MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis

Article abstract of DOI:10.3762/bjoc.14.272

Highly active olefin metathesis catalysts were prepared by thermal spreading MoO₃ and/or MoO₂(acac)₂ on MWW zeolites (MCM-22, delaminated MCM-56) and on two-dimensional MFI (all in NH₄⁺ form). The catalysts‘activities were tested in the metathesis of neat 1-octene (as an example of a longer chain olefin) at 40 °C. Catalysts with 6 wt % or 5 wt % of Mo were used. The acidic character of the supports had an important effect on both the catalyst activity and selectivity. The catalyst activity increases in the order 6MoO₃/HZSM-5(25) (Si/Al = 25) << 6MoO₂(acac)₂/MCM-22(70) < 6MoO₃/2D-MFI(26) < 6MoO₃/MCM-56(13) < 6MoO₃/ MCM-22(28) reflecting both the enhancing effect of the supports‘acidity and accessibility of the catalytic species on the surface. On the other hand the supports‘acidity decreases the selectivity to the main metathesis product C14 due to an acid-catalyzed double bond isomerization (followed by cross metathesis) and oligomerization. 6MoO₃/2D-MFI(26) with a lower concentration of the acidic centres resulting in catalysts of moderate activity but with the highest selectivity.

Full text of DOI:10.3762/bjoc.14.272

MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as
catalysts for 1-octene metathesis
Hynek Balcar*, Martin Kubů, Naděžda Žilková and Mariya Shamzhy
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2931–2939.
J. Heyrovský Institute of Physical Chemistry of the Czech Academy of
Sciences, v.v.i., Dolejškova 3, 182 23 Prague 8, Czech Republic
doi:10.3762/bjoc.14.272
Received: 30 August 2018
Accepted: 15 November 2018
Published: 27 November 2018
Email:
Hynek Balcar* - hynek.balcar@jh-inst.cas.cz
* Corresponding author
This article is part of the thematic issue "Progress in metathesis
chemistry III".
Keywords:
metathesis; molybdenum oxide; 1-octene; thermal spreading; zeolites
Guest Editors: K. Grela and A. Kajetanowicz
© 2018 Balcar et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Highly active olefin metathesis catalysts were prepared by thermal spreading MoO3 and/or MoO2(acac)2 on MWW zeolites (MCM-
22, delaminated MCM-56) and on two-dimensional MFI (all in NH4+ form). The catalysts‘ activities were tested in the metathesis
of neat 1-octene (as an example of a longer chain olefin) at 40 °C. Catalysts with 6 wt % or 5 wt % of Mo were used. The acidic
character of the supports had an important effect on both the catalyst activity and selectivity. The catalyst activity increases in the
order 6MoO3/HZSM-5(25) (Si/Al = 25) << 6MoO2(acac)2/MCM-22(70) < 6MoO3/2D-MFI(26) < 6MoO3/MCM-56(13) < 6MoO3/
MCM-22(28) reflecting both the enhancing effect of the supports‘ acidity and accessibility of the catalytic species on the surface.
On the other hand the supports‘ acidity decreases the selectivity to the main metathesis product C14 due to an acid-catalyzed double
bond isomerization (followed by cross metathesis) and oligomerization. 6MoO3/2D-MFI(26) with a lower concentration of the
acidic centres resulting in catalysts of moderate activity but with the highest selectivity.
Introduction
Molybdenum oxide on silica, alumina or silica-alumina belongs tetrahedra were proved as the main precursors of the catalytic
to the well-known and long-time used metathesis catalysts [1]. species [6,7], thus the perfect dispersion of MoO3 on the sur-
Albeit typical ill-defined catalysts they are still popular as rela- face is a crucial precondition for a high catalytic activity. The
tively cheap catalysts finding industrial applications especially mechanisms of transformation of these precursors to the sur-
in the treatment of low olefins [2-5]. Their catalytic activity face Mo carbenes as real catalytically active species has been
depends on many factors, especially on Mo loading, support suggested [6,7]. The replacement of ordinary silicas for meso-
acidity, and pre-reaction activations. Surface isolated MoO4 porous molecular sieves SBA-15 or MCM-41 increased the
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
catalyst activity substantially, which allowed performing the optimal Mo loading. On the other hand, x MoO3/MCM-22(70)
metathesis of long chain olefins under mild reaction conditions catalyst with x = 6 wt % Mo exhibited slight MoO3 signals,
[8-10]. The positive effect of these supports on the catalyst ac- when prepared from MoO3 probably due to the lower surface
tivity was ascribed to their high surface areas enhancing the area (especially external one) comparing with MCM-22(28).
spreading of MoO3 molecules on the surface and large pores in- However, when MoO2(acac)2 was used as a source of Mo, cata-
creasing the substrates/products transport rate.
lysts with 6 wt % (and lower) content of Mo did not exhibit any
MoO3 signals. It is consistent with the previous observation that
Microporous zeolites like HZSM-5 impregnated by ammonium MoO2(acac)2 provided better catalyst than MoO3 [9]. XRD
heptamolybdate solutions were used for the metathesis of low patterns of 6MoO3/MCM-56(13) and 6MoO3/2D-MFI(26) indi-
olefins (ethylene, propylene, butenes) [11-13]. In the case of cated also a good MoO3 spreading, contrary to 6MoO3/HZSM-
bulkier substrates they suffer, however, of micropore size limi- 5(25) where MoO3 signals were clearly visible, probably as a
tations. To overcome these limitations a decrease in crystal size result of lower external surface area.
and the application of two-dimensional zeolites can be used
[14-17]. Two dimensional 2D-MFI and MWW delaminated
Table 1: Texture properties of catalysts and corresponding supports.
zeolite MCM-56, which have been prepared recently [18-21],
catalyst
SBET
Sext
V
represent two types of these materials, which exhibit relatively
high surface areas and high accessibility of catalytic sites on the
surface as well [22]. Therefore, we supported MoO3 and/or
MoO2(acac)2 on (i) 2D-MFI (and ordinary HZSM-5 for com-
parison) and similarly on (ii) MCM-56 and its 3D analogue
MCM-22 (both in NH4+ form) and examined their activity in
the metathesis of neat 1-octene (Scheme 1) under ambient pres-
sure and 40 °C. According to our best knowledge, none of these
materials have been tested as supports for MoO3 based cata-
lysts for metathesis of higher alkenes up to now. MoOx on
MCM-22 combined with γ-Al2O3 was used in cross metathesis
of 2-butene and ethylene in a stream (125 °C, 1 MPa) [23].
MCM-22, and MCM-56 were also used as supports for
Hoveyda–Grubbs type hybrid catalysts active in metathesis of
long-chain unsaturated esters [24].
(m2/g) (m2/g) (cm3/g)
1
2
3
4
5
6
7
8
9
MCM-22(28)
455
423
426
421
180
355
565
478
469
269
410
388
119
119
94
0.59
0.38
0.57
0.29
0.25
0.24
0.61
0.57
0.57
0.55
0.23
0.23
6MoO3/MCM-22(28)
6MoO2(acac)2/MCM-22(28)
MCM-22(70)
58
6MoO3/MCM-22(70)
6MoO2(acac)2/MCM-22(70)
2D-MFI(26)
39
41
343
221
164
129
44
6MoO3/2D-MFI(26)
MCM-56(13)
10 6MoO3/MCM-56(13)
11 HZSM-5(25)
12 6MoO3/HZSM-5(25)
38
SBET = BET area, V = total void volume (p/p0 = 0.95), Sext = external
surface (from t-plot).
Results and Discussion
Catalyst preparation and characterization
Contrary to the all-siliceous mesoporous sieves (like SBA-15)
XRD patterns and texture properties (Table 1, Figure 1 which are neutral, zeolites are acidic and their acidity (both
A,B,C,D) of prepared MCM-22, MCM-56 and 2D-MFI zeolites Brønsted and Lewis-type) plays an important role for catalysis.
proved a high quality of these supports. For catalyst labelling The acid site concentrations of zeolitic supports and the corre-
following the mode has been adopted: x MoO3/MCM-22(y), sponding catalysts measured using FTIR spectroscopy of
where x = Mo concentration in wt % Mo, y = Si/Al molar ratio. adsorbed pyridine are shown in Table 2, while the relevant
After spreading Mo compounds over the support surface areas IR spectra are shown in Supporting Information File 1 (Figures
(SBET, Sext) as well as void volumes (V) decreased. Similar S1–S5). It is seen that all supports contained both Brønsted and
reduction of these quantities has been already observed earlier Lewis acid sites of various strength. MCM-22(28) and MCM-
[9,10,24]. For x MoO3/MCM-22(28), XRD patterns of cata- 56(13) exhibited the highest concentrations of acid sites (both
lysts are similar to those of their parents approximately up to Brønsted and Lewis) in accord with their highest Al concentra-
x = 6 wt % of Mo (0.9 Mo atoms per nm2). At higher Mo con- tions. The acid sites concentrations of MCM-22(70) and
centrations signals of crystalline MoO3 appeared (marked with 2D-MFI(26) were lower and close to each other. The Brønsted
* in Figure 1 A,B,D). It suggests 6 wt % of Mo being the acid site concentration of HZSM-5(25) was as high as that of
Scheme 1: 1-Octene metathesis reaction.
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
Figure 1: A,B,C,D: XRD patterns of parent supports and catalysts used. Asterisk marks MoO3.
Table 2: Acid site concentrations in catalysts and corresponding supports.a
sample
c(B)b, mmol/g
c(L)c, mmol/g
150 °C
250 °C
350 °C
450 °C
150 °C
250 °C
350 °C
450 °C
MCM-22(70)
0.11
0.02
0.06
0.29
0.07
0.09
0.13
0.06
0.29
0.14
0.23
0.04
0.12
0.01
0.06
0.32
0.09
0.07
0.13
0.04
0.28
0.12
0.17
0.03
0.09
–
0.02
–
0.06
0.10
0.16
0.13
0.31
0.23
0.07
0.10
0.04
0.17
0.18
0.17
0.04
0.01
0.07
0.09
0.10
0.08
0.07
0.03
0.03
0.07
0.12
0.06
0.03
0.01
0.04
0.08
0.03
0.03
0.06
0.01
0.02
0.04
0.09
0.03
0.03
–
6MoO3/MCM-22(70)
6MoO2(acac)2/MCM-22(70)
MCM-22(28)
0.05
0.30
0.07
0.05
0.08
0.03
0.22
0.09
0.13
0.01
0.02
0.19
0.04
0.02
0.02
0.01
0.07
0.03
0.04
–
0.03
0.06
0.01
0.01
0.04
–
6MoO3/MCM-22(28)
6MoO2(acac)2/ MCM-22(28)
2D-MFI(26)
6MoO3/2D-MFI(26)
HZSM-5(25)
0.02
0.04
0.08
0.02
6MoO3/HZSM-5(25)
MCM-56(13)
MoO3/MCM-56(13)
aDetermined by FTIR. bBrønsted acid site. cLewis acid site.
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
MCM-22(28), however, its Lewis acid site concentration was trimers, respectively; mC8 is weight amount of octene (all
significantly lower. After supporting Mo compounds the con- isomers) and MC8 is the molecular weight of octene.
centrations of Brønsted acid sites decreased significantly which
may indicate that MoOx species reacted predominantly with
Brønsted acid sites of the supports. It is manifested by intensity
decrease of the band in the region 3609–3625 cm−1, ascribed to
OH vibration in the Si–O(H)–Al acid site (see Supporting Infor-
mation File 1, Figures S1–S5) [25]. On the other hand, the con-
centrations of Lewis acid sites in the catalysts was slightly
higher compared to the parent supports. It may be explained by
the formation of some amount of Mo in a lower oxidation state
which has been already described for siliceous supports (MCM-
41, SBA-15) [9,26].
Catalytic activity
MCM-22-based catalysts
Na+ forms of zeolites turned out to be unsuitable supports for
metathesis catalysts. For example, by supporting MoO3 on
MCM-22(28) in Na+ form (6 wt % of Mo) we obtained materi-
al providing only 0.5% 1-octene conversion in 19 h
(1-octene/Mo = 320, t = 40 °C). Therefore, we converted Na+
forms to NH4+ forms, which were used for supporting Mo com-
pounds by thermal spreading method.
Figure 2: Conversion vs time curves for 1-octene metathesis over
6MoO3/MCM-22(28) and 6MoO3/SBA-15. Neat 1-octene,
1-octene/Mo = 320, t = 40 °C.
For comparison, the conversion curve over 6MoO3/SBA-15 is
added in Figure 2. 6MoO3/SBA-15 was prepared from all-
siliceous SBA-15 (SBET = 877m2/g, V = 1.07cm3/g, pore diame-
ter D = 6.4 nm) and it is known as a very active and selective
catalyst [9,10]. Under reaction conditions applied the selec-
tivity to tetradecene was about 98% during the whole experi-
ment, and therefore only Ktot is plotted in Figure 2 in this case.
The time development of 1-octene conversion over 6MoO3/
MCM-22(28) is shown in Figure 2. The GC chromatogram of
the final product is shown in Figure S6 (in Supporting Informa-
tion File 1). It is seen that in addition to the main metathesis
product (7-tetradecene), alkenes from C13 to C9 are present in
considerable amounts. It is a consequence of the 1-octene
double bond isomerization followed by cross metathesis. More- Both Ktot and Kmet for 6MoO3/MCM-22(28) were significantly
over, a certain amount of oligomers (mainly dimers) were also higher than the total conversion for 6MoO3/SBA-15 (Figure 2).
observed in the reaction mixtures. Both isomerization and olig- Conversions to oligomers (Kol = Ktot − Kmet) were about 12%
omerization are due to the acidic character of the support (vide (at 2 h) and practically did not change in the further course of
infra). In addition to the total conversion of 1-octene (Ktot), the the reaction. However, the conversions to tetradecene were
conversion to all metathesis products (Kmet), and the conver- rather low (maximum conversion about 20% was achieved).
sion to tetradecene (KC14) calculated according to the following Higher catalytic activity of molybdenum oxide on zeolitic
equations are plotted in Figure 2.
support in metathesis may be ascribed to the higher acidity of
supports. The enhancing effect of Brønsted acidity on the cata-
lytic activity has been already described [6] and it assumed that
most of Mo active species in zeolite-based catalysts are formed
by reacting molybdenum oxide with Si-O(H)-Al groups [12,27].
Similarly, Lim et al. showed recently [28], that Brønsted acid
sites improve dispersion of molybdenum oxide on the surface.
Moreover, for related system based on tungsten oxide in zeolite,
it was suggested using high resolution STEM that Brønsted acid
sites in proximity to metathesis active sites facilitate olefin
adsorption and metallocycle formation [29]. Such mechanism
where mi and Mi (i = 9–14) are weight amounts and molecular may be effective also for Mo catalysts. The decrease in the
weights of alkenes from C9 to C14; md, mt and Md, Mt are selectivity due to isomerization and/or oligomerization seems to
weight amounts and molecular weights of octene dimers and be an unavoidable cost for this activity enhancement.
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
It is known for molybdenum oxide catalysts, that with increas- bulk MoO3 appeared in the XRD pattern of 10MoO3/MCM-
ing Mo loading the catalytic activities increase up to maximum 22(28) (see Figure 1A).
value [6,10]. At higher loadings the molybdenum oxide
spreading on the surface became imperfect and catalytically To reduce isomerization and oligomerization ability of MCM-
inactive bulk MoO3 appears. The effect of increasing Mo 22-based catalysts we prepared zeolite with Si/Al = 70 (and
loading on catalyst activity for MCM-22(28)-based catalyst is therefore with lower acidity – vide supra): MCM-22(70).
shown in Table 3.
The results showing the catalytic behavior of the prepared
MCM-22(70)-based catalysts 6MoO2(acac)2/MCM-22(70),
For 8MoO3/MCM-22(28) XRD pattern shows a small amount 5MoO2(acac)2/MCM-22(70), and 4MoO2(acac)2/MCM-22(70)
of bulk MoO3 (marked with asterisk in Figure 1A). In accord are collected in Table 4.
with this, the conversions fell down in comparison with
6MoO3/MCM-22(28), the selectivity, however, slightly in- XRD pattern of 6MoO3/MCM-22(70) exhibited some amount
creased: the amount of oligomers was reduced and the selec- of bulk MoO3 (Figure 1B). Evidently on this less acidic support
tivity to the tetradecene approximately doubled. It suggests that the MoO3 spreading is not perfect, which explains its negli-
more acid sites were covered by MoOx species and oligomeriza- gible activity in metathesis reaction. However, using bis(acetyl-
tion and isomerization ability of catalysts decreased. However, acetonate) complex MoO2(acac)2 as a source of Mo we ob-
further increase in the Mo loading to 10 wt % in 10MoO3/ tained 6MoO2(acac)2/MCM-22(70), 5MoO2(acac)2/MCM-
MCM-22(28) led nearly to the lost of catalytic activity, which is 22(70), and 4MoO2(acac)2/MCM-22(70) exhibiting no signals
explained by deposition of Mo in the catalytically inactive bulk of bulk MoO3 in XRD pattern (Figure 1B) and showing a mild
MoO3. Correspondingly, very intensive diffraction lines of the metathesis activity. The highest conversion Ktot = 35% (after
Table 3: The effect of Mo loading on catalyst activity in 1-octene metathesis.a
catalyst
reaction time, h
Ktot, %
Kmet, %
Kol, %
KC14, %
6MoO3/MCM-22(28)
2
4
6
45
86
92
33
75
79
12
11
13
12
20
20
8MoO3/MCM-22(28)
2
4
6.5
22
21
41
58
85
15
35
51
77
6
6
7
8
10
18
25
36
10MoO3/MCM-22(28)
2
4
6
2.6
3.4
4
0.6
0.7
1
2
2.7
3
0.6
0.7
1
a50 mg Catalyst, 1.5 mL 1-octene, 40 °C.
Table 4: 1-Octene metathesis over MCM-22(70)-based catalysts.a
catalyst
reaction time, h
Ktot, %
Kmet, %
Kol, %
KC14, %
6MoO3/MCM-22(70)
2
4.5
6
2
3
2
–
–
–
6MoO2(acac)2/MCM-22(70)
5MoO2(acac)2/MCM-22(70)
4MoO2(acac)2/MCM-22(70)
2
4
22
8
9
11.5
7.5
8
10.5
0.5
1
1
5
6
7
2.3
3.3
20
8
11
35
7
10
32
1
1
3
5
7
17
2
4
21
11
16
16
11
15
15
0
1
1
10
14
14
a50 mg Catalyst, 1.5 mL 1-octene, 40 °C.
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
20 h) was achieved over 5MoO2(acac)2/MCM-22(70). Oligo- MFI-based catalysts
merization activity of all these catalysts was considerably lower The comparison of conversion curves for 1-octene over 6MoO3/
in comparison with that of 6MoO3/MCM-22(28) (Kol = 1% 2D-MFI(26) and 6MoO3/HZSM-5(25) under standard condi-
only). However, the isomerization was not suppressed and tions is given in Figure 4. It is seen that 6MoO3/HZSM-5(25)
conversion to tetradecene KC14 was low.
exhibited only negligible activity (Ktot = Kmet = 3% after 20 h)
in accord with poor MoO3 spreading (see Figure 1D). Despite
the high acidity of the support, a poor accessibility of relevant
MCM-56-based catalysts
Conversion curves for the 1-octene metathesis over 6MoO3/ surface OH groups during the thermal spreading process and a
MCM-56(13) under standard conditions are displayed in poor accessibility of possible active sites by substrate molecule
Figure 3. In spite of the 2D character of support the conver- during metathesis may cause 6MoO3/HZSM-5(25) to be practi-
sions over 6MoO3/MCM-56(13) were significantly lower in cally inactive. On the other hand, over 6MoO3/2D-MFI(26)
comparison with 6MoO3/MCM-22(28): the initial reaction rate about 90% conversion was achieved for the same reaction time
(calculated at reaction time = 2 h) being about a half of the (20 h). The initial reaction rate over 6MoO3/2D-MFI(26) was
initial reaction rate over 6MoO3/MCM-22(28). On the other only slightly lower than that over 6MoO3/MCM-56(13) and
hand the extent of oligomerization was practically the same (for about one half of that over 6MoO3/MCM-22(28). Contrary to
final product the oligomerization selectivity was 14%) and the 6MoO3/MCM-22(28) the oligomerization activity of 6MoO3/
extent of cross metathesis was even higher (the selectivity to 2D-MFI(26) was reduced (Kol was from 1% to 5%) and the
tetradecene was only 15%). The crystals of MCM-22 (see SEM selectivity to tetradecene was higher (for final conversions
image in Supporting Information File 1, Figure S7) consist of KC14/Kmet = 0.41 and 0.25 for 6MoO3/2D-MFI(28) and
very thin platelets and therefore a great amounts of 12-mem- 6MoO3/MCM-22(28), respectively). Lower acidity of 6MoO3/
bered ring cups of MWW structure are on crystal exterior [18]. 2D-MFI(28) may explain the lower extent of oligomerization
These cups as we assume host MoOx species. Although MCM- and isomerization reactions and increased tetradecene selec-
56(13) as 2D zeolite consists of very thin layers, these layers tivity. Lower acidity may also bring about the reduced activity
may be curled and packed, which prevents the access of sub- as compared with 6MoO3/MCM-22(28); however, different
strate molecules to the most of 12MR cups (for MCM-56(13) structures of MCM-22 and MFI do not allow simple compari-
morphology see Supporting Information File 1, Figure S8). This son.
may explain the lower activity of 6MoO3/MCM-56(13) com-
pared with 6MoO3/MCM-22(28). Similarly, a higher activity of
MCM-22 in comparison with MCM-56 has been observed in
toluene disproportionation [18] and also for RCM of citronel-
lene over immobilized Ru catalysts the activity of catalyst based
on MCM-56 was not higher than that based on MCM-22 [24].
Figure 4: Conversion vs time curves for 1-octene metathesis over
6MoO3/2D-MFI(26) and 6MoO3/HZSM-5(25). Neat 1-octene,
1-octene/Mo = 320, t = 40 °C.
The accompanying oligomerization activity
The experiments with Mo-free zeolites (Figure 5a,b) confirmed
that the oligomerization activity was connected with the support
Figure 3: Conversion vs time curves for the 1-octene metathesis over
6MoO3/MCM-56(13). Neat 1-octene, 1-octene/Mo = 320, t = 40 °C.
itself. In these “blank” experiments the reaction conditions, as
well as pretreatment mode were the same as for Mo oxide cata-
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
Figure 5: Conversion to oligomers for 1-octene over MCM-22(28) and MCM-22(70) (a) and conversion to oligomers for 1-octene, 2-octene, 3-octene,
and 4-octene over MCM-22(28) (b). 50 mg Support, 1.5 mL octene, 40 °C.
lysts. No metathesis products were observed, only 1-octene Conclusion
oligomerization and double bond isomerization occurred. 3D and 2D zeolites of MWW (MCM-22 and MCM-56) and
Figure 5a shows 1-octene oligomerization over MCM-22(28) MFI topologies were used for the first time as supports for the
and MCM-22(70). Families of dimers and trimers (in weight preparation of highly active molybdenum oxide metathesis cata-
ratio dimers/trimers approximately 20:1 for the final conver- lysts. The catalysts, prepared by thermal spreading of MoO3
sions) were detected, isolation and characterization of indi- and/or MoO2(acac)2 on these supports in NH4+ forms (6 wt %
vidual dimers/trimer was not possible. It was visible from GC, and/or 5 wt % of Mo) were tested in neat 1-octene metathesis
that isomerization of starting 1-octene also occurred, however, under mild conditions (batch reactor, atmospheric pressure,
the exact quantification was not possible. The oligomerization 40 °C).
rate was higher for MCM-22(28) in accord with its higher
acidity as compared with MCM-22(70). The extent of oligomer- The catalyst activity (expressed as Ktot values at the reaction
ization in these blank experiments is several times higher than time = 2 h) decreased in the order 6MoO3/MCM-22(28) >
that achieved over metathesis catalysts: it may be due to the 6MoO3/MCM-56(13) > 6MoO3/2D-MFI(26) > 6MoO2(acac)2/
partial capping of support acid sites with Mo species catalysts MCM-22(70) >> 6MoO3/HZSM-5(25). This activity order
and also due the parallel consumption of 1-octene in metathesis. reflects two effects enhancing the activity: (i) support acidity
and (ii) structure characteristics ensuring good accessibility of
Figure 5b shows oligomerization of 1-octene, 2-octene active species by substrate molecules. The most active 6MoO3/
(cis + trans), 3-octene (trans), and 4-octene (trans) over MCM- MCM-22(28) exhibited a significantly higher activity than that
22(28). It is seen that the initial reaction rate decreases in the of a similar catalyst supported on siliceous mesoporous molecu-
order 1-octene ≈ 2-octene > 3-octene > 4-octene. The low-tem- lar sieve SBA-15.
perature oligomerization of alkenes over zeolite was studied as
concerns industrially important low alkenes oligomerization Due to the catalyst acidity accompanying reactions occurred:
and lower reactivity of internal alkenes in comparison with (i) 1-octene double bond isomerization followed by cross me-
1-alkenes was also recognized [30,31]. The reduced activity of tathesis and (ii) 1-octene oligomerization (mainly dimerization).
3- and 4-octenes in oligomerization might explain the fact, that The extent of these reactions depends strongly on the support
in our metathesis experiments the accompanying oligomeriza- acidity. Highly acidic supports MCM-22(28) and MCM-56(13)
tion occurred practically only in the beginning of the reaction. delivered a catalyst of rather low selectivity (up to 14% conver-
In later stages when most of 1-octene was isomerized to 3- and sion to oligomers, 15–20% conversion to tetradecene at about
4-octenes only little increase in oligomer amounts was ob- Ktot = 90%). Less acidic supports – MCM-22(70) and
served.
2D-MFI(26) gave rise catalysts of significantly higher selec-
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Beilstein J. Org. Chem. 2018, 14, 2931–2939.
tivity: conversion to oligomers was reduced to 1%, double bond The spectra were recalculated using a wafer density of
isomerization and cross metathesis proceeded in less extent, so 10 mg·cm–2. cL and cB were evaluated from the integral inten-
selectivity to tetradecene increased (e.g., for 2D-MFI(26) to sities of bands at 1454 cm–1 (cL) and 1545 cm–1 (cB)
35% at Ktot = 90%).
using extinction coefficients, ε(L)=2.22 cm·mmol–1 and
ε (B)=1.67 cm·mmol–1[34].
It is seen that for the metathesis of longer chain hydrocarbons
like 1-octene, supports ensuring a good access of bulkier sub- For elemental analysis ICP OES (iCAP 7000, Thermo Scien-
strate to the active centers are necessary. The acidity of the tific) was used. About 50 mg of the catalyst was digested in a
support increases the catalyst activity, however, simultaneously mixture of HF, HCl, and HNO3 (1:2:2). The samples were
with decrease of the catalyst selectivity. 2D-MFI(26) due to its placed in a Berghof microwave in a closed vessel at T = 140 °C
moderate acidity and 2D character results in catalysts of moder- for 35 min. Saturated solution of H3BO3 was then added for
ate activity but of the highest selectivity.
complexation of the excess of HF. After digestion solutions
under analysis were collected in 250 mL flasks and diluted with
With the described catalysts 1-octene was converted into a mix- ultra pure water.
ture of higher olefins: in addition to tetradecene as a
homometathesis product, olefins of 9–13 C atoms from cross Catalytic experiments
metathesis and C16 dimers were formed in various extent. Catalytic experiments were carried out in an argon atmosphere
Therefore, the described catalysts may find application espe- using a vacuum argon line. 1-Octene (Aldrich, 98%) was passed
cially if a mixture of higher olefins is desired, for example in through alumina and stored with Na. The content of water in
the preparation of detergents, lubricants etc.
1-octene was about 5 ppm. 2-Octene (Alfa-Aesar, 98%), trans-
3-octene (Alfa-Aesar, 97%) and trans-4-octene (Aldrich) were
purified in a similar way. In a typical experiment 50 mg of cata-
lyst (6 wt % of Mo) was used. Before reaction catalyst was
Experimental
Catalyst preparation and characterization
The zeolite supports MCM-22 and MCM-56 were prepared ac- pretreated in vacuo at 500 °C for 30 min. After cooling to
cording to [32,33], 2D-MFI was synthesized according to [21]. 40 °C, the reactor was filled with Ar and neat 1-octene
HZSM-5 (CBV 5524) was purchased from Zeolyst. Na+ forms (1-octene/Mo ratio = 320) was added under stirring. The reac-
of zeolites were converted to NH4+ form by three-fold treat- tion progress was followed by GC analysis of reaction mixture
ment with 1.0 M NH4NO3 solution at room temperature for 3 h. samples taken at given intervals. Individual compounds were
The supports were characterized by XRD (Bruker AXS D8 identified by GC/MS. A high-resolution gas chromatograph
Advance diffractometer with a graphite monochromator and a Agilent 6890 with a DB-5 column (length: 50 m, inner diame-
Vantec-1 position sensitive detector using Cu Kα radiation ter: 320 μm, stationary phase thickness: 1 μm), equipped with
in Bragg−Brentano geometry) and by N2 adsorption a 7683 Automatic Liquid Sampler and a FID detector
(77 K, Micromeritics GEMINI II 2370 volumetric Surface and GC/MS (ThermoFinnigan, FOCUS DSQ II single Quadru-
Area Analyzer). Molybdenum(VI) oxide (Sigma-Aldrich) pole) were used. Conversions were calculated from the mass
and bis(acetylacetonato)dioxomolybdenum(VI) (Aldrich) balance.
as sources of Mo oxide species were used for catalyst
preparation using the thermal spreading method (500 °C, 8 h).
Supporting Information
SEM images were recorded using a JEOL JSM-5500LV micro-
scope.
Supporting Information File 1
IR spectra of catalysts, GC of reaction products, and SEM
The concentrations of Lewis (cL) and Brønsted (cB) acid sites
were determined by FTIR spectroscopy of adsorbed pyridine
(Py) using a Nicolet 6700 with a transmission MCT/B detector.
The zeolites were pressed into self-supporting wafers with a
density of 8.0–12 mg·cm–2 and activated in situ at T = 450 °C
and p = 5·10–5 torr for 4 h. Pyridine adsorption was carried out
at 150 °C and a partial pressure of 3.5 torr for 20 min followed
by desorption for 20 min at 150, 250, 350 or 450 °C. Before
adsorption, pyridine was degassed by freeze–pump–thaw
cycles. All spectra were recorded with a resolution of 4 cm–1 by
collecting 128 scans for a single spectrum at room temperature.
images of catalysts.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-272-S1.pdf]
Acknowledgements
The authors thank J. Přech (J. Heyrovský Institute) for the prep-
aration of MFI support samples, and Valeryia Kasneryk
(J. Heyrovský Institute) for the SEM images. Financial support
from the Grant Agency of the Czech Republic (project No.
17-01440S) is gratefully acknowledged.
2938

Beilstein J. Org. Chem. 2018, 14, 2931–2939.
26.Topka, P. Molybdenum oxide supported on mesoporous molecular
sieves – new catalysts for alkene metathesis and alkyne
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