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Particular selectivity of m-xylene isomerization over MCM-41 mesoporous aluminosilicates
i
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 159 (1997) 317-331
Particular selectivity of m-xylene isomerization over MCM-41 mesoporous aluminosilicates S. Morin, P. Ayrault, S. E1 Mouahid, N.S. Gnep, M. Guisnet* URA CNRS 350 - Laboratoire de Catalyse en Chimie Organique, Universit~ de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received 21 November 1996; received in revised form 29 January 1997; accepted 30 January 1997
Abstract While over silica alumina and non-shape selective zeolite catalysts such as FAU, m-xylene isomerizes at similar rates into o- and p-xylenes as expected from the classical intramolecular mechanism, over MCM-41 mesoporous aluminosilicates o-xylene is preferably formed (o/p>_2.5). A bimolecular pathway involving m-xylene disproportionation followed by transalkylation between trimethylbenzenes and m-xylene would be responsible for this preferential isomerization into oxylene. In agreement with this proposal the addition of methylcyclohexane to the reactant causes an identical decrease in the rates of isomerization and disproportionation while over silica alumina the xylene isomerization which occurs through the classical intramolecular mechanism is not affected, The particular behaviour of MCM-41 aluminosilicates seems to be due to their unidirectional mesopore structure (shape selectivity effect) rather than their weak acidity. Keywords: rn-Xylene isomerization; Intramolecular mechanisms; Bimolecular mechanisms; MCM41; Shape
selectivity
1. Introduction
Over acid catalysts, xylene isomerization generally occurs through a unimolecular mechanism involving transformation of benzenium ion intermediates through a methyl shift.
* Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0926- 860X(97)00057-4
S. Morin et aL/Applied Catalysis A: General 159 (1997) 317-331
318
CH 3
•
C}_I3+ H+~
H - H*
H CH3
~3
CH3 (1)
As this transformation is the limiting step, no direct isomerization ofp-xylene into o-xylene (or vice versa) can occur in the absence of diffusion limitations, thus m-xylene leads to o- and p-xylenes at approximately similar rates [ 1,2]. With shape selective zeolites, in particular those with average pore size such as HMFI, direct p-xylene-o-xylene isomerization can be observed and m-xylene is preferably transformed into p-xylene, the smallest isomer [3-5]. Another complication appears in the case of large pore zeolites, such as HFAU, due to secondary disproportionation-transalkylation processes [6-8]: with certain zeolite samples, the high rate of transalkylation renders possible, besides the unimolecular isomerization process, another isomerization mechanism involving successive bimolecular reactions: xylene disproportionation followed by transalkylation between the trimethylbenzenes (TMB) and the xylene reactant.
+
~
Cl--I3
~ C H3C
+
Cl'~
CH3
(2)
H3C
1-13 +
~ CI'~
H3 +
CI-~ H3C
(3) Through this bimolecular pathway, there is a direct o-xylene-p-xylene isomerization and a preferential formation of o-xylene from m-xylene: the p/o xylene ratio has been estimated to be equal to 0.275 [8]. However with zeolites, the significance of this bimolecular pathway is limited: 20-30% at the most with HFAU zeolites and practically zero with HBEA, HMOR [7] and HEMT zeolites [9]. In this paper we investigate the transformation of xylenes over the mesoporous crystalline aluminosilicates of the MCM-41 family recently developed by Mobil
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
319
[10,11]. It is well demonstrated that the acidity of these mesoporous aluminosilicates is weaker than that of HFAU zeolites and quite similar to that of amorphous silica alumina [12-15]. Furthermore their activity in various acid catalysed transformations is close to that of silica alumina and much smaller than that of HFAU zeolites [12,14-18]. This is also the case for m-xylene transformation. However, we show here that the isomerization selectivity, hence the isomerization mechanism, is completely different with MCM-41: bimolecular isomerization pathway (reactions (2) and (3)) and with silica alumina the classical unimolecular mechanism.
2. Experimental 2.1. Catalysts Tile aluminosilicate MCM-41 samples with Si/A1 ratios of 10, 30 and I00 were synthesized at the Laboratoire des Mat6riaux Min6raux (LMM), in Mulhouse, following the procedure given in [11] using cetyltrimethylammonium bromide (Fluka) as the template, sodium silicate (Crossfield) and sodium aluminate (Carlo Erba) as the silicon and aluminium sources. In order to eliminate the templating agent, MCM-41 were calcined under dry air at 873 K. Silica alumina (14 wt% alumina) was supplied by Ketjen. Before use, all the samples were calcined in situ under dry air flow at 500°C for 10 h.
2.2. Catalyst characterization X-ray powder diffraction patterns of the MCM-41 samples were carried out with a Siemens D500 diffractometer (40KV, 30mA) using Cu K,~ radiation (A=1.5418 A). The nitrogen adsorption--desorption isotherms were obtained with an ASAP 2010 (Micromeritics) apparatus. Surface areas and average pore sizes were calculated using the BET procedure and the BJH method based on the desorption isotherm, respectively. IR spectra were recorded with a NICOLET MAGNA IR 550 spectrometer using thin wafers of 4-8 mg/cmz activated in situ in the IR cell. The activation conditions were the following: under air flow (1 cm3/s) at 500°C for 16 h and then in vacuum (10 -3 Pa) at 400°C for 1 h. Pyridine was adsorbed on the samples at 150°C. The IR spectra were carried out after thermodesorption in vacuum (10 .3 Pa) at temperatures between 150°C and 500°C.
2.3. Xylene transformation Xylene transformation was carried out in a flow reactor under the following conditions: 623 K, p(xylene)=0.0625 bar, p(N2)=0.9375 bar, WWH (weight of
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S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
reactant per hour and per weight of catalyst)= 1-11 h -1. The procedure developed for limiting catalyst deactivation has been previously described [8]. Analysis of reaction products was performed on line by FID gas chromatography using a 30 m fused silica J&W DB WAX capillary column.
3. Results
3.1. Physicochemical characterization of MCM-41 samples XRD spectra of the three MCM-41 samples are typical of these materials as described by the researchers from Mobil [11]. Their BET surface area was equal to 1350 m2/g for MCM-41 (10), 1020 and 1050 m2/g for MCM-41 (30) and (100), respectively. An example of the adsorption-desorption isotherm (MCM-41 (100) sample) is given in Fig. l(a). The sharp inflection (for P/Po,~0.35) is characteristic of uniform pores. The diameter of the pores (Fig. l(c)) is equal to 28 A. The average pore size of the other samples is slightly different: 34 A for MCM-41 (10) (pore diameters between 25 and 40 A) and 30.5 for MCM-41 (30). Silica alumina has a BET surface area of 365 m~/g. Its isotherm for nitrogen adso~tion presents a hysteresis (Fig. l(b)); the size of the pores is between 30 and 80 A (average pore diameter of approximately 40 ,~). The IR spectra of the MCM-41 samples after pretreatment in vacuum (10-3 Pa) at 400°C shows a very intense and narrow silanol band (at 3745 cm -1) and a very small band at 3606 cm -1 (more than 100 times smaller than the silanol band) indicating the presence of acidic SiOHA1 hydroxyls [12]. The greater the Si/A1 ratio of the sample the more intense the silanol band and the smaller the band at 3606 cm -~ (Fig. 2). With silica alumina only the band at 3475 cm -1 can be observed (Fig. 2). The sorption of pyridine over the MCM-41 samples results in the disappearance of the band at 3606 cm -1 and in a small decrease in the silanol band. The silanol band is practically restored after a desorption treatment at 200°C while the band at 3606 cm -1 is not restored even after desorption at 500°C. Fig. 3 shows the infrared spectra of pyridine in the 1650--1440 cm -1 region following its adsorption at 150°C and subsequent desorption treatment in vacuum (10 -3 Pa) at temperatures between 150°C and 500°C. The expected bands due to Lewis-bound pyridine (1456, 1578 and 1624 cm-1), pyridine bound with Br6nsted acid sites (1546 and 1639 cm -1) and with both Lewis and Br6nsted acid sites (1492 cm -1) are observed with all the samples. Shoulders at approximately 1450 and 1600cm -1 which correspond to hydrogen-bonded pyridine can also be observed at a desorption temperature of 150°C. Finally bands are also found at 1462 and 1496cm -1 with silica alumina as well as with the MCM-41 samples. These bands have already been observed with dealuminated mazzite samples [ 19].
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
321
0.90 0.4E
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0.30
.
.
.
.
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Relative Pressure (P/Po)
0.16
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(P/;',,)
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~
-
,
;.,,,~
100
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' ' Pore Diameter (A)
*
0.00010
4O
100 Pore Diameter (A)
Fig. 1. Isotherm of nitrogen adsorption at 77 K for the MCM-41 (100) sample (a) and for silica alumina (b). Distribution of mesopores given by the BJH method (desorption step); dV/dD (cm3/g ~,) versus the pore diameter D (A) for MCM-41 (100) (e) and for silica alumina (d).
The concentrations of Br6nsted and Lewis acid sites were determined in quantitative IR studies of pyridine sorption. They were calculated from the integrated intensities of pyridinium ions (1546 cm- l) and of Lewis-bound pyridine (1456cm -1) and from the extinction coefficients of these bands (1.13 and 1.28 ~tmol/cm, respectively) determined in a previous study [20]. The densities of Br6nsted and Lewis acid sites of the three MCM-41 samples and of silica alumina are plotted in Fig. 4 as a function of the temperature of desorption. The total number of protonic sites (estimated after desorption at 150°C) of MCM-41 (10) is similar to that of the silica alumina sample and approximately 2.5 times greater than those of MCM-41 (30) and MCM-41 (100). However, silica alumina has stronger protonic sites than the MCM-41 samples: some pyridine remains adsorbed at 350°C over silica alumina but not over the MCM-41 samples.
322
S. Morin et aL /Applied Catalysis A: General 159 (1997) 317-331
a
b 0.5
0.003
100 Silica alumina
3O
10 30
\ lo
'60
3740
3720
Wavenumbers (cm-1)
:
3615
3610 3805 3600 Wavenumbers (crn- 1)
Fig. 2. IR spectra of hydroxyl groups of MCM-41 samples (normalized arbitrary units of absorbance) (a) silanol band (b) band at 3606 cm -l.
Similar remarks can be made concerning the Lewis acidity of the MCM-41 samples and of silica alumina (Fig. 4): quasi identical densities of Lewis acid sites in MCM-41 (10) and in silica alumina, the densities in MCM-41 (30) and (100) being about 2 times lower; greater strength of the acid sites of silica alumina. With the three MCM-41 samples and with silica alumina, the ratio between the densities of the Brtnsted and of the Lewis acid sites (B/L) is very small (0.250.35). Quasi constant values of B/L have already been found by Mokaya et al. [15] for MCM-41 samples with different Si/A1 ratios. However, our values are smaller because of the higher desorption temperature (150°C instead of 100°C).
3.2. Xylene transformation With all the catalyst samples (MCM-41 and silica alumina) xylenes are transformed by isomerization and disproportionation. Toluene and trimethylbenzenes (in proportion close to that of thermodynamic equilibrium) are formed in quasi equimolar amounts. Fig. 5 shows the change with time-on-stream in the activity of silica alumina and of the MCM-41 samples for m-xylene transformation. In the experiments, the amount of catalyst was chosen so as to obtain quasi identical initial values of m-xylene conversion, which allows direct comparison of the catalyst stability: MCM-41 (100)>MCM-41 (30)>MCM-41 (10)~silica alumina. To obtain with a good accuracy the initial activities and to determine the selectivity of the fresh catalysts (with no coke deposit) the procedure described
S. Morin et al,/Applied Catalysis A: General 159 (1997) 317-331
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MCM41 (10)
323
MCM41 (30)
0.051 L ]
A
H
L
°'-IA
B
8
LA.___
165o
16oo 1~o ~oo Wavenumbers (crn-1)
MCM41 (100)
145q J
I I
A. 8
1650
_
1600 1550 1500 Wavenumbers(era-.1)
Silica alumina
L
145C
L
I
5 1650
1600
1550
1500
Wavenumbers (cm-1)
145(
1650
1600
1550
Wavenumbers
1500 (era-l)
145
Fig, 3. IR spectra of pyridine adsorbed on MCM-41 samples with various bulk Si/A1 ratios and on silica alumina following desorption treatment at (1) 150°C (2) 250°C (3) 350°C (4) 450°C (5) 500°C. H denotes hydrogenbonded pyridine, B, Br~nsted-bound pyridine; L, Lewis-bound pyridine and I, iminium ions.
in [8] was applied. Short time-on-stream values were used and the catalysts were regenerated under dry air flow at 500°C for 4 h after each experiment. This regeneration treatment allows one to keep a constant activity value for at least 10 experiments. The analysis of the effluents was carried out for a long enough timeon-stream (15 to 65 s depending on the reactant flow rate) to obtain steady state conditions [8]. Whatever the xylene reactant the lower the Si/A1 ratio of the MCM-
324
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
Br6nsted acidity (micromoles g-l) 60
•
a
\
50
I" MoM,I10/
•\
• MCM41 (30) / " MCM41 (100)
~
40 "7,
O1
e
=o
30
E
20
10
0 100
200
300
400
500
Desorption Temperature (°C) Lewis acidity (micromoles g-l) 250 •
b
•MCM41(10) ] • MCM41(30) / x MCM41 (100) • Silica alumln~
\ \
2OO
N
150
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•
.~ 100
0
100
,
,
150
200
~
250
300
350
'
400
450
500
Desorption Temperature (°C) Fig. 4. Concentrations of Br6nsted (a) and Lewis (b) sites of MCM-4! and silica alumina samples on wbic| pyridine remains adsorbed as a function of the temperature of desorption treatment.
s. Morin et al./Applied Catalysis A: General 159 (1997) 317-331 1.2
325
a
' II MCM-41(lO) 1 • MCM-41 (30)
[XMCM-41(lO0) 0.6
x q
0
0
50
100
150
200
TOS (mn)
b 4
\
..C 0
E 3 E >
2 0
I-
1
0
50
1O0
150
200
T O S (mn)
Fig. 5. Activities of MCM-41 samples (a) and of silica alumina (b) for m-xylene transformation versus time-onstream TOS.
41 sample the greater the initial activity. Thus, in m-xylene transformation the initial activity of MCM-41 (10) is approximately 2 times greater than that of MCM-41 (30) and 4 times that of MCM-41 (100). However, MCM-41 (10) is approximately 4 times less active than silica alumina (Fig. 5). The selectivity of m-xylene transformation depends on the Si/AI ratio of the MCM-41 samples. The greater the Si/A1 ratio the lower the disproportionation/
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
326
Table 1 Selectivity of m-xylene transformation (values extrapolated at zero conversion) MCM-41
D/I p/o
(lo)
(30)
(lOO)
0.5 0.4
0.4 0.2
0.3 0.15
SA
HFAU (10)
0.2 1.4
0.8 1.1
1.2 1 0.8
O
0.6 0.4
• MCM41(30) 1 • SilicaAlumina
0.2 0
'r
0
-1-
5
10
15
%MCH
0.8
O
a 13
0.6 0.4 0.2 b 0
t
I
5
10
15
%MCH Fig. 6. Influence of methylcyclohexane on the rates of m-xylene isomerization (a) and disproportionation (b). I and Io: rates of isomerization in the presence and in the absence of methylcyclohexane; D and Do rates of disproportionation in the presence and in the absence of methylcyclohexane.
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
327
isomerization (D/I) and the para/ortho xylene ratios (Table 1). With silica alumina the D//ratio is lower than that found for the MCM-41 samples. However, the para/ ortho ratio is higher. In order to discuss the eventual participation of disproportionation and transalkylation reactions in xylene isomerization over MCM-41 and silica alumina samples, the effect of the addition of methylcyclohexane to the reactant on the rate and on the selectivity of m-xylene transformation was determined. Indeed, it has been demonstrated that adding isoalkanes to xylenes allows the reduction of the disproportionation rate without affecting the unimolecular isomerization [8,21]. Fig. 6 shows that with MCM-41 (30) methylcyclohexane decreases both the rates of m-xylene isomerization (I) and disproportionation (D). Moreover, it has the same reducing effect (i) on those rates and (ii) on the rates of p- and o-xylene formation: no change can be observed in the D/I and p/o ratios. This is quite different from what is observed with silica alumina: larger reducing effect on the rate of disproportionation and no effect on the rate of isomerization (and on the p/o ratio) with consequently a large decrease in the D/I ratio.
4. Discussion
4.1. Acidic, properties In agreement with the literature, we show here that the acidity of the MCM-41 samples is similar to that of amorphous silica alumina. All these materials possess weak and middle strength Brrnsted acid sites and strong Lewis acid sites (able to retain pyridine adsorbed above 500°C); the ratios between the densities of the Brrnsted and of the Lewis acid sites (B/L) are quite comparable (0.25-0.35). These acidity characteristics are very different from those of HFAU zeolites [9]. Thus for a Si/AI ratio of 10, the Brrnsted acid sites density of a HFAU zeolite is 10 times greater than that of the MCM-41 sample, while the Lewis acid sites densities are similar. B/L is therefore much higher in HFAU (10) (about 3 times) than that in MCM-41 (10). Moreover, the protonic acid sites of HFAU (10) are stronger than those of HMCM-41 (10). Indeed they are able to retain pyridine adsorbed above 450-500°C while those of HMCM-41 (10) can retain pyridine adsorbed up to 300°C only. It should be emphasized that the protonic acid sites of silica alumina are slightly stronger than those of the MCM-41 samples (Fig. 4). With MCM-41 and silica alumina samples bands at 1462 and 1496 cm -1 appear at high desorption temperatures in the spectra of pyridine adsorbed. The band at 1462 cm-1 has also been observed with B-MCM-41 [22], the band at 1496 cm - l is also present (Fig. 6(A), [22]) but not shown. The origin of these bands remains debated. Thus, the band at 1462 cm-1 in B-MCM-41 has been assigned to strong Lewis acid sites generated by the polarization of hydroxyl nests and/or of electrophilic B atoms [22]. The
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S. Morin et aL /Applied Catalysis A: General 159 (1997) 317-331
bands at 1462 and 1496 cm -1 found with dealuminated mazzites have been attributed to iminium ions formed on strong Lewis/Br6nsted pairs of sites (superacid sites) [19]. In agreement with this latter proposal the corresponding bands remain present even after a desorption treatment at 500°C (Fig. 3). The change in acidity of MCM-41 samples with the bulk Si/A1 ratio is similar to the one found in the literature [15]: a decrease in the densities of Br6nsted and Lewis acid sites with increasing Si/A1 ratio, i.e. with a decrease in the amount of aluminium; the B/L ratio is practically independent of the Si/A1 ratio. According to Klinowski et al. [15], this indicates that the proportion of framework aluminium removed during the calcination treatment at 600°C does not change with the total amount of aluminium. However, as emphasized by one of the referees, this statement is quite questionable. Indeed the concentration of Lewis acid sites is not proportional to the amount of A1 removed; it also depends on the nature (cationic or neutral) and the dispersion of extra framework A1 (EFAL) species. Obviously as the Si/A1 ratio increases (hence the framework A1 density decreases) the dispersion of EFAL species should increase. The proportion of cationic EFAL species (which neutralize the Br6nsted acid sites) can also be different with the three MCM-41 samples. Furthermore, if it is assumed that the concentration of Br6nsted acid sites on which pyridine remains adsorbed above 150°C (Fig. 4) is proportional to the concentration of framework aluminium, the Alfrarnework/Altota I ratio values found for the MCM-41 samples are practically identical for MCM-41 (10) and (30) but 3 times greater for MCM-41 (100). Hence, as is the case with zeolites, the higher the Si/A1 ratio the greater the difficulty in the dealumination of the mesoporous aluminosilicates framework.
4.2. Catalytic properties In agreement with the higher strength of its protonic sites (which are the active ones [23]), silica alumina is more active than the MCM-41 samples for m-xylene transformation. Furthermore the greater the Si/A1 ratio of the MCM-41 samples the lower their activity. The activity of silica alumina and of MCM-41 samples is much lower than that of HFAU zeolites. Thus m-xylene is transformed 500 times more slowly over MCM-41 (10) than over HFAU (10). This can be related both to the lower density of the protonic acid sites of the MCM-41 samples and also to their lower strength. Similar observations have been made for the transformation of simple hydrocarbons [ 12,14-18]. Differences exist between the selectivities of silica alumina, MCM-41 and HFAU samples. Thus, the disproportionation/isomerization rate ratio (D/I) is lower with silica alumina and greater with HFAU zeolites than with the MCM-41 samples. With these samples the greater the Si/A1 ratio the lower D/I (Table 1). However, the main difference in selectivity concerns the p/o ratio, which is much lower with the MCM-41 samples than with silica alumina and HFAU zeolites
S. Morin et al. /Applied Catalysis A: General 159 (1997) 317-331
329
(Table 1). The value of p/o found with silica alumina is slightly higher than that attributed to a purely intramolecular mechanism of m-xylene isomerization over HFAU zeolites (1.18). Values of 1.1 and 1.0 have been found with HFAU (10) and (30) indicating the participation in m-xylene isomerization of a bimolecular pathway in addition to the intramolecular one [8]. Furthermore from a simple model it has been estimated that the p/o selectivity of a bimolecular pathway (through reactions (2) and (3)) was equal to 0.275 [8]. As the values obtained with the MCM-41 samples were close to this value it can be proposed that over these mesoporous silicoaluminates m-xylene isomerizes mainly through the bimolecular pathway. In agreement with this proposal a direct isomerization of o-xylene into pxylene (and vice versa) can be observed. Indeed, an initial m-/p-xylene ratio of 12 is obtained with MCM-41 (30) while with the HFAU zeolite (HFAU (20)) for which 20% of the isomerization occurs through the bimolecular pathway, a value of 25 is found. With silica alumina the m/p ratio becomes very high at low conversion, which shows that, in agreement with a purely unimolecular isomerization mechanism, no direct transformation of o-xylene into p-xylene occurs. The existence of different mechanisms for xylene isomerization over silica alumina (and HFAU zeolites) on the one hand and over MCM-41 samples on the other is confirmed by the very different effect which methylcyclohexane has on the transformation of m-xylene on these catalysts. With silica alumina, methylcyclohexane decreases very much the rate of m-xylene disproportionation but does not change the rate of m-xylene isomerization. This confirms that over this catalyst mxylene isomerization does not involve disproportionation or transalkylation steps whose rates are affected by methylcyclohexane [8,21], thus occurs through the classical unimolecular pathway. With MCM-41 (30) methylcyclohexane decreases to the same extent both the rates of m-xylene disproportionation and isomerization. This decreasing effect of methylcyclohexane on the rate of xylene isomerization is a strong argument in favour of a bimolecular pathway involving successive disproportionation and transalkylation steps (reactions (2) and (3)). The identical effect of methylcyclohexane on the rates of m-xylene isomerization and disproportionation is in favour of a quasi pure bimolecular isomerization pathway. This particular behaviour of the MCM-41 samples cannot be attributed to the characteristics of their acidity: weak protonic acidity, presence of a large number of strong Lewis acid sites and maybe a very small number of superacid sites (evidenced by iminium ions in the spectrum of adsorbed pyridine). Indeed, with silica alumina which presents the same acidity characteristics, no bimolecular isomerization of xylene can be observed. This behaviour is most likely due to the presence of uniform non-interconnected pores in the mesopore range in MCM-41. The mesopores of approximately 30 ,~ in diameter would favour the bimolecular mechanism of xylene isomerization, xylene molecules undergoing successive reactions of disproportionation and transalkylation when passing through the mesopores (shape selectivity effect). With this bimolecular pathway, different values of D/I can be obtained. Indeed trimethylbenzenes (TMB) resulting from
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S. Morin et al. /Applied Catalysis A: General 159 (1997) 317-331
disproportionation (reaction (2)) can undergo during their passage through the noninterconnected channels, one or several successive reactions of transalkylation (known to be faster than disproportionation [8]) with production of xylene (X) isomers.
2 m-X
~
T +
m p
X (4)
m-X
Therefore the differences between the D/I values of the MCM-41 samples would be due to differences in the number of transalkylation steps undergone by trimethylbenzene molecules. Thus the increase in the D/I ratio with the decrease of the Si/AI ratio of the MCM-41 samples would be related to a decrease in the number of transalkylation steps. This decrease could be due to a decrease in the length of the channels caused by the degradation of the framework during the thermal treatment. Obviously, the lower the Si/A1 ratio, hence the greater the concentration of AI atoms, the more significant the probability of defects, hence the shorter the diffusion path of organic molecules before their desorption. Besides this direct effect of the aluminium concentration there is most likely an indirect effect due to the increase in the percentage of dealumination with the aluminium concentration which was found. It should be emphasized that the D/I value does not have the same meaning in the case of silica alumina and HFAU samples as in the case of MCM-41 samples. With the former catalysts, isomerization is only (silica alumina) or mainly (HFAU) monomolecular, hence D/I is the ratio between the rates of bimolecular and monomolecular reactions while in the case of the MCM-41 catalysts, D/I depends on the number of transalkylation steps involved in reaction (4).
5. Conclusion The acidity characteristics of MCM-41 samples and of an amorphous silica alumina are very similar in what concerns the densities of protonic and Lewis acid sites and their strength. In particular the protonic acid sites are weaker and are in smaller amount than the Lewis acid sites. The activities for m-xylene transformation and the disproportionation/isomerization rate ratios are not very different with MCM-41 samples and with silica alumina. However, there exists a large difference in the relative rates of formation of the para and of the ortho isomer: p/o is much lower with MCM-41 (0.15 to 0.4) than with silica alumina (1.4), which indicates that the isomerization mechanisms are different: classical intramolecular mechan-
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
331
ism over silica alumina and bimolecular mechanism through successive disproportionation and transalkylation steps over MCM-41. The effect of adding methylcyclohexane to m-xylene on the rates of isomerization and disproportionation is the one expected from these reaction mechanisms: with MCM-41 an identical inhibiting effect for isomerization and disproportionation was found whereas with silica alumina there was an inhibiting effect for disproportionation but not for isomerization. The particular selectivity of MCM-41 samples is most likely due to the presence of regular non-interconnected long channels in which xylene molecules undergo, before desorption, successive reactions of disproportionation and transalkylation. This shape selectivity cannot be observed in the narrow pores of unidirectional zeolites, such as mordenites, because of steric constraints limiting the formation of the bulky intermediates of disproportionation and transalkylation reactions.
References [1] A. Cortes and A. Corma, J. Catal, 51 (1978) 338. [2] 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, vol. 80, Martinus Nijhoff, The Hague, 1984, p. 571. [3] N.S. Gnep, J. Tejada and M. Guisnet, Bull. Soc. Chim. Fr. (1982) 5. 14] J. Martens, J. Perez-Pariente, E. Sastre, A. Corma and P.A. Jacobs, Appl. Catal. 45 (1988) 85. 15] N.Y. Chen, W.E. Garwood, EG. Dwyer, Shape Selective Catalysis in Industrial Applications, Chemical Industries, vol. 36, Marcel Dekker, New York and Basel, 1989, p. 65. [6] M.A. Lanewala and A.P. Bolton, J. Org. Chem. 34 (1969) 3107. [7] A. Corma and E. Sastre, J. Catal. 129 (1991) 177. [8] S. Morin, N.S. Gnep and M. Guisnet, J. Catal. 159 (1996) 296. [9] S. Morin, unpublished results. [10] C.T. Kresge, M.E. Leouowisz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. [11] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, Chu C.T,-W, D.H. Olson, E.W. Sheppard, S.B. Mc Cullen, J.B. Higgins and J.E. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [12] A. Corma, V. Fornes, M.T. Navarro and J. Perez-Pariente, J. Catal. 148 (1994) 569. [13] A. Corma, A. Martinez, V. Martinez-Soria and J.B. Monton, J. Catal. 153 (1995) 25. [14] J.M. Dominguez, E Hernandez, E. TorTes, A. Toledo, J. Navarrette and M.L. Occelli, Preprints Symposia 21 h h National ACS Meeting 41 (1996) 415. [15] R. Mokaya, W. Jones, Z. Luan, M.D. Alba and J. Klinowski, Catal. Lett. 37 (1996) 113. [16] J.B. Kim and T. lnui, Catal. Lett. 36 (1996) 255. [17] R.B. Borade and A. Clearfield, Catal. Lett. 31 (1995) 267. [18] X.S. Zhao, G.Q. Lu, G.J. Millar and X.S. Li, Catal. Lett. 38 (1996) 33. [19] B.H. Chiche, E Fajula and E. Garrone, J. Catal, 146 (1994) 460. [20] M. Guisnet, P. Ayrault, J. Datka, submitted for publication. [21] N.S. Gnep and M. Guisnet, React. Kinet. Catal. Lett. 22 (1983) 237. [22] D. Trong On, P.N. Joshi, G. Lemay and S. Kaliaguine, Stud. Surf. Sci. Catal. 97 (1995) 543. [23] J.W. Ward and R.C. Hansford, J. Catal. 13 11969) 154.
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 159 (1997) 317-331
Particular selectivity of m-xylene isomerization over MCM-41 mesoporous aluminosilicates S. Morin, P. Ayrault, S. E1 Mouahid, N.S. Gnep, M. Guisnet* URA CNRS 350 - Laboratoire de Catalyse en Chimie Organique, Universit~ de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received 21 November 1996; received in revised form 29 January 1997; accepted 30 January 1997
Abstract While over silica alumina and non-shape selective zeolite catalysts such as FAU, m-xylene isomerizes at similar rates into o- and p-xylenes as expected from the classical intramolecular mechanism, over MCM-41 mesoporous aluminosilicates o-xylene is preferably formed (o/p>_2.5). A bimolecular pathway involving m-xylene disproportionation followed by transalkylation between trimethylbenzenes and m-xylene would be responsible for this preferential isomerization into oxylene. In agreement with this proposal the addition of methylcyclohexane to the reactant causes an identical decrease in the rates of isomerization and disproportionation while over silica alumina the xylene isomerization which occurs through the classical intramolecular mechanism is not affected, The particular behaviour of MCM-41 aluminosilicates seems to be due to their unidirectional mesopore structure (shape selectivity effect) rather than their weak acidity. Keywords: rn-Xylene isomerization; Intramolecular mechanisms; Bimolecular mechanisms; MCM41; Shape
selectivity
1. Introduction
Over acid catalysts, xylene isomerization generally occurs through a unimolecular mechanism involving transformation of benzenium ion intermediates through a methyl shift.
* Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0926- 860X(97)00057-4
S. Morin et aL/Applied Catalysis A: General 159 (1997) 317-331
318
CH 3
•
C}_I3+ H+~
H - H*
H CH3
~3
CH3 (1)
As this transformation is the limiting step, no direct isomerization ofp-xylene into o-xylene (or vice versa) can occur in the absence of diffusion limitations, thus m-xylene leads to o- and p-xylenes at approximately similar rates [ 1,2]. With shape selective zeolites, in particular those with average pore size such as HMFI, direct p-xylene-o-xylene isomerization can be observed and m-xylene is preferably transformed into p-xylene, the smallest isomer [3-5]. Another complication appears in the case of large pore zeolites, such as HFAU, due to secondary disproportionation-transalkylation processes [6-8]: with certain zeolite samples, the high rate of transalkylation renders possible, besides the unimolecular isomerization process, another isomerization mechanism involving successive bimolecular reactions: xylene disproportionation followed by transalkylation between the trimethylbenzenes (TMB) and the xylene reactant.
+
~
Cl--I3
~ C H3C
+
Cl'~
CH3
(2)
H3C
1-13 +
~ CI'~
H3 +
CI-~ H3C
(3) Through this bimolecular pathway, there is a direct o-xylene-p-xylene isomerization and a preferential formation of o-xylene from m-xylene: the p/o xylene ratio has been estimated to be equal to 0.275 [8]. However with zeolites, the significance of this bimolecular pathway is limited: 20-30% at the most with HFAU zeolites and practically zero with HBEA, HMOR [7] and HEMT zeolites [9]. In this paper we investigate the transformation of xylenes over the mesoporous crystalline aluminosilicates of the MCM-41 family recently developed by Mobil
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
319
[10,11]. It is well demonstrated that the acidity of these mesoporous aluminosilicates is weaker than that of HFAU zeolites and quite similar to that of amorphous silica alumina [12-15]. Furthermore their activity in various acid catalysed transformations is close to that of silica alumina and much smaller than that of HFAU zeolites [12,14-18]. This is also the case for m-xylene transformation. However, we show here that the isomerization selectivity, hence the isomerization mechanism, is completely different with MCM-41: bimolecular isomerization pathway (reactions (2) and (3)) and with silica alumina the classical unimolecular mechanism.
2. Experimental 2.1. Catalysts Tile aluminosilicate MCM-41 samples with Si/A1 ratios of 10, 30 and I00 were synthesized at the Laboratoire des Mat6riaux Min6raux (LMM), in Mulhouse, following the procedure given in [11] using cetyltrimethylammonium bromide (Fluka) as the template, sodium silicate (Crossfield) and sodium aluminate (Carlo Erba) as the silicon and aluminium sources. In order to eliminate the templating agent, MCM-41 were calcined under dry air at 873 K. Silica alumina (14 wt% alumina) was supplied by Ketjen. Before use, all the samples were calcined in situ under dry air flow at 500°C for 10 h.
2.2. Catalyst characterization X-ray powder diffraction patterns of the MCM-41 samples were carried out with a Siemens D500 diffractometer (40KV, 30mA) using Cu K,~ radiation (A=1.5418 A). The nitrogen adsorption--desorption isotherms were obtained with an ASAP 2010 (Micromeritics) apparatus. Surface areas and average pore sizes were calculated using the BET procedure and the BJH method based on the desorption isotherm, respectively. IR spectra were recorded with a NICOLET MAGNA IR 550 spectrometer using thin wafers of 4-8 mg/cmz activated in situ in the IR cell. The activation conditions were the following: under air flow (1 cm3/s) at 500°C for 16 h and then in vacuum (10 -3 Pa) at 400°C for 1 h. Pyridine was adsorbed on the samples at 150°C. The IR spectra were carried out after thermodesorption in vacuum (10 .3 Pa) at temperatures between 150°C and 500°C.
2.3. Xylene transformation Xylene transformation was carried out in a flow reactor under the following conditions: 623 K, p(xylene)=0.0625 bar, p(N2)=0.9375 bar, WWH (weight of
320
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
reactant per hour and per weight of catalyst)= 1-11 h -1. The procedure developed for limiting catalyst deactivation has been previously described [8]. Analysis of reaction products was performed on line by FID gas chromatography using a 30 m fused silica J&W DB WAX capillary column.
3. Results
3.1. Physicochemical characterization of MCM-41 samples XRD spectra of the three MCM-41 samples are typical of these materials as described by the researchers from Mobil [11]. Their BET surface area was equal to 1350 m2/g for MCM-41 (10), 1020 and 1050 m2/g for MCM-41 (30) and (100), respectively. An example of the adsorption-desorption isotherm (MCM-41 (100) sample) is given in Fig. l(a). The sharp inflection (for P/Po,~0.35) is characteristic of uniform pores. The diameter of the pores (Fig. l(c)) is equal to 28 A. The average pore size of the other samples is slightly different: 34 A for MCM-41 (10) (pore diameters between 25 and 40 A) and 30.5 for MCM-41 (30). Silica alumina has a BET surface area of 365 m~/g. Its isotherm for nitrogen adso~tion presents a hysteresis (Fig. l(b)); the size of the pores is between 30 and 80 A (average pore diameter of approximately 40 ,~). The IR spectra of the MCM-41 samples after pretreatment in vacuum (10-3 Pa) at 400°C shows a very intense and narrow silanol band (at 3745 cm -1) and a very small band at 3606 cm -1 (more than 100 times smaller than the silanol band) indicating the presence of acidic SiOHA1 hydroxyls [12]. The greater the Si/A1 ratio of the sample the more intense the silanol band and the smaller the band at 3606 cm -~ (Fig. 2). With silica alumina only the band at 3475 cm -1 can be observed (Fig. 2). The sorption of pyridine over the MCM-41 samples results in the disappearance of the band at 3606 cm -1 and in a small decrease in the silanol band. The silanol band is practically restored after a desorption treatment at 200°C while the band at 3606 cm -1 is not restored even after desorption at 500°C. Fig. 3 shows the infrared spectra of pyridine in the 1650--1440 cm -1 region following its adsorption at 150°C and subsequent desorption treatment in vacuum (10 -3 Pa) at temperatures between 150°C and 500°C. The expected bands due to Lewis-bound pyridine (1456, 1578 and 1624 cm-1), pyridine bound with Br6nsted acid sites (1546 and 1639 cm -1) and with both Lewis and Br6nsted acid sites (1492 cm -1) are observed with all the samples. Shoulders at approximately 1450 and 1600cm -1 which correspond to hydrogen-bonded pyridine can also be observed at a desorption temperature of 150°C. Finally bands are also found at 1462 and 1496cm -1 with silica alumina as well as with the MCM-41 samples. These bands have already been observed with dealuminated mazzite samples [ 19].
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
321
0.90 0.4E
0.75 03
~
u
E U
0.60
0.3C
0.45 w, ,
0.30
.
.
.
.
0.15
%
0.]5
>
0
57.5
0
~rT
0.0
Relative Pressure (P/Po)
0.16
- -
--i--
- - - 7
k~,~ative Pressure
(P/;',,)
0.016
,
i 0.12
i i
i ~E
0.012
. . . . .
,
....
.
! i
,
.
.
.
-
,
i
O.O8
0.008
0.04
~. 0.004
2
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o
.
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~
-
,
;.,,,~
100
......
' ' Pore Diameter (A)
*
0.00010
4O
100 Pore Diameter (A)
Fig. 1. Isotherm of nitrogen adsorption at 77 K for the MCM-41 (100) sample (a) and for silica alumina (b). Distribution of mesopores given by the BJH method (desorption step); dV/dD (cm3/g ~,) versus the pore diameter D (A) for MCM-41 (100) (e) and for silica alumina (d).
The concentrations of Br6nsted and Lewis acid sites were determined in quantitative IR studies of pyridine sorption. They were calculated from the integrated intensities of pyridinium ions (1546 cm- l) and of Lewis-bound pyridine (1456cm -1) and from the extinction coefficients of these bands (1.13 and 1.28 ~tmol/cm, respectively) determined in a previous study [20]. The densities of Br6nsted and Lewis acid sites of the three MCM-41 samples and of silica alumina are plotted in Fig. 4 as a function of the temperature of desorption. The total number of protonic sites (estimated after desorption at 150°C) of MCM-41 (10) is similar to that of the silica alumina sample and approximately 2.5 times greater than those of MCM-41 (30) and MCM-41 (100). However, silica alumina has stronger protonic sites than the MCM-41 samples: some pyridine remains adsorbed at 350°C over silica alumina but not over the MCM-41 samples.
322
S. Morin et aL /Applied Catalysis A: General 159 (1997) 317-331
a
b 0.5
0.003
100 Silica alumina
3O
10 30
\ lo
'60
3740
3720
Wavenumbers (cm-1)
:
3615
3610 3805 3600 Wavenumbers (crn- 1)
Fig. 2. IR spectra of hydroxyl groups of MCM-41 samples (normalized arbitrary units of absorbance) (a) silanol band (b) band at 3606 cm -l.
Similar remarks can be made concerning the Lewis acidity of the MCM-41 samples and of silica alumina (Fig. 4): quasi identical densities of Lewis acid sites in MCM-41 (10) and in silica alumina, the densities in MCM-41 (30) and (100) being about 2 times lower; greater strength of the acid sites of silica alumina. With the three MCM-41 samples and with silica alumina, the ratio between the densities of the Brtnsted and of the Lewis acid sites (B/L) is very small (0.250.35). Quasi constant values of B/L have already been found by Mokaya et al. [15] for MCM-41 samples with different Si/A1 ratios. However, our values are smaller because of the higher desorption temperature (150°C instead of 100°C).
3.2. Xylene transformation With all the catalyst samples (MCM-41 and silica alumina) xylenes are transformed by isomerization and disproportionation. Toluene and trimethylbenzenes (in proportion close to that of thermodynamic equilibrium) are formed in quasi equimolar amounts. Fig. 5 shows the change with time-on-stream in the activity of silica alumina and of the MCM-41 samples for m-xylene transformation. In the experiments, the amount of catalyst was chosen so as to obtain quasi identical initial values of m-xylene conversion, which allows direct comparison of the catalyst stability: MCM-41 (100)>MCM-41 (30)>MCM-41 (10)~silica alumina. To obtain with a good accuracy the initial activities and to determine the selectivity of the fresh catalysts (with no coke deposit) the procedure described
S. Morin et al,/Applied Catalysis A: General 159 (1997) 317-331
-"-]"
MCM41 (10)
323
MCM41 (30)
0.051 L ]
A
H
L
°'-IA
B
8
LA.___
165o
16oo 1~o ~oo Wavenumbers (crn-1)
MCM41 (100)
145q J
I I
A. 8
1650
_
1600 1550 1500 Wavenumbers(era-.1)
Silica alumina
L
145C
L
I
5 1650
1600
1550
1500
Wavenumbers (cm-1)
145(
1650
1600
1550
Wavenumbers
1500 (era-l)
145
Fig, 3. IR spectra of pyridine adsorbed on MCM-41 samples with various bulk Si/A1 ratios and on silica alumina following desorption treatment at (1) 150°C (2) 250°C (3) 350°C (4) 450°C (5) 500°C. H denotes hydrogenbonded pyridine, B, Br~nsted-bound pyridine; L, Lewis-bound pyridine and I, iminium ions.
in [8] was applied. Short time-on-stream values were used and the catalysts were regenerated under dry air flow at 500°C for 4 h after each experiment. This regeneration treatment allows one to keep a constant activity value for at least 10 experiments. The analysis of the effluents was carried out for a long enough timeon-stream (15 to 65 s depending on the reactant flow rate) to obtain steady state conditions [8]. Whatever the xylene reactant the lower the Si/A1 ratio of the MCM-
324
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
Br6nsted acidity (micromoles g-l) 60
•
a
\
50
I" MoM,I10/
•\
• MCM41 (30) / " MCM41 (100)
~
40 "7,
O1
e
=o
30
E
20
10
0 100
200
300
400
500
Desorption Temperature (°C) Lewis acidity (micromoles g-l) 250 •
b
•MCM41(10) ] • MCM41(30) / x MCM41 (100) • Silica alumln~
\ \
2OO
N
150
•~
•
.~ 100
0
100
,
,
150
200
~
250
300
350
'
400
450
500
Desorption Temperature (°C) Fig. 4. Concentrations of Br6nsted (a) and Lewis (b) sites of MCM-4! and silica alumina samples on wbic| pyridine remains adsorbed as a function of the temperature of desorption treatment.
s. Morin et al./Applied Catalysis A: General 159 (1997) 317-331 1.2
325
a
' II MCM-41(lO) 1 • MCM-41 (30)
[XMCM-41(lO0) 0.6
x q
0
0
50
100
150
200
TOS (mn)
b 4
\
..C 0
E 3 E >
2 0
I-
1
0
50
1O0
150
200
T O S (mn)
Fig. 5. Activities of MCM-41 samples (a) and of silica alumina (b) for m-xylene transformation versus time-onstream TOS.
41 sample the greater the initial activity. Thus, in m-xylene transformation the initial activity of MCM-41 (10) is approximately 2 times greater than that of MCM-41 (30) and 4 times that of MCM-41 (100). However, MCM-41 (10) is approximately 4 times less active than silica alumina (Fig. 5). The selectivity of m-xylene transformation depends on the Si/AI ratio of the MCM-41 samples. The greater the Si/A1 ratio the lower the disproportionation/
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
326
Table 1 Selectivity of m-xylene transformation (values extrapolated at zero conversion) MCM-41
D/I p/o
(lo)
(30)
(lOO)
0.5 0.4
0.4 0.2
0.3 0.15
SA
HFAU (10)
0.2 1.4
0.8 1.1
1.2 1 0.8
O
0.6 0.4
• MCM41(30) 1 • SilicaAlumina
0.2 0
'r
0
-1-
5
10
15
%MCH
0.8
O
a 13
0.6 0.4 0.2 b 0
t
I
5
10
15
%MCH Fig. 6. Influence of methylcyclohexane on the rates of m-xylene isomerization (a) and disproportionation (b). I and Io: rates of isomerization in the presence and in the absence of methylcyclohexane; D and Do rates of disproportionation in the presence and in the absence of methylcyclohexane.
S. Morin et al./Applied Catalysis A: General 159 (1997) 317-331
327
isomerization (D/I) and the para/ortho xylene ratios (Table 1). With silica alumina the D//ratio is lower than that found for the MCM-41 samples. However, the para/ ortho ratio is higher. In order to discuss the eventual participation of disproportionation and transalkylation reactions in xylene isomerization over MCM-41 and silica alumina samples, the effect of the addition of methylcyclohexane to the reactant on the rate and on the selectivity of m-xylene transformation was determined. Indeed, it has been demonstrated that adding isoalkanes to xylenes allows the reduction of the disproportionation rate without affecting the unimolecular isomerization [8,21]. Fig. 6 shows that with MCM-41 (30) methylcyclohexane decreases both the rates of m-xylene isomerization (I) and disproportionation (D). Moreover, it has the same reducing effect (i) on those rates and (ii) on the rates of p- and o-xylene formation: no change can be observed in the D/I and p/o ratios. This is quite different from what is observed with silica alumina: larger reducing effect on the rate of disproportionation and no effect on the rate of isomerization (and on the p/o ratio) with consequently a large decrease in the D/I ratio.
4. Discussion
4.1. Acidic, properties In agreement with the literature, we show here that the acidity of the MCM-41 samples is similar to that of amorphous silica alumina. All these materials possess weak and middle strength Brrnsted acid sites and strong Lewis acid sites (able to retain pyridine adsorbed above 500°C); the ratios between the densities of the Brrnsted and of the Lewis acid sites (B/L) are quite comparable (0.25-0.35). These acidity characteristics are very different from those of HFAU zeolites [9]. Thus for a Si/AI ratio of 10, the Brrnsted acid sites density of a HFAU zeolite is 10 times greater than that of the MCM-41 sample, while the Lewis acid sites densities are similar. B/L is therefore much higher in HFAU (10) (about 3 times) than that in MCM-41 (10). Moreover, the protonic acid sites of HFAU (10) are stronger than those of HMCM-41 (10). Indeed they are able to retain pyridine adsorbed above 450-500°C while those of HMCM-41 (10) can retain pyridine adsorbed up to 300°C only. It should be emphasized that the protonic acid sites of silica alumina are slightly stronger than those of the MCM-41 samples (Fig. 4). With MCM-41 and silica alumina samples bands at 1462 and 1496 cm -1 appear at high desorption temperatures in the spectra of pyridine adsorbed. The band at 1462 cm-1 has also been observed with B-MCM-41 [22], the band at 1496 cm - l is also present (Fig. 6(A), [22]) but not shown. The origin of these bands remains debated. Thus, the band at 1462 cm-1 in B-MCM-41 has been assigned to strong Lewis acid sites generated by the polarization of hydroxyl nests and/or of electrophilic B atoms [22]. The
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S. Morin et aL /Applied Catalysis A: General 159 (1997) 317-331
bands at 1462 and 1496 cm -1 found with dealuminated mazzites have been attributed to iminium ions formed on strong Lewis/Br6nsted pairs of sites (superacid sites) [19]. In agreement with this latter proposal the corresponding bands remain present even after a desorption treatment at 500°C (Fig. 3). The change in acidity of MCM-41 samples with the bulk Si/A1 ratio is similar to the one found in the literature [15]: a decrease in the densities of Br6nsted and Lewis acid sites with increasing Si/A1 ratio, i.e. with a decrease in the amount of aluminium; the B/L ratio is practically independent of the Si/A1 ratio. According to Klinowski et al. [15], this indicates that the proportion of framework aluminium removed during the calcination treatment at 600°C does not change with the total amount of aluminium. However, as emphasized by one of the referees, this statement is quite questionable. Indeed the concentration of Lewis acid sites is not proportional to the amount of A1 removed; it also depends on the nature (cationic or neutral) and the dispersion of extra framework A1 (EFAL) species. Obviously as the Si/A1 ratio increases (hence the framework A1 density decreases) the dispersion of EFAL species should increase. The proportion of cationic EFAL species (which neutralize the Br6nsted acid sites) can also be different with the three MCM-41 samples. Furthermore, if it is assumed that the concentration of Br6nsted acid sites on which pyridine remains adsorbed above 150°C (Fig. 4) is proportional to the concentration of framework aluminium, the Alfrarnework/Altota I ratio values found for the MCM-41 samples are practically identical for MCM-41 (10) and (30) but 3 times greater for MCM-41 (100). Hence, as is the case with zeolites, the higher the Si/A1 ratio the greater the difficulty in the dealumination of the mesoporous aluminosilicates framework.
4.2. Catalytic properties In agreement with the higher strength of its protonic sites (which are the active ones [23]), silica alumina is more active than the MCM-41 samples for m-xylene transformation. Furthermore the greater the Si/A1 ratio of the MCM-41 samples the lower their activity. The activity of silica alumina and of MCM-41 samples is much lower than that of HFAU zeolites. Thus m-xylene is transformed 500 times more slowly over MCM-41 (10) than over HFAU (10). This can be related both to the lower density of the protonic acid sites of the MCM-41 samples and also to their lower strength. Similar observations have been made for the transformation of simple hydrocarbons [ 12,14-18]. Differences exist between the selectivities of silica alumina, MCM-41 and HFAU samples. Thus, the disproportionation/isomerization rate ratio (D/I) is lower with silica alumina and greater with HFAU zeolites than with the MCM-41 samples. With these samples the greater the Si/A1 ratio the lower D/I (Table 1). However, the main difference in selectivity concerns the p/o ratio, which is much lower with the MCM-41 samples than with silica alumina and HFAU zeolites
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(Table 1). The value of p/o found with silica alumina is slightly higher than that attributed to a purely intramolecular mechanism of m-xylene isomerization over HFAU zeolites (1.18). Values of 1.1 and 1.0 have been found with HFAU (10) and (30) indicating the participation in m-xylene isomerization of a bimolecular pathway in addition to the intramolecular one [8]. Furthermore from a simple model it has been estimated that the p/o selectivity of a bimolecular pathway (through reactions (2) and (3)) was equal to 0.275 [8]. As the values obtained with the MCM-41 samples were close to this value it can be proposed that over these mesoporous silicoaluminates m-xylene isomerizes mainly through the bimolecular pathway. In agreement with this proposal a direct isomerization of o-xylene into pxylene (and vice versa) can be observed. Indeed, an initial m-/p-xylene ratio of 12 is obtained with MCM-41 (30) while with the HFAU zeolite (HFAU (20)) for which 20% of the isomerization occurs through the bimolecular pathway, a value of 25 is found. With silica alumina the m/p ratio becomes very high at low conversion, which shows that, in agreement with a purely unimolecular isomerization mechanism, no direct transformation of o-xylene into p-xylene occurs. The existence of different mechanisms for xylene isomerization over silica alumina (and HFAU zeolites) on the one hand and over MCM-41 samples on the other is confirmed by the very different effect which methylcyclohexane has on the transformation of m-xylene on these catalysts. With silica alumina, methylcyclohexane decreases very much the rate of m-xylene disproportionation but does not change the rate of m-xylene isomerization. This confirms that over this catalyst mxylene isomerization does not involve disproportionation or transalkylation steps whose rates are affected by methylcyclohexane [8,21], thus occurs through the classical unimolecular pathway. With MCM-41 (30) methylcyclohexane decreases to the same extent both the rates of m-xylene disproportionation and isomerization. This decreasing effect of methylcyclohexane on the rate of xylene isomerization is a strong argument in favour of a bimolecular pathway involving successive disproportionation and transalkylation steps (reactions (2) and (3)). The identical effect of methylcyclohexane on the rates of m-xylene isomerization and disproportionation is in favour of a quasi pure bimolecular isomerization pathway. This particular behaviour of the MCM-41 samples cannot be attributed to the characteristics of their acidity: weak protonic acidity, presence of a large number of strong Lewis acid sites and maybe a very small number of superacid sites (evidenced by iminium ions in the spectrum of adsorbed pyridine). Indeed, with silica alumina which presents the same acidity characteristics, no bimolecular isomerization of xylene can be observed. This behaviour is most likely due to the presence of uniform non-interconnected pores in the mesopore range in MCM-41. The mesopores of approximately 30 ,~ in diameter would favour the bimolecular mechanism of xylene isomerization, xylene molecules undergoing successive reactions of disproportionation and transalkylation when passing through the mesopores (shape selectivity effect). With this bimolecular pathway, different values of D/I can be obtained. Indeed trimethylbenzenes (TMB) resulting from
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disproportionation (reaction (2)) can undergo during their passage through the noninterconnected channels, one or several successive reactions of transalkylation (known to be faster than disproportionation [8]) with production of xylene (X) isomers.
2 m-X
~
T +
m p
X (4)
m-X
Therefore the differences between the D/I values of the MCM-41 samples would be due to differences in the number of transalkylation steps undergone by trimethylbenzene molecules. Thus the increase in the D/I ratio with the decrease of the Si/AI ratio of the MCM-41 samples would be related to a decrease in the number of transalkylation steps. This decrease could be due to a decrease in the length of the channels caused by the degradation of the framework during the thermal treatment. Obviously, the lower the Si/A1 ratio, hence the greater the concentration of AI atoms, the more significant the probability of defects, hence the shorter the diffusion path of organic molecules before their desorption. Besides this direct effect of the aluminium concentration there is most likely an indirect effect due to the increase in the percentage of dealumination with the aluminium concentration which was found. It should be emphasized that the D/I value does not have the same meaning in the case of silica alumina and HFAU samples as in the case of MCM-41 samples. With the former catalysts, isomerization is only (silica alumina) or mainly (HFAU) monomolecular, hence D/I is the ratio between the rates of bimolecular and monomolecular reactions while in the case of the MCM-41 catalysts, D/I depends on the number of transalkylation steps involved in reaction (4).
5. Conclusion The acidity characteristics of MCM-41 samples and of an amorphous silica alumina are very similar in what concerns the densities of protonic and Lewis acid sites and their strength. In particular the protonic acid sites are weaker and are in smaller amount than the Lewis acid sites. The activities for m-xylene transformation and the disproportionation/isomerization rate ratios are not very different with MCM-41 samples and with silica alumina. However, there exists a large difference in the relative rates of formation of the para and of the ortho isomer: p/o is much lower with MCM-41 (0.15 to 0.4) than with silica alumina (1.4), which indicates that the isomerization mechanisms are different: classical intramolecular mechan-
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ism over silica alumina and bimolecular mechanism through successive disproportionation and transalkylation steps over MCM-41. The effect of adding methylcyclohexane to m-xylene on the rates of isomerization and disproportionation is the one expected from these reaction mechanisms: with MCM-41 an identical inhibiting effect for isomerization and disproportionation was found whereas with silica alumina there was an inhibiting effect for disproportionation but not for isomerization. The particular selectivity of MCM-41 samples is most likely due to the presence of regular non-interconnected long channels in which xylene molecules undergo, before desorption, successive reactions of disproportionation and transalkylation. This shape selectivity cannot be observed in the narrow pores of unidirectional zeolites, such as mordenites, because of steric constraints limiting the formation of the bulky intermediates of disproportionation and transalkylation reactions.
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