Greenhouse gas (GHG) emission is increasing significantly due to rapid urbanization and industrialization, which contributes largely to global warming and climate change. To combat this issue, it is crucial to control GHG emissions, particularly carbon dioxide (CO2), whose concentration in the atmosphere has risen significantly [1]. Various methods, including absorption, adsorption, and cryogenic separation, have been developed to capture CO2 emissions, but they may not be cost-effective or efficient [2]. Therefore, researchers have been exploring other CO2 capture methods, such as membrane technology, which offers several advantages over traditional methods. Membrane technology responds quickly to changes, has a short start-up time, requires minimal energy and control, and is reliable and easy to expand [3]. Additionally, membrane technology is more energy efficient, has a smaller carbon footprint, and is simpler to operate and maintain, potentially reducing costs [4].
Many types of membranes have been reported to separate CO2, such as polymeric membranes, inorganic membranes and mixed matrix membranes. Polydimethylsiloxane (PDMS) is one of the membrane materials that is most commonly employed among industrially adopted polymeric materials due to its effectiveness, which attributes to its gas permeability, low chemical reactivity, high free volume, excellent mechanical and dielectric properties as well as flexibility at low temperature [5]. Besides exhibiting excellent CO2 permeability, PDMS also possess high thermal stability and present no aging issues [6]. However, PDMS-based membranes have a major drawback in which they suffer from relatively low CO2/nitrogen (N2) selectivity of approximately 6.3–9.5 [7]. As most of the industrial flue gases contains both CO2 and N2, the gas selectivity of these PDMS-based membranes must be enhanced in order for them to be employed for practical usage [6].
Few researchers have proposed ways to modify the surface of PDMS membrane to enhance its gas separation performance, in which surface modification using 3-aminopropyltriethoxysilane (APTES) has been proposed in several studies [4]. Such modification was done by Beal et al. (2012) via a rapid yet cost-effective method by immersing the PDMS membrane into APTES solution [8]. In the work of Zhimin et al. (2017), the PDMS membrane modified using APTES as the crosslinking agent has demonstrated selectivity as high as 95.0 [2]. This shows that APTES has great potential in enhancing the gas separation performance of PDMS membrane through chemical grafting.
APTES is a silane coupling agent and it can be used to immobilize biomolecules. By modifying PDMS membrane with APTES, amine functional groups which are capable forming hydrogen bonds with water are formed on the membrane surface, thus decreasing its water contact angle and increasing its hydrophilicity [9]. Other than decreased hydrophobicity, membrane modified using APTES demonstrates enhanced CO2 separation characteristics [2].
The capability of APTES in enhancing CO2 separation is attributed to its reaction with CO2 which leads to the formation of carbamate. According to Yu et al. (2017), CO2 membranes or sorbents use 3 main types of amines, namely primary, secondary and tertiary amines, in which APTES belongs to primary amines [10]. Zwitterion mechanism describes this kind of reactions between amines and CO2, where CO2 undergoes reaction under unhindered amines and leads to the formation of an intermediate known as the zwitterion, which will then be deprotonated by another amine to give a stable anhydrous carbamate ion. The relevant reactions are given in Eqs. (1) and (2):
$${RNH}_{2} + {CO}_{2}\leftrightarrow {{RNH}_{2}}^{+}{COO}^{-}$$
1
$${{RNH}_{2}}^{+}{COO}^{-}+{RNH}_{2} \leftrightarrow {RNHCOO}^{-}+{{RNH}_{3}}^{+}$$
2
Thus far, limited research has been reported for PDMS membrane surface modification for gas separation. In this study, a flat sheet PDMS membrane was fabricated via casting method and surface modified using APTES through chemical grafting. After the membrane has been fabricated and surface modified, its CO2 separation performance was evaluated in terms of gas permeability and selectivity by a gas permeation test. There are several parameters in the gas permeation test that can influence the CO2 separation efficiency of the membrane. So far, limited research has studied the CO2 gas separation performance of PDMS membrane surface modified using APTES through chemical grafting. In this study, the effect of transmembrane gas pressure difference and surface modification using APTES on CO2 separation efficiency was investigated.