Cellulose 8: 29–33, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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High-yield carbonization of cellulose by sulfuric acid impregnation

Dae-Young Kim, Yoshiharu Nishiyama, Masahisa Wada & Shigenori KugaDepartment of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo,Yayoi 1-1-1, Bunkyo-ku, Tokyo, Japan 113-8657 (e-mail: d-kim@sbp.fp.a.u-tokyo.ac.jp)

Received 23 May 2000; accepted 8 January 2001

Key words: cellulose, carbonization, sulfuric acid, thermogravimetry

Abstract

The carbonization of cellulose with sulfuric acid impregnation was studied by thermogravimetric analysis andscanning electron microscopy. The mass yield of carbon after 800◦C treatment in nitrogen increased to 2–3 timesby addition of small amounts of sulfuric acid. The sulfuric acid is considered to work as dehydration catalyst, thussuppressing the release of volatile organic substances. The shrinkage of the sample during carbonization was alsosignificantly reduced by the addition of sulfuric acid.

Introduction

Cellulosic materials such as wood, cotton and rayonare important starting materials for the productionof carbons, including charcoal and activated carbon.Graphite fibers were also produced from rayon inearly days. Since cellulose has chemical formula of(C6H10O5)n, the maximum carbon yield is 44.4% ifhydrogen and oxygen are removed as water. In reality,the mass yield of carbon from cellulose is some 15% at400◦C (Ramiah, 1970; Shafizadeh & Bradbury, 1979;Fengel & Wegener, 1983; Hon, 1991; Williams &Besler, 1996), because volatile compounds such asmethanol, acetic acid, carbon dioxide and tar sub-stances will be released (Nikitin, 1966; Shafizadeh &Bradbury, 1979; Williams & Besler, 1996). When wetreated cellulose at above 800◦C, the weight loss waseven greater depending on the type of cellulose andheating conditions, giving final yields of as low as12%. An improvement in carbon yield is reported tobe achieved by lower heating rate as low as 1◦C/min(Mackay & Roberts, 1982), but still the yield does notexceed 30% at 500◦C.

Since cellulose is the most abundant renewablematerial, improvement of its carbonization yield canhave impacts on practical carbon production. Toachieve a high yield, we need to enhance dehydration

in the course of pyrolysis. Addition of dehydratingagent, such as zinc chloride, seems to be adopted inindustrial processes, but detailed descriptions cannotbe found in existing literatures. Therefore, we un-dertook quantitative examination of the phenomenon,choosing sulfuric acid as an inexpensive, nonvolatiledehydrating agent, and examined the influence of itsaddition on pyrolysis of cellulose.

Methods

The cellulose samples used were: absorbent cotton(air-dried, 7.3% water regain). The weight averagedegree of polymerization was about 4300 by sizeexclusion chromatography (Kuga et al.,1989)

Impregnation of sulfuric acid

The dry sample was weighed and immersed in dilutesulfuric acid for a few minutes and excess liquid wasremoved by pressing the sample between filter papers.The amount of remaining solution was determined byweighing, and the sulfuric acid content was calcu-lated as a percentage to cellulose weight. The ratiowas controlled by changing the concentration of theimmersing solution between 1% and 20%.

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Thermogravimetric analysis

The sulfuric acid-impregnated cellulose samples wereplaced in a platinum holder of a thermogravimet-ric/differential thermal analyzer (TGD-9600, Ulvac,Tokyo) and pyrolyzed up to 800◦C at a heating rateof 10◦C/min under nitrogen flow of 200 ml/min.

Scanning electron microscopy and energy-dispersiveX-ray analysis (SEM/EDXA)

Original cellulose sample dried at 80◦C for 1 day wascoated with platinum by an ion sputter coater, andobserved with a field emission scanning electron mi-croscope (Hitachi S-4000). Energy dispersive X-rayanalysis (EDXA) was carried out on the sample toconfirm that the carbonized sample was free of sulfur(Horiba EMAX-5770).

Results and discussion

Figure 1 shows the TG curves of original and sulfuricacid-impregnated cotton cellulose. The weight lossesat below 100◦C represent evaporation of imbibed wa-ter. After that, the original cotton sample (no H2SO4)shows essentially no weight loss up to 300◦C; thenthe weight decrease rapidly between 300◦C and 360◦Cfollowed by slow weight losses up to 800◦C. This isa typical gravimetric course of pyrolysis of cellulose

Figure 1. Thermogravimetric curves of original and sulfuricacid-impregnated cotton cellulose.

(Hirata & Abe, 1973a; Shafizadeh & Bradbury, 1979;Hon & Shiraishi, 1991; Varma & Chavan, 1995).

The sulfuric acid-impregnated samples, on theother hand, showed remarkably different behavior.The cellulose sample containing 0.4% of H2SO4showed significant weight loss beginning at around150◦C. The weight curve almost leveled off at 350◦C,and slow decrease continued up to 800◦C. With thesamples containing more H2SO4 (2.3–5.6%), pyro-lysis starts at ca. 120◦C and the TG curve shows aplateau region between 200◦C and 350◦C.

Namely, the pyrolysis of pure cellulose is appar-ently a one-step reaction, whereas that of sulfuricacid-impregnated cellulose proceeds in two steps: alow-temperature process at 110–200◦C and a high-temperature process at 300–600◦C. Similar resultswere reported for the pyrolysis of cellulose with addi-tion of inorganic material (Hirata & Abe, 1973b) and aseries of oxidized cellulose (Varma & Chavan, 1995).

An important feature with the sulfuric acid-impregnated samples is remarkably higher carbonyields at 800◦C than that of pure cellulose. By normal-izing the TG curves in Figure 1 to the starting celluloseweight, we obtain the remaining weight fraction atselected temperatures as in Figure 2.

Figure 3 shows the final yield at 800◦C plottedagainst the added H2SO4 per dry cellulose. While theyield from cellulose is ca. 12%, addition of 0.4% sul-furic acid resulted in 28% yield; 5.6% sulfuric acid

Figure 2. Mass yield as weight fraction of starting cellulose atselected temperatures.

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Figure 3. Final yield at 800◦C plotted against added H2SO4 per drycellulose.

gave 38%. Further increases in sulfuric acid addition,7.8% and 10%, resulted in slight decreases in yield.The maximum yield here, 38%, corresponds to 86%of the theoretical yield of 44.4%.

To confirm completeness of carbonization, weexamined elemental composition of the carbonizedmaterials.

A likely mechanism of this remarkable improve-ment in yield is the action of sulfuric acid as dehyd-rating catalyst. Most of water in the initial sulfuricacid solution evaporates at below 100◦C, leaving smallamount of concentrated sulfuric acid distributed ho-mogeneously around the cellulose microfibrils. Byvigorous thermal motion at above 120◦C, sulfuric acidwould chemically extract water from cellulose and thewater would evaporate immediately. This cycle wouldlead to an efficient reaction of Cn(H2O)m → mH2O +nC. Since the loss of carbon in pyrolysis of cellulose isdue to emission of mainly CO2 and CO, the removal ofoxygen in the form of water would effectively preventweight losses and would result in neary theoreticalyield.

Figure 4 shows the differential thermal analysis(DTA) curves recorded simultaneously with thermo-gravimetry. Because the features were similar for allthe sulfuric acid-impregnated samples, the curves forthe original (no H2SO4) and that containing 7.8% ofsulfuric acid are shown. The endothermic peak below100◦C corresponds to evaporation of liquid and sorbedwater. The cellulose shows a large endothermic peak at320–360◦C corresponding to the one-step weight lossin Figure 1. The curve for the 7.8% H2SO4 contain-

Figure 4. Differential thermal analysis (DTA) curves recorded atthe same time as thermogravimetry.

ing sample has a small endothermic peak between 100and 200◦C corresponding to the low-temperature re-action, presumably dehydration catalyzed by sulfuricacid. Remarkably, no endothermic peak was observedat 320–360◦C; instead, a small exothermic peak ap-peared at ca. 360◦C. These features were the same forall the sulfuric acid containing samples.

Figure 5 show EDXA spectra of 800◦C carbon-ized cellulose sample with sulfuric acid impregnation.

Figure 5. EDXA signal of sulfuric acid-impregnated (5.6%) cottoncellulose carbonized up to 800◦C. Excitation energy of each elementis indicated in parentheses.

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The spectra show that the materials are nearly purecarbon, with trace amounts of oxygen and no sulfur.Sulfuric acid was probably completely decomposedand volatilized below 800◦C as water and sulfur diox-ide/trioxide. This is a desirable feature since it makeslengthy washing unnecessary after carbonization.

Figure 6 shows the scanning electron micrographsof the starting cotton (A), carbonized cotton without

Figure 6. Scanning electron micrographs of the starting cot-ton (A), carbonized cotton without (B) and with (C) sulfuricacid-impregnation (5.6% H2SO4).

(B) and with sulfuric acid impregnation (C, 5.6%H2SO4). The fibers became very thin when carbon-ized without sulfuric acid, whereas those of sul-furic acid-impregnated sample have widths nearly thesame as the original. The appearance of fiber alsochanges drastically when carbonized without sulfuricacid leading to a shriveled structure. Thus the sulfuricacid treatment not only improves the carbon yield,but also has an effect of conserving the fiber morpho-logy. These features can be advantageous in carbonmanufacture from cellulosic materials.

Cellulose materials vary in degrees of polymer-ization and crystallinity, and we here chose cottonas representative cellulose material. Our preliminarilyexamination showed similar effects of sulfuric acidon carbon yield from other materials, such as algaland bacterial cellulose. The crystallinity was foundto strongly affect the structure of resultant graphite(without sulfuric acid) (Kim et al., in press). Thesefindings combined may lead to novel processes andcarbon materials by pyrolytic carbonization of cellu-lose.

Acknowledgements

The authors thank Ung-Jin Kim for measurement ofdegree of polymerization. This work was supported bythe Grant-in-Aid for Scientific Research from Ministryof Education, Science, Sports and Culture, Japan (No.11460077). D.-Y. Kim was a Japanese GovernmentScholarship Student.

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