Daniel Dittmann

Zusammenfassung

The amount of BTEX aromatics obtained from the conversion of ethanol (ETA) is increased by combining ZSM-5 catalysts having optimum acidity with desilication and zinc ion exchange. Zinc leads to preferred dehydrogenation instead of hydrogen transfer. It decreases the share of paraffin products and increases BTEX contents (up to SBTEX = 50%) at the cost of lifetime. The latter can be increased via desilication. An ethylene feed increases lifetime and BTEX production as result of oxygenate absence. Combination of improvements resulted in a C2 conversion capacity of 206 g g−1 and a total yield of BTEX aromatics of 31.6 g g−1, which is about a factor of 2–3 times better than the respective values found for microporous, mesoporous, or microporous Zn-exchanged materials. In situ UV/vis spectra reveal that desilicated samples coke significantly slower than microporous samples, whereas Zn exchange supports the formation of coke. Thus, by a clever combination of suitable post-modifications, a significantly higher BTEX production from the primary source ethanol can be achieved.

Zusammenfassung

Herein, we apply three different copper-exchanged materials (Na–AlSBA-15, silica, Na–MCM-22) as hosts for a direct synthesis of CuI(1,1′-bis(diphenylphosphino)ferrocene = dppf) complexes in cationic ion exchange position. Using 31P MAS NMR spectroscopy, we show that identical complexes as after ion exchange are generated if the solids are applied as reactants directly. The homogeneity of copper exchanges is evaluated by EDX spectroscopy. Both CuI and CuII result in the formation of complexes, thereby oxidizing dppf. Cu-particles were not reactive. Optimized conditions for a maximized complex formation are identified applying quantitative 31P MAS NMR spectroscopy and ICP-OES. Only accessible copper in cationic position of the solids forms the complexes. This enables a quantification of the amount of copper in mesopores vs. the total copper amount. Thus, besides a new synthesis of the complex a suitable method for quantitative elucidation of the location of copper cations is demonstrated herein.

Zusammenfassung

Herein, we apply three different copper-exchanged materials (Na–AlSBA-15, silica, Na–MCM-22) as hosts for a direct synthesis of CuI(1,1′-bis(diphenylphosphino)ferrocene = dppf) complexes in cationic ion exchange position. Using 31P MAS NMR spectroscopy, we show that identical complexes as after ion exchange are generated if the solids are applied as reactants directly. The homogeneity of copper exchanges is evaluated by EDX spectroscopy. Both CuI and CuII result in the formation of complexes, thereby oxidizing dppf. Cu-particles were not reactive. Optimized conditions for a maximized complex formation are identified applying quantitative 31P MAS NMR spectroscopy and ICP-OES. Only accessible copper in cationic position of the solids forms the complexes. This enables a quantification of the amount of copper in mesopores vs. the total copper amount. Thus, besides a new synthesis of the complex a suitable method for quantitative elucidation of the location of copper cations is demonstrated herein.

Zusammenfassung

Crystal size is a key parameter of zeolites applied as catalysts. Herein, ZSM-5 crystals with similar physicochemical and acid properties, few defects, and aluminum exclusively in tetrahedral coordination are synthesized and the influence of the crystal size on the MTO and ETA conversion is investigated. Short olefins are the main products of the MTO conversion, whereas larger olefins and aromatics dominate the products after ETA conversion. In the case of both feeds, an increased crystal size decreases the catalyst’s lifetime. The MTO conversion over larger ZSM-5 altered the product distribution, which was not the case for the ETA conversion. The reason is that the instantly available aromatics during ETA conversion lead to fast coking and zeolite crystals only active in the outer layers. Thus, the different reactivity of different-sized ZSM-5 is direct proof of a different conversion mechanism for both alcohols.

Zusammenfassung

Water and alcohols like methanol are key substrates in catalysis. Thus, adsorption on catalysts plays a key role in chemical reactions. Herein we investigate how surface sites and confinement influence the formation of water surface species, their adsorption strength, and complex stability. We compare adsorption on microporous MFI zeolites, mesoporous SBA-15 materials and silica A200 as supports for silicotungstic acid (STA) in their isostructural silica, Na-, and H-forms. A systematic variation of confinements, surface sites and adsorbates enables a separation of confinement from other effects. In saturation, the quantity of bound water is determined by the available surface area and maximized for materials with high Si(OH) densities on the surface. The strong binding of water at Si(OH) explains why the Si(OH) density influences heterogeneous catalysis. Complexes between water and counter ions (Na+) or acid sites (H+) form under micropore confinement in Na- and H-ZSM-5. Complexes are absent or decompose quickly on aluminated SBA-15 and STA in Na- or H-forms. More methanol than water molecules bind to counter ions and acid sites on ZSM-5. This explains why acid sites in the methanol-to-olefin conversion are just partially blocked by water. We identify confinement as the key parameter for the formation of surface complexes and for adjusting the efficiency of their protonation. Our finding helps to rationalize reactions with water and/or alcohol reactants.

Zusammenfassung

We herein investigate methanol adsorbates on a variety of heterogeneous catalysts. We quantitatively desorb methanol from saturated MFI zeolites, SBA-15 materials and silicotungstic acid (STA) supported on silica, all in the respective siliceous, Na- and H-forms. Surface species are identified by 1H and 13C MAS NMR and DRIFTS. On saturated surfaces, we find liquid-like methanol in weak surface interaction. For siliceous materials, adsorption on silanol Si(OH) groups is dominant, especially on materials with amorphous pore walls like SBA-15. Weak methanol binding on microporous silicalite is caused by a repulsive effect due to micropore confinement. For Na-form materials, methanol complexes at Na+ counter ions dominate the adsorption of methanol in Na-ZSM-5 micropores. The strong confinement leads to stronger methanol adsorption compared to less confined systems. Without confinement, no complex at Na+ is observed and Si(OH) groups dominate adsorption. On H-form materials, methanol complexes at acid sites form in a higher quantity under confinement in H-ZSM-5. After treatment at 423 K, the formation of dimethyl ether (DME) was evidenced by IR and 13C MAS NMR spectroscopy and the acid site proton peak found at δ1H = 14.4 ppm. Si(OH) groups bind methanol stronger than counter ions and acid sites (Na+ and H+). This explains why defects and the Si(OH) density influence heterogeneous reactions. Our findings show that confinement in micropores is crucial for the stabilization of methanol complexes at counter ions Na+ and acid sites H+.

Zusammenfassung

Herein, we describe a method for the quantification of Brønsted acid sites located on surfaces and in pores of hierarchical zeolite catalysts. The probe triphenylphosphine (TPP) accesses only pores bigger 0.72 nm. The signal of protonated TPP is baseline separated from other signals and can be directly quantified by 31P MAS NMR spectroscopy. Results are robust and are not affected by the total TPP loading nor by remaining solvent traces. The error of the Brønsted acid site density evaluation is below ±10\% for amorphous silica–alumina and below ±5\% for probing crystalline materials like MCM-22 or hierarchical zeolites. On amorphous silica–alumina, only 12.5\% of all acid sites were accessible by TPP, which binds near tetrahedral and pentahedral aluminum. The 47 ± 2 μmol/g acid sites on the surface and in pore mouths of zeolite MCM-22 represent 12\% of the total acidity. On TNU-9, 2\% of the total acidity is located on the surface. On commercial zeolite ZSM-5, no surface acidity was found. Desilication of ZSM-5 and TNU-9 zeolites introduced an additional 20 ± 1 and 29 ± 1 μmol/g of Brønsted acid sites, respectively. These additional acid sites are located in introduced mesopores of hierarchical ZSM-5 and TNU-9 zeolites and account for 6–7\% of the total sites present. The location in mesopores can cause undesired byproducts in catalysis due to the absence of shape selectivity effects. The techniques described herein will aid the understanding of the acid site density in hierarchical systems and lead to improvements of catalyst synthesis and performance.

Zusammenfassung

We herein investigate the alumination mechanism of siliceous micro- and mesoporous materials (SBA-15, SBA-16, and dealuminated Y-zeolite) with NaAlO2 to synthesize new ion exchangers and acid catalysts. We show that in aqueous alkaline solution, the materials’ surface is partially dissolved to form Si(OH)x groups (x = 1, 2, 3) that react with tetrahedral aluminum sites. The amount of introduced aluminum depends on the aqueous treatment temperature, pore diameters and structure of siliceous parents. The incorporation works best for hexagonal SBA-15 mesopores at the cost of micropores. Zeolite Y micropores remain accessible upon alumination. All Na-forms can quantitatively be ion exchanged. They are thus potential carriers for catalytically active metal ions. The presence of different Al(OH) groups is proven by 1H27Al TRAPDOR. The 27Al MQMAS NMR spectra indicate the formation of octahedral and pentahedral aluminum upon transformation into H-forms. On AlSBA-15 up to 18% of ion exchange sites can be transformed into Brønsted acid sites. The acid properties were investigated using the probe molecules NH3, acetonitrile-d3, and TMPO. On mesoporous SBA-15 and SBA-16, Brønsted acid sites show a flexible coordination between Si(OH) and tetrahedral aluminum. The sites are weak and form exclusively in the presence of a strong base or counter ion. Zeolite acid site strength was found for re-aluminated Y. TMPO loading combined with 27Al and 31P MAS NMR spectroscopy indicates the presence of Lewis acidic framework aluminum and the presence of several distinct Brønsted and Lewis acid sites on H-forms.

Zusammenfassung

Abstract We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM-41 and SBA-15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. 31P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM-41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 \% of NMs are located on the external surface of MCM-41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites.