Adsorption of Carbon Dioxide on Porous Catalyst Materials at Near-Critical Conditions

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1 491 Adsorption of Carbon Dioxide on Porous Catalyst Materials at Near-Critical Conditions Fabio Capezzuoli 1,, Frantisek Stepanek 1 and David Chadwick 1 (1) Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K. (Received 16 April 2012; revised form accepted 17 July 2012) ABSTRACT: Herein, the construction of a manometric adsorption apparatus has been described and an experimental method has been developed to measure the adsorption of carbon dioxide on porous materials at sub-critical pressures. The materials used are silica gel and silica loaded with 5 mol% ZnO; γ-alumina and alumina with 5 mol% ZnO. It was observed that the addition of zinc oxide causes a % increase of the adsorbed amount at 350 K. However, this manometric apparatus is only suitable for relatively large adsorbed amounts due to its sensitivity and precision limitations. 1. INTRODUCTION The solvent properties (Smith 1999), phase equilibria and chemical reactivity of dense and supercritical carbon dioxide are being studied intensively because of the potential of carbon dioxide as a green solvent for the extraction of disparate substances from different matrices (Beckman 2004) and as a feedstock for the production of useful chemicals (methanol, higher alcohols, etc.) (Baiker 2000). High-pressure reactors for the hydrogenation of CO/CO 2 mixtures coupled with underground gasification of coal and oil may provide an economically viable method for the exploitation of fossil fuel reserves that cannot be economically extracted using conventional methods. The investigation of the adsorption of carbon dioxide on porous materials is also of interest for the carbon capture and storage technologies (Friedmann 2007) being developed to reduce atmospheric emissions of CO 2 sequestrating it in suitable geological formations. However, the adsorption of CO 2 on dispersed metal catalysts and support oxides has received attention only at relatively low pressures despite the relevant practical importance of adsorption equilibria in a range of industrial processes (including purification, separation and catalysis). The adsorption equilibria on hydrotalcite of multi-component systems containing carbon dioxide and water have been studied in a fixed-bed system at technical conditions (Ding and Alpay 2000) but at low pressures. As a part of a larger programme investigating the hydrogenation of CO 2 at high pressures, we have measured the adsorption curves of this gas on a small number of relevant porous materials: high surface area silica gels and alumina commonly used as supports for the copper-based catalysts used in methanol synthesis and water-gas shift reactions. We have used the manometric method* because it requires a smaller investment than the gravimetric method, which uses a magnetic suspension microbalance to measure the adsorbed amount (Di Giovanni et al. 2001). Author to whom all correspondence should be addressed. fabcap2008@hotmail.it. *This method is also called volumetric.

2 492 F. Capezzuoli et al./adsorption Science & Technology Vol. 30 No The manometric adsorption system operates with the following principle (Roquerol et al. 1999): the first section of the rig (dosing volume) is loaded with a known amount of gas, which is then expanded at constant temperature into the second section (adsorption volume) where a known weight of adsorbent has been placed. The difference between the pressure the gas would have in the absence of adsorption and the actual measured pressure is proportional to the Gibbsian adsorption excess n exc (Sircar 1999). This relation is usually written in terms of the bulk fluidphase molar density, void volume of the system and total amount of fluid: n exc = (n tot V 0 ρ m )/m a (1) It is necessary to assume that ρ m is constant in the whole volume, and a proper equation of state (EOS) is used to estimate density from P, T data. Another assumption is that helium, used to measure V 0, is not adsorbed in any significant amount onto the material being examined (Menon 1968). The system we designed and built allows for the measurement of adsorption up to supercritical pressures and temperatures up to 393 K. 2. MATERIALS AND METHODS 2.1. Description of the Apparatus A number of practical difficulties are needed to be overcome in the construction of the instrument: the high pressures involved up to moderate temperatures, and the aggressive nature of supercritical CO 2, require the use of an all-stainless steel system and high performance polymers [polyether ether ketone (PEEK), Viton, polytetrafluoroethylene (PTFE)] for valve seats, seals and gaskets. All valves and fittings were purchased from Swagelok Company (Ohio, U.S.A.). The final system (Figure 1) has the form of one off-the-shelf pressure vessel (PT1) with an isolation valve (V1), and one custom-made smaller vessel (AV1) where the adsorbent will be placed within an appropriate basket. A needle valve (V2) allows to control the expansion of the gas and another isolation valve is placed downstream the adsorption vessel; two relief valves are RV1 190 bar Oven PT1 RV2 190 bar Filter V1 V2 AV1 V3 Vacuum vent CV PI TI TI CYL V11 To water cooler Insulated line Vent Figure 1. Schematic diagram of the adsorption apparatus.

3 Adsorption of carbon dioxide on porous catalyst materials at near-critical conditions 493 added for safety (RV1 and RV2). A rotary vane vacuum pump (BOC Edwards, U.K.) serves to evacuate the rig, and the main vessel is fitted with a coil for coolant circulation from an external chiller; CO 2 (BOC CP grade, %) is supplied from a liquid withdrawal cylinder and helium (for purging/testing) comes from a standard compressed gas bottle (BOC, CP grade). A slightly modified common laboratory oven with circulation fan (Sanyo-Gallenkamp) was used to control the temperature within 0.4-K reproducibility. The custom-made adsorption vessel (Figure 2) has an internal dimension of mm 2, a groove-type flanged lid and was designed with ample safety margin (1.5 times the maximum working pressure); machined from solid 316SS and finally tested at 240 bar (Stansted Fluid Power, U.K.). The seal is normally a rectangular section PTFE ring, but a nitrile rubber O-ring can also be used. A PEEK seal was considered but then discarded due to machining difficulties and satisfactory performance of PTFE. A cylindrical adsorbent basket that fits inside the adsorption vessel with small tolerance was also machined from stainless steel. Fluid-phase pressure is measured using a high-accuracy transducer with MPa range (Super TJE, RDP Electronics, U.K.) with ±0.05% full-scale uncertainty and a high-resolution digital indicator (RDP E725), and finally logged on a personal computer (PC). Temperature is measured with two thin stainless steel sheathed Type-T thermocouples (TC Inc., U.K.) having ±0.5 C accuracy, reaching inside the two vessels through appropriate fittings and connected to the same control PC using a datalogger and relative software (TC-08, Pico Technology Inc., U.K.). Finally, to improve the accuracy of the temperature measurement, the thermocouples and datalogger together were calibrated to give a reading in Kelvin using a highaccuracy platinum-resistance thermometer; the uncertainty on the reading was thus reduced to ±0.1% in the K range. Results of preliminary tests showed that temperature in the oven oscillates with amplitude of 0.3 K and a period of about 20 min and pressure closely follows the same temporal trend. At supercritical pressures even slight variations of temperature are enough to cause considerable changes in pressure, while below P c this effect is less pronounced. Figure 2. Section of the adsorption vessel (AV1 in Figure 1). This vessel has axial holes in the flange lid and bottom for threaded tube fittings, a 6-bolt flange and o-ring groove on the body.

4 494 F. Capezzuoli et al./adsorption Science & Technology Vol. 30 No Calibration and Commissioning Recalling equation (1), it is clear that the use of manometric adsorption methods requires a very accurate measure of the apparatus volume (Roquerol et al. 1999). In this case, the volume was measured as follows: a small, valve-equipped pressure bomb (Swagelok, 75 ml) is loaded with argon at approximately 60 bar and weighed on a balance with a ±0.001 g precision (METTLER- TOLEDO PR5003); the bomb is then connected directly to V1 and used to load the dosing volume (V d, between V1 and V2) of the adsorption apparatus. After the temperature in the system has reached the chosen value, pressure and temperature are recorded for 40 min, while the bomb is weighed again in order to calculate the amount of argon introduced in the rig. The volume of the fittings between the bomb and valve was calculated by allowing gas from the bomb into that space (with V1 closed), closing the bomb valve, venting and weighing the bomb a third time, while temperature and pressure were assumed to be the same as in the rig. With these data, the volume of the first section is calculated using the virial EOS (V-EOS) P= RT/V m RTB/V m 2 (2) with the following parameters (Reid et al. 2001) (the subscript c denotes critical constants): B = B 0 RT c /P c (3) B 0 = /T r 1.6 (4) This procedure gave a V d of ± 0.1 ml at 40 C; the measurement was repeated at 100 C to obtain the volume temperature relation: V d = T (5) The volume of the whole apparatus (V a ) was measured directly because it is subject to change with the amount of adsorbent used. Thus, at the beginning of each experimental run the ratio V a /V d is measured with low-pressure expansion of helium (assumed to be an ideal gas) at the operative temperature using equation (6): V a /V d = P i T f /P f T i (6) where the subscripts i and f indicate initial and final states respectively. V a is then used in place of V 0 in equation (1). In order to calculate the fluid-phase density at the experimental conditions, it is necessary to use an appropriate EOS. For this work, the Soave Reidlich Kwong (SRK) (Soave 1993) and Peng Robinson (PR) (Peng and Robinson 1976) cubic EOS were considered. The SRK EOS was discarded after a cursory examination for its insufficient accuracy, while the PR EOS was used to elaborate a first batch of experimental data. However, the performance of this EOS was also not satisfactory: the excess amounts calculated in this way were inconsistent particularly at low temperature for P r > 1. Finally, we chose the interactive database REFPROP 7.0 developed by NIST, which encodes the EOS for CO 2 proposed by Span and Wagner (SW EOS) (Span and Wagner 1996). A comparison of density values calculated with PR and SW EOS is shown in Figure 3: the sensible deviation of the PR values is clearly visible.

5 Adsorption of carbon dioxide on porous catalyst materials at near-critical conditions (ρ PR ρ SW )/ρ SW K 330 K 350 K 370 K 390 K Figure 3. Deviation of the density calculated using PR EOS from the SW EOS predictions Experimental Procedure For each adsorbent material, surface area, average pore diameter and Barret Joiner Halenda pore volume were determined using N2 Brunauer Emmett Teller method, and the results are summarized in Table 1. A typical adsorption experiment is run as follows: a batch of adsorbent ( g) is weighed accurately and placed inside the basket (eventually capped with quartz wool to avoid dispersion of fine powders) and dried and degassed in a vacuum oven at 150 C for 4 h. The basket is then quickly transferred into the adsorption vessel, and the vessel is sealed and connected to the adsorption system that is briefly flushed with helium to eliminate air and other impurities, then evacuated while heating to the operative temperature, and at this point the V a /V d ratio is measured. The system is evacuated again while cooling PT1 down to C to allow the vessel to be loaded with the estimated correct amount of liquid CO 2 ; subsequently, the temperature is increased to the operative value. AV1 is kept under vacuum while temperature and pressure reach equilibrium (a process that can take up to 1 h); however, it is possible to start acquiring temperature and pressure data: when working at P r > 1, 1200 data points are recorded over a 40-min period for pressure and temperature; at P r < 1, it is sufficient to record 600 data points over a 20-min period. Once the recording of the initial conditions is completed, AV1 is isolated by closing V3; V2 is then opened slowly (and of a fixed number of turns for repeatability) to let the carbon dioxide expand. Equilibration to the new conditions takes up to 25 min, and at this point it is possible to record the TABLE 1. Properties of the Adsorbent Materials Material BET surface Particle size BET avg. pore BJH ads. pore area (m 2 /g) (µm) diameter (nm) volume (cm 3 /g) Silica gel Type Silica gel EP mol% ZnO on silica gel mol% ZnO on silica gel a Alumina Norton mol% ZnO on alumina a Adsorption has not been measured for this material due to the low surface area.

6 496 F. Capezzuoli et al./adsorption Science & Technology Vol. 30 No final conditions with the same modalities described above. At the end of this process, V2 is closed again and V3 is cautiously opened to vent the CO 2 from AV1, which is then evacuated again to be ready for another adsorption run at lower pressure. Repeating this procedure, raw PT data are recorded for a wide range of pressures. The arithmetic average over all of the data points of temperature in PV1 and pressure are taken as initial conditions; for pressure, the instrumental error is generally smaller than the standard deviation of the dataset and for this reason the standard deviation (σ P ) is taken as the error of the measurement (not less than MPa). In the case of temperature, the error (δ T ) is taken as 0.1% of the measurement or the standard deviation of the dataset, whichever the greatest. The final temperature after expansion of the fluid is taken as the average of the PT1 and AV1 measurements. The estimation of the experimental errors on the adsorbed amount required careful evaluation. We defined the following relative errors: s T = δ T /T s P = σ P /P (7a) (7b) The accuracy of the SW EOS in the range of interest is less than or equal to ±0.05%, so we defined the relative error on the calculated (molar) density as follows: s ρ = s T + s P (8) Errors on the volumes are calculated with standard error propagation formulas, and the balance used to weigh the catalyst has a precision of ± g. With all these data, the error on the adsorbed amount has been calculated and will be displayed on the plots as error bars Materials The materials used for our experiments are silica gel Merck Type 60 (Aldrich), for comparison with literature data and used as purchased, silica Crosfield EP10 (Crosfield Organics) sieved to exclude the <106-µm particles and γ-alumina Norton 6175 (Norton/Saint-Gobain), ground and sieved to µm particle size. A series of silica-supported zinc oxide materials was prepared by wet impregnation of silica gel (Yoshida et al. 2003). Typically, 3 g of silica gel EP10 were suspended in 120 ml of aqueous solution containing the appropriate amount of zinc nitrate and boiled to dryness under stirring; the catalyst was dried in oven at 120 C for 12 h, and then calcined at 500 C for 5 h in a moderate flow of clean air and finally sieved again to eliminate the fines. One material had a zinc loading of 5 mol%. With the same method, a material with 5 mol% Zn on alumina powder was prepared; this material was sieved to separate the µm fraction before calcination. 3. RESULTS AND DISCUSSION Using the procedure and method described in Experimental Procedure, we have measured the adsorption isotherms of CO 2 on the materials listed below; their physical properties are summarized in Table 1 (BET, Brunauer Emmett Teller method; BJH, Barret Joiner Halenda method). Three isotherms were measured with silica gel Type 60 at 320, 353 and 373 K and compared with the data reported by Di Giovanni et al. (Di Giovanni et al. 2001) in order to establish the

7 Adsorption of carbon dioxide on porous catalyst materials at near-critical conditions 497 performance of the apparatus and procedure described here. Our isotherms break down at supercritical pressures, where unjustifiably high and even negative adsorbed amounts have been measured. This is probably due to condensation of carbon dioxide in the pressure transducer cavity, which is placed outside of the oven to prevent the zero point from drifting with temperature changes. Attempts to measure and subtract the background did not produce significant improvements. A typical blank run can be seen in Figure 4: there is considerable apparent adsorption above 7 MPa, consistent with localized condensation: this violates the required condition of constant density for equation (1) (n t n i ) (mol) Figure 4. Apparent CO 2 amount variation for a blank run at 352 K. At sub-critical pressures, the agreement between our curves and the ones described in literature becomes more satisfactory, but deviations up to 30% are still possible and our system appears to overestimate the adsorbed amount consistently: the accuracy of manometric systems at high pressure is limited by the large quantity of CO 2 in the void volume. However, it is not clear whether these deviations can be at least in part caused by a difference in surface area of different batches of the adsorbent. Using nm 2 as the cross-section of adsorbed carbon dioxide (Roquerol et al. 1999), we obtain mol/g as the monolayer capacity for silica gel Type 60: adsorption is multi-layer for low temperature and pressure in the high sub-critical range. Two isotherms, at 351 and 371 K, were measured on silica gel EP10 in the MPa range (Figure 5). For a material with this surface area, the CO 2 monolayer capacity is estimated at mol/g; the curves level out at approximately the same adsorbed amount of silica gel Type 60, according to a multi-layer adsorption model. According to Sircar (Sircar 1999), the isoexcess heat of adsorption (q) is calculated from q = RT 2 δ ln( P) δτ n exc (9) taking the 351 K isotherm as reference, q is estimated as a function of P (Table 2).

8 498 F. Capezzuoli et al./adsorption Science & Technology Vol. 30 No n exc (mol/g) EXP (this work) 320 K EXP 373 K DG 353 K EXP 320 K DG (DiGiovanni) 320 K DG 373 K Figure 5. Comparison between adsorption isotherms obtained with this work (EXP) and those by Di Giovanni (2001) (DG). TABLE 2. Heat of Adsorption on Silica Gel EP10 at 351 K n exc (mol/g) Q (kj/mol) The 350 K adsorption isotherm has also been measured for 5 mol% ZnO on silica (Figure 6) and compared with the results for silica gel EP10 in Figure 7. According to Yoshida et al. (Yoshida et al. 2003), a sensible decrease of surface area is observed only for zinc loading above 30%; we have observed this phenomenon (Table 1) for a EP K EP K n exc (mol/g) Figure 6. CO 2 Isotherms on Silica Gel EP10.

9 Adsorption of carbon dioxide on porous catalyst materials at near-critical conditions % ZnO on EP K 5% ZnO on EP K n exc (mol/g) Figure 7. Isotherms for 5 mol% ZnO on silica mol% loading. Even after considering the 1-K difference between the average temperatures of the two isotherms under comparison, it is evident that the adsorbed excess is sensibly higher for silica with 5 mol% zinc oxide, while carbon dioxide is only physisorbed on silica, and it is chemisorbed on ZnO with the formation of carbonate and carboxylate (CO 2 ) species (Freund and Roberts 1996). The isothermal adsorption curves of CO 2 for alumina Norton and 5 mol% ZnO on alumina follow a similar trend, illustrated in Figure 8. Also, in this case, the presence of zinc oxide produces a noticeable increase of the adsorbed surface excess. Applying the Langmuir model to 5 mol% zinc oxide on alumina, monolayer capacity of mol/g has been calculated. Adsorption on alumina without zinc oxide barely reaches complete coverage for the pressures studied. Only the isotherms at 350 K (Figure 9) have been constructed for these materials Silica gel EP10,351 K 5 mol% ZnO on silica, 350 K Monolayer n exc (mol/g) Figure 8. Isotherms for CO 2 on silica and silica-supported zinc oxide (at 350 K).

10 500 F. Capezzuoli et al./adsorption Science & Technology Vol. 30 No Alumina norton 5 mol% ZnO on alumina Monolayer n exc (mol/g) Figure 9. Isotherms for alumina Norton and 5 mol% ZnO on alumina at 350 K. 4. CONCLUSIONS From the data we collected during our experimental campaign, we conclude that our manometric adsorption apparatus is suitable for the measurement of carbon dioxide adsorption isotherms on high surface area porous materials at sub-critical pressures. Regrettably, around and above the critical pressure, the performance of the system degrades to the point that it cannot yield usable isotherms. Examining the isotherms we obtained, it appears that the adsorption of carbon dioxide on porous silica and alumina becomes multi-layer above a threshold pressure that increases with temperature and surface area of the material. When supported zinc oxide is used, the adsorbed amount increases sharply due to the chemisorption of CO 2 on zinc oxide with the formation of surface carbonate and carboxylate. The measured isotherms presented in this study are the first step towards the full characterization of catalytic processes in near-critical and supercritical CO 2, which is the ultimate objective of our work. NOMENCLATURE B Second virial coefficient, cm 3 /mol B 0 Simple second virial coefficient, cm 3 /mol m a Adsorbent mass, g n exc Gibbsian adsorption excess, mol g 1 n tot Total fluid amount, mol P Pressure, MPa P c Critical pressure, MPa P r Reduced pressure, P/P c, adimensional T Temperature, K T c Critical temperature, K T r Reduced temperature, T/T c, adimensional V 0 Void volume, m 3 V a Apparatus volume, m 3

11 Adsorption of carbon dioxide on porous catalyst materials at near-critical conditions 501 V d Volume of the dosing section, m 3 δ T Error on temperature, K ρ m Molar density, mol m 3 σ P Pressure standard deviation, MPa ACKNOWLEDGEMENTS This work is supported by EPSRC. The authors are grateful to the Chemical Engineering Workshop (Imperial College, London) for their assistance with construction of the experimental apparatus. We also thank G. Saville for his discussion; M. Trusler and F. Peleties for their assistance and the use of their facilities. REFERENCES Baiker, A. (2000) Appl. Organomet. Chem. 14, 751. Beckman, E.J. (2004) J. Supercrit. Fluids 28, 121. Di Giovanni, O., Dorfler, W., Mazzotti, M. (2001) Langmuir 17, Ding, Y. and Alpay, E. (2000) Chem. Eng. Sci. 55, Freund, H.J. and Roberts, M.W. (1996) Surf. Sci. Rep. 25, 225. Friedmann, S.J. (2007) Carbon Capture and Sequestration Technologies: Status and Future Deployment, Lawrence Livermore National Laboratory, Livermore. Menon, P.G. (1968) Chem. Rev. 68, 277. Peng, D. and Robinson, D.B. (1976) Ind. Eng. Chem. Fundam. 15, 59. Reid, R.C., Prausnitz, J.M. and Poling, B.E. (2001) The Properties of Gases and Liquids, McGraw-Hill, New York. Roquerol, F., Roquerol, J. and Sing, K. (1999) Adsorption by Powders and Porous Solids, 1st Ed., Academic Press, London. Sircar, S. (1999) Ind. Eng. Chem. Res. 38, Smith, R.M. (1999) J. Chromatogr. A 856, 83. Soave, G. (1993) Fluid Phase Equilibria 82, 345. Span, R. and Wagner, W. (1996) J. Phys. Chem. Ref. Data 25, Yoshida, H., Shimizu, T., Murata, C. (2003) J. Catal. 220, 226.