Transition metal exchanged  zeolites: characterization of the metal state and catalytic application in the methanol conversion to hydrocarbons R.Vijaya Shanthi ( ) Dolores Esquivel, Aurora J. Cruz-Cabeza, César Jiménez-Sanchidrián, Francisco J. Romero-Salguero, Microporous and Mesoporous Materials 179 (2013) 30–39.

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 Co 2+, Fe 3+, and Cu selective catalytic reduction (SCR) of NO by ammonia. Ni isomerization and ring-opening of styrene oxide. Cr oxidative dehydrogenation of propane. Mn liquid phase epoxidation of alkenes with aqueous hydrogen peroxide. Zn 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, 27 Al 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.

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. Sample preparation (Ion exchanged method)

DRIFT 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. 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.

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.

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 Fe 3+ 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 O 2- to Cr 6+ 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 Cr 3+, e.g. at 420 and 600–620 nm, are not detected in the sample, thus indicating a complete oxidation of Cr 3+ to Cr 6+ during the calcination. For the Co- β sample : Co 2+ 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 Co 2+ ions. The absence of bands at 530–650 and 410 nm ruled out the presence of tetrahedrally coordinated Co 2+ and Co 3+ ions. In our case, the color of the Co-β sample was pale pink, thus confirming the presence of Co 2+ 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 O 2- to Mn 2+ charge-transfer transition. In fact, the absence of bands at ca.500 nm seemed to rule out the presence of Mn 3+ species. The absorption of MnO 2 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 O 2- to Ni 2+ 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 Ni 2+ ions. For the Cu-β sample : ca. 220 nm - charge-transfer between mononuclear Cu 2+ ions and oxygen, ca. 800 nm assigned to d–d transitions of isolated distorted octahedral Cu 2+ 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. UV–Vis spectra of zeolites

27 Al NMR 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. 55 and 0ppm - framework tetrahedral & octahedral aluminum Species. 35 ppm and -10 ppm - pentacoordinated aluminum (A lV ) & to Al VI in very (distorted octahedral)

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 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 Zn 2+, Mn 2+, Ni 2+ and Co 2+, 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 Me 2+ cations with an open coordination sphere and, as a consequence, these transition cations exhibit strong Lewis acid properties. Pyridine TPD adsorption

Product composition (%wt) in the methanol conversion to hydrocarbons over the exchanged zeolites at 400 °C.

Overall CH 3 OH–CH 3 OCH 3 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 wt(%)Cu-β CO Acetaldehyde2.4 CH 3 OCH CH 3 OH12.1 Products composition (% wt) in the methanol conversion to hydrocarbons over the Cu-β zeolite at 400 ° C.

Cation exchange with transition metals (Cr 3+,Mn 2+, Fe 3+, Co 2+, Ni 2+, Cu 2+ & Zn 2+ ) 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 Fe 3+, which is mostly introduced to the zeolite as oxide-like clusters & Fe 2 O 3 nanoparticles. Cr 3+ species retained in zeolite beta were completely oxidized by calcination, thus giving rise to Cr 6+ 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 Mn 2+, 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, Ni 2+ and Co 2+ exchanged on zeolite beta seemed to have predominantly neutralized Bronsted acid sites and so their activity was low. The same happened for Cu 2+, which gave a very low conversion due to its high exchange degree, but it also yielded some oxidation products. Remarkably, the Mn 2+, Zn 2+, Fe 3+ and Cr 6+ species provided higher activities than expected because these cations seem to promoteoligomerization (Mn 2+, Fe 3+ and Cr 6+ ) or aromatization (Zn 2+ ) 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 Cr 6+ species, in virtue of its excellent performance.

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