenka, B. Taraba, Sorption of CO2 on low-rank coal: Study of influence of various drying methods on microporous characteristics, Int. J. Coal Geol. 132 (2014) 1–5. doi:10.1016/j.coal.2014.07.006. X. Zhao, X. Bu, T. Wu, S.-T. Zheng, L. Wang, P. Feng, Selective anion exchange with nanogated isoreticular positive metal-organic frameworks, Nat. Commun. 4 (2013). doi:10.1038/ncomms3344.
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[25] E. Hinteregger, A.B. Pribil, T.S. Hofer, B.R. Randolf, A.K.H. Weiss, B.M. Rode, Structure and Dynamics of the Chromate Ion in Aqueous Solution. An ab Initio QMCFMD Simulation, Inorg. Chem. 49 (2010) 7964–7968. doi:10.1021/ic101001e. [26] J.L. Figueiredo, M.F.R. Pereira, The role of surface chemistry in catalysis with carbons, Catal. Today. 150 (2010) 2–7. doi:10.1016/j.cattod.2009.04.010. Table 1 Synthesis parameters of resorcinol-formaldehyde xerogels
1000 2000 3000
Mass of H2O (g) 6.93 14.93 17.60
Table 2 The surface characterizations of xerogels
Smicro,N2 (m2/g) 485 522 524
Vmicro,N2 (cm3/g) 0.191 0.204 0.205
Smicro,CO2 (m2/g) 1002 1041 1074
Vmicro,CO2 (cm3/g) 0.362 0.374 0.388
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Vmeso (cm3/g) 0.218 0.057 0.038
Iodine number IN (mmol/g) 0.981 1.954 1.931
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RC1000 RC2000 RC3000
Smeso (m2/g) 116 44 39
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Mass of 0.1% Na2CO3 (g) 16.00 8.00 5.33
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Molar ratio R/C
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S0040-6031(17)30327-1 https://doi.org/10.1016/j.tca.2017.12.017 TCA 77899
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
31-8-2017 8-12-2017 12-12-2017
Please cite this article as: Eva Kinnertov´a, V´aclav Slov´ak, Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels, Thermochimica Acta https://doi.org/10.1016/j.tca.2017.12.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels Eva Kinnertová, Václav Slovák Faculty of Science, University of Ostrava, 30. dubna 22, 701 03, Ostrava, Czech Republic Email: [email protected], [email protected] Highlights Catalyst amount affects the structure and properties of organic and carbon RF xerogels. Less catalyst – better adsorption properties, microporosity. More catalyst – better mechanical properties, ink-bottle shaped mesopores. No effect of catalyst on pyrolysis and oxyreactivity.
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Abstract
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The effect of catalyst amount on properties of resorcinol-formaldehyde xerogels was investigated. The samples were prepared by sol-gel polycondensation of resorcinol with formaldehyde at three different levels of catalyst (Na2CO3, molar ratios of resorcinol/catalyst R/C = 1000, 2000, 3000). The influence on mechanical, thermal and thermomechanical properties of organic xerogels and on properties of carbon xerogels (surface, adsorption and oxyreactivity) was evaluated. The higher R/C ratio leads to increased particle size of the materials, which affects mechanical strength, residual humidity and shrinkage during heating (all parameters decrease). The R/C ratio does not affect significantly pyrolysis process and microporosity of the materials, the effect on oxyreactivity was negligible. Contrary, higher catalyst content (R/C = 1000) lead to relative high volume of ink-bottle shaped mesopores, but this pores are practically unavailable for adsorption of large molecules of methylene blue or hydrated chromate anions, which negatively influenced adsorption of both adsorbates from aqueous solution on the carbon xerogels. Keywords: Resorcinol-formaldehyde xerogels; catalyst amount; mechanical properties; surface; adsorption; oxyreactivity.
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1. Introduction
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Resorcinol-formaldehyde (RF) carbon xerogels are monolithic amorphous materials with the interesting properties such as high porosity, high surface area, low density, and chemical resistance [1]. These materials are promising for many applications due to the possibility of controlling their properties via the synthesis, drying or pyrolysis conditions [2]. The high specific surface areas and electric conductivity make these materials good candidates for the elaboration of electrodes for supercapacitors, batteries or fuel cells [3–5]. Their low thermal conductivity is interesting for the development of thermal insulators [6]. The ability to tailor the pore texture of the carbon materials is a great advantage in the fields of adsorption [7– 9], catalysis [1] including photocatalysis [10,11] or hydrogen storage [12]. RF xerogels are produced by sol-gel polycondensation of resorcinol with formaldehyde in the presence of alkaline or acidic catalyst. The concentration of reactants (R and F) and the initial pH of aqueous solution [13] are the most important factors affecting the properties of organic gels in this stage of synthesis. The dried polymers, xerogels, are formed by evaporation of the solvent at the ambient pressure. However, the presence of the capillary pressure inside the pores during the drying step may cause changes/degradation of the pore structure of xerogels 1
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[1]. To produce the carbon xerogels, the dried gels are carbonized to form a highly porous carbon network. One of the most important factor influencing properties of resorcinol-formaldehyde xerogels is the molar ratio of resorcinol and used catalyst (R/C), which is in relation with initial pH of polycondensation mixture. In general, low R/C ratio (high catalyst content) results in polymeric gels, which consist of small particles in a well-connected configuration. High R/C ratios (low catalyst content) give rise to colloidal gels containing spherical particles connected by narrow necks [1]. It was reported that increasing R/C leads to higher values of pore volumes and BET surface area [14], but also opposite trend has been observed [15]. Other authors found out that mesoporosity of xerogels increases [16] whereas mesoporosity of aerogels decreases [17] with increasing R/C ratio. The increasing of R/C leads also to increase of aerogels microporosity [17]. The influence of catalyst amount on the mechanical properties of gels was also observed. Some authors reported that the shrinkage of xerogels (during drying [18]) and aerogels (during drying and carbonization [17]) increases with decreasing ratio R/C. In terms of the adsorption properties, the adsorption capacity of xerogels for methylene blue increases with increasing R/C [19]. Although a studies demonstrating ability of carbon xerogels for adsorption of heavy metals from aqueous media exist [8,9], the effect of R/C ratio in this field has not been described yet. Most of the presented (and many other) literature sources deals with effect of R/C to specific properties of the carbon xerogels, but there is a lack of information enabling assessment of the effects on the complex set of properties (often mutually interconnected). That is the reason for our investigation of the influence of catalyst amount (ratio R/C = 1000 - 3000) on a wide range of properties of resorcinol-formaldehyde xerogels (mechanical, thermal, thermomechanical, surface, adsorption, oxyreactivity) presented in this work. 2. Experimental procedures 2.1 Preparation of xerogels
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Preparation of resorcinol-formaldehyde xerogels was based on the sol-gel polycondensation of resorcinol (R) (99 %, p. a., Penta) and formaldehyde (F) (38 %, p. a., Mach Chemikálie s.r.o.) described elsewhere [20]. Resorcinol (16.5 g) was dissolved in 38% formaldehyde (24.32 g) and appropriate amount of 0.1% solution of Na2CO3 (C) (anhydrous, Lachema) and demineralized water was added to obtain molar ratios R/C = 1000, 2000 and 3000 (Table 1). The resulting solution was mixed and poured into the glass tubes (diameter 10 mm) which were sealed by rubber stoppers. The glass tubes containing the RF solution were kept at 21 °C for 11 days. Then the wet organic gels were cut to small cylinders and dried at room temperature for 48 hours. Part of the obtained organic xerogels was pyrolyzed in the atmosphere of nitrogen (30 minutes at 100 °C, 1 hour at 500 °C, heating rate 10 K/min).
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2.2 Mechanical properties of xerogels Softness of organic xerogels was measured by Setsys Evolution, Setaram with thermomechanical (TMA) module. The minimal (5 g) and maximal (150 g) load was applied on the samples (cylinders size 2 – 4 mm) by quartz hemispherical probe at room temperature. The movement of the probe between both used loads was recorded as the measure of softness. The measurement of softness was carried out three times, the average values are presented. 2.3 Thermal and thermomechanical analysis
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The dimensional changes of xerogels were monitored during heating. Measurements were carried out using Setsys Evolution, Setaram with thermomechanical (TMA) module with flat quartz probe. The organic xerogels in the form of cylinders (height about 3 mm) were analysed in the flow of argon (20 mL/min). The samples were tested with load of 5 g. Temperature program consisted of heating from 15 °C to 150 °C (dimensional changes during drying) or 900 °C (dimensional changes during pyrolysis) with the heating rate of 1 K/min followed by cooling back to 15°C with the same rate. Pyrolysis of organic xerogels was studied by the simultaneous thermogravimetric analysis and differential scanning calorimetry (TG/DSC) using Setsys Evolution. Experiments were carried out in the alumina crucibles without standard in the inert argon atmosphere with the flow rate of 20 mL/min. About 20 mg of sample was used for analysis. The samples were equilibrated at 15 °C for 30 minutes and then were heated with a rate of 10 K/min to the final temperature 1000 °C. Oxidative reactivity of the samples was observed thermoanalytically. The simultaneous TG/DSC oxidative experiments were performed using STA 449 C Jupiter, Netzsch. Oxidation was carried out in the alumina crucibles without standard in the dynamic atmosphere containing 20 % O2 and 80 % N2 with the total flow rate of 100 mL/min. The mass of samples was in the range 7.5 – 9.0 mg. The samples were kept at 30 °C for 30 minutes and then they were heated with a rate of 10 K/min to 1000 °C. 2.4 Adsorption from aqueous solution
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Adsorption of methylene blue (MB), iodine, and hexavalent chromium was tested in aqueous solutions. 10 mL of MB solution with various initial concentrations was added to 10 - 12 mg of powdered carbon xerogel sample. The initial concentration of solutions ranged from 0.25 to 2.5 mmol/L. Adsorption was realized at room temperature in the plastic tubes for 0.5 hours to reach equilibrium. The concentration of MB was determined using spectrophotometry (Spectronic 200, Thermo Fischer Scientific) at wavelength of 665 nm. The adsorbed amount of MB was calculated according to equation (1), (𝑐0 −𝑐𝑅 )∙𝑉 𝑚𝑎𝑑𝑠
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where a is the adsorbed amount of adsorbate [mmol/g], c0 is the initial concentration of adsorbate [mmol/L], cR is the equilibrium concentration [mmol/L], V is the volume of solution used for adsorption [L] and mads is the mass of adsorbent [g].
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Iodine number (IN) was determined as the amount of iodine adsorbed on 1 g of sample under specific conditions. Experiment was carried out as follows: 25 mL of iodine solution (I2 + KI) (c = 0.0243 mol/L) was mixed with 0.2 g of carbon sample, Erlenmayer flask with this mixture was slowly shaken for 90 minutes at room temperature, then the solution was filtered and 20 mL of filtrate was titrated with Na2S2O3 (p. a., Sigma Aldrich) standardized solution (c = 0.0395 mol/L) using starch as indicator. The value of iodine number was calculated according to the equation (1). Chromium(VI) was chosen as a representative of the toxic heavy metal ions. The stock Cr(VI) solution with concentration of 1 mmol/L was prepared from potassium chromate (99 %, p. a., Lachema), 0.01 M hydrochloric acid (p. a., Mach Chemikálie s.r.o.) was used as a solvent to obtain pH about 2. Adsorption of Cr(VI) was carried out by adding 250 mg of the carbon sample 3
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to 50 mL of Cr(VI) solution (1 mmol/L). This mixture was stirred using a magnetic stirrer for 24 hours to equilibrium. The stirring was stopped and after sedimentation of adsorbent 0.5 mL of the supernatant was taken for analysis. 0.5 mL of Cr(VI) solution (100 mmol/L) was then added to the mixture and this process was repeated several times. The equilibrium concentration was in the range 1 – 8 mmol/L, the pH was during the whole experiment in the range 2.0 – 2.5. The concentration of metal solution was determined using atomic absorption spectrophotometer (FS 240, Varian). The adsorbed amount was calculated according to the equation (1). The data from adsorption of MB and Cr(VI) were interpolated by Langmuir isotherm and the maximal adsorption capacities were calculated. 2.5 Characterization of surface properties
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The surface morphology of samples was investigated using scanning electron microscope (SEM) JSM-6610LV, JEOL. Samples were coated with gold before observation. The accelerating voltage was 30 kV.
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The porosity and surface characteristics of samples were determined by nitrogen adsorptiondesorption isotherms analysis at 77 K using 3Flex Surface Characterization Analyser, Micromeritics. The cumulative mesopore surface area Smeso and mesopore volume Vmeso were obtained from desorption branch of the N2 isotherms using BJH method [21]. The micropore volume Vmicro,N2 and micropore surface area Smicro,N2 were derived by the t-plot method using Carbon Black STSA thickness curve [22].
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3 Results and discussion 3.1 Softness of organic xerogels
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The micropore surface area Smicro,CO2 and micropore volume Vmicro,CO2 were determined by adsorption of CO2 at 30 °C using sorption analyser PCTPro E&E, Hy Energy, Setaram. The surface area of micropores was evaluated from Medek’s equation and volume of micropores from Dubinin-Radushkevich equation, calculation details are described elsewhere [23].
The mechanical properties of organic xerogels determine possibilities of their application in various physical forms as films, cylinders, spherical particles etc.
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Different amount of catalyst used during preparation of samples affected the mechanical properties and appearance of organic xerogels. The samples RC2000 and RC3000 were softer and with a similar appearance of ochre colour. The RC1000 sample was brown and substantially stiffer.
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3.2 Dimensional changes of organic xerogels during heating
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Fig. 1 Dependence of intrusion of probe into the RF xerogels on the catalyst content
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The shrinkage of organic xerogels with different catalyst content was measured during heating up to 150 °C. During heating, the shrinkage of xerogels is caused by capillary tension inside the pores of wet material [1]. The Fig. 2 illustrates that the shrinkage of the organic xerogels increased with increasing catalyst content (decreasing the R/C ratio) in the range 3.3 - 4.6 %. Using more catalyst, the gel microstructure becomes finer, composed of the smaller particles and pores, which leads to an increase of the capillary drying stresses and to the collapse of the organic structure [1]. In order to confirm the influence of moisture in the samples on their shrinkage, the samples were dried at 70°C for 24 hours in the oven, kept in the desiccator with dry silica gel (denoted as “dry”) and examined in the same manner. The dimensional changes of the dried samples were minimal and the influence of the catalyst content was negligible in this case (Fig. 2).
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Comparison of the shrinkage and moisture content (determined by thermogravimetric experiments, see Fig. 4) for all three samples (Fig. 3) shows that the relation between these parameters is evident. Increasing the catalyst content during preparation (decreasing the R/C ratio) leads to the higher content of residual moisture after drying at the ambient conditions, which causes more intense shrinkage of the materials during consequent heating. This behaviour can be explained by the differences in the structure of materials prepared with the different amount of catalyst. The higher concentration of catalyst (RC1000) leads to more compact and closed structure which keeps more water during drying at the ambient conditions. In this case, evolution of the moisture (and the beginning of shrinkage) becomes at the higher temperature in comparison with RC2000 or RC3000 (Fig. 2).
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Fig. 2 Shrinkage of the organic xerogels during heating to 150 °C
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Fig. 3 Dependence of shrinkage on the content of moisture 3.3 Pyrolysis of organic xerogels
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Pyrolysis of organic xerogels is one of the key steps of their transformation to carbonaceous materials. The process is accompanied with increasing of carbon content, chemical and thermal stability and enhancement of surface properties at the micropore level [14].
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The process of pyrolysis of the organic xerogels is illustrated in Fig. 4 (mass changes) and Fig. 5 (dimensional changes). The release of residual moisture (up to 200 °C) is the first step of process and was discussed together with the dimensional changes in chapter 3.2. The pyrolysis process continues with two reaction steps recorded as two peaks on DTG curves (300 – 450 °C and 490 – 800 °C). The dimensional changes had a similar course but with different intensity. The mass change (Fig. 4) in the first step of pyrolysis was more significant than the dimensional change (Fig. 5). Therefore it can be assumed that this step occurs without the considerable structural changes of carbonaceous skeleton of the organic xerogels and only the surface groups and heteroatoms are released.
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The carbonization process occurs at the second step of pyrolysis. This is associated with the massive shrinkage and changes of mass (changes occur in the carbonaceous skeleton of xerogel). From the TMA measurements it can be presumed that the second pyrolysis step is at least a two-stage process because the derived thermomechanical peaks indicate two maxima.
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The influence of the R/C ratio on the mass or the dimensional changes during pyrolysis of the organic xerogels is not significant. The mass loss slightly increases with decreasing R/C ratio, which is in agreement with more compactly crosslinked structure in the presence of higher concentration of catalyst.
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Fig. 4 TG and DTG curves of pyrolysis of the organic xerogels
Fig. 5 TMA and derivation TMA curves of pyrolysis of the organic xerogels 3.4 Surface properties of carbon xerogels The surface characteristics of carbonaceous samples are included in Table 2, the pore size distribution is depicted in Fig. 6. It is clear that surface characteristics of mesopores (surface 7
Smeso and volume Vmeso) for sample RC1000 are considerably higher than these values of the other samples. In fact, only the RC1000 sample (highest content of catalyst during condensation) displays significant mesoporosity. The characteristics of microporosity (surface Smicro,N2 and Smicro,CO2, volume Vmicro,N2 and Vmicro,CO2) are nearly the same for all samples. These results indicate that all samples have very similar microporosity, which does not depend on catalyst content. The microporous structure is formed during pyrolysis of xerogels (not in the polycondensation step) and therefore the micropores are independent on the R/C ratio.
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Surprisingly, the iodine number is considerable lower for RC1000 sample. The higher values of Smeso and Vmeso and considerably lower adsorption of iodine for RC1000 in comparison with other samples can be caused probably by different shape of pores. To understand the situation, the kinetics of nitrogen adsorption (time required to reach the equilibrium) was inspected in details for all three materials. It was found that the adsorption of nitrogen took three times longer time for RC1000 than for the other samples. This finding suggests that mesopores present in RC1000 sample are poorly accessible and they have probably ink-bottle shape with very narrow openning at micropore level. Due to this shape the pores are partially blocked for nitrogen molecules, which require longer time for adsorption and attainment of equilibrium during measurement. The mesopores openning has to be so small that it acts as a barrier for iodine molecules (under conditions used for iodine number determination), which are not able to be adsorbed in mesopores and micropores connected with them. Thus the iodine number is smaller for RC1000 sample despite the fact that its micropore characteristics are very close to samples RC2000 and RC3000.
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The different size and shape of pores caused by catalyst amount (as in this case) can be the reason for contradictory observation of the influence of R/C ratio to surface characteristics of carbon xerogels (e.g. [14,15]).
Fig. 6 The pore size distribution of studied samples determined by desorption branch of the N2 adsorption isotherm by BJH analysis SEM images of the surface morphology of the carbon xerogels are shown in Fig. 7. It was observed that the size of particles increased with increasing molar ratio R/C. This dependence 8
is in agreement with literature [1]. The sample with the highest catalyst content was characterized by compactly crosslinked structure and the particles are small, not observable at the given magnification. Reduction of the catalyst amount resulted in the increase of particle size. The particles exhibited size from several tens to hundreds of nanometers in diameter for the sample RC2000. The highest R/C value (RC3000) leads to the particles about a micrometer size. b)
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Fig. 7 SEM images of the carbon xerogels a) RC1000, b) RC2000, c) RC3000 3.5 Adsorption properties of carbon xerogels The adsorption properties of prepared RF carbon xerogels were tested on adsorption of cationic dye methylene blue (MB) and chromate anions from aqueous media (Fig 8).
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The observed similarity of the adsorption of both adsorbates indicates a similar mechanism of adsorption based probably on the same active sites on the xerogels surface. The comparison with the surface characteristics (see chapter 3.4) and expected porosity of the samples shows that both adsorbates are adsorbed in large micropores. The reason for lower adsorbed amount of MB and Cr(VI) on the sample RC1000 is connected with expectation that this sample has the micropores partly present in ink-bottle shaped mesopores which are probably unavailable for studied adsorbates (at least part of them).
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The methylene blue is adsorbed in the form of cations with size 0.400 x 0.793 x 1.634 nm [24], which can be hydrated by up to 3 water molecules. Contrary, anion Cr(VI) is much smaller, but intensively hydrated – the first hydration shell contains 14 molecules of water and its diameter is 1.05 nm [25]. Considering the anion aggregation in acidic conditions (the adsorption experiments were performed at pH 2) and formation of bichromate anion Cr2O72– leads to conclusion that the size of adsorbed Cr(VI) particles exceeds the size of methylene blue molecules. Thus, the Cr(VI) particles are not able to enter all microporosity occupied by better sized and shaped MB molecules and its adsorption is at slightly lower level for samples RC2000 and RC3000. The size of adsorbates can also enable to understand the more distinct difference of adsorption of MB and Cr(VI) on RC1000 – smaller molecules of MB are able to enter and be adsorbed in part of ink-bottle mesopores with wider entrance while this surface is unavailable for bigger hydrated bichromate particles.
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According to [19], the adsorption capacity of alkaline catalyzed xerogels increases with the increasing R/C ratio in the range of R/C 50 - 1000. The increasing of the R/C ratio of our samples led to growth of adsorption capacity for series RC1000 - RC2000. This findings are in agreement with the changes of porosity type when the catalyst content is decreased from R/C = 1000 to R/C = 2000. Further increasing of the R/C ratio did not cause improvement of the adsorption properties of the carbon xerogels, the materials stays mostly microporous.
Fig. 8 Adsorption capacities of the carbon xerogels for methylene blue (white) and Cr(VI) (black) 3.6 Reactivity of carbon xerogels with oxygen
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containing surface groups (chemisorption of oxygen). The extent of oxygen chemisorption increased in a series RC3000 – RC1000 – RC2000. The mass increase did not correlate with the surface characteristics of the xerogels and the influence of the catalyst concentration on chemisorption of oxygen was not observed. The process continued as normal oxidation (combustion) with the mass decrease above 350°C. The onset temperature of burning corresponded about 354 °C for all samples. The burning was completed at 535 – 565 °C and the rate increased with the decrease of catalyst content. Slightly higher oxidation resistance of sample RC1000 is connected with expected ink-bottle shaped mesopores which are worse available for interaction with oxygen due to diffusion barrier under dynamic heating.
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Fig. 9 TG curves of oxidation of xerogels in 20 % O2
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In this work the influence of the different catalyst amount (R/C ratio in a range 1000 3000) on the properties of resorcinol-formaldehyde xerogels was investigated. The effect of R/C especially on mechanical, thermal, thermomechanical, surface, adsorption properties and oxyreactivity of xerogels was evaluated. The increasing of the R/C ratio led to the increase of the xerogels particle size from nanoscale level at R/C = 1000 to micrometer size for R/C = 3000. The observed morphology is in agreement with mechanical strength of the material. Small and well connected particles form very compact and rigid structure, while increasing of particle size leads to not so stiffer material. The morphology is also related to shrinkage of organic materials (before carbonization) during thermal treatment. The extent of shrinkage is proportional to moisture content after drying at ambient conditions. Decreasing size of particles (connected with increasing catalyst content) leads to higher content of residual moisture and higher shrinkage of the materials during heating. Contrary, the influence of the R/C ratio on the mass or dimensional changes during pyrolysis of the organic xerogels was not significant. The mass loss slightly increased with 11
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decreasing R/C ratio, which is in agreement with more compact structure in the presence of higher concentration of catalyst. The catalyst amount had negligible effect on microporosity of the materials, but considerable mesoporosity was found only at high catalyst content R/C = 1000. Comparing the surface characteristics from gas sorption and adsorption from aqueous media (iodine, methylene blue and Cr(VI)) lead to conclusion that the mesopores of RC1000 sample are of ink-bottle shape with very narrow openings. The mesopores are mostly unavailable for adsorption from aqueous solutions and the adsorption capacity of RC1000 sample is the lowest. Decreasing of the catalyst content (RC2000 and RC3000) lead to disappearing of the mesoporosity and only available micropores are present. The differences in surface of samples RC1000 and the others have negligible effect on the interaction with oxygen during heating, where only micropores plays important role. Acknowledgements This work was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic in the “National Feasibility Program I”, project LO1208 “TEWEP”, by EU structural funding Operational Programme Research and Development for Innovation, project No. CZ.1.05/2.1.00/19.0388 and by students grant no. SGS01/PřF/2017 (University of Ostrava). The authors gratefully acknowledge Miloslav Lhotka from the Department of Inorganic Technology (University of Chemistry and Technology Prague) for performing the surface characterization of studied samples by nitrogen adsorption. References
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[25] E. Hinteregger, A.B. Pribil, T.S. Hofer, B.R. Randolf, A.K.H. Weiss, B.M. Rode, Structure and Dynamics of the Chromate Ion in Aqueous Solution. An ab Initio QMCFMD Simulation, Inorg. Chem. 49 (2010) 7964–7968. doi:10.1021/ic101001e. [26] J.L. Figueiredo, M.F.R. Pereira, The role of surface chemistry in catalysis with carbons, Catal. Today. 150 (2010) 2–7. doi:10.1016/j.cattod.2009.04.010. Table 1 Synthesis parameters of resorcinol-formaldehyde xerogels
1000 2000 3000
Mass of H2O (g) 6.93 14.93 17.60
Table 2 The surface characterizations of xerogels
Smicro,N2 (m2/g) 485 522 524
Vmicro,N2 (cm3/g) 0.191 0.204 0.205
Smicro,CO2 (m2/g) 1002 1041 1074
Vmicro,CO2 (cm3/g) 0.362 0.374 0.388
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Vmeso (cm3/g) 0.218 0.057 0.038
Iodine number IN (mmol/g) 0.981 1.954 1.931
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RC1000 RC2000 RC3000
Smeso (m2/g) 116 44 39
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Mass of 0.1% Na2CO3 (g) 16.00 8.00 5.33
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14