Depolymerization, aromatic

Zavitsas et al. added terms for the extent of hemiformal and paraformaldehyde formation. Hemiformal formation slows the methylolation reaction as does the presence of paraformaldehyde. They report that only monomeric methylene glycol appears to methylolate. They point out that the terms for the two polyoxy-methylene species partially cancel one another, as depolymerization of paraformaldehyde naturally occurs while hemiformal formation is increasing due to methylolation. They observe that hemiformals form only on the methylolphenol hydroxyls and not on the aromatic hydroxyl. They calculate that the average number of methoxy groups involved in each of the hemiformals is about two in addition to the original methylol. There is no selectivity for ortho versus para positions in hemiformal formation. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

The polyester domains of suberized walls can also be depolymerized using chemical and/or enzymatic approaches similar to those used for cutin. The aromatic domains are far more difficult to depolymerize as C-C and C-O-C crosslinks are probably present in such domains. Therefore, more drastic degradation procedures such as nitrobenzene, CuO oxidation, or thioglycolic... 

Most lignin is now burnt for heat and power, but process options are available to depolymerize the phenolic material by thermal cracking or using base treatments [7]. In consecutive steps the products can be converted into aromatic hydrocarbon feeds. 

Transalkylation involves the transfer of alkyl groups between aromatic nuclei, usually in the presence of strong Lewis acids. Heredy and Neuworth used this reaction to "depolymerize" coal. As a result of the reaction of coal with BF3 and phenol, the solubility of coal in phenol or pyridine increased substantially. Various modifications of this reaction have since been reported . Transall lation reactions in the presence of trifluoromethane sulfonic acid and aromatic hydrocarbons have recently been used by Benjamin et al. and Farcasiu et al. to identify structural features in coals and heavy petroleum ends, respectively. 

Suberized Cell Walls. An analogous set of CPMAS experiments is presented for suberin in Figure 6. Because this polymer is an integral part of the plant cell wall, the 13C NMR spectrum had contributions from both polysaccharide and polyester components. Chemical-shift assignments, summarized in Table IV, demonstrated the feasibility of identifying major polyester and sugar moieties despite serious spectral overlap. Semiquantitative estimates for the various carbon types indicated that, as compared with cutin, the suberin polyester had dramatically fewer aliphatic and more aromatic residues. A similar observation was made previously for the soluble depolymerization products of these plant polymers (1,8,11). 

Thus, for the degradation of polymeric lignin by the enzyme, two major questions were left (i) Can lignin peroxidase, by itself, depolymerize polymeric lignin without repolymerization or not (ii) Can lignin peroxidase cleave aromatic rings and /J-0-4 bonds of polymeric lignin, or not ... 

In the depolymerized scrap mbber (DSR) experimental process, ground scrap mbber tires produce a carbon black dispersion in oil (35). Initially, aromatic oils are blended with the tire crumb, and the mixture is heated at 250—275°C in an autoclave for 12—24 h. The oil acts as a heat-transfer medium and swelling agent, and the heat and oil cause the mbber to depolymerize. As more DSR is produced and mbber is added, less aromatic oil is needed, and eventually virtually 100% of the oil is replaced by DSR. The DSR reduces thermal oxidation of polymers and increases the tack of uncured mbber (36,37). Depolymerized scrap mbber has a heat value of 40 MJ/kg (17,200 Btu/lb) and is blended with No. 2 fuel oil as fuel extender (38). 

The formation of the coal extract was interpreted as a free-radical chain reaction leading to the depolymerization of coal and aromatization of some of the hydroaromatic structures. It was suggested that phenanthrene possibly plays the role of a chain carrier in this process. 

Jhe distribution of hydrogen types in coals continues to be a subject of considerable interest in coal structure studies. Published data indicate that the fraction of aromatic hydrogens usually increases with increasing rank, but the absolute values depend on the specific analytical method used (7). Hydrogen type analysis of a single coal based on the application of NMR spectroscopy to the soluble fraction from depolymerization with phenol-BFa has been reported by us (3). The conversion of coal to soluble fragments in substantial yields under very mild conditions permits a reliable determination of the hydrogen types by NMR analysis, and these results can be extrapolated to the parent coal with considerable confidence. 

These values are summarized in Figure 2 and compared with our results on the soluble fractions from depolymerization. Van Krevelen (9) reported aromatic hydrogen values ranging from 23 to 54% for coals containing 76 to 89% carbon. Aromatic hydrogen content varies directly with rank. Brown (i) obtained values of 19-42% for a series of vitrains and showed a similar relationship with rank displaced toward lower aromatic values. Ladner and Stacey (5) analyzed two additional coals using the procedure developed by Brown, and the results fit Brown s correlation. 

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