Environmental Catalysis

The following sections are included:

• Preface

• Contents

• List of Contributors

An introductory overview of chemistry-related environmental problems is given with a focus on catalytic solutions. A background to the reasons of damaging effects of particular chemicals to the environment is given.

The production of energy from fossil fuels gives rise to the emission of CO₂, NO_x, and SO₂. The amounts of these compounds formed during the production of energy from renewables such as wind, sun and biomass are less than those in case of fossil fuels. Therefore, research and development is carried out for the application of renewables now and in the near future. This chapter focuses on the application of biomass, and more specific on the use of catalysts for the conversion technologies of biomass. Promising technologies, which are in a demonstration phase, arc pyrolysis, gasification and hydrothermal upgrading The products from these types of conversion technologies are: gas, fuel oil or bio-oil transport fuels and chemicals for the food and chemical industry. The production of hydrocarbons is a major objective of most bio-oil upgrading research. Flash and fast pyrolysis are techniques combining high heating rates and short reaction times. Rapid quenching preserves valuable products. Most of the technologies are studied on lab-scale or pilot-scale. For a number of conversion routes catalysts are applied. Char formation and coke deposition are major problems in upgrading of pyrolysis oils Coke deposition deactivates the catalyst and has to be avoided or minimised. Pyrolysis oils are highly oxygenated, viscous, corrosive, relatively unstable and very complex. However, catalytic upgrading may produce a high valuable chemical feedstock. Oxygen in the oxygenated compounds is, therefore, converted to CO and CO₂ rather than H₂O to preserve hydrogen for hydrocarbon reactions This paper discusses the use of catalysts in the field of thermal conversion of biomass.

The principles and present-day embodiments of fuel cells are discussed. Nearly all cells are hydrogen/oxygen ones, where the hydrogen fuel is usually obtained on-site from the reforming of methane or methanol. There exists a tension between the promise of high efficiency in the conversion of chemical into electrical energy and of very low emissions of noxious compounds, and the enormous difficulty of manufacturing the fuel cells cost-effectively. After three decennia of widespread effort to adapt the fuel cell to terrestrial applications, it is still too early to say whether their large-scale introduction will prove to be viable. However, a very considerable effort is presently being undertaken to make it happen.

The growing use of cars has stimulated concern about the impact on ambient air quality. The introduction of exhaust catalysts has reduced considerably the emissions per gasoline car but the growing number of cars, especially in cities, has largely offset this. Continued reduction of emissions is therefore pursued with concomitant demands on both hardware (engine and exhaust catalyst) and fuel (gasoline and diesel). The latter demand involves lower sulphur and aromatics levels in fuels. In this paper we discuss a selection of catalytic routes to cope with these reductions. Following developments in hydrotreating we discuss light alkanes and alkenes conversions for the production of (branched) alkanes and ethers as gasoline components Finally, synthetic fuels are dealt with focussing on innovations in synthesis gas generation and the Fischer Tropsch technology for conversion of H₂/CO into alkanes for middle distillates.

The oxidative coupling of methane to form ethane and ethylene provides a direct method for the production of valuable chemicals from a simple, abundant hydrocarbon. The reaction is an example of heterogeneous-homogeneous catalysis in that methyl radicals, formed on an active surface, couple in the gas phase to give the products. Yields of ethylene obtained in single-pass flow reactors are rather small (∼ 10%); however, by using a recycle reactor with continuous removal of ethylene, yields in excess of 70% may be attained. Nevertheless, the economics for producing ethylene by this process currently are not favourable Cross coupling reactions involving methane and another molecule are also possible. One such example is the reaction of methane and toluene to form ethylbenzene and styrene.

Concern for the diminishing oil reserves has turned attention to the utilisation of natural gas as an alternative. However, the remote location of the gas fields requires on-site conversion of the natural gas to easily transportable chemicals to be commercially attractive. Current industrial option for this is the indirect routes via synthesis gas generation. The chemistry involved and the technologies used for the different options in the generation of synthesis gas, i.e., steam reforming, carbon dioxide reforming and partial oxidation are discussed.

Bio- and chemocatalysis are important tools, but not the only tools in the fine chemical toolbox to develop sustainable processes on an industrial scale. Catalysis may play an important role in improving multistep syntheses of products more complicated in molecular architecture. Additionally, fine chemical technology is an essential element in a sustainable development of next-century society, enabling responsiveness to an ever increasing demand for highly tuned application products. An improvement factor of nearly a factor of 20 in clean and green technology will be required to realise economical growth by accommodating the ecological targets. The present position of the fine chemical industry will not be adequate on the long run to succeed. Sustainable technology development will be required to prepare this industry for the future. Examples in catalysis are discussed demonstrating that a promising start has been made, but an excellent performance in fine chemical technology development is prerequisite to meet the sustainability targets.

It is argued that the construction of reactors for catalytic oxidation is determined by the desired temperature level. When temperature below about 800°C are desired, the thermal conductance of the catalyst bed is highly important. At temperatures above about 1000°C, the oxidation can be performed either completely catalytically or the homogeneous gas phase reaction can be ignited by the catalytic reaction. The development of oxidation reactors for gas turbines, where the requirements are very severe, is reviewed as well as that for radiant heaters With gas turbines a commercial application has been realised. It is argued that catalytic oxidation is the procedure of choice to deal with volatile organic compounds and badly smelling or poisonous compounds present in gas flows. The fuel economy is important with this application, where reverse flow reactors are applied. Water vapour has an adverse effect on the activity of most oxidation catalysts. Since a dry feed leads to a strongly varying water concentration within the catalyst bed, reliable measurements of the kinetics of catalytic oxidations call for utilisation of wet feeds.

Photocatalysis is based on the double aptitude of the photocatalyst (essentially titania) to simultaneously adsorb both reactants and to absorb efficient photons. The basic fundamental principles are described as well as the influence of the main parameters governing the kinetics (mass of catalyst, wavelength, initial concentration, temperature and radiant flux). Besides various types of photocatalytic reactions (selective mild oxidation of organics, dehydrogenation of alcohols, D₂-alkane isotopic exchange), UV-irradiated titania becomes a total oxidation catalyst once in water because of the photogeneration of OH° radicals by neutralisation of OH− surface groups by positive photo-holes. A large variety of organics could be totally degraded and mineralised into CO₂ and harmless inorganic anions. Any attempt of improving titania's photoactivity by noble metal deposition or ion-doping was derimental. In parallel, heavy toxic metal ions (Hg²⁺, Ag⁺, and noble metals) can be removed from water by photodeposition on titania. Several water detoxification photocatalytic devices have already been commercialised. Solar platforms are working with large scale pilot photoreactors, in which are degraded pollutants with quantum yields comparable to those determined in the laboratory with artificial light.

Technologies for the removal of nitrate from drinking and waste water will be required further in the near future. The catalytic nitrate reduction process offers the sustainable nitrate reduction to nitrogen. The development of active and selective bimetallic nitrate and monometallic nitrite reducing catalysts is described. The influence of different reaction conditions on activity and selectivity is also discussed. The use of formic acid instead of hydrogen as reducing agent offers a possibility to circumvent the negative impact of the pH gradient on activity and selectivity.

In this article a number of problems concerning the environment are addressed. The enhanced greenhouse effect, which is believed to be responsible for global warming, is caused by the accumulation in the lower atmosphere of mainly carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons, emitted as a result of human activity. The latter two are also the main contributors to ozone depletion in the stratosphere. Reducing emissions of these gases is the main topic of this article. Solutions for the CO₂ problem can be found in better energy efficiency, capture and storage of CO₂, chemical conversion into useful products, and the use of non-fossil fuel alternatives such as biomass. Recovery and conversion of stored chlorofluorocarbons into less harmful refrigerants is a viable option for tackling both the greenhouse and the ozone depletion problem. An integrated approach based on a kinetic/mechanistic study combined with process development is presented. N₂O is present in off-gases from fluid-bed combustors. Reducing emissions from this source is problematic since this often leads to enhanced NO_x emissions, while the concentration of N₂O in these off-gases is low. N₂O in the off-gas from adipic acid plants, on the other hand, is a relatively easy target because it is very much concentrated. This article briefly discusses the mechanistic aspects of N₂O destruction, together with some process opportunities. Other issues touched briefly are the formation of ozone at ground level, leading to smog, and its prevention, and the reduction of the ozone concentration in aircraft cabins through a catalytic process.

The subject of this article is the emission of undesired compounds from mobile sources, and their reduction, with the emphasis on emissions from internal combustion engines in passenger cars. The main pollutants resulting from combustion in these otto and diesel engines are hydrocarbons, carbon monoxide, nitric oxides, and particulates. Most countries have developed legislation over the past decades to control these emissions. In order to meet current and future standards many strategies can be employed, varying from engine modifications to catalytic treatment of exhaust gas. Emissions of otto engines were the first to become regulated, and nowadays very effective monolithic three-way catalysts are fitted to most new otto-engine cars. These three-way catalysts simultaneously convert hydrocarbons, CO, and NO_x to harmless compounds. Current developments with respect to reducing emissions from otto engines are aimed at further improving the catalysts, and at the implementation of so-called lean burn engines, which inherently produce less emissions. Diesel engines were long considered clean, but since some time emission standards also apply to the exhaust from these engines. The main concerns are NO_x and paniculate matter, consisting of primarily soot, soluble organics, and sulphuric acid. Activities to reduce these emissions started with the development of paniculate traps that collect the paniculate matter, which can subsequently be combusted. Although research is still going on in this direction, a faster but less efficient way to success has been the development of flow-through oxidation catalysts that convert the soluble organics in the gas phase. The removal of NO_x from diesel exhaust gases is still subject of much research efforts. A different approach to tackling the pollution problems is the use of alternative fuels, such as alcohols or electricity. The latter seems promising in combination with engines or fuel cells for the on-board generation of electricity, leading to so-called hybrid-electricity vehicles. Large-scale applications of these alternatives can not be expected in the near future, although promising results have already been obtained.

In this chapter the removal of NO and is NH₃ emphasised. In the troposphere NO contributes to smog formation and acid deposition. Stratospheric ozone depletion is catalysed by NO in the stratosphere.

The application of catalysts for the selective catalytic reduction (SCR) of NO with NH₃ is highlighted. The technology for catalytic NO removal from exhausts of stationary sources is now in a mature state, whereas the removal of ammonia is still in its infancy. Stationary sources are industrial boilers, power plants, waste and biomass incinerators, engines and gas turbines. Issues such as the catalyst types, the behaviour of these catalysts in power plants, the mechanism of NO and ammonia over the catalysts are discussed. Ammonia removal from biomass gasification is also reviewed in this chapter.

Millions of tons of various thermoplastic and thermoset polymers are yearly discarded from post-use consumer and industrial applications. Re-use of the polymer wastes by means of back to chemical feedstocks recycling may contribute to a large extent to the conservation of our eco-system and to a sustainable development of the continuing growth of the use of polymers in our society. Many promising processes for recycling of polymer wastes to gasolines, kerosines, dieseloils, monomers and other chemicals of high economic value have been developed in Europe, Japan and the USA and are reviewed here. It is demonstrated that catalysis plays a vital role in the technical and economic feasibility of these recycling processes by reduction of the energy consumption and improvement of the yields and quality of the recovered chemical products.

The present paper discusses the deactivation, regeneration and recycling of sulfidic Ni/Co-Mo/W hydroprocessing catalysts. These catalysts are commercially used in different refinery applications over a broad range of conditions. That is why their life cycle is very complex and includes many different aspects. As these catalysts contain (toxic) transition metals, there are numerous environmental issues involved in their handling during their entire life cycle (production, presulfiding, use, regeneration, recycling, transport, loading).

The following sections are included:

• Index