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J. Phys. Chem. C 2010, 114, 1190–1193
Diethylated Diphenylethane Species: Main Reaction Intermediates of Ethylbenzene Disproportionation over Large-Pore Zeolites Hyung-Ki Min, Velusamy Chidambaram, and Suk Bong Hong* Department of Chemical Engineering and School of EnVironmental Science and Engineering POSTECH, Pohang 790-784, Korea ReceiVed: October 1, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009
GC/MS evidence for the build-up of diethylated diphenylethane species, as well as of monoethylated diphenylethane derivatives, inside the LaNa-Y cavities during ethylbenzene disproportionation is presented. The role of the former species as reaction intermediates was found to become greater than that of the latter ones with increasing time on stream. A new dual-cycle mechanism for diethylbenzene formation over largepore zeolites is proposed based on the overall results presented here. Introduction The use of zeolites as catalysts is now well established in the petroleum and petrochemical industries. However, understanding the mechanisms in zeolite-catalyzed reactions remains an issue of intense study, not only because of the practical relevance for improving current processes and facilitating entirely new ones, but also because of the fundamental interest in refining modern theories of shape-selectivity in heterogeneous catalysis.1 The observation and identification of reaction intermediates on working zeolite catalysts is, of course, prerequisite for more explicitly addressing this complicated issue. Disproportionation of aromatic hydrocarbons is among the most important petrochemical processes. Ethylbenzene (EB) disproportionation to benzene (B) and the three diethylbenzene (DEB) isomers is of particular importance, because p-DEB is a highly value-added species used as a desorbent for the recovery of p-xylene in UOP Parex and IFP Eluxyl processes.2 This reaction can also distinguish between medium- and large-pore zeolites,3-6 most notably based on the occurrence of an induction period, which has led the Catalysis Commission of the International Zeolite Association to recommend it as a test reaction for the characterization of zeolite acidity.7 About 50 years or longer ago two major types of reaction pathways were proposed for the acid-catalyzed EB disproportionation: (i) the monomolecular ethyl-transfer reaction mechanism and (ii) the bimolecular diphenylethane-mediated reaction mechanism.8-10 Until a very recent date, however, no attempts to observe its reaction intermediates have been successful. In 2008, Hunger and co-workers presented in situ 13C MAS NMR evidence for the generation of monoethylated diphenylethane (mEDPE) species in large-pore zeolites AlNa-X and AlNa-Y during EB disproportionation,11 allowing them to support the bimolecular reaction pathway given in Supporting Information Scheme S1. Large organic compounds formed via hydrocarbon conversions usually end up entrapped inside the void spaces of the acid zeolite catalysts employed. Hence, the careful characterization of such species frequently provides valuable insights into both reaction and deactivation mechanisms of heterogeneous catalysis.12,13 Here we report on the observation of diethylated diphenylethane (dEDPE) derivatives, as well as of mEDPE * To whom correspondence should be addressed. Phone: (+82) 54-2798277. Fax: (+82) 54-279-8299. E-mail:
[email protected].
derivatives, in the EB disproportionation over LaNa-Y by gas chromatography/mass spectrometry (GC/MS). We have also been able to clarify the roles of these two different types of bulky aromatic species during the reaction, which provides unprecedented insights into the DEB formation over large-pore zeolites. Experimental Section A Na-Y zeolite (Si/Al ) 2.6, Tosoh) was converted into La3+exchanged form (i.e., LaNa-Y with a exchange degree of 78%, according to ICP analysis). Powder X-ray diffraction and 29Si MAS NMR measurements revealed no signs of dealumination during the La3+ ion exchange step. By IR spectroscopy after pyridine adsorption at room temperature followed by desorption at 300 °C for 2 h, the concentration of Brønsted acid sites in our LaNa-Y was determined to be 0.32 mmol g-1, with no detectable Lewis acid sites. The EB disproportionation over LaNa-Y was performed under atmospheric pressure in a fixed-bed flow-type apparatus at 130-200 °C, following a protocol recommended by the IZA Catalysis Commission.7 Prior to the catalytic experiments, the catalyst was activated under flowing N2 (50 cm3 min-1) at 250 °C for 12 h and kept at the desired reaction temperature. EB (99.8%, Aldrich) with a partial pressure of 12.2 kPa in N2 was first purified by passing through a column filled with alumina previously activated at 450 °C, to remove oxygenated aromatic impurities, and then fed into a microreactor containing LaNa-Y catalyst at 5.2 h-1 WHSV. The reaction products were analyzed online in a Varian CP-3800 gas chromatograph equipped with a CP-Chirasil-Dex CB capillary column (25 m × 0.25 mm) and a flame ionization detector. Flushing experiments, in which the EB-containing stream was replaced by a pure N2 feed at 130 °C, were also performed as a function of time at 150 °C. Prior to these experiments, the catalyst was reacted with EB at 130 °C for 30 h, cooled 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 cm3 min-1) at 150 °C for times ranging from 0 to 720 min. The flushed catalyst was subjected to exactly the same HF dissolution procedures given below, to follow the evolution of the organic compounds accumulated within the zeolite pores with increasing flushing time. When necessary, EB was again reacted over the flushed catalyst at 150 °C and 31.2 h-1 WHSV.
10.1021/jp9094408 2010 American Chemical Society Published on Web 12/07/2009
Ethylbenzene Disproportionation over Large-Pore Zeolites
J. Phys. Chem. C, Vol. 114, No. 2, 2010 1191
Figure 2. GC/MS total ion chromatograms of the CCl4 extracts of LaNa-Y after EB disproportionation at 130 °C and 5.2 h-1 WHSV for 30 h followed by flushing with N2 (40 cm3 min-1) at 150 °C for (from rear to front) 0, 5, 10, 20, 40, 80, 160, 240, 360, 540, and 720 min.
Figure 1. GC/MS total ion chromatogram of the CCl4 extract of LaNa-Y after EB disproportionation at 130 °C and 5.2 h-1 WHSV for 30 h, showing the assignment of each organic compound. The asterisk represents the mass signal of C2Cl6 produced by pyrolysis of CCl4.
GC/MS analysis was carried out by a modification of the procedure originally developed by Guisnet group.12 In a typical analysis, 50 mg of the used catalyst were completely dissolved in 3 mL of 10% HF solutions and neutralized with K2CO3. CCl4 (Aldrich, 99.5%) was employed to extract the organic species from the resulting solutions. Water in the organic phase was removed by adding a small amount of Na2SO4 that was subsequently recovered using an Advantec DISMIC-13JP syringe filter. The GC/MS total ion chromatograms of extracted organic phases were recorded on a Varian CP 3800 gas chromatograph equipped with a Varian 320-MSD mass selective detector, using electron impact ionization at 70 eV. The split ratio was 30:1, and the column used was a VF-5 capillary column (30 m × 0.25 mm) with flowing He (0.3 cm3 min-1). The temperature program ramped the column from 70 to 280 °C at a rate of 4 °C min-1. Results and Discussion With LaNa-Y at 130-200 °C and 5.2 h-1 WHSV, an induction period in EB conversion, whose duration becomes shorter with increasing reaction temperature, is observed (Supporting InformationFigure S1). Also, there is a significant DEB deficit during the induction period, due to the formation of polyalkylated aromatics that stay adsorbed inside the zeolite cavities, characteristic of large-pore zeolites.3-7 Figure 1 shows the GC/MS total ion chromatogram of the CCl4 extract of LaNa-Y after EB disproportionation at 130 °C and 5.2 h-1 WHSV for 30 h on stream, where EB conversions around 3% in steady state were achieved. The two weak peaks 1a,b have mass spectra and retention times corresponding to m- and p-DEB isomers, respectively. Although hydrocarbons
