Thermodynamics of Molybdenum Trioxide Encapsulated in Zeolite Y

Zeolites with encapsulated transition metal species are extensively applied in the chemical industry as heterogenous catalysts for favorable kinetic pathways. To elucidate the energetic insights into formation of subnano-sized molybdenum trioxide (MoO) encapsulated/confined in zeolite Y (FAU) from constituent oxides, we performed a systematic experimental thermodynamic study using high temperature oxide melt solution calorimetry as the major tool. Specifically, the formation enthalpy of each MoO/FAU is less endothermic than corresponding zeolite Y, suggesting enhanced thermodynamic stability. As Si/Al ratio increases, the enthalpies of formation of MoO/FAU with identical loading (5 Mo-wt%) tend to be less endothermic, ranging from 61.1 ± 1.8 (Si/Al = 2.9) to 32.8 ± 1.4 kJ/mol TO (Si/Al = 45.6). Coupled with spectroscopic, structural and morphological characterizations, we revealed intricate energetics of MoO – zeolite Y guest – host interactions likely determined by the subtle redox and/or phase evolutions of encapsulated MoO.

Thermodynamics of Molybdenum Trioxide (MoO3) Encapsulated in Zeolite Y

Zeolites with encapsulated transition metal species are extensively applied in the chemical industry as heterogenous catalysts for favorable kinetic pathways. To elucidate the energetic insights into formation of subnano-sized molybdenum trioxide (MoO) encapsulated/confined in zeolite Y (FAU) from constituent oxides, we performed a systematic experimental thermodynamic study using high temperature oxide melt solution calorimetry as the major tool. Specifically, the formation enthalpy of each MoO/FAU is less endothermic than corresponding zeolite Y, suggesting enhanced thermodynamic stability. As Si/Al ratio increases, the enthalpies of formation of MoO/FAU with identical loading (5 Mo-wt%) tend to be less endothermic, ranging from 61.1 ± 1.8 (Si/Al = 2.9) to 32.8 ± 1.4 kJ/mol TO (Si/Al = 45.6). Coupled with spectroscopic, structural and morphological characterizations, we revealed intricate energetics of MoO – zeolite Y guest – host interactions likely determined by the subtle redox and/or phase evolutions of encapsulated MoO.

First and Second Dissociation Enthalpies in Bi-Component Crystals Consisting of Maleic Acid and L-Phenylalanine

The energetics of the stepwise dissociation of a A:B2 bi-component crystal, according to A:B2(cr) → A:B(cr) + B(cr) and A:B(cr) → A(cr) + B(cr), was investigated using MA:Phe2 and MA:Phe (MA = maleic acid; Phe = L-phenylalanine) as model systems. The enthalpy changes associated with these sequential processes and with the overall dissociation reaction A:B2(cr) → A(cr) + 2B(cr) were determined by solution calorimetry. It was found that they are all positive, indicating that there is a lattice enthalpy gain when MA:Phe2 is formed, either from the individual precursors or by adding Phe to MA:Phe. Single-crystal X-ray diffraction (SCXRD) analysis showed that MA:Phe2 is best described as a protic salt containing a maleate anion (MA−) and two non-equivalent L-phenylalanine units, both linked to MA− by NH···O hydrogen bonds (H-bond): one of these units is protonated (HPhe+) and the other zwitterionic (Phe±). Only MA− and HPhe+ molecules are present in the MA:Phe lattice. In this case, however, NH···O and OH···O H-bonds are formed between each MA− unit and two HPhe+ molecules. Despite these structural differences, the enthalpy cost for the removal of the zwitterionic Phe± unit from the MA:Phe2 lattice to yield MA:Phe is only 0.9 ± 0.4 kJ mol−1 higher than that for the dissociation of MA:Phe, which requires a proton transfer from HPhe+ to MA− and the rearrangement of L-phenylalanine to the zwitterionic, Phe±, form. Finally, a comparison of the dissociation energetics and structures of MA:Phe and of the previously reported glycine maleate (MA:Gly) analogue indicated that parameters, such as the packing coefficient, density, hydrogen bonds formed, or fusion temperature, are not necessarily good descriptors of dissociation enthalpy or lattice enthalpy trends when bi-component crystals with different molecular composition are being compared, even if the stoichiometry is the same.

Thermodynamics of Molybdenum Trioxide (MoO3) Encapsulated in Zeolite Y

Zeolites with encapsulated transition metal species are extensively applied in the chemical industry as heterogenous catalysts for favorable kinetic pathways. To elucidate the energetic insights into formation of subnano-sized molybdenum trioxide (MoO3) encapsulated/confined in zeolite Y (FAU) from constituent oxides, we performed a systematic experimental thermodynamic study using high temperature oxide melt solution calorimetry as the major tool. Specifically, the formation enthalpy of each MoO3/FAU is less endothermic than corresponding zeolite Y, suggesting enhanced thermodynamic stability. As Si/Al ratio increases, the enthalpies of formation of MoO3/FAU with identical MoO3 loading tends to be less endothermic, ranging from 61.1 ± 1.8 (Si/Al = 2.9) to 32.8 ± 1.4 kJ/mol TO2 (Si/Al = 45.6). Coupled with spectroscopic, structural and morphological characterizations, and catalytic performance tests, we revealed intricate energetics of MoO3 – zeolite Y guest – host interactions and catalytic performance governed by the phase evolutions of encapsulated MoO3.

Drop Solution Calorimetric Studies of Interface Enthalpy of Cubic Silver (I) Oxide (Ag2O) Nanocrystals

The nanoscale, cubic silver (I) oxide (Ag2O.nH2O) with different particles sizes and surface areas were synthesized by a wet chemical technique. The prepared crystallite size ranges were from (33.3±0.3 to 39.4±0.4 nm). Interface areas were estimated by comparing the surface areas measured by N2 adsorption to the crystallite sizes refined from X-ray diffraction data. The interface enthalpy of Ag2O.nH2O nanocrystal was measured using isothermal acid solution calorimetry in 25%HNO3 at 26°C. The interface enthalpy was verified by utilizing thermodynamic cycle. The enthalpies of drop solution (ΔHds) for Ag2O.nH2O are exothermic and range from (-62.228±0.197) to (-64.025±0.434 kJ/mol), while its interface enthalpy is (0.842±0.508 J/m2). This work provides the first calorimetric measurement of the interface enthalpy of nanocrystalline silver (I) oxide (Ag2O.nH2O).

Calorimetric Studies of Magnesium-Rich Mg-Pd Alloys

Solution calorimetry with liquid aluminum as the bath was conducted to measure the enthalpy of a solution of magnesium and palladium as well as the standard formation enthalpies of selected magnesium-palladium alloys. These alloys were synthesized from pure elements, which were melted in a resistance furnace that was placed in a glove box containing high-purity argon and a very low concentration of impurities, such as oxygen and water vapor. A Setaram MHTC 96 Line evo drop calorimeter was used to determine the energetic effects of the solution. The enthalpies of the Mg and Pd solutions in liquid aluminum were measured at 1033 K, and they equaled −8.6 ± 1.1 and −186.8 ± 1.1 kJ/mol, respectively. The values of the standard formation enthalpy of the investigated alloys with concentrations close to the Mg6Pd, ε, Mg5Pd2, and Mg2Pd intermetallic phases were determined as follows: −28.0 ± 1.2 kJ/mol of atoms, −32.6 ± 1.6 kJ/mol of atoms, −46.8 ± 1.4 kJ/mol of atoms, and −56.0 ± 1.6 kJ/mol of atoms, respectively. The latter data were compared with existing experimental and theoretical data from the literature along with data calculated using the Miedema model.