Article pubs.acs.org/EF
Adsorption Capacity Enhancement by Activation with CO2 of Monolithic Adsorbents Made of KOH-Activated Carbon and PolymerDerived Binder Jacek Machnikowski,* Krzysztof Kierzek, and Kamila Torchała Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wroclaw University of Technology, Gdańska 7/9, 50-344 Wrocław, Poland ABSTRACT: Disc-shaped monoliths molded from KOH-activated carbon powder (AC) and novolac resin (N) or poly(furfuryl alhohol) (P) binder were activated with CO2 to open an access to the microporosity that was blocked by the binder char. The variation in the porous texture of monoliths was characterized by the N2 adsorption at 77 K and mercury porosimetry. The results show that using N allows the monoliths to be activated to a higher burnoff compared to P (30 vs 15 wt %) without noticeable loss of the integrity and mechanical strength. The increase in the surface area and micropore volume on activation is accompanied by a considerable development of meso- and macropores and a decrease in bulk density of monoliths. The behavior has been discussed as an effect of excessive reactivity to carbon dioxide of KOH-activated carbon compared to binder derived char. The preferential burnoff of accessible activated carbon particles limits the enhancement of volumetric storage capacity with activation progress of monolith. The maximum methane uptake of 10.7 mmol g−1 at 25 °C and 3.5 MPa was measured for the activated monolith made using novolac binder.
1. INTRODUCTION Powder nature and low packing density are serious drawbacks for gas-phase application of microporous high surface area activated carbons produced using activation with alkaline hydroxide. Tight packing of fine particles is crucial in cases of adsorbents dedicated to natural gas storage in vehicular applications,1−6 but also to cooling systems7 and helium compressors.8 One of the approaches which has been applied to prepare adsorbent of reduced interparticle voids is pressing the activated carbon powder with a suitable polymeric binder. An unavoidable side effect of using binder is a considerable loss of microporosity compared to that which is accessible in the original powder. The loss is attributed primarily to blocking of pore mouths in activated carbon particles, but the presence of poorly porous binder in the system cannot be neglected. The selection of suitable binder to ensure sufficient mechanical properties of the monolith while minimizing both the binder proportion and pore blockage is therefore of great importance to adsorbent performance. The monolithic adsorbents reported in the literature consist of activated carbon particles which are bound together by a resin4,5,9−11 or, when baking is applied to molded pellets, by the resin-derived coke/char.1,5,10 The second approach seems to have some advantages, including a better resistance to various environmental conditions, improved thermal conductivity, and, possibly, enhanced methane adsorption due to better affinity of adsorbate to more hydrophobic adsorbent. However, the number of suitable binders is restricted to those which give a considerable residue yield on carbonization, like phenolic resin,1 poly(furfuryl alcohol),2 cellulose derivative, and a proprietary binder.5 In the former work12 we have demonstrated that the porosity of baked monolith made of KOH-activated carbon and resin derived binder could be enhanced to a certain extent by a © 2012 American Chemical Society
moderate activation with CO2. A consequence was an increase in volumetric CH4 storage capacity despite decreased monolith bulk density. It has been anticipated that the primary effect of activation is opening the access to the porosity which was blocked during monolith pressing. However, the baked monolith is a complex system of activated carbon particles and resin-derived char of varied accessibility and reactivity toward CO2 of both phases. Binder properties, including wettability of active component and char yield and reactivity, are therefore, of great importance not only for monolith strength and the extent of pore blockage but also for its behavior during activation. Hence, the present work is focused on understanding the potential and limitations in the porosity enhancement during activation of composite adsorbent with carbon dioxide. The monoliths studied consist of KOH-activated carbon and phenolic resin or poly(furfuryl alcohol) derived chars. The porous texture and reactivity toward CO2 of constituting phases, i,e, activated carbon particles and resin-derived char are assessed to anticipate their contribution to porosity generation in the monolith as a whole.
2. EXPERIMENTAL SECTION 2.1. Materials. The laboratory sample of activated carbon (AC) was made by activation of pitch semicoke, which was prepared from commercial coal-tar pitch by heat-treatment at 520 °C, with potassium hydroxide. The physical mixture of KOH powder and the semicoke, particle size 100−630 μm, at the weight ratio of 3:1 was heat-treated at 750 °C for 1 h using a muffle furnace equipped with a horizontal nickel retort. Details of the procedure has been given elsewhere.12 Received: January 2, 2012 Revised: May 6, 2012 Published: May 7, 2012 3697
dx.doi.org/10.1021/ef300008y | Energy Fuels 2012, 26, 3697−3702
Energy & Fuels
Article
Novolac type phenol-formaldehyde resin with 7 wt % of hexamethylenetetramine added as a hardener, N, was provided by the “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 molded from 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 a horizontal tube furnace under nitrogen. The activation of the baked monoliths, AC/N and AC/P, with carbon dioxide was performed at 850 °C in a horizontal tube furnace. Activation time was fixed experimentally so to get burnoff required in the range of 10−30 wt %. For the activated monoliths, the number giving a nominal burnoff is added. 2.3. Evaluation of Reactivity toward CO2. To assess the possible contributions of individual constituents to the weight loss on monolith activation, the activated carbon, binder chars (particle size 0.6 nm) and mesopores and CO 2 adsorption at 273 K (NOVA 2200, Quantachrome) in the narrow micropores (
