Gold-catalyzed aerobic oxidation of dibenzylamine: Homogeneous or heterogeneous catalysis?

Au(OAc)₃ is applied as an effective catalyst of the selective oxidation of dibenzylamine to dibenzylimine using molecular oxygen as the only oxidant. When Au(OAc)₃ was preadsorbed onto CeO₂, the supported catalyst was more active than any homogeneous or heterogeneous catalyst known for this reaction. Although, some fascinating color changes in the early stage of the reaction indicated the formation of an amine complex, conventional filtration experiments proved the heterogeneity of the system. The fate of the active gold component was studied by in situ X-ray absorption spectroscopy (XANES) using a specially designed cell. These investigations revealed that in the early stage of the reaction Au(OAc)₃ is dissolved and subsequently reduced by the amine and the in situ formed gold nanoparticles are the real active species of the reaction. Formation of gold nanoparticles during dibenzylamine oxidation was proved independently by transmission electron microscopy. Our findings lead to a simple synthetic procedure using a commercially available gold salt, which upon interaction with the amine forms highly active and selective gold nanoparticles.     

金负载量对低温水煤气变换Au/α-Fe2O3催化剂结构和性能的影响

采用共沉淀法制备了低温水煤气变换Au/α-Fe2O3催化剂。通过正交实验优化催化剂的还原活化条件，考察了金负载量对催化剂性能的影响。采用BET、XRD、UV-VIS、XRF、H2-TPR和O2-TPO等表征手段对催化剂的结构进行分析，并与其催化性能进行关联。结果表明，（1）采用10%-H2/N2还原气将催化剂在150 ℃原位还原9 h，其催化活性最高；（2）金的最佳负载量为8.00%，此时在催化剂制备过程中金的流失量较少，金粒子较小，也有利于抑制催化剂在反应过程中烧结；（3）TPR-TPO结果表明，金的负载量为8.00%时，Au/α-Fe2O3催化剂具有较易被还原、不易被氧化的性质，从而显示出最高催化活性。（4）Au/α-Fe2O3催化剂中的金以单质金（Au0）形式存在；其高活性与Au0-Fe3O4间的协同作用有关。     

Three-component Au—Chitosan—SiO<Subscript>2</Subscript> systems as heterogeneous catalysts for intramolecular cyclization of 2-(2-phenylethynyl)aniline

A series of three-component heterogeneous catalysts Au—Ch—SiO₂ with Ch for chitosan was prepared. The size of gold nanoparticles immobilized on the chitosan matrix depends on the method of sample preparation. The particles with the size <3 nm are formed when the preliminary prepared homogeneous Au—Ch complex is supported on SiO₂. The catalysts were tested in the intramolecular cyclization reaction of 2-(2-phenylethynyl)aniline to for 2-phenylindole. The maximum conversion of 2-(2-phenylethynyl)aniline (25% within 35 h) was obtained on the Au(0.4%)—Ch(2.9%)/SiO₂ catalyst.     

Formation of nonclassical carbonyls of Au3+ in zeolite NaY: characterization by infrared spectroscopy

Adsorption of CO on gold supported in zeolite NaY at 85 K led to the formation of (i) various carbonyls and isocarbonyls typical of the zeolite and (ii) carbonyls formed at cationic gold sites (observed in the 2186-2171 cm(-1) region). Analysis of the behavior of the bands allows their assignment to carbonyls of Au(3+) ions. At temperatures higher than 220 K, CO adsorption led to the formation of a new type of Au(3+)-CO species (2207 cm(-1)). Once formed, these complexes could be transformed into the dicarbonyls Au(3+)(CO)(2) when the sample was cooled to 85 K in the presence of CO. The results are explained by migration of Au(3+) ions to more accessible positions within the zeolite at increasing temperatures. When a CO molecule is already adsorbed, it stabilizes the Au(3+) ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species. These results are the first evidence of Au(3+)(CO)(2) complexes.     

Caffeine gold complex supported on magnetic nanoparticles as a green and high turnover frequency catalyst for room temperature A3 coupling reaction in water

A new heterogeneous catalyst derived from gold (III) and supported on caffeine‐coated magnetic nanoparticles, Fe₃O₄@Caff‐Au, has been prepared and characterized using different techniques. This magnetic gold composite shows high catalytic activity in A³ coupling reactions of terminal alkynes, aldehydes and secondary amines. Using this green catalyst, propargylamines are obtained in high turnover frequency in short reaction times using water as solvent at room temperature. This stable and ready accessible catalyst can be easily recycled magnetically for at least nine consecutive runs without significant loss of activity and with slight aggregation of Au species.     

Gold‐Catalyzed Heterogeneous Aerobic Dehydrogenative Amination of α,β‐Unsaturated Aldehydes to Enaminals

Although enaminals (β‐enaminals) are very important compounds and have been utilized as useful synthons for various important compounds, they have been synthesized through non‐green and/or limited procedures until now. Herein, we have successfully developed a green synthetic procedure using a heterogeneous catalyst. In the presence of gold nanoparticles supported on manganese‐oxide‐based octahedral molecular sieves OMS‐2 (Au/OMS‐2), dehydrogenative amination of α,β‐unsaturated aldehydes with amines proceeded efficiently, with the corresponding enaminals isolated in moderate to high yields (50–97 %). The catalysis was truly heterogeneous, and Au/OMS‐2 could be reused. Furthermore, the formal Wacker‐type oxidation of α,β‐unsaturated aldehydes to enaminones has been realized.     

Highly Efficient Dehydrogenative Coupling of Hydrosilanes with Amines or Amides Using Supported Gold Nanoparticles

Hydroxyapatite‐supported gold nanoparticles (Au/HAP) can act as a highly active and reusable catalyst for the coupling of hydrosilanes with amines under mild conditions. Various silylamines can be selectively obtained from diverse combinations of equimolar amounts of hydrosilanes with amines including less reactive bulky hydrosilanes. This study also highlights the applicability of Au/HAP to the selective synthesis of silylamides through the coupling of hydrosilanes with amides, demonstrating the first example of an efficient heterogeneous catalyst. Moreover, Au/HAP shows high reusability and applicability for gram‐scale synthesis.     

Quick XAFS studies on the Y-type zeolite supported Au catalysts for CO-O2 reaction

Au/zeolite catalysts prepared with a deposition-precipitation method were characterized with quick XAFS (QXAFS) in combination with IR. The data were correlated with the catalytic performance in the CO-O(2) reaction conducted at 273 K. On the basis of the XANES analysis of Au loaded on H-Y, the deposited Au(2)O(3) was observed at the initial stage. The transformation of Au(2)O(3) to form metal Au clusters was observed at 473 K in a H(2) atmosphere. The fact was supported by the IR measurement of adsorbed CO and the subsequent reaction with O(2). Detailed clustering process of Au supported catalysts could be directly followed by EXAFS analysis. The growth of metal Au proceeded via the formation of a Au(55) cluster at 473 K. Then it agglomerated to give metal Au with diameter of 2 nm at 723 K. The addition of H(2) was effective to retard the sintering of Au clusters. A similar phenomenon was observed over Au loaded on USY zeolite. In marked contrast to the H-Y and USY supports, significantly agglomerated Au particles generated on Na-Y zeolite, indicating the importance of the presence of acid sites in keeping the Au clusters with highly dispersed form. The performance of 5 wt % Au loaded on H-Y and USY in the CO-O(2) reaction was remarkably sensitive to the pretreatment temperature and the gas atmosphere. The catalyst pretreated with hydrogen showed a two-spike pattern with respect to the pretreatment temperature. Namely, the optimum activity was observed after the pretreatment at 373 and 723 K, where the temperatures corresponded to the generation of Au(2)O(3) and metal Au clusters with 2 nm diameter as evidenced by QXAFS analysis, respectively. The reason for enhancement of the activity of Au/H-Y by the addition of H(2) in the pretreatment step could be attributed to the formation of metal Au with appropriate size. In contrast to the H-Y and USY support, Au loaded on Na-Y prepared under the same condition was almost inactive in the reaction due to the formation of aggregated metal Au.     

Single‐Atom Gold Catalysis in the Context of Developments in Parahydrogen‐Induced Polarization

A highly isolated monoatomic gold catalyst, with single gold atoms dispersed on multiwalled carbon nanotubes (MWCNTs), has been synthesized, characterized, and tested in heterogeneous hydrogenation of 1,3‐butadiene and 1‐butyne with parahydrogen to maximize the polarization level and the contribution of the pairwise hydrogen addition route. The Au/MWCNTs catalyst was found to be active and efficient in pairwise hydrogen addition and the estimated contributions from the pairwise hydrogen addition route are at least an order of magnitude higher than those for supported metal nanoparticle catalysts. Therefore, the use of the highly isolated monoatomic catalysts is very promising for production of hyperpolarized fluids that can be used for the significant enhancement of NMR signals. A mechanism of 1,3‐butadiene hydrogenation with parahydrogen over the highly isolated monoatomic Au/MWCNTs catalyst is also proposed.     

Preparation of highly active silica-supported Au catalysts for CO oxidation by a solution-based technique

Although Au catalysts can be readily prepared on titania via the deposition-precipitation (DP) method, the direct application of the method similar to the preparation of silica-supported Au catalysts only results in diminished success. This paper reports a novel, efficient method to synthesize highly active Au catalysts supported on mesoporous silica (SBA-15) through a gold cationic complex precursor [Au(en)2]3+ (en = ethylenediamine) via a wet chemical process. The gold cationic precursor was immobilized on negatively charged surfaces of silica by a unique DP method that makes use of the deprotonation reaction of ethylenediamine ligands. The resulting mesoporous catalyst has been demonstrated to be highly active for CO oxidation at room temperature and even below 273 K, the activity of which is much superior to that of silica-supported Au catalysts previously prepared by various solution techniques. The pH value of the gold precursor solution plays a key role in determining the catalytic activity through the regulation of [Au(en)2]3+ deprotonation reaction and the surface interaction of silica with the gold precursor. This mesoporous gold silica catalyst has also been shown to be highly resistant to sintering because of the stabilization of Au nanoparticles inside mesopores.     

Time-resolved in situ XAS study of the preparation of supported gold clusters

Incipient-wetness impregnation of gamma-Al(2)O(3) with HAuCl(4) and subsequent removal of chlorine with NaOH, and deposition-precipitation of HAuCl(4) on TiO(2) at pH 7 resulted in supported Au(3+) species. Time-resolved in situ XAS at the Au L(3) edge showed that the Al(2)O(3)-supported oxidic or hydroxidic species were reduced in hydrogen at 440 K to yield small metallic gold clusters. The Au(3+) precursor decomposed to metallic gold in inert atmosphere at 573 K and in oxidizing atmosphere above 623 K. In all atmospheres, initially small clusters were formed that gradually grew with increasing temperature. The TiO(2)-supported species were considerably less stable. In hydrogen and carbon monoxide, Au(0) clusters of 1 to 1.5 nm were formed at room temperature, which was the lowest temperature studied. In inert and oxidizing atmosphere, the Au(3+) precursor decomposed fully to metallic gold at 530 K, as shown by XAS and temperature-programmed experiments. Large clusters were obtained already in the initial stage of reduction. Residual chlorine inhibited the reduction and led to sintering of the gold clusters. Exposure of the TiO(2)-supported catalyst precursor to light or the X-ray beam led to partial reduction, and STEM showed that storage of the reduced gold clusters under ambient conditions led to agglomeration and bimodal cluster-size distributions.     

Au/Al2O3催化剂的制备及其在CH4/CO2重整反应中的催化活性

采用浸渍法和沉积－沉淀法制备了四种不同的Au/Al2O3催化剂，测定了它们在氢气还原前后及催化反应后的金含量及比表面积，结果表明，制备方法明显影响催化剂的金含量，应用X－光粉末衍射技术研究了这些催化剂经还原处理及反应后的物相变化，金以Au^0物相存在，没有发现氧化态的金物相，考察了该催化剂在CH4／CO2重整反应中的催化活性，发现金催化剂的活性取决于金粒子的大小，浸渍法制备的金催化剂具有较大的金晶粒尺寸，催化活性低，沉积－沉淀法制备的金催化剂金晶粒尺寸较小，催化活性较高，以尿素为沉淀剂制备的催化剂给出1073K时的CH4和CO2转化率分别为8．1％和17．6％，高温反应不仅导致金晶粒的聚集，而且存在明显的金流失现象。     

Gold nanoparticles stabilized by modified halloysite nanotubes for catalytic applications

A highly sustainable prototype of a flow system based on gold nanoparticles (4.2 nm) supported on thiol‐functionalized halloysite nanotubes (HNTs) was developed for catalytic applications. The catalytic performances were evaluated using the reduction of 4‐nitrophenol to 4‐aminophenol as a model system. Under the best experimental conditions (0.0001 mol%, 1.97 × 10^?8 mg of Au nanoparticles), an impressive apparent turnover frequency value up to 2 204 530 h^?1 was achieved and the halloysite‐based catalyst showed full recyclability even after ten cycles. The high catalytic activity confirms the importance of the use of HNTs as support for Au nanoparticles that can exert a synergistic effect both as medium for transfer of electrons from borohydride ions to 4‐nitrophenol and by modulating interfacial electron transfer dynamics. With the application of flow technology, the obtained heterogeneous HNT@Au catalyst was fully recovered and reused for at least one month.     

A coordination cage hosting ultrafine and highly catalytically active gold nanoparticles

Ultrafine metal nanoparticles (MNPs) with size <2 nm are of great interest due to their superior catalytic capabilities. Herein, we report the size-controlled synthesis of gold nanoparticles (Au NPs) by using a thiacalixarene-based coordination cage CIAC-108 as a confined host or stabilizer. The Au NPs encapsulated within the cavity of CIAC-108 (Au@CIAC-108) show smaller size (∼1.3 nm) than the ones (∼4.7 nm) anchored on the surface of CIAC-108 (Au/CIAC-108). The cage-embedded Au NPs can be used as a homogeneous catalyst in a mixture of methanol and dichloromethane while as a heterogeneous catalyst in methanol. The homogeneous catalyst Au@CIAC-108-homo exhibits significantly enhanced catalytic activities toward nitroarene reduction and organic dye decomposition, as compared with its larger counterpart Au/CIAC-108-homo and its heterogeneous counterpart Au@CIAC-108-hetero. More importantly, the as-prepared Au@CIAC-108-homo possesses remarkable stability and durability.

The size-controlled synthesis of Au NPs was achieved by using a coordination cage CIAC-108 as a support. The Au NPs encapsulated within the cavity of CIAC-108 show smaller size (∼1.3 nm) than the ones (∼4.7 nm) anchored on the surface of CIAC-108.     

Synthesis of silica supported AuCu nanoparticle catalysts and the effects of pretreatment conditions for the CO oxidation reaction

Supported gold nanoparticles have generated an immense interest in the field of catalysis due to their extremely high reactivity and selectivity. Recently, alloy nanoparticles of gold have received a lot of attention due to their enhanced catalytic properties. Here we report the synthesis of silica supported AuCu nanoparticles through the conversion of supported Au nanoparticles in a solution of Cu(C(2)H(3)O(2))(2) at 300 °C. The AuCu alloy structure was confirmed through powder XRD (which indicated a weakly ordered alloy phase), XANES, and EXAFS. It was also shown that heating the AuCu/SiO(2) in an O(2) atmosphere segregated the catalyst into a Au-CuO(x) heterostructure between 150 °C to 240 °C. Heating the catalyst in H(2) at 300 °C reduced the CuO(x) back to Cu(0) to reform the AuCu alloy phase. It was found that the AuCu/SiO(2) catalysts were inactive for CO oxidation. However, various pretreatment conditions were required to form a highly active and stable Au-CuO(x)/SiO(2) catalyst to achieve 100% CO conversion below room-temperature. This is explained by the in situ FTIR result, which shows that CO molecules can be chemisorbed and activated only on the Au-CuO(x)/SiO(2) catalyst but not on the AuCu/SiO(2) catalyst.     

Synthesis of P(MBA-co-MAA) microsphere-grafted PAMAM dendrimers and their application as supporters for gold nanoparticles

Poly(N,N′-methylenebisacrylamide-co-acrylic acid) microsphere-supported polyamidoamine (PAMAM) dendrimers up to third generation (G) were grown onto the surface as well as the gel-layer of P(MBA-co-MAA) microspheres by a divergent method. The P(MBA-co-MAA) supported PAMAM dendrimers were used as heterogeneous stabilizers for the gold nanoparticles by an in situ reduction of HAuCl₄ via the efficient coordination interaction between the amino groups of the supported PAMAM dendrimers and the gold atoms. The effects of the generations of the P(MBA-co-MAA) supported PAMAM dendrimer on the loadings and the catalytic activity of the heterogeneous Au nanoparticles were systematically investigated with the reduction of 4-nitrophenol to 4-aminophenol as a model reaction.     

Influence of pretreatment conditions on low-temperature CO oxidation over Au/MOx/Al2O3 catalysts

Composite oxide MO_x/Al₂O₃ supported gold catalysts for low-temperature CO oxidation were prepared and investigated. The presence of transition metal oxide was proved to be beneficial to the improvement of catalytic performance of Au/Al₂O₃ catalysts for low-temperature CO oxidation. Furthermore, the influence of various pretreatment conditions on Au/MO_x/Al₂O₃ catalysts was studied carefully. The image of TEM showed that gold catalyst with small gold particles only in the form of a fine dispersion exhibited highly catalytic activity. The XPS, Fourier transform infrared (FTIR) spectroscopy characterization results of Au/FeO_x/Al₂O₃ catalyst showed that gold catalysts having partially oxidized gold species have the best catalytic performance. One possible pathway for CO oxidation on Au/FeO_x/Al₂O₃ catalyst is that the CO adsorbed on gold particles reacts with adsorbed oxygen, which is possible to occur on oxygen vacancies on the support or at the metal–support interface.     

A Molecular mechanism for the chemoselective hydrogenation of substituted nitroaromatics with nanoparticles of gold on TiO2 catalysts: a cooperative effect between gold and the support

Nanoparticles of gold on TiO2 are highly chemoselective for the reduction of substituted nitroaromatics, such as nitrostyrene. By combining kinetics and in situ IR spectroscopy, it has been found that there is a preferential adsorption of the reactant on the catalyst through the nitro group. IR studies of nitrobenzene, styrene, and nitrostyrene adsorption, together with quantum chemical calculations, show that the nitro and the olefinic groups adsorb weakly on the Au(111) and Au(001) surfaces, and that although a stronger adsorption occurs on low-coordinated atoms in gold nanoparticles, this adsorption is not selective. On the other hand, an energetically and geometrically favored adsorption through the nitro group occurs on the TiO2 support and in the interface between the gold nanoparticle and the TiO2 support. Such preferential adsorption is not observed with nanoparticles of gold on silica which, contrary to the Au/TiO2 catalyst, is not chemoselective for the reduction of substituted nitroaromatic compounds. Therefore, the high chemoselectiviy of the Au/TiO2 catalyst can be attributed to a cooperation between the gold nanoparticle and the support that preferentially activates the nitro group.     

高分散的炭载Au纳米催化剂的制备、表征和催化活性

采用柠檬酸钠还原-胶体负载法, 制备了高分散的炭载Au纳米催化剂, 并以液相催化氧化葡萄糖为葡萄糖酸钠的反应评价了Au/C催化剂的活性. 研究结果表明, 金溶胶制备过程中柠檬酸钠的用量对粒子尺寸以及所获催化剂的催化活性有重要影响; 催化剂在多次使用之后活性的降低可能是由于在活性炭表面金粒子活性位点上形成的Au^δ＋-O^δ－化合态减少的缘故. 同时比较了制备的Au/C和商业Pd/C催化剂对葡萄糖的液相催化氧化反应, 证明Au/C催化剂明显优于Pd/C催化剂.     

Zeolite-supported gold nanoparticles for selective photooxidation of aromatic alcohols under visible-light irradiation

With new photocatalysts of gold nanoparticles supported on zeolite supports (Au/zeolite), oxidation of benzyl alcohol and its derivatives into the corresponding aldehydes can proceed well with a high selectivity (99?%) under visible-light irradiation at ambient temperature. Au/zeolite photocatalysts were characterised by UV/Vis, X-ray photoelectron spectroscopy (XPS), TEM, XRD, energy-dispersive spectroscopy (EDS), Brauner-Emmet-Teller (BET) analyses, IR and Raman techniques. The surface plasmon resonance (SPR) effect of gold nanoparticles, the adsorption capability of zeolite supports and the molecular polarities of aromatic alcohols were demonstrated to have an essential correlation with the photocatalytic performances. In addition, the effects of light intensity, wavelength range and the role of molecular oxygen were investigated in detail. The kinetic study indicated that the visible-light irradiation required much less apparent activation energy for photooxidation compared with thermal reaction. Based on the characterisation data and the photocatalytic performances, we proposed a possible photooxidation mechanism.