Investigations into the Mechanisms of Zeolite-Catalyzed

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Investigations into the Mechanisms of Zeolite-Catalyzed Transalkylation of iso-Propylbenzene with Toluene Seung Hyeok Cha, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06069 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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The Journal of Physical Chemistry

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Investigations into the Mechanisms of Zeolite-Catalyzed Transalkylation of iso-Propylbenzene with Toluene

Seung Hyeok Cha and Suk Bong Hong*

Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea

ABSTRACT

The catalytic properties of four large-pore (H-Y, H-beta, H-mordenite, and

H-UZM-35) and three medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites for the transalkylation of iso-propylbenzene (iPB) with toluene are investigated. Among the zeolite catalysts employed here, H-UZM-35 with a 12 × 10 × 10-ring channel system was found to exhibit a comparable cymenes yield to that of H-beta with a 12 × 12 × 12-ring channel system, the most widely studied catalyst for this reaction. GC-MS analysis reveals that monomethylated 2,2,-diphenylpropane species, whose existence has not been experimentally verified yet, are serving as the main reaction intermediates of the bimolecular iPB-toluene transalkylation. Also, the intrazeolitic build-up of dimethylated 2,2-diphenylpropane and 2methylphenyl-2-iso-propylphenylpropane species, which must be involved in the formation of 2-tolylpropanylium cations and thus in the simultaneous consumption and production of the reactant molecules (i.e., toluene and iPB), was observed. The formation of these three different groups of diphenylpropane species, which has been further supported by the DFT calculation results, allowed us to propose a new bimolecular reaction mechanism for this transalkylation. To our knowledge, our study is the first example where the repetitive mechanism is ascertained in zeolite-catalyzed reactions.

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INTRODUCTION

Zeolites are a class of microporous, crystalline solids with unique shape-selective properties that are intrinsically related to their framework structures. While their use as catalysts and catalyst supports in the chemical industry is now well established,1-5 the knowledge accumulated so far is still not enough to satisfactorily understand the mechanisms of zeolite-catalyzed reactions. This is also the case for aromatic hydrocarbon conversions, many of which have already been commercialized,6,7 which has led us to be involved in an investigation to observe and identify their reaction intermediates and mechanisms over the past 7 years. The zeolite-catalyzed reactions of aromatic hydrocarbons we have studied includes the isomerization and disproportionation of m-xylene, disproportionation of ethylbenzene, npropylbenzene, iso-propylbenzene, and 1,2,4-trimethylbenzene, and transalkylation of 1,2,4trimethylbenzene with toluene.8-13 Ex situ gas chromatography-mass spectrometry (GC-MS) was the main tool we used in the characterization of bimolecular aromatic compounds formed during each reaction within the void spaces of zeolite catalysts. Contrary to our expectation, this simple analytical method has always allowed us to find unprecedented type(s) of bimolecular reaction intermediates and thus to propose a new reaction pathways for the aromatic hydrocarbon conversions described above. In our recent work on the reaction mechanism of iso-propylbenzene (iPB) disproportionation, for example, we were able to observe mono-iso-propenylated 2,2-diphenylpropane species, as well as mono-iso-propylated ones are acting as intermediates.12 This study was the first to show that the alkenyl groupcontaining bicyclic aromatic compounds play a role as reaction intermediates of the transformation of alkylaromatic hydrocarbons.


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As an extension of our investigations into the mechanisms of zeolite-catalyzed conversions of alkylaromatics, we have focused on the transalkylation of iPB with toluene into the three cymene isomers and benzene. This transalkylation has long been recognized as an important technology for the production of cymenes, valuable raw materials in the polymer industry.14 Two different reaction pathways have been proposed: one is the monomolecular iso-propyl transfer reaction pathway, and the other the bimolecular monomethylated diphenylpropane (1mDPP)-mediated one.14-16 To our knowledge, however, no studies have been successful to experimentally observe or evidence the 1mDPP-type intermediates yet. Here we demonstrate that not only 1mDPP species but also dimethylated diphenylpropane (2mDPP) and 2-methylphenyl-2-iso-propylphenylpropane (mipPP) species are real reaction intermediates of iPB-toluene transalkylation over large-pore zeolites. We also propose a new bimolecular reaction pathway based on the overall GC-MS results, the validity of which has been further established by density functional theory calculations.

EXPERIMENTAL SECTION

Catalyst Preparation and Characterization. H-Y (framework type FAU; Si/Al = 15) and NH4-beta (BEA*; Si/Al = 13) were obtained from PQ, while H-mordenite (MOR; Si/Al = 10) and NH4-ZSM-5 (MFI; Si/Al = 14) were purchased from Tosoh. To ensure that these zeolites were completely in their proton form, they were refluxed twice in 1.0 M NH4(NO3)3 solutions (2.0 g solid per 100 mL solution) for 6 h followed by calcination at 823 K for 4 h. UZM-35 (MSE; Si/Al = 9), NU-87 (NES; Si/Al = 25), and TNU-9 (TUN; Si/Al = 18) were synthesized, converted into their proton form, and characterized by powder X-ray diffraction,



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scanning electron microscopy, elemental and thermal analyses, N2 adsorption, and IR measurements of adsorbed pyridine, as described elsewhere.11-13 GC-MS analyses of the organic compounds formed within zeolite catalysts after transalkylation of iPB with toluene were carried out following the procedures given in our previous papers.8,9 The GC-MS total ion chromatograms were recorded on an Agilent 7890A gas chromatograph equipped with an Agilent 5975C mass selective detector, and the organic compounds extracted were identified by comparing with the NIST database.17 The characterization data for all zeolite catalysts studied here are summarized in Table 1. Catalysis. Transalkylation of iPB with toluene was performed under atmospheric pressure in a continuous-flow apparatus with a fixed-bed microreactor. Prior to use as a catalyst, the zeolite powder was pressed without a binder under a maximum pressure of 2.8 × 107 Pa, crushed, and sieved to obtain particles with a diameter of 0.2 - 0.3 mm. Then, the resulting pellets were activated under flowing N2 (50 mL min-1) at 723 K for 2 h and kept at the desired reaction temperature, allowing time for the reactant/carrier gas distribution to be stabilized. A reactant stream containing a mixture of 1.0 kPa iPB (98%, Aldrich) and 14.0 kPa toluene (99%, Aldrich) in N2 fed into a microreactor containing 0.3 g of zeolite catalyst at 523 K with a weight hourly space velocity (WHSV) of 5.4 h-1. The reaction was also carried out using a reactant stream of iPB and toluene with partial pressures of 2.1 and 29.8 kPa, respectively, at 14.8 h-1 WHSV in the relatively lower temperature region (403 – 523 K) in order to minimize the decomposition of reaction intermediates formed, as well as to reduce the secondary reactions. The reaction products were analyzed on-line in a Varian CP-3800 gas chromatograph equipped with a CP-Chirasil-Dex CB capillary column (0.25 mm × 25 m) and a flame ionization detector for 10 h, and the first analysis was performed after 5 min on stream.


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Flushing experiments were carried out at a given temperature for different times up to 400 min. Before these experiments, the zeolite catalyst was reacted with iPB and toluene at 403 K for 10 h, cooled quickly to room temperature, and divided into a series of batches with exactly the same amount (50 mg). Then, each batch was flushed in a pure N2 stream (40 mL min-1) at 403 K for times ranging from 0 to 400 min. The flushed catalyst was subjected to exactly the same HF dissolution procedures given elsewhere8,9 in order to follow the evolution of the organic compounds accumulated within zeolite pores with increasing flushing time. Computational Methods. All calculations were performed in the theoretical hybrid model, a combination of the method (B3LYP/6-31G(d,p) and ωB97XD/6-31G(d,p)) with a semi-empirical

MNDO

level,

using

the

Gaussian

09

software

package.18 The

ONIOM(B3LYP/6-31G(d,p):MNDO) level of theory was applied to calculate the relative energies of all species (i.e., reactant, reaction intermediates, and/or products) that can be formed during the transalkylation reaction over the 84T H-Y model.19 It has been repeatedly shown that the B3LYP functional can accurately predict the structures of hydrocarbon molecules on zeolites and their relative energies.20,21 The organic molecule and the 8 tetrahedral atoms (T-atoms) in the zeolite framework surrounding the adsorbed species were treated at a high level and allowed to relax during the calculation. Frequency calculations were not carried out because of the high computational cost. The strain energies of 1mDPP derivatives in zeolite Y, beta, mordenite, UZM-35, NU-87, TNU-9, and ZSM-5 were also calculated using the 84T, 168T, 68T, 112T, 64T, 62T, and 72T cluster models, respectively. During the geometric optimization, in general, the 24 T-atoms surrounding the adsorbed molecule within the zeolite framework, as well as the adsorbed molecule itself, were treated at the high ωB97XD level. The rest of T-atoms in the theoretical model were treated at the low MNDO level, while being kept fixed at their crystallographic



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positions. The single-point energy calculations were further refined using the ωB97XD/631G(d,p) method on the optimized structures.


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RESULTS AND DISCUSSION

Figure 1 shows iPB conversion and cymenes yield as function of time on stream (TOS) in transalkylation of iPB with toluene over H-Y, H-beta, H-mordenite, H-UZM-35, H-NU-87, H-TNU-9, and H-ZSM-5 measured at 523 K and 5.4 h-1 WHSV. As there are no significant differences in the Al content and crystallite size of the zeolite catalysts (Table 1), the catalytic results in Figure 1 could exhibit the shape selective effects influenced by the unique pore structure of each zeolite (Supporting Information Table S1). It is worth noting that the transalkylation reaction was carried under toluene-rich conditions (toluene/iPB = 14) in order to mainly focus on the formation of cymene isomers. Since iPB is more active than toluene, its disproportionation reaction when rich can lead to undesired products like iso-propenyl-isopropylbenzene species.12 No significant differences in the initial conversion of zeolite catalysts studied here were observed. As shown in Figure 1, however, all of them are characterized by fast catalyst deactivation. This is particularly true for H-mordenite that should be one-dimensional (1D) as for both iPB and toluene reactant molecules which cannot pass through the small 8-rings. Figure 1 also shows that over the period of TOS studied, the iPB conversion and cymenes yield of H-UZM-35 remain similar to those of H-beta, known as the best catalyst for this reaction.14 Although the pore structure of the former large-pore zeolite consists of one straight 12-ring (6.4 × 6.8 Å) channel and two interesting 10-ring (5.2 × 5.8 and 5.2 × 5.2 Å) channels, it has a large oval-shaped 24-hedral ([465866104]) mse-2 cage with approximate dimensions of 6.4 × 6.5 × 18.8 Å3.22 This may allow H-UZM-35 to accommodate the bulky reaction intermediates formed during iPB-toluene transalkylation, making it more selective to cymenes formation. Despite the presence of the large mse-2 cavity, however, its p-/m-cymene ratio was



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found to be almost constant (ca. 0.5) over 10 h on stream at 523 K (Supporting Information Table S2), like the case of the channel-based large-pore zeolite H-beta. We also note that the coke formation on H-UZM-35 during this transalkylation is not so severe compared to the other zeolites including medium-pore ones (Table 1). Figure 2 shows iPB conversions as a function of TOS in transalkylation of iPB with toluene over a series of zeolite catalysts with different pore topologies at 403-523 K and 14.8 h-1 WHSV. An induction period in iPB conversion is hardly observed for H-NU-87, H-TNU-9, and H-ZSM-5, in contrast to the transalkylation of 1,2,4-TMB with toluene over the same zeolites,13 suggesting that cymenes formation over medium-pore zeolites is dominated by the monomolecular propyl-transfer mechanism.23-26 This can be further supported by the fact that the selectivity to various aliphatic hydrocarbons (< C6) is considerably higher over mediumpore zeolites than over large-pore ones (Supporting Information Table S2). While this can be attributed to the slightly larger kinetic diameter (6.7 vs 6.1 Å) of iPB compared to 1,2,4-TMB, all large-pore zeolites exhibit an induction period, indirect evidence for the intrazeolitic formation of polyalkylated biphenyl iso-propane species.27,28 Figure 2 also shows that the duration time in induction period is longer in the order 1D H-mordenite < 3D H-UZM-35 < 3D H-beta < 3D H-Y. It thus appears that the pore dimensionality of large-pore zeolites, as well as their pore size, is a critical factor governing the extent of bimolecular iPB-toluene transalkylation. Figure 3 shows the GC-MS total ion chromatogram of the CH2Cl2 extract from H-Y after transalkylation of iPB with toluene at 403 K and 14.8 h-1 WHSV for 10 h on stream, where iPB conversions around 2% in steady state were achieved. This low reaction temperature was selected to minimize the decomposition of instable reaction intermediates (Supporting Information Figure S1). While the peaks 1a and 1b in Figure 3 represent the cymene isomers,


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the major product of transalkylation of iPB with toluene, peak 2 is identified as 2phenylpropan-2-ol. Because there are no detectable amounts of oxygenated impurities, 2phenylpropan-2-ol appears to be produced from chemisorbed iPB molecules on the Brønsted acid sites in H-Y during HF dissolution, like the case of acetophenone formed during ethylbenzene disproportionation over the same zeolite catalyst.8 It is also remarkable that all the organic species corresponding to peaks 3a-c have a molecular mass of 210, equivalent to a C16H18 hydrocarbon. When compared with the NIST database,17 their ion mass distributions (Supporting Information Figure S2) were found to be identical to the three monomethylated 2,2-diphenylpropane (1mDPP) isomers. Of particular interest is the two compounds represented by peaks 4a and 4b, both of which have a molecular mass of 224 and thus a molecular formula of C17H20. Comparison with the NIST database reveals that their ion mass distributions (Supporting Information Figure S2) are the same as those of two out of six dimethylated 2,2-diphenylpropane (2mDPP) isomers, although the exact positions of their methyl groups remain unknown owing to the lack of the authentic compounds. On the other hand, peaks 5a-d are characterized by a molecular mass of 252, assignable to 2-methylphenyl-2-isopropylphenylpropane (mipPP) species. To check whether 1mDPP, 2mDPP, and mipPP species play a role as reaction intermediates, we reacted H-Y with iPB and toluene at 403 K for 10 h on stream, where the steady-state conversion of ca. 2% was obtained. Then, we divided the resulting catalyst into a series of batches with exactly the same amount (50 mg) and flushed them in a pure N2 stream (40 mL min-1) at the same temperature for times up to 400 min. As shown in Figure 3, peaks 1a and 1b, which are from the cymene isomers generated within the H-Y supercages, respectively, increase in intensity at the beginning of flushing time, and then decrease after 20 min. This is consistent with the rapid intensity decrease of peaks 3a-c, 4a and 4b, and 5a-d



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corresponding to the 1mDPP, 2mDPP, and mipPP derivatives, respectively, with increasing flushing time. Thus, these three types of diphenylpropane species appear to be so unstable to easily decompose into benzene and cymenes even at 403 K. In fact, they were hardly detectable upon reaction with iPB and toluene over H-Y at 473 K or higher (Supporting Information Figure S1). We also monitored their generation as a function of TOS within H-Y at 403 K and 14.8 h-1 WHSV. As shown in Figure 3, all the 1mDPP, 2mDPP, and mipPP signals grow rapidly at early TOS and then level off after 720 min on stream. This clearly shows the continuous consumption of these diphenylpropane species and thus their role as real reaction intermediates during transalkylation of iPB with toluene. Figure 4 compares the GC-MS chromatograms from four large-pore zeolites with different framework structures after transalkylation of iPB with toluene at 403 K and 14.8 h-1 WHSV for 10 h on stream. The signals from peaks 3a-c assigned to the three 1mDPP isomers are detectable in the chromatograms from H-beta and H-UZM-35, both of which are 3D channel-based. Also, two 2mDPP (peaks 4a and 4b) and three mipPP (peaks 5b-d) signals are clearly resolved in their chromatograms. Recall that although H-beta and H-UZM-35 have different pore sizes and shapes, there are no detectable differences in their transalkylation activities (Figure 1 and Supporting Information Table S2). This suggests that differences in their channel intersection or cavity dimension (Supporting Information Table S1) have no great influence on the type and relative distribution of reaction intermediates formed, although the precise reason remains unclear. As shown in Figure 4, however, only two 1mDPP (peaks 3b and 3c), one 2mDPP (peak 4b), and two mipPP (peaks 5c and 5d) signals are observable from 1D H-mordenite. It should be noted here that when N2 flushing (40 mL min-1) experiments at 403 K after this transalkylation over H-beta, H-mordenite, and H-UZM35 at the same temperature and 14.8 h-1 WHSV for 10 h on stream (Supporting Information


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Figure S3), all these species display a rapid decrease in intensity with increasing flushing time. However, the cymene signals from each zeolite was found to increase at the beginning of flushing, level off, and then decrease after 20 min. Therefore, it is clear that while the bimolecular iPB-toluene transalkylation can occur over the large-pore zeolite catalysts with 12-ring channels, the type of diphenylpropane derivatives formed is strongly influenced by not only the pore topology of the zeolite catalyst, but also by its pore dimensionality. As shown in Supporting Information Figure S4, on the other hand, we hardly observed any diphenylpropane peak in the GC-MS chromatograms from H-NU-87 containing 12-ring channels limited by 10-ring channels and H-TNU-9 with 12-ring cavities accessible only through 10-ring windows, as well as in the chromatogram from H-ZSM-5 with two intersecting 10-ring channels. When correlating with the lack of an induction period with these medium-pore zeolites (Figure 2), it is not difficult to conclude that the monomolecular iso-propyl transfer mechanism is generally prevailing with medium-pore zeolites. On the basis of the GC-MS results presented so far, we propose a new bimolecular reaction pathway of transalkylation of iPB with toluene over large-pore zeolites as illustrated in Scheme 1. This mechanism starts with hydride abstraction upon adsorption of iPB molecules (A) on the Brønsted acid sites in zeolites to form phenyl-iso-propanylium ions (B), which can be confirmed by the existence of the iso-propylphenol signal (peak 2) in Figure 3. Then, the monomethylated benzenium-type carbenium ions (C) are formed upon reaction of species B with a toluene molecule, being in turn converted to their neutral 1mDPP species (D) by proton migration to adjacent Brønsted acid sites and another type of benzenium-type carbenium ions (E) on nearby but unoccupied Brønsted acid sites. Species E may be further split into benzene (F) and the 2-tolylpropanylium ion (G). Finally, species G should be converted to its neutral cymene isomer (H) by hydride abstraction of an iPB molecule so that



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cycle I continues to run. However, the appearance of 2mDPP (J) and mipPP (J′′) species in Figure 3 indicates that besides their hydride abstraction reaction, species G react not only with toluene (T) but also with iPB (A), making it initiate cycle II and III. Therefore, we speculate that the energy barriers for the formation of dimethylated and 2-methylphenyl-2iso-propylphenylpropane benzenium-type carbenium ions (I and I′′, respectively) may be similar to each other, but cannot be higher than the barrier of the hydride abstraction and transfer (i.e., 2-tolylpropanylium ions (G) to species B) in cycle I. Theoretical evidence to support our speculation will be given below. Another interesting point obtained from the bimolecular reaction pathway in Scheme 1 is that although the cymene isomers cannot be produced via cycles II and III, these two cycles are not unwanted side reactions. This is because their products are toluene (T) and iPB (A), respectively, the two reactant molecules of the reaction under study. Therefore, both cycles should be considered to be the repetitive reactions of iPB-toluene transalkylation. If such is the case, this transalkylation would then be first example where the repetitive nature is observed during the zeolite-catalyzed hydrocarbon conversions. It is also worthwhile to note that the existence of cycles II and III cannot be beneficial for cymenes formations, since their reaction intermediates will become the source of coke in the long run. Of course, coke formation during this bimolecular reaction may be faster at higher reaction temperatures where the side reaction products could be more abundantly formed (Figure 2). Scheme 1 also shows that species I, which are formed upon reaction of 2tolylpropanylium ions (G) with toluene (T), can be deprotonated to neutral species J (2mDPP) and again protonated to another series of benzenium-type cations K in a manner similar to that described above. Finally, species K can be split into toluene (T) and 2tolylpropanylium (G), allowing cycle I to continue. As described above, species G also react


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with iPB (A) to form methyl-iso-propylated benzenium-type carbenium ions (I′′). These species are converted to their neutral mipPP species (J′′) and then to another type of carbenium ions (K′′), leading the formation of iPB and 2-tolylpropanylium ions (G). It is worth noting that although cycles II and III cannot start without cycle I, they operate in parallel upon formation of 2-tolylpropanylium (G). To gain further insights into the bimolecular mechanism of the zeolite-catalyzed transalkylation of iPB with toluene, we calculated the relative energies of the reactant, reaction intermediates, transition states, and/or products embedded in the 84T model of zeolite H-Y. The relative energy level diagram for these species embedded in the 84T HY model is shown in Figure 5, and their relative energies are listed in Table 2. Here, all the energies are relatively based on the energy of “the two iPBs and two toluene molecules in zeolite H-Y” which reasonably satisfy the bimolecular reaction pathway in Scheme 1. Among all the possible isomers of 1mDPP, 2mDPP, and mipPP species, on the other hand, 3-1mDPP, 3,3′-2mDPP, and 3,3′-mipPP can split into m-cymene and benzene, respectively, which are the major products of iPB-toluene transalkylation (Supporting Information Table S2). Thus, we selected them as the representative isomer of these different types of diphenylpropane derivatives, respectively. As shown in Figure 5, the relative energy barrier of transition state 1 (TS1; for its structure within the 84T H-Y model, see Supporting Information Figure S6) to the generation of phenyl-iso-propanylium ions (B) was calculated to be 180 kJ mol-1. This value is the highest among the elementary steps in Scheme 1 and is similar to the well-known example of m-xylene disproportionation over various zeolite catalysts.20,21 Once species B are formed, they are converted into monomethylated benzenium-type carbenium ions (C). Then, species C must overcome two energy barriers (TS2 and TS3) to produce cymenes (H) and 2-



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tolylpropanylium cations (G). TS2 was calculated to have an energy barrier of 67 kJ mol-1. As can be found in Supporting Information Figure S6, The proton in its structure is located at the center of the carbon atom of the aromatic ring with a C-H bond distance of 1.72 Å and with an O-H bond distance of 1.34 Å where O is the zeolite oxygen atom. Therefore, it is not difficult to expect that TS2 is thermodynamically stable within the 84T H-Y model. To further proceed the transalkylation of iPB with toluene over this large-pore zeolite, the proton of Brønsted acid sites has to shift to the other aromatic ring of neutral 1mDPP species (i.e., 3-1mDPP). We note here that species I in cycle II and I′′ in cycle III are formed via TS4 or TS7 (Supporting Information Figure S6) with a relatively lower energy barrier (144 and 150 kJ mol-1, respectively) than that (180 kJ mol-1) of the elementary step G to B (i.e., hydride abstraction) in cycle I, unlike other alkylaromatic conversions such as m-xylene isomerization and ethylbenzene and iso-propylbenzene disproportionations.8,9,12 These results support the repetitive nature of the bimolecular reaction pathway in Scheme 1, because they thermodynamically explain why the neutral forms (i.e., 2mDPP (J) and mipPP (J′′) species, respectively) of I and I′′ are observed in the GC-MS chromatograms from four different largepore zeolites after transalkylation of iPB with toluene at 403 K for 10 h on stream (Figures 3 and 4). We next calculated the strain energies of 2-1mDPP, 3-1mDPP, and 4-1mDPP isomers within the 84T zeolite Y, 168T beta, 68T mordenite, 112T UZM-35, 64T NU-87, 62T TNU-9, and 72T ZSM-5 models in order to understand the effects of zeolite pore topology on the type of reaction intermediates formed during the bimolecular iPB-toluene transalkylation. As shown in Figure 6, their strain energies when embedded in NU-87, TNU-9, and ZSM-5 were calculated to be always higher than 20 kJ mol-1. Recall that none of these three medium-pore zeolites gave noticeable GC-MS peaks assignable to bicyclic aromatic species even after


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transalkylation of iPB with toluene at 403 K for 10 h on stream (Supporting Information Figure S4). Therefore, it can be concluded that if the strain energy of a particular 1mDPP isomer is higher than 20 kJ mol-1, its intrazeolitic formation may be energetically unfavorable. Another interesting result obtained from Figure 6 and Supporting Information Table S3 is that differences (7 vs ≤ 3 kJ mol-1) in the strain energy between 2-1mDPP, which is the fattest among the three different 1mDPP isomers (Supporting Information Figure S5), and the other two isomers are rather higher within mordenite than within the other three large-pore zeolites. Moreover, the strain energy of 2-1mDPP embedded in mordenite was calculated to be 23 kJ mol-1, suggesting the difficulty of its formation within this 1D large-pore zeolite during the bimolecular transalkylation of iPB with toluene. As described above, only two 1mDPP isomers (peaks 3b and 3c in Figure 4) are observable in the GC-MS chromatogram from Hmordenite after transalkylation of iPB with toluene at 403 K for 10 h on stream, unlike the case of the other three large-pore zeolites. Therefore, peak 3a can be unequivocally assigned to the fattest 2-1mDPP isomer. These calculation results again show that the formation of particular type of 1mDPP isomers during this transalkylation is influenced by the pore dimensionality of large-pore zeolite catalysts employed, as well as their pore architecture.



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CONCLUSIONS

The mechanisms of transalkylation of iPB with toluene over a series of large-pore (H-Y, H-beta, H-mordenite, H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites have been investigated. The cymenes yield of H-UZM-35, the pore structure of which consists of one type of straight 12-ring channel and two types of tortuous 10-ring channels, is similar to that of H-beta, known as the best catalyst for this reaction, over the period of time on stream tested here. All zeolite catalyst studied were analyzed by ex situ GC-MS to identify the reaction intermediates of this transalkylation. There are no signs of the formation of bimolecular diphenylpropane species during iPB-toluene transalkylation over medium-pore zeolites. However, it was found that while 1mDPP species act as the main reaction intermediates of the bimolecular transalkylation of iPB with toluene over large-pore zeolites, 2mDPP and mipPP species also play a role in the formation of 2-tolypropanylium cations, and thus simultaneously consuming and producing the reactant molecules (i.e., toluene and iPB). Among the zeolite-catalyzed reactions studied thus far, therefore, transalkylation of iPB with toluene is the first to show the repetitive nature. On the basis of the overall GC-MS results of our study, a new bimolecular diphenylpropane-mediated reaction pathway is proposed. This mechanism has been further supported by theoretical calculations on the strain and relative energies for its bicyclic aromatic reaction intermediates embedded in different zeolite structures.


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ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author [email protected]. ORCID Suk Bong Hong: 0000-0002-2855-1600

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea.

Supporting Information

Structural features of zeolites studied in this work and their catalytic results for transalkylation of iPB with toluene, strain energies and structures of species involved in the bimolecular transalkylation of iPB with toluene over various zeolite models, and GC-MS data of used zeolite catalysts. This information can be found on the internet at http://pubs.acs.org.



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Table 1. Physicochemical properties of all zeolite catalysts employed in this study. BET surface areac (m2 g-1) acidityd (µmol pyridine g-1) amount of organics NH4+ exchange a b catalyst Si/Al degree (%) total microporous external Brønsted Lewis total depositede (wt%) crystal shape and size (µm) H-Y 15 98 polygonal platelets, 0.7 × 0.7 × 0.2 800 690 110 113 18 131 13.5 H-beta 13 97 grains, 0.1 540 360 180 116 59 175 10.5 H-mordenite 10 97 rods, 0.3 × 1.0 620 570 50 124 14 138 15.1 H-UZM-35 9 91 platelets, 0.2 × 0.05 470 380 90 154 38 192 11.2 H-NU-87 25 98 rods, 0.3 × 1.0 460 420 40 109 19 128 11.0 530 480 50 143 16 159 12.4 H-TNU-9 18 99 rods, 0.3 × 1.0 H-ZSM-5 14 98 rods, 0.3 × 1.0 390 330 60 158 17 175 10.1 a Determined by elemental analysis. b Determined by SEM. c Calculated from N2 adsorption data. d Determined from the intensities of the IR bands of retained pyridine at 1545 and 1455 cm-1 after desorption at 473 K after 2 h, respectively, by using the extinction coefficients. The acidity of H-mordenite may be underestimated here because pyridine cannot diffuse into its small 8-ring channels. e The exothermic weight loss by TGA/DTA at 523-1073 K after transalkylation of iPB with toluene at 523 K and 5.4 h-1 WHSV for 10 h on stream. a



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Table 2. Relative energies for the reactants, reaction intermediates, and transition states of the bimolecular transalkylation of iPB with toluene over the 84T H-Y model cylcle I cycle II cycle III a,b -1 a,c -1 a,d species energy (kJ mol ) species energy (kJ mol ) species energy (kJ mol-1) two iPB and 0 83 83 G G two toluene in H-Y TS4 227 TS7 233 132 180 121 TS1 I I′′ 102 TS5 197 TS8 189 B 98 88 91 C J J′′ TS2 165 TS6 177 TS9 172 95 117 123 D K K′′ TS3 151 91 E 83 G a The same as the species given in Scheme 1. b D is 3-1mDPP. c J is 3,3′-2mDPP. d J′′ is 3,3′-mipPP.


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FIGURE CAPTIONS

Figure 1. iPB conversion and cymenes yield as a function of time on stream in transalkylation of iPB with toluene over H-Y, H-beta, H-mordenite, H-UZM-35, H-NU87, H-TNU-9, and H-ZSM-5 at 523 K and 5.4 h-1 WHSV.

Figure 2. iPB conversion in iPB-toluene transalkylation at 14.8 h-1 WHSV over seven zeolites with different framework topologies at 403 (■), 423 (●), 443 (▲), 473 (▼), 493 (◀), and 523 (▶) K.

Figure 3. (a) GC-MS total ion chromatogram of the CH2Cl2 extract from H-Y after transalkylation of iPB with toluene at 403 K and 14.8 h-1 WHSV for 10 h on stream. The structures annotated onto the chromatogram from H-Y are peak identifications having compared the mass spectra with those in the NIST database.18 (b) GC-MS chromatograms of the CH2Cl2 extracts from H-Y after transalkylation of iPB with toluene at 403 K and 14.8 h-1 WHSV for 10 h on stream followed by flushing with N2 (40 mL min-1) for (from back to front) 0, 5, 10, 20, 40, 80, 160, 240, 360, and 400 min, respectively. (c) GC-MS chromatograms of the CH2Cl2 extracts from H-Y after transalkylation of iPB with toluene at 403 K and 14.8 h-1 WHSV for (from front to back) 5, 10, 20, 40, 80, 160, 300, 600, 720, 900, and 1200 min on stream, respectively.

Figure 4. GC-MS total ion chromatograms of the CH2Cl2 extracts from (from top to bottom) H-Y, H-beta, H-mordenite, and H-UZM-35 after iPB-toluene transalkylation at 403 K and 14.8 h-1 WHSV for 10 h on stream. The parts covering 1mDPP, 2mDPP, and mipPP species only are shown for simplicity.



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Figure 5. Relative energy level diagram for the reactant, reaction intermediates, and transition states involved in the formation of 1mDPP (black), 2mDPP (red), and mipPP (blue) species during the bimolecular transalkylation of iPB with toluene over the 84T H-Y model, respectively (Scheme 1).

Figure 6. Strain energies of three 1mDPP isomers that can be formed during the bimolecular transalkylation of iPB with toluene over the 84T zeolite Y, 168T beta, 68T mordenite, 112T UZM-35, 62T NU-87, 64T TNU-9, and 72T ZSM-5 models.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Scheme 1 Bimolecular diphenylpropane-mediated reaction pathway of the zeolite-catalyzed transalkylation of iPB with toluene proposed based on the overall GC-MS results of this work.



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Investigations into the Mechanisms of Zeolite-Catalyzed

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