Shaping amine-based solid CO2 adsorbents: Effects of pelletization pressure on the physical and chemical properties

Accepted Manuscript Shaping amine-based solid CO2 adsorbents: effects of pelletization pressure on the physical and chemical properties Fateme Rezaei, Miles A. Sakwa-Novak, Sumit Bali, Daniel M. Duncanson, Christopher W. Jones PII: DOI: Reference:

S1387-1811(14)00640-4 http://dx.doi.org/10.1016/j.micromeso.2014.10.047 MICMAT 6851

To appear in:

Microporous and Mesoporous Materials

Received Date: Revised Date: Accepted Date:

22 July 2014 27 October 2014 30 October 2014

Please cite this article as: F. Rezaei, M.A. Sakwa-Novak, S. Bali, D.M. Duncanson, C.W. Jones, Shaping aminebased solid CO2 adsorbents: effects of pelletization pressure on the physical and chemical properties, Microporous and Mesoporous Materials (2014), doi: http://dx.doi.org/10.1016/j.micromeso.2014.10.047

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Shaping amine-based solid CO2 adsorbents: effects of pelletization pressure on the physical and chemical properties Fateme Rezaei1, Miles A. Sakwa-Novak, Sumit Bali, Daniel M. Duncanson, Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA Corresponding Author: [email protected]

ABSTRACT: Amine-based solid adsorbents are promising candidates for the separation of CO2 from dilute gas streams. Here we report the effect of pelletization pressure on the physical and chemical properties of an array of supported amine adsorbents, based on mesoporous silica and γ-alumina supports. The virgin powders based on poly(ethyleneimine) (PEI) and 3aminopropyltrimethoxysilane (APS) functionalized oxides are pressed at 1000 and 5000 psig pressure to form self-supporting pellets and their physical and chemical properties are compared. No change in chemical structure of the adsorbents is observed after pelletization, though the porosity of each material changes to some degree. Of the three mesoporous supports examined in this study, the commercial porous silica was the most stable support for both class 1 and class 2 adsorbents, whereas lab synthesized mesoporous SBA-15 silica and lab synthesized mesoporous γ-alumina are found to be the least stable supports for class 1 and class 2 adsorbents, respectively. The CO2 uptake results show a significant drop in equilibrium capacity for the pellets pressed at 5000 psig, regardless of the support used, while the 1000 psig pellets retain their capacity and show comparable performance to their powder counterparts. CO2

1

Current address: Department of Chemical and Biochemical Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA

breakthrough experiments suggest an increase in mass transfer resistance for the pellet samples as compared with virgin powders, resulting in the dispersion of the concentration fronts during CO2 breakthrough using a fixed bed. These findings suggest that shaping solid supported amine adsorbents into binderless pellets requires pressing the powders at low to moderate pressures to ensure that these materials retain their performance in processes that require pelletized samples. Keywords: Amine adsorbents, Pelletization, PEI, APS, CO2 adsorption

1. Introduction CO2 capture from exhaust gas streams is considered a possible way of decreasing anthropogenic CO2 emissions that are contributing to global climate change. As an alternative to conventional liquid amine scrubbing technology, adsorption-based processes using solids have been proposed as an alternative approach for CO2 capture, possibly reducing the energy demand relative to current capture processes [1,2]. Among the wide array of adsorbent materials studied for the removal of CO2 from both ambient air and flue gas streams, solid-supported amines have been identified as among the most promising materials due to higher loading capacity, similar to liquid amines, acceptable kinetics, high selectivity, simple and scalable synthesis and more importantly, improved capacity in the presence of water compared to conventional physisorbants [3–13]. Shaping the solid adsorbents for use in practical contactors is a major challenge in facilitating the utilization of the materials in industrial processes and correspondingly bringing the adsorptive gas separation processes closer to commercialization. In addition, preparing pelletized powder samples is a common step used in preparing adsorbents for lab scale testing in fixed bed 2

processes [14,15]. Often for use in practical contactors, fine powders are formed into granules, pellets, beads or extrudates with an inert binder for improved ease of handling and mechanical strength. In addition to the conventional approach in making pellets or beads, other approaches have also been considered for the incorporation of solid adsorbents into practical configurations such as monoliths [16–18] and hollow fibers [19–21]. Such proposed approaches have been investigated to address practical issues associated with using pellets/beads in fixed or fluidized beds, such as gradual material loss due to wear, excessive pressure drop, or inadequate mass and heat transfer kinetics. Previous work has shown that upon pelletization, the adsorption properties of silica supported poly(ethyleneimine) (PEI) could be impacted [22,23]. Song and co-workers showed that for a CO2 adsorbent based on PEI impregnated MCM-41, the adsorption capacity of the pelletized adsorbent was about 10% lower than that of the powder adsorbent when the adsorbent was pressed to 18-35 mesh [22]. In another study conducted by Chaffee and co-workers, the performance of SBA-15 supported PEI pellets was evaluated for the selective capture of CO2 from simulated post-combustion flue gas via a pressure / vacuum swing adsorption (VSA) process. The powders were pelletized under 7600 psig pressure for 10 min and it was found that the pelletized SBA-PEI material exhibited the same sorption kinetics as the powdered adsorbent, however the CO2 working capacity, determined via TGA, decreased by 18 % (from 3.4 to 2.5 wt%) when exposed to 5-15 % CO2 in air. On the basis of their results, it was concluded that pelletization leads only to a small reduction in sorbent capacity [23]. It should also be noted that most recently, a similar study was carried out by Peterson et al., whereby the effect of pelletization pressure on the physical and chemical characteristics of the metal-organicframeworks Cu3(BTC) and UiO-66 was investigated [24]. These MOFs were pressed at 1000 and

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10000 psig and the authors showed that both CuBTC and UiO-66 could tolerate these pressures and be engineered into pellets, although some degradation in porosity was observed for these materials as well. Despite the preliminary investigations described above, there is no systematic study in the literature that suggests to what extent the pelletization pressure will impact the physical or chemical properties of supported amine adsorbents. Furthermore, no such systematic study exists to elucidate how the choice of support material, and the properties thereof, will affect changes in material properties and performance upon pelletization. With the significant advantages offered by amine functionalized mesoporous adsorbents in CO2 capture [6], it is worthwhile to investigate the adsorption characteristics of pelletized materials to elucidate how they compare with their powder counterparts that are so often discussed in the literature. Thus, the motivation behind the current work is therefore to identify suitable pressures for pressing solid amine-based adsorbents and making binder-less amine-supported pellets to facilitate the utilization of such materials in practical CO2 capture processes. In this work, the impact of pelletization pressure on the physical and chemical properties of PEI impregnated-(class 1)[6] and APS grafted-(class 2) silica and γ-alumina mesoporous supports has been investigated. The materials are characterized in detail in both powder and pellet form, and the CO2 adsorption characteristics are characterized by equilibrium capacity measurements via TGA, as well as via CO2 breakthrough measurements using a packed bed of pelletized samples.

2. Experimental section 2.1. Materials synthesis

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Silica- and alumina- supported amine adsorbents were synthesized according to procedures reported in previous papers from our group [25,26]. A commercial silica support (PD-09024 from PQ Corporation) as well as laboratory prepared SBA-15 [27] and γ-alumina [25] were used as support materials. In a typical synthesis, the physically impregnated (class 1) adsorbents were prepared using a conventional wet impregnation method, whereby a desired amount of PEI was first dissolved in methanol for 1 h, and subsequently, dried silica/alumina was added and stirred for an additional 12 h. The methanol solvent was later removed by rotary evaporation (rotovap), and the resulting adsorbent powder was further dried under a vacuum at 105 °C overnight before testing. For preparation of aminosilane grafted (class 2) adsorbents, a desired amount of toluene and silica/alumina was first mixed for 1 h and then a small amount of water (0.3 mL/g support) and a desired amount of 3-aminopropyltrimethoxysilane (APS) with a ratio of 1g silane/1g support, were added into the mixture. The mixture was kept under vigorous stirring for 24 h at 85 °C. The resulting adsorbent was recovered by filtration, washed with toluene, and then dried under vacuum at 105 °C. 2.2. Pellet preparation A Carver press was used to prepare pellets. A desired amount of as-synthesized powder material was loaded into a 13 mm die and pressed at either 1000 or 5000 psig for about 1 minute. The materials were then crushed and sieved between 250 and 500 µm for further characterization. 2.3. Materials characterization Nitrogen physisorption measurements were carried out on a Micromeritics TRISTAR-II at 77 K. Surface areas and pore volumes were calculated from the collected isotherm data. Surface areas were calculated using the BET method [28] while pore volumes were estimated using the 5

BdB-FHH method [29]. A Netzsch STA409PG thermogravimetric analyzer (TGA) was used to determine the organic loading of the materials. Fourier Transform Infra-Red (FT-IR) spectroscopy measurements using KBr pellets were performed on a Bruker Vertex 80v optical bench. Powder X-ray diffraction (XRD) patterns were collected on a PANalytical X-ray diffractometer using a Cu−Kα radiation source.

2.4. CO2 equilibrium uptake measurements A TA instruments Q500 thermogravimetric analyzer (TGA) was used to measure dry CO2 adsorption uptakes. A typical adsorption run consisted of pretreating the material for 1 h at 120 °C in flowing helium to remove pre-adsorbed CO2 and water, followed by ramping down the temperature to 35 °C, the desired temperature for adsorption measurements, with the rate of 10 °C/min, and then adsorbing the CO2 gas for 3 h. The CO2 uptake curves were constructed from data obtained using a gas flow rate of 90 mL/min (at ambient conditions) containing 10% CO2 balanced with helium.

2.5. CO2 breakthrough measurements CO2 breakthrough measurements were carried out in a packed bed column as schematically shown in Fig. 1. A 7.0 cm-long Pyrex tube with a diameter of 1.0 cm equipped with a fine-sized frit was loaded with at least 500 mg adsorbent pellets for each experiment. A heating tape was used to heat the column during the desorption step and a thermocouple was used to probe the column temperature. The outlet gas concentrations were measured by a mass spectrometer (Pfeiffer Vacuum Omnistar, QMG 220). First, helium was passed through the bed at 90 mL/min for pretreatment while the bed temperature was raised to 120 °C for 1 h, after which it was

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lowered back to 35 °C and allowed to equilibrate at that temperature for 30 min. The flow was then switched from helium to 10% CO2 (balance helium), while the outlet gas concentration was continuously monitored.

3.

Results and discussion

3.1. N2 physisorption measurements The textural properties of the powder and pelletized samples were characterized by nitrogen physisorption measurements at 77 K. The corresponding BET surface area and total pore volume (at P/P0 = 0.99) for the bare supports and virgin powders are presented in Table 1,while the corresponding nitrogen adsorption/desorption isotherms are shown in Fig. 2 for both the class 1 and 2 amine adsorbents. The data presented in Table 1 indicate that upon functionalizing the γalumina and silica supports, the BET surface areas and total pore volumes decreased, indicating the successful loading of organic amines into the pore space. It should be noted that the powder samples exhibit physisorption characteristics consistent with the similar samples reported in the literature [14,21,30]. One other thing to note here is that all class 1 and class 2 materials have about the same amine loadings, a widely different change in pore volume and BET surface area for class 1 versus class 2 materials could be observed. We hypothesized that the PEI evenly distributed into the pore space of the support, thereby causing similar pore volume reductions for each material. Much larger pore volume reductions were observed for the class 2 sorbents, likely due to the larger molecular weight of the APS fragment, as well as the possibility that APS preferentially grafted near to the pore mouths, making some pores inaccessible, thus contributing to a much smaller surface area and pore volume than those of PEI impregnated samples with almost similar amine loadings.

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All the nitrogen isotherms shown in Fig. 2 are type-IV isotherms, indicating a typical mesoporous structure, with type H2 hysteresis according to IUPAC classification. It is apparent from these profiles that the BET surface area and total pore volume decrease after pressing the powders and forming pellets; however, the change is relatively small for the pellets pressed at 1000 psig. For example, considering the AL-PEI-1000 sample, the surface area and pore volume dropped from 157 to 151 m2/g and 0.68 to 0.44 cm3/g, respectively, pelletization, indicating that the material retained the majority of its porous nature after pressing under 1000 psig pressure. While the drop in nitrogen uptake was less profound for the1000 psig pellets, the pellets pressed at 5000 psig experienced a dramatic decrease in the uptake compared with the powder samples. For instance, the surface area and pore volume decreased by 85 and 81 %, respectively, for SBA-PEI-5000 compared to SBA-PEI. Moreover, the AL-APS-5000 sample lost its porosity completely upon pressing, exhibiting virtually zero pore volume. Another striking feature in the profiles displayed in Fig. 2 is the change in the hysteresis loop in the isotherms of the pelletized samples in comparison to those of the original powders. A rather sharp step in the desorption branch of the isotherm in the hysteresis region for the powder samples can be observed. On the contrary, the step change becomes broader for the pellet samples, indicative of a change in the pore network of the materials. In addition, the shift of the hysteresis loop to lower partial pressures in the case of pellets implies a reduction in the effective pore size induced by pelletization. Noteworthy is that these changes are most pronounced for samples pelletized at 5000 psig, compared to those treated at 1000 psig. Thus, in general, pressing the amine-based adsorbents not only changes the pore size but also influences the pore network and interconnectivity of the pores. In fact, shaping the pellets creates a new hierarchical porosity containing macropores, mesopores and the newly generated

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mesopores that are larger than the intrinsic mesoporosity of the pristine materials. As is apparent from the obtained results, a rather low pressing pressure (i.e., 1000 psig) has little impact on the overall nitrogen uptake and porosity, whereas applying a high pressing pressure to pelletize the samples severely impacts the textural properties of solid amine adsorbents, resulting in a very low nitrogen uptake.

3.2. Organic loading measurements As a general rule for amine based solid adsorbents, the organic loading measurements are typically carried out to investigate if the incorporation of amine functionality into the mesoporous supports is successful. However, in this work we are also interested in elucidating if, upon pressing, the organic content of the samples changes. The amine loadings are presented in Table 2 for both the powder and pellet materials. As can be clearly observed, for all materials, the TGA data do not reveal any change in the organic content of the materials before and after pelletization, suggesting that the amine moieties did not leach out of the adsorbent upon pressing. This is an important finding, especially for class 1 adsorbents, as the physically impregnated alumina/silica supports with PEI are more susceptible to amine loss due to the lack of strong chemical bonds between the surface of the support and the PEI.

3.3. XRD measurements The small angle XRD patterns of the SBA-15 samples impregnated with PEI and grafted with APS are given in Fig. 3a, respectively, in powder and pellet form (produced at both pressures). Analogous characterization of the alumina and PD supported materials are not reported, as the pore structure of these materials is disordered and hence does not show an XRD

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pattern. The dominant peak at 2θ of 0.88 represents a d(100) spacing of 10.0 nm for the ordered pore structure of the unfunctionalized SBA-15. The 110 and 200 reflections are also observed in the bare SBA-15 and the PEI impregnated powder, as well as the 1000 psig pellets. In both the PEI impregnated and APS grafted powders, there was a small shift in the 100 reflection to lower values of 2θ, indicative of a slightly larger d spacing in the functionalized materials. SBA-15 materials are known to show similar differences in 100 reflections when the synthesis template (Pluronic 123) is not completely removed. [31]. This has been attributed to removal of ethylene oxide from the P123 that has penetrated into the framework of the silica walls, hence creating void space which further promotes structural shrinkage. It is possible that in these postsynthetically modified materials, the incorporated molecule (either APS or PEI) penetrates into these smaller void spaces and swells, causing the silica pore framework to expand slightly. Additionally, the adsorption of water from the ambient atmosphere onto the hydrophilic amine groups would be expected to enhance this effect. Nonetheless, the prominence of the 100 reflection suggests that the ordered structure of the material was retained after incorporation of the amines. After pelletizing at 1000 psig, the 100 plane shifts to higher 2θ and a correspondingly smaller d spacing. This is consistent with a slight compression of the pores that occurs upon pelletization at this moderate pressure. At pressures of 5000 psig, the 100 reflection is significantly reduced in intensity, indicating significant disruption to the ordered pore structure of the material due to pore collapse, as corroborated by the nitrogen physisorption data. Fig. 3b shows the wide angle XRD patterns of the γ-alumina samples impregnated with PEI and grafted with APS, respectively, in powder and pellet form, at both pressures. Diffraction peaks at 2θ of 37, 46, 66 represent the 311, 400 and 440 planes characteristic of the γ phase alumina. The patterns of all the amine functionalized materials are similar to that of the bare γ10

alumina, aside from a broad peak that emerges between 18-28 2θ. This peak has been previously observed on amine functionalized alumina and has been attributed to diffraction arising from average bond distances in amorphous regions in the sample, which were hypothesized to arise from interactions between the amine moieties and the surface of the alumina [32]. Nonetheless, the materials remained predominantly in the γ phase after functionalization and pelletization, as expected.

3.4. FT-IR measurements To probe if the chemical structure of solid amine adsorbents changed by shaping them into pellets under different pellitization pressures, IR measurements were carried out. The FT-IR spectra of the powder and pellet samples are presented in Fig. 4. In the FT-IR spectra of the Albased samples, most of the bands that appeared in the spectra of the powders were nearly replicated for the pellets, with the location of the peaks remaining unchanged. The samples exhibited bands at 1320 and 1397 cm-1, which could be attributed to C-H deformations. The stretching vibrations of C–H bonds were present in the range 2840 and 2970 cm-1. Bands associated with N–H bonds were also present at 1581, 1590 and 1630 cm-1, whereas the peaks at 1482 and 1494 cm-1 were associated with C–N bonds. The presence of two bands at 3289 and 3345 cm-1 and one peak at 3428 cm-1 in the spectra of alumina supported APS and PEI samples, respectively, can be correlated with symmetric N–H stretching vibrations. No extra bands were observed in the spectra of pellet samples, indicating no change in the chemical structure after pressing. The silica supported materials (both SBA- and PD-based adsorbents) showed a strong absorption band near 1100 cm-1 that is clearly attributed to Si–O–Si asymmetric stretching 11

vibrations. The peaks of the N–H bonds and C–N bonds were present in the ranges 1572-1676 and 1390-1497 cm-1, respectively, and the stretching vibrations of the C–H bonds were present in the range of 2936 and 2834 cm-1, while bands between 2840-2970 cm-1 were associated with C–H stretching vibrations. As with alumina supported samples, no additional peaks in the spectra of silica supported pellets were evident. Overall, all the major IR peaks in the powder spectra were essentially replicated in the spectra of the pellets, with the 5000 psig pellets showing decreased IR band intensities in their spectra. In conclusion, all the FT-IR spectra clearly reveal no distinct changes in the chemical structure of the materials before and after pelletization, indicating that the amine functionalized silica and γ-alumina adsorbents retained their chemical structure after shaping them into pellets by applying pressure.

3.5. CO2 uptake measurements Carbon dioxide equilibrium uptake measurements at a partial pressure of 0.1 atm CO2 were carried out via TGA to evaluate and quantify changes in equilibrium CO2 uptake induced by pelletization. CO2 equilibrium capacities and amine efficiencies, defined as moles of CO2 adsorbed per mole of amine (mol CO2/mol N) are presented in Fig. 5 for both PEI- and APSbased materials. In comparing the performance of class 1 powder and pellet samples in Fig. 5a, it can be seen that for all samples pelletized at 1000 psig, the uptake of CO2 was not affected by shaping the powder into pellets and the pellets retained their capacities, showing similar amine efficiencies as the powders. However, at a pelletization pressure of 5000 psig, the γ-alumina, SBA and PD supported materials exhibited 21, 44 and 13 % loss in their capacity and amine efficiency relative to that of analogous powders, respectively. The moderate decrease in CO2

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uptake for 5000 psig pellets of γ-alumina and SBA can be attributed to the loss of porosity and pore blockages, as suggested by nitrogen physisorption profiles, making the amine sites less accessible for CO2 molecules. For the PD samples, it appears that the pelletization made some pores completely inaccessible, resulting in the decreased CO2 uptake. The latter argument is supported by the kinetic profiles shown in Fig. 6, where the uptake kinetics were not affected by pelletization and all the profiles laid on top of each other for the PD-based samples. Fig. 5b shows the capacity and amine efficiency results for the class 2 samples. The γalumina and SBA supported class 2 materials showed a slight decrease in CO2 uptake and amine efficiency for the 1000 psig pellets, and a more dramatic decrease for the 5000 psig pellets. For example, in the case of AL-APS, the capacity and amine efficiency decreased from 1.09 to 0.30 mmol/g and 0.27 to 0.08 mol CO2/mol N, respectively, after pelletization at 5000 psig, indicating less accessibility of the amine functionalities for CO2 molecules during adsorption, again attributed to pore blockage and reduced porosity. This is in agreement with the physisorption profiles (shown in Fig. 2), where this sample showed virtually no porosity. On the contrary, PDsupported class 2 pellets pressed at 1000 and 5000 psig exhibited only 2 and 13 % capacity losses. The results related to CO2 capacity loss are summarized in Table 3. Overall, these data show that pelletization affects APS samples more than PEI samples. It can be hypothesized that PEI distributes more evenly on the surface of the supports, potentially enhancing stability, whereas APS may preferentially graft near a pore mouths. In addition to evaluating the equilibrium capacities, the TGA data were used to examine the adsorption response times for the adsorbent materials as well. The uptake curves for PEI- and APS-based powders and pellets are shown in Fig. 6. Differences in uptake rate and response time with these materials can most easily be observed in the results for the class 2 adsorbents. For 13

each of the three supports, the 1000 psig pellets showed almost similar uptake rates to the powder samples and achieved greater than 90 % equilibration in less than 2 min for class1 and approximately 20-30 min for class 2 materials, similar to their powder counterparts, while the 5000 psig pellets showed a reduction in apparent response time. This is most likely due to the slow diffusion through the pores as a result of pore blockage, loss of interconnectivity or limited pore volume as compared to the powders. The uptake results for the class 1 materials show quite rapid adsorption equilibration for γalumina and PD supported samples in both powder and pellet forms, indicating that the reduced porosity that was discussed earlier (section 3.1) did not affect the adsorption rate in these TGA experiments. Noticeably however, significant differences were observed in the uptake curves of SBA supported PEI in comparison to the analogous γ-alumina and PD supported materials. The SBA-PEI-5000 showed a significant reduction in the apparent response time, and displayed poor kinetics in the 90 % equilibrium capacity, taking ~ 60 min. In comparison, the SBA-PEI powder sample equilibrated in less than 2 min. This further confirms the enhancement of internal diffusional resistances through the pores of pellet samples and therefore a slower uptake rate. Furthermore, Fig. 6 shows a generally similar trend to Fig. 5, where for all three supports, the class 2 samples were more affected than the class 1 materials and showed reduced uptake kinetics. These results indicate that for most of the pellets pressed at 1000 psig, the pelletization had limited effects on both uptake capacity and adsorption rate and the pellets retained their capacity and showed fast kinetics during CO2 adsorption. On the other hand, the capacity and uptake rate of pellets that were pelletized at higher pressure were significantly influenced by pelletization pressure and these pellets showed decreased performance as compared with the original

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powders. Comparing the pelletization stability of the different supports investigated in this study, the PD silica was the most stable support for both class 1 and class 2 amine adsorbents, while SBA-15 and γ-alumina were found to be the least stable supports for class 1 and class 2 materials, respectively.

3.6. CO2 breakthrough measurements It is important to understand the kinetics of adsorption to facilitate adsorber design and operation. The fixed bed breakthrough profiles of some of the samples are shown in Fig. 7. To make a valid comparison between these profiles, the total CO2 capacity was kept constant and therefore the amount of adsorbent in the bed was varied based on their equilibrium capacities, as measured in the TGA. In this way, the breakthrough time can be used as a quantitative metric to assess the adsorption dynamics, where a shorter breakthrough time would directly indicate slower adsorption mainly due to hindered diffusion. In addition, the gas flow rate was chosen such that the pressure drop across the fixed-bed was negligible. For all runs, a CO2 containing gas stream with the flow rate of 10 mL/min was fed to the adsorbent bed. With such operating conditions, it has been shown previously that heat effects were negligible in comparing adsorption dynamics for packed bed experiments of this scale using supported amine adsorbents [14]. Furthermore, since all the pellets were of the same size, differences in external mass transfer resistance between the experiments with the various pellets can be assumed to be negligible. Therefore, we assert that reductions in kinetic performance are necessarily due to increases in intrapellet mass transfer resistance, induced by pelletization. What can be clearly seen from the profiles presented in Fig. 7 is that the effect of pressing on adsorption kinetics is twofold, namely, it changes the breakthrough time and working capacity,

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with the pelletized materials showing earlier breakthrough time. This is a result of slow kinetics (mass diffusion), as the total capacity of the bed was kept constant, and in this way, the breakthrough time was only a function of the kinetics. Secondly, it affects the shape of the fronts and results in broadening of concentration profiles. This is due to the slower diffusion and increased mass transfer resistance through the less porous pellets. Specially, the effect was more pronounced in the case of AL-APS-5000 pellets, as shown in Fig. 7c. This sample exhibited a sharp increase in the outlet CO2 concentration up to about 5 %, followed by a gradual increase to the feed concentration after 450 min, while AL-APS and AL-APS-1000 samples reached to equilibrium after ~120 min. These results are clearly in line with the nitrogen physisorption and TGA data, which indicate that pressing the supported amine powders at higher pressures will deteriorate the performance of the resulting pellets.

4. Conclusions The impact of pelletization on the physical and chemical properties of amine functionalized mesoporous supports were evaluated by comparing the performance of class 1 and class 2 powder and pellet adsorbents pelletized under 1000 and 5000 psig pressure. For three different γalumina and silica supports investigated in this study, physisorption data revealed a drastic drop in both surface area and pore volume for pellet samples prepared at 5000 psig compared to virgin powders, attributed to the pore blockage and collapse of the pore structure under pressing, whereas pellets prepared at 1000 psig exhibited similar porosity to their virgin powder counterparts. As a result of reduced porosity and less accessibility to the amine sites, the pelletized samples that were pressed at 5000 psig lost some of their CO2 adsorption capacity, although the amine loadings remained the same. The fixed-bed CO2 breakthrough profiles in

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conjunction with TGA CO2 uptake results indicated slow uptake kinetics and poor internal mass transfer for CO2 adsorption using the 5000 psig pellets, indicating slow diffusion through the less porous samples. In addition, on the basis of XRD and FT-IR spectroscopy results, no significant chemical changes were found and the chemical structure of the samples remained intact after pressing. Considering pressing solid powders to make binder-less pellets is a practical way of forming solid adsorbents for laboratory applications, our results indicate that amine functionalized adsorbents can be effectively pelletized under 1000 psig pressure, with the resulting pellets showing adsorption properties akin to their powder counterparts.

Acknowledgment This work was financially supported by DOE-NETL through grant number: DE-FE0007804. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE.

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19

Table 1 Physical properties of bare supports and amine-functionalized powders. Abbreviation

SBET (m2/g)

Vpore (cm3/g)

γ-Alumina-Bare

AL

238

1.12

SBA-15-Bare

SBA

839

0.98

PD-09024-Bare

PD

340

0.99

Material Bare Supports

PEI Impregnated Sorbents (Class 1)

γ-Alumina-PEI

AL-PEI

181

0.77

SBA-15-PEI

SBA-PEI

373

0.66

PD09024-PEI

PD-PEI

197

0.68

γ-Alumina-APS

AL-APS

41

0.18

SBA-15-APS

SBA-APS

98

0.23

PD09024-APS

PD-APS

65

0.25

APS Grafted Sorbents (Class 2)

Table 2 Organic content of amine-functionalized powders and pellets. Material Class 1

Amine Loading (mmolN/g)

Material Class 2

Amine Loading (mmolN/g)

AL- PEI

4.3

AL-APS

4.0

AL- PEI-1000

4.3

AL-APS-1000

4.0

AL- PEI-5000

4.3

AL-APS-5000

4.0

SBA- PEI

4.2

SBA-APS

4.1

SBA- PEI-1000

4.2

SBA-APS-1000

4.1

SBA- PEI-5000

4.2

SBA-APS-5000

4.1

PD-PEI

4.3

PD-APS

3.3

PD- PEI-1000

4.3

PD-APS-1000

3.3

PD- PEI-5000

4.3

PD-APS-5000

3.3

20

Table 3 CO2 capacity loss upon pelletization for class 1 and class 2 aminosilica adsorbents. CO2 Capacity Loss (Powder to 1000 psig) (% )

CO2 Capacity Loss (1000 to 5000 psig) (%)

CO2 Capacity Loss (Powder to 5000 psig) (% )

PEI Impregnated Sorbents (Class 1) AL- PEI SBA-APS PD- PEI

4.7 0 1.4

16.6 43.9 11.4

20.6 43.9 12.7

APS Grafted Sorbents (Class 2) AL-APS SBA-APS PD-APS

16.5 13.6 1.6

32.9 31.4 12.1

72.5 40.7 13.5

Material

21

Figure Captions Fig. 1. Schematic of packed bed system used for the CO2 breakthrough experiments. Fig. 2. Nitrogen adsorption/desorption isotherms at 77 K of powder and pellets (a) γ-alumina, (b) SBA and (c) PD silica supported adsorbents. Fig. 3. (a) Small angle XRD patterns of the bare SBA-15 support and SBA-15 functionalized materials and (b) wide angle XRD patterns of the bare γ-alumina support and γ-alumina functionalized materials in powder and pellet form, after pelletization at both pressures. Fig. 4. FT-IR spectra for powders and pellets with γ-alumina, SBA and PD silica supports for both class 1 and class 2 adsorbents. Fig. 5. CO2 capacity and amine efficiency of (a) PEI-impregnated and (b) APS-grafted powder and pellets. Fig. 6. CO2 uptake response time comparisons between powders and pellets with γ-alumina, SBA and PD silica supports for both class 1 and class 2 adsorbents. The profiles were obtained via TGA. Fig. 7. CO2 breakthrough profiles of (a) PEI-impregnated SBA, (b) APS-grafted SBA and (c) APS-grafted γ-alumina powder and pellets.

22

Fig. 1.

23

800

200 0 AL-APS AL-APS-1000 AL-APS-5000

200 150 100 50

200

0 SBA-APS SBA-APS-1000 SBA-APS-5000

200 150 100 50

0

0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

Relative Pressure (P/P0)

600

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

a)

b)

PD-PEI PD-PEI-1000 PD-PEI-5000

400

Quantity Adsorbed (cm³ N 2 /g)

SBA-PEI SBA-PEI-1000 SBA-PEI-5000

400

400

Q uantity Adsorbed (cm ³ N 2 /g)

Q uantity A dsorbed (cm ³ N 2/g )

600

AL-PEI AL-PEI-1000 AL-PEI-5000

600

200

0 300 PD-APS PD-APS-1000 PD-APS-5000

200

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

c) Fig. 2.

24

Intensity (a.u.)

SBA-PEI-5000

SBA-APS-5000

SBA-PEI-1000

SBA-APS-1000

SBA-PEI

SBA-APS

SBA

SBA

0.5

1.0

1.5

2.0

2.5

3.0

0.5

1.0

1.5

2.0

2.5

3.0

2θ θ / degrees

2θ θ / degrees

a) X Axis Title

Al-PEI-5000

Intensity (a.u.)

Al-APS-5000

Al-PEI-1000

Al-APS-1000

Al-PEI

Al-APS

Al

0

20

40

60

80

Al

100

0

20

40

60

80

100

2θ θ / degrees

2θ θ / degrees

b)

Fig. 3.

25

AL-APS-1000

AL-PEI-1000

2923 2891

3350

1632 1494

2970 2845

3428

1676 1580 1497 1394

AL-APS-5000

AL-PEI-5000

Absorbance (a.u.)

1678 1581 1492 1397

2924 2893

3345 3289

1630 1590 1494 1320

3428

2970 2841

AL-APS

AL-PEI

SBA-PEI SBA-APS SBA-PEI-1000

2931 2892

3350

2970 2840

3430

1630 1572 1490

PD-APS

PD-PEI PD-PEI-1000 PD-PEI-5000 4000

3500

3000

2500

2000

1676 1580 1487 1390

SBA-APS-1000 SBA-APS-5000

SBA-PEI-5000

PD-APS-1000 PD-APS-5000 1500

Wavenumber (cm-1)

1000

500 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig. 4.

26

0.6

0.20

0.15 0.4 0.10 0.2 0.05

AL-PEI SBA-PEI PD-PEI 0.0

0.00

Powder

Pellet-1000

a)

Pellet-5000

0.5

0.4 1.5

0.3 1.0 0.2

0.5 0.1

AL-APS SBA-APS PD-APS 0.0

Amine Efficiency (mol CO2/mol N)

0.25

2.0

Capacity (mmol CO2/g)

Capacity (mmol CO2/g)

0.8

Amine Efficiency (mol CO2/mol N)

0.30

0.0

Powder

Pellet-1000

Pellet-5000

b)

Fig. 5.

27

Normalized CO2 Capacity

1.0 0.8 0.6 0.4 0.2

Normalized CO2 Capacity

0.0

AL-APS AL-APS-1000 AL-APS-5000

SBA-PEI SBA-PEI-1000 SBA-PEI-5000

SBA-APS SBA-APS-1000 SBA-APS-5000

1.0 0.8 0.6 0.4 0.2 0.0

Normalized CO2 Capacity

AL-PEI AL-PEI-1000 AL-PEI-5000

1.0 0.8 0.6 0.4 PD-APS PD-APS-1000 PD-APS-5000

PD-PEI PD-PEI-1000 PD-PEI-5000

0.2 0.0 0

20

40

60 80 Time (min)

100

120

140

0

20

40

60 80 Time (min)

100

120

140

Fig. 6.

28

8

8

CO 2 Concentration (%)

10

CO 2 Concentration (% )

10

6

4

SBA-PEI SBA-PEI-1000 SBA-PEI-5000

2

6

4

SBA-APS SBA-APS-1000 SBA-APS-5000

2

0

0 0

20

40

60

0

80

20

40

60

80

Time (min)

Time (min)

a)

b)

CO2 Concentration (%)

10

8

6

4

10 8 6

2

AL-APS AL-APS-1000 AL-APS-5000

0

4 2 0 0

0

50

100

150

200

250

20

40

300

60

350

80

400

Time (min)

c)

Fig. 7.

29

Highlights: • • • • •

Pelletization effects on physiochemical properties of amine adsorbents assessed No change in chemical structure of the adsorbents is observed after pelletization CO2 uptake capacities drop for the pellets pressed at 5000 psig Increased internal mass transfer resistance for the pellets compared to powders Shaping solid amine adsorbents into binderless pellets requires low pressures

30

Graphical Abstract:

31