Aliphatics Separation by Adsorption: New Sorbents for

3856

Ind. Eng. Chem. Res. 2000, 39, 3856-3867

Aromatics/Aliphatics Separation by Adsorption: New Sorbents for Selective Aromatics Adsorption by π-Complexation Akira Takahashi, Frances H. Yang, and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

New sorbents for benzene/cyclohexane separation based on π-complexation were prepared by dispersion of transition metal salts on a high-surface-area substrate. PdCl2 or AgNO3 dispersed on SiO2 gel exhibited high equilibrium adsorption ratios of benzene over cyclohexane. PdCl2 loading of 0.88 g/g of SiO2 showed the best benzene/cyclohexane ratio of 3.2. The heats of adsorption of benzene on these sorbents were in the range 7-12 kcal/mol and followed the rank order CuCl > PdCl2 > AgNO3 > AuCl3 > PtCl4, compared to 5-7 kcal/mol for cyclohexane. Molecular orbital calculations for the bonding of benzene and chlorides of these metals were performed at the Hartree-Fock (HF) and density functional theory (DFT) levels using effective core potentials. The theoretical bond energies were (in kcal/mol) 12.5 (CuCl), 10.8 (PdCl2), 8.6 (AgCl), 6.5 (AuCl3), and 5.2 (PtCl4), in fair agreement with the experimental results. The M-C interactions followed classical π-complexation in most cases, and differences in the bonds with these metal ions are explained by natural bond orbital results. Introduction Separations of aromatics/aliphatics are crucially important in the chemical and petrochemical industries. They are also a most difficult class of separations. Simple fractional distillation is not useful because of the close relative volatilities between aromatics and aliphatics; for example, the boiling point of benzene is 80 °C, while that of cyclohexane is 81 °C. Aromatics/aliphatics separation is a key step in the production of the basic chemical feedstock, BTX (i.e., benzene, toluene, and xylene), as well as in the production of fuels. Aromatics are premium blending stocks for motor fuels, as the high-octane-number hydrocarbons in the gasoline boiling range are primarily aromatic hydrocarbons. In these processes, high-purity aromatics must be separated from the catalytic reformates containing 45-60% aromatics.1-4 In a typical process, the reformate from the catalytic reactor (e.g., “Platforming”) is sent to a reformate splitter column. The C7- fractions from the overhead are sent to a separation process to separate benzene or toluene from aliphatics, while the C8+ fractions from the bottom are sent for xylene recovery. Aromatics/aliphatics separations are being accomplished by solvent extraction. A number of solvents have been used.1,5-7 The first efficient method for recovery of aromatics was the Udex process of Union Carbide (in 1952) using a glycol-based solvent. Since then, several other solvents, such as sulfolane and N-methyl pyrrolidone, have also been developed. These solvent extraction processes generally consist of three parts: extractor, extractive distillation column, and solvent stripper column. Although these separation processes are efficient, they are highly energy intensive, and more importantly, the solvents are increasingly posing as environmental hazards. One of the most representative solvents, sulfolane, is potentially hazardous and is now regulated by the Federal Toxic Substances Control Act * Corresponding author. Telephone: (734)936-0771. Fax: (734)763-0459. E-mail: [email protected].

of EPA, Section 8(a) (http://www.epa.com). It was recently reported that the groundwater in Brisbane, Australia, was contaminated by sulfolane from dumping that occurred over 20 years ago.8 Another possible separation technique is fractional distillation. As mentioned, it is difficult because of the close relative volatilities. For benzene/cyclohexane, the mixture has a minimum azeotrope at about 53%. Therefore, acetone is added as an entrainer, and a complex hybrid system (distillation combined with extraction in this case) can be used for separation.9 Because of the importance of aromatics/aliphatics separation and the problems associated with solvent extraction, possible alternatives have been studied. These include pervaporation,10 liquid membranes,11 and the use of liquid inclusion complexes.12 Purification of dilute aromatics from aliphatics (e.g., toluene and/or xylene in heptane) by temperature swing adsorption (TSA) was also studied in the liquid phase.13 The adsorbents considered by Matz and Knaebel were silica gel, activated alumina, activated carbon, zeolite 13X, and polymeric adsorbent (XAD-7). They claimed that silica gel was the best sorbent because of its superior thermal property. Among the separation technologies, adsorption is playing an increasingly important role.14 It is energyefficient and does not produce wastes. However, its utility depends entirely on the availability of selective sorbents. The conventional sorbents and separations using these sorbents are restricted to van der Waals and electrostatic interactions between the sorbent and sorbate. Studies of new sorbents/separations using weak chemical bonds such as chemical complexation have begun only recently in our laboratory. As suggested by King,15 chemical complexation bonds are generally stronger than van der Waals interaction, yet weak enough to be reversible. Therefore, tremendous opportunity exists for developing new sorbents by using weak chemical bonds, including various forms of complexation bonds.

10.1021/ie000376l CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3857

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.16 These metals or their ions can form the normal σ bond to carbon, and in addition, the unique characteristics of the d orbitals in these metals or ions enable them to form bonds with unsaturated hydrocarbons in a nonclassical manner. This type of bonding is broadly referred to as π-complexation. This π-complexation was seriously considered for olefin/ paraffin separation and purification by employing liquid solutions containing silver or cuprous ions.17-22 These involved gas-liquid operations. Although gas-solid operation can be simpler as well as more efficient, particularly by pressure swing adsorption, the list of attempts for developing solid π-complexation sorbent is a short one. CuCl, which is insoluble in water, has been considered in the powder form for olefin/paraffin separations.23-25 However, CuCl, in the solid particulate form, dose not have a high surface area for the desired high olefin sorption capacity. Other attempts at developing solid sorbents for π-complexation included silver salts supported on anion-exchange resins,26,27 Ag+-Y Zeolite, 28 and Cu-Y Zeolite.29 However, these were met with limited success. The only commercially available sorbent based on π-complexation is CuCl/γ-Al2O3 for CO separation.30-33 Selectivities for olefins of the known sorbents are not high enough to be used for commercial PSA processes. Hence, their use would require additional, substantial operations.34 More recently, several new sorbents based on π-complexation were prepared for selective olefin adsorption. These include Ag+-exchanged resins and monolayer CuCl/γ-alumina,28,35 monolayer CuCl on pillared clays,36 and monolayer AgNO3/SiO2.37-39 No selective sorbents are known for aromatics/aliphatics separations. Based on our understanding of π-complexation, it is certainly possible to develop such sorbents based on π-complexation. In the benzene molecule, the carbon atom is sp2 hybridized. Hence, each carbon has three sp2 orbitals and another pz orbital. The six pz orbitals in the benzene ring form the conjugative π bond. The pz orbitals also form the antibonding π* orbitals, which are not occupied. When benzene interacts with transition metals, the π orbitals of benzene can overlap with the empty outer-shell s orbital of the transition metal to form a σ bond. Moreover, it is possible that the vacant antibonding π* orbital of benzene can overlap with the d orbitals in the transition metal, in a manner similar to the formation of the olefin-Cu+ bond.40 In this work, monolayer salts containing transition metals spread on high-surface-area substrates were prepared and studied as selective sorbents for aromatics for use in aromatics/aliphatics bulk separations. Benzene and cyclohexane form an ideal pair of model compounds for developing selective sorbents for aromatics. These molecules have similar shapes, and as mentioned, the boiling point of benzene is 80 °C, while that of cyclohexane is 81 °C. The kinetic diameter of benzene, which is calculated from the minimum equilibrium cross-sectional diameter, is estimated to be 5.85 Å, compared with 6.0 Å for cyclohexane.41 Therefore, benzene and cyclohexane were used in this work.

surface-area substrates. Based on the results of selective olefin sorbents for olefin/paraffin separations,28,35-39 Cu+, Ag+, Pt4+, and Pd2+ cations were the most promising sorbents because of their strong interactions with the olefin molecules. From the results of molecular orbital calculations, Cu+ cation would form stronger π-complexation bonds than Ag+.40 PdCl2 (Strem Chemicals), CuCl2 (Aldrich), AgNO3 (Strem Chemicals), PtCl4 (Strem Chemicals), and AuCl3 (Strem Chemicals) were selected for transition metal salts because of their solubilities to water. The highsurface-area substrates were SiO2 gel (8-20 mesh or 100-200 mesh, Strem Chemicals), Al2O3 (PSD-350, ALCOA), and activated carbon (BPL 4 × 10, Calgon). Dispersion of salts on the substrates was performed by the incipient wetness method, which is used in the manufacturing of supported catalysts, except for PdCl2/ activated carbon. An aqueous solution of the desired salt was first prepared. A volume of the solution equal to the total pore volume of the substrate was brought into contact with the substrate. After the substrate had imbibed the solution containing the salt into its pore structure, the sample was heated at 100 °C to remove the solvent. This procedure was repeated several times until the predetermined loading amount was achieved.39 As for the Cu+ sorbent, it was prepared by two steps: (1) impregnation of Cu2+ salt from solution on the substrates by incipient wetness and (2) reduction of Cu2+ to Cu+ by temperature-programmed reduction (TPR). This procedure was necessary because stable Cu+ salts were not water-soluble. The TPR conditions will be explained shortly. PdCl2/activated carbon was prepared by the impregnation method, as described by Tamon et al.42 Because activated carbon has a low affinity for water, the incipient wetness method is not suitable in the aqueous system because of poor wetting. A predetermined amount of PdCl2 was magnetically stirred in 15 cm3 of 1 N HCl aqueous solution at room temperature, and 4 g of activated carbon was added to the mixture. The carbon was impregnated with the solution for 1 day. It was washed by deionized water and was dried at 110 °C in air. The adsorbents prepared and studied in this work are summarized in Table 1. BET surface areas and pore volumes determined by N2 isotherms at 77 K are also provided. Adsorption Isotherms and Uptake Rates. Singlecomponent isotherms for benzene and cyclohexane were measured using a standard gravimetric method. A Shimadzu TGA-50 automatic recording microbalance was used. Helium (Prepurified grade, Metro welding 99.995%) was used as the carrier gas and was first passed through two consecutive gas-wash bottles that contained benzene (HPLC grade, Sigma-Aldrich, >99.9%) or cyclohexane (HPLC grade, Aldrich, >99.9%) in order to obtain saturated benzene or cyclohexane vapor. After the concentration was diluted to the desired value by being blended with additional helium (total flow volume: 250 cm3/min), the mixture was directed into the microbalance. Isosteric heats of adsorption were calculated using the Clausius-Clapeyron equation, eq 1, from isotherms at different temperatures.

Experimental Section Sorbent Preparation. The sorbents in this work were transition metal salts that were dispersed on high-

ln P (d dT ) ) RT

qst 2

(1)

3858

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000

Table 1. Adsorbents Examined in This Work salt

substrate

salt loading (g/g of SiO2)

sample preparation

BET area (m2/g)

pore volume (cm3/g)

PdCl2 PdCl2 PdCl2 PdCl2 PdCl2 AgNO3 CuCl CuCl PtCl4 AuCl3 PdCl2 none none none none

SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (8-20 mesh) Al2O3 SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (100-200 mesh) activated carbon SiO2 (100-200 mesh) SiO2 (8-20 mesh) Al2O3 activated carbon

0.045 0.121 0.305 0.600 0.880 0.330 0.500 0.500 0.460 1.000 0.305 0 0 0 0

incipient wetness incipient wetness incipient wetness incipient wetness incipient wetness incipient wetness incipient wetness+TPR incipient wetness + TPR incipient wetness incipient wetness Impregnation as-received as-received as-received as-received

599 485 306 341 284 298 214 437 271 356 673 669 650 340 891

0.35 0.28 0.18 0.20 0.17 0.18 0.28 0.24 0.14 0.20 0.39 0.38 0.40 0.57 0.52

Adsorption isotherms of propylene (CP grade, Matheson minimum purity 99.0%) and propane (CP grade, Matheson minimum purity 99.0%) were measured volumetrically using a Micromeritics ASAP 2010 system. Nitrogen isotherms at 77 K, also measured with the Micromeritics ASAP 2010, were used for BET surface area and pore volume determinations. BET surface areas were calculated from the isotherms at a relative pressure (P/Po) of 0.06-0.20. Pore volumes were calculated at 0.95 P/Po. The overall diffusion time constant values, D/r2, were calculated from the uptake rates.43 In this work, the short time region (Mt/Mi < 0.3) was used for the calculation of D/r2 by plotting Mt/Mi vs t1/2 based on following relation

( )

Mt 6 Dt ) Mi xπ r2

1/2

(2)

Molecular Orbital Computational Details. Molecular orbital (MO) studies on π-complexation bonding for olefins and sorbent surfaces have been extensively investigated.40,44-45 In this work, similar MO studies were extended to benzene and sorbent surfaces. The Gaussian 94 progam46 in the Cerius2 molecular modeling software47 from Molecular Simulation, Inc., was used for all calculations. MO calculations for benzene and sorbent surfaces were performed at the HartreeFock (HF) and density functional theory (DFT) levels using effective core potentials (ECPs).48-50 The LanL2DZ basis set51 is a double-ζ basis set containing ECP representations of electrons near the nuclei for postthird-row atoms. The reliability of this basis set has been confirmed by the accuracy of calculation results as compared with experimental data. Therefore, the LanL2DZ basis set was employed for both geometry optimization and natural bond orbital (NBO) analysis. Geometry Optimization and Energy of Adsorption Calculations. The restricted Hartree-Fock (RHF) theory with the LanL2DZ level basis set was used to determine the geometries and the bonding energies of benzene on AgCl and CuCl, as Ag+ and Cu+ have filled d orbitals with spin ) 1. However, the unrestricted Hartree-Fock (UHF) theory with the LanL2DZ level basis set was used to determine the geometries and bonding energies of benzene on PdCl2, AuCl3, and PtCl4, because Pd2+, Au3+, and Pt4+ have unfilled d orbitals with spin > 1. Two of the computational models used for the adsorption systems are shown in Figures 17 and 18. The simplest models with only a single metal chloride

interacting with a benzene molecule were chosen for π-complexation studies. The optimized structures were then used for bond energy calculations according to the following expression:

Eads ) Eadsorbate + Eadsorbent - Eadsorbent-adsorbate (3) where Eadsorbate is the total energy of benzene; Eadsorbent is the total energy of the bare adsorbent, i.e., the metal chloride; and Eadsorbent-adsorbate is the total energy of the adsorbate/adsorbent system. A higher value of Eads corresponds to a stronger adsorption. Natural Bond Orbital (NBO) Analysis. The optimized structures were also used for NBO analysis at the B3LYP/LanL2DZ level. The B3LYP52 approach is one of the most useful self-consistent hybrid (SCH) approaches,53 which is Beck’s three-parameter nonlocal exchange functional54 with nonlocal correlation functional of Lee, Yang, and Parr.55 The NBO analysis performs population analysis that pertains to localized wave-function properties. It gives a better description of the electron distribution in compounds of high ionic character, such as those containing metal atoms.56 It is known to be sensitive for calculations of localized weak interactions, such as charge transfer, hydrogen bonding, and weak chemisorption. Therefore, the NBO program57 was used for studying the electron density distribution of the adsorption system. Results and Discussion Temperature-Programmed Reduction of CuCl2/ Al2O3. Because CuICl is not water soluble, CuI sorbent was prepared by incipient wetness of CuIICl2 on Al2O3 followed by temperature-programmed reduction (TPR) to convert Cu2+ to Cu+. To understand its reduction behavior, CuCl2/Al2O3 was first heated in a flow of 5.3% H2 in He up to 700 °C at a heating rate of 10 °C/min. The TPR effluent from the bed of the sample was analyzed by a gas chromatograph (Shimadzu, GC-14A). The intensity measured by the thermal conductivity detector (TCD) indicated H2 consumption. The result is shown in Figure 1a. During the heat treatment of CuCl2/ Al2O3 (0.50 g/g) in H2, two distinct peaks, at 270 °C and 410 °C, were detected. These two peaks corresponded to two reduction steps. The first peak at 270 °C indicated the change from Cu2+ to Cu+, and the second peak indicated the change from Cu+ to Cu0. The difference in intensities of the two peaks was likely caused by incomplete conversion of Cu+ to Cu0. Consequently, a long tail from 450 to 700 °C appeared. From


Figure 2. Pure-component equilibrium isotherms for propylene and propane adsorption on PdCl2/SiO2 (0.88 g/g) and AgNO3/SiO2 (0.33 g/g) at 70 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Figure 1. Temperature-programmed reduction (TPR) of CuCl2 supported on Al2O3 in H2 (5.3% H2 in He). The intensity shows H2 consumption. (a) H2 consumption at 10 °C/min. (b) H2 consumption during sample preparation for CuCl/SiO2. Table 2. Calculated Monolayer Dispersion Amounts salt

substrate

monolayer loading (g/g)

PdCl2 PdCl2 CuCl CuCl PtCl4 AuCl3

SiO2 activated carbon Al2O3 SiO2 SiO2 SiO2

0.88 1.16 0.49 0.96 0.83 1.00

the result above, it was determined that 270 °C was the appropriate temperature for reduction to CuCl. The intensity change of the TCD detector during the CuCl2/ Al2O3 (0.50 g/g-SiO2) sorbent preparation is shown in Figure 1b. The intensity at 270 °C began to decrease after 70 min, indicating that most of the Cu2+ was converted to Cu+. Subsequently, CuCl2/Al2O3 sorbents with different salt loadings were reduced at 270 °C for 1 h in the presence of 5.3% H2. Propane/Propylene Isotherms. Before isotherms for benzene and cyclohexane were measured, isotherms for propylene and propane were determined in order to confirm the π-complexation capabilities of the sorbents. The optimum loading amounts of transition metal salts, at which propylene/propane adsorption ratios (at 1.0 atm and 70 °C) reached maxima, were determined experimentally by changing the salt loadings. The experimental optimum loading amounts were found to be almost the same as the theoretical monolayer loading amounts, as shown in Table 2. XRD analysis by Padin revealed that the diffraction peaks of AgNO3 disappear completely after incipient wetness dispersion on SiO2, indicating monolayer dispersion.58 Additional evidence for monolayer dispersion was the absence of the adsorption characteristics of alumina, indicating the lack of any bare surface. Theoretical monolayer loading amounts for these chlorides were calculated using the assumption that Cl- (radius: 0.181 nm) was dispersed on the

Figure 3. Pure-component equilibrium isotherms for propylene and propane adsorption on CuCl/Al2O3 (0.50 g/g) at 70 °C and CuCl/SiO2 (0.50 g/g) at 25 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

surface as a monolayer in a close-packing manner.59 In the case of PtCl4/SiO2, 0.83 g/g loading of PtCl4 showed a slightly higher adsorption ratio of propylene over propane than 0.46 g/g loading. However, the BET surface area was reduced to 137 m2/g with the higher loading. Therefore, PtCl4/SiO2 (0.46 g/g) was used for benzene/cyclohexane adsorption. The isotherms of propane and propylene on sorbents at optimum loadings are shown in Figures 2-4, and they are compared with those on bare SiO2 and Al2O3 (Figure 5). Isotherms fit by eqs 4 and 5, which will be discussed later, are also plotted in these figures as solid curves. Here,a heterogeneity factor of 5 was used for propylene isotherms except for that of AuCl3/SiO2. For AuCl3/SiO2, s ) 3 was used for proper fitting. Pure-component adsorption ratios of propylene over propane at 70 °C by bare Al2O3 and SiO2 were near 1.5. In contrast, those by transition metal salts dispersed on SiO2 or Al2O3 were around 3-4 (except for CuCl/SiO2 and AuCl3/SiO2), indicating π-complexation with the olefin. PdCl2/SiO2 (0.88 g/g) and PtCl4/SiO2(0.46 g/g) showed the highest ratios of 4.0 at 70 °C. However, CuCl/SiO2 (0.50 g/g) showed a poor ratio of 1.4.

3860


Figure 4. Pure-component equilibrium isotherms for propylene and propane adsorption on PtCl4/SiO2 (0.46 g/g) and AuCl3/SiO2 (1.00 g/g) at 70 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Figure 5. Pure-component equilibrium isotherms for propylene and propane adsorption on bare SiO2 and Al2O3 at 70 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Benzene/Cyclohexane Isotherms. Benzene and cyclohexane adsorption isotherms were measured for the selected sorbents that showed π-complexation capability for propylene. PdCl2 impregnated on activated carbon (0.305 g of PdCl2/g of SiO2) was also used for benzene/cyclohexane adsorption measurements, as this sorbent exhibited excellent sorption capability to carbon monoxide, also by π-complexation.42 Adsorption isotherms are shown in Figures 6-11. Isotherms fit by eqs 4 and 5, which will be discussed later, are also plotted in the figures. For comparison, the isotherms for benzene and cyclohexane on the bare substrates are shown in Figures 12-14. PdCl2/SiO2 (0.88 g/g) showed the best benzene/cyclohexane adsorption ratios among the sorbents examined (Table 3). The pure component adsorption ratio, at 0.1 atm pressure, was 3.2 at 120 °C and 2.9 at 100 °C. Benzene and cyclohexane isotherms were also measured on this sorbent at 70 °C (results not shown here). The amounts adsorbed were higher but the benzene/cyclohexane ratios were lower, being 1.5 at 0.1 atm. The reason for the higher adsorption ratios at higher temperatures is related to nonselective pore filling by

Figure 6. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on PdCl2/SiO2 (0.88 g/g) at 100 and 120 °C.

Figure 7. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on CuCl/Al2O3 (0.50 g/g) at 100 and 120 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

benzene and cyclohexane. At low temperatures, pore filling of benzene and cyclohexane occurs readily even in the larger pores. According to the Horvath-Kawazoe theory,60 the pore diameters for pore filling by benzene and cyclohexane at 0.1 atm and 70 °C are 3.2 and 3.1 nm, respectively. Those at 120 °C are 1.8 and 1.7 nm, respectively. These calculation results indicate that both benzene and cyclohexane are adsorbed in the pores of the sorbent at lower temperatures, leading to lower adsorption ratios. At higher temperatures, pore filling does not occur, and benzene is selectively adsorbed by π-complexation, leading to the higher adsorption ratios. The ratios of 2.9 and 3.2 are encouraging for separation by cyclic processes. Furthermore, isotherms in this pressure range were not steep, which is also desirable for cyclic adsorption/desorption. The pure-component ratio on AgNO3/SiO2 (0.33 g/g) at 120 °C was also higher than that on bare SiO2. However, the ratio of 2.0 for AgNO3/SiO2 (0.33 g/g) was not high enough for bulk separation. PtCl4/SiO2 and CuCl/Al2O3 showed low benzene/cyclohexane adsorption ratios, although these sorbents had excellent propylene/propane adsorption ratios. In conclusion, PdCl2/SiO2 and AgNO3/SiO2 showed enhanced adsorption ratios due to π-complexation with


Figure 8. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on AgNO3/SiO2 (0.33 g/g) at 100 and 120 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Figure 10. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on PtCl4/SiO2 (0.46 g/g) at 85 and 100 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Figure 9. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on PdCl2/activated carbon (0.305 g/g) at 120 and 180 °C.

Figure 11. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on AuCl3/SiO2 (1.00 g/g) at 100 and 120 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

benzene, though PtCl4/SiO2 and CuCl/Al2O3 did not yield high ratios. PdCl2/activated carbon showed adsorption behavior that was different from that of other sorbents. Cyclohexane adsorption amounts in the low-pressure range below 0.03 atm were very small, hence leading to very large benzene/cyclohexane adsorption ratios in this pressure range, indicating π-complexation with benzene by PdCl2. The high benzene/cyclohexane ratios are excellent for purification purposes. The stepped isotherm behavior was very similar to the adsorption behavior of bare activated carbon, although the adsorption amounts for bare activated carbon were larger than those for PdCl2-impregnated carbon. This reduction was apparently caused by the reduction of surface area by PdCl2 dispersion. In this experiment, the PdCl2 impregnation amount to activated carbon was 1/4 of a monolayer (Table 2). Much larger loading might be necessary to enhance the benzene/cyclohexane ratio in a higher pressure range. The reversibility of the isotherms of benzene and cyclohexane on PdCl2/SiO2 (0.88 g/g) at 120 °C was also measured (results not shown here). Both benzene and

cyclohexane isotherms were reversible between 0 and 0.1 atm. PdCl2/SiO2 sorbents with different salt loadings also showed reversible isotherms. The step-shaped isotherms of cyclohexane on activated carbon and also on PdCl2/carbon merit an explanation. This behavior has also been observed for cyclohexane on Y zeolites (results not shown here). This behavior is best explained with the Horvath-Kawazoe theory,60 which predicts the threshold relative pressure at which pores begin to fill, on the basis of the sorbatesurface interaction potential. Using a reasonable Lennard-Jones potential for cyclohexane and carbon, the threshold pressure of 0.02-0.03 atm (from the experimental data) corresponds to a pore size around 1 nm. This is a reasonable size for the pore constrictions in activated carbon. This phenomenon did not occur for benzene, because the interaction is much stronger with benzene; hence, the threshold pressure is much lower and is not seen in the isotherms as plotted. Effects of PdCl2 Loading on Benzene/Cyclohexane Selectivity. To understand the effect of PdCl2 loading on benzene/cyclohexane adsorption, the loading

3862


Figure 12. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on bare SiO2 at 100 and 120 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

Figure 13. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on bare Al2O3 at 100 and 120 °C. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5.

amount of PdCl2 was varied from 0.045 g/g to 0.88 g/g SiO2. Adsorption isotherms for benzene and cyclohexane are compared in Figure 15. It is seen that both benzene and cyclohexane adsorption amounts were decreased with increasing PdCl2 loading because of decreased surface areas. However, the pure-component benzene/ cyclohexane ratio continued to increase with PdCl2 loading, which was clearly due to π-complexation of benzene with Pd2+. The pure-component adsorbed amounts at 0.1 atm and the adsorption ratios are plotted in Figure 16. The adsorption ratio reached to 3.2 at a PdCl2 loading of 0.88 g/g-SiO2. This loading corresponded to the calculated monolayer value for PdCl2 on SiO2 (Table 2). As mentioned, the amount adsorbed continued to decrease as the loading was increased. The surface area also decreased (Table 1). After normalization of the adsorbed amounts by surface area, the amount adsorbed of benzene was always higher (by 15-75%) than that on bare SiO2 because of π-complexation. At a PdCl2 loading of 0.305 g/g of SiO2 in particular, a 75% increase was obtained. On the other hand, the cyclohexane

Figure 14. Pure-component equilibrium isotherms for benzene and cyclohexane adsorption on activated carbon at 120 and 180 °C.

adsorption was decreased at loadings between 0.60 and 0.88 g/g when the surface area remained unchanged. This decrease was caused by the lower affinity of PdCl2 for cyclohexane because of the nonpolar nature of this molecule. For both benzene and cyclohexane, the maximum amount adsorbed per surface area reached a maximum at the loading of 0.33 g/g. Ideally, benzene adsorption should reach its maximum at the monolayer loading (0.88 g/g). This result was attributed to the poor dispersion of PdCl2 on SiO2 at high loadings, i.e., loadings > 0.33 g/g. The poor dispersion was also related to pore blockage at high loadings. Pore blockage would limit diffusion of the benzene and cyclohexane molecules into the sorbent. N2, on the other hand, is a smaller molecule; hence, it was not limited in the measurements of surface area. Equilibrium Isotherm Model. The adsorption of cyclohexane molecules includes physical adsorption only; thus, it can be modeled well by the Langmuir isotherm with two parameters shown in eq 4.

q)

qmpbpP 1 + bpP

(4)

On the other hand, the adsorption of benzene molecules on PdCl2/SiO2 and AgNO3/SiO2 includes both physical adsorption and chemisorption by π-complexation. Therefore, a different model is required to account for chemisorption. The isotherm model developed by Yang and Kikkinides28 to account for both interactions is shown in eq 5.

q)

(

qmpbpP qmc 1 + bcPes + ln 1 + bpP 2s 1 + bcPe-s

)

(5)

The first term accounts for physical adsorption, while the second term represents contributions from chemisorption. The second term also takes into account the energetic heterogeneity of the surface ion sites available for complexation. When a chemical bond is formed, the electron distribution in the molecular orbital is altered, resulting in substantial weakening of the van der Waals forces. This weakening is considered to be mainly due to the reduced

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3863 Table 3. Pure-Component Adsorption Ratios for Benzene/Cyclohexane and Propylene/Propane equilibrium adsorption ratios of benzene and cyclohexane at 0.1 atm salt

substrate

PdCl2 AgNO3 CuCl CuCl PtCl4 AuCl3 PdCl2 none none none

SiO2 (100-200 mesh) SiO2 (8-20 mesh) Al2O3 SiO2 (100-200 mesh) SiO2 (100-200 mesh) SiO2 (100-200 mesh) activated carbon SiO2 (100-200 mesh) Al2O3 activated carbon

loading (g/g) 0.880 0330 0.500 0.500 0.460 1.000 0.305 0 0 0

equilibrium adsorption ratios of propylene and propane at 1.0 atm

100 °C

120 °C

25 °C

70 °C

2.9 1.3 1.5 1.9 1.1 1.4 1.5 1.1

3.2 2.0 1.3 1.1 1.6 1.5 1.1

1.9 3.0 2.3 1.4 2.7 1.7 1.4 1.3 -

4.1 3.9 3.4 4.0 2.1 1.6 1.6 -

Figure 16. Effects of PdCl2 loading on adsorption ratio of benzene over cyclohexane and adsorbed amounts at 120 °C.

Figure 15. Dependence of adsorption at 120 °C on PdCl2 loading on SiO2. Symbols are experimental data, and lines are isotherms fit by eqs 4 and 5, except for 0.880-g loading.

polarizability, as electrons are localized and/or transferred because of chemisorption. For this reason, the first term in eq 5 represents the adsorption on a site without π-complexation, whereas the second term arises from that with π-complexation. Although eq 5 contains five parameters, certain constraints must be imposed on some of the parameters in order for them to have physical meaning. For example, empirical values for s are available from the literature.61,62 The value for s generally falls within the range 0-12 and increases with the carbon number for hydrocarbons as adsorbates. For physical adsorption, the values of the Langmuir constant (bp) are approximately equal for aromatics and aliphatics with the same carbon number. Consequently, the b value from the cyclohexane isotherm is imposed as the upper bound for benzene. Therefore, the cyclohexane isotherms were first used to obtain the two parameters in eq 4. Because

an excellent fit was obtained, an isotherm with heterogeneity was not needed for cyclohexane. Subsequently, eq 5 was used to fit the data on π-complexation with imposed values or constraints on qmp, bp, and s, leaving only two parameters, qmc and bc, as the true fitting parameters. The estimated parameters using eqs 4 and 5 are shown in Table 4. The isotherms for PdCl2/SiO2 (0.88 g/g) could not be fit by the Langmuir isotherm and, hence, could also not be fit by eqs 4 and 5. The pure-component adsorption ratios at 1 atm were estimated using the parameters in Table 4, as the commercial PSA process is usually operated at 1 atm or higher pressure. The adsorption ratios at 1.0 atm were 4.1 for PdCl2/SiO2 (0.305 g/g) and 3.3 for AgNO3/ SiO2 (0.33 g/g) compared to 1.2 for SiO2 and 1.4 for Al2O3. Although the extrapolation to 1 atm using the data at less than 0.1 atm might contain a certain degree of error, these high ratios are encouraging for commercial applications. Experimental investigations of pure and binary component adsorption up to 1 atm are planned for future studies. Computer simulations of PSA processes will also give considerable insight into the applicability of these sorbents and the optimum operating conditions. Isosteric Heats of Adsorption. Isosteric heats of adsorption for selected sorbents were calculated by the Clausius-Clapeyron equation, eq 1, using the isotherms at different temperatures, and the results are shown in Table 5. Heats of adsorption for cyclohexane were nearly the same for all sorbents in this study, ranging from 4 to 8 kcal/mol. On the other hand, heats of

3864


Table 4. Fitting Parameters for Isotherms (eqs 4 and 5) of Benzene and Cyclohexane adsorbent

adsorbate

temp (°C)

qmp (mmol/g)

bp (atm-1)

qmc (mmol/g)

bc (atm-1)

s (-)

PdCl2/SiO2 (0.305 g/g) CuCl/Al2O3 (0.50 g/g) AgNO3/SiO2 (0.33 g/g) AuCl3/SiO2 (1.00 g/g) SiO2

benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane benzene cyclohexane

120 120 120 120 120 120 120 120 120 120 120 120 100 100 100 100 100 100

0.23 0.23 0.30 0.30 0.30 0.30 0.91 0.91 1.27 1.29 0.59 0.45 0.45 0.45 1.31 1.30 0.79 0.84

9.74 20.49 16.32 35.38 3.63 29.78 1.52 2.87 4.44 2.27 10.13 7.84 1.50 11.78 6.61 3.99 10.60 5.09

3.38 0.69 1.92 12.42 3.20 -

0.031 0.075 0.232 0.028 0.066 -

5.7 7.0 5.7 4.3 5.6 -

Al2O3 PtCl4/SiO2 (0.46 g/g) SiO2 Al2O3

Table 5. Isosteric Heats of Adsorptiona

adsorbent PdCl2/SiO2 (0.88 g/g) AgNO3/SiO2 (0.33 g/g) CuCl/Al2O3 (0.50 g/g) PtCl4/SiO2 (0.46 g/g) AuCl3/SiO2 (1.00 g/g) PdCl2/activated carbon (0.305 g/g) SiO2 Al2O3 activated carbon

benzene heat of adsorption (kcal/mol)

cyclohexane heat of adsorption (kcal/mol)

9.3-10.9 (0.10-0.19) 9.2-10.1 (0.15-0.25) 10.1-11.0 (0.10-0.15) 7.2-9.0 (0.15-0.25) 8.8-10.1 (0.10-0.30) 9.5-10.6 (0.20-0.95)

4.9-7.0 (0.025-0.050) 4.4-5.8 (0.10-0.20) 3.9-5.9 (0.60-0.88)

6.3-6.4 (0.10-0.30) 4.7-7.3 (0.10-0.30) 4.1-6.8 (0.50-0.75)

5.4-6.4 (0.05-0.15) 7.4-7.9 (0.15-0.20) 6.5 (1.5)

a Values in parentheses indicate the adsorption amounts (mmol/ g) for calculation.

adsorption for benzene on sorbents with salts were higher than those on bare substrates. The heats of adsorption for the PdCl2, AgNO3, CuCl, and AuCl3 sorbents were in the range of 9-12 kcal/mol. The value for the PtCl4 sorbent was slightly lower than those for the other sorbents. The order of heats of benzene adsorption were CuCl > PdCl2 > AgNO3 > AuCl3 > PtCl4. This trend was consistent with the theoretical calculation results, which will be discussed shortly. Heats of adsorption for other π-complexation systems are available in the literature. For π-complexation with olefins, the heats of adsorption are 11.5 kcal/mol for propylene on AgNO3/SiO2,37 14.2 kcal/mol for propylene on CuCl/Al2O3,28 11.7 kcal/mol for ethylene on CuCl/ Al2O3,28 and 9.4 kcal/mol for ethylene on Ag+/Amberlyst 35.35 The heats of adsorption of corresponding paraffins on the same sorbents are 3.4 kcal/mol for propane on AgNO3/SiO2,37 5.6 kcal/mol for propane on CuCl/Al2O3,28 5.3 kcal/mol for ethane on CuCl/Al2O3,28 and 4.6 kcal/ mol for ethane on Ag+/Amberlyst 35.35 The latent heats of condensation for benzene and cyclohexane are almost the same (7.2-7.3 kcal/mol at 80 °C). The higher heats of benzene adsorption by salt-dispersed sorbents were caused by π-complexation. Furthermore, it should be noted that no substrate effect on the heat of adsorption was observed, as the heats of benzene adsorption between PdCl2/SiO2 and PdCl2/activated carbon were the same. The calculation of heats of adsorption was conducted using the data at small adsorption amounts, as shown in the Table 5. In these regions, the calculated heat of adsorption is based solely on the interaction between adsorbate and adsorbent. Therefore, experimental heats

of adsorption showed good agreement with molecular orbital results, in which only adsorbate-adsorbent interactions were taken into consideration. However, energetic heterogeneity largely influences the shape of adsorption isotherms and the adsorption amounts at 0.1 atm. CuCl/Al2O3 had the largest heterogeneity factor among the sorbents investigated. One possible reason for the largest s value for CuCl/Al2O3 is the coexistence of Cu+ and Cu2+. CuICl exhibits strong π-complexation capability, whereas CuIICl does not. This might result in the poor adsorption ratios at 0.1 atm, even though the heat of adsorption of benzene on CuCl/Al2O3 was high. Diffusion Time Constants. Uptake rates for benzene and cyclohexane were measured for all sorbents. The uptake curves consisted of two portions: a fast uptake portion at 0-0.7 fractional uptake and a slow uptake (long-tail) portion at 0.7-1.0 fractional uptake. The long-tail portion was probably due to the crowding effects of adsorbate molecules inside the pores at the large fractional uptakes. The diffusion time constants calculated from 0 to 0.3 fractional uptake are shown in Table 6. The values for olefin/paraffin adsorption are taken from the literature and also given in Table 6 for comparison. The diffusion time constants of benzene and cyclohexane in these sorbents were on the order of 10-4 s-1, which is an order of magnitude lower than those of propylene and propane in AgNO3/SiO2.37 However, they were higher or at least the same as those of ethylene and ethane for Ag+ ionexchanged resins.35 Because the diffusion time constants were not different for benzene and cyclohexane, π-complexation of benzene did not influence the diffusion rates. The fairly fast uptake rates at low fractional coverages are favorable for cyclic adsorption processes. Theoretical Adsorption Bond Energies and Molecular Geometries. Adsorption bond energies and structural geometries were calculated from molecular orbital theory. The energies of adsorption calculated using eq 3 along with the experimental data are shown in Table 7. The theoretical and experimental values follow the same trend, i.e., Cu > Pd > Ag > Au > Pt. The optimized structural data are also summarized in Table 7. The optimized geometries of the benzene-CuCl and benzene-PdCl2 complexes are shown in Figures 17 and 18. The structure for benzene-CuCl has a C6v symmetry; that for benzene-AuCl3, a C3v symmetry; and those for benzene-PdCl2 and benzene-PtCl4, a C2v

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3865 Table 6. Diffusion Time Constants (D/r2) of Benzene and Cyclohexane adsorbent PdCl2/SiO2 (0.305 g/g) PdCl2/SiO2 (0.88 g/g) AgNO3/SiO2 (0.33 g/g) CuCl/Al2O3 (0.50 g/g) PtCl4/SiO2 (0.46 g/g) AuCl3/SiO2 (1.00 g/g) PdCl2/activated carbon (0.305 g/g) SiO2 (100-200 mesh) Al2O3 activated carbon

temp (°C)

benzene (s-1)

pressure change (atm)

cyclohexane (s-1)


120 100 120 120 120 100 100 120 180 100 120 100 120 120 180

4.2E-04 1.2E-04 6.0E-04 1.3E-04 7.1E-04 8.2E-04 3.6E-04 2.6E-04 6.9E-04 5.7E-04 2.8E-04 4.7E-04 8.4E-04 5.7E-04 7.1E-04

(0.021 f 0.039) (0.020 f 0.037) (0.021 f 0.039) (0.020 f 0.037) (0.022 f 0.041) (0.023 f 0.042) (0.024 f 0.043) (0.021 f 0.031) (0.022 f 0.032) (0.019 f 0.027) (0.025 f 0.036) (0.040 f 0.064) (0.037 f 0.060) (0.021 f 0.039) (0.022 f 0.040)

4.9E-04 1.5E-04 7.8E-04 2.9E-04 9.0E-04 9.1E-04 1.6E-03 1.1E-04 9.7E-04 3.7E-04 3.1E-04 7.4E-04 9.4E-04 7.8E-04 7.8E-04

(0.020 f 0.038) (0.037 f 0.046) (0.022 f 0.032) (0.020 f 0.037) (0.021 f 0.039) (0.023 f 0.042) (0.024 f 0.043) (0.023 f 0.042) (0.036 f 0.047) (0.018 f 0.026) (0.021 f 0.030) (0.040 f 0.064) (0.039 f 0.061) (0.023 f 0.042) (0.022 f 0.042)

propylene (s-1)

temp (°C) g/g)37

AgNO3/SiO2 (0.33 AgNO3/SiO2 (0.33 g/g)37

3535

Ag-Amberlyst Ag-Amberlyst 3535

25 70

2.3E-03 3.5E-03

M

2.835 2.827 3.219 3.342 3.645


(0 f 0.1) (0 f 0.1)

8.7E-03 -

(0 f 0.1) -

ethylene (s-1)


ethane (s-1)

25 70

1.0E-04 1.5E-04

1.1E-04 1.4E-04

-

r(C-C) r(M-C) r(M-Cl)b r(M-Cl)c 1.401 1.403 1.400 1.399 1.400 1.396

propane (s-1)

temp (°C)

Table 7. Summary of Energies of Adsorption and Geometries for Benzene-MClx Systema Cu Pd Ag Au Pt benzene


2.226 2.324 2.451 2.593 2.613

2.210 2.301 2.440 2.578 2.595

∆H

exptl ∆H

12.5 10.1-11.0 10.8 9.3-10.9 8.6 9.2-10.1 6.5 8.8-10.1 5.2 7.2-9.0

a The bond lengths (r) are in angstroms and the energies (∆H) are in kcal/mol. b MCl in complex. c In free MClx.

Figure 17. Optimized structure for benzene-CuCl.

Figure 18. Optimized structure for benzene-PdCl2.

symmetry, with the metal atom approaching the π orbital of benzene along the midpoint of the benzene ring. The flat benzene ring is on the xy plane, and the metal atom is on the z axis. Six equal-length metalcarbon bonds were formed between each metal chloride


and the six carbons of the benzene ring. A comparison of the bond length of free benzene with those of benzene in the complexes shows that, upon adsorption, the C-C bond length increases slightly. Also, a comparison of the bond length of free metal chlorides with those of the corresponding metal chlorides in the complexes shows that, upon adsorption, the M-Cl bond length also increases, indicating that the C-C bonds and M-Cl bonds are weakened upon adsorption. The amounts of positive charge on the metal chlorides are determined in the NBO analysis and are given as follows: PdCl2, 0.732; CuCl, 0.644; AgCl, 0.625; AuCl3, 0.576; and PtCl4, 0.539. Metals with a more positive charge will be a better electron acceptors for π-complexation with benzene; thus, the bond distance between M and C will be shorter. This trend is confirmed in Table 7, where the M-C distances follow the order: benzene-PdCl2 < benzene-CuCl < benzene-AgCl < benzene-AuCl3 < benzene-PtCl4. Natural Bond Orbital (NBO) Results. The nature of the metal-benzene bond can be better understood by analyzing the NBO results. The main feature of the bonding can be seen from the population changes in the vacant outer-shell s orbital of the metal and in the d orbitals of the metal upon adsorption.63 The slight stretching of the M-Cl bond is due to chemisorption of benzene onto the metal. This is a reversible process and will not cause decomposition or reaction. The NBO analysis, summarized in Table 8, is in general agreement with previously reported results of metal-olefin complexation.40,44,45,64 In most cases, the M-C interaction is a dative bond, i.e., donation of electron charges from the π orbital of benzene to the vacant s orbital of the metal and, simultaneously, back-donation of electron charges from the d orbitals of metals to the π* orbital of benzene. Table 8 shows that, upon adsorption, the electron occupancy of the valence s orbitals of Cu, Ag, Au, and Pt increases; this is caused by σ donation. Pd appears to be an exceptional case, for which the electron occupancy of the valence s orbital decreases upon adsorption. As for Ni,17 where the same phenomenon was attributed to electron redistribution from 5s to 4dyz

3866


Table 8. Summary of NBO Analysisa of π-Complexation between Benzene and MClx C f M interaction

M f C interaction

(σ donation)

(d-π* back-donation)

net change

MCl

q1

q2

q1 + q 2

CuCl PdCl2 AgCl AuCl3 PtCl4

0.1342 -0.0269 0.0033 0.0045 0.0234

-0.0125 -0.0199 -0.0071 -0.0443 -0.0182

0.1217 -0.0468 -0.004 -0.0398 0.005

a q is the amount of electron population change in valence s 1 orbitals of the metal, and q2 is the total amount of electron population change in the valence d orbitals of the metal.

because the energy level of 4dyz is closest to that of 5s in Pd, and upon adsorption, the electron occupancy of 4dyz increases and that of 5s decreases by electron redistribution within the Pd atom. Table 8 also shows that, upon adsorption, the total occupancy of the metals’ 3d, 4d, and 5d orbitals (i.e., outer-shell d orbitals of the respective metal atoms) all decrease because of backdonation of electrons from the metals to benzene. It appears that, in the case of benzene, σ donation plays a more important role than π back-donation. Pd has the shortest M-C bond distance, which leads to more overlap of π* orbitals of benzene with the d orbitals of the metal; therefore, Pd has the highest d-π* backdonation; in contrast, Cu has the highest σ donation and the highest heat of adsorption. Acknowledgment This work was supported by National Science Foundation under Grant CTS-9819008. We are grateful to NGK Insulators, Ltd., Nagoya, Japan, for financial support to A.T. during his study at the University of Michigan. We also thank Dr. Joel Padin for preparing the AgNO3/SiO2 and some of the PdCl2/SiO2 sorbents. Notation b ) Langmuir constant D ) Diffusion coefficient Mt/Mi ) Fractional uptake at time t p ) Pressure qm ) Monolayer or saturated amount adsorbed qst ) Isosteric heat of adsorption R ) Gas constant r ) Particle radius s ) Heterogeneity parameter T ) Temperature t ) Time Subscripts c ) Chemisorption or π-complexation p ) Physical adsorption

Literature Cited (1) Bailes, P. J. Petroleum and Petrochemicals Processings Introduction and Aromatics-Aliphatics Separation. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hansen, C., Eds.; Wiley: New York, 1983; p 517. (2) Little, D. M. Catalytic Reforming; Pennwell Publishing Company: Tulsa, OK, 1985. (3) Wells, G. M. Handbook of Petrochemicals and Processes; Gower Publishing Company: Brookfield, VT, 1991. (4) Meyers, R. A. Handbook of Petroleum Refining Process; McGraw-Hill: New York, 1997.

(5) Muller, E. NMP (AROSOLVAN) Process for BTX Separation. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hansen, C., Eds.; Wiley: New York, 1983; p 523. (6) Vidueira, J. A. Union Carbide Tetra Process. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hansen, C., Eds.; Wiley: New York, 1983; p 531. (7) Kosters, W. C. G. Sulfolane Extraction Process. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hansen, C., Eds.; Wiley: New York, 1983; p 541. (8) Kim, C. G.; Clarke, W. P.; Lockington, D. Competitive Adsorption of Sulfolane and Thiolane on clay material. Korean J. Chem. Eng. 1999, 16 (2), 215. (9) Stichlmair, J. G.; Fair, J. R. Distillation: Principle and Practice; Wiley-VCH: New York, 1998. (10) Hao, J.; Tanaka, T.; Kita, H.; Okamoto, K. The pervaporation properties of sulfonyl-containing polyimide membranes to aromatic/aliphatic hydrocarbon mixture. J. Membr. Sci. 1997, 132, 97. (11) Goswami, A. L.; Rawat, B. S. Studies on the Permeation of Aromatic Hydrocarbons through Liquid Surfactant Membranes. J. Membr. Sci. 1985, 24, 145. (12) Atwood, J. L. New Inclusion Methods for Separations Problems. Sep. Sci. Technol. 1984, 19, 751. (13) Matz, M. J.; Knaebel, K. S. Criteria for selection of an Adsorption for a Temperature Swing Process: Applied to Purification of an Aliphatic Solvent Contaminated with Aromatics Solutes. Sep. Sci. Technol. 1990, 25, 961. (14) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, 1987. (15) King, C. J. Separation processes based on reversible chemical complexation. In Handbook of Separation Process Technology; Wiley: New York, 1987. (16) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Interscience: New York, 1966; Chapters 25 and 28. (17) Quinn, H. W. Hydrocarbon Separations with Silver(I) Systems. In Progress in Separation and Purification; Perry, E. S., Ed.; Interscience: New York, 1971; Vol. 4. (18) Ho, W. S.; Doyle, G.; Savage, D. W.; Pruett, R. L. Olefin Separation via Complexation with Cuprous Diketonate. Ind. Eng. Chem. Res. 1998, 27, 334. (19) Keller, G. E.; Marcinkowsky, A. E.; Verma, S. K.; Williamson, K. D. Olefin Recovery and Purification via Silver Complexation. In Separation and Purification Technology, Li, N. N., Calo, J. M., Eds.; Marcel Dekker: New York, 1992; pp 59-83. (20) Blytas, G. C. Separation of Unsaturates by Complexing with Nonaqueous Solutions of Cuprous Salts. In Separation and Purification Technology; Li, N. N., Calo, J. M., Eds.; Marcel Dekker: New York, 1992; Chapter 2. (21) Eldridge, R. B. Olefin/Paraffin Separation Technology: Review. Ind. Eng. Chem. Res. 1993, 32, 2208. (22) Safarik, D. J.; Eldridge, R. B. Olefin/Paraffin Separations by Reactive Adsorption: A Review. Ind. Eng. Chem. Res. 1998, 37, 2571. (23) Gilliland, E. R.; Bliss, H. L.; Kip, C. E. Reactions of Olefins with Solid Cuprous Halide. J. Am. Chem. Soc. 1941, 63, 2088. (24) Gilliland, E. R. Concentration of Olefins, U.S. Patent 2,369, 559, 1945. (25) Long, R. B. Separation of Unsaturates by Complexing with Solid Copper Salts. In Recent Developments in Separation Science; Li, N. N., Ed.; CRC Press: Cleveland, OH, 1972; Vol. 1, p 35. (26) Hirai, H.; Hara, S.; Komiyama, M. Polystylene-Supported Aluminum Silver Chloride as Selective Ethylene adsorbent. Angew. Makromol. Chem. 1985, 130, 207. (27) Hirai, H.; Kimura, K.; Wada, K.; Komiyama, M. Selective Ethylene Adsorbents Composed of Cu(I)Cl and Polystyrene Resins having Amino Groups. Chem. Lett. (Japan) 1985, 1513. (28) Yang, R. T.; Kikkinides, E. S. New Sorbents for Olefin/ Paraffin Separations by Adsorption via π-Complexation. AIChE J. 1995, 41, 509. (29) Cen, P. L. Simultaneous Physical and Chemical Adsorption of Ethylene and Cu(I) Na-Y Zeolite. In Separation and Purification; Li, N. N., Calo, J. M., Eds.; Marcel Dekker: New York, 1992. (30) Kumar, R.; Golden, T. C.; White, T. R.; Rokicki, A. Novel Adsorption Distillation Hybrid Scheme for Propylene/Propane Separation. Sep. Sci. Technol. 1992, 27, 2157. (31) Golden, T. C.; Guro, D. E.; Kratz, W. C.; Occhialini, J. M.; Sabram, T. E. APCI’S CO VSA, Scale-up and Plant Performance. Proc. Fundam. Adsorpt. 6 1998, 695.

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3867 (32) Golden, T. C.; Kratz, W. C.; Wilhelm, F. C. Highly Dispersed Cuprous Compositions. U.S. Patent 5,126, 310, 1992. (33) Golden, T. C.; Kratz, W. C.; Wilhelm, F. C.; Pierantozzi, R.; Rokicki, A. Highly Dispersed Cuprous Compositions. U.S. Patent 5,175, 137, 1992. (34) Javelin, H.; Fair, J. R. Adsorptive Separation of Propylene-Propane Mixtures. Ind. Eng. Chem. Res. 1993, 32, 2201. (35) Wu, Z.; Han, S. S.; Cho, S. H.; Kim, J. N.; Chue, K. T.; Yang, R. T. Modification of Resin-type Adsorbents for ethane/ ethylene Separation, Ind. Eng. Chem. Res. 1997, 36, 2749. (36) Cheng, L. S.; Yang, R. T. Monolayer Cuprous Chloride Dispersed on Pillared Clays for Olefin-Paraffin Separation by π-Complexation. Adsorption 1995, 1, 61. (37) Rege, S. U.; Padin, J.; Yang, R. T. Olefin-Paraffin Separation by Adsorption: Equilibrium Separation by π-Complexation vs Kinetic Separation. AIChE J. 1998, 44, 799. (38) Padin, J.; Yang, R. T. Tailoring New Adsorbents based on π-Complexation: Cation and Substrate Effects on Selective Acetylene Adsorption. Ind. Eng. Chem. Res. 1997, 36, 4224. (39) Padin, J.; Yang, R. T. New Sorbents for Olefin-Paraffin Separation by Adsorption via π-Complexation: I. Synthesis and Effects of Anions and Substrates. Chem. Eng. Sci. 2000, 55, 2607. (40) Huang, H. Y.; Padin, J.; Yang, R. T. Anion and Cation Effects on Olefin Adsorption on Silver and Copper Halides: Ab Initio Effective Core Potential Study of π-Complexation, J. Phys. Chem. B 1999, 103, 3206 (41) Breck. D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley: New York, 1974. (42) Tamon, H.; Kitamura, K.; Okazaki, M. Adsorption of Carbon Monoxide on Activated Carbon Impregnated with Metal Halide. AIChE J. 1996, 42, 422. (43) Yeh, Y. T. Diffusion and Adsorption of Gases in Molecular Sieves. Ph.D. Dissertation, University of New York at Buffalo, Buffalo, New York, 1989. (44) Huang, H. Y.; Padin, J.; Yang, R. T. Comparison of π-Complexations of Ethylene and Carbon Monoxide with Cu+ and Ag+. Ind. Eng. Chem. Res. 1999, 38, 2720. (45) Chen, N.; Yang, R. T. Ab Initio Molecular Orbital Study of Adsorption of Oxygen, Nitrogen, and Ethylene on Silver-Zeolite and Silver Halides. Ind. Eng. Chem. Res. 1996, 35, 4020. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robs, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; DeFrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision B.3; Gaussian, Inc., Pittsburgh, PA., 1995. (47) Bowie, J. E. Data Visualization in Molecular Science: Tools for Insight and Innovation; Addison-Wesley: Reading, MA, 1995; Chapter 9.

(48) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potential for Molecular Calculations. Potentials for the Transition Metals Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270. (49) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potential for Molecular Calculations: Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 92, 284. (50) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potential for Molecular Calculations: Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299. (51) Russo, T. V.; Martin, R. L.; Hay, P. J. Effective Core Potentials for DFT Calculations. J. Phys. Chem. 1995, 99, 17085. (52) Becke, A. D. Density Functional Thermochemistry. III. The Role of Exact Exchanges. J. Chem. Phys. 1993, 98, 5648. (53) Becke, A. D. A New Mixing of Hartree-Fock and Local Density-Functional Theories. J. Chem. Phys. 1993, 98, 1372. (54) Becke, A. D. Density-Functional Thermochemistry. II. The Effect of the Perdew-Wang generalized-gradient correlation correction. J. Chem. Phys. 1992, 97, 9173. (55) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. 1988, B37, 785. (56) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural Population Analysis. J. Chem. Phys. 1985, 83 (2), 735. (57) Glendening, E. D.; Reed, A. E.; Carpenter, J. E., Weinhold, F. NBO, version 3.1; University of Wisconsin: Madison, WI, 1995. (58) Padin, J. New Sorbents for Enhanced Separation via π-Complexation: Synthesis, Characterization and Tailoring of Sorbents for the Separation of Olefins and Paraffin. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI, 2000. (59) Gui, L.; Guo, Q.; Xie, Y.; Tang, Y. An Investigation of the Dispersion of CuCl on γ-Al2O3 Surface and the Adsorption of Ethylene on CuCl/γ-Al2O3 and Other Supported Transition Metal Salts. Sci. Sin., Ser. B 1984, 27, 445. (60) Horvath, G.; Kawazoe, K. Method for Calculation of Effective Pore Size Distribution in Molecular Sieve Carbon. J. Chem. Eng. Jpn. 1983, 16, 470. (61) Valenzuela, D. P.; Myers, A. L. Adsorption Equilibrium Data Handbook; Prentice Hall: Englewood Cliffs, NJ, 1989. (62) Kapoor, A.; Yang, R. T. Surface Diffusion on Energetically Heterogeneous Surfacesan Effective Medium Approximation Approach. Chem. Eng. Sci. 1990, 45, 3261. (63) Nicolas, G.; Spielgelmann, F. Theoretical Study of Ethylene-Noble Metal Complexes. J. Am. Chem. Soc. 1990, 112, 5410. (64) Huang, H. Y.; Yang, R. T.; Chen, N. Anion Effects on the Adsorption of Acetylene by Nickel Halides. Langmuir 1999, 15, 7647.

Received for review April 5, 2000 Revised manuscript received July 18, 2000 Accepted July 20, 2000 IE000376L

Aliphatics Separation by Adsorption: New Sorbents for

Recommend Documents