Novel freeze-dried amine functionalised 2D/3D assemblies of graphene and mesoporous silica for CO2 capture

Abstract

The anthropogenic emission of carbon dioxide (CO2) contributes adversely to global warming and climate change. Since fossil fuels will still be widely and continuously utilized as the dominant energy source, carbon capture and storage (CCS) is an acknowledged solution for the reduction of CO2 emission in the short to medium term. While there are several conventional sorbent technologies to capture CO2, many of these technologies require improvements for CCS deployment. In other words, there is a pressing need to develop advanced solid sorbents to meet the requirements of real-world application.This thesis focuses on novel advanced amine functionalised sorbents for CO2 capture. There are three main hypotheses associated with four contributions to knowledge in this thesis. Firstly, it is postulated that the freeze-drying method avoids the common problem of structural collapse as in conventional drying methods of sorbents derived from mesoporous silica. Secondly, it is postulated that novel 2D/3D assemblies containing 2D graphene and 3D mesoporous silica have morphological features that can confer higher CO2 sorption capacity. The last postulation focuses on a one-pot facile method to synthesise 2D/3D assemblies containing 2D graphene and 3D templated silica aerogel where total pore volumes are improved by the molecular cavity from the template’s carbon chain.The first contribution of this thesis shows proof that the freeze-drying method was effective in retaining the mesoporous silica structure during aqueous amine impregnation. This was attributed to the freeze-drying conditions which hindered the reaction of liquid water with silanol groups, avoiding silica surface de-hydroxylation and structural collapse. As the mesoporous structure of silica was preserved, the freeze-drying method resulted in higher total pore volume, thus accommodating larger amounts of amine. Consequently, the freeze-dried sorbents delivered significantly higher CO2 sorption capacities up to a factor of 18 times as compared to analogous sorbents prepared by conventional evaporation methods.The second contribution of this thesis is directly related to the successful preparation of novel 2D/3D assemblies containing 2D graphene and 3D mesoporous silica. The unique morphological features of the novel 2D/3D assemblies allowed for the intercalation of 2D graphene sheets within the 3D rod-like SBA-15 mesoporous silica. Consequently, the overall pore volume of the 2D/3D assemblies increased as compared to the pure 3D mesoporous silica, accommodating a higher content of amine and resulting in 51 % increase of CO2 sorption capacity.The third contribution of this thesis is associated with the development of 2D/3D assemblies by a two-step method containing 2D/3D graphene silica aerogel and 3D SBA-15 mesoporous silica. The 2D/3D graphene silica aerogel formed flake-like particles which allowed their intercalation within the 3D rod-like SBA-15. This type of 2D/3D assembly resulted in high total porosity derived from those in both mesoporous and aerogel silica, and compounded with the inter-particle spacing. Upon amine functionalisation, the CO2 sorption capacity reached 6.02 mmol g-1, one of the highest values reported in literature.The final and fourth contribution of this thesis is the preparation of 2D/3D assemblies by a single-pot facile synthesis. In this synthesis, graphene sheets were enveloped by templated silica aerogel, thus forming 2D/3D structures. Upon burning off the templates, molecular imprinting cavities were formed in the silica aerogel matrix, which increased with the length of the carbon chain of the template used. As a result, the surface area and total pore volume increased significantly by over 80 % as compared to the samples without templates. This increase was benefial to accommodate more amine, and leading to CO2 sorption capacity of 4.9 mmol g-1, equivalent to 11.67 mmol cm-3. The latter is almost one order of magnitude higher than the best results reported for amine functionalised silica aerogels.In all above contributions, the CO2 sorption of the resultant sorbents increased in some cases significantly as compared to analogous sorbents. Concomitantly, the amine efficiency increased, whilst the cyclic sorption/desorption cycles showed that the amine functionalised mesoporous silica and 2D/3D assemblies were stable. Further, the heat of sorption reduced owing to the weak sorption (physisorption) of CO2 on graphene sheets, whilst mass transfer limitations for CO2 to access the amine groups were reduced. All these attributes are desirable for the engineering deployment of this technology to capture CO2 and avert climate change.