Ind. Eng. Chem. Res. 1992,31, 2503-2516
2503
Catalysis and Mechanism of Hydrodenitrogenation: The Piperidine Hydrogenolysis Reaction George C. €ladjiloizou,+ John B. Butt,* and Joshua S. Dranoff Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60,208
The chemistry of hydrodenitrogenation catalysis is discussed with special attention being focused on the nature of the catalytic sites active for piperidine hydrogenolysis and the mechanism operative in the C-N bond cleavage during piperidine conversion. Reactions such as hydrogenolysis, alkylation, cyclization, cleavage, dehydrogenation, and hydrogenation occurred when piperidine was hydrogenated over a sulfided CoMo commercial hydrocracking catalyst. Reaction mechanisms are proposed to account for the formation of the identified products from these reactions. Both acidic (Bransted or Lewis) and basic sites are proposed to participate in the amine (such as piperidine) denitrogenation reactions.
Introduction The declining of the petroleum reserves brought, as a result, an increased interest in the production of synthetic fuels and chemical feedstocks from coal and shale oil (Katzer and Sivasubramanian, 1979). Such synthetic feedstocks and heavier petroleum fractions contain higher concentrations of nitrogen than light petroleum stocks and, being more difficult to process, will place increasing demands on hydroprocessing catalysts and processes. Nitrogen removal from thme liquids is neceeeary to meet both product quality and environmental constraints, and the need for more active and more selective hydrodenitrogenation (HDN)catalysta to process such synthetic feedstocks and heavier petroleum fractions is steadily increasing. The conventional HDN catalysts constituted of promoted molybdenum or tungsten sulfide supported on alumina are not selective in that they require very high hydrogen consumptions to achieve nitrogen removal (Katzer and Sivasubramanian, 1979). Hydrogen consumption represents a large fraction of the cost of hydrotreating heavy liquids and of coal liquefaction, so improved catalysts are needed to reduce the hydrogen requirement in these processes, especially in HDN. Moat of the nitrogen-containing compounds present in these liquids are heterocyclic in nature (Rollmann, 1977; Katzer and Sivasubramanian, 1979). Nonheterocyclic nitrogen-containing compounds are also present in liquid fuels but in smaller concentrations. Nitrogen removal from heterocyclic molecules requires hydrogenation of the aromatic heterocyclic rings prior to carbon-nitrogen (C-N) bond scission (Rollmann, 1977). Therefore, the catalyst must be bifunctional, having both hydrogenation and C-N bond scission functions, and it is very important to understand the nature of the catalytic s i b involved in these reactions and the detailed mechanism of the C-N bond cleavage in order to rationally develop better catalysts that will operate under more severe process conditions and maximize selectivity toward the desired end products. This article provides a review of the nature of the catalytic sites and the different mechanisms proposed to be operative for C-N bond cleavage during HDN. Emphasis is placed on piperidine hydrogenolysis and detailed mechanisms are proposed to account for the formation of products from this reaction over a commercial hydrocracking catalyst.
Discussion Chemistry and Nature of Catalytic Sites in Hydrodenitrogenation (HDN) and Hydradesulfurization (HDS) Reactions. It is generally proposed that the catalytic sites for activating nitrogen or sulfur compounds and hydrogen are different. To explain HDN and HDS kinetic data, several studies have postulated the existence of two kinds of catalytic sites on sulfided CoMo/A1203or NiMo/A1203catalysts. In a study of the thiophene HDS reaction network over a sulfided CoMo/A1203catalyst, Desikan and Amberg (1964)proposed two kinds of sites to exist on the catalyst surface: Site I, which is strongly acidic, is responsible for olefin hydrogenation and thiophene HDS and has strong affinity for adsorption of thiophene, H2S, and pyridine; site 11, which is weakly electrophilic (weaker acidic), facilitates hydrogenolysis of thiophenes, hydrothiophenes, and thiol and can be poisoned by NH3 or alkali. Two kinds of catalytic sites were also proposed to exist on a sulfided CoMo/A1203catalyst by Satterfield et al. (1975)during a simultaneousstudy of thiophene HDS and pyridine HDN. Site I is active for HDS and very sensitive to poisoning by nitrogen bases, while site I1 is much less active for HDS and is less susceptible to pyridine poisoning. To explain the effect of H,S on the HDN of quinoline, Yang and Satterfield (1984)postulated the existence of two kinds of catalytic sites on a sulfided NiMo/A1203catalyst: Site I is a sulfur vacancy associated with the molybdenum atom, while site I1 is a Bransted acid site. It was also concluded that sulfur vacancies are responsible for mostly hydrogenation and possibly hydrogenolysis reactions, while Br0nsted acid sites are responsible for hydrogenolysis and ring isomerization reactions. As Muralidhar et al. (1982) suggested, there may be different types of vacancies on the surface. Corner as well as edge vacancy sitea can be present with the former having a higher degree of uncoordination. The adsorption and dissociation of a H2S molecule can convert a sulfur vacancy (0) to a Bransted acid site (H+) and a sulfhydryl group (SH), as shown in scheme 1, but the adsorption is readily reversible if Ha is removed from the reaction system.
17 4s Mo
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* To whom correspondence ehould be addreeaed.
Present addreaa: Ekxon Reeearch and EngineeringCompany, P.0. Box 101, Florham Park,NJ 07932.
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H'Mo /
S
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(1)
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A24
In another study, Kwart et al. (1982)proposed two kinds
0888-588519212631-2503$03.00/00 1992 American Chemical Society
2604 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992
of sites to be present on a sulf‘ided CoMo/A1203catalyst during the hydroprocessing of phenothiazine in the presence of H2S. Site I is an anion vacancy and is responsible for hydrogenolysis of phenothiazine and site I’ (note the difference with site I1 above) is responsible for hydrogenation. These sites are illustrated in scheme 2. 0)
0Mo HS /
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H
’
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H2S
(1’)
~MO (2) A‘
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Ai24 Similar conclusions with the above studies were reached by Shih et al. (1977)during a quinoline HDN study over a sulfded NiMo/A1203catalyst. To explain the observations that hydrogenation of the quinoline benzenoid ring was suppressed by the presence of H2Swhile hydrogenation of the nitrogen-containing ring was promoted, they concluded that two kinds of active sites were present on the NiMo catalyst surface: one strongly acidic (I) and the other weakly acidic (1’) with the former adsorbing H2S more strongly. Thus, hydrogenation of the benzenoid ring was ascribed mostly to the more acidic sites while hydrogenation of the nitmgen ring was ascribed to the other kind of sites (weaker acidic). The idea of separate sites for hydrogenolysis and hydrogenation is also supported by the work of Satterfield and Cocchetto (19811,who found indirect experimental evidence, during the HDN of quinoline over a presulfided NiMo/A1203catalyst, that the hydrogenolysis and hydrogenation catalyst functionalities were associated with different sites. This conclusion was based on catalyst deactivation data, where, for the same time period, the hydrogenolyais activity of the catalyst decreased by nearly 60% while there was only a 20% loss in the hydrogenation activity. MaternovH (1982,1983)has established the existence of SH groups on a sulfided commercial CoMo/A1203catalyst and on sulfided or reduced MoS2by a technique that involved adsorption of silver ions from a pyridine solution. The dissociation of H2S is similar to the dissociative adsorption of water onto a zeolite or alumina, in which a surface vacancy is converted to a Brernsted acid site (Yang and Satterfield, 1984). The sulfhydryl group can also be considered to act as a Brernsted acid, but the electron affinity of a H+ site should be stronger than that of a H atom in a SH group (Yang and Satterfield, 1983). Therefore, H+ sites should more readily promote reactions which involve a carbocation mechanism. On the beeis of the above information, the characteristics of the different pro@ catalytic sites can be summarized as listed below (Yang and Satterfield, 1983): Type I Sites. (a) These are sulfur vacancies associated with the molybdenum atom. (b) They can facilitate hydrogenation and dehydrogenation reactions (coupled with sites 1’) as well as the direct extrusion of sulfur from thiophene (Lipsch and Schuit, 1969rqb). Muralidhar et al. (1982)propoeed that the corner vacancy sites are the sites active for HDS while the edge vacancy sites, having less uncoordination, are involved in hydrogenation. The hydrogenation of a heterocyclic molecule adsorbed on a vacancy should be facilitated by nearby chemisorbed hydrogen atoms and/or by hydrogen from a SH group (site 1’). (c) They are easily poisoned by nitrogen bases such as piperidine which may adsorb on them either associatively or dissociatively, as shown in reaction 3. 42%
‘ k ‘
S
(d) They might facilitate hydrogenolysis, Le., breakage of a single C-N, C-S, or C-0 bond. Type I1 Sites. (a) These are Brernsted acid sites, consisting of H+ on the surface from the dissociation of H2S on the surface (scheme 1)or from the support, as discussed elsewhere (Hadjiloizou, 1989). (b) These sites facilitate hydrogenolysis, cracking, and isomerization reactions that involve a carbocation mechanism. (c) They are less vulnerable to poisoning by nitrogen bases than sites I, though, as discussed by Corma et al. (1986,1987a),nitrogen compounds easily poison the protonic acid centers of commercial acidic catalysts. The above framework model indicates that the presence of H2S during reaction reduces the number of sulfur vacancies and increases the number of Brernsted acid sites (H+)and sulfhydryl groups (SH). In the presence of H2S, a slight reduction in the hydrogenation rates and a significant increase in the hydrogenolysisrates were observed (Satterfield and Giiltekin, 1981; Yang and Satterfield, 1984) when quinoline was hydrodenitrogenated over a presulf‘ided NiMo/A1203catalyst. The explanation given was that hydrogenation reactions can be retarded by competitive adsorption between H2S and nitrogen compounds for hydrogenation sites, while hydrogenolyeisreactions are enhanced due to the increase of surface acidity caused by the H2S. In addition, Yang and Satterfield (1983)utilized the above ideas and successfully explained phenomena observed in various HDN, HDS, and hydrodeoxygenation (HDO) reaction studies. The above theory, however, is not consistent with the results of Hanlon (1987),who studied the effects of the partial pressure of hydrogen (PHJ,the partial pressure of hydrogen sulfide ( P H 1, and the ratio P H / P H 1 on the HDN of pyridine anfpiperidine at 310 OEover a commercial NiMo/A1203catalyst. Hanlon’s data showed that the initial hydrogenation of pyridine to piperidine was fmt order in P H 2 and was unaffected by the PH while the hydrogenolysis of piperidine was unaffected either the absolute value of PH&or PH,as long as the value of the ratio P H ~ / Premained H ~ constant. The conversion of piperidine was found to increase though, with increasing values of the ratio PHfi/PHz. To explain thee observations, Hanlon suggested that an equilibrium reaction between H2S, H2, and the catalyst surface takes place as follows:
fi
H2S + 0 E
D - S + H2
(4)
where the surface sulfidic species formed (0-S) plays an active role in the C-N bond breaking reaction. This sulfidic species was proposed to be of the form NiMoS based on the model proposed by Topsm and co-workers (Topsere and Topsere, 1983;Clausen et al., 1984;Topsere et al., 1985;Topsore and Clausen, 1985;Topsore and Clausen, 1986). For example, T o p m and Topspre (1983)found that the promotion of the thiophene HDS reaction over a sullided NiMo/AI2O3catalyst was Wed to the presence of NiMoS. Furthermore, Hanlon (1987)pointed out that MWbauer emission spectroscopyexperiments by Clausen et al. (1976)provide evidence that the number of such
Ind. Eng. Chem. Res., Vol. 31, No. 11,1992 2606 sulfidic sites can be controlled by the gaseous environment (e.g., the H,S/Hz ratio), as proposed in reaction 4. In particular, these experiments showed that upon changing the H a / H z ratio, reversible changes in the valence state of the Co promoter atoms in CoMoS, which should behave analogouslyto the Ni atoms in NiMoS, may occur. Hanlon (1987)also speculated that since the hydrogenation of pyridine was unaffected by P H a , and the hydrogenolysis of piperidine was unaffected by P H 8 and P H 2 , as long as the ratio of the two remained constant, it is possible that HzS adsorbs on sites different from those adsorbing nitrogen compounds. It is also possible that the inherent activity of the sites responsible for hydrogenolysis on the NiMo/AlZO3catalyst is much greater than that of the ratio hydrogenation sites; a small increase in the PH+/PH, could cause a small increase in the number of hqhly active NiMoS species, thus, enhancing hydrogenolysis while not decreasing significantly the number of sites available for N-compound adsorption, as reflected by an almost constant hydrogenation rate (Hanlon, 1987). The nature of the catalytic sites for hydrogen activation ie much less understood (Hanlon, 1987;Ho, 1988). In the study by Hanlon (1987),the assumption that Hz did not adsorb competitivelywith nitrogen compounds fit the data quite well over a wide range of P H 2 values. Whether Hz is adsorbed on those sites active for N-compound adsorption, or it adsorbs on different sites, was addressed in some adsorption studies of H2 and nitrogen bases on Mo catalysts by Sonnemans et al. (1.973).From their results they postulated that Hz adsorbs strongly on the catalyst and on sites different from thow adsorbing nitrogen bases. However, as Hanlon (1987)pointed out, the adsorption studies by Sonnemans et al. were conducted in a nonreactive environment, thus making it difficult to determine the adsorption characteristics of the Hz specifically involved in the reaction. Of course, another possibility is that Hz reacts directly from the gas phase in an EleyRideal mechanism. It has also been proposed that hydrogen may be dissodatively bound to sulfur (creating sullhydryl groups) rather than to Mo, although some suggested reaction mechanisms involve both (see scheme 15 below). Not only can the sulfhydryl groups serve as hydrogen donors but they can ala0 impart Br0nsted acidity to the catalyst surface (Yang and Satterfield, 1983),as discussed further by Hadjiloizou (1989). Piperidine Hydrogenolyeie Reaction Chemistry: Review of Prior Studies. McIlvried (1970,1971) reported that the first principal step in the hydrodenitrogenation of pyridine ie the formation of piperidine via saturation of the heterocyclic ring. Subsequent C-N bond hydrogenolysis reactions result in the formation of n-pentylamine, pentane, and NH3 as follows:
sible. Their suggested reaction mechanisms were valid; however, the reaction definition used ie not entirely correct, because Hofmann degradation (exhaustive methylation) is the formation of an olefin and a tertiary amine by pyrolysis of a quaternary ammonium hydroxide (Hofmann, 18811,as shown in scheme 6. Using the ideas suggested
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