Enhancement of catalytic properties and lifetime of nanostructured SAPO-34 by la isomorphous substitution and alteration of Si/Al ratio used in methanol conversion to light olefins

Article abstract of DOI:10.1039/c6ra02664h

Successful synthesis of nanostructured LaAPSO-34 catalysts with different Si/Al ratios of 0.1, 0.2, 0.3 and 0.4 were used in methanol conversion to light olefins. The catalysts were prepared via a hydrothermal method and characterized by XRD, FESEM, PSD, EDX, TEM, BET, NH₃-TPD and FTIR techniques. The XRD results showed that the applied La ions preserve and complete the crystallite framework of SAPO-34 resulting in higher crystallinity. It was found that the crystallinity of SAPO-34 increased upon Si incorporation up to a Si/Al ratio of 0.3 followed by a decrease in intensity with further increases in silicon content. The FESEM micrographs for all the samples depict cubic shaped typical crystals of CHA type zeotype structure. The Si incorporation value was decreased by increasing the Si content in the initial gel. A stability test was conducted maintaining constant operational conditions to distinguish the effect of time on stream. Consequently, coke formation was delayed. The outstanding catalytic lifetime of the La modified catalyst with optimum Si content was reported to be 5500 min sustaining light olefin selectivity at higher values at a diminished reaction temperature of 350 °C.

Full text of DOI:10.1039/c6ra02664h

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Page 1 of 44
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Enhancement of Catalytic Properties and Lifetime of Nanostructured
SAPO-34 by La Isomorphous Substitution and Alteration of Si/Al Ratio
Used in Methanol to Light Olefins
Abolfazl Amoozegar1,2, Mohammad Haghighi*,1,2, Sogand Aghamohammadi1,2
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
Successful synthesis of nanostructured LaAPSOꢀ34 catalysts with different Si/Al ratios being
0.1, 0.2, 0.3 and 0.4 were taken into account and used in methanol to light olefins. The
catalysts were prepared via hydrothermal method and characterized by XRD, FESEM, PSD,
EDX, TEM, BET, NH3ꢀTPD and FTIR techniques. The XRD results showed that the applied
La ions preserve and complete the crystallite framework of the SAPOꢀ34 resulted in higher
crystallinity. It was found that the crystallinity of SAPOꢀ34 increased upon Si incorporation
up to Si/Al ratio of 0.3 and then followed by a decrease in intensity with further increase in
silicon content. The FESEM micrographs for all the samples depict the cubic shaped typical
crystals of CHA type zeotype structure. Si incorporation value was decreased by increasing
the Si content in the initial gel. The stability test was conducted maintaining at constant
operational conditions to distinguish the effect of time on stream. Consequently, coke
formation has been delayed. The outstanding catalytic lifetime of the La modified catalyst
with optimum Si content was reported to be 5500 min sustaining light olefins selectivity at
higher values at diminished reaction temperature of 350ºC.
Keywords: LaAPSOꢀ34, Si/Al ratio, Methanol, Olefins, MTO.
1

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1 Introduction
Ethylene and propylene are important petrochemicals feedstock, useful in a variety of
processes for making plastics and other chemical compounds such as ethylene dichloride,
ethylene oxide, propylene oxide, oxoꢀalcohol, polystyrene, acrylic acid and acrylonitrile.
Demand for these key components is forecasted to increase over the next decade 1ꢀ3. The
demand by 2015 is projected to grow up to 160 and 105 million tons for ethylene and
propylene, respectively. At present, 97% of ethylene is produced from thermal steam
cracking. Moreover, it produces propylene as a byꢀproduct which contributes to 66% of
worldwide propylene production 1, 2
.
The preferred methanol conversion process is generally referred to methanolꢀtoꢀolefins
(MTO) process, where methanol is converted to primarily ethylene and/or propylene in the
presence of a molecular sieve 4ꢀ8. The oil crisis in 1970s caused a great interest in developing
the methanol to hydrocarbons (MTH) technology as a vital alternative to the petroleum based
route for the production of needed chemicals 1, 2, 9. Efforts were made to develop technologies
for selective production of light olefins from methanol either on the mediumꢀpore ZSMꢀ5
zeolite 10, 11, or later on smallꢀpore SAPOꢀ34 zeotype 12ꢀ14. SAPOꢀ34 molecular sieve with
CHA structures, shows excellent performance in catalytic conversion of methanol to light
olefins due to its moderate acidity and small 8ꢀring pore opening 15ꢀ17. The small (about 4.7
Å) cages of SAPOꢀ34 restrict the diffusion of heavy hydrocarbons, and this leads to high
selectivity to the desired small light olefins 18ꢀ21. The structure, acidity and catalytic
performance of SAPOꢀ34 catalyst directly depend on the number and distribution of Si in the
framework 22ꢀ24. The acidity of SAPO molecular sieves is generated by the hydroxyl groups,
which originates from the protons for compensating the unbalanced electric charges due to Si
incorporation into the neutral framework of AlPOs 25. Many researchers have studied the
mechanism of Si incorporation into AlPOs framework and the Si substitution mechanism is
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generally recognized in the crystallization process of SAPO molecular sieve. First, AlPOs
structure is formed, then Si atoms is incorporated into the AlPOs framework by isomorphous
substitution. It is a general rule that the linkage of the type SiꢀOꢀP is avoided. It is recognized
that there exist two kinds of Si substitution mechanisms in the crystallization of SAPO
molecular sieves. One is the substitution of P by Si (denoted as SM2), which would form Si
(4Al) entities. The other is the double substitution of adjacent P and Al by two Si atoms
(denoted as SM3), which gives rise to Si(nAl) (n = 3ꢀ0) structures and stronger Brønsted acid
sites 26. Hence, the catalytic properties of the SAPO should depend at least to some extent on
the relative occurrence of both substitution mechanisms. Therefore, the silica content is the
controlling variable which determines the relative ratio of acid sites of different strength. It
was proposed that Si incorporation by the first mechanism formed Si(4Al) structures when
the SiO2/Al2O3 molar ratio was low in starting gel. While for starting gel with increasing
SiO2/Al2O3 molar ratio, Si(3Al), Si(2Al), Si(1Al) and Si(0Al) structures were formed
simultaneously by the second mechanism 26ꢀ28
.
A main problem associated with the applied SAPOꢀ34 molecular sieve is the rapid
deactivation due to coke deposition during MTO reaction, leading to a serious decrease in
activity and selectivity of SAPOꢀ34 29, 30. For this reason, increasing the ability of the catalyst
to resist coke formation is a key challenge that is urgently required for the practical MTO
application 31, 32. Metal doping as a convenient way to control or alter the catalytic behaviour
was extensively investigated for MTO reaction with the goal of modifying the acidity of the
SAPOꢀ34 and adjusting the strength of TꢀO bond in the TO4 tetrahedral unit of the SAPOꢀ34
framework . A method of enhancing the catalyst lifetime and simultaneously avoiding the
formation of methane during the MTO process is metal introduction into the SAPOꢀ34
framework 33ꢀ35. However, the previously published articles mainly paid attention to the
transition metals but, effect of rare earth metal doping on the lifetime of the SAPOꢀ34
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catalyst have not been taken into account in detail especially with the isomorphous
substitution method. It has been reported that Laꢀdoped SAPOꢀ34 catalysts with solid state
ion exchange method resulted in higher selectivity of light olefins and reduced amount of
methane compared to the ones prepared without metal addition 36. In the present work,
lanthanum was chosen to be used for SAPOꢀ34 modification by the isomorphous substitution
method.
In this paper, a series of SAPOꢀ34 molecular sieves with Laꢀincorporation were synthesized
with different Si/Al ratios. In addition, the catalyst obtained by optimum Si content was
synthesized without metal addition to explore the metal influence. The physicochemical
properties of the molecular sieves were characterized using XRD, FESEM, PSD, EDX, BET,
NH3ꢀTPD and FTIR techniques. The characterization of materials followed by the testing of
samples for catalytic performance qualities such as reactivity (activity and selectivity) and
stability (lifetime) allows us to gain deeper insights into the relationship between structure
and catalytic properties.
2 Materials and Methods
2.1 Materials
The reactants used for preparation of LaAPSOꢀ34 samples were aluminium triꢀ isopropylate
(Aldrich, 98+ %), fumed silica (Aldrich, 99.9%), phosphoric acid (Merck, 85% aqueous
solution), lanthanum nitrate hexahydrate (Merck, 99%) as the sources of Al, Si, P and La,
respectively. TEAOH (Aldrich, 20%) was used as the organic template.
2.2 Preparation and Procedures
The details of LaAPSOꢀ34 synthesis with different Si contents are shown in Figure 1. The
synthesized catalysts with Si/Al ratio of 0.1, 0.2, 0.3 and 0.4 were denoted as LaAPSOꢀ34
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(0.1), LaAPSOꢀ34 (0.2), LaAPSOꢀ34 (0.3) and LaAPSOꢀ34 (0.4), respectively. To elucidate
the effect of metal addition, the LaAPSOꢀ34 (0.3) sample was synthesized without metal
introduction and named SAPOꢀ34 (0.3). The molar composition of synthesis solution was
2TEAOH: 1Al2O3: xSiO2: 0.05La2O3: 1P2O5: 70H2O. The parameter x, took the values of
0.2, 0.4, 0.6 and 0.8. In detail, weighted aluminium triꢀ isopropylate was dissolved in
TEAOH for 90 min. Fumed silica and lanthanum nitrate were added to the solution in turn.
Finally, phosphoric acid was gradually added and it was continually stirred for 24 h. The
resulting gel was transferred into a stainless autoclave and heated at 200ºC for 72 h. The
resulting solids were collected by centrifugation, washed with water and dried overnight at
110ºC. Finally, the catalyst sample was calcined at 550ºC for 12 h to remove organic
template and trapped water within the micro pores of the asꢀsynthesized solid.
(Figure 1)
2.3 Characterization Techniques
XRD patterns were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation
(1.54178˚A) to identify crystal phases. The phase identification was made by comparison to
the Joint Committee on Powder Diffraction Standards (JCPDSs). The average crystal size
was calculated using the halfꢀwidth at halfꢀheight of most intense peaks of diffraction pattern
and wellꢀknown DebyeꢀScherrer equation. The morphology of the samples was observed by
the Field Emission Scanning Electron Microscopy (FESEM) on a HITACHI Sꢀ4160 (Japan).
For reporting particle size distribution we used ImageJ software. Using this image analyser,
we find particles size in selected FESEM micrograph which shown in the figure. Some statics
have been performed to find the minimum, maximum and average particle size of the
samples by this software. The number of particles have been taken into account was 60. The
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FESEM was equipped with an Energy Dispersive Xꢀray (EDX) analyzer to carry out dot
maps for dispersion analysis. Transmission electron microscopy (TEM) was carried out on a
Philips CMꢀ200. Samples for TEM measurements were ultrasonically dispersed in ethanol.
The specific surface area of the sample was determined by N2 adsorptionꢀdesorption method
at ꢀ196ºC on a surface area analyzer (Quantachrome ChemBETꢀ3000). FTIR spectra of the
samples were recorded on a UNICAM 4600 FTIR spectroscopy using KBr pellet. Catalysts
acidity was measured by ammonia temperature programmed desorption using BELCAT
analyser with a TCD detector. Before analysis, 0.1g of calcined sample was preheated at
550ºC for 60 min under a 50 cm3/min helium gas flow. Ammonia adsorption was made from
a mixture of 5% (molar basis) of ammonia in helium under total flow rate of 50 cm3/min at
100ºC. After adsorption of ammonia, the samples were kept under a helium gas flow at 100ºC
to remove physically adsorbed gases. Finally, the helium flow (50 cm3/min) was passed
through the sample with increasing temperature up to 800ºC at a rate of 10ºC/m.
2.4 Experimental Setup for Catalytic Performance Test
The experimental setup used in this study is presented in Figure 2. Catalytic reaction studies
were performed in a fixed bed Uꢀshaped Pyrex reactor under atmospheric pressure. The
certain amount of catalyst (0.4 g) was loaded and prior to the reaction preheated using 70
ml/min argon at 500ºC for 60 min to remove the adsorbed water and then cooled to reaction
temperature. The liquid mixture of methanol and water with molar ratio of 30:70 was fed to
the reactor by passing argon flow through the saturator. All reactions were carried out at
constant gas hourly space velocity (GHSV= 4200 cm3/g.h). The saturator temperature was
kept at 8ꢀ9ºC by using an ice bath. The rate of carrier gas is adjustment with a mass flow
controller (MFC).The products were analyzed on gas chromatograph (GC Chrom, Teif
Gostar Faraz, Iran) with flame ionization detector (FID) using PlotꢀU (Agilent) column by
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programming the oven temperature between 40 and 180ºC. Argon was used as a carrier gas
and hydrogen was used in FID. Typical material balance was carried out for some of the
experiments and it was found that the value is within the error limit (±3%).
(Figure 2)
3 Results and Discussions
3.1 Physicochemical Characterizations
3.1.1 XRD Analysis
XRD patterns of the catalysts synthesized via hydrothermal method are shown in Figure 3.
All the five samples possess SAPOꢀ34 rhombohedral structure with the strongest XRD
diffraction at 2θ = 9.5 (JCPDS 01ꢀ087ꢀ1527) 3, 26, 37, 38. But, some special diffractions at 2θ =
19.8, 21.0 and 22.4 corresponding to SAPOꢀ5 phase (JCPDS 00ꢀ049ꢀ0659) 39, 40 can be found
for the sample with lower Si content (Si/Al = 0.1). The SAPOꢀ5 phase content is negligible
for LaAPSOꢀ34 (0.1). It will be demonstrated that the typical diffractive peaks and
crystallites of SAPOꢀ5 cannot be easily monitored by the XRD and FESEM analyses. By
further increase of Si content, the minor peaks of SAPOꢀ5 were disappeared for the LaAPSOꢀ
34 (0.2) sample. Furthermore, no assignable peak to metal oxide (La2O3) was observed for
the metal containing samples. The effect of Si/Al ratio was estimated from the XRD spectra
and it was found that the crystallinity of SAPOꢀ34 increased upon Si incorporation up to
LaAPSOꢀ34 (0.3) and then followed by a decrease in intensity with further increase in the
silicon content. To elucidate the effect of metal addition, the LaAPSOꢀ34 (0.3) sample was
synthesized without metal introduction. The XRD results show that the applied La ions
preserve and complete the crystallite framework of SAPOꢀ34 resulted in higher crystallinity.
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Thermal stability of the samples can be confirmed by the observation of all characteristic
peaks of SAPOꢀ34.
Relative crystallinity and crystallite size of the samples were calculated based on the peak
intensity at 2θ = 9.5 and Scherrer equation, respectively (Figure 4). The obtained crystallite
size was in a range of 23 to 43 nm, confirming the nanostructure framework of the
synthesized samples. It should be noticed that nanostructure and wellꢀdefined framework of
the catalysts can resultantly make them highly desired material for catalytic processes.
Relative crystallinity was calculated to be 49, 54.9, 87.4, 100 and 43% for LaAPSOꢀ34 (0.1),
LaAPSOꢀ34 (0.2), SAPOꢀ34 (0.3), LaAPSOꢀ34 (0.3) and LaAPSOꢀ34 (0.4), respectively
confirming the observed intensity variation of diffractive peaks. Most of the crystallinity
reduction occurred when the Si content increased from 0.3 to 0.4 for which the obtained
relative crystallinity is lower than that of the sample prepared with the lowest Si. It can be
explained in a way that the further increase of Si ions in the initial gel cannot incorporate into
the crystallite framework, leading to recrystallization and/or formation of amorphous phase.
The lattice parameters of SAPOꢀ34 and LaAPSOꢀ34 samples are shown in Error! Reference
source not found.. As seen, by introducing La into structure of SAPOꢀ34, lattice parameters
changed and volume increased. It confirmed isomorphously substitution of La into SAPOꢀ34
in SAPOꢀ34 sample. As, due to the low lanthanum oxide on the surface of crystals and well
dispersion of it, related peaks to lanthanum oxide were not observed in the patterns of
samples with La loading.
(Figure 3)
(Figure 4)
(Error! Reference source not found.)
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3.1.2 FESEM Analysis
Figure 5 shows the FESEM images of the synthesized catalysts to investigate the effect of
increasing Si/Al ratio on morphology. The scanning electron micrographs for all the samples
depict the cubic shaped typical crystals of CHA type zeotype structure reported in literature
41ꢀ43. It is obvious that variation of particle sizes and morphology arises from various Si
loadings into the SAPOꢀ34 structure. XRD pattern of the LaAPSOꢀ34 (0.1) sample confirmed
the presence of SAPOꢀ5 structure which is not observed by the corresponding typical
hexagonal like particles. It can afford additional support for the minor formation of the
SAPOꢀ5 impurity phase evidenced by the XRD analysis. As indicated, metal addition
resulted in smoother and a little bit larger particles. Thus, it can be implied that the presence
of La heteroatoms in the precursors solution did not result in any distribution of the SAPOꢀ34
framework and also enhance the relative crystallinity.
At lower Si content, crystallite framework of the SAPOꢀ34 structure did not formed
completely and some regions of amorphous particles can be easily detected from the FESEM
images. By gradual increase of Si/Al ratio, much more smoother and cubic like particles
appeared diminishing the roughness of the particles surface. However, the increase of Si/Al
ratio from 0.3 to 0.4 leads to the formation of some nonꢀcubic particles confirming the
crystallinity loss due to the recrystallization process and/or amorphous phase formation.
Completing the SAPOꢀ34 structure by Si increasing happens with the different Si substitution
mechanisms (SM2 and SM3). At lower Si loadings, the SM2 is dominant but the SM3 also
becomes dominant at higher Si contents. As discussed, the relative crystallite phase
decomposition can be the disadvantageous side effect of the further Si introduction. It can be
explored in terms of catalytic performance during the MTO reaction.
(Figure 5)
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3.1.3 PSD Analysis
44
Particle size distribution (PSD) of the samples has been calculated using ImageJ software
and illustrated in Figure 6. The average particle size was calculated to be 0.7 and 1.3 μm for
the SAPOꢀ34 (0.3) and LaAPSOꢀ34 (0.3) samples, respectively. It seems that the samples
with higher crystallinity appeared with much more smoother and larger particles in FESEM
micrographs that it confirms the accordance of the results of XRD and FESEM analyses.
Particle size and its distribution have great influences on the performance of SAPOꢀ34
catalysts applied for the MTO reaction.
(Figure 6)
3.1.4 EDX Analysis
Figure 7 shows the EDX dotꢀmapping of Al, Si, P and La for the synthesized catalysts.
Presence of all used elements in the corresponding EDX spectrums and results of former
analyses proves the successful hydrothermal synthesis. The determination of metal amount
was carried out with EDX analysis and compared with the initial gel composition as depicted
in Figure 8. The obtained materials and the initial gel compositions are relatively close to
each other. The Al and P elements of LaAPSOꢀ34 samples were the most scattered elements
which is simply observable by the EDX spectra and their dotꢀmappings. Si dotꢀmapping
exhibited with the highest dispersion for the LaAPSOꢀ34 (0.3) sample compared to the Si
distribution of the other samples. Another detectable difference may be attributed to the La
heteroatoms distribution on the surface which is responsible for enhancing the stability of
SAPOꢀ34. It has the same trend as mentioned for the Si dispersion on the surface of the
catalysts and higher dispersion of the La was obtained for LaAPSOꢀ34 (0.3). Thus, applying
Si/Al ratio of 0.3 resulted in higher dispersion of Si and La. It is better to make a comparison
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between the Si content of the initial gel and the synthesized samples to discuss the Si
incorporation. The Si/(Al+P+Si+La) value of all the starting gels and the values for the
prepared solid products are quite different, indicating that different amount of Si was
incorporated in to the SAPOꢀ34 framework by applying different Si/Al ratio. Si incorporation
was defined as the ratio of the [Si/(Al+P +Si+La) Solid]/[Si/(Al+P+Si+La)gel]. It is calculated
to be 2.33, 1.25, 1.11 and 0.97 for the LaAPSOꢀ34 (0.1), LaAPSOꢀ34 (0.2), LaAPSOꢀ34
(0.3) and LaAPSOꢀ34 (0.4), respectively. Si incorporation value was decreased by increasing
the Si content in the initial gel. It is indicative of the incapability of the crystallite framework
to incorporate higher amounts of Si. Si content and its distribution in the SAPOꢀ34
framework might have great impact on the catalytic behaviour of the catalysts.
(Figure 7)
(Figure 8)
3.1.5 TEM Analysis
TEM images of the LaAPSOꢀ34 (0.3) catalyst synthesized were illustrated in Figure 9. The
production of some nanometer scale particles alongside bigger ones can be clearly observed
from the corresponding TEM images. Nanocatalysis research can be explained as the
preparation of heterogeneous catalysts in the nanometer length scale. They are very
promising and it can be expected that use of them can decrease the energy usage in the
chemical processes results in a greener chemical industry.
(Figure 9)
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3.1.6 BET Analysis
The total specific surface area of the synthesized samples was measured and reported in
Figure 10. It is obvious that the use of La heteroatom caused the increase of specific surface
area comparing to the one prepared without metal. It can be explained by the further growth
of the SAPOꢀ34 crystallite framework in the presence of metal ions in the initial gel. The
increase of Si/Al ratio from 0.1 to 0.3 resulted in an increase of specific surface area from
425 to 528 m2/g and then followed by a decrease to 443 m2/g for the LaAPSOꢀ34 (0.4). The
observed trend for the variation of surface area was seen for the relative crystallinity. It can
be implied that the increase of Si/Al ratio up to 0.3 leads to the structure completeness.
Consequently, enhancement of crystallinity and particles smoothness cause the increase of
specific surface area. The highest specific surface area was achieved for the catalyst with the
highest crystallinity so results of BET analysis are in well agreement with XRD analysis.
Active sites availability for the MTO reaction has a relationship with the specific surface area
of the catalysts. It is expected that the LaAPSOꢀ34 (0.3) with the highest specific surface area
and much more availability of active sites is going to have excellent catalytic performance.
(Figure 10)
3.1.7 FTIR Analysis
Figure 11 illustrates the FTIR spectra of the calcined catalysts that synthesized with different
Si contents. Characteristic peaks of SAPOꢀ34 were identified at wave numbers 480, 635, 715
and 1110 cmꢀ1 for all the samples 45, 46. The peaks at 480, 530 and 570 cmꢀ1 are assignable to
the bending vibrations of SiO4, AlO4 and PO4, respectively 26, 47. Furthermore, peak located at
635 cmꢀ1 can be attributed to the TꢀO bending in Dꢀ6 rings 37, 45. Symmetric and asymmetric
vibration of the OꢀPꢀO can be observed at wavenumber of 715 and 1110 cmꢀ1, respectively 48.
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Major peaks positions did not change with La addition which is in accordance with the
corresponding XRD analysis. Moreover, no displacement of the characteristic peaks of
SAPOꢀ34 was monitored by increasing the Si/Al ratio. Peaks at wavenumber of 2350 and
2450 cmꢀ1 can be attributed to the physically adsorption of CO and CO2 from surrounding 38,
49, 50. The peak at wavenumber of 3450 cmꢀ1 can be attributed to bridging hydroxyl groups 51ꢀ
53. SiꢀOHꢀAl bridges position and intensity of them in the IR spectra can provide information
to judge about the products acidity 17, 54, 55. The peak at wavenumber of 1650 cmꢀ1 is assigned
to physical adsorption of the H2O molecules on the samples surface 52, 56, 57
.
(Figure 11)
3.1.8 TPD-NH3 Analysis
The TPD profiles of ammonia desorption on LaAPSOꢀ34 (0.2), SAPOꢀ34 (0.3) and LaAPSOꢀ
34(0.3) catalysts are presented in Figure 12 to determine the concentration and strength of
acid sites. Two desorption peaks are clearly observed for all of the samples. The stronger acid
sites exhibits with a higher desorption temperature resulting in higher values of temperature
maxima in TPD patterns. Furthermore, the area underlying the curves indicates the amount of
ammonia indicative of acid sites concentration. Ammonia desorption peaks approximately
centered at 176ꢀ179ºC for weak acid sites and 392ꢀ423ºC for strong acid sites. The first peak
attributes to the weak acid sites representing the defect structural OH groups (including Siꢀ
OH, PꢀOH and AlꢀOH). The second desorption peak belongs to the strong acid sites, related
to SiꢀOHꢀAl groups (Brønsted acidic sites). As indicated, weak acid sites density was
decreased by the increase of Si/Al ratio from 0.2 to 0.3. But its strength did not modify
considering the peaks maxima for the LaAPSOꢀ34 (0.2) and LaAPSOꢀ34 (0.3) catalysts. The
same phenomenon occurred for high temperature peaks by the increase of Si/Al ratio. The
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SAPOꢀ34 (0.3) sample showed higher desorption temperatures for both weak and strong acid
sites than the LaAPSOꢀ34 (0.3) catalyst. Furthermore, it indicates that metal introduction
resulted in decrease of strong acid sites strength compared to the SAPOꢀ34 by the observation
of high temperature peak shift to lower temperatures. It has been proved that weak acid sites
do not have any strong effect in MTO reaction and these sites can only produce DME and
water from dehydration of methanol. On the other hand, strong acid sites (located in zone 2)
have a critical role in the formation of light olefins from DME. However, stronger acid sites
would also promote coke formation reactions and lead to the fast catalyst deactivation. As
seen, the concentration of both weak and strong acid sites for the LaAPSOꢀ34 (0.3) catalyst is
lower than that of SAPOꢀ34 catalyst. NH3ꢀTPD results indicate that La incorporation leaded
to the decrease of acid sites concentration and strength.The high concentration of acid sites
has positive effect on olefins selectivity and increasing lifetime. On the other hands,
increasing strength of acid sites has negative effect on MTO performance especially in
viewpoint of lifetime.
(Figure 12)
3.2 Catalytic Performance Study toward Methanol to Light Olefins Conversion
3.2.1 Effect of Temperature on Methanol Conversion and Products Selectivity
Methanol conversion and products distribution at temperatures ranging from 300 to 500°C
over LaAPSOꢀ34 (0.1) catalyst was studied as shown in Figure 13. All the reactions were
carried out at constant feed composition and GHSV. Methanol conversion increased and
reached a maximum value of 100% at 400°C. The observed trend has been reported for the
other samples too. Ethylene selectivity increased up to 400°C and then decreased. Moreover,
propylene and higher hydrocarbons selectivity followed a continuous decreasing trend by the
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increasing temperature. Methane is considered an undesired byꢀproduct of the MTO reaction.
Methane formation has been enhanced at higher temperatures. The highest value of the
methane selectivity was obtained at 500°C to be 74%. Thus, lower temperatures are favour to
achieve the highest selectivity of the light olefins. Propylene and higher hydrocarbons
conversion to ethylene occurs dominantly resulting in higher ethylene selectivity. But,
methane formation side reactions play a major role at higher temperatures and consequently,
ethylene selectivity starts decreasing. As a result, operational temperature can be a key
parameter to achieve the desired selectivity of the light olefins.
(Figure 13)
3.2.2 Effect of La Addition on Methanol Conversion and Products Selectivity
Methanol conversion at various temperatures for SAPOꢀ34 (0.3) and LaAPSOꢀ34 (0.3)
catalysts is depicted in Figure 14. At higher temperatures, above 400°C, methanol conversion
was complete for both of the catalysts. However, the SAPOꢀ34 catalyst with metal introduced
gains higher values of methanol conversion even at low temperatures of 300 and 350°C. It
was proved that the La addition during the synthesis has a positive effect on crystallinity and
specific surface area. Therefore, active sites availability towards feed molecules would be
enhanced.
Figure 15 illustrates the effect of La addition on olefins selectivity, C2H4 (a) and C3H6 (b). At
400°C, ethylene selectivity reaches a maximum of 70 and 63 % for the LaAPSOꢀ34 (0.3) and
SAPOꢀ34 (0.3) samples, respectively. Afterward, it followed a decreasing trend. In addition,
propylene selectivity reached the maximum at 300°C for both of the catalysts. For all the
examined temperatures, light olefins selectivity for the metal containing sample was
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enhanced compared to the sample synthesized without metal. It is due to the increased
crystallinity and specific surface area of the La modified SAPOꢀ34.
Figure 16 shows the effect of the La addition on methane selectivity for the SAPOꢀ34 (0.3)
and LaAPSOꢀ34 (0.3) catalysts. Methane formation was limited for the catalyst synthesized
with metal. For instance, there was no methane at 300 and 350°C for the LaAPSOꢀ34 (0.3)
catalyst. The best operating temperature with respect to light olefins selectivity was 350 and
400°C for the LaAPSOꢀ34 (0.3) and SAPOꢀ34 (0.3) samples, respectively. In the other
words, by incorporation of La into the structure of SAPOꢀ34, optimum temperature for the
process was reduced. This point is significant from an economic point of view. However, the
stability test was carried out at 350°C to make a good comparison.
(Figure 14)
(Figure 15)
(Figure 16)
3.2.3 Effect of Si/Al Ratio on Methanol Conversion and Products Selectivity
The time dependence selectivity of the major products and methanol conversion over the
synthesized samples was shown in Figure 17. The stability test has been carried out for all the
five samples at 350°C and constant GHSV. Long term behaviour of the catalysts can be
investigated in terms of Si/Al ratio and also La heteroatom introduction. As indicated,
methanol conversion and light olefins selectivity reached at about 85 and 60%, respectively
after about 2400 min for the SAPOꢀ34 (0.3). Moreover, LaAPSOꢀ34 (0.3) catalyst exhibited
superior activity for which methanol conversion and light olefins selectivity reached at about
90 and 60%, respectively at TOS = 5500 min. The time of reaction sustaining light olefins
selectivity at about 60% was selected as a criterion to evaluate the long term behaviour of the
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catalysts. Stability test results of the LaAPSOꢀ34 (0.3) and SAPOꢀ34 (0.3) samples can
provide excellent information about the effect of La addition on the catalytic performance of
SAPOꢀ34. Higher specific surface area, proper acid sites density and proper crystallinity of
the LaAPSOꢀ34 (0.3) can provide much more active sites for the MTO reaction. Thus, it
takes longer time for the LaAPSOꢀ34 (0.3) to deactivate at constant operational conditions.
Moreover, La incorporation into the crystalline framework would diminish the effect of side
reactions by probably adjustment of the active sites density and their strength. Consequently,
coke formation would be delayed. Appearance of DME in the products can illustrate
beginning of catalyst deactivation. With catalyst deactivation by coke deposition, the strong
acid sites poison with coke. But, DME can be produced on weak acid sites that these active
sites are not capable of converting DME to light olefins. As seen, for the SAPOꢀ34 (0.3)
sample DME appeared after 1350 min of reaction and then increased. For LaAPSOꢀ34 (0.3),
DME produced at later times (2200 min) and also increased with a lower rate for which
milder Brønsted acid sites can be the main reason.
Conversion elimination is not so much and can be neglected. It can be attributed to the
conversion of methanol to dimethyl ether (DME) on weakened acid centres by the further
coke deposition. DME can be easily obtained from methanol dehydration by using catalysts
with lower acidity. The reaction time at which light olefins selectivity reaches at about 60%
was reported to be 2400, 2700, 5500 and 3300 min for the LaAPSOꢀ34 (0.1), LaAPSOꢀ34
(0.2), LaAPSOꢀ34 (0.3) and LaAPSOꢀ34 (0.4), respectively. The higher specific surface area
and crystallinity evidenced by BET and XRD techniques can be the reasons for the best
catalytic performance of the LaAPSOꢀ34 (0.3). Furthermore, it seems that catalyst lifetime
was prolonged by increasing Si/Al ratio from 0.1 to 0.3. But, further increase of Si content
had an adverse effect on the stability of the catalyst. It can be a meaningful relationship
between the physiochemical characteristics of the catalyst and their lifetime during the
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reaction. Increase of Si/Al ratio up to 0.3 resulted in specific surface area and crystallinity
enhancement. Consequently, stable MTO reaction was expected for the LaAPSOꢀ34 (0.3). As
mentioned before, all the stability tests were conducted at 350°C. Therefore, negligible
methane formation can be due to the applied low temperature. Another noticeable point may
be attributed to the variation of ethylene and propylene selectivity toward the time progress.
Ethylene selectivity showed an increasing trend at earlier times of reaction and then
decreased. However, propylene started decreasing from the onset of reaction to the end of
analysis. Channels and pore mouths of the applied molecular sieve can be partially blocked
by the higher hydrocarbon molecules known as hydrocarbon pool intermediates like
methylnaphthalenes, phenanthrene and pyrene. Consequently, propylene diffusivity would be
decreased and resultantly its cracking to ethylene could be happened. However, due to further
coke deposition and complete blockage of some channels, selectivity of both C2H4 and C3H6
decreased at longer times.
(Figure 17)
4 Conclusions
The higher specific surface area and crystallinity evidenced by BET and XRD techniques can
be the reasons for the best catalytic performance of LaAPSOꢀ34 (0.3). It seems that catalyst
lifetime was prolonged by increasing Si/Al ratio from 0.1 to 0.3. But, further increase of Si
content had an adverse effect on the stability of the catalyst. The reaction time at which light
olefins selectivity reaches at about 60% was reported to be 2400, 2700, 5500 and 3300 min
for the LaAPSOꢀ34 (0.1), LaAPSOꢀ34 (0.2), LaAPSOꢀ34 (0.3) and LaAPSOꢀ34 (0.4)
samples, respectively. The excellent stability of the synthesized catalyst with Si/Al ratio of
0.3 and simultaneous metal introduction can be known as a novel one for the MTO reaction
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which gains higher values of desired products at complete methanol conversion and lower
temperature of 350°C.
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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Figure Captions
Figure 1. Preparation steps of nanostructured LaAPSOꢀ34 catalyst with different Si/Al
ratios............................................................................................................ 26
Figure 2. Experimental setup for activity test of nanostructured LaAPSOꢀ34 catalyst used
in conversion of methanol to light olefins..................................................... 27
Figure 3. XRD patterns of nanostructured LaAPSOꢀ34 catalyst synthesized with different
Si/Al ratios: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c) SAPOꢀ34 (0.3),
(d) LaAPSOꢀ34 (0.3) and (e) LaAPSOꢀ34 (0.4)............................................ 28
Figure 4. Crystallite size and relative crystallinity of nanostructured LaAPSOꢀ34 catalyst
synthesized with different Si/Al ratios.......................................................... 29
Figure 5. FESEM images of nanostructured LaAPSOꢀ34 catalyst synthesized with
different Si/Al ratios: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c) SAPOꢀ
34 (0.3), (d) LaAPSOꢀ34 (0.3) and (e) LaAPSOꢀ34 (0.4).............................. 30
Figure 6. Particle size histogram of Laꢀdoped SAPOꢀ34 catalyst: (a) SAPOꢀ34 (0.3) and
(b) LaAPSOꢀ34 (0.3).................................................................................... 31
Figure 7. EDX dotꢀmapping analysis of nanostructured LaAPSOꢀ34 catalyst synthesized
with different Si/Al ratios: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c)
LaAPSOꢀ34 (0.3) and (d) LaAPSOꢀ34 (0.4)................................................. 32
Figure 8. Gel vs. crystal composition of nanostructured LaAPSOꢀ34 catalyst synthesized
with different Si/Al ratios: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c)
LaAPSOꢀ34 (0.3) and (d) LaAPSOꢀ34 (0.4)................................................. 33
Figure 9. TEM images of nanostructured LaAPSOꢀ34 catalyst: LaAPSOꢀ34 (0.3)......... 34
Figure 10. BET surface area analysis of nanostructured LaAPSOꢀ34 catalyst synthesized
with different Si/Al ratios............................................................................. 34
Figure 11. FTIR spectra of nanostructured LaAPSOꢀ34 catalyst synthesized with different
Si/Al ratios: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c) SAPOꢀ34 (0.3),
(d) LaAPSOꢀ34 (0.3) and (e) LaAPSOꢀ34 (0.4)............................................ 35
Figure 12. TPDꢀNH3 analysis of nanostructured Laꢀdoped SAPOꢀ34 catalysts synthesized
at different crystallization time: (a) LaAPSOꢀ34 (0.2), (b) SAPOꢀ34 (0.3) and
(c) LaAPSOꢀ34 (0.3).................................................................................... 36
Figure 13. Effect of temperature on (a) methanol conversion and (b) products selectivities
over LaAPSOꢀ34 (0.1), GHSV=4200 cm3/gcat.h, temperature=300ꢀ500 ºC,
feed: 30% methanol, 1 g catalyst. ................................................................. 37
Figure 14. Effect of La addition on methanol conversion: (SAPOꢀ34 (0.3) vs. LaAPSOꢀ34
(0.3)), GHSV=4200 cm3/gcat.h, temperature=300ꢀ500 ºC, feed: 30% methanol,
1 g catalyst................................................................................................... 38
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Figure 15. Effect of La addition on olefins selectivities (SAPOꢀ34 (0.3) vs. LaAPSOꢀ34
(0.3)): (a) C2H4 and (b) C3H6, GHSV=4200 cm3/gcat.h, temperature=300ꢀ500
ºC, feed: 30% methanol, 1 g catalyst. ........................................................... 39
Figure 16. Effect of La addition on methane selectivity (SAPOꢀ34 (0.3) vs. LaAPSOꢀ34
(0.3)),........................................................................................................... 40
Figure 17. Effect of Si/Al ratio on methanol conversion and products selectivity over
nanostructured LaAPSOꢀ34: (a) LaAPSOꢀ34 (0.1), (b) LaAPSOꢀ34 (0.2), (c)
SAPOꢀ34 (0.3), (d) LaAPSOꢀ34 (0.3) and (e) LaAPSOꢀ34 (0.4), GHSV=4200
cm3/gcat.h, temperature=350 ºC, feed: 30% methanol, 1 g catalyst. ............... 41
Tables Caption
Table 1. Unit cell parameters and unit cell volume of samples....................................... 26
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Figures and Tables
Table 1. Unit cell parameters and unit cell volume of samples.
Catalyst
a = b (
̊
c (̊
V (̊
A3)
LaAPSOꢀ34 (0.1)
LaAPSOꢀ34 (0.2)
SAPOꢀ34 (0.3)
LaAPSOꢀ34 (0.3)
LaAPSOꢀ34 (0.4)
13.69
13.60
13.62
13.64
13.56
14.53
15.10
14.71
14.76
14.62
2356.21
2420.77
2364.87
2376.88
2329.03
(a) Precursor preparation
Pꢀprecursor
Phosphoric acid: 85 wt% (aq.)
Alꢀprecursor
ATP: Aluminum tri isopropylate
Laꢀprecursor
Lanthanum(III) nitrate hexahydrate
Reaction medium
TEAOH: 20 wt% (aq.) Deꢀionized water Fumed silica
Template
Siꢀprecursor
Mixing of aq. solution for 90 min, TEAOH/ATP molar ratio = 2.0
Gradually addition of deꢀionized water and fumed silica and mixing for 60 min
Mixing for 60 min
Gradually addition of phosphoric acid aq. solution
Aging for 24 h while mixing
Gel formation in appropriate molar ratio of
Al2O3/SiO2/P2O5/La2O3/TEAOH/H2O = 1.0/0.2,0.4,0.6 and 0.8/1.0/0.05/2.0/70 (pH = 7ꢀ8)
(b) Hydrothermal synthesis
Hydrothermal synthesis: in a teflonꢀlined stainless steel autoclave reactor at 200ºC for 72 h
(c) Post treatment
Filtration and washing with deꢀionized water
Drying at 110°C for 24 h under air flow
Calcination at 550°C for 12 h under air flow
Powder forming: LaAPSOꢀ34
Figure 1. Preparation steps of nanostructured LaAPSO-34 catalyst with different Si/Al ratios.
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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 2. Experimental setup for activity test of nanostructured LaAPSO-34 catalyst used in conversion
of methanol to light olefins.
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■
■
■ SAPOꢀ34
SAPOꢀ5
♦
■
♦
■
■
■
♦
■
♦
■
♦
■
♦
■
■
■
■
(a) LaAPSOꢀ34 (0.1)
(b) LaAPSOꢀ34 (0.2)
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
(c) SAPOꢀ34 (0.3)
■
■
■
■
■
■
■
♦
♦
♦
■
■
♦
■
■
■
♦
♦
♦
■
(d) LaAPSOꢀ34 (0.3)
(e) LaAPSOꢀ34 (0.4)
■
♦
■
■
♦
■
♦
Reference patterns
SAPOꢀ34 (Rhombohedral, 01ꢀ087ꢀ1527)
SAPOꢀ5 (00ꢀ049ꢀ0659)
La2O3 (Cubic, 00ꢀ004ꢀ0856)
5
10
15
20
25
30
35
40
45
50
2θ (degree)
Figure 3. XRD patterns of nanostructured LaAPSO-34 catalyst synthesized with different Si/Al ratios: (a)
LaAPSO-34 (0.1), (b) LaAPSO-34 (0.2), (c) SAPO-34 (0.3), (d) LaAPSO-34 (0.3) and (e) LaAPSO-34 (0.4).
28