Alexander G. Stepanov

Dynamics of Benzene Rings in MIL‐53(Cr) and MIL‐47(V) Frameworks Studied by 2H NMR Spectroscopy

Metal–organic frameworks (MOFs) combine metal oxide clusters and organic linkers in almost infinite manners. Because the variability in pore dimensions and chemical composition is larger than in zeolites, this class of hybrid porous solids has major potential applications in the fields of adsorption or separation of gases and liquids, catalysis, drug delivery, and others. 5] A remarkable feature of some MOFs is their flexibility. The MIL-53 type (MIL: Materials of Institut Lavoisier) is one of the best representatives of the “breathing” MOFs. This series of metal(III) terephthalates of formula (M(OH)·O2CC6H4CO2) (M = Al, Cr, Fe, Ga), is built up from chains of metal-centered octahedra sharing OH vertices, which are linked in the two other directions by terephthalate groups to create one-dimensional (1D) lozenge-shaped tunnels. Depending on the guest entrapped in the pores, MIL-53(Cr) has been shown to exhibit different crystalline states, corresponding to different pore openings, while the framework topology remains unchanged. The assynthesized form contains disordered terephthalic acid molecules in the pores and has a cell volume of 1440 . Upon calcination, the free acid is removed and the cell volume increases to 1486 , while it decreases to 1012 3 on hydration. This transition between large-pore (LP) and narrow-pore (NP) forms corresponding to anhydrous and hydrated states, respectively, is reversible. On the other hand, MIL-47(V), which is isostructural to MIL-53-LP but without the OH groups, has a rigid framework. 8] As a consequence, MIL-47(V) exhibits only type I adsorption isotherms, as expected for gas adsorption in a rigid nanoporous material. In contrast, steps in the adsorption isotherms of CO2 and various hydrocarbons occur in MIL-53(Cr) at room temperature, and are associated with two consecutive structural transitions. The transition from the LP to the NP form is observed at low concentration, and the NP-to-LP transition at higher loadings. The structural switching were evidenced by X-ray powder diffraction, adsorption microcalorimetry, and simulations. 10] This phenomenon is guest-dependent; for example, MIL-53(Cr) behaves as a rigid framework (LP form) on adsorption of certain small species (H2 and CH4), but is flexible for others (Xe). The magnitude of breathing can be related to the van der Waals volume of the guest molecule: the smaller the molecule the more MIL-53(Cr) is able to breathe. 11] The largest amplitude of breathing (ca. 40%) is obtained for the empty material. Surprisingly, the LP–NP transition can occur without any guest, simply by changing the temperature. This conclusion was drawn from elastic and inelastic neutron scattering measurements, which also showed the existence of a large temperature hysteresis in MIL-53(Al). It was suggested that the low-energy librational modes of the aromatic ring are coupled to the structural transition. In MOF-5, the softest twisting or torsional modes of benzene were calculated at similar energies. Although the energy of these modes is very low (20–80 cm ), no rotation of benzene was observed by quasi-elastic neutron scattering (QENS), the timescale of which ranges typically between 10 13 and 10 8 s. The energy barrier for 1808 (p) flips was indeed found to be relatively large, with estimates varying between 51.8 and 63 kJ mol . On the longer timescale of H NMR spectroscopy (> 10 7 s), the benzene rings in MOF-5 were found to be stationary at temperatures below 298 K, but p flips were observed at higher temperatures, and all benzene rings execute this motion at 373 K. More recently, an activation energy of 47.3 kJ mol 1 was obtained for the p-flip rate constant in MOF-5. In MIL-47 and MIL-53 frameworks, which have 1D pore systems, the dynamics of the benzene rings could more strongly influence the adsorption and transport properties compared to a MOF with 3D pore connectivity. For small molecules, 1D diffusion has been evidenced in MIL-47(V) and MIL-53(Cr) by QENS. Moreover, in molecular simulations of these two MILs, the framework was taken to be rigid, and no switching of molecules from one channel to another was observed. On a much longer timescale, which is relevant for macroscopic measurements, the rotational motion of benzene could play a role. We used solidstate H NMR to determine the flipping rate of benzene rings in MIL-47(V) and MIL-53(Cr) frameworks. An additional [*] D. I. Kolokolov, Dr. H. Jobic Universit Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de LYON 2. Av. A. Einstein, 69626 Villeurbanne (France) E-mail: herve.jobic@ircelyon.univ-lyon1.fr

Understanding Methane Aromatization on a Zn-Modified High-Silica Zeolite†

Methane is the principle constituent of natural gas and also the most inert of the saturated hydrocarbons. Its conversion into more commercially useful chemicals and liquid fuels represents one of the most important challenges in modern catalysis. Coaromatization of methane and light hydrocarbons (paraffins and olefins) at 700–800 K is one of the alternative methods for the conversion of methane. It has been reported recently that the conversion of methane during coaromatization with higher alkanes or alkenes (C2–C6) at 670–870 K in the presence of bifunctional catalysts (mainly, high-silica ZSM-5 or ZSM-11 zeolites, modified with gallium or zinc) may reach 20–40%. However, previous experiments in which C-labeled methane was used did not confirm the presence of the C-labeled atoms from the methane in the aromatization products. This result gave rise to scepticism as to whether methane-involved aromatization occurred at all. Herein we report that transfer of isotopically C-labeled atoms from methane into the aromatic products does occur to a high degree during the co-conversion of methane and propane on the Zn-modified high-silica zeolite BEA. We have identified the nature of the intermediates formed during the activation of methane and established how the conversion of methane into aromatic compounds occurs. Figure 1 shows the C CP/MAS NMR spectra of the products (in their adsorbed state on the zeolite catalyst) which are formed from methane and propane at 823–873 K. The spectrum of the products formed from unlabeled CH4 and C3H8 exhibits only a weak signal at d = 8.5 ppm from methane (Figure 1a). When unlabeled CH4 was replaced with CH4, the spectrum of the reaction products showed two new signals, which undoubtedly belong to hydrocarbons containing the C labels from the CH4 (Figure 1b). The carbon atoms of the C-labeled methane molecules are incorporated into both methyl groups (signal at d = 20 ppm) and aromatic rings (d = 130 ppm) of the methyl-substituted aromatic compounds (Figure 1b,c). According to GC-MS analysis of the products extracted from the zeolite, a mixture of benzene and toluene, as well as mand p-xylenes (BTX) with C enrichment is formed from CH4 and unlabeled propane at 773–823 K (Figure 2). The presence of singly (C1), doubly ( C2), and triply ( C3) labeled molecules of BTX (Figure 2b) provides proof for the incorporation of C-labeled methane into both the methyl groups and the carbon atoms of the aromatic rings of BTX. Neat propane converts on Zn/H-BEA into a mixture of aromatic products and methane at lower temperature (573– 723 K; Figure 1d). According to the H MAS NMR spectra, approximately 1.6–1.7 methane molecules are produced per reacted propane molecule. The possible overall reaction which describe the aromatization of propane can be described by Equation (1). Figure 1. C CP/MAS NMR spectra of products in the adsorbed state formed from methane and propane on zeolite Zn/H-BEA: a) from CH4 and C3H8 at 823 K for 15 min; b,c) from CH4 and C3H8 at 823 K for 15 min (b) and at 873 K for 15 min (c); d) from [1-C]C3H8 at 723 K for 15 min. Asterisks (*) in Figures 1, 3, and 4 denote the spinning side bands.

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with13C in CH3 or C-O groups (t-BuOH[2−13C2] andt−BuOH[1-13C]), as well as selectively deuterated in methyl groups (t-BuOH[2-2H9]), was studied on H-ZSM-5 zeolite simultaneously with13 C CP/MAS and2H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2−13C1] andt-BuOH[1−13C] give rise to identical13C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the “side” reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to2H NMR, the proton of the acidic ≡Al-OH-Si≡ group of the zeolite is transferred into the CH2= group of the octene-1 molecule. (2) According to13C NMR the13C label inserted into the terminal CH2= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.

Significant influence of Zn on activation of the C-H bonds of small alkanes by Bronsted acid sites of zeolite.

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.

13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a13C nucleus in the CH2 group (i-BuOH[1−13C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using13C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.

In situ high temperature MAS NMR study of the mechanisms of catalysis. Ethane aromatization on Zn-modified zeolite BEA

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.

In situ 1H MAS NMR studies of the H/D exchange of deuterated propane adsorbed on zeolite H‐ZSM‐5

The hydrogen exchange in the propane‐d8 loaded zeolite H‐ZSM‐5 was monitored by in situ 1H MAS NMR spectroscopy within the temperature range 457–543 K. Measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that both methyl and methylene groups of alkane are involved in the exchange. The comparison of the experimentally obtained apparent activation energies for the exchange in methyl groups (108 ± 7 kJ mol-1) and methylene groups (117 ± 7 kJ mol-1) with theoretical values for methane and ethane supports the assumption that the H/D exchange for methyl and methylene groups takes place via a pentavalent transition state.

13C solid state NMR evidence for the existence of isobutyl carbenium ion in the reaction of isobutyl alcohol dehydration in H-ZSM-5 zeolite

Using two-dimensionalJ-resolved and CP/MAS13C NMR, the pathway for the transfer of the13C label from the CH2 group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydration inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH2 group of the isobutyl silyl ether reaction intermediate (IBSE), and then into the CH and CH3 groups of the isobutyl fragment (-CH2CH(CH3)2) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl carbenium ion is formed as a reaction intermediate or transition state during the transformation of isobutyl silyl ether into butene oligomers.

n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13C-label scrambling as studied by in situ 13C MAS NMR and ex situ GC-MS

Abstract Using 13 C MAS NMR, conversion of selectively 13 C-labeled n -butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13 C label in the n -butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13 C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13 C-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n -butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation.