Atomically precised Aun(SR)m Nanoclusters for aerobic oxidation of alkenes

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Atomically well-defined Au-25(SR)(17/18) nanoclusters deposited on silica supports for the aerobic epoxidation of trans-stilbene

MATERIAUX+CLA:ADM:ATUTwo atomically well-defined thiolate gold nanoclusters, Au-25(SC2H4Ph)(18) and Au-25(SPhNH2)(17), having the same gold core but different thiolate ligands have been deposited on silica supports and the obtained catalysts have been tested in the aerobic oxidation of trans-stilbene. By properly selecting the support to maximize cluster/surface interactions, it was possible to prepare materials with small nanoparticles and very narrow particle size distributions. Au-25(SPhNH2)(17)@SBA-1 5 catalyst showed an activity and a trans-stilbene epoxide selectivity comparable to the best heterogeneous catalysts. Elimination of the thiolate molecules by calcination did not significantly modify the particle size but decreased the activity, pointing out the critical role of the ligands in the reaction. (C) 2014 Elsevier B.V. All rights reserved

Atomically Well-Defined Thiolate Gold Nanoclusters for Heterogeneous Catalysis

National @ MATERIAUX+CLA:ADM:ATUInternational audienceGold nanoparticles exhibit catalytic activity in many chemical processes.1 However, fundamental investigations on the structure-catalytic activity relationships still lag behind, partly due to the polydispersity issue of gold nanoparticles. Indeed polydisperse particles obscure the interesting size-dependent catalytic activity of nanogold and preclude an in-depth understanding of the origin of this size dependence. Recently, atomically monodisperse, thiolate-capped Au nanoclusters (denoted as Aun(SR)m) have been successfully isolated and their catalytic properties have been demonstrated.2 These well-defined gold clusters hold promises as a new generation of catalysts and, more importantly, permit in-depth studies on the subtle correlation of structure and catalytic activity, since these nanoclusters are atomically well defined and some of their structures have been solved by single crystal X-ray diffraction.3 To investigate the influence of the size, the type of ligands at the surface and also the support effect, different nanoclusters have been synthesized. Here we present the synthesis of new clusters made of 4-aminothiophenol (HSPhNH2). Among them Au10(SPhNH2)9/10 and Au25(SPhNH2)17 have been isolated and fully characterized by mass spectrometry, X-ray diffraction and XPS.4 Moreover these clusters exhibit characteristic absorption behaviors related to their molecular state. Catalytic activity for oxidation of alkene derivatives of these colloidal or supported clusters were investigated and compared to commonly used Aun(SCH2CH2Ph)m nanoclusters or bared nanoparticles. (1) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem.-Int. Edit. 2006, 45, 7896. (2) Jin, R. C.; Zhu, Y.; Qian, H. Chem. Eur. J. 2011, 17, 6584. (3) Zhu, Y.; Qian, H. F.; Zhu, M. Z.; Jin, R. C. Adv. Mater. 2010, 22, 1915. (4) Lavenn, C.; Albrieux, Bergeret, G.; F.; Chiriac, R., Delichčre P.; Tuel, A.; Demessence, A. Nanoscale , 2012, 4, 7334

Oxidation Reactions Catalyzed by Atomically Well-Defined Gold Clusters

National @ +CLA:ADM:ATUInternational audienceGold nanoparticles have demonstrated interesting catalytic activity in many chemical processes.[1] However, fundamental investigations on the structure-catalytic activity relationships still lag behind, partly due to the polydispesity issues of gold nanoparticles. Sub-2 nm gold clusters have attracted more attention since atomically well-defined and monodisperse thiolate capped ones have been successfully isolated.[2] These clusters, with Aun(SR)m formula, are formed by a precise number n of gold atoms comprised between ~10 and few hundred and stabilized by m thiolate molecules. What renders these clusters unique is the discrete electronic structure of the gold core due to quantum size effects, leading to original properties intermediate between those of molecular and bulk states. In addition, the total structure determination by single crystal X-ray crystallography permits to know exactly the position of the atoms forming the gold core and the organic layer (Fig 1).[3] More importantly, their catalytic performance have been demonstrated, thus, gold nanoclusters hold promises as a new generation of catalysts which permit in-depth studies on the subtle correlation of structure and catalytic activity.[4] In this communication we investigate the influence of the number of gold atoms, the type of ligand at the surface and also the support effect on different aerobic oxidation reactions. 2 Results and discussion In a first part, we will present the synthesis of clusters made of different thiolate ligands, such as 2- phenylethanethiol (HSC2H4Ph) or functionalized thiophenol (HSPh-X with X = H, NH2 or CO2H). Two synthetic approaches are used to get pure atomically well-defined thiolate gold clusters, which are by exchange reaction from Aun(SC2H4Ph)m or by direct synthesis starting with HAuCl4. To obtain the exact Aun(SR)m formula of the clusters, electrospray mass spectrometry is a technique of choice to prevent their fragmentation. In addition all the synthesized clusters are characterized by small and large angle X-ray scattering, X-ray photoelectron, infra-red and UVvisible spectroscopies, thermogravimetric and elemental analyses. Among those clusters, an unprecedented Au25(SPhNH2)17 cluster has been isolated and the different characterizations in solid and liquid phases show that the 18th position is occupied by an amine or DMF molecules to close its electronic shell.[5] This well-defined cluster is really appealing for further experiments, because it offers possibilities of post-functionalization or deposition on substrates by coupling with the amino groups. Once synthesized, the clusters have been deposited by impregnation on silica supports, such as the mesoporous SBA-15 and the hydrophobic ~16 nm SiO2 particles. These choices have been made to enhance the dispersability of the so-prepared catalysts in the solvents (methylcyclohexane and toluene) and be able to compare them directly with more commonly used supported gold catalysts. We studied the catalytic activity of the gold nanoclusters in two different aerobic oxidation catalytic tests: the epoxidation of t-stilbene and the oxidation of benzyl acohol to form benzaldehyde. The first observation for trans-stilbene epoxidation is that the bare Au25(SCH2CH2Ph)18 and Au25(SPhNH2)17 are precipitating, resulting in a lack of accessibility on the gold catalyst. This is why these nanoclusters are deposited on hydrophobic silica (Amorphous silica from Degusa, AerosilŽ R972). This way, the dispersion is enhanced and the high selectivity of 97 % obtained is, to our knowledge, the best reported up to now. For comparison the catalytic test carried out in the same conditions with the reference gold nanoparticles from the World Gold Council (3.5 ą 0.9 nm) deposited on TiO2 gives a selectivity of 75 %. This observation shows two major points (i) the hybrid clusters are chemically stable for catalytic oxidation reaction, (ii) the presence of the ligands around the clusters does not prevent the catalytic reaction and may even be responsible for such high selectivity. This conclusion is supported by the comparison between the activity of two Au25 clusters surrounded by different thiolate molecules: Au25(SCH2CH2Ph)18 and Au25(SPhNH2)17 . Both show a selectivity for the formation of the epoxide > 97 %, but the activity is much higher for clusters surrounded by phenylethanethiolate that aminothiophenol. This may be due to the affinity of the gold nanoclusters with the support or to the flexibility of SCH2CH2Ph, which would lead to a better access of the substrates to the gold atoms. At the opposite when the clusters are deposited inside hydrophilic mesoporous silica (SBA-15) for oxidation of benzyl alcohol, there is no activity. The ligands have to be burned at 300°C to observe the formation of benzaldehyde. This observation shows a third point (iii) the catalytic activity of gold nanoclusters is dependent of the presence of the ligands as for the type of reaction. Fig. 2. Au25(SPhNH2)17 nanoclusters used as catalyst for aerobic t-stilbene epoxidation. 3 Conclusions We investigated the catalytic properties in aerobic oxidations of sub-2 nm gold compounds. By comparing the results for the different gold clusters and supported nanoparticules, we pointed out that while still being functionalized; gold nanoclusters are active in catalytic oxidation of trans-stilbene. For the oxidation of benzyl acohol, we showed that the ligand presence is inhibiting the catalytic activity. This ligand effect present with functionalized gold clusters makes possible to tune the overall catalytic selectivity and specificity to have chimio specific reaction on multi-modal sites. We showed here that thiolated gold nanoclusters compounds hold great interests and promises as a new class of catalysts thanks to their well-defined structure and to the influence of the ligands in the catalysis. References 1 A. S. K. Hashmi; G. J. Hutchings, Angew. Chem.-Int. Edit. 45 (2006) 7896. 2 R. Jin; Y. Zhu, H. Qian, Chem. Eur. J. 17 (2011) 6584; T. Tsukuda, Bull. Chem. Soc. Jpn 85 (2012) 151. 3 M. Zhu; W. T. Eckenhoff; T. Pintauer; R. Jin, J. Phys. Chem. C 112 (2008) 14221. 4 Y. Zhu, H. Qian, R. Jin, J. Mater. Chem. 21 (2011) 6793; J. Oliver-Meseguer; J. R. Cabrero-Antonino; I. Dominguez; A. Leyva-Perez; A. Corma Science 338 (2012) 1452. 5 C. Lavenn, F. Albrieux, G. Bergeret, R. Chiriac, A. Tuel, A. Demessence, Nanoscal

Au-25(SPh-pNH(2))(17) nanoclusters deposited on SBA-15 as catalysts for aerobic benzyl alcohol oxidation

SSCI-VIDE+CDFA:ING+CLA:ADM:ATUInternational audienceAu-25(SPh-PNH2)(17) clusters deposited on mesoporous silica SBA-15 (mean particle size 0.9 +/- 0.2 nm) have been used as catalysts in the oxidation of benzyl alcohol. In the absence of radicals, fresh thiolate clusters are not active even in the presence of caesium carbonate. The activity strongly increases with calcination temperature and reaches a maximum when all ligands have been removed to form gold nanoparticles of ca. 1.8 nm. In the presence of radical initiators, functionalized clusters deposited on SBA-15 are very active, but the benzaldehyde formed is rapidly consumed by over-oxidation. The activity is attributed to the presence of -S-Au-I-S-bridges at the surface of the clusters. Upon calcination, the activity decreases due to reduction of Au-I species and the absence of radicals in the reaction medium. However, approximately 45% of alcohol is converted into benzaldehyde, with a selectivity of ca. 65% after 30 h. (C) 2014 Elsevier Inc. All rights reserved

Oxidation Reactions Catalyzed by Atomically Well-Defined Gold Clusters

International @ MATERIAUX+CLA:ATU:ADMInternational audienceSub-2 nm gold clusters have attracted more attention since atomically well-defined and monodisperse thiolate capped ones have been successfully isolated. These clusters, with Aun(SR)m formula, are formed by a precise number n of gold atoms comprised between ~10 and few hundred and stabilized by m thiolate molecules. In this communication we investigate the influence of gold atoms number, the type of ligand at the surface and also the support effect on different aerobic oxidation reactions. More importantly, we pointed out that while still being functionalized; gold nanoclusters are active and highly selective in epoxidation of trans-stilbene

Atomically well-defined gold clusters: Oxidation reactions catalized by ultra-small gold core

International @ MATERIAUX+CLA:ATU:ADMInternational audienceRecently, atomically well defined thiolate-capped gold nanoclusters (denoted Aun(SR)m with (10,10) < (n,m) < (333,79)) have been successfully isolated and few structures have been solved. Thus, these monodispersed functionalized clusters, with precise atomic positions and gold core less than 2 nm, hold promises as a new generation of catalysts. More importantly, these nanoclusters permit in-depth studies on the subtle correlation of the structure, core diameter, thiolate ligands effect and electronic state of gold nanoparticles with their catalytic activity. New clusters made of 4-aminothiophenol (HSPhNH2) have been isolated, such as Au25(SPhNH2)17, and fully characterized by mass spectrometry, X-ray diffraction and XPS.3 Moreover these clusters exhibit absorption bands related to their molecular state. Those clusters have then be used to investigate in aerobic oxidations reactions and compared to the Au25(SC2H4Ph)18 cluster, having the same Au core but different thiolate ligand. They have been deposited on silica supports (hydrophobic 16 nm-SiO2 nanoparticles and SBA-15 mesoporous silica) and by properly selecting the support to maximize clusters/support interactions, it was possible to prepare materials with ultra-small gold particles (<2nm, fig. 1) and very narrow size distribution. We investigated the catalytic properties in aerobic oxidations of these sub-2 nm gold catalysts in (i) the aerobic epoxidation of trans-stilbene and (ii), the oxidative dehydrogenation of benzyl alcohol. By comparing the results for the different supported gold clusters, we pointed out that while still being functionalized, gold nanoclusters exhibit a good catalytic activity for the trans-stilbene epoxidation and that this catalytic activity is linked to the presence of the thiolate moiety.4 For the benzyl acohol oxidation, we showed that the presence of ligands is inhibiting the catalytic activity. This ligand effect observed with functionalized gold clusters makes possible to tune the overall catalytic selectivity and specificity to have chimio specific reactions on multi-modal sites. We showed here that thiolated gold nanoclusters compounds hold great interests and promises as a new class of catalysts thanks to their well-defined structure and to the ligands effect over the catalysis. [1] Qian et al., Acc. Chem. Res., 2012, 45, 1470 [2] R. Jin et al., Chem. Eur. J., 2011, 17, 6584. [3] C. Lavenn et al., Nanoscale, 2012, 4, 7334. [4] C. Lavenn et al., Cat. Tod., Accepte

Synthesis, characterization and optical properties of an amino-functionalized gold thiolate cluster: Au-10(SPh-pNH(2))(10)

MATERIAUX+CLA:ATU:ADMResearch interest in ultra small gold thiolate clusters has been rising in recent years for the challenges they offer to bring together properties of nanoscience and well-defined materials from molecular chemistry. Here, a new atomically well-defined Au-10 gold nanocluster surrounded by ten 4-aminothiophenolate ligands is reported. Its synthesis followed the similar conditions reported for the elaboration of Au-144(SR)(60), but because the reactivity of thiophenol ligands is different from alkanethiol derivates, smaller Au-10 clusters were formed. Different techniques, such as ESI-MS, elemental analysis, XRD, TGA, XPS and UV-vis-NIR experiments, have been carried out to determine the Au-10(SPh-pNH(2))(10) formula. Photo-emission experiment has been done and reveals that the Au-10 clusters are weakly luminescent as opposed to the amino-based ultra-small gold clusters. This observation points out that the emission of gold thiolate clusters is highly dependent on both the structure of the gold core and the type of the ligands at the surface. In addition, ultra-small amino-functionalized clusters offer the opportunity for extended work on self-assembling networks or deposition on substrates for nanotechnologies or catalytic applications. (C) 2013 Elsevier Inc. All rights reserved

Atomically Well Defined Thiolate Gold Nanoclusters for Heterogeneous Catalysis

International @ MATERIAUX+CLA:ATU:ADMInternational audienceGold nanoparticles, less than 5 nm, exhibit a catalytic activity in many chemical processes. However polydisperse particles obscure the interesting size-dependent catalytic activity of nanogold. Recently, atomically well defined thiolate-capped Au nanoclusters (denoted as Aun(SR)m) have been successfully isolated and their catalytic properties have been demonstrated. These monodispersed functionalized clusters, with gold core between less than 1 nm and more than 2 nm, hold promises as a new generation of catalysts. More importantly, these nanoclusters permit in-depth studies on the subtle correlation of structure and catalytic activity, since they are well defined and their crystallographic structures start to be solved. To investigate the influence of the size, the type of ligands at the surface and also the support effect, different nanoclusters have been synthesized. New clusters made of 4-aminothiophenol (HSPhNH2) have been synthesized, such as Au25(SPhNH2)17, and fully characterized by mass spectrometry, X-ray diffraction and XPS. Moreover these clusters exhibit absorption bands related to their molecular state. Catalytic activity for oxidation of alkene and alcohol derivatives of these colloidal or supported clusters were investigated and compared to the commonly used Aun(SCH2CH2Ph)m nanoclusters. At the opposite of the bare gold nanoparticles, the presence of the ligands around the clusters leads to a much better selectivity of the product

Oxidation Reactions Catalyzed by Atomically Well-Defined Gold Clusters

National @ +CLA:ADM:ATUInternational audienceGold nanoparticles have demonstrated interesting catalytic activity in many chemical processes.[1] However, fundamental investigations on the structure-catalytic activity relationships still lag behind, partly due to the polydispesity issues of gold nanoparticles. Sub-2 nm gold clusters have attracted more attention since atomically well-defined and monodisperse thiolate capped ones have been successfully isolated.[2] These clusters, with Aun(SR)m formula, are formed by a precise number n of gold atoms comprised between ~10 and few hundred and stabilized by m thiolate molecules. What renders these clusters unique is the discrete electronic structure of the gold core due to quantum size effects, leading to original properties intermediate between those of molecular and bulk states. In addition, the total structure determination by single crystal X-ray crystallography permits to know exactly the position of the atoms forming the gold core and the organic layer (Fig 1).[3] More importantly, their catalytic performance have been demonstrated, thus, gold nanoclusters hold promises as a new generation of catalysts which permit in-depth studies on the subtle correlation of structure and catalytic activity.[4] In this communication we investigate the influence of the number of gold atoms, the type of ligand at the surface and also the support effect on different aerobic oxidation reactions. 2 Results and discussion In a first part, we will present the synthesis of clusters made of different thiolate ligands, such as 2- phenylethanethiol (HSC2H4Ph) or functionalized thiophenol (HSPh-X with X = H, NH2 or CO2H). Two synthetic approaches are used to get pure atomically well-defined thiolate gold clusters, which are by exchange reaction from Aun(SC2H4Ph)m or by direct synthesis starting with HAuCl4. To obtain the exact Aun(SR)m formula of the clusters, electrospray mass spectrometry is a technique of choice to prevent their fragmentation. In addition all the synthesized clusters are characterized by small and large angle X-ray scattering, X-ray photoelectron, infra-red and UVvisible spectroscopies, thermogravimetric and elemental analyses. Among those clusters, an unprecedented Au25(SPhNH2)17 cluster has been isolated and the different characterizations in solid and liquid phases show that the 18th position is occupied by an amine or DMF molecules to close its electronic shell.[5] This well-defined cluster is really appealing for further experiments, because it offers possibilities of post-functionalization or deposition on substrates by coupling with the amino groups. Once synthesized, the clusters have been deposited by impregnation on silica supports, such as the mesoporous SBA-15 and the hydrophobic ~16 nm SiO2 particles. These choices have been made to enhance the dispersability of the so-prepared catalysts in the solvents (methylcyclohexane and toluene) and be able to compare them directly with more commonly used supported gold catalysts. We studied the catalytic activity of the gold nanoclusters in two different aerobic oxidation catalytic tests: the epoxidation of t-stilbene and the oxidation of benzyl acohol to form benzaldehyde. The first observation for trans-stilbene epoxidation is that the bare Au25(SCH2CH2Ph)18 and Au25(SPhNH2)17 are precipitating, resulting in a lack of accessibility on the gold catalyst. This is why these nanoclusters are deposited on hydrophobic silica (Amorphous silica from Degusa, AerosilŽ R972). This way, the dispersion is enhanced and the high selectivity of 97 % obtained is, to our knowledge, the best reported up to now. For comparison the catalytic test carried out in the same conditions with the reference gold nanoparticles from the World Gold Council (3.5 ą 0.9 nm) deposited on TiO2 gives a selectivity of 75 %. This observation shows two major points (i) the hybrid clusters are chemically stable for catalytic oxidation reaction, (ii) the presence of the ligands around the clusters does not prevent the catalytic reaction and may even be responsible for such high selectivity. This conclusion is supported by the comparison between the activity of two Au25 clusters surrounded by different thiolate molecules: Au25(SCH2CH2Ph)18 and Au25(SPhNH2)17 . Both show a selectivity for the formation of the epoxide > 97 %, but the activity is much higher for clusters surrounded by phenylethanethiolate that aminothiophenol. This may be due to the affinity of the gold nanoclusters with the support or to the flexibility of SCH2CH2Ph, which would lead to a better access of the substrates to the gold atoms. At the opposite when the clusters are deposited inside hydrophilic mesoporous silica (SBA-15) for oxidation of benzyl alcohol, there is no activity. The ligands have to be burned at 300°C to observe the formation of benzaldehyde. This observation shows a third point (iii) the catalytic activity of gold nanoclusters is dependent of the presence of the ligands as for the type of reaction. Fig. 2. Au25(SPhNH2)17 nanoclusters used as catalyst for aerobic t-stilbene epoxidation. 3 Conclusions We investigated the catalytic properties in aerobic oxidations of sub-2 nm gold compounds. By comparing the results for the different gold clusters and supported nanoparticules, we pointed out that while still being functionalized; gold nanoclusters are active in catalytic oxidation of trans-stilbene. For the oxidation of benzyl acohol, we showed that the ligand presence is inhibiting the catalytic activity. This ligand effect present with functionalized gold clusters makes possible to tune the overall catalytic selectivity and specificity to have chimio specific reaction on multi-modal sites. We showed here that thiolated gold nanoclusters compounds hold great interests and promises as a new class of catalysts thanks to their well-defined structure and to the influence of the ligands in the catalysis. References 1 A. S. K. Hashmi; G. J. Hutchings, Angew. Chem.-Int. Edit. 45 (2006) 7896. 2 R. Jin; Y. Zhu, H. Qian, Chem. Eur. J. 17 (2011) 6584; T. Tsukuda, Bull. Chem. Soc. Jpn 85 (2012) 151. 3 M. Zhu; W. T. Eckenhoff; T. Pintauer; R. Jin, J. Phys. Chem. C 112 (2008) 14221. 4 Y. Zhu, H. Qian, R. Jin, J. Mater. Chem. 21 (2011) 6793; J. Oliver-Meseguer; J. R. Cabrero-Antonino; I. Dominguez; A. Leyva-Perez; A. Corma Science 338 (2012) 1452. 5 C. Lavenn, F. Albrieux, G. Bergeret, R. Chiriac, A. Tuel, A. Demessence, Nanoscal