Convective Regime of Filtration Combustion of Energetic Materials in a Cocurrent Flow of Their Combustion Products

The convective regime of filtration combustion of energetic materials in a cocurrent flow of their combustion products is studied using a model with extremely simplified kinetics and heat transfer, which shows instability of the process. It is shown that the more accurate twotemperature model describes a steadystate regime. In this regime, the gas temperature on the hot boundary of the heating zone is well below the combustion temperature, and the solidphase temperature is well below the temperature proposed in recent studies on this topic. It is pointed out that the twotemperature approach is unjustified and intragranular nonisothermicity must be taken into account for convective regimes. It is shown that the threetemperature model, which takes into account this effect, does not give a stable steadystate solution.     

A study of the acidic and catalytic properties of pure and sulfated zirconia–titania and zirconia–silica mixed oxides

Protonated pyridine (PyH⁺) was not found on ZrO₂ (Z) or ZrO₂–TiO₂ (ZT), but was detected on sulfated oxides (ZS, ZTS) by IR spectroscopy. In contrast, ZrO₂–SiO₂ samples containing about 30–80 mol% ZrO₂ showed Brønsted acidity both in nonsulfated (ZS) and sulfated (ZSS) forms. The total acidity was determined by NH₃TPD. Introduction of sulfate ions increased the sitespecific catalytic activity (TOF) in the conversion of cyclopropane or nhexane. The effect of sulfate ions was more significant on samples rich in zirconia. Results suggest that Zr is homogeneously distributed in ZS samples rich in silica. Zirconiabound dimeric sulfate, generating strong acidity, could not be formed in these preparations due to the absence of fairly large ZrO₂ domains.     

Variable‐temperature IR spectroscopic studies of CO adsorbed on Na‐ZSM‐5 and Na‐Y zeolites

CO interacts with extraframework alkali metal cations (M⁺=) of zeolites to form both M⁺CO and M⁺OC species. By using variabletemperature FTIR spectroscopy, these Cbonded and Obonded species were found to be in a temperaturedependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa⁺=CO ZNa⁺OC, where Z represents the zeolite framework, H ⁰ was found to take the values 3.8 and 2.4 kJ mol^– for CO/NaZSM5 and CO/NaY, respectively. The Cbonded species show always the highest cation–CO interaction energy.     

Location of Mn in MnAPO‐11: influence of synthesis from aqueous and non‐aqueous media

MnAPO11 samples were synthesized from aqueous (MnAPO11(A)) and ethylene glycol (MnAPO11(NA)) media. The crystallinity of the samples was more when the synthesis was carried out in ethylene glycol. Chemical and thermogravimetric analyses reveal greater incorporation of Mn in the framework of MnAPO11(NA) than in MnAPO11(A). At least five different types of Mn(II) species are detected in the samples by ESR. The studies suggest that Mn is more homogeneously distributed in MnAPO11(NA) than in MnAPO11(A).     

Lean NOx catalysis over alumina‐supported catalysts

aluminasupported catalysts show promise as lean NO_x catalysts. The role of alumina in influencing the structural and chemical properties of the active phase supported on it is discussed using some effective aluminabased lean NO_x catalysts. These include Ag/Al₂O₃, CoO_x/Al₂O₃ and SnO₂/Al₂O₃. Alumina plays an important role in stabilizing Ag in the oxidic phase and cobalt in the 2+ oxidation state. For SnO₂/Al₂O₃, alumina increases the SnO₂ surface area. On both Ag/Al₂O₃ and SnO₂/Al₂O₃, alumina also participates actively in the NO_x reduction reaction. An active organic intermediate is formed on Ag or Sn oxide which reacts with NO_x subsequently on alumina to form N₂.     

Impact of Structural Factors on Unsteady Combustion Modes of Gasless Systems

Based on the twotemperature, twovelocity timedependent model of gasless combustion, taking into account structural transformations related to the force action of the gas filtering in the pores and vitrification and volume variation of the condensed phase during the chemical transformation, selfoscillatory combustion modes are studied. Structural transformations are shown to have a pronounced effect on the propagation pattern of combustion waves and can either stabilize or destabilize combustion. The major structural parameters appreciably affecting combustionwave stability are the initial porosity, particle size, and pressure.     

Effect of framework aluminum in proton‐ or copper‐exchanged MFI ferrialuminosilicates on their activity for the reduction of nitric oxide with ammonia

Reduction of NO with ammonia in excess oxygen has been carried out on protonform and Cu²⁺exchanged MFI ferrialuminosilicate. Though HZSM5 showed very low activity, the framework Al in Hferrialuminosilicate greatly enhanced the activity. The framework Al in Cu²⁺exchanged ferrialuminosilicate also enhanced the activity to some extent.     

Quasi‐Homogeneous and Pseudospin Modes of Zirconium‐Wire Combustion in Air

A novel experimental technique is proposed for examining the transition mechanism from quasihomogeneous to heterogeneous combustion — burning of a variablepitch spring. Depending on the pitch of aircombustible zirconium springs, two combustion modes are possible. Quasihomogeneous (layerbylayer) combustion is observed in the case of smallpitch springs; as the spring pitch increases, quasihomogeneous combustion transforms into heterogeneous (pseudospin) combustion. Conditions for the occurrence of various combustion modes, depending on the spring diameter and pitch, are studied.     

Synthesis of nanosized platinum cluster in cubic mesoporous material via a direct introduction method

Platinumclustercontaining cubic mesoporous material (PtMCM48) has been synthesized by direct introduction of chloroplatinic acid during the synthesis of MCM48. In addition, we have also studied the incipient wetness impregnation and ionexchange method to obtain the platinumcontaining cubic mesoporous material. The nature of the platinumMCM48 catalyst has been characterized by different techniques such as XRD, N₂ adsorption, TEM, XPS, and NMR. The catalyst obtained by direct introduction of platinum in the synthesis gel shows higher activity in the hydrogenation of benzene and toluene.     

A study of the influence of the synthesis conditions upon the catalytic properties of silicoaluminophosphate molecular sieves

The skeletal isomerization of 1butene was performed over a series of silicoaluminophosphate molecular sieves with AEL structure (SAPO11). The results were compared with those obtained over an aluminophosphate molecular sieve (AlPO₄11). The three SAPO11 samples, with different acidic properties, were synthesized by either varying the chemical composition of the synthesis gel or by varying the preparation time of the aforementioned gel. The catalytic results indicate that irrespective of how the acidity of the SAPO11 samples is changed (viz., independently of the method elected to modify the acidic properties of the samples), a close parallelism between the selectivity towards the skeletal isomerization and the number of (moderate + strong) Brønsted acid sites (sites retaining pyridine above 623 K) was observed for the SAPO11 solids. These results, definitively, indicate the participation of these acid sites in the skeletal isomerization process.     

Fabrication and characterization of the Ag‐based high‐technology model nanocluster catalyst for ethylene epoxidation manufactured by electron beam lithography

Nanocluster catalysis is an area where greater fundamental knowledge is needed to understand the behavior of aggregates of metal atoms in determining product selectivity of chemical reactions. While catalysis is practiced industrially with economic success there is still a great need to eliminate wasteful sidereactions which hurt overall yields. Here we report on fabrication of a Agbased hightechnology model nanocluster catalyst by using electron beam lithography (EBL) designed for systematic studies of the ethylene epoxidation reaction. The catalyst is made of a square array of cylindershaped Ag nanoclusters that are 200 Å in diameter, deposited on a four inch silicon wafer, precovered with a 100 Å thick film of alumina. The height of the particles and interparticle distance can vary, and were chosen to be 150–300 and 1000 Å, respectively. The high technology catalyst was characterized by Xray photoelectron spectroscopy (XPS), highresolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The thermal stability of Ag nanoclusters in vacuo was investigated.     

Gasless Combustion of Ti–Al Bimetallic Multilayer Nanofoils

Ti–Al multilayer foils were produced magnetron vacuum deposition. The microstructure period varied in the range of 5–110 nm, the number of layers was 150–4700, and the total thickness of a multilayer foil reached 15–20 m. The gasless combustion of the foils was studied. Steadystate and pulsating combustion regimes were revealed; combustion temperatures were determined for both regimes. It was shown that the most probable mechanism of the selfpropagating reaction is the diffusion of Al in Ti at a temperature close to the temperature of the transition.     

Possible Ignition of Particles Ejected into the Gap in Explosive Welding of Titanium

Processes that occur in the welding gap during explosive welding are analyzed. It is shown that metal particles flying out into the gap due to the jetformation effect can ignite in shockcompressed air. For most metals, the energy released thereby is small and has no significant influence on weld formation. In titanium welding on large areas, surface sections located far from the place of detonation initiation, which experience a longterm action of a hot air flow, can dissolve a large amount of oxygen and nitrogen. If particles from these sections enter the gap, it can lead to chemical reactions with formation of TiO₂ and TiN by the mechanism of internal combustion. The energy released in the gap per unit area is commensurable with and even greater than the kinetic energy of the accelerated (flyer) plate. Local bulging and rupture of metal observed in practice can be explained by ignition and combustion of gassaturated titanium particles in the welding gap.     

Reductive transamination of methoxyacetone with benzylamine over Pd/SiO2 catalyst modified with anchored chiral compounds

Methoxyacetone was transaminated with benzylamine to methoxyisopropylamine over a Pd/SiO₂ catalyst modified with Lalaninol or Lphenylalaninol covalently anchored to the surface of the support via an organosilicon spacer group. In the first step of transamination a Schiff base was formed from the ketone and benzylamine, and then it was hydrogenated in the second step on the chirally modified Pd/SiO₂ catalysts to an asymmetric secondary amine (Nbenzylmethoxyisopropylamine). In the third step the hydrogenolysis of the asymmetric secondary amine resulting in methoxyisopropylamine and toluene was carried out over a 10 wt% Pd/C catalyst. The highest enantiomeric excess of (S)methoxyisopropylamine was observed in cyclohexane (ee = –20–21%) using anchored Lalaninol as a chiral modifier.     

Surface of a Titanium Target After Interaction with Shaped‐Charge Jet Particles

Coatings containing WC_1-x and W₂C_1-x tungsten carbides were produced on titanium targets by a shapedcharge explosion. The experiment used conical liners with a cone angle of 30° manufactured from a mixture of fine powders of tungsten, graphite, and ammonium nitrate. A comparative quantitative xray analysis of the top and bottom layers of the coating was performed. For the phase WC_1-x in the bottom layer, the value of the cubic unit cell parameter falls outside the range known in the literature. Microhardness in different segments of the target was studied.     

Ignition of β‐Azidoethanol Droplets in Air

The behavior of a droplet of azidoethanol, a liquid explosive, in heated air was studied. It was shown that droplet ignition is a gasphase process limited by heat release due to the vaporphase oxidation of azidoethanol. An increase in the ambient temperature with distance from the ignition limit eads to enhancement of the role of the thermal decomposition of azidoethanol molecules. In the range of ambient temperatures of 560–620 K, the droplet ignition is determined by the kinetics of the parallel reactions — the gasphase oxidation and thermal decomposition of azidoethanol.     

Chemistry of Combustion Waves in Substances with a Complicated Structure of Reagent Molecules. 1. Structure of the Front of Rich Isopentane Flame

Results on the structure of the lowtemperature relaxation zone of the front of a laminar Bunsen flame of isoC₅H₁₂ (2methylbutane) under atmospheric pressure are presented. The flame of a premixed mixture isoC₅H₁₂ + O₂ + Ar with a fueltoair equivalence ratio of 1.7 is examined. The mass fluxes, total rates of reactions of matter consumption and expenditure, balance of substances, and profiles of bulk heatrelease rates are calculated on the basis of the experimental concentration and temperature profiles. The results obtained indicate that there is a vast region of lowtemperature conversion of isopentane in the flame front. It is found that only part of the products sampled by the microprobe from different points of the flame front results from transformations in the lowtemperature region, namely, oxygen, isopentane, water, carbon monoxide, propane, methane, and methanol. Ethylene, propylene, hydrogen dioxide, and formaldehyde are present in the lowtemperature zone in insignificant amounts; they are secondary products of conversion of methyl and propyl radicals. It is assumed that the observed feature is a result of the competing interaction of two mechanisms of fuelmixture conversion: selfcatalysis and thermal selfacceleration. Based on the previously suggested mechanism of oxidation pyrolysis by the scheme of intramolecular quadratic destruction, experimentally observed fragmentation of the isopentane molecule is demonstrated. In contrast to npentane, formation of methyl alcohol has been found in isopentane convection products.     

Failure of a Steel Spherical Particle upon Impact on High‐Porous Sheets

The failure of a 2mmdiameter steel particle upon normal impact with a velocity of 1.5–7.3 km/sec onsheets 1 mm thick was studied. Two types of sheets were used: continuous sheets of Duralumin and highporous sheets of a copper powder. The depth of the largest crater and the positions, areas, and effective diameters of all craters were measured on the witness plate. Values of the threshold impact velocities for the beginning of failure of the steel particle and distributions of the number of craters in size and radius from the impact point were obtained for various impact velocities. It is shown that at identical impact pressures near the failure threshold, the lowdensity highporous sheets break up the steel particle better than the Duralumin sheets. This is possibly caused by the thermal explosion of the sheet material in the collision region, which increases the time of interaction of the impactor with the sheet.     

Thrust Performance of an Ideal Pulse Detonation Engine

Quasisteady and twodimensional unsteady formulations of the problem on the operation cycle of a pulse detonation engine are derived. A formula for the specific impulse is obtained, and the thrust performance of the engine is calculated. It is found that the thrust performance of this engine for flight Mach numbers M [0; 3.6] and compression ratios p ₂/p ₁ [1; 80] are always higher than those of the ramjet and onespool turbojet. As the compression ratio increases, the advantage of the pulse detonation engine becomes less noticeable.     

Dehydration of 2‐methylbutanal to isoprene using aluminium phosphate catalysts

The synthesis of isoprene from the dehydration of 2methylbutanal is described using aluminium phosphates as catalysts. Two samples of aluminium phosphate are studied prepared from the reaction of phosphoric acid with aluminium chloride or sulphate. The chloride route gives a mixed cristobalite/tridymite AlPO₄ and this is shown to be more active than a catalyst containing only the tridymite form of AlPO₄ formed from the sulphate route. The AlPO₄ catalysts are also shown to be active catalysts for the synthesis of isoprene from 3methylbutan2one, which is the major byproduct formed from the reaction of 2methylbutanal. The AlPO₄ catalysts are deactivated due to the deposition of coke in addition to loss of phosphorus from the surface. Catalytic activity can be totally restored by a simple calcination procedure at 800°C.