Long-Term Effect of Steam Exposure on CO

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Long-Term Effect of Steam Exposure on CO2 Capture Performance of Amine-Grafted Silica Mohammadreza Fayaz, and Abdelhamid Sayari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15463 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Long-Term Effect of Steam Exposure on CO2 Capture Performance of Amine-Grafted Silica Mohammadreza Fayaz and Abdelhamid Sayari∗ Centre for Catalysis Research and Innovation, Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

ABSTRACT This study investigates the hydrothermal stability of triamine-grafted CO2 adsorbentbased on a commercial-grade silica (CARiACT, P10). Grafting was conducted in dry and wet conditions at 85 °C. At optimum grafting conditions using 0.2 cm3 water and 1.5 cm3 aminosilane per gram silica, the highest CO2 uptake of 1.93 mmol/g at 50 °C was obtained. This material was exposed to steam at 120 °C for up to 360 h. It was observed that increasing the duration of steam exposure from 3 to 24 h reduced adsorption uptake at 25 °C by 56 %. However, the CO2 uptake reduction was much less severe at higher adsorption temperatures, reaching 21% at 50 °C and only 4 % at 75 °C. Conducting steam treatment for 360 h, reduced adsorption uptake at 25, 50, and 75 °C by 83, 61, and 26 %, respectively. For this extreme steaming experiment, the decrease in CO2 uptake at all adsorption temperatures was attributed to the reduction of the sorbent average pore width, increasing diffusional mass transfer resistance. The results revealed that steam exposure did not reduce the amine loading or deactivate the amine groups; however, increasing exposure time decreased the average pore width, until partial collapse of material structure. Nevertheless, the large average pore width (21 nm) of the P10 silica led to higher hydrothermal stability of the amine-grafted sorbent compared to those with ordered pore structure supports, such as SBA-15 silica.

KEYWORDS Steam stability – CO2 capture – Commercial-grade silica – SBA-15 silica – Amine-grafted silica sorbent – Mesoporous material – Adsorption and desorption – Steam stripping

∗

Corresponding Author. Email: [email protected]; Tel: 613-562-5483 (A. Sayari)

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1. INTRODUCTION As a major greenhouse gas, carbon dioxide is an important contributor to climate change.1, 2 According to the US Environmental Protection Agency (EPA), CO2 represents ca. 82.2 % of all US anthropogenic greenhouse gas emissions.3 Therefore, significant research has been conducted on CO2 capture using different technologies.4, 5 Amine-based aqueous solutions such as monoethanolamine are extensively used for CO2 capture through absorption.6 Although amine scrubbing is a mature technology, it suffers from several drawbacks, such as large energy demand for regeneration and the corrosive nature of amine that increases capital cost.7-9 Compared to amine scrubbing processes, adsorption has been recognized as a more energy-efficient method.10 Potential CO2 adsorbents include metal organic frameworks (MOFs)11-13, coordination organic polymers (COPs)14, 15, zeolites16, 17, clays, carbons18, 19, and amine containing porous materials.20, 21 Recent progress in this field has been documented in authoritative reviews.20-26 Amine compounds may be incorporated into porous supports through two methods: (i) physical loading of the amine into or onto the support (amine impregnation)25, 27-30

(ii) covalent linking of amine-containing compounds to the support (amine grafting).10, 23, 24

Amine-grafted sorbents have several advantages over their impregnated counterparts. For instance, interactions between support and occluded species for amine impregnated sorbents are weaker than for amine-grafted sorbents, leading to possible losses by evaporation.31 Moreover, because of diffusion limitation, adsorption over amine-impregnated sorbents is often slower than on amine-grafted materials.20 Cyclic adsorption-desorption process for CO2 capture can be performed using different techniques, such as temperature swing adsorption (TSA), pressure swing adsorption (PSA), concentration swing adsorption (CSA)32, and steam-aided vacuum swing adsorption (SA2 ACS Paragon Plus Environment

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VSA).33 Most of research studies conducted adsorption-desorption cycles using TSA with an inert purge gas. However, this technique is not effective in concentrating and separating CO2, as both inlet and outlet streams are dilute CO2-containing gas mixtures.34 In contrast, steam regeneration was demonstrated to be an effective strategy35,

36

, particularly because low-

temperature steam is available in power plants as waste heat. The advantage of this regeneration method is that concentrated CO2 can be obtained by simple condensation of the moisture.37 Depending on the physical properties of the support and the method for adding amine, sorbents demonstrate different tolerance towards steam exposure.36 Sandhu et al.37 performed steam treatment at 110 °C on a polyethylenimine (PEI) impregnated silica (CARiACT Q10, from Fuji Silysia Chemical Ltd.) for a total of 5 h and observed 5 wt. % loss in adsorption capacity of the sorbent. Jones group34 conducted steam treatment at 105 °C on an alumina and a mesoporous silica supported PEI. Steam treatment for 24 h reduced CO2 uptake for the alumina-based sorbent by 16.3 %, and for the silica-based sorbent by 67.1 %, when adsorption was conducted at 25 °C using 10% CO2/N2. Furthermore, when using dilute CO2 stream (400 ppm), the uptake losses were 25.2 % and 81.3 % for the alumina and silica-based sorbents, respectively. In another study35, the same group found that for PEI-impregnated mesocellular silica foam (MCF) with thin pore walls, steam treatment resulted in structural collapse and consequently, severe reduction in adsorption capacity. Hammache et al.38 observed that after eight adsorption/steam regeneration cycles of a PEI-impregnated CARiACT G10 silica (total steam exposure of 3.5 h at 105 °C), the CO2 adsorption capacity decreased by 12 %. Based on their observations, they attributed the reduction in CO2 uptake to reagglomeration of the impregnated PEI inside the pores, leading to pore blockage and consequently, decrease in adsorption capacity. Jahandar et al.39 investigated steam stability of amine-grafted SBA-15. They found for the first time that the



effect of steam depends on the adsorption temperature. They observed that steam treatment at 120 °C up to 6 h reduced CO2 uptake at low adsorption temperature (25 °C), however, at mild (50 °C) and high adsorption temperature (75 °C), it did not have any negative effect on CO2 uptake. Further steaming up to 48 h had little effect on the material performance. They ruled out the possibility of structural collapse of steamed sorbent and attributed the reduction in CO2 capacity of steam treated sorbent to amine restructuring inside the pores. Min et al.40 compared the hydrothermal stability of four PEI-impregnated silicas, namely a macroporous silica (MacS) and three mesoporous silicas (MCF, SBA-15, and MCM-41). After exposing the sorbents to steam for 14 days at 120 °C, they observed that CO2 uptakes at 40 °C for MCM-41-, SBA-15-, and MCF-based sorbents reduced by 61, 50, and 42 %, respectively, with PEI losses ranging from 4 to 34 %. Nevertheless, in the presence of PEI-MacS the CO2 uptake decreased by only 10%, whereas the PEI loading surprisingly remained unchanged even though it was of relatively low molecular weight, MW=1200. The enhanced stability of PEI-MacS to steam was attributed to the thick pore walls of the support, and the lack of PEI leaching. With the exception of Min et al.40 who applied steam for 14 days, in all other instances, the adsorbents were exposed to steam for a limited time ranging from few hours21, 37, 38 to 48 h.39 Therefore, in the instances where the material seemed to be stable37-39, such a conclusion could be premature. The longest reported steaming for an amine-grafted material being only 48 h,39 raises the following question: what would happen upon significantly lengthier steam treatments of amine-grafted silica? Therefore, there is a need to investigate the effect of long exposure to steam for typical amine-grafted adsorbents. The current work is a contribution to fill this knowledge gap. As support we used a commercial-grade CARiACT, P10 silica from Fuji Silysia Chemical Ltd., similar to the material used by Sandhu et al.37 and Hammache et al.38 In addition


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to its interesting textural properties (vide infra), this silica seemed to be rugged enough.37, 38 Moreover, we chose grafting over impregnation, presumably for added stability, and to specifically observe the actual impact of long-term steaming on grafted materials. The grafting condition was optimized based on earlier findings by our group24, that controlled addition of water has extremely beneficial effect on (i) amine loading, (ii) amine efficiency for CO2 adsorption, i.e., CO2/N ratio, and (iii) adsorption kinetics. Steam treatment of such material for periods up to 15 days (360 h) was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials A commercial-grade silica (CARiACT, P10) from Fuji Silysia Chemical Ltd. was used throughout this study. SBA-15 mesoporous silica was used for comparison. The synthesis of SBA-15 was performed using triblock copolymer Pluronic P123 (Aldrich), hydrochloric acid (Fisher), tetraethyl orthosilicate (TEOS; 98 %, Aldrich), and distilled water. 3-[2-(2aminoethylamino) ethylamino]propyl trimethoxysilane (Tech) (TRI; Aldrich), pentane (99.6 %, Fisher), toluene (99.9 %, Fisher), and distilled water were used to perform grafting of silica samples. To conduct adsorption measurements, ultra-high purity (99.999 %) nitrogen and 5 % CO2 in nitrogen, supplied by Linde, were used.



2.2. SBA-15 synthesis The procedure for the synthesis of SBA-15 was thoroughly reported elsewhere.41 Briefly, Pluronic P123 (14 g) was added to a mixture of water (105 mL) and 2 M HCl (420 mL) aqueous solution in a Teflon-lined container. The mixture was continuously stirred for several hours at 40 °C until P123 was completely dissolved. Subsequently, TEOS (29.75 g) was added to the solution under vigorous stirring. After mixing for 5 min, the mixture was kept under static condition at 40 °C for 20 h, then for 40 h at 100 °C in an autoclave. The obtained solid was separated by filtration and washed with sufficient amount of distilled water until neutral pH. The solid product was subsequently dried, and calcined at 550 °C in flowing air for 5 h.

2.3. Triamine grafting The silica support was dried at 120 °C for 4 h. Then, dried silica (1 g) was stirred in toluene (30 mL) at ambient temperature for 1 h. A specific volume of water (0.0 to 0.6 cm3, in 0.1 cm3 increments) was added to the mixture. After 2-3 h mixing, TRI (1.5 cm3) was added to the suspension and the temperature was increased to 85 °C. Mixing was continued for 17-18 h. The solution was then filtered and the excess amine was washed off the grafted silica using toluene and pentane. The grafted samples were labelled as X-Y-Z, where X, Y, and Z correspond to the name of silica support, amount of added water, and amount of added TRI (cm3) per gram silica, respectively. In addition, SBA-15 and P10 grafted using 0.2 cm3/g water and 1.5 cm3/g triamine, i.e., SBA-15-0.2-1.5 and P10-0.2-1.5, were denoted as SG and PG, respectively.


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2.4. Steam treatment To assess their hydrothermal stability, steam treatment was performed on the silica supports (P10 and SBA-15) and two grafted materials (PG and SG). Steaming was conducted for different durations including 3, 6, 24, and 360 h. The sample was placed in a stainless steel tube (outer diameter: ½”) using glass wool plugs and was preheated in nitrogen at 120 °C for 2 h, then exposed to flowing steam. Distilled water (ca. 3 g/h) was pumped and injected into an evaporator to generate steam at 110 °C. To prevent any condensation, all the lines were wrapped by heating tapes and their temperature was kept at 120 °C. After steam treatment, the reactor was purged by flowing nitrogen for 30 min at 120 °C. While maintaining the nitrogen flow, the reactor was then cooled to ambient temperature. Calcination of the grafted and steamed samples was performed under flowing air at 550 °C for 5 h. Steam treated and calcined samples were denoted by adding “S” and “C” to their name, respectively.

2.5. Adsorption measurements CO2 uptake for different grafted materials was determined using a thermogravimetric analyzer (Q500 TGA, TA Instruments). Analysis was conducted on powdered samples (20 to 30 mg). The sample was heated at 120 °C in N2 for 90 min to remove any adsorbed species. Then, it was cooled down to 25 °C, at which point adsorption began by switching the purge gas to 5 % CO2 in N2. CO2 uptake was measured at low (25 °C), moderate (50 °C), and high (75 °C) temperature and adsorption time of 1 h using the corresponding mass balance. After completing adsorption at 75 °C, the purge gas was switched to nitrogen and the temperature was increased to 800 °C to decompose the amine-containing moieties. The purge gas was then switched to air and the sample was kept at 800 °C for 10 min to burn out any carbonaceous residue on the support. The amine content was calculated based on the weight loss beyond 200 °C.24



2.6. Sorbent characterization The textural properties of the samples were determined by nitrogen adsorption at -196 °C using a surface characterization analyzer (3Flex, Micromeritics), as described elsewhere.42 Before starting adsorption, the samples were degassed for 5 h at 120 °C. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method in the range of relative pressure of 0.06 to 0.2. The pore diameter was determined using the Kruk-Jaroniec-Sayari (KJS) method from the adsorption branch.43 The total pore volume was determined based on the volume of liquid nitrogen adsorbed at a relative pressure of 0.99. To investigate possible changes in the pore structure of SBA-15 upon steaming, TEM measurements were conducted using a FEI Tecnai G2 F20 operating at 120 kV and equipped with an Oxford X-ray detection systems – Aztec EDS. Samples were prepared by dispersing 10 mg of material (SBA-15, SG, and calcined SG) in ca. 5 mL isopropanol using an ultrasound bath for few minutes. Then, a drop of the mixture was placed onto a carbon-coated copper grid and dried in air.

3. RESULTS AND DISCUSSION 3.1. Triamine grafting Conducting grafting in wet conditions facilitated surface grafting and increased amine loading (Figure 1), because adding water results in hydrated surface with increased surface density of hydroxyl groups. Therefore, more alkoxy ligands can react with the silica surface. Moreover, the presence of water molecules in the solution facilitate the reaction between free silanes and those already attached to the support through Si-O-Si bridging.24 As illustrated in Figure S1, increasing water addition beyond 0.2 cm3/(g silica), did not significantly change grafting efficiency (molar ratio of grafted amine to the total amine used), as it reached a 8 ACS Paragon Plus Environment

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maximum of 67 %. Increasing water content of the grafting solution up to 0.3 cm3 per gram silica resulted in decreasing BET surface area and pore volume of the samples. After 0.3 cm3/(g silica), however, further increase in water content did not change the BET surface area and pore volume significantly, because the organic content of the samples did not change (Figure S2). The increase in amine content after addition of water resulted in decrease of samples mesoporosity. The progressive reduction in the mesoporosity of the grafted samples was also reflected in their pore size distributions (Figure S3). 10

50

8

40 6 30 4 20 2

10 0

Nitrogen Content (mmol/g)

60 Organic Content (wt%)

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


0 0

0.1

0.2

0.3

0.4

0.5

0.6

Water Added (cm3/g silica)

Figure 1. Organic (diamond) and amine (triangle) content of the grafted samples as a function of added water to the grating mixture for 1.5 cm3/(g silica) TRI addition Figure 2a depicts the CO2 uptake by all adsorbents at three different temperatures as a function of added water in the materials synthesis. It is seen that up to 0.2 cm3 added water per gram of silica, the CO2 uptake generally increases as a result of increasing amine loading as indicated in Figure 1. However, there is evidence that as the amount of added water increases, stronger diffusion resistance takes place. Up to 0.1 cm3 water per gram of silica, CO2 uptake 9 ACS Paragon Plus Environment


decreased as the adsorption temperature increased, indicating that diffusion resistance is limited, and the adsorption is thermodynamically controlled. In the presence of as little as 0.15 cm3 added water per gram of silica, the CO2 uptake at 50 ºC was higher than at 25 ºC, whereas the CO2 uptake at 25 ºC started decreasing at 0.2 cm3 added water per gram of silica, despite the higher amine loading. These findings indicate that adsorption became kinetically controlled because of increased diffusion resistance, consistent with the decreasing pore volume and pore width (Figure S2). As seen in Figure 2a, such resistance increases further in the presence of larger amounts of added water, while the amine content remained constant (Figure 1). In fact, water addition beyond 0.2 cm3/g resulted in increased diffusive mass transfer resistance and consequently, a larger decrease in CO2 uptake at low (25 °C) versus mild (50 °C) temperature. Nevertheless, over the same range of water added, the CO2 uptake at 75 °C changed only marginally, highlighting the role of high temperature in overcoming mass transfer limitation. Moreover, the increasing trend for amine efficiency at 75 °C (Figure 2b) implies that CO2 adsorption became kinetically controlled; hence increasing adsorption temperature led to more efficient utilization of amine species. The highest CO2 uptake of 1.93 mmol/(g silica) was achieved at 50 ºC in the presence of P10-0.2-1.5 adsorbent. This performance is comparable to that reported for triamine-grafted mesoporous silica.24


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2.0

25 C 50 C 75 C

(a)

Amount Adsorbed (mmol CO2/g)

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


1.6

0.4

25 C 50 C 75 C

(b)

0.3

1.2 0.2

0.8 0.1

0.4

Amine Efficiency (mmol CO2/mmol N)

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0.0

0.0 0

0.2

0.4

0.6

0

0.8

0.2

0.4

0.6

0.8

3

Water Added (cm /(g silica))

Figure 2. (a) CO2 uptake and (b) amine efficiency for P10 samples grafted using various amount of water added to the grafting solution and 1.5 cm3/(g silica) TRI addition Adding 0.1 cm3/(g silica) water to the grafting solution improved adsorption kinetics, which is possibly because of higher amine loading (Figure 3). As mentioned earlier, increasing water up to 0.3 cm3 progressively increased diffusional resistance, which is consistent with the decreased maximum adsorption rate and percentage of CO2 uptake at 1 min after starting adsorption (referred to as uptake percentage) up to 64 and 42 %, as shown in Figure 3. Further increase in added water from 0.3 to 0.6 cm3 did not have any additional effect on the kinetic parameters, as it did not increase the amine loading.


80

Uptake Percentage (%)

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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2.5

2

60

1.5 40 1 20

0.5

0

Maximum Adsorption Rate at 25 ̊C (mmol CO2/(g.min))


0

Figure 3. Percentage of CO2 uptake at 1 min after starting adsorption (filled bars) and maximum adsorption rate for P10 samples (patterned bars) grafted using various amounts of water added to the grafting solution and 1.5 cm3/(g silica) TRI

3.2 Steam treatment PG was exposed to steam at 120 °C for 3, 6, 24, and 360 h. Figure S4 shows that steam treatment resulted in negligible change in amine content; even after 360 h steaming the change in amine content was less than 6 %. This is consistent with Jahandar et al.39 who reported that the amine content in triamine-grafted SBA-15 was not affected by steam at 120 °C up to 48 h. In contrast, Jones group35 found that steam treatment resulted in reduction of organic content of several amine-grafted MCF samples. This was attributed to the collapse of the support and amine degradation under steam treatment. Increasing the duration of steam exposure to 24 h, reduced CO2 uptake at 25 °C by 56 % (Figure 4a). The reduction was significantly smaller at higher adsorption temperatures, as it 12 ACS Paragon Plus Environment

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reached only 21 and 4 % at 50 and 75 °C, respectively. Extending steam treatment to 360 h reduced adsorption uptake at 25, 50, and 75 °C by 83, 61, and 26 %, respectively. It is inferred that steaming increases the diffusion resistance, whose effect decreases at higher adsorption temperature. Since steaming did not change amine content, the amine efficiencies for each steam-treated sample have the same trend as CO2 uptakes for the corresponding sample (Figure 4b). 0.30

(b)

(a)

0.25

1.6

0.20 1.2 0.15 0.8

0h 3h 6h 24 h 360 h

0h 3h 6h 24 h 360 h

0.4 0.0 20

40

60

20

80

40

60

0.10 0.05

Amine Efficiency (mmol CO2/mmol N)

2.0 Amount Adsorbed (mmol CO2/g)

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


0.00 80

Temperature (°C)

Figure 4. (a) CO2 uptake and (b) amine efficiency as a function of adsorption temperature for PG after exposure to steam for different durations To further support the occurrence of increased diffusional resistance, adsorption rates for samples exposed to steam for different durations were calculated. As illustrated in Figure 5, exposing PG to steam for 360 h reduced the maximum adsorption rate at 25 °C by 88 %, indicating the occurrence of significant diffusional mass transfer resistance, albeit more strongly at low adsorption temperature.


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Adsoprtion Rate (mmol CO2/(g.min))


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1.6

0h 3h 1.2

6h 24 h 360 h

0.8

0.4

0 0

0.3

0.6

0.9

1.2

1.5

Time (min)

Figure 5. Adsorption rate of CO2 at 25 °C over PG after exposure to steam for different periods of time The results in this study are in agreement with earlier findings by Jahandar et al.39 showing that steaming of amine-grafted SBA-15 reduces CO2 uptake mostly at low adsorption temperature. They also showed that increasing steaming duration from 3 to 48 h, did not result in any further loss in CO2 uptake. However, the current results (Figure 4) show that, conducting steam treatment for sufficiently long time (360 h), resulted in decrease of CO2 uptake, even at adsorption temperature of 75 °C, indicating that some pores on PG were no longer accessible by CO2 molecules and likely blocked. Table 1 shows that 24 h steaming of the blank P10 did not significantly change its porosity, as after treatment, the BET surface area and pore volume of the sample reduced only by 5 and 2 %, respectively. This is consistent with the strong similarity between the nitrogen adsorption isotherms (Type IV), before and after steam treatment (Figure 6a). The stability of


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P10 structure towards steam treatment can be attributed to its thick silica walls as previously reported for a similar commercial-grade silica (Q10) with high hydrothermal stability.37 Moi et al.40 also showed that silicas with thick pore walls exhibit higher hydrothermal stability. The results in Table 1 also show that steam treatment progressively reduced the BET surface area and pore volume of PG, as steam exposure for 24 h resulted in 67 and 61 % reduction in BET surface area and pore volume, respectively. Although steam treatment reduced the porosity of PG, the mesoporous nature of material was preserved, as indicated by their Type IV nitrogen adsorption isotherms (Figure 6 a and b). Prolonged steam exposure (360 h) further reduced the BET surface area and pore volume by 71 and 97%, respectively, turning the sorbent into a material with limited porosity, as reflected by the shape of its nitrogen adsorption isotherm (Figure 6a and b). Interestingly, it was observed that the calcined sample (PG-C) has 45 % higher BET surface area and 55 % lower pore volume than P10 silica. For 24 h steam-treated sample, even larger differences were observed, as PG-24h S-C has 87 % higher BET surface area and 80 % lower pore volume than P10. Also PG-360h S-C has 64 % higher BET surface area and 93 % lower pore volume than P10. As discussed later, one possible reason for increasing BET surface area of PG-24h S-C and PG-360h S-C is the occurrence of a silica deposit upon decomposition of the grafted aminosilane, which supposedly exhibits higher roughness than the original silica surface. The lower BET surface area of PG-360h S-C compared to PG-24h S-C could be attributed to blockage of some pores during prolonged steaming as indicated by the much lower pore volume. The results show that as steam exposure time extends, the pores shrink to the point of structural collapse. Occurrence of structural collapse for PG-360h S-C is consistent with the lower CO2 uptake of PG-360h S at temperature as high as 75 °C. Although the overall amine 15 ACS Paragon Plus Environment


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content remained unchanged, a portion of amine can no longer be accessed by CO2 molecules even at high temperature. Table 1. BET Surface Area and Pore Volume of Various Samples sample

BET surface area (m2/g)

pore volume (cm3/g)

P10

327

1.53

P10-24h S

310

1.50

PG

48

0.31

PG-C

475

0.68

PG-3h S

34

0.22

PG-6h S

29

0.18

PG-24h S

16

0.12

PG-360h S

14

0.05

PG-24h S-C

611

0.41

PG-360h S-C

537

0.11

As seen in Figure 6a, the nitrogen adsorption isotherms corresponding to PG-C and PG24h S-C are no longer of Type IV, as at low relative pressures nitrogen uptake increased strongly before reaching a plateau, with little increase at relative pressures beyond 0.8. This behavior is similar to isotherm Type I, rather than IV. The shape of the isotherms as well as the high BET surface area of the calcined samples suggest the development of mesoporosity in smaller pore range, or possibly microporosity, within the sample. For PG-360h S-C, however, the nitrogen adsorption isotherm is clearly of Type I.


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Quantity Adsorbed (cm3/g STP)

1000

(a) PG-24h S-C PG-360h S-C PG-C P10-Blank P10-Blank-24h S PG PG-3h S See Figure 6b PG-6h S PG-24h S PG-360h S

750

500

250

0 0

200 Quantity Adsorbed (cm3/g STP)

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0.2

0.4 0.6 Relative Pressure (p/p°)

0.8

1

0.8

1

(b) PG PG-3h S PG-6h S PG-24h S PG-360h S

150

100

50

0 0

0.2


Figure 6. Nitrogen adsorption-desorption isotherms for (a) different materials, and (b) PG and PG steam-treated materials



Figure 7 shows that PG and steam-treated PG samples are mesoporous in nature; however, they have low mesoporosity. Likewise, PG-C and PG-24h S-C exhibit low mesoporosity, however, their pore size distributions show significant increase of pores smaller than 5 nm. The development of narrow pores, increases the BET surface area as indicated in Table 1. Similarly, pore size distribution of PG-360h S-C implies that when steaming extended to 360 h, larger mesopores completely vanished in favor of smaller mesopores and possibly micropores.

0.25 dV/dw Pore Volume (cm³/(gnm))

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PG-24h S-C PG-360h S-C PG-C P10-Blank P10-Blank-24h S PG PG-3h S PG-6h S PG-24h S PG-360 h S

0.2

0.15

0.1

0.05

0 0

10

20

30 40 Pore Width (nm)

50

60

Figure 7. Pore size distribution for various samples In the next step, the samples grafted in the presence of different amounts of water, were calcined and their textural properties were measured. Figure 8 shows that with increasing amount of added water, the total volume of larger pores decreased in favor of narrower pores. With increasing water addition from 0 to 0.6 cm3/(g silica), the average pore decreased from 18.1 to 18 ACS Paragon Plus Environment

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3.4 nm. Moreover, as illustrated in Figure S5 the samples prepared in the presence of large amounts of water then calcined, have higher BET surface area and lower pore volume.

dV/dw Pore Volume (cm³/(gnm))

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


0.25

P10-Blank P10-0-1.5-C P10-0.1-1.5-C P10-0.2-1.5-C P10-0.3-1.5-C P10-0.4-1.5-C P10-0.5-1.5-C P10-0.6-1.5-C

0.2 0.15 0.1 0.05 0 0

10

20

30 40 Pore Width (nm)

50

60

Figure 8. Pore size distribution for samples grafted in the presence of different amounts of water, then calcined Based on the results depicted in Figures 6 and 7, it is inferred that steam treatment and/or calcination of the grafted silicas significantly change their textural properties. For the calcined samples, the increasing trend in BET surface area is in agreement with the decreasing trend in pore volume and pore width. On the other hand, the larger BET surface area of PG-24h S-C relative to PG-C can be attributed to the pore shrinkage caused by steaming with possible increase of surface roughness, and consequently to larger BET surface area of PG-24h S-C. The 12 % lower BET surface area of PG-360h S-C relative to PG-24h S-C implies the partial collapse of PG-360h S-C and is in agreement with the significant capacity loss of the sorbent at all adsorption temperatures. 19 ACS Paragon Plus Environment


To gain further insights into the effect of steaming on the grafted species as well as the silica matrix, we thought of using a highly ordered mesoporous silica to take advantage of easier TEM observations of possible structural changes. Hence, an ordered SBA-15 mesoporous silica was synthesized at 100 °C and grafted with triamine (herein referred to as SG) using the optimum grafting conditions used for P10. SG was exposed to steam for 24 h, then both grafted and steam treated samples were calcined. Figure 9 shows that SBA-15 before and after aminegrafting exhibit nitrogen adsorption isotherm of Type IV, typical of ordered mesoporous materials. It is seen that capillary condensation in the presence of SG-C takes place at lower relative pressure than for SBA-15, indicating that the average pore size decreased. It is also seen that the total pore volume has decreased upon triamine-grafting, consistent with the functionalization of the internal surface. Unfortunately, upon 24 h steaming, the SG- 24h S-C exhibited a Type I adsorption isotherm, indicating that the material evolved from a mesoporous to a microporous material, most likely due to structural collapse.

Quantity Adsorbed (cm³/g STP)

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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800

SBA-15 600

SG-C 400

200

SG-24h S-C

0 0

0.2



0.8

1

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Figure 9. Nitrogen adsorption-desorption isotherms for SBA-15 based materials The ordered tubular pore structure of SBA-15 is illustrated in Figure 10a. As observed in Figure 10b, the ordered structure of the pores did not change after grafting and calcination. Steam treatment of the grafted sample, however, resulted in complete structural collapse as shown in Figure 10c.

(b)

(a)

(c)

Figure 10. TEM pictures for (a) SBA-15, (b) SG-C, (c) SG-24h S-C Similar to PG, exposing SG to steam for 24 h resulted in significant decrease in CO2 uptake at 25 °C, moderate decrease in CO2 uptake at 50 °C, and negligible decrease in CO2 uptake at 75 °C (Table 2). However, compared to PG, steam treatment resulted in larger drop in CO2 uptake of SG at adsorption temperature of 25 °C and 50 °C. Steam treatment of PG reduced its CO2 uptake at 25 °C and 50 °C by 52 and 24 %, respectively. The corresponding values for SG were 71 and 43 % (Table 2). The larger decrease in adsorption uptake for SG compared to PG is associated with the more extensive structural collapse. Therefore, in addition to being cheaper than SG materials, the grafted commercial-grade P10 silica demonstrates superior hydrothermal stability. Table 2. CO2 Uptake and Amine Efficiency for SG and SG Exposed to Steam for 24 h



sample

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adsorption at 25 °C



uptake (mmol/g)

amine efficiency

uptake (mmol/g)

amine efficiency

uptake (mmol/g)

amine efficiency

SG

0.55

0.11

1.42

0.27

1.15

0.21

6.84

SG-24 h steam

0.16

0.03

0.80

0.13

1.13

0.21

6.86

amine loading (mmol/g)

4. CONCLUSIONS The commercial-grade silica (P10) grafted under optimum condition demonstrated promising CO2 uptake (1.93 mmol/g at adsorption temperature of 50 °C), comparable with grafted ordered mesoporous silica sorbents. Steam treatment of the amine-grafted P10 silica progressively reduced its average pore width and consequently, increased the diffusional mass transfer resistance. Therefore, after steam treatment, the drop in CO2 uptake was more severe at low adsorption temperatures. Prolonged steam treatment (360 h) of the grafted material led to blockage of a fraction of pores, and partial structural collapse of the sorbent. Nonetheless, the triamine-grafted P10 demonstrated higher hydrothermal stability than the sorbent based on ordered structure silica such as SBA-15.

ASSOCIATED CONTENT Supporting Information Grafting efficiency, BET surface area, pore volume and pore size distribution for different amine-grafted silicas; BET surface area and pore volume for different calcined amine-grafted silicas; Organic and amine content for an amine-grafted sample exposed to steam for different durations 22 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC).



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TOC 450

Quantity of Adsorbed N2 (cm³/g STP)

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


Silica (Grafted + Calcined)

300

Silica (Grafted + 24 h steamed + Calcined) 150

Silica (Grafted + 360 h steamed + Calcined) 0 0

0.2

0.4

0.6

Relative pressure (p/p°)


0.8

1

Long-Term Effect of Steam Exposure on CO

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