About reactions occurring during chemical activation
with hydroxides
M.A.Lillo-R o
denas,J.Juan-Juan,D.Cazorla-Amor o s,A.Linares-Solano *
Departamento de Qu ımica Inorg a nica,Universidad de Alicante,Ap.Correos 99,E-03080Alicante,Spain
Received 10July 2003;accepted 28December 2003
Available online 18February 2004
发布会邀请函Abstract
The chemical activation of anthracites with hydroxides has been shown to be of interest for the production of activated carbons
with a highly microporous structure.In a previous paper,attention was placed on the reactions occurring
during the chemical activation of an anthracite by NaOH and KOH.In the prent work,the process of chemical activation by hydroxides has been extended to different coal precursors to confirm that such a chemical activation process starts through a solid–solid reaction and continues as a solid–liquid reaction.In such a solid reaction,the reactivity of the solid (precursor)should be a key parameter.The importance of the carbon reactivity on its reaction with hydroxides has been confirmed:the lowest rank coal reacts much easily and has a much lower temperature for the beginning of reaction than the highest rank coal.Ó2004Elvier Ltd.All rights rerved.
Keywords:A.Activated carbon;B.Activation;C.Adsorption,Infrared spectroscopy,Temperature programmed desorption
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1.Introduction
Chemical activation with hydroxides has recently been of great interest as it permits the preparation of activated carbons with a highly developed porosity.Therefore there is an increasing number of studies re-lated to the preparation of activated carbons by chem-ical activation with KOH [1–8]and,more recently,also with NaOH [1,3,5,6,9].
It has been shown [1–4]that some experimental variables have a great influence on the porosity of th
e activated carbons prepared by chemical activation with hydroxides:the ratio activating agent/coal,the method of mixing of the activating agent and coal,the temper-ature and flow of gas during the carbonisation,etc.Although there exist some studies regarding the effect of some experimental variables on chemical activation with hydroxides,there is a lack of information about how the chemical activation occurs.In a previous study [10],we analyd some aspects relative to the reactions
occurring during the chemical activation with hydrox-ides,KOH and NaOH,using anthracite as precursor.This study led to conclusions related to the solid–hydroxide reaction,to the products of the reaction and to the temperature of the process.It also permitted to understand why some experimental variables have such an important effect on the final porosity.
The purpo of this work is to further explore the reactions occurring during chemical activation with ei-ther NaOH or KOH by extending the study to other carbon precursors:lignite,subbituminous coal and anthracite.The activation of lignite will be compared with that of carbonid lignite,whereas the behaviour of anthracite is compared with that of the anthracite car-bonid up to 1000°C and higher temperatures,to vary the reactivity behaviour of the precursor.2.Experimental
TPD experiments have been carried out in a quartz reactor to study the carbon–hydroxide reactions occur-ring during the heat-treatment of the sample.The con-ditions of the heating were:a helium flow (60ml/min),a heating rate of 20°C/min,a maximum temperature of about 750°C and a soaking time of 1h at the maximum
*
Corresponding author.Tel.:+34-9-6590-3545;fax:+34-9-6590-3454.
E-mail address:linares@ua.es (A.Linares-Solano).0008-6223/$-e front matter Ó2004Elvier Ltd.All rights rerved.
doi:10.1016/j.carbon.2004.01.008
Carbon 42(2004)
1371–1375
/locate/carbon
temperature.The evolved gas from the experiments have been analyd by mass spectrometry.All the samples were identically prepared by a physical mixing of the hydroxide and the carbon in a1/1molar ratio, following cloly the experimental conditions ud dur-ing the activation process[3].
The experiments provide information about the temperature for the beginning of the reaction and about the percentage of hydroxide reacted during the process of activation.Hence,different variables have been analyd:the nature of the hydroxide(KOH and NaOH),the nature of the precursor(lignite,subbitu-minous and anthracite)and the effect of heat-treatment of lignite and anthracite(lignite versus carbonid lignite and anthracite versus heat-treated ones,the temperature of which is indicated by the number next to A).
Becau the heat-treatment of the carbonaceous so-lid–hydroxide involves esntially a solid–liquid
reac-tion,it has to be related to the reactivity of the carbon precursor.To confirm such a hypothesis,a study of the behaviour of the carbon precursor in air has been car-ried out.Thermogravimetric analysis(TG)of the dif-ferent precursors has been done in air(60ml/min),using a heating rate of5°C/min up to650°C over different precursors:anthracite(A),anthracite treated up to1800°C(A-1800),subbituminous coal(SB),lignite(L)and carbonid lignite(LC).
3.Results and discussion
3.1.TPD experiments for the mixtures precursor–hydroxide
Our study of the reactions occurring during activa-tion of an anthracite by KOH or NaOH[10]has shown that chemical activation consists of a reaction between the carbon and the hydroxide.The reaction products are the metal carbonate,hydrogen and some metallic com-pound,in good agreement with the results of Otowa et al.[7,8]obtained by mixing petroleum cokes and KOH.From the reaction products our propod reactions,different from tho of Otowa et al.[7]are: 6NaOHþ2C$2Naþ3H2þ2Na2CO3ð1Þ6KOHþ2C$2Kþ3H2þ2K2CO3ð2ÞIn this type of reaction(solid–hydroxide)the reactivity of the solid has to be a key factor,as will be shown next by analyzing the different carbon precursors ud(lig-nite,subbituminous,anthracite,heat-treated carbons). It should b
e noted that the above reactions are global reactions that do not describe the detailed reaction mechanism.
Over different precursor–hydroxide mixtures,TPD experiments have been carried out,in order to determine their different reactivities(deduced from the temperature for the beginning of the reaction,T i)and the extents of the reaction(carbon reacted deduced from the hydrogen formation applying reactions(1)and(2)).The experi-ments were carried out both with NaOH and KOH to simulate the chemical activation in a furnace.
As an example,Fig.1shows the total hydrogen evolved from a TPD experiment carried out with a subbituminous coal/NaOH mixture and Figs.2–4pres-ent the H2evolved up to the activation temperature(750°C)from different precursor/activating agent mixtures.
From the TPD experiments the data of Table1 have been obtained for some of the samples studied with both hydroxides.The beginning of the reaction in Table 1corresponds to the start of hydrogen appearance.
As shown in Table1,in all the precursors studied the starting temperature of the reaction is lower for KOH than for NaOH,confirming that KOH is a more reactive agent,in agreement with our previous re
sults[10].
From Table1and Fig.2it can be en that the chemical activation process depends on the rank/reac-tivity of the coals.As expected,the higher the reactivity
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1372M.A.Lillo-R o denas et al./Carbon42(2004)1371–1375
of the coal,the lower is the temperature for the begin-ning of the reaction both by NaOH and KOH.
Such hydrogen evolution is in agreement with the for-mer studies of Yamashita and Ouchi [11,12]obtained by studying the carbonisation process of different precursors heat-treated with NaOH.However,their propod reac-tions,bad on dehydrogenation through active methy-lene groups causing condensation reactions,differ considerably from the one we propo in reaction (1).
The effect of the heat-treatments,which was to modify the reactivity,has also been studied.Figs.3and 4prent the evolution of hydrogen versus temperature for the pairs lignite–carbonid lignite and anthracite–anthracite treated up to 1000°C,respectively (e also Table 1).
As it is well-known [13],the heat-treatment of a given carbon material caus a decrea in its reactivity,in agreement with data compiled in Table 1and with data plotted in Figs.3and 4(shown by an increa in the initial reaction temperature).In the ca of anthracite heat-treated up to temperatures higher than 1000°C,the hydrogen evolution can not be followed,nor can it be activated,as it can be en from Table 2which includes the BET surface areas for the samples prepared using either NaOH or KOH.The higher the temperature of the treatment,the lower the amount of H 2evolved and the less the resulting activation.
The data corroborate the suggestion that the beginning of the reaction depends on the reactivity of the materials ud in the mixture and that the activation process of a coal–hydroxide mixture is a reaction be-tween a solid (the coal)and the hydroxide (which according to its melting point will be liquid)producing hydrogen.
From TPD experiments the quantification of hydro-gen evolved during the activation process can be
as-sd.Table 3compiles the results obtained in the ca of NaOH,for some of the samples studied.We can obrve that the H 2evolution decreas with the rank/reactivity.
If we assume that the hydrogen evolved from the mixtures comes from the conversion of the hydroxides,according to reactions (1)and (2),the percentage of
Table 2
BET surface area for anthracite and heat-treated anthracites chemi-cally activated with NaOH and KOH Sample
BET surface area,m 2/g Samples activated with NaOH
Samples activated with KOH A
16302326A-10005877A-1800109A-2500
4
6
Table 3
Hydrogen evolved in the TPD experiments carried out over the mix-ture precursor–NaOH and %of reacted carbon Precursor H 2evolution,l mol/g %wt.of reacted carbon L 22,7599.0SB 13,563 6.0A
3860 2.0A-1000
680
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0.3
Table 1
Temperatures for the start of the reaction for different precursor–hydroxide mixtures Precursor Temperature
(NaOH reaction),°C Temperature
(KOH reaction),°C L 250225LC 450250SB 375325A
475375A-1000
575
550
M.A.Lillo-R o denas et al./Carbon 42(2004)1371–1375
1373
reacted carbon has been calculated for the different
mixtures.From this quantification,compiled in Table 3for the mixtures precursor/NaOH,we obrve that the percentage of reacted carbon decreas as the rank in-creas.The same trend is obrved for KOH activation.It should be noted that carbon is only removed from the solid carbon sample as a carbonate of sodium,or potassium.In this paper,and in a previous one [10]no CO and CO 2evolution were detected at temperatures below 750°C but only H 2was measured.Only at higher temperatures does carbonate decomposition occur pro-ducing CO and CO 2.
3.2.Study of the reactivity of the different precursors To confirm that the carbon–hydroxide reaction,in terms of the importance of the solid reactivity,does not differ from the well-known air–carbon reaction,TG experiments have been done on the different samples.Fig.5includes the TG curves for the anthracite (A),
subbituminous coal (SB),lignite (L)and carbonid lignite.
As obrved,and following the logical order for the well-known air–coal reaction [14],the most reactive is the lignite coal,whereas the anthracite is the less reactive one.The air reaction of the lignite (L)has been com-pared with that of the carbonid lignite (LC).As ob-rved,the carbonid lignite is less reactive than the pristine one,showing similar behaviour to that of the subbituminous coal.
The air reaction of the pristine anthracite (A)has also been compared with that of the anthracite carbonid to 1000°C (A-1000)and 1800°C (A-1800).The order,as expected,shows that the pristine anthracite is the most reactive among the three (e Fig.6).
4.Conclusions
An analysis of the chemical activation of different precursors (coals)with hydroxides (NaOH and KOH)has confirmed that the heat-treatment of a carbon in the prence of an hydroxide,produces hydrogen and oc-curs via a solid–liquid reaction in which the reactivity of the starting solid is a key parameter that controls the extent of the reaction and the starting temperature for the reaction.Such hydrogen evolution,examined by TPD,allows us to asss both the initial reaction tem-perature and the extent of the carbon reacted during the activation process.
It has been en that for the more reactive coals (the lower rank ones),the carbon–hydroxide reaction begins at lower temperatures than for higher rank coals.The extent of such a reaction also depends on the reactivity of the starting carbon,decreasing from lignite to sub-bituminous coal to anthracite.In addition,heat-treat-ment prior to activation reduces considerably the ea of the reaction,as also happens in the carbon–gas reactions,and hence it makes its activation process difficult.
12月16号是什么星座Acknowledgements
The authors thank MCYT (Project MAT 2000-0621)
for financial support.M.A.Lillo-R o
denas thanks GV for a thesis grant.
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References
[1]Ill a n-G o mez MJ,Garc ıa-Garc ıa A,Salinas-Mart ınez de Lecea C,
Linares-Solano A.Activated carbons from Spanish coals. 2.Chemical activation.Energy Fuels 1996;10(5):1108–14.
[2]Lozano-Castell o
D,Lillo-R o denas MA,Cazorla-Amor o s D,Linares-Solano A.Preparation of activated carbons from Spanish anthracite.I.Activation by KOH.Carbon
2001;39(5):741–9.
短柄斧1
1374M.A.Lillo-R o denas et al./Carbon 42(2004)1371–1375
[3]Lillo-R o denas MA,Lozano-Castell o D,Cazorla-Amor o s D,
Linares-Solano A.Preparation of activated carbons from Spanish anthracite.II.Activation by NaOH.Carbon2001;39(5):751–9.
[4]Teng H,Hsu L.High-porosity carbons prepared from bituminous
coal with potassium hydroxide activation.Ind Eng Chem Res 1999;38(8):2947–53.
[5]Evans MJB,Halliop E,MacDonald JAF.The production of
chemically-activated carbon.Carbon1999;37(2):269–74.
[6]Lee SH,Choi CS.Chemical activation of high sulfur petroleum
cokes by alkali metal compounds.Fuel Process Technol 2000;64(1):141–53.
[7]Otowa T,Tanibata R,Masao I.Production and adsorption
characteristics of MAXSORB:high-surface-area active carbon.
Gas Separation Purificat1993;7(4):241–5.
[8]Otowa T,Nojima Y,Miyazaki T.Development of KOH activated
high surface area carbon and its application to drinking water purification.Carbon1997;35(9):1315–9.
[9]Hayashi J,Watkinson AP,Teo KC,Takemoto S,Muroyama K.
Production of activated carbon from Canadian coal by chemical activation.Coal Sci1995;1:1121–4.
[10]Lillo-R o denas MA,Cazorla-Amor o s D,Linares-Solano A.
Understanding chemical reactions between carbons and NaOH and KOH:an insight into the chemical activation mechanism.
Carbon2003;41(2):267–75.
[11]Yamashita Y,Ouchi K.Influence of Alkali on the carbonization
process.I.Carbonization of3,5-dimethylphenolformaldehyde resin with NaOH.Carbon1982;20(1):41–5.
[12]Yamashita Y,Ouchi K.Influence of Alkali on the carbonization
process.II.Carbonization of various coals and asphalt with NaOH.Carbon1982;20(1):47–53.
八年级下册语文教学计划
[13]Radovic LR,Walker Jr PL,Jenkins RG.Importance of carbon
active sites in the gasification of coal chars.Fuel1983;62:849–56.
[14]Mahajan OP,Walker Jr PL,Karr Jr C,editors.Reactivity of heat
treated coals.Analytical methods for coal and coal products,vol.
II.New York:Academic Press Publisher;1978.p.465–94.
M.A.Lillo-R o denas et al./Carbon42(2004)1371–13751375
