Liquefaction mechanism of Wandoan coal using tritium and 14C tracer methods: 2. Liquefaction using 3H labelled gaseous hydrogen

Tritium labelled gaseous hydrogen was used to clarify the role of gaseous hydrogen in coal liquefaction. Wandoan coal was hydrogenated under 5.9 MPa (initial pressure) of ³H-labelled hydrogen and in unlabelled solvents such as tetralin, naphthalene and decalin at 400 °C and for 30 min in the presence or absence of NiMoAl₂O₃ catalyst. Without a catalyst, liquefaction proceeded by addition of the hydrogen from donor solvent. The NiMoAl₂O₃ catalyst enhanced both hydrogen transfer from gas phase to coal and hydrocracking of coal-derived liquids. With NiMoAl₂O₃ catalyst, liquefaction in naphthalene solvent proceeded through the hydrogen-donation cycle: naphthalene → tetralin → naphthalene. The amount of residues showed that this cycle was more effective for coal liquefaction than the direct addition of hydrogen from gas phase to coal in decalin solvent. The ³H incorporated in the coal-derived liquids from gas phase was found to increase in the following order: oil < asphaltenes < preasphaltenes < residue.     

Chemistry of hydrogen sulphide under coal liquefaction conditions: 1. Modelling hydrogen transfer catalysis

The work reported here represents initial attempts to develop a complete kinetic and mechanistic understanding of the reaction chemistry of H₂S under coal liquefaction conditions, using both model systems and coal. Hydrogen sulphide was found to promote/catalyse the transfer of hydrogen from tetralin to 2-hydroxyquinoline (2-HOQ). The presence of H₂S can increase the rate of hydrogen transfer from tetralin to 2-HOQ by a factor of 10 compared with the same reaction run in the absence of H₂S. The energy of activation for hydrogen transfer was found to decrease by ≈5 kcal mol⁻¹ in the presence of H₂S. The presence of H₂S was also found to promote loss of oxygen from 2-HOQ to form small amounts of quinoline. No evidence of CC or CN bond cleavage in 2-HOQ was noted under any of the reaction conditions studied. These results suggest that the presence of H₂S reduces the temperatures necessary to promote effective hydrogen transfer from tetralin by 50–75 °C. Moreover, they imply that similar effects occur in H₂S-promoted coal liquefaction.     

Hydroliquefaction of low-sulfur coals using iron carbonyl complexes—Sulfur as catalysts

Hydroliquefaction of low-sulfur Australian coals (Wandoan and Yallourn) was studied using iron carbonyl complexes as catalyst. The addition of Fe(CO)₅ (2.8 wt% Fe of coal) increased coal conversion from 48.6 to 85.2% for Wandoan coal, and from 36.7 to 69.7% for Yallourn coal in 1-methylnaphthalene at 425°C under an initial hydrogen pressure of 50 kg cm^?2. When molecular sulfur was added to iron carbonyls (Fe(CO)₅, Fe₂(CO)₉ and Fe₃(CO)₁₂), higher coal converions ( > 92%) and higher oil yields (>46%) were obtained, along with an increase in the amount of hydrogen transferred to coal from the gas phase (0.2 to 2.8%, d.a.f. coal basis). In the liquefaction studies using a hydrogen donor solvent, tetralin, Fe(CO)₅S catalyst increased the amount of hydrogen absorbed from the gaseous phase and decreased the amount of naphthalene dehydrogenated from tetralin. The direct hydrogen transfer reaction from molecular hydrogen to coal fragment radicals seems to be a major reaction pathway. Organic sulfur compounds, dimethyldisulfide and benzothiophene, and inorganic FeS₂ and NiS were found to be good sulfur sources to Fe(CO)₅. From X-ray diffraction analyses of liquefaction residues, it is concluded that Fe(CO)₅ was converted into pyrrhotite (Fe_1?xS) when sulfur was present, but into Fe₃O₄ in the absence of sulfur.     

Hydrogenation of a brown coal pretreated with water-soluble nickel/molybdenum and cobalt/molybdenum catalysts

Loy Yang brown coal treated with cobalt acetate/ammonium molybdate (Co/Mo) gave lower conversions than the very high values obtained for the same coal treated with nickel acetate/ammonium molybdate (Ni/Mo) when reacted with hydrogen at 400°C. The difference in conversions obtained between the two catalyst systems decreased with increasing time. Addition of sulphur as carbon disulphide (CS₂) eliminated the difference between the Co/Mo and Ni/Mo catalyst systems, but neither system was more active than a sulphided Mo catalyst. Addition of a hydrogen donor solvent, tetralin, to a reaction in the absence of sulphur decreased conversion for the Ni/Mo catalysed system, but increased that for the Co/Mo system. The order of activity in reactions without solvent or added sulphur for the coal treated with the individual metals was CoMo < Ni. In the presence of sulphur the order was Co Ni < Mo; the addition of sulphur led to no significant improvement with Co catalysts.     

Liquefaction of sawdust for liquid fuel

Pressurized liquefaction of sawdust was carried out in an autoclave in the presence of solvent under cold hydrogen pressure ranging from 2.0 to 5.5 MPa at the temperature range of 150C–450°C. The reaction time varied from 5 to 30 min. Investigations were made on the effects of temperature, reaction time, cold hydrogen pressure and solvent on the liquefaction process. Results indicate that liquefaction of sawdust can start at a temperature of 200°C, much lower than that for coal in a hydrogen-donor solvent, e.g., tetralin which was used in this run of experiment. Oil yield increase with the rise either in temperature and in cold hydrogen pressure or with the longer reaction time.     

Reaction of dinaphthyl and diphenyl ethers at liquefaction conditions

The reactions of 2,2′-dinaphthyl ether and diphenyl ether were studied at 375–425°C using 6.9 MPa (cold) hydrogen or nitrogen, 9,10-dihydrophenanthrene (DHP) and decalin as solvents, and a molybdenum sulfide catalyst. We chose to examine these compounds as models for the cleavage of diaryl ether bridges during coal liquefaction. The molybdenum sulfide was added to the reaction as MoS₃, which should transform to the active MoS₂ catalyst. Cleavage of the C_arO in 2,2′-dinaphthyl ether, at reaction temperatures of 375 and 400°C, proceeded in the sequence H₂ < DHPN₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < MoS₃H₂ < Dec.MoS₃H₂. At 425°C, the MoS₃H₂ and Dec.MoS₃H₂ systems exchange places in this order. Diphenyl ether is less reactive than dinaphthyl ether toward hydrogenolysis reactions under these conditions. The conversion rate of diphenyl ether increases in the order H₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < Dec.MoS₃H₂ < MoS₃H₂. Although the rates of conversion of the two ethers are different, the relative effects of using a reactive gaseous atmosphere, donor solvent, catalyst - or some combination of these factors - are the same for both compounds. In liquefaction experiments, hydrogen donor solvent or hydrogen shuttling solvent seems necessary to reduce retrogressive reactions. However, a solvent interacting strongly with catalyst and scavenging hydrogen atoms can reduce the activity of catalysts in hydrocracking reactions.     

Catalytic effects of MoCl3- and NiCl2-containing molten salts in hydroliquefaction of Akabira bituminous coal with and without hydrogen-donor vehicle

Catalytic effects of MoCl₃-LiCl-KCl and NiCl₂-LiCl-KCl molten salts in hydroliquefaction of Akabira bituminous coal were studied. In the absence of solvent, both catalysts showed high coal conversion activity and high selectivity for the formation of hexane-soluble oil product. Oil yields from the catalytic runs were 3.4–3.0 times that from a non-catalytic run. Addition of hydrogen-donor tetralin considerably increased the oil yield and conversion and reduced the total hydrogen consumption. About 95 and 91 wt% daf coal was converted into pyridine-solubles and 59 and 54 wt% into oil with relatively low total hydrogen consumption (3.5 and 3.1 wt% daf coal) with the MoCl₃ and NiCl₂ catalysts respectively, in the presence of tetralin. Thermogravimetric analysis indicated that these catalysts enhanced the depolymerization of the coal organic matrix. Analysis of the liquefaction products suggested that the catalysts effectively catalysed the hydrocracking of polyaromatic structures contained in heavy products to hydroaromatics with relatively small ring sizes, explaining the high oil selectivity.     

Coal liquefaction using iron complexes as catalysts

Hydroliquefaction of Japanese Miike and Taiheiyo coals was carried out using various iron complexes as catalysts in tetralin at 375–445 °C. Iron pentacarbonyl (Fe(CO)₅) showed the highest catalytic activity, increasing coal conversion by about 10% at 425 °C under an initial hydrogen pressure of 5 MPa. Amounts of hydrogen transferred to coal increased from 1.4–2.3 wt% of daf coal in the absence of the catalyst to 2.5–4.2 wt% of daf coal in the presence of Fe(CO)₅ at 425 °C.     

13C, 2H, 1H n.m.r. and gpc study of structural evolution of a subbituminous coal during treatment with tetralin at 427 °C

Subbituminous Kaiparowitz coal was treated with 1,1-d₂-tetralin or tetralin in sealed tubes at 427 °C and 500 °C for varying periods of time and rates of temperature rise (15 °C/s, 3 °C/s, and 1 °C/s). The time dependence of yields, average molecular weights, molecular-weight distributions and changes in hydroxyl-group content and elemental composition were determined. Deuterium FT n.m.r. was used to monitor the incorporation of deuterium from 1,1-dideuteriotetralin into aliphatic and aromatic structures in the fractionated products. The exchange of ²H with phenol in the presence of the coal was examined to aid in the interpretation of ²H n.m.r. results. ¹³C and ¹H FT n.m.r. and i.r. spectroscopy were used to monitor the fractionated products over time. The ultimate obtainable yields of THF-soluble product were not significantly altered by the shorter temperature-rise times. The highoxygen-containing subbituminous coal undergoes an extremely rapid loss of about 20% of its oxygen by dehydration, and was found to enhance the rate of reduction of acetophenone and the scrambling of deuterium label in tetralin. The growth of benzylic aliphatic hydrogen at the expense of β (ArCH₂C$\text{H}$₂) hydrogen was rapid in the early stages of reaction. In spite of apparently labile aliphatic structures, the preasphaltenes exhibited products above MW 1200 that were stable for more than 2 h at 500 °C.     

The use of various metal carbonyls as catalyst precursors for coal hydroliquefaction

The catalytic activity of metal carbonyl complexes of chromium, molybdenum, tungsten, manganese, iron, cobalt, and nickel in the liquefaction of coal (Illinois No. 6, Wandoan and Mi-ike) was investigated. The carbonyl compounds of molybdenum, tungsten, iron, cobalt, and nickel acted as highly active catalysts for the liquefaction of Illinois No. 6 coal, resulting in high coal conversion (>90%) and high oil yield (>32%), under hydrogen pressure of 50 kg cm^?1 in a nonhydrogen-donating solvent at 425°C for 60 min. Among the catalysts surveyed, Mo(CO)₆ gave the highest oil yield (57.7%) and the largest amount of hydrogen transferred to coal (3.1 wt.% of d.a.f. coal). However, the molybdenum and tungsten carbonyls did not exhibit high catalytic activity for low sulfur Wandoan coal in the absence of added sulfur. On the other hand, cobalt and nickel carbonyls showed high catalytic activity irrespective of the amount of sulfur in the reaction system. Fe(CO)₅Mo(CO)₆ binary catalyst promoted hydroliquefaction of Wandoan coal, resulting in increases in oil yield and transfer of hydrogen to coal in the presence of sulfur.     

Carbon-14 tracer study of the fate of tetralin under simulated SRC-1 coal liquefaction conditions

A 28.4 wt% slurry of Illinois No. 6 coal in Wilsonville recycle solvent containing 0.83% of 1-¹⁴C-tetralin as a tracer for hydroaromatic solvent components has been hydroliquefied in a bench-scale, continuous-flow system under simulated SRC-I conditions (454 °C dissolver outlet; 13.9 MPa H₂). A combination of solvent fractionation, distillation, g.l.p.c. separation, and radio-assay procedures allow determination of the chemical fate of the labelled tetralin. Of the tetralin which is consumed (47 wt% of the feed), only 85% is converted to the ‘ideal’ dehydrogenation product, naphthalene. The remaining 15% is divided in a 1:2 ratio between conversion to structurally altered C₉–C₁₁ hydrocarbons (l-methylindane, indane, methylnaphthalene) and bonding to much heavier coal- and solvent-derived products. These side reactions would be expected to decrease both the solvent donor quality and the inventory. Although the mechanisms for grafting of tetralin-derived fragments to heavy materials cannot yet be described in detail, the chemical processes responsible must be quite indiscriminant towards functional groups because the specific radioactivities of the oils, the asphaltenes, the preasphaltenes, and even the insoluble organic matter are very similar.     

Free radical chemistry of coal liquefaction: role of molecular hydrogen

Model compounds containing the types of carboncarbon bonds thought to be present in coal were pyrolyzed in the presence of tetralin and molecular hydrogen at 450 °C. The relative rates of conversion of the model structures are predictable from the bond dissociation energies of the compounds. Conversion of dibenzyl in the presence of both tetralin and molecular hydrogen or in the presence of hydrogen alone proceeds along two parallel reaction paths. Toluene is produced by a thermal cracking reaction in which the rate-controlling step is the thermal cleavage of the β-bond in dibenzyl. Benzene and ethylbenzene are produced by a hydrocracking reaction. The rate of the hydrocracking reaction is directly proportional to the hydrogen pressure. The strong bond in diphenyl is hydrocracked in a system containing both molecular hydrogen and a source of free radicals. These studies on model coal structures offer firm evidence that molecular hydrogen can participate directly in free radical reactions under coal liquefaction conditions. Under some conditions molecular hydrogen can compete with a good donor solvent to stabilize the thermally produced free radicals. Molecular hydrogen can also promote some hydrocracking reactions in coal liquefaction that do not occur to an appreciable extent in the presence of only donor.     

Deuterium as a tracer in coal liquefaction Part 2. Non-catalytic studies

A bituminous Australian coal (Liddell) was liquefied in the absence of catalyst using tetralin as vehicle, and molecular deuterium and hydrogen—deuterium gas mixtures. The structures of the liquid and gaseous products were investigated by mass spectroscopy, ¹H-and ²H-NMR spectroscopy and gel permeation chromatography (GPC). The proportion of ²H to ¹H in the liquid products was found to be higher at 425°C than at 400°C because deuterium preferentially enters more aromatic rings at the higher temperature.The distributions of deuterium in the deuteromethanes formed during liquefaction show that deuterium randomly enters the structural groups in the coal which produce methane before the methane is released to the gas phase. This illustrates the extreme mobility of hydrogen, including the hydrogen that originates from the coal. As a consequence, it is proposed that hydrogen released as methane arises from a pool in which memory of the original bonding is lost.     

Behaviour of tetralin in coal liquefaction: Examination in long-run batch-autoclave experiments

The decomposition of tetralin in the presence and absence of coal was investigated in batch-autoclave experiments. The effect of temperature, atmosphere and reaction time on tetralin dehydrogenation, isomerization and hydrocracking was studied. At 400 and 450 °C, coal accelerates the formation of 1 - methylindan and n-butylbenzene (as primary products) changing the tetralin into compounds with reduced hydrogen donor capacity. The 1 -methylindan and n-butylbenzene are subsequently (hydro)-cracked to smaller products. At low hydrogen pressure the conversion of tetralin into naphthalene and hydrogen becomes considerable, making uncertain the calculation of hydrogen transfer from the tetralin to the coal on the basis of tetralin/naphthalene ratios.     

Reaction mechanism of coal hydrogenation: 1. Two-ring solvent system

Taiheiyo coal was hydrogenated in naphthalene, tetralin and decalin under 10 MPa (initial pressure) of hydrogen or nitrogen with stabilized nickel as catalyst at 400 °C for 15 min. Preasphaltene, asphaltene and oil conversions and the conversion of the solvents were measured. The hydrogen absorbed by coal from molecular hydrogen and from the donor solvent was calculated. The main reaction route appears to be the direct hydrogenation of coal by molecular hydrogen, with the side reaction via solvent by molecular hydrogen occurring only slightly, when an active catalyst such as stabilized nickel is present.     

Liquefaction of subbituminous coals with a basic nitrogenous model process solvent

Liquefaction reactions in a tubing-bomb reactor have been carried out as a function of coal, coal sampling source, reaction time, atmosphere, temperature, coal pre-treatment, SRC post-treatment and process solvent. Pyridine as well as toluene conversions ranging from 70 to > 90 wt% involving both eastern bituminous and western subbituminous coals are obtained. 1,2,3,4-Tetrahydroquinoline (THQ) has been extensively used as a process solvent under optimized liquefaction conditions of 2:1 solvent: coal, 7.5 MPa H₂, 691 K and 30 min reaction time. Comparisons of THQ with other model process solvents such as methylnaphthalene and tetralin are described. Liquefaction product yield for conversion of subbituminous coal is markedly decreased when surface water is removed from the coal by drying in vacuo at room temperature prior to liquefaction. The effect of mixing THQ with Wilsonville hydrogenated process solvent in the liquefaction of Wyodak and Indiana V coals is described.     

Effect of hydrogen sulphide on the deactivation of alumina-supported ruthenium methanation catalysts

The deactivation of a 0.5% ruthenium-alumina catalyst for the methanation reaction has been studied at 250 and 400 °C in the absence and presence of small concentrations of hydrogen sulphide (H₂S). The adequacy of a suggested mechanistic model for the methanation reaction has been confirmed and the model has been applied to the deactivation in the absence of H₂S, which is significantly improved with respect to an empirical methanation rate equation. The effect of the presence of 10 mg dm^?3 H₂S in the reactant stream on the catalyst deactivation rate has also been investigated.     

Hydrogenolysis of coal-related model compounds by carbon monoxide-water mixture

Model organic compounds have been subject to hydrogenolysis reactions using CO and H₂O under industrial conditions as used for lignite and residual oil. Twenty-eight compounds, including diphenylmethane, indane, tetralin, anthracene, diphenyl ether, indene, benzophenone and xanthene were used. The purpose is to study the hydrogenolysis behaviour of compounds with structures as found in coal and residual oil. Using a Co-Mo catalyst, scission of the CC chain connecting two aromatic rings is enhanced by increased chain length of the molecule. Ether bonds are broken more easily than CC bonds. With the exception of decalin, cyclic CC bonds are more resistant to cleavage than linear bonds. Alcohol hydroxyl, and carbonyl groups of stilbene are the easiest to reduce but phenolic hydroxyl groups are stable. The Co-Mo catalyst promoted hydrolysis of ether bonds as well as hydrogenolysis. The method of the study is considered to be more effective for hydrogenolysis of organic compounds than the reported use of alkali and tetralin because of milder reaction conditions which are required.     

Pyrolysis of tetralin

The pyrolysis of tetralin has been studied at 500 °C. The pyrolysis products resulting from the decomposition of 1-¹³C-tetralin were analysed by ¹³C n.m.r. spectroscopy to determine the carbon redistribution in these substances. Some results were confirmed by experiments with ¹⁴C-labelled compounds. Under the conditions employed, carbon scrambling is observed only in the aliphatic portion of the products. The primary mode of decomposition of tetralin proceeds via homolysis of the 1–8a bond. No equilibrium between tetralin and naphthalene plus hydrogen takes place in the absence of a catalyst.     

Liquefaction of coal in hydrogen-donor and non-donor vehicles

Coal, slurried with tetralin, and heated rapidly to 400 °C, was converted to benzene-soluble, liquid products through a reaction path which appears to involve thermal cleavage of chemical bonds in the coal (so-called depolymerization). Free radicals formed pyrolytically were stabilized in the early stages by autogenous hydrogen transfer, and in later stages by abstraction of hydrogen from the hydrogen-donating tetralin (which was converted to dihydronaphthalene and then to naphthalene). Vitrinite of the coal particles became almost completely soluble in pyridine after 5 min reaction, and dispersed in the vehicle (tetralin) within about 10 min. Reaction in non-donor vehicles (naphthalene, dodecane) resulted in dispersion, but ‘repolymerization’ formed a benzene-insoluble material. Hydrogen transfer from tetralin increased exponentially with increased conversion of the coal to benzene-soluble material. Negligible influence of particle size (below 2–3 mm) implies that mass transfer is not a rate-limiting factor. High-volatile c bituminous coal and coals of lower rank converted at about the same rate and to the same extent (given equivalent petrographic compositions); approximately two-thirds of the ultimately achievable conversion occurs within 20 min. Higher-rank coals converted at a lower rate. Oxidation of the coal was deleterious to ultimate conversion.