Preparation of carbon molecular sieves from carbon micro ... - IIT Kanpur

chemical engineering research and design 8 9 ( 2 0 1 1 ) 1737–1746

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Preparation of carbon molecular sieves from carbon micro and nanofibers for sequestration of CO2 Mekala Bikshapathi a , Ashish Sharma a , Ashutosh Sharma a,b,∗ , Nishith Verma a,∗∗ a b

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208106, India Department of Chemical Engineering and DST Unit on Nanosciences, Indian Institute of Technology Kanpur, Kanpur 208106, India

a b s t r a c t In this study, a hierarchal web of carbon micro and nanofibers was used as a precursor for the synthesis of a carbon molecular sieve (CMS). CMSs were prepared by thermal treatment of carbon fibers using a microwave heating device. The heating power and treatment time were optimized for the maximum performance of the prepared CMS for the separation of CO2 at low concentrations from the gaseous mixture of CO2 and air under dynamic (flow) conditions. Based on the experimental data, microwave power input of 240 W and treatment time of 15 min were found to be suitable for the maximum uptake of CO2 by CMS. Adsorption breakthrough curves were obtained at different gas flow rates and CO2 concentrations. CMSs prepared from the hierarchal web of carbon micro and nanofibers were found to be superior to those prepared from ACF. The CO2 uptake was determined to be approximately 0.88 mg/g and 10 mg/g at concentrations of 500 ppm and 5000 ppm, respectively, in air. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Carbon molecular sieve (CMS); Activated carbon fiber (ACF); Carbon nanofiber (CNF); Microwave heating; CO2 ; Adsorption

1.

Introduction

In this study, we describe synthesis of carbon molecular sieves (CMSs) by the microwave heating of a hierarchal web of carbon micro and nanofibers used as a precursor, and the efficacy of these CMSs in adsorptive sequestration of CO2 at low concentrations from the binary mixture of CO2 and air under flow conditions in a tubular packed bed adsorber. Literature is replete with preparation of CMS from coal and the other carbonaceous materials such as tar pitch, walnut and coconut shell for several applications, such as production of oxygen and nitrogen from air (Juntgen et al., 1981; Ruthven et al., 1986; Schroter, 1993; Kapoor et al., 1993; Miguel et al., 2003; Tan and Ani, 2004; Arriagada et al., 2005), storage of hydrogen and compressed natural gas (Sircar et al., 1996; Dillon and Heben, 2001), and separation of CO2 from the binary mixture of CO2 and CH4 (Warmuzinski and Sodzawiczny, 1995; Gomes and Hassan, 2001; Carrott et al., 2006; Nabais et al., 2006; Kouvelos et al., 2007).

CMS has also been prepared from micron size activated carbon fiber (ACF) by surface modifications. In this method, chemical vapor deposition (CVD) of organic compounds such as CH4 or benzene is carried out at high temperatures (∼700–900 ◦ C) (Kawabuchi et al., 1997; Freitas and Figueiredo, 2001; de la Casa-Lillo et al., 2002; Villar-Rodil et al., 2002; Lozano-Castello et al., 2005; Wan Daud et al., 2007). Deposition of organics results in the constriction of the pore mouths, thereby inducing sieving action. The molecules with size smaller than that of the pore mouth diffuse into the pore and are adsorbed on the pore-walls. Thus, the relatively smaller size molecules are sequestered. The method, however, has a drawback. If the CVD conditions (temperature, duration of deposition, vapor flowrate and composition) are not accurately optimized, the excess vapor deposition often blocks the pore mouth, or renders the separation pore-diffusion controlled (Kawabuchi et al., 1997; Freitas and Figueiredo, 2001). Fig. 1 describes different morphologies of pores that can develop following CVD. As shown, the separation may be controlled by

∗ Corresponding author at: Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208106, India. Tel.: +91 512 2596124; fax: +91 512 2590104. ∗∗ Corresponding author. E-mail addresses: [email protected] (A. Sharma), [email protected] (N. Verma). Received 3 May 2010; Received in revised form 28 August 2010; Accepted 15 September 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.09.009

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Fig. 1 – Preparation of CMS by chemical vapor deposition: (a) precursor, (b) kinetic controlled, (c) diffusion controlled and (d) pore mouth blockage. diffusion or adsorption kinetic rate, depending on the extent of vapor deposition. Constriction of pore mouth resulting in molecular sieving action can also be created by chemical activation of ACF (Wang and Hong, 2005; Vilaplana-Ortego et al., 2008). The preparation of CMS by microwave heating of ACF has recently drawn considerable attention (Carrott et al., 2001, 2004; Jhung et al., 2003). The method is simple without requiring chemical reagents. In addition, the treatment time is relatively small. The CMSs thus prepared showed promising results in the separation of the binary mixtures of O2 /N2 and CH4 /CO2 during batch adsorption tests. Fig. 2 schematically describes separation by CMS prepared by the microwave heating of ACF. As shown, thermal heating results in the constriction of the pore mouth, as the carbon particles migrate to the pore-mouth on heating from the surface or the bulk volume. There are three salient features of the present study. (1) We have used a hierarchal web of carbon micro/nanofibers as a precursor material to prepare CMS. Synthesis and applications of the hierarchal carbon web as an adsorbent in several environmental remediation applications have been discussed (Singhal et al., 2008; Gupta et al., 2009). Due to improved pore size distribution (PSD), the hierarchal web is a potential candidate as a precursor for preparing CMS. (2) We have optimized microwave heating rate and treatment time, and (3) we have

carried out adsorption tests (separation of CO2 from the binary mixture of CO2 and air) under flow (dynamic) conditions in a packed bed column, rather than batch conditions. The test of CMS under flow condition is the realistic representation of the performance of the prepared material, if used on a large scale in sequestration of CO2 .

2.

Experiment

2.1.

Materials

The phenolic resin precursor based micron size carbon fiber was procured in non-activated form from Kynol Inc., Japan. The precursor is made of phenol formaldehyde. The carbon fibers were carbonized and activated using steam as the activating agent according to the procedure described in a previous study (Gaur et al., 2006). Before preparing CMS from ACF, ACF sample was pre-heated at 200 ◦ C for 6 h under a low nitrogen flow rate (∼100 cc per min) in an oven. CNF was grown on ACF by CVD at 800 ◦ C using Ni as the metal catalyst and benzene as the source of carbon. The detailed procedure of growing CNF on ACF and the description of the type of the CVD reactor and that of the especially designed tubular sample holder used in the synthesis are available in a previous study (Singhal et al., 2008). A direct growth of CNF on ACF support

Fig. 2 – CMS prepared by microwave heat treatment: (left) precursor, (right) CMS with constriction at pore mouth, large size molecules of O2 (0.346 nm) and N2 (0.364 nm) are retained in the gas phase, whereas CO2 (0.33 nm) are sequestered inside the pores of CMS.


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obviates the need for the post-synthesis step of transferring CNF to another substrate. As a consequence, the prepared hierarchal web of carbon micro and nanofibers (ACF/CNF) can be used directly in an end-application, for example, for preparing the CMS of the present study.

2.2.

Experimental set-up

Fig. 3 is the schematic of the experimental set-up designed and fabricated for the preparation of CMS by microwave heating. The microwave heating device operates at 2450 MHz under varying power inputs (120–1200 W). As shown in the figure, a quartz shell (L = 100 mm, ID = 25 mm) is vertically installed in the microwave oven. Another quartz tube (ID = 2.0 mm, OD = 4 mm, L = 45 mm) is coaxially and vertically fitted in the shell, with provisions made for a gas inlet and outlet. The surface of the tube was perforated with holes of diameter 0.5 mm at center-to-center distance of 1 mm. The bottom end of the tube was closed. The sample was wrapped over the perforated section of the tube. Heating was carried out in the inert atmosphere using N2 at a constant flow rate of 100 cc per min. After heating was switched off, the sample was cooled under a small N2 flow rate (∼50 cc per min). The temperature of the sample was varied by varying the input power wattage. The operating variables were time of heating (5–30 min), power input (240–1200 W), and the amount (0.1–1 g) of the sample. As schematically shown, the samples absorb electromagnetic radiation energy leading to the Joule-heating of carbon particles. Some samples of ACF based CMS were also prepared for comparison with the ACF/CNF web. Temperature in the oven was recorded using an optical pyrometer camera (Model Raynger 3i, Raytek, USA). Under the experimental conditions, it was observed that the maximum temperature was reached

Fig. 3 – Schematic diagram of experimental set-up for the preparation of CMS by microwave heating. within less than 60–90 s of starting the microwave heating. The maximum temperatures recorded were 1000 ◦ C and 900 ◦ C corresponding to the heating powers of 1200 W and 240 W, respectively. Fig. 4 is the schematic of the experimental set-up designed and fabricated to study the adsorptive removal of CO2 from the gaseous mixture of CO2 and air by CMS. Air was prepared by mixing N2 and O2 in the required proportion (79:21) in a

Fig. 4 – Schematic diagram of experimental set-up for adsorption breakthrough.

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500 450 400 350

Volume (cc/gm)

tubular mixer. All gaseous streams from cylinders were passed through gas purifiers containing silica gel and alumina to remove moisture. The Bronkhorst made mass flow controllers (MFC) were used to regulate the flowrate of different gases. The challenge gas consisting of CO2 in air at pre-calibrated concentration level was passed through a tubular adsorber. The detailed configuration of the adsorber may be obtained from our earlier study (Gupta et al., 2009). All adsorption tests were carried out at 30 ◦ C. The effluent gaseous stream from the adsorber was passed to the analytical section consisting of a gas chromatography (Model 5700, Nucon Eng. Co., India) equipped with the thermal conductivity detector (TCD) and a data station.

300 250

ACF ACF-CMS ACF/CNF CNF-CMS

200 150 100 50 0 0.0

3.

Surface characterization

3.1.

BET surface area and PSD data analysis

0.1

0.0 20

a’ ACF

0 .1 0

Volume (cc/A /g)

0.0 10

0 .0 8 0.0 05

0.0 00

ACF-CMS

0 .1 0

o

o Volume (cc/A /g)

0.7

0 .01 5

0

2

4

6

8

10

0 .01 0

0 .0 8 0 .0 6

0 .00 5

0 .0 4 0 .00 0

0 .0 2

0

2

4

6

8

10

0 .0 2

0 .0 0

0 .0 0 0

5

10 15 20 25 30 35 40

0

5

Pore width (nm) 0 .0 6

b

10 15 20 25 30 35 40

Pore width (nm) 0 .0 6

0 .0 20

b’

ACF/CNF

0 .0 2 0

CNF-CMS 0 .0 1 5

o Volume (cc/A /g)

0 .0 15

o Volume (cc/A /g)

0.6

0 .02 0

0 .1 2

0.0 15

0 .0 4

0.5

0 .0 4 0 .0 10

0 .0 05

0 .0 2

0 .0 4 0 .0 1 0

0 .0 0 5

0 .0 2 0 .0 0 0

0 .0 00

0

2

4

6

8

5

10 15 20 25 30 35 40

Pore width (nm)

0

2

4

6

8

10

10

0 .0 0 0

0.8

0.9

1.0

low relative pressure ( ACFCMS > ACF. The CMS prepared from the hierarchal web of ACF and CNF had the largest saturation and breakthrough times, about 1500 s and 320 s, in comparison to 990 s and 220 s, respectively, for the CMS prepared from the traditional precursor, ACF. The adsorption characteristic times for the precursor, ACF/CNF and ACF were also observed to be smaller than those of CNF-CMS. The equilibrium loading (mg/g) of CO2 on different adsorbents may be obtained by determining the solute uptake (mg) from the corresponding experimentally obtained breakthrough curve. The uptake is determined by calculating the area above the breakthrough curve as:

⎛ Uptake(mg) = Q ⎝Cin T −

T

⎞ Cexit dt⎠

0

where, T is the total time of adsorption until the bed is saturated with the solute in the influent stream at the flow rate, Q. Table 1 (last column) summarizes the equilibrium loading obtained for different materials. The data are in agreement with the observed performance. The CO2 equilibrium loading on CNF-CMS is approximately 0.88 mg/g in comparison to 0.58 mg/g obtained for ACF-CMS (1.5 times larger loading in CNF-CMS than that in ACF-CMS), corresponding to 500 ppm of CO2 in air. The reason of the relatively larger uptake by CNF-CMS is attributed to the fact that its precursor, CNF has narrower pores and hence, the CNF-CMS has narrower pore mouth than that of ACF-CMS, which facilitates sieving actions for separating CO2 from the mixture. It is also important to point out that CNF walls contain exposed edges with unsaturated dangling bonds of and ␴ which mainly consist of unsaturated delocalized electrons. Due to the unsaturated delocalized electrons, CNF is more reactive and superior adsorbent than ACF (Lim et al., 2004). This also explains the superior performance of the precursor, ACF/CNF (with CO2 uptake of 0.72 mg/g) than ACF (with CO2 uptake of 0.16 mg/g only), as observed from Table 1. There is an additional feature of the CMS prepared from ACF/CNF. As pointed out earlier in the section on the results from the FT-IR analysis, the microwave heating of CNF-CMS resulted in the decrease in the acidity or increase in the basicity of the prepared surface. Thus, the modified basic surface of CNF-CMS also helps in retaining the CO2 molecules by adsorption once sequestered in the pores. The experiments were also carried out for different influent concentrations and flow rates. Fig. 12 summarizes the break-


through and saturation times obtained for different operating conditions. As expected, breakthrough and saturation times decrease with increasing gas flow rate and inlet gas concentration. From the analysis of the breakthrough data, the uptakes of CO2 were calculated as 0.88, 1.19, 4.13 mg/g at the CO2 concentration levels of 500, 1000, and 5000 ppm, respectively. We have pointed out earlier in the manuscript that this study focuses on the CO2 adsorption at low concentration levels (500–5000 ppm) and under dynamic (flow) conditions at the total pressure of 1 atm and temperature of 30 ◦ C. In this context, most of the literature data pertain to the adsorption of CO2 by CMS under batch conditions and at relatively larger pressure. As calculated from the breakthrough curve, the equilibrium loading of CO2 on CNF-CMS is approximately 0.9 mg/g corresponding to the CO2 concentration of 500 ppm (or partial pressure of 0.05 kPa) in the binary mixture of CO2 and air at 30 ◦ C. The equilibrium loading increases to approximately 4 mg/g at the concentration level of 5000 ppm (0.5 kPa). These data compare to approximately 86 mg/g of loading obtained for pure CO2 at the pressure of 20 kPa on NaX zeolites at 25 ◦ C under batch conditions (Walton et al., 2006) and approximately 10 mg/g at100 kPa for pure CO2 on LDH at 100 ◦ C, also under the batch conditions (Ram Reddy et al., 2006). In general, the adsorption capacity increases with increasing pressure, CO2 concentration and decreases with increasing temperature. We close the discussion by pointing out that the equilibrium loading under batch conditions may not also reflect the true sequestration efficiency of the material, which should be ascertained under dynamic (flow) conditions, taking into account the influence of various mass transfer resistances and propensity for flow mal-distribution.

5.

Conclusions

CMSs prepared by microwave heating of the hierarchal web of carbon micro and nanofibers were found to be effective even at very low CO2 concentrations and superior to those prepared from a traditional precursor, ACF in separating CO2 from the mixture of CO2 and air. The heating power of microwave treatment and treatment time were shown to influence the efficiency of the prepared CMS. The relative superior performance of CNF-CMS, evaluated under dynamic (flow) conditions by calculating the uptake from the corresponding breakthrough curve, is due to the narrower pores in the micro/nano web. The increased basicity of the surface of CNF-CMS due to the reduction of acidic function groups following microwave heating also helps in the increased surface adsorption capacity for CO2 . CNF-CMS prepared from the hierarchal web of carbon micro and nanofibers may also find applications in separation and purification of the other gaseous mixtures.

Acknowledgement The authors acknowledge the partial financial support from the Council of Scientific and Industrial Research (Scheme number: 22(0430)/07/EMR-II, 2007).

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