View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Universiteit Twente Repository JOURNAL OF CATALYSIS 34, 215-229 (1974) The Mechanism of Pyridine 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 ammonia, hydrocracking to pentane 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- amine 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- amines 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 nitrogen 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 chromatography; 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 hydrogenation 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 0 PRODUCT K - 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. Isopentane 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 alkyl 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 (PPRODUCT Q,MPA, T 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.