Materials Transactions, Vol. 46, No. 12 (2005) pp. 2673 to 2678 Special Issue on Growth of Ecomaterials as a Key to Eco-Society II #2005 The Japan Institute of Metals
Gas Adsorption Properties of Woodceramics Riko Ozao1; * , Toshihiro Okabe2 , Tadashi Arii3 , Yuko Nishimoto4 , Yan Cao5 , Nathan Whitely5 and Wei-Ping Pan5 1
SONY Institute of Higher Education, Atsugi 243-8501, Japan Aomori Industrial Research Center, Aomori 030-0113, Japan 3 Rigaku Corporation, Tokyo 196-8666, Japan 4 Kanagawa University, Department of Chemistry, Hiratsuka 259-1295, Japan 5 Western Kentucky University, Department of Chemistry, 1 Big Red Way, Bowling Green, KY 42101, USA 2
Gas adsorption properties of woodceramics prepared from cedar (Cryptomeria japonica, abbreviated as CE), apple waste (AP), and chicken waste (CH) were studied. Depending on the starting material, woodceramics differed in physisorption properties as evaluated by specific surface area (SSA) obtained by BET (Brunauer–Emmett–Teller) method, in pore structure and size, and in chemical adsorption properties for perfumery substances. CH and AP yielded lower SSA as compared with CE, however, they showed higher gas selectivity for oxygen vs nitrogen gas molecules. The adsorption ability of CE on perfumery essential oil components was evaluated for the first time using the rapid measuring method for VOCs; i.e., the adsorption capacity of cedar-based woodceramics for perfumery materials (essential oil extracts: carvone, pulegone, geraniol, citronellol, menthone, nerol, and citral) was examined. CE showed particularly strong affinity with geraniol and citral, in which more than 99.9% of the compounds were adsorbed. About 99.7–99.8% of pulegone and menthone, 99.0% of nerol, 97.9% of carvone, and 94.0% of citronellol were adsorbed by CE. The adsorption affinity was also influenced by the particle size; particles coarser than 4.0 mm in size showed stronger adsorption. (Received August 22, 2005; Accepted November 1, 2005; Published December 15, 2005) Keywords: woodceramics, cedar, adsorption, ecomaterial, VOC
1.
Introduction
Wood-based materials are attracting attention as good absorbers of volatile organic compounds (VOCs). Woodceramics are porous carbon/carbon composites or hybrid materials consisting of plant-originated amorphous carbon reinforced by glassy carbon generated from resin. Originally, woodceramics were produced by impregnating wooden materials with thermo-setting resin, such as phenolic resin, and by then carbonizing the resin-impregnated material in a vacuum furnace.1) Woodceramics obtained from wood are generally porous, and have macro-pores with diameter ranging from 1 to 50 mm (See Fig. 1). The process for producing woodeceramics is widely applicable to plantbased materials; so far, woodceramics had been produced from wood-based (lignocellulosic) materials, such as hiba (Thujopsis dolabrata var. hondae), cedar (Cryptomeria japonica), pine (Pseudotsuga menziesii), MDF (medium density fiber) boards, etc. Their specific surface area (BET) is generally greater than about 500 m2 g1 . Woodceramics are also known as environmentally benign materials in that they can be produced from carbonaceous industrial wastes, such as waste paper,2) olive pomace and olive stones,3) and apple pomace.4) By making woodceramics from such wastes, a mechanically stronger functional material having higher heat resistance can be obtained. Furthermore, by varying the starting material or by changing the carbonizing temperature, the properties such as electric resistance or dielectric properties,5) mechanical strength,6) tribological or friction properties,7) chemical adsorption properties,8) and so forth, can be tailored. Woodceramics prepared from apple pomace showed potential for gas adsorbents.4) Although woodceramics *Corresponding
author, E-mail:
[email protected]
produced from apple pomace typically do not have pores as wood-based woodceramics as shown above (Fig. 1) and has a small BET specific surface area (nitrogen) lower than 0.5 m2 g1 , they are expected to have oxygen and nitrogen gas adsorption capacities well comparable to molecular sieve carbon (MSC). Attempts have been made to produce woodceramics from other carbonaceous materials. Since broiler litter generally contains about 40 mass% (dry basis) carbon,9,10) woodceramics were produced from biomass based on chicken wastes.11) As compared with carbonized chicken wastes, the product (woodceramics) is advantageous in that it is free of unfavorable smell.12) Perfumery essential oil components are generally extracted from plants and woods and used in various ways. For instance, citronellol and citral, which are the terpenes, are also well known for their antibacterial activity on MRSA (methicillin-resistant Staphylococcus aureus).13) Other perfumery components, such as menthone, pulegone, geraniol, nerol, carvone, etc., are well known for their bioactivity on insects; they are cockroach repellents.14) These plant-extracted perfumery materials can be added to adsorbents having chemical affinity with them, and may be allowed to release in gas phase, i.e., in their most effective form. Accordingly, this paper reports preliminary experimental results on the selectivity of gas species of the woodceramics and on the adsorptivity of the perfumery materials on woodceramics for practical applications. 2.
Experimental
2.1 Samples The following samples were prepared and used in the experiment: (1) CE-Woodceramics prepared from cedar. Saw dust of cedar was mixed at a weight ratio of 6:4 with phenolic
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Fig. 1 FE-SEM photograph of a typical cedar-based woodceramics (CE). (a) Vertical section. micro-pores less than 1 mm in diameter are seen on the wall. (b) Cross section. Cylindrical pores 2–30 mm in size run perpendicular to the section.
resin (BELLPEARL S890, a product of Kanebo, Ltd.), and the resulting product was carbonized at 800 C in a rotary continuous heating kiln while flowing gaseous nitrogen in such a manner that the residence time within the kiln should be 6 min. Where necessary, the particles were sieved to obtain particles 1.0–1.7 mm, 1.7– 4.0 mm, and >4:0 mm in size. (2) AP-Apple fiber (product of Nichiro Corp.) 250 mm or smaller in size was mixed at a weight ratio of 6:4 with phenolic resin (BELLPEARL S890), and the resulting product was carbonized at 800 C in the same manner as CE. (3) CH-Woodceramics prepared from chicken wastes provided from Japan Agricultural Cooperatives (JA). 600 g of chicken wastes (originally containing about 40 mass% water) was bone-dried at 105–110 C, and was mixed with 400 g of BELLPEARL S890 to be molten at 300 C for 3 h. CH800 was finally obtained by carbonizing the resulting product at 800 C. Other raw wood materials and phenolic resin were used as comparative samples where necessary. 2.2 Specific surface area (SSA) Adsorption isotherms and SSA (Multipoint Brunauer– Emmett–Teller (BET) method) were obtained using nitrogen molecules as adsorbate at 196 C (Autosorb 1, Quantachrome Instruments) and where necessary, CO2 molecules as adsorbate at 78 C (BELSORP-mini, BEL Japan). 2.3 Oxygen and nitrogen gas adsorption capacity Gas adsorption capacity of the sample was measured using the system described before.12) Thus, oxygen and nitrogen gas adsorption profile with passage of time was obtained under controlled constant pressure of 0.26 MPa. 2.4
Rapid measuring method for volatile organic compounds (VOCs) The adsorption properties of Woodceramics were evaluated by the rapid measuring method for volatile organic
compounds (VOCs) evolved from wood materials recently developed by Nishimoto et al.15) This method had been developed by using thermogravimetry (TG) and gas chromatography (GC), based on the findings that the sum of the peak areas obtained in a predetermined duration of time in the chromatogram is directly related to the amount of gas species evolved from the sample. As shown in Fig. 2, a small amount (50 mg) of the sample was sealed in a vial (20 mL) with 0.1 to 0.5 mL of the adsorbates dropped onto glass fiber, and they were kept at a certain temperature (110 C) for 1 h. Then, 1.0 mL of the gas phase in the vial was taken and measured by gas chromatography (HP 6890, equipped with flame ionization detector (FID)). Experimental conditions were as follows: gas flow rates: He (carrier gas), 0.8 mL min1 ; H2 , 40.0 mL min1 ; air, 450 mL min1 ; split ratio; 10.0, split flow; 7.8 mL, detector temperature, 250 C; injector temperature, 250 C; oven temperature program, from 40 to 240 C at 10 C min1 . A DB-1 100% dimethylpolysiloxane separation l column (30 m length, 0.32 mm i.d., and 0.25 mm film thicknesses) was used. In this manner, adsorption capacity of woodceramics for the perfumery materials shown in Fig. 3, i.e., carvone, pulegone, geraniol, citronellol, menthone, nerol, and citral, was evaluated. 3.
Results and Discussion
3.1 Specific surface area of the samples The SSA of the samples is shown in Table 1. It can be understood that woodceramics greatly differ in SSA depending on the starting material, and that CE, which was obtained from wood (cedar), has a large SSA; it is 185 times and about 9 times as large as those of AP and CH. The porous structure observed by SEM (Fig. 1) may account for high SSA. However, this SSA is about one-third or half of a commercially available active carbon. Furthermore, it is to be noted that commercially available molecular sieve carbon (MSC) useful for separating oxygen and nitrogen from air, has very low SSA near to zero.11)
Gas Adsorption Properties of Woodceramics
Fig. 2
Fig. 3
Rapid Measuring Method for VOCs-Head space GC.
Perfumery essential oil components tested: menthone, pulegone, citronellol, gerniol, citral, nerol, carvone.
Table 1 Specific surface area (SSA) of the samples and comparative samples. Samples
SSA m2 g1
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Comparative
MSC1
CE
AP
CH
Super active carbon2
Active carbon3
Cedar wood
2
555
3
63
1687
913
500
1
Molecular sieve carbon, Bellpearl S890, product of Air Water Bellpearl, Inc.; 2 BELLFINE, product of Air Water Bellpearl, Inc.; 3 FLUKA05120, product of Sigma-Aldrich Corp.
3.2 Oxygen and nitrogen gas adsorption properties This method is effective for evaluating the selective adsorption on gas species. Since oxygen and nitrogen gas molecules are different in size (e.g., 0.141 and 0.162 nm2 at 77 K), the size and shape of micropores of the adsorbent
determines the gas adsorption behavior. The gas adsorption curves for CE samples are shown in Fig. 4. In order to examine the effect of particle size, the CE samples classified in three sizes were used: 1.0–1.7 mm, 1.7–4.0 mm, and over 4.0 mm in size. From the results shown in Fig. 4, saturation occurs in adsorption capacity for all the samples though the capacity depends on the particle size and the gas species. Thus, it can be understood that the smaller the size, the higher the maximum adsorption capacity. The difference in maximum adsorption capacity is about 3 mgg1 at highest, and the difference is negligible for coarser samples. Similarly, measurements were made on AP, and the maximum adsorption capacity observed on AP for O2 was similar to that of CE, i.e., about 30 mgg1 . However, the response time on taking up the gas species differed from that of CE. Commercially available molecular sieve carbon (MSC) has slit-like micropores not easily accessible by larger size molecules, and takes advantage of the difference in response time.11)
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Fig. 4
Oxygen and nitrogen gas adsorption profile for CE. Fig. 6 Evaluation of gas adsorption selectivity. The bold straight line indicates that the time constant is equal for oxygen and nitrogen. The steeper the gradient of the line connecting to the origin, the better is the selectivity for nitrogen.
dQðtÞ S ¼ : dt0
Fig. 5 Schematically shown adsorption curve and the parameters.
In order to evaluate the selectivity of the gases, the following parameter, time constant, , is defined as follows. Referring to the schematically drawn Fig. 5, QðtÞ represents the adsorbed amount of gas depending time t, which approaches a constant value S with passage of time. QðtÞ also depends on the applied pressure p, but is omitted from the discussion because p is maintained constant (0.26 MPa) in this case. Thus, the observed curve can be fitted empirically to: t
QðtÞ ¼ S ð1 " Þ
ð1Þ
where, S is the maximum adsorption capacity, " is a constant, and t is time. Thus, is a material-dependent time constant which can be derived from the time vs adsorption curve as shown in Fig. 5; or, the curve can be represented by the derivative at time t0 ,
Table 2
Table 2 shows the S and values of the samples. Figure 6 is a graph showing the time constant values obtained from oxygen and nitrogen adsorption curves, plotted on the abscissa and the ordinate, respectively. The bold straight line connects points on which the time constant is equal for oxygen and nitrogen. The steeper the gradient of the line connecting to the origin, the better is the selectivity for nitrogen. It can be seen that CE samples have no selectivity on gas species. CH has some selectivity, and this may be attributed to the metal impurities.11) The results suggest that AP also has some selectivity on gas species; i.e., that there are micropores capable of segregating the molecules differing in diameter. However, AP needs a longer time for gas separation and therefore not as effective as the commercial MSC. 3.3
Gas selectivity evaluation using rapid measuring method for volatile organic compounds (VOCs) Adsorption characteristics of CE for perfumery compounds shown in Fig. 3 was evaluated. Since discussions were made above on SSA (3.1), which provides an indication of physisorption, and on nitrogen and oxygen gas adsorption behavior under pressure (3.2), which indicates difference in micropore structure, in this section, chemical adsorption
Time constant and maximum adsorption capacity S of the samples for oxygen and nitrogen gases. CE
Oxygen
Nitrogen
/s S/mg g1 /s S/mg g1
AP
CH
MSC
109.09
133.33
103.23
30.00
35.6
31.1
35.0
44.6
169.70
169.70
224.24
409.68
293.33
32.3
32.3
26.8
27.8
37.0
1.0–1.7
1.7–4.0
>4:0 mm
109.09
109.09
39.4
36.4
133.33 35.6
Gas Adsorption Properties of Woodceramics
behavior of the adsorbents is discussed. The boiling points of the perfumery compounds tested are shown in Table 3. The boiling points of the compounds are approximately the same except for citronellol. This method is a kind of accelerated test, and hence, the temperature for keeping the adsorbate with the adsorbent was set to 110 C, i.e., a temperature higher than the generally used temperature of 40 C for absorbing VOCs. It is also confirmed by TG-DTA/FTIR (thermogravimetry-differential thermal analysis coupled with evolved gas analysis using Fourier-transform infrared
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spectroscopy) that gas evolution behavior is the same for the temperature range of from 25 to 150 C.12) Figure 7 shows the actual gas chromatogram of menthone in gas phase, and of the gas sampled according to the present method using CE differing in particle size. It can be understood that CE with larger particle size has higher adsorption affinity for menthone. The peak area obtained in the chromatogram was converted into TVOC value according to the following definition: TVOC ¼
Table 3
Boiling point ( C) of the perfumery compounds tested. Boiling point ( C)
Gas phase Retention Time
peak area (105 )
Menthone Pulegone
207 224
14.0 15.7
33 37
Citronellol
225
16.0
24
Nerol
227
15.9
29
Citral
229
16.4
52
Geraniol
230
16.3
24
Carvone
230
16.4
32
Fig. 7
Table 4
Aobs Atoluene,std
qtoluene,std
where, TVOC is the total area of chromatogram converted to the amount equivalent of toluene; Aobs is the peak area observed on the sample, Atoluene,std is the peak area on the chromatogram of toluene taken at a standard amount, and qtoluene,std is the amount of toluene taken as the standard. In Table 4 is shown the adsorption capacity of CE differing in particle size, CE(1) consists of particles with size range of 1.0–1.7 mm, CE(2) 1.7–4.0 mm, and CE(3) above 4.0 mm. Figure 8(a) shows that chemical adsorption behavior is influenced by the particle size. In general, for the essential oil components, coarser particles 4.0 mm or larger showed higher adsorption capacities.
Actual gas chromatogram of menthone and menthone adsorbed by cedar-based woodceramics (CE) differing in particle size.
Gas adsorption ability of CE differing in particle size compared with other wood based materials, expressed by TVOC/nL. (in TVOC/nL; n ¼ 3) perfume only 0.1 mL
with CE(1)
with CE(2)
with CE(3)
with cedar wood
with bamboo charcoal
with cedar charcoal (0.05 g)
(0.05 g)
(0.05 g)
(0.05 g)
(1.0 g)
(0.2 g)
Menthone
111.4
16.9
2.5
0.3
0.4
24.3
9.8
Pulegone
124.5
22.7
3.2
0.4
2.9
5.7
12.9
81.9
11.5
5.1
4.9
7.0
10.5
18.0
96.6 176.8
7.8 0.1
2.6 2.6
1.0 3.4
7.4 6.9
17.6 7.0
17.6 15.9 30.8
Citronellol Nerol Citral Geraniol
80.5
3.3
2.3
0.1
3.2
10.2
Carvone
106.9
0.3
1.2
2.2
1.4
33
(1) particle size 1.0–1.7 mm, (2) 1.7–4.0 mm, (3) above 4.0 mm
6.4
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composites and ecomaterial prepared from cedar (CE), apple waste (AP), and chicken waste (CH), were evaluated from the viewpoint of gas adsorption abilities. Physisorption properties as evaluated by specific surface area (SSA) was higher for CE. Micropores useful for segregating oxygen gas molecules from nitrogen gas molecules were present in CH and in AP. Thus, woodceramics showed different gas adsorption properties depending on the starting material. A first report was made on the adsorption ability of CE on perfumery essential oil components, which was evaluated by using the rapid measuring method for measuring VOCs developed by Nishimoto et al. In particular, the adsorption behavior of cedar-based woodceramics on perfumery materials (essential oil extracts: carvone, pulegone, geraniol, citronellol, menthone, nerol, and citral) was examined. CE showed particularly strong affinity with geraniol and citral, in which more than 99.9% of the compounds were adsorbed. About 99.7–99.8% of pulegone and menthone, 99.0% of nerol, 97.9% of carvone, and 94.0% of citronellol were adsorbed by CE; this tendency was somewhat similar to the original wood, cedar. Acknowledgements RO thanks SONY Institute of Higher Education for financial support. RO is also grateful to Kakuhiro Co. Ltd. for sample preparation and Mr. Yoshinaga of Air Water Bellpearl, Inc.
REFERENCES
Fig. 8 (a) Influence of particle size of CE on adsorption capacity on essential oil components. (b) Adsorption capacity of CE for perfumery compounds, compared with that of bamboo charcoal and cedar charcoal.
Figure 8(b) shows the adsorption ability of CE compared with other wood-based materials. It should be noted that cedar wood and bamboo charcoal are each used at an amount 20 times and 4 times as large as the amount used of CE. Cedar-based woodceramics showed strong adsorption ability particularly on geraniol and citral, in which more than 99.9% of the compounds were adsorbed. About 99.7–99.8% of pulegone and menthone, 99.0% of nerol, 97.9% of carvone, and 94.0% of citronellol were adsorbed by the cedar-based woodceramics. Furthermore, CE had similar tendency on the preference as the original wood, cedar. The reason for this preference is yet to be clarified. 4.
Conclusions Gas adsorption properties of woodceramics, which are c/c
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