Reactive Deposition of Palladium Nanoparticles onto Zeolite

Aug 10, 2010 - Jugal Kishore Das and Nandini Das. ACS Applied Materials ... Chandra Babu Putta , Sutapa Ghosh. Advanced Synthesis & Catalysis 2011 353...
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Reactive Deposition of Palladium Nanoparticles onto Zeolite Membranes in Supercritical CO2 Xiufeng Liu,†,§ Wei Liu,†,‡ Jian Li,† Yushan Zhang,*,‡ Lin Lang,† Laibo Ma,† and Baoquan Zhang*,† State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, The institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration, Tianjin 300192, China, and State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China

Pd/Si-MFI membranes are fabricated by depositing palladium nanoparticles onto Si-MFI membranes via opposing reactants chemical fluid deposition in supercritical carbon dioxide. Palladium hexafluoroacetylacetonate [Pd(hfac)2] and ethanol are used as the Pd precursor and the reducing agent, respectively, in the deposition process. The resulting Pd/Si-MFI membrane is characterized using SEM, XRD, EPMA, TEM, and EDX techniques. It is demonstrated that a continuous layer of Pd particles with size of 10 nm is covered on the matrix surface while 30-nm Pd particles are plugged into the defects within the matrix. The H2/N2 permselectivity has been significantly increased due to the deposition of Pd nanoparticles in the Si-MFI membrane, which could keep stable below the critical temperature. This bifunctional membrane and similar membranes possess the application potential to achieve various one-step syntheses. I. Introduction Palladium-loaded zeolites have been widely utilized to intensify a variety of catalytic reactions. The application of Pdloaded zeolites in catalysis arises by considering the shapeselectivity of zeolitic pores or the bifunctional nature of the composites. For examples, Kranich et al. employed silicalite-1 or ZSM-5 supported Pd catalysts in acetylene hydrogenation to minimize the formation of byproduct under the space constraint of zeolitic pores.1 Over SAPOs (SAPO-5, -11, and -34) supported Pd catalysts, hydrocarbons, especially C2-C4 alkanes, were yielded from CO hydrogenation via a bifunctional pathway.2 The hydroisomerization of n-heptane to dibranched isomers was enhanced on CoAPO supported Pd catalysts in both catalytic activity and selectivity.3 The hydroisomerization of n-octane on Pd/β-zeolite also occurred via both hydrogenatingdehydrogenating and isomerization functions.4 One-step synthesis of methyl isobutyl ketone was achieved using AlPO4-11 or SAPO-11 supported Pd catalysts to displace the traditional three-step manufacturing process.5 Recently, the Pd/zeolite catalysts were applied in microreactors for hydrogenation of o-nitroanisole to o-anisidine for process intensification.6 In the viewpoint of practical applications, more benefits would be expected for Pd-loaded zeolite films and membranes. As a typical example, the film of zeolite-Y loading Pd nanoparticles fabricated via self-assembly on the (3-mercaptopropyl)trimethoxysilane-modified Au electrode exhibited higher electrocatalytic activity for oxidation of adsorbed CO than the pure zeolite-Y film.7 It has been well recognized that the Pd membranes possess the application potential for hydrogenselective separation processes.8-10 The thin membranes of Pdloaded zeolites with selective permeability to hydrogen should * Authors to whom correspondence should be addressed. Tel.: +86 22 27405165. Fax: +86 22 87898959. E-mail: [email protected] (Y.Z.), [email protected] (B.Z.). † State Key Laboratory of Chemical Engineering, Tianjin University. ‡ State Oceanic Administration. § State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology.

be advantageous to intensify a hydrogen-related catalytic reaction in the membrane reactor.10,11 The fabrication of Pd-loaded zeolite membranes can be performed through self-assembling postsynthetic Pd-loaded zeolite crystals.7 However, the membranes synthesized as such may contain large crystal boundaries, resulting in low separation efficiencies.12 On the other hand, depositing Pd nanoparticles onto targeted zeolite membranes is another route to acquire the Pd-loaded zeolite membranes. Impregnation has been the most widely used method of preparing Pd/molecular sieve bifunctional catalysts. However, some problems exist for this traditional method.13 In general, it is difficult to obtain nanosized particles with high dispersivity. When a liquid solution is used as the processing medium, the agglomeration of particles and the collapse of fragile matrices inevitably occur due to the high surface tension of the liquid solution. Besides, the metal-support interaction is weak. Nevertheless, these problems can be conquered by using chemical fluid deposition (CFD) involving the chemical reduction of a metal organic precursor dissolved in a supercritical fluid. The properties of a supercritical fluid can be adjusted by slight change in temperature and/or pressure. Supercritical carbon dioxide is particularly attractive for a variety of applications because it is chemically inert and nontoxic, leaving no residue in the treated medium.14 As a pioneering work, the continuous films of nanosized Pd particles were synthesized at controlled depths within porous alumina substrates by chemical fluid deposition in supercritical carbon dioxide (scCO2).15 In this case, the trapped film made of nanosized Pd particles demonstrated satisfactory mechanical strength, durability, and toleration to operational temperatures. Due to the use of hydrogen as the reducing agent, the Pd layer within the porous substrate could be concentrated in a relatively narrow region. On the contrary, the well-dispersed Pd nanoparticles within a porous substrate might be acquired using a weak reducing agent to allow Pd precursors available everywhere within the matrix prior to reduction. Therefore, we are going to deposit Pd nanoparticles into polycrystalline zeolite membranes through opposing reactants chemical fluid deposition (ORCFD) using

10.1021/ie100655m  2010 American Chemical Society Published on Web 08/10/2010

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Figure 1. (a) Top-view and (b) cross-sectional-view SEM images of the h0h-oriented Si-MFI membrane after the second-time seeded growth.

at room temperature.19 Furthermore, it was reported that alcohols including methanol and ethanol could assist the reduction of Cu(β-diketonate)2 during chemical fluid deposition.20 Therefore, ethanol, a relatively weak reducing agent, was selected in this experiment to achieve well-dispersed and nanosized Pd particles inside the matrix. The reduction of Pd(hfac)2 that contains the functional group β-diketonate using ethanol as the reducing agent may proceed as20 Pd(hfac)2 + CH3CH2OH f Pd0 + 2F3C(β - diketonate)CF3 + CH3COH Figure 2. Pore size distribution in the h0h-oriented Si-MFI membrane.

an organometallic palladium as the Pd precursor and ethanol as the reducing agent. The Pd precursors and the reducing-agent dissolved in scCO2 get contact on the surface and within the targeted zeolite membrane via counter-diffusion configuration followed by the reduction of Pd precursors. II. Experimental Section II.1. Materials. Carbon dioxide, nitrogen, and hydrogen (99.95%, BOC Gases Co., Ltd.) were used without further purification. Palladium hexafluoroacetylacetonate (Pd(hfac)2, 99%, Aldrich) was employed as the Pd precursor due to its satisfactory solubility and stability in scCO2. In this study, the h0h-oriented pure silica MFI (Si-MFI) membrane supported on the R-Al2O3 substrate was used as the matrix to which the deposition of Pd nanoparticles is conducted. The h0h-oriented Si-MFI membrane (the matrix), fabricated by performing the seeded growth twice, is continuous and uniform (Figure 1a), the thickness of which is ca. 10 µm (Figure 1b).16,17 The permoporometry-based method was used to evaluate the pore size distribution of the used h0h-oriented Si-MFI membrane.18 According to the relationship between the He permeance and the relative vapor pressure of n-hexane in the feed stream (P/PS), the pore size distribution within the h0h-oriented Si-MFI membrane was further calculated. As shown in Figure 2, the pore size of the membrane ranged from ca. 0.4 to 12 nm, and is concentrated at ca. 0.8 nm. II.2. Pd Deposition onto the Matrix. A concentrated Pd membrane could be fabricated inside the porous substrate using hydrogen as the reducing agent due to its strong ability to reduce Pd ions in scCO2.15 On the contrary, a weak reducing agent such as methanol was the prerequisite for the formation of spherical aggregates of Pd nanoparticles that had been selforganized in the solution of palladium acetate and a cubic linker

The particular details of this surface reaction are not known at present. One possibility involves the formation of an intermediate being reacted from ethanol and Pd(hfac)2, which are further self-reduced to metallic Pd. Another possibility is that ethanol is oxidized to aldehyde and hydrogen, which is used to reduce Pd(hfac)2. Actually, the introduced ethanol acts as the reducing agentsit is used to enhance the precursor solubility in scCO2 as well. The Pd deposition within the matrix includes (1) making Pd precursors/scCO2 and ethanol/scCO2 get contact on the surface or inside the defects of the matrix by introducing them from the two sides, and (2) dispersion of Pd precursors and ethanol accompanied by Pd deposition as indicated in Figure 3. In contrast to hydrogen, ethanol molecules would not react with Pd precursors immediately when meeting each other, allowing enough time to form well-dispersed Pd precursors and ethanol within the matrix. Pd deposits could grow outward on the surface of the matrix after the internal defects are filled with Pd particles. The thickness and position of the well-dispersed region mainly depend on the concentration and feed rate of both Pd precursors and ethanol, and operational temperature and pressure. II.3. Apparatus and Operation. The continuous ORCFD system used in this experiment is schematically shown in Figure 4. The reservoir of Pd precursors with a volume of 80 cm3 and the membrane module were made of stainless steel. A small magnetic bar was employed in the reservoir to ensure a uniform concentration of Pd precursors. The two chambers of the module were separated by the matrix with O-rings seals. The pressure in each chamber could be controlled by a back-pressure regulator. Two syringe pumps together with two mass flow controllers were used to continuously deliver Pd precursors/ scCO2 and ethanol/scCO2 with controlled flow rates. The reservoir of Pd precursors was controlled at 60 °C, while the membrane module was kept at 80 °C using a water jacket heater.

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Figure 3. Deposition of Pd nanoparticles onto the Si-MFI membrane: (1) supported Si-MFI membrane as the matrix, (2) introduction of the Pd precursor and the reducing agent to the matrix, (3) Pd deposition onto the matrix. A: zeolite membrane, B: R-Al2O3 substrate.

Figure 4. Flowsheet of experimental setups: 1. CO2 cylinder, 2. ethanol tank, 3. cooling unit, 4. syringe pumps, 5. pressure-reducing regulator, 6. mass flow controllers, 7. stop valves, 8. reservoir, 9. stir and heater, 10. the matrix and permeation module, 11. back-pressure regulators, 12. water jacket heater.

Initially, an excessive amount of Pd(hfac)2 (ca. 80 mg) was added into the reservoir, followed by the introduction of pure CO2 passing through the whole system to exhaust air. Then, the whole system was pressurized and heated to the required temperatures. It was necessary to increase the pressure in each chamber to ca. 10 MPa, and to purge with CO2 synchronously and slowly. Afterward, the reservoir of Pd precursors was isolated for a couple of hours by shutting off the two attached stop valves to dissolve Pd(hfac)2. Furthermore, the scCO2 stream saturated with Pd precursors was continuously fed into the chamber with the side of the Si-MFI membrane in a constant flow rate using the syringe pump after opening the stop valves. Meanwhile, the other chamber was fed with pure scCO2 at the same flow rate. Approximately 30 min was allowed for Pd precursors to diffuse well within the matrix before alcohol/scCO2 was introduced. In this experiment, the amount of Pd precursors is excessive, only a small quantity of Pd precursors could be dissolved at first. Since a steady-state flow of fresh scCO2 was supplied, the remaining Pd precursors in the reservoir would be dissolved unceasingly to ensure a constant concentration of Pd precursors/scCO2 in the chamber. The Pd deposition process lasted for 10 h. II.4. Gas Permeation Measurements. The single-gas permeation test was carried out. The permeation fluxes of hydrogen and nitrogen through the matrix and the resulting Pd/Si-MFI membrane were measured under 303 and 373 K with a prescribed pressure difference. The matrix or the Pd/Si-MFI membrane was positioned in the stainless steel module and sealed by a PTFE O-ring on each end, the top-layer of which was facing to the permeating gas. As the steady state was reached, the flow rate of permeated gas was measured at atmospheric pressure and ambient temperature with a soap-

bubble flow meter. The single-gas permselectivity was defined as the flux ratio of hydrogen to nitrogen under the same pressure difference and temperature.16,17 II.5. Instrumentation. The morphology of the matrix and the Pd/Si-MFI membrane was investigated by scanning electron microscopy (SEM, Philips Xl30ESEM) at an accelerating voltage of 20 kV. Pd deposit close to the surface was analyzed by X-ray diffraction (XRD, Rigaku D/max 2500v/pv) using Cu KR radiation. Electron probe microanalysis (EPMA, Shimadzu EPMA-1600) was employed to determine Si and Pd profiles along the cross-section of the Pd/Si-MFI membrane. Before EPMA measurements, the cross-section of the Pd/Si-MFI membrane was polished with SiC sandpaper (grit sized 2000) in water and ultrasonicated in alcohol to eliminate possible powders. The size of Pd particles in the membrane was observed through transmission electron microscope (TEM, Tecnai G2 F-20) operated at an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by scratching small particles from the Pd/Si-MFI membrane surface and crosssection respectively, followed by dispersion in ethanol and deposition onto a carbon-coated copper grid. The elemental distributions on the Pd/Si-MFI membrane surface and the crosssection area were measured using the Philips XL30ESEM equipped with an Oxford-Isis300 energy dispersive X-ray (EDX) spectrometer. III. Results and Discussion The surface morphology of the Pd/Si-MFI membrane is illustrated in Figure 5. Compared with the matrix shown in Figure 1a, a rough Pd top-layer in black is formed on the matrix surface after Pd deposition. Direct observation indicates that

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Figure 5. Top-view SEM image of the Pd/Si-MFI membrane.

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Figure 7. Qualitative EPMA elemental analysis in the Pd/Si-MFI membrane. SEM image and elemental profiles in normalized atomic signal intensity vs distance of the cross-section of the Pd/Si-MFI membrane for the area between the two white lines. Blue: Pd; pink: Si.

Figure 8. TEM images of dispersed Pd with Cu grid for the sample collected from the top layer (a) and the cross-section (b). Figure 6. XRD patterns of R-Al2O3 substrate (A), h0h-oriented Si-MFI membrane (B), and Pd/Si-MFI membrane (C). The asterisks mark the peaks originating from the R-Al2O3 substrate.

Pd particles are fully covered on the surface of the matrix. According to XRD measurements given in Figure 6, the characteristic diffraction peaks corresponding to the matrix (h0horiented Si-MFI membrane) and the R-Al2O3 substrate are significantly reduced after Pd deposition, and simultaneously the palladium face-centered cubic phase is identified from Pd (111) and Pd (200) peaks (Figure 6C), indicating the formation of a continuous metallic Pd layer on the surface of the matrix. The elemental profiles for both Pd and Si along the cross section of the Pd/Si-MFI membrane were measured using the EPMA technique. The SEM image and the elemental composition profile along the cross section of the 10-µm thick membrane are given in Figure 7, where the Pd and Si contents along the cross-sectional distance for the span between the two white lines are plotted in blue and pink curves, respectively. Figure 7 shows that the Pd/Si-MFI membrane mainly consists of Si element. The deposited metallic Pd is well distributed along the crosssection of the membrane from the substrate at ca. 65 µm deep to the outer surface of the membrane where the Pd concentration reaches its maximal value. This observation is coincides with the SEM and XRD measurements as shown in Figures 5 and 6. The TEM observation is employed to check whether the deposited Pd particles are nanosized. The TEM image of the sample scratched from the outer surface of the Pd/Si-MFI membrane indicates the particles are uniformly dispersed with an average size of ca. 10 nm (Figure 8a). The EDX data shown in Figure 9a prove that the top layer contains only Pd together with a little C and O elements. Therefore, the Pd/Si-MFI membrane is covered with a layer of 10-nm sized Pd particles, consistent with the EPMA measurement in Figure 7. The TEM

image and corresponding EDX data of the sample scraped from the cross section of the Pd/Si-MFI membrane are shown in Figures 8b and 9b. It is demonstrated that Pd particles with size of ca. 30 nm are dispersed on ca. 100-nm sized silica ellipsoidal pellets or sandwiched between neighboring silica ellipsoidal pellets, indicating that Pd nanoparticles have been plugged into the defects of the matrix. It should be noted that the EDX spectrum contains strong characteristic C and Cu peaks due to the sample preparation. The single-gas permeation fluxes through the matrix and the Pd/Si-MFI membrane (as-synthesized and annealed) were measured with respect to both H2 and N2, the results of which are shown in Figure 10. For the matrix and the Pd/Si-MFI membrane, the single-gas permeation flux is linearly proportional to the transmembrane pressure difference. The Pd deposition leads to a significant reduction in the single-gas permeation flux because the defects in the matrix have been filled with Pd nanoparticles and a Pd top layer has been formed as well. Based on the fluxes of H2 and N2, the corresponding H2/N2 permselectivities are calculated and supplied in Figure 11. The H2/N2 permselectivity of the matrix is ca. 3.5 at both 303 and 373 K, however, it is increased to 45 at 303 K and to 60 at 373 K after Pd deposition. Besides, the permselectivity of the Pd/Si-MFI membrane is decreased after being annealed. We suspect that the deposited Pd nanoparticles grow bigger when annealing is applied. To check the stability and durability of the Pd/Si-MFI membrane, the single-gas permeation was performed at 373 K for 48 h by alternately using hydrogen and nitrogen. It has been demonstrated that the resulting Pd/Si-MFI membrane could remain stable below the critical temperature (566 K),8 and almost the same hydrogen and nitrogen fluxes were observed.

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Figure 9. Elemental distributions of the Pd/Si-MFI membrane: (a) top-layer analysis, (b) cross-sectional analysis.

Figure 10. Hydrogen and nitrogen fluxes through the Si-MFI and Pd/Si-MFI membranes (as-synthesized and annealed) plotted against transmembrane pressure difference at 303 and 373 K.

analyses demonstrate the Pd layer on the matrix surface consists of 10-nm sized Pd particles while the 30-nm sized Pd particles are plugged into the defects within the matrix. The single-gas permeation measurements with respect to hydrogen and nitrogen show that the H2/N2 permselectivity through the Pd/Si-MFI membrane has been significantly increased due to the deposition of Pd nanoparticles. The resulting Pd/Si-MFI membrane could remain stable below the critical temperature. The methodology established is applicable to the deposition of Pd nanoparticles in porous matrices to fabricate various bifunctional membrane materials. Acknowledgment Figure 11. H2/N2 permselectivity of Si-MFI and Pd/Si-MFI membranes plotted against tranmembrane pressure difference at 303 and 373 K.

With no doubt, the availability of this type of bifunctional membranes will trigger their applications in the one-step synthesis of a variety of chemical products. IV. Conclusions We have fabricated the Pd/Si-MFI membrane through opposing reactants chemical fluid deposition using Pd(hfac)2 and ethanol as the Pd precursor and the reducing agent. Pd(hfac)2 and ethanol dissolved in scCO2 are first introduced into the SiMFI membrane (the matrix) via counter-diffusion configuration, and well-dispersed Pd precursors are further reduced to metallic Pd particles. SEM and XRD measurements together with EPMA data confirm that a continuous metallic Pd layer has been formed on the matrix surface, and simultaneously Pd particles are well distributed within the matrix due to the use of a weak reducing agent. Furthermore, TEM observations associated with EDX

This work is supported by the National Natural Science Foundation of China (Grants 20636030 and 20776108), the Natural Science Foundation of Tianjin (06YFJZJC01400) and Taishan Scholars Program of Shandong Province, China (ts20081119). Literature Cited (1) Kranich, W. L.; Weiss, A. H.; Schay, Z.; Guczi, L. Acetylene hydrogenation using palladium zeolite catalysts. Appl. Catal. 1985, 13, 257– 267. (2) Thomson, R.; Montes, C.; Davis, M. E.; Wolf, E. E. Hydrocarbon synthesis from CO hydrogenation over Pd supported on SAPO molecular sieves. J. Catal. 1990, 124, 401–415. (3) Hochtl, M.; Jentys, A.; Vinek, H. Acidity of SAPO and CoAPO molecular sieves and their activity in the hydroisomerization of n-heptane. Microporous Mesoporous Mater. 1999, 31, 271–285. (4) de Lucas, A.; Ramos, M. J.; Dorado, F.; Sa´nchez, P.; Valverde, J. L. Influence of the Si/Al ratio in the hydroisomerization of n-octane over platinum and palladium beta zeolite-based catalytsts wth or without binder. Appl. Catal. A-Gen. 2005, 289, 205–213. (5) Yang, S. M.; Wu, Y. M. One-step synthesis of methyl isobutyl ketone over palladium supported on AlPO4-11 and SAPO-11. Appl. Catal., A 2000, 192, 211–220.

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ReceiVed for reView March 17, 2010 ReVised manuscript receiVed July 27, 2010 Accepted July 28, 2010 IE100655M