π-complexation Mesoporous Adsorbents Cu-MCM-48 for Ethylene-Ethane Separation *

Transcription

1 Chinese Journal of Chemical Engineering, 16(4) (2008) π-complexation Mesoporous Adsorbents Cu-MCM-48 for Ethylene-Ethane Separation * CHEN Le ( 陈 乐 ) and LIU Xiaoqin ( 刘 晓 勤 )** State Key Laboratory of Material-Oriented Chemical Engineering, School of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing , China Abstract Copper incorporated MCM-48 molecular sieve adsorbents with different Cu content have been hydrothermally synthesized. The samples have been characterized by various physicochemical methods, including X-ray diffraction (XRD), nitrogen adsorption (N 2 ) and X-ray photoelectron spectroscopy (XPS). The results reveal that Cu-MCM-48 with mass fraction of copper up to 10 % can still retain the uniform mesoporous framework of MCM-48. The copper in the framework of MCM-48 was easily auto-reduced to Cu(I) in N 2 at high temperature, which did not alter the mesoporous structure of MCM-48. The adsorption equilibrium isotherms of ethylene and ethane on these molecular sieve adsorbents have been measured at 30 C. At 100 kpa, the adsorption capacities of ethylene on 5Cu-MCM-48 and 10Cu-MCM-48 are higher than those on MCM-48. The 10Cu-MCM-48 molecular sieve adsorbent has a higher selective adsorption ratio of ethylene to ethane, the separation factor is 3.8, and the amount of ethylene adsorbed is 11.1 ml g -1. Keywords mesoporous molecular sieve, Cu-MCM-48, adsorption separation, ethylene, ethane 1 INTRODUCTION Olefin-paraffin separation is important and expensive in the petrochemical industry. The most challenging olefin-paraffin separation is the binary mixture of ethylene-ethane, because of the similar size and the close relative volatility of both hydrocarbon molecules. Cryogenic distillation, which has been used for about 60 years for this separation, is energyconsuming; however, a number of alternative technologies have been investigated. The adsorption technique is a promising alternative because it is a simple engineering operation and an energy-saving process. The search for an adequate and selective adsorbent is a significant primary research activity. The group of Yang [1-3] has tested many olefinparaffin separation adsorbents based on the π-complex bond formation between olefins and some transition metal ions. In their articles, different substrates (SiO 2, Al 2 O 3, Y-zeolites, MCM-41) have been used to disperse silver or copper by some methods such as ion exchange, thermal dispersion, and wet impregnation. Recently, some new molecular sieve materials have been reported to adsorb ethylene. These include highly porous metallo-organic molecular sieves, Cu-BTC, synthesized by Bülow s group [4] and Ag + -impregnated clay-based alkene-selective adsorbent, prepared by Choudary and his coworkers [5]. Al-Baghli et al. [6] have reported the equilibrium adsorption data of ethane, ethylene, and ethane on titanosilicate ETS-10. MCM-48 is a member of the M41S mesoporous silica family. It consists of a cubic array with a three-dimensional pore network and has a high surface area and hydrothermal stability. MCM-48 has already been used in many applications, including adsorption, ion exchange, catalysis, and so on. In catalysis, the surface modification of M41S has been studied extensively. The incorporation of transition metals, such as, Ti, Cr, Fe or others, into M41S materials has been reported by a number of researchers [7-10]. In this article, copper incorporated MCM-48 molecular sieve adsorbents have been synthesized hydrothermally, which involved cuprammonia and tetraethyl orthosilicate with cetyltrimethyl-ammonium bromide as a template. Cu-MCM-48, with a copper mass amount up to 10%, can still retain the uniform mesoporous framework of MCM-48. The nitrogen treatment with Cu-MCM-48, under high temperature, results in the reduction of Cu(II) to Cu(I). Cu (I) in the adsorbent lattice leads to favorable selectivity toward ethylene. All samples have been characterized by XRD, XPS, and N 2 adsorption (77K) methods. Adsorption equilibrium of ethylene and ethane in the adsorbents (MCM-48 and Cu-MCM-48) have been studied at 30 C. 2 EXPERIMENTAL 2.1 Synthesis of MCM-48 MCM-48 was synthesized by the conventional hydrothermal pathway. CTAB (cetyltrimethyl-ammonium bromide, template, Aldrich) was dissolved in water and NaOH. After heating slightly, to dissolve the surfactant, the silicate source TEOS (tetraethyl orthosilicate, Aldrich) was added. The ratios of the reactants SiO 2.. Na 2 O..CTAB..H 2 O were The resulting mixture was stirred for about 30 min at room temperature before being loaded into a Teflon-lined steel autoclave, where the synthesis solution was heated for 3 d at 110 C. The product was then filtered, washed with water, and calcined at 550 C for 6 h. 2.2 Synthesis of Cu-MCM-48 Cuprammonia solution composed of Cu(NO 3 ) 2 Received , accepted * Supported by the National Natural Science Foundation of China ( ) and the Specialized Research Fund for the Doctoral Program of Higher Education of China ( ). ** To whom correspondence should be addressed. liuxq@njut.edu.cn

2 Chin. J. Chem. Eng., Vol. 16, No. 4, August and 25% aqueous ammonia was used as a copper precursor, to load copper into the pores of MCM-48. A typical synthesis procedure was as follows: CTAB was dissolved in water, NaOH, and TEOS. After stirring the mixture for about 30 min, a certain concentration of cuprammonia solution was added to it. The stirring was continued for a further 4 h, before the mixture was loaded into a Teflon-lined steel autoclave, where the synthesis solution was heated for 3 d at 110 C. The product was then filtered, washed with water, and calcined at 550 C for 6 h. The products were designated as 5Cu-MCM-48, 10Cu-MCM-48, and 20Cu-MCM-48 [5, 10, and 20 indicate the nominal mass content (%) of copper in the products]. 2.3 Characterization of the samples The XRD patterns of the samples were recorded using a Bruker D8 Advance diffractometer, using Cu K α radiation and a Ni filter, at 40 kv, 30 ma, with a scan speed of 0.25( ) s -1. The N 2 adsorption isotherms and BET surface area were measured at 77 K on a Micromeritics ASAP2010 sorption analyzer. The pore size distribution was calculated using the desorption branch of the N 2 adsorption-desorption isotherm and the Barrett- Joyner-Halenda (BJH) formula. The samples were degassed at 423 K, under a vacuum, before analysis. A Perkin Elme Optima 2000 DV inductively coupled plasma atomic emission spectrometer (ICP-AES) was used to determine the total content of copper incorporated in the mesoporous silica MCM-48. The samples were completely dissolved in suitable acid (HF) before analysis. The X-ray photoelectron spectroscopy (XPS) analyses were conducted on a Physical Electronics PHI-550 spectrometer equipped with an Al K α X-ray source, at 10 kv, 35 ma. Figure 1 Powder XRD patterns of the calcined samples a MCM-48; b 5Cu-MCM-48; c 10Cu-MCM-48; d 20Cu-MCM-48 in the XRD pattern decrease with an increase in Cu loading because of the partial collapse of the cubic ordered phase, after the introduction of the copper species. The ordered mesoporous structures cannot be obtained with a nominal Cu content of 20% in this synthesis. Fig. 2 shows the high-angle XRD patterns of the Cu-MCM-48 samples. On account of copper oxide phases, no additional peaks are observed for 5Cu-MCM-48 and 10Cu-MCM-48 samples at the high 2θ range. In the case of 20Cu-MCM-48, CuO diffraction peaks corresponding to 2θ=36.18 and can be observed, indicating that cupric oxide agglomerates are presented in the 20Cu-MCM-48 sample. 2.4 Adsorption isotherms measurements The equilibrium adsorption isotherms of pure ethylene and ethane on the adsorbents were measured using a Micromeritics ASAP2010 at 30 C. Before equilibrium measurements, the samples were first treated with nitrogen at 500 C. 3 RESULTS AND DISCUSSION 3.1 Characterization of samples The XRD patterns obtained for calcined MCM-48 and Cu-MCM-48, with different Cu loading, are depicted in Fig. 1. Intense diffraction (211), (220) peaks at about 2θ=2.5, 2θ=3.0, and additional (420), (332), and (431) peaks are clearly observed for sample a, indicating an MCM-48 structure. For 5Cu-MCM-48 and 10Cu-MCM-48, the higher intense diffraction (211) peaks are still observable, maintaining the ordered cubic mesoporous structures. However, the (211) peak of Cu-MCM-48 samples shifts to lower 2θ values compared to MCM-48. The peak intensities Figure 2 High-angle XRD patterns of 5Cu-MCM-48, 10Cu-MCM-48, and 20Cu-MCM-48 Nitrogen treatment at a high temperature can result in the reduction of Cu(II) to Cu(I), in the framework of a molecular sieve. Fig. 3 shows the comparison of XRD patterns for the calcined 5Cu-MCM-48 sample before and after N 2 treatment. N 2 treatment does not alter the mesoporous structure of the molecular sieve. The (211) peak is still reserved for the Cu-MCM-48 sample. Besides XRD, nitrogen physisorption is usually employed to characterize mesoporous adsorption materials. This method gives information about the specific surface area and the pore diameter as seen in Table 1. The Cu contents of the Cu-MCM-48 adsorbents verified by ICP analysis are also given there. Figs. 4 and 5 show N 2 adsorption-desorption isotherms and pore

3 572 Chin. J. Chem. Eng., Vol. 16, No. 4, August 2008 Figure 3 XRD patterns of the calcined 5Cu-MCM-48 after N 2 treatment a the calcined 5Cu-MCM-48; b the calcined 5Cu-MCM-48 after N 2 treatment Table 1 Adsorbents Textural properties of MCM-48 and Cu-MCM-48 adsorbents from 77 K N 2 adsorption data Cu mass content(icp) /% Surface area (BET) -1 /m 2 g Total pore volume -1 /cm 3 g Desorption pore size/nm MCM Cu-MCM Cu-MCM Cu-MCM Figure 5 BJH pore diameter distribution of MCM-48 and Cu-MCM-48 MCM-48; 5Cu-MCM-48; 10Cu-MCM-48 the range of pore size distribution broader. The results are in good agreement with those observed from the XRD tests. The authors also measured the valence state of Cu in Cu-MCM-48 samples after N 2 treatment using X-ray photoemission spectroscopy (XPS) analysis. Fig. 6 shows the XPS spectra for Cu2p 3/2 in the Cu-MCM-48 samples before and after N 2 treatment. The binding energies of Cu2p 3/2 are listed in Table 2. The Cu2p 3/2 binding energy of the calcined 5Cu-MCM-48 has shifted to a lower energy after N 2 treatment, the peak of which has become sharper, similar to the 10Cu- MCM-48 sample. This shift is clearly caused by the autoreduction of Cu(II) to Cu(I) in the Cu-MCM-48 under heated N 2. Cu(I) and Cu(II) coexist in the N 2 treated Cu-MCM-48 samples. The Cu2p 3/2 XPS spectra of 5Cu-MCM-48 and 10Cu-MCM-48 samples can be split into two parts: one is for Cu(I) at about ev; another is for Cu(II) at about ev. The two peaks for Cu(I) and Cu(II) can be fitted by the XPS peak software, the results are shown in Fig. 7 and Table 2. Figure 4 Nitrogen adsorption-desorption isotherms for MCM-48 and Cu-MCM-48 adsorption: MCM-48; 5Cu-MCM-48; 10Cu-MCM-48 desorption: MCM-48; 5Cu-MCM-48; + 10Cu-MCM-48 diameter distributions for MCM-48, 5Cu-MCM-48, and 10Cu-MCM-48, respectively. As depicted in Fig. 4, all these N 2 adsorption isotherms show type IV isotherms (IUPAC), with a characteristic step at around P/P 0 = of capillary condensation of N 2 molecules, in the pores, which is typical of ordered mesoporous materials. The physisorbed amount of N 2 decreases with increasing Cu content, causing a reduction in surface area and pore volume, as listed in Table 1. The narrow and sharp pore size distribution curve of MCM-48 indicates uniform mesoporosity. In Fig. 5, the maximum of the pore size distribution shift, however, is slightly to the right, as the Cu content increases. The excess Cu in the adsorbent lattice makes Figure 6 Cu2p 3/2 XPS spectra of Cu-MCM-48 adsorbents a the calcined 5Cu-MCM-48; b the calcined 5Cu-MCM-48 after N 2 treatment; c the calcined 10Cu-MCM-48 after N 2 treatment 3.2 Adsorption equilibrium of ethylene and ethane on the adsorbents Adsorption of ethylene and ethane on MCM-48 and Cu-MCM-48 adsorbents was studied under static

4 Chin. J. Chem. Eng., Vol. 16, No. 4, August Sample Table 2 XPS analyses of Cu-MCM-48 adsorbents Cu2p 3/2 /ev Standard binding energy of some compounds Cu2p 3/2 /ev 5Cu-MCM CuSiO Atomic concentrations/% Cu2p C1s O1s Si2p Na1s 5Cu-MCM-48 after N 2 treatment 933.3, CuO Cu-MCM-48 after N 2 treatment 933.2, Cu 2 O Figure 8 Adsorption equilibrium isotherms of C 2 H 4 and C 2 H 6 on MCM-48 (30 C) Figure 7 Cu2p 3/2 XPS spectra of N 2 treated 5Cu-MCM-48 and 10Cu-MCM-48 samples conditions at 30 C. These adsorbents were first outgassed and the Cu-MCM-48 samples were treated with nitrogen at 500 C for the autoreduction of Cu (II) to Cu(I). The equilibrium isotherms of pure ethylene and ethane on different adsorbents are shown in Figs. 8 to 11. The amount of ethylene adsorbed on 5Cu-MCM-48 and 10Cu-MCM-48 samples is higher than that adsorbed on the pure MCM-48 sample, because of the interaction of the π-complexation bond between ethylene and Cu-MCM-48 adsorbents. At 100 kpa, the adsorption capacities for ethylene have increased in the order 10Cu-MCM-48>5Cu-MCM-48>MCM-48> 20Cu-MCM-48. Their respective capacity values are 11.1, 10.0, 8.5, and 6.7 ml g -1. The ratio of the adsorbed concentration of ethylene and ethane are 3.8, 3.1, 2.2, and 1.9, respectively. Increasing Cu content in the adsorbent lattice leads to a corresponding improvement in the selectivity toward ethylene. On the other hand, the lowest amount of ethylene is adsorbed on 20Cu-MCM-48 because of the blockage of channels by excessive CuO, a result expected from the characterization of XRD and N 2 adsorption, analyzed earlier in this article. The 10Cu-MCM-48 adsorbent Figure 9 Adsorption equilibrium isotherms of C 2 H 4 and C 2 H 6 on 5Cu-MCM-48 (30 C) Figure 10 Adsorption equilibrium isotherms of C 2 H 4 and C 2 H 6 on 10Cu-MCM-48 (30 C) has a higher adsorption amount and selectivity to ethylene. It is known that the more the content of Cu in

5 574 Chin. J. Chem. Eng., Vol. 16, No. 4, August 2008 Figure 11 Adsorption equilibrium isotherms of C 2 H 4 and C 2 H 6 on 20Cu-MCM-48 (30 C) the sample, the less the percentage of Cu that is reduced. However, with the same mass of adsorbent samples, the total Cu content of 10Cu(I)-MCM-48 is higher, so the total amount of Cu (II) reduced to Cu(I) is probably more than that of 5Cu(I)-MCM-48, which may explain why the adsorbed amount of ethylene on 10Cu(I)- MCM-48 is larger than that on 5Cu(I)-MCM-48. All adsorption isotherms of C 2 H 4 and C 2 H 6 on Cu-MCM-48 adsorbents can be well described by the Freundlich equation, namely, q=kp 1/n, where q is the amount adsorbed at equilibrium pressure P, and k and 1/n are Freundlich constants. In the case of 5Cu-MCM- 48 and 10Cu-MCM-48, the fitted values of k and 1/n for both gases are reported in Table 3, with R being the fitting level given by the statistic software. Furthermore Fig. 12 shows the adsorption equilibrium data and fitting curve of C 2 H 4 on 10Cu-MCM-48. Table 3 Fitted values of parameter k and 1/n for ethylene and ethane on 5Cu-MCM-48 Freundlich Freundlich Adsorbent constant k constant 1/n Fitting level R C 2 H 4 C 2 H 6 C 2 H 4 C 2 H 6 C 2 H 4 C 2 H 6 5Cu-MCM Cu-MCM CONCLUSIONS MCM-48 incorporated with copper was synthesized and characterized by XRD, HRTEM, nitrogen adsorption, and XPS. The Cu(II) in the framework of Cu-MCM-48 was reduced to Cu(I) in heated N 2, and the adsorbent still retained the mesoporous structure. The adsorption Figure 12 Adsorption equilibrium data and fitting curve of C 2 H 4 on 10Cu-MCM-48 equilibrium isotherms of ethylene and ethane on these molecular sieve adsorbents were measured at 30 C. At 100 kpa, the adsorption capacities for ethylene increased in the order 10Cu-MCM-48>5Cu-MCM-48> MCM-48>20Cu-MCM-48. The 10Cu-MCM-48 adsorbent had a higher adsorption amount and selectivity to ethylene. All the adsorption isotherms of C 2 H 4 and C 2 H 6 on Cu-MCM-48 adsorbents could be well described by the Freundlich equation. REFERENCES 1 Yang, R.T., Kikkinides, E.S., New sorbents for olefin/paraffin separations by adsorption via π-complexation, AIChE J., 41 (3), (1995). 2 Rege, S.U., Padin, J., Yang, R.T., Olefin/paraffin separations by adsorption: π-complexation vs. kinetic separation, AIChE J., 44 (4), (1998). 3 Padin, J., Yang, R.T., New sorbents for olefin/paraffin separations by adsorption via π-complexation: Synthesis and effects of substrates, Chem. Eng. Sci., 55 (14), (2000). 4 Wang, M.Q., Shen, D.M., Bülow, M., Lau, M.L., Deng, S.G., Fitch, F.R., Lemcoff, N.O., Semanscin, J., Metallo-organic molecular sieves for gas separation and purification, Microporous Mesoporous Mater., 55, (2002). 5 Choudary, N.V., Kumar, P., Bhat, T.S., Adsorption of light hydrocarbon gases on alkene-selective adsorbent, Ind. Eng. Chem. Res., 41, (2002). 6 Al-Baghli, N.A., Loughlin, K.F., Adsorption of methane, ethane, and ethylene on titanosilicate ETS-10, J. Chem. Eng. Data, 50, (2005). 7 Kawi, S., Te, M., MCM-48 supported chromium catalyst for trichloroethylene oxidation, Catal. Today, 44, (1998). 8 Echchahed, B., Moen, A., Nicholson, D., Iron-modified MCM-48 mesoporous molecular sieves, Chem. Mater., 9, (1997). 9 Anpo, M., Yamashita, H., Ikeue, K., Photocatalytic reduction of CO 2 with H 2 O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts, Catal. Today, 44, (1998). 10 Shao, Y.F., Wang, L.Z., Zhang, J.L., The photoluminescence of rhodamine B encapsulated in mesoporous Si-MCM-48, Ce-MCM-48 and Fe-MCM-48 and Cr-MCM-48 molecular sieves, J. Photochem. Photobiol. A: Chem., 180, (2006).