Kinetics of Fischer-Tropsch selectivity

Fuel Processing Technology, 18 (1988) 293-304 293 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

Kinetics of Fischer-Tropsch Selectivity*

HANS SCHULZ, KLAUS BECK and EGON ERICH

Engler-Bunte-Institut, University of Karlsruhe, Kaiserstrafle 12, 7500 Karlsruhe (F.R.G.)

(Received December 8th, 1987; accepted January 22nd, 1988)

ABSTRACT

The kinetic model of Fischer-Tropsch chain growth as a non-trivial surface polymerization is developed and applied to discuss olefin/paraffin selectivity. The probability of the olefin chemidesorption-reaction, which controls primary olefin selectivity, is derived from experimental data as a function of temperature and total pressure. Secondary olefin hydrogenation and olefin double bond shift are identified in the system as interrelated reactions.

INTRODUCTION AND BASIC KINETIC MODEL

Due to the ordered multiplicity of the composition of a Fischer-Tropsch reaction product, the kinetic scheme of its formation has to be considered as a complex system of combined reactions. A serious complication emerges from the peculiarity that most of its basic reaction steps proceed in the adsorbed state between chemisorbed intermediates on the catalyst surface and are thus not accessible for kinetic measurements by means of gas phase concentration determination. A simplified kinetic scheme of Fischer-Tropsch chain growth and product formation is visualized in Fig. 1. The surface species " S p l " - containing one carbon atom - is being formed from CO and hydrogen and grows further through stepwise prolongation by one carbon atom (growth steps are termed as "g"). This growth reaction has recently been classified as a "non: trivial surface polymerization" [ 1 ]. This definition acknowledges the unique character of the system in which one step of chain prolongation (by one CH2- unit) is only performed through a set of reactions of hydrogen transfer, elim- ination of oxygen and formation of a new C/C-bond. In the model of Anderson [2] the addition of the Cl-monomer to the penult imate carbon atom of the chain is also admitted in order to explain methyl branching (steps "gbr" in Fig. 1).

Desorption is regarded in Fig. 1 as leading to 3 types of product compounds: paraffins, olefins and alcohols (plus aldehydes). In a most idealized model all

*Paper presented at the International Coal and Gas Conversion Conference, Pretoria, August 1987.

0378-3820/88/$03.50 © 1988 Elsevier Science Publishers B.V.

294

Pr{.,sat Prl..ol Prl.,ox

Products dl,.,,ot~ d/.,ot//~, ox \ T / / g,

=:f> !Pi --SP2 ° Sp3 " Pa " S.P5 SPc"

m storling of choins I [g6.br ~'X with o Cl-species g3. br g/,. br gS. br

",,,t/' \ " Sp6

, b r

",,,t/ ",,,,t/' \ ' SPS.br ' SPS.br '

",,,t/" ",,,t/" ",,,t/" ",,, • Sp&,b r = SpS, b r ' SPc, b r =

t / Sp 7

SP7, br

?,I SPT.br

?7 SPT, br

SP7,br

Fig. 1. Kinetic scheme of Fischer-Tropsch surface polymerization regarding chain branching and product desorption as paraffins, olefins and oxygen compounds (alcochols plus aldehydes). For- mation of dimethyl branched compounds is being omitted.

values of reaction rate constants for repetitive reaction steps are regarded as being carbon number independent. No chain branching and formation of only one type of product are being assumed. The distribution of products then can be described with only one parameter, i.e. the probability of chain prolongation "pg". The sum of probabilities of a surface species "Sp," to grow further or to be chemidesorbed is defined as 1:

Pgn +Pan = 1 ( 1 )

These reaction probabilities of a surface species Spn are related to the respective rate constants (kgn and kdn)

pgn =kgn/ (kgn-[-kdn) (2a)

and

Pd,, = kdn / ( kgn -Jr" kdn ) (2b)

Thus relative values of rate constants are easily obtained from the correspond- ing reaction probabilities:

Pgn/Pdn = kgn/kdn (3)

Normalizing the sum of product moles as 1.00 i = ~

Mi = 1 (4) i=1

295

leads to the molar distribution of reaction products eqn. (5)

M. =p ~n 1 "Pdn (5)

respectively, with definition ( 1 ) to eqn. (6)

Mn =p~Z 1" (1-pgn) (6)

Equation (5) has been used to examine experimental product distributions by Schulz and Elstner [3]. This model of "ideal non-trivial surface polymerization" is identical in its formalism to that developed earlier for homogeneous polymerization by Flory [4]. The logarithmic form of eqn. (6) given as eqn. (7) and particularly its graphic representation according to Anderson [5] are suited to examine experimental distributions for deviations from ideality:

logM~ = ( n - l ) log (Pgn) -I- log (1--pg.) (7)

The nature of the C 1-building blocks for chain prolongation is not discussed in this paper and it seems that several species are capable of chain prolongation. The kinetic aspect of establishing a Fischer-Tropsch (FT) regime is not so much a question of supplying "monomers" respectively their chemical nature but is a kinetic demand: the slowing down of the rate of chemidesorption reactions so that chain growth reactions can become predominant [6 ].

The product distribution parameters - probabilities, respectively rate constants of chain growth and chain termination - are easily combined with the equations which describe the macro kinetics of the FT-synthesis:

The rate of CO consumption dnc. con~/dt is equal to the rate of product formation on a carbon basis dnc. form/dt (eqn. 8) )

- dnc .... ~/dt + dnc,form/dt = 0 (8a)

Generally the rate of product formation dnc,~orJdt consists of two terms the rate of formation of organic compounds dnc,form,o,g./dt and the rate of formation of C02 dnc,form,Co2/dt through the water gas shift reaction.

dnc,fo~m/ dt = dnc,fo~m,o~g./ dt + dnc,form,co2dt (8b)

Thus the rate of CO-consumption is easily corrected for CO2-formation by substracting the rate of CO-consumption for this reaction. This point should be recognized in systems where CO2 is formed together with organic compounds. In this paper the following equations relate to systems without CO2 formation respectively to rates of CO consumption, which have been corrected for CO2-formation.

To calculate the rate of product formation on a molar basis dnpJdt, the rate of product formation on the carbon basis must be divided by the average carbon number of the product molecules (/Vc), which has to be determined experi- mentally (eqn. (9)).

296

dnpr dnc,form 1 d ~ - d ~ Nc (9)

The molar rate of formation of one individual component dni,er/dt is then obtained by multiplication of the rate of total molar product formation with the molar fraction of product compound "i" (Fri) (eqn. (10)).

dni,pr/dt= F r i . d n p r / d t (10)

The final equation for calculating individual product formation rates is (eqn. (11)).

dni,pr dnc .... s 1 d---~- d-----~ /Vc Fri (11)

Actual cases of FT-synthesis are now compared with the ideal model of non- trivial surface polymerization. The model is then extended to describe individual experimental distributions adequately, and with the help of the model, reaction probabilities and reaction rate constants of particular reaction steps are calculated and evaluated as a function of catalyst characteristics and reaction parameters.

In the present paper only paraffin/olefin-selectivity is considered. Changes in chain growth probability as a function of carbon number and carbon chain branching probability as a function of carbon number and reaction parameters are treated separately [1, 7]. Paraffin/olefin-selectivity also has been inves- tigated recently by Wojciechowski et al. [8].

EXPERIMENTAL

CO hydrogenation has been accomplished in a small fixed bed flow-type reactor. The catalysts were prepared from coprecipitation in aqueous solution. Product analysis was performed mainly by means of capillary gas chromato- graphy [9, 10].

RESULTS AND DISCUSSION - APPLICATION OF THE MODEL TO PARAFFIN/OLEFIN SELECTIVITY

Main products of Fischer-Tropsch CO-hydrogenation are paraffins and olefins. However, in most of the actual cases the primarily formed olefins are in part converted into paraffins through secondary hydrogenation reactions [ 11 ] and the primary selectivity is thus obscured. FT-systems with no olefin secondary hydrogenation have been reported by Schulz et al. [11, 12]. Primary formation of paraffins and olefins through dissociative or associative chemidesorption of an alkyl species is visualized by the scheme in Fig. 2.

Olefin/paraffin selectivity is discussed below on the basis of the simplified kinetic model shown in Fig. 3 assuming carbon number independent rate con-

297

R - - CH---CH 2

R - - C H 2 - - C H 3

Fig. 2. Kinetic scheme of primary formation of olefins and paraffins during Fischer-Tropsch synthesis via dissociative and associative chemidesorption.

Prsat,1 Prsat,2 Prol2 Prsat,3 ProL3 Prsat.n Prol,n

I / ' \ / o . \ So Sp1 g ~ Spe g -- Sp3 g " ' S p . -

Fig. 3. Simplified kinetic scheme of Fischer-Tropsch synthesis with carbon number independent rate constants for repetitive reaction steps: no branching reactions and olefins and paraffins as the only products are assumed.

stants for different types of reaction and only paraffins and olefins as the products.

Calculated Anderson-plots for a product consisting of olefins and paraffins according to the kinetic scheme in Fig. 3 are presented in Fig. 4 (left). The distributions of the olefins and paraffins are parallel straight lines in this representation. Accordingly, the olefinic portion in the carbon-number-product- fractions represents a straight horizontal line when plotted against carbon number, as shown in Fig. 4 (right) for three products with values of olefin desorption probabilities of Pd,ol = 0.90; (0.80 and 0.70). This corresponds to ratios of rate constants of desorption for olefins and paraffins as

kd,o|/kd,sat ----Pd,ol/Pd,sat ---- 9 (respectively 4 or 2.33 )

In Fig. 5 an experimental distribution with mainly primary selectivity of olefins is presented in the same mode as the calculated distribution in Fig. 4. Obviously the simplified model is a good approach and the assumption of carbon number independence of the rate constants of olefin- and paraffin-product chemidesorption is basically justified.

The model can be refined in order to account for the small decrease of Pd,o~ with carbon number:

Pd,ol,n =Pd,ol,3 "an-3,

a=pd,ol(n+ 3) /Pd,ol,n, and

n > 3 because this dependence only starts with propene as the smallest olefin. Due to Pd,ol,. " l -pd , sa t ,n = 1 one obtains

Pd,sat,n ~-~ 1 --Pd,sat,3 "a - (n--3)

The new parameter "a" is introduced in order to account for a linear decrease

298

2

I,

0 t-

.--~-I-

-2-

-3

~ UM DF PRODUCIS

PARAFFINSI

CARBON

IO0

= = 8 o =.__, ,

6o '

~ 20

C )

'~" 0 . . . . . . . . . . . . . . . 2 z, 6 8 10 12 1/.,

NUMBER OF PRDDUCI FRACIlON N c

(Pdot -o.go

.,Pdo= • O.BO

.,pdo[ • 070

Fig. 4. Calculated product distributions according to the kinetic scheme in Fig. 3 (pg = 0.6; Pd = 0.4; Pd.o] ---- 0 . 7 ; Pd ,mt = 0 . 3 ; individual parameters for C 1: Pg~ -- 0.83; Pd~ = 0.17 ).

2- .~ ~oo,

OF PRDDUClS ~=~ ~ o

~. O- ! ~ 6 o ~ t . I . .

._m_ 1. u_ PARAFFINS ~ ~ L,O-~

-2.

-3 ' z ' ~ ' ~ ' ~ ' ~ ' 1 2 ' l Z ' b ~ 0~ . . . . . . . . . . . . . . . z ~ 6 e 10 12 1~

CARBON NUMBER DF PRDDUCI FRACTION N c

Fig. 5. Experimental distribution of a Fischer-Tropsch synthesis product represented as an An- derson-graph (left diagram) for paraffins, olefins and both the hydrocarbons. Right hand of Fig. 5: molar olefin content as a function of carbon number of product fraction: Small amounts of oxygenates have been neglected for this investigation. Experimental conditions: 100 Co: 500 Mn: 100 Aerosil; 225°C, 26 bar; GHSV=250 h -1, H2/CO= 1.74 [14].

of primary olefin selectivity with increasing carbon number is the negative slope of the straight lines in Fig. 5 and equal to the ratio of the olefin desorption probabilities at carbon number "n + 1" and "n".

The value for the parameter of "a" which can be taken from Fig. 5 for the respective product distribution is a--0.997 which defines a decrease of olefin content in the carbon number hydrocarbon-product-fractions of about 0.3% per carbon number.

Secondary olefin-hydrogenation

Simultaneous partial hydrogenation of olefins with different carbon num- bers has been performed in a GC-pulse reactor [13]. These mixtures had been

299

obtained through FT-synthesis. A typical result is reproduced in Fig. 6, show- ing the degree of olefin hydrogenation as a function of carbon number of the olefins. The curve has a minimum at C3/C5 which can be explained as reflecting an extraordinarily high reactivity of ethylene due to its small size and great mobility in the adsorbate phase and the increase of adsorbability of the olefins with increasing carbon number. This curve is very indicative of competitive hydrogenation of aliphatic olefins and thus also for secondary olefin hydrogenation in Fischer-Tropsch systems. In Fig. 7 the olefin content as a function of carbon number is represented for several series of Fischer-Tropsch experiments. The extent of secondary olefin hydrogenation is quantified from how much the olefin content in C2 is lower than that in C3 (with no secondary olefin hydrogenation the olefin content in C2 is even higher than in C3) and also from the decrease of olefin content with carbon number in the range from C5 up- wards. It is seen in Fig. 7 (left) that secondary olefin hydrogenation is sup- pressed when iron catalysts are modified with large amounts of manganese. This behaviour has been explained as a matrix effect of the manganese oxide [ 6 ]. Fig. 7 (middle) shows secondary olefin hydrogenation to increase towards higher reaction temperature. The effect of total pressure in Fig. 7 (right) is complex.

The olefin content in the range CJC4 is least affected through secondary olefin hydrogenation and therefore best suited to determine the olefin-desorption-probabilities from Fischer-Tropsch catalyst sites. Primary olefin selectivity as a function of temperature and total pressure, Pd,ol,3/4 as derived from Fig. 7 is plotted as a function of temperature and total pressure in Fig. 8: The desorption probability for an olefin (instead of for a paraffin) increases with increasing temperature and decreasing pressure.

The temperature dependence is easily understood favouring the dissociative

.

°

CARBON NUMBER Nc

Fig. 6. Degree of hydrogenation conversion of olefins during competitive hydrogenation in a pulse reactor; Sample: product of FT-synthesis. Hydrogenation conditions: Pt/Chromosorb P, 100 ° C, 3 bar.

300

100

• ' u _ _ 100 Fe 1080 Mn 250'C .7.Sbor 15bar I

~ 20"

" i 0 ' ~' ~' g' e ' lb ' fz ' lZ ' '~ ' ~ ' g ' d '1'o'1~'1~' ' ~ ' / ; ' g ' ~'lb'4z'lZ'

I:ARBON NUMBER . N c

Fig. 7. Olefin content of hydrocarbon-carbon number fractions of various Fischer-Tropsch reaction products. Left: Reaction temperature 250°C, 10 bar, H2/CO=2 [15]; middle: 100 Fe:540 Mn: 50 Aerosil: 10 A1203:13.3 K20; 10 bar; H2/CO = 1.9, ca. 600 h - 1 [5 ]; right: 100 Fe : 877 Mn : 29 KzO; 235°C, H2/CO= 1.7; 100 h -1 [19].

1.0

0.6-

~0,6- o -

"6 0.~- .

0,2-

0 I I i I i i i i

z00 z~o 300 0 ~ lb lb zb is 3b l ' . °C P. b0r

Fig. 8. Olefin-chemidesorption (Pd,ol) a8 a function of temperature (left) and pressure. Values derived from Fig. 7 (middle and right).

desorption reaction on account of the associative desorption reaction of the alkyl species (Fig. 2).

Increase in pressure favours the associative chemidesorpt ion of the alkyl together with hydrogen and can be interpreted as an increase in availability of hydrogen with increasing pressure (the more strongly adsorbed CO will reach saturation on active sites at relatively low pressure).

Olefin double bond isomerization

It has been observed tha t pr imary Fischer -Tropsch selectivity generates olefins with terminal double bonds, thus making the system attractive for direct product ion of c~-olefins from synthesis gas [ 11 ].

301

In Fig. 9 the molar fraction of c~-olefins among the straight chain olefins of the same carbon number is plotted as a function of carbon number for the same experiments as used in Fig. 7. Comparison of the curves in Figs. 7 and 9 easily leads to the conclusion that double bond shift and secondary olefin hydrogenation proceed via a common intermediate as pictured in the kinetic scheme of Fig. 10.

The addition of the first hydrogen atom to an olefin then is a fast and re- versible reaction and the addition of the second hydrogen is comparatively slow and irreversible in Fischer-Tropsch hydrogenation systems.

The double bond isomerization in Fischer-Tropsch systems should proceed stepwise e.g. with a gas phase intermediate 2-pentene for producing 3-pentene. This is in agreement with many observations which show a great predomi- nance offl-olefins, the first product of c~-olefins isomerization. The concentration of fl-olefins in products which show the characteristics of primary FT- selectivity is only about 3 hundredths of that of the ~-olefins. It cannot pres- ently be decided if this small amount of fl-olefins is of primary or secondary nature.

The shape of the curves in Fig. 9 which indicate secondary double bond isomerization is interpreted as reflecting increasing conversion-probability with increasing carbon number due to an increasing adsorbability. Because of this

100

" "

60-

n . . ¢ ,n

• ~__ , 20-

T ~ o

. . . . . . . . n m

" - " " ; ' o ; , - , o o o M° " "

! | i v i i i i i w i v

/. 6 g 10 12 14

,

2 7 e ' ¢ ~

t |

~ ' g ' i ' 1 b ' 1 ~ ' 4 ~ ' CARBON NUHBER .

~. ' ~ ' ~ 'Ib' Ii' ~'~'

N¢ Fig. 9. Molar fractions of ~-olefins among the straight chain olefins of the same carbon number. Catalysts and reaction conditions as in Fig. 7.

+ H + H R ~ C H 2 - - C H ' ~ - C H 2 ~ R ~ C H 2 - - C H 2 - - C H 2 ~ R - - C H 2 - - C H 2 - - C H 3

R ~ C H z ~ C H ~ C H 3

R - - C H ~ C H ~ C H 3

Fig. 10. Kinetic scheme of secondary olefin hydrogenation and double bond shift during Fischer-Tropsch synthesis.

302

systematic behaviour a misinterpretation of Anderson-plots - regarding the fl- olefins as primary products - is possible.

CONCLUSION

On the basis of the presented kinetic model of FT-CO hydrogenation as a non-trivial polymerization-conversion, the repetitive elementary steps of surface reaction within the reaction network are linked to the overall rate of CO consumption. Now from an experimental product distribution for each of the reaction steps its reaction rate, the probability of reaction and the relative rate constant can be calculated. Thus, it is possible now to investigate how the rate constants of elementary reaction steps vary with carbon number of the products, respectively carbon number of the chemisorbed species and how they vary with reaction conditions (partial pressures of reactants and products, reaction temperature) and with catalyst properties (as active metals, promotors, mode of preparation and pretreatment, respectively surface area, dispersion, phase composition, etc. ). This paper has been restricted to the formation of olefins and paraffins. Recently we have succeeded in making Fischer-Tropsch products of actually primary nature with respect to the olefin and paraffin compounds [ 11 ]. The olefins of these product slates are those which originate from desorption from Fischer-Tropsch active sites. They belong almost exclusively to the class of olefins with terminal double bond. Only these products of primary nature allow for conclusions with respect to reaction rates of individual reaction steps within the reaction network of the FT-conversion. In a preced- ing publication [ 1 ] the chain growth reaction step and its deviation from an ideal polymerization system has been treated. In a future paper chain branching will be discussed [ 7 ].

LIST OF SYMBOLS AND DEFINITIONS

Sp Sp5 SP~,b~ g

g5 gs,br

Organic surface species Surface species with 5 carbon atoms Methylbranched-surface species with 5 carbon atoms Growth step of chain prolongation of an organic surface species by one carbon atom. Stoichiometrically it concerns the consumption of 1 CO and 2 H2 and the formation of one CH2 and one H20 Linear growth step of the surface species with 5 C-atoms Growth step of the surface species with five C-atoms under formation of a methyl side group

ds,sat

d5 ,ol ds,ox

P Pgn

Pal,

kg, k d M,

dnc ..... /dt dnc,form/ dt

dnc,fo ..... ,/dt dnc,fo~,co2 / dt dnpJdt

dni,pJ dt NC Fr~

303

Reaction step of desorption of a surface species. Physide- sorption is assumed to be generally fast as compared with the rate of chemidesorption. Thus physisorption is neglected in the kinetic scheme and in order to stress the point that product formation through desorption concerns a chemical reaction, the more specific term chemidesorption is used in this paper. In the case of paraffin chemidesorption this reaction step is of associative nature: the reaction between an alkyl- with an H-species. In case of olefin chemidesorption a dissociative reaction of the alkyl species to form an olefin and an alkyl is very probable. Chemidesorption step of surface species of carbon number 5 to form a paraffin C5 Chemidesorption step to form an olefin C5 Chemidesorption step to form an organic oxygen compound C5 (alcohol or aldehyde) Probability of reaction of a distinct surface species Probability of chain prolongation of a surface species with n C-atoms Probability of chain termination of the species with n C- atoms Rate constants of reaction through growth or desorption Moles of product compounds "n" (in a normalized product distribution ) Molar rate of consumption of CO Formation rate of all carbon containing compounds on a carbon basis Formation rate of organic compounds on a carbon basis Formation rate of CO2 on a carbon basis

Molar formation rate of the sum of all organic product compounds Molar formation rate of a product compound "i" Average carbon number of all organic product compounds Molar fraction of compound "i" among the sum of all organic product compounds

REFERENCES

Schulz, H., Beck, K. and Erich, E., 1987. Methane Conversion, Proc. Symp. on the Produc- tion of Fuel and Chemicals from Natural Gas, Auckland, April 1987. Bibby, D.M. Chang, C.D., Howe, R.F. and Yurchak, S. (Eds.). Elsevier Science Publishers, Amsterdam.

304

2 Storch, H.H., Golumbic, N. and Anderson, R.B., 1951. The Fischer-Tropsch and Related Synthesis, Wiley, New York.

3 Pichler, H., Schulz, H. and Elstner, M., 1967. Brennstoffchem., 48: 78. 4 Flory, P.J., 1936. J. Am. Chem. Soc, 58: 1877. 5 Anderson, R.B., 1956. In: Emmett, P.H. {Ed.). Catalysis IV. Reinhold, New York. 6 Schulz, H., 1985. C~ Mol. Chem., 1: 231. 7 Schulz, H., Beck, K. and Erich, E., 1988. Proc. 9th Int. Congr. on Catalysis, Calgary. 8 Egiebor, N.O., Cooper, W.C. and Wojciechowski, B.W., 1985. Can. J. Chem. Eng., 63: 826. 9 Schulz, H., Bamberger, E. and Gorre, H., 1984. Proc. 8th Int. Congr. on Catalysis, Berlin

1984. Dechema (Eds.) Verlag Chemie. Weinheim. 10 Schulz, H., BShringer, W., Kohl, C.P., Rahman, N.M. and Will, A., 1986. ErdSl und Kohle,

39: 333. 11 Schulz, H. and GSkcebay, H., 1982. In: Kosak, F.R. (Ed). Catalysis of Organic Reactions.

Proc. Ninth Conference on Catalysis of Organic Reactions. Charleston, S.C., U.S.A., April. Marcel Dekker, NY, 1984, p. 153.

12 Schulz, H., RSsch, S. and GSkcebay, H., 1981. Proc. Int. Conf. on Coal Science, Dtisseldorf, F.R.G., September. Verlag Gliickauf GmbH. Essen.

13 Schulz, H. and Parsche, G., 1978. Unpublished results. 14 Schulz, H. and Erich, E., 1983. Unpublished results. 15 GSkcebay, H., 1982. Dissertation, Universi~t Karlsruhe. 16 Schulz, H. and Beck, K., 1985. Unpublished results.

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Kinetics of Fischer-Tropsch selectivity