Nonvolatile Colloidal Dispersion of MgO Nanoparticles in Molten Salts

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Non-volatile colloidal dispersion of MgO nanoparticles in molten salts for continuous CO2 capture at intermediate temperatures Takuya Harada, Paul Brown, and T. Alan Hatton ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00911 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Sustainable Chemistry & Engineering

Non-volatile colloidal dispersion of MgO nanoparticles in molten salts for continuous CO2 capture at intermediate temperatures Takuya Harada, Paul Brown, and T. Alan Hatton*. Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 USA. *Corresponding Author E-mail: [email protected]

KEYWORDS: CO capture, utilization and storage (CCUS), Global warming, Non-aqueous colloidal absorbent, 2

MgO nanoparticles, Molten salts, Intermediate temperature, Continuous separation.

ABSTRACT:

The establishment of advanced CO capture, utilization and storage (CCUS) technology is a crucial 2

challenge for the mitigation of serious on-going climate change. Herein, we report non-aqueous

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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

Page 2 of 30

colloidal dispersions of MgO nanoparticles in molten salts as a new class of fluid absorbents for continuous CO capture at intermediate temperatures ranging from 200 to 350 ºC. The colloidal 2

absorbents were developed by dispersion of the nanoparticles in three different types of thermally stable

low-melting

point

salts:

ternary-eutectic

alkali-metal

nitrates

((Li-Na-K)NO ), 3

tetraphenylphosphonium bis(trifluoromethane)sulfonimide ([P(Ph) ][NTf ]), and their mixtures. 4

2

The new absorbents show high CO uptake performance with acceptable rheological properties at 2

the target temperatures. The analysis of reaction rate kinetics in the uptake of CO revealed that 2

CO can diffuse quickly into the molten salts to initiate the rapid formation of carbonates on the 2

surfaces of MgO nanoparticles dispersed in these molten salts. These results demonstrate that the new colloidal dispersions could be used as fluid absorbents for advanced continuous CO capture 2

processes at the temperatures of exhausts from fossil fuel combustion reactors without the energy losses incurred upon cooling of the gases as required for traditional absorption systems.

INTRODUCTION: The establishment of advanced high-efficiency CO capture processes is a crucial challenge for the 2

mitigation of serious on-going global warming issues. The benchmark industrial process for CO 1–4

2

capture is the amine-scrubbing technique, where aqueous solutions of alkanol-amines (typically ~25-30 wt%) such as MEA (Monoethanolamine), DEA (Diethanolamine) and TEA (Triethanolamine) are utilized as the absorbents to separate CO from the exhaust stream. The 5

2

circulation of aqueous amine solutions between the absorber and the stripper, which are maintained at different temperatures allows, the continuous operation of CO capture units. In recent years, 2

5–7

molten salts with melting points lower than 100 ºC, called “Ionic liquids (ILs)”, have been studied


2

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widely as CO absorbents.

8–14

2

The IL-based absorbents have the advantages of negligible vapor

pressure, non-flammability and thermal stability, which contribute to the lowering of energy and capital costs for CO capture operations.

15,16

2

For the capture of CO at high temperatures, various 2

types of solid adsorbents, such as hydrotalcites (HTLs),

17–19

oxides (CaO) , lithium-zirconates (Li ZrO ), 25–27

2

3

28–31

magnesium oxides (MgO),

lithium-silicates (Li SiO ),

32–34

4

4

20–24

calcium

and lithium-borates

(Li BO ) have been proposed. These solid adsorbents show superior uptake performance at 35

3

3

temperatures greater than 200 ºC when used in a temperature-swing mode. Nevertheless, it is difficult to develop low-cost continuous CO capture systems using the solid adsorbents due to the 2

lack of rheological fluidity required for the smooth circulation of the adsorbents between the absorber and the stripper. Here, we propose a non-aqueous colloidal dispersion of nanoparticles of solid adsorbents in molten salts as a new class of fluidic CO absorbents that can be used at high 2

temperatures. The colloidal absorbents were prepared by dispersing MgO nanoparticles in three different types of

molten

ionic

salts,

a

ternary

([Li]:[Na]:[K]=0.30:0.18:0.52),

mixture

of

alkali-metal

tetraphenylphosphonium

nitrates,

(Li-Na-K)NO

3

bis(trifluoromethane)sulfonimide

([P(Ph) ][NTf ]), and a mixture of the two salts. MgO is a well-known basic metal oxide that can 4

2

work as a CO adsorbent with high theoretical uptake capacity (24.8 mmol·g ) over an intermediate −1

2

temperature range of 200 to 350 ºC. It was shown recently that the reactivity of CO on the surface 36

2

of MgO is enhanced dramatically by a surface coating of molten nitrate/nitrite salts.

20–24

The molten

salts used as the dispersants in the current work possess low melting points at around 130 ºC. (LiNa-K)NO works also as a valid reaction mediator to enhance the CO uptake by MgO. 3

2

[P(Ph) ][NTf ] is an organic salt with high thermal stability, which was selected because 4

2

tetraphenylphonsphonium, [P(Ph )], cations are known to form stable salts and are relatively inert 4


3


Page 4 of 30

compared to imidazolium, tetraalkylammonium (TAA) and tetraalklyphosphonium (TPP) analogues. CO uptake by the colloidal adsorbents was examined through measurements of weight 37

2

variations in the colloidal dispersions under a flow of 100% CO at atmospheric pressure. CO 2

2

uptake rates were analyzed further in terms of the kinetic rate equation for gas-liquid-solid interfaces. The rheological behavior of the colloidal absorbents was examined with a rheometer equipped with a temperature- controlled sample chamber.

EXPERIMENTAL SECTION: Materials. Magnesium acetylacetonate dehydrate (98%), 1,2-tetradecandiol (90%), oleic acid (90%), oleylamine (70%), benzyl ether (98%), lithium nitrate (LiNO , Reagent-Plus), sodium 3

nitrate (NaNO , 99%), potassium nitrate (KNO , 99%), tetraphenylphosphonium chloride 3

3

([P(Ph) ]Cl, 98%), and bis(trifluoromethane)sulfonamide lithium salt (99.95%) were purchased 4

from Sigma Aldrich. Methanol (anhydrous, MACRON AR ACS Reagent grade), ethanol (200 proof anhydrous, KOPTEC, 99.5%), and hexane (MACRON AR ACS Reagent grade) were purchased from VWR. All chemicals were used as received without further purification. Preparation of MgO nanoparticles. Nanoparticles of MgO were prepared by a non-hydrolytic sol-gel process, followed by calcination in air to remove organic species adsorbed on the surfaces of the particles. Typically, magnesium acetylacetonate dehydrate (20 mmol), 1,2-tetradecandiol (20 mmol), oleic acid (10 mmol), and oleylamine (20 mmol) were mixed in 20 mL of benzyl ether and stirred vigorously under flowing nitrogen. The mixture was heated at 200 ºC for 2 h, followed by reflux at 300 ºC for 1 h. The solution was cooled down to room temperature and centrifuged after the addition of methanol (~40 mL). The precipitate was rinsed with hexane (~40 mL),


4

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dispersed in ethanol (~40 mL), and centrifuged again. The precipitate was dried overnight and ground in an agate mortar, before calcination in air at 500 ºC for 6 h to obtain the nanoparticles of MgO without adsorbed surfactants. Morphological, chemical and crystallographic features of the nanoparticles are summarized in the Supporting Information (Figure S1). Preparation of (Li-Na-K)NO and [P(Ph) ][NTf ]. A ternary mixture of alkali-metal nitrates at 3

4

2

the eutectic composition of (Li-Na-K)NO ([Li]:[Na]:[K] = 0.30:0.18:0.52) was prepared by the 3

thermal fusion of LiNO , NaNO , and KNO . In brief, powders of LiNO , NaNO , and KNO were 3

3

3

3

3

3

mixed at a molar ratio of 0.30:0.18:0.52 in an agate mortar, and melted in an aluminum crucible at 500 ºC for 4 h in an oven. After cooling to room temperature, the block salt was crushed and ground in the agate mortar to obtain the eutectic salt powder. [P(Ph) ][NTf ] was synthesized 4

2

according to literature reports. A typical synthesis involved mixing of the [P(Ph ]Cl (0.051 moles) 38

4)

with LiNTf (1 mol. eq.) in distilled water. The precipitate of the newly formed compound 2

tetraphenylphosphonium bis(trifluoromethane)sulfonimide, [P(Ph) ][NTf ] was filtered off as a 4

2

white solid and washed five times with distilled water. The compound was then dried in vacuo for 72 hrs at 80 C. Elemental analysis indicated the loss of chloride ions and P-NMR (CDCl , 300 o

31

3

Hz, 298 K) indicated a peak at δ = 24.49 (s). Thermo-physical and rheological behavior of the salts obtained by the above procedures are summarized in the Supporting Information (Figure S2, Figure S3, and Table S1). Preparation of colloidal dispersions of MgO nanoparticles in molten salts.

Colloidal

dispersions of MgO nanoparticles in molten salts were prepared by the heating of mixed powders of MgO nanoparticles and salts beyond the melting points of the salts. For the preparation of the colloidal dispersion in (Li-Na-K)NO or in [P(Ph) ][NTf ], the appropriate amounts of MgO 3

4

2

nanoparticles and salts were dispersed in methanol to make a mixed suspension. Typically, 80 mg


5


Page 6 of 30

of MgO nanoparticles and appropriate amounts of the salts were dissolved in 30 ml of methanol. After ultrasonication, the solvent was evaporated. The precipitate was re-dispersed in ethanol and dried again at 90 ºC overnight. The dried precipitants were ground again in an agate mortar to obtain a mixed powder of MgO nanoparticle clusters and the salts. The colloidal dispersions in mixed salts of (Li-Na-K)NO and [P(Ph) ][NTf ] were prepared by two methods. In the first, 3

4

2

appropriate amounts of (Li-Na-K)NO (20 mol% of MgO) were deposited on MgO nanoparticles 3

prior to dispersion in the salts (“pre-coated”) by the evaporation-induced precipitation of the nitrate salts from the mixed dispersion of MgO nanoparticles and alkali metal nitrates in methanol. After ultrasonication, the solvent was evaporated and the particles were re-dispersed in ethanol with [P(Ph) ][NTf ], followed by ultrasonication and solvent evaporation to obtain mixed precipitates of 4

2

the MgO nanoparticles and salts. The precipitate was re-dispersed in methanol or ethanol and dried again at 90 ºC overnight. The dried solid was ground in an agate mortar and heated to over 150 ºC to obtain the colloidal dispersion. In the second method, appropriate amounts of MgO nanoparticles and two types of salt, (Li-Na-K)NO and [P(Ph) ][NTf ], were dissolved in methanol 3

4

2

to make a mixed suspension. The mixed powder precipitated out upon solvent evaporation. The precipitate was re-dispersed in methanol or ethanol and dried again at 90 ºC overnight. The dried solid was ground again in an agate mortar and heated to over 150 ºC to obtain the colloidal dispersion, dubbed “co-mixed” in the discussions that follow. Sample characterization. Size and morphological features of MgO colloidal nanoparticles assynthesized were examined using Transmission Electron Microscopy (TEM, JEM-2010; JEOL). The samples for TEM were prepared by drop casting of the particle dispersion in ethanol on a carbon coated TEM grid and then allowing the grid to dry. Phase composition and crystallographic states of the particles were identified by powder X-ray diffractometry (XRD, X’Pert Pro


6

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Multipurpose Diffractometer; PANalytial). Molecular specification on the surface of the particles was examined by Fourier Transform Infrared Spectroscopy (FT-IR, NEXUS 470; Nicolet). Particle size distributions of the nanoparticles in the polar solvent were measured by dynamic light scattering (DLS, NanoBrook Omni, Brookhaven). Viscosities of the colloidal dispersions were measured on a temperature controlled rotational rheometer equipped with an Environmental Testing Chamber (AR 2000-ETC; TA Instrument). CO uptake was measured by the weight 2

variation in the colloidal dispersions placed in the sample pan under a flow of 100 % dry CO

2

(Airgas) at atmospheric pressure using thermogravimetric analysis (TGA, TGA Q50; TA Instrument). The appropriate operation conditions for repeated cycles of CO absorption and 2

desorption were determined by TGA under 100% N for a colloidal dispersion of 20 wt% MgO 2

nanoparticles in mixtures of (Li-Na-K)NO and [P(Ph) ][NTf ] (pre-coated) at a 1 : 8 ratio by 3

4

2

weight, as shown in Figure S4.

RESULTS AND DISCUSSION: The CO uptake performance of colloidal dispersions of MgO nanoparticles dispersed in molten 2

salts was examined first with dispersions of 20 wt% MgO nanoparticles in salts of three different compositions: (Li-Na-K)NO , [P(Ph) ][NTf ], and their mixture, as shown in Figure 1. Here, the 3

4

2

average size of MgO nanoparticles was 130 nm (see Figure S1). It was clarified that the uptake of CO proceeded when the nanoparticles were dispersed in solvents containing nitrate salts, but not 2

in the absence of these salts. In pure (Li-Na-K)NO , the uptake initiated after a delay of a few 3

minutes, and then increased at a constant rate. The uptake capacity exceeded 4.7 wt% based on the weight of the colloidal dispersion, corresponding to 23.5 wt% based on the weight of MgO


7


Page 8 of 30

nanoparticles in the dispersion, after 4 h of reaction with 100% CO . When the nanoparticles were 2

dispersed in a mixture of (Li-Na-K)NO and [P(Ph) ][NTf ], the uptake increased more rapidly than 3

4

2

for the dispersion in pure (Li-Na-K)NO , with no initial delay, and reached ~8.4 wt% (for “co3

mixed”) and ~9.6 wt% (for “pre-coated”) based on the weight of the colloidal dispersion, corresponding to 42.0 wt % and 47.9 wt% based on the weight of MgO nanoparticles, respectively. Figure 2 (a) and (b) show the variations in external appearance of the dispersions in (Li-NaK)NO and in the (Li-Na-K)NO - [P(Ph) ][NTf ] mixture (pre-coated); as the solid crystals of the 3

3

4

2

salts transformed into molten salts at temperatures above their melting points of 130~135 ºC (see Figure S2), the nanoparticles mixed with the salts dispersed in the molten salts to yield colloidal nanoparticle dispersions. The dark brown color of the colloidal absorbents could be ascribed to the photonic absorption by carbon on the surfaces of the MgO nanoparticles generated during the calcination of surfactants and/or by surface oxygen deficiencies on the MgO nanoparticles. The viscosities of the colloidal dispersions with different nanoparticle loadings at 300 ºC are shown in Figure 3 (a) and (b). These dispersions, in particular those with low nanoparticle loadings (5 and 10 wt%), exhibited low viscosities under moderate shear rates. The viscosities of the colloidal dispersions in the (Li-Na-K)NO - [P(Ph) ][NTf ] mixture were lower than those of the dispersions 3

4

2

in pure (Li-Na-K)NO . In both cases, the suspensions showed “shear-thinning” behavior. The 3

variations were described well by a power-law model, indicating that the suspensions behaved as typical pseudo-plastic liquids without flocculation of the dispersed nanoparticles. The viscosity 39

increased with increasing nanoparticle loading, although the viscosity value was low (10.0 cp in (Li-Na-K)NO and 1.4 cp in (Li-Na-K)NO - [P(Ph) ][NTf ] mixture) at high shear rate (1000 s ) -1

3

3

4

2

even for the dispersions of 20 wt% MgO nanoparticles. The difference in the viscosities of the colloidal dispersions in the two salts can be attributed primarily to differences in the viscosities


8

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and densities of the pure [P(Ph) ][NTf ] and (Li-Na-K)NO salts where the values for the organic 4

2

3

salts are much lower than those for the nitrates, as shown in Figure S3 and Table S1. The CO

2

uptake performance of the colloidal dispersions was examined further for dispersions with different loading amounts of MgO nanoparticles under different uptake conditions, as summarized in Figure 4 (a)-(c) for the dispersions in pure (Li-Na-K)NO , and in Figure 5 (a)-(c) for the 3

dispersions in (Li-Na-K)NO -[P(Ph) ][NTf ] mixtures (pre-coated, w 3

4

2

(Li-Na-K)NO3)

/w =0.43, w: weight). MgO

The results revealed that the uptake capacity increases with increasing MgO loading, as shown in Figure 4(a) and Figure 5(a). It is clear that dispersions with higher nanoparticle loadings absorb CO at higher rates to attain larger uptake capacities. The initial delay of a few minutes in the CO 2

2

uptake observed with the 20 wt% MgO nanoparticles in (Li-Na-K)NO , could be due to partial 3

aggregation of the nanoparticles at high concentration, with restricted diffusion of CO into the 2

aggregates. A slight decrease in weight of the colloidal dispersion in [P(Ph) ][NTf ] and similar 4

2

behavior for the molten salts without MgO could be due to evaporation of trace amounts of residual water dissolved in the molten salts and/or a slight thermal decomposition of the molten salts. CO uptake at different temperatures with 5 wt% MgO nanoparticles in (Li-Na-K)NO revealed 2

3

that the highest uptake is recorded at 300 ºC, as shown in Figure 4(b). The cyclic regenerability and stability under repeated cycles of CO absorption and desorption were also examined, as shown 2

in Figure 4 (c). CO uptake by the colloidal dispersion in (Li-Na-K)NO shows good cyclic 2

3

regenerability with a slight lowering of the uptake over the ten cycles shown. The nanoparticle dispersion in the mixture of (Li-Na-K)NO and [P(Ph) ][NTf ] exhibited higher CO uptake than 3

4

2

2

did the dispersions with the same loading of MgO nanoparticles in pure (Li-Na-K)NO . The highest 3

uptake capacity by the sample with 20wt% MgO nanoparticles exceeded 10.7 wt% over 4 h of reaction with CO at 280 ºC. The desorption of CO during regeneration at a fairly low temperature 2

2


9


Page 10 of 30

of 300 ºC under N , was accelerated by the nitrate salts which facilitated the dissociation of the 2

carbonates.

23,40,41

The cyclic absorption-desorption test in this case, however, shows a lowering of

uptake capacity with increasing cycle number, as shown in Figure 5(c). The results demonstrate that colloidal dispersions of MgO nanoparticles in the molten nitratecontaining salts possess high CO uptake reactivity and rheological fluidity at around 300 ºC. As 2

reported previously,

23,24

(Li-Na-K)NO works as an effective reaction mediator to promote the 3

uptake of CO by MgO at temperatures above the melting point of the nitrate salt. The effects were 2

ascribed to the prevention of the formation of a rigid, impermeable product layer on the surface of the MgO and the facilitation of the generation of porous MgCO through the inter-dissolution of 3

CO and MgO in the molten salts to initiate the nucleation of carbonate crystals.

23,24

2

These

observations suggest that the nitrate salts also function as a dispersing medium to realize the colloidal dispersion of MgO nanoparticles when the volume fraction of the nitrates salts relative to the nanoparticles is high. When the mixed salts of (Li-Na-K)NO and [P(Ph) ][NTf ] are utilized 3

4

2

as the dispersing medium, the uptake of CO is enhanced further. [P(Ph) ][NTf ] has a lower mass 2

4

2

density and lower viscosity than (Li-Na-K)NO . The lower mass density of the salts results in the 3

lowering of viscosity through a decrease in the volume fraction of MgO nanoparticles in the molten salts. There are several reports demonstrating that nanoparticles of metal oxides can be dispersed well in molten salts by solvation forces.

42,43

The strong interaction of the nitrate anions with MgO

nanoparticles may induce the strong solvation forces needed to realize the high particle dispersibility observed in the molten salts. In addition, the nitrate anions could also facilitate the partial dissolution of MgO in the molten salts to accelerate the formation of carbonate crystals. On the other hand, the dissolution of CO may proceed faster with the high concentration of organic 2

salts. The CO uptake rate by the colloidal absorbents could be determined by a balance between 2


10

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the inter-dissolution of MgO and that of CO in the molten salts. CO uptake by the colloidal 2

2

dispersion in the mixed salts deteriorated over repeated cycles of absorption and desorption, possibly due to dissolution of the nitrate salts coating the surfaces of the MgO nanoparticles and partial decomposition of the organic [P(Ph) ][NTf ] salts in the presence of the highly oxidative 4

2

nitrates with a subsequent lowering of CO solubility in these molten salts. 2

A more detailed understanding of the reaction processes for the uptake of CO by the colloidal 2

dispersions can be gained by a kinetic rate analysis of the transport processes occurring at the gasliquid-solid interface.

44,45

First, we defined the colloidal dispersion as a typical gas-liquid-solid

double interface system, as shown in Figure 6 (a) and (b). Here, for simplicity, the molten salts layer was assumed to be a single liquid phase, over which the concentration of CO varies only in 2

thin interfacial boundary layers (δ , δ , δ ) and is constant in the bulk layers. In addition, there is a g

L

s

concentration jump (ΔC = C - C ) at the gas-liquid interface due to the difference in physical gL

g,i

L,i

solubility of molecules in the two phases. In this model, the CO reaction rate equation at steady 2

state (J ) can be written as, CO2

𝐽#$% = 𝑘( 𝑎* +𝐶( − 𝐶(,/ 0

(Bulk gas to Gas-Liquid interface)

(1)

= 𝑘* 𝑎* +𝐶*,/ − 𝐶* 0

(Gas-Liquid interface to bulk liquid)

(2)

= 𝑘1 𝑎2 (𝐶* − 𝐶4 )

(Bulk liquid to particle)

(3)

= 𝑘6 𝑎2 𝐶4 (𝐶* )

(Solid surface reaction)

(4)


11


Page 12 of 30

where, J is the molar flux of CO , a , and a are the gas-liquid and liquid-particle interfacial areas, CO2

2

L

p

respectively, k is the surface reaction rate coefficient, and k , k , and k are mass transfer s

g

L

c

coefficients. Combining these equations, we obtain,

8

#7 9:;

=

=

=

+>

7 ?@

@ B?@

=

=

+>

=

C B?D

=

+>

=

E B?D

=

F , (γ = CL,i/Cg,i).

=

=

C

E

= ? + > B F + B? N> + > O @

7

@

D

(5)

With d as the particle diameter of particles and m as the loading of the particles in the colloidal p

dispersion, we have, 𝑎2 = 6𝑚⁄𝜌𝑑2

(6)

Thus, the rate equation becomes, #7 89:;

=

=

=

UV

=

=

C

E

= ? + > B F + WBXD N> + > O @

7

@

(7)

Here, the first term represents the reaction at the gas-liquid interface and the second term gives the reaction at the particle surfaces. For nanoparticles smaller than the thickness of the liquid interface layer (δ ), the reaction can be treated as if it takes place in a pseudo-homogeneous L

solution; if this reaction proceeds by a fast first order reaction, the mass transfer coefficient in the liquid (k ) is replaced by Ek , where E is an enhancement factor written as, L

L

46

𝐸~ 𝐻? btanh 𝐻? , 𝐻? =

√>a >@

(8)

in which H is the Hatta number, D is the diffusion coefficient in the liquid phase, and k is reaction a

rate constant. Then, the reaction rate equation is modified to be,


12

Page 13 of 30 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


#7 89:;

=

=

UV

=

=

=

C

E

= ? + c> BF + WBXD N> + > O @

7

@

(9)

Thus, the reciprocal of J should vary linearly with 1/m. The model predictions are confirmed by CO2

the linearity of the plot of J of CO uptake at 260 ºC at 2 h after the reaction starts versus particle CO2

2

loading (m) on the double reciprocal scale shown in Figure 6(c). The intercept is small, suggesting that the overall reactions are controlled primarily by the reaction at the surface of the MgO particles in the colloids. The result indicates that CO is dissolved rapidly in the molten salt dispersing 2

medium, which quickly becomes saturated. The generation of the carbonate ions (CO ) is 23

accelerated by the reaction of CO and MgO dissolved in the molten salts to nucleate MgCO 2

3

crystals on the surface of nanoparticles. The enhancement of uptake rates with nitrate salts mixed with [P(Ph) ][NTf ] can be explained by the increase in the maximum solubility of CO in the salts. 4

2

2

This increase in CO concentration on the surface of nanoparticles (C ) attributed to the increase of 2

s

CO solubility in the salts results in the enhancement of CO uptake reactivity by MgO 2

2

nanoparticles in the dispersions. The initial delay in the uptake observed with the dispersions in (Li-Na-K)NO at high uptake temperatures may be due to the lower CO solubility in the salts at 3

2

these higher temperatures, such that saturation of the molten salts with CO takes a few minutes 2

under these conditions.

CONCLUSION Colloidal suspensions of MgO nanoparticles dispersed in molten salts that include alkali-metal nitrates are proposed as a new class of liquid CO absorbents that can operate over intermediate 2

temperature ranges from 200 to 350 ºC. The colloidal dispersions showed high CO reactivity and 2


13


Page 14 of 30

rheological fluidity at the target temperatures, with good working capacity and cyclability. The partial replacement of the nitrate salts by [P(Ph) ][NTf ] led to the further enhancement of CO 4

2

2

uptake reactivity and rheological fluidity, although the uptake performance deteriorated over repeated absorption-desorption cycles. A detailed kinetic analysis of the rate of CO uptake by the 2

colloidal dispersions indicated that CO diffused into and saturated the molten salts to initiate the 2

formation of the carbonates at the surfaces of the dispersed MgO nanoparticles. These new fluidlike CO absorbents can be used over intermediate temperature ranges and show promise for the 2

design of advanced continuous CO capture units for hot exhaust streams thereby avoiding the need 2

for cooling of the hot gases required by conventional absorption systems, and minimizing the associated energy penalties.


14

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Figure 1. CO uptake under a flow of 100% CO at 260 ºC by colloidal dispersions of 20 wt% MgO 2

2

nanoparticles in (Li-Na-K)NO (green), in [P(Ph) ][NTf ] (blue), in a mixture of (Li-Na-K)NO and 3

4

2

3

[P(Ph) ][NTf ] (co-mixed) at a 1 : 8 ratio by weight (orange), and in a mixture of (Li-Na-K)NO 4

2

3

and [P(Ph) ][NTf ] (pre-coated) at a 1 : 8 ratio by weight (red). 4

2


15


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Figure 2. Variations in external appearance of mixed powders of MgO nanoparticles and the salts at room temperature, their colloidal dispersions at 300 ºC obtained at temperatures beyond the melting points of the salts, and TEM images of the colloidal dispersions after quick quenching; (a) MgO nanoparticles mixed with (Li-Na-K)NO (loading of MgO nanoparticles: 10 wt%), (b) MgO 3

nanoparticles mixed with (Li-Na-K)NO and [P(Ph) ][NTf ] (pre-coated, loading of MgO 3

4

2

nanoparticles: 10 wt%).


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Figure 3. Viscosities as a function of shear rate at 300 ºC for colloidal dispersions of MgO nanoparticles in (a) (Li-Na-K)NO (10 wt%) and (b) a mixture of (Li-Na-K)NO and [P(Ph) ][NTf ] 3

3

4

2

(pre-coated).


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Figure 4. CO uptake by colloidal dispersions of MgO nanoparticles in (Li-Na-K)NO . (a) CO 2

3

2

uptake at 260 ºC for colloidal dispersions with different MgO loadings; (b) CO uptake for a 5 wt% 2


18

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MgO colloidal dispersion at different temperatures; (c) CO uptake over repeated cycles of CO 2

2

absorption in 100% CO at 300 ºC and desorption in 100% N at 350 ºC. 2

2


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Figure 5. CO uptake by colloidal suspensions of MgO nanoparticles in mixtures of (Li-Na-K)NO 2

3

and [P(Ph) ][NTf ] (pre-coated). (a) CO uptake at 260 ºC by colloidal dispersions with different 4

2

2


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MgO loadings; (b) CO uptake by colloidal dispersions with 20 wt% MgO at different 2

temperatures; (c) CO uptake over repeated cycles of CO absorption in 100% CO at 260 ºC and 2

2

2

desorption in 100% N at 300 ºC. 2


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Figure 6. Schematic illustration of the model colloidal dispersion system for the kinetic analysis of the reaction rates with CO and the results of the experimental data fitted to the model equation. 2

(a) Schematic illustration of the colloidal dispersion; (b) Variation of CO concentration in the gas2


22

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liquid-solid double interface system; (c) Reciprocal of J (at 2 h) plotted as a function of the CO2

reciprocal of particle loading (m).

ASSOCIATED CONTENT Supporting Information Morphological, chemical and crystallographic features of MgO nanoparticles, thermo-physical and rheological behavior of molten salts used for the preparation of colloidal CO absorbents are 2

summarized in Supporting Information. The following file is available free of charge. 1). Supporting Information.pdf

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written with contributions from all authors, who have approved the final version of the manuscript. Funding Sources This work was supported financially by Saudi Aramco under the MIT Energy Initiative program.


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ACKNOWLEDGMENT: The authors acknowledge useful discussions and comments on this work by Dr. Esam Z. Hamad and Dr. Fritz Simeon. .

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Nonvolatile Colloidal Dispersion of MgO Nanoparticles in Molten Salts

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