1、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（英文） 车建伟 郝梦佳 易武中 Hisayoshi Kobayashib 周宇恒 肖丽萍 范杰 浙江大学化学系浙江省应用化学重点实验室 京都工艺纤维大学 摘 要： 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,
2、 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
3、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
4、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 CO 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 CH 键生成, 从而抑制甲苯的生成.关键词： 钯-镍; 双金属纳米颗粒; 苯甲醇; 甲苯; 无溶剂; 氧化; 作者简介：范杰 电话/传真: (0571) 87952338;电子信箱:收稿日期：12 July 2017基金：supported by National Natural Science Foundation of China (21271153, 21373181, 21222307, U14022
5、33) ;Major Research Plan of National Natural Science Foundation of China (91545113) Selective suppression of toluene formation in solvent-free benzyl alcohol oxidation using supported Pd-Ni bimetallic nanoparticlesJianwei Che Mengjia Hao Wuzhong Yi Hisayoshi Kobayashi Yuheng Zhou Liping Xiao Jie Fan
6、 Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University; Department of Chemistry and Materials Technology, Kyoto Institute of Technology; Abstract： The solvent-free oxidation of benzyl alcohol was studied using supported Pd-Ni bimetallic nanoparticles. Compar
7、ed with monometallic Pd, the addition of Ni to Pd was found to be effective in suppressing the nondesired product toluene, thereby enhancing the selectivity towards benzaldehyde. This result was attributed to a dual effect of Ni addition: the weakening of dissociative adsorption of benzyl alcohol an
8、d the promotion of oxygen species involved in the oxidation pathway.Keyword： Palladium-nickel; Bimetallic nanoparticle; Benzyl alcohol; Toluene; Solvent-free; Oxidation; Received： 12 July 20171. IntroductionSelective oxidation of alcohols to corresponding aldehydes or ketones is of fundamental impor
9、tance in functional group transformation in both laboratorial and industrial applications.Traditionally, alcohol oxidations are performed by using stoichiometric and toxic permanganate or chromate as oxidants.From the viewpoint of green chemistry, the liquid-phase oxidation of alcohols with molecula
10、r oxygen or air as oxidants under mild solvent-free reaction conditions is highly desirable.Over the past few years, a variety of heterogeneous catalysts have been developed for this transformation1, 2.Among the reported catalysts, supported palladium has been recognized as one of the best catalysts
11、 owing to its excellent activity37.Although supported palladium catalysts can be very active for the oxidation of benzyl alcohol, which is a model reaction for selective oxidation, the selectivity towards the desired product benzaldehyde is difficult to control.Previous investigations on the solvent
12、-free oxidation of benzyl alcohol using palladium catalysts have demonstrated the formation of many products, including (a) benzaldehyde, benzoic acid, and benzyl benzoate from oxidation, dehydrogenation, and esterification, respectively (b) benzene from decarbonylation, (c) dibenzyl ether from dehy
13、dration, (d) dibenzyl acetal from condensation, and (e) toluene from disproportionation or hydrogenolysis of benzyl alcohol810.Among these products, toluene is the major byproduct under solvent-free conditions, which can lower the selectivity towards the oxidation product6, 7.To switch off toluene f
14、ormation and achieve high aldehyde selectivity, one widely adopted approach is to use intrinsically basic or modified supports.Sankar et al.9reported Mg O-and Zn O-supported catalysts did not produce any toluene.They interpreted that the basicity of the supports inhibited the cleavage of the CO bond
15、 of benzyl alcohol that gives rise to toluene generation.Furthermore, N-doped mesoporous carbon11, amino-group-functionalized TUD-1 and CNT12, 13, and alkali-treated titanate nanobelts14also showed enhanced selectivity towards the desired oxidation product owing to the basic sites grafted on the sur
16、face of the supports.Another approach to suppress toluene formation involves the use of supported bimetallic or trimetallic catalysts.Pd-Au bimetallic nanoparticles are most frequently used, which improve the overall selectivity compared with monometallic Pd15.However, a considerable amount of tolue
17、ne is inevitably formed when it comes to a higher reaction temperature or conversion9, 16.Recently, He et al.17demonstrated that the addition of Pt to the Pd-Au alloy catalysts could significantly minimize toluene formation.However, the full complexity of ternary alloys can be missed because the com
18、position of such catalysts varies systematically with particle size.It is generally accepted that bimetallic catalysts exhibit superior catalytic performance to monometallic catalysts owing to the synergetic interactions between the metals1820.In terms of selectivity control, the introduction of a l
19、ess active metal to a more active metal may selectively nullify some active sites for side reactions21.In the case of Pd-Au bimetallic catalysts, the catalytically more active Pd is perturbed by the inferior activity of Au, which makes Pd more atomic-like and suppresses the formation of byproducts22
20、.Additionally, analogous Pd-M (M=Co, Cu, Zn, Ga, Ag, Sn, Pb, Bi) catalysts have been used in many Pd-catalyzed reactions and has resulted in enhanced selectivity2329.For example, Pd-Ag and Pd-Cu bimetallic nanoparticles dramatically increased the selectivity towards the target alkenol in the semi-hy
21、drogenation of alkynol reported by Yarulin et al.24.In an attempt to prevent the formation of nonselective reaction products, especially toluene, in solvent-free benzyl alcohol oxidation, we adopted a bimetallic catalysis strategy by alloying Pd with nonprecious transition metals instead of other no
22、ble metals such as Au and Pt.Herein, we report that a Pd-Ni bimetallic catalyst displayed high selectivity and overall productivity to the corresponding aldehyde.2. Experimental2.1. Catalyst preparation2.1.1. Synthesis of EP-FDU-12The EP-FDU-12 support was synthesized as reported in the literature30
23、.Typically, 0.50 g of Pluronic F127, 0.60 g of1, 3, 5-trimethylbenzene (TMB) , and 1.25 g of KCl were dissolved in 50 m L of HCl (1 mol/L) at 140.1C.After stirring for 1 h, 2.08 g of tetraethyl orthosilicate (TEOS) was added to this solution.After stirring at 14C for 24 h, the mixture was transferre
24、d into an autoclave and heated at 140C for 24 h.The product was obtained by filtration and dried at room temperature in air.The organic templates were removed by annealing at 350C.2.1.2. Synthesis of Pd nanoparticlesPd nanoparticles (Pd NPs) were synthesized according to a reported method31.In a typ
25、ical synthesis, 112 mg of palladium acetate was mixed with 3.3 m L of oleic amine with stirring at 50C to form a clear solution.Oleic acid (3.2 m L) was then added and the mixture was kept at 50C for 1 h.Then, 386mg of tetrabutylammonium borohydride (TBAB) dissolved in 2m L of CHCl3 was added in one
26、 portion.The mixture was heated for 1 h before it was cooled to room temperature.The Pd NPs were precipitated by the addition of 15 m L of ethanol.The precipitate was separated by centrifugation and washed with ethanol.Finally, the precipitate was dried in a vacuum oven overnight.The size of the Pd
27、NPs was examined by transmission electron microscopy (TEM) and determined to be 2.830.27nm.2.1.3. Synthesis of supported Pd-Ni bimetallic nanoparticlesThe synthesis of supported Pd-Ni bimetallic nanoparticles (BMNPs) was similar to our previous study with some modifications32.First, the required amo
28、unt of nickel nitrate and500 mg of mesoporous silica (EP-FDU-12) were added to 5 m L of ethanol.The mixture was stirred until the ethanol evaporated to form a powder, which was further dried under vacuum oven overnight.The as-obtained solid powder was added to a predetermined volume of a chloroform
29、solution of Pd NPs.After24 h of stirring, the solid product was centrifuged and dried in air.Finally, the powder was calcinated under 5%H2/95%Ar at500C for 4 h.The loading mass of Pd was 1 wt%for all the samples.For comparison, supported monometallic Pd and Ni nanoparticles were prepared in a simila
30、r manner to the above procedure by omitting the deposition of nickel nitrate and Pd NPs, respectively.2.2. CharacterizationsThe TEM images were recorded on a Hitachi HT-7700 operated at 80 k V.High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive
31、X-ray spectrometry (EDX) analysis were performed on a Tecnai G2 F20 operated at 200 k V.The sample was embedded in epoxy resin and then microtomed into a sub-100-nm ultrathin film at room temperature.These thin film samples floated on water or other solvents and were collected by a copper mesh with
32、a polymer microgrid for HAADF-STEM imaging and elemental line scanning.X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV with Cu K radiation.X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultra-h
33、igh vacuum (UHV) chambers.All binding energies were referenced to the C 1s peak at 284.8 e V of the surface adventitious carbon.Diffuse reflectance Fourier transform infrared spectra of adsorbed CO (CO-DRIFT) were obtained on a Bruker Vertex 70 spectrometer equipped with a mercury-cadmium-tellurium
34、(MCT) detector, a diffuse reflectance accessory, and a Harrick HVC-DRP cell.The sample loaded into the cell was purged with flowing argon (20 m L/min) and a spectrum was recorded as a background.Then, the CO adsorption on the sample was performed at room temperature by switching the gas flow to CO (
35、20 m L/min) and then argon (20 m L/min) .All the spectra were recorded with 64 scans at a4 cm-1 resolution.2.3. Catalytic reactionBenzyl alcohol oxidation was carried out in a 50 m L glass stirred reactor.In a typical reaction, 20 mg of catalyst and 3 g (27.7 mmol) of benzyl alcohol were charged int
36、o the reactor, which was then purged with gas (O2, H2, or N2) three times before closing.The pressure was maintained at 0.1 MPa (relative pressure) by connecting the reactor to a gas line to ensure that any consumed gas was replenished.The mixture was then heated and stirred vigorously at 120C in an
37、 oil bath.After a specific time, the stirring was stopped and the reactor was immediately cooled in an ice bath.After cooling to room temperature, the solid catalyst was removed by centrifugation and the organic products were analyzed by a gas chromatograph (Fu Li GC-9790) equipped with a KB-5 capil
38、lary column and a FID detector.The structures of the products were confirmed by comparison with standard samples and by GC-MS (Shimadzu GCMS-QP2010) .A standard normalization method was used to quantify the composition of the reaction mixtures.The conversion of benzyl alcohol and product selectivity
39、 were calculated using the following equations:in which A is the corrected FID chromatographic peak area, and r, p, and i represent benzyl alcohol, all the products, and a certain product, respectively.2.4. Density functional theory calculationsDensity functional theory (DFT) calculations were carri
40、ed out using a plane-wave-based program, Castep33, 34.Model clusters, Pd25 and Ni21Pd4, were used in the calculations.Whole structures were truncated from Pd and Ni crystals along the (111) orientation.For Ni21Pd4, the central four Ni atoms were replaced by Pd atoms.The lattice constants of the unit
41、 cell were a=b=2 nm, c=3 nm, and=90.The direction of the c-axis includes the vacuum region.Only the central four atoms and adsorbate were optimized.The Perdew-Burke-Ernzerhof (PBE) functional35, 36was used together with the ultrasoft-core potentials37.The basis set cutoff energies were set to 300 e
42、V for geometry optimization, and 340 e V for the energy re-evaluation.The electron configurations of the atoms were:H, 1s1;C, 2s22p2;O, 2s22p4;Ni, 3d84s2;and Pd, 4d10.3. Results and discussionSupported Pd-Ni BMNPs were initially synthesized as an example of bimetallic catalysts.We prepared Pd-Ni BMN
43、Ps loaded on mesoporous silica (EP-FDU-12) by a general solid-state alloying method according to our previous study32.In brief, nickel nitrate was first loaded onto EP-FDU-12 by impregnation, and then colloidal Pd nanoparticles were deposited on the support as a seed for nickel growth before high-te
44、mperature reduction and solid-state alloying.The as-synthesized bimetallic catalysts with different Ni/Pd molar ratios in the precursors are denoted as Pd1Nix (x=120) .The overall compositions of the bimetallic catalysts were confirmed by ICP-MS analysis.As shown in Table 1, the actual Ni/Pd molar r
45、atios were slightly lower than the nominal ratios.Electron microscopy techniques were employed to reveal the morphology and structure of the Pd-Ni bimetallic catalysts.The particle size distributions of the monometallic Pd and bimetallic Pd-Ni samples were obtained by TEM, as depicted in Fig.1.The T
46、EM images show that the Pd nanoparticles in Pd/EP-FDU-12 were mostly at the outer edge of the mesoporous channels, whereas Pd-Ni BMNPs were uniformly dispersed in the EP-FDU-12channels.This suggested that interparticle heteroatom migration occurred between Pd nanoparticles on the periphery and Ni cl
47、usters in the inner channels during the solid-state alloying process.The corresponding particle size distribution for each sample reveal that the average particle size was approximately5.8 nm for monometallic Pd catalyst and 6.3 and 6.5 nm for the Pd1Ni5 and Pd1Ni20, respectively, for the bimetallic
48、 catalysts.These slight differences in the overall particle size distributions could rule out the effect of the particle size on the catalytic performances.To confirm the formation of the bimetallic nanoparticles, HRTEM, HAADF-STEM, and EDX analysis were employed.Fig.2 (a) presents a HRTEM image of
49、an individual Pd-Ni BMNP from a Pd1Ni5 sample.The lattice fringes of Pd1Ni5 display a fcc crystalline structure with interplanar spacings of 0.210 nm in the particle, which is between the interplanar spacing of pure Pd (0.224 nm) and pure Ni (0.203 nm) particles.Thus, the Ni/Pd molar ratio calculated according to Vegards law is 2/1, which is lower than the actual ratio of 4.2/1 obtained by ICP analysis.This is mainly because some monometallic Ni clusters formed through self-nucleation and growth when ther
