An environmental friendly catalyst for the high temperature shift reaction

An e n v i r o n m e n t a l f r i e n d l y c a t a l y s t for t h e h i g h t e m p e r a t u r e

s h i f t r e a c t i o n

G. C. Araujo a n d M. C. Rangel

Instituto de Quimica, Universidade Federal da Bahia, C a m p u s UniversitY_rio de Ondina, Federa~gto. 40170-280 Salvador, Bahia, Brazil

The need for environmental protection h a s d e m a n d e d the searching non- toxic catalysts easily handled and discarded. This work deals with the use of a l u m i n u m as a s u b s t i t u t e of chromium, in iron- a n d copper-based catalysts for the high t e m p e r a t u r e shift reaction. Catalysts were prepared in the commercial form (hematite) and was evaluated in more severe operational conditions t h a n the u s u a l industrial one (e.g. s t e a m to carbon ratios, R, lower t h a n 0.6). It w a s found that a l u m i n u m can replace c h r o m i u m in these catalysts leading to better catalytic properties as compared to chromium.

The catalyst h a s the advantage of being non toxic a n d can work in low

a m o u n t s of steam.

I. INTRODUCTION

The production of hydrogen from the n a t u r a l gas or n a p h t h a feedstock is usually increased by m e a n s of the water gas shift reaction (WGSR) [1]:

CO(g) + H20(g} ~ CO2 + H20(~) AH = - 4 1 . 1 kJ.mo1-1

which is also an i m p o r t a n t stage in the commercial production of hydrogen. This reaction is a reversible and exothermic one and t h u s is favored by low t e m p e r a t u r e s a n d excess of steam. However, it requires high t e m p e r a t u r e s to achieve rates high enough for industrial applications. Therefore, the WGSR is often carried out in two steps, the first being performed in the range of 320-450oC (named high t e m p e r a t u r e shift, HTS). In the second stage (known as low t e m p e r a t u r e shift, LTS), carbon monoxide is removed from the feed s t r e a m in thermodynamically favorable conditions in the range of 200-250~ [ll.

The HTS stage is often carried out over catalysts of c h r o m i u m - d o p e d iron oxides, available as hematite. The catalyst is reduced in situ to produce magnetite which is found to be the active phase. In a m m o n i a plants, the

reduction is performed with a gaseous mixture of carbon monoxide and

dioxide, hydrogen and nitrogen. This reaction is highly exothermic and should be carefully controlled to prevent damage to the reactor or to the catalyst. The production of metallic iron should be particularly avoided since it may catalyze the hydrocarbon formation [1,2]. In order to a s s u r e the magnetite stability in industrial processes, large a m o u n t s of steam are used. However, this procedure increases the operational costs and t h u s new

s y s t e m s t h a t do not generate metallic iron, even at low c o n t e n t s of steam, are d e m a n d e d [3].

In spite of the c h r o m i u m - d o p e d h e m a t i t e catalysts have shown high stability in performance, they have been recently modified by the addition of small a m o u n t s of copper, resulting in even more active a n d selective catalysts. However, b e c a u s e of e n v i r o n m e n t a l restrictions c o n c e r n i n g the discarding of c h r o m i u m c o m p o u n d s , the search for non-toxic catalysts which could be easily h a n d l e d a n d discarded is m u c h needed.

With this goal in mind, this work deals with the r e p l a c e m e n t of c h r o m i u m by a l u m i n u m in HTS catalysts. In order to save the energy related to the s t e a m c o n s u m p t i o n , the catalysts were also evaluated in more severe operational conditions t h a n the industrial one, e.g. lower s t e a m to carbon ratios.

2. E X P E R I M E N T A L

The r e a g e n t s u s e d were analytical grade.

The catalysts were p r e p a r e d by copreciptation t e c h n i q u e s at room t e m p e r a t u r e , followed by h e a t i n g at 500~ for 2h u n d e r nitrogen flow (100 ml. min-1). Four s a m p l e s were prepared: (i) with a l u m i n u m a n d copper (HAC sample); ( i i ) w i t h only a l u m i n u m (HA sample); (iii) with only copper (HC sample) a n d (iv)without any d o p a n t (H sample). For all s a m p l e s an iron to the d o p a n t molar ratio of 10 was used.

The a l u m i n u m and c o p p e r - b a s e d catalyst was p r e p a r e d by adding a q u e o u s solutions of Fe(NO3)3.9H20(1N) a n d AI(NO3)a.9H20(0.1N) a n d a c o n c e n t r a t e d (25% w/w) a q u e o u s solution of a m m o n i u m hydroxide to a beaker with water, u n d e r stirring. The final pH was a d j u s t e d to 11 a n d the system was kept u n d e r stirring for additional 30 min. The sol produced was centrifuged {2000 rpm, 5 min) followed by a water rinsing to remove the nitrate ions from the starting material. After a second centrifugation step, the gel was i m p r e g n a t e d with an a q u e o u s solution of Cu(NO3)2.3H20 (0.06N) for 24h u n d e r stirring, centrifuged again a n d dried in an oven at 120~ The s a m e procedure were used to prepare the other samples. For the catalysts without copper, the gel was kept in p u r e water for 24h, u n d e r stirring, in order to simulate the s a m e experimental conditions u s e d to prepare the other samples.

The qualitative analysis of nitrate was performed by adding a b o u t 1 ml of c o n c e n t r a t e d sulfuric acid to 10 ml of the s o b r e n a d a n t after centrifugation. The formation of [Fe(NO)I 2§ was detected by a brown ring [4]. The a b s e n c e of nitrate in the solid was confirmed by infrared spectroscopy in the range of 4 0 0 0 - 6 5 0 cm -1 using a S h i m a d z u model IR-430 spectrometer a n d KI discs. The metal c o n t e n t s were d e t e r m i n e d by inductively coupled p l a s m a atomic emission by using an model Arl 3410 equipment. X-ray diffractograms were recorded at room t e m p e r a t u r e with a S h i m a d z u model XD3A i n s t r u m e n t u s i n g Cu Kcz radiation generated at 30 kV a n d 20 iliA. The surface a r e a (BET method) was m e a s u r e d in a Micromeritics model TPD/TPO 2900 e q u i p m e n t on s a m p l e s previously heated u n d e r nitrogen (150~ 2h). The t e m p e r a t u r e

p r o g r a m m e d r e d u c t i o n (TPR)was performed in the s a m e e q u i p m e n t , u s i n g a 5% H2/N2 mixture. The copper a r e a w a s m e a s u r e d in a CG-2001 e q u i p m e n t u s i n g the N20 pulse t e c h n i q u e [5,6]. X-rays m i c r o a n a l y s i s were performed in a Noran microprobe coupled to a model JSM-T300 microscope operating at 20-30KV.

The catalyst p e r f o r m a n c e w a s evaluated u s i n g 0.2 cm 3 of powder within - 50 a n d + 325 m e s h size a n d a fixed bed microreactor consisting of a stainless tube, providing there is no diffusion effect. All e x p e r i m e n t s were

carried out u n d e r i s o t h e r m a l condition (370~ a n d at a t m o s p h e r i c pressure,

employing a gas m i x t u r e with composition a r o u n d 10% CO, 10 % CO2, 60% H2 a n d 20% N2. It w a s u s e d several s t e a m to gas molar ratios: 0.2, 0.4 a n d

0.6. The g a s e o u s effluent w a s analyzed by on line gas c h r o m a t o g r a p h y ,

u s i n g a CG-35 i n s t r u m e n t . A commercial catalyst, b a s e d on c h r o m i u m , copper a n d iron oxides, w a s u s e d to c o m p a r e the p e r f o r m a n c e of the s a m p l e s prepared in this work. In order to determine the role of copper in these catalysts, a m e c h a n i c a l mixture of the sample with a l u m i n u m (HA sample) a n d copper oxide w a s also evaluated. In this case, the catalyst tests were r u n at 200~ b e c a u s e of the high t e n d e n c y of copper of sintering at 370oC [1]. After each experiment, the Fe(II) c o n t e n t in the c a t a l y s t s w a s d e t e r m i n e d to follow the iron r e d u c t i o n u n d e r reaction a t m o s p h e r e . S a m p l e s were dissolved in c o n c e n t r a t e d chloridric acid, u n d e r carbon dioxide a t m o s p h e r e a n d t h e n titrated with p o t a s s i u m d i c h r o m a t e [7].

3. R E S U L T S AND D I S C U S S I O N

As s h o w n in Figure 1, h e m a t i t e w a s detected in the flesh c a t a l y s t s while magnetite w a s found in the u s e d catalysts; a l u m i n u m t e n d s to impair crystallization w h e r e a s copper t e n d s to favor it. No other p h a s e besides h e m a t i t e a n d m a g n e t i t e w a s detected.

The p r e s e n c e of either a l u m i n u m or the two d o p a n t s increased the surface a r e a of the solids (Table 1), which w a s not affected by doping the solid with copper alone. For the u s e d catalysts, a l u m i n u m increased the

surface a r e a while copper decreased it. The c h a r a c t e r i s t i c s of the s p e n t

c a t a l y s t s did not show a n y significant d e p e n d e n c e on the s t e a m to gas

molar ratio (R) u s e d in the test reaction. The catalyst r e d u c t i o n strongly decreased the surface area, showing t h a t the p h a s e c h a n g e is followed by a coalescence of particles a n d / o r porous. A l u m i n u m i n c r e a s e d the copper a r e a s showing t h a t this metal prevents copper sintering. The a l u m i n u m presence on the surface w a s confirmed by X-ray microanalysis.

The TPR curves of h e m a t i t e (Figure 2) showed one p e a k at 510oC, due to magnetite formation a n d a n o t h e r a r o u n d 770oC, a t t r i b u t e d to the formation of metallic iron [8]. In the a l u m i n u m - d o p e d sample, the first p e a k w a s

shifted to lower t e m p e r a t u r e s (400~ w h e r e a s the higher t e m p e r a t u r e p e a k

w a s not affected; this behavior s h o w s t h a t this metal e a s e s the formation of

the active p h a s e b u t does not affect its stability. The copper-doped

copper [9] a n d other p e a k s a r o u n d 230 a n d 600oC; these TPR p e a k s show t h a t copper favors the active p h a s e formation a n d destruction. The a l u m i n u m a n d copper-doped solid showed a curve with the c h a r a c t e r i s t i c p e a k due to magnetite formation displaced to 300oC a n d the p e a k related to metallic iron at 770 oC. It m e a n s t h a t the synergetic effect of the d o p a n t s is

>,, t - c - HAC HC HA H 5 HACR - - - l i t . . . . i . . . 9 . . . i . . . I . . . i . . . 9 . . . i . . . i . . . a . . . n . . . i . . . n . . . 9 . . . 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 20 (degrees) 20 (degrees)

(a)

Figure 1. X-ray diffractograms of (a) fresh a n d (b) u s e d catalysts. H= hematite; A = a l u m i n u m a n d C = copper

Table 1

Surface a r e a {Sg) and copper a r e a of the s a m p l e s Sample H HA HC HAC Sg (m2.g -I) (before reaction) 27 59 26 86 R = s t e a m / p r o c e s s gas (molar) Sg (m2.g -1) (after reaction) R=0.6 R=0.4 R=0.2 8 9 9 24 25 28 6 4 6 17 22 24 Copper a r e a (fresh catalysts) ---- __ 39 44

to ease magnetite formation b u t not to affect the production of metallic iron. The commercial catalyst showed a TPR curve with p e a k s a r o u n d 270, 340 and 750~ showing t h a t c h r o m i u m is less effective in prevent the active p h a s e formation a n d destruction.

All c a t a l y s t s were active to HTS reaction, a s s h o w n in Table 2. A l u m i n u m leads to a slight i n c r e a s e in activity, w h i c h is d u e to a t e x t u r a l effect. On the o t h e r h a n d , c o p p e r i n c r e a s e d both the activity a n d the activity per area, showing t h a t it a c t s a s a s t r u c t u r a l promoter. The c o p p e r a r e a values, a s

well a s the TPR r e s u l t s , s u g g e s t t h a t the h i g h e r activity of the copper

p r o m o t e d c a t a l y s t is d u e to a n i n c r e a s e of the n u m b e r of active sites; it is well k n o w n t h a t c o p p e r is also active in the HTS reaction [1,2]. However, the m e c h a n i c a l m i x t u r e of the s a m p l e with a l u m i n u m {HA) a n d c o p p e r oxide (3x10 -4 mol.g-lh -~) s h o w e d a lower activity t h a n the HAC s a m p l e {7x10 -4 mol. g-~.h -I) at 200oC, s u g g e s t i n g t h a t there is p r o b a b l y a n electronic effect of copper. "-7. :3 v C 0 . I E C 0 0 7- Commercial ~ HAC . . . . k J k . . . " . . . " . . . . 9 II HC I, ti

...'"" "'".

(~

Figure 2. TPR catalysts. H = hematite; A = a l u m i n u m a n d C = c o p p e r

The e x p e r i m e n t s carried out at lower s t e a m to gas ratio s h o w e d t h a t the c a t a l y s t s c a n w o r k at R=0.4 w i t h o u t a n y d a m a g e to both activity or selectivity to c a r b o n dioxide. At R=0.2, however, the activity a n d selectivity strongly d e c r e a s e d with the a m o u n t the steam. The a n a l y s i s of the u s e d c a t a l y s t s s h o w e d t h a t this behavior w a s likely d u e to the lower Fe(II)/Fe(III) ratio, w h i c h m e a n s a lower a m o u n t of the active sites. Therefore, one can a s s u m e t h a t u n d e r lower R larger a m o u n t s of metallic iron w a s p r o d u c e d a n d h y d r o c a r b o n formation w a s catalyzed.

The c a t a l y s t with b o t h d o p a n t s showed higher activity t h a n a c h r o m i u m a n d c o p p e r - d o p e d c o m m e r c i a l catalyst (25x10 -4 mol.g-l.h-1). This sample p r o d u c e s the active p h a s e more easily t h a n the o t h e r catalysts, shows r e s i s t a n c e to a f u r t h e r m a g n e t i t e r e d u c t i o n a n d c a n w o r k at severe conditions, t h a t is, low s t e a m to p r o c e s s gas ratio.

Table 2

Catalytic activity (a) a n d selectivil

Sample axl04 (mol.g-l.h -1) R=0.6 R=0.4 R=0.2 H 7 6 5 HA 9 8 6 HC 24 23 16 HAC 34 34 17

R = s t e a m to gas molar ratio

selectivity (S) of the s a m p l e s a / S g x l 0 s (mol.m-2. h-~) R=0.6 R=0.4 R=0.2 9 7 6 4 3 2 40 58 27 20 15 7 Selectivity (%) R=0.6 R=0.4 R=0.2 50 50 20 60 20 10 80 60 60 70 70 80 4. C O N C L U S I O N S

The presence of both a l u m i n u m a n d copper in h e m a t i t e - b a s e d catalysts to HTS reaction favors the formation a n d stability of the active phase. A l u m i n u m acts as textural p r o m o t e r a n d copper acts as a s t r u c t u r a l one. The a l u m i n u m , copper-doped h e m a t i t e catalyst is more active t h a n a commercial catalyst. Therefore, a l u m i n u m is a promising d o p a n t to replace c h r o m i u m in HTS catalysts since it shows two i m p o r t a n t features: first, it is non toxic, in c o m p a r i s o n to the high c h r o m i u m toxicity, a n d second it leads to better HTS catalyts as c o m p a r e d to the c h r o m i u m - d o p e d ones.

A C K O W L E D G E M E N T

The a u t h o r s t h a n k the financial s u p p o r t from CNPq, FINEP and

CNPq/PADCT.

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