Catalysis Science &

a Conversion of 1-hexene or olefins obtained by fluid catalytic cracking (FCC) to propylene via isomerization-metathesis (ISOMET) were investigated using ethylene, as cross-coupling agent. Zeolite H-beta (HBEA) was applied as isomerization catalyst. The olefin metathesis (OM) catalysts were about 12 wt% molybdena, supported on zeolite beta (MoO 3 /HBEA), and γ-alumina (MoO 3 /Al 2 O 3 ). HBEA supported catalyst with a lower molybdena content (6 wt%) was also investigated. The catalysts were characterized by X-ray diffractometry (XRD), H 2 -temperature-programmed reduction (H 2 -TPR), Fourier Transform Infrared (FT-IR), visible Raman, in-situ Ultraviolet-Visible (UV/VIS) and XPS spectroscopies. It was shown that HBEA is highly active and robust catalyst of double bound isomerization. Applying a physical mixture of HBEA and 12MoO 3 /Al 2 O 3 catalyst at 150 °C and 3 bar ethylene pressure 60 % conversion of 1-hexene to propylene was attained. Interestingly, quantitative conversion to propylene was achieved after reactivation of the deactivated catalyst in argon atmosphere at 550 °C. It has been found that the pre-treatment of the catalyst with olefins such as ethylene before inert gas activation resulted in significant catalyst activity improvement. This suggests that the adsorbed olefins may play key role in the formation of active metal centers during the catalyst reactivation process. The catalyst mixture had also good performance in the conversion of FCC olefins to propylene. The MoO 3 /HBEA catalysts have rendered reasonable activity, however, the catalyst showed significantly shorter lifetime than the alumina-containing catalyst mixture.


árszki 
Insti
ute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Blanka Szabó 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Róbert Auer 
Hungarian Oil and Gas Public Limited Company
Október huszonharmadika u. 181117BudapestHungary

Katalin Tóth 
Hungarian Oil and Gas Public Limited Company
Október huszonharmadika u. 181117BudapestHungary

László Leveles 
Hungarian Oil and Gas Public Limited Company
Október huszonharmadika u. 181117BudapestHungary

Róbert Barthos 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Gábor Turczel 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Zoltán Pászti 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


József Valyon 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Magdolna R Mihályi 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja


Róbert Tuba tuba.robert@ttk.hu 
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences Magyar tudósok körútja



P.O. Box 2861519BudapestHungary


Catalysis Science & Technology Catalysis Science & Technology


Propylene Synthesis via Isomerization-Metathesis of 1-Hexene and FCC olefins
19 July 20218B054488A5D3A99384CDE447AC502A3810.1039/x0xx00000xReceived 00th January 20xx, Accepted 00th January 20xx
Conversion of 1-hexene or olefins obtained by fluid catalytic cracking (FCC) to propylene via isomerization-metathesis (ISOMET) were investigated using ethylene, as cross-coupling agent.Zeolite H-beta (HBEA) was applied as isomerization catalyst.The olefin metathesis (OM) catalysts were about 12 wt% molybdena, supported on zeolite beta (MoO 3 /HBEA), and γ-alumina (MoO 3 /Al 2 O 3 ).HBEA supported catalyst with a lower molybdena content (6 wt%) was also investigated.The catalysts were characterized by X-ray diffractometry (XRD), H 2 -temperature-programmed reduction (H 2 -TPR), Fourier Transform Infrared (FT-IR), visible Raman, in-situ Ultraviolet-Visible (UV/VIS) and XPS spectroscopies.It was shown that HBEA is highly active and robust catalyst of double bound isomerization.Applying a physical mixture of HBEA and 12MoO 3 /Al 2 O 3 catalyst at 150 °C and 3 bar ethylene pressure 60 % conversion of 1-hexene to propylene was attained.Interestingly, quantitative conversion to propylene was achieved after reactivation of the deactivated catalyst in argon atmosphere at 550 °C.It has been found that the pre-treatment of the catalyst with olefins such as ethylene before inert gas activation resulted in significant catalyst activity improvement.This suggests that the adsorbed olefins may play key role in the formation of active metal centers during the catalyst reactivation process.The catalyst mixture had also good performance in the conversion of FCC olefins to propylene.The MoO 3 /HBEA catalysts have rendered reasonable activity, however, the catalyst showed significantly shorter lifetime than the alumina-containing catalyst mixture.

Introduction

Olefin metathesis (OM) is a powerful and versatile method of organic chemistry.During OM carbon atoms of two C=C double bonds are reorganized to new double-bond-containing molecules.These reactions are very selective and require mild reaction conditions.The atom economy of the synthetic procedures is often 100%, i.e., all the starting materials are incorporated into the products.Nature is abundant in bio-based materials, containing olefin bond.Low value olefins can also be found in vast amounts in petrochemical by-products.These chemicals often appear as an underutilized feedstock.With the advent of alternative fuels and electric cars, the worldwide demand for mineral oil-based fossil fuels will certainly decrease.][3] Unfortunately, at present, these demands cannot be met by using renewable feedstocks only.Therefore, there is still a constant need for the development of high atom economic chemical procedures for the efficient and environmental benign conversion of petrochemicals to high value materials.Propylene is an emerging bulk chemical, the key monomer of polypropylene plastic and other commodity chemicals.It is mainly obtained as by-product of ethylene production in steam cracker units. 4,5Possibilities to control the ratio of ethylene to propylene in the cracked product mixture are limited.1][12] Propylene can be produced selectively also via OM under moderate reaction conditions.][13][14] There are several industrial examples for OM of olefinic hydrocarbon over heterogeneous catalyst.Such as the Shell higher olefin process (SHOP) for detergent production, 15 the Philips Triolefin Process for synthesis of 2-butene 16 and the Olefin Conversion technology used by ABB Lummus Global for propylene production. 17The most widely used catalyst systems includes Mo 11,14,18-22 W 3,4,6,8,9,12,18,[23][24][25] or Re 2,26,27 oxides.Mo and W-based catalysts are widely used in the petrochemical industry.Some new OM reactions, catalysed by metal-oxide, have been reviewed recently. 10,28 special case of OM is the isomerization-metathesis (ISOMET).During ISOMET, the olefinic bonds migrate along the hydrocarbon chain (isomerization), which is followed by a crossmetathesis reaction with another olefin, a cross-coupling agent.The special case of OM, when the cross-coupling agent is ethylene is called ethenolysis. 10,29In case of linear mono-olefins, the complete ISOMET using ethylene as cross-coupling agent theoretically ends up in propylene as the only end product (Scheme 1).The ISOMET of C4 and C5 olefins producing propylene is one of the most widely investigated area.1][32] The ISOMET process of higher olefins (C4-C9) and the mixtures thereof, has also been patended. 33Re 2 O 7 , WO 3 and MoO 3 were used as OM catalysts, whereas RuO 2 , MgO and K 2 CO 3 were applied as isomerization catalysts.Another patent application describes a process for converting an olefin feed containing butenes, diolefins and polyolefins to propylene with ethylene using molybdenum as metathesis catalyst and MgO, K 2 CO 3 , K 2 O as isomerisation catalyst. 34In both cases, the use of FCC mixture (fluid catalytic cracking) is mentioned as a source of olefin feed, but only after appropriate refining. 35,36he FCC and FCC light fractions are abundant in C>5 olefin components.Furthermore, not only petrochemical streams

ut also materials deriv
d from renewable feedstocks may contain C>5 olefins.For example, the ethenolysis of oleic acid produces 1-decene, 29,37 while the ethenolysis of linoleic acid gives 1hexene.Both vegetable oils are highly abundant in the nature. 38ight olefins (propylene, butenes, butadiene) are also available from biomass originated bioethanol 5,39 , biobutanol 40 and from biomass pyrolysis 6 .][22] In the present study zeolite beta was chosen as isomerization catalyst for ISOMET of 1-hexene model compound and FCC fractions.Beta is a large-pore zeolite with a three-dimensional structure of 12-membe

d ring channels. 41Owing t
its large pore size, strong acid sites and high chemical and thermal stability it is used as catalyst in the petrochemical industry and fine chemistry.It is also utilized as adsorbent. 42Zeolite beta has already been applied as a support itself, 43,44 or mixed with Al 2 O 3 in the OM of 2-butene with ethylene to propylene. 45However, it has not been used in the ISOMET of long-chain olefins yet, where the role of zeolite is the double bond isomerization of the reactant and intermediate alpha-olefin products.The dispersed MoO x, the active component of the oxide supported catalysts, was shown to be present as isolated or oligomeric surface species, as well as crystalline particles on a high-surface-area oxide support. 28apers reporting isomerization metathesis using C5+ alkenes as raw materials for propylene synthesis are rare. 46,47Especially there is no information in the literature about the synthesis of propylene from C5+ alkenes via ISOMET using HBEA as isomerization and MoO x /Al 2 O 3 as olefin metathesis catalysts.This paper describes a zeolite-supported molybdena (MoO 3 /HBEA) catalyst systems having isomerization (HBEA), as well as metathesis (MoO 3 ) activity.The mixed bed of aluminasupported molybdena and zeolite beta (MoO 3 /Al 2 O 3 +HBEA) ISOMET catalyst is also reported.MoO 3 loading of about 12 wt% was applied on both supports.Based on the work of Li and coworkers, for HBEA, a catalyst containing lower MoO 3 loading (6 wt%) was also investigated. 48The catalysts were characterized and investigated in ISOMET of 1-hexene model compound with ethylene.Conversion of FCC having high olefin content to propylene was also studied.It has been found that the initial catalytic activity can be significantly improved by simple high temperature heat treatment of the deactivated ISOMET catalyst in an inert gas.


Results and discussion

The molybdena content of the catalysts, determined by ICP-OES, is

own in Table 1.The ISOMET activity of 13MoO 3 /HBEA and
he physical mixture of 12MoO 3 /Al 2 O 3 and HBEA were higher than that of 6MoO 3 /HBEA (vide infra).The effect of the amount of MoO 3 loading on HBEA zeolite (70%)/Al 2 O 3 (30%) has been investigated by Li et al.Using the above-mentioned solid support mixture, a 6-8% molybdena loading was found to be optimal. 48The characterization of 13MoO 3 /HBEA and 12MoO 3 /Al 2 O 3 is presented below, whereas the properties of 6MoO 3 /HBEA are included in the ESI.


Catalyst characterization

Structure and texture The XRD patterns of Al 2 O 3 , HBEA supports, and 12MoO 3 /Al 2 O 3 , 13MoO 3 /HBEA catalysts are shown in Fig. 1.Gamma-alumina is the only detectable phase of MoO 3 /Al 2 O 3 containing about 12 wt% MoO 3 .This amount of molybdena is about two-third of the monolayer capacity.The monolayer coverage of the used -Al 2 O 3 , having a specific surface area of 192 m 2 g -1 corresponds to about 19.2 wt % MoO 3 content. 49The absence of the MoO 3 XRD reflections suggests that MoO 3 crystallites are well dispersed on the alumina surface, thus they are X-ray amorphous species.The obtained results confirmed that alumina-supported MoO 3 catalysts of a high dispersion can be obtained below monolayer coverage or even at MoO 3 contents, higher than that corresponding to the monolayer coverage. 50,51Upon Mo loading (13.5 wt% MoO 3 ) the characteristic reflections of HBEA at 2 = 7.8 and 22.6 significantly decreased, as it was also shown by others. 43,52


Catalysis Science & Technology Accepted Manuscript

Open reflection, was about 90 nm.At lower MoO 3 loading (6MoO 3 /HBEA) no crystalline MoO 3 phase was detected by XRD and the diffraction lines of the zeolite support was more intense (Fig. S1).No crystalline Al 2 (MoO 4 ) 3 phase was detected in these samples.Formation of this phase was found in samples treated at high-temperature (680 °C). 43In our ca e, however, the MoO 3 content was not high and the decomposition temperature of the Mo precursor was lower, 500 °C, so the formation of the aluminium molybdate phase i

unlikely in any of the supports.As expec
ed, the specific surface area of the molybdena-loaded catalyst was smaller than those of the corresponding support.

The difference in the surface areas depends on the pore size of the support (Table 1).For γ-Al 2 O 3 , which is mesoporous, the specific surface area decreased only slightly, (from 192 to 184 m 2 g -1 ).The specific surface area of the microporous zeolite catalyst with 12 wt% MoO 3 loading, however, was only half of that of the zeolite support (235 and 480 m 2 g -1 ).At 6 wt% of MoO 3 content the SSA of HBEA did not decrease (476 m 2 g -1 ).


Temperature-programmed reduction by hydrogen (H 2 -TPR)

Reducibility of the molybdenum species in 12MoO 3 /Al 2 O 3 and 13MoO 3 /HBEA was investigated by temperature-programmed H 2 reduction (Fig. 2).Two reduction peaks were observed on the H 2 -TPR curve of the 12MoO 3 /Al 2 O 3 catalyst.The lowtemperature peak in the range of 300-600 °C with a maximum of 450 °C represents the reduction of multilayered and octahedral Mo(VI) to Mo(IV). 50,53The tetrahedral Mo (IV) species has stronger interaction with the Al 2 O 3 support leading to a reduction temperature in the range of 600-800 °C.In this higher temperature region the reduction of Mo(IV) to Mo(0) takes place.For the 13MoO 3 /HBEA catalyst, two reduction peaks were observed in the temperature range of 300-650 °C and 650-800 °C.The peak at lower temperature is assigned to reduction of Mo(VI) to Mo(IV) as in case of the 12MoO 3 /Al 2 O 3 catalyst.The second peak represents the complete reduction of Mo(IV) to Mo(0). 54Similar TPR curve was observed for 6MoO 3 /HBEA (Fig. S2).Table 2 shows that the hydrogen uptake, expressed in H/Mo ratio, in the Al 2 O 3 -supported sample was 5.9.This amount of hydrogen consumption is near to the amount needs to the total reduction of MoO 3 to Mo(0) (6).Molybdena has been shown to react with the zeolitic protons during oxidative deco position of the heptamolybdate precursor. 55At 500 °C, MoO x oligomers migrate into the channels and react with the Brønsted acid sites of the zeolite to form ditetrahedral Mo species, i.e. (Mo 2 O 5 ) 2+ cations.These cationic Mo species can only be reduced at temperatures above 800 °C. 54,56Table 1 shows that H/Mo ratio was 5.6 for both MoO 3 /HBEA catalysts, suggesting that some Mo may occupy cationic positio

and cannot be redu
ed under our TPR conditions.In line with the above results, the loss of Brønsted acid sites was also observed for these samples (vide infra).

Before catalytic experiments, the calcined catalysts were activated in-situ in a flow of Ar at 550 °C.Thermal autoreduction of Mo(VI) cannot be detected by H 2 -TPR.


FT-IR spectroscopy of adsorbed pyridine

For the ISOMET reaction, the catalyst must contain Brønstedacid sites that promote double-bond isomerization of terminal olefins.Pyridine adsorption, followed by FT-IR spectroscopy, was used to characterize the acidity of the supported molybdena catalyst (Fig. 3).Brønsted and Lewis acid sites of the catalysts can be distinguished by the characteristic 19b/8a ring vibrations of pyridinium ions and pyridine bound to Lewis acid sites.These vibrations of the two species appear in different regions of the spectrum, i.e., at 1545/1637 cm -1 , and around 1455/1620 cm -1 , respectively.Fig. 3 shows that the HBEA support has both Brønsted and Lewis acid sites.In zeolites, Brønsted acidity is due to protons compensating negative charge on the zeolite framework generated by tetrahedral framework aluminum.Extra-framework aluminum represents Lewis acid sites.High amount of defect sites caused by crystallographic faults is typical for the structure of zeolite Beta. 57So, trigonal framework aluminum near the silanol nests shows Lewis acid

y.Upon Moloading, the number
of both Brønsted and Lewis acid sites decreased significantly.The FT-IR results suggest that molybdenum reacts with zeolitic protons and occupies cationic positions as ditetrahedral Mo ions (Mo 2 O 5 ) 2+ suggested by Ding et al. 58 The amount of Brønsted acid sites in the zeolite sample was 0.71 mmol g -1 , measured by the ammonium ion exchange capacity.Comparing the band intensities of the pyridinium ions around 1545 cm -1 the number of Brønsted acid sites decreased by 36 %, indicating that 0.12 mmol g -1 Mo is in the cationic position, which is about 15% of the total Mo content.In addition, surface MoO x species can be bound in the zeolite silanol nests both in the micropores and on the outer surface of the zeolite crystallites.These species cause a significant decrease in the number of Lewis acid sites.


Catalysis Science & Technology Accepted Manuscript

In the spectrum of 12MoO 3 /Al 2 O 3 the intensities of the bands characteristic of Lewis acid sites slightly increase at 1453 and 1621 cm -1 compared to the bare support (Fig. 3).Highly polymerized MoO x species were suggested to be responsible for the additional Lewis acid sites at this Mo content. 59App. a 10 % decrease in Brønsted sites was observed on 6MoO 3 /HBEA relative to the HBEA support (Fig. S3).


Raman spectroscopy

The structure of supported molybdenum oxide was investigated by Raman spectroscopy under ambien

conditions, in hy
rated state.The Raman spectra of 12MoO 3 /Al 2 O 3 and 13MoO 3 /HBEA are presented in Fig. 4. In the spectrum of 12MoO 3 /Al 2 O 3 , the bands observed at 953, 910 and 355 cm -1 are assigned to the symmetric stretching, asymmetric stretching, and bending vibrations of the terminal MoO bond of octahedral MoO 6 species in hydrated heptamolybdate, respectively. 60In addition, the bands at 566 and 222 cm -1 are due to the Mo-O-Mo symmetric stretching and deformation vibrations of these molybdena species, respectively.These results confirm that at 12 wt% of molybdena loading octahedral species dominate on alumina.It should be mentioned that tetrahedral MoO 4 units is also present as the Raman band at 910 cm -1 is typical of the symmetrical stretching vibrations of the terminal MoO in tetrahedral molybdena species. 61This observation was also confirmed by UV-VIS spectroscopy.The intense and narrow bands in the spectrum of 13MoO 3 /HBEA is due to crystalline MoO 3 , 62 indicating that this catalyst contains MoO 3 phase, which is also supported by the XRD data (Fig. 1).At 6 wt% molybdena loading, the intensity of the 970 cm -1 -band increased, compared to the zeolite support (Fig. S4).This indicates that the catalyst contains tetrahedral MoO x species.


In-situ UV-VIS spectroscopy

In-situ UV-VIS experiments were performed to determine the type of MoO x species in the calcined 12MoO MoO x , respectively.4][65] For 12MoO 3 /Al 2 O 3, the intensity of the absorption band at 250 nm becomes weaker above 200 °C, while the band at 330 nm gains strength.This result suggests that during thermal treatment in inert atmosph

e tetrahedral MoO x is transformed to poly
erized octahedral MoO x species over alumina-supported molybdena.

Comparing the relative intensity of the bands below 300 nm and at 330 nm in the spectra of 13MoO 3 /HBEA, similar but smaller changes were observed, indicating that a smaller portion of tetrahedral MoO x species is transformed to octahedral MoO x species on the zeolite-supported molybdena.MoO 3 located in microporous channels is less capable to form longer MoO x chains.They can be formed only on the outer surface of the zeolite crystals, but in small amounts only because the outer surface is only approx.4-5% of the specific surface area of the zeolite.In the spectrum of the calcined 13MoO 3 /HBEA the presence of a weak absorption band around 400 nm is due to the crystalline MoO 3 phase, which was also confirmed by XRD.Crystalline MoO 3 was shown to be catalytically inactive. 66In the spectrum of 6MoO 3 /HBEA, at higher temperatures, those tetrahedral MoO x species that are surrounded by other Mo atoms cannot be detected (Fig. S5).This result suggests that during heat treatment in Ar, MoO x species migrate into the microporous channels and occupy cationic position in the zeolite.


XPS measurements

Quantitative evaluation of the XPS data indicated that the Mooxide content of the MoO 3 /Al  531.4 eV all pointed to an oxidized/hydroxylated environment for the Al(III) cations, 67,68 in agreement with literature data on similar catalysts. 69No significant changes in the chemical environment of Al were observed during the treatments.In addition, no sign of dissolution of Mo into the alumina was detected.The Mo 3d 5/2-3/2 spin orbit doublet of well-dispersed MoO 3 on alumina has a rather broad line shape with a 3d 5/2 binding energy around 233 eV [69][70][71] which is somewhat higher than the value characteristic for bulk MoO 3 (232.5 eV) 72 .While the shift is attributed to the strong interaction of the oxidized Mo species with the alumina support, 70 the broadening can be the result of a charging effect, 70 but may also indicate the existence of a range of slightly different environments for the adsorbed Mo(VI) ions. 73As it is shown in Fig. 6a, these features are well reproduced in the Mo 3d spectrum of the calcined 12MoO 3 /Al 2 O 3 sample exposed to air.The spectrum can be well modelled by a single peak pair with the 3d 5/2 component at 233.2 eV, indicating the exclusive presence of Mo(VI) ions.

Treatment in Ar at 550 °C resulted in a marginal shift of the maximum of the spectrum towards lower binding energies and further apparent broadening (Fig. 6b).Spectral modelling revealed that these changes can be interpreted as the result of the appearance of a new Mo 3d doublet with its 3d 5/2 peak around 231.7-231.9eV.According to its binding energy, this new component was attributed to Mo(V) species. 72The abundance of the Mo(V) species clearly increased upon ethylene

posure (Fig. 6c) and some further increase w
ter th subse uent tr atment in Ar at 550 °C (Fig. 6d).Although the combination of the Mo(VI) and Mo(V) states adequately modeled the observed line shapes, the broad peaks can easily shadow weak Mo(IV) contributions, so their presence -especially after the ethylen exposure and the subsequent re-activation -cannot be completely ruled out.


Catalysis Science & Technology Accepted Manuscript

Open


1-Hexene isomerization over HBEA catalyst

Preliminary experiments have been carried out to investigate the isomerization of 1-hexene model compound using HBEA catalyst at 75 °C and atmospheric pressure in inert Ar gas atmosphere.At nearly 90 % 1-hexene conversion, the yields of cisand trans-2-hexene and cisand trans-3-hexene were 70 nd 20 %, respectively.(Fig. 7).This result indicates that double bond shift and cis-trans rearrangement are the main reactions in the conversion of 1-hexene over zeolite beta, consistent with earlier results on large-and medium-pore zeolites, i.e., H-Y and H-ZSM-5.5][76] It was demonstrated that under these conditions, HBEA is highly robust.No catalyst deactivation was observed during a time on stream (TOS) of 10 h.The ISOMET of 1-hexene with ethylene was performed at 3 bar total pressure of and at ethylene/1-hexene molar ratio of 10.In order to make sure that the isomerization is not affected by ISOMET condition the isomerization reaction was performed in the presence of 3 bar of ethylene (optimal ethylene pressure for OM, Fig. S7), as well.The results showed that neither the catalytic activity nor the product selectivity was affected.Similar conversion and reaction product distribution was observed as in the absence of ethylene.The influence of reaction temperature on the isomerization conversion and product distribution has been investigated at 75 (optimal reaction temperature for OM), 100, 125 and 150 °C and 3 g cat g 1- H -1 h space time under 3 bar ethylene pressure.It could be concluded that even at 75 °C neither the conversion of 1-hexene nor the product distribution have been changed significantly.By using the optimized OM (ethenolysis) condition, the catalyst activity did not decrease within ten hours of the reaction (Fig. 7).The yield of 2-hexenes achieved 66 % (55 % trans-2-hexene and 11 % cis-2-hexene) after 1 hour TOS.HBEA is a highly robust isomerization catalyst showing high activity and stability at optimized metathesis reaction conditions.The 1-hexene isomerization activity of the MoO 3 -containing HBEA catalysts was also investigated.The catalysts were pretreated in O 2 flow at 550 °C for 2h, to keep Mo in Mo(VI) state.It was found that the isomerization activity of HBEA significantly drops upon imp egnating with MoO 3 .With increasing MoO 3 content, the number of Brønsted acid sites decreases (shown by FT-IR spectroscopy) therefore the isomerization activity also decreases.Actually, at 6 wt% loading 80 % 1-hexene isomerization was observed (versus 90 % on neat HBEA), meanwhile at 12 wt% loading the isomerization activity was around 60 % after one hour of time on stream.At 12 wt% molybdena content the isomerization activity was halved after 5 hours of time on stream, wher

s 6MoO 3 /HBEA shows good stability in 1-hexene isomerization.


1
Hexene ISOMET using MoO 3 /HBEA catalysts

Theoretically, terminal olefins (not only) such as 1-hexene can be completely converted to propylene by using ethylene in sequential isomerization and ethenolysis steps (ISOMET) (Scheme.2).Two types of sites are required, Brønsted sites for the C=C double-bond isomerization of 1-hexene and the intermediate alpha-olefins formed (1-pentene, 1-butene), and active MoO x sites for the conversion of isomerized olefins (2-, 3hexenes, 2-pentenes, 2-butenes) with ethylene to give propylene and lower alpha-olefins (1-pentene, 1-butene).

The pathway of 3-hexene isomer to propylene is shorter, as its reaction with ethylene gives 1-butene, which is converted to propylene in the isomerization and ethenolysis steps (not shown in Scheme 2).

In the following series of experiments, HBEA was used not only as an isomerization catalyst but also as a support for MoO 3 .The reactions have been carried out under 3 bar ethylene pressure and 3 g cat g 1-H -1 h space time in the temperature range of 75-150°C.The catalyst was pre-treated in inert gas at 550 °C and the activated catalyst thus obtained was tested in ISOMET reaction for 3 h of time-on-stream (TOS).After activation, the catalyst was regenerated under the same conditions as the pretreatment.The activity of the reactivated catalyst was also studied in ISOMET reaction.a Activation: The ex-situ calcined catalyst was in-situ pre-treated in a flow of Ar (50 ml min -1 ) at 550 °C for 2 h.The activated catalyst was cooled to the target temperature in a flow of Ar and the ISOMET reaction was performed.

b Reactivation: After 3 h of TOS, the reactant feed was stopped, the total pressure was reduced to atmospheric pressure.The catalyst was purged with a flow of Ar (50 ml min -1 ) for 30 min at the reaction temperature to remove olefins, then heated to 550 °C at a ramp rate of 10 °C min -1 , and maintained at this temperature for 2 h.The reactivated catalyst thus obtained was cooled to the target temperature in a flow of Ar and the ISOMET reaction was performed again.

On the activated 13MoO 3 /HBEA catalyst, the propylene yield was low, ranging from 10 to 29 % at 0.5 h of TOS (Table 2).It was 10 % at 75 °C, with increasing reaction temperature reached its maximum (29 %) at 125 °C and then at 150 °C decreased to 19 %.Reactivation, i.e., heat treatment in an inert atmosphere at high temperature (550 °C) after the ISOMET reaction, however, resulted in a significantly increase in the initial activity of the 13MoO 3 /HBEA catalyst.At 75 °C, the propylene yield doubled, while at higher temperatures the amount of propylene was 3 and 4 times as high n the reactivated catalyst as on the freshly activated catalyst.Thus, at 125 and 150 °C more than 80 % propylene yield can be obtained over reactivated 13MoO 3 /HBEA at 0.5 h of TOS.However, the catalyst is rapidly deactivated under all conditions.The rate of deactivation was higher at higher temperatures, and higher on the reactivated catalyst.During 3 h of TOS, at 75 °C the propylene yield decreased from 10 to 6 % and from 20 to 16 % on the activated and reac

vated catalyst, respectively.At 125 °C, the initial activity of the activat
d and reactivated catalyst decreased from 29 to 5 % and from 86 to 17 %, respectively, in three hours (Table 2).The 13MoO 3 /HBEA catalyst, in which the active sites of both the isomerization and OM reactions are located in a microporous aluminosilicate support, is rapidly deactivated due to the polymerization of olefins. 77Especially at 150°C, when after three hours of TOS the propylene yield dropped significantly, from 85 to 10 %.This catalyst contains 90 nm-size MoO 3 crystals, which blocking the micropores, therefore we have halved the MoO 3 content of the catalyst.No crystalline MoO 3 phase was detected by XRD in 6MoO 3 /HBEA and only the low-intensity Raman bands indicate the presence of small amount of MoO x species, however, the catalyst showed lower activity in 1-hexene ISOMET (Table S1).

At 75 °C the activity of the two catalysts were about the same, but as the temperature increased, the propylene yield on 6MoO 3 /HBEA hardly changed.The number of active sites is presumably decreasing during the reaction as carbonaceous deposits may gradually block the micropores, thus the reactants molecules cannot access the active sites located inside the pores.Not only coking, but also the low oxidation state of Mo is also a reason of the decreased activity (vide infra).8][79] Zhang et al. 11 reported that both the number of Brønsted acid sites and the high dispersion of MoO 3 species has an important role in determining the OM activity.Brønsted acid sites not only catalyse double-bond isomerization of alpha-olefins, but are also involved in the OM reactions, i.e., participate in the generation of active metal carbene species, which are the active site of the OM reactions.Previous work suggested that Brønsted acid OH groups interact with adjacent metal oxides, for example, MoOx, WOx, or ReOx for the generation of OM active carbene species. 80,81Recent results showed that using MoO x /SBA-15 catalyst, surface Brønsted acidic OHs coordinated to Mo(VI) protonate propylene and isopropoxide species are formed upon propylene adsorption. 82uch species are further oxidiz d by lattice oxygen of MoO x to gas-phase acetone yielding reduced Mo(IV).This species reacts with gas-phase propylene to form the OM active Mo(VI)alkylidene species.When Brønsted acid sites and MoO x species were located on the microporous support, rapid deactivation occurred during the ISOMET reaction.

The complementary characterization techniques revealed that in the 13MoO 3 /HBEA sample Mo exists in highly dispersed tetrahedral and polymerized octahedral MoO x species, which are active in the OM reaction.However, a fraction of Mo is present as catalytically inactive crystalline MoO 3 phase.

Tentatively it is presumed that the lower activity of 6MoO 3 /HBEA is most probably due to the formation of hardly accessible MoO x species located in the microporous channels.

In order to achieve a more active and stable catalyst for the ISOMET of 1-hexene with ethylene, molybdena was loaded onto a gamma-alumina support (12MoO 3 /Al 2 O 3 ), because at 12 wt% loading only highly dispersed MoO x is formed.Further experiments were performed with a mixed bed of 12MoO 3 /Al 2 O 3 (50 %) and zeolite HBEA (50 %) catalysts.


1-Hexene ISOMET using 12MoO 3 /Al 2 O 3 and HBEA catalyst mixture

The activity and stability of the physical mixture of 12MoO 3 /Al 2 O 3 and HBEA was studied in the ISOMET reaction of 1-hexene at 75 °C, ethylene pressure of 3 bar and, 6 and 12 g cat g 1-H -1 h space time (Fig. 8A and A'; B and B') and compared with MoO 3 /HBEA.In these experiments, we compared the properties of catalysts with the same molybdena content but different weights, i.e., a mixture of 1 g of 12MoO 3 /Al 2 O 3 + 1 g of HBEA as well as 1 g of 13MoO 3 /HBEA.Over the freshly activated catalyst, the propylene yield was 33 % after 0.5 h of TOS.However, after reactivation of the catalyst the propylene yield more than doubled and reached a level of nearly 80 %.


Catalysis Science & Technology Accepted Manuscript

Open  2 for detailed conditions.)After activation and reactivation the catalyst was cooled to the reaction temperature in a flow of Ar (50 ml min -1 ) and the ISOMET reaction was performed again.

Repeated reactivation gave similar high propylene yield (Fig. S9).Not only the initial activity of the catalyst improved significantly but the lifetime of the catalyst also increased.(Fig. 8A and A').

The propylene yield on the activated catalyst mixture decreased from 33 to 18 % in 3 h of TOS.Deactivation of the reactivated catalyst is slower, the propylene yield dropped from 79 only to 57 % in 3 h.Significantly lower initial activity and faster deactivation were observed for 13MoO 3 /HBEA (Table 2).Over the activated and reactivated 13MoO 3 /HBEA catalyst in three hours of TOS the propylene yield decreased from 10 to 6 % and 20 to 16 %, respectively.The higher activity of the 12MoO 3 /Al 2 O 3 catalyst compared to 13MoO 3 /HBEA is due to the higher concentration of active MoO x species on alumina than on zeolite (microporous aluminosilicate) confirmed by XRD, Raman and UV-VIS spectroscopies.Higher space time resulted in a significantly higher propylene yield.The catalyst reactivation has shown significantly higher (>90 %) propylene yield and longer catalyst lifetime (Fig. 8B and B').Further experiments have been carried out to investigate the influence of the reaction temperature on the overall propylene yield (Fig. 9).At 125 °C, the propylene yield was around 50 %.The catalyst activity did not change as a function of TOS.After reactivation of the catalyst, a propylene yield of more than 90 % can be obtained.Some catalyst deactivation was observed, however after 3 h TOS still 80 % propylene yield could be measured.2 for detailed conditions.)After activation and reactivation, the catalyst was cooled to the reaction temperature in a flow of Ar (50 ml min -1 ) and the ISOMET reaction was performed at conditions described above.

By increasing the reaction temperature up to 150 °C even higher yield and longer catalyst lifetime was observed.After the catalyst reactivation approximately 100 % propylene yield was found, which was maintained up to two hours.From the third hour of the reaction the propylene yield slightly decreased, however it was still high (> 80%).As it was observed after the first run and catalyst reactivation, the catalyst performance was always significantly higher.


Activation and reactivation of 12MoO 3 /Al 2 O 3 and HBEA catalyst mixture

Conventional pre-treatment of the heterogeneous OM catalysts includes high-temperature calcination and inert gas purging.It ha

also been shown that olefin pre-tr
atment at low or high temperatures can improve the initial activity.Amakawa et al. 82 reported that heat treatment in inert gas at 550 °C for 2 hours after the room-temperature olefin adsorption doubled the catalytic activity of MoO 3 /SBA-15.Surface isopropoxide species were supposed to form and activate surface Mo(VI) sites by reduction to Mo(IV) and formation of C3 oxygenate (acetone).This reduced Mo species react with propylene and gives active Mo(VI)-alkylidene species.Two orders of magnitude increase in activity was observed over silica-supported MoO 3 and WO 3 when propylene adsorption was performed at high temperature, 550 °C and 700 °C, respectively. 83The hightemperature activation was explained by Pseudo-Wittig mechanism.


Catalysis Science & Technology Accepted Manuscript

Open Activation in H 2 or Ar: the ex-situ calcined catalyst was in-situ pre-treated in 5 % H 2 /Ar or Ar flow (50 ml min -1 ) at 550 °C for 2 h.Reactivation in Ar: after 3 h of TOS in 1-hexene ISOMET the catalyst was purged in a flow of

(50 ml min -1 ) for 0.5 h
at 75 °C to desorb olefins, then heated to 550 °C at a ramp rate of 10 °C min -1 , and maintained at this temperature for 2 h.Reactivation in O 2 : after 3 h of TOS in 1hexene ISOMET the catalyst was purged in a flow of O 2 (50 ml min -1 ) for 0.5 h at 75 °C to desorb olefins, then heated to 550 °C at a ramp rate of 10 °C min -1 , and maintained at this temperature for 2 h, and then purged with Ar (50 ml min -1 ) for 0.5 h and cooled down to the reaction temperature.Activation in ethylene: after Ar-activation ethylene was fed into the reactor for 1 h at 75 °C then changed to Ar flow for 0.5 h and heated to 550 °C at a ramp rate of 10 °C min -1 , and maintained at this temperature for 2 h.Activation: the mixture of ex-situ calcined catalyst was in-situ pre-treated in a flow of Ar (50 ml min -1 ) at 550 °C for 2 h.Reactivation: after 3 h of TOS in ISOMET reaction the catalyst was purged with Ar flow (50 ml min -1 ) at 550 °C for 2 h.(See Table 2 for detailed conditions.)After activation and reactivation, the catalyst was cooled to the reaction temperature in a flow of Ar (50 ml min -1 ) and the ISOMET reaction was performed at conditions described above.

Others state that the active species is Mo(V). 84Thus, there is still some debate in the scientific community as to which catalyst oxidation state results in high catalytic activity.The surface Mo(VI) oxide species are known to be catalytically inactive, Mo(V) and Mo(IV) are shown to be active, however lowering the oxidation state results in catalytically inactive species again, therefore the appropriate activation is crucial for the high catalyst activity.Fig. 10 shows the effect of different activation and reactivation on propylene yield in 1-hexene ISOMET over the 12MoO 3 /Al 2 O 3 -HBEA catalyst mixture.No propylene was formed over the 12MoO 3 /Al 2 O 3 and HBEA mixture oxidized in-situ at 550 °C, confirming that surface Mo(VI) oxide species are not active in OM reaction.The experiment was stopped after 0.5 h of TOS, not waiting for the catalyst to be activated in-situ during the reaction, as it was proved by Amakawa et al. 62 The catalyst pretreated in 5% H 2 /Ar mixture at 550 °C for 2 hours has the lowest activity (Figs. 10 and S8).At 75 °C and 3 bar ethylene pressure the propylene yield was about 15 %.Higher propylene yield (~ 28 %) was obtained over Ar-activated catalyst (Figs. 8 and 10).However, performing ISOMET reaction for 3 of TOS and treatment again in inert gas flow at 550 °C for 2 h, improved activity was observed.The propylene yield increased to 80 %.When the catalyst was reactivated in O 2 flow at 550 °C for 1 h followed by purging with Ar flow for 2 h no catalyst activity improvement was observed.It remained the same as those of the Ar-activated catalyst.Fig 10 shows that improved ISOMET activity was also observed when only the cross-coupling agent, ethylene was fed into the reactor at 75 °C (actually self-metathesis proceeds) and the catalyst was heattreated in Ar at 550 °C (ethylene-activated catalyst).These results suggest that after 1-hexene ISOMET or ethylene OM, during the Ar-reactivation the remaining olefins may participate in the generation of the catalytically active molybdenum species also. 83Calcined the catalyst in O 2 flow at 550 °C Mo-carbene species are decomposed and Mo is oxidized to Mo(VI).The XPS results shows that during inert gas treatment at 550 °C part of Mo(VI) are reduced to Mo(V) species.The ethyleneactivated catalyst contains more Mo(V) species.The high catalyst performance can also be explained by the partial reduction of Mo initiated by the hydrocarbon residues remaining on the catalyst surface.

Propylene Synthesis via ISOMET of FCC fractions using 12MoO 3 /Al 2 O 3 and HBEA catalyst mixture Our further aim was to develop OM based chemical process for a mixture produced by FCC cracking (further named as crude FCC and FCC light).The analysis of FCC light and FCC

actions reveale
that the olefin content is about 37 wt% in FCC light and 22 wt% in crude FCC and approximately 50 % of the olefins are 2-olefins (Fig. S10).The main olefin components of FCC light fraction are C5-C7 olefins, however some C4 and C8 components could also be detected.Experiments have been carried out to synthetize propylene from crude FCC fraction.Theoretically, regardless of the nature of the linear mono olefins (either terminal or internal) all olefins can be converted to propylene via ISOMET using ethylene (Scheme 1).In terms of weight of olefins approximately 100 tons of C4-C8 olefin mixture can be converted to 200 tons of propylene by using 100 tons ethylene.The ISOMET of FCC light and FCC fractions were carried out without any pretreatment at 75 °C and 3 bar ethylene pressure.


Catalysis Science & Technology Accepted Manuscript

The FCC light fraction containing mainly C5-C7 olefin components.Due to the relatively high olefin content (35-37%) and purity, high propylene yield (70 %) and long catalyst lifetime have been reached (Fig. 11).Following the reactivation significantly higher (90 %) propylene yield and longer catalyst lifetime were observed.After 8 hours of TOS the catalyst remained still active and the propylene yield achieved 50 %.Compared to the 12MoO 3 /Al 2 O 3 -HBEA catalyst mixture significantly lower p

pylene yield could be obtained us
ng the 12MoO 3 /HBEA catalyst (Fig. S11).

The ISOMET of FCC shows that after 0.5 h TOS slightly higher than 45 % propylene yield have been obtained, which has been declining steadily in the next two hours (Fig. 11).The catalyst reusability was also investigated.It was found that experiments carried out with reactivated catalyst resulted in significantly higher (75%) propylene yield and longer catalyst lifetime.München) with ammonium heptamolybdate solution.Before impregnation zeolite HBEA was calcined at 500 °C for 8 h.After impregnation, the solvent was evaporated by keeping the preparation at 120 °C for 6 h.The samples were then heated to 500 °C in air and kept at this temperature for 12 h.The aircalcined samples are designated as xMoO 3 /HBEA.The MoO 3 content of the catalysts were 13.5 and 6.1 wt%.


Experimental


Catalyst characterization

X-ray powder diffraction patterns of the catalysts were recorded by a Philips PW 1810/3710 diffractometer, equipped with a graphite monochromator.The CuKα radiation (λ = 0.1541 nm) was used.The X-ray tube was set at 40 kV and 35 mA current.The scan step size was 0.02 degrees 2-theta, whereas the scan time was five seconds in each step.The specific surface area (SSA of the catalysts was calculated from the N 2 a sorption isotherms using the Brunauer-Emmett-Teller (BET) method.The adsorption isotherms were measured at −196 °C using a Thermo Scientific Surfer Automated Gas Sorption instrument.Before measurement, samples were evacuated at 250 °C for 2 h in high vacuum (~10−6 mbar).

Reducibility of the catalysts were studied by temperatureprogrammed H 2 -reduction (H 2 -TPR) method.About 100 mg of ample was treated in 30 ml/min O 2 flow at 500 °C for 1 h in a quartz reactor tube (6 mm ID).In order to obtain TPR curve the samples were cooled to 25 °C, exposed to a 30 ml/min flow of 9.0 vol% H 2 /N 2 mixture, and then heated up to 800 °C at a rate of 10 °C/min.The reactor effluent was passed through a trap cooled by liquid nitrogen (−196 °C) to remove water from the gas flow.

e rate of H
uptake was recorded using a thermal conductivity detector (TCD).Acidity of the catalysts were characterized with a Nicolet Impact Type 400 spectrometer applying the self-supported wafer technique and using an in-situ cell.The acid site concentrations were determined using Transmission Fourier-transform infrared (FT-IR) spectra of the adsorbed pyridine (Py) on Brønsted-and/or Lewis acid sites of the catalysts.IR spectra of the sample were recorded at room temperature averaging 32 scans at a resolution of 2 cm −1 .Spectra were normalized to a wafer thickness of 5 mg/cm


Catalytic test

The catalytic experiments were carried out in a down-stream, fixed-bed, and stainless-steel tube reactor (12 mm ID).The reactor was loaded with 2 g of catalyst grains prepared by compression of the sample powder, crashing and sieving (0.315-0.63 mm) and flow rate of reactant liquid mixture (1,5-6 ml/h) were varied.The reactor temperature was controlled by an Omron E5CN controller.The liq id mixture (1-hexene in hexane solution (30%), FCC; FCC light) was fed into the reactor by high-pressure micro pump (Teledyne Isco, Model 100DM).

The gas was introduced by mass flow controller (Aalborg GFC17).The total pressure and the reaction temperature were varied in the range of 1-50 bar and 25-150 °C, respectively.Using water cooling, the reactor effluent was separated to liquid and gas products.The liquid product mixtures were analysed by GC-MS (Shimadzu QP-2010) using a 60 m ZB-WAX PLUS capillary column.The gaseous reactor effluent was analysed for detection of propylene and light hydrocarbons using an on-line gas chromatograph (Varian 3300)

th flame ionization de
ector (FID) and applying 30 m Supel