Technologies for the removal of phenol from fluid streams: a short review of recent developments.

It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Catalytic oxidation of volatile organic compounds (VOCs) – A review

Chromium-, manganese- and cobalt-doped titanium dioxide photocatalysts containing 0.2, 0.5 or 1 at.% of metal-dopant were investigated by UV–VIS, FT-IR, near-IR and electron paramagnetic resonance (EPR) spectroscopic techniques. The presence of the doping ions in the titania structure caused significant absorption shift to the visible region compared to pure TiO 2 powder (P25 Degussa). The EPR spectra of TiO 2 powders containing chromium showed the superposition of three types of individual paramagnetic species (β-, γ- and δ-signals), whose relative EPR intensity is significantly dependent on the dopant concentration, as well as on the photocatalysts preparation and treatment. The EPR spectra of the chromium-doped photocatalysts heated in H 2 /N 2 atmosphere corresponded to the Cr(III) ions occupying vacated cation sites in the rutile or anatase crystal lattice. The characteristic feature of the EPR spectra of the Mn/TiO 2 samples is a sharp six-line Mn(II) component centered on g eff =1.99, flanked by shoulders with a weak feature, which appeared on g eff =2.66 and 4.32. The photocatalytic activity of the various metal-doped TiO 2 samples was tested in aqueous or dimethylsulfoxide (DMSO) suspensions using EPR spin trapping technique with 5,5-dimethyl-1-pyrroline N -oxide (DMPO) spin trap. The ability of the irradiated photocatalysts to generate reactive oxygen species, namely hydroxyl radicals and super-oxide anion radicals, which were trapped as the corresponding DMPO-adducts was investigated.

Investigations of metal-doped titanium dioxide photocatalysts

Catalytic combustion of volatile organic compounds (VOCs: benzene and toluene) was studied over manganese oxide catalysts (Mn3O4, Mn2O3 and MnO2) and over the promoted manganese oxide catalysts with alkaline metal and alkaline earth metal. Their properties and performance were characterized by using the Brunauer Emmett Teller (BET), temperature programmed reduction (TPR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The sequence of catalytic activity was as follows: Mn3O4 > Mn2O3 > MnO2, which was correlated with the oxygen mobility on the catalyst. Each addition of potassium (K), calcium (Ca) and magnesium (Mg) to Mn3O4 catalyst enhanced the catalytic activity of Mn3O4 catalyst. Accordingly, K, Ca and Mg seemed to act as promoters, and the promoting effect might be ascribed to the defect-oxide or a hydroxyl-like group. A mutual inhibitory effect was observed between benzene and toluene in the binary mixture. In addition, the order of catalytic activity with respect to VOC molecules for single compound is benzene > toluene, and the binary mixture showed the opposite order of toluene > benzene.

Catalytic combustion of VOCs over a series of manganese oxide catalysts

The catalytic oxidation of the C3 organic compounds: propane, propene, 1-propanol, 2-propanol, prop-2-en-1-ol (allyl alcohol), propanal, propanone (acetone) and propenal (acrolein) has been investigated over Mn3O4 in the presence of excess oxygen. Total combustion to CO2 occurs at temperatures >623 K (673 K for propane). However, the production of pollutant products (such as propene from propane, CO from propene, acetaldehyde from propanols, propanal and propanone, carboxylic acids) persists even at very high volatile organic compounds (VOC) conversions. On the basis of the reaction products, a tentative reaction pathway for propane and propene combustion is proposed.

Catalytic combustion of C3 hydrocarbons and oxygenates over Mn3O4

Mn2O3–Fe2O3 powders have been prepared by a coprecipitation method. The pure compounds have been characterized as constituted of α-Fe2O3 (haematite) and α-Mn2O3 (bixbyite) while the mixed oxides are constituted of a mixture of haematite- and bixbyite-structure solid solutions. The observed reciprocal solubilities calculated from the XRD patterns approach the thermodynamic ones. α-Mn2O3 is more active than α-Fe2O3 as catalyst for both propane and propene oxidation. However, α-Mn2O3 is less active than Mn3O4 powder. Propene oxidation is in all cases very selective to CO2 while propane oxidation gives rise to significant amounts of propene on α-Fe2O3. Mn2O3–Fe2O3 powders are slightly more active than α-Mn2O3 as combustion catalysts. The selectivities to propene upon propane oxidation decrease with increasing Mn content.

Characterization of manganese and iron oxides as combustion catalysts for propane and propene

Mn-Ti mixed oxides with composition Ti
 1–x
Mn
 x
 
O
 y
 (x=0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1) have been prepared. A Mn-TiO
 2
 monolayer type sample has also been prepared by impregnation, for comparison. Manganese is found to speed up the anatase-to-rutile phase transition, more clearly in the impregnated sample, while titanium tends to slightly hinder the thermodynamically reversible hausmannite-to-bixbyite phase transition upon cooling. The catalytic activity of all samples in propene oxidation decreases by increasing the Ti content. Conversely, the catalytic activity in propane oxidation shows a maximum at intermediate composition.

A study of Mn-Ti oxide powders and their behaviour in propane oxidation catalysis

IR studies on the activation of C–H hydrocarbon bonds on oxidation catalysts

The oxidation of propane over a Mn 3 O 4 catalyst has been investigated with oxygen to propane ratios up to 2:1. High selectivities to the oxy-dehydrogenation product propene have been obtained at low conversions, with yields up to 5% and productivities up to above 1.5 Kg C 3 H 6 /Kg cat  h. Data on the mechanism and on the reaction pathways have been derived from an evaluation of the reaction kinetics and from FT-IR experiments. The main by-reaction with respect to oxy-dehydrogenation is propene over-oxidation to CO 2 . Direct total oxidation of propane to CO 2 can become predominant only in oxygen excess. Isopropoxide species act probably as surface intermediate species in propene synthesis.

Marco Baldi

Papers

Catalytic combustion of C3 hydrocarbons and oxygenates over Mn3O4

Characterization of manganese and iron oxides as combustion catalysts for propane and propene

A study of Mn-Ti oxide powders and their behaviour in propane oxidation catalysis

IR studies on the activation of C–H hydrocarbon bonds on oxidation catalysts

Evaluation of the mechanism of the oxy-dehydrogenation of propane over manganese oxide