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The preparation of activated carbon discs from tar pitch and coal powder for adsorption of CO2, CH4 and N2
Accepted Manuscript The preparation of activated carbon discs from tar pitch and coal powder for adsorption of CO2, CH4 and N2 Shuai Gao, Lei Ge, Thomas E. Rufford, Zhonghua Zhu PII:
S1387-1811(16)30319-5
DOI:
10.1016/j.micromeso.2016.08.004
Reference:
MICMAT 7843
To appear in:
Microporous and Mesoporous Materials
Received Date: 15 September 2015 Revised Date:
3 August 2016
Accepted Date: 4 August 2016
Please cite this article as: S. Gao, L. Ge, T.E. Rufford, Z. Zhu, The preparation of activated carbon discs from tar pitch and coal powder for adsorption of CO2, CH4 and N2, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.08.004. 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.
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THE PREPARATION OF ACTIVATED CARBON DISCS FROM TAR PITCH AND COAL POWDER FOR ADSORPTION OF CO2, CH4 AND N2 Shuai Gao, Lei Ge, Thomas E. Rufford* and Zhonghua Zhu* School of Chemical Engineering. The University of Queensland, St Lucia 4072 Australia
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* Corresponding authors. Tel: +61 7 336 53528. E-mail: [email protected] (Zhonghua Zhu),
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Tel: +61 7 336 54165. Email: [email protected] (Thomas Rufford)
Abstract
We used a pitch foaming process to prepare high surface area activated carbon discs ACD
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with a relatively high mechanical strength (4.3 MPa) from mixtures of tar pitch and coal powders, with potassium hydroxide as a chemical activator. The two primary parameters investigated in this study were the amount of nitric acid used in the pitch pre-treatment and the size of the coal particles (0 – 53 µm, 150 – 250 µm, and 355 – 550 µm). The pre-treatment of the pitch was observed to be critical in the production of the carbon discs and in this study
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the preferred ratio of 15 M nitric acid to pitch in weight was 17/100. The coal size of 0 – 53 µm produced the activated carbon discs with the highest gas adsorption capacity, which we attribute to the stabilization of the pitch foaming process. Equilibrium adsorption capacities of CO2, N2, and CH4 on the carbons were measured by a gravimetric sorption
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method at pressures up to 3500 kPa. At 298 K the adsorption capacities of carbon foam disc prepared with coal particle size of 0 - 53 µm and pitch pre-treated with 18 ml HNO3 per 100 g
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pitch were 3.08 mol/kg N2, 5.48 mol/kg CH4 and 8.88 mol/kg CO2 at about 3500 kPa.
Key words: activated carbon, chemical activation, coal particle size, pitch, carbon foam
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1
Introduction
Carbon monoliths with foam-like hierarchical pore structures and connected macropores have potential applications in high temperature insulation, electrodes for electric energy storage devices, thermal energy storage systems, and as structured catalyst supports and adsorbents
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[1]. In packed reactors and adsorbent towers some of the potential advantages of structured foam and honey-comb type monoliths over random packings (e.g. beads, extruded pellets, and granules) include lower bed pressure drops and higher packing densities of the catalytic or adsorption sites in a vessel [2-4]. Foam-like carbon monoliths with open cellular structures
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may also provide better radial mixing of fluids in packed beds than the typical linear macro channels in honeycomb monolith, and any enhanced heat and mass transfer in carbon foams
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could be utilized to reduce contact times in reactors or pressure swing adsorption systems. The current limitations to development of carbon foams for industrial catalyst-support, electrochemical and adsorption applications include mechanical strength, and the optimization of macropore structure and micropore surface area (to balance fast macroscale fluid transport and density of active catalytic or adsorption sites).
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In a recent review paper Inagaki et al.[1] describe five classifications of the state-of-the-art methods to produce carbon foams including blowing of carbon precursors followed by carbonization, template carbonization, compression of exfoliated graphite, assembly of
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graphene nanosheets and assembly of carbon-foam precursors from oil-water emulsions. Blowing of carbon precursors either by pyrolysis under pressure or the use of chemical
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additives are the most commonly applied methods to produce carbon foams. A wide range of precursors have been reported for carbon foam synthesis including coal [5-7], coke [8, 9], pitch [6, 10-15]
, polymer [16-19] and biomass [20-23]. Of these materials, bituminous tar pitch and coal tar
pitch are some of the most commonly used carbon precursors because pitch is already used in more-conventional activated carbon production processes, low cost pitch is widely available from the petrochemicals industry, and pitch has a high carbon content [24]. However, foams produced directly from tar pitch by pyrolysis at low pressure (close to atmospheric pressure) typically have a low bulk density and poor mechanical strength. Carbon foams with improved mechanical strength have been achieved by (1) blowing the precursor at high pressures (e.g. 2
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30 – 100 bar) or (2) incorporating filler particles such as clay [25], coal [26], graphite [27], carbon nanofibers [28] or carbon nanotubes [29] with the carbon foam. Both approaches have been used together: for example, Zhu et al. [27] reported that the addition of graphite particles to mesophase pitch blown at a pressure 8 MPa and carbonized at 1200 °C increased the
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compressive strength of the carbon foam from 3.7 MPa to 12.5 Mpa. However, foaming processes at high pressures introduces extra complexity, costs and risks to an industrial scale carbon production process. In this paper, we focus on the second approach to incorporate coal filler particles in carbon foams to produce activated carbon discs (ACD).
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Carbon filler particles may serve multiple functions in composite foams as filler particles can (i) provide additional surface areas, (ii) modify the density of the produced foam, and (iii) act
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to stabilize the pith in low pressure foam preparation processes. In this context, the term of stabilization of the foam refers to restriction of bubble growth in the pitch during heat treatment to control the expansion and prevent overflow from the foaming mould (others have used the term stabilization in reference to maximizing bubble size). We selected coal particles as a low cost filler particle (relative to the cost of carbon nanotubes or graphite) and
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investigated the effect of the coal particle size on the foaming process. The coal particles affect the development of the mesopore and macropore structure during foaming and carbonization. As the pitch viscosity and reactivity are the other key parameters affecting the
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stability of the pitch + coal mixture during pyrolysis and carbonization we also investigated the effect of treating the pitch with nitric acid prior to the foaming procedures on the carbon
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foam monolith properties. A chemical activation agent, KOH, was used to enhance the microporosity of the ACDs for increased adsorption of CO2, CH4 and N2.
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2
Experimental
2.1 Materials and pitch pretreatment BP Bitumen Class 170 pitch with a softening point of 320 K and density of softening1.03 g/cm3 [30] was used as the carbon foam precursor in this study. The pitch was
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pretreated using nitric acid (HNO3) and a thermal treatment step; this process increased the softening point of the pitch which improves the foaming properties
[6]
and allows better
physical mixing of the pitch with the coal powder. We investigated the effect of pitch
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pretreatment conditions on the final carbon foam properties by treating the Class 170 pitch with three ratios of HNO3 to pitch: 12 mL, 18 mL and 24 mL of 15 M HNO3 to 100 g of pitch.
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The pitch was heated to 393 K using an oil bath, and stirred with an impellor at 150 rpm, before the HNO3 was added dropwise and the temperature of the oil bath increased to 433 K. The acid pretreatment step was allowed to continue until the impellor could no longer stir the pitch at 433 K. Then the acid-treated pitch was transferred to a tube furnace and thermally treated at 623 K for 2 hours in Argon (heating rate 5 K/min).
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Coal from the Blackwater coal mine in Queensland’s Bowen Basin (Australia) was ground and screened into three size fractions: 0 - 53 µm, 150 - 250 µm and 355 - 550 µm.
2.2 Carbon foam preparation
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The pretreated pitch was ground by hand in a mortar and pestle, then mixed with the selected size of coal powder and potassium hydroxide at a pitch: coal: KOH mass ratio of 1:1:1. To
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improve the dispersion of the KOH through the solids, the pitch + coal + KOH mixture was sonicated for 5 minutes in 50 mL of distilled water. This free water was subsequently evaporated at 353 K for 2 hours over a hot plate with the mixture continuously stirred at 350 rpm. Discs (20 mm diameter) were formed from about 1 g of the pitch + coal + KOH blend in a hydraulic press and these discs were dried overnight in oven at 353 K. The foaming of the discs was performed under Argon in a horizontal tube furnace at 1073 K (heating rate 10 K/min) with a soaking time 1 h. Foamed discs were washed with 0.2 M HCl in a vacuum vessel for one day, dried overnight in oven at 353 K, rinsed with distilled water several times to remove KOH until a filtrate pH of 7 was achieved, and finally dried in the 4
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oven at 353 K overnight. Activated carbon discs (ACD) obtained from the different synthesis conditions are labeled (see Table 1) with the HNO3 pretreatment volume (in mL) and the coal particle size fraction added to the pitch (in micron). For example, the sample labeled ACD18053 was prepared with 18 ml of 15 M HNO3 added to 100 g pitch in the pretreatment and with
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the 0 – 53 µm coal fraction. Photographs of ACD18-150250 are shown in Figure 1a-b. In addition to the ACDs produced from the method described above we also (1) activated 0 53 µm coal particles with KOH to estimate the contribution of the coal to the porosity of the ACDs, and (2) attempted to foam the pre-treated pitch in the tube furnace without any coal
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particles. In the experiment performed with pitch without any coal particles led to the pitch overflowing the furnace crucible (Figure 1c) and did not produce a stable carbon disc.
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The carbon yield of the ACDs was calculated from the treated-pitch weight (mp), coal weight (mc), and the weight of the washed ACD end product (mf) according to: % =
× 100%
(1)
The carbon yield increased from 45.08% to 48.17 % with increasing HNO3 to pitch ratio from
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12 ml to 24 ml/100g pitch. This result is attributed to a greater removal of volatiles, for example by polymerization of shorter chain carbon molecules, at higher concentrations of HNO3 during the pitch pretreatment. Subsequently, during the foaming there is a smaller mass
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of volatiles to be evolved from the pitch treated with 24 mL HNO3/100 pitch than pitch treated with 12 mL HNO3/100 g pitch.
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2.3 Characterization
Scanning electron microscope (SEM) images were collected on a JSM-7100F instrument (JEOL Ltd., Japan). X-ray photoelectron spectroscopy (XPS) was performed on a PHI-560 ESCA (Perkin Elmer) at 15 kV. The C1s peak position was set to 284.8 eV and taken as an internal standard. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were performed with a Perkin Elmer STA 6000 instrument at heating rates of 10 K/min and a nitrogen flow of 20 mL/min. Compressive strength was measured by a uniaxial compression test with an Instron 5584 computer controlled material testing system. The bulk or apparent density (ρHg) of the carbon discs was measured by mercury porosimetry 5
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(Micromeritics PoreSizer 9320) and the skeletal density (ρHe) by helium pycnometer (Micromeritics AccuPyc II 1340). The disc porosity (ϕ) was calculated with Equation 2: % =
× 100
(2)
Sorption isotherms of N2 at 77 K and CO2 at 273 K were measured with a TriStar II 3020
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apparatus (Micromeritics, USA) after degassing the activated carbon discs at 473 K and a pressure of 10-5 torr for 24 h. The N2 isotherms were used to determine the Brunauer Emmett Teller (BET) specific surface area at a relative pressure in the range of the P/P0 = (0.05 - 0.35);
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total pore volumes at P/P0 = 0.99; micropore volumes using the t-plot method and mesopore volumes from the Barrett Joyner Halenda (BJH) method. The pore size distributions (PSD)
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were also calculated from the N2 adsorption on TriStar II 3020. Limiting micropore volumes were also calculated from the CO2 isotherms measured at 273 K with the Dubinin-Astakhov (DA) equation [31]. We also measured, with the TriStar II 3020 apparatus, adsorption isotherms on ACD18-053 for CO2 at 283 K and 303 K, N2 at 273 K, 283 K and 303 K and CH4 at 273 K and 303 K. These low pressure isotherms were used to determine isosteric heats
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of adsorption (qst).
Adsorption equilibrium capacities of CH4, CO2 and N2 on ACD18-053 were measured at 298 K with pressure up to 3500 kPa on a Belsorp-BG instrument (BEL, Japan) equipped with
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a magnetic floating balance (Rubotherm, Germany). The activated carbon discs were
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degassed at 423 K for 24 hour prior to high pressure adsorption measurements.
Results and discussion
3.1 Bulk properties of the activated carbon discs A summary of the bulk properties of the activated carbon discs is provided in Table 1. No density or yield results are reported in Table 1 for the foamed tar pitch prepared without coal powder (shown in Figure 1c) as this sample overflowed the crucible and a monolithic disc was not produced. In addition, we were not able to measure the bulk density of ACD12150250 by mercury intrusion porosimetry because after washing in HCl this disc broke apart. 6
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Instead, the ACD12-150250 bulk density of 0.39 g/cm3 was estimated from the dimensions of the cylindrical disc before washing and the weight of the carbon obtained after washing. The low strength of ACD12-150250, which was prepared from pitch treated only with a low HNO3 to pitch ratio, is consistent with results reported in the literature [6].
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The bulk densities of the other four ACDs measured by mercury intrusion porosimetry were in the range of (0.48 to 0.56) g/cm3, which is within the typical range of densities reported for activated carbon foams prepared by other researchers [32]. Chemical activation of the carbon foams with KOH develops microporosity in the carbon walls, and as a result our ACDs can be
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expected to have a lower bulk density than carbon foams that have not been chemically activated. The bulk density of ACD24-150250 was greater than that of ACD18-150250 and
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ACD12-150250, which shows in general the bulk density increased with the ratio of HNO3 used in the pitch pretreatment. When more acid was used, the molecular weight of pitch and softening point increase as well as the viscosity of precursor. In such case, the total bubbling volume decreases and leads to the increment of bulk density. The ratio of HNO3 to pitch was not observed to have a large effect on the skeletal density of the ACDs. In contrast, the
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particle size of coal powder had a significant effect on skeletal density; we will discuss this result in the following section.
Table 2 compares the mechanical strength of ACD18-053 and the strength of three carbon
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foams reported in the literature [33-35]. The compressive strength of ACD18-053 measured by an Instron instrument was 4.3 MPa. This is comparable with low density carbon foams
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produced by Narasimman et al. [33], but less than some high density foams that have been reported. The aim of our current study was not to maximize compressive strength. Instead, our aim is to produce a high surface area carbon monolith with sufficient strength to be used in a pressure swing adsorption process, and for this purpose a crushing strength of 4.3 MPa may be sufficient. Detailed investigations of the stresses induced by pressure swings in an adsorbent bed are recommended but such investigations are beyond the scope of this paper. The relative concentrations of oxygen functional groups on the surface of activated carbon disc ACD18-053 were examined by deconvolution of the XPS C1s spectra (Figure 2). Curves fitted to the C1s spectra was for (1) C–C bonds of graphite at 284.6 eV, (2) phenol and ether 7
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groups (C–O) at 285.3 eV, (3) carbonyls, quinone and aldehydes (C=O) at 286.9 eV, (4) carboxyls, carboxylic anhydrides and esters (O=C–O) at 289.1 eV, and (5) the characteristic shake-up line of aromatic carbon (π=π*) at 290.5 eV [36]. The results of the deconvolution analyses are summarized in Table 3. This result showed that C-O bond is the most dominant
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oxygen functionality on the surface of ACD18-053. The surface chemistry of ACD18-053 is consistent with that of other carbon materials prepared from pitch and coal and carbon materials prepared at temperatures up to 1073 K [37-39].
3.2 Effect of coal filler particles on the morphology of carbon discs
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Figure 3 shows SEM images of the KOH activated coal produced from the 0 to 53 µm coal particle size fraction without tar pitch, and the activated carbon discs ACD18-053 produced
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from coal and pitch mixtures. The activated coal particles (Figure 3 a and b) feature large cracks and interparticle void spaces, but there are no open macropore cells observed in the surfaces of the coal particles. In Figure 3c-d the shape of coal particles is still identified in the ACDs, but the texture of the ACDs is clearly different to that of the coal particles. In the SEM images of ACD18-053 the coal particles are coated in a foam derived from the pitch and this
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foam forms bridges between coal particles. Two types of pore structures between the coal particles are observed: (1) Irregular shaped pores approximately 50 µm in diameter, which result from partial filling of the voids between the coal particles, and (2) open cells and
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channels of 100 – 200 nm width with bubble-like shapes (for example in Figure 3d). The bimodal distribution of macropores in ACD18-053 is confirmed with the pore size
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distributions determined by mercury intrusion porosimetry (Figure 4). The 100 – 200 nm macropore structures developed in the pitch component of the ACDs during the foam blowing process, and these pores are created in the pitch by the evolution and expansion of volatile hydrocarbons in the pitch as the furnace temperature is increased. As the temperature is increased further during foaming and carbonization reactions occur, the remaining pitch components begin to solidify and form solid carbon walls around the bubbles [6, 40, 41]. The pore macropore structures observed in the ACDs are consistent with structures of foams derived from pitch and other precursors that have been reported [20, 25, 34]. We propose that the coal filler particles have two main mechanistic roles during the low8
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pressure pitch foaming process: (1) the particles act to stabilize bubble growth in the pitch during the heating and volatilization, and (2) at temperature greater than about 800 K the coal particles provide a skeleton to the foam structure during carbonization. The TGA curves in Figure 5 show the weight loss from a mixture of pitch (pretreated in
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HNO3) and coal particles in an experiment that replicated the conditions used for foaming and carbonization in the tube furnace to produce ACD18-053. There are four distinct stages in the thermal treatment of the pitch + coal mixture. (i) In the first stage of the heat treatment the temperature is increased from room temperature to the pitch softening point of approximately
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623 K, and only a small mass of moisture and any dissolved light hydrocarbon gases (that remained after pitch pretreatment) is lost from the pitch + coal mixture. A weight loss of
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moisture and adsorbed volatiles is also observed from the coal in the TGA curves. (ii) In the second stage at temperatures above 623 K, the pitch starts to soften and then melt, which allows the pitch to fill the interparticle voids between coal particles. This phenomenon is not observed directly in our TGA measurements, but can be assumed to lead to a reduction in the total volume of the composite as the molten pitch fills void spaces between coal particles.
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Since the pitch has been pre-treated at 623K before foaming, no bubbling occurred at this step due to the absence of volatile in pitch. (iii) In the third stage, bubbles and pitch polymerization begin to occur at temperatures above the pitch pre-treatment temperature of
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623 K, and an increase in the rate of weight loss from pitch + coal is observed. During this stage, the coal particles act as bubble nucleation sites, increase the density and viscosity of the
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mixture and reducing the bubbling expansion rate [25, 34]: the combined effects are that a larger number of small bubbles are formed than in a pitch that is foamed without any coal filler particles, the latter tends to form large bubbles that overflow the foaming crucible (Figure 1c). The derivative thermogravimetric analysis (DTG) curve in Figure 5 shows the maximum rate of weight loss from pitch + coal occurred at approximately 760 K, which is nearly the same as the temperature (763 K) at which Wang et al. observed a maximum rate of weight loss from mesophase pitch [15]. The significance of the temperature of maximum weight loss in the foaming process is that this temperature corresponds to the point at which the pitch has maximum fluidity and expansion [40]. (iv) The fourth and final stage in the 9
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carbon foam process is observed above 800 K as a steady weight loss due to pyrolysis and carbonization reactions.
3.3 Adsorption of N2, CO2 and CH4 on the activated carbon discs The N2 sorption measurements at 77 K show that all the ACDs exhibited TypeⅠisotherms
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(Figure 6a). The ACDs adsorbed large volumes of N2 a P < 10 kPa, which is attributed to adsorption in the micropores that were predominantly created by KOH activation of the ACDs. Table 4 summarizes the pore textural properties of the ACDs and the Blackwater coal samples. The BET surface areas of the ACDs were all greater than 1000 m2/g. The as-received
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Blackwater coal has a BET surface area of 3.1 m2/g but this specific area was increased to 1182 m2/g after KOH activation. This suggests that the KOH activated coal filler particles
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provide significant contributions of micropores to the total surface area of the composite ACDs and the BET surface area associated with the micropore volume in coal filler particles is larger for ACDs prepared with smaller size fractions of coal particles. Due to the kinetic limitations associated with the sorption of N2 in narrow micropores at 77 K [42], we also
included in Table 4.
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probed the ACD structures with CO2 at 273 K and results from CO2 sorption analysis are
Activated carbon disc ACD18-053 had the highest CO2 and N2 adsorption capacities at a pressure of 100 kPa; so we selected this ACD for further adsorption studies at high pressure
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including measurements of CH4 uptake. Absolute equilibrium adsorption capacities for CH4, CO2 and N2 on ACD18-053 measured with the Belsorp-BG at 298 K and pressure up to
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3500 KPa are reported in Table 5 and Figure 7. As expected for an activated carbon the adsorption capacities on ACD18-053 were in the order CO2 > CH4 > N2. The CO2 adsorption capacity of ACD18-053 is comparable to the CO2 uptakes reported for activated carbons at high pressures [43, 44]. There are few reports of high pressure adsorption measurements on carbon foams, but at 100 kPa the adsorption of 3.51 mol/kg CO2 on ACD18-053 is comparable to the 2.5 mol/kg CO2 reported for carbon foams prepared by Narasimman, Vijayan and Prabhakaran [23]. Table 6 presents further comparisons of adsorption capacities of ACD18-053 with other carbon foams and commercial activated carbons.
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To provide additional data to readers that may be useful for the evaluation of gas adsorption processes based on ACD18-053 we calculated isosteric heats of adsorption (qst) at different CO2, CH4 and N2 loadings by the direct method [45] of applying the Clausius-Clapeyron equation (Equation 3). For this part of the study adsorption isotherms were measured at
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(273.15, 283.15 and 303.15) K and pressures up to 120 kPa on the Tristar II instrument. These equilibrium adsorption isotherms are included in the Supplementary Information.
= −
!" # $
%
&
(3)
gas constant. Integrating Equation (3) gives: + ,-
0
/ *1/ + 3
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ln ) & = − *
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where p is the pressure, T is the temperature, Q is the amount adsorbed, and R is the universal
.
(4)
where C is a constant. To determine qst values we fitted a high-order polynomial equation to each isotherms to obtain an expression for Q as a function of p, and used these fitted isotherm equations to interpolate for values of p at a given Q for each temperature so that ln(P)Q could
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be plotted against 1/T. Then with these plots, qst was obtained by linear regression of Equation 4. Figure 8 summaries the isosteric heat of adsorption results. The heats of adsorption were in the ranges 22 – 26 kJ/mol for CO2 at loadings of 2 – 3 mmol/g, 21 – 22.7 kJ/mol for N2 at
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loadings of 0.1 – 0.6 mmol/g, and 6 – 8 kJ/mol for CH4 at loadings of 0.2 – 1.2 mmol/g. The values of CO2 isosteric heat of adsorption on ACD18-053 in this study similar to the range of
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reported values for other activated carbons [46] and isoteric heats of adsorption for N2 are also similar to other reported ranges for similar pressures and coverages [47]. However, our calculated isosteric heat of adsorption for CH4 is at a lower range than some other reports for activated carbons [47, 48]. To reduce the uncertainty in these results for CH4, and confirm the enthalpies of adsorption for the other two adsorbates, additional adsorption measurements at the range of temperatures and a wider pressure range may be required. Those additional measurements are beyond the scope of this current paper and will feature in our continued efforts to develop carbon foam materials for gas separation and storage applications.
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4
Conclusions
Petroleum tar pitch and coal filler particles were used as precursors to prepare activated carbon discs with high surface area, hierarchical pore structures, and relatively strong mechanical strength. The coal particles act to stabilize bubble growth during the pitch
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foaming and carbonization, with smaller coal particles providing more bubble nucleation sites and KOH-activated surface areas than larger coal particles. We found that a HNO3 to pitch ratio of 17/100 in weight for pretreatment of the pitch produced the ACDs with the best mechanical strength and highest bulk density. The ACD18-053 with a surface area of
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1401 m2/g adsorbed 3.51 mol/kg of CO2 at 100 kPa and 298 K, which is comparable to other activated carbons in the literature. The high selectivity of CO2 over N2 and CH4 shows that the
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ACDs prepared in this work have a good potential for pressure swing adsorption.
Acknowledgments
This research was funded by the Australian Research Council (FT120100720 and DE140100569). S. Gao received financial support from the China Scholarship Council.
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Mengran Li and Rijia Lin are acknowledged for their technical assistance with SEM, and Arash Arami Niya for assistance with the high pressure adsorption experiments. The authors acknowledge the facilities and technical assistance of the Australian Microscopy &
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Microanalysis Research facility at the Centre for Microscopy & Microanalysis at the
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University of Queensland.
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and high compressive strength, Carbon, 48 (2010) 2644-2646.
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(2013) 992-997.
[19] J. Seo, H. Park, K. Shin, S.H. Baeck, Y. Rhym, S.E. Shim, Lignin-derived macroporous carbon foams prepared by using poly(methyl methacrylate) particles as the template, Carbon, 76 (2014) 357-367.
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[20] M.G. Plaza, I. Durán, F. Rubiera, C. Pevida, CO2 adsorbent pellets produced from pine sawdust: Effect of coal tar pitch addition, Applied Energy, 144 (2015) 182-192. [21] M.G. Plaza, A.S. González, C. Pevida, F. Rubiera, Green coffee based CO2 adsorbent with high performance in postcombustion conditions, Fuel, 140 (2015) 633-648. [22] R. Narasimman, K. Prabhakaran, Preparation of low density carbon foams by foaming molten sucrose using an aluminium nitrate blowing agent, Carbon, 50 (2012) 1999-2009. [23] R. Narasimman, S. Vijayan, K. Prabhakaran, Carbon foam with microporous cell wall and strut for CO2 capture, RSC Advances, 4 (2014) 578. [24] B. Petrova, T. Budinova, N. Petrov, M.F. Yardim, E. Ekinci, M. Razvigorova, Effect of different oxidation treatments on the chemical structure and properties of commercial coal tar pitch, Carbon, 43 (2005) 261-267. [25] X. Wang, J. Zhong, Y. Wang, M. Yu, A study of the properties of carbon foam reinforced by clay, Carbon, 44 (2006) 1560-1564. [26] B. Nagel, S. Pusz, B. Trzebicka, Review: tailoring the properties of macroporous carbon foams, Journal of
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[27] J. Zhu, X. Wang, L. Guo, Y. Wang, Y. Wang, M. Yu, K.-t. Lau, A graphite foam reinforced by graphite particles, Carbon, 45 (2007) 2547-2550. [28] W. Fawcett, D.K. Shetty, Effects of carbon nanofibers on cell morphology, thermal conductivity and crush strength of carbon foam, Carbon, 48 (2010) 66-80. [29] C.M. Leroy, F. Carn, R. Backov, M. Trinquecoste, P. Delhaes, Multiwalle-carbon-nanotube-based carbon [30] BP Bitumen Class 170, MSDS SDS no.0000003723, in: BP (Ed.) 2014.
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foams, Carbon, 45 (2007) 2307-2320. [31] M.M. Dubinin, Physical Adsorption of Gases and Vapors in Micropores, in: D.A. Cadenhead, J.F. Danielli, M.D. Rosenberg (Eds.) Progress in Surface and Membrane Science, Academic Press, New York, 1975, pp. 1-70. [32] M. Inagaki, F. Kang, M. Toyoda, H. Konno, Carbon Foams, (2014) 189-214.
[33] R. Narasimman, K. Prabhakaran, Preparation of carbon foams by thermo-foaming of activated carbon powder dispersions in an aqueous sucrose resin, Carbon, 50 (2012) 5583-5593.
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[34] W.Q. Li, H.B. Zhang, X. Xiong, F. Xiao, A study of the properties of mesophase-pitch-based foam/graphitized carbon black composites, Materials Science and Engineering: A, 528 (2011) 2999-3002. [35] H. Liu, T. Li, X. Wang, W. Zhang, T. Zhao, Preparation and characterization of carbon foams with high
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[36] L. Wang, L. Ge, T.E. Rufford, J. Chen, W. Zhou, Z. Zhu, V. Rudolph, A comparison study of catalytic oxidation and acid oxidation to prepare carbon nanotubes for filling with Ru nanoparticles, Carbon, 49 (2011) 2022-2032.
[37] A. Sarkar, D. Kocaefe, Y. Kocaefe, D. Sarkar, D. Bhattacharyay, B. Morais, J. Chabot, Coke–pitch interactions during anode preparation, Fuel, 117, Part A (2014) 598-607.
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[38] Y. Guo, Z.-q. Shi, M.-m. Chen, C.-y. Wang, Hierarchical porous carbon derived from sulfonated pitch for electrical double layer capacitors, Journal of Power Sources, 252 (2014) 235-243. [39] B. Wu, H.-q. Hu, Y.-p. Zhao, L.-j. Jin, Y.-m. Fang, XPS analysis and combustibility of residues from two coals extraction with sub- and supercritical water, Journal of Fuel Chemistry and Technology, 37 (2009) 385392.
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[40] C. Chen, E.B. Kennel, A.H. Stiller, P.G. Stansberry, J.W. Zondlo, Carbon foam derived from various precursors, Carbon, 44 (2006) 1535-1543.
[41] Z.-h. Min, M. Cao, S. Zhang, X.-d. Wang, Y.-g. Wang, Effect of precursor on the pore structure of carbon
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foams, New Carbon Materials, 22 (2007) 75-79. [42] H. Marsh, Adsorption methods to study microporosity in coals and carbons—a critique, Carbon, 25 (1987) 49-58.
[43] F. Dreisbach, High pressure adsorption data of methane, nitrogen, carbon dioxide and their binary and ternary mixtures on activated carbon, Adsorption : journal of the International Adsorption Society, 5 (1999) 215227.
[44] S. Garcia, J.J. Pis, F. Rubiera, C. Pevida, Predicting mixed-gas adsorption equilibria on activated carbon for precombustion CO2 capture, Langmuir, 29 (2013) 6042-6052. [45] R.J. Li, M. Li, X.P. Zhou, D. Li, M. O'Keeffe, A highly stable MOF with a rod SBU and a tetracarboxylate linker: unusual topology and CO2 adsorption behaviour under ambient conditions, Chem Commun (Camb), 50 (2014) 4047-4049. [46] S. Himeno, T. Komatsu, S. Fujita, High-Pressure Adsorption Equilibria of Methane and Carbon Dioxide on Several Activated Carbons, Journal of Chemical & Engineering Data, 50 (2005) 369-376.
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[47] S.A. Al-Muhtaseb, Adsorption and desorption equilibria of nitrogen, methane, ethane, and ethylene on datepit activated carbon, Journal of Chemical and Engineering Data, 55 (2010) 313-319. [48] A.A. García Blanco, A.F. Vallone, S.A. Korili, A. Gil, K. Sapag, A comparative study of several microporous materials to store methane by adsorption, Microporous and Mesoporous Materials, 224 (2016) 323331. [49] F. Dreisbach, R. Staudt, J.U. Keller, High pressure adsorption data of methane, nitrogen, carbon dioxide and their binary and ternary mixtures on activated carbon, Adsorption, 5 (1999) 215-227.
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[50] S. Cavenati, C.A. Grande, A.E. Rodrigues, Adsorption Equilibrium of Methane, Carbon Dioxide, and Nitrogen on Zeolite 13X at High Pressures, Journal of Chemical & Engineering Data, 49 (2004) 1095-1101. [51] Z. Bao, L. Yu, Q. Ren, X. Lu, S. Deng, Adsorption of CO2 and CH4 on a magnesium-based metal organic framework, J Colloid Interface Sci, 353 (2011) 549-556.
[52] D. Saha, Z. Bao, F. Jia, S. Deng, Adsorption of CO(2), CH(4), N(2)O, and N(2) on MOF-5, MOF-177, and zeolite 5A, Environ Sci Technol, 44 (2010) 1820-1826.
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[53] R.P. Ribeiro, T.P. Sauer, F.V. Lopes, R.F. Moreira, C.A. Grande, A.E. Rodrigues, Adsorption of CO2, CH4, and N2 in activated carbon honeycomb monolith, Journal of Chemical & Engineering Data, 53 (2008) 23112317.
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[54] A.B. Bourlinos, T.A. Steriotis, M. Karakassides, Y. Sanakis, V. Tzitzios, C. Trapalis, E. Kouvelos, A. Stubos, Synthesis, characterization and gas sorption properties of a molecularly-derived graphite oxide-like
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foam, Carbon, 45 (2007) 852-857.
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Figure 1 Photographs of (a，b) the carbon foam disc ACD18-053 and (c) the overflow of pitch from the crucible observed
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when attempting to foam the pitch without any coal powder.
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Figure 2 Curve fitting of C1s XPS spectra of ACD18-053053 prepared from mixture of coal and tar pitch.
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Figure 3 SEM images of (a, b) KOH activated coal without adding pitch and (c, d) ACD18-053prepared from mixture of coal
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and tar pitch.
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Figure 4 Pore size distribution in activated carbon disc ACD18-053 determined by (a) main figure showing the distribution determined by mercury injection porosimetry (Hg) and (b) inset the size distribution of micropores and mesopores determined
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by NLDFT from the N2 sorption isotherm measured at 77 K.
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Figure 5 Weight loss (TGA) and derivate weight loss (DTG) curves measured for Pitch + coal 1:1:1 weight mixture of pitch (pretreated with 18 mL 15 M HNO3 to 100 g pitch), 0 – 53 µm coal particles and KOH; and KOH activated coal. .Heating
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rate 10 K/min. Nitrogen flow rate 20 mL/min.
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Figure 6 Adsorption of (a) N2 at 77 K and (b) CO2 at 273 K on activated carbon discs prepared from BP Class 170 pitch and
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Blackwater coal powders of different size fractions 0 - 53 µm, 150 - 250 µm and 355 - 550 µm.
Figure 7 Adsorption equilibrium of ACD18-053 sample at 298K.
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Figure 8 Isosteric heat of adsorption of CO2, CH4 and N2 on ACD18-053 calculated via the Clausius-Clapeyron equation.
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.
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ACCEPTED MANUSCRIPT IMMS-9, 17-20 August 2015, Brisbane, Australia Table 1 The bulk density, skeletal density, porosity and carbon yield of the coal and activated carbon discs.
Coal particle size
Bulk density
porosity
Carbon yield
%
%
1.38±0.009
6.01±0.62
-
1.92±0.24
58.92±5.88
46.99
2.07±0.16
81.03±1.58
45.08
1.85±0.07
73.94±1.02
43.58
2.05±0.09
79.23±0.96
46.70
0.56
1.59±0.13
64.61±3.14
44.40
0.49
2.05±0.10
75.96±1.23
48.17
3
µm
g/cm
Black Water Coal
-
-
1.30
KOH Activated Coal
-
0-53
0.79
ACD12-150250
12
150-250
0.39
ACD18-053
18
0-53
0.48
ACD18-150250
18
150-250
0.43
ACD18-355550
18
355-550
ACD24-150250
24
150-250
g/cm
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ml/100g
Skeletal density
22
3
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HNO3/Pitch
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Sample
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Table 2 Bulk properties of the carbon foam disc ACD18-053 and representative carbon foams in literature (GF0
[27]
was
made from graphitized carbon black particles and mesophase pitch, CFS carbon foam made by Narasimman and Prabhakaran [33]was prepared from activated carbon powder and aqueous sucrose resin and CTP3 [35] was prepared using coal tar pitches modified with cinnamaldehyde (CMA) and boric acid).
strength/MPa Bulk density(g/cm3)
GF0 [34]
CFS [33]
CTP3 [35]
4.3
5.39
3.4
21.27
0.48
0.76
0.22
0.76
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Compressive
ACD18-053
C-O
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Sample
C=O
O=C-O
π=π*
286.1
287.4
288.8
290.1
15.0
6.2
2.0
5.8
Table 3 Relative concentration (at. %) of oxygen functional groups on ACD18-053. Calculated from deconvolution of high
Group
C-C
Binding energy (eV)
284.8
ACD18-053
71.0
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resolution XPS C1s spectra.
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Table 4 Main textural parameters of the obtained activated carbon discs (BET surface area was evaluated at p/p0 ~ 0.99, micropore volume was evaluated by the t-plot method applied to N2 adsorption isotherms and mesopore volume was evaluated by the BJH method applied to N2 adsorption isotherms at 77K. DA Micropore surface area and narrow micropore volume determined from CO2 adsorption at 273K).
Sample
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N2 Adsorption
SBET 2
Vtotal
Vmicro
Vmeso
3
3
3
(cm ·g )
(cm ·g )
(m ·g )
(cm3·g-1)
3.1
-
-
-
-
-
KOH activated coal
1182
0.505
0.429
0.049
-
-
ACD12-150250
1221
0.515
0.430
0.035
1053
0.419
ACD18-053
1401
0.600
0.489
0.047
1584
0.647
ACD18-150250
1302
0.534
0.437
0.049
1310
0.527
ACD18-355550
1055
0.436
0.364
0.029
1181
0.469
ACD24-150250
1269
0.542
0.435
0.054
1547
0.665
23
-1
-1
2
-1
Vmicro
(cm ·g )
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-1
SD-R
(m ·g )
Blackwater coal
-1
CO2 Adsorption
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Table 5 Methane, carbon dioxide, nitrogen adsorption equilibrium data on ACD18-053 sample at 298 K.
CH4
CO2
N2
Qads(mol/kg)
P(kPa)
Qads(mol/kg)
P(kPa)
Qads(mol/kg)
9.22
0.19
5.74
0.52
8.64
0.02
29.65
0.52
28.41
1.68
28.12
0.09
71.60
1.00
68.56
2.87
102.88
1.22
99.13
3.51
303.96
1.72
297.51
5.74
701.54
2.57
698.30
7.31
1001.06
2.89
998.61
7.87
1500.87
3.24
1497.94
8.35
1499.77
1.90
2000.50
3.98
1995.99
8.66
1998.41
2.32
2500.35
4.70
2495.47
8.76
2497.84
2.65
2999.30
5.11
2994.94
8.84
2997.72
2.89
3500.07
5.43
8.87
3495.76
3.09
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P(kPa)
0.21
101.77
0.28
302.47
0.57
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70.99
1.10
1000.63
1.39
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701.18
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3493.96
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Table 6 Equilibrium capacity of adsorbents in literature and ACD for N2, CO2 and CH4 at 100 and 1000 kPa and ambient temperature.
Adsorbent
Type
CH4 capacity
N2 capacity
(mol/kg)
(mol/kg)
(mol/kg)
1000kPa
100kPa
1000kPa
100kPa
1000kPa
ACD
3.51
7.87
1.22
3.76
0.28
1.39
AC
2.20
7.60
1.22
2.89
0.39
2.02
Zeolite
3.3
6.40
0.59
2.90
0.28
1.83
MOF
1.59
-
0.63
-
0.17
-
ACHM [53]
Monolith
3.0
-
1.4
-
0.5
-
CA300[54]
Foam
-
2.0
-
0.7
-
0.23
Norit RB1 extra[49] 13X [50, 51]
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MOF-177 [52]
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ACD18-053
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100kPa
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Pressure
CO2 capacity
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ACCEPTED MANUSCRIPT THE PREPARATION OF ACTIVATED CARBON DISCS FROM TAR PITCH AND COAL POWDER FOR ADSORPTION OF CO2, CH4 AND N2 Shuai Gao, Lei Ge, Thomas E. Rufford* and Zhonghua Zhu*
Highlights
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Carbon discs with hierarchical pore structures prepared from pitch and coal powder. Nitric acid pretreatment of pitch improved foaming properties. Smaller coal particles produce carbon discs with better mechanical strength.
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• • •
S1387-1811(16)30319-5
DOI:
10.1016/j.micromeso.2016.08.004
Reference:
MICMAT 7843
To appear in:
Microporous and Mesoporous Materials
Received Date: 15 September 2015 Revised Date:
3 August 2016
Accepted Date: 4 August 2016
Please cite this article as: S. Gao, L. Ge, T.E. Rufford, Z. Zhu, The preparation of activated carbon discs from tar pitch and coal powder for adsorption of CO2, CH4 and N2, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.08.004. 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.
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THE PREPARATION OF ACTIVATED CARBON DISCS FROM TAR PITCH AND COAL POWDER FOR ADSORPTION OF CO2, CH4 AND N2 Shuai Gao, Lei Ge, Thomas E. Rufford* and Zhonghua Zhu* School of Chemical Engineering. The University of Queensland, St Lucia 4072 Australia
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* Corresponding authors. Tel: +61 7 336 53528. E-mail: [email protected] (Zhonghua Zhu),
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Tel: +61 7 336 54165. Email: [email protected] (Thomas Rufford)
Abstract
We used a pitch foaming process to prepare high surface area activated carbon discs ACD
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with a relatively high mechanical strength (4.3 MPa) from mixtures of tar pitch and coal powders, with potassium hydroxide as a chemical activator. The two primary parameters investigated in this study were the amount of nitric acid used in the pitch pre-treatment and the size of the coal particles (0 – 53 µm, 150 – 250 µm, and 355 – 550 µm). The pre-treatment of the pitch was observed to be critical in the production of the carbon discs and in this study
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the preferred ratio of 15 M nitric acid to pitch in weight was 17/100. The coal size of 0 – 53 µm produced the activated carbon discs with the highest gas adsorption capacity, which we attribute to the stabilization of the pitch foaming process. Equilibrium adsorption capacities of CO2, N2, and CH4 on the carbons were measured by a gravimetric sorption
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method at pressures up to 3500 kPa. At 298 K the adsorption capacities of carbon foam disc prepared with coal particle size of 0 - 53 µm and pitch pre-treated with 18 ml HNO3 per 100 g
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pitch were 3.08 mol/kg N2, 5.48 mol/kg CH4 and 8.88 mol/kg CO2 at about 3500 kPa.
Key words: activated carbon, chemical activation, coal particle size, pitch, carbon foam
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1
Introduction
Carbon monoliths with foam-like hierarchical pore structures and connected macropores have potential applications in high temperature insulation, electrodes for electric energy storage devices, thermal energy storage systems, and as structured catalyst supports and adsorbents
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[1]. In packed reactors and adsorbent towers some of the potential advantages of structured foam and honey-comb type monoliths over random packings (e.g. beads, extruded pellets, and granules) include lower bed pressure drops and higher packing densities of the catalytic or adsorption sites in a vessel [2-4]. Foam-like carbon monoliths with open cellular structures
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may also provide better radial mixing of fluids in packed beds than the typical linear macro channels in honeycomb monolith, and any enhanced heat and mass transfer in carbon foams
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could be utilized to reduce contact times in reactors or pressure swing adsorption systems. The current limitations to development of carbon foams for industrial catalyst-support, electrochemical and adsorption applications include mechanical strength, and the optimization of macropore structure and micropore surface area (to balance fast macroscale fluid transport and density of active catalytic or adsorption sites).
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In a recent review paper Inagaki et al.[1] describe five classifications of the state-of-the-art methods to produce carbon foams including blowing of carbon precursors followed by carbonization, template carbonization, compression of exfoliated graphite, assembly of
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graphene nanosheets and assembly of carbon-foam precursors from oil-water emulsions. Blowing of carbon precursors either by pyrolysis under pressure or the use of chemical
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additives are the most commonly applied methods to produce carbon foams. A wide range of precursors have been reported for carbon foam synthesis including coal [5-7], coke [8, 9], pitch [6, 10-15]
, polymer [16-19] and biomass [20-23]. Of these materials, bituminous tar pitch and coal tar
pitch are some of the most commonly used carbon precursors because pitch is already used in more-conventional activated carbon production processes, low cost pitch is widely available from the petrochemicals industry, and pitch has a high carbon content [24]. However, foams produced directly from tar pitch by pyrolysis at low pressure (close to atmospheric pressure) typically have a low bulk density and poor mechanical strength. Carbon foams with improved mechanical strength have been achieved by (1) blowing the precursor at high pressures (e.g. 2
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30 – 100 bar) or (2) incorporating filler particles such as clay [25], coal [26], graphite [27], carbon nanofibers [28] or carbon nanotubes [29] with the carbon foam. Both approaches have been used together: for example, Zhu et al. [27] reported that the addition of graphite particles to mesophase pitch blown at a pressure 8 MPa and carbonized at 1200 °C increased the
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compressive strength of the carbon foam from 3.7 MPa to 12.5 Mpa. However, foaming processes at high pressures introduces extra complexity, costs and risks to an industrial scale carbon production process. In this paper, we focus on the second approach to incorporate coal filler particles in carbon foams to produce activated carbon discs (ACD).
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Carbon filler particles may serve multiple functions in composite foams as filler particles can (i) provide additional surface areas, (ii) modify the density of the produced foam, and (iii) act
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to stabilize the pith in low pressure foam preparation processes. In this context, the term of stabilization of the foam refers to restriction of bubble growth in the pitch during heat treatment to control the expansion and prevent overflow from the foaming mould (others have used the term stabilization in reference to maximizing bubble size). We selected coal particles as a low cost filler particle (relative to the cost of carbon nanotubes or graphite) and
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investigated the effect of the coal particle size on the foaming process. The coal particles affect the development of the mesopore and macropore structure during foaming and carbonization. As the pitch viscosity and reactivity are the other key parameters affecting the
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stability of the pitch + coal mixture during pyrolysis and carbonization we also investigated the effect of treating the pitch with nitric acid prior to the foaming procedures on the carbon
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foam monolith properties. A chemical activation agent, KOH, was used to enhance the microporosity of the ACDs for increased adsorption of CO2, CH4 and N2.
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2
Experimental
2.1 Materials and pitch pretreatment BP Bitumen Class 170 pitch with a softening point of 320 K and density of softening1.03 g/cm3 [30] was used as the carbon foam precursor in this study. The pitch was
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pretreated using nitric acid (HNO3) and a thermal treatment step; this process increased the softening point of the pitch which improves the foaming properties
[6]
and allows better
physical mixing of the pitch with the coal powder. We investigated the effect of pitch
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pretreatment conditions on the final carbon foam properties by treating the Class 170 pitch with three ratios of HNO3 to pitch: 12 mL, 18 mL and 24 mL of 15 M HNO3 to 100 g of pitch.
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The pitch was heated to 393 K using an oil bath, and stirred with an impellor at 150 rpm, before the HNO3 was added dropwise and the temperature of the oil bath increased to 433 K. The acid pretreatment step was allowed to continue until the impellor could no longer stir the pitch at 433 K. Then the acid-treated pitch was transferred to a tube furnace and thermally treated at 623 K for 2 hours in Argon (heating rate 5 K/min).
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Coal from the Blackwater coal mine in Queensland’s Bowen Basin (Australia) was ground and screened into three size fractions: 0 - 53 µm, 150 - 250 µm and 355 - 550 µm.
2.2 Carbon foam preparation
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The pretreated pitch was ground by hand in a mortar and pestle, then mixed with the selected size of coal powder and potassium hydroxide at a pitch: coal: KOH mass ratio of 1:1:1. To
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improve the dispersion of the KOH through the solids, the pitch + coal + KOH mixture was sonicated for 5 minutes in 50 mL of distilled water. This free water was subsequently evaporated at 353 K for 2 hours over a hot plate with the mixture continuously stirred at 350 rpm. Discs (20 mm diameter) were formed from about 1 g of the pitch + coal + KOH blend in a hydraulic press and these discs were dried overnight in oven at 353 K. The foaming of the discs was performed under Argon in a horizontal tube furnace at 1073 K (heating rate 10 K/min) with a soaking time 1 h. Foamed discs were washed with 0.2 M HCl in a vacuum vessel for one day, dried overnight in oven at 353 K, rinsed with distilled water several times to remove KOH until a filtrate pH of 7 was achieved, and finally dried in the 4
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oven at 353 K overnight. Activated carbon discs (ACD) obtained from the different synthesis conditions are labeled (see Table 1) with the HNO3 pretreatment volume (in mL) and the coal particle size fraction added to the pitch (in micron). For example, the sample labeled ACD18053 was prepared with 18 ml of 15 M HNO3 added to 100 g pitch in the pretreatment and with
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the 0 – 53 µm coal fraction. Photographs of ACD18-150250 are shown in Figure 1a-b. In addition to the ACDs produced from the method described above we also (1) activated 0 53 µm coal particles with KOH to estimate the contribution of the coal to the porosity of the ACDs, and (2) attempted to foam the pre-treated pitch in the tube furnace without any coal
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particles. In the experiment performed with pitch without any coal particles led to the pitch overflowing the furnace crucible (Figure 1c) and did not produce a stable carbon disc.
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The carbon yield of the ACDs was calculated from the treated-pitch weight (mp), coal weight (mc), and the weight of the washed ACD end product (mf) according to: % =
× 100%
(1)
The carbon yield increased from 45.08% to 48.17 % with increasing HNO3 to pitch ratio from
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12 ml to 24 ml/100g pitch. This result is attributed to a greater removal of volatiles, for example by polymerization of shorter chain carbon molecules, at higher concentrations of HNO3 during the pitch pretreatment. Subsequently, during the foaming there is a smaller mass
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of volatiles to be evolved from the pitch treated with 24 mL HNO3/100 pitch than pitch treated with 12 mL HNO3/100 g pitch.
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2.3 Characterization
Scanning electron microscope (SEM) images were collected on a JSM-7100F instrument (JEOL Ltd., Japan). X-ray photoelectron spectroscopy (XPS) was performed on a PHI-560 ESCA (Perkin Elmer) at 15 kV. The C1s peak position was set to 284.8 eV and taken as an internal standard. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were performed with a Perkin Elmer STA 6000 instrument at heating rates of 10 K/min and a nitrogen flow of 20 mL/min. Compressive strength was measured by a uniaxial compression test with an Instron 5584 computer controlled material testing system. The bulk or apparent density (ρHg) of the carbon discs was measured by mercury porosimetry 5
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(Micromeritics PoreSizer 9320) and the skeletal density (ρHe) by helium pycnometer (Micromeritics AccuPyc II 1340). The disc porosity (ϕ) was calculated with Equation 2: % =
× 100
(2)
Sorption isotherms of N2 at 77 K and CO2 at 273 K were measured with a TriStar II 3020
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apparatus (Micromeritics, USA) after degassing the activated carbon discs at 473 K and a pressure of 10-5 torr for 24 h. The N2 isotherms were used to determine the Brunauer Emmett Teller (BET) specific surface area at a relative pressure in the range of the P/P0 = (0.05 - 0.35);
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total pore volumes at P/P0 = 0.99; micropore volumes using the t-plot method and mesopore volumes from the Barrett Joyner Halenda (BJH) method. The pore size distributions (PSD)
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were also calculated from the N2 adsorption on TriStar II 3020. Limiting micropore volumes were also calculated from the CO2 isotherms measured at 273 K with the Dubinin-Astakhov (DA) equation [31]. We also measured, with the TriStar II 3020 apparatus, adsorption isotherms on ACD18-053 for CO2 at 283 K and 303 K, N2 at 273 K, 283 K and 303 K and CH4 at 273 K and 303 K. These low pressure isotherms were used to determine isosteric heats
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of adsorption (qst).
Adsorption equilibrium capacities of CH4, CO2 and N2 on ACD18-053 were measured at 298 K with pressure up to 3500 kPa on a Belsorp-BG instrument (BEL, Japan) equipped with
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a magnetic floating balance (Rubotherm, Germany). The activated carbon discs were
3
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degassed at 423 K for 24 hour prior to high pressure adsorption measurements.
Results and discussion
3.1 Bulk properties of the activated carbon discs A summary of the bulk properties of the activated carbon discs is provided in Table 1. No density or yield results are reported in Table 1 for the foamed tar pitch prepared without coal powder (shown in Figure 1c) as this sample overflowed the crucible and a monolithic disc was not produced. In addition, we were not able to measure the bulk density of ACD12150250 by mercury intrusion porosimetry because after washing in HCl this disc broke apart. 6
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Instead, the ACD12-150250 bulk density of 0.39 g/cm3 was estimated from the dimensions of the cylindrical disc before washing and the weight of the carbon obtained after washing. The low strength of ACD12-150250, which was prepared from pitch treated only with a low HNO3 to pitch ratio, is consistent with results reported in the literature [6].
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The bulk densities of the other four ACDs measured by mercury intrusion porosimetry were in the range of (0.48 to 0.56) g/cm3, which is within the typical range of densities reported for activated carbon foams prepared by other researchers [32]. Chemical activation of the carbon foams with KOH develops microporosity in the carbon walls, and as a result our ACDs can be
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expected to have a lower bulk density than carbon foams that have not been chemically activated. The bulk density of ACD24-150250 was greater than that of ACD18-150250 and
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ACD12-150250, which shows in general the bulk density increased with the ratio of HNO3 used in the pitch pretreatment. When more acid was used, the molecular weight of pitch and softening point increase as well as the viscosity of precursor. In such case, the total bubbling volume decreases and leads to the increment of bulk density. The ratio of HNO3 to pitch was not observed to have a large effect on the skeletal density of the ACDs. In contrast, the
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particle size of coal powder had a significant effect on skeletal density; we will discuss this result in the following section.
Table 2 compares the mechanical strength of ACD18-053 and the strength of three carbon
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foams reported in the literature [33-35]. The compressive strength of ACD18-053 measured by an Instron instrument was 4.3 MPa. This is comparable with low density carbon foams
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produced by Narasimman et al. [33], but less than some high density foams that have been reported. The aim of our current study was not to maximize compressive strength. Instead, our aim is to produce a high surface area carbon monolith with sufficient strength to be used in a pressure swing adsorption process, and for this purpose a crushing strength of 4.3 MPa may be sufficient. Detailed investigations of the stresses induced by pressure swings in an adsorbent bed are recommended but such investigations are beyond the scope of this paper. The relative concentrations of oxygen functional groups on the surface of activated carbon disc ACD18-053 were examined by deconvolution of the XPS C1s spectra (Figure 2). Curves fitted to the C1s spectra was for (1) C–C bonds of graphite at 284.6 eV, (2) phenol and ether 7
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groups (C–O) at 285.3 eV, (3) carbonyls, quinone and aldehydes (C=O) at 286.9 eV, (4) carboxyls, carboxylic anhydrides and esters (O=C–O) at 289.1 eV, and (5) the characteristic shake-up line of aromatic carbon (π=π*) at 290.5 eV [36]. The results of the deconvolution analyses are summarized in Table 3. This result showed that C-O bond is the most dominant
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oxygen functionality on the surface of ACD18-053. The surface chemistry of ACD18-053 is consistent with that of other carbon materials prepared from pitch and coal and carbon materials prepared at temperatures up to 1073 K [37-39].
3.2 Effect of coal filler particles on the morphology of carbon discs
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Figure 3 shows SEM images of the KOH activated coal produced from the 0 to 53 µm coal particle size fraction without tar pitch, and the activated carbon discs ACD18-053 produced
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from coal and pitch mixtures. The activated coal particles (Figure 3 a and b) feature large cracks and interparticle void spaces, but there are no open macropore cells observed in the surfaces of the coal particles. In Figure 3c-d the shape of coal particles is still identified in the ACDs, but the texture of the ACDs is clearly different to that of the coal particles. In the SEM images of ACD18-053 the coal particles are coated in a foam derived from the pitch and this
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foam forms bridges between coal particles. Two types of pore structures between the coal particles are observed: (1) Irregular shaped pores approximately 50 µm in diameter, which result from partial filling of the voids between the coal particles, and (2) open cells and
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channels of 100 – 200 nm width with bubble-like shapes (for example in Figure 3d). The bimodal distribution of macropores in ACD18-053 is confirmed with the pore size
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distributions determined by mercury intrusion porosimetry (Figure 4). The 100 – 200 nm macropore structures developed in the pitch component of the ACDs during the foam blowing process, and these pores are created in the pitch by the evolution and expansion of volatile hydrocarbons in the pitch as the furnace temperature is increased. As the temperature is increased further during foaming and carbonization reactions occur, the remaining pitch components begin to solidify and form solid carbon walls around the bubbles [6, 40, 41]. The pore macropore structures observed in the ACDs are consistent with structures of foams derived from pitch and other precursors that have been reported [20, 25, 34]. We propose that the coal filler particles have two main mechanistic roles during the low8
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pressure pitch foaming process: (1) the particles act to stabilize bubble growth in the pitch during the heating and volatilization, and (2) at temperature greater than about 800 K the coal particles provide a skeleton to the foam structure during carbonization. The TGA curves in Figure 5 show the weight loss from a mixture of pitch (pretreated in
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HNO3) and coal particles in an experiment that replicated the conditions used for foaming and carbonization in the tube furnace to produce ACD18-053. There are four distinct stages in the thermal treatment of the pitch + coal mixture. (i) In the first stage of the heat treatment the temperature is increased from room temperature to the pitch softening point of approximately
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623 K, and only a small mass of moisture and any dissolved light hydrocarbon gases (that remained after pitch pretreatment) is lost from the pitch + coal mixture. A weight loss of
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moisture and adsorbed volatiles is also observed from the coal in the TGA curves. (ii) In the second stage at temperatures above 623 K, the pitch starts to soften and then melt, which allows the pitch to fill the interparticle voids between coal particles. This phenomenon is not observed directly in our TGA measurements, but can be assumed to lead to a reduction in the total volume of the composite as the molten pitch fills void spaces between coal particles.
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Since the pitch has been pre-treated at 623K before foaming, no bubbling occurred at this step due to the absence of volatile in pitch. (iii) In the third stage, bubbles and pitch polymerization begin to occur at temperatures above the pitch pre-treatment temperature of
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623 K, and an increase in the rate of weight loss from pitch + coal is observed. During this stage, the coal particles act as bubble nucleation sites, increase the density and viscosity of the
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mixture and reducing the bubbling expansion rate [25, 34]: the combined effects are that a larger number of small bubbles are formed than in a pitch that is foamed without any coal filler particles, the latter tends to form large bubbles that overflow the foaming crucible (Figure 1c). The derivative thermogravimetric analysis (DTG) curve in Figure 5 shows the maximum rate of weight loss from pitch + coal occurred at approximately 760 K, which is nearly the same as the temperature (763 K) at which Wang et al. observed a maximum rate of weight loss from mesophase pitch [15]. The significance of the temperature of maximum weight loss in the foaming process is that this temperature corresponds to the point at which the pitch has maximum fluidity and expansion [40]. (iv) The fourth and final stage in the 9
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carbon foam process is observed above 800 K as a steady weight loss due to pyrolysis and carbonization reactions.
3.3 Adsorption of N2, CO2 and CH4 on the activated carbon discs The N2 sorption measurements at 77 K show that all the ACDs exhibited TypeⅠisotherms
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(Figure 6a). The ACDs adsorbed large volumes of N2 a P < 10 kPa, which is attributed to adsorption in the micropores that were predominantly created by KOH activation of the ACDs. Table 4 summarizes the pore textural properties of the ACDs and the Blackwater coal samples. The BET surface areas of the ACDs were all greater than 1000 m2/g. The as-received
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Blackwater coal has a BET surface area of 3.1 m2/g but this specific area was increased to 1182 m2/g after KOH activation. This suggests that the KOH activated coal filler particles
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provide significant contributions of micropores to the total surface area of the composite ACDs and the BET surface area associated with the micropore volume in coal filler particles is larger for ACDs prepared with smaller size fractions of coal particles. Due to the kinetic limitations associated with the sorption of N2 in narrow micropores at 77 K [42], we also
included in Table 4.
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probed the ACD structures with CO2 at 273 K and results from CO2 sorption analysis are
Activated carbon disc ACD18-053 had the highest CO2 and N2 adsorption capacities at a pressure of 100 kPa; so we selected this ACD for further adsorption studies at high pressure
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including measurements of CH4 uptake. Absolute equilibrium adsorption capacities for CH4, CO2 and N2 on ACD18-053 measured with the Belsorp-BG at 298 K and pressure up to
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3500 KPa are reported in Table 5 and Figure 7. As expected for an activated carbon the adsorption capacities on ACD18-053 were in the order CO2 > CH4 > N2. The CO2 adsorption capacity of ACD18-053 is comparable to the CO2 uptakes reported for activated carbons at high pressures [43, 44]. There are few reports of high pressure adsorption measurements on carbon foams, but at 100 kPa the adsorption of 3.51 mol/kg CO2 on ACD18-053 is comparable to the 2.5 mol/kg CO2 reported for carbon foams prepared by Narasimman, Vijayan and Prabhakaran [23]. Table 6 presents further comparisons of adsorption capacities of ACD18-053 with other carbon foams and commercial activated carbons.
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To provide additional data to readers that may be useful for the evaluation of gas adsorption processes based on ACD18-053 we calculated isosteric heats of adsorption (qst) at different CO2, CH4 and N2 loadings by the direct method [45] of applying the Clausius-Clapeyron equation (Equation 3). For this part of the study adsorption isotherms were measured at
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(273.15, 283.15 and 303.15) K and pressures up to 120 kPa on the Tristar II instrument. These equilibrium adsorption isotherms are included in the Supplementary Information.
= −
!" # $
%
&
(3)
gas constant. Integrating Equation (3) gives: + ,-
0
/ *1/ + 3
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ln ) & = − *
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where p is the pressure, T is the temperature, Q is the amount adsorbed, and R is the universal
.
(4)
where C is a constant. To determine qst values we fitted a high-order polynomial equation to each isotherms to obtain an expression for Q as a function of p, and used these fitted isotherm equations to interpolate for values of p at a given Q for each temperature so that ln(P)Q could
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be plotted against 1/T. Then with these plots, qst was obtained by linear regression of Equation 4. Figure 8 summaries the isosteric heat of adsorption results. The heats of adsorption were in the ranges 22 – 26 kJ/mol for CO2 at loadings of 2 – 3 mmol/g, 21 – 22.7 kJ/mol for N2 at
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loadings of 0.1 – 0.6 mmol/g, and 6 – 8 kJ/mol for CH4 at loadings of 0.2 – 1.2 mmol/g. The values of CO2 isosteric heat of adsorption on ACD18-053 in this study similar to the range of
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reported values for other activated carbons [46] and isoteric heats of adsorption for N2 are also similar to other reported ranges for similar pressures and coverages [47]. However, our calculated isosteric heat of adsorption for CH4 is at a lower range than some other reports for activated carbons [47, 48]. To reduce the uncertainty in these results for CH4, and confirm the enthalpies of adsorption for the other two adsorbates, additional adsorption measurements at the range of temperatures and a wider pressure range may be required. Those additional measurements are beyond the scope of this current paper and will feature in our continued efforts to develop carbon foam materials for gas separation and storage applications.
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4
Conclusions
Petroleum tar pitch and coal filler particles were used as precursors to prepare activated carbon discs with high surface area, hierarchical pore structures, and relatively strong mechanical strength. The coal particles act to stabilize bubble growth during the pitch
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foaming and carbonization, with smaller coal particles providing more bubble nucleation sites and KOH-activated surface areas than larger coal particles. We found that a HNO3 to pitch ratio of 17/100 in weight for pretreatment of the pitch produced the ACDs with the best mechanical strength and highest bulk density. The ACD18-053 with a surface area of
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1401 m2/g adsorbed 3.51 mol/kg of CO2 at 100 kPa and 298 K, which is comparable to other activated carbons in the literature. The high selectivity of CO2 over N2 and CH4 shows that the
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ACDs prepared in this work have a good potential for pressure swing adsorption.
Acknowledgments
This research was funded by the Australian Research Council (FT120100720 and DE140100569). S. Gao received financial support from the China Scholarship Council.
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Mengran Li and Rijia Lin are acknowledged for their technical assistance with SEM, and Arash Arami Niya for assistance with the high pressure adsorption experiments. The authors acknowledge the facilities and technical assistance of the Australian Microscopy &
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Microanalysis Research facility at the Centre for Microscopy & Microanalysis at the
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University of Queensland.
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Figure 1 Photographs of (a，b) the carbon foam disc ACD18-053 and (c) the overflow of pitch from the crucible observed
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when attempting to foam the pitch without any coal powder.
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Figure 2 Curve fitting of C1s XPS spectra of ACD18-053053 prepared from mixture of coal and tar pitch.
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Figure 3 SEM images of (a, b) KOH activated coal without adding pitch and (c, d) ACD18-053prepared from mixture of coal
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and tar pitch.
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Figure 4 Pore size distribution in activated carbon disc ACD18-053 determined by (a) main figure showing the distribution determined by mercury injection porosimetry (Hg) and (b) inset the size distribution of micropores and mesopores determined
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by NLDFT from the N2 sorption isotherm measured at 77 K.
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Figure 5 Weight loss (TGA) and derivate weight loss (DTG) curves measured for Pitch + coal 1:1:1 weight mixture of pitch (pretreated with 18 mL 15 M HNO3 to 100 g pitch), 0 – 53 µm coal particles and KOH; and KOH activated coal. .Heating
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rate 10 K/min. Nitrogen flow rate 20 mL/min.
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Figure 6 Adsorption of (a) N2 at 77 K and (b) CO2 at 273 K on activated carbon discs prepared from BP Class 170 pitch and
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Blackwater coal powders of different size fractions 0 - 53 µm, 150 - 250 µm and 355 - 550 µm.
Figure 7 Adsorption equilibrium of ACD18-053 sample at 298K.
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Figure 8 Isosteric heat of adsorption of CO2, CH4 and N2 on ACD18-053 calculated via the Clausius-Clapeyron equation.
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.
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ACCEPTED MANUSCRIPT IMMS-9, 17-20 August 2015, Brisbane, Australia Table 1 The bulk density, skeletal density, porosity and carbon yield of the coal and activated carbon discs.
Coal particle size
Bulk density
porosity
Carbon yield
%
%
1.38±0.009
6.01±0.62
-
1.92±0.24
58.92±5.88
46.99
2.07±0.16
81.03±1.58
45.08
1.85±0.07
73.94±1.02
43.58
2.05±0.09
79.23±0.96
46.70
0.56
1.59±0.13
64.61±3.14
44.40
0.49
2.05±0.10
75.96±1.23
48.17
3
µm
g/cm
Black Water Coal
-
-
1.30
KOH Activated Coal
-
0-53
0.79
ACD12-150250
12
150-250
0.39
ACD18-053
18
0-53
0.48
ACD18-150250
18
150-250
0.43
ACD18-355550
18
355-550
ACD24-150250
24
150-250
g/cm
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ml/100g
Skeletal density
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Sample
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Table 2 Bulk properties of the carbon foam disc ACD18-053 and representative carbon foams in literature (GF0
[27]
was
made from graphitized carbon black particles and mesophase pitch, CFS carbon foam made by Narasimman and Prabhakaran [33]was prepared from activated carbon powder and aqueous sucrose resin and CTP3 [35] was prepared using coal tar pitches modified with cinnamaldehyde (CMA) and boric acid).
strength/MPa Bulk density(g/cm3)
GF0 [34]
CFS [33]
CTP3 [35]
4.3
5.39
3.4
21.27
0.48
0.76
0.22
0.76
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Compressive
ACD18-053
C-O
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Sample
C=O
O=C-O
π=π*
286.1
287.4
288.8
290.1
15.0
6.2
2.0
5.8
Table 3 Relative concentration (at. %) of oxygen functional groups on ACD18-053. Calculated from deconvolution of high
Group
C-C
Binding energy (eV)
284.8
ACD18-053
71.0
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resolution XPS C1s spectra.
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Table 4 Main textural parameters of the obtained activated carbon discs (BET surface area was evaluated at p/p0 ~ 0.99, micropore volume was evaluated by the t-plot method applied to N2 adsorption isotherms and mesopore volume was evaluated by the BJH method applied to N2 adsorption isotherms at 77K. DA Micropore surface area and narrow micropore volume determined from CO2 adsorption at 273K).
Sample
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Vtotal
Vmicro
Vmeso
3
3
3
(cm ·g )
(cm ·g )
(m ·g )
(cm3·g-1)
3.1
-
-
-
-
-
KOH activated coal
1182
0.505
0.429
0.049
-
-
ACD12-150250
1221
0.515
0.430
0.035
1053
0.419
ACD18-053
1401
0.600
0.489
0.047
1584
0.647
ACD18-150250
1302
0.534
0.437
0.049
1310
0.527
ACD18-355550
1055
0.436
0.364
0.029
1181
0.469
ACD24-150250
1269
0.542
0.435
0.054
1547
0.665
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-1
-1
2
-1
Vmicro
(cm ·g )
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SD-R
(m ·g )
Blackwater coal
-1
CO2 Adsorption
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Table 5 Methane, carbon dioxide, nitrogen adsorption equilibrium data on ACD18-053 sample at 298 K.
CH4
CO2
N2
Qads(mol/kg)
P(kPa)
Qads(mol/kg)
P(kPa)
Qads(mol/kg)
9.22
0.19
5.74
0.52
8.64
0.02
29.65
0.52
28.41
1.68
28.12
0.09
71.60
1.00
68.56
2.87
102.88
1.22
99.13
3.51
303.96
1.72
297.51
5.74
701.54
2.57
698.30
7.31
1001.06
2.89
998.61
7.87
1500.87
3.24
1497.94
8.35
1499.77
1.90
2000.50
3.98
1995.99
8.66
1998.41
2.32
2500.35
4.70
2495.47
8.76
2497.84
2.65
2999.30
5.11
2994.94
8.84
2997.72
2.89
3500.07
5.43
8.87
3495.76
3.09
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P(kPa)
0.21
101.77
0.28
302.47
0.57
SC
70.99
1.10
1000.63
1.39
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701.18
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3493.96
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Table 6 Equilibrium capacity of adsorbents in literature and ACD for N2, CO2 and CH4 at 100 and 1000 kPa and ambient temperature.
Adsorbent
Type
CH4 capacity
N2 capacity
(mol/kg)
(mol/kg)
(mol/kg)
1000kPa
100kPa
1000kPa
100kPa
1000kPa
ACD
3.51
7.87
1.22
3.76
0.28
1.39
AC
2.20
7.60
1.22
2.89
0.39
2.02
Zeolite
3.3
6.40
0.59
2.90
0.28
1.83
MOF
1.59
-
0.63
-
0.17
-
ACHM [53]
Monolith
3.0
-
1.4
-
0.5
-
CA300[54]
Foam
-
2.0
-
0.7
-
0.23
Norit RB1 extra[49] 13X [50, 51]
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MOF-177 [52]
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ACD18-053
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100kPa
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Pressure
CO2 capacity
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ACCEPTED MANUSCRIPT THE PREPARATION OF ACTIVATED CARBON DISCS FROM TAR PITCH AND COAL POWDER FOR ADSORPTION OF CO2, CH4 AND N2 Shuai Gao, Lei Ge, Thomas E. Rufford* and Zhonghua Zhu*
Highlights
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Carbon discs with hierarchical pore structures prepared from pitch and coal powder. Nitric acid pretreatment of pitch improved foaming properties. Smaller coal particles produce carbon discs with better mechanical strength.
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• • •