Capacity and Heat of Sorption on a Polyethylenimine-Impregnated

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

CO Capacity and Heat of Sorption on a Polyethylenimine-Impregnated Silica under Equilibrium and Transient Sorption Conditions Linxi Wang, Seyed Mehdi Kamali Shahri, and Robert M. Rioux J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02860 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

CO2 Capacity and Heat of Sorption on a Polyethylenimine-Impregnated Silica under Equilibrium and Transient Sorption Conditions

Linxi Wang1, Seyed Mehdi Kamali Shahri1,2, Robert M. Rioux1,3,* 1

2

Department of Chemical Engineering

Department of Energy and Mineral Engineering 3

Department of Chemistry

The Pennsylvania State University University Park, PA, 16802 [email protected]; (814)867-2503

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Amine-based solid sorbents represent promising replacements for aqueous alkanolamines for CO2 capture since they reduce significantly the sorbent regeneration energy and avoid instrument corrosion encountered with aqueous amine solutions.

CO2 capture by solid

amine sorbents is often achieved by pressure-temperature swing adsorption (PTSA) technology, which selectively sorbs CO2 from a gas mixture, and desorbs concentrated CO2 by pressure swing and/or heating.

CO2 capacity and heat of sorption on different types of

amine-based solids has been evaluated predominantly under equilibrium conditions using thermogravimetric analysis (TGA), with emphasis on parametric studies of temperature, amine loadings and supporting substrates on CO2 sorption kinetics and capacity.

Due to

differences in the configuration between a TGA apparatus and a PTSA column, CO2 capacity and heat of sorption may vary under transient sorption condition versus equilibrium sorption condition.

In this work, we constructed a laboratory-scale breakthrough reactor (BTR) to

simulate the industrial PTSA process, evaluated CO2 capacity and heat of sorption at different temperatures under transient sorption conditions, and compared these values with equilibrium values determined by volumetric sorption analysis.

We found the temperature effects on

CO2 capacity and the heat of sorption differ under the two sorption modes.

The BTR

experiments simulate a typical PTSA process; capacity and heat of sorption data not only provide useful information for the design of sorption and regeneration process, but also enable spatiotemporal modeling of the CO2 sorption process in a packed bed.

2


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Introduction Large-scale burning of fossil fuels has led to a dramatic increase in the atmospheric CO2 concentration, which contributes to global warming, ocean acidification, and anthropogenic climate change.

Carbon capture and storage (CCS) is a strategy to capture CO2 from large point sources –

such as fossil fuel power plants – to mitigate emission of greenhouse gas, and store or utilize CO2 through mineral carbonation1,2, enhanced oil recovery3, (electro)catalytic reduction4 and artificial photosynthesis5.

CO2 can be captured and sequestrated by separation technologies, including

scrubbing with aqueous amine6, or strong bases7, pressure/temperature-swing adsorption8,9 and membrane gas separation10,11. While chemical absorption using aqueous alkanolamine solutions is the state-of-art technology, there are inherent problems associated with this process such as equipment corrosion due to the use of caustic amine solutions and the high energy cost of regeneration due to the large volumetric fraction of water in the capture solutions, which could be avoided with pressure-swing adsorption technology12. It is estimated sequestered coal-based power plants in 2025 could be less costly than unsequestered ones today, with the successful development of either solid- or liquid-based CO2 sorbents13.

Compared to liquid amine solutions, solid amine sorbents alleviate the corrosive nature of the amine solutions that leads to fouling of equipment14, reduce the regeneration energy of CO215 and have lower capital cost16. Among various types of solid CO2 sorbent materials including activated carbons17, zeolites18, metal-organic frameworks (MOFs)19 and amine-impregnated solids20–26, the latter possess distinct advantages such as high capacity at high temperatures, fast sorption kinetics, extremely high selectivity towards CO2 over N2, multi-cycle stability, and tolerance to water17,27. These traits make 3



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solid amine materials ideal candidates for industrial CO2 capture applications. For example, PEI-based solid amines supported on mesoporous silica (SBA-15 and MCM-41 type) have been applied by RTI International in a bench-scale and a large prototype-scale fluidized-bed reactor, capturing CO2 through cyclic PTSA process, which achieved high CO2 capacity, good thermal stability, and long-term performance28. For the sorbent used by RTI, the cost of the silica support was identified as challenge to commercial sorbent production; in this work we utilized a microporous silica support with suitable physical and chemical properties for CO2 sorption, suggesting the potential application of these materials in large scale operations given they satisfy other process constraints. Song et al23,25 developed prototypical type I sorbents through the impregnation of pre-synthesized polyethyleneimine (PEI) to silica supports. While the capacity increases with gas-phase CO2 partial pressure under isothermal conditions, the influence of temperature on capacity is more complex. Several groups measured CO2 capacity at various temperatures with thermogravimetric analysis (TGA)20,22,24,29 and the entirety of the results demonstrated CO2 capacity maximizes at an intermediate temperature (between 45-70℃), rather than constantly decreasing with temperature as would be expected from LeChatelier’s principle.

The maxima behavior in CO2 capacity is due to a

compromise between diffusion-controlled effects versus thermodynamics (i.e., LeChatelier’s principle), which has been discussed elsewhere20,24,30.

A few reports on the simultaneous measurement of CO2 sorption capacity and released heat via combined thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) have been published21,31. The dynamics in this set-up differ from that in a fixed-bed reactor, i.e. BTR, due to 4


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substantial gas bypass and heat flow losses due to the constrained configuration of sample container and heat sensors respectively. Simultaneous measurement of CO2 capacity and heat flow in a BTR provides a more relevant measurement for spatiotemporal modeling of industrial-scale fixed-bed reactors32. We measured the CO2 capacity and heat of sorption in different instrumental setups to enable a direct comparison of CO2 sorption performance under transient and equilibrium sorption conditions.

In this work, we used a 40 wt. % PEI423 impregnated silica obtained from the National Energy Technology Laboratory (NETL) as the CO2 sorbent and measured the capacity and heat flow under both transient and equilibrium conditions.

We combined a breakthrough reactor (BTR) and a

differential scanning calorimetry (DSC) to replace a TGA-DSC apparatus. The combined BTR-DSC replicates the (1) interaction between adsorptive gas and the solid sorbent, analogous to interactions in an industrial packed-bed reactor since gas msut pass through the bed; 2) high accuracy in heat measurements due to a 3D capture of the emitted heat by a large number of thermopiles, 3) operation time within 10 min, typical for a PSA process, which requires fast cyclic operation33. Coupled BTR-DSC systems have been applied in the study of other reactions and sorption34–38, but the application to the investigation on CO2 capture with BTR-DSC measurement has yet to be reported. Herein we report direct measurements of the enthalpy of CO2 sorption and desorption via combined BTR-DSC instruments on impregnated PEI sorbents at constant CO2 partial pressure (0.1 bar) and various temperatures (25 – 80℃). The effect of temperature on the capacity and heat of sorption is explored using the 40 wt. % PEI423 in a volumetric sorption apparatus to demonstrate the difference in CO2 sorption under equilibrium and transient conditions. 5



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Experimental 1. Materials and synthesis

The sample used in this work was supplied by National Energy Technology Laboratory (NETL) and used as-is.

The material consists of 40 wt. % polyethyleneimine (PEI423) supported on SiO2

(CARiACT-G10, Fuji Silysia Chemical Company) prepared by wet impregnation.

PEI423

(Sigma-Aldrich) was dissolved in methanol and mixed with silica in a weight ratio of 0.67 (PEI): 1 (silica).

PEI423 is a linear polymer with an average molecular weight is 423 g/mol.

The

impregnated 40 wt. % PEI423/SiO2 and the silica support were characterized by thermogravimetric analysis (Figure S1), BET surface area and pore volume analysis (Table S1 and Figure S2).

2.

Breakthrough

reactor

-

differential

scanning

calorimetry-mass

spectrometer

(BTR-DSC-MS)

The sorption behavior was tested in a BTR coupled with a differential scanning calorimeter (DSC, SENSYS Evo TG-DSC, Setaram). The instrument set-up is shown in Figure S3. We loaded the sample into a ¼” O.D. quartz tube between two pieces of quartz wool. The breakthrough tube and an empty quartz reference tube were inserted into the sample and reference channel of the DSC. The total flow rate of gas was fixed at 30 cc/min by mass flow controllers and the upstream pressure was monitored by a pressure transducer. Sorbent temperature was controlled by the DSC, while the DSC simultaneously measured the heat flow during sorption and desorption of CO2. A Hiden HPR-20 QIC R&D mass spectrometer recorded the H2O, CO2 and Ar concentration in the effluent gas.

Prior to transient CO2 sorption, the sample was pretreated at 105℃ under pure He flow for 1 h to 6


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remove previously sorbed CO2 and water, as monitored by the mass spectrometer. The temperature was stabilized at 25, 40, 50, 60, 70, 80℃, followed by exchanging the He flow to a gas mixture containing 10% CO2, 1% Ar and He balance using an electronic 10-way switching valve.

We

introduced Ar as an inert tracer because Ar tracks the gas flow, accounts for switching valve dynamics and does not sorb to the sample. Additionally, Ar assists in the calculation of CO2 breakthrough capacity, which is the difference in the Ar and CO2 mass spectrometer signals.

The Ar signal also

indicates a change in the total volumetric flow rate as CO2 sorbs. Detailed calculations of the breakthrough capacity (qs,BTR) is described in the Supporting Information S3, which is based on the difference between the integrated molar flow rates of Ar and CO2 within a time period of 500 s39. During this period, both the Ar and CO2 signals start at zero and stabilize at a plateau, indicating the saturation of the sorbent by CO2.

There was a delay in the DSC heat flow due to slow conduction through the bulk sample and the tube wall, which takes about an additional 300 s to reach baseline.

We waited much longer time (150 min)

before starting the pressure-swing desorption process. Since the purpose is to obtain the heat of sorption, in contrast to spatiotemporal modeling where the shape of heat flow curve is important, we did not correct for the time delay and integrated the peak area from the appropriate baseline points to obtain a value for the heat of sorption. Future temperature-dependent spatiotemporal modeling will account for time constant of the Tian-Calvet DSC.

After the heat flow curve reached the baseline, we switched the gas flow back to He to initiate pressure-swing desorption and recorded the endothermic heat flow for 240 min. The amount of CO2 desorbed during this stage is called the pressure-swing capacity (qp,BTR) and corresponds to the weakly 7



Page 8 of 35

bond CO2 species. This was followed by heating the sample up to 105℃ for temperature-swing desorption., while the desorbed CO2 signal was recorded by the mass spectrometer.

The

temperature-swing capacity (qt,BTR) corresponds to the strongly bond CO2 that desorbed only upon heating to higher temperature, which was calculated by comparing the integrated areas during the temperature swing and the total sorption process, where this ratio was then multiplied by the sorption capacity (qs,BTR).

As the total sorption capacity and desorption capacity (pressure- and

temperature-swing) should be equal, pressure-swing capacity (qp,BTR) was determined by taking the difference between total capacity and temperature-swing capacity (eqn. (1)).

, = , − , 3.

(1)

Volumetric apparatus – differential scanning calorimetry (VA-DSC)

We combined a volumetric analyzer (3Flex Surface Characterization Analyzer, Micromeritics) and a differential scanning calorimetry (DSC, SENSYS Evo TG-DSC, Setaram) to measure CO2 sorption isotherms and heat of sorption. Sorption isotherms were measured with the volumetric analyzer at different temperatures (25, 40, 50, 60, 70, 80℃), while the DSC controlled the sorbent bed temperature and recorded the transient heat flow during the equilibrium sorption experiments. A home-made forked tube with closed ends was employed to house the sample and enable heat flow measurements40. We weighed the sample on a wet basis and loaded on one side of the fork tube, which was later connected to the 3Flex instrument. We then inserted the fork tube into the DSC channels and aligned the sample location with the thermopile of the DSC.

The samples were then

pretreated under vacuum at 80℃ for 12 h to remove moisture and adventitious sorbed CO2. A lower pretreatment temperature was used under vacuum in order to reduce evaporative losses of PEI. 8


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Sorption isotherms were measured at 25, 40, 50, 60, 70, 80 ℃ from 0 to 760 torr of CO2. The heat flow peaks were integrated to calculate the exothermic heat at each equilibrium uptake increment. We summed the heat flow up to 76 torr, which was equal to the partial pressure of CO2 in the breakthrough experiment, and divided that by the sample mass and CO2 capacity (qs,VA) at 76 torr to determine an integral heat of sorption,

∆ =

( )

×,

(2)

where t0 is the time where pressure reaches 76 torr (P0), q(t) is the transient heat flow, m is the mass of the sample, and qs,VA is the CO2 equilibrium capacity at 76 torr.

9



Results and Discussion 1. BTR-DSC-MS

Figure 1 is an example of the normalized signal of CO2 and Ar tracer at 60 ℃, as well as the transient heat flow during pressure swing sorption.

After switching the inlet gas from He to 10% CO2/1%

Ar/He, an abrupt increase in the Ar signal appears indicating breakthrough of Ar, while the CO2 sorbs to the sample. After approximately 75 s, CO2 breakthrough is observed. Both CO2 and Ar signals reach a plateau region within 200 s, when the sample becomes saturated with CO2. The heat flow curve increases as exothermic CO2 sorption proceeds, maximizes and then decreases as the sorption slows down. The DSC signal takes longer to reach the baseline due to heat transfer from the sample to the DSC sensor.

The time scale for desorption is much longer due to the slow kinetics of

pressure-swing desorption.

Table 1 summarizes CO2

capacity, pressure/temperature swing capacity, and heat of

sorption/desorption in the breakthrough measurements. CO2 capacity increases with temperature, maximizes at 50℃ (Tmax), and decreases at higher temperatures.

The heat of CO2 sorption (∆Hs) falls

within a small range (67 – 73 kJ/mol) and is relatively independent of temperature. The axial heat transfer is estimated in to be negligible (Supporting Information S4). The ∆Hs results show CO2 sorption at different temperatures generates a similar amount of heat under the transient sorption. Considering the short sorption time in a breakthrough measurement, it is likely CO2 mainly sorbs to exposed amine sites instead of buried amines due to the slow diffusion of the CO2 in amine-filled pores. The measured ∆Hs will be compared with that under equilibrium conditions and the origin of their differences discussed. The enthalpy of desorption (∆Hd) during pressure swing increased with 10


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temperature, suggesting the desorbed CO2 at higher temperatures has a higher average binding energy with amines than the CO2 desorbed at lower temperatures.

The BTR simulates a laboratory-scale pressure swing adsorption (PSA) process in several aspects. In terms of flow dynamics, the PEI/silica sorbent is fixed in a tube and the inlet gases are required to pass through the sample bed.

The operation time scale for CO2 sorption in the breakthrough

experiment is typical for a normal PSA process, which is often in the order of 10 min33. The coupled DSC allows real-time monitoring of transient heat flow, which represents the heat transfer in an operation cycle and assists in the design of heat management systems33,41. Finally, the capacity and heat flow data provide useful information for spatiotemporal modeling of transient CO2 sorption in a fixed-bed column32.

11



Figure 1.

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Normalized molar flow rates of CO2 (red), Ar (blue) and transient heat flow (black)

during (a) isothermal CO2 sorption (b) pressure swing desorption in a breakthrough measurement at 60 ℃.

12


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Table 1. CO2 uptake, pressure-swing capacity, temperature-swing capacity and heat of sorption of CO2 to PEI sorbents determined from experiments. Temperature

CO2 capacitya

Pressure-swingb

Temperature-swing

(℃)

qs,BTR

capacity, qp,BTR

capacity, qt,BTR

(mmol/g)

(mmol/g)

25

1.99

40

Δ Hs

Δ Hd c

(mmol/g)

(kJ/mol CO2)

(kJ/mol CO2)

1.13

0.86

-72.6

23.7

2.10

2.07

0.13

-69.8

45.2

50

2.21

2.20

0.01

-67.2

63.9

60

2.13

2.13

0

-71.2

69.5

70

1.81

1.81

0

-70.9

70.3

80

1.45

1.45

0

-72.8

72.2

a

Inlet gas flow was controlled at 30 cc/min by electronic mass flow controllers. The pressure and temperature were set at 1 bar and 25°C and an initial CO2 concentration was 40.9 mol/m3 (as calculated by ideal gas law). The PEI/silica sorbent weighed 65 mg (wet basis). b Pressure swing capacity (qp,BTR) was obtained by taking the difference between the total CO2 capacity and (qs,BTR) temperature-swing capacity (qt,BTR). c Pressure-swing heat of sorption calculated by dividing the pressure-swing endothermic heat flow by the pressure-swing capacity.

13



2.

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VA-DSC

We studied sorption of CO2 at equilibrium conditions in the VA-DSC for comparison with the results from the transient measurements.

Figure 2 is an example of CO2 isotherms at 60℃ and the

corresponding differential heat of sorption (∆Hθ) at various surface coverage.

For each isotherm

point, the transient heat flow data versus time is shown in Figure S5. Initially, ∆Hθ=0 is ~95 kJ/mol at low coverage, and begins to decrease at CO2 coverage > 2 mmol/g. The decreasing trend of ∆Hθ with coverage has been observed on grafted amines42,43 and amine-functionalized MOFs44. According to Hahn at al., the decrease in ∆Hθ is due to decreasing probability for CO2 to form ammonium carbamate through pairwise interaction, as an increasing number of amines are occupied by previously sorbed CO243.

Mechanistically, CO2 first interacts with an amine to form carbamic acid or zwitterion, which can further form an ammonium carbamate ion pair by protonation of a neighboring amine. At low coverage, ammonium carbamate is the dominant sorbed CO2 species due to the abundance of free amine groups.

With increasing saturation of amines, additionally sorbed CO2 may exist as carbamic

acid due to the decreased concentration of neighboring amine pairs.

This hypothesis is supported by

a number of spectroscopic studies45–47; Didas et al applied FTIR spectroscopy on a highly loaded solid amine sorbent and identified carbamate ion pairs as the dominant sorbed CO2 species at low CO2 pressure (0.01 bar)45.

At higher CO2 pressure (1 bar), Mafra et al. discovered an unstable

chemisorbed CO2 species with

13

C CPMAS NMR, which they assigned to the isolated carbamic

acids46. The assignment was supported by the identical 13C CPMAS NMR signal on another sample with ultra-low amine loading, where the amines resided far from each other and isolated carbamic 14


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acid became the dominant sorbed CO2 species46. An in situ IR study by Tumuluri et al. observed higher intensity of ammonium carbamate ions and lower intensity of carbamic acid at a higher amine density, indicating the favored formation of ammonium ions47.

The CO2 isotherm and coverage dependent heat of sorption at other temperatures are shown in Figure S6. At zero CO2 coverage, all the measured ∆Hθ=0 values are between 95 and 100 kJ/mol, which are comparable to low CO2 capacity results on a 33.3 wt% PEI/SBA-15 reported by Potter et al48. Although ∆Hθ decreases with CO2 coverage, the extent is smaller at higher temperatures. This suggests at the same surface coverage, CO2 is more strongly bound at elevated temperatures, which is likely because the sorbed CO2 forms carbamate throughout the bulk PEI. A larger portion of CO2 forms less stable carbamic acid on the exposed surface of PEI at low temperatures, due to diffusion resistance and limited accessibility of CO2 to buried amines.

As listed in Table 2, the equilibrium capacity increases slightly with temperature, maximizes at 60℃ (Tmax), followed by a decrease at higher temperatures.

This trend shows maximum behavior in CO2

capacity, similar to the breakthrough results, except for the difference in Tmax (50℃ in BTR-DSC and 60℃ in VA-DSC). Previously, the low CO2 capacity at low temperatures was attributed to kinetic issues, which apparently should not exist in true equilibrium measurements, however, the maximum behavior is still observed.

This can be understood if the CO2 diffusion becomes extremely slow that

the system is not in true equilibrium (i.e., quasi-static).

The influence of temperature on the heat of CO2 sorption is shown in Table 2; the ∆Hθ at 0.1 bar CO2 increases with temperature from 70 to 93 kJ/mol. The temperature-dependent heat of CO2 sorption has also been observed by Kim et al. using aqueous amines49, and by Zhang et al. using 15



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PEI-impregnated silica21, the trend of which is in consistent with our results. However, the reason of temperature effects on the heat of CO2 sorption to amines has not been thoroughly discussed in these studies. Considering ∆Hθ decreases with capacity as discussed above, we attribute the increasing ∆Hθ at higher temperatures to the penetration of CO2 into the bulk PEI, allowing more CO2 to form the more stable ammonium carbamate intermediate.

It is probable under equilibrium sorption

conditions, primary-CO2-primary interactions (Figure S8) are maximized, since they are enthalpically favored over interactions involving secondary and tertiary amines, exhibiting higher heat of sorption49–52. Another contribution to the higher heat of sorption is the stabilization of sorbed CO2 via hydrogen bonding, which also increases with amine density as suggested by Tumuluri et al47.

16


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Figure 2. (a) Isotherms for CO2 sorption and (b) heat of sorption at different CO2 coverage at 60℃.

17



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Table 2. Equilibrium CO2 capacity on 40%PEI423/SiO2 and heat of CO2 sorption 76 Torr and different temperatures.

Temperature (℃)

a b

qs,VAa (mmol/g)

∆Hs (kJ/mol CO2)b

25

2.16

-70.8

40

2.28

-75.2

50

2.38

-79.6

60

2.41

-86.2

70

2.03

-90.1

80

1.85

-92.8

qs,VA refers to CO2 capacity at 76 Torr under equilibrium condition. Integral heat of sorption up to a CO2 pressure of 76 Torr.

() ∆ = × , where t0 is the time where pressure reaches 76 torr (P0), q(t) is the transient heat flow, m is the mass of the sample, and qs,VA is the CO2 equilibrium capacity at 76 torr.

18


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3.

Comparison between BTR-DSC and VA-DSC

The amine efficiency (

!"# : % )

under both equilibrium and transient sorption conditions are plotted

in Figure 3. The sorption under both conditions show maximum behavior, which has also been observed in sorption to solid amines measured by thermogravimetric analysis20,22,24,29,30,53.

The

temperature of maximum capacity varies with amine loadings. For example, Li et al. reported the optimal temperature increases from 45 ℃ at 10 wt. % PEI loading to 115℃ at 60% PEI loading24. In the current study, the transient sorption capacity is lower than the equilibrium capacity, which has been reported by Zhang et al.20. They compared breakthrough capacity measured in a fluidized bed versus equilibrium capacity in TGA analysis at similar conditions. The lower capacity in transient sorption conditions is attributed to short sorption time scale and rapid increase in viscosity, leading to increased diffusion resistance.

While the time period for CO2 sorption in the breakthrough

experiment is 500 s, it took days for the VA-DSC measurements to reach equilibrium (quasi-static) at 76 Torr CO2, which is longer than all reported CO2 sorption measurements.

The extended time to

reach equilibrium allows CO2 to maximize pairwise interactions. Although convection due to bulk gas flow facilitates CO2 mass transfer from the gas phase to the surface of the impregnated aminosilica, micropore diffusion into the bulk PEI is much slower and independent of the bulk gas flow. Tong et al. suggested CO2 can diffuse among the amine sites through “hopping” mechanism11, and Wilfong et al. revealed readsorption of CO2 among the amine sites inside the pores with in situ DRIFTS technique54. As the VA-DSC instrument doses a small amount of CO2 and waits a long period for equilibration, the diffusion limitation increases slowly as well as the CO2 capacity, which allows CO2 to diffuse into bulk PEI at elevated temperatures. In the BTR-DSC measurement, the 19



PEI was immediately exposed to 76 Torr of CO2 during the pressure upswing.

Page 20 of 35

As shown in the mass

spectrometer signal in Figure 1, the CO2 signal reached the plateau region within 200 s. In the transient sorption scenario, CO2 instantly binds to amines on the exterior surface of the sample and reaches an “apparent” saturation state. In the transient sorption scenario, CO2 instantly binds to amines on the exterior surface of the sample and reaches an “apparent” saturation state. This rapid sorption results in the formation of bulky amine-CO2 groups (carbamic acids and carbamates) and ionic interconnected networks54.55 throughout the surface, creating increasing diffusion barriers that CO2(g) must overcome to gain access to free amines buried in the bulk of the PEI polymer. It is likely in a breakthrough experiment, a significant fraction of buried amines are never accessed due to slow diffusion on the time scale in this experiment; a similar phenomenon is possible in any relevant practical PSA operation. Therefore, the incorporation of capacity and heat of sorption results in the breakthrough experiments should be considered as a reference for a real CO2 capture scenario, instead of those from equilibrated volumetric sorption, since the BTR situation better simulates the dynamics of gas and transient sorption scenario.

The amine efficiency under equilibrium condition (VA-DSC) is still lower than the theoretical efficiency of 0.5, indicating a large fraction of amines not accessible to CO2. The reduced efficiency has been observed previously56–59 and two predominant explanations have been put forth. Amines comprising the first monolayer of PEI (determined to be 10-15 wt. % PEI on SBA-15 support56,57) participate in hydrogen bonding with silanol groups, which eliminates their ability to bind CO260. Another viable reason is the small(est) pores filled with PEI are unable to accommodate CO2 to stoichiometric equivalent due to steric hindrance. In this work, the pore filling by PEI leads to a 20


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significant reduction (~74%) in the total pore volume of the silica support, as shown in Table S1.

The different patterns in heat of sorption also implies the different CO2 sorption performance under transient and equilibrium conditions.

The ∆Hθ increases with temperature in VA-DSC experiments,

but ∆Hs is relatively independent of temperature in BTR-DSC experiments.

We hypothesize the

buried sites61 – which are subject to diffusion limitations and are only accessible under equilibrium conditions and at high temperatures – have higher affinity towards CO2 due to the higher amine density in the bulk PEI. The three-dimensional packing of amines in the buried layers, compared to amines on the exposed surface distributed two-dimensionally leads to a higher probability of pairwise interaction.42

The long sorption time also allows for the optimization of primary-primary interaction

that are the most stable form of sorbed CO249,50,52.

In summary, both the capacity and ∆Hs is smaller under transient sorption conditions (BTR-DSC) than that under equilibrium conditions (VA-DSC) due to slowed diffusion and the short cycle time of transient operation.

CO2 mainly interacts with the surface amine sites under transient sorption

conditions, while buried amine sites with a greater CO2 affinity under equilibrium conditions, especially at elevated temperatures become available leading to increased CO2 capacity and heat of sorption.

21



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Figure 3. Amine efficiency in BTR-DSC (■) and VA-DSC (●) measurements at different temperatures. The theoretical maximum efficiency under dry conditions is 0.5 where each CO2 molecule binds two amine groups.

22


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Conclusion Compared to a breakthrough reactor operating on fast and cyclic operation, equilibrium sorption measurement renders higher CO2 capacity as well as higher heat of sorption values.

The increased

time of the equilibrium sorption measurement allows the diffusion of CO2 into the buried parts of the sorbent under equilibrium conditions, while there is very little penetration under transient sorption conditions. Hindered CO2 diffusion in transient sorption experiments is most likely important. The abrupt increase in PEI surface coverage under transient sorption conditions quickly forms numerous bulky carbamic acid and carbamate groups, leading to barriers for the diffusion of CO2 into buried layers. In addition, pairwise interactions with stronger binding towards CO2 are more relevant in the buried regions due to the greater local amine density than on the density on exposed surface. Thus, amine sites in the buried layers have higher CO2 affinity towards, leading to a higher heat of sorption. Thermodynamic and kinetic parameters obtained from BTR-DSC combination should increase our fundamental understanding of the nature of sorbate-solid interactions under transient sorption conditions, and could be applied in variety of fields such as catalysis, corrosion, mapping different chemical sites, etc. The breakthrough reactor is similar to fixed-bed sorption columns used in industrial pressure-swing and temperature-swing sorption processes, which provides essential information to predict the sorption process at larger scale.

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Supporting Information Material preparation, TGA and N2 adsorption characterization at 77 K; schematic of the combined breakthrough reactor (BTR), mass spectrometer (MS) and differential scanning calorimetry (DSC); calculation of the breakthrough capacity; estimation of the axial heat transfer in the breakthrough reactor; an example of transient heat flow versus time in a VA-DSC measurement at 60℃; and four possible formation of carbamate ion pairs through primary and secondary amines.

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Acknowledgements The authors acknowledge the US National Science Foundation (NSF grant # CBET-1551119) for financial support of this work. Additional support of this work was provided to RMR by the Institutes for Energy and the Environment at the Pennsylvania State University. We acknowledge McMahan Gray and James Hoffman from the National Energy Technology Laboratory for the PEI423 material.

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Page 26 of 35

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Capacity and Heat of Sorption on a Polyethylenimine-Impregnated

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