Catalytic Oxidations Using Nanosized Octahedral Molecular Sieves

Chapter 41 Catalytic Oxidations Using Nanosized Octahedral Molecular Sieves 1

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Young-ChanSon ,JiaLiu ,RumaGhosh ,Vinit D. Makwana , and Steven L. Suib Downloaded by UNIV OF TENNESSEE KNOXVILLE on August 15, 2015 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch041

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Department of Chemistry, University of Connecticut, U-3060 55 North Eagleville Road, Storrs, CT 06269-3060 Institute of Materials Science, University of Connecticut, Storrs, CT 06269 Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269 Corresponding author: [email protected]

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Introduction and Background Considerable efforts have been invested to accomplish hydrocarbon oxidation using heterogeneous catalysts with excellent properties [1-4]. Readily available starting chemicals add valuable versatile chemicals are the main targets for the petroleum industry. Oxidations are widely accepted procedures for synthesizing intermediates for the bulk chemical and pharmaceutical industry. However, this reaction is hard to perform and requires extreme conditions. High pressures and high temperatures are usually required. Gas phase reactions are non-selective, and carbon monoxide and carbon dioxide are formed due to complete oxidation. Environmentally friendly reactions of oxidation have received much attention by industry. Cost efficient processes such as solvent free, low energy consumption, and non-pressurized methods are desirable. The development of a catalytic, highly selective, energy efficient, and environmentally friendly process of oxidation is highly desirable. Stoichiometric oxidations of chemical compounds by active manganese oxides have been known for a long time. Stoichiometric oxidation is unattractive and catalytic oxidations will replace traditional stoichiometric methods. The synthetic OMS (Octahedral Molecular Sieves) materials, which occur as cryptomelane in nature, have been reported for alcohol oxidations [5]. Alcohols can be easily converted to aldehydes and ketones. The kinetics and mechanism of alcohol oxidation have also been also reported [6]. Based on these reports, alcohol oxidations can be broadened to hydrocarbon oxidations. OMS © 2005 American Chemical Society

In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Downloaded by UNIV OF TENNESSEE KNOXVILLE on August 15, 2015 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch041

materials can be utilized for the oxidation of hydrocarbons. Various stoichiometric oxidations of hydrocarbons were reported. Potassium permanganate adsorbed on a solid support oxidized alkylbenzene at the benzylic position [7]. These oxidations mostly require stoichiometric oxidants. Strong acids, halogen compounds, and halogenated solvents are usually required.

Material and Methods OMS has been synthesized and applied for a variety of catalytic applications [8]. OMS has a 2 χ 2 tunnel structure and dimensions of 4.6 À χ 4.6 A (Figure 1). The OMS-2 has variable oxidation states of Mn , M n , and M n [9]. The average oxidation state of OMS-2 is 3.8. The mixed valence of manganese is known to be a critical factor, which is responsible for carrying out the oxidation. OMS-2 materials are known to be microporous materials, which act like zeolite-like materials. Methods of synthesizing well-defined OMS-2 materials are reported and characterized [9,10]. OMS synthesis is easy and inexpensive. By inserting divalent cations into OMS, electronic, catalytic, and structural properties can be varied. Nano-sized OMS materials have been synthesized using cross-linking reagents (PVA, glycerol and glucose) [11]. High resolution SEM (scanning electron microscopy) shows that these nanosized OMS materials are nanofibers or nanorods. 2+

3+

4+

c*>

H0 2

Figure 1. Structure ofKOMS-2

Liquid phase oxidations of toluene or α,β unsaturated ketones with or without peroxide have been carried out depending on substrate. When peroxide (mostly TBHP) was not used, oxygen was the oxidant for the reaction. The oxidation of toluene with molecular oxygen in the liquid phase is carried out by radical initiators. Propagation and termination are major factors to determine the reaction rate and product distribution. The presence of radicals has not been confirmed for OMS system. In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Results and Discussion OMS has Lewis and Brônsted acid and base sites which help the oxidation process, depending on reaction conditions. The characteristics of porosity of OMS are also important parameters for these reactions. Toluene and α,β unsaturated ketones were selected as substrates. Toluene was converted to benzyl alcohol, benzaldehyde, and benzoic acid using AIBN. Oxidation of toluene under reflux conditions using nano-sized OMS materials gave 10 - 15 % conversion without any solvent. Selectivity was changed depending on catalyst, reaction temperature, and the amount of catalyst, α,β unsaturated ketones gave 1,4 diketone product using TBHP. Using TBHP as an oxidant, more valuable chemicals were formed. Fluorene was exclusively transformed to 9-fluorenone using OMS and TBHP. a- isophorone was oxidized to 4-ketoisophorone using nano-sized OMS and TBHP in 30 % conversion and 98 % selectivity. Different kinds of OMS catalysts gave various products. The selectivity was excellent for certain conditions. Nanosized OMS gave higher activity. Conventional OMS materials, which are made by the reflux method, provide higher selectivity but less activity.

R—Η

+

R-

+

A0

2ROO-

RO- +

*~

R-

AH

* ROO-

2

2RO

R—Η

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-

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0

ROH

2

+

R-

R: Alkyl or Benzyl A: Radical Initiator Figure 2. General free radical mechanism of hydrocarbon oxidation

Oxidation of aromatic hydrocarbons may be due to the propagation of mixed free radicals as shown in the above example. The mixed valence of manganese species and oxygen also plays an important role (Figure 3). Oxidation with TBHP as well as OMS materials also gave very promising results. OMS catalyst is the key, which gave oxygen-containing product. High selectivity may be due to the porosity of the OMS catalysts.


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o

2

Hydrocarbon

Figure 3. General mechanism of hydrocarbon air oxidation using OMS

This preliminary study gave a fundamental view of the catalytic oxidation using nano-sized OMS materials. The conversions of easily available raw materials to industrially useful chemicals using environmentally friendly nanosized catalysts are very important.

References 1. 2. 3. 4. 5. 6. 7.

Sheldon, R.A., Kochi, J.K. (1981). Metal-Catalyzed Oxidations of Organic Compounds: Academic Press: New York. Shilov, A.E., Shul'pin, G.B. (1997). Chem. Rev. 97, 2879-2932. Yu, J.Q., Corey, E.J. (2003). J. Am. Chem. Soc. 125, 3232-3233. Parvulescu, V., Anastasescu, C., Constantin, C., Su, B.L. (2003). Catal. Today 78, 477-485. Son, Y.C., Makwana, V.D., Howell, A.R. and Suib, S.L. (2001). Angew. Chem. Int. Ed. 40, 4280-4283. Makwana, V.D., Son, Y.C., Howell, A.R. and Suib, S.L. (2002). J. Catal. 210, 46-52. Noureldin, N.A., Zhao, D., Lee, D.G. (1997). J. Org. Chem. 62, 87678772. In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Brock, S.L., Duan, Ν., Tian, Z.R., Giraldo, O., Zhou, H., Suib, S.L. (1998). Chem. Mater. 10, 2619-2628. 9. DeGuzman, R.N., Shen, Y.F., Neth, E.J., Suib, S.L., O'Young, C.L., Levine, S., Newsam, J.M. (1994). Chem. Mater. 6, 815-821. 10. Yin, Y.G., Xu, W.Q., DeGuzman, R., Suib, S.L., O'Young, C.L. (1994).


Inorg. Chem. 33, 4384-4389.

11. Liu, J., Cai, J., Shen, X., Suib, S. L., Aindow, M., (2003). MRS sym. Proc. 755, DD6.24.


Catalytic Oxidations Using Nanosized Octahedral Molecular Sieves

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