Solid State Chemistry in Catalysis - American Chemical Society

18 Alkali-Promoted Copper-Zinc Oxide Catalysts for Low Alcohol Synthesis

Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 20, 2013 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch018

G. A. VEDAGE, P. B. HIMELFARB, G. W. SIMMONS, and K. KLIER Center for Surface and Coatings Research and Department of Chemistry, Lehigh University, Bethlehem, PA 18015

Alkali and barium hydroxide promotion of the Cu/ZnO methanol synthesis catalysts has been investigated with the following results: (i) at low temperatures, high H /CO ratios, water-free and CO -free synthesis gas, the alkali promote methanol synthesis rates in the order Cs>Rb>K>Na>Li; (ii) the alkali hydroxides are highly dispersed on the catalyst surface; ( i i i ) Ba hydroxide agglomerates and shows little effects; (iv) at temperatures above 280°C and low H /CO ratios, the synthesis of higher alcohol is promoted by the alkali via a mechanism in which the addition of C intermediate on the beta carbon of the growing alcohol chain dominates; and (v) methyl esters are side products of the higher alcohol synthesis. A method is given for a quantitative treatment of X-ray photoelectron spectra for the determination of the surface concentration of the dopant. 2

2

2

1

It has been known since the 1930's that the addition of a l k a l i to high temperature ( 4 0 0 - 4 5 0 ° C ) , high pressure (200-250 atm) methanol synthesis catalysts promotes the synthesis of higher alcohols and oxygenates from carbon monoxide and hydrogen. One of the e a r l i e s t studies was conducted by Morgan et a l . (1) using Ο^Οβ/ΜηΟ catalysts. These investigators concluded that the most effective promoters for higher oxygenates were K, Rb and Cs with the l a t t e r being the most active. They postulated the formation of higher alcohols to occur by successive processes of aldolation of the C-^ intermediate with other aldehydic intermediates, followed by p a r t i a l dehydration and hydro génation. Graves (2), on the other hand, proposed that higher a l c o hols originate by a d i r e c t dehydrocondensation of two lower alcohol molecules. Natta et a l . (3) investigated the promotion effect of a l k a l i on higher alcohol synthesis using several catalysts derived from calcined smithsonite, ZnCO^. These catalysts were found to exhibit poor s t a b i l i t y , p a r t i c u l a r l y those containing copper. The high temperature, high pressure catalysts are of l i t t l e pract i c a l interest today because of the extreme conditions required for 0097-6156/85/0279-0295$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


296

S O L I D STATE C H E M I S T R Y IN CATALYSIS

t h e i r use. The more r e c e n t l y developed methanol s y n t h e s i s c a t a l y s t s o p e r a t e under m i l d e r c o n d i t i o n s a t temperatures below 300°C and p r e s s u r e below 100 atm. The e f f e c t s o f a l k a l i promoters on the s y n t h e s i s o f h i g h e r a l c o h o l s over the low p r e s s u r e - l o w temperature methanol s y n t h e s i s c a t a l y s t s have not been s t u d i e d e x t e n s i v e l y . Only r e c e n t l y , Smith and Anderson (4) have s y s t e m a t i c a l l y s t u d i e d the e f f e c t of K2CO3 on the Cu/ZnO/Al203 c a t a l y s t i n the s y n t h e s i s o f h i g h e r a l c o h o l s . T h i s potassium-promoted c a t a l y s t produced h i g h e r a l c o h o l s and hydrocarbons w i t h a d i s t r i b u t i o n s i g n i f i c a n t l y d i f f e r e n t from t h a t of F i s c h e r - T r o p s c h s y n t h e s i s . Smith and Anderson developed a m a t h e m a t i c a l model of the a l c o h o l c h a i n growth i n c l u d i n g b r a n c h i n g w h i c h d i d not r e q u i r e a d e t a i l e d knowledge of the mechanism. A l t h o u g h such models have p r o v e n u s e f u l i n c h a r a c t e r i z i n g and i n p r e d i c t i n g the p r o d u c t c o m p o s i t i o n over a s i n g l e c a t a l y s t , i t i s s t i l l d e s i r a b l e to determine the c h e m i c a l m e c h a n i s t i c f e a t u r e s i n o r d e r t o f u r n i s h the c a t a l y s t w i t h s u r f a c e dopants t h a t p e r f o r m a s p e c i f i c m e c h a n i s t i c f u n c t i o n a t an optimum r a t e . The p r i n c i p a l o b j e c t i v e o f t h i s study was t o a d d r e s s the mechan i s t i c q u e s t i o n s i n c l u d i n g the r o l e of the d i s t r i b u t i o n and c h e m i c a l s t a t e o f the a l k a l i by i n v e s t i g a t i n g the i n f l u e n c e o f d i f f e r e n t a l k a l i h y d r o x i d e s on the a c t i v i t y and s e l e c t i v i t y o f a l o w - t e m p e r a t u r e , l o w - p r e s s u r e methanol c a t a l y s t . The 30/70 Cu/ZnO methanol s y n t h e s i s c a t a l y s t t h a t has been p r e v i o u s l y c h a r a c t e r i z e d i n terms of a c t i v i t y , s t a b i l i t y , c o m p o s i t i o n , morphology and s t r u c t u r e ( 5 , 6 ) was chosen f o r t h i s i n v e s t i g a t i o n . The p r o d u c t d i s t r i b u t i o n s were determined f o r each o f the a l k a l i - d o p e d c a t a l y s t s a t 250°C and 288°C w i t h d i f f e r e n t r a t i o s o f H2/CO and were compared w i t h the p r o d u c t d i s t r i b u t i o n o b t a i n e d w i t h undoped c a t a l y s t under the same t e s t i n g c o n d i t i o n s . To i d e n t i f y the p r e c u r s o r s t h a t may be i n v o l v e d i n the c h a i n growth t o h i g h e r a l c o h o l s , d i f f e r e n t a l c o h o l s were added t o the H2/CO s y n t h e s i s g a s . The y i e l d o f the a l c o h o l s f o r w h i c h the added a l c o h o l i s a p r e c u r s o r i s e x p e c t e d to i n c r e a s e . The e f f e c t s o f CO2 i n the syngas on the a c t i v i t y and s e l e c t i v i t y of the most a c t i v e a l k a l i - d o p e d c a t a l y s t were a l s o d e t e r m i n e d . The a c t i v i t i e s and s e l e c t i v i t i e s o f the c a t a l y s t s were c o r r e l a t e d w i t h the c o m p o s i t i o n and m i c r o s t r u c t u r e o f the c a t a l y s t s as d e t e r mined by X - r a y powder d i f f r a c t i o n , a n a l y t i c a l s c a n n i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y and X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y . R e s u l t s and D i s c u s s i o n The s u r f a c e d o p i n g o f the 30/70 Cu/ZnO c a t a l y s t was performed a f t e r the c a t a l y s t was reduced i n 2% H2/N2 at 250°C by an a d d i t i o n and t o t a l e v a p o r a t i o n to dryness under f l o w i n g n i t r o g e n of an a l k a l i o r an a l k a l i n e e a r t h h y d r o x i d e s o l u t i o n . I n t h i s manner, specimens o f the 30/70 Cu/ZnO c a t a l y s t were doped w i t h O.4 atomic % o f L i O H , NaOH, KOH, RbOH, CsOH and B a ( 0 H ) and one specimen was doped w i t h O.2 a t . % Ba(0H)2« These samples were then charged i n t o the r e a c t o r under n i t r o g e n , t e s t e d , removed from r e a c t o r under n i t r o g e n and s u b j e c t t o X - r a y powder d i f f r a c t i o n a n a l y s i s . The XPS and e l e c t r o n m i c r o s c o p i c a n a l y s e s were c a r r i e d out on a i r - e x p o s e d samples. S i m i l a r analyses were c a r r i e d out on u n t e s t e d c a t a l y s t s . An i n c r e a s e i n the z i n c o x i d e and copper p a r t i c l e s i z e s due t o the doping by a l k a l i was apparent from the X - r a y powder d i f f r a c t i o n 2 >


18.

297

Alkali-Promoted Cu-ZnO Catalysts

VEDAGE ET AL.

r e s u l t s i n T a b l e T. F u r t h e r e v i d e n c e f o r change i n the p a r t i c l e s i z e s and m o r p h o l o g i e s of z i n c o x i d e and copper was found by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . F o r example, comparison of the dark f i e l d e l e c t r o n m i c r o g r a p h o f ZnO i n an undoped c a t a l y s t i n F i g u r e l a , w i t h a s i m i l a r m i c r o g r a p h of ZnO i n a KOH-doped c a t a l y s t i n F i g u r e l b , shows t h a t the dopant d i d i n d u c e growth o f ZnO p a r t i c l e s . These r e s u l t s were f u r t h e r c o r r o b o r a t e d by the BET s u r f a c e a r e a measurements w h i c h showed t h a t the t e s t e d a l k a l i - d o p e d c a t a l y s t s , except L i O H , 32.6 m / g , had s u r f a c e a r e a s between 20 and 23 m / g , compared to the B a ( 0 H ) doped c a t a l y s t s w i t h 28-29 m / g and the undoped t e s t e d c a t a l y s t w i t h 36.5 m / g . The p a r t i c l e s i z e measurements p r o v i d e a q u a l i t a t i v e e v i dence t h a t the z i n c o x i d e and the copper p a r t i c l e s have grown as a consequence of doping and t e s t i n g . 2

2

2

2


2

both

Table

I.

The P a r t i c l e Dimensions of ZnO and Cu as Determined from X-Ray D i f f r a c t i o n L i n e B r o a d e n i n g 3

Tested Catalyst

Dimension (nm) Zn0 Zn0

Zn0

Cu

Cu/ZnO (30/70)

13.4

15.9

13.0

9.5

LiOH/Cu/ZnO (30/70/O.4)

16.9

19.8

15.2

14.0

Cu/ZnO/NaOH (30/70/O.4)

23.2

20.5

17.4

17.6

Cu/ZnO/KOH (30/70/O.4)

23.1

24.8

20.8

13.7

Cu/Zn0/Rb0H (30/70/O.4)

25.2

26.5

22.0

13.7

Cu/ZnO/CsOH (30/70/O.4)

17.4

20.5

15.8

11.6

2

14.9

18.5

14.7

13.0

2

15.1

18.2

13.8

13.0

Cu/ZnO/Ba(OH) (30/70/O.4) Cu/ZnO/Ba(OH) (30/70/O.2) Λ

The p a r t i c l e s i z e s were c a l c u l a t e d u s i n g the S c h e r r e r e q u a t i o n i n the form t = O.89X/cos9/(w - w )^* where θ i s the Bragg a n g l e , w the h a l f - w i d t h o f the measured r e f l e c t i o n and w the i n s t r u m e n t a l h a l f - w i d t h i n r a d i a n s , c f . J . R. A n d e r s o n , " S t r u c t u r e o f M e t a l l i c C a t a l y s t s , " AP 1975, ρ 366. 2

2

x

Q

^ C o n c e n t r a t i o n s i n terms o f o v e r a l l atomic % ( C u / Z n O / a l k a l i ) given i n parentheses.

are

The s u r f a c e a n a l y s e s o f the c a t a l y s t s were performed i n the P h y s i c a l E l e c t r o n i c s Model 548 X-Ray P h o t o e l e c t r o n Spectrometer w i t h a 400W Mg X - r a y source i n the r e q u i r e d 50eV range w i t h 100 eV pass




298

F i g u r e 1. Dark f i e l d e l e c t r o n m i c r o g r a p h s of ZnO i n (a) a t e s t e d Cu/ZnO (30/70 mol%) c a t a l y s t and (b) a t e s t e d Cu/ZnO/KOH ( 3 0 / 7 0 / O.4 mol%) c a t a l y s t . The dark f i e l d images were o b t a i n e d u s i n g the ZnO(0002) r e f l e c t i o n ( 6 ) .


18.

VEDAGE ET AL.

299


energy a t a s c a n r a t e o f O.2eV/s. M u l t i p l e scans were averaged by a N i c o l e t Model 1072 m u l t i c h a n n e l a n a l y z e r and i n t e g r a t e d i n t e n s i t i e s were o b t a i n e d w i t h the use o f a Z e i s s MOP-3 a u t o m a t i c i n t e g r a t o r . The measured i n t e g r a t e d i n t e n s i t i e s were c o n v e r t e d t o s u r f a c e c o n c e n t r a t i o n r a t i o s by a method o u t l i n e d i n the A p p e n d i x . The r e s u l t s o f the XPS a n a l y s e s a r e summarized i n T a b l e I I . Table I I .


X-Ray P h o t o e l e c t r o n Data and A n a l y s e s

w

a

Catalyst Tested: Cu/ZnO

O.0

Cu/&nO/LiOH

o.oo

Cu/ZnO/NaOH

O.0640

O.22

Cu/ZnO/KOH

O.0045

O.24

Cu/ZnO/RbOH

O.0084

O.29

Cu/ZnO/CsOH

O.0662

O.22

O.0145

O.05

O.0151

O.05

Cu/ZnO

O.0

0

Cu/ZnO/NaOH

O.0247

O.09

Cu/ZnO/RbOH

O.0023

O.09

Cu/ZnO/CsOH

O.0138

O.05

O.0123

O.04

Cu/ZnOBa(OH)

2

Cu/ZnO/Ba(OH) (O.2) 2

0 c

0

Untested:

Cu/ZnO/Ba(OH)

a

2

The molar r a t i o s i n a l l c a t a l y s t s were Cu/ZnO/MOH = 3 0 / 7 0 / O . 4 e x c e p t f o r the undoped Cu/ZnO sample and the Ba(0H)o-doped sample l a b e l e d (O.2) w h i c h had the molar c o m p o s i t i o n 3 0 / 7 0 / O . 2 .

^ C a l c u l a t e d as o u t l i n e d i n Appendix u s i n g E q u a t i o n ( A - 5 ) . L i t h i u m was n o t d e t e c t e d because o f the v e r y low p h o t o i o n i z a t i o n cross s e c t i o n , c f . Table A - I .

s r i o w

The atomic s u r f a c e c o n c e n t r a t i o n r a t i o s ( X ^ / X z n ^ t h a t the a l k a l i a r e accumulated on the s u r f a c e i n the t e s t e d Cu/ZnO/MOH (M = Na,K,Rb,Cs) c a t a l y s t s i n approximately equal concentrations. These c o n c e n t r a t i o n s a r e near the expected ones i f a l l the a l k a l i i o n s a r e u n i f o r m l y spread i n t o a submonolayer as i s e v i d e n t from the f o l l o w i n g example: the atomic c o n c e n t r a t i o n o f C s , X ^ = X § g

where x|

n

= 1.1744 χ 10

19

S . Zn

n

Here ( x £ / x | ) i s the r a t i o o f s u r s

n


300


i y

2

face c o n c e n t r a t i o n s from T a b l e 2,1.1744 χ 1 0 atoms/m i s the s u r face d e n s i t y of z i n c atoms i n the (1010) ZnO face and S z i s the a r e a of z i n c o x i d e p e r gram o f c a t a l y s t , S = 10 m / g o f c a t a l y s t c a l c u l a t e d from the t o t a l measured BET a r e a 23 m / g of the t e s t e d Cu/ZnO/ CsOH c a t a l y s t and the a r e a o f c o p p e r , S = 13 m / g o f c a t a l y s t o b t a i n e d from the p a r t i c l e s i z e i n T a b l e 1 assuming h e m i s p h e r i c a l p a r t i c l e shape. The p a r t i c l e shape o f the ZnO c r y s t a l l i t e s i s complex (6) and f o r t h i s r e a s o n was not used i n the e s t i m a t e o f S . With x f = 1.1744 χ 1 0 atoms/g o f c a t a l y s t and ( X § / x | ) = O . 2 2 from Table I I , X Q = 2.58 χ Ι Ο ^ s u r f a c e Cs atoms p e r gram of c a t a l y s t . T h i s v a l u e i s c l o s e to the t o t a l amount o f cesium doped onto the c a t a l y s t , 3.17 χ 1 0 - ^ atoms p e r gram o f c a t a l y s t , and shows t h a t over 80% o f the cesium dopant i s found on the s u r f a c e . Nearly equal s u r face c o n c e n t r a t i o n s were found on the N a , K , R b , and C s - doped catalysts. On the c o n t r a r y , the s u r f a c e c o n c e n t r a t i o n s o f these e l e ments on c a t a l y s t s were found to be much s m a l l e r , i n d i c a t i n g t h a t the a l k a l i h y d r o x i d e s were f i r s t o c c l u d e d o r agglomerated imme d i a t e l y a f t e r the d o p i n g and then spread i n t o a n e a r l y a t o m i c a l l y dispersed surface overlayer during t e s t i n g . The b a r i u m h y d r o x i d e doped c a t a l y s t s showed much l o w e r s u r f a c e c o n c e n t r a t i o n of B a , how e v e r , even a f t e r t e s t i n g , and a g g l o m e r a t i o n of Ba(0H)2 i n t o l a r g e p a r t i c l e s was c o n f i r m e d by e l e c t r o n m i c r o s c o p i c e v i d e n c e i n F i g u r e 2 . The a l k a l i , on the o t h e r hand, were d i s p e r s e d on a s c a l e below the e l e c t r o n m i c r o s c o p e r e s o l u t i o n l i m i t o f c a . 5nm. The tendency o f the u n i v a l e n t i o n h y d r o x i d e s t o spread can be understood i n terms o f t h e i r low l a t t i c e energy compared to t h a t o f d i v a l e n t i o n h y d r o x i d e s , and i t i s c o n c l u d e d t h a t more e f f e c t i v e c a t a l y t i c e f f e c t s w i l l be g e n e r a l l y a c h i e v e d w i t h a l k a l i r a t h e r than a l k a l i n e e a r t h compound d o p i n g , p r i m a r i l y because of the a g g l o m e r a t i o n of the l a t t e r . A l t h o u g h the q u a n t i t a t i v e XPS a n a l y s e s may be burdened by a c o n s i d e r a b l e s y s t e m a t i c e r r o r ( 8 ) , the r e s u l t s summarized i n T a b l e I I demonstrate c o n v i n c i n g l y t h a t n e a r l y e q u a l s u r f a c e c o n c e n t r a t i o n s o f N a , K , R b and Cs were a c h i e v e d by the employed p r e p a r a t i o n method. The d i f f e r e n c e s i n s u r f a c e c a t a l y t i c e f f e c t s summarized below a r e t h e r e f o r e caused by the d i f f e r e n t n a t u r e o f the a l k a l i i o n s and not by t h e i r d i f f e r e n t surface concentrations. A l t h o u g h methanol was the dominant p r o d u c t over a l l the p r e s e n t l y s t u d i e d a l k a l i and a l k a l i n e e a r t h - d o p e d Cu/ZnO c a t a l y s t s as shown i n F i g u r e s 3 - 5 , the r e s u l t s demonstrate t h r e e e f f e c t s o f the a l k a l i p r o m o t e r s , namely: ( i ) i n c r e a s e o f the a c t i v i t y o f the Cu/ZnO c a t a l y s t f o r methanol p r o d u c t i o n ; ( i i ) change of the C02~dependence of the methanol c o n v e r s i o n ; and ( i i i ) change o f s e l e c t i v i t y i n f a v o r o f the C2-C4 a l c o h o l s and e s t e r s . The most s i g n i f i c a n t e f f e c t s were d i s p l a y e d by the CsOH-doped c a t a l y s t , i n d i c a t i n g t h a t b a s e - c a t a l y z e d r e a c t i o n s are b e i n g promoted i n the system. The p r o m o t i o n e f f e c t s i n the Ba(0H)2~doped c a t a l y s t s were s m a l l , most l i k e l y due to the a g g l o m e r a t i o n of B a ( 0 H ) p a r t i c l e s shown i n F i g u r e 2 , w h i c h r e s u l t e d i n a l o w s u r f a c e c o n c e n t r a t i o n o f barium as i n d i c a t e d i n Table I I , One o f the most s t r i k i n g e f f e c t s of the a l k a l i h y d r o x i d e s was the change of methanol c o n v e r s i o n a t 250°C i n carbon d i o x i d e - f r e e s y n t h e s i s gas summarized i n F i g u r e 3 and i n T a b l e I I I . The CsOH doping had the g r e a t e s t e f f e c t , w i t h a t w o - f o l d enhancement o f the s y n t h e s i s rate. The p r o m o t i o n e f f e c t s i n the s y n t h e s i s gas C 0 / C 0 / H = 0 / 3 0 / 7 0 f o l l o w e d the o r d e r n

2

Z n

2

2

C u

Z n

2 0

n

s

n

1


s

tested

untested

2

2

2



18.


VEDAGE ET AL.

301

F i g u r e 2. E l e c t r o n m i c r o g r a p h of a Ba(0H)2 p a r t i c l e i n the t e s t e d Cu/ZnO/Ba(OH) ( 3 0 / 7 0 / O . 4 mol%) c a t a l y s t . The i n s e t i n the lower l e f t c o r n e r i s the convergent beam (40 nm d i a m e t e r ) d i f f r a c t i o n p a t t e r n from an a r e a i n the B a ( 0 H ) c r y s t a l w h i c h i s c o n s i s t e n t w i t h t h a t of a - B a ( 0 H ) (7). 2

2

2


302


500 METHANOL YIELD

in


g/Kg cat/hr 250°C 400

75 atm

H /CO = 70/30 2

300

200»

100 Ο

F i g u r e 3 . The e f f e c t o f a l k a l i on the i n i t i a l methanol y i e l d s a t 250°C., 75 atm and a t o t a l gas f l o w of 1 5 . 0 £ ( S T P ) / h r o v e r 2.45 g of the impregnated Cu/ZnO = 30/70 mol% reduced c a t a l y s t .


18.

V E D A G E E T AL.


303

Cs > Rb > Κ > Ba * undoped > Na > L i w i t h e q u i m o l a r amounts o f the a l k a l i o r b a r i u m h y d r o x i d e s added t o the Cu/ZnO c a t a l y s t . F u r t h e r , a t 250°C and 75 atm the a l k a l i h y d r o x i d e s d i d not s i g n i f i c a n t l y a l t e r the h i g h s e l e c t i v i t y t o methanol o f the Cu/ZnO c a t a l y s t .


Table I I I . The E f f e c t of CO2 on Carbon C o n v e r s i o n to Methanol f o r the Cu/ZnO/CsOH and Cu/ZnO C a t a l y s t s Temperature 235°C.; P r e s s u r e 75 atm; T o t a l gas f l o w = = 15 l i t e r s ( S T P ) / h r over 2.45 g o f reduced c a t a l y s t . % Carbon C o n v e r s i o n t o CH 0H 3

Gas C o m p o s i t i o n C0 /CO/H

2

= 0/30/70

C0 /CO/H

2

2

2

2

C0 /CO/H 2

Cu/ZnO/CsOH

Cu/ZnO

13.7

9.0

= 2/28/70

22.4

51.0

= 6/24/70

17.0

37.0

Only 2% s i d e p r o d u c t s , i n d e c r e a s i n g o r d e r m e t h y l f o r m a t e , meth ane and e t h a n o l , appeared over the CsOH-doped c a t a l y s t d e s p i t e the t w o - f o l d i n c r e a s e i n a c t i v i t y , and no s i d e p r o d u c t s were observed over the KOH doped c a t a l y s t d e s p i t e the 30% enhancement of the meth a n o l a c t i v i t y compared to the undoped c a t a l y s t . When 00 was added to the s y n t h e s i s g a s , the i n c r e a s e o f methanol y i e l d was s m a l l e r over the CsOH/Cu/ZnO c a t a l y s t ( T a b l e I I I ) t h a n over the undoped Cu/ZnO c a t a l y s t (10), r e s u l t i n g i n a r e v e r s a l o f the o r d e r o f a c t i v i t i e s t o undoped Cu/ZnO>(greater than) Cu/ZnO/CsOH i n the C 0 / C O / H = 2 / 2 8 / 7 0 synthesis gas. The enhancement o f the methanol s y n t h e s i s r a t e i n the C02-free gas and the r e v e r s a l of t h i s t r e n d i n the C 0 - c o n t a i n i n g s y n t h e s i s gas f o r the CsOH-promoted c a t a l y s t i s suggested to o c c u r i n the f o l l o w i n g manner. CsOH r e a c t s w i t h carbon monoxide to produce a s u r f a c e f o r m a t e , HCOOCs, w h i c h i s then hydrogenated to methanol by hydrogen a c t i v a t e d on the Cu/ZnO s u r f a c e i n the i n t i m a t e neighborhood o f the CsOH s i t e s . T h i s r e a c t i o n a c c o u n t s f o r the methanol s y n t h e s i s r a t e enhancement a t low c o n c e n t r a t i o n s o f CO2. As the CO2 c o n c e n t r a t i o n i s i n c r e a s e d , the normal mechanism i n v o l v i n g the whole a c t i v e s u r f a c e of the Cu/ZnO components t a k e s over w i t h the p o s s i b l e w i t h d r a w a l o f cesium from the a c t i o n by the f o r m a t i o n o f s u r f a c e c a r b o n a t e . Be cause the s u r f a c e a r e a o f the Cs-doped c a t a l y s t i s o n l y a f r a c t i o n of t h a t o f the undoped c a t a l y s t , i t s o v e r a l l a c t i v i t y i s l o w e r a t i n t e r m e d i a t e CO2 c o n c e n t r a t i o n s i n the s y n t h e s i s g a s . The CsOH s u r f a c e m o i e t y i s more r e a c t i v e than the o t h e r a l k a l i h y d r o x i d e s because the l a r g e i o n r a d i u s o f C s makes the 0H~ groups the most a c c e s s i b l e as w e l l as the most b a s i c . The c o n d i t i o n s employed i n t h i s work a t w h i c h the s y n t h e s i s o f 2

2

2

2

+


SOLID STATE CHEMISTRY IN CATALYSIS

304

higher oxygenates, i . e . alcohols and esters was favored, were temper atures above 280°C., low l ^ / C O r a t i o s , and gas hourly space v e l o c i t i e s 2600-5000. The results are summarized i n Figures 4 and 5 . Methyl esters which were also produced are not shown i n these figures. A formal c l a s s i f i c a t i o n of the points of attack involved i n the synthe s i s of the C alcohols and esters i s introduced below. The i n j e c tion of alcohols indicates that higher alcohols and esters were form ed from lower alcohols, as documented i n Table IV, and therefore our c l a s s i f i c a t i o n scheme i s based upon a stepwise attachment of a C-^ intermediate to a growing alcohol chain. The C-^ intermediate i s assumed to have an intact C-0 bond and to be capable of attaching i t s e l f by either the carbon atom or the oxygen atom to the growing alcohol chain as depicted below. +


2

° 0.-

'c-o

ι O-H

^ IΓ

R-C-C-C-O-H t t t The processes a , &Q and are attachments of the C-^ intermediate by i t s carbon at the a, 3 or γ carbon of the reacting chain, a i s the attachment of the intermediate by i t s oxygen at the α-carbon, i ç 0 and Î Q - H insertions of the carbon end of the C-^ intermediate into the C - 0 and the 0-H bonds of the growing alcohol chain. The products obtained by these i n d i v i d u a l processes are shown i n the diagram below with a star l a b e l on the carbon atom that originates from the C^ intermediate. c

0

a

r

e

—R-C-C-C-OH

-R-C-C-C-OH C* ι R-C-C-C-OH R-C-C-C-O-H

Ο R - C - C - CII- 0 - C * H , R-C-C-C-C*-OH Ο ^^R-C-C-C-O-C^H



4.

2.45 g

of

the

2-M1-P = 2 - m e t h y l

over

the

alcohol

and

1-butanol.

of

= 1-P

=

1-propanol,

Z(STP)/hr

Cu/ZnO 8.0 Ε = ethanol,

flow

impregnated gas

over total

M = methanol, 1-B =

catalyst.

1-propanol,

a

yields,

2 8 8 C., 7 5 a t m a n d

of

reduced

at

A comparison

3 0 / 7 0 mol% c a t a l y s t ,

Figure


^ ^


F i g u r e 5 . A c o m p a r i s o n o f the a l c o h o l y i e l d s , o v e r impregnated Cu/ZnO = 30/70 mol% c a t a l y s t s , a t 288 C., 75 atm and a t o t a l gas f l o w o f 10.4 Z ( S T P ) / h r over 2.45 g o f the reduced c a t a l y s t . M = m e t h a n o l , Ε = e t h a n o l , 1-P - 1 - p r o p a n o l , 2-M1-P « 2 - m e t h y l 1 - p r o p a n o l , and 1-B » 1 - b u t a n o l .


K

£j £


3.01 O.37 o and i c - o f ° the dependence o f the p r o d u c t c o m p o s i t i o n on the H 2 / C O r a t i o and on the b a s i c i t y o f the a l k a l i h y d r o x i d e p r o m o t e r . The 3Q a t t a c k dominates a t low H 2 / C O r a t i o s and i n the p r e s e n c e o f CsOH, i n d i c a t i n g t h a t a l d o l s y n t h e s i s o f aldehyde p r e c u r s o r s o r p r o ducts of a l c o h o l dehydrogenation i s i n v o l v e d . As shown b e l o w , o n l y p r i m a r y o r secondary but not t e r t i a r y 3-carbon i s a t t a c k e d by the 3c p r o c e s s , a f e a t u r e c h a r a c t e r i s t i c of a l d o l c o n d e n s a t i o n . a n Q l

R O

a

t

0

a

c

c

o

u

n

t

r

RCH CH CH OH 2

2

RCH CH CHO + H

2

2

2

2

Θ

RCH CH CHO + CsOH Ξ = Ξ RCH CHCHO + H 0 2

2

2

H H

2

+ CO — *

\

Η

Q

// C

Η

3 = ^

2

Ο®

\ C. /

Η


18.

VEDAGE ET AL.

H Θ

CT

\ ® /

RCH CHCHO + C

- RCH CH-CHO

2

2

m

a

- R C H C H - C H O H + CsOH 2

2

v

The i n s e r t i o n i p r o c e e d by the mechanism proposed by N a t t a and coworkers (3) i . e . , by the r e a c t i o n of s u r f a c e a l k o x i d e s w i t h CO as shown b e l o w : c


309


i

0

R C H d % CO

*RCH CO(f

2

C-0

RCH COO°+ 2H 2

2

*RCH CH OH + ΟΗ Θ

2

2

2

Since t h i s r e a c t i o n r e q u i r e s hydrogen, i t w i l l occur at h i g h e r r a t e s when the H^/CO r a t i o i s i n c r e a s e d , as o b s e r v e d . The m e t h y l e s t e r - f o r m i n g a t t a c k a c o u l d be due t o the C a n i z z a r o type c o n d e n s a t i o n of two a l d e h y d e s , to the r e a c t i o n o f s u r f a c e carboxyl a t e s w i t h f o r m y l o r formaldehyde, o r t o the r e a c t i o n o f s u r f a c e metho x i d e w i t h an aldehyde w i t h a h y d r i d e e l i m i n a t i o n . To be c o n s i s t e n t w i t h the e x p e r i m e n t a l o b s e r v a t i o n s , we s h a l l g i v e an example f o r the l a s t mechanism because i t r e q u i r e s the fewest number o f s t e p s , u t i l i zes methoxide w h i c h may be f a v o r e d i n hydrogen r i c h s y n t h e s i s gas and does not r e q u i r e a s t r o n g base c a t a l y s t . Q

a

f

0

CH cF+,C® H R 3

Ο > CH3-O-C-R H θ

Ο > CH3OCR + H®

M e t h y l formate i s the p r o d u c t o f t h i s r e a c t i o n when R = H . I f m e t h y l formate were the r e s u l t o f the i n s e r t i o n Î Q J J , one would expect t h i s r e a c t i o n to produce a l s o h i g h e r formate e s t e r s , c o n t r a r y t o the f i n d i n g t h a t o n l y m e t h y l e s t e r s were p r o d u c e d . The p r i n c i p a l d i f f e r e n c e between the a l d o l a d d i t i o n 3^, the i n s e r tion i o > 0 e d i t i o n i s t h a t the f i r s t o f these t h r e e r e a c t i o n types r e q u i r e s a s t r o n g base c a t a l y s t w h i l e the r e m a i n i n g two u t i l i z e a l k o x i d e s which are merely h e t e r o l y t i c a l l y d i s s o c i a t e d a l c o h o l s and a r e expected to be common s u r f a c e i n t e r m e d i a t e s i n a wide range of H /C0 r a t i o s and o f s u r f a c e b a s i c i t y . I t i s t h e r e f o r e e v i dent t h a t the s u r f a c e c o n c e n t r a t i o n s of the a l k a l i promoters and the H2/CO r a t i o can be used as n e a r l y independent v a r i a b l e s t o a c h i e v e s e l e c t i v i t i e s g i v e n i n F i g u r e s 4 and 5. The s e l e c t i v i t y p a t t e r n c o n t r o l l e d by the p r o c e s s e s 3^, i-c-0 * CXQ has an i n h e r e n t l i m i t a t i o n , however, w h i c h i s apparent from the f o l l o w i n g c o n s i d e r a t i o n . I f one w i s h e s to suppress the f o r m a t i o n o f h i g h e r l i n e a r a l c o h o l s , the i n s e r t i o n i ^ - o roust be m i n i m i z e d but as a consequence a l s o the f i r s t C-C bond f o r m i n g r e a c t i o n from to C w i l l be suppressed s i n c e n e i t h e r 3ς; n o r OLQ can be i n v o l v e d i n t h i s a

n

d

t

n

e

a

c

2

a n c

2


310


reaction. Hence the s u p p r e s s i o n o f i ç o r e s u l t s i n s e l e c t i v e methanol s y n t h e s i s no m a t t e r how e f f e c t i v e the c a t a l y s t may be f o r the 3ç a d d i tion. On the o t h e r hand, when the i ^ Q growth mechanism does o p e r a t e , the &Q a d d i t i o n must be s i g n i f i c a n t l y f a s t e r i n o r d e r t h a t the a l c o h o l s y n t h e s i s may be k i n e t i c a l l y r e s t r i c t e d so t h a t o n l y low a l c o h o l s a r e formed. I f one t a k e s the r a t i o o f ( l - b u t a n o l : l - p r o p a n o l ) i n the p r o duct as a crude measure of Î Q - Q * i ° °f (2-methyl-l-propanol: 1-propanol) as a measure o f 3 ç , then i t appears from F i g u r e s 4 and 5 t h a t the e f f e c t o f a l k a l i , p a r t i c u l a r l y CsOH, has been to i n c r e a s e the &C *C-0 ^ ° * comparison t o the undoped Cu/ZnO c a t a l y s t . a n c


:

r a t

t

n

e

r

a

t

n

Appendix C a l c u l a t i o n s o f the s u r f a c e c o n c e n t r a t i o n s XPS i n t e n s i t i e s

o f a l k a l i and barium from

The model f o r the c a l c u l a t i o n of s u r f a c e c o n c e n t r a t i o n s here i s t h a t o f D r e i l i n g ( 1 1 ) .

The measured p h o t o e l e c t r o n

utilized intensity

1^ due t o s p e c i e s i t h a t i s e v e n l y d i s t r i b u t e d over a specimen o f t h i c k n e s s t^ i s g i v e n by τ

±

=

^ X^gX (l-exp[-t /gX ])/E i

i

i

i

(A-l)

i

where Κ i s an i n s t r u m e n t a l c o n s t a n t ,

i s the p h o t o i o n i z a t i o n c r o s s

s e c t i o n , X? i s the atomic volume c o n c e n t r a t i o n o f element i , g i s escape a n g u l a r f a c t o r ,

and λ . i s the escape depth o f

of k i n e t i c energy E^ i n the specimen.

the

photoelectrons

The i n t e n s i t y 1^ may be a t t e n

uated by a s u r f a c e o v e r l a y e r o f t h i c k n e s s t.. c o n t a i n i n g s p e c i e s j by a f a c t o r e x p ( - t ^ / g X ) , w h i c h f o r v e r y t h i n l a y e r s approximates i

as

D e f i n e the a t o m i c s u r f a c e c o n c e n t r a t i o n X? = X ? t ? where t ? i s t h i c k n e s s o f a m o n o l a y e r , and take e f f e c t i v e t^. = t ^ X ^ / X ^ .

the

Then the

a t t e n t u a t e d p h o t o e l e c t r o n s i g n a l o f s p e c i e s i p a s s i n g through an o v e r l a y e r of t h i c k n e s s t.. i s g i v e n by 1

±

= Ka X^gX (l-t?xJ/(gX.X^))/E i

i

where we have t a k e n i n t o account l a r g e t ^ / g X ^ . photoelectrons Ij - K a

j X

from Jt°/

E j

(A-2)

i

The i n t e n s i t y I j o f

s p e c i e s j i n the t h i n o v e r l a y e r i s g i v e n by - Κσ.Χ*/Ε.

(A-3)

w h i c h f o l l o w s from ( A - l ) f o r s m a l l t ° / g X . . . The r e l a t i v e i n t e n s i t i e s o f the o v e r l a y e r s p e c i e s j and the b u l k s p e c i e s i a r e ,

from (A-2) and

(A-3),


18.

I

σ

311


VEDAGEETAL.

X?

Ε.

t?

4

^*

*r

/

(

i

t

x

- ;

/

'

(

x

i

g

x

i

)

)

(

a

4

-

)

where we used X ^ t ° = X ^ . The

r a t i o o f s u r f a c e c o n c e n t r a t i o n s f o l l o w s from


S

X.

I.

gX,

X? i

T

t? i

i

σ.Ε, a

E

i j

J

t?

I.

t° ι

\

(A-4).

E q u a t i o n (A-5) p e r m i t s the i n t e n s i t y 1^ o f the b u l k s p e c i e s t o be used as an i n t e r n a l r e f e r e n c e

f o r the d e t e r m i n a t i o n o f the s u r f a c e c o n c e n

t r a t i o n o f the s p e c i e s j t h a t i s p r e s e n t i n the o v e r l a y e r o n l y . In

the p r e s e n t work i n t e r e s t

c e n t e r s upon o v e r l a y e r s o f a l k a l i

compounds on the Cu/ZnO c a t a l y s t .

The Zn 2p^ p h o t o e l e c t r o n i n t e n s i t y

is

s i g n a l anâ hence the s p e c i e s i

t a k e n as the i n t e r n a l r e f e r e n c e

Zn and s p e c i e s j i s an a l k a l i i o n .

is

The escape depth X ^ was e s t i m a t e d n

from a r e l a t i o n g i v e n by Chang ( 1 2 ) ,

λ

Ζη

" ° .

where t £

n

2

( A

^ Ζ η

= O.2824 nm was t a k e n t o be the s p a c i n g between the

p l a n e s o f ZnO (13) and E

Z

= 232.1 e V , y i e l d i n g X

n

l o w i n g D r e i l i n g ( 1 1 ) , g = O.75 was u s e d .

Z

n

"

6 )

(10Ϊ0)

« O.86 nm.

Other d a t a used i n the

Fol cal

c u l a t i o n s o f the a l k a l i s u r f a c e c o n c e n t r a t i o n s a r e g i v e n i n T a b l e A - I . Table A - I XPS

Data f o r P h o t o e l e c t r o n s o f Elements A n a l y z e d i n the P r e s e n t Work

Element i

Photoelectron

o

O.0593

±

Li

Is

1198.1

Na

Is

182.2

Κ

2p£

Rb

2

3d

>

3

2

Cs 2

Ba Zn

.

2

2

P1

a

E (eV)

±

7.99

960.7

2.67

1143.1

4.44

529.6

22.93

473.6

24.75

232.1

18.01

b

From r e f e r e n c e (14). 'Sum o f p h o t o i o n i z a t i o n c r o s s s e c t i o n s f o r the Rb 3 d | and 3 d | photoelectrons. The e x p e r i m e n t a l i n t e n s i t y ( c f . T a b l e I I ) i s the i n t e g r a t e d i n t e n s i t y o f t h e s e two p h o t o e l e c t r o n e m i s s i o n s . In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

312

SOLID STATE CHEMISTRY IN CATALYSIS

The a l k a l i monolayer thickness t ° was taken equal to the sum of the diameters of the OH +

group, O.272 nm, and of the a l k a l i i o n , O.12 +

+

+

+

nm for L i , O.19 for N a , O.266 for K , O.296 for R b , O.338 for C s , 44

and O.27 for Ba *, y i e l d i n g t ° t°

b

t

>

A

= O.568, t ° a n (

^ ' ^Zn

(I^/l )

= O.610 and t °

* *"Zn

a

S

n o t e c

= O.392, t °

a

« O.542 nm.

= O.462, t £ « O.538, Using these values of

* above, the measured i n t e n s i t y r a t i o s

shown i n column 2 of Table I I » as well as the parameters S S

Z n


s

±

x

X

l i s t e d i n Table A - I , the surface concentration r a t i o s ( ^ Z n ^

w

e

r

e

calculated from equation (A-5) and summarized i n column 3 of T a b l e I I . Acknowledgments This work was supported by the U.S. Department of Energy (Subcon tract No. XX-2-02173 under prime contract No. EG-77-C-01-4092). References Cited 1. Morgan, G. T., Hardy, D. V. N. and Procter, R. H. J., Soc. Chem. Ind. Trans. and Comm. 51, 1T (1932). 2. Graves, G. D., Ind. and Eng. Chem. 1381 (1931). 3. Natta, G., Colombo, U. and Pasquon, I., "Catalysis," Reinhold, New York, NY, 5, p 131 (1957). 4. Smith, K. J., and Anderson, R. B., "The Higher Alcohol Synthesis over Promoted Methanol Catalysts," presented at the 8th Can. Symp. on Catalysis, May 26-29, 1982, U. of Waterloo, Ont., Canada. 5. Herman, R. G., Klier, Κ., Simmons, G. W., Finn, B. P., Bulko, J. B. and Kobylinski, T. P., J. Catal. 56, 407 (1979). 6. Mehta, S., Simmons, G. W., Klier, K. and Herman, R. G., J. Catal. 57 (1979). 7. Buch, Von P., Bärnighausen, H., Acta Cryst., B24, 1705 (1968). 8. The systematic errors are likely to originate, in the order of decreasing magnitude, from (a) the uncertainties in λ (if Seah's (9) λ = 1.4 nm were used in the present calculations, there would ensue unrealistic surface alkali concentrations higher by relative 30% than the total amount of the dopant alkali introduced into the system; (b) the values of S ; (c) the average escape angular factor g; (d) the estimated thickness tÅ, and (e) the possibly uneven distribution of the alkali between the surface and the bulk. 9. Seah, M. P., Dench, W. Α., Surf. Interface Analysis 1(1), 2(1979). 10. Klier, Κ., Chatikavanij, V., Herman, R. G., and Simmons, G. W., J. Catal. 74, 343 (1982). 11. Dreiling, M. J., Surface Sci. 71, 231-246 (1978). 12. Chang, C. C., Surface Sci. 48, 9 (1975). 13. Bulko, J. B., Thesis, Lehigh University, 1980. 14. Scofield, S. Η., J. Electron Spectroscopy 8, 129 (1976). Zn

Zn

Zn

RECEIVED

October 26, 1984


Solid State Chemistry in Catalysis - American Chemical Society

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