Tailoring New Adsorbents Based on π-Complexation: Cation and

Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109. Ind. Eng. Chem. Res. , 1997, 36 (10), pp 4224–4230. DOI: 10.1...
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Ind. Eng. Chem. Res. 1997, 36, 4224-4230

Tailoring New Adsorbents Based on π-Complexation: Cation and Substrate Effects on Selective Acetylene Adsorption Joel Padin and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109

Sorbents were prepared by dispersing various metal cations, M2+ (Fe2+, Co2+, Ni2+), by thermal monolayer dispersion of MCl2 on various high surface area substrates (γ-Al2O3, SiO2, MCM-41 mesoporous zeolite). All sorbents showed reversible and highly selective adsorption for acetylene (C2H2) over other hydrocarbons. This was due to the formation of π-complexation bonds between the M2+ cations and C2H2. The following trend was observed for all substrates studied: Fe2+ > Co2+ > Ni2+. Substrates played an important role in acetylene adsorption, with SiO2-based sorbents outperforming those based on Al2O3. For example, the ratio of C2H2/M2+ for FeCl2/ SiO2 was 1.35, while for FeCl2/Al2O3 the ratio was 0.9. This result is due to the fact that SiO2 has less surface oxygen vacancies, and consequently there are more four-coordinated M2+ ions (which can bond C2H2) and less five- and six-coordinated M2+ (which cannot bond C2H2 for stereochemical reasons) on SiO2. This work demonstrates the effects of cations and substrates on selective acetylene adsorption. Introduction Separation of acetylene is an important industrial process in acetylene production and as a preseparation for other separation processes. Modern processes for the manufacture of acetylene from hydrocarbons began in the 1920s when Badische Anilin- und Soda-Fabrik (BASF) investigated the conversion of low molecular weight aliphatic hydrocarbons to acetylene using thermal cracking (Kirk-Othmer Concise Encyclopedia of Chemical Technology, 1978). However, the first commercial plant for the manufacture of acetylene from hydrocarbons used BASF’s electric arc process. Regardless of the manufacturing method, the isolation of acetylene from other stream components can be complicated. Current acetylene separations are accomplished by solvent extraction processes employing liquid solvents such as dimethylformamide (DMF) (Kirk-Othmer Concise Encyclopedia of Chemical Technology, 1978). An alternative to the current technique would be to utilize pressure-swing adsorption processes to separate the acetylene from the other cracked gases. However, there are no commercially available sorbents that can reversibly and selectively adsorb acetylene over other hydrocarbons. A new class of highly efficient sorbents based on π-complexation has recently been developed for olefin-paraffin separations and for acetylene separations which can fulfill this role (Yang and Kikkinides, 1995; Yang and Foldes, 1996). These sorbents, based on reversible chemical complexation of which π-complexation is a subgroup, can have high capacities for dilute solutes and also high selectivities (King, 1987). This is attributed to the strength of the bonds formed in chemical complexation, which are stronger than those formed by van der Waals forces alone. However, these types of bonds remain weak enough to allow the bonds to be broken by changing the reaction conditions, e.g., by increasing the temperature or decreasing the pressure. Separation of olefins based on the phenomenon of π-complexation was * To whom all correspondence should be addressed. Telephone: (313) 936-0771. Fax: (313) 763-0459. E-mail: yang@ umich.edu. S0888-5885(97)00207-8 CCC: $14.00

proposed as early as 1941 (Gilliland et al., 1941). There have been several studies involving gas-liquid systems employing solutions of silver (Ag+) or cuprous (Cu+) ions (Quinn, 1971; Ho et al., 1988; Keller et al., 1992; Blytas, 1992; Eldridge, 1993). There have also been some studies dealing with gas-solid systems (Gilliland et al., 1941; Long, 1972; Xie and Tang, 1990; Kumar et al., 1992; Yang and Kikkinides, 1995; Yang and Foldes, 1996). Although there have been many studies aimed at obtaining sorbents for olefin/paraffin separations, the same cannot be said of acetylene-specific sorbents. This work is based on the new class of sorbents for acetylene separations proposed in Yang and Foldes (1996). These sorbents, based on π-complexation, are able to selectively adsorb acetylene over other hydrocarbons. The reversibility of these sorbents has already been established in Yang and Foldes (1996). The sorbents contained Ni2+ cations highly dispersed over a high surface area substrate. In this work, the concepts introduced by Yang and Kikkinides (1995) and Yang and Foldes (1996) are further developed to include sorbents based on other cations (Fe2+, Co2+, Ni2+) and other high surface area substrates (γ-Al2O3, SiO2, and MCM-41). This was done so as to observe the effect of various cations on acetylene adsorption. Any differences in adsorptive properties encountered can be utilized to manipulate the adsorptive properties of a sorbent to tailor it to a particular application. Experimental Section Preparation of Sorbents. The sorbents described in this study contained metal cations (M2+) dispersed over a high surface area substrate. This dispersion was accomplished by taking advantage of a technique known as spontaneous monolayer dispersion. This technique has been described in detail in the literature (Xie and Tang, 1990; Xie et al., 1992; Linlin et al., 1984). Also, it was successfully utilized by Yang and Foldes (1996) to prepare NiCl2/γ-Al2O3 sorbents that were selective for acetylene. Thermal monolayer dispersion involves mixing a metal salt or oxide with the substrate at a certain ratio. This ratio is determined by the surface area of the substrate and the size of the metal salt molecule. The © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4225

first can be obtained by simple nitrogen adsorption at liquid nitrogen temperatures (77 K). However, the latter is empirically calculated assuming a closed-pack monolayer model (Xie et al., 1992). Experimental evidence by Xie and Tang (1990) showed that this technique will provide an approximation to the threshold dispersion capacity. This is the maximum amount of salt that can be dispersed by heat as a monolayer on the surface of the substrate so that no residual amount of salt remains in the crystalline form. Once the sample has been thoroughly mixed in the appropriate ratio, it needs to be heated. The temperature for heating should be between the Tammann temperature and melting point of the salt. However, excessive temperatures would cause the metal salt to oxidize or react with the substrate. When heating, care needs to be taken so as to avoid oxidizing any of the metal salts which will inactivate the sorbent. As a rule, smaller particles and thorough mixing will decrease the amount of time required to disperse the salt on the substrate. The supports utilized in this study were γ-Al2O3, SiO2, and mesoporous zeolite (MCM-41). The BET surface areas of the samples were calculated at 340, 675, and 770 m2/g, respectively. Several metal salts were utilized in this work (FeCl2, CoCl2, NiCl2). All sorbents prepared with FeCl2, CoCl2, and NiCl2 were heated at 70 °C for 5 days. The ratios of metal salt to substrates in grams were as follows: alumina-based, 0.36; SiO2-based, 0.88; MCM41-based, 1.27. Equilibrium Isotherms, Uptake Rate, and Surface Area Measurements. Isotherms and uptake rates were measured utilizing both Cahn TG-121 and Shimadzu TGA-50 microbalances following the procedures described by Ackley and Yang (1991). Equilibrium time for isotherm measurements was about 15 min per equilibration point. Surface area measurements were made using a Micromeritics ASAP 2010. Also, measurements were made at two temperatures (25 and 70 °C) in order to obtain isosteric heat of adsorption data. The overall time diffusion constants, D/R2, were calculated from the uptake curves by methods and assumptions described in detail Yeh (1989). The gases were dried using a column filled with 3A molecular sieve. Helium was used as a carrier gas and as the regeneration gas for the drying columns. The hydrocarbons used were ethane (CP grade, Matheson minimum purity 99.0%), ethylene (CP grade, Matheson minimum purity 99.5%), propane (CP grade, Matheson minimum purity 99.0%), propylene (CP grade, Matheson minimum purity 99.0%), and acetylene (CP grade, minimum purity 98.0%). The gases were used without further purification. Results and Discussion Monolayer Dispersion: Nature of the Dispersed Species. The phenomenon of thermal monolayer dispersion is well-known. It has been successfully used to synthesize sorbents based on the phenomenon of π-complexation (Xie and Tang, 1990; Xie et al., 1992; Linlin et al., 1984; Yang and Kikkinides, 1995; Yang and Foldes, 1996). As mentioned earlier, the dispersion involves heating the mixture of metal salts and substrate in air. This could lead to the oxidation of some of the metal salts to an oxidation state less than optimal for π-complexation to occur. One example of an easily oxidized metal salt is FeCl2, which can be oxidized to FeCl3. This problem can be avoided by performing the dispersion at a lower temperature. However, this

Figure 1. Equilibrium isotherms of C2H2 on MCl2 salts.

lengthens the time required to carry out the dispersion. Alternatively, this problem was avoided by first blanketing the sample with an inert atmosphere, followed by evacuation of the sample (>50 µmHg). This was done so that any remaining atmosphere in the chamber will be inert and thereby prevent the oxidation of the salt. While more sophisticated methods exist for establishing oxidation number, the simplest method to determine whether oxidation has occurred is to look for any color changes in the salt. For example, CoCl2‚6 H2O salt is red. However, upon spreading, the sample changed to blue, which is the color of CoCl2 without the hydrates. This observation provided evidence of the oxidation state of the cobalt cations. If the Co2+ cations would have been oxidized to Co3+, the color of the sample should have changed to either red or yellow. From this observation, it was found that the species spread on the surface of the substrate was CoCl2 and not CoCl2‚6H2O. Similarly, NiCl2‚6H2O is a medium shade of green. However, upon spreading, the sample color changes to yellow, which is the color of NiCl2 without the hydrates. It can be concluded that spreading CoCl2 and NiCl2 salts at 70 °C in air would not cause any oxidation of the cations. However, a different situation occurred with FeCl2‚2H2O. Upon heating FeCl2‚2H2O, it should change from green to a yellow-green. However, the sample changed to a red-brown color, which points to the salt oxidizing to some mixture of FeCl3 and its oxides. Further analysis would be needed to determine the exact oxidation and nature of the resulting salt. However, this is not desired because the important conclusion has already been established. The sample is not FeCl2, which is what is needed. Therefore, in order to prevent oxidation, the spreading was repeated in vacuum with a balance atmosphere of nitrogen and the results were as expected. The sample color turned from a green to a dark-yellow, meaning a transition from FeCl2‚2H2O to FeCl2. Metal-Chloride (MCl2) Salts. Equilibrium isotherms for acetylene at 25 °C for various metal-chloride salts are given in Figure 1. The data were fitted using the isotherm equation developed by Yang (et al., (1995) for chemisorbed species. The isotherm model has the following form:

q)

qmc 1+b h cP exp(s) qmpbpP + ln 1 + bpP 2s 1+b h cP exp(-s)

(1)

The first term accounts for physical adsorption, while the second term represents contributions by chemical adsorption. The second term also takes into account the energetic heterogeneity of the surface ion sites for

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Figure 2. Normalized C2H2 equilibrium data on MCl2 salts at 25 °C. Table 1. Equilibrium Isotherm Parameters for Acetylene on MCl2 Salts at 25 °C FeCl2 CoCl2 NiCl2

qmc (mmol/g)

bc (1/atm)

s

3.41 3.49 2.75

0.016 0.016 0.018

4.37 4.40 4.42

(2)

However, a more meaningful method for reporting the isotherm data for comparison purposes would be to report it as molecules of acetylene per metal cation (M2+). In order to accomplish this, surface area measurements of the various salts were made by the BET method. The surface area values obtained for CoCl2, FeCl2, and NiCl2 were 27.1, 17.4, and 20.9 m2/g, respectively. The normalized isotherms are reported in Figure 2. The weakly chemisorbed acetylene on the surface metal cations can be represented by the following formula where M stands for the particular metal cation: M2+(C2H2)n. The values of n for the metal chloride salts at 1 atm ranged from 1.5 to 3.0, assuming surface reaction only. However, metal halide salts have been shown to have bulk reactions with olefins at high pressures (Gilliland et al., 1941, who used solid CuCl crystals as the sorbent for ethylene sorption at pressures up to 50 atm). Bulk reaction will increase the number of cations that can possibly interact with the acetylene and thereby reduce the calculated values of n. The ratio of olefin molecules to bulk cations was reported by Gilliland et al. (1941) to be around 1 at high pressures. Once the data were normalized, acetylene molecules demonstrated a greater affinity for the Fe2+ cations of the FeCl2 salt. The corrected trend for the adsorption of acetylene on metal cations is the following:

FeCl2 > CoCl2 > NiCl2

Table 2. Equilibrium Isotherm Parameters for Acetylene on Alumina-Based Sorbents at 25 °C FeCl2/Al2O3 CoCl2/Al2O3 NiCl2/Al2O3

complexation. Due to the strong dominance of chemisorption over physical adsorption, acetylene isotherms were fitted using only the second term of eq 1 (Yang and Foldes, 1996). Fitting parameters for the isotherm shown in Figure 1 are reported in Table 1. From this figure, it appears that acetylene adsorbed more strongly on the Co2+ of the CoCl2 salt and that the following trend can be observed:

CoCl2 > FeCl2 > NiCl2

Figure 3. Equilibrium isotherms of C2H2 on alumina-based sorbents at 25 °C.

(3)

It is noted that the bare supports also adsorb the hydrocarbons; however, the adsorption is very strong and irreversible and has little or no selectivity for different hydrocarbon, as shown by Yang and Foldes (1996).

qmc (mmol/g)

bc (1/atm)

s

3.64 3.5 3.51

0.106 0.07 0.076

7.16 6.42 6.95

Table 3. Overall Diffusion Time Constant (D/R2) for Acetylene on Alumina-Based Sorbents D/R2 (1/s) Al2O3 FeCl2/Al203 CoCl2/Al2O3 NiCl2/Al203

25 °C

70 °C

2.40 × 10-4 1.27 × 10-3 1.56 × 10-3 1.20 × 10-3

1.72 × 10-3 2.58 × 10-3 1.36 × 10-3

Table 4. Pure-Component Adsorption Ratios for Alumina-Based Sorbents at 70 °C and 1 atm FeCl2 CoCl2 NiCl2

C2H2/C2H4

C2H2/C2H6

4.4 4.5 4.5

20.3 7.3 23.2

MCl2/γ-Al2O3 Sorbents. FeCl2, CoCl2, and NiCl2 can be spread into a near monolayer on the surface of the γ-Al2O3 substrate upon heating (Xie and Tang, 1990). NiCl2/γ-Al2O3 sorbent was successfully shown by Yang and Foldes (1996) to be able to selectively adsorbed acetylene over other hydrocarbons. Equilibrium isotherms for acetylene adsorption on all three aluminabased sorbents are shown in Figure 3. The amounts of acetylene adsorbed at 1 atm on the FeCl2/γ-Al2O3, CoCl2/ γ-Al2O3, and NiCl2/γ-Al2O3 sorbents at 25 °C were 1.30, 1.18, and 1.16 mmol/g, respectively. The parameters obtained from fitting the data to eq 1 are shown in Table 2. Good fittings were obtained using the model developed in Yang and Foldes (1996). Also, BET surface area values for the above sorbents were measured at 190, 176, and 192 m2/g, respectively. Diffusion time constants for acetylene adsorption on the three sorbents and the support are given in Table 3. Uptake rates for all the sorbents tested were fast, with 90% completion achieved in less than 5 min for most sorbents. Purecomponent ratios of acetylene adsorption over other hydrocarbons at 1 atm and 70 °C are shown in Table 4. While the other alumina-based sorbents were selective for acetylene, selectivity data are shown only for NiCl2/ γ-Al2O3 in Figure 4. The pure-component adsorption ratios were excellent for industrial separation processes (Yang, 1987). Also, normalized isotherm data are given in Figure 5. The values of n for alumina-based sorbents at 1 atm were substantially lower than those of the metal-chloride salts. They ranged from 0.75 to 0.90. Also, from Figure 5, the adsorption trend for the

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Figure 4. NiCl2/Al2/O3: C2H2 selectivity over other hydrocarbons at 70 °C.

Figure 7. C2H2 uptake rates on silica gel-based sorbents at 25 °C. Table 5. Equilibrium Isotherm Parameters for Acetylene on Silica Gel Based Sorbents at 25 °C FeCl2 CoCl2 NiCl2

qmc (mmol/g)

bc (1/atm)

s

8.907 8.805 8.434

0.646 0.407 0.354

7.28 7.01 6.99

Table 6. Overall Diffusion Time Constant (D/R2) for Acetylene on Silica Gel Based Sorbents D/R2 (1/s)

Figure 5. Normalized C2H2 equilibrium data on alumina-based sorbents at 25 °C.

SiO2 FeCl2/SiO2 CoCl2/SiO2 NiCl2/SiO2

alumina-based sorbents was observed as follows:

FeCl2 > CoCl2 > NiCl2

(4)

This was the same trend observed for the metalchloride salts. This seems to point to either none or minimal substrate effects on the affinity of acetylene for the metal cations. MCl2/SiO2-Based Sorbents. In the previous section the metal salts being studied (FeCl2, CoCl2, NiCl2) were spread on activated alumina (PSD-350, Alcoa) with a surface area of 340 m2/g. As a method of increasing sorbent capacity, the salts were spread on a silica gel (Strem chemicals) substrate with a surface area of 675 m2/g. This proved to be a very effective combination. Figure 6 shows the equilibrium isotherm data for acetylene adsorption on all three sorbents. Fitting parameters for the isotherms are shown in Table 5. The amounts of acetylene adsorbed at 1 atm for FeCl2/SiO2, CoCl2/SiO2, and NiCl2/SiO2 were 4.29, 3.94, and 3.68 mmol/g at 25 °C. The BET surface areas of the sorbents were as follows: 403, 411, and 430 m2/g. This was a

70 °C 7.66 × 10-4 5.64 × 10-4 4.98 × 10-4

Table 7. Pure-Component Adsorption Ratios for Silica Gel Based Sorbents at 70 °C and 1 atm FeCl2 CoCl2 NiCl2

Figure 6. Equilibrium isotherms of C2H2 on silica gel based sorbents at 25 °C.

25 °C 2.16 × 10-4 7.41 × 10-4 4.95 × 10-4 4.71 × 10-4

C2H2/C2H4

C2H2/C2H6

10.7 12.7 12.3

28.6 35.8 19.4

significant increase over the surface area values for the alumina-based sorbents. The effectiveness of using silica gel as substrate lies in the fact that while the total surface area of the substrate was double, the amount of acetylene adsorbed at 1 atm was increased 3.5 times. However, this increase in sorbent capacity came at the expense of a lower diffusion time constant. Uptake curves are shown in Figure 7, and diffusion time constants are shown in Table 6. A 60-80% reduction in diffusion time constants was observed in changing from γ-Al2O3 to SiO2. Although diffusion was somewhat hindered by the different substrates, pure-component ratios of silica gel based sorbents were significantly higher than those obtained for alumina-based sorbents. Pure-component ratios of acetylene over other hydrocarbons at 1 atm and 70 °C are shown in Table 7. Figure 8 shows selective adsorption of acetylene over other hydrocarbons on FeCl2/SiO2 at 70 °C. Also, normalized isotherm data for all three sorbents are shown in Figure 9. The values of n for silica gel based sorbents at 1 atm ranged from 1.10 to 1.35. These values were higher than those observed for γ-Al2O3based sorbents. This can be attributed to the effect the sorbent has on the epitaxy of the monolayer of metal salt. Again, the adsorption trend for silica gel based sorbents is as follows:

FeCl2 > CoCl2 > NiCl2

(5)

MCl2/MCM41-Based Sorbents. In the previous sections, metal chloride salts were spread on γ-Al2O3

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Figure 8. CoCl2/SiO2: C2H2 selectivity over other hydrocarbons at 70 °C.

Figure 9. Normalized C2H2 equilibrium data on silica gel based sorbents at 25 °C.

Figure 11. FeCl2/MCM-41: C2H2 selectivity over other hydrocarbons at 70 °C.

Figure 12. Normalized C2H2 equilibrium data on MCM41-based sorbents at 25 °C. Table 9. Overall Diffusion Time Constant (D/R2) for Acetylene on MCM41-Based Sorbents D/R2 (1/s) MCM-41 FeCl2/MCM-41 CoCl2/MCM-41 NiCl2/MCM-41

25 °C

70 °C

2.19 × 10-4 8.49 × 10-4 7.04 × 10-4 2.74 × 10-4

8.69 × 10-4 8.52 × 10-4 5.91 × 10-4

Table 10. Pure-Component Adsorption Ratios for MCM41-Based Sorbents at 70 °C and 1 atm Figure 10. Equilibrium isotherms of C2H2 on MCM41-based sorbents. Table 8. Equilibrium Isotherm Parameters for Acetylene on MCM41-Based Sorbents at 25 °C FeCl2 CoCl2 NiCl2

qmc (mmol/g)

bc (1/atm)

s

5.98 6.22 6.79

0.158 0.194 0.305

7.09 8.93 7.27

and SiO2. A large increase in sorbent capacity was observed for silica gel based sorbents. However, a decrease in diffusion time constants was observed. In order to increase the diffusion time constant while maintaining high sorbent capacity, MCM-41 was utilized as the substrate. This material is one of a new family of mesoporous silicate molecular sieves with hexagonal arrangement of unidimensional channels with uniform sizes in the range of 20-100 Å (Kresge et al., 1992; Beck et al., 1992). The MCM-41 utilized had a BET surface area of 770 m2/g and an average pore size measured by the BJH method of 26 Å. Figure 10 shows the equilibrium data for acetylene on MCM41based sorbents at 70 °C. Fitting parameters for the isotherms are shown in Table 8. Again, good fits were obtained with eq 1. The amounts of acetylene adsorbed at 1 atm for FeCl2/MCM-41, CoCl2/ MCM-41, and NiCl2/

FeCl2 CoCl2 NiCl2

C2H2/C2H4

C2H2/C2H6

7.7 8.8 10.3

25.6 21.3 15.2

MCM-41 were 2.25, 2.55 and 2.88 mmol/g at 25 °C, respectively. The BET surface areas of the sorbents were as follows: 304, 371, and 455 m2/g, respectively. Diffusion time constants are shown in Table 9. Purecomponent ratios of acetylene over other hydrocarbons at 1 atm and 70 °C for MCM41-based sorbents are shown in Table 10. While diffusion time constant values for MCM-41 sorbents were better than those based on SiO2, sorbent capacity and selective adsorption were lower. Equilibrium data for ethylene and ethane are shown in Figure 11. Normalized isotherm data are shown in Figure 12. The values of n for MCM41-based sorbents ranged from 0.8 to 0.9, which are very similar to those of silica gel based sorbents. The trend for MCM41-based sorbents is as follows:

FeCl2 > CoCl2 > NiCl2

(6)

The above trend was maintained in all three substrates used in this work. Also, as mentioned above, the reason for utilizing MCM-41 as a substrate was to improve diffusion rates while maintaining high sorbent capacity.

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Figure 13. Model of metal cation bonding to acetylene via π-complexation.

However, while time diffusion constants for MCM41based sorbents are higher than those based on silica gel as a substrate, sorbent capacity was lower, even though values for n are very similar for both sorbents. This behavior was attributed to substrate effects such as epitaxy. A note should be made on the D/R2 values on all sorbents. The uptake rates were actually enhanced by the impregnation of metal salts in the porous supports in all cases (Figure 7 for SiO2 and SiO2 supported salts, Table 3 for the case of Al2O3, and Table 9 for MCM-41). This result is a further demonstration that chemisorption, rather than pore diffusion, was the rate-limiting step in the uptake, as shown earlier by Yang and Foldes (1996). Nature of π-Complexation Bonding: Effects of Different Cations and Substrates. The basic concept of π-complexation involving transition metals and unsaturates was described in a qualitative theory by Dewar (1951) utilizing molecular orbital theory. Dewar’s explanation involved a Ag+-C2H4 system in which a filled 2pπ orbital from the olefin overlaps with an empty 5s orbital of the transition metal, forming a σ-bond. Also, it involved the overlap of a filled 4d orbital of the transition metal with a vacant 2pπ* antibonding orbital of the olefin referred to as backdonation. This concept was supported by the work of Chen and Yang (1995, 1996). In this work an extended Hu¨ckel molecular orbital (EHMO) study was undertaken to gain a better understanding of the phenomenon of π-complexation. It was established that the σ-bond was the major contribution to π-complexation, while backdonation played a minor role. The model described by Dewar was generalized for acetylene by Chatt et al. (1957). Later, it was extended by Blizzard and Santry (1968). In this latter model electrons from mainly the πu and somewhat from the σg orbital of acetylene are donated to a vacant hybrid orbital (dsp2) of the Fe2+, Co2+, and Ni2+ cations. Also, backdonation from a filled d orbital of the metal to a mixture of πg and σu antibonding orbitals of acetylene occurs (Blizzard and Santry, 1968). A representation of this model as described by Hopkinson (1979) is shown in Figure 13. As mentioned in the previous sections, the trend observed for acetylene adsorption on the various transition-metal cations was the following: Fe2+ > Co2+ > Ni2+. This trend was supported by heat of adsorption data calculated for the various sorbents. For example, isosteric heat of adsorption (-∆H) values for FeCl2/

MCM-41, CoCl2/MCM-41, and NiCl2/MCM-41 sorbents were calculated at 11.7, 11.1, and 10.4 kcal/mol, respectively. Heat of adsorption data values for FeCl2/SiO2, CoCl2/SiO2, and NiCl2/SiO2 were calculated at 12.2, 11.5, and 11.4 kcal/mol, respectively. The values for Ni2+ are in agreement with those obtained in Yang and Foldes (1996). One possible mechanism explaining this trend is based on the well-known fact that electron configurations involving half- or full-filled orbitals have an extra degree of stability (McQuarrie, 1983). For example, an Fe2+ cation has a 4s03d6 electron configuration in its valence shell. In backdonation, the transfer of electron charge from the 3d orbitals results in a configuration that is close to half-filled. This provides the resulting complex with an extra degree of stability. However, this situation does not occur with Ni2+ cations, which have the configuration 4s03d8. It can be seen that transfer of electron charge from the 3d orbitals for Ni2+ will not lead to the favorable configuration, 4s03d5. In the case of Co2+, the electron valence configuration is such that a net transfer of charge leads to an arrangement that is closer to a half-filled orbital than Ni2+. Therefore, the complex formed will be more stable than the one formed by Ni2+ cations but not as stable as those formed by Fe2+ cations. While the above mechanism is not the only possible explanation for the trend observed in this series of experiments, it is a likely possibility. It is a good model to understand the interaction between the metal cations and the acetylene molecules. A detailed molecular orbital analysis will help in the elucidation of the mechanism for this observed phenomenon. Aside from the obvious characteristics such as surface area and pore size, it is clear that the substrate plays an important role in acetylene adsorption as observed in the comparison of SiO2 and Al2O3 sorbents. While the surface area of SiO2 is almost twice that of Al2O3, SiO2-based sorbents adsorbed 3.5 times more acetylene and were more selective than those prepared with Al2O3. This is also demonstrated by the heat of adsorption data for both sorbents. At equal loadings, different heats of adsorption were observed for the same cation on different substrates. For example, heats of adsorption for NiCl2 on SiO2 and Al2O3 were calculated at 11.4 and 9.3 kcal/mol, respectively. Also, the change in substrates affected the ratio of acetylene molecules to cations. For example, the ratio of C2H2/M2+ for FeCl2/ SiO2 was 1.35, while for FeCl2/Al2O3 the ratio was 0.9. This result may be understood from the difference in the surface chemistry of SiO2 and Al2O3 and from the coordination between M2+ and the substrate. All three cations (Fe2+, Co2+, Ni2+) have three coordination numbers: 4, 5, and 6. The surfaces of SiO2 and Al2O3 are both filled with oxygen atoms. Pure SiO2 surface has no acidity, whereas Al2O3 has acidity due to oxide vacancies. Consequently, there are more M2+ ions that are four-coordinated on the SiO2 surface as compared to the Al2O3 surface, and there are more five- and sixcoordinated M2+ on the Al2O3 surface. It is more favorable for the four-coordinated M2+ ions to bond C2H2 molecules. The five- and six-coordinated M2+ ions are unlikely to bond to C2H2 due to stereochemical reasons. The experimental results indeed showed that SiO2 is significantly better as a substrate. Molecular orbital calculations are in progress for better understanding the mechanism of substrate effects on acetylene adsorption. From the results presented in this work, it is clear that cations and substrates play a major role in selective

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acetylene adsorption. It is possible that this difference in behavior can be used to manipulate the adsorptive properties of sorbents to better suit a particular application. Acknowledgment We are grateful for the MCM-41 sample provided by Professor Mark Davis and Dr. Cong-Yan Chen at Caltech. This work was supported by NSF Grant CTS9520328 and by a GEM consortium graduate fellowship for J.P. Nomenclature b ) Langmuir constant D ) diffusivity M ) metal cation (Fe2+, Co2+, Ni2+) P ) pressure q ) equilibrium amount adsorbed qm ) monolayer or saturated amount adsorbed R ) gas constant or particle radius in D/R2 s ) heterogeneity parameter T ) absolute temperature Subscripts c ) chemisorption or π-complexation p ) physical adsorption

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Received for review March 11, 1997 Revised manuscript received July 7, 1997 Accepted July 8, 1997X IE970207+

X Abstract published in Advance ACS Abstracts, September 1, 1997.