CO2 sponge from plasma enhanced seeded growth of metal organic frameworks across carbon nanotube bucky-papers
Introduction
Adsorption onto solid sorbents hold promise for carbon dioxide (CO2) capture due to their high efficiency, low energy requirements, and often ease of regeneration [1]. However, most of the currently used adsorbents such as molecular sieves or zeolites, activated carbons [2], silica gel [3], and hydrotalcites [4] have relatively low adsorption capacities (0.3–4.4 mmol CO2/g) for CO2 at atmospheric pressure and require high operating pressures and temperature [1]. Therefore, alternative adsorbents, performing at low temperature and pressure needs to be considered.
Metal organic frameworks (MOF) [5] have recently drawn great attention mainly for their high surface area and tailorable pore structure, and are suitable for the capture, release, permeation, storage, or sensing [6] of gas molecules. The presence of open metal sites across MOF crystals enables for selective adsorption of target gases such as CO2 [7], and offers very specific functionalities and properties relevant to carbon capture. MOF-74 series were shown to offer strong adsorption sites for CO2 [8], [9], [10] with the adsorption capacity of 26 and 30 wt% for Ni-MOF-74 and Mg-MOF-74 at 423 K, respectively. The Mg-MOF has a higher heat of adsorption compared to MOFs of other metal ions and take up more CO2 molecules per unit cell than its analogues [8]. This behavior was attributed to the smaller molecular size of Mg that increases the ionic character of Mg-O bond between unsaturated Mg2+ ions and oxygen atoms in CO2 molecule [9].
Micron-sized MOF crystals formed from traditional solvothermal MOF synthesis reaction are not necessarily best fitted for separation/adsorption applications due to difficulty to control their growth into shapes other than their particulate form [11]. The development of MOF thin film materials where MOF crystals are intimately interconnected to form defect-free membranes has therefore been extensively investigated [12], [13]. A promising method is to embed MOF crystals across a polymer membrane to form mixed matrix membranes (MMM). MOF-based MMMs currently outperform pure MOF from economic point of view, since only a small amount of MOF is required to cover the support and no expensive support is necessarily needed for MOF-based MMM [14]. A variety of polymer materials such as electrospun fiber mats including poly(acrylonitrile) (PAN) [15], polypropylene fiber mats [16] have been applied for supporting MOF crystals. MOF-crystals on polymer fiber matrices have been integrated through different methods including encapsulation in electrospun fibers [17] or immobilization on fiber through solvothermal synthesis [18]. Unfortunately these methods resulted in small MOF loading fraction and poor MOF crystal quality [16]. Also, an important challenge that needs to be addressed is interface compatibility between the MOF and the polymer [19], which results in physical agglomeration of MOFs upon manufacturing. Therefore, new strategies to develop continuous MOF membranes supported onto nano-porous materials have been investigated. Additionally, the lack of chemical diversity of these materials has opened the route to the design of more chemically versatile platforms.
Carbon nanotubes (CNTs) recently attracted great attention as growth platform materials for their high chemical stability, tuneable chemistry, and excellent mechanical strength [20], [21]. A hybrid MIL-101/MWCNT has been fabricated [22] where the crystal structure and morphology was identical to that of virgin MIL-101 but CO2 adsorption capacity was increased from 0.84 to 1.35 mmol/g at 298 K and 10 bar. An improvement in H2 storage was achieved by incorporating MOF-5 into Pt-loaded multi-walled carbon nanotubes (MWCNTs), at both 77 K and 298 K, and over a wide range of pressure [23]. ZIF-8 seeded across CNT Bucky-paper (BP) supports were also produced via a secondary growth method. The mixed gas permeation performance of synthesized hybrid metal organic framework membranes towards CO2, and N2 revealed high selectivity of N2 over CO2 (Selectivity = 33) [24]. In situ growth of NH2-MIL-101(Al) on the external surface of CNTs, introduced amino groups and active sites for selective separation of CO2 from CH4 (Selectivity = 25.4) [25]. The developed CNT − MOF composites can also be promising for application in adsorption such as CO2 capture. The major limitation with the incorporation of CNTs across any composite material relates to the control of the interface between the different phases which may lead to poor heterogeneous nucleation sites for MOFs to grow on the surface of bare substrate [26], [27], [28]. Specifically, when considering the growth of nanostructures from the surface of CNTs, the surface-active sites should be optimized to ensure appropriate surface coverage density. Different methods, including chemical reaction in solution, exposure to radiative sources such as x-ray, gamma-ray or ultra violet-ozone and plasma have been developed to introduce functional groups on the surface of CNT [29], [30], [31]. Exposure to strong acids, such as nitric acid (HNO3) and sulfuric acid (H2SO4) [32], [33], ozone oxidation [34], [35] and plasma oxidation have been investigated as routes to anchor oxygenated groups such as hydroxyl or carboxylic groups across the surface on CNTs. Although chemical treatments are by far the most effective route, wet-oxidation was shown to lead to loss in crystallinity and damage of the CNT walls [36]. Plasma gas treatments, on the other hand, are versatile routes to generate specific functional group densities and do not affect the bulk properties of materials [36]. The plasma treatment process has been well studied since the 1960s [37] and has quickly evolved into a valuable technique to engineer the surface properties of CNTs via appropriate plasma parameters such as type of gas, treatment time and input power [38]. Several research studies on plasma treatment of CNTs revealed the enhanced chemical performance and interfacial adhesion by embedding oxygen-containing functional groups on the surface of CNT [39], [40], [41].
The purpose of this study is to investigate the synthesis of a new class of porous materials working at low pressure and possessing high adsorption capacity for CO2 gas. The seeding and growth of a thin, active, and selective layer of Mg-MOF-74 across the surface of plasma functionalized CNT-BP was performed and both the synergistic impact of functional groups grafting through the plasma treatments and CO2 adsorption performance correlated to the MOF seeding density across the graphitic plans.
Section snippets
Chemicals and reactants
All chemicals in this study were purchased from Sigma Aldrich, including 2,5 dihydroxyterephtalic acid (98%), N,N, dimethylformamide (99.8% anhydrous), Ethanol (90% anhydrous), and Isopropanol (99.5%), but with the exception of Magnesium nitrate hexahydrate (98%) which was acquired from Fisher Scientific.
CNTs growth and preparation
The growth of the multiwall CNTs was performed by Chemical Vapor Deposition (CVD), as detailed elsewhere [42]. Fe2O3 used for catalysis was deposited on silicon wafers (100 mm dia.,
Carbon nanotube surface conditioning and functionalization
The functionalization of CNTs to introduce oxygen rich groups, such as hydroxyl or carboxylic species, are extremely valuable routes not only to improve the wettability of the graphitic structures, but also to create anchoring points for subsequent specific grafting. Various parameters, including the choice of feed gas [40], [49] and plasma treatment time were used to achieve specific levels of functionalization [41].
A concentration breakdown of specific atomic configurations and groups from
Conclusions and prospects
The engineering of MOF seeding across CNT-BPs was found to be largely related to surface functional groups generated by specific plasma gas treatments. A significant improvement in adsorption capacity of the MOF-CNT-BP samples was ascribed to the positive features deriving from compositing the parent materials. In this respect, growth of MOF on CNT-BP platforms was found to benefit their formation and functionality through number of ways including increased dispersive forces, preventing
Acknowledgement
LD acknowledges the Australian Research Council for his DECRA fellowship (DE180100130). The support of Prof. Lingxue Kong is also acknowledged.
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