Charlotte M. Wentz would like to acknowledge funding through NIST Award # 70NANB8H165. Zois Tsinas would like to acknowledge funding through NIST Award # 70NANB22H140.

Impregnation and Grafting of Aminated Compounds onto Silica Substrates

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<li>Ünveren, E. E., Monkul, B. O., Sarioğlan, S., Karademir, N., Alper, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Solid+amine+sorbents+for+CO2+capture+by+chemical+adsorption%3A+A+review%5BTitle%5D)+AND+%22Petroleum%22%5BJournal%5D)">Solid amine sorbents for CO2 capture by chemical adsorption: A review.</a> Petroleum. 3 (1), 37-50 (2017).</li>
<li>Demir, H., Aksu, G. O., Gulbalkan, H. C., Keskin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((MOF+membranes+for+CO2+capture%3A+Past%2C+present+and+future%5BTitle%5D)+AND+%22Carbon+Capture+Science+%26+Technology%22%5BJournal%5D)">MOF membranes for CO2 capture: Past, present and future.</a> Carbon Capture Science & Technology. 2, 100026(2022).</li>
<li>Xu, X., Song, C., Andresen, J. M., Miller, B. G., Scaroni, A. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Novel+polyethylenimine-modified+mesoporous+molecular+sieve+of+MCM-41+type+as+high-capacity+adsorbent+for+CO2+capture%5BTitle%5D)+AND+%22Energy+%26+Fuels%22%5BJournal%5D)">Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41 type as high-capacity adsorbent for CO2 capture.</a> Energy & Fuels. 16 (6), 1463-1469 (2002).</li>
<li>Gelles, T., Lawson, S., Rownaghi, A., Rezaei, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Recent+advances+in+development+of+amine+functionalized+adsorbents+for+CO2+capture%5BTitle%5D)+AND+%22Adsorption%22%5BJournal%5D)">Recent advances in development of amine functionalized adsorbents for CO2 capture.</a> Adsorption. 26 (94), 5-50 (2020).</li>
<li>Rao, N., Wang, M., Shang, Z., Hou, Y., Fan, G., Li, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((CO2+adsorption+by+amine-functionalized+MCM-41%3A+A+comparison+between+impregnation+and+grafting+modification+methods%5BTitle%5D)+AND+%22Energy+Fuels%22%5BJournal%5D)">CO2 adsorption by amine-functionalized MCM-41: A comparison between impregnation and grafting modification methods.</a> Energy Fuels. 32 (1), 670-677 (2018).</li>
<li>Anyanwu, J. T., Wang, Y., Yang, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Amine-grafted+silica+gels+for+CO2+capture+including+direct+air+capture%5BTitle%5D)+AND+%22Industrial+%26+Engineering+Chemistry+Research%22%5BJournal%5D)">Amine-grafted silica gels for CO2 capture including direct air capture.</a> Industrial & Engineering Chemistry Research. 59 (15), 7072-7079 (2020).</li>
<li>Anyanwu, J. -T., Wang, Y., Yang, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((CO2+capture+%28including+direct+air+capture%29+and+natural+gas+desulfurization+of+amine-grafted+hierarchical+bimodal+silica%5BTitle%5D)+AND+%22Chemical+Engineering+Journal%22%5BJournal%5D)">CO2 capture (including direct air capture) and natural gas desulfurization of amine-grafted hierarchical bimodal silica.</a> Chemical Engineering Journal. 427 (14), 131561(2022).</li>
<li>Sanz, R., Calleja, G., Arencibia, A., Sanz-Pérez, E. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Amino+functionalized+mesostructured+SBA-15+silica+for+CO2+capture%3A+Exploring+the+relation+between+the+adsorption+capacity+and+the+distribution+of+amino+groups+by+TEM%5BTitle%5D)+AND+%22Microporous+and+Mesoporous+Materials%22%5BJournal%5D)">Amino functionalized mesostructured SBA-15 silica for CO2 capture: Exploring the relation between the adsorption capacity and the distribution of amino groups by TEM.</a> Microporous and Mesoporous Materials. 158, 309-317 (2012).</li>
<li>Moon, H. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Understanding+the+impacts+of+support-polymer+interactions+on+the+dynamics+of+poly%28ethyleneimine%29+confined+in+mesoporous+SBA-15%5BTitle%5D)+AND+%22Journal+of+the+American+Chemical+Society%22%5BJournal%5D)">Understanding the impacts of support-polymer interactions on the dynamics of poly(ethyleneimine) confined in mesoporous SBA-15.</a> Journal of the American Chemical Society. 144 (26), 11664-11675 (2022).</li>
<li>Xu, X., Song, C., Andresen, J. M., Miller, B. G., Scaroni, A. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Preparation+and+characterization+of+novel+CO2+%26%2334%3Bmolecular+basket%26%2334%3B+absorbents+based+on+polymer-modified+mesoporous+molecular+sieve+MCM-41%5BTitle%5D)+AND+%22Microporous+and+Mesoporous+Materials%22%5BJournal%5D)">Preparation and characterization of novel CO2 &#34;molecular basket&#34; absorbents based on polymer-modified mesoporous molecular sieve MCM-41.</a> Microporous and Mesoporous Materials. 62 (1-2), 29-45 (2003).</li>
<li>Sousa, J. A. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((H2S+and+H2O+combined+effect+on+CO2+capture+by+amino+functionalized+hollow+microsphere+silicas%5BTitle%5D)+AND+%22Industrial+%26+Engineering+Chemistry+Research%22%5BJournal%5D)">H2S and H2O combined effect on CO2 capture by amino functionalized hollow microsphere silicas.</a> Industrial & Engineering Chemistry Research. 60 (28), 10139-10154 (2021).</li>
<li>Rim, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Sub-ambient+temperature+direct+air+capture+CO2+using+amine-impregnated+MIL-101%28Cr%29+enables+ambient+temperature+CO2%5BTitle%5D)+AND+%22JACS+Au%22%5BJournal%5D)">Sub-ambient temperature direct air capture CO2 using amine-impregnated MIL-101(Cr) enables ambient temperature CO2.</a> JACS Au. 2 (2), 380-393 (2022).</li>
</ol>

All authors disclose no competing conflicts of interest. The full description of the procedures used in this paper requires the identification of certain commercial products and their suppliers. The inclusion of such information should in no way be construed as indicating that such products or suppliers are endorsed by NIST, or are recommended by NIST, or that they are necessarily the best materials, instruments, software or suppliers for the purposes described.

The methods described herein are intended to provide a protocol for preparing impregnated and grafted amine silica-composite adsorbents. The procedures we have documented are based on review of techniques reported in the literature and those refined in our laboratory.1,2,3. Preparation of these materials is useful within the field of carbon dioxide removal research to develop or benchmark other materials that may be used to lower CO2 emissions either in the atmosphere (direct air capture) or in industrial processes (point source capture). Among solid-amine adsorbents, mesoporous silica is commonly used. Silica substrates tend to have simple syntheses or can be purchased commercially, with structural properties that make them a good choice for the impregnation or grafting of amine-based solid adsorbents21. Within this procedure, MCM-41 is used as the silica substrate due to its large surface area and narrow pore size distribution between 35 &#197; and 38 &#197;. However, another commonly studied silica support is SBA-15, which is also utilized in carbon capture research due to its large surface area, pore volume, and uniform mesopore size24. Systematic studies utilizing MCM-41 and PEI via impregnation have shown an increase in CO2 adsorption with an increase in PEI loading amount26. Prior results detailing the weight loss for impregnated MCM-41 silica composites are given by Xu and coworkers26, and for grafted silica composites by Sousa and coworkers27,&#160;who experimented with H2S and H2O effect on CO2 capture. Xu and coworkers26 found that their MCM-41 at 50 % (wt %) loading of PEI has an adsorption capacity of 112 mg CO2/g-adsorbent. Weight loss TGA of these impregnated materials is also presented within the manuscript. Comparatively, the MCM-41 59.9 % (wt %) impregnated PEI within this manuscript has a capacity of 61.6 mg CO2/g-adsorbent. While this appears to be much less than the reported amount, consider that the parameters of measurement are different - Xu and coworkers heated the sample to 75 &#176;C, had a 99.8 % blend of CO2 in N2, and a 100 mL/min flow rate for 150 min. The increased temperature and nearly 100 % CO2 experimental conditions likely both aided in the adsorption of CO2. This highlights the challenges in comparing studies in which experimental procedures differ.
Even for a seemingly simple method such as impregnation, the selection of solvent, silica/solvent concentration, amine/solvent concentration, stirring/mixing method, temperature, and mixing time vary greatly in the literature. This paper is intended to describe one method of making these materials, but each individual researcher can make selections that are appropriate for their own research goals. Additionally, these methodologies are adaptable to other adsorbent substrates for carbon capture. It is our intent to help facilitate research into materials for carbon dioxide removal by providing a starting point for such work with this method.
Many studies have conveyed that the location of the amines within the structure of a silica composite can change for impregnated versus grafted amines. Impregnated materials loaded with PEI have higher degrees of functionalization (i.e., amine content) on the surface, which can prevent CO2 diffusion into the pores1,2,3,24. Pore blockage has been linked to colder reaction temperature conditions, higher amine loading, and steric hindrance, consequently decreasing the accessibility of amine sites and adsorption capabilities28.
Grafted materials have been shown to have amines located inside pore channels, and thus CO2 diffusion can readily occur through the whole structure, maximizing capture efficiency24,25.&#160;Comparing grafted amine silica-composites to one another poses a difficulty due to the variation in amine content both in the grafted material and within the aminosilane.
However, comparability between studies within the &#34;same material(s)&#34; still has significant limitations, such as variations in (1) synthesis, (2) materials, and (3) measurement. First, while many studies and review articles describe the preparation of impregnated or grafted silica materials, there tend to be varied protocols for synthesis with no clear guidelines as to the most critical steps in the synthesis. Second, there are variations in performance between silica substrates sourced from external suppliers and those prepared through synthetic routes utilized during these protocols. With variations in PEI molar masses, amine content in aminosilanes, and grafted or loaded percentages, there poses difficulty in comparing measured CO2 adsorption capacities between these varied materials. Third, CO2 adsorption capacity can be measured using many different types of commercial or specially engineered measurement tools, all of which have different associated measurement uncertainties. The overall percentage of CO2 within the gas source, the flow rate of gas, the selection of activation, adsorption, desorption temperatures, and the humidity within the gas source can vary between individual studies and individual instruments. All such experimental parameters are important to consider when comparing measured adsorption capacities for individual materials.
While this procedure focuses on silica substrates, the overall goal of eliminating carbon emissions and reducing atmospheric CO2 concentration is a complex, multifaceted problem that will require material innovations to tackle, consensus on measurement parameters to report, and a clear indication of critical procedural steps. Continued research to investigate novel solid-support adsorbents with CO2 adsorbent functionality is thus increasingly important to meet stated climate goals. This article highlights the need for a community consensus on relative parameters and demonstrates the need for benchmark materials.

The anthropogenic CO2 emissions over the past several decades have been widely implicated as the main factor driving the greenhouse gas effect and consequently, related climate change1,2,3,4. There are two general methods for CO2 capture, point source and direct air capture. For more than 50 years, wet-scrubbing CO2 capture technologies have been utilized for point source capture within the industry to mitigate CO2 emissions5,6. These technologies are based on liquid-phase amines that react with CO2 to form carbamates under dry conditions and hydrogen carbonates in the presence of water7,8, see Figure 1. The main reason that carbon capture and storage is utilized at large point (industrial) sources is to prevent the further release of large quantities of CO2, thus having a neutral effect on total CO2 concentration in the atmosphere. However, point-source carbon capture systems suffer from several drawbacks, such as equipment corrosion, solvent degradation, and high energy requirements for regeneration9. Direct air capture (DAC) goes beyond emission reduction and can facilitate the removal of CO2 from the atmosphere. The removal of this existing CO2 is necessary to limit continued climate change. DAC is an emerging methodology and must address the difficulties of removing low concentrations of CO2 in atmospheric conditions (400 to 420 ppm), operate in a variety of different environmental conditions, and address the need for cost-effective materials that can be reused many times1,2,3. Significant work is needed to identify materials that meet these requirements, which will accelerate the adoption of DAC and improve its economic feasibility. Most importantly, community consensus on critical parameters of measurement needs to be established, which is essential for benchmark materials to be developed.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65845/65845fig01.jpg" /> 
Figure 1: Schematic of the expected liquid amine adsorbent CO2 capture mechanism. The top reaction is in dry conditions, and the bottom reaction is in the presence of moisture. <a href="https://www.jove.com/files/ftp_upload/65845/65845fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In an effort to remedy these drawbacks, considerable research and development of novel porous material technology have resulted in a wide array of promising materials that have the potential to be utilized as either capture materials or substrates for DAC. Some examples of such materials include mesoporous silica species10,11,12,13, zeolites14,15, activated carbon16,17, and metal-organic frameworks18. Many solid-supported amine adsorbents also show a higher tolerance to water, which is a vital consideration in CO2 removal through DAC approaches. For DAC applications, researchers must consider wet/dry environmental conditions, hot/cold temperatures, and an overall dilute atmospheric CO2 concentration. Amongst the various substrate materials, silica is commonly used because of its adjustable pore sizes, ability to surface functionalize, and large surface area1,2,3. Typical synthetic procedures and primary features of both impregnated and grafted amine-silica composites are described within this work (Figure 2). Direct synthesis, where the material is made in situ with both components, substrate and amine, is another commonly used methodology2.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/65845/65845fig02.jpg" /> 
Figure 2: Schematic representations of impregnation. Mixing of PEI and silica substrate in methanol through diffusion (top) and grafted amine-silica composites through covalent tethering (bottom). <a href="https://www.jove.com/files/ftp_upload/65845/65845fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Impregnation&#160;is a method in which an amine is physically adsorbed onto a surface, in this case, a porous silica medium, through van der Waals forces and hydrogen bonding between the amine and silica surface19,&#160;see Figure 2. Solvents such as ethanol and methanol are commonly used to promote the diffusion of the molecules into the porous structure of the substrate material. The solution can also be heated to increase the solubility of the high molar mass polyamines, thereby increasing the homogeneity of amine penetration within the pores. In the case of impregnated materials, the amount of amine introduced to a silica substrate is determined by the initial quantity of the amine and the surface area of the substrate. If the amount of amine introduced exceeds the available surface area of the silica substrate, the amine species will agglomerate on its surface. This agglomeration is readily apparent, as the impregnated material will appear to have a gel-like coating, often yellow, rather than the expected white and powdery appearance1. Among the many types of amine-base solid adsorbents, polyethyleneimine (PEI) and tetraethylene pentamine (TEPA) are the most widely used due to their high stability and high nitrogen content20. For physically impregnated systems, the theoretical loading quantity of amine can be calculated from the pre-weighted amounts of the substrate and the density of the amine. The obvious advantage of physical impregnation lies in the straightforward synthesis procedure to prepare it, as well as the potential for a large amine content due to the high porosity of the silica substrate. Conversely, the stability of the amine within the silica is limited because there is no covalent bonding between the amine and silica support. Therefore, after multiple cycles of CO2 uptake and regeneration through heat or steam, the amine can leach out of the pores. Despite these drawbacks, the implementation of such materials for DAC holds great promise for removing CO2 from the atmosphere.
Another option for the preparation of DAC materials is grafting. Grafting is a method through which amines are immobilized onto a porous silica substrate through a chemical reaction, as shown in Figure 2. This reaction proceeds by reacting an aminosilane with the surface&#39;s silanol functional group, resulting in a covalent bond. Therefore, the number of functional groups on the surface of the silica substrate impacts the grafted amine density21,22. Compared to amine-impregnated adsorbents, chemical grafting methods have had lower CO2 adsorption capacity mainly due to the low amine loading21. Conversely, chemically-grafted amines have increased thermal stability due to their covalently bound structure. This stability can be useful in the regeneration of the material as adsorbents (such as grafted silica) are heated and pressurized to remove the captured CO2 for reuse to save material and cost. In a typical synthesis procedure, the mesoporous silica substrate is dispersed in a solvent (e.g., anhydrous toluene), which is then followed by the addition of aminosilanes. The resulting sample is then washed to remove unreacted aminosilanes. Improvements in aminosilane density are reported to have been achieved through water addition, specifically with SBA-15, to expand pore size23. The procedure for grafting that will be described herein uses moisture-sensitive techniques. Therefore, additional water will not be used. Implementation of grafted aminosilane materials for DAC is promising due to their expected stability during CO2 adsorption and desorption processes. However, the major drawbacks of this methodology include the complex reactions/preparation of these materials, leading to increased cost, and their overall low CO2 adsorption capacity, meaning larger quantities are required.
Overall, results of many previous studies indicate that the structure of the substrate and amine-related modification has a significant impact on the adsorption performance with specific studies utilizing techniques such as transmission electron microscopy (TEM) and quasi-elastic neutron scattering (QENS) to fully characterize these materials24,25. In other words, the structural properties (e.g., porosity and surface area) of the substrate material determine the amine loading, so increasing these parameters can improve CO2 capacity24,25. Continued research into the optimization and design of substrate materials and preparation processes is critical to the development of high-performance adsorbents for DAC. The goal of this work is to provide guidance on impregnation and grafted amine synthesis in hopes of facilitating better transparency of synthetic techniques. Within the literature, specific details on the amounts of solvent, substrate, and amines are not always described, making it difficult to understand the correlation between experimental loading amounts and quantitative measurements of amine-silica composites. The exact loading amounts and a detailed description of the experimental procedures will be provided herein to better facilitate these types of comparisons.

<table><tbody><tr><td>Anhydrous methanol</td><td>Sigma-Aldrich</td><td>322415</td><td>Does not come with sure-seal</td></tr><tr><td>Anhydrous toluene</td><td>Sigma-Aldrich</td><td>244511</td><td>Comes with sure-seal</td></tr><tr><td>Ceramic Stirring Hot Plate</td><td>NA</td><td>NA</td><td>The size, watage, and thermal capabilities of the stirr plate will differ depending on individual lab facilities.</td></tr><tr><td>Fourier Transform Infrared Spectroscopy (FTIR)&amp;nbsp;</td><td>Nicolet i550 series spectrometer</td><td>NA</td><td>Run on OMNIC standard software</td></tr><tr><td>Gastight syringe&amp;nbsp;</td><td>NA</td><td>NA</td><td>As long as the gas tight syringe has a PTFE plunger and luer tip, is suited for air sensitive technique and can be used in this protocol.&amp;nbsp;</td></tr><tr><td>Glass vial</td><td>NA</td><td>NA&amp;nbsp;</td><td>As long as the vial is made if borosilicate glass and has a screw based cap the brand name, size, or general shape does not matter for the protocol.</td></tr><tr><td>MCM-41 silica</td><td>ACS Material&amp;nbsp;</td><td>MSM41A01&amp;nbsp;</td><td>Cas no. 7631-86-9</td></tr><tr><td>Metal needle</td><td>NA</td><td>NA</td><td>Syringe needles need to be stainless steel. It is recommended to determine length and outerdiameter of needle by what will be transferred using the gas tight syringe. For large quantities of liquid a larger outer diameter will improve transfer rates.&amp;nbsp;</td></tr><tr><td>N&amp;rsquo;-(3-trimethylsilyl propyl) diethyleneamine (DAS)</td><td>Sigma-Aldrich</td><td>104884</td><td>Comes with sure-seal&amp;nbsp;</td></tr><tr><td>Polyethyleneimine (PEI)</td><td>Sigma-Aldrich</td><td>408719</td><td>Does not come with sure-seal</td></tr><tr><td>Schlenk round bottom flask</td><td>ChemGlass AirFree</td><td>NA</td><td>As long as the flask is suited for high pressure and temperture but the brand name, size, or general shape does not matter for the protocol</td></tr><tr><td>Thermogravemetric Anlysis (TGA)&amp;nbsp;</td><td>TA Advantage</td><td>NA</td><td>550 series from Waters and TA Instruments</td></tr></tbody></table>

a synthetic methodology for preparing impregnated and grafted amine-based silica composites for carbon capture

Recently, there has been a significant effort towards reducing or mitigating CO2 emissions through the use of carbon capture materials for point source or direct air capture (DAC) methods. This work focuses on amine-functionalized CO2 adsorbents for DAC. These materials show promise for CO2 removal because they have low regeneration energy consumption and high adsorption capacity. The incorporation of amine species into a porous substrate combines the advantages of the amine species&#39; affinity to CO2 with the large pore volumes and surface areas of the porous substrate. There are three methods commonly used to prepare amine-based CO2 sorbents, depending on the selection of the amine species, material support, and preparation method. These methods are impregnation, grafting, or chemical synthesis. Silica is a prevalent choice of substrate material because of its adjustable pore size, moisture tolerance, temperature stability, and ability to adsorb CO2 in low concentrations for DAC applications. Typical synthetic procedures and primary attributes of both impregnated and grafted amine-silica composites are described herein.

TGA is commonly used to quantify the amount of amine loaded or grafted to the silica surface for these materials. The obtained TGA curves show a loss of residual solvent and water between 60 &#176;C to 100 &#176;C, which is shown in the derivate weight (weight %/&#176;C) curve as the first peak, and a loss of amine, which is shown in the derivate weight curve (weight %/&#176;C) as the second peak. For PEI-impregnated silica, this loss of amine is expected to appear around 200 &#176;C to 300 &#176;C, which appears as the second peak in the derivate weight curve, and for DAS grafted silica, the loss of amine is expected to appear around 350 &#176;C to 550 &#176;C (Figure 6). The overall weight loss is indicative of the amount of amine loaded or grafted onto the silica substrate and is an important characterization parameter for judging the quality of the synthesis. For PEI-impregnated specimens, wamine by TGA (wamine_TGA_imp) = 59.2 % &#177; 0.3 % (n = 3) in contrast to the experimental (wamine_exp_graft)= 59.9 % (Figure 6B). For DAS-grafted specimens, wamine_TGA_graft = 22.3 % &#177; 0.1 % (n = 3) in contrast to wamine_exp_graft = 90.0 % when the synthesis is repeated three times (Figure 6A).
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/65845/65845fig06.jpg" /> 
Figure 6: TGA. (A) PEI impregnated at wamine_exp_imp = 59.9 % observed, wamine_TGA_imp = 59.2 % &#177; 0.3 % (n = 3), and (B) Grafted materials at wamine_exp = 90.0 % observed, wamine_TGA = 22.3 % &#177; 0.2 % (n = 3). <a href="https://www.jove.com/files/ftp_upload/65845/65845fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In Figure 7, overall CO2 adsorption was measured using TGA at 5 % CO2 in N2, a 60 mL/min flow rate, and held at 40 &#176;C for 60 min. In Figure 7A, CO2 adsorption curves are shown for PEI-impregnated specimens with an average CO2 percent adsorption by weight (wCO2_imp) of 6.16 % &#177; 0.2 % (n = 3). In Figure 7B, CO2 adsorption curves are shown for DAS-grafted specimens with wCO2_graft = 2.03 % &#177; 0.04 % (n = 3). Within these TGA curves, the baseline is corrected to begin at 100 %.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/65845/65845fig07.jpg" /> 
Figure 7: TGA curves of adsorption of CO2. (A) Impregnated PEI MCM-41 specimen wCO2_imp= 6.16 % &#177; 0.3 % (n = 3). (B) Grafted DAS MCM-41 specimen wCO2_graft = 2.03 % &#177; 0.04 % (n = 3). <a href="https://www.jove.com/files/ftp_upload/65845/65845fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
FTIR-ATR spectroscopy is a type of vibrational spectroscopy commonly used to understand the chemical structure of a material. Figure 8 shows FTIR-ATR spectra of neat PEI, DAS, and MCM-41 as compared to PEI-impregnated or DAS-grafted MCM-41 materials. Distinct peaks ranging from 2500 cm-1 to 3600 cm-1 are attributed to the amine-based N-H signals expected from the aminated materials. When comparing the spectra of the impregnated and grafted materials, a decrease in peak intensity is observed, which is attributed to a lower quantity of amine in the grafted material. Strong peaks corresponding to Si-O-Si range from 400 cm-1 to 1200 cm-1 can be seen within the spectra. The spectra presented corrected for attenuated total reflectance and then automatically baseline corrected through the instrument&#39;s software.
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/65845/65845fig08.jpg" /> 
Figure 8: FTIR spectra. Representative FTIR spectra for neat PEI, MCM-41, DAS, DAS grafted specimen and PEI-impregnated material specimen. <a href="https://www.jove.com/files/ftp_upload/65845/65845fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In Figure 9, SEM micrographs of unaltered MCM-41 are compared with MCM-41 impregnated with PEI at wamine_TGA_imp = 59.2 % and MCM-41 grafted with DAS at wamine_TGA_graft = 22.3 % for morphological and surface differences.
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/65845/65845fig09.jpg" /> 
Figure 9: SEM images. (A) PEI impregnated MCM-41, (B) neat MCM-41, and (C) DAS grafted MCM-41. <a href="https://www.jove.com/files/ftp_upload/65845/65845fig09large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In Figure 10 and Figure 11, neat starting materials such as MCM-41, DAS, and PEI 800 are measured using TGA for baseline reference of weight loss and CO2 adsorption. In Figure 10A, neat MCM-41 TGA curves can be seen as a slow, gradual loss of weight but with no clear peaks in the derivate weight (weight %/&#176;C). In Figure 10B, neat PEI 800 TGA curves with a total weight loss ranging from 200 &#176;C to 370 &#176;C can be seen.
<img alt="Figure 10" class="xfigimg" src="/files/ftp_upload/65845/65845fig10.jpg" /> 
Figure 10: TGA curves. (A) Neat MCM-41 wMCM41 = 8.58 % &#177; 0.5 % (n =3 ). (B) PEI 800 molecular weight wPEI = 98.9 % &#177; 0.9 % (n = 3).. <a href="https://www.jove.com/files/ftp_upload/65845/65845fig10large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 11" class="xfigimg" src="/files/ftp_upload/65845/65845fig11.jpg" /> 
Figure 11: TGA curves. (A) Adsorption of CO2 of neat MCM-41 wCO2 = 0.223 % &#177; 0.2 % (n = 3). (B) Adsorption of CO2 of neat PEI 800 wCO2 = 0.879 % &#177; 0.3 % (n = 3). (C) Adsorption of CO2 of neat DAS wCO2 = 0.247 % &#177; 0.1 % (n = 3). <a href="https://www.jove.com/files/ftp_upload/65845/65845fig011large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In Figure 11A, CO2 adsorption curve are shown for neat MCM-41 with minimal CO2 adsorption wCO2 = 0.222 % &#177; 0.2 % (n = 3). In Figure 11B,&#160;neat PEI 800&#39;s CO2 adsorption curve is shown with minimal CO2 adsorption wCO2 = 0.879 % &#177; 0.3 % (n = 3). In Figure 11C, a&#160;CO2 adsorption curve is shown for neat DAS with minimal CO2 adsorption wCO2 = 0.247 % &#177; 0.1 % (n = 3). Note the adsorption for the neat materials is low because the CO2 adsorption is only occurring at the surface that is exposed to the blended gas environment. Notably, DAS is air and moisture sensitive and is exposed to air during the loading process into the TGA, which may affect its capacity to adsorb CO2.
In Figure 12, CO2 adsorption and desorption cycle curves are shown for impregnated MCM-41 (Figure 12A), and grafted MCM-41 (Figure 12B). Within this protocol, the sample is first activated by heating to 105 &#176;C under 100 % N2 for 5 min, then ramped down to 40 &#176;C and held for 1 min before a blend of 5 % CO2 in N2 is administered for 35 min. This procedure is then repeated (Figure 12C). The figure shows ten repeat cycles with decreasing adsorption capabilities of the material as time progresses. Data can be accessed through the Management of Institutional Data Assets (MIDAS) at&#160;https://doi.org/10.18434/mds2-3017.
<img alt="Figure 12" class="xfigimg" src="/files/ftp_upload/65845/65845fig12.jpg" /> 
Figure 12: Cyclic TGA adsorption and desorption study. (A) PEI impregnated MCM-41, (B) DAS grafted MCM-41, and (C) temperature profile of the procedural runs for (A) and (B). <a href="https://www.jove.com/files/ftp_upload/65845/65845fig12large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Standardizing the Development of Amine-Based Silica Composites as CO2 Adsorbents for Direct Air Capture at JoVE.com

Author Spotlight: Standardizing the Development of Amine-Based Silica Composites as CO2 Adsorbents for Direct Air Capture 

This work aims to facilitate the development of standardized techniques for impregnating or grafting aminated compounds onto silica substrates, which are often broadly described in the literature. Specific amounts of solvent, substrate, amines, and the values of other important experimental parameters will be discussed in detail.

A Synthetic Methodology for Preparing Impregnated and Grafted Amine-Based Silica Composites for Carbon Capture

Recently, there has been a significant effort towards reducing or mitigating CO2 emissions through the use of carbon capture materials for point source or ...

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2.3K Views.  National Institute of Standards and Technology (NIST). This work aims to facilitate the development of standardized techniques for impregnating or grafting aminated compounds onto silica substrates, which are often broadly described in the literature. Specific amounts of solvent, substrate, amines, and the values of other important experimental parameters will be discussed in detail. 

Video: Author Spotlight: Standardizing the Development of Amine-Based Silica Composites as CO2 Adsorbents for Direct Air Capture 

Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle diameters ranging from 30-250 nm by varying the reaction conditions. Through the selection of a microwave compatible solvent, silicic acid precursor, catalyst, and microwave irradiation time, these microwave-assisted methods were capable of overcoming the previously reported shortcomings associated with synthesis of silica nanoparticles using microwave reactors. The siloxane precursor was hydrolyzed using the acid catalyst, HCl. Acetone, a low-tan &#948; solvent, mediates the condensation reactions and has minimal interaction with the electromagnetic field. Condensation reactions begin when the silicic acid precursor couples with the microwave radiation, leading to silica nanoparticle sol formation. The silica nanoparticles were characterized by dynamic light scattering data and scanning electron microscopy, which show the materials&#39; morphology and size to be dependent on the reaction conditions. Microwave-assisted reactions produce silica nanoparticles with roughened textured surfaces that are atypical for silica sols produced by St&#246;ber&#39;s methods, which have smooth surfaces.

Silica nanoparticles were prepared using acid-catalysis of a siloxane precursor and microwave-assisted synthetic techniques resulting in the controlled growth of nanomaterials ranging from 30-250 nm in diameter. The growth dynamics can be controlled by varying the initial silicic acid concentration, time of the reaction, and temperature of reaction.

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle ...

Towards Biomimicking Wood: Fabricated Free-standing Films of Nanocellulose, Lignin, and a Synthetic Polycation

Woody materials are comprised of plant cell walls that contain a layered secondary cell wall composed of structural polymers of polysaccharides and lignin. Layer-by-layer (LbL) assembly process which relies on the assembly of oppositely charged molecules from aqueous solutions was used to build a freestanding composite film of isolated wood polymers of lignin and oxidized nanofibril cellulose (NFC). To facilitate the assembly of these negatively charged polymers, a positively charged polyelectrolyte, poly(diallyldimethylammomium chloride) (PDDA), was used as a linking layer to create this simplified model cell wall. The layered adsorption process was studied quantitatively using quartz crystal microbalance with dissipation monitoring (QCM-D) and ellipsometry. The results showed that layer mass/thickness per adsorbed layer increased as a function of total number of layers. The surface coverage of the adsorbed layers was studied with atomic force microscopy (AFM). Complete coverage of the surface with lignin in all the deposition cycles was found for the system, however, surface coverage by NFC increased with the number of layers. The adsorption process was carried out for 250 cycles (500 bilayers) on a cellulose acetate (CA) substrate. Transparent free-standing LBL assembled nanocomposite films were obtained when the CA substrate was later dissolved in acetone. Scanning electron microscopy (SEM) of the fractured cross-sections showed a lamellar structure, and the thickness per adsorption cycle (PDDA-Lignin-PDDA-NC) was estimated to be 17 nm for two different lignin types used in the study. The data indicates a film with highly controlled architecture where nanocellulose and lignin are spatially deposited on the nanoscale (a polymer-polymer nanocomposites), similar to what is observed in the native cell wall.

The objective of this research was to form synthetic plant cell wall tissue using layer-by-layer assembly of nanocellulose fibrils and isolated lignin assembled from dilute aqueous suspensions.&#160; Surface measurement techniques of quartz crystal microbalance and atomic force microscopy were used to monitor the formation of the polymer-polymer nanocomposite material.

Woody materials are comprised of plant cell walls that contain a layered secondary cell wall composed of structural polymers of polysaccharides and ...

Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10-3 molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The ...

Manufacturing of Three-dimensionally Microstructured Nanocomposites through Microfluidic Infiltration

Microstructured composite beams reinforced with complex three-dimensionally (3D) patterned nanocomposite microfilaments are fabricated via nanocomposite in&#64257;ltration of 3D interconnected microfluidic networks. The manufacturing of the reinforced beams begins with the fabrication of microfluidic networks, which involves layer-by-layer deposition of fugitive ink filaments using a dispensing robot, filling the empty space between filaments using a low viscosity resin, curing the resin and finally removing the ink.&#160;Self-supported 3D structures with other geometries and many layers (e.g.&#160;a few hundreds layers) could be built using this method. The resulting tubular micro&#64258;uidic networks are then infiltrated with thermosetting nanocomposite suspensions containing nanofillers (e.g.&#160;single-walled carbon nanotubes), and subsequently cured. The infiltration is done by applying a pressure gradient between two ends of the empty network (either by applying a vacuum or vacuum-assisted microinjection). Prior to the infiltration, the nanocomposite suspensions are prepared by dispersing nanofillers into polymer matrices using ultrasonication and three-roll mixing methods. The nanocomposites (i.e.&#160;materials infiltrated) are then solidified under UV exposure/heat cure, resulting in a 3D-reinforced composite structure. The technique presented here enables the design of functional nanocomposite macroscopic products for microengineering applications such as actuators and sensors.

Three-dimensional (3D) microstructured composite beams are fabricated through the directed and localized infiltration of nanocomposites into 3D porous microfluidic networks. The flexibility of this manufacturing method enables the utilization of different thermosetting materials and nanofillers in order to achieve a variety of functional 3D reinforced nanocomposite macroscopic products.

Microstructured composite beams reinforced with complex three-dimensionally (3D) patterned nanocomposite microfilaments are fabricated via nanocomposite ...

Synthesis and Characterization of Supramolecular Colloids

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical properties for applications in photonics, drug delivery and coating technology. Here we present a new family of colloidal building blocks, coined supramolecular colloids, whose self-assembly is controlled through surface-functionalization with a benzene-1,3,5-tricarboxamide (BTA) derived supramolecular moiety. Such BTAs interact via directional, strong, yet reversible hydrogen-bonds with other identical BTAs. Herein, a protocol is presented that describes how to couple these BTAs to colloids and how to quantify the number of coupling sites, which determines the multivalency of the supramolecular colloids. Light scattering measurements show that the refractive index of the colloids is almost matched with that of the solvent, which strongly reduces the van der Waals forces between the colloids. Before photo-activation, the colloids remain well dispersed, as the BTAs are equipped with a photo-labile group that blocks the formation of hydrogen-bonds. Controlled deprotection with UV-light activates the short-range hydrogen-bonds between the BTAs, which triggers the colloidal self-assembly. The evolution from the dispersed state to the clustered state is monitored by confocal microscopy. These results are further quantified by image analysis with simple routines using ImageJ and Matlab. This merger of supramolecular chemistry and colloidal science offers a direct route towards light- and thermo-responsive colloidal assembly encoded in the surface-grafted monolayer.

A protocol for the synthesis and characterization of colloids coated with supramolecular moieties is described. These supramolecular colloids undergo self-assembly upon the activation of the hydrogen-bonds between the surface-anchored molecules by UV-light.

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical ...

Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three preparation methods are described: (a) the incorporation of metal salts into silica or alumina wet gels using an impregnation method; (b) the incorporation of metal salts into alumina wet gels using a co-precursor method; and (c) the addition of metal nanoparticles directly into a silica aerogel precursor mixture. The methods utilize a hydraulic hot press, which allows for rapid (&#60;6 h) supercritical extraction and results in aerogels of low density (0.10 g/mL) and high surface area (200-800 m2/g). While the work presented here focuses on the use of copper salts and copper nanoparticles, the approach can be implemented using other metal salts and nanoparticles. A protocol for testing the three-way catalytic ability of these aerogels for automotive pollution mitigation is also presented. This technique uses custom-built equipment, the Union Catalytic Testbed (UCAT), in which a simulated exhaust mixture is passed over an aerogel sample at a controlled temperature and flow rate. The system is capable of measuring the ability of the catalytic aerogels, under both oxidizing and reducing conditions, to convert CO, NO and unburned hydrocarbons (HCs) to less harmful species (CO2, H2O and N2). Example catalytic results are presented for the aerogels described.

Here we present protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms. Methods for preparing materials using copper salts and copper-containing nanoparticles are featured. Catalytic testing protocols demonstrate the effectiveness of these aerogels for three-way catalysis applications.

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three ...

Preparation of Functional Silica Using a Bioinspired Method

The goal of the protocols described herein is to synthesize bioinspired silica materials, perform enzyme encapsulation therein, and partially or totally purify the same by acid elution. By combining sodium silicate with a polyfunctional bioinspired additive, silica is rapidly formed at ambient conditions upon neutralization. 
The effect of neutralization rate and biomolecule addition point on silica yield are investigated, and biomolecule immobilization efficiency is reported for varying addition point. In contrast to other porous silica synthesis methods, it is shown that the mild conditions required for bioinspired silica synthesis are fully compatible with the encapsulation of delicate biomolecules. Additionally, mild conditions are used across all synthesis and modification steps, making bioinspired silica a promising target for the scale-up and commercialization as both a bare material and active support medium.
The synthesis is shown to be highly sensitive to conditions, i.e., the neutralization rate and final synthesis pH, however tight control over these parameters is demonstrated through the use of auto titration methods, leading to high reproducibility in reaction progression pathway and yield. 
Therefore, bioinspired silica is an excellent active material support choice, showing versatility towards many current applications, not limited to those demonstrated here, and potency in future applications.

Here, we present a protocol to synthesize bioinspired silica materials and immobilize enzymes therein. Silica is synthesized by combining sodium silicate and an amine &#39;additive&#39;, which neutralize at a controlled rate. Material properties and function can be altered either by in situ enzyme immobilization or post-synthetic acid elution of encapsulated additives.

The goal of the protocols described herein is to synthesize bioinspired silica materials, perform enzyme encapsulation therein, and partially or totally ...

Surface Properties of Synthesized Nanoporous Carbon and Silica Matrices

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size. This work describes the surface properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the differently ordered porous materials with similar pore sizes. For this purpose, OMC and SBA-15 with highly ordered nanoporous structures are synthesized via impregnation of the silica matrix by applying a carbon precursor and by the sol-gel method, respectively. The porous structure of investigated systems is characterized by an N2 adsorption-desorption analysis at 77 K. To determine the electrochemical character of the surface of synthesized materials, potentiometric titration measurements are conducted; the obtained results for OMC shows a significant pHpzc shift toward the higher values of pH, relative to SBA-15. This suggests that investigated OMC has surface properties related to oxygen-based functional groups. To describe the surface properties of the materials, the contact angles of liquids penetrating the studied porous beds are also determined. The capillary rise method has confirmed the increased wettability of the silica walls relative to the carbon walls and an influence of the pore roughness on the fluid/wall interactions, which is much more pronounced for silica than for carbon mesopores. We have also studied the melting behavior of D2O confined in OMC and SBA-15 by applying the dielectric method. The results show that the depression of the melting temperature of D2O in the pores of OMC is about 15 K higher relative to the depression of the melting temperature in SBA-15 pores with a comparable 5 nm size. This is caused by the influence of adsorbate/adsorbent interactions of the studied matrices.

Here we report the synthesis and characterization of ordered nanoporous carbon (with a 4.6 nm pore size) and SBA-15 (with a 5.3 nm pore size). The work describes the surface and textural properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the materials.

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) ...

Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is designed and prepared according to the methodology developed by surface organometallic chemistry (SOMC). In this framework, catalytic cycles can be determined by isolating crucial intermediates. All air-sensitive materials are handled under inert atmosphere (using gloveboxes or a Schlenk line) or high vacuum lines (HVLs, &#60;10-5 mbar). The preparation of SiO2-700 (silica dehydroxylated at 700 &#176;C) and subsequent applications (the grafting of complexes and catalytic runs) requires the use of HVLs and double-Schlenk techniques. Several well-known characterization methods are used, such as Fourier-transform infrared spectroscopy (FTIR), elemental microanalysis, solid-state nuclear magnetic resonance spectroscopy (SSNMR), and state-of-the-art dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS). FTIR and elemental microanalysis permit scientists to establish the grafting and its stoichiometry. 1H and 13C SSNMR allows the structural determination of the hydrocarbon ligands coordination sphere. DNP SENS is an emerging powerful technique in solid characterization for the detection of poorly sensitive nuclei (15N, in our case). SiO2-700 is treated with about one equivalent of the metal precursor compared to the amount of surface silanol (0.30 mmol&#183;g-1) in pentane at room temperature. Then, volatiles are removed, and the powder samples are dried under dynamic high vacuum to afford the desired materials [(&#8801;Si-O-)Hf(&#951;2&#960;-MeNCH2)(&#951;1-NMe2)(&#951;1-HNMe2)]. After a thermal treatment under high vacuum, the grafted complex is converted into metal imido silica complex [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)]. [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] effectively promotes the metathesis of imines, using the combination of two imine substrates, N-(4-phenylbenzylidene)benzylamine, or N-(4-fluorobenzylidene)-4-fluoroaniline, with N-benzylidenetert-butylamine as substrates. A significantly lower conversion is observed with the blank runs; thus, the presence of the imido group in [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is correlated to the catalytic performance.

A new group IV metal catalyst for imine metathesis is prepared by grafting amine metal complex onto dehydroxylated silica. Surface metal fragments are characterized using FT-IR, elemental microanalysis, and solid-state NMR spectroscopy. Further dynamic nuclear polarization surface enhanced NMR spectroscopy experiments complement the determination of the coordination sphere.

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is designed and prepared ...

Aesthetically Enhanced Silica Aerogel Via Incorporation of Laser Etching and Dyes

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid supercritical extraction method, large silica aerogel monolith (10 cm x 11 cm x 1.5 cm) can be fabricated in about 10 h. Dyes incorporated into the precursor mixture result in yellow-, pink- and orange-tinged aerogels. Text, patterns, and images can be etched onto the surface (or surfaces) of the aerogel monolith without damaging the bulk structure. The laser engraver can be used to cut shapes from the aerogel and form colorful mosaics.

This protocol describes a method for etching text, patterns, and images onto the surface of silica aerogel monoliths in native and dyed form and assembling the aerogels into mosaic designs.

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid ...