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Besides preparation of chiral ligand modified Pt and Pd nanoclusters another aim of the current thesis is in determination the geometrical models of adsorption of chiral ligands (Fig. 3-2) on the surface of Pt and Pd nanoclusters via DRIFTS (Diffuse Reflectance Infra-red Fourier Transform Spectroscopy) and molecular modeling. Since the ability of the adsorbed cinchonidine to induce enantioselectivity e.g. in hydrogenation of α-ketoesters is well known in heterogeneous enantioselective catalysis we placed high emphasis on the examination of cinchonidine modified Pt and Pd nanoclusters in ethyl pyruvate hydrogenation. The work is also focused on obtaining active, highly enantioselective stable catalyst for the performing hydrogenation under mild conditions that is very important from application point of view. Furthermore we investigated catalytic and enantioselective activity of all synthesized samples in typical Orito reaction. 27
Chapter 4 Cinchonidine modified Pt colloidal nanoparticles: characterization and catalytic properties 4.1 Introduction Cinchonidine modified transition metal surfaces (primarily Pt) represent one approach towards asymmetric heterogeneous catalysis. Since the first report by Orito [45, 46], numerous experimental and theoretical investigations into the cinchona on Pt and Pd systems have revealed a great deal of insight into the system and its applicability. As was mentioned above, adsorption geometry of the cinchonidine molecule on metal surface has a significant role in the enantioselectivity. It is known from the work of Ferri [76], Zaera [77] and Wells [74], that cinchonidine can adsorb on the surface of macroscopic Pt (plane, single crystal with sizes are much bigger than the size of the cinchonidine molecule) in three different orientations (Fig. 1-19): “π-bonded”, “α-H abstracted” and “N lone pair bonded”. It is also considers, that only cinchonidine in the flat “π-bonded” orientation is able to induce enantioselective hydrogenation. However adsorption mode of cinchonidine on the surface of nanosized Pt crystals has never been investigated and it attracts special attention, for instance, because the maximum reported enantiomeric excess (98 %, in the hydrogenation of methyl pyruvate) was obtained on small (1.2 nm) Pt colloids. Thus the beginning of this chapter is dedicated to the determination of adsorption modes of cinchonidine on Pt nanoclusters. The initial transient period (ITP) is an interesting phenomenon which consists of an increase of enantiomeric excess in the beginning of e.g. ethyl pyruvate hydrogenation. The ITP effect was observed in the examination of enantioselectivity of cinchonidine modified Pt nanoclusters and discussed in the second part of the current chapter. The initial transient period (ITP) for the Pt/Al 2 O 3 system has been reported and investigated by numerous groups and has been the subject of intense discussions [164, 165]. ITP was observed by Baiker and co-workers [166] and was attributed to an impurity effect (from destructive adsorption of EP and alcoholic solvents on Pt) which could be eliminated by pre-activation with hydrogen. Blackmond and co-workers proposed the “reaction-driven equilibration" explanation [167] and demonstrated the significant influence of mass transport (gas-liquid diffusion of hydrogen) limitation on the enantioselectivity [168]. Recently, it was shown that ITP is not only the effect of impurities (lactates in ethyl pyruvate) but could also be a result of competitive adsorption of reactant, modifier and solvent [169]. From the work of Hutchings et al. [170], we know, that the ITP effect was found with cinchonidine premodified conventional Pt/Al 2 O 3 in ethanol and dichloromethane solution and explanation through “removal of adsorbed oxygen species from the surface” was proposed. Here, we investigate the behavior of enantiomeric excess (ee) during hydrogenation of ethyl pyruvate with conventional Pt/Al 2 O 3 and with cinchonidine Pt nanoclusters prepared by two different methods, an aqueous (first reported by Bönnemann [114]) and a novel organometallic method through decomposition of Pt 2 (DBA) 3 (DBA is bisdibenzylidene acetone) complex. The nature of the increase in ee at initial reaction time, further decreases and differences in the behavior of conventional Pt/Al 2 O 3 and 28
Page 1 and 2: Preparation and Characterization of
Page 3 and 4: 5.4 Summary 66 Chapter 6 Cinchonidi
Page 5 and 6: Abstract Studies of the interaction
Page 7 and 8: many compounds are biologically act
Page 9 and 10: Fig. 1-7. Hexahelicene [3]. 7. Chir
Page 11 and 12: R H COOH C OH CH 3 H COO - Bricine-
Page 13 and 14: Catalyst activity, expressed in the
Page 15 and 16: In fact, there are only two heterog
Page 17 and 18: similar way as relative abundance o
Page 19 and 20: surface concentration of cinchonidi
Page 21 and 22: site on Pt surface formed by a adso
Page 23 and 24: Chapter 2 Introduction to the nanos
Page 25 and 26: Triangular prism 5 4 Comments: A -
Page 27 and 28: precursors and building-up. This me
Page 29 and 30: 2.3 Preparation of nanoclusters on
Page 31: Paris Marseille Ljubljana Synphos Q
Page 35 and 36: dihydrocinchonidine (355 mg or 1.2
Page 37 and 38: Table 4-1. Comparison between 5 % P
Page 39 and 40: widening can be used in estimation
Page 41 and 42: 1 Absorbance 0.1 2 1600 1400 1200 1
Page 43 and 44: the focus of these prior experiment
Page 45 and 46: Fig. 4-11. Tilted adsorbed cinchoni
Page 47 and 48: sci., QL. comp. 768 (str) 756 (str)
Page 49 and 50: Conversion, % 100 80 60 40 ITP time
Page 51 and 52: 6A c 10 19.5 B, 20 61 0 3 0.18 0.51
Page 53 and 54: pressure, exposing time, cinchonidi
Page 55 and 56: Fig. 4-16. Linear relationship betw
Page 57 and 58: 1 , 1209 cm -1 and 1382 cm -1 remai
Page 59 and 60: Our results are in line with previo
Page 61 and 62: eaction. The stability of the catal
Page 63 and 64: Table 5-1. Ee and reaction rate ind
Page 65 and 66: Enantiomeric excess, % 85 80 75 A2
Page 67 and 68: 35 30 Before After Percent of parti
Page 69 and 70: catalyst activation seems to indica
Page 71 and 72: Table 5-3. Assignment of some vibra
Page 73 and 74: 6.2 Experimental Materials Cinchoni
Page 75 and 76: Fig. 6-3. TEM image of cinchonidine
Page 77 and 78: From the results of EP and EB hydro
Page 79 and 80: enhancement. However the exact fact
Page 81 and 82: Further, this family of man-made li
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Fig. 7-2. TEM of quiphos modified P
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of quiphos adsorbed on Pt and Pd an
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Fig. 7-8 Flat adsorbed quiphos via
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comp., H 11,12,13,14 2 C b., 2 P 11
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Chapter 8 “Quiphos-spider” modi
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flat crystal surface due to the hig
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Chapter 9 Diphos and diop modified
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Table 9-1. Average crystal size of
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Fig. 9-4. Conformation of diphos mo
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ings. Due to these reasons we can n
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Fig. 9-9. Proposed orientation of a
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The model of adsorption of diop on
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The calculation of energy of formin
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For the preparation of synphos modi
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1 2 20 40 60 80 2Θ Fig. 10-3. XRD
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The DRIFT spectra of synphos (Fig.
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peaks which can be used for analysi
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References [1] G.E. Tranter, J. Che
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[64] A. Szabo, N. Kunzle, T. Mallat
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[128] J.D. Aiken III, R.G. Finke, J
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[189] K. Umezawa, T. Ito, M. Asada,
Page 125:
I declare that the current Ph.D. th
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