PD A Simplified Kinetic Scheme

M. Guisnet et al. (Editors), ~etnogenaous&tdysis a d Fine Chemicals III aD 1993 Elnevier Science Publishers B.V. All rights reserved.

329

CATALYTIC SYNTHESIS OF 2-METHYLPYRAZINE OVER ZN-CR-OPD A SIMPLIFIED KINETIC SCHEME

*

Lucio Forni and Roberta Miglio Dipartimento di Chimica Fisica ed Elettrochimica Universita' di Milano. Via C.Golgi, 19 20133 Milano. Italy ABSTRACT

The kinetics of cyclisation of ethylenediamine and propylene glycol to 2-methylpyrazine has been studied at 623 to 663 K and in excess of steam. under the optimised reaction conditions put in evidence by a previous extended research on this process. In spite of the complexity of the reaction, the present results allowed to define a very simple kinetic model, by means of which the behaviour of the system can be satisfactorily described through a set of only five pseudo-first order rate equations.

INTRODUCTION The most important usually accepted mechanisms of alcohol amination with ammonia or amines are: i) The so called dehydroamination, in which the dehydrogenation of the alcohol to a carbonyl derivative takes place first on a hydro-dehydrogenation catalyst, followed by reaction of the latter with the amino group to give an imino intermediate, further hydrogenated to the amine. An excellent review paper [l] has been published quite recently on this subject. ii) The so called dehydrative amination, taking place on a dehydration catalyst 12-41, in which a dehydration between the amino and the hydroxyl groups leads directly to the amine. A dehydrogenation with formation of a A1-dehydrogenated product may follow. quickly proceeding to a final heterocyclic compound, if the reaction is carried out on an a,@-aminoalcohol of the proper chain length 141. A n extended explorative study [5-101 carried out in our laboratory put in evidence the practical feasibility of a two-step catalytic route for the preparation of 2-amido-pyrazine. The first step of the process may be considered a double alcohol amination and consists in the cyclisation of ethylenediamine (ED) and propylene glycol (FG) to 2-methylpyrazine (MP), which can be carried out quite satisfactorily in vapour phase over a Zn-Cr-O/Pd catalyst. A further, detailed mechanistic study [11,121 showed that the main reaction can be described by a dehydration, followed by multiple quick dehydrogenation steps, perfectly in line with what suggested in literature [1,41.That study indicated also the role played by Pd in increasing selectivity. by favouring the dehydrogenation following the initial dehydration and suggested a possible Rideal-Eley step between adsorbed PG and ED coming from the gas phase, as rate-determining, at least at low ED partial pressure. However, the overall reaction proved to be quite complex, since up to ca.30 different species could be detected in the reactor effluent. Fortunately, most of them form only in trace, so that, by properly choosing the reaction conditions, a selectivity to MP exceeding 80% at total conversion can be attained. Hence, a simplified reaction scheme could be forecast, able to describe the general behaviour of the system by taking into account only the most important species. The present paper

reports on a kinetic study of the dehydration-dehydrogenation of ED+PG to MP, aiming at defining the principal kinetic equations of such a simplified scheme and at determining the values of the relative kinetic parameters, useful for the development of a more detailed pilot-plant study.

EXPERIMENTAL The Zn-Cr-O/Pd catalyst was prepared as described in previous work ill]. The principal characteristics y e : Z d C r atomic ratio33/1, Pd concentration 1 wt %, BET surface area 52.9 m /g, pore volume 0.4 cm /g, bulk density 1.12 g/cm 3 , particle size 0.15 to 0.18 mm. Details of the fixed-bed, continuous, microreactor assembly and of the GC analysis of the reactor effluent are given elsewhere [61. The identification of the various species was done by GC-MS. The overall mass balance around the reactor was ca. 100% (99f3. including the experimental error). The principal products, covering more than 98% of the organic matter, with respect to the reactants transformed, were MP, dihydro-2-methylpyrazine (DHMP), acetone ( A ) , pyrazine (PI and dimethylpyrazine (DMP). Due to the practical purpose of the present study, all of the analytical data take into account only these species, besides reactants, neglecting minor byproducts. RESULTS AND DISCUSSION

Kinetic data have been usually expressed in terms of conversion Ci (mol % of the reactant i transformed into products) and yield Yj (mol % of the j-th product/mol of converted ED), referred to the principal reactant ED. Differential reactor technique runs. Two series of runs have been carried out at 643 K and atmospheric pressure, by feeding 2 cm3/h of a 10 wt % aq. Fig.1 - Initial reaction rate ro vs. partial pressure of each reagent. ConFtant valueos (Torr) of: (01 p,= 7.6 (p,, in abscissa), (01 piG= 11.7 (piD in abscissa).

solution of ED+PG and charging 0.05 g of catalyst, diluted with pyrex beads. Some blank runs in the absence of catalyst, but in the presence of such beads, showed no conversion at any temperature. The range of partial

331 pressure explored for each of the two reagents was ca. one order of magnitude wide. Of course, the analytical samples for every run were collected only after the system had attained the steady state, f.e. ca. 4 hours after starting the reactants feeding. The results are shown in Fig.1.

The trend of the data for constant piD is not in contrast with a Rideal-Eley mechanism, controlled by the surface reaction between adsorbed PG and ED coming from the gaseous phase. A possible interpretation is given in Fig.2. In it the adsorption of PG takes place through the interaction of the couple of electrons present on the oxygen of the terminal hydroxyl with a Lewis acid site of the catalyst. The slow process is the attack of the carbon atom adjacent to the activated hydroxyl group by the amino group of the ED. The kinetic equation describing such a mechanism can be written as

100 -2

c

60

: L

I

0.1

0.3

I

1

2

3

4

Fig.4. Dependeace of reaction rate on p p,,= 11.7 Torr ED.

l/PK

Fig.3. Fitting of eq. l/r=m/pPc+q to exptl. data. k (mol of conv. ED/h x g cat.) being the kinetic constant, bi (Tom-', 1 Torr = 133 Pa) the adsorption equilibrium constant and pi (Torr) the partial pressure of the f-th species, respectively, the subscript w indicating water. In the present case p , is constant and, due to the excess of water, also bRw is practically constant, so that we have

l/r = m/pPc + q , where

332 pmbm and q = bpG/k pDbED. The fitting of eq. l/r = m/ppc+ q to our experimental data is shown in

m = (l+b#,,)/k

Fig.3. From the best straight line (least-squares method, corr. factor = 0.992) the values of m = 222.2 (h x g cat. x Torr/mol of conv. ED) and q = 12.94 (h x g cat./mol of conv. ED) were calculaoted. As for the results obtained at constant p (see Fig.1). the range of PG

experimental data should be divided into two parts. When <,p

12 Tori-, Eq.1

is still able to describe the behaviour of our system, since, for constant values of both ,p and pw, (l+b,,p,,+bpGp,) is also constant and r = k"p,. Fig.4 shows a slope 1 for the best straight line (corr. factor = 0.999) drawn through the experimental points for p < 12 Torr. However, beyond this ED

value, the data deviate markedly from linearity, indicating that the previous mechanism is accompanied by a competitive adsorption of ED. Therefore, the secondary reactions, especially the condensation of two molecules of ED to P, are no more negligible. Integral reactor technique runs. Our previous research 16,101 showed that the ED/PG molar ratio in the feeding mixture has to be controlled, in order to get a good yield to MP. Those results limit considerably the range of experimental conditions, under which integral reactor technique data, useful for practical purposes, can be collected. So it was decided to $ollect suFh data at the maximum value of the reactants partial pressure p and p, ED

40

r (hr.9 cat./rnol ED fed1

Fig.5. Integral reactor data. (A) D W , ( 0 ) MP. ( 0 ) P. (A) A, (01 DMP. Solid lines calc. by Model 3 eq.s with optim. parameters of Table 1. T = 623 K.

60

r 1hr.g cat./mol ED fed1

Fig.6. As for Fig.5. T = 643 K.

(7.64 and 11.67 Torr, respectively), still avoiding unacceptable wasting of runs were reactants to byproducts. Therefore, all the integral react:r carried out at atmospheric total pressure and by feeding 2 cm /h of an aq. solution containing 3.55 and 6.45 wt X of ED and PG, respectively, together

333 3

with the usual 16,101 small flow (3 Ncm /min) of N, as carrier gas. Catalyst weight ranged from 0.005 to 0.100 g and temperature from 623 to 663 K. Under these conditions. diffusive intrusions are negligible, as verified previously [lo]. The results, expressed in terms of Yi, are shown as experimental points in Fig.5-7. The trend of data agrees with the principal reaction sequence ED + PG + DHMP + MP, with DMP forming through a parallel reaction. Furthermore, as a first approximation, A has been assumed to form exclusively by dehydration of PG. As for P, several pathways can be imagined, e.g. by demethylation of MP, or by condensation of two molecules of ED, etc. Hence, the different models considered take into account one or more pathways to this species. A simplified kinetic scheme, taking into account all the mentioned hypotheses, takes the form ED+PG A D H M P 4

2

M

P &P

(2)

'L5

&4

&6

P

A DMP + P in which all pseudo-first order reactions with respect to the principal reactant can be assumed, except reaction 6, which is of second order with respect to ED and. of course, reaction 1, which is of overall second order, being first-order with respect to both ED and PG. I

0

20

40

60

80

1.50

-

I

1.55

Fig.8. Arrhenius plot, Table 3 data. (0) k ,. (A) k,, (A) k,, (01 k,, ( 0 ) k,.

The stoichiometric equations referring to this scheme are

-

ED + PG DHMP + 2 H,O + 2 H, D+PG - M P + 2 H , O + 3 H 2 P + H,O + H, + CH,OH ED + PG FG A + H,O ED + PG d 0 . 5 DMP + 0.5 P + 2 H,O 2ED - P + 2 N H 3 + 3 H ,

1.60

lo3/ T , K

r (hr 1 g cet./md ED fed)

Fig.7. As for Fig.5. T = 663 K.

'

+ 3 €I,

334 By referring to 100 mol of ED fed and by taking into account the feeding rates of the various substances (vide supra), the molar feeding ratios of the various species are ED:PG:H20:N2 = 100:142.9:8417.7:617.9. Hence =,p ( 1oO-nDm-nw-np3-2n -2n I/& i , =,p ( 142.9-nDm-nw-np -2n -nA)an i '5

3

6'

( J = A,P.MP.DHMP,DMP), where n

and pj= nj/Zni

p5

represents the mol of P

k'

formed through the k-th reaction. Furthermore, since nMeOH=n , n n = H

2nDHnp+3n +n +6n +3n UP

2

'3

5 '

6'

and

nW =

8417.7+2nDm+2nUP+nA+np+4np ,

have Xn i = 9278.5+3nDHnp+4nw+nr+nDw+2np+ 7np +4np . 3

=2n ,

NH 3

p3

5

3

'6

we

5

6

From our definition of yield Yi, we have ni = mol of ED fed to the reactor. S o , for no 400, ni ED summation including the "yield" of ED and PG, 1 . e . reactants, calculated by difference with respect system of rate equations takes the form

YiniD/lOO. n;, being the = Yi and Zni = ZYi, the the mol % of unconverted to the feed. Hence, the

dYDm/dt = (klYEDYPC-k2YDm-k5YDm )/I?f i dYw/d.c = (k2YDm-k3YUP)/ZY i dYDw/dt = k5YDm/ZY i dYA/dz = k4Y,/ZY i dYp/dr = (k3Yw+k5YDm)/ZY i

+

k6(YED/ZYi l2

(4)

The present scheme defines the molar ratios among the various species. However, it cannot give any information about the quantitative ratio between the amounts of P produced through different routes. This has been determined by formulating an hypothesis about these relative amounts and by verifying through a non-linear regression-optimisation procedure the agreement between calculated and experimental data. Three hypotheses have been formulated within the framework of the general scheme 2. The first one, referred to as Model 1, is based on the results of some auxiliary runs, carried out under properly chosen experimental conditions, so to isolate the particular reaction under study. For instance, by feeding an aqueous solution of pure ED, it was observed that the amount of P formed by condensation of 2 molecules of ED (reaction 6 in scheme 51 is about l(10 of the overall amount of P found under the same conditions, but feeding the standard solution. On the other hand, by feeding an aqueous solution of MP only, it has been noticed that the amount of P forming by dealkylation of MP is about 2/10 of the total obtained with the standard solution. As a consequence, in Model 1 the following assumption was done: n :n :n = 2:7:1. In this way, no explicit relation between P and DMP is '3

'5

6'

can be taken into account

needed, since the previous ratios among the n,

-implicitly through Eq.s 3 and the following expressions: p, (lOO-nDHnp-nUP-l. 8np)/Zni and p, = (142.9-nDm-nw-nA-l. 6np)/Zni. The second hypothesis (Model 2 ) gives more weight to the transalkylation (reaction 5 of scheme 2). by assuming that only the moles of P exceeding 1

33s DMP

those of

can form

through

( 100-nDHwp-nIIP-2/3nDIIP-4/3np )/Zn i

reactions and

-2/3np)/Xn i .

3

=,p

or 6. Therefore, pm= ( 142.9-nDm-nw-nA-4/3nDw

It may be observed that with Model 1 the error affecting YDWp is less important than that affecting Yp. while the opposite occurs with Model 2. The third hypothesis (Model 3) assumes that the condensation of ED to P (reaction 6 of scheme 2) is negligible and that the excess of P, with respect to DMP, forms exclusively by dealkylation of MP. Therefore, and n = 0. n =n -n DIIP’

Pg P

nP = nDIIP

P

5

6

After modification of the system 4 of rate equations, according to the Table 1. Optimised kinetic parameters of Model 3 (Scheme 6) Parameter

k, k, k, k, k,

623

(mol/h x (mol/h x (mol/h x (mol/h x (mol/h x

g

g

g g g

cat. cat. cat. cat. cat.

x x x x x

2

atm 1 atm) atm) atm) atm)

Temperature, K 643 663

330. 176. 3.43 6.53 0.017 0.033 0.084 0.280 0.079 0.121

980. 7.40 0.046 0.790 0.136

Table 2. Arrhenius parameters for the reactions of Model 3 (Scheme 6).

Ea (kcal/mol)

1nA

ED+PG%DHMP

3526

33f5

D W % M P

16f6

14f5

M P 5 P

20f4

12f3

PG

4621

3521

1154

7f3

Reaction

A‘

D W ”-, DMP + P

various models, the kinetic parameters (kl to k6) have been evaluated by minimising the objective function @ =

N

X 1=1

7 max

X (YE,l,JYc,l,,)2/D

J=O

indicating experimental and calculated data, respectively. The best fitting was obtained in any case with D = Models 1 and 2 gave practically identical results, with k6 values of the (mol/h x g cat. x ah2). This means that reaction 6 in scheme order of lo-' 2 is negligible and that Model 3, the scheme of which is P &P

ED+PG L D H M P L M J'r J.5 A DMP + P

in spite of the lower number of parameters. can represent satisfactorily our reacting system. The optimised parameters of Model 3 are shown in Table 1. The agreement between experimental data and the curves, calculated through Model 3 equations, together with the latter parameters, is shown in Fig.5-7. By means of the well-known equation In k = In A - Ea/RT and of Table 1 parameters, the Arrhenius plots of Fig.8 were drawn, from which the values of the apparent activation energy E, (kcal/mol) and of the preexponential factor A (mol/h

x g

cat.

x

atm") were evaluated (Table 2).

The principal conclusions one can draw from the present results are: i) The differential reactor technique runs showed that, within the explored range of reaction conditions, the overall reaction rate is practically of first order with respect to both reactants. ii) This allows to write an extremely simplified reaction model, in which only five reactions are considered. showing a satisfactory interpretation of the whole set of data, collected by the integral reactor technique. iii) No nore complex models are needed for practical purposes, since the reaction conditions cannot be very different from the present ones, in order to get good yield and satisfactory catalyst life, as reported [6,101. iv) The kinetic equations and parameters so obtained may constitute a safe basis for the design of a pilot plant for the further development of the process. REFERENCES

1) A.Baiker and J.Kijenski, Catal.Rev.-Sci.Eng., 27 (1985) 653. 2) Y.Takita, Y.Nishida and T.Seiyama, Bull.Soc.Chem.Jpn., 49 (1976) 3699. 3) W.W.Kaeding, US Pat. 4082805 (1978). 4) W.Hammerschrnidt, A.Baiker, A.Wokaun and W.Fluhr, Appl.Catal., 20 (1986) 305. 5) L.Forni, Appl.Catal., 20 (1986) 219. 6) L.Forni, G.Stern and M.Gatti, Appl.Catal., 29 (1987) 161. 7) L.Forni, C.Oliva and C.Rebuscini, J.Chem.Soc., Faraday I, 84 (1988) 2397. 8) L.Forni, J.Catal., 111 (1988) 199. 9) L.Forni, Appl.Catal., 37 (1988) 305. 10) L.Forni and S.Nestori, in M.Guisnet et al.(Eds.) Heterogeneous Catalysis and Fine Chemicals, Elsevier. Amsterdam 1988, p.291. 11) L.Forni and P.Pollese1, J.Catal., 130 (1991) 403. 12) L.Forni and R.Miglio, in M.Guisnet et al. (Eds.) Heterog.Cata1. and Fine Chemicals 11, Elsevier. Amsterdam, 1991, p.367.