Abstract—This article reports the fabrication of porous

carbon by the direct carbonization of waste cigarette filters at high temperatures. Applying a new set of operational parameters compared to previous work, the Brunauer, Emmet, and Teller (BET) surface area could increase by about 37%.This was done by increasing the carbonization period and heating rate from 7 to 9 hours and 1 to 3 °C/min, respectively. Scanning electron microscopy (SEM) could very well demonstrate the formation of pores on the surface of synthetic carbon and an obvious change from the initial carbonaceous material. However, the surface morphology of the pyrolyzed carbon was completely different when changing the synthesis parameters compared to earlier reports. To further investigate the pyrolysis process, thermal gravimetric analyses (TGA) were carefully carried out. Energy dispersive x-ray (EDX) analysis was also performed for the elemental analysis of raw cigarette filter and the final carbonized product. Since cellulose acetate is a major constituent in many products, recycling such waste in order to synthesize high-surface-area porous carbons would be a promising technology in the future.

Index Terms—Cellulose acetate, cigarette filter, porous carbon, pyrolysis

I. INTRODUCTION Porous carbon materials have been prepared in different

ways to produce task-specific carbonaceous materials. These synthetic rich-in-carbon matters can then be used in industrial or scientific realms. They can be utilized in separation processes as adsorbents, as catalysts and catalytic supports, and in electrical and energy storage devices. In recent years use of various types of polymers as precursors for carbon materials has led to activated carbons with new or improved porosity characteristics [1].

Carbonaceous materials can be prepared as carbon black, activated carbon, activated carbon fiber, activated carbon cloth, carbon molecular sieves, charcoal, mesoporous carbon, ordered mesoporous carbon, mesoporous molecular sieves and etc [2]. To synthesize any carbonaceous matter, specific procedures should be applied to reach the desired pore characteristics, internal surface area and other physical and chemical features. The most direct approach to produce

Manuscript received June 13, 2012; revised July 16, 2012. S. Masoudi Soltani S. KazemiYazdi is with the department of Chemical

& Environmental Engineering, The University of Nottingham, Broga Road, Semenyih, and 43500 Selangor, Malaysia. (e-mail: keyx1smt@nottingham.edu.my)

S. KazemiYazdi is with the department of Chemical and Environmental Engineering, The University of Nottingham, BrogaRoad, Semenyih, 43500 Selangor, Malaysia (e-mail: sara.yazdi@nottingham.edu.my)

carbonaceous materials is called carbonization. Carbonization is the extreme form of pyrolysis in which the carbonaceous material goes through thermal decomposition at elevated temperature in the absence of oxygen. The remaining material is mostly made of elemental carbon. Moreover, activation (heat treatment using and oxidizing agent) of the resulted carbon using some gas such as CO2 or O2 will produce carbons with desired properties such as higher surface area and specific pore size distribution [3]. Through activation the oxidizing gas will open up the blocked pores that might have been formed during carbonization step. Moreover, mesoporous carbons can be synthesized using direct carbonization, soft-templating approach and hard-templating approach [4] among which direct carbonization is the only one-step and cost-effective technique. Traditionally, activated carbons have been produced by direct carbonization of nearly all carbon- rich materials: wood, nutshells and fruit stones, peat, charcoal, lignite, bituminous coal, brown coal, petroleum coke and etc [2].

As much as 766,571 metric tons of butts wind up every year as litter worldwide [5]. The trillions of cigarette butts generated each year throughout the world pose a significant challenge for disposal regulations, primarily because there are millions of points of disposal, along with the necessity to segregate, collect and dispose of butts in a safe manner, and cigarette butts are toxic, hazardous waste [6]. Numerous proposals have been made to prevent or mitigate cigarette butt pollution, but none has been effective; cigarette butts are consistently found to be the single most collected item in beach clean-ups and litter surveys [5]. Cigarette filters are made of 95% non-biodegradable cellulose acetate (a plastic), and the balance are made from papers and rayon. The cellulose acetate tow fibers are thinner than sewing thread, white and packed tightly together to create a filter; they can look like cotton. Cigarette butts contain fibrous matter. Spreading apart the matrix reveals more than 12,000 white fibers. Microscopically they are Y shaped. Although they are mainly made of cellulose acetate, a plasticizer, triacetin (glycerol triacetate) is applied to bond the fibers [7]. To date, very few investigatetions have been made on recycling cigarette butts as porous carbon materials. Polarz et al. [8] reported the synthesis of porous carbon from cigarette filters with a surface area of 262 m2/g. However, they did not study the effect of carbonization duration nor heating rate effect on the resulted porous carbon. In this work we could observe a 37% increase in BET surface area by simply elevating pyrolysis time and heating rate. Doing so, carbon yield could also reach the maximum amount of 15% which is the highest value reported by [9].

An Investigation into the Effects of Temperature and Heating Rate on Porous Carbon Characteristics Obtained from the Carbonization of a Cellulosic-Based Solid Waste

Salman Masoudi Soltani and Sara KazemiYazdi

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228DOI: 10.7763/IJCEA.2012.V3.191

II. MATERIALS AND METHODOLOGY Newly smoked cigarettes were collected all belonging to

the same cigarette manufacturer. The butts were then removed and unwrapped from within the cigarette paper. They were then placed into a ceramic furnace crucible and next placed inside a steel tube furnace (Carbolite Co.). N2 gas flow was supplied from a nearby nitrogen capsule which was directly connected to a flow meter and then the furnace tube. Prior to any experiment, nitrogen was allowed to pass through the furnace for several minutes to make sure the furnace tube was free of any oxygen. Nitrogen flow rate was set to 100 cm3/min in all of the experiments. Carbonization temperature, carbonization duration and heating rate were set to 800 °C, 9 hr and 3 °C/min, respectively. Before taking out the sample after its pyrolysis was complete, the furnace was left unopened overnight with the same nitrogen gas flow rate, until it reached the ambient temperature. Then the sample was taken out to prevent any unwanted oxidation at elevated temperatures.To characterize the synthesized carbon, adsorption-desorption isotherms were obtained using a Micromeritics Co. surface analyzer using liquid nitrogen (77 K) as the analysis gas. Prior to the analysis the sample was degassed for 6 hours at 300 °C at a heating rate of 3 °C/min to make sure no gas molecules were trapped inside the pores. An FE-SEM (Quanta) was used to take surface images of the materials. Also, a Mettler Toeldo (Star System) Co. thermo gravimetric analyzer was used to study the thermal decompositionof the smoked cigarette filters. To simulate the synthesis conditions, the temperature increased at a heating rate of 3 °C/min and was kept at 800 °C for 5 hours in a nitrogen flow at 100 cm3/min.

III. RESULTS AND DISCUSSIONS

A. Thermal Gravimetric Analyses (TGA) Fig 1 shows the TGA/DSC results obtained by the

thermal decomposition of smoked cigarette filters in an inert atmosphere. In the initial stage of heating, a mass loss of around 10% is observed up to 280 °C. This could be due to the evaporation of the water content in the used cigarette filters. The weight loss trendfrom here on shows that sample’ssharp weight loss starts at about 280 °C and is completed at 370 °C. This weight loss can be due to dehydration and acetic acid removal and corresponds to a mass loss of 70 % [9]. By increasing the temperature to 800 °C another weight loss of 8% is observed. This may be due to the incomplete degree of carbonization (CxHy species) [9]. After being held at 800 ºC for a few hours, no further decrease in mass was observed on the TGA control panel. The final weight loss shows a final carbon yield of 12% which is in good agreement with our synthesized carbon [9].

B. Adsorption-Desorption Isotherms The isotherm obtained in fig 1 is marginally classified as

adsorption-desorption isotherm type IV by the International Union of Pure and Applied Chemistry (IUPAC) for porous materials [10]. This isotherm is the characteristics of a mesoporous material [11].However, the quite broad desorption branch of the isotherm is a sign of a pore system with a distinct polydispersity in size [8]. Polarz, et al, also

reported similar isotherm with the open-end desorption branch. This phenomenon is known for certaincarbon materials and might be due to changes in the bulk density upon nitrogen sorption or the irreversible adsorption of nitrogen. Moreover, this effect might also be connected with retarded adsorption, where the adsorbing and desorbing molecules have to overcome a certain energy barrier, which might be a consequence of the presence of narrow pores, penetration into which is difficult because of steric considerations [8]. Using BET model to measure micro and meso pore surface area, a surface area of 360 m2/gwas calculated.The linear BET adsorption range was found to be between 0.05 to 0.3 P/P0. This BET value is around 37% higher than the value reported by Polarz et al. Also the micro pore surface area calculated using t-plot method, shows that the synthesized material has 317 m2/g microporous surface area. Also, total pore volume was calculated to be 0.184 cm3/g considering total amount of nitrogen adsorbed at P/P0 =0.984. The higher value of micropore area compared to that of mesopores can be attributed to the increased carbonization time which leads to the shrinkage of the carbon and the collapses of mesopores carbon walls and as a consequence, the creation of smaller micropores [12]. The faster heating rate, on the other hand, might hinder full deterioration of carbon walls and mesopores at elevated carbonization time.

Fig. 1. TGA graph of the smoked cigarette filter (heating rate: 3 °C/min,

N2 gas flow rate: 100 ml/min)

Fig. 2. Nitrogen adsorption-desorption isotherm of smoked cigarette filter

at 77 K

(Sample was degassed at 300 °C for 6 hours before analysis)

Meanwhile, using BJH method applied to the adsorption section of the isotherm, the average pore width is estimated

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Relative pressure (P/P°)

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to be 3 nm compared to 4 nm reported by Polarz et al. This may also be due to the increased carbonization time which alternatively may give a chance to the carbon walls to collapse and form smaller pores. Total pore volume and t-plot micropore volume are calculated to be 0.185 cm3/g and 0.164 cm3/g, respectively.

C. Scanning Electron Microscopy (SEM) Fig 3 and Fig 4 show the surface morphology of the

smoked cigarette filters and the carbonized cigarette filters, respectively.The long cellulose acetate chains are very well illustrated in fig 1. Fig 1 shows the lack of any porosity in the initial material. However, when carbonized at mentioned conditions, a totally different surface structure was obtained. The SEM image can successfully show the presence of macro pores and porous structure on the surface of the synthesized carbon.

Fig. 1. Raw cigarette filter

Fig. 2. Carbonized filters at 800 ºC for 9 hr at 3 ºC/min

Fig. 3. Carbonized filters at 800 ºC for 7 hr at 1 ºC/min (Adopted from [8] )

Fig 3, for comparison, shows the SEM image reported by

Polarz et al when cigarette filters were carbonized at 800 °C for 7 hours at a very slow heating rate of 1 °C/min. Fig 2 and fig 3 show the different morphology resulted from two different operational conditions. It appears that the higher carbonization time results in the partial destruction of the surface pores and could possibly generate higher surface area.

D. Energy Dispersive X-Ray (EDX) To further analyze the prepared carbon, EDX analysis

was utilized. The EDX spectrum of the carbon, carbonized at 800 ºC for 9 hr at 3 ºC/min as is shown below.

Fig. 4. Smoked cigarette filter EDX spectrum.

The elemental compositional analysis of used filter and carbonized filter are also presented in fig 5 and fig 6. It is evident that the final porous material is composed of more than 94% of elemental carbon.

TABLE I: ELEMENTAL ANALYSIS OF USED CIGARETTE FILTER Element Weight% Atomic%

C 55.01 62.07

O 44.66 37.83

TABLE II: ELEMENTAL ANALYSIS OF CARBONIZED CIGARETTE FILTER AT

800 ºC FOR 9 HR AT 3 ºC/MIN

Element Weight% Atomic%

C 94.13 96.43

O 3.82 2.94

IV. CONCLUSION The results highlight the interesting effect of

carbonization time and heating rate on the BET surface area of the porous carbon synthesized from the pyrolysis of cigarette filters. The results demonstrate the potential capabilities of future studies in order to optimize the current process and yielding higher porous surface area by the pyrolysis of cigarette filters. Also, as cigarette filters mainly consist of cellulose acetate, other cellulose acetate and cellulosic waste can accordingly be nominated to be recycled via the production of porous carbons. Since cigarette filters are made of cellulosic-based materials, using acid as in acid pretreatment of the carbonaceous initial material before and/or during the pyrolysis stage may result in the elimination of water from the raw material al still lower temperatures. This may lead to a higher carbon content and higher carbon efficiency and consequently improved process yield.

ACKNOWLEDGMENT The authors would like to thank the University of

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Nottingham Malaysia Campus (UNMC) for providing us with the required facilities to accomplish such experiments.Also, special thanks to the laboratory technicians, Mr. Andrew Yakin Sinit and Ms. Farrawahida Mohtar for their professional experience and supports.

REFERENCES [1] M. B. V. Santos, A. C. Muniz, A. M. Alonso, and J. M. Tascon,

“Porous texture evolution in activated carbon fibers prepared from poly (p-phenylene benzobisoxazole) by carbon dioxide activation,” Microporous and Mesoporous Materials, vol. 116, pp. 622-626, 2008.

[2] F. R. Reinoso and A. S. Escribano, Porous carbons in adsorption and catalysis. In Handbook of Surfaces and Interfaces of Materials Academic Press, vol. 5.

[3] L. R. Radovic, “Chemistry and Physics of Carbon,” Taylor and francis Group, vol. 30, 2008.

[4] W. Shen, X. Yang, Q. Guo, Y. Liu, Y. Song, and Z. Han, “The effect of carbon precursor on the pore size distribution of mesoporous carbon during templating synthesis process,” Materials Letters, vol. 60, pp. 3517-3521, 2006.

[5] E. A. Smith and T. E. Novotny, “Whose butt is it? Tobacco industry research about smokers and cigarette butt waste,” Tobacco Journal, pp. 2-9, 2011.

[6] R. Barnes, “Regulating the disposal of cigarette butts as toxic hazardous waste,” Tobacco Control, pp. 45-48, 2011. Longwood Education. [Online]. Available: http://www.longwood.edu/cleanva/cigbuttfilters.htm

[7] S. Polarz, B. Smarsly, and J. H. Schattka, “Hierarchical porous carbon structures from cellulose acetate fibers,” Chemistry of Materials, pp. 2940-2945, 2002.

[8] K. Laszlo, A. Bota, and L.G. Nagy, “Comparative adsorption study on carbons from polymer precursors,” Carbon, 38, pp. 1965-1976, 2000.

[9] S. Sing, D. Everette, R. Haul, L. Moscou, R. Pierotti, and J. Rouquerol, “Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and pososity,” Pure and Applied Chemistry, vol. 57 (4), pp. 603, 1985.

[10] H. Marsh, R. Reinoso, and F. Activated, Carbon. Elsevier, 2006. [11] C. Liu, L. Li, H. Song, and X. Chen, “Facile synthesis of ordered

mesoporous carbons from F108/resorcinol-formaldehyde composites obtained in basic media,” Chemical Communications, vol. 7, pp. 757-759, 2007.

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