Surface Functionalization of Mesoporous Silica-Based Sorbents for CO2 Capture

Investigators Jennifer Wilcox, Assistant Professor, Energy Resources Engineering; T. Daniel P. Stack, Associate Professor, Chemistry; Zhenan Bao, Associate Professor, Chemical Engineering; Jiajun He, Graduate Researcher, Energy Resources Engineering; Erik Rupp, Research Assistant, Energy Resources Engineering; J. Brannon Gary, Post-Doctoral Researcher, Chemistry, Stanford University

Abstract Sorbent technologies for CO2 capture have several advantages over the traditional amine-based solvent absorption approaches. For instance, within an adsorption-based approach water is absent, which decreases the energy requirements associated with regeneration since heating water is the greatest energetic expense associated with CO2 capture using solvent-based approaches. Another benefit of sorbents is the flexibility associated with the choice in pore size and connectivity, in addition to the favorable heat conduction properties of materials such as carbon. Mesoporous carbon-based frameworks will allow for heat to be dissipated readily during the adsorption process, which will lead to maximum capacity, in addition to the ease of heat transfer into the system for regeneration. In this proposal we aim to develop affordable high surface area mesoporous carbon-based materials with covalently attached functional model complexes of carbonic anhydrases (CA) that can selectively adsorb and regenerate CO2. CA are natural enzymes found in the red blood cells of mammals, and are used to capture CO2 as bicarbonate, which can easily dissolve in blood and can be removed subsequently and transported to the lungs. In the lungs, the enzymes are “regenerated,” thereby releasing CO2, which is exhaled. The kinetics of bicarbonate formation via CA are ca. 8 orders of magnitude faster than its formation in an aqueous solution and ca. 6 orders of magnitude faster than CO2 binding via an amine-based solvent, which is currently the most advanced process for CO2 capture. It is our aim to apply this approach to sorbent-based technologies to enhance the mass transfer of CO2 from the gas to adsorbed phase, in addition to enhancing the kinetics associated with adsorption and desorption (regeneration) processes. Introduction The current state-of-the art technology for selective CO2 capture at scale is amine scrubbing, which is a chemical absorption-based technology. Another option for CO2 capture is adsorption-based technologies. There are several benefits associated with adsorption versus absorption for CO2 capture including, regeneration savings, since water is not present, in addition to minimizing the environmental hazards associated with corrosive solvents such as MEA. Research and development into sorbent-based technologies are expanding as CO2 capture is anticipated to be a primary component of the CO2 mitigation portfolio. A challenge with many solid sorbents, such as zeolites and metal-organic frameworks (MOFs) are competition for water, which is present at ca. 10-

20 mol.% in the flue gas.† While pre-dehydration of the flue gas is considered, such a process only increases the separation complexity with attendant energy costs. The mesoporous carbon-based biomimetic sorbent technology proposed circumvents these common challenges, as will be highlighted next. Background Ideal sorbent characteristics include:

• high CO2 selectivity, • fast adsorption kinetics, • low heat of adsorption, • steep adsorption isotherm, and • high working capacity

The sorbents that we will focus on will be mesoporous carbon-based materials with discrete biomimetic surface complexes. Both structure and pore surface chemistry will be tuned within the sorbents of the proposed study. The pore structure, porosity, tortuosity, and pore size are parameters that will be tuned for optimal mass transfer by minimizing diffusion limitations. The surface chemistry of the pore will be tuned through the choice of the macrocyclic amine, the tether for covalent attachment, and coadsorbents to assure a charge neutral environment similar to that found in CA. Based on the size of the discrete complexes to be used (ca. 1 nm), the ideal pore size of the mesoporous carbon supports (MCS) would be 3-6 nm to assure facile diffusion of the gases to and from the adsorbing complexes. We want to select a chemistry that will allow tunable pore size so that we can synthesize several pore sizes to test the optimal conditions. Moreover, to maximize the usable surface area and reduce dead end pores, an ordered MCS is desired. The surface functionality will include a hydroxyl group bonded to a zinc cation that will selectively interact with CO2 to form bicarbonate, not unlike the mechanism that occurs in solution with CA. The rate of bicarbonate formation between CO2 in CA is ca. 8 orders of magnitude faster than the non-catalyzed reaction in aqueous solution; the best biomimetic complexes are ca. 100-1000 times slower than CA. Fast adsorption kinetics along with the mesoporous carbon support structure will allow for enhanced sorption kinetics for both CO2 uptake and regeneration. It is anticipated that mesopore adsorption of CO2 will include both physical and chemical mechanisms. More specifically, in addition to the chemical bonded CO2 in the form of bicarbonate, there will be ca. 2 nm of pore space remaining between the pore surface functional groups that will allow for densely packed CO2 to reside during the uptake stage. Density functional theory (DFT) calculations suggest that the heat of adsorption of bicarbonate is ca. 34 kJ/mol, which is significantly reduced from typical amines, which is ca. 80 kJ/mol depending upon the type of amine.[1] Additionally, zeolites and MOFs have heats of adsorption that can range between 35 to 90 kJ/mol.[2, 3] This weak chemical interaction will allow for minimal heat of regeneration. An additional thermodynamic consideration is the shape of the adsorption isotherm. A steeper adsorption isotherm leads to a lower temperature or

                                                                                                               †flue gas from a coal-fired utility boiler can reach up to 12 mol% depending upon coal rank and up to 18 mol% in the case of natural gas combustion

pressure swing required to obtain a desired working capacity, which further reduces the energy penalty. The working capacity is the capacity of the sorbent after undergoing many cycles, which is different from the sorbent capacity on its first exposure to a given feed gas.

Preliminary Results Materials and Methods Brand E silica gel (230-400 mesh) was purchased from Dynamic Adsorbents, Inc. All other chemicals and organic solvents were purchased from Sigma Aldrich. p-aminophenyl phosphonic acid[4] and the mesoporous silica (SBA-15)[5] were synthesized according to published procedures.

The surface functionalization of silica gel was based on a modified method reported by Duchateau et al.[6] The as-received silica gel was either dried at 200 °C under vacuum for 3 h (labeled as zinc silica 1) or directly used without being dried (labeled as zinc silica 2). Typically, 2 g of silica gel was dispersed in 10 mL anhydrous heptane, which was pretreated by activated molecular sieves with an average pore size of 4 Å. Diethyl zinc (10 mL, 1 M solution in hexane) was slowly added to the suspension, which was stirred overnight, vacuum filtered, washed with 3 doses of 20 mL of heptane to remove unreacted diethyl zinc, and dried in vacuum for 15 min.

In order to analyze the zinc loading, unmodified silica gel (40.7 mg), zinc silica 1

(39.4 mg) and zinc silica 2 (40.3 mg) were each extracted in 10 mL of 1% HNO3 at room temperature for 1 h. The resulting solutions were diluted, filtered by 0.45 µm membranes, and analyzed by ICP-MS (Thermo Scientific XSERIES 2 ICP-MS). The concentrations of zinc in unmodified silica gel, zinc-silica 1 and zinc-silica 2 were observed to be 3.38 × 10-5, 1.63 and1.82 mmol g-1, respectively.

Phosphonic acid was deposited on SBA-15 by mixing 200 mg of SBA-15 and 60 mg

(0.35 mmol) of p-aminophenyl phosphonic acid, which was suspended in 30 mL anhydrous ethanol and refluxed for 4 hours. After cooling to room temperature, the precipitate was collected by filtration and washed with ethanol (2 x 10 mL). The solid was dried in a vacuum oven at 60 °C for 2 hours to yield 145 mg of silica material.

The phosphonic acid loading was analyzed by digesting 18.2 mg of SBA material in 2 mL of 10% KOH solution and heating to completely dissolve the material. The pH was adjusted to ~1 with concentrated hydrochloric acid and diluted to 10 mL. The concentration of phosphonic acid was then determined by absorbance with UV-vis spectroscopy (λmax = 261 nm; ε = 208). The material was found to have a loading of 1.22 mmol g-1.

Breakthrough experiments were performed in a quartz reactor with packed beds

ranging from 100 mg to 200 mg of samples. The experimental schematic can be seen in Figure 1. Nitrogen was passed through a bubbler containing H2O (the bubbler is bypassed in dry experiments) at 20 mL/min and passed through the packed bed. At a designated time, 2 mL/min of CO2 was passed into the flow stream. Breakthrough was measured

downstream of the packed bed using an Extrel 300Max-LG quadrupole MS. Both CO2 and H2O concentrations were monitored, and CO2 was not sent to the bed until the H2O concentration has stabilized. Typical experimental parameters were 9.09% CO2 and 2.1% H2O with a balance of N2. The bed was held at a constant temperature of 40 ˚C for adsorption experiments. The reactor could be rapidly heated to 120 ˚C for desorption measurements. The capacity of the sorbent was determined by measuring the difference between CO2 breakthrough curves through an empty reactor and through the packed bed. Experiments were performed to determine the effect of pressure drop across the bed using a 500 mg bed of Sigma Aldrich 50-70 mesh quartz sand. Changes due to pressure drop were determined to be minimal, and within the experimental error.

Figure 1: Schematic for CO2 breakthrough experiments.

Porosity and surface analysis was performed using a Quantachrome AutosorbiQ gas

sorption analyzer. Each sample was outgassed at 0.03 torr with a 1˚C/min ramp to 50 ˚C, where the sample was held overnight and tested for continuing outgassing. If the sample was still outgassing, as measured by pressure change, it was held at 50 ˚C for up to 3 additional hours, with a test every 15 minutes. The sample was then held at vacuum (0.03 torr) until the analysis was performed. Pore analysis was performed using CO2 at 273 K (P/P0 range of 3×10-7 to 3×10-2). Micropore distributions were estimated using CO2 adsorption isotherms in combination with the Dubinin-Astakhov (DA) method. CO2 Breakthrough Analysis Breakthrough experiments were performed on zinc-functionalized silica and aniline SBA. The samples were initially tested at 40 °C in 9.1% CO2 without the addition of water. This ideal state approximates the CO2 concentration in flue gases, but eliminates the effects of other flue gas constituents, e.g., SO2, H2O and NOx . It should be noted that CO2 and N2 were not dried before being introduced into the pack-bed reactor, which means even without the additional water, there was a non-negligible amount of water in the gas stream. It was observed from the MS that there was H2O breakthrough at the time

when the CO2 breakthrough occurred. As a proof of concept, several known materials (soda lime, the commercial metal-organic framework ZIF-18 and the commercial zeolite 13X) were tested in the system before analysis of the zinc-functionalized silica was commenced. One of these materials can be seen in Figure 2, which can also serve as a discussion of the breakthrough calculations in general.

Figure 2: CO2 breakthrough experiment for 197.3 mg of zeolite 13X without water bubbler.

The black line in Figure 2 is the empty bed breakthrough curve for CO2. It is

averaged over 4 experimental runs, with the error bars indicating the standard deviation of 0.12%. This corresponds to an error of 0.1 mmol CO2 g-1 sorbent in the final calculation. The red line is the CO2 breakthrough curve for a 197.3 mg bed of powdered 13X zeolite (obtained from Sigma Aldrich in pellet form). The difference between the areas under these two curves is equivalent to a CO2 capacity of 0.97 mmol g-1of 13X sorbent. A recent paper by Mulgundmath et al. put the capacity for a 13X material from a different supplier at 0.8 mmol g-1 sorbent in N2/CO2 binary systems at a CO2 partial pressure of 0.1 and 40 °C.[7] When factoring in the unknown impact of the pellet binding material and system error, this is a strong indication that the system is performing as expected.

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As can be seen in Figure 3, all three samples (ca. 100 mg each) including silica gel, zinc silica 1 and zinc silica 2 captured CO2 under relatively dry conditions without the water bubbler. The CO2 capacities of the sorbents obtained from the breakthrough curves, were 0.51, 0.38, and 0.42 mmol g-1 for silica gel, zinc silica 1 and zinc silica 2, respectively. It can be seen that surface modification with zinc did not improve the CO2 capacity of silica gel under the experimental conditions. To further validate the data from breakthrough measurements, the analysis was repeated under the same conditions using approximately twice the amounts previously used. The results are given in Figure 4, which shows that with increased bed size, the breakthrough times of CO2 were delayed slightly for silica gel and zinc silica 1, but significantly for zinc silica 2. The CO2 capacities of the sorbents were calculated as 0.30, 0.26 and 0.56 for silica gel, zinc silica 1 and zinc silica 2, respectively. The discrepancy in CO2 capacities of different amounts of the same sorbents might be induced by the heterogeneously packing of the sorbents leading to possible tunneling effects of the gas stream. To obtain accurate CO2 adsorption capacity, the breakthrough experiments need to be re-run with a better bed characteristics, including a tighter and homogenous packing. A route towards tighter and more homogeneous beds might include dispersing the samples into sieved sands, which have similar particle sizes compared to the sorbent particles.[8]

Figure 3: CO2 breakthrough experiments for silica gel (103.2 mg), zinc silica 1 (98.4 mg) and zinc silica 2 (100.7 mg) without water bubbler.

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Figure 4: CO2 breakthrough experiments for silica gel (207.9 mg), zinc silica 1 (198.5 mg) and zinc silica 2 (203.0 mg) without the water bubbler.

To simulate conditions closer to the flue gas and evaluate the effects of water on the sorbent performance, the zinc-functionalized silica sorbents were also tested under humid conditions, where the N2 was humidified by passing the stream through a water bubbler. Water concentration in the gas is expected to be 2.1%, and this has been confirmed by observation on the MS. The H2O concentration was allowed to equilibrate before the bed was exposed to CO2. Approximately 100 mg of each sorbent were tested and plotted in Figure 5. The exact weight of each sorbent was recording in the caption of Figure 5. All three sorbents continued to capture CO2 with varied capacities, which were 0.27, 0.43 and 0.52 mmol g-1 for silica gel, zinc silica 1 and zinc silica 2, respectively. The CO2 capacity of silica gel decreased significantly compared to the capacity obtained for ~100 mg silica tested without water bubbler. The decrease can be attributed to the strong interaction of H2O with the silanol groups on the silica gel surfaces. Therefore, H2O molecules preferentially adsorbed on the silica gel surfaces and competed with the adsorption of CO2. Interestingly, it can be seen that for ~100 mg zinc silica 1 and zinc silica 2 the capacities increased by 0.05 and 0.10 mmol/g, respectively, compared to the capacities for ~100 mg sorbents tested without the water bubbler. The enhancement in sorbent capacities might indicate the hydration of the zinc sites under humid conditions and the interaction between hydrated zinc and CO2.

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Figure 5: CO2 breakthrough experiments for silica gel (100.9 mg), zinc silica 1 (99.5 mg) and zinc silica 2 (99.7 mg) with water bubbler.

Moreover, increased amounts of sorbents (~200 mg) were also tested with a bubbler set up and plotted in Figure 6. Similar discrepancy in CO2 capacities by different amounts of the same sorbent was observed. Again, the data needs further verification by re-running the analysis using an improved bed preparation method, e.g., dispersing the sorbents in sand. CO2 capacities for silica gel, zinc silica 1 and zinc silica 2 under different conditions are summarized in Table 1. In general, under relatively dry conditions (0.1% H2O) surface functionalization with zinc groups did not improve CO2 capacities of the silica sorbent. However, under humid conditions (2.1% H2O), the zinc-modified sorbentsshowed improved capacities compared to unmodified silica gel. Furthermore, zinc silica 2 (1.82 mmol zinc g-1 of sorbent) exhibited higher CO2 capacities than zinc silica 1 (1.63 mmol zinc g-1 of sorbent) under all conditions tested, which suggests that it is possible to tune the CO2 adsorption capacities of the sorbents by varying the loading of surface functional groups. Note that the exact CO2 adsorption capacities need to be verified by further analysis.

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Figure 6: CO2 breakthrough experiments for silica gel (200.9 mg), zinc silica 1 (201.5 mg) and zinc silica 2 (199.9 mg) with water bubbler.

Sorbent Humidity Sorbent amount (mg) CO2 capacity (mmol/g)

Silica gel 0.1% 103.2 0.51

207.9 0.30

2.1% 100.9 0.27 200.9 0.27

Zinc silica 1 0.1% 98.4 0.38

198.5 0.26

2.1% 99.5 0.43 201.5 0.32

Zinc silica 2 0.1% 100.7 0.42

203.0 0.56

2.1% 99.7 0.52 199.5 0.35

Table 1: Silica gel-based sorbent capacities for CO2 uptake under different humidities (i.e., 0.1% without water bubbler and 2.1% with water bubbler) with different sorbent amounts (i.e., ~100 mg and ~200 mg).

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Moreover, CO2 breakthrough experiments were performed with aniline-modified SBA-15 and unmodified SBA-15 (as a control). Breakthrough curves and calculated CO2 uptake capacities are shown in Figure 7 and Table 2, respectively. In the experiments without the bubbler (~0.1% H2O), CO2 broke through at similar times with both aniline SBA-15 and unmodified SBA-15 beds. In fact, aniline SBA-15 yields a slight lower capacity than that of unmodified SBA-15, probably due to reduced pore size caused by the aniline groups. However, introducing water vapor into the gas flow reduced the capacity of SBA-15 from 0.68 mmol g-1 to 0.39 mmol g-1, while that of aniline SBA-15 remained almost unchanged. This implies that under humid conditions, CO2 preferentially interacts with the surface aniline functionalities over the silanol functionalities. However, at this time we cannot confirm any possible formation of carbamate or bicarbonate groups. Therefore, the reaction mechanisms are still unclear. FTIR analysis of the sorbents after CO2 adsorption may give some hints.

Figure 7: CO2 breakthrough experiments for aniline SBA-15 with or without water bubbler. 89.7 mg of aniline SBA-15 was used in the experimental with bubbler while 103.6 mg was used in the experimental without bubbler. Unmodified SBA-15 was tested as control. 70.8 mg of SBA-15 was used in the experimental with bubbler while 90.2 mg was used in the experimental without bubbler.

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Sorbent Humidity CO2 capacity (mmol/g)

SBA-15 0.1% 0.68 2.1% 0.39

Aniline SBA-15 0.1% 0.54 2.1% 0.51

Table 2: Unmodified and aniline-functionalized SBA-15 sorbent capacities for CO2 uptake under different humidities (i.e., 0.1% without water bubbler and 2.1% with water bubbler). Porosity Analysis

Micropore distributions of silica gel-based sorbents were measured by performing isothermal sorption and desorption measurements using CO2 at 273 K. The micropore distributions are presented in Figure 8. First, the micropore data obtained from using the DA method and the adsorption isotherm indicates that functionalization with zinc groups decreased the micropore volume of the sorbents from 0.047 cm3/g for silica gel to 0.016 cm3/g for zinc silica 1 and 0.020 cm3/g for zinc silica 2. Furthermore, zinc silica 1 possessed a mean pore radius of 0.9 nm, which is smaller than that of silica gel, i.e., 1.4 nm. In this case the zinc functional groups likely occupied all the micropores and reduced the diameter of each pore. However, zinc silica 2 has a pore size distribution with the same mean pore radius as silica gel, but smaller total volume. In this case the functional groups might block some of the micropores and allow other micropores to remain unmodified. Future work will include micropore analysis using argon to avoid chemical reaction between the gas probe and surface functionalities.

Figure 8: Micropore distribution measured by CO2 sorption at 273 K. The micropore distribution is estimated using the Dubinin-Astakhov method. Discussion

Materials characterization of the zinc-functionalized silica gel and aniline-functionalized SBA-15 were carried out to highlight the characteristics that might play

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important roles in CO2 capture. The loading density of the surface functional groups is a crucial aspect. This study varied the loading of the zinc functional groups by treatment of the pure silica gel prior to surface modification. ICP analysis shown that the sample (zinc silica 1) which was heated up to 200 °C and held for 3 h under vacuum had a lower loading of zinc than the sample (zinc silica 2), which was directly functionalized without pretreatment. A possible reason involves the dehydration of the silica surface leading to the transformation of two silanol groups into one disiloxane group. The resulting disiloxane groups are less reactive to diethyl zinc than silanol groups are, leading to lower zinc loading after surface functionalization. This is a rough control of the loading density of the surface functional groups. Future work will explore more sophisticated approach for the control over surface group coverage, e.g., click chemistry. And the relationship between surface group coverage and CO2 adsorption will be studied to optimize uptake capacity.

Breakthrough experiments indicate considerable change in CO2 uptake after surface

functionalization. Under relatively dry conditions (~0.1% H2O), zinc-functionalized silica gel adsorbed less CO2 than pure silica gel, which indicates zero to weak interaction between CO2 and the zinc groups. The decrease in capacity might come from the loss of pore space due to the existence of the surface functional groups. However, under more realistic humid flue gas conditions with a water concentration of ~2.1%, the zinc-functionalized silica gel samples exhibited larger capacities than pure silica gel. This is probably due to the hydration of the zinc sites forming zinc-bonded hydroxyl functionalities, which are reactive to CO2. Interestingly, the sample zinc silica 2 with higher zinc loading displayed higher CO2 uptake capacity than the sample zinc silica 1 with lower zinc loading. This also indicates the interaction between CO2 and the hydrated zinc sites.

Another attempt at fabricating readily regenerable sorbents, aniline-functionalized

SBA-15 exhibited lower CO2 capacity than pure SBA-15 under dry conditions, probably due to similar reasons as those in the case of zinc-functionalized silica gel. However, under humid conditions, the pure SBA-15 dropped in CO2 capacity down to about the half of the original value while the aniline-functionalized SBA-15 maintained its capacity, which was higher than that of pure SBA-15 in this case. This might serve as indication that CO2 interacts preferentially with the aniline sites over the surface silanol sites and the presence of water does not impact the CO2 - aniline interaction. However, at this time, there is not enough information to reach the conclusion that CO2 was chemisorbed onto the aniline sites and there is no direct evidence of the formation of carbamate species.

Conclusions

Two types of silica-based sorbents including zinc-functionalized silica gel and aniline-functionalized SBA-15 were synthesized and characterized. For zinc-functionalized silica gel, the zinc loading was characterized by ICP-MS. It was found that by preheating the silica gel prior to surface functionalization the amount of zinc loading was reduced. Porosity measurements on zinc silica samples indicate considerable changes in the micropore volume and mean pore radius. More importantly, the CO2 adsorption

capacities of the sorbents were measured by breakthrough experiments and the effects of water on the sorbent capacities were investigated. For silica gel-based sorbents, it was found that under relatively dry conditions (0.1% H2O) surface modification with zinc functionalities did not improve capacities, while under humid conditions (2.1% H2O) the surface zinc functionalities resulted in relative high capacities compared to pure silica gel. Additionally, aniline-functionalized SBA-15 was tested by breakthrough experiments and exhibited higher CO2 capacity than pure SBA-15 under the same humid conditions.

Progress Several surface functionalized sorbents have been synthesized and tested using SBA as the sorbent support due to its ease in synthesis for quick and efficient proof-of-concept testing. Breakthrough experiments have been carried out on these sorbents to determine if carbonate chemistry is occurring as anticipated. Thus far, it has been found that the Zn is bound too strong to the support to be able to form a carbonate with CO2 in a mixed CO2/N2/H2O environment. At this stage, these results are quite preliminary and we anticipate the synthesis of a Zn-functionalized sorbent that will be tunable to fast CO2 uptake and regeneration. Future Plans

Various functionalization techniques for SBA materials using Zn will be synthesized in the Stack group and tested for proof of concept in the Wilcox group. The Bao group will continue their efforts on carbon-based mesoporous supports with controlled porosity and pore size distribution. Scanning electron microscopy including scanning electron microcopy (SEM) and transmission electron microscopy (TEM) will be used to characterize the morphology of the sorbents. Porosity and surface characterization will be performed using argon as the probe gas. Breakthrough experiments will be repeated using improved packing method of the sorbents by dispersing sorbents in sieved sand with similar particle size. Moreover, gas purifiers for drying gas will be used in the breakthrough experiments to further evaluate the role of water in the adsorption process. The durability of the sorbents will be investigated by regenerating the sorbents after CO2 adsorption and repeating the CO2 adsorption process. Publication and Patents None to report at this time

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aqueous amine systems from experimentally determined reaction enthalpies. Energ. Proc., 2011. 4: p. 1542-1549.

2. Ruthven, D.M., Principles of adsorption and adsorption processes1984: Wiley-Interscience. 3. D' Alessandro, D., B. Smit, and J.R. Long, Carbon dioxide capture: prospects for new materials.

Angew. Chem., Int. Ed, 2010. 49: p. 6058-6082. 4. Cooper, R.J., et al., The assembly of rotaxane-like dye/cyclodextrin/surface complexes on aluminium

trihydroxide or goethite. Dalton Transactions, 2006(23): p. 2785-2793. 5. Zhao, D., et al., Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom

Pores. Science, 1998. 279(5350): p. 548-552. 6. Duchateau, R., et al., Silica-Grafted Diethylzinc and a Silsesquioxane-Based Zinc Alkyl Complex as

Catalysts for the Alternating Oxirane−Carbon Dioxide Copolymerization. Organometallics, 2007. 26(17): p. 4204-4211.

7. Mulgundmath, V.P., et al., Adsorption and separation of CO2/N2 and CO2/CH4 by 13X zeolite. The Canadian Journal of Chemical Engineering, 2012. 90(3): p. 730-738.

8. Hicks, J.C., et al., Designing Adsorbents for CO2 Capture from Flue Gas-Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. Journal of the American Chemical Society, 2008. 130(10): p. 2902-2903.

Contacts Jennifer Wilcox: wilcoxj@stanford.edu T. Daniel P. Stack: stack@stanford.edu ZhenanBao: zbao@stanford.edu Jiajun He: jiajunhe@stanford.edu Erik Rupp: ecrupp@stanford.edu J. Brannon Gary: jbgary@stanford.edu