the kinetics of quinoline hydrodenitrogenation through reaction intermediate products

Tine Kinetics of Quinoline Hydrodenitrogenation through Reaction Intermediate Products

AHMED KADRY ABOUL-GHEIT Petroleum Department, NationalResearch Centre, Dokki, Cairo, Egypr

Received September 17, 1974'

AHMED KADRY ABOUL-GHEIT. Can. J. Chem. 53, 2575 (1975). Under petroleum hydrotreating conditions and in the presence of a Co-Mo-alumina

catalyst, quinoline is hydrodenitrogenated through a consecutive first order reaction involving a hydrogenation step followed by two hydrocracking steps. The first step is too fast to measure whereas the second step is the slowest. The ratio of the rate constants of the second to the third step ranges between 0.33 and 0.62 at reaction temperatures between 350 and 400°C. The second step can be considered responsible for the relatively high activation energy and enthalpy values obtained for the overall reaction. The activation entropy obtained for the overall reaction is -38.2 e.u. mol-', while the second and the third steps have values of -43.9 and -58.0 e.u. mol-', respectively. These values indicate the progressive complexity of the steps involved in the reaction sequence as it proceeds towards ammonia production.

AHMED KADRY ABOUL-GHEIT. Can., J. Chem. 53, 2575 (1975). Dans les conditions de traitement du petrole et en presence d'un catalyseur Co-Mo-alumine,

la quinoleine est hydrodenitrogenie en passant par une reaction du premier ordre impliquant une Btape d'hydrogenation suivie de deux ttapes d'hydrocraquage. La premiere etape est trop rapide pour Ctre mesuree alors que la deuxieme est la plus lente. Le rapport des constantes de vitesse de la deuxitme a la troisieme Ctape se situe entre 0.33 et 0.62 pour des temperatures de reaction de 350 a 400 "C. La seconde Btape peut &tre considCree comme etant responsable des valeurs relativement ClevCes de I'energie d'activation et de I'enthalpie obtenues pour la reaction totale. L'entropie d'activation obtenue pour la reaction globale est -38.2 u.e. mol-', alors que les deuxieme et troisieme etapes ont respectivement les valeurs de -43.9 et - 58.0 u.e. mol-'. Ces valeurs indiquent la complexite progressive des etapes impliquees dans la sequence des reactions qui conduisent a la production d'ammoniac. [Traduit par le journal]

Introduction Hydrodenitrogenation (HDN) is a petroleum

hydrotreating reaction of prime importance since nitrogen compounds cause serious prob- lems in the processing and storage of petroleum products. Under hydrotreating conditions, HDN is much more difficult than hydrodesulfurization (1, 2). For high HDN levels, high hydrogen pressures are employed (3, 4). Temperatures above 400 "C are undesirable in hydrotreating since they enhance hydrocracking reactions that result in lower liquid yields, higher hydrogen consump- tion, and lower viscosity of lubricating oil stocks.

Hydrodenitrogenation is a stepwise process involving hydrogenation and hydrocracking reactions (4-9). The catalysts employed in this process possess both hydrogenation-dehydro- genation and cracking functions (dual functional catalysts). Over such catalysts, the petroleum nitrogen compounds are converted through

'Revision received April 15, 1975.

intermediate products to ammonia. Each of these intermediates possesses different adsorptivity on the catalyst surface (9).

The subject of this paper is to study the kinetic and thermodynamic parameters for quinoline HDN involving the formation and conversion of intermediate products in order to clarify the reaction mechanism.

Experimental Catalyst

A catalyst containing 3% cobalt oxide and 10% molyb- denum oxide was prepared by impregnating 4 in. extrudates of y-alumina with cobalt nitrate solution, drying at 110 "C overnight, and calcinating at 550 "C for 4 h. Another impregnation with ammonium molyb- date solution was then carried out followed by drying and calcination as above. The catalyst was then ground into 60-80 mesh particles because this has been shown to be the optimum size (5). The surface areas (measured by the BET method) of the alumina support and the prepared Co-Mo-alumina catalyst were 228 and 196 m2/g, respectively.

Feedstock Extra-pure grade Merck quinoline was redistilled under

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CAN. J . CHEM. VOL. 53, 1975

k o

(The overall HDN

step I I I step2 1 Hydrogenation Hydrocracking H ydrocracking

reduced pressure, then dissolved in a high purity paraffin oil. The oil exhibited no absorbance in the ultraviolet spectral region. The ebullioscopically determined average molecular weight of the oil was 185. The concentration of quinoline in the oil was 4.6 wt.%. i.e., equivalent to 0.5 wt.% nitrogen. The quinoline solution was preserved in a dark container under nitrogen atmosphere and used as the feedstock.

Hydrogenation Procedure The HDN experiments were carried out in a 2-1 batch

autoclave. After closing and purging twice with nitrogen and twice with hydrogen, the autoclave was filled with high purity hydrogen to a predetermined initial hydrogen pressure such that the operating pressure at the working temperature was 200 atm. The pressure dropped l?y only 3 atm after a reaction period of 8 h at 400 "C. The pressure drop was even less at lower temperatures and shorter reaction periods, The autoclave was heated to the reaction temperature, then a mixture of 250 g of the feedstock and 25 g of the catalyst was introduced into the reactor via a high pressure injector. At this moment, the autoclave's magnetic stirrer was switched on at a velocity of 1500 r.p.m. and the time was considered as zero. This stirring velocity has been shown to be optimum (5). Liquid samples of 2 ml volume were withdrawn from the system every hour, except for the first sample that was takcn after a reaction period of + h. The catalyst was removed from the sample by filtration while the dissolved ammonia was removed by bubbling a gentle current of nitrogen gas.

Analysis The product samples were analyzed to determine the

nitrogen content using a micro-Dumas technique via a olem man Nitrogen ~ n a l ~ z e r model 29. The individual nitrogen comvonents were isolated from the vroduct sampies with 5% aqueous HCI solution then freed with 10% NaOH solution and extracted by ether. The ether was evaporated and the concentrate of nitrogenous com- ponents was analyzed by gas-liquid chromatography using an SE-30 column. The apparatus and conditions were described elsewhere (6, 7). Lower anilines (aniline, o-toluidine, and o-ethylaniline) in the products were always less than 2.0% and were calculated as o-propylaniline, whereas alkylquinolines were always less than 3.5% and were calculated as unreacted quinoline. A spectrophotometric search for pyrrolic nitrogen compounds in the product samples (10) showed the presence of negligible quantities only.

Results and Discussion The 'hydrotreating process is carried out at

relatively mild temperatures and high pressures, using supported or unsupported metal oxide or sulfide catalysts. Under such conditions, the hydrocarbon composition of the oil to be hydro- treated is only slightly affected, whereas the hetero compounds, including sulfur, nitrogen, and oxygen compounds, are greatly transformed. In this investigation, the paraffin-oil diluent was slightly hydrocracked, whereas the quinoline molecule was hydrodenitrogenated through a series of hydrogenation and hydrocracking reactions to yield ultimately ammonia and the "clean" hydrocarbon n-propylbenzene as en- visaged in Scheme 1, where the subscripts 0, 1,2, and 3 designate the overall HDN reaction, the first, second, and third step in the HDNsequence, respectively.

The overall process rate constant, k,, has been s h k n (5) to be first order with respect to quinoline via both integral and differential approaches, hence, k, is herein graphically calculated from a plot of log (1 - MD) us. t , as given in Fig. 1 .

From Fig. 2, it is evident that virtually all of the quinoline reacted is converted to 1,2,3,4-

I I I I I I I I I I I 0 1 2 3 4 5 6 7 8

REACTION TIME, h

FIG. 1. First order plot for the overall hydrodenitrogenation of quinoline.

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ABOUL-GHEIT: KINETICS OF QUlNOLME HYDRODENITROGENATION 2577

REACTION T I M E , h

FIG. 2. Kinetic curve for quinoline hydrodenitrogenation at 350 "C.

tetrahydroquinoline at an early stage in the HDN process. The conversion is so fast that the molar concentration of the tetrahydroquinoline reaches 96% after + h of reaction. In this case, it appears reasonable to calculate k2 from a plot of log , (1 - MB) us. t, as given in Fig. 5. The straight

I line relationship obtained confirms that step 2 in

i Scheme 1 follows first order kinetics and sub- stantiates the feasibility of the approach used to calculate k,.

The instantaneous rates of component concentration changes, considering first order reaction of the consecutive type are given by the following

I series of equations :


REI\CTIOII T I M E , h


TABLE 1. Reaction rate constants at hydrotreating temperaturesa

[1 1 dMA/dt = - klMA Temperature ("C)

[2 I Rate constant

dMB/dt = klMA - k2MB x lo4 (s-') 350 375 400

[3] dMc/dt = k2MB - k3Mc klb Fast Fast Fast

[41 kz 0.204 0.454 0.908 dMD/dt = k3MD k , 0.623 0.884 I .475

where M is the compo~ent concentration, mole fraction, and the subscripts A, B, C, and D designate quinoline, 1,2,3,4-tetrahydroquinoline, o-propylaniline, and ammonia, respectively. The approach of quasi-stationary concentrations (steady state) is used to calculate k,, whereby the term dMc/dt in [3] is set equal to zero at a definite reaction time, t,,,, where tmaX is the reaction period at which o-propylaniline is of maximum concentration. From Figs. 2-4, it is evident that t,,, is 9, 7, and 6 h at reaction temperatures of 350, 375, and 400 "C, respectively. The constants obtained for steps 2 and 3, as well as the overall HDN rate constant, at hydrotreating temperatures, are given in Table 1.

The ratio of k2/k3 is 0.33 at 350 "C, 0.51 at 375 OC, and 0.62 at 400 "C. The fact that step 2 is the slowest step in the HDN of quinoline is

ko 0.137 0.332 0.767

ORate constants rcfer to the use of 25 g of catalyst. bk, is too large to be measured by the methods used in this study.

also supported by Larson (9) who has found the k2/k, ratio to be 0.80. However, Larson does not indicate his method for calculating these rate constants.

The temperature dependence of the rate constants gives linear Arrhenius plots as shown in Fig. 6. Accordingly, the rate constants can be represented by [5-71.

Since the HDN reaction is carried out in the liquid phase (ll), [8] will relate the energy of

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2578 CAN. J. CHEM. VOL. 53, 1975

0.0

Value Value Parameter (kcal mol-I) Parameter (e.u. mol-I)

23.330 AS, * -43.9 13.265 AS3 * -58.0 27.445 ASo * -38.2

-1 -08 - The values of Ea and AH* obtained for step 2 (the slowest step) do not significantly differ from those obtained for the overall HDN reaction; thus step 2 can be considered responsible for the

1 1 b relatively higher values of Ea and AH* obtained REACTION T I M E , h

for the overall reaction (5). The magnitude of the entropy values obtained

FIG. 5. ~irst 'order plot for the second step in quino- in this study shows that the activated complex is line hydrodenitrogenation. an immobile molecule attached to the catalyst

-a,2 surface and the product molecule is much like the activated complex. Since the entropy of activation for desorption is expected to be small and positive, or nil, the entropy values obtained cannot be associated with desorption as the rate controlling process. The high hydrogen pressure employed in this study should greatly influence

-1.0 - the product desorption. The magnitude of the

-1.2 - activation energies and enthalpies obtained also suggests that diffusion is not the rate controlling

-1.4 - process. I I I I I I

1.475 1.500 1 .525 1.510 1.575 1 .6m Since step 1 in Scheme 1 is too fast to measure, l / ~ 103 K it is not possible to calculate AS,* from the

FIG. 6. Arrhenius plot for the overall reaction, the second, and third step.

activation to an enthalpy of activation

and accordingly the calculated average AH* values, in the temperature range of 350-400 "C, are as follows: AH2* = 23.400 kcal mol-'; AH3* = 12.880 kcal mol-'; AHo* = 27.290 kcal mol- ' .

According to the theory of absolute reaction rates (12), the rate constant is related to the entropy and enthalpy of activation by [9].

PI k T AS* AH* k h (T- --) R T

where k, = Boltzmann's constant, 1.380 x 10-l6 erg/K. The following values of enthalpy and entropy of activation are calculated from plots of log k/T us. 1/T (Fig. 7) by applying [9] :

experimental data. Nevertheless, AS,* value of -38.2 e.u. mol-' compared with a value of -43.9 e.u. mol-' for AS2* and -58.0 e.u. mol-' for AS3*, is indicative of a much smaller negative value (or even positive) for AS, *.

When the quinoline molecule is first hydro- genated to the tetrahydroquinoline (which is expected to be more strongly basic than quinoline) (9), this product competes for the catalyst's

- 3 . 0 , I

FIG. 7. Eyring plot for the overall reaction, the second step, and third step.

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~ ABOUL-GHEIT: KINETICS OF QUINOLINE HYDRODENITROGENATION 2579

I active sites. The complexity encountered in orienting and adsorbing on the surface of the catalyst gives rise to a loss of several degrees of freedom whereby an entropy change of -43.9 e.u. mol-l is obtained.

As the tetrahydroderivative is hydrocracked to o-propylaniline, the latter competes to attach to the catalyst and the reaction becomes more complex due to the loss of additional degrees of freedom. Hence, a much larger negative entropy change results (AS, * = - 58.0 e.u. mol- l).

The effect of basicity of the intermediates produced during the HDN reactions cannot be ignored. However, its influence is not yet clari- fied. For instance, Larson (9) assumes that the most adsorbed phase on the catalyst is the aniline-type compound. McIlvried (4), on the other hand, assumes that all of the intermediate basic compounds produced during the HDN of pyridine appear to be equally strongly adsorbed 1 on the denitrogenation sites.' The basicity of the

I nitrogen compounds at the reaction temperatures

I should be greatly different from that measured at normal temperatures, and no decisive explana- 1 tion for the behavior of the quinoline intermediates, based on their basicities, could be achieved. However, the results of this study seem to agree with Larson's assumption.

1 The hydrogenation function of the catalyst I employed in this investigation appears high

enough to achieve fast hydrogenation of the , quinoline-type compounds (Figs. 24 ) . On the

other hand, the cracking function of the catalyst I

'McIlvried differentiates the active sites of his Ni-Co- Mo-alumina catalyst into hydrogenation and denitrogenation sites.

is relatively less active (step 2 is the slowest step). Therefore, if the cracking function of this catalyst is activated through some treatment, a more active HDN catalyst is obtained. For instance, when the catalyst under investigation has been treated with HCl gas, the quinoline solution employed becomes completely hydrodenitrogenated after a reaction period of 3.5 h at 400 "C.,

The author wishes to thank Dr. Sami A. Shama of the Lowell Technological Institute, Lowell, U.S.A., for his interest and valuable discussions.

1 . A. K . ABOUL-GHEIT and I. K. ABDOU. J. Inst. Pet. London, 58,305 (1972).

2. E. M. BLUE and B. SPURLOCK. Chem. Eng. Prog. 56, 54 (1960).

3. R. A. FLINN, 0. A. LARSON, and H. BEUTHER. Hydrocarbon Process. Pet. Refiner,42, 129 (1963).

4. H. G. MCILVRIED. Ind. Eng. Chem. Process Des. Dev. 10,125 (1971).

5. A. K . ABOUL-GHEIT and I. K. ABDOU. J. Inst. Pet. London, 59, 188 (1973).

6. A. K. ABOUL-GHEIT, I. K. ABDOU, and A. MUSTAFA. 6th Arab Sci. Congr. Proc. Vol. 5. Damascus. 1969. p. 77.

7. A. K . ABOUL-GHEIT, I. K. ABDOU, and A. MUSTAFA. Egypt. J. Chem. In press.

8. J. DOELMAN and J. C. VLUGTER. 6th World Pet. Congr. Proc. Sect. 111, Frankfurt. 1963. p. 247.

9. 0. A. LARSON. Preprints, Div. Pet. Chem., Am. Chem. Soc., 12, B-123 (1967).

10. M. A. MUHS and F. T. WEISS. Anal. Chem. 30,2591 (1958).

1 1 . J. M. SMITH. Chemical engineering kinetics. McGraw-Hill, Inc., New York. 1956. p. 378.

12. S. GLASSTONE, K. J. LAIDLER, and H. EYRING. Theory of rate processes. McGraw-Hill, Inc., New York. 1941. p. 9.

3A. K. Aboul-Gheit. Unpublished results.

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the kinetics of quinoline hydrodenitrogenation through reaction intermediate products

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