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Synthesis of ionic liquid-SBA-15 composite materials and their application for SO2 capture from flue gas Lei Zhang, Lirong Xiao, Yanke Zhang, Liam John France, Yinghao Yu, Jinxing Long, Dawei Guo, and Xuehui Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02946 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017
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Energy & Fuels
Synthesis of ionic liquid-SBA-15 composite materials and their application for SO2 capture from flue gas Lei Zhang,a Lirong Xiao,a Yanke Zhang,a Liam John France,*a Yinghao Yu,a Jinxing Long,a Dawei Guob and Xuehui Li*a a
School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper
Engineering, South China University of Technology, Guangzhou 510641, China. b
Research Institute of Petroleum Processing Sinopec, Beijing 100083, P. R. China.
Abstract A series of 1,3-bispropyltriethoxysilane-imidazolium chloride (FILs)-modified SBA-15 adsorbents have been prepared, characterized and applied in SO2 capture for the first time. At low FILs loadings significant levels of grafting was observed, while higher loading levels resulted in retention of a greater fraction of FILs precursors, as evidenced by FTIR spectroscopy. Textural properties gradually declined as FILs content increased, in conjunction with an apparent change in the regular nature of the SBA-15 pore structure and bulk particle morphology, as demonstrated by XRD, TEM and SEM. SO2 adsorption breakthrough curves indicated that all materials possessed rapid and facile adsorption properties, with t0.9 values 1 to 2 orders of magnitude lower than those obtained over comparable materials under more favorable conditions. The largest total SO2 adsorption was achieved over 10%FILs@SBA-15, which exhibited a maximized FILs adsorption contribution which remained constant at higher loadings. Considering the importance of surface area, corrections demonstrated that a continual increase in adsorption per surface area unit was apparent, verifying the importance of the change in nature of the FILs at higher loadings. The optimized adsorbent demonstrated few strong SO2 binding sites, as indicated by excellent stability during adsorption-desorption cycles. Keywords： ：Functionalized ionic liquids, Sol-gel process, SBA-15, Flue gas, SO2 capture 1 Introduction The generation of NOx, SOx and CO2, from the combustion of fossil fuels has led to significant efforts from oil and coal industries to reduce these emissions, where possible, due to more stringent 1
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environmental regulations. The latter (CO2) receiving much attention due to its increasing atmospheric concentration caused by human activities. To combat this issue, two approaches may be utilized, either the carbon cycle must become closed in a sustainable manner, where CO2 is cycled back to fuels and commodity chemicals, or it must be removed entirely, leaving the cycle open.1 At present, the removal and sequestration of CO2 from flue gases is seen as the most obvious choice to solve the associated environmental issues. But, the presence of SO2 in the flue gas has a detrimental effect on this technology, in part, due to its preferential adsorption, it is also a significant pollutant in its own right, causing serious environmental and health related issues.2 Traditional flue gas desulfurization (FGD) technologies have focused upon employing limestone or liquid adsorbents, in particular, the use of numerous amine-based solvents and physical solvents have been found to be particularly efficient.3,4 However, due to the relatively high GHSV values and vapor pressure of organic liquids, losses are inevitable in power plant and FCC flue gas processing. Some amine-based adsorbents are not thermally stable and may degrade during regeneration processes, whereas others may form thermally stable sulfate species,4,5 which are difficult to separate. The largest detrimental factor is the relative toxicity of many of these compounds and their resulting degradation products, which may be released in the “treated” flue gas. Several other FGD technologies have been developed; semidry, dry, ammonium and electron beam. While they each offer unique advantages, they inherently have their own issues, such as, formation of low value byproducts, additional pollution, low desulphurization efficiencies and high cost of operation.5-8 Therefore, novel FGD technology should be highly efficient, exhibit an excellent energy balance and avoid significant byproduct formation. Ionic Liquids (ILs) are considered a promising class of materials, which possess a number of special physical and chemical properties; low vapor pressure, high thermal/chemical stability (depending upon IL choice) and excellent solvating power.9,10 In the last few decades they have received significant attention from researchers in a number of fields, in particular, gas separation and capture technologies.11,12 Four major IL classes can be identified from literature sources as suitable for the adsorption of SO2; guanidinium-, hydroxyl ammonium-, and imidazolium-based ILs.13-16 The latter of which, has been demonstrated to possess significant adsorption capacity and reasonable thermal stability.16,17 While encouraging results have been realized with the use of bulk ILs, their poor mass/heat transfer characteristics, low surface area and associated cost make their 2
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industrial use challenging.18,19 In an effort to resolve these issues a number of studies have been devoted to the preparation and use of supported ILs. Zhao et al. found that supporting ether functionalized imidazolium-based ILs on porous silica gel led to a significant improvement of the composites SO2 adsorption properties compared to the bulk.20 Severa and co-workers demonstrated that some activated carbon supported ILs lose their high SO2 removal performance in the presence of humid air, whereas, [C2mim][Ac] does not.21 Li et al. showed that the bulk IL, tetramethylguanidinium lactate, was unsuitable as a bulk SO2 adsorber due to its hyper viscosity, but supporting it on MCM-41 led to a superior adsorbent than either individual material.22 Perdikaki et al. found that the grafting of the silylated IL [SPMIM][PF6-] did not considerably influence its properties or affinity towards SO2.23 Previously, we have synthesized [CnMIM]Br ILs immobilized in NaY super cages, by employing the “ship-in-a-bottle” approach. The resulting adsorbent was found to possess superior thermal stability and improved cyclic CO2 adsorption-desorption performance compared to the bulk ILs.18 [APMIM]Br-NaY prepared by the same approach was also found to be a suitable material for CO2 adsorption. It was determined to possess both physisorbed and chemisorbed ILs, while exhibiting suitable cycling stability at 200 oC.19 Xu and co-workers demonstrated that the immobilization of TMG (used as a template) in hierarchical AlPO-5/cordierite honeycomb gave a material which exhibited significantly enhanced rates of adsorption with negligible pressure drop, compared to more traditional supported systems.24 Karousos and co-workers compared physically bound and chemically grafted acetate ILs on activated carbon supports, they found that regardless of the immobilization route the resulting materials exhibited low thermal stability. The best performing system was found for [Tf2N]- SILPs, which exhibited some cyclic behavior, albeit with reduced performance. Interestingly, chemical grafting gave significant improvement in the selectivity of SO2 adsorption compared to CO2 with approximately 60% less IL.25 Clearly there is significant potential for the application of FILs-framework composites, however, some grafted FILs appear to suffer stability issues under cyclic operation, while long adsorption times are generally found for other systems.20,22,25 On the basis of our own previous work with CO2 adsorption18,19 and the substantial body of literature describing the potential of imidazolium-based ILs,14,17,20,25 it was decided that this initial study would focus on a relatively simple imidazolium-based ILs with identical C3 side chain lengths. The framework (SBA-15) choice was 3
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made based upon numerous different factors, namely, the compatibility of synthesis conditions with the FILs precursor, in addition to the successful predictive modelling of the pore structures adsorption contribution for acidic gases.26 A series of FILs-SBA-15 composite materials were prepared and characterized for the first time. Key differences between materials allowed for the rationalization of an apparent enhancement effect for 10%FILs@SBA-15. While cyclic stability was found to be significant, as evidenced by the minimal loss in adsorption capacity even after 9 desorption cycles at 200 oC. Relatively small (
