Probing Intramolecular versus Intermolecular CO₂ Adsorption on Amine-Grafted SBA-15

Abstract

A mesoporous silica SBA-15 is modified with an array of amine-containing organosilanes including (i) propylamine, SiCH2CH2CH2NH2 (MONO), (ii) propylethylenediamine, SiCH2CH2CH2NHCH2CH2NH2 (DI), (iii) propyldiethylenetriamine, SiCH2CH2CH2NHCH2CH2NHCH2CH2NH2 (TRI), and (iv) propyltriethylenetetramine, SiCH2CH2CH2NHCH2CH2N(CH2CH2NH2)2 (TREN) and the low loading silane adsorbents (∼0.45 mmol silane/g) are evaluated for their CO2 adsorption properties, with a focus on gaining insight into the propensity for intramolecular vs intermolecular CO2 adsorption. Adsorption isotherms at low CO2 coverages are measured while simultaneously recording the heat evolved via a Tian-Calvet calorimeter. The results are compared on a silane molecule efficiency basis (mol CO2 adsorbed/mol silane) to assess the potential for intramolecular CO2 adsorption, employing two amine groups in a single silane molecule. As the number of amines in the silane molecule increases (MONO < DI < TREN ∼ TRI), the silane molecule efficiency is enhanced owing to the ability to intramolecularly capture CO2. Analysis of the CO2 uptake for samples with the surface silanols removed by capping demonstrates that cooperative uptake due to amine-CO2-silanol interactions is also possible over these adsorbents and is the primary mode of sorption for the MONO material at the studied low silane loading. As the propensity for intramolecular CO2 capture increases due to the presence of multiple amines in a single silane molecule (MONO < DI < TREN ∼ TRI), the measured heat of adsorption also increases. This study of various amine-containing silanes at low coverage is the first to provide significant, direct evidence for intramolecular CO2 capture in a single silane molecule. Furthermore, it provides evidence for the relative heats of adsorption for physisorption on a silanol laden surface (ca. 37 kJ/mol), a silanol-capped surface (ca. 25 kJ/mol), via amine-CO2-silanol interactions (ca. 46 kJ/mol), and via amine-CO2-amine interactions at low surface coverages (ca. 65 kJ/mol).

CO2adsorption on different organo-modified SBA-15 silicas: a multidisciplinary study on the effects of basic surface groups

“…In particular, the higher is the length of the grafted alkylamine chain, the higher is the amount of formed carbamic acid. Comparing our results to those reported by Yoo et al [15] for similar sorbents (i.e. SBA--15 grafted with APTS, EAPTS and PAPTS), but containing lower amount of organosilane, we can see a significant effect of the silane density.…”

“…on passing from the A--SBA--15 to the P--SBA--15 sample), thus suggesting that the higher the concentration of amine groups, the higher the reactivity towards CO 2 . 15 A similar behavior was observed for samples prepared by one--pot procedure. In this specific case, however, although the detected IR bands are in similar positions to those observed for grafted samples (thus suggesting the formation of the same species upon contact with CO 2 ), the intensity of absorptions appeared significantly lower with respect to the analogous post--synthesis grafted samples.…”

“…If, on the other hand, the alkyl chain contains another amino group, the base B is present in the same molecule, and therefore the reaction can also occur via intramolecular mechanism. 15 Ordered porous silica--based materials like HMS, mesoporous silica microspheres, mesocellular silica foam, MCM--41, MCM--48, SBA--12, SBA--15, SBA--16 and KIT--6, have been widely investigated as solid sorbent for CO 2 capture. 11 In fact, they are good candidates for CO 2 capture for their structural and textural properties, such as large pore size, high specific surface area and pore volume.…”

“…Even if several studies in the literature reported the use of silanes with different concentration of basic species, the CO 2 capture performances of these materials is difficult to be compared in that in many cases different conditions of pressure and temperature have been adopted. Recently, Yoo et al 15 compared SBA--15 functionalized with amine--containing organosilanes with different chain length (including 3--aminopropyltrimethoxysilane (APTS), 3--(2--aminoethyl)aminopropyltrimethoxysilane (EAPTS) and 3--[2--(2--aminoethyl)aminoethyl] aminopropyltrimethoxysilane (PAPTS)) with low loading of silane adsorbents (∼0.45 mmol silane/g). In their work, particularly focused on the determination of CO 2 adsorption mechanism with functionalized silica surface, they suggested the occurrence of intramolecular mechanism for CO 2 adsorption on EAPTS and PAPTS, whereas for samples grafted with shorter silane species (APTS) an intermolecular mechanism was proposed.…”

Preparation and Properties of A Hyperbranch-Structured Polyamine adsorbent for Carbon Dioxide Capture

“…Among solid adsorbents, solid amine adsorbents such as amino-functionalized MCM-41 3 , SBA-15 4, 5 , Zeolite 13X 6, 7 and activated carbon 8, 9 have been used, having attracted much attention due to their physical and chemical adsorption of CO 2 . Solid amine adsorbents have advantages such as lower energy consumption for regeneration, higher adsorption capacity and efficiency, higher stability and less erosion for equipments.…”