Mechanism and Kinetic of Coke Oxidation by Nonthermal Plasma in

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mechanism and Kinetic of Coke Oxidation by Nonthermal Plasma in Fixed Bed Dielectric Barrier Reactor Amir Astafan, Catherine Batiot-Dupeyrat, and ludovic Pinard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00743 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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The Journal of Physical Chemistry

Mechanism and Kinetic of Coke Oxidation by Nonthermal Plasma in FixedBed Dielectric Barrier Reactor A. Astafan, C. Batiot Dupeyrat, L. Pinard*

Institut de Chimie des Milieux et Matériaux de Poitiers (ICM2P), UMR 7285 CNRS, 4 Rue Michel Brunet, Bâtiment B27, 86073 Poitiers Cedex – France.

Corresponding author: [email protected]

Abstract The formation of coke resulting from propene transformation at 623 K on a FAU zeolite occurs according to a product shape selectivity mechanism and yields to the formation of highly alkylated polyaromatic molecules such as naphthalene, pyrene and coronene. The main part of them are trapped in the inner cavities (supercages), poison Brønsted acid sites, and plug micropores. With a common thermal regeneration process, coke burns at 800 K, while it is shown in this study that a complete regeneration of zeolite (i.e. total recovery of the native acidity and microporosity) can be achieved at 293 K, by using a nonthermal plasma, with a low energy consumption, in a fixed bed dielectric barrier reactor: a geometry suitable for an industrial scaling. The kinetic rates of coke oxidation and the recovery of acidity and microporosity are similar. The active species (e.g. O*, O2+) are able to diffuse within the zeolite micropores and oxidize the light molecules, 36 times faster than the heavier ones. Thanks to a complete characterization of both the regenerated catalyst and the remaining coke molecules, a reaction scheme is proposed. We claim that catalyst regeneration assisted by nonthermal plasma is a real alternative to the thermal combustion.

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Introduction Zeolites are used in numerous commercial catalytic processes for hydrocarbon conversion such as cracking, transalkylation, disproportionation, isomerization, etc, but these acidic catalysts lose activity and/or selectivity, through various deactivation mechanisms, the major being the coking1. As catalyst ages with increasing time-on-stream, therefore more severe operating conditions, such as higher temperatures and/or lower through-puts, are usually required to maintain activity and/or selectivity. When the maximum reactor temperature and/or maximal contact time is reached, the catalyst needs to be replaced, rejuvenated or regenerated in and/or ex situ. The spent catalyst, at the end of its useful life, may contain a significant amount of coke, exceeding up to 20 wt.% and sometimes even more than 40 wt.%. Moreover, at elevated process temperatures (> 623 K), the coke formed is of very complex nature, ranging from 2 up to more 15 aromatic rings2-5, hence more difficult to eliminate6-7. The most common regeneration process is to burn the coke at elevated temperature in an oxidative environment (air or oxygen), but severe and irreversible damage can occur such as zeolite dealumination. Irreversible modifications are due to the high temperatures (Tox), that implies a more frequent catalyst replacement, increasing its cost substantially. Tox can be decreased by increasing the oxygen content during the process1, adding few ppm of metal such as Pt to catalyze the oxidation reaction1 and tuning the textural properties of the zeolite.3,4. There is therefore a need to propose a method to regenerate at moderate temperature deactivated catalysts in order to lengthen time on stream, or cycle length. The off-site regeneration of many catalysts is now accepted and promoted by industry due to safety and time considerations, and in particular better recovery of catalyst activity. The off-site regeneration allows to develop alternative and novel processes. Among alternative techniques to overcome the difficulties associated with thermal regeneration, nonthermal plasma can be considered as a promising technology since it enables the activation of gas molecules by electron impact at room temperature8-10. Recently, we reported that it was possible to obtain a complete elimination of coke from a ZSM-5 zeolite, when the material was shaped as a thin wafer and placed in a Dielectric Barrier Discharge (DBD) plasma reactor with a pin to plate geometry11-12. Yet, this reactor geometry is unsuitable for applications at industrial scale,. In this study, the regeneration of coked zeolite (FAU) was carried out in a coaxial DBD plasma reactor. This geometry is potentially suited for industrial up-scale as demonstrated by the industrial ozone generator13, and which further allows to establish the kinetics and mechanisms of zeolite regeneration. 2 ACS Paragon Plus Environment

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1. Experimental part Fresh and coked FAU zeolites. The material used is a commercial FAU zeolite (CBV 720, Zeolyst) with a Si/Al molar ratio of 17. The coking of the protonic zeolite was carried out in a fixed bed reactor at 623 K under 1 bar of propene for 6 h by setting a gas hourly space velocity (GHSV) of 67000 h-1 . Before introduction of the coking agent, the zeolite was pressed, crushed and sieved between 0.2 and 0.4 mm then pretreated under nitrogen at 723 K for 12 h. The temperature of coke combustion was measured with a SDT Q600 TA thermogravimetric analyser under a 100 mL/min flow of air up to 1173 K. The amount of carbon in the spent catalyst, was measured using a C.E. Instruments NA2100 PROTEIN elementary analyzer. Textural properties were determined by sorption measurements of nitrogen at 77 K, on a Micromeritics ASAP 2000 gas adsorption analyzer. Prior to analysis, the samples were pretreated at 573 K under vacuum for 15 h. Coked samples were outgassed at 423 K for 1 h to avoid coke elimination. The microporous volumes (Vmicro) were calculated using the t-plot method. The total pore volume was calculated at p/p0 = 0.9. The mesopore volume (Vmeso) were determined by the difference between the total pore volume and the micropore volume. Fourier transform Infrared spectra (FT-IR) of pyridine adsorbed samples were recorded on a Nicolet Magna 550-FT-IR spectrometer with a 2 cmL9 optical resolution. The zeolites were first pressed into self-supporting wafers (diameter: 1.6 cm, M 20 mg) and pretreated from room temperature to 723 K (heating rate of 1.5 K min-1 for 5 h under a pressure of 1.33 10L< Pa) in an IR cell connected to a vacuum line (coked samples were outgassed at 363 K for 1 h and 423 K for 1 h to avoid coke elimination). Pyridine adsorption was carried out at 423 K. After establishing a pressure of 133 Pa at equilibrium, the cell was evacuated at 623 K to remove all physisorbed species. The amount of pyridine adsorbed on the Brønsted and Lewis sites was determined by integrating the band areas at respectively 1545 cm-1 and 1454 cmL9 and using the following extinction coefficients measured at 293 K:

1545

= 1.13 and

1454

= 1.28 cm

molL9.14 Coke composition: The coke present on the zeolite external surface (Cexternal) was extracted by washing the spent catalyst with methylene chloride (CH2Cl2) at 313 K under 10.0 MPa nitrogen pressure during 10 min on a Dionex ASE 350 apparatus. The coke molecules trapped in the zeolite pores were recovered after digestion of the spent catalyst using a concentrated hydrofluoric acid solution (51 vol. %) at room temperature during 20 min2. The low temperature of HF treatment, the short contact time with the acid solution and the coke components as well 3 ACS Paragon Plus Environment


as the very small contact area between the mineral and organic phases allows to avoid any change of the coke composition. One fraction of coke was soluble in CH2Cl2 (Csoluble), the other one insoluble (Cinsoluble) which consists of black particles that can be totally collected by simple filtration. The soluble coke fraction obtained after CH2Cl2 evaporation was analyzed and quantified by GC–MS (Thermoelectron DSQ) and GC-FID (Agilent), respectively. The insoluble coke fraction was further characterized by MALDI-TOF MS on a Brüker Autoflex Speed mass spectrometer in a reflectron positive mode where ions were generated by a 337 nm wavelength nitrogen laser. Samples preparation, analysis and calibration methods were performed by following the same methodology described in ref.2. Fixed bed dielectric barrier reactor: The reactor is a cylindrical fixed bed reactor consisting of two co-axial electrodes separated by a Pyrex tube (i.d. = 9.5 mm, thickness = 0.5 mm) as dielectric barrier (Fig. 1). The outer electrode is a copper sheet of 20 mm length wrapped around the glass tube, and the inner electrode is an inox rod (diameter = 1.3 mm). The electrodes are connected to a high voltage (HV) bi-polar pulse generator (A2E-Enertronic).

Gas inlet

1.3 inner electrode,

Spent catalyst ( 0.2 550 K. The main difference between a thermal oxidation of coke and that assisted by NTP is the absence of a condensation reaction of intermediates owing to the low temperature, avoiding the formation of insoluble coke. Scheme 2 provides an apparent reaction path of coke oxidation by NTP, and table 2 summarizes all the kinetic rates measured in this contribution. Contrariwise to the thermal regeneration7, the coke oxidation by NTP depends on the coke composition.



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CO2

CO2

Regenerated Zeolite

Coke Intermediate products

Poisoning + partial pore blocking

Intermediate products

time NTP : 12 W at 293 K

Scheme 2. Apparent oxidation path of coke Conclusion The regeneration of a coked HFAU zeolite was investigated using a DBD plasma reactor. We show in this study that a fixed bed dielectric barrier reactor was effective to remove high coke content at room temperature with a deposited power as low as 12 W. The coke oxidation proceeds step by step, i.e. it begins by oxidize totally the lighter molecules, then partially the more complex ones creating some carboxylic groups which transform and desorb into volatile organic compounds. Regardless of both the location and the nature of coke, the oxygenated species generated by the non-thermal plasma diffuse within the zeolite micropores and completely regenerate the zeolite, recovering both the native acidity and microporosity of the catalyst. A nonthermal plasma carried out in a fixed bed reactor is an alternative to the common regeneration by air combustion.

Supporting information: MALDI TOF spectrum of insoluble coke in dichloromethane, thermal analysis of coked zeolite, images of sodalite cages and inner cavities in the FAU zeolite framework.


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Acknowledgements: Amir Astafan, thanks the Programme PAUSE and the University of Poitiers for their financial supports. The authors acknowledge financial support from the European Union (ERDF) and "Région Nouvelle Aquitaine". Reference (1)

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Mechanism and Kinetic of Coke Oxidation by Nonthermal Plasma in

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