Michael Dyballa

Zusammenfassung

In molecular heterogeneous catalysis knowledge about the location and accessibility of the immobilized metal complex inside porous solids is important to assess the catalytic efficiency. Here, we developed a method to quantify accessible Rh complexes in SBA-15 mesopores via solid state NMR spectroscopy. In order to identify suitable probe molecules several phosphines were screened in solution and their diameters were determined by DFT calculations. The ligation of phosphines at immobilized Rh complexes was then monitored by quantitative 31P MAS NMR spectroscopy. For the first time a polymer too large to enter the mesopores, poly(styrene-co-vinyltriphenylphosphine), PS-co-PVPP, was used to probe selectively Rh complexes immobilized on the external surface, in pore mouths or defects. By combining 31P MAS NMR and ICP-OES the P/Rh ratio was determined revealing that only up to 39% of the immobilized Rh complexes were accessible. Therefore, corrected turnover numbers (TONs) of up to 55 could be calculated for the asymmetric 1,2-addition with immobilized Rh catalysts. With the herein presented methodology the accessibility and spatial distribution of immobilized molecular catalysts can for the first time be evaluated quantitatively.

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

ABSTRACTBiochar is a promising material for environmental amendment. However, the adsorption mechanism of nitrogen-based ionic species, particularly the pH dependence, is still under debate because of its high susceptibility to experimental conditions. In this study, the adsorption of ammonium sulfate by rice-husk biochar produced by low-temperature carbonization as an eco-friendly approach was studied using Fourier-transform infrared (FT-IR) spectroscopy, 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR), and open-space analysis using positrons coupled with molecular simulations. In the aqueous solution with pH of 5.38, graphenes are isolated exposing fresh surfaces, which physisorb NH4+ via interaction with π electrons rather than surface functional groups. Furthermore, the low-temperature carbonization leads to a lower degree of graphitization with grain boundary defects in graphene, triggering the fracture of graphene during the shaking process in an aqueous solution. SO42− physisorption occurs as an outer-sphere surface complex on the positive charge caused by a charge transfer of ∼2.5\% from the terminal hydrogen at the graphene edge. A decrease in aqueous pH by ∼0.7\% significantly changes the above adsorption properties both on the surfaces and at the edges: diminishment of physisorption and transition to chemisorption by the formation of an inner-sphere surface complex.

Zusammenfassung

Abstract Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA-15 with 1,1,1-trimethyl-N-(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3-azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh-catalyzed 1,2-addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA-15.

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.

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

We herein investigate the selective dehydration of ethanol at high conversions (>99.5%) and a moderate reaction temperature of 493 K over a silicotungstic acid (STA) catalyst supported on silica. The variation of STA loadings resulted in a linearly decreasing BET surface area with increasing STA content and the formation of negligible quantities of crystalline STA. In the 1H MAS NMR spectra, two distinct NH4+ species were identified after NH3 adsorption, i.e. Brønsted acid sites on the external surface of the STA nanoparticles corresponding to a chemical shift of δ1H = 6.6 ppm and Brønsted acid sites within the pseudo-liquid phase (PLP) corresponding to a chemical shift at δ1H = 5.6 ppm. At 36 wt% loading, all Brønsted acid sites were easily accessible, while at higher STA-loadings the intensity at 5.6 ppm increased slowly with exposure time to NH3 due to diffusion limitations. The lifetime of the STA catalyst has a maximum at a STA loading of 36 wt% exhibiting a TOS of 15.7 h. Impregnation with CsOH led to a slightly increased BET surface area. The preferred inhibition of external Brønsted acid sites is reflected by a decreasing intensity of the 6.6 ppm 1H MAS NMR peak of NH4+ ions. The inhibition increases the lifetime to up to 30 h at conversions above 99.5%, as a result of the suppressed formation of higher olefins as by-products. Thus, the impregnation with CsOH is an efficient way to reduce external Brønsted acid sites of supported STA catalysts and leads to a longer lifetime and selectivity in the ethanol dehydration process under industrially relevant conditions.

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

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D 27Al, 31P MAS NMR, and 2D 31P-27Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Zusammenfassung

Abstract Ru/Al2O3 is a highly stable, but less active catalyst for methanation reactions. Herein we report an effective approach to significantly improve its performance in the methanation of CO2/H2 mixtures. Highly active and stable Ru/γ-Al2O3 catalysts were prepared by high-temperature treatment in the reductive reaction gas. Operando/in situ spectroscopy and STEM imaging reveals that the strongly improved activity, by factors of 5 and 14 for CO and CO2 methanation, is accompanied by a flattening of the Ru nanoparticles and the formation of highly basic hydroxylated alumina sites. We propose a modification of the metal–support interactions (MSIs) as the origin of the increased activity, caused by modification of the Al2O3 surface in the reductive atmosphere and an increased thermal mobility of the Ru nanoparticles, allowing their transfer to modified surface sites.

Zusammenfassung

The direct stepwise transformation of CH4 to CH3OH over Cu-exchanged zeolites has been an intensively researched reaction as it can provide a solution for the utilization of this abundant feedstock. Up to date a commercial process is far from realization, which is why an understanding of the Cu speciation in zeolites as a function of reaction conditions as well as the development of a mechanistic view of the reaction are necessary to further advance the field. Herein we study Cu-exchanged ferrierite zeolite for the direct CH4 to CH3OH conversion by utilizing X-ray absorption spectroscopy (XAS), in order to assess the local structure and electronic properties of Cu through the reaction. A Cu-FER sample with a Cu/Alþinspace=þinspace0.20 and Si/Alþinspace=þinspace11 was subjected to three reaction cycles yielding ultimately 96 µmol\$\$\_\\\\backslashtext\C\\\\\backslashtext\H\\\_3\\\backslashtextØH\\\\/\\\backslashtext\g\\\_\\\backslashtext\zeolite\\\\\$\$CH3OH/gzeolite. Normalized to the Cu loading, this accounts for 0.33 mol\$\$\_\\\\backslashtext\C\\\\\backslashtext\H\\\_3\\\backslashtextØH\\\\\$\$CH3OH/molCu, making the sample comparable to very active Cu-MOR materials reported in the literature. During O2 activation, a transient self-reduction regime of CuII to CuI ions was identified; eventually leading to mostly framework interacting CuII species. CH4 loading leads to a reduction of these CuII containing species; which are finally partially reoxidized during H2O-assisted CH3OH extraction. The speciation after CH4 activation as well as H2O-assisted CH3OH extraction was assessed via linear combination fitting analysis of the XAS data.

Zusammenfassung

Low-silica AlPO-34 materials with similar crystal sizes but different Brønsted acid site densities were prepared and investigated as catalysts in methanol-to-olefin (MTO) conversion. The effect of Brønsted acid site density on catalyst activity and the dominant reaction mechanism during the MTO conversion was investigated via TGA, GC-MS, solid-state NMR spectroscopy, and in situ UV/vis spectroscopy together with the catalytic performance. For the catalysts with lower Brønsted acid site densities, the olefin-based cycle mechanism is the dominant mechanism during the MTO conversion. Long-chain alkenes, e.g., C5–C6 alkenes, act as intermediates that are cracked to lower olefins, or are converted to dienes via hydride transfer reactions, and can also diffuse out of the cages of low-silica AlPO-34 catalysts as the products. With decreasing Brønsted acid site density or reaction temperature, the methylation route of the olefin-based cycle was found to be much more favored than the cracking route. Therefore, a higher selectivity to C5–C6 alkenes (∼50%) is achieved. Simultaneously, dienes are the predominant deposits occluded in the used catalysts. For catalysts with slightly higher Brønsted acid site densities, the long-chain alkenes are rapidly transformed to aromatics and, subsequently, an aromatic-based cycle mechanism contributes to the MTO conversion. Interestingly, the catalyst with the most suitable Brønsted acid site density can well balance the above-mentioned two reaction cycles accompanied by a low deactivation rate, leading to a long catalyst lifetime of up to 15 h.