Ab Initio and Microcalorimetric Investigations of Alkene Adsorption on

Apr 13, 2005 - Previous analysis has demonstrated this HPA formulation possesses a low surface area of ∼6 m2 g-1.47 The alkene gases were supplied b...
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Ab Initio and Microcalorimetric Investigations of Alkene Adsorption on Phosphotungstic Acid Kimberly A. Campbell, Michael J. Janik, Robert J. Davis,* and Matthew Neurock* Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904 Received October 21, 2004. In Final Form: March 3, 2005 The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 kJ mol-1, respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1°) carbenium-like transition states followed by secondary (2°) and tertiary (3°) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately -100 kJ mol-1, which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1° carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.

Introduction Industrial, acid-catalyzed processes have often relied on liquid acid catalysts that are corrosive, noxious, and require costly separation steps, which have thus motivated the development of heterogeneous catalytic materials. The use of solid acid catalysts would eliminate the bulk volume of the liquid acid and facilitate separation in heterogeneous reaction conditions. Heteropolyacids (HPAs) offer a potential replacement for liquid acids on account of their high acid strength and their effectiveness in multiple types of reaction media, such as gas-solid or liquid-solid systems.1,2 Presently, supported HPAs are not utilized to the same extent as solid acid zeolites and ion-exchange resins in industrial processes. This is partly due to their rapid deactivation.3 In this work, we analyze the modes and the corresponding energies for the adsorption of different alkenes on phosphotungstic acid and compare the results with those found for alkene adsorption on zeolites. Understanding the interaction of alkenes with phosphotungstic acid is an important first step to probing hydrocarbon surface chemistry before deactivation or regeneration can be considered. HPAs are strong Brønsted acids, which have low proton affinities as a result of a significant delocalization of charge over their conjugate base.4,5 Figure 1 shows the primary * Authors to whom correspondence should be addressed. Tel: (434) 924-6284 (R.J.D.); (434) 924-6248 (M.N.). E-mail: [email protected] (R.J.D.); [email protected] (M.N.). Fax: (434) 982-2658 (M.N.). (1) Misono, M. C. R. Acad. Sci. Paris, Ser. IIc: Chim./Chem. 2000, 3, 471-475. (2) Sheldon, R. A.; Downing, R. S. Appl. Catal. A 1999, 189, 163183. (3) Kozhevnikov, I. V.; Holmes, S.; Siddiqui, M. R. H. Appl. Catal. A 2001, 214, 47-58. (4) Corma, A. Chem. Rev. 1995, 95, 559-614. (5) Janik, M. J.; Campbell, K. A.; Bardin, B. B.; Davis, R. J.; Neurock, M. Appl. Catal. A 2003, 256, 51-60.

Figure 1. Keggin unit of phosphotungstic anion, PW12O403-.

Keggin unit (KU) of the phosphotungstic anion, which is the most commonly studied heteropolyanion due to its high acid strength.6,7 The Keggin structure (XM12O40n-) is comprised of central (Oa), bridging, and terminal (Od) oxygen atoms.8 The bridging oxygen atoms are further subdivided into the classifications of corner-sharing oxygen atoms (Ob) and edge-sharing oxygen atoms (Oc).9 The Keggin unit has a negative charge (n-), which must be balanced by oxonium ions, protons, or other cations. The primary units arrange into a three-dimensional secondary structure. The most stable secondary structure is the hexahydrate form, which has a body-centered cubic structure (bcc) with protons located in H5O2+ water bridges.10 Herein we analyze the chemisorption of alkenes (6) Bardin, B. B.; Bordawekar, S. V.; Neurock, M.; Davis, R. J. J. Phys. Chem. B 1998, 102, 10817-10825. (7) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113252. (8) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: New York, 1983. (9) Misono, M. Catal. Rev.-Sci. Eng. 1987, 29, 269-321. (10) Brown, G. M.; Noe-Spirlet, M. R.; Busing, W. R.; Levy, H. A. Acta Crystallogr. 1977, B33, 1038-1046.

10.1021/la047395a CCC: $30.25 © 2005 American Chemical Society Published on Web 04/13/2005

Investigations of Alkene Adsorption on Phosphotungstic Acid Scheme 1. Illustration of Alkenea Adsorption on an Acid Catalyst

a Cp is the protonation site of the alkene. C* is the adsorption site.

to the anhydrous H3PW12O40 structure, where the protons are assumed to be located on a single KU. Particular to heteropolyacids, small polar molecules are able to penetrate the secondary structure and react with the exterior of the primary unit. The ability of the fluid phase to interact with all the active sites of the catalyst has been termed Bulk I, “pseudo-liquid” behavior.9,11 Hydrocarbons and other nonpolar species will only interact with surface sites of the HPA. The interactions of alkenes on HPAs likely demonstrate similar trends as those found on other solid Brønsted acids, such as proton-exchanged zeolites. Computational studies of alkene adsorption on zeolites indicate that the alkene initially forms a π-bound state, in which electrons in the π bond participate in a hydrogen bond with the acidic proton. The physisorption of the π-bound intermediate is slightly exothermic and is followed by a concerted reaction that results in formation of the chemisorbed alkoxide, as depicted in Scheme 1. Alkene adsorption proceeds via the simultaneous electrophilic attack of the acidic proton (Ha) on a carbon atom of the alkene (Cp) and formation of the alkoxide with the remaining alkene carbon atom (C*) and a neighboring surface oxygen atom. From the π complex to the alkoxide state, the alkene passes through a carbenium-like transition state.12 On the basis of the stability of gas-phase carbenium ions, adsorption through a tertiary (3°) carbenium-like transition state is thought to have a lower activation barrier (Eact) than processes with a secondary (2°) or primary (1°) transition state.13 Because of the differences among zeolite structures, as well as the different computational models and methods chosen, the reported energies for alkene adsorption vary widely. Larger cluster, embedded clusters, and periodic calculations typically constrain the outer region of the zeolite to their crystallographic positions and optimize only the local region near the adsorption site.14-16 Rozanska et al. demonstrated that imposing constraints on the zeolite atoms neighboring the active site can lead to inaccurate, endothermic values for alkoxide formation.16 The comparison of computed adsorption energies on zeolites to experimental results can be problematic, since alkenes have been observed to readily oligomerize above room temperature.17 The diffusion of alkenes through the zeolite micropores presents another challenge for modeling this system.18 (11) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199-217. (12) Kazansky, V. B.; Senchenya, I. N. J. Mol. Catal. 1992, 74, 257266. (13) Gates, B. C. Catalytic Chemistry; John Wiley and Sons: New York, 1992. (14) Bhan, A.; Joshi, Y. V.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. B 2003, 107, 10476-10487. (15) Kasuriya, S.; Namuangruk, S.; Treesukol, P.; Tirtowidjojo, M.; Limtrakul, J. J. Catal. 2003, 219, 320-328. (16) Rozanska, X.; Demuth, T.; Hutschka, F.; Hafner, J.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 3248-3254. (17) Kondo, J. N.; Domen, K. J. Mol. Catal. A: Chem. 2003, 199, 27-38.

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The previous body of literature on alkene interactions with heteropolyacids is composed of solely experimental work. There are no computational studies that we are aware of. For example, the formation of alkoxides on heteropolyacids has been investigated using catalytic studies and IR spectroscopy.19,20 Małecka et al. observed isobutene adsorbed on the surface of H4SiW12O40 in the catalytic reaction of isobutene and methanol to form methyl t-butyl ether. Similarly, Ivanov et al. concluded propene did not interact with bulk sites on phosphotungstic acid, as the IR spectra showed no modification of the KU vibrational frequencies. The differential heats of adsorption determined by calorimetric experiments is one of the most common ways to characterize the solid acid sites.21 Basic probe molecules, such as ammonia (NH3) and pyridine, are often employed.22-26 We previously reported the average sorption enthalpy of NH3 on H3PW12O40 as -153 ( 5 kJ mol-1, independent of uptake.5 The uniform differential heat of adsorption suggests the acid sites on phosphotungstic acid are equivalent. Zeolites also show equivalent acid sites in microcalorimetric studies.27,28 However, differences in NH3 adsorption enthalpies cannot be directly used to rank the acid strength of H3PW12O40 in comparison to zeolites.29 To the best of our knowledge, no microcalorimetry studies have been reported in the literature that address alkene adsorption on the surface of HPAs. Alkene probes have been applied to study the surface acidity of Na+-exchanged zeolites,30 Ziegler-Natta catalysts,31 bismuth molybdates,32 and other metal catalysts.33 However, metalalkene π complexes will differ in nature to the alkoxides formed on the surface oxygen atoms of HPAs. The adsorption enthalpy determined from microcalorimetric experiments can be compared to energies calculated from DFT studies. Several of the factors complicating the modeling of alkene adsorption on zeolites are not a concern for heteropolyacids. The primary unit of HPAs has a well-defined molecular structure, whereas small cluster models of zeolites face the challenge of how to terminate the Si and Al tetrahedra. Furthermore, the unsupported H3PW12O40 does not possess micropores or demonstrate extensive oligomerization of alkenes in comparison to the zeolite H-ZSM-5.20 This study combines computational and microcalorimetric methods to investigate the adsorption of alkenes on the H3PW12O40 structure. The results are subsequently compared to those associated with zeolites. Experimental (18) Rigby, A. M.; Kramer, G. J.; van Santen, R. A. J. Catal. 1997, 170, 1-10. (19) Małecka, A.; Poz´niczek, J.; Micek-Ilnicka, A.; Bielan´ski, A. J. Mol. Catal. A: Chem. 1999, 138, 67-81. (20) Ivanov, A. V.; Zausa, E.; Taaˆrit, Y. B.; Essayem, N. Appl. Catal. A 2003, 256, 225-242. (21) Auroux, A. Top. Catal. 1997, 4, 71-89. (22) Bardin, B. B.; Davis, R. J. Appl. Catal. A 2000, 200, 219-231. (23) Jozefowicz, L. C.; Karge, H. G.; Vasilyeva, E.; Moffat, J. B. Microporous Mater. 1993, 1, 313-322. (24) Lefebvre, F.; Liu-Cai, F. X.; Auroux, A. J. Mater. Chem. 1994, 4, 125-131. (25) Liu-Cai, F. X.; Sahut, B.; Faydi, E.; Auroux, A.; Herve´, G. Appl. Catal. A 1999, 185, 75-83. (26) Misono, M. Chem. Commun. 2001, 1141-1152. (27) Parrillo, D. J.; Gorte, R. J. J. Phys. Chem. 1993, 97, 8786-8792. (28) Parrillo, D. J.; Lee, C.; Gorte, R. J. Appl. Catal. A 1994, 110, 67-74. (29) Kresnawahjuesa, O.; Ku¨hl, G. H.; Gorte, R. J.; Quierini, C. A. J. Catal. 2002, 210, 106-115. (30) Thamm, H.; Stach, H.; Fiebig, W. Zeolites 1983, 3, 95-97. (31) Auroux, A.; Bujadoux, K.; Jannel, J. C.; Lalart, D. React. Solid 1989, 7, 67-74. (32) Stradella, L. New J. Chem. 1988, 12, 835-838. (33) Cardona-Martinez, N.; Dumesic, J. A. Adv. Catal. 1992, 38, 149244.

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adsorption studies will detail the enthalpy of adsorption and the level of alkene uptake. Computational methods can then elucidate the nature of the adsorbed species, the energetics of the adsorption process, and the adsorbate characteristics that affect the energetics. Computational Section Adsorption energies of ethene, propene, 1-butene, trans-2butene, and isobutene on phosphotungstic acid were calculated using quantum-chemical methods. Gradient-corrected, DFT calculations were carried out using the Vienna ab initio Simulation Package (VASP),34-36 as well as the Amsterdam Density Functional (ADF)37 program. VASP was used for geometry optimizations of the reactant (physisorbed), transition, and product (chemisorbed) states. The chemisorbed states determined from ADF were used simply for comparison. All calculations were performed on the Keggin unit with three charge-balancing protons, located at their optimal positions.5 Molecular systems were deemed sufficient to represent the alkene-HPA interface, as alkenes were expected to react only with surface acid sites. The adsorption energy, ∆Eads, of the alkene is calculated as (eq 1)

∆Eads ) Eadsorbate-H2PW - Eadsorbate - EH3PW

(1)

where EH3PW is the energy of H3PW12O40 (H3PW), Eadsorbate is the energy of the gas-phase alkene molecule, and Eadsorbate-H2PW is the energy of the adsorbed complex. The physisorption energy, ∆Eads(phys), is defined as the energy difference between the π complex of the alkene and the gas-phase alkene (Eadsorbate) along with the gas-phase HPA structure (EH3PW). The chemisorption energy, ∆Eads(chem), is defined as the energy difference between the alkoxide and both the gas-phase alkene and the gas-phase HPA structure. Activation energies are reported with respect to the physisorbed state. VASP. Electron-ion interactions are described by using ultrasoft pseudopotentials.38 The exchange and correlation energies (XC) were calculated using the Perdew-Wang (PW91) functional form of the generalized gradient approximation (GGA).39 A conjugate-gradient algorithm was chosen to relax the ions to their optimum geometry. The structural optimization was conducted until the forces on all ions were less than 0.05 eV Å-1. A 20 × 20 × 20 Å3 supercell was used to represent the molecular system within the periodic code. A 1 × 1 × 1 Monkhorst-Pack mesh40 was chosen to sample the first Brillouin zone. A cutoff energy of 396.0 eV was selected for the plane-wave basis set. Transition states were located using the nudged elastic band (NEB) transition-state search method.41 The absolute value of the tangential force for the transition state was optimized to less than 0.08 eV Å-1. Harmonic frequencies were calculated for trans2-butene and ethene adsorption transition states, and the species were confirmed to have a single, imaginary frequency along the reaction coordinate. The Hessian was calculated only for the atoms of the adsorbate molecule and the nearby atoms of phosphotungstic acid within the binding area. Zero-point vibrational energy (ZPVE) corrections were generally not calculated due to the extensive computational requirements of creating the Hessian for these systems. ZPVE corrections shifted the calculated physisorption and chemisorption energies of ethene by 8.5 and 0.4 kJ mol-1, respectively, to more exothermic values. These values are approximate but indicate that the magnitude of the ZPVE corrections is small, especially for (34) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558-561. (35) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169. (36) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (37) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931-967. (38) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892-7895. (39) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 66717895. (40) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188-5192. (41) Mills, G.; Jo´nsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305337.

chemisorption. Hence, their absence from our calculations should not significantly affect comparisons made with experimental microcalorimetry values. ADF. In the ADF molecular calculations, triple-ζ basis sets were used for all atoms.37,42 The core electrons were frozen up to (and including) the 2p, 4d, 1s, and 1s orbital for P, W, O, and C atoms, respectively. Scalar relativistic corrections using the Pauli formalism were incorporated into the frozen core potentials.43 Geometry optimizations were converged to energies of less than 0.027 eV and to energy gradients less than 0.14 eV Å-1 with respect to the nuclear displacement of each atom. The total bonding energy was corrected using the Vosko-Wilk-Nusair (VWN)44 local density approximation (LDA) and either the Becke-Perdew45,46 or PW91 GGA functionals for the XC energies.

Experimental Section Materials. Phosphotungstic acid hydrate was obtained from Aldrich (12501-23-4). The solid acid was first recrystallized by dissolution in distilled, deionized water. The solution was subsequently dried under moderate heating. The resulting powder was ground to reduce particle size and minimize mass transport limitations of the adsorbate. Hereafter, the solid acid will be termed H3PWrec. Previous analysis has demonstrated this HPA formulation possesses a low surface area of ∼6 m2 g-1.47 The alkene gases were supplied by Aldrich in the following purities: ethene 99.5% (74-85-1), propene 99% (115-07-1), and isobutene 99% (115-11-7). Microcalorimetry. Adsorption studies were performed using a Calvet-type, differential heat-flow microcalorimeter, detailed elsewhere.48 Approximately 1 g of H3PWrec was pretreated in situ, under vacuum, at 573 K for 2 h. The system was cooled to room temperature prior to adsorption. Ethene, propene, and isobutene were studied as probe gases. Other C4 alkenes, such as 1-butene or trans-2-butene, were not studied since these reactants would likely undergo double-bond isomerization in addition to adsorption. Starting with an alkene pressure of ∼1.5 Torr, the gas was delivered into the system in individual doses. After investigating multiple pressures, 1.5 Torr was selected to minimize oligomerization. All thermograms were terminated below δQ ≈ 40 kJ mol-1. By convention, the δQ is reported as a positive quantity, although it equates to an exothermic enthalpy. The alkene is expected to interact only with surface protons (HS+). The amount of surface HPA units was estimated using reported values of the surface area47 and lattice parameter10 of the bcc secondary structure. Although there are three protons per H3PWrec unit, not all of the protons on surface HPA units are expected to be accessible. A factor of 1.5 surface protons per surface H3PWrec unit was used to maintain charge neutrality of the surface plane. The proposed ratio of 8.4 µmol HS+ g-1 [0.024 mol (mol KU)-1] coincides with the range of 8-16 µmol HS+ g-1 presented by Okuhara et al.49 In contrast, the estimated ratio of bulk protons (HB+) is 1042 µmol HB+ g-1 [3.001 mol (mol KU) -1].

Results GGA Functional. A series of calculations was carried out to compare the results from the different computational methods and functionals considered. Table 1 presents the chemisorption binding energy of 1-butene over various oxygen binding sites, using several GGA functionals with periodic (i.e., VASP)- and atomic-orbital (i.e., ADF)-based models. The thermodynamically favored chemisorption site is the terminal oxygen atom (Od), as indicated by the (42) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391-403. (43) Ziegler, T.; Tschinke, V. J. Phys. Chem. 1989, 93, 3050-3056. (44) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 12001211. (45) Becke, A. D. Phys. Rev. A 1988, 38, 3098-3100. (46) Perdew, J. P. Phys. Rev. B 1986, 33, 8822-8824. (47) Bardin, B. B.; Davis, R. J. Top. Catal. 1998, 6, 77-86. (48) Bordawekar, S. V.; Doskocil, E. J.; Davis, R. J. Langmuir 1998, 14, 1734-1738. (49) Okuhara, T.; Nishimura, T.; Watanabe, H.; Misono, M. J. Mol. Catal. 1992, 74, 247-256.

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Table 1. DFT-Calculated Chemisorption Energiesa and Alkoxide Bond Lengthsb for 1-Butene on H3PW12O40 VASP, PW91 site Od Oc Ob a

C*

subst.c 1° 2° 1° 2° 1° 2°

∆Eads(chem) -88.1 -102.6 -75.3 -79.7 -36.1 -36.2

ADF, Becke-Perdew C*-O

∆Eads(chem)

1.454 1.492 1.501 1.549 1.519 1.567

-89.4 -99.5 -73.2 -85.6 -

ADF, PW91

C*-O

∆Eads(chem)

C*-O

1.451 1.482 1.497 1.541 -

-82.1 -111.3 -83.4 -99.7 -

1.449 1.480 1.495 1.540 -

Energies are in kJ mol-1. b Bond lengths are in Å. c Subst. represents the degree of substitution of the C* atom.

Figure 2. Formation of ethyl group on H3PW12O40. (a) physisorbed, π-bound state, (b) transition state, and (c) chemisorbed, alkoxide state.

most exothermic ∆Eads(chem) in Table 1. The adsorption energies calculated in VASP and ADF show good agreement between the atomic-orbital-based and plane-wavebased codes. The energy differences primarily fall in the range of 1-10 kJ mol-1, which is within the error of the computational methods.50 Subsequent calculations were performed using VASP. Reaction Path. Analogous to the computational work on zeolites, alkene adsorption on phosphotungstic acid was modeled by determining the initial physisorbed states, the transition states, and the final chemisorbed states. While multiple reaction paths can be envisioned, several assumptions were made to rationalize the most-likely adsorption process. The physisorption of the alkene would presumably begin with the proton located at the energetically favored Oc site.5 A neighboring Od atom was explored as the chemisorption site, in accordance with the relative adsorption energies in Table 1. This reaction path is illustrated in Figure 2 for the example of ethene adsorption on H3PW. The entire process occurs nearly in the same plane, defined by the Oc-W-Od positions. Distorting the dihedral angle of Cp-C*-Od-W from its equilibrium position changed the binding energy by only a few kiloJoules per mole. Physisorbed States. Identifying the physisorbed state is important to accurately report the activation barrier for alkene adsorption. The C*-Cp bond of the alkene is found to be perpendicular to the Oc-Ha bond of H3PW, as seen in Figure 2a. For all of the alkenes, the calculated physisorption energies listed in Table 2 are within the narrow range of -42 to -49 kJ mol-1. The double bond between C* and Cp is maintained, and the acidic proton is still strongly associated with the heteropolyacid. The C* atom is orientated toward the same Od adsorption site for various adsorbate configurations. For this reason, slight differences in physisorption energies and geometries are seen for the same alkene (Table 2). The physisorption energy is not indicative of the most-favorable binding configuration for the alkoxides. Chemisorbed States. The DFT results can be used to rationalize which configuration of the final alkoxide structures is formed on phosphotungstic acid by comparing the chemisorption energies of the respective alkenes. Table 3 gives the chemisorption energies of the alkenes along (50) Ziegler, T. Chem. Rev. 1991, 91, 651-667.

with the bond lengths of the surface alkoxy species that form on the H3PW unit, as illustrated in Figure 2c. The configuration with a higher degree of substitution at C* is thermodynamically favored. Transition States. The DFT-calculated activation barriers are listed in Table 4 for the transition state involved in the conversion of the physisorbed to the chemisorbed state. At the transition state, there is a definite transfer of the proton from bonding strongly to the Oc atom to the Cp atom. The C*-Od bond has not yet formed, although the C*-Cp bond has elongated. Eact does decrease in the order of C* substitution, as 1° > 2° > 3°, following the expected trends of carbenium ion stability in the gas phase. The reaction coordinate for alkoxide formation from ethene is shown in Figure 3. In the transition state, the bond angles of the alkyl species are similar to a carbenium ion. However, the ethene case illustrates the trouble in regarding the transition state entirely as a carbenium ion, interacting solely through Coulombic attraction with the H2PW12O40- anion. The protonation of the alkene is not fully independent of the alkoxide formation since the transition state species is stabilized by the interaction of the C* atom with the Od atom. The imaginary, vibrational mode shows no movement of the Cp atom, revealing that ethene first “slides” in the direction of the double bond to bring the C* atom closer to the Od atom before protonation. The lateral translation of an “uneven” physisorbed state was located along the flat region of the reaction coordinate. This state is 13.1 kJ mol-1 higher in energy than the symmetric physisorbed state. The two-site, Oc and Od, structure helps to facilitate a concerted path, as seen by the distances given in Table 4, especially in the case of ethene. Thermochemical Cycle of Alkene Adsorption. The DFT-calculated alkene chemisorption energies on phosphotungstic acid can be used to construct a thermochemical cycle describing the hypothetical steps that govern adsorption, as illustrated in Scheme 2.51 The calculated energies for each step can be used to rank the interaction energy (∆Eint) and proton affinity of H3PW with those reported in the literature for zeolites. The three steps of this cycle provide a formalism to separate the contributions of the solid acid, adsorbate, and acid-adsorbate complex to the overall adsorption energy. Previous sections have already detailed that the transition state is not a classical carbenium ion. An example of this cycle is discussed next for propene adsorption. The proton affinity of a molecule is the negative of the exothermic reaction energy to add a proton to the molecule. The proton affinity of the H2PW12O40- anion was previously reported as 1079 kJ mol-1.5 The gas-phase proton affinity for propene is calculated as 783.7 kJ mol-1 to form the s-propyl carbenium ion. The proton affinity values and the chemisorption energy of -90.3 kJ mol-1 (51) Gorte, R. J.; White, D. Top. Catal. 1997, 4, 57-69.

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Table 2. DFT-Calculated Physisorption Energiesaand Key Interatomic Distancesbfor Alkene π Complexes on H3PW12O40 alkene ethene propene 1-butene trans-2-butene isobutene a

C* subst.c

∆Eads(phys)

Oc-Ha

Cp-Ha

C*-Ha

C*-Cp

C*-Od

1° 1° 2° 1° 2° 2° 1° 3°

-41.5 -43.0 -47.1 -47.9 -44.5 -43.7 -48.5 -45.8

1.013 1.019 1.018 1.021 1.017 1.019 1.025 1.027

2.018 2.097 2.001 2.058 1.986 1.990 2.184 1.899

2.028 1.970 2.063 1.995 2.126 2.127 1.899 2.133

1.338 1.343 1.343 1.343 1.343 1.347 1.350 1.350

3.232 3.231 3.353 3.246 3.213 3.331 3.247 3.370

Energies are in kJ mol-1. b Distances are in Å. c Subst. represents the degree of substitution of the C* atom.

Table 3. DFT-Calculated Alkene Chemisorption Energiesa and Key Interatomic Distancesb for the Surface Alkoxides Products Formed on H3PW12O40 alkene reactant ethene propene 1-butene trans-2-butene isobutene a

alkoxide

C* subst.c

∆Eads(chem)

Cp-Ha

C*-Cp

C*-Od

ethyl n-propyl s-propyl n-butyl s-butyl s-butyl i-butyl t-butyl

1° 1° 2° 1° 2° 2° 1° 3°

-86.8 -76.6 -90.3 -88.1 -102.6 -79.9 -63.9 -91.4

1.093 1.097 1.094 1.098 1.097 1.097 1.100 1.093

1.508 1.516 1.512 1.517 1.522 1.522 1.526 1.518

1.455 1.453 1.476 1.454 1.492 1.492 1.451 1.499

Energies are in kJ mol-1. b Distances are in Å. c Subst. represents the degree of substitution of the C* atom.

Table 4. Summary of Activation Barriersa and Key Interatomic Distancesb for Transition States Isolated by DFT Calculations alkene ethene propene 1-butene trans-2-butene isobutene

C* subst.c

Eact

1° 1° 2° 1° 2° 2° 1° 3°

69.1 81.5 50.5 77.0 36.3 49.4 101.4 23.9

Oc-Ha Cp-Ha C*-Cp C*-Od 1.410 1.431 1.654 1.689 1.658 1.542 1.569 1.651

1.242 1.248 1.164 1.119 1.168 1.212 1.227 1.167

1.395 1.397 1.420 1.462 1.415 1.404 1.403 1.427

2.177 2.166 2.365 1.867 2.464 2.503 2.248 2.634

a Energies are in kJ mol-1. b Distances are in Å. c Subst. represents the degree of substitution of the C* atom.

are used to calculate a ∆Eint of -385.6 kJ mol-1 between the s-propyl carbenium ion and the H2PW12O40- anion. Microcalorimetry. The experimental results elucidating H3PW-alkene interactions are introduced next for comparisons with computational studies. Thermograms of ethene, propene, and isobutene adsorption on H3PWrec are plotted in Figure 4 as differential heat released versus alkene uptake. The initial differential heat, δQ0, was 46, 103, and 104 kJ mol-1 for ethene, propene, and isobutene, respectively. H3PWrec showed a minimal adsorption for ethene of 1.9 µmol g-1 [0.0055 mol (mol KU)-1]. The total uptake of propene was 9.4 µmol g-1 [0.027 mol (mol KU)-1]. For isobutene, the adsorption was 10.5 µmol g-1 [0.0302 mol (mol KU)-1]. Discussion Comparison of Alkene π Complexes on H3PW to Zeolites. Physisorption is often considered a precursor to chemisorption. There was little distinction between the formation energies of the various π complexes, as only a 7 kJ mol-1 difference is seen in Table 2 for all of the alkenes. Rozanska et al. reported a physisorption energy of -21 kJ mol-1 for propene on chabazite.16 Benco et al. found the ∆Eads(phys) on gmelinite was -34, -33, and -38 kJ mol-1 for ethene, propene, and 1-butene, respectively.52 The differences in physisorption energies on gmelinite are minor, as is the case for H3PW. (52) Benco, L.; Hafner, J.; Hutschka, F.; Toulhoat, H. J. Phys. Chem. B 2003, 107, 9756-9762.

Comparison of Alkoxide Complexes on H3PW to Zeolites. The chemisorption energies of alkenes on H3PW are compared next to reported values for alkene adsorption on zeolites. The formation of the s-propyl alkoxide corresponded to a ∆Eads(chem) of only -48 mol-1 on chabazite.16 On gmelinite, the ∆Eads(chem) for ethene, propene (forming s-propyl), and 1-butene (forming s-butyl) was -70, -62, and -59 kJ mol-1, respectively, showing a decreasing trend with growing alkene chain length due to steric repulsions.52 Our experimental and computational results for alkenes adsorbed onto H3PW do not follow this same trend, probably because anhydrous H3PW is nonporous and the steric effects seen in zeolite micropores do not apply. The chemisorption energies from Table 3 thus demonstrate a stronger adsorption of alkenes on H3PW than on the zeolite cases discussed. The computational values of ∆Eads(chem) for propene and isobutene are slightly lower than the experimental adsorption enthalpies. One of the major shortcomings of DFT is its inability to accurately predict the weak van der Waals interactions, which are the result of many-body dispersion effects. The interactions between fully saturated CH2 or CH3 fragments and the oxygen atoms of a zeolite are approximately 5 kJ mol-1 for each direct interaction. The number of direct contacts between the neighboring CH2 and CH3 groups for phosphotungstic acid are at most two since the Keggin structure is convex. The error here would be on the order of 5-10 kJ mol-1. The primary interaction is between the alkene and the proton, which is significantly stronger. The trends that rank activation barriers and chemisorption energies should be preserved. Previous researchers have supported the preservation of energy trends in DFT studies.16,53 Hence, the configuration of alkene adsorption can still be inferred by comparing the experimental and computational values despite the anticipated underestimation. As illustrated by Scheme 2, the alkene adsorption strength can be related to both the proton affinity of the heteropolyanion and the interaction energy between the catalyst and adsorbate ions. For H-ZSM-5 and faujasite zeolites, the proton affinity has been reported to range (53) Demuth, T.; Rozanska, X.; Benco, L.; Hafner, J.; van Santen, R. A.; Toulhoat, H. J. Catal. 2003, 214, 68-77.

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Figure 3. The reaction coordinate and the energies involved in the chemisorption of ethene. Images are enlarged around the adsorption site. The C*-Cp bond lengths shown are in Angstroms. Scheme 2. Thermochemical Cycle for the Chemisorption of Propene on Phosphotungstic Acid to Form an s-Propyl Alkoxide

from around 1200 to 1400 kJ mol-1.54 Bhan et al. calculated a ∆Eads(chem) of -77.4 kJ mol-1 for s-propyl on H-ZSM-5.14 In the case of propene adsorption, the corresponding interaction energy for the secondary carbenium ion with the zeolite anion could be estimated at between -494 and -694 kJ mol-1. As previously stated, the ∆Eint on H3PW is -385.6 kJ mol-1 for propene. The DFT-predicted proton affinity for propene of 783.7 kJ mol-1 is higher than the typical experimental value of 752 ( 3 kJ mol-1 reported in the literature.55 Despite the variance, the trends in interaction energy are preserved. Thus, the greater ∆Eads(chem) of light alkenes on H3PW can be related to the lower proton affinity and ∆Eint in comparison to zeolites. The computational and experimental results suggest ethene adsorbs on H3PW in a different mode than propene and isobutene. The microcalorimetric enthalpy for ethene of -46 kJ mol-1 is closer to the DFT physisorption energy of -41.5 kJ mol-1 (Table 2) than to the chemisorption energy of -86.8 kJ mol-1 (Table 3). The lower experimental uptake of ethene compared to propene and isobutene (Figure 4) is a result of the larger activation barrier to form a primary carbenium-like ion. The measured uptakes support the claim that nonpolar adsorbates cannot access bulk acid sites.11 (54) Kramer, G. J.; van Santen, R. A. J. Am. Chem. Soc. 1993, 115, 2887-2897. (55) Hunter, E.; Lias, S. J. Phys. Chem. Ref. Data 1998, 27, 413-6??

Significant contributions of the chemisorption energy may be separated into the energy required to break the Oc-Ha and C*-Cp bonds and to form the Cp-Ha and C*Od bonds. The Keggin unit structural bonds in H3PW also adjust upon reaction, but the energy effects are likely comparable for the same adsorption site (Od). The energy to break the Oc-Ha bond can be assumed to have a similar contribution if the proton originates from the same Oc atom. As only one CdC bond dissociation energy is generally reported in the literature, regardless of the alkene chain length, it was anticipated that the bonds formed were the principal factors affecting the binding energy. The differences in ∆Eads(chem) are investigated in the following sections with respect to the Cp-Ha and C*Od bond energies. Substitution of Cp Atom. The binding energy and bond distances of the primary (C*) ethyl, n-propyl, n-butyl, and i-butyl alkoxide species are summarized in Table 5 (taken from Table 3). Because all of the surface groups are bound to the same type of oxygen atom (Od), it was expected that the C*-Od bond contribution would be similar. This indicates that the Cp-Ha bond is the primary factor that controls the differences in adsorption energies between the various primary alkoxides. A significant decrease in ∆Eads(chem) was observed in the order 1° > 2° > 3° Cp atom if the n-butyl case is excluded.

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Campbell et al. Table 6. Summary of Chemisorption Binding Energiesa and Key Interatomic Distancesb Illustrating the Effect of C* Substitution alkene reactant alkoxide C* subst.c ∆Eads(chem) C*-Cp C*-Od ethene propene 1-butene isobutene

ethyl s-propyl s-butyl t-butyl

1° 2° 2° 3°

-86.8 -90.3 -102.6 -91.4

1.508 1.512 1.522 1.518

1.455 1.476 1.492 1.499

a Energies are in kJ mol-1. b Distances are in Å. c Subst. represents the degree of substitution of the Cp atom.

Figure 4. Comparison of differential heats of alkene adsorption on H3PWrec (1 g). Probe gases (1.5 Torr): ethene ([), propene (2), and isobutene (9). Pretreatment at 573 K for 2 h, under vacuum. Adsorption temperature ∼ 295 K. Table 5. Summary of Chemisorption Binding Energiesa and Key Interatomic Distancesb Illustrating the Effect of Cp Substitution alkene reactant alkoxide Cp subst.c ∆Eads(chem) C*-Cp C*-Od ethene propene 1-butene isobutene

ethyl n-propyl n-butyl i-butyl

1° 2° 2° 3°

-86.8 -76.6 -88.1 -63.9

1.508 1.516 1.517 1.526

1.455 1.453 1.454 1.451

a Energies are in kJ mol-1. b Distances are in Å. c Subst. represents the degree of substitution of the Cp atom.

The experimental C-H bond dissociation energy for ethane [CH3-CH2sH], propane [(CH3)2CHsH], butane [CH3CH2(CH3)CHsH], and isobutane [(CH3)3CsH] is 423.0 ( 1.7, 412.5 ( 1.7, 410.9 ( 2.1, and 403.8 ( 1.7 kJ mol-1, respectively.56 This series illustrates that the C-H bond dissociation energy varies with the substitution of the carbon atom (analogous to Cp). The neighboring carbon atoms are primary (analogous to C* carbon atoms). One would then expect the ∆Eads(chem) of the alkoxides to follow the same trend: ethyl > n-propyl > n-butyl > i-butyl, as the newly formed Cp-Ha bond is strongest for the ethene reactant. The differences seen in Table 5 are close to the differences in the bond dissociation energies from the literature,56 with the exception of the n-butyl complex. The n-butyl alkoxide replaces a H atom with a larger ethyl ligand. The increased electronegativity of this substitution alters the electron density around the C* atom, making it more susceptible to nucleophilic attack by the Od atom. The ∆Eads(chem) is thus greater, moving the n-butyl ligand out of the trend established by the other species. This difference in chemisorption energies is somewhat surprising giving the very similar C-H bond dissociation energies of the gas-phase propane and butane molecules. The C-H bond dissociation and alkene chemisorption energy trends suggest that the Cp-Ha bond is weaker and the ∆Eads(chem) is more favorable when electronegative groups are added to the Cp atom. The t-butyl adsorbate is preferred over i-butyl due to the increased strength of the Cp-Ha bond, indicating that there is a thermodynamic driving force for formation of the tertiary alkoxide. Still, the ∆Eads(chem) for ethyl, s-propyl, and t-butyl H2PW complexes are not the same. The dependence of the chemisorption energy on the C*-Od bond is discussed in the next section. Substitution of C* Atom. The binding energy of the ethyl, s-propyl, s-butyl, and t-butyl ligands is summarized in Table 6. Although these ligands possess C* atoms of varying degrees of substitution, the Cp atom is primary (56) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255263.

in each case. Any significant changes in the chemisorption energies from Table 6 would suggest a contributing factor from the C*-Od bond formation. The increased substitution at the C* atom corresponds to a slight increase in adsorption energy of ethene, propene, and isobutene. The differences are smaller than that seen for Cp substitution in Table 5, and it is difficult to conclude whether this trend is due to the degree of substitution or uncertainty in the computational method. Comparison of Activation Barriers on H3PW to Zeolites. Despite the thermodynamic arguments, the activation barrier must also be considered in determining the most-likely adsorption path. In the case of isobutene, the lower Eact of the tertiary species compared to the primary species is the kinetic driving force for the adsorbed t-butyl configuration. The previous sections suggest that the activation barrier depends on the substitution of C*, the substitution of Cp, and steric effects. The primary factor is the degree of substitution of the C* atom, with the Eact decreasing in the C* substitution order: 1° > 2° > 3°. In addition, the activation barrier parallels the strength of the Cp-Ha bond being formed, so that the Eact to form an ethyl ligand is lower than that for the n-propyl and i-butyl species. Therefore, the substitution of the Cp atom is a secondary factor that contributes to the activation barrier. Finally, it may be expected that longer-chain alkenes than those used in this study could lead to minor changes in Eact due to steric considerations. The activation barriers for alkene adsorption on H3PW (Table 4) are comparable and almost always lower than the barriers on zeolites. For propene adsorption on H-ZSM514 and chabazite,16 the Eact of the secondary transition state was calculated to be 65 and 56 kJ mol-1, respectively, whereas the Eact of the primary transition state was 122 and 128 kJ mol-1, respectively. The Eact for the tertiary transition state of isobutene was 24-45 kJ mol-1, and the barrier was 90-138 kJ mol-1 for the primary transition state on chabazite, mordenite, and H-ZSM-22 in a periodic DFT study.57 In bare-cluster and embedded-cluster models of chabazite, the Eact was 110-123, 87-95, and 60-83 kJ mol-1 for ethene, propene, and isobutene, respectively (for the most highly substituted C* configuration).58 On H3PW, the π complex of ethene was observed to move toward the Od atom in the NEB approach. This concerted mechanism could explain why the activation barrier is significantly lower than the barrier seen in zeolites for other primary alkoxides. The geometric structure of the Oc and Od area may be more favorable for a concerted adsorption path, allowing for a transition state with less charge separation. Conclusions Density functional theoretical calculations for alkene adsorption on phosphotungstic acid were found to be (57) Rozanska, X.; van Santen, R. A.; Demuth, T.; Hutschka, F.; Hafner, J. J. Phys. Chem. B 2003, 107, 1309-1315. (58) Sinclair, P. E.; de Vries, A.; Sherwood, P.; Catalow, C. R. A.; van Santen, R. A. J. Chem. Soc., Faraday Trans. 1998, 94, 3401-3408.

Investigations of Alkene Adsorption on Phosphotungstic Acid

comparable to microcalorimetric experiments. In addition, our computational results for phosphotungstic acid are consistent with energy trends observed for the interactions between alkenes and zeolites. However, the activation barriers are primarily lower and the chemisorption energies are more exothermic on phosphotungstic acid than on zeolites. The molecular structure of phosphotungstic acid enables dual oxygen sites on the catalyst to

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favorably interact with the adsorbate, possibly accounting for the lower activation barriers than that typically estimated for zeolites. Acknowledgment. We thank Dr. Billy Bardin for his helpful comments, as well as the National Science Foundation for support (CTS-0124333). LA047395A