Transcript MS PowerPoint - Catalysis Eprints database

Transition metal exchanged  zeolites:
characterization of the metal state and catalytic
application in the methanol conversion to
hydrocarbons
Dolores Esquivel, Aurora J. Cruz-Cabeza , César Jiménez-Sanchidrián,
Francisco J. Romero-Salguero, Microporous and Mesoporous Materials
179 (2013) 30–39.
R.Vijaya Shanthi
(06-07-2013)
Zeolites are widely used as catalysts for a variety of organic reactions.
Ion exchange has been used to introduce different metal cations in zeolites. These
species can act as Lewis acid sites and redox active centers, thus providing new
functionalities to zeolites.
Whilst the catalytic activity in acid catalysis is usually related to the Bronsted acid
sites, the influence of the Lewis acid sites cannot be overruled and is still widely
debated .
Uses of zeolites exchanged with transition metals
Co2+, Fe3+, and Cu2+ ---- selective catalytic reduction (SCR) of
NO by ammonia.
Ni2+
---- isomerization and ring-opening of styrene oxide.
Cr3+
---- oxidative dehydrogenation of propane.
Mn2+
---- liquid phase epoxidation of alkenes with aqueous
hydrogen peroxide.
Zn2+
---- hydroamination reactions
Zeolite beta is an outstanding catalyst for a great variety of organic processes.
Ion exchange has been used for the introduction of different metal cations in this material to
improve its activity and/or selectivity. Beside some previously referred examples, alkaline and alkalin
eearth metals have been usually chosen for that purpose. Generally, they reduce the concentration of
Bronsted acid sites, thus favoring the selectivity toward different products.
The highly interesting methanol to hydrocarbons process proceeds by a complex Mechanism and it
is well known that it requires the presence of Bronsted acid sites and indeed it has been proposed as a
test reaction for different molecular sieves.
We have previously reported that the conversion to hydrocarbons over alkaline and alkaline-earth
exchanged beta zeolite is mostly dependent on the exchange degree and that the Lewis acidity
generated by the exchanged cations does not influence the reaction but it modulates the materials acid
strength and product selectivity.
In this work we study different transition metal exchanged beta zeolites using complementary
techniques such as FTIR, UV–Vis, XPS, 27Al NMR and chemisorption experiments, in order to
determine the nature of the metal species. We also test the materials performance as catalysts in the
methanol conversion to hydrocarbons and elucidate the possible participation of the exchanged metals
(Lewis acid sites) in this reaction.
Sample preparation (Ion exchanged method)
The protonic form of zeolite (Si/
Al = 12.5) was stirred in a 0.3 M aqueous solution of
the metal salt (6 ml/g) at 80 C during 24 h.
Then the exchanged zeolites were dried at 100 °C
overnight and calcined at 600 °C for 3 h.
Denoted as: (a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β,
(f) Ni-β,(g) Cu-β and (h) Zn-β.
DRIFT spectra of zeolites
872 cm-1 - O–Al–O vibrations (aluminum
defects)
960 and 880 cm-1- asymmetric framework
vibrations (T–O–T) perturbed by the presence
of different cations.
(a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β, (f) Ni-β,(g) Cu-β and (h) Zn-β.
Exchange degree (%) and surface atomic ratios for the
metal-exchanged zeolites from XPS data.
a
By EDAX analysis (bulk composition).
b By XPS analysis (surface composition).
Diffuse reflectance UV–Vis spectra of zeolites
(a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β, (f) Ni-β,(g) Cu-β and (h) Zn-β.
UV–Vis spectra of zeolites
230–240 and 270–280 nm - Al–O charge-transfer transition of the tetrahedral framework
aluminum and octahedral aluminum atoms with different environments.
For the Fe- β sample : 220–245 and 270 nm – ligand to- metal O–Fe charge-transfer
transitions of Fe3+ ions in a tetrahedral and/or octahedral environment of oxygen atoms,
which means that single Fe ions were present in cationic sites.
For the Cr- β sample :260, 360 and 460 nm - charge transfer transitions from O2- to Cr6+
of chromate and dichromate species. The presence of polymerized chromates can be ruled
out since the corresponding IR band at 948 cm-1 is absent. Bands associated to Cr3+, e.g. at
420 and 600–620 nm, are not detected in the sample, thus indicating a complete oxidation
of Cr3+ to Cr6+ during the calcination.
For the Co- β sample : Co2+ cations in zeolite beta are reported to prefer exchange
positions where two Bronsted sites are located in the vicinity, thus resulting as observed in
the disappearance of the band at 872 cm-1 The broad band at about 515 nm in the UV–Vis
spectrum could be assigned to octahedrally coordinated Co2+ ions. The absence of bands at
530–650 and 410 nm ruled out the presence of tetrahedrally coordinated Co2+ and Co3+
ions. In our case, the color of the Co-β sample was pale pink, thus confirming the presence
of Co2+ ions as the dominant species.
UV–Vis spectra of zeolites
For the Zn-β sample : zinc cations are best stabilized when the divalent charge is directly
balanced by two framework Si–O–Al groups, that is, coordinated to 6-membered rings of
oxygen atoms, as well as in a second site consisting of [Zn–O–Zn]2+ balanced by aluminum
atoms which are further apart.
For the Mn-β sample : More specifically, Mn2+-exchanged zeolites exhibit a band at 255
nm assigned to O2- to Mn2+ charge-transfer transition. In fact, the absence of bands at ca.500
nm seemed to rule out the presence of Mn3+ species. The absorption of MnO2 would have
covered the whole visible region and so it was not probably present in the exchanged zeolite
For the Ni-β sample : 245 nm - charge transfer from O2- to Ni2+ and a shoulder at ca. 320
nm characteristic of NiO . Additional broad bands centered at 390 and 650 nm were referred
to d–d bonding of octahedrally coordinated Ni2+ ions.
For the Cu-β sample : ca. 220 nm - charge-transfer between mononuclear Cu2+ ions and
oxygen, ca. 800 nm assigned to d–d transitions of isolated distorted octahedral Cu2+ ions,310
and 370 nm - ligand-metal charge transfer band for square plane clustered copper oxide
species similar to highly dispersed CuO.
The interaction between the zeolitic framework and these cations follows the sequence:
Mn, Ni < Co, Zn < Cu.
27Al
NMR spectra of zeolites
55 and 0ppm - framework tetrahedral
& octahedral aluminum Species.
35 ppm and -10 ppm pentacoordinated aluminum (AlV) &
to AlVI in very (distorted octahedral)
(a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β, (f) Ni-β,(g) Cu-β and (h) Zn-β.
Pyridine TPD curves
(a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β, (f) Ni-β,(g) Cu-β and (h) Zn-β.
Acetonitrile TPD curves
(a) H-β, (b) Cr-β, (c) Mn-β, (d) Fe-β, (e) Co-β, (f) Ni-β,(g) Cu-β and (h) Zn-β.
Pyridine TPD adsorption
Pyridine adsorbs at both Bronsted and Lewis acid sites whereas acetonitrile, a much
weaker base, interacts more selectively with Lewis acid sites.
Upon ion exchange with Zn2+, Mn2+, Ni2+ and Co2+, a decrease in the number of
medium and strong acid sites while the population of weak acid sites increased.
In the case of Cu- β, all strong acid sites were lost due to its high exchange degree
while those of weak and medium strength became the most important.
The samples Fe- β and Cr- β due to higher dealumination produced in the former
weak acid sites predominated in Fe- β , whereas medium acid sites prevailed in Cr- β.
In the acetonitrile TPD : 265 and 400 °C (zeolite H- β ) - acetonitrile chemisorbed on
Bronsted acid sites and on Lewis acid Sites. All divalent cations showed desorption
bands at ca. 600 °C which can be ascribed to acetonitrile adsorbed on bare Me2+
cations with an open coordination sphere and, as a consequence, these transition
cations exhibit strong Lewis acid properties.
Product composition (%wt) in the methanol conversion to
hydrocarbons over the exchanged zeolites at 400 °C.
Overall CH3OH–CH3OCH3 conversion vs. exchange degree for different
transition metal exchanged β zeolites
The linear trend (dotted line) for alkaline and alkaline-earth exchanged
β zeolites is also given for comparison.
Products composition (% wt) in the methanol conversion
to hydrocarbons over the Cu-β zeolite at 400 ° C.
Products wt(%)
Cu-β
CO2
2.1
Acetaldehyde
2.4
CH3OCH3
83.3
CH3OH
12.1
Cation exchange with transition metals (Cr3+,Mn2+, Fe3+, Co2+, Ni2+, Cu2+ & Zn2+)
Constitutes an easy and versatile way of modulating the acid-base and redox
properties of zeolite beta.
Great differences have been observed in the final materials upon exchange with divalent
and trivalent cations, trivalent cations caused an extensive dealumination, particularly in the
case of Fe3+ , which is mostly introduced to the zeolite as oxide-like clusters & Fe2 O3
nanoparticles. Cr3+ species retained in zeolite beta were completely oxidized by
calcination, thus giving rise to Cr6+ as chromate and dichromate species.
Unlike, divalent cations remained exchanged preferentially, although they also appeared
in the form of oxides to some extent, depending on the particular metal. Except for Mn2+,
transition metal cations favored the incorporation of Al defects back into the zeolite beta
framework, as described for alkaline and alkaline-earth cations.
The presence of all these metal species influenced the activity and selectivity on the
transformation of methanol to hydrocarbons, which is typically catalyzed by Bronsted
acid sites. Therefore, Ni2+ and Co2+ exchanged on zeolite beta seemed to have
predominantly neutralized Bronsted acid sites and so their activity was low. The same
happened for Cu2+, which gave a very low conversion due to its high exchange
degree, but it also yielded some oxidation products.
Remarkably, the Mn2+, Zn2+, Fe3+ and Cr6+ species provided higher activities than
expected because these cations seem to promoteoligomerization (Mn2+ , Fe3+ and Cr6+ )
or aromatization (Zn2+) reactions of the lower alkenes produced through the Bronsted
catalysis during the first stages of the methanol conversion. They are promising
candidates for future investigations in these reactions, in particular those materials
possessing Cr6+ species, in virtue of its excellent performance.
Thank You