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Author ORCID ID is 0000000152451426
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  1. In this article, we report on a new route to produce hexane-1,2,5,6-tetrol (tetrol) from cellulose-derived levoglucosanol (lgol). We investigate the reaction intermediates formed over metal and acid catalysts, and propose a reaction network for this process. Lgol is converted to tetrol in up to 90% yield over a bifunctional Pt/SiO 2–Al 2O 3 catalyst at 150 °C. High tetrol yields are maintained at lgol concentrations of up to 21 wt% in water. threo- and erythro-lgol first undergo hydrolysis to 3,4-dideoxymannose (DDM) and 3,4-dideoxyglucose (DDG), respectively. This reaction can be carried out selectively over an Amberlyst 70 acid catalyst at amore » temperature of 100 °C. At a higher temperature of 150 °C with no added catalyst, DDM and DDG undergo aldose–ketose isomerization to 3,4-dideoxyfructose (DDF). DDM is hydrogenated to cis-tetrol over a Pt/SiO 2 catalyst, while DDG is hydrogenated to trans-tetrol. Formation of DDF erases the stereocenter at the C 2 position of lgol, and hydrogenation of DDF produces a nearly 1:1 mixture of cis- and trans-tetrol. Lastly, this catalytic approach to produce tetrol from biomass opens the door to sustainable chemicals derived from tetrol.« less
  2. Zeolites, having widespread applications in chemical industries, are often synthesized using organic templates. These can be cost-prohibitive, motivating investigations into their role in promoting crystallization. Herein, the relationship between framework structure, chemical composition, synthesis conditions and the conformation of the occluded, economical template tetraethylammonium (TEA +) has been systematically examined by experimental and computational means. The results show two distinct regimes of occluded conformer tendencies: 1) In frameworks with a large stabilization energy difference, only a single conformer was found (BEA, LTA and MFI). 2) In the frameworks with small stabilization energy differences (AEI, AFI, CHA and MOR), less thanmore » the interconversion of TEA + in solution, a heteroatom-dependent (Al, B, Co, Mn, Ti, Zn) distribution of conformers was observed. Our findings demonstrate that host–guest chemistry principles, including electrostatic interactions and coordination chemistry, are as important as ideal pore-filling.« less
    Cited by 13Full Text Available
  3. In this study, the average and the local structure of phosphorus-treated HZSM-5 zeolites were investigated by means of atom probe tomography, powder X-ray diffraction (at ambient and cryogenic temperatures) and 1H, 29Si, 27Al, and 31P magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy. Phosphatation to yield a product with P/Al ≤ 1 followed by thermal treatment leads to breaking of the Si–OH–Al bridging groups, and subsequent partial dealumination of the zeolite framework, as shown by the contraction of the orthorhombic unit-cell volume and by the loss of tetrahedral framework Al, as observed in the 27Al Multiple Quantummore » (MQ) MAS NMR spectrum. Most of the framework Al is present in an electronic environment distorted by the presence of phosphorus and appears not to be involved in classic Si–OH–Al Brønsted acid sites. The 31P MAS NMR signals indicate that phosphorus interacts with the zeolitic framework to locally form silico-aluminophosphate (SAPO) domains and the presence of a new kind of acidic site was confirmed by the resonance at ~8.6 ppm in the 1H MAS NMR spectra, attributed to P–OH groups. Increasing the phosphorus loading (P/Al >> 1) promotes further dealumination of the framework and cross-dehydroxylation between P–OH and Si–OH species, leading to the formation of a crystalline silicon orthophosphate phase. With decreasing Al content, the monoclinic HZSM-5 structure becomes preferred, especially at 85 K where the strain relaxation is higher. Nevertheless, the presence of a higher amount of silicophosphate impurities hinders the low-temperature strain release of the framework, indicating that some of these species are localized in the zeolite pores and contribute to the strain build up.« less
  4. Understanding the 3-D distribution and nature of active sites in heterogeneous catalysts is critical to developing structure–function relationships. However, this is difficult to achieve in microporous materials as there is little relative z-contrast between active and inactive framework elements (e.g., Al, O, P, and Si), making them difficult to differentiate with electron microscopies. We have applied atom probe tomography (APT), currently the only nanometer-scale 3-D microscopy to offer routine light element contrast, to the methanol-to-hydrocarbons (MTH) catalyst SAPO-34, with Si as the active site, which may be present in the framework as either isolated Si species or clusters (islands) ofmore » Si atoms. 29Si solid-state NMR data on isotopically enriched and natural abundance materials are consistent with the presence of Si islands, and the APT results have been complemented with simulations to show the smallest detectable cluster size as a function of instrument spatial resolution and detector efficiency. We have identified significant Si–Si affinity in the materials, as well as clustering of coke deposited by the MTH reaction (13CH3OH used) and an affinity between Brønsted acid sites and coke. A comparison with simulations shows that the ultimate spatial resolution that can be attained by APT applied to molecular sieves is 0.5–1 nm. Finally, the observed 13C clusters are consistent with hydrocarbon pool mechanism intermediates that are preferentially located in regions of increased Brønsted acidity.« less
  5. Understanding structure–composition–property relationships in zeolite-based materials is critical to engineering improved solid catalysts. However, this can be difficult to realize as even single zeolite crystals can exhibit heterogeneities spanning several orders of magnitude, with consequences for, for example, reactivity, diffusion as well as stability. Great progress has been made in characterizing these porous solids using tomographic techniques, though each method has an ultimate spatial resolution limitation. Atom probe tomography (APT) is the only technique so far capable of producing 3D compositional reconstructions with sub-nanometer-scale resolution, and has only recently been applied to zeolite-based catalysts. Herein, we discuss the use ofmore » APT to study zeolites, including the critical aspects of sample preparation, data collection, assignment of mass spectral peaks including the predominant CO peak, the limitations of spatial resolution for the recovery of crystallographic information, and proper data analysis. All sections are illustrated with examples from recent literature, as well as previously unpublished data and analyses to demonstrate practical strategies to overcome potential pitfalls in applying APT to zeolites, thereby highlighting new insights gained from the APT method.« less
    Cited by 1
  6. Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NO X with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000more » mile simulation, Cu-SSZ-13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl2O4 spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale heterogeneities that lead to catalytic activity and material deactivation.« less

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