Altering C2H4/C3H6 yield in methanol to light olefins over HZSM-5, SAPO-34 and SAPO-34/HZSM-5 nanostructured catalysts: Influence of Si/Al ratio and composite formation

Article abstract of DOI:10.1039/c6ra00432f

A series of nanostructured catalysts composed of HZSM-5 zeolite with various ratios of Si/Al (120, 140, 180 and 220) was successfully synthesized by a hydrothermal method. The HZSM-5 sample with optimum ratio of Si/Al = 180 was then added into SAPO-34 synthetic gel to synthesize SAPO-34/HZSM-5 composite. The physicochemical properties and catalytic performance of SAPO-34/HZSM-5 composite were then compared to HZSM-5 (Si/Al = 180) and SAPO-34 catalysts. The nanostructured catalysts were characterized by XRD, FESEM, EDX, BET and FTIR techniques. The XRD patterns indicated that the extent of crystallization and crystallite size were dependent on the Si/Al ratio and type of structure phase (CHA and/or MFI). The EDX images show high distribution of Si for the SAPO-34/HZSM-5 composite. The highest BET surface area (388 m² g^-1) was obtained for HZSM-5 zeolite catalyst with Si/Al ratio of 220. For the MTO reaction, comparing to the HZSM-5 synthesized with different Si/Al ratios, the HZSM-5 (Si/Al = 180) sample possesses highest crystallinity and therefore exhibits the highest selectivity to ethylene. The SAPO-34/HZSM-5 composite is an active catalyst with high selectivity toward light olefins of around 80%.

Full text of DOI:10.1039/c6ra00432f

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Altering C2H4/C3H6 Yield in Methanol to Light Olefins over HZSM-5, SAPO-34
and SAPO-34/HZSM-5 Nanostructured Catalysts: Influence of Si/Al Ratio and
Composite Formation
Bahman Mohammadkhani1,2, Mohammad Haghighi*,1,2, Parisa Sadeghpour1,2
1. Chemical Engineering Faculty, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town,
Tabriz, Iran.
2. Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O.Box 51335-1996,
Sahand New Town, Tabriz, Iran.
*
Corresponding author:
Reactor and Catalysis Research Center, Sahand University of Technology, P.O. Box 51335ꢀ1996, Sahand New Town, Tabriz,
Iran. Email: haghighi@sut.ac.ir, Tel: +98ꢀ41ꢀ33458096 & +98ꢀ41ꢀ33459152, Fax: +98ꢀ41ꢀ33444355, web: http://rcrc.sut.ac.ir

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Abstract
A series of nanostructured catalysts composed of HZSMꢀ5 zeolite with various ratios of Si/Al
ratio (120, 140, 180 and 220) was successfully synthesized by hydrothermal method. The
HZSMꢀ5 sample with optimum ratio of Si/Al=180 was then added into SAPOꢀ34 synthetic
gel to synthesize SAPOꢀ34/HZSMꢀ5 composite. The physicochemical properties and
catalytic performance of SAPOꢀ34/HZSMꢀ5 composite compared to HZSMꢀ5(Si/Al=180)
and SAPOꢀ34 catalysts. The nanostructured catalysts were characterized by XRD, FESEM,
EDX, BET and FTIR techniques
.
The XRD patterns indicated that the extent of
crystallization and crystallite size were dependent on the Si/Al ratio and type of structure
phase (CHA and/or MFI). The EDX images show high distribution of Si for SAPOꢀ
34/HZSMꢀ5 composite. The highest BET surface area (388 m2/g) was obtained for HZSMꢀ5
zeolite catalyst with Si/Al ratio of 220. For the MTO reaction, comparing to the HZSMꢀ5
synthesized with different Si/Al ratios, HZSMꢀ5(Si/Al=180) sample possesses high
crystallinity and therefore exhibits the highest selectivity to ethylene. The SAPOꢀ34/HZSMꢀ5
composite is an active catalyst with high selectivity toward light olefins around 80%.
Keywords: SAPOꢀ34, HZSMꢀ5, Catalyst, Methanol, Olefins.
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1 Introduction
Methanol to olefins process over microporous zeolite or zeotype catalysts have been attracted
as one of potential technologies for the production of specific fuels and platform
petrochemicals, i.e., ethylene and propylene 1ꢀ4. These materials are in increasing demand in
worldwide and have significant financial in the marketplace 5, 6. The anticipated annual
growth in demand, equal to around 4% for ethylene and 6% for propylene, could create an
imbalance in the future supplies 7ꢀ10. Hence, the control of propylene/ethylene ratio as one of
the key issues in MTO, necessitates an inꢀdepth fundamental understanding on the extremely
reaction network 7, 11, 12. As it is well known, microporous acidic zeolites are widely used as
solid catalysts to investigative and commercially apply in the field of the methanol
conversion 13ꢀ15. In particular, HZSMꢀ5 zeolite, and the silicoaluminophosphate molecular
sieve SAPOꢀ34 have been attracting an enormous interest as the most promising candidates
14, 16, 17
.
HZSMꢀ5 catalyst (with MFI structure) is a channel structure, with straight channels in one
direction and sinusoidal channels in another, which represents a family of microporous,
crystalline, acidic aluminosilicates 18ꢀ20. The structure of SAPOꢀ34 is analogous to the natural
zeolite chabazite (CHA) which is small pore 3ꢀdimensional framework, built from chabazite
cage connected through double 6ꢀmembered rings 21ꢀ23. It has been well established that the
small pore silicoaluminophosphate, SAPOꢀ34, is an extremely sufficient shape selective
catalyst for the MTO process which has high stability and ethylene yield 24ꢀ26. However, it is
rapidly deactivated by coke formation 27ꢀ30. On the other hand, HZSMꢀ5 zeolite has high
propylene yield and long catalyst lifetime, but the selectivity toward total olefins is low 18, 31,
32. Therefore, the composite of SAPOꢀ34 and HZSMꢀ5 catalysts is good candidate for
achieving high performance features including high activity, remarkable selectivity of desired
olefins and high resistance to deactivation32ꢀ34. According to the combination method for the
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components, there are several kinds of composite catalysts. In this study, a certain amount of
HZSMꢀ5 powder was added to the SAPOꢀ34 synthesized gel. As can be seen, the most recent
researches concerning composite CHA/MFI framework which combine the structures of both
zeolites and microporous molecular sieves, in the presence of molecular template for
conversion of methanol to olefins 32, 33, 35. Chae et al. synthesized SAPOꢀ34/HZSMꢀ5
composites and used it as catalyst for the MTO reaction 32. The SAPOꢀ34/HZSMꢀ5
composites exhibited very different physicochemical and catalytic properties with respect to
HZSMꢀ5 and SAPOꢀ34 catalysts. Several parameters including gel composition, synthesis
procedure, weight proportion and framework Si/Al ratio can affect the properties of SAPOꢀ
34/HZSMꢀ5 composites as well as catalytic activity 36ꢀ38. The number and distribution of Si
into the framework of zeolites may affect the structure, acidity and catalytic performance of
catalysts. Hence, The Si/Al ratio presents in the reactant gel believed to be a dominant factor
in governing which structure is obtained 15, 36, 39ꢀ41. The optimum ratio of Si/Al in the starting
gel of composite can adjust the acidity as it reduces the rate of coke formation to produce the
highly desired ethylene and propylene derived products 36, 42, 43. Wang et al. investigated the
effect of Si/Al ratio on the conversion of methanol to olefins over HZSMꢀ5 and SAPOꢀ34
molecular sieves 36. It was reported that the samples with higher Si/Al ratio exhibit higher
catalytic activity in MTO reaction over HZSMꢀ5. In contrast, the SAPOꢀ34 sample with
lower Si content is more active.
The main objectives of this paper are synthesis of series HZSMꢀ5 catalyst with different Si/Al
ratios (Si/Al= 120, 140, 180 and 220). The HZSMꢀ5 sample with optimum Si/Al ratio
combined with SAPOꢀ34 to obtain the composite of the catalysts features. Although, there
are several reports on the impact of Si/Al ratio on HZSMꢀ5 catalyst applied to MTO reaction,
but the impact of optimum Si/Al ratio used on the composite of SAPOꢀ34/HZSMꢀ5 has not
been reported yet. In fact, The MTO reaction was investigated over the catalysts to address
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the effects of Si/Al ratio and its optimum ratio application with SAPOꢀ34 on the catalytic
performance. Physiochemical properties of the nanostructured catalysts were identified by
XRD, FESEM, EDX, BET, and FTIR techniques. In addition, the catalytic performance of
the samples in the MTO reaction is examined.
2 Materials and Methods
2.1 Materials
In a typical synthesis to get pure HZSMꢀ5, sodium aluminate (99%, Riedelꢀde Haën) and
fumed silica (99.9%, Aldrich) were used as sources of Al and Si, respectively. Tetraꢀ
propylammonium bromide (TPABr, 99.9%, Merck) and NaOH (97%, Merck) were also used
for the formation of HZSMꢀ5 catalysts. In order to prepare chabazite silicoaluminophosphate
SAPOꢀ34, aluminium triisopropylate (98%, Merck), phosphoric acid (85% aqueous solution,
Merck) and fumed silica (99.9%, Aldrich) were applied as the sources of Al, P and Si,
respectively. In addition, tetraethyl ammonium hydroxide aqueous solution (TEAOH, 20%,
Aldrich) was added to gel of SAPOꢀ34 as template. Deionized water purchased from Kasra
Company was used to prepare solutions.
2.2 Preparation of HZSM-5
HZSMꢀ5 catalysts with different Si/Al ratios were synthesized by hydrothermal
crystallization method from a gel composition of xAl2O3:1.0SiO2:0.1Na2O:0.1TPABr:35H2O,
(x= 0.0023ꢀ0.0042). A typical procedure of preparation of nanostructured samples is
schematically shown in Figure 1s in supplementary materials. A mixture of TPABr, NaOH
and sodium aluminate was dissolved in deionized water under stirring condition for 1 h.
Fumed silica was added into the solution while stirring. Then the mixture was stirred for 24 h.
The hydrothermal crystallization of the synthetic gel was carried out at 150ºC for 144 h in an
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autoclave reactor. The solid product was recovered by filtration, washed several times with
deionized water and dried overnight at 110ºC. NaZSMꢀ5 sample was calcined at 550ºC for 15
h to remove organic template. The HZSMꢀ5 catalysts were obtained by ammonium ion
exchange of calcined NaZSMꢀ5 samples which refluxed two times for 12 h each at 80ºC in
1.0 M NH4NO3 solution. Finally, the samples were dried at 110ºC, and calcined at 500ºC for
4 h.
2.3 Preparation of SAPO-34
Nanostructured SAPOꢀ34 catalyst was synthesized by hydrothermal method which is shown
in Figure 2s in supplementary materials. The chemical composition of synthetic gel was
1.0Al2O3:0.6SiO2:1.0P2O5:2.0TEAOH:70H2O. In a typical preparation of SAPOꢀ34,
aluminum triisopropylate was mixed with TEAOH and deionized water at room temperature
and stirred for 1.5 h. In the next step, aqueous solution of phosphoric acid was added
dropwise to this solution and stirred for 1 h. Then, TEOS was added to the solution under
stirring condition and aged at room temperature for 24 h. The obtained gel was then placed in
a stainless steel autoclave where it was heated at 200ºC for 36 h. The solid product was
removed by filtration, washed several times with deionized water and dried at 110ºC for 24 h.
Finally, the nanostructured catalyst was calcined at 550ºC for 12 h to remove organic
template and the water content trapped within the micro pores of the asꢀsynthesized solid.
2.4 Preparation of SAPO-34/HZSM-5
The SAPOꢀ34/HZAMꢀ5 composite was prepared by hydrothermal method as shown in
Figure 3s in supplementary materials. A certain amount of synthesized HZSMꢀ5 powder with
optimum ratio of Si/Al (Si/Al=180) was added to the SAPOꢀ34 gel and aged for 24 h for
synthesis of SAPOꢀ34(50%)/HZSMꢀ5(50%) composite. For crystallization, the resulting gel
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was transferred into a stainless steel autoclave and heated at 200ºC for 36 h. The solid
product was recovered by filtration, washed several times with deionized water and dried
overnight at 110ºC. Afterward, the catalyst sample was calcined at 550ºC for 12 h to remove
organic template.
2.5 Characterization Techniques
The Xꢀray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer
(Cu Kα radiation 1.54178˚A) to identify the crystal phases in the range of 2θ = 5ꢀ50°. The
phase identification was made by comparing the measured data with the Joint Committee on
Powder Diffraction Standards (JCPDSs). The morphology of the nanostructured catalyst was
investigated using the Field Emission Scanning Electron Microscopy (FESEM), HITACHI Sꢀ
4160. The studied samples were covered with a thin gold film to improve its conductivity.
The surface chemical composition of the nanostructured catalysts was determined by
scanning electron microscope equipped with an Energy Dispersive Xꢀray (EDX)
spectrometer for dot maps, Scan MV 2300 Cam. The BrunnerꢀEmmettꢀTeller (BET) surface
area was measured using nitrogen adsorption/desorption isotherms by a micrometric
Quantachrome (ChemBET 3000) analyzer. The infrared analysis of the nanostructured
catalyst was carried out by a UNICAM 4600 FTIR spectroscopy addressing the surface
functional groups.
2.6 Experimental Setup for Catalytic Performance Test
MTO catalytic performance was carried out in a fixed bed reactor at atmospheric pressure
(Figure 1). For a typical test, 0.4 g of the nanostructured catalyst was packed into the tubular
reactor with 0.8 cm inner diameter. Ar gas, as carrier, flow turned on to precondition the
catalyst at 500ºC for 1 h and then cooled to reaction temperature. During this time, the
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methanol aqueous solution (90 mol% methanol, 10 mol% H2O) was prepared and then
poured into the saturator. Ar gas has been bubbled through the saturator which was held in a
temperate ice bath at 10ºC and carried the methanol feed with a gaseous space velocity of
10000 hꢀ1 to the reactor. For each test, feed was replaced with a fresh one in order to insure
for constant feed concentration of the reactant by bubbling the methanol solution. The
products of the MTO reaction were analyzed using a gas chromatograph (GC Chrom, Teif
Gostar Faraz, Iran) equipped with PlotꢀU capillary column (Agilent) as well as flame
ionization detector (FID).
(Figure 1)
3 Results and Discussions
3.1 Physicochemical Characterizations
3.1.1 XRD Analysis
The Xꢀray diffraction patterns of asꢀsynthesized samples are shown in Figure 2. The XRD
patterns of the HZSMꢀ5 samples reveals the formation of tetragonal structure (JCPDS: 00ꢀ
044ꢀ0002) without any competing phases. This indicates the different crystallinity of the
samples by diffraction peak at 2θ = 8.0, 8.9, 20.9 and 23.2° which is similar to those HZSMꢀ
5s reported in the literature 44, 45. The obtained XRD result of SAPOꢀ34 molecular sieve
possesses characteristic peaks at 2θ = 9.5, 12.9, 20.6 and 31° related to rhombohedral
structure (JCPDS: 01ꢀ087ꢀ1527) 46ꢀ48. Moreover, the XRD results reveal that the synthesized
SAPOꢀ34(50%)/HZSMꢀ5(50%) catalyst is a real composite and not a physical mixture of the
separated phases. First of all, it exhibited all characteristic XRD peaks of both SAPOꢀ34 and
ZSMꢀ5. Moreover, the composite exhibits relatively weak HZSMꢀ5 and SAPOꢀ34
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characteristic peaks. This is due to the crystals with an interaction between HZSMꢀ5 and
SAPOꢀ34 and as well as not individual crystals 32.
Therefore, the intensities of peaks differ based on MFI, CHA and CHA/MFI structure,
indicating that the extent of crystallization and crystallite size were dependent on the Si/Al
ratio and type of structure phase. It is clear that the crystalline structure affects the
crystallization rate and also crystallite size 23.
Table 1 presents the results of relative crystallinity and crystallite size taking place during
hydrothermal synthesis of HZSMꢀ5 zeolites with different Si/Al ratios, SAPOꢀ34 and SAPOꢀ
34(50%)/HZASMꢀ5(50%) composite. The relative crystallinity was calculated using the
intensity of the most important peak at 2θ = 9.5° attributed to SAPOꢀ34 and 2θ = 23.2°
attributed to HZSMꢀ5 on the basis of the best prepared samples with highest crystallinity as
the references. The relative crystallinity of nanostructured HZSMꢀ5 catalysts with different
ratio of Si/Al was found to be 55.4%, 49.1%, 100% and 64.3%, respectively. Hence, the
crystallinity of HZSMꢀ5(Si/Al=180) nanostructured catalyst is higher than that other samples.
This indicates that proper distribution of Si for HZSMꢀ5 sample with Si/Al ratio of 180
increased the number of crystal nuclei. The appropriate crystallinity may be an important
factor for choosing the optimum ratio of Si/Al for HZSMꢀ5 synthesis because of its high
activity in MTO process 36. The relative crystallinity of SAPOꢀ34 catalyst was similar to
HZSMꢀ5 with Si/Al ratio of 180. In the case of SAPOꢀ34(50%)/HZSMꢀ5(50%) composite,
HZSMꢀ5 crystallinity is higher than SAPOꢀ34. This means that the HZSMꢀ5 powder which is
added to the SAPOꢀ34 synthetic gel decrease the nucleation rate for SAPOꢀ34. The SAPOꢀ
34(50%)/HZSMꢀ5(50%) sample appeared low crystallinity in comparison to SAPOꢀ34 and
HZSMꢀ5.
The average crystallite size of synthesized catalysts is calculated from the XRD spectra by
the Scherrer equation. The crystallite size of all samples was in the range of 15ꢀ30 nm. It can
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be seen that different Si/Al ratio for synthesis of HZAMꢀ5 samples can have low effect on
crystal growth rates resulting in the production of crystals with similar sizes. The average
crystallite size of SAPOꢀ34 catalyst was about 26 nm. Therefore, pure SAPOꢀ34 leads to
rapid crystal growth rates which results in larger crystallite sizes. Moreover, SAPOꢀ
34(50%)/HZSMꢀ5(50%) composite exhibited two kinds of crystallites. The large crystallites
assigned to HZSMꢀ5 and small ones related to SAPOꢀ34.
(Figure 2)
(Table 1)
3.1.2 FESEM Analysis
Figure 3 illustrates FESEM images of HZSMꢀ5 Si/Al= 120, 140, 180 and 220), SAPOꢀ34 and
SAPOꢀ34(50%)/HZSMꢀ5(50%) nanostructured catalysts. The HZSMꢀ5 samples show the
coffin morphology of zeolites without obvious aggregation 49. But, the SAPOꢀ34 sample
exhibits the presence of particles similar to cubic morphology like natural chabazite with
small grains on zeolite 50ꢀ52. In particular, the catalyst pore size and pore size distribution,
intracrystalline diffusion as well as catalytic activity was strongly affected by catalyst
morphology 27, 53. The average particle size of HZSMꢀ5 samples with different ratio of Si/Al
and SAPOꢀ34 is <1 ꢁm. While, SAPOꢀ34(50%)/HZSMꢀ5(50%) sample consists of two kinds
of particles, in which the small particles of SAPOꢀ34 were aggregated on HZSMꢀ5.
Therefore, it is obvious that we can expect the interaction among them and formation of a
real composite. As indicated in the figure, with increase of silicon content, HZSMꢀ5 particles
became larger and more uniform than others. In addition, The SAPOꢀ34 particles with narrow
size distribution are smaller than HZSMꢀ5 samples. It can be attributed to the existence of
more nucleation centres, which generated more molecular sieves with small particle size54ꢀ56
.
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(Figure 3)
3.1.3 EDX Analysis
The chemical composition of nanostructured samples was investigated by using EDX
analysis (Figure 4). It was found that the crystalline structure of HZSMꢀ5 samples comprised
of Si and Al, while the elements of Si, Al and P were detected for SAPOꢀ34 and SAPOꢀ
34(50%)/HZSMꢀ5(50%) samples. The presence of oxygen in local EDX images indicates the
formation of molecular components of SiO2, Al2O3 and P2O5. In addition, the proper
distribution of the elements on the surface of all catalysts is shown. In HZSMꢀ5 samples
synthesized with different Si/Al ratios, higher incorporation of oxygen for HZMꢀ
5(Si/Al=180) catalyst confirmed the formation of SiO4 and AlO4 structures. This means that
the region around HZSMꢀ5 sample synthesized with Si/Al=180 contains a small amount of
amorphous silica. Therefore, this observation expected good catalytic performance for this
sample. The micrograph shows more appropriate distribution of Si for SAPOꢀ
34(50%)/HZSMꢀ5(50%) composite. As a result, creating new acid sites with proper
distribution of Si in crystal lattice structures significantly improve the catalytic performance
of SAPOꢀ34(50%)/HZSMꢀ5(50%) composite. Furthermore, during the synthesis and
maintenance of synthetic samples no impurities were observed.
(Figure 4)
3.1.4 BET Analysis
The BET surface area of prepared nanostructured catalysts are shown in Figure 5. The results
show that when Si/Al ratio increased, the surface area increased from 337 to 388 m2/g, which
correspond to those obtained in previous studies 36. According to the results, the SAPOꢀ34
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sample exhibited the highest surface area (592 m2/g) due to its high crystallinity and small
particle sizes. The surface area of pure SAPOꢀ34 is very larger than that of the nanostructured
HSZMꢀ5 (Si/Al=180) sample with almost the same crystallinity. This difference between the
two samples may be attributed to the morphology of catalysts. Unlike the HZSMꢀ5
(Si/Al=180) sample with coffin particles, the cubic SAPOꢀ34 grains with uniform
morphology leading to more cell crystals and resulting in the increase of BET surface area.
Especially, comparison of specific surface area of the samples reveals the reduction of SBET
for these catalysts as SAPOꢀ34>SAPOꢀ34(50%)/HZSMꢀ5(50%)>HZSMꢀ5(Si/Al=180). The
moderate specific surface area of the SAPOꢀ34/HZSMꢀ5 composite can be attributed to the
low surface area of HZSMꢀ5 (Si/Al=180).
(Figure 5)
3.1.5 FTIR Analysis
The FTIR spectra of nanostructured samples shown in Figure 6 confirming the formation of
aluminosilicate MFI structure and silicoaluminophosphate CHA framework 27, 31, 52, 57. The
58ꢀ60
band around 3450 cmꢀ1 was related to structural OꢀH
for Si(OH)Al bridging hydroxyl
group. The high intensity of this peak for pure SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ5(50%)
composite indicates proper hydroxyl group as well as better catalytic performance, which
have also been confirmed by BET analysis. The stretching vibration at 1650 cmꢀ1 can be
attributed to the physically adsorbed water 51, 61, 62. The peak in wave number of 460 cmꢀ1 can
be attributed to TꢀO bending Si incorporated into the tetrahedral framework of the catalysts
63ꢀ65. The significant difference in the intensity of this peak for all samples is in excellent
agreement with the result of EDX analysis which is presented earlier. Moreover, the
absorption band at 560 cmꢀ1 is related to asymmetric stretching vibration of five membered
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rings similar to that of pure HZSMꢀ5 which can be seen in HZSMꢀ5 with different Si/Al ratio
and SAPOꢀ34(50%)/HZSMꢀ5(50%) composite. The peaks at wave numbers about 1390,
1110, 810, 690 and 640 cmꢀ1 are assigned to the asymmetric stretching of TO4 tetrahedra, Tꢀ
OꢀT symmetric stretching, protonated template, symmetric and TꢀO bending in Dꢀ6 rings,
respectively 27, 66ꢀ68. The stretching vibration around 2020, 2345 and 2390 cmꢀ1 illustrates
physical adsorption of CO and CO2 from surrounding 62, 69, 70
.
(Figure 6)
3.2 Catalytic Performance Study toward Methanol to Light Olefins Conversion
3.2.1 Effect of Si/Al Ratio of HZSM-5 on Methanol Conversion
The methanol conversion over HZSMꢀ5 nanostructured catalysts with different Si/Al ratio in
MTO reaction was tested at 235ꢀ415ºC and shown in Figure 7. It is observed that the
methanol conversion for all catalysts was enhanced by increasing temperature. On the other
hand, the conversion of methanol decreased slightly with increasing the Si/Al ratio in HZSMꢀ
5 samples. Therefore, the HZSMꢀ5 sample with Si/Al ratio of 120 showed high activity in all
reaction temperatures. The total amount of acid sites on catalyst surface plays a key role for
surpassing side reactions and leading to coke formation 36. According to FTIR analysis, the
catalysts with high ratio of Si/Al indicate low intensity of Si(OH)Al bridging hydroxyl peak
as well as low active sites. It can be conclude that the small amount of acid sites covered by
coke and would lead to decrease in catalyst activity. In particular, the value of methanol
conversions at all reaction temperature showed the low effect of different Si/Al ratio on
methanol conversion.
(Figure 7)
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3.2.2 Effect of Si/Al Ratio of HZSM-5 on Products Selectivity
The selectivity of ethylene and propylene for HZSMꢀ5 nanostructured catalysts with different
Si/Al ratio are presented in Figure 8 (a) and (b). According to the figures, in all HZSMꢀ5
nanostructured catalysts, the production of ethylene increased gradually, while propylene
selectivity decreased by increasing temperature. These results seem to be the cracking of
larger molecules such as propylene and butylene in the crystalline space which forms the
ethylene molecules. When Si/Al ratio increased, the ethylene selectivity of HZSMꢀ5 samples
decreased expect for Si/Al of 180, while propylene selectivity increased. The maximum
selectivity of ethylene is achieved on the sample with Si/Al ratio of 180 and that of propylene
is on the ones Si/Al ratio of 180 and 220 in the whole reaction temperature. The reason for
this observation is attributed to high specific surface area, complete crystallinity and proper
distribution of Si into the framework of HZSMꢀ5(Si/Al=180) catalyst which improve the
catalytic performance. In addition, the ratio of ethylene to propylene can be changed by
increasing Si/Al ratio from 120 to 220.
(Figure 8)
Figure 9 shows the selectivity of CH4 (a) and C4+ (b) produced over nanostructured HZSMꢀ5
catalysts with different Si/Al ratios. Methane is an undesired product for MTO reaction;
hence, large methane formation coupled with significant boost in lower olefins. It is observed
that at lower temperatures the amount of methane formed is negligible while it increased at
higher temperatures. The HZSMꢀ5(Si/Al=180) catalyst was found to produce less methane in
comparison to other samples. This is due to high amount of Si which existed inside the
framework and low decomposition of DME into methane. The different methane selectivity
13

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confirmed the quite different deactivation rates for HZSMꢀ5 samples synthesized with Si/Al
+
+
ratio in the range of 120 to 220. In the case of C4 , by increasing temperature, cracking of C4
to ethylene caused the decrease in C4+ selectivity for all HZSMꢀ5 samples. The HZSMꢀ5
(Si/Al=120) catalyst with low selectivity toward ethylene showed high amount of C4+ during
+
the reaction temperatures. As a result, the low selectivities of methane and C4 products
improved the catalytic performance of HZSMꢀ5(Si/Al=180).
(Figure 9)
Figure 10 shows the olefin/nonꢀolefin hydrocarbons at temperature range of 235ꢀ415°C over
nanostructured HZSMꢀ5 catalysts with Si/Al ratios of 120, 140, 180 and 220. When the
reaction temperature increased from 235 to 415°C, the ratio of olefins to nonꢀolefin
hydrocarbons was increased. This observation is due to increasing the rate of olefins
production in comparison to nonꢀolefin hydrocarbons. Evidently, on HZSMꢀ5(Si/Al =120)
the ratio of olefins to nonꢀolefin hydrocarbons is the lowest at all reaction temperatures. This
+
coincides with the high amounts of undesirable products such as methane and C4 presented
earlier. Therefore, increasing the ratio of Si/Al caused to obtain high ratio of olefin/nonꢀolefin
hydrocarbons. By considering methanol conversion as well as catalytic performance for the
nanostructured HZSMꢀ5(Si/Al=120, 140, 180 and 220) catalysts, the HZSMꢀ5(Si/Al =180)
was selected as optimum sample to compared with SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ
5(50%) composite.
(Figure 10)
3.2.3 Methanol Conversion over HZSM-5, SAPO-34 and SAPO-34/HZSM-5
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Figure 11 compares the methanol conversion at different temperatures on HZSMꢀ
5(Si/Al=180), SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ5(50%) composite. At low
temperatures, all the catalysts present low methanol conversion indicating their low initial
activity. It is evident that, the increasing of temperature enhances the methanol conversion.
As the temperature increased to 370 and 415°C, the methanol conversion becomes complete
for pure SAPOꢀ34 catalyst. In particular, the maximum methanol conversion (about 81ꢀ
100%) was observed for SAPOꢀ34(50%)/HZSMꢀ5(50%) composite. This is due to
appropriate distribution of Si in crystal lattice structures which significantly improves the
catalytic activity of SAPOꢀ34(50%)/HZSMꢀ5(50%) composite.
(Figure 11)
3.2.4 Products Selectivity over HZSM-5, SAPO-34 and SAPO-34/HZSM-5
Figure 12 shows the effect of reaction temperature on products selectivity over HZSMꢀ5,
SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ5(50%) catalysts. When the temperature rose, C2H4
+
gradually increased whereas the selectivity of C3H6 and C4 decreased for all samples. As
mentioned earlier, oligomerization of propylene and butylene which cracks to form ethylene
allows the diffusion of ethylene molecules out of the pores. Among the catalysts, pure
HZSMꢀ5 gave the highest propylene selectivity with constant value around 70ꢀ77%, while
SAPOꢀ34 had the highest selectivity toward ethylene reached a maximum of 75% at 415°C.
This result can be addressed by catalyst structure which well matched with those of HZSMꢀ5
and SAPOꢀ34 products in MTO process reported in the literature 32, 71, 72. Over SAPOꢀ
34(50%)/HZSMꢀ5(50%) composite, both ethylene and propylene are prominent products.
Among the light olefins products, three dimensional CHA structure of SAPOꢀ34 favours the
ethylene production. The combination of HZSMꢀ5 powder, with two dimensional MFI
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structure, with SAPOꢀ34 is helpful for propylene production. As a result, the C2H4/C3H6 yield
can be altered with using HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ5(50%) as
nanostructured catalysts for light olefins production. The methane production is very small at
these reaction temperatures, because of the adsorption of DME on the active sites of catalysts
to prevent formation of CH4. By considering CH3OH conversion as well as the light olefins
selectivity for the nanostructured catalysts, the temperature of 370°C was selected to
investigate the time on stream performance.
(Figure 12)
3.2.5 Time on Stream Performance of HZSM-5, SAPO-34 and SAPO-34/HZSM-5
Figure 13 (aꢀc) show the product distribution over HZSMꢀ5, SAPOꢀ34 and SAPOꢀ
34(50%)/HZSMꢀ5(50%) nanostructured catalysts with reaction time on stream (TOS). All the
catalysts were kept at the respective reaction temperature for a given reaction times. The
samples have different catalytic performance at the given condition and also their lifetime
varies significantly. The SAPOꢀ34 and SAPOꢀ34(50%)/HZSMꢀ5(50%) samples have the
highest methanol conversion over the course of 22 h, but they lose their activity afterward. In
contrast, the HZSMꢀ5 catalyst showed no distinct reduction in the methanol conversion
(fairly stable at 62%) during the reaction time of 33 h. Therefore, the SAPOꢀ34 catalyst and
interaction of HZSMꢀ5 and SAPOꢀ34 occurred in SAPOꢀ34(50%)/HZSMꢀ5(50%) composite,
favoured the high coke formation and blockage of catalysts pore which limited the active
sites for methanol conversion. After 5 h, the maximum selectivity toward ethylene (57 %)
over SAPOꢀ34(50%)/HZSMꢀ5(50%) composite decreased substantially with subsequent
increases in the other olefin products, including propylene. It is concluded that much higher
C3H6 yield of SAPOꢀ34(50%)/HZSMꢀ5(50%) composite must be responsible for the
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enhancement of olefins production (about 82ꢀ85%) at 33 h time on stream. When the SAPOꢀ
34 was employed as catalyst, it was found that the high selectivity toward ethylene from 60%
(after 17 h) was reduced to 38% (after 20 h) and also showed a decrease in selectivity toward
propylene. The observations suggest that pure SAPOꢀ34 showed poor catalytic stability under
the reaction conditions. HZSMꢀ5(Si/Al=180) was the only material which was found to have
a moderately stable production of olefins, in which the selectivity towards ethylene (17 %)
and propylene (65 %) stayed relatively constant over 33 h, owing to its ability to not form
deactivating coke over a long lifetime. Furthermore, the ethylene to propylene ratio remained
constant around 2.5 during 33 h time on stream for this catalyst. Overall at 370°C and
TOS=30 h, the order in olefins selectivity with different ratios of ethylene/propylene was
followed as:
SAPOꢀ34(50%)/HZSMꢀ5(50%) ≥ HZSMꢀ5 > SAPOꢀ34
This is indicated that the rate of ethylene and propylene production altered during this period
which is related to type of catalyst used in MTO reaction. The selectivity for methane and
DME was found to be negligible for SAPOꢀ34(50%)/HZSMꢀ5(50%) and HZSMꢀ5 catalysts
due to their high selectivity toward light olefins. It can also be observed that the production
of methane and DME increased after 17 h for SAPOꢀ34. The appearance of DME and
methane corresponded to a decrease in light olefins production because of channels blocking
by coke formation. The catalytic performance confirmed that the SAPOꢀ34(50%)/HZSMꢀ
5(50%) sample is the real composite. Therefore, it is expected that the real composite
synthesized in this study with appropriate properties have a good catalytic performance and
control the composition of light olefins in comparison to the sample like physical mixture.
Table 2 illustrates different SAPOꢀ34, HZSMꢀ5 and SAPOꢀ34/HZSMꢀ5 catalysts reported in
olefins production. The methanol conversion and olefins selectivity of synthesized catalysts
compared to reported results. As shown, using the optimum ratio of Si/Al for synthesis of
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HZSMꢀ5 and SAPOꢀ34/HZSMꢀ5 catalysts demonstrates excellent performance in
comparison to those catalysts reported in literature.
(Figure 13)
(Table 2)
4 Conclusions
It is found that the Si/Al ratio has influential effect on crystallinity, particle size, surface area,
acidity as well as catalytic performance of HZSMꢀ5 samples. It is concluded that the HZSMꢀ
5 (Si/Al=180) catalyst considering by physicochemical properties and catalytic performance
could be an optimum sample interacted with SAPOꢀ34 synthetic gel to produce SAPOꢀ
34/HZSMꢀ5 composite. The SAPOꢀ34/HZSMꢀ5 composite exhibited two kinds of crystals
which large crystals assigned to HZSMꢀ5 and small ones related to SAPOꢀ34. Among the
light olefins products over composite, the SAPOꢀ34 favours the ethylene production and
coating of HZSMꢀ5 powder with SAPOꢀ34 is helpful for propylene production. As a result,
the C2H4/C3H6 yield can be altered with using HZSMꢀ5, SAPOꢀ34 and SAPOꢀ
34(50%)/HZSMꢀ5(50%) as nanostructured catalysts for light olefins production.
Acknowledgements
The authors gratefully acknowledge Sahand University of Technology for the financial
support of the research as well as Iran Nanotechnology Initiative Council for complementary
financial supports.
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Figures Caption
Figure 1. Experimental setup for performance test of nanostructured HZSMꢀ5, SAPOꢀ34
and SAPOꢀ34/HZSMꢀ5 catalysts used in conversion of methanol to light
olefins.......................................................................................................... 26
Figure 2. XRD patterns of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts: (a) HZSMꢀ5(Si/Al=120), (b) HZSMꢀ5 (Si/Al=140), (c) HZSMꢀ5
(Si/Al=180), (d) HZSMꢀ5 (Si/Al=220), (e) SAPOꢀ4 and (f) SAPOꢀ
34(50%)/HZSMꢀ5(50%). ............................................................................. 27
Figure 3. FESEM images of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts: (a) HZSMꢀ5(Si/Al=120), (b) HZSMꢀ5 (Si/Al=140), (c) HZSMꢀ5
(Si/Al=180), (d) HZSMꢀ5 (Si/Al=220), (e) SAPOꢀ34 and (f) SAPOꢀ
34(50%)/HZSMꢀ5(50%). ............................................................................. 28
Figure 4. EDX analysis of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts: (a) HZSMꢀ5(Si/Al=120), (b) HZSMꢀ5 (Si/Al=140), (c) HZSMꢀ5
(Si/Al=180), (d) HZSMꢀ5 (Si/Al=220), (e) SAPOꢀ34 and (f) SAPOꢀ
34(50%)/HZSMꢀ5(50%). ............................................................................. 29
Figure 5. BET surface area analysis of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ
34/HZSMꢀ5 catalysts.................................................................................... 30
Figure 6. FTIR spectra of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts: (a) HZSMꢀ5(Si/Al=120), (b) HZSMꢀ5 (Si/Al=140), (c) HZSMꢀ5
(Si/Al=180), (d) HZSMꢀ5 (Si/Al=220), (e) SAPOꢀ34 and (f) SAPOꢀ
34(50%)/HZSMꢀ5(50%). ............................................................................. 31
Figure 7. Effect of Si/Al ratio of HZSMꢀ5 on methanol conversion. .............................. 32
Figure 8. Effect of Si/Al ratio of HZSMꢀ5 on light olefins selectivity: (a) C2H4 and (b)
C3H6............................................................................................................. 33
Figure 9. Effect of Si/Al ratio of HZSMꢀ5 on side products selectivity: (a) CH4 and (b)
+
C4 . .............................................................................................................. 34
Figure 10. Effect of Si/Al ratio on olefin/nonꢀolefinic products ratio. ............................ 35
Figure 11. Methanol conversion over HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts. ...................................................................................................... 35
Figure 12. Products selectivity over HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts. ...................................................................................................... 36
Figure 13. Time on stream performance of HZSMꢀ5, SAPOꢀ34 and SAPOꢀ34/HZSMꢀ5
catalysts. ...................................................................................................... 37
Tables Caption
Table 1. Structural properties of nanostructured HZSMꢀ5, SAPOꢀ34 and SAPOꢀ
34/HZSMꢀ5 catalysts used in conversion of methanol to light olefins........... 25
Table 2. The methanol conversion and products selectivity of typical SAPO-34,
HZSM-5 and SAPO-34/HZSM-5 catalysts................................................ 25
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Tables and Figures
Table 1. Structural properties of nanostructured HZSM-5, SAPO-34 and SAPO-34/HZSM-5 catalysts
used in conversion of methanol to light olefins.
Relative Crystallinitya
Crystallite sizeb (nm)
SAPOꢀ34 HZSMꢀ5
Catalyst
SAPOꢀ34
HZSMꢀ5
55.4
49.1
100
SAPOꢀ34c HZSMꢀ5d
(wt. %)
(wt. %)
100
100
100
100
ꢀ
HZSMꢀ5(Si/Al=120)
HZSMꢀ5 (Si/Al=140)
HZSMꢀ5 (Si/Al=180)
HZSMꢀ5 (Si/Al=220)
SAPOꢀ34
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
18.8
19.5
17.9
16.2
ꢀ
ꢀ
ꢀ
64.3
ꢀ
50.9
ꢀ
100
100
25.5
26.6
16.9
SAPOꢀ34/HZSMꢀ5
50
50
18.1
a. Relative crystallinity: XRD pattern relative intensity.
b. Crystallite size estimated by Scherre's equation.
c. Crystallite phase: Rhombohedral (JCPDS: 01ꢀ087ꢀ1527, 2θ= 9.5, 12.9, 16.0, 17.7, 20.6, 24.9, 25.9, 30.6, 31.0 º)
d. Crystallite phase: Tetragonal (JCPDS: 00ꢀ044ꢀ0002, 2θ= 8.0, 8.9, 9.1, 14.9, 20.9, 23.2º)
Table 2. The methanol conversion and products selectivity of typical SAPO-34, HZSM-5 and SAPO-
34/HZSM-5 catalysts.
Reaction
time (min
SAPOꢀ34/HZSMꢀ5
(weight ratio)
Si/Al(molar ratio)
in HZSMꢀ5
Conversion
(%)
)
Olefins
selectivity (%)
T(°C)
Catalysts
SAPOꢀ34
SAPOꢀ34
SAPOꢀ34
HZSMꢀ5
HZSMꢀ5
HZSMꢀ5
SAPOꢀ34/HZSMꢀ5
SAPOꢀ34/HZSMꢀ5
SAPOꢀ34/HZSMꢀ5
SAPOꢀ34/HZSMꢀ5
Ref.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
370
425
400
370
500
500
370
500
400
500
800
360
190
1800
120
960
1200
180
90
99
100
80
86
40
68
This work
29
52
180
120
120
180
25
80
25
70
82
This work
43
98.8
98.6
80
100
100
100
59.1
58.5
82
37.4
55.6
37.4
43
ꢀ
1
1
1
1
This work
35
32
33
180
25

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RSC Advances
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DOI: 10.1039/C6RA00432F
PIꢀ02
Ar
FM (02)
Notations:
C:
Cylinder
PRV: Pressure Regulator Valve
R: Reactor
TIC: Temp. Indicator & Controller
3WV: 3ꢀWay Valve
GC: Gas Chromatography
MFC: Mass Flow Controller
PI:
Pressure Indicator
PIꢀ01
Ar
Ar
FM (01)
Ar+CH3OH
Feed
to reactor
Feed
to GC
Feed
to GC
Ar
Vent
CH3OH
Reactor
outlet
Reactor
inlet
Saturator
PRVꢀ02
PRVꢀ01
Cooler
Ar
Vent
Air
H2
Electrical heating
Products
to GC
furnace with
temperature
controller
Vent
Temperatur
e Controller
Controlled
power supply
Ar
Ar
GC
Ar Air H2
TICꢀ01
Rꢀ01
Furnace/Reactor
Cꢀ1
Cꢀ2
Cꢀ3
Cꢀ5
Cꢀ4
Figure 1. Experimental setup for performance test of nanostructured HZSM-5, SAPO-34 and SAPO-
34/HZSM-5 catalysts used in conversion of methanol to light olefins.
26

RSC Advances
Page 28 of 39
View Article Online
DOI: 10.1039/C6RA00432F
■
■
●
SAPOꢀ34
■ ZSMꢀ5
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■ ■
■
■
■
■
(a) HZSMꢀ5(Si/Al=120)
■
■
■
■
■
■ ■
■
■
(b) HZSMꢀ5(Si/Al=140)
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■ ■
■
■
■
■
■
■
■
(c) HZSMꢀ5(Si/Al=180)
■
■
■
■
■
■
■
■
■
■
●
■
■
■
■ ■
(d) HZSMꢀ5(Si/Al=220)
●
●
●
●
●
●
●
●
●
●
(e) SAPOꢀ34
■
■
■
●
■
■
●
●
●
●
●
■
■
●
■
■
■
■
■
■ ■
■
■
■
■
■
(f) SAPOꢀ34(50%)/ZSMꢀ5(50%)
Reference patterns
SAPOꢀ34 (Rhombohedral, 01ꢀ087ꢀ1527)
HZSMꢀ5 (Tetragonal, 00ꢀ044ꢀ0002)
5
10
15
20
25
30
35
40
45
50
2θ (degree)
Figure 2. XRD patterns of nanostructured HZSM-5, SAPO-34 and SAPO-34/HZSM-5 catalysts: (a)
HZSM-5(Si/Al=120), (b) HZSM-5 (Si/Al=140), (c) HZSM-5 (Si/Al=180), (d) HZSM-5 (Si/Al=220), (e)
SAPO-4 and (f) SAPO-34(50%)/HZSM-5(50%).
27

Page 29 of 39
RSC Advances
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DOI: 10.1039/C6RA00432F
(a) HZSMꢀ5(Si/Al=120)
(b) HZSMꢀ5(Si/Al=140)
(c) HZSMꢀ5(Si/Al=180)
(d) HZSMꢀ5(Si/Al=220)
(f) SAPOꢀ34
(e) SAPOꢀ34(50%)/ZSMꢀ5(50%)
Figure 3. FESEM images of nanostructured HZSM-5, SAPO-34 and SAPO-34/HZSM-5 catalysts: (a)
HZSM-5(Si/Al=120), (b) HZSM-5 (Si/Al=140), (c) HZSM-5 (Si/Al=180), (d) HZSM-5 (Si/Al=220), (e)
SAPO-34 and (f) SAPO-34(50%)/HZSM-5(50%).
28