Functionalized organosilicate materials for irritant gas removal
Graphical abstract
Highlights
► Under dry conditions, organosilicates remove more ammonia than the carbon material. ► Isocyanate functional groups improve ammonia removal under humid conditions. ► Amine functional groups improve cyanogen chloride removal. ► Sulfur dioxide removal is enhanced in materials with amine functional groups. ► Octane adsorption by all sorbents is negatively impacted by increased humidity.
Introduction
Traditional air purification materials often rely on porous carbons such as activated carbon or activated charcoal (Carey, 1998). Through modification of these types of carbon materials, functional moieties can be combined with high surface area and adsorptive properties to provide air purification materials with improved characteristics or novel function (Bashkova and Bandosz, 2009, Seredych et al., 2010). Ongoing efforts seek to improve the performance of carbon materials in air purification applications as well as to provide alternatives to these materials (Bae et al., 2010, Glover et al., 2011, Mu et al., 2010, Mulfort et al., 2010, Yazaydin et al., 2009). Here, we seek to demonstrate the potential of porous organosilicate materials to provide the type of functionality desired in new adsorptive and reactive sorbents for air purification applications.
Periodic mesoporous organosilicas (PMOs) (Asefa et al., 1999, Inagaki et al., 1999, Melde et al., 1999, Yohsina-Ishii et al., 1999) combine organic and inorganic components mixed at the molecular scale with the ordered porosity of surfactant-templated mesoporous silicates (Hatton et al., 2005, Hoffmann et al., 2006a). During synthesis polysilsesquioxane precursors are condensed around surfactant micelles, which act as a template for the structure and organization of the pores (Burleigh et al., 2001, Goto and Inagaki, 2002, Kresge et al., 1992). The silica component of these materials provides the structural rigidity required to employ surfactant templating methods while the organic groups provide characteristics normally associated with organic polymers. The mesoporous nature of PMOs can limit their applicability when flow of targets through the materials is desired as in, for example, chromatography or filtration applications. One approach to address this problem is to incorporate hierarchical structure into the sorbents including both macro- (>50 nm) and meso-scale (2–50 nm) morphology (Brandhuber et al., 2006, Johnson et al., 2010a, Johnson et al., 2010b, Melde et al., 2010, Nakanishi, 1997, Nakanishi et al., 2004). The macropores provide improved flow through the sorbents and enhanced access to the mesopore volume which is advantageous to diffusion dependent applications.
Porous organosilicates have been utilized in a wide range of applications from catalysis to sensing (Hoffmann et al., 2006b, Melde et al., 2008, Weitkamp et al., 2001). They have also been applied to the capture and/or detection of gases (Jansat et al., 2007, Palaniappan et al., 2006a, Palaniappan et al., 2008a, Palaniappan et al., 2008b, Palaniappan et al., 2006b, Yuliarto et al., 2009). Selectivity in the materials has been altered through the introduction of active groups providing discrimination of hydrocarbons and alcohols (Palaniappan et al., 2006b) and coordinated metals have been used for detection of nitric oxide (Palaniappan et al., 2008a, Palaniappan et al., 2008b) and carbon monoxide (Jansat et al., 2007). Alteration of the bridging groups in organosilicates has also been shown to impact the binding characteristics of the sorbents for gases. (Johnson et al., 2011, Yuliarto et al., 2009).
We have previously reported on our efforts directed at optimizing the characteristics of organosilicate materials for capture of nitroenergetic (Johnson et al., 2008, Johnson et al., 2010a), hydrocarbon (Johnson et al., 2011), and organophosphate targets (Johnson et al., 2010b). Those efforts focused on the development of a scaffold with high binding capacity and selectivity for the targets through alteration of the bridging groups comprising the pore walls. Here, we have taken a different approach in that a simple scaffold was utilized and alterations to the characteristics of that scaffold were achieved through post-synthesis modification. While organic functionalities can be incorporated in high amounts by one-step co-condensation of silanes during synthesis, this can significantly affect the final pore structure of a product (Hoffmann et al., 2006b). Post-synthesis grafting also increases the likelihood that pendant functional groups are located on the surface. This study details the impact of post-synthesis grafting on the interaction of the organosilicate scaffold with the irritant gases ammonia, octane, sulfur dioxide, and cyanogen chloride. These targets were selected as a result of the 2009 document released by the Toxic Industrial Chemical/Toxic Industrial Material (TIC/TIM) Task Force (Force, 2009), which focuses on inhalation hazards in an operational environment and provides a list of compounds prioritized based on toxic hazard and the likelihood of an encounter.
Section snippets
Reagents
3-Aminopropyltrimethoxysilane (APS), 3-isocyanatopropyltriethoxysilane (ICS), and 1,2-bis(trimethoxysilyl)ethane (BTE) were obtained from Gelest, Inc. (Tullytown, PA). Pluronic®P123 was generously donated by BASF. Dichloromethane (≥99.5%), magnesium turnings (98%), and mesitylene (1,3,5-trimethylbenzene or TMB) were purchased from Sigma-Aldrich (St. Louis, MO). All chemicals were used as received. Water was deionized to 18.2 MΩ cm using a Millipore Milli Q UV-Plus water purification system.
Materials synthesis and grafting
Our
Materials characterization
Nitrogen adsorption–desorption characterization indicates that E50 had an average BET surface area of 1096 m2/g and total pore volume of 1.04 cm3/g. The isotherm is type IV-like in shape, characteristic of mesoporous materials, but with the capillary condensation step occurring at high relative pressures (P/P0 0.5–0.99) and a pronounced H3- or H4-type hysteresis between the adsorption and desorption branches, which suggests a complex pore structure (Fig. 1) (Kruk and Jaroniec, 2001). A wide pore
Conclusions
The study presented here demonstrated the potential of the organosilicate sorbents in irritant gas capture and the impact of functionalization of the sorbents. We have shown that it is possible to significantly alter interactions between irritant gases and the sorbents through modification of the surface chemistry via post-synthesis grafting. Other materials studied for air purification applications have found improved performance upon inclusion of metal ions and functional moieties (Bae et
Acknowledgements
The authors would like to thank Dr. A.P. Malanoski for his invaluable insights. This research was sponsored by the U.S. Defense Threat Reduction Agency's Chemical & Biological Technologies Directorate (DTRA-CB) Physical Science & Technology Division under the topic Protection and Hazard Mitigation (BA08PRO015). We applied the SDC approach (“sequence-determines-credit”) for determining the sequence of authors (Tscharntke et al., 2007). The views expressed here are those of the authors and do not
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