Yashodhan Bhawe

California Institute of Technology, Pasadena, CA, United States

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Publications (5)39.02 Total impact

  • Yuewei Ji · Mark A. Deimund · Yashodhan Bhawe · Mark E. Davis ·
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    ABSTRACT: Chabazite (CHA)-type zeolites are prepared from the hydrothermal conversion of faujasite (FAU)-type zeolites, dealuminated by high-temperature steam treatments (500–700 °C), and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The effects of temperature and partial pressure of water vapor during steaming are investigated. Powder X-ray diffraction (XRD) and Ar physisorption data show that the steam treatments cause partial structural collapse of the zeolite with the extent of degradation increasing with steaming temperature. 27Al MAS NMR spectra of the steamed materials reveal the presence of tetrahedral, pentacoordinate, and octahedral aluminum. NH3 and i-propylamine temperature-programmed desorption (TPD) demonstrate that steaming removes Brønsted acid sites, while simultaneously introducing larger pores into the CHA materials that make the remaining acid sites more accessible. Acid washing the steamed CHA-type zeolites removes a significant portion of the extra-framework aluminum, producing an increase in the bulk Si/Al ratio as well as the adsorption volume. The proton form of the as-synthesized CHA (Si/Al = 2.4) rapidly deactivates when tested for MTO at a reaction temperature of 400 °C and atmospheric pressure. CHA samples steamed at 600 °C performed the best among the samples tested, showing increased olefin selectivities as well as catalyst lifetime compared to the unsteamed CHA. Both lifetime and C2–C3 olefin selectivities are found to increase with increasing reaction temperature. At 450 °C, CHA steamed at 600 °C reached a combined C2–C3 olefin selectivity of 74.2% at 100% methanol conversion, with conversion remaining above 80% for more than 130 min of time-on-stream (TOS) before deactivating. More stable time-on-stream behavior is observed for 600 °C-steamed CHA that underwent acid washing: conversion above 90% for more than 200 min of TOS at 450 °C with a maximum total C2–C3 olefin selectivity of 71.4% at 100% conversion.
    ACS Catalysis 07/2015; 5(7):4456-4465. DOI:10.1021/acscatal.5b00404 · 9.31 Impact Factor
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    ABSTRACT: High resolution, multi-nuclear solid state nuclear magnetic resonance (NMR) characterizations are carried out in order to obtain insights into the structural features of Sn-beta zeolites that catalyze glucose isomerization or epimerization reactions in water and methanol solvents. In particular, we focus on investigating the local structural changes to catalytically-active framework Sn sites of different 119Sn-labeled beta zeolites, including the calcined, dehydrated, rehydrated, and post-sugar isomerization catalysis forms. Magic angle spinning (MAS) and cross polarization MAS (either from 1H or 19F) 119Sn NMR spectra provide evidence for changes to the local framework Sn coordination in the presence of water and sugar molecules, and provide insights into structural features of adsorbed intermediates that may be relevant in sugar isomerization reaction pathways.
    Topics in Catalysis 05/2015; 58(7-9):435-440. DOI:10.1007/s11244-015-0388-7 · 2.37 Impact Factor
  • Bingjun Xu · Yashodhan Bhawe · Mark E. Davis ·
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    ABSTRACT: A manganese oxide-based, thermochemical cycle for water splitting below 1000 °C has recently been reported. The cycle involves the shuttling of Na+ into and out of manganese oxides via the consumption and formation of sodium carbonate, respectively. Here, we explore the combinations of three spinel metal oxides and three alkali carbonates in thermochemical cycles for water splitting and CO2 reduction. Hydrogen evolution and CO2 reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe3O4 > Mn3O4 > Co3O4, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li2CO3 > Na2CO3 > K2CO3. While hydrogen evolution and CO2 reduction reactions occur at a lower temperature on the combinations with the more reactive metal oxide and alkali carbonate, higher thermal reduction temperatures and more difficult alkali ion extractions are observed for the combinations of the more reactive metal oxides and alkali carbonates. Thus, for a thermochemical cycle to be closed at low temperatures, all three reactions of hydrogen evolution (CO2 reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na2CO3/Mn3O4 combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °C.
    Chemistry of Materials 04/2013; 25(9):1564–1571. DOI:10.1021/cm3038747 · 8.35 Impact Factor
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    ABSTRACT: Zeolites that contain eight-membered ring pores but different cavity geometries (LEV, CHA, and AFX structure types) are synthesized at similar Si/Al ratios and crystal sizes. These materials are tested as catalysts for the selective conversion of methanol to light olefins. At 400 °C, atmospheric pressure, and 100% conversion of methanol, the ethylene selectivity decreases as the cage size increases. Variations in the Si/Al ratio of the LEV and CHA show that the maximum selectivity occurs at Si/Al = 15–18. Because lower Si/Al ratios tend to produce faster deactivation rates and poorer selectivities, reactivity comparisons between frameworks are performed with solids having a ratio Si/Al = 15–18. With LEV and AFX, the data are the first from materials with this high Si/Al. At similar Si/Al and primary crystallite size, the propylene selectivity for the material with the CHA structure exceeds those from either the LEV or AFX structure. The AFX material gives the shortest reaction lifetime, but has the lowest amount of carbonaceous residue after reaction. Thus, there appears to be an intermediate cage size for maximizing the production of light olefins and propylene selectivities equivalent to or exceeding ethylene selectivities.
    ACS Catalysis 10/2012; 2(12):2490–2495. DOI:10.1021/cs300558x · 9.31 Impact Factor
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    Bingjun Xu · Yashodhan Bhawe · Mark E Davis ·
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    ABSTRACT: Thermochemical cycles that split water into stoichiometric amounts of hydrogen and oxygen below 1,000 °C, and do not involve toxic or corrosive intermediates, are highly desirable because they can convert heat into chemical energy in the form of hydrogen. We report a manganese-based thermochemical cycle with a highest operating temperature of 850 °C that is completely recyclable and does not involve toxic or corrosive components. The thermochemical cycle utilizes redox reactions of Mn(II)/Mn(III) oxides. The shuttling of Na(+) into and out of the manganese oxides in the hydrogen and oxygen evolution steps, respectively, provides the key thermodynamic driving forces and allows for the cycle to be closed at temperatures below 1,000 °C. The production of hydrogen and oxygen is fully reproducible for at least five cycles.
    Proceedings of the National Academy of Sciences 05/2012; 109(24):9260-4. DOI:10.1073/pnas.1206407109 · 9.67 Impact Factor