Nanoconfined Amine-Functionalized Silicone Oil Sorbents for Rapid

Article pubs.acs.org/IECR

Nanoconfined Amine-Functionalized Silicone Oil Sorbents for Rapid CO2‑Capture David W. Palm, Robert M. Enick, and Götz Veser* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ABSTRACT: A novel CO2 sorbent system involving the confinement of aminated silicone oil-based liquid sorbents in hollow, microporous silica shells was investigated. In TGA experiments with three silicone oil sorbents (GAP-0, M′D′M′, and M3′T′), the nanoconfined materials exhibited initial CO2 sorption rates between 44 and 90 times faster than the corresponding unconfined, bulk liquid silicone oils. However, the liquids proved to be volatile at temperatures as low as 45 °C in the gaseous flow environment, resulting in a continuous decrease in CO2 sorption capacities over multiple sorption/desorption cycles. In contrast to their volatility in their CO2-free state, the nanoconfined sorbents exhibited excellent stability upon sorption of CO2, suggesting that the unique CO2 sorption-induced liquid-to-solid phase transition that characterizes these sorbents in their unconfined bulk form is not significantly altered through nanoconfinement. Overall, while the stability of these nanoconfined amine-functionalized silicone oils suffers from their high volatility at flue gas temperatures, the observed drastic acceleration of the CO2 sorption kinetics with very little energy penalty for the overall composite sorbent suggests that nanoconfinement is a flexible way to accelerate the sorption kinetics of liquid CO2 sorbents.

1. INTRODUCTION The ever increasing concerns about climate change, recently fueled by yet another report quantifying the ecological, economic, and quality-of-life impact predicted by current climate models, make the reduction of anthropogenic emissions of “greenhouse gases” a key concern of our times. In recent years, 57% of global greenhouse gas emissions have been due to the release of carbon dioxide from fossil fuel use.1 At the same time, however, the worldwide dependence on fossil fuels to deliver the vast majority of our energy needs makes a sufficiently rapid transition to carbon-free energy sources unlikely and economically prohibitive. Hence, cleanup of our fossil fuel emission streams via CO2 capture and sequestration (CCS) continues to receive attention as a potential solution for mitigating climate change over the near to medium future. Within CCS technologies, the separation of CO2 from postcombustion gas streams accounts for approximately three-fourths of the cost of the CCS sequence,2 thus motivating continued efforts to advance technologies for capturing CO2. Among carbon capture technologies, the most widely studied and most mature method for CO2 capture is absorption by aqueous amine solutions such as monoethanolamine (MEA). While this technology is rather effective in chemically absorbing CO2, its application is hampered by two main disadvantages. The first is the corrosiveness of the material, which leads to the requirement of expensive scrubber construction materials. The second, even more important drawback is the high viscosity of the sorbent material which, combined with the corrosiveness, requires the use of MEA in a very dilute form. However, the use of an inert solvent, typically water, carries a very large energy penalty as it not only constitutes a large thermal mass that must be heated during the temperature swing required for regeneration of the sorbent (i.e., CO2 desorption) but also results in significant evaporation of water, adding the very large latent heat of evaporation to the before mentioned sensible heat.3 Finally, increasing scarcity of water supply strongly © 2014 American Chemical Society

motivates solutions to our energy crisis that do not put an additional burden on our water consumption (the so-called “energy-water nexus”). Carbon capture efforts that avoid the use of a bulk solvent have largely focused upon solid adsorbents such as zeolites and activated carbon. These materials are effective CO2 adsorbents that utilize the large number of active sites made available by the extensive surface area of these microporous materials. However, these materials suffer from the disadvantage of poor CO2 selectivity over other components of flue gas (N2 and H2O), since these methods are based upon nonselective physical, rather than chemical, adsorption, thus reducing their effective capture capacity, resulting in energy penalties for desorption of these undesired components and potentially even diluting the captured CO2 stream.4 Recent research thrusts in CO2 capture have hence sought to combine the chemical selectivity of amine-functionalized absorbents with the stability and increased active surface area of nanoscale solid supports. Approaches have included impregnating mesoporous silica particles with polyethylenimine (PEI),5 covalently anchoring PEI onto mesoporous silica,6 and loading PEI and tetraethylenepentamine (TEPA) into hollow mesoporous silica capsules.3,5a These hybrid organic−inorganic liquid−solid nanosorbent systems have demonstrated high CO2 sorption capacities, along with stability across multiple sorption and regeneration cycles and thus constitute a promising research direction. For PEI-impregnated mesoporous silicas in particular, a series of systematic studies in particular by Sayari and co-workers have demonstrated the importance of the pore length of silicas7 as well as the dispersion of the PEI sorbent,8 Received: Revised: Accepted: Published: 16476

May 28, 2014 September 22, 2014 September 30, 2014 September 30, 2014 dx.doi.org/10.1021/ie502175w | Ind. Eng. Chem. Res. 2014, 53, 16476−16484

Industrial & Engineering Chemistry Research

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application, CO2 uptake has been measured via material balance on the CO2 and aminosilicone, and the results have been expressed as the percentage of the maximum stoichiometric uptake of CO2 by the particular aminosilicone (e.g., 1 molecule of CO 2: 1 molecule GAP-0). Of the three aminosilicones used in this study, GAP-0 has been proven to be the most effective aminosilicone for quickly generating a dry, easily recoverable powder. All three aminosilicones have achieved CO2 uptake of 98−100% in lab-scale spray towers using gas-atomized nozzles and hydraulic nozzles.10b The same study reported GAP-0 carbamate desorption curves, providing an indication of the temperature required for the regeneration of the GAP-0 carbamate solid powder in a high pressure desorption chamber, as well as the vapor pressure of aminosilicones at temperatures between 75−180 °C, demonstrating that all of the aminosilicones are significantly less volatile than monoethanol amine in this temperature range.10b In the present work, these liquid absorbents (in the CO2-free state) are loaded into hollow, microporous silica shells, socalled “nanobubbles”, allowing us to investigate the effect of nanoconfinement on CO2-sorption capacity and rate and probe whether this confinement influences the substances’ phasechange behavior.

and thus the crucial role of mass transport within the hybrid sorbent materials, which was further corroborated most recently through transient IR studies by Wilfong and Chuang.9 However, to-date, these approaches have been virtually exclusively applied to the above-mentioned organic amines (PEI and TEPA). The present study seeks to explore the applicability of the hybrid amine-functionalized nanoparticle approach for CO2 capture beyond the use of oligomeric amines like PEI and TEPA. Here, we employ a recently discovered new class of amine-based CO 2 sorbents: the amino-silicone oil CO2 sorbents (designated by GE Global Research as GAP-0, M′D′M′, and M′3T′; Figure 1), which are liquid at room

2. EXPERIMENTAL SECTION 2.1. Silica Nanobubble Synthesis. Hollow silica core− shell materials (“silica nanobubbles”) were synthesized in a simple one-pot reverse microemulsion-template approach developed previously in our lab. In a typical synthesis, an aqueous nickel nitrate solution (1.5 mL, 1.0 M, Aldrich) is added to a Brij-58 (10 g, ≥99% Aldrich)/cyclohexane (50 mL, ≥99% Aldrich) mixture and reduced via addition of hydrazine hydrate (1.5 mL, Aldrich) at 50 °C. After addition of ammonia (3 mL) and 10 g of tetraethyl orthosilicate (TEOS, ≥99% Aldrich), the mixture is hydrolyzed at 323 K for 2 h. The resulting particles are precipitated with 2-propanol, collected via centrifugation, dried in air, and finally calcined in the tube furnace at 500 °C in air for 2 h. The Ni salts aid in stabilizing the micelle-templated cavity within the nascent porous silica shell, resulting in a material composed of ∼10 nm thick porous silica walls surrounding a ∼8−10 nm wide cavity (see Figure 2A). As a result, the central cavity is lined with a layer of Ni nanoclusters, which are bystanders for the present application (i.e., they do not play a functional role in the present sorption process).

Figure 1. Chemical structures for (A) GAP-0, (B) M′D′M′, and (C) M′3T′.

temperature but exhibit a unique phase-transition to a solid carbamate phase upon absorption of carbon dioxide.10 Each of these compounds forms a waxy or dry solid upon exposure to CO2. Therefore, they are not suitable for conventional gas− liquid absorption processes, and their uptake of CO2 cannot be expressed in terms of isothermal gas−liquid bubble point curves. Rather, earlier investigations have proposed exposing a fine spray mist of these liquid droplets to a cocurrent gas-phase of dilute CO2, which results in the formation of a powder when the CO2 and aminosilicone react. In this “spray tower”

Figure 2. (A) TEM of hollow microporous silica “nanobubbles” showing pronounced central cavity lined with Ni clusters (black). (B) Pore size distribution for this material. While the cavity is ∼10 nm in diameter, the large peak around 20−30 nm is due to interstitial porosity between aggregated silica “nanobubbles”. 16477

dx.doi.org/10.1021/ie502175w | Ind. Eng. Chem. Res. 2014, 53, 16476−16484


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in pure He (30 mL/min). For a given experiment, uptake and desorption were run isothermally within a temperature range of 45−90 °C. The balance purge gas for all experiments was Ar (60 mL/min).

Nitrogen sorption measurements were performed at liquid nitrogen temperature (77 K; Micromeritics ASAP 2020). Samples are degassed at 200 °C for 12 h under high vacuum prior to each test. The surface area was determined from the adsorption branch of the isotherm according to the BrunauerEmmett-Teller (BET) method in the relative pressure range 0.1 < P/P0 < 0.35. The pore size distribution (Figure 2B) shows three distinct maxima: a peak in the micropore range (d ≤ 1 nm) represents the porosity in the silica shells, while the peaks at 8−10 nm and 20−30 nm represent the central cavity and interstitial spaces between the aggregated silica nanobubbles, respectively. 2.2. Silicone Oil Sorbent Loading. For loading the nanobubbles with the sorbents, any one of the three aminosilicone oil sorbents [i.e., 1,3-bis(3-aminopropyl)-1,1,3,3tetramethyldsiloxane (GAP-0), 1,3,5-tris(3-aminopropyl)1,1,3,5,5-pentamethyltrisiloxane (M′D′M′), or tris(3-aminopropyldimethylsiloxy)-3-aminopropylsilane (M3′T′)] was impregnated into the above-described materials via a simple wet impregnation procedure adapted from Xu et al.4 In a typical preparation, 50 mg of sorbent was mixed with 250 mg MeOH and stirred until completely uniform. Next, 50 mg of the hollow porous Ni@SiO2 “nanobubble” material was added, the mixture was sonicated and then dried overnight at 75 °C. The resulting dry gray-white powder material was then crushed into a fine powder form. This 1:1 (by mass) sorbent-to-particles loading was calculated to yield complete filling of the nanobubbles based upon the cumulative pore volume of the nanobubbles and the liquid density of the respective sorbent. Successful loading of the nanobubbles was confirmed by the observation that the final mass equaled the combined mass of the sorbent and silica material (i.e., complete take-up of the liquid sorbent) and that a completely dry solid was formed (i.e., no external viscous liquid was detectable). A calculation based on the density (0.901 g/mL) and molecular weight (248.51 g/mol) of GAP-0 estimated that around 1100 molecules of GAP-0 reside in each central nanobubble cavity at maximum capacity; similar values are expected for the other sorbents, although exact density data was unavailable for M′D′M′ and M′3T′. 2.3. TGA Measurements. CO2 sorption/desorption kinetics and capacities were measured via thermogravimetric analysis (TGA) using a PerkinElmer TGA-7, in which ∼10 mg of sample was suspended in a platinum pan (5 mm diameter) from a balance wire. For bulk liquid experiments, the liquid aminosilicone oils were loaded into the center of the pan using disposable transfer pipettes; they formed a film about 0.5 mm thick with 0.2 cm2 exposed surface area in the pan, which was transformed into a solid film during the experiment. For nanoconfined aminosilicones, the dry powder was placed in the pan. As each silica “nanobubble” contains a droplet of aminosilicone oil with ∼10 nm diameter, the exposed surface area of silicone oil can be roughly estimated to be 105 times increased compared to the liquid film (neglecting the effect of the shielding of a significant part of the aminosilicone surface by the porous silica shell, which is impossible to estimate reliably based on experimental data). The mass of the sample and temperature of the chamber right above the sample pan are tracked over the duration of the experiment. In a typical experimental run, initial activation of the material (removing moisture and ambiently absorbed CO2) was carried out by heating the sample to the desired temperature for 10 min in He (30 mL/min). CO2 uptake was then measured in a high-purity CO2 flow (30 mL/min, PCO2 = 1 bar), followed by desorption

3. RESULTS AND DISCUSSION The effect of nanoconfinement on the kinetics of CO2 uptake by the three silicone oil sorbents was investigated using TGA experiments to monitor the mass gain associated with CO2 uptake from a pure CO2 gas stream over time. From these experiments, uptake kinetics, uptake capacity, and cyclic stability were determined. 3.1. Uptake Kinetics. For each silicone oil, the CO2 uptake kinetics of the free (i.e., unconfined) liquid sorbent were determined first, and then the uptake kinetics of the same silicone oil after confinement in the hollow mesoporous silica nanobubbles were measured. The results are summarized in Figures 3, 4, and 5, where the kinetic uptake curves for the free

Figure 3. Comparison of CO2 uptake kinetics for the nanoconfined GAP-0 and the unconfined liquid GAP-0 measured via TGA at 75 °C under pure CO2 flow.

Figure 4. Comparison of CO2 uptake kinetics for the nanoconfined M′D′M′ and the unconfined liquid M′D′M′ measured via TGA at 75 °C under pure CO2 flow.

liquid and nanoconfined material are shown in direct comparison for GAP-0, M′D′M′, and M′3T′, respectively. In each of these figures, the cumulative CO2 uptake (in mol CO2/ mol sorbent) for the material is plotted against the uptake time (in minutes). Clearly, a drastic acceleration of the uptake kinetics can be observed for all three materials. In each graph, the nanoconfined silicone oil exhibits rapid CO2 uptake, reaching its final uptake capacity in ∼1 min; in contrast to that, 16478



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the maximum sorption capacities for the various nanoconfined silicone oils (i.e., the point at which the uptake curve ceased to increase with continued CO2 flow). This molar uptake capacity (in mol CO2/mol silicone oil) is most useful in comparing the actual uptake of these amine-functionalized materials to their theoretical capacities. Since two primary amine groups are required to bind one CO2 molecule (eq 1), the number of moles of CO2 that each of these sorbents can capture ought to be proportional to the number of primary amine groups present in the sorbent molecule. The molecular structures of the three amino-silicone oil sorbents under investigation show that GAP0 possesses two primary amine groups, and that M′D′M′ and M′3 T′ possess three and four primary amine groups, respectively. Thus, we would expect M′3T′ to exhibit the greatest molar uptake capacity (theoretically two moles CO2 per mole of M′3T′) and GAP-0 the smallest (one mole CO2 per mole GAP-0). As Table 2 shows, this is indeed the case,

Figure 5. Comparison of CO2 uptake kinetics for the nanoconfined M′3T′ and the unconfined liquid M′3T′ measured via TGA at 75 °C under pure CO2 flow.

the uptake curves for the free liquid silicone oils show a much more gradual mass increase, which asymptotically approachs the final capacity at timescales well beyond the duration of the experiments. This effect is quantified in Table 1, where the initial rate of uptake (both on a molar and a mass base) has been calculated using the time required for each system to achieve a cumulative uptake of 0.6 mol CO2/mol sorbent. This metric was chosen for comparison across all three sorbent systems in both their nanoconfined and their free form because it represented a capacity that it achieved by all sorbents within the relatively rapid initial uptake phase (i.e., avoiding the very slow asymptotic uptake phase of the unconfined sorbents), which would result in much increased acceleration factors for the nanoconfined materials). These results show that the initial molar uptake rate of the nanoconfined GAP-0 material was 40 times greater than that of the free GAP-0 liquid. Similarly, strongly accelerated uptake kinetics were observed for M′D′M′ and M′3T′, with the nanoconfined sorbents achieving initial molar uptake rates 67 times and 90 times greater than those of the corresponding free liquids, respectively. Interestingly, the initial molar uptake rate for the free silicone oils is quite comparable (0.024 ± 20% mol CO2/mol silicone oil/min), while the uptake kinetics after nanoconfinement vary much more significantly. This suggests that the uptake kinetics for all three free sorbents is entirely limited by the diffusion through the viscous liquids (which are expected to have similar density and viscosity), hence masking the significant difference in functional chemical structure between the three sorbents, while the nanoconfinement allows observation of a true uptake kinetics, which is expected to increase with increasing number of amine groups per mole of silicon oil sorbent. 3.2. Uptake Capacity. Beyond rate, the total uptake capacity was calculated from the uptake time traces, utilizing

Table 2. CO2 Uptake Capacities for the Nano-Confined Silicone Oils material nanoencapsulated GAP-0 nanoencapsulated M′D′M′ nanoencapsulated M′3T′

uptake capacity (mol CO2/mol silicon oil)

uptake capacity (mg CO2/g tot sorbent)

0.74

54.33

1.17

59.57

1.48

57.67

although none of the three silicone oils achieved the maximum theoretical adsorption: GAP-0 adsorbed 0.74 mol CO2/mol silicone oil, M′D′M′ adsorbed 1.17, and M′3T′ adsorbed 1.48. Remarkably, the maximum observed uptake capacity for all three materials corresponds quite closely to 75% of the maximum theoretical uptake capacity. At this point, we do not have an explanation for this observation. 2(R − NH 2) + CO2 ⇌ R − NH3+ + R − NHCO2− (1)

In view of the fact that the nanoconfinement adds inert mass (the silica) to the sorbent, the uptake data were also calculated on a total sorbent mass basis (i.e., unlike the molar uptake data, the entire mass of the confined sorbent plus the silica host was utilized) and is included in Tables 1 (for uptake kinetics) and 2 (for uptake capacity). The uptake kinetics data in Table 1 show that the scaling with the added inert mass reduces the calculated acceleration factor in the kinetics by a factor of ∼2.2 but still yields accelerations well in excess of 1 order of magnitude. As expected, the use of the silica shell yields a penalty on a per mass basis but still results overall in a significant enhancement of CO2 uptake. More important than the effect of the added mass on the uptake kinetics is the potential effect that this added mass can

Table 1. Comparison of CO2 Uptake Kinetics for the Nano-Confined Silicone Oils and the Unconfined Liquid Silicone Oils material

initial rate (mol CO2/mol silicone oil/min)

acceleration molar basis

initial rate (mg CO2/g sorbent/min)

acceleration mass basis

GAP-0 liquid nanoencapsulated GAP-0 M′D′M′ liquid nanoencapsulated M′D′M′ M′3T′ liquid nanoencapsulated M ′3T′

0.019 0.767 0.029 1.921 0.024 2.147

− 40× − 67× − 90×

3.09 56.61 3.15 97.65 2.09 83.54

− 18× − 31× − 40×

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temperature change, the energy requirement for heating the GAP-0 is 91.7 kJ/mol CO2, and heating the silica nanobubble requires 28.5 kJ/mol CO2. Despite the rather large, and hence conservative, assumed temperature swing of 100 °C, the combined sensible heats of the sorbent materials are comparable to the heat of the desorption reaction, which is 110.3 kJ/mol CO2.10b More importantly, the silica nanoshell required to confine the silicone oil is responsible for only 12.4% of the overall heat requirement for the desorption process. 3.3. Cyclic Stability. Beyond uptake kinetics and sorption capacity, the most important characteristic of a suitable CO2 sorbent is stability in continued cyclic uptake and desorption of CO2. Destruction or loss of the CO2 capture species is neither economically nor practically viable, since the valuable material would have to be constantly replenished. Thus, the ideal sorbent is thermally stable (to both decomposition and volatilization) and resistant to fouling (both physically and chemically). Perry et al. previously demonstrated that the silicone oils GAP-0, M′D′M′, and M′ 3 T′ are thermally stable to decomposition and exhibit lower vapor pressures than the traditional MEA sorbent at 180 °C.10 In order to verify these results at the conditions of our experiments, a set of TGA experiments was conducted for each material in which the nanoconfined sorbent system was subjected to an extended period (typically 90 min) of pure He (inert) gas flow at temperatures ranging from 45 to 75 °C. The results of these experiments for nanoconfined GAP-0 are shown in Figure 7,

have on the energy balance of the CO2 sorption process: the silica shell constitutes an inert thermal mass, yielding an energy penalty that must be paid when changing the temperature of the sorbent. In an industrial CO2 capture process, CO2 desorption from the sorbent material will not be induced via change in CO2 partial pressure (as done here and in most fundamental CO2 sorption studies due to the ease of the experimental procedure) but rather via thermal desorption of the CO2 into an already concentrated CO2 stream (which is then ready for sequestration). Since the heat capacity of the system, and thus the energy required to raise the temperature of the material during desorption, is proportional to the entire sorbent mass, minimizing the total (thermal) mass required to capture a given amount of CO2 must be a priority for any operational CO2 capture apparatus. We hence calculated the relative contributions to the thermal desorption process for one of the nanoconfined sorbent materials. Figure 6 illustrates the comparative heat require-

Figure 6. Contribution of heat of reaction, GAP-0 sensible heat, and SiO2 shell sensible heat to overall desorption energy requirement, given a 100 °C temperature swing. Figure 7. Demonstration of the volatility of nanoconfined GAP-0 at moderate temperatures. In three separate experiments, an initial pure He flow phase (light overhead bar) endured for 90 min before the material was subjected to pure CO2 flow (dark overhead bar).

ments for the three endothermic processes required for desorption: the heat of the desorption reaction, the sensible heating of the silicone oil, and the sensible heating of the silica nanoshell. The calculation was conducted on a “per mole of CO2” basis, assuming a 100 °C temperature swing for desorption and a 1:1 loading ratio of GAP-0 silicone oil to silica shell material. In order to calculate the sensible heat per mole of CO2, the amount of material required to take up one mole of CO2 was determined using the GAP-0 uptake capacity identified in the above TGA experiments (i.e., 1.23 mmol CO2/ g total sorbent material). Since a 1:1 ratio of GAP-0 and silica nanobubble was used in these experiments, this is the uptake capacity per 0.5 g of each sorbent component. Thus, in this scenario 406 g of GAP-0 and 406 g of silica nanobubble materials are required to take up one mole of CO2. Perry et al. determined the heat capacity of GAP0 to be 2.26 J/g °C over the temperature range from 40 to 140 °C.10a The silica nanobubbles were assumed to have the same heat capacity as bulk SiO2 (0.703 J/g °C). Applying these heat capacities and mass values over the assumed 100 °C

which shows the sorbent mass (normalized to the initial mass at t = 0 min) plotted against time for the three experimental temperatures. The bars shown at the top of the graph indicate the gas flow that the system was being subjected to at a given time, with the light bar representing pure He and the dark bar representing CO2. One can see that during the pure He flow phase of the three experiments, the mass of the samples is described by a fast initial mass loss followed by a slow, linear decrease. The initial rapid mass loss is expected from the evaporation of moisture from the material, as the microporous silica shell is prone to take up moisture when exposed to ambient air. However, the sustained mass decrease was unexpected. Note that the effect is more pronounced at higher temperatures, with the curve at 75 °C exhibiting the fastest rate of mass loss. 16480



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Figure 8. (A) Normalized sorbent mass over multiple uptake/desorption cycles for nanoconfined GAP-0; alternating He flow and CO2 flow periods denoted by light and dark overhead bars, respectively. (B) CO2 uptake capacity over multiple cycles during the same experiment. The entire experiment was conducted at 75 °C after an initial ramp from room temperature.

The continuously decreasing maximum and minimum sample weights during the ten subsequent sorption/desorption cycles indicate already that even during this cyclic operation, loss of the silicone oil sorbent from the nanoconfinement could not be avoided, despite an attempt to minimize the desorption time (and hence the time that the sorbent would spend in the volatile liquid form). However, one can see that the trends in the mass decrease for the maximum and the minimum mass are not identical (i.e., they reflect two different trends; this is most obvious during the first few cycles where the maximum mass already shows an almost linear decrease while the minimum mass is almost constant). The continually decreasing minimum mass (i.e., mass after desorption of CO2) during He exposure is explained by the silicone oil volatility discussed above; as the total time under inert He flow environment increases, silicone oil is continually volatilized, and the mass of the sample decreases accordingly. Understanding the decreasing maximum mass during CO2 cycles is less straightforward. If constant CO2 uptake across cycles is achieved, then the maximum mass should decrease in an identical manner to the minimum mass. However, the maximum mass decreases more quickly than the minimum mass for the first ten He/CO2 cycles (the last cycle underwent a slightly different procedure and will be discussed in the following paragraphs), so that the net mass increase due to uptake in each CO2 cycle lessens with each further cycle. In order to quantify this, we determined the molar uptake capacity of the nanoconfined silicone oil sorbent for each cycle (i.e., moles CO2 per mole of silicone oil, Figure 8B). This value was calculated by assessing the amount of silicone oil remaining in the sample at the start of each CO2 cycle, with the assumption made that all mass loss (at the sample minimum mass) over the course of the experiment is due to the volatilization of silicone oil. The CO2 adsorption of the sample decreases continuously from 0.74 mol CO2/mol GAP-0 in the first CO2 cycle to ∼0.50 in the sixth, after which it seems to stabilize. Two effects that are believed to be occurring with each He cycle account for this decrease in adsorption capacity. First, the loss of silicone oil molecules to volatilization decreases the capacity of the material to take up CO2; second, accumulation of CO2 molecules in the sample with each successive cycle decreases the number of silicone oil molecules that are free to take up CO2, suggesting that the short 3 min desorption cycle (chosen in order to minimize sorbent loss due to volatilization in this phase) is insufficient to successfully desorb the CO2 quantitatively.

This continued loss of mass can be explained as the result of volatilization of the liquid GAP-0 sorbent over time, a notion supported by the fact that the effect is increasingly noticeable with increasing temperature, as expected for an endothermic process like volatilization. The free liquid GAP-0 sorbent, as well as both the nanoconfined and free liquid M′3T′ and M′D′M′ sorbents, showed similar linear mass-loss trends in similar experiments. Thus, we conclude that the aminefunctionalized silicone oil materials exhibit volatility in the temperature range from 45 to 75 °C, both in nanoconfined and in free liquid form. At this point, we cannot explain the contradiction between our experiments and the observations by Perry et al., who reported low volatility at temperatures well in excess of those in our experiment. It may be possible that the use of a closed system (rather than the open flow system in our case) might have masked some of the volatility of the sorbent, but this remains pure speculation. However, since the present class of sorbents shows the remarkable feature of undergoing a phase transition to a solid state upon sorption of CO2, and since this solid state is likely to be associated with a much lower vapor pressure than the liquid, we further investigated whether it would be possible to stabilize the sorbent in cyclic uptake/desorption via sufficiently rapid cycling times (i.e., by minimizing the time that the sorbent material would spend in the CO2-free liquid state). Results of these experiments are shown in Figure 8. Figure 8A shows a multicycle TGA experiment using nanoconfined GAP-0, where the total sorbent material mass (normalized to the initial sample mass) is plotted against time for multiple cycles of switching the gas flow between pure He (for CO2 desorption) and pure CO2 (for uptake). As seen above, there is an initial rapid mass decrease due to the initial removal of moisture from the material under pure He gas flow, followed by the onset of the linear mass decrease associated with silicone oil volatilization. Upon switching the gas to pure CO2 flow, we observe an immediate, rapid mass increase associated with the chemical adsorption of CO2 by the sorbent material; this adsorption curve is steep, achieving final sorption capacity in approximately 1 min. Upon switching back from CO2 to He flow (after about 5 min of exposure to CO2), an immediate mass decrease is observed; this mass decrease is attributed to the desorption of CO2 from the sample as the CO2 sorption equilibrium (eq 1) shifts toward the reactants in response to the absence of CO2 in the environment, thus releasing the bound CO2. 16481



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Figure 9. (A) Normalized sorbent mass over multiple uptake/desorption cycles for nanoconfined M′D′M′; alternating He flow and CO2 flow periods denoted by light and dark overhead bars, respectively. (B) CO2 uptake capacity over multiple cycles during the same experiment. The entire experiment was conducted at 75 °C after an initial ramp from room temperature.

Figure 10. (A) Normalized sorbent mass over multiple uptake/desorption cycles for nanoconfined M′3T′; alternating He flow and CO2 flow periods denoted by light and dark overhead bars, respectively. (B) CO2 uptake capacity over multiple cycles during the same experiment. The entire experiment was conducted at 75 °C after an initial ramp from room temperature.

This is supported by an analysis of the cycle labeled “ED” in Figure 8. After the tenth He/CO2 cycle, the sample was subjected to an extended 10 min He flow segment (still at a constant 75 °C) in order to allow sufficient time for CO2 desorption. As seen in Figure 8A, the characteristic linear mass decrease associated with the volatilization of silicone oil occurs over the course of these 10 min. However, despite this added sorbent loss, the uptake of CO2 during the subsequent exposure to CO2 is significantly greater than that of the preceding cycle (0.57 mol CO2/mol GAP-0 vs 0.50). Since the actual amount of silicone oil in the sample is certain to have decreased from the tenth to the “ED” cycle, the increased adsorption is attributed to an increased proportion of amine groups in the silicone oil molecules being made available for CO2 uptake due to the extended time allowed for the desorption of previously taken up CO2 molecules. The same cyclic experiments were also conducted for the other two silicone oils (M′D′M′ and M′3T′) in silica nanoconfinement. Both exhibit a similar decay of uptake capacity with multiple cycles under similar experimental conditions, as shown in Figures 9 and 10. For M′D′M′, the uptake capacity decreases from 1.17 mol CO2/mol M′D′M′ for the first cycle to 0.83 for the fifth, after which it again stabilizes; for M′3T′, the decrease is from 1.48 mol CO2/mol M′3T′ to 1.09 (again over the first five cycles). Both of these samples also recover some of their sorption capacity per mole of silicone oil retained in the sample for the CO2 cycle following an extended “ED” desorption segment.

3.4. Effect of Nano-Confinement. While the abovedescribed experiments thus unfortunately point toward an unexpected volatility of the silicone oils at the experimental conditions, they confirm the promise that nanoconfinement holds for drastic acceleration of the sorption kinetics of typically viscous liquid CO2 sorbents. Beyond these application-oriented aspects, however, it is tempting to see whether the present experiments can yield insights on a fundamental level into how these liquid molecules interact with CO2 in a size-constrained nanoenvironment. Especially of interest is the effect of nanoconfinement on the phase-change phenomena that the silicone oils are known to exhibit upon interaction with CO2. Related effects of nanoconfinement on phase change behavior have been studied before, including changes in density11 and in the solid−liquid phase transition for nanoconfined fluids.12 For example, computational studies by Maddox and Gubbins on solid−liquid phase transitions of methane in cylindrical nanopores of ∼3.5 nm report a significant freezing point depression13 in agreement with prior experimental results which had shown that CO2 confined to ∼2 nm cylindrical pores exhibits a freezing point depression of 12 K.14 While these results are intriguing, the pore sizes employed in these studies are smaller than the central cavity of the nanobubbles used in our study; thus, it remains uncertain whether any similar effect would be observed in the ∼8−10 nm cavities of our silica nanobubbles. On the other hand, the pores (∼1 nm) in the silica shells that run from the shell exterior to the internal cavity are well below those studied by Maddox and Duffy, so that some effect could be expected for these regions. 16482



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Figure 11. Normalized sorbent mass of nanoconfined GAP-0 during (A) extended CO2 uptake in pure CO2 and (B) desorption into pure He. The uptake curve shows thermally stable behavior after CO2 uptake (indicated by the steady sample mass), while the desorption curve indicates a transition to a continuous slow mass loss due to volatilization after CO2 desorption. Both experiments were conducted isothermally at 75 °C.

Also, the presence of an ensemble of ∼1000 molecules puts our system in a range which should be below the thermodynamic (bulk) limit but larger than a cluster approximation, leaving us in a particularly poorly understood and rarely studied mesoscale regime. Furthermore, while the above cited previous findings indicate that phase transitions are likely to be affected by nanoconfinement, we were not able to find any studies in the literature that consider phase transitions induced by chemical reaction rather than temperature change. Since microporous materials, such as zeolites, have long been known to impact reactivity (guiding, for example, the formation of aromatic products via the spatial confinement in their porous network), it stands to reason that if nanopores can affect both thermally induced phase transition as well as reactivity, they might impact reactive phase changes as well. Obviously, our experiments were not designed (nor intended) to yield fundamental insights into such phase behavior. Yet, the results of our experiments can yield at least some “coarse grained” insight, indicating that the phase transition from liquid silicone oil to solid carbamate salt upon CO2 uptake that is observed in the bulk liquid is not significantly affected by the nanoconfinment in our silica nanobubbles. This claim is based on the volatility of the liquid sorbent versus the stability of the solid sorbent after CO2 sorption. Figure 11A displays a typical adsorption segment of a TGA experiment, studying nanoconfined GAP-0 under pure CO2 flow, with the total mass of the sample plotted against uptake time. As shown in preceding figures, the initial mass increase due to the adsorption of CO2 is very rapid, with the sample reaching near full capacity (i.e., near maximum mass) in about 1 min. Following this initial uptake, the mass of the sample is stable over an extended period of time; some minor mass increase occurs due to further CO2 uptake, but no indication of any mass decrease is observed, suggesting that the sorbent is thermally stable and thus implying the presence of the solid carbamate salt. Figure 11B depicts a separate He gas flow (i.e., desorption) segment from the same TGA experiment. After an initial period of relatively rapid mass decrease due to desorption of CO2, the slope of the curve changes to a constant linear mass decrease, which can be attributed to the volatilization of the liquid silicone oil. Hence, the transition in the shape of the curve appears to occur when most of the CO2 has been desorbed from the sample.

These observations thus suggest the transition between a thermally stable form of the sorbent after CO2 sorption and a thermally labile form after CO2 desorption, consistent with the behavior of the free liquid where this behavior is caused by the solid−liquid phase transition. It hence seems reasonable to suggest that the phase transition is not significantly affected by the nanoconfined environment in the silica nanobubbles.

4. CONCLUSIONS In this study, a novel CO2 sorbent system was investigated, in which silicone oil sorbents were confined in solid, hollow, microporous silica nanobubbles. Experiments were conducted in order to determine whether nanoconfinement has any significant effect upon CO2 uptake kinetics, CO2 uptake capacity, and, finally, on the intriguing phase-change properties of these sorbent materials. In TGA experiments with nanoconfined silicone oil sorbents (GAP-0, M′D′M′, and M3′T′), the nanoconfined materials exhibited initial CO2 uptake rates that were accelerated up to 90-fold in comparison to the corresponding unconfined, bulk liquid silicone oils. However, the silicone oils were found to be volatile at temperatures as low as 45 °C in the gaseous flow environment of the TGA. Thus, continuously decreasing CO2 uptake capacities were observed in multiple uptake/desorption cycles for each of the three silicone oil sorbents studied. Although volatile in the unbound liquid oil form, the nanoconfined silicone oil sorbents exhibited thermal stability upon sorption of CO2; this behavior resembles the liquid-tosolid phase transition that occurs in the bulk silicone oils upon sorption and suggests that this phase transition is not significantly affected by the nanoconfinement. While the volatility of these nanoconfined silicone oils at operating conditions makes their use for industrial CO2 capture applications questionable, the kinetic uptake acceleration accompanying the nanoconfinement of these sorbent materials constitutes as an exciting observation that can be expected to apply to many other CO2 sorbent materials. Given the rather low energy penalty for the added thermal mass (calculated to be ∼12% for the systems studied here, well below the large energy penalty due to conventional aqueous dilution for these materials), nanoconfinement seems a promising and highly flexible way to enhance the uptake kinetics of established and emerging CO2 sorbents with the added benefit of turning a viscous (and often corrosive) liquid into a much more easily handled solid material without altering its sorbent properties. An ongoing extension of this work is the study of room 16483



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of Adsorbed CO2 and C6H6. Ind. Eng. Chem. Res. 2014, 53, 4224− 4231. (10) (a) Perry, R. J.; G, S. E.; Farnum, R. L.; Spiry, I.; Perry, T. M.; O’Brien, M. J.; Xie, H.; Chen, D.-L.; Enick, R. M.; Johnson, J. K.; Alshahrani, S. S. A Combined Experimental and Computational Study on Selected Physical Properties of Aminosilicones. Ind. Eng. Chem. Res. 2014, 53, 1334−1341. (b) Perry, R. J.; Wood, B. R.; Genovese, S.; O’Brien, M. J.; Westendorf, T.; Meketa, M. L.; Farnum, R.; McDermott, J.; Sultanova, I.; Perry, T. M.; Vipperla, R.-K.; Wichmann, L. A.; Enick, R. M.; Hong, L.; Tapriyal, D. CO2 Capture Using Phase-Changing Sorbents. Energy Fuels 2012, 26, 2528−2538. (11) (a) Guegan, R.; M, D.; Alba-Simionesco, C. Interfacial Structure of an H-Bonding Liquid Confined into Silica Nanopores with Surface Silanols. Chem. Phys. 2005, 317, 236−244. (b) Schneider, M. S.; G, J.D.; Baiker. Near-critical CO2 in mesoporous silica studied by in situ FTIR spectroscopy. Langmuir 2004, 20, 2890. (12) Christenson, H. K. Confinement Effects on Freezing and Melting. J. Phys.: Condens. Matter 2001, 13, r95. (13) Maddox, M. W.; Gubbins, K. E. A molecular simulation study of freezing/melting phenomena for Lennard-Jones methane in cylindrical nanoscale pores. J. Chem. Phys. 1997, 107, 9659. (14) Duffy, J. A.; Wilkinson, N. J.; Fretwell, H. M.; Alam, M. A.; Evans, R. Phase transitions of CO2 confined in nanometer pores as revealed by positronium annihilation. J. Phys: Condens. Matter. 1995, 7, l713.

temperature ionic liquids (ILs) in a similar nanoconfined setup; these materials are known to exhibit significant potential for CO2 capture combined with a dramatically lower volatility at flue gas operating temperatures and are hence expected to overcome the main drawback of the present sorbent system.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (412) 624-1042. Fax: (412) 624-9639. E-mail: gveser@ pitt.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the University of Pittsburgh’s Swanson School of Engineering through a Summer Research Internship to D.W.P. is gratefully acknowledged. We would like to thank Robert Perry of GE Global Research for providing samples of the amino-silicone liquids.

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REFERENCES

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Nanoconfined Amine-Functionalized Silicone Oil Sorbents for Rapid

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