No abstract

The formation and nature of active sites for methanol conversion over solid acid catalyst materials are studied by using a unique combined spectroscopic and theoretical approach. A working catalyst for the methanol‐to‐olefin conversion has a hybrid organic–inorganic nature in which a cocatalytic organic species is trapped in zeolite pores. As a case study, microporous materials with the chabazite topology, namely, H‐SAPO‐34 and H‐SSZ‐13, are considered with trapped (poly)aromatic species. First‐principle rate calculations on methylation reactions and in situ UV/Vis spectroscopy measurements are performed. The theoretical results show that the structure of the organic compound and zeolite composition determine the methylation rates: 1) the rate increases by 6 orders of magnitude if more methyl groups are added on benzenic species, 2) transition state selectivity occurs for organic species with more than one aromatic core and bearing more than three methyl groups, 3) methylation rates for H‐SSZ‐13 are approximately 3 orders of magnitude higher than on H‐SAPO‐34 owing to its higher acidity. The formation of (poly)aromatic cationic compounds can be followed by using in situ UV/Vis spectroscopy because these species yield characteristic absorption bands in the visible region of the spectrum. We have monitored the growth of characteristic peaks and derived activation energies of formation for various sets of (poly)aromatic compounds trapped in the zeolite host. The formation–activation barriers deduced by using UV/Vis microspectroscopy correlate well with the activation energies for the methylation of the benzenic species and the lower methylated naphthalenic species. This study shows that a fundamental insight at the molecular level can be obtained by using a combined in situ spectroscopic and theoretical approach for a complex catalyst of industrial relevance.

The optical absorption properties of (poly)aromatic hydrocarbons occluded in a nanoporous environment were investigated by theoretical and experimental methods. The carbonaceous species are an essential part of a working catalyst for the methanol-to-olefins (MTO) process. In situ UV/Vis microscopy measurements on methanol conversion over the acidic solid catalysts H-SAPO-34 and H-SSZ-13 revealed the growth of various broad absorption bands around 400, 480, and 580 nm. The cationic nature of the involved species was determined by interaction of ammonia with the methanol-treated samples. To determine which organic species contribute to the various bands, a systematic series of aromatics was analyzed by means of time-dependent density functional theory (TDDFT) calculations. Static gas-phase simulations revealed the influence of structurally different hydrocarbons on the absorption spectra, whereas the influence of the zeolitic framework was examined by using supramolecular models within a quantum mechanics/molecular mechanics framework. To fully understand the origin of the main absorption peaks, a molecular dynamics (MD) study on the organic species trapped in the inorganic host was essential. During such simulation the flexibility is fully taken into account and the effect on the UV/Vis spectra is determined by performing TDDFT calculations on various snapshots of the MD run. This procedure allows an energy absorption scale to be provided and the various absorption bands determined from in situ UV/Vis spectra to be assigned to structurally different species.

Die Umgebung muss passen: Welche Olefine aus Methanol in Zeolithen gebildet werden, hängt stark von spezifischen Kombinationen der intermediär gebildeten Spezies des Pools organischer Kohlenwasserstoffe und der Zeolithtopologie ab (siehe Bild). Wenn der Käfig zu groß ist, sind neutrale Spezies günstiger als reaktive Kationen. Wenn der Käfig zu klein ist, bringt die Formselektivität des Übergangszustands erhebliche Einschränkungen für die Reaktivität sperrigerer Spezies.

The polymerization of acrylamide (AA) and methacrylamide (MAA) was studied by an extensive set of computational methods with a particular focus on the possible influence of water molecules on the propagation reaction. An extensive set of electronic structure methods was tested, consisting of B3LYP, BMK, MPWB1K, MP2, and B2-PLYP of which some include dispersion effects. The effect of water on the transition state is modeled in two different ways. Explicit water molecules are added to the system, showing that replacing the hydrogen bond that dominates the transition state structure by a water-mediated hydrogen bond, results in more stable, more feasible transition states. This effect is the largest for AA polymerization, a monomer that is known to experience a larger solvent effect than MAA. Additionally, a conductor-like polarizable continuum model (C-PCM) is applied on both the transition states in gas phase and the ones bearing explicit water molecules. This model has a dramatic effect on all the propagation rates, raising them by about 3 orders of magnitude. The inclusion of explicit water molecules gives insight into the role of water molecules and the formation of prereactive complexes. The relative rate of polymerization of AA with regard to MAA is well reproduced for a trimeric propagating radical with inclusion of explicit water molecules or by using an implicit solvation model at the BMK and MPWB1K level of theory.