Study On The Triethylenetetramine Based Adsorbents: Preparation, Characterization And Applications In CO₂ Capture

CO₂ adsorption, one of important technologies for CO₂ capture and separation, is reciving great attention because of the big adsorption capacity, fast adsorption rate and less corrosion to adsorption equipment. Adsorbent is the core part of CO 2 adsorption. So far, many solid adsorbents have been developed, such as lithium-based salts, metal oxides, molecular sieves, carbon and metal-organic frameworks（MOFs） materials. The former two types of materials exhibit great adsorption capacity at high tempera ture. However, CO₂ desorption temperature is too high for the regeneration of adsorbents. Other types of adsorbents show good adsorption-desorption and regeneration performance at low temperature. In addition, it is possible to improve their adsorption per formance by modifying the solid materials with organic-amines. Despite this, most of organic-amine functionalized porous materials cannot achieve good adsorption performance in a low CO 2 concentration. Consequently, development of solid adsorbents with efficient adsorption performance at a low CO₂ concentration is still a great issue in CO₂ adsorption.Considering that the properties of solid adsorbents can be easily modified and organicamines functionalized solid adsorbents exhibit good CO₂ adsorption performance, a series of triethylenetetramine（TETA） based solid adsorbents were synthesized in the present dissertation. The physical and chemical properties of the as-synthesized adsorbents, such as the structure and composition, were characterized by various techniques. CO₂ adsorption performance was evaluated by exposing the adsorbents into a stream of 5% CO 2 and the relationship between strucuture and adsorption performance was invesitigated. Listed below are the results:1) ZrO₂, a mesoporous metal oxide with Lewis acid sites, was prepared by hydrothermal synthesis method using polyethylene glycol as surfactant. ZrO 2-TETA-n was synthesized by functionalizing ZrO₂ with triethylenetetramine through the interaction between N and Zr species. After modification, the content of amino-groups was increased and the number of basic sites was enhanced, leading to the improvement of CO₂ adsorption capacity. In addition, the capacity and weight of the adsorbent were stable across a test of 10 cycles of adsorption-desorption procedure. Based on the results of characterization and CO₂ adsorption, a possible mechanism for the enhancement of CO₂ adsorption capacity was proposed: on the surface of ZrO₂-TETA-n, N species originating from triethylenetetramine reacted with CO₂ molecules to form carbamates, leading to the improvement of CO₂ adsorption capacity.2) Mesoporous double-metal complexes of Fe-Zr was first synthesized with ZrOCl₂·8H2O and K4Fe（CN）6·3H2O as raw materials. Then, the adsorbent of Fe-Zr-TETAn was synthesized by introducing triethylenetetramine into the surface of Fe-Zr through the interaction between N and Zr species. The possible structures of Fe-Zr and Fe-Zr-TETA-n were proposed on the basis of the literatures and the characterization results of the materials. After modification, CO₂ adsorption capacity was enhanced as a result of the interaction between CO₂ and amino-groups. The maximum capacity of 3.31 mmol·g-1 was achieved at 75 oC and 10 cm3·min-1 and the capacity was stable across 10 runs of adsorption-desorption process. This is a facile and new method for the preparation of organic-amine functionalized adsorbent which shows good adsorption performance in low CO 2 concentration.3) Zr-MCM-41, a mesorporous molecular sieve with Lewis acid sites, was synthesized by doping Zr into MCM-41 using hydrothermal synthesis method with cetyltrimethylammonium bromide（CTAB） as a surfactant. Zr-MCM-41-TETA-n was synthesized by introducing triethylenetetramine to the surface of Zr-MCM-41 through the interaction between N and Zr species. By doping Zr, the acid content was improved, which increased the triethylenetetramine content and thus led to the enhancement of CO₂ adsorption capacity. On the surface of Zr-MCM-41-TETA-n, CO₂ molecules reacted with amine-groups to form carbamates, leading to the enhancement of CO₂ adsorption capacity. The maximum capacity of 3.36 mmol·g-1 was achieved at 50 oC and 5 cm3·min-1. In addition, the capacity and the mass of spent sample were stable across 10 times of cyclic adsorption-desorption process.4) Mesoporous Zr-SBA（P） was synthesized by doping Zr into SBA-15 via hydrothermal synthesis method with polyethylene-ploypropylene glycol（P123） as a surfactant. In addition, Zr-SBA（C） was prepared from Zr-SBA（P） by removing P123 into the pore channels. Both of Zr-SBA（P） and Zr-SBA（C） were modified with TETA（ZrSBA（P）-n and Zr-SBA（C）-n） and tested in CO₂ adsorption. Compared with Zr-SBA（P） and Zr-SBA（C）, Zr-SBA（P）-n and Zr-SBA（C）-n exhibited great adsorption capacities on account of the interaction between amine-groups and CO₂ molecules. In addition, The maximum capacity of Zr-SBA（P）-n（4.27 mmol·g-1） was higher than that of Zr-SBA（C）-n at 50 oC and 5 cm3·min-1. This could be ascribed to the existence of hydroxyl in P123, which was able to participate in the adsorption procedure. In the adsorption, the molar ration of CO₂/amine-group was enhanced because of the presence of P123, leading to the further enhancement of adsorption capacity.5) Nonporous materials of Zr-TETA-n were prepared by one-step facile synthesized method only using ZrOCl₂·8H2O and TETA as raw materials. Zr-TETA-n exhibited better thermal stability and CO₂ adsorption performance in comparison with ZrO₂-TETA-n、FeZr-TETA-n、Zr-MCM-41-TETA-n and Zr-SBA（P）-n. The adsorbent of Zr-TETA-n was stable even at 270 oC. In the adsorption, Zr-TETA-n exhibited high amine utilization and adsorption capacity. Interestingly, the adsorption temperature had little effect on the adsorption capacity. The maximum capacity of 4.71 mmol·g-1 and amine utilization of 81.6% were achieved at 75 oC and a flow rate of 10 cm3·min-1. In addition, there was no apparent loss of adsorption capacity across 10 runs of adsorption-desorption process. The adsorbents would find promising applications in CO₂ capture due to the simply preparation procedure, short preparation time, cheap and available raw materials and the high adsorption capacity in a low CO₂ concentration.