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Zeolite-Catalyzed Disproportionation of Iso-Propylbenzene: Identification of Reaction Intermediates and Mechanism Seung Hyeok Cha, Kyounghwan Lee, Youngchul Byun, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02910 • Publication Date (Web): 14 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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The Journal of Physical Chemistry

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Zeolite-Catalyzed Disproportionation of iso-Propylbenzene: Identification of Reaction Intermediates and Mechanism

Seung Hyeok Cha, Kyounghwan Lee, Youngchul Byun, and Suk Bong Hong*

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

ACS Paragon Plus Environment


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ABSTRACT

The catalytic properties of a series of large-pore (H-Y, H-beta, H-mordenite, and H-UZM35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites are compared in isopropylbenzene (iPB) disproportionation. Among the zeolite catalysts studied here, H-UZM-35 with a three-dimensional framework consisting of one type of straight 12-ring channels and two types of tortuous 10-ring channels was found to show a comparable di-isopropylbenzenes (DiPBs) yield to that of H-beta with two intersecting 12-ring channels, the best catalyst tested for this reaction so far. GC-MS analysis of used zeolite catalysts demonstrates that while mono-iso-propylated 2,2-diphenylpropane derivatives are serving as real reaction intermediates of iPB disproportionation over large-pore zeolites, mono-isopropenylated 2,2-diphenylpropane species, which contains a double bond in the alkyl chain, are intermediates of its side reaction. Unlike that of other aromatic hydrocarbons such as mxylene, ethylbenzene, and n-propylbenzene, the formation of di-iso-propylated derivatives was not observed as reaction intermediates. A new bimolecular diphenylpropane-mediated reaction pathway, which includes both intermediates of main and side reactions of iPB disproportionation, is proposed based on the experimental and theoretical results.


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INTRODUCTION

Zeolites are crystalline, microporous aluminosilicates that have found a wide variety of commercial applications as catalysts in the petrochemical and chemical industries since the early 1960’s.1-3 Despite this, however, the exact mechanisms of many chemical processes catalyzed by these materials, especially those of the conversion of simple monoalkylaromatics into various branched ring compounds, require further clarification.4-11 One example is npropylbenzene (nPB) disproportionation, the detailed mechanism of which has remained until a very recent date.12 A new bimolecular diphenylpropane (DPP)-mediated reaction pathway, in which both mono- and dipropylated DPP derivatives are serving as intermediates, has been proposed based on the ex situ gas chromatography-mass spectrometry (GC-MS) results from used zeolite catalysts. This reaction was also found to be useful for assessing the framework topology of large-pore zeolites, because the type of not only its main reaction intermediates but also that of side reaction ones formed during nPB disproportionation can differ notably according to the size and shape of zeolite void spaces.12 Although looking similar to nPB disproportionation, on the other hand, iso-propylbenzene (iPB) disproportionation is more interesting in practice, probably due to the importance of its main products, i.e., di-iso-propylbenzenes (DiPBs). For example, oxidation of m-DiPB and pDiPB leads to resorcinol and hydroquinone, respectively, that are valuable monomers for numerous polymers.13 In addition, iPB disproportionation has long been known as a useful test reaction for characterizing the internal pore structure of zeolites.14,15 It has been repeatedly shown that while medium-pore zeolites like H-ZSM-5 (framework type MFI) with two intersecting 10-ring channels inhibit the formation of diphenyl-iso-propane species during the reaction, these bulky reaction intermediates can be easily created in large-pore



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materials.13-15 Like the case of nPB disproportionation, as a result, two major types of reaction pathways have been proposed for iPB disproportionation: i) the monomolecular iso-propyltransfer mechanism and ii) the bimolecular mono-iso-propylated 2,2-diphenylpropane (mipDP)-mediated one.14-16 However, no studies have provided clear evidence for the formation of mipDP species during this reaction. In the present study we report on the catalytic properties of four different large-pore (H-Y (FAU), H-beta (*BEA), H-mordenite (MOR), and H-UZM-35 (MSE)) and three different medium-pore (H-NU-87 (NES), H-TNU-9 (TUN), and H-ZSM-5) zeolites for iPB disproportionation and the GC-MS results from used zeolite catalysts. These experimental results allowed us to confirm the formation of not only mipDP derivatives but also mono-isopropenylated 2,2-diphenylpropane (mip=DP) ones inside the void spaces of all large-pore zeolites employed here and thus to propose a new bimolecular reaction pathway for iPB disproportionation. While the latter group of bicyclic aromatic compounds was found to be intermediates of iPB dehydrogenation, a side reaction of the iPB disproportionation, both groups of them are hardly observed over medium-pore zeolites, regardless of their structure type. However, we were not able to detect any di-iso-propylated 2,2-diphenylpropane (dipDP) species in this study, unlike the disproportionation of other aromatic hydrocarbons, including m-xylene, ethylbenzene, and n-propylbenzene,12,17,18 in which formation of dialkylated diphenylalkane species as reaction intermediates has been clearly confirmed. To better understand the results described above, we have calculated the strain energies of three mipDP, three mip=DP, or six dipDP isomers embedded within the structures of zeolites studied, together with the relative energies of all species (i.e., reactant, reaction intermediates, transition states, and products) in the 84T H-Y model, using the mixed quantum-mechanical and semi-empirical ONIOM method.


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EXPERIMENTAL SECTION

Catalyst Preparation. Zeolites H-Y (Si/Al = 15) and NH4-beta (Si/Al = 13) were purchased from PQ, and H-mordenite (Si/Al = 10) and NH4-ZSM-5 (Si/Al = 14) were obtained from Tosoh. To ensure that these zeolites were completely in the 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, NU-87, and TNU-9 were synthesized and converted into their proton form according to the procedures reported in the literature.19-21 Prior to use as catalysts, zeolites were granulated by pressing 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. Analytical Methods. Elemental analysis was carried out on a Jarrell-Ash Polyscan 61E inductively coupled plasma spectrometer in combination with a Perkin-Elmer 5000 atomic absorption spectrophotometer. Crystal morphology and size were determined by a JEOL JSM-6510 scanning electron microscope. N2 sorption experiments were made with a Mirae SI nanoPorosity-XQ analyzer. Thermogravimetric analyses (TGA) were performed in air on an SII EXSTAR 6000 thermal analyzer, where the weight loss related to the combustion of organic species formed during iPB disproportionation was further confirmed by differential thermal analyses (DTA) using the same analyzer. The acidic properties of zeolites employed here were characterized by IR spectroscopy using pyridine as a probe molecule. Details of the IR measurements with adsorbed pyridine can be found elsewhere.17,18,22 After desorption at 473 K for 2 h, the concentrations of Brønsted and Lewis acid sites in zeolites were determined from the intensities of the IR bands around 1550 and 1450 cm-1, respectively, using the



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equations of Emeis.23 The characterization data for zeolite catalysts studied here are given in Table 1. GC-MS analyses of the organic compounds formed within zeolite catalysts after iPB disproportionation were carried out following the procedures described in our previous papers.17,18,22 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.24 Catalysis. iPB disproportionation was conducted under atmospheric pressure in a continuous-flow apparatus with a fixed-bed microreactor. Prior to the experiments, the zeolite catalyst was 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. Then, iPB (98%, Aldrich) with a partial pressure of 14.0 kPa in N2 was fed into a microreactor containing 0.2 g of zeolite catalyst at 523 K with a weight hourly space velocity (WHSV) of 7.8 h-1. The reaction was also performed using a reactant stream of iPB with a partial pressure of 24.6 kPa in the relatively lower temperature region (403 – 523 K) and a WHSV of 15.5 h-1 to avoid the decomposition of reaction intermediates and to reduce the secondary/side reactions. The reaction products were analyzed on-line in a Agilent 7890A gas chromatograph equipped with a HP-5 capillary column (0.25 mm × 30 m) and a flame ionization detector for 10 h, and the first analysis was carried out after 5 min on stream. Flushing experiments, in which the iPB-containing stream was replaced by a pure N2 feed, were carried out under a given temperature as a function of time. Before these experiments, the catalyst was reacted with iPB 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


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flushed catalyst was subjected to exactly the same HF dissolution procedures given elsewhere17,18,22 in order to follow the evolution of the organic compounds accumulated within zeolite pores with increasing flushing time. Computational Methods. The 84T, 168T, 68T, 112T, 64T, 62T, and 72T cluster models, which were extracted from their crystallographic data,25 were applied to calculated the strain energies of mipDP and mip=DP derivatives in zeolite Y, beta, mordenite, UZM-35, TNU-9, NU-87, and ZSM-5, respectively. Since these models include the complete pore structures of the corresponding zeolites (Figure S1, Supporting Information), it was possible to rationally consider the confinement effects on the reaction intermediates during iPB disproportionation. The theoretical hybrid model, a combination of the ωB97XD/6-31G(d,p) and B3LYP/631G(d,p) methods with a semi-empirical MNDO level, was applied in all calculations using the Gaussian 09 software package.26 During the geometric optimization, the 24T 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 tetrahedral atoms (Tatoms) in the theoretical model were treated at the low MNDO level, while being kept fixed at their crystallographic positions. The single-point energy calculations were further refined using the ωB97XD/6-31G(d,p) method on the optimized structures. This method has previously been shown to accurately predict the structure of adsorbed aromatic hydrocarbon species in zeolites and their strain energies.27 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 bimolecular iPB disproportionation over the 84T H-Y model. This functional has proved to accurately predict the structures of adsorbed hydrocarbon molecules in zeolites and their relative energies.22,27 The organic molecule and the 8T atoms in the



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zeolite framework surrounding the adsorbed species were treated at a high level and allowed to relax during the calculation. Because the zeolite Y structure has only one crystallographically distinct T-site, there are only four distinct O sites at which the proton can be located upon Al substitution. Among these O sites, thermodynamically the most stable site, site O4, was selected as the proton location for the Brønsted acid sites in H-Y. Frequency calculations were not performed due to the high computational cost.

RESULTS AND DISCUSSION

Catalytic Activity. Figure 1 shows iPB conversion as a function of time on stream (TOS) in iPB disproportionation over various large-pore (H-Y, H-beta, H-mordenite, and H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites at 7.8 h-1 WHSV and 523 K. Since there are no significant differences in their Al content and crystallite size (Table 1), the catalytic results in Figure 1 would illustrate the effects of zeolite pore structure on this aromatic transformation (Table S1, Supporting Information). It can be seen that H-UZM-35 and H-ZSM-5 exhibit the highest initial conversions among the large- and medium-pore zeolites employed in this work, respectively. All zeolite catalysts are rapidly deactivated, but the rate of deactivation over the period of TOS studied is faster for H-Y and H-mordenite than for the other zeolites. The one-dimensional (1D) nature of H-mordenite as far as iPB disproportionation is concerned can rationalize its fast deactivation behavior. Although the reason 3D cage-based H-Y with 12-ring windows shows a larger decrease in iPB conversion than 3D channel-based H-beta is unclear at this time, in addition, one possible explanation is that the presence of bottlenecks in the former zeolite is responsible for its higher coke forming


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propensity as a solid acid catalyst. Another interesting finding obtained from Figure 1 is that the DiPBs yield of H-UZM-35 is comparable with that of H-beta, the best catalyst known for iPB disproportionation,15 during 10 h on stream. UZM-35, an MSE-type large-pore zeolite crystallized in the presence of Na+, K+, and dimethyldipropylammonium ions as structure-directing agents,19 contains a 3D pore system consisting of one straight 12-ring channels that intersect with two twisted independent 10-ring channels.28-30 This zeolite also contains oval-shaped supercages that are accessible through 10-ring windows only, providing a large void pore volume for large molecules to be produced inside the pore system. Such unique structural features may in our view render H-UZM-35 more selective for the formation of bulky DiPB molecules than H-Y and H-mordenite, as well as than the medium-pore zeolites used here. The m-DiPB/p-DiPB ratios in iPB disproportionation over seven zeolites with different framework topologies at 523 K and initial conversion levels near 20% can be found in Table S2 (Supporting Information). Comparison of these values to one another could reveal differences in the shape selectivity imposed by the particular pore structure of each zeolite catalyst. The cage-based large-pore zeolite H-Y gives an m-DiPB/p-DiPB ratio of 2.1, which is the highest value among the zeolites studied in this work and is similar to the thermodynamic equilibrium value (2.1) at 523 K.16 By contrast, the lowest ratio (1.2) is observed for 3D medium-pore zeolite H-ZSM-5 with two intersecting 10-ring channels. As previously reported,14,15 therefore, iPB disproportionation is useful to distinguish between large- and medium-pore pore zeolites. We should note here that despite the presence of large supercages in this zeolite, H-UZM-35 shows an m-DiPB/p-DiPB ratio of 1.8 that is even slightly smaller than the ratio (1.9) of 1D H-mordenite. Figure 2 shows iPB conversions as a function of TOS at different reaction temperatures in



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iPB disproportionation over a series of large- and medium-pore zeolites at 15.5 h-1 WHSV. With all four large-pore zeolites, an induction period in iPB conversion is clearly observed, which should be attributed to the intrazeolitic formation of polyalkylated aromatic species.31,32 The duration time in induction period was found to become longer in the order of Hmordenite < H-UZM-35 ≈ H-beta < H-Y, which matches well with their DiPBs deficits at 443 K. This appears to have relevance mainly to the pore dimensionality of zeolites, as well as to the extent of bimolecular iPB disproportionation over these four large-pore zeolites. Also, there are no significant differences in their steady-state DiPBs yield/benzene yield ratio (Table S2, Supporting Information). As shown in Figure 2, however, none of medium-pore zeolites used here gave no detectable induction period, although they contain 12-ring channels (H-NU-87) or cavities (H-TNU-9) that are accessible only through 10-ring pores. It thus appears that the prevailing mechanism in the DiPBs formation over medium-pore zeolites is monomolecular.

GC-MS Analyses and Reaction Mechanism. Figure 3 shows the GC-MS total ion chromatogram of the CH2Cl2 extract from four large-pore zeolites with different framework structures after iPB disproportionation at 403 K and 15.5 h-1 WHSV for 10 h on stream, where iPB conversions around 3% in steady state were achieved. Here we carried out this disproportionation at a temperature as low as possible to avoid the decomposition of unstable reaction intermediates formed within the zeolite void spaces (Figure S2, Supporting Information). While the peaks 1a and 1b observed in the GC-MS chromatograms from all these zeolites have mass spectra and retention times corresponding to the iso-propenyl-isopropylbenzene (i=iPB) isomers, the major by-product of iPB disproportionation, peaks 2a and 2b represent the DiPB ones, the major products of iPB disproportionation. Peak 3 is identified


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as 2-phenylpropan-2-ol and 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 within the same zeolite catalyst after ethylbenzene disproportionation at 403 K for 30 h on stream.17 On the other hand, the compounds responsible for peak 4a and 4b have a molecular mass of 236, equivalent to a C18H20 hydrocarbon. When compared with the NIST database,24 their ion mass distributions (Figure S3a, Supporting Information) were found to be the same as two of the three mono-iso-propenylated 2,2-diphenylpropane (mip=DP) isomers. To our knowledge, no studies have proposed or experimentally demonstrated the formation of such bicyclic aromatic compounds containing a double bond in their alkyl chain during the zeolitecatalyzed aromatic hydrocarbon conversions including iPB disproportionation. Another important compounds found in Figure 3 are represented by peaks 5a-c, all of which have a molecular mass of 238 and thus a molecular formula of C18H22. The NIST database reveal that they are assignable to the three mipDP isomers (Figure S3b, Supporting Information), although the exact positions of their iso-propyl groups is still unclear due to the lack of the authentic compounds. Unlike the disproportionation of m-xylene, ethylbenzene, or nPB,12,17,18 however, we were not able to observe dialkylated diphenylalkane species, i.e., di-iso-propylated 2,2diphenylpropane (dipDP) ones, during iPB disproportionation over all large-pore zeolite catalysts employed. The precise reason for this remains unknown, because di-n-propylated 1,1-diphenylpropane derivatives, the molecular dimensions of which cannot be smaller than those of dipDP species, can be formed within the supercages of H-Y during nPB disproportionation.12 Figure 3 also shows that while the peak 5c due to one mipDP isomer is barely detectable from the cage-based, large-pore H-Y only, the peaks 4a,b and 5a,b assigned



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to two of the three mip=DP and three mipDP isomers, respectively, are clearly visible from all four large-pore zeolites used in this study. This suggests that the shape-selective effects of large-pore zeolites are not strong enough to prevent the bimolecular iPB disproportionation, even over the 1D channel-based H-mordenite. However, the mipDP and mip=DP signals were hardly observable in the GC-MS chromatograms from all of H-NU-87, H-TNU-9, and HZSM-5 (Figure S4, Supporting Information). To elucidate the role of mipDP and mip=DP species during iPB disproportionation, we reacted H-Y with iPB at 403 K and 15.5 h-1 WHSV for 10 h on stream, where the steady-state conversion of ca. 3% was obtained. Then, we divided the used catalyst into a series of batches with exactly the same amount (50 mg), and then flushed them in a pure N2 stream (40 mL min-1) at the same temperature for up to 400 min. As shown in Figure 4, peaks 1a and 2a,b due to the i=iPB and DiPB isomers generated within the H-Y supercages, respectively, increase in intensity at the beginning of flushing time, and then decrease after 40 min. This goes well with the rapid decrease in intensity of peaks 4a,b and 5a-c corresponding to the mip=DP and mipDP derivatives, respectively, with increasing flushing time. Therefore, it is clear that these two different types of DPP species are considerably unstable to decompose into benzene and i=iPBs or DiPBs even at 403 K. In fact, they are hardly detectable upon reaction with iPB over H-Y at 473 K or higher (Figure S2, Supporting Information). We also monitored the generation of both types of DPP derivatives as a function of TOS in H-Y during iPB disproportionation at the same reaction conditions described above. All mip=DP and mipDP signals grow rapidly at early TOS. As shown in Figure 4, however, they become almost constant after 160 min on stream (Figure 4). This indicates the continuous consumption of mip=DP and mipDP derivatives and thus their role as real reaction intermediates during the reaction. The same conclusion can also be drawn from the GC-MS


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results data of H-beta, H-mordenite, and H-UZM-35 flushed with N2 as a function of TOS at 403 K after iPB disproportionation at the same temperature and 15.5 h-1 WHSV for 10 h on stream (Figure S5, Supporting Information). Here, we note that the intensity change pattern of GC-MS signals from H-UZM-35 is similar to that observed for the other three large-pore zeolites, despite its smallest 12-ring pore size (Table S1, Supporting Information). Therefore, the bimolecular iPB disproportionation over H-UZM-35 appears to occur within its supercages which can be accessed through 10-ring windows only, and as well as in its 12-ring channels. Scheme 1 illustrates a new dual-cycle mechanism involving both mip=DP and mipDP derivatives as intermediates of iPB disproportionation and its side reaction (i.e., iPB dehydrogenation), respectively, over large-pore zeolites proposed based on the GC-MS results in Figures 3 and 4. The formation of phenyl-iso-propanylium ions (B) by hydride abstraction upon adsorption of iPB molecules (A) on the Brønsted acid sites in zeolites initiates this reaction, as clearly evidenced by the presence of the iso-propylphenol signal (peak 3) in Figure 3. Species B react with another iPB molecule, yielding iso-propylated benzenium-type carbenium ions (C) that can in turn be converted to neutral mipDP species (D) and another benzenium-type carbenium ions (E). Then, species E split into benzene molecules (F) and (iso-propylphenyl)propanylium ions (G). Finally, species G react with another iPB molecules to produce not only neutral DiPB isomer (H) but also species B, allowing the main reaction cycle in Scheme 1 to continue. While the main reaction cycle explains why the mipDP signals are observed in Figures 3 and 4, the appearance of the mip=DP ones indicates that it is accompanied with a side reaction. This is particularly true when considering the fact that i=iPBs, decomposition products of mip=DPs, are major by-products (Table S2, Supporting Information). It thus appears that the



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energy barriers of elementary reactions are not much higher in the side reaction of iPB disproportionation than in its main reaction. The density functional theory (DFT) calculation results to support this speculation will be given below. If such is the case, species B would then react with iso-propenylbenzene (I) to form species J. These benzenium-type carbenium ions can be deprotonated to neutral species K (mip=DPs) by proton migration to nearby Brønsted acid sites and again protonated to another series of benzenium-type cations L on nearby but unoccupied Brønsted acid sites. Finally, species L can split into benzene (F) and (iso-propylphenyl)propenylium ions (M), thereby which was changed to neutral i=iPB isomers (N) by hydride abstraction of iPB molecules. Scheme 1 also shows that the side reaction can start without the main reaction, although it should of course be suppressed to more selectively produce DiPBs.

DFT Calculations. To better understand the eﬀects of zeolite pore topology on the bimolecular reaction mechanism for the iPB disproportionation proposed above, we first calculated the strain energies of three mipDP and three mip=DP isomers in seven zeolites with diﬀerent framework structures employed in this study. As shown in Figure 5 and Table S3 (Supporting Information), the strain energies of these bicyclic compounds all were calculated to be lower at least by 5 kJ mol-1 in the cage-based, large-pore zeolite Y than in the other three large-pore zeolites, indicating their easy formation within the former structure. We also note that the ortho-isomer of mipDP and mip=DP species always has a higher strain energy than their meta- and para-isomers. Differences in the strain energy were found to become larger when the pore dimensionality of zeolites changes from 3D to 1D, as well as when their pore system is channel-based. This implies that the intrazeolitic formation of o-mipDP and omip=DP as a reaction intermediate of cycles I and II in Scheme 1, respectively, is


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energetically less favorable than that of their meta- and para-isomers. Therefore, we tentatively assign the weak peak 5c appearing in the GC-MS chromatogram from H-Y (Figure 3) as o-mipDP among the three mipDP isomers, because they are fatter (Figure S6, Supporting Information). The conclusion can also be drawn for the two peaks 4a,b so that they are assignable to m-mip=DP and p-mip=DP, respectively, or vice versa. When embedded in the medium-pore zeolites NU-87 and TNU-9 containing 12-ring channels/cavities, on the other hand, the strain energies of three mip=DP isomers are always higher by > 30 kJ mol-1 than those calculated for the large-pore zeolites. Moreover, their strain energies become higher at least by 50 kJ mol-1 upon embedment in ZSM-5 free of 12ring pores. A similar trend was also observed for the mip=DP species, although an increase in their strain energy was smaller. Therefore, it is most likely that the formation of such bicyclic aromatic reaction intermediates in medium-pore zeolites are unfavorable due to severe steric constraints, in excellent agreement with the GC-MS results in Figure S5 (Supporting Information). This reveals that in contrast to that of large-pore zeolites, the catalytic action of medium-pore zeolites is dominated by the monomolecular reaction pathway. As described above, we were able to observe no dipDP isomers in the GC-MS chromatograms from a series of large-pore zeolites after iPB disproportionation at 403 K for 10 h on stream. This led us to consider the probability that the absence of dipDP isomers could be due to their large molecular dimensions, leading to severe geometric constraints and van der Waals interactions with the zeolite framework. To clarify the confinement effects on the type of dipDP isomers, we first calculated the strain energies of six different dipDP derivatives embedded in the 84T H-Y model. As shown in Figure S7 (Supporting Information), however, there are no significant differences in the calculated strain energy. This implies that the H-Y supercage is already large enough to allow the formation of any of



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dipDP isomers (Table S1, Supporting Information). Therefore, we moved our attention to the relative energy calculations of dipDP isomers within the 84T H-Y models. Here we selected m,m-dipDP as a representative dipDP isomer, because it splits off m-DiPB, the major products of iPB disproportionation over H-Y (Table S2, Supporting Information). The calculation results presented in Figure S8 (Supporting Information) reveal that m,m-dipDP, the existence of which has not been experimentally confirmed in this work, has a higher energy barrier (409 kJ mol-1; from species G to TS(G to O)) than mipDP and mip=DP, the main reaction intermediate of the bimolecular iPB disproportionation and that of its side reaction, respectively (see below). This strongly suggests that the intrazeolitic formation of dipDP isomers during iPB disproportionation is energetically much less favorable than the formation of mipDP and mip=DP species, rationalizing why iPB dehydrogenation should also take place as a side reaction of iPB disproportionation. We next calculated the relative energies of the reactant, reaction intermediates, transition states, and/or products within the 84T H-Y. Figure 6 shows the relative energy level diagram for all these species of the bimolecular iPB disproportionation over the 84T H-Y model, and Table 2 lists their relative energies. Here all the energies are referenced relative to the energy of “the five iPB molecules in zeolite H-Y” which can reasonably meet the bimolecular reaction pathway in Scheme 1. Among the mipDP and mip=DP species observed in our study, m-mipDP and m-mip=DP were selected as representative mipDP and mip=DP species, respectively, because they split into m-i=iPB and m-DiPB, which are the major products of iPB disproportionation and its side reaction, respectively (Table S2, Supporting Information). The relative energy calculation results in Figure 6 and Table 2 reveal that the energy barrier of TS1 to the generation of phenyl-iso-propanylium ion (B) by hydride abstraction of


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iPB molecules (for details, see Figure S9, Supporting Information) is 180 kJ mol-1, which is the highest energy value among all the elementary steps in Scheme 1. This value is also similar to that calculated for m-xylene disproportionation over various zeolite catalysts.33,34 Once species B are formed, they can be converted into the iso-propylated bezenium-type carbenium (C) and iso-propenylated bezenium-type carbenium ions (J) by reacting with species A and I, respectively. Then, species C must overcome two energy barriers (TS3 and TS4) to produce benzene (F) and the (iso-propylphenyl)propanylium ion (G). Of particular interest is species I that can be formed via TS2´ state with a relatively low energy barrier (36 kJ mol-1) compared with that (74 kJ mol-1) of elementary step B to C, as illustrated in Figure 6. It is worth noting that the iPB dehydrogenation cycle, which has been newly identified in this work, is thermodynamically favorable enough to operate by itself, supporting the bimolecular reaction pathway proposed in Scheme 1.

CONCLUSIONS

The mechanisms of iPB disproportionation over four large-pore (H-Y, H-beta, Hmordenite, and H-UZM-35) and three medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites are investigated. While the DiPBs yield of H-UZM-35 containing 12 × 10 × 10-ring channels is quite similar to that of H-beta, known as the best catalyst for this reaction, over the period of time on stream studied here, the GC-MS results from used zeolite catalysts demonstrate the intrazeolitc build-up of mipDP and mip=DP species, whose existence during the iPB disproportionation over any acidic catalyst has not been experimentally observed yet. These two groups of bicyclic aromatic compounds were ascertained to serve as the key



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reaction intermediates of the bimolecular iPB disproportionation and its side reaction (i.e., iPB dehydrogenation) over large-pore zeolites, respectively. To our knowledge, our work is the first to show that the alkenyl group-containing bicyclic aromatic compounds (i.e., mip=DP species) can play a role as reaction intermediates of the zeolite-catalyzed transformation of alkylaromatic hydrocarbons. The formation of both mipDP and mip=DP derivatives in largepore zeolites during iPB disproportionation has been further supported by their strain and relative energy calculation results, and a new bimolecular reaction pathway is proposed based on the overall results obtained.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected]. Fax: +82 54 279 8299

ACKNOWLEDGMENTS

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

Supporting Information


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Structural features of zeolites studied in this work and their catalytic results for iPB disproportionation, strain energies and structures of species involved in the bimolecular iPB disproportionation 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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(20) Shannon, M. D.; Casci, J. L.; Cox, P. A.; Andrews, S. J. Structure of the TwoDimensional Medium-Pore High-Silica Zeolite NU-87. Nature 1991, 353, 417-420. (21) Hong, S. B.; Min, H.-K.; Shin, C.-H.; Cox, P. A. Warrender, S. J.; Wright, P. A. Synthesis, Crystal Structure, Characterization, and Catalytic Properties of TNU-9. J. Am. Chem. Soc. 2007, 129, 10870-10885. (22) Cha, S. H.; Byun, Y.; Min, H.-K.; Hong, S. B. 1,2,4-Trimethylbenzene Disproportionation over Large-Pore Zeolites: An Experimental and Theoretical Study. J. Catal. 2015, 323, 145-157. (23) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347-354. (24) NIST Chemistry Web book; NIST Standard Reference Database 69; National Institute of Standards

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(29) Dorset, D. L.; Weston, S. C.; Dhingra, S. S. Crystal Structure of Zeolite MCM-68: A New Three-Dimensional Framework with Large Pores. J. Phys. Chem. B 2006, 110, 2045-2050. (30) Koyama, Y.; Ikeda, T.; Tatsumi, T.; Kubota, Y. A Multi-Dimensional Microporous Silicate That is Isomorphous to Zeolite MCM-68. Angew. Chem. Int. Ed. 2008, 47, 1042-1046. (31) Weitkamp, J.; Ernst, S. Catalytic Test Reactions for Probing the Pore Width of Large and Super-Large Pore Molecular Sieves. Catal. Today 1994, 19, 107-149. (32) Weiß, U.; Weihe, M.; Hunger, M.; Karge, H. G.; Weitkamp, J. The Induction Period in Ethylbenzene Disproportionation over Large Pore Zeolites. Stud. Surf. Sci. Catal. 1997, 105, 973-980. (33) Clark, L.; Sierka, M.; Sauer, J. Computational Elucidation of the Transition State Shape Selectivity Phenomenon. J. Am. Chem. Soc. 2004, 126, 936-947. (34) Clark, L. A.; Sierka, M.; Sauer, J. Stable Mechanistically-Relevant Aromatic-Based Carbenium Ions in Zeolite Catalysts. J. Am. Chem. Soc. 2003, 125, 2136-2141.



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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 catalyst Si/Al crystal shape and size (µm) total microporous external Brønsted Lewis total depositede (wt%) H-Y 15 polygonal platelets, 0.7 × 0.7 × 0.2 800 690 110 113 18 131 16.1 H-beta 13 grains, 0.1 540 360 180 116 59 175 13.1 H-mordenite 10 rods, 0.3 × 1.0 620 570 50 124 14 138 15.8 H-UZM-35 9 platelets, 0.2 × 0.05 470 380 90 154 38 192 12.5 H-NU-87 25 rods, 0.3 × 1.0 460 420 40 109 19 128 12.3 H-TNU-9 18 rods, 0.3 × 1.0 530 480 50 143 16 159 12.2 H-ZSM-5 14 rods, 0.3 × 1.0 390 330 60 158 17 175 11.5 a b c d Determined by elemental analysis. Determined by SEM. Calculated from N2 adsorption data. 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 given by Emeis.32 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 iPB disproportionation at 523 K and 7.8 h-1 WHSV for 10 h on stream. a

b


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Table 2. Relative energies of the reactant, reaction intermediates, transition states, and products of the main and side reactions in Scheme 1 that can be formed during the bimolecular iPB disproportionation over the 84T H-Y model. main reaction speciesa,b energy (kJ mol-1) five iPB molecules in H-Y 0 TS1 180 108 B TS2 182 110 C TS3 157 101 D TS4 225 196 E 108 G TS2´ 144 124 I a

side reaction speciesa,c energy (kJ mol-1) five iPB molecules in H-Y 0 TS1 180 108 B TS5 162 129 J TS6 178 111 K TS7 152 118 L 106 M

The same as the species given in Scheme 1. b D is m-mipDP. c K is m-mip=DP.



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FIGURE CAPTIONS

Figure 1. iPB conversion and DiPBs yield as a function of time on stream in iPB disproportionation over 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 with different framework topologies at 523 K and 7.8 h-1 WHSV. DiPBs indicates di-isopropylbenzene isomers.

Figure 2. iPB conversion in iPB disproportionation at 15.5 h-1 WHSV over seven zeolites with different framework topologies (H-Y, H-beta, H-mordenite, H-UZM-35, HNU-87, H-TNU-9, and H-ZSM-5, from left to right) at 403 (■), 423 (●), 443 (▲), 473 (▼), 493 (◀), and 523 (▶) K. The DiPBs yield/benzene yield ratios at 443 K are given below each of the iPB conversion data.

Figure 3. GC-MS total ion chromatograms of the CH2Cl2 extracts from (a) H-Y, (b) Hbeta, (c) H-mordenite, and (d) H-UZM-35 after iPB disproportionation at 403 K and 15.5 h-1 WHSV for 10 h on stream. The structures annotated above the chromatogram from H-Y are peak identifications made by comparing the mass spectra with those in the NIST database.24

Figure 4. GC-MS ion chromatograms of the CH2Cl2 extracts from H-Y after iPB disproportionation at 403 K and 15.5 h-1 WHSV (a) for 10 h on stream followed by flushing with N2 (40 mL min-1) for 0, 5, 10, 20, 40, 80, 160, 240, 360, and 400 min (from rear to front) and (b) for 5, 10, 20, 40, 80, 160, 300, 600, 720, 900, and 1200 min (from front to rear) on stream.


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Figure 5. Strain energies of three mipDP (top) and three mip=DP (bottom) isomers that can be formed during the bimolecular iPB disproportionation over the 84T H-Y, 168T Hbeta, 68T H-mordenite, 112T H-UZM-35, 62T H-NU-87, 64T H-TNU-9, and 72T HZSM-5 models.

Figure 6. Relative energy level diagram for the reactant, reaction intermediates, and transition states involved in the formation of m-mipDP (left) and m-mip=DP (right) species during the bimolecular iPB disproportionation over the 84T H-Y model and its side reaction, respectively (Scheme 1).



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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 iPB disproportionation proposed based on the overall GC-MS results of this work.


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