Porous solids for low temperature CO2 adsorption

Carbon dioxide (CO2) is seen as one of the major global contributors to global warming. CO2 is produced naturally in the environment, however, with the advent of climate change, CO2 emissions from manmade sources has come under scrutiny as a driver of manmade climate change. Combustion of fossil fuels and related practices is seen as the primary source of such emissions. Current CO2 capture technologies tend to be limited by cost and effectiveness. The basis of this project was to design a novel adsorbent material capable of removing significant levels of CO2 from gaseous streams. As a fundamental requirement, the developed adsorbent material was synthesised using economically viable materials, such as amine modified microporous zeolites which have not been extensively studied to date compared to amine modified mesoporous solids such as SBA-15. This study looks primarily at CO2 adsorption from low temperature exhaust streams with two primary factors of interest; CO2 adsorption capacity and regeneration energy requirements. Various microporous solids, including zeolite-Y and zeolite- , were chosen with the view of examining their potential as supports for liquid amine. A number of techniques were employed for surface modification and subsequently tested using a gas adsorption rig with online mass spectrometry. The most successful technique for synthesising amine modified adsorption, which showed preferable properties in terms of CO2 adsorption and regeneration, was chosen for further testing. The solid which performed best during CO2 adsorption/desorption testing was aminopropyltriethoxysilane (APTES) modified zeolite - 25, prepared using a amine solution impregnation technique whereby APTES was dissolved in toluene, zeolite- was then added to the mixture and subsequently heat treated. This solid was found to have a CO2 adsorption capacity of 216 mg CO2 g-1 solid adsorbent. Other factors investigated included the effects of silica-alumina ratios on amine functionalisation, the effects of surface pre-treatments, such as acid treatment, in boosting surface modification and finally, the use of alternative amines to modify microporous solids. These tests showed that with decreasing silica/alumina ratio the adsorption capacity of each solid similarly decreased. Dilute acid pre-treatment was found to boost surface hydroxyl groups for subsequent amine bonding in some samples. The weight percentage amine loading of solid supports was also found to be an important factor as too much amine was found to be detrimental to the CO2 adsorption properties. A number of amine modified mesoporous materials were also prepared for comparative purposes. APTES, polyethyleneimine (PEI) and tetraethylenepentamine (TEPA) were used to modify a number of MCM-type mesoporous silicas. MCM-41 and porous silica spheres were synthesised for this investigation. The TEPA modified MCM-41 samples produced an adsorbent with the most favourable CO2 adsorption/desorption properties. The adsorption capacities obtained were up to 196 mg CO2 g-1 for modified MCM-41 with desorption occurring at temperatures as low as 75°C. Co-condensed silicas were prepared by incorporating aminosilanes into the synthesis mixtures in order to synthesise MCM-type materials with a homogeneous distribution of functional amines on the solids surface. CO2 TPD studies showed that the adsorption capacity of the 15% TRI amino silane co-condensed spheres was found to be 65 mg CO2 g-1. This sample was subsequently exposed to secondary post synthesis modification. This sample achieved a CO2 adsorption capacity of 211.3 mg CO2 g-1 sorbent. The amine cocondensed solids showed promising results as both supports and adsorbents for CO2 capture. Solid characterisation was carried out using N2 adsorption isotherm analysis to establish the surface area and pore sizes. The morphologies of the silica supports were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The organic contents of solid adsorbents were examined using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and C.H.N. elemental analysis. The two most adsorbent samples were examined using CO2 adsorption isotherms and kinetics studies. These were used to show the effects of flue gas parameters on the CO2 adsorption properties of APTES and TEPA modified solid supports. CO2 partial pressure, exhaust temperature and exhaust constituents such as moisture and other flue gases can impact on the CO2 adsorption characteristics of the amine modified adsorbent. The adsorption isotherms for each sample follow the assumptions of the Langmuir model which suggests that there are a fixed amount of adsorption sites which CO2 can interact with during the adsorption process. Qst values of 131 kJ mol-1 and 161 kJ mol-1 were obtained for 40% TEPA/MCM41(R)EtOH and APTES/N -25(WI) Tol, respectively. The findings of this thesis clearly demonstrate that amine modified zeolites show significant potential as an alternative adsorbent in the removal of CO2 from flue gas streams when compared with more expensive mesoporous materials.