Effects of Lewis acid catalysts on the hydrogenation and cracking of three-ring aromatic and hydroaromatic structures related to coal

Little is known about the hydrogenation and cracking of fused aromatic nuclei during the liquefaction of coal under the influence of Lewis acid catalysts. This study was conducted to establish the effects of catalyst acidity on the activity and selectivity of Lewis acid catalysts, the sources of hydrogen involved in hydrogenation and cracking, and the relationships between reactant structure and reactivity. Three-ring aromatic and hydroaromatic compounds were used to simulate some of the structural units present in coal. The catalysts examined were ZnCl₂ and AlCl₃. It has been established that the rates of both processes are strongly influenced by the Brönsted acidity of the active catalyst, e.g. H⁺ (MX_nY)^?, and the Brönsted basicity of the aromatic portions of the reactant. The source of the hydrogen used for hydrogenation depends on the choice of catalyst. In the presence of AlCl₃, Scholl condensation of aromatic nuclei serves as the principal source of hydrogen. Molecular hydrogen is used exclusively, however, when hydrogenation is catalysed by ZnCl₂. The formation of reaction products and the trends in reactant reactivity are discussed on the basis of cationic mechanisms. The results of this study contribute to an understanding of the processes which occur during the liquefaction of coal using ZnCl₂ or AlCl₃.     

Effects of Lewis acid catalysts on the cleavage of aliphatic and aryl-aryl linkages in coal-related structures

ZnCl₂ and AlCl₃ catalyse the cleavage of aliphatic linkages between aromatic nuclei but not the cleavage of direct aryl-aryl bonds between such nuclei. The rate of alkyl-aryl bond breakage depends on the Brönsted acidity of the active catalyst (e.g. H⁺(ZnCl₂X)^? or H⁺(AlCl₃X)^?) and the Brönsted basicity of the aromatic portions of the reactant. AlCl₃ is significantly more active than ZnCl₂, and reactants containing hydroxyphenyl or napthyl groups are more reactive than those containing phenyl groups. The distribution of final products is strongly affected by the reactions of the aryl-alkyl carbonium ion formed upon cleavage of an alkyl-aryl bond. If the alkyl portion of the carbonium ion contains three or more carbon atoms, the ion preferentially undergoes an intramolecular reaction to form a hydroaromatic product. With only one or two carbon atoms the carbonium ion reacts via either hydride abstraction or electrophilic substitution, the hydride abstraction producing an alkylaromatic product, which may in turn undergo dealkylation. Reactant and products produced by a Scholl condensation act as the principle sources of hydride ions. Molecular H₂ also contributes some hydride ions. The reaction of carbonium ions by electrophilic substitution leads either to the regeneration of the initial reactant or to the formation of high-molecular-weight tars.     

Interaction between coal and liquefaction catalysts studied by high temperature electron spin resonance

The high temperature electron spin resonance technique has been used to obtain in situ information on the behaviour of liquefaction catalysts during coal pyrolysis. The spin concentration in coal was induced in the presence of a catalyst at the pyrolysis temperature. ZnCl₂ drastically increased the spin concentration of coal. The order of activity of the catalysts with respect to the increase in spin concentration was: ZnCl₂ (impregnated) ?ZnCl₂ (dispersed) >ZnCl₂/KCl>SnCl₂ > SbCl₃≈AlCl₃ ≈CaCl₂ > coal alone.     

Microwave assisted solvent-free synthesis of dihydropyrimidinones by Biginelli reaction over Si-MCM-41 supported FeCl3 catalyst

Among the Si-MCM-41 or montmorillonite K 10 clay supported ZnCl₂, AlCl₃, GaCl₃, InCl₃ and FeCl₃ catalysts, FeCl₃/Si-MCM-41 shows best performance for the microwave-assisted synthesis of dihydropyrimidinones by the Biginelli reaction involving multicomponent condensation of aromatic aldehyde, ethyl acetoacetate and urea in the absence of any solvent. It is a promising catalyst for the microwave-assisted reaction providing high product yield in a short period (3.0–5.0 min).     

Activity of modified SnO2 catalysts for acid-catalysed reactions

The effect of Lewis and Brønsted acid modification on the catalytic behaviour of SnO₂ was studied. Modified AlCl₃, FeCl₃, ZnCl₂ and H₂SO₄ were used for this purpose. Catalytic activity was tested for Friedel–Crafts alkylation, acylation and Pechmann condensation. It was observed that the alkylation activity of SnO₂ was improved upon treatment with Lewis acids and Brønsted acids. However the acylation activity was observed only when SnO₂ was treated with H₂SO₄. Moderate improvement in the activity for Pechmann condensation reaction was observed in the case of all the modified catalysts. It was inferred that this method of modification resulted in an increase in acid strength as well as acidity of the parent oxide catalyst. © 1998 SCI     

Effects of zinc chloride on sulphur removal from coal-related structures

Compounds containing sulphur in structures similar to those in coal were reacted with ZnCl₂ which promotes the removal of sulphur from sulphides and disulphides in which sulphur is bonded to a methylene group. Reaction is initiated either by proton transfer from the Brönsted acid form of the catalyst or by direct complexing of the reactant to ZnCl₂. During subsequent steps a carbonium ion is produced which can participate in electrophilic substitution with an aromatic centre. It is observed that a significant portion of the sulphur removed is retained by the ZnCl₂ as a Lewis acid/base adduct or reacts with the ZnCl₂ to form ZnS. These studies demonstrate that ZnCl₂ does not promote the removal of sulphur from diphenyl sulphide diphenyl disulphide, thiophene, or dibenzothiophene.     

Solubility increase of coal by alkylation with various alcohols

A vitrinite concentrate of Taiheyio coat has been reacted with various alcohols using ZnCl₂ as a catalyst under nitrogen pressure. The reaction conditions such as the reaction time, reaction temperature, initial nitrogen pressure and the effect of other Lewis acids as catalysts have been examined using methanol. The product from the reaction with methanol at 400 °C is fully soluble in pyridine, 57% in benzene and 26% in straight-chain hexane. This result is similar to that obtained when coal is hydrogenated under 10 MPa hydrogen pressure. Higher temperature, higher initial nitrogen pressure and longer reaction times give increased solubility. ZnCl₂ and FeCl₃ are the best catalysts. Of the various alcohols, branched alcohols give better results.     

Study of the preasphaltenes of coal liquefaction and its hydro-conversion kinetics catalyzed by SO4/ZrO2

The investigation of hydro-conversion behavior of the heavy intermediate products derived from coal direct liquefaction is advantageous to optimize the technological conditions of direct coal liquefaction and improve the oil yield. In this paper, the hydro-conversion of preasphaltenes catalyzed by SO₄²⁻/ZrO₂ solid acid was investigated based on the structural characterization of preasphaltenes and its hydro-conversion products, and the determination of products distribution and the kinetics of preasphaltenes hydro-conversion. The results indicated that the content of condensed aromatic rings increased, and the contents of hydrogen, oxygen and aliphatic side chains of preasphaltenes decreased with the increase of coal liquefaction temperature. The preasphaltenes showed higher hydro-conversion reactivity while SO₄²⁻/ZrO₂ solid acid was used as catalyst. Higher temperature and longer time were in favor of increasing the conversion and the oil + gas yield. The conversion of preasphaltenes hydro-conversion under 425 °C, for 40 min reached 81.3% with 51.2% oil + gas yield. SO₄²⁻/ZrO₂ solid acid was in favor of the catalytic cracking rather than the catalytic hydrogenation in the hydro-conversion of preasphaltenes. The activation energy of preasphaltenes conversion into asphaltenes was 72 kJ/mol. The regressive reactions were only observed at a higher temperature.     

Effects of petrographic constituents in three Yallourn brown coal lithotypes on hydroliquefaction processes

Pale, medium-light and medium-dark lithotypes of Yallourn coal were hydrogenated with and without ZnCl₂-containing catalysts (400 °C, 9.8 MPa H₂ and 3 h). The degree of hydroliquefaction was examined petrographically. Without catalyst, the amounts of water produced can be correlated with the amounts of humodetrinite; whereas with catalyst, either humotelinite or humocollinite may contribute to coal liquefaction in addition to humodetrinite; with pale lithotypes in the presence of catalyst, three submaceral groups may be converted.     

Conversion of solvent-refined coal to liquid products in the presence of Lewis acids

The formation of liquid products from a solvent-refined coal (SRC) was studied using Lewis acid catalysts in the presence of either benzene or cyclohexane as an extracting solvent. The soluble products were characterized by elemental analysis, ¹H-n.m.r., and, in some experiments, by gel permeation chromatography. Only ZnCl₂ and SnCl₂, the weakest Lewis acids examined, enhanced the dissolution of SRC over that observed in the absence of a catalyst. Increases in solubility were accompanied by increases in $\text{H}\text{C}$ and aliphatic character and by decreases in average molecular weight. These changes are ascribed to the ability of ZnCl₂ and SnCl₂ to promote depolymerization and hydrogenation of the SRC. Alkylation with isopropanol was also found to enhance the solubility of SRC in benzene and cyclohexane.     

Biodiesel synthesis via homogeneous Lewis acid-catalyzed transesterification

Lewis acids (AlCl₃ or ZnCl₂) were used to catalyze the transesterification of canola oil with methanol in the presence of terahydrofuran (THF) as co-solvent. The conversion of canola oil into fatty acid methyl esters was monitored by ¹H NMR. NMR analysis demonstrated that AlCl₃ catalyzes both the esterification of long chain fatty acid and the transesterification of vegetable oil with methanol suggesting that the catalyst is suitable for the preparation of biodiesel from vegetable oil containing high amounts of free fatty acids. Optimization by statistical analysis showed that the conversion of triglycerides into fatty acid methyl esters using AlCl₃ as catalyst was affected by reaction time, methanol to oil molar ratio, temperature and the presence of THF as co-solvent. The optimum conditions with AlCl₃ that achieved 98% conversion were 24:1 molar ratio at 110 °C and 18 h reaction time with THF as co-solvent. The presence of THF minimized the mass transfer problem normally encountered in heterogeneous systems. ZnCl₂ was far less effective as a catalyst compared to AlCl₃, which was attributed to its lesser acidity. Nevertheless, statistical analysis showed that the conversion with the use of ZnCl₂ differs only with reaction time but not with molar ratio.     

Alkyl aryl ethers in lignite solubilization: 2. Analysis of oil fractions

The FT-i.r. and ¹H n.m.r. spectroscopic analyses of oils or maltenes from a Spanish lignite (Utrillas, Teruel), are reported. These oils were obtained by depolymerization with alkyl aromatic ethers (anisole, 3-methylanisoleand 1,3-dimethoxybenzene) catalyzed by Lewis acids ZnCl₂, AlCl₃, SbCl₃and BF₃ (as boron trifluoride etherate), at atmospheric pressure and temperatures <220 °C. Bands due to aromatic ethers in the i.r. and n.m.r. spectra of the oils obtained by depolymerization indicate solvent incorporation. Oils obtained by direct lignite extraction showed 25% aromatic H and some H_α (≈3%) without OH groups. These appeared in some oils obtained by depolymerization with AlCl₃ and were due to secondary reactions with the aromatic extract. Oils derived from processes with good yields showed increases in aromaticity. The extent of substitution of aromatic rings in oils obtained by depolymerization was less than for oils directly extracted. All the oils studied show a low degree of condensation.     

Polystyrene and silica gel supported AlCl3 as highly chemoselective heterogeneous Lewis acid catalysts for Friedel–Crafts sulfonylation of aromatic compounds

Crosslinked polystyrene supported AlCl₃ (Ps–AlCl₃) and silica gel supported AlCl₃ (SiO₂–AlCl₃) have shown to be mild and chemoselective heterogeneous Lewis acid catalysts for the sulfonylation of different kinds of aromatic compounds with p-toluene sulfonyl chloride (TsCl) and benzene sulfonyl chloride (PhSO₂Cl) as sulfonylating agents. These solid acid catalysts are stable (as bench top catalysts) and can be easily recovered and reused without appreciable change in their efficiency.     

Hydrogenation of Yūbari coal coated with ZnCl2-MCln (CuCl,CrCl3 and MoCl5) catalyst melts

Hydrogenation of Yūbari coal coated with 5 wt% of ZnCl₂-MCl_n (CuCl, CrCl₃ and MoCl₅) was carried out at 400 °C for 3 h with an initial hydrogen pressure of 9.8 MPa. The binary melt catalysts showed relatively higher activity in increasing the yield of hexane-solubles than did ZnCl₂ alone. The ZnCl₂-MoCl₅ catalyst melt was best in view of its highest conversion (90.9 wt%), the highest yield of hexane-soluble material (50.5 wt%) and the smallest hydrogen consumption. In terms of yield of monoaromatics of the hexane-soluble material relative to polar materials, ZnCl₂-CrCl₃ catalyst was superior to ZnCl₂-MoCl₅ although its hydrogen consumption was high. The roles of CuCl, CrCl₃ and MoCl₅ in association with ZnCl₂ in coal-hydrogenation are discussed.     

Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Brønsted acid sites

Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael reaction between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted acidic sites loading in [P₄VPy-BuSO₃H]Cl-X(AlCl₃) were found to be 2.15 and 0.9 mmol per gram of catalyst, respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The catalyst is stable (as a bench top catalyst) and reusable.     

Mechanism of coal hydrogenation—liquefaction; Effect of temperature and coal particle size

The effect of coal particle size, hydrogen pressure and temperature on the extent of coal conversion in an entrained flow reactor is presented. Coal hydrogenation is done by feeding dry coal with ZnCl₂ catalyst into a continuous stream of hydrogen. The hydrogen-coal stream enters a long, small internal-diameter reactor (coiled tube reactor) controlled at about 500°C and 12.4 MPa hydrogen. At these conditions the coal particles become plastic and sticky. The hydrogen provides the energy to force the sticky coal particles through the reactor. Conversion of 85% of the coal to liquids and gases is easily attained. A physical mechanism is presented based on the unreacted-core-shrinking model. This mechanism aids in the explanation of the effect of process variables on reaction rates. Projections beyond the range of the variables studied are presented. These projections indicate that the pressure of coal liquefaction processes may be reduced by (1) the use of dry coal particles and (2) the reduction of the particle size. Significant reaction rates may be attained at pressures as low as 0.7 MPa by proper adjustment of particle size and temperature.     

Catalysts with noble metals based on super-cross-linked polystyrene for the hydrogenation of aromatic hydrocarbons

A series of catalysts containing noble metals on a super-cross-linked polystyrene (SCP) support with a developed specific surface area (>1000 m²/g) and high thermal stability are prepared and studied to develop an effective catalyst for the low-temperature hydrogenation of aromatic hydrocarbons. A study of Pt- and Pd-containing catalysts based on SCP, carbon supports, and alumina in the hydrogenation of simple (benzene, toluene), branched (n-butylbenzene) and polycyclic (terphenyl) aromatic compounds is conducted. In the hydrogenation of aromatic hydrocarbons, the activity of the catalysts on SCP is comparable to or surpasses analogous catalysts based on Al₂O₃ and Sibunit in the content of noble metals; it is established that catalysts on SCP have greater selectivity in the hydrogenation of benzene in a benzene-toluene mixture. The electronic state of metals in the Pt(Pd)/SCP catalysts is studied by the IR spectroscopy of adsorbed CO. In testing the catalysts in the hydrogenation of terphenyl, it is found that Pt-containing catalyst on the SCP can operate in reversible hydrogenation-dehydrogenation cycles (terphenyl-tercyclohexane); this is promising for the use of such catalyst systems in creating composite materials for hydrogen storage.     

Low-temperature hydrogen transfer and cracking catalysis in molten SbCl3-AlCl3

Molten SbCl₃-10 mol% AlCl₃ has been found to catalyse unusual hydrogen transfer chemistry for the polycyclic aromatic hydrocarbon, naphthalene, under extremely mild, aprotic conditions. At 130 °C and in the absence of added hydrogen gas, naphthalene is rapidly hydrogenated and isomerized in the melt to form 1-methylindene, 1-methylindane, and tetralin. The hydrogen used in the formation of these products is generated internally from the coupling of a portion of the naphthalene to form condensed aromatics such as binaphthalenes, benzofluoranthenes, and perylene. Furthermore, many of these condensed products not only undergo similar hydrogenation and isomerization, but they are also severely cracked in this medium. Under these aprotic, low-temperature reaction conditions, the redox properties of SbCl₃ are believed to play a key role in the catalytic process, the effect of added AlCl₃ being to enhance the oxidizing power of the SbCl₃ in addition to increasing the Lewis acidity of the melt.     

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.     

In-situ monitoring of the stabilization with hydrogen of free radicals thermally induced from coal using high pressure e.s.r., d.t.a. and p.d.a. equipment

High temperature, high pressure e.s.r. measurements of the hydrogenation reaction of Taiheiyo coal in the presence of catalysts were carried out to understand the stabilization of thermally and/or catalytically induced free radicals. A decrease in free radical concentration with increasing temperature was observed for ZnCl₂ and SnCl₂ · 2H₂O catalysts at 10MPa under hydrogen gas. High pressure modified single-cell d.t.a. and p.d.a. equipment augmented the uniquely designed high temperature, high pressure e.s.r. cell. The hydrogenation reaction was monitored under the same experimental conditions as for e.s.r. From the results of the combination of high temperature, high pressure e.s.r. with high pressure d.t.a. and p.d.a., it was established that H₂ molecules can react efficiently with free radicals from coal molecules created by the presence of ZnCl₂ and SnCl₂ · 2H₂O catalysts.