Green Chemical Syntheses and Processes - American Chemical Society

Chapter 18

Oxygenation of Hydrocarbons Using Nanostructured TiO as a Photocatalyst: A Green Alternative 2

Downloaded by COLUMBIA UNIV on September 20, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0767.ch018

Endalkachew Sahle-Demessie and Michael A. Gonzalez ORD, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH 45268

High-value organic compounds have been synthesized successfully from linear and cyclic hydrocarbons by photocatalytic oxidation using the semiconductor material, titanium dioxide (TiO ). Various hydrocarbons were partially oxygenated in both aqueous and gaseous phase reactors using ultraviolet light and titanium dioxide under mild conditions. Gas phase reaction conditions eliminate the need for a separation step involving with liquid solvents and minimizes the adsorption of products to the catalyst. The conversions and selectivities obtained for partial oxidation of hydrocarbons have been comparable to those achieved with conventional methods. Initial life-cycle analysis showed that the technology has the potential to reduce water contaminants and eliminate the use of toxic catalysts. Light-induced catalysis opens up possibilities of the use of oxygen in partial oxidation reactions now being conducted with far more expensive polluting oxidants. The high selectivity of the mild photochemical routes are especially attractive for the manufacture of fine chemicals. This chapter describes the chemistry of TiO catalyzed reactions for environmentally beneficial chemistry based on the work done in our laboratory and others. 2

2

The chemical industry is a significant component of the domestic economy, generating over $250 billion in sales and maintaining a trade surplus of more than $15 billion for each of the last 5 years. The industry is also a major source of industrial waste and is the dominant source of hazardous waste in the United States. The costs of handling, treating and disposing of wastes generated annually in the United States has climbed to 2.2% of the gross domestic product, and continues to rise (/). As a consequence, the chemical industry spends billions of dollars annually on managing pollutants and has hundreds of U.S. government work. Published 2000 American Chemical Society In Green Chemical Syntheses and Processes; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

217


218 billions of dollars invested in pollution control equipment. Costs to clean up existing toxic sites are estimated to approach $750 billion over a 30 year period. Cleaning up our water and air and preventing future deterioration of the environment is estimated to cost even more. The average environmental cost per firm in various industries is broken down as follows: high-tech firms, $2 million (6.1% of revenues); utilities, $340 million (6.1 % of revenues); steel and metals, $50 million (2.9% of revenues); and oil companies, $430 million (1.9% of revenues). Given that the average pre-tax profits of the 500 largest U.S. manufacturing companies were 7.7 percent of sales in 1996, these figures are staggering. These costs will only grow when one considers "superfund legislation", which provides government support and mandate to clean up existing sites. The chemical manufacturing industry generates more than 1.5 billion tons of hazardous waste and 9 billion tons of non-hazardous waste annually. Roughly half of the releases and transfers of chemicals reported through the Toxic Release Inventory and 80-90% of hazardous waste generation reported through the Resource Conservation and Recovery Act (RCRA) are due to chemical manufacturing (2,3,4). Organic chemicals constitute the largest source of the toxic releases (2). Many of these releases can be minimized by improving conventional house keeping methods. Other strategies for pollution prevention include better management of material and energy, more efficient process control, optimized process conditions, and recycle and reuse of waste and byproducts (5). However, cleaner production methods can be achieved by adopting "green synthesis" methods. Cleaner chemical processes, involving both evolutionary and revolutionary technologies, could generate less waste and emission of toxic substances, use materials that are less toxic, and require less energy than their predecessors. These new technologies, which can emerge from advances in basic research, will help the chemical industry address the dual goals of global competitiveness and environmental stewardship. Partial Oxidation Reactions. Oxidation is used in industry for producing aliphatic and aromatic alcohols, aldehydes, ketones and acids. Generally, oxidation involves splitting of C-C or C-H bonds and formation of C-0 bonds. For example, the partial oxidation of hydrocarbons by molecular oxygen, to form oxygenates that are used as building blocks in the manufacturing of plastics and synthetic fibers, is an important process in the chemical industry. Oxidation of alkanes is especially important both industrially and in organic synthesis. Among the products, aldehydes and ketones are useful synthetic intermediates both industrially and in organic synthesis. Oxidation reactions are usually catalyzed and carried out in liquid or gas phase. Stoichiometric oxidation is widely used and large quantities of by-products are formed. Replacement of stoichiometric oxidation processes with new catalytic, low or no salt technologies could avoid the creation of such byproducts. The current processes are energy intensive, have low conversion efficiencies and generate environmentally hazardous waste and by-products. In addition, the desired oxygenated product is subject to further oxidation, which must be prevented or minimized. In particular, autooxidation of small alkanes, alkenes, or aromatics is inherently unselective, whether conducted in the gas or liquid phase or catalyzed by transition metals. The major reason why selectivities are low is that the desired products

In Green Chemical Syntheses and Processes; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

219 (such as aldehydes or alcohols) are more easily oxidized by 0 than the parent hydrocarbon. Most oxidations are highly exothermic and may generate high localized temperatures such that the catalyst surface is degraded. Careful control of partial oxidation reactions to prevent further oxidation of desired products is difficult. Overoxidation can be minimized only by keeping conversions low, a serious disadvantagefroma chemical processing standpoint. Therefore, it is a major challenge tofinda reaction pathway that affords the primary product with high selectivity and high conversion of the hydrocarbon. In addition, several commonly used catalysts for oxidation employ toxic heavy metals (like chromium, vanadium) or strong acids (like H S0 , HN0 ) increasing the environmental hazards of these reactions. Pollution is inevitable in loading, recovering, and regeneration of these catalysts. Thus, a cleaner alternative is needed. We describe here our investigations of an alternative method for the activation and oxidation of hydrocarbons. The substrate is reacted using air under ambient conditions in a photochemical reactor that uses ultraviolet (preferably solar) light and a specially prepared semiconductor catalyst. The use of photocatalysis has many advantages because of its environmentally friendly nature, its high oxygen atom efficiency, and its versatility. This research incorporates the principles of Green Chemistry and Engineering (6) by allowing the production of oxygenates in a selective manner and producing less by-products and pollutants than comparable conventional techniques. 2


2

4

3

Photocatalysis Principles. A photocatalyst is a substance that is activated by a photon. Activation of the semiconductor photocatalyst for reaction is achieved through the absorption of a photon of ultra-band energy, which results in the promotion of electrons from the filled level (i.e. the valence band edge) to the vacant level (i.e. the conduction band edge), generating an electron-hole pair (Figure 1). This electron-hole pair is the primary photoproduct formed upon photoexitation of a semiconductor and has a finite lifetime to allow a separate site for the oxidative and reductive half-reactions to occur. Semiconductor photocatalysis can be more attractive than the more conventional chemical oxidation methods because semiconductors are inexpensive, nontoxic, and capable of repeated use without loss of photoactivity. Therefore, the development of efficient photocatalysis for selective oxidation of alkanes with T i 0 is very appealing. 2

Mechanism of Photocatalyzed Oxidations. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst. Upon irradiation, a semiconductor generates electron/hole pairs withfreeelectrons produced in the nearly empty conduction band (CB) and positive holes remaining in the valency band (VB) (7). The holes migrate to the semiconductor surface and react with organic compounds, acting as strong oxidizing agents. Figure 1 shows the typical reaction scheme of a η-type semiconductor such as Ti0 . Depending on the ambient conditions, 2


220 the lifetime of an electron/hole separation process can befroma few nano-seconds to a few hours (8). Absorption of light energy greater than or equal to the band gap (E ) of the semiconductor results in a shift of electrons from the valence band(VB) to the conduction band (CB) and the creation of holes (h+) in the valence band: bg


SC + Λν-> VVB

(1)

+ e'cB

These charge carriers recombine, radiatively and/or nonradiatively, in competition with rapid diffusion to the surface where the resulting nonequilibrium distribution of electrons and holes gives rise to reduction or oxidation processes of adsorbed species, surface groups, and the semiconductor component. If the irradiated semiconductor is in contact with a suitable redox system, a redox reaction may take place. The process occurring on the irradiated semiconductor is dependent on pH, temperature, the concentration of reactants, and a redox potential that lies between E and E . From the thermodynamic point of view, possible oxidative processes are those characterized by potentials lower than the valence band energy. A semiconductor is chosen based on the potentials of its valence and conduction band edges as measured against the reference electrode. These positions govern the oxidizability and reducibility of the half-reactions to be conducted. The band positions of several commonly used semiconductor photocatalysts are listed in Table I (9). Chemical transformation can occur, only if the process competes kinetically with electron-hole recombination The recombination of electron/hole pairs can take place either between energy bands or on the surface. As a result the photocatalytic efficiency is reduced. To impede the recombination process, conducting materials such as noble metals can be incorporated into the semiconductor to facilitate the electron transfer and prolong the lifetime of the electron/hole separation process (70). Although, there has been considerable efforts in using photocatalysis for complete oxidation of organic compounds in air and water streams, incomplete or partial oxidation has been reported (77, 72). The valence band positions of most of these semiconductors lie higher than the oxidation potentials of many organic compounds, indicating that oxidation of many functional groups can be facilitated. The redox potential of hydrogen evolution (H H ) and oxygen evolution (0 /H 0) are -0.3 eV and leV, respectively (75). Many conjugated groups or functional groups bearing non-bonded electrons should participate in photoinduced single-electron oxidation (14). The band gap energy for T i 0 varies from 3.0 to 3.2 eV depending on the environment in which it is found (75). Titanium dioxide is the most commonly used semiconductor since it has many advantages: it is inexpensive, widely available, insoluble in water and many other solvents, is stable over a wide range of pH and has fewer problems with photocorrosivity (16,17). Photocatalytic oxidation processes involving the use of semiconductors have gained increased attention for innovative treatment of hazardous wastes as well as photoreduction of toxic compounds and purification and disinfection of drinking water. c b

vb

+/

2

2

2

2



221

Figure L Illustration of major processes occurring on semiconductor particle following electronic excitation

Table I. Band Position* of Semiconductor Photocatalyst (9) Semiconductor Ti0 Sn0 ZnO W0 CdS CdSe GaAs GaP SiC 2

2

3

Valence Band +3.1 +4.1 +3.0 +3.0 +2.1 +1.6 +1.0 +2.2 +1.6

Conduction Band -0.1 +0.3 -0.2 +0.2 -0.4 -0.1 -0.4 -1.0 -1.4

* Band position in water at pH 1 There is also great interest at present in photocatalytic degradation of organic molecules in air and water streams. The focus of the research described here, however, is the photocatalyzed partial and selective oxidation of a hydrocarbon to alcohol and ketone, derived by kinetic control of the reaction in gas phase. Semiconductor Catalysis for Chemical Synthesis. In recent years, the use of semiconductor photocatalysts have been investigated in a variety of applications. Anatase phase titania has applicability as a photocatalyst for several problems of environmental interest (18,19), including use as catalyst for sulfur removal (20), for toxic



222 metal capture (20,21) and as an additive in cosmetics due to its efficient sunscreen properties. Theoretically, photocatalysis function as a pool of electron holes, providing efficient charge separation at the interface between the semiconductor and a gas or liquid. Two critical features that a photocatalyst must posses in order to be effective are sufficiently long excited-state lifetime to allow for interaction with the organic substrate of interest and sufficient oxidizing and reducing power. We found that aqueous phase photooxidaton of hydrocarbons (such as cyclohexane) using a batch reactor for two hours resulted in 3 to 5 % initial conversion to their respective alcohols and aldehydes (23). Similar work was done on the photoinduced conversion of acetic acid to methane and carbon dioxide as an alternative to Kolbe reaction (24). When water was used as a solvent, deep oxidation, or the complete oxidative mineralization of the organic substrate, usually predominated due to the generation of highly oxidizing hydroxyl radicals. Because aqueous phase reactions encounter separation problems, it is better to carry out organic chemical synthesis using photocatalysis either in an inert solvent, such as acetonitrile, in the neat organic solvent for liquid substrates or in gas phase. We performed gas phase photocatalytic oxidation reactions of hydrocarbons by flowing a known mixture of heated humid air with organic vapor through an annular reactor (25). We used nanostructured T i 0 coated using flame aerosol technique at optimized process conditions, which enabled us to achieve higher conversions and selectivities than obtained by coating prepared by other methods. 2

Photoxidations. The photocatalytic oxidation of many organic molecules, including saturated hydrocarbons, by optically-excited semiconductor oxides is thermodynamically allowed in the presence of oxygen at room temperature. UV light assisted reactions have been promising for oxidation and epoxidation of small olefins (26, 27). Selectivities for ketones and aldehydes were higher than those obtained by other oxidation means have been reported (9, 28). Some of the substrates investigated, the products formed, and the conversion and selectivities are shown in Table II. Distinction between photocatalytic oxidations and dehydrogenations, both of which give oxidized organic products, have been made (28). A photocatalytic oxidation involves a primary electron transfer to the photogenerated hole, producing, at least transiently, a single electron oxidized intermediate. Incontrast, a photocatalytic dehydrogenation, may involve loss of hydrogen atomsfromthe molecule without forming intermediate radicals. Some interesting examples of oxidative cleavage of photoreactions sensitized by semiconductors include the photoxidation of cyclohexane and toluene. Photoxidation of toluene in water resulted in the formation of cresols (11, 30). In acetonitrile toluene oxidation resulted in benzaldehyde and then benzoic acid. In our laboratory, gas-phase photocatalyzed oxidation of toluene resulted primarily in benzaldehyde and secondarily in benzyl alcohol (Table II). The high selectivity of toluene-to-benzaldehyde oxidation by 0 without overoxidation or side reaction is remarkable. The selectivity and the efficiency with which the photoreactor operates were influenced by the oxygen concentration, the illumination, the properties of the photocatalytic coating, and the 2


223 Table II. Results of photoxidation of hydrocarbons.

Conversion Reactants

perpass%

Products

CHpH CHO


à à H

6-12

96-98% benzaldehyde

2-6

65-84% cyclohexanone

V \ A /C\/y\

2-4

M

0

Λα

Selectivity

80% pentanones

rw

ΛΤ

m

Ο

ÇH3

OVOVOH

HO

CH3OH

HCHO

ΛΛ

OH

HO

/V

\ /

CH3CHO

OW

2-5

86-89% methylcyclo hexanones

ÇH3

Green Chemical Syntheses and Processes - American Chemical Society

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