C. Morterra

IR studies of carbons—II: The vacuum pyrolysis of cellulose

Abstract The pyrolysis of cellulose in vacuum from 22 to 765°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra recorded at various stages of pyrolysis showed that although the main decomposition occurred near 300°C, some decomposition occurred as low as 190°C with the formation or highly absorbing aromatic systems. Mixtures of predominantly aliphatic material predominate in the 300–400°C range but decline in extent to be replaced by mixtures of aromatic nature above 500°C. Further degassing causes discrete spectral features to decline and disappear near 700°C; a continuum absorption remains. Band assignments are discussed. In particular, the 1600 cm −1 band of spectra of carbons is shown by isotopic study not to be ascribed to carbonyls. but is thought to be a C = C mode made IR active by asymmetry caused by bound oxygen.

I.R. studies of carbons. VII: The pyrolysis of a phenol-formaldehyde resin

Abstract Infrared spectra were recorded of the chars produced by pyrolyzing a Novolac phenol-formaldehyde resin in vacuum and in nitrogen, using photothermal beam deflection spectroscopy. The two pyrolysis techniques led to the same results. Thermal branching and cross-linking occurred near 350°C, with the formation of diphenyl ether structures. These reactions continued at higher temperatures when aryl-aryl ethers were formed. Autooxidation is not an important degradation pathway. Although some changes occur, the polymer network remains essentially intact until 500°C, the aromatic systems being held apart and stabilized by aliphatic bridges. In the 500–560°C range, however, drastic changes occur in that the network collapses, aliphatic bridges are destroyed, hydrocarbonaceous residues are eliminated and those remaining are altered, polyaromatic domains form, and the resulting char is much like other intermediate temperature chars.

IR studies of carbons—III: The oxidation of cellulose chars☆

Abstract Carbons prepared by charring cellulose in vacuum were oxidized under the conditions leading to the formation of acid or basic carbons. The reactions were followed with IR phothermal beam deflection spectroscopy. The oxidation of chars at temperatures leading to the formation of acid carbons resulted in the formation of several surface species to which acid properties may be ascribed. Some band assignments to definite surface structures were made. Spectra were recorded for the first time of high temperature carbons oxidized at the high temperatures leading to the formation of basic carbons, and the interconversion of acid and basic carbons could be followed. No oxygen-containing species to which basic properties could be attributed were observable.

Infrared studies of carbons. XII The formation of chars from a polycarbonate

Abstract Series of infrared spectra were recorded of chars produced by heating a pure bisphenol-A polycarbonate resin (PC) in vacua at successively increasing temperatures, up to 750°C. FT-IR photothermal beam deflection spectroscopy was used. PC, after remaining essentially intact over a wide range of temperatures, underwent massive and dramatic changes upon heating in a narrow temperature interval (440–490°C). Thermal branching and cross-linking occurred near 440°C with the formation of diaryl ether, ester and unsaturated hydrocarbonaceous bridges. Small, still discrete aromatic entities were formed on pyrolysis above 490°C. Fusion of the aromatic rings into larger polyaromatic structures, however, occurred to a significant extent only at 590°C, when the polymeric network collapsed completely; all aliphatic bridges were destroyed and the oxygen bridges were incorporated in the polyaromatic domains which were formed. The resulting char was similar to other intermediate temperature chars. The continuum of the carbon absorption extends throughout the spectral region at 750°C.

IR studies of carbons—VI. The effects of KHCO3 on cellulose pyrolysis and char oxidation☆

The pyrolysis in vacuum of cellulose containing 5 wt. % of KHCO3 (KHC) was followed by IR photothermal beam deflection spectroscopy, and compared with observations made with pure cellulose (PC) [Carbon 21, 283(1983)]. The KHCO3, and KOH and K2CO3 formed from it, drastically altered the cellulose carbonization and the subsequent oxidation of the KHC chars. KHC became charred at 140 °C and the cellulose disrupted at 220 °C. The surface layer was a mixture of weak acid K salts. K2CO3 was not formed until 310 °C and persisted until about 500 °C, when metallic K was evolved. Unlike PC, the surface layer contained no aromatic CH structures. The carbonization of KHC proceeded more rapidly and to a greater extent than with PC. The oxidation of KHC chars was also markedly affected; burn-off occurred at much lower temperatures, and the gasification temperature range was narrower, than with PC chars.

IR studies of carbons—V Effects of nacl on cellulose pyrolysis and char oxidation∗

The pyrolysis in vacuum of cellulose containing 5 wt.% of NaCl (NCC) was followed by i.r. photothermal beam deflection spectroscopy, and compared observations made with pure cellulose (PC) [Carbon21, 238 (1983)]. Although the overall aspects of NCC and PC pyrolysis were similar, NaCl accelerated the cellulose decomposition, the relative amounts of aliphatic and aromatic residues were changed, and i.r.-detectable species disappeared about 100°C lower than with PC, all implying changes in the decomposition and charring mechanisms. The reactions of NCC chars with O2 was also followed and compared with those of PC chars. NaCl caused the ratios of oxidation products to change (some band assignments are discussed) but the behavior of NCC chars produced at 650°C, when NaCl evaporated, was similar to high-temperature PC chars.

IR studies of carbons—IV The vacuum pyrolysis of oxidized cellulose and the characterization of the chars

Abstract The pyrolysis of an NO2-oxydized cellulose (NOC) in vacuum from 22 to 700°C was followed by IR photothermal beam deflection spectroscopy. Series of spectra were recorded at various stages of pyrolysis and compared with and contrasted to similar data previously obtained with cellulose. Also, NOC-derived chars were oxidized, and the effects of treating NOC and NOC char with KOH were observed. The detailed IR spectroscopic data indicate that NOC degrades more easily than cellulose and yields chars that possess different functional groups than cellulose chars, up to charring temperatures of near 500°C. Above that temperature the chars have similar chemical and physical properties.

Some Aspects of the Use of Infrared Photothermal Beam Deflection Spectroscopy for Studies of Chemisorbed Species

Abstract Photothermal bean deflection spectroscopy (PBDS) was used to record infrared (IR) spectra of monolayers of pyridine adsorbed on particulate adsorbers which absorb IR radiation weakly, and of pyrene adsorbed on medium- and high-temperature carbons which are very strong IR absorbers. The various results show that although some IR data about surface layers on weak absorbers can be obtained, the data are generally inferior in quality than those obtainable with IR transmission/ absorption measurements. PBDS results decrease in quality as the transparency of the sample increases. Increasing the specific surface area and the degree of compaction of the sample can lead to an improvement, but a high surface area per se is neither a necessary nor sufficient condition. The scattering of IR radiation appears to be the most important of the parameters which can affect the quality of the PBD spectra, and a favourable combination of surface area, compaction and especially scattering effects can result. In general, the PBD technique has little to offer for use with non-scattering weak absorbers, but can be usefully applied with highly-scattering weak absorbers. With the other extreme type of sample, the very strong absorbers, PBDS has been used to observe monolayers of pyrene on medium- and high-temperature carbons. Although the spectra of the adsorbed species are relatively poor in comparison to those obtained with medium or strong absorbers, improvements in technique seem possible, so that PBDS will become a unique method for the study of surface effects with very strong absorbers.

Infrared studies of carbons. XIII, The oxidation of polycarbonate chars

Abstract IR photothermal beam deflection spectroscopy was used to record IR spectra of chars, produced by the in vacuo pyrolysis of a polycarbonate resin (PC), at several stages of oxidation. The high-temperature chars seem to be more susceptible to oxidation than the low-temperature PC chars. This behavior is quite unlike that of coals, cellulose based chars, and other carbons. Oxidation of the low temperature chars proceeds mainly through attack on the hydrocarbonaceous side chains and the elimination of carbonate functionality, accompanied by an extensive formation of benzophenone, lactones, and acidic carbonylic species. However, the chars obtained at temperatures higher than 590°C, when polyaromatic domains first form to a significant extent, exhibit the normal oxidation behavior found with chars derived from several types of precursors.

Infrared studies of carbons. XVI. The carbonization of an aliphatic allyl polycarbonate

Abstract Infrared photothermal beam deflection spectroscopy was used to record spectra of pyrolyzates produced by heating an aliphatic allyl polycarbonate in vacuo and in the air. It was found that this resin would not char to an appreciable extent upon vacuum pyrolysis unless it had been first subjected to an initial oxidation at low temperature (250°C). The char derived after such a treatment, and after its subsequent pyrolysis at high temperature, exhibits all the infrared spectral characteristics associated with the intermediate-temperature carbons.