Liquefaction reaction of coal: 2. Structural correlation between coal and its liquefaction products

The structural correlation between coal and its liquefaction products has been examined using cross-polarization, magic angle spinning (CP/MAS) ¹³C n.m.r. and field ionization mass spectrometry (f.i.m.s.). The CH₂/aromatic carbon ratios of all solid products (asphaltene, preasphaltene and residue) were close to the corrected +CH₂/aromatic carbon ratio for the coal. This suggests that the ring structure of the structural unit of each solid product is essentially similar to that of the parent coal, except for a difference in the degree of polymerization of the structural units. The CH₂/aromatic carbon ratios of aromatic ring-type oil fractions also correlated with the corrected ratio for the coal, although they were larger. The z series distribution obtained from the f.i.m.s. of oil fractions revealed that coal with a higher CH₂/aromatic carbon ratio produced an oil rich in naphthenic structures.     

Formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis

The formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis were investigated. In short residence time coal hydrogenolysis, preasphaltenes are the major product. The maximum yield for this parametric study was obtained under reaction conditions of 500 °C and 21 s. The formation of preasphaltenes reached the maximum value in the initial stage of the liquefaction reaction. As the liquefaction reaction continued, the deoxygenation of preasphaltenes proceeded. However, the decrease in aromatic atoms bound to the hydroxy, methoxy and oxygen atoms of the diphenyl ether group (Arz.sbnd;O) is small, and the ArO functionality still remains abundant in preasphaltenes. Preasphaltene-I is characterized by carbon aromaticity (f_a) of 0.6–0.7, aromatic rings of from 1 to 3–5 per condensed aromatic ring system, 55–70% substitution of aromatic ring carbons and C_2–3 aliphatic substituents. The molecular weight ranges from 500 to 650, and is not much different from that of the asphaltenes. The f_a values based on the Brown-Ladner method and on solid state CP/MAS ¹³C n.m.r. spectra data agree closely.     

Relaxation of 13C and 15N nuclei by solvent-refined coal

The effects of changing various spectroscopic parameters on the solid-state and solution ¹³C-nuclear magnetic resonance (n.m.r.) spectra of solvent-refined coal have been investigated. Solution spectra were obtained with the use of broad band decoupling, inverse gated decoupling and coupling techniques. Conventional relaxation reagent (Cr(acac)₃) was sometimes added to the solvent-refined coal. The effects of pulse delay on the total signal intensity, and on the intensity of the signal from aromatic carbon have been measured. The results show that inverse gated decoupling with pulse delays of 10 s is needed for complete relaxation of solvent-refined coal, but pulse delays of 6 s can give accurate estimations of aromaticity. However, it is recommended that conventional relaxation reagent such as Cr(acac)₃ be added to ensure relaxation if shorter pulse delays are used. The effect of solvent-refined coal on relaxation of some pure compounds in solution has also been studied. Solvent-refined coal acts as a relaxation reagent on ¹³C nuclei in benzene, toluene and ethylbenzene. It can also relax ¹⁵N nuclei in aniline, N,N-dimethylaniline and nitromethane. Spin-lattice relaxation times (T₁'s) of selected nuclei have been measured and the contribution of solvent-refined coal to relaxation (T₁SRC) has been calculated. Solid-state 13C-n.m.r. spectra have been obtained using the cross-polarization (CP) technique with magic-angle sample spinning (MASS). A variety of cross-polarization times and recycle times have been used. The results show that no serious errors in measurement of aromaticity (f_a) are caused by using a contact time as short as 1 ms and a recycle time of 0.3 s. There is good agreement between f_a's obtained by solution and solid-state n.m.r., and as solid-state spectra can be obtained in only a fraction of the time needed to obtain a solution spectrum (≈$\text{1}\text{20th}$), CP-n.m.r. is the method of choice for analysis of f_a of solvent-refined coal. The results also show that CP-MASS n.m.r. can be used to estimate the fraction of aromatic carbon which is unprotonated in solvent-refined coal and, hence, indirectly, the fraction of hydrogen which is aromatic.     

Determination of Algerian Hassi-Messaoud asphaltene aromaticity with different solid-state NMR sequences

Algerian oil well deposit derived asphaltene fraction was characterized by different MAS/NMR sequences to investigate asphaltene aromaticity and the best cross-polarization contact time. The aromaticity was estimated by single pulse sequence (SP), Hahn-echo (HE), cross-polarization (CP) and variable cross-polarization (VACP) sequences. The values found ranging from 0.58 to 0.48 are of the same order of magnitude as ones published in the literature. The discrepancies between the values are thought to be relevant to both the specificity of each sequence and the asphaltene structure. Spectra band de-convolution enables us the determination of the average number of carbon atoms per side chain according to each sequence. The obtained values spanning from 3 to 7 are also sequence nature dependent.     

The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl₂O₄) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO₃), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO₃ intermediate phase was not observed and La₄Al₂MgO₁₀ was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.     

Characterization of whole coals by 13C CP/MAS spectroscopy at high field

Solid-state 50-MHz ¹³C spectra essentially free of spinning sidebands have been constructed for three bituminous coals by the addition of echo spectra having phase-altered spinning sidebands (PASS). The echo spectra are produced by a modified version of the Dixon pulse sequence. Quantitative analysis of the aromatic carbon content (f_a) from PASS spectra for the three coals compares favourably with results obtained by other methods. Values of f_a are found in the range 0.69–0.73. Removal of the unwanted spinning bands allows absorptions for specific structural units present in the coals to be distinguished and assigned. Spectra show an upfield shoulder at 13–15 ppm, and moderately intense absorptions at 20–24 ppm and ≈30 ppm which are characteristic of several aliphatic structures in different steric environments. In addition to the main aromatic band at ≈120 ppm, absorptions for substituted aromatic carbons appear at ≈140 and ≈155 ppm. Less intense signals from several carbonyl functional groups (160–190 ppm) and oxygen- and nitrogen-substituted aliphatic groups (50–90 ppm) are also present.     

Characterization of coals and coal oxidation by magic-angle 13C n.m.r. spectroscopy

Magic-angle ¹³C n.m.r. spectra have been obtained for a series of vitrinite concentrates. Proper modification of the cross-polarization pulse sequence allows separation of protonated and nonprotonated carbon resonances. This technique is used to determine the relative fraction of nonprotonated aromatic carbons for each of the vitrinites, a parameter observed to decrease with increasing rank. Another parameter, related to the aromatic hydrogen content, is also calculated from these data and the results correlate with those from Fourier transform i.r. spectroscopy. The methods used for analysis of the vitrinite concentrates were then applied to the low-temperature oxidation of coal. The fractional aromaticity as determined by n.m.r. increases with longer oxidation times, indicating preferential attack on aliphatic structures. Here the FT-i.r. results are in quantitative agreement with those from n.m.r. Finally, the advantages of using various pulse sequences to extend the range of magic-angle n.m.r. and of combining FT-i.r. and n.m.r. results are discussed in the context of their potential for coal science.     

Characterization of organic material in coal by proton-decoupled 13C nuclear magnetic resonance with magic-angle spinning

¹³C n.m.r. spectra have been obtained on ten solid coal samples of various types and on three coal-derived materials, using high-power proton decoupling, cross polarization and magic-angle spinning, and provide valuable information on the carbon distribution between aromatic and non-aromatic structures in the sample. Apparent carbon aromaticities, f_a′, have been determined and have been correlated with H/C ratios and as one factor in fuel values. Both solvent refining and reverse combustion (see introduction) are found to increase the aromatic fraction. These techniques should be very useful in characterizing and optimizing coal-conversion processes.     

Seed oil determination without weighing and drying the seeds by combined free induction decay and spin-echo nuclear magnetic resonance signals

The free induction decay (FID) and echo signals in a spin-echo pulse sequence have been used for seed oil determination without weighing and drying the seeds with nuclear magnetic resonance (NMR) equipment that has a magnetic field of low homogeneity. Earlier known methods, based on use of the FID signal to determine seed oil, become inapplicable when the magnetic field homogeneity is poor, because the angular position of seed significantly affects the signal. The present method, which elegantly eliminates the angular dependence, involves sampling the FID signal at 10 μs after a 90° pulse and the subsequent echo signal at 100 μs formed by applying a 180° pulse at 50 μs. Such short pulse spacing in spin-echo sequence produces almost a full oil signal. It also eliminates the effects of sample-to-sample variation inT ₂ and molecular diffusion on oil signal. The oil values obtained by this method are in good agreement (correlation for mustard: 0.952; linseed: 0.99; and for peanut: 0.912) with the values obtained by the well established and accurate pulsed NMR method, which is based on the measurement of the FID signal of oil in dried and weighed seeds.     

A calorimetric study on the thermal desorption of naphthalene from CχS and activated carbon

A quantitative calorimetric study on the thermal desorption of naphthalene from CχS and activated carbon has been performed in a Calvet-type twin calorimeter. The heat of desorption of naphthalene was measured to be 31.1 kcal/mole for Filtrasorb 300(activate carbon) and 21.8 kcal/mole for a CχS sample according to the following equation: Adsorbent-Naphthalene (24°C) = Adsorbent (24°C) + Naphthalene (377°C). By studying the desorption of adsorbed water from the adsorbents, the binding energy between water and the adsorbent was measured to be 18.7 kcal/mole for Filtrasorb 300 and 4.6 kcal/mole for CχS according to the following equation: Adsorbent-H₂O(24°C) = Adsorbent (24°C) + H₂O (24°C). Based on the results of the present study and previous adsorption measurements, it is concluded that the modification of the carbon surface with sulfur groups decreases the binding energy between the adsorbate and the adsorbent very significantly.     

Liquefaction reaction of coal: 1. Depolymerization of coal by cleavages of ether and methylene bridges

The roles of ether and methylene bridges in the depolymerization of coal have been re-evaluated on the basis of the results of a mild liquefaction reaction (400 °C, 30 min) and the distributions of oxygen and carbon atoms obtained by cross-polarization, magic angle spinning (CP/MAS) ¹³C n.m.r. spectrometry. Coals ranging from 66.2 to 87.4 wt% C (dmmf) were used as sample coals. The content of etheric oxygen was < 3.7 per 100 carbon atoms and the cleavage of ether bridges contributed to the formation of preasphaltene. The conversion to hexane solubles in the mild liquefaction reaction correlated well with CH₂ carbon content of coal, though the conversion to pyridine solubles did not. These results suggest that the formation of oil from preasphaltene is caused by the scission of CH₂ bridges and some naphthenic CH₂ bonds. The phenolic OH oxygen-rich portions in coal tended to remain as a residue formed by the condensation reaction of phenolic OH groups.     

Propane conversion over a Ru/CGO catalyst and its application in intermediate temperature solid oxide fuel cells

A Ru/CGO catalyst was investigated in combination with a Cu current collector for the direct electro-oxidation and internal reforming of propane in a solid oxide fuel cell. The electrochemical power densities for the direct oxidation were larger than in the internal reforming process at 750 °C. The electrochemical performance in the presence of propane was significantly affected by the polarization resistance which was about three times larger than that obtained for the SOFC fed with hydrogen at 750 °C. However, out-of-cell steam reforming tests showed a C₃H₈ conversion to syngas approaching 90% at 800 °C. Thus, significant enhancements may be achieved by properly optimizing the anode structure. No formation of carbon deposits was observed both upon operation of the anode in the direct oxidation and internal reforming processes at 750 °C.     

Conductivity and phase transitions in a solid electrolyte RbAg4I5

The electrical conductivity has been investigated of the solid electrolyte RbAg₄I₅ in the vicinity of phase transitions taking place at temperatures T₁  121.8°K and T₂  209°K. Measurements were carried out both on single crystals and on polycrystalline specimens.The transition at T₁ was found to be extended over a certain temperatures range, and hysteresis was observed. In the case of the transition at T₂ a conductivity jump was observed in single crystal specimens. The result obtained are discussed in terms of a theoretical model, suggested for describing the transition of solid electrolytes into the superionic state.     

Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures

The separation of aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) from C₄ to C₁₀ aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. In this work, we investigated the separation of toluene from heptane by extraction with ionic liquids.Several ionic liquids are suitable for extraction of toluene from toluene/heptane mixtures. The toluene/heptane selectivities at 40 °C and 75 °C with several ionic liquids, [mebupy]BF₄, [mebupy]CH₃SO₄, [bmim]BF₄ (40 °C) and [emim] tosylate (75 °C), are a factor of 1.5–2.5 higher compared to those obtained with sulfolane (S_tol/hept = 30.9, D_tol = 0.31 at 40 °C), which is the most industrially used solvent for the extraction of aromatic hydrocarbons from a mixed aromatic/aliphatic hydrocarbon stream. From these five ionic liquids, [mebupy]BF₄ appeared to be the most suitable, because of a combination of a high toluene distribution coefficient (D_tol = 0.44) and a high toluene/heptane selectivity (S_tol/hept = 53.6). Therefore, with [mebupy]BF₄ also extraction experiments with other aromatic/aliphatic combinations (benzene/n-hexane, ethylbenzene/n-octane and m-xylene/n-octane) were carried out. The aromatic/aliphatic selectivities were all in the same range, from which it can be concluded that the toluene/heptane mixture is a representative model system for the aromatic/aliphatic separation.     

Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures

The separation of aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) from C₄ to C₁₀ aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. In this work, we investigated the separation of toluene from heptane by extraction with ionic liquids.Several ionic liquids are suitable for extraction of toluene from toluene/heptane mixtures. The toluene/heptane selectivities at 40 °C and 75 °C with several ionic liquids, [mebupy]BF₄, [mebupy]CH₃SO₄, [bmim]BF₄ (40 °C) and [emim] tosylate (75 °C), are a factor of 1.5–2.5 higher compared to those obtained with sulfolane (S_tol/hept = 30.9, D_tol = 0.31 at 40 °C), which is the most industrially used solvent for the extraction of aromatic hydrocarbons from a mixed aromatic/aliphatic hydrocarbon stream. From these five ionic liquids, [mebupy]BF₄ appeared to be the most suitable, because of a combination of a high toluene distribution coefficient (D_tol = 0.44) and a high toluene/heptane selectivity (S_tol/hept = 53.6). Therefore, with [mebupy]BF₄ also extraction experiments with other aromatic/aliphatic combinations (benzene/n-hexane, ethylbenzene/n-octane and m-xylene/n-octane) were carried out. The aromatic/aliphatic selectivities were all in the same range, from which it can be concluded that the toluene/heptane mixture is a representative model system for the aromatic/aliphatic separation.     

Estimation of coal aromaticities by proton-decoupled carbon-13 magnetic resonance spectra of whole coals

A recently developed nuclear magnetic resonance (n.m.r.) cross-polarization technique has been applied to obtain high-resolution carbon-13 spectra of vitrains from two coals. The method, which overcomes the problem of dipolar line broadening by protons, permits estimates of the carbon aromaticities of solid coals. Preliminary results support the classical views that coals are highly aromatic materials and that the aromaticity of coal increases with increasing rank. Limitations of the technique, the accuracy of representation of the distribution of carbons in the sample, and consequently the reliability of the resulting f_a values are discussed.     

Application of new 13C n.m.r. techniques to the study of products from catalytic hydrodeoxygenation of SRC-II liquids

A middle-heavy SRC-II distillate (b.p. 230–455 °C), containing 3.0 wt% of oxygen, has been studied by means of ¹³C n.m.r. at 75, 100 and 125 MHz. The magnetization refocussing techniques INEPT and J-resolved two-dimensional Fourier transform have been utilized to demonstrate methods by which resonance line multiplicities may be determined in complex liquid mixtures. Products derived from the above coal liquid by hydrodeoxygenation at temperatures from 200 to 370 °C, using sulphided Co—Mo and Ni-W catalysts, were also examined. The fraction of aromatic carbon in the hydrotreated liquids was found to correlate directly with their C/H atomic ratio and inversely with the hydrogen content. Comparison of O/C atomic ratios with f_a values for these liquids indicates that hydrogen uptake < 260 °C is associated primarily with hydrogenolytic oxygen removal without attendant ring hydrogenation, while at temperatures between 260 and 350 °C hydrodeoxygenation is accompanied by ring hydrogenation and dealkylation reactions.     

Quantitative analyses of oxygen release/storage and CO2 adsorption on ceria and Pt–Rh/ceria

The CO/O₂ and CO₂ pulse experiments were carried out to acquire useful information about oxygen release/storage and CO₂ adsorption on ceria and Pt–Rh/ceria. In the CO pulse experiments at 500 °C, ca. 60% of CO uptake was released as CO₂ while the rest of CO uptake was retained as carbon residuals on the surfaces of both samples. The carbon residuals could be removed when O₂ was provided. In the CO₂ pulse experiments, the adsorption of CO₂ was found to relate to the temperatures and the oxidation states of surface cerium. The reduced Ce³⁺ sites (O vacancies) were responsible for the adsorption of CO₂ at the temperature of 500 °C. In addition, the molar ratios of CO₂ adsorption to O vacancies (38–39%) were in agreement with the ratios of carbon residuals to CO uptake ( ca. 40%) measured in the CO pulse experiments. Quantitative analyses of oxygen release/storage and CO₂ adsorption implied that in the process of oxygen release, carbon residuals were possibly in the form of a carbonate-like species due to the adsorption of CO₂ onto the reduced Ce³⁺ sites.     

Effect of temperature on tacticity in thermal polymerization of p-methylstyrene by 13C NMR spectroscopy

Poly(p-methylstyrene) (Pp-MS) was synthesized at three different temperatures of 50, 150 and 250?°C via bulk thermal polymerization method. The assignment of all stereosequences at triad and pentad levels for two quaternary aromatic carbons and hexad level for methylene carbon was carried out by ¹³C liquid nuclear magnetic resonance spectroscopy (NMR) in deuterated chloroform at similar conditions. The probability of meso addition (P _m) was calculated from second quaternary aromatic carbon and used to predict the relative intensities of methylene and first quaternary aromatic carbon by Bernoullian and first-order Markov statistical models. The results were compared with experimental data. It is shown that the Bernoullian statistics fit better than first-order Markov model for all three samples. The results indicate that P _m increases by increasing polymerization temperature. The corresponding P _m values determined for synthesized Pp-MSs at 50, 150 and 250?°C were 0.383, 0.392 and 0.404, respectively. It was observed that higher resolutions and better splitting patterns were achievable by increasing the NMR acquisition temperature from 20 to 50?°C. When temperature increased during NMR acquisition, the resolution improved for the first and second quaternary aromatic carbons and methylene carbon, though there was no splitting pattern observed for methyl carbon atom at the para-position of the aromatic ring.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.