Ind. Eng. Chem. Res. 2003, 42, 5737-5744
5737
KINETICS, CATALYSIS, AND REACTION ENGINEERING Coking Characteristics of Chromium-Exchanged ZSM-5 in Catalytic Combustion of Ethyl Acetate and Benzene in Air Ahmad Zuhairi Abdullah, Mohamad Zailani Bakar, and Subhash Bhatia* School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, Nibong Tebal, 14300 Pulau Pinang, Malaysia
The coking characteristics during ethyl acetate and benzene combustion, in the presence and absence of water vapor, are reported. Coked Cr-ZSM-5 was prepared by exposing the catalyst to 32000 ppm of respective VOC at a GHSV of 3800 h-1 at 400 °C and with the addition of 9000 ppm of water vapor in humid feed. Stable residual activity at longer time on stream suggested that organic combustion and coking occurred at different sites. More coke was formed during ethyl acetate combustion than benzene. Water vapor promoted monolayer coke formation but weakened the formation of whisker-type coke in the combustion of ethyl acetate. Coke obtained from ethyl acetate reaction was made up of oxygenated aromatics which were softer but caused higher deactivation. It formed on Brønsted acid sites and silanols while that of benzene reaction formed preferentially on Brønsted acid sites. Softer coke formed was below 300 and 400 °C for ethyl acetate and benzene, respectively. 1. Introduction Volatile organic compounds (VOCs) are an important class of air pollutants, emitted from many industrial processes and transportation activities. For the elimination of these substances, catalytic combustion is reported to be the most promising technology due to its low-energy requirement.1 However, catalyst deactivation stands as the main drawback. In VOC combustion process where lower operating temperatures are normally used, the most dominant cause of catalyst deactivation is the formation of coke.2 Coke formation in zeolites results from undesirable side reactions occurring on strong acid sites responsible for hydrogen-transfer reactions.3 Accumulated coke may cover some of the acid sites (preferentially strong acid sites) or block the channel of the zeolite. ZSM-5 catalysts show low coking tendency, attributed to the steric hindrance which limits the condensation of polyaromatic rings in the internal channels of the crystals.4 Because of the limited activity of these hydrogen-transfer reactions, coke of high H/C ratio corresponding to nonaromatic (oligomers) and monoaromatic structures form.3 There are numerous investigations on the effect of coke deposition on catalytic activity in hydrocarbon reactions.3,5,6 In many cases, poisoning of the active sites or blocking of pores leading to catalyst deactivation were reported. However, complete deactivation was never observed.7 Often, after a period of fast coke formation, a slower growth rate and subsequently a pseudo-steady state is achieved in which a residual constant activity remains. Until recently, this phenomenon was unsatisfactorily explained. * To whom correspondence should be addressed. Tel.: +604 593 7788. Fax: +604 594 1013. E-mail:
[email protected].
Reports on types and chemical characteristics of coke, and how they relate to the catalytic reactor operational conditions, are quite scarce. High concentration of water vapor, despite inhibitions to the VOC combustion process, affects the distribution of combustion products. Thus, the coking reaction might proceed through different sets of coke precursors, leading to different types of coke. Reaction temperature also has significant influence on coking reaction as its individual step is affected differently by temperature. At higher temperatures, thermal stability of coke precursors can play a significant role in determining the amount and characteristics of coke accumulated. In the present study, ethyl acetate and benzene were used as VOC model compounds of different chemical natures. Chromium is the most active metal among the transition metals7-9 and chromium-exchanged ZSM-5 (Cr-ZSM-5) produced a catalyst of high activity, high stability, and less coking tendency. The objectives of the research are to characterize and compare the coke accumulation process during the combustion of ethyl acetate and benzene. Emphases are given on the identification of the coking sites and the effect of coking on the activity and acid site distribution of Cr-ZSM-5. The relationship between reaction temperature and water vapor present in the feed with the amount and chemical characteristics of coke depositing on the catalyst are also presented. 2. Experimental Section 2.1. Preparation of Cr-ZSM-5. Na-ZSM-5 (Si/Al ) 240) sample was obtained from Sud-Chemie and was used as received. Chromium-exchanged ZSM-5 was prepared in two steps. In the first step, NH4+ exchange of Na-ZSM-5 was performed in 2.25 M of NH4Cl
10.1021/ie030427p CCC: $25.00 © 2003 American Chemical Society Published on Web 10/18/2003
5738 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 1. Characteristics of Cr- ZSM-5 Catalysts as Compared to Its Parent Na-Form catalyst Cr-ZSM-5(EAc)b characteristics
Na-ZSM-5
Cr-ZSM-5
dry
humidd
SBET (m2/g) micropore area (m2/g) mesopore area (m2/g) chromium loading (%) crystallinity (%)a acidity (mmol of NH3/g)
393 321 72
366 272 94 0.98 96 0.14
326 183 143 0.98 78 0.08
314 198 116 0.98 65 0.06
100 0.18
Cr-ZSM-5(Bz)c dry
humidd
338 213 125 0.98 85 0.10
320 195 125 0.98 73 0.12
a Relative to parent Na-ZSM-5. b Exposed to 32000 ppm ethyl acetate for 12 h at 400 °C. c Exposed to 32000 ppm benzene for 12 h at 400 °C. d With 9000 ppm of water vapor.
solution for 6 h. The chromium-exchange step was done in acidified (to pH 4) aqueous Cr(NO3)3 solution at 0.086 M, for 6 h followed by filtration, drying, and calcination at 500 °C for 6 h. Before use in the reactor, the catalysts were pelletized, crushed, and sieved between 0.25 and 0.30 mm. 2.2. Experimental Set-up. The preparation of coked Cr-ZSM-5 and activity study was performed using an 11-mm i.d. glass reactor. The VOC-laden air stream as fed to the reactor was generated by bubbling nitrogen gas through the VOC saturators. Water vapor in the feed was introduced by passing the same gas through water saturator and another flow of high-purity air was used to make up the total flow rate to give the desired gas hourly space velocity (GHSV). The accurate control of flow rate was achieved by means of Aalborg (AFC 2600) mass flow controllers. The reactor was operated at atmospheric pressure and the reaction temperature of the catalyst bed was monitored using a K-type thermocouple temperature probe. The feed and product gases were analyzed using an off-line Shimadzu GC8A gas chromatograph. A Porapak Q column was used for separation of carbon dioxide and organic components while the separation of carbon monoxide was achieved by means of a molecular sieve 5A column. All the experiments were conducted under oxygen-rich conditions. 2.3. Preparation of Coked Cr-ZSM-5. Feeds containing 32000 ppm of either ethyl acetate or benzene were passed through the reactor at a GHSV of 3800 h-1 and reaction temperatures between 100 and 600 °C. Experiments under humid conditions were carried out with the inclusion of 9000 ppm of water vapor in the feed. After reaching the intended reaction times, the coked catalysts were taken out for analysis and activity study. 2.4. Activity Study. The conversion of organic reactants was measured in integral reactor mode conducted at a GHSV of 32000 h-1 while the rate of reaction was measured in differential mode at a GHSV of 78900 h-1. For both reactor modes, the feed concentrations for both organics were kept at 2000 ppm. The rate of reaction was calculated as
(-r) )
Fvoc,in - Fvoc,out (1 - C)Wcat
(1)
in which
(-r) ) reaction rate (mol/s‚gcat) Fvoc,in and Fvoc,out are the VOC molar flow rates at the inlet and outlet of the reactor, respectively (mol/s), C is
the coke content of the catalyst (g/gcat), and Wcat is the weight of the catalyst (g). 2.5. Catalyst Characterization. The catalyst samples were characterized for surface area using Quantachrome Autosorb-1 and for acidity via thermal programmed desorption of ammonia (NH3-TPD) using Chembet 3000. Crystallinity was determined using a Siemens D2000 X-ray diffractometer by comparing the intensity of the four most intense peaks with that of parent ZSM-5, which was considered to be 100% crystalline.8 Thermogravimetry experiments on coked catalysts were performed using a Perkin-Elmer TGA7 thermogravimetry analyzer under 20 mL/min of pure oxygen flow and at a temperature ramping rate of 10 °C/min. Weight loss upon heating to 700 °C was taken as coke content of the sample. Infrared spectroscopy characterization of coked samples was performed using a Perkin-Elmer 2000 FTIR system. In acid sites characterization, the method proposed by Triantafillidis et al.11 and Fonseca et al.12 was used. In this method, prior to scanning with FTIR, the samples were adsorbed with pyridine followed by desorption at 150 °C for 1 h to remove any physically adsorbed pyridine. 3. Results 3.1. Characteristics of the Catalysts. The characteristics of Cr-ZSM-5 catalyst prepared are presented in Table 1. The chromium-exchange process and coking of Cr-ZSM-5 catalyst resulted in the drop of 22-32% of its micropore area while the mesopore area was increased. Chromium exchange was also found to cause a 4% drop in relative crystallinity attributed to heat treatment during preparation. A significant loss of crystallinity was observed after treatment with ethyl acetate, but with benzene, the loss was slightly lower. The inclusion of 9000 ppm of water in the feed was shown to further speed up loss of crystallinity. Data on acidity in Table 1 suggests more reduction in acidity when humid ethyl acetate was used as the feed. Coking by benzene reaction, especially under humid conditions, resulted in the least drop in acidity. Changes in surface area characterization were ascribed to two reasons. Exposure at 400 °C for 12 h caused some changes in the framework as suggested by lower crystallinity. The transformation could be due to partial breakdown of Si-O-Si and Si-O-Al bonds, which was accelerated by water, or through thermal dexydroxylation of Brønsted acid sites. These reactions led to the formation of framework defects which were responsible for increasing mesoporosity. Elimination of micropores was also the consequence of clogging by coke deposition. This deposit further changed the surface characteristics of the catalyst by forming secondary surfaces.
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5739
Figure 1. Conversion of ethyl acetate (EAc) and benzene (Bz) under dry and humid feed conditions (GHSV ) 32000 h-1, Cvoc ) 2000 ppm). Table 2. Product Distribution (in ppm) at Reaction Temperature 400 °C in the Reactor Outlet Stream (Cvoc ) 2000 ppm, GHSV ) 32000 h-1) ethyl acetate
benzene
products
dry
humid
dry
humid
acetaldehyde acetic acid formaldehyde formic acid carbon monoxide carbon dioxide
180 788 1220 750 1166 3120
176 740 1126 695 1072 2878
2956 1360
1884 868
3.2. Role of Water Vapor in VOC Combustion. The presence of water vapor as co-feed was found to negatively affect the conversion of ethyl acetate and benzene as shown in Figure 1. The effect of water was strongly temperature-dependent as the reduction in the conversion decreased with an increase in reaction temperature. The negative effect of water vapor on the conversion of organic was attributed to competitive adsorption of these two types of compounds on active sites (chromium). In competitive adsorption of VOC and water, the force of adsorption plays a dominant role.13 Physical adsorption involving interactions of sorbate molecules (VOC and water) with sorbent (Cr-ZSM-5), which can be accounted for by van der Waals forces and electrostatic forces. The magnitude of the forces depends on the polar nature of the sorbate and the sorbent. On polar surfaces, such as cation-exchanged zeolites, electrostatic forces dominate over van der Waals forces. Since water is a polar molecule, the adsorption is favored and consequently caused blockage of cationic sites9 or solvation of nucleophile, hence, slowing down the reaction. Sinquin et al.14 reported that, at low concentrations, water molecules can favor the reaction by increasing the mobility of the leaving group. Water molecules also have a cluster-forming ability around active sites, thereby creating a diffusion block for VOC molecules. At higher temperatures, this localization might be prevented and consequently the deactivation effect by water molecules diminished as observed in Figure 1. Catalytic combustion of ethyl acetate yielded acetyldehyde, acetic acid, formaldehyde, formic acid, carbon monoxide, and carbon dioxide as the products while benzene gave rise to carbon monoxide and carbon dioxide as the only carbon-containing products. Table 2 shows product distribution at a reaction temperature of 400 °C. At this temperature, carbon dioxide presented as the predominant product in ethyl acetate combustion
Figure 2. Profiles of coke content with time on stream for dry and humid ethyl acetate (EAc) and benzene (Bz) (reaction temperature ) 400 °C, GHSV ) 32000 h-1, Cvoc ) 32000 ppm).
while upstream products such as acetaldehyde and acetic acid presented at lower concentrations. With benzene feed, most of the products were in the form of carbon monoxide. No obvious trend difference was observed between products distribution under dry and humid feed except lower concentrations of these compounds when water vapor presented as co-feed. 3.3. Coke Accumulation Process. Ethyl acetate combustion was found to produce more coke than benzene, both under dry and humid feed as shown in Figure 2. It was partly due to higher reactivity of ethyl acetate over Cr-ZSM-5 compared to benzene. At the same time, it also produced more products of incomplete combustion to provide more reaction pathways toward coke formation. The high polarity of these oxygenated intermediates could also play a role in coke formation and the retention process. Figure 2 also shows the slowing down of the coke accumulation process after a period of fast formation during early hours of time on stream. Under humid feed, rates of coke formation (or slope in Figure 2) seemed to slow more rapidly than those of dry feed for both organics. Significant growth of coke content was still observed with dry feed at the end of the 12th hour on stream, at which the coke-time profiles with humid feed were almost flat. With ethyl acetate feed, coking of Cr-ZSM-5 was found to be more active while the negative effect of water vapor was obtained with benzene feed. Coking resulted from hydrogen-transfer reactions occurring on strong acid sites. Fresh Cr-ZSM-5 having a higher concentration of these sites consequently produced a high coking rate, especially during the first 6 h. Deposition of coke poisoned these sites for further coke-forming reactions, thus stabilizing the catalyst. The effect manifested as a gradual decrease in coking rate. Slow coking rate at longer time on stream signified the exhaustion of coking sites actively participating in coking reaction. These sites might have been poisoned by earlier formed coke or the blockage of certain channels, making them unavailable for subsequent coking reactions. Participation of water molecule in the coking process with both organics was observed as a rapid decrease in coking rate (or slope in Figure 2) at longer time on stream. This result suggested that water limited the activity of some acid sites which were involved in coking reactions. The mechanism could be via the transformation of Brønsted acid sites to Lewis acid sites in high humidity conditions. Cr-ZSM-5 also accumulated more
5740 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 3. Activity profile vs coke content for dry and humid ethyl acetate (EAc) and benzene (Bz) (reaction temperature ) 400 °C, GHSV ) 32000 h-1, Cvoc ) 2000 ppm).
coke with humid ethyl acetate feed despite being less active due to inhibition by water molecules. This observation suggested that coking activity in ethyl acetate combustion was enhanced under high humidity. In this case, the effect of enhancement in coking activity was more dominant than the effect by transformation of Brønsted acid sites to Lewis acid sites. In contrast, water molecule reduced the coking tendency of CrZSM-5 during the combustion of benzene. It could be due to lower reactivity of benzene for combustion, especially in humid feed, thus producing less coking precursors. 3.4. Effect of Coke Content on Cr-ZSM-5 Activity. During coking, coke was heterogeneously distributed along the interior of the channels and the intersections between straight channels and zigzag channels.4 As shown in Figure 3, coking caused less than 10% activity decrease after 12 h time on stream. This was attributed to high branching of ZSM-5 channels that allowed the accessibility of internal active sites through lateral channels even when the progressive blockage of internal channels occurred. The accumulation of coke during early hours of time on stream (when the coke content was below about 2%) caused a drop in the activity of the Cr-ZSM-5 catalyst. This drop was attributed to active and preferential accumulation of coke at pore intersections, creating diffusion limitation to the active sites. Because of geometrical constraint in the internal channels, only monolayer coke formed to produce site covering while further propagation of coke was retarded. Multilayer coke formed especially on the external surfaces or in the mesopores did not deactivate the catalyst. The activity drop was further contributed by the masking of metal (chromium) sites adjacent to acid sites by coke. However, at longer time on stream, about constant residual activity remained with no regard to the coke content. This result suggested that organic combustion and coking occurred on two different sites. An initial sharp drop in activity was therefore due to physical phenomena rather than chemical phenomena. Acid site poisoning was feed condition-dependent as the activity drop varied with type of organic compound and with or without the presence of water vapor. CrZSM-5 experienced more activity drop with humid ethyl acetate while the least drop was observed with humid benzene. Therefore, coke from ethyl acetate reaction had more effect on the activity. This was due to the formation of lighter or lower density coke, which occupied more spaces (at the same weight) compared to coke from benzene reaction. Because of higher volume, this type
of coke had lower mobility and preferentially accumulated at the external surfaces. 3.5. Kinetic Modeling of Coking Process. In zeolites, coking initiates on the Brønsted acid sites to form the primary or monolayer coke.3 Subsequent coking can form either on the new (different) acid sites or on the primary layer to form the secondary layer, normally in the form of whiskerlike coke.15 These two types of coke play different roles in the activity and selectivity, and their relative concentrations depend on the reactions leading to their formation. At early stages of the reaction, while the organic conversion is still high, coke forms rapidly on acidic sites (monolayer-type coke) while high amounts of coke precursors are formed. These sites will gradually self-deactivate and lead to the drop in rate of its formation. Simultaneously, the whisker-type coke (Cw) grows in weak interaction with and anchored on monolayer coke (Cm). Thus, total coke content (C) and the overall rate of coke formation (rc) can be written as
rc )
C ) Cm + Cw
(2)
dC dCm dCw ) + dt dt dt
(3)
The rate of monolayer coke formation is considered to be a function of available coking sites on the primary catalytic surface and is calculated as15
dCm ) km(Cm,max - Cm)h dt
(4)
where km stands for the constant rate of monolayer-type coke formation, Cw is the rate constant of whisker-type coke formation, Cm,max is the maximum amount of coke in the monolayer, Cm is the actual concentration of coke in the monolayer, and h is the number of active sites involved in the controlling step of coke formation. The rate of formation of whiskers (kw) is assumed to be constant at a given temperature:
dCw ) kw dt
(5)
Several values for h were tested on a trial-and-error basis and it was found that the best results were obtained with h ) 2. Thus, integrating eqs 4 and 5 yields
C ) Cm + Cw )
kmCm,max2t + kwt 1 + kmCm,maxt
(6)
The activity (a) can be related with the coke content using the equation7
a)
[
]
Cm,max - Cm rt ) r0 Cm,max
p
(7)
in which r0 and rt are reaction rates at zero and t time on stream, respectively, and p is a factor. Coking parameters were estimated using Polymath software and the results are summarized in Table 3. With humid ethyl acetate feed, Cr-ZSM-5 demonstrated a higher rate of monolayer coke formation (km) while the reverse was observed with benzene feed. Water vapor retarded the formation of whisker-type coke during the combustion of both organics as sug-
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5741 Table 3. Coking Parameters for Ethyl Acetate (EAc) and Benzene (Bz) at 400 °C feed parameters
EAc (dry)
EAc (humid)
Bz (dry)
Bz (humid)
km (gcat/g‚h) kw (g/gcat‚h) Cm,max (g/gcat) p
6.4 2.5 × 10-3 6.4 × 10-2 0.05
9.6 1.0 × 10-3 1.0 × 10-1 0.07
5.7 1.4 × 10-3 5.3 × 10-2 0.04
3.1 6.0 × 10-4 7.9 × 10-2 0.05
gested by lower values of kw, whereas the capacity of monolayer coke (Cm,max) was found to increase. All km, kw, and Cm,max values were in the same order of magnitude with the corresponding parameters obtained from the acetylene hydrogenation process as reported by Romeo et al.15 Both organics gave a p value less than 1, indicating nonlinear relation between available coking sites and activity, and it further suggests that coking and combustion of both organics occurred on different sites. Monolayer coke primarily formed on the Brønsted acid sites of Cr-ZSM-5.3 The higher value of km with humid ethyl acetate could be translated as more overall coke accumulation since km was 3 orders of magnitude larger compared to kw. This result suggested activation of the acid sites for coking reaction by water molecule. Monolayer coke was rather hydrophilic owing to the presence of hydroxyl-containing components.5 Therefore, the formation of whisker-type coke was retarded by water molecule due to preferential adsorption of the latter on the monolayer coke. This coking model suggested that, with the addition of water vapor in the feed, monolayer coke of higher density and/or more surface coverage was formed. This type of coke was also more toxic to the activity of CrZSM-5 for the combustion of both organics. In general, this characteristic of coke was associated with coke of higher H/C ratios.3 With higher surface coverage, this type of coke might cause the masking of metal sites located adjacent to the coking sites to cause more drop in the activity. This was supported by the higher values of p with humid feed for both organics. 3.6. Effect of Water Vapor on Characteristics of Coke. As shown in Figure 4, coke resulted from ethyl acetate and benzene reaction displayed characteristic absorption bands at 1110 and 1230 cm-1 ascribed to secondary C-OH and aromatic C-OH, respectively. Additional distinct band at 1144 cm-1 by coke from benzene reaction was attributed to in-plane stretching of adjacently substituted aromatic C-H. Aromatic CdC in-plane skeletal vibrations resulted in a weak band that appeared at 1621 cm-1 while out-of-plane bending of aromatic C-H gave rise to a band occurring at 821 cm-1. With humid ethyl acetate feed, more oxygenated coke was formed as indicated by a more intense peak at 1110 cm-1. At the same time, a reduced concentration of aromatic C-H was observed, as suggested by the weakening of absorbance at 821 cm-1. Antunnes et al.5 suggested that oxygenated, but less aromatic coke generally formed softer coke, which oxidized easily in an oxygen-rich environment. In benzene combustion, less oxygenated coke formed under humid conditions but it was more aromatic as the band at 1144 cm-1 was stronger than that occurring at 1110 cm-1. As shown in Figure 5, there were two general groups of coke forming on the catalyst. The softer coke, mainly built up of oxygenated polyaromatic compounds, oxi-
Figure 4. Infrared absorption spectra of coke produced in the combustion of ethyl acetate (EAc) and benzene (Bz) under dry and humid conditions.
Figure 5. Effect of water on thermogravimetric behavior of coked Cr-ZSM-5 with ethyl acetate and benzene (Bz) (coking conditions: temperature ) 400 °C, GHSV ) 3800 h-1, Cvoc ) 2000 ppm, CH2O ) 9000 ppm in humid feed).
dized rapidly below 250 °C. It accumulated preferentially on silanols in the extraframework phase, in defects, mesopores, and on the external surface of the zeolite.3 The oxidation of harder coke fraction, mainly made up of less oxygenated polyaromatic hydrocarbons, started to take place from around 500 °C until complete oxidation, which occurred above 650 °C. This type of coke normally formed on Brønsted acid sites.5 These results were in agreement with conclusions drawn from coke infrared absorption study. Ethyl acetate combustion produced a significant amount of softer coke. Decomposition of ethyl acetate was also found to form more coke while benzene combustion produced mostly harder coke. In humid conditions, combustion of ethyl acetate accumulated more coke, but the reverse was observed in the case of benzene combustion. The harder portion of the coke was oxidized between 500 and 650 °C for ethyl acetate but it started at about 50 °C lower for benzene. This result suggested that harder coke forming during ethyl acetate and benzene combustions were chemically different, although forming on the same site. The effect of water molecule on the overall coke formation was dictated by three mechanisms. First, it competed with organic molecules for adsorption on active sites to lower the conversion of these organics. Thus, less coke precursors were produced, leading to less coke formation. Second, water might take part in cokeforming reactions to produce coke of different chemical characteristics. Last, it increased the concentration of surface hydroxyls on the catalyst so that, upon hydroly-
5742 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 6. Infrared spectra of coked Cr-ZSM-5 samples under different conditions.
sis, it resulted in the formation of Brønsted acid sites.6 In ethyl acetate combustion, the second and third mechanisms presented more dominant effects while the first and second mechanisms were more dominant in the case of benzene combustion. 3.7. Effect of Coking on Hydroxyl Groups in CrZSM-5. In ZSM-5 zeolites, hydroxyl belongs to two major groups, viz. bridging hydroxyl groups belonging to Brønsted acid sites and silanols associated with infrared absorption band at 3595 and 3735 cm-1, respectively.16 Since infrared absorption spectra for these two hydroxyl groups as shown in Figure 6 did not show well-separated peaks, qualitative comparison between samples was based on the shape of the peaks between 3500 and 3800 cm-1. The majority of the hydroxyl groups in fresh CrZSM-5 were in the form of the bridging hydroxyl as the main peak occurred at 3595 cm-1. Upon coking with ethyl acetate, nonselective poisoning of some of the hydroxyl groups of both types was observed. In humid conditions, more hydroxyl groups were left unpoisoned by coke after 12 h of reaction time. The disappearance of silanols was attributed to deposition of oxygenated polyaromatic coke, which formed softer coke on CrZSM-5. A hump at 3735 cm-1 upon coking with benzene signified a bigger proportion of silanol groups in CrZSM-5. Hence, coke from the benzene reaction selectively deposited on Brønsted acid sites and had lower tendencies to poison silanols in both dry and humid conditions. 3.8. Effect of Coking on Acid Sites under Dry and Humid Feed. Infrared spectra with chemisorbed pyridine as the probe molecule was used to characterize types of acid sites sitting in Cr-ZSM-5 catalyst (Figure 7). The peak at 1446 cm-1 was attributed to pyridine attached to Lewis acid sites. The absorption that occurred at 1545 cm-1, belonging to pyridinium ions (PyH+), was used as an indicator of the presence of Brønsted acid sites on the external and internal surface of zeolite crystallites. The peak at 1440 cm-1, indicative of interaction between pyridine and metal cations, was not observed, probably due to overlapping with Lewis acid sites. Upon coking with ethyl acetate, especially in humid conditions, significant elimination of Brønsted acid sites was observed but the effect on Lewis acid sites was minimal. The same trend was also observed with benzene but relatively more Brønsted acid sites were still available after 12 h of the coking process. The accumulation of coke over Cr-ZSM-5 was concentrated
Figure 7. Infrared spectra of Cr-ZSM-5 catalyst (a) without pyridine chemisorbed, and (b) with chemisorbed pyridine. (i) Fresh Cr-ZSM-5, (ii) EAc (dry), (iii) EAc (humid), (iv) Bz (dry), and (v) Bz (humid).
Figure 8. Thermogravimetry analyses on coked Cr-ZSM-5 obtained at different reaction temperatures. (a) Coked with ethyl acetate and (b) coked with benzene (coking conditions: Cvoc ) 32000 ppm, GHSV ) 3800 h-1, 12 h).
around stronger Brønsted acid sites while Lewis acid sites had no or little role in coke formation during ethyl acetate or benzene combustion. 3.9. Effect of Reaction Temperature. The TGA results of coked Cr-ZSM-5 at different reaction temperatures for both feed organics are shown in Figure 8. Reaction temperatures of 300 and 400 °C were found to be the most favorable for coke formation for ethyl acetate and benzene, respectively. In general, ethyl acetate accumulated more coke than benzene at all reaction temperatures. Low coke formation at lower temperatures was due to low activity so that a higher fraction of the feed organic passed through the reactor unreacted. Therefore, less coke precursors formed to participate in the coking reactions. At higher temperatures, lower coke content was obtained with increasing
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5743
reaction temperature. This trend suggested that oxygen activated by metal sites played a role in coke formation. Lower coking activity at high temperature of its formation was ascribed to several mechanisms. Rodrı´guez et al.7 suggested reversible coke formation with negative dependence of coking with reaction temperature. In the coke formation process, among the key reaction steps were dehydrogenation steps,5 which were normally endothermic processes.6 Thus, the temperature increase did not favor the coke formation process. In an oxygen-rich environment, the coke might undergo oxidation in conditions of its formation as elaborated by Antunes et al.5 and Bulushev et al.17 Guisnet et al.18 suggested that higher reaction temperature resulted in higher mobility and faster diffusion of coke precursors to avoid condensation and subsequent coke growth. Again, the presence of coke precursors at high temperature was short-lived and they could not participate in rate-limiting steps, to the detriment of the overall coke formation. Figure 8 also suggests a strong influence of reaction temperature on the oxidative decomposition of coke formed during TGA analysis. For both organics, coke formed at 200 °C was readily oxidizable between 250 and 350 °C, suggesting softer coke with high H/C ratios. At 300 and 400 °C, the presence of two different groups of coke was noticeable, especially with ethyl acetate as the feed organic. When the reaction was conducted at higher temperatures, only a weight loss profile corresponding to harder coke appeared in a decreasing amount. This observation suggested that as the reaction temperature was increased from 200 to 500 °C, coke accumulated on the catalyst gradually changed from oxygenated polyaromatic to polyaromatic hydrocarbon with a corresponding increase in the temperature required for its oxidation. 4. Conclusions Coking resulted in elimination of micropores and loss of crystallinity of Cr-ZSM-5 between 65 and 85% depending on coking conditions. Initial rapid coke formation was attributed to more available acid sites and the rate progressively declined with time on stream. Reduced Cr-ZSM-5 acidity upon coking was caused by poisoning of acid sites by coke deposits. Decomposition of ethyl acetate produced more coke than that of benzene combustion. Water vapor enhanced coking in ethyl acetate combustion by promoting monolayer coke formation but weakened the formation of whisker-type coke. Coke from ethyl acetate reaction, especially under humid conditions, was made up of oxygenated aromatics, which was softer but caused more deactivation. It formed on both Brønsted acid sites and silanols while that of benzene reaction formed preferentially on Brønsted acid sites. Reaction temperatures of 300 and 400 °C for ethyl acetate and benzene, respectively, resulted in the highest coke formation. Softer coke formed below these temperatures, while above these temperatures, less but harder coke formed. Acknowledgment An IRPA research grant (08-02-05-1039 EA 001) from The Ministry of Science, Technology and Environment of Malaysia (MOSTE) and zeolite samples from Su¨d Chemie AG are gratefully acknowledged.
Nomenclature a ) activity C ) coke content of the catalyst (wt %) Cm ) actual amount of coke in the monolayer (g/gcat) Cm,max ) maximum amount of coke in monolayer (g/gcat) Fvoc,in, Fvoc,out ) molar flow rate at the inlet and outlet of the reactor, respectively (mol/h) h ) number of active sites involved in the controlling steps of coke formation km ) rate constant of monolayer-type coke formation (gcat/ g‚h) kw ) rate constant of whisker-type coke formation (g/gcat‚ h) p ) factor rc ) the rate of coke formation (g/gcat‚h) r0, rt ) initial reaction rate and reaction rate at time on stream at which the coke content was determined, respectively t ) time on stream (h) Wcat ) weight of the catalyst (g).
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Received for review May 19, 2003 Revised manuscript received September 5, 2003 Accepted September 17, 2003 IE030427P
