Marit Jagtoyen

Activated carbons from yellow poplar and white oak by H3PO4 activation

Abstract Results are presented from continuing investigations of the phosphoric acid activation of hardwoods. Earlier work with white oak has been extended to include yellow poplar. It is found that the same general chemical and physical changes occur with both precursors. A discussion is presented on the possible mechanisms of phosphoric acid activation, drawing upon extensive research on the use of phosphorous compounds as fire retardants for wood and cellulose. Phosphoric acid appears to function both as an acid catalyst to promote bond cleavage reactions and the formation of crosslinks via processes such as cyclization, and condensation, and to combine with organic species to form phosphate and polyphosphate bridges that connect and crosslink biopolymer fragments. The addition or insertion of phosphate groups drives a process of dilation that, after removal of the acid, leaves the matrix in an expanded state with an accessible pore structure. It is considered that activation of the amorphous polymers produces mostly micropores, while activation of crystalline cellulose produces a mixture of pore sizes. The different response of crystalline cellulose is attributed to a much greater potential for structural expansion than is possible with the amorphous polymers due, among other factors, to its higher density and its chemical structure that allows for a more extensive degree of combination with phosphoric acid, and hence “bulking” of the cell walls. The pore size distribution obtained from crystalline cellulose can be altered by increasing the HTT and/or the ratio of acid to precursor such that, eventually, the structure is dominantly mesoporous. At temperatures above 450 °C, a secondary contraction of the structure occurs when the phosphate linkages become thermally unstable. The reduction in crosslink density allows the growth and alignment of polyaromatic clusters, producing a more densely packed and less porous structure.

Evolution of carbon structure in chemically activated wood

Abstract 13C NMR and FTIR analyses have been employed to follow the evolution of chemical structure in relation to porosity development, as a function of heat treatment temperature (HTT), for activated carbons produced from white oak by phosphoric acid activation. The chemical changes effected by acid treatment at low HTT are: by 50 °C there is significant alteration of the lignin structure; by 100 °C a significant portion of the cellulose has reacted, with the formation of ketones and esters; the formation of phosphate esters becomes apparent around 150 °C; crosslinking reactions are initiated below 150 °C, consistent with the higher carbon yield obtained in chemical activation; and generally there is an increase in aromaticity and loss of aliphatic, carboxyl, and carbonyl groups. The low temperature phenomena precede, and relate to, the development of porosity and structural dilation that commences around 250 °C, and attains a maximum between 350 and 450 °C. Up to 450 °C, pore volume is found to correlate with crosslink density. Above 450 °C, there is a dimensional contraction and a reduction in porosity. Among the accompanying phenomena are: the elimination of cellulose phosphates and oxygen functionalities; and a dramatic increase in the estimated aromatic cluster size. The latter would require a reduction in crosslink density to facilitate cluster growth, and the resulting structural rearrangement and increased alignment of clusters would produce a more densely packed structure with reduced porosity.

Some considerations of the origins of porosity in carbons from chemically activated wood

Abstract Investigations have been made of the conversion of white oak to activated carbons by reaction with phosphoric acid at temperatures up to 650°C. At low temperatures, reaction with the acid promotes dehydration reactions, and the redistribution of biopolymers. It is considered that the subsequent formation of crosslinks leads to an increase in carbon yield above about 300°C. Detailed observations by optical microscopy, and measurements of dimensional changes, have established for the first time a link between the origins of porosity and reaction mechanisms. Following an initial contraction, the structure undergoes considerable expansion between 250 and 450°C, which corresponds to the development of high surface area. Above 450°C, there is secondary contraction and an accompanying loss of accessible porosity. It is concluded that porosity development is directly related to the retention and dilation of cellular material, which creates an extensive surface accessible to adsorbent molecules. The results may be relevant to understanding equivalent processes with other biomass and coal precursors that contain biopolymers or altered biopolymers.

Adsorbent carbon synthesis from coals by phosphoric acid activation

The chemical activation of a bituminous coal by reaction with phosphoric acid has been followed by studying chemical and structural changes to the activate and the development of porosity. There are clear differences in porosity development after thermal treatment in the range 350–650°C and after heat treatment in the same temperature range following reaction with phosphoric acid. It is proposed that chemical treatment causes rupture of weaker linkages in the coal structure and early development of a rigidly cross-linked product through the formation of new stronger linkages. This process is accompanied by development of a mainly microporous structure which reaches a maximum BET surface area of ~ 750 m2g−1 HTT ~ 550°C. Pore structure development can be tailored to some extent by varying both phosphoric acid strength and final heat-treatment temperature.

Activated extrudates by oxidation and KOH activation of bituminous coal

An investigation has been made of the synthesis of formed (extruded) activated carbons from a bituminous coal by KOH activation. The coal was first pretreated by oxidation with nitric acid, at different levels of oxidation severity, in an attempt to introduce properties similar to those possessed by lowrank coals that can be directly processed in this way. It is found that hard, high-surface-area activated extrudates can be successfully produced from pre-oxidized bituminous coal. The effects of nitric acid oxidation are to regenerate humic acids and introduce oxygen and NO2 functional groups, that are more concentrated in the humic acids. The magnitude of these changes increases with the severity of oxidation, the extent of which can be followed by optical microscopy using a dye technique. At nitric acid normalities above 0.25, the treated coals formed gel-like mixtures with KOH solution, and were easily extruded. Preoxidation was found to enhance the development of surface area in the heat-treated products, which also increased with the ratio of KOH to fixed carbon in the precursor. These parameters also strongly influenced the hardness of the extrudates. For a given oxidation severity, the hardness at first increased with the ratio of KOH to fixed carbon, then passed through a maximum. At high ratios, the extrudates were weak. The maximum hardness increased with the level of oxidation: at higher severities, extrudates were produced that were harder, and had higher surface areas, than samples of commercial carbons. The reduced hardness at high ratios is attributed to the dilution of the mixture of dispersed coal and coal particles by excess KOH, reducing the ability to form an extensive network of bridging linkages. Correspondingly, the optical texture of the harder extrudates was found to be homogeneous, whereas at high KOH ratios there was reduced fusion between coal particles and the appearance of cracks.

Synthesis of isotropic carbon fibers and activated carbon fibers from pitch precursors

The influence of pitch precursor composition on the formation and properties of isotropic pitch fibers from non-conventional precursors has been examined. Changes in fiber weight that occur during the oxidative stabilization of green fibers are inversely related to the observed axial contraction. The weight gain upon stabilization also increases, and the contraction decreases with increasing pitch carbon content and aromaticity while the opposite occurs with increasing hydrogen and hetero-atom (H, N, O and S) content. Similar trends are found for fiber carbonization. The combined effects of stabilization and carbonization give a fiber yield of between 50 to 86% of the green fibers, with axial contractions of 12 to 28%, the highest yield corresponding to the smallest contraction. The net yield increases with pitch carbon content and aromaticity, and decreases with hetero-atom content, while the overall axial contraction decreases. The fiber tensile strength was found to increase with precursor carbon content, carbon yield and aromaticity. The activation rate of the derived fibers increased with increasing heteroatom content, especially oxygen content. While most fibers were microporous upon activation, fibers from shale oil and the sub-bituminous coal extract, developed a more mesoporous structure.

Activated carbons from bituminous coals by reaction with H3PO4: The influence of coal cleaning

Abstract A study has been made to examine the effects of coal cleaning by column flotation on the properties of carbons prepared by the phosphoric acid activation of bituminous coals. Earlier work has shown that while moderate surface areas are generated by the reaction of coals with H 3 PO 4 at temperatures around 550°C, phosphoric acid also reacts with coal sulfur and mineral matter. These side reactions consume reagent that is otherwise available for reaction with the organic structure, and result in the formation of insoluble phosphates, increasing the carbon ash content and limiting the amount of recoverable reagent. Lowering the mineral matter content of the coals prior to carbon synthesis is found to have a direct influence on reducing the ash content of the derived carbons and the extent of phosphorus retention. Coal cleaning also increases the extent of sulfur removal (most of which is liberated as H 2 S), the BET and mesopore surface areas, and the carbon pore volume. Possible causes of these effects are discussed. The assumption that the ash in the carbon has negligible porosity, and hence that the reduction in mineral matter content of the coal will automatically increase the specific surface area of the carbon by lowering the ash content, does not appear to fully explain the results. The consumption of phosphoric acid by side reactions lowers the effective reagent to coal ratio and limits the amount of available reagent, which can cause a reduction in surface area. Further, it is possible that ash constituents can block the pore structure, and that lowering the coal mineral matter content will improve access. It seems probable that all three explanations have some validity and can contribute to the observed changes.

Fabrication of carbon fibre composites for gas separation

Abstract The fabrication of monolithic activated carbon fibre composites using isotropic pitch based carbon fibres, and phenolic resin as binder, is briefly described. The dimensional changes during drying, curing, baking and activation stages are presented and discussed. Data on other physical properties of the composites including their permeability and surface area are presented. With respect to gas separation, a technique developed to assess the potential of the composites to separate methane and carbon dioxide is described; the effects of some of the fabricating process variables on performance in CH 4 /CO 2 separation is presented and discussed. In particular the effect of the extent of weight loss during activation on the final composites properties is described.

Relationship between reflectance and structure of high surface area carbons

Abstract Reflectance measurements have been used to try to track the structural alterations during activated carbon synthesis by chemical activation. H3PO4 has been used as a chemical reagent with white oak, subbituminous coal and a bituminous coal, and KOH with a bituminous coal. The reflectance of thermally treated carbons follows a single correlation with heat treatment temperature (HTT), consistent with thermally induced increases in aromaticity and structural order. For chemically activated carbons, the relationship between reflectance and HTT depends upon the precursor-activant combination that is used. At low HTT, chemically activated carbons have higher reflectance than their thermally treated counterparts, due to accelerated chemical change. At higher HTT this situation is reversed, despite further increase in aromaticity. It is proposed that the development of significant porosity (principally in pores 2–50 nm in diameter) reduces the measured reflectance by contributing to the scattering of incident light.

Carbon catalysts for reactions relevant to coal liquefaction

Publisher Summary This chapter discusses the preparation of catalysts based on chemically activated coals. The catalytic activity of the activated coals are measured in microtests for carbon–carbon bond cleavage on 4-(1-naphthylmethyl)bibenzyl (I) and for dehydroxylation and hydrogenation on 2-hydroxynaphthalene (II). The activation of coal or other materials with potassium hydroxide (KOH) give high surface area active carbons developed on a limited industrial basis, but the materials usually exhibit very low activity as catalysts in the reactions of interest for coal liquefaction. A procedure for coal activation with KOH, which increases the catalytic activity and gives materials with higher activity than the much more expensive carbonized polymers and carbon blacks, is described in the chapter.