M. Catherine Depew

Applications of high power micro wave catalysis in chemistry

Several applications of microwave induced catalysis in chemical reactions are discussed including the synthesis of higher aliphatic and aromatic hydrocarbons from methane, the oxidation of hydrocarbons with water as oxidant, and the destruction of SO2 and NO. A model describing the concept of the technology is briefly introdueed with a discussion of several of the parameters affecting the interactions of the microwaves with the catalysts and the chemical reagents in the process.

CIDEP studies of the photolysis of the lignin model compound α-guaiacoxylacetoveratrone: the role of triplet reactions in aqueous and hydroxylic solvents

Free radical reactions induced by the photolysis of the lignin model compound α-guaiacoxylacetoveratrone have been studied by conventional and time-resolved ESR spectroscopy. In the presence of efficient hydrogen donors such as aqueous and hydroxylic solvents the primary reaction involves photoreduction of the triplet phenacyl ether to form the ketyl radical followed by rapid cleavage to the phenacyl radical and guaiacol. Subsequent formation of polarized guaiacoxyl radicals is due to secondary photo-oxidation. The cleavage of the ketyl radical is retarded in basic media and accelerated in acetic acid. Minor reaction pathways involving excited singlets cannot be ruled out by the current CIDEP observations.

Spin adducts between thioketones and phosphorus centred radicals : Part II. An electron spin resonance study of the addition of phophinyl, phosphonyl and thiophosphonyl radicals to diaryl and dialkyl thioketones

Abstract The paramagnetic adducts between a variety of phosphinyl, phosphonyl or thiophosphonyl radicals and thiobenzophenone, 9,H-xanthen-9-thione, 9, H-thiaxanthen-9-thione, 9, H-selenaxanthen-9-thione and di-tert-butyl thioketone have been generated and studied by means of ESR spectroscopy. Attention has been focused on the variations of a (31P) with the nature of the ligands L in the attacking PLn radicals and with the nature of the thiocarbonylic substrates. In all cases the reactions with the precursors of phosphinyl radicals afforded, beside the expected adducts, additional paramagnetic species, identified as the corresponding phosphonyl and thiophosphonyl adducts: while the formation of the former reflects the high sensitivity of tervalent phosphorus compounds to oxidizing agents, the latter adducts are believed to originate from desulfuration of the thioketones by the starting phosphines.

Catalytic conversion of methane to acetylene induced by microwave irradiation

Abstract Methane decomposition over Ni, Cu and W containing catalysts to form acetylene was examined under microwave treatment. Three types of catalyst have been developed and tested in order to obtain an electric discharge on the surface while irradiating with microwave and, therefore, to initiate the chemical reaction. The effects of catalyst conformation, reaction time and microwave parameters were investigated. Methane decomposition efficiency, yield of acetylene formation and the amount and modality of coke deposition inside the microwave reactor were measured. The cooling of the surface of the catalyst was found to increase the value of methane conversion and acetylene selectivity and also helped to maintain the catalyst activity during longer processing times.

Microwave enhanced non-alkaline bleaching of mechanical pulps: A new solution to an old problem

Since the granting of our first US patent on microwave induced catalytic decomposition of PCBs some 18 years ago, microwave technology has slowly begun to attract some industrial attention in the last few years. There was no doubt that microwaves had been applied to facilitate the conventional bleaching process of wood pulps in the past but with little or no success. The myth that microwaves can be regarded and manipulated as a rapid heat source probably caused most of the failures of unsuccessful microwave applications. It is therefore necessary to understand the real potential of microwaves as an energy source and to identify the many critical electromagnetic properties of the load (chemical systems), before a final choice of a proper microwave system be designed and installed. In this preliminary report, we demonstrate the real potential of pulsed microwave radiation for non-alkaline bleaching of mechanical pulps. Typically, very high consistency TMP up to 93% can be treated by pulsed microwave in the order of 90 seconds with an average increase of 20 to 25 points in brightness.

Applications of ESR and CIDEP to mechanistic studies of lignin chemistry

Using time-resolved CIDEP and conventional ESR as well as optical studies much can be elucidated concerning the initial photochemical reactions of lignin model compounds such as α-guaiacoxyacetoveratrone and α-guaiacoxy-β-propioveratrone. The resulting transient radicals are models for intermediates in the processes of photoyellowing of lignin containing mechanical pulps and papers. Model solution experiments which produce evidence for the roles of free radicals in reductive bleaching of mechanical pulps and in the oxidative aging of papers are also described. The more thorough understanding of these complex processes allows the development of strategies to try to “protect” mechanical pulps and papers from light and thermally induced damage and to “repair” any damage which invevitably occurs.

Esr and cidep studies of the photooxidation of phenols in single crystals and in solutions

Photooxidation of single crystals of 2,6-di-tert-butyl-4-methylphenol and 2,6-di-tert-butylphenol, respectively, led to localized pair-wise trapping of triplet phenoxyl radical pairs. Electron ejection is not implicated in the primary photooxidation of phenols in single crystals, but in liquid solutions time-resolved CIDEP observations for the 2,4,6-tritert-butylphenol in hexaftuorobenzene suggest an initial formation of a phenol radical cation from the photoexcited triplet state.

The Role of Fluorine Substituted Acetic Acid in Stabilization of Organosulfur Cation Radicals: A Kinetic E. S. R. Study

Abstract Trifluoroacetic acid, the widely used solvolytic agent, is generally acknowledged to be a good solvent in stabilizing organic cations due to its acidity and low nucleophilicity1. Although some years ago Dannenberg2 has carried out INDO calculations on the potential surfaces of such probable interactions between the CF3 group and the carbenium ion, little experimental evidence of the actual molecular interactions has been known. In our continuing study of the CIDEP (chemically induced dynamic electron polarization) applications to chemistry, we have recently initiated a model charge transfer reaction between benzoquinone cation radical and organosulfur compounds in trifluoroacetic acid3. Photochemically induced electron polarization of the benzoquinone cation radical is followed by a well defined secondary polarization transfer in the charge transfer reaction leading to a polarized organosulfur cation radical. One of the interesting observations is that the organosulfur cation radical polarization...

AN ELECTRON SPIN RESONANCE STUDY OF THE REACTIONS OF PHOSPHORUS CENTERED RADICALS WITH THIOKETONES

Abstract The utility of thioketones as traps for intermediate phosphorus-centered radicals is illustrated by an e.s.r. study of the phosphorus-thioketone radical adducts. The direct evidence of the P[sbnd]S bond formation in the adduct is clearly supported by the large phosphorus hyperfine splittings which vary with the nature of the organophosphorus moiety. The method also readily detects impurities in the organophosphorus compounds which are susceptible to thermal oxidation. The direct photochemical reactions of thioketones with dialkoxyphosphites are shown to involve free radical mechanisms.

Non-alkaline bleaching of wood pulp using pulsed RF dielectric heating: comparison with pulsed microwave enhanced bleaching

Non-alkaline bleaching of thermo-mechanical pulps using 40 MHz RF dielectric capacitor heating is described. Experimental data obtained under RF irradiation are compared with non-alkaline bleaching under pulsed microwave at 2.45 GHz. Optimal processing time is determined. Effectiveness of non-alkaline bleaching of TMP in RF is shown to be close to that in microwave. Neither microwave nor RF irradiation, under non-alkaline bleaching conditions, is shown to result in detriment to the physical properties of the pulp.