The catalytic cracking of sterically challenging plastic feedstocks over high acid density Al-SBA-15 catalysts

Article abstract of DOI:10.1016/j.apcata.2018.11.020

The catalytic cracking of polyolefinic waste materials over solid acid catalysts, such as zeolites, is a promising process for the production of useful fuels and chemicals. However, the inherent diffusional constraints of the microporous zeolites restrict the access of bulky polyolefin molecules to the active site, therefore limiting their effectiveness. To address this, a simple yet effective method of producing mesoporous Al-SBA-15 materials with a high density of Br?nsted acid sites has been employed. These catalysts are shown to be very active for the catalytic cracking of low density polyethylene (LDPE), a common waste plastic. The acidic and textural properties of the catalysts were characterised by ICP-OES, XPS, XRD, N₂ physisorption, propylamine-TPD, pyridine-FTIR and STEM and have been correlated with their catalytic activity. The product distribution from the catalytic cracking of LDPE has been shown to depend strongly on both the pore architecture and the Al content of the SBA-15 and thus the density and strength of Br?nsted acid sites. Fine-tuning the Al content of the SBA-15 materials can direct the product distribution of the hydrocarbons. The Al-SBA-15 materials display increased cracking orientated towards aliphatic hydrocarbons compared to ZSM-5, attributed to the mesoporous nature of SBA-15, overcoming diffusional limitations.

Full text of DOI:10.1016/j.apcata.2018.11.020

Accepted Manuscript
Title: The catalytic cracking of sterically challenging plastic
feedstocks over high acid density Al-SBA-15 catalysts
Authors: Joseph Socci, Amin Osatiashtiani, Georgios
Kyriakou, Tony Bridgwater
PII:
S0926-860X(18)30575-1
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.11.020
APCATA 16889
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
17 September 2018
21 November 2018
23 November 2018
Please cite this article as: Socci J, Osatiashtiani A, Kyriakou G, Bridgwater
T, The catalytic cracking of sterically challenging plastic feedstocks over
high acid density Al-SBA-15 catalysts, Applied Catalysis A, General (2018),
https://doi.org/10.1016/j.apcata.2018.11.020
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The catalytic cracking of sterically challenging plastic feedstocks
over high acid density Al-SBA-15 catalysts
Joseph Socci, Amin Osatiashtiani, Georgios Kyriakou, Tony Bridgwater*
European Bioenergy Research Institute (EBRI), School of Engineering and Applied Science,
Aston University, Aston Triangle, Birmingham, B4 7ET, United Kingdom.
* Corresponding Author: a.v.bridgwater@aston.ac.uk
Graphical Abstract
Highlights




Synthesis of Al-SBA-15 catalysts with similar acidic properties to zeolites.
Al-SBA-15 catalysts overcomes diffusional limitations of ZSM-5 in LDPE cracking.
Aluminium rich SBA-15 exhibits high selectivity to gasoline range hydrocarbons.
Selectivity can be fine-tuned by controlling the incorporation of Al into SBA-15.
Abstract
1

The catalytic cracking of polyolefinic waste materials over solid acid catalysts, such as
zeolites, is a promising process for the production of useful fuels and chemicals. However, the
inherent diffusional constraints of the microporous zeolites restrict the access of bulky
polyolefin molecules to the active site, therefore limiting their effectiveness. To address this, a
simple yet effective method of producing mesoporous Al-SBA-15 materials with a high density
of Brønsted acid sites has been employed. These catalysts are shown to be very active for
the catalytic cracking of low density polyethylene (LDPE), a common waste plastic. The acidic
and textural properties of the catalysts were characterised by ICP-OES, XPS, XRD, N2
physisorption, propylamine-TPD, pyridine-FTIR and STEM and have been correlated with
their catalytic activity. The product distribution from the catalytic cracking of LDPE has been
shown to depend strongly on both the pore architecture and the Al content of the SBA-15 and
thus the density and strength of Brønsted acid sites. Fine-tuning the Al content of the SBA-15
materials can direct the product distribution of the hydrocarbons. The Al-SBA-15 materials
display increased cracking orientated towards aliphatic hydrocarbons compared to ZSM-5,
attributed to the mesoporous nature of SBA-15, overcoming diffusional limitations.
Keywords: Mesoporous materials, Al-SBA-15, Catalytic cracking, LDPE, Pyrolysis
1. Introduction
The catalytic cracking of polyolefin waste for the production of transportation fuels and
chemicals is receiving increased attention due to the environmental and economic benefit
associated with these technologies [1].
Several studies have been carried out to examine the catalytic cracking of pure polyolefins
over solid acid catalysts such as zeolites, clays and ordered mesoporous materials (OMMs)
including MCM-41 and SBA-15 [2–4]. Typically, high catalytic activity is observed with
materials possessing high acid strength/densities, high surface areas and large pore sizes
which facilitate the accessibility of bulky plastic derivatives to the active sites [5].
Zeolites possess high stability and strong acidity due to their highly crystalline framework and
have been employed industrially in large-scale applications such as in refining and
petrochemistry [6]. However, their microporous nature presents severe mass transfer
limitations of the bulky polyolefin derivatives to the internal pores, where the majority of the
acid sites can be found. Contrary to this, silica (SiO2) based mesoporous materials, such as
MCM-41 and SBA-15, possess an ordered arrangement of pores with diameters between 2
and 50 nm compared to 0.5 nm for ZSM-5. This overcomes the limitation of diffusional
constraints associated with microporous zeolites. However, the application of MCM-41 is
limited in many catalytic processes due to its relatively low thermal and hydrothermal stability
compared to zeolites and alumina [7]. Mesoporous silica SBA-15 is structurally very similar to
2

MCM-41 but has thicker pore walls (4-8 nm compared to 1-2 nm for MCM-41) and larger pore
sizes (2-15 nm compared to 2-5 nm for MCM-41) [8]. The thicker pore walls of SBA-15 results
in improved framework crosslinking compared to MCM-41, leading to higher thermal and
hydrothermal stability. These properties have opened up many new opportunities in
separation, adsorption, as drug delivery carriers and as heterogeneous catalysts [9–12].
The insertion of aluminium ions into the SBA-15 structure allows the creation of acid sites
which are essential for acid catalysed reactions. However, it is difficult to achieve high levels
of aluminium incorporation into Al-SBA-15, due to the difference in hydrolysis rates of Al and
Si at the low pH required for SBA-15 synthesis [13]. Various synthetic methods have been
proposed for the incorporation of higher amounts of aluminium to achieve higher surface
acidities [14–16]. A literature survey on the acidity evaluation for a variety of Al-SBA-15
synthesis methods is presented in Table 1.
Table 1. Comparison of Al-SBA-15 synthesis and acidity evaluation referenced in literature.
References are presented in chronological order.
Al
Lowest Acidity
Author
Year Ref.
Brief evaluation
incorporation
Si/Al
(µmol/g)
Luan et al.
1999
2001
[17]
[18]
Direct synthesis
Post synthesis
8.5
N/A
Acidity not evaluated.
Py-IR confirms existence of
Brønsted acid sites and
confirmed by cumene cracking
reaction, though not quantified.
Py-IR shows increase in Brønsted
and Lewis acidity with a decrease
in Si/Al, not quantified.
Sumiya et
al.
4.8
N/A
Direct
synthesis/two
step
Y. Li et al.
2004
[13]
[16]
[19]
22
2.4
7.2
N/A
N/A
430
S. Wu et al. 2004
Zeng et al. 2005
pH adjustment
Acid sites not measured.
Acidity quantified by Py-IR,
highest Brønsted acidity achieved
at Si/Al = 9.8 (81.2 µmol g-1).
Acid sites evaluated by probe
molecules and NMR. Acid site
strength is dependent on the
probe molecule.
Acid properties measured by NH3
TPD, shows acidity is 0.11
mequiv.NH3 g-1 (Si/Al = 30).
Acidity evaluated by NH3, linear
relationship between acidity and
cumene cracking. Highest total
acidity achieved at Si/Al = 11
(402 µmol g-1).
Post synthesis
Post synthesis
pH adjustment
W. Hu et al. 2006
Van
Grieken et
al.
[20]
[21]
10
31
160
110
2006
Muthu
Kumaran et 2008
al.
[22]
[23]
Direct synthesis
11.4
8
402
491
Acidity evaluated by both Py-IR
and NH3 TPD, highest acidity
achieved at Si/Al = 8 (470 and
491 µmol g-1, Py-IR and NH3 TPD
respectively).
Direct and post
synthesis
Dragoi et al. 2009
Acidity quantified and evaluated
by Py-IR and NH3 TPD, highest
Brønsted acidity achieved with
post synthesis method. Si/Al = 15
(128 and 622 µmol g-1, Py-IR and
NH3 TPD respectively).
pH adjustment
and post
Gallo et al.
2010
[24]
13.2
417
synthesis
3

Acidity quantified by NH3 TPD.
Highest total acidity was seen at
Si/Al = 20 (440 µmol g-1)
decreasing Si/Al decreased
quantity of Brønsted sites.
Acidity evaluated by the catalytic
activity of n-heptane conversion.
Highest Brønsted acidity at Si/Al
= 14 (3.6 µmol g-1).
Acidity quantified and evaluated
by Py-IR, no difference in
Brønsted/Lewis ratios with Al
incorporation.
Acidity quantified and evaluated
by NH3 TPD, highest total acidity
at Si/Al = 34 (2960 µmol g-1).
Acidity evaluated by NH3 TPD,
highest total acidity at Si/Al = 3.4
(438 µmol g-1).
Acidity evaluated by Py-IR,
highest Brønsted acidity achieved
at Si/Al = 13 (58 µmol g-1).
Acidity quantified by propylamine
-TPD and evaluated by Py-IR.
Bhange et
al.
2011
2012
[25]
Direct synthesis
20
440
Koekkoek
et al.
[26]
[27]
Direct synthesis
pH adjustment
14
4
Ungureanu
et al.
2012
2015
5.2
471
S. Lin et al.
[28]
[29]
[30]
Direct synthesis
pH adjustment
34
2960
438
Meloni et al. 2016
3.4
Xing et al.
Socci et al.
2017
2018
Direct synthesis
pH adjustment
13
5
58
This
paper
564
According to the literature, the quality of aluminium incorporation for the generation of acid
sites is highly dependent on the synthesis procedure. The direct synthesis of Al-SBA-15
results in Al species that are not retained in the silica framework and often end up as extra-
framework aluminium. Likewise, post-synthesis techniques require additional preparation
steps and further calcination also leading to a high amount of extra-framework aluminium [24].
On the other hand, the “pH adjustment” method is a simple, but effective, technique for the
incorporation of high amounts of aluminium (Si/Al = < 25) into SBA-15, with high Brønsted
acidity, similar to those found in zeolites [27]. Furthermore, results for the characterisation and
quantification of acidic sites can vary between methods. Therefore, caution must be taken
when comparing values measured by different techniques [31].
Structural/textural properties are also heavily dependent on Al-SBA-15 synthesis parameters
such as ageing temperature and time: generally higher temperatures and longer hydrothermal
treatment steps lead to larger mesopore pore diameters and higher surface areas [32].
Previous work reported on the catalytic activity of Al-SBA-15 catalysts, for the cracking of
waste plastics, have either used direct synthesis methods or post-synthesis grafting at medium
Si/Al ratios which are unable to yield large quantities of aluminium in the tetrahedral
environment. This has led to an unreasonable comparison of both the catalytic activity and
distribution of products by OMMs, in the cracking of waste polyolefins, with much lower
acidities compared to that of zeolites [5,33,34].
Polyolefin plastics in particular, low density polyethylene (LDPE) in Fig. 1, accounts for the
majority of plastics consumed and find use in many applications, such as packaging and
4

insulation [1]. The catalytic cracking of polyolefins proceeds through the formation of a
carbonium ion or carbenium ion upon reaction with a Brønsted or Lewis acid site, respectively
[35]. Following this, several other acid-catalysed reactions may occur on the acid sites such
as isomerisation, oligomerisation, cyclisation, aromatisation and further cracking [1]. The
numerous side branches of varying chain length, present in LDPE, cause its low density and
low amounts of crystallinity compared to high density polyethylene (HDPE). Furthermore,
although the presence of side branches in LDPE facilitates cracking through the formation of
tertiary carbocations, the existence of cross-linking in LDPE reduces cracking rates compared
to HDPE. This is likely due to steric hindrances which limits access to active sites of the
catalyst [36].
Fig. 1. Repeatable monomer of low density polyethylene (LDPE).
In this work, a series of Al-SBA-15 catalysts have been synthesised with Si/Al ranging from
(100-5:1), using the alternative “pH adjustment” method, in order to produce acidities
comparable to zeolites. The newly-created catalysts were characterised by ICP-OES, XPS,
XRD, N2 physisorption, propylamine-TPD, pyridine-FTIR and STEM. They were subsequently
applied in the catalytic cracking of the sterically challenging feedstock LDPE, to investigate
the relationship between their physicochemical properties and their cracking effectiveness.
The catalytic properties of the Al-SBA-15 materials were compared to those of microporous
ZSM-5 catalyst in order to assess diffusion limitations and reactant accessibility of the active
sites.
2. Experimental methods
2.1 Chemicals and Materials
Pluronic P123 (Mn= 5800, EO20PO70EO20), tetraethyl orthosilicate (TEOS, >98%), Al
(NO3)39H2O (>98%), hydrochloric acid (37 wt. %) and ammonium hydroxide solution (28-30%
NH3 basis) were all purchased from Sigma-Aldrich. ZSM-5 (SM-27 (Si/Al=12) reference
material was purchased from Alsi-Penta Zeolithe GmbH. LDPE was kindly supplied by Sabic
Europe.
2.2 Synthesis of SBA-15
SBA-15 was synthesised following the procedure reported by Zhao and co-workers [37].
Approximately 4 g of Pluronic P123 triblock copolymer was dissolved in 125 ml of 2 M HCl
5

solution and stirred at 35 °C for 4 h. 8.5 ml of TEOS was added dropwise and left stirring for
a further 20 h. The resulting gel was aged at 100 °C for 48 h. The solid product was separated
by vacuum filtration, washed with water and dried at 60 °C, then heated to 550 °C at a ramp
rate of 1.5 °C min-1 and held for 6 h.
2.3 Incorporation of aluminium
A series of Al-SBA-15 catalysts was synthesised with Si/Al ratios ranging from 100:1 to 5:1
using a modified two-step “pH adjustment method” developed by Wu and co-workers [16]. The
method was modified by using Al(NO3)39H2O as the aluminium source instead of
Al2(SO4)318H2O; as well as incorporating a longer hydrothermal treatment in the second step
(72 h instead of 48 h to increase the pore size) [27]. Approximately 4 g of Pluronic P123 triblock
copolymer was dissolved in 125 ml of 2 M HCl solution and stirred at 35 °C for 4 h. 8.5 ml of
tetraethyl orthosilicate (TEOS) was added dropwise and left to stir for 3 h. Then, the amount
of Al(NO3)39H2O required to give the specific Si/Al ratio was added to the mixture and left to
stir for 20 h. The resulting gel was aged at 100 °C for 48 h before being cooled to room
temperature. The pH value of the mixture was increased to 7.5 by the dropwise addition of 4
M NH4OH with stirring, and the mixture was then subjected to a second hydrothermal
treatment at 100 °C for 72 h. The solid product was separated by vacuum filtration, washed
with water and dried at 60 °C, before being calcined in static air at a ramp rate of 1.5 °C min-1
to 550 °C with a hold time of 6 h. The resulting Al-SBA-15 materials were designated as
AlSBA15(X), where X indicates the molar Si/Al ratio in the synthesised catalyst.
2.4 Materials Characterisation
Low angle X-ray diffraction patterns were recorded for 2θ = 0.5-5° with a step size of 0.01, on
a Bruker D8 Advance diffractometer fitted with an X’celerator and Cu Kα (1.54 Å) radiation
source and nickel filter.
The Si and Al contents of the bulk of the samples were determined by ICP-OES on a Thermo
Scientific iCAP 7000 instrument. Prior to analyses, samples were digested in a mixture of nitric
acid, sulphuric acid and deionised water using a CEM SP-D discover microwave (300 W).
Ammonium fluoride was added to generate hydrofluoric acid in-situ before neutralisation with
boric acid and HCl. The samples were subsequently diluted with deionised water by a factor
of 10.
XPS analysis was carried out on a Kratos Supra employing a monochromated Al Kα X-ray
source (hv = 1486.7 eV). The data was processed using CasaXPS version 2.3.14, with binding
energies corrected to the C 1s peak at 284.6 eV and surface compositions quantified by
application of element and instrument specific response factors.
6

Nitrogen physisorption analyses were measured at -196 °C on a Quantachrome Nova 4000
porosimeter and analysed with NovaWin software. Samples were degassed under vacuum at
120 °C for 10 h prior to analysis. Surface areas (S) were calculated using the Brunauer, Emmet
Teller (BET) model and the de Boer “t-plot” method was applied to calculate the micropore
volume (Vμ). The single point gas adsorption volume (VT) was calculated using the amount of
gas adsorbed at a relative pressure of 0.98 in the desorption branch using the Barrett, Joyner
and Halenda (BJH) method. The volume of the mesopores (Vm) was calculated using the
equation Vm = VT - Vμ. The BJH method was also applied to the adsorption branch to calculate
mean pore diameters (dp).
Acid site densities were determined by propylamine adsorption followed by thermogravimetric
analysis coupled with mass spectrometry (TGA-MS), where chemisorbed propylamine
decomposes into propene and ammonia over acid sites. Prior to analyses, propylamine was
added dropwise to samples and then excess physisorbed propylamine was removed by
vacuum drying at 35 °C overnight. Temperature programmed desorption was performed on a
Mettler Toledo TGA/DSC 2 StarSystem between 40 and 800 °C under a He flow of 20 ml min-
1, using a ramp rate of 10 °C min-1. Propene and ammonia were detected using a Pfeiffer
Vacuum, ThermoStar MS at m/z = 41 and 17 respectively.
The Brønsted/Lewis acid character of the catalysts was studied by means of Diffuse
Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) using pyridine as a probe
molecule. Ex-situ pyridine adsorption was performed by the saturation of diluted samples (10
wt. % in KBr). Excess physisorbed pyridine was removed under vacuum at 35 °C prior to
spectral acquisition on a Thermo Scientific Nicolet iS50 FT-IR spectrometer with a mercury
cadmium telluride (MCT-A) photo detector at -196 °C, cooled by liquid nitrogen. Analyses were
performed in an environmental cell at 110 °C, to remove physisorbed water/moisture.
High-resolution S/TEM images were taken using an FEI Philips TECNAI F20 at 200 kV
equipped with an Oxford instruments ISIS Energy Dispersive Spectroscopy (EDS) detector,
located at The Centre for Electron Microscopy, University of Birmingham. The sample was
diluted in ethanol and then deposited by drop casting onto a carbon coated mesh copper grid
and then dried under ambient conditions.
2.5 Catalytic Cracking of LDPE
2.5.1 Thermogravimetric Analysis
Both thermal and catalytic cracking of LDPE was investigated using a Perkin-Elmer Pyris 1
thermogravimetric analyser (TGA) equipped with a 20 position autosampler. The powdered
polymer and powdered catalyst, with similar particle sizes (2-10 µm), were thoroughly mixed
using a mortar and pestle in a weight ratio of LDPE:catalyst = 3:1. Then, 5 mg of the sample
mixture was loaded into the ceramic crucibles for analysis. The sample was heated from 50
7

to 600 °C at 10 °C min-1, under a nitrogen inert atmosphere with a flow rate of 20 ml min-1.
Each experiment was performed in triplicate and an average of the measured values was
calculated and presented on a percentage conversion basis (Eq. 1).
퐼푛푖푡푖푎푙 퐿퐷푃퐸 푚푎푠푠−퐹푖푛푎푙 푚푎푠푠
( )
퐶표푛푣푒푟푠푖표푛 % =
Eq. 1
× 100
퐼푛푖푡푖푎푙 퐿퐷푃퐸 푚푎푠푠
2.5.2 Py-GC/MS Analysis
The selectivity of the thermal and catalytic cracking of LDPE was investigated by Py-GC/MS
on a CDS analytics (Chemical Data Systems, Oxford, PA) 5200 series pyrolyser, close-
coupled to a PerkinElmer Clarus 680 gas chromatograph (GC) and Clarus 600S mass
spectrometer. A small amount of LDPE (approximately 0.7 mg) was placed inside a 25 mm
quartz tube between quartz wool. For each catalytic experiment, the powdered polymer and
powdered catalyst, with similar particle sizes (2-10 µm), were thoroughly mixed using a mortar
and pestle until homogeneous to give a weight ratio of LDPE:catalyst = 3:1. A small mass of
catalyst/LDPE mixture (approximately 1 mg) was pyrolysed.
For each experiment, the sample was heated by a platinum coil probe to 700 °C at 10 °C ms-
1 with a hold time of 10 s under a He carrier gas flow. The pyrolysis interface was maintained
at 290 °C to prevent prior condensation. Volatilised compounds were immediately trapped on
a Tenax ®-TA adsorbent trap at 45 °C to avoid secondary/recombination reactions. The Tenax
®-TA adsorbent trap was then gradually heated to 295 °C over 2 mins and the pyrolysis
products were transferred to the GC column via a heated transfer line kept at 310 °C. A
PerkinElmer Elite-1701 column (cross-bond: 14% cyanopropylphenyl and 85% dimethyl
polysiloxane; 30 m, 0.25 mm i.d., 0.25 mm df) was used to separate the products using helium
as a carrier gas. The GC injection port was kept at 275 °C and a 1:125 split ratio was used.
The GC oven was heated at 5 °C min-1 from 45 °C to 280 °C. Proposed peak assignments
(m/z = 45-300) were made from mass spectra detection using the NIST 2011 MS library. The
results are presented in area %, corresponding to the percentage area of the chromatogram
peak with regards to the total peak area of the chromatogram. Thermal and catalytic Py-
GC/MS experiments were performed in duplicate to confirm the reproducibility of the reported
procedure.
Due to the elevated pyrolysis temperature, the thermal stability of the Al-SBA-15 catalysts was
first confirmed by XRD after calcination at 700 °C at 1.5 °C min-1 for 5 h. The corresponding
XRD patterns are displayed in the supporting information (Fig. S-2).
3. Results and discussion
8

3.1 Materials characterisation
Elemental analysis data of the Al-SBA-15 catalysts are given in Table 2. The experimentally
determined Al content (given as Si/Al ratio) is close to the planned composition. This highlights
the advantage of using the “pH-adjustment” method, which offers a high level of control of the
incorporation of aluminium from the initial synthesis gel to the final solid, in agreement with
prior work [16,27,29].
Table 2. Physicochemical properties of the studied catalysts.
Bulk
Si/Al
(at.)a
Surface
Si/Al
Vµ
Vm
(cm3
g-1)
VT
Acid site
loading
S
B
L
dp
Sample
(cm3
g-1)d
(cm3
g-1)e
B/L
(nm)
(m2 g-1)c
(µmol g-1)
(µmol g-1)
(at.)b
(µmol g-1)
AlSBA15(5)
AlSBA15(15)
AlSBA15(35)
AlSBA15(100)
SBA-15
5
17
27
75
-
5
12
15
30
-
414
412
432
485
857
304
0.00
0.00
0.02
0.03
0.06
0.11
1.0
0.8
1.3
1.3
1.2
0.0
1.0
0.8
1.4
1.3
1.3
0.2
6.8
6.0
7.4
9.5
6.0
0.9
564
409
232
124
17
347
194
103
29
217
215
129
95
1.6
0.9
0.8
0.3
0.1
1.3
2
15
ZSM-5
12*
0.2
672
380
292
Determined by a ICP-OES, b XPS, c BET, d t-plot, e BJH, *commercial specification
XPS analysis of the catalysts shows a deviation of the Si/Al ratios on the surface compared to
the bulk of the materials derived from ICP-OES (Table 2). The surface and bulk Si/Al ratio of
the aluminium rich catalysts (nominal Si/Al = 5 and 15) are quite similar, suggesting that
aluminium is uniformly dispersed within the SBA-15 structure. However, at higher Si/Al ratios
(nominal Si/Al > 35), larger quantities of aluminium were found at the surface of the material,
indicating that the aluminium is not homogeneously distributed throughout the material as was
also noted in a previous study on the synthesis of ZSM-5 [38]. The increase in aluminium at
the surface could possibly be due to extra-framework aluminium, leading to the generation of
Lewis acid sites as opposed to Brønsted acid sites. Although an excess of Al in the synthesis
procedure has not been investigated in this study, it can be assumed further incorporation of
Al will lead to the formation of predominantly extra-framework Al.
Analysis of the Si 2p region in Fig. 2A shows a perturbation of the peaks to lower binding
energies with increasing aluminium incorporation, attributed to the generation of Si-O-Al
species in agreement with previous work [39]. The emergence of a peak at high aluminium
levels is also evident at a binding energy of around 74 eV (Fig. 2B). This shifts slightly to
higher binding energies, due to the formation of Si-O-Al species [40].
9

Fig. 2. High resolution XP spectra of (A) Si 2p and (B) Al 2p for Al-SBA-15 catalysts: (a) SBA-
15, (b) AlSBA15(100), (c) AlSBA15(35), (d) AlSBA15(15) and (e) AlSBA15(5)
Fig. 3 shows the low-angle XRD patterns of the Al-SBA-15 catalysts and pure SBA-15, as a
reference. The diffraction pattern in the low angle region displays an intense peak associated
with the (100) plane and two less intense peaks associated with the (110) and (200) planes of
the lattice. The diffraction patterns are all consistent with well-ordered 2D hexagonal structures
typically displayed by SBA-15 mesoporous silica. A slight shift of the peak towards lower
angles is evident in the (100) plane (B to D, Fig. 3 inset), due to an increase in lattice spacing
(d). However, at the highest aluminium loading, i.e. AlSBA15(5), the angle of the peak is of
similar value to the Al-free SBA-15 (a). The evidence of three distinct reflections in the low-
angle XRD patterns in the series confirms that the short and long-range order of the hexagonal
structure is unaffected by the incorporation of aluminium.
(100)
(100)
e
d
c
b
a
(110)
(200)
e
0.50
0.75
1.00
d
c
b
a
1
2
3
4
5
2 (°)
10

Fig. 3. Low angle XRD patterns of Al-SBA-15 catalysts: (a) SBA-15, (b) AlSBA15(100), (c)
AlSBA15(35), (d) AlSBA15(15) and (e) AlSBA15(5) (inset: amplification of 2θ in the region
between 0.5 and 1.1°).
The textural properties of the synthesised materials were analysed by N2 physisorption (Fig.
4). All SBA-15 type catalysts exhibit type IV isotherms with H1 hysteresis, which are
characteristic of mesoporous solids possessing cylindrical pore geometry and a high degree
of pore size uniformity [41]. It appears that in the isotherms of the Al-SBA-15 catalysts with
increasing aluminium content (b to e), the hysteresis loops are shifted to lower relative
pressures, indicative of a decrease in pore sizes; with the exception of AlSBA15(5), having a
similar pore size to the Al-free counterpart. At high Si/Al ratios (such as AlSBA15(35)), irregular
hysteresis loops are evident, indicative of broad pore size distributions and large cylindrical
pores. This is in agreement with the measurements obtained by XRD and indicate that
structural ordering decreases at high Si/Al. Previous work by Ungureanu et al. [27], also noted
an increase in structural ordering with a decrease in Si/Al. Their explanation for this
observation was due to the addition of aluminium cations (from aluminium nitrate) in the
synthesis solution. This resulted in increased ionic strength in the synthesis solution and hence
a higher degree of ordering of the hexagonal structure of the SBA-15.
160
e
d
120
c
80
b
a
40
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 4. N2 physisorption isotherms of the Al-SBA-15 catalysts: (a) SBA-15, (b) AlSBA15(100),
(c) AlSBA15(35), (d) AlSBA15(15) and (e) AlSBA15(5) (isotherms were offset for clarity).
The structural properties derived from the physisorption isotherms are summarised in Table 2
above. It is clear from the BET results that increasing the incorporation of aluminium
dramatically decreases the surface area in the aluminium-rich catalysts compared with the Al-
11

free SBA-15. This is due to the disappearance of the micropore volume (Vµ) in the series, i.e.
the micropores disappear with the increasing Al content; the Al-free material has a micropore
volume of 0.06 cm3 g-1 in comparison to 0.00 cm3 g-1 for the AlSBA15(5) material. Hence, the
introduction of Al species into the SBA-15 structure inhibits the formation of micropores in the
synthesis. As such, micropore volume is below the detection limit for the AlSBA15(15) and
AlSBA15(5) materials, therefore leading to a purely mesoporous material at higher Al
concentrations.
Decreasing the Al content of the materials causes a slight decrease in the volume of
mesopores (Vm) and total pore volume (VT). On the other hand, the average pore (dp) diameter
of all the SBA-15 materials are in the region of 6-9 nm. These values are consistent with
reported values in the literature for SBA-15 using similar preparation procedures [42–44]. It is
worth noting that the pore diameter of the Al-rich materials appears to be larger than the Al-
free SBA-15. The increase in pore diameter is associated with the longer hydrothermal
treatment experienced by the Al-rich materials, which were treated for an additional 72 h in
comparison to Al-free SBA-15 [45]. Fig. S-1 (in supporting information) shows the BJH pore
size distributions of the Al-SBA-15 catalysts. SBA-15 exhibits the typical narrow pore size
distribution centred at 6.0 nm. Incorporating Al up to a Si/Al ratio of 35 broadens the pore size
distribution and shifts its centre to larger pore diameters. Further increase of the Al content
(AlSBA15(5)) narrows the pore size distribution and centres at a lower diameter of 6.8 nm.
N2 physisorption analysis of the ZSM-5 confirms the microporous nature of crystalline zeolite
, with a micropore volume of 0.11 cm3 g-1 and an average pore diameter of 0.9 nm, which is
approximately an order of magnitude smaller than the Al-SBA-15 materials.
To further elucidate the structure of the Al-rich SBA-15 material, STEM was performed on
AlSBA15(5). Fig. 5 shows well-ordered channels within the particle structure with an
approximate length of 700-750 nm, which suggests that the reactant molecules will be
travelling a substantial internal length of the mesopore whilst being exposed to the reactive
acid sites. In addition to STEM, EDS analysis was also carried out. Fig. 6 shows EDS maps
of the main elements (Si, O, and Al), confirming high dispersion of Al, Si and O atoms.
12

Fig. 5. High-resolution STEM image of an AlSBA15(5) particle. (Inset) increased
magnification.
Fig. 6. Energy Dispersive X-Ray Spectroscopy (EDS) analysis of AlSBA15(5).
The acidic properties of all materials were investigated via n-propylamine chemisorption
followed by TGA-MS analysis. N-propylamine is a weak base that reacts with accessible acid
sites to form propene and ammonia under thermal conditions. The desorption temperature of
reactively-formed propene is inversely proportional to the strength of the acid site. Fig. 7 shows
that increasing the Al content of the material barely changes the propene desorption
13

temperature of 435 °C, suggesting that all the Al-SBA-15 materials possess acid sites of
similar strength. In contrast to Al-SBA-15s, ZSM-5 exhibits two types of acid sites; strong acid
sites (major peak centred at 407 °C) and weak acid sites (small peak centred at 473 °C). The
acid site loadings were calculated and are presented in Table 2 above. The results show that
the acid site loading closely mirrors the aluminium content of the catalysts. The aluminium
species withdraw electrons from the framework hydroxyl groups, therefore increasing the
dissociation capability of the proton [46]. A small number of acid sites were identified in SBA-
15 which could be attributed to surface silanol groups. AlSBA15(5), with the highest aluminium
incorporation, displays the highest total acidity which appears to be exponentially proportional
to bulk aluminium incorporation as displayed in Fig. S-3.
SBA-15
407
AlSBA15(100)
AlSBA15(35)
AlSBA15(15)
AlSBA15(5)
ZSM-5
439
473
435
100
200
300
400
500
600
700
Temperature (C)
Fig. 7. Temperature programmed desorption of reactively formed propene from propylamine
decomposition over Al-SBA-15 and ZSM-5 catalysts.
The adsorption of pyridine was studied using FTIR spectroscopy to assess the nature of the
acid sites of the materials. The DRIFT spectra of the Al-SBA-15 catalysts and ZSM-5 zeolite,
following chemisorption of pyridine, are shown in Fig. 8. Absorption features at 1445 and 1595
cm-1 are assigned to pyridine interacting by electron donation to Lewis acid sites, due to
partially coordinated Al atoms in the extra-framework aluminium and silanol groups. This band
is present in all samples and decreases in intensity with an increase in aluminium along the
series of Al-SBA-15. An absorption band at 1545 cm-1 is more visible in AlSBA15(15),
AlSBA15(5) and ZSM-5, which is commonly assigned to pyridinium ions (PyH+) coordinated
to a Brønsted acid site. The absorption band at 1490 cm-1 is assigned to pyridine adsorbed
over Brønsted and Lewis sites [47]. The ratio of Brønsted to Lewis acidity present in the
materials was calculated by integration of the 1445 and 1545 cm-1 absorption bands and are
14

presented in Table 2 above, along with the individual contributions. It is apparent that the
Brønsted/Lewis ratio is positively correlated with the incorporation of aluminium, though a very
small quantity of Brønsted and Lewis sites are present in the Al-free SBA-15. It is worth noting
that 27Al NMR combined with acid site analysis on similar materials suggests that the
Brønsted acid sites are generated by framework Al. Extra-framework Al typically led to Lewis
acid sites [27].
PyL
PyH+ PyH+/PyL PyL
f
e
d
c
b
a
1700
1600
1500
1400
Wavenumber (cm-1)
Fig. 8. DRIFT spectra of chemisorbed pyridine over Al-SBA-15 and ZSM-5 catalysts: (a) SBA-
15, (b) AlSBA15(100), (c) AlSBA15(35), (d) AlSBA15(15), (e) AlSBA15(5) and (f) ZSM-5
TPD of ammonia is widely used for the characterisation of acid sites. However, due to the high
basicity of ammonia, weak sites that may not contribute to the acidity of the catalyst are
capable of being titrated. This can lead to an overestimation of the number of acid sites by this
method. In addition, the small molecular size of ammonia enables the accessibility of all pores
of the solid which may be impenetrable by larger molecules commonly found in cracking
reactions [48]. In contrast, propyl amine, used in this study, gives a more realistic insight as it
only titrates strong and medium strength acid sites. Furthermore, due to its larger size, it is
only able to access pore sizes in the range required for catalytic cracking reactions.Therefore,
the acid sites characterised in this study by propyl amine are all expected to be accessible
and active in the cracking of LDPE.
In summary, the quantity of Brønsted acid sites generated in the Al-SBA-15 materials
synthesised in this work is comparable with previously reported values where the “pH
adjustment” method was employed. These observations highlight that the “pH adjustment”
15

method allows precise control over the incorporation of Al into the silica framework. Therefore,
materials derived from this method afford acidic properties comparable to zeolites. It is worth
noting, that the “pH adjustment” method is highly reproducible offering consistent results
across studies.
3.2 Catalyst Testing
3.2.1 Thermogravimetric analysis
Thermogravimetric analysis was carried out to compare the activity of Al-SBA-15 catalysts
with different Si/Al ratio on cracking of LDPE. Additionally, the protonic form of ZSM-5 (Alsi-
Penta SM-27) with a nominal Si/Al of 12, was employed as a reference material.The results
of the thermal and catalytic degradation of LDPE derived from thermogravimetric analysis are
plotted in Fig. 9.
Thermal
100
SBA-15
AlSBA15(100)
AlSBA15(35)
AlSBA15(15)
AlSBA15(5)
80
ZSM-5
60
40
20
A
0
200
300
400
500
600
Temperature (°C)
Thermal
SBA-15
AlSBA15(100)
AlSBA15(35)
AlSBA15(15)
AlSBA15(5)
ZSM-5
B
200
300
400
500
600
Temperature (°C)
Fig. 9. (A) Conversion plot for the thermal and catalytic degradation of LDPE measured by
thermogravimetric analysis. (B) Derivative thermogravimetric analysis plot of the thermal and
catalytic degradation of LDPE.
16

Thermal degradation of pure LDPE polymer commenced around 400 °C, reaching a maximum
reaction rate at 464 °C. In the presence of solid acid catalysts, the temperature of maximum
degradation rate is shifted much lower due to the catalytic cracking over Brønsted acid sites
[49]. The presence of pure SBA-15 causes a decrease of 7 °C to the temperature of maximum
degradation rate suggesting that even with a low quantity of acid sites a noticeable catalytic
effect is observed. Furthermore, Fig. 9 shows that LDPE degradation temperature decreases
continuously with increasing Al loading of the catalyst. This can be attributed to the increasing
Brønsted/Lewis acid site ratio. Fig. 10 shows temperatures of conversions at 5, 50 and 80%,
derived from TGA data using Eq. 1, in relation to the Brønsted/Lewis ratio of the Al-SBA-15
catalysts. There is a clear linear correlation between the lower temperatures of LDPE
degradation and increasing Brønsted acidity.
Since the Brønsted/Lewis ratio for the Al-SBA-15 catalysts increases with the incorporation of
aluminium, a similar correlation can be seen for the number of acid sites. Nevertheless, it is
difficult to distinguish the individual contribution of the two types of acid sites. However, based
on the acidity analysis, it can be observed that the two Al-rich catalysts (AlSBA15(5) and
AlSBA15(15)) possess very similar proportion of Lewis acid sites (Table 2) and similar acid
strength (Fig. 7). Despite these similarities, the activity of AlSBA15(5) is considerably greater
than that of AlSBA15(15) (Fig. 10 and Table S-1). Hence, the greater activity of AlSBA15(5)
can be attributed to the higher Brønsted acid site loading. This hypothesis is supported by the
literature, based on theoretical modelling performed by Li et al. [50], it was shown that Lewis
acid catalysed cracking has a higher energy barrier compared to Brønsted acid cracking.
Therefore, it can be concluded that both types of acid sites may contribute to the catalytic
cracking of LDPE, however the contribution of Brønsted acid sites are more significant.
17

500
450
400
350
300
250
T5
T50
T80
ZSM-5
0.0
0.4
0.8
1.2
1.6
Brønsted/Lewis ratio
Fig. 10. Temperatures at conversions of 5, 50 and 80% of LDPE pyrolysis in relation to
Brønsted to Lewis ratio of Al-SBA-15 catalysts and ZSM-5 (hollow squares).
An assessment of catalytic activity can be determined by a simple measurement of the
temperature at which maximum cracking is observed (see Fig. 9B above), using
thermogravimetric analysis. The catalyst lowers the activation energy of the cracking reaction;
therefore, catalysts with higher activity perform maximum cracking at lower temperatures. As
expected, ZSM-5 was highly active for the cracking reaction, due to its high acid site loading
(672 µmol g-1 ) and high Brønsted/Lewis ratio. However, ZSM-5 performed the degradation at
a slightly higher temperature than the AlSBA15(15) catalyst, with a comparative Si/Al ratio,
much higher than the AlSBA15(5) catalyst. This comparatively lower activity can be associated
with the microporous nature of the ZSM-5 structure, which restricts the diffusion of sterically
challenging molecules into its internal acid sites. This highlights the importance of evaluating
a combination of increased Brønsted acidity and acid site accessibility for cracking sterically
challenging molecules.
Previous investigations by Aguado et al. [5] into the catalytic degradation of LDPE, by a variety
of catalysts, reported a maximum degradation rate at 396 °C using nanometer crystal-sized
ZSM-5. A more recent investigation by Ding et al. [51], on the synthesis of mesoporous ZSM-
5, reported a maximum degradation temperature of 348 °C, much lower than that of
conventional ZSM-5. In this work, the lowest temperature of LDPE degradation was achieved
with the AlSBA15(5), with a maximum degradation rate at around 390 °C. Therefore, it is clear
the accessibility of the reactant molecules has a profound effect on the catalytic activity of the
cracking of polyolefins. However, from a commercial perspective, the selectivity of the catalyst
may be of more value, allowing the conversion to be tailored towards more desirable products.
18

3.2.2 Py-GC/MS analysis
Pyrolysis gas chromatography/mass spectroscopy (Py-GC/MS) was employed to investigate
the distribution of the products i.e. the proportion of desired products compared to total
products. The product distribution is highly dependent on pyrolysis temperature, heating rate
and also on the occurrence of secondary reactions. A scheme of the reactor set-up is provided
in Fig. 11. With the introduction of a volatile trap, secondary reactions are minimised due to
the short residence time of the generated vapours. While this work was only able to consider
volatile products with molecular weights below 300 g mol-1, Py-GC/MS presents a simple
analytical technique for the qualitative assessment of catalyst selectivity in LDPE catalytic
pyrolysis.
Fig. 11. Scheme of the pyroprobe reactor and GC/MS setup.
It is well known that the catalytic cracking of hydrocarbons is initiated through either the
generation of a carbonium ion or a carbenium ion by a Brønsted or Lewis acid site,
respectively, as shown in Fig. 12 [49]. However, due to the complexity of pyrolysis, with
multiple reactions happening simultaneously, a more in depth reaction mechanism cannot yet
be presented.
19

Fig. 12. Mechanism of acid catalysed cracking of hydrocarbons.
Fig. 13 shows the product distribution by the carbon number (Cn) of identifiable products,
corresponding to the peak area % in the chromatogram, for the thermal and catalytic pyrolysis
of LDPE as studied by Py-GC/MS (a more detailed breakdown is available in the supporting
information, Fig. S-4). Characteristic triplet peaks are apparent in the GC chromatogram (see
Fig. S-5), corresponding to the production of alkanes, alkenes and dienes. The molecules of
each subsequent triplet peak contains an additional carbon atom as the peaks evolve. The
product distribution of thermal LDPE pyrolysis yields a wide range of products spread between
4-20 Cn. Thermal pyrolysis mainly yielded C7 products, this contributed to ca. 14 % of the
overall identifiable products. Furthermore, as is evident in Fig. 13, the thermal pyrolysis of
LDPE produced a higher percentage of products with longer carbon chain length (C11-20
)
compared to the catalytic experiments.
20

70
60
50
40
30
20
10
0
Thermal
SBA-15
AlSBA15(100)
AlSBA15(35)
AlSBA15(15)
AlSBA15(5)
ZSM-5
C1-C5
C6-10
C11-C15 C16-C22
Fig. 13. The relative abundance of identified products from the thermal and catalytic cracking
of LDPE analysed by Py-GC/MS, grouped by number of carbon atoms.
In the catalytic fast pyrolysis of LDPE at 700 °C, SBA-15 showed the least activity, with a
selectivity similar to thermal pyrolysis, consistent with the low quantity of Brønsted and Lewis
acid sites. Comparing the product distribution of LDPE cracking over SBA-15 with simple
thermal cracking reveals that the selectivity to small hydrocarbon compounds with 1 < Cn < 10
has slightly increased while lowering the selectivity of Cn > 10 products.
Critically, the product distribution from the catalytic cracking of LDPE appears to depend
strongly on the Al content of the SBA-15 materials. In other words, by tuning the Al content of
SBA-15 the selectivity of the LPDE catalytic cracking can be altered, directing the reaction
towards a different range of products. The results in Fig. 13 show that with increased
aluminium content of the Al-SBA-15 catalysts, a higher degree of cracking is achieved. More
specifically, increasing the Al content of the materials lead to a significant reduction in the
formation of Cn > 10 products in favour of forming Cn <10 compounds in the range of gasoline
(C6-C12), as shown in Fig. 13. At increased aluminium incorporation (Si/Al < 35), there was a
complete absence of the characteristic triplicate peak corresponding to an alkane, alkene and
diene in the GC chromatogram. This suggests that the typical products from thermal cracking
of LDPE are further cracked or converted in the presence of strong acid catalysts. This led to
an irregular product distribution for each catalyst, dependent on individual catalyst selectivity.
ZSM-5 also resulted in a high level of cracking, reducing the number of products with C11-22
,
and increasing the number of lower Cn products, particularly between C7-8. Interestingly, ZSM-
5 displayed a greater percentage of aromatic products in comparison to the Al-SBA-15
catalysts. This is due to its shape-selective ability from its complementary pore size and
21

structure [1,52]. This suggests that shape selectivity is much more important for the production
of aromatics than acidity, although low acidity in the Al-SBA-15s appeared to be preferential
for the production of aromatics (Fig. S-4, C6-8). Furthermore, although ZSM-5 had a much
higher acidity than AlSBA15(15), it produced a similar distribution of products in the region of
C11-22 to the AlSBA15(15) catalyst. Again, this could be due to the microporous nature of the
material and thus the diffusional hindrances of the long-chained polymers in comparison to
the mesoporous SBA-15 type materials.
Based on the Py-GC/MS analysis, it is evident the degree of LDPE cracking is a function of
aluminium loading and in particular the quantity of Brønsted acidity. Hence, Al-rich SBA-15
materials produced predominantly short chain-length products, greater than that of ZSM-5.
Therefore, due to the increased pore size and increased achievable acidities of Al-SBA-15,
comparable to that of zeolites, the application of these catalysts for the catalytic cracking of
sterically challenging feedstocks is promising. However, it is evident that ZSM-5 is highly
shape selective towards the production of aromatics, preventing further cracking to lower
molecular weight molecules. This presents an opportunity for further investigation into a multi-
step catalytic approach, with high acidity mesoporous catalysts being initially used to crack
the bulky feedstock molecules followed by ZSM-5 to produce aromatics. Research is
underway to identify quantifiable product yields and to investigate scale-up challenges of Al-
SBA-15 such as catalyst lifetime and reusability and also to investigate a close coupled
sequential process.
Conclusions
In summary, a series of Al-SBA-15 catalysts were synthesised and applied to the catalytic
cracking of bulky polyolefin molecules, LDPE, and compared to ZSM-5. The product
distribution from the catalytic cracking of LDPE depends strongly on the Al content of the SBA-
15 materials. The maximum rate of catalytic LDPE degradation (calculated by
thermogravimetric analysis) was achieved by AlSBA15(5), with a Si/Al of 5, approximately 70
°C lower than that of simple thermal degradation. The catalytic activity of all the synthesised
Al-SBA-15 catalysts was demonstrated to be directly proportional to acidity and, more
specifically, Brønsted to Lewis ratio. Due to the enhanced diffusional efficiency of reactants in
the mesoporous structure of the Al-SBA-15 catalysts compared to ZSM-5, a higher selectivity
of molecules in the gasoline range was produced in the case of AlSBA15(5) as demonstrated
by Py-GC/MS analysis. Moreover, the pore architecture of the catalyst had a significant effect
on the product selectivity. The micropore openings of ZSM-5 were more selective towards
aromatic products, preventing further cracking while the Al-SBA-15 materials allow for a
broader distribution of aliphatic products. In conclusion, high acidity Al-SBA-15 catalysts are
promising for the catalytic cracking of sterically challenging feedstocks such as waste plastics.
This work paves the way for the development of sequential reactor systems which include
22

efficient catalytic cracking units followed by catalytic upgrading of the resultant catalytic
pyrolysis products.
Acknowledgements
Joseph Socci would like to gratefully acknowledge Aston University for financing his PhD
scholarship. The authors would also like to acknowledge Jinsen Tian from the School of
Metallurgy and Materials, University of Birmingham for his assistance with STEM-EDS
measurements. Amin Osatiashtiani and Georgios Kyriakou would like to thank The British
Council for funding through the Newton Institutional Links scheme.
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