Adsorption performance of VOCs in ordered mesoporous silicas with different pore structures and surface chemistry

Abstract

Ordered mesoporous silicas with different pore structures, including SBA-15, MCM-41, MCM-48 and KIT-6, were functionalized with phenyltriethoxysilane by a post-synthesis grafting approach. It was found that phenyl groups were covalently anchored onto the surface of mesoporous silicas, and the long-range ordering of the mesoporous channels was well retained after the surface functionalization. The static adsorption of benzene and the dynamic adsorption of single component (benzene) and bicomponent (benzene and cyclohexane) on the original and functionalized materials were investigated. As indicated by the adsorption study, the functionalized silicas exhibit improvement in the surface hydrophobicity and affinity for aromatic compounds as compared with the original silicas. Furthermore, the pore structure and the surface chemistry of materials can significantly influence adsorption performance. A larger pore diameter and cubic pore structure are favorable to surface functionalization and adsorption performance. In particular, the best adsorption performance observed with phenyl-grafted KIT-6 is probably related to the highest degree of surface functionalization, arising from the relatively large mesopores and bi-continuous cubic pore structure which allow great accessibility for the functional groups. In contrast, functionalized MCM-41 exhibits the lowest adsorption efficiency, probably owing to the small size of mesopores and 1D mesoporous channels.

Introduction

Volatile organic compounds (VOCs) have been extensively involved in many industrial processes, whereas such compounds are hazardous to human health, contributing to serious environmental problems such as the destruction of the ozone layer, photochemical smog and global warming [1], [2]. Hence the emission of VOCs has triggered increasing awareness focused on the development of abatement technologies to comply with the latest environmental regulations. Among the different alternatives available to remove VOCs, adsorption technology with no chemical degradation has been generally recognized as a preferred strategy, especially in cases where the organic pollutants to be captured have alternative uses [3]. The primary requirements of an adsorbent adopted in the industry process include sufficient adsorptive capacity, high adsorption rate, and strong hydrophobicity [4]. Activated carbon is widely employed in adsorption process owing to its high micropore volume and low operating cost [5]. Unfortunately, activated carbon often suffers from added fire risk, pore blocking, hygroscopicity and a lack of regenerative ability [6].

As a result, alternative adsorbents such as hydrophobic zeolite and mesoporous silica have attracted considerable attention to control VOCs. Hydrophobic zeolites have been found to be effective in VOCs removal [7], [8]. However, serious diffusion restrictions imposed by the micropores (<2 nm) tend to inhibit its ability to adsorb large VOC molecules [9]. Ordered mesoporous silicas offer excellent features such as large pore volume and high surface area, narrow pore size distribution, open pore structure and reliable desorption performance [10], [11], [12]. Siliceous SBA-15, MCM-41, MCM-48 and KIT-6 are considered as representatives of mesoporous materials. SBA-15 and MCM-41 possess a two-dimensional (2D) hexagonal array of uniform cylindrical mesopores (p6m), while MCM-48 and KIT-6 consist of three-dimensional (3D) bicontinuous cubic arrangement of mesopores (Ia3d). In particular, studies found that the porous structure of SBA-15 not only consists of ordered mesopores but also of much smaller complementary pores connecting adjacent mesopore channels [13], [14]. Similarly, the two intertwined systems of mesoporous channels in KIT-6 can also be connected through irregular micropores in the walls [15]. Previous studies have indicated that the bimodal materials (SBA-15 and KIT-6) have high affinity for various VOCs due to their complementary micropores, which are favorable to the diffusion process [10], [16].

In fact, the silanols (Si–OH) on the amorphous walls surface make original mesoporous silicas less hydrophobic and decrease the adsorption capacity of VOCs in the presence of water vapor, which limit their applications in the field of adsorption. Furthermore, surface siloxanes of original silicas may hydrolyze to silanols by water adsorption [17], [18]. Therefore, surface chemistry tailoring is required for mesoporous silicas in order to improve hydrophobicity and selectivity for VOCs. One important way of modifying the surface chemistry is organic functionalization [13]. It has been established that the introduction of organic groups in the framework of nanoporous material has a positive effect on VOCs removal [19].

Surface functionalization can be conducted in three various ways such as subsequent surface functionalization of pre-fabricated silica materials (“grafting”), the simultaneous condensation of corresponding silica and organosilica precursors groups (“co-condensation”) and the incorporation of organic groups as bridging components into the pore walls (“production of periodic mesoporous organosilica”) [20]. Compared with the other two pathways, the grafting method, carried out by reaction of organosilanes with surface silanol groups, can maintain the original mesoporous structure. Adjustment of the surface chemistry of porous silicas with organic groups, such as alkyl chains or phenyl groups, can enhance the surface hydrophobicity [13]. Much progress focusing on the post-synthetic functionalization of mesoporous materials with organic groups and the corresponding applications has been made [17], [21], [22], [23], [24]. Zhao and Lu [21] demonstrated that surface functionalization of siliceous MCM-41 by grafting with trimethylchlorosilane (TMCS) was an effective technique in tailoring adsorbents for the selective abatement of VOCs from wastewater. However, there is very limited information available in the literature relating to the application of such inorganic-organic mesoporous solids in the field of VOCs removal, especially in gas steams. Furthermore, the effect of pore structure on the surface functionalization is essential for the synthesis of functionalized silicas and the corresponding VOCs adsorption.

The objective of this work is to investigate the surface functionalization and adsorption performances of ordered mesoporous silicas with different pore structures, including SBA-15, MCM-41, MCM-48 and KIT-6. The functionalized mesoporous silicas were synthesized by post-synthetic treatment with phenyltriethoxysilane (PTES). The dynamic and static adsorption behaviors of VOCs on the original pure silicas and functionalized silicas were investigated by continuous-flow adsorption measurements and digital microbalance, respectively. With regard to the dynamic adsorption, besides single component adsorption of benzene, competitive adsorptions of benzene with water or cyclohexane were also conducted to explore the affinity between VOCs molecules and functional groups on the mesoporous silica surface.