Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

Dynamic Nuclear Polarization (DNP) solid-state NMR has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and Magic Angle Spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow EPR line radical, BDPA, tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electronic spin relaxation times on one hand, and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP Surface Enhanced NMR spectra (DNP SENS) of amorphous aluminosilicates and 17O NMR on silica nanoparticles.

α-Ga2O3, β-Ga2O3, and γ-Ga2O3 as well as the silica-supported catalysts γ-Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga­(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO2-supported Ga2O3 catalysts were studied using FTIR, 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in γ-Ga2O3 and β-Ga2O3 (the latter obtained from colloidal nanocrystals of γ-Ga2O3 via thermal treatment at 750 °C) are similar, except that β-Ga2O3 contains a larger relative fraction of weak Ga3+ Lewis acid sites. In contrast, α-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: i.e., weak Lewis acid surface sites are more abundant in β-Ga2O3 relative to α-Ga2O3 and γ-Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C3H6 m–2 (Ga2O3) h–1 at 550 °C, for respectively β-, α-, and γ-Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of γ-Ga2O3 or β-Ga2O3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The γ-Ga2O3/SiO2 catalyst was slightly more active than β-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol Ga–1 h–1 (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga–1 h–1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga2O3 surface show that tetra- and pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of β-Ga2O3.

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.

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 1 H 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 13 C 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.