(13f) Effect of the Pore Structure of Zeolite On Pyrolytic Behavior of Low-Density Polyethylene(LDPE)

Effect of the
pore structure of zeolite on pyrolytic behavior

of low-density
polyethylene(LDPE)

                  
Li Yang*, Chen Wang, Jialin Cai, Daoni Pan, Qinwei Gao

Jiangsu Key Lab of
Biomass-Based Green Fuels and Chemicals,

                 Nanjing 210037, China

College of Chemical Engineering, Nanjing Forestry University,

                  Nanjing 210037, China

 

^* Corresponding author
at: College of Chemical Engineering, Nanjing Forestry

University, Nanjing 210037,
China. Tel.: +86-25-85427635; fax: +86-25-85418873.

E-mail address: haxia2010@gmail.com (Li Yang, Assistant Professor).

Abstract

 
Polyethylene is by far the world's most used plastic, which brings huge environment
problem as waste. Among the various plastic waste recycling methods, feedstock
recycling is becoming a alternative for recovering valuable gasoline or
diesel-range hydrocarbons. Pyrolysis of polyolefins is a good method, which can
produce paraffins, olefins, and aromatics [1-10]. Catalytic pyrolysis of
polymer wastes has been widely studied to improve gasoline yield. During
catalytic pyrolysis, the solid polymer melts and disperses into the catalysts.
Surface reaction produces very complex compounds. As a result of this complex
process, product distributions reflect characteristics of the catalysts related
to their pore systems and chemical compositions. In general, the carbon atom
number distribution of products obtained from catalytic pyrolysis is narrower
in comparison with the distribution produced by non-catalytic pyrolysis.

  
In this work, HZSM-5, H-beta, and H-Y zeolite catalysts with varying pore size
was used to catalytically pyrolyze LDPE. The effect of type of zeolite,
reaction temperature, the mass ratio of catalyst to LDPE, and amount of
Brönsted acid sites on the product yields and the component of liquid was also
investigated. The composition of liquid fractions, variations with the carbon
number and compound type, and the yields of the most important compounds were
analyzed by GC-MS and NMR. The pyrolysis process was also investigated by
TG-DTG technique and the zeolites were characterized by IR spectra of pyridine
adsorption and NH₃-TPD.

Tab.1 shows the yield of gas,
liquid, and coke produced from catalytic pyrolysis of LDPE using H-ZSM-5,
H-beta, and H-Y as catalysts. As can be seen, H-beta catalyst gave a higher
yield of liquid, but three catalysts produced a little coke. It should be noted
that non-catalytic pyrolysis of LDPE only produced wax (the yield > 90%).
Zeolite catalysts such as H-ZSM-5 has Brönsted acid sites and micro-pores,
promoting the cracking of C-C of LDPE into small molecular.

Table
1 The
yield of gas, liquid, and coke produced from catalytic pryolysis of LDPE ^a

Yield	H-beta + LDPE	H-Y + LDPE	H-ZSM-5 + LDPE
Gas	42.5	65	61
Liquids	55	30	35
coke	2.5	5	4

a-The
reaction condition: the mass ratio of catalyst to LDPE is 1:4; the reaction was
carried out in N₂ flow in a quartz fix-bed reactor; The yield s of
liquid and coke were determined by weight, while the gas yield was determined
by means of an overall mass balance.

  
Table 2 presents the relative percentages of olefins, paraffin, and aromatics
of liquid obtained from catalytic pyrolysis of LDPE. It can be seen that three
liquid products all contain olefins, paraffins, and aromatics. However, H-ZSM-5
catalyst gets the highest yield of aromatics with the content being 61.47%. One
of the possible interpretations is that H-ZSM-5 has better shape selectivity
and higher aromatization performance than those of H-Y and H-beta. Fig.3
depicts that the distribution of the carbon number of aromatics in the liquid
product. It can be seen that the carbon number of the aromatics is mainly range
from 6 to 11 due to the shape selectivity of micropore zeolite. The aromatic
components include toluene, o-, m-, p-xylene, ethylbenzene, tri-methyl benzene,
etc. It should be noted that the pyrolysis of LDPE catalyzed by three catalysts
produced a little naphthalene. H-ZSM-5 got the highest xylene yield compared
with two other catalysts due to the pore size of HZSM-5 favoring the formation
of xylene. 

Table
2 The
relative percentages of olefins, paraffin, and aromatics of liquid produced
from catalytic pyrolysis of LDPE^a

Contents	H-beta + LDPE	H-Y + LDPE	H-ZSM-5 + LDPE
olefins	32.87	35.96	12.04
paraffin	39.67	21.48	26.49
aromatics	27.46	42.56	61.47

a-the
liquid components were performed by Agilent GC-MS 7890A-5975C with a quadrupole
detector and a HP-5MS capillary column (30 m °Á 0.25
mm i.d., 0.25 mm film thickness).

Figure 1 The distribution of
the carbon number of aromatics in the liquid obtained from catalytic pyrolysis
of LDPE on H-beta, H-Y, and H-ZSM-5.

In summary, these results show that zeolite
catalyst could effectively promote the degradation of LDPE to valuable aromatic
compounds. H-ZSM-5 got the high aromatic yield of 61.47% and the high xylene
yield of 26% because its pore size and structure favors the formation of
aromatics.

References

[1]    M.S.
Renzini, U. Sedran, L.B. Pierella, H-ZSM-11 and Zn-ZSM-11 zeolites and their
applications in the catalytic transformation of LDPE, Journal of Analytical and
Applied Pyrolysis, 2009, 86:215-20.

[2]    G.
Elordi, M. Olazar, G. Lopez, M. Amutio, M. Artetxe, R. Aguado, J. Bilbao, Catalytic
pyrolysis of HDPE in continuous mode over zeolite catalysts in a conical
spouted bed reactor, Journal of Analytical and Applied Pyrolysis, 2009,
85:345-51.

[3]    A.
Marcilla, M.I. Beltran, R. Navarro, Thermal and catalytic pyrolysis of
polyethylene over HZSM5 and HUSY zeolites in a batch reactor under dynamic
conditions, Applied Catalysis B-Environmental, 2009, 86:78-86.

[4]    G.
Elordi, G. Lopez, R. Aguado, M. Olazar, J. Bilbao, Catalytic pyrolysis of high
density polyethylene on a HZSM-5 zeolite catalyst in a conical spouted bed
reactor, International Journal of Chemical Reactor Engineering, 2007, 5.

[5]    A.
Marcilla, A. Gomez-Siurana, F. Valdes, Catalytic pyrolysis of LDPE over H-beta
and HZSM-5 zeolites in dynamic conditions - Study of the evolution of the
process, Journal of Analytical and Applied Pyrolysis, 2007, 79:433-42.

[6]    A.
Marcilla, A. Gomez-Siurana, F. Valdes, Catalytic cracking of low-density
polyethylene over H-Beta and HZSM-5 zeolites: Influence of the external
surface. Kinetic model, Polymer Degradation and Stability, 2007, 92:197-204.

[7]    I.
Torre, J.M. Arandes, P. Castano, M. Azkoiti, J. Bilbao, H. Ignacio de Lasa, Catalytic
cracking of plastic pyrolysis waxes with vacuum gasoil: Effect of HZSM-5
zeolite in the FCC catalyst, International Journal of Chemical Reactor
Engineering, 2006, 4.

[8]    A.
Durmus, S.N. Koc, G.S. Pozan, A. Kasgoz, Thermal-catalytic degradation kinetics
of polypropylene over BEA, ZSM-5 and MOR zeolites, Applied Catalysis
B-Environmental, 2005, 61:316-22.

[9]    G.
Elordi, M. Olazar, M. Artetxe, P. Castano, J. Bilbao, Effect of the acidity of
the HZSM-5 zeolite catalyst on the cracking of high density polyethylene in a
conical spouted bed reactor, Applied Catalysis a-General, 415:89-95.

[10]  A.
Coelho, L. Costa, M.M. Marques, I.M. Fonseca, M.A.N.D.A. Lemos, F. Lemos, The
effect of ZSM-5 zeolite acidity on the catalytic degradation of high-density
polyethylene using simultaneous DSC/TG analysis, Applied Catalysis a-General,
413:183-91.