Pore-expanded MCM-41 Research Articles

Advancing Low-CO2 Sorption Technology: Density Adjustment of Urea-Resistant Secondary Monoamine for Improved Adsorption Performance

Amine-containing adsorbents, recognized for their lower desorption energy in comparison to aqueous amine, have encountered challenges stemming from poor stability and inadequate working capacity under realistic operational conditions, impeding their adoption as viable CO2 capture options. The alteration of amine groups using epoxy-based agents has been demonstrated to effectively inhibit undesirable urea formation, but decreases amine density and thereby the CO2 adsorption capacity. Although secondary monoamine structures have been found to possess anti-urea stability, their CO2 uptake has never been optimized. Consequently, we systematically adjusted the density of N-methylaminopropyltrimethoxysilane on pore-expanded MCM-41 by optimizing the water and aminosilane levels for grafting. Additionally, an expression was developed to estimate the location of aminosilane on the mesoporous silica. The adsorbent with the highest amount of aminosilane grafted inside the pores showed the best capacity (1.94 mmol/g under 40 °C and 5 v% CO2), and an adsorption efficiency close to the theoretical maximum (i.e., 0.5, dry feed). The adsorbent also demonstrated rapid adsorption (1.76 mmol/g in 3 min) and no loss during long-term stability tests (366 and 423 h) performed under industrially relevant conditions.

A combined experimental and simulations assessment of CO2 capture and CO2/H2 separation performance of aminosilane-grafted MCM-41 and pore-expanded MCM-41

Amine-functionalized mesoporous silica materials have attracted considerable attention as robust adsorbents for large-scale CO2 capture and direct air capture (DAC). Amine grafting increases selective affinity at low pressures, yet also often reduces the adsorbent's pore volume and surface area, affecting the kinetics and capacity upon CO2 adsorption. MCM-41 and pore-expanded MCM-41 (PE-MCM-41) were developed here, functionalized using 3-aminopropyl triethoxy silane (APTES), and comparably evaluated for their carbon capture performance at different conditions by a combinational experimental, equilibrium and kinetic modelling, and molecular simulations approach. In specific, CO2 adsorption capacity, CO2/N2 selectivity, CO2 adsorption at humid conditions, heat of adsorption, and regeneration/cyclability were assessed, coupled to molecular simulation insights on the physisorption and chemisorption contribution to the overall adsorption uptake. Indicatively, a capacity value of 3.07 and 2.44 mmol/g was obtained for APTES-PE-MCM-41, and 2.27 and 1.89 mmol/g for APTES-MCM-41, under wet and dry conditions, respectively. The CO2 selective adsorption from H2 rich gas streams was also examined over APTES-MCM-41 in order to assess the potential application in H2 stream purification process. The latter is of high significance in order to use hydrogen as fuel e.g. in anion exchange membrane fuel cells for power production since the presence of CO2 in the feed composition is considered a major drawback in their commercialization.

Amino-Functionalized Pore-Expanded MCM-41 for CO2 Adsorption: Effect of Alkyl Chain Length of the Template

Solid adsorbents modified with N-containing functional groups have been extensively studied due to their excellent CO2 adsorption performance. In this work, an adsorbent was obtained to capture CO2 by impregnating polyethylenimine (PEI) into MCM-41 with a reserved template that had different alkyl chain lengths [myristyltrimethylammonium bromide (C14), cetyltrimethylammonium bromide (C16), and stearyltrimethylammonium bromide (C18)]. The thermogravimetric analysis, small-angle X-ray diffraction, N2 adsorption–desorption, Fourier transform infrared (FTIR) spectroscopy analysis, and in situ FTIR spectra were employed to study the properties of adsorbents. The results showed that the template had been successfully introduced into the supports in the synthetic process, and the CO2 adsorption performance was quite sensitive to the alkyl chain length of the template. Besides, the adsorbent with the template (C16) exhibited the highest CO2 adsorption capacity (2.49 mmol/g) and amine utilization rate (0.28 mmol CO2/mmol N) under 25 °C and pure CO2. It is believed that the appropriate alkyl chain length of the template could act as an important role in the enhancement of PEI distribution and decrease the CO2 transfer resistance, thereby improving the CO2 adsorption capacity.

Study of rice husk ash derived MCM-41-type materials on pore expansion, Al incorporation, PEI impregnation, and CO2 adsorption.

Conventional MCM-41 (M41), silica-pure pore-expanded MCM-41 (PM41), and Al-containing pore-expanded MCM-41 (PM41Ax) were synthesized from rice husk ash and tested as polyethyleneimine (PEI) supports for CO2 capture. Samples were characterized by small-angle X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, granulometric analysis, and nitrogen adsorption techniques. The PEI loading rate and CO2 adsorption-desorption performance were determined via thermogravimetric analysis. The effects of pore expansion, heteroatom Al incorporation, PEI loading rate, and Si/Al ratio on CO2 adsorption performance were examined. For the first time, the amount of PEI impregnated in PM41 was increased beyond 55 wt%, and the low-Si/Al-ratio PM41Ax support was used to load PEI in a novel procedure. Results show that at the same PEI loading rate, PM41 is always superior to M41 regarding adsorption capacity and adsorption rate. For a PEI loading rate >50 wt%, the superiority is amplified, reaching 15.9% and 21.3%, respectively. The use of the high-Al-containing PM41Ax support further increases adsorption capacity and adsorption rate by 13.4% and 9.6%, respectively. The presented reaction has a hybrid adsorption characteristic that includes both chemisorption and physisorption. Avrami’s fractional-order kinetic model describes the adsorption best. Over the entire time scale, the adsorption rate is determined by several kinetic diffusion-controlled processes. The intraparticle diffusion and equilibrium adsorption are two predominant rate-limiting steps, and their control ranges change with temperature. After five cycles of adsorption and desorption, the desorption ratio was as high as 99%, and the working capacity still retained 96.5% of the original capacity. In addition, the presence of water vapor increased the adsorption capacity of the adsorbents presented in this study. These advantages make them successful iin capturing CO2 in the post-combustion scenario.

Pentaethylenehexamine-Loaded Hierarchically Porous Silica for CO₂ Adsorption.

Recently, amine-functionalized materials as a prospective chemical sorbent for post combustion CO2 capture have gained great interest. However, the amine grafting for the traditional MCM-41, SBA-15, pore-expanded MCM-41 or SBA-15 supports can cause the pore volume and specific surface area of sorbents to decrease, significantly affecting the CO2 adsorption-desorption dynamics. To overcome this issue, hierarchical porous silica with interparticle macropores and long-range ordering mesopores was prepared and impregnated with pentaethylenehexamine. The pore structure and amino functional group content of the modified silicas were analyzed by scanning electron microscope, transmission electron microscope, N2 adsorption, X-ray powder diffraction, and Fourier transform infrared spectra. Moreover, the effects of the pore structure as well as the amount of PEHA loading of the samples on the CO2 adsorption capacity were investigated in a fixed-bed adsorption system. The CO2 adsorption capacity reached 4.5 mmol CO2/(g of adsorbent) for HPS−PEHA-70 at 75 °C. Further, the adsorption capacity for HPS-PEHA-70 was steady after a total of 15 adsorption-desorption cycles.

Amine tethered pore-expanded MCM-41: A promising adsorbent for CO 2 capture

The current work investigates the influence of pore size as well as pore volume of MCM-41 supports on amine tethering. Initially, monoamine tethering on MCM-41-3 as well as MCM-41-30 is carried out under dry conditions to screen the best support. The CO2 adsorption capacity and amine efficiency results revealed the beneficial effect of utilizing MCM-41-30, which is a pore-expanded support fabricated by our research group in a cost-effective manner for amino silane tethering. In order to increase the amine loading and further enhance the CO2 uptake in the important low pressure regime (0.1–0.2 bar), wet grafting process is followed for amino silane tethering on MCM-41-30 support. The present work also elucidates the performance of amine tethered MCM-41-30 in terms of post combustion CO2 capture metrics such as (i) CO2 adsorption capacity (ii) amine efficiency (iii) operating window (iv) CO2 adsorption kinetics (v) selectivity (vi) cyclic adsorption-desorption studies and (vii) cost. Comparative analysis of the same with other adsorbents such as activated carbon, zeolites, metal organic frameworks, amine impregnated and amine tethered mesoporous silica is highlighted to rank the performance for post combustion CO2 capture application. The mono and triamine tethered MCM-41 adsorbents developed in the current work, exhibited a maximum adsorption capacity of ∼1.2 and ∼2.1 mmol/g (75 °C, 0.2 bar) respectively. This in turn, indicated that MCM-41-30 support developed herein serves as a promising support for amine tethering application and demonstrates effective CO2 capture characteristics.

Enhanced Adsorption Efficiency through Materials Design for Direct Air Capture over Supported Polyethylenimine.

Until recently, carbon capture and sequestration (CCS) was regarded as the most promising technology to address the alarming increase in the concentration of anthropogenic CO2 in the atmosphere. There is now an increasing interest in carbon capture and utilization (CCU). In this context, the capture of CO2 from air is an ideal solution to supply pure CO2 wherever it is needed. Here, we describe innovative materials for direct air capture (DAC) with unprecedented efficiency. Polyethylenimine (PEI) was supported on PME, which is an extra-large-pore silica (pore-expanded MCM-41) with its internal surfaces fully covered by a uniform layer of readily accessible C16 chains from cetyltrimethylammonium (CTMA+ ) cations. The CTMA+ layer plays a key role in enhancing the amine efficiency toward dry or humid ultradilute CO2 (400 ppm CO2 /N2 ) to unprecedented levels. At the same PEI content, the amine efficiency of PEI/PME was two to four times higher than that of the corresponding calcined mesoporous silica loaded with PEI or with different combinations of C16 chains and PEI. Under humid conditions, the amine efficiency of 40 wt % PEI/PME reached 7.31 mmolCO2 /gPEI , the highest ever reported for any supported PEI in the presence of 400 ppm CO2 . Thus, amine accessibility, which reflects both the state of PEI dispersion and the adsorption efficiency, is intimately associated with the molecular design of the adsorbent.

Amine tethered pore-expanded MCM-41 for CO2 capture: Experimental, isotherm and kinetic modeling studies

At present, research activities based on different separation technologies are vigorously carried out with the aim to capture carbon dioxide (CO2), which remains as the major contributor for global warming problem. As a part of such research activity, mono and tri amine tethered pore-expanded MCM-41 (AP-PE-MCM-41 and TP-PE-MCM-41) mesoporous silica adsorbents are developed in the present work. The TP-PE-MCM-41 adsorbent exhibited an excellent CO2 adsorption capacity of ∼2.1mmol/g under flue gas conditions (Pressure=0.2bar, Temperature=75°C), which is the highest reported CO2 adsorption capacity for a tri-amine tethered mesoporous silica adsorbent so far to the best of our knowledge. The experimental CO2 uptake data are fitted to Langmuir and Langmuir dual-site models. Langmuir-dual site model elucidated that chemisorption controls the initial stages of adsorption followed by physisorption during the latter stages of adsorption process. Kinetic uptake measurements are performed for both the adsorbents and the uptake profile is further analyzed by pseudo-first order, pseudo-second order and double-exponential models. Double-exponential model provided an excellent fit to the kinetic data indicating that CO2 adsorption on amine tethered PE-MCM-41 samples is governed by both chemisorption and physisorption.

Pore-expanded MCM-41 for CO2 adsorption: Experimental and isotherm modeling studies

In the current scenario, pore-expanded mesoporous silica materials are explored as promising supports for the art of amine tethering. Recently, our research group fabricated a pore-expanded MCM-41 (PE-MCM-41) through a cost-effective approach for carbon capture application. In the current work, adsorption isotherm measurements of pure CO2 on PE-MCM-41 are carried out at four different temperatures (30, 45, 60 and 75°C) under the pressure regime of 0–25bar using a Rubotherm magnetic suspension balance. Diffuse reflectance infrared spectroscopy (DRIFTS) analysis is carried out in order to get an insight into various surface functionalities that can exist on PE-MCM-41. The experimental CO2 uptake data are fitted to Langmuir, Sips, Toth and Langmuir multi-site models. To figure out the potential surface groups responsible for CO2 adsorption, patch-wise topography hypothesis is developed. The Langmuir multi-site isotherm model together with heat of adsorption calculations enlightens that CO2 adsorption on PE-MCM-41 occurs via both dispersion forces and hydrogen bonding.

Evaluation of Optimal Pore Size of (3-Aminopropyl)triethoxysilane Grafted MCM-41 for Improved CO2Adsorption

An array of new MCM-41 with substantially larger average pore diameters was synthesized through adding 1,3,5-trimethylbenzene (TMB) as the swelling agent to explore the effect of pore size on final adsorbent properties. The pore expanded MCM-41 was also grafted with (3-Aminopropyl)triethoxysilane (APTES) to determine the optimal pore size for CO2adsorption. The pore-expanded mesoporous MCM-41s showed relatively less structural regularity but significant increments of pore diameter (4.64 to 7.50 nm); the fraction of mesopore volume also illustrated an increase. The adsorption heat values were correlated with the order of the adsorption capacities for pore expanded MCM-41s. After amine functionalization, the adsorption capacities and heat values showed a significant increase. APTES-grafted pore-expanded MCM-41s depicted a high potential for CO2capture regardless of the major drawback of the high energy required for regeneration.

CO2 capture with pore-expanded MCM-41 silica modified with amino groups by double functionalization

A pore expanded MCM-41 mesostructured silica (PE-MCM-41) was modified by double-functionalization with molecules containing amino groups in order to obtain CO2 adsorbents for industrial post-combustion applications. PE-MCM-41 was modified by grafting with aminopropyl (AP), ethylenediamino (ED) and diethylenetriamino (DT) organosilanes, by impregnation with polyethyleneimine (PEI), pentaethylenehexamine (PEHA) and tetraethylenepentamine (TEPA), and by double functionalization, impregnating PEI, PEHA and TEPA over samples previously grafted with AP and DT. CO2 adsorption properties of these materials were found to depend on the kind, amount and viscosity of the organic compound loaded. Grafted samples showed amine efficiencies in CO2 capture up to 0.38mol CO2/mol N, leading to CO2 uptakes ranging from 38.2 to 76.9mg CO2/g ads (pure CO2, 45°C, 1bar). Impregnation yielded adsorbents with higher organic content but lower efficiency (between 0.20 and 0.25mol CO2/mol N), and with a net CO2 adsorption capacity up to 96.9mg CO2/g ads. By using the double-functionalization technique, the nitrogen content was increased while maintaining high amine efficiency (0.37mol CO2/mol N) and consequently exhibiting higher CO2 adsorption capacity, up to 104mg CO2/g ads. Thus, this functionalization route was proven successful. However, at high organic loading a sharp drop in the amine efficiency was observed, being ascribed to the blocking of the MCM-41 pores by amines.

CO2 adsorption kinetics on mesoporous silica under wide range of pressure and temperature

Abstract Adsorption kinetics is a critical parameter to assess the performance of an adsorbent. Therefore, from a practical standpoint, it is important to characterize the rate-controlling steps associated with an adsorption process. More recently, our research group synthesized a pore-expanded MCM-41 (MCM-41) for CO 2 capture application. In the current work, kinetic measurements of pure CO 2 on MCM-41 at four different temperatures (30, 45, 60, and 75 °C) are performed under wide pressure regime (0.2–11) bar. The experimental data of CO 2 uptake as a function of time at different pressure and temperature conditions are analyzed by both pseudo-first order and the pseudo-second order kinetic models. The type of interaction between CO 2 and MCM-41 as well as adsorption rate behavior of CO 2 on MCM-41 is deduced from the best fitted model. To further investigate the mechanism of CO 2 adsorption on MCM-41, interparticle diffusion, intraparticle diffusion and Boyd’s models are also selected. The CO 2 diffusion mechanism in MCM-41 is elucidated based on the information derived from all the three models. Film diffusion mainly governed the rate-controlling process of CO 2 uptake on MCM-41 at low pressure and low temperature conditions. Interestingly, it is also interpreted that pore diffusion becomes more prominent at higher pressure and higher temperature conditions.

Supported Polytertiary Amines: Highly Efficient and Selective SO2 Adsorbents

Tertiary amine containing poly(propyleneimine) second (G2) and third (G3) generation dendrimers as well as polyethyleneimine (PEI) were developed for the selective removal of SO2. N-Alkylation of primary and secondary amines into tertiary amines was confirmed by FTIR and NMR analysis. Such modified polyamines were impregnated on two nanoporous supports, namely, SBA-15PL silica with platelet morphology and ethanol-extracted pore-expanded MCM-41 (PME) composite. In the presence of 0.1% SO2/N2 at 23 °C, the uptake of modified PEI, G2, and G3 supported on SBA-15PL was 2.07, 2.35, and 1.71 mmol/g, respectively; corresponding to SO2/N ratios of 0.22, 0.4, and 0.3. Under the same conditions, the SO2 adsorption capacity of PME-supported modified PEI and G3 was significantly higher, reaching 4.68 and 4.34 mmol/g, corresponding to SO2/N ratios of 0.41 and 0.82, respectively. The working SO2 adsorption capacity decreased with increasing temperature, reflecting the exothermic nature of the process. The adsorption capacity of these materials was enhanced dramatically in the presence of humidity in the gas mixture. FTIR data before SO2 adsorption and after adsorption and regeneration did not indicate any change in the materials. Nonetheless, the SO2 working capacity decreased in consecutive adsorption/regeneration cycles due to evaporation of impregnated polyamines, rather than actual deactivation. FTIR and (13)C and (15)N CP-MAS NMR of fresh and SO2 adsorbed modified G3 on PME confirmed the formation of a complexation adduct.

Direct synthesis of bi-modal porous structure MCM-41 and its application in CO2 capturing through amine-grafting

A bi-modal porous structure MCM-41 (BPS-MCM-41) was synthesized and functionalized by 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane (TRI); also, its performance in amine grafting and CO2 capturing was compared with that of pore-expanded MCM-41 [1]. To create larger pores beside the mesoporous structure of MCM-41, carbon black nanoparticles were used as the solid template. Characterizing the BPS-MCM-41 using the BET and BJH techniques resulted in the surface reduction of 29.3 percent and volume increase of 68.46 percent. The pore size distribution showed two peaks: a narrow peak at 2.24 nm diameter, which belonged to micelles, and a wide one at about 50 nm due to the presence of used nanoparticles. The functionalization confirmed that BPS-MCM-41 is capable of accommodating a large quantity of amine groups. The CO2 adsorption measurement indicated that internal volume of the adsorbent was a critical factor affecting the adsorption capacity of the amine grafted adsorbents.

Molecular‐Level Insights into the Oxidative Degradation of Grafted Amines

The oxidative degradation of CO2 adsorbents consisting of amine-grafted pore-expanded mesoporous MCM-41 silica was investigated. The adsorbents were treated under flowing air at various temperatures, and the degree of deactivation was evaluated through the measurement of their CO2 adsorption capacity prior and subsequent to exposure to air. To decipher the chemical structure of surface species upon air-deactivation of grafted amines, a solvent extraction procedure was developed using a deuterated basic solution. The obtained solutions were analyzed by a variety of 1D and 2D NMR spectroscopy techniques, such as (29)Si, (13)C, (1)H, [(1)H,(15)N] HMBC, [(1)H,(13)C] HMQC, COSY and DOSY. The surface species generated by oxidative degradation of amine-grafted silica were found to contain functional groups such as imine, amide and carboxylic groups. Several structural units were conclusively demonstrated.

Novel Pore-Expanded MCM-41 for CO2 Capture: Synthesis and Characterization

Recently, amine-functionalized mesoporous silica materials have attracted considerable attention as a promising chemical sorbent for postcombustion CO2 capture applications. However, the grafting of amines in the conventional MCM-41 support induces the subsequent reduction of surface area and pore volume of the sorbents, affecting the CO2 adsorption-desorption kinetics significantly. To mitigate this problem, expensive pore expansion agents have been used to increase the pore size as well as the pore volume. The present study provides an innovative approach to the development of novel pore-expanded MCM-41 without the application of any swelling agent. The average pore size (~30 nm) obtained in our work is remarkably higher than the values (9 to 10 nm) reported in the literature. On the basis of the fundamental understanding of micelle properties under different alkaline conditions, a mechanism for the pore expansion process is proposed. The outcome (1.2 mmol/g) of the preliminary CO2 adsorption studies carried out on the novel support material grafted with monoamine silane is very promising.

CO2 Deactivation of Supported Amines: Does the Nature of Amine Matter?

Adsorption of CO(2) was investigated on a series of primary, secondary, and tertiary monoamine-grafted pore-expanded mesoporous MCM-41 silicas, referred to as pMONO, sMONO, and tMONO, respectively. The pMONO adsorbent showed the highest CO(2) adsorption capacity, followed by sMONO, whereas tMONO exhibited hardly any CO(2) uptake. As for the stability in the presence of dry CO(2), we showed in a previous contribution [J. Am. Chem. Soc.2010, 132, 6312-6314] that amine-supported materials deactivate in the presence of dry CO(2) via the formation of urea linkages. Here, we showed that only primary amines suffered extensive loss in CO(2) uptake, whereas secondary and tertiary amines were stable even at temperature as high as 200 °C. The difference in the stability of primary vs secondary and tertiary amines was associated with the occurrence of isocyanate as intermediate species toward the formation of urea groups, since only primary amines can be precursors to isocyanate in the presence of CO(2). However, using a grafted propyldiethylenetriamine containing both primary and secondary amines, we demonstrated that while primary amines gave rise to isocyanate, the latter can react with either primary or secondary amines to generate di- and trisubstituted ureas, leading to deactivation of secondary amines as well.

Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption

Poly(ethyleneimine) (PEI) supported on pore-expanded MCM-41 whose surface is covered with a layer of long-alkyl chains was found to be a more efficient CO(2) adsorbent than PEI supported on the corresponding calcined silica and all PEI-impregnated materials reported in the literature. The layer of surface alkyl chains plays an important role in enhancing the dispersion of PEI, thus decreasing the diffusion resistance. It was also found that at low temperature, adsorbents with relatively low PEI contents are more efficient than their highly loaded counterparts because of the increased adsorption rate. Extensive CO(2) adsorption-desorption cycling showed that the use of humidified feed and purge gases affords materials with enhanced stability, despite limited loss due to amine evaporation.

Effect of the Pore Length on CO2 Adsorption over Amine-Modified Mesoporous Silicas

Carbon dioxide adsorption was investigated in the presence of polyethylenimine (PEI)-impregnated mesoporous silicas with different pore lengths, namely, pore-expanded MCM-41, conventional SBA-15 with different pore diameters (7.2 and 10.5 nm), and SBA-15 with platelet morphology. The pore lengths of the silica supports were ca. 25, 1.5, and 0.2 μm, respectively. Under comparable conditions, the adsorption performance was found to be strongly dependent upon the pore length. The materials with the shortest channels showed the highest capacity and fastest adsorption. These findings were associated with diminished diffusion resistance and enhanced amine accessibility inside the pores.

CO 2 capture on polyethylenimine-impregnated hydrophobic mesoporous silica: Experimental and kinetic modeling

CO 2 adsorption measurements for polyethylenimine (PEI)-impregnated pore-expanded MCM-41 were conducted by gravimetry to investigate the effects of (i) amine loading, (ii) CO 2 partial pressure, (iii) adsorption and desorption temperatures. Amine impregnation was conducted on ethanol-extracted pore-expanded MCM-41, referred to as PME which is a mesoporous silica whose internal surface is laden by a layer of cetyltrimethylammonium cations (CTMA). The well-dispersed PEI inside the PME hydrophobic channels exhibited a CO 2 adsorption capacity at 75 °C as high as 206 mg/g for 55 wt.% PEI loading. Moreover, the current PEI-impregnated PME materials had high CO 2 adsorption efficiency (g CO 2/g PEI) than any other PEI-containing adsorbent reported in the literature. In contrast to most PEI-impregnated materials reported in the literature, which because of diffusion resistance showed little or no CO 2 adsorption at room temperature, the PEI-impregnated PME material showed high potential for CO 2 removal at ambient temperature. Also a new adsorption kinetic model was proposed to describe the adsorption of CO 2 over amine-impregnated materials. The model was found to be in good agreement with experimental data under a wide range of conditions including different PEI loadings, CO 2 pressures and adsorption temperatures.