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Carbon silica composites for sulfur dioxide and ammonia adsorption
Microporous and Mesoporous Materials 165 (2013) 48–54
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Carbon silica composites for sulfur dioxide and ammonia adsorption Amanda M.B. Furtado, Yu Wang, M. Douglas LeVan ⇑ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA
a r t i c l e
i n f o
Article history: Received 6 April 2012 Received in revised form 14 July 2012 Accepted 15 July 2012 Available online 4 August 2012 Keywords: Composite adsorbent Carbon Mesoporous silica Sulfur dioxide Ammonia
a b s t r a c t This work focuses on creating nanoporous carbon silica composites from MCM-41 and two carbon sources, sucrose and furfuryl alcohol. The carbon silica composite with sucrose as the carbon phase was synthesized using a novel low temperature procedure. These novel, biphasic materials were tested for their ability to adsorb two distinct types of gases: sulfur dioxide, an acidic gas, and ammonia, a basic gas. The materials are characterized by XRD, nitrogen adsorption isotherms, high resolution TEM, and TGA. The characterization techniques show that impregnation with the carbon phases does not disrupt the hexagonal mesoporous silica structure. Equilibrium breakthrough results show that the presence of the carbon phase enhances both ammonia and sulfur dioxide adsorption capacities compared to the parent MCM-41 and BPL activated carbon. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The carbon silica composite (CSC) introduced by Glover et al. [1] in 2008 consists of carbonized polyfurfuryl alcohol within the pores of MCM-41. This was the first in a series of studies reported by our group to produce biphasic composite materials targeting light gas adsorption. The CSC provides a large surface area and two distinct phases in which adsorption can occur: a nonpolar carbonaceous phase and a polar siliceous phase. The CSC material is of interest since it is well documented that interactions between the adsorbate and adsorbent are affected by the polarity of each; nonpolar surfaces such as carbon show greater attraction for adsorbate molecules of low polarity, whereas polar surfaces have higher affinity for polar molecules [2]. Materials that provide both polar and nonpolar surfaces for adsorption have an advantage over single-phase adsorbents in broad scale applications. In today’s society, light gases are widely used as industrial chemicals, yet they can also be hazardous to human health [3]. Adsorbent materials designed for air purification applications must have the ability to remove low concentrations of a broad spectrum of toxic gases. For air purification in industrial settings, activated carbons such as BPL AC are generally used [4]. Such carbons have a large pore size distribution that provides macropores and mesopores to enhance the transport properties throughout the adsorbents and micropores that provide capacity for physical adsorption due to strong potential wells [5]. The unique transport properties and physical characteristics of biphasic materials have been studied extensively [6–8]. Biphasic materials such as carbon ⇑ Corresponding author. Tel.: +1 615 343 1672; fax: +1 615 343 7951. E-mail address: [email protected] (M.D. LeVan). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.032
silica composites have the potential to be more efficient at air purification than single phase adsorbents such as activated carbon. Each phase in the biphasic composite can be tailored to remove one type of gas; i.e., silicas can be used to target the removal of basic gases such as ammonia and an organic phase can be used to target the removal of acidic gases such as sulfur dioxide. Mesoporous materials with ordered pore structures and large surface areas have shown great promise for use in industrial applications ranging from air to water purification. MCM-41, which is a member of the M41S family of siliceous materials, is one popular example of this type of structured mesoporous material. This siliceous material, which was first created by Mobil scientists in the early 1990s [9], forms a hexagonal close-packed structure composed of unidirectional channels arranged in a hexagonal manner [10]. It has a high surface area and a repeating structure of cylindrical pores, which is an ideal backbone for a well characterized adsorbent material. Extensive research by Foley and others [11–17] has provided the foundation for the use of carbonized furfuryl alcohol as one carbon phase in a composite material. Carbonized furfuryl alcohol has been extensively studied as a carbonaceous adsorbent material because it has a small pore size distribution centered at an average pore size of 4–5 Å [11]. Pore formation by furfuryl alcohol depends on the carbonization temperature [11–13,15]. Burket et al. [11] determined that both mesopores and micropores begin to appear within the material at carbonization temperatures as low as 300 °C. They proposed that mesopores are formed from the incomplete carbonization of polymer remnants and aromatic cores. As the carbonization temperature was increased from 300 °C to 600 °C, the initial mesoporosity collapsed, leaving only micropores ranging from 4 to 5 Å after carbonization at 500–600 °C. With such
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a narrow and controlled pore size distribution, furfuryl alcoholbased CMS materials are useful for separating two gaseous compounds of different sizes [18]. Carbonized furfuryl alcohol within the pores of a siliceous material has been studied for its molecular sieving properties. De Clippel et al. [16,17] reported a furfuryl alcohol-based carbon silica composite that showed remarkable properties for separating linear and branched alkanes due to the narrow pore size distribution provided by the impregnated furfuryl alcohol phase. The small pore sizes and large surface area of this CSC-furfuryl alcohol (CSC-FA) material make it a good candidate for the removal of light gases for air purification [19]. Carbonized sucrose is another carbon phase that could be of interest in a CSC material. Sucrose as a carbon phase is different from carbonaceous furfuryl alcohol since the pores are larger, and the temperature necessary for carbonization is much lower than that of furfuryl alcohol. Exploring a CSC containing a carbon phase that can be carbonized at a lower temperature than furfuryl alcohol could be beneficial for future work on these adsorbents, since many functionalization approaches are best performed at low temperatures. [20,21]. Sucrose has been shown to form a mesoporous carbon phase with high surface areas (on the order of 1000 m2/g) and a pore size of approximately 20 Å [22,23]. Extensive work on sucrose carbonization has been performed at different temperatures and using different synthesis procedures. Ting et al. [23] performed a one-pot synthesis using sucrose catalyzed with sulfuric acid and carbonized at 900 °C. The resulting mesoporous carbon had a surface area of 1200 m2/g and 44 Å pores. Peng et al. [24] synthesized mesoporous carbonaceous materials at carbonization temperatures ranging from 400 to 600 °C. Zhuang and Yang [25] successfully produced carbonaceous spheres from aqueous sucrose solutions carbonized under high pressure at 175 °C. Zheng et al. [26] produced CMK-3 type materials by taking advantage of low temperature carbonization of sucrose in ethanol at 200 °C in a high pressure reactor. Banham et al. [22] templated sucrose into hexagonal mesoporous silicas. Sucrose has also been templated into silica gel and carbonized at 800 °C [27]. Bimodal porous carbons have been produced by impregnating silica spheres with sucrose and carbonizing at high temperatures [28]. This research focuses on carbonaceous CSC materials as biphasic adsorbents for adsorption of light acidic and basic gases. Although not all acidic and basic gases will adsorb in the same way, sulfur dioxide and ammonia are used as representative acidic and basic gases, respectively, to understand the overall trends associated with adsorption on these composites. It builds on previous research from our group published in this journal that introduced a CSC material based on MCM-41 and furfuryl alcohol [1], optimized its synthesis [29], characterized its light gas adsorption properties for carbon dioxide, nitrogen, methane, and ethane [1,29], and optimized the silica phase of the CSC for basic gas adsorption [21]. In this paper, the carbonization method for CSC materials with two different carbon phase precursors, furfuryl alcohol (CSC-FA) and sucrose (CSC-S), is investigated, and these composites are tested for their feasibility for air purification applications. The CSC containing sucrose as the carbon phase was synthesized using a novel low carbonization temperature synthesis procedure. The furfuryl alcohol-based CSC was synthesized at different carbonization temperatures, and the pore sizes and structure are examined after carbonization at different temperatures to increase surface area in the CSCs. The materials are characterized using nitrogen adsorption isotherms, X-ray diffraction, thermogravimetric analysis, and tested for their ammonia and sulfur dioxide adsorption capacities. To the best of our knowledge, this is the first study of a carbon silica composite synthesized to target the adsorption of acidic and basic gases for air purification.
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2. Experimental methods 2.1. Materials 2.1.1. MCM-41 Tetramethylammonium hydroxide pentahydrate, TMAOH, (97%), tetramethylammonium silicate solution, TMASi, (99.99%, 15–20 wt.% in water), sulfuric acid (95.0–98.0%), and furfuryl alcohol (99%) were purchased from Sigma Aldrich. Hexadecyltrimethylammonium chloride, CTAC, (25%) in water was purchased from Pfaltz and Bauer. A solution of ammonium hydroxide (29 wt.%) in water and Cab–O–Sil M5 were purchased from Fisher Scientific. 2.1.2. CSC-S Sucrose was purchased from Fisher Scientific, and dry ethanol (200 proof) was purchased from Pharmco-aaper. 2.1.3. CSC-FA Furfuryl alcohol (99%) and toluene (99%) were purchased from Sigma Aldrich. 2.2. MCM-41 synthesis All CSC materials include MCM-41 as the silica phase. Hexagonally-ordered MCM-41 with a 37 Å pore was synthesized according to the procedure described in a previous study [21]. 2.3. CSC-S 2.3.1. CSC-S synthesis The novel sucrose-impregnated CSC material was synthesized using a low temperature carbonization technique. Equal parts ethanol and water were mixed in a Teflon-lined Parr reactor at room temperature. One gram of sucrose was added to the mixture, which was then covered and stirred vigorously for thirty minutes. Next, 0.2 g of calcined MCM-41 was added to the mixture and stirred for an additional 1 h. The Parr reactor was sealed and put into the oven at 200 °C for either 24 h (to produce CSC-1) or 48 h (to produce CSC-2). After the reaction was complete, the Parr reactor was removed from the oven and cooled to room temperature. The CSC-S material was separated from the brown solution via vacuum filtration and rinsed with distilled water. The CSC-S was air dried overnight. 2.4. CSC-FA 2.4.1. CSC-FA synthesis The furfuryl alcohol-impregnated carbon silica composite material was synthesized using the procedure outlined by Glover et al. [1]. After impregnation with the furfuryl alcohol phase, samples were carbonized at temperatures of 300, 500, and 600 °C, following the method previously described [1]. 2.5. Materials characterization 2.5.1. Textural characterization Adsorption isotherms were measured using a Micromeritics ASAP 2020 at 196 °C with UHP nitrogen as the analysis gas. Prior to measurement, approximately 0.1 g of each sample was degassed with heating to 90 °C and vacuum to 10 lbar. After reaching 10 lbar, the samples were heated to 100 °C under vacuum for an additional 6 h. Density functional theory (DFT) provided in the ASAP 2020 software was used to calculate pore volumes and pore size distributions. Pore volumes reported correspond to P=P 0 ¼ 0:999.
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2.5.2. X-ray diffraction (XRD) XRD spectra were used to confirm the long range structure of the native and impregnated MCM-41 samples. The spectra were measured using a Scintag X1h/h automated powder diffractometer with Cu target, a Peltier-cooled solid-state detector, a zero background Si(5 1 0) support, and a copper X-ray tube as the radiation source. Spectra were collected from 1.2° to 7° 2h using a step size of 0.02°.
2.5.4. Transmission electron microscopy (TEM) High resolution TEM was performed on the CSC materials to investigate the MCM-41 mesoporous network using a Philips CM20 electron miscroscope operating at 80 kV. The samples were prepared by dispersing approximately 0.1 g of CSC powder into approximately 0.5 mL isopropanol via sonication for 1 min. The dispersion was then placed onto Lacey carbon copper grids and air dried. 2.5.5. Target gas capacity Equilibrium capacities for room temperature light gas adsorption of both ammonia and sulfur dioxide were measured for all samples using a breakthrough apparatus, a schematic of which has been given by Furtado et al. [21,30]. Although measured under dynamic conditions, full breakthrough capacities were measured at low flow rates and are equilibrium capacities, as they end after the feed concentration exits the bed [31,32]. The equilibrium capacities agree with equilibrium results obtained using a gravimetric (Cahn) balance [30]. Prior to analysis, all samples were regenerated under vacuum at 120 °C for 2 h. The flow rate of gas across the adsorbent bed was kept constant at 1133 mg/m3 for ammonia (1500 ppm in helium) and 1428 mg/ m3 for sulfur dioxide (500 ppm in helium). The capacity of the adsorbent material, n (mol ammonia or sulfur dioxide/kg adsorbent), was calculated via material balance using [21,32]
n¼
F m
Z
1
ðc0 cÞ dt
ð1Þ
0
where c0 is the feed molar concentration, and c is the effluent concentration at time t. The volumetric flow rate of gas through the adsorbent bed, F, was adjusted to yield a breakthrough time of approximately 1 h. The mass of the sample, m, was approximately 10 mg and was contained in a small cylindrical adsorbent bed. The capacities calculated using the breakthrough apparatus have a standard deviation on the order of 3%, and different batches of CSC are repeatable to within 5%. 3. Results 3.1. Carbon silica composites synthesis and characterization 3.1.1. CSC-S carbonized for different reaction times Two novel CSC materials using sucrose as the carbon phase were successfully produced by impregnating MCM-41 with sucrose using different reaction times. Using thermogravimetric analysis, the amount of carbon loaded into the MCM-41 scales with the reaction time, as shown in Fig. 1. The initial mass loss corre-
Weight Percent
80
60
40
20
CSC-1 CSC-2 0 0
100
200
300
400
500
600
Temperature (K) Fig. 1. Thermogravimetric analysis of CSC-S samples.
Table 1 Physical properties of the CSC-S samples. Sample
Reaction time (h)
wt.% C
BET SA (m2/g)
Vpore (cm3/g)
Predominant pores (Å)
MCM-41 CSC-1 CSC-2
– 24 48
– 26 46
836 436 263
1.2 0.56 0.45
37 12–30 12–30
sponds to the liberation of adsorbed water and the second gradual mass loss represents the loss of carbon from the samples. Table 1 summarizes the amount of carbon loaded into each sample. Nitrogen isotherms are shown in Fig. 2 for both CSC-S materials and for the unimpregnated MCM-41. The parent MCM-41 exhibits
700
3 Amount Adsorbed (cm /g at STP)
2.5.3. Thermogravimetric analysis (TGA) Thermogravimetric analysis was performed on the CSC materials to determine the amount of carbon impregnated into the MCM41 using a TA Instruments Q600 SDT, a simultaneous DSC-TGA. Samples were heated in zero air from room temperature to 600 °C using a ramp rate of 5 °C per minute with an air flow rate of 10 ml per min. To fully burn off the carbonaceous phase, the samples were maintained at 600 °C for 3 h.
100
600
MCM-41 CSC-1 CSC-2
500
400
300
200
100
0 0.0
0.2
0.4
0.6
0.8
Relative Pressure (P/P0) Fig. 2. Nitrogen adsorption isotherms for CSC-S samples.
1.0
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MCM-41
3
Differential Pore Volume (cm /g)
8
6
4
2
0 10
15
20
25
30
35
Pore Width (Å)
3
Differential Pore Volume (cm /g)
1.0
CSC-1 CSC-2 CSC300 CSC500 CSC600
0.8
51
materials compared to the parent MCM-41, as shown by the column in Table 1 summarizing the predominant pores in the samples and by the pore size distributions shown in Fig. 3. This is due to filling the mesopores with a carbon phase, which results in an increase in microporosity and a decrease in mesoporosity. After filling the MCM-41 pores with different amounts of sucrose carbon, density functional theory predicts lower predominant pore sizes compared to the initial 37 Å pore of the parent MCM-41. Consequently, it is evident that sucrose carbon fills the MCM-41 mesopores and produces micropores and small mesopores in the material after carbonization. High resolution TEM images were obtained to compare MCM41 to the CSC-S samples. Fig. 4 compares TEM images of the parent MCM-41 with CSC-1. In image a, which shows the parent MCM-41, the pores are clearly visible. Image analysis using the Philips CM20 software calculated the pore diameters to be 38.5 Å, which compares well with the expected value of 37 Å. Image b shows the aligned sucrose carbon-impregnated MCM-41 pores of 38.5 Å in diameter. Also apparent during analysis of this CSC-1 sample were carbon spheres ranging from 500 nm to 1 micron in diameter. These spheres, shown in image c alongside the aligned CSC-S cylindrical pores, are similar to those found by Zhuang and Yang [25] in their low temperature carbon synthesis, and are structures forming around carbon filled MCM-41 pores.
0.6
0.4
0.2
0.0 10
15
20
25
30
35
Pore Width (Å) Fig. 3. Pore size distributions calculated via DFT for MCM-41, CSC-S, and CSC-FA samples. The curve for CSC300 lies just above the x-axis.
Type IV behavior according to the IUPAC classification [34]. The CSCs are microporous and mesoporous, and exhibit Type I behavior due to the sharp increase in the amount adsorbed at low relative pressures, which levels off as the relative pressure increases. BET surface areas and pore volumes calculated from the isotherms are summarized in Table 1. Impregnating MCM-41 with the sucrose phase decreases the surface area and pore volume of the
3.1.2. CSC-FA carbonized at different temperatures The CSC-FA materials were characterized by X-ray diffraction to verify that the impregnation of the silica phase with carbon did not disrupt the MCM-41 structure. Fig. 5 shows the XRD spectra of the base CSC-FA materials carbonized at different temperatures and the MCM-41 base material. Despite the decrease in intensity of the base CSC materials compared to MCM-41, which is due to interference from the carbon phase, the MCM-41 peaks are identifiable in the CSC-FA spectra. Thus, carbon impregnation does not alter the MCM-41 hexagonal order. Long range XRD scans were also run to identify evidence of graphitization, which can be expected to occur during carbonization at higher temperatures. No graphitic peaks were detected in these scans. Characterization of the CSC-FA materials was performed by measuring nitrogen isotherms at 77 K. These are shown in Fig. 6 and are Type I isotherms. The nitrogen isotherm for the CSC300 sample is not shown in Fig. 6 because it adsorbs much less nitrogen than the other samples and is on a much lower scale. All CSC-FA materials are nanoporous and exhibit Type I behavior due to the sharp increase in the amount adsorbed at low relative pressures, which levels off as the relative pressure approaches unity. The
Fig. 4. High resolution transmission electron microscopy images of MCM-41 and CSC-1.
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A.M.B. Furtado et al. / Microporous and Mesoporous Materials 165 (2013) 48–54 Table 2 Physical properties of the CSC-FA samples.
Intensity (counts per second)
MCM-41 CSC300 CSC500 CSC600
2
3
4
5
6
7
Degrees 2θ Fig. 5. X-ray diffraction spectra for MCM-41 and CSC-FAs carbonized at different temperatures.
Sample
Carbonization temperature (K)
wt.% C
BET SA (m2/g)
Vpore (cm3/g)
Predominant pores (Å)
MCM-41 CSC300 CSC500 CSC600
– 300 500 600
– 51 40 44
836 24 466 505
1.2 0.07 0.39 0.42
35 Minimal 8–20 8–25
TGA analysis was performed on the CSC-FA samples produced at different carbonization temperatures. As summarized in Table 2, data show that the CSC-FA carbonized at 300 °C has the highest mass loss, 51 wt.%, due to incomplete polyfurfuryl alcohol carbonization at the lower temperature [11–13,15]. The average carbon content for the remaining samples is approximately 40%. High resolution TEM images were also obtained on these samples. Fig. 7 compares TEM images of MCM-41 and CSC500, a representative CSC-FA sample. Image a shows the parent MCM-41 material. Image b shows the carbon-filled MCM-41 arranged in an ordered manner. In image c, the hexagonal pores are visible. It is difficult to distinguish between the carbon filled MCM-41 pores and the base MCM-41. It is evident from these images that the furfuryl alcohol polymerizes well within the MCM-41 mesopores rather than forming a pure carbon phase outside of the MCM-41. 3.2. CSC target gas adsorption
700
3
Amount Adsorbed (cm /g at STP)
600
MCM-41 CSC500 CSC600
500
400
300
200
100
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 6. Nitrogen adsorption isotherms for CSC-FAs carbonized at different temperatures.
hysteresis during desorption is characteristic of a Type IV isotherm [19]. Surface area and pore information were calculated from the nitrogen adsorption isotherms of the CSC-FA materials. BET surface areas were calculated according to the procedure for microporous materials outlined by Rouquerol et al. [33] and are summarized in Table 2 with pore size distributions given in Fig. 3. When all samples are compared, those carbonized at mid-range temperatures (500–600 °C) have much higher surface areas and larger pore volumes than the sample carbonized at 300 °C, which has minimal porosity. Also, as shown in Fig. 3, when compared with the sucrose-based materials, the FA-based materials have pore size distributions shifted more towards the micropore region.
3.2.1. CSC-S As shown in Table 3, the sucrose-based carbon silica composites were tested for ammonia and sulfur dioxide adsorption capacity and compared to MCM-41 and BPL activated carbon. This table includes two columns presenting ammonia and sulfur dioxide adsorption capacities calculated per kg sample, and two columns presenting these capacities calculated on a per kg silica and carbon basis, respectively. Calculating the capacities per kg of each phase was done to emphasize that the carbon phase in the composite targets sulfur dioxide adsorption and the silica phase targets ammonia adsorption. It should be emphasized that the formation of the composite gives a material with a different pore size distribution and surface chemistry than either the parent MCM-41 or a sucrose-based carbon created outside of the MCM-41 mesopores. The pore size distribution of the carbon phase can impact the adsorption of ammonia targeted for the silica phase and the templating silica support can impact the carbon phase targeted for sulfur dioxide adsorption. Thus, there are synergistic interactions in the formation of the two phases of the composite material for adsorption of the two target gases. It is obvious from the table that the presence of the sucrose carbon phase increases both the ammonia and sulfur dioxide adsorption capacities compared to the parent MCM-41. The MCM-41 provides hydroxyl groups that enhance ammonia adsorption through hydrogen bonding [21,35–37], and these sites provide much of the ammonia adsorption capacity of the CSC material. The parent MCM-41 has minimal sulfur dioxide adsorption capacity. However, after impregnation, the carbon phase introduces micropores into the composite that promote adsorption of sulfur dioxide and also ammonia. The microporous carbon phase enhances the sulfur dioxide capacity of the composite by 400% and it enhances the ammonia adsorption capacity by 9%. The drastic increase in sulfur dioxide capacity after impregnation results from the basic nature of the sucrose carbon phase [38,39], thereby enhancing the acidic gas adsorption of the composite. The much smaller increase in ammonia adsorption capacity is due to the increased presence of micropores throughout the sample. Compared to BPL activated carbon, both CSC samples show much higher
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53
Fig. 7. High resolution transmission electron microscopy images of MCM-41 and CSC500.
Table 3 Gas adsorption on the CSC-S samples. Sample
BPL AC MCM-41 CSC-1 CSC-2
SO2 adsorption capacity
NH3 adsorption capacity
mol/kg sample
mol/kg carbon
mol/kg sample
mol/kg SiO2
0.20 0.02 0.10 0.15
0.20 – 0.38 0.33
0.10 2.01 2.18 2.10
– 2.01 2.95 3.93
ammonia capacities. For sulfur dioxide, the carbon phase provides base capacity for the composite, whereas the silica phase does not. On a mol/kg carbon basis, the sulfur dioxide adsorption capacity for both CSC-S materials is higher than that of BPL AC.
3.2.2. CSC-FA Target gas adsorption capacities are shown in Table 4 for the CSC-FA samples, the parent MCM-41, and BPL activated carbon. The presence of the carbon phase enhances the sulfur dioxide adsorption compared to the parent MCM-41, with CSC600 having the highest SO2 adsorption capacity. Similar to the sucrose carbon phase, the carbon phase produced by the furfuryl alcohol is basic in nature [38,39], and it enhances the sulfur dioxide adsorption capacity over that of the parent MCM-41. When compared on a mol/kg silica basis, the carbon phase produced at the 300 °C carbonization temperature causes a decrease in the ammonia adsorption capacity compared to MCM-41; however, the carbon phases in CSC500 and CSC600 result in an increase in the ammonia adsorption capacity compared to the base MCM-41. For the CSC-FA samples, it is obvious that higher adsorption capacities correspond to higher carbonization temperatures. The nitrogen isotherms of Fig. 6 show much higher surface areas and well developed pore structures for the samples carbonized at 500 and 600 °C compared to the sample carbonized at the lower temperature. Analyses of the samples show incomplete furfuryl alcohol carbonization at the low
Table 4 Gas adsorption on the CSC-FA samples. Sample
BPL AC MCM-41 CSC300 CSC500 CSC600
SO2 adsorption capacity
NH3 adsorption capacity
mol/kg sample
mol/kg carbon
mol/kg sample
mol/kg SiO2
0.20 0.02 0.20 0.25 0.34
0.20 – 0.55 0.71 0.95
0.10 2.01 0.76 1.38 1.48
– 2.01 1.19 2.14 2.30
carbonization temperature of 300 °C, giving a decrease in ammonia and sulfur dioxide adsorption capacity compared to the samples carbonized at 500 and 600 °C. All CSC samples show much higher ammonia adsorption capacities than BPL activated carbon. On a mol/kg carbon basis, all CSC materials also have higher sulfur dioxide adsorption capacities than the BPL activated carbon. When compared on a mol/kg sample basis, CSC500 and CSC600 show higher sulfur dioxide adsorption capacities than the commercial carbon.
4. Conclusions A series of carbon silica composites with MCM-41 as the silica phase and carbonized sucrose or furfuryl alcohol as the carbon phase have been synthesized using different carbonization temperatures and reaction times. These materials have been characterized via adsorption isotherms, XRD, and TGA. They were also tested for their ammonia and sulfur dioxide capacities, a basic and an acidic gas, respectively. Impregnation of MCM-41 with carbonized sucrose results in a novel CSC material with potential as an air purification adsorbent. CSC-S shows an increase in the sulfur dioxide and ammonia adsorption capacities compared to the unimpregnated MCM-41. The carbonized sucrose phase of the CSC-S composite enhances the sulfur dioxide adsorption capacity of the adsorbent compared to the unimpregnated silica phase. The presence of the sucrose phase also results in an increase in ammonia adsorption capacity. The increases in ammonia and sulfur dioxide adsorption capacities occur for samples impregnated using both 24 and 48 h reaction times. Similar to the CSC-S material, impregnation of MCM-41 with furfuryl alcohol to form the CSC-FA results in a composite with high toxic gas capacities. The CSC-FA maintains the ammonia adsorption capacity and enhances the sulfur dioxide adsorption capacity compared to MCM-41. The CSC-FA carbonized at 600 °C has the highest surface area, followed by the samples carbonized at 500 and 300 °C. Carbonization of the furfuryl alcohol polymer is a rate process, and the sample heated to 300 °C is not fully carbonized, whereas the sample carbonized at 600 °C has the most well-developed pore structure. The development of the pore structure correlates with the ammonia and sulfur dioxide capacities; in general, the samples carbonized at higher temperatures have higher target gas capacities than the sample carbonized at 300 °C. Carbon silica composites produced by impregnating MCM-41 with sucrose and furfuryl alcohol carbons show promise as adsorbent materials for air purification. These materials provide a carbon phase for physical adsorption and a silica phase for hydrogen
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bonding with electronegative atoms. These biphasic materials have large capacities for two very different types of gases, ammonia and sulfur dioxide. Their adsorption capacities for these gases are higher than those of BPL activated carbon.
[13] [14] [15] [16] [17]
Acknowledgements [18]
We are grateful to the US Army Edgewood Chemical and Biological Center and the Defense Threat Reduction Agency for the support of this research under contract number W911SR-08-C-0028. We are also grateful to Dr. James McBride for his help with the transmission electron microscope.
[19] [20] [21] [22]
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Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Carbon silica composites for sulfur dioxide and ammonia adsorption Amanda M.B. Furtado, Yu Wang, M. Douglas LeVan ⇑ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA
a r t i c l e
i n f o
Article history: Received 6 April 2012 Received in revised form 14 July 2012 Accepted 15 July 2012 Available online 4 August 2012 Keywords: Composite adsorbent Carbon Mesoporous silica Sulfur dioxide Ammonia
a b s t r a c t This work focuses on creating nanoporous carbon silica composites from MCM-41 and two carbon sources, sucrose and furfuryl alcohol. The carbon silica composite with sucrose as the carbon phase was synthesized using a novel low temperature procedure. These novel, biphasic materials were tested for their ability to adsorb two distinct types of gases: sulfur dioxide, an acidic gas, and ammonia, a basic gas. The materials are characterized by XRD, nitrogen adsorption isotherms, high resolution TEM, and TGA. The characterization techniques show that impregnation with the carbon phases does not disrupt the hexagonal mesoporous silica structure. Equilibrium breakthrough results show that the presence of the carbon phase enhances both ammonia and sulfur dioxide adsorption capacities compared to the parent MCM-41 and BPL activated carbon. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The carbon silica composite (CSC) introduced by Glover et al. [1] in 2008 consists of carbonized polyfurfuryl alcohol within the pores of MCM-41. This was the first in a series of studies reported by our group to produce biphasic composite materials targeting light gas adsorption. The CSC provides a large surface area and two distinct phases in which adsorption can occur: a nonpolar carbonaceous phase and a polar siliceous phase. The CSC material is of interest since it is well documented that interactions between the adsorbate and adsorbent are affected by the polarity of each; nonpolar surfaces such as carbon show greater attraction for adsorbate molecules of low polarity, whereas polar surfaces have higher affinity for polar molecules [2]. Materials that provide both polar and nonpolar surfaces for adsorption have an advantage over single-phase adsorbents in broad scale applications. In today’s society, light gases are widely used as industrial chemicals, yet they can also be hazardous to human health [3]. Adsorbent materials designed for air purification applications must have the ability to remove low concentrations of a broad spectrum of toxic gases. For air purification in industrial settings, activated carbons such as BPL AC are generally used [4]. Such carbons have a large pore size distribution that provides macropores and mesopores to enhance the transport properties throughout the adsorbents and micropores that provide capacity for physical adsorption due to strong potential wells [5]. The unique transport properties and physical characteristics of biphasic materials have been studied extensively [6–8]. Biphasic materials such as carbon ⇑ Corresponding author. Tel.: +1 615 343 1672; fax: +1 615 343 7951. E-mail address: [email protected] (M.D. LeVan). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.032
silica composites have the potential to be more efficient at air purification than single phase adsorbents such as activated carbon. Each phase in the biphasic composite can be tailored to remove one type of gas; i.e., silicas can be used to target the removal of basic gases such as ammonia and an organic phase can be used to target the removal of acidic gases such as sulfur dioxide. Mesoporous materials with ordered pore structures and large surface areas have shown great promise for use in industrial applications ranging from air to water purification. MCM-41, which is a member of the M41S family of siliceous materials, is one popular example of this type of structured mesoporous material. This siliceous material, which was first created by Mobil scientists in the early 1990s [9], forms a hexagonal close-packed structure composed of unidirectional channels arranged in a hexagonal manner [10]. It has a high surface area and a repeating structure of cylindrical pores, which is an ideal backbone for a well characterized adsorbent material. Extensive research by Foley and others [11–17] has provided the foundation for the use of carbonized furfuryl alcohol as one carbon phase in a composite material. Carbonized furfuryl alcohol has been extensively studied as a carbonaceous adsorbent material because it has a small pore size distribution centered at an average pore size of 4–5 Å [11]. Pore formation by furfuryl alcohol depends on the carbonization temperature [11–13,15]. Burket et al. [11] determined that both mesopores and micropores begin to appear within the material at carbonization temperatures as low as 300 °C. They proposed that mesopores are formed from the incomplete carbonization of polymer remnants and aromatic cores. As the carbonization temperature was increased from 300 °C to 600 °C, the initial mesoporosity collapsed, leaving only micropores ranging from 4 to 5 Å after carbonization at 500–600 °C. With such
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a narrow and controlled pore size distribution, furfuryl alcoholbased CMS materials are useful for separating two gaseous compounds of different sizes [18]. Carbonized furfuryl alcohol within the pores of a siliceous material has been studied for its molecular sieving properties. De Clippel et al. [16,17] reported a furfuryl alcohol-based carbon silica composite that showed remarkable properties for separating linear and branched alkanes due to the narrow pore size distribution provided by the impregnated furfuryl alcohol phase. The small pore sizes and large surface area of this CSC-furfuryl alcohol (CSC-FA) material make it a good candidate for the removal of light gases for air purification [19]. Carbonized sucrose is another carbon phase that could be of interest in a CSC material. Sucrose as a carbon phase is different from carbonaceous furfuryl alcohol since the pores are larger, and the temperature necessary for carbonization is much lower than that of furfuryl alcohol. Exploring a CSC containing a carbon phase that can be carbonized at a lower temperature than furfuryl alcohol could be beneficial for future work on these adsorbents, since many functionalization approaches are best performed at low temperatures. [20,21]. Sucrose has been shown to form a mesoporous carbon phase with high surface areas (on the order of 1000 m2/g) and a pore size of approximately 20 Å [22,23]. Extensive work on sucrose carbonization has been performed at different temperatures and using different synthesis procedures. Ting et al. [23] performed a one-pot synthesis using sucrose catalyzed with sulfuric acid and carbonized at 900 °C. The resulting mesoporous carbon had a surface area of 1200 m2/g and 44 Å pores. Peng et al. [24] synthesized mesoporous carbonaceous materials at carbonization temperatures ranging from 400 to 600 °C. Zhuang and Yang [25] successfully produced carbonaceous spheres from aqueous sucrose solutions carbonized under high pressure at 175 °C. Zheng et al. [26] produced CMK-3 type materials by taking advantage of low temperature carbonization of sucrose in ethanol at 200 °C in a high pressure reactor. Banham et al. [22] templated sucrose into hexagonal mesoporous silicas. Sucrose has also been templated into silica gel and carbonized at 800 °C [27]. Bimodal porous carbons have been produced by impregnating silica spheres with sucrose and carbonizing at high temperatures [28]. This research focuses on carbonaceous CSC materials as biphasic adsorbents for adsorption of light acidic and basic gases. Although not all acidic and basic gases will adsorb in the same way, sulfur dioxide and ammonia are used as representative acidic and basic gases, respectively, to understand the overall trends associated with adsorption on these composites. It builds on previous research from our group published in this journal that introduced a CSC material based on MCM-41 and furfuryl alcohol [1], optimized its synthesis [29], characterized its light gas adsorption properties for carbon dioxide, nitrogen, methane, and ethane [1,29], and optimized the silica phase of the CSC for basic gas adsorption [21]. In this paper, the carbonization method for CSC materials with two different carbon phase precursors, furfuryl alcohol (CSC-FA) and sucrose (CSC-S), is investigated, and these composites are tested for their feasibility for air purification applications. The CSC containing sucrose as the carbon phase was synthesized using a novel low carbonization temperature synthesis procedure. The furfuryl alcohol-based CSC was synthesized at different carbonization temperatures, and the pore sizes and structure are examined after carbonization at different temperatures to increase surface area in the CSCs. The materials are characterized using nitrogen adsorption isotherms, X-ray diffraction, thermogravimetric analysis, and tested for their ammonia and sulfur dioxide adsorption capacities. To the best of our knowledge, this is the first study of a carbon silica composite synthesized to target the adsorption of acidic and basic gases for air purification.
49
2. Experimental methods 2.1. Materials 2.1.1. MCM-41 Tetramethylammonium hydroxide pentahydrate, TMAOH, (97%), tetramethylammonium silicate solution, TMASi, (99.99%, 15–20 wt.% in water), sulfuric acid (95.0–98.0%), and furfuryl alcohol (99%) were purchased from Sigma Aldrich. Hexadecyltrimethylammonium chloride, CTAC, (25%) in water was purchased from Pfaltz and Bauer. A solution of ammonium hydroxide (29 wt.%) in water and Cab–O–Sil M5 were purchased from Fisher Scientific. 2.1.2. CSC-S Sucrose was purchased from Fisher Scientific, and dry ethanol (200 proof) was purchased from Pharmco-aaper. 2.1.3. CSC-FA Furfuryl alcohol (99%) and toluene (99%) were purchased from Sigma Aldrich. 2.2. MCM-41 synthesis All CSC materials include MCM-41 as the silica phase. Hexagonally-ordered MCM-41 with a 37 Å pore was synthesized according to the procedure described in a previous study [21]. 2.3. CSC-S 2.3.1. CSC-S synthesis The novel sucrose-impregnated CSC material was synthesized using a low temperature carbonization technique. Equal parts ethanol and water were mixed in a Teflon-lined Parr reactor at room temperature. One gram of sucrose was added to the mixture, which was then covered and stirred vigorously for thirty minutes. Next, 0.2 g of calcined MCM-41 was added to the mixture and stirred for an additional 1 h. The Parr reactor was sealed and put into the oven at 200 °C for either 24 h (to produce CSC-1) or 48 h (to produce CSC-2). After the reaction was complete, the Parr reactor was removed from the oven and cooled to room temperature. The CSC-S material was separated from the brown solution via vacuum filtration and rinsed with distilled water. The CSC-S was air dried overnight. 2.4. CSC-FA 2.4.1. CSC-FA synthesis The furfuryl alcohol-impregnated carbon silica composite material was synthesized using the procedure outlined by Glover et al. [1]. After impregnation with the furfuryl alcohol phase, samples were carbonized at temperatures of 300, 500, and 600 °C, following the method previously described [1]. 2.5. Materials characterization 2.5.1. Textural characterization Adsorption isotherms were measured using a Micromeritics ASAP 2020 at 196 °C with UHP nitrogen as the analysis gas. Prior to measurement, approximately 0.1 g of each sample was degassed with heating to 90 °C and vacuum to 10 lbar. After reaching 10 lbar, the samples were heated to 100 °C under vacuum for an additional 6 h. Density functional theory (DFT) provided in the ASAP 2020 software was used to calculate pore volumes and pore size distributions. Pore volumes reported correspond to P=P 0 ¼ 0:999.
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2.5.2. X-ray diffraction (XRD) XRD spectra were used to confirm the long range structure of the native and impregnated MCM-41 samples. The spectra were measured using a Scintag X1h/h automated powder diffractometer with Cu target, a Peltier-cooled solid-state detector, a zero background Si(5 1 0) support, and a copper X-ray tube as the radiation source. Spectra were collected from 1.2° to 7° 2h using a step size of 0.02°.
2.5.4. Transmission electron microscopy (TEM) High resolution TEM was performed on the CSC materials to investigate the MCM-41 mesoporous network using a Philips CM20 electron miscroscope operating at 80 kV. The samples were prepared by dispersing approximately 0.1 g of CSC powder into approximately 0.5 mL isopropanol via sonication for 1 min. The dispersion was then placed onto Lacey carbon copper grids and air dried. 2.5.5. Target gas capacity Equilibrium capacities for room temperature light gas adsorption of both ammonia and sulfur dioxide were measured for all samples using a breakthrough apparatus, a schematic of which has been given by Furtado et al. [21,30]. Although measured under dynamic conditions, full breakthrough capacities were measured at low flow rates and are equilibrium capacities, as they end after the feed concentration exits the bed [31,32]. The equilibrium capacities agree with equilibrium results obtained using a gravimetric (Cahn) balance [30]. Prior to analysis, all samples were regenerated under vacuum at 120 °C for 2 h. The flow rate of gas across the adsorbent bed was kept constant at 1133 mg/m3 for ammonia (1500 ppm in helium) and 1428 mg/ m3 for sulfur dioxide (500 ppm in helium). The capacity of the adsorbent material, n (mol ammonia or sulfur dioxide/kg adsorbent), was calculated via material balance using [21,32]
n¼
F m
Z
1
ðc0 cÞ dt
ð1Þ
0
where c0 is the feed molar concentration, and c is the effluent concentration at time t. The volumetric flow rate of gas through the adsorbent bed, F, was adjusted to yield a breakthrough time of approximately 1 h. The mass of the sample, m, was approximately 10 mg and was contained in a small cylindrical adsorbent bed. The capacities calculated using the breakthrough apparatus have a standard deviation on the order of 3%, and different batches of CSC are repeatable to within 5%. 3. Results 3.1. Carbon silica composites synthesis and characterization 3.1.1. CSC-S carbonized for different reaction times Two novel CSC materials using sucrose as the carbon phase were successfully produced by impregnating MCM-41 with sucrose using different reaction times. Using thermogravimetric analysis, the amount of carbon loaded into the MCM-41 scales with the reaction time, as shown in Fig. 1. The initial mass loss corre-
Weight Percent
80
60
40
20
CSC-1 CSC-2 0 0
100
200
300
400
500
600
Temperature (K) Fig. 1. Thermogravimetric analysis of CSC-S samples.
Table 1 Physical properties of the CSC-S samples. Sample
Reaction time (h)
wt.% C
BET SA (m2/g)
Vpore (cm3/g)
Predominant pores (Å)
MCM-41 CSC-1 CSC-2
– 24 48
– 26 46
836 436 263
1.2 0.56 0.45
37 12–30 12–30
sponds to the liberation of adsorbed water and the second gradual mass loss represents the loss of carbon from the samples. Table 1 summarizes the amount of carbon loaded into each sample. Nitrogen isotherms are shown in Fig. 2 for both CSC-S materials and for the unimpregnated MCM-41. The parent MCM-41 exhibits
700
3 Amount Adsorbed (cm /g at STP)
2.5.3. Thermogravimetric analysis (TGA) Thermogravimetric analysis was performed on the CSC materials to determine the amount of carbon impregnated into the MCM41 using a TA Instruments Q600 SDT, a simultaneous DSC-TGA. Samples were heated in zero air from room temperature to 600 °C using a ramp rate of 5 °C per minute with an air flow rate of 10 ml per min. To fully burn off the carbonaceous phase, the samples were maintained at 600 °C for 3 h.
100
600
MCM-41 CSC-1 CSC-2
500
400
300
200
100
0 0.0
0.2
0.4
0.6
0.8
Relative Pressure (P/P0) Fig. 2. Nitrogen adsorption isotherms for CSC-S samples.
1.0
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MCM-41
3
Differential Pore Volume (cm /g)
8
6
4
2
0 10
15
20
25
30
35
Pore Width (Å)
3
Differential Pore Volume (cm /g)
1.0
CSC-1 CSC-2 CSC300 CSC500 CSC600
0.8
51
materials compared to the parent MCM-41, as shown by the column in Table 1 summarizing the predominant pores in the samples and by the pore size distributions shown in Fig. 3. This is due to filling the mesopores with a carbon phase, which results in an increase in microporosity and a decrease in mesoporosity. After filling the MCM-41 pores with different amounts of sucrose carbon, density functional theory predicts lower predominant pore sizes compared to the initial 37 Å pore of the parent MCM-41. Consequently, it is evident that sucrose carbon fills the MCM-41 mesopores and produces micropores and small mesopores in the material after carbonization. High resolution TEM images were obtained to compare MCM41 to the CSC-S samples. Fig. 4 compares TEM images of the parent MCM-41 with CSC-1. In image a, which shows the parent MCM-41, the pores are clearly visible. Image analysis using the Philips CM20 software calculated the pore diameters to be 38.5 Å, which compares well with the expected value of 37 Å. Image b shows the aligned sucrose carbon-impregnated MCM-41 pores of 38.5 Å in diameter. Also apparent during analysis of this CSC-1 sample were carbon spheres ranging from 500 nm to 1 micron in diameter. These spheres, shown in image c alongside the aligned CSC-S cylindrical pores, are similar to those found by Zhuang and Yang [25] in their low temperature carbon synthesis, and are structures forming around carbon filled MCM-41 pores.
0.6
0.4
0.2
0.0 10
15
20
25
30
35
Pore Width (Å) Fig. 3. Pore size distributions calculated via DFT for MCM-41, CSC-S, and CSC-FA samples. The curve for CSC300 lies just above the x-axis.
Type IV behavior according to the IUPAC classification [34]. The CSCs are microporous and mesoporous, and exhibit Type I behavior due to the sharp increase in the amount adsorbed at low relative pressures, which levels off as the relative pressure increases. BET surface areas and pore volumes calculated from the isotherms are summarized in Table 1. Impregnating MCM-41 with the sucrose phase decreases the surface area and pore volume of the
3.1.2. CSC-FA carbonized at different temperatures The CSC-FA materials were characterized by X-ray diffraction to verify that the impregnation of the silica phase with carbon did not disrupt the MCM-41 structure. Fig. 5 shows the XRD spectra of the base CSC-FA materials carbonized at different temperatures and the MCM-41 base material. Despite the decrease in intensity of the base CSC materials compared to MCM-41, which is due to interference from the carbon phase, the MCM-41 peaks are identifiable in the CSC-FA spectra. Thus, carbon impregnation does not alter the MCM-41 hexagonal order. Long range XRD scans were also run to identify evidence of graphitization, which can be expected to occur during carbonization at higher temperatures. No graphitic peaks were detected in these scans. Characterization of the CSC-FA materials was performed by measuring nitrogen isotherms at 77 K. These are shown in Fig. 6 and are Type I isotherms. The nitrogen isotherm for the CSC300 sample is not shown in Fig. 6 because it adsorbs much less nitrogen than the other samples and is on a much lower scale. All CSC-FA materials are nanoporous and exhibit Type I behavior due to the sharp increase in the amount adsorbed at low relative pressures, which levels off as the relative pressure approaches unity. The
Fig. 4. High resolution transmission electron microscopy images of MCM-41 and CSC-1.
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A.M.B. Furtado et al. / Microporous and Mesoporous Materials 165 (2013) 48–54 Table 2 Physical properties of the CSC-FA samples.
Intensity (counts per second)
MCM-41 CSC300 CSC500 CSC600
2
3
4
5
6
7
Degrees 2θ Fig. 5. X-ray diffraction spectra for MCM-41 and CSC-FAs carbonized at different temperatures.
Sample
Carbonization temperature (K)
wt.% C
BET SA (m2/g)
Vpore (cm3/g)
Predominant pores (Å)
MCM-41 CSC300 CSC500 CSC600
– 300 500 600
– 51 40 44
836 24 466 505
1.2 0.07 0.39 0.42
35 Minimal 8–20 8–25
TGA analysis was performed on the CSC-FA samples produced at different carbonization temperatures. As summarized in Table 2, data show that the CSC-FA carbonized at 300 °C has the highest mass loss, 51 wt.%, due to incomplete polyfurfuryl alcohol carbonization at the lower temperature [11–13,15]. The average carbon content for the remaining samples is approximately 40%. High resolution TEM images were also obtained on these samples. Fig. 7 compares TEM images of MCM-41 and CSC500, a representative CSC-FA sample. Image a shows the parent MCM-41 material. Image b shows the carbon-filled MCM-41 arranged in an ordered manner. In image c, the hexagonal pores are visible. It is difficult to distinguish between the carbon filled MCM-41 pores and the base MCM-41. It is evident from these images that the furfuryl alcohol polymerizes well within the MCM-41 mesopores rather than forming a pure carbon phase outside of the MCM-41. 3.2. CSC target gas adsorption
700
3
Amount Adsorbed (cm /g at STP)
600
MCM-41 CSC500 CSC600
500
400
300
200
100
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 6. Nitrogen adsorption isotherms for CSC-FAs carbonized at different temperatures.
hysteresis during desorption is characteristic of a Type IV isotherm [19]. Surface area and pore information were calculated from the nitrogen adsorption isotherms of the CSC-FA materials. BET surface areas were calculated according to the procedure for microporous materials outlined by Rouquerol et al. [33] and are summarized in Table 2 with pore size distributions given in Fig. 3. When all samples are compared, those carbonized at mid-range temperatures (500–600 °C) have much higher surface areas and larger pore volumes than the sample carbonized at 300 °C, which has minimal porosity. Also, as shown in Fig. 3, when compared with the sucrose-based materials, the FA-based materials have pore size distributions shifted more towards the micropore region.
3.2.1. CSC-S As shown in Table 3, the sucrose-based carbon silica composites were tested for ammonia and sulfur dioxide adsorption capacity and compared to MCM-41 and BPL activated carbon. This table includes two columns presenting ammonia and sulfur dioxide adsorption capacities calculated per kg sample, and two columns presenting these capacities calculated on a per kg silica and carbon basis, respectively. Calculating the capacities per kg of each phase was done to emphasize that the carbon phase in the composite targets sulfur dioxide adsorption and the silica phase targets ammonia adsorption. It should be emphasized that the formation of the composite gives a material with a different pore size distribution and surface chemistry than either the parent MCM-41 or a sucrose-based carbon created outside of the MCM-41 mesopores. The pore size distribution of the carbon phase can impact the adsorption of ammonia targeted for the silica phase and the templating silica support can impact the carbon phase targeted for sulfur dioxide adsorption. Thus, there are synergistic interactions in the formation of the two phases of the composite material for adsorption of the two target gases. It is obvious from the table that the presence of the sucrose carbon phase increases both the ammonia and sulfur dioxide adsorption capacities compared to the parent MCM-41. The MCM-41 provides hydroxyl groups that enhance ammonia adsorption through hydrogen bonding [21,35–37], and these sites provide much of the ammonia adsorption capacity of the CSC material. The parent MCM-41 has minimal sulfur dioxide adsorption capacity. However, after impregnation, the carbon phase introduces micropores into the composite that promote adsorption of sulfur dioxide and also ammonia. The microporous carbon phase enhances the sulfur dioxide capacity of the composite by 400% and it enhances the ammonia adsorption capacity by 9%. The drastic increase in sulfur dioxide capacity after impregnation results from the basic nature of the sucrose carbon phase [38,39], thereby enhancing the acidic gas adsorption of the composite. The much smaller increase in ammonia adsorption capacity is due to the increased presence of micropores throughout the sample. Compared to BPL activated carbon, both CSC samples show much higher
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53
Fig. 7. High resolution transmission electron microscopy images of MCM-41 and CSC500.
Table 3 Gas adsorption on the CSC-S samples. Sample
BPL AC MCM-41 CSC-1 CSC-2
SO2 adsorption capacity
NH3 adsorption capacity
mol/kg sample
mol/kg carbon
mol/kg sample
mol/kg SiO2
0.20 0.02 0.10 0.15
0.20 – 0.38 0.33
0.10 2.01 2.18 2.10
– 2.01 2.95 3.93
ammonia capacities. For sulfur dioxide, the carbon phase provides base capacity for the composite, whereas the silica phase does not. On a mol/kg carbon basis, the sulfur dioxide adsorption capacity for both CSC-S materials is higher than that of BPL AC.
3.2.2. CSC-FA Target gas adsorption capacities are shown in Table 4 for the CSC-FA samples, the parent MCM-41, and BPL activated carbon. The presence of the carbon phase enhances the sulfur dioxide adsorption compared to the parent MCM-41, with CSC600 having the highest SO2 adsorption capacity. Similar to the sucrose carbon phase, the carbon phase produced by the furfuryl alcohol is basic in nature [38,39], and it enhances the sulfur dioxide adsorption capacity over that of the parent MCM-41. When compared on a mol/kg silica basis, the carbon phase produced at the 300 °C carbonization temperature causes a decrease in the ammonia adsorption capacity compared to MCM-41; however, the carbon phases in CSC500 and CSC600 result in an increase in the ammonia adsorption capacity compared to the base MCM-41. For the CSC-FA samples, it is obvious that higher adsorption capacities correspond to higher carbonization temperatures. The nitrogen isotherms of Fig. 6 show much higher surface areas and well developed pore structures for the samples carbonized at 500 and 600 °C compared to the sample carbonized at the lower temperature. Analyses of the samples show incomplete furfuryl alcohol carbonization at the low
Table 4 Gas adsorption on the CSC-FA samples. Sample
BPL AC MCM-41 CSC300 CSC500 CSC600
SO2 adsorption capacity
NH3 adsorption capacity
mol/kg sample
mol/kg carbon
mol/kg sample
mol/kg SiO2
0.20 0.02 0.20 0.25 0.34
0.20 – 0.55 0.71 0.95
0.10 2.01 0.76 1.38 1.48
– 2.01 1.19 2.14 2.30
carbonization temperature of 300 °C, giving a decrease in ammonia and sulfur dioxide adsorption capacity compared to the samples carbonized at 500 and 600 °C. All CSC samples show much higher ammonia adsorption capacities than BPL activated carbon. On a mol/kg carbon basis, all CSC materials also have higher sulfur dioxide adsorption capacities than the BPL activated carbon. When compared on a mol/kg sample basis, CSC500 and CSC600 show higher sulfur dioxide adsorption capacities than the commercial carbon.
4. Conclusions A series of carbon silica composites with MCM-41 as the silica phase and carbonized sucrose or furfuryl alcohol as the carbon phase have been synthesized using different carbonization temperatures and reaction times. These materials have been characterized via adsorption isotherms, XRD, and TGA. They were also tested for their ammonia and sulfur dioxide capacities, a basic and an acidic gas, respectively. Impregnation of MCM-41 with carbonized sucrose results in a novel CSC material with potential as an air purification adsorbent. CSC-S shows an increase in the sulfur dioxide and ammonia adsorption capacities compared to the unimpregnated MCM-41. The carbonized sucrose phase of the CSC-S composite enhances the sulfur dioxide adsorption capacity of the adsorbent compared to the unimpregnated silica phase. The presence of the sucrose phase also results in an increase in ammonia adsorption capacity. The increases in ammonia and sulfur dioxide adsorption capacities occur for samples impregnated using both 24 and 48 h reaction times. Similar to the CSC-S material, impregnation of MCM-41 with furfuryl alcohol to form the CSC-FA results in a composite with high toxic gas capacities. The CSC-FA maintains the ammonia adsorption capacity and enhances the sulfur dioxide adsorption capacity compared to MCM-41. The CSC-FA carbonized at 600 °C has the highest surface area, followed by the samples carbonized at 500 and 300 °C. Carbonization of the furfuryl alcohol polymer is a rate process, and the sample heated to 300 °C is not fully carbonized, whereas the sample carbonized at 600 °C has the most well-developed pore structure. The development of the pore structure correlates with the ammonia and sulfur dioxide capacities; in general, the samples carbonized at higher temperatures have higher target gas capacities than the sample carbonized at 300 °C. Carbon silica composites produced by impregnating MCM-41 with sucrose and furfuryl alcohol carbons show promise as adsorbent materials for air purification. These materials provide a carbon phase for physical adsorption and a silica phase for hydrogen
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bonding with electronegative atoms. These biphasic materials have large capacities for two very different types of gases, ammonia and sulfur dioxide. Their adsorption capacities for these gases are higher than those of BPL activated carbon.
[13] [14] [15] [16] [17]
Acknowledgements [18]
We are grateful to the US Army Edgewood Chemical and Biological Center and the Defense Threat Reduction Agency for the support of this research under contract number W911SR-08-C-0028. We are also grateful to Dr. James McBride for his help with the transmission electron microscope.
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