Porous noble metal/ceramic catalytic membranes ...

BULLETIN

Porous noble metal/ceramic catalytic membranes prepared by modification of the surface of sol particles ZHAO Hongbin, LI Anwu, GU Jinghua, SHENG Shishan and XlONG Guoxing * State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics. Chinese Academy of Sciences, Dalian 116023, China intarctlaa at liqoid/sdid interface, catalytic memhmne, membrane reactor.

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AN impetus has been given to the investigation of high-temperature membrane reactors since asymmetric y-A2O3 membrane was commercially available in the early 1980s. Since then many catalytic reactions including dehydrogenation, hydrogenation, oxidation, etc. have been intensively demonstrated with inorganic membrane reactors[']. The driving forces behind the interest in this topic continue to be the use of inorganic membranes to carry out important separations and also to catalyze reactions. For an equilibrium-limited reaction, the membrane is *

To whom correspondence shodd be addressed.

Chinese Science Bulletin

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used to remove one of the products from the reaction zone as they form, thus shifting the equilibrium and leading to a higher conversion in a single pass. On the other hand, the membrane is used to keep bulk reactants separated while allowing the controlled addition of the active reactant to the reaction zone, improving the product selectivity of the reaction where parallel or sequential by-product reactions are a problem. There are basically two types of inorganic membranes which can be used for membrane reactors: dense and porous membranes. According to catalysis, inorganic membranes can also be classified into active and inert membranes. For catalysis applications, catalytically active phases can be deposited in an inert inorganic membrane by physical vapor deposition, chemical vapor deposition, electriless plating[21, impregnationr3]and i o n - e ~ c h a n ~ e ' ~ ] . In this study, the modification of the surface of sol particles was proposed to prepare porous noble metal/ceramic catalytic membranes['] . The membrane integrity was examined by gas permeation measurements. It should be emphasized that the location of the active phase was determined.

I Experimental 1 . 1 The procedure for determination of adsorption percentage of the metal ions versus pH curves Fifteen samples of 0.300 0 g 7-A1203.particleswere weighed and put into conical flasks, respectively. Then a given volume of distilled water was added to the above samples. These samples were aged for about 24 h in order to hydrate the surface of y-A1203 particles. And then a given volume of the standard solution containing the metal ions was added to the conical flasks in which the concentration of the metal ions attained mol/L and the total volume of each sample was 100 mI-. The pH values of the samples were adjusted to the pH range of 110 with diluted acid and base, and the pH difference of the two neighboring samples remained a pH unit of 0 . 5 or so. The samples were shaken at room temperature for about 24 h in order to attain the chemical equilibrium of the adsorption of the metal ions on y-A1203particles. And finally the pH values of the samples were measured using a Cole Parmer 5986-50 pH-meter. The solution was separated by centrifugation and the concentration of the metal ions was analyzed by the spectrophotometic method. The content of the adsorbed metal ions can be calculated by subtraction of the equilibrium concentration of the metal ions from the total concentration of the added metal ions. The adsorption percentage is defined as the ratio of the adsorbed metal ions to the added metal ions. The adsorption percentages of the metal ions were plotted against the corresponding pH values. 1 . 2 Preparation of noble metal/ceramic catalytic membranes The preparation of noble metal/ceramic catalytic membranes is illustrated in fig. 1. AlOOH sol was produced by peptization of the boehmite powders[61. Adsorption of the metal ions on A l 0 0 H sol particles was employed to obtain the metal ion-modified AlOOH sol. The casting sol consisted of the metal ion-modified AlOOH sol ( 0 . 5 mol/L), polyvinylalcohol (PVA) and polyethylenglycol (PEG) (2-5 % (weight percentage)). PVA and PEG can enhance the strength of the gel-coating to prevent crack formation during drying and calcination. Using spin-coating, the sol was deposited onto the flat-shaped a-A1203 ceramic substrate of 1 .6-pm average pore size and 48 % porosity. The coating was dried at 5 C and 65 % relative humidity for two days, followed by calcination at 600'C for 3 h with the heating and cooling 8 18


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rate being smaller than 1°C /min.

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AIOOH suspension

1

Organic additives

Fig. 1 . The schematic of preparation of noble metal/ceramic catalytic membranes.

2 Results and discussion

2 . 1 The modification of AlOOH sol by adsorption of the metal ions The effect of ligands on the adsorption of Pd( II ), Pt( IV ) and Rh( 1 ) on 7-A1203particles as a function of pH value was investigated in order to find the metal complex-ions for modification of AlOOH sol. AlOOH sol remains dynamically stable in an aqueous solution of 3-4 pH. Therefore, adsorption of the .5- 8" metal complex-ions is required to take place 5 51 60 significantly in a pH range of 3-4. The modified AlOOH sol sill remains dynamically B stable, and thereafter can be used to produce 40 the catalytic membranes by sol-gel method. Jt As an example, the result of Pd( II ) is shown c in fig. 2. From fig. 2, the adsorption percentages of P ~ E D T A- ~and Pd ( NH3 ): + in 0 I I I I I I I I , the pH range of 3-4 attained more than 1 2 3 4 5 6 7 8 9 1 0 85 % . In the same way, it was found that adPH sorption of ~ t c 1 ; - and R ~ C G - became signifiFig. 2. Adsorption of Pd( I1 ) as a function of pH valw of cant at a pH value of 3-4. suspension. 1 , Pd( U ) X I ; 2, Pd( U )-NH,;3, Pd( U )-EDThe surface of y-A1203 particles was hyTA. drated in an aqueous solution, and the lyogel at liquid/solid interface was developed["]. The surfacial hydroxy-groups of Y-A1203particles are comparable to those of AlOOH sol particles. Therefore, adsorption of the metal complexions on 7-A1203 particles are similar to that on AlOOH sol particles. Thus the above results were used for the modification of AlOOH sol by adsorption of the metal ions. On the basis of the above results, Pd ( NH3 )$+ was used to produce Pd ( I1 )-modified AlOOH sol. Quasielastic light scattering showed that the Pd( I1 )-modified AlOOH sols up to 4 % (weight percentage) (Pd/y-A1203) exhibited a narrow particle size distribution and an average particle size of 60 nm. It needs to be mentioned that the sols still kept their dynamic stability. It can be said that adsorption of the metal ions can be used for modification of AlOOH sol.

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2 . 2 Preparation of noble metal/ceramic catalytic membranes The SEM micrograph of the Pd/y-A1203 membrane prepared by six-time dipping is given in fig. 3 . As shown in fig. 3, a uniform layer of Pd/y-A1203 with 2 0 - ~ mthickness was Chinese Science Bulletin

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Fig. 3 .

The SEM micrograph of the Pd/y-AI2O3 membrane produced by six-time dipping.

deposited on the substrate, and the Pdly-A1203layer adhered to the substrate well. The integrity of the Pd/y-M203 mem7,-. branes prepared by three-time dipping was ex6 amined by gas permeation measurements, and the result is shown in fig. 4 . From fig. 4, the i permeation rates of helium and nitrogen re1.21 mained unchanged as the transmembrane pressure increased. According to the theory of gas transport through porous medium, the transportation of helium and nitrogen through the membrane took place in Knudsen diffusion mechanism. It shows that the gas-pass of the membrane is smaller than the mean free path of I I I

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helium and nitrogen. Therefore, the Pd/yA1203 membrane prepared by three-time dip-

ping can be defect-free. The desorption isotherm of nitrogen at 77 K showed that the unsupported Pd/y-A1203 membrane showed a narrow pore size distribution and an average pore size of 6 nm. The location of palladium in the Pdly-M203membrane produced by six-time dipping was analyzed by scanning electron microscope-energy dispersive X-ray (SEM-EDX), and the result Fig. 4 . Gas permeation rate as a function of transmembrane pressure. 0, Nitrogen; 0 , helium.

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is shown in fig. 5 . The SEM micrograph is on the right side, the EDX result on the left side and the white dots represent the palladium distribution. As seen in fig. 5, palladium was mainly dispersed in the 7-A1203layer rather than the substrate. This is because Pd( II ) was fixed on the surface of AlOOH sol particles by adsorption before the sol-casting, and prevented from infiltration to the substrate during the sol-casting.

Fig. 5 .

The SEM-EDX micrograph of the Pdly-A1203 membrane produced by six-time dipping.

(Received August 12, 1996)

References 1 Armor, J . N. , Catalysis with permselective inorganic membranes, Applied Catalysis A: General, 1989, 49: 1 . 2 Shu, J . , Grardjean, B. P. A . , Van ~ e s t e ,A. et a1 . , Catalytic palladium-based membrane reactors: A review, The Canadian J . Chemical Engineering, 1991, 69: 1036. 3 Uhlhorn, R . J . R . , Zaspalis, V. T . , Keizer, K . et a l . , Synthesis of ceramic membranes. part U . modification of alumina thin film: reservoir method, J . Material S c i . , 1992, 27: 538. 4 Uzio, D. , Peureux, J . , Giroir-Fendler, A. et a l . , Platinumly-AlnOs catalytic membrane: preparation, morphological and catalytic characterization, Applied Catalysis A; General, 1993, 96: 83. 5

Xiong, G . X . , Zhao, H. B. , Li, A. W. et a l . , An in-situ method of modification of the surface of sol particles, Chinese Patent , 951 13920. 7, 1995-12-05.

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Xiong, G . X. , Zhao, H. B. , Lu, M . C . , Novel catalytic materials from sol-gel technique, 1 . preparation and characterization of stable and monodispersive AlOOH sol, J . Petrochemical Industry, 1994, 23 : 1 .

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Stumm, W . , Morgan, J . J . , Aquatic Chemistry, 2nd ed., New York: John Wiley & Sons Inc., 1981, 204.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 29392003 and No. 29273134).


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May 1997