Publikationsliste des Instituts für Technische Chemie

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

Many Cu catalyzed ATRC reactions suffer from low catalyst activity and stability. We have synthesized five 1,10-phenanthroline ligands substituted in the 5-position with α-aminophosphonate groups, through which the corresponding Cu complexes can be immobilized on Al2O3. These catalysts show similar activity and higher selectivity than the homogenous catalysts while being recyclable.

Zusammenfassung

Acrylic acid is an important basic chemical and a key starting compound for a variety of consumer products. Today, acrylic acid is still produced from fossil-based propene. If acrylic acid were produced from bio-based lactic acid, this would be an important step towards sustainability. The gas-phase dehydration reaction of lactic acid to acrylic acid was performed over eight-membered ring PHI-type zeolites in the Na+ and K+-form. A few variations in the synthesis procedure of PHI-type zeolite made a big difference in the performance during the catalytic reaction due to differences in the physical and chemical properties, especially the accessibility of the pores. The catalysts were characterized with ICP-OES, XRD, CO2 physisorption, SEM and 27Al MAS NMR. The calcination resulted in a partial collapse of the PHI structure. In the case of Na,K-PHI with a low surface area, the catalysis tends to take place on the outer surface, while in the case of Na,K-PHI with a high surface area the catalysis can also take place within the pore system. This has a considerable influence on the selectivity of the catalysts.

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

Exploring advanced catalysts and reaction systems operated at mild reaction conditions is crucial for conducting the direct methane oxidation reaction toward oxygenate products. Many efforts have been put into research on pentasil−type (MFI) zeolites based on mononuclear and/or binuclear iron sites, using H2O2 as the oxidant. In this work, we present a modified liquid ion−exchange method to better control Fe loading in a mordenite−type (MOR) zeolite with a Si/Al molar ratio of 9. The optimized Fe/MOR catalyst showed excellent performance in the direct methane oxidation reaction with turnover frequencies (TOFs) of 555 h−1 to C1 oxygenates, significantly better than the reported activity. Multiple comparative experiments were conducted to reveal the mechanism behind the performance. Strikingly, the active sites in the Fe/MOR catalyst were found to be mononuclear iron sites, confirmed by transmission electron microscopy (TEM), ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS), and X-ray absorption spectroscopy (XAS). Increasing the iron loading led to the aggregation of the iron sites, which tend to trigger undesirable side reactions (i.e., H2O2 decomposition and over−oxidation), resulting in a significant decrease in TOFs to C1 oxygenates.

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 2D metal–organic-framework (MOF) based composites have emerged as promising candidates for electrocatalysis due to their high structural flexibility and fully exposed active sites. Herein, a freestanding metal–organic layer (MOL) with a 2D kgd (kagome dual) lattice was constructed with abundant surface oxygenate groups serving as anchoring sites to immobilize diverse guests. Taking Bi as an example, tetragonal Bi2O3 nanowires can be uniformly grown on MOLs after solvothermal treatment, the structural evolution of which was followed by ex situ electron microscopy. The as-prepared Bi2O3/MOL exhibits excellent CO2 electroreduction activity towards formate reaching a specific current of 2.3 A mgBi−1 and Faradaic efficiencies of over 85 \% with a wide potential range from −0.87 to −1.17 V, far surpassing Bi2O3/UiO (a 3D Zr6-oxo based MOF) and Bi2O3/AB (Acetylene Black). Such a post-synthetic modification strategy can be flexibly extended to develop versatile MOL composites, highlighting the superiority of optimizing MOL-based composites for electrocatalysis.

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

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

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

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

Electrochemical CO2 reduction to oxalic acid in aprotic solvents could be a potential pathway to produce carbon-neutral oxalic acid. One of the challenges in aprotic CO2 reduction are the limited achievable current densities under standard conditions, despite the increased CO2 solubility compared to aqueous applications. The application of aprotic solvents can reduce CO2 rather selectively to oxalate, and faradaic efficiencies (FEs) of up to 80% were achieved in this study with a Pb catalyst in acetonitrile, the FE being mainly dictated by the local CO2 concentration at the electrode. This process was integrated into a flow cell employing a two-layered carbon-free lead (Pb) gas diffusion electrode (GDE) and a sacrificial zinc (Zn) anode. With the application of this GDE the applicable current densities could be improved up to a current density of j = 80 mA cm−2 at a FE(oxalate) = 53%, which is within the range of the highest j reported in the literature. In addition, we provide an explanation for the deactivation mechanism of metal catalysts observed in the aprotic CO2 reduction literature. The deactivation is not related to a mass transport limitation but to cathodic corrosion observed at highly negative potential when employing quaternary ammonium supporting electrolyte cations, promoting catalyst leaching.

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

Abstract The ensemble effect, i.e., the fact that the reaction requiring the largest ensemble of surface metal atoms will be most sensitive to promoting with a second metal, has rarely been studied in bifunctional catalysis. In this contribution, it is shown that, during the dehydroalkylation of toluene with ethane on Pd-H-ZSM-5 catalysts with zinc as promoter, the ensemble effect has only an influence on the side reactions on the metal such as hydrogenolysis, but not on the desired acid-catalyzed alkylation reaction.

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 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

Lactic acid and alkyl lactates are widely applied in the production of food, cosmetics, pharmaceuticals, organic synthesis and biodegradable polymers. They can be prepared via one-pot synthesis from renewable trioses, such as dihydroxyacetone (DHA). Brønsted–Lewis bifunctional solid acids (BAS & LAS) can promote the reaction via a two-step cascade reaction mechanism. BAS catalyses the dehydration of DHA, resulting in the formation of pyruvaldehyde (PA) via the rearrangement of the enol form. Upon alcohol addition, PA can be converted to the desired alkyl lactates at LAS or to pyruvaldehyde hemiacetal (PAHA) at strong BAS. The density and strength control of Brønsted acid sites (BAS) and Lewis acid sites (LAS) and the optimization of their cooperation are essential for the efficient conversion of trioses to the target products. Here, we prepared a series of Sn-containing mesoporous MCM-41 catalysts with various BAS/LAS ratios by room temperature techniques. Sn-doped SiMCM-41 having a lower BAS/LAS ratio in this research shows a high initial selectivity to ethyl lactate (EL) and similar EL yield in 6 hours as the reported best Sn catalyst Sn-grafted SiMCM-41/carbon network materials in DHA conversion. A relatively large density of LAS in Sn-doped SiMCM-41 causes a fast conversion of PA to EL, while the overall yield has been limited by the BAS density for the DHA conversion. New H-form SnMCM-41, having a suitable density of LAS and weak BAS and an optimized BAS/LAS ratio, provides a 100% yield of ethyl lactate in the catalytic conversion of DHA in ethanol within 30 min, showing a superior performance hitherto.

Zusammenfassung

Nano-ZSM-5/SBA-15 analog composites (ZSC) were prepared in a two-step process from ZSM-5 precursors with different Si/Al molar ratios (10--50) via high-temperature synthesis in mildly acidic media (200 °C, pH 3.5) aiming to evaluate the influence of the initial Si/Al ratio on their structural, acidic and catalytic properties. The resulting materials were characterized by SAXS, XRD, FTIR, TEM, N2 sorption, 27Al solid state-NMR, NH3-TPD, FTIR spectroscopy of adsorbed pyridine, AAS and ICP-AES. Under the applied synthesis conditions, a ZSC material with controlled distribution of nano-ZSM-5 and SBA-15 analog phases can be prepared from ZSM-5 precursors by adjusting the initial Si/Al ratio in the range of 20--30. Increasing the initial Si/Al ratio to 50, only ZSM-5 nanocrystals were obtained whereas reducing the initial Si/Al ratio to 10 led to the formation of a disordered mesoporous SBA-15 analog. The total acidity increases with the crystallinity of the ZSM-5 phase as varying the Si/Al ratio from 10 to 30 despite the decreased amount of incorporated aluminum. However, the acidity declines slightly when raising the Si/Al ratio to 50 because of the low incorporated aluminum. The catalytic performance of the ZSC materials compared to the reference materials, i.e. purely mesoporous Al-SBA-15 and purely microporous H-ZSM-5 was assessed in the gas phase cracking of cumene and 1,3,5-tri-isopropylbenzene (TIPB) as test reactions. The results show that a balanced ratio of nano-ZSM-5 and SBA-15 analog phases obtained by tuning the initial Si/Al ratio is crucial to achieve superior catalytic performance of the ZSC materials in the cracking of both cumene and TIPB.

Zusammenfassung

Radioactive Cs released into a soil environment migrates along with groundwater in a manner dependent on Cs concentration. Data on the variation of Cs adsorption as a function of solution concentration are an essential prerequisite to successful decontamination work in Fukushima. To aid the ongoing decontamination work, the adsorption of Cs in aqueous solution across a wide Cs+ molarity range is studied for the case of saponite clay as adsorbent, an inorganic layered material that is an abundant mineral in the soil environment. The local molecular structures, i.e. nanosheet surfaces, nanosheet edges, and oncoming hexagonal cavities, participating in Cs adsorption are qualitatively highlighted by means of a recently developed analytical method using data from a conventional elution test, 133Cs magic-angle-spinning nuclear magnetic resonance (MAS NMR), and the radiocesium interception potential (RIP) K. Sato, et al., J. Phys. Chem. C, 2016, 120, 1270. The concentrations of nanosheet edges amount to between 100 and 400 mmol kg-1, which are not substantially different from those of the nanosheet surfaces, generally regarded as the main decontamination sites. This unambiguously implies that the nanosheet edges should be targeted as the molecular sites for decontaminating radioactive Cs, in addition to the nanosheet surfaces.

Zusammenfassung

Homologous series of solid catalysts (silica, SBA-15, dealuminated zeolite DeA–Y, zeolite H,Na–Y) with different pore sizes and loaded with different noble metals (Pt, Rh, Ir, Pd) were applied for the gas phase hydrogenation of propene with para-enriched hydrogen (p-H2) with the aim to produce parahydrogen-induced polarization (PHIP). This hyperpolarization formed by the pairwise incorporation of p-H2 into propene was studied via in situ 1H MAS NMR spectroscopy under continuous-flow conditions. By evaluating the characteristic 1H NMR antiphase signals of the hyperpolarized reaction products, the experimental parameters and properties of noble-metal-containing porous catalysts were investigated, which affect the formation and relaxation of PHIP. For the catalysts under study, small pore diameters and interactions of the hyperpolarized product molecules with nuclei inside the pores, such as framework aluminum atoms and extra-framework sodium cations, were found to enhance the relaxation of the hyperpolarization formed by the pairwise incorporation of p-H2 into olefinic reactants. Furthermore, a very high hydrogenation activity of the noble-metal-containing catalyst (Pd/H,Na–Y) decreases the formation of PHIP, possibly caused by a too large number of reactive spillover H species without spin correlation, which hinders the pairwise incorporation of p-H2 into the olefinic reactants.

Zusammenfassung

Solid-state NMR spectroscopy was utilized for investigating the post-synthetic formation of surface acid sites via AlCl3 modification of dealuminated zeolite Y (DeaY) and siliceous mesoporous SBA-15 and subsequent thermal treatment. Upon calcination at 723 K, aluminum atoms introduced by the AlCl3 modification coordinate at Q2 (Si(2Si,2OH)) and Q3 (Si(3Si,1OH)) sites and form tetrahedrally coordinated (AlIV) framework aluminum species. Most of the spectroscopically observed pentacoordinated (AlV) and octahedrally coordinated (AlVI) aluminum atoms are extra-framework species, partially acting as Lewis acid sites (LAC). The aluminum coordination at the framework is accompanied by the formation of Brønsted acidic SiOH groups with weak acid strength. The weak Brønsted acidity of these SiOH groups is explained by neighboring framework aluminum atoms with strongly disturbed tetrahedral oxygen coordination. For zeolite DeaY, in addition to the aluminum incorporation into silanol nests, also the aluminum incorporation via surface reactions of AlCl3 with the intact SiO2 framework occurs. In the case of mesoporous SBA-15, with a fivefold higher density of SiOH groups of the parent material compared with that of the parent zeolite DeaY, the aluminum incorporation into silanol nests is the dominating mechanism. By solid-state NMR spectroscopy of the dehydrated samples loaded with probe molecules, significantly larger densities (factor of 3--8) of LAC compared with those of Brønsted acid sites were determined for the AlCl3-modified and calcined zeolite DeaY and mesoporous SBA-15.

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.

Zusammenfassung

Size-confined Pt nanoparticles of about 1.5 nm have been introduced into AlMCM-41 supports with similar acid strength but various population densities of acid sites by means of wet impregnation. The Pt nanoparticles covered preferentially the surface Brønsted acid sites (BAS) of the supports or were located near acid sites rather than on the bigger free space between acid sites even at a very low acid density (6 BAS per 1000 nm2). The free BAS around the Pt particles did not interact with Pt atoms and the electronic properties of the Pt nanoparticles as probed by DRIFTS combined with CO adsorption were similar for Pt/AlMCM-41 with and without nearby free BAS. Ionic effects were generated by the Pt-covered acid sites, whereas the population of BAS did not contribute significantly to the ionic effects induced on the Pt nanoparticles. The coverage of BAS of similar strength by platinum nanoparticles led to similar chemoselectivity and product distribution in acetophenone (Aph) hydrogenation, though the density of BAS on the supports increased by more than 11 times. However, increasing the number of BAS on the supports significantly changed the hydrogenation rate. TOFs continuously increased from 125 h−1 up to 534 h−1, when the population of free BAS increased from 18.2 BAS per 1000 nm2 to 39.9 BAS per 1000 nm2. When the free BAS density was further increased to 70.4 BAS per 1000 nm2, the TOF then dropped to 176 h−1. The hydrogenation pathway is similar for both monofunctional (Pt covering all BAS) and bifunctional catalysts (Pt with free BAS), and the reaction was initiated on the Pt surface. This finding indicates that proper tuning of the population density of acid sites on the support can significantly improve the catalytic performance of the supported metal catalysts while keeping similar product selectivities.

Zusammenfassung

Mesoporous zirconosilicate, stannosilicate, and titanosilicate with the BEA structure framework have been prepared from the commercially available Beta zeolite via acid–alkaline treatments and subsequent dry impregnation of appropriate organometallic precursors. N2 adsorption–desorption isotherms and TEM observations confirm that alkaline treatment can induce desilication to create intra-crystalline mesopores from the partially dealuminated Beta sample. The incorporation of Zr species into the zeolite framework at formed silanol defect sites is monitored by infrared and 1H MAS NMR spectroscopy. Characterization results from UV-vis and XPS reveal that the majority of the incorporated Zr species exist in the form of tetrahedral Zr(iv) in the zeolite framework. The creation of Lewis acid sites with moderate acid strength upon Zr incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The as-prepared mesoporous Zr-Beta exhibits a remarkable catalytic activity and regio-selectivity to β-amino alcohols in the ring-opening aminolysis of epoxides, and the presence of mesopores can promote the reaction to a great extent through enhanced mass transfer. The impacts of the Lewis acidity of the catalysts, the basicity of amines and adsorption of reactants on the ring-opening aminolysis of epoxides are discussed in detail.

Zusammenfassung

A new technique is introduced allowing simultaneous in situ MAS NMR investigations of hydrocarbon conversions on solids under flow conditions and on-line gas chromatography. For adsorption of methanol on zeolite HBeta, equal amounts of adsorbed molecules were determined by both analytical methods. Studying the synthesis of methyl tertiary-butyl ether (MTBE) on zeolite HBeta using an MAS NMR rotor reactor, a constant yield of MTBE of Ymtbe= 27\% was obtained up to a weight hourly space velocity of 1.4 h-1. The variation of the reaction temperature led to a simultaneous change of the 13C MAS NMR signals of isobutoxy species and of the yield of MTBE determined by on-line gas chromatography which indicates that isobutoxy species act as chemically active compounds. In this first application, the new in situ technique has demonstrated its advantage for a simultaneous investigation of compounds with a long residence time on the catalyst surface and of compounds rapidly leaving the catalyst surface.

Zusammenfassung

The synthesis of the high-silica zeolite NU-87 and its physicochemical characterization are reported. The catalytic properties of zeolite HNU-87 were investigated in the conversion of 1- and 2-methylnaphthalene. Both isomerization to the corresponding methylnaphthalene and transalkylation to naphthalene and dimethylnaphthalenes are observed. The influence of reaction temperature and the modified residence time on the product distributions is reported and discussed in terms of shape selectivity effects in the peculiar pore system of NU-87. For comparison, typical results obained with zeolite HMCM-22, which possesses a similar pore architecture, are also included. The results of methylnaphthalene conversion over HNU-87 are compared to those obtained in the alkylation of naphthalene with methanol on the same zeolite catalyst.