# Patent

## Title
Materials, methods and systems for selective capture of CO2 at high pressure

## Abstract

The present invention provides methods and systems for carbon dioxide, hydrogen sulfide and other acid gases capture via adsorption at high pressure using mesoporous materials.

## Background
FIELD OF THE INVENTION
The present invention pertains to the field of adsorption methods and systems for selective capture of carbon dioxide and other acid gases, more particularly, to the field of adsorption methods and systems that employ mesoporous silica for the separation of carbon dioxide and other acid gases at high pressure.
BACKGROUND
Carbon dioxide (CO2) is a major greenhouse gas with significant contribution to global warming (Halmann and Stenberg 1999). Removal of CO2 from different gas streams is becoming increasingly important for various applications like treatment of flue gas, natural gas, biogas, and hydrogen purification as well as closed-circuit breathing systems (CCBS) for use in confined spaces such as manned space shuttles (Satyapal et al. 2001), and in emergency situations. The recovered CO2, with different degrees of purity, also has numerous applications in the chemical industry.
Separation, capture and storage of carbon dioxide (CO2) have received significant attention in recent years. Liquid phase absorption in amine solutions has been widely used to treat gases with medium to high CO2 concentration, but due to the high regeneration cost of the absorbent and corrosion problems (Veawab et al. 1999), it is highly desirable to develop less energy intensive technologies like adsorption (Ruthven 1994) and membrane separation (Hong et al. 2008).
Many of CO2 adsorbents have been developed in recent years including metal oxides (Wang et al. 2008), zeolites (Goj et al. 2002; Cavenati et al. 2006; Akten et al. 2003; Belmabkhout et al. 2007), carbon (Himeno et al. 2005), metal-organic frameworks (MOFs) (Millward and Yaghi 2005; Bourrelly et al. 2005; Yang et al. 2008; Yang and Zhong 2006; Li and Yang 2007), organo-silicas and surface-modified silicas (Harlick and Sayari 2007; Comoti et al. 2007) as well as membrane technology (Sridhar et al. 2007; Hong et al. 2008).
Ideally, an adsorption medium for CO2 removal at ambient temperature should combine (i) high CO2 uptake, (ii) complete regeneration under mild condition, (iii) high thermal stability, and (iv) favourable adsorption-desorption kinetics.
The discovery of periodic mesoporous materials like MCM-41 silica has resulted in extensive research activity on their synthesis and applications, particularly for separation and catalysis (Sayari 1996; Sayari and Jaroniec 2008). It is intriguing that despite the significant growth in the area of periodic mesoporous materials (for a review see Sayari (2003) and references therein), there are only few studies devoted to CO2 adsorption on materials like MCM-41 silica (Branton et al. 1995; Morishige et al. 1997; Morishige and Nakamura 2004; Sonwane et al. 1998). The early studies by Morishige et al. (1997, 2004) and Sonwane et al. (1998) focused on high pressure CO2 adsorption at temperature below 273 K for the purpose of structural characterization. He and Seaton (2006) studied low pressure adsorption of pure CO2 and CO2—CH4 mixture for the characterization of MCM-41 surface heterogeneity. Although, the use of organically-modified silica materials for CO2 removal was extensively studied using different mesoporous silica supports such as MCM-41, SBA-15, MCM-48 and pore-expanded MCM-41 (for a review see Harlick and Sayari (2007) and reference therein); adsorption of CO2 was investigated in a limited range of CO2 concentration, temperature and pressure. The patent application WO 2008/081102 (Pirngruber et al. 2008) discloses the use of metal-organic frameworks (MOFs) having a pore diameter in the range of 0.5-5 nm and surface area the range of 2000-4000 m2/g, for hydrogen purification and carbon dioxide recovery at pressure higher than 4 bar.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide methods and systems for selective CO2, H2S, SO2 and other acid gases adsorption using mesoporous silica. In accordance with one aspect of the present invention, there is provided a process for the removing CO2 from a gas stream containing CO2, which process comprises conducting said gas stream through an adsorbent containing a mesoporous material under high pressure to adsorb said CO2 onto said adsorbent and produce a substantially CO2-free gas stream (Stage 1). Advantageously, the process additionally comprises the step of reducing the pressure on said adsorbent having CO2 adsorbed thereon to a moderate pressure to desorb at least a fraction of the adsorbed CO2 (Stage 2). When the two Stages 1 and 2 take place at the same temperature, the process is a pressure swing adsorption referred to as PSA-H/M where H in bar is the adsorption pressure (Stage 1) and M in bar is the desorption pressure (Stage 2).
In accordance with another aspect of the present invention, there is provided a method for selectively removing or recovering CO2, as well as H2S, SO2 and other acid gases from a gaseous stream or atmosphere containing CO2, H2S, SO2 and other acid gases, comprising the step of contacting the gaseous stream or atmosphere with an adsorbent comprising ordered or disordered mesoporous silica having a pore volume of between 0.4 and 4 cm3/g, a median pore diameter of between 2 and 50 nm and a BET surface area of between 500 and 2000 m2/g.
In accordance with another aspect of the invention, there is provided a system for selectively removing or recovering CO2, H2S, SO2 and other acid gases from an gaseous stream or atmosphere containing said CO2, H2S, SO2 and other acid gases using a system comprising: (a) a sorbent bed comprising a mesoporous silica; (b) means for contacting the gaseous stream or atmosphere with the sorbent bed; and (c) means of removing the CO2, H2S, SO2 and other acid gases from the sorbent bed.
In accordance with another aspect of the present invention there is provided a mesoporous silica adsorbent having a high gravimetric and volumetric CO2 adsorption capacity, high efficiency for selective CO2 adsorption, fast CO2 kinetics with a low energy requirement for regeneration.
In one example, the gravimetric and volumetric CO2 adsorption capacities for mesoporous MCM-41-100 silica was 64.7 wt % (14.7 mmol/g) and 234.2 cm3/cm3 at 45 bar and room temperature.
In another example, the CO2 selectivity vs. N2 in CO2:N2=20:80 mixture over MCM-41-100 was 15 at 45 bar and room temperature.
In another example, the CO2 selectivity vs. O2 in CO2:O2=95:5 mixture over MCM-41-100 was 22 at 45 bar and room temperature.
In another example, the CO2 selectivity vs. CH4 in CO2:CH4=50:50 mixture over MCM-41-100 was 7 at 45 bar and room temperature.
In another example, the CO2 selectivity vs. H2 in CO2:H2=20:80 mixture over MCM-41-100 was 63 at 45 bar and room temperature.
In accordance with another aspect of the present invention there is provided a PSA-H/M process using mesoporous silica for bulk CO2 separation process with the dual purpose of separation at high pressure (e.g., H=45 bar) and recovery of CO2 at moderate pressure (M=10 bar for example) from gas streams.
In one example, the CO2 PSA-45/10 operating capacity in CO2:N2=20:80 mixture over MCM-41-100 was 11.13 wt % (2.58 mmol/g).
In another example, the CO2 PSA-45/10 operating capacity in CO2:CH4=50:50 mixture over MCM-41-100 was 23.7 wt % (5.40 mmol/g).
In another example, the CO2 PSA-45/10 operating capacity in CO2:H2=20:80 mixture over MCM-41-100 was 13.3 wt % (3.1 mmol/g).
In accordance with another aspect of the present invention there is provided a mesoporous silica adsorbent having a high capacity of CO2 at high pressure with and without the presence of water vapour.
In another example, the gravimetric CO2 adsorption capacity for mesoporous PE-MCM-41 silica in dry and humid (40% relative humidity, RH) conditions was 100 wt % (22.8 mmol/g) and 102 wt % (23.2) at 60 bar and room temperature.
In accordance with another aspect of the present invention there is provided a hydrated mesoporous silica adsorbent having an enhanced selectivity toward CO2 vs. supercritcal gases such as N2, CH4, O2 and H2.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 schematically depicts the synthesis of MCM-41 mesoporous silica and post-synthesis pore expansion to PE-MCM-41.
FIG. 2 shows N2 adsorption isotherms for materials; the inset figure represents the pore size distributions.
FIG. 3 graphically depicts fractional CO2 uptake (nt/ne) at 1 bar and 298 K for MCM-41-100, PE-MCM-41.
FIG. 4 graphically depicts gravimetric CO2 excess adsorption uptake of MCM-41-100 in comparison with other adsorbents.
FIG. 5 shows volumetric CO2 excess adsorption uptake for MCM-41-100 in comparison with other adsorbents.
FIG. 6 shows volumetric CO2 excess adsorption uptake per unit surface area for MCM-41-100 in comparison with other adsorbents.
FIG. 7 depicts CO2 excess adsorption isotherms for MCM-41-100 and MaxsorbAC at 298 K showing PSA-45/10 working CO2 capacity when adsorption and desorption stages take place at 45 bar and 10 bar, respectively.
FIG. 8 depicts the adsorption isotherms of CO2, N2, CH4, H2 and O2 on MCM-41-100 at 298 K.
FIG. 9 shows the molar selectivity ratio of CO2 to CH4 adsorbed on MCM-41-100, 13X zeolite, MaxsorbAC and NoritAC at 298 K vs. pressure.
FIG. 10 shows IAST prediction compared to experimental data for adsorption of CO2:N2=20:80 mixture on MCM-41-100 at 298 K.
FIG. 11 shows IAST CO2 selectivity over N2 for CO2:N2=20:80 mixture over MCM-41-100 compared to NoritAC and 13X at 298 K vs. pressure.
FIG. 12 shows IAST CO2 selectivity over CH4 vs. pressure for CO2:CH4=50:50 mixture on MCM-41-100 compared to NoritAC, MaxsorbAC and 13X at 298 K.
FIG. 13 shows IAST CO2 selectivity over H2 for CO2:H2=20:80 mixture on MCM-41-100 compared to IAST CO2 selectivity over H2 for CO2:H2=1.4:98.6 mixture for NaA zeolite at 298 K vs. pressure
FIG. 14 shows IAST CO2 selectivity over O2 for CO2:O2=95:5 mixture for MCM-41-100 at 298 K vs. pressure.
FIG. 15 schematically depicts the general procedure for CO2 capture.
FIG. 16 schematically depicts the proposed procedure for CO2 capture using PSA-H/M with H=45 bar and M=10 bar.
FIG. 17 depicts gravimetric CO2 excess adsorption uptake of PE-MCM-41-100 in dry and hydrated conditions.



DETAILED DESCRIPTION OF THE INVENTION
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
The present invention provides methods and systems for CO2 adsorption that take advantage of the selective CO2 adsorption capabilities of mesoporous silica, particularly when adsorption is performed under high pressure. In one preferred embodiment, the system and process or method of the invention includes the use of mesoporous silica as a sorbent.
Mesoporous Silica
Mesoporous silicas exhibit ordered or disordered pore systems. These mesoporous silicas include those prepared in the presence of surfactants or polymer solutions via different pathways including the so-called cooperative organization mechanism and the liquid crystal templating mechanism (For review see Sayari 2003). Typically, the surfactants or polymers are removed by calcination of mesoporous silica precursor at high temperature. Other procedures for surfactant or polymer removal such as solvent extraction or microwave treatment may also be applied. Mesoporous silicas may exhibit different structures and pore systems, the most prominent being the so-called MCM-41 with a two-dimensional hexagonal symmetry. Table 1 provides a non-exhaustive list of mesoporous silicas, prepared under different pH conditions using different amphiphile molecules, that can be used in the present invention. The pore size of such material may be adjusted from a low of 1 nm to well into the macropore regime, i.e. >50 nm.







TABLE 1







Mesoporous Silicas and Organosilicas











Mesophase
Amphiphile template
pH
Structure
Ref.





MCM-41
CnH2n+1(CH3)3N+
basic
2D hexagonal (p6mm)
 [1]


MCM-48
CnH2n+1(CH3)3N+
basic
cubic (Ia 3d)
 [1]



Gemini Cn−s−na


 [2]


FSM-16
C16H31(CH3)3N+
basic
2D hexagonal (p6mm)
 [3]


SBA-1
C18H37N(C2H5)3+
acidic
cubic (Pm 3n)
 [2]


SBA-2
Divalent Cn−s−1b
acidic/
3D hexagonal (P63/mmc)
 [2]




basic




SBA-3
CnH2n+1N(CH3)3+
acidic
2D hexagonal (p6mm)
 [4]


SBA-6
Divalent 18B4−3−1c
basic
cubic (Pm 3n)
 [5]


SBA-8
Bolaformd
basic
2D rectangular (cmm)
 [6]


SBA-11
Brij ® 56; C16EO10
acidic
cubic (Pm 3n)
 [7]


SBA-12
Brij ® 76; C18EO10
acidic
3D hexagonal (P63/mmc)
 [7]


SBA-14
Brij ® 30; C12EO4
acidic
cubic
 [7]


SBA-15
P123; EO20PO70EO20
acidic
2D hexagonal (p6mm)
 [8]


KIT-6
P123 + Butanol
acidic
cubic (Ia 3d)
 [9]


JLU-11
CF3(CF2)5(EO)14
acidic
disordered
[27]


JLU-12
CF3(CF2)5(EO)14
neutral
disordered
[27]


JLU-14
CF3(CF2)4(EO)10
acidic
2D hexagonal (p6mm)
[30]


JLU-15
CF3(CF2)4(EO)10
neutral
2D hexagonal (p6mm)
[30]


JLU-20
P123 + FC-4e
acidic
2D hexagonal (p6mm)
[10]


JLU-21
FC-4 and F127
acidic
cubic Im3m
[28]


JLU-30
(>160°) DIHABj
basic
2D hexagonal (p6mm)
[29]


PSU-1
P123 + CTACl
acidic
2D hexagonal (p6mm)
[11]


Mesocellular
P123 + TMBf
acidic
disordered
[12]


SBA-16
F127; EO106PO70EO106
acidic
cubic (Im 3m)
 [7]


KIT-5
F127
acidic
cubic (Fm 3m)
[13]


FDU-12
F127 + additivesg
acidic
cubic (Fm 3m)
[14]


FDU-1
B50-6600; EO39BO47EO39
acidic
cubic (Im 3m)
[15]


FDU-2
RN+N+N+h
basic
cubic (Fd3m)
[16]


FDU-5
P123 + additivesi
acidic
cubic (Ia 3d)
[17]


FDU-18
PEO-b-PS
acidic
cubic (Fm 3m)
[26]


FDU-12
F127 + TMB
acidic
cubic (Fm 3m)
[25]





AMS-1: 3D hexagonal
[18, 19]





AMS-2: 2D cubic






AMS-3: 2D hexagonal



AMS-n
Anionic surfactant
basic
AMS-4: 3D cubic






AMS-6: 3D cubic






AMS-7: 3D disordered






AMS-8: 3D cubic






AMS-10: cubic Pn3m
[31]


MSU-1
Tergitol; C11-15(EO)12
neutral
disordered
[20]


MSU-2
TX-114; C8Ph(EO)8
neutral
disordered
[20]



TX-100; C8Ph(EO)10





MSU-3
P64L; EO13PO30EO13
neutral
disordered
[20]


MSU-4
Tween ®-20, 40, 60, 80
neutral
disordered
[21]


MSU-V
H2N(CH2)nNH2
neutral
lamellar
[22]


MSU-G
CnH2n+1NH(CH2)2NH2
neutral
lamellar
[23]


HMS
CnH2n+1NH2
neutral
disordered
[24]





EO = ethylene oxide; PO = propylene oxide.


aGemini surfactants Cn−s−n: CnH2n+1N+(CH3)2(CH2)sN+(CH3)2CnH2n+1.


bDivalent surfactants Cn−s−1: CnH2n+1N+(CH3)2(CH2)sN+(CH3)3.


cDivalent surfactant 18B4−3−1: C18H37O—C6H4—O(CH2)4N+(CH3)2(CH2)3N+(CH3)3.


dBolaform surfactants: (CH3)3N+(CH2)nO—C6H4—C6H4—O(CH2)nN+(CH3)3.


eFC-4: (C3F7O(CFCF3CF2O)2CFCF3CONH(CH2)3N+(C2H5)2CH3I−.


fTMB: trimethylbenzene.


gAdditives = TMB and KCl.


hTri-head group surfactant: C16H33N+(CH3)2(CH2)2N+(CH3)2(CH2)3N+(CH3)3.


iAdditives = 3-mercaptopropyl-trimethoxysilane (MPTS) and benzene, or a benzene derivative (methyl-, ethyl-, dimethyl-, or trimethylbenzene).


j(1,3-dimethyl-2-imidazolidin-2-ylidene)hexadecylmethylammonium bromide


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Following the initial preparation steps, the mesoporous silica can be calcined or solvent extracted to remove surfactant and, if necessary, characterised using X-ray diffraction, N2 adsorption, scanning electron microscopy, and/or transmission electron microscopy.
The mesoporous silicas of the present invention include, but are not limited to, all mesoporous silicas described in Table 1. They are prepared in the presence of a structure directing agent which consists of a surfactant, oligomer, or polymer. The mesoporous material is then treated to remove the structure directing agent, either by heat treatment or by extraction.
Mesoporous silicas that are suitable for use in the present invention exhibit preferably high surface area, large pore volume and high degree of pore ordering. Such material shows a suitable combination of adsorption uptake, adsorption kinetics, separation efficiency and ease of regeneration using pressure swing adsorption (PSA).
Mesoporous silicas that are suitable for use in the present invention exhibit high surface areas and provide sufficiently large pores to enable relatively unhindered flow of CO2, or other acid gases, containing gaseous streams inside the pore system. The resulted modified mesoporous silicas exhibit a high adsorption uptake, fast adsorption kinetics, high separation efficiency and ease of regeneration using temperature swing (TSA), pressure swing (PSA) adsorption or a combination of both temperature and pressure swing adsorption.
Adsorption Methods and Systems
The present invention further provides methods and systems for removing CO2 and/or other acid gases, such as H25 and SO2, using mesoporous silicas. For simplicity, the following discussion specifically refers to CO2 as the acid gas.
Mesoporous silicas can be used successfully as an adsorbent for CO2 under high pressure with desorption under moderate pressure. The terms “high pressure” and “moderate pressure”, as used herein, refers to the operational pressure of greater than 10 bar and 2 bar for both adsorption and desorption stages, respectively, but preferably higher than 20 bar and 5 bar, respectively. It is noteworthy that conventional pressure swing adsorption (PSA) processes operate between a high loading pressure and 1 bar or vacuum for the desorption stage. Mesoporous silica adsorbents can be used for CO2 bulk separation from different pre-dried gaseous streams. The proposed PSA-H/M using mesoporous silica is particularly suitable for simultaneous separation and recovery of CO2 at high (e.g, H=45 bar) and medium (e.g., M=10 bar) pressures, respectively.
In accordance with another aspect of the present invention, there is provided a system for CO2 adsorption. The system comprises a sorbent bed that includes a mesoporous silica and a means for contacting a gaseous stream containing CO2 with the sorbent bed for a sufficient amount of time to permit adsorption of the CO2 by the mesoporous silica.
Once the mesoporous silica adsorbent has been synthesized, it can be employed in a sorbent bed for use in an adsorption process, such as a cyclic adsorption-regeneration process. To apply the adsorbent of the present invention to such an adsorption process, it must be formed into a stable, mechanically strong form. These forms may include, but are not limited to, powder forms, pellet forms and monolithic structures or foams. In the case of pellet forms, the adsorbent is mixed with a suitable inert or active secondary material as a binder. Criteria for selecting a suitable binder can include (i) achieving pellets or extrudates with minimum amount of binder; (ii) enhanced mechanical stability; (iii) preservation of adsorbent porosity and accessibility of adsorption sites; and (iv) affordability. For example, siloxanes and siloxane derivatives can be employed with the appropriate weight percentage as binders for mesoporous silica to form structured pellets, extrudates or spheres. The selection of the appropriate form and, if necessary, additive, is based on the application of the adsorbent and the type of equipment used in the acid gas removal process. The selection and manufacture of the adsorbent form is well within the ordinary abilities of a worker skilled in the art.
Once the adsorbent form is selected and manufactured, it is used in a sorbent bed where a gaseous stream containing CO2, and possibly water vapour, contacts the adsorbent. In the presence of mesoporous silica, the CO2 interacts with the silica surface and is physically adsorbed.
According to a specific embodiment of the present invention, once the mesoporous silica is loaded with CO2 to a satisfactory level, or at a designated cycle time, the sorbent bed can be regenerated. Regeneration comprises ceasing the flow of the acid gas containing stream through the bed and desorbing the adsorbed acid gas. The desorption is accomplished by pressure gradient means or by the use of a sweeping or purge gas, or any combination thereof. During this step, the adsorbed CO2 is released and flushed or washed out of the sorbent bed. The adsorbent is then ready for re-use. In a specific example, in which the mesoporous silica is MCM-41-100 with pore diameter of 3.3 nm, CO2 is removed at medium pressures, typically 2 to 5 bar or vacuum and the regenerated material is ready for re-use.
The CO2 removed from the sorbent via a desorption process can be collected at low or medium pressure purge. The CO2 thus recovered can be reused in a variety of applications or can be compressed for sequestration. As such, the present invention further provides a method of manufacturing CO2, which method comprises the steps of adsorbing CO2 on mesoporous silica and collecting the adsorbed CO2 following desorption from mesoporous silica.
In one embodiment of the present invention, the use of the adsorbent to remove CO2, another acid gas, or a combination thereof, can comprise utilising two or more sorbent beds operating cyclically such that the first bed is in the adsorption cycle while the second bed is in the desorption cycle. This system comprises two or more sorbent beds and computer or manually controlled valves and pumps allowing for continuous CO2 and other acid gases removal from the gaseous stream.
In one embodiment of the present invention, mesoporous silicas can be used for the removal and recovery of CO2, or other acid gases from streams containing in addition to CO2, or other acid gases, other gases including, but not limited to, H2, N2, O2, CO, CH4 and other hydrocarbons using PSA-H/M. Gaseous streams include, but are not limited to, natural gas, biogas, syngas, stack gas and air.
In one embodiment of the present invention, if necessary, different amounts of humidity may be added during adsorption and/or desorption operation in PSA-H/M in fixed, moving or fluidized beds, to optimize the capture of CO2.
In one embodiment of the present invention, mesoporous silicas can be used for the removal and recovery of CO2, or other acid gases from streams containing in addition to CO2, other gases including, but not limited to, H2, N2, O2, CO, CH4 and other hydrocarbons using wet (i.e., added moisture) adsorption processes, i.e., WPSA-H/M. Gaseous streams include, but are not limited to, natural gas, biogas, syngas, stack gas and air.
To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in any way.
EXAMPLES
Example 1
Preparation of MCM-41-X Mesoporous Silica
FIG. 1 shows the procedure for the synthesis of periodic mesoporous MCM-41 silica. MCM-41-X silica where X is the synthesis temperature in degree celsius was prepared in the presence of cetyltrimethylammonium bromide (CTAB) using the overall mixture composition: 1.0 SiO2:0.29 TMAOH:0.21 CTAB:60 H2O. In a typical synthesis, 1.76 g of tetramethylammonium hydroxide (TMAOH) (25%) was diluted with 72 g of water before adding 5.1 g of CTAB under vigorous stirring. After 15 min, 4 g of Cab-O-Sil silica was added. The gel obtained after stirring for an additional 30 min was transferred into a Teflon-lined autoclave, and heated statically under autogenous pressure for 40 h at a temperature within the range of 298 to 403 K. The obtained materials were filtered washed extensively, dried, and calcined at 813 K. The structural properties of MCM-41-100 as determined by nitrogen adsorption were: 1490 m2/g, 0.99 cm3/g, 3.3 nm for the surface area, pore volume and pore diameter, respectively (FIG. 2).
Example 2
Preparation of Pore-Expanded MCM-41 (PE-MCM-41) Silica
FIG. 1 shows also the procedure for the post-synthesis pore expansion of MCM-41. The expander agent used for the preparation of PE-MCM-41 was dimethyldecylamine (DMDA). More details about the procedure may be found elsewhere (Serna-Guerrero and Sayari 2007; Harlick and Sayari 2007). Under appropriate conditions, i.e., DMDA/MCM-41 ratio, temperature and time of the post-synthesis hydrothermal stage, the pore size of MCM-41 can be expanded from ca. 3 nm up to ca. 25 nm. As shown earlier (Harlick and Sayari 2007), pore size tuning is critical for improved CO2 adsorptive properties at high pressure. The structural properties for a PE-MCM-41 sample as determined by nitrogen adsorption were: 1230 m2/g, 3.09 cm3/g, 11.7 nm for the surface area, pore volume and pore diameter, respectively (FIG. 2).
Example 3
Method for Measurement of Adsorption Properties and Kinetics
Adsorption equilibrium and kinetics measurements of pure CO2 were performed using a Rubotherm gravimetric-densimetric apparatus (Rubotherm, Bochum Germany), composed mainly of a magnetic suspension balance (MSB) and a network of valves, mass flowmeters and temperature and pressure sensors. It operates both in closed and open loops. In a typical adsorption experiment, the adsorbent was weighed and placed in a basket suspended by a permanent magnet through an electromagnet. The cell in which the basket is housed was then closed, and vacuum or high pressure was applied. This system is able to perform adsorption measurements in a wide range of gas pressure from 0 to 60 bar. The adsorption temperature may also be controlled within the range of 298 to 423 K. The clean (outgassed) adsorbent is exposed to flowing pure CO2 at constant temperature at a rate of 100 ml/min. In a typical experiment for kinetic measurements, the gas was introduced in such a way to reach the desired pressure in 5-10 s. The change in the weight of the adsorbent sample as well as the pressure and temperature were measured continuously until the thermodynamic equilibrium was reached. The change in the weight of the adsorbent sample as well as the pressure and temperature were monitored continuously until the thermodynamic equilibrium was reached. The gravimetric method allows the direct measurement of the reduced mass Ω. Correction for the buoyancy effect is required to determine the excess adsorbed amount mexcess (Belmabkhout et al. 2004; Dreisbach et al. 2003) using equation 1, where Vadsorbent and Vss refer to the volume of the adsorbent and the volume of the suspension system, respectively. These volumes were determined using the helium isotherm method by assuming that helium penetrates in all the open pores of the materials without being adsorbed (Sircar 2002; Belmabkhout et al. 2004). The density of the gas ρgas was determined experimentally using a volume-calibrated titanium cylinder. By weighing this calibrated volume in the gas atmosphere, the local density of the gas was also determined. Simultaneous measurement of gas uptake and gas phase density as a function of pressure and temperature was thus possible.

Ω=mexcess−ρgas(Vadsorbent+Vss)  (1)

Example 4
Kinetics of CO2 Adsorption
FIG. 3 shows the kinetic curve for adsorption at 298 K and 1 bar over MCM-41-100, PE-MCM-41 materials determined using pure CO2 flowing at 200 mL/min.
The CO2 adsorption kinetic curves were fitted to Linear Driving Model (LDF) (Murcia et al. 2003), to estimate the kinetic rate constant of CO2 adsorption. The LDF model is described by the equation 2:




  
    
      
        
          
            n
            t
          
          
            n
            e
          
        
        =
        
          1
          -
          
            e
            
              
                -
                k
              
              ⁢
              
                
              
              ⁢
              t
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




where ne is the equilibrium uptake at 298 K and 1 bar, nt is the uptake at time t and k is the kinetic rate constant. The results of the fit are shown in FIG. 7 and Table 2. The CO2 kinetic rate constant was significantly higher upon pore expansion, most likely due to the larger pores and higher pore volume of PE-MCM-41 in comparison to MCM-41-100. The PE-MCM-41 has higher kinetic rate constant than MCM-41-100, up to 0.5 fractional uptake nt/ne. The sequence in terms of LDF kinetic rate constant was PE-MCM-41>MCM-41-100.








TABLE 2







LDF kinetic rate constant of CO2 adsorption










Material
k (LDF kinetic rate constant)/s−1






MCM-41-100
4*10−2



PE-MCM-41
6*10−2









Example 5
Comparison of MCM-41 Silica with Other Adsorbents
Extensive investigations have been carried out on CO2 adsorption using well known benchmark industrial adsorbents such as zeolites and carbon-based materials or the rapidly evolving hybrid materials, MOFs. Among these materials, the most promising CO2 adsorbents were selected and compared with the current MCM-41-100 silica for CO2 adsorption up to 45 bar pressure at ambient temperature. Pertinent properties of the selected materials are shown in Table 3. FIGS. 4, 5 and 6 show the CO2 gravimetric, volumetric and volumetric per surface area excess uptakes of CO2 on the above-mentioned materials in comparison to MCM-41-100 at ambient temperature. The comparison on a volume basis was made by multiplying the density of the corresponding material shown in Table 3 by the gravimetric CO2 capacity in cm3 STP/g. The particle density (ca. 0.71 g/cm3) of MCM-41-100 was calculated from the experimentally determined skeletal density (2.34 g/cm3) and the pore volume (ca. 0.99 cm3/g).







TABLE 3







Surface area and density of the selected materials











SBET
Density



Materials
(m2/g)
(g/cm3)
Reference













13X
685
1.13(a)
Belmabkhout et al. 2007;





Cavenati et al. 2004


MaxsorbAC
3250
0.29(b)
Himeno et al. 2005


NoritAC
1450
0.43(b)
Himeno et al. 2005


MOF-177
4508
0.43(c)
Millward and Yaghi





2005; Yang et al. 2008


IRMOF-1
2833
0.59(c)
Millward and Yaghi





2005; Yang et al. 2008


MCM-41-100
1490
0.71(a)
This work





(a)particle density,


(b)packed density,


(c)crystallographic density






In terms of CO2 gravimetric capacity, as shown in FIG. 4, MCM-41-100 exhibited the lowest capacity at low pressure but exceeded 13X zeolite and NoritAC carbon at a pressure of ca. 20 bar and 30 bar, respectively. At 45 bar, the CO2 adsorption capacity for MCM-41-100 was 14.7 mmol/g vs. ca. 7.37 mmol/g and 11.28 mmol/g for 13X and NoritAC, respectively. The sequence of the gravimetric uptake at 45 bar was as follows: MOF-107>MaxsorbAC>IRMOF-1>MCM-41-100>NoritAC>13X.
In terms of CO2 volumetric capacity, as shown in FIG. 5, MCM-41-100 outperformed 13X zeolite as well as NoritAC and MaxsorbAC carbons at high pressure, but exhibited lower volumetric capacity than MOF-177 and IRMOF-1. The sequence of the volumetric uptake at 45 bar was as follows MOF-107>IRMOF-1>MCM-41-100>MaxsorbAC>NoritAC>13X. Nevertheless, mesoporous silicas materials have the advantage of being very stable during prolonged exposure to ambient air and moisture (Cassiers et al. 2002). This is in contrast to MOF-177 and IRMOF-1 as reported recently (Li and Yang 2007; Bahr et al. 2007).
Comparison in terms of volumetric uptake on a surface area basis is provided in FIG. 6. MCM-41-100 exhibited comparable capacity at high pressure (ca. 45 bar) as 13X and exceeded slightly all the other aforementioned materials, indicative of the high surface efficiency of MCM-41-100 for CO2 adsorption. Moreover, as shown in Table 4, MCM-41-100 exhibited one of the weakest adsorbent-CO2 interactions, reflected by lower isosteric heat of adsorption, allowing CO2 to desorb at very mild conditions, in contrast to 13X.







TABLE 4







Isosteric heat of CO2 adsorption at low loading


for MCM-41-100 and the benchmark adsorbents











Material
Qisos (kJmol−1)
References














13X
37.2
Cavenati et al. 2004



NoritAC
22
Himeno et al. 2005



MCM-41-100
21.6
This work



MaxsorbAC
16.2
Himeno et al. 2005









The low gravimetric CO2 adsorption capacity of MCM-41-100 at low to moderate pressures (1-10 bar) may seem to be unattractive for CO2 separation in comparison to the benchmark commercial materials. It is however important to notice that the current MCM-41-100 exhibited ca. 43.6 wt % pure CO2 operating PSA capacity (designated as ΔCO2) as shown in FIG. 7 based on 45 and 10 bar as pressures for the adsorption and desorption stages, respectively. This CO2 uptake is lower than for MaxsorbAC (ca. 58.6 wt %) but significantly higher than for NoritAC (ca. 13.2 wt %) and 13X (3.7 wt %). Thus, MCM-41-100 can be used for example in PSA separation processes with the dual purpose of separation and recovery of CO2 at moderate pressure (10 bar for example) from gas streams with medium to high CO2 concentrations as shown in FIG. 8. This PSA configuration has the advantage to reduce the recompression cost of CO2 prior the storage step. This process was designated as PSA-H/M where H and M stand for the high pressure adsorption and medium pressure desorption. It is noteworthy that conventional PSA processes operate between a high loading pressure and vacuum or 1 bar for the desorption stage
Example 6
Adsorption of CO2, N2, CH4, O2 and H2 on MCM-41-100
Adsorption isotherms of CO2, N2, CH4, H2 and O2 onto MCM-41-100 at 298 K and up to 25 bar are shown in FIG. 8. The shape of the isotherms is reminiscent of Type I according to the IUPAC classification, with a much higher CO2 adsorption capacity than other adsorbates over the whole pressure range. It is inferred that MCM-41-100 exhibits strong preferential adsorption of CO2 compared to the other species. From the pure CO2 and CH4 data shown in FIG. 8, the molar selectivity ratio of the adsorbed CO2 to CH4 (CO2/CH4) was calculated as a function of pressure and plotted in FIG. 9. The corresponding molar selectivity ratios for 13X zeolite (Siriwardane et al. 2001, Cavaneti et al. 2004), MaxsorbAC and NoritAC (Siriwardane et al. 2001, Himeno et al. 2005) from literature data were also plotted in FIG. 9 for comparison.
At low pressure, the molar selectivity ratio CO2/CH4 for MCM-41-100 was lower than 13X but higher than both activated carbons. At pressures above ca. 3 bar, the molar selectivity ratio was higher for MCM-41-100 in comparison to all the other adsorbents, indicative of the higher efficiency of MCM-41-100 for separation of CO2 from CO2—CH4 mixtures at moderate to high pressure. The sequence in terms of CO2/CH4 molar selectivity ratio at high pressure was MCM-41-100>NoritAC≈Maxsorb AC>13X. Similar trends were observed by comparing the molar selectivity ratio CO2/N2 on MCM-41-100 to the corresponding molar selectivity ratios for 13X (Siriwardane et al. 2001, Cavaneti et al. 2004), and NoritAC (Dreisbach et al. 2005), and by comparing the molar selectivity ratio CO2/H2 on MCM-41-100 to that for NaA (4A) zeolite (Akten et al, 2003).
Example 7
Comparison Between IAST CO2—N2 Binary Mixture Results and Experimental Data on MCM-41-100
FIG. 10 presents the pure gas adsorption isotherms for CO2 and N2 on MCM-41-100, successfully fitted to Toth model equation, along with the results of IAST prediction for CO2:N2=20:80 mixture. The total amount adsorbed of CO2—N2 mixture is in excellent agreement with the experimental data over a wide range of pressure, indicative of the suitability of IAST, combined with Toth model, for the prediction of binary adsorption equilibria on MCM-41-100 as already recognized by other workers (He and Seaton 2006; Yun et al. 2002). Therefore, the selectivity of CO2 over N2, CH4, H2 and O2, as function of pressure, has been mapped systematically using IAST. The CH4, O2 and H2 adsorption isotherms were also fitted to Toth model. The overall results of the fit for the pure gas adsorption of CO2, N2, CH4, H2 and O2 are presented in Table 5.







TABLE 5







Parameters of Toth equation for adsorption


of pure gases on MCM-41-100 at 298 K









Toth model parameters












Pure gas
qs (mmol/g)
b (1/bar)
m















CO2
145.9
5.8 * 10−3
0.44



N2
4.2
1.7 * 10−2
1.23



CH4
10.4
1.4 * 10−2
0.85



H2
434.2
  1 * 10−4
0.22



O2
14.5
5.2 * 10−3
0.64









Example 8
CO2 Adsorption Capacity and Selectivity on MCM-41-100 for CO2:N2=20:80 Mixture
The most important binary system involved in flue gas separation is CO2—N2 mixture with a typical molar composition of 10-20% of CO2 and ca. 80% N2. FIG. 11 shows the selectivity of MCM-41-100 for CO2 vs. N2 for 20 mol % CO2 in N2 as a function of pressure. The corresponding data for NoritAC (Dreisbach et al. 2005) carbon and 13X (Cavenati et al, 2004) zeolite were also included for comparison.
The selectivity of MCM-41-100 for CO2 over N2 in the presence of CO2:N2=20:80 mixture was found to be around 11 in the range of 1 to 10 bar range with a tendency to increase up to ca. 15 as the pressure increased to 45 bar. The sequence in terms of CO2 selectivity versus N2 at high pressure was as follows: NoritAC>MCM-41-100>>13X. At very low pressure, 13X zeolite exhibited higher CO2 vs. N2 selectivity than all the other materials; however, the selectivity decreased steeply at increased pressure (Cavenati et al. 2004). Separation of CO2 from CO2—N2 mixtures using other nanoporous materials has also been widely investigated both experimentally and theoretically. For example, at ambient temperature and moderate pressure, CO2 vs. N2 selectivity was found to be 12-18 for carbonaceous materials with slit-shaped pores (Cracknell and Nicholson 1996), 100 for ITQ-3 (Goj et al. 2002), 14 for MFI-type zeolites (Bernal et al. 2004), 4 for MOF-508b (Bastin et al. 1996) and 20 for Cu-BTC MOFs (Yang et al. 2007).
Table 6 shows the PSA-45/10 CO2 removal capacity for MCM-41-100 and NoritAC in the presence of CO2:N2=20:80 mixture calculated using IAST. Although NoritAC exhibited somewhat higher CO2 selectivity, MCM-41-100 still has a slightly higher PSA-H/M CO2 adsorption capacity in the presence of CO2:N2=20:80 mixture. Thus, MCM-41-100 has suitable properties for CO2 separation from flue gas at high pressure.







TABLE 6







PSA-H/M removal capacity of CO2 in CO2:


N2 = 20:80 mixture for MCM-41-100 and


NoritAC (adsorption at 45 bar, desorption at 10 bar)










Adsorbent
PSA-45/10 CO2 capacity in mmol/g and (wt %)






MCM-41-100
2.58 (11.13 wt %)



NoritAC
2.37 (10.4 wt %) 









Example 9
CO2 Adsorption Capacity and Selectivity on MCM-41-100 for CO2:CH4=50:50 Mixture
The most important binary system involved in biogas separation, purification processes is CO2—CH4 mixture with a molar composition of 25 to 50% and 50 to 75% for CO2 and CH4, respectively. FIG. 12 shows the selectivity of MCM-41-100 for CO2 versus CH4 in the presence of CO2:CH4=50:50. The corresponding literature data for benchmark materials like NoritAC, MaxsorbAC carbons and 13X zeolite were also included for comparison.
The MCM-41-100 CO2 vs. CH4 selectivity for CO2:CH4=50:50 mixture was found to be around 5 at low pressure, and showed an upward tendency up to ca. 7 as the pressure increased to 45 bar. The experimental data for NoritAC (Dreisbach et al. 2005; Himeno et al. 2005) were in good agreement with the IAST prediction based on pure CO2 and CH4 data (Himeno et al. 2005). MCM-41-100 had the highest CO2 vs. CH4 selectivity at moderate to high pressure for CO2:CH4=50:50 ca. >5 bar. Zeolite 13X exhibited higher CO2 selectivity than all the other materials in the low pressure range (ca. <5 bar), but the selectivity decreased drastically by increasing the pressure (Cavenati et al. 2004). The sequence in terms of CO2 vs. CH4 selectivity for CO2:CH4=50:50 at high pressure was MCM-41-100>NoritAC≈Maxsorb AC>13X, similar to that observed in FIG. 10 based on the molar CO2/CH4 selectivity ratios. The separation of CO2 from CO2—CH4 mixtures has also been investigated experimentally and theoretically for other nanoporous materials including MOFs and carbon nanotubes. For example, under similar conditions of pressure, temperature and composition, the CO2—CH4 selectivity was reported to be 3 for IRMOF-1 (Yang and Zhong 2006; Babarao et al. 2007) and MOF-508b (Bastin et al. 1996), 10 for Cu-BTC (Yang and Zhong 2006) and 11 for carbon nanotubes (Huang et al. 2007). Llewellyn et al. (2006) reported molar CO2/CH4 selectivity ratio of 1.8 and 38.5 at 20 bar and 304 K on dehydrated and hydrated MIL-53(Cr), respectively. Llewellyn et al. also (2008) reported molar CO2/CH4 selectivity ratio of ca. 3 at 50 bar and 303 K on Mil-101c.
Table 7 shows the CO2 PSA-45/10 capacity for CO2:CH4=50:50 mixture over MCM-41-100 and other benchmark adsorbents calculated using IAST. The sequence of CO2 PSA-H/M removal capacity using CO2:CH4=50:50 mixture was in good agreement with the pure CO2 capacity sequence mentioned previously.







TABLE 7







PSA-H/M removal capacity of CO2 from CO2:


CH4 = 50:50 mixture for MCM-41-100, NoritAC


and MaxsorbAC (adsorption at 45 bar, desorption at 10 bar)










Adsorbent
PSA-45/10 CO2 capacity in mmol/g and (wt %)






MCM-41-100
5.40 (23.7 wt %)



NoritAC
3.44 (15.2 wt %)



MaxsorbAC
9.44 (41.5 wt %)









Example 10
CO2 Adsorption Capacity and Selectivity on MCM-41-100 for CO2:H2=20:80 Mixture
The most important binary system involved in pre-dried synthesis gas for hydrogen production is CO2—H2 mixture. The typical molar composition of dry synthesis gas after the water gas shift process in typically 20 to 30% CO2 and 70 to 80% H2. FIG. 13 shows the CO2 vs. H2 selectivity for CO2:H2=20:80 mixture as a function of pressure for MCM-41-100 compared to the corresponding literature data, available for NaA zeolite (Akten et al. 2003).
NaA zeolite exhibited higher selectivity than MCM-41-100 at pressure up to ca. 18 bar. However at higher pressure, MCM-41-100 outperformed NaA reaching a CO2 vs. H2 selectivity of 63 for CO2:H2=20:80 at 45 bar. The PSA-45/10 CO2 removal capacity in the presence of CO2:H2=20:80 for MCM-41-100, calculated using IAST was 3.1 mmol/g (13.3 wt %). Notice that neglecting the buoyancy effect on the adsorbed phase in pure H2 adsorption data may lead to a slight overestimation of the selectivity using IAST. Separation of equimolar mixture of CO2 and H2 has also been performed on other nanoporous materials like carbon and MOFs. At 50 bar and room temperature, the CO2 vs. H2 selectivity was reported to be 35 for activated carbon (Cao and Wu 2005), 25 for MOFs-5 (IRMOF-1) (Yang and Zhong 2006) and 60 for Cu-BTC (Yang and Zhong 2006). Thus, MCM-41-100 is also a promising material for carbon dioxide removal from synthesis gas at high pressure.
Example 11
CO2 Adsorption Capacity and Selectivity on MCM-41-100 for CO2:O2=95:5 Mixture
Although the CO2—N2 mixture is the most dominant in flue gas, investigation of CO2—O2 mixtures is also important. The molar composition of O2 in flue gas is typically 2 to 5%. Ideally the selectivity of CO2 in CO2—O2 mixtures should be as high as for CO2—N2 mixtures. FIG. 14 representing the CO2 vs. O2 selectivity for CO2:O2=95:5 as a function of pressure for MCM-41-100 shows a linear tendency with pressure. A CO2 vs. O2 selectivity of 22 was obtained at 45 bar. The PSA-45/10 CO2 removal capacity for MCM-41-100 in the presence of a CO2:O2=95:5 mixture for MCM-41-100, calculated using IAST, was 8.9 mmol/g (39.3 wt %). Adsorption of CO2—O2 mixtures was rarely studied in the literature. At 50 bar and room temperature, the CO2 selectivity in CO2:O2=77.8:22.2 mixture in the presence of Cu-BTC was reported to be 35 (Yang et al. 2007).
Example 12
CO2 Capture Using PSA-H/M with Mesoporous Silica
A simplified general scheme for CO2 capture, from different gas streams, is presented in FIG. 15. It is composed of a CO2 removal stage using suitable technology (e.g., absorption, membrane, adsorption using PSA, etc), and a CO2 compression step before the final CO2 storage. In this scheme, the capture step operates generally at atmospheric to moderate pressure and the CO2 is recovered at low pressure when PSA is used.
FIG. 16 illustrates the proposed CO2 capture scheme incorporating PSA-H/M using mesoporous silica as adsorbent. The proposed scheme involves two compression stages. Initially the gas feed is compressed (e.g., 45 bar), the CO2 is removed at high pressure and recovered at moderate pressure (e.g., 10 bar) before the final compression (if necessary) and storage steps.
Example 13
CO2 Adsorption on Dry and Hydrated PE-MCM-41
FIG. 17 shows the CO2 adsorption isotherms of dry and hydrated (40% RH) PE-MCM-41 at room temperature and high pressure. The CO2 adsorption uptake was 100 wt % (22.8 mmol/g) and 102 wt % (23.2) at 60 bar and room temperature. The pure CO2 PSA-60/10 operating capacity for the dry and hydrated material was ca. 80 wt % and 81 wt %, respectively.
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All publications, patents and patent applications mentioned in this Specification are indicative of the level of skill of those skilled in the art to which this invention pertains and are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

## Claims

1. A process for the removing CO2 from a gas stream containing CO2, which process comprises:
(a) conducting said gas stream through an adsorbent containing a mesoporous material under high pressure to adsorb said CO2 onto said adsorbent and produce a substantially CO2-free gas stream.

2. The process of claim 1 additionally comprising:
(b) reducing the pressure on said adsorbent having CO2 adsorbed thereon to a moderate pressure to desorb at least a fraction of the adsorbed CO2.

3. A process according to claim 1 wherein said adsorbent is a mesoporous silica with pore diameter in the range of 2-50 nm, having a pore volume in the range of 0.4 to 4 cm3/g, and a surface area in the range of 500-2000 m2/g.

4. A process according to claim 3, wherein the pore diameter is in the range of 3 to 15 nm, and the pore volume is in the range of 1 to 3 cm3/g.

5. A process according to claim 1 wherein said adsorbent is a mesoporous metallosilica with pore diameter in the range of 2-50 nm, having a pore volume in the range of 0.4 to 4 cm3/g, and a surface area in the range of 500-2000 m2/g.

6. A process according to claim 5, wherein the pore diameter is in the range of 3 to 15 nm, and the pore volume is in the range of 1 to 3 cm3/g.

7. A process according to claim 1 wherein said adsorbent is a mesoporous metal or mixed metal oxide with pore diameter in the range of 2-50 nm, having a pore volume in the range of 0.3 to 4 cm3/g, and a surface area in the range of 200-2000 m2/g.

8. A process according to claim 7, wherein the pore diameter is in the range of 3 to 15 nm, and the pore volume is in the range of 1 to 3 cm3/g.

9. A process according to claim 1 wherein said adsorbent is a cation exchanged mesoporous metallosilica with pore diameter in the range of 2-50 nm, having a pore volume in the range of 0.4 to 4 cm3/g, and a surface area in the range of 500-2000 m2/g.

10. A process according to claim 9, wherein the pore diameter is in the range of 3 to 15 nm, and the pore volume is in the range of 1 to 3 cm3/g.

11. A process according to claim 5 wherein said metallosilica is aluminosilica, titanosilica, borosilica or iron-silica.

12. A process according to claim 7 wherein said metal or mixed metal oxide is alumina, titania, zirconia or a combination thereof.

13. A process according to claim 1 wherein metals and/or metal oxides are added to said adsorbent.

14. A process according to claim 1 wherein said adsorbent exhibits a CO2 adsorption isotherm corresponding to high adsorption capacity at high pressure (H) and low capacity at medium pressure (M), suitable for use in bulk separation applications based on PSA-H/M pressure-swing adsorption, with adsorption at pressure H (bar) and desorption at pressure M (bar).

15. A process according to claim 14 wherein said high pressure H is between about 20 and about 65 bar, and wherein said medium pressure M is between about 5 and about 25 bar.

16. A process according to claim 15, wherein said high pressure H is between about 35 and about 55 bar, and wherein said medium pressure is in between about 10 and about 20 bar.

17. A process according to claim 3 wherein said mesoporous silica exhibits a CO2 adsorption capacity at room temperature in dry and humid conditions in the range of 40 wt %-120 wt % at 60 bar and PSA-60/10 capacity of 30 to 100 wt %.

18. A process according to claim 3 wherein said mesoporous silica exhibits favourable CO2 selectivity over N2, CH4, O2, H2 and CO.

19. A process according to claim 3 wherein said mesoporous silica exhibits enhanced CO2 selectivity over N2, CH4, O2, H2 and CO in the presence of moisture.

20. A process according to claim 3 wherein said mesoporous silica exhibits a CO2 adsorption rate at room temperature in the range of 3×10−2-6×10−2 s−1.