Production of liquid fuels from recycled plastics using acidic HNaY catalysts

Maja Jaśkiewicz

ABSTRACT

This study investigates thermal and catalytic cracking of high -density polyethylene and low-density

polyethylene ( HDPE and LDPE) using simultaneously thermogravimetry (TG) and

differential scanning calorimetry (DSC) techniques.

The catalyst chosen for this work were Ultra Stable acidic Y zeolites (USHY). For the purpose of the study,

the catalysts were subjected to ion exchange with sodium nitrate in order to obtain a set of different acid

contents catalysts. The amount of acid sites was calculated by resorting to temperature programmed

desorption (TPD) with ammonia. The relationship between the acidity and activity of the zeolites was

investigated.

The TG-DSC analysis allowed studying the conditions of thermal, as well as catalytic degradations. It was

found that the degradation temperatures decreased during the reactions involving catalysts and also that

the changes in acidity of the zeolites did not have a significant influence on the cracking process. Moreover

it was found that the structure of the polymer itself induces changes in cracking conditions. More

branched molecule – LDPE, presented lower mel ting temperatures as expected, and lower pre-cracking

temperatures than linear HDPE.

A kinetic model was fitted to the signals from TG-DSC in order to describe the kinetics of the process,

allowing the estimation of several kinetic and thermodynamic parameters. The estimated values showed a

decreas e in activation energy for the catalytic processes.

The degradation products were analyzed using gas chromatography (GC). It was observed that there is a

significant difference in product distribution between thermal and catalytic process. The presence of the

catalyst favoured the formation of C4-C6 hydrocarbons, whereas after thermal degradation the majority of

products were in C1-C4 range. The acidity of the zeolites had no significant influence on the product

distribution. Overall it was found that the amount of olefins was higher than paraffins for all of the

analyses.

1. Introduction

One of the problems of modern society

is waste management, and the management of

the plastic wastes became a major concern in

recent years, because of our consumer lifestyle.

The amount of plastics on Earth is enormous,

mostly since plastics have replaced materials

such as glass, ceramic or wood in the

manufacture of everyday use objects. Another

concern of our and future generations is

depletion of natural crude oil resources, and

what’s involved – constant increase of gasoline

prices.

Studies presented in this work present the

possibility of contributing to the solution of

these global problems, by recycling of plastic

wastes by incorporating them in petrochemical

industry, as a feed to FCC process. [1-5, 7-12]

The main objective of this work is to

study the production of liquid fuels by thermal

and catalytic degradation of polyethylene using

acidic HNaUSY zeolites as catalysts.

Furthermore the influence of zeolite acidity on

the catalytic degradation was investigated.

Degradations were performed on a

TG/DSC apparatus and the data collected was

evaluated with the use of kinetic model that

helped to understand better the differences

between degradation parameters for each

catalyst.

Part of this work was used for writing the

extended abstract “Catalytic Cracking of High

Density Polyethylene Using USY zeolites – a

TG/DSC Study” that will be presented at

CHEMPOR 2011 conference, held in Lisbon,

Portugal.

2. Experimental

2.1. Materials

The polymer, used for thermal and

catalytic decomposition in these studies, was

low and high density polyethylene. The material

was supplied by Borealis and it doesn’t possess

any additives.

The low density PE is in the form of small pellets

with the average molecular weight of

Mw =376000. High density PE has a powdery

form and its average molecular weight

Mw=290000.

The parent catalyst used in this work was a

commercial Y-82 zeolite with Si/Al ratio of 4.5

supplied by UOP.

2.2. Catalyst preparation and

characterization

To obtain different acidic strengths of

the catalyst ion exchange was performed. The

original Y-82 was subject to ion-exchange with

0.5M and 1M solutions of sodium nitrate. During

this step some of the protons were replaced with

sodium ions and by this modification the total

amount of Bronsted acid sites was decreased.

The ion exchanges were performed by stirring

the zeolite with sodium nitrate solution at room

temperature for four hours. The amount of

NaNO3 solution used was 4ml per gram of

catalyst. Afterwards the catalyst was filtrated

and washed with distilled water.

The catalysts after the exchanges were

designated with the names : Y-82 for the original

zeolite, Y-82_1 for zeolite exchanged with 0.5M

NaNO3 , Y-82_2 for zeolite exchanged with 1M

NaNO3 , Y-82_3 for zeolite two times exchanged

with 1M NaNO3 and Y-82_4 for zeolite three

times exchanged with 1M NaNO3.

To purify the catalyst material from any

trace compounds that might be adsorbed on its

surface, it underwent a calcination process that

was held in a tubular reactor by heating the

zeolite at 10oC/min up to the temperature of

500oC, temperature which was kept for 8 hours.

The calcination took place under a dry air flow of

0.5l/h per gram of catalyst.

The acidity of catalyst used in this work

was measured by ammonia Temperature

Programmed Desorption (TPD). This method

involves saturating the sampl e with an

adsorbate (in this case – ammonia) and heating

it according to a previously established

temperature profile. The procedure uses

ammonia because it is a strong base and a small

molecule that may access most of the acid sites

in zeolites. The TG-DSC apparatus also was used

for desorption measurements.

The saturation step, prior to TG-DSC

experiments, was carried-out by placing the

catalyst sample (20mg) and ammonia in Schlenk

thermostated at 40oC using a water bath for 8

hours.

2.3. TG and DSC analysis

Thermogravimetry and Differencial

Scanning Calorimetry analysis were used in this

study to obtain signals referring to the weight

change of the sample as well as the change in

heat flux, which allows us to followthe heat

changes in the sampl e, including the amount of

heat involved in the brakeage of polymer bonds.

The experiment consisted of two degradation

runs ( 1st and 2nd cycle respectively) and catalyst

regeneration run ( 3rd cycle, only when the

catalyst was present).

During the first cycle, the polymer

sample (pure or mixed with the catalyst) was

placed in a quartz pan and the whole system was

purged for 30 minutes with ni trogen, to wash

out all the oxygen from the heating chamber. The

sample was then heated according to the

temperature profile – 10oC/min up to 600oC.

After the first run was complete, and the sample

was cold again, a second cycle begun without

changing the conditions inside the furnace and

no new polymer added; this 2nd run was

performed to obtain the baseline for the DSC

signal. Both cycles were carried out under

continuous nitrogen flow rate of 80ml/min.

Catalyst regeneration was performed to

measure the amount of coke deposited on the

catalyst. Throughout the 3rd cycle, air, with the

flow of 75ml/min, was used to burn the coke at a

given temperature profile.

Thermogravimetric (TG) and

Differential Scanning Calorimetric (DSC)

analyses were performed in SDT 2960 DSC-TGA

apparatus

The amount of PE used for thermal and catalytic

degradation was around 10 mg, whereas the

amount of zeolite added was around 1mg.

2.4. GC analysis

The gas chromatograph used in the

experiments was Shimadazu GC-9A, working

under the ni trogen pressure of 2 bar, equipped

with flame ionizing detector (FID) and PLOT

column (KCl/Al2O3). The machine was connected

to Shimadazu C-R3A integrator. The gas

Chromatograph is equipped with the column

oven, flow control section, split/splitless injector

system and FID detector.

Gases for the analysis were collected

directly from the outlet of the TA Universal

Analysis 2000 apparatus. A Teflon tube

connected the apparatus to the opening of

volumetric flask filled with water (V=1000ml),

that was set up inverted on water container.

After the collection the gases produced during

polymer degradation were sampled using a

100µl syringe and injected to the GC.

3. Resuls and discussion

3.1. Acidity of catalysts

It was expected to obtain a TPD curve

with peaks only around 200-300oC, however

after running the blank tests and subtracting the

experimental and blank signals, at high

temperatures around 700oC another peak is

visible. Usually this peak corresponds to

dehydroxylation of the zeolite sample. Yet on the

blank test no peaks were observed for the

catalyst at these higher temperatures, which

indicates that this third peak in the ammonia

TPD curve seems to be related to very strong

acid sites.

As it can be noted in figure 1, the most

acidic catalyst, the parent zeolite, presents three

very distinctive peaks with a large area under

the curves, corresponding to the amount of

adsorbed ammonia, therefore the amount of acid

sites. The first two peaks refer to less acidic acid

sites, whereas the last peak seems tocorrespond

to very strong acid sites, because the higher

activation energy is needed for the desorption of

ammonia from those sites.

The behaviour of the ion-exchanged

catalysts is as expected. The higher the sodium

content in the zeolite the lesser the amount of

ammonia absorbed; however the strong acid

sites peak still occurs, albeit smaller, but it is

shifted towards higher temperatures, meaning

the catalyst still possesses strong acid sites.

It might be concluded from the graph that the

USHY family of zeolites present a very strong

catalytic activity due to having acid sites that

perform well at high activation energies.

Figure 1 Comparison between the ammonia TPD model curves obtained for different zeolites

To obtain the acid strength distribution

an activation energy grid was chosen to fit a

model for the desorption to the experimental

using the “Solver” tool in an Excel ( ©Microsoft

Corp.) spreadsheet. Energy values are in kJ/mol

and are the same for all of the catalysts; the grid

is as follows: 50, 60, 70, 80, 90, 110, 150, 160,

180, 200, 250, 280.

Calculating the quantity of ammonia per

each decomposition curve for all of the catalysts,

led to the information about the total number of

acid sites in catalysts. Those values are shown in

table 1. The most acidic catalyst presents the

highest amount of acid sites, as expected. The

zeolites which were exchanged with sodium ions

0,0E+00

5,0E-06

1,0E-05

1,5E-05

2,0E-05

2,5E-05

3,0E-05

150 400 650 900

dm

/dt(

mm

ol.

s-1

)

Temperature(ºC)

Y-82

Y-82_1

Y-82_2

Y-82_3

Y-82_4

in different concentrations, show smaller

number of acid sites than the parent catalyst.

Yet, it needs to be noted that the values of acid

sites do not vary significantly between the

exchanged catalysts, so it might be assumed that

the degree of exchange did not change much

with the exchange procedureand, thus, the

amount of available acid sites for this particular

catalyst.

Table 1 Total amount of acid sites

catalyst

acid sites

(mmol/mgzeolite)

Y-82 3.37x10-3

Y-82_1 1.21x10-3

Y-82_2 1.00x10-3

Y-82_3 1.07x10-3

Y-82_4 9.77x10-4

3.2. Polymer degradation

The DSC curves for degradation both

HDPE and LDPE are very similar: they consist of

one melting peak and two cracking peaks. The

first cracking peak might correspond to pre-

cracking of the polymer taking pl ace at the

surface of the catalyst. The second peak is the

cracking of the species formed during the pre-

cracking, probably taking place in the acid sites

inside the zeolite pores. [13]

The temperature of melting of LDPE is lower

than for HDPE.

Analyzing the data presented in table 2

it may be observed that there is little influence

between of the number of ion-exchanges and the

cracking temperatures of HDPE, because for all

of the zeolites the degradation temperatures are

similar. In relation to the temperatures of 1st and

2nd cracking peaks for LDPE there is a noticeable

trend of increasing the temperature needed for

the cracking process with decreasing acidity.

The Y-82 zeolite cracks LDPE at 402oC and then

at 432oC, the less acidic catalysts present higher

temperatures.

Degradation of LDPE with Y-82_4 zeolite

does not follow the increased temperature for

decreased acidi ty pattern. Although this catalyst

presents the least acidic sites and is the most

exchanged, it presents a cracking temperature of

LDPE comparabl e to those of Y-82_1 zeolite.

The degradation temperature for the

pre–cracking of LDPE is slightly lower than for

HDPE; the 1st cracking occurs at 402oC and

407oC for Y-82 zeolite for LDPE and HDPE

respectively. Neverthel ess, the temperatures of

pre-cracking for both polymers are more or less

in the same range 402-418o C.

The second catalytic cracking peak

occurs at wider distributions of temperatures,

especially for the degradation of low density PE,

and the values obtained for particul ar catalyst

are hard to compare. In general the 2nd cracking

occurs at lower temperatures for HDPE.

When investigating the TG curves, it

may be noticed that only during thermal

degradation the final weight of the sample was

zero, for the catalytic processes the final mass

equalled mass of the catalyst plus the deposited

coke. Also the weight loss for reactions involving

catalysts started more rapid, comparing with the

thermal degradation, the catalytic curves have

much more rigid appearance when the polymer

starts to vaporize.

Table 2 Degradation temperatures obtained in thermal and catalytic cracking for HDPE and LDPE

Melting

temperature (oC)

1st cracking temperature(oC) 2nd cracking

temperature (oC) E HDPE 134 483 - HDPE+ Y-82 133 407 430

HDPE + Y-82_1 133 411 433 HDPE + Y-82_2 134 406 433 HDPE + Y-82_3 134 418 427 HDPE + Y-82_4 133 410 435

LDPE 113 476 - LDPE + Y-82 113 402 432

LDPE + Y-82_1 112 415 435 LDPE + Y-82_2 112 418 447 LDPE + Y-82_3 113 418 452 LDPE + Y-82_4 113 412 439

3.3. Kinetic model

The degradation process occurs through

successive breakages of bonds and to describe

this process kinetic model was used in this

studies. The model illustrates the reactions

taking place during the pyrolysis in a certain

way – for each broken bond, some energy is

required, and this energy should be seen in the

signal of heat flow. Using only TG analysis it is

only possibile to observe the mass loss of a

sample and this only occurs when the products

are light enough to vaporize. Smaller, shorter

compounds produced during degradation are

volatile, so they will be detected when they leave

the system both by the mass signal and by the

heat flux signal, since they also require energy to

be vaporized. However, breakages leading to

heavier, non-volatile molecules, can be seen only

using the DSC signal. Only when combining the

two analysis, it is possibile to obtain a more

complete information about the degradation

process.

Hence, the model used was constructed

using a material balance, to the number of

bonds, and an energy balance of the sampl e in

time. If we assume that the polymer is a long

alkane chain, that has n carbon atoms, the

number of C-C bonds per unit mass, N, will be

given by the following expression:

N=

Taking into account that n is a very large

number, the amount of C-C bonds at the

beginning of the run will be close to 1/14, and

this value is assumed to be the bond density of

the sample.

(1) (1)

However, the number of the actual bonds that

may be broken is only half of this value,

supposing that the process is very effective.

According to this argument, the total breakable

bond density used for polyethylene in this study

was 1/28.

To perform the bal ance to the number of

breakable bonds we will have to take under

considerations all processes that lead to loss of

breakable bonds. That is, thermal and/or

catalytic cracking, and the evaporation process,

during which small molecules will carry to the

gas phase a certain amount of unbroken bonds.

Additionally to properly balance the number of

bonds, it was assumed that the process of

breaking the bonds is first-order in relation to

bond concentration.

Therefore the balance equation for the number

of breakable bonds is expressed as :

where dm/dt is the rate of weight loss and α is

the average number of bonds lost to the gas

phase.

The k(T), temperature dependent rate constant

is described by the Arrhenius law:

where Tref is a reference temperature (573 K in

this case), Ea is the reaction activation energy

and kref is the kinetic constant at the reference

temperature.

To estimate the heat flow in the experiment an

energy balance to the pan was performed, with

the assumption that the apparatus is capable of

correctly compensating the required flows.

Following this, the heat flow is given as:

Heat Flow =

where m is the weight of the sample at any given

time, obtained experimentally, Cp is the average

heat capacity of the mixture, ΔHC-C is the average

C-C bond enthalpy and ΔHvap the average

vaporization enthalpy per unit mass.

The model was fitted within the range of

the degradation temperatures obtained from the

experiments. The fi tting parameters were kref, Ea,

α, Cp, ΔHC-C, and ΔHvap. The last three parameters

were initially guessed from published data.

Equations 2 and 4 were solved

numerically, using the Euler method for each

run, and the model parameters were estimated

by least-squares procedure, using the sum of the

squares of the residues on the heat flow as the

objective function (O.F) to be minimized:

O.F =

The optimization was carried out using the

“Solver” tool in an Excel (©Microsoft Corp.)

spreadsheet. [5]

Due to reasons that have not yet been

identified the parameters fitted with the model

show little consistency with the results made in

other works.

Analyzing the data provided in table 3 it

is possible to conclude that presence of catalyst

decreases the activation energy as it was

expected and the values for the less acidic

catalysts are lower.

The second important conclusion is the

relationship between the branching of polymer

and the amount of heat needed for the cracking

to occur. HDPE being more linear polymer

presents higher activation energy than LDPE.

(4)

(5)

(5) (2)

(3)

Table 3 Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking

of HDPE with catalyst of different acidity

HDPE

Thermal

Degradation

HDPE+

Y-82

HDPE +

Y-82_1

HDPE +

Y-82_2

HDPE +

Y-82_3

HDPE +

Y-82_4

k ref(min-1 ) 3x10-5 3.72x10-7 5.40x10-6 2.92x10-7 5.83x10-6 5.19x10-6

Ea (kJ/mol) 111 74 81 77 82 80

ΔH c-c(kJ/mol) 145 41 51 46 48 61

alfa (Bondmol/g) 1,35x10-4 9.53x10-4 1.40x10-4 2.22x10-4 1.34x10-4 3.21x10-4

Eaα (kJ/mol) 114 89 236 311 153 166

ΔH vap (J/g) 276 431 485 477 399 416

Cp (J/g) 2.99 2.02 4.23 4.30 3.92 3.76

Table 4 Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking

of LDPE with catalyst of different acidity

LDPE Thermal

Degradation

LDPE +

Y-82

LDPE +

Y-82_1

LDPE +

Y-82_2

LDPE +

Y-82_3

LDPE +

Y-82_4

k ref(min-1 ) 6.27x10-5 6.27 x10-5 3.41 x10-7 5.69 x10-5 9.82 x10-5 6.18 x10-5

Ea (kJ/mol) 89 67 68 67 70 83

ΔH c-c

(kJ/mol) 196 19 20 176 315 192

alfa (Bond

mole/g) 2.25 x10-4 1.19 x10-4 1.28 x10-4 2.41 x10-4 1.47 x10-4 2.37 x10-4

Eaα (kJ/mol) 108 89 111 141 148 139

ΔH vap (J/g) 302 431 471 463 392 409

Cp (J/g) 2.50 3.26 3.37 3.62 3.39 2.90

3.4. Product distribution

It was noticed that the presence of

catalyst shifted the formation of products

towards longer and more branched compounds

with C4-C6 in catalytic cracking of HDPE as well

as LDPE.

Figures 3 and 4 illustrate the product

distribution as a func tion of carbon atoms.

The highest yield for all of the catalysts

was obtained for C4 products. Among them

there can be distinguished paraffin compounds

such as butane and isobutane, as well as olefins

with butene aas the most relevant.

Other products that were produced in

significant amounts for the catalytic processes

were C3 (propane, propene), C5 (pentane and

isopentane, pentene), C6 (hexane and hexane),

C7 (heptane and heptene) and C8 (octane and

octene). Hydrocarbons with more than 8 carbon

atoms consti tuted to less than 5% molar of total

obtained products.

The acidity of the catalysts didn’t

influence in a significant way the product

distribution for HDPE or LDPE cracking.

Figure 3 Product distribution obtained for catalytic cracking of HDPE

Figure 4 Product distribution obtained for catalytic cracking of LDPE

Table 5 presents the olefin to paraffin

and hydrogen to carbon ratio as well as molar

percent of aromatic compounds formed. It is

noticed that thermal processes for HDPE and

LDPE produce higher amounts of olefins than

catalytic processes.

Comparing only the catalytic reactions

of HDPE the highest production of olefins is

observed for Y-82_4 zeolite, with relatively high

concentrations of butene. The least amount of

olefins are produced by the Y-82_2 zeolite, O/P

ratio equals 1.4, meaning that there were almost

as much alkanes as alkenes.

The hydrogen to carbon proportion for

all of the catalysts was around 2.1.

Analysis of the aromatic compounds

content leads to the conclusion that there is also

no significant influence of acidity on their

formation. All of the catalysts present rather

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct d

istr

ibu

tio

n (

% m

ola

r)

Number of carbon atoms

HDPE + Y-82

HDPE + Y-82_1

HDPE + Y-82_2

HDPE + Y-82_3

HDPE + Y-82_4

HDPE Thermal Degradation

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct d

istr

ibu

tio

n (

% m

ola

r)

Number of carbon atoms

LDPE + Y-82LDPE + Y-82_1LDPE + Y-82_2LDPE + Y-82_3LDPE + Y-82_4

poor aromatic yields, with no more than 2% of

overall products.

Catalytic cracking of LDPE l eads to the

highest production of olefins for Y-82 zeolite, 2.4

olefin/paraffin ratio, this is connected with high

concentrations of propene, isobutene and

pentene ; and the lowest for Y-82_3 zeolite, with

the an olefin/paraffin ratio of 1.3.

Most of the catalyst produced only small fraction

of aromatics from LDPE during cracking, except

for Y-82_1 zeolite. The molar % of cyclic

aromatic compounds obtained in this latter case

is 8%.

Table 5 Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for thermal and catalytic

degradation of HDPE and LDPE

Sample O/P ratio H/C ratio % aromatic

HDPE 6.1 2.1 1

HDPE +Y-82 1.8 2.3 1.7

HDPE +Y-82_1 1.9 2.1 0.6

HDPE +Y-82_2 1.4 2.1 0.4

HDPE +Y-82_3 1.6 2.1 1.5

HDPE +Y-82_4 2.6 2.1 0.1

LDPE 3.6 2.5 3

LDPE + Y-82 2.4 2.1 0.8

LDPE + Y-82_1 2.3 3.1 8

LDPE + Y-82_2 1.5 2.2 1

LDPE + Y-82_3 1.3 2.2 1

LDPE + Y-82_4 1.9 2.1 1.2

4. Conclusions

The experimental part of this work involved

preparation of Y-82 zeolites with decreasing

acidity and the evaluation of their performance

in catalytic cracking of two polymers – HDPE

and LDPE using TG and DSC analysis as well as

gas chromatography with comparison to thermal

degradation process.

The TG-DSC signals were fitted to previously

made kinetic model, which would calculate the

kinetic and thermodynamic parameters . This

step was done to better understand the kinetics

of the processes i tself and to distinguish the

major differences between catalytic and thermal

cracking.

During the thermogravimetric and differential

scanning calorimetric experiments it was

possible to observe the influence of the polymer

type and presence or absence of catalyst on the

cracking temperatures. First of all it was noticed

that the degradation temperatures for thermal

cracking were different for HDPE and LDPE. This

difference originates in the structure of the

polymers. As expected compounds with higher

degree of branching present lower cracking

temperatures, and what is connected with this,

lower activation energies.

For the catalytic cracking reactions the most

important observation was the occurrence of

two endothermic peaks relating to polymer

cracking. The first cracking peak is assumed to

be the pre-cracking happening on the catalyst

surface, whereas the second cracking is expected

to occur inside the zeolite pores. Another

observation was the expected decrease in

degradation temperatures during the reactions

involving catalysts.

The acidity of the catalyst had some influence on

the cracking temperatures, the zeolite with the

least amount of sodium required the lowest

temperature to crack the polymer. The

temperatures for catalysts with less active sites

were slightly higher, but comparable with each

other.

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