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Influence of Na exchange on the acidic and catalytic properties of an HMOR zeolite
Microporous and Mesoporous Materials 51 (2002) 211–221 www.elsevier.com/locate/micromeso
Influence of Na exchange on the acidic and catalytic properties of an HMOR zeolite F. Moreau a, P. Ayrault a, N.S. Gnep a, S. Lacombe b, E. Merlen b, M. Guisnet a,* a
UMR CNRS 6503, Catalyse en Chimie Organique, Facult e des Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France b Institut Francßais du P etrole, 1 et 4, avenue de Bois-Pr eau, 92852 Rueil-Malmaison Cedex, France Received 7 May 2001; received in revised form 23 November 2001; accepted 26 November 2001
Abstract IR spectroscopy was used to characterize the hydroxyl groups of a series of NaHMOR samples resulting from sodium exchange of an HMOR sample with an Si/Al ratio of 10. Na exchange causes a significant decrease in the intensity of the band corresponding to bridged OH groups. This band presents two components, the first one at 3608– 3611 cm1 corresponding to bridged OH groups in the large channels, the second one at 3583–3585 cm1 corresponding to bridged OH groups in the side pockets. At low exchange rate, this latter is more affected than the former demonstrating a preferential exchange of the protonic sites of side pockets. Pyridine adsorption shows that part of the protonic sites of the side pockets of HMOR are accessible to pyridine molecules; sodium exchange causes the preferential disappearance of the non-accessible sites of the side pockets. Furthermore, part of the hydroxyl groups of the large channels do not interact with pyridine, suggesting a blockage of their access. Sodium exchange of HMOR causes a decrease in the activity for m-xylene transformation at 623 K, this decrease being initially more significant. Sodium exchange also affects the product distribution, causing an increase in the selectivity to isomers at the expense of disproportionation products. This effect is proposed to be due to the decrease in the proximity of the protonic acid sites. The substitution of nitrogen as carrier gas by hydrogen does neither affect the activity nor the selectivity of the parent HMOR sample, but decreases significantly the disproportionation activity of a sodium-exchanged sample. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mordenites; Sodium exchange; Acidity; m-Xylene; Isomerization; Disproportionation
1. Introduction Mordenite catalysts are used in refining and petrochemical processes such as:
* Corresponding author. Tel.: +33-5-49453905; fax: +33-549453779. E-mail address: [email protected] (M. Guisnet).
• isomerization of light gasoline [1–4] in order to improve the octane number, • selective synthesis of alkylbenzenes such as cumene [5–7] which is used for phenol preparation, or long-chain linear alkylbenzenes [8] which are precursors of biodegradable surfactants, • isomerization of the C8 aromatic cut (xylenes þ ethylbenzene) [9–11] for the production of paraxylene which is the precursor of polyester fibers.
1387-1811/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 4 8 4 - X
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On the bifunctional Pt mordenite catalysts which are used in the latter processes, secondary reactions of disproportionation and transalkylation accompany xylene isomerization and ethylbenzene isomerization into xylenes causing a loss in the production of para-xylene. According to various authors [12–16], partial exchange of the protonic sites of mordenite by sodium cations limits preferentially disproportionation and transalkylation, hence improves significantly the selectivity to xylenes. The influence of Na/H exchange on the acidity of mordenite was determined by various authors [13,16–25] using ammonia and/or pyridine adsorption followed by microcalorimetry, IR spectroscopy, etc. The location of the bridged hydroxyl groups (or protonic acid sites), either in the large channels or in the side pockets, as well as their strength depend on the degree of exchange. At high sodium content, the hydroxyl groups would be essentially located in the large channels; at high degree of exchange of sodium cations by protons, hydroxyl groups located in the side pockets would be mainly formed [16,22,23]. The acid strength of both bridged hydroxyl groups increases with the degree of protonic exchange [23,24]. Unfortunately, both hydroxyl groups appear in only one band. However, deconvolution into two components, one at high frequency (HF) corresponding to OH groups of the large channels, the other at low frequency (LF) corresponding to OH groups of side pockets can be carried out [22,25]. This allows a discrimination between these OH groups and a semiquantitative evaluation of their relative significance. It should be mentioned that most of the studies deal with the protonic exchange of NaMOR samples having no secondary porosity. Therefore, with the obtained NaHMOR samples, pyridine molecules have no access to the OH groups located in the side pockets (LFOH ) hence react only with those of the large channels (HFOH ). In this paper, NaHMOR samples were prepared through exchange by sodium cations of the protons of an HMOR10 zeolite sample containing a small amount of mesopores which should favor the access to the side pockets as well as a small
amount of extra-framework aluminum species [26]. Pyridine adsorption followed by IR spectroscopy and deconvolution of the OH groups were carried out in order to characterize the acidity of the NaHMOR samples. The activity and selectivity for disproportionation and isomerization of m-xylene were also determined, the objective being to specify if the apparent increase in selectivity to isomers caused by sodium exchange was genuine or simply due to differences in reaction temperature, conversion, degree of catalyst deactivation etc. between the NaHMOR samples.
2. Experimental section 2.1. Catalysts The initial H-mordenite sample, HMOR10, (total Si/Al ratio of 10, framework ratio of 12) was provided by ‘‘Institut Francßais du Petrole’’ RueilMalmaison. This sample resulted from exchange with NHþ 4 of a NaMOR sample (HSZ-640 NaA from TOSOH, Amsterdam, The Netherlands) followed by calcination under dry air flow at 550 C. Scanning electron microscopy showed that HMOR10 was mainly constituted of small crystallites (100–200 nm) and of plates of approximately 1 lm, both associated in aggregates of 50–70 lm. Na-exchanged H-mordenites were prepared by ion exchange at room temperature of HMOR10 with NaNO3 solution (100 ml/g of mordenite) with different sodium concentrations. The sodium contents were determined by two ways: elemental analysis of the mordenite samples and from the difference between Naþ concentrations in the solution before and after exchange measured by atomic absorption using a Perkin– Elmer 3300 system. Six NaHMOR samples were prepared with Na exchange degrees of 14%, 28%, 35%, 45%, 50% and 63%, the corresponding samples being denominated 14NaHMOR, 28NaHMOR, 35NaHMOR, 45NaHMOR, 50NaHMOR and 63NaHMOR, respectively. Sorption isotherms for nitrogen at 77 K over HMOR samples were recorded using a gas ad-
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3. Results and discussion
sorption system ASAP 2010 (Micromeritics), the samples being previously pretreated at 653 K for 2 h under vacuum (103 Pa). The acidity of HMOR samples was characterized by pyridine adsorption followed by IR spectroscopy using a Nicolet MAGNA IR 550 spectrometer. Thin wafers of about 8 mg cm2 were pretreated in situ in the IR cell under air flow: 60 ml min1 at 773 K for 12 h and then in vacuum (103 Pa) at 673 K for 1 h. Pyridine was adsorbed on the samples at 423 K. The IR spectra were recorded at room temperature after the activation period and after pyridine thermodesorption in vacuum (103 Pa) at various temperatures: 423, 523, 623, and 723 K. To discriminate between the bridged acidic OH groups in the main channels and side pockets, a procedure of deconvolution of the corresponding band was performed by using the Peak Solve Software (1990) from Jandel Scientific. Extinction coefficients for the high (HF) and low (LF) frequency bands (i.e. 2.5 and 3.8 lmol1 cm for OH(HF) and OH(LF) respectively) were used for quantification. m-Xylene transformation was carried out at 623 K in a flow reactor under atmospheric pressure generally with nitrogen as carrier gas (N2 /m-xylene molar ratio ¼ 15); some experiments were also carried out with hydrogen as carrier gas (H2 /mxylene molar ratio ¼ 15). The reaction products were analyzed on line by gas chromatography (GC). In order to obtain with a good accuracy the value of the activity of the fresh catalysts, the procedure developed by Morin et al. [27] was used.
3.1. Influence of Na exchange on the physicochemical properties of mordenite HMOR10 presents less than 0.01 Na atoms per unit cell, which corresponds to a degree of protonic exchange of approximately 99.8%. Nitrogen adsorption showed the presence of supermicropores (0.8–2 nm) and of mesopores (> 2 nm) in this sample. This suggests that a partial dealumination of the framework probably occurred during the calcination at 550 C, which is confirmed by the presence of a small amount of extra-framework aluminum species [26]. Sodium exchange of HMOR10 does not cause any significant change in the isotherm for nitrogen adsorption, hence on the total pore volume and on the pore size distribution. With all the samples, the pore volume distribution is the following : 78–81% of ultramicropores, 6.5–7% of supermicropores and 12–14% of meso-and macropores (Table 1). Fig. 1 shows that sodium exchange has practically no effect on the silanol groups (band at 3745 cm1 ) and on the OH linked to extra-framework aluminum species (band at 3655 cm1 ) which are present in small amounts in HMOR10:0.35 extraframework aluminum atoms per unit cell compared to 3.7 framework aluminum atoms [28]. On the other hand, sodium exchange causes a significant decrease in the intensity of the band corresponding to bridged OH groups (Fig. 2, curve a) as well as a slight shift to higher frequencies: from 3607 cm1 with HMOR10 to 3611 cm1 with 63NaHMOR. This OH band can be deconvoluted
Table 1 Characteristics of mordenite samples: pore volumes (cm3 g1 ) of HMOR10 and of NaHMOR samples determined by N2 adsorption at , supermicropores: 8–20 A , mesopores: >20 A 77 K; ultramicropores: <8 A Samples HMOR10 14NaHMOR 28NaHMOR 45NaHMOR 63NaHMOR
Pore volume (cm3 g1 )
Concentration of acidic sites
Total
Ultramicropores
Supermicropores
Mesopores
NHþ
NL
0.227 n.d. n.d. 0.216 0.224
0.179 n.d. n.d. 0.168 0.181
0.014 n.d. n.d. 0.015 0.016
0.033 n.d. n.d. 0.032 0.027
2.54 2.01 1.56 1.29 0.93
0.18 0.14 0.17 0.20 0.18
NHþ , NL : number of protonic sites (Hþ ) and of Lewis (L) sites per unit cell able to retain pyridine adsorbed at 423 K.
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Fig. 1. IR spectra of the OH groups in mordenite samples: A: HMOR10; B: 14NaHMOR; C: 45NaHMOR. Spectrum a: activated samples; Spectrum b: after pyridine adsorption at 423 K and removal of physisorbed pyridine: OH not accessible by pyridine molecules or non enough acidic to retain pyridine adsorbed at 423 K (OHNA ), Spectrum c: difference spectra (a–b): OH accessible and acidic (OHA ).
in two bands, the first one at 3608–3611 cm1 corresponding to OH groups of the large channels, the second one at 3583–3585 cm1 corresponding to those located in the side pockets. With HMOR10, the integrated intensities of the LF and HF bands are practically similar (Fig. 2). By using the extinction coefficients previously estimated [29], i.e. 1.8 and 3.8 cm lmol1 respectively, the percentages of OH in the side pockets and in the large channels were found to be 31% and 69%, respectively, which is close to what was obtained by Makarova et al. [22]. The integrated intensity of the HF band (Fig. 2, curve c) decreases quasilinearly with the percentage of sodium exchange,
whereas the introduction of the first sodium cations in HMOR10 causes a very significant decrease in the intensity of the LF band (curve b). This confirms the preferential exchange of the protonic sites located in the side pockets which was proposed by various authors [22,23]. This preferential exchange in the side pockets is also demonstrated by pyridine adsorption experiments. With all these mordenite samples, pyridine adsorption has practically no effect on the bands at 3745 cm1 and at 3655 cm1 , indicating that the corresponding OH groups, i.e., silanols, and OH linked to extra-framework aluminum atoms are not acidic. On the other hand, pyridine causes a
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Table 2 OHNA /OHA : Ratio between the integrated intensities of the band corresponding to non-acidic or non-accessible OH groups (OHNA ) and to acidic and accessible ones (OHA ) Samples
OHNA /OHA
OHLF /OHHF
HMOR10 14NaHMOR 28NaHMOR 45NaHMOR 63NaHMOR
0.66 0.32 0.45 0.43 0.80
0.94 0.49 0.35 0.19 0.26
OHLF /OHHF : ratio between the integrated intensities of the band corresponding to the OH groups located in the side pockets (OHLF ) and in the large channels (OHHF ) obtained by deconvolution of the bridged OH bands. Fig. 2. Integrated intensity of the bridged OH band in NaHMOR samples as a function of the percentage of sodium exchange; curve a: all the OH groups; curve b: OH groups of the side pockets; curve c: OH groups of the large channels. The intensities of the bands corresponding to the OH groups of the side pockets and of the large channels were obtained by deconvolution of the IR band corresponding to all the bridged OH groups.
large decrease of the band at 3607–3611 cm1 corresponding to bridged OH groups. However, with all the samples, part of the bridged OH groups are not able to retain pyridine adsorbed at 423 K (Fig. 1, curve b). This could be due to an inaccessibility of these OH groups by pyridine molecules, i.e. due to their location in the side pockets. However, another reason could be that part of the corresponding protonic sites located either in the large channels or in the side pockets are too weak to retain pyridine adsorbed at 423 K. This is certainly the case at high sodium contents. Indeed, for the corresponding samples, the ratio between the integrated intensities of the bands corresponding to non-acidic or non-accessible OH groups (OHNA ) and to the acidic and accessible ones (OHA ) is greater than the LF/HF intensity ratio obtained by deconvolution of the bridged OH bands (Table 2). The change with Na exchange in the intensities of the bands corresponding to all the bridged OH groups (OHT ), to OHNA and OHA is shown in Fig. 3. It should be noticed that, for degrees of Na exchange >28%, only one large band is obtained for OHNA and hydroxyls linked to extra-framework Al species, hence that the values of the intensity of the OHNA band obtained by deconvolution
Fig. 3. Integrated intensity of the bridged OH band in NaHMOR samples as a function of the percentage of sodium exchange: curve a: all OH groups; curve b: OH groups not affected by pyridine adsorption (OHNA ), curve c: OH groups disappearing upon pyridine adsorption (OHA ).
lack precision. The intensity of OHT (curve a) decreases strongly by introduction of the first sodium cations then more weakly and quasi-linearly. Similarly, the intensity of OHNA (curve b) first decreases strongly, then remains quasi-constant for degrees of Na exchange from 14 to 63%. Lastly, the intensity of OHA (curve c) decreases linearly with the degree of sodium exchange. The significant initial decrease of the intensity of the OHNA band confirms that Na exchange occurs preferentially in the side pockets. However, the quasi-constant values of the intensity found for degrees of Na exchange between 14 and 63% is more difficult to explain. The explanation could
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come from the large imprecision in the determination of the band intensity for highly exchanged samples because of the presence of extra-framework aluminum species and of a significant shift of the OHNA band towards high frequencies: from 3592 cm1 with HMOR10 to 3611–3612 cm1 with the Na exchanged samples. According to Datka et al. [23], this shift is due to a significant decreasing effect of Na exchange on the acid strength of the protonic sites located in the side pockets. Another explanation could be that the OHNA band corresponds to OH groups located in the large channels, the access of which by pyridine molecules would be blocked by extra-framework Al species. A deconvolution of the OHNA and OHA bands into HF and LF components (supposed to correspond to bridged OH groups in the large channels and in the side pockets, respectively), was attempted. For OHNA , the deconvolution was only carried out with HMOR10, 14NaHMOR and 28NaHMOR, because with the more highly exchanged samples the imprecision was too large. All the protonic sites of the side pockets which are not accessible by pyridine molecules are immediately exchanged (Fig. 4, curve b). On the other hand, sodium exchange has no significant effect on the number of OHNA groups of the large channels (Fig. 4, curve c), which suggests that the corre-
Fig. 5. Integrated intensity of the bridged OH groups interacting with pyridine at 423 K (OHA ): curve a: all the OHA ; curve b: OHA located in the side pockets (LF), curve c: OH located in the large channels (HF).
sponding acid sites are not accessible by Na cations. Part of the protonic sites of HMOR10 which are located in the side pockets (LF band) seem to be accessible by pyridine molecules (Fig. 5, curve b). This is not surprising, because the HMOR10 sample used in this work has, in addition to ultramicropores, supermicropores and mesopores which are known to facilitate the access of organic molecules to the protonic sites located in the side pockets. A quasi-regular decrease in the LF and HF components of the OHA band is found (Fig. 5, curves b and c), which indicates a similar ease in the exchange of the corresponding protonic sites. However, small differences appear at high percentages of Na exchange, but these differences could come from the imprecision of the deconvolution treatments. When Figs. 4 and 5 are compared, it can be concluded that: (i) the protonic sites located in the side pockets (LF band) are more easily exchanged when they are not accessible by pyridine molecules, (ii) the protonic sites corresponding to OHNA groups of the large channels are not exchanged (Fig. 4) whereas those corresponding to OHA can be exchanged by Na cations.
Fig. 4. Integrated intensity of the bridged OH groups incapable to retain pyridine adsorbed at 423 K (OHNA ): curve a: all the OHNA ; curve b: OHNA located in the side pockets (LF), curve c: OHNA located in the large channels (HF).
The concentrations of Br€ onsted and Lewis acid sites able to retain pyridine at 423 K as pyridinium ions (band at 1545 cm1 ) and as coordinated
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Fig. 6. Theoretical and measured concentrations of protonic sites (CHþ ) as a function of the percentage of sodium exchange.
pyridine (band at 1455 cm1 ) were calculated from the integrated intensities of the corresponding bands. The values of the extinction coefficients (1.15 and 1.28 lmol1 cm respectively) are those previously determined in our laboratory with mordenite samples having practically no Lewis acid sites and with alumina (only Lewis sites), respectively. With HMOR10, approximately 70% of the protonic sites (number estimated from the unit cell formula) are capable of retaining pyridine adsorbed at 423 K; the number of Lewis acid sites is 14 times lower than the number of protonic sites (Table 1). Sodium exchange causes a significant decrease in the protonic acidity, but no change in the Lewis acidity. The decrease in the protonic acidity, very pronounced at low exchange rate (Fig. 6), becomes linear above 14% exchange. This large initial effect of sodium exchange is quite unexpected, since exchange occurs preferentially with protonic sites of side pockets incapable to adsorb pyridine molecules as pyridinium ions. This observation suggests that the strongest protonic sites of the large channels are also exchanged and probably that sodium cations decreases the accessibility of pyridine molecules to the remaining protonic sites [16].
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p- and o-xylene and disproportionation into toluene and trimethylbenzenes (TMB). A very fast initial deactivation followed by a quasi-plateau in activity is observed as shown for the example of HMOR10 Fig. 7. The activity of the fresh samples was estimated from the initial slope of the curve representing conversion at very short time-on-stream (TOS) versus contact time, taken here as the reverse of the weight hourly space velocity (WHSV), e.g. with HMOR10 and 28NaHMOR samples in Fig. 8. The values of TOS were chosen just long enough to obtain steady state conditions: constant value of the GC peak area, hence of the reactant pressure in the reactor while limiting deactivation [27].
Fig. 7. Transformation of m-xylene over HMOR10 zeolite. Conversion (X) versus TOS for 25, 10 and 5 mg of catalyst.
4. Influence of Na exchange on the activity of mordenite samples With all the mordenite samples, m-xylene undergoes two main reactions: isomerization into
Fig. 8. Transformation of m-xylene over HMOR10 and 28NaHMOR samples. Conversion (X) as a function of contact time (taken as the reverse of the WHSV).
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Fig. 9. Transformation of m-xylene over HMOR10 zeolite samples. Activity as a function of the percentage of sodium exchange.
The change in activity with the percentage of Na exchange (Fig. 9) is qualitatively similar to the change in the protonic acidity (estimated from the intensity of the PyHþ band (Fig. 6)); a very pronounced effect of the exchange of the first sodium cations followed by a linear decrease with the degree of Na exchange. However, the turnover frequency of the protonic sites able to retain pyridine adsorbed as pyridinium ions at 423 K is 1.7 times greater with HMOR10 (2100 h1 ) than with sodium-exchanged catalysts (1150–1270 h1 ). This indicates that the exchange of the first protons of HMOR by sodium cations eliminates, in addition to protonic sites of the side pockets inaccessible to pyridine molecules hence probably inactive, very active sites hence very strong acid sites located in the large channels. For higher exchange rates, the practically constant value of TOF suggests a similar acid strength for the remaining protonic active sites.
5. Influence of Na exchange on the selectivity of mordenite With all the zeolite samples, para- and orthoxylenes are formed in a ratio of approximatively 1.1, which is very close to thermodynamic equilibrium [27]. This ratio is practically independent on conversion and on the degree of deactivation. The value of this ratio is the one found in the
absence of product shape selectivity indicating that even with the most active sample (HMOR10), mxylene isomerization is not limited by intracrystalline mass transfer. The distribution of the TMB resulting from xylene disproportionation depends on conversion but is practically not affected by deactivation. Thus, with HMOR10, the TMB distribution passes from 78% 1,2,4-TMB 10% 1,2,3-TMB and 12% 1,3,5-TMB at zero conversion to 68%/8%/24% at 30% conversion, i.e. close to thermodynamic equilibrium [30]: 65.5%/7%/ 27.5%. The initial distribution is characteristic of an electrophilic substitution (preferential methyl substitution in the activated ortho- and parapositions), the change with conversion is due to secondary isomerization. For identical values of m-xylene conversion, there is also a change in TMB distribution with the exchange of HMOR10 by sodium cations: the percentage of 1,3,5-TMB decreases at the expense of 1,2,4-TMB, which suggests a decrease in the significance of secondary isomerization. The toluene/TMB ratio is practically independent on the conversion and on the sodium content on the fresh samples. The molar ratio, which is higher than the expected value of 1 on the fresh samples, tends to 1 on the deactivated samples. This can be explained by difficulties in the desorption of the bulky molecules of TMB from the mordenite channels. The selectivity of m-xylene transformation into isomers and disproportionation products depends on the conversion and on the degree of deactivation. Thus, on the fresh samples, the disproportionation/isomerization (D=I) ratio decreases with conversion, and the D=I values are lower on the deactivated than on the fresh samples (e.g. HMOR10 in Fig. 10). The D=I ratio depends also on the catalyst: for identical values of conversion on the fresh and as well as on the deactivated sample, this ratio decreases when the Na content increases (Fig. 11). This observation was already made by other authors [13–16], the effect observed here being, however, less significant. The larger apparent effect observed by Bankos et al. [14,15] can be related to an effect of conversion; indeed the experiments were carried out at
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Fig. 10. Transformation of m-xylene over HMOR10 samples. Disproportionation/isomerization rate ratio (D=I) versus mxylene conversion (X) on fresh and on deactivated catalysts; HMOR10 25 mg (D), HMOR10 10 mg (), HMOR10 5 mg ().
Fig. 11. Transformation of m-xylene over HMOR10 and NaHMOR samples. Disproportionation/isomerization rate ratio (D=I) versus m-xylene conversion (X) on fresh and on deactivated catalysts; HMOR10 (), 14NaHMOR (), 28NaHMOR (4), 45NaHMOR (þ), 63NaHMOR ( ).
the same space velocity with all the zeolite samples. Therefore the conversion decreases when the sodium content increases with a concomitant a decrease in the D=I ratio. This is not the case with the experiments reported by Ratnasamy et al. [13] in which the selectivities are compared for identical values of conversion. The difference in the sodium effect on D=I between Ratnasamy et al. [13] and us is most likely related to differences in the operating conditions: o-xylene was used instead of m-xylene, the change in conversion was obtained by chang-
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Fig. 12. Conversion of m-xylene (X) over HMOR10 and 45NaHMOR10 under nitrogen: HMOR10 () 45NaHMOR10 ( ) and under hydrogen: HMOR10 (j) 45NaHMOR ( ) as a function of contact time (taken as the reverse of the WHSV).
ing the temperature instead of contact time and the carrier gas was hydrogen instead of nitrogen. Experiments carried out with o-xylene with HMOR10 and 45NaHMOR show that the effect of Na is quantitatively the same as in m-xylene transformation. The effect of the substitution of nitrogen for hydrogen on the selectivity to disproportionation and isomerization products was also examined with HMOR10 and a 45NaHMOR (Fig. 12). The activity and selectivity of HMOR10 were found to be quite independent on the diluent, nitrogen or hydrogen. On the other hand, the activity of 45NaHMOR was lower under hydrogen than under nitrogen, the effect of the carrier gas being particularly pronounced for disproportionation (three times lower). Therefore, the use of hydrogen instead of nitrogen is responsible for a part of the effect of Na on D=I observed by Ratnasamy et al. [13]. This effect of hydrogen on the rate of disproportionation suggests an activation of hydrogen over Na cations. This possibility to activate hydrogen has been previously demonstrated by Minachev et al. [31]: Na mordenite samples were shown to be very active for benzene hydrogenation, their activity increasing linearly with the Naþ concentration. No mechanism was proposed for this hydrogenation. The effect of hydrogen activated over Na cations on xylene transformation is similar to the effect of hydrogen
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activated over nickel mordenite [32] which was interpreted by a reaction between activated hydrogen and benzylic carbocations, intermediates in disproportionation:
This reaction leads to a decrease in the concentration of benzylic carbocation intermediates, hence in the rate of disproportionation. The different temperatures used with the mordenite samples in Ref. [13] seem also to be responsible for the larger effect of Na on the D=I ratio. Indeed the higher the temperature the lower the D=I ratio. Thus, at 20% conversion of m-xylene on HMOR10, the D=I ratio is equal to 0.55 at 573 K and to 0.4 at 623 K. Therefore, as the reaction temperature is higher with the less active NaHMOR samples, the effect of the Na content is apparently more pronounced. From this study, it can be concluded that the decrease in the disproportionation/isomerization ratio observed here by exchange of HMOR10 by sodium is due to the modification of the physicochemical properties of mordenite (porosity, acidity) rather than to differences in the operating conditions with the various samples. N2 adsorption shows no change in the adsorption properties. Nor can any change be observed in the para-/ ortho-ratio and in the toluene/TMB ratio, which would indicate modifications of the porosity. Therefore, the decrease in the D=I selectivity caused by Na exchange is most likely related to the decrease which was found in the density and in the strength of the protonic sites. No clear conclusion can be drawn from the literature on the effect of the acid strength on the D=I ratio. Indeed, according to certain authors, disproportionation demands stronger acid sites than isomerization [13,33,34], whereas the reverse is proposed by other authors [35,36]. Furthermore, a positive effect of the density of acid sites (or of their
proximity) can be expected from the reaction mechanisms. Indeed, xylene isomerization occurs through a monomolecular mechanism, xylene disproportionation through a complex bimolecular mechanism involving six successive steps in which two different protonic sites probably participate [28]. Therefore it can be suggested that the decreasing effect of the Na content on the D=I selectivity of mordenites is probably due for a large part to the decrease in the proximity of the protonic sites. The same proposal was recently advanced to explain the decrease in the ratio between the rates of ethylbenzene disproportionation and isomerization caused by dealumination of a HMOR sample [28].
6. Conclusions From this study of the exchange by Na cations of the hydroxyl groups of a HMOR sample with an Si/Al ratio of 10 (HMOR10), the following conclusions can be drawn: 1. Na exchange occurs preferentially with the OH groups of the side pockets. 2. The exchange of the first Na cations causes a significant decrease in the activity of mordenite for m-xylene transformation and a decrease in the turnover frequency of the protonic sites, which suggests the disappearance of the strongest protonic sites. Afterwards, the activity decreases linearly with the percentage of exchange. 3. Sodium exchange of mordenite causes an increase in the isomerization selectivity (decrease in the disproportionation/isomerization ratio), even when the selectivities are compared at the same temperature, at identical conversions and with nitrogen as carrier gas. 4. The substitution of hydrogen for nitrogen as carrier gas has no effect on the activity and selectivity of HMOR10, but decreases significantly the disproportionation activity of a sodium-exchanged sample, thereby improving the selectivity to isomers. This effect of hydrogen could be related to its activation by Na cations.
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Acknowledgements O. Marie and F. Thibault-Starzyk (ISMRA, Caen) are gratefully acknowledged for fruitful discussions. References [1] H.W. Kouwenhoven, in: W.M. Meier, J.B. Uytterhoeven (Eds.), Molecular sieves, Adv. Chem. Ser., vol. 121, American Chemical Society, Washington, 1973, p. 529. [2] A.P. Bolton, in: J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington, 1976, pp. 714–779, Chapter 13. [3] I.E. Maxwell, W.H.J. Stork, in: J.C. Jansen, H. Van Bekkum, E.M. Flanigen (Eds.), Introduction to Zeolite Science and Practice, vol. 58, Elsevier, Amsterdam, 1991, p. 571 (Chapter 15). [4] A. Corma, A. Martinez, in: E.G. Derouane, J. Haber, F. Lemos, F.R. Ribeiro, M. Guisnet (Eds.), Catalytic Activation and Functionalisation of light alkanes, Advances and Challenges, NATO ASI series, vol. 44, Kluwer Academic Publishers, Dordrecht, 1998, p. 35. [5] J.S. Beck, W.O. Haag, in: G. Ertl, H. Kn€ ozinger, J. Weitkamp (Eds.), Handbook of the Heterogeneous Catalysis, vol. 5, VCH, Weinheim, 1997, p. 2123. [6] G.R. Meima, M.J.M. van de Aalst, M.S.U. Samson, J.M. Garces, J.G. Lee, in: R. von Ballmoos, J.B. Higgins, M.M.J. Treacy (Eds.), Proc. 9th Intern. Zeolite Conf., Butterworth, Boston, 1992, p. 327. [7] J.M. Garces, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proc. 12th Intern. Zeolite Conf., Material Research Society, Warrendale, PA, 1998, p. 551. [8] S. Sivasanker, A. Thangaraj, R.A. Abdulla, P. Ratnasamy, in: L. Guczi, F. Solymosi, P. Tetenyi (Eds.), New Frontiers in Catalysis, Proc. 10th Intern. Congress Catal, Part C, Akademiai Kiado, Budapest, 1993, p. 397. [9] J.S. Beck, W.O. Haag, in: G. Ertl, H. Kn€ ozinger, J. Weitkamp (Eds.), Handbook of the Heterogeneous Catalysis, vol. 5, VCH, Weinheim, 1997, p. 2136. [10] M. Guisnet, N.S. Gnep, in: F.R. Ribeiro, A.E. Rodrigues, L.D. Rollmann, C. Naccache (Eds.), Zeolites Science and Technology-NATO ASI Series, Series E, no. 80, Martinus Nijhoff Publishers, The Hague, 1984, p. 571. [11] S.T. Sie, A.F. de Vries, C.M.A.M. Mesters, A.Q.M. Boon, K. Bottenberg, B. Trautrims, Erd€ ol Erdgas Kohle 11 (1996) 463. [12] T. Terlouw, J.P. Gilson, Shell Internationale Research, Maatschappij B.V., EP no. 458 378, 1991.
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Influence of Na exchange on the acidic and catalytic properties of an HMOR zeolite F. Moreau a, P. Ayrault a, N.S. Gnep a, S. Lacombe b, E. Merlen b, M. Guisnet a,* a
UMR CNRS 6503, Catalyse en Chimie Organique, Facult e des Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France b Institut Francßais du P etrole, 1 et 4, avenue de Bois-Pr eau, 92852 Rueil-Malmaison Cedex, France Received 7 May 2001; received in revised form 23 November 2001; accepted 26 November 2001
Abstract IR spectroscopy was used to characterize the hydroxyl groups of a series of NaHMOR samples resulting from sodium exchange of an HMOR sample with an Si/Al ratio of 10. Na exchange causes a significant decrease in the intensity of the band corresponding to bridged OH groups. This band presents two components, the first one at 3608– 3611 cm1 corresponding to bridged OH groups in the large channels, the second one at 3583–3585 cm1 corresponding to bridged OH groups in the side pockets. At low exchange rate, this latter is more affected than the former demonstrating a preferential exchange of the protonic sites of side pockets. Pyridine adsorption shows that part of the protonic sites of the side pockets of HMOR are accessible to pyridine molecules; sodium exchange causes the preferential disappearance of the non-accessible sites of the side pockets. Furthermore, part of the hydroxyl groups of the large channels do not interact with pyridine, suggesting a blockage of their access. Sodium exchange of HMOR causes a decrease in the activity for m-xylene transformation at 623 K, this decrease being initially more significant. Sodium exchange also affects the product distribution, causing an increase in the selectivity to isomers at the expense of disproportionation products. This effect is proposed to be due to the decrease in the proximity of the protonic acid sites. The substitution of nitrogen as carrier gas by hydrogen does neither affect the activity nor the selectivity of the parent HMOR sample, but decreases significantly the disproportionation activity of a sodium-exchanged sample. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mordenites; Sodium exchange; Acidity; m-Xylene; Isomerization; Disproportionation
1. Introduction Mordenite catalysts are used in refining and petrochemical processes such as:
* Corresponding author. Tel.: +33-5-49453905; fax: +33-549453779. E-mail address: [email protected] (M. Guisnet).
• isomerization of light gasoline [1–4] in order to improve the octane number, • selective synthesis of alkylbenzenes such as cumene [5–7] which is used for phenol preparation, or long-chain linear alkylbenzenes [8] which are precursors of biodegradable surfactants, • isomerization of the C8 aromatic cut (xylenes þ ethylbenzene) [9–11] for the production of paraxylene which is the precursor of polyester fibers.
1387-1811/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 4 8 4 - X
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On the bifunctional Pt mordenite catalysts which are used in the latter processes, secondary reactions of disproportionation and transalkylation accompany xylene isomerization and ethylbenzene isomerization into xylenes causing a loss in the production of para-xylene. According to various authors [12–16], partial exchange of the protonic sites of mordenite by sodium cations limits preferentially disproportionation and transalkylation, hence improves significantly the selectivity to xylenes. The influence of Na/H exchange on the acidity of mordenite was determined by various authors [13,16–25] using ammonia and/or pyridine adsorption followed by microcalorimetry, IR spectroscopy, etc. The location of the bridged hydroxyl groups (or protonic acid sites), either in the large channels or in the side pockets, as well as their strength depend on the degree of exchange. At high sodium content, the hydroxyl groups would be essentially located in the large channels; at high degree of exchange of sodium cations by protons, hydroxyl groups located in the side pockets would be mainly formed [16,22,23]. The acid strength of both bridged hydroxyl groups increases with the degree of protonic exchange [23,24]. Unfortunately, both hydroxyl groups appear in only one band. However, deconvolution into two components, one at high frequency (HF) corresponding to OH groups of the large channels, the other at low frequency (LF) corresponding to OH groups of side pockets can be carried out [22,25]. This allows a discrimination between these OH groups and a semiquantitative evaluation of their relative significance. It should be mentioned that most of the studies deal with the protonic exchange of NaMOR samples having no secondary porosity. Therefore, with the obtained NaHMOR samples, pyridine molecules have no access to the OH groups located in the side pockets (LFOH ) hence react only with those of the large channels (HFOH ). In this paper, NaHMOR samples were prepared through exchange by sodium cations of the protons of an HMOR10 zeolite sample containing a small amount of mesopores which should favor the access to the side pockets as well as a small
amount of extra-framework aluminum species [26]. Pyridine adsorption followed by IR spectroscopy and deconvolution of the OH groups were carried out in order to characterize the acidity of the NaHMOR samples. The activity and selectivity for disproportionation and isomerization of m-xylene were also determined, the objective being to specify if the apparent increase in selectivity to isomers caused by sodium exchange was genuine or simply due to differences in reaction temperature, conversion, degree of catalyst deactivation etc. between the NaHMOR samples.
2. Experimental section 2.1. Catalysts The initial H-mordenite sample, HMOR10, (total Si/Al ratio of 10, framework ratio of 12) was provided by ‘‘Institut Francßais du Petrole’’ RueilMalmaison. This sample resulted from exchange with NHþ 4 of a NaMOR sample (HSZ-640 NaA from TOSOH, Amsterdam, The Netherlands) followed by calcination under dry air flow at 550 C. Scanning electron microscopy showed that HMOR10 was mainly constituted of small crystallites (100–200 nm) and of plates of approximately 1 lm, both associated in aggregates of 50–70 lm. Na-exchanged H-mordenites were prepared by ion exchange at room temperature of HMOR10 with NaNO3 solution (100 ml/g of mordenite) with different sodium concentrations. The sodium contents were determined by two ways: elemental analysis of the mordenite samples and from the difference between Naþ concentrations in the solution before and after exchange measured by atomic absorption using a Perkin– Elmer 3300 system. Six NaHMOR samples were prepared with Na exchange degrees of 14%, 28%, 35%, 45%, 50% and 63%, the corresponding samples being denominated 14NaHMOR, 28NaHMOR, 35NaHMOR, 45NaHMOR, 50NaHMOR and 63NaHMOR, respectively. Sorption isotherms for nitrogen at 77 K over HMOR samples were recorded using a gas ad-
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3. Results and discussion
sorption system ASAP 2010 (Micromeritics), the samples being previously pretreated at 653 K for 2 h under vacuum (103 Pa). The acidity of HMOR samples was characterized by pyridine adsorption followed by IR spectroscopy using a Nicolet MAGNA IR 550 spectrometer. Thin wafers of about 8 mg cm2 were pretreated in situ in the IR cell under air flow: 60 ml min1 at 773 K for 12 h and then in vacuum (103 Pa) at 673 K for 1 h. Pyridine was adsorbed on the samples at 423 K. The IR spectra were recorded at room temperature after the activation period and after pyridine thermodesorption in vacuum (103 Pa) at various temperatures: 423, 523, 623, and 723 K. To discriminate between the bridged acidic OH groups in the main channels and side pockets, a procedure of deconvolution of the corresponding band was performed by using the Peak Solve Software (1990) from Jandel Scientific. Extinction coefficients for the high (HF) and low (LF) frequency bands (i.e. 2.5 and 3.8 lmol1 cm for OH(HF) and OH(LF) respectively) were used for quantification. m-Xylene transformation was carried out at 623 K in a flow reactor under atmospheric pressure generally with nitrogen as carrier gas (N2 /m-xylene molar ratio ¼ 15); some experiments were also carried out with hydrogen as carrier gas (H2 /mxylene molar ratio ¼ 15). The reaction products were analyzed on line by gas chromatography (GC). In order to obtain with a good accuracy the value of the activity of the fresh catalysts, the procedure developed by Morin et al. [27] was used.
3.1. Influence of Na exchange on the physicochemical properties of mordenite HMOR10 presents less than 0.01 Na atoms per unit cell, which corresponds to a degree of protonic exchange of approximately 99.8%. Nitrogen adsorption showed the presence of supermicropores (0.8–2 nm) and of mesopores (> 2 nm) in this sample. This suggests that a partial dealumination of the framework probably occurred during the calcination at 550 C, which is confirmed by the presence of a small amount of extra-framework aluminum species [26]. Sodium exchange of HMOR10 does not cause any significant change in the isotherm for nitrogen adsorption, hence on the total pore volume and on the pore size distribution. With all the samples, the pore volume distribution is the following : 78–81% of ultramicropores, 6.5–7% of supermicropores and 12–14% of meso-and macropores (Table 1). Fig. 1 shows that sodium exchange has practically no effect on the silanol groups (band at 3745 cm1 ) and on the OH linked to extra-framework aluminum species (band at 3655 cm1 ) which are present in small amounts in HMOR10:0.35 extraframework aluminum atoms per unit cell compared to 3.7 framework aluminum atoms [28]. On the other hand, sodium exchange causes a significant decrease in the intensity of the band corresponding to bridged OH groups (Fig. 2, curve a) as well as a slight shift to higher frequencies: from 3607 cm1 with HMOR10 to 3611 cm1 with 63NaHMOR. This OH band can be deconvoluted
Table 1 Characteristics of mordenite samples: pore volumes (cm3 g1 ) of HMOR10 and of NaHMOR samples determined by N2 adsorption at , supermicropores: 8–20 A , mesopores: >20 A 77 K; ultramicropores: <8 A Samples HMOR10 14NaHMOR 28NaHMOR 45NaHMOR 63NaHMOR
Pore volume (cm3 g1 )
Concentration of acidic sites
Total
Ultramicropores
Supermicropores
Mesopores
NHþ
NL
0.227 n.d. n.d. 0.216 0.224
0.179 n.d. n.d. 0.168 0.181
0.014 n.d. n.d. 0.015 0.016
0.033 n.d. n.d. 0.032 0.027
2.54 2.01 1.56 1.29 0.93
0.18 0.14 0.17 0.20 0.18
NHþ , NL : number of protonic sites (Hþ ) and of Lewis (L) sites per unit cell able to retain pyridine adsorbed at 423 K.
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Fig. 1. IR spectra of the OH groups in mordenite samples: A: HMOR10; B: 14NaHMOR; C: 45NaHMOR. Spectrum a: activated samples; Spectrum b: after pyridine adsorption at 423 K and removal of physisorbed pyridine: OH not accessible by pyridine molecules or non enough acidic to retain pyridine adsorbed at 423 K (OHNA ), Spectrum c: difference spectra (a–b): OH accessible and acidic (OHA ).
in two bands, the first one at 3608–3611 cm1 corresponding to OH groups of the large channels, the second one at 3583–3585 cm1 corresponding to those located in the side pockets. With HMOR10, the integrated intensities of the LF and HF bands are practically similar (Fig. 2). By using the extinction coefficients previously estimated [29], i.e. 1.8 and 3.8 cm lmol1 respectively, the percentages of OH in the side pockets and in the large channels were found to be 31% and 69%, respectively, which is close to what was obtained by Makarova et al. [22]. The integrated intensity of the HF band (Fig. 2, curve c) decreases quasilinearly with the percentage of sodium exchange,
whereas the introduction of the first sodium cations in HMOR10 causes a very significant decrease in the intensity of the LF band (curve b). This confirms the preferential exchange of the protonic sites located in the side pockets which was proposed by various authors [22,23]. This preferential exchange in the side pockets is also demonstrated by pyridine adsorption experiments. With all these mordenite samples, pyridine adsorption has practically no effect on the bands at 3745 cm1 and at 3655 cm1 , indicating that the corresponding OH groups, i.e., silanols, and OH linked to extra-framework aluminum atoms are not acidic. On the other hand, pyridine causes a
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215
Table 2 OHNA /OHA : Ratio between the integrated intensities of the band corresponding to non-acidic or non-accessible OH groups (OHNA ) and to acidic and accessible ones (OHA ) Samples
OHNA /OHA
OHLF /OHHF
HMOR10 14NaHMOR 28NaHMOR 45NaHMOR 63NaHMOR
0.66 0.32 0.45 0.43 0.80
0.94 0.49 0.35 0.19 0.26
OHLF /OHHF : ratio between the integrated intensities of the band corresponding to the OH groups located in the side pockets (OHLF ) and in the large channels (OHHF ) obtained by deconvolution of the bridged OH bands. Fig. 2. Integrated intensity of the bridged OH band in NaHMOR samples as a function of the percentage of sodium exchange; curve a: all the OH groups; curve b: OH groups of the side pockets; curve c: OH groups of the large channels. The intensities of the bands corresponding to the OH groups of the side pockets and of the large channels were obtained by deconvolution of the IR band corresponding to all the bridged OH groups.
large decrease of the band at 3607–3611 cm1 corresponding to bridged OH groups. However, with all the samples, part of the bridged OH groups are not able to retain pyridine adsorbed at 423 K (Fig. 1, curve b). This could be due to an inaccessibility of these OH groups by pyridine molecules, i.e. due to their location in the side pockets. However, another reason could be that part of the corresponding protonic sites located either in the large channels or in the side pockets are too weak to retain pyridine adsorbed at 423 K. This is certainly the case at high sodium contents. Indeed, for the corresponding samples, the ratio between the integrated intensities of the bands corresponding to non-acidic or non-accessible OH groups (OHNA ) and to the acidic and accessible ones (OHA ) is greater than the LF/HF intensity ratio obtained by deconvolution of the bridged OH bands (Table 2). The change with Na exchange in the intensities of the bands corresponding to all the bridged OH groups (OHT ), to OHNA and OHA is shown in Fig. 3. It should be noticed that, for degrees of Na exchange >28%, only one large band is obtained for OHNA and hydroxyls linked to extra-framework Al species, hence that the values of the intensity of the OHNA band obtained by deconvolution
Fig. 3. Integrated intensity of the bridged OH band in NaHMOR samples as a function of the percentage of sodium exchange: curve a: all OH groups; curve b: OH groups not affected by pyridine adsorption (OHNA ), curve c: OH groups disappearing upon pyridine adsorption (OHA ).
lack precision. The intensity of OHT (curve a) decreases strongly by introduction of the first sodium cations then more weakly and quasi-linearly. Similarly, the intensity of OHNA (curve b) first decreases strongly, then remains quasi-constant for degrees of Na exchange from 14 to 63%. Lastly, the intensity of OHA (curve c) decreases linearly with the degree of sodium exchange. The significant initial decrease of the intensity of the OHNA band confirms that Na exchange occurs preferentially in the side pockets. However, the quasi-constant values of the intensity found for degrees of Na exchange between 14 and 63% is more difficult to explain. The explanation could
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come from the large imprecision in the determination of the band intensity for highly exchanged samples because of the presence of extra-framework aluminum species and of a significant shift of the OHNA band towards high frequencies: from 3592 cm1 with HMOR10 to 3611–3612 cm1 with the Na exchanged samples. According to Datka et al. [23], this shift is due to a significant decreasing effect of Na exchange on the acid strength of the protonic sites located in the side pockets. Another explanation could be that the OHNA band corresponds to OH groups located in the large channels, the access of which by pyridine molecules would be blocked by extra-framework Al species. A deconvolution of the OHNA and OHA bands into HF and LF components (supposed to correspond to bridged OH groups in the large channels and in the side pockets, respectively), was attempted. For OHNA , the deconvolution was only carried out with HMOR10, 14NaHMOR and 28NaHMOR, because with the more highly exchanged samples the imprecision was too large. All the protonic sites of the side pockets which are not accessible by pyridine molecules are immediately exchanged (Fig. 4, curve b). On the other hand, sodium exchange has no significant effect on the number of OHNA groups of the large channels (Fig. 4, curve c), which suggests that the corre-
Fig. 5. Integrated intensity of the bridged OH groups interacting with pyridine at 423 K (OHA ): curve a: all the OHA ; curve b: OHA located in the side pockets (LF), curve c: OH located in the large channels (HF).
sponding acid sites are not accessible by Na cations. Part of the protonic sites of HMOR10 which are located in the side pockets (LF band) seem to be accessible by pyridine molecules (Fig. 5, curve b). This is not surprising, because the HMOR10 sample used in this work has, in addition to ultramicropores, supermicropores and mesopores which are known to facilitate the access of organic molecules to the protonic sites located in the side pockets. A quasi-regular decrease in the LF and HF components of the OHA band is found (Fig. 5, curves b and c), which indicates a similar ease in the exchange of the corresponding protonic sites. However, small differences appear at high percentages of Na exchange, but these differences could come from the imprecision of the deconvolution treatments. When Figs. 4 and 5 are compared, it can be concluded that: (i) the protonic sites located in the side pockets (LF band) are more easily exchanged when they are not accessible by pyridine molecules, (ii) the protonic sites corresponding to OHNA groups of the large channels are not exchanged (Fig. 4) whereas those corresponding to OHA can be exchanged by Na cations.
Fig. 4. Integrated intensity of the bridged OH groups incapable to retain pyridine adsorbed at 423 K (OHNA ): curve a: all the OHNA ; curve b: OHNA located in the side pockets (LF), curve c: OHNA located in the large channels (HF).
The concentrations of Br€ onsted and Lewis acid sites able to retain pyridine at 423 K as pyridinium ions (band at 1545 cm1 ) and as coordinated
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Fig. 6. Theoretical and measured concentrations of protonic sites (CHþ ) as a function of the percentage of sodium exchange.
pyridine (band at 1455 cm1 ) were calculated from the integrated intensities of the corresponding bands. The values of the extinction coefficients (1.15 and 1.28 lmol1 cm respectively) are those previously determined in our laboratory with mordenite samples having practically no Lewis acid sites and with alumina (only Lewis sites), respectively. With HMOR10, approximately 70% of the protonic sites (number estimated from the unit cell formula) are capable of retaining pyridine adsorbed at 423 K; the number of Lewis acid sites is 14 times lower than the number of protonic sites (Table 1). Sodium exchange causes a significant decrease in the protonic acidity, but no change in the Lewis acidity. The decrease in the protonic acidity, very pronounced at low exchange rate (Fig. 6), becomes linear above 14% exchange. This large initial effect of sodium exchange is quite unexpected, since exchange occurs preferentially with protonic sites of side pockets incapable to adsorb pyridine molecules as pyridinium ions. This observation suggests that the strongest protonic sites of the large channels are also exchanged and probably that sodium cations decreases the accessibility of pyridine molecules to the remaining protonic sites [16].
217
p- and o-xylene and disproportionation into toluene and trimethylbenzenes (TMB). A very fast initial deactivation followed by a quasi-plateau in activity is observed as shown for the example of HMOR10 Fig. 7. The activity of the fresh samples was estimated from the initial slope of the curve representing conversion at very short time-on-stream (TOS) versus contact time, taken here as the reverse of the weight hourly space velocity (WHSV), e.g. with HMOR10 and 28NaHMOR samples in Fig. 8. The values of TOS were chosen just long enough to obtain steady state conditions: constant value of the GC peak area, hence of the reactant pressure in the reactor while limiting deactivation [27].
Fig. 7. Transformation of m-xylene over HMOR10 zeolite. Conversion (X) versus TOS for 25, 10 and 5 mg of catalyst.
4. Influence of Na exchange on the activity of mordenite samples With all the mordenite samples, m-xylene undergoes two main reactions: isomerization into
Fig. 8. Transformation of m-xylene over HMOR10 and 28NaHMOR samples. Conversion (X) as a function of contact time (taken as the reverse of the WHSV).
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Fig. 9. Transformation of m-xylene over HMOR10 zeolite samples. Activity as a function of the percentage of sodium exchange.
The change in activity with the percentage of Na exchange (Fig. 9) is qualitatively similar to the change in the protonic acidity (estimated from the intensity of the PyHþ band (Fig. 6)); a very pronounced effect of the exchange of the first sodium cations followed by a linear decrease with the degree of Na exchange. However, the turnover frequency of the protonic sites able to retain pyridine adsorbed as pyridinium ions at 423 K is 1.7 times greater with HMOR10 (2100 h1 ) than with sodium-exchanged catalysts (1150–1270 h1 ). This indicates that the exchange of the first protons of HMOR by sodium cations eliminates, in addition to protonic sites of the side pockets inaccessible to pyridine molecules hence probably inactive, very active sites hence very strong acid sites located in the large channels. For higher exchange rates, the practically constant value of TOF suggests a similar acid strength for the remaining protonic active sites.
5. Influence of Na exchange on the selectivity of mordenite With all the zeolite samples, para- and orthoxylenes are formed in a ratio of approximatively 1.1, which is very close to thermodynamic equilibrium [27]. This ratio is practically independent on conversion and on the degree of deactivation. The value of this ratio is the one found in the
absence of product shape selectivity indicating that even with the most active sample (HMOR10), mxylene isomerization is not limited by intracrystalline mass transfer. The distribution of the TMB resulting from xylene disproportionation depends on conversion but is practically not affected by deactivation. Thus, with HMOR10, the TMB distribution passes from 78% 1,2,4-TMB 10% 1,2,3-TMB and 12% 1,3,5-TMB at zero conversion to 68%/8%/24% at 30% conversion, i.e. close to thermodynamic equilibrium [30]: 65.5%/7%/ 27.5%. The initial distribution is characteristic of an electrophilic substitution (preferential methyl substitution in the activated ortho- and parapositions), the change with conversion is due to secondary isomerization. For identical values of m-xylene conversion, there is also a change in TMB distribution with the exchange of HMOR10 by sodium cations: the percentage of 1,3,5-TMB decreases at the expense of 1,2,4-TMB, which suggests a decrease in the significance of secondary isomerization. The toluene/TMB ratio is practically independent on the conversion and on the sodium content on the fresh samples. The molar ratio, which is higher than the expected value of 1 on the fresh samples, tends to 1 on the deactivated samples. This can be explained by difficulties in the desorption of the bulky molecules of TMB from the mordenite channels. The selectivity of m-xylene transformation into isomers and disproportionation products depends on the conversion and on the degree of deactivation. Thus, on the fresh samples, the disproportionation/isomerization (D=I) ratio decreases with conversion, and the D=I values are lower on the deactivated than on the fresh samples (e.g. HMOR10 in Fig. 10). The D=I ratio depends also on the catalyst: for identical values of conversion on the fresh and as well as on the deactivated sample, this ratio decreases when the Na content increases (Fig. 11). This observation was already made by other authors [13–16], the effect observed here being, however, less significant. The larger apparent effect observed by Bankos et al. [14,15] can be related to an effect of conversion; indeed the experiments were carried out at
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Fig. 10. Transformation of m-xylene over HMOR10 samples. Disproportionation/isomerization rate ratio (D=I) versus mxylene conversion (X) on fresh and on deactivated catalysts; HMOR10 25 mg (D), HMOR10 10 mg (), HMOR10 5 mg ().
Fig. 11. Transformation of m-xylene over HMOR10 and NaHMOR samples. Disproportionation/isomerization rate ratio (D=I) versus m-xylene conversion (X) on fresh and on deactivated catalysts; HMOR10 (), 14NaHMOR (), 28NaHMOR (4), 45NaHMOR (þ), 63NaHMOR ( ).
the same space velocity with all the zeolite samples. Therefore the conversion decreases when the sodium content increases with a concomitant a decrease in the D=I ratio. This is not the case with the experiments reported by Ratnasamy et al. [13] in which the selectivities are compared for identical values of conversion. The difference in the sodium effect on D=I between Ratnasamy et al. [13] and us is most likely related to differences in the operating conditions: o-xylene was used instead of m-xylene, the change in conversion was obtained by chang-
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Fig. 12. Conversion of m-xylene (X) over HMOR10 and 45NaHMOR10 under nitrogen: HMOR10 () 45NaHMOR10 ( ) and under hydrogen: HMOR10 (j) 45NaHMOR ( ) as a function of contact time (taken as the reverse of the WHSV).
ing the temperature instead of contact time and the carrier gas was hydrogen instead of nitrogen. Experiments carried out with o-xylene with HMOR10 and 45NaHMOR show that the effect of Na is quantitatively the same as in m-xylene transformation. The effect of the substitution of nitrogen for hydrogen on the selectivity to disproportionation and isomerization products was also examined with HMOR10 and a 45NaHMOR (Fig. 12). The activity and selectivity of HMOR10 were found to be quite independent on the diluent, nitrogen or hydrogen. On the other hand, the activity of 45NaHMOR was lower under hydrogen than under nitrogen, the effect of the carrier gas being particularly pronounced for disproportionation (three times lower). Therefore, the use of hydrogen instead of nitrogen is responsible for a part of the effect of Na on D=I observed by Ratnasamy et al. [13]. This effect of hydrogen on the rate of disproportionation suggests an activation of hydrogen over Na cations. This possibility to activate hydrogen has been previously demonstrated by Minachev et al. [31]: Na mordenite samples were shown to be very active for benzene hydrogenation, their activity increasing linearly with the Naþ concentration. No mechanism was proposed for this hydrogenation. The effect of hydrogen activated over Na cations on xylene transformation is similar to the effect of hydrogen
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activated over nickel mordenite [32] which was interpreted by a reaction between activated hydrogen and benzylic carbocations, intermediates in disproportionation:
This reaction leads to a decrease in the concentration of benzylic carbocation intermediates, hence in the rate of disproportionation. The different temperatures used with the mordenite samples in Ref. [13] seem also to be responsible for the larger effect of Na on the D=I ratio. Indeed the higher the temperature the lower the D=I ratio. Thus, at 20% conversion of m-xylene on HMOR10, the D=I ratio is equal to 0.55 at 573 K and to 0.4 at 623 K. Therefore, as the reaction temperature is higher with the less active NaHMOR samples, the effect of the Na content is apparently more pronounced. From this study, it can be concluded that the decrease in the disproportionation/isomerization ratio observed here by exchange of HMOR10 by sodium is due to the modification of the physicochemical properties of mordenite (porosity, acidity) rather than to differences in the operating conditions with the various samples. N2 adsorption shows no change in the adsorption properties. Nor can any change be observed in the para-/ ortho-ratio and in the toluene/TMB ratio, which would indicate modifications of the porosity. Therefore, the decrease in the D=I selectivity caused by Na exchange is most likely related to the decrease which was found in the density and in the strength of the protonic sites. No clear conclusion can be drawn from the literature on the effect of the acid strength on the D=I ratio. Indeed, according to certain authors, disproportionation demands stronger acid sites than isomerization [13,33,34], whereas the reverse is proposed by other authors [35,36]. Furthermore, a positive effect of the density of acid sites (or of their
proximity) can be expected from the reaction mechanisms. Indeed, xylene isomerization occurs through a monomolecular mechanism, xylene disproportionation through a complex bimolecular mechanism involving six successive steps in which two different protonic sites probably participate [28]. Therefore it can be suggested that the decreasing effect of the Na content on the D=I selectivity of mordenites is probably due for a large part to the decrease in the proximity of the protonic sites. The same proposal was recently advanced to explain the decrease in the ratio between the rates of ethylbenzene disproportionation and isomerization caused by dealumination of a HMOR sample [28].
6. Conclusions From this study of the exchange by Na cations of the hydroxyl groups of a HMOR sample with an Si/Al ratio of 10 (HMOR10), the following conclusions can be drawn: 1. Na exchange occurs preferentially with the OH groups of the side pockets. 2. The exchange of the first Na cations causes a significant decrease in the activity of mordenite for m-xylene transformation and a decrease in the turnover frequency of the protonic sites, which suggests the disappearance of the strongest protonic sites. Afterwards, the activity decreases linearly with the percentage of exchange. 3. Sodium exchange of mordenite causes an increase in the isomerization selectivity (decrease in the disproportionation/isomerization ratio), even when the selectivities are compared at the same temperature, at identical conversions and with nitrogen as carrier gas. 4. The substitution of hydrogen for nitrogen as carrier gas has no effect on the activity and selectivity of HMOR10, but decreases significantly the disproportionation activity of a sodium-exchanged sample, thereby improving the selectivity to isomers. This effect of hydrogen could be related to its activation by Na cations.
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