Kinetic analysis of an anion exchange absorbent for CO2 capture from ambient air

This study reports a preparation method of a new moisture swing sorbent for CO2 capture from air. The new sorbent components include ion exchange resin (IER) and polyvinyl chloride (PVC) as a binder. The IER can absorb CO2 when surrounding is dry and release CO2 when surrounding is wet. The manuscript presents the studies of membrane structure, kinetic model of absorption process, performance of desorption process and the diffusivity of water molecules in the CO2 absorbent. It has been proved that the kinetic performance of CO2 absorption/desorption can be improved by using thin binder and hot water treatment. The fast kinetics of P-100-90C absorbent is due to the thin PVC binder, and high diffusion rate of H2O molecules in the sample. The impressive is this new CO2 absorbent has the fastest CO2 absorption rate among all absorbents which have been reported by other up-to-date literatures.


Introduction
As the announcement of Intergovernmental Panel on Climate Change (IPCC), the CO 2 emissions will rise between 48 and 55 Gt/yr by 2050, with the energy demands of 40% to 150% increase [1]. Atmospheric CO 2 will be ranging from 535 to 983 parts per million (ppm) by 2100, roughly double current value, 406 ppm. CO 2 concentration increment leads to a global mean temperature change from 1990 to 2100 of between 1.4˚C and 6.1˚C [2]. The significance and urgency of the development of CO 2 capture from ambient air has been discussed elsewhere [3][4][5][6].
In order to compensating for CO 2 emission to ambient air, a moisture-swing sorbent for CO 2 capture from ambient air was proposed [7], which provides a novel approach to absorb CO 2 in dilute streams. The moisture-swing CO 2 absorbent is an anion exchange resin [8][9][10][11][12][13][14] (IER). IER acts like a strong base, analogous to NH 4 + , where each hydrogen has been replaced by an organic carbon chain attached to a polymer matrix. The chemical structure is shown in Fig 1. The isothermal [15,16] and kinetic [13,17] performance of the resin-based sorbent have been revealed systematically. The novel principle of the CO 2 absorption/desorption process over IER was well illustrated and clarified [8,18]. The reason is that reduction of the number of water molecules presenting in the pore space promote the hydrolysis of CO 3 2to HCO 3 and OH -8 . This phenomenon enables a nano-structured CO 2  spontaneously in the ambient air when the surrounding is dry, while releasing it when exposed to moisture [8,9]. In the dry condition, a CO 3 2ion splits a H 2 O molecule to form a HCO 3 and a OHion which both bind tightly to their respective NR 4 + cations. OHion absorbs CO 2 even at a low partial pressure of CO 2 . This results in a CO 2 -loaded state which is entirely HCO 3 -. In the wet condition to regenerate the full-loaded absorbent, each two HCO 3 ions react to produce a CO 3 2ion, a H 2 O and a CO 2 . The released CO 2 in the wet condition can be collected collectively. The underlying mechanism has been revealed by Shi [9] and the phenomenon can be applied to a series of counter-intuitive chemical reactions which is related to the hydrolysis of basic ions [10]. Meanwhile, the discovery opens a new approach for the technology of gas separation. For these conditional CO 2 capture methods, like thermal-swing CO 2 absorbents [19], increasing absorption capacity is a significant task due to the high cost (the regeneration process consumes heat) on the absorbent regeneration. However, for the novel water-driven CO 2 absorbent, kinetics improvement is a more interesting factor due to the low cost (the regeneration process consumes water) of the regeneration part [17]. The energy consumption and cost can be reduced significantly according to improving the absorption rate of the water-driven CO 2 sorbent. The objective of this study is to propose a new moisture-swing CO 2 sorbent (P-100) by using ion exchange resin (IER) as a functional group and polyvinyl chloride (PVC) as a binder. The kinetic characterization of the new CO 2 sorbent has been enhanced significantly comparing to sorbent I-200 [15], which is manufactured by Snowpure LLC, California. The preparation process of this new sorbent is introduced first, and the analysis of kinetic performance is presented next based on the studies of material structure, CO 2 absorption/desorption process and water diffusion experiments.

Materials
A heterogeneous ion-exchange material in a flat sheet was prepared in this study. The material includes: 1) ion exchange resin (IER) [7]. The IER is composed of a polystyrene backbone with attached quaternary amine ligands. 2) Polyvinyl chloride (PVC). PVC is a widely produced synthetic plastic polymer which was used as a binder. 3) Tetrahydrofuran (THF). THF is an organic compound with the formula (CH 2 ) 4 O which was employed as a solvent to mix IER and PVC.

Preparation of anion-exchange sorbent
The heterogeneous CO 2 absorbent was prepared by using dip-coating technique [20]. First, the IER particles were grinded in a ball mill and then filtered using a mesh with 44~74 micrometer openings. Then, PVC was dispersed into THF in a glass reactor and stirred mechanically for more than 5 hours. The weight ratio of PVC to THF is 1:20. Next, powdered resin particles (44~74um) were added into the mixture of PVC and THF. The mechanical stirrer stirred vigorously at room temperature for 30 minutes to mix IER and PVC uniformly. The IER to PVC weight ratio is 1:1 and the total solid to THF ratio is 1:10 (w/v) [21]. After completed mixing, the dip coating method was conducted by using dry clean glass plate. The thickness of the produced membrane was 100 micron. The membrane was dried in the ambient air at temperature 25˚C for 30 min, and then was immersed in distilled water. Last, three absorbent samples were treated by different temperatures of water for 48 hours (25˚C water (P-100-25C), 50˚C water (P-100-50C), 90˚C water (P-100-90C)). I-200 sample was only treated by 90˚C water as a reference. Three P-100 samples and an I-200 sample, containing the same load of IER, were immersed in 1.0 M sodium carbonate solution for 2-4 hours [15]. Samples were washed 4-5 times, and then washed by plenty of deionized water (DI water) to flush away the sodium carbonate solution residues on the samples. Afterwards, the samples were ready-to-use.

The absorption capacity of CO 2 sorbent
The Mohr method was used to determine the effective ion charge density ρ c of the IER. The ρ c is 1.58 mol/kg. CO 2 capacity Q est is 17.69 L/kg, which was estimated by ρ c at standard condition. The CO 2 capacity Q 1 was also measured by experiments under the condition of 1000 ppm CO 2 partial pressure. The value of Q 1 is 16.4L/kg. The effective charge density and CO 2 capacity of the absorbent can be both enhanced according to increasing the weight ratio of IER to PVC during the sample preparation process.

Absorption experiment
The experimental device with humidity control was set up to measure the half-time (the time when the absorbent reaches half of its capacity) of the moisture-swing CO 2 absorbents. A layout of the device is shown in Fig 2. The CO 2 concentration change was measured by two infrared gas analyzers (IRGA). Measurements were recorded once per second. The wet and fresh samples (each sample contains 0.30g IER) were put into a sealed chamber one by one and flushed with 2L/min CO 2 -free dry air. The water concentration in the air at outlet was monitored to determine whether the samples were sufficiently dried. (The water concentration in the CO 2 -free dry air at inlet and water concentration in the CO 2 -free dry air at outlet were same, when samples had been sufficiently dried. The water concentration was about 2% relative humidity,). The 1 L/min air containing 400 ppm CO 2 , went through a dew point generator (MODEL LI-610). The air contained 30% relative humidity and flowed over all sorbent samples. Entire absorption process would last until CO 2 concentrations were same at inlet and outlet with 1% error.

Desorption experiment
The absorbed CO 2 by absorbents will be released when the absorbents are exposed to a high humidity or liquid water. Meanwhile, the absorbents continue to absorb water vapor from air when they are put into a higher humidity until the system reaches to an equilibrium state. The absorbed water molecules increase the weight of the absorbents. Here, the desorption experiment is to analyze the kinetic characteristics of the absorbents by studying the equilibrium time of the process of H 2 O absorption and CO 2 desorption. The diffusion coefficients of water molecules in the CO 2 absorbents were derived by calculating the weight change of the samples. The diagram of the experimental device is shown as Fig 3 The total amount of CO 2 in the closed-loop experimental device is constant. We can track the amount of absorbed/desorbed CO 2 by measuring the CO 2 concentration in the air. The device can control the water concentration in the air by heating or cooling the condenser. The weight change of the absorbents can also be measured by weight scale in the device.
Four samples (25˚C-water-treated P-100, 50˚C-water-treated P-100, 90˚C-water-treated P-100, and 90˚C-water-treated I-200) containing the same resin load, were firstly exposed to pretreated dry air (dew point -18˚C) for two hours to be fully dried and loaded. Next, put the fullloaded samples into experimental device (as Fig 3) subsequently and increased the humidity level to 15˚C. The increase of sample weight and CO 2 concentration in the experimental device were measured separately. The increase of the sample weight was mainly due to the water absorption on the samples. A humidity controller was employed to ensure a constant humidity in the chamber by PID control. CO 2 concentration change was recorded every second by infrared gas analyzers (IRGA).

Absorption kinetics of sorbent
The absorption characteristics of P-100-90C sorbent was depicted by Lagergren pseudo firstorder model [22], which has been most frequently employed to present absorption dynamic process under various conditions.
q is absorption quantity at time t, k is constant number, q e is equilibrium isotherm absorption capacity. Integrating Eq 1 with boundary conditions (a) t = 0, q = 0; (b) t = t, q = q e q ¼ q e ð1 À e À kt Þ ð2Þ

Absorbent structure analysis
The structures of the P-100 sorbents, which were treated by different-temperature hot water, were studied by scanning electron microscope SEM (Agilent Technologies, SE 1000V) as shown in Fig 4. Obviously, hot water treatment results in the formation of narrow cavities between anion exchange particles and PVC matrix, as well as larger amount of pores in PVC materials. Micro-structure schematic of P-100 sorbent can express the percolation structure of P-100 sorbent. After the treatment of 90˚C hot water, some small islands of interconnected particles appear and these connections form extended pathways. More and more IER particles are connected by channels if the connections keep growing. The chance of the appearance of percolation threshold can increase the rate and range of gas diffusion inside the sorbent. According to the observation of SEM, much more resin particles in the new sorbent P-100 are exposed into air than the ones held by I-200 [15] because of the thinner thickness of membrane and the more continuous channels inside P-100. Moreover, percolation threshold may further promote the conduction level between surrounding air and resin particles. Therefore, treating P-100 by hot water may promote IER particles to be exposed to ambient air, thereby further improving the performance of the CO 2 absorbent.

Absorption half-time
The kinetic characteristics are significant factors for moisture-swing CO 2 capture sorbent. Absorption kinetics of the sorbent are determined by mass diffusivity in the materials, heat transfer into and out of the pores, and intrinsic chemical reaction rates [17,23,24]. As a preliminary assessment for the CO 2 absorption kinetics, absorption half time is an assessment factor 23 to evaluate the absorption rate of CO 2 absorbent. The absorption half time is expressed by Eq 3: T CO 2 is the time for CO 2 absorption by sorbent from fresh-empty status to full-loaded status,  [23,25] in literatures have also been presented.
The sample P-100-90C owns the best kinetic characteristics comparing with the other three moisture-swing CO 2 sorbents. The 31.8 min half time is also the shortest half time in all air capture sorbents which have been reported by other up-to-date literatures. Obviously, the kinetics of P-100-90C is better than the other two P-100 sorbents, because the hot water treatment enlarges the surface area of IER to be exposed to the ambient air. This leads to a much faster absorption/desorption rate of water molecules in IER than the other two P-100 samples, meaning the smaller T H 2 O value. The reaction rates of moisture-swing CO 2 absorbents were estimated similar. I-200 moisture-swing CO 2 absorbent was also treated by 90˚C hot water but still had a longer half time than those of three P-100 samples. The reason is the thickness of I-200 sorbent is 640 microns which is much thicker than the 100 microns thickness of P-100 sorbent. Plenty of time is consumed by water vapor to permeate into I-200 sorbent to contact the inside IER particles wrapped by polypropylene matrix binder. Fig 6 shows that pseudo first order model as Eq 1 and 2 fits the absorption kinetic data of P-100-90C absorbent with the coefficient of determination 0.98. The amount of absorbed CO 2 by P-100-90C sorbent was recorded per second, and the k values were determined by Eq 2.

Desorption kinetic performance
Moisture-swing CO 2 absorbent can release CO 2 back to the air in a wet surrounding 8 . The desorption kinetic characteristics of the moisture-swing absorbents are mainly influenced by the diffusion rates of H 2 O and CO 2 in sorbent. This study focuses on analyzing the impact of these two factors on the desorption rate.
To determine the diffusion coefficients of water molecules in the four absorbents, the moisture uptake percentage was determined from the equation: Where M t is the total amount of water content absorbed by sorbent samples at time t, M d is the original weight of the dry samples. The diffusivity D was determined from the slope (K) of the initial linear region of the plot of the percentage moisture uptake M t M d versus ffiffi t p curve [26,27].
Where h is the thickness of the sample, t is exposure time and M 1 is the maximum moisture gain. The samples increasingly absorb water molecules over time, which leads to their weights increase to equilibrium values under a certain water vapor partial pressure. Absorbed water molecules are conducive to desorb CO 2 from the full-loaded absorbents. The released CO 2 from absorbent increases the CO 2 concentration in the experimental device. The coefficient of water diffusion, as well as the equilibrium time of T w (the equilibrium time of absorbed H 2 O by absorbent) and T c (the equilibrium time of desorbed CO 2 by absorbent) of the four absorbents have been listed in Table 1.
Comparing Table 1 (A), (B) and (C), the diffusion rate of water is higher when sorbent is treated by higher temperature hot water. The higher diffusion rate is due to larger number of pores in PVC matrix, as well as larger narrow cavities between IER particles and PVC binder. These characteristics greatly promote the diffusion rates of H 2 O and CO 2 in sorbents. Higher diffusion rate of water can help ions in the absorbents to move more quickly and help desorbed CO 2 to release back to air more rapidly. For the P-100-90C sorbent, the equilibrium time difference ΔT between T w and T c is smallest ΔT = 66s. It means the desorbed CO 2 diffuses out of the sorbent costing 66s after water reaches to equilibrium in the sorbent. However, ΔT of P-100-25C is as long as 3428s. It means the released CO 2 is still trapped in sorbent for a certain amount of time.
Comparing (A) and (D), both sorbents had been treated by 90˚C hot water. Though the diffusion coefficient of water in I-200 is much larger than the one of P-100-90C, the water equilibrium time of P-100-90C is much smaller than those of I-200 due to the different binder materials of two sorbents. The reason is hydrophilic polypropylene binder of I-200 may also contribute to the water diffusion than the hydrophobic PVC binder of P-100-90C. However, the polypropylene of 640 micron thickness costs more time for water diffusion than the 100 micron PVC. The polypropylene binder of I-200 (provided directly by Snowpure LLC, California) is very difficult to be thinned due to the essential characteristics of this polymer.
I-200 also needs longer time for CO 2 to release back to the ambient air. For the I-200 absorbent, the IER particles of 44~74um diameters are inlayed in polypropylene of 640 micron thickness. The narrow, tortuous and long paths among resin particles and polypropylene are against to CO 2 diffusion. However, the same IER particles of 44~74um diameters can almost penetrate the 100 micron PVC in P-100-90c sorbent. IER in P-100-90c sorbent can contact atmosphere directly is benefit to desorption performance. Hydrophilic polypropylene binder  Table 1. Diffusion coefficients of water, equilibrium times of water and CO 2 in four samples.

D(m 2 /s × E-12) T w (s) T c (s) ΔT (s)
in I-200 can also attract some absorbed water molecules. This part of water molecules can not contact functional IER particles to release CO 2 .

Conclusion
This study introduces a new moisture-swing CO 2 absorbent by employing polyvinyl chloride (PVC) as binder for ion exchange resin (IER). The manuscript analyzes the preparation process, absorbent structure, kinetic model, absorption and desorption characteristics of this CO 2 absorbent. The CO 2 absorption rate of this new produced absorbent P-100 is nearly three times as fast as the one of I-200, and also three to ten times as fast as amine-tethered solid CO 2 absorbents. This fast absorption/desorption rate is with the benefit of a thin bind holder and fast diffusion rate of H 2 O. This new CO 2 absorbent provides a way of designing a moistureswing CO 2 absorbent with a high absorption/desorption rate in the future.
Supporting information S1