J.M. Martín-Martínez

The combined use of different approaches in the characterization of microporous carbons

Abstract Adsorption of N2 (77 K), CO2 (273 K), and several hydrocarbons (273 or 298 K) and n-nonane preadsorption for two series of activated carbons with more-or-less distorted type I isotherms have been analyzed by the Dubinin-Radushkevich (D-R), Langmuir, and αs methods. When the microporosity is narrow and uniform, all theoretical and experimental approaches lead to values of micropore volume that are in good agreement. For carbons with wide micropore size distributions the Langmuir, αs and D-R methods applied to N2 (77 K) (if for the latter a straight portion in the D-R plot can be drawn in the 0.05–0.3 range of relative pressure) yield the total micropore volume (as the conventional D-R plots for hydrocarbons) whereas n-nonane preadsorption and CO2 give the volume corresponding to narrow micropores, in this way allowing a more complete characterization of the microporosity.

ADSORPTION OF SUBSTITUTED PHENOLS ON ACTIVATED CARBON

The adsorption of phenol and the substituted phenols, 4-nitrophenol, 2,4-dinitrophenol, 4-chlorophenol, and 2,4-dichlorophenol in aqueous solution, has been determined at 298 K on a series of activated carbons, prepared from olive stones, having a wide range of burn-off (8–52%) and micropore size distributions. The adsorption process is controlled predominantly by the porosity of carbon when the microporosity is narrow in range. If the ragne of microporosity is wide then the adsorption process is affected by the chemical nature of the carbon and by the nature of the substituent group in the phenol. The adsorption isotherm of 2,4-dinitrophenol is a step function, which is interpreted as being due to the coexistence of neutral and anionic adsorbate species, and not due to a change in the orientation of the neutral adsorbate species.

Modified activated carbons for the selective catalytic reduction of NO with NH3

Abstract Activated carbons, modified with nitrogen- and oxygen-containing organic compounds by wet impregnation, followed by pyrolysis and CO2 activation, have been used for the low temperature selective catalytic reduction (SCR) of NO with NH3 (385–550 K). Of all the additives studied, only glucosamine results in an outstanding increase of the activity, which might be ascribed to the creation of stable surface oxygen complexes. These complexes can only be removed at relatively high temperatures and probably affect the carbon surface in such a way that adsorption of the reactants is improved, resulting in higher NO conversions compared to the original carbon. The explicit role of the incorporated nitrogen is not unequivocally clear. Apparently, the overall SCR activity is the result of a combination of the following factors: interaction of oxygen with the carbon, the presence of stable oxygen groups, the nitrogen content and the accessibility of the pores. For the modified carbons the SCR reaction is zeroth order in NH3 and first order in NO. Furthermore, the oxygen dependency can be modelled by a Langmuir type of adsorption. Over the unmodified carbons a second type of selective NO reduction can be distinguished in which the carbon only acts as an adsorbents and for which a negative apparent activation energy is observed.

A comparison of the porous texture of two CO2 activated botanic materials

Abstract Two botanically similar agricultural by-products, plum and peach stones have been used as precursors for the preparation of activated carbons. Both carbonization followed by activation in CO2 and direct activation in CO2 are compared. The adsorption of N2 (77 K), CO2 (273 K), i-butane (273 K) and paranitrophenol and methylene blue (both from aqueous solution, at 298 K) have been used to study the microporosity of all carbons. Peach stones yield activated carbons with a molecular sieving effect toward i-butane; these carbons exhibit a very narrow microporosity. However, the microporosity and macroporosity is much more developed in carbons prepared from plum stones; this development is more marked when using the direct activation with CO2.

The controlled reaction of active carbons with air at 350°C—II: Evolution of microporosity

Abstract The evolution of the porosity in two series of samples prepared by the reaction of two activated carbons (from almond shells and olive stones) with air at 350°C has been followed by adsorption of CO2 (273 and 298 K) and n-butane (273 K). The results have been compared with the adsorption of N2 at 77 K [1] and the anomalous behaviour of samples prepared from almond shells in respect to the adsorption of CO2 is discussed. The analysis of the isotherms in terms of percentages of pores filled at different relative pressures gives a good picture of the evolution of the microporosity in the two series of carbons.

Mechanisms of adsorption of CO2 in the micropores of activated anthracite

The combined use of n-nonane preadsorption and CO2 adsorption at 273 K allows interpretation of the mechanism of adsorption of CO2 in narrow micropores of a series of CO2-activated carbons prepared from an anthracite. The mechanism depends on the shape and size of the pores. In narrow micropores of molecular dimensions (1σ), CO2 is probably adsorbed by micropore filling — as proposed for the adsorption of N2 at 77 K — which is associated with curved CO2 isotherms at 273 K. With increasing degree of activation, the microporosity widens and the adsorption of CO2 in narrow micropores of supramolecular dimensions (> 2σ) probably occurs by surface coverage, which is associated with rectilinear isotherms.

Surface modification of synthetic vulcanized rubber

Surface modifications produced by treatments (mainly halogenation) of synthetic vulcanized styrene-butadiene rubber (SBR) leading to increased adhesion properties with polyurethane adhesives have been studied. T-peel tests, scanning electron microscopy (SEM), advancing contact angle measurements, infra-red (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and differential scanning calorimetry (DSC) were used to analyze the nature of surface modifications produced in the rubber. Although some surface heterogeneities were created, physical treatments (ultrasonic cleaning, solvent wiping, abrasion) did not noticeably increase the adhesion strength because certain abhesive substances (e.g. zinc stearate, paraffin wax) cannot be removed from the rubber surface by such treatments. Chemical treatment (chlorination) was carried out using ethyl acetate solutions of trichloroisocyanuric acid (TCI) (1,3,5-trichloro-1,3,5-triazine-2,4,6-trione). Chlorination of SBR with trichloroisocyanuric acid produced a si...

Activation of lignocellulosic materials: a comparison between chemical, physical and combined activation in terms of porous texture

Abstract The influence of the chemical activating agent:raw material ratio in the preparation of activated carbons from almond shells by chemical activation with ZnCl 2 has been studied. Selected aqueous mixtures were kept in contact for 5 h and after filtering the remaining solution and drying, the impregnated almond shells were pyrolysed under N 2 flow. The main effect of this impregnation was the development of microporosity in the activated carbons as a result of the pyrolysis process increasing the activating agent:raw material ratio to a value of 2 enhancing the micropore volume. By this procedure carbons with surface areas ranging from 420 to 1211 m 2 g −1 have been obtained. The subsequent activation of one of the low surface area carbons with CO 2 produces a development in the overall range of porosity giving rise to apparent surface areas of > 2300 m 2 g −1 . Comparing activation with CO 2 of carbonized almond shells and ZnCl 2 -activated carbons from almond shell it is shown that the differences are not in the range of porosity developed but in the global yields, always being larger in the latter.

Structural modification of sepiolite (natural magnesium silicate) by thermal treatment: effect on the properties of polyurethane adhesives

Abstract A sepiolite silicate was heat-treated at 550 and 1000°C to modify its structure, and was used as a filler in a solvent-based polyurethane (PU) adhesive. The treated sepiolites were characterized by X-ray diffraction and infra-red spectroscopy, and it was observed that the water was irreversibly removed from the structure and pores of the sepiolite, changing the structure. The increase of temperature produced a collapse of the sepiolite structure. The rheological, mechanical, thermal and adhesion properties of the filled PU adhesives were measured. In general, the addition of treated sepiolite to PU adhesives resulted in a loss of adhesive properties with respect to the blank (PU adhesive with untreated sepiolite). The loss in properties was more noticeable as the treatment temperature increased. Thus the PU adhesives containing treated sepiolite had reduced rheological properties (lower viscosity, lower storage and loss moduli, and they did not provide thixotropy and pseudoplasticity to the solutions) with respect to the PU adhesive filled with untreated sepiolite. On the other hand, the addition of treated sepiolite decreased the mechanical and thermal mechanical properties of PU films. The T-peel strength of roughened and roughened + chlorinated (with 1 wt% trichloroisocyanuric acid in 2-butanone) styrene-butadiene rubber/PU adhesive joints was improved if the PU adhesive contained untreated sepiolite, but it decreased if the sepiolite was heat-treated. Interactions between the untreated sepiolite, the solvent and the polyurethane were responsible for the improved properties of PU adhesives. These interactions disappeared when the sepiolite was heat-treated, because of the destruction of the structure of the sepiolite and the removal of surface silanol groups.

Microporosity development by CO2 activation of an anthracite studied by physical adsorption of gases, mercury porosimetry, and scanning electron microscopy

A series of CO2-activated carbons (18%–97% burn-off) prepared from a low-ash-content anthracite has been characterized by macerai analysis, physical adsorption of gases (Ar/77 K, N2/77 K, n-butane/273 K, CO2/273 K), n-nonane preadsorption, mercury porosimetry and scanning electron microscopy. Apparent surface areas greater than 1600 m2/g and pore volumes near 1 cm3/g were obtained. There is a continuous increase in adsorption capacity with the burn-offdue to the enlargement of the micropores of the carbons. The meso- and macroporosity were poorly developed and almost exclusively microporosity was created in the carbons. The development of this microporosity was produced in three consecutive stages: for burn-off values smaller than 35% only small micropores were created and the carbons exhibited molecular sieve properties towards n-butane; at burn-off percentages between 35% and 60% medium micropores were developed and for greater activation degrees large micropores were produced. The small micropores can be evaluated from the CO2/273 K. adsorption isotherms by application of the Dubinin-Radushkevich (DR) eqn. Medium micropores can be obtained from application of the DR eq to the adsorption isotherms of Ar/77 K, N2/77 K or n-butane/273 K. Large micropores are evaluated either by n-nonane preadsorption or from the αs, plots of Ar/77 K, N2/77 K or n-butane/273 K adsorption. On the other hand, the total pore volumes of carbons evaluated from the He and Hg densities were in good agreement with the gas volumes adsorbed at P/P0 = 0.95 (Ar/77 K, N2/77 K and n-butane/273 K). The results obtained for these carbons are very similar to those previously reported for different series of CO2 activated carbons prepared from lignocellulosic materials.