A Frequency-Response Study on Sorption of Thiophene and Benzene on NiY Zeolite

Transcription

1 Scientific Research China Petroleum Processing and Petrochemical Technology 2011,Vol. 13, No. 2, pp June 30, 2011 A Frequency-Response Study on Sorption of Thiophene and Benzene on NiY Zeolite Lai Junling 1,2 ; Song Lijuan 1,2 ; Sun Zhaolin 1,2 (1. Colledge of Chemistry & Chemical Engineering, China University of Petroleum (East China), Dongying ; 2. Liaoning Key Laboratory of Petrochemical Engineering, Liaoning Shihua University) Abstract: The adsorption behavior of thiophene and benzene on NiY zeolite has been investigated by the frequency response (FR) method. The FR spectra of thiophene and benzene on NiY zeolite were recorded at K in a pressure range of Pa. The results showed that adsorption was found to be the rate-controlling process for the thiophene/ NiY zeolite system, and there were two different adsorption processes. Two kinds of adsorption models have been proposed, namely: the S-M interaction (low frequency adsorption) and the π-complexation (high frequency adsorption). High frequency adsorption obeyed the Langmuir model. By combining the FR method and Langmuir model, the adsorption site of high frequency adsorption process at 302 K and 335 K was mmol/g and mmol/g, respectively, and on the combined adsorption isotherms, the adsorption site of the low frequency adsorption process at 302 K and 335 K was mmol/g and mmol/g, respectively. The low frequency adsorption process (S-M interaction) was the main adsorption process. The diffusion process was the rate-controlling process for the benzene-niy zeolite system. Key words: frequency response; thiophene; benzene; NiY zeolite; adsorption site 1 Introduction Selective adsorption is one of the most promising ultradeep desulfurization methods realized at ambient temperature and pressure [1]. But current researches have shown that the development of a selective adsorbent with highly selective adsorption capacity for sulfur was the most challenging task of desulfurization. One of the decisive factors restricting the development of this kind of adsorbents was that the researchers could not reach a consensus on the mechanism of selective adsorption [2-11]. The frequency response (FR) method is a very powerful means for determining the intra-crystalline mass transfer of molecules through zeolite crystals. The FR method can distinguish multi-kinetic processes in the same system, and can delve into the rate-controlling process and the kinetics formation [12-14]. In this study, the NiY zeolite was prepared by liquid phase ion-exchange method, and the spectra of thiophene and benzene on NiY zeolite were investigated by the FR method. Then the FR spectra were analyzed and interpreted according to the adsorption isotherms and Langmuir rate equation, and the study on the adsorption mechanism of thiophene and benzene on NiY zeolite could establish a theoretical foundation to interpret the selective adsorption mechanism. 2 Experimental 2.1 Materials NaY zeolite sample (with the Si/Al ratio equating to 2.55) was provided by the Nankai University in Tianjin, China. Thiophene (more than 99% pure) and benzene (more than 99% pure) were obtained from the Sigma-Aldrich Co. Ltd. of the UK. 2.2 Sorbent preparation The liquid phase ion-exchange method was used. Under the atmospheric pressure, the NaY zeolite sample was mixed with a nickel nitrate solution (0.2 mol/l) according to a set ratio of solid/liquid, and then the mixture was put into a 250-mL flask, heated under reflux at 373 K for 8 h, then washed with the de-ionized water, and filtered. The sample was dried at 373 K for 12 h and calcined at 773 K for 4 h. The solid material was again subjected to ion- 24

2 Lai Junling, et al. A Frequency-Response Study on Sorption of Thiophene and Benzene on NiY Zeolite exchange with the nickel nitrate solution for two times according to the above-mentioned steps and finally the NiY zeolite sample was obtained. 2.3 FR method and theory The principles and technical details of the FR apparatus have been described previously [15-16]. An accurate amount of sorbent was carefully distributed in a plug of glass wool and the sample was then outgassed at a pressure of < 10-3 Pa. The temperature was raised to 623 K at a rate of 2 K per minute and the sample was activated for 5 h at that temperature, which was afterwards decreased to the measurement temperature. A dose of purified sorbate was brought into the sorption chamber at the chosen pressure. A perturbation of ±1% of the gasphase volume was applied. The perturbation frequency was changed from 0.01 Hz to 100 Hz. Experiments were carried out with and without sorbents under the same conditions. The phase lag and amplitude were measured, and the response function was derived by a Fourier transformation of the volume and pressure square-wave signals. The in-phase and out-of-phase functions were plotted against the frequency to generate the FR spectra. The in-phase function is: (1) The out-of-phase function is: (2) (3) where k j is an intensity parameter of the FR spectra, which is proportional to the gradient of the adsorption isotherm, and k -j is the time constant of the adsorption/ desorption process on the site j. After having combined with the Langmuir rate equation, the rate constants of the adsorption process (k (j) a) and of the desorption process (k (j) d) on j site can be obtained from the FR parameters using the formula (4). (4) If the gradient of log(k j /k -j ) vs log(k j ) is -2, the Langmuir rate equation for the adsorption process is valid. The adsorption sites available for j site (N (j) s) can be obtained from the formula (5). (5) 3 Results and Discussion 3.1 Adsorption of thiophene on NiY zeolite The FR spectra of thiophene adsorption on NiY at 302 K, 335 K and 373 K, respectively, are presented in Figure 1. The curves are combined by means of the fit of Yasuda Figure 1 The FR spectra of thiophene adsorption on NiY at 302 K, 335 K and 373 K in-phase curve; out-of-phase curve 25

3 China Petroleum Processing and Petrochemical Technology 2011,13(2):24-28 adsorption model. The in-phase curves and out-of-phase curves intersected at half the step height and at the maximum values of the out-of-phase curves in Figure 1, which suggested that the sorption process was a rate controlling process. There were two parallel adsorption processes (the low frequency adsorption and the high frequency adsorption), i. e.: two kinds of adsorption sites for the adsorption of TH on the NiY zeolite. It can be seen from Figure 1 that the time constant of low frequency adsorption k -1 was smaller than that of high frequency adsorption, k -2, the response intensity of low frequency adsorption was lower than that of high frequency adsorption, so the interaction between thiophene and the low frequency adsorption site was stronger than that of high frequency adsorption site. Two kinds of interaction models between thiophene and the NiY zeolite were ascribed to the S-M interaction and the π-complexation [17], respectively. Hence, the interaction model of the low frequency adsorption process and that of high frequency adsorption process might be the S-M interaction and the π-complexation, respectively. The lines of log(k j /k -j ) vs log(k -j ) at 302 K and 335 K are presented in Figure 2(a) and Figure 3(a), respectively. The results suggested that the gradient of log(k j /k -j ) vs log(k -j ) for the high frequency adsorption process was -2, denoting that the high frequency adsorption process obeyed the Langmuir rate model. Therefore the high frequency adsorption was a kind of single adsorption and the energy of adsorption center was balanced. But the low frequency adsorption process did not obey the Langmuir rate model. The lines of P e vs k -j are shown in Figure 2(b) and Figure 3(b) at 302 K and 335 K, respectively. By combining formula (4) with formula (5), the adsorption sites of high Figure 3 Fit of the FR parameters of high frequency adsorption process for thiophene-niy system at 335 K by Langmuir frequency adsorption process (Ns(2)) at 302 K and 335 K were mmol/g and mmol/g, respectively. Figure 4 shows the adsorption isotherm for thiophene adsorption on the NiY zeolite. According to the adsorption isotherm and adsorption sites of high frequency adsorption process, the adsorption sites of low frequency adsorption process (Ns(1)) at 302 K and 335 K were mmol/g and mmol/g, respectively. It can be learned that the low frequency adsorption process was the main adsorption process from the adsorption sites. That is, the S-M action was the main interaction model. The result of the FR method was consistent with that of the molecular simulation technique [17]. The two kinds of adsorption energy of thiophene adsorption on NiY zeolite were calculated by means of the molecular simulation technique. The adsorption energy ( kj/mol) of the S-M action model was stronger than that of π-complexation ( kj/mol), and therefore the S-M action model was regarded as the dominating action. Figure 2 Fit of the FR parameters of high frequency adsorption process for thiophene-niy zeolite system at 302 K by Langmuir equation Figure 4 The adsorption isotherm of thiophene adsorption on the NiY zeolite 302 K; 335 K; 373 K 26

4 Lai Junling, et al. A Frequency-Response Study on Sorption of Thiophene and Benzene on NiY Zeolite Figure 5 Benzene FR spectra on NiY zeolite at 133 Pa and 302 K, 335 K and 373 K, respectively 3.2 Benzene diffusion on NiY zeolite The FR spectra of benzene adsorption on the NiY zeolite at 133 Pa and different temperatures are presented in Figure 5. The symbols and are the experimental data points, with the curves being fitted via the single intracrystalline diffusion model. It can be seen from Figure 5 that the high frequency end of the out-of-phase curve approaches the high-frequency tail of the in-phase curve asymptotically. The diffusion process was the rate controlling process for the benzene-niy zeolite system, and there was only one diffusion process with the diffusivity equating to about 10-9 m 2 /s. The rate of benzene adsorption on the NiY zeolite was faster than that of benzene diffusion on the NiY zeolite. The interaction between benzene molecules and NiY zeolite was only ascribed to the π-complexation. The interaction between benzene molecules and NiY zeolite (π-complexation) was weaker than that between thiophene molecules and NiY zeolite (S-M action and π-complexation), which can be learned from the FR spectra. The pore dimension of the supercage of Y zeolite is 0.74 nm 0.74 nm, and the dynamic diameter of a benzene molecule is nm, which means that the pore diameter of the supercage is close to that of the benzene molecule. So benzene molecule can diffuse into the intracrystalline space of cages. 4 Conclusions The behavior of thiophene and benzene adsorption on the NiY zeolite was studied by the FR method. The ratecontrolling processes of thiophene-niy zeolite system and benzene-niy zeolite system were ascribed to the adsorption process and the diffusion process, respectively. Different rate-controlling processes showed that the interaction between thiophene and NiY zeolite was stronger than that of between benzene and NiY zeolite. The two kinds of adsorption models for the thiophene- NiY zeolite system belonged to the π-complexation (high frequency adsorption process) and the S-M action (low frequency adsorption process), respectively. The high frequency adsorption process obeyed the Langmuir rate equation, and the low frequency adsorption process did not obey the Langmuir rate equation. The low frequency adsorption process was the main process. Benzene diffusion on the NiY zeolite was a single intracrystalline diffusion process. Acknowledgements: Thanks are expressed to the National Natural Science Foundation of China (NSFC. Contract No and No ) for financial support. Reference [1] Henamdeez-Maldonado A J, Yang R T. New sorbents for desulfurization of diesel fuels via π-complexation[j]. AIChE Journal, 2004, 50(4): [2] Ju Xiufang, Jin Lingling, Ma Tao, et al. Effects of 1,5-hexadiene and benzene on the desulfurization property of NiY zeolites [J]. Acta Physico-Chimica Sinica, 2009, 25(11): (in Chinese) [3] Wang Hongguo, Jiang Heng, Xu Jing, et al. Effects of benzene and 1-octene on desulfurization by selective adsorption with Ce(Ⅳ)Y zeolite [J]. Acta Physico-Chimica Sinica, 2008, 24(9): (in Chinese) [4] Zhou Danhong, Wang Yuqing, He Ning, et al. The π-complexation mechanisms of Cu(I), Ag(I)/zeolites for desulfurization[j]. Acta Physico-Chimica Sinica, 2006, 27

5 China Petroleum Processing and Petrochemical Technology 2011,13(2): (5): (in Chinese) [5] Cheng Zhilin, Liu Xuesong, Lu Jiqing, et al. Deep desulfurization of FCC gasoline by selective adsorption over nanosized zeolite-based adsorbents[j]. React Kinet Catal Lett, 2009, 97 (1): 1-6 [6] Duan Linhai, Meng Xiuhong, Shi Yan, et al. Selective adsorptive desulfurization performance and mechanism of sulfur compounds over modified Y zeolite[j]. Acta Petrolei Sinica (Petroleum Processing Section), 2009, 25(2): (in Chinese) [7] Hernandez-Maldonado A J, Yang R T. Desulfurization of diesel fuels by adsorption via π-complexation with vaporphase exchanged Cu(I)-Y zeolites[j]. J Am Chem Soc, 2004, 126(4): [8] Henámdez-Maldonado A, Yang R T. Desulfurization of liquid fuels by adsorption via π-complexation with Cu(I)-Y and Ag-Y zeolites[j]. Ind Eng Chem Res, 2003, 42(1): [9] Takahashi A, Yang F H, Yang R T. New sorbents for desulfurization by π-complexation: Thiophene/benzene adsorption[j]. Ind Eng Chem Res, 2002, 41(10): [10] Liu Dan, Gui Jianzhou, Liu Shi, et al. A density functional study of the chemsorptions of thiophene on Cu(I)Y zeolite [J]. Fuel Chem Divi Prep, 2006, 51(1): [11] Liu Dan, Song Lijuan, Gui Jianzhou, et al. Adsorption structures of heterocyclic sulfur compounds on Cu(I)Y zeolite: A first principle study [J]. Sur Sci and Catal Stud, 2007, 170: [12] Shen D, Rees L V C. Adsorption and diffusion of n-butane and 2-butyne in silicalite-1[j]. Zeolites, 1991, 11(7): [13] Shen D, Rees L V C. Chemical frequency response technique measurements of p-xylene diffusion in silicalite-1 and silicalite-2[j]. Chem Soc Faraday Trans, 1993, 89(7): [14] Song L, Rees L V C. Adsorption and transport of n-hexane in silicalite-1 by the frequency response technique [J]. Chem Soc Faraday Trans, 1997, 93(4): [15] Song Lijuan, Rees L V C. Diffusion of propane in theta-1 and silicalite-1 zeolites [J]. Microporous and Mesoporous Materials, 2000, 41(1/3): [16] Song Lijuan, Rees L V C. Frequency response diffusion of propane in silicalite-1[j]. Microporous Materials, 1996, 6(5/6): [17] Chen Xiaolu. DFT studies of selective adsorptive desulfurization of ultra-clean fuels [D]. Fushun: Liaoning Shihua University, 2009 (in Chinese) Commercial Technology for Manufacture of Coal-Based EG Developed by CAS Passed Acceptance Tests On March 29, 2011 the State Science and Technology Support Project Development of commercial technology for manufacture of EG from coal organized by CAS and undertaken by the CAS Fujian Institute of Material Structure had passed the acceptance tests. The expert team attending the meeting has admitted that this project team has in the past two years made strenuous efforts to successfully scale up the EG production from a 10-kt/a pilot unit to a 200-kt/a commercial demonstration unit. It is learned that this project team has innovatively solved the key problems related with improvements in the stability of catalyst made by mass production, effective control over reaction rate and heat transfer in catalytic dehydrogenation and cleanup of technical CO gas, and upgrading of simultaneous oxidation and esterification of CO gas, which have led to the emergence of technology for mass production of catalyst for coal-based EG. Furthermore, this project team, after having accomplished the technical scale-up and systematic integration of a 10-kt/a coalbased EG pilot unit to match the excellent catalyst performance, can provide a package technology for manufacture of 200-kt/a EG from cheap brown coal while cranking out quality grade ethylene glycol product meeting the national standard GB The relevant experts besides showing consent on acceptance of this project have suggested to further improve and make breakthroughs in key commercial production technology. 28