Catalytic conversion of ethanol on carbon catalysts

Carbon, Vol. 32, No. 2, pp. X5-271. 1994 ~opy~ht 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved ooO8-6223/94 $6.00 + .OO

Pergamon

CATALYTIC

CONVERSION OF ETHANOL CARBON CATALYSTS

ON

GRZEGORZ S. SZYMAI~SKI, GERARD RYCHLICKI, and ARTUR P. TERZYK Institute of Chemistry, Nicolas Copernicus University, ul. Gagarina 7, 87-100 Toruli, Poland (Received

14 May 1993; accepted

in revised.form

20 August 1993)

Abstract-The decomposition of ethanol was studied using carbon catalysts with a different nature of the surface. They were prepared from poly(furfuryl alcohol). The cataiytic tests were performed in a flow reactor in the temperature range 323-633 K. Dehydration and dehydrogenation of ethanol, as well as acetal formation, occur under the conditions applied. The products are ethene plus diethyl ether, acetaldehyde and 1,l ‘-diethoxyethane, respectively. The study supports previous findings that oxidation with nitric acid enhances dehydration activity and selectivity, whereas introduction of Ni*+ ions increases dehydrogenation activity and selectivity. The supported N?+ cations, in the hydrated form, act as additional active centers for dehydrogenation. In addition, both these surface modifications also enhance carbon catalytic activity and selectivity for the acetal formation. The acetal results from a secondary reaction between the formed acetaldehyde and ethanol on acidic centers located on the outer surface of the catalyst. Key Words-Carbon,

surface oxide, catalysis, dehydration,

1. INTRODU~IDN

As known, activated carbons catalyze dehydration and/or dehydrogenation of aliphatic alcohols[ I-51. It was shown that their catalytic activity results from the presence of acidic and basic sites[3-51. Aldehydes, the products of dehydrogenation of primary alcohois, in the presence of acidic or basic solids, may react with alcohols forming acetals, industrially important chemicals. Thus, it seems reasonable to study the decomposition of primary alcohols on carbon catalysts with the objective of obtaining information about the prospects of synthesis of acetals from alcohols in one step. This paper presents the results of catalytic decomposition of ethanol over carbon catalysts with different surface chemical nature. To avoid the poison effects and unwanted side reactions caused by inorganic impurities, high-putty polymeric carbon was used.

2.2

dehydrogenation,

alcohols, acetals

Catalyst c~~racferization

The pore structure of the carbon catalysts was determined by physical adsorption of Ar at 77 K and by mercury porosimetry (Carlo Erba 1500, Milan). The adsorption measurements were conducted volumetricalIy in a semiautomatic sorption manostate[7]. The contents of oxygenated surface groups were determined, according to the method of Boehm@], by performing titrations with bases of increasing strength, or with 0.05 M HCl. The amount of nickel ions on the carbon surface was determined by the AAS method[9]. The C-Ni and NiiC catalysts contain 4.34 * 1O-2 and 3.54 * 10m2 mmol * g-’ of nickel cations, respectively. Thermal analysis of the carbon catalysts was conducted in an inert atmosphere (N,, 20 dmi x h-l), using the derivatograph of Paulik-Paulik-Erdy, type OD 102 (Budapest), in the temperature range of 293 to 773 K. The heating rate was 300 K x h-l. 2.3 Catalyst activity

2. EXPERIMENTAL

Poiy(furfuryl alcohol) was used for the preparation of polymeric carbon[ti]. The carbon was activated with carbon dioxide at 1233 K (26.7% burnoff), followed by annealing in a flow of hydrogen (10 dm3 x h-l) at the same temperature for one hour(C). The chemical nature of the carbon surface was changed by oxidation with concentrated nitric acid at 358 K for two hours (C-ox) and subsequent introduction ofNi*+ ions by ion-exchange (C-Ni)[4]. In addition, some samples of the activated carbon (C) were impregnated with nickel(H) nitrate (NilC).

The decomposition of ethanol was conducted in a fixed-bed flow type reactor working at atmospheric pressure, using nitrogen as carrier gas. The product analysis was performed by on-line gas chromatography[3-51. The catalytic tests were conducted in the temperature range of 323 to 633 K at an ethanol partial pressure of 91 hPa, using 0.5 g of catalyst. 3. RESULTS AND DISCUSSION

3.1 Catalyst characterization As we have shown earlier[4,5], no substantia1 changes in the PF carbon (C) result from the nitric acid treatment (Table 1). It has been indicated that only a smalt increase in apparent surface area and

266

G. S. SZYMA~~SKI et Table 1. Physicochemical

Catalyst

al.

properties of the carbon catalysts studied

Apparent surface area (m2 * g-t)

V (I$ > 7.5 nm)

V,

NaHCO,

1152 1302 831

0.07 0.07 0.07

0.40 0.41 0.37

0.0 37.5 -

C c-ox C-Ni

Pore volume (cm3 * g-‘)

pore volume occurred

due to opening of formerly closed microporesl51. The lower apparent surface area for carbon containing nickel(D) cations results from the presence of partially hydrated nickel cations[5,10]. The introduced, partially hydrated cations hinder the access of argon molecules into the micropores as a result of narrowing their entrances or even blocking them. In contrast to PF carbon (C), which reveals mainly basic properties, the oxidized carbon (C-ox) demonstrates acidic as well basic properties. The basic properties probably result from the presence of adsorbed O- and 0, species[ll-131 or pyronelike structures[l4], whereas the surface acidity results from the presence of carboxylic, lactone, and phenolic groups[ E-171.

Base uptake * 102 (mmole * g-l) Na2C03 0.0 32.7 -

NaOH

Acid uptake * 102 (mmole * g-l) HCl

1.5 109.6 -

42.7 18.5 -

The thermal analysis data (Fig. 1) indicate quite good thermal stability of the activated carbon (C) in the investigated range of temperature (303-773 K) and the oxidized carbon (C-ox) below 573 K. The observed minimum on DTG curves (Fig. 2) in the temperature range of 368 to 388 K corresponds to a mass loss of the carbon samples due to desorption of physically adsorbed water1181. In the case of Ni/C samples, there was also observed a second minimum at about 403 K, caused by decomposition of supported nickel(D) nitrate hexahydrate[ 191. The higher water sorption (mass loss) for the Cox and C-Ni catalysts results from the presence of additional adsorption centers, i.e., surface oxides and coordinatively unsaturated Ni*+ ions. These surface oxides are also responsible for the lower

TG

C :

:

I

“’ ..

:

’

,

‘t

I...

-.. --__ ----.--.

. . -.

-.._

..

.-..__

..

.... ..,.

+. .-._

.’

.. --.

/

WC

‘:~

, 390

-. -.

I 490

-_--_\

*-._

I 590

--__ -. -. I 690

‘.

-.

c-ox

*. C-Ni I 790 T (K)

Fig. 1. TG curves of the investigated carbon catalysts.

I

3

390

I

490

I

590

I

690

I

790

T (K) Fig. 2. DTG curves of the investigated carbon catalysts.

267

Catalytic conversion of ethanol on carbon catalysts Ni / C W / F =

P

QQMD w aappD

-

732.5

kg

s /

mole

acetal aldehyde ether ethene

C m umun .a=. QQQ!.X -

f

acetal aldehyde ether ethene

fl

173

3-

:

123

473

(K)

Fig. 4. Catalytic activity of the impregnated carbon NiiC in decomposition of ethanol (W/F = space time).

423

473

523

573 T

(K)

thermal stability of the carbons. At higher temperatures thermal decomposition of surface functional groups occurs[l7,20].

3.2 Catalytic activity The nonoxidized activated carbon (C) exhibits very low catalytic activity in ethanol decomposition. It promotes both dehydration and dehydrogenation of the alcohol (Fig. 3); ethene, diethyl ether, and acetaldehyde, respectively, are the products. In addition, formation of an acetal, 1,l’-diethoxyethane, was observed. The products of dehydrogenation dominate in the whole range of temperature. The impregnation of the activated carbon (C) with nickel(I1) nitrate hexahydrate (Ni/C) enhances its catalytic activity in the dehydrogenation and the acetal formation (Fig. 4, Table 2). As we have shown earlier for secondary alcohols[3-51, the oxidation of carbon samples with nitric acid results in higher catalytic activity, especially in the dehydration activity (Figs. .5a and b. Table 2). It also enhances formation of the acetal. At lower temperatures, the dehydrogenation of ethanol

dominates, while at higher temperatures dehydration is predominant. An increase in the reaction temperature above 593 K causes a decrease of the catalytic activity. The thermal analysis data, mentioned above (Fig. 2), indicate that the decrease in catalytic activity results from the thermal desorption of the catalytically active surface oxides. The introduction of nickel(U) ions on the surface of the oxidized carbon (C-ox) by ion exchange drastically diminishes its dehydration activity. It results in an increase in reactivity to acetaldehyde and I,1 ‘diethoxyethane (Fig. 6, Table 2). Besides changes in the catalytic activity, the modification of the chemical nature of the carbon surface is accompanied also by changes in the product selectivity. The oxidation leads to an increase in the dehydration selectivity, whereas the introduction of nickel(I1) ions results in a higher selectivity for the aldehyde. However, in both cases an increase in the selectivity for I, 1 ‘-diethoxyethane is also observed (Table 3). An increase of the reaction temperature enhances the dehydration selectivity. The dehydrogenation selectivity also increases initially, but at higher temperatures passes through a maximum and then decreases (Figs. 7-10). The maximum of dehydrogenation selectivity is followed by a minimum selectivity for acetal formation. It suggests that the acetal results from a secondary

Table 2. Catalytic activity of the carbon catalysts in decomposition

C NiiC c-ox C-Ni

57 T

Fig. 3. Catalytic activity of the activated carbon C in decomposition of ethanol.

Catalyst

523

Nickel cation content * 102 (mmole * g-‘)

Ethene

Diethyl ether

3.54 4.34

3.39 3.94 50.80 5.22

1.30 I .57 28.80 1.68

Conversion

of ethanol at 533 K. * 10’

I, 1‘-diethoxyAcetaldehyde 4.51 7.11 9.76 10.01

ethane

2

0.81 3.30 9.69 13.89

IO.01 15.92 99.05 30.89

268

G. S.

SZYMA~~SKI et al.

C -

E

Uw

2

z

;

-

clQLlW CLCY QQOQO -

‘G0.8 amu

0.6

uunn~

-

Ni acetal aldehyde &her elhcne

acelat

sldehyde

- elhene 0.4

0.2

0.0

350

300

450

400

500 T (K)

3

T

(K)

Fig. 6. Catalytic activity of the Ni2+ ion-exchanged oxidized carbon C-Ni in decom~sition of ethanol.

W

-

CUQQ.Q -

metal ethene

T (K) Fig. 5. Cataiytic activity of the oxidized carbon C-ox in decomposition of ethanol.

reaction between the formed acetaldehyde and the alcohol on acidic centers. This is confirmed by the effect of introducing gaseous acetaldehyde into the catalyst bed. After adding a portion of the aldehyde a temporary increase in the acetal concentration was observed (Fig. 11).

The formation of l,l’-diethoxyethane during the ethanol decomposition, whose molecule is large compared to the carbon pore dimensionsf41, suggests that the process of the acetal formation occurs on the outer surface of the catalyst. It has been previously demonstratedL51 that the dehydration takes place on acidic centers located on the outer surface of the catalyst. Thus, the observed maximum dehydration selectivity with simultaneous minimum selectivity for the acetal (Figs. 7-10) may confirm our suggestion. In contrast to the dehydration, the dehydrogenation also occurs within the pores of the catalyst, and it is limited by the desorption of the product[4,5]. So, the observed increase in the selectivity for acetaldehyde and its acetal with an increase in space time W/F (W = weight of catalyst in kg, F = feed rate in mole x s-l) at the expense of the selectivity for ethene (Fig. 12), indicates that the process of acetal formation requires desorption of acetaldehyde from pores and its subsequent readsorption on acidic centers where it reacts with the alcohol. The observed disappearance of acetaldehyde as well as of l,l’-diethoxyethane in the products at low values of space time W/F (high feed rate F)

Table 3. Product selectivity for decomposition of ethanol at 493 K. Selectivitv * lo* Catalyst C NilC c-ox C-Ni

Ethene

Diethyl ether

Acetaldehyde

1,l ‘-diethoxyethane

27.86 2.09 37.48 19.21

19.05 15.99 26.34 7.82

44.52 71.96 23.31 47.88

8.57 9.96 12.67 25.10

Cataiytic conversion of ethanot on carbon catalysts

aBm#

-

w QQQQG -

acclal

aldehyde ether ethtne

ZO-

0

323

313

423

473

1

523

573 T

fW

Fig. 7. Jkpendence of product selectivity on temperature in decompasition of ethanol on the activated carbon C.

Fig. 9. Dependence of product selectivity on temperature in decomposition of ethanal on the oxidized carbon C-ox.

(Fig. 131, results from the microporous structure of the carbon catalysis. At high concentrations, the adsorbed ethanol molecules hinder the formation and the desorption of acetaldehyde from pores as a result of narrowing or blocking pore entrances. An increase of the acetal concentration upon injecting gaseous acetaldehyde into the catalyst bed, during the partia1 decomposition of ethanol, indicates that the carbon catalysts effectively promote formation of the acetal from a mixture of ethanol with acetaldehyde (Fig. 11). But this is fallowed by the presence of products of other acetaldehyde t~nsformations~ e.g., 3-hydroxybutanal~ 2-butenaI (crotonic aldehyde), and ethyl acetate (Fig. 14).

The formation of 3-hydroxybu~n~ oniy over the most basic catalysts (C, NYC) indicates that it is formed as a result of an aldol condensation. The increase in concentration of surface acidic centers of Broensted ar Lewis type (Ni/C, C-Nil leads to dehydration of the aldol and formation of Zbutenal (crotonic aldehyde). According to references 121-231, the formation of ethyl acetate during ethanol decomposition over heterogeneous catalysts based on transition metal compounds, at high ethanol conversion, results from the reaction of the acetaidehyde initial@ formed with surface ethoxy group or ethanol molecules. This mechanism was a&so suggested for the

‘1

- aldehydde Q!Z?QQQ-

373

423

ethene

473

523

573 T

Fig. 8. Dependence of product selectivity on temperature in decomposition of ethanol on the impregnated carbon NilC.

WI

Fig. 10. Dependence of praduct selectivity on temperature in decomposition of ethanol on the Ni2’ ion-exchanged carbon C-Ni.

G. S.

270

SZYMA~SKI

et al.

553 K , C -

ox

w - aceta1 clpppp - aldehyde

.I QQQ.QC -

ether ethene

I

2.5

3.0 t (h)

Fig.

11. Effect of the presence of acetaldehyde in ethanol feed on the acetal formation at 553 K.

01

323

d

,

I

573

W /

ethyl acetate formation during oxidation of ethanol with air on carbon catalysts[24]. The acetate formation decreases with an increase in catalyst acidity[23]. This may explain the formation of ethyl acetate only over the Ni/C catalyst with the highest dehydrogenation activity and low surface acidity (Figs. 4 and 14, Table 1). According to our earlier findingsp-51, the dehydrogenation occurs with the simultaneous participation of Lewis acid and base sites. Introduced transition metal cations (Lewis acids) act as additional active centers. This is supported by the similar shape of curves of dehydrogenation activity versus temperature for Ni/C and C-Ni catalysts (Figs. 4 and 6). The fall of their dehydrogenation activity at temperature close to the temperature of the decomposition of surface nickel(I1) aqua complexes (Fig.

c -

1073

F

(kg

1

I

I

623

1323

s /

1573

mole)

Fig. 13. Decomposition products of ethanol at 553 K for different space time W/F on C-ox.

2) indicates that only nickel(I1) cations in the hydrated form are catalytically active. The necessity of the presence of hydrated transition metal cations

Ni/C

ox

umm QQJ.UD -

acelal

QQQCC -

ethene

aldehyde

i/ ;:_i

! ‘.,j

11 .,,, ; :\,,

C

Ill VJ

a

Fig. 12. Dependence of product selectivity __.I_. . ^. - on space time W/P

m decomposltlon

ot ethanol on C-ox at 553 K.

I

I

I,

12345676

I

I

I

I

t

I1

9 (min)

10

Fig. 14. Chromatograms of products of ethanol decomposition at 533 K, after introduction of acetaldehyde to ethanol feed; 1-ethene, 2-diethyl ether, 3-acetaldehyde, 4-1,l ‘-diethoxyethane, S-ethanol, 6-3-hydroxybuta._. .^ nal, 7-2-butenal, 8-ethyl acetate.

Catalytic conversion of ethanol on carbon catalysts

for alcohol dehydrogenation was earlier reported for silica catalysts impregnated with transition metal nitrates[25].

REFERENCES I.

A. Turuizumi, Nippon Kagaku Zasshi 82, 545 (1961).

2. A. Turuizumi, Nippon Kagaku Zusshi 82, 1111 (1961). 3. G. S. Szymariski and G. Rychlicki. Reuct. Kinet. Catal.

4. CONCLUSIONS

The results show that the investigated carbons also promote an acetal formation, besides dehydration and dehydrogenation of ethanol. Ethene, diethyl ether, acetaldehyde, and 1,l ‘-diethoxyethane, respectively, are the products. As previously observed with secondary alcohols[3-51, oxidation with nitric acid enhances dehydration activity and selectivity, whereas introduction of Ni’+ ions increases dehydrogenation activity and selectivity. The introduced Ni*+ cations, in the hydrated form, act as additional active centers for dehydrogenation. Simultaneously, both of these surface modifications result in high-carbon catalytic activity and selectivity for the acetal formation. The oxidized carbon C-ox, after ion-exchange with Ni*+ ions, is the most active and selective catalyst for the acetal synthesis. The acetal results from a secondary reaction between the formed acetaldehyde and the alcohol on acidic centers located on the outer surface of the catalyst. Upon adding acetaldehyde to the ethanol, besides an increase in the acetal concentration, products of other acetaldehyde transformations e.g., 3-hydroxybutanal. 2-butenal, and ethyl acetate, were also observed. The carbon catalytic activity increases with an increase of the decomposition temperature. However, above 593 K it decreases due to thermal decomposition of the catalytically active surface oxides. For all catalytically active carbons, an increase of temperature enhances their dehydration selectivity.

271

Lett.

43, 475 (1991).

4. G. S. Szymaliski and G. Rychlicki. Carbon

29, 489

(1991). 5. G. S. Szymadski

and G. Rychlicki, Carbon 31, 247 (1993). 6. H. Marsh and W. F. K. Wynne-Jones, Curbon 1,269 (1964).

7. A. Ciembroniewicz and M. Lasori, Polish J. Chem. 46, 703 (1972). 8. H. P. Boehm, Adv. Catulysis 16, 179 (1966). 9. R. J. Guidoboni, In Analytical Methods for Coal und Coal Products (Edited by C. Karr, Jr.), Vol. I, p. 421. Academic Press, New York (1978). IO. G. Rychlicki, Polish J. Chem. 62, 1175 (1988). Il. G. Rychlicki, Polish J. Chem. 60, 171I (1988). 12. J. Zawadzki, Curbon 18, 281 (1980). 13. J. Zawadzki and S. Biniak. Polish J. Chem. 62. 195 (1988). 14. H. P. Boehm and M. Voll, Carbon 8, 227 (1970). 15. J. B. Donnet, Carbon 6, 161 (1968). 16. J. Zawadzki, Curbon 26, 627 (1988). und Physical 17. K. Kinoshita, Carbon, Electrochemical Propevlies, p. 86. Wiley, New York (1988). 18. C. Karr, Jr, Analytical Methods for Coal and Coul Products, Vol. 2, p. 619. Academic Press, New York (1978). Analysis, p. 19. C. Duval, Inorgunic Thermogravimetric 360. Elsevier, Amsterdam (1963). 20. D. T. Faean and T. Kuwana. Anal. Chem. 61. 1017 (1989). 21. K. Kawamoto and Y. Nishimura, Bull. Chem. Sot. Jpn. 44, 819 (1971). 22. Y. Matsumara, K. Hashimoto, and S. Yoshida. .I. Coral. 122, 352 (1990). 23. N. Iwasa and N. Takezawa, Bull. Chem. Sot. Jpn. 64, 2619 (1991).

24. G. C. Grunewald and R. S. Drago, J. Am. Chem. Sot. 113, 1639 (1991). 25. T. Nishiguchi and F. Asano, J. Org. Chem. 54, 1531 (1989).