- Email: [email protected]
n-propylbenzene
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2239
INFLUENCE OF ZEOLITE PORE S T R U C T U R E ON B E N Z E N E P R O P Y L A T I O N TO iso-/n-PROPYLBENZENE Perego, C., Millini, R., Parker, Jr., W.O., Bellussi, G. and Romano, U. EniTecnologie S.p.A., Via Felice Maritano 26, 120097 San Donato Milanese (MI), Italy. E-mail: [email protected] ABSTRACT The isomerization of cumene (isopropylbenzene, i-PB) to n-propylbenzene (n-PB), considered to proceed via a 1,2-diphenylpropane intermediate (1), was studied by theoretical and experimental methods to investigate the importance of shape selectivity effects for different zeolites. The location and energetics of 1, and also of the intermediate 2,2-diphenylpropane (2) involved in transalkylation of i-PB, were determined by molecular mechanics and dynamics calculations to evaluate the influence of the pore architecture on the two reactions. Only zeolites with 1D linear channel systems (e.g. MTT, MTW) were predicted to preferentially stabilize intermediate 1 with respect to 2, while limited differences were observed for 3D interconnected porous systems (e.g. *BEA, MOR, MFI, FAU). Preliminary in-situ 13C NMR studies (without MAS) of benzene and cumene (3"1) adsorbed on H + form zeolites found n-PB production (at 473 K) to increase as: Beta < ZSM-5 (MFI) < USY (FAU) ~ ERB-1 (MWW) < MOR. Keywords: cumene, in-situ 13C NMR, isomerization, molecular modeling, n-propylbenzene, transalkylation
INTRODUCTION The direct alkylation of benzene with propene to form cumene (isopropylbenzene, i-PB) is applied on a large scale in the petrochemical industry. The homogeneous (e.g. A1C13 [1]) or heterogeneous (e.g. solid phosphoric acid, SPA [2]) catalysts used in the old industrial processes have recently been replaced with the more selective, regenerable and environmentally friendly zeolite catalysts such as FAU, MWW, MOR and Beta [3]. This apparently simple process is, however, accompanied by several side (i.e. propene oligomerization) and consecutive (i.e., polyalkylation, isomerization to n-propylbenzene (n-PB)) reactions which may seriously affect the economics of the overall process. For instance, n-PB formation is detrimental since it cannot be separated from i-PB by distillation. Among the by-products, only polyalkylates (mainly diisopropylbenzenes, DIPBs) are considered useful because they can be transalkylated with benzene to yield iPB [4]. While the formation of propene oligomers can be limited by choosing suitable operating conditions (e.g. high benzene/propene molar ratio, optimal reaction temperature), this is not true for DIPBs and n-PB. Their production depends mainly on the pore structure of the zeolite, in particular the dimension and shape of the pores in the vicinity of the active site [5, 6]. In fact, a comparison of catalytic selectivities for different zeolites (i.e., Beta, MOR, MWW, FAU and MTW) in cumene synthesis revealed that DIPBs production ranged from 4.58 (for MTW) to 21.49 wt% (for FAU), while n-PB production ranged from 107 (for MOR) to 406 ppm/cumene (for MTW) [7]. Oligomers were produced in the range 0.10 (for Beta) to 0.95 wt% (for MTW) [7]. Molecular mechanics calculations were made to explain the experimental results in terms of reactant and product shape selectivities resulting from the zeolite structure [7]. The diffusional energy barriers for i-PB, and the three DIPB isomers, were consistent with the selectivities observed for DIPBs and the anomalous behavior of MWW (ERB-1), a medium pore zeolite behaving as a large pore one. It was concluded that the alkylation reactions which occur inside the hemi-supercages located on the [001 ] surface of the platelet-like crystallites of MWW are under steric control and have no diffusion barriers [7]. However, the formation of n-PB was not addressed. It is the focus of the present work.
2240 The monomolecular isomerization mechanism initially proposed for the formation of n-propyl aromatics [8] was recently discharged in favor of a reaction pathway involving intermolecular transalkylation between cumene and benzene [9] (Figure 1). In part A, the transalkylation of cumene occurs through the formation of a 2,2-diphenylpropane intermediate, derived from electrophillic attack of a benzylic carbocation on a benzene ring. Obviously, the disproportionation of two cumene molecules to form benzene and DIPB, as well as the transalkylation of DIPB with benzene to form two cumene molecules, occur by the same mechanism. In part B, the isomerization of i-PB to n-PB occurs via a 1,2-diphenylpropane intermediate originating from the electrophillic attack of a primary carbocation on a benzene ring. The lower stability of the primary carbocation, with respect to the benzylic one, accounts for the low n-PB yields generally obtained with zeolite catalysts. However, Perego et al. found that the observed kinetics for both alkylation and isomerization reactions cannot be accounted for by this mechanism alone. It cannot explain that at low contact time the mole ratio n-PB/i-PB, produced by alkylation, depends on the temperature but is practically independent of propylene conversion [10]. To rationalize this behavior, the authors hypothesized that n-PB is not only formed by subsequent isomerization of i-PB, but also by primary alkylation of benzene [10].
lJ
""
.............
""
I()1
2 ........
0
i
I
+
9 A
13
Figure 1. Bimolecular mechanism proposed for the formation of n-PB. t~ejka et al. provided evidence that the benzene propylation selectivity to cumene/n-PB by zeolite catalysts is controlled by the size and architecture of the microporous structure [11,12]. In particular, they found that n-PB did not form over MOR, MTW zeolites. MFI, MFI/MEL zeolites provided the proper reaction space for n-PB formation, while with Beta and much more with FAU zeolites the n-PB selectivity decreased [11]. Recently, Ivanova et al. examined cumene transformation over different MOR catalysts by in-situ 13C MAS NMR spectroscopy [13]. Unlabeled i-PB and ISC labeled benzene were adsorbed on the activated catalyst in a sealed NMR cell, which was then heated off-line (298 to 473 K) for the desired time. Intermolecular transalkylation and disproportionation were observed under mild conditions (T = 298 to 393 K, weak acidity), while more severe conditions (T > 423 K, strong acidity) were required for isomerization of i-PB to n-PB [13]. Apart from these interesting results, the experimental approach they used is certainly suitable for better understanding the catalytic selectivities of zeolites and for exploring the hypothesis that the pore architecture influences isomerization [11,12]. Thus, a similar approach will be employed here. In this work, the importance of shape selectivity effects in n-PB production is examined theoretically. Molecular mechanics and dynamics calculations were made to differentiate the zeolite structures in terms of transition state shape selectivity. The locations and energetics of the molecules which best approximate the structures of the two carbocation intermediates involved in the reaction mechanisms (1,2- and 2,2diphenylpropane in Figure 1) were evaluated. In addition, a simple experimental approach which is sensitive enough to rank zeolites in terms of their propensities to produce n-PB was sought. In-situ 13C NMR
2241 spectroscopy in its "static" form (without the complications of rapidly spinning a sealed NMR cell) was applied for the first time to quantify n-PB formation over various zeolites. EXPERIMENTAL
Computations Location and energetics of 1,2-diphenylpropane (1) and 2,2-diphenylpropane (2) were determined using a procedure based on the Quench Dynamics (QD) protocol [14]. This procedure has been successfully applied to locate the complex organic molecules used as structure directing agents in zeolite synthesis [15], as well as the 1,l-diarylmethane intermediates formed in the transalkylation of 1,2,4-trimethylbenzene with naphthalene to form methyl-substituted naphthalenes [16]. In the case of bulky and flexible molecules, such as the diphenylpropane intermediates 1 and 2, this procedure is preferred to the Monte Carlo docking proposed by Freeman et al. [ 17], which fails because few of the possible conformations actually fit the pore structure.
1
2
The QD protocol is a combination of constrained high-temperature Molecular Dynamics (MD) and Energy Minimization (EM) techniques. After building the zeolite model (a supercell with P1 symmetry and periodic boundary conditions), the intermediate was manually docked inside the pores in a random orientation with respect to the framework and its energy and geometry were optimized before starting the MD simulation. To assure complete exploration of the conformational space, pass over energy barriers between conformers and allow translations of the molecule within the pores, the MD simulations were run in the canonical NVT ensemble at 3000 K for 500 ps, with 1 fs steps. Every 500 fs, the MD simulation was interrupted and the resulting conformation was energy minimized and archived for successive elaboration. When limited or no movements were observed, the procedure was repeated starting from a different initial position of the intermediate molecule. For each intermediate/zeolite system, the minimum energy conformation was selected and further optimized until the maximum derivative was less than 0.001 kcal.mol-l.A -1. Purely siliceous structures of medium (MFI, MEL, MTT, MWW) and large pore (*BEA, FAU, ISV, MOR, MTW) zeolites were built and kept fixed during the simulations. Electrostatic (Coulombic) interactions were neglected. All simulations were performed with the modules and functionalities contained in the Accelrys Cerius 2 (release 4.2MS) software package [18], employing the COMPASS forcefield [19]. By convention, the minimized energy (E) is related to that of the isolated intermediate and zeolite systems as follows.
E = Eintermediate/zeolite - Eintermediate - Ezeolite Zeolites Zeolite Beta (Si/A1 = 10) and ERB-1 (MWW, Si/AI =15) were synthesized according to the procedure reported in references [20] and [21], respectively. Samples were transformed into the H + form by ion-exchange with ammonium acetate solution, followed by calcination in air at 823 K. Mordenite (MOR) was purchased from Zeolyst in the ammonium form (CBV 20A, Si/A1 = 10) and transformed into the H + form by calcination in air at 823 K. ZSM-5 (MFI) named CBV 3020 (Si/AI = 15) and USY (FAU) named TSZ HUA 330 (Si/A1 = 3) in the H+ form, were used as received from Zeolyst and Tosoh.
13C NMR H + form zeolites (300 mg) were thoroughly dried overnight (at 773 K) under vacuum (10 -3 mbar) in 10 mm o.d. glass tubes prior to adsorption of gaseous reagent molecules with natural abundance 13C (1%). Benzene (0.36 mmol, from Merck) and cumene (0.12 mmol, from Aldrich) were dosed sequentially onto the catalyst (by cold-finger transfer) prior to flame-sealing the tube. 13C NMR spectra were collected at 298 K on a Bruker ASX-300 (operating at 75 MHz) using: static (non-spinning) samples, a solution state probe, 15 ~ts (70 ~ tip) rf pulses, 1H decoupling, ls re-cycle time and ca. 60,000 scans. Chemical shifts were referenced
2242
externally to benzene (129 ppm) dissolved in acetone-d6. The 13C N M R shifts given here for static samples are slightly greater (2 to 4 ppm) than those reported by Ivanova et al. [13] obtained using rapid sample spinning. This is attributed to differences in magnetic susceptibility, which are averaged out by spinning. Samples were allowed to equilibrate for 2 days at 298 K prior to collecting the first spectrum, n-PB formation was detected after each sequential step of the off-line heating protocol ( 3 hours at 423 K, followed by a total of 1, 2 and 4 hours at 473 K). Signal assignments for two spectra of the USY analysis are shown in Figure 2. The amount of n-PB produced, relative to cumene, was measured from the de-convoluted spectral areas (A) of the peaks arising from the methyl carbons of cumene (ca. 26 ppm) and n-PB (ca. 18 ppm). Since the methyl carbons of cumene have equivalent chemical shifts, the signal near 26 ppm represents 2 carbons (per molecule) and thus, the following formula was used to calculate the yield of n-PB in % mole. n-PB yield/100 = [n-PB]/[cumene] = (An-PB)/(An-PB + (A cumene/2) ) The accuracy of this yield calculation is affected somewhat by the uncertainty in the amount of DIPB produced. Due to limited spectral resolution, the methyl carbon peaks of DIPB overlaps with those of cumene. Thus, for comparison with n-PB, the peak at 26 ppm due to DIPB should be divided by 4. Neglecting DIPB formation, and division by 2 is expected to cause a relatively small error due to the minor presence of DIPB (compared to cumene). Only the methyl signals can be used for analysis under the present spectral conditions. Notice in Figure 2, that the relative intensities are not correct for all peaks (non methyl signals of cumene are too small) and the tx carbon signal of n-PB is missing. This is attributed to rapid transverse relaxation for these carbons. 26
42[ J ~ ~
18
6
423 K, 3 h
147 j
36 31
26 36
298 K, 48 h
18
~
36 152
.....'.......'.......~ :1 . . . . . . 150
"'! ......' .....'--~' . ......~.......!............. .... .................r..........' ...... 100 50 ppm
Figure 2. In-situ 13C NMR spectra of USY dosed with benzene-cumene (3:1). The large peak of benzene (ca. 131 ppm) is plotted off scale to facilitate visualization of the cumene and n-PB peaks.
RESULTS AND DISCUSSION
Computer simulations The simulations involved not only the zeolite structures studied by in-situ 13C N M R (i.e. MOR, MWW, FAU, MFI, and BEA, see below) but also other medium and large pore zeolites for which alkylation/isomerization data are available in the literature. The structures were selected to be representative of most varied porous structures, ranging from the 1D linear channel systems (e.g. MTT, MTW) to 3D
2243 interconnected porous systems (e.g. MFI, FAU). The aim of this computational study is, in fact, to verify if the pore architecture influences the formation of n-PB. The QD protocol proved to be an efficient tool for locating the intermediate molecules within the porous structures. A high temperature MD run assured the complete inspection of the conformational space of the molecule and the zeolite potential energy surface. It permitted the most energetically stable occupancy sites to be identified. As an example, the lowest energy sites for 1 and 2 in MFI are shown in Figure 3, where it is seen that both molecules are located at the channels intersection. The valence (Eval) and van der Waals (Evaw) contributions to the total energy (Etot) of the different intermediate/zeolite systems are listed in Table 1, the latter being illustrated in Figure 4 together with the differences between the values (2 minus 1). Eval values are the sum of the bond, valence and torsion angles, out-of-plane and cross-terms energy contributions. They are a measure of the degree of distortion for a molecule docked inside the porous structure, with respect to the minimum energy conformation in-vacuo. On the contrary, the nonbonding terms (Evdw, coulombic interactions were neglected) give an indication of the steric compatibility between a guest molecule and a given host framework. It must be pointed out that since intermediates 1_ and 2 are configurational isomers (i.e. they have the same number of atoms), the energy values can be directly compared. Table 1. Valence (Eval), van der Waals (Evdw), total (Etot) and difference in total energies (AEtot) for the two intermediates in the most energetically stable position within the zeolite (kJ-mol-1). 1
2
Etot
AEtot i
-122.2
-112.5
+53.6
-144.3
-143.7
+6.7
-127.9
-127.3
+21.1
0.3
-133.5
-133.2
+13.5
Etot
Eval
-166.8
-166.1
9.7
-151.3
-150.4
0.6
5.4
-153.8
-148.4
0.6
ISV
0.0
-146.7
-146.7
Eval
EvdW
MTW
0.7
MOR
0.9
MEL
EvdW
MFI
6.0
-146.5
-140.5
2.0
-140.9
-138.9
+1.6
*BEA
0.0
-140.3
-140.3
0.0
-127.9
-127.9
+12.4
M W W ii
8.7
-115.8
-107.1
0.0
-115.5
-115.5
-8.4
MTT
11.1
-117.7
-106.6
38.7
-84.9
-46.2
+60.4
FAU
3.0
- 105.6
- 102.6
0.3
-99.4
-99.1
+3.5
' difference between the total energy of 2 and 1; ii in the hemisupercages on the [001 ] crystal surface On the basis of the results reported in Table 1, it is possible to obtain some interesting indications. In general, the systems intermediate l (n-PB)/zeolite are more stable than those involving the intermediate 2 (i-PB), with energy differences (AEtot) varying from +1.6 (for MFI) to +60.4 (for MTT) kJ.mol 1. MWW is an exception since, it is the only zeolite displaying a higher stabilization of 2 (Figure 4). However, the simulations were performed considering the intermediate molecules to be located inside the hemi-supercages present on the [001 ] surface of the platelet-like crystals, where the alkylation/isomerization reactions likely occur [7]. In any case, the AEtot value observed (-8.4 kJ-mo1-1, Table 1) is too small to conclude a significant influence of the surface pore architecture on the reaction pathway. The other zeolites can be divided into three groups, depending on the AEtot values computed. First, those characterized by a mono-dimensional linear channel system without any cages displayed the largest AEtot values (+53.6 and +60.4 kJ-mo1-1 for MTW and MTT, respectively, Table 1). These differences are due to the lower steric compatibility of intermediate 2 with the two frameworks structures (low Evdw contribution) and also to distortion of the molecular geometry, particularly high in the case of 2 docked inside the medium pore zeolite MTT (Table 1). Therefore, in these structures the isomerization reaction (pathway B, Figure 1) should be preferred over the transalkylation (pathway A, Figure 1).
2244
Figure 3. Lowest energy location of 1 (left) and 2 (right) in MFI.
Figure 4. Total energies, and their difference, for the intermediates (1 for n-PB, 2_for i-PB) within zeolites.
In the second group are zeolites displaying relatively high AEtot values: MEL (+21.1 kJ.mol-1), ISV (+13.5 kJ.mo1-1) and *BEA (+12.4 kJ.mol -l) (Table 1). In these cases, a certain influence of the pore structure on the reaction pathway is indicated. And the path leading to n-PB is slightly preferred. Both intermediate molecules are hosted inside the pores without any significant distortion of their geometries (apart from MEL, with Eval - +5.4 kJ.mol 1, Table 1), but they exhibit different steric compatibilities with the zeolite framework (high Evdw differences). Finally, in the third group belong MOR, MFI and FAU, which display AEtot values in the range +1.6 - +6.7 kJ.mo1-1 (Table 1). These zeolites are characterized by the presence of large channels and or large cages where both intermediate molecules can be easily hosted. Therefore, no influence on the reaction pathway is expected. It is interesting to note the behavior of MFI, particularly when compared with the closely related MEL structure. Both are characterized by the presence of cages at the 10-membered ring channels intersections. But intermediate 2 is predicted to be better hosted by MFI because the 2,2diphenylpropane molecule has a shape which fits well the cage at the intersection between the linear and the sinusoidal channels (Figure 3).
13C NMR The implementation of static (non spinning) in-situ NMR spectroscopy to study catalytic reactions has the distinct advantage of simplicity. Fast rotation of the sealed glass containers is not required. However, the spectral resolution is compromised and only highly mobile physi-sorbed molecules are detected. These conditions do not compromise our goal, in-situ quantification of n-PB production. n-PB yields, obtained for the various zeolites using the off-line heating protocol described, are shown in the histogram of Figure 5. In general, higher amounts of n-PB were produced (%) compared to the liquid-phase alkylation of benzene with propylene in a continuous flow reactor (ppm) [7]. This is attributed to the infinitely long contact times employed for NMR, the higher temperature (473 vs. 323 K) and the high thermodynamic stability of n-PB, which increases with the temperature. In fact, thermodynamic equilibrium was reached in the case of mordenite (61% mole of n-PB at 473 K) after 4 hours. The "ranking" of catalysts in Figure 5, according to n-PB production within a closed system, needs interpretation. The NMR measurement clearly reflects the total catalytic behavior, with the combined effects of pore constraints and acidity. Evidently, pore size is not the determinant factor, since Beta and MOR both have large pores but exhibit widely different propensities to form n-PB. Excluding the importance of pore constraints for a moment, let us consider these catalysts from the perspectives of acidity, and yields in a relative way. Al content: Broensted acid strength [22]: n-PB yield during alkylation [7]"
(ZSM-5 = ERB-1) < (Beta = MOR) < USY USY < MOR < Beta < ERB-1 < ZSM-5 MOR < USY < Beta < ERB-1
2245
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.s.,/:H
E 9
40
a-m
20
.....: S .............
Beta
f
ZSM-S
USY
ERBd
IMOR
4z3 K:,:~ h O ~:73 K 2 :h
U: 4'73 K 1 ih ~ 473 K 4 h Figure 5. Product yields for n-PB, produced over H+ form zeolites by off-line heating, determined by in-situ 13C NMR spectroscopy. Neither the A1 content which is related to the overall acidity, nor the Broensted acid strength are able to explain NMR experiments. Also, the "ranking" in Figure 5 is different from that reported for the liquid-phase alkylation of benzene with9 propylene [7]. During alkylation experiments, n-PB comes from both direct alkylation and cumene isomerization [ 10]. This could be the cause for such different trends. In addition, it is known that alkylation and transalkylation reactions require acid sites with different strengths. Besides the acidic strength, also the acid site distribution should have a relevant role. A more detailed study of these considerations is currently in progress. CONCLUSIONS The isomerization of i-PB to n-PB was examined theoretically, considering that the reaction proceeds via a 1,2-diphenylpropane intermediate (_1) and is in competition with the transalkylation of i-PB with benzene to produce i-PB via a 2,2-diphenylpropane intermediate (2). In zeolite catalysts, the pore size and architecture may influence the reaction pathway in the sense that only reactions involving a sterically compatible intermediate (or transition state) are favored. This effect, known as transition state shape selectivity, can be invoked in the reactions under investigation here because they involve the bulky intermediates I and 2. Modeling tools proved to be an efficient means for differentiating zeolite structures in terms of transition state shape selectivity. In particular, microporous structures with different pore sizes and architectures, ranging from 1D linear channel systems (MTT and MTW) to 3D interconnected channel systems (e.g. MFI, FAU, *BEA), were chosen. The locations and energetics of 1 and 2, computed by molecular mechanics and dynamics calculations, lead to following conclusions: 9 1) All the structures considered can host 1 and 2. But systems intermediate 1/zeolite were more stable than those with intermediate 2. Energy differences (AEtot) varied from +1.6 (for MFI) to +60.4 kJ'mol 1 (for MTT). 9 2) Largest AEtot values were observed for the structures MTT (AEtot - +60.4) and MTW (AEtot = +53.6 kJ-mo1-1) with linear channels, predicting a clear transition state shape selectivity effect in favor of reaction pathway B to form n-PB. 9 3) The other zeolites can be divided into two groups. MEL, ISV and *BEA displayed relatively high AEtot values (+12.4 to +21.1 kJ'mo1-1) which indicate a slight preference for pathway B. MOR, MFI and FAU, with large pores and/or large cages gave AEtot values from +1.6 to +6.7 kJ-mol 1 predicting no shape selectivity effects. Preliminary in-situ 13C NMR experiments under non-flowing conditions found the ranking for n-PB production in H-form zeolites to be" Beta < ZSM-5 (MFI) < USY (FAU) ~ ERB-1 (MWW) < MOR. The
2246 ranking obtained with this simple approach indicates that factors other than just pore constraints are dominant. These findings will be developed further in future work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Canfield R.C., Unruh T.L., Chemical Engineering, March 21 (1983) 32. Pujado P.R., Salazar J.R. and Berger C.V., Hydroc. Proc., 55 (1976) 91 Perego, C., and Ingallina, P., Catal. Tod. 73 (2002) 3. Pradhan, A. R., and Rao, B. S., Appl. Catal. A 106 (1993) 143. Wichterlovfi, B., and (~ejka, J., J. Catal. 146 (1994) 523. (~ejka, J., Kapustin, G. A., and Wichterlov~i, B., Appl. Catal. 108 (1994) 187. Perego, C., Amarilli, S., Millini, R., Bellussi, G., Girotti, G., and Terzoni, G., Microporous Mater. 6 (1996) 395. Best D., Wojciechowski B.W., J. Catal. (1977) 11. Ivanova, I. I., Brunel, D., Nagy, J.B., and Derouane, E.G., J. Mol. Catal. A: Chemical 95 (1995) 243. Perego, C., Amarilli, S., Bellussi, G., Cappellazzo, O., and Girotti, G., Proc. 12th International Zeolite Conference, vol. 1 (1998) 575. (~ejka, J., Sponer, J. E., Zilkovfi, N., and Wichterlovfi, B., Proc. 12th International Zeolite Conference, vol. 2 (1998) 1419. (~ejka, J., and Wichterlovfi, B., Catal. Rev. 44(3) (2002) 375. Ivanova, I. I., Montouillout, V., Fernandez, C., Marie, O., and Gilson, J.-P., Microporous Mesoporous Mater. 57 (2003) 297. Millini, R., Carluccio, L. C., Carati, A., and Parker, Jr., W. O., Microporous Mesoporous Mater. 46 (2001) 191. Millini, R., Stud. Surf. Sci. Catal. 135 (2001) 264. Millini, R., Frigerio, F., Bellussi, G., Pazzuconi, G., Perego, C., Pollesel, P., and Romano, U., J. Catal. 217 (2003) 298. Freeman, C. M., Catlow, C. R. A., Thomas, J. M., and Brode, S., Chem. Phys. Lett. 186 (1991) 137. Cerius 2 - Release 4.2MS - Accelrys, San Diego (CA), 2000. Sun, H., J. Phys. Chem., 102 (1998) 7338. Kiricsi, I., Flego, C., Pazzuconi, G., Parker Jr., W. O., Millini, R., Perego, C., and Bellussi, G., J. Phys. Chem. 98 (1994) 4627. Bellussi, G., Perego, G., Clerici, M. G., and Giusti, A., Eur. Patent No. 293,032 (1988), assigned to Eniricerche S.p.A. Flego, C., Perego, C., Appl. Catal., 192 (2000) 317.
2239
INFLUENCE OF ZEOLITE PORE S T R U C T U R E ON B E N Z E N E P R O P Y L A T I O N TO iso-/n-PROPYLBENZENE Perego, C., Millini, R., Parker, Jr., W.O., Bellussi, G. and Romano, U. EniTecnologie S.p.A., Via Felice Maritano 26, 120097 San Donato Milanese (MI), Italy. E-mail: [email protected] ABSTRACT The isomerization of cumene (isopropylbenzene, i-PB) to n-propylbenzene (n-PB), considered to proceed via a 1,2-diphenylpropane intermediate (1), was studied by theoretical and experimental methods to investigate the importance of shape selectivity effects for different zeolites. The location and energetics of 1, and also of the intermediate 2,2-diphenylpropane (2) involved in transalkylation of i-PB, were determined by molecular mechanics and dynamics calculations to evaluate the influence of the pore architecture on the two reactions. Only zeolites with 1D linear channel systems (e.g. MTT, MTW) were predicted to preferentially stabilize intermediate 1 with respect to 2, while limited differences were observed for 3D interconnected porous systems (e.g. *BEA, MOR, MFI, FAU). Preliminary in-situ 13C NMR studies (without MAS) of benzene and cumene (3"1) adsorbed on H + form zeolites found n-PB production (at 473 K) to increase as: Beta < ZSM-5 (MFI) < USY (FAU) ~ ERB-1 (MWW) < MOR. Keywords: cumene, in-situ 13C NMR, isomerization, molecular modeling, n-propylbenzene, transalkylation
INTRODUCTION The direct alkylation of benzene with propene to form cumene (isopropylbenzene, i-PB) is applied on a large scale in the petrochemical industry. The homogeneous (e.g. A1C13 [1]) or heterogeneous (e.g. solid phosphoric acid, SPA [2]) catalysts used in the old industrial processes have recently been replaced with the more selective, regenerable and environmentally friendly zeolite catalysts such as FAU, MWW, MOR and Beta [3]. This apparently simple process is, however, accompanied by several side (i.e. propene oligomerization) and consecutive (i.e., polyalkylation, isomerization to n-propylbenzene (n-PB)) reactions which may seriously affect the economics of the overall process. For instance, n-PB formation is detrimental since it cannot be separated from i-PB by distillation. Among the by-products, only polyalkylates (mainly diisopropylbenzenes, DIPBs) are considered useful because they can be transalkylated with benzene to yield iPB [4]. While the formation of propene oligomers can be limited by choosing suitable operating conditions (e.g. high benzene/propene molar ratio, optimal reaction temperature), this is not true for DIPBs and n-PB. Their production depends mainly on the pore structure of the zeolite, in particular the dimension and shape of the pores in the vicinity of the active site [5, 6]. In fact, a comparison of catalytic selectivities for different zeolites (i.e., Beta, MOR, MWW, FAU and MTW) in cumene synthesis revealed that DIPBs production ranged from 4.58 (for MTW) to 21.49 wt% (for FAU), while n-PB production ranged from 107 (for MOR) to 406 ppm/cumene (for MTW) [7]. Oligomers were produced in the range 0.10 (for Beta) to 0.95 wt% (for MTW) [7]. Molecular mechanics calculations were made to explain the experimental results in terms of reactant and product shape selectivities resulting from the zeolite structure [7]. The diffusional energy barriers for i-PB, and the three DIPB isomers, were consistent with the selectivities observed for DIPBs and the anomalous behavior of MWW (ERB-1), a medium pore zeolite behaving as a large pore one. It was concluded that the alkylation reactions which occur inside the hemi-supercages located on the [001 ] surface of the platelet-like crystallites of MWW are under steric control and have no diffusion barriers [7]. However, the formation of n-PB was not addressed. It is the focus of the present work.
2240 The monomolecular isomerization mechanism initially proposed for the formation of n-propyl aromatics [8] was recently discharged in favor of a reaction pathway involving intermolecular transalkylation between cumene and benzene [9] (Figure 1). In part A, the transalkylation of cumene occurs through the formation of a 2,2-diphenylpropane intermediate, derived from electrophillic attack of a benzylic carbocation on a benzene ring. Obviously, the disproportionation of two cumene molecules to form benzene and DIPB, as well as the transalkylation of DIPB with benzene to form two cumene molecules, occur by the same mechanism. In part B, the isomerization of i-PB to n-PB occurs via a 1,2-diphenylpropane intermediate originating from the electrophillic attack of a primary carbocation on a benzene ring. The lower stability of the primary carbocation, with respect to the benzylic one, accounts for the low n-PB yields generally obtained with zeolite catalysts. However, Perego et al. found that the observed kinetics for both alkylation and isomerization reactions cannot be accounted for by this mechanism alone. It cannot explain that at low contact time the mole ratio n-PB/i-PB, produced by alkylation, depends on the temperature but is practically independent of propylene conversion [10]. To rationalize this behavior, the authors hypothesized that n-PB is not only formed by subsequent isomerization of i-PB, but also by primary alkylation of benzene [10].
lJ
""
.............
""
I()1
2 ........
0
i
I
+
9 A
13
Figure 1. Bimolecular mechanism proposed for the formation of n-PB. t~ejka et al. provided evidence that the benzene propylation selectivity to cumene/n-PB by zeolite catalysts is controlled by the size and architecture of the microporous structure [11,12]. In particular, they found that n-PB did not form over MOR, MTW zeolites. MFI, MFI/MEL zeolites provided the proper reaction space for n-PB formation, while with Beta and much more with FAU zeolites the n-PB selectivity decreased [11]. Recently, Ivanova et al. examined cumene transformation over different MOR catalysts by in-situ 13C MAS NMR spectroscopy [13]. Unlabeled i-PB and ISC labeled benzene were adsorbed on the activated catalyst in a sealed NMR cell, which was then heated off-line (298 to 473 K) for the desired time. Intermolecular transalkylation and disproportionation were observed under mild conditions (T = 298 to 393 K, weak acidity), while more severe conditions (T > 423 K, strong acidity) were required for isomerization of i-PB to n-PB [13]. Apart from these interesting results, the experimental approach they used is certainly suitable for better understanding the catalytic selectivities of zeolites and for exploring the hypothesis that the pore architecture influences isomerization [11,12]. Thus, a similar approach will be employed here. In this work, the importance of shape selectivity effects in n-PB production is examined theoretically. Molecular mechanics and dynamics calculations were made to differentiate the zeolite structures in terms of transition state shape selectivity. The locations and energetics of the molecules which best approximate the structures of the two carbocation intermediates involved in the reaction mechanisms (1,2- and 2,2diphenylpropane in Figure 1) were evaluated. In addition, a simple experimental approach which is sensitive enough to rank zeolites in terms of their propensities to produce n-PB was sought. In-situ 13C NMR
2241 spectroscopy in its "static" form (without the complications of rapidly spinning a sealed NMR cell) was applied for the first time to quantify n-PB formation over various zeolites. EXPERIMENTAL
Computations Location and energetics of 1,2-diphenylpropane (1) and 2,2-diphenylpropane (2) were determined using a procedure based on the Quench Dynamics (QD) protocol [14]. This procedure has been successfully applied to locate the complex organic molecules used as structure directing agents in zeolite synthesis [15], as well as the 1,l-diarylmethane intermediates formed in the transalkylation of 1,2,4-trimethylbenzene with naphthalene to form methyl-substituted naphthalenes [16]. In the case of bulky and flexible molecules, such as the diphenylpropane intermediates 1 and 2, this procedure is preferred to the Monte Carlo docking proposed by Freeman et al. [ 17], which fails because few of the possible conformations actually fit the pore structure.
1
2
The QD protocol is a combination of constrained high-temperature Molecular Dynamics (MD) and Energy Minimization (EM) techniques. After building the zeolite model (a supercell with P1 symmetry and periodic boundary conditions), the intermediate was manually docked inside the pores in a random orientation with respect to the framework and its energy and geometry were optimized before starting the MD simulation. To assure complete exploration of the conformational space, pass over energy barriers between conformers and allow translations of the molecule within the pores, the MD simulations were run in the canonical NVT ensemble at 3000 K for 500 ps, with 1 fs steps. Every 500 fs, the MD simulation was interrupted and the resulting conformation was energy minimized and archived for successive elaboration. When limited or no movements were observed, the procedure was repeated starting from a different initial position of the intermediate molecule. For each intermediate/zeolite system, the minimum energy conformation was selected and further optimized until the maximum derivative was less than 0.001 kcal.mol-l.A -1. Purely siliceous structures of medium (MFI, MEL, MTT, MWW) and large pore (*BEA, FAU, ISV, MOR, MTW) zeolites were built and kept fixed during the simulations. Electrostatic (Coulombic) interactions were neglected. All simulations were performed with the modules and functionalities contained in the Accelrys Cerius 2 (release 4.2MS) software package [18], employing the COMPASS forcefield [19]. By convention, the minimized energy (E) is related to that of the isolated intermediate and zeolite systems as follows.
E = Eintermediate/zeolite - Eintermediate - Ezeolite Zeolites Zeolite Beta (Si/A1 = 10) and ERB-1 (MWW, Si/AI =15) were synthesized according to the procedure reported in references [20] and [21], respectively. Samples were transformed into the H + form by ion-exchange with ammonium acetate solution, followed by calcination in air at 823 K. Mordenite (MOR) was purchased from Zeolyst in the ammonium form (CBV 20A, Si/A1 = 10) and transformed into the H + form by calcination in air at 823 K. ZSM-5 (MFI) named CBV 3020 (Si/AI = 15) and USY (FAU) named TSZ HUA 330 (Si/A1 = 3) in the H+ form, were used as received from Zeolyst and Tosoh.
13C NMR H + form zeolites (300 mg) were thoroughly dried overnight (at 773 K) under vacuum (10 -3 mbar) in 10 mm o.d. glass tubes prior to adsorption of gaseous reagent molecules with natural abundance 13C (1%). Benzene (0.36 mmol, from Merck) and cumene (0.12 mmol, from Aldrich) were dosed sequentially onto the catalyst (by cold-finger transfer) prior to flame-sealing the tube. 13C NMR spectra were collected at 298 K on a Bruker ASX-300 (operating at 75 MHz) using: static (non-spinning) samples, a solution state probe, 15 ~ts (70 ~ tip) rf pulses, 1H decoupling, ls re-cycle time and ca. 60,000 scans. Chemical shifts were referenced
2242
externally to benzene (129 ppm) dissolved in acetone-d6. The 13C N M R shifts given here for static samples are slightly greater (2 to 4 ppm) than those reported by Ivanova et al. [13] obtained using rapid sample spinning. This is attributed to differences in magnetic susceptibility, which are averaged out by spinning. Samples were allowed to equilibrate for 2 days at 298 K prior to collecting the first spectrum, n-PB formation was detected after each sequential step of the off-line heating protocol ( 3 hours at 423 K, followed by a total of 1, 2 and 4 hours at 473 K). Signal assignments for two spectra of the USY analysis are shown in Figure 2. The amount of n-PB produced, relative to cumene, was measured from the de-convoluted spectral areas (A) of the peaks arising from the methyl carbons of cumene (ca. 26 ppm) and n-PB (ca. 18 ppm). Since the methyl carbons of cumene have equivalent chemical shifts, the signal near 26 ppm represents 2 carbons (per molecule) and thus, the following formula was used to calculate the yield of n-PB in % mole. n-PB yield/100 = [n-PB]/[cumene] = (An-PB)/(An-PB + (A cumene/2) ) The accuracy of this yield calculation is affected somewhat by the uncertainty in the amount of DIPB produced. Due to limited spectral resolution, the methyl carbon peaks of DIPB overlaps with those of cumene. Thus, for comparison with n-PB, the peak at 26 ppm due to DIPB should be divided by 4. Neglecting DIPB formation, and division by 2 is expected to cause a relatively small error due to the minor presence of DIPB (compared to cumene). Only the methyl signals can be used for analysis under the present spectral conditions. Notice in Figure 2, that the relative intensities are not correct for all peaks (non methyl signals of cumene are too small) and the tx carbon signal of n-PB is missing. This is attributed to rapid transverse relaxation for these carbons. 26
42[ J ~ ~
18
6
423 K, 3 h
147 j
36 31
26 36
298 K, 48 h
18
~
36 152
.....'.......'.......~ :1 . . . . . . 150
"'! ......' .....'--~' . ......~.......!............. .... .................r..........' ...... 100 50 ppm
Figure 2. In-situ 13C NMR spectra of USY dosed with benzene-cumene (3:1). The large peak of benzene (ca. 131 ppm) is plotted off scale to facilitate visualization of the cumene and n-PB peaks.
RESULTS AND DISCUSSION
Computer simulations The simulations involved not only the zeolite structures studied by in-situ 13C N M R (i.e. MOR, MWW, FAU, MFI, and BEA, see below) but also other medium and large pore zeolites for which alkylation/isomerization data are available in the literature. The structures were selected to be representative of most varied porous structures, ranging from the 1D linear channel systems (e.g. MTT, MTW) to 3D
2243 interconnected porous systems (e.g. MFI, FAU). The aim of this computational study is, in fact, to verify if the pore architecture influences the formation of n-PB. The QD protocol proved to be an efficient tool for locating the intermediate molecules within the porous structures. A high temperature MD run assured the complete inspection of the conformational space of the molecule and the zeolite potential energy surface. It permitted the most energetically stable occupancy sites to be identified. As an example, the lowest energy sites for 1 and 2 in MFI are shown in Figure 3, where it is seen that both molecules are located at the channels intersection. The valence (Eval) and van der Waals (Evaw) contributions to the total energy (Etot) of the different intermediate/zeolite systems are listed in Table 1, the latter being illustrated in Figure 4 together with the differences between the values (2 minus 1). Eval values are the sum of the bond, valence and torsion angles, out-of-plane and cross-terms energy contributions. They are a measure of the degree of distortion for a molecule docked inside the porous structure, with respect to the minimum energy conformation in-vacuo. On the contrary, the nonbonding terms (Evdw, coulombic interactions were neglected) give an indication of the steric compatibility between a guest molecule and a given host framework. It must be pointed out that since intermediates 1_ and 2 are configurational isomers (i.e. they have the same number of atoms), the energy values can be directly compared. Table 1. Valence (Eval), van der Waals (Evdw), total (Etot) and difference in total energies (AEtot) for the two intermediates in the most energetically stable position within the zeolite (kJ-mol-1). 1
2
Etot
AEtot i
-122.2
-112.5
+53.6
-144.3
-143.7
+6.7
-127.9
-127.3
+21.1
0.3
-133.5
-133.2
+13.5
Etot
Eval
-166.8
-166.1
9.7
-151.3
-150.4
0.6
5.4
-153.8
-148.4
0.6
ISV
0.0
-146.7
-146.7
Eval
EvdW
MTW
0.7
MOR
0.9
MEL
EvdW
MFI
6.0
-146.5
-140.5
2.0
-140.9
-138.9
+1.6
*BEA
0.0
-140.3
-140.3
0.0
-127.9
-127.9
+12.4
M W W ii
8.7
-115.8
-107.1
0.0
-115.5
-115.5
-8.4
MTT
11.1
-117.7
-106.6
38.7
-84.9
-46.2
+60.4
FAU
3.0
- 105.6
- 102.6
0.3
-99.4
-99.1
+3.5
' difference between the total energy of 2 and 1; ii in the hemisupercages on the [001 ] crystal surface On the basis of the results reported in Table 1, it is possible to obtain some interesting indications. In general, the systems intermediate l (n-PB)/zeolite are more stable than those involving the intermediate 2 (i-PB), with energy differences (AEtot) varying from +1.6 (for MFI) to +60.4 (for MTT) kJ.mol 1. MWW is an exception since, it is the only zeolite displaying a higher stabilization of 2 (Figure 4). However, the simulations were performed considering the intermediate molecules to be located inside the hemi-supercages present on the [001 ] surface of the platelet-like crystals, where the alkylation/isomerization reactions likely occur [7]. In any case, the AEtot value observed (-8.4 kJ-mo1-1, Table 1) is too small to conclude a significant influence of the surface pore architecture on the reaction pathway. The other zeolites can be divided into three groups, depending on the AEtot values computed. First, those characterized by a mono-dimensional linear channel system without any cages displayed the largest AEtot values (+53.6 and +60.4 kJ-mo1-1 for MTW and MTT, respectively, Table 1). These differences are due to the lower steric compatibility of intermediate 2 with the two frameworks structures (low Evdw contribution) and also to distortion of the molecular geometry, particularly high in the case of 2 docked inside the medium pore zeolite MTT (Table 1). Therefore, in these structures the isomerization reaction (pathway B, Figure 1) should be preferred over the transalkylation (pathway A, Figure 1).
2244
Figure 3. Lowest energy location of 1 (left) and 2 (right) in MFI.
Figure 4. Total energies, and their difference, for the intermediates (1 for n-PB, 2_for i-PB) within zeolites.
In the second group are zeolites displaying relatively high AEtot values: MEL (+21.1 kJ.mol-1), ISV (+13.5 kJ.mo1-1) and *BEA (+12.4 kJ.mol -l) (Table 1). In these cases, a certain influence of the pore structure on the reaction pathway is indicated. And the path leading to n-PB is slightly preferred. Both intermediate molecules are hosted inside the pores without any significant distortion of their geometries (apart from MEL, with Eval - +5.4 kJ.mol 1, Table 1), but they exhibit different steric compatibilities with the zeolite framework (high Evdw differences). Finally, in the third group belong MOR, MFI and FAU, which display AEtot values in the range +1.6 - +6.7 kJ.mo1-1 (Table 1). These zeolites are characterized by the presence of large channels and or large cages where both intermediate molecules can be easily hosted. Therefore, no influence on the reaction pathway is expected. It is interesting to note the behavior of MFI, particularly when compared with the closely related MEL structure. Both are characterized by the presence of cages at the 10-membered ring channels intersections. But intermediate 2 is predicted to be better hosted by MFI because the 2,2diphenylpropane molecule has a shape which fits well the cage at the intersection between the linear and the sinusoidal channels (Figure 3).
13C NMR The implementation of static (non spinning) in-situ NMR spectroscopy to study catalytic reactions has the distinct advantage of simplicity. Fast rotation of the sealed glass containers is not required. However, the spectral resolution is compromised and only highly mobile physi-sorbed molecules are detected. These conditions do not compromise our goal, in-situ quantification of n-PB production. n-PB yields, obtained for the various zeolites using the off-line heating protocol described, are shown in the histogram of Figure 5. In general, higher amounts of n-PB were produced (%) compared to the liquid-phase alkylation of benzene with propylene in a continuous flow reactor (ppm) [7]. This is attributed to the infinitely long contact times employed for NMR, the higher temperature (473 vs. 323 K) and the high thermodynamic stability of n-PB, which increases with the temperature. In fact, thermodynamic equilibrium was reached in the case of mordenite (61% mole of n-PB at 473 K) after 4 hours. The "ranking" of catalysts in Figure 5, according to n-PB production within a closed system, needs interpretation. The NMR measurement clearly reflects the total catalytic behavior, with the combined effects of pore constraints and acidity. Evidently, pore size is not the determinant factor, since Beta and MOR both have large pores but exhibit widely different propensities to form n-PB. Excluding the importance of pore constraints for a moment, let us consider these catalysts from the perspectives of acidity, and yields in a relative way. Al content: Broensted acid strength [22]: n-PB yield during alkylation [7]"
(ZSM-5 = ERB-1) < (Beta = MOR) < USY USY < MOR < Beta < ERB-1 < ZSM-5 MOR < USY < Beta < ERB-1
2245
::/ ....... ...... : : s ~ ...... . s ......,....
.s.,/:H
E 9
40
a-m
20
.....: S .............
Beta
f
ZSM-S
USY
ERBd
IMOR
4z3 K:,:~ h O ~:73 K 2 :h
U: 4'73 K 1 ih ~ 473 K 4 h Figure 5. Product yields for n-PB, produced over H+ form zeolites by off-line heating, determined by in-situ 13C NMR spectroscopy. Neither the A1 content which is related to the overall acidity, nor the Broensted acid strength are able to explain NMR experiments. Also, the "ranking" in Figure 5 is different from that reported for the liquid-phase alkylation of benzene with9 propylene [7]. During alkylation experiments, n-PB comes from both direct alkylation and cumene isomerization [ 10]. This could be the cause for such different trends. In addition, it is known that alkylation and transalkylation reactions require acid sites with different strengths. Besides the acidic strength, also the acid site distribution should have a relevant role. A more detailed study of these considerations is currently in progress. CONCLUSIONS The isomerization of i-PB to n-PB was examined theoretically, considering that the reaction proceeds via a 1,2-diphenylpropane intermediate (_1) and is in competition with the transalkylation of i-PB with benzene to produce i-PB via a 2,2-diphenylpropane intermediate (2). In zeolite catalysts, the pore size and architecture may influence the reaction pathway in the sense that only reactions involving a sterically compatible intermediate (or transition state) are favored. This effect, known as transition state shape selectivity, can be invoked in the reactions under investigation here because they involve the bulky intermediates I and 2. Modeling tools proved to be an efficient means for differentiating zeolite structures in terms of transition state shape selectivity. In particular, microporous structures with different pore sizes and architectures, ranging from 1D linear channel systems (MTT and MTW) to 3D interconnected channel systems (e.g. MFI, FAU, *BEA), were chosen. The locations and energetics of 1 and 2, computed by molecular mechanics and dynamics calculations, lead to following conclusions: 9 1) All the structures considered can host 1 and 2. But systems intermediate 1/zeolite were more stable than those with intermediate 2. Energy differences (AEtot) varied from +1.6 (for MFI) to +60.4 kJ'mol 1 (for MTT). 9 2) Largest AEtot values were observed for the structures MTT (AEtot - +60.4) and MTW (AEtot = +53.6 kJ-mo1-1) with linear channels, predicting a clear transition state shape selectivity effect in favor of reaction pathway B to form n-PB. 9 3) The other zeolites can be divided into two groups. MEL, ISV and *BEA displayed relatively high AEtot values (+12.4 to +21.1 kJ'mo1-1) which indicate a slight preference for pathway B. MOR, MFI and FAU, with large pores and/or large cages gave AEtot values from +1.6 to +6.7 kJ-mol 1 predicting no shape selectivity effects. Preliminary in-situ 13C NMR experiments under non-flowing conditions found the ranking for n-PB production in H-form zeolites to be" Beta < ZSM-5 (MFI) < USY (FAU) ~ ERB-1 (MWW) < MOR. The
2246 ranking obtained with this simple approach indicates that factors other than just pore constraints are dominant. These findings will be developed further in future work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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