Peter Branton

Physisorption of argon, nitrogen and oxygen by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of argon, nitrogen and oxygen have been determined at 77 K on a sample of MCM-41, a novel form of aluminosilicate. All the isotherms are Type IV in the IUPAC classification. The argon and oxygen isotherms exhibit well defined hysteresis loops, whereas the nitrogen isotherm is completely reversible. This unusual character is attributed to capillary condensation taking place within a narrow range of tubular pores of effective width 3.3–4.3 nm. MCM-41 shows considerable promise as a model mesoporous adsorbent.

Physisorption of nitrogen and oxygen by MCM-41, a model mesoporous adsorbent

Physisorption measurements of nitrogen and oxygen reveal that MCM-41 has a well defined mesoporous structure and is a highly promising model adsorbent.

Reduction of aldehydes and hydrogen cyanide yields in mainstream cigarette smoke using an amine functionalised ion exchange resin

BackgroundCigarette smoking is a well recognized cause of diseases such as lung cancer, chronic obstructive pulmonary disease and cardiovascular disease. Of the more than 5000 identified species in cigarette smoke, at least 150 have toxicological activity. For example, formaldehyde and acetaldehyde have been assigned as Group 1 and Group 2B carcinogens by IARC, and hydrogen cyanide has been identified as a respiratory and cardiovascular toxicant. Active carbon has been shown to be an effective material for the physical adsorption of many of the smoke volatile species. However, physical adsorption of acetaldehyde, formaldehyde and also hydrogen cyanide from smoke is less effective using carbon. Alternative methods for the removal of these species from cigarette smoke are therefore of interest. A macroporous, polystyrene based ion-exchange resin (Diaion®CR20) with surface amine group functionality has been investigated for its ability to react with aldehydes and HCN in an aerosol stream, and thus selectively reduce the yields of these compounds (in particular formaldehyde) in mainstream cigarette smoke.ResultsResin surface chemistry was characterized using vapour sorption, XPS, TOF-SIMS and 15N NMR. Diaion®CR20 was found to have structural characteristics indicating weak physisorption properties, but sufficient surface functionalities to selectively remove aldehydes and HCN from cigarette smoke. Using 60 mg of Diaion®CR20 in a cigarette cavity filter gave reductions in smoke formaldehyde greater than 50% (estimated to be equivalent to >80% of the formaldehyde present in the smoke vapour phase) independent of a range of flow rates. Substantial removal of HCN (>80%) and acetaldehyde (>60%) was also observed. The performance of Diaion®CR20 was found to be consistent over a test period of 6 months. The overall adsorption for the majority of smoke compounds measured appeared to follow a pseudo-first order approximation to second order kinetics.ConclusionsThis study has shown that Diaion®CR20 is a highly selective and efficient adsorbent for formaldehyde, acetaldehyde and HCN in cigarette smoke. The reductions for these compounds were greater than those achieved using an active carbon. The results also demonstrate that chemisorption can be an effective mechanism for the removal of certain vapour phase toxicants from cigarette smoke.

Adsorption of carbon dioxide, sulfur dioxide and water vapour by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of carbon dioxide, sulfur dioxide and water vapour have been determined on a well characterized sample of MCM-41. The shape of the carbon dioxide isotherm (at 195 K) is similar to that given by mesoporous hydroxylated silica, whereas the water isotherm (at 303 K) is of Type V in the IUPAC classification and its shape is therefore indicative of a more hydrophobic behaviour. Isosteric enthalpies of adsorption, calculated from the sulfur dioxide isotherms (at 254 and 273 K), reveal strong energetic heterogeneity in the adsorbent-adsorbate interactions and it appears that the high-energy sites occupy ca. 30% of the total surface area.

Adsorption of carbon tetrachloride and nitrogen by 3.4 nm pore diameter siliceous MCM-41

Adsorption isotherms of nitrogen at 77 K and carbon tetrachloride at temperatures between 273 and 323 K have been determined on the pure silica form of MCM-41 of pore diameter ca. 3.4 nm. The nitrogen isotherm is of a reversible type IV in the IUPAC classification and shows a first-order change in the physisorption mechanism below a relative pressure of 0.4. Carbon tetrachloride isotherms were of type V, the isotherms at 273, 288 and 303 K showing distinctive hysteresis loops, whereas the isotherm at 323 K was completely reversible. Despite the questionable validity of the Kelvin equation when applied to narrow mesopores, good agreement was achieved for pore diameter calculations for all the isotherms measured.

Use of Classical Adsorption Theory to Understand the Dynamic Filtration of Volatile Toxicants in Cigarette Smoke by Active Carbons

The ability of two very different active carbons, a polymer-derived carbon (with ultramicropores and supermicropores, and a large volume of “transport” pores) and a coconut shell-derived carbon (predominantly ultramicroporous), to reduce the levels of volatile toxicants in cigarette smoke has been measured and compared. The polymer-derived carbon was found to be approximately twice as effective in removing the majority of measured smoke vapour-phase toxicants compared to the coconut shell-derived carbon in three different cigarette formats and with two different smoking regimes. Single-component dynamic breakthrough experiments were conducted with benzene, acrylonitrile and 2-butanone at 298 K for beds of each carbon under dry (0% RH) and wet (60% RH) conditions. Longer breakthrough times were found with the polymer-derived carbon, and breakthrough times recorded under wet conditions were found to be up to 20% shorter than those obtained under dry conditions. Correlations between micropore volume, dynamic adsorption volume and filter bed breakthrough time have been demonstrated.

Structure and dynamics of hydrogen sorption in mesoporousMCM-41

Adsorption isotherms taken at temperatures ranging from 20 to 77 K show a large pore volume and surface area of 980 m2 g -1 for the physical adsorption of molecular hydrogen on MCM-41. The adsorbed hydrogen behaves more like a solid than a liquid and isosteric heats of adsorption reveal a heterogenous surface. The evaporation behaviour of the adsorbed hydrogen indicates that the hexagonally packed tubes in MCM-41 may be effectively interconnected into a single void space. Neutron inelastic scattering shows that molecular hydrogen exists in two sites in the pores of MCM-41. We designate as surface states those with a variety of weakly hindered rotational excitations centred at 11.8(2) meV and as bulk states, at high doping of H 2 , those which have narrow rotational excitations at 14.7(3) meV. These excitations are hardly changed from those of the free crystal in energy, energy width or momentum transfer dependence. Each type accounts for ca. 50% of the total pore volume. Strong hydrogen recoil scattering is also observed at momentum transfers above ca. 3 A -1 . Neutron diffraction from the filled sample at 1.9 K shows a single peak at 3.1 A, characteristic of the H 2 –H 2 correlations in bulk hydrogen. This peak is much weaker for the surface adsorbed species. The concentration dependence of this peak also shows a 1:1 division of void space between surface-adsorbed and bulk-like species. In addition we observe a separate peak at 14.38(3) A, the hexagonal (21) of MCM-41 involving the silica framework, whose diffracted intensity changes dramatically with hydrogen doping, in a way inconsistent with a smooth walled, hexagonally packed mesopore. These data, together with previous X-ray diffraction data, provide information on silica density, absorbing surface and free volume projected onto the basal plane. The detail and complexity of the projection were unexpected. The data agree well with a model for which the silica in the walls is loosely interwoven, and occupies only 40% of the wall volume. The 35% silica surrounding the empty 7 A hole as a 12.7 A lining has a projected density of ca. 90% that of the walls. Either highly divided, ‘hairy’ walls or a structure like a highly defective variant of the MCM-48 structure would fit the data. The projection of smooth-walled but highly twisted channels onto a horizontal plane could give a density distribution equivalent to the highly porous silicate walls in straight channels.

The effect of doping transition metal oxides on copper manganese oxides for the catalytic oxidation of CO

Abstract A series of copper manganese oxides doped with transition metal oxides were prepared by co-precipitation using copper acetate and manganese acetate as precursors, ammonium bicarbonate as precipitant, and metal nitrates as dopants. The catalysts were characterized by N2 adsorption-desorption, X-ray powder diffraction, temperature-programmed reduction, and in situ diffuse reflectance infrared Fourier transform spectroscopy. The results showed that doping transition metal oxides into copper manganese oxides can modify the CO adsorption ability of the catalyst and thus affect the catalytic oxidation of CO.

Activated Carbons for the Adsorption of Vapours from Cigarette Smoke

The main requirements for the adsorption of cigarette smoke vapours using active carbons and the methods currently being used to characterise and select carbons for this application are reviewed. Emphasis is placed on the total volume of smoke, the pulsed characteristics and relatively short challenge times, the compounds in the smoke and the environment in which adsorption takes place. Using experimental data, the types of carbons most suitable for cigarette filter application are considered, as are the adsorption kinetics required to meet this challenge. The concept of carbon ageing and the effects of water within the smoke are also reviewed.

Ethyl Acetate Adsorption onto Activated Carbon

The adsorption and desorption of ethyl acetate on a coconut-based activated carbon has been studied using static and flow adsorption methods at different temperatures from 295 K to 325 K. The adsorbed state of ethyl acetate was studied using nitrogen adsorption at 77 K after the pre-adsorption of ethyl acetate. The adsorption isotherms of ethyl acetate were of Type I in the IUPAC classification, suggesting micropore filling by ethyl acetate molecules. The micropore volume of the activated carbon as evaluated from ethyl acetate adsorption using the liquid density of ethyl acetate agreed well with that from nitrogen adsorption. Nitrogen adsorption after pre-adsorption of ethyl acetate indicated considerably packed molecular states of ethyl acetate molecules within the micropores. Thus, ethyl acetate molecules can migrate to the deeper parts of micropores in the case of static adsorption measurements. However, the specific surface area evaluated from ethyl acetate adsorption was nearly 40% smaller than that from nitrogen adsorption under the assumption of an isotropic structure. The assumption of a highly orientated adsorption structure for ethyl acetate molecules on the pore wall led to a good correlation even as far as the surface area was concerned. In addition, 26 kJ/mol of the excess stabilization energy determined from the temperature dependence of the ethyl acetate adsorption isotherm supports the favourable molecular alignment of ethyl acetate molecules which increases the molecule–pore wall interactions.