Preparation of hierarchical HZSM-5 zeolites with combined desilication with NaAlO2/tetrapropylammonium hydroxide and acid modification for converting methanol to propylene

Article abstract of DOI:10.1039/c8ra08624a

In the present work, mesoporosity is introduced into highly siliceous HZSM-5 zeolites (SiO₂/Al₂O₃ = 400) by a two-step path including desilication using NaAlO₂ and TPAOH (tetrapropylammonium hydroxide) mixtures, followed by acid washing treatment. The physicochemical properties of conventional microporous HZSM-5 and all treated samples were characterized by ICP-OES, XRD, FE-SEM, BET and NH₃-TPD methods. The catalytic performance of the HZSM-5 samples was determined in methanol to propylene conversion reaction at 460 °C and methanol WHSV of 0.9 h^?1 using feed containing 50 wt% methanol in water. The results showed that the porosity of the desilicated samples has been mainly blocked by sodium aluminate derived deposits and silicon-containing debris. After a subsequent acid washing step with hydrochloric acid, the blocking species were removed which resulted in improving the mesoporosity generated in the desilication step. It was found that alkaline-acid treatment in a NaAlO₂/TPAOH solution with TPAOH/(NaAlO₂ + TPAOH) = 0.4 followed by acid washing, leads to the formation of narrow and uniform mesoporosity without severely destroying the crystal structure. Also, it exhibits higher selectivities to propylene (37.7 vs. 30.7%) and total butylenes (21.2 vs. 16.1%), propylene to ethylene ratio (4.0 vs. 2.7), as well as total light olefins (68.4 vs. 57.9%) compared to the parent catalyst, while its selectivities to C₁-C₄ alkanes (9.6 vs. 13.7%) and heavy hydrocarbons (13.8 vs. 28.4%) are relatively lower. The lifetime of the optimum alkaline-acid treated sample (640 h) showed a significant increase compared to that of the parent catalyst (425 h). The results exhibited that desilication process leads to a considerable mesoporosity development, while acid washing treatment mostly influences on the catalyst acidity. Therefore, the combination of the alkaline-acid treatment leads to hierarchical HZSM-5 catalyst formation with tailored pore architecture and surface acidic properties.

Full text of DOI:10.1039/c8ra08624a

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Preparation of hierarchical HZSM-5 zeolites with
combined desilication with NaAlO2/
Cite this: RSC Adv., 2018, 8, 41131
tetrapropylammonium hydroxide and acid
modification for converting methanol to propylene
a
Fatemeh Gorzin,a Jafar Towfighi Darian,
Fereydoon Yaripourb
*
and Seyyed Mohammad Mousavia
In the present work, mesoporosity is introduced into highly siliceous HZSM-5 zeolites (SiO2/Al2O3 ¼ 400) by
a two-step path including desilication using NaAlO2 and TPAOH (tetrapropylammonium hydroxide)
mixtures, followed by acid washing treatment. The physicochemical properties of conventional
microporous HZSM-5 and all treated samples were characterized by ICP-OES, XRD, FE-SEM, BET and
NH3-TPD methods. The catalytic performance of the HZSM-5 samples was determined in methanol to
propylene conversion reaction at 460 ꢀC and methanol WHSV of 0.9 hꢁ1 using feed containing 50 wt%
methanol in water. The results showed that the porosity of the desilicated samples has been mainly
blocked by sodium aluminate derived deposits and silicon-containing debris. After a subsequent acid
washing step with hydrochloric acid, the blocking species were removed which resulted in improving the
mesoporosity generated in the desilication step. It was found that alkaline-acid treatment in a NaAlO2/
TPAOH solution with TPAOH/(NaAlO2 + TPAOH) ¼ 0.4 followed by acid washing, leads to the formation
of narrow and uniform mesoporosity without severely destroying the crystal structure. Also, it exhibits
higher selectivities to propylene (37.7 vs. 30.7%) and total butylenes (21.2 vs. 16.1%), propylene to
ethylene ratio (4.0 vs. 2.7), as well as total light olefins (68.4 vs. 57.9%) compared to the parent catalyst,
while its selectivities to C1–C4 alkanes (9.6 vs. 13.7%) and heavy hydrocarbons (13.8 vs. 28.4%) are
relatively lower. The lifetime of the optimum alkaline-acid treated sample (640 h) showed a significant
increase compared to that of the parent catalyst (425 h). The results exhibited that desilication process
leads to a considerable mesoporosity development, while acid washing treatment mostly influences on
the catalyst acidity. Therefore, the combination of the alkaline-acid treatment leads to hierarchical
HZSM-5 catalyst formation with tailored pore architecture and surface acidic properties.
Received 17th October 2018
Accepted 27th November 2018
DOI: 10.1039/c8ra08624a
rsc.li/rsc-advances
yield, since methanol can be easily synthesized by steam
reforming of natural gas or coal gasication.4,5
1. Introduction
Chang and Silvestri6 proposed a reaction path for methanol
Propylene is one of the key components for the production of
propylene oxide, polypropylene, acrylic acid, acrylonitrile, and
isopropyl alcohol in the petrochemical industry.1 It is mainly
produced as a byproduct of steam cracking of hydrocarbons and
uid catalytic cracking units which have high energy
consumption and lead to greenhouse gas emission.2 Increasing
oil prices, shortage of petroleum resources, and consumption
demands for propylene, lead to considerable attention toward
new processes with high propylene to ethylene ratios.3 The
methanol to propylene (MTP) process based on acidic catalysts
is a promising way to produce light olens with high propylene
conversion to hydrocarbons reaction over ZSM-5 catalyst as
follows:
ꢁH2O
þH2O
ƒƒƒƒƒ!
2CH OH
3
CH OCH /light olefins
3 3
ƒƒƒƒƒ
/higher olefins alkanes paraffins aromatics
(1)
At rst, methanol dehydrates to dimethyl ether (DME), which
reacts further to produce ethylene and propylene. Dimethyl
ether formation is directly related to weak acid sites and strong
acid sites which are favor for secondary reactions as well as
+
aDepartment of Chemical Engineering, Tarbiat Modares University, P. O. Box
14115-143, Tehran, Iran. E-mail: towghi@modares.ac.ir; Fax: +98 218 288 3311;
Tel: +98 218 288 3311
MTO reaction. Butanes, higher olens, alkanes, C5 heavy
hydrocarbons, and aromatics are usually produced as by-
products in the MTO process. It is well established that the
bCatalysis Research Group, Petrochemical Research & Technology Company, National
Iranian Petrochemical Company, P. O. Box: 1493, Tehran, Iran
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products distributions in the MTO process strongly depend on results showed that aer desilication, the external surface area
the catalyst properties and operating conditions.7,8
of the hierarchical increased 3 to 4 times higher than in the
ZSM-5 is a common catalyst for converting methanol to parent one, while crystallinity and acidity of the desilicated
propylene due to high resistance against coke deposition, high samples were largely preserved. The results showed that acid
acidic activity, and excellent shape selectivity. However, the washing treatment aer desilication process removed the
relatively small micropores of this catalyst considerably lead to blocking species and regained the native microporosity. Groen
mass transfer limitation.9,10 Therefore, ZSM-5 modication with et al.22 investigated mesoporosity development and acidity
good performance in MTP reaction still remains a great chal- adjustment by sequential desilication–dealumination of ZSM-5
lenge in the catalyst science.
zeolites (Si/Al ¼ 15, 35 and 200). Desilication and deal-
Introducing an additional mesopore network besides the umination were done by 0.2 M NaOH solution at 65 ꢀC for
intrinsic micropore system, which referred to hierarchical ZSM- 30 min and steam treatment at 600 ꢀC for 5 h, respectively. They
5, can be a promising method to overcome fast deactivation and showed that appropriate modication of structure and acidity
develop a catalyst with high stability for the MTP reaction. This can be obtained using alkaline treatment followed by steaming.
leads to decrease in inner diffusion paths, increase in catalyst Ding et al.23 introduced mesoporosity in highly siliceous ZSM-5
external surface area, increase in accessibility and mass transfer zeolite (SiO2/Al2O3 ¼ 417) by alkaline treatment using mixtures
of molecules to/from the active sites.11,12
of NaOH and TEAOH (tetraethylammonium hydroxide). A
A wide variety of strategies have been developed to generate subsequent phosphorus modication was carried out to stabi-
the hierarchical porous ZSM-5, such as direct synthesis in the lize the acidity properties and porosity of the desilicated
presence of various hard or so templates and post treatment samples. The catalytic performance of the phosphorus modied
approaches such as desilication and dealumination, which are mesoporous ZSM-5 catalysts in catalytic hydrocarbons cracking
13
14
¨
presented in the reviews of Serrano et al. , Moller and Bein,
reaction showed higher conversion in the bulky molecule
cracking reaction compared to the parent one.
15
ˇ
and Cejka and Mintova.
Among all the approaches, desilication of ZSM-5 in an
To the best of our knowledge, the catalytic performance of
alkaline medium (typically NaOH), seems to be a promising a highly siliceous mesoporous HZSM-5 catalyst prepared by
route due to its simplicity operations, versatility, and efficiency. desilication with different ratios of NaAlO2/TPAOH mixtures in
According to the literatures,16,17 the mesoporosity formation by MTP reaction have not been previously investigated. This work
NaOH operates in the optimum ratio of Si/Al of 25–50 and focuses on using different alkaline solutions of NaAlO2/TPAOH
majority of studies have been done in this ratio, while desili- mixtures in mesoporosity formation over HZSM-5 zeolite. A
cation studies of highly siliceous ZSM-5 have been less frequent. conventional microporous HZSM-5 catalyst was used to
Sadowska et al.,18,19 investigated the inuence of alkaline compare the effect of mesoporosity in the MTP reaction using
treatments with NaOH and NaOH/TBAOH mixtures on the a xed-bed ow reactor under the same operating conditions.
physicochemical properties and catalytic performance of two Also, the physicochemical properties of these catalysts were
different ratio of ZSM-5 (Si/Al ¼ 164 and 31.5) zeolite in cracking characterized by ICP-OES, XRD, FE-SEM, BET and NH3-TPD
reaction. The results showed that the mixture of NaOH and methods.
TBAOH as an alkaline agent, leads to form mesoporous ZSM-5
with smaller diameter and narrower pore size distribution
with higher accessibility to active sites of catalyst respect to the
case of using NaOH alone. Ahmadpour and Taghizadeh10
2. Experimental
2.1 Materials
studied the MTP reaction over desilicated ZSM-5 (Si/Al ¼ 175)
All the chemical reagents were the analytical grade and used
samples by different ratio of NaOH/TPAOH mixtures. They
without further purication. Silicic acid (SiO2$xH2O, $99 wt%),
found that NaOH/TPAOH solution with 0.4 ratio of TPAOH in
ammonium nitrate (NH4NO3, 99 wt%), sulfuric acid (H2SO4,
the mixture shows higher propylene selectivity (%47.2) and
98 wt%), sodium hydroxide (NaOH, 99.6 wt%), hydrochloric
longer catalyst life time (80 h) compared to the parent one (%
acid (HCl, 37 wt%), and tetrapropylammonium hydroxide
(TPAOH, 40 wt% aqueous solution) were purchased from Merck
Company (Germany) while sodium aluminate (NaAlO2,
35.7 propylene selectivity and 43 h life time). Fathi et al.20
desilicated ZSM-5 (Si/Al ¼ 15) zeolite by alkaline treatment
(CaCO3, Na2CO3 and NaOH solution) at 75 ꢀC for 3 h and
Al2O3 wt% ¼ 55) was purchased from Riedel de Haen
catalytically tested for the methanol to gasoline reaction. The
results showed that desilicated sample by NaOH produced
mesoporous structure more than other desilicated samples.
(Germany).
2.2 Catalysts synthesis
They also showed that the alkaline treated sample by Na2CO3
+
solution provided the highest C5 selectivity (43%) and the
2.2.1 Parent zeolite synthesis. High silica H-ZSM-5 catalyst
´
longest catalytic lifetime (4 h). Caicedo-Realpe and Perez- (nominal Si/Al ratio of 200) was synthesized by the hydro-
21
´
Ramırez prepared mesopore ZSM-5 zeolites (Si/Al ¼ 15, 25, thermal method using silicic acid and sodium aluminate as Si
and 40) by desilication and subsequent acid washing treatment. and Al sources, respectively. The gel composition of
They studied the effect of different conditions of desilication 20SiO2 : 0.05Al2O3 : 1TPAOH : 1.5Na2O : 200H2O was used for
and acid washing parameters with respect to concentration, the reaction mixture. The synthesis procedure to prepare the gel
temperature, and time on the mesoporosity formation. The is as bellow. At rst sodium aluminate was added to NaOH
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aqueous solution and mixed for 30 min. Then TPAOH was drop- spectrometer. Powder X-ray diffraction (XRD) analysis was used
wise added to the mixture for 30 min (solution A). Simulta- to recognize the type of crystalline phase and crystallinity of
neously, silicic acid added slowly to NaOH aqueous solution samples. Patterns were recorded on a Philips PW1730 diffrac-
and mixed for 30 min (solution B). Solution A was added tometer (Philips Analytical, Almelo, The Netherlands) using Cu
ꢀ
ꢀ
ꢀ
dropwise to solution B during 120 min under vigorous stirring Ka radiation (l ¼ 1.5406 A ) over a 2q range of 5 to 50 with
and then concentrated H2SO4 was added to the mixture to a step size of 0.05ꢀ and a step time of 1 s at room temperature.
adjust gel pH. The crystallization was carried out at 187 ꢀC The crystal size and morphology of HZSM-5 catalysts were
under autogenously pressure for 48 h without stirring in determined by eld-emission scanning electron microscopy
stainless-steel Teon-lined autoclave. The solid product was (FE-SEM) on MIRA3 TESCAN, USA instrument, operating at 15
recovered by ltration and then was washed with deionized kV. The textural properties of all samples including the specic
water several times until the pH value of the decanted water surface areas and pore diameters were determined using N2
ꢀ
reached to neutral. Aer that, it was dried overnight at 105 C. adsorption–desorption isotherms at 77 K on NOVA2000 Quan-
Finally, the catalyst sample was calcined to remove the organic taChrome, USA instrument. Before measuring, the samples
template in a muffle furnace under air ow at 530 ꢀC for 12 h at were evacuated at 300 ꢀC under N2 ow for 3 h. The Brunauer–
a heating rate of 3 ꢀC minꢁ1. Ion exchange of as-synthesized Na- Emmett–Teller (BET) equation was used to calculate the specic
ZSM-5 sample was carried out four times with 1.0 M NH4NO3 surface area using adsorption data at p/p0 ¼ 0.05–0.25. Micro-
solution for 10 h at 90 ꢀC under continuous agitation. The pore volume and micropore surface area were determined by t-
sample was then washed and dried at 105 ꢀC for 12 h, followed plot curve at thickness range between 3.5 and 5.4 A. The pore
˚
by calcination at 530 ꢀC for 12 h (at heating rate of 3 ꢀC minꢁ1
)
size distribution was obtained from adsorption isotherm using
to obtain the H-form of the zeolite. The resulting powder was Barrett–Joyner–Halenda (BJH) method. Mesopore volume was
formed by tableting and then catalyst with a mesh size in the calculated by using difference between the total pore volume (p/
range of 16–25 was made by crushing and sieving for catalytic p0 ¼ 0.99) and micropore volume. The surface acidity of the
evaluation in the reactor. This sample was denoted as parent.
catalysts was determined by temperature-programmed desorp-
2.2.2 Alkaline treatment. Desilication treatments on the tion of ammonia (NH3-TPD) with an online thermal conduc-
parent catalyst were carried out in the alkaline solution of tivity detector (TCD) using a conventional ow apparatus
NaAlO2, and NaAlO2/TPAOH mixture in different ratios. The (BELCAT-A, BEL Japan, Inc.). 35 mg of the sample was pre-
alkaline samples were named as DeSi-NaAlO2-TPA-R which R is heated under helium ow at 300 ꢀC for 2 h with a heating rate of
ꢁ1
ꢀ
ꢀ
the molar ratio of the TPAOH/(NaAlO2 + TPAOH) which varied 10 C min . Aer cooling to 60 C, the sample was saturated
from 0 to 0.6. The concentration of each solution was 0.2 M. with NH3 in the micro reactor for 1 h. To remove the physically
300 ml of the alkaline solution were heated to 65 ꢀC in the ax adsorbed NH3 on the catalyst surface; helium was passed over
under reux, aer then, 30 g of the parent catalyst were added to the sample for 30 min. The temperature ranges of ammonia
ꢀ
the heated solution and stirred vigorously for 30 min. Then, the desorption was 35 to 850 C.
suspension was cooled down immediately in ice bath. Aer
that, it was ltered, and then washed with deionized water until
neutral pH. The ltrated cake was dried at 105 C for 12 h and
converted into the H-form following the procedure described The performance of the samples for conversion of methanol to
for the parent catalyst.
propylene (MTP) reaction was investigated in a xed-bed
2.4 Catalytic performance tests
ꢀ
2.2.3 Acid treatment. Aer alkaline treatment, acid continuous ow reactor (L ¼ 60 cm, I.D. ¼ 11 mm, S.S. 316)
ꢀ
washing step was used to remove silicate debris and alkali ions at 460 C under atmospheric pressure. The reactor was heated
which deposited on the zeolite pores and to modify the porosity by a temperature-controlled three-zone furnace and a K-type
of desilicated samples in the presence of NaAlO2. Acid washing thermocouple probe was placed coaxially in the center of the
treatment as an extraction step was performed by contacting the catalyst bed to measure and monitor the reaction temperature.
HZSM-5 desilicated samples with an aqueous solution of A schematic ow diagram of the lab scale set-up has been
3
ꢀ
hydrochloric acid (3.0 M) at 80 C for 3 h with ratio of 10 cm
shown in Fig. 1. In each run, 4 g of the catalyst was loaded in the
HCl solution/1
g of desilicated sample. Aerwards, the center section of the reactor and a liquid mixture containing
suspension washed several times with deionized water, dried at 50 wt% methanol in water with methanol WHSV of 0.9 hꢁ1 was
105 ꢀC for 12 h, followed by calcination at 530 ꢀC for 12 h with injected to catalyst bed with a WellChrom K-120 HPLC pump.
heating rate of 3 ꢀC minꢁ1. Aer acid washing, the samples were Prior to the MTP reaction, the sample was activated at 300 ꢀC for
denoted as DeSi-NaAlO2-TPA-R-AW which R is the molar ratio of 2 h under N2 ow with a heating rate of 3 ꢀC minꢁ1. Then, the N2
the TPAOH/(NaAlO2 + TPAOH) varying from 0 to 0.6 and AW gas ow was stopped and the mixture of methanol and water
refer to acid washing step.
was pumped from the feed tank to the preheater. The reactor
outlet stream was cooled to 6 ꢀC in a refrigerating bath and then
the gas and liquid products were separated. In order to prevent
heavy components condensation, the transfer line from the
2.3 Catalyst characterization techniques
The molar ratio of Si/Al in the parent and all alkaline treated reactor outlet to the refrigerating bath was externally heated and
samples before and aer acid washing step were determined by maintained at 170 ꢀC. On-line analysis of the gas phase was
the ICP OES method using a Perkin Elmer Optima 2000 DV carried out using a micro-GC (Varian CP-4900) equipped with
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a TCD detector. Four channels of this instrument were used to where N is the number of moles. Superscripts i, o, and x
separate the products [(CH4, C2–C5 olen/paraffins, DME, C6- represent components at the inlet of reactor, components at the
cut) and (H2, CO and CO2)]. The separated aqueous parts of the outlet of reactor, and the number of carbon atoms, respectively.
liquid products were analyzed using Varian CP-3800 gas chro-
matograph equipped with FID and TCD detectors in which a CP-
3. Results and discussion
3.1 Catalysts characterization
Wax 52 CB column was used to separate the components
(methanol and water). The organic part was also analyzed using
Varian CP-3800 gas chromatograph equipped with a FID
detector in which a PONA column was used to separate the
components (C6–C16 hydrocarbons).
The methanol conversion and selectivity of products were
evaluated using eqn (1) and (2), respectively.
3.1.1 Chemical composition. The amount of Si/Al molar
ratio for the parent and all the alkaline treated samples before
and aer acid washing step, have been listed in Table 1.
According to the literature,10,16 alkaline treatment over ZSM-5,
results in both silicon and aluminum dissolution from the
zeolite framework. However, due to easier hydrolysꢁis of Si–OH–
Si than Si–OH–Al bond in the presence of Al(OH)4 attack, the
dissolution of silicon is more favorable than aluminum from
the zeolite framework in alkaline treatment.24,25 As listed in
Table 1, more reduction in Si/Al molar ratio is observed for the
alkaline treated sample with pure NaAlO2 compared to the
desilicated samples in the presence of TPAOH. At low Si/Al
À
Á
NMi eOH ꢁ NMo eOH þ 2NDo ME
Methanol conversion ¼
Selectivity ¼
ꢂ 100 (2)
NMi eOH
o
xN
CxHy
À
Á
(3)
NMi eOH ꢁ NMo eOH þ 2NDo ME
Fig. 1 Schematic flow diagram of the experimental setup for the catalytic activity tests.
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Table 1 Chemical composition and textural properties of the parent, all desilicated HZSM-5 samples before and after acid washing
Si/Ala SBETb (m2 gꢁ1
)
SMicro (m2 gꢁ1
)
SExtd (m2 gꢁ1
)
VTotale (cm3 gꢁ1
)
VMicro (cm3 gꢁ1
)
VMeso (cm3 gꢁ1
)
HFh
c
f
g
Sample
Parent
DeSi-NaAlO2
183
44
72
84
58
398.8
350.9
351.8
360.7
339.1
367.7
375.1
383.1
357
368.7
280.3
229.3
238.2
255.1
244.8
231.9
214.5
221.4
30.1
70.6
119.4
122.5
84
122.9
143.2
168.6
135.6
0.202
0.174
0.186
0.198
0.191
0.225
0.231
0.235
0.228
0.137
0.117
0.119
0.124
0.121
0.121
0.124
0.127
0.121
0.065
0.057
0.067
0.074
0.07
0.104
0.107
0.108
0.107
0.051
0.135
0.217
0.212
0.157
0.180
0.205
0.238
0.201
DeSi-NaAlO2-TPA0.2
DeSi-NaAlO2-TPA0.4
DeSi-NaAlO2-TPA0.6
DeSi-NaAlO2-AW
DeSi-NaAlO2-TPA0.2-AW 176
DeSi-NaAlO2-TPA0.4-AW 162
DeSi-NaAlO2-TPA0.6-AW 170
155
a
Determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). b Total surface areas were obtained by the BET method using
adsorption data in P/P0 ranging from 0.05 to 0.25. c Measured by t-plot method using adsorption data in P/P0 ranging from 0.19 to 0.39. d External
surface area, measured by t-plot method using adsorption data in P/P0 ranging from 0.19 to 0.39. Total pore volumes were estimated from the
e
f
g
adsorbed amount at P/P0 ¼ 0.99. Measured by t-plot method using adsorption data in P/P0 ranging from 0.19 to 0.39. VMeso ¼ Vads,p/p ¼0.99
ꢁ
0
h
VMicro
.
The hierarchical factor, dened as (VMicro/VTotal) ꢂ (SExt/SBET).
ratios, aluminum acts as a pore directing agent and prevents the structure without considerable mesoporosity.24,28 This result
framework from base attack, while in highly siliceous ZSM-5 was veried by the BJH curve of the sample (Fig. 3b) which does
(nominal Si/Al ratio of 200), because of low concentration of not indicate any clear peak at mesoporous range (2–50 nm).
aluminum on the zeolite framework, uncontrolled excessive Moreover, as presented in Table 1, the parent catalyst has
silicon extraction occurred, which resulted in massive zeolite relatively low contribution of mesoporosity (SExt of 30.1 m2 gꢁ1
dissolution. Less reduction in Si/Al ratio for the alkaline treated and VMeso of 0.065 cm3 gꢁ1) compared to its total BET surface
samples in the presence of TPAOH is related to its high area (398.8 m2 gꢁ1) and total pore volume (0.202 cm3 gꢁ1). As
adsorption affinity towards the zeolite surface, which protects can be seen in Fig. 3a, all the alkaline-acid treatment HZSM-5
zeolite against dissolution.26,27 However, the molar ratio of Si/Al samples exhibit both types
I
and VI behavior with
for all the alkaline treated samples is very low compared to the a pronounced type H3 hysteresis at high relative pressures (P/P0
parent one, which is related to the Al2O3 deposits present in the > 0.4). This event indicates a porous system coupling micro and
sodium aluminate treated samples.21 It can be seen from Table mesoporosity, conrming by the t-plot results listed in Table 1.
1 that aer acid washing step with concentrated hydrochloric The alkaline-acid treated HZSM-5 samples verify the generation
acid, the Si/Al molar ratio of all the alkaline-acid treated cata- of the narrow and uniform mesopore size distribution centered
lysts is very close to that of the parent one. These results at ca. 3 nm (Fig. 3b). While pore size distribution curves of the
conrmed that a preferential elimination of alumina deposits sample desilicated by pure NaAlO2 show a narrow peak at 3 nm
and siliceous species occurred during the acid washing step.
and a broad peak at around 9 nm. On the other words, the
3.1.2 XRD analysis. Fig. 2 shows XRD patterns of the parent affinity of TPA+ cations to the catalyst surface, as a pore growth
zeolite and all desilicated samples prepared with mixture of moderator, can avoid the formation of large pores and excessive
NaAlO2/TPAOH at different ratios aer acid washing step. The silicon extraction from the zeolite framework during desilica-
comparison between the XRD patterns of the alkaline-acid tion process.29,30 As can be seen, there is no considerable
treated samples with those of the parent zeolite showed two difference in BJH pore size distributions for the samples treated
main peaks at 2q ¼ 7–10ꢀ and three peaks at 2q ¼ 23–25ꢀ, which with NaAlO2/TPAOH mixtures at different ratios of both bases.
can be attributed to the preservation of the MFI lattice structure As listed in Table 1, for all desilicated samples, the total BET
during alkaline-acid treatment without any additional phases surface areas have been decreased compared to the parent
formation. A slight decrease in the peak intensity related to the catalyst. This reduction for the alkaline treated sample with
desilicated samples is associated with partial extraction of Si pure NaAlO2 is much higher than other desilicated samples.
and Al from zeolite framework during alkaline-acid treatment. These results are related to the catalyst framework destruction
This event conrmed that the hierarchical samples crystallinity due to silicon extraction during desilication process.
has not destroyed.
Moreover, because of pore blockage caused by NaAlO2-
3.1.3 Nitrogen adsorption/desorption analysis. Nitrogen derived deposits, the total pore volume and micropore volume
adsorption–desorption isotherms and corresponding BJH pore of the all desilicated samples are decreased.21 While the
size distribution curves of the parent and all the hierarchical micropore volume decreased, the external surface area for all
HZSM-5 samples prepared by two-step path of alkaline-acid treated samples increased. As presented in Table 1, aer acid
treatment with different ratios of the NaAlO2/TPAOH mixtures washing step and removing silicate debris and sodium alumi-
have been shown in Fig. 3a and b, respectively. The nitrogen nate deposits, a considerable increase of total surface area and
isotherm of the parent catalyst shows isotherm type I without total pore volume observed for the all samples. The signicant
any recognizable hysteresis loop and with a high uptake at increase in external surface area (more than 4 times) and
relative pressures (p/p0 < 0.2), conrming its microporous mesopore volume with a slight drop in micropore volume
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Fig. 2 The XRD patterns of the parent and all desilicated HZSM-5
samples after acid washing.
(ꢃmax. 10%) observed for all the desilicated samples in the
presence of TPAOH, compared to the parent one. While for the
desilicated sample by pure NaAlO2, the external surface area
enhance twice in comparison by the parent catalyst. Among
these samples with different ratios, DeSi-NaAlO2-TPA0.4-AW
(ratio ¼ 0.4) illustrated the largest mesopore surface area
(168.6 m2 gꢁ1), and the lowest micropore volume reduction
(0.127 cm3 gꢁ1). It can be concluded that the alkaline treatment
by different rations of NaAlO2/TPAOH solution is absolutely
responsible for the mesoporosity formation in the parent
zeolite, while the acid washing treatment step removes the pore
blocking deposits which had been accumulated in the catalyst
pores in the rst step treatment.
3.1.4 FE-SEM analysis. Fig.
4
shows the surface
morphology before and aer acid washing step for the parent
and all the desilicated samples. As shown in Fig. 4a, a relatively
spherical aggregate shaped morphology observes for the parent
catalyst. The crystal size of the sample is large with quite clear
and smooth surface which conrms low presence of mesopores
in the structure. As can be clearly seen in the FE-SEM micro-
graphs of the alkaline treated sample with pure NaAlO2 (Fig. 4b)
Fig. 3 (a) N2 adsorption/desorption isotherms of the parent, all desi-
licated HZSM-5 samples after acid washing (b) BJH pore size distri-
butions derived from the adsorption branch of the isotherms of the
samples.
and the treated sample with NaAlO2/TPAOH solution with TPD method. Fig. 5 presents the NH3-TPD patterns of the parent
TPAOH/(NaAlO2 + TPAOH) ¼ 0.4 (Fig. 4d), the pore porosity of zeolite and all desilicated samples before and aer acid washing
the samples has been mainly blocked by sodium aluminate- treatment. The NH3-TPD results including the amount of acid
derived deposits and silicon-containing debris. In addition, sites along with maximum desorption temperature which was
aer desilication process, the external surface area of the parent calculated by desorption peak area and desorption peak
catalyst becomes rough and rugged. Even though, as can be temperature, respectively have been listed in Table 2. As shown
seen in Fig. 4c and e, aer acid washing step with hydrochloric in Fig. 5a, in comparison with the parent catalyst, total acid
acid, the blocking species were removed which result in amount for the desilicated samples with pure NaAlO2 and
improving the intracrystalline mesoporosity generated in the different ratios of mixture of NaAlO2 and TPAOH solution
desilication step. These results are conrmed by the t-plot increases remarkably respect to the parent catalyst. It can be
results listed in Table 1. Similar observations have been ob- explained by the fact that for desilicated samples in the pres-
tained by other researchers on mesoporous zeolites aer ence of sodium aluminate, aluminate ions and Al2O3 deposited
dealumination.22,31
on the acid sites of zeolite surface are responsible for increasing
3.1.5 NH3-TPD analysis. The acidity changes including the total acidity of the zeolite. As depicted in the gure, all
surface concentration and strength of the acid sites in the treated samples present similar NH3-TPD patterns with three
parent catalyst and all treated samples were studied by the NH3- desorption peaks which the lower temperature peak (TP1) at
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Fig. 4 FE-SEM images of the (a) parent, (b) DeSi-NaAlO2, (c) DeSi-NaAlO2-AW, (d) DeSi-NaAlO2-TPA0.4, and (e) DeSi-NaAlO2-TPA0.4-AW.
around 165–195 ꢀC and the higher temperature peak (TP2) at alkaline treatment. They showed that aer alkaline treatment,
around 350–390 ꢀC, are related to ammonia desorption from both Al and Si were leached from the framework, but the
weak/moderate and moderate/strong acid sites, respectively. concentration of Al was more than two orders lower than that of
Aer desilication process with NaAlO2/TPAOH mixtures, weak Si, which conrmed that in alkaline treatment Si dissolution
and strong acid sites show a shi to lower temperatures respect was favored over that of Al. Some parts of aluminum species
to the parent one. This means that all the desilicated samples extracted during alkaline treatment have been reinserted into
have lower acidic strength than the parent one because of the zeolite surface from the alkaline solution. This phenom-
aluminum atoms extraction from the zeolite framework and enon has been previously reported by other researchers based
more mesoporosity of the desilicated samples. It is generally on 27Al MAS NMR and FTIR measurements.32,33 Verboekend and
accepted that alkaline treatment over ZSM-5, results in both Perez-Ramırez27 reported that extracted aluminum species from
silicon and aluminum dissolution from the zeolite framework. the zeolite in alkaline solutions can be partially inserted into the
Groen et al.16 investigated the role of aluminum for meso- zeolite surface, which leads to increase both Brønsted and Lewis
porosity development of ZSM-5 zeolite in desilication by acid sites. You et al.34 showed that aer desilication by alkaline
´
´
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treatment, the amount of framework aluminum decreased parent and all the treated samples in steady state conditions
owing to extra framework aluminum formation. So it can be (time on stream of 48 h) when methanol conversion is above
concluded that the generation of extra framework aluminum 99%. Parent catalyst in MTP reaction shows a catalytic perfor-
species is an inherent characteristic of desilication by alkaline mance with propylene selectivity of 30.7% and a total light
solution. Aer acid washing treatment with concentrated olens (C]2 –C4]) selectivity of 57.9% along with 13.7% selectivity
hydrochloric acid, by removing the Al2O3 deposits and extra to C1–C4 saturated hydrocarbons and a lot of heavy hydrocar-
framework aluminum which reinserted to the zeolite surface at bons formation (28.4%).
rst step, the amount of total acidity decreased mostly even
It can be clearly seen from the Table 3 that all the alkaline
lower than the parent one (see Fig. 5b and Table 2). This is in treated samples before acid washing represent higher heavy
line with previous studies which have been obtained similar hydrocarbons selectivity and lower light olens selectivity as
observations on mesoporous zeolites aer dealumination.21,22
well as lower propylene to ethylene ratio compared to the parent
According to easier hydrolysis of Si–OꢁH–Si bond than Si– one, which these results are not appropriate for MTP process. As
OH–Al bond in the presence of Al(OH)4 attack in alkaline listed in Table 3, propylene and total light olens selectivity
solution, the amount of Si–OH groups (known as weak acid decrease from 30.7 and 57.9% for the parent catalyst to 21.4 and
sites) on the external surface of the zeolite increase which is 47.4% for the desilicated sample with pure sodium aluminate,
expected to have a good performance in MTP reaction.24,35 It is respectively. Among all the desilicated samples with different
generally accepted that methanol to DME conversion, alkylation ratio of NaAlO2 and TPAOH, the DeSi-NaAlO2-TPA0.4 sample
and methylation reactions were carried out by weak acid sites. represents better catalytic performance in terms of higher
They can also prevent many side reactions such as hydrogen propylene selectivity (26.8%), higher light olens selectivity
transfer reactions in MTP process. But the strong acid sites have (53.8%), and lower heavy hydrocarbon formation (32.4%).
been known as main acid sites for olen production in MTH However, there is no improvement in the catalytic performance
reaction.7,36,37 Therefore, in order to improve catalyst stability of this sample and other desilicated samples compared to the
and propylene selectivity in MTP reaction, it is necessary to parent one.
adjust the concentration amount of weak and strong acid sites.
These reactor test results show that only mesoporosity
creation in the parent catalyst is not sufficient for improving the
catalyst performance in MTP reaction. As discussed in BET and
FE-SEM results, for all the desilicated samples pore blockage is
caused by sodium aluminate-derived deposits. Moreover,
precipitation of aluminate ions on the acid sites of the catalyst
surface results in appearing Al2O3 aer calcination step, which
is responsible for huge acidity of the desilicated samples.21 It is
generally believed that the catalyst with high acidity on the
zeolite surface converts more light olens to higher olens,
paraffins, and heavy hydrocarbons, which leads to low selec-
tivity of light olens, more coke formation and short catalytic
3.2 Catalytic performance
In order to investigate the effect of mesoporosity formation
associated with alkaline-acid treatment using different ratios of
the NaAlO2/TPAOH mixtures followed by acid washing, all the
treated samples and the parent one were tested in the conver-
sion of methanol to propylene reaction under similar opera-
tional conditions (T ¼ 460 ꢀC, P ¼ 1 atm, feed: 50% wt methanol
in water, WHSV ¼ 0.9 hꢁ1). Table 3 represents full description of
the average product distributions in MTP reaction over the
Fig. 5 NH3-TPD profiles of the parent, all desilicated HZSM-5 samples (a) before acid washing (b) after acid washing.
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Table 2 NH3-TPD data for the parent, all desilicated HZSM-5 samples before and after acid washing
Characteristic
Acidity (mmol NH3 gꢁ1
)
Peak temperature (ꢀC)
Low temp. peak
High temp. peak
Catalyst name
Weak
Medium
Strong
Total
(TP1
)
(TP2)
Parent
DeSi-NaAlO2
0.083
0.185
0.172
0.182
0.195
0.065
0.05
0.139
0.421
0.301
0.322
0.378
0.127
0.094
0.1
0.114
0.204
0.260
0.219
0.218
0.133
0.091
0.107
0.115
0.336
0.810
0.733
0.723
0.791
0.356
0.235
0.262
0.248
191
186
189
187
184
168
167
162
176
391
396
387
397
387
357
366
347
379
DeSi-NaAlO2-TPA0.2
DeSi-NaAlO2-TPA0.4
DeSi-NaAlO2-TPA0.6
DeSi-NaAlO2-AW
DeSi-NaAlO2-TPA0.2-AW
DeSi-NaAlO2-TPA0.4-AW
DeSi-NaAlO2-TPA0.6-AW
0.055
0.048
0.085
lifetime. So, it is necessary to adjust the acidity of the desilicated probably due to higher external surface area and mesopore
samples to obtain better catalytic performance in MTP process. volume compared to the other ratios of NaAlO2/TPAOH mixtures,
Our experimental results of the catalytic performance of the which leads to a shorter diffusion path length. Therefore,
desilicated catalysts before acid washing are in consistent with a shorten residence time of heavy hydrocarbon products in the
BET, FE-SEM and NH3-TPD analysis.
crystal pores of this sample before diffusing out to the gas phase
+
As listed in Table 3, all the alkaline-acid treated samples results in less heavy hydrocarbons products (C5 ) and less satu-
exhibit higher propylene and butylene selectivity and lower rated hydrocarbons (C1–C4) selectivities in this sample. In this
+
selectivity to ethylene, C1–C4 saturated hydrocarbons and C5
study, C4 Hydrogen Transfer Index (C4-HTI) was also used to
heavy hydrocarbons compared to the parent catalyst due to their evaluate the catalytic performance of the parent and all the
high external surface area and mesopore volume. alkaline-acid treated samples in MTP reaction. This parameter is
According to Table 3, for alkaline-acid treated samples in the dened as a ratio between the yields of butane (n-C4 and i-C4) and
presence of TPAOH, propylene and butylene selectivity increase the total yield of C4 hydrocarbons (n-C4, i-C4 and total C]4 ) which
from (35.8 and 19.1%) to (37.7 and 21.3%), respectively by announce the progress of the hydrogen transfer reactions over
increasing TPAOH/(NaAlO2 + TPAOH) ratio from 0.2 to 0.4. But the ZSM-5 catalysts.38 Therefore, a high value of C4-HTI indicates
a more increase in the ratio (DeSi-NaAlO2-TPA0.6-AW sample) extensive aromatization and cyclization reactions along with high
leads to decrease in propylene selectivity to 34.3% which is not alkane generation.39 As listed in the nal column of the Table 3,
benecial. It can be clearly seen from the different ratios of the for all the alkaline-acid treated samples C4-HTI is lower than the
NaAlO2/TPAOH mixtures, the DeSi-NaAlO2-TPA0.4-AW compared parent one. This parameter for the DeSi-NaAlO2-TPA0.4-AW
to the parent catalyst exhibits higher selectivities to propylene sample is 0.17 which is less than the other alkaline-acid treated
(37.7 vs. 30.7%) and total butylenes (21.3 vs. 16.1%), as well as samples and less than half of the parent catalyst (0.33). This trend
total light olens (68.4 vs. 57.9%), while its selectivities to C1–C4 can be expected by the NH3-TPD results for this sample due to
alkanes (9.6 vs. 13.7%) and heavy hydrocarbons (13.8% vs. 28.4%) their high weak and medium acid sites which result in reducing
are relatively lower. The catalytic performance of this sample is the hydrogen transfer reactions in the MTP reaction.
Table 3 Product distribution of MTP reaction over the parent, all desilicated HZSM-5 samples before and after acid washing measured at steady
state conditions (reaction condition: T ¼ 460 ꢀC, P ¼ 1 atm, WHSV ¼ 0.9 hꢁ1, feed: 50 wt% methanol in water, time on stream of 48 h.)
Selectivity (%)
Conversion
Sample
(%)
C1–C4
C2]
C3]
Total C]4
(C]2 –C]4
)
C]3 /C]2
C5
C4-HTI
a
+b
Parent
DeSi-NaAlO2
99.9
99.9
99.7
99.8
99.9
99.7
99.8
99.8
99.7
13.7
17.6
16.4
15.1
16.1
11.4
10.8
9.6
11.1
12.6
11.4
11.7
12.1
10.7
10.1
9.4
30.7
21.4
23.3
26.8
25.2
33.7
35.8
37.7
34.3
16.1
13.4
14.2
15.3
13.8
17.6
19.1
21.2
19.4
57.9
47.4
48.9
53.8
51.1
62.0
65.0
68.3
64.3
2.7
1.7
2.0
2.3
28.4
36.5
34.6
32.4
35.5
21.8
15.1
13.8
16.6
0.33
0.40
0.36
0.35
0.38
0.29
0.22
0.17
0.19
DeSi-NaAlO2-TPA0.2
DeSi-NaAlO2-TPA0.4
DeSi-NaAlO2-TPA0.6
DeSi-NaAlO2-AW
DeSi-NaAlO2-TPA0.2-AW
DeSi-NaAlO2-TPA0.4-AW
DeSi-NaAlO2-TPA0.6-AW
2.1
3.15
3.54
4.01
3.22
10.1
10.6
a
b
C1–C4 saturated hydrocarbons. C5 and higher hydrocarbons.
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Fig. 6 Conversion of methanol as a function of time on stream over the parent and optimum ratio of NaAlO2/TPAOH before and after acid
washing step.
In order to quantitative estimate of the catalyst stability, the value in MTP reaction was obtained 192 h for DeSi-NaAlO2-
catalyst life time is considered as the time on stream aer which TPA0.4 sample, which is lower than 425 h for the parent one. As
the methanol/DME conversion drops below 90% which is shown in Fig. 6, the initial activity of the DeSi-NaAlO2-TPA0.4-
indicated by the dashed line in Fig. 6. The reactor test results AW catalyst is similar to that of the parent one, but methanol
over the parent catalyst showed that the methanol/DME conversion remains constant at approximately 100% for 520 h
conversion remained approximately 100% up to 250 h on on stream. The methanol conversion of the DeSi-NaAlO2-
stream and then dropped slowly to 90% aer 425 h on stream. It TPA0.4-AW sample falls below 90% aer 640 h on stream. The
should be noted the similar long life time has been recently catalytic lifetime of this catalyst is nearly 1.5 times higher than
reported by Yaripour et al.7 According to literature,22,40 it is ex- that of the parent one. These results conrmed that aer
pected that improving mesoporosity and acid modications alkaline-acid treatment over the microporous ZSM-5 catalyst,
over microporous ZSM-5 catalyst surface results in better cata- the stability was noticeably improved. A comparison of MTP
lytic performance and longer life time.
reactor test results of the DeSi-NaAlO2-TPA0.4-AW catalyst with
As it was expected from the product distribution data over those of other reported mesoporous ZSM-5 catalysts have been
the alkaline treated samples (see Table 3), the catalyst life time presented in Table 4. As presented in this table, this catalyst
Table 4 Comparison of MTP reactor test results of the optimum alkaline-acid treated catalyst with literature
Selectivity
Si/Al MeOH : water WHSV (hꢁ1
)
Ethylene Propylene C2]–C4
P/E
Lifetimea Application Ref.
]
Preparation method
Dealumination
Desilication
Desilication
200
175
76
200
175
175
23
45
27
100
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
Pure MeOH
__
0.9
1
1
0.9
1
6
1.8
20
6.65
2
4
40
42.9
80.8
67.7
__
73.9
72.6
66
45
__
74
68.3
11.2 384 h
4.3 80 h
10.1 100 h
5.4 75 h
3.5 83 h
3.0 75 h
MTP
MTP
MTP
MTO
MTP
MTP
MTP
MTH
MTH
MTP
MTP
5
10
11
41
42
43
44
45
10.3
4.1
8
11.6
12.5
7
17
25
14
9.4
44.4
42.2
43
40.6
38.0
36
28
37
36
37.7
Desilication
So template
Dealumination
Fluoride medium route
Hard template
Dealumination
Seed-induced
5.1
1.6 10 h
1.5
—
__
1 : 1
1 : 1
—
46
47
2.6 190 h
4.0 640 h
DeSi-NaAlO2-TPA0.4-AW 200
0.9
This study
a
The catalyst life time is considered as the time on stream aer which the methanol/DME conversion drops below 90%.
41140 | RSC Adv., 2018, 8, 41131–41142
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shows long life time and good catalytic performance in terms of
high propylene and total light olens selectivity compared to
literature. Although the numerical value of our catalyst lifetime
is higher than the reported values in the table, but since the
operating conditions of the tests presented in the table are not
the same, therefore we cannot claim that our catalyst shows
a better performance compared to other catalyst in the same
3 H. M. Torres Galvis and K. P. de Jong, ACS Catal., 2013, 3,
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4 F. Yaripour, Z. Shariatinia, S. Sahebdelfar and A. Irandoukht,
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4. Conclusion
A two-step route comprising alkaline treatment in different
ratios of NaAlO2 and TPAOH, followed by acid washing was
successfully developed to introduce mesoporosity in highly
siliceous HZSM-5 zeolites. The porosity of the alkaline treated
samples which is mostly blocked by Al2O3 and silica debris
signicantly improved aer acid washing treatment. The char-
acterization results conrm that the acid washing step aer
desilication of HZSM-5 catalyst leads to a reduction in the acid
sites strength without harshly destructing the crystal structure
and its intrinsic acidity. Moreover, the hierarchical catalysts
prepared by alkaline-acid treatment have the Si/Al ratio very
similar to that of the parent one. The catalytic performance of
the alkaline treated samples did not show any improve in
propylene and light olens selectivity, due to pore blockage and
huge acidity while aer acid washing step, a considerable
increase in light olens selectivity and life time was achieved.
Alkaline-acid treated sample with TPAOH/(NaAlO2 + TPAOH)
ratio of 0.4, represents the highest propylene and light olens
selectivities (37.7 and 68.4%, respectively), as well as the lowest
selectivity to heavy hydrocarbons (13.8%), among all the
alkaline-treated samples. Moreover, the lifetime of this catalyst
(640 h) showed a signicant increase compared to the parent
one (425 h). The lower strength of its acid sites and higher
mesoporosity may be the reasons for better catalytic perfor-
mance of DeSi-NaAlO2-TPA0.4-AW catalyst in MTP reaction.
13 D. Serrano and P. Pizarro, Chem. Soc. Rev., 2013, 42, 4004–
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14 K. Moller and T. Bein, Chem. Soc. Rev., 2013, 42, 3689–3707.
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15 J. Cejka and S. Mintova, Catal. Rev., 2007, 49, 457–509.
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´
16 J. C. Groen, L. A. A. Peffer, J. A. Moulijn and J. Perez-Ramırez,
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¨
18 F. Schmidt, M. R. Lohe, B. Buchner, F. Giordanino,
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´
19 K. Sadowska, A. Wach, Z. Olejniczak, P. Kustrowski and
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´
´
21 R. Caicedo-Realpe and J. Perez-Ramırez, Microporous
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´
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22 J. C. Groen, J. A. Moulijn and J. Perez-Ramırez, Microporous
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23 J. Ding, M. Wang, L. Peng, N. Xue, Y. Wang and M.-Y. He,
Appl. Catal., A, 2015, 503, 147–155.
24 M.-L. Gou, R. Wang, Q. Qiao and X. Yang, Microporous
Conflicts of interest
Mesoporous Mater., 2015, 206, 170–176.
ˇ
´
25 A. Ciˇzmek, B. Subotic, R. Aiello, F. Crea, A. Nastro and
There are no conicts to declare.
C. Tuoto, Microporous Mater., 1995, 4, 159–168.
´
´
26 D. Verboekend and J. Perez-Ramırez, Catal. Sci. Technol.,
2011, 1, 879–890.
27 D. Verboekend and J. Perez-Ramırez, Chem.–Eur. J., 2011, 17,
Acknowledgements
´
´
The authors gratefully acknowledge Tarbiat Modares University
(Tehran, Iran) and the Petrochemical Research and Technology
Company (Tehran, Iran) for their nancial supports of the
research.
1137–1147.
28 T. Fu, J. Chang, J. Shao and Z. Li, J. Energy Chem., 2017, 26,
139–146.
29 K. Sadowska, K. Gora-Marek, M. Drozdek, P. Kustrowski,
J. Datka, J. Martinez Triguero and F. Rey, Microporous
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