Immersion Calorimetry: Molecular Packing Effects in Micropores

Abstract

Repeated and controlled immersion calorimetry experiments were performed to determine the specific surface area and pore-size distribution (PSD) of a well-characterized, microporous poly(furfuryl alcohol)-based activated carbon. The PSD derived from nitrogen gas adsorption indicated a narrow distribution centered at 0.57±0.05 nm. Immersion into liquids of increasing molecular sizes ranging from 0.33 nm (dichloromethane) to 0.70 nm (α-pinene) showed a decreasing enthalpy of immersion at a critical probe size (0.43-0.48 nm), followed by an increase at 0.48-0.56 nm, and a second decrease at 0.56-0.60 nm. This maximum has not been reported previously. After consideration of possible reasons for this new observation, it is concluded that the effect arises from molecular packing inside the micropores, interpreted in terms of 2D packing. The immersion enthalpy PSD was consistent with that from quenched solid density functional theory (QSDFT) analysis of the nitrogen adsorption isotherm.

Decoding gas-solid interaction effects on adsorption isotherm shape: II. Polar adsorptives

“…The PSD calculated from a QSDFT application confirmed this supposition, indicating a narrow, primary distribution of pores centred at 0.57 ± 0.05 nm, also consistent with previous studies based on repeated isotherm measurements [22] and on model-independent methods such as calorimetry and isosteric heat analyses [20,34,35].…”

“…derives from the proximity of the mean pore size to the adsorptive's kinetic diameter. These effects were proposed by Brunauer et al [36], supported by Rege and Yang [37], and more recently confirmed by immersion calorimetry studies of this PFA-based carbon (mean pore width = 0.57 ± 0.05 nm) into MeOH and iso-PrOH [20]. These enthalpy measurements suggested there was a negligible influence of packing effect during adsorption filling for MeOH since its kinetic diameter (0.43 nm) was considerably smaller than the mean pore size.…”

“…Interestingly, only DCM and pyridine produced pore filling to an extent statistically coincident with the accepted Gurvitsch volume, and thus exhibited independence of the criterion classified as molecular sieve exclusion effects, criterion 1. Previous immersion calorimetry analyses showed no molecular packing effects for probes with similar dimensions, including DCM [20], implying that these adsorptives were also independent of the criterion defined above as The kinetic diameters for iso-PrOH and 2M2B are close to the mean pore width of 0.57 nm, suggesting one would expect some molecular sieve exclusion effects to prevail. The Gurvitsch volume for iso-PrOH was marginally lower than the accepted pore volume whereas that for 2M2B…”

Using in-situ adsorption dilatometry for assessment of micropore size distribution in monolithic carbons

“…activated carbons, activated carbon fibers, carbon molecular sieves, carbon xerogels/aerogels and coals) is one of the fundamental steps in understanding and improving their respective performance in technical applications including separations, sequestrations, gas storage, supercapacitors and batteries, adsorption/desorption heat pumps, catalysis, and many others [1][2][3][4][5][6][7]. Various experimental and theoretical studies conducted on this subject concluded that disordered carbonaceous materials could be described as nano-scale pores of various sizes embedded into a disordered carbon matrix [8][9][10][11][12][13][14][15][16]. For most carbonaceous materials the majority of relevant pore volume is related to micropores, i.e.…”

Molecular simulation aided nanoporous carbon design for highly efficient low-concentrated formaldehyde capture

“…Yet, the previous studies cannot draw solid conclusions about the pore size and surface chemistry that maximize the capture of formaldehyde. It is not surprising because carbonaceous materials are both structurally and energetically heterogeneous [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. The intrinsic distribution of pore sizes and diversity of various surface functional groups hamper our ability to understand microscopic details of the mechanism of dynamic formaldehyde adsorption at ~ppm concentrations.…”