Discrimination of active palladium sites in catalytic liquid-phase oxidation of benzyl alcohol

Knowledge of the structure of active sites is a prerequisite for the rational design of solid catalysts. Using site-selective blocking by CO and isotope labeling combined with in situ attenuated total reflection infrared (ATR-IR) spectroscopy, we were able to discriminate the different sites involved in the liquid-phase oxidation of benzyl alcohol on Pd/Al(2)O(3). The main reaction, that is, the oxidative dehydrogenation of the alcohol to the corresponding aldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas the undesired product decarbonylation occurred preferentially on hollow sites on (111) Pd faces. This explains why specific blocking of the latter sites, as realized in the industrially used Pd-Bi/Al(2)O(3) catalysts, leads to improved catalytic performance.     

Mn-Al和Cu-Mn-Al复合氧化物催化苯甲醇选择氧化反应

用溶胶-凝胶法制备了不同组成的Mn-Al和Cu-Mn-Al复合氧化物两组催化剂,用于苯甲醇选择氧化反应.用X射线衍射(XRD)、N2物理吸附(BET)、扫描电镜(SEM)、H_2程序升温还原(H_2-TPR)、O_2程序升温脱附(O_2-TPD)和X射线光电子能谱(XPS)技术对催化剂进行了结构表征,考察了催化剂组成对催化活性的影响.结果表明:以甲苯为溶剂,O_2为氧化剂,353 K反应5 h,Mn_2Al和Cu_(0.3)Mn_(0.7)Al_2催化剂上的苯甲醇转化率分别为36.6%和40.9%,苯甲醛选择性均为100%.进一步研究表明:催化剂活性与其H2还原性和O_2吸附性有关,高活性的催化剂吸附氧多,生成的活性氧易参与反应.     

Initiation Chemistries in Hydrocarbon (Aut)Oxidation

For the (aut)oxidation of toluene to benzyl hydroperoxide, benzyl alcohol, benzaldehyde, and benzoic acid, the thermochemical profiles for various radical‐generating reactions have been compared. A key intermediate in all of these reactions is benzyl hydroperoxide, the heat of formation of which has been estimated by using results from CBS‐QB3, G4, and G3B3 calculations. Homolytic O?O bond cleavage in this hydroperoxide is strongly endothermic and thus unlikely to contribute significantly to initiation processes. In terms of reaction enthalpies the most favorable initiation process involves bimolecular reaction of benzyl hydroperoxide to yield hydroxy and benzyloxy radicals along with water and benzaldehyde. The reaction enthalpy and free energy of this process is significantly more favorable than those for the unimolecular dissociation of known radical initiators, such as dibenzoylperoxide or dibenzylhyponitrite.     

Pt／Al_2O_3，Pd／Al_2O_3催化剂上CO氧化与表面氧脱出－恢复性能

Ｐｔ／Ａｌ_２Ｏ_３，Ｐｄ／Ａｌ_２Ｏ_３催化剂上ＣＯ氧化与表面氧脱出－恢复性能周仁贤，郑小明（杭州大学催化研究所，杭州３１００２８）关键词铂，钯，氧化铝，氧化锆，负载型催化剂，一氧化碳，氧化，氧脱附ＡＩ。Ｏ。负载的贵金属（Ｐｔ，Ｐｄ或Ｒｈ等）催化剂对有...     

Supercritical carbon dioxide: an inert solvent for catalytic hydrogenation?

Various surface species originating from the reaction between CO2 and H2 over Al2O3-supported Pt, Pd, Rh, and Ru model catalysts were investigated by attenuated total reflection infrared (ATR-IR) spectroscopy under high-pressure conditions. Two different spectroscopic cells were used: a variable-volume view cell equipped with ATR-crystal and transmission IR windows (batch reactor) and a continuous-flow cell also equipped with a reflection element for ATR-IR spectroscopy. The study corroborated that CO formation from dense CO2 in the presence of hydrogen occurs over all Pt-group metals commonly used in heterogeneous catalytic hydrogenations in supercritical CO2 (scCO2). In the batch reactor cell, formation of CO was detected on all metals at 50 and 90 degrees C, with the highest rate on Pt. Additional surface species were observed on Pt/Al2O3 at 150 bar under static conditions. It seems that further reaction of CO with hydrogen is facilitated by the higher surface concentration at higher pressure. In the continuous-flow cell, CO coverage on Pt/Al2O3 was less prominent than that in the batch reactor cell. A transient experiment in the continuous-flow cell additionally revealed CO formation on Pt/Al2O3 at 120 bar after switching the feed from a H2-ethane to a H2-CO2 mixture. The in situ ATR-IR measurements indicate that CO formation in CO2-H2 mixtures is normally a minor side reaction during hydrogenation reactions on Pt-group metal catalysts, and dense ("supercritical") CO2 may be considered as a relatively "inert" solvent in many practical applications. However, blocking of specific sites on the metal surface by CO and consecutive products can affect structure sensitive hydrogenation reactions and may be at the origin of unexpected shifts in the product distribution.     

Controlling the duality of the mechanism in liquid-phase oxidation of benzyl alcohol catalysed by supported Au-Pd nanoparticles

In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.     

Cu-BTC及其衍生物在苯甲醇选择氧化反应中的催化活性

以硝酸铜和均苯三甲酸(BTC)为原料,水热合成了一种金属有机骨架化合物Cu-BTC,在水相中催化苯甲醇选择氧化反应,H2O2氧化剂,优化了Cu-BTC的晶化条件.70℃反应1 h,Cu-BTC(110C/24 h)上的苯甲醇转化率为75.4%、苯甲醛选择性83.5%,但反应后Cu-BTC骨架完全塌陷.在氮气中高温焙烧Cu-BTC,制得衍生物Cu@C,也用于催化苯甲醇氧化反应.结果表明:Cu@C催化剂重复使用5次,可维持较高的苯甲醇转化率,但苯甲醛选择性有所下降.用X射线衍射(XRD)、N2物理吸附(BET)、热分析(TG-DSC)、红外光谱(FTIR)、能量色散X射线光谱分析(EDX)等技术对催化剂进行了结构表征,发现:Cu@C在反应中生成的Cu2O促进了苯甲醛的深度氧化.     

In situ extended X-ray absorption fine structure study during selective alcohol oxidation over Pd/Al2O3 in supercritical carbon dioxide

High-pressure in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are reported during the selective oxidation of benzyl alcohol to benzaldehyde in supercritical carbon dioxide over a Pd/Al(2)O(3) catalyst (shell impregnated). For this purpose, a continuous-flow system with a spectroscopic cell suitable for in situ X-ray absorption studies on heterogeneous catalysts up to 200 degrees C and 200 bar has been developed. Due to the high contribution of the dense fluid to the overall X-ray absorption, high stability of the process pressure is mandatory, particularly when recording EXAFS spectra. According to EXAFS and XANES results, the palladium particles were fully reduced after exposure to benzyl alcohol in scCO(2). In contrast to Pd-catalyzed liquid-phase oxidation, a higher oxygen tolerance of the catalyst was observed. Palladium was partially oxidized on the surface under typical reaction conditions (0.9 mol % benzyl alcohol/0.5 mol % O(2) in carbon dioxide), which gradually increased when the concentration of oxygen in the feed was raised. Both XANES and EXAFS data uncovered that palladium is mainly oxidized on the surface or within the outermost layers. These results are in accordance with simulations of the XANES data using the FEFF8.20 code (program for ab initio calculations on multiple scattering XAS) and EXAFS data fitting/simulation.     

Role of Bi promotion and solvent in platinum-catalyzed alcohol oxidation probed by in situ X-ray absorption and ATR-IR spectroscopy

Modification of 5 wt% Pt/Al(2)O(3) by Bi (0.9 wt%) affords a drastic improvement of catalytic activity in the liquid phase aerobic oxidation of benzyl alcohol. The nature of the solvent employed, cyclohexane or toluene, seems to influence the catalytic activity as well. We have investigated the catalysts under working conditions using in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR), aiming at uncovering the roles of the metal promoter and the reaction medium. XAS confirms that Bi is oxidized more easily than Pt, maintaining the catalytic activity of the metallic Pt sites for a longer period of time. Interestingly, toluene contrary to cyclohexane reduced Pt to a large extent. The freshly reduced noble metal sites seem to directly interact with the solvent, inducing an immediate poisoning of the material and limiting its performance. This behaviour is not observed in the presence of Bi, whose geometric effect (site blocking) is interpreted as additionally limiting the adsorption of toluene and the premature deactivation of Pt. ATR-IR spectroscopy during CO adsorption on Pt and during reaction indicates that Bi is located rather on extended surfaces than on step or kink sites. Side products, CO and benzoate species, appearing during the reaction reveal that the geometric suppression of undesired reactions does not occur to the same extent on Pt-based catalysts as on Pd, suggesting that decarbonylation of the produced aldehyde on Pt may occur also on sites other than the (111) terraces.     

Catalytic Conversion of Methanol by Oxidative Dehydrogenation

This study investigates the effects of addition of oxygen on the oxidative dehydrogenation (ODH) of methanol when a fluorotetrasilicic mica ion-exchanged with palladium (Pd2 -TSM) was used as the catalyst. The reaction proceeded at a very low temperature in the presence of oxygen, and HCOOCH3 was obtained at high selectivity. By calculating the equilibrium conversion, it has been shown that substantial ODH took place for HCOOCH3 production. Consequently, this reaction would make dehydrogenation the dominant reaction at equilibrium. Not all the H dissociated from CH3OH was converted to H2O by oxidation. It has been shown that the H2O was not produced from oxidative dehydrogenation by the direct reaction of CH3OH and O2 when an attempt was made to carry out oxidative dehydrogenation using an isotope oxygen trace method in the gas phase. Therefore, when CH3OH was converted to CO2 and dehydrogenated to HCOOCH3, the C—O bonds were not dissociated.     

Mechanistic characterization of aerobic alcohol oxidation catalyzed by Pd(OAc)(2)/pyridine including identification of the catalyst resting state and the origin of nonlinear [catalyst] dependence

The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.     

氧化铈对Pd／Al2O3表面上CO氧化性能的影响

采用ＴＰＤ－ＭＳ及ＴＰＳＲ－ＭＳ技术研究了添加ＣｅＯ２对Ｐｄ／Ａｌ２Ｏ３催化剂上ＣＯ脱附、表面反应及表面氧脱附等性能的影响，考察了不同含氧量的气氛下ＣＯ的氧化活性，结果表明，Ｐｄ－Ｃｅ间的相互作用有利于各自原子上表面氧的吸脱附及ＣＯ的表面反应，并发现ＣＯ２脱附量大小及峰温次序与对ＣＯ的催化氧化活性有一致的对应关系。     

分子筛催化苯甲醛与醇缩合反应的研究

1　引言分子筛作为催化剂由于其特殊的酸性和择形性在精细有机合成中获得了广泛的应用[1],在缩醛化反应中也表现出了良好的催化性能,如Joshi[2]等用HY和HZSM5分子筛作催化剂,将苯甲醛与甲醇缩合,获得了83%的苯甲缩甲醛,并考察了不同种类的醛在HZSM5分子筛上的择形性。王存...     

负载型Pd-Ni双金属纳米颗粒在无溶剂苯甲醇氧化中有效抑制甲苯的生成

醇类化合物的选择性氧化是实验室和工业应用中一类重要的官能团转化反应.以分子氧为氧化剂,在液相无溶剂条件下温和氧化符合绿色化学的要求.负载型Pd基催化剂因其优异的催化活性而在该反应中得到广泛应用.但是,单金属Pd催化剂对反应目标产物醛类化合物的选择性还有待提高.例如,在苯甲醇液相无溶剂氧化中,甲苯是在单金属Pd催化剂上的主要副产物.针对这一问题,除了对载体进行改性和修饰外,开发双金属Pd基催化剂也是一种有效的选择性调控策略.虽然已有的Pd-Au双金属催化剂可以在一定程度上降低甲苯的选择性,但是在较高温度和较高转化率下仍然难以控制甲苯的大量生成.本文采用固相合金化法合成了负载型Pd-Ni双金属纳米颗粒.该方法首先以硝酸镍为镍的前驱体浸渍介孔二氧化硅,然后负载钯纳米颗粒.在高温固相还原条件下,作为种子的钯纳米颗粒和镍通过原子迁移和生长,形成Pd-Ni双金属纳米颗粒.扫描透射电镜、能量色散X射线光谱、X射线衍射和X射线光电子能谱等表征证实了Pd-Ni双金属纳米颗粒的生成.上述催化剂用于苯甲醇液相无溶剂氧化,催化结果显示Ni的加入可以抑制副产物甲苯的生成,并且随Ni负载量增加,甲苯的选择性(在80%等转化率下)由22.6%(单金属Pd)降低至1.6%(双金属Pd1Ni20).尽管Ni的加入降低了单金属Pd的活性,但是由于提高了目标产物苯甲醛的选择性,醛的最终产率得到提升.进一步催化研究表明,Ni的加入可以抑制无氧氛围下甲苯的生成,说明Ni可以抑制歧化反应和降低表面氢浓度.这种作用可归结于Pd-Ni双金属的协同效应.该效应得到了CO吸附的傅里叶变换漫反射红外光谱和密度泛函理论研究的证实.双金属的几何效应和电子效应均减弱了苯甲醇在双金属纳米颗粒表面的解离吸附和相互作用,导致苯甲醇的吸附减弱,同时C–O键断裂不易进行.另外,由于Ni的亲氧性,双金属纳米颗粒表面有利于氧的吸附,降低吸附氢的浓度,减少C–H键生成,从而抑制甲苯的生成.     

Imidazolium ionic liquids as solvents for cerium(IV)-mediated oxidation reactions

Use of imidazolium ionic liquids as solvents for organic transformations with tetravalent cerium salts as oxidizing agents was evaluated. Good solubility was found for ammonium hexanitratocerate(IV) (ceric ammonium nitrate, CAN) and cerium(IV) triflate in 1-alkyl-3-methylimidazolium triflate ionic liquids. Oxidation of benzyl alcohol to benzaldehyde in 1-ethyl-3-methylimidazolium triflate was studied by in-situ FTIR spectroscopy and 13C NMR spectroscopy on carbon-13-labeled benzyl alcohol. Careful control of the reaction conditions is necessary because ammonium hexanitratocerate(IV) dissolved in an ionic liquid can transform benzyl alcohol not only into benzaldehyde but also into benzyl nitrate or benzoic acid. The selectivity of the reaction of cerium(IV) triflate with benzyl alcohol in dry ionic liquids depends on the degree of hydration of cerium(IV) triflate: anhydrous cerium(IV) triflate transforms benzyl alcohol into dibenzyl ether, whereas hydrated cerium(IV) triflate affords benzaldehyde as the main reaction product. Reactions of ammonium hexanitratocerate(IV) with organic substrates other than benzyl alcohol have been explored. 1,4-Hydroquinone is quantitatively transformed into 1,4-quinone. Anisole and naphthalene are nitrated. For the cerium-mediated oxidation reactions in ionic liquids, high reaction temperatures are an advantage because under these conditions smaller amounts of byproducts are formed.     

聚苯并噁嗪固载钯基纳米催化剂在芳香醇氧化反应中的应用

以球状聚苯并噁嗪为载体,采用浸渍热解法合成了钯炭纳米催化剂.通过透射电子显微镜观察发现,钯纳米粒子几乎全部均匀分布在载体上,且尺寸均一,平均直径约为3. 5 nm.结果表明,载体表面含有丰富的含氮含氧官能团,氮和氧原子与钯之间存在相互作用,从而使聚苯并噁嗪能够有效固载钯纳米粒子.采用相同的方法进一步合成Pd-Au/C和Pd-Pt/C双金属催化剂,Pd-Au和Pd-Pt纳米粒子也展现出良好的分散性,无明显团聚现象,平均直径分别为4. 3和4. 2 nm,进一步说明聚苯并噁嗪对金属活性组分的有效固载.将催化剂应用于苯甲醇氧化反应,其中Pd₁-Au₁/C在2 h的转化率为98%,对产物苯甲醛的选择性大于99%,该催化剂经过焙烧可恢复催化活性,表现出良好的循环稳定性,并能将不同取代基的芳香醇氧化为相应的醛,是一种良好的醇氧化催化剂.     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.     

Light at the interface: the potential of attenuated total reflection infrared spectroscopy for understanding heterogeneous catalysis in water

IR spectroscopy has been an important tool for studying detailed interactions of reactants and reaction-intermediates with catalyst surfaces. Studying reactions in water is, however, far from trivial, due to the excessive absorption of infrared light by water. One way to deal with this is the use of Attenuated Total Reflection spectroscopy (ATR-IR) minimizing the path length of infrared light through the water. Moreover, ATR-IR allows for a direct comparison of reactions in gas and water on the same sample, which bridges the gap between separate catalyst investigations in gas and liquid phase. This tutorial review describes recent progress in using ATR-IR for studying heterogeneous catalysts in water. An overview is given of the important aspects to be taken into account when using ATR-IR to study heterogeneous catalysts in liquid phase, like the procedure to prepare stable catalyst layers on the internal reflection element. As a case study, CO adsorption and oxidation on noble metal catalysts is investigated with ATR-IR in gas and water. The results show a large effect of water and pH on the adsorption and oxidation of CO on Pt/Al(2)O(3) and Pd/Al(2)O(3). From the results it is concluded that water affects the metal particle potential as well as the adsorbed CO molecule directly, resulting in higher oxidation rates in water compared to gas phase. Moreover, also pH influences the metal particle potential with a clear effect on the observed oxidation rates. Finally, the future outlook illustrates that ATR-IR spectroscopy holds great promise in the field of liquid phase heterogeneous catalysis.     

Holey Lamellar High-Entropy Oxide as an Ultra-High-Activity Heterogeneous Catalyst for Solvent-free Aerobic Oxidation of Benzyl Alcohol

The development of noble-metal-free heterogeneous catalysts is promising for selective oxidation of aromatic alcohols; however, the relatively low conversion of non-noble metal catalysts under solvent-free atmospheric conditions hinders their industrial application. Now, a holey lamellar high entropy oxide (HEO) Co_0.2Ni_0.2Cu_0.2Mg_0.2Zn_0.2O material with mesoporous structure is prepared by an anchoring and merging process. The HEO has ultra-high catalytic activity for the solvent-free aerobic oxidation of benzyl alcohol. Up to 98 % conversion can be achieved in only 2 h, to our knowledge, the highest conversion of benzyl alcohol by oxidation to date. By regulating the catalytic reaction parameters, benzoic acid or benzaldehyde can be selectively optimized as the main product. Analytical characterizations and calculations provide a deeper insight into the catalysis mechanism, revealing abundant oxygen vacancies and holey lamellar framework contribute to the ultra-high catalytic activity.     

多金属氧酸盐催化苯甲醇的选择性氧化：一个绿色可循环的高效催化氧化体系

以Keggin结构的几类杂多酸和三乙胺（TEA）为原料,通过简单的酸碱反应合成了相应杂多酸的TEA盐。 并以它们作为催化剂,30％H2O2作氧化剂,在不使用长链相转移剂的条件下,研究了它们催化苯甲醇选择氧化制备苯甲醛的反应性能。 结果表明,该类催化剂在苯甲醇的选择氧化反应中具有比相应杂多酸更高的催化活性或选择性。 其中[TEAH]H2PW12O40为最佳催化剂,在适宜的反应条件下,该催化剂上苯甲醇转化率可达99.5％以上,苯甲醛选择性达~100％。 催化剂可以被分离和循环使用多次,活性、选择性基本不变。 用水作溶剂,避免了有机溶剂的使用,是一个高效、绿色的苯甲醛选择氧化体系。