Chirally modified platinum generated by adsorption of cinchonidine ether derivatives: towards uncovering the chiral sites

The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employed as chiral modifiers for heterogeneous enantioselective hydrogenations on supported Pt catalysts, has been investigated by using attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory (DFT) electronic structure calculations. The ATR-IR spectroscopic investigation provided detailed insight of the adsorbed modifiers under conditions close to those employed during catalytic processes, and electronic structure calculations were used as a complement to the experiments to uncover the implications of conformational changes in generating the topology of the surface chiral site. The structural investigation of the adsorbed modifiers revealed a relationship between the spatial positions of the ether substituents and the enantiodifferentiation induced by the modified catalyst observed in the hydrogenation of alpha-activated ketones. Experiments and calculations corroborate a model, according to which the addition of a bulky ether group to CD reshapes the chiral sites, thus generating catalytic chiral surfaces with different and, in some cases (e.g. hydrogenation of ketopantolactone), even opposite enantioselective properties to those obtained with CD without altering the absolute configuration of the modifier. The study also confirms that active surface conformations of cinchona modifiers are markedly different from those existing in vacuum and in solution, thus underlying the necessity of investigating the surface-modifier interaction in order to understand enantioselectivity.     

Time-lapse STM studies of diastereomeric cinchona alkaloids on platinum metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.     

Solvent-induced conformational changes of O-phenyl-cinchonidine: a theoretical and VCD spectroscopy study

The conformational analysis of the synthetic chiral modifier O-phenyl-cinchonidine (PhOCD) used in enantioselective hydrogenations over noble metal catalysts has been performed at a PM3 semiempirical level in vacuum. The minimum energy conformations calculated at the DFT level with a medium-size basis set have been compared to those of the parent alkaloid cinchonidine (CD). PhOCD behaves similarly to CD and shows four main conformers, denoted as Closed(1), Closed(2), Open(3), and Open(4). Open(3) is found to be the most stable in vacuum and in CH2Cl2 and CCl4 solvents. A comprehensive normal-mode analysis has been performed for these conformers, and assignment of the infrared spectrum of PhOCD in CCl4 (epsilon = 2.2) has been performed using the calculated spectrum of Open(3), which appears to be the most populated in this solvent. A combined theoretical-experimental VCD spectroscopy approach was used to increase the spectroscopic sensitivity toward changes in the distribution of conformers upon change of solvent polarity. The VCD spectra confirm that Open(3) is by far the most stable conformation in CCl4 (epsilon = 2.2) and indicate that an excess Closed(2) conformer has to be expected in CD2Cl2 (epsilon = 8.9). The possible influence of this conformational behavior is discussed on the basis of available catalytic data and in relation to the enantioselective potential of PhOCD as a chiral modifier on supported metal catalysts.     

Chiral Modification of Platinum by Co‐Adsorbed Cinchonidine and Trifluoroacetic Acid: Origin of Enhanced Stereocontrol in the Hydrogenation of Trifluoroacetophenone

Cinchonidine (CD) adsorbed onto a platinum metal catalyst leads to rate acceleration and induces strong stereocontrol in the asymmetric hydrogenation of trifluoroacetophenone. Addition of catalytic amounts of trifluoroacetic acid (TFA) significantly enhances the enantiomeric excess from 50 to 92 %. The origin of the enantioselectivity bestowed by co‐adsorbed CD and TFA is investigated by using in situ attenuated total reflection infrared spectroscopy and modulation excitation spectroscopy. Molecular interactions between the chiral modifier (CD), acid additive (TFA) and the trifluoro‐activated substrate at the solid–liquid interface are elucidated under conditions relevant to catalytic hydrogenations, that is, on a technical Pt/Al₂O₃ catalyst in the presence of H₂ and solvent. Monitoring of the unmodified and modified surface during the hydrogenation provides an insight into the phenomenon of rate enhancement and the crucial interactions of CD with the ketone, corresponding product alcohol, and TFA. Comparison of the diastereomeric interactions occurring on the modified surface and in the liquid solution shows a striking difference for the chiral preferences of CD. The spectroscopic data, in combination with calculations of molecular structures and energies, sheds light on the reaction mechanism of the heterogeneous asymmetric hydrogenation of trifluoromethyl ketones and the involvement of TFA in the diastereomeric intermediate surface complex: the quinuclidine N atom of the adsorbed CD forms an N?H?O‐type hydrogen‐bonding interaction not only with the trifluoro‐activated ketone but also with the corresponding alcohol and the acid additive. Strong evidence is provided that it is a monodentate acid/base adduct in which the carboxylate of TFA resides at the quinuclidine N‐atom of CD, which imparts a better stereochemical control.     

An in situ attenuated total reflection infrared study of a chiral catalytic solid-liquid interface: cinchonidine adsorption on pt.

An in situ attenuated total reflection study of the chiral solid-liquid interface created by cinchonidine adsorption on a Pt/Al(2)O(3) model catalyst is presented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydrogenation of alpha-functionalized ketones. Cinchonidine adsorbs via the quinoline moiety. The adsorption mode is coverage dependent and several species coexist on the surface. At low concentration (10(-6)M) a predominantly flat adsorption mode prevails. At increasing coverage two different tilted species, alpha-H abstracted and N lone pair bonded cinchonidine, are observed. The latter is only weakly bound and in a fast dynamic equilibrium with dissolved cinchonidine. At high concentration (10(-4)-10(-3) M) all three species coexist on the Pt surface. A slow transition from an adsorbate layer with a high fraction of alpha-H abstracted cinchonidine to one with a high fraction of N lone pair bonded cinchonidine is observed with the cinchonidine concentration being the driving force for the process. The reverse transition in the absence of dissolved cinchonidine is fast. Cinchonidine competes with solvent decomposition products for adsorption sites on the Pt, which may contribute to the observed solvent dependence of the heterogeneous enantioselective hydrogenation of ketones by cinchonidine-modified Pt.     

Surface processes occurring on Rh/alumina during chiral modification by cinchonidine: an ATR-IR spectroscopy study

Cinchona alkaloids are frequently used for chiral modification of supported noble metal catalysts employed in heterogeneous enantioselective hydrogenation. In order to gain molecular insight into the surface processes occurring at the metal/liquid interface, cinchonidine (CD) adsorption on vapor-deposited Rh/Al2O3 films has been studied in the presence of solvent and hydrogen by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The spectrum of CD adsorbed on Rh exhibited two dominant signals at 1593 and 1511 cm(-1), which are characteristic of a surface species having a quinoline ring tilted with respect to the metal. Interestingly, no adsorbed modifier in the flat geometry (quinoline parallel to the metal plane) was observed. During desorption, these signals vanished, and a new prominent signal appeared at 1601 cm(-1) which belongs to a species with the quinoline ring hydrogenated on the heteroaromatic side. Concentration-dependent experiments and the reversibility of the observed phenomenon indicate that CD was readily hydrogenated to 1',2',3',4',10,11-hexahydrocinchonidine (CDH(6)) on Rh. The ATR-IR spectra also reveal that the flat species was indeed immediately hydrogenated when CD was provided from solution, and the only visible adsorbed species was the tilted species, which displaced the hydrogenation product from the metal surface. In the absence of dissolved CD, during desorption, the tilted species was converted to the flat species and rapidly hydrogenated. The hydrogenation product was stable on the metal surface only in the absence of CD. Therefore, the adsorption strength of the different species is as follows: flat > tilted > CDH(6). Evidence for the formation of the flat species and its role as an intermediate to the hydrogenation product is given by an experiment in which CD was adsorbed in the absence of dissolved hydrogen after surface cleaning. The adsorption and hydrogenation of CD on Rh deviate significantly from that observed earlier on Pt and Pd under similar conditions, where the flat species could be observed even in the presence of hydrogen. This difference is attributed to the weaker interaction and lower hydrogenation rate occurring on Pt and Pd.     

1-(1-Naphthyl)ethylamine adsorption on platinum surfaces: on the mechanism of chiral modification in catalysis

The adsorption of 1-(1-naphthyl)ethylamine (NEA) on platinum surfaces has been characterized by reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) both under ultrahigh vacuum and in situ from liquid solutions. The main focus of this study was to identify the mechanism by which single enantiomers of NEA bestow chirality on the platinum surface. Evidence was acquired for both of the prevailing explanations available in the literature for the NEA behavior: formation of supramolecular chiral templates and complexation of individual modifiers with the reactant. Indeed, TPD titrations of NEA-modified Pt(111) using propylene oxide (PO) as a chiral probe point to a relative enhancement in the adsorption of one enantiomer over the other at intermediate NEA coverages, which is the behavior expected from the templating mechanism. However, a difference in adsorption energetics was also observed. Both the TPD and RAIRS data suggest possible interactions between the adsorbed NEA and adjacent PO that differ according to the relative chirality of the two compounds. The NEA uptake from solution displays additional enantioselectivity, in particular when the adsorption of enantiopure compounds is compared with that of racemic mixtures, and also points to possible adsorption changes induced by ethyl pyruvate, a common reactant in chiral hydrogenation processes.     

Mechanistic insights into the proline-directed enantioselective heterogeneous hydrogenation of isophorone

The adsorption rates onto a range of platinum single-crystal surfaces of key species involved in the proline-directed heterogeneous enantioselective hydrogenation of isophorone were investigated by electrochemical means. Specifically, the uptakes of the prochiral reactant (isophorone), the chiral hydrogenation product (3,3,5-trimethylcyclohexanone), and the chiral directing agent ((R)- and (S)-proline) were examined. The effects of R,S chiral kink sites on the adsorption of (R,S)-proline were also studied. The reactant adsorbs approximately 105 times faster than the chiral modifier so that under conditions of competitive adsorption the latter is entirely excluded from the metal surface. Supplementary displacement and reaction rate measurements carried out with practical Pd/carbon catalysts show that under certain reaction conditions isophorone quickly displaces preadsorbed proline from the metal surface. Thus both kinetics and thermodynamics ensure that the chiral modifier can play no role in any surface-mediated process that leads to enantiodifferentiation. These results are fully consistent with the recent proposal1 that the crucial step leading to enantiodifferentiation occurs in the solution phase and not at the metal surface. In addition, it is found that there is no preferred diastereomeric interaction between (R,S)-proline and R,S step kink sites on Pt{643} and Pt{976}, implying that such sites do not play a role in determining the catalytic behavior of supported metal nanoparticles.     

Investigating the mechanism of chiral surface reactions: the interaction of methylacetoacetate with (S)-glutamic Acid modified Ni{111}

The enantioselective hydrogenation of beta ketoesters over Ni-based catalysts is a rare example of a heterogeneously catalyzed chiral reaction. The key step in catalyst preparation is the adsorption from solution of chiral molecules (modifiers). One particularly interesting modifier is (S)-glutamic acid because the dominant enantiomeric product in the catalytic reaction depends upon the modification temperature. We report a reflection absorption infrared spectroscopy (RAIRS) study of the adsorption of methylacetoacetate (the simplest beta ketoester) onto (S)-glutamic acid modified Ni{111} surfaces as functions of the modifier coverage and modification temperature. We show that the sticking probability of methylacetoacetate is close to 0 on saturated (S)-glutamic acid covered surfaces. At lower modifier coverage, methylacetoacetate adsorption can occur. Adsorption of methylacetoacetate onto a Ni{111} surface modified by (S)-glutamic acid at 300 K results in the diketo tautomeric form, with evidence being observed for a 1:1 interaction between zwitterionic (S)-glutamate and methylacetoacetate. In contrast, adsorption of methylacetoacetate onto a Ni{111} surface modified by (S)-glutamic acid at 350 K occurs exclusively in the enol tautomeric form. The implications for the heterogeneous catalytic reaction are discussed.     

Role of guiding groups in cinchona-modified platinum for controlling the sense of enantiodifferentiation in the hydrogenation of ketones

Systematic structural variations of cinchona-type modifiers used in the platinum-catalyzed hydrogenation of ketones give insight into the adsorption mode of the modifier and its interaction with the substrate on the platinum surface under truly in situ conditions. The performance of a new modifier, O-(2-pyridyl)-cinchonidine, is compared to that of O-phenyl-cinchonidine and cinchonidine (CD). In the hydrogenation of ethyl pyruvate, ketopantolactone, and 2-methoxyacetophenone, CD gives the (R)-alcohol in excess. Introduction of the bulky O-phenyl group favors the (S)-enantiomer, whereas upon replacement of the phenyl by a 2-pyridyl group the (R)-alcohol is again the major product. This finding is particularly striking, because the two ether groups have virtually identical van der Waals volumes. A catalytic study including the nonlinear behavior of modifier mixtures, and attenuated total reflection infrared spectroscopy of the solid-liquid interface in the presence of hydrogen, revealed the adsorption mode and strength of the modifiers on Pt. Theoretical calculations of the modifier-substrate interactions offered a feasible explanation for the different role of the bulky ether groups: repulsion by the phenoxy and attraction by the 2-pyridoxy groups. Simulation of the interaction of o-pyridoxy-CD with ketopantolactone on a model Pt surface suggests that formation of two N-H-O-type H-bonds--involving the quinuclidine and pyridine N atoms, and the two keto-carbonyls in the substrate--controls the adsorption of the substrate during hydrogen uptake. This mechanistic study demonstrates the potential of insertion of suitable substituents into CD and their influence on adsorption and stereocontrol on the platinum surface.     

Chiral Polymeric Coordination Polymer:A Potent Asymmetric Catalyst?

Owing to the recent environmental consideration and safety concerns,the uses of homogeneous and heterogeneous asymmetric methods for the synthesis of chiral product such as enantioselective hydrogenation are especially preferable.¹ Up to now, only a few homogeneous and heterogeneous enantioselective hydrogenation catalytic systems have been investigated in some detail. One of these is the enantioselective hydrogenation of activated carbonyl compounds over a-hydroxy acid modified Raney nickel catalyst.² Systematic variation of the modifier may prove as an efficient tool to shed light on the reaction mechanism. It has been proposed that tartaric acid adsorbs as a chelate to form binuclear Ni tartarate surface complex. However, the exact adsorption state of the modifier is unknown exactly and requires further attention. Direct investigation of the adsorption mode of the modifier over Raney nickel is not yet possible. Any indirect evidence for the adsorption mode is therefore of crucial importance for a better understanding of the reaction.³     

Asymmetric hydrogenation on chirally modified Pt: origin of hydrogen in the N-H-O interaction between cinchonidine and ketone

An understanding of the chiral site-substrate interaction is a necessary prerequisite for the rational design and development of efficient heterogeneous asymmetric catalysts. For the enantioselective hydrogenation of α-ketoesters on cinchona-modified platinum, it has earlier been proposed that the crucial interaction is an N-H-O type hydrogen bonding between the quinuclidine N atom of cinchonidine and the α-carbonyl O atom of the substrate. The involved hydrogen atom has been proposed to originate either from protonation (in protic solvent) or from dissociatively adsorbed hydrogen (in aprotic solvent), but experimental evidence for the latter was lacking so far. In this study, in situ attenuated total reflection infrared spectroscopy combined with modulation excitation spectroscopy and phase sensitive detection provides clear evidence that in aprotic media, hydrogen dissociated on Pt is involved in the N-H-O interaction between the chiral modifier, cinchonidine, and the ketone. In the absence of Pt (pure alumina support), no such interaction occurs, indicating the crucial role of dissociated hydrogen in the formation of the diastereomeric transition complex.     

Theoretical and spectroscopic study of the effect of ring substitution on the adsorption of anisole on platinum

The adsorption of anisole, 3,5-dimethylanisole, and 3,5-bis-(trifluoromethyl)-anisole on Pt(111) was studied theoretically and compared to the adsorption of benzene using relativistically corrected density functional theory. A cluster of 31 platinum atoms was used to simulate the surface. The three anisoles were found to be less strongly adsorbed than the parent molecule benzene, 3,5-bis-(trifluoromethyl)-anisole showing weakest adsorption, with an adsorption energy of only one-third that of benzene. The theoretical study was complemented by in situ ATR-IR spectroscopy of the adsorption of the anisole derivatives on a polycrystalline Pt film. The spectroscopic study indicated that the adsorption strength of the anisoles follows the same order as predicted by the calculations. In addition, catalytic hydrogenation tests showed that the propensity to aromatic ring hydrogenation can also be correlated to the mode and strength of adsorption of the anisoles. The degree of saturation followed the same order as the adsorption strength found by the calculations and indicated by spectroscopy. Although 3,5-dimethyl substitution on anisole resulted in only a partial loss of adsorption energy and reactivity toward ring hydrogenation as compared to anisole, the substitution by CF(3) groups led to a large loss of adsorption energy and complete loss of reactivity toward aromatic ring saturation. Along with the study of the substituent effect on the adsorption of aromatic molecules, the correlation between adsorption and propensity to saturation of aromatic substrates could be corroborated.     

Aspects of heterogeneous enantioselective catalysis by metals

Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific reference to four different systems where the phenomena that control enantioselection appear to be very different. In the case of glucose electro-oxidation, it is clear that any intrinsic chirality present at the metal surface plays a vital role. With α-keto hydrogenation, achiral surfaces modified by the adsorption of chiral agents become effective enantioselective catalysts and the formation of extended arrays of chiral species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. Hydrogen bonding also plays a central role in β-ketoester hydrogenation, but here fundamental studies indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. Asymmetric C═C hydrogenation, though relatively little studied, has the potential for major impact in synthetic organic chemistry both on the laboratory scale and in the manufacture of fine chemicals and pharmaceuticals. The structural attributes that determine whether a given chiral ligand is effective have been identified; the ability to form strong covalent bonds with the metal surface while also resisting hydrogenation and displacement by the strongly adsorbing reactant under reaction conditions is an essential necessary condition. Beyond this, ligand rigidity in the vicinity of the chirality center coupled with resistance to SAM formation is a critically important factor whose absence results in racemic chemistry.     

The origin of chemo- and enantioselectivity in the hydrogenation of diketones on platinum

In the Pt-catalyzed hydrogenation of 1,1,1-trifluoro-2,4-diketones, addition of trace amounts of cinchonidine, O-methyl-cinchonidine, or (R,R)-pantoyl-naphthylethylamine induces up to 93% ee and enhances the chemoselectivity up to 100% in the hydrogenation of the activated carbonyl group to an OH function. A combined catalytic, NMR and FTIR spectroscopic, and theoretical study revealed that the two phenomena are coupled, offering the unique possibility for understanding the substrate-modifier-metal interactions. The high chemo- and enantioselectivities are attributed to the formation of an ion pair involving the protonated amine function of the chiral modifier and the enolate form of the substrate. DFT calculations including the simulation of the interaction of a protonated amine with the enolate adsorbed on a Pt 31 cluster revealed that only the C-O bond next to the CF3 group of the substrate is in direct contact with Pt and can be hydrogenated. The present study illustrates the fundamental role played by the metal surface and indicates that also the enol form can be the reactive species in the hydrogenation of the activated ketone on chirally modified Pt.     

In situ studies of butyronitrile adsorption and hydrogenation on Pt/Al2O3 using attenuated total reflection infrared spectroscopy

The adsorption and hydrogenation of butyronitrile (BN) in hexane on a 5% Pt/Al2O3 catalyst has been studied using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. ATR-IR measurements were conducted on thin ( approximately 10 mum) films of catalyst deposited on Ge wave guides. Multivariate analysis involving classical lease-squares (CLS) and partial least-squares (PLS) modeling was used to aid in the interpretation of the spectroscopic data. During the adsorption of BN over a concentration range from 4 to 40 mM in hexane, no clear evidence for adsorbed N-bound end-on species could be detected. However, a feature at approximately 1635-1640 cm-1 indicated the presence of an adsorbed imine species, with the C=N group existing in a tilted configuration involving a strong degree of pi interaction with the surface. This assignment is bolstered by the detection of N-H stretching bands that are consistent with imine vibrations. This imine-type intermediate is very prominent and shows transient behavior in the presence of solution-phase hydrogen, suggesting that, once formed, it can be converted into amine products that adsorb on the catalyst surface. Evidence for amine formation was observed in the form of N-H stretching and NH2 bending vibrations, with assignments confirmed through comparison studies of butylamine adsorption under identical conditions. Comparisons between Pt/Al2O3 and Al2O3 suggest that there may be some adsorption of these amines on the support surface. The mechanistic implications with regard to heterogeneous nitrile hydrogenation on transition metals under mild conditions are briefly discussed in light of these findings.     

A theoretical investigation of the enantioselective hydrogenation mechanism of α-ketoesters

The enantioselective hydrogenation of α-ketoesters to α-hydroxyesters over Pt/Al₂O₃ catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the α-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.     

The effect of alloying on the H-atom adsorption on the (100) surfaces of Pd-Ag,Pd-Pt,Pd-Au,Pt-Ag,and Pt-Au. A theoretical study

The effect of alloying on the adsorption of atomic hydrogen was studied using density functional theory (DFT). In the study the (100) surfaces of Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag, and Pt-Au alloys were considered by means of a cluster model. The structural and energetic properties of the H atom adsorbed on the Pd₄Me (Me = Ag, Pt, Au) and Pt₄Me (Me = Pd, Ag, Au) clusters were calculated and compared with the H-atom adsorption on monometallic clusters. The effect of alloying on the H-atom adsorption is evident for all the investigated bimetallic systems. However, it strongly depends on the second metal atom, Me, is placed in the surface layer or in the subsurface one. In general, the H atom adsorbed in a site containing the second metal exhibits different properties from those characteristic of its adsorption on Pd(100) and Pt(100). Hence, the modified interaction between atomic hydrogen and the alloyed surfaces may increase the selectivity of the catalytic hydrogenation reactions on such surfaces.     

Synthesis,characterization and catalytic performance in enantioselective reactions by mesoporous silica materials functionalized with chiral thiourea-amine ligand

Chiral heterogeneous catalysts have been synthesized by grafting of silyl derivatives of (1R, 2R)- or (1S, 2S)-1,2-diphenylethane-1,2-diamine on SBA-15 mesoporous support. The mesoporous material SBA-15 and so-prepared chiral heterogeneous catalysts were characterized by a combination of different techniques such as X-ray diffractometry (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), and Brunauer–Emmett–Teller (BET) surface area. Results showed that (1R, 2R)- and (1S, 2S)-1,2-diphenylethane-1,2-diamine were successively immobilized on SBA-15 mesoporous support. Chiral heterogeneous catalysts and their homogenous counterparts were tested in enantioselective transfer hydrogenation of aromatic ketones and enantioselective Michael addition of acetylacetone to β-nitroolefin derivatives. The catalysts demonstrated notably high catalytic conversions (up to 99%) with moderate enantiomeric excess (up to 30% ee) for the heterogeneous enantioselective transfer hydrogenation. The catalytic performances for enantioselective Michael reaction showed excellent activities (up to 99%) with poor enantioselectivities. Particularly, the chiral heterogeneous catalysts could be readily recycled for Michael reaction and reused in three consecutive catalytic experiments with no loss of catalytic efficacies.

     

Pt在MgAl2O4载体上的分散程度对催化苯甲醛加氢反应影响

构建催化剂特别是在亚纳米尺度下分散的贵金属催化剂的构效关系是多相催化研究领域中的主要任务之一.我们采用与金属Pt具有强相互作用的MgAl2O4尖晶石作为载体,通过简单浸渍法制备了在纳米、亚纳米和单原子尺度上分散的Pt催化剂.首先利用X射线衍射和原子分辨的球差校正电镜,确定了Pt在MgAl2O4尖晶石载体表面上随负载量增大逐渐形成孤立的和相邻的单原子Pt,然后逐渐形成无定形Pt聚集体和小晶粒;然后利用电感耦合等离子体光谱和CO化学吸附测定了催化剂中Pt的含量和分散度;进一步通过测定CO在Pt表面吸附的红外光谱,区分了载体表面单原子和金属颗粒表面原子的CO吸附特征结构,并据此对不同结构的Pt原子进行了半定量估算.考察了具有不同Pt分散结构的Pt/MgAl2O4催化剂的催化苯甲醛选择性加氢能力,发现以载体表面Pt单原子物种为主的催化剂,可在较宽的温度区间内保持较高的部分加氢产物苯甲醇的选择性(60–150oC,苯甲醇选择性99.4–97.9％,甲苯选择性～0.4％),而以Pt纳米颗粒为主的催化剂上苯甲醇选择性降低显著,同时生成较多深度加氢产物甲苯(60–150oC,苯甲醇选择性99.0–93.1％,甲苯选择性0.7–5.0％).此外,我们测定了各催化剂在不同转化率(～20–90％)时催化剂加氢反应的质量比活性和转化频率(TOF),并在较低苯甲醛转化率(～20％)时,估算了不同结构Pt物种对苯甲醛加氢反应的本征活性,发现Pt纳米颗粒表面原子比MgAl2O4载体表面Pt单原子本征活性更高(4807 h–1 versus 3277 h–1).综上,Pt单原子催化剂具有贵金属原子利用率高,本征活性和加氢选择性高等优点;Pt纳米催化剂表面原子深度加氢能力强,加氢选择性较差,虽本征活性更高,但不足以补偿贵金属原子利用率降低带来的活性损失,Pt质量比活性显著低于单原子催化剂.此外,MgAl2O4尖晶石负载的单原子Pt催化剂也具有良好的催化反应循环稳定性,是一种较为理想的催化苯甲醛选择性加氢制苯甲醇催化剂.