Ind. Eng. Chem. Res. 1992,31,2633-2635
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KINETICS AND CATALYSIS Sol-Gel-Derived Nickel/?-Alumina Catalyst for a Methanol-to-Hydrogen Converter Masayuki Watanabe,' Tatsuya Okubo,*il Katsuki Kusakabe, and Shigeharu Morooka Department of Chemical Science and Technology, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812, Japan
Nickel/ boehmite composite gels are prepared from stable boehmite sols of different nickel compositions. By firing the gels, nickelly-alumina catalysts are prepared, and its catalytic activity in methanol decomposition (CH30H CO 2Hz) is studied. The catalytic performance strongly depends on the reduction temperature and the nickel content. At lower reduction temperature the dehydration of methanol to dimethyl ether is mainly observed. As the reduction temperature is raised, the dehydration activity is reduced, and the methanol decomposition is enhanced. The temperature where hydrogen evolved significantly decreases as the nickel content increases.
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1. Introduction Methanol is one of the most hopeful candidates as an energy transport carrier for the future. The conversion of methanol to hydrogen and the reverse will be, therefore, an important chemical process. In this sense a membrane reactor having a function of methanol decomposition as well as produced hydrogen separation is strongly expected. CHSOH 2H2 +CO (1) CHSOH + H2O 3& + CO2 (2) The concept of this reactor, which we call a methanol-tohydrogen converter, is illustrated in Figure 1. In the inorganic membrane preparation techniques reported so far, the sol-gel method is advantageoussince thin membranes with pores on the scale of nanometers can be prepared (Leenaars and Burggraaf, 1985; Asaeda and Du, 1986; Okubo et al., 1990). Alumina membrane is the most popular among the sol-gel-derived ones since the alumina thin film is formed on the support with fewer defects than silica or titania. We studied the performance of the membrane reactor consisting of the thin membrane and the catalyst particles (Okubo et al., 1991). We are also interested in an introduction of a catalytic activity to a sol-gel-derived thin membrane. In view of the nanostructural control, the sol-gel method with a sol including catalytic species is preferable to the postimpregnation of the catalyst into an inert membrane. In another paper (Okubo et al., in press) we demonstrated that a homogeneous nickel-alumina composite sol and gel could be prepared and that the nanostructure of the gel was finely controllable via a modification of sol by nickel nitrate. In this paper the catalytic activity of the sol-gel-derived nickel/ y-alumina is studied in methanol decomposition (1). +
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* All correspondence should be addressed to this author.
t Present address: Ceramics Research Laboratory, Onoda Cement Co. Ltd., 2-4-2, Osaku, Sakura, Chiba 285, Japan. Present address: Engineering Research Institute, The University of Tokyo, 2-11-16, Yayoi, Bunkyo-ku, Tokyo 113, Japan.
*
+
Table I. Catalytic Reaction Condition reactor: quartz glass tube amount of catalyst catalyst size methanol concn at inlet reduction temperature reaction temperature
6 mm i.d. 0.16 g
149-210 pm 15-18% 573-873 K
573 K
2. Experimental Procedure 2.1. Sample Preparation. Nickel, active in methanol decomposition,was introduced to boehmite sol prepared by Yoldas' procedure (Yoldas, 1975). The preparation scheme of nickel/boehmite (AlOOH) sol and ita gel are as follows (Okubo et al., in press). Nickel nitrate was dissolved in distilled water, and aluminum isopropoxide was added at 353 K. Nitric acid was used as a peptizer, the molar ratio to aluminum being fixed at 0.1. The molar ratio of nickel to aluminum was changed from 0 to 0.30. The gel was obtained by evaporating the water and was fired at 773 K. A reference catalyst was prepared by impregnation. Nickel nitrate was impregnated to y-alumina obtained by firing boehmite particles at 773 K. 2.2. Thermal Analysis. The composite gels were characterized by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The measurements were carried out at a rate of 5 K/min in a nitrogen stream. Prior to the analpis the residual water was removed at 393 K in the instrument. 2.3. Catalysis. After firing at 773 K, the catalyst particles of 149-210 pm in size were packed in a silica glass tube of 6-mm i.d. and were reduced in a pure hydrogen stream for 1h. The reduction temperature were changed from 573 to 873 K. The samples were then tested on methanol decomposition (1) in the same reactor. The reaction condition is summarized in Table I. Methanol diluted with nitrogen was fed at 573 K to the catalyst. The reaction products were analyzed by gas chromatography. 3. Results and Discussion 3.1. Thermal Analysis. The DTA curves are illustrated in Figure 2. Boehmite, Ni/Al = 0, showed an
0888-5885/92/2631-2633$03.00/00 1992 American Chemical Society
2634 Ind. Eng. Chem. Res., VoL 31,No. 12.1992 CH,OH
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CH,OH
CH,OH + H,O
-co
catalyn
-
- CO + 2 4
,
-
co,
calalyst
membrane
CH,OH
+ H,O
- CO* + 3H,
Figure 1. conesptual illustration of methanol-tehydmgen con-
Figure a. = 0.10.
,I
. 4 , , , , 600 700 8M) 900 Reduction Temperature [Kl Cbaugm m product with reduction tanpera!am at W/Ai 0
H,
1
verter.
k N i l 4 (mol I mol
c
I
.... ........ 5
_..--.
lMPREGNATlON
......*
ReductionTemperature 1Kl
't
... 300
"........-......
.... ... .................. ...-.. 0.00
400
500 800 700 Temperature M
800
Pigam 2. DTA cur?= of mmpoaits gels.
endothermic change to y-alumina a t about 700 K (Leenaars et aL, 1984). As the nickel content increased, this peak shiftad to the lower temperature d d i o n and overlapped with the peak caused by the demmpcaition of nickel nitrate to nickel oxide. The peak due to the decomposition was rather vague a t Ni/M = 0.10 but was dearly observed at Ni/AI = 0.20 and 0.30. Similar resulta were obtained also from TGA. The crystal phase was determined by X-ray diffraction (XRD) with Cu Kor radiation. Peaks due to 7-alumina were observed for boehmite of Ni/AI = 0 when the firing temperature was higher than 723 K. On the other hand, the boehmite in Ni/AI = 0.30 was converted into y-alumina at 623 K,which indicated that nickel addition enhanced the dehydration of boehmite to y-alumina. These correspond with the results obtained from the thermal d y w a as mentioned above. The peak due to nickel oxide was obtained only for the sample of Ni/AI = 0.30. On the other hand, the peaka were observed at Ni/AI = 0.10 for the impregnated sample. 3.2. Catalysis. Figure 3 shows the changes in the reaction product with the reduction temperature a t Ni/AI = 0.10. At lower reduction temperatures the dehydration of methanol to dimethyl ether (3)is mainly observed, and 2CH30H CH,0CH3 + H20 (3)
+.
only a amall amount of hydrogen is obtained. The contribution of y-alumina was teated without nickel loading, and it was shown that the dehydration was mainly c a d
Figure 4. Changw m selectivity of hydrogen with duction tamprature for catalwta prepared hy the &-gel method and impregnation.
by the support as reported by Mizuno et aL (1981). As the reduction temperature is raised, the dehydration activity is reduced, and the methanol decomposition is, on the contrary, enhaucd. This tendency was observed at Ni/AI = 0.05,0.10,0.20,and 0.30. The changes in the selectivity of hydrogen with the reduction temperature is illustrated in Figure 4. The temperature where hydrogen evolved decreases as the nickel content increases. The changes in the selectivity of hydrogen for nickelimpregnated catalysts are also shown in Figure 4. Hydmgen was evolved at a much lower reduction temperature although the manner of the change was B i m i to that of eol-gel-derivedcatalyst. Wu and Hercules (1979)studied the state of impregnated nickel and concluded that nickel showed little interaction with silica but strong interaction with y-alumina The present results of catalysis as well as thermal analysis and XFtD indicate that impregnated nickel has a weaker intaraction with the support than that in the sol-gel-derived catalyst. On the basis of the results mentioned above, it is suggeated that the state of nickel at higher loadingsis different from that at lower loadings. In another work (Okubo et al., in press) we demonstrated that the agglomerationin a sol as well as the reduction of the pore size in the oxide was e n h a n d by nickel addition only at Ni/M < 0.10-0.16. On the contrary, the catalytic activity was mainly improved at the higher loadings. The nickel at lower loadings would have a strong interaction with the support and be effective in the nanostructural control. On the other hand, nickel at higher loadings would weakly bond to the surface and be catalytically active in the methanol decomposition. 4. Conclusion
The catalytic activity of sol-gel-derived nickel/y-aluAs the
mina strongly depended on the nickel content.
Ind. Eng. Chem. Res. 1992,31, 2635-2642
nickel content increased, the temperature where hydrogen evolved decreases. On alumina support the sol-gel processing was preferable to the impregnation in view of nanostructural control, while the impregnation was advantageous in view of catalytic performance. Based on the alumina membrane, it is important to reduce the interaction between the catalyst and the support membrane.
Acknowledgment This work was partially supported by the Ministry of Education, Science and Culture, Japan (Grant No. 01750879),and the Nippon Sheet Glass Foundation for Materials Science. Registry No. H2, 1333-74-0; MeOH, 67-56-1; Ni, 7440-02-0; A1203, 1344-28-1; MeOMe, 115-10-6; Al(OH)O, 24623-77-6.
Literature Cited Asaeda, M.; Du, L. D. Separation of Alcohol/Water Gaseous Mixtures by Thin Ceramic Membrane. J. Chem. Eng. Jpn. 1986,19, 12-17.
Leenaare, A. F. M.; Keizer, K.; Burggraaf, A. J. The Preparation and Characterization of Alumina Membranes with Ultra-Fine Pores, Part 1 Microstructural Investigations on Non-Supported Mem-
2635
branes. J . Mater. Sci. 1984, 19, 1077-1088. Leenaars, A. F. M.; Burggraaf, A. J. The Preparation and Characterization of Alumina Membranes with Ultra-Fine Porea, 2. The Formation of Supported Membranes. J. Colloid Interface Sci. 1986,105,27-40.
Mizuno, K.; Iwasaki, Y.;Hsing, C. T.; Suzuki, M. Decomposition of Methanol over Alumina Supported Rhodium Catalysta. Nenryokyokakhi 1981,60,836-841. Okubo, T.; Watanabe, M.; Kusakabe, K.; Morooka, S. Preparation of y-Alumina Thin Membrane by Sol-Gel Processing and Ita Characterization by Gas Permeation J. Mater. Sci. 1990, 25, 4822-4827.
Okubo, T.; Haruta, K.; Kusakabe, K.; Morooka, S.; Anzai, H.; Akiyama, S. Equilibrium Shift of Dehydrogenation at Short SpaceTime with Hollow Fiber Ceramic Membrane. Znd. Eng. Chem. Res. 1991,30,614-616. Okubo, T.; Watanabe, M.; Kusakabe, K.; Morooka, S. Nanoetructural Control of Sol-Gel Derived Porous Alumina via Modification of Sol. J. Mater. Sci. Lett., in press. Wu, M.; Hercules, D. M. Studies of Supported Nickel Catalysta by X-ray Photoelectron and Ion Sputtering Spectroscopies. J. Phys. Chem. 1979,83, 2003-2008. Yoldas, B. E. Alumina Sol Preparation from Alkoxides. Am. Ceram. SOC.Bull. 1975,54,28%290.
Receiued for review June 8, 1992 Accepted Auguet 28,1992
A Medium-Temperature Process for Removal of Hydrogen Sulfide from Sour Gas Streams with Aqueous Metal Sulfate Solutions Robert R. Broekhuis, David J. Koch, and Scott Lynn* Department of Chemical Engineering and Lawrence Berkeley Laboratory, Uniuersity of California, Berkeley, California 94720
A process is proposed for removing hydrogen sulfide from coal gas a t ita adiabatic saturation temperature. The coal gas would be contacted with a metal sulfate solution in a Venturi scrubber, leading
to the formation of a solid metal sulfide. This metal sulfide would be oxidized with ferric ion, forming sulfur and regenerating the metal sulfate solution. Ferrous ion would be reoxidized with air. Experimental work was done on the absorption of hydrogen sulfide into zinc and copper sulfate solutions. The rate of absorption into zinc sulfate is a strong function of pH and is unsatisfactory a t the pH proposed for this process. The rate of absorption into copper sulfate solutions is high over a large range of pH and may be modeled as an instantaneous reaction with liquid- or gas-phase mass transfer controlling. Calculations indicate that a Venturi scrubber can reduce hydrogen sulfide to a concentration below 10 ppm using a copper sulfate solution. The oxidation of zinc and copper sulfide with ferric ion at temperatures 80-150 "C yielded conversions as high as 99%; however, complete conversion of either sulfide to elemental sulfur was not achieved.
Introduction A promising method of producing clean power from coal couples coal gasification and a combustion turbine. In the coal gasification process, coal is partially oxidized with air or oxygen and reacted with water vapor at high temperature. This reaction produces a gas consisting mostly of carbon monoxide, hydrogen, carbon dioxide, and water. Under the reducing conditions of the gasifier, the sulfur content of the coal is converted to hydrogen sulfide. This is a corrosive gas that forms sulfur dioxide upon combustion. To prevent corrosion of the blades of the combustion turbine to which the coal gas is fed and to prevent emission of sulfur-containing compounds to the environment, the hydrogen sulfide must be removed before the coal gas enters the turbine. As the treatment temperature decreases, there is a loss in power plant efficiency. A hydrogen sulfide removal process operating at an elevated temperature is therefore deeirable. When the hot fuel gas is quenched with water, the loss in efficiency is minimized
since the increased mass flow due to water vaporization partially offsets the lower temperature. This paper describes a process designed to carry out the removal of hydrogen sulfide from coal gas already quenched with water to the adiabatic saturation temperature of the coal gas, typically ca. 200 O C . In this process elemental sulfur is formed as a product. In the first step of the process, the coal gas is contacted with an aqueous solution of a metal sulfate. Hydrogen sulfide is absorbed into the solution and reacta with the metal ion to form an insoluble metal sulfide. The resulting metal sulfide slurry is then reacted with ferric ion to produce sulfur and ferrous ion and to regenerate the original metal sulfate. The sulfur is separated as a salable product. The ferous ion is oxidized with air or oxygen to regenerate ferric ion. A schematic of the process is shown as Figure 1. In the first aqueous scrubber the coal gas is quenched to the adiabatic saturation temperature and particulates are removed. A substoichiometricamount of
0888-5885/92/2631-2635$03.00/00 1992 American Chemical Society
