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Degradation of Low-Density Polyethylene over Modified Zeolites.

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Dev. Chem. Eng. Mineral Process. 14(1/2)*pp. 203-218, 2006.
Degradation of Low-Density Polyethylene
over Modified Zeolites
P. Tu, K. Pratt, E. Kosior' and F. Malherbe"
Environment and Biotechnology Centre, Faculty of Life and Social
Sciences, Swinburne University of Technology, PO Box 21 8, Hawthorn,
Victoria 3122, Australia
Visy Technical Centre, 2 Davis Road, Wetherill Park, New South
Wales 21 64, Australia
'
The potential of catalytic processing as an effective method for polymer recycling was
studied using various modfled zeolites catalysts in the degradation of low-density
polyethylene (LDPE). Particular attention was paid to catalytic activity, and
selectivity and yield of liquid products. Two types of catalysts were evaluated: acid
catalysts (HX and HY), and, pure NaX and its base-modified derivatives (NaX
impregnated with MgO, CaO, SrO and BaO). As a benchmark, thermal degradations
were performed at 623 K, 673 K, 698 K, and 723 K, to identify optimum operating
conditions for the catalytic reactions. Thermal degradation at 623 K showed no
conversion during the first 7 hours run, while at other temperatures conversion was
100% and liquid yields at 673 K, 698 K, and 723 K were 21, 77 and 80wt%
respectively. Catalytic cracking tests were performed at 673 K to improve liquid
yield, and showed significant increases in the following catalyst order: HX (SOwtN)
> HY (40wt%) > modified NaX (33wt%) NaX (30wt%) > thermal (21wt%).
Introduction
The degradation of plastic waste provides a method for the recovery of hydrocarbon
sources, particularly with the present depletion in petroleum reserves [ 11. Incineration
of plastic wastes can eliminate the landfill option, but also releases greenhouse gases.
Traditional recycling methods, such as thermal degradation producing gas and liquid
products, provide a more sustainable way of recovery compared to incineration.
However, pure thermal degradation is energy intensive, requiring high temperatures.
Moreover, the hydrocarbon liquid product formed is heavy, thus requiring further
processing to improve its quality.
FeedstocWchemical recycling of plastic waste over catalysts i s of great interest as
an alternative to the traditional recycling procedures because the process is less
energy intensive, and produces mainly hydrocarbon products that are in the motor fuel
boiling range. Extensive worldwide studies have been performed using solid acid
catalysts, mainly zeolites [2]. The strong acidic properties of zeolites have the
tendency for over-cracking, resulting in more gaseous products than liquid products.
~~
* Author for correspondence
Cfmalherbe@swin.edu.au)
2 03
P. Tu.K. Pratt, E. Kosior and F. Malherbe
As a novel approach to polymer cracking, we have investigated the performance
of some solid-base catalysts, obtained by the preparation of zeolite-X containing
occluded alkali species. Various authors [3,4, 5 , 61 have shown that basicity could be
induced in zeolites through ion exchange and impregnation, with the final material
being extensively tested in base-catalysed heterogeneous reactions.
The main objective of this paper is to investigate how the nature of the alkali-earth
oxide (MgO, CaO, SrO and BaO) impregnated on NaX zeolite will affect the catalytic
behaviour of the parent compound. Typical loading was 0.48 mmol of oxide per 1 g
of catalyst. The influence of loading was also studied through preparation of catalysts
impregnated with various amounts of MgO (0.48 mmol, 0.96 mmol and 1.8 mmol per
1 g of catalyst). The alkali-modified zeolite catalysts were then compared to HY and
HX zeolites (representative solid acid catalysts), and to catalyst-free thermal
degradation.
Experimental Details
(0 Catalyst Preparation
Commercial NaX (13X), NH4Y and LDPE were obtained from Aldnch Pty Ltd. HX
was prepared by exchange of NaX in a 1M solution of NH,' at 333 K overnight,
followed by calcination for 20 hours at 773 K with a ramping rate of 1 Wrnin.
The modified zeolites were prepared by impregnation as follows: equimolar
amounts of alkali-earth nitrate solutions were prepared and stirred with 5 g of NaX in
250 mL solution. The suspension was left under magnetic stirring for a few hours, and
left to stand overnight. The water was then evaporated under vacuum using a rotary
evaporator at 358 K for 1 hour. The samples were subsequently oven-dried at 373 K
overnight and firther calcined at 773 K for 16 hours with a ramping rate of 1 Wmin.
During the course of calcination, the alkali-earth nitrates are decomposed to give the
corresponding oxides. For the same loading of 0.48 mmol per gram of NaX, several
catalysts were prepared using solutions of magnesium, calcium, strontium and barium
nitrates. These samples were denoted as 0.48MgO/NaX, 0.48CaO/NaX,
0.48MgO/NaX, 0.48SrO/NaX, 0.48BaO/NaX. Catalysts containing increasing
concentrations of MgO were also prepared in order to measure the effects of loading
on activity. These samples were denoted as 0.96MgONaX and 1.8MgODJaX.
(ii) X-ray Diffraction Measurements
X-ray investigation of the solids was carried out using a Philips powder
diffractometer (Model PW 1130) using a monochromated Cu K a source with a
wavelength of 1.5406 A, operated at 40 kV and 25 mA. The samples, as unoriented
powders, were scanned from 5" to 76" (20) at a speed of 2"/min.
(iii) Surface Area Measurements
The nitrogen adsorption-desorption isotherms were measured at 77 K on a
Micromeritics ASAP 2000 using standard adsorption techniques. The samples were
first calcined at 773 K for 16 hours followed by an overnight outgassing at 623 K.
204
Degradation of Low-Density Polyethylene over Modified Zeolites
H e Gas
T h e r m o c o u p le
Furnace
P o l y m e r & catalyst
Figure 1. Schematic diagram of a semi-batch reactor system.
(iv) Degradation of LDPE
Degradations of LDPE were performed in a semi-batch reactor as shown in Figure 1,
comprising a Pyrex glass reactor operated under atmospheric pressure. In all
experiments, 3 g of dehydrated LDPE and 0.03 g of catalyst (Iw-t??)were tested. In a
typical experimental run, the polymer and catalyst mixture was heated to the
degradation temperature with a ramping rate of 5 Wmin. The degradation products
were removed with a continuous helium flow of 25 ml/min. The effluent passed from
the reactor via a glass tube, to a condenser maintained at 258 K in order to trap
condensable liquids. The waxy products condensed at ambient temperature in the
glass tube between the reactor and the cold trap, while the gaseous products were
stored in Tedlar gasbags for further analysis.
All reactions ran for 7 hours and liquid and gaseous products were sampled
hourly. Analysis of liquid products was carried out using a Varian 3400 GC equipped
with a BPX-1 column, while the gaseous products were identified using a Shimadzu
17A GC fitted with a CP-AI203 PLOT.
The yields of products were defined as the percentage of the initial mass of
polymer reacted. The amount of gaseous products was determined by subtracting the
weight of liquid and waxy products. Almost no carbonaceous materials were formed;
the highest level observed being with the acid catalysts and was less than Iwt%.
Considering that polymer conversion for most runs was loo%, the rate of liquid
production is the chosen criterion to measure cracking activity.
Results and Discussion
(i) Pliysical Characterisation of the Catalysrs
The specific surface areas of the materials were calculated using the BET equation
from the adsorption isotherm of nitrogen obtained at 77 K. Experimental specific
surface areas and other surface properties of the catalysts are summarised in Table 1.
205
P. Tu,K. Pratt, E. Kosior and F. Malherbe
Table 1. Physical properties of catalysts.
Quartz
0.48BaOINaX
HX
I
74 1
16
108
393
254
446
203
412
HY
568
MgO*
8
NaX
I .8MgO/NaX
0.96MgOINaX
0.48MgO/N aX
0.48CaOllNaX
0.48 SrO/N aX
42
0.326
13
13
0.00 1
24
0.147
28
27
24
0.106
41
0.062
126
7
0. I77
0.044
0. I96
0.072
* Obtained by calcination of Mg(NOJ2under conditions used to prepare the modified NaX
A systematic observation for all modified samples was the decrease in surface area
following impregnation. While the parent NaX exhibits a high surface area of
74 1 m2g-', this property is lost in the impre nated derivatives, this latter group having
surface areas ranging from 203 to 398 m g I . This decrease in the values of the
surface area is almost directly proportional to the increase in the ionic radius of the
non-framework-exchanged alkali from MgO to BaO, except for the discrepancy
observed with strontium. A similar trend is observed with the micropore volumes: a
significant decrease compared to the parent zeolite, while a gradual decrease related
to atom size when only the alkali-oxides impregnated zeolites are considered.
These data suggest that the loss in surface area is more likely to be related to the
occupancy of the pores by the impregnated alkali oxides. During the impregnation
step the pores of the parent zeolite are gradually filled with the nitrate solution. The
relatively slow drying process that follows will leave the metallic salt trapped in the
channels, thus blocking accessibility to the pores. However, as indicated by the
decrease in external surface areas (compared to the parent NaX), the impregnated
oxide species are not located exclusively inside the porous structure but they also
cover, to a non-negligible extent, the zeolite framework. This seems to be consistent
with all alkali-earth oxides as they exhibit very similar external surface areas.
The only exception to these trends is observed with the strontia-impregnated NaX,
for which the surface properties are very close to those of the magnesia-modified
NaX. In view of these experimental data, it is reasonable to suggest that not all
strontium species have infiltrated the pores. However, taking into consideration the
surface area of the CaO- and BaO-modified NaX, exclusion from the pores cannot be
assigned solely to atom size. Other parameters such as ion exchange efficiency, or a
limited affinity for strontium, may lead to incomplete occupancy of the pore cavities.
9 -
206
Degradation of Low-Density Pohethylene over Modified Zeolites
IIU
I
1
-
-
I1 4 U M g O / N z X
0 Y6M yo i N ax
I t M y0IN.X
NIX
r
.(11_:--------8
R e la I I v
t
P rerru re, p
Ip.
Figure 2. N: isotherms of parent NaX and MgO-loaded NaX.
To further analyse the effect of alkali oxide loading on the surface properties, the
N2 adsorption isotherms of the parent NaX and samples containing different
concentrations of MgO are shown in Figure 2. The volume of gas adsorbed is a direct
indication of the surface area, which can be seen to decrease dramatically with
increasing loading of magnesia. The parent NaX exhibits a Type I isotherm typical of
a microporous material, and by increasing the concentration of impregnated magnesia
the resulting materials progressively evolve to a non-porous material. It can be
concluded that complete filling of the pore cavities is achieved with a concentration of
MgO between 0.96 and 1.8 mmol. As noted for the highest loaded sample, the surface
area measured is extremely small (compared to the parent NaX) and the equally low
external surface area suggests further coverage of the zeolite framework.
The X-ray diffraction patterns of the parent and modified samples are summarised
in Figure 3. It can be observed that in most cases the zeolite structure is largely
retained; all of the peaks conesponding to pure NaX are present in the impregnated
materials and their position unchanged. There are two important observations: (i) the
significant decrease in the intensity of the (1 11) reflection; and (ii) the appearance of
an amorphous phase in the region between 30 and 40 degrees (20).
The changes in the intensity are most probably caused by the different scattering
coefficient of the impregnated alkali species or may be explained by a variation in the
water content, as suggested by Lasperas et al. [7]. For all samples with a 0.48 mmol
loading, the decrease in intensity (counts) is proportional to the atomic number, an
exception within this trend is again noted for SrO.
(ii) Thermal Cracking of LDPE
The thermal degradation of low-density polyethylene was investigated at various
temperatures (623 K, 673 K, 698 K and 723 K) using 3 g of polymer mixed with
0.03 g of finely ground quartz. These preliminary tests were carried out to determine
the optimum temperature for the study of the catalysts. The cumulative liquid yields
from these tests are reported in Figure 4. Not reported on the graph is thermal
cracking at 623 K, for which no product was collected and the material recovered was
clearly a hardened molten polymer.
207
P. Tu,K. Pratt, E. Kosior and F. Malherbe
A
Intensity = 1100 counts
10
20
30 40
50
2 Theta, degrees
60
70
10
20
30
40
50
2 Theta, degrees
Figure 3. X-ray diffraction patterns ofparent and modfled N d .
0
60
120
180
240
300
360
Reaction T i m e , min
Figure 4. Rate of liquid production from thermal cracking of LDPE.
208
420
60
70
Degradation of Low-Density Polyethylene over Modified Zeolites
Increasing the temperature to 673 K resulted, as expected, in a slight increase in
the yield of liquid products. However, the rate of production over the 7 hours is rather
slow with an average value of 0.09 g h-'. Starting with a rate of around 0.021 g h i for
the first hour, to liquid production at a maximum of 0.13 g h-' during the last hour.
The mass of liquid obtained at the end of the reaction was only 20wt% of the initial
polymer mass. The selectivity in liquid would most probably increase had the reaction
been maintained for a longer time. The residue left in the reactor after this run was a
waxy product, much softer and greasier than the residue obtained after thermal
degradation at 623 K. The material was definitely not polyethylene and was classified
as hydrocarbon wax.
At 698 K, the reaction rate is significantly improved and a maximum of 77wt% of
liquid is produced. The rate of liquid production progressively increased from
0.42 g h" to a maximum of 0.57 g h-' at 180 minutes, and gradually decreased to
0.18 g h' towards the end of the reaction.
A further increase of the crackmg temperature to 723 K showed a sharp increase
in the rate of liquid formation, with a value of 1.86 g h-' for the first hour. When
compared to the run at 673 K, it can be noted that in the present case a yield of 60wt%
of liquid is obtained, while for the previous test a similar level of conversion is
achieved only after 300 minutes. An increase in temperature by 25 K is translated into
a saving in reaction time of 4 hours. At this temperature, the random cracking of
polymer chains into oligomers was vigorous. These oligomers were then readily
cracked into smaller hydrocarbons, as evidenced by the relatively high liquid yield in
the first hour. An interesting feature of Figure 4 relates to the early formation of liquid
(after 5 minutes).
The starting point of each experiment (t = 0) is normally taken to be the time when
the set temperature is reached. Due to the relatively high temperature, a significant
amount of liquid started to be formed. As indicated in Figure 3, thermal cracking
seems to be initiated at around 673 K and is fbrther enhanced with gradual increase of
the reaction temperature. By the time the set reaction temperature is attained, the
polymer would have been subjected to a long enough heat treatment to undergo
thermal degradation and generate liquid products.
It was also observed during the reaction that part of the vapour being camed away
by the sweeping gas would condense on the upper cooler walls of the reactor (outside
the furnace). The condensate is probably a mixture of high melting point
hydrocarbons, which will then slowly slide along the walls of the reactor to reach the
heating zone. The liquefied solid will then undergo further cracking to yield lighter
hydrocarbons. This would explain the dramatic decrease in the reaction rate noted
during the second hour. In fact, it is can be considered that at t = 60 minutes, the
cracking reaction is complete with a product selectivity of 60wt% liquid. Here again
the improvement in reaction time is considerable.
At any given cracking time, the liquid yield at 673 K was significantly lower than
at the corresponding cracking temperatures of 698 K and 723 K, while a relatively
larger amount of wax is obtained. The liquid yield obtained at this temperature
(673 K) was subsequently chosen as the benchmark for comparison with catalytic
cracking. The product yields for thermal and catalytic cracking at 673 K are
summarised in Table 2.
209
P. Tu,K. Pratt, E. Kosior and F. Malherbe
Table 2. Product yields for the thermal and catalytic cracking of LDPE at 678 K.
Cracking yield (wt%)
Catalyst
Liauid
Gas
WaX
Quartz
21
4
75
NaX
26
5
69
0.48MgO/NaX
30
4
66
0.96MgO/NaX
33
4
63
1.8MgONaX
32
5
63
0.48CaO//NaX
30
4
66
0.48SrO/NaX
32
5
63
0.48BaOmaX
29
5
66
HX
50
4
46
HY
40
5
55
(iii) Catalytic Cracking of LDPE with Acidic HXand HY compared to NaX
Compared to NaX and thermal degradation at 673 K, both the acidic HX and HY
zeolites exhibited significantly higher catalytic activities as shown in Figure 5. The
strong Bronsted acid sites of these two catalysts are instrumental in cleaving carboncarbon bonds. Craclung occurs both on the catalyst's external surface and in the
acidic cavities. The channels in HX and HY are slightly bigger than those in NaX due
to the small atomic size of the non-framework hydrogen (compared to Na'). The
larger pore openings allowed larger oligomers to enter the cavities for W h e r
cracking, thus producing lighter hydrocarbons. HX achieved higher liquid yield than
HY due to its higher external area and a slower deactivation rate.
50
0
fd
Quartz
--c N a X
4 0 -
0
-HX
HY
60
I20
180
240
300
360
420
R e a c t i o n T i m e , rnin
Figure 5. Rate of liquid production from thermal and catalytic (NaX, HX and HY)
cracking of LDPE at 673 K.
210
Degradation of Low-Density Polyethylene over Modlfied Zeolites
However, based on the high selectivity in liquid with the acid catalysts, and
provided that NaX also possesses a significant porous volume, the high catalytic
activity observed in the former materials would be mainly due to the acidity rather
than surface area or porosity. As suggested by Grieken et al. [8], strong acid sites
have the tendency to facilitate an end-chain cracking mechanism, thus favouring the
formation of lighter hydrocarbons.
Surprisingly, the liquid yield with NaX was only 5wt% higher than that of thermal
degradation, thus exhibiting almost no catalytic activity. Although polyethylene is too
bulky to fit inside the pores, the initial cracking oligomers would be expected to be
able to diffuse through the channel and benefit from the large surface area in order to
increase the cracking rate. It would appear that this diffusion is somehow hindered in
NaX, and cracking of the polymer is done only on the catalyst surface. The difference
in external surface area of NaX (42 m2/g) and the pulverised quartz (1 m'/g) would be
too small to induce substantial activity. It can therefore be concluded that the activity
observed with NaX resulted essentially from the result of thermal cracking on the
catalyst surface.
The rate of liquid production with the acid catalysts is also improved from an
average of 0.09 g h-' during thermal degradation to 0.21 g h-' with HX. This
difference between HX and HY is an indication of a slight deactivation of the former
within the first hour. This will be discussed further in a later section.
(iv) Catalytic Cracking of LDPE with Alkali Earth Oxides
Basic sites on the surface of zeolite catalysts are of the Lewis type, i.e. high electronic
density capable of proton abstraction. The basicity is associated with the framework
oxygen bearing a negative charge of the lattice, and consequently the density of the
negative charge on any given oxygen [ 9 ] . Therefore, it can be predicted that the
proton abstraction capacity of the studied zeolites would increase with its intrinsic
basic strength. The basicity of the NaX was systematically increased through
impregnation of the alkali-earth oxides, MgO, CaO, SrO, and BaO. Kavacheva et al.
[lo], studying the methylation of toluene using similar catalysts, have demonstrated
the existence of strong basic sites in these materials, with increased basicity from Mg
to Sr, based on microcalometric studies.
The liquid yields from the cracking on the alkali-earth oxides impregnated-NaX,
compared to NaX and thermal degradation, are reported in Figure 6. The catalysts
impregnated with MgO, CaO and BaO all exhibit similar activity (-30wt%),
representing an increase of 10% compared to thermal cracking. Although there are
significant variations in the BET and external area of the materials, this IS not
reflected in their catalytic activity. In fact, the modified sample offers less external
surface for polymer cracking to take place.
A further slight improvement is noted with the strontia-modified zeolite, which is
expected to be the most basic of all materials discussed here, yielding a total of 32%
liquid after 7 hours. We have shown earlier that a significant amount of SrO seems to
be excluded from the porous structure. In this respect, the slight increase observed
with the strontia-impregnated zeolite may be a result of greater availability of the
oxide species.
P. Tu,K. Pratt, E. Kosior and F. Malherbe
048Mp01NaX
0 48Ca01NaX
0.4SSrOlNaX
+0 . 4 8 B a 0 1 N a X
--c
s
.-0-
-
.E
-
lo
a
5
CI
0
0
60
120
180
240
300
360
1120
Reaction T i m e , rnin
Figure 6. Rate of liquid production from thermal and catalytic (pure and modlfied
NaX) cracking of LDPE at 673 K,
As discussed in the case of NaX compared to the acid catalysts, accessibility to the
internal pore structure does not appear to influence the catalytic activity. This
assumption is further confirmed here with the alkali-oxide modified-Wax, where
although the reference material exhibits a higher surface area and porosity, the
modified ones, with diminished properties, proved to be better catalysts. Moreover,
with the additional decrease in external surface area, these observations would
suggest that the gain in activity is to be attributed to the basicity.
In general, the rate of liquid production for the base-rnodified NaX, averaging
about 0.12 g h' over 7 hours, was significantly lower than the value determined for
HX (0.21 g h-') but showed an improvement from thermal cracking (0.09 g K'). This
leads us to suggest that catalysts impregnated with different alkali-earth oxides show
cracking mechanisms close to the parent NaX.
(v) Catahtic Cracking of LDPE with Different Loadings of MgO in NaX
Figure 7 summarises the results for the catalytic activity of modified NaX with
increasing loadings of MgO at 673 K. The liquid yield increased with increasing
loading from 0.48MgOINaX to 0.96MgOmaX. However, further increase in the MgO
content to 1.8MgO/NaX had no effect on the catalytic properties. This may be related
to the availability of the MgO surface for the catalytic reaction to occur.
Considering surface area and porosity, we have discussed the localisation of the
impregnated alkali earth oxides which was assumed to be distributed both inside the
pores and on the framework of the zeolite. Species trapped in the pores would not
normally participate in the reaction and doubling the loading would at the same time
increase the concentration outside the pores, hence, it would have a positive effect on
catalytic behaviour.
212
Degradation of Low-Density Polyethylene over Modrfied Zeolites
0
----C
-
30
----t
--m-
Thermal
NaX
0 PSMgOINaX
0 96MgOINaX
I8MgO/NaX
20
LO
0
0
60
120
180
240
300
360
420
Reaction T i m e , m i n
Figure 7. Rate of liquid production from thermal and catalytic (NaX, and with
different loadings of MgO) cracking of LDPE at 673 K.
A further increase in MgO content has almost no effect on the catalytic activity. It
is believed that due to the higher concentration of the impregnating solution, the
probability for ion exchange to occur will also increase, with Mg” replacing Na’ as
the charge compensating cation. This would result in less MgO being available than
initially expected. Some authors have suggested that depending on its amount, MgO
would be dispersed differently [l 13. In view of the nitrogen adsorption data, we can
ascertain that at higher concentration, the oxide species are more likely to be confined
in the porous structure, thus affecting the amount of active basic sites available for
cracking.
It has also been shown elsewhere that the basic strength of the active sites is not
contingent on the amount of MgO [ 121. This means that the catalytic activity will be
dependent on the number of sites available rather than upon the strength of these sites.
(vi) Acid-Base Properties of the Catalysts
The product distribution of the catalysed reaction can provide some indication of the
acid-base properties of the materials under investigation. It is generally agreed that
thermal cracking occurs via a free radical mechanism and results in the production of
many oligomers [ 131. On the other hand, acid-catalysed cracking is known to proceed
by carbonium ion mechanism, often characterised by the presence of substantial
levels of branched hydrocarbons in the products. A base-catalysed reaction is
expected to involve a carbanion intermediate resulting in lower levels of branched
compounds.
Adopting the rationales described by Seddegi et al. [ 141, and Garforth et al. [ 151,
the acid-base characteristics of the materials were monitored by the transient changes
in the levels of iso-butane during the cracking experiments. Figure 8 shows the
evolution of the levels of iso-butane with different catalysts.
213
P. Tu. K. Pratt, E. Kosior and F. Malherbe
E
3
6
18
-
16
-
14
-
12
-
0
-
Thermal
--e N a X
-o0.48MgOiNaX
10 -
HX
HY
m
4
J?
-P
8 -
-
6 4 2 O&--+--!
60
I20
-
-
-
I80
240
300
3 60
420
Reaction T i m e , m i n
Figure 8. Levels of iso-butane in thermal and catalytic cracking of LDPE at 673 K.
With HY, relatively high levels of iso-butane were observed during the first hour,
indicative of its strong acidic nature and of the substantial involvement of tertiary
carbonium ions in bimolecular hydrogen transfer reaction [ 161. However, the relative
percentage sharply decreased to lower values then to a constant lower level,
suggesting a rapid deactivation of the HY catalyst most probably associated with coke
formation. The HX zeolite exhibits a much lower deactivation rate during the first
five hours run with a significant drop thereafter. Not surprisingly, the levels of isobutane obtained with NaX and the alkali-exchanged NaX were extremely low,
confirming the absence of acidic protons in these catalysts. In view of these results,
we would expect the mechanisms involved with the two classes of materials to differ
significantly.
I2
10
723 K
.-2 ’ 8
.->
-8
4-
6
$
2 4
U
3
2
I
0
5
10
1.5
20
25
Carbon N u m b e r
Figure 9. Carbon number distribution in liquid products from thermal cracking of
LDPE at various temperatures.
214
Degradation of Low-Density Polyethylene over Modified Zeolites
(vii) Liquid Product Distribution from Thermal Cracking
Figure 9 shows the carbon number distribution of straight chain paraffins and olefins
of liquid products from thermal cracking at different temperatures. There is a clear
shift in the product distribution towards lower carbon numbers with a gradual increase
in the reaction temperature. An increase in temperature is thus accompanied by the
production of a lighter liquid. While at 673 K the product distribution is in the range
C8-C2., (based on the bell-shaped graph), at 723 K this range is C6-Cl0.The random
cracking of the polymer chain into oligomers is increased at higher temperatures,
together with the probability for these oligomers to be further cracked into light
hydrocarbons.
(viii) Liquid Product Distribution from Catalytic Cracking of LDPE using HX, HY
and NaX
Figure 10 compares the liquid product distribution from thermal cracking to that
obtained with the acidic HY and HX, and the basic NaX. The carbon number
distribution clearly shows that the liquid fraction from zeolite catalytic crachng is
much lighter than that obtained during thermal cracking. Acid catalysed cracking has
been extensively studied, and it is generally agreed that oligomers produced were able
to enter the larger channels of HX, into the active cavities and be further cracked into
smaller hydrocarbons [ 171.
-
NaX
+HX
5
10
15
20
25
Carbon Number
Figure 10. Carbon number distribution in the product of thermal and catalytic ( N d ,
HXand HY) cracking of LDPE at 673 K.
With HY, the product distribution was surprisingly in the higher carbon-number
range although, due to its intrinsic acidity, a similar distribution to HX was expected.
Although the cracking process was rather vigorous in the early stages of the process
(high levels of iso-butane), the HY seemed to undergo a rapid partial deactivation.
The resulting material would then lose an important part of its catalytic potential. The
greater amounts of higher hydrocarbon may result from partial blockage of the pore
system following cokmg of the catalyst.
215
P. Tu, K . Pratt, E. Kosior and F. Malherbe
The main observation made from HX craclung of LDPE was that there was a
significantly high level of branched hydrocarbons produced. Liquid fraction from HX
contained 18wt% branched hydrocarbons compared to the low level of 1wt% obtained
with the other catalysts. The low deactivation rate of HX allowed a large degree of
isomerisation to take place in the active sites. The large channels of HX allowed the
branched hydrocarbons to exit the active sites. In contrast, the branched isomers from
HY underwent further cracking due to its higher acidity. This had two significant
effects; first, lowering the amount of branched hydrocarbons produced, and second,
enhancing the deactivation process.
(h) Liquid Product Distribution from Catalytic Cracking of LDPE with Alkali
Earth Oxides
The liquid products from NaX impregnated with different alkali-earth oxides showed
similar distributions as shown in Figure 11. This result is consistent with the similar
liquid yield observed, further proof that cracking occurred mainly on the catalyst
surface and not within the active sites. Although the increased basicity of the
impregnated materials seems to have a minimal effect on the amount of liquid
produced, the carbon number distribution of the liquid products are very close to
thermal cracking. As discussed in section (iv), all impregnated samples exhibited a
higher activity compared to NaX, although they possessed poorer surface properties.
It is assumed that the extra activity would be mostly a result of the increased basicity,
as all other physical parameters (surface area, pores) are negatively affected.
The fact that higher hydrocarbons are obtained with the impregnated zeolites
indicates that the porous system in NaX does provide a means to improve the quality
of the liquid mixture. With less pores available, smaller amounts of heavier products
were able to diffuse into the cavities to be further cracked (size-selectivity due to
diminished pore sizes may also be an issue) into lighter products. In all impregnated
samples, there is an unexpectedly high percentage of hydrocarbons in the range
C19-C24.The impregnated samples seem to give a product distribution which is
intermediate between thermal degradation and cracking using NaX.
14
12
F
-
10
2:
.-5 8
-5
$
6
4
g 4
.-
-1
2
0
5
10
IS
20
2s
Carbon Number
Figure 11. Liquid product distribution of thermal and catalytic (NaX and with
different alkali-earth oxides) cracking of LDPE at 673 K.
216
Degradation of Low-Density Polyethylene over Modlfied Zeolites
5
10
15
20
2s
Carbon N u m b e r
Figure 12. Liquid product distribution of thermal and catalytic ( N d , and with
diflerent loadings of MgO) cracking of LDPE at 400°C.
(x) Liquid Product Distribution from Catalytic Cracking of LDPE with Different
Loadings of MgO in NaX
Liquid product distribution from the catalytic cracking of LDPE with different
loadings of MgO in NaX was lighter than that from thermal crackmg as shown in
Figure 12. However, increasing the loading of MgO resulted in an increase in heavier
hydrocarbons. An increase in the loading of MgO increased the occupancy of the
active sites and channels, resulting in less cracking of the heavy oligomers into
shorter chain hydrocarbons. Interestingly, the distribution from 0.96 and 1.8 mmol of
MgO showed similar liquid distributions, indicating similar cracking mechanisms.
This is in agreement with the BET data showing similar values of the measured
external surface area (13 mz/g), as shown in Table 1. The channels of NaX with these
two loadings were saturated with MgO, thus resulting in the same external area,
hence, subsequently giving similar liquid distribution.
Conclusions
Various alkali-earth oxides impregnated into NaX zeolite have been examined and
compared to acidic HY and HX zeolites as catalysts for the cracking of LDPE. Of all
the tested catalysts, HX showed superior activity to produce high quality liquid
products, with high levels of Cs-CI2hydrocarbons. The highly acidic active sites of
HX combined with the large channels allowed the selective production of branched
hydrocarbons, compared to the relatively low levels of branched hydrocarbons with
NaX, and the active but readily deactivated HY.
Compared to thermal cracking, catalytic cracking using pure NaX did not seem to
significantly improve the product distribution. However, a slight increase in activity,
measured using the rate of liquid production, is observed. Following impregnation
into NaX of different types of alkali-earth oxides, the surface properties of the
resulting materials were significantly changed, a systematic decrease in surface area
and porous volume was observed for all impregnated samples.
217
P. Tu, K . Pratt, E. Kosior and F.Malherbe
BET adsorption experiments indicated that impregnation resulted in the
occupancy of the pores and channels of the catalyst by the oxide species. No
significant improvement in the catalytic properties of NaX was observed, and this was
lmked to both the blockage of the porous system and lack of acidity on the catalyst
surfaces. Random cracking on the catalyst external surfaces yielded heavier
hydrocarbon liquid fractions that are lower in quality compared to liquid products
fiom LDPE cracking over HX.
Acknowledgments
We gratefully acknowledge the financial support of Visy Recycling and the
Australian Research Council (ARC).
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