Evaluating pore characteristics and acid site locations in hierarchical SAPO-11 by catalytic model reactions

Microporous and Mesoporous Materials 329 (2022) 111550

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Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso

Evaluating pore characteristics and acid site locations in hierarchical
SAPO-11 by catalytic model reactions
Daniel Ali a, Zhihui Li a, Muhammad Mohsin Azim a, Hilde Lea Lein b, Karina Mathisen a, *
a
b

Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491, Trondheim, Norway
Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491, Trondheim, Norway

A R T I C L E I N F O

A B S T R A C T

Keywords:
Hierarchical
SAPO-11
Zeotype
Hydrothermal synthesis
Model reactions

Hierarchical SAPO-11 molecular sieves were synthesized with three different mesopore structure directing agents
(meso-SDAs): cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA) and [3-(trimethoxysilyl)
propyl] dimethyloctadecylammonium chloride (TPOAC). Two model reactions, methanol-to-hydrocarbons
(MTH) and the Beckmann rearrangement (BMR) of cyclohexanone oxime, were employed to evaluate the pore

topology and acid site locations of the hydrothermally synthesized hierarchical SAPO-11s. Initially, the modified
porosity of the hierarchical SAPO-11s was thoroughly probed by employing a set of general characterization
methods and by comparing the results to the conventional microporous C-SAPO-11. The nitrogen physisorption
results revealed that CTAB-11 had a uniform distribution of mesopores centered at 2.8 nm, whereas the presence
of mesopores in PVA-11 could not be convincingly resolved through conventional methods. Instead, the pore
topology of PVA-11 was determined by utilizing model reactions, where the shape selective MTH model reaction
revealed that the sample had mesopores present through an increased production of large products compared to
the conventional C-SAPO-11. Additionally, the MTH model reaction showed that while PVA shifted the location
of the Brønsted acid sites (BAS) towards the mesopores, CTAB did not affect the BAS location of SAPO-11. Finally,
the BMR model reaction elucidated the excellent intrapore connectivity of the hierarchical SAPO-11s through an
increased lifetime compared to the conventional C-SAPO-11.

1. Introduction
The silicoaluminophosphate-11 (SAPO-11) is a microporous, acidic
SAPO with the AEL framework which consists of 10-membered elliptical
rings with 0.40 × 0.65 nm pore openings aligned in a one-dimensional
array [1]. Compared to other SAPOs such as SAPO-34 (CHA) and
SAPO-5 (AFI), SAPO-11 is known to be mildly acidic, i.e. it has a low
density of Brønsted acid sites [2,3]. Still, SAPO-11 is a well-studied
material and is catalytically active for many reactions, including
hydroisomerization of n-alkanes [4–6], methanol-to-hydrocarbons [2,3,
7] and the vapor phase isomerization of cyclohexanone oxime (Beck­
mann rearrangement) [8]. Due to its narrow micropores however, the
SAPO suffers from deactivation over time, largely due to diffusion and
mass transfer limitations which result in coke formation due to trapped
hydrocarbons [3,5,8]. In order to mitigate this, the introduction of an
auxiliary large-pore system (most often mesopores) to make what is
known as hierarchical SAPO-11 has recently gained attention [4,5,
9–11]. Here, the larger pores are thought to function as super-highways

that transport molecules to the micropores and improve the accessibility
to active sites. This would simultaneously reduce the diffusion limita­
tions and enhance the lifetime of the catalyst.
Several approaches for synthesizing hierarchical SAPO-11 have been
reported [6,11,12], where hydrothermal synthesis [4,10,13,14] is
possibly the most frequently employed synthesis method. For hydro­
thermal synthesis, the auxiliary pore system is introduced during the
initial synthesis process either by hard-templating [15], or by
soft-templating with mesopore structure directing agents (meso-SDAs)
such as surfactants or polymers [9,10]. Indeed, quaternary ammonium
surfactants, organosilane surfactants and polymers have all previously
been successfully applied as meso-SDAs for the hydrothermal synthesis
of hierarchical SAPO-11 [10,15,16]. However, there has yet to be direct
comparison of how these soft-template meso-SDAs may produce
different types of hierarchy in hierarchical SAPO-11.
Two model reactions which have already shown promise for the
evaluation of hierarchy in SAPOs, are the methanol-to-hydrocarbons
reaction (MTH) [17] and the isomerization of cyclohexanone oxime

* Corresponding author.
E-mail address: (K. Mathisen).
/>Received 3 June 2021; Received in revised form 13 September 2021; Accepted 2 November 2021
Available online 6 November 2021
1387-1811/© 2021 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license ( />

D. Ali et al.

Microporous and Mesoporous Materials 329 (2022) 111550

(Beckmann rearrangement) [18]. While SAPO-11 typically has low hy­

drocarbon yields during the MTH reaction due to its mild acid properties
(vide supra) [3], it is known to be a highly active catalyst for the Beck­
mann rearrangement [8]. The Beckmann rearrangement (BMR) of
cyclohexanone oxime rearranges the oxime into its lactam oligomer, the
industrially relevant nylon-6 precursor, ε-caprolactam (CPL) [19]. Pre­
vious reports have already shown that the introduction of auxiliary
mesopores into microporous SAPOs leads to a longer lifetime for the
reaction due to alleviation of diffusion limitations [18,20]. Furthermore,
the BMR also shows promise for demonstrating the presence of con­
nected micro- and mesopores in both one- and three-dimensional
SAPOs, such as the SAPO-5 and SAPO-34, respectively [18].
In this study, one-step hydrothermal syntheses of hierarchical SAPO11 molecular sieves were attempted with three different types of mesoSDAs (soft-templates) in order to study the effects of template type on
the resulting hierarchical SAPO-11. Specifically, a quaternary ammo­
nium surfactant (cetyltrimethylammonium bromide, CTAB), a polymer
(polyvinyl alcohol, PVA) and an organosilane surfactant ([3-(trime­
thoxysilyl) propyl] dimethyloctadecylammonium chloride, TPOAC)
were employed as meso-SDAs. After a comprehensive characterization,
the MTH and BMR model reactions were utilized respectively to eval­
uate the acid site locations and pore topologies of the hierarchical SAPO11s compared to the conventional, microporous SAPO-11.

Table 1
An overview of the mesopore structure directing agents (meso-SDAs) employed
in this study.
Parameter

Meso-SDA property

Short
name

CTAB

PVA

TPOAC

Name

Cetyltrimethylammonium
bromide

Polyvinyl alcohol

[3(trimethoxysilyl)
propyl]
dimethyloctadecylammonium
chloride

Quaternary
ammonium
surfactant

Polymer

Organosilane
surfactant

Structure

2. Experimental

2.1. Synthesis of samples

Meso-SDA
type

2.1.1. Conventional SAPO-11
The conventional SAPO-11 was hydrothermally synthesized using a
single-SDA modification of the procedure described by Zhao et al. [10]
in order to obtain phase pure SAPO-11. An initial solution of aluminium
isopropoxide (Al(O-i-Pr)3, 11.03 g, Sigma Aldrich, ≥98%) in deionized
water (H2O, 57.39 g) was stirred until homogeneous, after which
phosphoric acid (H3PO4, 6.22 g, Merck, 85%) was added dropwise and
the resulting mixture was stirred for 1 h. Subsequently, tetraethyl
orthosilicate (TEOS, 1.13 g, Sigma Aldrich, 98%) was added and the
mixture was stirred for 2 h before dropwise addition of the micropore
SDA (micro-SDA), dipropylamine (DPA, 3.28 g, Fluka, ≥99%). After
stirring for an additional 2 h, the final mixture, with a theoretical
composition of 1.0Al: 1.0P: 0.1Si: 0.6DPA: 60H2O, was adjusted to pH
6.0 using phosphoric acid before being poured into a 60 mL Teflon-lined
stainless-steel autoclave for crystallization at 170 ◦ C for 48 h. After
quenching, the resulting powder was washed three times with deionized
water and once with ethanol. The final product, C-SAPO-11, was ob­
tained after drying for 3 h at 110 ◦ C and finally calcining for 6 h at
600 ◦ C in air.

acronym of the meso-SDA used for the synthesis, resulting in the ma­
terials CTAB-11, PVA-11 and TPOAC-11. To optimize the crystallinity
and phase purity of the hierarchical SAPO-11s, a parameter study of the
crystallization times and temperatures was carried out and has been
detailed in the supplementary information, Tables S1–1. Accordingly,

the crystallization temperature for CTAB-11 and PVA-11 was set to
170 ◦ C, whereas the crystallization times were 84 h and 48 h, respec­
tively. For TPOAC-11, the crystallization temperature and time was
200 ◦ C and 48 h, respectively.
2.2. Characterization
The experimental information regarding the characterization tech­
niques is based on previous reports [17] and has been detailed below.
X-ray powder diffraction (XRD) was performed on a Bruker D8 Focus
X-ray Diffractometer with a CuKα radiation source (1.5406 Å) and
LynxEye™ SuperSpeed Detector. The diffractograms were recorded
from 5 to 60◦ with a step size of ~0.01◦ . A fixed 0.2 mm divergence slit
was used throughout the run. Relative crystallinities were calculated
according to previously reported methods [21,22] using the sum of the
following reflections of 2θ: 8.1◦ , 9.4◦ , 13.1◦ , 15.6◦ , 20.3◦ and 21.0◦ .
Nitrogen physisorption analyses were carried out on a Micromeritics
Tristar 3000 Surface Area and Porosity Analyzer at − 196 ◦ C. Prior to
measurements, the materials were degassed under vacuum at 250 ◦ C
using a Micromeritics VacPrep 061 Sample Degas System in order to
remove water and other volatile adsorbates. The specific surface area
was determined by the BET (Brunauer-Emmett-Teller) method while the
micropore and external area were estimated using the t-plot method.
Finally, the specific pore volumes were obtained by BJH (Barrett-Joy­
ner-Halenda) analysis.
Scanning electron microscopy (SEM) was performed on a Hitachi
S–3400 N where the samples were gold coated by sputtering using an
Edwards Sputter Coater (S150B) prior to imaging. Images were captured
in secondary electron (SE) mode while particle sizes were determined

2.1.2. Hierarchical SAPO-11
Hierarchical SAPO-11 was synthesized by adding a certain equiva­

lent of mesopore SDA (meso-SDA) to the synthesis procedure of the
conventional material (vide supra). Specifically, the meso-SDA was
either the polymer polyvinyl alcohol (PVA, Alfa Aesar, 86–89%), the
organosilane surfactant [3-(trimethoxysilyl) propyl] dimethyloctadecy­
lammonium chloride (TPOAC, Sigma Aldrich, 42 wt%), or the quater­
nary ammonium surfactant cetyltrimethylammonium bromide (CTAB,
Sigma Aldrich, >99%) (see also Table 1). The meso-SDA was added to
the initial solution of Al(O-i-Pr)3 and H2O. In accordance to previous
reports [10], an additional micro-SDA, diisopropylamine (DIPA, 0.82 g,
Sigma Aldrich, ≥99.5%), was added in conjunction with DPA in order to
produce phase pure hierarchical samples. The final mixtures for poly­
mer- and surfactant-based syntheses had theoretical compositions of
1.0Al: 1.0P: 0.1Si: 0.3DPA: 0.3DIPA: 60H2O: 15 gPVA LH2O− 1 PVA and
1.0Al: 1.0P: 0.1Si: 0.3DPA: 0.3DIPA: 60H2O: 0.025 surfactant, respec­
tively. The washing, drying and calcination procedures were identical to
that of the conventional C-SAPO-11 and labelling was done by using the
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Microporous and Mesoporous Materials 329 (2022) 111550

from single images constituting at least 100 particles per sample using
the software ImageJ (version 1.52a) [23].
Thermogravimetric analyses coupled with mass spectrometry (TGAMS) were carried out with 10–15 mg of filtered particle size (212–425
μm) on a Netzsch Jupiter STA 449 equipped with a QMS 403 Aăelos
quadrupole mass spectrometer. The flow consisted of 45 mL min− 1 air
and 25 mL min− 1 argon while the temperature program started at 35 ◦ C,
subsequently heated to 850 ◦ C at a rate of 2 ◦ C min− 1, and held for 6 h

before finally cooling down to room temperature at a rate of 2 ◦ C min− 1.
Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) was
conducted using an Agilent 8800 Triple Quadropole ICP-MS (ICP-QQQ)
with a SPS 4 Autosampler. The samples (20–40 mg) were decomposed
with concentrated nitric acid (HNO3, 1.5 mL, 65%) and concentrated
hydrofluoric acid (HF, 0.5 mL, 40%). The final solution was diluted with
deionized water and filled into a 16 mL sample tube. Before analysis, the
samples were re-diluted in 5% HNO3 and 115In was added as an internal
standard. Standards from Inorganic Ventures were used for
quantification.
Carbon monoxide (CO) adsorption was performed with a Bruker
Vertex 80 FTIR spectrometer equipped with an LN-MCT detector from
Kolmar Technologies and a custom-built transmission cell. Measure­
ments were conducted at an aperture setting of 2 mm, a scanner velocity
of 20 kHz and a resolution of 4 cm− 1. Samples were pressed into selfsupported wafers (5–7 mg) and were pre-treated for 1 h at 500 ◦ C
under vacuum to remove adsorbed water and impurities. Subsequently,
the cell was cooled to − 196 ◦ C before slowly introducing CO (AGA).
Finally, stepwise desorption of CO was conducted by gradually lowering
the pressure in the system until the initial spectrum was recovered.

3. Results & discussion
3.1. General characterization
To investigate the effects of utilizing different meso-SDAs for syn­
thesizing hierarchical SAPO-11, a thorough general characterization
was conducted. Initially, XRD was employed to evaluate the phase pu­
rity as well as the relative crystallinity of the samples. Following this,
ICP-MS was conducted in order to determine the elemental composition
of the samples, while SEM was carried out to assess morphologies and
particle sizes. Finally, nitrogen physisorption was performed in order to
evaluate the presence of incorporated mesopores in the hierarchical

SAPO-11 systems.
The X-ray diffractograms of the calcined conventional and hierar­
chical SAPO-11s are stacked together with the simulated AEL pattern in
Fig. 1 [1]. The major crystalline phase for C-SAPO-11, CTAB-11 and
PVA-11 was the AEL phase, indicating that the SAPO-11 structure was
successfully obtained for these samples. For TPOAC-11 however, the
AEL reflection at 21.2◦ (denoted with a circle in Fig. 1) was missing and
instead, several impurities (marked with asterisks in Fig. 1) had
appeared. These impurities were also present in the as-synthesized
sample and due to the low crystallinity (<50%) and phase purity of
the sample, TPOAC-11 was not included in further analyses in this study.
In contrast to this, CTAB-11 was phase pure, whereas the conventional
C-SAPO-11 and PVA-11 only had minor impurities (indicated with as­
terisks in Fig. 1). Specifically, PVA-11s impurity was also present in the
as-synthesized sample whereas C-SAPO-11s impurity was only present
in the calcined sample. These impurities were ascribed to cristobalite
and the AFI framework for C-SAPO-11 and PVA-11, respectively [24,
25]. Furthermore, the relative crystallinities of C-SAPO-11 and PVA-11
were equally high (100%), whereas CTAB-11 had a slightly reduced
crystallinity (84%), as given in Table 2. The high relative crystallinity of
PVA-11 as well as the reduced relative crystallinity of CTAB-11 matches
previous reports on hierarchical SAPO-11 synthesized with PVA and
CTAB respectively [10,26]. In summary, utilizing PVA and CTAB as
meso-SDAs resulted in successful syntheses of highly crystalline
SAPO-11 structures. TPOAC on the other hand, could not be utilized as a
meso-SDA for the hydrothermal synthesis of hierarchical SAPO-11 ac­
cording to the methods described in this study.
ICP-MS results (Table 2) revealed that the hierarchical SAPO-11s
contained slightly more Si than the conventional C-SAPO-11, with the
amount of incorporated Si increasing in the order of C-SAPO-11≤PVA11

only sample where the theoretical Si/Al ratio matched the actual ratio of

2.3. Model reactions
In accordance to previous reports [17], the methanol to hydrocar­
bons (MTH) model reaction was carried out in a tube reactor (ID: 4 mm).
The reaction products were analyzed with a gas chromatograph equip­
ped with a flame ionizing detector (FID) coupled to a mass spectrometer
(GC-MS, Agilent 7890A coupled to an Agilent 5975C inert XL MSD).
In a typical experiment, 30 mg of filtered particle size (212–425 μm)
of calcined SAPO-11 was loaded into the reactor before water and other
adsorbed impurities were removed by heating the reactor to 500 ◦ C for 1
h. The reaction was performed at 400 ◦ C by sending chilled methanol
(0 ◦ C, VWR, ≥99.8%) carried by helium into the reactor at a Weight
Hourly Space Velocity (WHSV) of 1.8 gMeOH gcat− 1 h− 1.
The vapor phase Beckmann rearrangement of cyclohexanone oxime
was carried out in a vertical tube reactor (ID: 4 mm) attached to a
condenser. The obtained liquid products were identified and analyzed
with a gas chromatograph equipped with a flame ionizing detector (FID)
coupled to a mass spectrometer (GC-MS, Agilent 7890A coupled to an
Agilent 5975C inert XL MSD). Standards of cyclohexanone oxime (CHO,
99%, Sigma Aldrich) and ε-caprolactam (CPL, 97%, Sigma Aldrich) were
initially obtained and used to identify these reaction constituents.
Following this, calibrations with the internal standard chlorobenzene
(CB, 99%, Sigma Aldrich) were conducted to determine the CHO and
CPL responses for the instruments used in the current study.
In a typical experiment, 30 mg of filtered particle size (212–425 μm)
of calcined SAPO-11 was loaded into the reactor before heating the
reactor to 500 ◦ C for 8 h in He to remove water and other adsorbed
impurities. The reaction was performed at 325 ◦ C with a helium flow of
14 mL min− 1. The liquid-feed consisted of 5 g L− 1 of cyclohexanone

oxime in ethanol and was fed via an electric syringe pump (KD Scientific
Legato 210) at a weight hourly space velocity (WHSV) of 0.79 gCHO
gcat− 1 h− 1. The conversion was calculated based on previously reported
methods [18] whereas the calibration, selectivity and mass balance
calculations have been detailed in the supporting information (Section
3.0).

Fig. 1. XRD of C-SAPO-11, CTAB-11, PVA-11 and TPOAC-11 with the AEL
structure as a reference. Asterisks denote impurities while circles denote
missing reflections.
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Microporous and Mesoporous Materials 329 (2022) 111550

Table 2
Summary of XRD, ICP-MS and nitrogen sorption characterization results for the SAPO-11s.
Sample

RCa
(%)

Equivalents

SBET

Smicro

Sext

Vmicro

Vmeso

C-SAPO-11

100

0.1

0.07

211

135

76

0.07

0.07

PVA-11
CTAB-11
TPOAC-11

100
84

40

0.1
0.1
0.1

0.08
0.1
–

208
202
–

139
78
–

69
124
–

0.07
0.04
–

0.06
0.14
–

a
b
c

Si/Altheoryb

Si/AlICPc

Surface area (m2 g-1)

Pore volume (cm3 g-1)

Relative crystallinity.
Theoretically calculated gel composition.
Sample composition obtained by ICP-MS element analysis for calcined samples.

incorporated silicon in the calcined sample. This is in contrast to an
earlier report on hierarchical SAPO-11 hydrothermally synthesized with
CTAB however [4], where the surfactant instead facilitated the incor­
poration of Al into the SAPO structure, resulting in a lower Si/Al ratio.
The SEM images of the SAPO-11s are shown in Fig. 2. Altogether, the
average particle sizes of the samples were within the previously reported
values for both hierarchical and conventional SAPO-11 [10,12,27].
Specifically for C-SAPO-11, CTAB-11 and PVA-11, an average particle
had a diameter of 10 ± 2, 8 ± 2 and 25 ± 6 μm, respectively. Coinciding
with the aforementioned reports, all samples displayed agglomerates of
smaller particles, where C-SAPO-11s particles consisted of small plates,
PVA-11 was composed of larger plates, and CTAB-11 had agglomerates
of relatively smaller spheres/plates compared to C-SAPO-11 and PVA-11
(Figs. S1–1).

The results from nitrogen adsorption analyses (Table 2) showed that
the SAPO-11 samples had comparable BET surface areas (202–211 m2
g− 1), which were within the previously reported range for hierarchical
and conventional SAPO-11 in the literature (130–250 m2 g− 1) [6,28].
Specifically, PVA-11 and C-SAPO-11 had comparable micropore areas
(139 and 135 m2 g− 1, respectively) and external surface areas (69 and
76 m2 g− 1, respectively), whereas CTAB-11 had a significantly lower
micropore area (78 m2 g− 1), but the largest external surface area (124
m2 g− 1) of the samples. It should also be mentioned that the micropore
areas of PVA-11 and CTAB-11 were in the lower range of previous

reports on hierarchical SAPO-11 [4,10,29,30]. Hence, the addition of
meso-SDAs to SAPO-11 does not significantly affect the total surface
area, but the distribution of the micropore and external surface area vary
significantly according to which meso-SDA is utilized. For CTAB, the
quaternary ammonium surfactant causes significant changes in the
surface area distribution, reducing the micropore area and significantly
increasing the external surface area of the resulting hierarchical
SAPO-11 compared to C-SAPO-11. For PVA on the other hand, the
polymer does not change the surface area distribution, and the resulting
hierarchical SAPO-11s texture properties are seemingly identical to that
of the conventional C-SAPO-11.
Regarding the pore volumes, the total pore volumes of the SAPOs
were similar (0.13–0.18 cm3 g− 1), and CTAB-11 had the largest total
pore volume (0.18 cm3 g− 1) followed by C-SAPO-11 (0.14 cm3 g− 1) and
PVA-11 (0.13 cm3 g− 1). Moreover, PVA-11 and C-SAPO-11 had identical
micropore volumes (0.07 cm3 g− 1) and comparable mesopore volumes
(0.06 and 0.07 cm3 g− 1 respectively). For CTAB-11, while the micropore
volume (0.04 cm3 g− 1) was somewhat lower than that of C-SAPO-11, the
mesopore volume was twice as large (0.14 cm3 g− 1). In general, the

micropore volumes coincided with previous reports on hierarchical and
conventional SAPO-11 [4,10,31], whereas the mesopore volumes were
in the lower range of what has previously been reported for these sys­
tems [4,9,16]. In summary, PVA produced a hierarchical SAPO-11 with
pore volumes that resembled the conventional C-SAPO-11, while CTAB

Fig. 2. SEM micrographs of C-SAPO-11 (1), CTAB-11 (2) and PVA-11 (3), showing a typical particle for each sample.
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Microporous and Mesoporous Materials 329 (2022) 111550

caused a reduction in the micropore volume, but a significant increase in
the mesopore volume compared to C-SAPO-11.
The nitrogen sorption isotherms of the SAPO-11 samples are given in
Fig. 3A, while Fig. 3B shows the pore size distributions. The isotherms
were generally in accordance with previous reports on hierarchical and
conventional SAPO-11 [6,10,26]. C-SAPO-11 displayed a type I(a)
isotherm with a small H4 hysteresis loop, characteristic for materials
with narrow micropores and aggregated SAPO crystals, as defined by
IUPAC [32]. Both CTAB-11 and PVA-11 displayed type IV(a) isotherms
with saturation plateaus, characteristic of mesoporous molecular sieves
[32], where CTAB-11 had an additional plateau at p∙p0− 1 0.9 and
PVA-11 had an additional, minor plateau at p∙p0− 1 0.7. These additional
plateaus are typically indicative of the presence of mesopores and
similar plateaus are often displayed by mesoporous materials such as
SBA-15 or MCM-41 [33]. Identical plateaus have also previously been
reported for hierarchical SAPO-11 where interconnectivity between

micropores and mesopores was found via TEM investigations [6].
Furthermore, both hierarchical SAPOs displayed composite hysteresis
loops of type H4 and H2(b), where PVA-11 had a larger influence of the
former hysteresis loop and CTAB-11 had a larger influence of the latter
H2(b) loop, which is characteristic of structures with ordered mesopores
[32]. As for the pore size distributions (Fig. 3B), the conventional
C-SAPO-11 displayed a pore size distribution with no prominent fea­
tures, as expected for purely microporous materials. On the other hand,

both CTAB-11 and PVA-11 displayed single distinct features at pore
diameters of 2.8 nm and 3.2 nm respectively, suggesting the presence of
a uniform size distribution of mesopores. While this coincides with
previous reports on hierarchical SAPO-11 synthesized with CTAB and
PVA [10,26], it should be noted that the feature for PVA-11 was
significantly smaller than for CTAB-11. In summary, the addition of
CTAB to SAPO-11 caused significant changes to both the isotherm and
pore size distribution of the SAPO-11 system, collectively indicating the
presence and incorporation of mesopores into the hierarchical structure.
Conversely, the addition of PVA to SAPO-11 only generated a minor
plateau in the isotherm and a miniscule feature in the pore size distri­
bution of the sample. Otherwise, the isotherm and pore size distribution
of PVA-11 were essentially identical to those of C-SAPO-11, making it
difficult to ascertain the presence of an additional pore system in hier­
archical PVA-11 by using standard characterization methods.
To summarize, the XRD results indicated that a highly crystalline
AEL phase was obtained for the samples synthesized with PVA and
CTAB, where PVA produced a hierarchical SAPO-11 with a higher
crystallinity than CTAB. The ICP-MS results showed that both mesoSDAs preferentially increased the amount of Si incorporated in the
SAPO-11 structure, where CTAB generated the hierarchical SAPO-11
with the largest amount of incorporated Si. The nitrogen physisorption

results indicated that CTAB had a larger effect than PVA on the texture
properties of the resulting hierarchical SAPO-11. The former clearly
yielded the hierarchical SAPO-11 with the largest external surface area,
while PVA did not significantly affect the texture properties of the
resulting hierarchical SAPO-11 compared to C-SAPO-11. Finally, CTAB
additionally caused a significant change in both the isotherm and BJH
pore size distribution of the hierarchical SAPO-11, collectively indi­
cating the presence and incorporation of uniform mesopores. For PVA
on the other hand, the main characteristics of the resulting isotherm and
pore size distribution were identical to that of the conventional SAPO11. Thus, from the general characterization methods, CTAB produces
a hierarchical SAPO-11 with an abundance of mesopores, whereas it is
unclear whether PVA generates mesopores in the resulting hierarchical
SAPO-11 or not.
3.2. Acid characterization by CO FTIR spectroscopy
In order to identify if the introduction of meso-SDAs affected the
acidity of the hierarchical SAPO-11s, the relative acid density and acid
strength was investigated with carbon monoxide (CO) FTIR
spectroscopy.
The results of the CO adsorption are depicted in Figs. S1–2 as
normalized difference spectra. All samples displayed the typically re­
ported bands for SAPO-11: silanol sites (Si-OH) at 3745 cm− 1, surface
groups (terminal P-OH/Al-OH) at 3678 cm− 1, high frequency Brønsted
acid sites (BAS) at approximately 3630 cm− 1 and low frequency
Brønsted acid sites (LF-BAS, bridging hydroxyl bands interacting with
lattice oxygens) at 3520 cm− 1 [34–37]. Notably and similar to other
SAPOs with LF-BAS (e.g. SAPO-5) [38,39], these sites were found to be
inaccessible to CO and will resultantly not be further discussed.
Tables S1–2 gives an overview of the location of the observed bands
for each sample and the shifts, ΔνOH, between the high frequency BAS
(νBrønsted) and the BAS perturbed by CO (νBrønsted*). The acid shifts of the

SAPO-11 samples were similar and matched previous reports on other
SAPOs with moderate acid strength [17]. Specifically for this study,
PVA-11 had the largest shift (267 cm− 1), followed by CTAB-11 (263
cm− 1) and lastly the conventional C-SAPO-11 (261 cm− 1). Clearly, the
modification of the AEL system with mesopores does not lead to any
significant changes in acid strengths, matching previous reports on hi­
erarchical SAPO-11 [4,6].
The relative estimate on the acid density in each sample is shown in
Tables S1–3 and was obtained by integrating the area under the bands of
the FTIR spectra and normalizing the values with respect to C-SAPO-11,
in accordance to previously reported methods [17]. It should be noted

Fig. 3. Nitrogen sorption isotherms (A) and pore size distributions (B) for all
synthesized SAPO-11s. For the pore size distributions, the adsorption branch of
the BJH method was used.
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Microporous and Mesoporous Materials 329 (2022) 111550

that the intensity of the BAS band was extremely low for all samples.
However, this is in accordance with previous reports on the SAPO-11
system, which state that SAPO-11 has a low acid density compared to
other SAPOs such as the high acid density SAPO-5 and SAPO-34 [2,3,
35]. In this study, the conventional C-SAPO-11 contained the most BAS
(1), followed by CTAB-11 and PVA-11 (0.93 and 0.45, respectively).
Previous studies on hydrothermally synthesized hierarchical SAPO-11
have shown that the acid density may decrease slightly compared to

the conventional SAPO-11 when CTAB is employed as a meso-SDA [4,9].
This is in accordance with the observation in this study, where CTAB-11
had a minor reduction in acid density compared to the conventional
C-SAPO-11. On the other hand, CTAB-11 had more than twice the
density of surface groups (2.23) and silanols (2.30) compared to
C-SAPO-11 (1). This is attributed to the increased amount of silicon
incorporated in CTAB-11 and has previously been reported to occur for
the SAPO-11 system [4,36]. Previous reports on other SAPOs (SAPO-34)
[40] have also mentioned that an increased density of weak acid sites
(WAS, silanols) is typically associated with a decrease in crystallinity,
which coincides with the drop in crystallinity for CTAB-11 reported in
this study. As for PVA-11, the densities of BAS (vide supra), surface
groups (0.43) and silanols (0.32) were significantly lower than that of
C-SAPO-11. While there are few studies on hierarchical SAPO-11 syn­
thesized with PVA, the drop in acid density is generally larger than what
is typically found for hierarchical SAPO-11 [4,41]. The reduced acid
density does however match one report on hydrothermally synthesized
hierarchical SAPO-11 with TPOAC as a meso-SDA [15], where the low
acid density hierarchical SAPO-11 also had highly accessible BAS in
mesopores.
To summarize, while the acid strengths were largely unaffected by
the introduction of mesopores, the acid densities varied based on the
type of meso-SDA that was used in the synthesis. Specifically, when
compared to the conventional C-SAPO-11, using the quaternary
ammonium surfactant CTAB as a meso-SDA caused a minor drop in the
density of BAS and a large increase in the density of WAS. On the other
hand, using the polymer PVA as a meso-SDA caused a large drop in the
density of both BAS and WAS compared to the conventional C-SAPO-11.

identical. A detailed account on the activity and lifetime of the samples

has been provided in the supporting information (Section 2.0).
The initial and 20-hour product selectivities of the MTH model re­
action have been provided in Fig. 4 while detailed values are listed in
Tables S2–1. Whereas the major product for all SAPO-11 samples
regardless of time on stream was dimethylether (DME), there were sig­
nificant differences between the initial product distributions of the hi­
erarchical and conventional SAPO-11s. Specifically, C-SAPO-11 mainly
produced DME (66%) as well as small amounts of aliphatics, which is in
accordance with previous reports on SAPO-11 [3,42]. For the hierar­
chical SAPO-11s however, CTAB-11 had a greatly increased selectivity
towards DME (95%) whereas PVA-11 produced less DME (52%) but
significant amounts of polymethylbenzenes (13%) and large products
(8%).
To evaluate the location of BAS, the acidities (acid shifts and den­
sities) and particle sizes should be considered, where a more detailed
overview of factors affecting the MTH reaction has been provided
elsewhere [17]. For CTAB-11, the sample had the lowest initial selec­
tivity to hydrocarbons and did not produce any significant quantities of
large or branched products. Furthermore, as CTAB-11 and C-SAPO-11
also had comparable acidities and particle sizes, BAS are most likely not
present in the mesopores of CTAB-11. Regarding the high DME selec­
tivity for the sample, previous reports on hierarchical SAPO-5 [18] have
noted that an increased incorporation of Si may lead to an increased
density of WAS in mesopores, thus facilitating the selectivity towards
DME. Indeed, CTAB-11 had slightly more incorporated Si than
C-SAPO-11 and the increased selectivity towards DME is therefore
ascribed to WAS being present in the mesopores of the hierarchical
sample.
As for PVA-11, the sample produced notable amounts of poly­
methylbenzenes as well as products that were larger than hexame­

thylbenzene (branched polymethylbenzenes and methylated azulenes,
Figs. S2–2). This is a significant result, as previous studies on SAPO-11 in
the MTH reaction have shown that the largest products were C5+ ali­
phatics, whereas methylated aromatics (e.g. trimethylbenzene and
dimethylnaphtalene) were retained as coke in the deactivated catalyst
[31]. The same authors attributed this to the 10-membered rings of the
AEL structure (0.45 × 0.65 nm [1]), which both suppressed the pathway
for producing aromatics and restricted the size of the products formed in
the micropores, regardless of acidity. Finally, while PVA-11 had the
largest particle size of the samples in this study, larger particles are in
fact expected to produce fewer quantities of aromatic products
compared to smaller particles [43]. Evidently, PVA-11 must have highly
accessible BAS situated in pores that are larger than the AEL micropores.
In other words, while it was not apparent from the nitrogen phys­
isorption measurements, the MTH model reaction clearly indicates that
PVA-11 has incorporated mesopores in its structure. The post-catalysis
nitrogen physisorption (Figs. S2–3) further corroborated this result by
revealing that the mesopores of PVA-11 were more congested than for
CTAB-11. This is most likely by virtue of PVA-11 having a higher density
of BAS in mesopores compared to CTAB-11. Conclusively, the MTH
model reaction revealed the hierarchical nature of the PVA synthesized
hierarchical SAPO-11 as well as how the meso-SDAs affected the loca­
tion of the BAS. Specifically, the CTAB surfactant did not affect the
location of the BAS, while the PVA polymer caused a significant portion
of the BAS to be shifted to the mesopores of the hierarchical SAPO-11.

3.3. Model reactions over SAPO-11
Through the general characterization of C-SAPO-11, CTAB-11 and
PVA-11, it has been established that while CTAB-11 contains a signifi­
cant portion of mesopores, PVA-11 displays pore characteristics that are

similar to that of the conventional C-SAPO-11. To further investigate
how different types of meso-SDAs may produce unique hierarchical
SAPO-11s, the pore topologies and acid site locations of the samples
were probed with model reactions. Specifically for this study, the
methanol-to-hydrocarbons model reaction was initially employed to
evaluate the BAS location in the SAPOs. Subsequently, the Beckmann
rearrangement of cyclohexanone oxime was conducted to evaluate the
pore topologies and to get an indication on the WAS locations in the
hierarchical SAPO-11s.
3.3.1. Methanol-to-hydrocarbons model reaction over SAPO-11
A straightforward assessment on the location of the BAS may be
obtained by utilizing the methanol-to-hydrocarbons (MTH) model re­
action [17]. Here, the presence of BAS in mesopores may be directly
probed by examining the product selectivity of the reaction, where
products that are larger than the micropores may only form if BAS are
present in the mesopores. The MTH model reaction was therefore
initially employed to get a preliminary indication on the location of the
BAS of CTAB-11 and PVA-11.
The results from the methanol-to-hydrocarbons (MTH) model reac­
tion for C-SAPO-11, PVA-11 and CTAB-11 are displayed in Figs. S2–1. As
the catalysts were deemed to be deactivated after falling below an
arbitrarily chosen conversion of 70%, the initial and 20-hour conver­
sions, and lifetimes for the SAPO-11s in this study were essentially

3.3.2. Beckmann rearrangement of cyclohexanone oxime model reaction
over SAPO-11
Although there are few studies on the Beckmann rearrangement
(BMR) of cyclohexanone oxime (CHO) for hierarchical SAPO-11, New­
land et al. [18] previously applied the reaction to elucidate the pore
topology as well as the location of weak acid sites in hierarchical SAPO-5

and SAPO-34. When compared to the conventional SAPOs, the same
authors suggested that an increased lifetime could be expected for the
hierarchical SAPOs through an interplay between WAS in mesopores
6


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Microporous and Mesoporous Materials 329 (2022) 111550

Fig. 4. Initial (top) and 20-hour (bottom) product distributions for the MTH model reaction, where ‘PolyMB’ are polymethylbenzenes (tetra-, penta- and hexam­
ethylbenzene (HMB)). Detailed values have been given in Table S2-1 in the supplementary information. The standard deviation (2σ) is given by error bars.

and BAS in micropores. In other words, the micropores should be
directly connected to the mesopores in order to observe a prolonged
lifetime in the BMR of CHO. Thus, the presence of an intraconnected
pore system where the mesopores also contain active sites for the re­
action (e.g. WAS), should contribute to a prolonged lifetime for hierar­
chical SAPO-11 in the BMR of CHO. In this study, the catalyst lifetime in
the Beckmann rearrangement of cyclohexanone oxime was utilized in
combination with post-catalysis characterization to elucidate the pore
topology and to get an indication on the WAS location in the hierarchical
SAPO-11s. Initially however, the selectivity was probed in order to
investigate if the product distribution of the hierarchical SAPOs was
altered by mesopore incorporation, by the type of meso-SDA, or by
differences in BAS accessibility as obtained from the MTH model
reaction.

Detailed values for the selectivity have been provided in the sup­
porting information, Tables S3–1. The initial selectivity towards

ε-caprolactam (CPL) was equally high for all samples, where C-SAPO-11
had the highest selectivity (85%) followed by CTAB-11 (83%) and PVA11 (82%). Moreover, the selectivity was more or less constant regardless
of conversion for the catalysts, which matches previous reports on
conventional SAPO-11 [8] and other hierarchical SAPOs as well [18,20].
In summary, this confirms three things for the SAPO-11 catalysts: firstly,
the presence of mesopores in CTAB-11 and PVA-11 did not affect the
product distribution of the rearrangement, which aligns with previous
reports on hierarchical SAPOs [18]. Secondly and thirdly, the type of
meso-SDA, and the presence of highly accessible BAS in the mesopores
of PVA-11, also did not influence the product distribution of the
reaction.
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Microporous and Mesoporous Materials 329 (2022) 111550

The conversion results from the BMR of CHO over C-SAPO-11, CTAB11 and PVA-11 are plotted in Fig. 5. The initial conversions of the SAPOs
mirrored the Brønsted acid densities of the samples, where C-SAPO-11
had the highest initial conversion (86%) and was closely followed by
CTAB-11 (82%), while PVA-11 had the lowest initial conversion (60%).
In accordance with previous reports [8], this confirms that while WAS
may be active for the rearrangement, the predominant part of the re­
action occurs at BAS. From Fig. 5, it is also clear that the conventional
C-SAPO-11 deactivated more rapidly than the hierarchical SAPO-11s
and specifically, the lifetime of the catalysts increased in the following
order: C-SAPO-11«PVA-11≤CTAB-11.
Post-catalysis XRD (Figs. S3–1) showed that the AEL structure was
intact for all samples, confirming that the SAPOs had not collapsed

during the reaction. Nitrogen physisorption on the spent samples
(Figs. S3–2) revealed that while the micropores of the catalysts were
completely congested, all SAPO samples retained ~40% or more of their
external surface area and mesopore volume. Specifically, the amount of
retained external surface area and mesopore volume increased in the
order of C-SAPO-11≤PVA-11«CTAB-11, where CTAB-11 notably
retained all of its external surface area as well as most of its mesopore
volume (86%). Lastly, the post-catalysis TGA-MS (Figs. S3–3) revealed
the amount of coke deposited on the SAPOs after the reaction
(Tables S3–2), where PVA-11 contained the most coke (4.0%), followed
by C-SAPO-11 (3.7%) and CTAB-11 (3.2%).
Due to the size of cyclohexanone oxime (0.58 × 0.41 nm [44]), there
have been discrepancies on whether the BMR of CHO occurs in the
micropores or on the external surface of 10-membered ring frameworks
[45,46]. Singh et al. [8] however, showed that the main catalytic centers
for SAPO-11 were Brønsted acid sites, which are mostly situated in the
micropores of the catalyst [31]. The same authors also demonstrated
that the conversion of CHO was still above 50% upon passivating the
external surface, indicating that the bulk of the reaction occurred in the
micropores of the catalyst. Thus, in this study the rearrangement was
assumed to take place inside the micropores of SAPO-11. Furthermore,
while both BAS and WAS are active for the rearrangement reaction, the
specific rearrangement to CPL proceeds preferentially on moderately
strong BAS or WAS [18,47]. Indeed, zeolitic-strength BAS (with a ΔνOH
of over 300 cm− 1 [48]) are frequently associated with a decreased CPL
selectivity due to formation of by-products and coke precursors, thus
decreasing the lifetime of the catalysts [49–51]. For SAPOs however
(with a ΔνOH of less than 300 cm− 1 [48]), Newland et al. [18] and Singh
et al. [8] did not observe any differences in CPL selectivity over SAPO-5
or SAPO-11 respectively, during the reaction. This most likely indicates

that the moderately strong BAS of SAPOs do not further catalyze the
products to coke precursors. Hence, specifically for one-dimensional
SAPOs, deactivation during the Beckmann rearrangement mainly

occurs due to diffusion hindrances resulting from having a
one-dimensional network [8,18,49].
To better understand the distinct catalytic performances of the
SAPO-11s in this study, the factors affecting a catalyst in the BMR of
CHO have been detailed and summarized in Tables S3–3. As the acid
strengths of the catalysts were essentially identical, there are mainly
three factors which need to be considered to clarify if the prolonged
lifetimes of the hierarchical SAPO-11s are related to pore topology and
WAS location. Specifically, the particle size, BAS density and the loca­
tion of BAS in the hierarchical systems need to be accounted for, where
Table 3 gives a brief summary of the most relevant information.
Large particles have previously been shown to accumulate more coke
in the BMR, leading to a rapid catalyst deactivation [50]. Specifically for
the BMR of CHO over SAPO-11 however, Singh et al. [8] did not report
any significant differences in oxime conversion, CPL selectivity or
catalyst lifetime for different particle sizes. Others have reported that
differences in particle size only become apparent when the size falls
below a critical value due to an increase in external surface area [51].
While the external surface area of PVA-11 (69 m2 g− 1) may also be due
to the presence of mesopores, it was still comparable to that of
C-SAPO-11 (76 m2 g− 1) even though the particle sizes (25 ± 6 μm and
10 ± 2 μm, respectively) were different. Indeed, while PVA-11 had the
largest particle size as well as the largest accumulation of coke (Table 3),
the sample still had a longer lifetime than the conventional C-SAPO-11,
clearly demonstrating the exceptional properties of PVA-11. Lastly,
while the particle sizes of CTAB-11 and C-SAPO-11 were essentially

identical, CTAB-11 had the longest lifetime for the BMR in this study.
Conclusively, the variation in particle size distributions cannot explain
the prolonged lifetimes of the hierarchical SAPO-11s.
The BAS density for the samples in this study increased in the
following order: PVA-11«CTAB-11≤C-SAPO-11, where an increased
BAS density is typically associated with a decreased lifetime in the BMR
due to coke formation [49]. As previously mentioned however, the BAS
of SAPOs are moderately strong and while coke is formed in the reaction,
the main reason for deactivation is diffusion limitations rather than coke
formation [8,18,49]. Indeed, Newland et al. [18] noted that variations
in BAS densities did not affect the lifetimes of hierarchical SAPO-34 and
SAPO-5. For the present study, while the conventional C-SAPO-11
certainly deactivated first, CTAB-11 and PVA-11 had comparably long
lifetimes despite having contrasting acid densities. Furthermore, the
accumulation of coke was clearly independent of the BAS density, as
PVA-11 (with the lowest acid density) formed the largest amount of coke
of the samples, as indicated in Table 3. In other words, while the relative
BAS density may be related to the deactivation of the conventional
C-SAPO-11, it falls short of explaining the prolonged lifetime of the hi­
erarchical SAPOs.
Regarding the location of the BAS, the MTH model reaction estab­
lished that BAS were only present in micropores for CTAB-11, whereas
for PVA-11, BAS were also present in highly accessible mesopores. For
the BMR, the lifetimes of the hierarchical samples were essentially
identical, signifying that the location of the BAS clearly did not have a
noteworthy effect on the lifetime of the hierarchical SAPOs.
As previously specified, an increased lifetime for the rearrangement
of cyclohexanone oxime may be expected due to an interplay between
WAS and BAS originating from interconnected micropores and meso­
pores. For CTAB-11, the increased incorporation of Si from ICP-MS as

well as the increased density of surface groups from the FTIR mea­
surements, indicated that the sample had WAS in its mesopores.
Furthermore, the MTH model reaction revealed that the BAS of CTAB-11
were located in its micropores, whereas the WAS were confirmed to be
situated in the mesopores of the sample. Evidently, the enhanced life­
time in the Beckmann rearrangement points towards the presence of
interconnected micropores and mesopores for CTAB-11, where the mi­
cropores principally contain BAS, and the mesopores accommodate
WAS. For PVA-11, whereas the MTH model reaction clearly evidenced
the presence of highly accessible BAS in mesopores, the WAS location

Fig. 5. Conversion of cyclohexanone oxime in the Beckmann rearrangement for
C-SAPO-11, CTAB-11 and PVA-11.
8


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Microporous and Mesoporous Materials 329 (2022) 111550

Table 3
Summary of SEM, FTIR and post-BMR TGA-MS results for the SAPO-11s.
Sample

Particle size (μm)

BAS shift (cm− 1)

ρa (BAS) (a.u.)

ρ (WAS) (a.u.)

Mass balance (%)

Cokeb (%, mg mgcat− 1)

C-SAPO-11
PVA-11
CTAB-11

10 ± 2
25 ± 6
8±2

263
267
261

1
0.45
0.93

1
0.33
2.3

87
76
99

3.7
4.0
3.2

a
b

Relative density of acid sites.
Total amount of coke (%) generated during the BMR reaction.

was not plainly accounted for. From the BMR however, the increased
amount of coke (4.0%) combined with the increased lifetime for PVA-11
indicates an improved accessibility to active sites and a high grade of
utilization of the pore system. This corresponds with several reports on
hierarchical systems with interconnected micropores and mesopores
[52,53], and necessitates that the active sites (WAS and BAS) contribute
to reduced diffusion limitations throughout the catalyst system by being
highly accessible. The WAS of PVA-11 are thus most likely located in the
mesopores of the catalyst, analogous to CTAB-11. The increased life­
times of the hierarchical SAPO-11s in the BMR of CHO are thus attrib­
uted to the presence of intraconnected pore systems and highly
accessible acid sites. This reaffirms the findings from the MTH model
reaction that PVA-11 clearly has incorporated mesopores in its structure.
As nitrogen physisorption could not differentiate between the conven­
tional C-SAPO-11 and the hierarchical PVA-11, this clearly shows that
model reactions are better suited than standard characterization
methods for demonstrating the presence of hierarchy in hierarchical
SAPO-11. In other words, model reactions allow for a considerably more
thorough analysis of the topological properties of hierarchical SAPO-11
compared to general characterization methods.

In summary, the MTH and BMR model reactions show that utilizing
the quaternary ammonium surfactant CTAB and polymer PVA as mesoSDAs, results in hierarchical SAPO-11s with a high degree of connec­
tivity between the micropores and mesopores. Furthermore, while CTAB
does not shift the location of the BAS, PVA causes BAS to be situated in
the mesopores of the hierarchical SAPO-11. Importantly, the hierar­
chical component of PVA-11 could only be discerned by utilizing model
reactions, as standard characterization methods could not sufficiently
distinguish between the characteristics of the hierarchical PVA-11 and
the conventional C-SAPO-11. This clearly demonstrates the importance
of utilizing model reactions in order to obtain a complete and compre­
hensive characterization of hierarchical SAPO-11.

that the application of these model reactions for investigation of the
properties of hierarchical SAPO-11 can be expanded to other onedimensional SAPOs and perhaps also more complicated systems.
CRediT authorship contribution statement
Daniel Ali: Conceptualization, Methodology, Writing – original
draft. Zhihui Li: Resources. Muhammad Mohsin Azim: Writing – re­
view & editing. Hilde Lea Lein: Writing – review & editing. Karina
Mathisen: Conceptualization, Writing – review & editing, Supervision,
Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the Norwegian University of
Science and Technology for financial support. SINTEF (Marianne Kjos) is
thanked for conducting ICP-MS experiments, Andrew Harvie is thanked
for lending us the syringe pump and Joakim Tafjord is thanked for many
fruitful discussions regarding the setup of the vertical reactor.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2021.111550.
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