A. C. Buchanan

Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways

Abstract The pyrolysis of surface-immobilized β-alkyl aryl ethers, m ≈ PhOCH 2 CH 2 Ph , ≈ PhCH 2 CH 2 OPh , and ≈ PhCH 2 CH 2 OPh - o - OCH 3 , has been studied with a dispersed acid catalyst at 300–450 °C in order to gain an insight into the relative importance of ionic reactions vs. free-radical reactions in the thermal depolymerization of lignin. In the absence of acid catalyst, surface-immobilization does not hinder the free-radical reaction pathway to produce phenols, alkenes, alkanes, and aldehydes as the major products. Pyrolysis in the presence of small particle sized (15 nm) SiO 2 -1% Al 2 O 3 at 300 °C, where free-radical reactions are suppressed, produces an acceleration in the rate of decomposition and a substantial alteration in the product distribution as a consequence of the solid-solid interactions between the catalyst and substrate. Products from the silica-alumina catalyzed reactions are characteristic of acid-catalyzed cracking reactions which involve carbocation intermediates to produce products from ether cleavage and aromatic alkylation and dealkylation reactions resulting most notably in the near absence of alkenes. The surface-immobilized primary products continue to undergo acid-catalyzed reactions to form larger aromatics and coke. At temperatures in which the free-radical and acid-catalyzed reactions can compete (> 375 °C), products from the acid-catalyzed reaction dominate. A comparison of the products from the pyrolysis of lignin, biomass, and the surface-immobilized model compounds provides new evidence that the thermal degradation of lignin occurs principally by a free-radical reaction pathway.

Computational Study of Bond Dissociation Enthalpies for Lignin Model Compounds. Substituent Effects in Phenethyl Phenyl Ethers

Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH(2)CH(2)OPh, PPE) is the simplest example representing the dominant beta-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen-carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen-carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon-carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen. On the other hand, the carbon-carbon bond dissociation enthalpy shows little variation with different substitution patterns on either phenyl ring.

Computational Prediction of α/β Selectivities in the Pyrolysis of Oxygen-Substituted Phenethyl Phenyl Ethers

Phenethyl phenyl ether (PPE; PhCH 2CH 2OPh) is the simplest model for the most common beta-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the alpha/beta product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The alpha/beta selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the alpha- and beta-carbons of PPE. We use density functional theory and employ transition state theory where we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semiclassical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para position on the phenethyl ring of PPE on the alpha/beta selectivities. The total alpha/beta selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent. The strongest effect of the substituents is observed for the alpha-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the beta pathway and the abstraction by the R-benzyl radical (R = OH,OCH 3) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.

Computational Study of Bond Dissociation Enthalpies for Substituted β‐O‐4 Lignin Model Compounds

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant β-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.

Kinetic analysis of the phenyl-shift reaction in β-O-4 lignin model compounds: a computational study.

The phenyl-shift reaction for the β-radical of phenethyl phenyl ether (PhCH(2)ĊHOPh, β-PPE) is an integral step in the pyrolysis of PPE, which is a model compound for the β-O-4 linkage in lignin. We investigated the influence of natural occurring substituents (hydroxy, methoxy) on the reaction rate by calculating relative rate constants using density functional theory in combination with transition state theory, including anharmonic correction for low-frequency modes. The phenyl-shift reaction proceeds through an oxaspiro[2.5]octadienyl radical intermediate and the overall rate constants were computed invoking the steady-state approximation (its validity was confirmed). Substituents on the phenethyl ring have only little influence on the rate constants. If a methoxy substituent is located in the para position of the phenyl ring adjacent to the ether oxygen, the energies of the intermediate and second transition state are lowered, but the overall rate constant is not significantly altered. This is a consequence of the dominating first transition from reactant to intermediate in the overall rate constant. In contrast, o- and di-o-methoxy substituents significantly accelerate the phenyl-migration rate compared to β-PPE.

A study of the reductive pyrolysis behaviour of sulphur model compounds

Abstract The difficulties inherent in the direct determination of sulphur functionalities in complex solid matrices by various techniques often make the need for reference compounds indispensable. One of the pyrolysis techniques used for sulphur determination is atmospheric pressure–temperature programmed reduction (AP-TPR). Experiments on sulphur model compounds have served successfully as a reference for both the temperature region in which the reduction or hydrogenation occurs and the efficiency of the reduction reaction. In this study, the pyrolysis behaviour of several organic and inorganic sulphur model compounds is investigated by AP-TPR using a mass spectrometer detector interfaced with the pyrolysis reactor (AP-TPR-MS). This technique permits a more complete description of the competitive and successive reactions that are occurring during the pyrolysis of the model compounds, providing new information regarding the reduction efficiency of oxidised and non-oxidised sulphur compounds.

Influence of steroid structure on the pyrolytic formation of polycyclic aromatic hydrocarbons

Abstract It is well known that polycyclic aromatic hydrocarbons (PAHs) are formed from the pyrolysis of biomass, but the precursors and pathways that lead to PAHs are not well characterized. In this investigation, the flash vacuum pyrolysis (FVP) and the atmospheric pressure flow pyrolysis of a series of structurally different plant steroids, including stigmasterol, stigmasteryl acetate, β-sitosterol, stigmasta-3,5-diene, cholesterol, cholesteryl acetate, dihydrocholesterol, and ergosterol was investigated at 700xa0°C to determine the impact of steroid structure on the formation of three, four, and five ring PAHs. FVP of the steroids revealed that PAHs, such as phenanthrene, anthracene, pyrene, chrysene, benz[a]anthracene and their monomethylated derivatives, were formed by a series of unimolecular reactions, and the PAH yields were dependent upon the steroid structure. PAH yields were most sensitive to the number of double bonds in the steroid B-ring. Ergosterol (two double bonds) produced 13-fold more PAHs than dihydrocholesterol (no double bonds) in FVP experiments and 4-fold more PAHs in the flow pyrolysis experiments. Increasing the temperature from 700 to 800xa0°C in the FVP experiments only slightly increased the PAH yields for dihydrocholesterol relative to cholesterol, but the yield of small aromatic hydrocarbons, such as benzene and toluene, increased approximately 2-fold. Small increases in PAH yields (10–40%) were found in the FVP experiments if a double bond was placed in the steroid A-ring by 1,2-elimination of an ester (as for cholesteryl acetate), but this trend was not observed in the flow pyrolysis experiments. Insight into the structural origins of the PAHs was gained by FVP of [4-13C]cholesterol, in which the 13C content of the products were determined by mass spectrometry. The PAH yields from the flow pyrolysis of steroids esterified with saturated and unsaturated long chain fatty acids, i.e. cholesteryl stearate, cholesteryl oleate, and cholesteryl linolenate, were 20–40% lower (per gram of steroid) than the free steroid. The yield of three to five ring PAHs did not correlate with the number of double bonds in the ester chain, but the yield of benzene increased as the number of double bonds in the ester chain increased. In general, this investigation has shown that structural differences in steroids can significantly alter the PAH yields formed from the vacuum pyrolysis or atmospheric pressure pyrolysis of steroids.

Confinement effects on product selectivity in the pyrolysis of phenethyl phenyl ether in mesoporous silica

Pyrolysis of phenethyl phenyl ether confined in mesoporous silicas by covalent grafting results in significantly increased product selectivity compared with fluid phases.

Investigations of restricted mass transport effects on hydrocarbon pyrolysis mechanisms through silica immobilization

Abstract In the pyrolysis of polymers and other macromolecular systems including coal, kerogen, lignin, and cellulose, the importance of mass transport in affecting pyrolysis kinetics, tar and char yields, and product molecular weight distribution is becoming increasingly appreciated. However, detailed molecular level information on the effects of restricted mass transport on the reaction mechanisms for organic constituents in these complex organic materials is understandably more limited. Well-designed, experimental studies of the pyrolysis of model compounds and polymers are capable of providing such insights. In this paper, research employing silica-immobilized compounds is reviewed and selected examples from other types of model systems that explore mass transport effects are presented.

Effect of cross-linking on the pyrolysis of diphenylalkanes

Abstract In the early stages of the thermal depolymerization of biopolymers, geopolymers, and synthetic polymers, their cross-linked macromolecular structure may restrict the diffusion of reactive intermediates and alter reaction pathways. In an effort to model the effects of restricted mass transport on the thermally induced free radical decomposition of polymethylene units bridging aromatic clusters in coal, 1,3-diphenylpropane (DPP) and 1,4-diphenylbutane (DPB) have been cross-linked to an inert silica surface by the condensation of the corresponding diphenol, p -HOC 6 H 4 (CH 2 ) n C 6 H 4 OH- p . Pyrolysis of a cross-linked DPP at 375°C yielded a 100-fold increase in secondary products as a consequence of cross-linking. Pyrolysis of a cross-linked DPB at 400°C afforded an altered product selectivity favoring products from the thermodynamically less stable aliphatic radical. The surface-attached compounds were also used as solid state probes to study the reactions of hydrogen donor solvents in a heterogeneous environment.