Patent Application
20130294991
N/A
1. Field of Invention
This invention relates generally to the use of species removal media for more efficient removal of an unwanted species from a process stream containing the unwanted species, and more specifically to a tunable species removal media comprising the introduction of heteroatoms in a silica matrix loaded with polymeric amines.
2. Background and Related Art
The increasing CO2 concentration in the atmosphere has been regarded as a leading contributor to global climate change witnessed over the last century. Approximately forty percent of anthropogenic CO2 emissions are attributed to burning fossil fuels (coal, natural gas, oil) for power generation, and the atmospheric CO2 level is anticipated to increase over the current concentration of approximately 395 ppm in the near future as fossil fuels remain the major source utilized to meet global energy demand. To reduce carbon emissions, worldwide efforts have been devoted to development of new and advanced materials and technologies/processes suited for efficient carbon capture and storage (CCS) and for improving the energy utilization efficiency.
CO2 capture from ambient air has been suggested as a means of lowering the atmospheric CO2 level as a mode of combating climate change. However, others have argued that some (or all) processes suggested to allow for CO2 capture from ambient air cannot compete with traditional post-combustion CO2 capture approaches, from an economic perspective. While the economics of processes have hardly been studied and are still ill-defined, supported amine materials have been shown to be promising materials that effectively remove CO2 from gases with CO2 concentrations similar to ambient air, and initial economic assessments are promising.
Absorption by amine-based aqueous solution has been a benchmark process for the post-combustion capture of CO2 from large stationary sources, such as flue gas (ca. 10% CO2) generated from coal-fired power plants. However, operational problems such as an energy-intensive regeneration step, oxidative degradation of the aqueous amines, and the corrosion of process equipment motivate alternative approaches.
The ability of supported amines to remove CO2 from the air has the potential to not only impact air capture for environmental purposes, but also to open doors for on-demand, on-site CO2 generation for productive purpose, such as feeding greenhouses, algae installations for bio-fuel production, or other industries.
A significant focus has recently been directed towards solid materials that can capture CO2 reversibly in repeated cycles. Among the array of available solid adsorbents, silica-supported amine materials (e.g., amine-impregnated porous silicas (class 1), amine-grafted silica materials (class 2), and materials prepared via in-situ polymerization of an amine-containing monomer on a silica support (class 3) etc.) have recently emerged as promising candidates for this use. The promise of these materials is associated with the following aspects:
(i) a large CO2 adsorption capacity;
(ii) an ability to reversibly adsorb CO2 even from humid gas streams at low temperature; and
(iii) the high tunability of the amine type and silicate structure, allowing for good control over adsorbent properties.
Adsorptive separation of CO2 from ambient air (ca. 400 ppm CO2, the so-called “air capture”) now represents one of the fastest growing application areas of these silica-supported amine materials. CO2 capture from ambient air has been suggested as a means of lowering the global atmospheric CO2 concentration as a mode of combating climate change. Air capture has a potential advantage over conventional CCS because it can, in principle, be installed anywhere and can capture CO2 from all sources, including small ubiquitous sources such as cars and homes, if the technology is operated on a sufficiently large scale.
Such solid adsorption materials are generally formed by contacting a solid support material with an amine source to absorb amine molecules on the surface of the solid material. The amine molecules on the solid material surface are then subjected to a process stream containing the unwanted species (for example, CO2) such that upon contact with the amine and the solid material, the unwanted species binds with the amine functional group of the amine molecule, reducing the potential for the interaction of the amine functional group with the solid surface and increasing amine sites available for unwanted species capture.
While the economics of processes have hardly been studied and scalable processes are still very early in their development, the collected work to date demonstrates that the silica-supported amine materials are promising materials that effectively remove CO2 from gases with CO2 concentrations similar to ambient air, and initial economic assessments of processes employing supported amine materials are believed to be promising.
The ability of silica-supported amines to remove CO2 from the air has the potential to not only impact air capture for environmental purposes, but also to open doors for on-demand, on-site CO2 generation for productive use, such as feeding greenhouses, creation of a C1 feedstock for chemical and polymer production, feeding enclosed algae installations for bio-fuel production, or other industries. A key objective in the design of silica-supported amine adsorbents is to increase working capacities and to improve adsorption/desorption kinetics and materials stability over multiple regeneration cycles to meet both the economic and technical requirements needed for use in flue gas, air capture, or other applications.
To date, the research in this area has been predominantly focused on altering the amount and the nature of the amine groups and tuning the porosity/morphology of the silicate support. Silica (SiO2) is the support material that has been used in the overwhelming majority of preparations of composite amine-oxide adsorbents. Examples of the use of other oxides such as alumina (Al2O3), titania (TiO2), or composite oxides such as aluminosilicates (composed of aluminum, silicon, and oxygen, plus countercations) or titanosilicates (some silica tetrahedra are replaced by titanium octahedra) are rare. However, the role of the surface properties of the oxide support in class 1 aminopolymer-oxide composite adsorbents has been largely overlooked. Thus, there is significant untapped opportunity to improve adsorption characteristics by manipulating the intrinsic properties of the support, such as the acidity or basicity.
To further improve the efficiency of the removal of an unwanted species with species removal media, the introduction of heteroatom sites onto aminopolymer-impregnated porous silica systems may also provide productive synergism at the interface between the aminopolymer and the solid surface, thus accordingly providing improved unwanted species adsorption performance. It is the intention of the present invention to provide such a beneficial media.
Briefly described, in a preferred form, the present invention comprises systems, methods, and media providing enhanced unwanted species uptake over conventional systems. The present invention improves upon the conventional prototypical (class 1) amine-based solid adsorbents, by presenting a tunable species removal media where the solid framework contains isolated heteroatoms.
The present invention preferably comprises a tunable species removal media comprising a polymer-impregnated porous material with the introduction of heteroatoms into the porous material during the synthesis of the oxide support. In a preferred embodiment, the polymer has a high amine density, the porous material is a mesoporous material, and the heteroatoms are metal atoms. In further preferred embodiments, the polymer is poly(ethyleneimine) (PEI), the porous material is a framework of silica nanoparticles with a hexagonal array of pores, and the heteroatoms are selected from the group consisting of atoms of Zr, Ti, Fe, Ce, Al, B, Ga, Co, Ca, P, and Ni.
The removal media can comprise a sorbent for CO2 capture. In a sorbent for CO2 capture having an amine efficiency of X mol CO2 mol−1N, defined as the number of moles of CO2 captured per mole of active amines, the present invention comprises doping the sorbent with heteroatoms such that the amine efficiency is at least 110% X, where X is the amine efficiency of the sorbent made from undoped support. In a more preferred embodiment, the present invention has an amine efficiency of at least 210% X. In a more preferred embodiment, the present invention has an amine efficiency of at least 400% X.
The present invention can comprise a nanocomposite sorbent, and the nanocomposite sorbent can comprise silica nanoparticles and polyethyleneimine (PEI).
The heteroatoms of the present invention are preferably selected from the group consisting of atoms of Zr, Ti, Fe, Ce, Al, B, Ga, Co, Ca, P, and Ni. In a more preferred embodiment, the heteroatoms are atoms of Zr.
In another embodiment of the present invention, a sorbent for species capture comprises a porous material comprising Silica, a polymer, and heteroatoms, wherein the polymer is impregnated in the porous material, and wherein the heteroatom/Si molar ratio is greater than 0.038.
The polymer can be an amine-containing polymer. The porous material can be a mesoporous material. The heteroatoms can be metal atoms.
In another embodiment of the present invention, a sorbent for species capture comprises a framework of silica nanoparticles, an amine-containing polymer, and heteroatoms, wherein the sorbent has a CO2 adsorption of greater than 0.19 mmol CO2/g when exposed to a 400 ppm CO2/Ar flow at a rate of 100 mL/min. The sorbent can also have a CO2 adsorption of greater than 0.65 mmol CO2/g when exposed to a 10% CO2/Ar flow at a rate of 100 mL/min. The sorbent can have a heteroatom/Si molar ratio greater than or equal to 0.002.
In another embodiment of the present invention, a sorbent for species capture comprises a framework of silica nanoparticles, poly(ethyleneimine) (PEI), and heteroatoms selected from the group consisting of atoms of Zr, Ti, Fe, Ce, Al, B, Ga, Co, Ca, P, and Ni. The PEI can be a low molecular weight, branched PEI. The framework can be SBA-15.
In another embodiment, the present invention comprises a method of increasing species capture comprising doping silica supports with heteroatoms. In yet another embodiment, the present invention comprises a method of enhancing material stability during species adsorption/desorption cycles comprising doping silica supports with heteroatoms. The species can be CO2, or other gases, for example H2S, NO2, SO2, and NO. The invention can further comprise capturing CO2 with the doped silica supports with heteroatoms from a stream containing CO2 with concentrations ranging from 1 ppm to 25% by volume. The heteroatoms can be metal atoms, and the silica supports can include polymeric amines. The heteroatoms can be selected from the group consisting of atoms of Zr, Ti, Fe, Ce, Al, B, Ga, Co, Ca, P, and Ni. The polymeric amines can be selected from the group consisting of poly(ethylenimine), poly(propylenimine), poly(allylamine), poly(vinylamine), and tetraethylenepentamine.
In another exemplary embodiment, the present invention comprises a polymer-impregnated porous material containing heteroatoms, where the polymer is low molecular weight, branched PEI, the porous material is Santa Barbara Amorphous type material (SBA-15), and the heteroatoms are Zr species.
One of the notable characteristics of silicate materials is an ability to create tunable or multi-functional porous solids via the introduction of heteroatoms (e.g., Al, Ti, V, Cr, Fe, Zr, Ce etc.) into the silica matrix by direct or postsynthetic procedures, through which highly elaborated functional materials and catalytic systems can be designed. However, the effectiveness of heteroatom-containing silicates such as aluminosilicates or titanosilicates as support materials for CO2 capture applications has been largely disregarded.
Yet, the present invention provides for the outstanding enhancement of CO2 adsorption performance of prototypical class 1, silica-supported aminopolymer adsorbents by the incorporation of Zr species within the silica framework, whereby the unique surface properties of the support surface play a critical, previously unrecognized role in creating efficient adsorbents.
Additionally, the present invention comprises mesoporous silica SBA-15 materials containing several different heteroatoms, including Al, Ti, Zr, and Ce as supports for poly(ethyleneimine) (PEI) to create an array of class 1 adsorbents. CO2 capacities and amine efficiencies were determined from adsorption experiments under both conditions simulating flue gas (10% CO2 in Ar) and ambient air (400 ppm CO2 in Ar). The associated material structures including local information of the incorporated heteroatoms are characterized by spectroscopic and sorption techniques, in detail, to elucidate the impact of heteroatom incorporation on the structural properties and the performance.
The surface properties of the support materials were studied by thermogravimetric analysis (TGA) and temperature programmed desorption using NH3 and CO2 (NH3/CO2-TPD), and molecular level insights into the CO2 adsorption/desorption process was obtained by in situ Fourier transform infrared (FT-IR) spectroscopy measurements. In addition, the stability and regenerability of the resultant amine-oxide composite materials, which are important for practical CO2 capture applications, are also investigated.
One of the advantageous characteristics of silicate materials is the ability to create tunable porous solids via the introduction of heteroatoms (e.g., Al, Ti, and Zr) into the silica matrix by direct or postsynthetic procedures. It is expected that electrophilic/nucleophilic sites created in this way can play a role in CO2 adsorption by acting as CO2 or amine activating sites.
For example, it has been reported that adenine-grafted Ti-SBA-15 can efficiently catalyze cycloaddition of CO2 with epoxides owing to its improved acid-base properties, coupling the Lewis acidity of the titanium sites with the basicity of grafted amines. Further investigation showed that aminopropyl-grafted silica including Ti sites spatially isolated from amine sites gives a material where both sites (i.e., Brønsted base and Lewis acid, respectively) retain their ability to perform independent chemistry.
PEI preferably is used as the polymer in the present invention as it is an amine source because of its high amine density and accessible primary amine sites on chain ends. Zr atoms preferably are the heteroatom species as Zr substitution creates more effective amine-stabilizing sites on the silica surface compared to, for example, Ti or other common heteroatoms. Indeed, the substitution of Zr atoms in the silicate matrix allows for creation of composite organic-inorganic adsorbents with dramatically enhanced CO2 adsorption performance (as a function of Zr loading) using mixed gas simulating both traditional flue gas capture and CO2 removal from ultra-dilute gas streams, such as ambient air.
The incorporation of Zr species into mesoporous silica creates a support material for low molecular weight polymeric amines to afford composite adsorbents with dramatically enhanced CO2 adsorption properties. Whereas most work on this class of adsorbent materials (class 1 materials) has focused on employing larger amounts of aminopolymer, different types of aminopolymer, or silica supports with altered porosities, the present invention utilizes the acid/base properties of the support, which play a critical, previously unrecognized role in creating more efficient adsorbents. The combination of adsorption experiments and detailed physicochemical characterization showed that these new adsorbents, having an optimal amount of Zr (Zr/Si˜0.07), offer significantly increased CO2 adsorption capacities, improved desorption kinetics and enhanced thermal stability and regenerability compared to conventional PEI/SBA-15 materials.
The present invention demonstrates that CO2 adsorption characteristics of prototypical poly(ethyleneimine)-silica composite adsorbents (class 1 materials) can be drastically enhanced by tuning the acid/base properties of the support via heteroatom incorporation into the silica matrix. More specifically, under feed gas conditions simulating both air capture and capture from flue gas (400 ppm CO2 and 10% CO2, respectively), PEI aminopolymers supported on heteroatom-containing silicates showed increased CO2 adsorption performance, with the extent of enhancement depending on the heteroatom species and their concentrations in the supports. Of the adsorbents studied, the material made from Zr-SBA-15 with moderate level of Zr (Zr/Si≈0.07) and still retaining its ordered mesoporosity was found to be the best material, followed by those composed of Ce-SBA-15 and Ti-SBA-15.
From adsorption experiments, combined with detailed structural characterization, incorporated heteroatoms are found to create an amphoteric surface that stabilizes PEI (probably via acid-base interactions), enabling the PEI to have an increased number of productive amine adsorption sites. This allows for easy access of incoming CO2 molecules to the amine sites, accordingly yielding significantly increased CO2 adsorption capacities, improved CO2 adsorption/desorption kinetics, and enhanced regenerabilities.
In addition, these materials can be easily prepared with only a few modifications of the standard synthetic protocol for the prototypical PEI/SBA-15 adsorbents and possess durabilities over multiple adsorption/desorption cycles under both dry and humid conditions. With these advantageous and feasible characteristics, the PEI supported on heteroatom-incorporated SBA-15 composite adsorbents investigated here are promising candidates for practical CO2 capture applications, including direct air capture, and warrant further studies. Further improvements to CO2 performance may be successful by changing the kinds and configurations of heteroatoms and/or aminopolymers in these class 1 adsorbents.
The above outlines the outstanding enhancement of CO2 adsorption performance of prototypical class 1, silica-supported aminopolymer adsorbents by the incorporation of Zr species within the silica framework, whereby it is demonstrated that the unique surface properties of the support surface play a critical, previously unrecognized role in creating efficient adsorbents.
However, the mechanism (whether the enhancement is associated with a change in the number of adsorption sites or the creation of adsorption sites with different strengths etc.) still remains to be elucidated. An extensive exploration of a variety of heteroatoms that could prove effective for enhancing CO2 adsorption properties may provide constructive insight into this issue.
To this end, mesoporous silica SBA-15 materials containing several different heteroatoms, including Zr, Ti, Fe, Ce, Al, B, Ga, Co, Ca, P, and Ni, were explored as supports for poly(ethyleneimine) (PEI) to create an array of class 1 adsorbents. CO2 capacities and amine efficiencies were determined from adsorption experiments under both conditions simulating flue gas (10% CO2 in Ar) and ambient air (400 ppm CO2 in Ar). The associated material structures including local information of the incorporated heteroatoms are characterized by spectroscopic and sorption techniques, in detail, to elucidate the impact of heteroatom incorporation on the structural properties and the performance. The surface properties of the support materials were studied by thermogravimetric analysis (TGA) and temperature programmed desorption using NH3 and CO2 (NH3/CO2-TPD), and molecular level insights into the CO2 adsorption/desorption process was obtained by in situ Fourier transform infrared (FT-IR) spectroscopy measurements. In addition, the stability and regenerability of the resultant amine-oxide composite materials, which are important for practical CO2 capture applications, are also discussed.
These and other objects, features and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.
Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:
To facilitate an understanding of the principles and features of the various embodiments of the invention, various illustrative embodiments are explained below. Although exemplary embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the exemplary embodiments, specific terminology will be resorted to for the sake of clarity.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. For example, reference to a component is intended also to include composition of a plurality of components. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named.
Also, in describing the exemplary embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
Ranges may be expressed herein as from “about” or “approximately” or “substantially” one particular value and/or to “about” or “approximately” or “substantially” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value.
Similarly, as used herein, “substantially free” of something, or “substantially pure”, and like characterizations, can include both being “at least substantially free” of something, or “at least substantially pure”, and being “completely free” of something, or “completely pure”.
By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.
It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components than those expressly identified.
The materials described as making up the various elements of the invention are intended to be illustrative and not restrictive. Many suitable materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of the invention. Such other materials not described herein can include, but are not limited to, for example, materials that are developed after the time of the development of the invention.
In a preferred embodiment, the present invention comprises a system of enhanced unwanted species uptake over conventional systems. In many of the following examples, the unwanted species is CO2. The invention can comprise a polymer-impregnated porous material containing heteroatoms, where the polymer is low molecular weight, branched PEI, the porous material is Santa Barbara Amorphous type material (SBA-15), and the heteroatoms are Zr species.
In a set of trials, structures of the obtained calcined Zr-SBA-15 materials with Zr/Si molar ratios ranging from 0.038 to 0.138 were characterized by means of X-ray diffraction (XRD), N2 physisorption, and transmission electron microscopy (TEM), which identified the materials as having periodic 2D hexagonal mesoporous structures (P6 mm) with large surface areas (SBET˜650 m2/g), large pore volumes (Vp˜1.2 cm3/g), and uniform pore sizes (dp˜9 nm) for the unmodified SBA-15, Zr4-SBA-15, and Zr7-SBA-15, as summarized in TABLE 1. See
aValues were determined from elemental analysis.
bDetermined by TG analysis.
cCalculated from the adsorption branch of the N2 iso-therms by BET (Brunauer-Emmett-Teller) method.
dValues at P/P0 = 0.99
eEstimated by the BdB-FHH (Frenkell-Halsey-Hill-modified Broekhoff-de Boer) method.
fDefined by an equation of [Occupancy rate (%)] = [calculated aminopolymer volume (cm3-polymer/g-SiO2)]/[Vtotal of bare adsorbent (cm3/g)] × 100 assuming the PEI density of 1.07 cm3/g.
gMeasured at 25° C. under dry conditions (adsorption time 12 h).
However, partial collapse of the mesostructure with appreciable loss of surface area and pore volume was observed when more than 15 mol % of Zr was used. Zr K-edge X-ray absorption fine structure (XAFS) coupled with XRD and FT-Raman spectroscopy confirmed that the Zr species are mostly present as highly dispersed and isolated sites within the silica framework in all cases, but causes a distortion of the silica network, being accompanied by creation of silica defect sites such as silanols, upon increasing the Zr content.
In addition, the effective charge of Zr was determined to be approximately 4+ in all cases based on the absorption edge energies of XAFS spectra. The energy shifts (ΔE0 (eV)); corresponding to the difference between the threshold absorption energy values and that of ZrOCl2.8H2O) of Zr-SBA-15 samples were determined to be ΔE0<±1.2 eV, indicating that the effective charge of Zr in the Zr-SBA-15 materials is approximately 4+ and the content of Zr3+ is quite small. Slight energy shifts towards lower energy positions upon increasing the Zr content seem to be reflecting the simultaneous creation of trace of oxygen defect sites and Zr3+ species, which is well consistent with the results obtained from Raman spectroscopy
These combined characterization results indicate that Zr atoms are effectively covalently-embedded within the silica matrix via isomorphous substitution, irrespective of the Zr content, but introduction of an excess of Zr (>15 mol %) leads to disordering of the mesostructure and results in reduced mesoporosity.
A series of PEI/Zr-SBA-15 composite materials was prepared by the physical impregnation of a low molecular weight PEI (MW=800) into the calcined Zr-SBA-15 samples using MeOH as solvent, in which the organic content was approximately adjusted to ca. 30 wt % for all samples. While larger adsorption capacities could undoubtedly be obtained using higher amine loadings (e.g., ca. 50 wt %), a more moderate loading was chosen here to promote close contact between the PEI polymers and the solid surface while still providing appreciable sample porosity.
Thermogravimetric analysis (TGA) confirmed the intended PEI loading levels in all cases, based on which amine loadings and occupancy rates were stoichiometrically calculated. As summarized in TABLE 1, PEI-impregnated SBA-15, Zr4-SBA-15, and Zr7-SBA-15 retained their ordered mesoporosity and sufficient pore volumes such that they could be considered porous adsorbents. In contrast, complete pore saturation was observed for PEI/Zr14-SBA-15 due to its smaller pore volume.
The CO2 adsorption performance of the PEI/Zr-SBA-15 composites was analyzed by exposing the solids to a 10% or 400 ppm CO2/Ar flow at a rate of 100 mL/min (both under dry conditions) in a TGA. The CO2 uptake of the conventional PEI/SBA-15 was 0.19 and 0.65 mmol/g at 400 ppm and 10% CO2 adsorption, respectively.
This sample provides the baseline performance to which the Zr substituted samples should be compared. The series of PEI/Zr-SBA-15 composite materials showed significantly increased CO2 capacities and thus higher amine efficiencies under both dilute and ultra-dilute CO2 conditions up to a Zr/Si ratio of 0.11, as shown in
The highest CO2 uptake was attained for PEI/Zr7-SBA-15, which showed amine efficiencies 2.1 times and 4.0 times higher than the conventional PEI/SBA-15 material under identical 10% and 400 ppm CO2 adsorption conditions, respectively. Zr7-SBA-15 is shown to have the highest adsorption capacity over a range of temperatures, as well.
It has been reported that at high PEI loadings (above 50 wt. % PEI, loadings that typically result in complete mesopore saturation with PEI), the CO2 adsorption capacity increases with increasing adsorption temperature and the optimal temperature is at around 75° C. This has been explained to be due to restricted diffusion effects of CO2 molecules diffusing through pores filled with dense PEI polymer. Although this trend is often associated with all PEI/silica composites, regardless of PEI-loading, others have shown that materials with PEI loadings that do not result in pore saturation have adsorption trends that track with thermodynamic expectations—reduced capacity upon increasing temperature (e.g. when 30 wt. % PEI-impregnated porous silica materials were used, a higher CO2 uptake was achieved at 25° C. rather than at 75° C. Thus, one may hypothesize that the adsorption optimum around 75° C. may be associated with diffusion effects in PEI-filled mesopores.
In the present invention, substantial mesoporosity still exists for all materials after PEI impregnation, except PEI/Zr14-SBA-15. A higher CO2 uptake was attained at 25° C. when PEI/SBA-15 and PEI/Zr7-SBA-15, both retaining high porosity (occupancy rates=40%, 46%, respectively, see TABLE 1) were evaluated over the range of 25-75° C.) were used as adsorbents. On the other hand, a higher CO2 uptake was obtained at 75° C. over PEI/Zr14-SBA-15, whose pores are completely filled with dense PEI polymer (Occupancy rate=143%, TABLE 1). Furthermore, the PEI/Zr7-SBA-15, which still has significant residual porosity, showed higher CO2 adsorption capacities over the whole range of temperature compared the other adsorbents, while following the same trend with temperature as other adsorbents with residual porosity, such as PEI/SBA-15. These results demonstrate that the enhancement associated with Zr sites in the silicate matrix is apparent at all adsorption temperatures.
Meanwhile, inclusion of excess Zr in the silica led to a reduced CO2 uptake and decreased amine efficiency. This trend coincides with the changes in the support structure induced by excess Zr, as observed by structural analyses (vide supra). It is well-documented that the CO2 capacity of these types of materials is strongly dependent on several structural factors, such as the pore diameter, remaining pore volume as well as the aminopolymer loading level. In this regard, the decreased performance of the PEI/Zr14-SBA-15 sample can be clearly associated with the poor structural characteristics of the support (e.g., surface area, porosity, etc.).
In contrast, the PEI loading, calculated occupancy rates, and structural parameters of the PEI/SBA-15, PEI/Zr4-SBA-15, and PEI/Zr7-SBA-15 adsorbents were all quite similar (see TABLE 1), yet the latter two materials presented significantly increased adsorption performance. This unambiguously demonstrates the positive effect of moderate levels of Zr incorporation in the silicate matrix on the CO2 adsorption properties.
In contrast, others have found that Ti sites present on the silica support surface diminished the ability of the amines to adsorb CO2 if the aminopropyl groups were directly grafted on its surface, since the Ti sites interacted with amines by acting as Lewis acid sites. This, coupled with the fact that the entire support surface is likely covered with aminopolymer in the present case, led to the hypothesis that strong, productive interactions between the PEI and Zr-containing oxide surface enhanced the ability of some of the amines to capture CO2.
This hypothesis is supported by TGA and IR analyses. The PEI decomposition temperature determined from TGA shifted to 30° C. higher temperatures on the Zr-SBA-15 materials compared to traditional pure silica SBA-15.
To quantitatively understand the effect of the incorporated Zr heteroatoms on the CO2 adsorption, the measured CO2 adsorption capacities of PEI/Zr-SBA-15 materials were plotted as a function of the surface density of the acid/base centers,
While Zr14-SBA-15 also possesses acid/base bifunctionality comparable to the other supports containing moderate amounts of Zr (Zr4-SBA-15 and Zr7-SBA-15), it deviated from these correlations due to its subpar porosity characteristics. At this stage, it is too early to unambiguously draw conclusions concerning the molecular level basis of the CO2 adsorption capacity improvement upon moderate levels of Zr incorporation in the silicate framework, but a plausible explanation arises via stabilizing interactions between the aminopolymer and the amphoteric zirconosilicate support.
Based on the above hypotheses, in situ FT-IR experiments were undertaken to clarify the CO2 adsorption kinetics on the adsorbents. As shown in
Regarding the CO2 adsorption kinetics, the observed absorption bands are all typical of those previously reported on conventional amine-based adsorbents, indicating that the amine groups in PEI are the main adsorption sites for CO2. The most noticeable differences observed via FT-IR were in the difference spectra upon CO2 desorption. As shown in
In the case of the conventional adsorbent, some fraction of the CO2 adsorbed could not be eliminated even after a prolonged period of vacuum at room temperature, indicating limited adsorption reversibility of the PEI in silica, perhaps due to an aggregation/degradation of the PEI during the CO2 adsorption. This is consistent with color changes of the adsorbent after multiple adsorption/desorption cycles. The PEI/SBA-15 changed from white to yellow after cycling, suggesting a substantial degradation/aggregation of the PEI, whereas PEI supported on Zr7-SBA-15 remained white even after repeated use. From the combined data, it was found that Zr atoms incorporated into the silicate stabilize the PEI, yielding enhanced adsorption capacities and limiting undesired degradation/aggregation of PEI.
This continual capacity reduction may be associated with the loss of CO2 adsorption sites, perhaps caused by amine degradation originating from urea formation, via amine loss, or by thermal conformational changes of the PEI during its repeated use. In contrast, the PEI/Zr7-SBA-15 composite, which was shown here to efficiently capture CO2, adsorbed CO2 reversibly in a temperature swing process while retaining most of its CO2 capacity during the same number of cycles (decreased by 2% after 4 cycles), demonstrating promising regenerability. These results support the conclusion that Zr atoms on the silica surface efficiently stabilize PEI, providing thermal stability and adsorbent longevity.
Expanding beyond Zr, tunable or multi-functional porous solids via the introduction of heteroatoms beyond Zr (e.g., Al, Ti, V, Cr, Fe Ce etc.) into the silica matrix by direct or postsynthetic procedures, through which highly elaborated functional materials and catalytic systems can be designed. To this end, mesoporous silica SBA-15 materials containing several different heteroatoms, were explored as supports for poly(ethyleneimine) (PEI) to create an array of class 1 adsorbents.
[a]Values were determined from elemental analysis.
[b]Determined by TG analysis.
[c]Calculated from the adsorption branch of the N2 isotherms by BET (Brunauer-Emmett-Teller) method.
[d]Values at P/P0 = 0.99
[e]Estimated by the BdB-FHH (Frenkell-Halsey-Hill-modified Broekhoff-de Boer) method.
[f]Defined by an equation of [Occupancy rate (%)] = [calculated aminopolymer volume (cm3-polymer/g-SiO2)]/[Vtotal of bare adsorbent (cm3/g)] × 100 assuming the PEI density of 1.07 cm3/g.
[g]Measured CO2 adsorption capacities at 25° C. under dry conditions (adsorption time was fixed at 12
[h]The values in parentheses are pseudo-equilibrium CO2 adsorption capacities calculated by fitting the CO2 adsorption curves with a pseudo-first-order adsorption model.
This characterization of the local environment of the Ti and Zr atoms in the SBA-15 samples are also corroborated by XAFS measurements.
The latter peak was scarcely observed for Ti0.2-SBA-15 and all the Zr-SBA-15 materials, indicating that the Zr species (or Ti species in Ti0.2-SBA-15) are mostly present as highly dispersed and isolated species in these samples. Contrary to these findings, distinct peaks were found in the second coordination shell for Ti-SBA-15 with more than 1.4 mol % of Ti content, reflecting the presence of agglomerated titanium oxide species. This result again well fits with the results of XRD and FT-Raman spectroscopy. In addition, a gradual increase in Zr—O interatomic distance was observed upon increasing the Zr content in Zr-SBA-15, implying the distortion of silica matrix by the insertion of Zr atoms with larger ionic radii. These combined characterization results demonstrate the incorporation of Zr atoms within the silica matrix via isomorphous substitution at all Zr levels examined in this study, but introduction of an excess amount of Zr (>15 mol %) leads a disordering of the mesostructure, and accordingly results in reduced mesoporosity.
Al and Ce incorporation into SBA-15, in all cases, yielded materials that show clearly resolved diffraction patterns with high intensities in XRD (
It should be noted that, for Al-SBA-15, an excess amount of Al precursor (more than twice as much as was incorporated) was added in the initial gels to achieve the incorporation of the desired amounts of Al into the silica matrix. For Ce-SBA-15, the incorporated amount of Ce was unfortunately significantly lower than expected (0.4-0.6 mol % Ce per Si). Thus, most of Al and Ce species added in the initial gel remained unincorporated into the silicate particles under the acidic synthesis conditions. Al MAS NMR analysis verified that the Al atoms are embedded within silica matrix mostly in tetrahedral coordination and, in part, in octahedral coordination (for Al MAS NMR spectra, see
Structural ordering of the support materials was further confirmed by TEM.
The thus synthesized and characterized Me-SBA-15 samples were used as supports for low-molecular weight, branched PEI (Mw≈800 Da) to prepare a series of PEI/Me-SBA-15 organic-inorganic composite adsorbents, whose organic content was approximately adjusted to ca. 30 wt % for all samples. It should be noted that the synthesized materials having average pore volumes of ca. 1.2 cm3/g could theoretically host up to 53 wt % of PEI within the pores, assuming a PEI density of 1.07 cm3/g. Of course, adsorbents with larger adsorption capacities could undoubtedly be obtained using higher amine loadings (e.g., ca. 50 wt %), however, a more moderate loading was chosen in this study to promote tight contact between the PEI polymers and the solid surface, as well as to compare all samples at a common amine content. As shown in TABLE 2, thermogravimetric analysis confirmed the intended PEI loading levels (30.8-34.7 wt %) in all cases, based on which amine contents and PEI occupancies were stoichiometrically calculated. As can be identified from the changes in nitrogen physisorption isotherms (
The pore characteristics calculated from the nitrogen adsorption-desorption isotherms provide direct information on the porosity of the PEI-impregnated samples. As summarized in TABLE 2, the pore parameters decreased substantially after the impregnation of 30 wt % PEI; however, most of the composites, except for PEI/Ti8.0-SBA-15, PEI/Zr11-SBA-15, and PEI/Zr14-SBA-15, retained considerable surface areas (SBET˜200-270 m2/g) and pore volumes (Vp˜0.46-0.74 cm3/g), which should be sufficient for effective mass diffusion of adsorbing and desorbing CO2 gases. These composite materials commonly showed 1-2 nm reduced pore size distributions upon PEI impregnation (
In these materials, considering the occupancy rate of PEI to be around 35-49% of the total porosity and the fact that the adsorption-desorption branches were essentially parallel and still exhibited a narrow hysteresis, most aminopolymers are likely well-dispersed inside the pores without significant pore blocking, and the entire support surface is likely covered with aminopolymer, leaving little or no bare silicate surface exposed. On the contrary, the PEI loaded on ill-defined porous silicates such as Ti8.0-SBA-15 and Zr11-SBA-15 showed markedly reduced surface areas after PEI-impregnation due to the filling of their subpar mesoporosity by PEI. Specifically, complete pore saturation was observed for PEI/Zr14-SBA-15. Such an overloaded sample is expected to behave less efficiently as an adsorbent because of the low accessibility of CO2 molecules to the interior amine sites and its physically sticky/tacky nature, as discussed below.
Pluronic P123 block copolymer (PEO20-PPO70-PEO20), zirconium oxychloride (ZrOCl2.8H2O, purity >99%), methanol (reagent grade) and low-molecular weight poly(ethyleneimine) (PEI, Mw≈800) were purchased from Sigma-Aldrich. All other chemicals used for material synthesis were also purchased from Sigma-Aldrich and used without further purification. Single-component gases were purchased from Airgas, Inc. Custom gas mixtures (10% and 400 ppm CO2 in Ar) were purchased from Matheson Tri-Gas, Inc.
Zr-SBA-15 Synthesis: Zr-SBA-15 supports were synthesized similar to previously reported methods with minor modifications. In a typical synthesis, 3.0 g of Pluronic P123 as a pore directing agent, 1.77 g of NaCl, and 120 g of H2O were stirred for 2 h in a 300 mL flask. To this micellular solution, an amount of zirconium oxychloride as a zirconium source and 6.44 g of tetraethylorthosilicate (TEOS) as a silicon source were sequentially added and stirred at 40° C. for 1 day. The molar ratio of the initial gel was adjusted to P123:Si:Zr:NaCl:H2O=0.017:1.0:(0-0.2):1.0:220. The solution was transferred to an oven and then hydrothermally reacted at 100° C. for another day under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Zr-SBA-15 samples were finally calcined at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The synthesized samples were named as ZrX-SBA-15, where X is the atomic ratio of Zr/Si in the final product.
Impregnation of PEI:
The PEI-impregnated adsorbents were prepared by a conventional wet impregnation method. Specifically, the silica support previously dried at 75° C. overnight was immersed into a mixed solution containing the desired amounts of PEI and methanol, and then stirred at room temperature for 1 day. The weight ratios of this slurry were PEI/methanol/silica=0.43:10.0:1.0. This slurry was then placed in a rotary evaporator, and the methanol was removed under vacuum to facilitate the incorporation of the aminopolymer into the silica support. The resulting solid product was collected and stored in an oven at 75° C. until further characterization.
Powder X-ray diffraction (XRD) patterns were recorded on an MO3X-HF (Bruker AXS) diffractometer with CuKα radiation (λ=1.54056 Å). Nitrogen adsorption-desorption isotherms were measured at −196° C. using Micromeritics TriStar II 3020. The calcined samples were degassed at 200° C. under vacuum for 12 h prior to the measurements, while the PEI-impregnated samples were degassed at 100° C. for at least 12 h to vaporize the physisorbed water. The specific surface area was calculated by the BET (Brunauer-Emmett-Teller) method using adsorption data ranging from P/P0=0.05 to 0.30. The pore size distributions were obtained from the adsorption branch of the N2 isotherms by the BdB-FHH (Frenkell-Halsey-Hill-modified Broekhoff-de Boer) method. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM100CX2 electron microscope. Elemental analysis was performed by Columbia Analytical Services. Amine loadings were determined by thermogravimetric analysis (TGA) using Netzsch STA409 instrument, in which the samples (ca. 20 mg) were subjected to continuous heating from room temperature to 900° C. at a heating rate of 10° C./min in an air flow (diluted with nitrogen) of 60 mL/min using α-Al2O3 as a standard.
Investigation of the local structure of Zr atoms in Zr-SBA-15 materials was performed by FT-Raman spectroscopy and XAFS (X-ray Absorption Fine Structure) measurement. FT-Raman spectroscopy was obtained on a Bruker Vertex 80v optical bench with a RAMII Raman module in the spectral range 200-1400/cm and with the spatial resolution of 4/cm. Zr K-edge XAFS spectra were recorded in the fluorescence mode at room temperature using the beam line BL01B1 at SPring-8, Hyogo, Japan. A Si(311) double crystal was used to monochromatize the X-rays from the 8 GeV electron storage ring. EXAFS data were examined using an EXAFS analysis program, Rigaku EXAFS. Fourier transformation of k3-weighted normalized EXAFS data (FT-EXAFS) was performed over the range 1.0<k/Å−1<11.0 to obtain the radial structure function.
The acidity and basicity of the materials were studied by temperature programmed desorption using NH3 and CO2 as probe molecules (NH3/CO2-TPD), respectively, using Micromeritics AutoChem II 2920 Chemisorption Analyzer. Approximately 100 mg of samples was preheated at 500° C. for 1 h under a He flow (10 mL/min), and subsequently were allowed to cool to 50° C., where it was exposed to flowing NH3 (2000 ppm in N2) or CO2 (bone dry CO2) with a flow rate of 30 mL/min for 1 h. Then, the system was purged at 50° C. for 30 min with He to remove weakly adsorbed NH3 or CO2 molecules. The NH3/CO2-TPD was carried out between 50 and 500° C. under a He flow (10 mL/min) with a ramp rate of 10° C./min, and the desorbed NH3/CO2 was detected by a thermal conductivity detector.
Fourier transform infrared (FT-IR) spectroscopy was performed by using a Thermo Scientific 8700 FT-IR instrument with a MCT/A detector. Spectra were typically collected with 64 scans at a resolution of 4/cm. For in situ FT-IR spectroscopy of CO2 adsorption/desorption, each sample was pressed into a self-supported wafer prior to analysis. The wafer was placed into a temperature controlled sample holder, which was loaded into a custom built, high-vacuum transmission FT-IR cell. Vacuum (less than 10−6 mbar) was applied to the wafer before it was heated to 105° C. for a duration of at least 12 h. After cooling to room temperature, the “activated” sample spectrum was collected. The background used for all samples was that of the empty transmission cell under high-vacuum and at room temperature. FT-IR difference spectra of CO2 adsorbed on the samples were obtained with increasing CO2 pressures from 0 to 11 mbar by subtracting the “activated” sample spectrum from the CO2 dosed spectrum. FT-IR difference spectra of CO2 desorbed from CO2-saturated samples were collected every 60 s during evacuation under high vacuum at room temperature for 1 h, and utilized the aforementioned spectral subtraction procedure. After the experiment, the wafer was removed from the cell, and a circle with an exact diameter was cut from the wafer, and the cut circle was weighed in order to obtain the area density (mg/cm2) of the sample. FT-IR spectra collected from wafers of different density were normalized where noted, in order to make direct comparison of absorbance values from different samples valid.
CO2 adsorption measurements under dry conditions were performed using a TA Q500 thermogravimetric analyzer using 10% CO2 or 400 ppm CO2 (balanced with argon). First, approximately 25 mg of the adsorbent was loaded into a platinum pan and subjected to pretreatment under an Ar flow (100 mL/min) at 110° C. for 180 min with a ramping rate of 5° C./min. Then the temperature was lowered to 25° C. and held for 60 min to stabilize the sample weight and temperature before introducing the CO2-containing gas. Adsorption experiments were started by exposing the samples to dry CO2 (10% or 400 ppm CO2 in Ar) at a flow rate of 100 mL/min for 12 h, during which the weight gains were monitored. In the regenerability tests of the adsorbent, the desorption/adsorption cycles were repeated four times to trace the changes of the adsorption capacities.
Materials
Pluronic P123 block copolymer (PEO20-PPO70-PEO20, Mw=5,800), tetraethylorthosilicate (Si(OEt)4, purity 98%), tetramethylorthosilicate (Si(OMe)4, purity 98%), titanium tetraisopropoxide (Ti(OiPr)4, purity >99%), zirconium(IV) oxychloride octahydrate (ZrOCl2.8H2O, purity >99%), aluminium isopropoxide (Al(OiPr)3, purity >99%), cerium(III) nitrate hexahydrate (Ce(NO3)3.6H2O, purity >99.9%), ammonium fluoride (NH4F, purity 98%), methanol (reagent grade) and low-molecular weight poly(ethyleneimine) (PEI, Mw≈800 Da) were purchased from Sigma-Aldrich. All other chemicals used for material synthesis were also purchased from Sigma-Aldrich and used without further purification. Single-component gases were purchased from Airgas, Inc. Custom gas mixtures (10% and 400 ppm CO2 in Ar) were purchased from Matheson Tri-Gas, Inc.
Synthesis
Al-SBA-15 Synthesis: Al-SBA-15 supports were synthesized similar to previously reported methods with minor modifications. In a typical synthesis, 4.0 g of Pluronic P123 as a pore directing agent and 100 g of 0.2 N HCl aqueous solution were stirred for 2 h in a 300 mL flask. To this micellular solution, an amount of aluminium isopropoxide as an aluminium source and 9.0 g of tetraethylorthosilicate (TEOS) as a silicon source were sequentially added and stirred at 40° C. for 1 day. The molar ratio of the initial gel was adjusted to P123:Si:Al:HCl:H2O=0.016:1.0:(0-0.25):0.468:130. The solution was transferred to an oven and then hydrothermally reacted at 100° C. for another 2 days under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Al-SBA-15 samples were finally calcined in air at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The synthesized samples were named as AlX-SBA-15, where X is the atomic ratio of Al/Si in the final solid.
Ti-SBA-15 Synthesis: Ti-SBA-15 supports were synthesized similar to previously reported methods with minor modifications. In a typical synthesis, 2.67 g of Pluronic P123 as a pore directing agent, 35.1 mg of NH4F, and 100 g of 0.1N HCl aqueous solution were stirred for 2 h in a 300 mL flask. To this micellular solution, a mixed solution containing an amount of titanium tetraisopropoxide (TPOT) as a titanium source and 4.7 g of tetramethylorthosilicate (TMOS) as a silicon source was added and stirred at 40° C. for 1 day. The molar ratio of the initial gel was adjusted to P123:Si:Ti:NH4F:H2O=0.015:1.0:(0-0.1):0.03:180. The solution was then hydrothermally reacted at 80° C. for another 2 days under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Ti-SBA-15 samples were finally calcined in air at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The synthesized samples were named as TiX-SBA-15, where X is the atomic ratio of Ti/Si in the final solid.
Zr-SBA-15 Synthesis: Zr-SBA-15 supports were synthesized similar to previously reported methods with minor modifications. In a typical synthesis, 3.0 g of Pluronic P123 as a pore directing agent, 1.77 g of NaCl, and 120 g of H2O were stirred for 2 h in a 300 mL flask. To this micellular solution, an amount of zirconium oxychloride octahydrate as a zirconium source and 6.44 g of tetraethylorthosilicate (TEOS) as a silicon source were sequentially added and stirred at 40° C. for 1 day. The molar ratio of the initial gel was adjusted to P123:Si:Zr:NaCl:H2O=0.017:1.0:(0-0.2):1.0:220. The solution was transferred to an oven and then hydrothermally reacted at 100° C. for another day under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Zr-SBA-15 samples were finally calcined in air at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The synthesized samples were named as ZrX-SBA-15, where X is the atomic ratio of Zr/Si in the final solid.
Ce-SBA-15 Synthesis: Ce-SBA-15 supports were synthesized according to previously reported methods with minor modifications. In a typical synthesis, 2.67 g of Pluronic P123 as a pore directing agent, 35.1 mg of NH4F, and 100 g of 0.1N HCl aqueous solution were stirred for 2 h in a 300 mL flask. To this micellular solution, an amount of cerium(III) nitrate hexahydrate as a cerium source and 6.44 g of tetraethylorthosilicate (TEOS) as a silicon source were sequentially added and stirred at 40° C. for 1 day. The molar ratio of the initial gel was adjusted to P123:Si:Ce:NH4F:H2O=0.015:1.0:(0-0.1):0.03:180. The solution was transferred to an oven and then hydrothermally reacted at 100° C. for another 2 days under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Ce-SBA-15 samples were finally calcined in air at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The synthesized samples were named as CeX-SBA-15, where X is the atomic ratio of Ce/Si in the final solid.
Synthesis of Materials
All chemicals used for material synthesis were purchased from Sigma-Aldrich and used without further purification. A variety of heteroatom-substituted SBA-15 supports, Me-SBA-15 (Me=Al, Ti, Zr, and Ce), with varied metal content were synthesized by sol-gel process using Pluronic P123 block copolymer (PEO20-PPO70-PEO20, Mw=5,800) as a pore directing agent and tetraethyl orthosilicate (TEOS, 98%) as a silicon source under acidic conditions according to previous reports with minor modifications. Briefly, Pluronic P123 was dissolved in HCl aqueous solution with vigorous stirring for 2 h in a flask, in which a portion of NH4F and NaCl was added as a catalyst for Ti-SBA-15 and Zr-SBA-15 syntheses, respectively, to facilitate heteroatom incorporation (no specific catalyst was added for Al- and Ce-SBA-15 syntheses). To this micellular solution, a designated amount of each metal source and TEOS were sequentially added and stirred at 40° C. for 1 day, in which titanium(IV) tetraisopropoxide (Ti(OiPr)4, >99%), zirconium(IV) oxychloride octahydrate (ZrOCl2.8H2O, >99%), aluminum(III) isopropoxide (Al(OiPr)3, >99%), and cerium(III) nitrate hexahydrate (Ce(NO3)3.6H2O, >99.9%) were used as metal precursors. The solution was then transferred to a pressure bottle, sealed, and hydrothermally reacted at 100° C. for another 2 days under static conditions. The resulting product was filtered, washed with deionized water, and dried overnight at 75° C. The as-synthesized Me-SBA-15 samples were finally calcined in air at 500° C. for 10 h at a ramping rate of 3° C./min to remove the organic template. The calcined samples were denoted as MeX-SBA-15 (Me=Al, Ti, Zr, and Ce), where X is the atomic ratio of Me/Si in the final solid. For detailed information on material synthesis, readers are referred to the supporting information.
Synthesis of PEI-Impregnated Adsorbents
A low-molecular weight, branched poly(ethyleneimine) (PEI, MW≈800 Da) was used as the amine functionality, as this aminopolymer has high amine density and accessible primary amine sites on chain ends. The PEI-impregnated adsorbents were prepared by a conventional wet impregnation method. Specifically, the silica support previously dried at 75° C. overnight was immersed into a mixed solution containing the desired amounts of PEI and methanol (>99.8%, reagent grade), and then stirred at room temperature for 1 day. The weight ratios of this slurry were always maintained constant at PEI/methanol/silica=0.43:10.0:1.0. This slurry was then placed in a rotary evaporator, and the methanol was removed under vacuum to facilitate the incorporation of the aminopolymer into the silica support. The resultant solid product was collected and stored in an oven at 75° C. until further characterization. The silica supported amine composites thus synthesized were named PEI/MeX-SBA-15 (Me=Al, Ti, Zr, and Ce).
Characterization
Powder XRD patterns were recorded on an M03X-HF (Bruker AXS) diffractometer with CuKα radiation (λ=1.54056 Å). Nitrogen adsorption-desorption isotherms were measured at −196° C. using Micromeritics TriStar II 3020. The calcined samples were degassed at 200° C. under vacuum for 12 h prior to the measurements, while the PEI-impregnated samples were degassed at 100° C. for at least 12 h to vaporize physisorbed water. The specific surface area was calculated by the BET (Brunauer-Emmett-Teller) method using adsorption data ranging from P/P0=0.05 to 0.30. The pore size distributions were obtained from the adsorption branch of the nitrogen isotherms by the BdB-FHH (Frenkell-Halsey-Hill-modified Broekhoff-de Boer) method, which has been shown to be more accurate than the Barret-Joyner-Halenda method (BJH) for practical application to mesoporous materials. TEM images were obtained with a JEOL JEM100CX2 electron microscope. Elemental analysis was performed by Columbia Analytical Services. Amine loadings were determined by TGA using Netzsch STA409 instrument, in which the samples (ca. 20 mg) were subjected to continuous heating from room temperature to 800° C. under a mixed gas stream of air (30 mL/min) and nitrogen (30 mL/min) at a heating rate of 10° C./min using α-Al2O3 as a standard. FT-Raman spectra were obtained on a Bruker Vertex 80v optical bench with a RAMII Raman module in the spectral range of 200-1400/cm with the spatial resolution of 4/cm. Ti K-edge and Zr K-edge X-ray absorption fine structure (XAFS) spectra were recorded in fluorescence mode at room temperature at BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba, Japan and at BL01B1 facility of SPring-8, Hyogo, Japan, respectively. All extended XAFS (EXAFS) data were examined using an EXAFS analysis program, Rigaku EXAFS, whereby Fourier transformation of k3-weighted normalized EXAFS data (FT-EXAFS) was performed over the range 1.0<k/Å−1<11.0 to obtain the radial structure function.
The acidity and basicity of the materials were studied by NH3/CO2-TPD using a Micromeritics AutoChem II 2920 Chemisorption Analyzer. Approximately 100 mg of each sample was preheated at 500° C. for 1 h under a He flow (10 mL/min), and subsequently was allowed to cool to 50° C., where it was exposed to flowing NH3 (2000 ppm in N2) or CO2 (bone dry CO2) with a flow rate of 30 mL/min for 1 h. Then, the system was purged at 50° C. for 30 min with He to remove weakly adsorbed NH3 or CO2 molecules. The NH3/CO2-TPD was carried out between 50 and 500° C. under a He flow (10 mL/min) with a ramping rate of 10° C./min, and the desorbed NH3/CO2 was quantified by a thermal conductivity detector.
FT-IR spectroscopy was performed by using a Thermo Scientific 8700 FT-IR instrument with a MCT/A detector. For in situ FT-IR spectroscopy of CO2 adsorption/desorption, each sample was pressed into a self-supported wafer prior to analysis. The wafer was loaded into a high-vacuum transmission FT-IR cell and was heated to 105° C. under vacuum (less than 10−6 mbar) for a duration of at least 12 h, and the “activated” sample spectrum was collected after cooling to room temperature. FT-IR difference spectra of CO2 adsorbed on the samples were obtained with increasing CO2 pressures from 0 to 11 mbar by subtracting the “activated” sample spectrum from the CO2 dosed spectrum. FT-IR difference spectra of CO2 desorbed from CO2-saturated samples were collected every 120 s during evacuation under high vacuum at room temperature for 1 h, and utilized the aforementioned spectral subtraction procedure. FT-IR spectra were normalized by the area densities of the wafers to make direct comparison of absorbance values from different samples valid.
Adsorption Tests
CO2 adsorption measurements under dry conditions were performed using a TA Instruments Model Q500 thermogravimetric analyzer using 10% CO2 or 400 ppm CO2 balanced with Ar. First, approximately 25 mg of the adsorbent was loaded into a platinum pan and subjected to pretreatment under an Ar flow (100 mL/min) at 110° C. for 180 min with a ramping rate of 5° C./min. The temperature was then lowered to 25° C. and held for 60 min to stabilize the sample weight and temperature before introducing the CO2-containing gas. Adsorption was initiated by exposing the samples to dry target gas of desired concentration (10% or 400 ppm CO2 balanced with Ar) at a flow rate of 100 mL/min for 12 h, during which the weight gains were monitored. This adsorption time was sufficient to consider the amount of adsorbed CO2 as a pseudo-equilibrium capacity. In the regenerability tests of the adsorbent, the desorption/adsorption cycles were repeated four times to trace the changes of the adsorption capacities.
Characterizations of the Materials:
The structural characteristics of a series of heteroatom-substituted SBA-15 support materials with varied metal concentrations were verified by X-ray diffraction (XRD) measurement, N2 physisorption, and Transmission electron microscopy (TEM). For example,
In the N2 adsorption-desorptipon isotherms, all of the samples, except for Ti8.0-SBA-15, displayed type IV isotherms with clear capillary condensation at around P/P0=0.8 along with a hysteresis loop (
In high angle XRD patterns, several diffraction planes associated with anatase TiO2 crystals were observed as the Ti/Si ratio increased above 0.014, evidencing the aggregation of Ti species. This is due to the different hydrolysis and polymerization rates of the titanium precursor and silicon precursor (tetraethyl orthosilicate; TEOS). Similar results have been reported in many publications, whereby the overall trend suggests that the addition of more than 2 mol % of Ti leads to creation of polymeric titanium oxide moieties. Such aggregated species could not be observed for the Zr-SBA-15 materials at all Zr loading levels, suggesting a different coordination geometry of the heteroatoms in this case.
Investigation of the local structure of the Ti and Zr atoms in Ti-SBA-15 and Zr-SBA-15 materials was performed by the combination of FT-Raman spectroscopy and X-ray absorption fine structure (XAFS) analysis.
On the other hand, the Zr-SBA-15 samples exhibited no distinct peaks assignable to contiguous Zr—O—Zr bonds at all Zr levels as seen in
CO2 Adsorption Performance:
The CO2 adsorption performance of the PEI/Me-SBA-15 composites was evaluated by exposing the solids to a simulated flue gas (ca. 10% CO2 balanced with Ar) or simulated ambient air (ca. 400 ppm CO2 balanced with Ar) at a flow rate of 100 mL/min (both under dry conditions at 25° C.) using a thermal gravimetric apparatus (TGA). Here, a sufficiently long adsorption time (12 h) was chosen to render kinetic effects less significant and to evaluate the measured capacity as a function of the accessible adsorption sites. It is noteworthy that, in the absence of water, only primary and secondary amines are suggested to react with CO2 to produce carbamates through the formation of zwitterionic intermediates. In this mechanism, an additional free base is required per mole of CO2 captured for deprotonating the zwitterions to form the carbamates. Thereby, under dry conditions, the theoretical maximum amine efficiency, defined as the number of moles of CO2 captured per mole of active amines, for the PEI-based adsorbent becomes 0.385, assuming an average amine ratio of PEI is primary:secondary:tertiary=44:33:23.
The measured CO2 adsorption capacities are summarized in TABLE 2. These values closely approximate the pseudo-equilibrium CO2 adsorption capacities calculated by fitting the adsorption curves with a pseudo-first-order adsorption model, thereby validating a direct comparison of these values. The measured CO2 uptakes of the PEI/SBA-15 (one of the most well-studied class 1 supported amine adsorbents) as a standard sample were 0.19 and 0.65 mmol/g under identical 400 ppm and 10% CO2 adsorption conditions, respectively, whose values seem slightly lower than those reported previously for similar materials. This is mainly because its lower PEI loading level, relatively lower concentrations of CO2 gases, and use of dry adsorption conditions where two amine sites are required to capture one CO2 molecule. Under both feed gas conditions (400 ppm CO2 and 10% CO2), the PEI aminopolymers supported on heteroatom-incorporated SBA-15 samples overall showed increased CO2 adsorption performances. Depending on the metal species used and their concentrations in the support materials, the effective capacity varied, and the Zr-SBA-15 with Zr/Si ˜0.07 was found to be optimal for improving the CO2 adsorption capacity, followed by Ce-SBA-15 and the series of Ti-SBA-15 materials.
Comparisons of the pseudo-equilibrium CO2 capture capacities and amine efficiencies over the series of PEI/Zr-SBA-15 composite adsorbents are shown in
The adsorption data for the series of PEI/Ti-SBA-15 composite adsorbents also displayed a similar trend, albeit to a lesser degree (
Cerium oxide nanocrystals dispersed in SBA-15 helped the composite adsorbent capture up to 3.6 times as much CO2 in 400 ppm CO2 conditions, demonstrating that Ce-SBA-15 can be a viable option as an efficient support material for PEI adsorbents, even though uniform incorporation of Ce into the SBA-15 framework was not achieved in this study (see
Al-substitution resulted in a material with only 1.5 times increased capacity for adsorbing CO2 from the simulated ambient air. The effect of Al-incorporation into silica supports using similar class 1 composite adsorbents has been investigated comprising 50 wt % PEI-impregnated Al-MCM-41, whereby the Al content was ca. 0.2-1.0 mol %. They reported that 12% larger CO2 adsorption capacity could be obtained by using the PEI supported on Al-MCM-41 compared with PEI/MCM-41 under the applied adsorption conditions (pure CO2 gas flow and at 75° C.). However, they attributed this improved CO2 uptake to the increased pore characteristics of the Al-MCM-41 supports (e.g., extended pore diameter and remaining pore volume). Compared to this, the PEI/Al-SBA-15 adsorbents examined in this study present a more marked difference in CO2 uptake, despite showing almost similar pore characteristics as the standard PEI/SBA-15 (see TABLE 2). Another aspect that needs to be taken into account for Al-substituted silica materials is the surface acidity, which may play a negative role if combined with PEI, as will be discussed below.
Thus, by varying the heteroatom species, it has been observed that both isolated and aggregated heteroatoms can drastically alter the ability of PEI to adsorb CO2. At optimal metal concentrations, the CO2 capacity using 400 ppm CO2 gases increases in the order of: PEI/SBA-15<PEI/Al-SBA-15<PEI/Ti-SBA-15<PEI/Ce-SBA-15<PEI/Zr-SBA-15.
Surface Properties of Supports:
TGA and FT-IR analyses preliminarily provided some insight into the aminopolymer-surface interactions.
To further elucidate this intricate aminopolymer-surface interaction, a quantitative study was performed by plotting the measured CO2 adsorption capacities of the PEI/Me-SBA-15 materials as a function of the surface density of the acid/base centres of the supports (
A deviation from the correlation by Zr14-SBA-15 is due to its subpar porosity characteristics, as mentioned above. Although the Al-SBA-15 materials possessed acid/base bifunctionality similar to the Zr-SBA-15 materials, the CO2 adsorption capacities of the resultant composite adsorbents were significantly lower. This might be due to the presence of strong Brønsted acidity associated with Al-substitution. Unlike the case of substitution by tetravalent cations such as Zr4+ and Ti4+, substituting silicon by trivalent cations such as Al3+ in the mesoporous silica wall provides Brønsted acid sites on the surface by compensating the negative framework charge with protons. Such surface Brønsted acidic sites do not seem to effectively function as productive sites for the aminopolymer. In comparison, Ti- and Ce-incorporated SBA-15 materials, both of which afford a mildly acidic surface by creating weak Lewis acid sites (basic sites are scarcely observed), effectively contributed to enhanced CO2 capture properties, despite their lower surface acid densities than the Al- or Zr-SBA-15 materials. It appears that the heteroatom incorporation reinforces the interaction between PEI and the silicate surface, thereby leading to the enhanced CO2 adsorption capacity when compared to unsubstituted SBA-15. A plausible explanation for this interaction arises via an acid-base interaction between primary amines, present on all chain ends in PEI, and the heteroatom-associated Lewis acid sites on the silicate surface, which may stabilize and/or change the structure of PEI and potentially enhance the accessibility of the rest of the amines to incoming CO2, whereas interactions between amines and a strong Brønsted acid sites created by Al-substitution may be unproductive for this purpose.
CO2 Adsorption Kinetics:
The kinetics of adsorption and desorption are also important in assessing an adsorbent's performance, since practical CO2 capture applications require short adsorption/desorption cycle times to reduce the total quantity of adsorbent in an installation.
The incorporation of heteroatoms into the silica matrix of the class 1 adsorbents made a significant impact on the adsorption kinetics as well. As mentioned above, the series of PEI/Zr-SBA-15 and PEI/Ti-SBA-15 samples exhibited faster CO2 adsorption kinetics, which were more obvious at the 400 ppm CO2 adsorption conditions than at the 10% CO2 conditions. At 400 ppm CO2, the adsorption curves of the series of PEI/Zr-SBA-15 and PEI/Ti-SBA-15 samples showed steep increases (exceeded 80% of the pseudo-equilibrium capacities within the first 90 min of adsorption), followed by much faster rates of saturation for the adsorption-equilibrium compared to the conventional PEI/SBA-15 (see
More importantly, there was a strong link between the CO2 adsorption capacities and the adsorption rates.
Among those, PEI/Zr-SBA-15 samples with Zr content of 7.0˜11, showing significantly increased adsorption capacities and shorter adsorption times, were found to be the most promising adsorbents for ultra-dilute CO2 capture applications, followed by PEI/Ti-SBA-15 adsorbents with Ti content of 1.4˜4.3. It has been well-established that the adsorbent with higher amine loadings generally require longer times to approach adsorption equilibrium, however, the amine loadings are all similar for the materials described here. The adsorption kinetics of supported amine adsorbents are influenced by other factors, such as CO2 diffusion into and out of the pores, CO2 accessibility to the active amine sites, and the type and intrinsic chemical reaction rate of the amines. Considering that all these materials have similar porosity, similar amine types and loadings, and the measurement variables are all similar, the improved CO2 adsorption kinetics observed in the PEI/Me-SBA-15 adsorbents are likely associated with differences in the accessibility of amines available, rather than with intrinsic chemical reaction rates.
To provide molecular level insight into CO2 adsorption on the materials, in situ FT-IR experiments were carried out using CO2 as the probe molecule.
More intriguing findings regarding the kinetics were obtained from in situ FT-IR difference spectra upon CO2 desorption (
All the above experimental results led us to a generic conclusion that heteroatoms incorporated into silica create an amphoteric surface that is responsible for stabilizing and/or changing the structure of PEI, limiting undesired degradation/aggregation of PEI via surface chemical interactions (possibly via acid-base interactions). Such a stabilized PEI is believed to have an increased number of productive amine adsorption sites and allow for easy access of incoming CO2 molecules to the amine sites, accordingly affording significantly increased CO2 adsorption capacities and improved CO2 adsorption/desorption kinetics. It is conceivable that the configuration of the PEI aminopolymer substantially influences the adsorption chemistry. The understanding about distribution and physical structure of the aminopolymer within composite adsorbents is still poorly developed, and additional studies focused on this aspect are clearly needed.
Regenerability Test:
Beside the adsorption capacities and adsorption rates, stability of the CO2 adsorbents is of key importance for practical CO2 capture applications as well, because the lifetime of the adsorbents strongly affects the cost of the overall process. To examine the regenerability of the adsorbents, several selected composite adsorbents were subjected to multiple CO2 adsorption/desorption cycles at 25° C. using 400 ppm CO2/Ar flow, in which the CO2 was desorbed from the surface in pure Ar atmosphere at 110° C. for 3 h.
CO2 Adsorption Under Humid Conditions:
Practical applications in CO2 capture from both ambient air and mixed gas including moisture requires robust materials that are stable even in humid conditions. It has been recognized that the stability of most silica-based materials is too limited for practical application in the presence of steam, hence hybrid materials based on oxide supports that are more steam stable, such as alumina and titania, are needed. It should be noted that Sayari and co-workers reported no adverse effects of the presence of moderate amounts of water vapor on the chemical stability of silica-supported amine groups during the adsorption/desorption cycles in their studies.
[a]Measured at 25° C. under dry conditions (adsorption time was fixed at 12 h).
[b]Measured at 25° C. under humid conditions (adsorption time was fixed at 12 h).
To address the impact of water on the material structure and the ability of the amines to capture CO2, an additional CO2 adsorption experiment was carried out by contacting the PEI/Zr7.0-SBA-15 sample, the most efficient adsorbent among those examined here, with a water-saturated 10% CO2/Ar flow of 100 mL/min for 12 h using a fixed-bed reactor equipped with an on-line mass spectrometer. As summarized in TABLE 3, under humid conditions at 25° C., the CO2 uptake increased 8% relative to the dry conditions and was fairly stable over two runs (1.68 mmol/g (cycle 1)→1.60 mmol/g (cycle 2)), showing only a modest enhancement of capacity under humid conditions. One might speculate that this unexpected low enhancement in the presence of humidity is the consequence of an alternate adsorption mechanism under dry conditions using the heteroatom-incorporated materials, i.e., the possibility that the surface-exposed heteroatoms function as sites to form carbamate-surface ion pairs, thus precluding the need for utilizing two amines to capture one CO2 molecule and accordingly increasing amine efficiency even under dry conditions. However, the calculated amount of Zr per gram of the adsorbent (ca. 0.76 mmol/g) is lower than the amine content (8.33 mmol/g) by an order of magnitude, thereby discounting this possibility.
As previously mentioned, under humid conditions where water molecules can act as an additional base, the maximum amine efficiency could theoretically increase up to 2-fold. However, the enhancements are usually less than this theoretical maximum. The promoting effect of moisture for CO2 capacity is known using 50 wt % PEI-impregnated MCM-41 adsorbent, where up to ca. 1.5 times larger CO2 capacity was observed under a flow of mixed gas containing CO2 and ˜16% moisture and at 75° C. Meanwhile, Sayari et al. have reported only a slight enhancement of CO2 capacity using a similar material under a flow of water-saturated CO2 and at 20° C. Goeppert et al. also found that 1.5 times increased CO2 capacity could be achieved over 33 wt % PEI-impregnated fumed silica at 25° C. under 400 ppm CO2 with 67% of relative humidity; however an adverse effect was observed for the adsorbent with a higher PEI loading (50 wt % PEI) where the adsorption is more diffusion-controlled, in which water molecules are considered to inhibit the access of incoming CO2 to reach amine adsorption sites. As seen in these reports, CO2 capacity in humid conditions can be strongly affected by several structural and operational factors, e.g, location/loading of amines within pores, pore blockage, steric hindrance from neighboring species, or concentration of moisture introduced, and these effects are more pronounced especially after long adsorption times where equilibrium is nearly reached and almost every accessible amine has captured a CO2 molecule. A plausible explanation for the only modest enhancement under humid conditions in this case is due to steric hindrance from adsorbed water molecules, which might inhibit the access of the entering CO2 to reach amine sites, although admittedly, further detailed studies are necessary to draw a more rational conclusion for this result.
Additionally, structural analyses of the used sample confirmed the crystallographic retention and structural stability of the material upon CO2 adsorption under humid conditions (i.e., 25° C. for 12 h under a flow of water-saturated 10% CO2 gas), in which phase-separation and porosity reduction of the zirconosilicate support originating from hydrolysis was negligible. Regarding the hydrothermal stability of zirconosilicate, a number of publications have commonly reported that they are highly stable against hydrothermal treatment (even in boiling water or in 100% steaming). These facts indicate that the PEI/Zr-SBA-15 and related materials can be potentially used under such humid conditions, and as such, these materials warrant further study under a variety of humid conditions.
Numerous characteristics and advantages have been set forth in the foregoing description, together with details of structure and function. While the invention has been disclosed in several forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions, especially in matters of shape, size, and arrangement of parts, can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims. Therefore, other modifications or embodiments as may be suggested by the teachings herein are particularly reserved as they fall within the breadth and scope of the claims here appended.
This application claims the benefit of U.S. Provisional Application No. 61/642,219 filed 3 May 2012, the entire contents and substance of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61642219 | May 2012 | US |
