Microporous crystalline silicoaluminophosphates: Effect of synthesis conditions on the physicochemical and catalytic properties in the reaction of methanol to C2-C4 olefins conversion

Article abstract of DOI:10.1134/S0965544114040069

Effect of the synthesis parameters (reaction mixture composition, crystallization temperature and duration) on the physicochemical and catalytic properties of microporous crystalline silicoaluminophosphates (SAPOs) has been studied. Methods for the directional control of phase composition, degree of crystallinity, morphology and size of SAPO crystals crystallized from colloidal silicoaluminophosphate sols stabilized in tetraethylammoniun hydroxide solution as a template have been developed. It has been determined that the use of more severe synthesis conditions (increase in temperature and duration of crystallization) leads to the formation of larger crystals and a growth in the concentration of medium strength sites in the samples, which causes a rapid deactivation of the samples in the reaction of methanol conversion to C ₂-C₄ olefins. Crystallization under milder conditions (a decrease in pH and temperature) promotes the acid formation of CHA/AEI intergrowth crystals exhibited a high and steady performance in the methanol conversion for more than 8 h at a total yield of olefins of 95 wt %.

Full text of DOI:10.1134/S0965544114040069

ISSN 0965ꢀ5441, Petroleum Chemistry, 2014, Vol. 54, No. 4, pp. 288–295. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.E. Knyazeva, S.V. Konnov, O.V. Shutkina, I.V. Dobryakova, O.A. Ponomareva, I.I. Ivanova, 2014, published in Neftekhimiya, 2014, Vol. 54, No. 4, pp. 287–
2
94.
Microporous Crystalline Silicoaluminophosphates:
Effect of Synthesis Conditions on the Physicochemical and Catalytic
Properties in the Reaction of Methanol to C –C Olefins Conversion
2
4
a, b
a
a
b
E. E. Knyazeva , S. V. Konnov , O. V. Shutkina , I. V. Dobryakova ,
a, b
a, b
O. A. Ponomareva , and I. I. Ivanova
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia
b
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
eꢀmail: eknyazeva62@mail.ru
Received February 18, 2014
Abstract—Effect of the synthesis parameters (reaction mixture composition, crystallization temperature and
duration) on the physicochemical and catalytic properties of microporous crystalline silicoaluminophosꢀ
phates (SAPOs) has been studied. Methods for the directional control of phase composition, degree of crysꢀ
tallinity, morphology and size of SAPO crystals crystallized from colloidal silicoaluminophosphate sols staꢀ
bilized in tetraethylammoniun hydroxide solution as a template have been developed. It has been determined
that the use of more severe synthesis conditions (increase in temperature and duration of crystallization) leads
to the formation of larger crystals and a growth in the concentration of medium strength sites in the samples,
which causes a rapid deactivation of the samples in the reaction of methanol conversion to C –C olefins.
2
4
Crystallization under milder conditions (a decrease in pH and temperature) promotes the acid formation of
CHA/AEI intergrowth crystals exhibited a high and steady performance in the methanol conversion for more
than 8 h at a total yield of olefins of 95 wt %.
Keywords: silicoaluminophosphates, methanolꢀtoꢀolefins conversion reaction
DOI: 10.1134/S0965544114040069
Over the past twenty years, fundamental and structureꢀforming template, ratio of the reaction mixꢀ
applied aspects of the production of lower olefins from ture components, degree of condensation (sol or gel)
alternative feedstock have been intensively developed of the reaction mixture, temperature and duration of
in view of the enormous practical importance of ethylꢀ crystallization. The acidic properties of silicoaluminoꢀ
ene and propylene as petrochemical intermediates [1]. phosphate depend on the method of the siliconꢀoxyꢀ
Oxygenates (methanol or dimethyl ether, DME), gen tetrahedra incorporation into the crystal lattice of
which are prepared from the products of either methꢀ the material during crystallization—in the form of
ane or coal processing, are used as a feedstock for the isolated tetrahedra (SM1 mechanism) or silicon–oxyꢀ
production of lower olefins [2]. Possibility to control gen islets (SM2 or SM3 mechanisms) [3, 5]. In the
ethylene/propylene ratio in the products of the conꢀ case of SM1 mechanism the acidic properties of siliꢀ
version of oxygenates primarily involves the use of varꢀ coaluminophosphates are due to isolated Brönsted
ious crystalline molecular sieve catalysts. When using centers that form the basis of SAPOꢀ34 acidity [3].
a zeolite with MFI structure, propylene predominates
Analysis of a significant number of publications on
in the reaction products; in the presence of SAPOꢀ34 SAPOꢀ34 shows that synthesis of silicoaluminophosꢀ
silicoaluminophosphate of the chabazite (CHA) phates may be performed from the reaction mixtures
structure ethylene and propylene are formed in an obtained by varied combination of precursors for
equimolar ratio [1]. The main problem when using Al O3 (aluminum isopropoxide [6, 8–10], pseudoboeꢀ
2
SAPOꢀ34 in the MTO (methanol to olefins) process is hmite [3–5, 7, 11]), P2O5 (concentrated phosphoric
its rapid deactivation during the course of the reaction. acid [3–11]), SiO2 (silica sol [3–5, 7–11], Aerosil [7],
As shown in [3, 4], the most important factors influꢀ silicic acid [10]), and templates (triethylamine [3, 5,
encing the rate of SAPOꢀ34 deactivation are the size of 11], diethylamine [9, 11], tetraethylammonium
its crystals and its acidic properties. A decrease in the hydroxide [6–8, 11], morpholine [7] or their mixꢀ
crystal size leads to an increase in the time of steady tures). The influence of synthesis conditions on the
performance of the catalyst. According to [4, 6, 7], the physicochemical properties of SAPOꢀ34 as reported
size of SAPOꢀ34 crystals is determined by the syntheꢀ in [3, 5, 8] was analyzed without relation to the cataꢀ
sis conditions, including type of starting reagents and lytic properties, and in the published data [4, 6, 7,
288

MICROPOROUS CRYSTALLINE SILICOALUMINOPHOSPHATES
289
9
⎯
11] on the SAPOꢀ34 catalytic activity in the reacꢀ mode at a rate of 10°C/min in the temperature range
tion of methanol conversion to lower olefins do not 20–800 in air flow (100 mL/min).
°C
make it possible to correctly assess the effect of syntheꢀ
sis conditions on the properties of the samples, since
the catalysts were prepared in various conditions from
different groups of precursors.
Details of the porous structure of silicoaluminoꢀ
phosphates were determined using lowꢀtemperature
nitrogen adsorption–desorption method. Registration
of the isotherms was performed by standard method
In this work, we comprehensively studied the influꢀ on an ASAP 2010 porosimeter (Micromeritics, USA).
ence of synthesis conditions on the physicochemical
and catalytic properties of microporous crystalline silꢀ
icoaluminophosphates prepared from silicoaluminoꢀ
phosphate sols stabilized in solution by tetraethylamꢀ
monium hydroxide as a template. The choice of the
reaction mixture type is caused by the possibility to
produce silicoaluminophosphates with a crystal size of
less than 500 nm from colloidal systems [8]. The ratio
of the reaction mixture components (P2O /Al O ,
Electron microscopic images of the samples were
obtained on a CAMSCAN electron microscope.
Before shooting, the surface of the samples was covꢀ
ered with a gold–iridium alloy layer by vacuum sputꢀ
tering.
The acid properties of the samples were studied by
the ammonia thermalꢀprogrammed desorption
(
TPD) technique. Experiments were conducted on an
5
2
3
USGAꢀ101 chemisorption analyzer produced by
UNISIT (Russia). A sample (0.15–0.20 g) was placed
in a quartz tubular reactor; standard automatic preꢀ
processing involved sequential calcination of the samꢀ
SiO /Al O3, template/Al2O , H O/Al O3), the pH of
2
2
3
2
2
the mixture, and the temperature and duration of crysꢀ
tallization were considered as factors influencing the
physicochemical and catalytic properties of the cataꢀ
lysts.
ple at 500
ammonia at 60
adsorbed ammonia in a helium flow at 100
°
C
for 1 h in a helium flow, saturation with
for 15 min, removal of physically
. The
°C
°C
experiment on NH3 TPD was carried out in a helium
EXPERIMENTAL
flow (30 mL/min) at a rate of temperature rise of
The silicoaluminophosphates were synthesized by
hydrothermal crystallization from the reaction mixꢀ
8 /min, the evolved ammonia was registered by a therꢀ
°
mal conductivity detector.
ture with a composition of Al O
·
(2–3)P O5
·
(0.3–
· (4–6) (C H ) NOH · (80–140)H O at temꢀ
2 5 4
2
3
2
The catalytic activity of the samples in the methaꢀ
nolꢀtoꢀolefins process was studied on a flow catalytic
reactor. Pretreatment of the catalyst (0.5–1 mm fracꢀ
1
.8)SiO2
2
peratures of 170–190°C for 20–60 h. Aluminum isoꢀ
propoxide, pseudoboehmite, silica sol (40 wt %),
phosphoric acid (85 wt %), tetraethylammonium
hydroxide (aqueous solution, 35 wt %), and morphoꢀ
line were used as reactants. The pH value of the reacꢀ
tion mixture was adjusted by titration with concenꢀ
trated HCl to a predetermined value. Crystallization
was conducted at 170–190°C for 20–60 h in autoꢀ
claves with stirring at a speed of 300 rpm. The solid
products were separated from the liquid phase by filꢀ
tion) included calcination at a temperature of 500
for 1.5 h and cooling down to an operating temperaꢀ
ture of 400 under nitrogen flow. The mass flow rate
°C
°C
–1
of methanol supply was 2 h . The sampling was perꢀ
formed every 35–40 min. Chromatographic analysis
of the reaction products was carried out on two chroꢀ
matographs. The reaction products were analyzed
using gas chromatography method on a Kristall
2
000 M instrument of Chromatec Analytic Company
tration or centrifugation, washed, dried at 100
2 h and calcined at 500 for 2 h in an air stream
heating rate, 2°C/min.)
Phase analysis of silicoaluminophosphates was
performed using diffraction patterns obtained on a
STOE STADI P ꢀdiffractometer Xꢀray diffractoꢀ
meter, Сu radiation. The diffraction patterns were
°C for
with a flameꢀionization detector and a capillary colꢀ
1
(
°C
umn with SEꢀ30 applied phase. Analysis of С –С
1
4
alkanes and alkenes was conducted on a Chromꢀ5
chromatograph with a flameꢀionization detector and
H2 carrier gas using a capillary column with the
θ θ
/
KCl/Al O3 applied phase.
2
K
α
recorded by rotating the sample in a horizontal plane
at angles of 5°–40° with a step of 0.05 deg, a slit
width of 1 mm, and a dwell time per point of 3 s.
2
θ
RESULTS AND DISCUSSION
Phase Composition
IR spectra of the samples were recorded using a
Nicolet Prot
é
g
é
408 IR spectrometer at frequencies of
A reaction mixture with a basic composition of
–
1
4
00–1400 cm . Before recording the spectrum, 1 mg Al O3
·
2P O5
·
0.6SiO2
of the sample was mixed with 150 mg of KBr and comꢀ crystallized to form silicoaluminophosphate with the
pressed into a tablet. The IR spectra obtained were chabazite (CHA) structure at 180 for 20 h (sample
· 4(C H ) NOH · 80H O was
2
2
2 5 4 2
°
C
processed using the OMNIC E.S.P. software package SAPOꢀ1, Table 1, Fig. 1). The data in Table 1 shows
(
Nicolet).
that the CHA phase was formed as a single crystalline
product in SAPOꢀ1, 2, 4–9 and SAPOꢀ13 samples.
Thermogravimetric (TGA) and differential therꢀ
mal (DTA) analyzes were performed on a SDT Q600
Under the chosen synthesis conditions, the phase
instrument (TA Instruments production). TGꢀDTA purity of the crystalline product was not affected by the
curves were recorded using linear temperature rise P2O5 and SiO2 concentration in the reaction mixture,
PETROLEUM CHEMISTRY Vol. 54
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2014

290
KNYAZEVA et al.
Table 1. Synthesis conditions and phase composition of silicoaluminophosphate samples
Reaction mixture composition,
Content
mol/mol Al O3
reaction
mixture pH
Phase
composition
+
2
Sample
Tcr, °C
τ
cr, h
(C H ) ,
2
5
4
P2O5
SiO2
Ra
μ
mol/g
8
SAPOꢀ1
SAPOꢀ2
SAPOꢀ3
SAPOꢀ4
SAPOꢀ5
SAPOꢀ6
SAPOꢀ7
SAPOꢀ8
SAPOꢀ9
SAPOꢀ10
SAPOꢀ11
SAPOꢀ12
SAPOꢀ13
2
3
2
2
2
2
1
2
2
2
2
2
2
0.6
0.6
0.3
1.2
1.8
0.6
1
4b
6b
4b
4b
4b
2b + 2c
2c
4b
4b
4b
4b
180
180
180
180
180
180
180
180
180
180
180
170
190
20 CHA
20 CHA
930
800
–
8
8
20 amorphous phase
20 CHA
8
850
890
970
800
900
670
1100
8
20 CHA
8
20 CHA
8
20 CHA
0.6
0.6
0.6
0.6
0.6
0.6
8
40 CHA
8
60 CHA + amorphous phase
20 CHA + AEI
20 AEI
7.4
7d
8
1200
1200
900
4b
4b
20 CHA + AEI
20 CHA
8
a, template; , tetraethyl ammonium hydroxide; , morpholine; , pH was adjusted by adding conc. HCl.
b
c
d
the template type, the crystallization time, and other ~1212 cm–1 (asymmetric vibrations of the P–O–Al
factors. In the case of varying the P2O5 concentration bonds) (Fig. 2b–d) [3, 8].
in the reaction mixture, it was taken into account that
According to Table 1, the CHA phase is formed
when compounds of different properties—tetraethyꢀ
lammonium hydroxide, morpholine and their
P2O : template molar ratio of 2 : 4 ensures the mainꢀ
5
tenance of the pH of reaction mixture at 8.0. Thereꢀ
fore, an increase in the phosphorus content was ineviꢀ equimolar mixtures are used as a template (SAPOꢀ1,
tably accompanied by an increase in the temꢀ SAPOꢀ6, and SAPOꢀ7 samples). However, it should
plate/Al2O3 and H O/Al O ratios (sample SAPOꢀ2, be noted that morpholine turned out inactive as an
2
2
3
Table 1).
individual template when aluminum isopropoxide was
used as an Al O3 precursor, and the synthesis of
SAPOꢀ7 sample was possible only by using pseudoꢀ
boehmite.
2
An increase in the SiO /Al O ratio in a wide range
2
2
3
from 0.3 to 1.8 did not affect crystallization selectivity,
but determined the crystallinity of the product. Moreꢀ
over, from the reaction mixture with the lowest SiO2
content the CHA phase could not be obtained
An increase of crystallization time from 20 to 60 h
samples SAPOꢀ1, SAPOꢀ8, and SAPOꢀ9) did not
(
result in a change in the phase purity of the product. At
the same time, the annealing of the reaction mixture at
180°C for 60 h was accompanied by partial amorꢀ
(
SAPOꢀ3 sample, Table 1.) The IR spectrum of the
sample (Fig. 2a) contained absorption bands (ABs)
indicating the presence of an amorphous phosphate
phization of the SAPOꢀ9 sample.
–1
material (ABs at 1108 and 1004 cm ), an amorphous
–1
A change in the crystallization selectivity was
achieved by variation of such parameters as pH of the
reaction mixture and crystallization temperature.
Together with the CHA phase or instead of it the AEI
phase was crystallized differing in the orientation
mode of the layers of hexagonal prisms forming the
silicate material (AB at 787 cm ), as well as a strucꢀ
–1
tured aluminosilicate material (AB at 533 cm correꢀ
sponding to vibrations of the T–O bands in the
(
(
Si,Al)O4 tetrahedra) containing sixꢀmembered rings
–1
AB at 640 cm ) [3, 8].
An increase in the silicon content in the samples SAPO crystal lattice [5]. Crystallization under milder
SAPOꢀ4—SAPOꢀ6 (Table 1) resulted in the formaꢀ conditions, namely, a decrease in the reaction mixture
tion of highly crystalline products containing in their pH value (sample SAPOꢀ10, Fig. 1b) and a decrease in
structure aluminum, phosphorus and silicon in a tetꢀ the crystallization temperature to 170°C (sample
SAPOꢀ12, Fig. 1c) promoted the AEI phase formaꢀ
tion.
rahedral oxygen environment. The presence of phosꢀ
phorus–oxygen tetrahedra in the crystalline frameꢀ
work of the samples is manifested by absorption bands
According to published data [12], changes in the
–1
at ~570 cm (T–O bonds in the PO4 tetrahedron), diffraction patterns of samples SAPOꢀ10 and SAPOꢀ
~
730 cm–1 (P–O bond symmetric vibrations) and 12 in the
2θ range of 15°–19°, namely, the disappearꢀ
PETROLEUM CHEMISTRY Vol. 54
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MICROPOROUS CRYSTALLINE SILICOALUMINOPHOSPHATES
291
а
b
e
d
c
c
d
b
а
1400
1200
1000
800
600
ν
, cm–1
Fig. 2. IR spectra of silicoaluminophosphates: (
a) SAPOꢀ3,
(b
) SAPOꢀ1, ( ) SAPOꢀ4, and ( ) SAPOꢀ5.
c
d
5
10
15
20
25
30
35
2␪
,
deg
(
SAPOꢀ11 sample) the AEI phase is formed as a single
crystalline product (Fig. 1d).
Fig. 1. Diffractograms of silicoaluminophosphates:
) SAPOꢀ1, ( ) SAPOꢀ10, ( ) SAPOꢀ12, ( ) SAPOꢀ11,
and ( ) SAPOꢀ13.
(a
b
c
d
In addition to Xꢀray diffraction and infrared specꢀ
troscopy, the quality of the molecular sieve material
obtained using the template synthesis can be characꢀ
terized by the quantitative content and the state of the
structureꢀforming template. The first property can be
determined from the TG curve by the mass loss value
e
ance of the reflection at
appearance of a reflection at
2
θ
18.8
2
θ
(Fig. 1a, *) and the
17.1° (Fig. 1b,
2
θ
᭢)
prove the formation of the CHA/AEI intergrowths. in the temperature range corresponding to the decomꢀ
Differences in the intensity ratios for the reflections at position of the template by heating. The second
2 16.0° and 17.1° make it possible to suggest that parameter can be evaluated by the type of the thermal
θ
samples SAPOꢀ10 and SAPOꢀ12 are formed by interꢀ effect and its position on the DTA curve. According to
growths having different CHA/AEI ratios. Thus, variꢀ [8, 11], highly crystalline samples with the CHA strucꢀ
ture contain in their composition from 11 to 15 wt %
of a template (840–1150 mol/g), the decomposition
of which upon heating of the samples is accompanied
by an exothermic effect with a maximum at 450
ation in the synthesis conditions makes it possible to
adjust the composition of SAPO CHA/AEI interꢀ
growths.
µ
°C.
An increase in the crystallization temperature to The study of the SAPO samples using TG–DTA analꢀ
90 resulted in the formation of a material, the difꢀ ysis (Table 1, Fig. 3) showed both quantitative and
fraction pattern of which contains reflections characꢀ qualitative differences between the samples with the
teristic of the CHA phase, namely, at
9.5°, 12.9°, CHA structure and the samples containing the AEI
phase.
1
°C
2
θ
1
6.1°, 17.9°, and 20.6° (Fig. 1e). At the same time,
changes in the reflection intensities ratio compared
with the SAPOꢀ1 sample indicate that the silicoalumiꢀ
nophosphate crystal phase formed at an elevated temꢀ
+
The content of a template, (C H ) N cation, in
2
5 4
the SAPOꢀ10, SAPOꢀ11, and SAPOꢀ12 samples is
100–1200 mol/g, which is higher than in the samꢀ
mol/g)
1
µ
perature has some structural features, the determinaꢀ ples with the CHA structure (800–970
µ
tion of which requires further investigation. At a (Table 1). In addition, the decomposition of the temꢀ
decrease in the pH of the reaction mixture to 7 plate by heating of the SAPOꢀ1 and SAPOꢀ11 samples
PETROLEUM CHEMISTRY Vol. 54
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292
KNYAZEVA et al.
2
(
480–580 m /g), the micropore volume in the samples
3
was 0.23–0.27 cm /g.
Crystal Morphology
The results of the scanning electron microscopy
show that the reference sample SAPOꢀ1 consists of
cubic crystals 0.3–0.5 m in size (Fig. 4, Table 2).
c
µ
Changes in the synthesis parameters such as the ratio
of the reaction mixture components, template type,
crystallization temperature and duration did not
change the morphology, but resulted in an increase in
the crystal size.
b
With an increase in the phosphorus and silicon
content in the reaction mixture the crystal size either
increased 1.5–2 times (SAPOꢀ2, SAPOꢀ4 samples) or
crystals with a more wide size distribution were formed
а
(
SAPOꢀ5) (Table 2). A partial or complete substitution
of tetraethylammonium hydroxide template for morꢀ
pholine (SAPOꢀ6 and SAPOꢀ7 samples, Table 2,
Fig. 4) and running the crystallization for 40 h
2
00
300
400
500
600
, °C
(
SAPOꢀ8 sample, Table 2) exert the greatest influence
t
on the size of the crystals. The crystal size increased by
an order of magnitude for SAPOꢀ6 and SAPOꢀ8 samꢀ
ples compared with the SAPOꢀ1 sample and by more
than thirty times for the SAPOꢀ7 sample.
Fig. 3. DTA curves of silicoaluminophosphates:
) SAPOꢀ1, ( ) SAPOꢀ10, and (c) SAPOꢀ11.
(a
b
A similar effect of complete or partial substitution
of templates in the reaction mixture based on pseudoꢀ
boehmite was described by in [2] for SAPOꢀ34 syntheꢀ
sized at 200°C. A crystallization time longer than 40 h
for SAPOꢀ9 sample, as shown above, was accompaꢀ
nied by its amorphization; as a result, the impact on
(
DTA curves in Fig. 3) is accompanied by exothermic
effects, the positions of the maxima for which differ by
15 . Probably, this fact is related to the peculiarities
of the template removal from a more open AEI strucꢀ
1
°C
ture, which is confirmed by the DTA curve for the the crystal size was leveled.
CHA/AEI intergrown crystalline sample (SAPOꢀ10,
Fig. 3), on which there are three exothermic effects— tion products changed the crystal morphology from
at 330 (similar to the AEI structure), 455
(simiꢀ cubic to a plateꢀlike one. In the samples of CHA/AEI
intergrowths (SAPOꢀ10, SAPOꢀ12, Table 2) crystal
The presence of the AEI phase in the crystallizaꢀ
°C
°C
lar to the CHA structure), and an intermediate effect
plates were squares with a side of 0.3–0.5 m and a
µ
at 360
thickness of 0.1 m. Crystals of SAPOꢀ11 sample with
As the study of the silicoaluminophosphate porous the AEI structure had the shape of a flat ellipse with
°C.
µ
structure showed, they all had an extended surface transverse dimensions 0.5
×
0.8
µm and 0.1
µm thick.
1
µ
m
3
µ
m
10 µm
(а)
(b)
(c)
Fig. 4. Electron microscopic images of silicoaluminophosphates: (a) SAPOꢀ1, (b) SAPOꢀ6, and (c) SAPOꢀ7.
PETROLEUM CHEMISTRY Vol. 54 No. 4
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MICROPOROUS CRYSTALLINE SILICOALUMINOPHOSPHATES
Table 2. Physicochemical properties of silicoaluminophosphates
293
Concentration of acid Part of medium strength
Sample
Crystal morphology
Crystal size, μm
centers a0
,
μ
mol/g
sites in the TPD pattern
SAPOꢀ1 Cubic
SAPOꢀ2 Cubic
SAPOꢀ4 Cubic
SAPOꢀ5 Cubic
SAPOꢀ6 Cubic
SAPOꢀ7 Cubic,
0.3– 0.5
0.5–1
0.5–1
0.2–0.8
2–4
1520
1550
1920
2040
1350
1450
0.35
0.38
0.40
0.47
0.60
0.70
10–20
crystalline aggregates
SAPOꢀ8 Cubic
4–6
800
900
0.60
0.33
0.15
0.05
0.16
0.46
SAPOꢀ9 Cubic
0.5–0.8
SAPOꢀ10 Square plates
SAPOꢀ11 Square plates
SAPOꢀ12 Square plates
SAPOꢀ13 Cubic
0.4–0.5, thickness 0.1
0.5
0.8, thickness 0.1
1140
1300
1660
1500
×
0.3–0.4, thickness 0.1
0.5–1
Acid Properties
medium strength centers decreased to 0.15–0.16, for
SAPOꢀ11 sample with the AEI structure, to 0.05. In
these samples the range of acidity is almost completely
represented by weak acid sites.
The acid properties of the synthesized silicoalumiꢀ
nophosphates were evaluated by the TPDꢀNH3
method. The values presented in Table 2 for the conꢀ
centration of acid sites а0, measured as the area under
the ammonia thermal desorption curve, show that
compared with the reference sample SAPOꢀ1, an
increase in the silicon content in the reaction mixture
Thus, changes in the synthesis conditions made it
possible to control the acidic properties of silicoalumiꢀ
nophosphates in a wide range.
resulted in a noticeable increase in а0 (samples SAPOꢀ4,
SAPOꢀ5) and an increase in the crystallization time
led to a marked decrease in а0 (samples SAPOꢀ8,
SAPOꢀ9). In addition to the observed quantitative difꢀ
ferences, qualitative changes in the acid properties of
the silicoaluminophosphates occurred as well. On the
TPDꢀNH3 patterns of the samples shown in Fig. 5,
there are two distinct peaks with thermal desorption
Catalytic Properties
Based on the results of the study of the physicoꢀ
chemical properties of silicoaluminophosphates, we
selected for catalytic testing a set of six samples,
SAPOꢀ1, 4, 6, 7, 10, and 11, differing in the crystal
structure, crystal size, and the proportion of mediumꢀ
strength acid sites (Figs. 6, 7; Table 2).
maxima around 200°C and 400°C corresponding to
the acid sites of low and medium strength. Analysis of
the TPDꢀNH3 patterns (Table 2, Fig. 5) shows that for
samples with the CHA structure change in the syntheꢀ
sis parameters such as the ratio of the reaction mixture
components, type of templating agent used and pH of
the reaction mixture led to an increase in the proporꢀ
tion of mediumꢀstrength sites in the TPD pattern. The
greatest increase in the concentration of these sites (up
to 0.6–0.7) was reached in the samples SAPOꢀ6–
SAPOꢀ8 that stand out because of their large crystals
1
.6
.4
e
d
c
1
1.2
.0
0.8
.6
0.4
1
0
b
а
(
Table 2). The effect of silicon concentration in the
reaction mixture is less pronounced—the а0 value
increased to 0.4 and 0.47 for SAPOꢀ4 and SAPOꢀ5
samples, respectively; in this case, SiO /Al O ratio in
0.2
2
2
3
the reaction mixture correlated with the proportion of
mediumꢀstrength sites in the samples (Tables 1, 2).
0
100
200
300
400
500
600
, °C
t
The presence of the AEI phase in the crystallizaꢀ
tion products has resulted in a drastic change in the
TPD patterns (Fig. 5). For CHA/AEI intergrowths
Fig. 5. Ammonia TPD curves for silicoaluminophosphates:
(
(
a
) SAPOꢀ1, (
b) SAPOꢀ4, (c) SAPOꢀ7, (d) SAPOꢀ10, and
(
samples SAPOꢀ10 and SAPOꢀ12) the proportion of
e) SAPOꢀ11.
PETROLEUM CHEMISTRY Vol. 54
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2014

294
KNYAZEVA et al.
ties of SAPOꢀ34 synthesized under conditions other
than those in the present study. In this case, the
SAPOꢀ7 sample was completely deactivated after
Methanol conversion, wt %
00
1
1
00 min of the catalytic experiment. The differences
80
60
40
20
0
in the selectivity in ethylene formation for the SAPOꢀ4
and SAPOꢀ6 samples (54 and 48 wt %, respectively;
Fig. 7) are related obviously to a decrease in the diffuꢀ
sion restrictions in crystals of a smaller size.
SAPOꢀ1
SAPOꢀ4
SAPOꢀ6
SAPOꢀ7
SAPOꢀ10
SAPOꢀ11
In the second group characterized by the lowest
values of crystal sizes and concentrations of medium
strength sites among the studied samples (Table 2) the
rate of deactivation in the SAPOꢀ1 > SAPOꢀ11 >
SAPOꢀ10 series did not correlate with the physicoꢀ
chemical properties of the samples. For individual
CHA (SAPOꢀ1 sample) and AEI (SAPOꢀ11 sample)
phases 100% methanol conversion was maintained for
1
00 200 300 400 500 600 700
Time, min
4
and 5 h, respectively. In this case, the SAPOꢀ1 samꢀ
Fig. 6. Change in the catalytic activity of silicoaluminoꢀ
ple showed a higher selectivity for ethylene (Fig. 7),
apparently due to strong stereoselective effect resultꢀ
ing from parallel packing of the layers of hexagonal
prisms in the crystal lattice of the CHA structure [5].
The SAPOꢀ11 sample that has a more open AEI strucꢀ
ture exhibited higher selectivities in the formation of
propylene and butenes than the SAPOꢀ1 sample
phosphates over time in the methanolꢀtoꢀolefins reaction.
As shown by the curves of change in the activity of
the samples over time (Fig. 6), the catalysts were
clearly divided into two groups according to their
steady operation time—the samples deactivated in a
short time (SAPOꢀ4, 6, and 7), and ones working
steadily (SAPOꢀ1, 10, and 11). In the first group, the
deactivation rate for the samples (SAPOꢀ7 > SAPOꢀ6 >
SAPOꢀ4) correlated with both the crystal size (10–
(
Fig. 7).
The onꢀstream time of the SAPOꢀ10 sample withꢀ
out loss of activity was more than 8 h. It was intermeꢀ
diate in the selectivity of the formation of lower indiꢀ
vidual olefins after 100 min of the reaction between the
SAPOꢀ1 and SAPOꢀ11 samples (Fig. 7), but surpassed
them in the selectivity in the formation of С –С olefins,
20
µm > 2–4
µm > 0.5–1.0
µm) and the proportion of
mediumꢀstrength sites in the samples (0.7 > 0.6 > 0.4).
2
4
The results are consistent with the relationships the total value of which for the sample was 95 wt %.
revealed in [4, 7, 10, 11] for the influence of the crystal Thus, as a result of the variation in silicoaluminophosꢀ
size and the acidity spectrum on the catalytic properꢀ phate synthesis conditions the preparation of
6
6
5
6
6
9
5
6
rest
11
15
butenes
propylene
3
2
32
3
5
3
8
6
43
ethylene
methane
54
54
48
4
34
SAPOꢀ1
SAPOꢀ4
SAPOꢀ6
SAPOꢀ10
SAPOꢀ11
Fig. 7. Selectivity of the formation of methanol conversion products (wt %) after 100 min of the reaction.
PETROLEUM CHEMISTRY Vol. 54 No. 4
2014

MICROPOROUS CRYSTALLINE SILICOALUMINOPHOSPHATES
295
AEI/CHA intergrowths made it possible to control the
composition of the products of the methanol to olefins
conversion process and time of steady catalyst perforꢀ tion for Basic Research, project no. 12ꢀ03ꢀ00624a.
mance.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
Thus, the effect of the synthesis conditions of
microporous crystalline SAPOs on their physicoꢀ
chemical and catalytic properties in the process of
methanol conversion to lower olefins was studied. The
variation in the composition of the reaction mixture,
temperature and duration of the crystallization of colꢀ
loidal silicoaluminophosphate sols stabilized in a soluꢀ
tion of tetraethylammonium hydroxide as a template
made it possible to develop the ways to the directional
control of the degree of crystallinity, morphology and
size of the crystals, as well as the acidic properties of
silicoaluminophosphates. An increase in the temperaꢀ
ture and duration of the synthesis resulted in the forꢀ
mation of larger crystals and in an increase in the proꢀ
portion of medium strength sites in the acidity spectra
of the samples. Crystallization under milder condiꢀ
tions (a decrease in the pH value and temperature of
the synthesis) led to the formation of CHA/AEI interꢀ
growths exhibited a high and steady performance in
the methanol to С –С olefins conversion process for
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Translated by V. Makhaev
PETROLEUM CHEMISTRY Vol. 54
No. 4
2014