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Liquid-phase alkylation of naphthalene by isopropanol over zeolites
Applied Catalysis A: General 241 (2003) 91–111
Liquid-phase alkylation of naphthalene by isopropanol over zeolites Part III. Mordenites M.G. Cutrufello a , I. Ferino a,∗ , R. Monaci a , E. Rombi a , V. Solinas a , P. Magnoux b , M. Guisnet b a
b
Dipartimento di Scienze Chimiche, Università di Cagliari, Complesso Universitario Monserrato, s.s. 554 Bivio Sestu, 09042 Monserrato (Ca), Italy URA-CNRS 350, Université de Poitiers, UFR Sciences, 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France Received 25 March 2002; received in revised form 22 July 2002; accepted 6 August 2002
Abstract The title reaction was investigated over three MOR zeolites with Si/Al ratios of 10, 60 and 80. Surface acidity was evaluated by adsorption microcalorimetry and FTIR analysis, using pyridine as probe molecule. Catalytic tests were carried out at 623 K and 40 bar in liquid-phase, with decalin as solvent, in a flow reactor. The formation of isopropylnaphthalenes (mainly monoisopropylnaphthalenes) was accompanied by fast oligomerisation/cracking of propene resulting from isopropanol dehydration and trapping of carbonaceous compounds (“coke”) in the zeolite pores. This caused a quasi-immediate blockage of the access to the micropores, suggesting that reactions occur on or near the outer surface of the crystallites. In agreement with this proposal, the activities of the three MOR samples, which have very different acidic properties but similar textural properties, were identical. A reaction scheme involving as a main step transalkylation between naphthalene molecules and the isopropyl aromatic molecules trapped in the MOR channels is proposed to explain the formation of isopropylnaphthalenes over the aged MOR samples. In agreement, isopropylnaphthalenes formation was found to remain significant for more than 1 h after substituting a decalin–naphthalene mixture for the normal feed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Naphthalene; Isopropylation; Transalkylation; Mordenites; Acidity of mordenites
1. Introduction The use of zeolites as non-conventional Friedel– Crafts catalysts is very interesting from both the environmental and economical points of view: no pollution problems are originated by these solid acids, whose pore structure is able to induce unique ∗ Corresponding author. Tel.: +39-70-675-4383; fax: +39-70-675-4388. E-mail address: [email protected] (I. Ferino).
selectivity effects. Among the Friedel–Crafts reactions, the selective alkylation of naphthalene to the  and  mono- and dialkylated products is of practical relevance. -Naphthol, key intermediate for dyes, pharmaceuticals and perfumes, can be obtained from 2-alkylnaphthalenes; 2,6-dialkylnaphthalenes are starting product for the manufacture of high performance engineering plastic materials (polyethylenenaphthalate) and liquid crystal polymers [1]. Both patent [2–8] and open literature [9–31] report about the use of zeolites for naphthalene alkylation.
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 6 1 - 1
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Investigations mostly focused on mordenites; propene [10,14,16,17,21,26], isopropanol [10,13,18,28], isopropylbromide [11] have been used as alkylating agents. In order to enhance shape selectivity effects, bulkier alkylating agents have also received attention [20,22,29,30]. It should be noted, however, that the bulkier the alkylating agent, the higher is the waste of carbon atoms when the alkyl groups are oxidised to carboxyl groups, a necessary step in the manufacture of polyethylenenaphthalate. Though semi-commercial (1000 t per year) plants have already been installed [31], a major drawback of zeolite catalysts is still their deactivation due to the formation of carbonaceous material. Despite their practical relevance (and fundamental interest as well), coking phenomena during naphthalene isopropylation have not always received the necessary attention. Papers dealing with this subject are rare [16]. Even when severe coking should be expected on the basis of the experimental conditions and remarkable catalyst deactivation is observed, the possible occurrence of coking is quoted only in generic terms [15,18] or completely neglected [26]. It seems to exist a need for further investigation of the coking phenomena, in order to elucidate their role in the complex interplay of the several processes which can occur during naphthalene isopropylation over zeolites. Polyalkylation of naphthalene, transalkylation between alkylated products and naphthalene, isomerisation between the isopropylated products, oligomerisation and subsequent cracking of the alkylating agent, as well as reactions involving the solvent, should be considered. Further complications arise from differences in the diffusion rate of the various products within the zeolite pores, which in turn depends on several factors: size of the products molecules, existence of mesopores, acid-sites density and strength, localisation and consistence of coke deposits.
In this light, a programme was started in the present authors’ laboratories, aimed to compare the behaviour of zeolites of different structural types, including those usually considered as no shape selective. Previous papers were devoted to the study of Y [32] and beta zeolites [33]. In the present work, the behaviour of mordenites in the liquid-phase isopropylation of naphthalene has been investigated. Samples with different Si/Al ratio were tested. Their acidity was assessed by adsorption microcalorimetry and FTIR technique using pyridine as probe molecule. The reaction was carried out at 623 K and 40 bar in a fixed-bed flow reactor; isopropanol and decalin were used as alkylating agent and solvent, respectively. Both the amount and the composition of coke were monitored during the reaction.
2. Experimental The mordenites were commercial samples from ZEOCAT. Their main characteristics are reported in Table 1. A Tian–Calvet heat flow calorimeter (Setaram) equipped with a volumetric vacuum line was used for microcalorimetric measurements. Each sample was pretreated overnigth at 673 K under vacuum (10−8 bar). Adsorption was carried out at 423 K, by admitting successive doses of pyridine and recording the thermal effect; the run was stopped at a final equilibrium pressure of 1.33 × 10−3 bar. After overnight outgassing of the sample at 423 K, a second adsorption was carried out, obtaining an adsorption isotherm parallel to the first one. From the difference between the adsorption isotherms the amount of pyridine irreversibly adsorbed was evaluated. FTIR measurements were carried out at 423 K by using the same probe
Table 1 Characteristics of the mordenite catalysts for naphthalene isopropylation Catalyst
MOR10 MOR60 MOR80 a b
Si/Al framework
10 59 79
Pore volumea (cm3 /g)
Concentration of acid-sitesb (mol/g)
Total
Micro
Meso
Brønsted
Lewis
0.263 0.272 0.281
0.186 0.178 0.186
0.077 0.094 0.095
372 226 96
63 15 32
From N2 adsorption–desorption measurements at 77 K; microporous volume determined by t-plot method. Determined by FTIR after pyridine desorption at 423 K.
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molecule (pyridine). The apparatus and procedure are described in detail elsewhere [34]. For catalytic testing, a small amount of catalyst (ca. 0.5 g) was placed into the tubular stainless-steel reactor and activated at 773 K for 2 h in N2 flow and then at 623 K overnight before rising the pressure up to 40 bar. N2 flow was then stopped and the liquid reaction mixture, composed of naphthalene, isopropanol and decalin as solvent (1:2:6 mol) was fed into the reactor; contact time was 5 × 10−2 gcat h/cm3feed . Each run lasted 6 h, with sampling of the reactor effluent every 20 min. GC analysis was carried out using N2 as carrier gas, an HP PONA capillary column (50 m × 0.21 mm × 0.57 mm) and an FID detector. Oven temperature programming was: 393 K for 1 min, heating rate 3 K/min up to 533 K, maintained for 10 min. Products identification was confirmed by GC/MS. Runs where both the flow rate and the catalyst amount were significantly changed while keeping the same W/F value gave practically the same results; accordingly, the occurrence of external diffusion limitations has been ruled out. The extent of coke deposition on the catalyst was determined by using a fresh portion of the same sample in runs lasting 1, 3, 4 and 6 h, respectively. The coked zeolite samples were treated at room temperature with a solution of HF (40%) in order to dissolve the zeolitic matrix and recover the coke; extraction by
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CH2 Cl2 then followed. The procedure is reported in detail elsewhere [35].
3. Results 3.1. Acidity measurements Calorimetric results are summarised in Figs. 1–3. The differential heat of adsorption, Qdiff. , depends on the coverage as shown in Fig. 1. The initial differential heat of adsorption is 215–220 kJ/mol. For MOR10, after a sudden drop caused by the first pyridine doses, Qdiff. changes slightly and stepwise over a relatively wide range of the pyridine uptake, n, and then steeply decreases. A similar trend is observed for the other two samples, whose stepped region is however considerably shorter (particularly for MOR80) in comparison with MOR10. As could be expected from the Si/Al ratio, the total pyridine uptake is in the order MOR10 > MOR60 > MOR80. At high coverage the adsorption is reversible and the released heat falls below 90 kJ/mol. Fig. 2 represents the site–energy distribution plots, −dn/dQdiff. versus Qdiff. , as obtained through graphical derivation of the curves in Fig. 1. Fig. 2 allows ranking of the sites according to their strength. For MOR10, the strongest sites are those with Qdiff. >
Fig. 1. Differential heat of pyridine adsorption, Qdiff. vs. uptake for mordenites. Open symbols refer to re-adsorption after evacuation.
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Fig. 2. Site–energy distribution plots for mordenites, as obtained through graphical derivation of Fig. 1 curves.
130 kJ/mol (region I), whose amount, n>130 , is 138 mol/g. In this region, a single narrow peak is visible, which indicates homogeneity of the sites. The narrow peak observed for MOR10 between 110 and 130 kJ/mol individuates another set of homogeneous sites (region II, n110–130 = 242 mol/g); between 90 and 110 kJ/mol (region III, n90–110 = 134 mol/g), no peaks are observed, i.e. the sites are continuously heterogeneous. A set of homogeneous, strong sites is also present on MOR60, where the amount of sites in region I is still high (n>130 = 127 mol/g); the concentrations of the sites in region II (n110–130 = 65 mol/g) and region III (n90–110 = 51 mol/g) are considerably lower in comparison with MOR10. On the most dealuminated sample (MOR80), a lower concentration of sites is observed also in region I (n>130 = 77 mol/g); most of these sites are grouped in a homogeneous set. The site population of regions II and III is very low (n110–130 = n90–110 = 16 mol/g). Below 90 kJ/mol (region IV) the adsorption is reversible for all the samples (cf. Fig. 1). Thermokinetic data are shown in Fig. 3, where t0 (the time over which the heat is evolved after each successive dose of pyridine) is plotted versus coverage. A maximum is observed for all the samples, which
indicates that surface diffusion is the mechanism by which the sites are selectively titrated during the sequential exposure of the sample to pyridine. When the run is started, the concentration of the sites is the highest, the strong sites are titrated first and t0 is relatively low. As coverage increases, the fraction of strong sites decreases and the adsorption of pyridine is initiated by its interaction with the largely available weak sites, from where it migrates within the pore system of the solid to reach the remaining strong sites. The time needed for the equilibration after each pyridine dose increases with the decrease in the available strong sites. Once the latter are completely filled, t0 decreases, because of the fast exchange of pyridine between weak sites. The maxima in Fig. 3 are attained in correspondence of the complete filling of the sites with Qdiff. > 110 kJ/mol (regions II and I in Fig. 2). The establishing of reversible adsorption is accompanied by a decrease of t0 , which attains very low values, i.e. very fast exchange between the sites occurs. Interaction of pyridine with weak Brønsted or Lewis sites, non-specific hydrogen bonding and physical adsorption could be simultaneously involved in the reversible adsorption, each contribution being difficult to assess. By FTIR it is possible to detect the
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interaction of pyridine with Brønsted and Lewis sites, whose concentrations are reported in Table 1. Fig. 4 shows that for each sample the total concentration of acid-sites (Brønsted + Lewis), nFTIR , compares well with the amount of pyridine irreversibly adsorbed during the calorimetric experiment, nA . Accordingly, reversible adsorption (corresponding to region IV in Fig. 2) seems mostly due to non-specific interaction of pyridine with the silica part of the zeolite and does not represent true acidity of the zeolite. 3.2. Catalytic runs
Fig. 3. Thermokinetic parameter, t0 , vs. coverage for mordenites.
Naphthalene alkylation by isopropanol gave monoisopropylnaphthalenes (IPNs) and diisopropylnaphthalenes (DIPNs); only traces of triisopropylnaphthalenes (TIPNs) were detected. Side reactions also took place. These involved isopropanol, which underwent dehydration, subsequent oligomerisation of propene and cracking of the oligomers, as well as the solvent decalin, which underwent cracking. Deposition of carbonaceous material (coke) also occurred on the catalysts. The conversions of isopropanol, XI , and decalin, XD , through the above parasite reactions were assessed according to [32]. The naphthalene conversion, XN , as well as XI and XD , are reported as a function of time-on-stream in Fig. 5. While decalin conversion is negligible, XI is remarkably high for all the catalysts and shows a decreasing trend as time-on-stream increases. With increasing Si/Al, the parasite reactions involving isopropanol appear to attain a higher extent. Naphthalene conversion is low and in a narrow range (XN = 25–28%) for the three catalysts and decreases during the run: after 6 h, XN is 78% of its initial value for MOR10, 58% for MOR60 and 50% for MOR80. Selectivities to mono- and diisopropylnaphtalenes (SIPN , SDIPN ) and the distribution of the isomers in the mono and dialkylated mixture were also determined as a function of time-on-stream for all the catalysts. In no case, SIPN and SDIPN were found to change with the reaction time (hence with XN ), the former being quite high (91%) in comparison with the latter. The evolution of the isomers distribution with time-on-stream is shown in Figs. 6–8. For all the catalysts and at any time-on-stream, 2-IPN is by far predominant in the monoisopropylated mixture (Figs. 6A–8A). However, the amount of 1-IPN
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Fig. 4. Acid-sites concentrations determined by microcalorimetry, nA , compared with acid-sites concentrations (Brønsted + Lewis) determined by FTIR, nFTIR .
increases with time-on-stream at the expense of 2-IPN. Among the dialkylated products, 2,6- and 2,7-DIPN are far more abundant in the case of MOR10 catalyst, for which small amounts of 1,3-, 1,6- and 1,7-DIPN are also detected (Fig. 6B–D). Only the 2,6- and 2,7-isomers are formed over MOR60 and MOR80 (Figs. 7B and 8B). For all the catalysts, the 2,6-/2,7-DIPN ratio remarkably increases with time-on-stream, passing from initial values close to 1 to values of 2.5–3.5 after 6 h on-stream, e.g. from 14% initially to 24% after 6 h for MOR10. The evolution of coke deposition with the reaction time is presented in Fig. 9. It shows that the catalysts are almost saturated by carbonaceous material since the first hour on-stream. HF attack followed by extraction with CH2 Cl2 was carried out on the catalysts after 1 and 6 h on-stream. After separation of the very heavy polyaromatic material (insoluble coke), the compounds present in the soluble fraction (soluble coke) were identified. Decalin,
naphthalene, mono- and diisopropylnaphthalenes as well as other alkylnaphthalenes (branched C4 and C5 side chains) were found in the soluble material after 1 h on-stream. Traces of alkylated (C3 and C4 ) phenanthrenes and alkylated (C4 and C5 ) chrysenes (or other compounds with 4 fused rings) were also detected. After 6 h on-stream these same species were found in the soluble coke. In the case of MOR60, traces of alkylated (C3 –C5 ) polyaromatics with five fused rings were also revealed in addition to alkylated phenanthrenes and chrysenes; traces of bulkier aromatics (six fused rings, C4 –C9 -alkylated) were detected in the soluble material from MOR80. Analytical data are summarised in Table 2, where the coke constituents have been grouped as follows: (i) naphthalene and decaline (reactant and solvent); (ii) isopropylated and other alkylated naphthalenes (products originated by reactions involving naphthalene); (iii) very heavy polyaromatic material (insoluble coke). It can be seen that the composition of the
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Fig. 5. Conversion of naphthalene, XN , isopropanol, XI , and decalin, XD , vs. time-on-stream for mordenites.
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Fig. 6. Isomers distribution with time-on-stream for MOR10.
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Fig. 7. Isomers distribution with time-on-stream for MOR60.
Table 2 Coke analysis dataa for mordenites Naphthalene + decalin (mg/gcat )
IPNs + DIPNs + other alkylnaphthalenes (mg/gcat )
Insoluble coke (mg/gcat )
5 –
55 –
20 65
1 6
36 –
24 –
27 64
1 6
38 –
19 –
31 68
Catalyst
Time-onstream (h)
A4251
1 6
A1583 A4841
a Due to the scarce amount of soluble coke at 6 h on-stream, the corresponding data were considered reliable only from a qualitative point of view (see text) and have been omitted in the present table.
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Fig. 8. Isomers distribution with time-on-stream for MOR80.
material retained after 1 h on-stream is different for the three catalysts, the major differences being observed between the slightly dealuminated sample on the one side and the two strongly dealuminated catalysts on the other. For each sample, the coke composition undergoes modification during the catalytic run: after 6 h on-stream, the insoluble coke is the most abundant component of the retained material, its amount being similar for the three samples. Nitrogen adsorption measurements carried out on MOR60 and MOR80 showed that coke deposition significantly affects the pore volume, especially the micropore volume. Since the first hour on-stream (≈8 wt.% coke), the access of nitrogen to the micropores, hence, to the acidic sites, is practically blocked (Fig. 10).
4. Discussion 4.1. Influence of catalyst acidity and porosity on naphthalene and isopropanol conversion According to the literature on the dehydration of alcohols [36], carbenium ions easily form on acidic solids by interaction of the alcohol molecules with either Brønsted or Lewis sites. In the present case, the isopropyl carbenium ion formed through this way can: (a) release a proton to give propene; (b) undergo attack by another isopropanol molecule from the liquid-phase to form a dimer ion; this process continues until the oligomer is large enough to undergo cracking; (c) undergo attack by a naphthalene
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Fig. 9. Amount of coke formed on mordenites after different time-on-stream values.
molecule from the liquid-phase, with formation of an adsorbed isopropylnaphthalenium ion; the latter can either evolve into monoisopropylnaphthalene or undergo further alkylation. The initial (50 min on-stream) extent of isopropanol transformation (XI ) through the (a) and (b) pathways and of naphthalene transformation (XN ) through the (c) pathway are plotted versus the concentration of the acid-sites, nA , of the present mordenite catalysts in Fig. 11. Formerly tested Y [32] and beta zeolites [33] are also included in Fig. 11 for comparison. Whatever the MOR samples, XI is approximately three times greater than XN , although the reactor feed contains isopropanol in excess compared to naphthalene (I/N = 2 mol/mol). XN is practically independent of the concentration of acid-sites (nA ), whereas XI slightly increases when nA decreases (Fig. 11). This shows clearly that the apparent activity of MOR samples is not determined by their acidity. For the beta zeolites neither XN nor XI significantly change with the acid-sites concentration. In spite of the wide difference in nA , the as-synthesised (Si/Al = 3.9) and the mildly dealuminated (Si/Al = 16) HY samples do not differ significantly as to both XN and XI ; however, naphthalene conversion is remarkably high (and XI correspondingly low) over the strongly dealuminated (Si/Al = 30) HY sample, for which the acid-sites concentration is the lowest. Whatever the concentration
of the acid-sites, XI is lower for HY than for both mordenite and beta zeolites. The trends in Fig. 11 reveal that the apparent activity of the catalysts cannot be traced back to acidity effects only. A complex interplay of several effects is probably underlying, among which coking should play a key role. Coking can be regarded as a complex sequence of “pathological” reactions involving carbenium ions and is governed by both structural and acidity effects [37,38]. In principle, a high density of the acid-sites would favour those bimolecular events which ultimately lead to coke. The presence of strong sites would enhance coke formation, in that it causes the ions to reside longer on the surface (instead of desorbing after proton release), and thus, offers a chance to consecutive reactions to occur. This possibility can be further enhanced by the confinement of large, “soluble coke” molecules in the vicinity of the active sites as a consequence of the slow diffusion of cumbersome reaction products, a phenomenon which could be favoured in the case of zeolites with a narrow pore system. It should be noted, however, that the composition of the retained material as well as its further evolution into insoluble, heavy polyaromatic coke will depend on the presence of suitable voids (cavities, channel intersections) large enough to allow for the growth processes. Furthermore, the coking rate and the micropore blockage (and hence the deactivating effect of coke)
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Fig. 10. Pore volume evolution as a function of coke amount.
are known to depend also on the secondary porosity of the zeolite [37,38]. The enhanced diffusion of reactant and product molecules induced by the presence of mesopores could have a positive role on the apparent activity and even result in some kind of compensation between the acidity and porosity effects. Though rather complex, such a picture allows one to suggest an interpretation of the XI and XN trends. For HY zeolites [32], 14% of carbonaceous material was found on the as-synthesised sample, i.e. the most acidic one, after 1 h reaction time. 20% of this material was insoluble (i.e. heavy polyaromatic in nature), the soluble fraction being composed by polyalkylated naphthalenes and very polyaromatic compounds, including polyisopropylpyrenic and in-
denopyrenic compounds. Trapping of these cumbersome products in the zeolite supercage strongly limits the diffusion of the isopropylated naphthalenes out of the pore system. In comparison, the products originated by the side reactions of isopropanol (dehydration–oligomerisation–cracking) can rapidly diffuse out, due to their smaller size. Isopropanol transformation through (a) and (b) pathways is hence favoured over naphthalene conversion. Severe dealumination of HY entrained two effects [32]: (a) the concentration of the acid-sites strongly decreased and (b) a mesoporous system (accounting for more than one-half of the total pore volume) was originated. Both factors determine rapid circulation of the reaction products within the pore system of the zeolite,
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Fig. 11. Initial (50-min on-stream) conversion of naphthalene XN , and isopropanol XI , vs. acid-sites concentration, nA , for mordenites (data from present work), Y (data from [32]) and beta zeolites (data from [33]).
thus, disfavouring coke deposition. Accordingly, no coke was found on the strongly dealuminated HY sample [32]. Under such conditions, no longer the side reactions of the alkylating agent are preferred (low XI value in comparison with XN , Fig. 11). The amount of carbonaceous material retained on beta catalysts after 1 h on-stream was 13–15%, most of which was soluble [33]. It was found that, at variance with HY, the development of mesopores upon increasingly severe dealumination was too modest for enhancing the diffusion rate of the cumbersome alkylated products. It seems that in the cageless microporous structure of the beta samples the spatial
constraints are such to determine significant retention of carbonaceous material (due to the slow diffusion of the alkylated products), regardless of the acid-sites concentration. This in turn greatly favours the parasite isopropanol transformation, which leads to small reaction products able to escape rapidly from the porous system, over the naphthalene conversion (Fig. 11). Restrictions to the diffusion of the cumbersome alkylated products should be particularly compelling in the non-intersecting channel system of mordenites, where at variance with the case of HY, dealumination does not induce significant differences in the mesopore volume among the three MOR samples
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(cf. Table 1). In this light, it is not surprising to observe that all the mordenite samples, regardless of their acid-sites concentration, are saturated by carbonaceous material since the first hour on-stream (Fig. 9), which in turn explains the very high XI values over the whole range of nA in Fig. 11. It is worthy of note (Fig. 11) that for mordenites XI has a tendency to increase with Si/Al. A tentative interpretation is suggested in the following. Besides the decrease in the acid-sites concentration, the dealumination process entrains a modification of the acid strength distribution of mordenites. From calorimetric data, the sites with Qdiff. > 130 kJ/mol (corresponding to region I in Fig. 2) account for 27, 52 and 71% of the total acid-sites, respectively for MOR10, MOR60 and MOR80. The presence of strong sites should be relevant to the composition of the carbonaceous ma-
terial. The ratio between the amount of insoluble coke and group (ii) products (isopropylated and other alkylated naphthalenes), calculated from coke analysis data at 1 h on-stream (Table 2), is plotted in Fig. 12 versus the fraction of sites with Qdiff. > 130 kJ/mol for the three mordenite samples. As expected, this ratio increases with the proportion in sites stronger than 130 kJ/mol, i.e. the building-up of very heavy polyaromatic material at the expense of the alkylated products is enhanced on the stronger sites, where the intermediate ions reside longer. Furthermore, Table 2 data for 1 h on-stream show that, besides the expected decrease of the alkylated products, an increase in naphthalene + decalin occurs as the insoluble coke amount grows. This suggests that increasing amounts of reactant and solvent remain trapped inside the porous system because of the increasingly severe spatial constraints
Fig. 12. Ratio between the amount of insoluble coke and the amount of (IPNs + DIPNs + other alkylnaphthalenes, ANs) in the soluble coke after 1 h on-stream vs. fraction of sites with Qdiff. > 130 kJ/mol for the three mordenite samples.
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induced by the deposition of insoluble coke. Such an effect is probably amplified by the location of insoluble coke in proximity of the pore mouth, owing to the lack of suitable space inside the non-intersecting channel system of mordenites. Along the series MOR10, MOR60 and MOR80 a progressive increase of diffusional resistances, originated by such a location of the insoluble coke, would be expected. Accordingly, formation of small-sized reaction products would be progressively favoured, which could explain the increasing trend of XI with Si/Al in Fig. 11. 4.2. Influence of coke formation on naphthalene conversion All the mordenite samples are rapidly saturated by carbonaceous material: after the first hour on-stream
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catalysed by the acid-sites. As it was proposed in the case of the transformation of naphthalene-isopropanol mixtures on FAU and BEA samples, alkylation could occur on the molecules trapped in the channels. This proposal is supported by the results reported in Fig. 13, which refer to a run where the feeding of isopropanol was interrupted after 3 h on-stream while naphthalene (in decalin) was kept flowing. The appreciable consumption of naphthalene occurring for more than 1 h after interrupting the feed of alkylating agent indicates that transisopropylation reactions actually occur between naphthalene in liquid-phase and isopropylderivatives trapped in the zeolite pores: IPNs, DIPNs and probably also insoluble coke molecules. It can therefore be proposed that after the rapid filling of the MOR pores by alkylated molecules, the formation of isopropylnaphthalene occurs through the following schematic reactions. (a) Transalkylation:
(b) Rapid re-isopropylation of coke molecules by isopropanol:
there is only a slight increase in the coke amount (Fig. 9). Moreover, the access of nitrogen to micropores is practically precluded (Fig. 10). It should be remarked that the blockage of the access to the micropores is not due to their complete filling by coke molecules. Indeed the micropore volume is more than three times greater than the volume estimated for the coke molecules retained in MOR samples after 1 h on-stream. The relatively high conversion of naphthalene which is found on MOR samples, whose access to micropores (hence to acidic sites) is blocked by carbonaceous material since the first hour on-stream, suggests that the formation of alkylated products is not directly
As it is generally the case with monodimensional zeolites [39], the carbonaceous material is formed and trapped near the entrance of the channels (at the pore mouth). Indeed the diffusion of molecules (especially of bulky molecules such as naphthalene and naphthalenic derivatives) in non-interconnected channels of monodimensional zeolites is generally slow in comparison with their transformation into more and more cumbersome molecules, which remain trapped in the channels. The evidence for micropore blockage obtained by the adsorption experiments strongly supports the view that preferential trapping of coke molecules occurs at the channel entrance. Assuming the occurrence of transalkylation at the pore mouth it is possible to explain why XN is
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Fig. 13. Naphthalene conversion, XN , vs. time-on-stream for MOR10. The open symbols refer to a situation where isopropanol is no more present in the feed.
independent of the MOR acidity. XN should be dependent on the number of pore mouths on the external surface and on the mesopore walls. As the crystallite sizes and the pore distribution are practically the same for the three MOR samples, similar values of XN should be obtained, which is actually the case (Fig. 11). 4.3. Influence of coke formation on isopropylation selectivity It is well known [40–42] that the ␣-positions at carbons 1 and 5 of naphthalene are more reactive than the -positions at carbons 2, 6 and 7. Two possible reasons can be invoked for explaining the observed predominance of 2-IPN over 1-IPN (Figs. 6A–8A): (i) isomerisation of the kinetically favoured 1-IPN into the thermodynamically stable 2-IPN, and/or (ii) shape selectivity effects leading to 2-IPN because of restrictions on the transition state and/or the products diffusion. In the first case, it should be supposed that at the pore mouth isomerisation is much faster than transalkylation between naphthalene and carbonaceous material. In the second case, the orientation of the naphthalene molecule at the entrance of the pore would be such that the formation of 2-IPN through transalkylation would be largely favoured. Scheme 1 shows how the naphthalene molecules can be
oriented if they have to enter the channels for reacting with carbonaceous compounds. In mode (a), leading to 1-IPN, the orientation of naphthalene is more hindered (7.2 Å) than in mode (b) (5.8 Å). Accordingly, preferential formation of 2-IPN should be expected, which is actually observed. The same proposal can be advanced for explaining why the initial value of the 2,6-/2,7-DIPN ratio is close to 1 on the three MOR samples. The main steps of the reaction pathways involving trapped coke molecules and already formed 2-IPN molecules can be summarised as in Schemes 2 and 3. Though qualitatively, they show that when position 6 of 2-IPN is
Scheme 1. Formation of 1-IPN (a) and 2-IPN (b) through transalkylation between naphthalene and isopropyl aromatics blocked in the mordenite channels.
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Scheme 2. Formation of 2,6-DIPN through transalkylation between 2-IPN and isopropyl aromatics blocked in the mordenite channels.
involved, the reaction intermediate (I) (Scheme 2) has a rather bent conformation in comparison with that of the reaction intermediate (III) (Scheme 3) originated by interaction of the trapped coke with 2-IPN at position 7. Due to its somewhat linear conformation, “nesting” of intermediate (III) at the pore mouth could be favoured, which would explain the otherwise unexpected significant formation of 2,7-DIPN.
It is worthy of note that intermediates (I) and (III) are also involved in coke-growth processes, through formation of species such as (II) and (IV). The two sets of aromatic rings in (I) and (III) are tilted of 19.8 and 7.5◦ , respectively. Due to the more co-planar character of (III) in comparison with (I), evolution of the former into (IV) should be more probable than evolution of (I) into (II). Insoluble coke formation can ultimately lead
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Scheme 3. Formation of 2,7-DIPN through transalkylation between 2-IPN and isopropyl aromatics blocked in the mordenite channels.
to coke protruding outside the pores, a well-known phenomenon occurring on mordenites [39]. Once this situation has been attained, spatial constraints would no more favour the formation of intermediate (I) and transalkylation would be oriented by the higher reactivity at position 6 in 2-IPN. Accordingly, 2,6-DIPN would become predominant over 2,7-DIPN, which is in agreement with the growing trend of 2,6-/2,7-DIPN with time-on-stream experimentally observed (Figs. 6D, 7B and 8B). Very recently, the higher reactivity at position 6 in 2-IPN has been questioned on the basis of computa-
tional results [42]. According to these authors, molecular electrostatics at ab initio level does not distinguish between carbons 6 and 7 in the 1- and 2-IPN molecules. It has also been proposed that, at variance with the case of dimethylnaphthalenes [43], the DIPN isomers do not form equilibrium groups. 2,6- and 2,7-DIPN are predicted to be the most stable among the DIPN isomers and may transform to each other via 1,2-isopropyl shift inside the main channel of mordenite. Under free isomerisation conditions they are expected to have the largest concentration (their ratio being ca. 1) in the equilibrium mixture. According to
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the calculated dimensions, 2,6-DIPN should move the easiest, i.e. diffuse the fastest in the main channel of mordenite [42,44]. Whether or not the high 2,6-/2,7-DIPN ratios reported in the literature for mordenites [10,13,14] have to be ascribed to diffusion-controlled shape selective catalysis (as claimed in [42] and formerly suggested in [14]) cannot be judged on the basis of the present results. Experimental evidence however shows that under the present reaction conditions coking, pore blockage and consequent transalkylation occur during the runs. Interconversion between 2,6- and 2,7-DIPN and achievement of equilibrium does not seem realistic, as there would be no reason for the observed increase of 2,6-/2,7-DIPN ratio from ca. 1 (at 50 min on-stream) to 2.5–3.3 during the run (cf. Figs. 6C and D, 7B and 8B). In this respect, it is worthy of note that values for the 2,6-/2,7-isomers ratio as low as 1 have been reported by some authors [11,20]. It might be possible that such a scattering in the literature values for the 2,6-/2,7-DIPN ratio has been originated by a different extent of the coking reaction in the runs carried out in the various laboratories, a phenomenon probably disguised because of the use of autoclave reactors. At variance with the above-cited papers, the present work has been carried out in a fixed-bed flow reactor at constant contact time, so that any influence of coking on the isomers distribution could be revealed by the on-stream behaviour. To check further the role of coke formation on the DIPNs distribution, an additional run was carried out over MOR80, in which 2,6-DIPN and 2,7-DIPN were monitored in the liquid reactor effluent since the very early reaction times. It was found that 2,6-DIPN predominates over 2,7-DIPN at 10 min on-stream (2,6-/2,7-DIPN = 3.3). This was followed by a decrease in the former isomer and a simultaneous increase in the latter, until, after 50 min on-stream, the 2,6-/2,7-DIPN ratio attained a value of ca. 1. From this moment onwards, 2,6-DIPN was found to grow continuously, while 2,7-DIPN decreased, with a trend nearly identical to that already observed in Fig. 8B. Though further experimental work involving all the catalysts is needed to confirm this point, it seems that when the reaction is not yet perturbed by coking (i.e. at its very beginning), predominance of the 2,6-isomer is observed. Whatever the reason for this predominance, i.e. diffusion-controlled shape selective
109
catalysis (as claimed in [42]), or frontier electron density on carbon 6 higher than on carbon 7 in 2-IPN (restricted electronic transition state selectivity [41,45]), it soon becomes perturbed by coke deposition. The complex evolution in the DIPNs distribution during the run can be reasonably interpreted as summarised in Schemes 2 and 3. Another point is worthy of note: only 2,6- and 2,7-DIPN are formed on MOR60 and MOR80 (Figs. 7B and 8B), while small amounts of 1,3-, 1,6and 1,7-DIPN are also detected in the case of MOR10 (Fig. 6B–D). This difference cannot be interpreted in detail on the basis of the present data only. It could be thought however that for MOR60 and MOR80 the only way towards DIPNs formation is pore-mouth transalkylation (as outlined in the preceding). Though this way is by far predominant also in the case of MOR10, it might be speculated that isopropylation processes leading to ␣ isomers can occur to some (very limited) extent on the external surface of the crystallites of this catalyst. According to some authors [16], the dealumination process preferentially removes the acid-sites at the external surface of mordenites. The (limited) presence of acid-sites on the external surface of MOR10 and their absence on the external surface of MOR60 and MOR80 would explain why the ␣ isomers are detected on the former but not on the strongly dealuminated samples. Probably the DIPNs formation at the external surface of MOR10 is not due to transalkylation with coke molecules, but involves acid-sites not yet covered by coke molecules. This is suggested by the acidity data (cf. Section 3.1). They show that after the dealumination process leading from MOR10 to MOR60 the concentration of the stronger sites (Qdiff. > 130 kJ/mol) remains nearly the same (138 and 127 mol/g, respectively), while the concentration of the intermediate-strength sites (110 kJ/mol < Qdiff. < 130 kJ/mol) dramatically drops from 242 to 65 mol/g. According to the view that surface sites are preferentially eliminated during this step, it should be concluded that the sites originally present on the surface of MOR10 do not belong to the stronger family. This would mean in turn that coking on the surface is not as fast as inside the pores, where, besides the strength of the sites, also limitations in the diffusion of the reaction products favour its formation. Accordingly, direct isopropylation (instead of transalkylation)
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would occur at the surface acid-sites, not yet covered by coke.
5. Conclusions The conclusions of the present work can be summarised as follows. Naphthalene isopropylation is accompanied by fast oligomerisation of propene resulting from isopropanol, followed by cracking of the oligomers. Trapping of carbonaceous compounds (coke) occurs in the pores. The formation of the alkylated products is not directly catalysed by the acid-sites, which are blocked by the carbonaceous material. Transisopropylation reactions occur between naphthalene in the liquid-phase and isopropylderivatives trapped in the zeolite pores, near the pore mouth. The predominance of 2-IPN over 1-IPN in the monoisopropylated product stems from the reaction of the carbonaceous material with incoming naphthalene molecules oriented in such a way that 2-IPN formation is favoured. Similarly, the significant formation of 2,7-DIPN in comparison with 2,6-DIPN is explained in terms of a more hindered conformation of the intermediate leading to the former during the transalkylation reaction between 2-IPN and the isopropylderivatives trapped in the pores. References [1] R.M. Gaydos, in: R.E. Kirk, D.F. Othmer (Eds.), Kirk–Othmer Encyclopaedia of Chemical Technology, vol. 15, Wiley, New York, 1981, p. 698. [2] K. Kasano, Y. Kano, K. Mishima, M. Oguri, Shin–Daikyowa Petrochemical Co. Ltd., Jpn. Kokai Tokkyo Koho JP 63,230,645 (1988). [3] S.F. Newman, J.D. Fellmann, P.H. Kilner, Catalytica Inc., PCT Int. Appl. WO 90,03,960 (1990). [4] J.D. Fellmann, R.J. Saxton, P.R. Wentrcek, E.G. Derouane, P. Massiani, Catalytica Inc., PCT Int. Appl. WO 90,03,961 (1990). [5] A. Katayama, M. Toba, F. Mizukami, Agency of Industrial Sciences and Technology; Nippon Steel Corp.; Nippon Steel Chemical Co. Ltd., Jpn. Kokai Tokkyo Koho JP 02,295,936 (1990). [6] M. Neuber, U. Dettmeier, E.I. Leupold. Hoechst A.-G. (Company), PCT Int. Appl. WO 92,07,810 (1992). [7] M. Neuber, U. Dettmeier, E.I. Leupold. Hoechst A.-G. (Company), Eur. Patent Appl. EP 545,307,09 (1993). [8] T. Yorisue, M. Furuya, Asahi Chemical Ind., Jpn. Kokai Tokkyo Koho JP 05,194,283 (1993).
[9] G.S. Lee, J.J. Maj, S.C. Rocke, J.M. Garcés, Catal. Lett. 2 (1989) 243. [10] A. Katayama, M. Toba, G. Takeuchi, F. Mizukami, S. Niwa, S. Mitamura, J. Chem. Soc., Chem. Commun. (1991) 39. [11] P. Moreau, A. Finiels, P. Geneste, J. Solofo, J. Catal. 136 (1992) 487. [12] Y. Sugi, M. Toba, Catal. Today 19 (1994) 187. [13] C. Song, S. Kirby, Micropor. Mater. 2 (1994) 467. [14] J.A. Horsley, J.D. Fellmann, E.G. Derouane, C.M. Freeman, J. Catal. 147 (1994) 231. [15] S.-J. Chu, Y.-W. Chen, Ind. Eng. Chem. Res. 33 (1994) 3112. [16] J.H. Kim, Y. Sugi, T. Matsuzaki, T. Hanaoka, Y. Kubota, X. Tu, M. Matsumoto, Micropor. Mater. 5 (1995) 113. [17] J.H. Kim, Y. Sugi, T. Matsuzaki, T. Hanaoka, Y. Kubota, X. Tu, M. Matsumoto, S. Nakata, A. Kato, G. Seo, C. Pak, Appl. Catal. A: Gen. 131 (1995) 15. [18] S.-J. Chu, Y.-W. Chen, Appl. Catal. A: Gen. 123 (1995) 51. [19] Z. Liu, P. Moreau, F. Fajula, J. Chem. Soc., Chem. Commun. (1996) 2653. [20] P. Moreau, A. Finiels, P. Geneste, J. Joffre, F. Moreau, J. Solofo, Catal. Today 31 (1996) 11. [21] A.D. Schmitz, C. Song, Catal. Today 31 (1996) 19. [22] Z. Liu, P. Moreau, F. Fajula, Appl. Catal. A: Gen. 159 (1997) 305. [23] E. Almengol, A. Corma, H. Garcia, J. Primo, Appl. Catal. A: Gen. 149 (1997) 411. [24] M. Toba, A. Katayama, G. Takeuchi, S. Niwa, F. Mizukami, S. Mitamura, in: C. Song, J.M. Garcés, Y. Sugi (Eds.), Shape-Selective Catalysis, ACS Symposium Series, vol. 738, 2000, p. 292. [25] C. He, Z. Liu, F. Faujula, P. Moreau, J. Chem. Soc., Chem. Commun. (1998) 1999. [26] R. Brzozowski, W. Tecza, Appl. Catal. A: Gen. 166 (1998) 21. [27] G. Kamalkar, S.J. Kulkarni, K.V. Raghavan, S. Unnikrisnan, A.B. Halgeri, J. Mol. Catal. A: Chem. 149 (1999) 283. [28] R. Gläser, J. Weitkamp, in: M.M.J. Treacey (Ed.), Proceedings of the 12th International Zeolite Conference, vol. 2, 1999, p. 1447. [29] K. Smith, S.D. Roberts, Catal. Today 60 (2000) 227. [30] P. Moreau, Z. Liu, F. Fajula, J. Joffre, Catal. Today 60 (2000) 235. [31] K. Tanabe, W.F. Hölderich, Appl. Catal. A: Gen. 181 (1999) 399. [32] G. Colón, I. Ferino, E. Rombi, E. Selli, L. Forni, P. Magnoux, M. Guisnet, Appl. Catal. A: Gen. 168 (1998) 81. [33] I. Ferino, R. Monaci, E. Rombi, V. Solinas, P. Magnoux, M. Guisnet, Appl. Catal. A: Gen. 183 (1999) 303. [34] M. Guisnet, P. Ayrault, C. Coutanceau, M.F. Alvarez, J. Datka, J. Chem. Soc., Faraday Trans. 93 (1997) 1661. [35] P. Magnoux, P. Roger, C. Canaff, V. Fouché, N.S. Gnep, M. Guisnet, Stud. Surf. Sci. Catal. 34 (1987) 317. [36] J.M. Winterbottom, Catalysis, vol. 4, The Royal Society of Chemistry, London, 1981, p. 141. [37] N.S. Gnep, P. Roger, P. Cartraud, M. Guisnet, B. Juguin, C. Hamon, C.R. Acad. Sci., Ser. II 309 (1989) 1743. [38] M. Guisnet, P. Magnoux, Catal. Today 36 (1997) 477.
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Liquid-phase alkylation of naphthalene by isopropanol over zeolites Part III. Mordenites M.G. Cutrufello a , I. Ferino a,∗ , R. Monaci a , E. Rombi a , V. Solinas a , P. Magnoux b , M. Guisnet b a
b
Dipartimento di Scienze Chimiche, Università di Cagliari, Complesso Universitario Monserrato, s.s. 554 Bivio Sestu, 09042 Monserrato (Ca), Italy URA-CNRS 350, Université de Poitiers, UFR Sciences, 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France Received 25 March 2002; received in revised form 22 July 2002; accepted 6 August 2002
Abstract The title reaction was investigated over three MOR zeolites with Si/Al ratios of 10, 60 and 80. Surface acidity was evaluated by adsorption microcalorimetry and FTIR analysis, using pyridine as probe molecule. Catalytic tests were carried out at 623 K and 40 bar in liquid-phase, with decalin as solvent, in a flow reactor. The formation of isopropylnaphthalenes (mainly monoisopropylnaphthalenes) was accompanied by fast oligomerisation/cracking of propene resulting from isopropanol dehydration and trapping of carbonaceous compounds (“coke”) in the zeolite pores. This caused a quasi-immediate blockage of the access to the micropores, suggesting that reactions occur on or near the outer surface of the crystallites. In agreement with this proposal, the activities of the three MOR samples, which have very different acidic properties but similar textural properties, were identical. A reaction scheme involving as a main step transalkylation between naphthalene molecules and the isopropyl aromatic molecules trapped in the MOR channels is proposed to explain the formation of isopropylnaphthalenes over the aged MOR samples. In agreement, isopropylnaphthalenes formation was found to remain significant for more than 1 h after substituting a decalin–naphthalene mixture for the normal feed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Naphthalene; Isopropylation; Transalkylation; Mordenites; Acidity of mordenites
1. Introduction The use of zeolites as non-conventional Friedel– Crafts catalysts is very interesting from both the environmental and economical points of view: no pollution problems are originated by these solid acids, whose pore structure is able to induce unique ∗ Corresponding author. Tel.: +39-70-675-4383; fax: +39-70-675-4388. E-mail address: [email protected] (I. Ferino).
selectivity effects. Among the Friedel–Crafts reactions, the selective alkylation of naphthalene to the  and  mono- and dialkylated products is of practical relevance. -Naphthol, key intermediate for dyes, pharmaceuticals and perfumes, can be obtained from 2-alkylnaphthalenes; 2,6-dialkylnaphthalenes are starting product for the manufacture of high performance engineering plastic materials (polyethylenenaphthalate) and liquid crystal polymers [1]. Both patent [2–8] and open literature [9–31] report about the use of zeolites for naphthalene alkylation.
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 6 1 - 1
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Investigations mostly focused on mordenites; propene [10,14,16,17,21,26], isopropanol [10,13,18,28], isopropylbromide [11] have been used as alkylating agents. In order to enhance shape selectivity effects, bulkier alkylating agents have also received attention [20,22,29,30]. It should be noted, however, that the bulkier the alkylating agent, the higher is the waste of carbon atoms when the alkyl groups are oxidised to carboxyl groups, a necessary step in the manufacture of polyethylenenaphthalate. Though semi-commercial (1000 t per year) plants have already been installed [31], a major drawback of zeolite catalysts is still their deactivation due to the formation of carbonaceous material. Despite their practical relevance (and fundamental interest as well), coking phenomena during naphthalene isopropylation have not always received the necessary attention. Papers dealing with this subject are rare [16]. Even when severe coking should be expected on the basis of the experimental conditions and remarkable catalyst deactivation is observed, the possible occurrence of coking is quoted only in generic terms [15,18] or completely neglected [26]. It seems to exist a need for further investigation of the coking phenomena, in order to elucidate their role in the complex interplay of the several processes which can occur during naphthalene isopropylation over zeolites. Polyalkylation of naphthalene, transalkylation between alkylated products and naphthalene, isomerisation between the isopropylated products, oligomerisation and subsequent cracking of the alkylating agent, as well as reactions involving the solvent, should be considered. Further complications arise from differences in the diffusion rate of the various products within the zeolite pores, which in turn depends on several factors: size of the products molecules, existence of mesopores, acid-sites density and strength, localisation and consistence of coke deposits.
In this light, a programme was started in the present authors’ laboratories, aimed to compare the behaviour of zeolites of different structural types, including those usually considered as no shape selective. Previous papers were devoted to the study of Y [32] and beta zeolites [33]. In the present work, the behaviour of mordenites in the liquid-phase isopropylation of naphthalene has been investigated. Samples with different Si/Al ratio were tested. Their acidity was assessed by adsorption microcalorimetry and FTIR technique using pyridine as probe molecule. The reaction was carried out at 623 K and 40 bar in a fixed-bed flow reactor; isopropanol and decalin were used as alkylating agent and solvent, respectively. Both the amount and the composition of coke were monitored during the reaction.
2. Experimental The mordenites were commercial samples from ZEOCAT. Their main characteristics are reported in Table 1. A Tian–Calvet heat flow calorimeter (Setaram) equipped with a volumetric vacuum line was used for microcalorimetric measurements. Each sample was pretreated overnigth at 673 K under vacuum (10−8 bar). Adsorption was carried out at 423 K, by admitting successive doses of pyridine and recording the thermal effect; the run was stopped at a final equilibrium pressure of 1.33 × 10−3 bar. After overnight outgassing of the sample at 423 K, a second adsorption was carried out, obtaining an adsorption isotherm parallel to the first one. From the difference between the adsorption isotherms the amount of pyridine irreversibly adsorbed was evaluated. FTIR measurements were carried out at 423 K by using the same probe
Table 1 Characteristics of the mordenite catalysts for naphthalene isopropylation Catalyst
MOR10 MOR60 MOR80 a b
Si/Al framework
10 59 79
Pore volumea (cm3 /g)
Concentration of acid-sitesb (mol/g)
Total
Micro
Meso
Brønsted
Lewis
0.263 0.272 0.281
0.186 0.178 0.186
0.077 0.094 0.095
372 226 96
63 15 32
From N2 adsorption–desorption measurements at 77 K; microporous volume determined by t-plot method. Determined by FTIR after pyridine desorption at 423 K.
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molecule (pyridine). The apparatus and procedure are described in detail elsewhere [34]. For catalytic testing, a small amount of catalyst (ca. 0.5 g) was placed into the tubular stainless-steel reactor and activated at 773 K for 2 h in N2 flow and then at 623 K overnight before rising the pressure up to 40 bar. N2 flow was then stopped and the liquid reaction mixture, composed of naphthalene, isopropanol and decalin as solvent (1:2:6 mol) was fed into the reactor; contact time was 5 × 10−2 gcat h/cm3feed . Each run lasted 6 h, with sampling of the reactor effluent every 20 min. GC analysis was carried out using N2 as carrier gas, an HP PONA capillary column (50 m × 0.21 mm × 0.57 mm) and an FID detector. Oven temperature programming was: 393 K for 1 min, heating rate 3 K/min up to 533 K, maintained for 10 min. Products identification was confirmed by GC/MS. Runs where both the flow rate and the catalyst amount were significantly changed while keeping the same W/F value gave practically the same results; accordingly, the occurrence of external diffusion limitations has been ruled out. The extent of coke deposition on the catalyst was determined by using a fresh portion of the same sample in runs lasting 1, 3, 4 and 6 h, respectively. The coked zeolite samples were treated at room temperature with a solution of HF (40%) in order to dissolve the zeolitic matrix and recover the coke; extraction by
93
CH2 Cl2 then followed. The procedure is reported in detail elsewhere [35].
3. Results 3.1. Acidity measurements Calorimetric results are summarised in Figs. 1–3. The differential heat of adsorption, Qdiff. , depends on the coverage as shown in Fig. 1. The initial differential heat of adsorption is 215–220 kJ/mol. For MOR10, after a sudden drop caused by the first pyridine doses, Qdiff. changes slightly and stepwise over a relatively wide range of the pyridine uptake, n, and then steeply decreases. A similar trend is observed for the other two samples, whose stepped region is however considerably shorter (particularly for MOR80) in comparison with MOR10. As could be expected from the Si/Al ratio, the total pyridine uptake is in the order MOR10 > MOR60 > MOR80. At high coverage the adsorption is reversible and the released heat falls below 90 kJ/mol. Fig. 2 represents the site–energy distribution plots, −dn/dQdiff. versus Qdiff. , as obtained through graphical derivation of the curves in Fig. 1. Fig. 2 allows ranking of the sites according to their strength. For MOR10, the strongest sites are those with Qdiff. >
Fig. 1. Differential heat of pyridine adsorption, Qdiff. vs. uptake for mordenites. Open symbols refer to re-adsorption after evacuation.
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Fig. 2. Site–energy distribution plots for mordenites, as obtained through graphical derivation of Fig. 1 curves.
130 kJ/mol (region I), whose amount, n>130 , is 138 mol/g. In this region, a single narrow peak is visible, which indicates homogeneity of the sites. The narrow peak observed for MOR10 between 110 and 130 kJ/mol individuates another set of homogeneous sites (region II, n110–130 = 242 mol/g); between 90 and 110 kJ/mol (region III, n90–110 = 134 mol/g), no peaks are observed, i.e. the sites are continuously heterogeneous. A set of homogeneous, strong sites is also present on MOR60, where the amount of sites in region I is still high (n>130 = 127 mol/g); the concentrations of the sites in region II (n110–130 = 65 mol/g) and region III (n90–110 = 51 mol/g) are considerably lower in comparison with MOR10. On the most dealuminated sample (MOR80), a lower concentration of sites is observed also in region I (n>130 = 77 mol/g); most of these sites are grouped in a homogeneous set. The site population of regions II and III is very low (n110–130 = n90–110 = 16 mol/g). Below 90 kJ/mol (region IV) the adsorption is reversible for all the samples (cf. Fig. 1). Thermokinetic data are shown in Fig. 3, where t0 (the time over which the heat is evolved after each successive dose of pyridine) is plotted versus coverage. A maximum is observed for all the samples, which
indicates that surface diffusion is the mechanism by which the sites are selectively titrated during the sequential exposure of the sample to pyridine. When the run is started, the concentration of the sites is the highest, the strong sites are titrated first and t0 is relatively low. As coverage increases, the fraction of strong sites decreases and the adsorption of pyridine is initiated by its interaction with the largely available weak sites, from where it migrates within the pore system of the solid to reach the remaining strong sites. The time needed for the equilibration after each pyridine dose increases with the decrease in the available strong sites. Once the latter are completely filled, t0 decreases, because of the fast exchange of pyridine between weak sites. The maxima in Fig. 3 are attained in correspondence of the complete filling of the sites with Qdiff. > 110 kJ/mol (regions II and I in Fig. 2). The establishing of reversible adsorption is accompanied by a decrease of t0 , which attains very low values, i.e. very fast exchange between the sites occurs. Interaction of pyridine with weak Brønsted or Lewis sites, non-specific hydrogen bonding and physical adsorption could be simultaneously involved in the reversible adsorption, each contribution being difficult to assess. By FTIR it is possible to detect the
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interaction of pyridine with Brønsted and Lewis sites, whose concentrations are reported in Table 1. Fig. 4 shows that for each sample the total concentration of acid-sites (Brønsted + Lewis), nFTIR , compares well with the amount of pyridine irreversibly adsorbed during the calorimetric experiment, nA . Accordingly, reversible adsorption (corresponding to region IV in Fig. 2) seems mostly due to non-specific interaction of pyridine with the silica part of the zeolite and does not represent true acidity of the zeolite. 3.2. Catalytic runs
Fig. 3. Thermokinetic parameter, t0 , vs. coverage for mordenites.
Naphthalene alkylation by isopropanol gave monoisopropylnaphthalenes (IPNs) and diisopropylnaphthalenes (DIPNs); only traces of triisopropylnaphthalenes (TIPNs) were detected. Side reactions also took place. These involved isopropanol, which underwent dehydration, subsequent oligomerisation of propene and cracking of the oligomers, as well as the solvent decalin, which underwent cracking. Deposition of carbonaceous material (coke) also occurred on the catalysts. The conversions of isopropanol, XI , and decalin, XD , through the above parasite reactions were assessed according to [32]. The naphthalene conversion, XN , as well as XI and XD , are reported as a function of time-on-stream in Fig. 5. While decalin conversion is negligible, XI is remarkably high for all the catalysts and shows a decreasing trend as time-on-stream increases. With increasing Si/Al, the parasite reactions involving isopropanol appear to attain a higher extent. Naphthalene conversion is low and in a narrow range (XN = 25–28%) for the three catalysts and decreases during the run: after 6 h, XN is 78% of its initial value for MOR10, 58% for MOR60 and 50% for MOR80. Selectivities to mono- and diisopropylnaphtalenes (SIPN , SDIPN ) and the distribution of the isomers in the mono and dialkylated mixture were also determined as a function of time-on-stream for all the catalysts. In no case, SIPN and SDIPN were found to change with the reaction time (hence with XN ), the former being quite high (91%) in comparison with the latter. The evolution of the isomers distribution with time-on-stream is shown in Figs. 6–8. For all the catalysts and at any time-on-stream, 2-IPN is by far predominant in the monoisopropylated mixture (Figs. 6A–8A). However, the amount of 1-IPN
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Fig. 4. Acid-sites concentrations determined by microcalorimetry, nA , compared with acid-sites concentrations (Brønsted + Lewis) determined by FTIR, nFTIR .
increases with time-on-stream at the expense of 2-IPN. Among the dialkylated products, 2,6- and 2,7-DIPN are far more abundant in the case of MOR10 catalyst, for which small amounts of 1,3-, 1,6- and 1,7-DIPN are also detected (Fig. 6B–D). Only the 2,6- and 2,7-isomers are formed over MOR60 and MOR80 (Figs. 7B and 8B). For all the catalysts, the 2,6-/2,7-DIPN ratio remarkably increases with time-on-stream, passing from initial values close to 1 to values of 2.5–3.5 after 6 h on-stream, e.g. from 14% initially to 24% after 6 h for MOR10. The evolution of coke deposition with the reaction time is presented in Fig. 9. It shows that the catalysts are almost saturated by carbonaceous material since the first hour on-stream. HF attack followed by extraction with CH2 Cl2 was carried out on the catalysts after 1 and 6 h on-stream. After separation of the very heavy polyaromatic material (insoluble coke), the compounds present in the soluble fraction (soluble coke) were identified. Decalin,
naphthalene, mono- and diisopropylnaphthalenes as well as other alkylnaphthalenes (branched C4 and C5 side chains) were found in the soluble material after 1 h on-stream. Traces of alkylated (C3 and C4 ) phenanthrenes and alkylated (C4 and C5 ) chrysenes (or other compounds with 4 fused rings) were also detected. After 6 h on-stream these same species were found in the soluble coke. In the case of MOR60, traces of alkylated (C3 –C5 ) polyaromatics with five fused rings were also revealed in addition to alkylated phenanthrenes and chrysenes; traces of bulkier aromatics (six fused rings, C4 –C9 -alkylated) were detected in the soluble material from MOR80. Analytical data are summarised in Table 2, where the coke constituents have been grouped as follows: (i) naphthalene and decaline (reactant and solvent); (ii) isopropylated and other alkylated naphthalenes (products originated by reactions involving naphthalene); (iii) very heavy polyaromatic material (insoluble coke). It can be seen that the composition of the
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Fig. 5. Conversion of naphthalene, XN , isopropanol, XI , and decalin, XD , vs. time-on-stream for mordenites.
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Fig. 6. Isomers distribution with time-on-stream for MOR10.
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Fig. 7. Isomers distribution with time-on-stream for MOR60.
Table 2 Coke analysis dataa for mordenites Naphthalene + decalin (mg/gcat )
IPNs + DIPNs + other alkylnaphthalenes (mg/gcat )
Insoluble coke (mg/gcat )
5 –
55 –
20 65
1 6
36 –
24 –
27 64
1 6
38 –
19 –
31 68
Catalyst
Time-onstream (h)
A4251
1 6
A1583 A4841
a Due to the scarce amount of soluble coke at 6 h on-stream, the corresponding data were considered reliable only from a qualitative point of view (see text) and have been omitted in the present table.
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Fig. 8. Isomers distribution with time-on-stream for MOR80.
material retained after 1 h on-stream is different for the three catalysts, the major differences being observed between the slightly dealuminated sample on the one side and the two strongly dealuminated catalysts on the other. For each sample, the coke composition undergoes modification during the catalytic run: after 6 h on-stream, the insoluble coke is the most abundant component of the retained material, its amount being similar for the three samples. Nitrogen adsorption measurements carried out on MOR60 and MOR80 showed that coke deposition significantly affects the pore volume, especially the micropore volume. Since the first hour on-stream (≈8 wt.% coke), the access of nitrogen to the micropores, hence, to the acidic sites, is practically blocked (Fig. 10).
4. Discussion 4.1. Influence of catalyst acidity and porosity on naphthalene and isopropanol conversion According to the literature on the dehydration of alcohols [36], carbenium ions easily form on acidic solids by interaction of the alcohol molecules with either Brønsted or Lewis sites. In the present case, the isopropyl carbenium ion formed through this way can: (a) release a proton to give propene; (b) undergo attack by another isopropanol molecule from the liquid-phase to form a dimer ion; this process continues until the oligomer is large enough to undergo cracking; (c) undergo attack by a naphthalene
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Fig. 9. Amount of coke formed on mordenites after different time-on-stream values.
molecule from the liquid-phase, with formation of an adsorbed isopropylnaphthalenium ion; the latter can either evolve into monoisopropylnaphthalene or undergo further alkylation. The initial (50 min on-stream) extent of isopropanol transformation (XI ) through the (a) and (b) pathways and of naphthalene transformation (XN ) through the (c) pathway are plotted versus the concentration of the acid-sites, nA , of the present mordenite catalysts in Fig. 11. Formerly tested Y [32] and beta zeolites [33] are also included in Fig. 11 for comparison. Whatever the MOR samples, XI is approximately three times greater than XN , although the reactor feed contains isopropanol in excess compared to naphthalene (I/N = 2 mol/mol). XN is practically independent of the concentration of acid-sites (nA ), whereas XI slightly increases when nA decreases (Fig. 11). This shows clearly that the apparent activity of MOR samples is not determined by their acidity. For the beta zeolites neither XN nor XI significantly change with the acid-sites concentration. In spite of the wide difference in nA , the as-synthesised (Si/Al = 3.9) and the mildly dealuminated (Si/Al = 16) HY samples do not differ significantly as to both XN and XI ; however, naphthalene conversion is remarkably high (and XI correspondingly low) over the strongly dealuminated (Si/Al = 30) HY sample, for which the acid-sites concentration is the lowest. Whatever the concentration
of the acid-sites, XI is lower for HY than for both mordenite and beta zeolites. The trends in Fig. 11 reveal that the apparent activity of the catalysts cannot be traced back to acidity effects only. A complex interplay of several effects is probably underlying, among which coking should play a key role. Coking can be regarded as a complex sequence of “pathological” reactions involving carbenium ions and is governed by both structural and acidity effects [37,38]. In principle, a high density of the acid-sites would favour those bimolecular events which ultimately lead to coke. The presence of strong sites would enhance coke formation, in that it causes the ions to reside longer on the surface (instead of desorbing after proton release), and thus, offers a chance to consecutive reactions to occur. This possibility can be further enhanced by the confinement of large, “soluble coke” molecules in the vicinity of the active sites as a consequence of the slow diffusion of cumbersome reaction products, a phenomenon which could be favoured in the case of zeolites with a narrow pore system. It should be noted, however, that the composition of the retained material as well as its further evolution into insoluble, heavy polyaromatic coke will depend on the presence of suitable voids (cavities, channel intersections) large enough to allow for the growth processes. Furthermore, the coking rate and the micropore blockage (and hence the deactivating effect of coke)
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Fig. 10. Pore volume evolution as a function of coke amount.
are known to depend also on the secondary porosity of the zeolite [37,38]. The enhanced diffusion of reactant and product molecules induced by the presence of mesopores could have a positive role on the apparent activity and even result in some kind of compensation between the acidity and porosity effects. Though rather complex, such a picture allows one to suggest an interpretation of the XI and XN trends. For HY zeolites [32], 14% of carbonaceous material was found on the as-synthesised sample, i.e. the most acidic one, after 1 h reaction time. 20% of this material was insoluble (i.e. heavy polyaromatic in nature), the soluble fraction being composed by polyalkylated naphthalenes and very polyaromatic compounds, including polyisopropylpyrenic and in-
denopyrenic compounds. Trapping of these cumbersome products in the zeolite supercage strongly limits the diffusion of the isopropylated naphthalenes out of the pore system. In comparison, the products originated by the side reactions of isopropanol (dehydration–oligomerisation–cracking) can rapidly diffuse out, due to their smaller size. Isopropanol transformation through (a) and (b) pathways is hence favoured over naphthalene conversion. Severe dealumination of HY entrained two effects [32]: (a) the concentration of the acid-sites strongly decreased and (b) a mesoporous system (accounting for more than one-half of the total pore volume) was originated. Both factors determine rapid circulation of the reaction products within the pore system of the zeolite,
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Fig. 11. Initial (50-min on-stream) conversion of naphthalene XN , and isopropanol XI , vs. acid-sites concentration, nA , for mordenites (data from present work), Y (data from [32]) and beta zeolites (data from [33]).
thus, disfavouring coke deposition. Accordingly, no coke was found on the strongly dealuminated HY sample [32]. Under such conditions, no longer the side reactions of the alkylating agent are preferred (low XI value in comparison with XN , Fig. 11). The amount of carbonaceous material retained on beta catalysts after 1 h on-stream was 13–15%, most of which was soluble [33]. It was found that, at variance with HY, the development of mesopores upon increasingly severe dealumination was too modest for enhancing the diffusion rate of the cumbersome alkylated products. It seems that in the cageless microporous structure of the beta samples the spatial
constraints are such to determine significant retention of carbonaceous material (due to the slow diffusion of the alkylated products), regardless of the acid-sites concentration. This in turn greatly favours the parasite isopropanol transformation, which leads to small reaction products able to escape rapidly from the porous system, over the naphthalene conversion (Fig. 11). Restrictions to the diffusion of the cumbersome alkylated products should be particularly compelling in the non-intersecting channel system of mordenites, where at variance with the case of HY, dealumination does not induce significant differences in the mesopore volume among the three MOR samples
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(cf. Table 1). In this light, it is not surprising to observe that all the mordenite samples, regardless of their acid-sites concentration, are saturated by carbonaceous material since the first hour on-stream (Fig. 9), which in turn explains the very high XI values over the whole range of nA in Fig. 11. It is worthy of note (Fig. 11) that for mordenites XI has a tendency to increase with Si/Al. A tentative interpretation is suggested in the following. Besides the decrease in the acid-sites concentration, the dealumination process entrains a modification of the acid strength distribution of mordenites. From calorimetric data, the sites with Qdiff. > 130 kJ/mol (corresponding to region I in Fig. 2) account for 27, 52 and 71% of the total acid-sites, respectively for MOR10, MOR60 and MOR80. The presence of strong sites should be relevant to the composition of the carbonaceous ma-
terial. The ratio between the amount of insoluble coke and group (ii) products (isopropylated and other alkylated naphthalenes), calculated from coke analysis data at 1 h on-stream (Table 2), is plotted in Fig. 12 versus the fraction of sites with Qdiff. > 130 kJ/mol for the three mordenite samples. As expected, this ratio increases with the proportion in sites stronger than 130 kJ/mol, i.e. the building-up of very heavy polyaromatic material at the expense of the alkylated products is enhanced on the stronger sites, where the intermediate ions reside longer. Furthermore, Table 2 data for 1 h on-stream show that, besides the expected decrease of the alkylated products, an increase in naphthalene + decalin occurs as the insoluble coke amount grows. This suggests that increasing amounts of reactant and solvent remain trapped inside the porous system because of the increasingly severe spatial constraints
Fig. 12. Ratio between the amount of insoluble coke and the amount of (IPNs + DIPNs + other alkylnaphthalenes, ANs) in the soluble coke after 1 h on-stream vs. fraction of sites with Qdiff. > 130 kJ/mol for the three mordenite samples.
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induced by the deposition of insoluble coke. Such an effect is probably amplified by the location of insoluble coke in proximity of the pore mouth, owing to the lack of suitable space inside the non-intersecting channel system of mordenites. Along the series MOR10, MOR60 and MOR80 a progressive increase of diffusional resistances, originated by such a location of the insoluble coke, would be expected. Accordingly, formation of small-sized reaction products would be progressively favoured, which could explain the increasing trend of XI with Si/Al in Fig. 11. 4.2. Influence of coke formation on naphthalene conversion All the mordenite samples are rapidly saturated by carbonaceous material: after the first hour on-stream
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catalysed by the acid-sites. As it was proposed in the case of the transformation of naphthalene-isopropanol mixtures on FAU and BEA samples, alkylation could occur on the molecules trapped in the channels. This proposal is supported by the results reported in Fig. 13, which refer to a run where the feeding of isopropanol was interrupted after 3 h on-stream while naphthalene (in decalin) was kept flowing. The appreciable consumption of naphthalene occurring for more than 1 h after interrupting the feed of alkylating agent indicates that transisopropylation reactions actually occur between naphthalene in liquid-phase and isopropylderivatives trapped in the zeolite pores: IPNs, DIPNs and probably also insoluble coke molecules. It can therefore be proposed that after the rapid filling of the MOR pores by alkylated molecules, the formation of isopropylnaphthalene occurs through the following schematic reactions. (a) Transalkylation:
(b) Rapid re-isopropylation of coke molecules by isopropanol:
there is only a slight increase in the coke amount (Fig. 9). Moreover, the access of nitrogen to micropores is practically precluded (Fig. 10). It should be remarked that the blockage of the access to the micropores is not due to their complete filling by coke molecules. Indeed the micropore volume is more than three times greater than the volume estimated for the coke molecules retained in MOR samples after 1 h on-stream. The relatively high conversion of naphthalene which is found on MOR samples, whose access to micropores (hence to acidic sites) is blocked by carbonaceous material since the first hour on-stream, suggests that the formation of alkylated products is not directly
As it is generally the case with monodimensional zeolites [39], the carbonaceous material is formed and trapped near the entrance of the channels (at the pore mouth). Indeed the diffusion of molecules (especially of bulky molecules such as naphthalene and naphthalenic derivatives) in non-interconnected channels of monodimensional zeolites is generally slow in comparison with their transformation into more and more cumbersome molecules, which remain trapped in the channels. The evidence for micropore blockage obtained by the adsorption experiments strongly supports the view that preferential trapping of coke molecules occurs at the channel entrance. Assuming the occurrence of transalkylation at the pore mouth it is possible to explain why XN is
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Fig. 13. Naphthalene conversion, XN , vs. time-on-stream for MOR10. The open symbols refer to a situation where isopropanol is no more present in the feed.
independent of the MOR acidity. XN should be dependent on the number of pore mouths on the external surface and on the mesopore walls. As the crystallite sizes and the pore distribution are practically the same for the three MOR samples, similar values of XN should be obtained, which is actually the case (Fig. 11). 4.3. Influence of coke formation on isopropylation selectivity It is well known [40–42] that the ␣-positions at carbons 1 and 5 of naphthalene are more reactive than the -positions at carbons 2, 6 and 7. Two possible reasons can be invoked for explaining the observed predominance of 2-IPN over 1-IPN (Figs. 6A–8A): (i) isomerisation of the kinetically favoured 1-IPN into the thermodynamically stable 2-IPN, and/or (ii) shape selectivity effects leading to 2-IPN because of restrictions on the transition state and/or the products diffusion. In the first case, it should be supposed that at the pore mouth isomerisation is much faster than transalkylation between naphthalene and carbonaceous material. In the second case, the orientation of the naphthalene molecule at the entrance of the pore would be such that the formation of 2-IPN through transalkylation would be largely favoured. Scheme 1 shows how the naphthalene molecules can be
oriented if they have to enter the channels for reacting with carbonaceous compounds. In mode (a), leading to 1-IPN, the orientation of naphthalene is more hindered (7.2 Å) than in mode (b) (5.8 Å). Accordingly, preferential formation of 2-IPN should be expected, which is actually observed. The same proposal can be advanced for explaining why the initial value of the 2,6-/2,7-DIPN ratio is close to 1 on the three MOR samples. The main steps of the reaction pathways involving trapped coke molecules and already formed 2-IPN molecules can be summarised as in Schemes 2 and 3. Though qualitatively, they show that when position 6 of 2-IPN is
Scheme 1. Formation of 1-IPN (a) and 2-IPN (b) through transalkylation between naphthalene and isopropyl aromatics blocked in the mordenite channels.
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Scheme 2. Formation of 2,6-DIPN through transalkylation between 2-IPN and isopropyl aromatics blocked in the mordenite channels.
involved, the reaction intermediate (I) (Scheme 2) has a rather bent conformation in comparison with that of the reaction intermediate (III) (Scheme 3) originated by interaction of the trapped coke with 2-IPN at position 7. Due to its somewhat linear conformation, “nesting” of intermediate (III) at the pore mouth could be favoured, which would explain the otherwise unexpected significant formation of 2,7-DIPN.
It is worthy of note that intermediates (I) and (III) are also involved in coke-growth processes, through formation of species such as (II) and (IV). The two sets of aromatic rings in (I) and (III) are tilted of 19.8 and 7.5◦ , respectively. Due to the more co-planar character of (III) in comparison with (I), evolution of the former into (IV) should be more probable than evolution of (I) into (II). Insoluble coke formation can ultimately lead
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Scheme 3. Formation of 2,7-DIPN through transalkylation between 2-IPN and isopropyl aromatics blocked in the mordenite channels.
to coke protruding outside the pores, a well-known phenomenon occurring on mordenites [39]. Once this situation has been attained, spatial constraints would no more favour the formation of intermediate (I) and transalkylation would be oriented by the higher reactivity at position 6 in 2-IPN. Accordingly, 2,6-DIPN would become predominant over 2,7-DIPN, which is in agreement with the growing trend of 2,6-/2,7-DIPN with time-on-stream experimentally observed (Figs. 6D, 7B and 8B). Very recently, the higher reactivity at position 6 in 2-IPN has been questioned on the basis of computa-
tional results [42]. According to these authors, molecular electrostatics at ab initio level does not distinguish between carbons 6 and 7 in the 1- and 2-IPN molecules. It has also been proposed that, at variance with the case of dimethylnaphthalenes [43], the DIPN isomers do not form equilibrium groups. 2,6- and 2,7-DIPN are predicted to be the most stable among the DIPN isomers and may transform to each other via 1,2-isopropyl shift inside the main channel of mordenite. Under free isomerisation conditions they are expected to have the largest concentration (their ratio being ca. 1) in the equilibrium mixture. According to
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the calculated dimensions, 2,6-DIPN should move the easiest, i.e. diffuse the fastest in the main channel of mordenite [42,44]. Whether or not the high 2,6-/2,7-DIPN ratios reported in the literature for mordenites [10,13,14] have to be ascribed to diffusion-controlled shape selective catalysis (as claimed in [42] and formerly suggested in [14]) cannot be judged on the basis of the present results. Experimental evidence however shows that under the present reaction conditions coking, pore blockage and consequent transalkylation occur during the runs. Interconversion between 2,6- and 2,7-DIPN and achievement of equilibrium does not seem realistic, as there would be no reason for the observed increase of 2,6-/2,7-DIPN ratio from ca. 1 (at 50 min on-stream) to 2.5–3.3 during the run (cf. Figs. 6C and D, 7B and 8B). In this respect, it is worthy of note that values for the 2,6-/2,7-isomers ratio as low as 1 have been reported by some authors [11,20]. It might be possible that such a scattering in the literature values for the 2,6-/2,7-DIPN ratio has been originated by a different extent of the coking reaction in the runs carried out in the various laboratories, a phenomenon probably disguised because of the use of autoclave reactors. At variance with the above-cited papers, the present work has been carried out in a fixed-bed flow reactor at constant contact time, so that any influence of coking on the isomers distribution could be revealed by the on-stream behaviour. To check further the role of coke formation on the DIPNs distribution, an additional run was carried out over MOR80, in which 2,6-DIPN and 2,7-DIPN were monitored in the liquid reactor effluent since the very early reaction times. It was found that 2,6-DIPN predominates over 2,7-DIPN at 10 min on-stream (2,6-/2,7-DIPN = 3.3). This was followed by a decrease in the former isomer and a simultaneous increase in the latter, until, after 50 min on-stream, the 2,6-/2,7-DIPN ratio attained a value of ca. 1. From this moment onwards, 2,6-DIPN was found to grow continuously, while 2,7-DIPN decreased, with a trend nearly identical to that already observed in Fig. 8B. Though further experimental work involving all the catalysts is needed to confirm this point, it seems that when the reaction is not yet perturbed by coking (i.e. at its very beginning), predominance of the 2,6-isomer is observed. Whatever the reason for this predominance, i.e. diffusion-controlled shape selective
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catalysis (as claimed in [42]), or frontier electron density on carbon 6 higher than on carbon 7 in 2-IPN (restricted electronic transition state selectivity [41,45]), it soon becomes perturbed by coke deposition. The complex evolution in the DIPNs distribution during the run can be reasonably interpreted as summarised in Schemes 2 and 3. Another point is worthy of note: only 2,6- and 2,7-DIPN are formed on MOR60 and MOR80 (Figs. 7B and 8B), while small amounts of 1,3-, 1,6and 1,7-DIPN are also detected in the case of MOR10 (Fig. 6B–D). This difference cannot be interpreted in detail on the basis of the present data only. It could be thought however that for MOR60 and MOR80 the only way towards DIPNs formation is pore-mouth transalkylation (as outlined in the preceding). Though this way is by far predominant also in the case of MOR10, it might be speculated that isopropylation processes leading to ␣ isomers can occur to some (very limited) extent on the external surface of the crystallites of this catalyst. According to some authors [16], the dealumination process preferentially removes the acid-sites at the external surface of mordenites. The (limited) presence of acid-sites on the external surface of MOR10 and their absence on the external surface of MOR60 and MOR80 would explain why the ␣ isomers are detected on the former but not on the strongly dealuminated samples. Probably the DIPNs formation at the external surface of MOR10 is not due to transalkylation with coke molecules, but involves acid-sites not yet covered by coke molecules. This is suggested by the acidity data (cf. Section 3.1). They show that after the dealumination process leading from MOR10 to MOR60 the concentration of the stronger sites (Qdiff. > 130 kJ/mol) remains nearly the same (138 and 127 mol/g, respectively), while the concentration of the intermediate-strength sites (110 kJ/mol < Qdiff. < 130 kJ/mol) dramatically drops from 242 to 65 mol/g. According to the view that surface sites are preferentially eliminated during this step, it should be concluded that the sites originally present on the surface of MOR10 do not belong to the stronger family. This would mean in turn that coking on the surface is not as fast as inside the pores, where, besides the strength of the sites, also limitations in the diffusion of the reaction products favour its formation. Accordingly, direct isopropylation (instead of transalkylation)
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would occur at the surface acid-sites, not yet covered by coke.
5. Conclusions The conclusions of the present work can be summarised as follows. Naphthalene isopropylation is accompanied by fast oligomerisation of propene resulting from isopropanol, followed by cracking of the oligomers. Trapping of carbonaceous compounds (coke) occurs in the pores. The formation of the alkylated products is not directly catalysed by the acid-sites, which are blocked by the carbonaceous material. Transisopropylation reactions occur between naphthalene in the liquid-phase and isopropylderivatives trapped in the zeolite pores, near the pore mouth. The predominance of 2-IPN over 1-IPN in the monoisopropylated product stems from the reaction of the carbonaceous material with incoming naphthalene molecules oriented in such a way that 2-IPN formation is favoured. Similarly, the significant formation of 2,7-DIPN in comparison with 2,6-DIPN is explained in terms of a more hindered conformation of the intermediate leading to the former during the transalkylation reaction between 2-IPN and the isopropylderivatives trapped in the pores. References [1] R.M. Gaydos, in: R.E. Kirk, D.F. Othmer (Eds.), Kirk–Othmer Encyclopaedia of Chemical Technology, vol. 15, Wiley, New York, 1981, p. 698. [2] K. Kasano, Y. Kano, K. Mishima, M. Oguri, Shin–Daikyowa Petrochemical Co. Ltd., Jpn. Kokai Tokkyo Koho JP 63,230,645 (1988). [3] S.F. Newman, J.D. Fellmann, P.H. Kilner, Catalytica Inc., PCT Int. Appl. WO 90,03,960 (1990). [4] J.D. Fellmann, R.J. Saxton, P.R. Wentrcek, E.G. Derouane, P. Massiani, Catalytica Inc., PCT Int. Appl. WO 90,03,961 (1990). [5] A. Katayama, M. Toba, F. Mizukami, Agency of Industrial Sciences and Technology; Nippon Steel Corp.; Nippon Steel Chemical Co. Ltd., Jpn. Kokai Tokkyo Koho JP 02,295,936 (1990). [6] M. Neuber, U. Dettmeier, E.I. Leupold. Hoechst A.-G. (Company), PCT Int. Appl. WO 92,07,810 (1992). [7] M. Neuber, U. Dettmeier, E.I. Leupold. Hoechst A.-G. (Company), Eur. Patent Appl. EP 545,307,09 (1993). [8] T. Yorisue, M. Furuya, Asahi Chemical Ind., Jpn. Kokai Tokkyo Koho JP 05,194,283 (1993).
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