Journal of Molecular Liquids 214 (2016) 1–6
Contents lists available at ScienceDirect
Journal of Molecular Liquids
(Mg–Zn)–Al layered double hydroxide as a regenerable catalyst for the
catalytic glycolysis of polyethylene terephthalate
,1
⁎
Gh. Eshaq, A.E. ElMetwally
Department of Petrochemicals, Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
(Mg–Zn)–Al layered double hydroxide (LDH) was prepared by the coprecipitation method at low super satura-
tion conditions. The prepared (Mg–Zn)–Al LDH was analyzed using XRD, FTIR, N2 adsorption–desorption, TGA
and DSC, confirming the formation of pure LDH phase. The extent of polyethylene terephthalate (PET) degrada-
tion by the glycolysis process was studied using the prepared (Mg–Zn)–Al LDH. The glycolysis process was opti-
mized in terms of catalyst concentration, temperature, time, ethylene glycol dosage. Under the optimal
conditions of 1.0 wt.% of catalyst with 20 g of ethylene glycol (EG) in the presence of 2.0 g of PET at 196 °C
after 3 h of glycolysis, complete PET conversion was achieved and the yield of bis (2-hydroxyethyl) terephthalate
(BHET) reached 75%.
Received 4 August 2015
Received in revised form 14 October 2015
Accepted 27 November 2015
Available online xxxx
Keywords:
Degradation
Glycolysis
© 2015 Elsevier B.V. All rights reserved.
Catalysis
Layered double hydroxide
Polyethylene terephthalate
1. Introduction
the kinetics show that without catalysts, PET glycolysis is very slow,
and complete conversion of PET to monomer BHET is almost impossible
Poly(ethylene terephthalate), more commonly known as PET, is an
indispensable material due to its excellent physical and chemical prop-
erties. It is an important material in the textile industry, and in food
packaging where it has noticeably become the choice for beverage con-
tainers [1]. The escalating production of PET today is, however, still lead-
ing to a global concern over the treatment of PET waste products and
their destructive impact to the environment [2]. PET does not create a
direct hazard to the environment, but due to its substantial fraction by
volume in the wastes stream and because it is a non-degradable mate-
rial in normal conditions, it is seen as a noxious material. Commonly
practiced mechanical recycling has been considered as a temporary so-
lution because the recycled PET, which is often used for secondary ma-
terials other than beverage bottles, ends up in landfills [3,4]. Great
attention is currently being paid to chemical recycling, which basically
involves the recovery of monomers and produces interesting value-
added chemicals or intermediates from the PET waste [5]. Chemical
recycling processes for PET are divided as follows: (i) glycolysis, (ii)
methanolysis, (iii) hydrolysis, and (iv) other processes such as
aminolysis or ammonolysis. Glycolysis can be described as a molecular
depolymerization process by transesterification between PET ester
groups and a diol, usually ethylene glycol (EG) in excess, to obtain the
monomer bis(2-hydroxyethyl) terephthalate (BHET) [6–8]. Studies on
[9,10]. Metal acetates were the first reported catalysts for PET glycolysis
[11–14]. Later on, researchers developed more environment-friendly al-
ternatives like mild alkalies, sulfates, metal chlorides, and zeolites; how-
ever, PET glycolysis using these catalysts still required long reaction
times, and gave low BHET monomer yields [15–19]. Wang et al. [20,
21] have reported that various ionic liquid catalysts for PET glycolysis
could be recovered, then used repeatedly; however, the conversion of
PET and selectivity toward BHET were very low, and the process
was very slow. They also studied the glycolysis of PET using an Fe-
containing magnetic ionic liquid, which exhibited higher catalytic activ-
ity than conventional ILs. However, the monomer produced was
very easily stained by the Fe-containing ILs [22]. Recently, Al-Sabagh
et al. [23] reported that 1-butyl-3-methylimidazolium acetate
([Bmim][OAc]) exhibited excellent catalytic activity and reusability
when used as a catalyst in the glycolysis of PET, compared with 1-
butyl-3-methylimidazolium chloride ([Bmim][Cl]). They also reported
that Cu- and Zn-acetate-containing ionic liquids could act as efficient
catalysts in PET glycolysis [24]. The catalytic activity with these homo-
geneous catalysts is high. However, they cannot be easily separated
from the reaction mixture. More recently, three series of solid catalysts
including SO24−/ZnO, SO42−/TiO2 and SO42−/ZnO-TiO2 have been tested
in PET glycolysis. The SO24−/ZnO-TiO2 exhibited a catalytic activity
with 100% conversion of PET and 72% selectivity of BHET after 3 h at
180 °C under atmospheric pressure [25]. Imran et al. reported that the
mixed-oxide spinels exhibited better catalytic performance than the
single metal oxides in PET glycolysis at 260 °C and 5.0 atm [26].
Bartolome et al. studied the glycolysis of PET using easily recoverable
⁎
Corresponding author.
Permanent address: Department of Petrochemicals, Egyptian Petroleum Research
1
Institute, Nasr City, Cairo, Egypt.
0167-7322/© 2015 Elsevier B.V. All rights reserved.