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Coke Aging and Its Incidence on Catalyst Regeneration Andres T. Aguayo,* Ana G. Gayubo, Javier Eren ˜ a, Alaitz Atutxa, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain
The effects of a sweeping treatment on the activity recovery, coke content, H/C ratio, and kinetics of coke combustion have been studied in the coke deposited on an HZSM-5 zeolite. The reaction test studied is the transformation of acetone into hydrocarbons at 400 °C, and the gases used in the sweeping are He, H2, and steam. Aside from the partial elimination of coke, which occurs with all these gases, each one has a different effect. Thus, sweeping with H2 causes catalyst rejuvenation. Sweeping with He promotes coke aging and a reduction in the H/C ratio. Steam is efficient for coke elimination but causes dealumination. The combustion of the remaining coke is disfavored (the activation energy is increased, and the kinetic constant is decreased) by the sweeping treatment that decreases the H/C ratio of the coke. 1. Introduction The main cause of deactivation of acidic zeolites is the deposition of coke on the active sites, which blocks these sites and, when the amount deposited is high, also blocks the crystal channels. Coke is an undesired carbonaceous byproduct produced by the degradation of reactants and/or products by simultaneous condensation-polymerization-dehydrogenation. Numerous reviews have been published on the deactivation of zeolites by coke deposition.1-7 Although deactivation is very rapid in several processes, it is fortunately reversible, and the catalyst completely recovers its activity upon coke combustion. This combustion is carried out in air or in an oxidizing atmosphere obtained by diluting oxygen with an inert gas. An accurate knowledge of coke combustion kinetics, of the heat associated with this combustion, and of the composition of the combustion gases is required to design the regeneration stage (if regeneration is carried out in situ in the reactor itself) or to design a regenerator [which operates independently or connected to the reactor, as in an FCC (fluid catalytic cracking) unit]. From studies of the regeneration of HY and HZSM-5 zeolites,8-13 the combustion of the coke on these microporous acid catalysts was shown to be a complex process. In this paper, the effect of the nature of the coke on this combustion is studied. In fact, this factor largely explains the diversity in the results encountered in the literature for coke combustion kinetics.11,12,14-22 Because of the small size of the channels in HY and HZSM-5 zeolites, the evolution of the hydrocarbon molecules that make up the coke is limited. Consequently, the coke is slightly evolved, with a high H/C ratio, which makes it highly unstable to any thermal treatment. Moreover, the combustion kinetics of any remaining coke will depend on the aging conditions. Schulz et al.23,24 have studied the relevant effects of the reaction conditions used in the conversion of methanol on the content and nature of the coke deposited on an HZSM-5 zeolite. Likewise, these authors have studied the effect of the aging thermal treatment on the remaining coke, and they have shown that the coke deposited at up to 290 °C can be completely eliminated * To whom correspondence should be addressed. Tel.: 3494-6012580. Fax: 34-94-4648500. E-mail:
[email protected].
by sweeping it with an inert gas. For higher reaction temperatures, between 325 and 375 °C, the coke is more evolved, and only partial elimination is attained with sweeping. This latter result was also obtained by Benito et al.25 A study on the same subject was also carried out by Magnoux et al. regarding the evolution of the composition of coke deposited over a USHY zeolite during thermal treatment prior to coke combustion.26 Given the larger size of the USHY zeolite channels, the coke studied was more evolved and had a polyaromatic structure depending on the reaction conditions and the conditions used for aging, which was carried out in a nitrogen atmosphere. Coke instability is attenuated by an aging treatment carried out by sweeping with an inert gas or steam. Through this treatment, light components of the coke, which are slightly evolved, have a high H/C ratio, and, consequently, are very reactive in the combustion, are eliminated from the catalyst. Simultaneously, the remaining coke evolves toward structures with lower H/C ratios through dehydrogenation, condensation, and cracking, in which hydrogen and methane are formed, as was shown by Nova´kova´ and Dolejsek for cokes deposited on HZSM-5 and HY zeolites used in the conversion of acetone.27 McLellan et al. found a similar evolution of the coke deposited on an HZSM-5 zeolite used in the MTG process.28 For supported metallic catalysts, in which the porous structure of the support is made up of mesopores without shape selectivity affecting coke molecule development, the coke is more developed and has a lower H/C ratio. Nevertheless, this coke has instability problems, which means that a treatment of thermal aging prior to combustion is advisable, as was demonstrated by Royo et al. for coke deposited on Cr2O3/Al2O3 catalysts deactivated in the dehydrogenation of butene in the range between 480 and 600 °C.29 In addition to affecting the nature of the coke, this aging treatment also influences the catalyst activity. Choudhary and Akolekar observed that, when zeolites deactivated at reaction temperatures of around 327 °C were heated after reaction in a flow of oxygen-free nitrogen to 427 °C, their activity was retained to some extent.30 This partial gain in activity was thought to be due to cracking of large hydrocarbon molecules that
10.1021/ie030085n CCC: $25.00 © 2003 American Chemical Society Published on Web 07/18/2003
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cannot leave the zeolite channels at reaction temperatures. During this cracking, hydrogen-rich C1-C4 fragments diffuse out of the zeolite channels, and the remaining carbonaceous deposits are highly polyaromatic or graphitic in nature, with a low H/C ratio. According to Magnoux et al., this “hard” coke increases the deactivation by pore blockage.26 The nature of the gaseous agent used for the aging treatment has a significant influence on both the structure of the remaining coke and the recovery of activity by the catalyst. Bauer et al. observed that hydrogen and alkenes had a positive effect on the stability of HZSM-5 catalysts during methanol conversion.31 Hydrogen and light alkanes prolonged the time on stream more than 2-fold compared to that seen with nitrogen. The suitable choice of a diluent agent for the reaction medium can function as an interesting method of attenuating the rate of catalyst deactivation. Henriques et al.32 observed that dilution with H2 or N2 gas has a different effect on the amount and composition of the coke deposited over two mordenite samples during o-xylene transformation at 350 °C. Coke formed in the presence of H2 consisted of soluble CH2Cl2 molecules, whereas under N2, the formation of insoluble coke was favored. Although in many cases the coke contents were greater under H2 than under N2, for similar conditions, the loss of activity was more severe in the latter case. This indicates that the large insoluble coke molecules formed under N2 had a greater poisoning effect. Wu et al. observed that a portion of the hydrocarbon layer that formed rapidly during metathesis of propylene at 607 °C on molybdenum and molybdenum oxide catalysts can easily be removed by heating the used catalyst in hydrogen. Nevertheless, the graphitic layer that formed slowly on the catalyst surface during reaction was difficult to remove by reaction with hydrogen.33 The addition of hydrogen to the metathesis reaction mixture reduced the thickness of the carbonaceous layer on the catalyst surface during reaction, thereby enhancing the observed metathesis rate. In this paper, a sweeping treatment of the catalyst (prepared from an HZSM-5 zeolite) and its effects on the coke structure and on the subsequent combustion stage are studied. Different gaseous agents (He, H2, and steam) are considered to evaluate the effects of sweeping with each one on both coke hydrogenation and coke gasification. The catalyst is used in the transformation of acetone into hydrocarbons. The rapid formation of coke in this process allows partially deactivated catalysts with different coke contents to be obtained in a reproducible way. 2. Experimental Section The ZSM-5 zeolite was synthesized with a Si/Al ratio of 24 from sodium silicate, ammonium sulfate, and tetra-n-propylammonium bromide, using ammonium nitrate for cation exchange. Patented Mobil techniques were used.34,35 The final catalyst, as used in the reactor, was obtained by agglomeration of the zeolite (25 wt %), under conditions of high humidity, with a binder (bentonite of Exaloid, 30 wt %) and an inert (fused alumina of Martinswerk, 45 wt %). The alumina inert is used to dilute the catalyst and provide sufficient mass to absorb the thermal variations that occur during the process. The purpose of the binder is to form particles of suitable
Table 1. Properties of the HZSM-5 Zeolite and of the Catalyst HZSM-5 zeolite Si/Al ratio Bronsted/Lewis ratio crystallinity crystal size, µm particle size, mm apparent density, g cm-3 surface area (BET), m2 g-1 pore volume, cm3 g-1 micropore volume, cm3 g-1 (99% with diameter < 0.7 nm)
catalysta
24 2.9 97% 6.3 0.3-0.5 1.21 124 0.43
0.94 420 0.65 0.17
pore volume distribution of the catalyst pore size (dp, µm)
content (vol %)
