Thermogravimetric Modulated Beam Mass Spectrometry Research Articles

An Investigation into the Hydrogen Storage Characteristics of Ca(BH4)2/LiNH2 and Ca(BH4)2/NaNH2: Evidence of Intramolecular Destabilization

We report a study of the hydrogen storage properties of materials that result from ball milling Ca(BH4)2 and MNH2 (M = Li or Na) in a 1:1 molar ratio. The reaction products were examined experimentally by powder X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS), and temperature-programmed desorption (TPD). The Ca(BH4)/LiNH2 system produces a single crystalline compound assigned to LiCa(BH4)2(NH2). In contrast, ball milling of the Ca(BH4)/NaNH2 system leads to a mixture of NaBH4 and Ca(NH2)2 produced by a metathesis reaction and another phase we assign to NaCa(BH4)2(NH2). Hydrogen desorption from the LiCa(BH4)2(NH2) compound starts around 150 °C, which is more than 160 °C lower than that from pure Ca(BH4)2. Hydrogen is the major gaseous species released from these materials; however various amounts of ammonia form as well. A comparison of the TGA/DSC, STMBMS, and TPD data suggests that the amount of NH3 released is lower when the desorption reaction is performed in a closed vessel. There is no evidence for diborane (B2H6) release from LiCa(BH4)2(NH2), but traces of other volatile boron–nitrogen species (B2N2H4 and BN3H3) are observed at 0.3 mol % of hydrogen released. Theoretical investigations of the possible crystal structures and detailed phase diagrams of the Li–Ca–B–N–H system were conducted using the prototype electrostatic ground state (PEGS) method and multiple gas canonical linear programming (MGCLP) approaches. The theory is in qualitative agreement with the experiments and explains how ammonia desorption in a closed volume can be suppressed. The reduced hydrogen desorption temperature of LiCa(BH4)2(NH2) relative to Ca(BH4)2 is believed to originate from intramolecular destabilization.

Thermodynamics and kinetics of NaAlH4 nanocluster decomposition

Reactive nanoparticles are of great interest for applications ranging from catalysis to energy storage. However, efforts to relate cluster size to thermodynamic stability and chemical reactivity are hampered by broad pore size distributions and poorly characterized chemical environments in many microporous templates. Metal hydrides are an important example of this problem. Theoretical calculations suggest that reducing their critical dimension to the nanoscale can in some cases considerably destabilize these materials and there is clear experimental evidence for accelerated kinetics, making hydrogen storage applications more attractive in some cases. However, quantitative measurements establishing the influence of size on thermodynamics are lacking, primarily because carbon aerogels often used as supports provide inadequate control over size and pore chemistry. Here, we employ the nanoporous metal-organic framework (MOF) Cu-BTC (also known as HKUST-1) as a template to synthesize and confine the complex hydride NaAlH(4). The well-defined crystalline structure and monodisperse pore dimensions of this MOF allow detailed, quantitative probing of the thermodynamics and kinetics of H(2) desorption from 1-nm NaAlH(4) clusters (NaAlH(4)@Cu-BTC) without the ambiguity associated with amorphous templates. Hydrogen evolution rates were measured as a function of time and temperature using the Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry method, in which sample mass changes are correlated with a complete analysis of evolved gases. NaAlH(4)@Cu-BTC undergoes a single-step dehydrogenation reaction in which the Na(3)AlH(6) intermediate formed during decomposition of the bulk hydride is not observed. Comparison of the thermodynamically controlled quasi-equilibrium reaction pathways in the bulk and nanoscale materials shows that the nanoclusters are slightly stabilized by confinement, having an H(2) desorption enthalpy that is 7 kJ (mol H(2))(-1) higher than the bulk material. In addition, the activation energy for desorption is only 53 kJ (mol H(2))(-1), more than 60 kJ (mol H(2))(-1) lower than the bulk. When combined with first-principles calculations of cluster thermodynamics, these data suggest that although interactions with the pore walls play a role in stabilizing these particles, size exerts the greater influence on the thermodynamics and reaction rates.

Synthesis, Structural Characterization, and Thermal Decomposition Study of Mg(H2O)6B10H10·4H2O

Compound 1 (Mg(H2O)6B10H10·4H2O) was synthesized and characterized using NMR, IR, XRD, and elemental analysis. Its thermal decomposition behavior was studied using Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry (STMBMS), TGA, DSC, IR, and 11B NMR. The crystal structure of 1 reveals multiple dihydrogen and hydrogen bonding interactions that form a 3D extended structure. A reaction network characterizing the thermal decomposition of 1 and its secondary products over a temperature range from 20 to 1000 °C has been developed. Thermal decomposition of 1 is primarily controlled by two competing branches in the reaction network, where coordinated water evolves as either H2O (dehydration) or H2 (dehydrogenation). The extent of reaction to form H2 depends on the fraction of the coordinated water remaining in the sample when its temperature is between 160 and 225 °C. The evolution of coordinated water is reversible and controlled by dissociative sublimation. For the release of coordinated water between 160 and 215 °C, the vapor pressure of water is given by LogeP (Torr) = 30.4561 – 12425.2/T (K) and ΔHs = 103.3 ± 0.3 kJ/mol. The nature of the condensed phase secondary product remaining after all coordinated water is removed by either dehydration and/or dehydrogenation depends strongly on the extent of reaction to form Mg(OH)xB10H10–x. Results of STMBMS experiments where x varies from 0.2 to ∼4 are used to develop the reaction network that characterizes the thermal decomposition process. Heating of 1 at 205 °C resulted in the formation of water-soluble Mg(OH)x(H2O)2–xB10H10–x, while prolonged heating of 1 at 270 °C and heating up to 1000 °C led to decomposition.

Metal−Organic Frameworks As Templates for Nanoscale NaAlH4

Metal-organic frameworks (MOFs) offer an attractive alternative to traditional hard and soft templates for nanocluster synthesis because their ordered crystalline lattice provides a highly controlled and inherently understandable environment. We demonstrate that MOFs are stable hosts for metal hydrides proposed for hydrogen storage and their reactive precursors, providing platform to test recent theoretical predictions that some of these materials can be destabilized with respect to hydrogen desorption by reducing their critical dimension to the nanoscale. With the MOF HKUST-1 as template, we show that NaAlH(4) nanoclusters as small as eight formula units can be synthesized. The confinement of these clusters within the MOF pores dramatically accelerates the desorption kinetics, causing decomposition to occur at approximately 100 degrees C lower than bulk NaAlH(4). However, using simultaneous thermogravimetric modulated beam mass spectrometry, we also show that the thermal decomposition mechanism of NaAlH(4) is complex and may involve processes such as nucleation and growth in addition to the normally assumed two-step chemical decomposition reactions.

Vapor Pressures, Mass Spectra and Thermal Decomposition Processes of Bis(2,2‐Dintropropyl)acetal (BDNPA) and Bis(2,2‐Dinitropropyl)formal (BDNPF)

AbstractSimultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) and Fourier‐transform ion cyclotron resonance (FTICR) instruments have been used to measure the mass spectra, measure vapor pressures and evaluate the thermal decomposition mechanism of bis(2,2‐dinitropropyl)acetal (BDNPA) and bis(2,2‐dinitropropyl)formal (BDNPF). The high mass accuracy FTICR mass spectra provide the chemical formulas of the ion fragments formed in the mass spectra of BDNPA, BDNPF and their decomposition products, and provide a basis for predicting possible structures of the ion fragments. The heat of vaporization (ΔvapH) and vapor pressure at 25 °C are 93.01±0.38 kJ/mol and 1.4532+0.40/−0.27 mPa for BDNPA, and 84.77±0.88 kJ/mol and 2.20+1.87/−1.07 mPa for BDNPF. STMBMS data support a nitro‐nitrite (NO2→ONO) rearrangement mechanism for both compounds. Upon rearrangement, both NO and NO2 are cleaved from the structure, thus producing a ketone radical. The nitro‐nitrite rearrangement begins to occur at appreciable rates between 160 and 180 °C. Additional decomposition products include amines, imines and amides, as well as CO2 and H2O at higher temperatures. STMBMS mass loss data suggest the formation of a residue during the decomposition of BDNPA and BDNPF. The major difference between the decomposition of the two compounds is the slower reaction rate of BDNPF. We postulate that the less sterically hindered formal carbon of BDNPF subjects it to interactions with an intermediate, thus forming a complex and delaying its release. Methods to elucidate complex thermal decomposition mechanisms from STMBMS data are illustrated.

A mechanism for the decomposition of dinitropropyl compounds

A decomposition mechanism is proposed for 2,2-dinitro-1-methoxypropane, a compound whose structure resembles the nitroplasticizer (NP) component of plastic-bonded explosive PBX 9501. A library of key reactions is presented and is based on the results of NP aging studies and existing decomposition mechanisms for similar nitro compounds. Density functional electronic structure calculations on these reactions were used to develop a decomposition mechanism at lower temperatures, which begins with HONO elimination and leads to intermediates that can produce CO, CO(2), NO, and N(2)O gases. These gases were observed in low temperature (48 to 64 degrees C) aging studies of NP. A high temperature mechanism involving NO(2) scission is compared to a thermal decomposition mechanism determined by simultaneous thermogravimetric modulated beam mass spectrometry. The calculated energy barriers for HONO elimination and NO(2) scission in the gas phase are reported and compared to experimental results.

Thermal decomposition mechanisms of bis(2‐fluoro‐2,2‐dinitroethyl) formal (FEFO) and bis(2‐fluoro‐2,2‐dinitroethyl) difluoroformal (DFF) from simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) measurements

AbstractThe simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) technique has been applied to measure the vapor pressures and evaluate the thermal decomposition chemistry of two energetic liquids, bis(2‐fluoro‐2,2‐dinitroethyl)formal (FEFO) and bis(2‐fluoro‐2,2‐dinitroethyl)difluoroformal (DFF). The resulting heat of vaporization (ΔHvap) and vapor pressure at 25°C are 20.3 ± 0.2 kcal/mol and 0.4 ± 0.1 millitorr for FEFO, and 17.3 ± 0.2 kcal/mol and 5.1 ± 1.1 millitorr for DFF. The thermal decomposition of FEFO indicates there are six major pyrolysis pathways. The results suggest that FEFO initially decomposes at 150°C by rearrangement of the nitro group (NO2) to the nitrite group (ONO), followed by loss of NO. Some NO2 is also formed at 170°C. Between 200°C–300°C, further pyrolysis occurs. In one pathway, the FEFO backbone remains intact and a high molecular‐weight product is formed. The other three pathways involve scission of the FEFO backbone; one yielding CO2 (possibly N2O), one yielding CH2O and CO, and one yielding C3H2NOF. Differences in the thermal decomposition behaviors in the liquid and gas phases are observed. In the thermal decomposition of DFF, the formal fluorine atoms stabilize the backbone structure. Numerous minor thermal decomposition products are also reported.

Mass spectra of 2,4-dinitroimidazole and its isotopomers using simultaneous thermogravimetric modulated beam mass spectrometry

The EI mass spectrum of 2,4-dinitroimidazole (2,4-DNI) was studied at 20 and 70 eV ionizing energy using simultaneous thermogravimetric modulated beam mass spectrometry. Product ion formulas and their origin from within the 2,4-DNI molecule were identified from the evaluation of perdeuterated and various 15 N-labeled 2,4-DNI mass spectra. The molecular ion (M +. ; m/z 158) is present in all the spectra and the NO + ion (m/z 30) is the major primary product ion. The base peak is formed by the M .+ ion at 20 eV and by the NO + ion at 70 eV. Other primary product ions are NO 2 + (m/z 46), CH 2 N + (m/z 28) and CHN 2 O 2 + (m/z 73). Fragmentation pathways are postulated based on the identity and relative abundance of the product ions, and on the change in relative abundance of the ions as a function of ionizing energy. Comparisons of the 70 eV mass spectra of 2,4-DNI, 4,5-dinitroimidazole (4,5-DNI) and 1,4-dinitroimidazole (1,4-DNI) show that the fragment product ions have the same m/z values in the three spectra but differ in their relative abundance and in the ion that forms the base peak. The base peak for 2,4-DNI and 4,5-DNI is formed by the ion at m/z 30 (NO + ) and for 1,4-DNI, the ion at m/z 46 (NO 2 + ). This difference is probably due to the nitramine functionality of 1,4-DNI

Thermal Decomposition Studies of a New, gem-Dinitroalkyl Nitramine 1,3,3-Trinitroazetidine ( TNAZ ).

The initial results from a study of the thermal decomposition of TNAZ and TNAZ-\.'sN02 using the simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) technique are presented. The major products formed in the decomposition of TNAZ are N02 and NO with slightly lesser amounts of H20, HCN, COIN2, C01!N20 and l-nitroso-Ld-dinitroazetidine (NDNAZ). The last mentioned product (NDNAZ) is shown to be an important intermediate in the decomposition of TNAZ and its decomposition has also been examined by synthesizing it independently. Major products formed in this decomposition of NDNAZare NO with smaller amounts of H20, HCN, COIN2 and C01!N20. The temporal behaviours of the ion signals associated with the various thermal decomposition products from TNAZ, TNAZ-I-15N02 and NDNAZ are also presented. They illustrate the evolution sequence of the various products that are associated with the different reaction pathways that control the decomposition of these materials. In particular, the study of the ISN-Iabeled sample revealed that N02 originates from both the likely sites in the TNAZ molecule and that the cleavage of the nitramine-N02 group precedes that of the C-N02 cleavage, resulting in similar sequences in the formation of NO and NDNAZ also.

Solid-Phase Thermal Decomposition of 2,4-Dinitroimidazole (2,4-DNI)

AbstractThe solid-phase thermal decomposition of the insensitive energetic aromatic heterocycle 2,4- dinitroimidazole (2,4-DNI: mp 265–274°C) is studied utilizing simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) between 200° and 247°C. The pyrolysis products have been identified using perdeuterated and N-labeled isotopomers. The products consist of low molecular-weight gases and a thermally stable solid residue. The major gaseous products are NO, CO2, CO, N2, HNCO and H2O. Minor gaseous products are HCN, C2N2, NO2, C3H4N2, C3H3N3O and NH3. The elemental formula of the residue is C2HN2O and FTIR analysis suggests that it is polyurea- and polycarbamate-like in nature. The rates of formation of the gaseous products and their respective quantities have been determined for a typical isothermal decomposition experiment at 235°C. The temporal behaviors of the gas formation rates indicate that the overall decomposition is characterized by a sequence of four events; 1) an early decomposition period induced by impurities and H2O, 2) an induction period where CO2 and NO are the primary products formed at relatively constant rates, 3) an autoacceleratory period that peaks when the sample is depleted and 4) a final period in which the residue decomposes. Arrhenius parameters for the induction period are Ea = 46.9 ± 0.7 kcal/mol and Log(A) = 16.3 ± 0.3. Decomposition pathways that are consistent with the data are presented.

ChemInform Abstract: Determination of the Rates of Formation of Gaseous Products from the Pyrolysis of Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) by Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry.

ChemInform Abstract: Determination of the Rates of Formation of Gaseous Products from the Pyrolysis of Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) by Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry.

ChemInform Abstract: Identification of Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) Pyrolysis Products by Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry and Time‐of‐Flight Velocity‐Spectra Measurements.

AbstractIn the frame of studies of physical processes that control the decomposition of energetic ingredients of explosives such as (I) its pyrolysis is studied.

Determination of the rates of formation of gaseous products from the pyrolysis of octahydro‐1, 3, 5, 7‐tetranitro‐1, 3, 5, 7‐tetrazocine (hmx) by simultaneous thermogravimetric modulated beam mass spectrometry

AbstractThe method for determining the rates of formation of gaseous pyrolysis products during thermal decompositions by simultaneous thermogravimetric modulated beam mass spectrometry is presented. The analysis procedure that handles both molecular and continuum flow from the reaction cell is described. The technique is illustrated with the isothermal decomposition of HMX. The temporal behaviors of the rates of formation of the pyrolysis products, H2O, HCN, CO, CH2O, NO, N2O, methylformamide, C2H6N2O, and octahydro‐1‐nitroso‐3, 5, 7‐trinitro‐1, 3, 5, 7‐tetrazocene, formed during the isothermal decomposition of HMX at 211°C, are presented. The results show that a complex condensed‐phase reaction mechanism controls the decomposition.

Identification of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) pyrolysis products by simultaneous thermogravimetric modulated beam mass spectrometry and time‐of‐flight velocity‐spectra measurements

AbstractThe pyrolysis products formed during the isothermal decomposition of HMX at 211°C are H2O, HCN, CO, CH2O, NO, N2O, methylformamide, C2H6N2O, octahydro‐1‐nitroso‐3,5,7‐trinitro‐1,3,5,7‐tetrazocine, and a nonvolatile residue. The temporal behaviors of these products during the decomposition are presented. The method for using time‐of‐flight (TOF) velocity spectra to assist mass‐spectrometry measurements in identifying the different gaseous products formed from the pyrolysis of a material by determining the approximate molecular weights of the different gaseous products contributing to the different m/z values in the mass spectrum of the mixture is described. The ion fragmentation of HMX as a function of electron energy shows complete fragmentation of the HMX molecular ion for electron energies ≥ 12.4 eV. No fragments from the pyrolysis of HMX other than those mentioned above are observed.