Structural information from progression bands in the FTIR spectra of long chain n-alkanes

The low temperature FTIR spectrum of long chain n-alkanes has been investigated in the region between the C-C stretching and CH₂ twisting fundamentals (1050-1133 cm⁻¹). With successive annealing and cooling stages, extended chain crystals of n-C₁₉₈H₃₉₈ show an improvement in the regularity of the progression bands observed. This is related to a ‘perfecting’ of the crystals. A once-folded sample of the same alkane shows additional features between 1050 and 1100 cm⁻¹, attributed to resonance modes from a tight (110) fold. These disappear on transformation to the extended form, to be replaced by progression bands. Assignment of the individual bands enables the length of the all-trans chain to be estimated and this method is used to show that centre-branched long chain n-alkanes have a folded conformation. It is also shown that the chain length derived from such FTIR data for a 1:1 molar mixture of n-C₁₆₂H₃₂₆ and n-C₂₄₆H₄₉₄ is consistent with a triple layer superlattice structure.     

Structural behaviour of molecular alloys: n-pentacontane (n-C50H102)-n-pentacosane (n-C25H52); n-pentacontane (n-C50H102)-n-tricosane (n-C23H48) at room temperature

This paper displays a study of binary mixtures of n-alkanes whose ratio of chain length is around two. The systems composed of n-tricosane (n-C₂₃H₄₈)-n-pentacontane (n-C₅₀H₁₀₂) and n-pentacosane (n-C₂₅H₅₂)-n-pentacontane (n-C₅₀H₁₀₂) have been studied by means of X-ray analyses. These latter, performed at room temperature, showed in both cases, the existence of a large domain where the phases characteristic of each pure component coexist. These mixtures obey Kravchenko's rule relative to the solubility of the n-alkanes according to the chain length of each component. The mixtures studied do not form an intermediate solid solution. In other words, there is no particular arrangement of the shorter molecules inside the crystallographic unit of the longer.     

Pure and mixed gas CH4 and n-C4H10 sorption and dilation in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C₄H₁₀ and CH₄ sorption and dilation in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures ranging from −20 to 35 °C. The presence of n-C₄H₁₀ in the mixture considerably reduces CH₄ solubility. For example, CH₄ solubility (in the limit of zero CH₄ fugacity) at 25°C decreases from 4.0 (pure gas) to 0.78 cm³(STP)/(cm³ polymer atm) in the presence of n-C₄H₁₀ at an activity of 0.60. At −20 °C, CH₄ solubility decreases by almost an order of magnitude, from 10.2 (pure gas) to 1.22 cm³(STP)/(cm³ polymer atm) in the presence of n-C₄H₁₀ at an activity of 0.61. In contrast, n-C₄H₁₀ mixture sorption properties are not measurably affected by the presence of CH₄. The dual mode sorption model parameters for CH₄ and n-C₄H₁₀ in PTMSP were determined from pure and mixed gas sorption measurements, and this model can adequately describe the sorption data. The n-C₄H₁₀/CH₄ mixed gas solubility selectivity in PTMSP decreases as temperature increases and as n-C₄H₁₀ activity increases. For example, at 25 °C, the n-C₄H₁₀/CH₄ solubility selectivity decreases from 250 to 120 as n-C₄H₁₀ activity increases from 0.02 to 0.25. At −20 °C and an n-C₄H₁₀ activity of 0.24, the n-C₄H₁₀/CH₄ solubility selectivity is 590. Penetrant-induced volume dilation of PTMSP can be adequately modeled by assuming that all swelling is caused by penetrant molecules sorbed in the polymer's dense equilibrium region (i.e., the Henry's law region) during sorption. However, the best fit partial molar volumes in the Henry's law region for the dilation data are considerably lower than the penetrant partial molar volumes in liquids, suggesting that further theoretical efforts are needed to develop predictive models of volume dilation in high free volume glassy polymers.     

Determination of critical properties of pure and multi-component mixtures using a “dynamic-synthetic” apparatus

In this work, we use an efficient and reliable “dynamic-synthetic” method for the measurement of critical properties of a variety of pure compounds, binary mixtures, and one ternary mixture. The critical phenomenon is observed via the critical opalescence in a view cell, which withstands conditions up to 543 K and 20 MPa. Excellent agreements were obtained between the measured pure compounds’ properties and those listed in recent databases. Among the pure compounds measured, several refrigerants less well described in open literature (C₃F₆, C₃F₆O, R365mfc) have been compared with conventional predictive models. The critical profiles of associating, azeotropic systems ethanol + n-alkanes (C₄-C₈) are presented, as well as the critical surfaces for the ternary system n-pentane + ethanol + n-hexane. We present forms of the Cibulka and Singh's equation suitable for correlating ternary critical properties.     

Preparation and spectroscopic properties of chlorofullerenes C60Cl24, C60Cl28, and C60Cl30

We report easy preparation of recently discovered highly chlorinated fullerenes T_h-C₆₀Cl₂₄, C₁-C₆₀Cl₂₈, and D_3d-C₆₀Cl₃₀ in high-temperature reactions of C₆₀ with PCl₅ and ICl. Formation and interconversion of chlorofullerenes was investigated in details for C₆₀-ICl system. C₆₀Cl₂₈ is the least stable chlorofullerene that undergoes rearrangement (accompanied by partial chlorine elimination) into more stable T_h-C₆₀Cl₂₄ under more drastic reaction conditions (increased temperature and time of chlorination). T_h-C₆₀Cl₂₄ yields D_3d-C₆₀Cl₃₀ at temperatures above 220 °C via a sequence of rearrangements and further addition of chlorine. In contrast to the fullerene reaction with ICl, interaction of C₆₀ with PCl₅ is very selective with respect to formation of C₆₀Cl₂₄ in a wide temperature range. Solid-state electronic (UV-Vis) and vibrational (IR) spectra of chlorinated fullerenes C₆₀Cl₂₄, C₆₀Cl₂₈, C₆₀Cl₃₀ and fluorinated fullerenes C₆₀F₁₈ and C₆₀F₃₆ were recorded in the spectral range between 30 and 45,000 cm⁻¹. Raman spectra were also acquired for all investigated compounds. Moreover, molecular geometry of the C₆₀Cl₂₄ and its theoretical IR-absorption spectrum were calculated using B3LYP/STO-3G chemistry model.     

Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water☆

Partial oxidative gasification of n-hexadecane (n-C₁₆) and organosolv-lignin (lignin) was studied by use of a batch type reactor in supercritical water: 673 K, 0.52 cm⁻³ of water density (40 MPa of water pressure at 673 K), and 0.3 of O/C ratio for the n-C₁₆ experiments; 673 K, 0.35 cm⁻³ of water density (30 MPa of water pressure at 673 K), and 1.0 of O/C ratio for the lignin experiments. The experiments without O₂ were also conducted for lignin (lignin decomposition). For all the cases (n-C₁₆ partial oxidation, lignin decomposition, lignin partial oxidation), NaOH or zirconia (ZrO₂) was added in the system as catalysts. Through n-C₁₆ studies, the catalytic effect of NaOH and ZrO₂ on partial oxidation in supercritical water were examined. In the case of lignin partial oxidation, we studied the possibility of partial oxidation in supercritical water for gasification technique of wastes. The yield of H₂ from n-C₁₆ and lignin with zirconia was twice as same as that without catalyst at the same condition. The H₂ yield with NaOH was 4 times higher than that without catalyst. Thus, a base catalyst has a positive effect on partial oxidation of n-C₁₆ and lignin to produce H₂. The catalytic effect of NaOH and ZrO₂ was found to be enhancement of decomposition of intermediate (aldehyde and ketone) into CO, through n-C₁₆ studies. In the case of lignin studies, the enhancement of decomposition of the carbonyl compounds by catalytic effect of NaOH and ZrO₂ inhibit char formation and promotes CO and thus H₂ formation.     

Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

The ignition and burnout of the volatiles in fluidized bed combustor are essential for its performance and emissions. NO_x are known to sensitize the oxidation of hydrocarbons, CO, and H₂. This effect is relevant especially for fluidized bed combustors, which are operated at relatively low temperatures (i.e. about 850 °C). Different reaction mechanisms and modifications to existing mechanisms have been proposed in the literature to account for these low temperature interactions of NO_x and hydrocarbons. In this work, an existing widely used reaction mechanism is adapted and tested for its capability to describe the NO sensitized oxidation of CH₄ under conditions relevant to fluidized bed combustion. NO lowers the ignition temperature to about 300 °C under the conditions investigated. Three different oxidation paths for the oxidation of CH₄ have been identified and discussed. Their relative importance strongly depends on combustion temperature, indicating that the presence of NO_x significantly affects the oxidation of the volatiles in fluidized bed combustion.     

Coal devolatilization and char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres

The aim of the present investigation is to examine differences between O₂/N₂ and O₂/CO₂ atmospheres during devolatilization and char conversion of a bituminous coal at conditions covering temperatures between 1173 K and 1673 K and inlet oxygen concentrations between 5 and 28 vol.%. The experiments have been carried out in an electrically heated entrained flow reactor that is designed to simulate the conditions in a suspension fired boiler. Coal devolatilized in N₂ and CO₂ atmospheres provided similar results regarding char morphology, char N₂-BET surface area and volatile yield. This strongly indicates that a shift from air to oxy-fuel combustion does not influence the devolatilization process significantly. Char combustion experiments yielded similar char conversion profiles when N₂ was replaced with CO₂ under conditions where combustion was primarily controlled by chemical kinetics. When char was burned at 1573 K and 1673 K a faster conversion was found in N₂ suggesting that the lower molecular diffusion coefficient of O₂ in CO₂ lowers the char conversion rate when external mass transfer influences combustion. The reaction of char with CO₂ was not observed to have an influence on char conversion rates at the applied experimental conditions.     

Thermodynamic and structural analyses of the solid phases in multi-alkane mixtures similar to petroleum cuts at ambient temperature

Structural analyses were carried out by X-ray diffraction, at ambient temperature, on multi-alkane samples whose mole fraction distribution shows a shape of the ‘exponential decreasing’ type, as observed in petroleum cuts. Nine samples, whose normal-alkane number varies from 15 to 23, have been studied with mole fraction continuous distributions of normal-alkanes going from C₂₂-C₃₆ to C₁₄-C₃₆: each mixture differs from the previous sample by the addition of a lighter n-alkane. At the solid state, the multi-alkane solid samples C₂₂-C₃₆ and C₂₁-C₃₆ are two-phase, C₂₀-C₃₆ to C₁₅-C₃₆, three-phase, and the broader distribution C₁₄-C₃₆, four-phase. In these polyphase solid systems, whose heaviest n-alkane is always C₃₆, the average composition of the heavy and middle phases are constant and their structure are isostructural to the β′ ordered intermediate solid solution, observed in n-alkane binary or ternary molecular alloys; the mean carbon atom number of the light phase decreases as the global average carbon atom number of the synthetic mixtures in relation to the addition of light n-alkanes and its structure simultaneously evolves from the β′ ordered intermediate solid solution towards the β-RI(Fmmm) and the disordered solid solutions, observed in pure n-alkanes: the light n-alkane added between each distribution intercalates itself into the structure whose molecule stacking period (thickness) is compatible with its own carbon chain length, in order to reduce the molecular gaps.     

The influence of pentavalent Nb substitution for Zr on electrical property of oxide-ion conductor Gd2Zr2O7

Gd₂(Zr_1−xNb_x)₂O_7+x (0 ≤ x ≤ 0.2) ceramics are prepared via the solid state reaction process at 1973 K for 10 h in air. Gd₂(Zr_1−xNb_x)₂O_7+x (x = 0.1, 0.2) ceramics exhibit an ordered pyrochlore-type structure, whereas Gd₂Zr₂O₇ has a defective fluorite-type structure. The electrical property of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics is investigated by electrochemical impedance spectroscopy over a frequency range of 10 Hz to 8 MHz from 623 to 923 K. The electrical conductivity obeys the Arrhenius equation. The grain conductivity of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics varies with doping different Nb contents, and exhibits a maximum at the Nb content of x = 0.1 in the temperature range of 623-923 K. The conductivity in hydrogen atmosphere is a little bit higher than in air in the temperature range of 723-923 K, which indicates that the doping of Zr⁴⁺ by Nb⁵⁺ can increase the proton-type conduction and reduce the oxide-ionic conduction. The conduction of Gd₂(Zr_1−xNb_x)₂O_7+x is not a pure oxide-ionic conductor.     

Spark plasma sintering of Al2O3-cBN composites facilitated by Ni nanoparticle precipitation on cBN powder by rotary chemical vapor deposition

Al₂O₃-cBN/Ni composites were consolidated by spark plasma sintering (SPS) using α-Al₂O₃ and Ni nanoparticle precipitated cBN (cBN/Ni) powders. The Ni nanoparticles, 10-100 nm in diameter and 0.5-2.2 mass% in content, were precipitated on cBN powder by rotary chemical vapor deposition. The effect of sintering temperature (T_SPS) and Ni content (C_Ni) on the densification, phase transformation, microstructure and hardness of the Al₂O₃-cBN/Ni composites were investigated. The highest relative density of Al₂O₃-30 vol% cBN composite was 99% at T_SPS = 1573 K and C_Ni = 1.7 mass%. At T_SPS = 1673 K, the relative density decreased due to the phase transformation of cBN to hBN. The Vickers hardness of Al₂O₃-30 vol% cBN/Ni at T_SPS = 1573 K and C_Ni = 1.7 mass% showed the highest value of 27 GPa.     

Liquid fuel production from syngas using bifunctional CuO-CoO-Cr2O3 catalyst mixed with MFI zeolite

CuO-CoO-Cr₂O₃ mixed with MFI Zeolite (Si/Al = 35) prepared by co-precipitation was used for synthesis gas conversion to long chain hydrocarbon fuel. CuO-CoO-Cr₂O₃ catalyst was prepared by co-precipitation method using citric acid as complexant with physicochemical characterization by BET, TPR, TGA, XRD, H₂-chemisorptions, SEM and TEM techniques. The conversion experiments were carried out in a fixed bed reactor, with different temperatures (225-325 °C), gas hourly space velocity (457 to 850 h⁻¹) and pressure (28-38 atm). The key products of the reaction were analyzed by gas chromatography mass spectroscopy (GC-MS). Significantly high yields of liquid aromatic hydrocarbon products were obtained over this catalyst. Higher temperature and pressure favored the CO conversion and formation of these liquid (C₅-C₁₅) hydrocarbons. Higher selectivity of C₅ + hydrocarbons observed at lower H₂/CO ratio and GHSV of the feed gas. On the other hand high yields of methane resulted, with a decrease in C₅+ to C₁₁+ fractions at lower GHSV. Addition of MFI Zeolite (Si/Al = 35) to catalyst CuO-CoO-Cr₂O₃ resulted a high conversion of CO-hydrogenation, which may be due to its large surface area and small particle size creating more active sites. The homogeneity of various components was also helpful to enhance the synergistic effect of Co promoters.     

Thermolysis of the polyyne C8H2 in hexane and methanol: Experimental and theoretical study

The mean lifetimes of polyyne C₈H₂ in hexane were determined at 50, 60, 80, and 100 °C and in methanol at 60 °C. The reactions are second order at all temperatures: ln k₂ = 20.5 ± 1.5-10303 ± 520T⁻¹ and the corresponding activation energy is 85.7 ± 6.3 kJ mol⁻¹ (7164 cm⁻¹). Extrapolation suggests that solutions at 1 mM concentration are significantly unstable at room temperature. Quantum chemical calculations show that polyynes C_mH₂ + C_nH₂ (m + n = 16) could be products, but these were not detected. Alternatively, C₁₆H₂ isomers could form. IR spectra of the solid residues from hexane and methanol solutions were obtained.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.     

Pure and mixed gas CH4 and n-C4H10 permeability and diffusivity in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C₄H₁₀ and CH₄ permeability coefficients in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures from −20 to 35 °C. CH₄ partial pressures range from 1.1 to 14.6 atm, and n-C₄H₁₀ partial pressures range from 0.02 to 1.8 atm. CH₄ permeability decreases with increasing n-C₄H₁₀ upstream activity (f/f_sat) in the feed. For example, at −20 °C, CH₄ permeability decreases by more than an order of magnitude, from 52,000 to 1700 Barrer, as n-C₄H₁₀ activity increases from 0 to 0.73. In contrast, n-C₄H₁₀ mixed gas permeability is essentially unaffected by the presence of CH₄. The depression of CH₄ permeability in mixtures is a result of competitive sorption and blocking effects, which reduce both CH₄ mixture solubility and diffusivity, respectively. Diffusion coefficients of n-C₄H₁₀ and CH₄ in mixtures were calculated from mixture permeability and mixture solubility data. The CH₄ concentration-averaged diffusion coefficient generally decreases as n-C₄H₁₀ activity increases. On the other hand, the n-C₄H₁₀ diffusion coefficient is essentially unaffected by the presence of CH₄. Pure and mixed gas activation energies of permeation and diffusion of CH₄ and n-C₄H₁₀ are reported. The mixed gas n-C₄H₁₀/CH₄ permeability selectivity increases with increasing n-C₄H₁₀ activity and decreasing temperature, and it is higher than pure gas estimates would suggest. Mixture diffusivity selectivity also increases with increasing n-C₄H₁₀ activity. The difference between pure and mixed gas permeability selectivity arises from both solubility and diffusivity effects. The dual mode mixed gas permeability model describes the mixture permeability data reasonably well for n-C₄H₁₀. However, the model must be modified to accurately describe the methane data by accounting for the decrease in methane diffusivity due to the presence of n-C₄H₁₀ (i.e., blocking). Even though the penetrant concentrations are rather significant at some of the conditions considered, no evidence is observed for phenomena such as multicomponent coupling that would require a model more complex than the binary form of Fick's law. That is, Fick's law in its simplest form adequately describes the experimental data.     

Infra-red spectrum of the tight (110) fold in n-C198H398

The alkane n-C₁₉₈H₃₉₈ has been crystallised in both extended chain and once-folded forms and annealed to produce materials with low concentrations of gauche bonds. The concentrations of the specific conformers detected by FTIR spectroscopy at −173 °C are calculated, using measurements on liquid n-hexadecane for calibration: values are all generally less than 2.0 per 100 carbon atoms, with extended chain samples showing values less than 1.0 per 100 carbon atoms. A subtraction spectrum (Once-folded chain sample minus Extended chain sample) shows positive bands at 1298, 1340, 1347 and 1369 cm⁻¹, which are predicted in earlier calculations for a (110) fold, while additional positive bands at 1353 and 1363 cm⁻¹ are assigned, respectively, to gg conformers and (tentatively) to strained gtg or gtg′ conformations.     

Influence of HCl on CO and NO emissions in combustion

The influence of HCl on CO and NO emissions was experimentally investigated in an entrained flow reactor (EFR) and an internally circulating fluidized bed (ICFB). The results in EFR show the addition of HCl inhibits CO oxidation and NO formation at 1073 K and 1123 K. At the lower temperature (1073 K) the inhibition of HCl becomes more obvious. In ICFB, chlorine-containing plastic (PVC) was added to increase the concentration of HCl during the combustion of coal or coke. Results show that HCl is likely to enhance the reduction of NO and N₂O. HCl greatly increases CO and CH₄ emission in the flue gas. A detailed mechanism of CO/NO/HCl/SO₂ system was used to model the effect of HCl in combustion. The results indicate that HCl not only promotes the recombination of radicals O, H, and OH, but also accelerates the chemical equilibration of radicals. The influence of HCl on the radicals mainly occurs at 800-1200 K.     

Hydroisomerization-cracking of gasoline distillate from Fischer-Tropsch synthesis over bifunctional catalysts containing Pt and heteropolyacids

A gasoline distillate from the Fischer-Tropsch synthesis (so-called as F-T gasoline) was collected in a cold trap set in a gas-flowed slurry reaction system. The F-T gasoline contained 92.8 wt.% n-alkanes (ranged from n-C₄H₁₀ to n-C₁₄H₃₀), 2.4 wt.% 1-alkenes, and 4.8 wt.% iso-alkanes. By the hydroisomerization-cracking in an atmospheric flowed fixed-bed reactor over the catalysts containing Pt and heteropoly compound Cs_2.5H_0.5PW₁₂O₄₀ (abbreviated as Cs2.5), the F-T gasoline was converted to gasoline distillated mixed alkanes with a high iso/n ratio. A mechanical mixture of Pt/Al₂O₃ and Cs2.5 (noted as Pt/Al₂O₃ + Cs2.5) showed the highest catalytic performance among various catalysts. The product over Pt/Al₂O₃ + Cs2.5 after 5 h on-steam at 523 K contained 94.4% C₅-C₉ (gasoline components) with a high iso/n ratio of 8.45. The co-grinding time of Pt/Al₂O₃ and Cs2.5 influenced the catalytic performance when the time was shorter than 10 min but gave little influence on the catalytic performance when the time was longer than 10 min. Because the iso/n ratio of products over Pt/Al₂O₃ + Cs2.5 increased by adding Pt in Cs2.5 and decreased with increasing H₂/feedstock, the reaction proceeded through a bifunctional mechanism in which Pt sites achieved a hydrogenation/dehydrogenation function and acid sites achieved an isomerization/cracking function. The balance of Pt and solid acids was important to obtain a high catalytic performance in the hydroisomerization-cracking of F-T gasoline. Because Cs2.5 possessed moderate acidic strength and uniformly distributed acidic sites, Pt/Al₂O₃ + Cs2.5 showed a higher catalytic stability than that over Pt/Al₂O₃ + SO₄/ZO₂ and showed a higher catalytic activity than that over Pt/Al₂O₃ + H-ZSM-5.     

Comparison of dual fluidized bed steam gasification of biomass with and without selective transport of CO2

A comparison of dual fluidized bed gasification of biomass with and without selective transport of CO₂ from the gasification to the combustion reactor is presented. The dual fluidized bed technology provides the necessary heat for steam gasification by circulating hot bed material that is heated in a separate fluidized bed reactor by combustion of residual biomass char. The hydrogen content in producer gas of gasifiers based on this concept is about 40 vol% (dry basis). Addition of carbonates to the bed material and adequate adjustment of operation temperatures in the reactors allow selective transport of CO₂ (absorption enhanced reforming—AER concept). Thus, hydrogen contents of up to 75 vol% (dry basis) can be achieved. Experimental data from a 120 kW_{Fuel input} pilot plant as well as thermodynamic data are used to determine the mass- and energy-balances. Carbon, hydrogen, oxygen, and energy balances for both concepts are presented and discussed.