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Effect of hydrochlorination and hydrofluorination of H-ZSM-5 on the catalytic hydroconversion reactions of cyclohexene
Applied Catalysis A: General 297 (2006) 102–110 www.elsevier.com/locate/apcata
Effect of hydrochlorination and hydrofluorination of H-ZSM-5 on the catalytic hydroconversion reactions of cyclohexene Ahmed K. Aboul-Gheit a,*, Sameh M. Aboul-Fotouh b, Sohair M. Abdel-Hamid a, Noha A.K. Aboul-Gheit a a
Egyptian Petroleum Research Institute, Process Development Department, 1 Ahmed Al-Zomor Street, P.O. Box 9540, Nasr City, Cairo 11787, Egypt b Faculty of Education, Chemistry Department, Ain Shams University, P.O. Box 7016, Nasr City, Cairo 11768, Egypt Received 17 July 2005; received in revised form 29 August 2005; accepted 30 August 2005 Available online 17 October 2005
Abstract Modifying the acid sites (number and strength) of H-ZSM-5 zeolite via doping with 3.0 wt.% HCl or HF for being used as catalysts for the hydroconversion of cyclohexene (CHE) in a flow-type fixed-bed reactor operated atmospherically in H2 carrier gas at temperatures of 50–400 8C. The acid sites strength distribution in these zeolite forms was evaluated using temperature programmed desorption (TPD) of presorbed ammonia in a differential scanning calorimeter (DSC). Either HCl or HF incorporation has increased both acid sites number and strength in the zeolite to varying extents. Hydrochlorination enhanced the acid sites number in the zeolite to a larger extent than hydrofluorination, whereas, the latter enhanced the acid sites strength to a larger extent than hydrochlorination. Nevertheless, not only the acid catalysed reactions; i.e., isomerisation of the six-membered-ring of CHE to the five-membered ring compounds (methylcyclopentenes (MCPEs) plus methylcyclopentane (MCPA)) and hydrocracking reactions to lower molecular weight components, were enhanced via both hydrohalogenation treatments of the zeolite, but also hydrogenation of CHE to cyclohexane and MCPEs to MCPA and dehydrogenation of CHE to cyclohexadienes (CHDEs) plus benzene were also enhanced. # 2005 Elsevier B.V. All rights reserved. Keywords: Cyclohexene; Hydroconversion; H-ZSM-5; Zeolitel; Catalyst
1. Introduction The role of CHE as a probe molecule for investigating catalyst preparation has been frequently evidenced [1–4]. CHE undergoes various conversions depending on catalytic functions and the experimental reaction conditions. In absence of hydrogen, disproportionation [5,6] and dehydrogenation [7,8] reactions take place. Disproportionation gives benzene and cyclohexane (hydrogen transfer) in presence of metal catalysts, since CHE behaves as hydrogen donor and acceptor. However, if the metal is supported on Al2O3 [9], the acid sites are of the weakly Lewis type, where the acid strength is not strong enough to promote carbonium ion formation, and hence isomerization and cracking reactions cannot occur. However, the H-forms of * Corresponding author. E-mail address: [email protected] (A.K. Aboul-Gheit). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.08.044
zeolites possess strong Bro¨nsted acid and Lewis sites that promote CHE isomerization to a mixture of MCPEs and cracking to lower molecular weight compounds whereby the reaction scheme becomes more complicated. Nevertheless, treatment of the cation-exchanged zeolites with aqueous HCl acid leads to decationation and partial dealumination. The results of isomerization of CHE over highly pure aluminas with or without Cl ions has been studied by Ozimek et al. [10,11]. Introducing Cl ions into aluminate aluminas in different ways creates Bro¨nsted acid sites that greatly increases skeletal isomerization and total conversion. The promoting effect of adsorbed Cl ions on the surface acidity of g-Al2O3 has been monitored by zero point charge (zpc) and CHE isomerization activity [12] shows a linear decrease of the zpc values with increasing chloride adsorption. Only few data are available for Cl treated g-alumina. Arena et al. [12] have indicated that Cl adsorbed on the surface of
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g-alumina results in a promotion due to substantial change in the electronic properties of the outermost layer of alumina. Ultimately, these effects result in weakening of the O–H bond that renders the proton more acidic. Doping H-ZSM-5 with a low concentration of F species followed by thermal activation enhances the surface acidity via formation of new Bro¨nsted acid sites and strengthening some acid sites of the parent zeolite [13]. There are no data available in the literature correlating HZSM-5(HCl) or H-ZSM-5(HF) zeolites as catalysts for cyclohexene (CHE) hydroconversion. In the present work, the effect of doping H-ZSM-5 with 3.0 wt.% HCl or 3.0 wt.% HF on hydroconverting CHE in a flow of hydrogen at temperatures ranging between 50 and 400 8C is investigated.
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quantity reaching the catalyst bed. The reaction temperature range investigated was 50–400 8C, with 25 8C decrements starting from 400 8C. After each catalytic run, the carrier gas was turned to air flow to burn off any coke deposited on the catalyst surface, then the hydrogen flow was resumed for 15 min before the next catalytic run (next reaction temperature) was carried out. The tube in which the reaction effluent passing from the reactor was continuously heated to prevent condensation of any produced components. This effluent was injected (twice at each reaction temperature) in a Perkin-Elmer Autosystem XL gas-chromatograph using a 15 m capillary column of Carbowax 20 M bonded in fused silica to be analysed. A flame ionization detector and a Turbochrom Navigator Programme were used. Each run was continued for 25 min at a given reaction temperature.
2. Experimental 2.3. Temperature programmed desorption (TPD) of NH3 2.1. Preparation of the catalysts 2.1.1. H-ZSM-5 catalyst The sodium form of ZSM-5 zeolite (Na-ZSM-5; Si/ Al = 24.5), kindly supplied by Su¨d-Chemie AG, Germany, was exchanged in 0.7 M NH4NO3 solution five times; each time with a fresh solution for 8 h at 70 8C. The zeolite was separated, washed with distilled water till free of the NO3+ ion, dried at 110 8C overnight then calcined in air at 530 8C for 3 h to deammoniate NH4-ZSM-5 to H-ZSM-5. The Si/Al ratio of the produced H-ZSM-5 was still 24.5. The Na+ content was <0.02 wt.%. The surface area and total pore volume were 400 m2 g1 and 0.16 cm3 g1, respectively. 2.1.2. Preparation of HCl and HF treated H-ZSM-5 zeolites A part of the H-ZSM-5 zeolite was doped with an aqueous solution containing 3.0 wt.% of HCl or HF. This solution possessed the retention volume of the zeolite sample used in each treatment. HCl and HF were in the form of the respective acids. These preparations were dried at 110 8C overnight then calcined in air at 530 8C for 3 h. Before carrying out the hydroconversion runs, the catalysts were heated in the catalytic reactor at 500 8C for 2 h in a flow of 20 cm3 min1 of pure hydrogen gas. 2.2. Cyclohexene hydroconversion procedure, reactor and analysis A silica-glass down-flow-type tubular reactor always containing 0.2 g of a catalyst was used in all hydroconversion runs. The reactor was heated in an insulated wider silica-tube jacket, thermostated to 1 8C. Hydrogen gas was used as a carrier and simultaneously as a reactant, at a flow rate of 20 cm3 min1 in all runs. The cyclohexene feed was introduced into the reactor via its continuous evaporation using the carrier hydrogen flow passing into a closed jar thermostated at a fixed temperature of 28 8C, whereby, the quantity of cyclohexene was always 8.33 103 mol h1. The quantity of condensed CHE feed was estimated and considered in calculating the true
The TPD procedure adopted by Aboul-Gheit [14,15] using highly sensitive differential scanning calorimetry (DSC) has been applied to detect and evaluate the desorption of presorbed ammonia from the acid sites of the current catalysts. Ammonia was initially adsorbed on the catalyst in a silica-tube furnace as follows; after evacuation of the catalyst at 1.33 103 Pa whilst heating for 30 min at 500 8C and subsequent cooling under vacuum to 50 8C, ammonia was introduced through the catalyst at a flow-rate of 50 cm3 min1 till saturation (normally 15 min.). The samples were then measured in a DSC Mettler TA-3000 unit using standard aluminium crucibles in a continuous current of highly pure N2 gas at a flow-rate of 30 cm3 min1. The heating rate was 10 K min1 and the full-scale range was 25 mW. The thermograms obtained for each sample gave two peaks; a lowtemperature peak corresponding to enthalpy (DHd) of ammonia desorption from the weak acid sites of the catalyst and a high temperature peak corresponding to DHd of ammonia desorbed from the strong acid sites (Fig. 1). The DHd values are proportional to the peak area and, hence, to the number of acid sites, whereas the peak temperature (Tmax) was used to correlate the acid sites strength of the catalysts; the higher the Tmax value, the stronger the acid sites in the examined catalyst. 3. Results and discussion The H-ZSM-5 zeolite and its HCl and HF promoted versions, possess slightly different values of Si/Al (24.5, 25.0
Fig. 1. TPD of ammonia from HCl and HF doped H-ZSM-5 zeolite.
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Table 1 Ammonia desorption enthalpy (DHdes) and desorption peak temperature for the zeolites Zeolite form
DHdes for ammonia desorption (J g1)
Peak temperature (8C)
H-ZSM-5 H-ZSM-5(HCl) H-ZSM-5(HF)
97.4 108.5 101.9
372.4 375.6 382.7
and 25.8, respectively), indicating that the current acid treatments does not significantly affect bulk dealumination of the H-ZSM-5 zeolite. Fig. 1 and Table 1 show that the acid sites number in the current zeolites is in order (3) H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5
ð3Þ
whereas, the acid sites strength of these zeolite catalysts can be arranged in order (4) H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ > H-ZSM-5
ð4Þ
In other words, both acid treatments of the zeolite have increased its acid sites number and strength, however, HCl increased acid sites number more than HF does, whereas, HF increased the acid sites strength more than HCl does. 3.1. The overall hydroconversion of cyclohexene The hydroconversion process of CHE includes several reactions (Scheme 1) taking place at varying extents depending
principally on their acid sites number and strength using the three current catalysts. However, the overall hydroconversion of CHE on the catalysts can be segmented into three regions according to the activities of the catalysts at three temperature ranges (Fig. 2).A low temperature region (50–200 8C): where the activities of the three catalysts are modestly influenced with temperature. Even low differences are recognised, the overall activities of the catalysts in this region can be arranged in the following order: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5
ð1Þ
A medium temperature region (200–275 8C): where a jump of CHE hydroconversion is accomplished. For instance, at 200 8C, using H-ZSM-5(HCl), H-ZSM-5(HF) and H-ZSM-5 catalysts, the conversion amounts to 26.5, 17.0 and 11.5%, respectively, and increases to as high as 78.5, 83.0 and 73.0%, respectively, at 275 8C. The activities of the catalysts during this temperature region remain as in order (1) for the low temperature region.At temperatures beyond 275 8C up to 400 8C: where the change in activities as a function of temperature is negligible, the catalysts activities in this temperature region can be arranged in the following order (2): H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ
ð2Þ
During the low temperature range, the catalytic activities are principally reflections of the intrinsic chemical reaction; controlled by acidity, whereas, during the higher temperature range, surface and physical factors, such as adsorption and diffusion in the catalytic pores, may mask the chemical reaction
Scheme 1. Proposed scheme for pathways of cyclohexene hydroconversion using the current catalysts.
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Fig. 2. Total hydroconversion of CHE using HZSM-5 catalysts. Fig. 5. MCPA in product using H-ZSM-5 catalysts.
rate. Hence at temperatures between 50 and 200 8C, the effect of physical factors may be excluded or at least minimized, whereas at temperatures between 200 and 400 8C, these factors are significantly effective. Evidently, throughout the low and medium temperature regions, the relatively higher activities of the H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, compared to the untreated one, can be attributed to their higher acid sites number and/or strength (Fig. 1 and Table 1). Comparatively, the larger number of acid sites in the HCl promoted catalyst than in the HF promoted one can be considered the influencing factor for
Fig. 3. Hydrogenation of CHE to CHA using H-ZSM-5 catalysts.
Fig. 4. MCPEs in product using H-ZSM-5 catalysts.
increasing the overall hydroconversion activity of the former catalyst than on the latter during low temperatures. Le Van Mao et al. [13] show that doping H-ZSM-5 with a low concentration of fluoride species enhances the surface acidity via: (1) formation of new Bro¨nsted acid sites, (2) strengthening some acid sites of the parent zeolite. Moreover, Arena et al. [12] have indicated that Cl adsorbed on the surface of Al2O3 results in a significant change in the electronic properties of the outer layer of Al2O3 that decreases the basic Lewis sites, or inducing a stronger ‘‘inductive effect’’ of Cl on the neighboring hydroxyl groups. This electronic effect weakens the O–H bond, rendering the proton more acidic. In this work, the most active catalyst at the low and medium temperatures, H-ZSM-5(HCl), evidently become relatively the least active during the higher temperature region due to pore diffusion restriction by chloro-aluminium debris resulting from slight dealumination of the zeolite by HCl [16]. However, HF doping not only similarly produces flouro-aluminium debris but also fluoro-aluminium debris that causes extra-diffusion retardation. On the contrary, the untreated H-ZSM-5 zeolite has evidently acquired the highest activity during the higher temperature region because of the absence of deposited debris in its channels. Summing up the use of the current catalysts, the first step in CHE hydroconversion (Scheme 1 and Fig. 2), taking place at low temperatures, is CHE hydrogenation to cyclohexane (CHA) (Fig. 3). At higher temperatures, CHE isomerises to the three possible methylcyclopentenes (MCPEs) (Fig. 4), which then partially hydrogenate to MCPA (Fig. 5) and the sum of MCPEs plus MCPA gives the overall isomers production (Fig. 6). Moreover, CHE dehydrogenates via forming 1,3- and 1,4-cyclohexadienes (CHDEs) (Fig. 7 and Fig. 8a–c) which dehydrogenate via a further increase of temperature producing benzene. Total CHE dehydrogenation (Fig. 9) will, hence, include production of CHDEs + benzene + aromatic ring in toluene + xylenes. Total aromatic hydrocarbons production (benzene, toluene and xylenes) are depicted in Fig. 10, whereas the alkylated products include only toluene and xylenes (Fig. 11). Most of the intermediates formed during CHE hydroconversion and CHE itself are susceptible to be hydrocracked to lower molecular weight hydrocarbons (C1–C5) at high temperatures
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Fig. 6. Overall isomerization products using H-ZSM-5 catalysts.
(Fig. 12). The hydrogenation reactions take place on the current unloaded catalysts at relatively higher temperatures, compared to those carried out using active metals loaded H-ZSM-5 catalysts [17,18]. 3.2. Individual hydroconversion reactions The left-hand limb in the volcano shape curves in Figs. 3–8 as well as the (left-hand) low temperature region in Figs. 9–11 show that the activities of the catalysts are in accordance with order (1), i.e., the H-ZSM-5(HCl) catalyst acquires the highest activity by virtue of acquiring the largest number of acid sites with moderate strength (Table 1). Comparatively, the untreated H-ZSM-5 catalyst, which acquires the lowest acid sites number and strength exhibits the lowest activities for the overall hydroconversion reaction. Correspondingly, the right-hand limb of Figs. 3–8 and the high temperature region of Figs. 9–11 show that the activity order of the current catalysts are as follows:
Fig. 8. (a) Individual cyclohexadienes in product using untreated H-ZSM-5 catalyst. (b) Individual cyclohexadienes in product using H-ZSM-5(HCl) catalyst. (c) Individual cyclohexadienes in product using H-ZSM-5(HF).
Arranging the current catalysts in orders (3) and (4) shows that all individual reactions in CHE hydroconversion during the high temperature region are most accelerated on the untreated H-ZSM-5 catalyst (relatively devoid of pore-diffusion restric-
tion), whereas the H-ZSM-5(HF) catalyst encounters the highest pore restriction (some fluoro-silicon debris in addition to fluoro-aluminium debris occur). Hence, the H-ZSM-5(HCl) catalyst can thus be said to acquire lower diffusion restriction than the H-ZSM-5(HF) catalyst (order (3)). Since the hydrogenation and isomerisation steps in CHE hydroconversion follow the same order of catalysts activities, the assumption that Bro¨nsted acid sites act as hydrogenation sites at higher temperatures is emphasized [19–21]. Sano et al. [19] report that they have heterogeneously acid catalysed
Fig. 7. CHDEs in product using H-ZSM-5 catalysts.
Fig. 9. Dehydrogenation products using H-ZSM-5 catalysts.
H-ZSM-5 > H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ ð3Þ Exceptionally, the decending limb in Fig. 7 deviates from order (3) and follows order (4): H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ ð4Þ
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Fig. 10. Total aromatics production using H-ZSM-5 catalysts.
Fig. 11. Alkylation products using H-ZSM-5 catalysts.
Fig. 12. Hydrocracked products in CHE hydroconversion using H-ZSM-5 catalysts.
ethene hydrogenation on protonated ZSM-5 zeolite (H-ZSM5). This zeolite was active as a hydrogenation catalyst such that at a temperature of 535 8C, 95% of ethane has been converted to ethane. Since no metal impurities in the zeolite could be detected, they concluded that the hydrogenation takes place at the acidic sites of the zeolite. Moreover, Jacobs and co-workers [20] have extensively investigated experimentally the nature of the active sites in zeolites by varying the reaction temperature and using different types of zeolites with changing Si/Al framework and Na+/H+ ratios and concluded that Na+ ions are the active sites at low temperatures, whereas Bro¨nsted acid sites become the active hydrogenation sites at high temperatures. Furthermore, Senger and Radom [21] have tried to provide a
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deeper insight into this type of heterogeneous catalytic hydrogenation in which the reaction proceeds without the participation of a transition metal. Our work proves, moreover, that the protons not only act as hydrogenation sites at higher temperatures but also can perform hydrogenation reaction at lower temperatures, as can be observed in Fig. 3. On the contrary, the largest formation of CHDEs is attained using the unhalogenated zeolite (which possesses the lowest acidity parameters). Hence, CHDEs conversion to benzene can be assumed rate controlling in the overall dehydrogenation route leading to benzene. Again, HZSM-5(HCl) is evidently the most active for benzene formation, followed by H-ZSM-5(HF), which indicates that Bro¨nsted acid sites number is the determining factor for CHE dehydrogenation, the same as it is for hydrogenation (Fig. 3). From a practical point of view, the majority of published data are concerned with promotion of the commercial Pt/Al2O3 catalysts, which are the conventional industrial hydroisomerisation catalysts, with Cl. Because g-Al2O3 possesses only weak Lewis acid sites, which cannot promote the isomerization of the low-octane number n-paraffins. A Cl species is injected during operation in these catalysts such that Cl ions adsorb on the alumina surface and the result will be the weakening of the O–H bond, rendering the proton more strongly acidic and new Bro¨nsted acid sites are formed. 3.2.1. Hydrogenation of CHE to cyclohexane (CHA) The present work shows that the unloaded H-ZSM-5 zeolite, whether treated or untreated with either HCl or HF acids are also capable of undergoing double-bond saturation (Fig. 3). Hydrogenation of CHE to CHA is highest on the H-ZSM5(HCl) catalyst. On this catalyst, CHA production amounts to 9.0% at 200 8C, compared to 1.4 and 2.0% on H-ZSM-5 and HZSM-5(HF), respectively. Even though, on the three catalysts, CHA production attains comparable maxima at 300 8C. The Bro¨nsted acid sites in these zeolites, whether originally present, strengthened or created via HCl or HF treatments, should become active hydrogenation sites for alkenes at higher temperatures [19–21]. Although Fig. 3 shows that the order of hydrogenation of CHE to CHA, using the catalysts under study is as follows: HZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM-5, the reverse reaction (dehydrogenation of CHA), evidently follows the order: HZSM-5(HF) > H-ZSM-5(HCl) > H-ZSM-5. These orders conclusively assume that the parent zeolite possesses the lowest hydrogenation and lowest dehydrogenation activities. Nevertheless, as observed from the ascending limbs of the curves, before the maximum point, and the descending limbs, after the maxima, it may be assumed that HCl on H-ZSM-5 promotes hydrogenation of the double bond more effectively than HF, whereas HF promotes dehydrogenation more effectively than HCl. 3.3. Methylcyclopentenes (MCPEs) production Fig. 4 shows that the most prevailing reaction using the current catalysts, is the isomerization of CHE to the possible
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five-membered ring olefins, methylcyclopentenes (MCPEs): 1MCPE, 3-MCPE and 4-MCPE. Bautista et al. [22] have shown that mainly 1- and 3-MCPEs are produced; of which 1-MCPE predominates. At lower reaction temperatures (50–250 8C), the HCl and HF treated catalysts are more active than the untreated H-ZSM-5 catalyst, which can be attributed to the stronger acid sites of the two former versions. These activity data are compatible with the acid sites strength data where H-ZSM5(HCl) is less strongly acidic than H-ZSM-5(HF). The H-ZSM5 zeolite acquires the lowest acid strength. Ozimek et al. [10,11] have shown that Cl introduction in Al2O3 in different ways causes Bronsted acid sites as strong as HR0 < 6.63 to appear and a large increase of both skeletal isomerization and total conversion of CHE. The work in [12] and [13] also support this finding. Fig. 4 shows that the HCl and HF-doped H-ZSM-5 attain comparable maxima of MCPEs at 275 8C of 52.56 and 54.79%, respectively. The untreated H-ZSM-5 gives 53.9% at this temperature. Even-though, the untreated catalyst is more active than the halogenated versions beyond the MCPEs maxima (at higher temperatures) (Fig. 4), which may be due to lower diffusion limitation. It is known that diffusion limitation is specially rate controlling at higher reaction temperatures where the isomerization activities of the current zeolites dominate. Some forms of extra-framework species formed and deposited in the zeolitic channels of the zeolites contribute to diffusion resistance. Moreover, in this high temperature region, the HF doped catalyst may encounter more resistance than the HCl doped version as explained above. Not only MCPEs are the products of CHE isomerization, but also their hydrogenation product methylcyclopentane (MCPA) (Fig. 5). The order of reactivity for producing MCPA over the three catalysts under investigation, before and after attaining the maximum yields, is exactly the same as for producing MCPEs (Fig. 4). The total isomers are actually the sum of unsaturated and saturated methyl-substituted five-membered ring compounds (MCPEs + MCPA) (Fig. 6) [11,23]. Evidently, isomerization of CHE is the major reaction taking place using the unloaded H-ZSM-5 zeolite, whether containing a halogen or not. Using the H-ZSM-5, H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, the maximum isomers of CHE amount to 65.29% (300 8C), 58.05% (275 8C) and 59.37% (275 8C), respectively. 3.4. Methylcyclopentane (MCPA) production MCPA is produced via hydrogenation of MCPEs (Fig. 5). However, Dwyer et al. [23] have considered MCPA as a product of CHE isomerisation to MCPEs followed by hydrogenation, and can thus be indicative of acquiring two catalytic functions (hydrogenation function and acid function). Both functions on the current catalysts are provided by strong Bro¨nsted acid sites. MCPA production using the catalysts under study indicates that the difference in activities of these catalysts for hydrogenating the five-membered ring cycloolefins (MCPEs) is compatible with the order of activities for hydrogenating the six-membered ring cycloolefin, CHE, following also the order; H-ZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM-5. The activity for MCPA production during the whole temperature show
parallelism on the HCl and HF promoted catalysts. However, only using the untreated H-ZSM-5 catalyst, MCPA production is the lowest at the low temperature range then becomes the highest at the higher temperature range. Evidently, the absence of diffusion limitation in the untreated catalyst overcompensates the activity difference. Nevertheless, it is evident that a broader maximum is attained (Fig. 5) via hydrogenating the five-membered ring such that the decline of the concentration of MCPA in product is shifted to a higher temperature of 350 8C, compared to that at which the six-membered cycloolefin, CHE, hydrogenates (Fig. 3), where the maximum is attained at 300 8C. It has been shown that the hydrogenation of the double bond in the five-membered ring is more difficult than that in the sixmembered ring; Aboul-Gheit and Abdou et al. [24–26] have found that the negative activation entropy values obtained for the hydrogenation of the double bonds in the five-membered nitrogen-containing ring compounds is almost double the values obtained for hydrogenating the corresponding sixmembered ring containing compounds. This is attributed to more difficult orientation and adsorption of the five-membered ring on the catalyst surface where the number of degrees of freedom is lower and hence the entropy change values are more highly negative negative. 3.5. Cyclohexadienes (CHDEs) production Peck and Koel [27] have indicated that chemisorbed CHDEs are the intermediates for benzene formation. Fig. 7 shows the sum of the two CHDEs as a function of temperature using the current catalysts. The order of the activities of these catalysts for the formation of CHDEs is as follows: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5 The order of magnitude of CHDEs beyond the maximum is in the reverse order: H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ This means that the rates of formation and disappearance of these compounds is highest on the H-ZSM-5(HCl) catalyst and lowest on the H-ZSM-5 catalyst. Evidently, this is compatible with the fact that formation and disappearance of CHDEs are both dehydrogenation reactions. CHE < ¼¼ > CHDEs and CHDEs < ¼¼¼ > benzene Two possible individual CHDEs, namely; 1,3- and 1,4-cyclohexadienes are formed during the hydroconversion of CHE using the catalysts under study (Fig. 8a–c). Somorjai and coworkers [28] assume that at high pressures, none of the CHDEs are formed on the catalyst surface. However, Peck and Koel [27] and Campbell et al. [29] show that in ultra-high vacuum, CHE dehydrogenates readily to benzene above 27 8C. In the present work, it is found that CHDEs start formation with very low yields at 50 8C at atmospheric pressure, using the untreated and HCl treated catalysts, whereas 1,3-CHDE does not form using the HF treated catalyst below 150 8C. As temperature
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increases, the two CHDEs increase up to a maximum at 350 8C using all catalysts, beyond which they decline with a further increase of temperature up to 400 8C. Using the untreated HZSM-5 catalyst (Fig. 8a), 1,3- and 1,4-CHDEs reach maxima of 2.57 and 2.43%, respectively, while using the H-ZSM-5(HCl) catalyst (Fig. 8b), these components reach maxima of 2.85 and 1.62%, respectively. However, using the H-ZSM-5(HF) catalyst (Fig. 8c), these CHDEs reach maxima of 3.30 and 1.98%, respectively. 3.6. CHE dehydrogenation The overall dehydrogenation activity of each of the current catalysts has been calculated as: CHDEs + benzene + benzene ring in toluene and xylenes, and these values have been plotted as a function of reaction temperature for the current catalysts in Fig. 9. At the lower temperature range in this figure, the order of the dehydrogenation activities of the current catalysts is as follows: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5 This is exactly the same order of the activities of the catalysts for the hydrogenation of CHE to CHA (Fig. 3) and hydrogenation of MCPEs to MCPA (Fig. 5). Hence, we can assume that both dehydrogenation and hydrogenation reactions occur on the same sites, i.e., Bro¨nsted acid sites as indicated before. However, at higher temperatures, the dehydrogenation activities of the catalysts become: H-ZSM-5 > H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ This order is compatible with the pore diffusion restriction regime. Using the H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, the dehydrogenation activities decline as temperature increases to 375 and 350 8C, respectively. This proves that the HF promoted catalyst encounters a larger diffusion restriction than the HCl promoted one. Moreover, the dehydrogenation activity of the latter is higher than the former. Another evidence, is acquiring a higher activity by the untreated H-ZSM-5 catalyst, that continues rising as a function of temperature up to 400 8C. 3.7. Aromatics formation The production of aromatics (Fig. 10), namely; benzene, toluene and xylenes in the reaction product of CHE hydroconversion is very slow at temperatures from 50 8C up to 250 8C, beyond which these aromatics increase measurably with temperature up to 400 8C reaching 20.4, 14.0 and 13.7% using the H-ZSM-5, H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, respectively. In aromatics production, the rates are controlled to a great extent by the hydrophilic/hydrophobic state of the catalysts, which affects the adsorption and desorption of the benzenoid structure. The higher the hydrophilicity of the zeolite, the larger is its capability for forming and adsorbing aromatics on its surface. The halogenated zeolites are known to be more hydrophobic than the untreated form [16]. Furthermore, the HF promoted zeolite is more hydrophobic than the HCl promoted one, which is
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compatible with the order of aromatics formation obtained. Another factor contributing to this observed catalytic activity order for aromatics formation, is the role played by the chloroand fluoro-aluminium debris precipitated in the zeolitic channels in case of the HCl and HF promoted catalysts. 3.8. Alkylation of aromatics The alkylation of CHE and CHDEs by methyl fragments produced via hydrocracking reactions during the hydroconversion process, are namely toluene and xylenes. Fig. 11 depicts the formation of these alkylation products using the current catalysts as a function of reaction temperature. Although this reaction is an acid catalysed one, it behaves similar to the dehydrogenation reaction (Fig. 9), which is generally supposed to be a metal catalysed reaction. Since the catalysts under investigation are, of course, unloaded with hydrogenating/ dehydrogenating metals, this also substantiates that the acid sites in all current H-ZSM-5 forms act as hydrogenation/ dehydrogenation sites. 3.9. Hydrocracked products The cracking activities of the current zeolite catalysts, are principally dependent on their acidities. n-Hexane cracking is traditionally used as a test for comparing the acidities of solid acid catalysts [30]. According to the data in Table 1, the acid sites number in the current catalysts is in the order: H-ZSM5(HCl) > H-ZSM-5(HF) > H-ZSM-5, whereas, the strength of these acid sites is in the order: H-ZSM-5(HF) > H-ZSM5(HCl) > H-ZSM-5. The data obtained for the CHE hydrocracking levels using these catalysts (Fig. 12) conform directly to the acid sites strength of catalysts. The hydrocracking products comprise 5.18, 7.95 and 17.5% at a reaction temperature of 300 8C and increase with increasing temperature to 40.7, 48.1 and 50.6%, respectively, at 400 8C using the HZSM-5(HF), H-ZSM-5(HCl) and H-ZSM-5 catalysts, respectively. The peak temperatures at which NH3 desorbs using these catalysts are 380.7, 376.6 and 372.4, respectively (Table 1). Since hydrocracked products are of smaller molecular sizes than the other molecular species present in contact with the catalyst, it appears that these hydrocracked products does not encounter significant diffusion restriction, such that they are most highly produced on the H-ZSM-5(HF) catalyst (acquiring highest restriction) and least produced on the untreated HZSM-5 catalyst (acquiring lowest or no restriction). The hydrocracking reaction rate control is thus attributed to the acid sites strength only. 4. Conclusion Unloaded H-ZSM-5 plays the role of bifunctional catalysts by virtue of its strong Bro¨nsted acid sites. Verification of this assumption has been realized via hydrochlorination and hydrofluorination of H-ZSM-5, which enhanced CHE hydrogenation reactions at relatively higher temperatures compared to those applied using specific bifunctional noble metals loaded
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H-ZSM-6 catalysts [17,18]. At low temperatures, the activities of the current catalysts for almost all hydroconversion reactions are in the order: H-ZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM5, by virtue of the higher acid sites number, whereas, at high temperatures, the activities are in the order: H-ZSM-5 > H-ZSM-5(HCl) > H-ZSM-5(HF), due to diffusion resistance in the zeolitic channels caused by deposited halo-aluminium debris. This debris is more accelerated by HF than by HCl. However, hydrocracking is the only reaction that is not influenced by diffusion, because of the low molecular weight fragments produced. The current catalysts activities for hydrocracking are compatible with their acid sites strength which is in the order: H-ZSM-5(HF) > H-ZSM-5(HCl) > H-ZSM-5. References [1] M. Bartok, A. Molnar, Sterochemistry of Heterogeneous Metal Catalysis, Wiley, Chichester, 1985, p. 53, Chapter 3. [2] E. Jacquinot, A. Mendes, F. Raatz, C. Marcilly, E. Ribeiro, J. Caeiro, Appl. Catal. 60 (1990) 101. [3] I. Palinko, Appl. Catal. A 126 (1995) 39. [4] Z. Xu, B.C. Gates, J. Catal. 154 (1995) 335. [5] D.V. Rebhan, V. Haensel, J. Catal. 111 (1988) 397. [6] C.M. McConica, M. Boudart, J. Catal. 117 (1989) 33. [7] D.W. Blakely, G.A. Somorjai, J. Catal. 42 (1976) 181. [8] S.M. Davis, G.A. Somorjai, J. Catal. 65 (1990) 78. [9] A.K. Aboul-Gheit, S.M. Aboul-Fotouh, Appl. Catal.: Gen. A 208 (2001) 55; A.K. Aboul-Gheit, S.M. Aboul-Fotouh, Proceedings of the Ninth International Symposium on Heterogen. Catal., Bulg. Acad. Sci., Varna, Bulgaria, September 2000, p. 163.
[10] B. Ozimek, J. Grezchowiak, B. Radomski, G. Szezyglowska, React. Kinet. Catal. Lett. 17 (1981) 139. [11] B. Ozimek, B. Radomiski, React. Kinet. Catal. Lett. 15 (1980) 407. [12] F. Arena, F. Frusterl, N. Mondeller, N. Giordano, J. Chem. Soc. Faraday Trans. 88 (1992) 3353. [13] R. Le Van Mao, T.S. Le, M. Fairbairn, A. Muntasar, S. Xiao, G. Denes, Appl. Catal.: Gen. A 185 (1999) 41. [14] A.K. Aboul-Gheit, Thermochim. Acta 191 (1991) 233. [15] A.K. Aboul-Gheit, Solid State Ionics 101–103 (1997) 893. [16] J. Scherzer, Catalytic materials, in: T.E. Whyte, Dalla Betta, Jr., E.G. Derouane, R.T.K. Baker (Eds.), Relationship between Structure and Reactivity, ACS Symposium Series No. 248, ACS, Washington, DC, 1984, p. 157. [17] A.K. Aboul-Gheit, S.M. Aboul-Fotouh, N.A.K. Aboul-Gheit, Appl. Catal.: Gen. A 283 (2005) 157. [18] A.K. Aboul-Gheit, S.M. Aboul-Fotouh, N.A.K. Aboul-Gheit, Appl. Catal.: Gen. A 292 (2005) 144. [19] T. Sano, H. Hajiwara, K. Okaba, H. Saito, T. Takaya, Sekiyu Gakkaishi 29 (1986) 89. [20] J. Kanai, J.A. Martens, P.A. Jacobs, J. Catal. 133 (1992) 527. [21] S. Senger, L. Radom, J. Am. Chem. Soc. 122 (2000) 2613. [22] F.M. Bautista, J.M. Campelo, A. Gracia, D. Luna, J.M. Marianas, A.A. Romero, J.A. Navio, M. Macisa, J. Catal. 145 (1994) 107. [23] J. Dwyer, J. Dewing, K. Karim, S. Homes, A. Fojo, A.A. Garforth, D.J. Rawlence, Zeolite chemistry, in: B. Delmon, J.T. Yates (Eds.), Studies in Surface Science and Catalysis, Vol.69, Elsevier, Amsterdam, 1991, p. 1. [24] A.K. Aboul-Gheit, I.K. Abdou, J. Inst. Petrol. (Lond.) 95 (1973) 188. [25] A.K. Aboul-Gheit, Can. J. Chem. 53 (1975) 2575. [26] A.K. Aboul-Gheit, Appl. Catal. 16 (1985) 39. [27] J.W. Peck, B.E. Koel, J. Am. Chem. Soc. 118 (1996) 2708. [28] X. Su, K.Y. Kung, J. Lahtinen, Y. Ron Shen, G.A. Somorjai, J. Mol. Catal. A. Chem. 141 (1999) 9. [29] J.M. Campbell, S. Seimanides, C.T. Campbell, J. Phys. Chem. 93 (1989), 806, 815. [30] A. Corma, B.W. Wojciechowski, Catal. Rev. 27 (1985) 29.
Effect of hydrochlorination and hydrofluorination of H-ZSM-5 on the catalytic hydroconversion reactions of cyclohexene Ahmed K. Aboul-Gheit a,*, Sameh M. Aboul-Fotouh b, Sohair M. Abdel-Hamid a, Noha A.K. Aboul-Gheit a a
Egyptian Petroleum Research Institute, Process Development Department, 1 Ahmed Al-Zomor Street, P.O. Box 9540, Nasr City, Cairo 11787, Egypt b Faculty of Education, Chemistry Department, Ain Shams University, P.O. Box 7016, Nasr City, Cairo 11768, Egypt Received 17 July 2005; received in revised form 29 August 2005; accepted 30 August 2005 Available online 17 October 2005
Abstract Modifying the acid sites (number and strength) of H-ZSM-5 zeolite via doping with 3.0 wt.% HCl or HF for being used as catalysts for the hydroconversion of cyclohexene (CHE) in a flow-type fixed-bed reactor operated atmospherically in H2 carrier gas at temperatures of 50–400 8C. The acid sites strength distribution in these zeolite forms was evaluated using temperature programmed desorption (TPD) of presorbed ammonia in a differential scanning calorimeter (DSC). Either HCl or HF incorporation has increased both acid sites number and strength in the zeolite to varying extents. Hydrochlorination enhanced the acid sites number in the zeolite to a larger extent than hydrofluorination, whereas, the latter enhanced the acid sites strength to a larger extent than hydrochlorination. Nevertheless, not only the acid catalysed reactions; i.e., isomerisation of the six-membered-ring of CHE to the five-membered ring compounds (methylcyclopentenes (MCPEs) plus methylcyclopentane (MCPA)) and hydrocracking reactions to lower molecular weight components, were enhanced via both hydrohalogenation treatments of the zeolite, but also hydrogenation of CHE to cyclohexane and MCPEs to MCPA and dehydrogenation of CHE to cyclohexadienes (CHDEs) plus benzene were also enhanced. # 2005 Elsevier B.V. All rights reserved. Keywords: Cyclohexene; Hydroconversion; H-ZSM-5; Zeolitel; Catalyst
1. Introduction The role of CHE as a probe molecule for investigating catalyst preparation has been frequently evidenced [1–4]. CHE undergoes various conversions depending on catalytic functions and the experimental reaction conditions. In absence of hydrogen, disproportionation [5,6] and dehydrogenation [7,8] reactions take place. Disproportionation gives benzene and cyclohexane (hydrogen transfer) in presence of metal catalysts, since CHE behaves as hydrogen donor and acceptor. However, if the metal is supported on Al2O3 [9], the acid sites are of the weakly Lewis type, where the acid strength is not strong enough to promote carbonium ion formation, and hence isomerization and cracking reactions cannot occur. However, the H-forms of * Corresponding author. E-mail address: [email protected] (A.K. Aboul-Gheit). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.08.044
zeolites possess strong Bro¨nsted acid and Lewis sites that promote CHE isomerization to a mixture of MCPEs and cracking to lower molecular weight compounds whereby the reaction scheme becomes more complicated. Nevertheless, treatment of the cation-exchanged zeolites with aqueous HCl acid leads to decationation and partial dealumination. The results of isomerization of CHE over highly pure aluminas with or without Cl ions has been studied by Ozimek et al. [10,11]. Introducing Cl ions into aluminate aluminas in different ways creates Bro¨nsted acid sites that greatly increases skeletal isomerization and total conversion. The promoting effect of adsorbed Cl ions on the surface acidity of g-Al2O3 has been monitored by zero point charge (zpc) and CHE isomerization activity [12] shows a linear decrease of the zpc values with increasing chloride adsorption. Only few data are available for Cl treated g-alumina. Arena et al. [12] have indicated that Cl adsorbed on the surface of
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g-alumina results in a promotion due to substantial change in the electronic properties of the outermost layer of alumina. Ultimately, these effects result in weakening of the O–H bond that renders the proton more acidic. Doping H-ZSM-5 with a low concentration of F species followed by thermal activation enhances the surface acidity via formation of new Bro¨nsted acid sites and strengthening some acid sites of the parent zeolite [13]. There are no data available in the literature correlating HZSM-5(HCl) or H-ZSM-5(HF) zeolites as catalysts for cyclohexene (CHE) hydroconversion. In the present work, the effect of doping H-ZSM-5 with 3.0 wt.% HCl or 3.0 wt.% HF on hydroconverting CHE in a flow of hydrogen at temperatures ranging between 50 and 400 8C is investigated.
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quantity reaching the catalyst bed. The reaction temperature range investigated was 50–400 8C, with 25 8C decrements starting from 400 8C. After each catalytic run, the carrier gas was turned to air flow to burn off any coke deposited on the catalyst surface, then the hydrogen flow was resumed for 15 min before the next catalytic run (next reaction temperature) was carried out. The tube in which the reaction effluent passing from the reactor was continuously heated to prevent condensation of any produced components. This effluent was injected (twice at each reaction temperature) in a Perkin-Elmer Autosystem XL gas-chromatograph using a 15 m capillary column of Carbowax 20 M bonded in fused silica to be analysed. A flame ionization detector and a Turbochrom Navigator Programme were used. Each run was continued for 25 min at a given reaction temperature.
2. Experimental 2.3. Temperature programmed desorption (TPD) of NH3 2.1. Preparation of the catalysts 2.1.1. H-ZSM-5 catalyst The sodium form of ZSM-5 zeolite (Na-ZSM-5; Si/ Al = 24.5), kindly supplied by Su¨d-Chemie AG, Germany, was exchanged in 0.7 M NH4NO3 solution five times; each time with a fresh solution for 8 h at 70 8C. The zeolite was separated, washed with distilled water till free of the NO3+ ion, dried at 110 8C overnight then calcined in air at 530 8C for 3 h to deammoniate NH4-ZSM-5 to H-ZSM-5. The Si/Al ratio of the produced H-ZSM-5 was still 24.5. The Na+ content was <0.02 wt.%. The surface area and total pore volume were 400 m2 g1 and 0.16 cm3 g1, respectively. 2.1.2. Preparation of HCl and HF treated H-ZSM-5 zeolites A part of the H-ZSM-5 zeolite was doped with an aqueous solution containing 3.0 wt.% of HCl or HF. This solution possessed the retention volume of the zeolite sample used in each treatment. HCl and HF were in the form of the respective acids. These preparations were dried at 110 8C overnight then calcined in air at 530 8C for 3 h. Before carrying out the hydroconversion runs, the catalysts were heated in the catalytic reactor at 500 8C for 2 h in a flow of 20 cm3 min1 of pure hydrogen gas. 2.2. Cyclohexene hydroconversion procedure, reactor and analysis A silica-glass down-flow-type tubular reactor always containing 0.2 g of a catalyst was used in all hydroconversion runs. The reactor was heated in an insulated wider silica-tube jacket, thermostated to 1 8C. Hydrogen gas was used as a carrier and simultaneously as a reactant, at a flow rate of 20 cm3 min1 in all runs. The cyclohexene feed was introduced into the reactor via its continuous evaporation using the carrier hydrogen flow passing into a closed jar thermostated at a fixed temperature of 28 8C, whereby, the quantity of cyclohexene was always 8.33 103 mol h1. The quantity of condensed CHE feed was estimated and considered in calculating the true
The TPD procedure adopted by Aboul-Gheit [14,15] using highly sensitive differential scanning calorimetry (DSC) has been applied to detect and evaluate the desorption of presorbed ammonia from the acid sites of the current catalysts. Ammonia was initially adsorbed on the catalyst in a silica-tube furnace as follows; after evacuation of the catalyst at 1.33 103 Pa whilst heating for 30 min at 500 8C and subsequent cooling under vacuum to 50 8C, ammonia was introduced through the catalyst at a flow-rate of 50 cm3 min1 till saturation (normally 15 min.). The samples were then measured in a DSC Mettler TA-3000 unit using standard aluminium crucibles in a continuous current of highly pure N2 gas at a flow-rate of 30 cm3 min1. The heating rate was 10 K min1 and the full-scale range was 25 mW. The thermograms obtained for each sample gave two peaks; a lowtemperature peak corresponding to enthalpy (DHd) of ammonia desorption from the weak acid sites of the catalyst and a high temperature peak corresponding to DHd of ammonia desorbed from the strong acid sites (Fig. 1). The DHd values are proportional to the peak area and, hence, to the number of acid sites, whereas the peak temperature (Tmax) was used to correlate the acid sites strength of the catalysts; the higher the Tmax value, the stronger the acid sites in the examined catalyst. 3. Results and discussion The H-ZSM-5 zeolite and its HCl and HF promoted versions, possess slightly different values of Si/Al (24.5, 25.0
Fig. 1. TPD of ammonia from HCl and HF doped H-ZSM-5 zeolite.
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Table 1 Ammonia desorption enthalpy (DHdes) and desorption peak temperature for the zeolites Zeolite form
DHdes for ammonia desorption (J g1)
Peak temperature (8C)
H-ZSM-5 H-ZSM-5(HCl) H-ZSM-5(HF)
97.4 108.5 101.9
372.4 375.6 382.7
and 25.8, respectively), indicating that the current acid treatments does not significantly affect bulk dealumination of the H-ZSM-5 zeolite. Fig. 1 and Table 1 show that the acid sites number in the current zeolites is in order (3) H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5
ð3Þ
whereas, the acid sites strength of these zeolite catalysts can be arranged in order (4) H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ > H-ZSM-5
ð4Þ
In other words, both acid treatments of the zeolite have increased its acid sites number and strength, however, HCl increased acid sites number more than HF does, whereas, HF increased the acid sites strength more than HCl does. 3.1. The overall hydroconversion of cyclohexene The hydroconversion process of CHE includes several reactions (Scheme 1) taking place at varying extents depending
principally on their acid sites number and strength using the three current catalysts. However, the overall hydroconversion of CHE on the catalysts can be segmented into three regions according to the activities of the catalysts at three temperature ranges (Fig. 2).A low temperature region (50–200 8C): where the activities of the three catalysts are modestly influenced with temperature. Even low differences are recognised, the overall activities of the catalysts in this region can be arranged in the following order: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5
ð1Þ
A medium temperature region (200–275 8C): where a jump of CHE hydroconversion is accomplished. For instance, at 200 8C, using H-ZSM-5(HCl), H-ZSM-5(HF) and H-ZSM-5 catalysts, the conversion amounts to 26.5, 17.0 and 11.5%, respectively, and increases to as high as 78.5, 83.0 and 73.0%, respectively, at 275 8C. The activities of the catalysts during this temperature region remain as in order (1) for the low temperature region.At temperatures beyond 275 8C up to 400 8C: where the change in activities as a function of temperature is negligible, the catalysts activities in this temperature region can be arranged in the following order (2): H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ
ð2Þ
During the low temperature range, the catalytic activities are principally reflections of the intrinsic chemical reaction; controlled by acidity, whereas, during the higher temperature range, surface and physical factors, such as adsorption and diffusion in the catalytic pores, may mask the chemical reaction
Scheme 1. Proposed scheme for pathways of cyclohexene hydroconversion using the current catalysts.
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Fig. 2. Total hydroconversion of CHE using HZSM-5 catalysts. Fig. 5. MCPA in product using H-ZSM-5 catalysts.
rate. Hence at temperatures between 50 and 200 8C, the effect of physical factors may be excluded or at least minimized, whereas at temperatures between 200 and 400 8C, these factors are significantly effective. Evidently, throughout the low and medium temperature regions, the relatively higher activities of the H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, compared to the untreated one, can be attributed to their higher acid sites number and/or strength (Fig. 1 and Table 1). Comparatively, the larger number of acid sites in the HCl promoted catalyst than in the HF promoted one can be considered the influencing factor for
Fig. 3. Hydrogenation of CHE to CHA using H-ZSM-5 catalysts.
Fig. 4. MCPEs in product using H-ZSM-5 catalysts.
increasing the overall hydroconversion activity of the former catalyst than on the latter during low temperatures. Le Van Mao et al. [13] show that doping H-ZSM-5 with a low concentration of fluoride species enhances the surface acidity via: (1) formation of new Bro¨nsted acid sites, (2) strengthening some acid sites of the parent zeolite. Moreover, Arena et al. [12] have indicated that Cl adsorbed on the surface of Al2O3 results in a significant change in the electronic properties of the outer layer of Al2O3 that decreases the basic Lewis sites, or inducing a stronger ‘‘inductive effect’’ of Cl on the neighboring hydroxyl groups. This electronic effect weakens the O–H bond, rendering the proton more acidic. In this work, the most active catalyst at the low and medium temperatures, H-ZSM-5(HCl), evidently become relatively the least active during the higher temperature region due to pore diffusion restriction by chloro-aluminium debris resulting from slight dealumination of the zeolite by HCl [16]. However, HF doping not only similarly produces flouro-aluminium debris but also fluoro-aluminium debris that causes extra-diffusion retardation. On the contrary, the untreated H-ZSM-5 zeolite has evidently acquired the highest activity during the higher temperature region because of the absence of deposited debris in its channels. Summing up the use of the current catalysts, the first step in CHE hydroconversion (Scheme 1 and Fig. 2), taking place at low temperatures, is CHE hydrogenation to cyclohexane (CHA) (Fig. 3). At higher temperatures, CHE isomerises to the three possible methylcyclopentenes (MCPEs) (Fig. 4), which then partially hydrogenate to MCPA (Fig. 5) and the sum of MCPEs plus MCPA gives the overall isomers production (Fig. 6). Moreover, CHE dehydrogenates via forming 1,3- and 1,4-cyclohexadienes (CHDEs) (Fig. 7 and Fig. 8a–c) which dehydrogenate via a further increase of temperature producing benzene. Total CHE dehydrogenation (Fig. 9) will, hence, include production of CHDEs + benzene + aromatic ring in toluene + xylenes. Total aromatic hydrocarbons production (benzene, toluene and xylenes) are depicted in Fig. 10, whereas the alkylated products include only toluene and xylenes (Fig. 11). Most of the intermediates formed during CHE hydroconversion and CHE itself are susceptible to be hydrocracked to lower molecular weight hydrocarbons (C1–C5) at high temperatures
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Fig. 6. Overall isomerization products using H-ZSM-5 catalysts.
(Fig. 12). The hydrogenation reactions take place on the current unloaded catalysts at relatively higher temperatures, compared to those carried out using active metals loaded H-ZSM-5 catalysts [17,18]. 3.2. Individual hydroconversion reactions The left-hand limb in the volcano shape curves in Figs. 3–8 as well as the (left-hand) low temperature region in Figs. 9–11 show that the activities of the catalysts are in accordance with order (1), i.e., the H-ZSM-5(HCl) catalyst acquires the highest activity by virtue of acquiring the largest number of acid sites with moderate strength (Table 1). Comparatively, the untreated H-ZSM-5 catalyst, which acquires the lowest acid sites number and strength exhibits the lowest activities for the overall hydroconversion reaction. Correspondingly, the right-hand limb of Figs. 3–8 and the high temperature region of Figs. 9–11 show that the activity order of the current catalysts are as follows:
Fig. 8. (a) Individual cyclohexadienes in product using untreated H-ZSM-5 catalyst. (b) Individual cyclohexadienes in product using H-ZSM-5(HCl) catalyst. (c) Individual cyclohexadienes in product using H-ZSM-5(HF).
Arranging the current catalysts in orders (3) and (4) shows that all individual reactions in CHE hydroconversion during the high temperature region are most accelerated on the untreated H-ZSM-5 catalyst (relatively devoid of pore-diffusion restric-
tion), whereas the H-ZSM-5(HF) catalyst encounters the highest pore restriction (some fluoro-silicon debris in addition to fluoro-aluminium debris occur). Hence, the H-ZSM-5(HCl) catalyst can thus be said to acquire lower diffusion restriction than the H-ZSM-5(HF) catalyst (order (3)). Since the hydrogenation and isomerisation steps in CHE hydroconversion follow the same order of catalysts activities, the assumption that Bro¨nsted acid sites act as hydrogenation sites at higher temperatures is emphasized [19–21]. Sano et al. [19] report that they have heterogeneously acid catalysed
Fig. 7. CHDEs in product using H-ZSM-5 catalysts.
Fig. 9. Dehydrogenation products using H-ZSM-5 catalysts.
H-ZSM-5 > H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ ð3Þ Exceptionally, the decending limb in Fig. 7 deviates from order (3) and follows order (4): H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ ð4Þ
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Fig. 10. Total aromatics production using H-ZSM-5 catalysts.
Fig. 11. Alkylation products using H-ZSM-5 catalysts.
Fig. 12. Hydrocracked products in CHE hydroconversion using H-ZSM-5 catalysts.
ethene hydrogenation on protonated ZSM-5 zeolite (H-ZSM5). This zeolite was active as a hydrogenation catalyst such that at a temperature of 535 8C, 95% of ethane has been converted to ethane. Since no metal impurities in the zeolite could be detected, they concluded that the hydrogenation takes place at the acidic sites of the zeolite. Moreover, Jacobs and co-workers [20] have extensively investigated experimentally the nature of the active sites in zeolites by varying the reaction temperature and using different types of zeolites with changing Si/Al framework and Na+/H+ ratios and concluded that Na+ ions are the active sites at low temperatures, whereas Bro¨nsted acid sites become the active hydrogenation sites at high temperatures. Furthermore, Senger and Radom [21] have tried to provide a
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deeper insight into this type of heterogeneous catalytic hydrogenation in which the reaction proceeds without the participation of a transition metal. Our work proves, moreover, that the protons not only act as hydrogenation sites at higher temperatures but also can perform hydrogenation reaction at lower temperatures, as can be observed in Fig. 3. On the contrary, the largest formation of CHDEs is attained using the unhalogenated zeolite (which possesses the lowest acidity parameters). Hence, CHDEs conversion to benzene can be assumed rate controlling in the overall dehydrogenation route leading to benzene. Again, HZSM-5(HCl) is evidently the most active for benzene formation, followed by H-ZSM-5(HF), which indicates that Bro¨nsted acid sites number is the determining factor for CHE dehydrogenation, the same as it is for hydrogenation (Fig. 3). From a practical point of view, the majority of published data are concerned with promotion of the commercial Pt/Al2O3 catalysts, which are the conventional industrial hydroisomerisation catalysts, with Cl. Because g-Al2O3 possesses only weak Lewis acid sites, which cannot promote the isomerization of the low-octane number n-paraffins. A Cl species is injected during operation in these catalysts such that Cl ions adsorb on the alumina surface and the result will be the weakening of the O–H bond, rendering the proton more strongly acidic and new Bro¨nsted acid sites are formed. 3.2.1. Hydrogenation of CHE to cyclohexane (CHA) The present work shows that the unloaded H-ZSM-5 zeolite, whether treated or untreated with either HCl or HF acids are also capable of undergoing double-bond saturation (Fig. 3). Hydrogenation of CHE to CHA is highest on the H-ZSM5(HCl) catalyst. On this catalyst, CHA production amounts to 9.0% at 200 8C, compared to 1.4 and 2.0% on H-ZSM-5 and HZSM-5(HF), respectively. Even though, on the three catalysts, CHA production attains comparable maxima at 300 8C. The Bro¨nsted acid sites in these zeolites, whether originally present, strengthened or created via HCl or HF treatments, should become active hydrogenation sites for alkenes at higher temperatures [19–21]. Although Fig. 3 shows that the order of hydrogenation of CHE to CHA, using the catalysts under study is as follows: HZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM-5, the reverse reaction (dehydrogenation of CHA), evidently follows the order: HZSM-5(HF) > H-ZSM-5(HCl) > H-ZSM-5. These orders conclusively assume that the parent zeolite possesses the lowest hydrogenation and lowest dehydrogenation activities. Nevertheless, as observed from the ascending limbs of the curves, before the maximum point, and the descending limbs, after the maxima, it may be assumed that HCl on H-ZSM-5 promotes hydrogenation of the double bond more effectively than HF, whereas HF promotes dehydrogenation more effectively than HCl. 3.3. Methylcyclopentenes (MCPEs) production Fig. 4 shows that the most prevailing reaction using the current catalysts, is the isomerization of CHE to the possible
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five-membered ring olefins, methylcyclopentenes (MCPEs): 1MCPE, 3-MCPE and 4-MCPE. Bautista et al. [22] have shown that mainly 1- and 3-MCPEs are produced; of which 1-MCPE predominates. At lower reaction temperatures (50–250 8C), the HCl and HF treated catalysts are more active than the untreated H-ZSM-5 catalyst, which can be attributed to the stronger acid sites of the two former versions. These activity data are compatible with the acid sites strength data where H-ZSM5(HCl) is less strongly acidic than H-ZSM-5(HF). The H-ZSM5 zeolite acquires the lowest acid strength. Ozimek et al. [10,11] have shown that Cl introduction in Al2O3 in different ways causes Bronsted acid sites as strong as HR0 < 6.63 to appear and a large increase of both skeletal isomerization and total conversion of CHE. The work in [12] and [13] also support this finding. Fig. 4 shows that the HCl and HF-doped H-ZSM-5 attain comparable maxima of MCPEs at 275 8C of 52.56 and 54.79%, respectively. The untreated H-ZSM-5 gives 53.9% at this temperature. Even-though, the untreated catalyst is more active than the halogenated versions beyond the MCPEs maxima (at higher temperatures) (Fig. 4), which may be due to lower diffusion limitation. It is known that diffusion limitation is specially rate controlling at higher reaction temperatures where the isomerization activities of the current zeolites dominate. Some forms of extra-framework species formed and deposited in the zeolitic channels of the zeolites contribute to diffusion resistance. Moreover, in this high temperature region, the HF doped catalyst may encounter more resistance than the HCl doped version as explained above. Not only MCPEs are the products of CHE isomerization, but also their hydrogenation product methylcyclopentane (MCPA) (Fig. 5). The order of reactivity for producing MCPA over the three catalysts under investigation, before and after attaining the maximum yields, is exactly the same as for producing MCPEs (Fig. 4). The total isomers are actually the sum of unsaturated and saturated methyl-substituted five-membered ring compounds (MCPEs + MCPA) (Fig. 6) [11,23]. Evidently, isomerization of CHE is the major reaction taking place using the unloaded H-ZSM-5 zeolite, whether containing a halogen or not. Using the H-ZSM-5, H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, the maximum isomers of CHE amount to 65.29% (300 8C), 58.05% (275 8C) and 59.37% (275 8C), respectively. 3.4. Methylcyclopentane (MCPA) production MCPA is produced via hydrogenation of MCPEs (Fig. 5). However, Dwyer et al. [23] have considered MCPA as a product of CHE isomerisation to MCPEs followed by hydrogenation, and can thus be indicative of acquiring two catalytic functions (hydrogenation function and acid function). Both functions on the current catalysts are provided by strong Bro¨nsted acid sites. MCPA production using the catalysts under study indicates that the difference in activities of these catalysts for hydrogenating the five-membered ring cycloolefins (MCPEs) is compatible with the order of activities for hydrogenating the six-membered ring cycloolefin, CHE, following also the order; H-ZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM-5. The activity for MCPA production during the whole temperature show
parallelism on the HCl and HF promoted catalysts. However, only using the untreated H-ZSM-5 catalyst, MCPA production is the lowest at the low temperature range then becomes the highest at the higher temperature range. Evidently, the absence of diffusion limitation in the untreated catalyst overcompensates the activity difference. Nevertheless, it is evident that a broader maximum is attained (Fig. 5) via hydrogenating the five-membered ring such that the decline of the concentration of MCPA in product is shifted to a higher temperature of 350 8C, compared to that at which the six-membered cycloolefin, CHE, hydrogenates (Fig. 3), where the maximum is attained at 300 8C. It has been shown that the hydrogenation of the double bond in the five-membered ring is more difficult than that in the sixmembered ring; Aboul-Gheit and Abdou et al. [24–26] have found that the negative activation entropy values obtained for the hydrogenation of the double bonds in the five-membered nitrogen-containing ring compounds is almost double the values obtained for hydrogenating the corresponding sixmembered ring containing compounds. This is attributed to more difficult orientation and adsorption of the five-membered ring on the catalyst surface where the number of degrees of freedom is lower and hence the entropy change values are more highly negative negative. 3.5. Cyclohexadienes (CHDEs) production Peck and Koel [27] have indicated that chemisorbed CHDEs are the intermediates for benzene formation. Fig. 7 shows the sum of the two CHDEs as a function of temperature using the current catalysts. The order of the activities of these catalysts for the formation of CHDEs is as follows: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5 The order of magnitude of CHDEs beyond the maximum is in the reverse order: H-ZSM-5 > H-ZSM-5ðHFÞ > H-ZSM-5ðHClÞ This means that the rates of formation and disappearance of these compounds is highest on the H-ZSM-5(HCl) catalyst and lowest on the H-ZSM-5 catalyst. Evidently, this is compatible with the fact that formation and disappearance of CHDEs are both dehydrogenation reactions. CHE < ¼¼ > CHDEs and CHDEs < ¼¼¼ > benzene Two possible individual CHDEs, namely; 1,3- and 1,4-cyclohexadienes are formed during the hydroconversion of CHE using the catalysts under study (Fig. 8a–c). Somorjai and coworkers [28] assume that at high pressures, none of the CHDEs are formed on the catalyst surface. However, Peck and Koel [27] and Campbell et al. [29] show that in ultra-high vacuum, CHE dehydrogenates readily to benzene above 27 8C. In the present work, it is found that CHDEs start formation with very low yields at 50 8C at atmospheric pressure, using the untreated and HCl treated catalysts, whereas 1,3-CHDE does not form using the HF treated catalyst below 150 8C. As temperature
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increases, the two CHDEs increase up to a maximum at 350 8C using all catalysts, beyond which they decline with a further increase of temperature up to 400 8C. Using the untreated HZSM-5 catalyst (Fig. 8a), 1,3- and 1,4-CHDEs reach maxima of 2.57 and 2.43%, respectively, while using the H-ZSM-5(HCl) catalyst (Fig. 8b), these components reach maxima of 2.85 and 1.62%, respectively. However, using the H-ZSM-5(HF) catalyst (Fig. 8c), these CHDEs reach maxima of 3.30 and 1.98%, respectively. 3.6. CHE dehydrogenation The overall dehydrogenation activity of each of the current catalysts has been calculated as: CHDEs + benzene + benzene ring in toluene and xylenes, and these values have been plotted as a function of reaction temperature for the current catalysts in Fig. 9. At the lower temperature range in this figure, the order of the dehydrogenation activities of the current catalysts is as follows: H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ > H-ZSM-5 This is exactly the same order of the activities of the catalysts for the hydrogenation of CHE to CHA (Fig. 3) and hydrogenation of MCPEs to MCPA (Fig. 5). Hence, we can assume that both dehydrogenation and hydrogenation reactions occur on the same sites, i.e., Bro¨nsted acid sites as indicated before. However, at higher temperatures, the dehydrogenation activities of the catalysts become: H-ZSM-5 > H-ZSM-5ðHClÞ > H-ZSM-5ðHFÞ This order is compatible with the pore diffusion restriction regime. Using the H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, the dehydrogenation activities decline as temperature increases to 375 and 350 8C, respectively. This proves that the HF promoted catalyst encounters a larger diffusion restriction than the HCl promoted one. Moreover, the dehydrogenation activity of the latter is higher than the former. Another evidence, is acquiring a higher activity by the untreated H-ZSM-5 catalyst, that continues rising as a function of temperature up to 400 8C. 3.7. Aromatics formation The production of aromatics (Fig. 10), namely; benzene, toluene and xylenes in the reaction product of CHE hydroconversion is very slow at temperatures from 50 8C up to 250 8C, beyond which these aromatics increase measurably with temperature up to 400 8C reaching 20.4, 14.0 and 13.7% using the H-ZSM-5, H-ZSM-5(HCl) and H-ZSM-5(HF) catalysts, respectively. In aromatics production, the rates are controlled to a great extent by the hydrophilic/hydrophobic state of the catalysts, which affects the adsorption and desorption of the benzenoid structure. The higher the hydrophilicity of the zeolite, the larger is its capability for forming and adsorbing aromatics on its surface. The halogenated zeolites are known to be more hydrophobic than the untreated form [16]. Furthermore, the HF promoted zeolite is more hydrophobic than the HCl promoted one, which is
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compatible with the order of aromatics formation obtained. Another factor contributing to this observed catalytic activity order for aromatics formation, is the role played by the chloroand fluoro-aluminium debris precipitated in the zeolitic channels in case of the HCl and HF promoted catalysts. 3.8. Alkylation of aromatics The alkylation of CHE and CHDEs by methyl fragments produced via hydrocracking reactions during the hydroconversion process, are namely toluene and xylenes. Fig. 11 depicts the formation of these alkylation products using the current catalysts as a function of reaction temperature. Although this reaction is an acid catalysed one, it behaves similar to the dehydrogenation reaction (Fig. 9), which is generally supposed to be a metal catalysed reaction. Since the catalysts under investigation are, of course, unloaded with hydrogenating/ dehydrogenating metals, this also substantiates that the acid sites in all current H-ZSM-5 forms act as hydrogenation/ dehydrogenation sites. 3.9. Hydrocracked products The cracking activities of the current zeolite catalysts, are principally dependent on their acidities. n-Hexane cracking is traditionally used as a test for comparing the acidities of solid acid catalysts [30]. According to the data in Table 1, the acid sites number in the current catalysts is in the order: H-ZSM5(HCl) > H-ZSM-5(HF) > H-ZSM-5, whereas, the strength of these acid sites is in the order: H-ZSM-5(HF) > H-ZSM5(HCl) > H-ZSM-5. The data obtained for the CHE hydrocracking levels using these catalysts (Fig. 12) conform directly to the acid sites strength of catalysts. The hydrocracking products comprise 5.18, 7.95 and 17.5% at a reaction temperature of 300 8C and increase with increasing temperature to 40.7, 48.1 and 50.6%, respectively, at 400 8C using the HZSM-5(HF), H-ZSM-5(HCl) and H-ZSM-5 catalysts, respectively. The peak temperatures at which NH3 desorbs using these catalysts are 380.7, 376.6 and 372.4, respectively (Table 1). Since hydrocracked products are of smaller molecular sizes than the other molecular species present in contact with the catalyst, it appears that these hydrocracked products does not encounter significant diffusion restriction, such that they are most highly produced on the H-ZSM-5(HF) catalyst (acquiring highest restriction) and least produced on the untreated HZSM-5 catalyst (acquiring lowest or no restriction). The hydrocracking reaction rate control is thus attributed to the acid sites strength only. 4. Conclusion Unloaded H-ZSM-5 plays the role of bifunctional catalysts by virtue of its strong Bro¨nsted acid sites. Verification of this assumption has been realized via hydrochlorination and hydrofluorination of H-ZSM-5, which enhanced CHE hydrogenation reactions at relatively higher temperatures compared to those applied using specific bifunctional noble metals loaded
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H-ZSM-6 catalysts [17,18]. At low temperatures, the activities of the current catalysts for almost all hydroconversion reactions are in the order: H-ZSM-5(HCl) > H-ZSM-5(HF) > H-ZSM5, by virtue of the higher acid sites number, whereas, at high temperatures, the activities are in the order: H-ZSM-5 > H-ZSM-5(HCl) > H-ZSM-5(HF), due to diffusion resistance in the zeolitic channels caused by deposited halo-aluminium debris. This debris is more accelerated by HF than by HCl. However, hydrocracking is the only reaction that is not influenced by diffusion, because of the low molecular weight fragments produced. The current catalysts activities for hydrocracking are compatible with their acid sites strength which is in the order: H-ZSM-5(HF) > H-ZSM-5(HCl) > H-ZSM-5. References [1] M. Bartok, A. Molnar, Sterochemistry of Heterogeneous Metal Catalysis, Wiley, Chichester, 1985, p. 53, Chapter 3. [2] E. Jacquinot, A. Mendes, F. Raatz, C. Marcilly, E. Ribeiro, J. Caeiro, Appl. Catal. 60 (1990) 101. [3] I. Palinko, Appl. Catal. A 126 (1995) 39. [4] Z. Xu, B.C. Gates, J. Catal. 154 (1995) 335. [5] D.V. Rebhan, V. Haensel, J. Catal. 111 (1988) 397. [6] C.M. McConica, M. Boudart, J. Catal. 117 (1989) 33. [7] D.W. Blakely, G.A. Somorjai, J. Catal. 42 (1976) 181. [8] S.M. Davis, G.A. Somorjai, J. Catal. 65 (1990) 78. [9] A.K. Aboul-Gheit, S.M. Aboul-Fotouh, Appl. Catal.: Gen. A 208 (2001) 55; A.K. Aboul-Gheit, S.M. Aboul-Fotouh, Proceedings of the Ninth International Symposium on Heterogen. Catal., Bulg. Acad. Sci., Varna, Bulgaria, September 2000, p. 163.
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