Adsorption Capacity Enhancement by Activation with CO2 ofMonolithic Adsorbents Made of KOH-Activated Carbon and Polymer-Derived BinderJacek Machnikowski,* Krzysztof Kierzek, and Kamila Torchała

Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wroclaw University of Technology, Gdan ska 7/9, 50-344Wrocław, Poland

ABSTRACT: Disc-shaped monoliths molded from KOH-activated carbon powder (AC) and novolac resin (N) or poly(furfurylalhohol) (P) binder were activated with CO2 to open an access to the microporosity that was blocked by the binder char. Thevariation in the porous texture of monoliths was characterized by the N2 adsorption at 77 K and mercury porosimetry. Theresults show that using N allows the monoliths to be activated to a higher burnoff compared to P (30 vs 15 wt %) withoutnoticeable loss of the integrity and mechanical strength. The increase in the surface area and micropore volume on activation isaccompanied by a considerable development of meso- and macropores and a decrease in bulk density of monoliths. The behaviorhas been discussed as an effect of excessive reactivity to carbon dioxide of KOH-activated carbon compared to binder derivedchar. The preferential burnoff of accessible activated carbon particles limits the enhancement of volumetric storage capacity withactivation progress of monolith. The maximum methane uptake of 10.7 mmol g−1 at 25 °C and 3.5 MPa was measured for theactivated monolith made using novolac binder.

1. INTRODUCTIONPowder nature and low packing density are serious drawbacksfor gas-phase application of microporous high surface areaactivated carbons produced using activation with alkalinehydroxide. Tight packing of fine particles is crucial in cases ofadsorbents dedicated to natural gas storage in vehicularapplications,1−6 but also to cooling systems7 and heliumcompressors.8 One of the approaches which has been applied toprepare adsorbent of reduced interparticle voids is pressing theactivated carbon powder with a suitable polymeric binder. Anunavoidable side effect of using binder is a considerable loss ofmicroporosity compared to that which is accessible in theoriginal powder. The loss is attributed primarily to blocking ofpore mouths in activated carbon particles, but the presence ofpoorly porous binder in the system cannot be neglected. Theselection of suitable binder to ensure sufficient mechanicalproperties of the monolith while minimizing both the binderproportion and pore blockage is therefore of great importanceto adsorbent performance.The monolithic adsorbents reported in the literature consist

of activated carbon particles which are bound together by aresin4,5,9−11 or, when baking is applied to molded pellets, by theresin-derived coke/char.1,5,10 The second approach seems tohave some advantages, including a better resistance to variousenvironmental conditions, improved thermal conductivity, and,possibly, enhanced methane adsorption due to better affinity ofadsorbate to more hydrophobic adsorbent. However, thenumber of suitable binders is restricted to those which give aconsiderable residue yield on carbonization, like phenolicresin,1 poly(furfuryl alcohol),2 cellulose derivative, and aproprietary binder.5

In the former work12 we have demonstrated that the porosityof baked monolith made of KOH-activated carbon and resinderived binder could be enhanced to a certain extent by a

moderate activation with CO2. A consequence was an increasein volumetric CH4 storage capacity despite decreased monolithbulk density. It has been anticipated that the primary effect ofactivation is opening the access to the porosity which wasblocked during monolith pressing. However, the bakedmonolith is a complex system of activated carbon particlesand resin-derived char of varied accessibility and reactivitytoward CO2 of both phases. Binder properties, includingwettability of active component and char yield and reactivity,are therefore, of great importance not only for monolithstrength and the extent of pore blockage but also for itsbehavior during activation.Hence, the present work is focused on understanding the

potential and limitations in the porosity enhancement duringactivation of composite adsorbent with carbon dioxide. Themonoliths studied consist of KOH-activated carbon andphenolic resin or poly(furfuryl alcohol) derived chars. Theporous texture and reactivity toward CO2 of constitutingphases, i,e, activated carbon particles and resin-derived char areassessed to anticipate their contribution to porosity generationin the monolith as a whole.

2. EXPERIMENTAL SECTION2.1. Materials. The laboratory sample of activated carbon (AC)

was made by activation of pitch semicoke, which was prepared fromcommercial coal-tar pitch by heat-treatment at 520 °C, with potassiumhydroxide. The physical mixture of KOH powder and the semicoke,particle size 100−630 μm, at the weight ratio of 3:1 was heat-treated at750 °C for 1 h using a muffle furnace equipped with a horizontal nickelretort. Details of the procedure has been given elsewhere.12

Received: January 2, 2012Revised: May 6, 2012Published: May 7, 2012

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© 2012 American Chemical Society 3697 dx.doi.org/10.1021/ef300008y | Energy Fuels 2012, 26, 3697−3702

Novolac type phenol-formaldehyde resin with 7 wt % ofhexamethylenetetramine added as a hardener, N, was provided bythe “Organika-Sarzyna” Chemical Works.Poly(furfuryl alcohol), P, purchased from Aldrich, was precured

using HCl as an initiator.2.2. Monolith Preparation. The procedure used for monolith

preparation follows, in principle, that of our previous work.12 Briefly,the pellets of 19 mm diameter and about 7 mm height were moldedfrom the mixture of activated carbon and a binder in the ratio of 2:1.The molding was performed at 160 °C under pressure of 300 MPa.The resultant pellets were dried and then baked at 900 °C for 1 h in ahorizontal tube furnace under nitrogen. The activation of the bakedmonoliths, AC/N and AC/P, with carbon dioxide was performed at850 °C in a horizontal tube furnace. Activation time was fixedexperimentally so to get burnoff required in the range of 10−30 wt %.For the activated monoliths, the number giving a nominal burnoff isadded.2.3. Evaluation of Reactivity toward CO2. To assess the

possible contributions of individual constituents to the weight loss onmonolith activation, the activated carbon, binder chars (particle size<0.1 mm), and baked pellets AC/N and AC/P were characterized interms of reactivity toward CO2. The reactivity was measured byheating at 850 °C for 5 h under CO2 flow, using a homemadethermobalance.13 The sample mass was about 0.5 g. Chars wereproduced for the test by heat-treatment of resins at 900 °C. AC wasalso annealed at the temperature. The experiments were performed induplicate with good reproducibility.2.4. Porous Texture Characterization. N2 adsorption at 77 K

(ASAP 2020, Micromeritics) was used to characterize porositydevelopment in a range of wider micropores (size >0.6 nm) andmesopores and CO2 adsorption at 273 K (NOVA 2200,Quantachrome) in the narrow micropores (<0.7 nm). The sampleswere degassed overnight at 300 °C before adsorption measurements.The N2 adsorption data were used to determine the total pore volume(VT), the BET specific surface area (SBET), and the microporositydevelopment in the range of 0.6−2.0 nm. The micropore volume(VDR) and the average micropore width (L0) were calculated byapplying the Dubinin−Radushkevich and Stoeckli equations,14

respectively, to the adsorption data up to p/p0 ≤ 0.015.Mercury porosimetry (PASCAL 440, CE Instruments) under

pressure from 0.1 to 400 MPa was used to characterize the porosityof monoliths in the diameter range of 3.6 nm−15 μm. Thecharacteristics measured from the relationship of accessible to mercurypore volume against pressure were total pore volume (Vtot), volumesof mesopores of size 3.6−50 nm (V<50), and pores larger than 50 nm(V>50) as well as bulk density dB and apparent density dA, measuredunder pressure of 0.1 and 400 MPa, respectively.Methane adsorption isotherms were measured under pressure up to

7 MPa using homemade high-pressure volumetric apparatus.15 Theamount of methane (mmol g−1) adsorbed at 3.5 MPa was used tocalculate volumetric storage capacity (V/V)A. Three monoliths weretypically used in an individual measurement so the storage capacityshould be considered as an average for a given series.

3. RESULTS AND DISCUSSION

3.1. Porosity Development on CO2 Activation ofMonoliths. Baked monoliths AC/N and AC/P molded usingKOH-activated carbon AC and novolac resin N or poly(furfurylalcohol) P were activated with carbon dioxide to differentburnoffs and the variation in porosity and density weremonitored using N2 and CO2 adsorptions at 77 and 273 K,respectively, and mercury porosimetry. It must be noted thatnovolac binder allows the monoliths to be activated to a higherweight loss, about 30%, compared to PFA (about 15%) withoutnoticeable loss of the integrity and mechanical strength. As aconsequence, the enhancement of micro- and mesoporosity,measured by N2 adsorption, is distinctly bigger for the formermonolith series (Figure 1). Corresponding increases in the

surface area SBET and micropore volume VDR amount to near400 m2 g−1 and 0.15 cm3 g−1 compared to about 200 m2 g−1

and 0.06 cm3 g−1 (Table 1). The CO2 adsorption data showthat only negligible changes of ultramicroporosity occur oncarbon dioxide activation.

Figure 2 shows bimodal distribution of meso- and macro-pores which is typical of all baked and activated monoliths. Themaximum in the mesopore region, corresponding to mesoporesof diameters up to 10 nm, seems to be related to the poroustexture of activated carbon particles. Much stronger maximumin the macropore range can be attributed to interpacticle voidscreated due to a limited compaction during molding and porescreated due to binder decomposition on baking. Activationenhances porosity in both the regions but the widening ofmacropores is the most striking effect.Properties of monoliths, which were calculated from mercury

porosimetry measurements, are consistent with the observa-tions. For both monolith series, the volume of pores withdiameter between 3.6 and 15 000 nm, which are accessible tomercury under pressure up to 400 MPa, increases graduallywith the burnoff (Table 2).Molding with novolac resin as a binder produces less porous

baked monolith, however, at the highest acceptable extent ofactivation, i.e. 30 wt % for N binder and 15 wt % for P binder,the total pore volumes are quite similar, about 0.66 cm3 g−1.Negative aspect of the activation is that the volume incrementis located in macropores rather which are useless for methaneadsorption. It is interesting to note, however, a distinctly higherincrease in mesopore volume when N is used as a binder than P(0.10 vs 0.05 cm3 g−1).

Figure 1. N2 adsorption isotherms at 77 K of baked monoliths built ofKOH-activated carbon powder and resin-derived char and thoseactivated with CO2 to different burnoffs.

Table 1. Characteristics of Porous Texture for ActivatedCarbon Monoliths Determined from N2 AdsorptionIsotherms at 77 K and CO2 Adsorption Isotherms at 273 K

monolithsymbol

SBET(m2 g−1)

VT(cm3 g−1)

VDR,N2(cm3 g−1)

L0,N2(nm)

VDR,CO2(cm3

g−1)S0,CO2

(m2 g−1)

AC/N 1142 0.52 0.42 1.27 0.33 822AC/N-15 1317 0.59 0.49 1.28 0.31 770AC/N-30 1517 0.69 0.57 1.41 0.34 817AC/P 1268 0.57 0.47 1.35 0.26 652AC/P-10 1313 0.60 0.49 1.32 0.29 726AC/P-15 1450 0.68 0.53 1.40 0.22 560

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The enhancement of porosity is associated with aconsiderable decrease in monolith density (Table 2). Thereduction of apparent density dA, measured at the maximumpressure of 400 MPa, is related to the development ofmicropores and mesopores smaller than 3.6 nm; however, thatof bulk density dB, measured at 0.1 MPa, is related to that oflarger pores. For N bound monoliths the decrease in dB and indA is rather comparable, 0.12 vs 0.10 g cm−3, but in the case ofusing P, the drop of the former density is much higher, 0.10 vs0.04 g cm−3. The data suggest an essential effect of binder typeon the activation behavior of the monolith. It should benoticed, however, that the bulk density of activated monolithsremains relatively high compared to values reported in theliterature which rarely exceed 0.6 g cm−3.2,5

The results of N2 adsorption at 77 K measurementsdemonstrate a continuous increase in micro- and mesoporevolume with activation progress. As the methane uptake at 3.5MPa is, generally, proportional to the surface area andmicropore volume,16−18 a growth of adsorption capacity withactivation depths could be expected. The estimations ofmethane adsorption capacity at 3.5 MPa and 25 °C for thebaked and activated monoliths (Table 3) show that the trend isfully preserved in the case of gravimetric uptake on N seriesmonoliths only. The activation enhances the amount ofmethane stored from 7.4 to 10.7 mmol g−1 for the monolith

of 30 wt % burnoff. When the adsorption data are given as thevolumetric storage capacity V/V, it appears that a maximum ofmethane storage, about 163 V/V, occurs for moderatelyactivated monoliths. The monoliths must combine suitablemicropores content with acceptable packing density. Appa-rently, the excessive burnoff results in pore widening ratherthan in opening the access to microporosity closed in theactivated carbon particles.Similar behavior was reported when carbon discs, prepared

from olive stones using ZnCl2 activation, were further activatedwith carbon dioxide.19 The amount of methane stored in thebinderless adsorbent at 3.4 MPa increased continuously withburnoff to be doubled at 40%, when expressed per unit weight;however, the volumetric storage capacity was enhanced, from96 to 110 V/V, at a moderate burnoff only. This means that inany case the activation extent has to be strictly controlled toavoid an excessive decrease in the adsorbent density.

3.2. Activation Behavior of Monolith Constituents.Monolithic adsorbents produced for the study consist ofactivated carbon particles which are embedded in char derivedfrom novolac resin or poly(furfuryl alcohol). The discussion inthe preceding chapter suggests that understanding theactivation behavior of the complex system requires moredeep insight into the monolith composition and reactivitytoward carbon dioxide of the constituting phases.The green monoliths were pressed from the mixture of

activated carbon powder (AC) and a binder in the weight ratioof 2:1. Baking changed the proportion of activated carbon tobinder char due to different coking yields of constituents. Theheat-treatment of AC, novolac, and poly(furfuryl alcohol)performed individually at 900 °C (i.e., at the temperature usedfor monolith baking) produced solid residues AC-900, N-900,and P-900 with yields of 87.8, 60.1, and 44.2 wt %, respectively.We used the data to assess the contributions of activated carbonand binder-derived char in baked monoliths. The calculationsgave the weight ratio of activated carbon to binder char ofabout 75/25 for AC/N and 80/20 for AC/P.N2 adsorption isotherms at 77 K (Figure 3) show, as

expected, a distinctly lower nitrogen adsorption capacity ofchars compared to that of activated carbon particles. Heat-treatment of activated carbon at 900 °C induces a noticeablereduction of porosity wider than ∼0.7 nm, which is accessibleto N2 molecules at 77 K. The knee extending up to p/p0 of 0.4on the isotherm indicates a significant contribution of relativelywide micropores and narrow (2−3 nm) mesopores, the featurethat is characteristic of KOH activated carbons.6,20 In contrast,the isotherms of chars indicate that adsorption occurs at verylow partial pressure, corresponding to the presence of narrowmicropores only. Moreover, in the case of P-900, the volume ofpores accessible to N2 molecules at 77 K is very low.

Figure 2. Pore size distribution for the baked and activated monolithmolded with novolac resin as a binder.

Table 2. Porosity and Density of Monoliths DeterminedUsing Mercury Porosimetry

monolithsymbol

Vtotcm3 g−1

V<50cm3 g−1

V>50cm3 g−1

dBg cm−3

dAg cm−3

AC/N 0.440 0.067 0.373 0.766 1.147AC/N-15 0.487 0.095 0.392 0.717 1.102AC/N-30 0.659 0.167 0.492 0.624 1.059AC/P 0.518 0.088 0.430 0.712 1.126AC/P-10 0.624 0.120 0.504 0.656 1.112AC/P-15 0.672 0.140 0.532 0.623 1.086

Table 3. Gravimetric and Volumetric Methane Uptake at 25°C and 3.5 MPa for Baked and Activated Monoliths

CH4 uptake

monolith mmol g−1 (V/V)A

AC/N 7.35 137AC/N-15 9.23 161AC/N-30 10.66 162AC/P 8.41 146AC/P-10 10.18 163AC/P-15 10.05 153

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The comparison of porosity parameters calculated fromisotherms of N2 adsorption at 77 K and CO2 adsorption at 273K (Table 4) shows that the heat-treatment of AC results insome reduction of different size pores including ultra-micropores. BET surface area decreases by about 300 m2 g−1.It is worth noticing that the mean size of micropores L0 (∼1.50nm) is larger than that measured for monolith (Table 1).Novolac derived char with its BET surface area of about 600 m2

g−1 located in narrow micropores (L0 = 0.66 nm) seems to havea valuable contribution to the total monolith porosity,conversely to PFA-derived char. Both binder derived charsare characterized by similar and well-developed ultramicro-porosity.The behavior of the monolith during activation seems to be

controlled mostly by the relative reactivity of activated carbonparticles and binder-derived char. To assess possible con-tributions from the gasification of the two phases to the burnoffof monolith as a whole, we measured the reactivity of individualconstituents and monoliths toward CO2 at 900 °C.Figure 4 shows kinetics of gasification plotted as the

relationship of the burnoff against reaction time. The moststriking observation from the study is extremely high reactivityof activated carbon particles, exceeding several times that ofbinder chars. P-derived char is slightly more reactive than theN-derived one. The gasification rates calculated from theexperiments (Table 5) show that at a comparable accessibilityof constituting phases the activated carbon consumption shouldoccur 16−19 times faster than that of char. As could beexpected, the gasification of monoliths occurs at anintermediate rate, and for AC/P it is faster than for AC/N.The respective gasification rates are about 7 and 11 timessmaller than the rate for AC. The difference can be caused, inpart, by a higher contribution of binder char in the lattermonolith.

The conditions of the reactivity test were rather mild. During4 h of treatment, the burnoff of polymer-derived chars was 5 wt% and that of monoliths was 8.5 and 12.5 wt %. In contrast, incase of AC, over 30 wt % burnoff was gained within 1.5 h. Thelinear weight loss with reaction time should be noted. Thebehavior is characteristic of slow reaction and limited burnoffrange.21 This proves that diffusion processes have no influenceon the gasification rate.We think that even at slight burnoff, the activation of binder

char can result in a noticeable increase in methane uptake. Ithas been reported22,23 that for some chars the pore mouthsbarriers inhibit the penetration of N2 at 77 K and CH4 atambient conditions to a large amount of microporosity. This isin contrast to Ar adsorption at 87 K. The mild activation withCO2 is believed to reduce the barriers and to open the initiallyinaccessible part of pores for CH4 storage.The enormous reactivity of the activated carbon can be

attributed to two features which are characteristic of KOH-activated carbons. First, there is a specific particle morhology, asshown on scanning electron micrographs of Figure 5. Stronglyexpanded, due to potassium intercalation, structure gives veryeasy access of CO2 molecules to highly defective graphenelayers. The second point is related to the presence of potassium

Figure 3. N2 adsorption isotherms at 77 K of as received (AC) andannealed at 900 °C (AC-900) activated carbon powder and chars (N-900 and P-900) derived from binders.

Table 4. Characteristics of Porous Texture for Activated Carbons and Binder Chars Determined from N2 Adsorption Isothermsat 77 K and CO2 Adsorption Isotherms at 273 K

sample SBET (m2 g−1) VT (cm3 g−1) VDR,N2 (cm3 g−1) L0,N2 (nm) VDR,CO2 (cm

3 g−1 S0,CO2 (m2 g−1)

AC 2163 1.02 0.80 1.49 0,19 588AC-900 1861 0.88 0.69 1.53 0.17 560N-900 595 0.25 0.24 0.66 0.30 806P-900 100 0.05 0.04 n.d. 0.28 786

Figure 4. Kinetics of gasification with CO2 at 900 °C for activatedcarbon (AC-900), binder derived chars (N-900 and P-900), andmonoliths (AC/N and AC/P).

Table 5. Gasification Rate Towards CO2 at 900 °C forActivated Carbon Particles, Binder Chars, and Monoliths

sample gasification rate [mg h−1 mg−1]

AC 0.3427N-900 0.0181P-900 0.0215AC/N 0.0319AC-P 0.0468

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atoms incorporated in the carbon network. The analysis byICP-AES showed 1.4 wt % of potassium in the activated carbon,which could not been removed from the material despiteexhaustive washing. It seems therefore a reasonable assumptionthat the perfectly dispersed metal can effectively catalyze thegasification reaction.The extremely high reactivity of activated carbon particles

toward carbon dioxide seems to be a main limitation in theenhancement of methane storage capacity by monolithactivation. The particles which are, or will become soon,accessible to CO2 molecules will be burnout very quicklyleaving relatively big, unsuitable for adsorption, voids.It seems that some contribution of catalytic gasification on

reaction of KOH-activated carbon with CO2 is unavoidable.Potassium is an efficient catalyst of carbon gasification by CO2,especially in that case of fine dispersion and metallic form.24

The preparation of potassium-free KOH-activated carbon isdoubtful. It seems, therefore, that a way to get more balancedburnoffs for both carbon phases constituting the monoliths isincreasing binder char reactivity. It has been demonstrated thatwell-dispersed sodium enhances about 16 times the gasificationrate of char derived from resole type resin compared to thatfrom novolac resin.13

4. CONCLUSIONS

Activation with carbon dioxide was successfully applied todevelop porosity in monolithic adsorbents built of KOH-activated carbon and polymer-derived binder. Novolac resin(N) seems to be a more suitable binder for monoliths thanpoly(furfuryl alcohol) (P) due to a higher carbonization yieldand more porous residue. The limit of burnoff, which isacceptable to preserve monolith integrity, amounts to 30 wt %for N but only 15 wt % for P. This means that the burnoffextent must be individually and carefully adjusted to themonolith composition to avoid “overactivation”.Activation clearly enhances the methane adsorption capacity

of monolith at 3.5 MPa. The highest uptake of 10.7 mmol g−1

was gained for a monolith molded with novolac resin as abinder and activated to the burnoff of 30 wt %. However, thevolumetric storage capacity reaches a maximum of 163 V/Valready at a moderate activation extent. The limitation is relatedto the decrease in bulk density due to excessive development ofmacropores. The reason for the drawback is extremely highreactivity of activated carbon particles toward CO2, severaltimes higher than that of binder char. In consequence, theparticles which become accessible to CO2 molecules are burnedout very quickly leaving empty space of macropore size.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: jacek.machnikowski@pwr.wroc.pl. Phone: +48713206350. Fax: +4871 3206506.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financed by a statutory activity subsidy from thePolish Ministry of Science and Higher Education for theFaculty of Chemistry of Wrocław University of Technology.

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Figure 5. Scanning electron micrographs of powdered, activated carbon AC.

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