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Investigation of Adsorption Process on Zeolites and Modified Zeolites by Frequency-Response Method

Author: LiFeiFei
Tutor: SunZhaoLin
School: Lanzhou University
Course: Analytical Chemistry
Keywords: Adsorption Mordenite Y zeolites Adsorption sites Adsorption mechanism
CLC: TQ028.15
Type: PhD thesis
Year: 2008
Downloads: 213
Quote: 1
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Abstract

Selective adsorption is a kind of the most promising methods because energy and capital savings and environment favorable, as much attention has been paid to ecotypic development. Selective adsorption is mainly used in desulfurization and separation of alkene. Success in this technique would, however, depend on the development of a highly selective sorbent with a high selective adsorption capacity to sorbate. So it is necessary for the design and improvement of sorbent to investigate the sorption mechanism and the influence factor to sorption process.Many previous studies on adsorption desulfurization and alkene separation were reviewed in detail, and the following originally innovative researches applying a new study system to investigate the adsorptions of thiophene and ethene on zeolites were carried out:1. Hmordenite was modified by Cs⁺ using ion-exchange method and CuO using impregnation method. The modified Hmordenite was used as the sorbent to adsorb ethene. It was firstly found that the adsorption ability of CuO/HMor and Cs/HMor to ethene can be enhanced by the introduction of proper quantity of CuO and Cs⁺, respectively, even though the diameter of CuO and Cs+ are large. This result laid a foundation of theory and experiment for the development of mordenite sorbent to separate ethene.NaY was firstly modified using two ion-exchange and two calcinations method which was heated by microwave. The modified NaY including Ce（Ⅲ）Y, Ce（Ⅳ）Y, CuY, AgY, KY, MgY and SrY were applied to adsorb thiophene and benzene. Ce（Ⅳ）Y is the most highly selective sorbent with high adsorption capacity than other sorbents. As the application of microwave to modification process increased the ion exchange degree of Y zeolites, so the sorbent with high selectivity and capacity of thiophene can be got.2. A new investigation method was established by combing the frequency response （FR） method, isotherms, Yasuda model and Langmuir equations. Then the adsorption processes of ethene in HMor、CuO/HMor and Cs/HMor were investigated by this method, meanwhile the adsorption processes of thiophene and benzene in Y zeolites were also studied by this way. This method is effective to study two parallel adsorption processes in one system, then the dynamic information and parameters of each process can be got.3. The method above has been applied to study adsorption of ethene in HMor, CuO/HMor and Cs/HMor. The dynamic information and sorption sites of each adsorption process are firstly achieved. The results as follows:（1） The adsorption of ethene in low frequency and in high frequency is detected and distinguished by FR method. At 252K, the adsorption sites in low frequency（SiOH） available to ethene is 0.692mmol/g and the adsorption sites in high frequency （H⁺） is 0.828mmol/g.（2） In CuO/HMor, low frequency adsorption sites which adsorbed ethene changed to be SiOH and Si-O-Cu, while high frequency adsorption sites are still H⁺. Ethene is mainly adsorbed by forming p-complex with low frequency sites in CuO/HMor.When the content of CuO is 5%, the number of low frequency adsorption sites increases to 1.5110 mmol/g, but the number of high frequency adsorption sites decreases to 0.223 mmol/g. When the content of CuO is more than 5%, the CuO molecular began to distribute in the surface of HMor channel, which hinder the diffusion of ethene and cause the number of low frequency sites begin to decrease.（3） In Cs/HMor, low frequency adsorption sites is still SiOH, while high frequency adsorption sites changes from H⁺ to be H⁺, Cs⁺ and Si-O-Cs. Ethene are mainly adsorbed by p electron interaction with high frequency sites in Cs/HMor.It is found that the hydrogen cation of SiOH has been exchanged by Cs⁺, which causes the high frequency adsorption site SiOH changing to be low frequency adsorption site Si-O-Cs. The introduction of Cs cation to the HMor channel also cause more H⁺ cation showing its adsorptive activity to ethene.The number of low frequency adsorption sites decreases to 0.220 mmol/g and the number of high frequency adsorption sites increases to 2.110 mmol/g, with the content of Cs cation increasing in Cs/HMor.4. The method also has been applied to study adsorption processes of thiophene and benzene on NaY zeolite and CeY zeolite, respectively. The dynamic information and sorption sites of each adsorption process are firstly achieved. The results as follows:（1） The mass transfer mechanism of thiophene in Y zeolites is different greatly from that of benzene in Y zeolites, which has been proved by experiment method the first time. In NaY, Ce（Ⅲ）Y and Ce（Ⅳ）Y, adsorption of thiophene in the supercage is found to be the rate controlling step but diffusion of benzene in the supercage is the rate controlling step.（2） In NaY supercage, adsorption of thiophene by p-electronic interaction reaches its balance earlier and adsorption of thiophen via pore-filling mechanism reaches its balance later. At 373K, 0.67 thiophene molecules are adsorbed by p-electronic interaction and 4.50 thiophene molecules are adsorbed via pore-filling mechanism in each supercage.The saturation loading of thiophene is higher than that of benzene in NaY, which is attributed to the p-electronic interaction between thiophene Na⁺.（3） In Ce（Ⅲ）Y and Ce（Ⅳ）Y, a few of thiophene molecules are adsorbed by forming p-complexation with Ce³⁺ and reach its adsorption balance quickly; meanwhile, a lot of thiophene molecules are adsorbed by forming S-Ce⁴⁺ bond with Ce⁴⁺ and reach its adsorption balance slowly. At 373K, 0.5 thiophene molecules is adsorbed by p-complexation and 4.8 thiophene molecules are adsorbed via S-Ce⁴⁺ bond in each Ce（Ⅲ）Y supercage; 0.1 thiophene molecules is adsorbed by p-complexation and 5.35 thiophene molecules are adsorbed via S-Ce⁴⁺ bond in each Ce（Ⅳ）Y supercage. Ce（Ⅲ）Y and Ce（Ⅳ）Y show high selectivity to thiophene, owing to the diffusion process of sorbate is hindered in space by the exchange of Ce³⁺ and Ce⁴⁺ to the supercage.These investigations above give a good theoretical demonstration of design of selective sorbent and adsorption processes for desulfurization and separation.

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CLC: > Industrial Technology > Chemical Industry > General issues > Chemical processes ( physical processes and physical and chemical processes ) > Separation process > Single- phase system the separation of gas mixtures > Adsorption and desorption
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