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To establish the effect of indium species introduced in the zeolite on protolytic activation of the alkane by zeolite Brønsted acid sites (BASs), the kinetics of H/D hydrogen exchange between n-butane-d10and the BAS of indium-modified zeolite H-BEA has been monitored by 1H magic-angle spinning nuclear magnetic resonance spectroscopy in situ at 455–573 K. Regioselectivity in the methyl groups of the alkane, high rate, and low activation energy (52–64 kJ mol–1) of the H/D exchange in the presence of either InO+or In+cations in the zeolite indicate that the mechanisms of protolytic activation and the H/D exchange for n-butane differ from that on pure acid-form zeolites. The promoting effect of InO+cations on the reaction kinetics is more essential than that of In+cations. It is proposed that InO+sites located in the vicinity of BASs provide fast exchange with the methyl groups of the alkane in the initial period of the kinetics. The BASs that are remote from InO+sites are involved in the H/D exchange with the alkane at the final period of the kinetics by protium transfer from the remote BAS to BAS vicinal to InO+sites. The transfer is realized by the H/D exchange between the remote BAS and BAS vicinal to InO+sites with the assistance of a minor quantity of molecular hydrogen (H2), evolved at n-butane dehydrogenation and working as the vehicle for this transfer.

In-modified ZSM-5 zeolites are promising catalysts for light alkane dehydrogenation and aromatization. The mechanism of alkane activation and transformation on In-containing zeolites is still under discussion. In this work, n-butane transformation on H-ZSM-5 zeolite modified with InO+sites has been investigated by 13C magic-angle spinning NMR and Fourier transform infrared (FTIR) spectroscopy at 296–673 K. It is established that n-butane conversion occurs by two parallel pathways: dehydrogenation to n-butene, followed by the formed alkene aromatization to simple aromatic hydrocarbons, and the direct oxidation of the alkane to C2–C6carboxylate surface species. For the dehydrogenation pathway, n-butane activation has been established by FTIR spectroscopy to occur through C–H bond dissociation on the reactive In═O bond of the InO+site. The oligomerization of the initially formed n-butene occurs by two pathways: (1) on InO+sites via the formation of the n-butene π-complex and carbanionic allyl-like species as the intermediates; (2) with the involvement of Brønsted acid site (BAS). The formed oligomers convert to aromatics on the BAS with the formation of methyl-substituted cyclopentenyl cations as the intermediates. As for the oxidation pathway, the formation of the carboxylate surface species implies the dissociative adsorption of both the C–C and C–H bonds of the alkane on InO+sites to give C2–C4oxyindium-alkyl species. The latter are oxidized by InO+to oxyindium-carboxylates, which can be further transformed to carboxylic acids by hydrolysis.

Selective dimerization of ethene to 2-butene on Zn2+-containing ZSM-5 zeolite (Zn2+/ZSM-5) at 296–523 K has been discovered. The intermediate but-3-en-1-ylzinc species is identified with 13C CP/MAS NMR and Fourier transform infrared spectroscopy. The density functional theory study of two alternative dimerization pathways reveals that the intermediate is formed with the involvement of the saturated bridged dimeric Zn–(CH2)4–O species. It is also shown that ethene conversion to 2-butene increases with the increase in the quantity of Brønsted acid sites in Zn2+/ZSM-5 zeolite; however, the selectivity of the reaction decreases. The results obtained are of potential interest for developing industrially relevant Zn-containing zeolite catalysts for the selective conversion of ethene to 2-butene.

With regard to possible involvement of zeolite Brønsted acid sites (BAS) in the activation of methane molecules for methane transformation to methanol, the effect of different Cu(II) species loaded in the zeolite on the kinetic parameters of the reaction of H/D hydrogen exchange of the alkane with BAS of Cu-modified ZSM-5 zeolites has been investigated with 1H MAS NMR in situ at 533–563 K. It is found that the acceleration of the H/D exchange by 1 order of magnitude occurs for zeolite containing Cu2+cations (Z2Cu(II) sites) or [Cu3(μ-O)3]2+oxo-clusters (Z2[Cu3(μ-O)3] sites) compared to pure H-form zeolite. It is thus inferred that both Z2Cu(II) and Z2[Cu3(μ-O)3] sites exhibit the promoting effect of copper on the activation of methane C–H bonds by BAS. Acceleration of the H/D exchange is rationalized by the change of the mechanism of the exchange accepted for the H-form zeolites for the mechanism that involves the formation of a transient molecular complex of methane with Cu(II) species, preceding the H/D exchange reaction. The formation of the complex of methane with both Z2Cu(II) and Z2[Cu3(μ-O)3] sites is confirmed by DRIFTS. BASs with a higher strength than in H-ZSM-5, generated in the zeolite at copper loading, are concluded to not be responsible for the H/D exchange reaction acceleration.

With the aim of clarifying the effect of Zn species of different nature (small clusters of ZnO and Zn2+cations) on the activation of alkane C–H bonds by Zn-modified zeolite, the mechanism of the H/D hydrogen exchange of n-butane-d10with Brønsted acid sites (BASs) of ZnO/H-BEA and Zn2+/H-BEA zeolites has been investigated with 1H MAS NMR in situ at 413–468 K. It is established that both ZnO and Zn2+species provide acceleration of the exchange in the methyl groups with a decreased activation energy compared to the exchange in n-butane on pure acid-form zeolite. The effect of Zn2+cation is more essential. The acceleration has been attributed to the formation of transient complexes of the alkane methyl groups with Zn species preceding the exchange with BAS. For ZnO/H-BEA, the involvement of the methylene groups in the exchange is related to the alkane dehydrogenation to form butene followed by protonation and hydride shift reaction in the formed butyl cation. For Zn2+/H-BEA zeolite, an enhanced rate of the exchange in the methylene groups is due to the contribution of two pathways: direct exchange with the assistance of Zn2+cations and through the intermediate formation of n-butene. We have concluded that the observed peculiarities of the kinetics of H/D hydrogen exchange of n-butane on Zn2+/H-BEA and ZnO/H-BEA zeolites could be rationalized in terms of a common kinetic scheme realized for two zeolite samples with different kinetic parameters, rate constants, and activation energies for identical reaction steps.

The intermediates formed upon the interaction of methane with Cu-modified ZSM-5 zeolites (Cu/H-ZSM-5) have been analyzed with solid-state NMR spectroscopy and DFT methods. Methane activation by Cu/H-ZSM-5 zeolites gives rise to three distinct surface methoxy-like species (−O–CH3) detected by 13C MAS NMR spectroscopy with specific chemical shifts in the range of 53–63 ppm. DFT calculations on representative cluster models of different sites potentially present in Cu/H-ZSM-5 have been used to assign these signals to (i) methanol adsorbed on two neighboring Cu sites (Cu–(HOCH3)–Cu, 62.6 ppm), (ii) methanol adsorbed on zeolite Brønsted acid sites (52.9 ppm), and (iii) lattice-bound methoxy groups (Si–O(CH3)–Al, 58.6). The formation of these methoxy-like intermediates depends on the Cu loading and, accordingly, the type of Cu species in the Cu/H-ZSM-5 zeolite. For the sample with low (0.1 wt %) Cu loading containing exclusively mononuclear isolated Cu species, only the intermediates ii and iii have been detected. The Cu-bound intermediate (i) is formed upon methane activation by multinuclear Cu sites featuring Cu–O–Cu bridging moieties present in the materials with relatively higher Cu loading (1.38 wt %). The presented results indicate that methane activation by Cu/H-ZSM-5 can be promoted by both mono- and multinuclear Cu species confined in the zeolite matrix.

With regard to establishing the effect of the nature of Zn species loaded in a zeolite on the aromatization of propane, the transformation of 13C-labeled propane (propane-1-13C and propane-2-13C) has been monitored by 1H and 13C (CP) MAS NMR at 296–623 K for two zeolite samples, containing exclusively either isolated Zn2+cations (Zn2+/H-BEA zeolite sample) or small (ZnO)nclusters of zinc oxide (ZnO/H-BEA zeolite sample). It has been established that similar intermediates, π-complex of propene and σ-allylzinc species, are formed at propane transformation to aromatics on both zeolite samples. However, the formation of the identified intermediates and their evolution to final aromatic products have been detected to occur on Zn2+/H-BEA at lower temperature (by 100 K) as compared to the same transformation on ZnO/H-BEA zeolite. Analysis of the kinetics of propane transformation by 1H MAS NMR in situhas shown similar regularities for propane aromatization and hydrogenolysis on the different zeolite samples, but the reactions on Zn2+/H-BEA occur 10 times faster than on ZnO/H-BEA. This allows us to conclude on a higher efficiency of Zn2+cations compared to ZnO species for propane aromatization on Zn-modified zeolites.

The peculiar role of either Zn2+cations or small ZnO clusters located inside the zeolite pores in propylene transformation to aromatic hydrocarbons on Zn2+/H-ZSM-5 and ZnO/H-BEA zeolites has been analyzed. 13C MAS NMR and FTIR spectroscopic investigation has revealed similar properties of different Zn-sites with respect to propylene oligomerization. Both Zn2+cations and ZnO species are capable to convert propylene to higher C6–C9olefins which represent the precursors of simple benzene-based aromatics. The mechanism of such oligomerization has been inferred to include the following stages: (a) adsorption of propylene on the Zn-sites in the form of a π-complex; (b) transformation of the π-complex of propylene to σ-allylzinc surface species at elevated temperature; (c) formation of C6+olefins; (d) transformation of oligomeric C6+olefins to simple aromatic molecules (benzene, toluene, and xylenes). NMR evidences that Brønsted acid sites (BAS) of both zeolite samples are not involved in the oligomerization to a large extent although definitely contribute to oligomers formation. Involvement of BAS in propylene oligomerization on ZnO/H-BEA zeolite is higher because of their larger quantity in the sample. The oligomerization has been monitored to proceed mainly via allylzinc species owing to the olefin insertion into the reactive Zn–C bond. In such a way, selective formation of the C6–C9olefins occurs which accounts for a particular set of aromatic products detected for propylene transformation on Zn-modified zeolites.

With respect to the potential application of indium-modified zeolites for light alkane conversion to simple aromatic hydrocarbons and their oxidation to carboxylic acids, the pathways of isobutane transformation on the In/H-ZSM-5 zeolite have been investigated by a combination of solid-state NMR spectroscopy, FTIR spectroscopy, and density functional theory (DFT) calculations. It is found that isobutane undergoes activation on InO+sites of In/H-ZSM-5 zeolites via the “alkyl” pathway to yield isobutylindium species, which precedes the formation of isobutene at 296 K. Analysis of isobutene transformation on In/H-ZSM-5 shows the formation of allyl-like intermediates, alkoxy species, and cyclopentenyl cations from the alkene. This implies that oligomerization and aromatization of isobutene formed from the alkane occurred with the involvement of both InO+sites and Brønsted acid sites (BASs) at 473–573 K. At higher temperatures (T≥ 623 K), C2–C4surface carboxylic species have been found as the products of isobutane oxidation. It is inferred that the direct oxidation of isobutane molecules to carboxylate species does not occur, contrary to the earlier found cases with propane and n-butane transformation on In-modified zeolites. Additionally, DFT calculations have been used to analyze the localization of indium species in the ZSM-5 zeolite, the energies of isobutane and isobutene adsorption on differently located active sites (InO+and BAS), and the mechanisms of C–H bond activation in the alkane.

With the aim of discovering the effect of indium species on the protolytic activation of isobutane by Brønsted acid sites (BAS), the kinetics of H/D hydrogen exchange between isobutane-d10and BAS of the InO+/H-BEA zeolite was monitored by 1H MAS NMR spectroscopy. The exchange has been established to occur exclusively between methyl groups of isobutane-d10and BAS and demonstrates a significant induction period. Dehydrogenation of isobutane to isobutene is observed alongside the H/D exchange in the alkane. Kinetics of protium transfer from BAS to both deuterated isobutane and isobutene are interpreted in terms of the indirect cyclic mechanism of H/D exchange with isobutene as a key intermediate. It is inferred that indium sites perform isobutane dehydrogenation and strongly retain isobutene formed. This accounts for the prolonged induction period in the H/D exchange reaction kinetics. Indium sites located in the zeolite pores decelerate protium transfer from zeolite BAS to isobutane via an indirect H/D exchange mechanism and, contrary to the case with linear alkanes, are not able to facilitate the direct hydrogen exchange between BAS and methyl groups in isobutane.

Fourier transform infrared (FTIR) spectroscopy is broadly applied nowadays for probing the concentration and strength of acid sites in zeolite catalysts. Accuracy of direct determination of the quantity of different hydroxyl groups by FTIR method suffers from uncertainty of the integrated molar absorption coefficients, ε. The values of ε reported by different authors might differ by an order of magnitude. This paper provides an approach for reliable determination of the integrated molar absorption coefficients by combining 1H magic-angle spinning (MAS) NMR and FTIR spectroscopy techniques. The concentration of Brønsted acid sites for the series of H-ZSM-5 and H-ZSM-23 zeolite samples with different Si/Al ratio has been reliably established with 1H MAS NMR using methane and benzene as internal standards adsorbed on the studied samples. The data on the obtained concentration were further used to analyze same zeolite samples with FTIR spectroscopy and derive the information on the values of the integrated molar absorption coefficients. The coefficients ε have been reliably determined to be 3.06 ± 0.04 and 1.50 ± 0.06 cm μmol–1for the IR bands at 3605–3615 and 3740–3747 cm–1, respectively. The ε values are similar for both H-ZSM-5 and H-ZSM-23 zeolites. It is also established that the ε values are constant with respect to the concentration of hydroxyl groups for H-ZSM-5 and H-ZSM-23 zeolites. The determined coefficients ε can be further used for reliable assessment of zeolite Brønsted acidity with the aid of the widely available and relatively simple methodology of FTIR spectroscopy.

In relation to the reported methane activation on Zn-modified zeolite ZSM-5 at room temperature to afford the surface methoxy species by Xu et al. (Chem. Sci.2012, 3, 2932), the activation of methane on Zn2+-exchanged H-ZSM-5 zeolite in the absence and the presence of molecular oxygen has been studied with 13C magic angle spinning (MAS) NMR spectroscopy. It has been established that the methane activation on zinc cationic sites under nonoxidative conditions occurs exclusively by an “alkyl” pathway to form the surface zinc-methyl species. The addition of the molecular oxygen (dioxygen) to methane adsorbed on the Zn2+-exchanged H-ZSM-5 zeolite results in the surface methoxy and other oxygen-containing species, such as formate, acetaldehyde, and acetic acid. The formation of the surface methoxy species occurs by the oxidation with molecular oxygen of zinc-methyl species primarily formed on the zeolite surface. The Zn2+/ZSM-5 zeolite with full substitution of Brønsted acid sites (BAS) by Zn2+cations offers zinc-methyl species from methane at T≥ 523 K, whereas Zn2+/H-ZSM-5 with partial substitution (60%) of BAS produces zinc-methyl at room temperature. BAS promotes the formation and decomposition (by the sample evacuation) of zinc-methyl species on Zn2+/H-ZSM-5 at room temperature. Zinc-methyl is readily oxidized by the dioxygen additive to offer methoxy species already at room temperature. Thus, it has been shown that pure methane forms only zinc-methyl species upon its interaction with zinc cationic sites of Zn2+-exchanged H-ZSM-5 zeolite, while the surface methoxide could be formed only by the interaction of zinc-methyl with dioxygen that might be contained in the reactive methane.

ATR-FTIR spectroscopy was used in situ to study nine unsaturated polyketones derived from cis-1,4-polybutadiene rubber, each containing a different concentration of carbonyl groups, under high-pressure CO2conditions (up to 100 bar). The study was aimed to systematically determine the relationship between the concentration of carbonyl groups in the polyketones and their ability to absorb CO2and swell. A linear relationship between increasing carbonyl concentration and the overall degree of swelling and CO2sorption was observed for polyketones with a concentration of carbonyl groups below a specific value based on quantitative analysis from the ATR-FTIR spectra. However, polyketones, which had the highest concentration of carbonyl groups, did not follow this correlation. Instead, there was evidence of intermolecular interactions between the carbonyl groups in the polymer chains, which decreases the total CO2sorption capacity and inhibited swelling. The effect of the different molecular weights of polymer was also studied with respect to polyketone swelling and CO2sorption. No correlation was observed when comparing polymers with different molecular weights but contained a similar concentration of carbonyl groups. Hence, the main physical properties that affect the overall swelling and CO2sorption into polyketone samples were quantitatively determined using ATR-FTIR spectroscopy.

Methane activation pathways as well as methane involvement in the reaction of ethylene aromatization on In-modified H-ZSM-5 zeolite (In/H-ZSM-5) have been studied with solid-state NMR spectroscopy. The state of indium in In/H-ZSM-5 in dependence of the zeolite activation procedure, reductive or oxidative, has been analyzed with X-ray photoelectron spectroscopy (XPS). On the basis of 1H MAS NMR analysis of the evolution of the quantity of Brønsted acid sites (BAS) and XPS analysis of the state of indium in dependence of zeolite activation procedure, it has been inferred that indium exists in the form of either In+or InO+isolated cationic species in the zeolite. Methane interaction with different indium cationic species has been analyzed with 13C MAS NMR spectroscopy. In+species has been concluded to be inactive, whereas InO+species provides dissociative adsorption of methane to afford primarily the oxyindium–methyl species. The secondary products of oxyindium–methyl species transformation are oxyindium–methoxy, ethane, formate, and acetaldehyde species. Methane can be involved in the reaction of ethylene aromatization on In-modified zeolite H-ZSM-5. This involvement is provided by the reaction of the surface oxyindium–methoxy species with simple aromatic molecules formed from ethylene. Preliminarily, oxyindium–methoxy species are generated by the interaction of oxyindium–methyl species with InO+cations.

With regard to a general interest in methane utilization in a rational way the activation and transformation of methane on Ag-modified zeolite ZSM-5 (Ag/H-ZSM-5) have been studied with solid-state NMR. The activation of methane occurs by dissociation of the C–H bond on silver cations via the “carbenium” pathway: methane C–H bond cleavage results in the methoxy groups (O–CH3) and possibly silver-hydride species (Ag–H). The formation of surface methoxy groups on Ag/H-ZSM-5 has been detected experimentally with 13C CP/MAS NMR at 508–623 K for the first time. A comparative analysis of the kinetics of the H/D exchange between methane and acid hydroxyl groups for H-ZSM-5 and Ag/H-ZSM-5 zeolites reveals a significant promoting effect of silver cations on the H/D exchange reaction and therefore on methane activation. This effect has been rationalized in terms of reversible methane dissociation on the surface of Ag/H-ZSM-5 zeolite and further involvement in the exchange of the methoxy groups and the silver-hydride species. Ethane represents the first intermediate product of methoxy group transformation. It is formed by the reaction of a methoxy group with methane. Further, dehydrogenation of ethane offers ethene, producing immediately π-complexes with Ag+cations, which are stable at temperature as high as 673 K. At 823 K π-complexes decompose and ethene undergoes oligomerization, cyclization, dehydrogenation, and aromatization to give benzene. In the presence of methane, ethene π-complexes decompose and become involved in oligomerization and aromatization reaction at lower temperature, already at 673 K. Methane is also involved in the reaction of coaromatization with ethene. This involvement occurs by the alkylation of aromatics, formed from ethene, with methane. Further demethanation of methylbenzenes in the presence of dihydrogen evolved at the stages of ethene transformation to aromatics produces benzene as the main reaction product.

The interaction of olefins with metal oxides affords surface species which are considered to be the intermediates in olefin isomerization reactions. The nature and the structure of these intermediates after earlier characterization by IR spectroscopy still remain debatable. In this paper by using 13C solid-state NMR spectroscopy we have characterized the surface intermediates formed from propene, n-butene, and isobutene on γ-Al2O3and α-Ga2O3, based on analysis of specific chemical shifts expected for similar organometallic or oxygenated compounds. NMR clearly shows that both allylic and alkoxy intermediates are simultaneously formed on two studied metal oxides. Propene affords isopropoxy and allylic intermediates on both oxides. Allyl formed on alumina is bound to the Al3+cations of metal oxide surface in a η1,η2-like fashion, whereas allyl on α-Ga2O3is bound to the Ga3+cation exclusively in a η1-like fashion. n-Butene gives 2-butoxy species for both metal oxides, whereas allylic species (σ-allyl) was identified for this olefin only for γ-Al2O3. Adsorption of isobutene results to the formation of tert-butoxy and allylic species on both metal oxides. π-Allyl with η3-like fashion of allyl bonding to the oxide surface is formed in the case of γ-Al2O3, whereas σ-allyl is formed in the case α-Ga2O3. Both allylic and alkoxy species can be involved as intermediates in a double bond shift reaction of olefins on metal oxide surfaces.

We declare after 40 years of a convention on the structure of allylic intermediates on ZnO surface that the surface allylzinc species exists in the form of σ,π-allyl rather than exclusively as the π-allyl complex as implied by Dent and Kokes (J. Am. Chem. Soc., 1970, 92, 1092). Olefin interaction with ZnO has been monitored by 13C cross-polarization magic angle spinning NMR. NMR shows that dissociative adsorption of propene and n-butene on ZnO surface results in the formation of carbanionic allylzinc complexes with a η1,η2-like fashion of allyl bonding to the zinc cations (σ,π-allyls). Formation of π-allylzinc complex (η3-like fashion of bonding to Zn) only as the minor species was also observed in case of propene.

The molecular mobility of deuterated tert-butyl alcohol (TBA) adsorbed in MFI framework type (silicalite-1 and ZSM-5 zeolite) has been studied by use of 2H NMR spectroscopy in the range of 106–453 K. In H-ZSM-5, the reorientation of the molecule as a whole is strongly restricted (τC≪ Q0–1≈ 10–6s) by hydrogen bonding to Brønsted acid sites (BAS). Being adsorbed to BAS, the motion of TBA molecules is described by intramolecular rotations around two successive C3and C3′ axes (CD3–C and C–O bonds). The activation energy for the methyl groups rotation around the C–O bond (Ea= 8.0 ± 1.6 kJ mol–1) is two times lower compared to that in solid TBA (J. Phys. Chem. A2011, 115, 7428). This shows that the motion of the butyl fragment of TBA in MFI framework is less restricted compared to the case of solid TBA. In silicalite-1, the TBA molecule is additionally involved into reorientational motions as a whole: one of the motions represents an exchange among orientations provided by the directions of four framework channels driven by the translational jump diffusion. The activation barriers for the jump diffusion between two zigzag channels (Ea= 7 ± 2 kJ mol–1) and between zigzag and straight channels (Ea= 5 ± 1 kJ mol–1) are of the similar values. The other motion represents a large-amplitude wobbling of the TBA molecule localized at a channel intersection site cavity. This motion is described by a fast restricted wobbling of the molecular axis in a sphere sector. The wobbling boundaries become gradually broader as temperature increases, allowing thus a larger accessible space for the TBA molecule to explore. The anisotropy of this motion persists even at 453 K, revealing the presence of a strong intracavity barrier that blocks the TBA from free rotational diffusion inside the cavity. This study demonstrates that the 2H NMR analysis of both line shape and spin–lattice relaxation represents a powerful tool to investigate the particular pore confinement effect on the molecular mobility of TBA adsorbed in the MFI framework.

Kinetics of H/D hydrogen exchange between C1–n-C4alkanes and Brønsted acid sites (BAS) of both the pure acid-form zeolite BEA (H-BEA) and Ga-modified zeolite BEA (Ga/H-BEA) was monitored by 1H MAS NMR spectroscopy in situ at 423–563 K. Comparative analysis of the rates of the exchange for H-BEA and Ga/H-BEA zeolites reveals a remarkable increase of the rate by 1–2 orders of magnitude, decrease of activation energy, and an appearance of regioselectivity of the exchange into the methyl groups of C3-n-C4alkanes upon modification of H-BEA zeolite with gallium. These data identify an evident promoting effect of Ga on activation of alkane C–H bonds by BAS. The effect has been rationalized by preliminary dissociative adsorption of alkanes on gallium(III) oxide species inside zeolite pores to form gallium-alkyl species, which are further involved in the exchange with neighbor BAS. Involvement of both BAS and Ga species in alkane activation accounts for earlier suggested synergistic effect (Buckles, G., et al. Catal. Lett.1991, 11, 89) of both BAS and gallium species in alkane activation and aromatization on Ga-modified high silica zeolites.

By using 13C solid-state NMR spectroscopy and GC-MS analysis, the activation of methane and coaromatization of methane and propane have been monitored on gallium-modified zeolite BEA at 573−823 K. A noticeable degree involvement of the 13C-label from methane-13C into the aromatic reaction products (benzene, toluene) has been demonstrated. The major intermediate of the methane activation represents gallium-methyl species, which are formed by methane dissociative adsorption on Ga2O3species of the zeolite. The minor species of methane activation, Ga-methoxy groups, provide the involvement of methane into aromatics by the methylation of aromatic molecules, which are generated exclusively from propane, by the mechanism of electrophilic substitution. Ga-methyl species can serve as methylating nucleophilic agent for the reaction of nucleophilic substitution with participation of aromatic molecules, which contain the electron-withdrawing substitutes.

The conversion of propane (propane-1-13C and propane-2-13C) on Zn/H-BEA zeolite at 520−620 K has been studied by 1H and 13C (CP) MAS NMR. Propene adsorption complex with zinc sites (π-complex) and σ-allylzinc species as intermediates have been identified in the course of propane conversion to aromatics. The mechanism leading to the formation of methane and ethane, which are constituents of an undesirable route in propane conversion, has been examined by kinetic modeling of the expected reaction network based on in situ 1H MAS NMR kinetic measurements of the reaction performance. The pathways for propane aromatization and hydrogenolysis have been proposed. Hydrogenolysis of propane has been concluded to occur with the involvement of both Brønsted acid sites and Zn sites.

In-modified zeolites possess promising catalytic properties for light alkane dehydrogenation and aromatization. However, the role of different indium species, which are present in zeolite pores after activation, remains unknown. Here, the transformation of propane on BEA zeolite containing either In+or InO+species in zeolite pores has been monitored with 13C MAS NMR at 298–773 K. It is inferred that In+/H-BEA zeolite with In+sites is inactive for alkane conversion at T< 723 K. The transformation of the alkane on InO+/H-BEA zeolite occurs by two parallel routes: dehydrogenation of propane followed by the formed alkene aromatization to simple aromatic hydrocarbons and the alkane oxidation resulting in C2–C3carboxylic acids. Propane activation by either C–H or C–C bond cleavage on InO+sites and pathways of carboxylic acid formation from the products of the alkane dissociative adsorption are discussed.

The pathways of propane molecule activation on indium-modified zeolite have been studied by 1H MAS NMR spectroscopy in situat temperatures of 453–573 K. The kinetics of H/D hydrogen exchange between propane-d8and Brønsted acid sites (BAS) of the H-BEA zeolite, modified with indium species in the form of either InO+or In+cations (InO+/H-BEA and In+/H-BEA samples), were monitored. It is shown that the presence of either InO+or In+species in zeolite pores provides regioselectivity of the H/D exchange in methyl groups of the propane molecule. For the InO+/H-BEA zeolite, the process is characterized by greater rates and a lower activation energy compared to those for the unmodified H-form zeolite. The two-exponential kinetics of the exchange for InO+/H-BEA is rationalized in terms of two mechanisms: a direct mechanism assisted by the InO+species and an indirect mechanism initiated by a propene intermediate. Both mechanisms provide the regioselectivity of the exchange and low activation energy. In+/H-BEA zeolite demonstrates less activity with regard to the rate of the exchange, but it shows a lower activation energy compared to H-BEA zeolite. It is inferred, for In+/H-BEA, that the mechanism of the direct regioselective exchange competes for the same BAS with the mechanism of unselective H/D exchange. The latter results in a specific growth of protium concentration in the methyl and methylene groups of propane-d8upon the increase of the reaction temperature.

Establishing the role of metal species in the reaction of alkane aromatization on metal-modified zeolites is a challenging task aiming at understanding the mechanism of the alkane transformation and improving the zeolite catalyst performance. Herein, the kinetics of isobutane transformation on BEA zeolites, modified with either Zn2+cations (Zn2+/H-BEA) or ZnO clusters (ZnO/H-BEA), has been monitored with 1H MAS NMR spectroscopy in situ at 543–573 K. It is established that the rates for the alkane dehydrogenation, hydrogenolysis, and aromatization steps are higher for Zn2+/H-BEA than for ZnO/H-BEA zeolite, implying higher efficiency of Zn2+/H-BEA compared to ZnO/H-BEA zeolite for isobutane transformation. An occurrence of the reaction of isobutane with methane to yield propane on both zeolites has been evidenced by kinetic analysis. The kinetic parameters of the reaction have been compared with earlier derived parameters for n-butane transformation on the same zeolites. It is inferred that Zn2+/H-BEA exhibits higher activity than ZnO/H-BEA for both n-butane and isobutane aromatization. At the same time, Zn2+/H-BEA is more active for n-butane aromatization than for isobutane transformation to aromatics.

The effect of Zn2+cations and small clusters of ZnO on isobutene transformation to aromatic hydrocarbons on Zn-modified zeolites (Zn2+/H-ZSM-5 and ZnO/H-BEA) has been investigated with 13C MAS NMR and FTIR spectroscopic techniques. It is inferred that isobutene is mainly stabilized in the form of a π-complex with Zn2+cations of the Zn2+/H-ZSM-5 zeolite at 296–573 K, whereas the oligomerization of the olefin on Brønsted acid sites (BAS) of ZnO/H-BEA occurs at 296 K. Using isobutene, selectively labeled with the 13C isotope in either ═CH2or ═C< groups, for NMR analysis of the evolution of the adsorbed species with temperature, the reaction intermediates have been identified within the temperature range of 296–623 K. At 296 K, an occurrence of the double-bond shift reaction in isobutene on Zn2+/H-ZSM-5 and a complete 13C-label scrambling in the oligomers formed on ZnO/H-BEA have been established. The detected intermediates include a π-complex of the olefin with Zn species, 2-methyl-σ-allylzinc species, isobutene oligomers, and delocalized carbanionic species. It is found that similar intermediates are formed in the course of the reaction on both zeolites at T≥ 473 K, which implies that similar pathways of isobutene transformation are realized on the zeolites with different Zn species. An analysis of FTIR data reveals that the intermediate species formed on the Zn2+/H-ZSM-5 zeolite interact with both BAS and Zn sites at each of the reaction steps of the olefin transformation to aromatic hydrocarbons.