Olefin Separation

Langmuir 2001, 17, 8405-8413

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Cu(I)-Y-Zeolite as a Superior Adsorbent for Diene/Olefin Separation Akira Takahashi and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

Curtis L. Munson and Daniel Chinn Separation Technology, Chevron Research and Technology Company, Richmond, California 94802-0627 Received July 30, 2001. In Final Form: September 28, 2001 Purification of normal R-olefins by removal of dienes has been demonstrated previously in our laboratories by π-complexation using Ag+ ion-exchanged zeolite (Ag-Y) or AgNO3/SiO2 sorbent. Although Ag-Y could purify 1-butene/1,3-butadiene effectively, the purification performance was degraded by H2 and/or H2S poisoning. A new sorbent for 1-butene/1,3-butadiene purification was developed in this study by ionexchange of Cu2+ cations into Y-zeolite followed by reduction of Cu2+ to Cu+. The performance of the Cu+-zeolite, Cu(I)-Y or Cu-Y, was found to be superior to that of Ag-Y. Cu-Y exhibited higher diene/ olefin separation factors than Ag-Y by approximately an order of magnitude. Furthermore, unlike Ag-Y, exposure to H2S/H2 at 120 °C had virtually no effect on 1,3-butadiene/1-butene adsorption, indicating the excellent poisoning resistance of Cu-Y. XPS and EPR analyses showed that half of the Cu2+ cations in Cu-Y underwent autoreduction to Cu+ either in vacuo or in He, resulting in the same purification capability with Cu-Y obtained by reduction with CO.

Introduction Adsorption is playing an increasingly important role in separation and purification.1 However, its application is limited by the availability of selective sorbents. The conventional adsorption processes are based on van der Waals and electrostatic interactions between the sorbate and the sorbent. Studies of new sorbents using weak chemical bonds such as chemical complexation have only begun recently in our laboratory. As suggested by King,2 chemical complexation bonds are generally stronger than van der Waals interactions, yet weak enough to be reversible by modest changes in temperatures or pressures. This point was also made clear by Keller.2 Therefore, tremendous opportunities exist for developing new sorbents and applications in separations by using weak chemical bonds, including various forms of complexation bonds. One of the most important application areas for adsorption technology is olefin/paraffin separations. Cryogenic distillation has been used for over 60 years for this separation.3 It remains the most energy-intensive distillation because of the close relative volatilities of olefin to paraffin.4 The most important olefin/paraffin separations are for the binary mixtures of ethane-ethylene and propane-propylene. A number of alternatives have been * Corresponding author. Tel: (734) 936-0771. Fax: (734) 7630459. E-mail: [email protected]. (1) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, 1987; reprinted (in paperback) by Imperial College Press: London and World Scientific Publishing Co.: River Edge, NJ, 1997. (2) King, C. J. In Handbook of Separation Process Technology; Rousseau, R. W., Ed.; Wiley: New York, 1987. (3) Keller, G. E.; Marcinkowsky, A. E.; Verma, S. K.; Williamson, K. D. In Separation and Purification Technology; Li, N. N., Calo, J. M., Eds.; Marcel Dekker: New York, 1992. (4) Humphrey, J. L.; Seibert, A. F.; Koort, R. A. DOE-ID-12920-1, 1991.

investigated, and the most promising one appears to be separation via π-complexation.5 π-Complexation is a subclass of chemical complexation. It pertains to the main group (or d-block) transition metals, i.e., from Sc to Cu, Y to Ag, and La to Au in the periodic table.6 The metals or their ions can form the normal σ-bond to carbon. In addition, the unique characteristics of the d orbital in these metals or ions can form bonds with unsaturated hydrocarbons in a nonclassical manner. This type of bonding is broadly referred to as π-complexation. π-Complexation has been seriously considered for olefin/ paraffin separation using liquid solutions containing silver or cuprous ions.3,5,7-10 While gas-solid operations (e.g., pressure swing adsorption) can be simpler as well as more efficient than gas-liquid operations, the list of attempts for developing a π-complexation sorbent is a short one. More recently, several new sorbents based on π-complexation were prepared for selective olefin adsorption: Ag+exchanged resins,11,12 monolayer CuCl on pillared clays,13 and monolayer AgNO3/SiO2.14-16 Several opportunities exist for the application of π-complexation sorbents in the separation of C4 streams. The (5) Eldridge, R. B. Ind. Eng. Chem. Res. 1993, 32, 2208. (6) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Interscience: New York, 1966; Chapters 25 and 28. (7) Quinn, H. W. In Progress in Separation and Purification; Perry, E. S. Ed.; Interscience: New York, 1971; Vol. 4. (8) Ho, W. S.; Doyle, G.; Savage, D. W.; Pruett, R. L. Ind. Eng. Chem. Res. 1998, 27, 334. (9) Blytas, G. C. In Separation and Purification Technology, Li, N. N., Calo, J. M., Eds.; Marcel Dekker: New York, 1992. (10) Safarik, D. J.; Eldridge, R. B. Ind. Eng. Chem. Res. 1998, 37, 2571. (11) Yang, R. T.; Kikkinides, E. S. AIChE J. 1995, 41, 509. (12) Wu, Z.; Han, S. S.; Cho, S. H.; Kim, J. N.; Chue, K. T.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 2749. (13) Cheng, L. S.; Yang, R. T. Adsorption 1995, 1, 61. (14) Padin, J.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 4224. (15) Rege, S. U.; Padin, J.; Yang, R. T. AIChE J. 1998, 44, 799. (16) Padin, J.; Yang, R. T. Chem. Eng. Sci. 2000, 55, 2607.

10.1021/la011196z CCC: $20.00 © 2001 American Chemical Society Published on Web 11/17/2001

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Langmuir, Vol. 17, No. 26, 2001

C4 streams obtained from various hydrocarbon-cracking operations contain many important chemicals. The absolute amounts and compositions of the C4 fraction obtained from cracking are substantially affected by the type of cracking, severity of cracking condition, and the feedstock.17 The C4 fraction cannot be separated into its components economically by simple distillation because of the close proximity of their boiling points. Several separation methods have been developed for this application, ranging from the selective reaction of isobutene to extractive distillation of 1,3-butadiene. The remaining C4 stream consists primary of n-butenes and butanes. This stream can be further separated by selective adsorption of the n-butenes on π-complexation sorbents. Another area where π-complexation sorbents can make an impact is in the purification of normal R-olefins (NAO). NAOs are chemical intermediates used to make a variety of products. The largest uses for NAOs are in the production of alcohols (via oxo chemistry), as comonomers for polyethylene production, and in the synthesis of poly (R-olefins) for synthetic lubricants. Also, oligomerization of n-butenes to more valuable octenes is an effective way of upgrading their value.18 A variety of catalysts have been developed for this reaction. The most common metalbased catalyst used in this application involves nickel. However, one common concern in all of the above applications is the need for ultrapure olefins with very low levels of 1,3-butadiene (C4H6). It has been shown by Podrebarac and co-workers19 that the nickel-based catalyst used in the oligomerization of n-butene can be severely deactivated in the presence of traces of C4H6 in the feed. Selective butadiene hydrotreating process is one option to cleanup the unwanted butadiene in a mixed-C4 stream.20 In this process, butadiene can be hydrogenerated to R-butene and β-butene in fixed bed reactors where hydrogen is fed in stoichiometric ratio with the diene content. Application of π-complexation sorbents to the separation of C4 hydrocarbons was successfully achieved by Padin et al.21 It was shown that the monolayer-dispersed AgNO3/ SiO2 sorbent had excellent selectivity and capacity for 1-butene over 1-butane. Furthermore, purification of 1-butene by removal of trace amounts of 1,3-butadiene was achieved by using Ag+-exchanged Y-zeolite. The effect of silver content in Ag-Y on 1,3-butadiene adsorption was systematically investigated using Ag-Y with different Si/ Al ratios and Ag+-Na+ mixed ion-exchanged zeolites (AgNa-Y).22 AgNa-Y with a Ag content of 34 Ag/uc (26 wt % Ag) exhibited excellent purification characteristics for this application. Since Ag is costly and Cu+ would form stronger π-complexation bonds with CO and C2H4 from molecular orbital calculations,23 1,3-butadiene/1-butene adsorption on Cu+ ion-exchanged zeolite (Cu-Y, Si/Al ) 2.43) was examined and was compared with that on Ag-Y in this work. It is known that hydrogen sulfide, acetylene, and hydrogen are coproduced by cracking and may be present in the process stream in an actual industrial process. They (17) Weissermel, K.; Arpe, H. J. Industrial Organic Chemistry; VCH Verlagesellschaft mbH: Weinheim, Germany, 1997. (18) Nierlich, F. Hydrocarbon Process. 1992, Feb, 45. (19) Podrebarac, G. G.; Ng, F. T.; Rempel, G. L. Appl. Catal. A 1996, 147, 159. (20) Meyers, R. A. Handbook of Petroleum Refining Process; McGrawHill: New York, 1986. (21) Padin, J.; Yang, R. T.; Munson, C. L. Ind. Eng. Chem. Res. 1999, 38, 3614. (22) Takahashi, A.; Yang, R. T.; Munson, C. L.; Chinn, D. Ind. Eng. Chem. Res. 2001, 40, 3979. (23) Huang, H. Y.; Padin, J.; Yang, R. T. Ind. Eng. Chem. Res. 1999, 38, 2720.

Takahashi et al.

are the three classical poisons to aqueous-based Ag+.10 In aqueous solution containing Ag+, hydrogen causes a gradual reduction of Ag+ and subsequent precipitation of metallic silver. Small quantities of an oxidizing agent, commonly hydrogen peroxide, are added to stabilize the solution and prevent silver loss. Hydrogen sulfide reacts irreversibly with Ag+ ions to form silver sulfide precipitate. Keller et al.3 suggested pretreatment to reduce sulfur compound concentrations to low levels. Acetylene also reacts with Ag+ to form silver acetylides (Ag2C2). Solid acetylides are unstable and explosive. Keller et al.3 recommended reducing the acetylene concentration below 1 ppm prior to feeding a gas stream to the aqueous-based process. The effects of H2 and H2S exposure on 1,3-butadiene/ 1-butene adsorption by Ag-Y were examined by Takahashi et al.22 and Jayaraman et al.24 H2S was irreversibly adsorbed on Ag-Y, causing a reduction of adsorption capacity. The purification capability of Ag-Y could be maintained by shifting the adsorption of both adsorbates to higher pressures. The separation factors for 1,3butadiene/1-butene were decreased somewhat by H2S exposure but were still high enough for the purification application to be viable. XPS analysis indicated that Ag+ in Ag-Y reacted with H2S to form Ag2S. On the other hand, H2 exposure to Ag-Y was detrimental, because the π-complexation capability of Ag-Y deteriorated significantly, owing to the reduction of Ag+ to Ag0. To circumvent this deterioration, rejuvenation by oxidation was successfully demonstrated and an optimum oxidation condition with respect to olefin adsorption was identified. In this paper, the effect of H2S and H2 exposure on 1,3butadiene/1-butene adsorption by Cu ion-exchanged zeolite was investigated. Experimental Section Sorbent Preparation. Cu+-Y was prepared by ion exchange of Na-Y-zeolites (Si/Al ) 2.43, 56 Al atoms/uc, Strem Chemical) with Cu(NO3)2 followed by reduction of Cu2+ to Cu+. First, asreceived Na-Y was exchanged twice using excess amounts [10fold cation-exchange capacity (CEC) assuming that one Cu2+ compensates two aluminum sites] of 0.5 M Cu(NO3)2 at room temperature for 24 h. After the exchange, the zeolite suspension was filtered and washed with copious amount of deionized water. The product was dried at 100 °C overnight. Several groups have reported reduction of Cu2+ to Cu+ in zeolite in a reducing atmosphere. Huang reduced Cu2+ in Y-zeolite at 400 °C for 36 h in 150 Torr CO in a closed chamber.25 He also claimed that the reduction process was enhanced by the presence of a small amount of ammonia (10 Torr). When ammonia was first adsorbed on dehydrated zeolite, most of the Cu2+ could be reduced in a few hours at temperatures as low as 100 °C. Rabo et al.26 and Pearce27 also used CO as the reducing agent, and reported complete reduction, respectively, at 300 °C for 6 h with 97% CO and 3% H2O and at 450 °C for at least 3 h with 1.4-2 bar of CO. Reduction was also achieved by ethylene at 250 °C for 4 h by Cen.28 In this study, reduction of Cu2+ to Cu+ was carried out either in CO/He mixture (CO reduction) or in He only (autoreduction). CO reduction was performed at 450 °C in 0.75 atm of CO/He for 12 h, which was the most severe condition described above to achieve complete reduction. Autoreduction of Cu2+ in ZSM-5 or in Y-zeolite at 100-500 °C has also been demonstrated by several groups.29,30 In this work, autoreduction was performed at 300 or 450 °C for 1 h. (24) Jayaraman, A.; Yang, R. T.; Munson, C. L.; Chinn, D. Ind. Eng. Chem. Res. In press. (25) Huang, Y.-Y. J. Catalysis 1973, 30, 187. (26) Robo, J. A.; Francis. J. N.; Angell. C. L. U.S. Patent 4,019,879, 1977. (27) Pearce, G. K. U. S. Patent 4,717,398, 1988. (28) Cen, P. L. Proc. Fundam. Adsorpt. 3 1989, 191.

Cu(I)-Y Zeolite as a Superior Adsorbent

Langmuir, Vol. 17, No. 26, 2001 8407

Chemical Analysis. The compositions of the samples were characterized using neutron activation analysis (NAA) in the research nuclear reactor of the Phoenix Memorial Laboratory at the University of Michigan. The sample was irradiated sequentially for 1 min at a core-face location with an average thermal neutron flux of 2 × 1012 n/cm2/s. Two separate γ-ray spectra were then collected for each sample with a high-resolution germanium detector: one after 13-min decay to determine the concentrations of Al, Ag, and Cu and a second after a 116-min decay to analyze for Na. γ-Energy lines at 1779, 632.99, 1039.20, and 1368.6 keV were used for the determination of Al, Ag, Cu, and Na concentration, respectively. Adsorption Isotherms and Uptake Rate. Single-component isotherms and uptake rates were measured at 120 °C using standard gravimetric methods following the procedure described in Ackley and Yang.31 A Shimadzu TGA-50 automatic recording microbalance was employed. The gases used were 1,3-butadiene (CP grade, Matheson, minimum purity 99.5%), 1-butene (CP grade, Matheson, minimum purity 99.5%), H2S (CP grade, Metro Welding, minimum purity 99.5%), H2 (Metro Welding, 99.999%), and helium (prepurified grade, Metro Welding, 99.995%). 1,3Butadiene/He certified gas mixture (1.00% 1,3-butadiene/balance helium, Matheson) was also used to measure isotherms at partial pressures lower than 6.0 × 10-3 atm. Prior to the measurement, the zeolite powders (binderless) were CO-reduced or autoreduced at 300 or 450 °C. To understand the influence of poisoning by H2S or H2, adsorption isotherms of butadiene/butene after H2S or H2 exposure were measured. The H2S and H2 exposure conditions were 0.7 atm of H2S at 120 °C for 10 min and 0.5 atm of H2 at 120 °C for 1 h. These conditions are extremely severe compared to the actual level of H2S and H2 present in process streams, so that the effects on the sorbent after long-time usage might be understood. Pore size distributions were determined by nitrogen isotherms at 77 K measured with a Micromeritics ASAP 2010 system. The Horvath-Kawazoe equation32-35 was used for the calculations. The diffusion time constants, D/r2 (s-1), were calculated from the uptake curves.36 In this work, short time region (up to 30% uptake) and spherical adsorbent model were used. Separation Factor. The pure-component adsorption data were first fitted by both D-A (Dubinin-Astakhov) and L-F (Langmuir-Freundlich) isotherms. The separation factors of 1,3butadiene over 1-butene were then obtained using the equivalent multicomponent isotherms. In the D-A equation of pure component gas, the volume adsorbed, V, at relative pressures P/Ps is expressed as

[(

)]

Ps P

V ) V0 exp - C ln

n

(1)

where

C)

RT βE0

(2)

Doong and Yang37 extended the D-A equation to mixed-gas adsorption in a simple way by using the concept of maximum available pore volume without any additional equations such as the Lewis relationship.38 For binary mixtures,

[( [(

)] )]

Ps1 P1

n1

Ps2 P2

n2

V1 ) (V01 - V2) exp - C1 ln V2 ) (V02 - V1) exp - C2 ln

(3)

(4)

Since eqs 3 and 4 are linear, V1 and V2 can be obtained in a (29) Larsen, S. C.; Aylor, A.; Bell, A. T.; Reimer, J. A. J. Phys. Chem. 1994, 98, 11533. (30) Kaushik, V. K.; Ravindranathan, M. Zeolites 1992, 12, 415. (31) Ackley, M. W.; Yang, R. T. AIChE J. 1991, 37, 1645. (32) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470. (33) Saito, A.; Foley, H. C. AIChE J. 1991, 37, 429. (34) Cheng, L. S.; Yang, R. T. Chem. Eng. Sci. 1994, 49, 2599. (35) Rege, S. U.; Yang, R. T. AIChE J. 2000, 46, 734.

straightforward fashion. After the calculation of V1 and V2, volumetric adsorbed amounts were converted to molar adsorbed amounts and the separation factors were calculated. The conversion of molar adsorbed amount to volumetric adsorbed amount and vice versa was performed using the liquid density data of the adsorbates. The liquid densities at 120 °C were obtained by the modified Rackett equation of Spencer and Adler.39 The liquid densities used in this work were 0.455 g/cm3 for 1,3butadiene, and 0.425 g/cm3 for 1-butene. The adsorbed molar volumes were assumed to be the same for both single- and mixedgas adsorption. For the L-F isotherm equation, the pure-component isotherm is written using three adjustable parameters:

q)

q0BP1/n 1 + BP1/n

(5)

Here, q0 is the saturated adsorbed amount, and B is the Langmuir constant. The L-F equation can also be extended for an n-component mixture:1

qi )

q0iBiPi1/ni n

1+

∑B P j

(6)

1/nj

j

j)1

The separation factors for 1,3-butadiene over 1-butene were calculated using the equilibrium mole fractions in the adsorbed (X) and gas phases (Y):

R)

X1,3-butadiene/Y1,3-butadiene X1-butene/Y1-butene

(7)

Separation factors were calculated for binary mixtures with ratios from 0.01/1 to 0.00001/1 (both in atmospheres) of 1,3butadiene/1-butene. Because the lowest data point for 1,3-butadiene adsorption isotherms was 3.7 × 10-5 atm, the separation factors for 0.00001(1,3-butadiene)/1(1-butene) were calculated by using the extrapolated data for using the fitted isotherm equations. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) is a widely used technique to investigate the oxidation states of surface species. According to standard binding energy values,40,41 Cu(I) exhibits the 2P3/2 peak at about 1 eV lower than Cu(II). XPS spectra were obtained from thin wafers of the zeolite using a Perkin-Elmer PHI 5400 ESCA system with a Mg anode. The binding energy of C 1s at 285 eV was used as the reference. Cu2+-exchanged zeolite powder was formed into a very thin wafer using a laboratory press. The wafer was degassed in the XPS stage cell at 10-8-10-9 Torr for several days at room temperature prior to analysis (dehydration at RT). Then, Cu2+ was treated at 450 °C for 1 h in a vacuum using a gas-phase reaction chamber and returned to the XPS stage for analysis without exposing to air (dehydration at 450 °C). Electron Paramagnetic Resonance. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for understanding the structural environment of paramagnetic Cu2+ sites.42,43 The ratio of Cu2+ over Cu+ in Cu-Y can also be determined by comparing the EPR intensity before and after reduction,29 since Cu2+ is EPR active and Cu+ is EPR inactive (diamagnetic). EPR spectra were obtained with glass-sealed zeolite powders (hydrated or dehydrated at 450 °C in a vacuum) (36) Yeh, Y. T. Ph.D. Dissertation, University of New York at Buffalo, Buffalo, New York, 1989. (37) Doong, S. J.; Yang, R. T. Ind. Eng. Chem. Res. 1988, 27, 630. (38) Lewis, W. K.; Gilliland, E. R.; Chertow, B.; Cadogan, W. P. Ind. Eng. Chem. 1950, 42, 1319. (39) Spencer, C. F.; Adler, S. B. J. Chem. Eng. Data 1978, 23, 82. (40) Bolis, V.; Maggiorini, S.; Meda, L.; D’Acapito, F.; TurnesPalomino, G.; Bordiga, S.; Lamberti, C. J. Chem. Phys. 2000, 113, 9248. (41) Romand, M.; Roubin, M.; Deloume, J. P. J. Electron Spectrosc. Relat. Phenom. 1978, 13, 229. (42) Chao, C.-C.; Lunsford, J. H. J. Chem. Phys. 1972, 57, 2890. (43) Carl, P. J.; Larsen, S. C. J. Phys. Chem. B 2000, 104, 6568.

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Figure 1. Pure-component equilibrium isotherms at 120 °C for 1,3-butadiene and 1-butene on Cu-Y and Ag-Y. Curves are fitted with Dubinin-Astakhov (solid line) and LangmuirFreundlich (dotted line) isotherms. using a Bruker EMX EPR spectrometer at -180 °C. The EPR spectral parameters were the following: microwave frequency ) 9.28 GHz, modulation amplitude ) 10.0 G, and modulation frequency ) 100 kHz.

Results and Discussion Chemical Composition. Neutron activation analysis showed that the Al, Cu, and Na contents in Cu-Y were 6.10 ( 0.25 wt %, 6.65 ( 0.05 wt %, and 1.50 ( 0.12 wt %, respectively. This meant that the Cu/Al molar ratio was 0.463 and the Na/Al ratio was 0.289. Since a certain amount of Na+ still remained in the zeolite, the ionexchange would exceed 100%, if it were assumed that Cu existed as Cu2+ and each Cu2+ compensated two aluminum sites. There are more than three possibilities to explain this result: (i) The excess Cu2+ are located outside the cationexchange sites, and the rest of the Cu2+ compensate two aluminum sites in addition to Na+ (which compensates one aluminum site). (ii) Some of the Cu2+ are exchanged into Y-zeolites in the form of [Cu2+OH-]+ and the other Cu2+ are exchanged as Cu2+. (iii) All of the Cu2+ ions exist as [Cu2+OH-]+ in addition to Na+, and the rest of the charge compensation is provided by proton (H+). Larsen et al.29 assumed the presence of proton (case iii) in their study of Cu-ZSM-5 (Si/Al ) 18) zeolites, because Parillo et al.44 have reported that Brφnsted acid sites are easily incorporated during ion-exchange. Further investigation is necessary to understand the states of Cu2+ in Y-zeolite. On the other hand, the Ag/Al and Na/Al ratios in Ag-Y were 1.13 and 0.01. So, more than 100% Ag ionexchange ratio was achieved in the case of Ag-Y, indicating that some Ag were located outside the chargecompensating sites. Comparison between Cu-Y and Ag-Y. Purecomponent adsorption isotherms of 1,3-butadiene/1-butene by Cu-Y and Ag-Y are compared in Figure 1. In the figure, isotherms were fitted by D-A and L-F equations, and the fitting parameters are summarized in Table 1. Cu-Y could adsorb larger amounts of both 1,3-butadiene and 1-butene than Ag-Y. The amounts of 1,3butadiene adsorbed by Cu-Y after CO reduction or autoreduction at 450 °C were higher by 40 and 100% (44) Parrillo, D. J.; Dolenec, D.; Gorte, R. J.; McCabe, R. W. J. Catal. 1993, 142, 708.

Takahashi et al.

compared to Ag-Y at pressures of 6 × 10-1 and 4 × 10-5 atm, respectively. On the other hand, the amounts of 1-butene adsorbed were the same at the lower pressures but were higher for Cu-Y at higher pressures, by 40% at 0.6 atm. One of the reasons for this enhancement in Cu-Y is simply the compositional differences. Because copper is lighter than silver and the copper content in Cu-Y (Cu/Al ) 0.463) was less than the silver content in Ag-Y (Ag/Al ) 1.13), the density of Cu-Y was lower by 26.3% (Na0.084Cu0.135Al0.292Si0.708O2; MW ) 70.27 vs Ag0.330Al0.292Si0.708O2; MW ) 95.36). Figure 2 shows that the cumulative pore volume up to 2.5 nm diameter was increased from 0.240 cm3/g in Ag-Y to 0.303 cm3/g in Cu-Y (a 26% increase). However, the increase in the adsorption amount by Cu-Y was more than 26%. It is considered that the further enhancement in the adsorbed amount beyond 26% especially for 1,3-butadiene was caused by the stronger interaction with Cu+. The differences in the C4 hydrocarbon adsorption amounts by various reduction procedures showed that reduction in He at 450 °C for 1 h (i.e., autoreduction) was enough to achieve the same 1,3butadiene adsorption by the Cu-Y obtained with CO reduction. However, autoreduction at 300 °C was not quite sufficient. 1.3-Butadiene and 1-butene adsorption amounts were examined from the viewpoint of zeolite pore volume and Cu site density in Cu-Y. The liquid densities of 1,3butadiene and 1-butene at 120 °C were calculated to be 0.455 and 0.425 g/cm3, respectively, by the modified Rackett equation of Spencer and Adler.39 Consequently, 4 mmol/g of 1,3-butadiene and 3.2 mmol/g of 1-butene corresponded to 0.47 and 0.42 cm3/g, respectively. These volumes were 40-60% larger than the pore volume of Cu-Y (0.303 g/cm3). Therefore, it was thought that all pores were occupied by adsorbates at the pressure of 0.6 atm. On the other hand, 26 Cu cations existed in one unit cell of Cu-Y (192 T atoms) on the basis of chemical analysis (56 Al and 0.463 Cu/Al ratio). Also, 4 mmol/g of 1,3butadiene corresponded to 53 molecules/uc, and 3.2 mmol/g of 1-butene corresponded to 42 molecules/uc. From the comparison of these numbers, it was found that more than one molecule of 1,3-butadiene/1-butene per Cu site was adsorbed by Cu-Y. It is worth pointing out that the calculated pore diameters of Cu-Y and Ag-Y (Figure 2) were quite different. This large difference could not be explained on the basis of differences in cation size and ion-exchange ratio. It was unreasonable for the pore size of Cu-Y to be 2-3 Å larger than that of Ag-Y. In the calculation of pore size distribution by the Horvath-Kawazoe (H-K) equation from nitrogen isotherms at 77 K, the same parameters of oxide ion for zeolite (i.e., diameter, polarizability, magnetic susceptibility and density) were used for Cu-Y and Ag-Y. However, the interaction between N2 molecules and Cu-Y was different (weaker than that between N2 and Ag-Y45), leading to the observed higher adsorption pressure of N2, which in turn gave rise to the larger calculated pore diameter. A new H-K model, which includes weak chemical interactions between cations in zeolite and adsorbates, is necessary to correct this difference. The separation factors of 1,3-butadiene over 1-butene are listed in Table 2. Calculated separation factors ranged from 10 to 100 000. These high separation factors originated from the fact that 1,3-butadine concentration is much lower than 1-butene in addition to the larger amounts 1,3-butadien adsorbed, since 1,3-butadiene is (45) Hutson, N. D.; Rege, S. U.; Yang, R. T. AIChE J. 1999, 45, 724.


Langmuir, Vol. 17, No. 26, 2001 8409

Table 1. Parameters for the D-A and L-F Equations D-A equation V0 (mL/g)

C

n

Ps (atm)

q0 (mmol/g)

0.494 0.512

0.0155 0.0168

1.04 0.97

23.81 23.81

4.211 4.256

13.5 14.6

8.71 7.53

0.538

0.0269

1.22

23.81

4.448

16.1

6.12

0.480 0.500

0.0915 0.1018

1.82 2.17

24.52 24.52

3.815 3.946

5.16 6.19

3.33 2.70

0.515

0.0756

2.17

24.52

4.056

9.87

3.28

1,3-butadiene 1,3-butadiene

0.437 0.476

0.0190 0.0234

0.93 1.79

23.81 23.81

3.726 4.150

8.95 16.9

8.89 8.96

none

1-butene

0.487

0.0909

1.61

24.52

3.721

5.03

3.45

none 120 °C for 1 h in 0.5 atm of H2/He 120 °C for 10 min in 0.7 atm of H2S/He none 120 °C for 1 h in 0.5 atm of H2/He 120 °C for 10 min in 0.7 atm of H2S/He

1,3-butadiene

0.397 0.009

0.0510 0.0783

1.19 14.90

23.81 23.81

3.727 0.072

3.90 5.64 × 106

6.53 0.60

0.250

0.0813

11.13

23.81

2.090

1.65× 105

0.72

0.371 0.172

0.0788 0.1066

1.38 3.59

24.52 24.52

2.520 1.592

9.83 6.82

3.25 2.24

0.247

0.1017

3.95

24.52

1.857

degas or reduction Cu-Y

Ag-Y

450 °C for 1 h in He

300 °C for 1 h in He 450 °C for 6 h in He followed by 450 °C for 12 h in 0.75 atm of CO/He 300 °C for 1 h in He

L-F equation

poisioning none 120 °C for 1 h in 0.5 atm of H2/He 120 °C for 10 min in 0.7 atm of H2S/He none 120 °C for 1 h in 0.5 atm of H2/He 120 °C for 10 min in 0.7 atm of H2S/He none none

adsorbate 1,3-butadiene

1-butene

1-butene

Figure 2. Cumulative pore volumes of Cu-Y and Ag-Y.

the impurity component in 1-butene product. Although the separation factors calculated by the two isotherm models (D-A and L-F) were not the same, both D-A and L-F equations led to similar dependence of separation factor on mixture composition. As intuitively expected from the adsorption isotherms, it was confirmed that Cu-Y exhibited higher separation factors than Ag-Y by an order of magnitude. The separation factors of Cu-Y after 450 °C autoreduction was the same with those after 450 °C CO reduction. Effect of H2S Exposure. The adsorption and desorption isotherms of H2S on Cu-Y and Ag-Y at 120 °C are compared in Figure 3. Similar to Ag-Y, H2S was irreversibly adsorbed on Cu-Y. However, the amount of H2S adsorbed on Cu-Y was smaller than Ag-Y. Hence the interaction between H2S and Cu-Y was weaker than that with Ag-Y. The effect of H2S exposure to Ag-Y was already reported in detail by Takahashi et al.22 Although H2S was irreversibly adsorbed on Ag-Y, its purification capability could be maintained by shifting the adsorption of both 1,3-butadiene and 1-butene to higher pressures. XPS analysis revealed that Ag in Ag-Y appeared to have reacted with H2S to form Ag2S.

B (1/atm)

50.0

n

1.50

1,3-Butadiene and 1-butene adsorption isotherms of Cu-Y before and after H2S exposures are shown in Figures 4 and 5. In the figures, the results on Ag-Y are also plotted for comparison. It was found that the excellent 1,3butadiene adsorption capability by Cu-Y was maintained completely after H2S exposure. The amounts of 1-butene adsorbed were actually increased slightly after H2S exposure. The enhancement of adsorption in the presence of a second component is not an entirely new phenomenon. Some previous work46-48 has reported the enhancement of adsorption of organic vapors on carbons with the coadsorption of H2O. The chemisorbed H2S apparently provided stronger interactions for 1-butene. The Cu ions clearly remained as cuprous ions for the strong π-complexation. The calculated separation factors in Table 2 indicated that they were decreased slightly by H2S exposure due to the slight increase in 1-butene adsorption. However, separation factors after H2S exposure are still better than Ag-Y before H2S exposure. This fact clearly demonstrates the superior poisoning resistance of Cu-Y against H2S compared to Ag-Y. Effect of H2 Exposure. H2 exposure to Ag-Y is detrimental because the π-complexation capability of Ag-Y is destroyed by H2 due to reduction of Ag+ to Ag0. To circumvent this problem, rejuvenation by oxidation was successfully demonstrated in a previous report.24 In this work, to understand the effect of H2 exposure to CuY, adsorption isotherms after H2 exposures are plotted in Figures 6 and 7. Again, the isotherms of Ag-Y are also included. As can be seen in the figure, both 1,3-butadiene and 1-butene isotherms of Cu-Y did not change at all by H2 exposure. The separation factors were maintained at a very high level. This outstanding H2 poisoning resistance is not surprising, when considering the fact that reduction of Cu-Y even at 450 °C in CO did not lead to any (46) Matsumura, Y.; Yamabe, K.; Takahashi, H. Carbon 1985, 23, 263. (47) Taqvi. S. M.; Appel, W. S.; LeVan, M. D. Ind. Eng. Chem. Res. 1999, 38, 240. (48) Kane, M. S.; Bushong, J. H.; Foley, H. C.; Brendley, W. H., Jr. Ind. Eng. Chem. Res. 1998, 37, 2416.

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Takahashi et al.

Table 2. Separation Factors for Cu-Y and Ag-Y estimated by the D-A and L-F Equations Cu-Y

Ag-Y

450 °C for 1 h in He for 1 h in He pressure (atm) 1,3-butadiene 1-butene D-A

L-F

0.01 0.001 0.0001 0.00001 0.01 0.001 0.0001 0.00001

1 1 1 1 1 1 1 1

degas or reduction poisoning

300 °C for 1 h in He

none

120 °C for1 h in 0.5 atm of H2/He

120 °C for 10 min in 0.7 atm of H2S/He

450 °C for 12 h in 0.75 atm of CO/He none

180 1200 9400 75000 170 1300 10000 77000

110 770 6000 49000 140 1000 7500 55000

88 560 4000 30000 84 580 4000 27000

190 1300 9800 74000 220 1700 13000 100000

none

120 °C for 10 min in 0.7 atm of H2S/He

82 530 3800 28000 29 200 1400 10000

240 130 2 620 250 100 42

Figure 3. Pure-component equilibrium isotherms at 120 °C for H2S adsorption and desorption on Cu-Y and Ag-Y. Figure 5. Pure-component equilibrium isotherms at 120 °C for 1-butene on Cu-Y and Ag-Y before and after H2S exposure. Curves are fitted with Dubinin-Astakhov (solid line) and Langmuir-Freundlich (dotted line) isotherms.

Figure 4. Pure-component equilibrium isotherms at 120 °C for 1,3-butadiene on Cu-Y and Ag-Y before and after H2S exposure. Curves are fitted with Dubinin-Astakhov (solid line) and Langmuir-Freundlich (dotted line) isotherms.

improvement or any degradation in terms of 1,3-butadiene/ 1-butene adsorption. XPS Results. Figure 8 shows X-ray photoemission spectroscopy (XPS) spectra for Cu 2P3/2 in Cu-Y dehydrated at room temperature and 450 °C. The binding energies of Cu 2P3/2 in Cu-Y and some copper compounds are listed in Table 3. The observed Cu 2P3/2 binding energies were shifted toward a lower energy after 450 °C dehydration in a vacuum. This shift by 0.7 eV is clearly caused by the autoreduction of some of the Cu2+ to Cu+ in Cu-Y. The coexistence of Cu2+ and Cu+, which will be shown shortly, resulted in the smaller shift (0.7 eV) of binding energy compared with the shift [933.4 and 935.4 eV for Cu(I) and Cu(II); 1.1 eV] in Cu-ZSM-5 (Si/Al ) 14)40 and that (0.9-1.3 eV) between CuO and Cu2O.41

Figure 6. Pure-component equilibrium isotherms at 120 °C for 1,3-butadiene adsorption on Cu-Y and Ag-Y before and after H2 exposure. Curves are fitted with Dubinin-Astakhov (solid line) and Langmuir-Freundlich (dotted line) isotherms.

Bolis et al. performed quantitative determination of Cu2+ content by splitting the XPS spectra of Cu 2P3/2 into several Gaussian curves.40 They showed that the XPS peak of copper centered at 933.4 eV became sharp during thermal activation at 450 °C and the content of Cu+ over total copper increased gradually from 2 h (Cu2+/(Cu2+ + Cu+) ) 0.397) to 6 h (0.171). However, the signal/noise ratios of XPS spectra obtained in this study were not good enough for quantitative determination. Instead of using XPS spectra, EPR analysis was used for the quantitative understanding of Cu2+ content in this study. As for the


Langmuir, Vol. 17, No. 26, 2001 8411

Figure 7. Pure-component equilibrium isotherms at 120 °C for 1-butene adsorption on Cu-Y and Ag-Y before and after H2 exposure. Curves are fitted with Dubinin-Astakhov (solid line) and Langmuir-Freundlich (dotted line) isotherms.

Figure 8. X-ray photoemission spectroscopy (XPS) spectra for Cu 2P3/2 in Cu-Y. Table 3. Binding Energies (eV) for Cu-Y material

BE (2P3/2)

Cu-Y dehydrated at rt Cu-Y dehydrated at 450 °C for 1 h Cu metal40 CuO40 Cu2O40

934.4 933.7 933.0 933.8-934.4 932.9-933.1

sulfide formation after H2S exposure, Ag2S formation could be detected by the peak shift of 0.7 eV compared to Ag+ in Ag-Y.22 However, because Cu2O and Cu2S have almost the same Cu 2P3/2 XPS binding energies, XPS analysis was not performed after H2S exposure. EPR Results. To understand the valence state of Cu in Cu-Y quantitatively, EPR analysis was performed for Cu-Y hydrated and dehydrated at 450 °C in a vacuum (Figure 9). Since EPR spectra are usually expressed as first derivatives of the absorption intensity, doubleintegrated areas are plotted in the figure (secondary axis). The ratio of EPR peak area of Cu-Y dehydrated at 450 °C over that of Cu-Y hydrated was 0.50, indicating that a half of Cu2+ was autoreduced to Cu+. This ratio of Cu+ over Cu2+ was in good agreement with previous work by Larsen et al.,29 in which EPR intensity in Cu ion-exchanged ZSM-5 (Si/Al ) 18) was reduced by 40% during the pretreatment in He at 410 °C. It may be useful to look more closely at the relationship between Cu+ content, Cu+ location in Cu-Y, and the

Figure 9. Electron paramagnetic resonance (EPR) spectra for Cu-Y.

amounts of 1,3-butadiene/1-butene adsorbed. The Na cation locations in Na-Y (Si/Al ) 2.43) and the locations of benzene adsorbed were investigated in detail by Fitch et al.49 in their powder neutron diffraction analysis. The location of the Na cations were 32 atoms on the SII site, 16 atoms on the SI site and 8 atoms on the SI′ site. It was also found that benzene was adsorbed preferentially near the SII cation site and the center of the 12-ring window. Until recently, the location of the Ag cation was not accurately determined. Using neutron diffraction data, Hutson et al.50 reported the Ag location and occupancies of Ag in Ag-Y (Si/Al ) 2.43) after 450 °C dehydration. Their results showed that the Ag cation sites were 28 atoms on the SII site, 4 atoms on the SII′ site, 11 atoms on the SI site, and 12 atoms on the SI′ site. Simultaneous occupancy of Ag cations at SII and SII′ as well as SI and SI′ sites was unlikely, due to the large repulsion. This location was very similar to the location of Na cations in Na-Y reported by Fitch et al. Takahashi and Yang51 showed in their Monte Carlo simulation study that the locations of adsorbed benzene in Ag-Y (Si/Al ) 2.43) were nearly the same as those in Na-Y. (The position of benzene adsorbed was slightly further away from the SII cation sites than that in Na-Y, due to the fact that the van der Waals radius of Ag+ is 30% larger than that of Na+.) The locations of Cu cations in Cu-Y (Si/Al ) 2.43) have not been thoroughly investigated yet. However, from the locations of Na or Ag cations and the adsorbed benzene, it may be assumed that Cu2+ at SII sites in Cu-Y can be reduced to Cu+ first, because SII sites are exposed to the supercage of Y-zeolite. (SIII cation-exchange site does not seem to exist in Y-zeolite with a Si/Al of 2.43.) It is likely that these Cu+ cations in the SII site are mainly responsible for the adsorption of 1,3-butadiene/1-butene. For this reason, 50% reduction of Cu2+ to Cu+ by autoreduction at 450 °C was enough for the 1,3-butadiene adsorption, hence giving the same adsorption amounts between CO reduction and autoreduction, although CO reduction might lead to much higher reduction ratios than autoreduction, as shown by Larsen et al.29 Larson et al. also investigated the coordination states of Cu2+ in ZSM-5 by using the characteristic structure (49) Fitch, A. N.; Jobic, H.; Renourez, A. A. J. Phys. Chem. 1986, 90, 1311. (50) Hutson, N. D.; Reisner, B. A.; Yang, R. T.; Toby, B. H. Chem. Mater. 2000, 12, 3020. (51) Takahashi, A.; Yang, R. T. New Adsorbents for Purification: Selective Removal of Aromatics. Submitted for Publication in AIChE J.

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Figure 10. Fractional uptake curves of 1,3-butadiene on Cu-Y and Ag-Y.

between 2600 and 3000 G, hyperfine coupling of the 3d unpaired electron to the copper (I ) 3/2) nuclear spin.29 By fitting the spectra, square-planar or square-pyramidal coordination of Cu2+ in zeolite were resolved. Carl and Larson obtained the hyperfine structure for Cu2+ in Cu-Y with a very small Cu loading (Si/Al ) 2, Cu/Al ) 0.01).43 If information on the coordination of Cu2+ in Cu-Y was available, it would be possible to examine the states of Cu2+ in more detail, which was discussed in the section of chemical compositions. However, good hyperfine structures were not obtained, even at the low temperature of -180 °C. This is because we used Cu-Y whose copper loading was so high that the formation of mobile copper complex hindered the formation of hyperfine structure.42 The mechanisms of autoreduction of Cu2+ in zeolites have been proposed by a number of groups. The following are two representative mechanisms:

(i) Mechanism 1 by Larson et al.29 [Cu2+OH-]+ T Cu+ + OH [Cu2+OH-]+ + OH T Cu2+O- + H2O 2[Cu2+OH-]+ T Cu+ + Cu2+O- + H2O (ii) Mechanism 252-54 2[Cu2+OH-]+ T [CuOCu]2+ + H2O [CuOCu]2+ T 2Cu+ + 1/2O2 Since good hyperfine structures could not be obtained, it is not possible to determine which mechanism is suitable in this work. However, it appears that mechanism 2 is generally accepted for Cu-Y, while mechanism 1 is suitable for Cu-ZSM5. Uptake Rates. Figures 10 and 11 show the fractional uptake curves of 1,3-butadiene and 1-butene. 1,3-Butadiene uptake in Cu-Y was slower than that in Ag-Y, while 1-butene uptake by Cu-Y was faster. This result is not favorable for the application of 1-butene purification by removal of 1,3-butadiene. In Table 4, diffusion time constants between 1,3-butadiene and 1-butene were compared. Although the diffusion time constants support (52) Iwamoto, M.; Yahiro, H.; Tanda, H.; Mizuno, N.; Mine, Y.; Kagawa, J. Phys. Chem. 1991, 95, 3727. (53) Sarkany, J.; d’Itri, J.; Sachtler, W. M. H. Catal. Lett. 1992, 16, 241. (54) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97, 7054.

Takahashi et al.

Figure 11. Fractional uptake curves of 1-butene on Cu-Y and Ag-Y. Table 4. Diffusion Time Constants (1/s) of 1,3-butadiene and 1-butene

adsorbate 1,3-butadiene 1-butene

pressure change (atm) 0 f 3.6 × 10-5 4.2 × 10-5 f 1.5 × 10-3 7.6 × 10-4 f 1.7 × 10-3

Cu-Y after 450 °C for 1 h in He

Ag-Y after 300 °C for 1 h in He

2 × 10-6 1 × 10-4

7 × 10-6-3 × 10-5 9 × 10-4

6 × 10-4

4 × 10-4

the tendency described above, the order of magnitude was the same between Cu-Y and Ag-Y. Thus, it appears that this disadvantage does not have much effect on the practical application of Cu-Y. Further investigations such as mixed-gas breakthrough experiments are necessary to judge the tradeoff between better separation factor and slower uptake. The uptake rates of 1,3-butadiene/1-butene were not different with different reduction methods (CO reduction vs autoreduction). Also, H2S and H2 did not show any dramatic effects on the uptake rates of 1,3-butadiene/ 1-butene. The reason for slower uptake was caused by the stronger interaction of Cu-Y to 1,3-butadiene, resulting in a longer residence time at the adsorption site. As described by Ka¨rger et al.,55 there was a parallel increase in the correlation time, reflecting the increase in the meanresidence time at the adsorption site, rather than any changes in the root-mean-square jump distance. Previous study of 1,3-butadiene adsorption on Ag-Y22 supported this fact. The diffusion time constants of 1,3-butadiene into Y-zeolites decreased as the Ag content in the mixedcation-exchanged AgNa-Y increased. In their work, the tradeoff between separation factor and uptake rate was also shown. AgNa-Y with an intermediate Ag loading (34 Ag/uc, 26wt %) had excellent adsorption performance in terms of both separation factor and uptake rate. In this context, CuNa-Y with less Cu content may be optimum. Acknowledgment. Neutron Activation Analysis (NAA) was conducted in the Ford Nuclear Reactor of the Phoenix Memorial Laboratory at the University of Michigan. The analysis was conducted by Dr. Leah Minc of the Michigan Memorial Phoenix Project. X-ray photoemission spectroscopy was conducted using the instrumentation in the University of Michigan’s Electron Microscopy Analysis Laboratory (EMAL). We would like to thank Dr. Corinna Wauchope in EMAL for teaching A.T. how to use the X-ray (55) Ka¨rger, J.; Michel, D.; Petzold, A.; Caro, J.; Pfeifer, H.; Scho¨llner, R. Z. Phys. Chem. (Leipzig) 1976, 257, 1009.


photoemission spectroscope and reaction chamber. Electron paramagnetic resonance (EPR) was performed using the instrumentation at the Chemistry Department in the University of Michigan. We would like to thank Dr. James Windak at the Chemistry Department for teaching A.T. how to use the EPR spectroscope. Partial support by NSF under CTS-9819008 and by NGK Insulators, Inc. (Nagoya, Japan) is gratefully acknowledged. Nomenclature B ) Langmuir constant C ) constant for Dubinin-Astakhov (D-A) equation D ) diffusivity E0 ) characteristic energy of sorbent n ) constant P ) pressure Ps ) saturated vapor pressure of adsorbate q ) molar adsorbed amount

Langmuir, Vol. 17, No. 26, 2001 8413 q0 ) molar saturated adsorbed amount R ) gas constant r ) radius T ) temperature V ) volumetric adsorbed amount V0) volumetric saturated adsorbed amount X ) equilibrium mole fraction in adsorbed phase Y ) equilibrium mole fraction in gas phase Greek Letters R ) separation factor β ) affinity coefficient Subscript i ) component in the mixture s ) saturation LA011196Z

Olefin Separation

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