Indian Journal o f Chemical Techno logy Vol.I O, May 2003, pp. 298-304

Articles

Carbon molecular sieves (CMS) from coconut shell by carbonization and carbon dioxide activation

KimTae- Hwan", S Vijayalakshmih*, Son Seok Jinc & Kim Jee Dong"

"Korea Institute o f Energy Research (K IER), P.O. Box No. I 03, Yusong, Taejon, Korea

bRS IC, Indi an Institute of Technology , Powai, Mumbai 400 076, Indi a

cChugnam National University , Dedok Science Town, Taejon , Korea

Received 5 March 2002 ; revised received 21 No vember 2002; accepred 30 December 2002

Carbon Molecular Sieves (CMS) are specially treated micro porous adsorbents derived from carbonaceous precursors. Due to the molecular sieving property, coupled with thermal, mechanical and chemical stability under acidic/a lkaline con­ditions, CMS have found many applications in the field of difficult gas, as well as liquid separations from their mixtures. CMS have been successfully synthesized from a wide variety of carbonaceous materials, by the carbonization followed by carbon diox ide or steam activation. The effect o f carbonization temperature and ac ti vation temperature on the surface area as well as pore volume, calculated using the carbon dioxide adsorption data at room temperature is studied. The most suit­able carbonization temperature is found to be in the range 750-850°C, beyond which the surface area is fo und to decrease due to si ntering of the particles at high temperature. The samples prepared were found to give good adsorption select ivity for C02 over H2 or N2• Kinetic and equilibrium adsorption data for gases like N2, 0 2, H2, CH4. and equilibrium adsorpt ion of selected organic vapours are also reported.

Sorbent materia ls like activated carbons, are used by different industries for a variety of applications. They are currently being evaluated for use in applications such as reduction of air and water emi ss ions, process improvements, product purificati on, and as catalysts. Activated carbon, which have been used as gas or liq­uid adsorbents for many years, find applications in a varie ty of fields due to their high surface areas with meso pores (0.2 to I 00 nm) and macro pores (500 to 2000 nm) . Such meso porous and macro porous ad­sorbents lack in the ability to separate gaseous com­ponents differing little in their molecular size, such as 0 2, N2, etc. The preparation of micro porous adsorb­ents like CMS has made the separation of 0 2-N2 from ai r feasible.

The various molecular sieve adsorbents currently in use are zeolites (ZMS), mordenites and recently de­veloped CMS and CMS membranes. Due to the hi ghly micro porous nature and excellent molecul ar sieving behav iour, the CMS is found to have increas­ing importance in both gas and liquid separations. The CMS is less hydrophyllic than zeolites and other similar micro porous materials. The advantages of

*For correspondence (E-mail: vlakshrni @cc.iitb.ac. in ; thkirn @ kier.re.kr)

CMS over the commercially availab le active carbons and zeolites are elaborately discussed elsewhere l.2. The excellent molecular siev ing effects of CMS are essentially due to the narrow pore size distribution3

responsible for large differences in the rate of adsorp­tion of molecules s lightly differing in their critical dimensions. The performance of CMS on separation of gas mixtures are attributed to the kine tic effects, which could not be explained by siev ing effect due to size differences alone. It may be affected by the in­termolecul ar force fields also4

. Different types of raw materials such as coconut shell5

, wood and cellulose, poly furfuryl alcohol, saran and various ranks of coal2

, etc . have been successfully used for synthesiz­ing CMS. The major steps involved in the sy nthes is of CMS are carbonization, activation with steam or C02,

followed by pore mouth narrowing by coke deposi­tion during hydrocarbon cracking.

In the present investi gation, the synthesis of CMS from coconut shell by carbonizatio n followed by activation with carbon dioxide has been achieved. The effect of both carbonization and acti vation temperature was studied on the C02 surface areas and pore volumes of granular coconut chars prepared in the laboratory. The kinetic and equilib rium adsorption data obtained were utili zed to characteri ze the

Tae-Hwan eta!.: Carbon molecular sieves from coconut she ll by carbonization and carbon diox ide ac tivation Articles

Thermocouple

Water trap Furnace - -+--tot

Insulation

Perforated Disc (M) ---11---+rli•

Gas distributor(D)

Manometer (M I ) Air-trap Gas pn:heater (G)

Fig. !-Experimental set-up for synthes is o f carbon molecular sieves

samples prepared. Molecular sieving effect was assessed by comparing the adsorption isotherms and kinetic adsorption curves of 0 2, N2, C02, H2, and CH4.

Experimental Procedure Coconut shells used in the sy nthesis of CMS were

crushed, sieved (particle size 2 - 3 mm), washed with 0.0 I N HCI, then washed with water and finally oven dried before subjecting to carbonization and activati on. The experimental set-up is shown in the Fig. I. About 250 g of acid washed and dried coconut she ll granules were charged into the reactor and were carboni zed in presence of N2 at a heating rate of 5°C/min. An isothermal conditio n was maintained for 30 min each at 200°C as well as at 400°C, when all the volati le and oily substance from the coconut she ll was removed. The carbon izatio n temperature range studi ed was between 350 to 950"C, and the duration of carbo nization was maintained for 30 min . At the end of the carbonizati on step, N2 flow was stopped and the C0 2 tlow was started fo r the activation step. The activati on temperature was kept same as th at o f the carboni zation temperature in each case. The time

of activation was 30 min for all samples. After the activation , the C02 flow was switched off and the 2

flow was started and the sample was baked for 30 min at that temperature. The reactor was cooled to about 60°C, before the sample was removed and stored in the desiccator.

Kinetic and equilibrium adsorption data for various adsorbates on the samples prepared were collected o n a ll the samples using Carlo-Erba Strumentazone­Series-1800 and Micromeritics-ASAP(2000) at 27°C.

f 7 ° The adsorbates studied '· were, 0 2 (3.46 A), N2 (3 .64 A), H2 (3.28 A) , C02 (3 .30 A), CH4 (3 .84 A), C2H2 (2 .95 A), and C3H6 (4.3A). In addition, N2 adsorpti on at - 196"C, was also carried out on each of the sam­ples. Adsorption of dye molecules o f diffe rent mo­lecular dimension on the CMS was carried out using impregnati on method . Vapour adsorption of few hy­drocarbons were carried out on sample S-6. by keep­ing the sample in a vacuum flask and care full y drop­ping the liquid hydrocarbon wi thout dropping directl y o n the surface of the C MS sample kept in side. The CMS sample was kept in the flask for I to 2 h and the increase in weight due to adsorpt ion of the hydrocar­bon was measured .

299

Articles Indian J. Chern. Techno!., May 2003

Table 1-Effect of carbonization/activation temperature on product yield

Duration of carbonization/activation: 30 min each step Activat ion agent: Carbon dioxide

Sample code S-1 S-2 S-3

Temp.°C 350 450 550

% yield 34.5 34.4 30.0

Results and Discussion

Effect of carbonization and activation temperature on the product yield is shown in Table I. The product yield decreased from 34.5% at 350°C to 20% at 950°C, due to increased loss of volatile matter at higher temperatures.

Surface area

The surface areas of all the samples prepared (S- 1 to S-7) were calculated using BET method by adsorbing N2 at -196°C on the samples. The surface area was found to increase with increase in carbonization and activation temperatures up to 850°C, beyond which a slight decrease was observed. This is due to the sintering effects occurring on the sample at high temperature. Moreover, due to the uncertainty of attainment of equilibrium for nitrogen adsorption on these samples, the BET surface calculated cannot be taken as the absolute value. The micro pore surface areas of the samples were calculated by t-method8

'9 also, and were found to be

about 80% of the total BET surface area (SN2). The average pore radius was calculated using the conventional method (r = 2Vg1Sg). The pore radii were found to be in the range, 7 to 8 A for all the samples. The equation used for calculating the pore volume using t-method was,

t = 3.64 (VNm) . . .. (1)

The pore volume, calculated using the t-method showed a sharp increase due to enhanced pore open­ing by C02 activation at higher temperatures up to 850°C. The estimated pore volume for samples ranged between 0.0 1 to 0.16 cc/g. The surface area of all the CMS samples calculated using BET method and t­method are shown in the Table 2. Table 3 summarizes the pore radii and micro pore volume of the samples calculated using t-method.

The surface area (Sc02) of all the samples was also calculated using Langmuir, BET and Dubinin-

300

S-4 S-5 S-6 S-7

650 750 850 950

29.2 27.0 24.3 20.0

Table 2-Surface area by BET method and micro pore area using !-method for N2 adsorption at -196°C

Sample code Total S.A. m2g Micro pore area m2g

S- 1 < I < l

S-2 24 18

S-3 335 195

S-4 341 257

S-5 374 289

S-6 530 447

S-7 427 342

Table 3-Micropore volumes and pore radii of samples S-1 to S-7

Sample code Temp. Micro pore vol. Pore radius oc (cc/g) cA)

S-1 350 7.003

S-2 450 0.0103 7.106

S-3 550 0.0619 7.116

S-4 650 0.1037 7.108

S-5 750 0.1362 7.120

S-6 850 0.1651 8.126

S-7 950 0.1190 7.810

Astakhov method (D-A method). The adsorption data for C02 at 27°C was used for the calculation. The ad­sorption data for all the gases studied were collected using the Carlo-Erba Strumentazone Series-1800 ma­chine. The results obtained by all the three methods are shown in Table 4. The equation used for calculat­ing Sc02 by Langmuir method was,

... (2)

A plot of PN versus P was found to be a straight line. The micro pore volume (V m) obtained from the graph was used for calculating the Sc02. Using the conven­tional BET equation, a plot of PI(P0 -P) V versus PIP0

was drawn, and the value of Vm was obtained.

Tae-Hwan eta/. : Carbon molecular sieves from coconut shell by carbonization and carbon dioxide activation Articles

In the case of Dubinin-Astakhov method for sur­face area calculation for micro porous adsorbents 10

, a graph was plotted using ln(WIW0 ) and A, from which V m is obtained for calculating the Sc02. A steep rise in surface area was observed up to 850°C. At 950°C, the surface area declined due to partial sintering effect. This phenomenon is also reported by Koresh et al. 7. A close look at the Table 4 shows that, BET, D-A and Langmuir surface area are very much comparable confirming the validity of all the equations on the pore volume.

Characterisation energy

Dubinin-Astakhov (D-A) equation for mtcro po­rous material has the form,

W = Wo exp. ( -A!Ef

W =alp* and A= RT ln(plp0 )

(3)

(4)

Considering n = 3 reported for CMS ll.l2, a plot of In W versus ( lnplpoP gave a straight line (Fig. 2) for all the samples, thereby confirming the homogeneous micro porous structure of the adsorbents. The value of W0 is obtained from the graph. Characteristic energy E values for all the samples were obtained from the graph, In ln(WofW) versus In A. The calculated values are given in the Table 5. The characteristic energy of the samples was found to have an increasing trend except for the sample S-7. This is due to the decrease

Table 4-Surface area of the CMS samples, calculated using C02 adsorption data at 27°C

Sample S.A (Lang- S.A. (BET) S.A. (D-A) code muir)

? m2g m-g m2g

S-1 251 239 272

S-2 236 314 350

S-3 356 353 411

S-4 395 387 440

S-5 421 480 484

S-6 581 501 581

S-7 403 395 431

in adsorption potential caused by high temperature sintering of micro pores.

Adsorption of dye molecules in the CMS samples prepared

Adsorption of a few selected dyes of various mo­lecular dimensions was studied on sample S-6 by us­ing dilute aqueous solutions of the respective dyes . The adsorption results are tabulated in Table 6. The adsorption of naphthalene sulphonic acid which has a molecular dimension 1.1 x0.8x0.5 (nm) was found to be less on S-6 sample when compared with ad­sorption of malachite green (1.5x 1.2x0.5 (nm) . This appears to be due to the fast desorption of rela­tively smaller size molecules, as well as the strong hydrophyllic nature of naphthalene sulphonic acid. Adsorption of fuschine, methyl violet and congo red

1·4

1 ·l

~

3

"'0 .8 E

0 .6

0 ·4

0 ·2

0 4

b. -87

0-86

0-85

* -84

X -83

• -82

.... -81

8 12 16 20

(Log Po/ Pl 3 -

Fig. 2-D-A plots for samples S1-S7

Table 5-Characteristic energy for samples S-1 - S-7

Sample code

£, Cal/g mol

S-1

2786

S-2

3064

S-3

3125

S-4

3 165

S-5

3284

S-6

3361

S-7

3026

301

Articles Indian J. Chern. Techno!. , May 2003

Table 6- Adsorpti on of dyes on S-6

Name of the dye Naphthalene sul­fonic acid

Malachite Fuschine Congo red

Cone. mg/L

Wt. adsorbed g/g dry adsorbent

700

0.0085

1 4 0,---~====:=====::::;;:::==::o:==~l

120

0.2 0-J 0 -4 0- ~ 0 - 6 0.7 Relative pres~u r• ( P/ Po) -

6 -57

0 -56

0-55

* -54

X -S3 • -52

.t.. - Sl

o.e 0.9

Fig. 3- Nitrogen ad sorption isotherms on samples S I-S7 at temp. 27"C

was found to be neg li g ible. Thi s is because, the CMS pore mouth is too small to allow these large molecular size dyes to pass through.

Adsorption isotherms

Adsorption isotherms of N2 at - 196"C and C02 at 27"C fo r samples S I to S7 are shown in Figs 3 & 4 . T ype I category o f the Breuner classification adsorpti on isotherms were observed with all the samples prepared. It is fo und th at all the sampl es showed a low ad.sorptio n for N2. The C02/N2 uptake ratio was found to be >7 for all the samples indi ca ting that separation of N2 from C02 is poss ible using the samples. The samples S5 , S6 o r S7 showed an adsorptio n capacity of 25, 43 and 35cc/g respectivel y for C02 at - 300 mm Hg adsorbing pressure . These data are useful for des igning a N2 removal unit using PSA . However, the res ults were not encouraging for 0 2-N2 separations.

Kinetic adsoqJtion curves

The kinetic adsorption curves for va ri ous gases arc plotted using the amount o f each gas adsorbed with respect to time on the C MS samples prepared (Fig. 5). Kinetic adsorptio n data co ll ec ted for N2, H2, C02 and CH~ o n S-6 at one min was used for calculating the

302

green

700

0035

u u

4 6

~ 3 2 , 1: ~ ~ 24

c , 0 E '6 <(

100

700 700

Not detectable Not detectable

6 - 57

0-56

0- S5 * -54

X - S3

• - 52

_. - S l

200 300 400 500 600 700 •oo 900

Pr~ssure (mrn Hg}--.

Fig. 4- C02 adsorpti on isotherms for samples S l -S7 at temp. 27"C

18.---------------------,

~--~~----------~ C~

____ _._ ________ ,~_.-CH

4

2.0 3.0 4.0 5-0 8 0 9-0 Time (min)->-

Fig. 5-Kineti c ad sorp ti on curves of H ~. N1• CH4 and C02 on S6 (Press ure: 300 mm Hg. Temperature: 27"C)

Tae-Hwan eta/.: Carbon molecu lar sieves from coconut she ll by carbonization and carbon dioxide activation Articles

Table 7-Uptake ratio calculated for various binary gas systems on S6

Adsorption pressure: 300 mm Hg, Temperature: 27°C

Amount adsorbed at I min . (cc/g)

C02

15.5

CH4 N2

6.6 2.2

Table 8-Adsorption data of vapours of organic compounds on S-6

Organic liq- Wt adsorbed Organic liquid Wt adsorbed uid (g/g) (g/g)

11-Hexane 0.052 Methano l 0.015

11-Heptane 0.050 Ethanol 0.018

Cyclohexane 0.12 Isopropyl a!- 0.082 coho!

p-Xylene 0.052 Isobuty l a leo- 0.110 hoi

Toluene 0.056 Water 0 . 140

Benzene 0.060 Chloroform 0.060

uptake ratios for a binary system of gases. The uptake ratios of various binary systems on S-6 are given in Table 7. It is seen that uptake ratio of COz- Hz system is 32 and that for CH4-Hz is 14. The separation of Hz from a mixture of CH4 or COz is possible with this CMS sample (S-6) using appropriate conditions on a Pressure Swing Adsorption (PSA) system.

Adsorption of organic vapours on CMS samples Adsorption of organic vapours was carried out in a

desiccator. Each sample was evacuated for 2 h at 160°C, prior to the vapour adsorption. The results of the adsorption of organic vapours on sample S-6 is shown in Table 8. The rate of adsorption of organic vapours studied are thus found to be,

n-Hexane = n-Heptane < Cyclohexane p-Xylene <Toluene < Benzene

Methanol < Ethanol < Isopropyl alcohol < Isobutyl alcohol

It is clear from the above observation that the ad­sorption capacity of the CMS sample with respect to the organic compounds studied are depended on the critical dimensions, polarity and hydrogen bonding capacity of the adsorbent molecules. The adsorption of cyclohexane vapours on S-6 is maximum compared to the other organic vapours studied, is due to its smallest molecular dimension , compared to the long

H2

0.5

Uptake ratio of

C02/N2

7.0

chain molecules, n-hexane and n-heptane. The metha­nol and ethanol molecule have very small molecular dimension and have a unhindered free diffusion in the pores of the CMS and hence is adsorbed less com­pared to isopropy l and isobuty l alcohol. The low ad­sorption capacity of p-xylene and toluene can be at­tributed to its specific directional nature and also larger molecular dimension compared to benzene. Water molecule, due to its hydrogen bonding nature exhibits a good adsorption capacity in the sample S-6.

Conclusions CMS prepared from coconut shell by COz activa­

tion have shown preferential adsorption of COz as compared to other gases. Hence separation of Hz, Nz. and Oz from COz is possible with these samples pre­pared . The average pore size and surface area ~f the CMS samples prepared are in the range 7-15 A and 300-550 mzlg respectively. The adsorption of vapours of organic compounds studied gives an encouraging result for separation of close boiling organic com­pounds.

Nomenclature

A =adsorption potential (cal/mo le)

a =amount of gas adsorbed in g/g

E =characteristic e nergy of adsorption (kJ/mo le)

P =adsorption equilibrium pressure (mm Hg)

P0 =saturation vapour pressure (mm Hg)

r =average pore radius (A)

R =gas constant (cal/mole °K)

s8 = surface area (m2/g)

= th ickness of the layer of adsorption

V =volume of gas adsorbed at STP, cc

V,., = lim iting pore volume

V, =pore volume (cc/g)

W =amount adsorbed (cc/g)

W0 =limiting pore volume (cc/g)

p • = density of the adsorbed phase

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