Langmuir 1995,11,3450-3456
3450
A Molecular Orbital Study of the Selective Adsorption of Simple Hydrocarbon Molecules on Ag+- and Cu+-ExchangedResins and Cuprous Halides J. P. Chent and R. T. Yang" Department of Chemical Engineering, State University of New Buffalo, New York 14260
York at Buffalo,
Received March 28, 1994. I n Final Form: June 21, 1995@ An extended Hiickel molecular orbital (EHMO) study is undertaken for the adsorption of CZand C3 hydrocarbon molecules on highly olefin-selectiveadsorbents, Ag+- and Cu+-exchanged sulfonic acid resins and copper(1) halides, by using a resin model and different crystal faces of halides. Consideration of the HOMO and LUMO energy levels of the adsorbate and adsorbent can give a qualitative indication of both the direction of electron transfer and the relative bond strength. Quantitative EHMO results show that for n complexation of olefins on resins, the strength of the bond follows the order Ag+ > Cu+ > H+. The bond stren&hs of olefins on Cu+ exposed on different crystal faces of the halides follow the order (111) > (001)> (lll),favoring Cu+with a lower coordination number. A strong anion effect on x-complexation is predicted by EHMO. For C2H4 on the (111)face of the coppert11halides, the bond strength order is CuCl > CuBr > CUI > CuS03C1jHs. The order is reversed for the (001)and (111)faces of the halides. From an analysis of the atomic orbital occupations and the net charges, a quantitative understanding of the n-complexation bond is obtained. The u donation to the bond is substantially higher than the d-n* backdonation. For the example of C2H4 on AgSOaCa5, the contribution from u donation accounts for 84% of the bond, whereas that by the d-n* back-donation accounts for only 16%. The EHMO results are in general agreement with the available experimental data.
Introduction Olefin-paraffin separations represent a class of the most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separati0ns.l Separation by JC complexation is a n attractive alternative and has been studied by using aqueous solutions1 or solid adsorbents2 containing Ag(1) or Cu(1). The most successful solid adsorbents have recently been prepared and studied in our l a b ~ r a t o r ythey ; ~ were formed by ion exchange on sulfonic acid resins and also by dispersing halides into nearly monolayer forms on y-Al203. On the Ag(1) resin, at 25 "C and 1 atm, the equilibrium adsorption ratio for C ~ H ~ / C=~9.2 H Iand ~ the C2H4 capacity = 1.15 mmollg. The corresponding values for C3HdC3H8 = 10.4 and C3H6 capacity = 1.29mmollg. The CuCllyA1203 showed equally promising results for olefin-paraffin ~eparation.~ Sulfonic acid resins have also been studied as catalysts for the dehydration of alcohol^^-^ and alkylation of benzene.8 Ag(1)-exchanged sulfonic acid resin was used as a bifunctional catalyst for the hydrolysis of allyl acetate: where Ag(1) served as the active site for adsorption of
* Author to whom all correspondence shouldbe addressed at the Departmentof Chemical Engineering, University of Michigan, Ann Arbor, MI 48109. Current address: Calsicat Division of Mallinckrodt Chemical, Inc., 1707 Gaskell Ave., Erie, PA 16503. Abstract published in Advance ACS Abstracts, September 1, 1995. (1)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;p 59. (2)Long, R. B.In Recent Developments in Separation Science; Li, N. N., Ed.; CRC: Cleveland, OH, 1976;Vol. 1, p 35. (3)Yang, R. T.;Kikkinides, E. S. MChE J . 1995,41, 509. (4) Thomton, R.; Gates, B. C. J . Catal. 1974,34, 275. (5)Gates, B. C.; Johanson, L. N. J . Catal. 1069,14, 69. (6)Gates, B.C.; Wisnouskas, J. S.; Heath, H. W., Jr. J . Catal. 1972, 24,320. (7)Gates, B. C.; Rodriguez, W. J. Catal. 1973,31, 27. (8)Wesley, R. B.; Gates, B. C. J . Catal. 1974,34, 288. (9)AfTrossman, S.;Murray, J. P. J . Chem. SOC.B 1968,1015. @
allyl groups by x complexation. In contrast, it was reported that no complexes were formed from liquid propylene on AgCl and Ag2S04.l' From the literature,it is likely that the anions or ligands have a strong influence on the ability of the cation to form JC complexes with hydrocarbons. The purpose of this study is to employ molecular orbital theory to investigate the effects ofthe anions and ligands on the adsorption. Since our understanding of x complexation remains primitive, it is also the goal of this study to obtain a better and quantitative understanding of the nature of the n-complexation bonds.
Model Selection and Computation Method T w o types of anion substrates are used: sulfonic acid resin and halides. Ag(1)- and Cu(1)-exchangedresins and copper(1) halides are included. The cation-exchange resin is synthesized by direct sulfonation of polystyrene that is cross-linked by divinylbenzene, so the monomeric unit may be represented by (vinyl)C&S03-H+. The proton may be readily exchanged by Ag(1) from, for example, AgN03 solution. The Ag(1)-exchanged resin may be represented by P-cS&-So&, where P stands for the polymer backbone. To minimize the computation, P is replaced by H. The model for the ion-exchangedresin is shown in Figure 1. The angle of Ag-0-S is 105",11and the angle of 0-S-0 is 113". The bond length of S-0 is 1.46A and is 1.79 A for the S-C bond." The C-H and C-C bonds in the aromatic ring are 1.084and 1.395 A,respectively,12and the bond length of Ag-0 is 2.1 A.ll When Ag+ is substituted by Cu+, the only change is the M-0 bond length;it is 1.86Afor the Cu-0 bond," while all other structural parameters remain the same. For cuprous halides, CuC1, C a r , and CUIare included in this study. CuF has not been prepared at ordinary temperatures hence, it is not included. In addition, different crystal faces of the halides are also considered. The faces are (0011,(lll),and (10)Francis,A. W. J . Am. Chem. Soc. 1951,72,3709and references cited therein. (11)Wells, A. F. Structural Inorganic Chemistry;Clarendon: Oxford, 1984;pp 125,721,724,978,1104. (12)CRC Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1986;p F-158.
0743-746319512411-3450$09.0OlO 0 1995 American Chemical Society
Hydrocarbon Molecules on Ag+- and Cu+-ExchangedResins
TM
Figure 1. Structure of the model cation-exchanged sulfonic acid resin, where M = Ag+ or Cu+.
(111)
(lil)
(001)
Figure 2. Schematic representation of the different crystal faces ofcuprous halides. Solid circles denote Cu and open circles halogen. A denotes adsorption site. The bond len hs of CuC1= 2.05 A, Cu-Br = 2.17 A, and Cu-I = 2.34
f
w bb
A
B
Figure 3. Configuration of the adsorbate-adsorbent adduct: (A) CZ& on resin and (B) CzH4 on CuX where the (111)face is shown as a n example.
(1111,and their structural parameters13 are shown in Figure 2. The cuprous halides have ghe zinc-blend structure. To obtain a representative charge distribution of the surface adsorption sites, the edge effects on the charges of surface atoms must be minimized. As shown in Figure 2, instead of using 1cuprous halide (CuX) molecule, 10 cuprous halide molecules are used as the substrate in this work, and only the central cuprous ion, denoted by A, is chosen as the adsorption site in computations. The surrounding cuprous ions are not used as adsorption sites due to their lower positive charges caused by edge effects. The effect of the model cluster size is judged by changes in the net charge of the central cuprous ion upon changing the surrounding cluster size. No further changes are seen in the net charge upon further increasing the cluster size beyond 20 atoms (as shown in Figure 2). This result is expected due to the structural symmetry and the fact that the edge atoms are already three lattices away from the central copper atom. Figure 3 shows the manner by which the hydrocarbon molecule is attached to the adsorption site ofAg or Cu, denoted by M. The M-C distance in each structure is obtained by optimization in the EHMO calculation for the lowest energy, while the structural parameters of the adsorbent remain unchanged. The geometry of the hydrocarbon is also optimized by varying the lengths of the C-C and C-H bonds and the angle of H-C-H. The extended Hiickel molecular orbital (EHMO) method14 is selected for this study. The principle ofEHMO and its application (13)Manson, E. L.; Lucia, F. C. D.; Gordy, W. J.Chem. Phys. 1975, 62, 1040; 1975, 62,4796; 1976, 63, 2724. (14)Hoffmann, R. J. Chem. Phys. 1963,39, 1397.
Langmuir, Vol. 11,No. 9, 1995 3451 Table 1. Atomic Orbital Parameters Used in the Calculations atom orbital exponent -Hi,eV H 1s 1.300 13.6 C 2s 1.625 18.43 2P 1.625 10.9 0 2s 2.275 31.32 2P 2.275 13.69 S 3s 1.817 20.00 3P 1.817 13.3 c1 3s 2.033 30.5 3P 2.033 15.0 Br 4s 2.638 26.94 4P 2.257 14.15 I 5s 2.681 23.3 5P 2.322 14.0 Ag 5s 1.606 11.0 5P 1.606 7.56 4d 3.81 12.5 cu 4s 2.05 11.4 4P 1.325 6.06 3d 5.95 14.0 to catalysis have been described in a comprehensive review.15 Due to the approximation involved, its predictions are not as accurate as the most rigorous ab initio methods. Like most of semiempiricalMO calculations, only valence orbitals are involved in the MO calculation. For EHMO, the Coulumb integrals are taken from the valence-state ionization potentials. The atomic orbitals of the inner shells are not included in the calculations. The resonance integrals (H$ are approximated as
where k = 1.75 in these calculations, Si is the overlap integral, and Hii and Hjj are the atomic orbital ionization potentials. Although it is an approximation, when EHMO is used for comparison of similar molecular models on the same basis, it provides valuable information, albeit semiquantitative, for complex systems in adsorption and catalysis.l6-20 The calculations are based on a program originally written by Hoffmann, and the version modified by Howell et aLZ1is used. The details for the EHMO computation used in this work are the same as those describedearlier and are available It should be noted that the version of EHMO used in this work includes the nucleus-nucleus and electron-electron repulsion terms, so the entire potential energy curves including the repulsion portion can be calculated. With this correction, the EHMO method can provide the correct potential energy curves with the repulsion portion which would otherwise be absent. The details of the formula for Hi with this correction can be found in the work of Howell et aLZ1 The atomic orbital parameters for C, 0, S, C1, Br, I, Ag, and Cu are listed in Table 1. All of the orbital exponents except those of Br and Ag are taken from ref 21. The Hiis of H, S, and C1 are also taken from ref 21. The exponents of Br and Ag and the Hi’sof C, 0,Br, and Ag are taken from Clementi and R ~ e t i . ~ ~ The parameters ofiodine are from Summerville and Hoffmann.26 (15)Baetzold, R. C. InAduances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic: New York, 1976; Vol. 25, p 1. (16)Saillard, J.-Y. S.; Hoffmann, R. J.Am. Chem. Soc. 1984, 106, 2006. (17) Kusuma, T. S.; Companion, A. L. Surf. Sci. 1988, 195, 59. (18)Casalone, G.; Merati, F.; Tantardini, G. F. Chem. Phys. Lett. 1987,137,234. (19)Mitchell, G. F.; Welch, A. J. J.Chem. Soc., Dalton Trans. 1987, 5 , 1017. (20)King, R. B. J. Comp. Chem. 1987,8, 341. (21) Howell, J.; Rosi, A.; Wallace, D.; Haraki, K.; Hoffmann, R.; Bartmess,J. E.; Thomas,D. Forticon8,QCMPO11,QuantumChemistry Exchange Program, IndianaUniversity,Departmentofchemistry, 1988. (22)Yang, R. T.; Chen, J. P. J. Catal. 1989, 115, 52. (23) Chen, J. P.; Yang, R. T. Surf Sci. 1989,216, 481. (24)Chen, J. P.; Yang, R. T. J. Catal. 1990, 125, 411. (25)Clementi, E.; Roeti, C. Atomic Data and Nuclear Data Tables 1974, 14 (3),177. (26) Summerville, R. H.; Hoffmann, R. J.Am. Chem. Soc. 1976,98 (23),7240.
Chen and Yang
3452 Langmuir, Vol. 11, No. 9, 1995
Table 2. Frontier Orbital Energy Level, Total Energy, and Net Charge of the Cation for the Adsorbate and the Adsorbent HOMO, eV LUMO, eV Et,t,l. eV 9.net charge -3.543 -237.5 -14.70 -201.6 -12.61 -7.944 -4.891 -168.1 - 13.33 -335.3 1.442 -13.13 -298.7 -12.46 -7.886 -12.23 -8.861 -954.5 0.383 (H) -1076 0.818 (Ag) -12.23 -10.013 -1092 0.715 (CU) -12.23 -9.874 Table 3. ExperimentalAmounts Adsorbed (mmoVg) at 25 "Con Ag+ Resin and H+ Resin Ag+ resin H+ resin C2H4 (0.5 atm) 0.84 0.10 C2H6 (0.5 atm) 0.080 0.070 C2H4 (1.0 atm) 1.02 0.14 C2H6 (1.0 atm) 0.12 0.11
-40
-35
-30
-25
-20
-15
-10
-5
Energy, eV
Figure 4. Density of occupied states of model resins: (A) HS03C6H5; (B) AgS03C6H5; ( c ) CUS03C6H5. o n l y occupied orbitals are shown. The Hi(s of Ag are determined by charge iteration on C2H4 adsorbed on AgSO&&.
Results and Discussion Molecular Orbital Properties of Adsorbents and Adsorbates. The electronic structures of the model resins, both ion exchanged and unexchanged, are shown in Figure 4. The densities of the occupied states are expressed by the number of occupied molecular orbitals at an interval of 0.5 eV. The global profiles of the density of states are approximately the same for Ag+- and Cu+exchanged resins in the entire energy region. However, in the region of -12 to -15 eV, the distributions of the density of states are quite different. In the Ag+-and Cu+exchanged resins, the density of states at around -12 eV is higher than that in the sulfonic acid form. In particular, Ag+exhibits the highest density of states at energy levels near that of the highest occupied molecular orbital (HOMO). Since n complexation involves donation of n electrons from the hydrocarbon molecule to the cation or metal surface, it is meaningful to consider the energy level of the HOMO of the hydrocarbon and that of the LUMO (lowest unoccupied molecular orbital) of the adsorbent. The hydrocarbon with a higher HOMO and the surface with a lower LUMO will facilitate electron transfer; therefore, it will be favored for forming n-complexation bonds. The HOMO and LUMO energy levels of the adsorbate molecules and the model resin adsorbents, along with the net charges of the cations in the resins, are given in Table 2. From Table 2, it is seen that the LUMO levels of the adsorbents (H+,Ag+,and Cu+resins) are higher than the HOMO levels of the adsorbates. The olefin molecules have higher HOMO levels than the paraffins. For example, the energy gap between the HOMO of C2H4 and the LUMO of MS03C6H5 is about 2.6 eV. This is much smaller than the energy gap between the HOMO of C2H6 and the LUMO of MS03C6H5, which is approximately 4.7 ev. Consequently, the energy transfer from the olefin to the adsorbent is much easier than that from the paraffin, and the large energy gap between the HOMO of C2H6 and the LUMO of MS03C6H5 makes it impossible to form a n
Table 4. Optimized Bond Distances (A)and Bond Angles (deg) of CzH4 C-H c-c H-C-H 1.07 1.34 122.8 free C2H4 0.85 1.35 119.3 C2H4 on CUS03C6H5 0.85 1.355 118.2 C2H4 on (111)of CuCl
complex. Also in Table 2, the HOMO level of C2H2 lies between those O f C2H6 and C2H4. However, experimental results showed1that C2H2 adsorbs considerably stronger than C2H4. This is because the frontier orbitals of C2H2 consist of two n bonds of equal energy levels, whereas C2H4 has only one n bond. Comparing the adsorbents, the LUMO energy levels of HSO3C6H5, AgSO3CsH5, and CUS03C6H5are, respectively, -8.861, -10.01, and -9.874 eV. A lower LUMO energy level is favorable for accepting electrons from the HOMO of the adsorbate. AgS03C6H5 has the lowest LUMO level and, hence, can form n complexes easier than the others. The LUMO level for the unexchanged resin (i.e., the proton form)is -8.86 eV, significantlyhigher than the exchanged forms, so adsorption on this surface is expected to be the most difficult. This is in agreement with our recent experimental results that show the adsorption of all hydrocarbon molecules considered here is very weak on the proton-form resin.3 Table 2 also reports the net charges of the cations, H+, Ag+, and Cu+, to be 0.383, 0.818, and 0.715, respectively. The cation with a higher positive charge will be a better electron acceptor to form n bonds with the hydrocarbon. Therefore, the ease with which to adsorb and its stability follow the order Ag+ > Cu+>> H+. This prediction is also in agreement with our recent experimental r e ~ u l t .Since ~ the H+-form resin is a very poor sorbent, no further computation will be performed on this resin. Table 3 shows the experimental data on equilibrium adsorption of C2H4 and C2H6 on Ag+ resin and H+ resin. It is clear that the bonding between C2H4 and Ag+ resin is much greater than that with H+resin. Cu+resin cannot be prepared at the present time since Cu+ salts are not water-soluble. So comparison with Cu+ resin cannot be made. Adsorption on Resins: Energies and Bond Lengths. The energy of adsorption, m a d s , can be calculated by
where E s u b is the total energy of the substrate, EHCis that of the hydrocarbon molecule, and Eaddu& is that of the
Hydrocarbon Molecules on Ag+- and Cu+-ExchangedResins Table 6. Relative Energy of Adsorption and Equilibrium Metal-Carbon Distances (D) for Ag+ and Cu+Model Resins and Experimental Heats of Adsorption (AZT) system
D,"A
CzHs+A CzH4+A C2Hz+A C3Hs+A C3Hs+A
2.24 2.32 2.24 2.25 2.36
CzHs+B CzH4t-B C2Hz+B
2.09 1.85 1.87
C3Ha+B C3Hcj+B
2.31 1.86
re1 energy of adsorption eV/molecule kcal/mol AgSO&& = A -0.718 -1.285 -1.296 -0.730 -1.110 cUso3csH5 = B -0.708 -1.203 -1.474 -0.576 -2.120
-16.55 -29.62 -29.87 -16.83 -25.58 -16.31 -27.64 -35.53 -13.28 -48.86
exptl m,
Langmuir, Vol. 11, No. 9, 1995 3453
,
8 ,
I
I
Ethylene
I
6
i
kcal/mol -4.8 -10.0 -5.1 -10.3 -5.3' -11.7' -1o.w - 12.5c -5.6' -14.2'
Experimental metal-carbon distances for Ag-C are in the range 2.09-2.50 -.&,11,28,29 and those for Cu-C are in the range 1.88-2.08 A.28-31 See text for discussion. The substrate is CuCV y-AlzO3. Experimental M v a l u e s for both Ag+ and Cu+ are taken from ref 3. Experimental M taken as the dissociation energies from complexes with CuCl crystals at 0 "C,from Gilliland et a1.39,40
'
adsorbed adduct. It should be noted that the energyvalues in the EHMO theory are dependent on one's choice of Coulomb integrals and formula for the resonance integral used in the calculation and are, therefore, relative. However, it should be stressed that the relative values from the same calculations are m e a n i n g f ~ l , ~as~ will , ~ ' be further discussed below. Literature data39,40on the experimental values of the dissociation energies of C2H2 and C2H4 from complexes with CuCl crystals at 0 "C are also included in Table 5. The dissociation energies are - 12.5 and - 10 kcal/mol for CzH2 and C2H4, respectively. The higher dissociation energy for C2H2 compared to C2H4 is also consistent with our calculations. The energy of adsorption is calculated for C2 molecules on Ag+ and Cu+ resins as a function of distance to the surface when the molecule approaches the cation site following the configuration shown in Figure 3. These potential energy curves are shown in Figures 5 and 6, for Ag+ and Cu+resins, respectively. Both figures show that the energies of interaction for C2H2 and C2H4 are substantially higher than that with C2Hs. This is, again, in agreement with the experimental result^.^ Both the metal-carbon bond length and the geometry of the hydrocarbon molecule are optimized. Examples of the optimized geometries of C2H4 at the energy minima (27) Turner, A. G. Methods in Molecular Orbital Theory; Prentice Hall: Englewood Cliffs, NJ, 1974. (28) Gheller, S. F.; Hambley, T. W.; Rodgers, J. R.; Bownlee, R. C. T.: O'Connor, M. J.; Snow, M. R.: Wedd, A. G. Znorg. - Chem. 1984,23, 2519. (29) Ferguson, G. Structure Reports; Kluwer Academic: Dordrecht, 1992; Vol. 51B, Part 2, pp 1266, 1491, 1924. (30) Herberhold,M. Metal n-Complexes;Elsevier: Amsterdam, 1972; Vol. 2, Part 1, p 245 and the literature cited therein. (31) Timmermans, P. J.A.; Macker, A.; Spek, A. L.; Kojic-Prodic,B. J . Organometal. Chem. 1984,276, 287. (32)Xie, Y.-C.;Tang, Y. Q.Adu. Catal. 1990, 37, 1. (33) Foldes, R.; Kikkinides, E. S.; Yang, R. T.Unpublished results, S U N Y at Buffalo, 1994. (34) Dewar, M. J. S. Bull. SOC.Chim. Fr. 1951, 18, C 79. (35) Chatt, J.; Duncanson, L.A. J . Chem. SOC.1963, 2939. (36)Solomon, J. L.; Madix, R. J.;Sttihr, J. Chem. Phys. 1990, 93, 8379. (37) Chin, Y.-H.;Ellis, P. E. J . Am. Chem. Soc. 1993, 115, 204. (38) Barteau, M. A.; Madix, R. J. J . Am. Chem. Soc. 1988,105,344. (39) Gilliland, E. R.; Seebold, J. E.; FitzHugh, J. R.; Morgan, D. S. J . Am. Chem. Soc. 1939,61,1939. (40) Gilliland, E. R.; Bliss, H. L.;Kip, C. E. J . Am. Chem.SOC.1941, 63, 2088.
I
0
1
2
3
4
5
6
Cu-C Bond Distance, A
Figure 6. Potential energy curves for C2H6, C2H4, and CzHz adsorption on Cu+-exchanged resin.
are shown in Table 4. The geometriesof the hydrocarbons are generally not significantly changed upon adsorption. From Figures 5 and 6 , one may take the energy minima as the energies of adsorption. These values are listed in Table 5 as the relative energies of adsorption. Also given in Table 5 are the experimental values for heat of ad~orption.~ For n complexation on CU+,Cu+-exchanged resin has not been prepared (because Cu+ salts are not soluble in water), so CuCl supported on yAl2O3 is used for comparison. The comparison between the relative theoretical values and the relative experimental data indeed shows a general agreement. Experimental results are not available for C2H2 since CzH2 dissociates on Ag+ and Cu+ to form acetylides as a result of very strong ads0rption.l The strong adsorption is also seen in the theoretical results. Despite the agreement between theory and experiment, it should be noted that the bond energies calculated from the EHMO are less reliable than that from other techniques such as CNDO and MINDO. The &+-exchanged resin has been used as a bifunctional Here the catalyst catalyst for allyl acetate hydroly~is.~ activity is due to the high concentration of allyl acetate as a result of n complexation with Ag+, even though the Ag+ is not the active site for reaction. Table 5 also includes the theoretical equilibrium distances for the adsorption and experimental bond lengths of&-C and Cu-C in organometallic compounds. The equilibrium distances, expressed in terms of the carbon-metal bond lengths, are obtained by optimizing
Chen and Yang
3454 Langmuir, Vol. 11, No. 9, 1995
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-2
I
. 0
a
! A
-2
1
2
3
4
5
0
6
CU-C Bond Distance, A
Figure 7. Potential energy curves for ethylene adsorption on the different crystal faces of CuC1.
1
2
3
4
5
6
Cu-C Bond Distance,
Figure 8. Potential energy curves for ethylene adsorption on the different crystal faces of CuBr. 8 ,
I
I
the total energy of the whole adsorption adduct. The calculated equilibrium distances of Ag-C, from 2.15 to 2.32 A, are in the range of the reported bond lengths in 6 Ag-organometallic compounds. In AgClO4C&, Ag+ being associated with the benzene ring, the closest Ag-C distance is 2.5 A.ll The same bond length for Ag-C is 3 4 found in AgN03C3H8.l' A shorter Ag-C bond length, s 2.093 A,is measured in [(C~H~)~IZ[(CN)A~SZWSZI .28*29 As w shown in Table 5, the calculated Cu-C bond length covers a wider range, from 1.85 to 2.096 A. A wider range for the Cu-C bond length is also seen in cuprous organometallic complexes. The Cu-C bond length is 1.880 A in 0 [(C~H~)~NIZ[(CN)CUSZMOSZI and 1.888 and 1.884 A in [(C3H7)&I2[(CN)CuS MoSzl Also, the Cu(I)-C bond I length of 2.06-2.076 exists in ( C ~ H ~ ~ ) C U O S ~ ~ C F ~ . ~ ~ -2, ~1 ~ 0 1 2 3 4 5 6 Again, in [CUNH(C~H~N)~CZ&)IC~O~ and [CUNH(C&LNz(CzHz)]BF4, ethylene and acetylene molecules are coorCu-C Bond Distance, dinated to the central Cu(1) with a 2.019-A Cu-C bond Figure 9. Potential energy curves for ethylene adsorption on length. In the simple n complex, C U C ~ G H ~ Othe N , Cu-C the different crystal faces of CUI. distance is 2.06 All the above experimental values are in close vicinity to the theoretical values in Table 5 . followingorderfor all three halides: (111)> (001) > (111). Adsorption of Ethylene on Cuprous Halides. CuThis order is in agreement with the coordination number; prous halides, in particular CuC1, can be dispersed with Cu+ with a lower coordination number forms a stronger ease on high-surface-area y-AlzO3 to form nearly monobond. layer species, and this type of sorbent is already being A comparison can also be made with the adsorption of used in industry for the separation of CO from COCZH4 on Cu+-exchanged resin (shown in Figure 6). The containing mixtures through JC c o m p l e x a t i ~ n They . ~ ~ ~also ~ adsorpiion on the Cu+ resin is stronger than on the (001) show promise for olefin-paraffin separation^.^ Compuand (111)faces of all three halides but weaker than the tation is therefore performed on copper(1)halide surfaces (111)faces of the three halides. in order to obtain an understanding of adsorption on these A direct comparison of CZH4 adsorption on the (111) surfaces. faces of the three CuX as well as the Cu+ resin is shown In the bulk structures of cuprous halides, both cuprous in Figure 10. This comparison shows the effects of the and halogen ions are 4-fold coordinated. As shown in associated anions on JC complexation. The order of the Figure 2, the Cu ions on different crystal faces have adsorption strength is CuCl> CuBr > CUI> CuS03C6H5. different coordination numbers. The cuprous ion on the However, the above order applies only to the (111) ( 111)face is the most unsaturated; each Cu atom is bonded crystal face. The EHMO computationresults for all three to only one halogen atom. On the (001) face, Each Cu faces and the resin are given in Table 6. Both relative atom is bonded to two halogen atoms. The (111)face energies of adsorption and the equilibrium m_etal-carbon exposes only 3-fold coordinated Cu atoms. The different bond lengths are listed. For the (001)and (111)faces, the coordination numbers will obviously give rise t o different anion effects are reversed, i.e., CuSOaCeH5 > CUI > CuBr adsorption properties. > CuC1. The experimental data ofequilibrium adsorption The potential energy curves for ethylene adsorption have been measured for C2H4 at 25 "C on the three coppercomputedfrom the EHMO theory are exhibited in Figures ~ ~1atm of CzH4, the (I) halides supported on y - f i l ~ 0 3 .At 7-9, for three different crystal faces on, respectively, CuC1, equilibrium adsorbed amounts are 0.73 mmoVg for CuC1, CuBr, and CUI. For all three halides, the (111)crystal 0.50 mmoUg for CuBr, and 0.32 mmoUg for CUI. This face is the clearly preferred one for ethylene adsorption. result is in agreement with the theoretical prediction for The energy of adsorption, hence, the strength of adsorpthe (111)faces, thereby indicating that the surfaces ofthe tion, is judged by the energy minimum in the potential supported cuprous halides are dominated by the (111) energy diagram. The strength of adsorption follows the faces. The EHMO results indicate that the mcomplexation
f,
x
Hydrocarbon Molecules on Ag+- and Cu+-ExchangedResins
'rac
t P;
:
w
T
7
7
-
1
-r
-
1
1
't
1 -
0 -
-1
-
0
i
Cu-C
a
a
4
i
e
Bond Dirtance, A
Figure 10. Potentialenergy curves for ethylene adsorption on the (111)faces of three halides and Cu-exchanged resin. The four curves, from top down, are CuS03C6H5, CUI,CuBr, and
cuc1.
Table 6. Relative Energy of Adsorption (a) and Equilibrium Carbon-Metal Bond (M-C) Distances for Ethylene Adsorption on Copper-Exchanged Resin and Cuprous Halides M-C,a AE, AE, exptl amt adsorbedb A eV kcaVmol at 1 atm, 25 "C, mmoVg CuSO&& 1.85 -1.203 -27.62 (111)face CUCl 1.88 -1.955 -45.06 0.73 CuBr 1.88 -1.714 -39.51 0.50 CUI 1.92 -1.522 -35.08 0.32 (001)face CUCl 2.06 -0.669 -15.42 CuBr ' 1.92 -0.972 -22.40 CUI 1.99 -0.983 -22.66 (iii)face CUCl 2.30 -0.420 -9.68 CuBr 2.21 -0.484 -11.16 CUI 2.30 -0.486 -11.20
Experimental Cu-C distances are in the range 1.88-2.08
A.28-31 See text for discussion. Amounts adsorbed on monolayer
cuprous halides supported on y-Al203 (thecrystal faces of cuprous halides are unknown).33
strength depends strongly on the manner by which the surfaces of the halides are exposed, i.e., the adsorption properties can be manipulated by adjusting the synthesis conditions to expose different crystal faces. One approach in this direction is to use different supports to provide the desired epitaxy. Further work is in progress in our laboratory. Nature of the n-ComplexationAdsorption Bond. The olefin n-complexation bond was first explained using the molecular orbital theory by Dewar.34 In his model, he separated the olefin-metal bond into two components. One is the a bond between the filled n orbital of the olefin molecule and the unoccupied s orbital of the metal atom. The other is the overlap between the occupied d orbital of the metal atom to the anti-bonding JZ orbital &e., n*) of the olefin molecule. This model was subsequently modified by Chatt and D u n ~ a n s o nwhere , ~ ~ the a-type bond is formed by the overlap of the hybrid orbital of the transition-metal atom with the n orbital of the olefin molecule, and this idea was applied to platinum-olefin complexes. A quantitative understanding of the n-complexation bonds with both Ag+ and Cu+is obtained from the EHMO calculations. The anion selected for this study is the model resin, C6HsS03-. Table 7 lists the overlap populations in the three C2 molecules before and after bonding to AgS03C6H5 and CuSO&,&. The overlap population between two atoms
Langmuir, Vol. 11, No. 9, 1995 3455 Table 7. Overlap Populations for Bonds (A)before and after Adsorption molecule c-c C-H C-Ag or C-Cu 0.559 0.8874 1.228 0.9105 2.158 0.9183 AgSO3CsHs = A 0.444 0.8914 0.0420 0.937 1.0288 0.1247 2.075 0.9308 0.0906 CuS03C6H5 = B 0.5534 0.8966 1.198 0.8418 2.075 0.9494
0.0028 0.0765 0.0447
is indicative of the covalent bond order. The results in Table 7 show that there are various degrees of overlap between carbon and Ag+or Cu+. However, they are small compared to those for C-H and C-C, meaning only weak interactions exist. It is clear that the interactions with Ag+ are stronger than Cu+ for all C2 molecules. It is not clear, however, why the interactions with C2H2 are weaker than that with C2H4. The results in Table 7 also show that upon adsorption, the C-H bonds are strengthened while the C-C bonds are weakened in the C2 molecules. The fact that C2 molecules interact more strongly with Ag+ resin than with Cu+ resin can be attributed partly to the higher net charge of Ag+. The net charges are given in Table 8. Experimental results reported in the literature indicate a direct relationship between the net charge of Ag and the n-complexation bond strength. On an oxygenfree Ag surface, there is only a-donation but no or little d-n* b a c k - d ~ n a t i o n . ~The ~ ~ ~study ' of C2H4 and C3H6 on clean and oxygen-covered Ag (110) surfaces using nearedge X-ray absorption fine structure showed no detectable n*resonance or changes in the C=C bond length.36 NMR results3' showed only a weak n bond between C2H4 and the Ag metal surface. The existence of oxygen (which is highly electronegative) on Ag will increase the net charge of nearby Ag atoms, which will in turn increase the electron donation from olefin. This conclusion has been supported by TPD experiments of C3H6 on the Ag surface with different oxygen coverages.38 The results in Table 8 show that for ethane adsorption on AgSO3C& or CuS03CdiI5, essentially no changes occur in the electron distributions in either adsorbate or adsorbent, indicating no or very little chemical interactions. As shown in Table 2, the HOMO energy level of ethane is 4.69 eV lower than the LUMO energy level of AgSo&H5. This large energy gap makes it difficult for electron transfer from ethane to Ag+. A quantitative understanding of the nature of the x complexation can be gained from the atomic orbital occupations given in Table 8. Since the bonding is very similar between AgS03C6H5 and CuS03CsH5, only AgS03C6H5 will be discussed. Only C2H4 bonding to AgS03C& is analyzed below. Upon bonding of C2H4 to AgS03C6H5, the net charge of Ag is decreased from 0.818 to 0.608 and that of C is increased from -0.071 to +0.048 (for each C atom in CzH.4). This transfer of electrons constitutes the a-n donation. Electrons are transferred from the 2p2 orbital of carbon (i.e., the n bond in C2H4) to the partially empty 5s orbital of Ag. The electron occupation in the 5s orbital of Ag is increased from 0.157 to 0.346, whereas that in the 2p2 orbital of carbon is decreased from 1.00 to 0.851. The total electron loss from the 2pz orbital is 0.298. The total electron loss from C2H4 is, however, less than this value. The total electron loss from the C2H4 molecule upon adsorption can be calculated from the sum of the electron
3456 Langmuir, Vol. 11, No. 9, 1995
Chen and Yang
Table 8. Outer-Shell Atomic Orbital Occupation and Net Charge before and after Adsorption molecule atom net charge S Px PY Pz dxz-yz dzz dxr dxz 1.044 0.948 1.044 C -0.070 1.071 1.000 0.979 -0.071 1.041 C 1.078 1.000 1.00 0.986 -0.074 1.088 C 2.000 1.997 1.988 0.004 1.999 0.018 0.020 0.157 0.818 Ag 2.000 1.987 0.040 1.999 0.038 0.062 1.983 0.213 0.759 Ag 0.998 0.955 1.029 1.088 -0.070 C 2.000 1.997 0.039 1.999 0.027 1.983 0.017 0.608 0.346 Ag 0.851 1.033 0.966 1.102 0.048 C 1.977 2.000 1.996 0.001 1.999 0.060 0.022 0.282 0.672 A + C2H2 Ag 0.902 0.979 0.987 1.105 0.027 C 1.999 1.990 0.003 1.997 0.001 0.00 1.914 0.413 0.695 cu 1.973 1.999 0.010 1.995 0.002 0.001 1.910 0.431 0.681 cu 1.042 1.000 0.929 1.103 -0.074 C 1.893 1.999 1.987 0.019 1.995 0.001 cu 0.003 0.628 0.579 0.860 0.945 1.156 0.153 C 0.886 0.001 1.997 0.002 0.013 1.891 1.999 1.990 0.562 0.560 cu B + C2H2 0.867 0.987 0.993 1.116 0.038 C
losses from the four H atoms (not shown in Table 8, occupation = 0.036 in free C2H4 and 0.039 in bonded C2H4) and the two C atoms (from -0.071 to 0.048). Thus, the total electron loss from C2H4 upon adsorption is 0.252. The difference of 0.048 electron is back-donated to ethylene. This 0-n complexation is illustrated in Figure 11, where the 0 donation is shown in a and the d-n* back-donation is shown in b. The net decrease of the electron occupationof the GZorbital ofAg upon adsorption is only 0.015, less than the 0.048 electron that is backdonated to the n* orbital of ethylene. The difference is attributable to electrons gained by the dy.orbital from the 5s orbital of Ag. In summary, for the 0-n complexation between C2H4 and AgS03C6H5, the contribution from 0 donation accounts for 84% of the bond, whereas that by the d-n* back-donation accounts for only 16%. Again, insights can be gained on the relative contribution to bonding from the HOMO and LUMO energy levels. From Table 2, the energy gap between the HOMO level of AgS03C6H5and the LUMO level of ethylene is 4.29 eV, which is much larger than the energy gap between the HOMO level of ethylene and the LUMO level of AgSO3-
d Y Z
2.000 1.999 1.985 1.991 1.988 1.998 1.896 1.985
Figure 11. Schematic representation of (a) a-donation of electronsfrom the C2& n-orbital to the 55 orbital of Ag and (b) d-n* back-donation from the Ag 4dyzto the n* of C2H4. C6H5,2.60ev. The large energy gap between the HOMO of the adsorbent and the LUMO of the adsorbate makes d-n* back-donation much weaker than 0 donation.
Acknowledgment. This work was supported by NSF Grant CTS-9212279. LA9402682
