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A basic feature common to all catalytic systems is that the catalytic reaction can be considered as a reaction cycle, in which catalytically active sites are initially consumed and at the end of the cycle are regenerated. There are many different catalytic systems. Of most basic mechanistic features are well understood. Here an attempt will be made to classify several catalytic systems which contains example of research works that were carried out by me and my colleagues and students. The catalytic systems are the oxidation and acid catalysis by heterogeneous catalysts. I classify our works into six classes:

1. Single center catalyst

2. Hydrophobic-hydrophilic catalytic system

3. Bifunctional catalyst

4. Synergetic multi reaction center catalyst

5. Photocatalytic system

6. Chiral catalyst.

The above classification was inspired by classification of catalytic system proposed by Prof. R. van Santen of Technische Universiteit Eindhoven [1]. In this web site, my research works are divided into six classes based on the above classification as described in the following sections.

| SINGLE CENTER CATALYST | HYDROPHOBIC-HYDROPHILIC CATALYTIC SYSTEM |

| BIFUNCTIONAL CATALYST | SYNERGETIC MULTI REACTION CENTER CATALYST |

| PHOTOCATALYTIC SYSTEM | CHIRAL CATALYST |

SINGLE CENTER CATALYST Catalytic reactions that one could define as belonging to single center catalyst, can be considered as local events. A single metal center or a cluster atoms is required for all of the elementary steps to occur. An example of such a catalytic reaction is dehydration and dehydrogenation of cyclohexanol by aluminophosphate molecular sieves [2-6]. The researches include synthesis, characterization and catalytic testing of aluminophosphate molecular sieves. These published papers were based on my PhD work in Department of Chemistry, Universiti Teknologi Malaysia under the supervision of Prof. Halimaton Hamdan. A large part of this works relates to metal-substituted aluminophosphates (MeAPO) molecular sieves. These materials with desired and controllable properties, be adsorptive or catalytic have been successfully synthesized and modified for the specific purposes such as dehydration and dehydrogenation of alcohols reactions. In this research, I substituted Al atom in the framework structure with the divalent metal (Me) atoms (Me = Mn, Mg, Co and Zn) and silicon atom to generate catalytic sites. It is clearly demonstrated that the conversion of cyclohexanol to cyclohexene (as a model reaction) involve the Me–O–P and Si–O–P sites in the framework of AlPO. This result also suggests that MeAPOs are potential catalysts for dehydrogenation of alcohols. It was demonstrated that MnAPSO-5 was the most active catalyst for dehydration and dehydrogenation reactions of alcohols. Based on our understanding on the fundamental factors in the catalytic activity of these materials, results of this research can open the innovation in applied catalysis and play role in industrial catalytic processes. This research was carried out in the period of 1996-1998. This period is an incredibly intense research work. I finally found out what independently doing good science. It was an exhilarating period. I finish my PhD project in two and half years in 1998. Research topic on single center catalyst also carried out in period of 2002-2006 at Universiti Teknologi Malaysia. The topics of researches include metal complexes encapsulated in Al-MCM-41 as catalysts in oxidation reactions [7-10], enhancement of catalytic activity of TS-1 in epoxidation of 1-octene [11], Ti-OMS-2 as catalyst in oxidation of cyclohexene [12], catalysis by zeolite beta [13, 14] and modification of surface of titania by attachment of silica nanoparticle for the enhancement of epoxidation of alkene [15]. The researches were assisted by undergraduate and postgraduate students which were guided together with Prof. Halimaton Hamdan, Dr. Zainab Ramli and Dr. Salasiah Endud.

HYDROPHOBIC-HYDROPHILIC CATALYTIC SYSTEM One important medium effect that has to be singled out is the hydrophobic-hydrophilic phase interplay possible in the liquid phase. Catalyst systems in which such effects play a role we propose to call hydrophobic-hydrophilic catalytic system [16-22]. In this section, a new hydrophobic-hydrophic catalytic system termed as phase-boundary catalyst is introduced. The catalyst has been designed in which the external part of the zeolite is hydrophobic, internally it is usually hydrophilic, notwithstanding to polar nature of some reactants. In this sense, the medium environment in this system is close to that of an enzyme. Click here for further explanation on Phase Boundary Catalysis (PBC). 

The work in this kind of catalytic system is based on my scientific work in the period of 1999-2002 [24-28]. In this period, I worked as Japan Society for Promotion of Science (JSPS) postdoctoral research fellow (1999-2001) and as a Center of Excellent (COE) visiting researcher (2001-2002) in the Prof. Bunsho Ohtani laboratory at Catalysis Research Center, Hokkaido University. I finished my postdoctoral stay which was due to the talented efforts of Dr. Shigeru Ikeda in the Ohtani group. I will always be indebted to Prof. Ohtani for showing me how to do science right. It is a debt that I can never repay, but showing new researchers the joy of chemical research is at least a small effort in this regard not to mention a lot of fun. The research was still continuing after I return to Universiti Teknologi Malaysia in 2002 [21-25].

BIFUNCTIONAL CATALYST Another type catalytic systems can be defined as bifunctional. The prototype chemocatalytic system is TS-1 loaded with sulfated zirconia as bifunctional oxidative and acidic catalyst for transformation of 1-octene to 1,2-octanediol [26-29]. The catalyst concerned contains two types of reactive centers, oxidative and acidic. The titanium act as active site for the transformation 1-octene to 1,2-epoxyoctane and the protonic sites hydrolyze the epoxide. The overall reaction consists of two steps, in which an intermediate formed in one reaction olefin is consumed on the other. In heterogeneous catalysis there is usually no control over the sequence of these steps. The control that exists is basically due to differences in the reactivity of the different sites. I gave the idea of this kind of catalytic system to Mr. Didik Prasetyoko, a PhD student at Universiti Teknologi Malaysia. His PhD work was supervised by me and Dr. Zainab Ramli and Dr. Salasiah Endud. Mr. Didik graduated in 2006.

SYNERGETIC MULTI RECTION CENTER CATALYST In reactions of synergetic multi reaction center catalyst, at least two different reaction centers that communicate are required. An example is heterogeneous catalyst for liquid-gas reaction system with Mars-van Krevelen type mechanism. A prototype reaction is the oxidation of cyclohexene with molecular oxygen, in the presence of hydrophobic niobium oxide/silica [30].

The oxygen that is inserted into the methylene part of the molecules at a niobium oxide active site is generated from molecular oxygen. The oxygen atom reaches the selective oxidation site via transport through the catalyst. The only report on this class of catalytic system proposed by me published in the Proceedings of Annual Fundamental Science Seminar 2003, Universiti Teknologi Malaysia [30]. This is only a preliminary study which shows the possibility of synergetic multi reaction center occurred in this catalytic system. Although all the results mentioned above seem consistent with this kind of catalytic action a detail mechanism is still not known.

PHOTOCATALYTIC SYSTEM One of important systems in heterogeneous catalysis is photocatalytic system. Photocatalysis can be defined as the catalysis processes that are induced by light. The study of photocatalysis is concerned with the interaction of light (in the form of photons) with photocatalysts. The photocatalys are usually semiconductor particles such as TiO₂ and CdS. Photocatalysis over TiO₂ is initiated by the absorption of a photon with energy equal to or greater than the band gap of TiO₂ (3.2 eV), producing electron-hole (e^-/h⁺) pairs. Consequently, following irradiation, the TiO₂ particle can act as either an electron donor or acceptor for molecules in the surrounding media. Recently, SnO₂-TiO₂ coupled semiconductor photocatalyst loaded with polyaniline (PANI), a conducting polymer, was synthesized in our laboratory [31]. We demonstrated that the attachment of PANI on the surface of SnO₂-TiO₂ composite will reduce the electron-hole recombination during the photocatalytic oxidation of 1-octene due to PANI´s electrical conductive properties. This work was just started in 2006 in our laboratory.

CHIRAL CATALYST The control of enantioselectivity by heterogeneous asymmetric catalysis remains a big challenge today. The main drive has been to find new, exciting routes to chiral molecules while achieving high enantiomer selectivity. Here, a new strategy to obtain active catalyst in the enantioselective hydration of epoxyclohexene is proposed [32]. The research strategy is based on the ideas that the enantioselective reactions could be induced by chiral amino acids and the use of heterogeneous catalysis for synthetic purposes will overcome practical separation problems. In order to realize these ideas, chiral amino acid needs to be attached to the hydrophilic part of hydrolyzed octadecyltrichlorosilane (OTS). Amino acids such as L-glutamic acid and L-phenylalanine have been chosen because of their water-soluble properties; hence they can be easily removed by treatment with water. It is expected that the attachment of amino acid would result in a chiral solid catalyst with bimodal hydrophobic-hydrophilic character.  

References

1. R. A. v. Santen, NRSC-Catalysis Newsletter, 2 (2000) 2-3.
2. H. Nur, H. Hamdan, Reaction Kinetics and Catalysis Letters, 66 (1999) 33-38.
3. H. Nur, H. Hamdan, Buletin Kimia, 13 (1998) 31-38.
4. H. Nur, H. Hamdan, Materials Research Bulletin, 36 (2001) 315-322.
5. S. Endud, H. Nur, H. Hamdan, Studies in Surface Science and Catalysis, 117 (1998) 453-459.
6. H. Nur, N. Y. Hau, M. N. M. Muhid, H. Hamdan, Physics Journal of the IPS., A7 (2004) 0218.
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12. F. Hayati, H. Nur, H. Hamdan, Buletin Kimia, 21 (2005) 49-54.
13. W. K. Man, H. Nur, A. R. Yacob, Z. Ramli, Physics Journal of the IPS., A7, (2004) 0211.
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30. H. Nur, Proceeding of Annual Fundamental Science Seminar 2003 (AFSS 2003), 20-21 May 2003, Puteri Pan Pacific Hotel, Johor Bahru, Malaysia. p. 92-95.
31. H. Nur, I. I. Misnon, L. K. Wei, International Journal of Photoenergy, vol. 2007, Article ID 98548, 6 pages, 2007. doi:10.1155/2007/98548
32. H. Nur, L. K. Wei, S. Endud, Reaction Kinetics and Catalysis Letters, 98 (2009) 157-164.