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JOURNAL OF CATALYSIS 34, 215-229 (1974)

The Mechanism of Hydrogenolysis on Molybdenum-Containing Catalysts III. Cracking, Hydrocracking, Dehydrogenation and Disproportionation of Pentylamine

J. SONNEMANS” AND P. MARS l’wente University of Technology, Enschede, The Netherlands

Received October 30, 1973

The conversion of pentylamine on a MoOrA1203 catalyst was studied between 250 and 350°C at various hydrogen pressures. The reactions observed were cracking to pentene and , hydrocracking to and ammonia, dehydrogenation to pentanimine and butylcarbonitrile, and disproportionation to ammonia and dipentylamine. The equilibrium between pentylamine, dipentylamine and ammonia appeared to be established under most of the experimental conditions. The equilibrium constant is about 9 at 250°C and about 5 at 320°C. The disproportionation reaction is zero order in hydrogen and of -1 order in the initial pentylamine pressure. Dehydrogenation was observed at low hydrogen pressures, and especially at higher temperatures; the reaction is first order in pentylamine. Both cracking and hydrocracking take place, mainly above 300°C. Hydrocracking appears to be half order in hydrogen; the rate of cracking is almost independent of the hydrogen pressure. The hydrocarbon formation is of zero order in pentyl- or dipentylamine. The same type of reactions (except hydrocracking) take place on alumina, but with a far lower reaction rate.

INTRODUCTION catalysts at hydrogen pressures of about One of the intermediates formed in the 60 atm (Z-4). They reported a high rate hydrogenolysis of pyridine is pentylamine of ammonia formation from the primary (1). Knowledge of the kinetics of the con- compared with heterocyclic nitro- version of this compound is necessary to gen compounds; for instance, the rate of distinguish whether deamination of pentyl- denitrogenation of hexylamine and piper- amine is rate determining in the hydro- idine differed by a factor of 15 (2). The genolysis of pyridine into pentane and conversion of amines into ammonia and ammonia. Moreover, this knowledge may hydrocarbons is generally considered to be increase the information about the mecha- a one step reaction. However, investiga- nism of dehydrogenation, disproportiona- tions at low pressures of hydrogen, mostly tion and (hydro)cracking of bases performed on alumina, showed the forma- on molybdenum-containing catalysts. tion of several intermediates. Brey and Several authors have investigated the Cobbledick (5) studied the conversion of rate of conversion of primary amines on, butylamine on alumina at 380430°C ; be- mostly sulfided, molybdenum-containing sides butene and ammonia, the products dibutylamine and propylcarbonitrile were * Present address : AKZO Chemie Nederland formed. Ebeid and PaF;ek (6) reported a b.v., Amsterdam-N. fast disproportionation of ethylamine to 215 Copyright @ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. 216 SONNEMANS AND MARS

ammonia and diethylamine on alumina temperature of the whole catalyst bed was above 300°C. constant within 1°C. The organic com- The kinetics of the conversion of amines pounds were supplied with the aid of two on molybdenum-containing catalysts is not saturators in series; the second one was well known. At high pressures of hydrogen thermostated within kO.l”C (separate a first order process was found (3). Devia- systems were used when two compounds tion of the first order kinetics was observed were supplied together). The composition above 95% conversion. The results ob- of feed and product has been deter- tained on alumina at low pressures of hy- mined by ; gas sam- drogen (mostly zero) are partly conflicting. ples were taken using gas sampling valves The reaction order for the disproportiona- thermostated at 60 and 14O”C, respectively. tion and that for the cracking reaction Defects in the mass balances were less were found to be one and zero, respec- than 10%. These are partly due to the tively, by Ebeid and Pagek (6). However, formation of high boiling compounds, like Catry and Jungers (7) concluded that the tripentylamine small amounts of which reaction orders are two and one, respec- were often observed in the products. tively. Separate experiments demonstrated that The aim of our study was to investigate no pentylamine conversion and pentene whether the types of reactions reported for took place when the wall the amine conversion on alumina also take temperature of the reactors, for both the place on the reduced Mo03-ALO catalyst low and high pressure equipment, was be- and to examine the kinetics of the several low 350°C and when the injection ports reaction types. of the gas chromatographs and the tubes were below 150°C. The influence of in- METHODS ternal and external diffusion on the rate of the reaction can be excluded as was Procedures shown by calculations. Detailed information about the equip- Some activity decline was observed for ment has been given elsewhere (1). In the the disproportionation and the (hydro) - high pressure equiment a stainless steel cracking reaction and found to be 30% in reactor with an internal diameter of 6 mm the first 20 hr and again 30% in the next has been used. The mean temperature of 80 hr (the total amount of pentanimine the catalyst bed was constant within 1°C and butylcarbonitrile remained constant). during the experiment. The temperature of Therefore the measurements were mostly the largest part of the catalyst bed was started 16 hr after feeding the pentyl- constant within 5°C; however, in the last amine. The experimental points have been 15% of the catalyst bed the temperature taken in arbitrary sequence to avoid the dropped by about 15°C. This may give a interference of the slight activity decline systematic error in the reaction rate eon- on t,he shape of the curve. The reaction stant of up to 10%. The pentylamine was time is defined as t = mP/+, in which fed with a thermostated (?0.5”C) satu- m = mass of catalyst (kg), P = total pres- rator. For the determination of the product sure (N m-2), + = total moles fed (mol composition gas samples were taken di- set-l) . rectly after the throttle valve and ana- lyzed by gas chromatography (8). Tubes Materials and gas sampling valve were heat.ed at a Catalyst: y-Al,O, (AKZO, 213 m2 g-l) temperature of at least 140°C t.0 prevent and 22% MOO,-Al,O, (195 m2 g-l). The condensation of the reaction products. latter catalyst has a high surface area of In the low pressure equipment a glass molybdenum oxide, spread out in a mono- reactor with an internal diameter of 9 mm molecular layer over the alumina surface has been used ; the diameter was 3 mm (9). .The particle diameter was 0.3-0.6 mm when less than 1 g catalyst was taken. The (0.21-0.30 mm in cases where less than 1 g PYRIDINE HYDROGENOLYSIS. III 217


- t (lo6 kg Nsec rnT2 Wled’)

FIG. 1. The conversion of’pentylamine at 250°C. (2.52g Mo03-A120a;Pa2= I.0 x 105N m-2; p,,, = 1.05 X 103 N m-?; (V) pentylamine; (v) dipentylamine; (A) ammonia; (a) butylcarbonitrile; (0) pentanimine. catalyst was used). Before use all the cat- butylcarbonitrile (BCN). (Both have been alysts were reduced in hydrogen at 450°C identified ,by mass spectroscopy and in- for at least 16 hr. frared spectroscopy after gas chromato- graphic separation). These products might RESULTS be formed by a direct dehydrogenation of The conversion of n-pentylamine was the monopentylamine. Pentane and pen- studied by varying the temperature, the tene were only observed in small amounts reaction Bime, the partial pressure of the in the products. The amount increased amine and the hydrogen pressure. The linearly with the reaction time up to 2.6% effect of other nitrogen bases on the rate at t = 12.2 X 106 (kg N set m-* mol-I). of the amine conversion was also inves- This result shows that the cracking rate of tigated. Most experiments were performed monopentylamine or dipentylamine must on the Moos-A1,O3 catalyst and only a be small at 250°C. few on alumina. Figure 2 gives the product distribution at 320°C. Pentane is taken as the total Effect of the Temperature amount of pentane + pentenes. and the Reaction Time was only present in very small amounts in The conversion of n-pentylamine (MPA) the products; next to pentene-1 a large on a Moos-ALO catalyst at a hydrogen amount of pentene-2 was observed, indicat- pressure of 1 atm was studied at 250, 320, ing a rather high rate of double bond 340 and 360°C. The result at 250°C is isomerization. given in Fig. 1. This figure shows that at Figures 1 and 2 show that at increasing low conversions equimolar amounts of am- temperatures the cracking and dehydro- monia and dipentylamine are rather selec- genation products are formed in larger tively formed by the following dispropor- amounts than dipentylamine. The relative tionation or transfer reaction: amounts of ammonia formed decreased at higher temperatures (50% at 250°C and 2-Pentylamine --f ammonia + dipentylamine (DPA). 30% at 320°C). At 360°C the conversion to ammonia was only 10% at total conver- Other products were pentanimine (PI) and sion of pentylamine; in this case even a 218 SONNEMANS AND MARS



i t ( lo5 kg N SW Wi2 mOd )

FIG. 2. The conversion of pentylamine at 320°C. (0.26 g MoOa-AhO*; pH$ = 1.0 X 106 N mw2; ~‘XPA~ = 0.88 X 1W N m”); for the symbols see Fig. 1; (A) pentane + pentenes. maximum was found for the partial was studied at low conversions to deter- pressure of ammonia as a function of the mine the reaction order in the amine for reaction time. the dehydrogenation and for the dispro- The ratio pentenes-pentane was about 1 portionation reaction. The results are at 320°C; an increase of this ratio at in- given in Table 1. creasing temperatures and reaction times was observed within a factor of 2 over the Efkd of the Hydrogen Pressure range of temperatures and the reaction To investigate the effect of the hydrogen times studied. pressure, experiments were performed in helium as well as at hydrogen pressures up Effect of the Initial Pentylamine Pressure to 60 atm. In the high pressure experi- The influence of the initial pentylamine ments temperature, reaction time and ini- pressure on the pentylamine conversion tial pentylamine pressure were also varied


Partial pressures of products

Initial Disproportion& Dehydrogenation pentylamine products products %I.+ PBCN Sequence of pressure PNE, + PDI'A PPI + PBCrj PYP& expts (102 N m-2) (102N m+) (lo” N m-9) (%I

3.3 0.66 0.13 3.9 6.0 0.6 0.22 3.7 9.2 0.5 0.32 3.5 9.3 0.9 0.34 3.7 15.0 0.85 0.46 3.1 20.5 0.8 0.59 2.9 23.9 0.5s 0.58 2.4 Q !!’ = 250°C; Pna = I.1 X 106N mw2; 0.27 g Moos-AhOx; t = 2.1 X 10” kg N set rn+ mol-r. PYRIDINE HYDROGENOLYSIS. III 219


Temp Pentylamine Dipentylamine Ammonia Pentane Pentenes Pa bol%) (ml%) (mol%) (mol%) (mol%)

250 15 46 40 0.5 0.3 320 13 29 58 28 3.5 350 1 1 98 80 0.5

a Par = 29 X lo5 N rne2; PYPA~ = 6 X lo3 N m-2; 1.43 g Mo03-A1~0~; t = 5.7 X 106 kg N set m-2 mol-1. The values are given as moles product/initial moles of pentylamine; pentanimine and butylcarbonitrile were not found in the products. Decane was found in small amounts (2% at 350°C). to investigate whether there were similar A few experiments have been carried out results at low and at high hydrogen to determine the conversion of pentylamine pressures. as a function of the reaction time and of the The influence of the temperature on the initial pentylamine pressure (t = 32O”C, product composition for the pentylamine P,, = 29 X lo5 N me2). In accordance conversion at a hydrogen pressure of 29 X with the results at a lower hydrogen pres- lo5 N m-* is given in Table. 2. Pentanimine sure (Fig. 2) we observed: and butylcarbonitrile could not be detected. A linear decrease of the dipentylamine At low temperatures only disproportiona- pressure as a function of the reaction time tion into ammonia and diamine takes (after its maximum) ) together with a linear place. (Hydro)cracking of the amines is increase of the partial pressure of pentane observed above 300°C. + pentenes. A few experiments were made at 250°C A total amount of pentane + pentenes, and 60 atm of hydrogen to investigate the which was independent of the initial pen- effect of the hydrogen pressure on the rate tylamine pressure (see also Table 4) ; the of the disproportionation reaction (Table ratio pentane-pentenes appeared to de- 3). The order in hydrogen of the (hydro) - crease with increasing initial pressure of cracking reaction was studied by varying the amine. the hydrogen pressure at reaction condi- tions in which the disproportionation equi- Apart from the experiments performed at libruim was established. The results of the hydrogen pressures from 1 to 60 X lo5 N experiments performed at two different m-” some were carried out in helium. (In initial pentylamine pressures are given in this case a small but increasing partial Table 4. pressure of hydrogen over the catalyst bed


Reaction condit, Rate constants t Prodrlcts (1OW mol kg-’ se+) PHZ PYPAo (lo5 kg N set PNHa + PUPA (105 N m-2) (103 N m-‘) m2 mol-I) (103 N m-2) ha kf’

0.01 1 .0.5 3.7 0.10 0.3 0.3 1.1 1.05 3 5 0.15 0. 5 0.5 60 40 31 3 4 1.2 1.2 60 13 1Y n 10.4 1.6 3 .9

a Two different rate equations were used: T = ~I(PMI~A/PYPA~) and T = ~~(PMI~A/~MI~A,)~. 220 SONNEMANS AND MARS


PVPA~ = 6.3 x i03 (N m-2) I%aO = 30 x IO3 (N m-2) Pentane + pentenes; PH, Pentane Pentenes Pentane Pentenes mean value (lOa N rn”) (103 N m-2) (103 N m-2) (103 N m-2) (103 N m-2) (103 N m-2)

20 1.2 0.2 1.1 0.3 1.4 29 1.56 0.15 1.3 0.3 1.6s 58 2.5 0.1 1.8 0.2 2.3

a 1.43 g MOOS-A1203; T = 320°C; t = 5.7 x lo6 kg N set m-2 mol-1.

can be expected). The product composition free alumina surface contributes to the as a function of the reaction time at 320°C total activity of a commercial COO-MOO,- is given in Fig. 3. Comparison with Fig. 2 Al,O, catalyst. The product composition shows a more selective conversion to butyl- as a function of the reaction time at 320°C carbonitrile. A similar experiment was has been given in Fig. 4. The effect of the made at 250°C. Comparison of the results initial pentylamine pressure on the amounts with those of the experiment at the hydro- of products formed is demonstrated in gen pressure of 1.0 X lo5 N m-2 showed Table 5. that the rate of disproportionation was 407% lower (Table 3) and that the rate The Influence of Nitrogen-Bases on the of dehydrogenation was about three times Pentylamine Convekon and the higher. In helium the decrease of the rate Pentene Hydrogenation on of formation at longer reaction MOO,-Al,O, times was not as striking as shown in The pentylamine conversion on the Fig. 1. MOO,-ALO catalyst at 250°C has been determined both with and without addition Conversion of Pentylamine on Alumina of about an equivalent amount of piperi- Some experiments have been performed dine and pyridine. The conversion de- on y-alumina to investigate whether the creased due to the addition of nitrogen

4.0 8.0 12.0 - t (105 kg N see m9 moles-‘)

FIG. 3. The conversion of pentylamine at very low hydrogen pressures. (0.26 g MoOa-AlsOa; T = 320°C; PH. = 1.0 x IO6 N m-; PYPA = 0.88 x 1W N m-“); for the symbols see Fig. 1; (A) pentane + pentenes. PYRIDINE HYDROGENOLYSIS. III 221



- t (d kg N .5.x rf~-~ moles-‘)

FIG. 4. The conversion of pentylamine on alumina at 320°C. (1.00 g catalyst, Pnn = 1.0 X lo6 N m-*; PYPA~ = 0.9 X [email protected] N m-*); for the symbols see Fig. 1; (A) pentane + pentenes; (X ) decane or decene. bases from 0.20 to 0.15. The effect of these absence of pentylamine a complete con- bases on the dehydrogenation appeared to version of pentene into pentane was found be less. (Table 7). Addition of pentylamine to the The influence of ammonia on the pentyl- feed showed a decrease in the pentane amine conversion is given in Table 6. Sim- formation to the same level as observed ilar experiments at 250 and 360°C in 1 in the amine conversion alone. Hence, pen- atm of hydrogen also showed a decrease of tylamine poisoned the hydrogenation of the dipentylamine and partial pres- pentene from a conversion of 100% to a sure and an increase of the formation of value below 2% (detection level). The butylcarbonitrile. isomerization of pentene-1 to pentene-2 To investigate whether the cracking of was only partly poisoned. These results pentylamine into pentane and ammonia is have been confirmed by a duplicate ex- a one step process (hydrocracking) or a periment in which also a slow recovery of two step process (cracking and hydrogena- the pentene hydrogenation was demon- tion of pentene), the following experiments strated when the pentylamine feed was were performed: stopped ; the conversion was 90% after 1 A Moos--Al,O, catalyst was fed succes- hr of purging with hydrogen with 1 ml see-l. sively with pentene, pentylamine with Experiments with the pulse technique, pentene and pentylamine alone. In the not reported in this paper, showed that the


Initial Partial pressures of the products (lo2 N m’) pentylamine pressure Dipentyl- Butylcarbo- (102 N m-2) Pentylamine amine Ammonia Pentene Pentanimine nit,rile

5.6 1.9 1.4 2.0 0.22 0.19 0.07 11.1 5.1 2.4 3.2 0.21 0.35 0.05 19.2 13.3 26 2.9 0.13 0.36 0.03

a 1.0 g AlsOa: 2’ = 320°C; PA? = 1.0 x 106 N m-2; t = 1.7 X 10’ kg N set m-* mol-I. 222 SONNEMANS AND MARS


Partial pressure Partial pressures of the products (lo2 N rnd2) Fl(Pi) for of ammonia the dis- in the feed Pentyl- Dipentyl- Am- Pent- Butylcar- propor- (102 N m-2) amine amine monia Pentane Pentenes animine bonitrile tionation

0 0.66 0.67 2.1 0.28 0.18 0.39 4.80 3.2 7.2 1.39 0.39 8.2 0.17 0.09 0.16 5.45 1.64

5 250 mg MoOa-AlzOa; !/’ = 320°C; Pae = 1.0 X 106 N rnm2; P MPAO= 9.6 X 102 N m-*; t = 6.2 X lo6 kg N set me2mol-1. hydrogenation of pentane was already at high pressures of hydrogen (Fig. 1, poisoned by small amounts of nitrogen Table 2). The more or less constant am- bases. monia and dipentylamine pressures at long reaction times (Fig. 1) suggest the estab- DISCUSSION lishment of an equilibrium. The results obtained suggest the reaction To investigate whether the dispropor- scheme given in Fig. 5. This figure shows tionation equilibrium is established, we disproportionation, dehydrogenation, crack- calculated the values of the following ing and hydrocracking reactions. The sys- pressure function: tem is very analogous to the conversion PDPAPN& of ; alcohols disproportionate selec- Fl(Pi) = PMPA2 ' tively into water and at low tempera- tures and dehydrate at higher temperatures Establishment of equilibrium demands (>2OO”C for alumina). Dehydrogenation that this function should have a constant of alcohols is observed on several catalysts. value at increasing reaction times, and We shall discuss below the kinetics of F, (Pi) then equals the equilibrium con- each of the various types of reaction and stant. Values of F,(Pi) given in Table 8 the influence of other amines on the ki- show that the disproportionation is near netics. We shall also give some suggestions equilibrium at 250 and 320°C. These about the mechanisms of the reactions, values, as well as those obtained in other discuss the results on alumina and the rate experiments suggest that the equilibrium determining step in the pentylamine de- constant may be 8-10 at 250°C and 4-6 composition. at 320°C. This is in accordance with the values of 10 at 250°C and 5 at 32O”C, Disproportionation estimated for the equilibrium constant by At low temperatures disproportionation the method of Van Krevelen and Chermin appears to be very selective at low and (IQ) and the value of 3 reported by Catry


Feed (lo2 N m-“) Partial pressures of the products (lo2 N rnmz)*

Pentyl- Am- Butylcar- Pentene-1 amine monia Pent,ane Pentene-1 Pentene-2 Decane bonitrile

0.8 - - 0.8

a 250 mg MoOa-A1203; T = 320°C; Pnr = 1.1 X lo5 N m -2; t = 5.8 X lo6 kg N set rnVz mol-‘. * Pentylamine and dipentylamine were not observed in the products. PYRIDINE HYDROGENOLYSIS. III

C4Hg- C = N

= NH (+i5H12)

--. k N 9,

FIG. 5. Reaction scheme for the conversion of pentylamine on a molybdenum oxide-containing catalyst*. and Jungers (7) for the equilibrium con- second order, because a possible stronger stant of the butylamine disproportionation of pentylamine than dipentyl- at 350°C. amine and ammonia, as well as the forma- Some information about the kinetics of tion of tripentylamine, might influence the the disproportionation in pentylamine may interpretation of the results. be obtained from Fig. 1. The shape of the The order of this disproportionation curves for ammonia and dipentylamine is reaction with respect to the initial pres- similar to that of a first order reaction; sure of pentylamine proves to be zero the same reaction order is reported for the (Table 1) ; (the difference in the values disproportionation of amines on alumina for the partial pressures of ammonia and (6, 11, 12). A reaction order of 2 is ex- dipentylamine can be explained by a slight pected when the reaction is of the Lang- activity decline of the catalyst; see the muir-Hinshelwood type. However, we were sequence of the experiments). Different unable to discriminate between a first and orders of reaction by varying the reaction


1.08 250 0.16 0.04 1.88 250 0.66 0.09 3.08 250 1.1 0.16 6.17 250 5.0 0.38 12.2 250 6.6 0.60 0.10 320 0.2 0.25 0.19 320 0.7 0.7 0.62 320 3.6 3.8 1.22 320 5.0 14 224 SONNEMANS AND MARS time and the initial partial pressure were volved in so many reactions (Fig. 5). A first also found for the conversion of ot%her question is whether the (hydro) cracking nitrogen ba,ses on molybdenum-containing rates of monopentylamine or dipentylamine catalysts (2, IS). differ. Only a few results are available on The order in hydrogen of the alkyl the dipentylamine conversion. The products transfer reaction is low, as can be con- formed from dipentylamine at 250 and cluded from Table 3. An increase of the 320°C were pentane, pentenes, ammonia, hydrogen pressure by a factor of 60, to- tripentylamine and small amounts of gether with an increase in the initial pen- monopentylamine and butylcarbonitrile. At tylamine pressure by a factor of 30, only 250°C the amount of pentane + pentenes gives an increase in the rate of the dis- appeared to be a factor of three higher proportionation by a factor of about 3 (the than the amount formed from monopen- rather fast rate of establishment of the tylamine at the same reaction time. disproportionation equilibrium of the ex- Hence, the (hydro)cracking rates of both periment described in Table 2 gives evi- amines are about the same, when calculated dence for a somewhat higher value). The per alkyl group. larger rate of disproportionation at high A second question is whether cracking pressures of hydrogen may be due to a or hydrocracking takes place. From Table smaller activity decline at these higher 7 the conclusion was drawn that at 320°C hydrogen pressures or to an additional con- and 1 at,m of hydrogen the hydrogenation version on other active sites on which pen- of pentene is completely poisoned by pen- tylamine is less strongly adsorbed. tylamine. Hence, under t,hose circum- stances reaction 6 may be excluded from Cracking and Hydrocracking the scheme of Fig. 5. This result., together The equilibrium constants of the crack- with the formation of pentene as well as ing and hydrocracking reactions are very pentane from pentylamine, suggest that large (> lo8 N m-2 and > 107, respectively, both cracking and hydrocracking of the in the temperature range studied). These amines occur. The rates of cracking and equilibria are not established in our ex- hydrocracking are about the same at a periments. The same applies to the hydro- hydrogen pressure of 1.0 X lo5 N m-?, as genation of pentene to pentane (Keg > 10-l can be concluded from the amounts of m2 N-l). pentane and pentene formed. These con- The kinetics for the total formation of clusions are only correct if reaction 9 in pentane and pentenes are discussed first. the scheme in Fig. 5 may be neglected. Figures 2 and 3, as well as the experiment Support for this is found in t,he small at 25O”C, show a linear relationship be- amounts of pentanimine and butylcar- tween this total amount and the reaction bonitrile formed when converting dipentvl- time. Evidence for an identical result at amine at 250 and 320°C and 1.0 X lo” N 29 atm of hydrogen and 350°C was also m-2 hydrogen pressure. obtained. This proves an overall zero order The (hydro) cracking reactions were for the cracking or hydrocracking reaction. found to have an overall order in hydro.gen The dependence of the rate of (hydro)- of about 0.5 at hydrogen pressures above cracking on the initial pentylamine pres- 29 X lo” N mm2 (Table 4). Extrapolation sure also appeared to be zero order, as can to a hydrogen pressure of 1.0 X lo5 N m-” be concluded from the results given in gives a rate constant of 0.54 X 10e3 moI Table 4. This table also demonstrates the kg1 set-’ (the used for the effect of the hydrogen pressure on the calculation of the rate constant k is r = rate of (hydro)cracking. The order in hy- k = k’ pH2s). The rate constant calculated drogen proves to be about 0.5. from Fig. 2 is 1.5 X 1P mol kg-l set-I. Discrimination between the several for- This difference, and the rather high amounts mation reactions of pentenes or pentane is of pentane + pentenes in the experiment difficult, because these products are in- in helium (Fig. 3)) suggest a lower order PYRIDINE HYDROGENOLYSIS. III 225 in hydrogen of the (hydro)cracking below dehydrogenation is first order in pentyl- 29 atm of hydrogen. This points to a crack- amine, as was indicated by computer ing reaction which is zero order in simulations of Figs. 1 and 2. Table 1 also hydrogen and a hydrocracking reaction shows a first order process by varying the which is half order in hydrogen. initial amine pressure, at least below lo3 N m-2. The order of the reaction in hydrogen Dehydrogenation is not well known. Figures 2 and 3 only Dehydrogenation of pentylamine into suggest a small effect, if any, of the hy- pentanimine and butylcarbonitrile was drogen pressure on the dehydrogenation. only observed at low pressures of hydro- gen. Pentanimine is an intermediate prod- Summary of the Kinetics Results uct in the dehydrogenation reaction, as is The results of the kinetics of the four shown in Figs. 1 and 2. types of reactions are summarized in An equilibrium between butylcarbonitrile Table 9. and pentylamine was not yet established The zero order behavior found for the under our reaction conditions as can be disproportionation and (hydro) cracking concluded from the increasing values of a reactions by varying the initial pentylamine pressure function, defined as F,(Pi) = PBc, pressure can be explained by assuming a PI122/Phm4, as a function of the reaction strong adsorption of the nitrogen bases time (Table 8). The equilibrium will be with identical adsorption constants of the established at somewhat longer reaction bases. Assuming the surface reaction as t.imes; from the ratio of t,he rate constants rate determining and a Langmuir type ad- of the conversion of pentylamine and sorption as an approximation of the ad- butylcarbonitrile, respectively, the equilib- sorption behavior, the following rate rium constant was estimated t.o be 2.7 at equation can be derived: 250°C (this value is higher than t’hat cal- culated for the but.ylaminc-propylcarbo- r = keAn = k nitrile equilibrium (IQ), i.e., 0.2 at. 250°C 1 + bAPA + Z&P; and 7 at 320°C). Pentanimine and butylcarbonitrile were where bi, Pi are the adsorption constant only observed in the experiments at low and partial pressure of product i. Because hydrogen pressures. The absence of these P, + ZPi = P&, bi = bA and bAPAo >> 1, products at a hydrogen pressure of 29 atm the rate equation can be simplified into: is due to the shift of the dehydrogenation T = k(bAP&%APAn)n = k(PA/PAo)” equilibrium to pentylamine at increasing hydrogen pressures. The supposition bAPAo>> 1 is confirmed by Regarding the kinetics of the dehydro- former investigations (13). genation reaction the conclusions are as The reaction whose rate can be described follows: as a function of reaction time the with the equation above will show the


Reaction order in Pentylamine by varying the Activat.ion Reaction order energies0 Type of reaction Reaction time P MPAo in hydrogen (kJ mol-1)

Disproportionation 0 0 100 (Hydro)cracking 0 0 0.5a 150 Dehydrogenation 1 1 Low 150

a The hydrocracking is about half order in hydrogen; the cracking reaction may be of zero order. 226 SOKNEMANS AND MARS

order n by varying the reaction time and The nitrogen bases have equal adsorp- the zero order by varying the initial par- tion constants. The results of the poisoning tial pressures of the nitrogen bases (at not experiments, in which the pentylamine too high conversions). conversion was poisoned by other nitrogen The results of our poisoning experiments bases, showed that this may be true only are also in accordance with this type of in rough approximation. kinetics. Table 6 shows that ammonia The (hydro)cracking rate of dipentyl- addition to the feed decreased both the amine is twice that of pentylamine. rate of disproportionation and that of the (hydro) cracking (the higher value obtained The zero order disproportionation (Table for butylcarbonitrile is due to the higher 1) is expected on this model, because in- pentylamine pressure in the reactor when crease of the pentylamine pressure [above ammonia is added to the feed). 1 X 10” N m-2 (IS)] does not increase the Corresponding experiments at 250°C with surface coverage of pentylamine. This zero addition of pyridine and to the order excludes an Eley-Rideal mechanism. pentylamine feed showed that their poison- As a function of the reaction time a second ing influence was about half that of am- order of reaction seems more probable monia. With the rate equation given above when the rate determining step is a surface (P,, = total pressure of the nitrogen bases) reaction. As stated before discrimination the ratio bMPA/bNH3is found to be about 1 between a first or a second order from the and the ratio bMPA/bpYRand bMPA/bPIPabout experimental results was impossible. 2 or 4 depending on the order of the dispro- A possible mechanism of this dispropor- portionation reaction. For a general de- tionation is an alkyl and hydrogen transfer scription of the kinetics the assumption of between two neighboring pentylamine the same order of magnitude of the ad- molecules:

C5Hll C5Hll \ \ H-N----H H-N-H C5Hll I I \ + - I I NH + NH3 I I / -N-H HllC5 H H11C5---:-H C5Hll L - - sorption constants may be useful. The nitrogen atom may be bonded to The activation energies given in Table 9, the molybdenum and the hydrogen and and those reported for the disproportiona- may have an interaction with tion and cracking of both amines and the oxygen atoms of the molybdenum alcohols on alumina (6, 16, 15, 16) show oxide. that cracking reactions may have higher The zero order kinetics found for (hydra) activation energies than disproportionation craclcing are expected as a function of the reactions and that for similar reactions the initial pentylamine pressure and not too activation energies for the nitrogen bases long reaction times, due to the more or less may be higher than those for alcohols. constant concentration of pentyl groups on the catalyst surface. A decrease in the Mechanistic Interpretation of the Results (hydro) cracking rate should be observed The main kinetic results obtained on the at long reaction times, because ammonia MOO,-Al,O, catalyst may be well ex- will cover an important part of the surface plained by the following model: (the same applies for the butylcarbonitrile, but this compound may have a lower ad- The surface is almost completely covered sorption constant). Table 6 shows that the with nitrogen bases (IS). (hydro) cracking rate indeed decreased by PYRIDINE HYDROGENOLYSIS. III 227

addition of ammonia to the feed, but we that more than one molybdenum site is did not observe a decrease of the pentane involved in this hydrogenation step. (pentene) formation at longer reaction times (Fig. 2). Computer simulations Conversion of Pentylamine on Alumina showed that this may point to a stronger Comparison between our data discussed adsorption or a higher cracking rate of so far and literature data is difficult be- dipentylamine. cause detailed kinetic information is only The cracking may take place as follows: reported- for the amine conversion on H2 I R \C/R ,/%qR H2c H2N H2N I\ / ‘\ HH I I : H cH . I +, 8’ 0 1 ’ 1 ,h, 1 ?\ I/“\ , MO MO ‘MO’ MO’ .Mo MO InL J An analogous mechanism was proposed for alumina. However, our investigations the dehydration (17) and recently showed the same selectivity pattern for for the deamination of triethylamine on alumina and a Mo03/A1,03 catalyst; the alumina (18). most important difference found between A possible mechanism of the hydro- the t,wo catalysts is the far lower activity cracking reaction is the donation of a hy- of the alumina for all types of reactions, drogen atom from an OH group of the especially for the dehydrogenation reaction molybdenum oxide to the nitrogen atom. (Figs. 2 and 4). The dehydrogenation was found to be For alumina we also observed a zero first order as a function of the reaction order disproportionation and cracking re- time, as well as a function of the initial action when the initial amine pressure was pentylamine pressure ; this is contrary to the varied (Table 5). This is partly in conflict disproportionation and cracking reactions. with kinetic data reported in the literature Another difference is that the conversion (6, 7, 12, 18), but in good agreement with into imine + nitrile appeared to be hardly our adsorption studies which showed a influenced by the observed initial activity high coverage of the alumina surface with decline of the catalyst. These phenomena nitrogen bases at low partial pressures of point to the presence of separate catalytic these bases (13). The difference between sites for the dehydrogenation of pentyl- our results and the literature data may be amine. due to a different pretreat.ment of the Evidence for the heterogeneity of sur- alumina. face sites on the molybdenum oxide on An interesting point is the far higher alumina was also obtained from the pulse activity of molybdena-alumina compared experiments. Total poisoning of the hydro- with alumina for the alkyl transfer and genation was observed when the amount cracking reaction (a factor of about 25; of poison injected corresponds with only the difference in the dehydrogenation rate about 25 mol% of the molybdenum oxide is about a fact,or of 60). In the mecha- present on the catalyst (19). Because all this nisms of disproportionation and cracking molybdenum oxide is exposed (9)) we may given above, the transfer of hydrogen conclude that either only a part of the atoms on the surface of the catalyst is molybdenum oxide forms active sites, or essential. This transfer may occur with a 228 SONNEMANS AND MARS far higher rate on molybdena-alumina, 2. A first order dehydrogenation of pen- due to the higher acidity of the MOO,- tylamine to butylcarbonitrile was observed A1,03 catalyst (SO), or to the interaction at hydrogen pressures of 1 atm and less. between hydrogen atoms and the oxygen The selectivity of the pentylamine con- of the molybdenum oxide in combination version in this compound increases at in- with a change in the valence state of the creasing temperatures. molybdenum ion. 3. Both cracking and hydrocracking of C-N bonds appear to take place. The Rate Determining Steps in the Pentyl- cracking and hydrocracking are of about amine Decomposition zero order in the amines ; the order in From the results obtained the following hydrogen is 0.5 for the hydrocracking. The conclusions can be drawn: activation energy is about 150 kJ mol-I. 4. Even at high pressures of hydrogen At low conversions the rate of ammonia (up to 60 atm) and high temperatures (up formation is mainly influenced by the dis- to 350°C) the disproportionation reaction proportionation reaction. is fast compared with the cracking reaction. At conversions above 60% (hydro)crack- The equilibrium constant of the dispropor- ing of pentylamine or dipentylamine is rate tionation is about 9 at 250°C and about 5 determining in the ammonia formation. at 320°C. The fast disproportionation reaction 5. A MOO,-A1,03 catalyst is consider- and the rather slow hydrocracking reactions ably more active than alumina for all the may give a gross rate of ammonia for- types of reactions observed. mation which approximately corresponds ACKNOWLEDGMENT to a first order reaction. First order de- nitrogenation of amines has been reported E. van Emmerik, L. Guntenaar and P. van by Flinn, Larson and Beuther (3) ; the Berkel are gratefully acknowledged for their ex- perimental contributions. deviation from first order which they ob- served at high conversions may be due to REFERENCES the occurrence of disproportionation or 1. S~NNEMANS, J., GOUDRIAAN, F., AND MARS, P., alkyl transfer reactions. Proc. Int. Congr. Catal. 6th, Palm Beach, According to several authors deamina- 1972, p. 16%. tion of primary amines is not rate deter- a. MCILVRIED, H. G., Ind. Eng. Chem., Process mining in hydrodenitrogenation of hetero- Des. Develop. 10, 125 (1971). cyclic nitrogen bases, because under the 23. FLINN, R. A., LARSON, 0. A., AND BEUTHER, same reaction conditions the rate of am- H., Hydrocarbon Process Petrol. Refiner monia formation from amines is high com- 42, 129 (1963). 4. DOELMAN, J., AND VLUGTER, J. C., Proc. World pared with those from heterocyclic nitrogen Petrol. Congr. 6th, Frankfurt, Germany, bases. However, the conversion of amines 1969, Sect. 3, Pap. 12-PD 6. turns out to be rather complex and this 6. BREY, W. S., AND COBBLEDICK, D. S., Znd. Eng. conclusion about the rate determining step Chem. 51, 1031 (1959). in the denitrogenation may be incorrect, 6. EBEID, M. F., AND PAEIEK, J., Collect. Czech. especially when other nitrogen bases are Chem. Commun. 35, 2166 (1970). concerned in alkyl transfer reactions (21). 7. CATRY, J. P., AND JUNGERS, J. C., Bull. Sot. Chim. Fr. 2317 (1964). CONCLUSIONS 8. BEUGELING, T., BODUSZYT%SKI, M., GOUDRIAAN, F., AND SONNEMANS, J., Anal. Lett. 4, 727 1. On both alumina and a MOO,-AlSO, (1971). catalyst a fast disproportionation of pen- 9. SONNEMANS, J., AND MARS, P., J. Catal. 31, tvlamine to ammonia and dipentylamine is 209 (1973). found. The rate equation for the dispro- 10. VAN KREVELEN, H. W., AND CHERMIN, portionation is: r = k (PMIPA/PMPAO)nPHZo. H. A. G., Chem. Eng. 6%. 1, 66 (1951). The activation energy is about 100 kJ 11. OGASAWARA, S., HAMAYA, K., AND KITAJIMA, mol-I. Y., J. Catal. 25, 105 (1972). PYRIDINE WDROGENOLYSIS. III 229

1% PABEK, J., TYRPEICL, J., AND MACHOV~, M., 16. KN~ZINGER, H., AXD RESS, E., 2. Phys. Chem. Collect. Czech. Chem. Commun. 31, 4108 (Frankfurt am Main) 54, 136 (1967). (1966). 1Y. NOTARI, B., Int. Congr. Chem. days, Ind. 1% SONNEMANS, J., VAN DEN BERG, G. H., AND Catal., ZOth, Milan, Italy, 1969. MARS, P., J. Catal. 31, 220 (1973). 18. HOGAN, P., AND PASEK, J., Collect. Czech. 14. STULL, D. R., WESTRUM, E. F., AND SINKE, Chem. Commun. 38, 1513 (1973). G. C., “The Chemical Thermodynamics of 19. SONNEMANS, J., AND MARS, P., unpublished Chemical Compounds.” Wiley, New York, data. 1967. $0. FRANSEN, T. unpublished data. 16. VISSEREN, W. J., Thesis, Tech. Univ. Delft, 21. SGNNEMANS, J., NEYENS, W. J., AND MARS, The Netherlands, 1968. P., J. Calal. 34, 230 (1974).