24 Evaluation of Some New Zeolite-Supported Metal Catalysts for Synthesis Gas Conversion GORDON A . MELSON, JANET E . CRAWFORD, JAMES W. CRITES, and K E T C H A J. M B A D C A M
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Virginia Commonwealth University, Department of Chemistry, Richmond, V A 23284 JOHN M . STENCEL and V . U D A Y A S. RAO U.S. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, PA 15236
Series of zeolite-supported iron-containing catalysts with weight percent iron (% Fe) varying from ~1 to ~17% Fe have been prepared from Fe (CO) and the synthetic zeolites ZSM-5, mordenite and 13X by an extraction technique. The zeolites ZSM-5 and mordenite were used i n the acid form, 13X i n the sodium form. The catalysts were characterized by a variety of techniques including infrared spectroscopy, X-ray powder diffractometry, X-ray photoelectron spectroscopy, ion-scattering spectrometry and Mössbauer spectroscopy. All catalysts contain highly dispersed, small p a r t i cle-sized γ-Fe O with a small amount of the iron (0.6-1.5% Fe depending on the support) located i n the pores of the support. Evaluation of the catalytic a b i l i t y of some of these materials for synthesis gas conversion was conducted at 280°C and 300°C by using a fixed-bed continuous flow microreactor. A l l catalysts evaluated produce significant quantities of hydrocarbons. The distribution of hydrocarbons varies, depending upon the support used, for catalysts with similar weight percent iron. For the liquid hydrocarbons, Fe/ZSM-5 produces the highest percentage of aromatics, Fe/mordenite produces the highest percentage of olefins, and Fe/13X produces the highest percentage of saturates. The effect of support acidity and pore structure on hydrocarbon product distribution i s discussed. 3
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The use of b i f u n c t i o n a l m e t a l / z e o l i t e c a t a l y s t s f o r the conv e r s i o n of s y n t h e s i s gas (carbon monoxide and hydrogen) t o gasol i n e range hydrocarbons has r e c e n t l y a t t r a c t e d much a t t e n t i o n . F o r example, the combination o f metal oxides w i t h the medium pore (-6A) z e o l i t e ZSM-5 and the use of a metal n i t r a t e impregnated ZSM-5 c a t a l y s t have been shown t o produce g a s o l i n e range hydrocar-
0097-6156/83/0218-0397$06.00/0 © 1983 American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
INTRAZEOLITE
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bons c o n t a i n i n g a h i g h percentage of aromatics from s y n t h e s i s gas (1-3). The e f f i c i e n c y and s e l e c t i v i t y of a supported metal c a t a l y s t is c l o s e l y r e l a t e d to the d i s p e r s i o n and p a r t i c l e s i z e of the metal component and to the nature of the i n t e r a c t i o n between the metal and the support. For a p a r t i c u l a r m e t a l , c a t a l y t i c a c t i v i t y may be v a r i e d by changing the metal d i s p e r s i o n and the support; t h u s , the method of s y n t h e s i s and any pre-treatment of the c a t a l y s t is Important i n the o v e r a l l process of c a t a l y s t e v a l u a t i o n . Supported metal c a t a l y s t s have t r a d i t i o n a l l y been prepared by i m pregnation techniques that i n v o l v e treatment of a support w i t h an aqueous s o l u t i o n of a metal s a l t followed by c a l c i n a t i o n (4). In the Fe/ZSM-5 system, the decomposition of the i r o n n i t r a t e during c a l c i n a t i o n produces a - F e 2 Û 3 of r e l a t i v e l y l a r g e c r y s t a l l i t e s i z e (>100 1). T h i s study was i n i t i a t e d i n an attempt to produce h i g h l y - d i s p e r s e d , thermally s t a b l e supported metal c a t a l y s t s that are e f f e c t i v e f o r s y n t h e s i s gas c o n v e r s i o n . The carbonyl Fe3(CO)^2 was used as the source of i r o n ; the supports used were the a c i d i c z e o l i t e s ZSM-5 and mordenite and the n o n - a c i d i c , l a r g e r pore z e o l i t e , 13X. Experimental The z e o l i t e supports were c a l c i n e d i n a i r p r i o r to u s e . ZSM5 ( S i 0 / A l 0 3 - 30) and mordenite ( S i 0 2 / A l 0 3 , 11.4) were used i n the a c i d form (H-ZSM-5, Η-Mord), and 13X ( S i 0 / A l 0 « 1) used i n the sodium form. S e r i e s of z e o l i t e - s u p p o r t e d i r o n c a t a l y s t s of weight percent i r o n (% Fe) v a r y i n g from -1% to ~17% Fe were p r e pared from Fe3(CO)i2 and the c a l c i n e d z e o l i t e s w i t h cyclohexane as solvent by an e x t r a c t i o n technique developed i n our l a b o r a t o r y ( 5 ) . A l l m a t e r i a l s were c h a r a c t e r i z e d by i n f r a r e d spectroscopy, X-ray powder d i f f r a c t o m e t r y (XRPD), X - r a y photoelectron s p e c t r o scopy (XPS), i o n - s c a t t e r i n g spectrometry (ISS), and Mossbauer spectroscopy. The weight percent of i r o n was determined by atomic a b s o r p t i o n spectroscopy. For the c a t a l y t i c a c t i v i t y e v a l u a t i o n , s e l e c t e d m a t e r i a l s were pressed i n t o 1/8 i n c h diameter p e l l e t s , reduced under H2 at 300 p s i g (20.7 bar) and 4 5 0 ° C f o r 24 hours, and then t r e a t e d w i t h synthesis gas ( H / C 0 , 1/1) at 100 p s i g (6.9 bar) and 250°C f o r 24 hours. C a t a l y t i c data were obtained by u s i n g a f i x e d - b e d c o n t i n u ous flow m i c r o r e a c t o r . Conditions employed were 300 p s i g (20.7 bar) pressure of 1/1 s y n t h e s i s gas, GHSV -1000 h " and r e a c t o r temperatures of 280°C and 3 0 0 ° C . The products obtained over a 48 to 72 hour p e r i o d from the microreactor were analyzed i n two f r a c tions. The gaseous e f f l u e n t , i n c l u d i n g C1-C4 hydrocarbons, was analyzed by use of a gas chromatograph that is an i n t e g r a l part of the m i c r o r e a c t o r . The l i q u i d product was c o l l e c t e d i n an i c e cooled trap l o c a t e d a f t e r a heated ( 1 7 5 ° C ) t r a p to c o l l e c t h i g h molecular weight (wax) p r o d u c t s . The l i q u i d product was separated i n t o o i l and aqueous f r a c t i o n s ; a n a l y s i s of the o i l f r a c t i o n was 2
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Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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New Zeolite-Supported Metal Catalysts
accomplished by FIA chromatography on s i l i c a g e l (6) lated d i s t i l l a t i o n (7).
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R e s u l t s and D i s c u s s i o n Characterization I n f r a r e d s p e c t r a of the prepared m a t e r i a l s do not show the presence of ν(CO) bands; t h u s , it is concluded t h a t decomposition of F e 3 ( C 0 ) accompanies a d s o r p t i o n onto the supports d u r i n g the p r e p a r a t i v e procedure. For c a t a l y s t s w i t h low (
