Water vapour adsorption on lignin‐based activated carbons

Abstract

Lignocellulosic wastes are interesting precursors for carbon products. The high amount of Na observed in kraft lignin makes it a promising precursor for the preparation of activated carbons for desiccant applications. Water adsorption capacity and kinetics of kraft lignin-based chars and activated carbons with different burn-off and inorganic matter content have been studied. CO 2 partial gasification of lignin char develops a wide porous structure. An increase of the micropore volume can be observed at low to medium burn-offs. At degrees of higher activation the mesoporous structure develops. For very high burn-off the porous structure is destroyed by coalescence of the pores and reduction of the carbon material. The carbons obtained show atomic surface concentrations of sodium from 7.6-15.4%, as revealed by XPS analysis. Water vapour adsorption isotherms have been obtained in a thermogravimetric system and have been fitted by a DS model, which properly represents the experimental data. The kinetics of water vapour adsorption follows a linear driving force mass transfer (LDF) model. The presence of sodium and oxygen surface groups on the carbon surface enhances water vapour adsorption at low relative pressure. Activated carbon produced at 41% burn-off shows the highest water vapour adsorption at low relative pressures, as a consequence of the high sodium dispersion on its surface. The sodium dispersed over the carbon surface undergoes clustering as gasification proceeds, decreasing the number of active centres. For burn-off higher than 41%, this behaviour produces a decrease in the water adsorbed at low relative pressures.  2007 Society of Chemical Industry Keywords: water vapour adsorption; lignin; CO 2 gasification; chars; activated carbon; sodium INTRODUCTION Activated carbons can be obtained from many raw materials, including carbonaceous wastes; they present a high capacity for water vapour adsorption at medium to high relative pressures, due to their large specific surface area and high porosity. Unfortunately, they show mostly a type V isotherm for water vapour adsorption, with very low uptake at low relative pressures. For desiccant applications, as in sorption cooling/heating systems, low-cost porous solids with relatively large water vapour adsorption at low relative pressures (type I moderate isotherms) are preferred.

Water vapour adsorption by a coffee-based microporous carbon: effect on CO2capture

“…The adsorption isotherms present a similar behaviour with two uptake zones: the first that takes place at very low relative pressures, is generally attributed to the adsorption of water molecules on the oxygen and nitrogen surface functional groups, and also on the inorganic matter present at the pore entrances. The total content of these primary adsorption sites on PPC is relatively large: 9 wt % O, 4.7 wt % N and 4.6 wt % inorganic matter (dry basis), which makes the initial upswing of the adsorption isotherms significant.…”

A Review on the Synthesis and Characterization of Biomass-Derived Carbons for Adsorption of Emerging Contaminants from Water

“…The literature contains many studies about the synthesis of carbon adsorbents from biomass agricultural waste, such as olive stones [39][40][41], rice husk [42,43], coconut husks [44][45][46][47] or bamboo [48,49]. Furthermore, many studies have also focused on the synthesis of carbon materials from biomass industrial wastes including lignin, (a by-product of the papermaking industry) [50][51][52][53][54], Tara gum (generated in the food industry) [55], wood waste or sawdust (from the furniture industry) [56,57], and so on. Figure 3 schematizes the synthesis routes to biochars and activated carbons from biomass.…”

Alcohol Dehydrogenation on Kraft Lignin-Derived Chars with Surface Basicity

“…This reduction would produce the oxidation of the carbon to carbonate surface in the form of groups, which decompose as CO2. Nevertheless, another feasible explanation could be the formation of sodium atom clusters in the external surface, decreasing the relative amount of sodium, as revealed by XPS [30]. The surface chemistry of the carbon catalysts was also analyzed by the temperature-programmed desorption (TPD) technique, which is used to characterize the carbon-oxygen groups of the samples.…”

“…This reduction would produce the oxidation of the carbon to carbonate surface in the form of groups, which decompose as CO 2 . Nevertheless, another feasible explanation could be the formation of sodium atom clusters in the external surface, decreasing the relative amount of sodium, as revealed by XPS [30].…”