Peter Mulder

The occurrence and reactivity of phenoxyl linkages in lignin and low rank coal

Abstract Nowadays crude oil and natural gas are the main sources for the production of fuels and feedstock chemicals. The resources are limited and the application of (renewable) alternatives will be needed in the future to sustain the progress of the global community. Coal is a clear option since the reserves are much larger than those of oil. Currently, the main use of coal is as a source of energy by direct combustion. A renewed interest exists in the production of petrochemicals and synthetic fuels by means of liquefaction of coal. The processes are a viable, although to date not economical, option [F.J. Derbyshire, D. Gray, Ullmann’s Encyclopedia of industrial Chemistry, 5th Ed., Vol. A7, VCH Verlagsgesellschaft, Weinheim, 1986, p. 197]. Coal is considered to originate from natural waste material (biomass). Biopolymers (cellulose, lignin, polypeptides) are fed into the soil, and along a geological time scale a complicated chemical interconversion follows in which the oxygen content is gradually reduced. These chemical transformations are brought about by means of bacteria and in a later stage by a combination of high pressures and temperatures. With an annual global production of around 172 billion metric tons [A. Heredia, A. Jimenez, R. Guillen, Z. Lebensm, Unters. Forsch. 200 (1995) 24], energy production from biomass seems to be an attractive option. The source is renewable and the atmospheric carbon dioxide is recycled. However, its use for feedstock production demands for a sequence of chemical conversions since the main building blocks in biomass are carbohydrates. Another constituent is lignin, which is removed from wood meal by the pulp and paper industry. In the USA alone about 50 million metric tons of lignin are produced each year and this waste stream is at present mainly burnt. However, in view of its chemical composition, lignin may serve as an interesting basic material for aromatic (phenolic) compounds with a high added value. The first part of this review will deal with lignin: its formation, structure and degradation under thermal and radiative conditions. The second part highlights the structure of coal and the mechanistic features of coal liquefaction. Phenoxyl linkages are important structural elements in both lignin and coal. In contrast to the chemistry of hydrocarbons under thermal liquefaction conditions, the fate of oxygen containing model compounds for lignin and coal is less well understood and will be surveyed in the third part of this review. A final section will deal with the experimental procedures to measure bond dissociation enthalpies (BDEs) and the thermochemical aspects of the phenoxyl linkages, including the effect of substitution.

Formation of polychlorinated benzenes during the catalytic combustion of chlorobenzene using a Pt/γ-Al2O3 catalyst

Abstract Pt/γ-Al2O3 has been used for the complete catalytic oxidation of chlorobenzene, a model for chlorinated aromatic compounds present in flue gases. Complete conversion of chlorobenzene is reached at ca. 440°C, but at this temperature substantial amounts of polychlorinated benzenes are formed. Only at 600°C complete selectivity to carbon dioxide is achieved. The addition of 1.75% of water to the carrier gas reduces formation of polychlorinated benzenes and improves conversion. The oxygen partial pressure has a remarkable effect on byproduct formation: the amounts of polychlorinated benzenes rise sharply with increasing oxygen concentration. Total conversion of chlorobenzene remains stable, hence the selectivity to carbon dioxide increases with decreasing oxygen pressure. When the γ-Al2O3 support alone is applied, complete conversion of chlorobenzene is reached only at ca. 550°C, without production of polychlorinated benzenes. The selectivity to carbon oxides however is poor. So platinum seems to be responsible for the formation of polychlorinated benzenes, which we propose is brought about by chlorination of adsorbed (chloro)benzene-species through platinum (oxy) chlorides.

Formation of Dibenzodioxins and Dibenzofurans in Homogenous Gas-Phase Reactions of Phenols

The operation of Municipal Waste Incinerators (MWI’s) results in the emission of organochlorine compounds including trace amounts of hazardous polychlorinated dibenzodioxins and dibenzofurans. Although the presence of PCDDs and PCDFs in stack gases and on fly ash is well established, little is known about the mechanisms and kinetics of formation and the phase(s) — e.g. pyrolysis, burning, or fly-ash catalysis — in which these compounds and/or their precursors are formed. Proper insight into these chemical features may learn how to improve a MWI installation so as to reduce, or eliminate, these emissions.

The role of the support and dispersion in the catalytic combustion of chlorobenzene on noble metal based catalysts

Polychlorinated benzenes (PhClx) are formed as byproducts in the combustion of chlorobenzene on Pt supported on γ-Al2O3, SiO2, SiO2–Al2O3, or ZrO2. The congener and isomer distribution of the PhClx differs for the various supports. The amounts of PhClx correlate with the dispersion of platinum. Thus, a Pt/γ-Al2O3 catalyst calcined at 500°C to yield very small Pt crystallites was more active in PhClx formation than Pt/γ-Al2O3 calcined at 800°C. In all cases T50% for chlorobenzene conversion is close to 300°C and appears to be independent of the crystallite size of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, in the presence of Cl and higher oxygen concentrations, small Pt crystallites are converted more easily into Pt(IV) species. These are less efficient in combustion, but can be more active in chlorination.

Lignin depolymerization in hydrogen-donor solvents

Summary The conversion of different types of lignin to monophenolic compounds has been studied between 500 and 650 K, under typical coal liquefaction conditions using 9,10-dihydroanthracene (AnH2) and 7H-benz[de]anthracene (BzH) as the hydrogen-donor solvents. The yield of phenolic compounds was found to increase with the capacity of the hydrogen donor. The application of a polar cosolvent appeared to be beneficial as well. The differences in product distribution could be related to the origin of lignin. The maximum yield amounted to 11 % after 4 h at 625 K using milled wood lignin in AnH2. It has been found that lignin itself is a hydrogen-donating substance and capable of cleaving aromatic ketones such as α-phenoxyacetophenone.

Catalytic combustion of chlorobenzene on Pt/γ-Al2O3 in the presence of aliphatic hydrocarbons

During the catalytic combustion of chlorobenzene on a 2% Pt/γ-Al2O3 catalyst, considerable amounts of polychlorinated benzenes are formed as by-products. The co-feeding of heptane practically eliminates this unwanted side-reaction. Moreover, the conversion of chlorobenzene occurs at much lower temperatures (the T50%drops from 305 to 225°C). Simultaneously, the conversion of heptane is retarded. The addition of other hydrocarbons have a similar effect. Water and heat produced by the combustion of the added hydrocarbon cannot explain the increase in destruction rate of chlorobenzene. Removal of Cl from the surface by the alkane appears to be the ruling factor.

Formation of PCDFs during chlorination and oxidation of chlorobenzene in chlorine/oxygen mixtures around 340 °C

Homogeneous gas-phase chlorination of chlorobenzene in the presence of oxygen at 330–350°C gives substantial amounts of PCDFs, up to 6% on the major product, the dichlorobenzenes. Chlorine atoms abstract H from chlorobenzene, the resulting chlorophenyl radicals reacting rapidly with Cl2, or with O2. The latter reaction leads to chlorophenoxyl radicals. Part of these appear to be further chlorinated before condensation occurs to mainly D2CDF, T3CDF and T4CDF. The simultaneous production of CO and (extra) HCl shows that (chlorine initiated) slow combustion also takes place, presumably by reaction of chlorinated phenylperoxyl- and/or phenoxyl radicals with oxygen.

Formation of dibenzodioxins and chlorobenzenes in fly ash catalyzed reactions of monochlorophenols

Abstract Partial autoxidation of monochlorophenols to carbon dioxide and carbon monoxide proceeds at 350 – 400 °C mediated by M unicipal W aste I ncinerator (MWI) fly ash. Moreover a wide range of (poly)chlorinated benzenes, mono benzofurans and dibenzo-p-dioxins is produced, with a considerable fraction of the original organic chlorine concentrated into these remaining aromatic rings.

Oxychlorination and combustion of propene on fly-ash. Formation of chlorinated benzenes, dibenzodioxines and mono- and dibenzofurans

Abstract Heterogenous gas phase reactions of propene on fly ash in the presence of hydrochloric acid and air between 300 – 580 °C have been investigated. At mild conditions only the formation of polychlorinated C 1 , C 2 and C 3 species takes place. At the high temperature end of this study substantial amounts polychlorinated benzenes, dibenzodioxines and mono- and dibenzofurans are observed. This is in contrast with similar experiments using ethene instead of propene. Clearly fly ash behaves not only as a potent catalyst for deep oxidation and chlorination, but accelerates condensation reactions of simple short chain olefins. Under comparable conditions but in the absence of a fly ash bed a low degree of homogeneous propene oxidation was observed at 590 °C.

Isomerization of Triphenylmethoxyl: The Wieland Free‐Radical Rearrangement Revisited a Century Later

yielded 2.3 g of a yellow oil from which benzophenone (almost 2 g) and phenol (0.2 g) were separated. Further heating gave a substantial, but not quantifiable, amount of Ph3COH. Wieland s interpretation was that triphenylmethoxyl radicals (Ph3COC, 2) had been formed and had isomerized to Ph2(PhO)CC radicals (3) which then coupled (Scheme 1). This was the first clearly demonstrated, and explicitly shown, free-radical rearrangement—a priority often overlooked. The rate constants and mechanisms of isomerization of triphenylmethoxyl (2) and the analogous isomerizations of Ph2C(Me)OC (5), [2–10] and related radicals (Scheme 2), have received considerable attention. Claims that discrete spiro intermediates (6) had been identified in the rearrangements of 2 and 5 3] have been disproven. However, computational studies on the rearrangement of 5 10] (and PhCH2OC) [11] do indicate stepwise processes with spiro radicals (6) as intermediates (Scheme 2). Consistent with these calculations, cumyloxyl radicals, PhC(Me)2OC, para substituted with a 2,2-diphenylcyclopropyl reporter group, have been demonstrated to be in equilibrium with spiro radicals 6. Rate constants (10 6 k2 s ) measured at room temperature by laser flash photolysis (LFP) in CH3CN were 2.5, [6]