Regeneration of Coked Zeolite from PMMA Cracking Process by Ozonation

.สุภาภรณ์ คางคำ


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Applied Catalysis B: Environmental

jo ur nal home p ag e: www.elsevier.com/locate/apcatb

Regeneration of coked zeolite from PMMA cracking process by ozonation

Supaporn Khangkhama,c, Carine Julcour-Lebiguea,b,, Somsak Damronglerdc,
Chawalit Ngamcharussrivichaic, Marie-Hélène Maneroa, Henri Delmasa

a Université de Toulouse, Laboratoire de Génie Chimique, 4 allée Emile Monso – BP 84234, 31030 Toulouse Cedex 4, France
b CNRS, Laboratoire de Génie Chimique, 31030 Toulouse, France
c Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand


A R T I C L E   I N F O


Article history:
Received 17 January 2013
Received in revised form 15 April 2013
Accepted 17 April 2013
Available online 24 April 2013


Keywords:
Coke
Ozone
ZSM-5
Acid sites P
lastic wastes

A B S T A C T


Regeneration of coked ZSM-5 zeolite was performed by oxidation with ozone at low temperature range (<150 C) so that to restore catalytic activity. Physicochemical properties of the samples were characterized by several techniques: thermogravimetry (nature of coke deposit), elemental analysis (carbon content), porosimetry (surface area and pore size), ammonia temperature-programmed desorption and pyridine adsorption followed by infrared spectroscopy (acidity). Reactions were carried out at various temperatures, gas hourly space velocities and inlet concentrations of ozone. They showed that partially coked samples (containing 3 wt.% of C) can be successfully regenerated by ozone with carbon removal up to 80%.

   Carbon removal is improved by increasing the inlet ozone concentration in the range 16–50 g/m3, with almost linear trend, and by increasing time on stream until it plateaus after 2 h. Coke oxidation with O3 starts at low temperature and exhibits an optimum at about 100 C. At higher temperatures, the rate of ozone decomposition becomes much faster than its pore diffusion rate, so that radical species are no longer available for the coke deposit within the particles and the overall oxidation yield decreases. Indeed, catalytic decomposition of ozone is found to occur significantly above 100 C: O3 decomposition reaches 90% with fresh ZSM-5 catalyst. Thus regeneration of coked zeolite particles involves both complex chemical reactions (coke oxidation and O3 decomposition to active but unstable species) and transport processes (pore diffusion to the internal coked surface).
Ozonation can restore both textural and acidic properties, allowing the catalyst to almost recover its initial activity in poly(methyl metacrylate) cracking. The activity results are well correlated with the carbon removal efficiency.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

       Zeolites are used in many industrial and petrochemical processes because of their unique properties of molecular sieving, acidity, high thermal stability, and shape selectivity [1]. For example, FAU-type zeolite, e.g. Y and Ultrastable Y zeolite, is extensively applied as catalyst in fluid catalytic cracking (FCC). In this process, MFI-type zeolite, e.g. ZSM-5, is frequently used as FCC catalyst additive either in the form of separate particles or within a composite to control product yield and/or to improve octane number [2]. Moreover, zeolites, and ZSM-5 in particular, are also applied
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∗ Corresponding author at: Université de Toulouse, Laboratoire de Génie Chimique, 4 allée Emile Monso – BP 84234, 31030 Toulouse Cedex 4, France. Tel.: +33 534323709; fax: +33 534323697. E-mail address: [email protected] (C. Julcour-Lebigue).

0926-3373/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2013.04.041

in recycling of plastic wastes to provide more valuable products: monomer and/or gas and liquid which can be reutilized as chemical reagents or fuels [3,4]. Previous results showed that MMA recovery by cracking of PMMA was successfully achieved on a zeolite fixed bed at temperatures below 300 C [5]. Such temperature is of significant advantage as compared to conventional thermal processes (400–450 C), as proposed by Kaminsky et al. [6–8] using fluidized bed reactors or more recently by Lopez et al. [9] using a spouted bed reactor.

Even if zeolite catalysts produce less coke than other materials, it is still a common feature which requires efficient and low-cost regeneration techniques for the process economy. Deposition of carbonaceous residues blocks the access of pores to the reacting molecules and/or poisons active acid catalytic sites [10–15]. Besides the loss of activity and/or selectivity, coking can also reduce heat transfer in the reactor, while increasing pressure drop and possibly even plugging the reactor.