Characterizations of Activated Carbon–Methanol Adsorption Pair

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Characterizations of Activated Carbon−Methanol Adsorption Pair Including the Heat of Adsorptions Jun W. Wu,*,†,‡ S. Hadi Madani,§ Mark J. Biggs,∥,⊥ Pendleton Phillip,§,# Chen Lei,† and Eric J. Hu*,† †

School of Mechanical Engineering, The University of Adelaide, South Australia, 5005, Australia Shanghai DFYH Tech Services Co., Ltd, 1255 Xikang Rd., Putuo District, Shanghai 200060, P.R. China § Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia ∥ School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ⊥ School of Science, Loughborough University, Leicestershire LE11 3TU, U.K. # School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, South Australia 5005, Australia ‡

S Supporting Information *

ABSTRACT: This paper presents adsorption isotherms and isosteric heats of adsorption for methanol vapor adsorption for two commercially available activated carbon samples207EA granules and WS-480 pellets (Calgon Carbon, U.S.A.)which were also fully characterized using nitrogen sorption at 77 K. The heat of adsorption of methanol as a function of loading was determined using the Clausius−Clapeyron approach with isotherms obtained at 5 °C, 15 °C, and 25 °C. The isosteric heats of adsorption increased sharply at the small coverage due to increasing effect of condensation heat with coverage. The heat reached a maximum and then varies little with loading, with the average values at around 46.6 kJ/mol for 207EA and 45.1 kJ/mol for WS-480. The higher heats of adsorption for the former activated carbon reflect its more microporous nature (around 78 % compare to 62 % for WS-480 activated carbon). The heat of adsorption data is also comparable to that obtained elsewhere for other activated carbons.

1. INTRODUCTION Adsorption cooling/heat pump systems have attracted considerable attention1 over the past few decades due to their promise to reduce greenhouse gas emissions and Oz-layer depletion problems. The thermophysical properties of the adsorbent/adsorbate pair significantly affect the performance of adsorption-based cooling systems.1 This observation has motivated much effort in determining these properties for systems including: zeolite/water, silica gel/water, activated carbon/methanol, activated carbon/ethanol, and carbon/ ammonia pairs.1,2 Of these, activated carbon/methanol offers considerable promise;3,4 methanol has a high latent heat of evaporation promoting small size systems and avoids corrosion issues for steel and copper at working temperatures below 120 °C. This system also has the advantage that methanol has a freezing point below that of water and, thus, systems based on it can be used to make ice.1 Extensive experimental studies have already been made to determine the efficacy of adsorption cooling systems using different combinations of adsorbent/ adsorbate working pairs.5−15 Few have addressed adsorption isotherms of the activated carbon/methanol pair, and the heat of adsorption data are scarce.8,13,14 In this paper, we report multiple temperature methanol adsorption isotherms and their resulting isosteric heats of © 2015 American Chemical Society

adsorption for two different commercial activated carbons: a granular activated carbon, 207EA, and a pelleted activated carbon, WS480. Although these carbons are used primarily for water treatment and volatile organic compound capture, respectively,16 each is also readily available and relatively inexpensive. We propose to assess their appropriateness for adsorption-based systems. In addition to the methanol data, we have determined for both activated carbons their porosity, BET specific surface area, and pore size distributions. The methanol results have been correlated with these characteristics as well as compared with data published elsewhere for methanol adsorption on other carbons.17

2. MATERIALS AND METHOD The coal-based, activated carbon 207EA, a granular material of particle size 4 × 10 mm and WS480, a pelleted activated carbon (diameter 4 mm) were sieved prior to use to remove any crushed carbon. Both activated carbons were obtained from Calgon Carbon, U.S.A.16 Both carbons were degassed at 250 °C and 10−5 kPa for 4 h prior to the adsorption experiments. Received: December 8, 2014 Accepted: May 18, 2015 Published: May 22, 2015 1727

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Figure 1. (a) Nitrogen adsorption isotherm comparison (at −196 °C) of 207EA and WS-480 activated carbons and (b) Lower pressure nitrogen adsorption isotherm comparison (at −196 °C) of 207EA and WS-480 activated carbons on a P/P0 axis.

The specific surface areas and pore volumes were derived from multiple nitrogen (BOC Australia; 99.999 %) adsorption isotherms obtained at −196 °C using a Belsorp-max gas adsorption apparatus (Bel Japan, Japan). The Brunauer− Emmett−Teller (BET) specific surfaces areas were evaluated using a weighted-mean least-squares method.18 Ultrahigh purity helium (BOC Australia; 99.999 %) was used for dead-space evaluation. Each data point was recorded after the system reached the equilibrium criteria settings of < 0.1 % change in system pressure over a 300 s time period. The quenched solid density functional theory method (QSDFT) method14 was used to evaluate pore size distributions (PSD).The αs method19 was used to calculate the micropore volume of each sample, and the total pore volume was identified as the amount adsorbed at 0.99P0. Analysis grade methanol (> 99.9 %, EMSURE) was used as the adsorptive after being degassed using five consecutive freeze−evacuate (77 K, 10−5kPa)−thaw (295 K) cycles. The methanol adsorption experiments were carried out at 5 °C, 15 °C, and 25 °C in a Belsorp-max adsorption instrument (Bel Japan, Japan) equipped with a vapor adsorption kit. Adsorption temperature was controlled by a Neslab refrigerated bath circulator. The isosteric heat of adsorption was evaluated using the Clausius−Clapeyron equation.20

Figure 2. Pore size distributions of both 207EA and WS-480 activated carbons.

0.0042 for 207EA, respectively, consistent with uncertainties reported elsewhere18 (see Table 1 for the values). Repeated nitrogen adsorption isotherm data on both adsorbents are reported in the Supporting Information (Table S1). The repeated isotherms essentially overlap, but for clarity, only the averaged isotherm is shown in Figure 1 with uncertainties based on standard deviations. PSDs were also calculated via the QSDFT method applied to each isotherm, averaged and reported in the Supporting Information (Table S2). Although slight differences exist in the calculated PSDs, they essentially show the same contribution from micropores and mesopores, with the compared results regarded as equivalent. Again, for clarity, the averaged PSD and its uncertainty are included in Figure 2. 3.2. Methanol Adsorption. The experimentally obtained methanol adsorption isotherms on the two activated carbons at the three temperatures considered here are shown in Figure 3. Because the isotherms at consecutive temperatures are relatively close, it is important to ensure the difference between two consecutive isotherms is statistically larger than the uncertainty in the isotherm data for either isotherm. To address this issue, adsorption isotherms for both adsorptives were repeated at 25 °C and the repeated isotherms are shown in Figure 3. The repeated adsorption isotherms essentially overlap and the difference between them is less than the difference between the amounts adsorbed at different consecutive temperatures. The methanol adsorption isotherm

3. RESULTS AND DISCUSSIONS 3.1. Nitrogen Adsorption. Figure 1a shows the nitrogen adsorption isotherms of the 207EA and WS-480 activated carbons. Both are Type 1 in character21 and are dominated by microporosity, although the relatively soft knees suggest the pore sizes are widely distributed. The presence of adsorption hysteresis suggests some mesoporosity is also present in both materials. Figure 1b shows the 207EA carbon is more effective at taking up nitrogen at low pressures compared to WS-480, which is in line with the smaller micropore size of this material, as seen in the PSD in Figure 2. Table 1 shows the total pore volume of the WS-480 material is larger, due to contributions from supermicropores and mesopores. This table also shows the BET specific surface area of the EA carbon is in line with that obtained by others, although the pore volumes reported here are somewhat higher. To assess the repeatability and uncertainty of the porous structure characterization, each isotherm was repeated three times keeping all the experimental parameters the same. The relative uncertainty in the BET surface area, total pore volume, and micropore volume determined from the repetitions are 0.0012, 0.0070, and 0.0129 for WS-480 and 0.0021, 0.0200, and 1728

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Table 1. Characterization Summary for 207EA and WS-480 Activated Carbonsa activated carbon sample

BET surface area22

total pore volumeb

micropore volumeb

2

(m /g)

3

(cm /g)

(cm3/g)

1231.3 ± 1.5 951.0 ± 2.0 950

0.663 ± 0.005 0.464 ± 0.005 0.37

0.411 ± 0.005 0.362 ± 0.002 0.25

WS-480 207EA 207EA

micropore volume percentage

reference

61.96 ± 0.84 78.11 ± 1.19 67.8

this study this study 23

a

Average values and their standard deviations for BET surface area, total pore volume, and micropore volume calculated from each of the repeated isotherms. bThree decimal places for pore volumes because uncertainty only arises in the third decimal place.

with each determined simultaneously from the experimental data. The parameter values obtained from this fit are shown in Table 2 for both activated carbons along with values published Table 2. D−A Equation Parameters for 207EA and WS-480 Activated Carbons As Determined Here and Elsewhere in the Literature carbon sample

x0

D

n

R2

reference

1.6 × 10−5 8.45 × 10−7 2.8 × 10−5 9.08 × 10−5

1.655 2.08 1.650 1.78

0.9815 0.9865 0.9963 0.9942

this work 13 this work 13

(kg/kg) 207EA 207EA WS-480 WS-480

previously. Though clearly the capacities of the two carbons considered here are different, the variation in the value of D suggests their microstructure is also different. The similarity of the small value of the exponent, n, on the other hand suggests both carbons are highly activated as they have broad pore size distributions.25 Comparing the results obtained here with those of Zhao et al.13 reveals significant differences. The maximum adsorbate capacities (x0) obtained for the 207EA and WS-480 activated carbons in their work were a fraction of that obtained here (85 % and 55 %, respectively). Their lower adsorbate capacity may be attributed to insufficient degassing of the carbons prior to adsorption, insufficient degassing of the methanol prior to adsorption measurements, or differences in the activated carbon batches provided by the supplier. None of these was a problem in the current work. Many methods have been used to evaluate the isosteric heat from the Clausius−Clapeyron equation via analysis of multiple temperature adsorption isotherms.26,27 In these cases, the isotherms were fitted to a model and inverted and the relative pressure determined for constant amounts adsorbed for each relevant temperature. Because the data can also be represented as a matrix, we interpolated the relative pressure for fixed amounts adsorbed via a Matlab program. The isosteric heats were then evaluated via a weighted-mean least-squares analysis for each coverage set. These heats and their calculated uncertainties are reported as a function of fraction surface coverage in the Supporting Information (Table S4). The variation of the isosteric heat of adsorption with coverage for the two carbons is shown in Figure 4. The figure also compares these heats with the methanol latent heat of condensation. The (isosteric) heat of adsorption is the summation of fluid− fluid and fluid−solid interactions, where the latter can be further divided into nonpolar and polar interactions. Adsorptives that tend not to interact with one another at low surface coverage would present negligible fluid−fluid interaction heat contributions to the overall isosteric heat of adsorption. The presence of polar surface sites would result in

Figure 3. Experimental methanol adsorption isotherms (points) with D−A equation fittings (lines) at T = 5 °C, T = 15 °C, and T = 25 °C for (a) 207EA activated carbon and (b) WS-480 activated carbon.

data, including the repeated isotherms, are reported in the Supporting Information (Table S3). The maximum adsorbate capacity of the 207EA carbon was estimated to be 330 mg/g, which is substantially less than that of the WS-480 carbon at 490 mg/g. Inspection of the PSDs in Figure 2 and the pore volume differences in Table 1 suggests that these lower values are due to the smaller levels of mesoporosity in the 207EA activated carbon. Because previous heat/cool cycle adsorption data have been fitted to the Toth isotherm model, we compared it with the Dubinin-Astakhov isotherm model. The latter was slightly superior for both carbons: R2 = 0.9815 and 0.9963 for 207EA and WS-480, respectively, compared to R2 = 0.9809 and 0.9896 for the Toth equation. The form of the Dubinin-Astakhov (D− A) equation,24 (eq 1) used here was ⎛ ⎛ P ⎞n ⎞ x = x0exp⎜ −D⎜T ln s ⎟ ⎟ ⎝ ⎝ P⎠ ⎠

0.33 0.28 0.49 0.27

(1)

where x0 is the adsorption capacity of the adsorbent, D is a constant determined by the adsorbent microstructure, and n is the characteristic parameter of the adsorbent−adsorbate pair, 1729

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ASSOCIATED CONTENT

S Supporting Information *

Repeated adsorption isotherm data for WS-480 and 207EA carbons, calculated PSDs for WS-480 and 207EA carbons based on repeated adsorption isotherm data and QSDFT model, methanol adsorption isotherm data on WS-480 and 207EA carbons including repetition of the isotherm at 25 °C, and isosteric heat of adsorption and its uncertainty for adsorption of methanol on WS-480 and 207EA activated carbons. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/je501113y.

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Figure 4. Isosteric heat of adsorption variation with methanol loading for the 207EA and WS-480 activated carbons. The heat of methanol condensation at the mean temperature (15 °C) is shown as a broken line for reference.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +61 8313 2235, Fax: +61 8303 4367. *E-mail: [email protected]. Tel.: +61 8313 0545. Fax: +61 8303 4367.

relatively large fluid-surface interaction heats at low surface coverage; these would decrease sharply with increasing coverage. Thus, isosteric heat at low coverage is a combination of competitive effects due to decreasing fluid−polar-site interactions and increasing fluid−fluid (adsorbed phase) interactions. The isosteric heat curves in Figure 4 show an increasing heat at low surface coverage suggesting dominant effects of fluid−fluid interaction heat compared with polarsurface-site−fluid interactions. Isosteric adsorption heat at relatively higher surface coverage would be (long-range or multiple-layer) fluid−solid interactions superimposed on condensation enthalpy, the latent heat of condensation. Differences in value between these illustrate the influence of long-range fluid−solid interactions, or possible effects due to porosity. These differences would be larger for adsorption by micropores compared with mesopores, but the latter often also affects the adsorption process. Sample 207EA exhibits a narrower and smaller primary pore range compared to WS-480, clarifying why it has a nominally but continuingly higher isosteric heat with coverage than the latter. Here, 78 % of the pore volume in 207EA is micropores compared with 62 % for WS-480. Similar observations have been reported for empirical and modeling studies.28−30

Funding

This work was supported by an Australian Research Council (ARC) and Orlando Wines Pty Ltd., Linkage Grant under the project no. LP120200352. The authors also thank the A u s t r a l i a n Re s e a r c h C o un c i l d i s c o v e r y p r o g r a m (DP110101293) for financial supports. We would also like to acknowledge Dr. Alexander Badalyan (from the University of Adelaide) for his assistance with the adsorption experiments. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS The nitrogen adsorption isotherms on 207EA and WS-480 activated carbons were measured (at −196 °C) and analyzed as Type I isotherms indicating microporosity. Sample WS-480 exhibited moderately more mesoporosity. Two repeats of the nitrogen isotherm measurements demonstrated homogeneity within the samples provided. The overall pore volume of WS480 exceeded that of 207EA by 42 %. Methanol adsorption isotherms were measured at 5, 15, and 25 °C exhibiting an Sshape suggesting possible specific adsorption followed by condensation adsorption. Repeats of the 25 °C methanol isotherms showed the uncertainty in the amounts adsorbed were considerably less than the difference between them and the 15 °C isotherms. Subtle differences were found in the methanol isosteric heat data but the primary difference was in the amounts adsorbed, where WS-480 greatly exceeded 207EA at higher relative pressures. The similar enthalpy of adsorption but greater capacity shown by WS-480 implies it would be a superior material for an activated carbon/methanol cool/heat pump system than 207EA; it would provide greater cooling per thermodynamic cycle.

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NOMENCLATURE D = constant in D−A equation n = characteristic parameter in D−A equation P = pressure (Pa) P0 = pressure (Pa) Qst = isosteric heat of adsorption (kJ·kg−1) T = temperature (K) V = volume (cm3) w = pore width (Å) X = fraction of amount adsorbate adsorbed by the adsorbent at equilibrium condition (kg·kg−1 dry adsorbent) x0 = adsorption capacity (g·g−1) x = amount adsorbed (g·g−1) REFERENCES

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